Electrochemical and Solid-State Letters, 2 (9) 443-445 (1999)
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Surfactant Mediated Electrochemical Deposition of Ag on Au(111)
S. R. Brankovic, N. Dimitrov, and K. Sieradzki*,z
Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, Arizona 85287-6106, USA
Electrochemical surfactant mediated growth of Ag on Au(111) was investigated experimentally using in situ scanning tunneling
microscopy. Almost ideal layer-by-layer growth of Ag was obtained using 0.8 of a monolayer of Pb as a surfactant. Atomically flat
epitaxial Ag thin films, 200 monolayer in thickness were grown on Au(111) using this technique. Energy dispersive X-ray analysis
and Rutherford backscattering spectrometry showed no traces of Pb in the Ag layers grown.
© 1999 The Electrochemical Society. S1099-0062(98)12-030-8. All rights reserved.
Manuscript submitted December 9, 1998; revised manuscript received May 27, 1999. Available electronically June 23, 1999.
Surfactants have been successfully employed in ultrahigh vacuum (UHV) deposition to modify homoepitaxial and heteroepitaxial
growth modes. Systems such as Ge/Si(100) and Cu/Cu(111), which
typically display 3D Stranski-Krastanov (S-K) or Volmer Webber
growth morphologies at 300 K displayed 2D layer-by-layer growth
by using appropriate surfactants.1-3 In Cu/Cu(111), Camarero et al.3
demonstrated that if the initial Cu(111) surface is covered by a full
monolayer (ML) of Pb, layer-by-layer growth of Cu on Cu(111)
occurred at 300 K.
It is now reasonably well understood that kinetic processes often
determine the growth mode in these systems.4-7 The relevant physics
can be described as follows. In order for growth to proceed in a 2D
mode, the interlayer transport of deposited adatoms must be rapid in
comparison to the intralayer transport, otherwise, nucleation of
islands occurs on as yet uncompleted growing monolayers which
results in 3D growth.8,a Intralayer mass transport occurs by surface
diffusion of adatoms on terraces. The barrier for adatom migration
transport over a descending step dominates the kinetics of interlayer
transport.9,10 The attempt frequency for migration over the barrier is
determined by the ratio D/L2 where D is the adatom diffusivity on a
growing terrace and L is a length scale characterizing the island size.
Experimental conditions favoring large D/L2 promote 2D growth.
These conditions correspond to either a high temperature or the
maintenance of a high density of small 2D islands. Surfactants act to
either enhance the nucleation density and/or reduce the magnitude of
the edge barrier to promote 2D growth.4-7 In order for 2D growth to
proceed for more than just a few layers, it is necessary for the surfactant to float on the surface as growth proceeds, otherwise the surfactant becomes incorporated into the overgrowth.5
The phenomenon of underpotential deposition (UPD) of metals
on foreign substrates provides a convenient electrochemical
approach to surfactant-mediated growth. Typically, ambient temperature homoepitaxial or heteroepitaxial growth of face-centered (fcc)
cubic metals on close-packed (111)-oriented substrates displays 3D
growth. A model system in this regard is Ag/Au(111) which grows
in a S-K mode forming two wetting layers prior to 3D cluster formation.11 Thermodynamically, a UPD layer will “wet” the substrate
if γse > γso + γoe. Here, γse is the interfacial free energy of the substrate/electrolyte interface, γso is interfacial free energy of the substrate/overlayer interface and, γoe is the interfacial free energy of the
overlayer/electrolyte interface. The energy difference between the
left and right sides of this inequality provides the thermodynamic
driving force for a UPD layer to serve as a surfactant. Additionally,
since UPD is a potential dependent process,12 the surface coverage
of a specific UPD layer can be accurately controlled.
Since the UPD system Pb2+/Ag(111) has received considerable
attention in the literature,13-25 we have chosen to examine the behavior of Pb as a surfactant for the electrochemical deposition of Ag.
Electrochemical results of the UPD system Pb2+/Ag (111) suggest
the formation of 1 UPD ML of Pb atoms on the substrate surface.1315 In the last few years, information on the atomic structure of the Pb
* Electrochemical Society Active Member.
z E-mail: [email protected]
UPD layer was obtained using in situ techniques such as electrochemical scanning tunneling microscopy (ECSTM)16-18 and grazing
incidence X-ray scattering.19,26 The general consensus that has
emerged is that at relatively low underpotentials, the close-packed
Pb adlayer is compressed (with respect to the Pb(111) spacing),
incommensurate, and rotated by an angle of >4.5° with respect to
the substrate.16-19 A specific feature of the Pb2+/Ag(111) system is a
surface alloy formation process that takes place during extended
polarization at underpotentials corresponding to an incomplete Pb
monolayer [θ = (10-85%)].20-23 This is believed to occur via a site
exchange process between Pb atoms in the UPD layer and Ag atoms
in the topmost substrate layer.21 The exchange results in surface
alloy formation with correspondingly different voltammetry (peak
positions of Pb stripping and deposition waves) with respect to the
behavior for shorter polarization times.20-23 The kinetics of the surface alloy formation process was found to depend strongly on the
initial Pb coverage,23,24 Ag(111) surface step density,22-24 and temperature.25 No alloying was detected at underpotentials, corresponding to coverage of a complete Pb monolayer.24 In this paper, we
report on results of experiments examining the electrochemical
growth of Ag/Au(111) using an incomplete Pb UPD layer as a surfactant.
Experimental
Electropolished and hydrogen flame annealed Au(111) single
crystals, 2 mm thick and 1 cm in diam, were used as a working electrode in the present investigation. The electropolishing procedure
involved anodic treatment of the gold crystal in a two-electrode cell
containing a mixture of ethanol, ethylene glycol, and hydrochloric
acid in the ratio 2.5:1.5:1, at a current of ~1.5 A cm-2 for ~30 s. A
platinum sheet (8 cm2 in area) was used as the cathode during polishing. Following electropolishing, the crystal was thoroughly rinsed
using Barnsted Nanopure water (>18 MΩ) and was hydrogen flame
annealed.
ECSTM studies were performed using a Molecular Imaging pico
STM with a 300S scanner and Molecular Imaging model 300S pico
bipotentiostat. The cell was made of Teflon, exposed an area of ca.
0.3 cm2, and had a volume of ca. 1.5 cm3. Prior to each experiment,
an STM tip was prepared by etching a 80:20 Pt + Ir wire in a CaCl2
solution and isolating with Apiezon wax.
The electrolyte used for the Ag growth consisted of 10-4 M
AgClO4 + 10-2 M Pb(ClO4)2 + 10-2 M HClO4. All the solutions were
made with high purity grade chemicals and >18 MΩ Barnsted
Nanopure water. An etched Pb wire was used as a pseudoreference
electrode. All potentials are quoted with respect to this electrode.
The growth experiments were performed at constant potentials.
Following growth, a voltammogram (10 mV s-1 sweep rate) was
recorded within the potential range of 0.600 ~ 0 V in order to obtain
the Pb coverage/potential behavior.
a In overlayer/substrate systems displaying 2D growth for more than just several
monlayers, the growth is never truly one layer at a time. Instead, there are several
layers growing laterally at the same time in a stable mode. Three-dimensional cluster growth can develop owing to an instability, possibly of the Mullins-Sekerka type.
444
Electrochemical and Solid-State Letters, 2 (9) 443-445 (1999)
S1099-0062(98)12-030-8 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 1. I-V behavior (cathodic sweep) of an Ag(111) electrode in 10-4 M
AgClO4 + 10-2 M HClO4 solution with and without 10-2 M Pb(ClO4)2.
Sweep rate, 10 mV s-1.
Results and Discussion
The experimental conditions for the ECSTM study of the growth
process are described with reference to Fig. 1. The solid line shows
a cathodic current-voltage (I-V) curve obtained at the end of a
growth experiment in a solution containing 10-4 M AgClO4 + 10-2 M
Pb(ClO4)2 + 10-2 M HClO4. The dashed line is the background
cathodic curve that illustrates the I-V behavior of the system
Ag+/Ag(111) within the same potential range but in absence of Pb2+
ions. Numerical subtraction of the dashed line from the solid line
allowed us to obtain information on the charge due only to the UPD
of Pb2+ on the Ag(111) surface. Time integration of this difference
curve allowed for determination of the charge/potential behavior in
this electrolyte. We obtained a charge of 365 ± 15 µC cm-2 for a
completed lead monolayer on the Ag(111) substrate. The deposition
wave was at 138 mV. These results are in a good agreement with
those obtained on polished and electrochemically grown Ag(111)
crystal surfaces.13,23 We chose three potentials (0.040, 0.125, and
0.145 V) for the growth experiments, as indicated by the vertical
lines in Fig. 1, corresponding, respectively, to Pb coverage of 1.0,
0.8, and 0.3 ML.
The growth morphology of Ag on the Au(111) surface in the
presence of a Pb UPD layer was monitored in situ with ECSTM.
Figure 2 shows a time sequence of images that follows the evolution
of the Ag surface morphology at a potential of 0.125 V where the Pb
UPD layer coverage was >0.8 ML. The mean growth rate was 3
ML/min.b The mean growth rate observed in this time sequence of
images is approximately a factor of four less owing to known tipscreening effects.27,c Figure 2a shows the initial morphology of the
Au(111) surface at a potential of 1 V vs. Pb2+/Pb where one complete UPD layer of Ag is formed. Small traces of a second Ag UPD
monolayer are present at this potential as 5-10 nm size clusters.
There are regions of high step density separated by atomically flat
terraces, 50-300 nm in width. A gradual increase of the number of
layers is apparent in the time sequence of images that shows a layerby-layer growth mode. The average step height in these images was
2 ML. It is also apparent that the growth process takes place simultaneously on different levels of the surface. There was approximately 30 ML of Ag grown after 2100 s on this portion of the surface.
b Anodic polarization was performed form 0.125 to 1 V at a sweep rate of 10 mV s-1
in order to strip the deposited layer. A charge density of ~25 mC cm-2 (correspond-
ing to a mean growth rate of 3 ML/min) was obtained by integration of the stripping
wave.
c The charge obtained by anodic stripping of the deposited Ag layer corresponds to
~120 ML, while the in situ STM results indicated that there was ~30 ML deposited, i.e., a factor of four less. This is the result of tip shielding effects that other
researchers have also noted.
Figure 2. In situ STM topographs showing the time evolution of the Ag layer
morphology. The potential was maintained constant at 0.125 V corresponding to a Pb coverage of 0.8 ML. Scan size on each image is 764 x 764 nm.
Figure 3. STM topograph of a Ag layer grown ex situ at constant potential:
0.125 V. Image acquired in electrolyte at the open-circuit potential of 0.745
V. Scan size is 6 x 6 µm.
This is a factor of 4 less than that grown on other regions of the surface that were not shielded by the STM tip.
Figure 3 shows results of ex situ growth of 200 MLd of
Ag/Au(111) under conditions identical to that of Fig. 2. Energy dispersive X-ray analysis and Rutherford backscattering spectrometry
were used to check for the quantity of Pb incorporated in the Ag
layer. No trace of Pb was found with either technique.e
The results of growth experiments at the other potentials (0.040
and 0.145 V vs. Pb2+/Pb) are shown in Fig. 4a and b. Ag growth proceeded in a 3D mode almost immediately for these growth conditions.
Our experimental results demonstrate that for the specific system
examined, the electrochemical growth of Ag on Au(111) in an electrolyte containing 10-4 M AgClO4 + 10-2 M Pb(ClO4)2 + 10-2 M
HClO4, optimum growth (at a deposition rate of ~3.0 ML/min) was
obtained using ~0.8 ML of Pb as a surfactant. The growth mode was
layer-by-layer as there was no evidence of step flow observed in our
d RBS and electrochemical (stripping wave) experiments were performed in order to
estimate the average thicknes of the Ag layer. In both cases a thickness of 200 ML
was measured.
e The compositional resolution of RBS and energy dispersive X-ray analysis for par-
ticular system was ~2 and ~ 1 atom %, respectively.
Electrochemical and Solid-State Letters, 2 (9) 443-445 (1999)
445
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Figure 4. STM topographs of Ag layers grown in situ at constant potentials:
0.040 V, corresponding to 1.0 ML of Pb (A) and 0.145 V, corresponding to
0.3 ML of Pb, (B). Scan size on each image is 764 x 764 nm.
in situ STM experiments. At present, the detailed mechanism of the
surfactant mediated growth is an open issue although we believe that
interlayer exchange occurs between Ag adatoms and the Pb atoms
comprising the adlayer.
Acknowledgments
The authors thank B. Wilkens for assistance with the RBS measurements and gratefully acknowledge the support of this work by
the National Science Foundation, Division of Materials Research
(contract DMR-9510663).
Arizona State University assisted in meeting the publication costs of this
article.
References
1. M. Copel, M. C. Reuter, E. Kaxiras, and R. M. Tromp, Phys. Rev. Lett., 63, 632
(1989)
2. R. M. Tromp and M. C. Reuter, Phys. Rev. Lett., 68, 954 (1992)
3. J. Camarero, J. Ferron, V. Cros, L. Gomez, A. L. Vasquez de Praga, J. M. Gallego,
J. E. Prieto, J. J. de Miguel, and R. Miranda, Phys. Rev. Lett., 81, 850 (1998)
4. J. Tersoff, A.W. Denier van der Gon, and R. M. Tromp, Phys. Rev. Lett., 72, 266
(1994).
5. Z. Zhang and M. G. Lagally, Phys. Rev. Lett., 72, 693 (1994).
6. J. A. Meyer, J. Vrijmoeth, H. A. van der Vegt, E. Vlieg, and R. J. Behm, Phys. Rev.
B, 51, 14790 (1995).
7. S. Harris, Phys. Rev. B, 52, 16793 (1995).
8. See, for example, A. Mazor, D. J. Srolovitz, P. S. Hagan, and B. J. Bukiet, Phys.
Rev. Lett., 60, 424 (1988).
9. G. Ehrlich and F. G. Hudda, J. Chem. Phys., 44, 1039 (1966).
10. R. L. Schwoebel and E. J. Shipsey, J. Appl. Phys., 37, 3682 (1966).
11. S. G. Corcoran, G. S. Chakarova, and K. Sieradzki, Phys. Rev. Lett., 71, 1585
(1993).
12. D. M. Kolb, in Advances in Electrochemistry and Electrochemical Engineering,
H. Gerischer and C. W. Tobias, Editors, Wiley, New York (1978).
13. H. Bort, K Juttner, W. J. Lorenz, and E. Schmidt, J. Electroanl. Chem., 90, 413
(1978).
14. A. Bewick and B. Thomas, J. Electroanal. Chem., 84, 127 (1977).
15. K Juttner and W. J. Lorenz, Z. Phys. Chem., 122, 163 (1980).
16. U. Muller, D. Carnal, H. Siegenthaler, E. Schmidt, W. J. Lorenz, W. Obretenov,
U. Schmidt, G. Staikov, and E. Budevski, Phys. Rev. B, 46, 12899 (1992).
17. W. Obretenov, U. Schmidt, W. J. Lorenz, G. Staikov, E. Budevski, D. Carnal, U.
Muller, H. Siegenthaler, and E. Schmidt, J. Electrochem. Soc., 140, 692 (1993).
18. D. Carnal, P. I. Oden, U. Muller, E. Schmidt, and H. Siegenthaler, Electochim.
Acta., 40, 1223 (1995).
19. O. R. Melroy, M. F. Toney, G. L. Borges, M. G. Samant, J. B. Kortright, P. N.
Ross, and L. Blum, J. Electroanal. Chem., 258, 403 (1989)
20. H. Siegenthaler and K. Juttner, Electrochim. Acta, 24, 109 (1979).
21. E. Schmidt and H. Siegenthaler, J. Electroanal. Chem., 150, 59 (1983).
22. T. Vitanov, A. Popov, G. Staikov, E. Budevski, W. J. Lorenz, and E. Schmidt,
Electrochim. Acta, 31, 981 (1986).
23. A. Popov, N. Dimitrov, O. Velev, T. Vitanov, E. Budevski, E. Schmidt, and
H. Siegenthaler, Electrochim. Acta, 34, 265 (1989).
24. N. Dimitrov, A. Popov, T. Vitanov, and E. Budevski, Electrochim. Acta, 36, 2077
(1991).
25. N. Dimitrov, A. Popov, D. Kashchiev, and T. Vitanov, Electrochim. Acta, 39, 957
(1994).
26. M. Toney, J. G. Gordon, G. L. Borges, O. R. Melroy, D. Yee, and L. B. Sorensen,
Phys. Rev. B, 49, 7793 (1994)
27. See, for example, D. M. Kolb, R. Ullmann, and T. Will, Science, 275, 1097
(1997); R. J. Nichols, D. M. Kolb, and R. J. Behm, J. Electroanal. Chem., 313,
109 (1991).