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This paper reports experimental studies of the development of bulk optical properties as a function of crystallite size for the inorganic direct gap semiconductor CdS. Small crystallites are synthesized via colloidal chemical techniques, and their optical properties are studied in situ at extreme dilution. The crystallites are characterized via high resolution transmission electron microscopy. Direct images show (111) lattice planes, and establish the crystallite structures as close to those of excised fragments of bulk CdS (zinc-blende cubic). Large crystallites (> 100 Å average diameter) show an optical absorption, in colloidal solution, close to that of bulk crystalline material. However, small crystallites of 30 Å average diameter show a large blue shift (∼0.8 eV) in absorption edge (effective band gap), and an intensification of edge absorption relative to absorption at higher energy regions. These observations can be understood as quantum size effects resulting from confinement of an electron and hole in a small volume. 40 Å average size crystallites show a smaller shift (∼0.25 eV), and corresponding changes in their fluorescence, and resonance Raman excitation, spectra.

Very small ZnS and CdS crystallites are made and stabilized in aqueous and methanolic media without organic surfactants. Low temperature (−77 °C) synthesis in methanol produces the smallest crystallites, ≈30 Å diameter cubic CdS and <20 Å diameter cubic ZnS. The crystallites are characterized by transmission electron microscopy and in situ optical spectroscopy (λ≳200 nm). The crystallites are too small to exhibit bulk band gaps in their optical spectra. In the band gap region, the small crystallites show a higher energy absorption threshold with a resolved spectral feature (quantum size exciton peak), not present in the spectra of larger crystals. The far ultraviolet spectra are unaffected by size at present resolution. These results can be understood in terms of the crystallite molecular orbitals, and an elementary confined electron and hole model.

Luminescence has been observed from dilute aqueous CdS colloidal solutions following irradiation above the semiconductor band gap. The emission is attributed to radiative recombination of short-lived (r < 5 ns) h+ with e~in small particles of hydrodynamic radius =¡200 A. The luminescence quantum yield is sensitive to surface-adsorbed species that are able to undergo reduction. PbS and p-benzoquinone cause 50% luminescence quenching at ^10"1 2345 M concentration, corresponding to far less than a monolayer surface coverage. These results demonstrate that recombination emission in small CdS particles can be used as a probe of the h+ and e" surface chemistry and physics.

Raman and infrared spectral data on the vibrational modes of the M-(η 5 -C 5 H 5 ) unit have been collected and the fundamental assignments revisited. Complexes belonging to the metallocene series M(η 5 -C 5 H 5 ) and the cyclopentadienide salts [η 5 -C 5 H 5 -][M + ] have been studied, together with some monometallic (η 5 -C 5 H 5 )ML n and bimetallic (η 5 -C 5 H 5 ) 2 M 2 L 2n compounds. Comparison of the spectra shows a common vibrational pattern of the intraring modes, with minor variations in frequency and intensity, which allows the establishment of a vibrational fingerprint of general validity. Raman spectra give rise to a firm assignment of the skeletal metal-ring modes; a correlation is noted between the metal-ring stretch force constant and the metal-carbon distance. The approximate CH out-of-plane force constants vary considerably and presumably reflect different H-ring electrostatic repulsions.