Abstract

Time resolved Raman scattering studies of the surface reactions of organic molecules adsorbed on colloidal semiconductor (TiO₂ and CdS) crystallites, following optical excitation of the crystallite, show that single electron redox reactions occur.¹ The vibrational structures of the initial transient species show negligible distortion due to adsorption on the crystallite surface. TEM micrographs of the CdS crystallites (zinc blende cubic) directly image the crystallite internal lattice. The crystallites are essentially excised fragments of the corresponding bulk lattices. As diameter decreases in the range 50 to 20Å, CdS and ZnS crystallites show an increasing blue shift of the optical absorption edge (exciton peak), by as much as 0.8 eV.^2,3 The exciton peak intensity also increases relative to the above gap absorption, as it shifts to higher energy. CdS crystallites of = 45Å average dimeter show corresponding exciton shifts in the LO phonon Raman excitation spectra, and the relaxed fluorescence spectra. We interpret these observations as quantum size effects. That is, these crystallites are too small for the bulk band gap to completely form. The size dependence of the crystallite lowest excited state is modelled, without adjustible parameters, at the same level of approximation as in the Wannier bulk exciton Hamiltonian.⁴ The exact Coulomb interaction between an electron and hole in a polarizable small crystallite is derived. The size dependences of the ionization potential and chemical redox potentials are also modelled.⁵ Materials with strong chemical bonds involving the valence electrons, and hence small effective masses for holes and electrons at the band edges, should show significant quantum size effects.

© 1984 Optical Society of America