pubs.acs.org/Langmuir © 2009 American Chemical Society
Electrolytic Gold Deposition on Dodecanethiol-Modified Gold Films Gyana Pattanaik,† Wenbo Shao,† Nathan Swami,‡ and Giovanni Zangari*,† †
Department of Materials Science and Engineering and Center for Electrochemical Science and Engineering and ‡Department of Electrical and Computer Engineering, University of Virginia, P.O. Box 400745, Charlottesville, Virginia 22904-4745 Received November 26, 2008. Revised Manuscript Received January 30, 2009
The electrochemical nucleation and growth of Au from a Au sulfite electrolyte onto dodecanethiol-modified Au surfaces is investigated by a combination of microscopy and chronoamperometry methods. The self-assembled dodecanethiol monolayers are continuous but exhibit defects in correspondence of the Au grain boundaries and on top of Au terraces. Nucleation of Au films occurs initially at these defect sites, but only a small fraction of these nuclei survive an initial competition process. The remaining nuclei expand through three-dimensional progressive nucleation followed by diffusion-limited growth. The resulting Au films exhibit microstructures which are widely different from those observed in the electrochemical growth of Au on Au and that depend on the applied potential: while at low overpotentials the film grows as an assembly of hemispherical clusters, at intermediate overvoltages the films are smooth and at high overvoltages become dendritic. Metal growth onto self-assembled monolayer-modified substrates can thus provide an alternative method for controlling film morphology for a wide range of applications.
Introduction The deposition of metallic layers onto surfaces modified with organic self-assembled monolayers (SAMs) finds various applications in device fabrication for contacts1,2 and interconnects,3,4 as well as for the formation of metallic clusters for electrocatalysis, charge quantization,5 localization of plasmonic effects,6 and surface-enhanced Raman spectroscopy.7 Electrochemical deposition (ECD) of metallic layers on SAMs is commonly utilized for these purposes. In device fabrication applications, it is necessary that metal penetration through the SAM be avoided; successful strategies for preventing this use selective metal complexation on top of the SAM,8 followed by electroless deposition in an appropriate plating solution,9 or ECD in a solution devoid of metal ions.10 In all other cases, ECD generally results in metal penetration through the SAM.11 Cluster formation, on the other hand, requires the penetration of metal through the SAM to be carefully controlled so that the morphology of the metallic overlayer can be modulated by balancing the influence of surface chemistry of the SAM over that of the substrate surface. ECD on SAMs can enable this modulation by controlling the relative rate of nucleation due to metal penetration at SAM-defect sites versus diffusion-limited growth on top of the SAM layer. In this way, a variety of shapes and patterns of nanoclusters can be generated,12 *Corresponding author. E-mail:
[email protected]. (1) Hipps, K. K. Science 2001, 294, 536–537. (2) McCreery, R. L. Chem. Mater. 2004, 16, 4477–4496. (3) Murthy, B. R.; Yee, W. M.; Krishnamoorthy, A.; Kumar, R.; Frye, D. C. Electrochem. Solid-State Lett. 2006, 9(7), F61–F63. (4) Ganesan, P. G.; Singh, A. P.; Ramanath, G. Appl. Phys. Lett. 2004, 85(4), 579–581. (5) Hicks, J. F.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 13322. (6) Ozbay, E. Science 2006, 311, 189–193. (7) Wang, J.; Zhu, T.; Song, J. Q.; Liu, Z. F. Thin Solid Films 1998, 327, 591. :: (8) Baunach, T.; Ivanova, V.; Kolb, D. M.; Boyen, H.-G.; Ziemann, P.; Buttner, M.; Oelhafen, P. Adv. Mater. 2004, 16, 2024–2028. (9) Camacho-Alanis, F.; Wu, L.; Zangari, G.; Swami, N. J. Mater. Chem. 2008, 18, 5459–5467. (10) Ivanova, V.; Baunach, T.; Kolb, D. M. Electrochim. Acta 2005, 50 4283–4288. (11) Schneeweiss, M. A.; Hagenstrom, H.; Esplandiu, M. J.; Kolb, D. M. Appl. Phys. A: Mater. Sci. Process. 1999, 69, 537–551. (12) Bittner, A. M. Surf. Sci. Rep. 2006, 61, 383–428.
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and novel nanofabrication processes can be developed.13 From a different perspective, the SAM modifies the surface properties of the electrode, enabling control of film morphology in a manner complementary to what can be achieved through the use of additives.14,15 The application of ECD methods in the prior work as described above can be greatly improved through a better understanding of the fundamental processes occurring during the electrochemical growth on SAM-modified conducting electrodes. For example, previous work has failed to correlate the grain size and morphology of the metallic overlayer to its nucleation and growth mode or to its growth kinetics. In this work, we study metal nucleation and growth onto SAM-modified electrodes using the electrodeposition of Au from a neutral electrolyte onto Au surfaces modified by moderately long chain (1.87 nm) dodecanethiol {C12 [CH3(CH2)10CH2SH]} SAMs. We chose this system to avoid potential complications resulting from (i) nucleation on a foreign substrate, where three-dimensional nucleation may result from surface energy or lattice mismatch effects, not necessarily from nucleation at SAM defects, or (ii) underpotential deposition (UPD) processes,16 where infiltration and reduction of metal species below the SAM complicate the interpretation of results. The initial stages of nucleation of Au clusters on the SAMmodified surface are studied through a combination of microscopy methods at various length scales and by potentiostatic current transients. The nucleation and growth kinetics are additionally studied as a function of the applied electrochemical potential to determine the growth mode, as well as the morphology of thick Au electrodeposits. We find that at an early growth stage the nuclei grow at SAM defect sites; successively, the diffusion zones feeding the growth of the various nuclei overlap, and nuclei coalesce or terminate their growth, leading to the formation of a lower density of Au clusters that eventually (13) Schilardi, P. L.; Dip, P.; Claro, P. C. D.; Benitez, G. A.; Fonticelli, M. H.; Azzaroni, O.; Salvarezza, R. C. Chem.;Eur. J. 2006, 12, 38–49. (14) Oniciu, L.; Muresan, L. J. Appl. Electrochem. 1991, 21, 565–574. (15) Franklin, T. C. Surf. Coat. Technol. 1987, 30, 415–428. (16) Silien, C.; Buck, M. J. Phys. Chem. C 2008, 112, 3881–3890.
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form a continuous film. The microstructure of such film is strongly dependent on the potential applied, leading to a variety of morphologies of potential interest in diverse applications.
Experimental Section Substrates. Gold films with a 100 nm thickness were evaporated on a Si wafer previously sputtered with a Ti adhesion layer 10 nm thick. The Au-coated wafer was cleaned in a piranha solution (a 3:1 mixture of H2SO4 and 30% H2O2), thoroughly rinsed in Milli-Q (18.2 MΩ cm) water, dried in a stream of nitrogen gas, and then cut into 1 cm 1 cm substrates. Caution: The piranha solution is a strong oxidant and reacts violently in presence of organic reagents. It should be handled with extreme care. SAM Assembly and Characterization. The Au film substrates were ultrasonically cleaned in ethanol (Suprapure, Aldrich) before being immersed in a 1 mM ethanolic solution of the dodecanethiol [CH3(CH2)11SH] (Aldrich) for 48 h for self-assembly of the molecule. The samples were then thoroughly rinsed in ethanol and dried in a stream of nitrogen gas. Cyclic voltammetry (CV) and ex situ scanning tunneling microscopy (STM) experiments were conducted on the SAMmodified Au substrates to verify the formation of a compact and uniform monolayer. Cyclic voltammograms were recorded on bare and SAM-covered Au substrates in an aqueous electrolyte containing 100 mM KNO3 and 1 mM K3Fe(CN)6 (Alfa Aesar). A saturated calomel electrode (SCE) was used as a reference electrode in all electrochemical experiments in this study, except in the in situ electrochemical STM (EC-STM) experiments, where a Au-coated Pt wire quasi-reference electrode was used. In the Au electrolyte, the potential of the wire showed a potential of 0 V versus SCE, reproducible within few millivolts before and after the experiments. All the potential values in this study are reported with respect to SCE, unless otherwise mentioned. Electrochemical experiments were controlled and recorded using an EG&G Princeton Applied Research model 273A potentiostat/galvanostat. Electrolyte and Film Growth. Electrochemical deposition of Au was carried out at ambient temperature from a non-cyanide, sulfite-based commercial electrolyte (Technic Inc., TechniGold 25E), containing 0.042 mol of Au+/L. In this solution, the Au ion is complexed by the sulfite anion: Au+ + 2SO23 = Au (SO3)32 . A commercial epoxy insulator (XP2000, Pyramid Plastics, Tobler Division, Hope, AR) was applied to mask the substrate, leaving an area of 5 mm 5 mm available for deposition. Electrochemical experiments and film deposition were carried out in a conventional three-electrode cell with a SCE as a reference and a Pt-coated Nb mesh as the counter electrode. The pH of the electrolyte was ∼6.0, adjusted with diluted sulfuric acid. The solution was not stirred during deposition. Microscopy. A scanning tunneling microscope (STM, Molecular Imaging Inc., model PicoPlus) was used to image the SAM layer using a Pt/Ir tip in air. In situ electrochemical STM (EC-STM) experiments were conducted in the commercial threeelectrode STM cell from Molecular Imaging Inc. using an Apiezon wax-coated Pt/Ir tip. A JEOL JSM 6700 scanning electron microscope (SEM) with a field emission source was used for the ex situ characterization of the morphology of the Au electrodeposits.
Results and Discussion SAM Assembly on Au. Self-assembly of the organic monolayer was investigated using ex situ STM imaging and cyclic voltammetry (CV). Figure 1 compares the CV of a bare Au and a SAM-covered Au electrode recorded at a scan rate of 20 mV/s in an aqueous electrolyte containing 100 mM KNO3 and 1 mM K3Fe(CN)6. The CV reveals the presence of a monolayer with low defect density, since the Fe2+-Fe3+ redox reaction 5032
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Figure 1. Cyclic voltammograms (scan rate of 20 mV/s) on bare and C12-covered Au electrodes in an aqueous electrolyte containing 100 mM KNO3 and 1 mM K3Fe(CN).
is strongly inhibited in the presence of the SAM. Ex situ STM images collected on SAM/Au samples (Figure 2) further confirmed the presence of a compact monolayer. In particular, stripes in the high-resolution image (Figure 2b) reveal an ordered arrangement of the molecules over a distance of several tens of nanometers, as well as the presence of various defects in the SAM, including voids and domain boundaries. A high density of pits is seen on gold terraces (Figure 2a); these have been associated with depressions in the Au surface,17 and their formation was ascribed to atomic surface rearrangement occurring during thiol adsorption. According to refs 11 and 17, the SAM should also be present in these depressions. Thus, the Au grain boundaries and the above-described defects in the SAM onto the Au terraces could act as preferential sites for the electrochemical nucleation of metal clusters. Cyclic Voltammetry of Au Electrodeposition. Figure 3 shows the CVs of bare and SAM-covered Au electrodes recorded at a scan rate of 20 mV/s in the Au electrolyte. The CV scans were recorded from the open circuit potential, which was approximately -50 mV for the SAM/Au electrode and 0 V for the bare Au, to -0.7 V and back. On the bare Au electrode, the onset potential for Au deposition is located at approximately -0.2 V. The current response in the forward scan shows a peak at approximately -0.5 V followed by a further increase in current. A nucleation loop is seen on the reverse scan, a signature of nucleation and growth of Au. The Au film electrodeposited in the cathodic scan was dissolved in the reverse anodic scan only at very high overpotentials (not shown). On the SAM-covered Au electrode, a large overpotential (∼300 mV more than that on bare Au) was necessary to start Au deposition, due to the presence of the SAM on the Au electrode. This observation is in agreement with previous reports, according to which the presence of SAM inhibits the metal electroreduction.18,19 The initial increase in current around -0.55 V is most probably due to Au reduction. The cathodic desorption potential (Edes) of dodecanethiol can in fact be estimated to be between -0.65 and -0.93 VSCE,20-22 assuming a shift in Edes of 59 mV per pH unit.23 The thermodynamic redox (17) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257–13267. (18) Langerock, S.; Menard, H.; Rowntree, P.; Heerman, L. Langmuir 2005, 21 (11), 5124–5133. (19) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11(12), 4823–4831. (20) Zhong, C.-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147–153. (21) Kondo, T.; Sumi, T.; Uosaki, K. J. Electroanal. Chem. 2002, 538-539 59–63. (22) Azzaroni, O.; Vela, M. E.; Andreasen, G.; Carro, P.; Salvarezza, R. C. J. Phys. Chem. B 2002, 106, 12267–12273. (23) Nelson, J. B.; Schwartz, D. T. Langmuir 2007, 23, 9661–9666.
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Figure 2. STM images of dodecanethiol monolayer-covered gold film substrates imaged in air showing various types of defects in the monolayer. Scan area: (a) 200 nm 200 nm and (b) 60 nm 60 nm.
Figure 3. Cyclic voltammograms (scan rate of 20 mV/s) on bare and C12-covered Au electrodes in the Au electrolyte (TechniGold 25E). Both first and second scan are reported for electrodeposition onto the C12-covered electrode.
potential for hydrogen evolution (HER) on the other hand is approximately -0.6 VSCE, and an overpotential of ∼200 mV is required to observe HER currents on the order of 10-4 A/cm2 at Au electrodes.24,25 A second CV scan on the SAM/Au electrode (Figure 3), which closely retraces the return segment of the CV collected at bare Au, showed a strong decrease in the overpotential for Au deposition, to a potential value more positive than that observed onto bare Au substrates, and an increased magnitude of the current. This suggests that the Au deposited in the first cycle (which was not stripped during the reverse cycle) is in electrical contact with the Au substrate through defects present in the monolayer, causing growth during the second cycle to effectively occur onto Au. The apparent positive shift in the onset potential for Au reduction with respect to the bare Au electrode may be due to the different characteristics of the freshly deposited Au and to the increase in the effective Au surface (vide infra). Langerock et al.18 and Sondag-Huethorst et al.19 have observed a similar decrease in deposition overpotential during a second scanning cycle when depositing rhodium and copper, respectively, on SAM/Au substrate. The latter authors19 hypothesized that this effect might be an indication of some “penetration mechanism” for Cu into the C12 monolayer. (24) Ohmori, T.; Enyo, M. Electrochim. Acta 1992, 37, 2021–2028. (25) Perez, J.; Gonzalez, E. R.; Villullas, H. M. J. Phys. Chem. B 1998, 102, 10931–10935.
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Figure 4. SEM images of Au electrodeposits on bare and C12-covered Au electrodes grown at ∼150 mV overpotential for 10 s: (a) Au electrodeposited at -0.4 VSCE on bare Au substrate and (b) Au electrodeposited at -0.7 VSCE on C12/Au substrate. The corresponding current transients are shown in panels c and d, respectively.
Nucleation and Growth of Au. To compare the morphology of the Au films on bare versus SAM-modified Au surfaces, we deposited two samples under potentiostatic conditions, at ∼150 mV more negative of the onset potential, i.e., at -0.4 V for 10 s on bare Au and -0.7 V for 10 s on SAM/Au. Although such a comparison may not be entirely rigorous, it nevertheless gives a qualitative picture of the growth in the two systems under similar applied current densities. Figure 4 shows the SEM images of the two samples described above along with the corresponding current transients. The difference in the growth mode is already evident from the shape of the transients; the transient on Au in fact shows no current maximum, indicative of growth on a very DOI: 10.1021/la803907p
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Figure 5. In situ electrochemical STM collected on a C12-covered electrode. Image a was obtained after immersion in the Au electrolyte, at the open circuit potential. (b-d) Images of the same sample at different scales, obtained at the open circuit potential after application of a potential pulse at -0.7 VSCE for 200 ms, to induce Au nucleation.
large number of nucleation sites, while the transient on SAM/Au shows the conventional current peak followed by an approach to diffusion-limited growth. The Au clusters on SAM/Au are three-dimensional (3D) and mound-like, with each mound likely comprised of several grains, whereas Au electrodeposited on bare Au presents a labyrinthine-like structure, typical of the growth of high-mobility metals on substrates with a large grain size. Nucleation and the early stages of growth of Au on SAM/Au surfaces were examined with in situ EC-STM as well as ex situ SEM techniques. Integration of the two techniques enables the observation of nucleation and growth processes at different length scales and potentially helps to clarify the process of morphology formation. Figure 5a shows an in situ EC-STM image of a SAM/Au surface at open circuit potential (OCP), ∼0 V. After this image had been collected, the tip was withdrawn and a potential pulse of -700 mV was applied for 200 ms to initiate Au nucleation on the SAM. Subsequently, the tip was approached again and the surface was imaged at OCP; panels b-d of Figure 5 show images collected in sequence at various magnifications. The Au clusters are preferentially nucleated at the defect sites of the monolayer, including grain boundaries and locations at terraces, the latter probably corresponding to the SAM defects at terraces observed in Figure 2b. By increasing the scan size, we verified that deposition was occurring uniformly across the entire electrode. The number density of Au nuclei, as determined by analysis of images such as Figure 5d, was ∼5 1011 cm-2. The electrodeposited Au clusters were stable over 5034
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hour-long scans at various magnifications as well as at different locations on the sample. We conclude, like other groups,11,18,19 that surface defects on the Au surface and corresponding defects in the SAM assembly play an important role in the initial stages of Au ECD on SAM/Au. Figure 6 shows ex situ SEM images of Au films electrodeposited onto SAM/Au substrate at -0.7 V for 1, 3, and 10 s, along with the corresponding current transients. These three deposition times represent three distinct stages of the nucleation and growth process, as evidenced by the current transients: (i) growth of nuclei before the current maximum (1 s), (ii) interactions among the ion diffusion fields around the different nuclei immediately after the current maximum (3 s), and, finally (iii) nuclei coalescence long after the current maximum (10 s). The SEM images show an array of metal clusters whose density and average size increase with time. Beyond the current maximum, the nuclei coalesce to form a dense deposit. These images reveal that Au electrodeposits on SAM/Au grow as 3D clusters through a progressive nucleation mode;26 i.e., the number density of nuclei increases with time. In addition, the Au clusters are nucleated mostly at visible grain boundaries of the Au film (see Figure 6a), but growth onto grains, presumably at defects in the monolayer, occurs as well. Supporting information on the nucleation and growth process can be gained by analysis of potentiostatic current transients. These were recorded by stepping the electrode (26) Scharifker, B.; Hills, G. Electrochim. Acta 1983, 28(7), 879–889.
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Figure 6. SEM images of Au electrodeposited on C12/Au at -0.7 VSCE for (a) 1, (b) 3, and (c) 10 s from the TechniGold 25E electrolyte. Below each image is reported the corresponding current transient.
Figure 7. Potentiostatic current transients for Au electrodeposition on fresh C12/Au substrates at various potentials in the range from -0.6 to -0.85 VSCE from the TechniGold 25E electrolyte.
potential from the OCP to various values between -0.6 and -0.85 V and are shown in Figure 7. All the transients show a rising part, pass through a maximum, and then decrease to finally reach a constant current value (idiff) at long times. The continuous variation in the magnitude and time of the current peak supports the hypothesis that no cathodic desorption of the SAM occurs in this range of potentials. idiff approximately doubles with the applied potential increasing from -0.6 to -0.85 V; this increase in current is not due to concurrent hydrogen evolution, which would account for only one-tenth of this value;24,25 we ascribe it instead to an increase in the effective surface area of the Au films (see Figure 11). Details of the growth process, in particular determination of the nucleation mode as well as an indirect estimate of the nucleation rate and saturation nucleus density, can be obtained through an analysis of the current transients as described by (27) Hyde, M. E.; Compton, R. G. J. Electroanal. Chem. 2003, 549, 1–12.
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Scharifker and Hills (S-H).26 Despite its shortcomings,27 the integration of this technique with microscopic observations of the films provides a simple and widely utilized method for assessing film growth characterictics. The normalized experimental transients (Figure 8) fit the theoretical trend for 3D progressive nucleation followed by diffusion-controlled growth, confirming the results derived from ex situ SEM images (Figure 6). Deviations from the theoretical curve observed after the current maximum (-0.65 and -0.75 V) are minimal and can be ascribed to electrode roughening. The nucleation rate Jnucl and saturation nucleus density NS can be calculated from the appropriate expressions for progressive 3D nucleation followed by diffusion-controlled growth:26-28 Jnucl ¼
dNðtÞ 1 ðzFcÞ2 ¼ kN N0 ¼ 0:2898 2 dt ð8πcVm Þ1=2 imax tmax 3 1=2 3 Ns ¼ f kN N0 =½ð8πcVm Þ1=2 Dg 8
ð1Þ
ð2Þ
In the expressions given above, N(t) is the number density of sites where nucleation has occurred, kN is the nucleation rate, N0 is the number of possible nucleation sites, z is the metal ion valence, F is the Faraday constant, D is the ion diffusivity in solution, c is its concentration, and Vm is the molar volume of gold. imax and tmax are the magnitude and time of the current maximum, respectively. Jnucl and NS are computed using the values of imax and tmax determined from the current transients at various potentials and are reported in Figures 9 and 10. Both quantities increase exponentially with the applied potential, (28) Ji, C.; Oskam, G.; Searson, P. C. J. Electrochem. Soc. 2001, 148, C746.
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Figure 8. Dimensionless plots for the normalized experimental current transients (dotted curve) along with the simulated curves obtained through the S-H model26 for the progressive (dashed line) and instantaneous (solid line) nucleation with diffusion-limited growth modes: (a) -0.65, (b) -0.7, (c) -0.75, and (d) -0.8 VSCE.
following a behavior similar to that of bare substrates; in particular, the values of NS calculated from eq 9 are between 4 104 and 6 105 cm-2. These values are 5 orders of magnitude lower than those obtained from ex situ SEM images such as those shown in Figure 6, which give values of 1.5 1010 cm-2 at 1 s and 1.95 1010 cm-2 at 3 s. A 2-3 order of magnitude difference has been reported for the electrochemical nucleation of Cu at hydrogen-terminated Si electrodes28-30 and has been ascribed to the inability of the S-H theory to include processes such as metal adsorption which involve charge transfer without leading to nucleus growth.29 The disagreement is much more apparent in this case, due probably to the fact that nuclei tend to form and grow at locations that are predefined by defects in the SAM, preferentially located at grain boundaries and thus invalidating the S-H assumption of random distribution of the nucleation centers.26 Langerock et al.18 using a modified S-H model also reported a large discrepancy in nucleation density for the electrocrystallization of rhodium clusters on alkanethiol-modified Au electrodes. Comparison of STM (Figure 5) and SEM (Figure 6) data provides further insight into the growth process. The nucleus density estimated by STM is in fact more than 1 order of magnitude larger than the same quantity estimated by SEM. Both data refer to growth occurring at -0.7 V, but Figure 5 shows a film grown for 200 ms while Figure 6a shows a film grown for 1 s. The current transient reported in Figure 6a reveals that the amount of Au deposited in the latter film is ∼75 times greater than that deposited in the films imaged by STM. We hypothesize that the inhomogeneous distribution of nuclei (mainly found at grain boundaries in Figure 5) causes overlapping of the diffusion fields feeding the growth of the islands (29) Radisic, A.; Vereecken, P. M.; Hannon, J. B.; Searson, P. C.; Ross, F. M. Nano Lett. 2006, 6(2), 238–242. (30) Shao, W.; Pattanaik, G.; Zangari, G. J. Electrochem. Soc. 2007, 154(7), D339–D345.
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Figure 9. Nucleation rate kNN0 vs applied potential for Au nucleation onto C12/Au electrodes. Dots are experimental points, and the line is an exponential fit.
Figure 10. Nucleus density as derived from the S-H model26 vs applied potential. Dots are experimental points, and the line is an exponential fit.
long before the current maximum, causing coalescence of some nuclei into clusters and termination of the growth of other nuclei. The film grown for 1 s thus shows only metal nuclei which survived an initial selection process; islands with an Langmuir 2009, 25(9), 5031–5038
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Figure 11. SEM images of Au electrodeposited on C12/Au substrates from the TechniGold 25E electrolyte: (a and b) -0.6, (c and d) -0.7, (e and f) -0.75, (g and h) -0.8, and (i and j) -0.85 VSCE.
initially larger growth rate in fact grow preferentially subtracting ions for the growth of smaller islands, effectively terminating their growth. The increase in the number of growth centers at longer times (Figure 6b) is probably due to the smaller nucleation and growth rate at sites with higher nucleation barriers. Nucleation at defects onto SAM/Au patterns has in fact been observed to occur at a fixed potential only after an incubation period.31 Two stages of growth are thus revealed by comparison of STM and SEM images; the first is the initial nucleation of Au clusters at SAM defect sites, and the successive one is a stage of coalescence and coarsening of selected islands, with termination of the growth of other islands. Morphology of Thick Films. A series of samples were potentiostatically grown for 100 s at constant potentials, set between -0.6 and -0.85 V, and characterized by SEM imaging. Samples grown at -0.6 and -0.65 V form large hemispherical clusters with a density of ∼1010 m-2 (Figure 11a). Nucleation rate and growth kinetics at these potentials are slow; the observed metal nuclei form at defects, survive an initial selection mechanism, and then mushroom out (Figure 11b). Deposition over 100 s ensures almost complete coverage by coalescence of the hemispherical clusters. At -0.7 V, the Au films exhibit a smaller apparent grain size and a smoother surface, probably as a consequence of the higher nucleation rate and density, which limits the lateral growth of the nuclei. These films are relatively small grained and homogeneous, a morphology which is difficult to achieve via deposition on a bare electrode (see Figure 4a). An applied potential of -0.75 V or higher yields clustered and dendritic morphologies (Figure 11e–j), despite the fact that the commercial Au electrolyte should produce smooth deposits and avoid dendritic growth. This is probably due to the fact that a (31) O’Brien, B.; Stebe, K. J.; Searson, P. C. J. Phys. Chem. C 2007, 111 8686–8691.
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high film growth rate occurring at relatively few defect sites results in very high current densities, presumably above the critical values where a leveler can inhibit dendritic growth.14 A change in dendritic morphology is observed with increasing potential. (i) Clusters and planar two-dimensional (2D) dendrites with dense branches at 180 are grown at -0.75 V (Figure 11e,f). (ii) 2D and 3D dendrites are observed at -0.8 V (Figure 11g,h). (iii) Fully developed 3D dendrites with side branches at 120 are present at -0.85 V (Figure 11i,j). Dendrite formation is determined both by ion diffusion processes toward the growth front and by details of the reduction kinetics at the interface; in particular, spatial anisotropy in the growth rate is a necessary condition for their presence.32 On grounds of symmetry, we hypothesize that the direction of the secondary branches in the 2D dendrites is Æ110æ, while that of the 3D dendrites is Æ211æ. This transition in the preferred growth direction may thus be due to a potential-dependent adsorption of the Au complex to the different planes, which would induce an anisotropic reduction and growth rate for Au. The crystallographic direction of the dendrites and the mechanism for dendrite formation are currently under study. These morphologies can be of interest, for example, in applications requiring high surface area, such as (electro)catalysts and superhydrophobic surfaces.
Conclusions We have electrodeposited Au from a commercial sulfitebased electrolyte on dodecanethiol-modified evaporated Au films. The presence of the organic monolayer inhibits Au electrocrystallization and results in a predominantly threedimensional nucleation followed by diffusion-limited growth. The Au clusters nucleate at surface defects in the SAM; only a small fraction of the nuclei survives an initial competition (32) Ben-Jacob, E.; Garik, P. Nature 1990, 343, 523.
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process, and the remaining nuclei grow in a progressive fashion. The nuclei eventually cover the whole monolayer at longer deposition times. The cluster density observed initially is ∼1011 cm-2, while at a later stage, the density is reduced to ∼1010 cm-2. These values are much larger than those calculated from potentiostatic current transients; this is partly a consequence of the nucleation sites being localized at surface defects and not randomly distributed. While films grown at low overpotentials consist of hemispherical clusters, films obtained at -0.7 VSCE are smooth and exhibit a grain size
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much smaller than that of Au films grown on bare Au, probably as a consequence of the higher nucleation density. Growth at larger cathodic potentials results in the formation of dendrites with morphology that can be controlled by the applied potential.
Acknowledgment. The financial support from the National Science Foundation through Grants NSF-DMR 0314233 and ECCS 0701505 is gratefully acknowledged.
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