Laser-induced nanoparticle ordering
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D.A. Blom and H.M. Meyer, III High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6064 (Received 23 May 2002; accepted 29 July 2002)
Nanoparticles were produced on the surface of silicon upon pulsed-laser irradiation in the presence of an inert gas atmosphere at fluences close to the melting threshold. It was observed that nanoparticle formation required redeposition of ablated material. Redeposition took place in the form of a thin film intermixed with extremely small nanoparticles possibly formed in the gas phase. Through the use of nonpolarized laser light, it was shown that nanoparticles, fairly uniform in size, became grouped into curvilinear strings distributed with a short-range ordering. Microstructuring of part of the surface prior to the laser treatment had the remarkable effect of producing nanoparticles lying along straight and fairly long (approximately 1 mm) lines, whose spacing equaled the laser wavelength for normal beam incidence. In this work, it is shown that the use of polarized light eliminated the need of an aiding agent: nanoparticle alignment ensued under similar laser treatment conditions. The phenomenon of nanoparticle alignment bears a striking similarity with the phenomenon of laser-induced periodic surface structures (LIPSS), obeying the same dependence of line spacing upon light wavelength and beam angle of incidence as the grating spacing in LIPSS. The new results strongly support the proposition that the two phenomena, LIPSS and laser-induced nanoparticle alignment, evolve as a result of the same light interference mechanism.
I. INTRODUCTION
Pulsed lasers are widely used in lithography, film deposition, nanoparticle formation, and surface modification. The possibility of using pulsed-laser ablation to produce nanoparticles into a self-organized array is very attractive because the laser beam can illuminate areas on a square-centimeter scale and thus ordering could be achieved on a very large scale. Material ejection from an illuminated surface is one of the important effects realized with a high-fluence beam from a pulsed-laser.1 The ablation mechanisms depend upon the beam energy. Photoablation, the ejection of ions or atoms at sufficiently low laser energies, is caused by direct interaction of the incoming photons with surface atoms. Thermal ablation, with possible ejection of microdroplets, takes place at higher energies, as the absorbed radiation produces a very high temperature increase in the near-surface region and thus profuse evaporation. Eventually, at very high laser energies, critical temperatures on the near surface can be reached and explosive evaporation can occur. J. Mater. Res., Vol. 17, No. 11, Nov 2002
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Generally, ablation takes place in a few tens of nanoseconds and, if the particle density in the gas phase ahead of the surface is sufficiently high, clusters may form by atomic collisions. The number of collisions is strongly enhanced when a buffer gas is introdu
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