Size-Induced Metal to Semiconductor Transition in a Stabilized Gold Cluster Ensemble

Abstract

Organic ligand stabilized metal clusters are a unique composite system possessing nanometer scale domains in which a metal core is encapsulated by an insulating organic monolayer. The confinement of a group of neutral metal atoms in such a small domain confers electronic properties that may be intermediate between continuous metals and quantized molecular species depending on the actual size of the domain. The insulating monolayer encapsulant is a conductivity barrier through which facile electron tunneling and/or hopping occurs. As a composite, both domains can exert strong influences on the macroscopic electrical conductivity. The strengths of these influences depend on their intrinsic nature and dimensions of each domain. Such materials are postulated to have exciting prospects for applications in microelectronics and possibly molecular electronics. 1 Our interest is in the development of these materials as the responsive component in a microelectronic chemical vapor sensor. In preparing and characterizing several series of these materials with variable core sizes and ligand shell thicknesses, we have ob-served a transition from a positive to a negative temperature coefficient of electrical conductivity for thin films of a homologous series of alkanethiol-stabilized gold clusters as described below and illustrated in Figures 1 and 2. As a model system, alkanethiol-stabilized gold clusters have a chemistry that is very amenable to systematic variation in both size of the gold core and thickness of the ligand shell. They are easily dispersed to concentrations as high as 10 wt % in nonpolar organic solvents and can be manipulated and characterized as soluble organic compounds. Recently, a synthesis of elegant simplicity was reported for this system where a nanometer scale gold cluster derivatized with a monolayer of dodecanethiol is described. 2 In this twophase system, gold chloride is dispersed by a phasetransfer agent in a toluene solution of the alkanethiol and is subsequently reduced by contact via rapid stirring with an aqueous sodium borohydride solution. As neutral gold particles nucleate and begin to grow, they are sequestered by a monolayer of the alkanethiol in a kinetically controlled fashion. Simple variation of the gold:alkanethiol stoichiometry generates a series of stabilized clusters with a large range in core size. 3 We have prepared such a series of dodecanethiol-stabilized gold clusters (see Table 1). As an abbreviated nomenclature for the normal alkanethiol-stabilized gold cluster we use the following general form: Au:C n (X:Y), where the subscript n denotes the number of carbon atoms in the alkane chain and (X:Y) denotes the gold:alkanethiol stoichiometric ratio used in the synthesis. As practical limits to desirable variations, a lower limit occurs where the electrical conductivity becomes very low and difficult to measure (e.g., X:Y ) 1:3 for the dodecanethiol system) (1) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; W...

Single‐Electron Transfer in Nanoparticle Solids

“…In scanning tunneling spectroscopic (STS) studies of isolated particles, [1][2][3] the resulting currentpotential (I-V) profile generally exhibits a Coulomb blockade in the central region, beyond which a Coulomb staircase (single-electron transfer; SET) may be identified. Such unique characteristics are the fundamental basis for the development of single-electron transistors.[4] By contrast, in studies of nanoparticle ensembles that form (sub)micrometer-thick solid films, [5][6][7][8] typically only linear (Ohmic) I-V behavior is observed, especially at a relatively high voltage bias, because of rampant structural defects within these particle solids that facilitate interparticle charge transfer (e.g., percolation effects). Fundamentally, the collective conductivity properties of organized assemblies of particles are found to be determined not only by the particle chemical structure (core size, shape, and surface ligands), but by the specific chemical environments and interparticle interactions as well.…”

Nanoelectronic Chemical Sensors for Chemical Agent and Explosives Detection

“…The relative rates of nanocluster growth and alkanethiol surface complexation are a function of the initial concentrations of Au(III) chloride and alkanethiol reagents. 11 In the cases described in this paper, a 1:1 molar ratio of reagents was employed which produced gold nanoclusters with an average core diameter of 1.7-nm. This translates into approximately 201 gold atoms per nanocluster (Au 201 ) as determined by others 12 from 1 H-NMR line-broadening, high-resolution transmission electron microscopy, small-angle X-ray scattering and thermogravimetric analysis.…”

Semiconductor and Metallic Core–Shell Nanostructures: Synthesis and Applications in Solar Cells and Catalysis

“…[22][23][24][25] Metallic NPs or clusters have also been emerging as a new type of important functional materials for optical, electronic, magnetic and catalytic applications. [26][27][28][29] They can exhibit interesting properties, such as band gap opening (responsible for metal-to-semiconductor transitions), size-dependent band gaps, strong surface plasmon resonance, and unusual catalytic behaviour. All of these properties are quite different from those of individual metal atoms and/or their bulk counterparts, due to the quantum-size and surface effects.…”