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,
Two new methods have been developed to precisely position gold nanoparticles on surfaces. The surface-active nanoparticles have a shell of a mixed monolayer comprised of alkanethiol and alkanedithiol molecules to anchor particles to gold surfaces via sulfur−gold chemisorption. In the first method, regions of an alkanethiol self-assembled monolayer (SAM) are shaved by the AFM tip under high force in a solution containing nanoparticles. Nanoparticles then adsorb onto the exposed areas defined by the shaving track of the tip. In a second method, the AFM tip is coated with nanoparticles. Under low force, AFM images are acquired and the nanoparticles remain on the tip. When higher load is applied, areas of the SAM matrix are uncovered and nanoparticles are deposited following the scanning track of the AFM tip. Thus, the 3D positions of the nanoparticles are precisely controlled. The nanostructures are characterized in situ with the same tip at reduced load. Individual particles within the nanopatterns can be resolved by AFM. In both methods, the matrix SAM effectively resists the nonspecific binding of nanoparticles, and prevents lateral diffusion of nanoparticles. The high spatial precision offered by AFM lithography is advantageous for fabrication of nanoparticle-based nanodevices.Metal nanoparticles exhibit size-dependent optical, 1 electronic, 2,3 and catalytic properties, 4 which have great potential for engineering new materials and sensors. 5,6 Prospective applications for nanoparticles include miniature electronic devices, 7-9 spin coatings, 10 and biosensing. [11][12][13][14] Prototype devices in molecular electronics, which incorporate gold nanoparticles as components, include single-electron transistors, 8,15-17 single-electron charging devices, photonic switches, 18 and quantum dots. 19 The 3D positions of nanoparticles on surfaces must be controlled precisely-hopefully at the level of individual particles. To make micro-and nanoscale devices functional, nanoparticles must be aligned precisely in nanowires 20,21 and nanoparticles must be positioned precisely at the gap of metal-insulator-metal junctions. 22 Methods are continuously being developed and improved for directing the organization of metal nanoparticles into thin film layers, 23,24 nanocrystal arrays, 25 and superlattices. 26 Hexagonal ordering has been achieved using approaches such
Magnetoelectrochemical studies of gold nanoparticle quantized capacitance charging were carried out at ambient conditions. The single electron transfer responses were found to be sensitive to external magnetic fields, reflected in the enhancement of voltammetric peak currents and shifts of peak formal potentials with increasing magnetic field intensities. Additionally, splittings of voltammetric peaks were also observed upon the application of an external magnetic field. These phenomena might be partly attributed to the paramagnetic characters (electron parity) of nanosized gold particles which are contingent upon their charge states. These novel observations suggest that the nanoparticle electronic energy structures can be varied by magnetic fields, leading to molecular manipulations of the nanoscale charge-transfer chemistry.
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