Single-electron tunneling in Au 55 cluster monolayers

Abstract

Metallic nanometer clusters composed of 55 gold atoms isolated from each other by a thin organic ligand film, with a total diameter of 2.1 nm, were fabricated as monolayers on various technically relevant substrates by using a two-step self-assembly (SA) procedure as well as combined Langmuir-Blodgett (LB) and SA techniques. Scanning tunneling microscopy (STM) and scanning force microscopy (SFM) studies on these systems reveal a short-range close packing of these clusters. Single-electron tunneling (SET) phenomena, including Coulomb blockade and Coulomb staircase, are studied by local tunneling spectroscopy (LTS) at room temperature as well as at low temperature (90-100 K). A narrow spread of the charge energy is observed on such prepared cluster monolayers.In recent years, nanometer-sized clusters of metals separated from each other by organic ligand shells have received increasing scientific interest [1]. These clusters have a welldefined size in the metal core, with a fixed number of metal atoms, and are stabilized by the organic ligand shell. Thus, they provide excellent model systems for monodispersed metal clusters, embedded in a dielectric matrix for the investigation of physical properties related to nanoscale particles. Among the various clusters, Au 55 (PPh 3 ) 12 Cl 6 (in short Au 55 ) and Pt 309 Phen 36 O 30 (in short Pt 309 ), have been intensely studied by different groups [2-6]. The possible electronic application of these metal clusters, with diameters of 1.4-2.1 nm, as "quantum dots" was first discussed by Schön and Simon [7].Scanning tunneling microscopy and local tunneling spectroscopy has been applied to measure the electronic properties of compact Au 55 cluster pellets [8] or solvent-evaporated Au 55 and Pt 309 clusters on various substrates [9, 10]. Singleelectron tunneling on these systems has been clearly observed in the current-voltage (I-V ) curves. In previous investigations, the cluster solutions were dropped on a substrate and then dried, or the clusters were compressed to a pellet. The * Corresponding author quantitative analysis of I-V curves is based to some extend on the undefined geometric packing of the clusters. The charge energy had a wider spread than expected [10]. Further, in LTS investigations, one interesting question is addressed: whether the additional structures on the Coulomb staircase measured at 4.2 K might be induced by the interaction between the substrate and the clusters or by that between the ligand molecules and the metal core [10]. A uniform packing of clusters on the substrate will help to rule out some unknown effects for the quantitative analysis, since modification of the charging energy of a cluster might be affected by its environment. In addition, achieving ordered arrays of these metal dots is an essential step for developing nanoscale electronic devices [11].Recently, the closed packing of gold clusters, each encapsulated by a monolayer of alkyl thiol molecules, has been successfully achieved by spin casting [12]. As is known, spin casting of suspens...

A study of charge quantization on ligand-stabilized Au55cluster monolayers

“…Finally, in these investigations the clusters were prepared at undefined geometric packing [12]- [14], which causes difficulties in a quantitative study of individual clusters. In other studies the clusters were deposited on a substrate which was covered with other molecules [15] which again complicates the investigation of specific cluster properties.…”

“…The employed variable-temperature UHV-STM set-up is described elsewhere [16]. The measurements were performed at 7 K. Due to the mobility of the clusters at room temperature, which makes the spectra not well reproducible [14,15], it is necessary to work under low-temperature conditions.…”

Tunneling time in nanostructures

“…Thus local microscopic capacitance C micro could be destined in accordance to later STM-measurements on single Au 55 clusters [100]. On the other hand, direct evaluation of local inter-particle resistance R T was not possible since sample resistance revealed no clear dependance from volume.…”

“…Even in compressed discs of ligand stabilized Au 55 clusters there are regions of high order with densest package [33] and SETfeatures (including fine structures due to energy quantization) were visible [98]. In monolayers of Au 55 clusters also were observed a short range close packing with a correlation length of the cluster arrays up to 40 nm (n = 20) [99] and SET-phenomena [100]. Regarding a single electron entering this chain, we are interested in the time or velocity relevant for the propagation of the electron which passes the array in a soliton-like state:…”

Towards production of novel catalyst powders from supported size-selected clusters by multilayer deposition and dicing

“…Nanoclusters display unique and highly size-dependent physical and chemical properties, providing potential for use in, e.g., electronics, optics, biology and catalysis applications [1][2][3][4][5][6][7][8][9]. There are three major cluster production routes: physical vapour (e.g.…”