Nanosecond pulsed laser ablation of gold with an excitation wavelength of 532 nm was conducted in supercritical CO2 to generate gold nanoparticles, which were then investigated by scanning electron microscopy and small-angle X-ray scattering, and their extinction spectra and simulated extinction spectra were studied. Both the morphology and amount of gold nanoparticles changed significantly with changes in the density of supercritical CO2 during laser ablation. In a gaslike density, a network structure consisting of nanonecklaces was the major product, whereas in a liquidlike density, large nanospheres with an average diameter (⟨D⟩) of 500 nm were produced. After absorption of multiphoton of excitation light, the gold nanonecklaces and large nanospheres were generated by the fragmentation and solidification, respectively, of liquid gold droplets with ⟨D⟩ = 500 nm. The amount of both products changed according to the branching ratio, which determined whether the liquid gold droplets followed the fragmentation or solidification channel. The local structure of supercritical CO2 in the vicinity of the gold nanoparticles determined the preferred reaction channel. A significant change in the branching ratio occurred near the density ρr = 0.7, where both the enhancement of the local density of supercritical CO2 and the degree of solvation of fluid molecules around the gold nanoparticles reached a maximum. To the best of our knowledge, this is the first study to observe the density dependence of morphological changes in gold nanoparticles fabricated by laser ablation in a supercritical fluid and the local structure of the supercritical fluid that determines the morphology and amount of nanoparticles.

Nanosecond pulsed laser ablation of bulk silicon crystal upon the excitation of 532 nm was conducted in supercritical CO 2 to generate silicon nanocrystals, whose properties were studied by seven experimental methods. According to the photoluminescence spectra and fluorescence microscope images, emissions of near-ultraviolet, violet, blue, green, and red were observed in air, at room temperature, and without cooling in liquid nitrogen or a helium cryogenic system. A preferable emission channel of carriers, generated by photoexcitation of Si/SiO 2 of core/shell structure, was responsible for interface states with defect sites. This luminescence process caused color changes and intensity increase, enhanced by a factor of 100, where thermal properties of supercritical CO 2 were maximized, due to critical anomaly. It was found that colors and intensities of photoluminescence of silicon nanocrystals are controlled by a cooling rate during ablation, whose quantity is manipulated by the supercritical fluid pressure.

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.