Oxidative Chloride Adsorption and Lead Upd on Cu(100

J. Phys. Chem. B , 1998, 102 (49), pp 10020–10026. DOI: 10.1021/jp9828888 ... A c(2 × 2) Cl adlayer is formed upon immersion of a Cu(100) electrode...
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J. Phys. Chem. B 1998, 102, 10020-10026

Oxidative Chloride Adsorption and Lead Upd on Cu(100): Investigations into Surfactant-Assisted Epitaxial Growth T. P. Moffat Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: July 7, 1998; In Final Form: September 28, 1998

The influence of chloride adsorption and lead upd on the step dynamics of Cu(100) has been examined in acid perchlorate solution. A c(2 × 2) Cl adlayer is formed upon immersion of a Cu(100) electrode leading to step faceting in the 〈100〉 direction. At more negative potentials an order-disorder transformation occurs leading to significant rearrangement of the steps. Alternatively, in an electrolyte containing Pb2+ the halide adlayer may be completely displaced by lead upd. Images of orthogonal surface steps in combination with an assessment of the coulometry suggest that the lead adlayer forms either a highly defective c(2 × 2), c(5x2 × x2)R45° or a disordered structure corresponding to a coverage ranging from 0.5 to 0.6. The transformation from the halide to the lead adlayer results in the formation of vacancies and adatoms which condense to form holes and islands, respectively. These features may be rationalized by the formation of an alloy phase at low coverage, which subsequently dealloys as the coverage approaches 0.5. The extent of the morphological changes associated with the alloying/dealloying processes is strongly path dependent. Voltammetry reveals that the stripping of the lead upd layer is associated with two oxidation waves. As the potential is increased beyond the peak of the first wave islands disappear which may be due to alloy formation occurring coincident with partial desorption of the lead. The second wave is associated with the nucleation 〈100〉-oriented rows of the c(2 × 2) Cl adlayer which propagate across the terraces displacing the lead phase. The use of metal upd and anions as surfactants in the electrochemical deposition of copper is likely to prove even more interesting than in vacuum deposition since the surfactant coverage and its effect on mesoscopic structure can be continuously manipulated by potential control.

1. Introduction In the past decade, vacuum surface science studies of metal on metal homoepitaxial and heteroepitaxial growth have begun examining the influence of “surfactants” on the mode of film growth.1 The “surfactants” of interest are substances that float on the surface during deposition and affect the relative rate of the individual surface processes, i.e., step edge diffusion and terrace diffusion, and possibly alter the overall mechanism of epitaxial growth. These investigations have also been extended to electrochemical deposition. For example, chloride ion has been shown to exhibit a strong effect on the step dynamics of immersed Cu(100) and Cu(111) surfaces.2,3 Recently, vacuum studies have demonstrated favorable effects of Pb on the growth of copper as well as copper/cobalt multilayers and/or spin valves all of which are materials of significant technical interest.4 Surface segregation of Pb during growth is supported by the low solubility of lead in bulk Cu and Co in combination with its lower surface energy. The ability of a monolayer of Pb to induce layer-by-layer growth of copper on Cu(111) has been demonstrated using atom scattering.5 The surfactant fundamentally changed the mechanism of surface transport by reducing in-plane diffusion via hopping while facilitating atomic exchange. The cooperative nature of the exchange process is thought to favor interlayer diffusion and thereby layer-by-layer growth.5 A preliminary investigation on the use of underpotential deposited (upd) lead as a surfactant to influence the step dynamics on immersed Cu(111) has also been reported.3 A

more detailed overview of lead upd on Cu(111)6 is available along with a recent X-ray scattering study7 of the adlayer structure. Interestingly, other vacuum studies of submonolayer deposition of Pb on Cu(100) and Cu(111) have revealed clear evidence of two-dimensional alloy formation in which the deposited metal intermixes with the topmost copper layer rather than forming a simple overlayer structure.8-12 Similar alloying effects have been reported for Pb and Tl upd on Au and Ag;13-15 however, these systems are distinct from Cu/Pb in the sense that the former elements are known to be somewhat miscible or form intermetallic compounds in bulk phase materials. Nevertheless, in vacuum studies the observation of two-dimensional or surface confined mixing has been found to be quite general even for immiscible system, particularly those which are dominated by atomic size mismatch such as Cu/Pb. The free surface allows relaxation of some of the strain energy associated with the mismatch thereby permitting limited alloy formation.16 In this paper Pb upd on Cu(100) in an acid chloride solution will be examined as a first step toward using lead as a surfactant to influence the electrodeposition of copper and Cu/Co multilayers on the 4-fold symmetric surface. A recent LEED, auger electron spectroscopy (AES), and electrochemical study of Pb upd on Cu(100) in the Cl/ClO4- system provides a useful overview of the system.17 At positive potentials, prior to Pb upd, the surface is covered with a c(2 × 2) Cl adlayer. Upon stepping the potential in the negative direction, upd lead displaces at least 90% of the Cl adlayer as determined by AES.

10.1021/jp9828888 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/12/1998

Surfactant-Assisted Epitaxial Growth LEED analysis of the lead adlayer indicated the absence of order in the as-deposited state although subsequent annealing at 423 K (0.71Tmelting) generated a c(2 × 2) Pb structure. Rotating ring-disk experiments were used to determine an electrosorption valency of 2 for a nearly complete lead upd layer while a value of ∼ 0.5 was derived for the c(2 × 2) halide layer. The latter value being derived from the total charge used to displace the Pb adlayer. The c(2 × 2) Cl adlayer on Cu(100) has been well studied in both electrochemical and vacuum environments by LEED, AES, and STM.2,17,18 Likewise, several vacuum studies of Pb adlayers on Cu(100) have been reported.9-11,19 This system has been studied for more than 25 years although the application of recently developed surface science tools has revealed unanticipated complexity such as alloying-dealloying phenomena and coverage-dependent mobility of surface species.9-11 From room temperature up to the melting point, at least three stable superstructures are known for submonolayer deposition of Pb on Cu(100): c(4 × 4), c(2 × 2), and c(5x2 × x2)R45°. The c(4 × 4) phase is formed at a coverage (θ) of 0.375 and corresponds to an ordered Pb3Cu4 alloy. Increasing the coverage to 0.5 yields a c(2 × 2) structure while further deposition up to 0.6 results in a c(5x2 × x2)R45° superstructure which may be derived by insertion of antiphase domain boundaries into the c(2 × 2) structure. An important consequence of Pb3Cu4 alloy formation is the generation of large number of Cu atoms. Indeed, large changes in morphology have been observed as the coverage is increased above 0.375 including the formation of islands and vacancy clusters.9-11 In addition to the three stable phases, low-temperature deposition studies19 have revealed two additional structures: x61 × x61R tan-1(5/6) for 0.1 < θ < 0.5 and a (5 × 5)R tan-1(3/4) for 0.55 < θ < 0.64. Between a coverage of 0.5 and 0.55 LEED indicates a strongly disordered phase. The higher order phases are metastable structures which transform to their stable counterparts upon heating to 273 K. However, little information is available concerning the kinetics of the transformation process. This paper reports some preliminary in situ STM observations of the deposition and stripping of upd Pb on Cu(100) in a Cl-/ClO4- electrolyte. 2. Experimental Section Single-crystal squares, 3-4 mm thick, were cut from a 2.5 cm diameter cylindrical single crystal copper ingot which had been previously aligned by Laue X-ray diffraction. The crystal substrates were mechanically polished to a 0.1 µm diamond finish followed by electropolishing in 85 vol % phosphoric acid. After extensive rinsing with water the substrate was dried and transferred to either a conventional three electrode cell for voltammetric studies or the STM electrochemical cell. Experiments were performed in deaerated solutions of 0.01 mol/L HClO4, 0.001 mol/L HCl, and 0.001 mol L Pb(ClO4)2 or some mixture thereof. The conventional electrochemical cell was first deaerated and then blanketed with Ar gas during operation. The electrolyte for the STM studies was deaerated for at least 30 min before injection into the STM cell which was also blanketed with Ar gas. The STM experiments were performed using a Molecular Imaging STM and a Digital Instruments Nanoscope III controller. The STM cell incorporated a platinum counter electrode, an polyethylene insulated tungsten tunneling probe, and a Cu/Cu+ quasi-reference electrode. A saturated calomel electrode (SCE) was used for the voltammetric experiments, and all potentials quoted in this paper are relative to the SCE. The experimentally determined reversible Pb/Pb2+ potential was

J. Phys. Chem. B, Vol. 102, No. 49, 1998 10021

Figure 1. Voltammetry revealing the oxidative adsorption and desorption of chloride on Cu(100). The sweep rate was 1 V/s.

used to verify the relationship between the Cu-based quasireference electrode and the SCE. 3. Results 3.1. Voltammetry in Cl-/ClO4-. An overview of the electrochemical behavior of Cu(100) in 0.01 mol/L HClO4 with and without 0.001 mol/L HCl is shown in Figure 1. The halide adsorption and desorption reaction is apparent at potentials between proton reduction and copper dissolution. Formation of a halide overlayer is congruent with LEED-AES experiments which demonstrate specific adsorption of chloride at potentials below E0Cu/Cu+.17,18 This result is also consistent with the negative potential of zero charge (pzc) reported for copper in KClO4 solutions20 and work function data21 as well as the wellknown tendency for underpotential reactions in group IB-halide systems.22 In this instance, the reaction may be viewed as

Cu + Cl- f CuClad + eThe adsorption process on Cu(100) is quite reversible judged by the symmetry of the three redox waves with respect to the potential axis, particularly in light of the fast scan rate. A residual dc offset of the voltammograms is apparent at slower sweep rates which may be attributed to proton reduction or reduction of residual oxygen. Prior LEED17,18 and STM2 studies indicate the formation of a c(2 × 2) Cl adlayer at potentials above -0.3 V. Below -0.3 V the adlayer becomes disordered coincident with at least partial desorption of the halide overlayer. Integrating and averaging the redox waves yields a charge of ∼0.070 mC/cm2 while complete reduction of the c(2 × 2) Cl structure would correspond to 0.123 mC/cm2 for an electrosorption valency of unity. However, an AES study18 of emersed electrodes indicates that only partial reduction of the adlattice occurs prior to the onset of hydrogen evolution. The multiple voltammetric peaks are presumably associated with structural alterations that occur within the diminished adlayer. 3.2. Voltammetry in Pb/Cl-/ClO4-. As shown in Figure 2, the addition of Pb2+ to 0.01 M HClO4- + 0.001 M HCl leads to a well-defined wave at -0.390 V due to upd of Pb on Cu(100). This is followed by the nucleation of threedimensional Pb crystallites at more negative potentials. The potential associated with the current crossover on the reverse sweep corresponds to the reversible value for the Pb/Pb2+ couple. This is followed, in sequence, by the immediate stripping of three-dimensional Pb clusters, displacement of the Pb upd layer by oxidatively adsorbed chloride at ∼-0.310 V, and finally the onset of copper dissolution at potentials above

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Figure 2. Voltammetry of lead upd and bulk deposition on Cu(100). [Pb2+] ) 0.001 mol/L.

-0.1 V. The overall upd reaction may be described as

x(CuCl)ad + (1 - x)Cu + Pb2+ + (2 + x)e- T

Moffat

Figure 3. Integration of the chronoamperometric transients yielding the charge associated with displacement of the chloride adlayer by upd lead during the deposition cycle as well as the charge of the reverse reaction. The initial and final potential in these measurements was -0.2 V.

(CuPb)upd + xClwhere the stoichiometric number accounts for the fact that the deposition of one Pb atom does not necessarily accompany desorption of exactly one chloride anion.17 Compared to the halide adsorption, outlined in Figure 1, the upd process is kinetically slow as indicated by the separation or displacement of the adsorption and desorption waves along the potential axis in Figure 2. The complexity of the upd reaction is also reflected in the breadth and shape of both waves. Close examination of the respective waves indicates the superposition of at least two processes. Integration of the deposition wave yields a total charge of ∼426 mC/cm2 while a value of ∼372 mC/cm2 was measured for the dissolution wave. This suggests that the process is chemically irreversible since ∼12% of the deposition charge is not recovered during the voltammetric experiment. However it is important to note the slight dc offset of the voltammograms at negative potentials which may be associated with some other faradaic process, such as proton or residual oxygen reduction, which might account for at least a portion of this discrepancy. Another view of the upd process was obtained by chronoamperometry. Measurements were performed by stepping the potential from -0.2 V to various negative values for deposition, followed by stripping at -0.2 V. A summary of the results of integrating the transients is given in Figure 3. Interestingly, a more favorable agreement between the deposition and stripping charge is found for deposition potentials between -0.370 and -0.440 V, while at more negative potentials the respective charge quantities become unequal. A simple minded comparison between the upd charge and possible adlayer structures indicates a favorable agreement for the displacement of the c(2 × 2) Cl adlayer by a c(5x2 × x2) Pb adlayer (i.e. Pb coverage of 0.6). This picture is based on assuming an electrosorption valency of 1 and 2 for the desorption and adsorption of the respective adlayers and ignoring the charge associated with the displacement of the pzc value between the respective surfaces. Surprisingly, the deposition charge measured in these experiments exceeds by ∼100 µC/ cm2 the value previously reported.17 The origin of this discrepancy is unclear; however, a voltammetric experiment performed following the chronoamperometric measurements revealed that the electrode response is a sensitive function of the electrode history as shown in Figure 4. STM experiments provide a rationale for this behavior in terms of the morphological evolution of the surface during the upd reaction.

Figure 4. Two voltammograms demonstrating the history or path dependence of the electrode response. The dashed line corresponds to the third upd cycle performed on freshly prepared electrode. The electrode was poised at -0.2 V for 30 s after each cycle. In contrast the solid line corresponds to the voltammogram obtained immediately following the chronoamperometric experiments described in the previous figure.

3.3. STM Results. Several prior STM studies have clearly revealed the c(2 × 2) Cl adlayer formed on Cu(100).2 Importantly, the halide adlayer leads to faceting of the steps in the [100] direction as shown in Figure 5a. This corresponds to the close packed direction of the adlayer stabilizing the underlying kink saturated metal step. The ordered halide adlayer may be displaced by Pb upd by stepping the potential from ∼-0.270 to -0.470 V. As shown in Figure 5b, the upd reaction nucleates and propagates from the kink saturated metal steps to cover the terraces. The image indicates that the faradaic process is completed within ∼0.5 s in good agreement with the representative transient shown in Figure 5c. The most notable characteristic of the reaction is the formation of a number of vacancy clusters in the terrace along with a limited number of new islands. Prior UHV studies of submonolayer lead deposition on Cu(100) have revealed similar morphological structures which were ascribed to an alloying-dealloying transition occurring with increasing lead coverage.9-11 Such transitions are typically associated with the ejection of adatoms onto the terraces and the injection of a high concentration of vacancies into the surface. In the experiments performed thus far, the atomic structure of the lead adlayer has not been unambiguously resolved. Limited data suggest a highly disordered c(2 × 2) structure is

Surfactant-Assisted Epitaxial Growth

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Figure 6. STM images of a higher order commensurate or incommensurate structure formed during the first upd voltammetric cycle for a freshly prepared Cu(100) crystal. This may be related to the (x61 × x61) tan-1(5/6) structure reported in a vacuum studies, but the limited data preclude a definite assignment at this time. Key: (a) 17.4 nm × 17.4 nm; (b) 8.1 nm × 8.1 nm area selected from (a). The image was collected immediately after sweeping the potential from -0.220 to -0.472 V at 10 mV/s. Etip ) -0.019 V, and itip ) 5 nA.

Figure 5. Two sequential STM images of lead upd along with a representative current transient. The first (29.3 nm × 29.3 nm) image (a) was collected at -0.12 V. The surface is covered with c(2 × 2) Cl adlayer which stabilizes the steps in the 〈100〉 direction. The line in the next image (b) corresponds to the time when the potential was stepped from -0.12 to -0.47 V. Nucleation of the upd layer at the steps followed by rapid propagation across the terrace is apparent. The terrace is covered within < 0.6 s which is consistent with the independently measured current transient shown in (c). Frame time ∼ 12.6 s, Etip ) -0.019 V, and itip ) 7.32 nA.

formed which would be related to either the c(2 × 2) or c(5x2 × x2) structure reported in a vacuum studies. The c(5x2 × x2) structure has been described as an alternation of “straight” and “zigzag” Pb atomic chains oriented in the 〈100〉 direction.11 The transition between the two structure may be facilitated by the insertion of antiphase boundaries into the c(2 × 2). Importantly, the absence of long range order in the STM images is consistent with a prior LEED study of an electrode emersed under similar conditions.17 While reliable atomically resolved imaging proved difficult, extended observation indicates that the lead adlayer preserved the [100] step faceting which suggests that the lead adlayer exerts a stabilizing influence similar to that associated with the c(2 × 2) Cl structure. A potentially significant exception to the above description follows from one experiment involving the first upd cycle on a freshly prepared crystal. An incommensurate or higher order commensurate structure was briefly observed after terminating the first negative potential sweep experiment at -0.472 V. This structure, shown in Figure 6, may be related to the x61 × x61R tan-1(5/6) adlayer structure reported for lead deposition in a vacuum at low temperature.19 The structure was subsequently displaced by a disordered or highly defective c(2 × 2) phase within ∼30 s. The metastable structure was only observed for a freshly prepared substrate perhaps highlighting the importance of defect density on the kinetics of structural evolution of the lead adlayer. Nonetheless, a degree of caution is warranted in light of the limited data available at this juncture. In a subsequent set of experiments the potential dependence of the upd reaction was investigated by progressively stepping the potential in the negative direction while imaging the surface. A typical experiment began with the surface being covered by a c(2 × 2) Cl adlayer at -0.2 V. Upon stepping of the potential

to -0.35 V, the image becomes noisy, as shown in Figure 7a, which may be related to vacancies generated in the halide overlayer, lead adatoms deposited on the terrace, or lead atoms incorporated into the terrace. In Figure 7b a new terrace (A) begins to expand across the original terrace (B). The step associated with the expanding terrace exhibits faceting in the 〈110〉 direction in addition to the 〈100〉 direction usually associated with the c(2 × 2) Cl and/or the saturated lead structure. In addition, as shown in Figure 7b-d, the steps were decorated with a band of enhanced electron density which must be associated with lead alloying. Preferential alloying at the step edge is not unexpected in light of the enhanced step edge mobility associated with disruption of the c(2 × 2) Cl adlayer.2,3 In Figure 7d the alloyed terrace is seen to expand further. The potential was then stepped to -0.45 V to saturate the lead coverage. As shown in Figure 7e-f, the terraces immediately become covered with a high density of islands or a monolayer deep channel pattern. The steps are largely faceted in the 〈100〉 direction which is a characteristic of the saturated lead adlayer. The morphological evolution of the terrace into the island or channel structure can be rationalized in terms of an alloyingdealloying transition analogous to that outlined earlier for vacuum deposition of submonolayer quantities of Pb on Cu(100).11 Accordingly, the expanded terrace corresponds to the growth of a two-dimensional Cu-Pb alloy at -0.35 V. Subsequently, this two-dimensional alloy layer undergoes dealloying when the potential is stepped to -0.45 V which leads to an increase in the lead coverage toward 0.5-0.6. If we assume, by analogy with the prior vacuum result, that the alloy corresponds to a Pb3Cu4 stoichiometry,10,11 dealloying of the terrace should reduce the total coverage of the terrace by 42%. This is in reasonable agreement with the channel area shown in Figure 7e-f. Further experiments performed under similar conditions, namely for low Pb upd overvoltages, provide additional evidence of two-dimensional alloy formation proceeding from the steps while much of the surface remains covered with the c(2 × 2) Cl adlayer. The veracity of this interpretation was investigated further by saturating the surface with lead. Notably, in these experiments the channel network ascribed to dealloying was concentrated in region associated with the growth of the two-dimensional alloy terraces. A series of experiments were also performed to investigate the dissolution of the upd lead adlayer. In the first experiment the potential was stepped from -0.45 to -0.2 which resulted in the displacement of the lead upd layer by the c(2 × 2) Cl adlayer. This was followed by rapid annealing of the surface leading to a minimization of the kink density in the halide

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Moffat

Figure 7. A sequence of (32.5 nm × 32.5 nm) STM images revealing the potential dependence of the upd reaction. The top half of the image a was collected at -0.2 V where the surface is covered with a c(2 × 2) Cl structure. The potential was then stepped to -0.35 V at the time denoted by the line. The image became somewhat noisy with the appearance of bright spots on the terrace which are most likely incorporated atoms of lead. In image b a new terrace (A) begins to expand across the surface with the steps being faceted in either the 〈100〉 or 〈110〉 direction in image c. Bands of enhanced electron density parallel to the 〈100〉 directions begin to develop, which are particularly noticeable near the step edge. The structure continues to expand in image d. The potential was then stepped to -0.45 V at the point denoted by the line in image e. Rapid development of a faceted channel network occurs immediately which may be ascribed to dealloying of the alloyed terrace. Comparing images e and f reveals a degree of coarsening of this structure. Etip ) 0.050 V, and itip ) 5 nA.

Figure 8. Atomically resolved (9.6 nm × 9.6 nm) image of the c(2 × 2) Cl layer that is formed at -0.2 V after several upd cycles. The bright spots which are coincident with the c(2 × 2) lattice may be ascribed to either residual lead incorporated into or beneath the terrace or possibly a vacancy in the copper and/or adlayer.

adlayer. This is similar to the coarsening behavior previously observed during the order-disorder transformation on the halide covered Cu(100) surface.2,3 However, close examination of the surface c(2 × 2) Cl adlayer reveals a significant defect population. As shown in Figure 8, certain sites of the c(2 × 2) lattice exhibit enhanced electron density which is probably associated with either a vacancy or lead atom incorporated into or beneath the adlayer or perhaps even beneath the first layer

of copper. Interestingly, the defects amount to ∼10% of the surface sites and might be associated with the discrepancy observed between the voltammetric deposition and stripping charge. The voltammogram shown in Figure 4 indicates at least two different processes occur as the potential is swept in the positive direction. Consequently, the surface was imaged while the potential was progressively stepped in the positive direction. The experiment was initiated by first depositing a upd lead layer at -0.45 V. As the potential was stepped to -0.320 V the islands decorating the terraces disappear first. This was followed by the nucleation and propagation of rows or stripes of a “dark” phase across the surface in the 〈100〉 direction as shown in Figure 9a-c. Repeated experiments indicate that the second voltammetric wave is clearly associated with the expansion of the “dark” phase across the surface. Atomically resolved imaging revealed the “dark” phase to be the c(2 × 2) Cl adlayer. The relatively high brightness of the other phase may be due to either topographic or electronic effects. In one experiment, shown in Figure 10, holes appeared in the terrace as the c(2 × 2) Cl layer displaced the “bright” phase. This suggests that the “bright” phase is an alloy of Pb and Cu which results in vacancy injection when the lead is selectively oxidized.

Figure 9. Sequence of (27 nm × 27 nm) STM images of the stripping of the upd lead. Image a was collected at -0.45 V. The potential was then stepped from -0.45 to -0.32 V as demarked by the black line in image b. The islands on the lower terrace disappear rapidly followed by the nucleation and propagation of rows of the c(2 × 2) Cl phase within the lead alloy phase. Image c reflects the progress of the phase transformation with time. Etip ) 0.050 V, and itip ) 5 nA.

Surfactant-Assisted Epitaxial Growth

J. Phys. Chem. B, Vol. 102, No. 49, 1998 10025 coincident with the copper lattice site giving a primitive cell dimension of ∼0.722 nm which agrees well with the STM observations. According to this model, two different 〈100〉oriented boundary structures can exist between the c(2 × 2) Cl and the Pb3Cu4 alloy phases. The different energetics associated with the respective boundaries probably account for the anisotropy of the reaction. Oxidation of the alloy phase leads to a supersaturation of vacancies which rapidly coalesce into monolayer deep pits as indicated. It is interesting to speculate if the striped alloy phase observed during stripping is the same structure which develops during submonolayer Pb deposition (e.g. Figure 7c). 4. Discussion

Figure 10. Sequence of (29.4 nm × 29.4 nm) STM images revealing the growth of the c(2 × 2) Cl phase at the expense of the lead-based phase. The lead-based phase is thought to be an alloy since as the reaction progresses the disappearance of the lead-phase is associated with the generation of vacancies which are annihilated at the steps or coalesce to form monolayer deep pits within the terrace. The rows of the alloy phase are 7 Å wide, and in certain instances corrugation of the electron density is apparent along the main axis of the rows and appears to be coincident with that of the c(2 × 2) Cl phase at the interphase boundary. Key: (a) the black line demarks the time at which the potential, E, was stepped from -0.31 to -0.30 V; (b-d) E ) -0.30 V, Etip ) 0.050 V, and itip ) 5 nA.

Subsequently, the vacancies coalesce to form the monolayer deep pits. Further analysis of the lead phase reveals that the width of the stripes corresponds to increments of ∼0.7 nm while inspection at higher resolution indicates that the alloy phase is coherent with the c(2 × 2) phase at least along the major interphase boundary. These observations in combination with prior vacuum work suggest that the lead phase corresponds to a c(4 × 4)-derived Pb3Cu4 alloy.10,11 Schematic drawings of the proposed structure are shown in Figure 11. Only the primitive cell of the Pb3Cu4 structure is shown with the different shading of the lead atoms reflecting the occupancy of different sites. The lead atoms (black species) on the boundary are

Coverage-dependent alloying-dealloying during upd is probably the most interesting finding in the present study. The morphological consequences of the reaction are path dependent such that different results are obtained for deposition via a potential sweep versus potential step or for a fresh electrode versus a repetitively cycled electrode, etc. Nonetheless, the general behavior of the immersed electrode appears to be closely related to Pb/Cu(100) vacuum interface,10,11 which is probably a consequence of the weak interaction between Pb and Cl. Important difference in the details of morphological development still remains due to different initial conditions, namely the 〈100〉 step faceted, c(2 × 2) Cl/Cu(100) versus the 〈110〉 step faceted, annealed Cu(100)/vacuum interface. A more precise comparison would be facilitated by experiments performed using ClO4-, BF4-, or some other anions that exhibit weak adsorption on copper. Similarly, in the last two years significant theoretical and experimental methods for quantifying defect generation and the role of steps in the kinetics of two-dimensional alloy formation23-28 have been described and the extension of these studies to electrochemical systems should be fruitful. An understanding of the influence of step dynamics on film growth is a subject of central importance to electrodeposition of metals. In this paper the influence of Cl- adsorption and Pb upd on the step dynamics of Cu(100) has been explored in the absence of copper deposition. In previous work, the impact of the ordered halide adlayer on the deposition of copper was clearly demonstrated.2,3 Specifically, the c(2 × 2) Cl adlayer floats on the surface and acts as a template guiding anisotropic

Figure 11. Schematic drawings of the two phase Pb3Cu4 alloy/c(2 × 2) Cl surface that develops during stripping of upd lead. Experiments indicate that the dissolution of this structure is intimately associated with the second voltammetric stripping wave. The “dark” lead atoms are coincident with sites on the substrate lattice while the “checkered” lead species are displaced from regular lattice position. Image a corresponds to the coexistence of the two phases while image c represents the coarsening of the surface following the displacement of the lead phase. Image b is a fictitious rendering of the copper sites of the Pb3Cu4 surface alloy while vacancies have been inserted in place of the lead atoms.

10026 J. Phys. Chem. B, Vol. 102, No. 49, 1998 growth in the 〈100〉 direction. In contrast, when the adlayer is disordered, at more negative potentials, the steps become more isotropic with perhaps a slight bias in the 〈110〉 direction. The ordered and disordered adlayers also present different barriers to intralayer and interlayer transport. Depending on the specific growth conditions, i.e., potential modulation, crystal miscut, etc., the order-disorder transformation of the halide adlayer may also lead to an increase in the step density. The increase is due to the nucleation of new islands which are quenched from the elevated adatom population that develops with the disordered adlayer.2,3 Importantly, the mode of homoepitaxial film growth is known to be a strong function of the ratio between the step density/surface diffusion/deposition flux.29,30 Thus, it is clear that the use of potential modulation to repetitively orderdisorder the halide adlayer in coordination with the deposition of copper offers a novel approach to controlling film growth. In fact, such phenomenon may already have manifested itself in certain pulse plating operations. In a completely analogous manner, the large morphological changes associated with repetitive alloying and dealloying of lead (i.e. Figure 7) may be used to influence homoepitiaxial growth dynamics. Alternatively, careful control of the upd alloying process may allow the formation of novel three-dimensional alloys.31 In this instance, precise potential control will be necessary in order to mediate the tendency for lead to float on the surface versus being incorporated in a growing Cu-Pb alloy deposit. Further work is in progress. 5. Conclusions In acid chloride solutions a c(2 × 2) Cl adlayer is formed upon an immersed Cu(100) electrode leading to step faceting in the 〈100〉 direction. At more negative potentials, the adlayer undergoes an order-disorder transformation which results in significant rearrangement of the steps. Alternatively, in an electrolyte containing Pb2+, the chloride adlayer may be completely displaced by lead upd. Images of orthogonal surface steps in combination with an assessment of the coulometry suggest that the lead adlayer forms either a highly defective c(2 × 2), c(5x2 × x2)R45° or a disordered structure corresponding to a coverage ranging from 0.5 to 0.6. The transformation from the halide to the lead adlayer results in the formation of vacancies and adatoms which condense to form holes and islands, respectively. These features may be rationalized by the formation of an alloy phase, at low coverage, which subsequently dealloys as the coverage approaches 0.5. Voltammetry reveals that the stripping of the lead upd layer is associated with two oxidation waves. As the potential is increased beyond the peak of the first wave, the islands disappear which may be due to alloy formation occurring coincident with partial desorption of the lead. The second wave is associated with the nucleation 〈100〉-oriented rows of the c(2 × 2) Cl adlayer which propagate across the terraces displacing the lead phase. The observation of vacancy clusters of holes in the terrace indicates that the dissolving lead phase corresponds to a two-dimensional Pb3Cu4 alloy. The STM studies provide clear evidence of an alloying/ dealloying transition associated with increasing Pb coverage during upd. The morphological consequences of this reaction are found to be path dependent. In certain instances the upd process may lead to a large increase in the effective step edge

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