Electrodeposition of Palladium onto a Pyridine-Terminated Self

Feb 21, 2011 - The electrodeposition of Pd onto self-assembled monolayers (SAMs) of 3-(4-pyridine-4-ylphenyl)propane-1-thiol on Au(111) has been ...
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Electrodeposition of Palladium onto a Pyridine-Terminated Self-Assembled Monolayer Christophe Silien,*,†,‡ Dorothee Lahaye,§ Marco Caffio,† Renald Schaub,† Neil R. Champness,§ and Manfred Buck*,† †

EaStCHEM School of Chemistry, University of St. Andrews, Scotland, U.K. Materials and Surface Science Institute and Department of Physics, University of Limerick, Limerick, Ireland § Department of Chemistry, University of Nottingham, Nottingham, U.K. ‡

ABSTRACT: The electrodeposition of Pd onto self-assembled monolayers (SAMs) of 3-(4-pyridine-4-ylphenyl)propane-1thiol on Au(111) has been investigated by scanning tunneling microscopy. Two schemes are compared: One involves an established two-step procedure where Pd2þ ions are first coordinated to the pyridine moieties and subsequently reduced in Pd2þ-free electrolyte. The second deposition routine involves electroreduction in an electrolyte containing low concentration of Pd2þ which merges both steps and, thus, significantly simplifies metal deposition onto pyridine-terminated SAMs. Both strategies produce identical Pd nanoparticles (NPs) which exhibit a narrow size distribution and an apparent STM height of ∼2.4 nm. The observation of a Coulomb blockade and easy displacement of the nanoparticles in STM experiments evidence deposition on top of the SAM. The NPs are concluded to be essentially spherical. Growth of the NPs is found to be selflimiting since repeating the complexation-deposition cycle increases the density of the nanoparticles rather than their size but only close to full coverage. At high concentration of the Pd2þ electrolyte, deposition on top of the SAM is impeded by a competitive mushroom-type growth.

’ INTRODUCTION Self-assembled monolayers (SAMs) of thiol-based molecules are attractive systems for electrochemical nanotechnology. Indeed, with patterning techniques covering length scales ranging from macroscopic to nanoscopic dimension,1,2 the interfacial charge transfer can be locally tuned and electrochemical processes such as electrometallization can be spatially confined and, thus, exploited to produce microscopic and nanoscopic structures.3-6 Because the SAMs can also minimize adhesion between substrate and metal deposit,6,7 the latter can even be lifted off and the original substrate reused.6,8 However, the precision at which metal nucleation and growth has to be controlled becomes increasingly demanding with continued downsizing of structures. A pertaining issue limiting the resolution of the SAM-templated deposition process is the diffusion of metal adatoms and ions through the SAMs. Typically, overpotential deposition (OPD) of metal on SAM-covered electrodes leads to formation of clusters that are above the organic film but are also in electrical contact with the substrate through a narrow wire at a defect in the SAM.6,8,9 Deposition at these socalled pinholes, commonly referred to as mushroom-type growth, becomes a limiting factor on the sub-50 nm scale. Another potential problem for the fidelity of metal electrodeposition is the underpotential deposition (UPD) of metal, i.e., r 2011 American Chemical Society

intercalation of a metal layer typically a monolayer thick, at the SAM-substrate interface at potentials positive of the onset of bulk deposition.10-12 Metal deposition from the vapor phase encounters similar issues. For example, Au penetrates easily through SAMs and forms monatomic-high islands at the interface with the substrate.13,14 Nonetheless, metal deposition on top of SAMs is possible to some extent provided that reactive moieties, such as thiol13 or carboxylic acid,14 are present as SAM terminating groups, or that cross-linked SAMs are used.15,16 Formation of clusters on top of the SAM was confirmed by STM measurements which showed a Coulomb gap in the conductivity,13 thus confirming the absence of direct electrical contact with the substrate, analogous to measurements of samples prepared by cluster-beam deposition.17,18 Electroless metal deposition has also been investigated,19-21 in particular on cross-linked aromatic SAMs which, again, show improved properties against metal penetration.22 Regarding electrodeposition, electrical separation of metal deposits from the substrate was only successfully demonstrated with the introduction of a two-stage strategy for Pd deposition onto mercaptopyridine (PyS) SAMs. The key point of the Received: November 16, 2010 Revised: January 13, 2011 Published: February 21, 2011 2567

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has been shown that the combination of rigid aromatic and flexible aliphatic moieties results in SAMs of excellent structural quality.27,28 In PyP3, a pyridine moiety is used instead of a second phenyl aromatic ring. Analogously, PyP3 SAMs also exhibit a high degree of orientational and translational order, with the pyridine exposed to the outer surface of the film, as verified by reversible protonation of the molecules.26 In this paper, we report on a comparative study of the electrodeposition of palladium on PyP3 SAMs, via the two-step scheme discussed above,23 and by reducing the surface directly with Pd2þ ions present in the electrolyte. STM data recorded ex situ (i.e., in air) as well as in situ (i.e., with the SAM in contact with the electrolyte) are used to discuss the Pd adlayer morphology. Cyclic voltammogramms (CVs) and current-voltage characteristics, recorded with a low-temperature STM (∼83 K), complement our analysis.

’ EXPERIMENTAL SECTION

Figure 1. (a, b) Representation of pyridine-terminated SAM on Au, prior to and after immersion in Pd2þ solution, triggering Pd2þ-pyridine complexation. (c) Electrochemical discharge leading to formation of two-dimensional Pd monolayer or Pd clusters on top of the SAM according to refs 23 and 25, respectively. (d) Schematic of PyP3 and PyS molecules.

scheme is that metal ions are first adsorbed by complexation onto a SAM which are then electrochemically reduced in a second step in an electrolyte free of the respective metal ion.23,24 The twostep scheme, illustrated in Figure 1, prevents mushroom-type cluster growth and insertion of material at the SAM-substrate interface by limiting the ions to those coordinated to the SAM. Scanning tunneling microscopy (STM) revealed monatomichigh islands, which were confirmed by electron spectroscopies to be confined to the region above the PyS SAM.23,24 Using the same system and protocol, three-dimensional clusters were also observed, and a gap in the STM conductance characteristic for a Coulomb blockade proved their electronic separation from the substrate.25 Notwithstanding variations in the morphology of the Pd deposits on PyS SAMs, the two-step scheme appears most promising for the electrochemically controlled deposition of ultrasmall metal structures since, unlike defect-dominated deposition in the overpotential region (OPD),6,8 it affords a true confinement of the metal deposit on top of a SAM. While the scheme was successfully demonstrated using PyS, an extension of the coordination-reduction strategy to thicker SAMs is of interest as a layer consisting of larger molecules will, in general, exhibit better barrier properties against mushroom-type metal growth.6 Hence, one could expect that it is possible to reduce the metal ions coordinated to the SAM also in the presence of metal ions in the bulk electrolyte without triggering mushroom-type OPD. This would be a decisive simplification of the two-step scheme as complexation and reduction can be performed in a single setup. To demonstrate the feasibility of this approach, we used 3-(4-pyridine-4-ylphenyl)propane-1-thiol (PyP3, see Figure 1d), a molecule recently introduced by us.26 It is based on the architecture of biphenylalkanethiols for which it

Material and Sample Preparation. Substrates, 300 nm thick epitaxial Au(111) films grown on mica, were purchased from Georg Albert PVD (Heidelberg, Germany) and stored in vacuum. Prior to SAM modification the substrates were flame-annealed. PyS SAMs were obtained by immersing Au/mica substrates at room temperature for 2 min in a PySSPy (aldrithiol, Aldrich) aqueous solution. PyP3 SAMS were formed overnight (∼15 h) at 345 K in 10 mM KOH ethanol solution, according to ref 26. The synthesis of PyP3 has been reported previously.26 All SAMs were rinsed with H2O or ethanol, depending on the solvent used for self-assembly, and blown dry with a gaseous stream N2. For analysis of the two-step scheme, Pd2þ-pyridine complexation was done in a 100 μM PdSO4/50 mM H2SO4 (aq), unless stated otherwise. Typical immersion times were 20 min for PyS and 2 min for PyP3. A systematic study of the relationship between complexation rate and immersion time was not carried out. However, it was verified that longer immersion times do not lead to an increase in the amount of Pd at the interface, suggesting that these times correspond to a saturation of all available pyridine moieties. After complexation, the samples were thoroughly rinsed with deionized H2O. For clarity, samples that underwent complexation prior to the experiment are labeled Pd2þ/PyS and Pd2þ/PyP3, respectively. CV and STM Measurements. A scanning tunneling microscope (Picoscan, Molecular Imaging) equipped with a bipotentiostat (Picostat) and a home-built Teflon electrochemical cell (EC cell) was used. For in situ experiments, tips were fabricated by electrochemical etching of a Pt/Ir (80:20) wire in a 2 M KSCN/0.5 M KOH (aq) with an ac voltage (∼6 V). The tips were subsequently coated with polyethylene to minimize leakage currents that are typically below 1 pA at the potentials used in our experiments. Both CV and in situ STM measurements were performed in the STM EC cell, in 50 mM H2SO4 (aqueous). Pt wires were used as pseudoreference and counter electrode. PdSO4 was mixed to the electrolyte when required, with concentration of 1 or 100 μM. In this case, a Pd wire was used as reference. However, all potentials are indicated with respect to a standard calomel electrode (SCE) for convenience. The experimental setup was thoroughly purged with N2 gas, prior to recording the CVs, but not for in situ STM measurements, in order to minimize evaporation of the electrolyte and subsequent drift of the scanner. For ex situ STM measurements, mechanically cut Pt-Ir tips were used. Sample potential (SP), tip bias (TB), and tunnel current are given in the respective figure captions. STM Conductance Measurements. I(V) curves were recorded using a low-temperature UHV-STM (LT-STM) from CreaTec. The temperature of the liquid N2 cooled sample was ∼83 K. Mechanically cut Pt-Ir tips were used. Samples were prepared with the EC cell setup 2568

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Figure 2. (a-c) Cyclic voltammograms of PyP3, Pd2þ/PyP3, and Pd2þ/PyS recorded in 50 mM H2SO4. (d) CVs of PyP3 recorded in 1 μM PdSO4/50 mM H2SO4. (e) CVs of PyP3 in 100 μM PdSO4/ 50 mM H2SO4. All CVs recorded at a scan rate of 10 mV/s. First (continuous line) and second cycle (dashed line). The inset in (b) highlights the background current (dotted line) removal from first experimental CVs (continuous line). The resulting data (dashed line) were used to determine reduction peaks integrated area. All sample potential values are expressed against SCE. and transferred into ultrahigh vacuum (UHV, 10-11 mbar) after rinsing and drying under a N2 gaseous stream. Samples were degassed in UHV at room temperature for 2 days prior to measurements. I(V) spectra were recorded in a single scan by ramping up the bias at 0.33 V/s starting from negative sample bias. The tunnel junction was set to 35 pA at þ0.50 V (sample bias).

’ RESULTS Cyclic Voltammetry. The stability of PyP3 SAMs was first verified in 50 mM H2SO4. As seen from Figure 2a, no Faradaic process is detected in CVs positive of -0.2 V, and only capacitive charging is observed when extending the potential range up to þ0.40 V. The SAM structure is thus not affected by the potential unlike PyS SAMs which exhibits a structural transition in this range.29 Cyclic voltammetry in the same electrolyte with a Pd2þ/ PyP3 SAM-modified electrode leads to a very different result (Figure 2b). In this case, the first cathodic sweep yields a distinct current peak at ∼-0.10 V, which is well below the potential for Pd bulk deposition.30 Its absence in the subsequent CV evidence that the associated reaction is complete and irreversible. This is the same behavior as reported in the literature for Pd2þ/ PyS23-25 and also reproduced in Figure 2c for comparison. Accordingly, the wave observed in the first sweep of Pd2þ/PyP3 is attributed to the reduction of the coordinated Pd2þ ions at the SAM/electrolyte interface. Notably, the same peak potential is observed for Pd2þ/PyP3 and Pd2þ/PyS. This is in line with voltammetric studies of SAMs of ferrocenyl-terminated thiol31,32 and highlights that the charge transfer across PyP3 and PyS SAMs is not rate limiting at the scan rates applied. Thus, reduction of the coordinated ions contrasts with defect-mediated

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OPD where the deposition potential depends strongly on the thickness of the SAM. For OPD, the reduction is indeed initiated at the SAM-electrode interface, and therefore, its rate is influenced by the ability of the SAM to prevent ion penetration.6 √ √ PyP3 SAMs exhibit a 2 3  3 molecular arrangement with two molecules per unit cell and thus an area of 21.7 Å2 per molecule.26 With the reduction of one Pd2þ involving two electrons and assuming that each PyP3 molecule carries a single ion,23,24 an ideal charge transfer of 148 μC/cm2 is expected, which is slightly larger than the value of ∼116 μC/cm2 measured from the CV in Figure 2b. For Pd2þ/PyS, a maximum experimental value of ∼100 μC/cm2 was reported for Au(111) single crystal,23 while ∼81 μC/cm2 is inferred here from the CV of Figure 2c. In line with ref 25, as-prepared PyS monolayers are not ordered and thus likely to be less compact than PyP3 SAMs, which in turn implies less Pd2þ available for reduction. Let us note that longer immersion in Pd2þ solution does not increase the measured charge transfer upon reduction both for Pd2þ/ PyP3 and Pd2þ/PyS. The presence of a background current following Pd2þ reduction, which becomes more obvious in the second cycle, limits the accuracy of the value of the charge associated with Pd reduction. The background current is seen for both Pd2þ/PyP3 and Pd2þ/PyS systems in our experiments but is absent in the original experiment.23 Its origin is not clear at present and may be ascribed to processes involving hydrogen as discussed below. Moreover, the values given above were extracted assuming a linear increase in the background current (see inset in Figure 2b) and are thus probably slightly underestimated. Compared to PyS, PyP3 SAMs are thicker26 and do not exhibit a potential induced structural transition.29 It seems thus that PyP3 layers feature an increased structural stability. This led us to explore whether the two-step deposition scheme involving changes of environment can be simplified to a procedure where both coordination of the metal ion to the SAM and subsequent reduction are performed in a single cell with a Pd2þ-containing electrolyte. For a concentration of 1 μM PdSO4, the CVs of PyP3 are shown in Figure 2d. Despite the low Pd2þ concentration in the electrolyte, the CVs exhibit features very similar to those seen above when reducing Pd2þ/PyP3 SAM in a Pd2þ-free environment. Prior to recording CVs the cell is purged for typically 30 min, which is sufficient to coordinate palladium to the SAM. This is confirmed below by the STM studies. Pd saturation of the SAM on this time scale is also inferred from the two-step protocol. Moreover, for PyP3 SAM exposed to a 1 μM PdSO4 solution for only 2 min, the reduction in 50 mM H2SO4 yields a charge of ∼46 μC/cm2, which corresponds already to ∼40% of the largest Pd2þ coverage measured. Even though Pd2þ is now present in the bulk electrolyte, the cathodic wave of the first CV peaks at the same potential (∼-0.10 V) as in the two-step scheme with an associated charge of 137 μC/cm2. The magnitude of the discharge, as well as its potential, strongly suggests that the process is of the same nature as discussed above; i.e., the Pd2þ ions coordinated to the pyridine moiety are reduced. There is no measurable indication of any other reduction route, in particular where Pd2þ ions are reduced directly at the Au substrate by penetration into the SAM at defects (i.e., mushroom-type OPD). This is fully supported by the second CV recorded immediately after the first one which is essentially identical to the one of the two-step scheme (Figure 2b). The lack of the Pd reduction wave despite the presence of Pd2þ in the electrolyte is very interesting as it reveals a kinetic limitation of the re-formation of 2569

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Figure 3. (a, b) STM images of a PyP3 SAM recorded in situ (50 mM H2SO4) prior to and after lowering SP down to -0.35 V (10 mV/s). Images were recorded at same location, with SP and TB at þ0.15 and þ0.20 V, and tunnel current of 2.5 and 3.5 pA, respectively. Inset with SP, TB, and current at þ0.05 V, þ0.45 V, and 3.5 pA. (c, d) Idem in 50 mM H2SO4 for a Pd2þ/PyP3 sample. (c) Prior to and (d) after discharge (achieved by lowering SP down to -0.35 V, 10 mV/s). Both images were recorded with SP at þ0.15 V and tunnel current at ∼2.0 pA. TB was þ0.30 and þ0.60 V, respectively. (c) and (d) were recorded at a different sample location.

the Pd2þ-pyridine complex after discharge. However, as demonstrated below, further Pd deposition is possible. For much higher concentrations of PdSO4, the first CV is substantially different as seen from data recorded for a PyP3 SAM in 100 μM PdSO4 sulfuric acid (Figure 2e). The reduction wave peaks at -0.07 V and is more intense. Moreover, during the course of the reduction that starts at ∼0.0 V, the curve is also altered with a sudden change in slope at -0.05 V. In contrast to the other CVs, a significant cathodic current of more than 15 μA/cm2 remains in the anodic scan, which indicates an ongoing Faradaic process. To minimize this contribution, the cathodic scan was stopped right at the end of the peak. The origin of the background current is not clear at present since hydrogen adsorption processes as well as continuing Pd deposition can occur, even though the latter may be kinetically hindered.30 In the second CV only the background current of 15-20 μA/cm2 is detected but not the reduction peak. Therefore, even at high palladium ion concentration, the discharge of Pd2þ-pyridine complexes remains an active and important supply of Pd in the first cycle, but a second complexation on the pyridine moieties appears hindered on the time scale of two subsequent CVs. in situ STM. In the previous section, it has been established that Pd2þ-pyridine complexes on PyP3 SAMs can be

discharged and that the density of available Pd adatoms scales with the molecular density in the SAMs, with one Pd adatom per 21.7 Å2 expected. The morphology of the palladium deposit is now addressed on the basis of STM images. Data were first recorded in situ on a PyP3 sample in order to confirm the stability of the SAM in sulfuric acid. Topographic images of a fresh PyP3 surface in 50 mM H2SO4 are shown in Figure 3a,b, both with the SP set at þ0.15 V. The second image was recorded after excursion of SP down to -0.35 V and back. From the image in Figure 3a, it is inferred that the SAM is not altered following immersion in the electrolyte, despite the fact 26 that all PyP3 molecules are now protonated. √ √Au vacancy islands are observed on terraces and the 2 3  3 molecular arrangement can be identified in the image presented in the inset. No changes are detected in Figure 3b, as expected from the CV discussed in Figure 2a. In particular, the vacancy islands remain intact, and no Au islands are detected. These would mark partial reductive desorption of the SAM.33,34 Furthermore, images recorded ex situ after the sample was rinsed with H2O and dried with N2 confirmed that the initial molecular arrangement remained the same. Hence, the PyP3 film is perfectly stable in the potential range relevant to our study. 2570

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Figure 4. STM images of PyP3 SAMs recorded in air (þ0.50 V, 1.5 pA) after (a) one (b) two, and (c) three deposition cycles in 1 μM PdSO4/50 mM H2SO4. NPs radius distribution in inset of (a) and (b). Inset in (c) (þ0.50 V, 1.0 pA) shows another sample after three deposition cycles and after scanning an area of 50  50 nm2 with the set point at þ0.10 V and 0.1 nA. (d) A different PyP3 sample after one deposition cycle in 100 μM PdSO4 (þ0.50 V, 2.5 pA). (e) Height profiles P1, P2, and P3 extracted from (a). (f) Idem P4 and P5 from (d).

An image of a Pd2þ/PyP3 sample, also recorded in 50 mM H2SO4, is presented in Figure 3c, with SP at þ0.15 V. A clear loss in resolution and increase in noise level are detected in comparison to the case of an unmodified PyP3 SAM. These observation are systematic and also made when measuring Pd2þ/PyP3 samples in air. It is thus attributed to the presence of the Pd2þ-pyridine complexes, which obviously affect the tunneling conditions. The image in Figure 3d was recorded after ramping SP down to -0.35 V and back to þ0.15 V (10 mV/s), which should promote the reduction of the Pd2þ-pyridine complexes (see CVs in Figure 2b). A large amount of nanoparticles (NPs) are now observed at the PyP3 surface. Since they are not formed in the absence of Pd2þ (Figure 3b), the NPs are assigned to reduced and coalescing Pd adatoms. ex situ STM. The NPs are clearly imaged in situ in the electrochemical environment. However, the images also reveal that, even if the tunneling current is kept at very low values of a few pA, the NPs are either displaced on the SAM or transferred to the tip, as revealed by streaks and partial imaging of NPs. With

the tip easily compromised, precise evaluation of the NPs size and density distributions is difficult. To avoid artifacts, a more thorough analysis has thus been performed ex situ, where STM tips are easily cleaned or replaced. The CVs presented in Figure 2b,d, showing the reduction of Pd2þ-pyridine complexes on PyP3 samples in Pd2þ-free and in 1 μM PdSO4 sulfuric acid, suggest the same process in both cases. This statement is confirmed by STM (Figure 4a). Because of the significantly simpler preparation protocol, the single-step procedure with the 1 μM Pd2þ sulfuric acid electrolyte was chosen to investigate multiple cycles of Pd deposition. After each reduction the sample was rinsed with H2O and blown dry with N2 for ex situ STM measurement, producing the images shown in Figure 4a-c. After the first discharge, Pd NPs cover the surface with a density of ∼8.0  103 per μm2. Their height distribution, with respect to the PyP3 background is relatively narrow, 2.4 ( 0.2 nm, as illustrated by three profiles P1, P2, and P3 extracted from Figure 4a and reproduced in Figure 4e. Moreover, the distribution of radius, as measured with a tip giving the smallest values, 2571

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Figure 5. (a-c) Air STM (þ0.50 V, 1.5 pA, 300  300 nm2) of three Pd2þ/PyP3 samples discharged in Pd2þ-free H2SO4 at þ0.10, þ0.00, and -0.20 V for 250, 254, and 277 s, respectively. Note that the topography of the Au substrate can be discerned in (a). (d) Typical current-voltage spectra recorded at ∼83 K above an isolated Pd NP on PyP3 (continuous line) and PyP3 background (dashed line) recorded with the same tip. The tip was preset at þ0.50 V, 35 pA for both curves. Sample bias scanned upward at 0.33 V/s. No average.

clearly peaks around 2.2 nm (see inset). The distribution is, however, not symmetric, with a noticeable bias toward smaller particles. Moreover, even though isolated particles are easily observed, Pd NPs tend to aggregate. After the second deposition cycle, the density of NPs is increased to ∼11  103 per μm2. The corresponding STM topography is shown in Figure 4b. The radius distribution remains centered around 2.2 nm and is again somewhat asymmetric toward the smaller values. The height of the NPs remains ∼2.4 nm. After the third cycle (Figure 4c), an increase in the number of particles is again observed (∼15  103 per μm2), with virtually no change in size. Hence, the data suggest that new NPs are generated but that those formed in previous deposition cycles are not affected. The NPs also exhibit weak adhesion onto the PyP3 SAM as demonstrated by the image shown in inset in Figure 4c. The latter was recorded after scanning a 50  50 nm2 area with the tip set at 0.1 nA and 0.1 V and shows that virtually all NPs have been removed and transferred, at least partially, onto the tip as evidenced by the multiple tip imaging artifact and by the absence of NPs accumulation at the left and right edges of the scanned area. The pronounced difference between the CVs of the 1 and 100 μM Pd solutions (Figure 2d,e) is also reflected by the STM images. In contrast to the characteristic formation of regular Pd NPs observed after reduction in the 1 μM Pd2þ solution (Figure 4a), the equivalent experiment at the higher Pd2þ concentration yields Pd clusters that are much bigger and less homogeneous, with heights varying from ∼7 to ∼20 nm, as inferred from the topographic image (Figure 4d) and respective height profiles (Figure 4f). Interestingly, no NPs can be seen

between the large Pd clusters, suggesting that Pd adatoms only aggregate into these larger clusters. The amount of palladium deposited at this high concentration appears above one monolayer equivalent, in line with the CVs presented in Figure 2e. While reduction of the Pd2þ-pyridine complexes occurs, the possibility of reducing ions from the solution needs also to be considered. The pronounced change in the slope seen at -0.05 V in the CV of Figure 2e is indeed consistent with a slow nucleation phase of Pd clusters at SAM defects (i.e., mushroom-type growth) followed by a fast discharge of the Pd2þ/PyP3 interface. In the experiments presented so far, the Pd deposition has been achieved during voltammetric scans at fixed scan rate. In order to assess the effect of SP on the Pd adlayer morphology, as well as possible dependence with reduction rate, a series of Pd2þ/ PyP3 samples have been reduced in Pd2þ-free H2SO4 with different SP. Samples were introduced at open circuit and SP immediately set at þ0.30 V. Reduction was then initiated by switching the potential to þ0.10, 0.00, and -0.20 V, for 250, 254, and 277 s, respectively. The corresponding STM images recorded after rinsing and drying are presented in Figure 5a-c. For all three samples, the topographies exhibit Pd NPs, very much like those presented in Figures 3d and 4a-c, albeit with a pronounced potential-dependent density of ∼0.2  103, ∼4.4  103, and ∼8.2  103 NPs per μm2, respectively. The last image is indistinguishable from a complete discharge and confirms that reduction of Pd2þ-pyridine complexes in Pd2þfree and 1 μM PdSO4 electrolyte gives the same Pd morphology. While the potential affects the density, it has obviously no significant effect on the size of the Pd NPs. 2572

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Langmuir Because of convolution with the tip, STM images alone cannot unambiguously clarify the exact shape of the Pd NPs. Yet, in our experiments, apart from deposition at high Pd concentration, the amount of Pd2þ-pyridine complexes determines the number of Pd adatoms available after completion of the discharge, as inferred from the CVs in Figure 2b,d. Assuming all Pd NPs are perfect spheres with a diameter of 2.4 nm, one infers that each NP contains ∼500 Pd atoms (i.e., with Pd density of 12.02 g per cm-3). For an ideally fully coordinated PyP3 SAM with a density of 4.7  106 molecules per μm2 this yields a density of 9.4  103 NPs per μm2. This value is very close to what is found experimentally (i.e., ∼8  103 NPs per μm2), the more so that the CVs indicate an amount of Pd slightly less than a nominal monolayer (Figure 2b,d). It is thus reasonable to identify the Pd NPs to rather spherical clusters with a diameter of ∼2.4 nm, close to the height measured by STM. Scanning Tunneling Spectroscopy. As mentioned above, STM imaging of Pd NPs on PyP3 requires tunneling currents of the order of a few pA in order to avoid tip-induced modifications such as displacement or removal of particles. This is a strong indication that reduction of Pd coordinated to the SAM results in NPs on top of the SAM and not in direct contact with the metal substrate. An STM tip above a metallic NP on a SAM represents a system with a double tunnel junction which for a sufficiently small size of the particle should give rise to a Coulomb blockade in the current-voltage characteristics.13,17,18,25,35 To verify this property, a NP/PyP3 sample was transferred to a UHV-STM system for spectroscopic measurements. A typical I(V) curve, acquired with the tunneling parameters set at 35 pA and þ0.5 V (sample bias) above an isolated 2.4 nm high Pd NP, is presented in Figure 5d (continuous line). For comparison, a I(V) curve acquired above the adjacent PyP3 SAM background is also shown (dashed line). STM imaging after recording the spectra confirmed that the NP was unperturbed by the measurement. It is clear from the comparison of both I(V) curves that the presence of the NP results in a gap of (0.20 V, whereas an ohmic behavior is found above the PyP3 background. Like for Pd clusters on PyS SAM,25 the gap is attributed to a Coulomb blockade, arising from the low capacitance of the metallic NP as well as the absence of strong coupling with the Au substrate due to the PyP3 film. We note that it was not possible to obtain reproducible results for larger bias ranges due to the weak interaction of the NP with the SAM which gave rise to instabilities of the NPs, despite working at ∼83 K. Thus, second current steps expected at about (0.6 V could not be verified. Repeating the measurements on other NPs produce the same Coulomb gap and, thus, confirm the homogeneity in particle size and in the supporting PyP3 layer seen in the topographic STM images. From the charging energy of 0.20 eV, using the same reasoning as in ref 25, the total capacitance associated with individual Pd NPs in the (double) tunnel barrier junction is estimated at ∼0.40 aF. These values are in full agreement with the literature17,18,25 and the size of our NPs.

’ DISCUSSION The Coulomb barrier and weak interactions between Pd NPs and PyP3 SAMs evidence that the NPs nucleate and grow on top of the monolayer and, therefore, are electronically coupled to the substrate only via tunneling. While in that respect the results for this type of SAM are analogous to the previous work on PyS,23-25 there are also substantial differences in a number of points.

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One refers to the morphology and size of the particles where the PyP3 and PyS studies vary. In the present work, particles of essentially uniform height of 2.4 nm form both in the two-step scheme and in the single-step procedure at low Pd2þ electrolyte concentration. In contrast, works on PyS by other authors revealed either bulklike NPs with substantial variation in size (i.e., heights ranging between 1.5 and 2.5 nm)25 or flat monatomic-high islands.23 Compared to our results for PyP3, the variations in the height distribution of bulklike NPs on PyS seen in ref 25 can be understood by considering the fact that after reduction atoms diffuse on the surface of the SAM and that nucleation is facilitated at sites where Pd atoms are preferentially pinned (e.g., structural inhomogeneities). Therefore, it can be expected that, compared to PyS, the higher structural quality of PyP3 SAM26-28 should yield a more uniform size of the NPs. In the context of the Pd morphology, we note that the observation in ref 23 of monatomic-high Pd islands on PyS for nominally the same experiment as in ref 25 remains surprising, in particular since bulklike NPs should be thermodynamically preferred considering the high surface tension of metals. While the origin of these differences is not clear at present, it suggests that the deposition process might be critically dependent on small (and currently unidentified) variations in the experimental conditions. Yet, it also emphasizes the potential of SAM-controlled electrodeposition for generating metal nanostructures. Related to the uniform NP size observed in our experiments is the finding that repeated deposition-reduction cycles do not result in a continuous growth of already existing particles but an increase in the density of particles of same size. This means that, once the particles have formed, a significant activation barrier appears that prevents further attachment of diffusing Pd adatoms. As no mechanistic study exists for Pd on SAMs and reference has thus to be made to Pd deposition on metal substrates, we tentatively propose hydrogen adsorption as an explanation for the activation barrier. Investigations of Pd deposition on Au(111) revealed hydrogen adsorption at potentials where Pd2þ/ PyP3 and PyS reduction occurs.30 Thus, it is conceivable that, after or during Pd NPs growth, hydrogen adsorption passivates the NPs against further growth. With both processes proceeding in parallel the overall growth mechanism is quite complex keeping in mind that the rate of hydrogen adsorption is likely to be critically dependent on Pd NP size and morphology.36 Notwithstanding its origin, the passivation is quite remarkable as for a sample like the one shown in Figure 4c no further growth of the NPs was detected even after holding the potential at -0.35 V for a period of 80 min in a 100 μM PdSO4 electrolyte. However, a few microscopic Pd clusters were then observed. Akin to metal electrodeposition on other SAMs,5,6,8,9 these large-scale deposits are understood to develop following mushroom-type OPD at defects in the PyP3 monolayer. The possibility that, for structurally robust SAMs such as those made of PyP3, Pd NP nucleation and growth can be reliably achieved at the SAM/electrolyte interface in the presence of the metal ion in the bulk solution is important from a technological point of view. First, this greatly simplifies the electrodeposition process as the original two-step scheme can now be carried out without changing the environment. Second, and maybe more importantly, two channels for metal deposition are then possible with Pd2þ ions available either from the complexes with the pyridine moieties or directly from the electrolyte. The respective contributions can be adjusted by controlling the concentration in the solution, as demonstrated by comparing depositions in 1 and 2573

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Langmuir 100 μm PdSO4 (see Figure 4), and by adjusting the delay between two consecutive reductions, thus controlling the amount of Pd2þ coordinated to the pyridine. However, with increasing electrolyte concentration the SAM quality becomes more and more critical as mushroom-type OPD is facilitated— first because the on-SAM grown Pd NPs exhibit passivation and, second, because the Pd mushrooms are in electrical contact with the Au electrode, which shifts the onset for metal deposition to significantly more positive potentials.

’ CONCLUSIONS The recently introduced PyP3 (3-(4-pyridine-4-ylphenyl)propane-1-thiol) molecule26 has been used to study the electrodeposition of Pd on top of a SAM using a complexationreduction sequence. Beyond the established two-step procedure23 consisting of Pd2þ-pyridine complex formation followed by reduction in a Pd2þ-free electrolyte, palladium deposition can also be very reliably accomplished by merging both steps, thus making the scheme more straightforward and flexible as deposition via complexation can be assisted by bulk deposition. NPs thus formed are also electronically decoupled from the Au substrate, as evidenced by their easy displacement and the observation of a Coulomb barrier of ∼0.20 V in STM experiments. A puzzling aspect is that morphology and size distribution of the Pd deposits on pyridine-terminated SAMs vary substantially across all the studies performed so far. They range from monatomic high islands for PyS23,24 to more spherical particles for both PyS25 and the PyP3 SAMs presented here. While further work is required to unravel the origin of these differences, the range of morphologies reveals a glimpse of the opportunities afforded by the combination of SAMs and electrodeposition to control metal nanostructures and tailor their electronic, optical, and catalytic properties. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (C.S.); [email protected] (M.B.).

’ ACKNOWLEDGMENT Financial support from the UK Engineering and Physical Sciences Research Council under grant no. EP/D048761/1 is gratefully acknowledged. C.S. also acknowledges funding from the Integrated Nanoscience Platform for Ireland (INSPIRE) program, initiated by the Higher Education Authority in Ireland (PRTLI4). N.R.C. gratefully acknowledges the receipt of a Royal Society Leverhulme Trust Senior Research Fellowship. ’ REFERENCES (1) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1–68. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (3) Langerock, S.; Menard, H.; Rowntree, P.; Heerman, L. Langmuir 2005, 21, 5124–5133. (4) Sondag-Huethorst, J. A. M.; van Helleputte, H. R. J.; Fokkink, L. G. J. Appl. Phys. Lett. 1994, 64, 285–287. (5) Felgenhauer, T.; Yan, C.; Geyer, W.; Rong, H. T.; G€olzh€auser, A.; Buck, M. Appl. Phys. Lett. 2001, 79, 3323–3325.

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