Efficient and dependable characterization methods of magnetic-plasmonic nanostructures
are essential towards the implementation of new nanoscale materials in magneto-optical
applications. Surface magneto-optic Kerr effect (SMOKE) is a powerful characterization
technique, because of its simplicity and high sensitive to even monolayer thick magnetic
materials. It relies on the measurement of polarization and absorption changes of reflected
light in the presence of a magnetic field. While SMOKE has been applied in the past to
investigate the magnetic information of continuous films, there is little work on applying it to
characterize arrays of nanoparticles with variable magnetic and optical properties. Here, we
have used it to investigate the magnetic behavior of nanoparticle arrays made by nanosecond
pulsed laser self-organization. This technique produces an array of single-domain magnetic
nanoparticles with size-dependent magnetic orientation. Nanoparticle arrays of Co and Ni
were prepared on SiO2 substrates. SMOKE measurements were performed for a variety of
different particle sizes and material. Systematic differences in saturation and coercivity were
observed for the different samples. These results demonstrated that SMOKE is a reliable
technique to rapidly characterize the magnetic behavior of nanoparticle arrays.
Laser-induced melting of ultrathin films can lead to self-organized arrays of hemispherical particles. We have applied
this procedure to assemble arrays of Fe nanomagnets on SiO2 substrates. Morphological studies showed presence of
spatial short range order (SRO) in the array. Magnetic properties were studied at room temperature using zero-field
magnetic force microscopy (MFM). The particles upto 55 nm in diameter showed in-plane (≤ 45°), compared to out-of-plane magnetization directions (≥ 45°) for the larger particles. The size-dependent orientation of magnetization for
these hemispherical particles, was attributed to the dominating magnetostrictive energy and a size-dependent residual
strain.
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.
Robust nanomanufacturing methodologies are crucial towards realizing simple and cost-effective products. Here we
discuss nanofabrication of ordered metal nanoparticles through pulsed-laser-induced self-organization. When ultrathin
metal films are exposed to short laser pulses, spontaneous pattern formation results under appropriate conditions. Under
uniform laser irradiation two competing modes of self-organization are observed. One, a thin film hydrodynamic dewetting
instability due to the competition between surface tension and attractive van derWaals interactions, results in nanoparticles
with well-defined and predictable interparticle spacings and sizes with short range spatial order. The second, thermocapillary
flow due to interference between the incident beam and a scattered surface wave, results in laser induced periodic surface
structures. Non-uniform laser irradiation, such as by 2-beam laser interference irradiation, initiates a tunable thermocapillary
effect in the film giving rise to nanowires, and continued laser irradiation leads to a Rayleigh-like breakup of the nanowires
producing nanoparticles with spatial long-range and short-range order. These self-organizing approaches appear to be
applicable to a variety of metal films, including Co, Cu, Ag, Fe, Ni, Pt, Zn, Ti, V and Mn. These results suggest that
laser-induced self-organization in thin films could be an attractive route to nanomanufacture well-defined nanoparticle
arrangements for applications in optical information processing, sensing and solar energy harvesting.
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.
We discuss the development of X-ray multilayer coatings for use as broad-band reflectors operating at energies above 100 keV. Such coatings can be used to produce hard X-ray telescopes that will make possible a variety of entirely new astronomical observations. We summarize our recent investigation into the growth, structure and hard X-ray performance of depth-graded W/Si multilayers, present follow-up information on Cu/Si multilayers, and discuss preliminary results obtained with Ni/Si, Ni.8Cr.2/Si, and Ni.93V.07/Si multilayers.
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