Bond Nature of Raman Shift in CdSe _xS_{1- x} Nanocrystals: A Bond-by-Bond Approach

Owing to the alloying effect, the bond composition of CdSe_xS_1-x nanocrystals is complex. To reveal the bond nature of the Raman red shift in complex-alloyed nanocrystals, we propose the concept of a basic bond unit for nanocrystalline alloy systems. An alloyed nanocrystal can be decomposed into basic bond units according to a bond-by-bond approach. A local-mode model of alloyed nanocrystals is developed, which is applied to determine the Raman red shift of the destination bonds in a basic bond unit. Using this theory, the Raman spectra of CdSe_xS_1-x nanocrystals are obtained. The calculated Raman curves are in good agreement with the experimental ones, indicating that the concept of basic bond unit can well demonstrate the chemical bond nature of the Raman spectra of the alloyed nanocrystals. Based on the strategy of breaking up the whole alloyed nanocrystal into basic bond units, the nanosize effect of Raman shifts may be accurately predicted even just by a pocket calculator.