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SCANNING TUNNELING MICROSCOPY STUDIES OF ELECTROCHEMICAL DEPOSITION

Michael P. Green and Karrie J. Hanson AT&T Bell Laboratories

600 Mountain Ave, Murray Hill NJ 07974

In this paper we describe the technique of in situ scanning tunneling microscopy (STM) and its application to electrochemical systems. The STM can provide real-space, time-resolved, atomic- scale images of topographic changes occurring a t an electrode during chemical reactions. We illustrate the technique with experimental examples of the deposition and stripping of Pb and Cu on Au (111) surfaces.

Inimductios

atomic structure of electrode surfaces, the fundamental steps in electrochemical reactions are receiving new attention. These techniques include traditional surface science probes such as x-ray scattering (1) and absorption (2) as well as the newly developed scanning probe microscopies, scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Among these only STM and AFM give direct (real-space) evidence of the surface structure.

sample surface at a distance close enough to allow quantum tunneling of electrons across the gap (3). A three-diqensional piezo-electric actuator maintains this distance, typically 10-20A, and also provides the scanning motion. The tunneling current, which is exponentially dependent on the gap spacing, is generally held constant by adjusting the perpendicular displacement of the tip in response to changes in surface topography or electronic structure. This displacement, in conjunction with the lateral position information, is used to create a three-dimensional map of the surface. This is usually displayed as a gray-scale image with the shade keyed to the vertical tip position. What this map shows is really the density of surface electronic states near the Fermi level meas1-d ~ l t the center nf CC-VE~C~P of the p-&p tip (4); in other wnrds, t,he eXt,ent. to which electron wave functions extend from the surface. For most surfaces, especially when atomic resolution is not sought, this accurately reflects the surface topography. In electrochemical systems, however, the interpretation of

With the development of several in situ techniques capable of probing the

The STM operates by rastering a sharpened conducting probe across a

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The Proceedings of the 79th AESF Annual Technical Conference SUWFM'N~

The American Electroplaters and Surface Finishers Society, Inc. (AESF) is an international, individual- membership, professional, technical and educational society for the advancement of electroplating and surface finishing. AESF fosters this advancement through a broad research program and comprehensive educational programs, which benefit its members and all persons involved in this widely diversified industry, as well as govemment agencies and the general public. AESF dissemi- nates technical and practical information through its monthly joumal, Plating and Surface Finishing, and through reports and other publications, meetings, symposia and conferences. Membership in AESF is open to all surface finishing professionals as well as to those who provide services, supplies, equipment, and support to the industry.

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Published by the American Electropiaters and Surface Finishers Society, Inc. 12644 Research Parkway Orlando, FL 32826-3298 Telephone: 4071281 -&I41 Fax: 407/281-6446

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the data must be considered carefully; the presence of adsorbates, the electrochemical double-layer, and the changing electrode potential can all affect the tunneling current. However, in situ electrochemical STM has an advantage over vacuum-based STM in the ability to control the voltage of the sample and tip independently. This allows chemical reactions to occur selectively, thus allowing imaging of the nucleation and growth of adsorbates in real-time on an intimately close substrate without having the tip structure affected.

The modifications to the STM necessary for its application to in situ electrochemistry are relatively straight-forward. Since electrochemistry involves charge transport via the motion of ions, the passage of electrons from the tunneling tip to the sample is still a tunneling process, even when both are immersed in a conducting solution. The main consideration is how to incorporate the STM tip into a standard three-electrode electrochemical cell. We configure the cell so that the tip is maintained a t a constant potential with respect to the reference electrode. This allows the tip to be held a t a voltage where no electrochemical activity takes place. In practice, this is usually set at the open circuit, or “rest” potential of the tip with respect to the reference electrode. While this creates the least interference to the tunneling signal from background electrochemical current, the varying electric field between the tip and sample could influence reactions being studied, especially if the tip penetrates the “double layer” region, the thickness of which is the effective screening length in the electrolyte. For 0.1 molar solution, the double layer has a thickness very close to the tunneling distance (5). We have looked for effects caused by the changing tunneling voltage and do not find a measurable effect. An additional precaution to minimize electrochemical current noise on the tunneling current signal is to coat all but the very point of the tip with a non-conductive coating. Noise levels can generally reduced to < O . l n A by these techniques.

demonstrated by considering the deposition of monolayer metal films, especially since there is little known about the microscopic details of the layer formation. We will discuss our recent investigation of the underpotential deposition (UPD) of Pb and Cu on Au(ll1) (6) to illustrate the technique.

Lead

the first and subsequent voltammograms in the UPD current peaks. It has been speculated that this change is due to some modification of the electrode surface (7), but nothing more was known. Fig. l a shows an evaporated Au(ll1) film immediately after immersion in the electrolyte (0.05M HClO4 + 5mM PbO), with

monoatomic steps (2.35A) repeat the morphology imaged in air. Fig. l b shows the result of the deposition of a Pb monolayer. Although the structure looks similar, terraces in Fig. l b are a monolayer higher than those of Fig. la ; the Pb

The utility of using the STM to study electrochemical systems can be

For the Pb/Au(lll) system, we noted that there was a variation between

the potentia! held at its epen-circuit va!ue. AtoE!!ca!!y n2t terraces separated by

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has deposited in an almost conformal layer. From a previous study of the deposition dynamics (S), we found that the Pb deposits directly into its final close- packed structure via two-dimensional nucleation and growth, and does not pass through a less dense superlattice as some have suggested (9). It was also determined that the growth of the Pb layer progressed outward from atomic steps at terrace edges and around small Pb islands which nucleated on the terrace planes. Removal of the Pb adlayer does not return the smooth Au surface. Fig. IC shows the roughened electrode surface which results from the deposition and stripping of the PbJayer. It is composed of 20-150A diameter monolayer-high islands, and -25A diameter monolayer-deep pits. Similar topographic changes were found to occur for sweep rates from 0.5 to 1000 mV/s.

In order to see if the roughening was a local effect of Pb deposition, we halted the first potential sweep at submonolayer Pb coverage (0=0.25ML), and then stripped the Pb to look for royghening in the regions of Pb coverage. Lead deposition was restricted to a -20A wide strip at terrace edges and a few islands on the terrace planes; this can be seen in Figs. 2a and b. During the potential hold, the close-packed structurp remained stable, but a less dense phase with an apparent height of only 1.3A slowly covered the Au surface (Fig. 2c). We speculate that this is a (./3x./3)R3Oo Pb superlattice, which is believed to form on Au( 111). Upon reversal of the potential sweep, the superlattice phase desorbed immediately; however, few of the deposited islands were removed, nor did the terrace edges retreat to their original positions (Figs. 2d and e). Instead, we observed the development of numerous, small, monolayer-deep pits clustered in a border near the terrace edges and around (and even within) the remaining islands. The pits can be seen most easily in Fig. 2f, after the surface has had a chance to anneal slightly and the very small vacancies have combined into larger, more easily imaged pits. The images of Figs. 2g-i show the results of the second partial (8=0.33ML) deposition and stripping sequence. The earliest topographic change we observed was ,Pb deposition into the pits. The terrace edges again extended by about a -20A strip of Pb, and more islands nucleated on the terraces. No islands nucleated over the border region which had contained the pits. Removal of this Pb layer again left the Au pitted (Fig. Zi), but as a result of the higher Pb coverage this effect was more pronounced.

We have suggested that these observations can be explained by a model in which the Pb deposition is followed by a surface alloy formation. The diagram in Fig. 3 illustrates how these effects occur. Through a series of Pb-Au place exchanges, both vertical and horizontal, a surface alloy forms in the vicinity of Pb deposits (Fig. 3a-d). Atomic vacancies which remain in the surface after Pb Stripping will quickly coalesce into small pits (Fig. 3e), and it is likely that these would continue t o grow as the surface annealed. Upon redeposition into these pits, the Pb atoms would be already partially dispersed in the plane, and therefore the alloy forms more rapidly (Fig. 3f). Since the overpotential for nucleation on the alloy is almost certainly greater than on bare Au, no new is lads would form over the alloy and a border, protected from island deposition results (Fig. 3g).

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In surnmary, we find that a Au(ll1) electrode is substantially roughened after a single monolayer of Pb is electroplated and stripped from the surface in a linegr sweep experiment. The roughened electrode surface rearranges into 20 to 150A diameter monolayer islands and smaller monolayer pits. This process is observed on all samples, for sweep rates ranging from 0.5 to lOOOmV/s. If a partial layer of Pb is plated and then stripped, pitting and an increased overpotential for Pb deposition are observed only in the areas adjacent to the plated sites. We suggest that these observations are consistent with the formation of a Pb-Au alloy in the surface layer.

Our in situ STM studies of Cu on Au (111) surfaces in sulfate electrolytes showed strikingly different behavior from the Pb system. No evidence of alloy formation was observed and the Cu adlayer was formed by a complicated sequence of intermediate superlattice arrangements. Recent in situ scanning tunneling microscopy (STM) (lo), atomic force microscopy (AFM) (111, and x-ray absorption spectroscopy (XAS) (12) studies of this system have emphasized the atomic arrangement of the Cu adatoms at various stages of the monolayer formation. Instead of this, we concentrated on larger substrate areas, and imaged continuously during the layer formation stages. This approach allows consideration of the film growth dynamics. At this scale we cannot determine an atomic lattice spacing for the phases we observe, but rely on comparison of our results with those of the previous in situ STM, AFM, and XAS studies of this system to assign a structure to the observed phases.

We observe three distinct Cu phases in our STM images during deposition and stripping. They are distinguished as regions of similar patterns of growth of a certain average height above the bare Au plane. The three phases are referred to as phase I (the lowest), phase I1 (approximately half of a monolayer hi h),

(5 x 51, and the (1 x 1) Cu overlayer structures, respectively, based on comparison with previous in situ results (10-12) and on calculations using a hard sphere model for the structures (13). Fig. 4 shows a deposition sequence consisting of a potential sweep from 500 to 30 mV (BmVls), which was interrupted and held at 75, 65, and’50mV. All images in this section were taken with an acquisition time of 51s, scanning from bottom to top. Three single atomic steps can be seen on the 750x750A2 region of the Au surface (Fig. 4a), shown during the initial part of the deposition wave. About halfway up the image, thc terrace surface begin t o show an additional layer having a height of 0.46 f 0.10A which we have termed phase I. This new phase can be seen more clearly in Fig, 4b, which is a closer view of the boxed-in in region of Fig 4a. Phase I deposits initially as patches (arrow A), in the center of the plane, and then grows rapidly over most regions of the Au surface. During the ramp from 200 to 75mV, addition$ Cu is deposited into two diffeTent phases having average heights of 1.23 +0.23A (phase I!) and 2.38 f 0.28A (phase 111). Phase I11 appears as small islands (10 to 25A in diameter) which nucleate in filaments across the surface of the terraces with no apparent

and phase I11 (one monolayer high). We assign these phases to the (43 x 3 31, the

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orientation. Phase 11, with an intermediate height between phases I and 111, is visible mostly between and around the phase I11 islands, giving a web-like appearance. Other domains of pha$e I1 appear, concentrated a t the top edge of the steps. This growth pattern gives the appearance of “ribbons,” which continue to widen and become more distinct as the phase I1 domains grow slowly outwards to replace phase I, finally merging until phase I disappears entirely. Thus, during the deposition sequence, we find that all three Cu phases exist simultaneously.

overlayer. Images of this process are presented in Fig. 5, with the current voltage data for Fig 5c included as Fig. 5d. Fig. 5a begins a t a potential of 175mV, mid-way through the adlayer removal. Between 50 and 175mV (not shown), phase I1 was stripped and many of the phase I11 islands disappeared. Phase I is removed with the main anodic current peak at 243mV. A zoomed view of a portion of Fig. 5a is presented in Fig. 5c with enhanced contrast, showing the removal of phase I. From the corresponding electrochemical data (Fig. 5d) it can be seen that phase I lifts in the potential window 230-243mV, and is completely dissolved by the time the current begins to drop. During the remainder of the potential sweep phase I11 is slowly removed by a process of island dissolution and coalescence. Fig. 5b shows the Au surface and the few remaining phase I11 islands at the end of the scan, with the potential held a t 500mV. We find that these islands are relatively stable at this potential, but that they can be removed if the electrode is cycled through a Au oxidation potential scan.

Reversing the direction of the potential sweep ( 75 to 500mV) strips the Cu

Numerous avenues exist in electrochemistry for the application of STM’s proven capabilities. Investigations of nucleation and growth structures can be applied to metal deposition on semiconductors and can be expanded t o follow the development of bulk plating on both semiconductor and metal substrates. In this regard, the role of additives - surfactants, leveling agents, inhibitors - can be assessed. A study of corrosion mechanisms at the atomic level could greatly increase our understanding of this process. Increased attention will certainly be given to the possibilities of using the tip to perform localized electrochemistry; lithography (by both deposition and etching) and catalysis are likely targets.

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1 M.G. Samant, M.F. Toney, G.L. Borges, L. Blum, and 0.R. Melroy, J. P b . c%-em. 92,220 (1988).

5

2 A. Tadjeddine, D. Guay, M. Ladouceur, and G. Tourillon, Phys. Rev. Lett. 66, 2235 (1991).

3 G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys. Rev. Lett. 49, 57 (1982).

4 J. Tersoff and D.R. Hamann, Phys. Rev. B 31, 2 (1985).

5 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications (Wiley, New York, 1980) p. 506.

6 M.P. Green and K.J. Hanson, Surf. Sci. Lett. 259, L743 (1991).

7 K. Engelsmann, W.J. Lorenz, and E. Schmidt, J. Electroanal. Chem. 114, 1

8. M.P. Green, K.J. Hanson, R. Carr, and I. Lindau, J . Electrochem. SOC.

(1980).

137(11), 3493 (1990).

9 K. Juttner and W.J. Lorenz, 2. Physik. Chem. NF 122, 163 (1980).

10 O.M. Magnussen, J. Hotlos, R.J. Nochols, D.M. Kolb, and R.J. Behm, Phys. Rev. Lett. 64, 2929 (1990); O.M. Magnussen, J . Hotlos, G. Beitel, D.M. Kolb, and R.J. Behm, J. Vac. Sci. Technol. B 9(2), 969 (1991); O.M. Magnussen, J . Hotlos, G. Beitel, D.M. Kolb, and R.J. Behm, unpublished.

11 S. Manne, P.K. Hansma, J. Massie, V.B. Elings, A.A. Gewirth, Science 251,

12 A. Tadjeddine, D. Guay, M. Ladouceur, and G. Tourillon, Phys. Rev. Lett. 66,

13 M.P. Green and K.J. Hanson, accepted for publication in J. Vac. Sci.

183 (1991).

2235 (1991).

Technol..

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CaDtions

Figure 1. In situ images of the deposition and stripping of Pb on pristine Au. Shading is keyed to height and five atomic levels are shown. Image a: Bare Au. Image b: After deposition of one monolayer of Pb. Image c: After Pb monolayer is stripped, first cycle. Image d Second deposition of Pb layer. Image e: Same region, as Pb is being stripped. (the direction of the STM scan is bottom-to-top). Image f: After Pb layer is stripped, second cycle.

Figure 2. Partial Pb coverages. Sequence of images from two sweep-and-hold, plate-strip cycles. Potential sweep is shown a t top of figure. Image a; Bare Au. Image b: Beginning of hold period. Terrace edges extend by 15 - 20 A and several islands appear. Image c: A new phase (arrow) is seen by the end of hold period. Image d: At the end of stripping, pits can be seen near terrace edges. Image e: A superposition of the original step edge positions (from Image a) over a portion of Image d. Image f: Pits are fewer and larger. Image g: Pits fill and new islands nucleate. Margins near the top edge of the steps are protected from nucleation at this potential (240 mV). Image h Step positions have rearranged slightly after hold. Image i: After second stripping sweep.

Figure 3. Schematic illustration of how a series of place-exchanges between Pb and Au could account for the observations shown in Figure 2.

Figure 4. Cu deposition sequence. Images scanned upwards. Image a: Bare Au surface and the formation of phase I adlayer; potential 270-188mV. Image b: Zoomed section of image a highlights phase I formation beginning at arrow; potential 230-188mV. Image c: Initial growth of phase I1 on top of step edges and around phase I11 islands; potential 75mV. Image d: Expansion of phase I1 in “ribbon” domains; potential 65mV. Image e: Continued filling of phase I1 domains leaving patches of phase I, potential 50mV.

Figure 5. Removal of first partial Cu adlayer. Images scanned upwards. Image a: Before this image phase I1 is removed, and here a lower number of phase I11 islands remain. Phase I is removed at arrow, box indicates area reproduced in image c; potential 200-275mV. Image b: Au substrate with several phase I11 islands remaining; potential 500mV. Image c: Zoomed view with enhanced contrast of boxed portion of image a showing phase I removal at arrow; potential 210-26OmV. Image d: Electrochemical current and potential for image c.

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-. .. July 25, 1992

Mr. 5. Howard Schumaker, Jr. Executive Director American Electroplaters and Surface Finishers Society 12644 Research Parkway Orlando, FL 32826-3298

Mr. J. Howard Schumaker Jr.

As a representative for the Waste Reduction Resource Center and an AESF member, I attended the SURF/FIN ' 92 technical conference in Atlanta on June 22-25. The conference was excellent and the proceedings are a major source of up-to-date information.

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L I

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