Surface Manipulation of the Electronic Energy of Subnanometer-Sized Gold Clusters:  An Electrochemical and Spectroscopic Investigation

Abstract

Stable undecagold clusters were synthesized and protected with a monolayer of alkanethiolates. The particles were found to exhibit semiconductor electronic characteristics with a band gap of about 1.8 eV, as evaluated from voltammetric and spectroscopic measurements. Photoluminescence in the visible range was also observed from the peak position at 840 nm. The indirect band-gap characteristics indicate that there exist substantial surface trap states in the nanoparticle molecules. Prior to exchange reactions with alkanethiols, however, no luminescence was observed with the gold particles, Au 11 Cl 3 (PPh 3 ) 7 . This was interpreted, at least in part, by the effect of ligand fields on the electronic (bandgap) energy splitting and the resulting electron distributions. These observations strongly suggest that surface chemistry plays a vital role in determining the electronic energy structure of these subnanometer-sized gold particles. Introduction.The intense research interest in nanosized materials is mainly fueled by unique properties that are sizetunable, the so-called quantum effects. For instance, for both semiconductor and (transition-) metal nanoparticles, one interesting property is the growth (increase) of the bandgap energy with decreasing particle dimensions. 1 This provides powerful control of the materials' properties and consequently their potential applications, in particular, in the fields of optoelectronics, nanocircuits, nanodevices, et cetera. 2 Certainly these effects are much more apparent with semiconductor nanomaterials, as demonstrated previously, for instance, by the variation in the color of quantum dots with their sizes, 3 whereas for the metal counterparts, a sizable band gap can be observed only with much smaller nanoparticle dimensions. 4,5 For instance, earlier studies of alkanethiolateprotected gold nanoparticles 5 demonstrated that when the gold cores were smaller than 1.2 nm in diameter a HOMO-LUMO band gap started to evolve, as evidenced by voltammetric as well as near-IR measurements. In other words, in this size range, the gold nanoparticles started to exhibit semiconductor electronic characteristics. In these previous studies, the gold particles were synthesized by the Brust reaction, 6 with the core diameter readily varied within the range of 1 to 5 nm. 7 It can be anticipated that for smaller (i.e., subnanometer-sized) gold particles the semiconductorlike behaviors will be much more pronounced, for instance,

Gold–silica quantum rattles for multimodal imaging and therapy

“…The NQRs presented the same extinction peak and fluorescent emission as the QRs in the NIR (Fig. S5) (3,(16)(17)(18). The QRs' quantum yield (2 × 10 −4 ) was comparable to the yields of other AuQDs reported in the literature (16).…”

“…The QRs displayed a distinct NIR extinction peak at 672 nm ( Fig. 2A), as expected from the photonic properties of the AuQDs hosted within their mesopores (Supporting Information) (3,(16)(17)(18). This peak was absent from the HS or from the AuNPs synthesized using the same protocol in the absence of HS.…”

Synthesis, Stability, and Photoluminescence Properties of PdAu10(PPh3)8Cl2 Clusters

“…Representative examples of these include [Au 11 (PPh 3 ) 8 Cl 2 ] ? [1][2][3][4][5][6][7][8], [Au 13 (PMe 2 Ph) 8 Cl 2 ] 3? [9], [Au 39 (PPh 3 ) 14 Cl 6 ] 2?…”

Electrochemistry of Metal Nanoparticles and Quantum Dots

“…Thus, electron-donating cappings such as phosphines would lead to an increase of the particle core electron density and hence a negative (upward) shift of the Fermi level. Opposite behavior is expected from electron-acceptor cappings, typically thiol-protecting ligands, resulting in a decrease of the electron density of the Au core [49].…”