Multiple Modes of Laser-Induced Pattern Formation in Nanoscopic Co Films
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0960-N02-02
Multiple Modes of Laser-Induced Pattern Formation in Nanoscopic Co Films Christopher Favazza1,2, Justin Trice1,2, Radhakrishna Sureshkumar2,3, and Ramki Kalyanaraman1,2 1 Department of Physics, Washington University in St. Louis, St. Louis, MO, 63130 2 Center for Materials Innovation, Washington University in St. Louis, St. Louis, MO, 63130 3 Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO, 63130 ABSTRACT Dewetting instabilities in nanoscopic Co films, induced by uniform multiple ns pulse laser irradiation, leads to a system of nanoparticles with robust spatial order. On the other hand, irradiation by non-uniform laser intensity, such as with a two beam laser interference pattern generates a quasi two-dimensional pattern of nanoparticles possessing long range order (LRO) and short range order (SRO). Here we discuss the various instabilities that are responsible for the production of these dissimilar patterns and length scales on the basis of their time scales. For the case of single beam irradiation, the film progresses in a manner that can be attributed to classical spinodal dewetting. Pattern formation from interference irradiation is the result of time scale-based selection of competing processes, which can be chosen by controlling the film thickness. This approach promises a simple and cost-effective means to self-assemble various nanostructures. INTRODUCTION In recent years there have been developments in understanding the dewetting nature of thin polymer films. Such studies have outlined two avenues for this process, nucleation of holes (heterogeneous and homogeneous) and spinodal dewetting [1,2,3]. According to linear spinodal theory [4,5], dewetting occurs because of a hydrodynamic instability resulting when attractive intermolecular forces exceed the stabilizing effect of the film's surface tension. Surface fluctuations in the film are amplified by the attractive intermolecular forces, whereby the wavelength of the surface roughness with the shortest rise time dictates the length scales formed in the film [4,5]. However, there have been relatively few studies detailing dewetting pattern evolution in metal films. In prior work, Bischof et al. observed that melting by a single ns laser pulse of thin Cu, Au and Ni films produced surface perturbations resembling the spinodal process [6]. However pattern evolution with continued laser irradiation and the final morphology, including formation of nanoparticles, was not explored. Recently, we have shown that multiple pulses of uniform ns laser irradiation of 1 ≤ h0 ≤ 8 nm thick Co films will dewet in a manner that is consistent with spinodal theory, leading to a final configuration of ordered nanoparticles [7]. We have also shown, recently, that non-uniform irradiation gives rise to quasi2D order [8]. Here we report on the instabilities and patterning morphologies, including nanowires and nanoparticles, which result from irradiating ultrathin metal films with a nonuniform ns pulsed laser i
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