The Thermodynamic Properties of Semiconductor Nanocrystals
Semiconductor nanocrystals or "quantum dots" exhibit changes in electronic band structure when their size becomes comparable to the bulk electronic delocalization lengths (tens to hundreds of Angstroms). The band structure modifications are manifest as a blue shift in the nanocrystal optical absorption spectrum. It is essential to characterize the size dependence of the absorption shift and the thermal stability before one may harness nanocrystal properties for device applications. Semiconductor nanocrystals are prepared by arrested precipitation techniques and as such the nanocrystal surface is passivated with an organic layer. Due to the colloidal growth process polydisperse particles result. The size distribution of CdSe nanocrystal samples ranging in radius from 8 A to 45 A are determined using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The results are compared with the tight binding and pseudo-potential calculations. Based on the size dependence measured and the single particle absorption spectrum, the sample size distribution is directly obtainable from the absorption spectrum of the sample. TEM and thermoanalytical melting studies of the II-VI and III-V semiconductors CdS and GaAs show that in the small size regime the melting temperature is size dependent and follows an inverse radius relationship and can be modelled with simple thermodynamic equations. CdS nanocrystals can be melted and recrystallized whereas GaAs decomposes upon phase transformation. The surface tensions of nanocrystals are greater than those of the bulk, in agreement with results for metallic clusters. The melting process involves particle annealing, organic desorption and melting. The melting mechanism appears to involve nucleation on the particle surface and a lobular progression of the melt into the solid.
- Pub Date:
- GALLIUM ARSENIDE;
- Chemistry: Physical; Physics: Condensed Matter