White-Light-Emitting Silicon Nanocrystal Generated by Pulsed Laser Ablation in Supercritical Fluid: Investigation of Spectral Components As a Function of Excitation Wavelengths and Aging Time

White-light-emitting silicon nanocrystals (Si-NCs) ranging from the near UV to the red region were fabricated by pulsed laser ablation (PLA) of a bulk silicon crystal in a supercritical fluid. The broad photoluminescence (PL) spectra, white light continuum, were investigated by measuring time evolution against aging in the atmosphere or oxygen ambience. The results show that the PL intensity of the higher-energy component increases, whereas that of the lower-energy component decreases as aging time increases. According to rate constants of PL intensity enhancement, the increase in the PL intensity was ascribed to the oxidation of the Si-NCs. This enhancement became significant when the sample was generated at the thermodynamic state, showing a critical anomaly of supercritical CO2. That is, rapid cooling of the hot Si-NC in supercritical CO2 immediately after PLA produces a luminescent Si-NC in the blue-green wavelength region. On the basis of PL spectral measurements at five excitation wavelengths, the lower- and higher-energy PL components were assigned to electronic structures arising from the quantum confinement effect of the Si-NC and the electron-hole recombination at the radiative centers at the surface of the Si-NC, respectively.