Nov 7, 2012 - Polydisperse diphosphine-capped gold clusters were synthesized in solution and introduced into the gas phase by electrospray ionization.
Article pubs.acs.org/JPCC
Coverage-Dependent Charge Reduction of Cationic Gold Clusters on Surfaces Prepared Using Soft Landing of Mass-Selected Ions Grant E. Johnson,* Thomas Priest, and Julia Laskin Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-88, Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: The ionic charge state of monodisperse multiply charged cationic gold clusters on surfaces may be controlled by selecting the coverage of mass-selected ions soft landed onto a substrate. Polydisperse diphosphine-capped gold clusters were synthesized in solution and introduced into the gas phase by electrospray ionization. Mass selection was employed to isolate a multiply charged cationic cluster species (Au11L53+, m/z = 1409, L = 1,3-bis(diphenylphosphino)propane) which was delivered to the surfaces of four different self-assembled monolayers on gold (SAMs) at controlled coverages of 1011 and 1012 clusters. Employing the spatial profiling capabilities of in situ time-of-flight secondary ion mass spectrometry (TOF-SIMS), it is shown that, in addition to the chemical functionality of the monolayer (as demonstrated previously: ACS Nano 2012, 6, 573), the coverage of cationic gold clusters on the surface may be used to control the relative abundance of different charge states of the soft landed multiply charged clusters. In the case of a 1H,1H,2H,2Hperfluorodecanethiol monolayer (FSAM) almost complete retention of charge by the deposited Au11L53+ clusters was observed at a lower coverage of 1011 clusters. In contrast, at a higher coverage of 1012 clusters, pronounced reduction of charge to Au11L52+ and Au11L5+ was observed on the FSAM. When soft landed onto 16- and 11-mercaptohexadecanoic acid surfaces on gold (16,11COOH-SAMs), the mass-selected Au11L53+ clusters exhibited partial reduction of charge to Au11L52+ at lower coverage and additional reduction of charge to both Au11L52+ and Au11L5+ at higher coverage. On the surface of the 1-dodecanethiol (HSAM) monolayer, the most abundant charge state was found to be Au11L52+ at lower coverage and Au11L5+ at higher coverage, respectively. A coverage-dependent electron tunneling mechanism is proposed to account for the observed reduction of charge of mass-selected multiply charged gold clusters soft landed on SAMs. The results demonstrate that one of the critical parameters that influence the chemical and physical properties of supported metal clusters, ionic charge state, may be controlled by selecting the coverage of charged species soft landed onto surfaces.
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INTRODUCTION The highly size-dependent physical and chemical properties1−5 of small metal clusters have made them the focus of considerable research attention in the chemistry and materials science communities. The eventual goal of much of this research is to develop economical techniques for the scalable synthesis of metal clusters of an exact size and composition. Such precise methods will enable the rational assembly of cluster-based materials6,7 with desired attributes from welldefined metal particles having distinctive optical,8−10 electronic,11−13 or catalytic properties.14,15 Indeed, metal nanoparticles have already shown considerable promise for applications in photothermal therapeutic treatments,16 as contrast agents in biological and cellular imaging,16 and as composite devices for threat detection through sensing of chemicals.17−19 In addition, a number of studies have demonstrated that the active species that improve the efficiency of chemical reactions in heterogeneous catalysis are small metal particles less than 1 nm in diameter.20,21 For instance, the catalytic oxidation activity of gold adsorbed onto iron oxide has been assigned to clusters containing ∼10 gold atoms that are ∼0.5 nm in diameter.20 In addition, 55-atom gold clusters (diameter ∼1.4 nm) deposited onto inert supports have been © 2012 American Chemical Society
demonstrated to be highly active toward the oxidation of styrene while clusters larger than 2 nm have been found to be completely inactive.21 Similar size-dependent behavior has been observed in studies of model catalysts prepared by the deposition of size-selected ionic metal clusters onto various supports. The pioneering work of Heiz and Landman demonstrated that clusters containing eight gold atoms supported on defect-rich magnesium oxide (MgO) surfaces are the smallest clusters to promote the low-temperature oxidation of carbon monoxide (CO) to carbon dioxide (CO2).22 In addition, Anderson and co-workers recently investigated the catalytic activity of size-selected palladium clusters (Pdn) deposited onto titanium dioxide (TiO2) toward the oxidation of CO to CO2.23 Using temperature-programmed reaction (TPR) and X-ray photoemission spectroscopy (XPS), they discovered a size-dependent variation in the catalytic activity that was strongly correlated with the Pd 3d electron binding energy of the clusters.23 In similar studies, Vajda and co-workers showed that size-selected three-atom silver clusters Received: September 4, 2012 Revised: November 6, 2012 Published: November 7, 2012 24977
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charge was reported by the same group 20 years later.54 Subsequent studies have shown that complex ions including peptides,55−58 proteins,59−65 organometallic complexes,66−70 and metal clusters50,71−82 may be soft landed onto substrates intact. For example, previous studies from our laboratory examined the charge reduction and neutralization of protonated peptides ions52,56−58,83−90 as well as cationic metal−salen91,92 and ruthenium tris(bipyridine)49,93 organometallic complexes soft landed onto different SAMs. Collectively, these studies demonstrated that the mechanism of charge reduction and neutralization depends on the nature of the deposited ion. Specifically, protonated molecules such as peptides and proteins undergo loss of charge by transferring a proton to the SAM while permanent ions such as organometallic complexes undergo neutralization through electron transfer from the surface. Theoretically, it is possible that certain protonated molecules may be neutralized by electron transfer from the surface as well as loss of a proton. This phenomenon, however, has not been confirmed experimentally for soft landed ions. More recently, we demonstrated that both the size and charge state of small multiply charged cationic gold clusters on surfaces may be controlled by soft landing of mass-selected ions onto carefully chosen monolayers.51 Mass selection allows species with precisely defined charge and chemical composition to be delivered to surfaces. In addition, for species that are deposited from the solution phase, soft landing avoids any contamination resulting from the presence of neutral clusters, residual reactants, solvent molecules, and counterions.52 Therefore, using soft landing of mass-selected ions, it is possible to study the size-, surface functionality-, and coveragedependent charge reduction of multiply charged gold cluster ions with an unprecedented level of control. Herein, we demonstrate that, in addition to the chemical functionality of the SAM, the surface coverage of mass-selected ions may be used to control the charge state of multiply charged cationic gold clusters soft landed onto surfaces. Gold clusters capped with diphosphine ligands were prepared in solution by reacting a gold precursor with a weak reducing agent. The resulting solution was introduced into the gas phase using electrospray ionization.94 Mass selection was utilized to select a single ionic cluster species, Au11L53+ m/z = 1409, which was delivered to SAMs terminated with CF3 (FSAM), COOH (COOH-SAM), and CH3 (HSAM) functional groups at defined coverages of 1011 and 1012 clusters. The spatial profiling capability of in situ TOF-SIMS was utilized to determine the distribution of ionic charge states of the multiply charged clusters across the deposited regions. We observe, for the first time, coverage-dependent reduction of charge of soft landed multiply charged cations that is consistent with electron tunneling through the SAM induced by the buildup of potential across the insulating monolayer. The results indicate that for substrates such as the FSAM and COOH-SAMs, which have significant barriers to electron transport, the coverage of soft landed ions may be used to control the relative abundance of different charge states on the surface.
deposited onto alumina (Al2O3) surfaces are highly selective toward the oxidation of propylene to propylene oxide.24 Collectively, these previous studies highlight the importance of having atomically precise control over the size of metal clusters supported on surfaces. While it is well accepted that size exerts a pronounced influence on the properties of small metal clusters, it has only recently become recognized that charge state also strongly affects the geometric and electronic structure as well as the chemical reactivity of these species.25−27 For example, ion mobility experiments conducted on ionic gold clusters in the gas phase have shown that a transition from two-dimensional to three-dimensional structures occurs at a size of 8 atoms for cationic clusters28 and 12 atoms for anionic clusters.29,30 With respect to chemical reactivity, it has been demonstrated that gas-phase anionic gold oxide clusters react with CO exclusively by an Eley−Rideal-like mechanism31 while oxidation of CO by cationic gold oxide clusters occurs by both an Eley−Rideal and a Langmuir−Hinshelwood-like mechanism.32 Charging effects also have been observed for metal clusters supported on surfaces. For instance, singly and doubly charged anionic linear gold clusters have been characterized on ultrathin MgO films supported on Ag.33,34 In addition, positively charged gold species have been isolated at specific binding sites on the surface of MgO.35 The high CO oxidation reactivity of Au8 clusters trapped at F-center defects on the surface of MgO has been attributed to the partial transfer of electron density from the support material to the cluster.22 More recently, similar charging effects have been observed for nonmetallic clusters such as WxOy and MoxSy deposited onto ultrathin supports such as alumina on Ni/Al, thereby expanding the relevance of interfacial charging to a broad new range of supported nanomaterials.36 The charge transport properties of self-assembled monolayers of organic molecules on surfaces have important implications for the fabrication of devices such as chemical electrodes functionalized with catalytically active nanoparticles,37,38 plasmonically enhanced photocatalysts,39 organic thin-film transistors,40 and organic memory.41 The local chemical and physical environment at the surface is known to exert a pronounced influence on the transport of electrons through nanometer-size metal particles in contact with SAMs.12,42 Moreover, the transport of charge in molecularscale devices has been shown to be highly dependent on the method of fabrication.43 Consequently, it is desirable to characterize the transport of charge through nanoparticle− molecule−metal junctions of a wide range of compositions and geometries. Furthermore, it is necessary to measure the transport of charge across monolayers at different length scales so that the influence of multiple surface domains and defects may be included. Because of these requirements, a technique that can expose a controlled area of a SAM surface to a defined potential and can estimate the magnitude of electron tunneling through the SAM offers a powerful analytical capability for the field of molecular electronics. While there are a variety of methods for preparing metal nanoparticles with a narrow distribution of sizes,44−48 only soft landing of mass-selected ions enables atomically precise “atomby-atom” control over the size, composition, and charge state of ionic metal clusters delivered to surfaces.49−52 Soft landing (SL) was first utilized by Cooks and co-workers in 1977 to deposit small sulfur-containing ions onto metal surfaces.53 Deposition of polyatomic ions onto surfaces with retention of
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EXPERIMENTAL METHODS Synthesis of Nanoparticles. Ligand-stabilized gold clusters were prepared in solution following procedures described in t he literature. 9 5 , 9 6 Brie fly, chlor o(triphenylphosphine)gold(I) (99.9%, Sigma-Aldrich) was dissolved in a 1:1 (v:v) mixture of methanol and chloroform (Sigma-Aldrich) to create a 0.1 mM solution. 1,3-Bis24978
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that all of the ions retain their charge once deposited onto the surface, the maximum potential resulting from the total charge delivered by the ion beam to the surface may be estimated58 and was calculated to be ∼2 V for 1 × 1012 triply charged Au11L53+ ions distributed over a spot 5 mm in diameter. Because this potential is much smaller than the kinetic energy of the ions, it does not interfere with ion deposition. It follows that all of the ions delivered to the substrate are able to approach the surface and may become immobilized. Previous studies by Nakajima and co-workers have demonstrated that small organometallic ions may penetrate into SAMs as opposed to staying on top of the monolayer when deposited from the gas phase.67,68,101 We propose that the significantly larger Au11L53+ clusters are less likely to penetrate into the monolayers than the smaller organometallic ions due to their greater steric repulsion. This proposition is supported by previous data which showed that cluster ions soft landed onto SAMs at low kinetic energy are readily desorbed from the surfaces, complete with retention of charge, without substantial fragmentation using TOF-SIMS.51 Analysis of Surfaces by in Situ TOF-SIMS. In situ analysis of the surfaces was performed using 15 keV Ga+ TOF-SIMS in a commercial PHI TRIFT II instrument (Physical Electronics, Eden Prairie, MN). In TOF-SIMS, the sample is bombarded by 15 keV primary gallium ions (Ga+, 500 pA, 5 ns pulse width, 10 kHz repetition rate) which induces desorption of material from the surface. The positive secondary ions ejected from the surface are extracted into the mass analyzer which consists of three electrostatic sectors. The mass spectrometer compensates for any energy dispersion of the secondary ions and achieves a mass resolution of around 4000 at 1000 m/z. The ion abundance line profiles shown in the figures were acquired by scanning the primary ion beam across the deposited spot and recording the abundance of Au11L53+, Au11L52+, and Au11L5+ secondary ions as a function of position on the surface.
(diphenylphosphino)propane (97%, Sigma-Aldrich) was added to a concentration 0.1 mM. After mixing of the gold precursor and diphosphine ligand, borane tert-butylamine (97%, SigmaAldrich) was added to a concentration of 0.5 mM. The solution was stirred at room temperature for 3 h until it turned a dark orange color, indicating the formation of diphosphine-capped gold clusters. The solution was stored in the dark at room temperature in a Pyrex bottle. Preparation of Self-Assembled Monolayer on Gold Surfaces. The gold surfaces used to prepare the SAMs were obtained from Platypus Technologies (Madison, WI) and have the following specifications: 1 × 1 cm, 525 μm thick Si, 50 Å Ti adhesion layer, 1000 Å Au layer. 1H,1H,2H,2H-Perfluorodecanethiol (FSAM), 11- and 16-mercaptohexadecanoic acid (C11,C16-COOH-SAM), and 1-dodecanethiol (HSAM) were purchased from Sigma-Aldrich. The FSAM, COOH-SAM, and HSAM surfaces were prepared following procedures described in the literature97,98 using ultrapure ethanol as the solvent which was also purchased from Sigma-Aldrich. The acetic acid used for the preparation of the COOH-SAM surfaces was purchased from Fisher Scientific. The gold substrates were ultrasonically washed in ethanol, cleaned using a Boekel (Boekel Scientific, Feasterville, PA) ultraviolet cleaner, and immersed in glass scintillation vials containing 1 mM solutions of thiol in ethanol solvent for the FSAM and HSAM and a 1 M acetic acid solution in ethanol for the COOH-SAMs. The monolayers were allowed to assemble for at least 24 h and then ultrasonically washed for 5 min in ethanol for the FSAM and HSAM and in 1 M acetic acid solution in ethanol for the COOH-SAMs. The surfaces were then rinsed with pure ethanol, dried with nitrogen (N2), and mounted in the SIMS sample holder for cluster deposition and subsequent analysis by in situ TOF-SIMS.49 Soft Landing of Mass-Selected Multiply Charged Gold Cluster Ions. The soft landing experiments were conducted employing a custom-built instrument coupled to a TOF-SIMS, which has been described in detail elsewhere.49 Briefly, triply charged Au11L53+ cations were generated through electrospray ionization,99 introduced into vacuum using an electrodynamic ion funnel,100 focused in a collision quadrupole, mass selected with a quadrupole mass filter, deflected 90° by a quadrupole bender, and transferred to the surface through two einzel lenses. An optimized ion current of ∼40 pA was directed at the surface for either 0.5 or 4 h, corresponding to a total delivery of 1.5 × 1011 or 1.2 × 1012 ions, respectively, to a circular spot ∼5 mm in diameter. During the experiments, positively charged ions are deposited onto partially insulating SAMs on Au on an underlying conductive Si surface. This substrate is mounted on a conductive metal bracket which is connected to a highresistance electrometer (Keithley 6517A, Keithley Instruments, Cleveland, OH). The ion current, measured using the electrometer, represents the current of electrons required to charge up the back plate of a parallel-plate capacitor in response to the current of 3+ ions charging up the front plate of the capacitor across the insulating SAMs.58 This current is measured continuously throughout the deposition to accurately determine the total number of ions deposited onto the surface. The current does not change significantly during the deposition and disappears immediately after the ion beam is switched off. The kinetic energy of the ions impacting the surface was controlled by adjusting the potentials applied to the second collision quadrupole and the surface and was set at ∼20 eV per charge for all of the experiments described herein. Assuming
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RESULTS AND DISCUSSION In this study we investigate the influence of the surface coverage of deposited ions on the charge retention, reduction, and neutralization behavior of mass-selected Au11L53+ clusters soft landed onto different SAMs. In a previous publication we demonstrated that at lower coverage (1011 clusters) triply charged monodisperse gold clusters retain their charge when deposited onto fluorinated monolayers. In contrast, partial and complete neutralization of charge was observed when the same gold clusters were soft landed onto COOH-SAM and HSAM monolayers, respectively.51 Based on these previous results, four different SAMs were selected for the current experiments because they are known to span a spectrum from surfaces that are charge reducing (HSAM), partially charge reducing (COOH-SAMs), and charge retaining (FSAM) toward soft landed ions. Reduction of charge and neutralization of ions on the surfaces of SAMs occurs either at defects in the monolayer or by the transfer of electrons through the intact layer. Consequently, when investigating how the terminal functionality of the SAM influences reduction of charge and neutralization of deposited ions, it is important to keep the thickness of the monolayer (length of the alkyl chain) similar. During the soft landing experiments, a mass-selected beam of Au11L53+ clusters is focused at a spot on the surface of a SAM for a predetermined period of time. The beam of ions has a Gaussian-like radial intensity profile as has been shown previously using the position-sensitive IonCCD detector.102,103 24979
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the center of spots of soft landed ions has been characterized previously for high coverages of protonated peptides on the surfaces of SAMs.84 The suppression of all secondary ion abundance was attributed to matrix effects that result in decreased abundance of all of the secondary ions at the center and increased abundance near the edges of the deposited spot. In comparison, for Au11L53+ deposited onto the FSAM surface at higher coverage, the singly charged Au11L5+ cluster exhibits increased relative abundance while the multiply charged Au11L53+/2+ clusters show decreased relative abundance at the center of the deposited spot (Figure 1b). Moreover, the suppression of abundance is more pronounced for the 3+ as compared to the 2+ ions. Because of the fact that the abundance of all of the secondary ions is not suppressed, a process involving reduction of charge by transfer of electrons through the monolayer rather than suppression of overall secondary ion abundance is consistent with the data presented in Figure 1b. The abundance line profiles presented in Figure 1 indicate that Coulombic effects due to charged clusters already landed on the surface do not influence the spatial distribution of ions arriving later in the deposition. If this were the case, arriving ions would experience an ever-increasing repulsion throughout the duration of the deposition which would result in a continuous expansion of the diameter of the deposited spot. Consequently, for Coulombic repulsion, there would not be a cluster coverage-dependent threshold at which the deposited charge would begin to defocus the incoming ion beam. Moreover, the relatively small potentials that build up on the surface during deposition (∼2 V), even at higher cluster coverage, will only effect ions with extremely low kinetic energy. Another possibility is that Coulombic repulsion between soft landed ions on the surface may result in an expansion of the spot following deposition. This is highly unlikely at the low surface coverage employed in these experiments. The Gaussian-shaped profiles in Figure 1a indicate that all three charge states of the deposited ions are most abundant at the center of the deposited spot, which is consistent with the radial intensity profile of the ion beam. Moreover, comparison of Figures 1a and 1b demonstrates that the overall width of the deposited spots is very similar at both low and high cluster coverage. Therefore, Coulombic repulsion between ions on the surface, which would create wide flat spots without depletion of the higher charge states at the center and would result in wider deposited spots at higher cluster coverage, may be excluded as an explanation for the line profiles observed in Figure 1b. In addition, the enrichment of the 1+ charge state at the center of the spot indicates that there is no net repulsive force preventing deposition of ions at that location. The voltage-induced tunneling of electrons across organic monolayers on metal surfaces has been characterized using several experimental techniques and also has been the subject of extensive theoretical modeling.43,104−108 It is generally agreed upon that the energy barrier at the metal−molecule interface, the molecular electronic structure, and the IP of the terminal group are the key parameters which determine the tunneling properties of alkanethiol monolayers.106 Of particular relevance, both the chemical functionality and length (thickness) of the monolayer may exhibit a pronounced influence on the magnitude of the tunneling currents observed at a given applied potential. For example, experimental dI/dV curves obtained using scanning tunneling microscopy (STM) have shown that conventional alkanethiols exhibit a linear increase in
Because of the Gaussian-like intensity profile of the ion beam, the surface coverage of soft landed ions is higher at the center than near the edges of the deposited spot. Consequently, the spatial profiling capabilities of in situ TOF-SIMS provide a straightforward way to determine the relative abundances of the different charge states of the Au11L5 clusters as a function of position and, thereby, cluster coverage on the surface. In this manner, the influence of cluster coverage on the distribution of ionic charge states of the soft landed Au11L53+ ions was systematically investigated on the surface of each SAM. Figures 1a and 1b show the in situ TOF-SIMS abundance line profiles obtained by scanning the primary Ga+ ion beam
Figure 1. In situ TOF-SIMS abundance line profiles of Au11L53+, Au11L52+, and Au11L5+ on the surface of the FSAM following deposition of (a) 1.5 × 1011 clusters and (b) 1.2 × 1012 clusters.
through the deposited spot of clusters at a coverage of 1011 and 1012 clusters on the surface of the FSAM, respectively. The spatially resolved in situ TOF-SIMS abundance profiles allow the distribution of ionic charge states of Au11L5 to be determined across the entire deposited spot of clusters as opposed to at only the center of the deposition, as was reported previously.51 At lower cluster coverage, the triply charged Au11L53+ ions are the most abundant molecular ions, followed by Au11L52+, and very minor singly charged Au11L5+ ions. The abundance ratios of the three different charge states of the Au11L5 clusters are observed to be constant across the entire deposited region at lower coverage, with no charge state showing enrichment over the others at any location. This result is consistent with the findings reported previously for the deposition of 1.5 × 1011 clusters onto the surface of the FSAM where the relative abundances of the charge states of Au11L5 were found to be 3+ > 2+ > 1+ at the center of the deposited spot.51 Moreover, the substantial retention of charge by Au11L53+ at lower coverage observed in the current spatial profiling experiments is consistent with previous results from our laboratory which demonstrated that cationic organometallic ions91,92 as well as singly and doubly protonated peptides52,56−58,85,88 retain their charge on the surface of the FSAM. At higher cluster coverage on the surface of the FSAM, however, the ion abundance line profiles are very different than at lower coverage. As shown in Figure 1b, the abundances of both Au11L53+ and Au11L52+ are dramatically suppressed at the center and enriched near the edges of the deposited spot. Of particular importance, at the center of the deposited spot the singly charged Au11L5+ cluster shows increased abundance. Suppression of the overall abundance of all secondary ions at 24980
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tunneling current with applied potential while fluorinated alkanethiols exhibit current suppression in the ±1 V potential range and an exponential increase in current above this voltage.104 In addition, Selloni and co-workers employed the Tersoff−Hamann approach to calculate the tunneling current between −1 and +1 V for various alkanethiol monolayers.107 The calculations revealed that tunneling currents may vary by up to a factor of 3 for SAMs of the same thickness depending on the identity of the terminal functional group. 107 Furthermore, calculations by Sanvito and co-workers have demonstrated that the interaction between the tip of a STM and the monolayer may result in configurational changes to the molecules in the monolayer that directly influence the measured tunneling current.108 Consequently, tunneling currents obtained using STM must account for this electrostatic interaction with the tip. Numerous studies have shown that longer (thicker) SAMs result in reduced electron tunneling currents at equivalent applied potentials compared to shorter (thinner) monolayers. Moreover, SAMs containing polar functional groups may create large overall surface dipoles that may either enhance or obstruct the transfer of electrons through the organic layer, depending on the direction of the net surface dipole.109,110 If tunneling of electrons is responsible for the reduced abundance of Au11L53+ and Au11L52+ and the increased abundance of Au11L5+ at the center of the deposited spot on the FSAM, then the extent of reduction of charge of ionic gold clusters soft landed onto SAMs should depend on both the chemical functionality and thickness of the monolayer. Both of these factors were examined experimentally and are discussed in the following paragraphs. To determine whether or not the observed charge reduction phenomenon is influenced by the chemical functionality of the monolayer, identical soft landing and in situ TOF-SIMS profiling experiments were conducted on two different carboxylic acid terminated (COOH-SAM) surfaces containing 11 and 16 carbon atoms in the alkyl chain. The abundance line profiles obtained from 1011 and 1012 Au11L53+ clusters soft landed onto the surface of the C11-COOH-SAM are shown in Figures 2a and 2b, respectively. Inspection of Figure 2a reveals that a much more pronounced reduction of charge of Au11L53+ to Au11L52+ occurs on the surface of the C11-COOH-SAM than
on the surface of the FSAM. This is manifested by the almost equal abundance of Au11L53+ and Au11L52+ across the entire deposited spot at lower cluster coverage. The relative order of abundance of the charge states is observed to be Au11L52+ > Au11L53+ ≫ Au11L5+, which is consistent with the results of our previous investigation that were obtained at the center of the deposited spot at similar coverage.51 The abundance line profiles obtained at higher cluster coverage, however, are very different and reveal a reduction in the abundance of multiply charged Au11L52+/3+ ions at the center but not near the edges of the deposited spot. Again, the abundance of the 3+ ions exhibits more substantial suppression at the center of the spot than the abundance of the 2+ ions. The two-peak profile with decreased abundance at the center is very similar to the profile observed at higher cluster coverage on the surface of the FSAM. In contrast, the increased abundance of Au11L5+ observed at the center of the spot on the surface of the FSAM at high cluster coverage (Figure 1b) is not observed on the C11-COOH-SAM (Figure 2b). To ascertain whether or not the efficiency of charge reduction depends on the length (thickness) of the monolayer, identical soft landing and TOF-SIMS profiling experiments were conducted on the longer C16-COOH-SAM containing 16 instead of 11 carbon atoms in the alkyl chain. The abundance line profiles are shown in Figures 3a and 3b for lower and
Figure 3. In situ TOF-SIMS abundance line profiles of Au11L53+, Au11L52+, and Au11L5+ on the surface of the C16-COOH-SAM following deposition of (a) 1.5 × 1011 clusters and (b) 1.2 × 1012 clusters.
higher cluster coverage, respectively. At lower cluster coverage the spatial profiles and relative abundances of the different charge states are similar for the C11 and C16-COOH-SAMs. At higher cluster coverage, however, the charge reduction of Au11L53+ and Au11L52+ at the center of the deposited spot is found to be far more pronounced on the shorter C11 than the longer C16-COOH-SAM. Therefore, the findings for the COOH-SAMs, combined with the results for the FSAM, strongly suggest that reduction of charge as opposed to suppression of secondary ion abundance is responsible for the observed profiles. As an additional test of the charge reduction phenomenon, soft landing and TOF-SIMS experiments were performed on a conventional C12-alkanethiol (HSAM). The abundance line profiles for 1011 and 1012 Au11L53+ clusters deposited onto the surface of the HSAM are shown in Figures 4a and 4b, respectively. The results obtained at lower cluster coverage
Figure 2. In situ TOF-SIMS abundance line profiles of Au11L53+, Au11L52+, and Au11L5+ on the surface of the C11-COOH-SAM following deposition of (a) 1.5 × 1011 clusters and (b) 1.2 × 1012 clusters. 24981
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the cluster beam (lower intensity beam over a defined deposition time). Consequently, at the end of soft landing, the area at the center of the deposited spot has the highest surface coverage of cationic clusters while the areas near the edges of the spot have the lowest cluster coverage. Because of the fact that each mass-selected Au11L53+ cluster soft landed onto the surface has three positive charges associated with it (assuming initial retention of charge on the surface), the Gaussian surface coverage profile of the clusters also results in a Gaussian potential (voltage) profile on the surface with the center of the deposited spot having the highest potential. This potential, ΔV, can be readily estimated using eq 1: ΔV = Figure 4. In situ TOF-SIMS abundance line profiles of Au11L53+, Au11L52+, and Au11L5+ on the surface of the HSAM following deposition of (a) 1.5 × 1011 clusters and (b) 1.2 × 1012 clusters.
ZeNionsd AεR ε0
(1)
where Z is the charge state of the ion, e is the elementary charge, Nions is the number of ions on the surface, d is the thickness of the film, A is the area exposed to the ion beam, ε0 is the vacuum permittivity, and εR is the relative permittivity of the SAM. Assuming that the dielectric constant of the SAM is ∼2 and the thickness of the SAM is ∼1 nm, a potential of ∼2 V may accumulate at the center of the deposited spot during soft landing of 1012 Au11L53+ clusters. On the surface of the FSAM, the 2 V potential at the center of the deposited spot of cationic clusters (area ∼ 20 mm2) is large enough to break down the charge barrier to electron transport and may enable reduction of Au11L53+ to Au11L52+/1+ through electron tunneling from the underlying gold surface. However, further from the center and closer to the edges of the deposited spot the coverage of triply charged clusters, and therefore the surface potential, decreases in a Gaussian-like manner. When the potential becomes lower than the ∼1 V threshold for electron transport across fluorinated monolayers, much less reduction of charge of Au11L53+ occurs, which is why Au11L52+/3+ both exhibit increased abundance near the edges of the deposited spots on the FSAM at higher cluster coverage. In the case of the carboxylic acid terminated monolayers on gold (C11/C16-COOH-SAMs) an interface dipole similar to that on the FSAM will exist due to the large dipole moment of the individual carboxylic acid groups (∼1.5 D) at the end of each alkyl chain in the monolayer. Collectively, this will create a charge barrier to electron transfer at the COOH/vacuum interface similar to that observed on the FSAM. In fact, theoretical calculations have shown that the strength of the dipole on COOH-SAMs depends not only on the dipole moment of the carboxylate group but also on the coupling between the dipole moments of the Au−S and COOH groups as well as the length of the carbon chains and the tilt angle of the overall monolayer.110 It is reasonable, therefore, that the reduction of charge of Au11L53+/2+ that is observed to be most pronounced at the center of the deposited spot on the COOHSAMs is due to the large potential that accumulates during soft landing of 1012 triply charged gold clusters. In contrast, near the edges of the spot the surface coverage and potential are not large enough to induce electron transport, and therefore, a larger abundance of the doubly and triply charged clusters is observed. The higher relative abundance of Au11L52+ on the surfaces of both the C11 and C16-COOH-SAM surfaces compared to the FSAM surface is due to more facile transfer of charge through the CH2 chains of the COOH-SAMs than the CF2 chains of the perfluorinated FSAM. Indeed, theoretical calculations have shown that in conventional alkanethiols the
reveal that all three charge states of Au11L5 are more abundant at the center as opposed to near the edges of the deposited spot on the surface of the HSAM. Similar to the results obtained on the COOH-SAM surfaces, the most abundant charge state at the center of the deposited spot on the HSAM is Au11L52+ at lower cluster coverage. However, the abundance ratio of Au11L52+ to Au11L53+ is much larger on the HSAM than on the COOH-SAM surfaces at lower cluster coverage. The charge reduction and neutralization is observed to be far more pronounced at higher surface coverage on the HSAM. In sharp contrast to the abundance profiles obtained on the surfaces of the FSAM and COOH-SAMs, almost all of the ions have been reduced to Au11L5+ at a coverage of 1012 clusters on the HSAM. A small abundance of Au11L52+ is also observed with only minor contributions from Au11L53+. Of particular importance, on the surface of the HSAM there is no enrichment of abundance of the higher charge states of Au11L5 near the edges of the deposited spot at higher cluster coverage as was observed on the FSAM and COOH-SAMs. The spatial distribution of the charge states of cationic gold clusters observed at higher cluster coverage on the FSAM as well as the COOH-SAM surfaces is consistent with a cluster coverage-dependent mechanism involving the tunneling of electrons from the underlying gold surface through the insulating monolayer and resulting in reduction of charge of the deposited multiply charged gold clusters. The slow transport of electrons through partially fluorinated SAMs has been characterized using several surface-science techniques and is generally attributed to interface dipoles which create a charge barrier to electron transport at the CF3/vacuum interface.104,109,111 Consequently, for SAMs with terminal CF3 groups electron tunneling in the ±1 V potential range is strongly suppressed and is not generally observed in scanning tunneling spectroscopy (STS) measurements.104 In comparison, voltage-dependent current I(V) curves obtained from a variety of different experiments have shown that relatively small potentials (
