Hot-carrier radiative recombination through phonon confinement in silicon nanocrystals embedded in colloidal xerogel matrix

Abstract

Colloidal suspension of free standing silicon/silicon oxide core/shell nanoparticles has been synthesized using a mechanochemical top-down approach. Quasi-mono-dispersed core size distribution of synthesized nanoparticles has been confirmed using different structural, morphological, and optoelectronic characterizations. Raman, continuous wave photoluminescence and time-resolved photoluminescence studies have been performed on synthesized colloidal nanoparticles in ethanol medium. Asymmetric broadening of the Raman peak (red shifted with respect to that of bulk silicon) has been observed. Intensities and positions of photoluminescence emission peaks are prominently dependent on excitation photon energy. Moreover, the photoluminescence decay time varies from sub-nanoseconds to tens of nanoseconds. The decay time also exhibits a strong dependence on the excitation wavelength, while the emission wavelength is kept unaltered. The abovementioned observations indicate the slow relaxation of photo-excited carriers in silicon quantum dots. This particular phenomenon takes place due to phonon mode discretization, which is further responsible for the radiative recombination of hot-carriers and consequent strong visible emission. The enhancement of hot-carrier lifetime in colloidal silicon quantum dots is the key requirement for the active material of the hot-carrier solar cell. Runny texture of the synthesized material inhibits practical device implementation; therefore, the synthesized nanoparticles have been embedded in the silica xerogel matrix. The impression of phonon mode confinement, in silicon quantum dots embedded in a hard matrix, has been observed, resulting in increased hot-carrier lifetime. The enhanced hot-carrier lifetime can lead to the realization of a silicon-based active material for the hot-carrier solar cell.