Epitaxial Electrodeposition of Cobalt on a Pt(111) Electrode Covered

ARTICLE pubs.acs.org/JPCC

Epitaxial Electrodeposition of Cobalt on a Pt(111) Electrode Covered with a Cu(111) Film Po-Yu Yen, Sihzih Chen, Hsin-Ling Tu, Hengliang Wu, and Shueh-Lin Yau* Department of Chemistry, National Central University, Jhongli, Taiwan 320, Republic of China

Jyh-Shen Tsay* Department of Physics, National Taiwan Normal University, Taipei, Taiwan 110, Republic of China

bS Supporting Information ABSTRACT: Electrodeposition is an inexpensive alternative to the conventional molecular beam epitaxy technique used to fabricate artificial magnetic materials, such as cobalt thin film. Reported here is a scanning tunneling microscopy (STM) study on the electrodeposition of Co on a Pt(111) single-crystal electrode precoated with a Cu thin film in 0.1 M KClO4 + 1 mM HCl + 0.04 M CoCl2 (pH 3). Deposition of Co started with the nucleation of nanometer-sized clusters preferentially at pits on the Cu support, followed by lateral expansion and coalescence of Co nuclei to form a uniform Co layer. Normally a few Co layers would grow simultaneously to produce a smooth Co deposit until the 12th layer. Cobalt grew in three dimensions afterward. Atomic-resolution STM imaging showed that the first Co layer assumed a double-lined pattern, which was lifted by the deposition of another layer of Co. The second Co layer exhibited a hollow-ring pattern, which transformed into a moir
e pattern and triangular pits at the third Co layer. The moir
e pattern gained prominence at the expense of the triangular pits as the Co deposit thickened. The amplitude of the intensity modulation of the moir
e pattern decreased with the thickness of the Co deposit and eventually became indiscernible at the 12th layer. These restructuring events resulted from a gradual release of the stress at the Co/Cu interface. Since the morphology of the copper substrate was hardly changed by the deposition of cobalt, mixing at the Co/Cu interface seems to be negligible. Similar to the deposition process, dissolution of Co deposit proceeded in layers also.

1. INTRODUCTION Nanometer-sized clusters, films, rods of nickel, cobalt, and iron have attracted great interest in the modern study of magnetic materials, with which exotic devices, such as microcapsules,1 switches,2and high-density data storage, are fabricated.3
5 The chemical and physical properties of these nanomaterials can vary with their atomic structure, composition, and dimension.6
10 In addition to the traditional means of X-ray and electron diffraction techniques,11,12 scanning tunneling microscopy (STM), renowned for its molecular resolution capability, has been used to examine the spatial and electronic structures of nanometersized cobalt features.13 Of close relevance to this study is the deposition of cobalt on copper surfaces, which has been examined intensively in the past two decades.14
19 Cobalt deposition normally starts with the production of bilayer or trilayer nuclei whose lowest layer becomes submerged in the copper substrate.14,15,17 It is shown that some Cu atoms in the uppermost layer diffuse to the perimeters and tops of Co nuclei as a way to lower the surface energy of the Co-on-Cu system. The first few layers of Co deposit follow the face-centered cubic (fcc) stacking habit of the Cu substrate, but thicker Co deposit gradually transforms to the hexagonal close-packed (hcp) arrangement known for the bulk Co.17 r 2011 American Chemical Society

Electrodeposition of cobalt has found applications in corrosion prevention,20 fuel cells,21 and nanomaterial fabrication.6,22,23 The standard reduction of cobalt is
0.25 V (versus normal hydrogen electrode), suggesting that this reaction would be coupled with hydrogen evolution in acidic solution. This concern prompted research into Co deposition in nonaqueous media.20,24 Electrodeposition of Co and Ni on gold single-crystal electrode are examined to reveal the possibility of making smooth Ni and Co films on Au(111) electrode in chloride-containing electrolyte.25
29 The magnetization direction of the as-formed Co deposit could change with its thickness.26,28,29 By contrast, Co deposit on Au(100) grows in three-dimensional (3D) clusters, whose orientations depend on the optimal lattice match between the Co deposit and the Au(100) substrate.12 While Co deposition on Cu surfaces has been examined extensively in vacuum, electrodeposition of Co on ordered Cu electrodes has received little attention. This study employed in situ STM to explore electrodeposition of Co on a Pt(111) electrode precoated with a Cu(111)-like thin film. It is worth Received: April 14, 2011 Revised: October 22, 2011 Published: October 30, 2011 23802

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noting that an artificial Co/Cu multilayer has been used to fabricate spin valves, the key device used to produce high-density hard drives.30,31 STM results obtained here indicate that electrodeposition of Co on the Cu(111) film could proceed in layers to produce a smooth Co film up to 12 layers thick. Substantial strains likely existed at the Co/Cu interface, which resulted in different atomic structures for the first three layers of the Co deposit.

2. EXPERIMENTAL SECTION The Pt(111) electrodes used for the STM and voltammetric experiments were homemade by melting one end of a polycrystalline Pt wire (o.d. 0.08 cm) with a mixed flame of H2 and O2. Details of this process were already reported.32,33 Typically, the pretreatment of a Pt(111) bead electrode involved annealing by a hydrogen torch and quenching in hydrogen-saturated Millipore water. It was then mounted onto the STM cell, which was equipped with Pt wires as the counterelectrode and quasireference electrode.34
36 All potentials reported here are converted to the scale of a Ag/AgCl electrode. The potential was scanned at 50 mV/s from the open-circuit potential (OCP) at ∼0.7 to
0.3 V in a 0.1 M KClO4 + 1 mM HCl (pH 3) medium. This procedure is needed to reduce thermally formed oxide on the Pt(111) electrode. Enough Cu(ClO4)2 was added to make the final [Cu2+] = 1 mM, and the potential was scanned to the onset potential for Cu deposition. The time for Cu deposition was controlled to ensure that a certain amount of Cu was deposited on the Pt(111) electrode. The electrolyte was then replaced with 0.1 M KClO4 + 1 mM HCl (pH 3) + 0.04 M CoCl2 under potential control at
0.15 V, where the copper deposit would be kept in a reduced state. The STM scanner was a typical A-head (Veeco, Santa Barbara, CA) with a maximal scan size of 500 nm. The tip was a tungsten tip etched by AC in 6 M KOH. Nail polish was applied to coat the whole tip for insulating purpose. The potential of the tip electrode and the feedback current were set normally at 0.4 V and 1 nA, respectively. Since Co deposition was visualized by the STM continuously, we could literally count the number of Co layers being deposited on the Cu substrate. In addition, pits or valleys frequently appeared next to Co deposit. These features could serve as references to estimate the thickness of Co deposit. The electrochemical cell used for voltammetry had a threeelectrode configuration, including a Ag/AgCl reference electrode and a Pt counterelectrode. The potentiostat was a CHI 600 (Austin, TX). Suprapure perchloric acid (HClO4) and hydrochloric acid (HCl) were purchased from Merck (Darmstadt, Germany). Potassium chloride (KCl) and cobalt chloride hexahydrate (CoCl2 3 6H2O) were obtained from Riedel-deHa€en (Honeywell). Triple-distilled water (Lotun Technology Co., Taipei) was used to prepare the electrolytes. Copper perchlorate [Cu(ClO4)2] and KClO4 was purchased from Alfa Aesar (Ward Hill, MA). All chemicals were used as received. 3. RESULTS 3.1. Cyclic Voltammetry. Shown in Figure 1 are cyclic voltammograms (CVs) recorded with a Pt(111)-supported copper film electrode immersed in 0.1 M KClO4 + 1 mM HCl (pH 3), without (red trace) and with (black trace) 0.04 M CoCl2. The copper film was ∼10 layers in thickness, estimated from the amount of charge involved in the stripping peak (not shown).

Figure 1. Cyclic voltammograms recorded at 50 mV/s with a Pt(111) electrode precoated with a Cu film in 0.1 M KClO4 + 1 mM HCl (pH 3) without (red) and with (black) 0.04 M CoCl2. Both profiles have a reduction peak, A and A0 , near
0.65 V, but only the black trace has a peak, B0 , at
0.77 V due to the reduction of Co2+. The oxidation peak at
0.25 V (D0 ) resulted from stripping of the Co deposit.

Figure 2. (a) Topo and (b) atomic-resolution STM images recorded with a Pt(111) electrode precoated with a Cu film at
0.1 V in 0.1 M KClO 1 mM HCl (pH 3). The structure seen in panel b is assigned as √ 4 +√ ( 3  3)R30
-chloride, represented by the ball model shown in panel c, where red and black circles represent chloride and Cu atoms, respectively. Scale bars are (a) 75 and (b) 1.2 nm.

The red trace was recorded as the potential was scanned between
0.1 and
1.2 V at 50 mV/s. The broad and irreversible peak at
0.65 V (A) could arise from reduction of proton (2H+ + 2e
f H2), followed by a more precipitous increase of current (B) due to reduction of water (2H2O + 2e
f H2 + 2OH
). The adsorption/desorption of chloride at the Cu film electrode did not yield a well-defined feature in this CV profile, as noted with a bulk Cu(111) electrode.37 Meanwhile, it is important to note that reduction of water did not occur until
1 V, a characteristic helpful to the deposition of Co at the Cu film electrode. The black trace was obtained similarly, and it reveals a broad peak at
0.65 V (A0 ), followed by another broad peak centered at
0.77 V (B0 ), before the current increased rapidly at
0.9 V (C0 ). These features are thought to arise from reduction of proton (A0 ), Co2+ (B0 ), and water (C0 ), respectively. The subsequent positive potential scan from
0.95 to
0.1 V produced only a broad peak at
0.25 V (D0 ), which resulted from stripping of the Co deposit. The deposition and stripping peaks of Co were separated by 0.52 V, indicating poor reversibility of these processes, mainly because Co2+ reduction was sluggish. Chloride did not seem to catalyze Co2+ reduction, as it does for the reduction of Cu2+.38 These results are similar to those found with Cu(111) and Au(111) electrodes in weakly acidic media (pH 3
5). At Cu(111) and glassy carbon electrodes, a crystallization overvoltage for Co deposition is noted.26,27,39 STM results obtained with Au(111) in 10 mM K2SO4 + 1 mM KCl + 1 mM H2SO4 reveal layered Co deposition, regardless of hydrogen evolution.28 23803

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Figure 4. STM images with successively finer resolution (from a to c), showing internal atomic structures of the first layer of cobalt deposited on a Pt(111) electrode precoated with a copper film at
0.81 V in 0.1 M KClO4 + 1 mM HCl (pH 3) + 0.04 M CoCl2. The atomic image shown in panel c was Fourier-transform-filtered. The arrows marked in panel c denote the 110 axis of Pt(111). Scale bars in panels a
c are 20, 14, and 4 nm, respectively. Figure 3. Time-dependent in situ STM images recorded consecutively in 2
3 min intervals with a Pt(111) electrode predeposited with a Cu film at
0.70 V (a
d) and
0.81 V (e, f) in 0.1 M KClO4 + 1 mM HCl + 0.04 M CoCl2. The arrows marked in panel a show the 110 axis of the Pt(111) substrate. Regions exhibiting different shades are numbered 0, 1, and 2 in panel f to denote the copper substrate and the first and second layers of the cobalt deposit, respectively. The scan sizes are all 206  206 nm. The inset in panel a is a 20  20 nm STM scan obtained under imaging conditions of 130 mV and 1 nA.

3.2. In Situ STM Imaging of Chloride Adsorbed on Pt(111) Precoated with a Cu Film. We first present in situ STM results

obtained with the copper film deposited on the Pt(111) electrode by holding its potential at
0.1 V for 10 min in 0.1 M KClO4 + 1 mM HCl (pH 3) + 1 mM Cu(ClO4)2. The amount of the as-deposited Cu was estimated from the amount of charge (not shown) associated with the subsequent stripping process. Typically, this yielded a 10-layer-thick Cu film, whose morphology seen in Figure 2a had well-defined terraces and monatomic steps (Δz = 0.23 nm), features of the Pt substrate also. Steps were frequently aligned in the 110 direction or the close-packed atomic directions of the Pt(111) substrate. On the other hand, pit or valleys presumably due to strains at the Cu/Pt(111) interface were always present. The higher-resolution STM scan (Figure 2b) resulted in an ordered hexagonal array with spots aligned in the 121 directions of the Pt(111) substrate. Two nearest neighbors are separated by 0.44 from each other. √ This ordered structure could fit the √ nm √ ( 3  3)R30
-Cl (or 3 in short) superlattice found on Cu(111).40 Although the lateral structure of the underlying √ Cu substrate could not be imaged directly by the STM, this 3 adlattice implies that it could be like Cu(111) with an in-plane spacing of 0.25 nm. Electrodeposition of Cu on Pt(111) has been examined, but researchers have focused attention on the structure of a monolayer deposit.35,41,42 The first Cu layer was found to be pseudomorphic; whereas the structure of multilayer Cu is not known. 3.3. Real-Time STM Imaging of Cobalt Electrodeposition on Cu Film. With the Cu film deposited on the Pt(111) electrode, the electrolyte was replaced with 0.1 M KClO4 + 1 mM HCl (pH 3) + 0.04 M CoCl2 under potential control at
0.1 V. The potential was then shifted negatively in 10 mV steps to induce deposition of Co. As revealed by the STM images shown in Figure 3a
d, shifting the potential from
0.69 to
0.70 V resulted in a marked increase of spots on an originally smooth Cu substrate. As judged from the locations of the step

ledges seen in these images, there was a slight upward drift in this experiment. Protruded dots popping up over a time span of ∼5 min are ascribed to nuclei of Co. They appeared preferentially at the lower terrace, rather than the two step ledges. We speculate that pits and impurities on the Cu substrate could act as nucleation sites, which could be distributed unevenly. A portion of the Co nuclei is highlighted by a high-resolution STM image shown as the inset in Figure 3a. They are mostly 1
3 nm wide and 0.20 nm in height. Hydrogen gas could be evolving in the course of STM imaging, but it had little effect on STM imaging. Interfacial mixing at the Co deposit and Cu substrate has been reported in vacuum. This phenomenon manifests in the formation of pits on a Cu(111) substrate upon Co nucleation, as some Cu atoms in the uppermost layer diffuse to the perimeters of Co nuclei.16,18 By contrast, STM images obtained here reveal a mostly smooth Cu substrate; no pits were found after Co nucleation. This difference indicates that interfacial mixing occurs in vacuum but not in electrochemistry, at least at the initial stage of deposition. The potential (
0.70 V) applied here could only yield nucleation of Co. To accelerate cobalt deposition, we shifted the potential further negative to
0.81 V, resulting in immediate changes at the electrode. In particular, as revealed by the sequenced STM images shown in Figure 3d
f, cobalt nuclei expanded laterally and isotropically, followed by coalescence to produce a smooth Co deposit. The second Co layer nucleated before the first layer was completed, which reflected quasi-layerby-layer growth of Co. Shown in Figure 4 are finer-resolution STM scans obtained with the first Co layer by use of 100 mV bias voltage and 1 nA tunneling current. The most distinct features seen here are the double-lined patterns disoriented by 60
from one another. They appeared to run parallel to the 121 axis of the Pt(111) substrate. The amplitude of the intensity modulation is ca. 0.035 nm. Paired lines are separated by 1.5 nm, and two neighboring pairs are 1.8 nm apart. This double-lined pattern is morphologically similar to reconstructed Au(111) and Pt(111).43,44 It was also seen with a monolayer of Cu on Ru(0001)45 and Co on Pt(111).46 The atomicresolution STM image shown in Figure 4c is somewhat distorted, revealing a pseudohexagonal array with a nearest-neighbor spacing of 0.36 nm. (Two atomic rows enclosed an angle of 60
( 5
.) It is likely that this is chloride adlattice, because the supporting electrolyte contained 1 mM HCl. The second Co layer emerged before the first layer was completed. It grew faster than the first Co layer, enabling deposition of bilayer Co, seen in the 200  200 nm STM scan shown in Figure 5a. In addition to real-time STM imaging of the 23804

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Figure 5. STM images showing (a, b) morphology and (c, d) internal atomic structures of the second Co layer (marked 2) on Pt(111) precoated with a Cu film. The image shown in panel d was a filtered version of that in panel c. Scale bars in panels a
d are 57, 21, 2.1, and 2.1 nm, respectively.

deposition event, the height of the Co deposit could be measured to infer its thickness. In this case the Co deposit was 0.4 nm higher than a neighboring terrace, suggesting a bilayer Co deposit. The bilayer cobalt deposit could span hundreds of nanometers, but its perimeters were always coarse, as also seen with the first and the thicker Co film (Figure 4a). It appears that there was essentially no preferred atomic structure at step sites, which contrasts markedly with Co deposition on Pt(111) in vacuum, as sharp and straight steps were typical.46 Finer resolution STM scans were obtained at a terrace site to reveal the atomic structures there (Figure 5b
d). Figure 5b shows a 60  60 nm STM scan, revealing the prominent 2D ring pattern punctured by pits on the second layer of the Co deposit. The internal atomic structure of this pattern was discerned by atomic-resolution STM imaging, represented by riginal and filtered STM images shown in Figure 5c,d. As seen with the first Co layer in Figure 4c, an ordered pseudohexagonal array with interatomic spacing of 0.36 nm was imaged, but it lacked a periodic modulation of intensity. The maximal corrugation height among atomic features is 0.035 nm, comparable to that seen in the first Co layer. This hollow ring pattern was not found in the cases of Co deposited on Pt(111)46 and Au(111),28 but it resembles the third layer of Cu deposited on Ru(0001)45 and Cuinduced reconstruction of Pt(111).47 Holding potential at
0.81 V enabled continuous deposition of Co. Shown in Figure 6a is an STM image collected over a region where the second (marked 2) and third (marked 3) layers of Co coexisted. The internal atomic structure of the third Co layer is revealed by a higher-resolution scan shown in Figure 6b, featuring a distinct, long-range modulation of intensity or socalled “moir
e pattern” along with aggregates of triangular pits. Both features were aligned in the 110 direction of the Pt(111) substrate, and they were in register with one another.

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Figure 6. In situ STM images collected over an area coated with (a, b) 2
3 layers or (c, d) 9
10 layers of Co, as denoted by the marked numbers. Experimental conditions were the same as those of Figure 4. Scale bars in panels a
d are 24, 12, 30, and 30 nm, respectively.

All protruded mounds and depressed triangles were equal in dimension. Depressed triangles could distribute sparsely or aggregate into 2D arrays. The surface morphology seen in Figure 6a indicates layered Co deposition, which prevailed until the 12th layer. Then 3D growth kicked in. Both the moir
e pattern and the triangular pit were seen in the 3rd to ∼10th layers of the Co film. However, since these structures remained mostly unchanged between the 3rd and 12th layers of Co, only STM images obtained with the 9th and 10th Co layers are described here (Figure 6c,d). The population of the moir
e pattern and the pit varied with the thickness of Co deposit. It seems that the number of mounds increased at the expense of pits. This phenomenon is illustrated by a series of STM images acquired as the Co deposit grew from the 9th to the 10th layer (these results are shown in the Supporting Information). We noted that the moir
e pattern had the same periodicity of 3.5 nm, but the amplitude of intensity modulation decreased with thicker Co deposit. Shown in Figure 7a,b are atomic-resolution STM images obtained at different regions on the third layer of the cobalt deposit. The long-range modulation of atomic intensity or the moir
e pattern seen in Figure 7a undulated in the 110 directions of the Pt(111) substrate periodically in every 3.5 nm. In essence, this structure is hexagonal with a nearest-neighbor spacing of 0.36 nm, ascribable to chloride adatoms adsorbed on a Co(0001)-like substrate with an in-plane atomic spacing of 0.25 nm. The close-packed atomic rows of chloride are rotated from the Æ110æ direction of Pt(111) by 30
. Figure 7b is a filtered STM image obtained over a pit-laden region. A section plot (Figure 7c) along the dotted line marked in Figure 7b reveals the atomic corrugation profiles. The pit was 0.04 nm lower and the mound was 0.06 nm higher than the terrace on which they were found. These values are notably smaller than 0.2 nm, the anticipated step height of a monatomic 23805

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Figure 8. Time- and potential-dependent STM images recorded at (a)
0.5 V, (b
e)
0.45 V, and (f)
0.15 V. The time differences from b to c, c to d, and d to e are 1, 2, and 3 min, respectively. These STM images reveal dissolution of a 12-layer-thick cobalt film (estimated from the depth of the valley c marked in panel a), yielding Cu substrate decorated by Co nuclei. The inset in panel a is a 10  10 nm STM scan revealing an ordered interface. Scale bars are (a
e) 25 and (f) 75 nm. Figure 7. (a, b, d) Atomic-resolution STM images acquired at the third layer of cobalt deposit under the conditions used for Figure 4. The crosssection profile shown in panel c reveals corrugation along the dotted line in panel b. The STM image shown in panel d highlights the atomic structure formed in the triangular pits. Scale bars are (a, b) 3 and (d) 1 nm.

step of the Co deposit, which means that difference in the thickness of Co deposit was not responsible for these features. Tentatively, they arose from stacking faults in the Co deposit, as established for the reconstructed Au(111) surface43 and the epitaxial Co film deposited on Pt(111).46 A further finer STM scan shown in Figure 7d reveals atomic features inside the pits. Layered-type Co deposition ended at the 12th layer of Co. The as-formed Co film is mostly smooth, although there were deep valleys, as revealed by the topo and atomic-resolution STM scans shown in Figure 8a. The thickness of the cobalt deposit estimated from the depth (∼2.4 nm) of the crater C is 12 layers. The width of the terrace spanned hundreds of nanometers in width, which could represent the grain size of the cobalt deposit. Co deposition beyond the 12th layer was also examined. A much rougher surface morphology reflects 3D Co deposition. The process of Co stripping triggered by positive shifts of potential was also imaged with the STM. A series of potentialdependent STM images were recorded and shown in Figure 8. The first image (Figure 8a) was obtained at
0.5 V, where the Co deposit was stable. The subsequent STM images shown in Figure 8b
e were recorded after the potential was shifted positively to
0.45 V. The Co film appeared to dissolve by retreats of steps in a time span of 10 min, revealing wide terrains and curvy step ledges. Crater c gradually opened up to unveil a stack of Co layers. These surface features were also seen in the deposition process, which suggests that the processes of dissolution and deposition of cobalt were reversible at the microscopic scale. Dissolution of a Co(0001) single-crystal electrode was examined in 0.05 M Na2SO4, pH 3.48 The last image (Figure 8f) was collected 20 min after the potential was changed to
0.15 V. At this stage of STM imaging

the Co deposit was mostly removed from the electrode, unveiling mainly the Cu film on the Pt(111) electrode. Although quite a few Co clusters remained, well-defined terraces and steps were evident. The nanometer-sized Co clusters could be removed by shifting potential to
0.10 V. Judged from the well-defined surface morphology of the Cu film, it seems that the 12 layers of Co deposit did not alter the texture of the Cu film, and Co and Cu did not mix under electrochemical conditions.

4. DISCUSSION Although the STM was imaging the chloride adlayers or an oxide phase throughout the deposition of Co, we contend that the atomic structure of the underlying Co film can be deduced from the atomic STM images shown in Figures 4
7. It would be useful to compare STM results obtained here with what has been found with Co(0001). Unfortunately, the spatial arrangement of chloride adatoms on Co(0001) is not known. On the other hand, a great many studies have addressed √ √the adsorption of chloride on coinage metals, yielding a ( 3  3)R30
structure on Cu(111) and Au(111)40,49 and a complicated (17  17) structure on Ag(111).50 The point is that none of these structures exhibits an STM appearance the same as the doublelined and hollow-ringed patterns seen in this study (Figures 4c and 5). Given the complexity of these structures identified by the STM, it is unlikely that the Co deposit adapted a pseudomorphic structure on the Cu(111) thin film. Thus, we contend that the buckled atomic structures presented in Figure 4c and 5 could be features due to the Co deposit, rather than the chloride adlayer.50 The morphology of the double-lined pattern seen with the first Co layer (Figure 4c) resembles those of reconstructed Au(111)43 and Pt(111).44,51 These structures have very different physical dimensions, as summarized in Table 1. The reconstructed Au(111) surface is shown to derive from a 4% uniaxial compression in the 121 direction of the substrate, resulting in registries of symmetric fcc and hcp 3-fold hollow sites and asymmetric types of sites for gold atoms in the uppermost layer. In analogy, the first Co adlayer deposited on the Cu(111)-like substrate could be considered as a layer of Cu atoms arranging in a compressed 23806

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Table 1. Average Physical Dimensions of the Paired Lines Seen with Reconstructed Au(111)43 and Pt(111)44 and the Deposited Co Film Prepared in This Studya Au(111)

Pt(111)

Co film

dr r (nm)

4.4

2.3

1.5

Δz (nm)

0.02

0.025

0.035

dr r and Δz refer to the spacing between paired lines and their corrugation heights, respectively.

a

Figure 9. Two ball models illustrating the evolution of moir
e patterns from two stacked hexagonal networks of Co with the same dimension. The two Co networks are rotated from each other by (a) 3
and (b) 4
, respectively.

structure on Cu(111), which rendered fcc, hcp, and asymmetric registers. This disparity in atomic register is the reason for the buckled atomic features seen with the Co deposit. Cobalt atoms residing at asymmetric types of sites would appear brighter than those adsorbed at hollow sites in the STM image. The doublelined pattern seen in Figure 4c derived from a random nucleation-and-growth process, making it difficult to establish longrange ordering. This stacking fault model can be applied also to explain the hollow-ringed pattern seen with the second Co adlayer. First, given the rather different STM appearances of the first and second Co layers, it would not be possible to have a hollowringed pattern sitting on a double-lined structure of the first Co layer. Thus, the first Co layer could rearrange into a pseudomorphic structure on the Cu(111) substrate if it was capped by an additional Co layer. The STM appearance of the hollow-ring pattern resembles that of trilayer Cu deposited on Ru(0001), where the Cu deposit is thought to be isotropically contracted.45 This view could apply to the present Co-on-Cu system, where Co atoms in the second layer sat at fcc and hcp sites on a Cu(111) substrate. It is, however, impossible to assign registries of Co adatoms in the second layer seen by the STM (Figure 5d). Starting from the third layer, a moir
e pattern intermixed with triangular pits appeared. This kind of moir
e pattern has been frequently observed with heteroepitaxial systems, where adlayer has a lattice constant smaller than that of the substrate; for example, Co on Au(111),27,28 Cu on Ru(0001),45 Co on Pt(111),46 etc. It is, however, not seen with Co deposited on Cu(111) in vacuum.16,17 To reconcile the moir
e pattern seen in this study, we consider the second and third Co layers as two stacked hexagonal lattices presumed to have the same lattice constants (dnn = 0.249 nm). If the close-packed atomic rows of these two lattices are aligned in parallel, operations of lateral shift of one with respect the other would not yield a moir
e pattern. However, rotating one plane with respect to the other by just a

few degrees resulted in moir
e patterns nearly the same as that shown in Figure 7a. For example, the model shown in Figure 9a results from of a 3
in-plane rotation, yielding a ∼20 nm modulation of intensity. However, a 4
rotation shown in Figure 9b shrinks the moir
e pattern to 3.5 nm. The latter is essentially identical to that found by STM (Figure 7a). The uppermost chloride adlayer is omitted to simplify the model. However, it is made clear that adding a hexagonal chloride adlattice (dnn = 0.36 nm) on the ball model would not alter the appearance of the moir
e pattern. Meanwhile, we also tried to superimpose a hexagonal chloride adlattice (dnn = 0.36 nm) on a hexagonal Co(0001) lattice (dnn = 0.249 nm) and found no moir
e pattern. On the other hand, we cannot ensure that the ball model shown in Figure 9b is a unique representation of the moir
e pattern seen in Figure 7a. Furthermore, we emphasize that the model proposed in Figure 9b does not mean that each Co layer would be rotated successively by 4
after the third layer. Rather, it is thought that only the uppermost layer of Co was rotated by 4
, and it could become aligned with the Cu(111) substrate once it was buried by another layer of Co. This idea of restructuring is conceivable, as the moir
e pattern seen at the third layer cannot be obtained by superimposing a Co(0001) plane on a complicated Co bilayer seen in Figure 5d. It is possible that the first and second layers of Co could adapt the structure of Cu(111) once they were covered by the third layer of Co. Unfortunately, the STM cannot probe the atomic structure of a buried interface. Other means such as surface X-ray scattering and reflection high-energy electron diffraction (RHEED) are more suitable for this task. The structure of Co deposited on Cu(111) has been extensively examined in vacuum.14
17 It is worth noting that none of the atomic structures seen in this study has been reported. This inconsistency likely arises from the fact that the electrified interface is inherently more complicated than the gas
solid interface, because of potential control and adsorbed anions on the electrode surface. As an example, Co films produced in SCN
- and Cl
-containing electrolytes exhibit rather different textures.29 Electrodeposited Co examined here seemed to have a minimal mixing with the Cu substrate, as opposed to a rather pronounced alloying found in vacuum at a temperature as low as 25
C.17,18 These differences speak to the crucial role of interfacial environment in determining how filmy materials are structured. Finally, chloride played a pivotal role in the present system, because without chloride the multilayer Cu film was mostly rough on Pt(111). Adsorbed chloride on the Co deposit could lower the interfacial energy to an extent that restructuring of the Cu substrate was inhibited.

5. CONCLUSION Electrodeposition of Co on a Pt(111) electrode precoated with a copper thin film was examined by real-time in situ STM imaging. The first 12 layers of Co were deposited via a nucleation-and-growth growth mechanism, where defects on the substrate acted as nucleation sites for Co adatoms. Subsequent Co deposition resulted in lateral expansion and coalescence of nuclei to produce a smooth cobalt deposit. The atomic structures of the first two layers of the Co deposit are identified respectively as double-line and hollow-ring patterns, which differ distinctively from those of the 3rd
12th layers of Co. From the 3rd to the 12th layer, a moir
e pattern emerged as the most important structure. The population of the moir
e pattern increased, while 23807

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The Journal of Physical Chemistry C the amplitude of intensity modulation decreased, as the amount of Co deposit increased. This evolution of the atomic structure of the Co deposit is indicative of a progressive relaxation of stress at the interface of Co and Cu. These results differ markedly from those observed in vacuum, thereby highlighting the critical roles played by potential control and adsorption of anions in guiding the growth mode and atomic structure of artificial metallic thin films.

’ ASSOCIATED CONTENT

bS

Supporting Information. One figure, showing time-dependent STM images obtained in the course of deposition of 9th and 10th Co layers on a Pt(111) electrode precoated with a Cu(111) film. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected], tel 886-3-4279573, fax 886-3-4227664 (S.-L.Y); E-mail [email protected], tel 886-2-77346010, fax 886-2-29326408 (J.-S.T.).

’ ACKNOWLEDGMENT We thank Professor C. C. Su (Institute of Organic and Polymeric Materials, National Taipei University of Technology) for technical help. Financial support was provided by the National Science Council of Taiwan (NSC 99-2113-M-008-001). ’ REFERENCES (1) Lu, Z.; Prouty, M. D.; Guo, Z.; Golub, V. O.; Kumar, C. S. S. R.; Lvov, Y. M. Langmuir 2005, 21, 2042. (2) Weber, W.; Back, C. H.; Bischof, A.; Pescia, D.; Allenspach, R. Nature 1995, 374, 788. (3) Song, Q.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 6164. (4) Thompson, D. A.; Best, J. S. IBM J. Res. Dev. 2000, 44, 311. (5) Chappert, C.; Fert, A.; Van Dau, F. N. Nat. Mater. 2007, 6, 813. (6) Ergeneman, O.; Sivaraman, K. M.; Pan, S.; Pellicer, E.; Teleki, A.; Hirt, A. M.; Bar, M. D.; Nelson, B. J. Electrochim. Acta 2011, 56, 1399. (7) Rafaja, D.; Schimpf, C.; Schucknecht, T.; Klemm, V.; Peter, L.; Bakonyi, I. Acta Mater. 2011, 59, 2992. (8) Mangen, T.; Bai, H. S.; Tsay, J. S. J. Magn. Magn. Mater. 2010, 322, 1863. (9) Haciismailoglu, M. S.; Alper, M.; Kockar, H. J. Electrochem. Soc. 2010, 157, D538. (10) Tsay, J. S.; Chen, Y. T.; Cheng, W. C.; Wang, K. C.; Yao, Y. D. Phys. Status Solidi B 2007, 244, 4507. (11) Ferrer, S.; Alvarez, J.; Lundgren, E.; Torrelles, X.; Fajardo, P.; Boscherini, F. Phys. Rev. B 1997, 56, 9848. (12) Wu, H.; Zei, M.; Yau, S. J. Phys. Chem. C 2011, 114, 20062. (13) Rastei, M. V.; Heinrich, B.; Limot, L.; Ignatiev, P. A.; Stepanyuk, V. S.; Bruno, P.; Bucher, J. P. Phys. Rev. Lett. 2007, 99, No. 246102. (14) de la Figuera, J.; Prieto, J. E.; Ocal, C.; Miranda, R. Phys. Rev. B 1993, 47, 13043. (15) Rabe, A.; Memmel, N.; Steltenpohl, A.; Fauster, T. Phys. Rev. Lett. 1994, 73, 2728. (16) de la Figuera, J.; Prieto, J. E.; Kostka, G.; Muller, S.; Ocal, C.; Miranda, R.; Heinz, K. Surf. Sci. 1996, 349, L139. (17) Muller, S.; Kostka, G.; Schafer, T.; de la Figuera, J.; Prieto, J. E.; Ocal, C.; Miranda, R.; Heinz, K.; Muller, K. Surf. Sci. 1996, 352
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dx.doi.org/10.1021/jp2078083 |J. Phys. Chem. C 2011, 115, 23802–23808