N. Shirato

Self-organization of nanoscale multilayer liquid metal films: experiment and theory.

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.

Self-organized bimetallic Ag-Co nanoparticles with tunable localized surface plasmons showing high environmental stability and sensitivity.

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.

Thermodynamic model for the dewetting instability in ultrathin films

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 eq...

Laser irradiated nano-architectured undoped tin oxide arrays: mechanism of ultrasensitive room temperature hydrogen sensing

Undoped nanostructured tin oxide (SnO(2)) arrays were prepared on oxidized Si substrates by nanosecond pulsed laser interference irradiation for hydrogen gas sensing applications. Scanning electron microscopy (SEM), in combination with Atomic Force Microscopy (AFM), showed that the SnO(2) surface consisted of periodic features of ∼130 nm width, ∼228 nm spacing, an average height of ∼8 nm along the periodicity and tens of microns length. The SnO(2) nanostructured arrays and precursor thin films were tested by cyclic exposure under dynamic conditions of hydrogen in the concentration range of 300-9000 ppm. The observed electrical response of SnO(2) towards hydrogen at low concentrations and room temperature drastically improved in the nanostructured array as compared to the thin film. The results suggest that this method to fabricate SnO(2) nanostructured arrays has the potential to produce nanodevices that have ultra-low detection limits, and fast response and recovery times, which are suited for practical hydrogen sensing applications.

Fe nanomagnets with unusual size-dependent magnetization directions produced by fast laser-induced self-organization

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.

Magnetic measurement of pulsed laser-induced nanomagnetic arrays using Surface Magneto-Optic Kerr Effect

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.