Molecular Orientation in Organic Thin Films
Prof. Kathy Rowlen
Department of Chemistry and Biochemistry
University of Colorado, Boulder
Why study molecular orientation in thin films?

interfacial properties
(optical, electronic and mechanical)

molecular interactions

organizational model for complex systems
Questions to be addressed:
 best means to probe molecular orientation?
 does the substrate affect thin film
characteristics?
 how does molecular structure affect
thin film characteristics?
 does molecular orientation and
organization vary with time and (or)
coverage?
Organization at Low Surface Coverage?
Photoacoustic Spectroscopy
A + h
A*
A*
A + heat
I oKa
S
C p
Angle-Resolved Absorbance with
Photoacoustic Detection (ARAPD)
Lab Z-axis
Incident
Beam
Molecular
z'-axis
z'Z

Lab Y-axis
90  
Lab
X-axis
Surface Plane
For a long-axis transition moment:


A(  )  ( Amax / 2 ) sin   K fZ 3 sin   2
2
2
K fZ  K z 'Z  cos2 z 'Z
*
zZ  fZ  cos
*
1
K  
1/ 2
fZ
Katz et al. Science (1991) 254, 1485-1487




Exhibited second harmonic generation
No linear dichroism
Apparent orientation angle (by SHG) ~ 45°
No change in orientation as layers added
Evolution of Orientation in Multilayer Film
Photoacoustic Signal (a.u.)
1
5 layers
3 layers
2 layers
1 layer
0 layers
0
0
10
20
Angle of Incidence
30
40
Molecular Long Axis Orientation
(as a function of number of layers)
Mean Tilt Angle
40
35
30
25
20
15
10
0
1
2
3
4
5
Number of Layers
6
7
“Self-Healing” ?
One “layer”
32° ± 2°
Six “layers”
15° ± 1°
Questions:
1) How does the angular distribution change?
2) What is the effect of surface roughness?
Substrate Roughness
Local
Surface
Normal, s
Lab Z-axis
sZ
sz'
DZ
L


  L  2   L 1  exp   L 
2

2
2H

cos2 sZ  K sZ  1  22  83 4  ...
Effect of surface roughness on ARAPD
(linear dichroism) measurements
sz' =90o
sz' =80o
Absorbance
o
Magic Angle (54.7 )
sz' =70o
80
70
sz' =60o
60
sz' =50o
50
sz' =40o
40
sz' =30o
30
sz' =20
 for Fused Silica,
L = 20 Е
o
sz' =10o
20
 for Air/Water,
L = 20 Е
sz' =0o
10
Apparent Molecular Tilt Angle, z'Z* (deg.)
90
0
0.0
0.2
Local Tilt
Angle
KzZ 
0.4
0.6
0.8
1.0
1.2
1.4
RMS Roughness, 
1
2
1  K
sZ
 Ksz  3KsZ Ksz 
in which each value of Kij is equal to <cos2ij> and the subscripts
indicate the relevant angle, such that z'Z is the angle between the
molecular orientation axis, z', and the macroscopic surface normal, Z.
Figure 2
Apparent Orientation Angle
Error as a function of length scale
80
60
L=5A
40
20
L = 30 A
0
0
20
40
60
80
Orientation w.r.t. Local Surface Normal
Second Harmonic Generation
1064 nm
532 nm
Molecular Orientation by SHG
cos
Z-axis

zzz
Molecular
Long Axis
sin
X-axis
cZZZ = Ns<cos3>zzz
cZXX = (1/2)Ns<sin2 cos >zzz
cos3 
c (zzz2 )
2
D
 (2)

cos

(2)
cos 
c zzz  2 c zxx
Effect of surface roughness on SHG
70

sz' = 70o
 for Fused Silica,
L = 20 Е
80
Apparent Orientation Angle, z'Z (deg.)
sz' = 80o
sz' = 60o
60
sz' = 50o
50
sz' = 40o
40
sz' = 30o
30
sz' = 20o
20
sz' = 10o
 for Water/Air,
L = 20 Е
sz' = 0o
10
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Local Tilt
Angle
RMS Roughness, 
Influence of Angular Distribution
(on SHG determination of zZ)
Common assumption:
cos3 
c (zzz2 )
2
D
 (2)

cos

2)
cos 
c zzz  2 c (zxx
Influence of Angular Distribution
(on SHG determination of zZ)
Common asumption:
cos3 
c (zzz2 )
2
D
 (2)

cos

2)
cos 
c zzz  2 c (zxx
Assume Gaussian distribution:
cos 
D ,  0  

cos 
3


 cos   exp    

0

cos3   exp     0  2 2 sin d

0
0
2
2

2 2 sin d
SHG “Magic Angle”


 cos   exp    

0

2
0
0

2 2 sin d
o=89o
90
o=80o
80
o=70o
70
o=60o
60
o=50o
50
o=40o
40
o=30o
30
o=20o
SHG magic angle
o=10o
20
10
o=0o
0
10
20

cos   exp     0  2 2 sin d
2
3
30
40
50
60
Apparent Molecular Tilt Angle (deg.)
cos 
D ,  0  

cos 
3
0
70
Width of Distribution ()
Figure 7.3. The apparent molecular tilt angle (calculated by incorrectly assuming a -
Legendre Polynomial (cos)
P0  12
P1  cos 
P2  12 3 cos2   1
P3  12 5 cos3   3 cos 

Pn   f Pn sin d
0
cos3 
D

cos 
2
5
P3  53 P1
P1
SHG Magic Angle
cos3 
D

cos 
2
5
P3  53 P1
P1
<P0>
<P1>
<P2>
1.0
0.8
<P3>
<P4>
0.4
<P5>
0.2
<P6>
0.0
<P7>
-0.2
<P8>
0
<Pn>
0.6
10
20
30
40
50
60
70
Width of Distribution (
<P1>
<P5>
0>  
0.3 broadens
Sequence of events<Pas
angular distribution
b)
<P6>
0.2
an
 as <P3> approaches zero, SHG apparent
0.1 tilt angle
converges
0.0
<P4> to 39.2°
-0.1
<P7>> approaches zero, loss of linear dichroism
 as <P
2
-0.2
<P8>
<P2>
 as <P1> approaches zero, loss of SHG-0.3intensity
<P3>
-0.4
Reported orientation angles (by SHG)
within 2 degrees of 39.2°
1)
Heinz, T. F.; Tom, H. W. K.; Shen, Y. R. Phys. Rev. A 1983, 28, 1883.
2)
Grubb, S. G.; Kim, M. W.; Rasing, Th.; Shen, Y. R. Langmuir 1988,
4, 452.
3)
Campbell, D. J.; Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1990,
94, 3681.
4)
Shirota, K.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl.
Phys. 1990, 29, 750.
5)
Li, DeQ.; Ratner, M. A.; Marks, T. J.; Zhang, C. H.; Yang, J.; Wong,
G. K. J. Am. Chem. Soc. 1990, 112, 7389.
6)
Bubeck, C.; Laschewsky, A.; Lupo, D.; Neher, D.; Ottenbreit, P.;
Paulus, W.; Prass, W.; Ringsdorf, H.; Wegner, G. Adv. Mater. 1991, 3, 54.
7)
Liu, X.; Liu, L.; Chen, Z.; Lu, X.; Zheng, J.; Wang, W. Thin Solid
Films 1992, 219, 221.
8)
Bell, A. J.; Frey, J. G.; VanderNoot, T. J. J. Chem. Soc. Faraday
Trans. 1992, 88,
2027.
9)
Yitzchaik, S.; Roscoe, T. B.; Kakkar, A. K.; Allan, D. S.; Marks, T.
J.; Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. J. Phys. Chem. 1993, 97, 6958.
10)
Nalwa, H. S.; Watanabe, T.; Nakajima, K.; Miyata, S. Thin Solid
Films 1993, 227, 205.
11)
Higgins, D. A.; Naujok, R. R.; Corn, R. M. Chem. Phys. Lett. 1993,
213, 485.
12)
Yokoyama, S.; Yamada, T.; Kajikawa, K.; Kakimoto, M.; Imai, Y.;
Takezoe, H.; Fukudo, A. Langmuir 1994, 10, 4599.
13)
Kezhi, W.; Chunhui, H.; Guangxian, X.; Xinsheng, Z.; Xiaming, X.;
Lingge, X.; Tiankai, L. Thin Solid Films 1994, 247, 1.
14)
Naujok, R. R.; Higgins, D. A.; Hanken, D. G.; Corn, R. M. J. Chem.
Soc. Faraday Trans. 1995, 91, 1411.
15)
Lin, W.; Yitzchaik, S.; Lin, W.; Malik, A.; Durbin, M. K.; Richter, A.
G.; Wong, G. K.; Dutta, P.; Marks, T. J. Angew. Chem. Int. Ed. Engl. 1995, 34,
1497.
16)
Marks, T. J.; Ratner, M. A. Angew. Chem. Int. Ed. Engl. 1995, 34,
155.
17)
Roscoe, S. B.; Kakkar, A. K.; Marks, T. J.; Malik, A.; Durbin, M. K.;
Lin, W.; Wong, G. K.; Dutta, P. Langmuir 1996, 12, 4128.
18)
Yokoyama, S.; Kakimoto, M.; Imai, Y.; Yamada, T.; Kajikawa, K.;
Takezoe, H.; Fukuda, A. Thin Solid Films 1996, 273, 254.
19)
Zhang, T.; Feng, Z.; Wong, G. K.; Ketterson, J. B. Langmuir 1996,
12, 2298.
Combined SHG and ARAPD
Total Internal Reflection Cell
2w
w
Piezo
Inlet
Outlet
ARAPD
DM
DM
Nd:YAG
PD
QF GLP
HWP
SHG
DM
Nd:YAG
V
DM
PD
QF GLP
HWP
L
IR P
IF
PMT
Physisorbed Stilbene Dye (DPB)
(CH 2)21CH3
-
Br
0.25
0.20
Absorbance
N+
0.15
0.10
50X
0.05
0.00
-0.05
250
300
350
400
450
500
550
600
Wavelength (nm)
OH
Figure 9.6. UV-visible absorbance spectra for DPB; monolayer film dipped in 8  104
M DPB (solid line), and 10-5 M DPB in chloroform (dashed line). The monolayer
absorbance has been multiplied by 50 for comparative purposes.
Ellipsometric Thickness (Е )
Ellipsometry yields an average
orientation angle of ~75°
12
10
8
6
4
2
0
0
2
4
6
8
10
Concentration (10-5 M)
Assuming a 45 Å rod-like molecular length
Angle-Resolved Photoacoustic Detection
Photoacoustic Amplitude (arb. units)
2.0
Multilayer
1.5
1.0
Monolayer
0.5
0.0
0
30
60
90
120
150
180
Polarization Rotation Angle (deg.)
If aFigure
narrow
distribution is assumed:
9.6. Averaged, normalized angle-resolved photoacoustic amplitudes acquired
for both monolayer films of DPB (solid triangles, 16 measurements), and multilayer
films consisting of a monolayer film with surface-aggregated DPB particulates (open
circles, 16 measurements). The solid lines are fits to the data using Eq. 9.6b with the
fitting coefficients given in Eq. 9.7. A representative error bar ( 1) is provided on
each curve.
Mean tilt angle ~ 72° ± 3° (monolayer)

Atot  a X  aY  a X  sin 2   KzZ sin 2  2a Z  a X  aY   a X  2a Z

Norm. SHG Intensity (arb. units)
SHG for Monolayer DPB
p-polarized
20
s-polarized
Monolayer DPB
10
0
0
30
60
90
120
150
180
Polarization Rotation Angle (deg.)
If a narrow angular distribution is assumed
9.8. Averaged, normalized SHG results acquired for monolayer films of DPB
theFigure
orientation
angle
is and
73°
3°
(4calculated
measurements). The solid
lines are fits to the data
using Eqs. 9.15
9.16 ±
for the
s-polarized (open circles) and p-polarized (solid triangles) second harmonics,
respectively, with the fitting coefficients given in Eq. 9.17. A representative error bar
( 1) is provided on each curve.
2
2 w
5 ZXX
2 XXZ
3 ZXX
4 ZZZ
5 ZXX
Ip  C s c
 cos 
s c
s c
w
I s  C s1 sin 2  c XXZ
s c
2
I 
w 2
s c

I 
w 2
Mean and Angular Distribution
For the DPB monolayer:
ARAPD yields a tilt angle of 72° ± 3°
SHG yields a tilt angle of 73° ± 3°
For DPB, molecular long axis tilted ~70°
with respect to surface normal, fairly
narrow angular distribution.
Photoacoustic Amplitude (arb. units)
DPB Multilayer by ARAPD
DPB Particles on Surface
1.65
1.50
Monolayer
1.35
0
30
60
90
120
150
180
Polarization Rotation Angle (deg.)
Figure 9.6. Averaged, normalized angle-resolved photoacoustic amplitudes acquired
If a narrow
distribution
is assumed:
for both monolayer
films of DPB (solid triangles,
16 measurements), and multilayer
films consisting of a monolayer film with surface-aggregated DPB particulates (open
circles, 16 measurements). The solid lines are fits to the data using Eq. 9.6b with the
fitting coefficients given in Eq. 9.7. A representative error bar ( 1) is provided on
each curve.
Mean tilt angle ~ 72° ± 3° (monolayer)
Mean tilt angle ~ 53° ± 0.9° (multilayer)
DPB Multilayer by SHG
Norm. SHG Intensity (arb. units)
40
p-polarized
s-polarized
30
20
10
0
0
30
60
90
120
150
180
Polarization Rotation Angle (deg.)
9.9. Averaged,angular
normalized SHGdistribution
results acquired for multilayer
films of DPB
IfFigure
a narrow
is assumed
consisting of an oriented monolayer film and surface-accumulated DPB (9
measurements).
The solid lines
are fits to the data using
Eqs. 9.15 is
and 9.16
the
calculated
orientation
angle
70°for the± s-3°
polarized (open circles) and p-polarized (solid triangles) second harmonics,
respectively, with the fitting coefficients given in Eq. 9.17. A representative error bar
( 1) is provided on each curve.
Mean and Angular Distribution
For the DPB monolayer:
ARAPD yields a tilt angle of 72° ± 3°
SHG yields a tilt angle of 73° ± 3°
For DPB multilayer:
ARAPD yields a tilt angle of 53° ± 0.9°
SHG yields a tilt angle of 70° ± 3°
Covalent Molecular System: Azo Dye
CH3
N
0.8
0.7
0.6
Absorbance
H3C
N
N
0.5
0.4
50X
0.3
0.2
0.1
0.0
200
300
400
500
600
700
Wavelength (nm)
O
S
O
HN
H3C
Si
O
Figure 9.9. UV-visible absorbance spectra for the azo dye; submonolayer film
prepared by surface reaction (solid line), and an 8  10-4 M solution in toluene
(dashed line). The monolayer absorbance has been multiplied by 50 for comparative
purposes.
CH3
Water Contact Angle and Ellipsometry
Advancing Contact Angle (deg.)
80
70
60
50
Ellipsometry indicates
only 6.5 Å thickness,
estimated 0.1 monolayer,
37 Å2 per molecule
40
30
20
0
0
20
40
60
80
100
120
Immersinon Time (min.)
140
160
Combined SHG and ARAPD Results
SH Intensity (arb. units)
p-polarized
s-polarized
-0
0
30
60
90
120
150
180
Polarization Rotation Angle (deg.)
Signal (arb. units)
1.05
Figure 10
0.90
0.75
0
30
60
90
120
150
Polarization Rotation Angle (deg.)
180
Mean and Angular Distribution
Assuming a narrow distribution
for ARAPD: 58° ± 2°
for SHG: 46° ± 2°
Mean and Angular Distribution
Assuming a narrow distribution
for ARAPD: 58° ± 2°
for SHG: 46° ± 2°
Linear Dichroism

KzZ  cos2  zZ 
2
cos
  zZ P zZ sin zZ d zZ
0

 P sin
z Z
z Z
d zZ
0
SHG

cos  zZ
DzZ 
cos  zZ
 cos
3
3

 zZ P zZ  sin  zZ d zZ
0

 cos 
0
z Z
P zZ  sin  zZ d zZ
Distribution Mean,
ARAPD
SHG
80Mean and Angular Distribution
75
Assuming a narrow distribution
for ARAPD: 58° ± 2°
70
Distribution Mean, o (deg.)
0
5
10 ± 2°
for SHG:
46°
15
20
Distribution Width,  (deg.)
80
70
60
ARAPD
50
SHG
40
0
10
20
30
o = 57o
 = 30o
40
Distribution Width,  (deg.)
50
Figure 9.11. SHG and ARAPD molecular orientation measurements are combin
yield both the means (o) and root mean square widths () of the orient
distributions for DPB (a) and the azo dye (b). Each curve indicates the rang
Gaussian distributions which can yield the experimental orientation parameter obt
by either the SHG or ARAPD orientation measurement alone. The point at whic
ARAPD and SHG curves cross represents the distribution mean and width wh
Azo Dye with aminopropyl silane linker
57 ° ± 30°
SHG for adsorption isotherms and
adsorption / desorption kinetics

SHG intensity depends on both
the number density and molecular orientation
I
2w
pp
 C s2cXXZ  s3cZXX  s4cZZZ I
2

w 2
cZZZ = Ns<cos3>zzz
cZXX = (1/2)Ns<sin2 cos >zzz

Experimental geometry can be used to minimize
sensitivity to orientation
Conventional SHG Adsorption Measurements
 Single polarization combination
(e.g., p-polarized w and 2w ), assume
molecular orientation does not change
with surface coverage
 Orientation angle corrected (OAC)
1) Measurement of several polarization
combinations at each concentration
2) Calculation of molecular orientation,
, at each concentration (assuming
narrow angular distribution)
3) Signal normalization
4) Construction of isotherm
Theoretical p-polarized SHG response curves as
a function of orientation angle (zzz dominant)
=0
4  = 10o
o
 = 20
I
1/2
(2w)
3
2
o
 = 30
o

 = 40o
 = 50

Re

o
1
 = 60
o
 = 70oo
0  = 80
0
10
20
30
40
50
60
70
80
90
Polarization Rotation Angle (deg.)
2w
p
2
I ( γ)  C s5 c ZXX  cos γ( s2 c XXZ  s3 c ZXX  s4 c ZZZ  s5 c ZXX ) ( I w ) 2
2
RMS deviation for p-polarized 2w
zzz dominant)
Orientation Angles
o
o
from 0 to 90
1.6
RMS deviation
1.4
1.2
1.0
0.8
*
0.6
0.4
Orientation Angles
o
o
from 0 to 50
0.2
0.0
0
10
20
30
40
50
60
70
80
Polarization Rotation Angle (deg.)


s5

γ  cos 
 3s 4  s 5  s 2  s 3 
*
1
1
2
90
Test Case:
Disperse Red 1 (in methylene chloride)
NO2
N
N
N
OH
Fused Silica
Adsorption isotherm for DR-1 as determined by
a variety of polarization conditions
Ipp
3
I*
Is45
Ips
ISH
1/2
2
1
0
0
10
20
30
40
50
60
-4
DR-1 Concentration (10 M)
70
Apparent orientation angle (deg.)
Apparent orientation angle
as a function of concentration
50
48
46
44
42
0
20
40
60
-4
DR-1 Concentration (10 M)
3.0
2.5
2.0
1.5
Ipp
Ips
1.0
1/2
1/2
Is45
1/2
Fit to entire
corrected data set
0.5
ISH
1/2
Corrected for Orientation Angle
Adsorption isotherm for DR-1 after correcting
for change in orientation
0.0
0
20
40
60
-4
DR-1 Concentration (10 M)
80
Experimental constants obtained from
Langmuir fit to adsorption isotherms
Keq (M-1)
DGads (kJ/mol)
Ipp
Ips
Is45
Ip63
OAC
940
± 40
410
± 40
470
± 50
540
± 60
500
± 40
-16.8
± 0.1
-14.7
± 0.3
-15.1
± 0.3
-15.4
± 0.3
-15.2
± 0.2
OAC = orientation angle corrected
Future Directions
Adsorption / Desorption Kinetics by SHG
3.5
I*
1/2
(arb. units)
3.0
2.5
2.0
1.5
1.0
0.5
0
500
Time (sec)
1000
1500
Thin Film Growth Mechanism
Thin Film Growth Mechanism
Thin Film Growth Mechanism
Orientation Angle
Angular Distribution
Time / Coverage
Thin Film Growth Mechanism
Thin Film Growth Mechanism
Thin Film Growth Mechanism
Orientation Angle
Angular Distribution
Time / Coverage
Acknowledgements for
Thin Film Work
Susan Doughty (Ph. D. 1996), Patent Lawyer
Garth Simpson (Ph. D. 2000), Post-doc
with Prof. Richard Zare at Stanford
Sarah Westerbuhr
Jessica Ekhoff
Funding from Beckman Foundation and the
National Science Foundation
For SHG measurements with an orientation
distribution which is isotropic within the surface plane,
the three nonzero, independent tensor elements of c(2)
are given by:62


c ZZZ  N s cos3  zzz  cos  sin 2  sin 2 Y zxx  2 xxz 
cZXX
 cos  sin 2  zzz  cos  zxx


 12 N s 
2
2
 cos  sin  sin Y zxx  2xxz 


c XXZ  c XZX
 cos  sin 2   z z z   cos   xxz 

1

 2 Ns 
2
2
 cos  sin  sin Y   z xx  2 xxz   
where Y is the Euler rotation angle about the
molecular z'-axis. Since the X- and Y-axes in the
surface plane are equivalent, cYYZ = cYZY = cXXZ and
cZYY = cZXX. The expressions in Eq. 4 are greatly
simplified if only a single tensor element of (2) is
dominant, which is often the case experimentally.
Once the apparent orientation angle has been
determined, it may be substituted into
<sin2cos> and <cos3> in Eq. 5, and the value
of Ns calculated by simple rearrangement of the
expressions in Eq. 3:
N s,ps  C  I
1/ 2 1
ps 2
N s,s 45  C  I
s5 sin  cos 

s sin  cos 

2
1/ 2 1
s 45 2 1
N s,pp  C  I1pp/ 2
1
2
2
* 1
*
*
* 1
s2  s3 sin 2 * cos *   s4 cos3 * 
1