Size And Temperature Dependence Of The Raman Shift Of Cdse Nanomaterials

It has long been puzzling regarding the physical origin of the dimensionally andthermally induced shift of Raman optical phonons in semiconductor nanostructuressuch as CdSe. It has been well observed that the optical Raman modes shift towardslower frequencies with reducing sample size or rising the experimental temperatureand it is expected that the magnitude of vibration for a surface atom is alwaysgreater than that in the bulk. At the same time, various shapes of device such asnanoparticles, nanowires, and nanotubes have become a focus area in condensedmatter physics and material science because of their tunable optical properties forapplications in optoelectronics, quantum dot lasers, high-density memory,bioengineering, etc.. Among these new interesting properties, electrical and opticalproperties are influenced directly by atomic vibration in terms of phonons. The opticalRaman modes often shift to lower frequencies when the structure is minimized downto the nanometer scale or the specimen is heated. The vibronic behavior of atoms in aspecimen is not only of fundamental significance to processes such aselectron-phonon coupling, phonon-phonon interaction and phonon transport, but alsoof technical importance to practical applications. Thus, it is important to obtainquantitative information from Raman spectroscopy and establish an effective functionto expound the physical mechanism.In terms of quantum confinement or phonon localization in finite-dimensionnanostructures, the size-and temperature-dependent Raman frequency shift has beenintensively investigated in past decades These models could replicate the experimentswell under given conditions despite the abundant adjustable parameters and theirlimitations in forecasting the trends of change and interpreting the intrinsicmechanism under external stimuli.In this communication, we devote to developing a new model to integrate the size,shape, and temperature effects on the Raman shifts of CdSe nanostructures in terms ofthe relaxation of a representative bond for the entire crystal. Based on the frameworkof bond order-length-strength (BOLS) and local bond average (LBA) approach, weshow that the Raman shift depends intrinsically on the order, length, and energy of therepresentative bond that is an average of all the bonds involved. From matching to the measurements, we are able to derive quantitative information about the bondrelaxation dynamics, which complements the available models.