In Situ Scanning Tunneling Microscopy of Electrodeposition of Indium

Nov 27, 2013 - Indium deposition is involved in preparing semiconducting thin films of copper indium gallium selenide (CIGS)—a material of great use i...
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In Situ Scanning Tunneling Microscopy of Electrodeposition of Indium on a Copper Thin Film Electrode Predeposited on Pt(111) Electrode Te Pao, YuYing Chen, Sihzih Chen, and Shuehlin Yau* Department of Chemistry, National Central University, Jhongli, Taiwan 320, Republic of China ABSTRACT: Indium deposition is involved in preparing semiconducting thin films of copper indium gallium selenide (CIGS)a material of great use in fabricating solar cells. In situ scanning tunneling microscopy (STM) was used to study electrodeposition of indium (In) on a copper (Cu) thin film electrode in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In2(SO4)3 (pH 3) electrolyte solutions without and with 1 mM chloride. A Cu thin film predeposited on platinum (111) comprised Cu(111) oriented layers stacked on the Pt(111) substrate. This highly ordered Cu(111)-like substrate rendered detailed characterization of underpotential and overpotential deposition (UPD and OPD) at potentials positive and negative of −0.60 V (vs Ag/AgCl), respectively. Both bisulfate and chloride anions preoccupied the Cu substrate impeded In UPD. Atomic resolution STM imaging revealed that In nucleated preferentially at surface defects, followed by lateral growth to form an organized indium adlayer, identified as a (√37 × √37)R25.3° structure. Indium adatoms aggregated, rather than distributing evenly on the Cu substrate. Indium deposit did not mix with the Cu substrate until the second stage of In UPD. This In/Cu interfacial mixing occurred first at steps and vacancy defects.

1. INTRODUCTION Electrodeposition of indium has been involved in producing low band gap semiconducting materials such as copper indium gallium selenide (CIGS),1−4 which can be used to fabricate solar cells with a quantum efficiency of 19% or higher.5−8 Compared to vacuum deposition, electrodeposition of indium (In) can yield high-quality thin films at a low cost in an easy-tooperate environment. Many groups have studied the electrodeposition of indium on Al,9 Cu,10 Mo,11 C,12 etc. in several different electrolyte solutions, addressing issues such as the nucleation and growth of bulk In thin films and the structures of Cu/In compound phases. Although Cu and In can mix at room temperature, a uniform alloy phase is possible only via annealing, leading to a CuIn2 phase characterized by X-ray diffraction.13 There are a number of reports addressing underpotential deposition (UPD) on Cu electrodes. It is found that anions such as bisulfate and chloride play important roles in the UPD processes. For example, chloride competes with metal adatoms such as Pb for surface sites, forcing metal deposition toward more negative potentials.14−18 On the other hand, chloride anion is capable of catalyzing the kinetics of Pb2+ reduction. Similarly, sulfate anions can impede Sn deposition on Cu(111).19 By contrast, UPD of Cd at Cu(111) electrode in the presence of chloride anions results in a Cd + Cl− bilayer structure.20 One of the possible explanations to account for the formation of metal + anion bilayer organization lies in the lattice match between the bilayer such as CdCl and the © 2013 American Chemical Society

Cu(111) substrate. This is similar to the well-studied Cu UPD on Pt(111), where a CuCl structure is identified.21−23 It is also noteworthy that metal deposition on Cu electrode, even in the early UPD stage, can result in intermetallic compounds, as noted with Pb/Cu(100)14 and Sn/Cu(111).19 Indium deposition on copper in a vacuum24 results in a number of alloyed phases, depending on the amount of indium deposited.25 From the perspective of Cu electrodeposition, In can act as a surfactant inducing layer-by-layer epitaxial growth of Cu on Cu(111) and suppress the formation of twin structures of Cu deposit.26 Given the importance of In deposition on Cu electrode, we were motivated to gain more understanding of this system. This study employed voltammetry and in situ scanning tunneling microscopy (STM) to explore the electrodeposition of In on a Cu thin film predeposited on a Pt(111) electrode. We focused on the UPD events occurring in sulfate- and chloride-containing media.

2. EXPERIMENTAL SECTION The Pt(111) electrodes used for STM and voltammetric measurements were homemade from melting the end of a Pt wire (φ = 0.8 mm) using a H2/O2 torch. The single-crystal bead electrode was treated with the standard annealing− Received: September 26, 2013 Revised: November 22, 2013 Published: November 27, 2013 26659

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quenching procedure.27,28 A Cu thin film predeposited on Pt(111) bead electrode was used as the substrate onto which In was deposited. First, the Cu thin film was made by Cu electrodeposition in 1 mM Cu(SO4)2 + 0.1 M K2SO4 + 1 mM HCl (pH 3) with the potential stepped (ΔE = 25 mV) negatively from 0 to −0.3 V (vs Ag/AgCl). The potential was paused for 5 min before it was shifted negatively. To be shown below, the as-prepared copper thin film consisted of layers of Cu(111) stacking on the Pt(111) electrode. The STM used here was a Nanoscope II (Santa Barbara, CA). The STM cell employed two Pt wires acting as the quasireference electrode and the counter electrode, respectively. All potentials reported are converted to a Ag/AgCl scale. The tip was made of tungsten wire (0.5 mm) electrochemically etched in 6 M KOH solution, and coated with a film of Apeazon wax to minimize the electrochemical interference. STM imaging was conducted under ambient condition (25 °C) in the constantcurrent mode with typical parameters of 1 nA feedback current and 200 mV bias voltage. All images shown in this article were treated with the “flatten” filter, which allows a clear view of a tilted plane. Suprapure sulfuric acid (H2SO4) and CuSO4 were purchased from Merck (Darmstadt, Germany). In2(SO4)3 was purchased from Alfa Aesar (Lancashire, U.K.). Triple-distilled water (Lotun Technology Co., Taipei) was used to make the electrolytes needed for STM and voltammetric experiments.

These voltammetric results differ greatly from those reported,13 which have focused mainly on the bulk deposition of In. Since UPD occurs only when deposited metal interacts favorably with the substrate, the occurrence of In UPD would depend on the chemical nature of the electrode. The sharp current spike C2 resulted from the first stage of bulk In deposition, which has not been observed in previous studies.11,13 This issue could be resolved by using STM imaging, but we will not address this point in this study. Anodic features seen at E > −0.2 V are ascribed to stripping of Cu deposit from the Pt electrode, including bulk and UPD Cu at 0.10 and 0.40 V, respectively. The inset in Figure 1 provides a closer look at the UPD process of In on the Cu film. In the negative scan from −0.2 to −0.55 V, a well-defined peak is seen at −0.38 V (U1), followed by a broad peak spanning between −0.4 and −0.5 V (U2). The subsequent positive scan led to two peaks (S1 and S2) at −0.4 and −0.31 V, which reflects clearly two-staged In UPD. The U1 and U2 had very different peak shapes, suggesting that In adatoms interacted differently in these two features. For example, the sharp peak shape of U1 (full width at halfmaximum = 20 mV) implies that In adatoms attracted one another, whereas the broad U2 peak means that In adatoms repelled each another. Meanwhile, the deposition and stripping processes (U1 and S1) differ by 80 mV in potential, an indication of poor reversibility. The most likely explanation of this result is the blocking effect of adsorbed anions (bisulfate in this case) on In deposition. The stronger the anion interacted with the substrate, the more negative potential indium UPD would occur. After correction for the double layer charging effect, the amounts of charge passed in U1 and U2 are 120 and 20 μC/ cm2, which implies a coverage of 0.14 (ratio of In adatom/Cu atoms) at the end of the first stage of UPD, which increased by 0.033 in the second stage of UPD. This estimation is based on the assumption that the Cu substrate was a defect-free Cu(111) lattice, at which an one-electron reduction would consume 284 μC/cm2 charge. Assuming the UPD event involved In3+ + 3e− → In, the deposition of a full monolayer of In would require 854 μC/cm2. To be described below, the 0.14 coverage of In described above is 13% smaller than that (0.162) determined from atomic STM images, which likely resulted from uncharacterized contribution of anion and uncertainty in the oxidation state of the In adatoms. The amount of charge integrated for the stripping peak A2 seen in Figure 1 was 2584 μC/cm2, which implies that the In deposit was three layers thick, assuming an oxidation reaction of In → In3+ + 3e−. The charge passed in the A1 peak was measured as 584 μC/cm2, which is 4 times larger than the sum of S1 and S2. This difference could result from interfacial mixing between the In deposit and the Cu substrate, if the potential was scanned to −0.6 V or more negative values. That In deposit mixing with the Cu substrate was more difficult to remove from the Cu electrode than that of unmixed In, which manifested in the ∼200 mV positive shift of its stripping potential. To study the effect of anion on the deposition of In, we obtained CV profiles by the same method but with HCl added to the electrolyte solution. The morphology of these profiles resembles those seen in Figure 1, except a few differences were noted. First, the main UPD feature of U1′, highlighted in the inset of Figure 2, shifted negatively from −0.38 to −0.50 V. This result reflects that chloride interacted more strongly with

3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry (CV). Figure 1 shows a CV recorded with a Pt(111)-supported Cu thin film electrode in

Figure 1. CVs recorded at 5 mV/s with a Cu thin film supported by Pt(111) in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In2(SO4)3. The potential was swept in the sequence of −0.2 V → −1.0 V → 0.8 V. The inset is a CV highlighting the In UPD, featuring two pairs of peaks U1/S1 and U2/S2.

0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In2(SO4)3. The potential was scanned at 5 mV/s in the following sequence: −0.3 V → −1.0 V → 0.8 V. The negative-going potential sweep resulted in a weak doublet peak around −0.38 V (C1), followed by a sharp current spike (C2) at −0.60 V, and then a broader peak at −0.65 V (C3). Since C1 lay at a potential more positive than the Nernst potential calculated for 1 mM In3+ (the standard reduction potential of In3+ to In0 is −0.53 V), this feature is ascribed to the underpotential deposition (UPD) of In. Meanwhile, C2 and C3 are ascribed to overpotential deposition (OPD) of indium. The subsequent positive-going potential scan resulted in stripping of bulk and UPD In at −0.58 and −0.33 V, respectively. 26660

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Figure 3. In situ STM images acquired with a Pt(111) electrode coated with multilayer Cu at −0.2 V in 0.1 M H2SO4 + 0.1 M Cu(SO4)2. Panel a is a topography scan revealing smooth Cu film decorated by a long-range-ordered moire pattern, whose internal structure, (√3 × √7)-HSO4−, was discerned by the high-resolution STM scan shown in panel b. Raising the tunneling current from 1 to 15 nA with a constant bias voltage of 80 mV yielded atomic resolution imaging of the Cu substrate revealed by panel c. The inset of panel a reveals the corrugation profile along the marked dotted line. Two step edges (Δz = 0.22 nm) are seen in this profile. Three vacancy defects marked by dotted circles are observed. Arrows marked in (a) indicate misalignment in the moire pattern. One of the close-packed rows of adsorbed bisulfate encloses an angle of 30° with one row of Cu atoms, as indicated by two arrows marked in panel b. Scale bars are (a) 40, (b) 4, and (c) 3 nm.

Figure 2. CVs recorded at 5 mV/s with Cu thin film/Pt(111) in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In2(SO4)3 + 1 mM HCl. The potential was swept in the sequence of −0.2 V → −1.0 V → 0.8 V. The inset highlights In UPD featuring two pairs of peaks negatively shifted as compared with those found in Figure 1. The peak A1′ due to stripping of UPD In was much larger than C1′ due to In UPD.

the Cu electrode than (bi)sulfate, forcing In deposition toward more negative potentials. It is mentioned however that adsorbed chloride anions do not always impede metal deposition. As mentioned earlier, chloride facilitates Cd deposition on Cu(111), yielding a sandwich structure of Cu− Cd−Cl. Deposition and stripping of Cd are more reversible than those seen with In.18 Second, the peak current of the In stripping peak A1′ increased if the potential was scanned to −1.0 V, which implies that In deposit mixed with the Cu substrate under these conditions. The charges associated with the stripping peaks A1′ and A2′ seen in Figure 2 are 3323 and 1806 μC/cm2, respectively, as compared with those (584 and 2584 μC/cm2) found in the sulfate media (Figure 1). The substantial difference in the charges of A1 and A1′ indicates a greater degree of interfacial mixing with the In deposit in the presence of chloride. This effect of anion on the formation of intermetallic compounds is novel. To our knowledge, this phenomenon has not been reported. In addition, the overall stripping charges including UPD and OPD of In were 3168 and 5129 μC/cm2 found in electrolyte solutions without and with 1 mM chloride. It seems that chloride promoted the reduction of In3+ by 1.62 times versus sulfate under the present conditions. This result is in line with reports showing the catalytic effect of halide on the reduction of In3+,29 along with a number of metal cations including Cu2+, Ni2+, Co2+, etc.30,31 It was proposed that chloride could act as a bridge facilitating the nucleation of In on carbon electrode.12 3.2. In Situ STM Imaging. 3.2.1. Copper Thin Film Deposited on Pt(111). A multilayer copper thin film prepared by the procedure described in the Experimental Section was first examined. Shown in Figure 3 are STM images revealing the surface morphology and atomic structure of a 10-layer-thick Cu thin film. The Cu thin film was atomically smooth with terraces spanning more than 100 nm (Figure 3a). The inset in Figure 3a reveals a corrugation profile along the dotted line marked in Figure 3a, with which two monatomic steps (Δz = 0.22 nm) were discerned. Step edges were randomly aligned. Vacancy defects were also observed, as marked by arrows. The moire pattern seen in Figure 3a,b is considered to be the signature of specifically adsorbed bisulfate anions on singlecrystal Cu(111) electrode.32 Thus, it is safe to state that the asprepared copper thin film consisted of epitaxially deposited Cu(111) planes on the Pt(111) electrode. Close inspection of these STM results discloses that not all moire patterns were aligned. Two neighboring moire patterns could be misaligned

by 16°, as marked by arrows in Figure 3a. It is known that adsorbed bisulfate anions results in surface reconstruction of Cu(111) single-crystal electrode.32 More specifically, bisulfate adsorbates induced lateral expansion of the Cu(111) plane by 12%. This surface change can be identified by acquiring atomicresolution STM images of the Cu(111) surface underlying the bisulfate adlayer. While STM imaging at 80 mV and 1 nA revealed a (√3 × √7) adlattice in Figure 3b, increasing the tunneling current to 15 nA allowed direct imaging of the Cu substrate, a hexagonal network with a nearest-neighbor spacing of 0.27 nm (Figure 3c). By comparing the images in panels b and c of Figure 3, one can see bisulfate anions were indeed misaligned with respect to the atomic row of Cu substrate by 30°. 3.2.2. The First Stage of In UPD. The as-prepared Cu thin film was then immersed in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In3(SO4)2 and the potential was shifted from −0.3 V negatively until In plating started. Shown in Figure 4 are a series of STM images recorded at −0.36 V, the onset of In deposition. Two islands I and II emerged first at the lower end of a step edge (lower end of Figure 4b), followed by lateral growth to displace the bisulfate moire pattern. In addition, indium could also nucleate at the upper ends of steps, as long as there were defects. This led to the production of In islands III seen at the upper right of the scan. Meanwhile, another patch of In marked “IV” appeared at a boundary of two moire patterns, another kind of surface defect. Shifting the potential to −0.4 V substantiated In deposition, as In nuclei grew laterally to wet the entire Cu substrate at the time mark of 114 s (Figure 4d). The surface was smooth momentarily, as In deposit continued to land on the electrode, yielding randomly distributed spots on the electrode. These features measuring 0.2 nm in height were first noted in Figure 4d. They grew rapidly to occupy the entire surface within 60 s, as revealed by Figure 4e,f. This “goose-bump” surface marked the onset of the second stage of In UPD. Although adsorbed randomly at the beginning, In adatoms accumulated with time and gradually aligned to produce locally ordered patterns (described below). The STM corrugation heights of the In deposit are observed with the cross-section plots shown in Figure 4g,h obtained 26661

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Figure 5. In situ STM images revealing the stripping process of In deposit from a Cu film electrode in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In3(SO4)2. Panel a acquired at −0.38 V revealed an In-plated Cu film, followed by a composite STM image (panel b), where the potential was changed abruptly from −0.38 to −0.3 V in the middle of the frame (dotted line). The bisulfate moire pattern appeared almost immediately in the lower half of panel b and over the entire panel c recorded 30 s later. The arrow marked at the right of each image indicates the slow scan direction. Scale bars = 30 nm.

later. The Cu substrate reconstructed from (1 × 1) to the moire structure, yielding marked changes of step edges and protruded islands. It is concluded that In did not mix with the Cu substrate in the first stage of UPD. The indium adlattice formed at −0.38 V (the end of the first UPD stage) was characterized by high-resolution STM imaging in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In3(SO4)2. As revealed by the 40 × 40 nm scan shown in Figure 6a, In adatoms were adsorbed in a highly ordered structure. Occasionally STM imaging revealed ordered In arrays (Figure 6a) sitting next to the (√3 × √7)-HSO4− structure, as found in Figure 6b. Aided by the well-characterized bisulfate structure, this In adlattice could be determined accurately. As indicated by

Figure 4. In situ STM images revealing the UPD process of In on Cu thin film in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM CuSO4 + 1 mM In3(SO4)2. Panel a reveals a pristine Cu thin film covered with a monolayer of bisulfate anions at −0.30 V. Shifting the potential from −0.3 to −0.36 V resulted in In islands marked “I”−“IV” in panels b and c. The potential was then shifted to −0.4 V, where panels d−f were obtained. The arrow marked on the right of each image indicates the slow scan direction. Panels g and h are corrugation profiles along the dotted lines marked in (c) and (d). A domain boundary in the bisulfate adlayer is highlighted in panel b (30 × 30 nm). Scale bars = 20 nm.

along the dotted lines marked in Figure 4c,d. Indium islands appeared 0.13 nm higher than the areas covered with adsorbed bisulfate anions, as compared to the monatomic step height of 0.25 nm seen in the same image. This result suggests that, to the STM, In deposit was more conducting than bisulfate anions. As In continued to deposit, pits one atom deep appeared next to the In islands, as seen in Figure 4c. This resulted from restructuring of the Cu(111)-like surface from a moire pattern to a (1 × 1) structure, as adsorbed bisulfate anions were displaced by indium adatoms. Meanwhile, the lattice constant of the Cu substrate shrank by 12%, leading to pit formation. These pits or an increase of surface roughness made it difficult to have long-range-ordered In adlattices. It is noteworthy that terraces stayed smooth and step lines were unchanged in the course of the first stage of In UPD (Figure 4d). These STM results suggest that In deposit resided on, rather than mixing with, the Cu substrate. To scrutinize further the first stage of In UPD, we examined the surface morphology of the electrode by conducting realtime STM imaging of In deposition/stripping processes. As suggested by the CV results, indium deposit once mixed in with the Cu substrate would be more difficult to come off. This would result in poor reversibility in the deposition and stripping of In, which should be realized by the STM. Shown in Figure 5 are a series of STM images collected as a function of potential over the same scan area. Figure 5a reveals an In-plated Cu electrode at −0.38 V. The electrode surface was mostly smooth until the potential was changed abruptly from −0.38 to −0.30 V when the tip traveled downward to the middle of the second scan shown in Figure 5b. The In adlattice was displaced rapidly by the bisulfate moire structure seen in the lower half of Figure 5b and over the entire scan area of Figure 5c, recorded 30 s

Figure 6. In situ STM images recorded at −0.38 V in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In3(SO4)2. Panel a reveals a long-range-ordered adlattice of In deposit, while panel b shows an area with mixed bisulfate and In adlattices. One of the atomic rows of the In array was misaligned with a row of bisulfate anions by 5°, which suggests a (√37 × √37)R25.3° structure revealed by the close-up view shown in panel c. The dotted circle marked in (c) indicates coadsorbed bisulfate anions. Panel d shows the corrugation profile along the dotted line in (c). Panel e shows a ball model of the In ordered array with all In adatoms assigned to 3-fold hollow sites. Scale bars are (a) 10, (b) 5, and (c) 1 nm. 26662

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expansions of step ledges as indicated by the dotted traces marked in Figure 7a,b. The higher resolution STM image shown in Figure 7c is intended to reveal the tier features seen at the perimeter of vacancy defects. Shown in Figure 8 are STM images collected over the same area 24 and 39 min after the potential was held at −0.50 V. In

arrows marked there, one row of In atoms representing a unit vector of the cell is 5° off from that of the close-packed bisulfate. In other words, this unit vector of the In structure was 25° rotated from the close-packed atomic rows of the Cu(111)like substrate. Thus, this information yielded a √37 unit vector, which is 25.3° rotated from the ⟨110⟩ axis of an fcc(111) plane. These STM results yield a (√37 × √37)R25.3° structure (hereafter referred as the √37 structure), whose internal atomic arrangement is revealed by the close-up STM image shown in Figure 6c. The unit cell of this structure in Figure 6c as marked is the rhombus, which contains six spots exhibiting the same intensity. If all spots are attributed to In adatoms, this STM result yields an In coverage of 6/37 = 0.16, which is 14% larger than that determined from the voltammetric results described above. On average, In adatoms were 0.1 nm higher than the background (or the copper substrate), as revealed by the cross-section profile shown in Figure 6d. In adatoms were arranged distinctively in a trio pattern with two nearest In adatoms separated by 0.43 nm. The same corrugation heights among In adatoms suggests their identical registries on the Cu substrate. This √37 structure is reconciled by the ball model shown in Figure 6e, where all In adatoms are assigned arbitrarily to 3-fold hollow sites. This tentative model also indicates that a large portion of the Cu substrate was unoccupied. Arguably, we attributed these In-free surface sites to adsorbed bisulfate anions, as they were frequently found to coadsorb with metal adatoms.33 Close examination of the image in Figure 6c discloses that there were weaker spots marked by a dotted circle, also seen in the corrugation profile in Figure 6d. This indium cluster arranged was predicted, but it has not been observed on the Cu(111) surface in a vacuum.34 3.2.3. The Second Stage of In UPD. Subsequently, the potential was shifted negatively from −0.38 to −0.42 V and then to −0.5 V to trigger the second stage of In UPD. These potential steps resulted in notable changes of the electrode, as revealed by Figure 7a,b. First, the √37 structure was displaced

Figure 8. In situ STM images recorded after the potential was held at −0.5 V for (a) 24 and (b) 39 min in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In3(SO4)2. These two images were acquired over the same area, using the marked circle as the reference. Ordered structures grew and nearly occupied the entire electrode after ∼40 min STM imaging. Scale bars = 30 nm.

strong contrast to the disarray seen in Figure 7, ordered structures gradually popped up on the terraces, as the amount of In adatom increased slowly until it was enough to form ordered structures. It took roughly 20 min to identify this disorder-to-order transition and 40 min to have a full monolayer of In. We reason that the √37 In structure formed in the first stage of UPD was rather stable and resisted against further addition of In adatoms. This view is supported by CV results showing sharp and broad peak shapes for the two UPD processes. While the electrode was gradually decorated by the ordered structure, the filled pits were mostly disordered. One could increase the rate of the second stage of UPD by lowering the overpotential to E < −0.6 V, but then bulk In deposition followed, leaving little chance to see ordered In adlayers like those of Figure 8. As Figure 8 reveals, In UPD in the sulfate media always resulted in a high density of pits, making it difficult to produce a long-range-ordered structure in the second stage of In UPD. By contrast, the In-plated Cu electrode was nearly pit-free in the presence of chloride, and In adlayer was much better ordered, as shown in Figure 9, obtained at −0.55 V in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl + 1 mM In2(SO4). This long-range ordering helped to achieve higher resolution STM images. One of the ordered arrays could form two rotational domains on a terrace (Figure 9b). These two ordered domains enclosing an angle of 15° (indicated by two arrows in Figure 9b), from which we infer that the unit vectors were rotated 7.5° from the ⟨110⟩ axis of the Cu(111)-like substrate. The unit vectors of the rhombus marked in Figure 9c are 1.7 nm in length, which indicates a (√43 × √43)R7.6° structure (hereafter referred as √43) consisting of distorted honeycombs connected by edges (Figure 9c). Since two neighboring spots seen in Figure 9c were separated by about 0.93 nm, it is likely that each spot was an aggregate of In atoms, rather than a single In adatom. This view was confirmed by higher resolution STM scans shown in Figure 10. Figure 10a shows an original STM image, whereas Figure 10b is a close-up view of a filtered version of Figure 10a. (A two-

Figure 7. In situ STM images revealing the second stage of In UPD on copper thin film in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM In3(SO4)2. Panel a was acquired 2 min after the potential was set to −0.42 V, whereas panel b was recorded 30 s after the potential was made furthermore negative to −0.5 V. Pits were filled and steps expanded, as marked by the dotted lines. Panel c highlights a pit incompletely filled by In deposit. Those edges of vacancy defects marked by arrows appear slightly dimer than terraces. Scale bars = 20 nm.

by disarray. Second, pits were nearly annealed and steps were frequently expanded. Close examination of filled pits reveals that they were not level with their neighboring terraces; they appeared 0.03 nm lower than their neighboring terraces. Although this difference could arise from unlike tunneling probability between Cu and In, it is also possible that Cu/In intermetallic compounds were produced at these defects. The In/Cu mixing scheme also explains slight and yet notable 26663

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Figure 9. In situ STM images with successively finer resolution acquired at −0.55 V in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl + 1 mM In2(SO4)3. The squares marked in (a) and (b) indicate regions of zoom-in. Smooth terraces seen in panel a were separated by rough edges. Two rotational domains of the ordered structure were found on a terrace (b). These two domains enclosed an angle of 15° with their molecule rows. This (√43 × √43)R7.6° ordered array has a rhombus unit cell marked in (c). Both (b) and (c) were treated with the 2D FT filtering method. Scale bars are (a) 30, (b) 10, and (c) 2 nm.

Figure 11. In situ STM images of the (10 × 10)-In adlayer structure recorded with the Cu film at −0.55 V in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl + 1 mM In2(SO4)3. Panels a and b reveal the long-range ordering and a close-up view of this structure. Arrows marked in (a) show the close-packed atomic directions of the Cu(111) substrate, and the rhombus marked in (b) is the (10 × 10) unit cell. Scale bars are (a) 12 and (b) 3 nm.

are considered as In adlattices under conditions approaching equilibrium states. We then consider the possibility of In/Cu mixing at the second stage of In UPD. Similar to the results shown in Figure 5, real-time STM imaging was used to probe the reversibility of the second stage of In deposition and stripping. Figure 12a

Figure 10. Atomic-resolution STM images (a and b) acquired at −0.55 V showing (√43 × √43)R7.6°-In structure formed on Cu(111) thin film. Panel b shows a portion of 2D filtered image of (a). Indium adatoms marked by white dots tended to aggregate. Scale bars are (a) 5 and (b) 1 nm.

dimensional (2D) Fourier transform (FT) filtering method was applied to remove signals closer than 0.3 nm.) Similar to In clusters seen Figure 6c, this structure consisted of aggregates of five and six In adatoms. If one assumes only the marked spots are In adatoms and ignore weaker spots in the background, the coverage of In is determined to be 10/43 or 0.23, as compared with 0.16 found with the √37 structure in the first stage. In line with the broad peak shape for the second stage of In UPD in the CV, STM revealed several In structures. For example, another ordered structure, identified as (10 × 10), is shown in Figure 11, which has a similar honeycomb feature noted also with the √43 structure described above (Figure 10). The ∼0.9 nm nearest-neighbor spacing means that each spot was a group of In atoms as found with the √43 structure. This structure was anomalously difficult to image with high-quality atomic resolution. Since this structure was seen simultaneously with the √43 structure, the coverage of indium is likely to be close to 0.23 determined above. Both the √43 and (10 × 10) structures were found in the sulfate medium (Figure 8), although their degrees of ordering were inferior to those seen in the presence of chloride. This finding indicates that interactions among In adatoms could determine how they arranged spatially on the Cu electrode. Anion, frequently involved in metal UPD, was relatively unimportant in this case. It is also emphasized that most atomic-resolution STM images presented in this article have been observed in more than three separate experiments. Also, because the In structures seen in this study were observed after many hours of STM imaging, they

Figure 12. In situ STM images recorded during stripping of In deposited at −0.55 V in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl + 1 mM In2(SO4)3. Panel a shows an In-plated Cu electrode at −0.55 V. Smooth terraces and rough steps were evident in (a). Panel b reveals stripping of In adlayer at the lowest terraces at −0.4 V, the end of the first stage of In UPD. Panels c−f show the subsequent In stripping at −0.3 V. In deposit was eventually stripped off to restore a smooth Cu substrate (f). Scale bars = 50 nm.

shows an In-covered Cu electrode at −0.55 V in 0.1 M K2SO4 + 1 mM H2SO4 + 1 mM KCl + 1 mM In2(SO4)3. Well-defined terraces and rough step edges are apparent. Shifting the potential to −0.4 V removed the (10 × 10) structure sitting on the terrace at the very right (Figure 12b). Shifting the potential toward a more positive value (−0.3 V) caused much more drastic changes, as revealed by Figure 12c. Indium deposited at step edges was the last to dissolve, as indicated by the protruding seams seen in Figure 12d. All the In deposit was nearly stripped off within 270 s (Figure 12f), resulting in a smooth Cu electrode covered by a chloride adlayer. Note that the step lines seen in these images changed notably upon the stripping of In deposit. Aided by these STM results, we conclude that In deposit could form compounds 26664

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(7) Jackson, P.; Hariskos, D.; Lotter, E.; Paetel, S.; Wuerz, R.; Menner, R.; Wischmann, W.; Powalla, M. New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%. Prog. Photovoltaics 2011, 19 (7), 894−897. (8) Ramanathan, K.; Contreras, M. A.; Perkins, C. L.; Asher, S.; Hasoon, F. S.; Keane, J.; Young, D.; Romero, M.; Metzger, W.; Noufi, R.; Ward, J.; Duda, A. Properties of 19.2% efficiency ZnO/CdS/ CuInGaSe2 thin-film solar cells. Prog. Photovoltaics 2003, 11 (4), 225− 230. (9) Saidman, S. B.; Muñoz, A. G.; Bessone, J. B. Electrodeposition of indium and zinc on aluminium. J. Appl. Electrochem. 1999, 29 (2), 245−251. (10) Chraibi, F.; Fahoume, M.; Ennaoui, A.; Delplancke, J. L. Influence of Citrate Ions as Complexing Agent for Electrodeposition of CuInSe2 Thin Films. Phys. Status Solidi A 2001, 186 (3), 373−381. (11) Valderrama, R. C.; Miranda-Hernández, M.; Sebastian, P. J.; Ocampo, A. L. Electrodeposition of indium onto Mo/Cu for the deposition of Cu(In,Ga)Se2 thin films. Electrochim. Acta 2008, 53 (10), 3714−3721. (12) Muñoz, A. G.; Saidman, S. B.; Bessone, J. B. Electrodeposition of Indium onto Vitreous Carbon from Acid Chloride Solutions. J. Electrochem. Soc. 1999, 146 (6), 2123−2130. (13) Huang, Q.; Reuter, K.; Amhed, S.; Deligianni, L.; Romankiw, L. T.; Jaime, S.; Grand, P.-P.; Charrier, V. Electrodeposition of Indium on Copper for CIS and CIGS Solar Cell Applications. J. Electrochem. Soc. 2011, 158 (2), D57−D61. (14) Moffat, T. P. Oxidative Chloride Adsorption and Lead UPD on Cu(100): Investigations into Surfactant-Assisted Epitaxial Growth. J. Phys. Chem. B 1998, 102 (49), 10020−10026. (15) Brisard, G. M.; Zenati, E.; Gasteiger, H. A.; Markovic, N.; Ross, P. N. Underpotential Deposition of Lead on Copper(111): A Study Using a Single-Crystal Rotating Ring Disk Electrode and ex Situ LowEnergy Electron Diffraction and Scanning Tunneling Microscopy. Langmuir 1995, 11 (6), 2221−2230. (16) Vilche, J. R.; Jüttner, K. Anion effects on the underpotential deposition of lead on Cu(111). Electrochim. Acta 1987, 32 (11), 1567−1572. (17) Chu, Y. S.; Robinson, I. K.; Gewirth, A. A. Properties of an electrochemically deposited Pb monolayer on Cu(111). Phys. Rev. B 1997, 55 (12), 7945−7954. (18) Hümann, S.; Hommrich, J.; Wandelt, K. Underpotential deposition of cadmium on Cu(111) and Cu(100). Thin Solid Films 2003, 428 (1−2), 76−82. (19) Yan, J.-W.; Wu, J.-M.; Wu, Q.; Xie, Z.-X.; Mao, B.-W. Competitive Adsorption and Surface Alloying: Underpotential Deposition of Sn on Sulfate-Covered Cu(111). Langmuir 2003, 19 (19), 7948−7954. (20) Stuhlmann, C.; Park, Z.; Bach, C.; Wandelt, K. An ex-situ study of Cd underpotential deposition on Cu(111). Electrochim. Acta 1998, 44 (6−7), 993−998. (21) Wu, Z.-L.; Yau, S.-L. Examination of Underpotential Deposition of Copper on Pt(111) Electrodes in Hydrochloric Acid Solutions with in Situ Scanning Tunneling Microscopy. Langmuir 2001, 17 (15), 4627−4633. (22) Stickney, J. L.; Rosasco, S. D.; Hubbard, A. T. Electrodeposition of Copper on Platinum (111) Surfaces Pretreated with Iodine: Studies by LEED, Auger Spectroscopy, and Electrochemistry. J. Electrochem. Soc. 1984, 131 (2), 260−267. (23) Markovic, N. M.; Gasteiger, H. A.; Ross, P. N., Jr. Copper Electrodeposition on Pt(111) in the Presence of Chloride and (Bi)sulfate: Rotating Ring-Pt(111) Disk Electrode Studies. Langmuir 1995, 11 (10), 4098−4108. (24) Klas, T.; Fink, R.; Krausch, G.; Platzer, R.; Voigt, J.; Wesche, R.; Schatz, G. Microscopic Observation of Step and Terrace Diffusion of Indium Atoms on Cu(111) Surfaces. EPL 1988, 7 (2), 151. (25) Wider, H.; Gimple, V.; Evenson, W.; Schatz, G.; Jaworski, J.; Prokop, J.; Marszałek, M. Surface alloying of indium on Cu(111). J. Phys.: Condens. Matter 2003, 15 (12), 1909−1919.

with the Cu substrate at surface defects such as steps. A pit was essentially not formed on the Cu electrode in the presence of chloride; otherwise, In could also mix in there, as seen in sulfate-containing medium (Figures 7 and 8). These results are in parallel with those reported with Sn deposited on Cu electrode.19 Overall, surface defects were just more accommodating than terrace sites, which enabled In/Cu compounds to form.

4. CONCLUSION Indium deposition on a Cu thin film electrode proceeded in UPD and OPD processes. Anions of bisulfate or chloride preadsorbed on the Cu electrode impeded In deposition. Judging from the amount of In deposited in the OPD region, chloride anions promoted the kinetics of In3+ to In0 reduction. Indium deposit nucleated mainly at surface defects, followed by lateral growth to occupy the entire Cu electrode. Indium adatoms were first arranged in a well-ordered (√37 × √37) R25.3° structure and then restructured slowly to (√43 × √43) R7.6° and (√10 × √10) as more In was deposited. All these structures consisted of indium clusters, indicating strong interactions among In adatoms. No In/Cu interfacial mixing occurred until the second stage of UPD, and this event occurred mainly at step and vacancy defects. The extent of this In/Cu mixing could vary with the chemical identity of anions, as chloride promoted the reduction of In3+, yielding 3 times more In/Cu compounds than that found in a sulfate solution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.:886-3-4279573. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSC (Contract No. NSC 1022120-M-008-002). We acknowledge technical assistant from Prof. C. Su (National Taipei University of Technology).

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REFERENCES

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