Interaction of photons with matter at length scales far below their wavelengths has given rise to many novel phenomena, including localized surface plasmon resonance (LSPR). However, LSPR with narrow bandwidth (BW) is observed only in a select few noble metals, and ferromagnets are not among them. Here, we report the discovery of LSPR in ferromagnetic Co and CoFe alloy (8% Fe) in contact with Ag in the form of bimetallic nanoparticles prepared by pulsed laser dewetting. These plasmons in metal-ferromagnetic nanostructures, or ferroplasmons (FP) for short, are in the visible spectrum with comparable intensity and BW to those of the LSPRs from the Ag regions. This finding was enabled by electron energy-loss mapping across individual nanoparticles in a monochromated scanning transmission electron microscope. The appearance of the FP is likely due to plasmonic interaction between the contacting Ag and Co nanoparticles. Since there is no previous evidence for materials that simultaneously show ferromagnetism and such intense LSPRs, this discovery may lead to the design of improved plasmonic materials and applications. It also demonstrates that materials with interesting plasmonic properties can be synthesized using bimetallic nanostructures in contact with each other.

In spontaneous pattern formation by spinodal dewetting, attractive intermolecular forces overcome surface tension and cause an ultrathin liquid film on a low energy substrate to produce ordered structures. Spinodal dewetting in single-layer film on a substrate is usually manifested by an early stage surface deformation and a highly nonlinear ripening stage that results in characteristic morphologies, typically bicontinuous- or holelike states. Here we have experimentally constructed the dewetting morphology phase diagrams for a bilayer (Ag, Co) liquid film system on SiO2. Nanosecond pulsed laser melting was used to initiate and foster the dewetting as a function of film thickness and arrangement. The early stage ripening morphology was observed by scanning electron microscopy from which the phase diagrams were constructed. Unlike single-layer films, which only show one morphology transition between the bicontinuous to hole states as the film thickness is increased, the bilayer system can have multiple transitions. We have utilized the thickness-dependent free energy curvature approach [Sharma and Khanna, Phys. Rev. Lett. 81, 3463 (1998)] to analyze the phase diagram. The location of the multiple transitions cannot be predicted from the curvature minima, as was the case for single-layer films. Nevertheless, despite the complexity from multiple interacting forces and different surface deformation mode in bilayer systems, the phase diagram can be completely generated by knowledge of the free energy curvature of the respective single-layer films. These results can permit improved modeling of the nonlinear dynamics in naturally driven self-organized phenomenon and help design nanomaterials for advanced applications.

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.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

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.

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.

Self-assembly of surfactant molecules into micelles of various shapes and forms has been extensively used to synthesize soft nanomaterials. Translucent solutions containing rod-like surfactant micelles can self-organize under flow to form viscoelastic gels. This flow-induced structure (FIS) formation has excited much fundamental research and pragmatic interest as a cost-effective manufacturing route for active nanomaterials. However, its practical impact has been very limited because all reported FIS transitions are reversible because the gel disintegrates soon after flow stoppage. We present a new microfluidics-assisted robust laminar-flow process, which allows for the generation of extension rates many orders of magnitude greater than is realizable in conventional devices, to produce purely flow-induced permanent nanogels. Cryogenic transmission electron microscopy imaging of the gel reveals a partially aligned micelle network. The critical flow rate for gel formation is consistent with the Turner-Cates fusion mechanism, proposed originally to explain reversible FIS formation in rod-like micelle solutions.

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.

Surfaces made from composite nanostructured materials are potential multifunctional platforms for detection, sensing, and energy harvesting in biological and inorganic systems. However, robust and cost-effective synthesis routes are required to create the required arrays of nanostructures with tailorable size, morphology, and composition. Here we show that self-organization via spontaneous pattern formation in nanometer thick bilayer liquid films could lead to such nanostructure arrays. Experimentally, bilayers of immiscible metallic liquids show different self-organized patterning characteristics based on their order of arrangement on a substrate. Energy rate theory based on equating the rate of free energy change to viscous dissipation was used to explain this result. The different bilayer arrangements change the signs of intermolecular interactions, which changes the mode of coupled deformations and the patterning characteristics. Patterning length scale characteristics from nanosecond pulsed laser induced self-organization of Ag and Co liquids on SiO₂ substrate were in good agreement with theory.

We show here that the morphological pathway of spontaneous dewetting of ultrathin Ag films on SiO2 under nanosecond laser melting is dependent on film thickness. For films with thickness h of 2 nm < or = h < or = 9.5 nm, the morphology during the intermediate stages of dewetting consisted of bicontinuous structures. For films with 11.5 nm < or = h < or = 20 nm, the intermediate stages consisted of regularly sized holes. Measurement of the characteristic length scales for different stages of dewetting as a function of film thickness showed a systematic increase, which is consistent with the spinodal dewetting instability over the entire thickness range investigated. This change in morphology with thickness is consistent with observations made previously for polymer films (Sharma and Khanna 1998 Phys. Rev. Lett. 81 3463-6; Seemann et al 2001 J. Phys.: Condens. Matter 13 4925-38). Based on the behavior of free energy curvature that incorporates intermolecular forces, we have estimated the morphological transition thickness for the intermolecular forces for Ag on SiO2. The theory predictions agree well with observations for Ag. These results show that it is possible to form a variety of complex Ag nanomorphologies in a consistent manner, which could be useful in optical applications of Ag surfaces, such as in surface enhanced Raman sensing.

Controlling the magnetic orientation of nanoparticles is important for many applications. Recently, it has been shown that single domain ferromagnetic hemispherical Co nanoparticles prepared by nanosecond laser-induced self-organization, show magnetic orientation that was related to the negative sign of the magnetostrictive coefficient λS [J. Appl. Phys. v103, p073902, 2008]. Here we have extended this work to the Fe50Co50 alloy, which has a positive λS and Ni, which has a negative λS. Patterned arrays of ferromagnetic nanoparticles of Fe50Co50, Ni, (and Co) were synthesized from their ultrathin metal films on SiO2 substrate by nanosecond laser-induced self-organization. The morphology, nanostructure, and magnetic behavior of the nanoparticle arrays were investigated by a combination of electron microscopy, atomic force microscopy, 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 hemispherical Co and Ni nanoparticles was predominantly out-of-plane while those for the Fe50Co50 alloy was in the plane of the substrate. Finite element analysis was used to estimate the average residual strain in the nanoparticles, following laser processing. The 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 λS is negative for polycrystalline Co and Ni, and positive for Fe50Co50, the tensile residual strain forces the magnetization direction to out-of-plane and in-plane, respectively. This work demonstrates a cost-effective non-epitaxial technique for the synthesis of magnetic nanoparticles with tailored magnetization orientations.

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 SiO_2 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 SiO_2 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.

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.

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, lambda(NN) was dependent on the initial film thickness, h, and varied as h(2). For Co films thicker than 8nm, 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 < R-rms < 2.8 nm as compared to smoother films examined in prior investigations, for which R-rms <= 0.2 nm. Laser induced dewetting of Co films with much large R-ms 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.

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 SiO 2 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.

It is well known that rodlike/wormlike micelles can self-organize under flow to form viscoelastic get phases. Flow-induced structure (FIS) formation is typically accompanied by an enhancement in the shear viscosity. While configurational dynamics of and collisions between micelles in flow and electrostatic inter-micelle interactions are recognized as the key factors that influence such phase transitions, there are no universally applicable criteria for the onset strain rate as function of salt/surfactant concentration. Further, FIS formation is generally considered reversible, i.e., the structure disintegrates quickly upon flow cessation. In this work, first, we examine the effect of salt concentration on the critical strain rate for CTAB/NaSal solutions of rodlike micelles and show that a "self-similar" phase transition regime, characterized by a constant critical strain, exists. Second, we show that under strong (elongational) flow conditions, the phase transition is irreversible, leading to the formation of permanent nanogels that are stable long after the flow is stopped. Atomic force microscopy shows that the gel phase obtained under strong flow conditions consists of highly aligned micelles.

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 30–250nm diameter were investigated by magnetic force microscopy under zero applied field in conjunction with simulations of the magnetic tip-particle interaction. Particles up to 180nm diameter were found to be single domain with the magnetization direction oriented predominantly in-plane for the smaller particles (⩽75nm) and out-of-plane for the larger particles (⩽180nm). Multidomain behavior was observed for particles larger than 180nm. 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.