SYNTHESIS AND OPTICAL-PROPERTIES OF MOS2 AND ISOMORPHOUS NANOCLUSTERSIN THE QUANTUM CONFINEMENT REGIME

Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and se veral of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to similar to 2 nm. Thes e clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band -structures for the bulk crystals. Key results of this work include: ( 1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clu sters similar to 2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters less than or si milar to 2.5 nm in size which are only two unit cells thick; (3) the d emonstration of the strong influence of dimensionality on the magnitud e of the quantum confinement. Specifically, three-dimensional confinem ent of the carriers produces energy shifts which are over an order of magnitude larger than those due to one-dimensional (perpendicular to t he layer planes) confinement emphasizing the two-dimensional nature of the structure and bonding; (4) the observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size, a feature that reflects quantum confinement as well as possible changes in the degree of hybridization of the electronic orbitals which make up the s tates at these points; and (5) the observation of photoluminescence du e to both direct and surface recombination. Several of these features bode well for the potential of these materials for solar photocatalysi s. (C) 1997 American Institute of Physics.