A2

Vnltammograin after 1000 s polarization

Transformed coverage transformation rates that were independent of the step densities, whereas a strong dependence of the rates of the step densities was found at high coverages, corresponding to the conditions of the STM studies described above. The measurements were related to kinetic site exchange models including surface inhomogeneities at low adsorbate coverages, and choosing a one-dimensional diffusion model without consideration of surface inhomogeneities for high coverages. However, there remain uncertainties about the dependencies of the transformation rates on the surface inhomogeneities that require further elucidation [7].

3 Conclusions and Outlook

The presented results demonstrate the relevance of the nanometer-scale morphology (stepped terrace domains, monoatomic islands and monoatomic pits) for the local progress of adsorbate formation and adsorbate stability. The stepwise formation of the Pb and T1 adsorbate coverages, combined with the slow formation of a surface alloy coverage, illustrates experimentally thermodynamic and kinetic aspects of various growth modes of metal deposits discussed recently [7, 20], In the two systems presented, the complete hep monolayer coverages formed during fast adsorption of Pb and T1 represent obviously metastable systems, whereas the surface alloy coverage formed during extended polari-zation of incomplete adsorbate layers is considered to be the thermodynamically stable coverage. The experiments described indicate that in-situ STM is specially suitable for local measurements. Further insight into the role of atomic-scale inhomogeneities in the local progress of electrochemical processes can be expected, e.g., from the use of nanostructured model electrodes.

Acknowledgements. The authors acknowledge gratefully the financial support by the Schweiz. Nationalfonds, and they thank F. Niederhauser for technical support.

4 References

[1] A.A. Gewirth, H. Siegenthaler (Eds.), Nanoscale Probes of the Solid/Liquid Interface, NATO Series E, Applied Sciences, Vol. 288, Kluwer Academic Publishers, Dordrecht, 1995.

[2] Scanning Tunneling Microscopy n, R. Wiesendanger, H.-J. Guntherodt (Eds.), Springer Series on Surface Sciences, Vol. 28, Springer-Verlag, Berlin, 1995.

[3] D. Carnal, P.I. Oden, U. Müller, E. Schmidt, H. Siegenthaler, Electrochim. Acta 40, 1223 (1995).

[4] E. Ammann, Diploma Thesis, University of Bern, 1995; E. Ammann H. Siegenthaler, submitted to J. Electrochem. Soc.

[5] U. Schmidt, S. Vinzelberg, G. Staikov, Surf. Sei. 348,261 (1996).

[6] J.X. Wang, R.R. Adzic, O.M. Magnussen, B.M. Ocko, Surf. Sei. 344, 11 (1995).

[7] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical Phase Formation and Growth -An Introduction to the Initial Stages of Metal Deposition. VCH, Weinheim, 1996.

[8] Y.T. Kim, H. Yang, A.J. Bard, J. Electrochem. Soc. 138, L71 (1991).

[9] R. Nyffenegger, C. Gerber, H. Siegenthaler, Synth. Metals 55-57,402 (1993).

[10] H.Yang, F.-RFan, Sh.-L.Yau, AJ.Bard, J. Electrochem. Soc. 139, 2182 (1992).

[11] R. Nyffenegger, E. Ammann, H. Siegenthaler, R. Kötz, O. Haas, Electrochim. Acta 40, 1411 (1995).

[12] H. Siegenthaler, K. Jüttner, Electrochim. Acta 24,109 (1979).

[13] W. Obretenow, M. Höpfiier, W.J. Lorenz, E. Budevski, G. Staikov, H. Siegenthaler, Surf. Sei. 271,191 (1992).

[14] U. Müller, D. Carnal, H. Siegenthaler, E. Schmidt, W.J. Lorenz, W. Obretenow, U. U. Schmidt, G. Staikov, E.Budevski, Phys. Rev. B 46,12899 (1992).

[15] U. Müller, D. Carnal, H. Siegenthaler, E. Schmidt, W.J. Lorenz, W. Obretenow, U. Schmidt, G. Staikov, E. Budevski, Phys. Rev. B 49,7795 (1994).

[16] M.F. Toney, J.G. Gordon, G.L. Borges, O.W. Melroy, D. Yee, L.B. Sorensen, Phys. Rev.B 45, 9362 (1992).

[17] H. Siegenthaler, K.. Jüttner, E. Schmidt, W.J. Lorenz, Electrochim. Acta 23, 1009 (1978).

[18] P.I. Oden, unpublished results.

[19] A. Popov, N. Dimitrov, D. Kashchiev, T. Vitanov, E. Budevski, Electrochim. Acta 38, 2173 (1992), and references by the same authors cited therein.

Electrochemical Nanotechnology

In-situ Local Probe Techniques at Electrochemical Interfaces Edited by W. J. Lorenz and W. Plieth © WILEY-VCH Verlag GmbH, 1998

Electrochemistry and Nanotechnology

Contents

1 Introduction 13

2 Analytical Nanoelectrochemistry 15

2.1 Substrate Surfaces 15

2.2 Growth Modes of 2D and 3D Metal-Phase Formation Processes 17

3 Preparative Nanotechnology 22

4 Conclusions 24

5 References 24

Summary. Electrochemical nanotechnology and its analytical and preparative aspects using local probe techniques such as STM and AFM are described. Typical examples for in-situ application of local probe methods in different electrochemical systems are discussed: UPD and OPD of metals and nanostructuring of metal, semiconductor, and superconductor surfaces.

1 Introduction

Future aspects of science and technology in many fields such as physics, chemistry, materials science, electronics, sensor technology, biology, medicine, etc., are characterized by miniaturization down to an atomic level. "Nanotechnology" dealing with single atoms, molecules or small clusters will take the place of the "micrometer technology" predominating during the last 150 years. In surface nanotechnology, the surfaces of solid-state materials such as metals, semiconductors, superconductors, and insulators have to be analyzed, structured, and modified in the nanometer range. This is only possible using local probe techniques such as STM, AFM and related methods which were developed during the last decade and are generally denoted as scanning probe microscopy (SPM) [1-9].

Analytical and preparative aspects of modern nanotechnology can be distinguished. Local probe investigations of surface thermodynamics, structure, dynamics, and reactions belong to the analytical aspect. On the other hand, surface nanostructuring or surface modification and the preparation of defined "nanoobjects" by local probe techniques represent the preparative aspect.

Local probe techniques are carried out "ex-situ", "non-situ" or "in-situ" with respect to applied environmental conditions. Ex-situ local probe investigations are performed under UHV conditions on well-defined substrates, e.g., single-crystal surfaces. Such ex-situ measurements are often made in far from real conditions, which are characterized by adsorption and film formation. Therefore, ex-situ UHV techniques are usually combined with appropriate transfer devices to switch substrates from the real environment to UHV and vice versa. Non-situ local probe measurements are also started under UHV conditions to characterize the bare substrate surface, but they are continued under a finite vapor pressure in order to form adsorbates or mono- or multi-atomic (-molecular) films modeling real environmental conditions. In-situ local probe measurements are carried out at solid/liquid or solid/gas interfaces under defined real conditions involving adsorption and film formation.

In-situ local probe investigations at solid/liquid interfaces can be performed by electro-chemical means if both phases are electronically and ionically conducting. In this case, electrochemistry offers a great advantage since the Fermi levels [10], Ef, of both substrate and tip (or metallized cantilever) can be adjusted precisely and independently of each other using bipotentiostatic control in a four-probe technique (substrate as working electrode; tip or conducting cantilever as local probe „sonde"; reference and counter electrodes) [8]. In STM studies, this Fermi level control leads to defined surface properties at tip and substrate and, therefore, to defined tunneling conditions for distance tunneling spectroscopy (DTS) and voltage tunneling spectroscopy (VTS). Without bipotentiostatic conditions, only the potential difference between tip and substrate, i.e., the tunneling voltage Et = Et¡p - E, can be held constant without control of the surface properties and, therefore, of the tunneling conditions.

A further advantage of electrochemical in-situ SPM studies of two- and three-dimensional phase formation processes is the possibility of controlling accurately the supersaturation or undersaturation, Afi, which can be correlated, in the absence of other kinetic hindrances with overpotential and underpotential, respectively [11]:

Am = ^,ads(£) - ^,ads(£Me/Me,+ ) = (E - EUe/UeZ+)

where n., ads(£) and ^,,ads(^Me/MeZ+) denote the chemical potentials of the adsorbed electroactive species i at the actual electrode potential E and at the Nernst equilibrium potential Erev, respectively. The potential difference E - Eudu<i + is defined as:

Consequently, under- or supersaturation can be adjusted precisely and varied rapidly under electrochemical conditions, in contrast to gas-phase investigations.

The combination of in-situ local probe techniques and classical steady-state and non-steady-state electrochemical measurements gives new information on the local and global behavior of electrified solid/liquid interfaces with respect to analytical and preparative nanotechnological aspects.

The electrochemistry group at the University of Karlsruhe, Germany, introduced in-situ local probe techniques in order to get more information about substrate surfaces and Faradaic reactions occurring at substrate/liquid electrolyte interfaces under different conditions. Single-crystal faces of metals and semiconductors as well as eptiaxially grown thin films of superconductors are used as substrates. Underpotential deposition (UPD) and overpotential deposition (OPD) of metals were used as Faradaic model reactions for 2D and 3D phase formation processes, respectively. First attempts are being made to use these processes and application of in-situ STM and AFM for a local structuring of solid-state surfaces. Current results will be briefly summarized in terms of analytical and preparative aspects.

2 Analytical Nanoelectrochemistry 2.1 Substrate Surfaces

The surface structure of single-crystal faces of noble metals such as Ag(hkl) and Au(hkl) were found to be unreconstructed under defined electrochemical conditions. For example, a unreconstructed Ag(lll) surface domain on a terrace in contact with perchloric acid solution is shown in Fig. 1. Surface reconstruction changing the interatomic distance and symmetry of surface atoms can be induced thermally or by potential, as observed by other authors [12]. This phenomenon can be lifted by potential or by adsorption processes. Surface defects such as monatomic steps and def

screw dislocations can be directly observed by in-situ SPM, as demonstrated in Fig. 2 [13].

Fig. 1. In-situ STM image of an Ag(lll) surface under electrochemical conditions. System: Ag( 111)/10"2 M HCIO4 at £h/h+ = 350 mV and T= 298 K with 7, = 10 nA and Pt-Ir tip.

Fig. 2. In-situ STM image of an Au(lll) surface under electrochemical conditions. System: Au( 111)/5 x 10"3 M AgC104 + 5 x 10"1 M HC104 at AE = 200 mV and T = 298 K. 7, = 10 nA, Pt-Ir tip.

2.2 Growth Modes of 2D and 3D Metal Phase Formation Processes

Three different growth modes (Volmer-Weber, Frank-van der Merwe, and Stranski-Krastanov) can be distinguished, depending on the vertical binding energy between a metal adatom, Meads, on a foreign substrate, S, and on the crystallographic Me-S misfit, as schematically illustrated in Fig. 3.

In the case of weak vertical Me-S interaction, only 3D Me cluster formation takes place in the OPD range according to the Volmer-Weber growth mode (Fig. 3(a)), as found experimentally in the system highly oriented pyrolytic graphite HOPG(OQO 1 )/Ag+) [14].

Fig. 3. Schematic representation of different growth modes in metal (Me) deposition on foreign substrate (S) depending on the binding energy (o-) of Meajs on S, ^Me^ -S > compared with that that of Meads on native substrate Me, g^ _ m e > and on the crystallographic misfit characterized by the interatomic distances i/o,Me and i/0,S of 3D Me and S bulk phases, respectively, (a) "Volmer-Weber" growth mode (3D Me island formation) for eads - S « e^ -M e independent of the ratio (rf0,Me - ¿o,S) (b) "Franck-van der Merwe" growth mode (Me layer-by-layer formation) for ^Me^-S^ ^Me^-Me and ratio (<i0,Me - c?o,S) I do,S ~ 0. (c) "Stranski-Krastanov" growth mode (3D Me island formation on top of predeposited 2D Meads overlayers on S for S^Me^-S^ ^Me^-Me and (i/0,Me - ¿o,S) /¿o,S > 0 (positive misfit) or (rf0,Me - ¿o,S) /¿o,S < 0 (negative misfit).

Fig. 3. Schematic representation of different growth modes in metal (Me) deposition on foreign substrate (S) depending on the binding energy (o-) of Meajs on S, ^Me^ -S > compared with that that of Meads on native substrate Me, g^ _ m e > and on the crystallographic misfit characterized by the interatomic distances i/o,Me and i/0,S of 3D Me and S bulk phases, respectively, (a) "Volmer-Weber" growth mode (3D Me island formation) for eads - S « e^ -M e independent of the ratio (rf0,Me - ¿o,S) (b) "Franck-van der Merwe" growth mode (Me layer-by-layer formation) for ^Me^-S^ ^Me^-Me and ratio (<i0,Me - c?o,S) I do,S ~ 0. (c) "Stranski-Krastanov" growth mode (3D Me island formation on top of predeposited 2D Meads overlayers on S for S^Me^-S^ ^Me^-Me and (i/0,Me - ¿o,S) /¿o,S > 0 (positive misfit) or (rf0,Me - ¿o,S) /¿o,S < 0 (negative misfit).

Strong vertical Me-S interaction (Figs. 3(b) and 3(c)) leads to the formation of two-dimensional Me phases in the UPD range prior to the formation of 3D Me phase in the OPD range. The systems Au(M/)/Ag+ [11,13,15,16], Au(M/)/Pb2+ [11,16,17], and Ag(hkI)fPb2+ [11,17,18] are typical examples of strong Me-S interaction. No Me-S misfit exists in the first system, whereas the second and third systems are characterized by a significant positive Me-S misfit.

2D Meads UPD overlayers were found to be formed stepwise [11]. At high AE, decoration of monatomic steps takes place. Expanded Meads overlayers with commensurate structures coexisting with bare substrate domains are observed at high and medium AE, as shown in Fig. 4 [11,17], At relatively low AE, 2D phase transitions take place and expanded Meads overlayers are found to be transformed into condensed 2D Me overlayers, which are higher-order commensurate or incommensurate depending on the crystallographic Me-S misfit. Figure 5 shows a compressed and internally strained incommensurate hep 2D Pb overlayer structure on Au(lll) giving rise to a moiré pattern of the surface structure [11, 16]. A 2D phase transition process at relatively low AE in the system Au(100)/Pb2+ is illustrated in Fig. 6 [11, 17],

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