To investigate the influence of pulse repetition rate on the average
size of the nanoparticles, nanocrystalline Si films were prepared by
pulsed laser ablation in high-purity Ar gas with a pressure of 10 Pa at
room temperature, under the pulse repetition rates between 1 and 40 Hz,
using a nanosecond laser. Raman, X-ray diffraction spectra, and scanning
electron microscopy images show that with increasing pulse repetition
rate, the average size of the nanoparticles in the film first decreases
and reach its minimum at 20 Hz, and then increases, which may be
attributed to the nonlinear dynamics of the laser-ablative deposition. In
our experiment conditions, the duration of the ambient restoration, a
characteristic parameter being used to distinguish nonlinear or linear
region, is about a few seconds from the order of magnitude, which is
consistent with the previous experimental observation. More detailed model
to explain quantitively the observed effect is under investigation.
Nanocrystalline silicon films were prepared by pulsed-laser ablation in high-purity He, Ar or Ne gas at room temperature under a deposition pressure of 10 Pa. The Raman and Xray diffraction spectra indicate that the films are nanocrystalline. Scanning electron microscopy images show that Ar or Ne gas, compared to He gas, yields smaller and more uniform-sized Si nanoparticles at the same deposition conditions, which is also confirmed by the blue-shifted and narrower peaks obtained in photoluminescence measurement. Ne gas induces the smallest and most uniform, in size, Si nanoparticles among all the three gases, which may be attributed to a more effective energy transfer between Si and Ne atoms resulting from the adjacent degree of the atomic weights.
We present a method to determine where the nanoparticles nucleate and grow during pulsed laser deposition in an ambient gas. Briefly, nanocrystalline Si films are systemically deposited on the substrates located at a distance from the plasma and placed in horizontal direction; meanwhile an external electric field is introduced perpendicularly to the plume. Based on the transportation dynamics of Si nanoparticles corresponding to different electric fields, the lateral nucleation range of 0.1 to 33.8 mm is determined for Si nanoparticles deposited in 10 Pa Ar gas at a laser fluence of 4 J/cm 2 . Further simulation of the mass and area density of Si nanoparticles demonstrates that both nucleation and growth probabilities in nucleation region are approximately Gauss-dependent of the lateral distance.
The transport dynamics of ablated particles produced by pulsed-laser deposition in an inert gas is investigated via the Monte Carlo simulation method. The splitting mechanism of ablated particles is discussed by tracking every ablated particle with their forces, velocities and locations. The force analysis demonstrates that whether the splitting appears or not is decided by the releasing way of the driving force acting on the ablated particles. The "average" drag force, which is related to the mass and radius of the ambient gas, determines the releasing way of the driving force. Our simulated results are approximately in agreement with the previous experimental data.
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