Previous studies on dewetting of ultrathin Co films by nanosecond pulsed laser melting have shown that the films dewet due to a thin film hydrodynamic instability and form a system of ordered nanoparticles with uniform average size and nearest neighbor particle spacing. For Co films less than 8 nm thick, the nanoparticle spacing, λNN was dependent on the initial film thickness, h, and varied as h2. For Co films thicker than 8 nm, the nanoparticle spacing decreased with increasing film thickness, due to a thermocapillary effect generated by the ns laser heating. Here we show the results from investigations on dewetting of Co films that had initially much rougher surfaces with root mean square roughness values, 0.9 < Rrms < 2.8 nm as compared to smoother films examined in prior investigations, for which Rrms ≤ 0.2 nm. Laser induced dewetting of Co films with much large Rrms values generated nanoparticles that were qualitatively similar to those created from smoother Co films. The size distribution of the nanoparticles was monodispersed and there was short range spatial order present in the system from the average nearest neighbor nanoparticle spacing; however, a drastic reduction in the characteristic length scales was observed in the nanoparticulate arrays created from the rougher Co films. This result suggests that knowledge of film thickness and roughness are important towards predicting characteristic length scales from metal film dewetting.
Metallic nanoparticles embedded in dielectrics permit enhanced capture absorption and/or scattering of light at specific wavelengths through excitation of plasmons, i.e. the quanta of coherent and collective oscillations of large concentrations of nearly free electrons. In order to maximize the potential of such enhanced absorption in useful tasks, such as the generation of carriers in photocatalysts and semiconductors, it is important to be able to predict and design plasmonic nanocomposites with desired wavelength-dependent optical absorption. Recently, a mixing approach formulated by Garcia and co-workers [Phys. Rev. B, 75, 045439 (2007)] has been successfully applied to model the experimentally measured broadband optical absorption for ternary nanocomposites containing alloys or mixtures of two metals (from Ag, Au or Cu) in SiO2 dielectric. In this work we present the broadband optical behavior of an important an optical coating dielectric, Si3N4, containing various configuration of nanoparticles of Al, Au, Ag, or Cu. The spectral behavior of various combinations of the metallic species in the dielectrics was optimized to show either broadband solar absorption or strong multiple plasmonic absorption peaks. The applications of such nanocomposite materials in solar energy harvesting and spectral sensing are also presented and discussed.
Computer models that accurately predict the dynamics of nanoscale self-organization are vital towards knowledge-based nanomanufacturing. Here we present a first principles computational model of laser induced self-organization of thin metallic films (thickness <= 30 nm ) into nanoscale patterns which eventually evolve into ordered nanoparticles. The pattern formation is initiated by a thin film hydrodynamic instability and the ensuing length scales are related to the intrinsic materials properties such as surface tension and van der Waal's dispersion forces. We discuss a fully implicit, finite-difference method with adaptive time step and mesh size control for the solution of the nonlinear, fourth-order PDE governing the thin film dynamics. These simulations capture
the changing morphology of the film due to the competition between surface tension and van der Waals forces.
Simulation results are used to understand the nonlinear amplifcation of film height perturbations ~(KT/γ)1/2, where K, T and γ represent the Boltzmann constant, absolute temperature, and surface tension respectively,leading eventually to film rupture.
Recently, a homogenization procedure has been proposed, based on the tight lower bounds of the Bergman-
Milton formulation, and successfully applied to dilute ternary nanocomposites to predict optical data without
using any fitting parameters [Garcia et al. Phys. Rev. B, 75, 045439 (2007)]. The procedure has been extended
and applied to predict the absorption coefficient of a quaternary nanocomposite consisting of Cu, Ag,
and Au nanospheres embedded in a SiO2 host matrix. Significant enhancement of the absorption coefficient is
observed over the spectral range 350-800 nm. The magnitude of this enhancement can be controlled by varying
the nanosphere diameter and the individual metal volume fraction with respect to the host matrix. We have
determined the optimal composition resulting in enhanced broadband (350nm-800nm) absorption of the solar
spectrum using a simulated annealing algorithm. Fabricating such composite materials with a desired optical
absorption has potential applications in solar energy harvesting.
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