The photoluminescence spectra were recorded at roomtemperature with a modular double grating excitation spectro-fluorimeter equipped with a TRIAX 320 Fluorolog-3, JobinYvon-Spex emission monochromator and coupled to a R928Hamamatsu photomultiplier, using the front-face acquisitionmode. The excitation source was a 450 W Xe arc lamp. Theemission spectra were corrected for detection and optical spec-tral response of the spectrofluorimeter and the excitation spectrawere corrected for the spectral distribution of the lamp intensityusing a photodiode reference detector.