The optical properties of Cu-Ni nanoparticles produced via pulsed laser dewetting of ultrathin films: The effect of nano
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J.D. Fowlkes Center for Nanophase Materials Sciences Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
P.D. Racka) Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996; and Center for Nanophase Materials Sciences Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 26 April 2010; accepted 6 August 2010)
Thin film Cu-Ni alloys ranging from 2–8 nm were synthesized and their optical properties were measured as-deposited and after a laser treatment which dewet the films into arrays of spatially correlated nanoparticles. The resultant nanoparticle size and spacing are attributed to a laser induced spinodal dewetting process. The evolution of the spinodal dewetting process is investigated as a function of the thin film composition which ultimately dictates the size distribution and spacing of the nanoparticles. The optical measurements of the copper rich alloy nanoparticles reveal a signature absorption peak suggestive of a plasmon peak that red-shifts with increasing nanoparticle size and blue-shifts and dampens with increasing nickel concentration.
I. INTRODUCTION
Pulsed laser induced dewetting has recently been shown to be a viable way to induce self-assembled pattern formation of metallic nanoparticles. The three dominant dewetting mechanisms include: (i) homogeneous nucleation, (ii) heterogeneous nucleation, and (iii) spinodal instabilities initiated by surface waves. The spinodal instability results in a characteristic length scale related to the original film thickness, the properties of the film, substrate, and gas ambient; thus it is suitable for forming spatially ordered metal nanoparticles. While historically several groups have investigated spinodal dewetting in polymeric thin films (e.g., Becker et al.1), more recently, several investigations have focused on thin metal films dewetted via pulsed laser heating.2–9 Most recent works have focused on the behavior of elemental metal thin films on inert (nonreactive) substrates.2–10 Bischoff et al. studied the dewetting of gold liquid metal films with a Q-swiched laser pulse and observed two coexisting mechanisms (nucleation of holes and spinodal dewetting) in 47 nm Au films on a fused silica substrate.2 Henley et al. reported the dewetting behavior of a range of metals on silicon dioxide coated Si substrates and analyzed the radius of nanoparticles induced by the Rayleigh instability criterion.3 Kalyanaraman et al. have recently a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.9 J. Mater. Res., Vol. 26, No. 2, Jan 28, 2011
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published a series of papers4–8 describing the spinodal dewetting of various thin elemental films and have correlated the time and length scale of the spinodal instability using thin film hydrodynamic theory and have also shown the influence that temperature gradients (both lateral and in the film thickness) have on the growth modes. More recently, they also performed an energy analysis of
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