On the origin of white photoluminescence from ZnO nanocones/porous silicon heterostructures at room temperature

Abstract

ZnO nanocones were grown on porous silicon (PS) by vapor transport at different temperatures and O2 flow rates without metal nanocatalysts. The ZnO/PS heterostructures were studied by electron microscopy, and by photoluminescence and electron resonance (EPR) spectroscopies. Samples emitted strong white PL, opening potential application for white light illumination. The photoluminescence spectra were found to include five main components centered at 1.84, 2.06, 2.30, 2.48 and 3.31 eV. The components at 2.48 and 2.06 eV were related to oxygen vacancies in ZnO, that at 2.3 eV to Zn vacancies and the one at 1.84 eV to the PS substrate. The low excitonic emission intensity at 3.31 eV and the observed rough ZnO nanocone morphology suggest the presence of surface trap states. EPR shows a g ∼ 2.006 line attributed to Zn divacancies. The absence of the g = 1.96 line, attributed to conduction band electrons, suggests strong carrier depletion.