Micro Raman Spectroscopy of Silicon Nanocrystals Produced by Picosecond Pulsed Laser Ablation
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Micro Raman Spectroscopy of Silicon Nanocrystals Produced by Picosecond Pulsed Laser Ablation M. H. Wu, R. Mu, A. Ueda and D. O. Henderson Department of Physics Fisk University Nashville, TN 37208 B. Vlahovic Department of Physics North Carolina Central University Durham, NC 27707 ABSTRACT Pulsed laser ablation has been used to produce silicon nanocrystals. Variation of the laser fluence, backing gas type and pressure result in nanocrystals with controllable size distributions. Properties of nanocrystals produced with this method also depend on the distance of the nanocrystal from the center of the laser plume. Correlated atomic force microscopy and in-situ micro-Raman measurements confirm that particle size decreases as distance from the plume center increases. Silicon peaks in the micro raman spectra taken at increasing distance from plume center show considerable differences in both center energy and width. Confocal micro raman spectra from thicker ( > 10 micron) samples show little variation with depth, in contrast with porous silicon samples. INTRODUCTION Silicon nanoparticles have long interested researchers due to their potential for use in silicon based photonic devices. Laser ablation of silicon has been used to generate silicon nanoparticles in an environment relatively free of contaminants.1 The vast majority of previous reports concerning pulsed laser ablation synthesis of silicon nanocrystals have involved the use of nanosecond pulse width lasers.2 These reports have conclusively shown that pulsed laser ablation is an efficient method of generating nanometer sized silicon particles. Nanoparticle production occurs in both forward and backward scattering configurations. In the first case, particles generated by ablation of a pure Si target grow through collisions with an inert backing gas (typical pressures 1 – 10 torr) and diffuse in a forward direction to a substrate typically located 1 – 5 cm from the target. Particles can also be collected on a substrate located in the plane of the target adjacent to the ablation site. Particle size has been shown to be a complex function of pulse energy, inert backing gas pressure and distance from plume center.3 Recent studies of ultrafast pulsed laser micromachining, however, have revealed that ultrafast pulses have significant advantages compared to longer pulses. Shorter pulses result in decreased thermal effects, which reduce the generation of large microparticles. Use of nanosecond pulses also inevitably results in laser plume interactions, which can also affect nanoparticle generation. We have previously reported that picosecond pulsed laser ablation can reproduce the results obtained with nanosecond pulsed laser ablation, but with lower laser pulse energy densities and inert backing gas pressures. This may
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help narrow particle size distributions and help clarify the mechanism of nanoparticle formation during the ablation process. We report here characterization of thin silicon nanoparticle films by confocal micro Raman scattering and atomi
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