Crystallization temperature dependency on Si nanoparticles size was studied by using Raman scattering spectroscopy. Si nanoparticles synthesized by pulsed laser ablation were annealed at various temperatures while they were suspended in helium background gas, and then were classified by a differential mobility analyzer. After the size classification, Si nanoparticles showed a narrow size distribution which enabled investigation of the size-dependent crystallization. The temperature threshold for the transition from amorphous to crystalline (Tc) decreased as the particle size decreased: the Tc values of the 10, 8, 6, and 4nm particles were 1273, 1173, 1073 and 773K, respectively.

Optical properties of silicon (Si) nanocrystallites prepared by excimer laser ablation in constant-pressure inert gas have been studied in relation to the particle size. Visible photoluminescence (PL) bands in the red and green spectral regions appear at room temperature after an oxidation process. The red PL band is independent of the particle size and is stable without degradation by excitation light irradiation. It is concluded that the red PL is emitted from the surface states of the oxidized Si nanocrystallites. In contrast, the green PL band depends on the particle size. The green PL intensity decreases during excitation light irradiation in air, and then recovers in the subsequent vacuum evacuation. These results suggest that the origin of the green PL is associated with a quantum confinement effect of Si nanocrystallites.

We have recently developed a method to fabricate monodispersed Ni/NiO core-shell nanoparticles by pulsed laser ablation. In this report, the size-dependent magnetic properties of monodispersed Ni/NiO core-shell nanoparticles were investigated. These nanoparticles were formed in two steps. The first was to fabricate a series of monodispersed Ni nanoparticles of 5 to 20 nm in diameter using a combination of laser ablation and size classification by a low-pressure differential mobility analyzer (DMA). The second step was to oxidize the surfaces of the Ni particles in situ to form core-shell structures. A superconducting quantum interference device (SQUID) magnetometer was used to measure the magnetic properties of nanostructured films prepared by depositing the nanoparticles at room temperature. Ferromagnetism was observed in the magnetic hysteresis loop of the nanostructured films composed of core-shell nanoparticles with core diameters smaller than the superparamagnetic limit, which suggests the spin of Ni core was weakly exchange coupled with antiferromagnetic NiO shell. In contrast, smaller nanoparticles with core diameters of 3.0 nm exhibited superparamagnetism. The drastic change in the hysteresis loops between field-deposited and zero-field-deposited samples was attributable to the strong anisotropy that developed during the magnetic-field-assisted nanostructuring process.

This paper presents the method for fabricating size-selected nickel nanoparticles (diameter:  5−20 nm) coated with nickel oxide shells (thickness:  about 2 nm). The core−shell particles were synthesized by a series of sequential gas-phase processes including pulsed laser ablation, aerosol postannealing, size-classification, compulsory oxidation, and aerosol-jet deposition. The change in the size distribution of the generated particles due to postannealing was measured by a low-pressure differential mobility analyzer (LP-DMA) coupled with a Faraday cup electrometer. The peak size decreased as the postannealing temperature rose, indicating that a sintering and restructuring of the agglomerates was taking place. In the high-resolution TEM (HRTEM) observation, the Ni particles of less than 15 nm in diameter were found to be spherical and single crystal after the annealing at 1073 K. The 20 nm particles were also spherical, but they remained polycrystalline due to insufficient annealing treatment required for recrystallization. The change in the surface morphology due to the compulsory oxidation was analyzed by the HRTEM and electron diffraction. Finally, monodispersed (geometric standard deviation < 1.2) Ni/NiO core−shell particles with oxidized surface layers of 2 nm and particle diameters ranging from 5 to 20 nm were successfully fabricated.

We have demonstrated narrow-band visible light emission from size selected silicon nanoparticles (np-Si), with a wavelength controlled by size tuning. The np-Si were synthesized by pulsed-laser ablation of a silicon single-crystal target in high-purity He background gas. A postannealing process improved morphology and crystallinity. Using a differential mobility analyzer, nanoparticles were classified with a diameter tunable from 3 to 6 nm. Monodispersed np-Si deposited on substrate exhibited a sharp photoluminescence band. The energy of this band increased from 1.34 to 1.79 eV with decrease in particle size, and narrowed to approximately 0.22 eV full width at half maximum due to highly resolved size-selection and improvement in crystallinity. The results suggest that tunable, narrow-band light emitting np-Si produced by gas phase synthesis have good possibilities for application as optoelectronic devices.