Biosynthesis of cadmium sulphide quantum semiconductor crystallites

NANOMETRE-SCALE semiconductor quantum crystallites exhibit size-dependent and discrete excited electronic states which occur at energies higher than the band gap of the corresponding bulk solid^1-4. These crystallites are too small to have continuous energy bands, even though a bulk crystal structure is present. The onset of such quantum properties sets a fundamental limit to device miniaturization in microelectronics⁵. Structures with either one, two or all three dimensions on the nanometer scale are of particular interest in solid state physics⁶. We report here our discovery of the biosynthesis of quantum crystallites in yeasts Candida glabrata and Schizosaccharomyces pombe, cultured in the presence of cad-mium salts. Short chelating peptides of general structure (γ-Glu-Cys)_n-Gly control the nucleation and growth of CdS crystallites to peptide-capped intracellular particles of diameter 20 Å. These quantum CdS crystallites are more monodisperse than CdS par-ticles synthesized chemically. X-ray data indicate that, at this small size, the CdS structure differs from that of bulk CdS and tends towards a six-coordinate rock-salt structure.