Surfaces of Nanosized Semiconductor Particles

Particles in the nanometer-size regime necessarily have large surface-to-volume ratios approximately one-third of the atoms are located on the surfaces of 40 A CdS particles, for example. Furthermore, colloid chemical preparations typically result in the development of surface imperfections and in the incorporation of adventitious or deliberately added dopants. Such surface defects act as electron and/or hole traps and, thus, substantially modify the optical and electro-optical properties of nanosized semiconductor particles. Altered photostabilities [595], fluorescence [579, 594, 596, 597], and non-linear optical properties [11, 598-600] are manifestations of the surface effects in colloidal semiconductors. 

Separation of bulk and surface properties in macroscopic semiconductors is less than straight forward and requires highly sensitive experimental techniques. In contrast, the large surface-to-volume ratios in nanosized semiconductor particles render the examination of surface processes in and/or on these colloids to be experimentally feasible. Advantage has been taken of pulse radiolysis to inject electrons (in aqueous, N20-saturated solutions which contained 2-propanol see Eqs. 22,23, and 25) or holes (in aqueous, N20-saturated solutions which did not contain 2-propanol see Eqs. 22 and 23) into nanosized semiconductor particles [601, 602], Electron injection into CdS particles, for example, decreased the extinction coefficient at 470 nm (the absorption onset) by — 5 x 104 M-1cm-1 (Fig. 98) [576]. Hole injection resulted in the appearance of a transient absorption band in the long-wavelength region and in much less 

The supporting medium (aqueous or organic solvents membrane-mimetic compartments) also has a profound influence on the optical and electro-optical properties of nanosized semiconductor particles. This dielectric confinement (or local field effect) originates, primarily, in the difference between the refractive indices of semiconductor particles and the surrounding medium [573, 604], In general, the refractive index of the medium is lower than that of the semiconductor particle, which enhances the local electric field adjacent to the semiconductor particle surface as compared with the incident field intensity. Dielectric confinement of semiconductor particles also manifests in altered optical and electro-optical behavior. 

Formation of composite and sandwich-type semiconductors constitutes an interesting development (Fig. 100) [576,605-609]. A composite semiconductor, like a cherry and its stone, contains one material as its core and a second material as its shell. CdS particles coated by Cd(OH)2, CdSe coated by ZnS, and HgS coated by CdS are examples of recently prepared composite semiconductors 

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