Future Projects on
MI Instrument
May 1, 2006
Ultimate Goal
While experiments done on our UHV/LT
STM provide great insight into chemical
systems, the operating conditions are not
practical for “real world” application.
 The advantages of the MI instrument is
that it works in an ambient environment
(i.e. room temp. and at 1 atm.), which
allows for easy application to industrial
processing conditions.

In situ STM



We are unable to achieve
atomic resolution (except for
HOPG) on the MI instrument
due to the ease with which the
metal surface can become
contaminated in air
(hydrocarbons and water).
Sonnenfield and Hansma in
1986 were the first to use STM
to study a surface immersed in
a liquid.1
In 1990, Magnussen et al.
achieved atomic resolution on
a metal surface.1
Figure from Ref. 2
Development of In Situ STM

Depended on three advances1:
 The
development of the STM by Binnig and Rohrer
 The development of surface preparation methods in
ambient conditions.
 The development of methods and materials to coat
the STM tip and to couple the STM with a
biopotentiostat.
This technique provides information on surface
processes such as phase transitions in adlayers on a
molecular and atomic level.
Comparing UHV and In Situ
Images of Au (herringbones)
Image of Au(111) under 0.1 M
HClO4 solution1
Image of Au(111) under UHV
Comparing UHV and In Situ
Images of Au (atomic res.)
Flame-annealed Au(111) under
clean mesitylene3
Image of Au(111) under UHV
Comparing Ambient and In Situ
Images of HOPG
Image of HOPG in air
File: 3-9-06HOPG009
Image of HOPG under
phenyloctane2
Comparing Ambient and In Situ
Images of Molecules on Au
L-cyseteine molecules on Au(111)
under perchlorate solution4
C10, C12 SAM on Au(111) in air
File: 3-15-06AuMicaSAMVap028
Electrochemistry in STM



Schematic of a sample molecule
coadsorbed with reference
molecules on a substrate as
probed by an STM tip.
RE and CE represent the
reference and counter electrodes,
respectively.
Vsub and Vbias are the substrate
potential (with respect to the
reference electrode) and the tipsubstrate bias voltage,
respectively, which are controlled
independently by a bipotentiostat.5
Electrochemistry in STM


Because the charge transfer event central to
electrochemical reactivity occurs within a few
atomic diameters of the electrode surface, the
detailed arrangement of atoms and molecules at
this interface strongly controls the corresponding
electrochemical activity1.
Cycling the potential causes significant changes
in the surface topography, from changing how
molecules adsorb to the surface to causing
reconstructions of the metal atoms themselves.
Insulating Tips



Because the faradaic background from a bare
metal wire immersed in solution can approach
several milliamps of current while tunneling
currents are typically on the order of nanoamps,
the STM tip must be insulated.
The tip is insulated by coating all but the very
end with an insulator so that the tunneling
current will not be overcome by the
electrochemical background.1
A variety of materials may be used to coat the
tip, specifically wax and nail polish.
Tip Etching
Extremely sharp tips with low aspect ratios
are prepared by chemically etching the tip
in a 1 M basic solution (KOH).
 The etching current, which depends on the
area of immersed wire and applied voltage
is adjusted to an initial value.
 This process produces a neck shape near
the air-solution interface.6

Tip Etching



As the etching proceeds,
the neck-like region
becomes thinner and
thinner, and eventually
the lower portion drops
off.
This causes an abrupt
decrease in the current.
A very sharp tip with a
small protrusion at the
end can be made by
switching off the circuit as
the current abruptly
drops.6
Wax Insulation of Tips





Most common method uses Apiezon-brand wax
The sharp etched tips are mounted vertically on
a manipulator.
A copper plate is heated and used to melt the
wax.
A rectangular slit in the plate provides a
temperature gradient for the melted wax.
The tip is brought from underneath the slit by
means of the manipulator.6
Wax Insulation of Tips
The tip is first moved slowly into the hot
wax and allowed to attain a thermal
equilibrium and uniform wetting.
 The tip is then raised through the wax and
allowed to break the top surface region of
the melt.
 The tip is moved sideways out of the slit
so as to leave the very end of the tip
unperturbed.6

Procedure for Wax Insulation of
Tips
From Ref. 6
Images of Wax Coated Tips
SEM image of EC STM tips, insulated with ‘double’ (a) and
‘single’ (b) pulling methods7
Nail Polish Insulation of Tips

Multiple articles cited using nail polish to
coat their tips, however the exact coating
procedure could not be found.
Reconstructions
Metal surfaces in UHV reconstruct in order
to minimize their surface energy.
 The extent of reconstruction is strongly
dependent on the work function of the
metal.
 The electrochemical environment offers an
opportunity to systematically vary the
electronic state of a surface, through the
application of potential and the influence of
adsorbed species in solution.1

Adsorption

Adsorption induces changes in the work
function
 modifications
of the surface dipolar layer
 particularly if significant charge transfer occurs
between the adsorbate and surface
 measurements of ΔΦ yield critical information
on the degree of charge reorganization upon
adsorption
ΔФ = Фadsorbate covered - Фclean
Au Reconstructions



Reconstructions can be removed electrochemically by
placing the electrode at sufficiently positive potential.
The removal of reconstruction can be attributed to the
adsorption of electrolyte anions at higher potentials.
Cycling the potential to a region where the herringbone
reconstruction is removed and then back reveals
changes in the shape of the step edges on the surface,
showing that the extra material required in the
compressed structure is taken from and returns to the
step edges.1
Images of Au Reconstructions
Typical Au(111) 23 X √3 reconstruction pattern.
The image was obtained for Au under pure
water at 0 mV.8
Typical image of Au(111) after the
transformation. The image was obtained for Au
under water after the surface potential was
raised to 400 mV.8
Sulfate on Au (111)



Sulfate is known to form a (√3 x √7)R19.1° structure on
Au(111)
The coadsorption of H3O+ ions is necessary to stabilize
the ordered oxoanion adlattices.
Both species in H2SO4, sulfate (10%) and bisulfate
(90%) have 3 free oxygen atoms to interact with the
surface. The distance between them (2.47 Å) is of the
same order of magnitude as the distance between Au
atoms (2.88 Å), so their geometrical arrangement
matches that of the Au (111) surface.9
Sulfate on Au (111)

The reason for the presence of nonuniform anion-anion distances is the
formation of H-bridge bonds between the
oxygen atoms of the oxoanions and the
coadsorbed H3O+ ions.9
Images of Sulfate on Au(111)



In situ STM image (10x10 nm2) of a
Au(111) electrode in 0.1 M H2SO4
showing both the (√3 x √7)R19.1°
sulfate structure, (upper and lower
parts) and the (1x1) substrate (middle
part).
The potential was switched from 0.80
to 0.65 V and then back to 0.80 V at
the points marked by the arrows.
The triangles and circles drawn on the
middle part of the image represent the
positions of the sulfate and hydronium
ions, respectively.9
Images of Sulfate on Au(111)



(B) Model of the(√3 x
√7)R19.1° sulfate
structure on Au(111) in
0.1 M H2SO4
The H3O+ ions are placed
on top of the Au atoms.
Every H3O+ adsorbed can
form 3 H-bridge bonds
with the oxygen atoms of
surrounding sulfate ions.9
Intro. To Cyclic Voltammetry

The voltage is swept
between two values
at a fixed rate, when
the voltage reaches
V2 the scan is
reversed and the
voltage is swept back
to V1.11
Intro. To Cyclic Voltammetry

In the forward sweep, as the voltage is swept
further to the right (to more reductive values) a
current begins to flow and eventually reaches a
peak before dropping. To rationalize this
behavior we need to consider the influence of
voltage on the equilibrium established at the
electrode surface. If we consider electrochemical
reduction, the rate of electron transfer is fast in
comparison to the voltage sweep rate.11 (i.e.
Fe3+  Fe2+)
Intro. To Cyclic Voltammetry

When the scan is
reversed we simply move
back through the
equilibrium positions
gradually converting
electrolysis product back
to reactant.(Fe2+  Fe3+)
The current flow is now
from the solution species
back to the electrode and
so occurs in the opposite
sense to the forward
sweep.11
Cyclic voltammogram of Au(111) in
0.1 M H2SO4


The peak at 0.55 V is
attributed to the lifting of
the (23 x √3)
reconstruction that takes
place in the lower
potential region.
The two sharp peaks
around 1.0 V are due to
the formation of an
ordered sulfate structure
at more positive
potentials.10
Underpotential Deposition

The electrodeposition of a metal on a foreign
metal at potentials less negative than the
equilibrium potential of the deposition reaction.
Such a process is energetically unfavorable and
it can occur only because of a strong interaction
between the two metals, with their interaction
energy changing the overall energetics to
favorable. Consequently, only one (very seldom
two) monolayer can be deposited this way, and
this is a very convenient way to produce wellcontrolled monolayer deposits.12
Underpotential Deposition



Upd monolayers are formed by the deposition of
low work function metals onto high work function
metals.
The monolayer originates from a relatively
strong adatom-substrate bond formed using less
energy than required for adatom-adatom bonds
formed during bulk deposition.
One of the most intriguing aspects of upd is the
anion dependence, which derives from
coadsoprtion of the anion and the adatom.1
Underpotential Deposition of Cu on
Au (111)
One of the first examples of atomic
resolution in the electrochemical
environment was Cu monolayers on Au
(111) in H2SO4.
 Three different structures are seen before
bulk Cu deposition.1

Images of Underpotential
Deposition of Cu on Au (111)

At positive potentials
(+300 mV), the bare
Au(111) surface is
seen.1
Images of Underpotential
Deposition of Cu on Au (111)


Ordered adlayer with
(√3 x √3)R30°
structure, ascribed to
coadsorbed sulfate.
Formed between 200
and 100 mV.1
Images of Underpotential
Deposition of Cu on Au (111)


Full Cu monolayer in
registry (1x1) with
Au(111).1
At 5 mV
Underpotential Deposition of Cu on
Au (111)



Different solutions of anions give rise to different
structures on the electrode surface.
Cl- anions form both (2 x 2) and (5 x 5)
incommensurate structures depending on the
conc. of the anion.
On other low Miller index faces of Au, Cu does
not exhibit the pronounced dependence on the
type and conc. of anion.1
Conclusions



In situ STM allows for atomic resolution under ambient
conditions.
Electrochemical STM can be used to understand the
electrochemical double layer and to correlate detailed
structure of the electrode surface with the double-layer
structure and ultimately with electrochemical response.
Studies of the upd processes reveal a rich structural and
reactive chemistry, the detailed nature of which is
dependent on potential, available anions, substrate
orientation, and substrate identity.1
References
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129-1162.
De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C.; De Schryver, F. C.; Meiners,
C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520-531.
Han, W.; Li, S.; Lindsay, S. M.; Gust, D.; Moore, T. A.; Moore, A. L. Langmuir. 1996, 12, 57425744.
Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Langmuir. 1996, 12, 2849-2852.
Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066-4069.
Nagahara, L. A.; Thundat, T.; Lindsay, S. M. Rev. Sci. Instrum. 1989, 60, 3128-3130.
Kazinczi, R.; Szocs, E.; Kalman, E.; Nagy, P. Appl. Phys. A. 1998, 66, S535-S538.
Tao, N.J.; Lindsay, S. M. J. Appl. Phys. 1991, 70, 5141-5143.
Cuesta, A.; Kleinert, M.; Kolb, D. M. Phys. Chem. Chem. Phys. 2000, 2, 5684-5690.
Climent, V.; Coles, B. A.; Compton, R. G. J. Phys. Chem. B 2001, 105, 10669-10673.
http://www.cartage.org.lb/en/themes/sciences/Chemistry/Electrochemis/Electrochemical/Cyclic
Voltammetry/CyclicVoltammetry.htm
http://www.corrosion-doctors.org/Dictionary/Dictionary-U.htm