ELECTROCHEMISTRY AND SURFACE CHEMISTRY OF
SELF-ASSEMBLED MONOLAYERS
by
Laura Bridget Goetting
Bachelor of Science (Honors)
Simon Fraser University, 1993
Submitted to the Department of Chemistry in Partial
Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 1999
© 1999 Massachusetts Institute of Technology. All rights reserved.
Signature of Author:
Department of Chemistry
November 24, 1998
Certified by:
i
I
Mark S. Wrighton
Thesis Supervisor
Certified by:
George M. Whitesides
Thesis Supervisor
Accepted by:
LIRI
L113RARIES
Dietmar Seyferth
talSCommittee on Graduate Students
2
This doctoral thesis has been examined by a committee of the Department of Chemistry as
follows:
Professor Timothy M. Swager
(U
Chairman
Professor Mark S. Wrighton
Thesis Supervisor
Professor George M. Whitesides
,
Thesis Supervisor
Professor Dietmar Seyferth
3
ELECTROCHEMISTRY AND SURFACE CHEMISTRY OF
SELF-ASSEMBLED MONOLAYERS
by
Laura Bridget Goetting
Submitted to the Department of Chemistry on November 24, 1998
in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy in Chemistry
ABSTRACT
Chapter 1
Exposure of a Pt or Au electrode to a solution of 1 1-ferrocenylundecyldiphenylphosphine, I, or 10-(ferrocenylcarbonyl)-decyl-diphenylphosphine, II, results in the
formation of a reversibly electroactive self-assembled monolayer (SAM) on the metal
surface. Kinetic desorption studies of SAMs of I or II on Pt into octane or SAMs of I into a
solution of 11 (1 mM in octane) were performed over the temperature range 85-115 'C. The
desorption of I from the Pt surface into octane was independent of the presence or absence of
II in solution, consistent with a first order dissociative mechanism. For the desorption of I
from the Pt surface into octane, AG298 = 98±11 kJ mo- 1 and for the desorption into a solution
of II in octane, AG298 = 98±3 kJ mol 1 . Kinetic desorption studies of SAMs of I on Au into
octane or a solution of 11 (1 mM in octane) were performed over the temperature range
115-125 'C. The desorption of I from the Au surface into octane was dependent on the
presence or absence of II in solution. For the desorption of I from the Au surface into
octane, AG298 = 117±18 kJ morl and for the desorption into a solution of II in octane,
AG298 = 96.4+8.3 kJ mol'.
Chapter 2
Microcontact printing, (gCP) has been used to pattern octadecanephosphonic acid on
the native oxide surface film of aluminum supported on silicon or on silicon nitride coated
silicon wafers. The patterned octadecanephosphonic acid protects the A12 0 3/Al film from
etching in a solution containing phosphoric, acetic, nitric acids and water in a ratio of
16:1:1:2, and allows the non-patterned film to be removed selectively. The patterned
A12 0 3/Al structures resulting from etching are continuous and electrically conductive within
each pattern, and separated patterns are electrically isolated. Resistance measurements of the
patterned structures are presented. Using gCP, Schottky diodes of aluminum have been
prepared on p-type Si (100). The Schottky diodes exhibit rectifying behavior; the forward
and reverse bias current-voltage (I-V) and reverse bias capacitance-voltage (C-V)
characteristics are presented.
4
Chapter 3
1-Alkanephosphonic acids, H3C(CH2)nPO(OH)2, (n=7, 9, 11, 13, 15, 17, 19, 21)
adsorb on the native oxide of aluminum to form ordered, oriented self-assembled monolayers
(SAMs). The SAMs were characterized by measurement of contact angles, by ellipsometry,
by X-ray photoelectron spectroscopy (XPS) and by polarized infrared external reflection
spectroscopy (PIERS). The monolayers expose a low-energy, methyl-terminated surface, as
determined from contact angle measurements. The XPS data shows there is a linear
relationship between thickness and the number of carbon atoms in the alkane chain. The
PIERS data indicates that the shorter chain molecules (n<13) form less ordered monolayers
than the longer chain molecules. The alkane chains are canted, with respect to the surface
normal, more than 130 but less than 300. 16-Phosphonohexadecanoic adsorbs on the native
oxide of aluminum to form SAMs that are bound predominantly through the phosphonic acid
head group, exposing a carboxylic acid terminus. Treatment of the carboxylic acid
terminated SAM with trifluoroacetic anhydride results in the formation of a mixture of
anhydrides on the surface, primarily, interchain carboxylic anhydride with a small amount of
mixed trifluoroacetic carboxylic anhydride. The anhydride surface reacts with alkylamines
to form a mixed SAM containing carboxylic acid and amide functionalities. The carboxylic
acid and anhydride terminated SAMs and the mixed SAMs were characterized by XPS and
PIERS.
Thesis Supervisor: Mark S. Wrighton
Title: Chancellor, Washington University
Thesis Supervisor: George M. Whitesides
Title: Mallinckrodt Professor of Chemistry, Harvard University
5
To my best friend and loving husband, Ben
6
One night I dreamed a dream.
I was walking along the beach with my Lord.
Across the dark sky flashed scenes from my life.
For each scene, I noticed two sets of footprints in the sand,
one belonging to me and one to my Lord.
When the last scene of my life shot before me
I looked back at the footprints in the sand.
There was only one set of footprints.
I realized that this was at the lowest and saddest times of my life.
This always bothered me and I questioned the Lord
about my dilemma.
"Lord, you told me when I decided to follow You,
You would walk and talk with me all the way.
But I'm aware that during the most troublesome
times of my life there is only one set of footprints.
I just don't understand why, when I needed You most,
You leave me."
He whispered, "my precious child,
I love you and will never leave you
never, ever, during your trials and testings.
When you saw only one set of footprints
it was then that I carried you."
@1964 Margaret Fishback Powers
7
Acknowledgements
It is with great appreciation and respect that I acknowledge my supervisors,
Mark S. Wrighton and George M. Whitesides. I feel very fortunate to have had the
opportunity to work in both of their labs while still in graduate school. Without their support,
none of the work in this thesis would have been possible.. .thank you!
I would like to thank Paul Laibinis for many helpful and stimulating conversations,
and for his generosity in allowing me to use his FTIR. I am grateful to Hanno zur Loye and
Alan Davison for their friendship and guidance during my time at MIT.
I would like to acknowledge all of the members of the Wrighton group that I
overlapped with, and some of those who I didn't overlap with, but told me tales of the good
times when they were members. I would like to acknowledge all of the members of the
Whitesides group that I have overlapped with, in particular, Ned Bowden, Joe Tien,
Francisco Arias, Bob Chapman, and Rainer Haag for their friendship and helpfulness.
Special thanks to Tracie Smart and Rich Ragin, they are the mainstay of the Whitesides
group and they make sure everything runs smoothly so we can concentrate on our work. I
owe many thanks to the staff at the MTL, especially, Paul Tierney, Pat Burkhart, Bernard
Alamariu, Debb Hodges, Vicky Diadiuk, Joe Walsh, Kurt Broderick, and Linus Cordes. I
would like to thank Libby Shaw (MIT) and Yuan Lu (Harvard) for help with surface
analysis. I would like to thank the Laibinis group, especially Kane Jennings and Mark
Angelino, for letting me into the lab whenever I needed to collect spectra. I also want to
acknowledge Bill Gates, I cursed him every time my computer crashed, but I could not
imagine writing-up without the benefits of silicon.
I am very grateful to my parents, Maureen and Hugh, for their complete support in
this venture, even though they had no idea why their daughter avoided the "real" world for so
long, for their constant love, for the great upbringing and for the sense of values they have
instilled in me. I want to thank my three older brothers, Bill, Dennis and Leo, for their love
and support, for making sure I keep a healthy sense of reality, and for teaching me how to
take care of myself. I want to thank my family-in-law, for welcoming me into the family and
for their love and support. Most importantly of all, I thank my husband Ben for his love,
guidance, support and help, but most of all, for being there when I needed him and for being
my best friend.
Finally, I want to thank the Lord for carrying me in the tough times.
8
Table of Contents
AB STR A CT .......................................................................-
.. ...--------------........................ 3
DED IC ATIO N ...............................................................................
.............................--.
Q U OT AT IO N ....................................................................................--
5
... .......... ............... 6
ACKNOWLEDGEMENTS....................................................................................7
.. . ..8
TABLE OF CONTENTS .....................................................................................
10
LIST O F FIGU RES ..........................................................................................................
. - ........ 16
LIST OF SCHEM ES .................................................................................
17
LIST O F TA B LES ............................................................................................................
CHAPTER 1. KINETIC DESORPTION AND EXCHANGE STUDIES OF
SELF-ASSEMBLED MONOLAYERS OF FERROCENE
TERMINATED ALKYLDIPHENYLPHOSPHINES ON PLATINUM
18
AND GOLD SURFACES .........................................................................
Introduction ..........................................................................................
.. .... 19
E xperim ental............................................................................................................20
Results and D iscussion..........................................................................................
C onclusions ..........................................................................................
References ................................................................................
23
....... 57
...... .................... 59
9
CHAPTER 2. MICROCONTACT PRINTING OF OCTADECANEPHOSPHONIC
ACID ON A120 3/A1: PATTERN TRANSFER BY WET CHEMICAL
ETCHING...................................................................62
Introduction ............................................................
Experimental.........................................................
63
............
.-
-
-
.
77
-
Results and Discussion........................................................83
..........
.... .....
Conclusions ............................................................
110
-...........
References ......................................................
109
CHAPTER 3. SELF-ASSEMBLED MONOLAYERS OF METHYL AND
CARBOXYLIC ACID TERMINATED ALKANEPHOSPHONIC
ACIDS ON A12 0 3/Al: FORMATION OF DERIVATIVES AT THE
115
CARBOXYLIC ACID TERMINUS ..........................................................
..... 116
Introduction ..................................................................
Experimental...............................................................................120
Results and D iscussion.........................................................................126
Conclusions ...........................................................
References .....................................--.....-
.
.
.
.......
........................
...178
179
10
LIST OF FIGURES
Number
Page
CHAPTER 1
1.1
1.2
1.3
1.4
1.5
Cyclic voltammograms of 0.25 cm 2 Pt electrodes derivatized with SAMs of I,
followed by the placement of the electrodes into octane at 95 'C for various
intervals. The voltammetry was performed under Ar at a scan rate of
500 mV s-. The center of the crossed lines in each voltammogram
corresponds to zero voltage, zero current. a) The initial SAM formed by
derivatization of the Pt electrode in a 1 mM solution of I in hexane at room
temperature, for -15 hours. b) 5 minutes in octane at 95 'C. c) 20 minutes in
octane at 95 'C. d) 45 minutes in octane at 95 'C. e) 90 minutes in octane at
9 5 C . ......................................................................................................................
26
Plot of coverage (x10 10 mol cm 2 ) of SAMs of I on Pt as a function of time in
octane at 95 'C. The inset is the corresponding plot of the natural logarithm
of coverage as a function of time and the solid line is the least squares fit of
the data. The rate constant corresponding to the least squares fit of the data is
(6.2±0.3)x 10 4 s-1. ....................................................................................................
28
Arrhenius plot of the natural logarithm of the rate constant for the desorption
of SAMs of I or II from a Pt surface, into octane, versus the reciprocal of
absolute temperature. The solid line is the least squares fit of the data. The
slope and y-intercept, corresponding to the least squares fit of the data are
-5460±520 K 1 and 7.4±1.4, respectively..............................................................
30
Cyclic voltammograms of 0.25 cm 2 Pt electrodes derivatized with SAMs of I,
followed by the placement of the electrodes into a solution of II (1 mM in
octane) at 95 'C for various intervals. The voltammetry was performed under
Ar at a scan rate of 500 mV s- . The center of the crossed lines in each
voltammogram corresponds to zero voltage, zero current. a) The initial SAM
formed by derivatization of the Pt electrode in a 1 mM solution of I in hexane
at room temperature, for -15 hours. b) 5 minutes in a solution of II (1 mM in
octane) at 95 'C. c) 15 minutes in a solution of II (1 mM in octane) at 95 'C.
d) 45 minutes in a solution of II (1 mM in octane) at 95 'C. e) 90 minutes in a
solution of II (1 mM in octane) at 95 C. .................................................................
33
Plot of coverage (x10
10
mol cm 2 ) of SAMs of I (filled squares) and II (open
squares) on Pt as a function of time in a solution of 11 (1 mM in octane) at
95 'C. The inset is the corresponding plot of the natural logarithm of
coverage of I, as a function of time, and the solid line is the least squares fit of
the data. The rate constant corresponding to the least squares fit of the data is
(6.4±0.3)x 104 s-1. .................................................................................................
35
11
Page
Number
1.6
1.7
Arrhenius plot of the natural logarithm of the rate constant for the desorption
of SAMs of I from a Pt surface, into a solution of II (1 mM in octane), versus
the reciprocal of absolute temperature. The solid line is the least squares fit of
the data. The slope and y-intercept, corresponding to the least squares fit of
the data are -5660±170 K' and 8.0±0.5, respectively. ..........................................
Cyclic voltammograms of 0.25 cm 2 Au electrodes derivatized with SAMs of
I, followed by the placement of the electrodes into octane at 115 'C for
various intervals. The voltammetry was performed under Ar at a scan rate of
500 mV s-. The center of the crossed lines in each voltammogram
corresponds to zero voltage, zero current. a) The initial SAM formed by
derivatization of the Au electrode in a 1 mM solution of I in hexane at room
temperature, for ~15 hours. b) 45 minutes in octane at 115 'C. c) 120
minutes in octane at 115 'C. d) 195 minutes in octane at 115 'C. e) 240
minutes in octane at 115 C .................................................................................
1.8
38
43
Plot of coverage (x1lO mol cm2) of SAMs of I on Au as a function of time in
octane at 115 'C. The inset is the corresponding plot of the natural logarithm
of coverage as a function of time and the solid line is the least squares fit of
the data. The rate constant corresponding to the least squares fit of the data is
(10 .0± 0 .5)x 10 5
1.9
1.10
1.11
s-.
....................................................
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Arrhenius plot of the natural logarithm of the rate constant for the desorption
of SAMs of I from a Au surface, into octane, versus the reciprocal of absolute
temperature. The solid line is the least squares fit of the data. The slope and
y-intercept, corresponding to the least squares fit of the data are
-12300±2100 K I and 22.7±5.4,respectively. .......................................................
47
Cyclic voltammograms of 0.25 cm2 Au electrodes derivatized with SAMs of
I, followed by the placement of the electrodes into a solution of II (1 mM in
octane) at 115 'C for various intervals. The voltammetry was performed
under Ar at a scan rate of 500 mV s-. The center of the crossed lines in each
voltammogram corresponds to zero voltage, zero current. a) The initial SAM
formed by derivatization of the Au electrode in a 1 mM solution of I in
hexane at room temperature, for ~15 hours. b) 5 minutes in a solution of II
(1 mM in octane) at 115 'C. c) 45 minutes in a solution of 11 (1 mM in
octane) at 115 'C. d) 195 minutes in a solution of II (1 mM in octane) at
115 'C. e) 240 minutes in a solution of II (1 mM in octane) at 115 C. .........
50
Plot of coverage (x10 10 mol cm-2 ) of SAMs of I (filled squares) and 1I (open
squares) on Au as a function of time in a solution of II (1 mM in octane) at
115 'C. The inset is the corresponding plot of the natural logarithm of
coverage of I, as a function of time, and the solid line is the least squares fit of
the data from 20 to 240 minutes. The rate constant corresponding to the least
squares fit of the data is (6.0±0.4)x10-5 s .............................................................
52
12
Page
Number
1.12
Arrhenius plot of the natural logarithm of the rate constant for the desorption
of SAMs of I from a Au surface, into a solution of II (1 mM in octane), versus
the reciprocal of absolute temperature. The solid line is the least squares fit of
the data. The slope and y-intercept, corresponding to the least squares fit of
the data are -870±260 K' and -7.5±0.7, respectively...........................................55
CHAPTER 2
2.1
Schematic representation of the gCP process. .......................................................
64
2.2
Mask design used for the resistance measurements. The wires were equally
spaced in the mask used; the different contrast that appears in the figure is a
printer artifact. .........................................................................................................
69
Mask design used to prepare Schottky diodes. The solid black features
correspond to the mask used for the ohmic contacts and the white features
outlined in black correspond to the mask used for the Schottky contacts. ............
72
2.3
2.4
Schematic representation of the process used to fabricate the Schottky diodes..........74
2.5
SEM images of octadecanephosphonic acid patterned by pCP on aluminum:
a) As stamped and b) After rinsing the surface with 2-propanol to remove the
excess octadecanephosphonic acid .......................................................................
2.6
a) AFM image of the stamp used to prepare structures for the resistance
measurements. b) AFM image of 300 nm thick aluminum, patterned by gCP
of octadecanephosphonic acid, followed by baking for 10 minutes at 70 'C
and wet-etching in Aluminum Etchant Type "A" at 35 'C....................................89
2.7
a) SEM image of the edge resolution of patterned aluminum wires prepared by
photolithography and lift-off. b) SEM image of the edge resolution of
patterned aluminum wires prepared by ptCP and wet-etching. We attribute the
bright edge of the wet-etched sample to charging (the substrate in both cases
was a silicon nitride coated silicon wafer, yet we observed more charging in
the wet-etched sample). We have defined the dark circular features in the
wet-etched image as "pinhole" defects (see text).................................................92
2.8
Plot of resistance versus length for 6 pim wide aluminum wires, patterned by
stamping, followed by baking and wet-etching (filled squares), compared to
the same wire patterned by photolithography (open squares). The solid line is
the least squares fit of the wet-etch data and the dashed line is the least
squares fit of the lift-off data................................................................................
86
95
13
Page
Number
2.9
Optical micrographs of the fabricated diodes. a) Optical micrograph of an
array of 32 diodes. b) Optical micrograph of a single diode with a 3 ptm gap
between the ohmic and Schottky contacts. ...........................................................
98
2.10
Current-voltage (I-V) characteristics of a fabricated Schottky diode. The
forward and reverse I-V scans were collected separately, with different current
scales. The band diagrams for the metal/semiconductor junction at
equilibrium, for reverse bias and for forward bias are shown at the top of the
figure, corresponding to the points A, B and C, respectively, on the I-V curve. ...... 101
2.11
Plot of the natural log of current density, ln(Jf), versus the forward bias
voltage, Vf, from 150 mV to 500 mV, for a fabricated Schottky diode
(solid line) and the associated linear fit over the range 150 mV to 350 mV,
extrapolated to V f=0 (dashed line)..........................................................................104
2.12
Capacitance-voltage (C-V) characteristics of a fabricated Schottky diode. The
upper trace corresponds to a plot of 1/(C-C0 )2 versus the reverse bias voltage,
Vr, and the associated linear fit (solid line) extrapolated to the x-axis intercept,
2
where C0= 17.0 nF cm-2. The lower trace corresponds to a plot of 1/C versus
the reverse bias voltage, Vr. ...................................................................................
107
CHAPTER 3
3.1
a) A typical contact angle for hexadecane on a methyl terminated surface,
where 0=45-46' and b) A typical contact angle for water on a methyl
term inated surface, where 0=111-115...................................................................127
3.2
Plot of the advancing (filled diamonds) and receding (open diamonds) contact
angle of water and the advancing (filled squares) and receding (open squares)
contact angle of hexadecane versus the number of methylene carbons, n, for a
series of SAMs of 1--alkanephosphonic acids formed on A12 0 3/Al. Each point
is the average of the measurement on the left and right side of at least three
drops of liquid on each surface and the error bars represent the standard
deviation associated with the average.....................................................................130
3.3
Plot of ellipsometric thickness versus the number of methylene carbons, n, for
a series of SAMs of 1-alkanephosphonic acids formed on A12 O3/Al. The
dashed line is an estimate of the thickness for an all-transalkane chain,
oriented norm al to the surface. ...............................................................................
134
Plot of (cos55)[(1+IC(m)/lP(m))/(1 +IC(m=12)IP(m=12))] versus the number of
carbons in the alkane chain, m, for a series of SAMs of 1-alkanephosphonic
acids formed on A12 0 3/Al. The solid line is the least squares linear fit of the
data. The slope of the linear fit is 0.062±0.003. .....................................................
139
3.4
14
Number
Page
3.5
Schematic illustration of the cant angle, (X,and chain twist angle, P, for SAMs
of 1-hexadecanephosphonic acid adsorbed on A12 0 3/Al, oriented in an
all-transconfiguration............................................................................................142
3.6
PIERS spectra of the high energy region (2700-3100 cm') for a series of
SAMs of 1-alkanephosphonic acids formed on A12 0 3/Al........................................145
3.7
Plot of the peak intensities (arbitrary units) of the symmetric (open squares)
and antisymmetric (filled squares) methyl stretches versus the number of
methylene carbons, n, for a series of SAMs of 1-alkanephosphonic acids
form ed on A 12 0 3/A l. ..............................................................................................
148
Plot of the peak intensities (arbitrary units) of the symmetric (open squares)
and antisymmetric (filled squares) methylene stretches versus the number of
methylene carbons, n, for SAMs of 1-alkanephosphonic acids formed on
A12 0 3 /A l. ...............................................................................................................
15 1
a) i) PIERS spectrum of the high energy region for a SAM of
16-phosphonohexadecanoic acid formed on A12 0 3/Al ii) IR spectrum-of the
high energy region for 16-phosphonohexadecanoic acid dispersed in a KBr
pellet. b) i) PIERS spectrum of the low energy region for a SAM of
16-phosphonohexadecanoic acid formed on A12 0 3/Al ii) IR spectrum of the
low energy region for 16-phosphonohexadecanoic acid dispersed in a KBr
pellet. The positions of the marked peaks are given in Table 3.3. ..........................
159
3.8
3.9
3.10
XPS survey spectrum of 16-phosphonohexadecanoic acid dispersed in a KBr
pellet (center), high resolution core level spectrum of the C(is) region
(left inset), and high resolution core level spectrum of the P(2p) region
(right inset)............................................................................................................164
3.11
XPS survey spectrum of SAMs of 16-phosphonohexadecanoic acid formed on
A12 0 3/Al (center), high resolution core level spectrum of the C(1s) region
(left inset), and high resolution core level spectrum of the P(2p) region
(rig ht in set)............................................................................................................167
3.12
Comparison of the XPS survey spectra of SAMs of: a) carboxylic acid
terminated SAMs of 16-phosphonohexadecanoic acid, b) a mixture of
interchain and mixed anhydrides formed by the reaction of the SAMs of
16-phosphonohexadecanoic acid with trifluoroacetic anhydride and c) mixed
SAMs terminated in carboxylic acid and hexadecylamide functionalities
formed by the reaction of hexadecylamine with the anhydride terminated
SA M s on A12 0 3/A l.................................................................................................172
15
Number
3.13
Comparison of the PIERS spectra of SAMs of: i) carboxylic acid terminated
SAMs of 16-phosphonohexadecanoic acid, ii) the mixture of interchain and
mixed anhydrides formed by the reaction of the SAMs of
16-phosphonohexadecanoic acid with trifluoroacetic anhydride and iii) the
mixed SAMs terminated in carboxylic acid and hexadecylamide
functionalities formed by the reaction of hexadecylamine with the anhydride
terminated SAMs on A120 3/Al. a) PIERS spectra of the high energy region
(2700-3 100 cm-1) showing the methylene (and methyl for the mixed SAMs)
stretching region and b) PIERS spectra of the C=O stretching region
(1500-1900 cm ')...................................................................................................175
Page
16
LIST OF SCHEMES
Page
Number
CHAPTER 3
3.1
Synthetic route for the formation of alkanephosphonic acids..................................117
3.2
Possible binding configurations for the SAMs of 16-phosphonohexadecanoic
acid formed on A12 0 3/Al films supported on Si. a) Binding to the surface
through the phosphonic acid head group. b) Binding to the surface through
the carboxylic acid head group. c) Binding to the surface by both the
phosphonic acid and carboxylic acid head groups on different molecules.
d) Binding to the surface by both the carboxylic acid and phosphonic acid
head groups on the same molecule. ........................................................................
156
Schematic representation for the formation of SAMs terminated in a mixture
of anhydrides and the reaction of the anhydride terminated SAM with
alkylamines to form a mixed SAM containing carboxylic acid and amide
functionalities. ........................................................--...........
..... 169
3.3
17
LIST OF TABLES
Page
Number
CHAPTER 3
3.1
Peak positions (cm-1) for the symmetric and antisymmetric methyl and
methylene vibrational modes for SAMs of H3C(CH 2)nPO(OH) 2 formed on
A12 0 3/Al and for comparison, SAMs of CH 3 (CH 2 )15SH formed on Au...................147
3.2
Peak positions (cm-I) for the CH 2 wagging and twisting progressional bands
for 1-alkanephosphonic acids adsorbed on A12 0 3/Al. .............................................
3.3
154
Peak positions (cm-1) for the vibrational modes of 16-phosphonohexadecanoic
acid dispersed in a KBr pellet and adsorbed as SAMs on A12 0 3/Al.........................161
18
CHAPTER 1
KINETIC DESORPTION AND EXCHANGE STUDIES OF SELF-ASSEMBLED
MONOLAYERS OF FERROCENE TERMINATED ALKYLDIPHENYLPHOSPHINES
ON PLATINUM AND GOLD SURFACES
19
Introduction
The ability to derivatize metal surfaces with self-assembled monolayers (SAMs) is an
area of active study.1-5 Considerable effort has been devoted to understanding how certain
molecules bind to surfaces, there is, however, relatively little known regarding the exchange
chemistry of neutral molecules bound to surfaces. 6 -10 This chapter will investigate the
desorption and exchange of SAMs bound through phosphine on Pt and Au surfaces,
analogous to the substitution chemistry of discrete metal complexes. Exposure of a Pt or Au
electrode to a solution of 1 1-ferrocenylundecyl-diphenylphosphine, I, or
10-(ferrocenylcarbonyl)-decyl-diphenylphosphine, II, results in the formation of a reversibly
electroactive SAM on the metal surface. 1 1
00
Fe
Fe
I
II
The different functional groups pendant to the ferrocene result in E
2
values for I and
II that are separated by 290 mV. Thus, rapid, quantitative assessment of the surface
coverage of the two electroactive molecules, on the same electrode, can be ascertained using
electrochemical measurements.
Pt complexes are commonly in oxidation states 0, II or IV. The zero oxidation state is
usually stabilized by phosphine, arsine or isocyanide ligands. 12 The chemistry of zerovalent
metal phosphine complexes, MLn, M=Ni(0), Pd(0) or Pt(0), L=phosphine ligand has been an
20
area of considerable study. 12-15 We will compare the desorption and exchange of SAMs of
I or II on Pt surfaces to the solution chemistry of the metal complex,
tetrakis(methyldiphenylphoshine)platinum(0).
Au complexes are most commonly in oxidation states I or III. Zerovalent gold forms
7c
complexes with CO, C 2 H 4 , and C6H6 , but Au(C 2 H 2 ), formed from Au atoms and acetylene,
can exist as a a-bonded radical species. 16 We will compare the desorption and exchange of
the neutral, phosphine bound SAMs on Au to the analogous studies on Pt.
Experimental
Materials. CH 3CN (anhydrous, Aldrich), 1 1-bromoundecanoic acid (Aldrich),
CH 2 Cl 2 (anhydrous, Aldrich), THF (Aldrich), Silica Gel (Merck grade 9385, 230-400 mesh,
Aldrich) H2 0 (Omnisolve, EM Science), AlCl 3 (99.999%, Alfa), SOC12 (Alfa),
diphenylphosphine (Alfa), Zn powder (99.999%, -100+200 mesh, Alfa), HgCl 2 (99.9995%,
Alfa), n-BuLi (2.16 M in hexanes, Aldrich), octane (Aldrich), hexanes (Mallinckrodt) and
EtOH (Pharmco, 200 proof) were used as received. Ferrocene (Aldrich) was sublimed prior
to use. [n-Bu 4 N]PF6 (Aldrich) was recrystallized three times from EtOH, dried in vacuo at
150 'C overnight, and stored under N2 in a dry box. The wafers used to support the
electrodes (Wafernet, Inc.) were test grade 4" diameter p-type Si(100), modified first with
500 nm of thermally grown Si0 2 followed by 150 nm of LPCVD Si 3 N4 .
11 -bromoundecanoylferrocene, and 11 -bromoundecylferrocene were prepared according to
previously published procedures. 17
11-Ferrocenylundecyl-diphenylphosphine, I. Diphenylphosphine (1.862 g,
10 mmol) was dissolved in THF (20 mL). The solution was cooled in a dry ice/acetone bath,
21
under Ar, with stirring. n-BuLi (4.5 mL, 10 mmol) was added dropwise with a syringe to the
cooled stirred solution. The solution was stirred an additional twenty minutes in the cooling
bath and then the cooling bath was removed and the solution was stirred for 60 minutes at
room temperature. The solution was cooled in the dry ice/acetone bath again and
1 1-bromoundecylferrocene (3.0 g, 6.9 mmol) in THF (10 mL) was added dropwise, via
syringe, to the stirred, cooled solution. The solution was stirred for 30 minutes in the cooling
bath and then 90 minutes at room temperature. The reaction was quenched with water
(100 mL), washed with NH 4 CI, and dried over MgSO 4 . The crude product was purified by
column chromatography on silica gel with ethyl acetate:hexanes (1:9) as the eluant. The
light orange oil was recrystallized from heptane at -33 'C to give a yellow solid. 1H NMR
(300 MHz, CD 2Cl 2) 8, ppm: 7.55-7.31 (m, 10 H), 4.07 (s, 5 H), 4.04 (t, 2 H), 4.01 (t, 2 H),
2.31 (t, 2 H), 2.04 (t, 2 H), 1.46-1.20, (m, 18 H).
31P
NMR (300 MHz, CD 30D) 6, ppm:
14.61 (relative to H3PO 4 , external standard). Mass Spectrometry (EI) calcd. (found) for
C 3 3 H4 1FeP (M+) 524.2295 (524.2297). Anal. calcd. (found) for C3 3H4 1FeOP:
C 75.60 (75.38); H 7.97 (7.86).
10-(Ferrocenylcarbonyl)-decyl-diphenylphosphine, II. This compound was
prepared from 11 -bromoundecanoylferrocene in a procedure analogous to that used to
prepare I. The orange oil was recrystallized from heptane at -33 'C to give an orange solid.
'H NMR (300 MHz, CD 2 Cl 2 ) 6, ppm: 7.41-7.30 (m, 10 H), 4.74 (t, 2 H), 4.48 (t, 2 H),
4.18 (s, 5 H), 2.68 (t, 2 H), 2.03 (t, 2H), 1.42-1.27 (m, 16 H).
31
P NMR (300 MHz, CDCl 3)
8, ppm: 33.84 (relative to H3 PO 4 , external standard). Mass Spectrometry (EI) calcd. (found)
for C3 3 H3 9FeOP (M+) 538.2088 (538.2084). Anal. Calcd. (found) for C33H 39FeOP:
C 73.60 (73.23); H 7.30 (7.28).
22
Fabrication of the electrodes. The electrodes consisted of two square pads, 0.5 cm
per side, separated by 0.5 cm, connected by three wires, 100 .1m wide and 0.5 cm long. This
electrode design was chosen in order to simultaneously prepare a series of electrodes with the
same geometrical area. The electrodes were fabricated using standard lithographic
techniques, producing electrodes with a macroscopic surface area of 0.25 cm 2 , of either Pt or
Au, on Si 3N4 coated Si wafers. Au electrodes were prepared by thermal evaporation of Au
(2000 A, 5 A s-) onto an adhesion layer of Cr (100 A, 2 A s-), in an Edwards Auto 306
thermal evaporator, under high vacuum (10-7 torr). Pt electrodes were prepared by electronbeam evaporation of Pt (2000
A, 5 A s-)
onto an adhesion layer of Ti (100
A, 2 A s-), in an
Temescal Semiconductor Products VES 2550 electron beam evaporator, under high
vacuum (10- torr). The electrodes were used immediately following their preparation.
Formation of SAMs. Adsorption of the surface-confined phosphines was
accomplished by immersion of the electrodes into the appropriate phosphine containing
solutions for ~15 hours. The modified surfaces were rinsed with the derivatizing solvent
prior to characterization by cyclic voltammetry. The dissociation or exchange did not depend
on the molecule forming the SAM and for convenience, I was typically used to form the
initial SAM.
Electrochemical methods. Electrochemical measurements were performed with a
Pine Instruments Model AFCBP1 computer controlled bipotentiostat, using PineChem
software, version 2.00. Linear sweep cyclic voltammetry of the functionalized electrodes
was performed under Ar in 0.1 M [n-Bu 4N]PF 6 in CH 3CN, at 298 K. Contact was made to
one of the square pads of the electrode with a flat clip, wrapped with gold foil. The counter
electrode was Pt mesh and the reference electrode was Ag/AgNO
3
(0.01M in CH 3CN).
23
Coverage measurements were determined from the average of the integrated charge from the
anodic and cathodic currents, for the ferrocene/ferricenium redox couple, after a background
subtraction to correct for charging. The coverage was determined from voltammograms
collected at a scan rate of 500 mV s-1. The cyclic voltammetry of II showed that it contained
a small impurity of I (2.2 %). In the exchange studies, the impurity of I in II was corrected
for by subtraction of 2.2% of the charge due to II from the charge due to I.
Ligand desorption and exchange experiments. Monolayers used in the ligand
desorption and exchange experiments were prepared by immersion of approximately 30 bare
Au or Pt electrodes into a 1 mM solution of I in octane for -15 hours. Electrodes were
removed from the derivatizing solution, rinsed with hexane, and cyclic voltammetry was
performed to determine the initial monolayer coverage. This process was repeated with at
least two electrodes to show that the coverage obtained was consistent. The remaining
electrodes were placed into individual culture tubes containing either octane or a solution of
1 mM of II in octane. The solutions in the culture tubes were equilibrated in a Brinkman
RC6 constant temperature silicone oil bath. The electrodes were then removed at various
time intervals, rinsed with hexane and characterized by cyclic voltammetry.
Results and Discussion
Electrochemical characterization of SAMs on Pt. The SAMs formed from I or II
on Pt show reversible, symmetrical cyclic voltammetric peaks with a full-width at half
maximum,
AEfwhm,
of -130 mV, at a scan rate of 500 mV s-.
The peak current was linearly
proportional to the potential scan rate, consistent with surface confined redox centers. 18 The
values of E%/ for the alkylferrocenyl centers of I are 290 mV more negative than
24
acylferrocenyl centers of II. The surface coverage, F, was determined from the average of
the integrated charge of the anodic and cathodic current for the ferrocene/ferricenium redox
couple,
Q, using
Equation 1.1, where n is the number of electrons in the redox couple, (n= 1),
F is the Faraday constant (96485 C mol') and A is the geometrical area of the electrode
(0.25 cm2). The initial coverage for SAMs of I or lIon Pt was (1.1±0.1)xO-10 mol cm.
F=
nFA
(1.1)
This coverage corresponds to approximately one surface bound molecule of I or II, per 25 Pt
surface atoms. This coverage is approximately 75% lower than the surface coverage of
(4±1)x10'
0
mol cm-2 for SAMs of 12-ferrocenyldodecyl isocyanide on Pt. 19
Desorption and exchange studies on Pt. A series of electrodes derivatized with
SAMs of I or II were prepared by immersion of the electrodes, supported on Si 3N4 coated Si
wafers, into a 1 mM solution of I or II in hexane for approximately 15 hours. The electrodes
were removed from the derivatizing solution, rinsed with hexane and either characterized by
cyclic voltammetry or placed into a culture tube containing octane or II in octane (1 mM) in
a constant temperature silicone oil bath. The culture tubes were removed from the constant
temperature bath at various intervals, in no particular order, the electrodes were rinsed with
hexane and characterized by cyclic voltamrnmetry. The dissociation or exchange was followed
for a minimum of four half-lives at all temperatures, with the exception of the dissociation of
I into octane at 115 'C which was followed for three half-lives. The coverage of I remaining
on the surface was determined using Equation 1.1.
Figure 1.1 shows a series of cyclic voltammograms for Pt electrodes derivatized with
I, followed by the placement of the electrodes into octane at 95 'C, for various intervals.
Figure 1.2 shows the plot of coverage, as a function of time in octane, and the corresponding
25
plot of the natural logarithm of coverage, as a function of time, for the loss of the SAMs of I
from the Pt surface at 95 0C. The plot of the natural logarithm of coverage as a function of
time is linear, which is consistent with a first order process. The rate constant for loss of I
from the Pt surface, at 95 OC, is (6 .2±0.3 )xlO~4 s-1. The desorption of I from the Pt surface
was measured at 85 'C, 95 'C, and 115 C and the desorption of II from the Pt surface was
measured at 103 'C. The loss of the SAM from the Pt surface was first order with respect to
the surface coverage of I or II, over the temperature range 85-115 'C.
Figure 1.3 shows the Arrhenius plot of the natural logarithm of the rate constant as a
function of the reciprocal of the absolute temperature. The plot is linear over the temperature
range 85-115 'C. The least squares fit of the data yields an activation energy for desorption,
Ea, of 45±4 kJ mor'I and a pre-exponential factor, A, of 1600±300 s- . Using Eyring Theory,
the parameters from the Arrhenius plot, Ea and A, are related to the enthalpy and entropy of
activation, AH* and AS* respectively, according to Equations 1.2 and 1.3, where R is the gas
constant (8.31441 J mol-1 K'), T is the absolute temperature (K), k is Boltzmann's constant
(1.38066x10-2 3 J K1) and h is Planck's constant (6.62618x10-34 J s-).
AH, = Ea -RT
(1.2)
AS. = R [InA - h](1.3)
The enthalpy of activation for the loss of SAMs of I or II from the Pt surface, into
octane is 43±4 kJ mol-1 and the entropy of activation is -180±30 J mol-1 K1. The Gibbs
energy of activation at 298 K, AG298 , for the formation of the activated complex that leads to
the desorption of I or II from the Pt surface is 98±11 kJ mol. The entropy and enthalpy of
activation have approximately equal contributions to AG
98 .
The entropy of activation is
26
Figure 1.1: Cyclic voltammograms of 0.25 cm2 Pt electrodes derivatized with SAMs of I,
followed by the placement of the electrodes into octane at 95 'C for various
intervals. The voltammetry was performed under Ar at a scan rate of 500 mV s~1.
The center of the crossed lines in each voltammogram corresponds to zero
voltage, zero current.
a) The initial SAM formed by derivatization of the Pt electrode in a 1 mM
solution of I in hexane at room temperature, for ~15 hours.
b) 5 minutes in octane at 95 'C.
c) 20 minutes in octane at 95 'C.
d) 45 minutes in octane at 95 'C.
e) 90 minutes in octane at 95 'C.
27
/
/
/
/
/
Pt/
~~Fe
a) Initial
Coverage
Fe
/
/
/
/
/
Fe
b) 5 minutes
octane, 95 0C
/
/
c) 20 minutes
d) 45 minutes
/
/
Pt/
/
/
/
/
/
e) 90 minutes
I
I
I
I
I
1
1
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Potential (V vs Ag/AgNO 3, 0.01M/CH 3 CN)
28
Figure 1.2: Plot of coverage (x10 10 mol cm-2) of SAMs of I on Pt as a function of time in
octane at 95 *C. The inset is the corresponding plot of the natural logarithm of
coverage as a function of time and the solid line is the least squares fit of the
data. The rate constant corresponding to the least squares fit of the data is
(6.2±O.3)x10
4
s-1.
29
1.2
-22
I
k = (6.2±0.3)x10
-24 -
1.0
s-
"
-26-
m0
0.8 -28
0
0
3600
1800
Time (seconds)
5400
0.6U
'-4
~I.
0
0.4
-
U
U
Ua
0.2 Ue
U=
0.0-
-r--------------~
0
1800
3600
Time (seconds)
I
5400
30
Figure 1.3: Arrhenius plot of the natural logarithm of the rate constant for the desorption of
SAMs of I or II from a Pt surface, into octane, versus the reciprocal of absolute
temperature. The solid line is the least squares fit of the data. The slope and
y-intercept, corresponding to the least squares fit of the data are -5460±520 K1
and 7.4±1.4, respectively.
31
-6.0
Ea =45+4kJmol1
A= 1600+300 s~1
-6.5
-7.0-
-7.5 -
-8.0-
-8.5 1
0.00254
0.00259
0.00264
0.00269
0.00274
Temperature-1 (K-1 )
0.00279
0.00284
32
negative which implies the activated complex has a greater degree of order than the ground
state complex. This result is not intuitively obvious because overall, the loss of I from the
surface results in an increase in entropy. The negative entropy of activation could result from
collisions between the solvent molecules and the ground state adsorbed complex which result
in the formation of the activated complex. These collisions correspond to molecules coming
together, resulting in a reduction in the randomness of the system. A second factor which
may contribute to the negative entropy of activation is a higher degree of charge in the
activated complex. The Pt surface and the "ligands", I or II, are neutral species and the
formation of SAMs of I or II on Pt results in a neutral, surface bound, "complex". The
P-Pt bond in the activated complex may be polarized, with a 6' charge on Pt and a 6- charge
on P. If this polarization of the P-Pt bond were to occur in the activated complex, it would
contribute to the observed negative entropy of activation.
Figure 1.4 shows the cyclic voltammograms for the desorption of SAMs of I into a
solution of II (1 mM in octane) at 95 'C. The plot of coverage of I and II on Pt, as a
function of time in a solution of II (1 mM in octane) and the corresponding plot of the natural
logarithm of coverage as a function of time for SAMs of I on Pt, are shown in Figure 1.5.
The plot of the natural logarithm of coverage of I on Pt, as a function of time in the solution
of II in octane, is linear, consistent with a first order process. The rate constant for loss of I
from the Pt surface into a solution of II (1mM in octane) is (6.4±0.3)x 4 s-1. Within
experimental error, the rate constant for loss of I from the Pt surface was unchanged by the
presence of II in solution, consistent with a dissociative mechanism. The desorption of
SAMs of I from a Pt surface into a solution of II (1 mM in octane) was performed at 85 'C,
33
Figure 1.4: Cyclic voltammograms of 0.25 cm2 Pt electrodes derivatized with SAMs of I,
followed by the placement of the electrodes into a solution of II (1 mM in
octane) at 95 'C for various intervals. The voltammetry was performed under Ar
at a scan rate of 500 mV s-1. The center of the crossed lines in each
voltammogram corresponds to zero voltage, zero current.
a) The initial SAM formed by derivatization of the Pt electrode in a 1 mM
solution of I in hexane at room temperature, for ~15 hours.
b) 5 minutes in a solution of II (1 mM in octane) at 95 'C.
c) 15 minutes in a solution of II (1 mM in octane) at 95 'C.
d) 45 minutes in a solution of II (1 mM in octane) at 95 'C.
e) 90 minutes in a solution of II (1 mM in octane) at 95 'C.
34
/
/
/
/
Pt/
9I
Fe
a) Initial
Coverage
/
/
/
/
/
Fe
b) 5 minutes
.11
Fe
c) 15 minutes
1 mM in octane, 95 0 C
y
/
/
/
/
Pt/
/
/
/
/
/
9
p
d) 45 minutes
e
e
9I
Fe
e) 90 minutes
P0
Fe
I
I
I
-0.4
-0.2
0.0
0.2
Potential (V vs Ag/AgNO 3, O.O1MICH 3CN)
0.4
0.6
0.8
35
Figure 1.5: Plot of coverage (x10
10 mol
cm-2) of SAMs of I (filled squares) and
II (open squares) on Pt as a function of time in a solution of II (1 mM in octane)
at 95 'C. The inset is the corresponding plot of the natural logarithm of coverage
of I, as a function of time, and the solid line is the least squares fit of the data.
The rate constant corresponding to the least squares fit of the data is
(6.40.3)x10 4 s-'.
36
2.4-22
k = (6.4+0.3)x10
2.0 -
4
s-i
-24 ~-26
1.6
-
-280
P
1.2
3600
1800
Time (seconds)
-
5400
0
"
0
0a
0D
0.8 - U
I
0.4
Pt-PPh 2(CH 2) 10 (CO)Fc
m Pt-PPh 2(CH 2)1 Fc
0
1
*
U
*
0.0 "0
1800
3600
Time (seconds)
U
5400
37
95 'C, 110 'C and 115 'C. The loss of I from the Pt surface into a solution of II is first order
with respect to the surface coverage of I, over the temperature range 85-115 'C.
Figure 1.6 shows the Arrhenius plot of the natural logarithm of the rate constant for
dissociation of I from Pt into a solution of II in octane, as a function of the reciprocal of the
absolute temperature. The Arrhenius plot is linear over the temperature range 85-115 'C.
The least squares fit of the data yields an activation energy for desorption, Ea, Of
47±1 kJ morl and a pre-exponential factor, A, of 3050±170 s-1. The enthalpy of activation
for the loss of SAMs of I from the Pt surface into a solution of II in octane, is 45+1 kJ mol'
and the entropy of activation is -180±10 J mol' K'. The Gibbs energy of activation at
298 K, AG
9 ,,
for the formation of the activated complex that leads to the loss of I from the
Pt surface, into a solution of II (1 mM in octane), is 98±3 kJ mol'. The entropy and enthalpy
of activation have approximately equal contributions to AG298 , similar to the loss of I from
Pt into octane, in the absence of II. The enthalpy, entropy and Gibbs energy of activation at
298 K, for the loss of I from the surface are, within experimental error, the same, regardless
of the presence or absence of II in solution. These results are consistent with a dissociative
mechanism.
Mann and Musco used
31P
NMR to measure the equilibrium constant, the standard
enthalpy, AH', the standard entropy, AS', the enthalpy of activation, AH, and the entropy of
activation, ASt, for the reaction shown in Equation 1.4.20 The temperature and
Pt(Ln+l)
O
Pt(Ln) + L
concentration were varied for a series of two, three and four coordinate Pt(0) phosphine
complexes. The predominant process for exchange was dissociative exchange, which
(1.4)
38
Figure 1.6: Arrhenius plot of the natural logarithm of the rate constant for the desorption of
SAMs of I from a Pt surface, into a solution of II (1 mM in octane), versus the
reciprocal of absolute temperature. The solid line is the least squares fit of the
data. The slope and y-intercept, corresponding to the least squares fit of the data
are -5660±170 K1 and 8.0±0.5, respectively.
39
-6.0Ea =47+l kJ mol
A = 3050+ 170 s~4
-6.5
S-7.0-
r -7.5 -
-8.0-
-8.5 0.00254
0.00259
0.00269
0.00274
0.00264
1
1
Temperature (K~ )
0.00279
0.00284
40
occurred for Pt(PMe 3)4, Pt(PMe 2Ph) 4, Pt(PMePh 2)4 , Pt(PBu")4, Pt(PPr3) and
Pt{P(C 6H,1 ) 3} 3 . Associative exchange was observed for Pt(PPr ) 2 and Pt{P(C6H 1 )3 } 2. In
two instances, Pt(PEt3 )3 and Pt(PBu3 )3, both associative and dissociative exchange
occurred, the relative rates of which depended on the concentration of the free tertiary
phosphine in solution and the temperature.
Over the temperature range, 0-70 'C, the rate of exchange of one
methyldiphenylphosphine ligand from tetrakis(methyldiphenylphoshine)platinum(0), was
independent of added ligand, consistent with a dissociative mechanism. The following
activation energies for this exchange process were determined: AH*=65.27±0.25 kJ mol-,
AS =26±1 J mol' K and AG3OO =57.3±0.4 kJ moP'.
The exchange of SAMs of I or II on a Pt surface is similar to the solution chemistry
of tetrakis(methyldiphenylphoshine)platinum(0), in that both processes are consistent with a
dissociative mechanism. The energetics are, however, different for the surface bound
molecules, as compared to that of the discreet metal complex. We attribute the differences in
the energies of activation to both steric and electronic effects. On the surface, the coverage
of SAMs of I or II corresponds to approximately one phosphine molecule of I or II, for
every 25 surface Pt atoms. In the complex tetrakis(methyldiphenylphoshine)platinum(0),
there are four, methyldiphenylphosphine ligands on each Pt atom, resulting in an eighteen
electron configuration. We believe that the greater steric crowding in the discreet complex,
as compared to the surface bound molecules accounts for the entropic differences, and the
tendency for Pt to form a sixteen electron complex contributes the enthalpic differences
between the solution and surface bound studies. 2 1
41
Electrochemical characterization of SAMs on Au. The SAMs formed from I or II
on Au, at room temperature or over the temperature range 110-120 'C, show reversible,
symmetrical cyclic voltammetric peaks with a full-width at half maximum, AEfwhm,
-100 mV at a scan rate of 500 mV s-.
Of
The peak current was linearly proportional to the
potential scan rate, consistent with surface confined redox centers. 18 The initial coverage for
SAMs of I or II on Au, formed at room temperature from a 1 mM solution in hexane, was
(1.1±0.2)x10' 0 mol cm 2 . This coverage corresponds to approximately one molecule per 23
Au surface atoms. This coverage is approximately 75% lower than the surface coverage for
SAMs of 11 -(ferrocenylcarbonyloxy)undecanethiol on Au. 1 8 The SAMs formed by
exchanging SAMs of I into a solution of II or the SAMs formed by placing bare Au into a
solution of I or II, or a mixture of I and II, over the temperature range 110-125 'C, exhibited
coverage that was approximately two and one half times greater than those formed at room
temperature. The coverage for SAMs formed at the higher temperatures was
(2.6i0.2)xlO0 mol cm-2 , which corresponds to approximately 10 surface Au atoms per
molecule. This coverage is approximately 50% less than that observed for SAMs of
11 -(ferrocenylcarbonyloxy)undecanethiol on Au. 1 8
Desorption and exchange studies on Au. The desorption and exchange studies on
Au were performed analogously to the studies on Pt. The rate of desorption of SAMs of I on
Au was, however, slower than that for the SAMs on Pt. The desorption studies of SAMs of I
on Au were performed over the temperature range 110-125 'C. We observed complete loss
of SAMs of I from the Au surface after ten minutes in octane at 130 'C. At 105 'C, the rate
of loss of I from Au into octane was too slow to provide reliable kinetic data. We measured
a half-life of 4.6 hours, but the fit to the data was poor (R2 =0.765). The dissociation of
42
SAMs of I from Au into either octane or II in octane was followed for a minimum of two,
and in most cases, three half-lives.
Figure 1.7 shows a series of cyclic voltammograms corresponding to the desorption
of SAMs of I on Au into octane at 115 'C. Figure 1.8 shows the plot of coverage as a
function of time and the corresponding plot of the natural logarithm of coverage as a function
of time for the loss of SAMs of I from the Au surface at 115 'C. The plot of the natural
logarithm of coverage as a function of time is linear, which is consistent with a first order
process. The rate constant for loss of I from the Au surface, into octane at 115 'C, is
(10.0±0.5)x10-5 s-'. The desorption of I from the Au surface was measured at 110 'C,
115 'C, 120 'C and 125 'C. The loss of SAMs of I, from the Au surface, was first order with
respect to the surface coverage of I, over the temperature range 110-125 'C.
Figure 1.9 shows the Arrhenius plot of the natural logarithm of the rate constant, as a
function of the reciprocal of the absolute temperature for the desorption of SAMs of I on Au
into octane. The plot is linear over the temperature range 110-125 'C, although the rate
constant measured for desorption at 120 'C did not fall, within experimental error, on the
least squares fit of the data. If the rate constant for desorption at 120 'C was excluded, all
other data points fell, within experimental error, on the least squares fit of the data. The least
squares fit of all the data yields an activation energy for desorption, Ea, of 103±18 kJ mol-1
and a pre-exponential factor, A, of (7.2±1.7)x10 9 s-.
The enthalpy of activation, AH*, for
desorption of SAMs of I on Au into octane is 100± 17 kJ mol-1 and the entropy of activation,
AS, is -56±13 J mo- K-1. The enthalpy of activation for loss of I from Au into octane is
approximately twice that for the same process on Pt, whereas the entropy of activation for the
43
Figure 1.7: Cyclic voltammograms of 0.25 cm 2 Au electrodes derivatized with SAMs of I,
followed by the placement of the electrodes into octane at 115 'C for various
intervals. The voltammetry was performed under Ar at a scan rate of 500 mV s-1.
The center of the crossed lines in each voltammogram corresponds to zero
voltage, zero current.
a) The initial SAM formed by derivatization of the Au electrode in a 1 mM
solution of I in hexane at room temperature, for -15 hours.
b) 45 minutes in octane at 115 'C.
c) 120 minutes in octane at 115 'C.
d) 195 minutes in octane at 115 'C.
e) 240 minutes in octane at 115 'C.
44
/
/
/
/
5 pAI
Fe
~
a) Initial
Coverage
Au~
/
/
/
/
/
~Fe
b) 45 minutes
octane, 115 0 C
c) 120 minutes
/
/
/
/
/
d) 195 minutes
Au/
/
/
/
/
/
e) 240 minutes
i
I
I
I
I
1
1
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Potential (V vs Ag/AgNO 3, 0.01M/CH 3CN)
45
Figure 1.8: Plot of coverage (x10 10 mol cmn2 ) of SAMs of I on Au as a function of time in
octane at 115 'C. The inset is the corresponding plot of the natural logarithm of
coverage as a function of time and the solid line is the least squares fit of the
data. The rate constant corresponding to the least squares fit of the data is
(10.0±.5)x10-5 s-1.
46
1.2
-22
I
1.0
k = (10.0±0.5)x10
5
s-1
-23 -
-
-240.8 -
U
-25 1
0
-s
3600
0
-o 0.6
7200 10800
Time (seconds)
14400
-
U
0
Q 0.4
-
Ue
Ua
U
0.2
-
0.0
0
3600
7200
Time (seconds)
10800
14400
47
Figure 1.9:
Arrhenius plot of the natural logarithm of the rate constant for the desorption of
SAMs of I from a Au surface, into octane, versus the reciprocal of absolute
temperature. The solid line is the least squares fit of the data. The slope and
y-intercept, corresponding to the least squares fit of the data are
-12300±2100 K' and 22.7±5.4, respectively.
48
-8.0
-
Ea =103+18 kJ molI
A =(7.21.7)x109 s -
-8.5 -
0
-9.0
-
-9.5
-
-10.0 -I
0.00247
I
0.00250
I
I
0.00259
0.00256
0.00253
Temperature" (K-')
0.00262
0.00265
49
loss of I on Au into octane, is approximately one third of the value observed for the same
process on Pt. The Gibbs energy of activation at 298 K, AG2'98 , for the formation of the
activated complex that leads to desorption of I from the Au surface into octane, is
117±18 kJ mol'. For the desorption of I from the Au surface, into octane, the main
contribution to AG29 8 ,, is the enthalpy of activation and the entropy term is a minor
component. This result is in contrast to the analogous process on Pt, in which the entropy
and enthalpy terms contributed approximately equally to AG
98
.
Figure 1.10 shows a series of cyclic voltammograms corresponding to the desorption
of SAMs of I on Au, into a solution of II (1 mM in octane), at 115 'C. The desorption of
SAMs of I on Au, into a solution of II, occurs by a distinctly different process than the
desorption of SAMs of I on Pt, into a solution of II or the desorption of SAMs of I on Au,
into octane. There is a portion of the SAM of I on Au that is rapidly exchanged with the
molecules in solution: the first half-life for loss of I from Au occurs in less than five
minutes, over the temperature range 110-125 'C. The rapid loss of I from the Au surface is
accompanied by a twofold increase in the total surface concentration of I and II on the
surface. At 115 'C, for exchange times greater than twenty minutes, the loss of I from the
Au surface into a solution of II (1 mM in octane) occurs more slowly than the exchange
occurring in the first twenty minutes.
Figure 1.11 shows the plot of coverage of I and II, as a function of time in a solution
of II in octane at 115 'C and the corresponding plot of the natural logarithm of coverage of I
on Au as a function of time. The solid line in the inset of Figure 1.11 corresponds to the least
squares fit of the coverage of I, as a function of time in the solution of II, from
50
Figure 1.10: Cyclic voltammograms of 0.25 cm2 Au electrodes derivatized with SAMs of I,
followed by the placement of the electrodes into a solution of II (1 mM in
octane) at 115 'C for various intervals. The voltammetry was performed under
Ar at a scan rate of 500 mV s-.
The center of the crossed lines in each
voltammogram corresponds to zero voltage, zero current.
a) The initial SAM formed by derivatization of the Au electrode in a 1 mM
solution of I in hexane at room temperature, for ~15 hours.
b) 5 minutes in a solution of II (1 mM in octane) at 115 *C.
c) 45 minutes in a solution of II (1 mM in octane) at 115 'C.
d) 195 minutes in a solution of II (1 mM in octane) at 115 'C.
e) 240 minutes in a solution of II (1 mM in octane) at 115 'C.
51
/
/
/
/
/
10 tA
a) Initial
Coverages
Au/
/
/
/
/
/
Fe
Fe
~
c) 45 minutes
1 mM in octane, 115 0C
/
/
/
/
/
r
d) 195 minutes
9w
Au/
/
/
/
/
/
Fe
e) 240 minutes
I
-0.4
I
-0.2
I
0.0
I
0.2
Potential (V vs Ag/AgNO 3, O.O1MICH 3 CN)
I
0.4
I
0.6
0
0.8
52
Figure 1.11: Plot of coverage (x1010 mol cm 2 ) of SAMs of I (filled squares) and II (open
squares) on Au as a function of time in a solution of II (1 mM in octane) at
115 'C. The inset is the corresponding plot of the natural logarithm of
coverage of I, as a function of time, and the solid line is the least squares fit of
the data from 20 to 240 minutes. The rate constant corresponding to the least
squares fit of the data is (6.0±0.4)x10-5 s-1.
53
2.4
Au-PPh 2 (CH 2)10(CO)Fc
I Au-PPh2 (CH 2)1 1Fc
O
2.0
03
a
0
a
a
03
1
03
1.6
13
D
3
-
0
0
-23
0
0
4
k = (6.0+0.4)x10~5 s~
0
-24 -
2
I
0
1.2
-
I
I-I
3600 7200 10800 14400
Time (seconds)
-26 0
0
0.8-?
0.4
Un
-25 -
-
O
w'pEnu
0
U
*
N
M0
0
M
*
U
E
0
0
0.0
0
3600
7200
Time (seconds)
10800
14400
54
20 to 240 minutes. The plot of the natural logarithm of coverage of I on Au, as a function of
time in a solution of II in octane, is linear, for exchange times greater than twenty minutes,
consistent with a first order process. The rate constant for loss of I from Au, for exchange
times greater than twenty minutes, is (6.0±0.5)x10-5 s-.
The loss of I from the Au surface
clearly occurs by different mechanisms in the presence and absence of II in solution.
The desorption of SAMs of I on Au, into a solution of II (1 mM in octane) was
performed at 110 *C, 115 'C, 120 'C and 125 'C. We believe that the molecules of I or II in
the SAMs on Au, exist in least two different environments: one in which rapid exchange
occurs, with the phosphine in solution, "rapid exchange sites" and one in which slow
exchange occurs, "slow exchange sites". At all temperatures investigated, the rate of
exchange for the "slow exchange sites" was less than the rate of loss of I on Au into octane,
in the absence of added II. Figure 1.12 shows the Arrhenius plot of the natural logarithm of
the rate constant for the dissociation of the "slow exchange sites" of I on Au, into a solution
of II (1 mM in octane), as a function of the reciprocal of the absolute temperature. The
Arrhenius plot is linear over the temperature range 110-125 'C. The least squares fit of the
data yields an activation energy for desorption, Ea, of 7.2±2.2 kJ mol1 and a pre-exponential
factor, A, of (5.6±0.5)x10-4 s-.
The enthalpy of activation for the loss of SAMs of I from the
Au surface into the solution of II is 4.8±1.4 kJ mol- and the entropy of activation is
-307±27 J mol' K1. The Gibbs energy of activation at 298 K, AG*98 , for the formation of
the activated complex that leads to desorption of I from the "slow exchange sites" on the Au
surface into a solution of 11 (1 mM in octane) is 96.4±8.3 kJ mol'. The value of AG298 for
the loss of I from Au is, within the uncertainty of the two calculated values, independent of
the presence or absence of II in solution, if we consider only the "slow exchange sites" for
55
Figure 1.12: Arrhenius plot of the natural logarithm of the rate constant for the desorption of
SAMs of I from a Au surface, into a solution of II (1 mM in octane), versus the
reciprocal of absolute temperature. The solid line is the least squares fit of the
data. The slope and y-intercept, corresponding to the least squares fit of the
data are -870±260 K' and -7.5+0.7, respectively.
56
-8.0Ea = 7.2+2.2 kJ mor
A =(5.6±0.5)xlO4 s~1
-8.5 -
-9.0-
-9.5 -
-10.002
0.00247
0.00250
0
0.00253
0 002
0.00256
0.00259
Temperature-' (K"l)
0.00262
0.00265
57
the loss of I into a solution of II. The entropy and enthalpy contributions are, however, very
different in the presence or absence of II in solution. For the desorption of I from Au into
octane, the major contribution to AGY*s , is the enthalpy of activation, whereas the major
contribution to AG298 , for the desorption of I from Au into a solution of II in octane, is the
entropy of activation.
The existence of at least two, kinetically different, exchange sites in SAMs has been
observed for SAMs of ferrocene terminated alkanethiols on Au. 1 8,22,23 The "rapid
exchange sites" are attributed to less ordered regions in the SAM, i.e., at domain boundaries
and defect sites within the monolayer. The "slow exchange sites" are attributed to exchange
occurring from the more ordered regions in the SAM.
Conclusions
Reversible, electroactive SAMs are formed from I or II on Pt and Au. The
desorption of I (or II) from the Pt surface, into octane or a solution of II in octane, is
consistent with a first order dissociative mechanism. The activation energies for the loss of I
(or II), on Pt, are independent of the presence or absence of added ligand in solution and the
entropy and enthalpy of activation have approximately equal contributions to AG298 . The
energies of activation for the desorption of the SAMs of I (or II) on Pt, into octane, are:
AH+ = 43±4 kJ mol', ASt = -180±30 J mol' K-, and AG29 8 = 98±11 kJ mol-.
For the
desorption of SAMs of I into a solution of II in octane, the energies of activation are:
AH* = 45±1 kJ mol', AS = -180+10 J molP K', and AGi 98 = 98±3 kJ moP'.
The desorption of I from the Au surface occurs by different mechanisms, depending
on the presence or absence of added ligand in solution. For the desorption into octane, the
58
enthalpy of activation is the major contributing factor to AG298 . For the desorption into a
solution of II in octane, the entropy of activation is the major contributing factor to AG298 .
The energies of activation for the desorption of the SAMs of I on Au, into octane, are:
AH* = 100+17 kJ mor', AS'= -56±13 J mol K-1, and AG298 = 117±18 kJ mor'. The
energies of activation for the desorption of SAMs of I into a solution of II in octane are:
AH* = 4.8+1.4 kJ molr, AS* = -307±27 J mor'1 K, and AG298 = 96.4±8.3 kJ molr'.
59
References
1)
Kumar, A.; Abbott, N. L.; Biebuyck, H. A.; Kim, E.; Whitesides, G. M. Acc. Chem. Res.
1995, 28, 219-26.
2)
Ulman, A. Chem. Rev. 1996, 96, 1533-1554.
3)
Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219-227.
4)
Ricco, A. J.; Crooks, R. M.; Osbourn, G. C. Acc. Chem. Res. 1998, 31, 289-296.
5)
Laibinis, P. E.; Palmer, B. J.; Lee, S.-W.; Jennings, G. K. Thin Films 1998, 24, 1-41.
6)
Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R.-G.
J.Am. Chem. Soc. 1989, 111, 321-35.
7)
Delamarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103-8.
8)
Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315-22.
9)
Schlenoff, J. B.; Li, M.; Ly, H. J.Am. Chem. Soc. 1995, 117, 12528-36.
10) Tirado, J. D.; Abruna, H. D. J.Phys. Chem. 1996, 100, 4556-63.
11) Kang, D. Photochemistry of Surface-Confined Derivatives of
CyclopentadienylmanganeseTricarbonyl and Self-Assembled Monolayers of
Ferrocenyl-BasedPhosphines on Gold and Platinum;Massachusetts Institute of
Technology: Cambridge, MA, 1993.
60
12) Roundhill, D. M. In Comprehensive Coordination Chemistry: the synthesis, reactions,
properties, & applicationsof coordinationcompounds, 1st ed.; Wilkinson, G.,
Gillard, R. D. and McCleverty, J. A., Ed.; Pergamon Press: Oxford, England, 1987;
Vol. 5, pp 351-531.
13) Malatesta, L.; Ugo, R.; Cenini, S. Adv. Chem. Ser. 1967, No. 62, 318-56.
14) Ugo, R. Coordination Chemistry Review 1968, 3, 319-344.
15) Malatesta, L.; Cenini, S. Zerovalent compounds of metals; Academic Press:
London, 1974.
16) Puddephatt, R. J. In Comprehensive CoordinationChemistry: the synthesis, reactions,
properties, & applicationsof coordination compounds, 1st ed.; Wilkinson, G.,
Gillard, R. D. and McCleverty, J. A., Ed.; Pergamon Press: Oxford, England, 1987;
Vol. 5, pp 861-923.
17) Okahata, Y.; Enna, G.; Takenouchi, K. J. Chem. Soc., Perkin Trans. 2 1989, 835-43.
18) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192-7.
19) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.;
Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357-9.
20) Mann, B. E.; Musco, A. J. Chem. Soc., Dalton Trans. 1980, 776-85.
21) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; 5th ed.; John Wiley &
Sons: New York, 1988.
61
22) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc.
1990, 112, 4301-6.
23) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M.
J. Am. Chem. Soc. 1991, 113, 1128-32.
62
CHAPTER 2
MICROCONTACT PRINTING OF OCTADECANEPHOSPHONIC ACID ON A12 0 3/Al:
PATTERN TRANSFER BY WET CHEMICAL ETCHING
63
Introduction
Microcontact printing (gCP) is a soft lithographic technique useful in patterning
self-assembled monolayers (SAMs). 1- 4 This chapter describes the gCP of alkanephosphonic
acids onto Al2 O3/Al films, and the performance of these patterned films in protecting
A12 0 3/Al films during wet etching. The minimum feature size achieved in this study was
6 gm, but the stamp used set this dimension. Electrical resistance measurements of the
patterned lines after etching confirmed that the lines were continuous and conductive.
Aluminum is commonly used as a metallization layer in the microelectronics
industry. 5 Currently aluminum is patterned by two methods: i) conventional
photolithography, followed by metal deposition and lift-off; ii) reactive ion etching (RIE) of
a continuous film (a procedure that also involves a step of photolithography for pattern
definition). In the case of metal deposition followed by lift-off, the pattern is transferred by
physical removal of the aluminum in the unwanted regions by dissolution of an underlying
polymer. No etching of the aluminum occurs; for larger features (on the order of
millimeters), however, complete removal of the unwanted aluminum can be difficult.
Conventional photolithography requires access to the appropriate facilities, and is not always
convenient, nor is it applicable to curved surfaces. This chapter describes a
non-photolithographic procedure for patterning aluminum.
Microcontact printing (gCP). Figure 2.1 shows a schematic representation of the
gCP process for octadecanephosphonic acid on supported A12 0 3/Al films.
Alkyltrichlorosilanes have been patterned on aluminum surfaces by gCP.6 -8 The use of
patterned alkylsiloxanes for the selective RIE etching of aluminum resulted in shallow etch
depths on the order of 5-10 nm. 6 ,7 In this work, using pCP of octadecanephosphonic acid
64
Figure 2.1: Schematic representation of the gCP process.
65
1) Ink stamp with C18PO(OH) 2 (5 mM in 2-propanol)
2) Remove excess "ink" under a stream of N2
4-
C18PO(OH) 2
+-
300 nm Al
3) Place stamp in conformal contact with Al
4) Stamp is removed and substrate is baked
on a hotplate at 70 0 C
C1 8 PO(OH)
5) Bare regions are selectively etched in
Aluminum Etchant Type A
2
66
(C1 8PO(OH)2 ) followed by wet etching, we have achieved etch depths of 300 nm without
loss of electrical properties in the patterned film.
The choice of alkanephosphonic acids. We chose to use alkanephosphonic acids
for this study because they are air stable compounds that are known to form stable, ordered
monolayers on metal oxide surfaces. 9 , 10 The use of alkanephosphonic acids to form SAMs
on metal oxide surfaces was motivated by the work of Mallouk et al. on
alkanebisphosphonate multilayers. 1 1 -13 SAMs on the native oxide of aluminum have been
9
formed from alkanecarboxylic acids,14-21 alkanehydroxamic acids, alkanephosphonic
acids 9 ,2 2 -2 4 and alkyltrichlorosilanes. 2 5
Alkanecarboxylic acids adsorb on A12 0 3/Al as the carboxylate anion, but this
15 Two studies have
interaction results in a weakly bound, easily displaced, adsorbate.
shown increased stability of SAMs of alkanecarboxylic acids on A12 0 3/Al films. Aronoff et
al. developed a method to increase the binding of SAMs of alkanecarboxylic acids on a
hydroxide terminated A12 0 3/Al surface by first priming the metal hydroxide surface with
zirconium alkoxide. 2 6 This study was, however, performed in ultra high vacuum (UHV) and
the reactivity of zirconium alkoxides may limit its application to monolayers formed in
solution.
Wallace et al. prepared and characterized SAMs of perfluorinated alkanecarboxylic
acids on A12 0 3/Al. The results of thermal desorption studies found that the molecule was
chemisorbed, with an apparent desorption activation energy of 126±13 kJ morl.
27
We
attribute the increase in binding ability of perfluorinated alkanecarboxylic acids on A12 0 3/Al,
28
as compared to their hydrocarbon counterparts, to the high acidity of these acids.
67
Ramsier et al. have studied the adsorption of several phosphorus oxy-acids on
alumina: phosphinic, phosphonic, methanephosphonic, hydroxymethanephosphonic,
aminomethanephosphonic and nitrilotris(methylene)triphosphonic acids by inelastic electron
tunneling spectroscopy (IETS). 2 9 Their results indicate that these acids chemisorb as the
deprotonated acids on alumina. They propose a mezhanism of adsorption involving a
condensation reaction between the anion of the deprotonated acid with surface oxide or
hydroxide groups of the alumina. This reaction results in the formation of P-O-Al bonds and
the authors concluded that the phosphonic acid precursors bind through a symmetrical,
tridentate, P0 3 group and the phosphinic acid precursors bind through a symmetrical,
bidentate P0 2 group.
A study of competitive binding between ferrocene terminated alkanecarboxylic acids
and alkanephosphonic acid on indium(tin) oxide (ITO) showed that the carboxylic acid
derivative could not compete for the surface in a solution of the two species. The energetics
of desorption of 1 1-ferrocenylundecanephosphonic acid on ITO were studied at various
temperatures and the activation energy for desorption was found to be 96±26 kJ mol'.
30
This binding energy is indicative of chemisorption. We hoped that this interaction would not
be limited to ITO but would also extend to other basic metal oxides. We liken the increased
stability of SAMs formed from alkanephosphonic acids, as compared to alkanecarboxylic
acids to the chelate effect observed for solution metal-ligand interactions. 3 1
A study of SAMs formed from octadecanephosphonic acid on porous ZrO 2 and TiO2
and non-porous SiO 2 and y-A12 O3 found that monolayers were formed on ZrO 2, TiO2 and
2 2 Elemental
SiO 2 ; a bulk aluminoalkyl(phosphonate), however, was formed on A12 0 3.
analysis indicated that the (aluminoalkyl)phosphonate was at least two layers thick. The
68
authors concluded that aluminum oxide was not a suitable substrate for SAM formation due
to the formation of bulk (aluminoalkyl)phosphonate. We speculate that the presence of a
monolayer of Al2 0 3, that is, the native oxide formed on aluminum metal, would limit bulk
(aluminoalkyl)phosphonate formation. Further, if a multilayer were to form spontaneously, it
might improve the etch resistance.
,
The choice of octadecanephosphonic acid. The results of our previous studies of
gCP with alkanethiols on gold showed that alkyl chains containing less than sixteen
methylene units did not provide adequate resistance during etching. 3 2 The defect density in
the resulting patterned structures was much higher for the shorter alkanethiols than for
hexadecanethiol. Hexadecanethiol became the standard for gCP on gold and silver because
it resisted etching, it is a liquid at room temperature and it adsorbs well into the PDMS
stamp. Alkanephosphonic acids are solids at room temperature and they do not adsorb into
the PDMS stamp but rather form a continuous, macroscopic film on the PDMS surface. We
used octadecanephosphonic acid for this study because it contains a long alkyl chain, which
provides a hydrophobic barrier to aqueous etching solutions, and it is soluble in 2-propanol,
the solvent we used to ink the stamps. Shorter chain alkanephosphonic acids can be
patterned on A1/A12 0 3 by pCP and they are protective upon exposure to the etching solution.
We did not, however, test the electrical performance of structures prepared from shorter
chain alkanephosphonic acids.
The mask design for the resistance measurements. The mask design shown in
Figure 2.2 was used to prepare patterned films for resistance measurements. The pattern
consists of a continuous line, 67.7 cm long and 6 gm wide, with 4 tm spacing between lines.
69
Figure 2.2: Mask design used for the resistance measurements. The wires were equally
spaced in the mask used; the different contrast that appears in the figure is a
printer artifact.
---------------
70
10 pm
I-1
4 pm
500 pm
6 pm
AL
2.5 mm
uu
4pm
6pm
71
There are contact pads at approximately 7.5 cm intervals along the length of the wire. This
design allows us to measure resistance as a function of length along the patterned wire.
Design of the Schottky diodes. Schottky diodes are rectifying devices, which allow
current flow under a forward bias and retard current flow under a reverse bias. Schottky
diodes are well suited for use in integrated circuits because they require fewer processing
steps than the p-n junction diode and because they are capable of high speed rectification. 5
A Schottky barrier is formed when a metal is deposited on a semiconductor and the work
function of the metal, (Dm, is greater than the work function of the semiconductor, Ps. 3 3
A Schottky diode consists of two metal-semiconductor contacts: an ohmic contact
and a Schottky barrier contact. Schottky diodes are most commonly prepared on n-type
silicon because of the greater energy difference between Ds and Dm
(Ds p-type Si >cFs n-type Si). We chose to use p-type silicon in this instance because of the
ease in preparing an ohmic contact on p-type silicon with aluminum. Any metal will form an
ohmic contact with a highly doped semiconductor. Aluminum is a p-type dopant in silicon
and the diffusion of aluminum into p-type silicon during an annealing step creates the highly
p-doped region necessary to form an ohmic contact. 5
The mask design for the Schottky diodes. The Schottky diodes were prepared
using the mask design shown in Figure 2.3. The solid black features in Figure 2.3
correspond to the mask used to define the ohmic contacts and the white features outlined in
black correspond to the mask used to define the Schottky contacts. The fabrication of the
diodes is a multi-step process that involves two metal deposition steps, two masks and
registration of the second layer with respect to the first layer. Figure 2.4 shows a schematic
illustratation of the process used to fabricate the Schottky diodes.
72
Figure 2.3: Mask design used to prepare Schottky diodes. The solid black features
correspond to the mask used for the ohmic contacts and the white features
outlined in black correspond to the mask used for the Schottky contacts.
73
Alignment
Marks
15 pm
Ohmic Schottky
Contact Contact
Pad Pad
Spacing
Between
Pairs of
+
, 150 pm
Pads:
lE
E
fl
100 pm
El]
20
pm
15 pm
lEl
S E
10 Pm
5 pmrn
I~~~ El L
L
4 pm
5m
2 pm
74
Figure 2.4: Schematic representation of the process used to fabricate the Schottky diodes.
75
a)
p-type Si(100)
AZ 5214
Photoresist
1) Deposit 100 nm Al, 10 nm Ti, 200 nm Pt
2) Lift-off in acetone with sonication
3) Anneal at 550 0 C for 5 minutes
b)
U
-Ohmic Contact
4
p-type Si(100)
1) 10 second etch in buffered HF
2) Deposit 200 nm Al
c)
200 nm Al
1) Register
2) Stamp 10 minutes with C 18 PO(OH)
2
d)
C 1 8 PO(OH) 2
200 nm Al
1) Bake 1 minute at 70 0 C
4r
e)
2) Etch in Aluminum Etchant Type "A"at 35 0 C
Ohmic Contact
-Schottky Contact
76
The ohmic contacts consist of a 2-dimensional array of square pads, 150 gm on a
side, separated by 350 pm in each direction. The ohmic contacts were defined in photoresist,
followed by metal deposition and lift-off. We used conventional photolithography in this
step, rather than soft lithography, because of limitations placed on us by the cleanroom
facilities where we carried out the initial fabrication steps. 34 Soft lithography is a viable
method to prepare devices such as Schottky diodes and the use of photolithographic
techniques in this instance should not be regarded as a limitation of soft lithography. 3 5
The ohmic contact consisted of three metal layers: Al (100 nm), Ti (10 nm) and
Pt (200 nm). The ohmic contact was formed at the Al/Si interface and the Ti layer was used
as an adhesion promoter between the Al and Pt layers. The Pt layer was used to protect the
Al contact during a hydrofluoric acid etching step.
The Schottky contacts consist of a 2-dimensional array of square pads, 157 gm on a
side, defined by gCP. The Schottky contacts were designed to be square pads, 150 pm on a
side. The features in the photoresist master used to make the PDMS stamp for the Schottky
contacts were 7 jim larger than designed. 3 6 In the horizontal direction of the array, there are
rows of Schottky contacts, equally spaced with respect to the ohmic contact pads. In the
vertical direction, there are columns of Schottky contacts, with a range of gaps between the
ohmic and Schottky contact pads. The gap between the ohmic and Schottky contact pads
was designed to vary from 100 jim to 2 gm (Figure 2.3). To aid in the registration,
alignment marks were placed at regular intervals on the masks used to prepare the ohmic and
Schottky contacts.
77
Experimental
Materials. 2-Propanol (HPLC grade, B&J Brand, Burdick & Jackson, 99.9%),
ethanol (Pharmco, 200 proof), water (HPLC grade, Omnisolve@, EM Science), Aluminum
Etchant Type "A" (Transene Co.), (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane
(United Chemical Technologies, Inc.) and polydimethylsiloxane SYLGARD ® 184 (Dow
Corning) were used as received. Octadecanephosphonic acid was available from a previous
study: It was prepared using the Michaelis-Arbuzov reaction according to published
procedures. 4 8 -5 0 Aluminum, titanium and platinum (Pure Tech) used for the e-beam
evaporations were of 99.999% purity. Wafers used for the resistance measurements
(Wafernet, Inc.) were test grade 4" diameter p-type Si(100) that were modified first with
500 nm of thermally grown Si0 2 followed by 150 nm of LPCVD Si 3N4. Wafers used for the
Schottky diodes (Wafernet, Inc.) were prime grade p-type Si(100), boron doped, 4" diameter,
thickness 500-550 gm, resistivity 0.500-2.000 Qcm. The following chemicals were provided
in the cleanroom facilities and were used as supplied: Buffered Oxide Etch (7:1 HF:NH 4 F
with surfactant), methanol, acetone, 2-propanol, sulfuric acid (concentrated) and hydrogen
peroxide (30%) ('Baker Analyzed'® Low Sodium CMOS Electronic Grade, J.T. Baker),
Nanostrip (Cyantek Corp.), hexadimethyldisilazane (Ultrapure Grade, MicroSi, Inc.), AZ®
5214-E IR Photoresist, AZ® P 4260 Photoresist, AZ® 422 MIF Developer and AZ® 440
MIF Developer (Hoechst Celanese).
Substrate preparation for resistance measurements. The substrates for the
resistance measurements were prepared at the Technology Research Laboratory, a class 100
cleanroom facility at MIT. The silicon nitride coated silicon wafers were cleaned for
10 minutes in Piranha solution (a mixture 7:3 (v/v) of 98% H2SO 4 and 30% H2 0 2 )5 1
78
followed by rinsing with de-ionized water and spin drying at 2000 rpm. The wafers were
divided into two batches at this point: the substrates for the lift-off samples were processed
further and the substrates for the stamping studies were stored in the cleanroom facility
during the preparation of the photoresist pattern on the lift-off samples. The lift-off samples
were first coated with hexamethyldisilazane vapor in a YES Vapor Prime Oven followed by
spin coating of AZ® 5214-E IR photoresist at 4000 rpm for 30 seconds with a Solitec 5110
Coater. The wafers were baked for 30 minutes in a 90 'C oven. The photoresist coated
wafers were exposed through a chrome on glass mask with a Karl Suss Model MA4 contact
aligner for 2 seconds (10 mW cm-2s-1 @ 365 nm), followed by baking at 120 'C for
90 seconds and a second exposure through a clear glass mask for 70 seconds. The
photoresist was developed for 75 seconds in AZ® 422 MIF developer, followed by rinsing
with de-ionized water and spin drying at 2000 rpm. The two sets of substrates were
combined and transferred to a Temescal Semiconductor Products Model VES 2550 e-beam
evaporator equipped with a cryopump. The system was evacuated to a pressure less than
9x10- 7 torr prior to deposition and the aluminum was deposited at a rate of 5
A s4.
The
samples were cooled under vacuum for 20 minutes after the deposition, followed by venting
with N2 gas. The substrates for the stamping studies were immediately placed in individual
4" Fluoroware@ wafer holders. The lift-off samples were sonicated in acetone, followed by
rinsing with acetone, methanol, 2-propanol and water. The lift-off samples were then dried
individually under a stream of filtered N2 gas and placed into wafer holders. The two sets of
samples were sealed in a plastic bag and transferred from the cleanroom to our laboratory.
Preparation of stamps for the resistivity measurements. The photoresist pattern
used to prepare stamps for the resistivity measurements was prepared in the same manner, at
79
the same time as the pattern for the lift-off samples described in the substrate preparation
section, with the following exception. The substrate for the stamping pattern was test grade
Si(100) used as received. The silicon wafer patterned with photoresist was placed in a wafer
holder and transferred to our laboratory. The wafer was placed under vacuum in a dessicator
with a vial containing (tridecafluoro- 1,1,2,2-tetrahydrooctyl)- 1 -trichlorosilane for 30
minutes. After silanization, the wafer was placed in a 15 cm petri dish and a degassed
mixture of PDMS prepolymer and curing agent (10:1 w/w) was poured over the wafer. The
PDMS was cured in a 60 *C oven for 2 hours. After curing, the PDMS stamped was peeled
away from the photoresist coated silicon master and cut into approximately 6 cm 2 pieces with
a razor blade.
Inking, stamping and etching. A solution of octadecanephosphonic acid (5 mM in
2-propanol) was filtered through a 0.2 pm Acrodisc® LC PVDF syringe filter onto the precut
PDMS stamp on a glass microscope slide. The solution remained in contact with the stamp
for 30 seconds and then the excess solution was removed under a stream of filtered N2 gas.
The stamp was placed onto the Al coated substrate with tweezers. After stamping for
10 minutes, the stamp was removed from the substrate with tweezers and the substrate was
baked on a stainless steel plate in a 70 'C oven for 10 minutes. After baking, the samples
were etched, with agitation, in Aluminum Etchant Type "A" at 35 'C. The entire sample
area was larger than the area stamped and the endpoint was judged by the complete
dissolution of the aluminum in the unstamped regions. Once the endpoint had been reached,
the samples were immediately transferred to a beaker of de-ionized water, and then rinsed for
1 minute under a stream of water, followed by drying with filtered N2 gas.
80
Resistance measurements. We used a Keithley 197 Autoranging Microvolt DMM
to measure the resistance of the patterned samples. The samples were secured on a stainless
steel plate with ScotchTM Double Stick Tape and contact was made to the various contact
pads with two sharpened tungsten wires mounted on Rucker and Kolls Model 448
micropositioners with magnetic bases.
Preparation of stamps for the Schottky diode contact pads. The photoresist
pattern used to prepare stamps for the Schottky contact pads was prepared using a thick resist
that gave a relief structure of 10 pm at a spin speed of 1500 rpm. The substrate used to
prepare the stamps for the Schottky diode contact pads was a test grade Si(100) wafer, used
as received. The procedure for patterning the photoresist was similar to that described
previously with the following differences: The photoresist (AZ® P 4260) was spin-coated at
1500 rpm, the exposure time was 5 seconds and the developing time was 180 seconds in
AZ®440 MIF developer. The preparation of the PDMS stamp from the silicon wafer
patterned with photoresist was performed in the same manner as described previously with
the following exception: After silanization, the wafer was coated with thin layer of degassed
PDMS (-0.5 mm). A 36 cm 2 square window was defined on a 58 cm2 clean glass plate with
two layers of masking tape. The glass plate was placed onto the PDMS coated wafer and
four binder clips were used to hold the assembly together. The entire assembly was placed in
a 60 'C oven for twelve hours to cure the PDMS. After curing, the glass supported PDMS
stamped was separated from the silicon wafer under de-ionized water. The tape was
removed from the glass plate, resulting in a thin (~40 jim) PDMS stamp supported on a glass
plate.
81
Diode preparation: 1. Ohmic contacts. The ohmic contacts for the Schottky
diodes were prepared at the Technology Research Laboratory, a class 100 cleanroom facility
at MIT. The prime, p-type Si wafers (Wafernet, Inc.) were used as received. The first step in
the fabrication of the ohmic contacts involved defining the pattern in photoresist. This
pattern transfer was accomplished with AZ® 5214-E IR photoresist in the same manner
described above for the lift-off samples prepared for the resistance measurements. The
samples were cleaned in an oxygen plasma cleaner for 3 minutes to remove any residual
undeveloped photoresist. The wafers were transferred from the plasma cleaner to a Temescal
Semiconductor Products Model VES 2550 e-beam evaporator equipped with a cryopump.
The system was evacuated to a pressure less than 9x10 7 torr prior to deposition. Aluminum
(100 nm) was deposited at a rate of 5
A s- , followed by titanium (10
nm, 2 A s-) and
platinum (200 nm, 5 A s-1). The samples were cooled under vacuum for 20 minutes after the
final deposition, followed by venting with N2 gas. The samples were sonicated in acetone,
followed by rinsing with acetone, methanol, 2-propanol and water. The samples were then
dried individually under a stream of filtered N2 gas. The samples were cleaned in Nanostrip
for 3 minutes to remove any residual photoresist or carbonaceous contaminants prior to the
annealing step. After cleaning in Nanostrip, the samples were rinsed with de-ionized water,
followed by spin drying at 2000 rpm. The wafers with the defined ohmic contacts were then
annealed for 3 minutes at 550 'C in an AG Associates Heatpulse 410 rapid thermal annealer.
The ohmic contacts were tested to confirm that they exhibited non-rectifying behavior prior
to the formation of the Schottky contacts.
Diode preparation: 2. Schottky contacts. The wafers with the defined ohmic
contacts were cleaned for 30 seconds in Buffered Oxide Etch (7:1 HF:NH 4F with surfactant)
82
to remove any silicon oxide on the silicon surface, followed by rinsing with de-ionized water
and spin drying at 2000 rpm. The samples were immediately transferred to the Temescal
Semiconductor Products Model VES 2550 e-beam evaporator. Aluminum (200 nm) was
deposited at 5 A s 1 over the entire wafer containing the ohmic contacts. The samples were
cooled under vacuum for 20 minutes after the aluminum deposition, followed by venting
with N2 gas. The samples were placed into wafer holders and sealed in a plastic bag for
transfer to the cleanroom facilities at Harvard. The definition of the Schottky contacts
involved one registration step and this step was carried out in a class 100 cleanroom. The
glass supported PDMS stamp was inked with octadecanephosphonic acid (5 mM in
2-propanol) in the same manner as the unsupported PDMS stamp. The glass plate supporting
the PDMS stamp was mounted on the mask holder of a Karl Suss Model MJB3 contact
aligner. The aluminum coated wafers with the defined ohmic contacts were cut into 1 cm 2
square pieces. A 1 cm2 sample was placed on the sample holder of the Karl Suss aligner and
the sample was raised to within 0.5 mm of the PDMS stamp for alignment. The registration
marks on the sample and the PDMS stamp were used for alignment. Once the alignment was
completed, the sample was brought into contact with the stamp and the sample was left in
contact with the stamp for 10 minutes. After stamping, the sample was removed from the
stamp with tweezers and the sample was baked for 1 minute on a stainless steel plate in a
70 'C oven. After baking, the sample was etched in a solution of Aluminum Etchant Type
"A" at 35 'C. After etching, the sample was rinsed with de-ionized water and dried under a
stream of filtered N 2 gas.
Current-voltage measurements. The dark current-voltage characteristics (I-V) were
measured with a Pine AFCBP1 computer controlled potentiostat. The data were collected at
83
a scan rate of 200 mV s4 . The samples were secured on a stainless steel plate with ScotchTM
Double Stick Tape and contact was made to the various contact pads with two sharpened
tungsten wires mounted on Rucker and Kolls Model 448 micropositioners with magnetic
bases. The reverse bias data in Figure 2.10 was collected with a Keithley Yieldmax 450
process monitor system.
Capacitance-voltage measurements. The dark capacitance-voltage characteristics
(C-V) were measured with a Keithley Yieldmax 450 process monitor system. The data were
collected at a frequency of 1 MHz.
AFM. The AFM measurements were performed with a Topometrix TMX 2010
scanning probe microscope, equipped with a linearized tripod scanner. The PDMS stamp
was imaged in non-contact mode using a high resonant frequency silicon cantilever and a
scan rate of 100 pm s-.
The aluminum patterns were imaged in contact mode using a silicon
nitride cantilever and a scan rate of 100 gm s-.
SEM. The SEM images were obtained with a LEO Digital Scanning Microscope,
Model 982.
Results and Discussion
Fabrication of the stamp. The elastomeric stamp was prepared by casting
polydimethylsiloxane (PDMS) against a relief pattern of interest (in this case, a photoresist
pattern on a silicon wafer, prepared by photolithography). 3 7 The PDMS was cured at 60 'C
for a minimum of 2 hours and then carefully peeled away from the master. The stamp used
to prepare the structures for the resistance measurements was a free standing, unsupported
PDMS stamp with a total thickness of approximately 4 mm; the thickness of the relief
84
structure was 1 pm. The stamp used to prepare the Schottky contact pads was a thin PDMS
film (-40 gm thick) supported on a glass substrate; the thickness of the relief structure
was 10 pm. A glass support was used to decrease the lateral distortions previously observed
for PDMS stamps. 3 8
Treatment and inking of the stamp. The surface of the PDMS stamp was untreated
and hydrophobic. A solution of octadecanephosphonic acid (5 mM in 2-propanol) was
filtered through a 0.2 pm filter onto the stamp. The solution remained in contact with the
stamp for at least 30 seconds. The excess solution was removed under a stream of filtered N2
gas. The octadecanephosphonic acid does not absorb into the stamp, but rather forms a
visible film on the surface of the stamp.
Oxidation of the PDMS stamp results in a hydrophilic surface.
Octadecanephosphonic acid also formed uniform films on the oxidized PDMS. Uniform
pattern transfer, however, did not occur after stamping with the oxidized PDMS stamp. 3 9
Preparation of the substrates. Aluminum was deposited onto either silicon or
silicon nitride-coated silicon wafers by electron beam (e-beam) evaporation. The highest
degree of reproducibility was obtained from substrates that had uniform aluminum thickness
across the entire wafer. This uniformity was achieved on 4" wafers by deposition of
aluminum in an e-beam evaporation chamber equipped with planetary sample holders. The
samples were rotated in three dimensions during the evaporation process; this rotation
substantially improved uniformity. If the depositions were performed in chambers equipped
with a sample holder that rotated by 3600 in only one dimension, the resulting coated 3" or
4" wafers exhibited a gradient in thickness across the wafer; upon etching, this gradient
resulted in over etching in some regions and incomplete etching in other regions. The
85
formation of the native oxide of aluminum occurred by air oxidation of the wafers after
removal from the evaporation chamber. In order to limit contamination of the aluminum by
airborne contaminants, the wafers were used after exposure to air for a minimum of 2 hours
but not more than 72 hours.
Stamping and baking. The inked stamp was placed in contact with the aluminum
surface and in cases where it did not make complete conformal contact with the surface, the
top of the stamp was touched lightly with tweezers to initiate the wetting of the stamp with
the surface. The stamp was left in contact with the substrate for 10 minutes. A pattern is
visible on the aluminum surface after stamping, indicative of a multilayer film on the surface.
The multilayer film of octadecanephosphonic acid on the surface is removed by rinsing with
2-propanol, leaving patterned surface. 4 0 The process used in gCP here is, thus qualitatively
different than that for alkanethiols on gold, in which a monolayer forms rapidly and cleanly.
Figure 2.5 shows scanning electron microscopy (SEM) images of
octadecanephosphonic acid stamped on aluminum before and after rinsing with 2-propanol.
The stamp used to prepare these patterns consisted of 6 pm wide lines, separated by 4 jim.
This difference in line widths between the stamped and unstamped regions allowed us to
assign the appropriate regions in the SEM micrographs. Figure 2.5a shows a SEM image of
the surface after stamping without rinsing. We attribute the dark, irregular lines in the center
of the stamped region to a thick film of octadecanephosphonic acid. Figure 2.5b shows the
same sample after rinsing with 2-propanol. The dark lines no longer appear in the stamped
region and the width of the bare aluminum region is the same in both images.
After stamping, the patterned substrate was baked at 70 'C for 10 minutes. 4 1 The
baking step was required for the stamped monolayers to act as resists during etching in
86
Figure 2.5: SEM images of octadecanephosphonic acid patterned by IiCP on aluminum:
a) As stamped and b) After rinsing the surface with 2-propanol to remove the
excess octadecanephosphonic acid.
.....
....
....
--------- -
87
Bare Al
Stamped Region
without rinsing
I
I
Bare Al
Stamped Region
after rinsing
I
I
88
Aluminum Etchant Type "A". We believe the baking step is a dehydration step that forms
alkyl(aluminophosphate) by reaction of the phosphonic acid head group with terminal
hydroxide groups and waters of hydration on the aluminum oxide surface.
Wet etching. Aluminum can be etched by both acidic and basic etching solutions, by
oxidizing solutions, and by electrochemical dissolution. 4 2 The aluminum etch rate is highly
dependent on the choice of etching solutions and the temperature. We tested several
solutions for their selectivity in etching A12 0 3/Al films patterned with octadecanephosphonic
acid: oxidizing solutions of ferricyanide (pH 13.6), sodium hydroxide solutions,
hydrochloric acid solutions, and various ratios of combinations of phosphoric, acetic and
nitric acids in water. The highest selectivity between the patterned octadecanephosphonic
acid and bare aluminum was obtained from Aluminum Etchant Type "A", which is a
combination of phosphoric, acetic and nitric acids and water in a ratio of 16:1:1:2.43
Figure 2.6a shows a three-dimensional atomic force microscopy (AFM) image
depicting the relief structure of the stamp used to prepare the structures for resistance
measurements and 6b shows an AFM image of 300 nm thick aluminum patterned by pCP
without rinsing, followed by baking at 70 0C for 10 minutes and wet etching in Aluminum
Etchant Type "A" at 35 'C. We measured the line width as the width of the features at half
the thickness of the relief structure. The line width of the stamped pattern, as compared to
the PDMS stamp, is unchanged as a result of stamping and baking: i.e., no reactive
spreading was observed upon pattern transfer by stamping or baking. The original film
thickness was 300 nm and, after etching, the thickness of the patterned aluminum was
295±5 nm. The decrease in thickness corresponds to a loss of less than 5%, without loss of
lateral resolution.
89
Figure 2.6: a) AFM image of the stamp used to prepare structures for the resistance
measurements. b) AFM image of 300 nm thick aluminum, patterned by gCP of
octadecanephosphonic acid, followed by baking for 10 minutes at 70 'C and
wet-etching in Aluminum Etchant Type "A" at 35 'C.
90
a)
,1200
t
600o300-
ATM
I
10
0
I
II
30
20
Distance (pn)
50
40
so
Lateral Distance (pm)
b)
500
375
-
250125A.
V -I
rrvflTnrx
I
10
0
I
III
30
20
Distance (pm)
40
0
Lateral Distance (pm)
50
91
Edge resolution and nature of defects. Figure 2.7a shows the edge resolution of the
patterns prepared by lift-off and Figure 2.7b shows the edge resolution of the patterns
prepared by gCP and wet-etching. The edge resolution of the patterns prepared by lift-off
was 50 nm and the edge resolution of the patterns prepared by gCP and etching was 150 nm.
We attribute 100 nm of the edge roughness in the etched patterns to the stamping and etching
process.
Pattern transfer occurs as a result of differing etch rates between the stamped and
unstamped regions. Prolonged exposure of a stamped pattern to the etching solution
ultimately results in overetching and a decrease in aluminum thickness in the patterned areas.
We observed two types of defects in the etched patterns. The first type of defects we
observed were large (>3 gm) circular holes in the etched pattern. We speculate that these
defects are due to inadequate pattern transfer in stamping, possibly due to air bubbles or
particles trapped between the surface and the stamp. If these defects are due to trapped
particles, stamping in a cleanroom should decrease the occurrence of these defects.
Figure 2.7b shows the second type of defects we observed. These defects appear as dark,
circular "pinholes" (-40 nm diameter) in high resolution SEM images of the etched pattern.
We were unable to determine if these "pinhole" defects result in holes extending through the
entire thickness of the patterned aluminum. 4 4 The SEM does not provide information on the
thickness of the films and the AFM images did not show these defects extending through the
entire thickness of the patterned aluminum. We note, however, that the defects are on the
order of the radius of curvature of the pyramidal AFM tip and the geometry and aspect ratio
of the AFM tip prevents us from determining their depth.
92
Figure 2.7: a) SEM image of the edge resolution of patterned aluminum wires prepared by
photolithography and lift-off. b) SEM image of the edge resolution of patterned
aluminum wires prepared by kCP and wet-etching. We attribute the bright edge
of the wet-etched sample to charging (the substrate in both cases was a silicon
nitride coated silicon wafer, yet we observed more charging in the wet-etched
sample). We have defined the dark circular features in the wet-etched image as
"pinhole" defects (see text).
93
a)
b)
94
Characterization of the electrical properties of the etched structures. We
measured the resistance as a function of length for structures prepared using the mask design
in Figure 2.2. The structures were patterned, by two different methods, on 300 nm thick
aluminum films supported on silicon nitride coated silicon wafers: i) Deposition of
evaporated aluminum onto a photoresist replica of the mask pattern, followed by lift-off in
acetone, and ii) Stamping octadecanephosphonic acid onto A12 0 3/Al with a PDMS stamp that
was cast against a photoresist replica of the mask, followed by baking and wet etching. The
silicon nitride provides an insulating layer between the metal and the silicon substrate and
isolates the patterned aluminum wires electrically.
. Figure 2.8 shows a plot of resistance versus length for the structures prepared by the
two methods described above. Three different samples, each containing at least twelve of the
resistance patterns shown in Figure 2.2 were measured for the plot in Figure 2.8. Each data
point represents the average of all the samples measured and the error bars represent the
standard deviation associated with the average. The data points for the shorter distances in
the plot (<22.6 cm) were the average of 13 to 71 measurements and the longer distances
(30.1-67.7 cm) were the average of 2 to 6 measurements. There were fewer measurements
for the longer distances because of defects in the stamped pattern, which resulted in breaks
along the patterned wire. On average, we observed approximately 10-15 breaks per pattern.
The position of the break determined the maximum length of the wire that we could assay.
The plots of the stamped pattern and the lift-off pattern are linear, indicating that the resulting
films are homogeneous over the length assayed.
Equation 2.1 relates resistance, (R, 0), resistivity, (p, i2m), length, (I , m) and
cross-sectional area, (A, m2 ). The slope for the patterned wires prepared by gCP and wet
95
Figure 2.8: Plot of resistance versus length for 6 gm wide aluminum wires, patterned by
stamping, followed by baking and wet-etching (filled squares), compared to the
same wire patterned by photolithography (open squares). The solid line is the
least squares fit of the wet-etch data and the dashed line is the least squares fit of
the lift-off data.
96
30000
m gCP and Wet Etching
25000
-
20000
-
o Photolithography and Lift-off
-'
15000ri~
1000005000-
0.0
-
I
0.1
I
0.2
0.3
I
0.4
Length (m)
I
0.5
I
0.6
0.7
0.8
97
R = " '(2.1)
A
etching is 32100±700 Qm'1 and the slope for those prepared by photolithography and lift-off
is 31300±100 Qm-1. The thickness of the structures prepared by lift-off is 300±3 nm, and the
average thickness of the samples prepared by gCP and wet etching is 295±5 nm. Using
Equation 2.1, the resistivity of the samples prepared by RCP and wet etching is
(5.6±0.2)x10-8 9m and the resistivity of the samples prepared by photolithography and
lift-off is (5.6±0.1)x10- Qm.
Preparation and characterization of Schottky diodes. The Schottky diodes were
prepared according to the fabrication scheme shown in Figure 2.4. The ohmic contacts were
defined in 1 cm 2 arrays, consisting of 19 columns and 39 rows of contacts, across a 4" wafer.
The wafer was cleaved into 1 cm2 pieces for stamping the Schottky contacts, resulting in an
array of 741 diodes. We defined the registration after stamping as the average of the
difference in the placement of the center of stamped alignment mark relative to the center of
the alignment mark prepared by photolithography. This average difference was 1 pm across
the 1 cm 2 array. The main factors contributing to the misalignment were larger than designed
features in the photoresist pattern used to prepare the stamp, and human error in judging the
center of the two alignment marks. The alignment marks on the mask for the Schottky
contacts were designed as crosses, 4 gm wide and 14 jim long; the fabricated registration
marks were distorted crosses, with rounded edges, approximately 6 pim wide and 18 jim
long. The fabricated diodes contained gaps which varied from 97 gm to 260 nm.
Figure 2.9a shows an optical micrograph of part of a 1 cm 2 array of Schottky diodes
(32 diodes) and Figure 2.9b shows a single diode with a gap of 3 pim.
98
Figure 2.9: Optical micrographs of the fabricated diodes. a) Optical micrograph of an array
of 32 diodes. b) Optical micrograph of a single diode with a 3 Im gap between
the ohmic and Schottky contacts.
----------
99
a)
b)
100
Forward current-voltage (I-V) characteristics. Figure 2.10 shows the
current-voltage characteristics of a representative Schottky diode. The forward and reverse
I-V scans were collected separately, with different current scales. The forward I-V
characteristics of a Schottky diode display an exponential relationship between current
density, Jf and forward voltage, Vf, at low forward biases. The Jt-Vf relationship of an ideal
33 45
Schottky diode can be modeled by the thermionic-emission theory for current transport. ,
This relationship, shown in Equation 2.2, relates the forward current density, Jf (A cm'), the
saturation current density, JO (A cm ), the magnitude of electronic charge, q (1.602x10-19 C),
the forward voltage, Vf (V), Boltzmann's constant, k (1.381xlO
3
J K-1), the absolute
temperature, T (K) and the ideality factor, n. The ideality factor takes into account the bias
dependence of the barrier height and the thermionicJf = J. e(qVf /nkT)
(2.2)
emission theory model is valid for n<l.1.33 The ideality factor is determined from the slope
of a linear plot of the natural log of current density versus forward bias voltage. If the diode
can be modeled by the thermionic emission theory, the effective barrier height, De (eV), is
calculated from the saturation current density, J0 , according to the relationship shown in
Equation 2.3, where A** is the modified Richardson constant (A**=32 A cm' K-2 for
p-type Si) and all other parameters are as defined above.
ln(Jo) = ln(A**T 2 ) - qIDe/kT
(2.3)
For n>1.1, the current-voltage characteristics cannot be modeled by the
thermionic-emission theory and a more complex model must be invoked. An ideality factor
greater than 1.1 is usually attributed to current arising from the recombination of electrons
101
Figure 2.10: Current-voltage (I-V) characteristics of a fabricated Schottky diode. The
forward and reverse I-V scans were collected separately, with different current
scales. The band diagrams for the metal/semiconductor junction at equilibrium,
for reverse bias and for forward bias are shown at the top of the figure,
corresponding to the points A, B and C, respectively, on the I-V curve.
102
B
ECB
A
-4V
ECB
E
EF
E9
I
C
4V
I
ECB
EV
Ir
-12
I
5 nA
-8
B
A
-4
0
Voltage (V)
4
20 mA
8
12
103
and holes in the depletion region. 3 3 The current-voltage characteristics of twenty four
arbitrarily chosen diodes with differing gaps between the ohmic and Schottky contacts were
measured. Figure 2.11 shows a typical plot of the ln(Jf)-Vf characteristics from 150 mV to
500 mV and the linear fit for the voltage region 150 mV to 350 mV. The deviation from
linearity above 350 mV is attributed to the series resistance of the silicon. 3 3 The ideality
factor ranged from 1.3 to 3.3, with the average equal to 2.4±0.6. There was no correlation
between the ideality factor and the gap between the contact pads, nor is such a correlation
expected from theory. The average value we obtained for the ideality factor is in agreement
46
with that of Adegboyega et al. for aluminum Schottky contacts prepared on p-type silicon.
The value of our measured ideality factor precludes us from determining the barrier height
using Equation 2.3.
Capacitance-voltage (C-V) measurements. The barrier height can be determined
from dark capacitance-voltage (C-V) measurements. The relationship between capacitance
and reverse bias is shown in Equation 2.4, where C is the capacitance per unit area (F cm
2
Es is the permittivity of the semiconductor (ss=1.04x10-14 F cm' for Si), Na is the acceptor
concentration for p-type semiconductors (cm-3), Vbi is the built-in potential at zero bias (eV),
V is the applied voltage (V) and all other terms are as defined previously. A plot of C-2
versus reverse bias should be a straight line with a slope equal to 2 /qasNa and the intercept on
-- =
2
C 2 qgsNa
V _.V_
y
q)
(2.4)
104
Figure 2.11: Plot of the natural log of current density, ln(Jf), versus the forward bias voltage,
Vf, from 150 mV to 500 mV, for a fabricated Schottky diode (solid line) and the
associated linear fit over the range 150 mV to 350 mV, extrapolated to Vf=0
(dashed line).
105
-6
-
-8
1
-12
-
-14
-
I44-
/
4-
/
-16
-I- 0
0.4
0.2
Vf (V)
0.6
106
the voltage axis, V1, equal to Vbi+kT/q. The barrier height, cDB, is given by Equation 2.5,
where , equal to (kT/q)ln(N,/Na), is the distance of the Fermi energy below the valence band
and Nv is the effective density of states in the valence band (Nv=1.OlxlO
cm3 for Si). 3 3
(2.5)
cDB = Vbi +
Non-ideal diodes with a thin interfacial oxide layer may contain an "excess
capacitance", C, that results in a non-linear C-V plot. This excess capacitance is an
empirical constant which accounts for the bias dependent charge on surface states and/or
deep bulk states. Vasudev et al. have shown that C0 can be determined from the y-intercept
4 7 Figure 2.12 shows the plot of 1/C
of a plot of capacitance versus (Vbi+V)-0. 5 .
2
and
2
1/(C-CO)2 as a function of reverse bias and the associated linear fit of the 1/(C-C 0 ) data
extrapolated to the x-axis intercept.
Using the linear fit from the plot of 1/(C-C.) 2 versus Vr, we calculated a barrier
height,
(DB,
of 1.17 eV and a majority carrier density of 1.04x1016 cm 3. The majority carrier
density corresponds to a resistivity value of 1.25 acm, which is within the specification
range of the wafers we used (0.5-2 Qcm). Adegboyega et al. attribute the high barrier height
values for aluminum p-type silicon Schottky contacts and the non-ideal behavior of the
4 6 Our results
current-voltage characteristics to the presence of a thin interfacial oxide layer.
are consistent with this hypothesis. Silicon forms a native oxide in air and it is likely that this
oxide formed on our samples during the time period between the silicon dioxide etching step
and the evaporation chamber achieving high vacuum.
107
Figure 2.12: Capacitance-voltage (C-V) characteristics of a fabricated Schottky diode. The
upper trace corresponds to a plot of 1/(C-Co)2 versus the reverse bias voltage, Vr,
and the associated linear fit (solid line) extrapolated to the x-axis intercept, where
CO=17.0 nF cmn.
bias voltage, Vr.
The lower trace corresponds to a plot of 1/C2 versus the reverse
108
16Co= 17.0 nF cm- 2
12-
8-
4Co= 0
0-14
-12
-10
-8
-6
Voltage (V)
-4
-2
0
2
109
Conclusions
We have shown that octadecanephosphonic acid can be patterned on Al 2 O3/Al by
gCP and that the patterned structures are electrically similar to samples prepared by
conventional photolithography and lift-off. We have not established the lower limit of the
features that can be produced, but we note that the edge roughness of the features we have
produced was 150 nm: a feature size of 600 nm might therefore be possible. We have
demonstrated that ptCP of octadecanephosphonic acid is compatible with semiconductor
device fabrication and that the Schottky diodes we prepared behave similarly to those
fabricated by conventional fabrication techniques.
110
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1)
Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550-575.
2)
Qin, D.; Xia, Y.; Rogers, J. A.; Jackman, R. J.; Zhao, X.-M.; Whitesides, G. M.
Top. Curr. Chem. 1998, 194, 1-20.
3)
Zhao, X.-M.; Xia, Y.; Whitesides, G. M. J. Mater. Chem. 1997, 7, 1069-1074.
4)
Wilbur, J. L.; Kumar, A.; Biebuyck, H. A.; Kim, E.; Whitesides, G. M. Nanotechnology
1996, 7, 452-457.
5)
Streetman, B. G. Solid State Electronic Devices; 4th ed.; Prentice Hall:
Englewood Cliffs, NJ, 1995.
6)
Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L.
J. Vac. Sci. Technol., A 1996, 14, 1844-1849.
7)
St. John, P. M.; Craighead, H. G. Appl. Phys. Lett. 1996, 68, 1022-4.
8)
Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir
1995, 11, 3024-6.
9)
Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.;
Nuzzo, R. G. Langmuir 1995, 11, 813-24.
10) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J.Am. Chem. Soc. 1995, 117, 6927-33.
11) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc.
1988, 110, 618-620.
111
12) Xu, X.-H.; Yang, H. C.; Mallouk, T. E.; Bard, A. J. J. Am. Chem. Soc.
1994, 116, 8386-7.
13) Mallouk, T. E.; Kim, H.-N.; Ollivier, P. J.; Keller, S. W. Ultrathinfilms based on
layered materials; Alberti, G. and Bein, T., Ed.; Elsevier, Oxford, UK., 1996;
Vol. 7, pp 189-217.
14) Golden, W. G.; Snyder, C. D.; Smith, B. J. Phys. Chem. 1982, 86, 4675-8.
15) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45-52.
16) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66.
17) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978-87.
18) Chen, S. H.; Frank, C. W. ACS Symp. Ser. 1990, 447, 160-76.
19) Chen, S. H.; Frank, C. W. Langmuir 1991, 7, 1719-26.
20) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350-8.
21) Tao, Y.-T.; Hietpas, G. D.; Allara, D. L. J.Am. Chem. Soc. 1996, 118, 6724-6735.
22) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir
1996, 12, 6429-6435.
23) Bram, C.; Jung, C.; Stratmann, M. Fresenius'J.Anal. Chem. 1997, 358, 108-111.
24) Maege, I.; Jaehne, E.; Henke, A.; Adler, H. J. P.; Bram, C.; Jung, C.; Stratmann, M.
Macromol. Symp. 1998, 126, 7-24.
112
25) Cave, N. G.; Kinloch, A. J. Polymer 1992, 33, 1162-70.
26) Aronoff, Y. G.; Chen, B.; Lu, G.; Seto, C.; Schwartz, J.; Bernasek, S. L.
J.Am. Chem. Soc. 1997, 119, 259-262.
27) Chen, P. J.; Wallace, R. M.; Henck, S. A. J. Vac. Sci. Technol., A 1998, 16, 700-706.
28) pKa CF 3COOH = 0.3; pKa CH 3COOH = 4.74.
29) Ramsier, R. D.; Henriksen, P. N.; Gent, A. N. Surface Science 1988, 203, 72-88.
30) Palmer, B. J.; Wrighton, M. S. Unpublished results.
31) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; 5th ed.; John Wiley &
Sons: New York, 1988.
32) Zhao, X.-M.; Wilbur, J. L.; Whitesides, G. M. Langmuir 1996, 12, 3257-3264.
33) Rhoderick, E. H.; Williams, R. H. Metal-Semiconductor Contacts; 2nd ed.; Clarendon
Press: Oxford, 1988; Vol. 19.
34) The e-beam we used to achieve uniform aluminum film thickness (see substrate
preparation section) was located in the Technology Research Laboratory at MIT and this
is a regulated cleanroom. The use of PDMS was not permitted in this facility, and
additionally, we were not permitted to bring in samples that had been processed with
PDMS outside of the cleanroom.
35) Hu, J.; Beck, R. G.; Westervelt, R. M.; Whitesides, G. M. Adv. Mater.
1998, 10, 574-577.
113
36) This increase in feature size could be due to inadequate exposure times or inherent
limitations of the thick resist used in this step.
37) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-511.
38) Rogers, J. A.; Paul, K. E.; Whitesides, G. M. J. Vac. Sci. Technol. B 1998, 16, 88-97.
39) This result might be due to a stronger interaction of the phosphonic acid head group with
the oxidized stamp, as compared to the hydrophobic, untreated stamp.
40) The rinsing step was employed in order to obtain SEM images of the patterned surface.
The etched films after rinsing, followed by baking contained many defects and exhibited
a decrease in aluminum thickness. There appeared to be a correlation between the
rinsing time and the quality of the etched samples: samples that were rinsed for ten
seconds contained fewer defects than those rinsed for thirty seconds. We did not
investigate this behavior further because in all cases, the rinsed samples contained too
many defects to be useful.
41) The initial baking time chosen was ten minutes. Subsequent studies varied the baking
time; there was no observable change in pattern definition, line width or aluminum
thickness after etching for samples which had been baked a minimum of one minute.
42) Vossen, J. L.; Kern, W. Thin Film Processes; Academic Press, Inc.: Orlando,
1978, pp 564.
43) Materials Safety Data Sheet for Aluminum Etchant Type "A", Transene Co. Inc.,
Danvers Industrial Park, 10 Electronics Avenue, Danvers, MA 01923.
114
32
44) We were unable to use the amplification procedure of Zhao et al. to quantify the size
and number of our defects because the etch solution used in that study is also an etchant
for aluminum.
45) Rhoderick, E. H. IEE proceedings. I, Solid-state and electron devices. 1982, 129, 1-14.
46) Adegboyega, G. A.; Poggi, A.; Susi, E.; Castaldini, A.; Cavallini, A. Appl. Phys.
1989, A48, 391-5.
47) Vasudev, P. K.; Mattes, B. L.; Pietras, E.; Bube, R. H. Solid-State Electronics
1976, 19, 557-559.
48) Kosolapoff, G. M. J. Am. Chem. Soc. 1945, 67, 1180-1182.
49) Bhattacharya, A. K.; Thyagarajan, G. Chem. Rev. 1981, 81, 415-430.
50) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626-3629.
51) Caution: Piranha solution is an extremely strong oxidant and should be handled with
care.
115
CHAPTER 3
SELF-ASSEMBLED MONOLAYERS OF METHYL AND CARBOXYLIC ACID
TERMINATED ALKANEPHOSPHONIC ACIDS ON A12 0 3/Al: FORMATION OF
DERIVATIVES AT THE CARBOXYLIC ACID TERMINUS
116
Introduction
This chapter describes the formation and characterization of self-assembled
monolayers (SAMs) on A12 0 3/Al films supported on Si substrates, of a series of
1-alkanephosphonic acids of increasing chain length, and the formation, characterization and
reaction of SAMs of 16-phosphonohexadecanoic acid on this substrate. The first part of the
chapter deals with SAMs of 1-alkanephosphonic acids: H3 C(CH2)nPO(OH)2; n=8, 10, 12,
14, 16, 18, 20, 22. The 1-alkanephosphonic acid monolayers were characterized by
measurements of contact angles, by ellipsometry, by X-ray photoelectron spectroscopy
(XPS) and by polarized infrared external reflection spectroscopy (PIERS). The second part
of the chapter addresses monolayers formed from 16-phosphonohexadecanoic acid and their
use in forming reactive SAMs terminated with a mixture of interchain carboxylic anhydride
and mixed anhydride groups. 1 The SAMs terminated with the mixture of anhydrides were
used to prepare mixed SAMs containing carboxylic and amide functionalities. The
monolayers formed from 16-phosphonohexadecanoic acid and the subsequent reaction
products were characterized by PIERS and XPS.
Synthesis of the alkanephosphonic acids. Scheme 3.1 shows the synthetic
methodology used for the preparation of the alkanephosphonic acids. The diethyl
alkanephosphonates were prepared from the corresponding bromoalkane and
triethylphosphite using the Michaelis-Arbuzov rearrangement. 2 We used acid hydrolysis to
convert the corresponding diethyl alkanephosphonic acid to the alkanephosphonic acid
because the functional groups at the o-terminus (CH 3, OH and COOH) were not subject to
decomposition under the acidic hydrolysis conditions. 3 The products were purified by
recrystallization.
117
Scheme 3.1: Synthetic route for the formation of alkanephosphonic acids.
118
0
A
R-X
+
P(OCH 2CH 3)3
R
(excess)
0
+
R
0
Hs)
(excess)
0
0
0
A
R
OH
OH
119
Preparation of substrates. The A120 3/Al substrates used in this study were prepared
by thermal evaporation of Al metal (200 nm) onto test grade, polished Si (100) wafers. The
oxidation of the Al surface was accomplished either by exposure of the wafers to oxygen in
the evaporation chamber after the evaporation was complete, or by ambient oxidation either
in air or in the derivatizing solutions in which the monolayers were formed. The quality of
the monolayers formed did not depend on the method of oxidation of the samples.
We chose to use evaporated Al films for this study because of the ease in preparation
of evaporated Al films and the literature precedence for the use of evaporated films in the
study of SAMs. 4 One drawback of this substrate system is its reactive nature. Aluminum
forms a native oxide in air; alumina is a high energy surface that is prone to contamination by
airborne contaminants. The oxidation and rapid adsorption of impurities on the surface of
the evaporated Al films complicate the characterization of the monolayers formed on these
substrates.
Formation of monolayers. The freshly evaporated substrates (200 nm of Al
supported on Si wafers) were placed into a 1 mM (50% aqueous ethanol) solution of the
corresponding alkanephosphonic acid. The 1-alkanephosphonic acids were derivatized for a
minimum of 16 h prior to characterization. The 1 mM (50% aqueous ethanol) solutions of
the three longest-chain 1-alkanephosphonic acids (m=18, 20, 22) were supersaturated. The
molecules were soluble in the 2 mM ethanol solutions from which the aqueous solutions
were prepared; the solutions became cloudy after the addition of water, but the compounds
did not precipitate in macrocrystalline form. After derivatization, the samples were removed
from solution and rinsed with ethanol or 2-propanol and dried under a stream of N2 gas.
120
Experimental
Materials. 2-Propanol (99.9%, HPLC grade, B&J Brand, Burdick & Jackson),
ethanol (200 proof, Pharmco), water (HPLC grade, Omnisolve®, EM Science), hydrochloric
acid (36.5-38%, EM Science), glacial acetic acid (99.75, EM Science), hydrobromic acid
(48%, JT Baker), hexanes (HPLC Grade, EM Science), triethylphosphite (98%, Aldrich),
1-propanephosphonic acid (95%, Aldrich), 1-butanephosphonic acid (98%, Alfa),
1-octanephosphonic acid (98%, Alfa), 1-bromodecane (98%, Aldrich), 1-bromododecane
(97%, Aldrich), 1-tetradecanephosphonic acid (98%, Alfa), 1-bromohexadecane (97%,
Aldrich), 16-hydroxyhexadecanoic acid (98%, Aldrich), octadecanoic acid-d 35 (98% D,
Cambridge Isotope Laboratories, Inc.), borane-tetrahydrofuran complex (1.0 M solution in
tetrahydrofuran, Aldrich), trifluoracetic anhydride (99+%, Aldrich), triethylamine (99+%,
Aldrich), NN-dimethylformamide (anhydrous, 99.8%, Aldrich), hexadecane (anhydrous,
99+%, Aldrich), 1-methyl-2-pyrrolidinone (Aldrich), phosphorus pentoxide (Fisher
Scientific), aluminum (99.999%, Alfa) and silicon (100) wafers (4" diameter, test grade,
Silicon Sense) were used as received. Tetrahydrofuran (Fisher Scientific) was distilled from
sodium/benzophenone under nitrogen. Dichloromethane (Mallinckrodt) was distilled from
calcium hydride under nitrogen. 1-octadecanephosphonic acid was available from a previous
study: It was prepared using the Michaelis-Arbuzov reaction according to published
procedures.2, 6 7 ,6 8 16-Bromohexadecanoic acid 6 9 and 16-bromohexadecanol 7 0 were
prepared according to published procedures.
General synthesis of alkanephosphonic acids. The 1-bromoalkane (27 mmol) was
refluxed in excess triethylphosphite (204 mmol, 35 mL) for three hours. The excess
triethylphosphite was removed by vacuum distillation. Concentrated hydrochloric acid
121
(50 mL) was added to the crude 1-diethylphosphonoalkane and the mixture was heated to
reflux for eight hours, followed by cooling and filtration through a glass frit. The crude
product was dried over phosphorus pentoxide in a vacuum dessicator (100 mtorr) overnight.
The products were recrystallized twice from boiling hexanes (decane, dodecane, and
hexadecanephosphonic acid), hot decane (-90 'C, eicosanephosphonic acid) or a mixture of
boiling hexanes with a minimum of 2-propanol (1 6-phosphonohexadecanol and
16-phosphonohexadecanoic acid) or a mixture of hot decane and 2-propanol (-90 *C,
docosanephosphonic acid). The NMR spectra were recorded on Bruker AM400 or AM500
spectrometers.
1-Decanephosphonic acid. M.P. 106 0C. 'H NMR (400 MHz, CD 30D) 6, ppm:
0.89 (t, J=7 Hz, 3 H), 1.30 (broad s, 12 H), 1.36-1.45 (m, 2 H), 1.53-1.72 (m, 4 H).
13C
NMR (400 MHz, CD 30D) 8, ppm: 14.45, 23.73, 23.91 (d, Jp-c=5 Hz),
28.09 (d, Jp-c=137 Hz), 30.32, 30.45, 30.57, 30.71, 30.79 (d, Jp-c=17 Hz), 33.06.
31P
NMR (500 MHz, CD 30D) 8, ppm: 31.58 (relative to H3 PO 4 , internal standard).
Mass Spectrometry (FAB) calcd. for CioH 2 3 0 3 P (M+H)+ 223.1463, found 223.1460.
1-Dodecanephosphonic acid. M.P. 104 0C. 'H NMR (400 MHz, CD 30D) 6, ppm:
0.89 (t, J=7 Hz, 3 H), 1.29 (broad s, 16 H), 1.37-1.44 (m, 2 H), 1.53-1.73 (m, 4 H).
13C
NMR (400 MHz, CD 30D) 6, ppm: 14.45, 23.74, 23.93 (d, Jp-c=5 Hz),
28.10 (d, Jp-c=137 Hz), 30.33, 30.49, 30.57, 30.76 (broad s), 31.80 (d, JP-c=17 Hz), 33.08.
3'P
NMR (500 MHz, CD 3 0D) 6, ppm: 31.64 (relative to H3 PO 4 , internal standard).
Mass Spectrometry (FAB) calcd. for C12 H 2 7 0 3 P (M+H)+ 251.1776, found 251.1775.
1-Hexadecanephosphonic acid. M.P. 98 0C. 'H NMR (500 MHz, CD 30D) 6, ppm:
0.89 (t, J=7 Hz, 3 H), 1.27 (broad s, 24 H), 1.37-1.41 (m, 2 H), 1.54-1.70 (m, 4 H).
122
"C NMR (500 MHz, CD 3 0D) 6, ppm: 14.46, 23.72, 23.92 (d, Jp-c=4 Hz),
28.11 (d, Jp-c=138 Hz), 30.30, 30.46, 30.54, 30.76 (broad s), 31.79 (d, JP-c=16 Hz), 33.05.
31
P NMR (500 MHz, CD 30D) 6, ppm: 31.53 (relative to H 3 PO 4 , internal standard).
Mass Spectrometry (FAB) caled. for C 16H 3 5 0 3 P (M+H)+ 307.2402, found 307.2408.
1-Eicosanephosphonic acid. M.P. 106 0C. 'H NMR (500 MHz, CD 3 0D) 6, ppm:
0.89 (t, J=7 Hz, 3 H), 1.28 (broad s, 32 H), 1.36-1.44 (m, 2 H), 1.54-1.71 (m, 4 H).
31P
NMR (500 MHz, CD 3 0D) 6, ppm: 31.58 (relative to H3PO 4 , internal standard).
Mass Spectrometry (FAB) calcd. for C 2 0H 4 30 3 P (M+H)+ 363.3028, found 363.3022.
1-Docosanephosphonic acid. M.P. 108 0C. 'H NMR (400 MHz, CD 3OD) 6, ppm:
0.89 (t, J=7 Hz, 3 H), 1.28 (broad s, 36 H), 1.36-1.46 (m, 2 H), 1.51-1.73 (m, 4 H).
3
1P
NMR (500 MHz, CD 30D) 6, ppm: 31.57 (relative to H 3PO 4 , internal standard).
Mass Spectrometry (FAB) calcd. for C 2 2 H4 70 3 P (M+H)+ 391.3341, found 391.3354.
16-Phosphonohexadecanoic acid. This compound was prepared from
16-bromohexadecanoic acid using the general synthesis described above, with the following
exceptions. The crude 16-diethylphosphonohexadecanoic acid was recrystallized twice from
boiling hexanes/2-propanol (a minimum of 2-propanol was added to dissolve the compound)
prior to the hydrolysis reaction. The hydrolysis product was recrystallized twice from boiling
hexanes/2-propanol (a minimum of 2-propanol was added to dissolve the compound).
M.P. 137 *C. 'H NMR (500 MHz, CD 30D) 6: 1.29 (broad s, 20 H), 1.36-1.44 (m, 2 H),
1.54-1.71 (m, 6 H), 2.27 (t, J=7 Hz, 2 H).
3C
NMR (400 MHz, CD 30D) 6, ppm:
24.08 (d, Jc-p=5 Hz), 26.25, 28.25 (d, JC-p=138 Hz), 30.39, 30.47, 30.58, 30.71, 30.76, 30.86,
30.91 (broad s), 31.94 (d, JC-p=17 Hz), 35.11, 177.86.
31
p
NMR (500 MHz, CD 3OD):
123
6, ppm: 31.64 (relative to H3 PO 4 , internal standard). Mass Spectrometry (FAB) calcd. for
C16H33 0 5P (M+H)+ 337.2144, found 337.2149.
16-Phosphonohexadecanol. This compound was prepared from
16-bromohexadecanol using the general synthesis described above, with the following
exception. The crude product was recrystallized from boiling hexanes/2-propanol
(a minimum of 2-propanol was added to dissolve the compound). M.P. 116 *C.
H NMR (500 MHz, CD 3 0D) 6, ppm: 1.29 (broad s, 22 H), 1.37-1.47 (m, 2 H),
1.48-1.78 (m, 6 H), 3.53 (q, J=6 Hz, 2 H). 13C NMR (400 MHz, CD 30D) 6, ppm:
23.94 (d, JC-p=5 Hz), 26.95, 27.93, 28.13 (d, Jc-p=137 Hz), 29.99, 30.31, 30.55, 30.60, 30.65,
30.73, 30.76 (broad s), 31.79 (d, Jc-p=17 Hz), 33.66, 33.83, 63.02.
31
P NMR (500 MHz, CD 30D) 6, ppm: 31.34 (relative to H3 PO 4 , internal standard).
Mass Spectrometry (FAB) calcd. for C16 H35 0 4 P (M+H)' 323.2351 found 323.2336.
Preparation of A/Al 2O3 substrates. Al (200 nm) was deposited onto test grade Si
wafers by thermal evaporation of Al under high vacuum (10-7 torr) at a rate of 5 A s-1 in an
Edwards Auto 306 thermal evaporator. The A12 0 3/Al coated Si substrates were used
immediately following deposition.
Monolayer formation. The A12 0 3/Al coated Si were placed into a 1 mM solution of
the alkanephosphonic acid in 50% aqueous ethanol. The samples were removed from
solution after derivatization, rinsed with ethanol and dried under a stream of N2 .
Formation of the anhydride terminated SAMs and mixed SAMs. Carboxylic acid
terminated SAMs were formed on freshly evaporated substrates (200 nm Al supported on Si
wafers) from a 1 mM solution of 16-phosphonohexadecanoic acid (50% aqueous ethanol).
After derivatization, the samples were removed from solution, rinsed with ethanol and dried
124
under a stream of N2 . The carboxylic acid terminated SAMs were placed into a freshly
prepared solution of trifluoracetic anhydride (0.1 M) and triethylamine (0.2 M) in anhydrous
N,N-dimethylformamide for twenty minutes, at room temperature. The samples were
removed from solution, rinsed with CH 2Cl 2 , and dried under a stream of N2. The samples
were either characterized immediately or placed into a solution of the appropriate alkylamine
(10 mM) in anhydrous 1-methyl-2-pyrrolidinone for thirty minutes, at room temperature.
The samples were removed from solution, rinsed with ethanol and dried with N2 .
Formation of trifluoroacetate terminated SAMs. Hydroxyl terminated SAMs were
formed on freshly evaporated substrates (200 nm Al supported on Si wafers) from a 1 mM
solution of 16-phosphonohexadecanol (50% aqueous ethanol). After derivatization, the
samples were removed from solution, rinsed with ethanol and dried under a stream of N2 .
The hydroxyl terminated SAMs were placed into a freshly prepared solution of trifluoracetic
anhydride (0.1 M) and triethylamine (0.2 M) in anhydrous N,N-dimethylformamide for
twenty minutes, at room temperature. The samples were removed from solution, rinsed with
CH2 Cl 2 , dried under a stream of N 2 and characterized immediately.
Ellipsometry measurements: The ellipsometry measurements were performed with
a Rudolf Instruments AutoEL computer controlled ellipsometer using a He-Ne laser
()=632.8 nm) at an incident angle of 70' relative to the surface normal of the substrate. The
optical constants of the surface with a preadsorbed layer of methanephosphonic acid were
measured in each of three areas marked on the sample. The monolayer thickness was
measured in each of the three areas marked on the sample twice and the thickness was
reported as the average of these six measurements and the standard deviation associated with
this average.
125
Contact angle measurements. Advancing and receding contact angles of water and
hexadecane were measured on static drops that had advanced or receded over the surface at a
constant flow rate (- 1 p.L s-) using a Matrix Technologies Electro-pipette. The contact
angles were measured using a Ramd-Hart goniometer; the pipette tip was not removed from
the drop. Each sample was characterized with at least three drops of liquid, and the contact
angles were measured on both sides of the drop. The data is presented as the average of all
measurements and the error bars represent the standard deviation associated with the average.
PIERS measurements. The PIERS spectra were obtained in single reflection mode
using a dry air purged Digilab Fourier transform infrared spectrometer (BioRad, Cambridge,
MA). The p-polarized light was incident at 800 relative to the surface normal of the
substrate. A spectrum of SAMs of octadecanoic acid-d 35 formed on A12 0 3/Al/Si or
A12 0 3/AI/Si, cleaned in an oxygen plasma immediately before the measurement, were taken
as the reference spectrum. The number of scans collected was 1024 at a resolution of 2 cm.
XPS measurements. XPS spectra were collected on a SSX-100 spectrometer
(Surface Science Instrument) using monochromatic Al Ka X-rays (1486.6 eV). The survey
spectra and the C(ls), P(2p) and F(ls) spectra recorded to obtain surface concentration ratios
were collected with a 150-eV pass energy and 1 mm spot size. The high resolution core level
spectra were recorded with a 50-eV pass energy and a 0.6 mm spot size. The binding
energies were referenced to the C(ls) peak, which was fixed at 286.6 eV. An electron flood
gun of 5 eV was used to dissipate sample charging.
126
Results and Discussion
Contact angles of SAMs of 1-alkanephosphonic acids on Al 2 O3/Al as a function
of chain length. If a drop of a liquid on a solid surface forms a sessile drop, the angle
between the liquid-vapor interface and the liquid-solid interface at the solid-liquid-vapor
5
three-phase contact line is defined as the contact angle. Figure 3.1 shows two
configurations in which the contact angle of a liquid on a solid surface is greater than zero.
6
Contact angle measurements are intrinsically a macroscopic surface property. The
theoretical treatment of contact angles, developed by Young for equilibrium contact angles
on ideal surfaces, is given by Equation 3.1.7 This relationship, known as Young's equation,
-ys =
civCos 0e -
(3.1)
relates ysv, the solid-vapor interfacial free energy, ysi, the solid-liquid interfacial free energy,
71V, the liquid-vapor interfacial tension and e, the equilibrium contact angle. The contact
angle measurements from this study are not equilibrium contact angles but rather exhibit
hysteresis: the contact angles measured were dependent on whether the liquid advanced or
receded over the surface prior to the measurement. Hysteresis is prevalent in contact angle
measurements, and is defined as the difference between the angle observed after a liquid has
advanced over the surface,
Oa,
and the angle observed after the liquid has retreated, Or.6
There are many factors contributing to hysteresis, some of which are: surface roughness,
surface heterogeneity, diffusion of the liquid into the surface, swelling of the surface by the
liquid and the rate of motion of the liquid over the surface. Hysteresis is a complex
phenomena, it can, however, provide additional information on surface properties. It is
generally accepted that the advancing contact angle is less sensitive to the effects of
roughness than the retreating contact angle6 and the advancing angles of a low energy
127
Figure 3.1: a) A typical contact angle for hexadecane on a methyl terminated surface, where
0=45-46* and b) A typical contact angle for water on a methyl terminated
surface, where O=111-115 o4
128
a)
129
surface with impurities of higher energy are more reproducible and possibly more
5
representative of the actual surface than the receding contact angles.
Figure 3.2 shows a plot of advancing, Oa, and receding, Or, contact angles of water
and hexadecane for a series of SAMs of 1-alkanephosphonic acids formed on A12 0 3/Al. The
shortest chain length molecule, octanephosphonic acid, exhibits the smallest advancing
contact angle of water, Oa, and the largest hysteresis. Previous studies of SAMs of
alkanethiols on Au, Ag and Cu found that the contact angles of chain lengths with n ;8 were
significantly lower than those for n>l 1.8 The results of our study show that the contact
angles for n=9, 17, 19, and 21 were similar and that the contact angles for n=11,13, and 15
were slightly higher than the others. The advancing contact angles of hexadecane were
invariant with chain length. In general, the wetting measurements suggest that all the
monolayers expose a low-energy, methyl-terminated surface. The larger advancing contact
9
angles observed for n=11,13 and 15 could be attributed to a rougher surface morphology.
Determination of monolayer thickness. 1. Ellipsometry. Ellipsometry is a
technique that measures the change in polarization for a beam of incident polarized light
upon reflection from an optically flat substrate. 10 The thickness is calculated from the
difference in the change in polarization of the reflected light from a "clean", reference
surface and the surface with the monolayer. In the case of Au, the surface resists oxidation
and it is possible to measure the optical constants of a "clean" Au surface (probably, in
actuality a gold surface with a thin, weakly bond organic layer) immediately after its
preparation, prior to the contamination of the surface by airborne contaminants. The relative
thicknesses of SAMs on Au measured by ellipsometry are highly reproducible within the
130
Figure 3.2: Plot of the advancing (filled diamonds) and receding (open diamonds) contact
angle of water and the advancing (filled squares) and receding (open squares)
contact angle of hexadecane versus the number of methylene carbons, n, for a
series of SAMs of 1-alkanephosphonic acids formed on A12 0 3/Al. Each point is
the average of the measurement on the left and right side of at least three drops
of liquid on each surface and the error bars represent the standard deviation
associated with the average.
131
140-
120I
100-
80-
60-
40-
20-
0-2
6
mm
8
10
12
14
n
16
18
20
22
132
limitations of the instrument. 11 -13 For reactive metals that form metal oxides or readily
react with airborne contaminants, it is difficult to obtain an accurate measurement of the
optical constants of the non-derivatized, "clean" surface. Ellipsometry has proven to be less
effective for the measurement of monolayer thickness on Ag, Cu and Al than on Au.8,14-18
If the surface oxidizes or if dissolution of the substrate occurs in the derivatizing solution, the
optical constants measured prior to the derivatization may not be representative of the surface
after derivatization. Allara and Nuzzo addressed the problem of contamination of the
A12 0 3/Al surface during the measurement of the optical constants of the "clean" surface by
preadsorbing acetic acid onto the surface, prior to the determination of the optical
constants. 14 After determination of the optical constants of the acetic acid coordinated
A12 0 3/Al surface, the monolayers were prepared by exchanging the acetic acid with a longer
chain n-alkanecarboxylic acid.
Instead of preadsorbing acetic acid, we preadsorbed methanephosphonic acid on the
surface prior to the determination of optical constants, and then exchanged the CH 3 POlayer with the longer chain 1-alkanephosphonic acids. Figure 3.3 shows a plot of thickness
versus the number of methylene units in the derivatizing molecule. Each point represents the
average of six measurements and the error bars represent the standard deviation associated
with this average. The dashed line represents the theoretical thickness of an all-trans
configuration, oriented normal to the surface. 1 9 The slope of the dashed line corresponds to
a thickness of 1.27 A per methylene unit 2 0 and the y-intercept corresponds to a thickness
of 4.3 A.21
The measured thickness always exceeded the calculated value for the monolayer by a
substantial amount. The data shown in Figure 3.3 have not been corrected for the
133
contribution of the displaced methanephosphonic acid. Allara and Nuzzo also reported their
data without correction for the displaced acetate layer. They noted, however, that during the
incubation time for monolayer formation, the oxide thickening compensated for the apparent
loss in thickness due to the acetate displacement. The data in Figure 3.3 suggest that we
cannot assume the same compensation under the conditions used here.
During the derivatization, there are several possible competing processes in solution:
oxide formation, multilayer formation, dissolution of the surface by the adsorbing molecule
and/or passivation of the surface as a result of monolayer formation. As the monolayer
becomes more ordered, passivation should be the dominant process. The contact angle
measurements suggest that the surface is methyl terminated and this conclusion implies that
multilayer formation is not occurring. If we assume that the molecules do not form
multilayers, then the data in Figure 3.3 are best rationalized by the formation of aluminum
oxide layers during the derivatization of the surface.
Determination of monolayer thickness. 2. Relative thickness determined from
XPS. XPS is a surface sensitive analytical technique that provides depth information on the
order of 10 nm or less. 2 2 -2 4 Unlike ellipsometry, the determination of monolayer thickness
by XPS does not require a measurement of the non-derivatized "clean" surface. It has been
previously shown that the thickness of SAMs of alkanethiols on Au, Ag and
Cu, 8 ,13,15,16,25,26 oligo(ethyleneglycol) terminated alkanethiols on Au, 2 7
alkyltrichlorosilanes on SrTiO 3 ,2 8 , alkyl monolayers on Ge(l 11),29 alcohols on silica 3 0 and
alkanehydroxamic acids on Cu and Ag18 can be estimated from the attenuation of the
substrate photoelectrons through the hydrocarbon overlayer. This technique did not apply to
134
Figure 3.3: Plot of ellipsometric thickness versus the number of methylene carbons, n, for a
series of SAMs of 1-alkanephosphonic acids formed on A12 0 3/Al. The dashed
line is an estimate of the thickness for an all-transalkane chain, oriented normal
to the surface.
135
80
1
70-
60
-
~50-
~40-
~30-
20
-
10
-
0 -1
0
I
2
4
I
I
I
6 8
12
10
n
14
16
18
20
22
136
alkanehydroxamic acids on Al 2O3/Al, possibly due to the differing ratios of Al and A120 3 on
different samples. 18
The 1-alkanephosphonic acids bind to the substrate through the phosphonic acid head
group and it may be possible to estimate the thickness of the monolayers using the
attenuation of the P(2p) signal as a function of alkane chain length. This treatment requires
that we choose a series of chain lengths where the monolayers form with equal coverage on
the surface. Previous studies of SAMs of alkanecarboxylic acids on Al and alkanethiols on
Au indicate that uniform coverage occurs for n>1 1.12,14
The determination of the thickness of a homogeneous overlayer A, on a substrate B is
given by Equation 3.2, where I, and Ib are the measured intensities of the overlayer and
substrate respectively, io and iB are the intensities for pure samples of A and B,
inelastic mean-free path of electrons from layer B in layer A,
XAA
B,A
is the
is the inelastic mean-free
path of electrons from layer A in layer A, q is the angle between the surface normal of the
sample and the analyzer and d is the thickness. 2 2 The application of this relationship
d
IBIA
= exp
IAIB e
0
requires a knowledge of I,
-exp ,
e
p
0
B,A
(cos
n
0
IB,
XB,A
d
- 0
B,A (COS 9)
-A,A
0
Ifwasueta
A,A.
If we assume that
d
(3.2)
(cOS T)
oA)0Ahnte
XB,A
A,A,
then the
thickness as a function of p is given by Equation 3.3.31,32
F0
d=
0
XB,A(cos
p) ln
IIBIA
0
IA IB
1
(3.3)
137
Equation 3.3 requires a knowledge of the attenuation length of the substrate electrons
in the overlayer,
XB,A
, and the intensities of the pure samples A and B, Ii and IB. The
literature contains examples where the measurement of the intensity of the C(is) signal from
a sample of polyethylene or paraffin wax has been used for IA, and either of those samples
3 0 3 3 The determination of
would provide a feasible method of obtaining IA in this study. '
IB, however, proves problematic since we are not dealing with a substrate of pure
phosphorus, but rather (alkyl)aluminophosphate. If we normalize our data to
1-dodecanephosphonic acid, then the requirement for the determination of I and 1B is no
longer necessary. The relationship between the thickness of a monolayer containing m
carbons, normalized to 1-dodecanephosphonic acid, is given in Equation 3.4, where dc(m) is
the thickness of a monolayer of 1-alkanephosphonic acid with m carbons,
dc(m=12)
is the
thickness of a monolayer of 1-dodecanephosphonic acid, Ic(m) and IP(m) are the intensities of
the C(ls) and P(2p) signals respectively for a monolayer with m carbons and
IP(m=12)
IC(m=12)
and
are the intensities of the C(1s) and P(2p) signals respectively for the monolayer
formed from 1-dodecanephosphonic acid. There are two unknowns in Equation 3.4: d and
1+ IC(m)
dc(m)- dC(m=12)= ',c (cos p) ln
(3.4)
IP(m)
1 + IC(m=12)
IP(m=12)
XB,A.
j
By varying p and d one can determine both unknowns. In our experimental setup, we
were limited to (p=55', and thus we can calculate dctm> -
dCm=12>
xP,C
to 1-dodecanephosphonic acid, in units of attenuation length.
,
the thickness, normalized
138
Figure 3.4 is a plot of (cos55)[(l+ICm)/IP(m))/(+IC(m=12)IP(m=12))] versus the number of
carbons in the alkane chain, m, and the linear least squares fit of the data. The slope of the
linear fit is 0.062i0.003. The data was collected from three independent XPS studies.
In order to determine the absolute thickness for the monolayers, we need to express
the slope in terms of thickness per methylene unit. This expression of the slope requires a
knowledge of the attenuation length of the P(2p) electrons in the hydrocarbon overlayer. The
attenuation length is a measure of the elastic and inelastic scattering of the exiting electrons
per unit path length. 3 4 The attenuation length is dependent on the kinetic energy of the
photoelectrons emitted from the surface and the nature of the surface. The accurate
34 4 1
determination of attenuation lengths is an area of active research and debate. -
Equation 3.5 is an empirical relationship for the determination of attenuation lengths
of metal substrates through hydrocarbon overlayers, where X is the attenuation length of a
metal substrate through a hydrocarbon overlayer and KE is the kinetic energy of the substrate
photoelectrons. 2 6 Equation 3.5 applies to the range of kinetic energies between
(3.5)
X(A) = 9.0 + 0.022 x KE(eV)
500-1500 eV. Using Equation 3.5, we calculate an attenuation length of 39
A for P(2p)
photoelectrons with a kinetic energy of 1354 eV. This value of X corresponds to an
incremental thickness per methylene unit of 2.4
A.
This incremental thickness exceeds that
of an all-trans hydrocarbon chain by 1.1 A and suggests that Equation 3.5 is not applicable to
the determination of X in our system.
The data in Figure 3.4 shows a linear relationship between the thickness and alkane
chain length. The model we chose to determine overlayer thickness does not involve a
139
Figure 3.4: Plot of (cos 5 5 )[(l+Ic(m)/IP(m))/(l +IC(m=I2)IP(m=12))] versus the number of carbons
in the alkane chain, m, for a series of SAMs of 1-alkanephosphonic acids formed
on A12 0 3/Al. The solid line is the least squares linear fit of the data. The slope
of the linear fit is 0.062±0.003.
140
0.8 U
0.70.6U
0.5
(cos55)In
1+IP(m)
- l")
0.4
-
1+ IC(m=12)
IPam=1>-
U
0.3 0.2
-
*
U
U
I
I
I
I
I
14
16
18
20
22
0.1 -
0- -1-- I
2
0
I
I
I
4
6
8
F~
10
~
12
m
24
141
substrate of infinite thickness (in terms of the sampling depth of XPS), but rather an
assumption of a monolayer of alkyl(aluminophosphate) where the coverage of the
phosphorus is constant with respect to differing monolayer thickness. As a result, it is not
unreasonable that the attenuation length calculated from Equation 3.5 does not apply to our
system. An angle dependent XPS study is required for the accurate determination of the
attenuation length for this system.3 1
Characterization of SAMs of 1-alkanephosphonic acids using PIERS. PIERS
provides spectral information for infrared active functional groups in which the transition
moment of the IR active group, or its component, is oriented normal to the surface. 4 2 PIERS
has been used to characterize SAMs of alkanethiols on Au, 1 1 ,43 Ag, 4 4 Cu 8 and for
submonolayers of Cu or Ag on Au; 4 5 alkanecarboxylic acids on Al, 4 6 ,4 7 Ag, and Cu;17,48
and alkanephosphonic acids on Al, 4 9 Au, Ag, and indium(tin)oxide (ITO). 5 0 The high
frequency region (2700-3100 cm-1) contains the information on the methyl and methylene
symmetric and antisymmetric stretches. The peak positions and intensities of these
vibrational modes can provide information on the degree of crystallinity and the packing
density of the monolayer on the surface. 1 1 If the monolayer forms a well-ordered,
crystalline layer, with the alkane chains in an all-trans conformation, and the thickness, d, of
the monolayer is known, then it is possible to estimate the orientation of the alkane chains,
specifically, the cant angle, a, and the chain twist angle, P, shown schematically in
Figure 3.5.51
The low energy region can provide information on the mode of binding of the head
group to the surface, e.g. for acids one can determine whether or not the acid binds as the
protonated or deprotonated acid. The low energy region also contains information on CH 2
142
Figure 3.5: Schematic illustration of the cant angle, a, and chain twist angle, P, for SAMs of
1-hexadecanephosphonic acid adsorbed on A12 0 3/Al, oriented in an all-trans
configuration.
143
A
d
A0
I
~
0/
AlU 3 O/Al (2 U nmj
0
~
I
144
deformation, twisting and wagging. Tao observed regularly spaced progressional bands in
the region between 1150 and 1350 cm-1 for alkanecarboxylic acids, in an all-trans
configuration, on Ag.
Figure 3.6 shows the high energy region of the polarized infrared external reflectance
spectra for a series of SAMs of 1-alkanephosphonic acids, (H3C(CH 2)nPO(OH) 2 , n=7, 9, 11,
13, 15, 17, 19) formed on A12 0 3/Al. Table 3.1 lists the peak positions and band assignments
corresponding to the spectra shown in Figure 3.6. The band assignments were made
according to the previously published work for alkanethiols on Au1 1 and alkanecarboxylic
acids on Al.4 6 The peak positions in Table 3.1 were calculated from the average peak
positions for a given chain length, determined from three spectra.
The PIERS spectra were recorded at a resolution of 2 cm-1 and were referenced to a
background spectrum of SAMs of octadecanoic acid-d 35 formed on A12 0 3/Al. For n>7, the
peak positions for the symmetric and in-plane antisymmetric methyl resonances in Table 3.1
are invariant with chain length. We note, however, that the antisymmetric, in-plane methyl
stretching mode in this study (2967 cm1) occurs at a higher frequency than that observed for
n-alkanethiols on Au (2965 cm-1),I for n-alkanecarboxylic acids on Cu and Al
(2964 cm-1),
17
(2964 cm-1).
50
or for Langmuir-Blodgett films of octadecanephosponic acid on Ag
The peak position is consistent with that observed for Langmuir-Blodgett
films of octadecanephosponic acid on Au (2967 cm'). 50
Figure 3.7 is a plot of the peak height of the symmetric and antisymmetric methyl
stretches, as a function of chain length. The intensity of the antisymmetric, in-plane stretch is
virtually independent of the chain length of the self-assembling molecule. The intensity of
the symmetric methyl stretch is invariant with respect to the chain length of the
145
Figure 3.6: PIERS spectra of the high energy region (2700-3100 cm-') for a series of SAMs
of 1-alkanephosphonic acids formed on A12 0 3/Al.
146
---
AY/A120
0.02
(CH2 CH3p
{
3
o-p-(CH2 )nCH3
0
21
19
3132
17
3100
3000
2900
Wavenumber (cm'1)
2800
2700
147
Table 3.1: Peak positions (cf)
for the symmetric and antisymmetric methyl and methylene
vibrational modes for SAMs of H3C(CH 2 )nPO(OH)2 formed on A12 0 3/Al and for
comparison, SAMs of CH 3(CH 2)15SH formed on Au.
peak frequency (cm-1)
mode descriptiona
7
9
11
13
15
17
19
21
15 b
2968
2967
2966
2967
2967
2967
2967
2967
2965
2937
2938
2937
n:
Va(CH3, i.p.)
vs(CH 3, FR)
2938
vs(CH 3)
2879
2879
2879
2879
2879
2879
2879
2879
2879
Va(CH2)
2927
2926
2923
2921
2920
2922
2921
2921
2918
vs(CH2)
2855
2852
2852
2851
2852
2852
2852
2852
2850
a Abbreviations used: Va: antisymmetric stretch; vs: symmetric stretch, i.p.: in plane;
FR: fermi resonance.
b Data for SAMs of CH 3 (CH 2) 15 SH on Au, taken from reference 11.
148
Figure 3.7: Plot of the peak intensities (arbitrary units) of the symmetric (open squares) and
antisymmetric (filled squares) methyl stretches versus the number of methylene
carbons, n, for a series of SAMs of 1-alkanephosphonic acids formed on
A12 0 3/Al.
149
20
0 v, (CH 3)
1.8 -
0
va
(CH 3 , ip)
1.6 1.41.20
1.0l
T
0.80.60.4-
0.20.0
6
8
10
8
10
.
12
.
14
n
I
I
I
16
18
20
22
150
self-assembling molecule for n 15; the intensity of this stretch is greater for n<15. The peak
positions of the methylene stretches indicate a less ordered monolayer for n<13 and this
decrease in order (greater randomness) may contribute to the observed higher intensities for
the symmetric methyl stretches with regards to the monolayers formed from shorter chain
length molecules.
For the three shortest alkanephosphonic acids (n=7-1 1), the peak position of the
antisymmetric methylene stretch decreases with increasing n; for n 13, it reaches the limiting
value of 2921±1 cm1. The peak position of the symmetric methylene stretch shows a
decrease in energy on going from octanephosphonic acid to the longer chain
alkanephosphonic acids; for n>7, the position (2852 cm') is not dependent on the molecule
forming the monolayer. The limiting values of the peak positions for both methylene
stretches occur at higher energies (-2-3 cm') than those for n-alkanethiols on Aul or for
that of Langmuir-Blodgett films of octadecanephosponic acid on Au, Ag or ITO.5 0 The
higher energies for the methylene stretches are indicative of a greater degree of gauche
conformations in the alkane chains of the alkanephosphonic acid SAMs. 1 8,45
Figure 3.8 is a plot of the peak height of the symmetric and antisymmetric methylene
stretches, as a function of chain length. The intensities of the SAMs formed from molecules
with n<13 appear scattered and exhibit as high or higher intensities than those for the
monolayers formed from n=13-17. We attribute these higher intensities to a greater degree
of disorder in the monolayer. For n=13-21, the antisymmetric methylene stretches show a
trend of increasing intensity with increasing n, albeit a slight increase for n=13-17. The
intensities of the corresponding symmetric methylene stretches appear to be invariant for the
range n=13-17 but show an increase with increasing n, for n 17. For an ordered surface of
151
Figure 3.8: Plot of the peak intensities (arbitrary units) of the symmetric (open squares) and
antisymmetric (filled squares) methylene stretches versus the number of
methylene carbons, n, for SAMs of 1-alkanephosphonic acids formed on
A42O3/Al.
152
6.0
-
Va (CH 2 )
o
5.0-
vs (CH 2)
4.0-
3.0-
2.0-
1.0-
0.0
-
6
8
1
10
2
12
4
14
n
6
16
8
18
20
22
153
all-trans alkane chains, oriented at an angle x, with respect to the surface normal, the
intensity of the methylene stretches should increase with increasing n. If the alkane chains
are oriented in an all-transconfiguration, oriented normal to the surface, the intensity of the
methylene stretches should be invariant with chain length, ideally exhibiting zero intensity. 17
Zero intensity was not observed for SAMs of alkanecarboxylic acids on Al or Cu, oriented
normal to the surface; the intensity of the methylene stretches was, however, invariant with
chain length. 17
We did not attempt to determine the absolute configuration of the alkane chains in the
monolayers in this study because we were unable to determine the absolute thickness of the
monolayers and the peak positions of the methylene stretches indicate some degree of
conformational disorder in the alkane chains.
The methylene stretches show an increase in intensity with increasing chain length
and this trend is indicative of a chain tilt angle, a, greater than zero. A comparison of the
intensities of the data in this study to that of alkanethiols on Au and Ag suggests that the
alkane chains of the SAMs in this study are canted from the surface normal more than that of
alkanethiols on Cu or Ag (-13') but less than that of alkanethiols on Au (~30').8,11,44,45
The tilt angle for Langmuir-Blodgett films of octadecanephosphonic acid varied from 9-12'
on Au to 16-17' on ITO, depending on the deposition pressure. 5 0
The low energy region of the PIERS spectra contains progressional peaks in the
region between 1350 and 1150 cm', which we attribute to the CH 2 wagging and twisting
modes. 17,52,53 These peaks, listed in Table 3.2, are present in all spectra, regardless of the
chain length of the self-assembling molecule. The number of these peaks varies from two for
n=7 to seven for n 19. The presence of these peaks suggests that portions of the alkane
154
Table 3.2: Peak positions (cm1) for the CH 2 wagging and twisting progressional bands for
1 -alkanephosphonic acids adsorbed on A12 0 3/Al.
7
9
11
13
15
17
19
21
o(CH2 )
1204
1197
1193
1190
1189
1188
1186
1184
y(CH 2)
1244
1231
1222
1214
1210
1206
1202
1199
1261
1248
1238
1231
1225
1219
1215
1297
1261
1261
1250
1243
1235
1230
1271
1260
1251
1244
1290
1278
1268
1259
1283
1288
n
155
chain are in an all-trans configuration. The peak positions of the methylene symmetric and
antisymmetric stretches indicated some degree of conformation disorder in the monolayer.
We speculate that the disordered region occurs for the alkane carbons nearest to the
phosphonic acid head group. A partially disordered region of the alkane chain near the head
group has been proposed to account for the intensities of the methylene stretches observed
for SAMs of alkanecarboxylic acids formed on aluminum oxide and copper oxide
surfaces.17
Formation and characterization of SAMs of 16-phosphonohexadecanoic acid.
The use of difunctionalized molecules to form SAMs provides a means to prepare surfaces
with a wide variety of surface properties. 12 ,43,54 In addition to the ability to tailor surface
properties by variation of the tail group in the self-assembling molecule, functional groups
capable of hydrogen bonding may impart increased order or stability to the SAM. Cooper
and Leggett reported a decrease in photooxidation of alkanethiols on Au when the
alkanethiol was terminated with hydroxyl or carboxylic acid groups, as compared to a methyl
terminus. 5 5
The main criteria for choosing the terminal group in the precursor molecule is the
choice of functional groups which do not bind as strongly to the surface as the head
group. 12,54 The phosphonic acid and carboxylic acid head groups will both form SAMs on
metal oxide surfaces. Scheme 3.2 shows four possible modes of binding for
16-phosphonohexadecanoic acid. Alkanecarboxylic acids adsorb on A12 0 3/Al as the
carboxylate anion, but this interaction results in a weakly bound, easily displaced,
adsorbate. 14 A study of competitive binding between ferrocene terminated alkanecarboxylic
acids and alkanephosphonic acid on indium(tin) oxide (ITO) indicated that in an equal molar
156
Scheme 3.2: Possible binding configurations for the SAMs of 16-phosphonohexadecanoic
acid formed on A12 0 3/Al films supported on Si. a) Binding to the surface
through the phosphonic acid head group. b) Binding to the surface through the
carboxylic acid head group. c) Binding to the surface by both the phosphonic
acid and carboxylic acid head groups on different molecules. d) Binding to the
surface by both the carboxylic acid and phosphonic acid head groups on the
same molecule.
157
b)
a)
AL02 /A1 (200 nm)
ALOJ/AI (200 nm)
d)
C)
A1,OV/A1 (200 nm)A10A1(0nm
A'203/Al (200 nm)
158
solution of 11-ferrocenoylundecanoic acid and 11-ferrocenylundecanephosphonic acid, the
predominant surface bound species was 1 1-ferrocenylundecanephosphonic acid. In a
solution containing 90% 1 1-ferrocenoylundecanoic acid and
10% 11 -ferrocenylundecanephosphonic acid, the surface composition was
92% 11-ferrocenylundecanephosphonic acid. 5 6 Based on these results, we expected that the
mode of binding shown in Scheme 3.2, part a, would be the favored configuration.
The SAMs were formed by immersion of Si wafers coated with freshly evaporated Al
(200 nm) into a 1 mM solution (50% aqueous ethanol) of 16-phosphonohexadecanoic acid.
Immersion times were varied from one hour to twenty-four hours. The quality of the
monolayers, as judged from the peak positions of the methylene stretches, did not vary
significantly for derivatization times longer than three hours.
Characterization of SAMs of 16-phosphonohexadecanoic acid using PIERS.
Figure 3.9 shows the PIERS spectrum of the SAM of 16-phosphonohexadecanoic acid
adsorbed on A12 0 3/Al and the IR spectrum of 16-phosphonohexadecanoic acid dispersed in a
KBr pellet. Table 3.3 lists the peak positions and assignments (where possible)
corresponding to the spectra shown in Figure 3.9.
The peak positions of the methylene stretches of the PIERS spectrum shown in
Figure 3.9 a) i) indicate a high degree of order and crystallinity in the monolayer alkane
chains. The PIERS spectrum shown in Figure 3.9 b) i) provides insight into the mode of
binding of the head group for the SAMs of 16-phosphonohexadecanoic acid.
Alkanecarboxylic acids bind to A12 0 3/Al as the carboxylate anion. 4 6 The peak positions for
the antisymmetric and symmetric carboxylate stretches for SAMs of hexadecanoic acid
formed on A12 0 3/Al occur at 1608 and 1475 cm-1, respectively. 4 6 Allara et al. showed that
159
Figure 3.9: a) i) PIERS spectrum of the high energy region for a SAM of
16-phosphonohexadecanoic acid formed on A12 0 3/Al ii) IR spectrum of the high
energy region for 16-phosphonohexadecanoic acid dispersed in a KBr pellet.
b) i) PIERS spectrum of the low energy region for a SAM of
16-phosphonohexadecanoic acid formed on A12 0 3/Al ii) IR spectrum of the low
energy region for 16-phosphonohexadecanoic acid dispersed in a KBr pellet.
The positions of the marked peaks are given in Table 3.3.
160
a)
I
0.0021
-o
.
ii)
3
3000
3100
2900
Wavenumber (cmn)
2700
2800
b)
0.002
rj~
i)
C
I
1850
I
1700
I
1550
I
1100
1250
1400
Wavenumber (cm")
II
950
800
650
161
Table 3.3: Peak Positions (cm') for the vibrational modes of 16-phosphonohexadecanoic
acid dispersed in a KBr pellet and adsorbed as SAMs on A12 0 3/A1.
KBr
SAM on A12 0 3/Al
Mode assignmenta,b
2919
2919
Va CH 2
2849
2851
Vs
1718, 1690
1710
v C=O
1468
1471
CH 2 , scissors, def.
1441, 1279, 1240
1436, 1279, 1239
v C-O + OH i.p. def.
1412
1412
c CH 2 , scissors, def.
1348, 1333, 1317, 1289
1351, 1318, 1298
CH 2 , wagging and twisting
V P=O
1216
1202,1116,1078
CH 2
1203, 1115, 1078 sh
1056 sh
Va po 3
942
vS pO3
2
2
1009, 953 sh
v P-O-(H)
933
C-OH o.p. def.
783
v P-C
723
CH 2 rocking
a Mode assignments were made according to references 53 and 54.
b Abbreviations used: Va antisymmetric stretch; vs symmetric stretch; v stretch;
def. deformation; i.p. in plane; sh, shoulder; o.p. out of plane.
162
SAMs of 1,32-dotriacontanedioic acid form monolayers similar to that shown in Scheme 3.2,
part d, where one of the carboxylic acid groups binds to the surface as the carboxylate anion
and one binds as the protonated acid. 57 In the SAMs of 1,32-dotriacontanedioic acid
adsorbed on A12 0 3/Al, the peak positions of the antisymmetric and symmetric carboxylate
stretches occur at 1537 and 1400 cm', respectively and the C=O stretch of the protonated
acid (hydrogen bonded) occurs at 1703 cm1. The peak at 1710 cm1 in PIERS spectrum
shown in Figure 3.9 b) i) indicates the presence of a hydrogen bonded carboxylic acid in the
SAM which is consistent with the binding configurations shown in part a or part c of
Scheme 3.2. There is also a weak peak, centered at 1611 cm-1 in Figure 3.9 b) i) which might
be indicative of an antisymmetric carboxylate stretch. The antisymmetric carboxylate stretch
for alkanecarboxylic acid SAMs adsorbed on A12 0 3/Al is less intense than the symmetric
carboxylate stretch. 17 ,4 6 ,5 7 If the peak at 1611 cm' in Figure 3.9 b) i) is indicative of a
carboxylate antisymmetric stretch, then we would also expect a more intense peak for the
symmetric carboxylate stretch in the region between 1400 and 1480 cm-1. There are three
peaks in this region in the PIERS spectrum of 16-phosphonohexadecanoic acid adsorbed on
A12 0 3/Al: 1471, 1436 and 1412 cm'. These peaks were assigned to the CH 2 scissors and
deformation, the C-O stretch coupled with the OH in plane deformation and the CH 2
scissors and deformation for the methylene group alpha to the carboxylic acid carbon,
respectively. The PIERS spectrum does not support the surface binding configuration
depicted in Scheme 3.2, part c, but rather it supports the conclusion that the phosphonic acid
head group is the predominant binding group.
The region between 1240 and 800 cm-1 in the PIERS spectrum shown in
Figure 3.9 b) i) contains three broad peaks and one peak of medium intensity. The
163
corresponding KBr spectrum shown in Figure 3.9 b) ii) contains several peaks, not nearly as
broad as those in the PIERS spectrum. We have assigned the P=O stretching frequency to
the peak in the KBr spectrum at 1216 cm'. Maege et al. observed the P=O stretching
frequency for octadecanephosphonic acid, dispersed in a KBr pellet, at 1225 cm-1. 5 8 We
attribute the lower energy for the P=O stretch in 16-phosphonohexadecanoic acid to
hydrogen bonding between the carboxylic acid proton and the P=O group. The low energy
region of the KBr spectrum also contains the P-O-(H) stretches which we assign to the
peak at 1009 cm' and the shoulder at 953 cm1. The PIERS spectrum does not contain the
peaks at 1216 and 1009 cm-1 or the shoulder at 953 cm-1. The absence of the peaks
associated with the protonated phosphonic acid head group in the PIERS spectrum is
consistent with the surface binding of the deprotonated phosphonic acid, presumably with
tridentate binding to the surface. 19 The region of the PIERS spectra between 1200 and
800 cm' resembles the spectrum Maege et al. reported for SAMs of octadecanephosphonic
acid adsorbed on A12 0 3/Al. 5 8 The PIERS data suggests that the mode of binding of
16-phosphonohexadecanoic acid occurs primarily through the phosphonic acid head group,
as depicted in Scheme 3.2, part a.
Characterization of SAMs of 16-phosphonohexadecanoic acid using XPS.
Figure 3.10 shows the XPS survey spectrum of 16-phosphonohexadecanoic acid dispersed in
a KBr pellet and two insets showing the high resolution core level spectra of the C(is) region
and P(2p) region. The survey spectrum contains only the elements in the sample: C, 0, P, K
and Br. The C(1s) high resolution spectrum exhibits a photoelectron peak centered at
284.6 eV which is assigned to the methylene carbons. A carboxylic acid carbon exhibits a
photoelectron peak at -289.5 eV. 2 2 There is a slight shoulder on the high energy side of the
164
Figure 3.10: XPS survey spectrum of 16-phosphonohexadecanoic acid dispersed in a KBr
pellet (center), high resolution core level spectrum of the C(is) region (left
inset), and high resolution core level spectrum of the P(2p) region (right inset).
P (2p)
K (2P 3/2)
294
291
C (1s)
279
282
285
288
Binding Energy (eV)
276
C (Is)
142
139
142
139
I
I
I
I
I
12'7
130
133
136
12 7
130
133
136
Binding Energy (eV)
124
K (2p 32 )
O(ls)
K ( 3 p 1/2)
Br (3p
K (2s)
12)
Br (3p
3/2)
r (3s)
w4
1000
900
800
700
I
I
600
500
- I
400
Binding Energy (eV)
Br (3d52)
(I2p)
~P
K(s
K(3s)
I
300
200
100
0
166
C(1s) peak at 284.6 eV, but it is not a well resolved peak. Figure 3.11 shows the
corresponding XPS spectra for SAMs of 16-phosphonohexadecanoic acid adsorbed on
A12 0 3/Al. The survey spectrum contains signals due to C, 0, P and Al. The C(1 s) high
resolution spectrum exhibits a photoelectron peak centered at 284.6 eV, which is assigned to
the methylene carbons and a small higher energy photoelectron peak centered at 289.0 eV,
which is assigned to the carboxylic acid carbon. The greater resolution of the peak due to the
carboxylic acid carbon in the XPS spectrum of the SAM compared to the XPS spectrum of
the molecule dispersed in the KBr pellet supports the conclusion that the mode of binding of
16-phosphonohexadecanoic acid to the surface occurs primarily through the phosphonic acid
head group.
Formation and reaction of SAMs terminated with a mixture of interchain
carboxylic anhydride and mixed trifluoroacetic carboxylic anhydride from SAMs of
16-phosphonohexadecanoic acid. SAMs terminated in reactive functional groups provides
a means to modify surface properties after SAM formation. 1 ,59-65 We have used the
method of Yan et al. to prepare mixed SAMs containing carboxylic acid and amide
functionalized SAMs. 1 Scheme 3.3 shows the methodology for the formation of the surface
anhydride mixture and the reaction of the anhydride terminated surface with alkylamines to
form a mixed SAM containing carboxylic acid and amide functionalities. The surface
reaction products were characterized by XPS and PIERS.
The anhydride terminated surface was formed by treatment of the
16-phosphonohexadecanoic acid SAMs with trifluoroacetic anhydride and triethylamine in
anhydrous NN-dimethylformamide and the mixed SAMs containing carboxylic acid and
167
Figure 3.11: XPS survey spectrum of SAMs of 16-phosphonohexadecanoic acid formed on
A12 0 3/Al (center), high resolution core level spectrum of the C(ls) region (left
inset), and high resolution core level spectrum of the P(2p) region (right inset).
0
P (2p)
c
('s)
I
294
291
279
282
285
288
Binding Energy (eV)
O (is)
276
139
142
I
I
I
I
124
127
130
133
136
Binding Energy (eV)
C ( s)
I-
P (2p)
Al (2s)
Al (2p)
P (2s)
0 (2s)
1000
900
800
700
600
I
I
I
I
I
500
400
300
200
100
0
Binding Energy (eV)
00
169
Scheme 3.3: Schematic representation for the formation of SAMs terminated in a mixture of
anhydrides and the reaction of the anhydride terminated SAM with alkylamines
to form a mixed SAM containing carboxylic acid and amide functionalities.
170
0
0
0
HO
0
HO
OH
0
OH
HO
0
0
0
0
(CF 3CO) 20,
Et3N, DMF,
rt
O
00
0 0
o
CF 3
o0
00
0
o
4-
nodN
oo'o ono o oo
A(1')/A
(200
flnm)
RNH 2,
NMP, rt
I
0
R
R
R
0
I
HN
0
HO
HN
HO
0
A1203/A1 (200 nm)
0
I
HN
HO
0
0
0
171
amide functionalities were prepared by placing the anhydride terminated surface into a
10 mM solution of hexadecylamine in anhydrous 1-methyl-2-pyrrolidinone.
Characterization of the anhydride terminated SAMs and the mixed SAMs by
XPS. Figure 3.12 shows the XPS survey spectra of SAMs of 16-phosphonohexadecanoic
acid, the anhydride terminated SAM, and the mixed SAM terminated with carboxylic acid
and hexadecylamide functionalities. The product of the anhydride formation reaction in this
study is different than that of Yan et al., who observed the conversion of the carboxylic acid
terminated SAM to a SAM terminated in an interchain anhydride. 1 In addition to the signals
due to P, C, 0, and Al, we observed a small signal for F in the XPS spectra of the anhydride
terminated SAM. Yan et al. postulated three plausible products of the reaction of the
carboxylic acid terminated SAMs with trifluoroacetic anhydride: an interchain anhydride, a
mixed trifluoracetic carboxylic anhydride, or a mixture of the two.1
In order to quantify the amount of the mixed trifluoracetic carboxylic anhydride
formed in the reaction, we prepared SAMs terminated with trifluoroacetate groups from the
reaction of SAMs of 16-phosphonohexadecanol with trifluoroacetic anhydride in anhydrous
N,N-dimethylformamide. 5 9 ,6 6 ,6 5 If we assume complete conversion of the hydroxyl
terminated SAMs to the trifluoroacetate terminated SAMs, then by comparing the F:P ratio
of the trifluoroacetate SAMs to that of the anhydride surface, we can estimate an upper limit
for the amount of the mixed trifluoroacetic carboxylic anhydride on the surface. 6 5 The F:P
ratio in the anhydride terminated SAM was 11±4% of the F:P ratio in the trifluoroacetate
terminated SAM. The mixed trifluoracetic carboxylic anhydride is, therefore, a minor
component of the anhydride terminated SAM.
172
Figure 3.12: Comparison of the XPS survey spectra of SAMs of: a) carboxylic acid
terminated SAMs of 16-phosphonohexadecanoic acid, b) a mixture of
interchain and mixed anhydrides formed by the reaction of the SAMs of
16-phosphonohexadecanoic acid with trifluoroacetic anhydride and c) mixed
SAMs terminated in carboxylic acid and hexadecylamide functionalities
formed by the reaction of hexadecylamine with the anhydride terminated
SAMs on A12 0 3/Al.
O(ls)
C (s)
a)
P (2p)
Al (2p)
Al (2s)
b)
F (Is)
c)
N (Is)
1000
900
800
700
600
500
Binding Energy (eV)
400
300
200
100
0
174
The XPS spectrum of the mixed SAMs containing carboxylic acid and
hexadecylamide functionalities contains signals due to P, C, 0, Al, and N. In addition to the
presence of the N( is) signal, the C:0 ratio is larger than that of the carboxylic acid or
anhydride terminated SAMs.
Characterization of the anhydride surface and the mixed SAMs by PIERS.
Figure 3.13 shows the PIERS spectra for the carboxylic acid, anhydride and mixed SAMs.
The high energy region of the carboxylic acid terminated SAMs shows two peaks at 2920
and 2851 cm-1, due to the antisymmetric and symmetric methylene stretches, respectively.
The antisymmetric and symmetric methylene stretches for the anhydride terminated SAM
occur at 2922 and 2852 cm-1, respectively. The slight shift to higher energies, as compared
to the carboxylic acid terminated SAMs, indicates that the alkane chains may be less ordered
in the anhydride terminated SAM. In addition to peaks due to the antisymmetric and
symmetric methylene stretches at 2927 and 2854 cm1, respectively, the mixed SAM contains
a shoulder on the high energy side of the antisymmetric methylene stretch at 2962 cm-I due
to the antisymmetric C-H stretch of the terminal methyl group in hexadecylamide. The shift
to higher energies for the methylene stretches is attributed to the disordered alkane chain of
the hexadecylamide.
The region between 1500 and 1900 cm-1 of the PIERS spectrum of the carboxylic
acid terminated SAM shows an asymmetric peak, centered at 1713 cm', which is assigned to
hydrogen bonded carboxylic acid, and a high energy shoulder at approximately 1736 cm-1,
which is assigned to free carboxylic acid. 5 2 ,5 3 The corresponding PIERS spectrum of the
anhydride terminated surface shows loss of the peaks due to the carboxylic acid terminated
SAM and the presence of two new peaks at 1750 and 1822 cm1. The position and relative
175
Figure 3.13: Comparison of the PIERS spectra of SAMs of: i) carboxylic acid terminated
SAMs of 16-phosphonohexadecanoic acid, ii) the mixture of interchain and
mixed anhydrides formed by the reaction of the SAMs of
16-phosphonohexadecanoic acid with trifluoroacetic anhydride and iii) the
mixed SAMs terminated in carboxylic acid and hexadecylamide functionalities
formed by the reaction of hexadecylamine with the anhydride terminated
SAMs on A12 0 3/A1. a) PIERS spectra of the high energy region
(2700-3100 cm 1 ) showing the methylene (and methyl for the mixed SAMs)
stretching region and b) PIERS spectra of the C=O stretching region
(1500-1900 cm').
176
a)2920
0.005
2851
2922
2852
2927
2854
2962
3100
3000
2900
Wavenumber
2800
(cm 1
2700
)
1713
b)
b)
0.001I
1736
1822
175
1559
1900
1800
1700
Wavenumber (cm-)
1600
1500
177
intensity of these two peaks are indicative of the antisymmetric and symmetric stretches of
52 5 3 The absence of a peak due
the carbonyl groups of an unstrained, open chain anhydride. ,
to the carboxylic acid terminated SAMs, combined with the XPS data for the anhydride
terminated SAM, suggests that the treatment of carboxylic acid terminated SAMs of
16-phosphonohexadecanoic acid with trifluoroacetic anhydride results in a mixture of
anhydrides on the surface, primarily an interchain anhydride.
The PIERS spectrum of the mixed SAM containing carboxylic acid and amide
functionalities shows loss of the peaks due to the anhydride carbonyl stretches and the
presence of four broad peaks, one band centered at 1559 cm-1, two peaks of weak intensity at
1623 and 1650 cm1, and a broad peak centered at 1723 cm-1. No absorption bands
associated with the carboxylic anion were observed, consistent with the study of Yan et al.,
who concluded that the carboxylic acids generated by the reaction of the anhydride with
hexadecylamine are protonated.1 The peak at 1559 cm-1 is assigned to the amide II band,
which is associated with both the C-N stretch and the C-N-H in plane bend in the stretch
bend mode. 5 2 ,5 3 The carbonyl stretch of amides (amide I) occurs as a very strong band
between 1630 and 1680 cm' .52,53 Yan et al. did not observe the amide I band in the
spectrum of the mixed SAMs and they concluded that the transition dipole moment of the
amide carbonyl was oriented parallel to the surface. The two peaks of low intensities at 1623
and 1650 cm' are assigned to the amide I band. The low intensities of these peaks suggest
that most of the amide carbonyl groups are oriented parallel to the surface, with a portion of
the amide carbonyl groups oriented away from the surface parallel, or that all the amide
carbonyl groups are oriented slightly away from the surface parallel, or some combination of
these two possibilities. We speculate that the two peaks attributed to the amide I band result
178
from the presence and absence of hydrogen bonding, between the amide carbonyl and either
neighboring carboxylic acid or amide groups, or both. The broad peak centered at 1726 cm-1
is assigned to the carbonyl stretch of the carboxylic acid groups in the mixed SAM. The
width of this peak suggests that it contains components due to free and hydrogen bonded
carboxylic acid. The broad absorptions due to the bound aluminophosphate group are
unchanged as a result of the anhydride and amide forming reactions.
Conclusions
1 -Alkanephosphonic acids self-assemble on the native oxide of evaporated aluminum
films to form ordered monolayers. The contact angle data indicates that the monolayers
expose a low-energy, methyl-terminated surface. The shorter chain molecules (n<13) are
less ordered than their longer chain analogues, as determined from PIERS. The monolayers
show an incremental increase in thickness with increasing chain length, although the absolute
thickness was not determined. The longer chain molecules are canted more than 13' but less
than 300, with respect to the surface normal.
16-Phosphonohexadecanoic acid self-assembles on the native oxide of aluminum to
form ordered monolayers which bind to the surface predominantly through the phosphonic
acid head group, exposing a carboxylic acid terminated monolayer. The carboxylic acid
terminus reacts with trifluoracetic anhydride to form a mixture of anhydrides on the surface,
predominantly an interchain anhydride with a small amount of mixed trifluoracetic
carboxylic anhydride. The anhydride terminated SAM reacts with alkylamines to form a
mixed SAM containing carboxylic and amide functionalities.
179
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