Gold-thiolate cluster emission from SAMs under keV ion bombardment: Experiments and molecular dynamics simulations (original) (raw)
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Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2002
Self-assembled monolayers (SAMs) of alkanethiols are an ideal model system to study the mechanisms that lead to emission of organic species under keV ion bombardment. In this contribution, we focus on the emission processes of gold-molecule cluster ions, which are not fully understood yet. To gain insights into these processes, monolayers of octanethiol CH 3 (CH 2) 7 SH adsorbed on gold are investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). First, the static SIMS conditions are verified using the degradation of the SAM signals as a function of ion fluence. Second, the kinetic energy distributions (KEDs) of fragment, parent and cluster ions ejected upon 15 keV Ga þ ion bombardment are measured. The peak maxima, FWHMs and high energy tails of the distributions are analyzed for Au-thiolate clusters, as well as thiol fragments. After calibration of the energy spectra using monoatomic ions, we find that the KEDs of all the clusters containing the thiolate molecule peak at about 1.2 eV. Besides, the distributions of the gold-molecule cluster ions including Au(M-H) À 2 , the most intense cluster peak in the spectrum, are significantly narrower than that of the hydrocarbon fragments.
The Journal of Physical Chemistry B, 2006
This article focuses on the emission of organometallic clusters upon kiloelectronvolt ion bombardment of self-assembled monolayers. It is particularly relevant for the elucidation of the physical processes underlying secondary ion mass spectrometry (SIMS). The experimental system, an overlayer of octanethiols on gold, was modeled by classical molecular dynamics, using a hydrocarbon potential involving bonding and nonbonding interactions (AIREBO). To validate the model, the calculated mass and energy distributions of sputtered atoms and molecules were compared to experimental data. Our key finding concerns the emission mechanism of large clusters of the form M x Au y up to M 6 Au 5 (where M is the thiolate molecule), which were not observed under sub-kiloelectronvolt projectile bombardment. Statistically, they are predominantly formed in highyield events, where many atoms, fragments, and (supra)molecular species are desorbed from the surface. From the microscopic viewpoint, these high-yield events mostly stem from the confinement of the projectile and recoil atom energies in a finite microvolume of the sample surface. As a result of the high local energy density, molecular aggregates desorb from an overheated liquidlike region surrounding the impact point of the projectile.
Quantum-chemical modeling of interaction between gold nanoclusters and thiols
2010
Ab initio calculations are used to model small Au n nanoclusters and Au m SH clusters. The results for the Au 6 , Au 8 , and Au 20 clusters demonstrate that the substitution of a SH group for a Au atom gives a stable cluster of the same geometry if the Au atom has an acute bond angle and a negative effective charge. The example of the Au 10 cluster suggests that SH substitution for Au has a stabilizing effect. The modeling results are discussed with application to self organizing thiol monolayers on gold clusters.
Journal of the American Chemical Society, 1995
Gold clusters stabilized by chemisorbed monolayers of octane-, dodecane-or hexadecanethiolate have been investigated in solution and in the solid phase. These materials can be pumped free of solvent to form a dark brown solid that can be re-dissolved in nonpolar solvents. Their exceptional stability suggests they be viewed as cluster compounds. The self-assembled alkanethiolate monolayers stabilizing the metal clusters can be investigated using techniques that are insufficiently sensitive for study of a monolayer on a flat surface, e.g., 'H and I3C NMR, elemental analysis, differential scanning calorimetry (DSC), thermogravimetry (TGA), and diffusion-ordered NMR spectroscopy (DOSY). Results from such measurements, combined with small-angle X-ray scattering (SAXS) data on solutions of the clusters and images from scanning tunneling (STM), and atomic force microscopy (AFM), are consistent with a small, monodisperse (12 8, radius) gold core, which modeled as a sphere contains -400 Au atoms and -126 alkanethiolate chains, or if modeled as a cuboctahedral structure contains 309 Au atoms and -95 alkanethiolate chains. High-resolution NMR spectra of cluster solutions display well-defined resonances except for methylenes nearest the gold interface; the absence of the latter resonances is attributed to a combination of broadening mechanisms based on the discontinuous change in magnetic susceptibility at the metal-hydrocarbon interface and residual dipolar interactions. Films of the dry, solid cluster compound on interdigitated array electrodes exhibit current-potential responses characteristic of electron hopping conductivity in which electrons tunnel from Au core to Au core. The electron hopping rate decreases and the activation barrier increases systematically at longer alkane chain length. The results are consistent with electron transport rate control being a combination of thermally activated electron transfer to create oppositely charged Au cores (cermet theory) and distance-dependent tunneling (8 = 1.2 A-1) through the oriented alkanethiolate layers separating them.
Dynamics of Thiolate Chains on a Gold Nanoparticle
Small, 2007
An important difference between the surfaces of bulk materials and nanoparticles is the percentage of surface atoms. In a gold nanoparticle % 2 nm in diameter more than 50 % of the atoms are on the surface. The surface chemical functionalization can both stabilize the particle and avoid aggregation. If the molecules at the surface are properly terminated, they can modulate the chemical, electronic, and photochemical properties of the particle. Here, molecular-dynamics simulations of Au 309 A C H T U N G T R E N N U N G (SC 6 H 13) 80 show that the thiolates located at the edge of the antipodal face of the gold nanoparticle are more accessible from the outside. The behavior is compared with experimental data and explained in simple terms. In one of the most investigated types of nanoparticle, thiolates form a protective layer and their terminal atoms are often connected to electro-or photoactive groups. Full surface coverage of a metal surface with linear thiolates ranges from 33 % of the gold atoms for a perfectly planar AuA C H T U N G T R E N N U N G (111) surface up to a maximum of 60 % for particles of the size studied here and even up to 70 % for smaller particles such as Au 25. [1] To achieve the best packing interactions, the ligands are tilted by at least 20-30 degrees with respect to the metal surface. A variety of experimental techniques allows the study of the coverage of the particles. These techniques can be of spectroscopic nature, such as IR and NMR spectroscopy, belong to the family of scanning probe microscopy, or involve chemical reactivity. [2] Molecular-dynamics simulations can also provide insight about the structure and dynamics that take place at the surface, in consideration of the roles played by the length of the thiolate chains and by their dynamics, roles that are now becoming apparent. For instance, nanoparticles easily exchange thiolates with solutions, [2-7] and the kinetics of the reactions vary as a function of the conditions and the timescale of the experimental methods used for assessment. [2-7] Here we examine the thiolate dynamics on a nanoparticle with a model that we have found useful to describe and explain Au-organic interactions. The thiolate chains are described by a standard force field. [8] The gold particle is de
Oxidation of Gold Clusters by Thiols
The Journal of Physical Chemistry A, 2013
The formation of gold−thiolate nanoparticles via oxidation of gold clusters by thiols is examined in this work. Using the BP86 density functional with a triple ζ basis set, the adsorption of methylthiol onto various gold clusters Au n Z (n = 1−8, 12, 13, 20; Z = 0, −1, +1) and Au 38 4+ is investigated. The rate-limiting step for the reaction of one thiol with the gold cluster is the dissociation of the thiol proton; the resulting hydrogen atom can move around the gold cluster relatively freely. The addition of a second thiol can lead to H 2 formation and the generation of a gold−thiolate staple motif.
Gas Phase Formation, Structure and Reactivity of Gold Cluster Ions
Structure and Bonding, 2014
With the advent of electrospray ionisation (ESI) and matrix-assisted laser desorption ionisation (MALDI), mass spectrometry (MS) is now routinely used to establish the molecular formulae of gold nanoclusters (AuNCs). ESI-MS has been used to monitor the solution phase growth of AuNCs when gold salts are reduced in the presence of phosphine or thiolate ligands. Beyond this analytical role, over the past 2 decades MS-based methods have been employed to examine the fundamental properties and reactivities of AuNC ions. For example, ion mobility and spectroscopic measurements may be used to assign structures; thermochemical data provides important information on ligand binding energies; unimolecular chemistry can be explored; and ion-molecule reactions with various substrates can be used to probe catalysis by AuNC ions. MS can also be used to monitor and direct the synthesis of AuNC bulk material either by guiding solution phase synthesis conditions or by soft landing a beam of mass-selected (i.e. monodisperse) AuNC ions onto a surface. This review showcases all areas in which mass spectrometry has played a role in AuNC science.
Background: The adsorption of organic molecules on metal surfaces has a broad array of applications, from device engineering to medical diagnosis. The most extensively investigated class of metal-molecule complexes is the adsorption of thiols on gold. Results: In the present manuscript, we investigate the dependence of methylthiol adsorption structures and energies on the degree of unsaturation at the metal binding site. We designed an Au 20 cluster with a broad range of metal site coordination numbers, from 3 to 9, and examined the binding conditions of methylthiol at the various sites. Conclusion: We found that despite the small molecular size, the dispersive interactions of the backbone are a determining factor in the molecular affinity for various sites. Kink sites were preferred binding locations due to the availability of multiple surface atoms for dispersive interactions with the methyl groups, whereas tip sites experienced low affinity, despite having low coordination numbers.
Theoretical Chemistry Accounts, 2012
Abounding potential technological applications is one of the many reasons why adsorption of aliphatic thiols on gold surface is a subject of intense research by many research groups. Understanding and exploring the nature of adsorbed species, the site of adsorption and the nature of interaction between adsorbed species and the gold surface using experimental and theoretical investigations is an active area of pursuit. However, despite a large number of investigations to understand the atomistic structures of thiols on Au(111), some of the basic issues are still unaddressed. For instance, there is still no clear information about the mechanism of adsorption of alkylthiol on gold surface. Furthermore, the reactivity and mechanism of adsorption of alkylthiol is likely to differ when gold adatoms and/or vacancies in the gold layers are considered. In this work, we have tackled these issues by computing the stationary states involved in the thiols adsorption in order to shed light on the kinetics aspects of adsorption process. In this respect, we have considered a variety of thiols into consideration such as methylthiol, dimethylsulfide, dimethyldisulfide, thioacetates, and thiocyanates. We have also considered the cleavage mechanism in the clean and the reconstructed surface scenario and the structure, energetics and spin densities have been computed using electronic structure calculations. For all the studied cases, an homolytic cleavage of CH 3 S-X (X = H, CH 3 , SCH 3 , CN, and COCH 3 ) bond has been found to occur upon adsorption on the gold surface.