Epitaxial growth of Ag on Au(111) by galvanic displacement of Pb and Tl monolayers

Article abstract of DOI:10.1149/1.2218769

The development of a new method for epitaxial growth of metals in solution by galvanic displacement of layers predeposited by underpotential deposition (UPD) is discussed and experimentally illustrated. Cyclic voltammetry and scanning tunneling microscopy are employed to carry out and monitor a quasi-perfect, two-dimensional growth of up to 35 monolayers of Ag on Au(111) by repetitive galvanic displacement of underpotentially deposited Tl and Pb monolayers. A complementary kinetic study of Pb and Tl UPD layer stability at open-circuit potential identifies the oxygen reduction reaction and hydrogen evolution reaction as key oxidative competitors of Ag in the proposed displacement protocol. Analysis of the morphology evolution during the growth of Ag by displacement Pb and Tl UPD layers suggests the one-to-one exchange scenario (Ag-Tl) as more efficient for longer maintenance of a layer-by-layer silver deposition. The excellent quality of layers deposited by monolayer-restricted galvanic displacement is manifested by a steady UPD voltammetry and ascertained by an overall flat and uniform surface morphology maintained during the entire growth process. An X-ray photoelectron spectroscopy analysis finds no traces of Pb and Tl in the Ag deposit.

Full text of DOI:10.1149/1.2218769

C648
Journal of The Electrochemical Society, 153 ͑9͒ C648-C655 ͑2006͒
0013-4651/2006/153͑9͒/C648/8/$20.00 © The Electrochemical Society
Epitaxial Growth of Ag on Au„111… by Galvanic Displacement
of Pb and Tl Monolayers
b
R. Vasilic,a, L. T. Viyannalage, and N. Dimitrov
*
a,b,**,z
aMaterials Science and Engineering Program, and bDepartment of Chemistry, State University of New
York at Binghamton, Binghamton, New York 13902, USA
The development of a new method for epitaxial growth of metals in solution by galvanic displacement of layers predeposited by
underpotential deposition ͑UPD͒ is discussed and experimentally illustrated. Cyclic voltammetry and scanning tunneling micros-
copy are employed to carry out and monitor a “quasi-perfect,” two-dimensional growth of up to 35 monolayers of Ag on Au͑111͒
by repetitive galvanic displacement of underpotentially deposited Tl and Pb monolayers. A complementary kinetic study of Pb and
Tl UPD layer stability at open-circuit potential identifies the oxygen reduction reaction and hydrogen evolution reaction as key
oxidative competitors of Ag in the proposed displacement protocol. Analysis of the morphology evolution during the growth of Ag
by displacement Pb and Tl UPD layers suggests the one-to-one exchange scenario ͑Ag–Tl͒ as more efficient for longer mainte-
nance of a layer-by-layer silver deposition. The excellent quality of layers deposited by monolayer-restricted galvanic displace-
ment is manifested by a steady UPD voltammetry and ascertained by an overall flat and uniform surface morphology maintained
during the entire growth process. An X-ray photoelectron spectroscopy analysis finds no traces of Pb and Tl in the Ag deposit.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2218769͔ All rights reserved.
Manuscript submitted April 4, 2006; revised manuscript received May 8, 2006. Available electronically July 13, 2006.
The growth of smooth, homo-, and heteroepitaxial metal layers
DMG and SMG,2-4 these methods are still subject to some limita-
tions and practical inconvenience that need to be addressed. For
instance, both methods are tunable over a variety of parameters,
including metal concentrations, scan rate, potential limits, and sub-
monolayer coverage that are mutually dependent and thus difficult
to balance. Also, the thickness control ͑via charge measurements͒
during growth is hindered by the interference of factors such as side
reactions and double-layer charging that, especially in DMG, are
manifested by currents comparable to ͑or even exceeding͒ the depo-
sition current level. Finally, although very low, the possibility for
incorporation of mediator into the deposit cannot be completely
ruled out.
In this paper we present an electrochemical and scanning tunnel-
ing microscopy ͑STM͒ study aimed at experimental validation of a
recently proposed deposition method6 realizing a monolayer-
restricted galvanic displacement ͑GD͒ as a “building block” for the
growth of smooth, epitaxial metal films. The employed strategy uti-
lizes a concept first proposed and then widely used for submono-
layer to monolayer surface modification, assisted by irreversible gal-
vanic displacement of underpotentially predeposited less-noble
metal by a more-noble metal of interest.7,8 The newly developed
protocol is enabled for heteroepitaxial systems in which the dis-
placed metal deposits underpotentially not only on the substrate, but
also on the growing metal, so that the “building block” reaction may
be repeated as many times as desired. Similarly, repetitive deposi-
tion loops have been technically used for years in electrochemical
atomic layer epitaxy ͑EC ALE͒ for growth of epitaxial, compound
semiconductor layers with wide application in the electronics
industry.9 Recent proof-of-concept results demonstrated excellent
growth uniformity achieved by multiple application of a galvanic
displacement step.6 A perfect thickness control during the deposition
is warranted by the redox exchange stoichiometry. The electroless
nature of the displacement reaction enables the decoupling of mutu-
ally dependent in a typical DMG or SMG scenario, growth control-
ling factors thus improving the overall deposition control. Lastly, a
potential monitoring during the displacement reaction serves to vir-
tually exclude incorporation of the UPD metal into the growing
layer.
has always been a major goal of electrodeposition. Such layers are
considered superior to any other kind of thin films as they strictly
reproduce the crystallography and ͑in general͒ the morphology of
the underlying substrate surface. Epitaxial metallic layers also fea-
ture reduced ohmic resistivity and electromigration that, along with
their continuity, render these layers highly desirable for applications
in the electronics industry and in the synthesis of low-metal loading
catalysts.
Thin-film growth modes are generally classified as Frank–van
der Merve or layer-by-layer ͑2D͒ growth, Volmer-Weber or 3D-
cluster growth, and Stranski-Krastanov ͑SK͒ mode, the latter asso-
ciated with a transition from 2D to 3D growth.1 Step-flow growth is
a multilayer variant of Frank–van der Merve mode and occurs at an
appropriate step density and deposition flux. As long as 2D mode is
operating during the growth ͑whether monolayer or multilayer͒, the
resulting film is in registry with the substrate structure. However, the
vast majority of characterized heteroepitaxial systems displays ei-
ther 3D or SK growth mode at ambient temperature. In addition,
owing to kinetic issues, 3D growth is often encountered even in
overlayer/substrate systems that should, according to thermody-
namic considerations, grow in a flat, 2D mode. As a result, 3D
clusters that are far apart grow predominantly in the vertical direc-
tion to merge eventually at a considerable layer thickness in a poly-
crystalline metal deposit.
A major advance toward electrodeposition of smooth, epitaxial
metal deposits was made by Sieradzki et al. with the development of
2
defect mediated growth ͑DMG͒ and surfactant mediated growth
͑SMG͒.3 The protocols employed in these techniques assume either
codeposition of the growing metal with a reversibly deposited me-
diator metal ͑Pb2+ or Cu2+͒ ͑DMG͒ or use a predeposited submono-
layer of “surfactant” metal ͑Pb2+͒ ͑SMG͒ that floats on the top of the
depositing metal and facilitates the 2D growth in the system of
interest. The applicability of these methods was successfully dem-
onstrated by growth of commercially thick metal deposits in the
4
systems Ag/Au͑111͒, Ag/Ag͑111͒,2,3 and Cu/Au͑111͒ with Pb2+
or Cu2+ as mediators. Recent X-ray specular reflectivity measure-
ments of Wang et al.5 demonstrated that nearly complete ͑“perfect
2D”͒ monolayer ͑ML͒ and bilayer Ag films can be grown by opti-
mized DMG procedures.
In this work the growth experiments are preceded by a concise
kinetic study of the stability of Tl and Pb UPD layers on Ag͑111͒ at
open-circuit potential ͑OCP͒. This effort is justified by arguments
͑discussed in more detail elsewhere10͒ associated with the competi-
tion between Ag+ as displacing ions and oxidative agents such as
oxygen reduction reaction ͑ORR͒ and hydrogen evolution reaction
͑HER͒. The ultimate goal of these experiments is to determine quan-
titatively a concentration of Ag+ that will guarantee maximum effi-
ciency and thus a stoichiometric yield of the employed displacement
Although results of structural characterization and elemental
analysis suggest smooth and inclusion-free epitaxial films grown by
*
Electrochemical Society Student Member.
Electrochemical Society Active Member.
**
z E-mail: dimitrov@binghamton.edu
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Journal of The Electrochemical Society, 153 ͑9͒ C648-C655 ͑2006͒
strategy. In the deposition part of this work we have chosen as a
C649
prototype the growth of Ag on Au͑111͒. In particular, a repetitive
displacement of a predeposited Pb UPD layer by silver ions serves
as a building block reaction. Without displacement, or additional
kinetics manipulation, the Ag/Au͑111͒ system grows in an SK
mode, forming two wetting layers prior to 3D cluster formation.11
This identifies Ag/Au͑111͒ as an excellent candidate for implemen-
tation of protocols that would facilitate and help maintain 2D growth
to a considerable thickness of the resulting layer. Another advantage
of that system is the well-documented ability of Tl and Pb to deposit
12-16
underpotentially on Au͑111͒ and Ag͑111͒
featuring distinct
voltammetric behavior that allows for quantitative discrimination
between silver and gold substrates.17
Experimental
Kinetic experiments (stability of UPD layers at OCP).— A
͑111͒-oriented silver crystal ͑Monocrystals Co.͒, 3 mm thick and
1 cm in diameter, was prepared using a modified procedure
originally described in Ref. 18. First, the crystal was mechanically
polished with water-based deagglomerated alumina suspensions
͑Buehler͒ down to 0.05 ␮m and rinsed with water. In the
next step Ag crystal was kept in concentrated H2SO4 for 5–10 min
and subsequently rinsed with Barnstead Nanopure
͑Ͼ18.3 M⍀͒ and dried in nitrogen. Then, it was transferred to
4 M CrO3 + 0.6 M HCl chemical polishing solution. The chemical
polishing took place ͑usually for 10–15 s͒ until the Ag surface
was uniformly covered with yellow-orange oxide film. Afterward,
the crystal was rinsed with Barnstead Nanopure water and soaked
in concentrated NH3 to completely remove the oxide film. This
step was followed by rinsing with Nanopure water and subsequent
immersion in concentrated H2SO4 for additional 5 min to dissolve
any oxide remnants on the crystal surface. After a final rinsing,
the crystal was transferred to the electrochemical cell under the
protection of deoxygenated water droplet. During the experiments
the Ag͑111͒ crystal was held in contact with the solution through
a hanging meniscus configuration.19 The UPD layer stability at
OCP was investigated by chronopotentiometry in Tl+ and Pb2+
perchlorate-based solutions with controlled pH and oxygen content.
A PAR-273 potentiostat coupled with a Nicolet 310 digital storage
oscilloscope was used to record chronopotentiograms at OCP and
to measure steady-state reduction currents prior to each experiment.
Figure 1. OCP transients illustrating the dependence of the Pb UPD layer
stability upon the oxygen concentration in 3 ϫ 10−3 M Pb͑ClO4͒ + 1
2
ϫ 10−2 M HClO4 + 1 ϫ 10−1 M NaClO4 solution. Inset legend: oxygen
concentration ͑mol L−1͒ prior to each transient registration.
A vertical mobility of the WE holder and an in-plane rotation of the
turret platform operated by an accessible shaft enable the multiple
solution immersion of the WE. All three compartments are incorpo-
rated in a clear acrylic jar with a sealable lid, equipped with purging
ultrahigh-purity ͑UHP͒ nitrogen gas connections for oxygen evacu-
ation. Cyclic voltammetry experiments were performed with a Cy-
press Systems 66-OMNI-101B potentiostat equipped with the 66-
Acquire101 software package to collect experimental data on a
computer. All solutions were made with highest purity grade chemi-
cals and Barnstead Nanopure water. For the sake of clarity, all po-
tentials are quoted as underpotentials ⌬E, ͑⌬E = E − EMez+/Me, and
E is the applied potential͒. MSE is used for potential reference
where underpotential cannot be defined ͑Fig. 3-two UPD metals and
Fig. 10-no UPD metal͒.
Characterization experiments (STM and XPS).— After the
growth, the Au crystal with the deposited Ag layer was rinsed
with Nanopure water and transferred with a small water droplet
to an environmental chamber steadily purged with UHP nitrogen,
where ex situ STM experiments were performed. A molecular
imaging ͑MI͒ Pico Scan 300S scanner, MI Pico Scan 2100
controller, and MI Pico Scan software were used for monitoring
the Ag layer surface morphology. Tips for the STM experiments
were made by etching of Pt 80%-Ir 20% wire in a 1:2 mixture
of saturated CaCl2 solution and water at 25 V ͑ac͒.
X-ray photoelectron spectroscopy ͑XPS͒ experiments were car-
ried out at the Center for Nanoscale Systems ͑CNS͒ at Cornell Uni-
versity. The instrument used was Surface Science SSX-100 ͓X-ray
source-Al K␣ ͑1486.6 eV͔͒ spectrometer operating at base pressure
of 1 ϫ 10−9 Torr with both incident beam and detector angle set to
55°. The spot size was 1000 ␮m.
Growth experiments (galvanic displacement).— A mechanically
polished, electropolished, and flame annealed Au͑111͒ single-
crystal ͑Monocrystals Co.͒, 2 mm thick and 1 cm in diameter, was
used as a working electrode ͑WE͒. The mechanical polishing down
to 0.05 ␮m was done using water-based, deagglomerated alumina
suspensions ͑Buehler͒. The gold surface was then electropolished by
anodization in 3:2:1 ethylene glycol, hydrochloric acid, and glacial
acetic acid for 10–15 s at a dc current density of 2.5 A cm−2 with a
platinum cathode. After a thorough rinsing with Barnsted Nanopure
water ͑Ͼ18 M⍀͒ the Au crystal was annealed to red heat in a pro-
pane flame for 10 min and cooled rapidly in nitrogen atmosphere.
The surface was terminated by a water droplet to prevent contami-
nation and mounted on a holder for work at a hanging meniscus
configuration.19 Finally, the holder with the crystal was placed in a
state-of-the-art, controlled environment, three-compartment electro-
chemical setup, which allows for multiple electrolyte immersion of
the WE.6 In this setup, a static, quartz glass Petri dish containing
1 ϫ 10−1M NaClO4 + 1 ϫ 10−2 M HClO4 + 3
Results and Discussion
Kinetic study: stability of Pb and Tl UPD layers at OCP.—
The influence of dissolved oxygen on the Pb UPD layer stability at
OCP was investigated by chronopotentiometry according to an ex-
perimental protocol described elsewhere.10 Figure 1 represents
potential-time dependences obtained in 2 ϫ 10−4 M Pb͑ClO4͒2
+ 1 ϫ 10−2 M HClO4 + 1 ϫ 10−1 M NaClO4 solution after differ-
ent times of solution deoxygenation. The oxygen concentrations
summarized in the legend of Fig. 1 are calculated based on a mea-
surement of steady-state current densities at the initial potential prior
ϫ 10−3 M Pb͑ClO4͒2 ͓or 5 ϫ 10−3 M TlClO4 instead of
Pb͑ClO4͒2͔ solution serves as a three-electrode cell where UPD ex-
periments are carried out using a Pb wire ͓or mercury sulfate elec-
trode ͑MSE͒ in the case of Tl͔ as a reference electrode and a Pt wire
serving as a counter electrode. Two other quartz glass compart-
ments,
containing,
respectively,
1 ϫ 10−2 M AgClO4 + 1
ϫ 10−2 M HClO4 solution for the galvanic displacement, and Barn-
sted Nanopure water for rinsing, are mounted on a turret assembly.
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C650
Journal of The Electrochemical Society, 153 ͑9͒ C648-C655 ͑2006͒
Figure 2. Dependence of the time to strip a Pb monolayer ͑measured at the
flat part of the curves in Fig. 1͒ upon the oxygen concentration in 3
ϫ 10−3 M Pb͑ClO4͒ + 1 ϫ 10−2 M HClO4 + 1 ϫ 10−1 M NaClO4 solu-
2
tion. The inset equation shows linear regression parameters.
to each chronopotentiometric experiment. Assuming a diffusion-
controlled ORR on a Pb covered Ag͑111͒ in perchlorate solution
over the entire oxygen concentration range20 one can attribute a
particular O2 concentration to each current value using the linear
dependence
ilim = k ϫ CO
͓1͔
2
where ilim is the oxygen diffusion-limited current density, k is a
constant of proportionality, and CO is the bulk oxygen concentra-
2
tion. A relevant reference number to quantify the proportionality in
Eq. 1 is the bulk concentration of O2 in naturally aerated solution,
21
CO = 2.25 ϫ 10−4 mol L−1
.
The chronopotentiometric experi-
2
ments were initiated at underpotential of 0.03 V by taking off the
potential control. The nitrogen purging was discontinued during the
registration of each transient and the cell was kept hermetically
closed to assure steady experimental conditions. The curves plotted
in Fig. 1 illustrate a strong dependence of the UPD layer stability
upon the presence of O2 in the solution. It is clear that the UPD
layer stripping time, tstr, increases up to three orders of magnitude as
the oxygen concentration in the solution decreases proportionally. A
power-law dependence with a slope ͑exponent͒ of −1 is clearly seen
Figure 3. ͑A͒ OCP transients registered in the systems Pb/Ag͑111͒ and
Tl/Ag͑111͒ in thoroughly deoxygenated 3 ϫ 10−3 M Pb͑ClO4͒ + 1
2
ϫ 10−1 M NaClO4 and 5 ϫ 10−3 M TlClO4 + 1 ϫ 10−1 M NaClO4 solu-
tions, respectively, at different pH ͑as detailed in the legend͒. ͑B͒ Cyclic
voltammetry curves of Pb/Ag͑111͒ and Tl/Ag͑111͒ processes in the above-
mentioned solutions at pH = 2. Sweep rate 10 mV s−1
.
in Fig. 2 for tstr as a function of CO . This result could be rational-
ized having in mind that
2
the stripping of Pb UPD layers in both systems at OCP.
The HER is another competitor to the metal ions in a galvanic
displacement process. We demonstrated elsewhere10 based on ki-
netic data that HER cannot be considered as a major competitor in
the case of Pb UPD layer stripping from Cu͑111͒ electrode at OCP.
In the present work a similar conclusion for the system Pb/Ag͑111͒
can be illustrated by the identity of potential transients registered at
pH 2 and 4.5 in solutions with the lowest possible oxygen concen-
tration ͑Fig. 3A͒. It could be clearly seen that there is no significant
change in the time for stripping a monolayer at OCP over more than
two orders of magnitude difference in hydronium ions concentra-
tion. The latter argument, however, does not hold for the OCP be-
havior in the system Tl/Ag͑111͒, where potential transients regis-
tered at pH 2, 3, and 5 in Tl+-containing solution with the lowest
possible oxygen concentration show clearly a pH dependence ͑Fig.
3A͒. This result could be qualitatively explained. While the ex-
change current densities for HER on bulk Pb and Tl are of the same
order of magnitude,23 the HER overpotential would be about
250 mV more negative on Tl-covered Ag͑111͒ in comparison with
qUPD
tstr
=
͓2͔
ilim
where qUPD is the charge density of the UPD layer ͑constant͒ and for
the sake of simplicity ilim is assumed to be constant over the entire
underpotential range. Now, combining Eq. 1 and 2 we get a math-
ematical expression for the inverse proportionality
1
tstr
ϳ
͓3͔
CO
2
This correlation explains the similarity between the OCP behavior in
Pb/Ag͑111͒ and Pb/Cu͑111͒ systems. The experimental results
10
obtained suggest the Pb monolayer as twice as stable on Ag͑111͒
substrate. The latter finding is in perfect agreement with the reduc-
tion currents generated by a two-electron ORR on Pb-covered
Ag͑111͒ vs four-electron ORR on Cu͑111͒.22 These arguments
20
identify the dissolved oxygen as a key oxidative agent that governs
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Journal of The Electrochemical Society, 153 ͑9͒ C648-C655 ͑2006͒
C651
Figure 4. Schematic illustrating the time-potential protocol employed for a
growth by galvanic displacement.
Figure 5. STM micrographs 570 ϫ 570 nm illustrating: ͑A͒ thermally re-
constructed Au͑111͒ surface ͑inset size 60 ϫ 60 nm͒ and ͑B͒ the same sur-
face after lifting of the reconstruction. Images ͑C͒ and ͑D͒ show the mor-
phology evolution of the Ag layer with the thickness increase after 1 and 40
replacements, respectively.
Pb-covered Ag͑111͒ as determined by the equilibrium potentials of
both metals ͑the arrows in Fig. 3͒. As a consequence a steady-state
reduction current density, of −400 nA/cm2, is measured prior to
potential transients at pH 2 in the system Tl/Ag͑111͒ that is nearly
ten times higher than the current density at the same pH for
Pb/Ag͑111͒ ͑−35 to −40 nA/cm2͒. Wrapping up the above discus-
sion, one needs to compare the potential transient registered for Tl
UPD stripping at pH 5 with the one registered at pH 2 for Pb. In
fact, such comparison clearly illustrates an identical rate of charge
compensation in both cases taking into account the state of oxidation
difference between Tl+ and Pb2+ ions. This means that pH of Tl+
solution needs to be as high as 5 so that the HER influence on the Tl
UPD layer stability at OCP becomes negligible. In summary, the
kinetic analysis suggests higher values of pH as more favorable for
galvanic displacement-based protocols because of the negligible im-
pact of HER on the UPD layer stability. However, in order to avoid
any possibility for surface oxidation or hydrolysis we decided to
perform our deposition experiments at pH 2, where even a Tl UPD
layer would be stable for at least 300 s before any stripping is ini-
tiated ͑Fig. 3A͒.
the Au͑111͒ surface uniformly as confirmed by the STM picture
presented in Fig. 5C, where the overall morphology, clusters size,
and average roughness are identical to those prior to the immersion
experiment ͑Fig. 5B͒. A stripping experiment ͑not presented here͒
following the STM observation suggests the presence of about 1.7
Ag MLs on the Au surface in the case considered. According to Ref.
7, the amount of Ag atoms deposited in a single building block
Galvanic displacement of Pb monolayer.— The time-potential
protocol employed for Ag deposition is schematically illustrated in
Fig. 4. Before the deposition, the thermally reconstructed Au͑111͒
surface ͑Fig. 5A͒ is subjected to a potential cycle in the Pb UPD
range until a stable voltammogram, consistent with previous
reports14,24 was registered ͑Fig. 6A͒. It has been shown that a Pb
UPD layer lifts the thermal reconstruction,25 thus populating the Au
surface with highly mobile small clusters of atoms. A subsequent
interaction between the relatively disordered Au surface and the Pb
UPD layer leads to a surface alloying,14 eventually resulting in a
morphology of Au͑111͒ similar to the one presented in Fig. 5B.
After the cycling, the Au surface was covered by a UPD layer of Pb
and upon discontinuing the potential control it was transferred into a
solution containing 1 ϫ 10−2 M Ag+ ions. The optimal immersion
time of 20 s to complete the stoichiometric redox exchange between
Pb and Ag was established based on our kinetic study reported on in
the previous paragraph. After the displacement, the surface was thor-
oughly rinsed by multiple immersions in deoxygenated water and
transferred back to the Pb UPD cell where a cyclic voltammogram
was registered ͑Fig. 6B͒. The peak potentials and shape of the curve
in Fig. 6B are associated with the formation of a hexagonal close-
packed monolayer of Pb on the Ag͑111͒ surface.13 According to
results of Pauling et al.,17 the remnants of features associated with
Pb UPD on Au͑111͒ suggest that less than two continuous MLs of
Ag are present on the Au substrate. Apparently the Ag atoms cover
Figure 6. CV of bare and Ag-covered Au͑111͒ surface after different num-
ber ͑n͒ of displacement cycles ͑A͒ n = 0; ͑B͒ n = 1; ͑C͒ n = 3; ͑D͒ n = 20 in
3 ϫ 10−3 M Pb͑ClO4͒ + 1 ϫ 10−2 M NaClO4 ͑pH 2͒ solution. x axis: Un-
2
derpotential ͑V͒; y-axis: current density ͑␮A cm−2͒. Sweep rateϭ10 mV s−1
.
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C652
Journal of The Electrochemical Society, 153 ͑9͒ C648-C655 ͑2006͒
reaction upon 100% displacement efficiency ͑no side reactions taken
into account͒ will depend upon the displacement reaction stoichiom-
etry according to the following equation
z2
ͩ ͪ
z1
z2
ͩ ͪ
z1
z +
z +
U0 + k
G
→ U + k
G0
͓4͔
1
2
where z1 and z2 are the oxidation states for the growing metal, G,
and the UPD metal, U, ions, respectively. The coefficient
k = dG/dU is introduced in our previous work6 takes into account the
different atomic densities, d ͑number of atoms per unit area͒, of G
and U layers participating in the displacement reaction. Based upon
grazing incidence X-ray scattering26 and in situ STM results14 sug-
gesting dPb Ϸ dTl in the UPD systems Pb2+/Ag͑111͒ and
Tl+/Ag͑111͒, respectively, the coefficient k should be within the
range ͑0.74–0.79͒. These arguments set a theoretical limit of up to
1.6 MLs of Ag grown by a single displacement of a predeposited Pb
UPD layer and up to 0.8 MLs of Ag when Tl is being displaced. The
reason for obtaining a slightly higher amount of deposited silver
͑Eq. 4 predicts 1.6 MLs͒ is most likely associated with a deviation
from the ideality for the system considered. Factors contributing to
such behavior could be the surface roughness, inaccuracy of back-
ground current correction, edge effects, or tiny portion of Ag+ ions
introduced into the UPD cell after displacement. In general, despite
these negligible differences, the building block procedure in our
experiments is in excellent agreement with the findings of Brank-
ovic et al. emphasizing a single-step Ag substrate modification by
displacement of a Cu UPD layer.7
Figure 7. ͑A͒ STM image 320 ϫ 320 nm with cross-sectional analysis il-
lustrating the morphology evolution after three displacement events with five
grown Ag layers. ͑B͒ STM image 700 ϫ 700 nm providing clues for pos-
sible growth mechanism. ͑C͒ Higher resolution STM image 3.6 ϫ 3.6 nm
illustrating the ͑111͒ atomic structure of the topmost Ag layer.
The new outcome that warrants the innovative aspect of our
study is associated with a multiple application of the building block
procedure for epitaxial metal growth. The galvanic displacement
step described in the previous paragraph was repeated up to 20 times
in the present study. The voltammetry curve displayed in Fig. 6C
and 6D was registered on Ag layers grown by 3 and 20 displace-
ments, respectively. The identical appearance of both curves with
the curve registered on Ag͑111͒ single-crystal ͑Fig. 3B-dashed line͒
suggests no qualitative difference in the surface morphology and
energetics. This conclusion, providing no hints for 3D growth, is
confirmed by the STM in Fig. 5D. Apparently, the surface morphol-
ogy consists of atomically flat terraces 20–200 nm in size, separated
by steps with monolayer height. A better justification for such state-
ment could be found in Fig. 7 ͑with the corresponding voltammetry
in Fig. 6C͒ that represents an intermediate morphology evolution
registered after three displacement events. It is evident from the
cross-sectional analysis in image A that the step height is always 1
atomic height. The higher resolution image C illustrates the ͑111͒
atomic structure of the topmost Ag layer, thus confirming the epi-
taxial growth in general. Also, the lower resolution image B sheds
more light on the mechanism of growth associated with the displace-
ment events. There is an apparent merging of clusters growing in a
lateral direction ͑emphasized by the framed areas͒ that eventually
leads to a layer completion. The lack of vacancy clusters deeper than
1 ML ͑Fig. 7, images A, B͒ and the relatively constant surface
roughness after growth of 35 Ag layers suggests a quasi-perfect
layer-by-layer growth mode.5
The unique ability of Tl to form two monolayers on Ag͑111͒
lends an opportunity to displace either one or two layers at once. For
the purposes of this study we decided to investigate only one-to-one
displacement. The cyclic voltammetry ͑CV͒ curves registered after
different number of replacements are shown in Fig. 8. A careful
Galvanic displacement of Tl monolayer.— Thalium UPD on
Ag͑111͒ is among very few processes producing two monolayers in
the underpotential range.27,28 A negative potential sweep results first
in formation of one monolayer of undepotentially deposited thalium
atoms on Ag͑111͒ at underpotential of ϳ260 mV as shown in Fig.
3B, solid line. A continuing sweep gives rise to another deposition
peak just before the equilibrium Tl/Tl+ potential. Careful consider-
ation of peak charges proves the association of most negative peak
with the formation of the second Tl monolayer on Ag͑111͒.27,28
Accordingly, two stripping peaks, respectively, associated with the
corresponding monolayers appear when potential is swept in the
positive direction ͑Fig. 3B, solid line͒.
Figure 8. CV curves for Tl UPD on Au͑111͒ and evolution of Tl UPD on
Ag͑111͒ after different number ͑n͒ of displacement cycles ͑A͒ n = 0;
͑B͒ n = 1; ͑C͒ n = 5; ͑D͒ n = 10 in 5 ϫ 10−3 M TlClO4 + 1
ϫ 10−1 M NaClO4 ͑pH 2͒ solution. x axis: Underpotential, ͑V͒; y axis: cur-
rent density, ͑␮A cm−2͒. Sweep rateϭ10 mV s−1
.
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Journal of The Electrochemical Society, 153 ͑9͒ C648-C655 ͑2006͒
C653
Figure 9. STM image of Au͑111͒ surface after potential cycling in 5
ϫ 10−3 M TlClO4 + 1 ϫ 10−1 M NaClO4 ͑pH = 2͒ solution at a sweep rate
of 10 mV s−1. Scan size 750 ϫ 750 nm.
Figure 10. STM micrographs showing the morphology evolution with the
thickness increase of Ag thin film on Au͑111͒: ͑A͒ after 5 displacements; ͑B͒
after 10 displacements; ͑C͒ after 20 displacements; and ͑D͒ after 30 displace-
ments. Scan size for all images is 750 ϫ 750 nm.
examination of the voltammetric features suggests a formation of at
least two Ag layers uniformly spread on the gold surface after the
fifth displacement.17 The Ag film acquires all properties of Ag͑111͒
single crystal after the tenth displacement. Apparently, after this
number of displacements, the whole crystal surface area acquires
entirely the chemical potential of a bulk Ag͑111͒.29 All voltammo-
grams in Fig. 8 are obtained right after the corresponding displace-
ment event ͑without additional cycling also known as electrochemi-
cal annealing30,31͒. However, a simple comparison suggests identical
qualitative and similar quantitative ͑peak height and width͒ appear-
ance of the CV curve obtained on as-deposited Ag film ͑after 10
displacements, Fig. 8D͒ with the one obtained on well-prepared
Ag͑111͒ single crystal ͑Fig. 3B, solid line͒. Having in mind that
well-prepared Au͑111͒ and Ag͑111͒ surfaces feature average ter-
race sizes of 200 and 50 nm, respectively,32,33 the above-mentioned
result rules out a substantial roughening and undoubtedly ascertains
the quasi-perfect mode of growth during our deposition experiment.
The discussion of our STM results begins with an analysis of the
Au͑111͒ surface after a few successive deposition and stripping
cycles of Tl UPD layer. Unlike the case of Pb UPD, where the
voltammogram changed quantitatively with cycling, the current
voltage curve in the presence of Tl demonstrated neither qualitative
nor quantitative changes under identical circumstances. This result
hints at substantially fewer changes taking place on the Au͑111͒
surface in the presence of Tl and as a result on a flatter appearance
͑Fig. 9͒ after lifting of the thermal reconstruction of the Au͑111͒
surface. In general, such behavior is not surprising, bearing in mind
that no evidence for surface alloying in the system Tl+/Au͑111͒ has
been reported so far. In addition, the lack of alloying not only pre-
vents the gold surface from roughening but also promotes the flat-
tening and rearranging functions typical for successively deposited
and stripped UPD layers.30,31
observed morphology ͑Fig. 10A and 10B͒ is almost identical to the
one of the gold substrate ͑Fig. 9͒. In fact, a comparison with Figin.
7B of the present work strengthens this argument, as the morphol-
ogy seen after 20 Tl GDs is quite similar to that observed after the
third Pb GD. Apparently it takes less to initiate the process of sur-
face roughening with using Pb as a displacement mediator.
Characterization by XPS.— The elemental composition of Ag
films grown by galvanic displacement of Pb and Tl UPD layers was
also analyzed. A two-point, depth-profile XPS analysis performed
for both Pb ͑Fig. 12͒ and Tl ͑Fig. 13͒ with a limit of detection
͑LOD͒ 0.1 atom % found no traces of those metals incorporated into
the deposit. This result is in concert with the elemental analysis of
layers grown by DMG and SMG where traces of mediating metals
were also not found.2-4 Indeed, the issue associated with surface
14
34,35
alloying in Pb/Au͑111͒ and Pb/Ag͑111͒
systems is addressed
Based on this reasoning, it can be expected that epitaxial growth
of Ag on Au͑111͒ assisted by galvanic displacement of Tl starts on
initially flatter gold surface than in the case of GD of Pb. However,
this does not necessarily mean that the initial roughness is the only
factor contributing to the overall smoother appearance of the silver
layer in the case of Tl GD ͑Fig. 9͒. As clearly illustrated by Fig.
10C, the first considerable morphological changes on the growing
Ag surface could be seen after 20 displacement events correspond-
ing to ϳ17 equivalent MLs ͑determined by integration of Ag strip-
ping peak registered anodically of Ag OCP-Fig. 11͒. Before that the
Figure 11. Anodic stripping voltammetry of Ag on Au͑111͒ after n displace-
ments ͑as detailed in the legend͒. Electrolyte: 5 ϫ 10−3 M TlClO4 + 1
ϫ 10−1 M NaClO4, ͑pH 2͒; Sweep rateϭ1 mV s.
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C654
Journal of The Electrochemical Society, 153 ͑9͒ C648-C655 ͑2006͒
able potential difference between the Nernst potential of the UPD
metal and the corrosion potential of the growing metal.
Proposed mechanism of deposition by galvanic displacement.—
At present, a quantitative view of the monolayer restricted displace-
ment mechanism is not available. However, well-known facts and
the experimental results of this work allow for building a working
hypothesis. The comparison of the above-described phenomenology
should start with a description of the GD mechanism. According to
our view, at a relatively high concentration ͑no diffusion limitations͒
of the displacing metal, its ions driven by the potential established
by the UPD metal ͑Tl or Pb͒, first uniformly cover the predeposited
UPD layer. Such flat layer deposition scenario satisfies the general
requirement for lowering of the surface free energy as interpreted
for a general UPD process in EC ALE9 and also following argu-
ments explaining the surfactant mediation role of various UPD lay-
ers. Subsequently, a stoichiometric redox exchange, initialized
mainly on the flat terraces, facilitates the formation of large clusters
that grow laterally and eventually merge to yield a continuous epi-
taxial deposit. The flat terraces are identified here as displacement
initialization centers because of the well-known fact that in a typical
UPD system the stripping process always starts on terrace sites
where the bonding to the substrate is weaker than on step and kink
sites.13,29 Apparently the stoichiometric factor will be impacting the
flux of the depositing metal in a single displacement event. Let us
assume, for the sake of this argument, that every displacing atom
will take the deposition site of the displaced one. Then, in the case
of Tl, where displacing and displaced metals manifest equal state of
oxidation ͑1:1͒, the odds for obtaining a layer that perfectly repro-
duces the substrate morphology are substantially higher. Following
this general sequence of events in the case of Pb͑2:1͒, there will be
always an extra atom of Ag per displacement event with uncertain
position in the growing layer. This extra atom will either occupy a
spot next to its neighbor into the growing layer or will contribute to
the nucleation of a second layer that takes place before the comple-
tion of the first one. Which of these two options takes place will also
depend upon other factors such as stress accumulation, surface dif-
fusivity, and energy to overcome the so-called Schwoebel’s
barrier.37 A mathematical modeling and numerical simulations could
help to quantitatively address this matter.
Figure 12. Characteristic XPS survey spectrum of 35 ML thick Ag layer
grown on Au͑111͒ by galvanic displacement of Pb UPD layer ͑main graph͒.
The inset represents a high-resolution spectrum of the same sample where
black markers indicate the expected peak positions presence of Pb exceeding
0.1 atom %.
by both ͑i͒ termination of the displacement step at a positive enough
potential ͑the stability of a surface alloy depends upon the applied
potential35͒ and ͑ii͒ selection of parameters employed in the cur-
rently developed procedure that assume a fast displacement reaction
͑the surface alloying is typically a slow process36͒. Such strict con-
trol makes virtually impossible the inclusion of any UPD metal par-
ticipating in the displacement reaction provided there is a consider-
Conclusions
This paper reports on the development and experimental valida-
tion of a new method for growth of epitaxial metal films that utilizes
a monolayer-restricted galvanic displacement as a building block
reaction. A complementary kinetic study identifies ORR as a key
oxidative competitor in the Pb displacement process, and both ORR
and HER when Tl layer is subjected to galvanic displacement. Elec-
trochemical and STM results illustrate a quasi-perfect, 2D growth of
up to 35 ML of Ag on Au͑111͒ assisted by repetitive galvanic dis-
placement of a predeposited Tl and Pb UPD layers. The stoichiom-
etry factor is found not only to control the film thickness but also to
influence the morphology evolution of the growing Ag layer. An
elemental analysis with LOD of 0.1 atom % suggests no traces of
Pb or Tl in the accordingly deposited Ag layer.
Acknowledgments
The authors acknowledge that Fig. 4, 7, and 12 are also presented
in Ref. 6 of this work. The authors thank Dr. Natasa Vasiljevic and
Dr. Miomir Vukmirovic for the helpful discussion, Dr. Jonathan Shu
from CNS at Cornell University for the assistance with the XPS
measurements, and George Shovlowsky from SUNY Binghamton
for the brilliant work in crafting the setup for controlled environ-
ment. The Research Foundation and IEEC at SUNY-Binghamton are
gratefully acknowledged for the financial support of this work.
Figure 13. Characteristic XPS survey spectrum of 25 ML thick Ag layer
grown on Au͑111͒ by galvanic displacement of Tl UPD layer ͑main graph͒.
The inset represents a high-resolution spectrum of the same sample where
black markers indicate the expected peak positions presence of Tl exceeding
0.1 atom %.
State University of New York at Binghamton assisted in meeting the
publication costs of this article.
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Journal of The Electrochemical Society, 153 ͑9͒ C648-C655 ͑2006͒
C655
149 ͑1981͒.
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