Weak Quantum Effects on the Acoustic Raman Mode in Strongly Confined Silicon: Consequence of Shallow Phonon Dispersion Near-Zone Edge

Abstract

Strong computational capabilities are required for precise prediction of Raman spectra from low-dimensional materials. A computational as well as analytical study of low-frequency Raman spectral studies have been carried out to propose a simpler way to understand the size-dependent variation in Raman spectral parameters with size for silicon (Si). The variation of peak position and peak width (FWHM) with the size of Si nanoclusters has been compared for optic and acoustic phonon branches of silicon. A decrease in the Raman peak position of acoustic phonon, as predicted by computational analysis, was found consistent with the analytically obtained Raman line shape using the phonon confinement quantum effect. It was also observed that the size-dependent variation in optic and acoustic phonons are not similar rather are opposite in nature. The optic phonon shows strong quantum effects in strongly confined systems, whereas acoustic phonons show a strong quantum effect in a weak confinement system. This apparent inconsistency has been explained by correlating the different natures of acoustic and optic phonon branches. An empirical formula has been proposed to validate the dependence of peak position with the size of nanostructures of Si, which fits well with the analytically obtained Raman line shapes. © 2023 American Chemical Society.