Article pubs.acs.org/JPCC
Structural Evolution of Gas-Phase Coinage Metal Clusters in Thiolate Self-Assembled Monolayers on Au Leila Costelle,†,⊥ Minna T. Raï san̈ en,‡ Jennifer T. Joyce,§ Christophe Silien,§ Leena-Sisko Johansson,∥ Joseph M. Campbell,∥ and Jyrki Raï san̈ en*,† †
Department of Physics, Division of Materials Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland Department of Chemistry, Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki , Finland § Department of Physics and Energy, and Materials and Surface Science Institute, University of Limerick, Castletroy, Co. Limerick, Ireland ∥ Forest Products Surface Chemistry Group, Department of Forest Products Technology, Aalto University, School of Chemical Technology, P.O. Box 16300, FI-00076 Aalto, Finland ‡
ABSTRACT: Metallization of organic surfaces is important especially for applications in molecular electronics. It can be realized by different means, one promising albeit less studied method being gas-phase deposition of metal clusters. Here, we report on the interactions of gas-phase Cu, Ag, and Au clusters with n-dodecanethiolate self-assembled monolayers (SAMs) on Au substrate. The morphology and composition of the deposited clusters and their impact on the interface structure of the SAM/Au substrate were investigated using scanning tunneling microscopy. The chemical and physical interactions between the clusters and thiolates were characterized using X-ray photoelectron spectroscopy. The Au clusters are found to penetrate through the monolayer as a whole and partially retain their spherical geometry, whereas atom-by-atom diffusion and/or defect-mediated penetration are proposed for the Cu and Ag clusters.
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reported a first series of experiments and simulations of gasphase Au cluster deposition on alkanethiolate SAM modified Au(111),13 which revealed a penetration mechanism of Au clusters through the monolayer (ML) fundamentally different from the ones reported for atomic vapor deposition. It was shown that, because of their low kinetic energy, the gold clusters initially compress the ML and that different cluster− surface binding patterns eventually prevail, ranging from nonbinding to full contact and including intermediate configurations in which the clusters are bound to the underlying Au(111) surface via molecular links and nanowires. The final configurations were shown to be strongly dependent on the choice of the physical parameters.13 In this new paper, we generalize our previous work and compare the interactions of gas-phase Cu, Ag, and Au clusters with n-dodecanethiolate [S(CH2)11CH3, DDT] SAMs on Au(111). The use of DDT SAMs in our study was motivated by the investigation of the cluster diffusion through the monolayer without the added complication of a reactive end group on the thiol, focusing thus on the penetration through the packed monolayer. Scanning tunneling microscopy (STM) was used to investigate the morphology of the deposited cluster and its impact on the SAM structure, and X-ray photoelectron
INTRODUCTION Metallization of organic surfaces has gained much interest in recent years because of its importance in molecular electronics. Thus, a thorough understanding of the interactions between gas-phase metal clusters and molecules is crucial. Alkanethiols form well-packed and ordered self-assembled monolayers (SAMs) on coinage metal substrates and enable tailoring of the interfacial properties of the surfaces. These layers are excellent model systems for both fundamental and applied research.1 Of special importance in metal surface−SAM experiments is the immobilization of metal clusters on top of the SAM for the fabrication of metal−SAM−metal structures. While the penetration of the metal clusters in the SAM can be controlled to some extent, e.g., by selecting SAM-forming molecules with particular functional groups,2,3 the mechanisms involved are poorly understood. Coinage metals were chosen in this study because of their catalytic, optical, and electrical properties, enabling numerous applications in catalysis4,5 and optoelectronics.6−8 In most studies reported in the literature, metallization is achieved by electrochemical3,9 or vapor deposition.10 The metal−molecule interactions are complex and strongly depend on the organic layer and the experimental conditions. Typically, the metal atoms penetrate through the monolayer via its structural defects and two-dimensional structures form at the SAM−substrate interface.11,12 In contrast, gas-phase cluster deposition allows more control over the properties of the deposited clusters. Recently, we have © 2012 American Chemical Society
Received: July 19, 2012 Revised: September 28, 2012 Published: October 2, 2012 22602
dx.doi.org/10.1021/jp307148p | J. Phys. Chem. C 2012, 116, 22602−22607
The Journal of Physical Chemistry C
Article
summarized to aid the reader to better follow the comparisons made between different metal cluster systems. Parts a and b of Figure 1 respectively show large-scale and high-resolution STM images of DDT SAM before cluster
spectroscopy (XPS) characterized the chemical and physical interactions between clusters, SAM, and substrate at different cluster coverages.
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EXPERIMENTAL SECTION SAM Preparation. n-Dodecanethiol (≥98%) was purchased from Sigma-Aldrich and used as received. Thiol SAMs were prepared at room temperature by immersing freshly flame-annealed Au(111)-coated mica slides (300 nm Au, Georg Albert, PVD-Beschichtung) in 1 mM ethanol solution of ndodecanethiol for 24 h, after which the samples were rinsed with ethanol and blown-dried with N2. Cluster Deposition. The Au, Ag, and Cu clusters were deposited under ultrahigh vacuum conditions using a gas aggregation source (NC200, Oxford Applied Research). In brief, a discharge is generated by a direct current magnetron equipped with the metal target and operated with a 20 sccm Ar flow to generate the clusters. The magnetron power and pressure in the chamber during deposition were kept at 30 W and 10 Pa, respectively. The cluster coverage was varied using different deposition times. The cluster size distribution was determined using the grain analysis function implemented in the scanning probe microscopy data analysis software Gwyddion.14 The mean size of the gas-phase cluster is about 2.5 nm in diameter.13 The preformed clusters impinge on the surface with a low kinetic energy. We consider, as a rough approximation, that the cluster velocity is close to the expanding gas velocity, namely, Vc ≈ VAr = {2γkT/[(γ − 1)mAr]}1/2, with γ = 5/3 for argon.15 Thus, the approximate kinetic energies of Au, Ag, and Cu clusters are ∼0.3, ∼0.2, and ∼0.1 eV/atom, respectively. STM Characterization. The samples were imaged by STM (PicoScan, Molecular Imaging) in constant-current mode under ambient conditions. The STM tips were mechanically cut Pt/Ir (80:20) wires of 0.25 mm diameter (Advent Research Materials Ltd. or Goodfellow). When imaging nanosized particles with STM, tip convolution effects led to enlargement of the apparent diameter of the particles. The cluster size has been corrected following the procedure described by Schiffmann et al.16 The zscale calibration of the STM was checked with the gold steps (height 0.25 nm) which are present on DDT SAM modified Au(111) surfaces.17 XPS Characterization. XPS data were obtained using an AXIS 165 system (KRATOS Analytical) equipped with an Al Kα monochromatic X-ray source operated at 100 W and 90° electron takeoff angle. Wide scans were recorded with a 1 eV step and 80 eV analyzer pass energy and high-resolution spectral regions with a 0.1 eV step and 20 eV analyzer pass energy. The samples were charge-neutralized during data acquisition with slow thermal electrons trapped in the vicinity of the sample surface. Since neutralization causes the surface Fermi levels to float, binding energies were corrected in the high-resolution spectra by setting the Au 4f7/2 peak to 84 eV.18 A more detailed description of the experimental procedure can be found elsewhere.19
Figure 1. Large-scale and high-resolution STM images of (a, b) DDT SAM modified Au(111) surface and (c) Au clusters deposited on DDT SAM modified Au(111) surface at low coverage (
