Lizhi Chu

Influence of pulse repetition rate on the average size of silicon nanoparticles deposited by laser ablation

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.

Width of Nucleation Region of Si Nanocrystal Grains Prepared by Pulsed Laser Ablation with Different Laser Fluence

Si nanocrystal grains were prepared by pulsed laser ablation with different laser fluence in Ar gas of 10 Pa at room temperature. The as-formed grains in the space deposited on the substrates and distributed in a certain range apart from target. According to the depositing position and radius of grains, the nucleation locations of grains in the space were roughly calculated. The results indicated that the width of nucleation region broadened with increasing of ion densities diagnosed by Langmuir probe, which increased with laser fluence from 2 J/cm2 to 6 J/cm2; that is, width of nucleation region broadened with addition of laser fluence. At the same time, the width broadened with the terminal formation position moving backward and the initial formation position of grains moving toward ablated spot. The experimental results were explained reasonably by nucleation thermokinetic theory.

Influence of Gas Sort on the Nucleation Region Width of Si Nanocrystal Grains Prepared by Pulsed Laser Ablation

We have calculated the nucleation region (NR) location of Si nanocrystal grains prepared by pulsed laser ablation (PLA) with fluence of 4 J/cm2 in 10 Pa gas at room temperature, and ambient gases were He, Ne, and Ar, respectively. Results of calculation indicated that NR width in Ne gas was narrowest, while it was widest in He gas. Maximum mean size of grains deposited on substrates under ablated spot, which were placed horizontally, was the smallest in Ne gas. It would be attribute to more effective energy transfer during the process of collision when atomic mass of Si and ambient gas Ne are more close to each other. In this work, an additional gas flow with the same element as ambient gas was introduced, which is vertical to the plume axis at different lateral positions above ablated spot.

First-Principle Calculation of Elastic Compliance Coefficients for BiFeO3

The elastic compliance coefficients of PbTiO 3 with/without geometric optimization are firstly calculated by using density-functional theory (DFT) under different exchange correlation functions. It is found that the best results are not obtained by LDA/CA-PZ and optimized structure although these induce the nearest lattice constants to experimental ones, however, the results of PbTiO 3 without geometry optimization under GGA/PW91 function are in great consistent with the experimental data. Based on the same calculating method of PbTiO 3 , the elastic compliance coefficients for BiFeO 3 are obtained. And the data are compared with previous results predicted by fitting experimental data to Landau-type phenomenological theory, which possessing the difference of an order of magnitude with our data.

The difference of energies of Si atoms with single-crystalline, amorphous, free and nanoparticle configurations

Nanocrystalline silicon (nc-Si) films were systematically prepared via three ways: a) laser anneal or b) thermal anneal of the amorphous silicon (α-Si) films deposited by pulsed-laser ablation (PLA) in base vacuum, c) direct PLA in high-purity Ar gas with pressure of 10 Pa. The anneal-laser fluence, thermal-anneal temperature and ablation-laser fluence thresholds corresponding to the beginning of nanoparticles formation were respectively determined by using scanning electron microscopy (SEM), Raman and X-ray diffraction (XRD) techniques. Incorporated with crystallization mechanism, energies compensated for the formation of one Si nanoparticle in the three ways were calculated approximately. The result shows that for different crystallization ways, the potential barriers during the formation of one ~16 nm nanoparticle are on the order of 10-9 mJ.

Angular distribution of damping coefficient of ablated particle in pure He, Ne, and Ar gases

To investigate the angular distribution of damping coefficient of ablated particle under various ambient gases, nanocrystalline silicon films are systemically deposited on a circular substrate by pulse laser ablation in pure He, Ne, and Ar gases, respectively. Scanning electron microscopy images and Raman and X-ray diffraction spectra indicate that the average size of Si nanoparticles decreases with the increase of the departure angle between the film and the plume, and Ne gas induces the smallest and most uniform Si nanoparticles in size among all the three gases. Further theoretical simulation demonstrates the bigger the departure angle, the smaller the damping coefficient of ablated particle, and the damping coefficient in Ne gas is largest for the same angle, implying the most effective energy transfer between Si and ambient atoms.

Dynamic mechanism of the velocity splitting of ablated particles produced by pulsed-laser deposition in an inert gas

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.

Influence of laser beam on transport dynamics of Si nanoparticles by laser ablation

To investigate nucleation area and transport dynamics of Si nanoparticles, nanocrystalline silicon films were prepared by pulsed laser ablation. Subsequently, the additional laser beam as energy source was introduced, which crossed vertically the plasma plume from the top down in front of the target at a distance of 0.5 cm under same experiment condition. In this region, due to collision between the photon and the plasma plume, the transport of Si nanoparticles was impacted by the cross-laser beam. The Raman and x-ray diffraction spectra (XRD), scanning electron microscopy (SEM) images of the films showed that Si nanoparticles were formed in a certain range, and the average size of Si nanoparticles monotonically decreases with the increase of distance. Obviously, the range of Si nanoparticles deposited in substrates became narrower due to the influence of additional laser beam. Experimental results were analyzed in terms of the nucleation area model.

The comparison of growing process of nanocrystalline Si films deposited by pulsed laser ablation in He, Ne, and Ar

In He, Ne or Ar gas under a deposition pressure of 10Pa, nanocrystalline silicon films were prepared by pulsed laser ablation, which the deposition time was 5, 7, 13, 15, 69 and 350min, respectively. A Lambda Pyhsik XeCl excimer laser (wavelength 308nm, pulse duration 15ns, laser fluence 4J/cm2, repetition rate 1Hz) was used, and the distance between Si target and the substrate was 3cm. The Raman spectra indicate that the films are nanocrystalline. Scanning electron microscopy images show that the discrete nanoparticles are first formed, more and more nanoparticles are obtained with increasing of deposition time, and then some nanoparticles start to aggregate and form continuous film, and finally the film ruptures due to the stress. It is the complicated interaction between nanoparticles as-formed in the film and those produced subsequently to lead to the phenomena mentioned above. The morphology of the films deposited in different ambient gases is compared. The result shows that aggregation between nanoparticles, film-formation and rupture take place in a lighter gas earlier than those in a heavier gas. This is related to the different growing rate of the films deposited in different gases.

A nucleation and growth model of silicon nanoparticles produced by pulsed laser deposition via Monte Carlo simulation

We simulated the nucleation and growth of Si nanoparticles produced by pulse laser deposition using Monte Carlo method at the molecular (microscopic) level. In the model, the mechanism and thermodynamic conditions of nucleation and growth of Si nanoparticles were described. In a real physical scale of target-substrate configuration, the model was used to analyze the average size distribution of Si nanoparticles in argon ambient gas and the calculated results are in agreement with the experimental results.