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ABSTRACT: We demonstrate a promising synthesis route based on pulsed laser dewetting of bilayer films (Ag and Co) to make bimetallic nanoparticle arrays. By combining experiment and theory we establish a parameter space for the independent control of composition and diameter for the bimetallic nanoparticles. As a result, physical properties, such as the localized surface plasmon resonance (LSPR), that depend on particle size and composition can be readily tuned over a wavelength range one order of magnitude greater than for pure Ag nanoparticles. The LSPR detection sensitivity of the bimetallic nanoparticles with narrow size distribution was found to be high-comparable with pure Ag (∼60 nm/RIU). Moreover, they showed significantly higher long-term environmental stability over pure Ag.
Nanotechnology 07/2012; 23(27):275604. · 3.98 Impact Factor
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ABSTRACT: Techniques for processing nanoscale metallic structures with spatial order and tunable physical characteristics, such as size
and microstructure, are paramount to realizing applications in the areas of magnetism, optics, and sensing. This paper discusses
how pulsed laser melting of ultrathin films can be a powerful but simple and cost-effective technique to fabricate functional
nanostructures. Ultrathin metal films (1 nm to 1,000 nm) on inert substrates like SiO2 are generally unstable, with their free energy resembling that of a spinodal system. Such films can spontaneously evolve
into predictable nanomorphologies with well-defined length scales. This study reviews this laser-based experimental technique
and provides examples of resulting robust nanostructures that can have applications in magnetism and optics.
JOM: the journal of the Minerals, Metals & Materials Society 04/2012; 60(9):37-42. · 1.42 Impact Factor
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ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.
Microscopy and Microanalysis 06/2011; 17:1634 - 1635. · 3.01 Impact Factor
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ABSTRACT: Reliable and cost-effective techniques to process surface nanoscale metallic structures with controllable and complex nanomorphologies is important toward progress in technologies related to sensing, energy harvesting, information storage, and computing. Here we discuss how pulsed laser melting and the ensuing self-organization by dewetting of ultrathin films can be utilized to fabricate various nanomorphologies in a predictable manner. Ultrathin metal films (1–100 nm) on inert substrates like SiO2 are generally unstable, with their free energy resembling that of a spinodal system. The energy rate theory of self-organization, which is based on balancing the rate of thermodynamic free energy change to the rate of energy dissipation, predicts the appearance of characteristic length scales. This is borne out in experiments of nanosecond pulsed laser melting of a variety of metal films. We review this laser-based self-organization technique with various examples from the behavior of Ag and Co metals on SiO2 substrates. Specifically, film thickness and film roughness can be used to control dewetting length scales, whereas knowledge of the intermolecular forces responsible for the free energy of the system control the type of morphology. Furthermore, novel dewetting is observed that is attributable to nanoscale heating effects resulting from the thickness-dependent pulsed laser heating. These results help elucidate the basic mechanisms of pulsed laser induced dewetting of metal films, but they also provide potential routes for cost-effective nanomanufacturing of metallic surfaces for applications in sensing, energy harvesting, and information processing.
Journal of Materials Research. 01/2011; 26(02):154 - 169.
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Journal of Magnetism and Magnetic Materials 01/2011; 323(3):356-362. · 1.78 Impact Factor
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Meeting Abstracts. 01/2011;
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ABSTRACT: The spontaneous pattern formation via the classical spinodal dewetting instability in ultrathin films is a nonlinear process. However, the physical manifestation of the instability in terms of characteristic length and time scales can be described by a linearized form of the initial conditions of the film’s dynamics. Alternately, the thermodynamic (TH) approach based on equating the rate of free energy decrease to the rate of frictional loss via viscous dissipation [de Gennes, C. R. Acad. Paris 298, 111 (1984)] can give similar information. Here we have evaluated dewetting in the presence of film-thickness- (h) dependent thermocapillary forces. Such a situation can be found during pulsed laser melting of ultrathin metal films where nanoscale effects lead to a local h -dependent temperature. The TH approach provides an analytical description of this thermocapillary dewetting. The results of this approach agree with those from linear theory and experimental observations provided the minimum dissipation is equated to the rate of free energy decrease. The flow boundary condition that produces this minimum dissipation is when the film-substrate tangential stress is zero. The physical implication of this finding is that the spontaneous dewetting instability follows the path of minimum rate of energy loss.
Journal of Applied Physics 08/2010; · 2.17 Impact Factor
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ABSTRACT: Patterned arrays of ferromagnetic nanoparticles of Co, Ni, and Fe_{\text{50}} Co_{\text{50}} have been synthesized from their ultrathin metal films on SiO_{\text{2}} substrate by nanosecond laser-induced self-organization. The morphology, nanostructure, and magnetic behavior of the nanoparticle arrays were investigated by a combination of electron, atomic force, and magnetic force microscopy techniques. Transmission electron microscopy investigations revealed a granular polycrystalline nanostructure, with the number of grains inside the nanoparticle increasing with their diameter. Magnetic force measurements showed that the magnetization direction of the Co and Ni nanoparticles was predominantly out-of-plane while those for the Fe_{\text{50}}Co_{\text{50}} alloy was in the plane of the substrate. This difference in behavior is due to the dominating influence of magnetostrictive energy on the magnetization as a result of residual thermal strain following fast laser processing. Since the magnetostriction coefficient is negative for polycrystalline Co and Ni, and positive for Fe_{\text{50}}Co_{\text{50}}, the tensile residual strain forces the magnetization direction of the negative magnetostriction materials out-of-plane and the positive magnetostriction materials in-plane. This demonstrates a cost-effective non-epitaxial technique for the fabrication of patterned arrays of magnetic nanoparticles with tailored magnetization orientations. Comment: 22 pages, 6 figures
01/2010;
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01/2010; Minerals, Metals and Materials Society/AIME, 420 Commonwealth Dr., P. O. Box 430 Warrendale PA 15086 USA.
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Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 01/2010
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ABSTRACT: Nanometre thick metallic liquid films on inert substrates can spontaneously dewet and self-organize into complex nanomorphologies and nanostructures with well-defined length scales. Nanosecond pulses of an ultraviolet laser can capture the dewetting evolution and ensuing nanomorphologies, as well as introduce dramatic changes to dewetting length scales due to the nanoscopic nature of film heating. Here, we show theoretically that the self-organization principle, based on equating the rate of transfer of thermodynamic free energy to rate of loss in liquid flow, accurately describes the spontaneous dewetting. Experimental measurements of laser dewetting of Ag and Co liquid films on SiO(2) substrates confirm this principle. This energy transfer approach could be useful for analyzing the behavior of nanomaterials and chemical processes in which spontaneous changes are important.
Physical Chemistry Chemical Physics 10/2009; 11(37):8136-43. · 3.57 Impact Factor
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ABSTRACT: The fluid dynamics of the classical dewetting instability in ultrathin films is a non-linear process. However, the physical manifestation of the instability in terms of characteristic length and time scales can be described by a linearized form of the initial conditions of the films's dynamics. Alternately, the thermodynamic approach based on equating the rate of free energy decrease to the viscous dissipation [de Gennes, C. R. Acad. Paris.v298, 1984] can give similar information. Here we have evaluated dewetting in the presence of thermocapillary forces arising from a film-thickness (h) dependent temperature. Such a situation can be found during pulsed laser melting of ultrathin metal films where nanoscale effects lead to a local h-dependent temperature. The thermodynamic approach provides an analytical description of this thermocapillary dewetting. The results of this approach agree with those from linear theory and experimental observations provided the minimum value of viscous dissipation is equated to the rate of free energy decrease. The flow boundary condition that produces this minimum viscous dissipation is when the film-substrate tangential stress is zero. The physical implication of this finding is that the spontaneous dewetting instability follows the path of minimum rate of energy loss. Comment: 8 pages, 3 figures. Under review
08/2009;
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ABSTRACT: Nanosecond pulsed laser melting of ultrathin metal films can lead to self-organized arrays of spherical nanoparticles. We have applied this technique to assemble arrays of nanoparticles of the soft elemental ferromagnet Co on SiO2. Surface morphology studies by using scanning electron microscopy and atomic force microscopy established that the nanoparticles were nearly hemispherical with an average contact angle of ∼ 104±22°. Magnetic properties of these nanoparticles in the size range of 30–250 nm diameter were investigated by magnetic force microscopy under zero applied field in conjunction with simulations of the magnetic tip-particle interaction. Particles up to 180 nm diameter were found to be single domain with the magnetization direction oriented predominantly in-plane for the smaller particles ( ⩽ 75 nm) and out-of-plane for the larger particles ( ⩽ 180 nm). Multidomain behavior was observed for particles larger than 180 nm. Magnetic hysteresis measurements at room temperature confirmed that the arrays consisted of a mixture of in-plane and out-of-plane orientations. Microstructural analysis by transmission electron microscopy revealed that the nanoparticles had a granular microstructure with the average grain size increasing with particle size. This size-dependent magnetic orientation is inconsistent with the expected in-plane orientation due to shape anisotropy. We suggest that a size-dependent residual strain and the microstructure formed by rapid laser processing determine the orientation of nanomagnets. This idea was supported by the significant increase in in-plane orientation of larger particles following thermal annealing. These studies suggest that anisotropic nanomagnets of near hemispherical polycrystalline particles with desired magnetic orientation can be prepared by fast laser thermal processing.
Journal of Applied Physics 03/2008; 103(7):073902-073902-11. · 2.17 Impact Factor
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Journal of Applied Physics. 01/2008; 103(7):073902-073902.
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Bulletin of the American Physical Society. 01/2008; 53.
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Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 01/2008
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Bulletin of the American Physical Society. 01/2008; 53.
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Proceedings of SPIE, the International Society for Optical Engineering. 01/2008;
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ABSTRACT: Templates are widely used to produce artificial nanostructures. Here, laser-assisted self-organization has been used to form one- and two-dimensional (D) nanoarrays of Cu nanocrystals. Using these nanoarrays as a template, a 2D patterned ferromagnetic nanostructure of FeCrSi nanocrystals has been produced by heterogeneous nucleation and growth of nanocrystals by partial devitrification from an amorphous Fe64.5Cr10Si13.5B9Nb3 alloy with the Cu nanoparticles acting as catalytic nucleation sites. The interaction among the ferromagnetic nanocrystals via the residual amorphous matrix can be controlled by suitable choice of the amorphous alloy composition. Although demonstrated for a ferromagnetic system, the processing method may have much wider applicability for producing artificial nanostructures of a wide variety of materials when materials-specific catalysts and amorphous alloy compositions are judiciously chosen.
Nanotechnology 10/2007; 18(48):485606. · 3.98 Impact Factor
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arXiv preprint cond-mat/0703016. 01/2007;
