Cadmium Vacancy Minority Defects as Luminescence Centers in Size and Strain Dependent Photoluminescence Shifts in CdS Nanotubes

Abstract

We have studied the absorbance and photoluminescence properties of cadmium sulfide nanotubes with overall size beyond the quantum confinement regime. While the absorption spectra are unaffected by the change in size there is an anomalous red-shift in the photoluminescence spectra with increase in size. Using density functional calculations, we have identified that the shift in the emission peak of the photoluminescence spectra is a result of the interplay between Cd vacancies on the surface of these nanotubes and the crystalline strain which was incorporated in these nanotubes during their growth process. Most importantly, our results show that rather than the defect concentration, it is the nature of the defect which plays a crucial role in determining the optical properties of these nanotubes. For this particular case of CdS nanotubes we find that though S interstitials are the most abundant ones, however, it is the Cd vacancies with second lowest formation energies which significantly affect the photoluminesence spectra.