Min Han

High-yield synthesis of uniform Ag nanowires with high aspect ratios by introducing the long-chain PVP in an improved polyol process

Polyvinyl pyrrolidone (PVP) with different molecular weights was used as capping agent to synthesize silver nanowires through a polyol process. The results indicated that the yields and aspect ratios of silver nanowires were controlled by the chain length of PVP and increased with increasing the molecular weight (MW) of PVP. When the long-chain PVP-K90 (MW= 800,000) was used, the product was uniform in size and was dominated by nanowires with high aspect ratios. The growth mechanism of the nanowires was studied. It is proposed that the chemical adsorption of Ag+ on the PVP chains at the initial stage promotes the growth of Ag nanowires.

Plume dynamics during film and nanoparticles deposition by pulsed laser ablation

The gas dynamics of pulsed laser ablation of silicon target in the helium gas ambient is investigated via direct simulation Monte Carlo method with a real physical scale of target-substrate configuration. A shock driven process is clearly observed. It is shown that the interaction of the shock front with the target surface and the vapor front induce significant backward flux of ablated particles and oscillating behavior of vapor front. A confined layer mixed with high density Si and He atoms is formed around the contact front. Its behavior is important to the nanoparticle formation and deposition.

Electronic transition and electrical transport properties of delafossite CuCr1−xMgxO2 (0 ≤ x ≤ 12%) films prepared by the sol-gel method: A composition dependence study

Highly transparent CuCr1−x Mg x O2 (0 ≤ x ≤ 12%) films were prepared on (001) sapphire substrates by sol-gel method. The microstructure, phonon modes, optical band gap, and electrical transport properties have been systematically discussed. It was found that Mg-doping improved the crystal quality and enhanced the (00l) preferred orientation. The spectral transmittance of films approaches about 70%–75% in the visible-near-infrared wavelength region. With increasing Mg-composition, the optical band gap first declines and climbs up due to the band gap renormalization and Burstein-Moss effect. The direct and indirect band gaps of CuCr0.94 Mg 0.06O2 film are 3.00 and 2.56 eV, respectively. In addition, it shows a crossover from the thermal activation behavior to that of three-dimensional variable range hopping from temperature-dependent electrical conductivity. The crossover temperature decreases with increasing Mg-doping composition, which can be ascribed to the change of spin-charge coupling between the hole and the local spin at Cr site. It should be noted that the electrical conductivity of CuCr1− x Mg x O2 films becomes larger with increasing x value. The highest electrical conductivity of 3.85 S cm−1 at room temperature for x = 12% is four-order magnitude larger than that (8.81 × 10−4 S cm−1) for pure CuCrO2 film. The high spectral transmittance and larger conductivity indicate that Mg-doped CuCrO2 films are promising for optoelectronic device applications.

Visualizing Plasmon Coupling in Closely Spaced Chains of Ag Nanoparticles by Electron Energy‐Loss Spectroscopy

Anisotropic plasmon coupling in closely spaced chains of Ag nanoparticles is visualized using electron energy-loss spectroscopy in a scanning transmission electron microscope. For dimers as the simplest chain, mapping the plasmon excitations with nanometer spatial resolution and an energy resolution of 0.27 eV intuitively identifies two coupling plasmons. The in-phase mode redshifts from the ultraviolet region as the interparticle spacing is reduced, reaching the visible range at 2.7 eV. Calculations based on the discrete-dipole approximation confirm its optical activeness, where the longitudinal direction is constructed as the path for light transportation. Two coupling paths are then observed in an inflexed four-particle chain.

Nanoscale ferromagnetic chromium oxide film from gas-phase nanocluster deposition

Ferromagnetic film of densely packing chromium oxide nanoparticles has been fabricated by vacuum deposition of chromium oxide clusters at room temperature. The clusters were generated with a magnetron plasma gas aggregation source by introducing a mixture of argon and oxygen as buffer gas. A magnetic hysteresis loop similar to that of bulk CrO2 was observed in a wide temperature range. The rise in the ferromagnetic property of the film was attributed to the nanoscale CrO2 composition. The work demonstrates a simple way to fabricate ferromagnetic films of chromium oxide nanoparticles under high-vacuum compatible low temperature condition.

Two-dimensional gradient Ag nanoparticle assemblies: multiscale fabrication and SERS applications.

We report a novel method for fabricating silver nanoparticle assemblies with a featured gradient of spatial organizations. The unique step is to generate a gradient of deposit mass by dynamical deposition on a mask-covered substrate with a collimated cluster beam in oblique incidence. Then such gradient can be translated to the gradients of sizes or number densities of the nanoparticles separately, depending on the nature of the substrate surface. Multiscale gradients are implemented from mesoscopic to macroscopic. One-chip rapid detection of the optimal structure for surface enhanced Raman scattering (SERS) is achieved on the gradient assembly with particle number densities.

Cluster-assembled Tb–Fe nanostructured films produced by low energy cluster beam deposition

Cluster-assembled Tb-Fe nanostructured films were prepared by the low energy cluster beam deposition method. The microstructure, magnetization and magnetostriction were investigated for the films. It is shown that the film is assembled by monodisperse spherical nanoparticles with average diameter of ∼30xa0nm which are distributed uniformly. The cluster-assembled Tb-Fe nanostructured films exhibit good magnetization and possess giant magnetostriction with saturation value of ∼1060 × 10(-6), much higher than that of the common Tb-Fe films. The origin of good magnetization and giant magnetostriction for the cluster-assembled Tb-Fe nanostructured film was discussed. The present work opens a new avenue to produce the nanostructured magnetostrictive alloy in application of a nano-electro-mechanical system.

Experimental Observation of Nanojets Formed by Heating PbO-Coated Pb Clusters

We are reporting the first experimental observation of nanojets formed by heating PbO-coated Pb clusters, which has been predicted theoretically by Moseler and Landman. During heating, the hot liquid is ejected through the broken orifice into a vacuum and forms a condensed trail in the shape of a tadpole, as shown in the transmission electron micrographs. The temperature-variable Raman spectra indicate that nanojet formation is closely related to the heating temperature and thus essentially to the pressure in the coated clusters. The pressure inside the shell rises from the inner cores melting and its confined volume expansion. It then drops after the final explosion, dominating the whole nanojetting process.

Nanofilms of Si clusters confined in SiO nanoparticles prepared by low energy cluster beam deposition

Abstract Cluster-based nanostructured films of Si clusters embedded in SiO nanoparticles have been prepared by low energy deposition of SiO cluster beam formed by inert gas aggregating method. The microstructures of the nanofilms have been characterized by TEM and the chemical components of the nanofilms have been analyzed by XPS spectra. It is shown that the film is assembled by the ultrafine spherical nanoparticles, which are distributed randomly and uniformly in the film, while the Si clusters produced by SiO disproportionation reaction are embedded dispersedly in the nanoparticles. Furthermore, the photoluminescence properties of the films have been measured and a blue PL band at wavelength of 400xa0nm with a shoulder peak at 393xa0nm has been observed. The PL band at 400xa0nm may be attributed to quantum confinement effect of the Si cores confined in SiO nanoparticles and energy potential barrier effect of outer SiO layer, while the 393xa0nm PL shoulder peak may be caused by the oxygen-deficient-associated defect centers.

Nanofilms with clusters of boron suboxide and their infrared absorption

Abstract Boron suboxide films containing B 6 O have been prepared by pulsed laser deposition. Different kinds of characterizations were performed: TEM and SEM for the observation of the structure morphology; IR absorption and Raman scattering for the analysis on the optical properties of the films. IR spectra show two peaks at 2850.27 and 2917.77xa0cm −1 , which result from the splitting vibration of B 1 –B 1 inter-icosahedral bonding in the rhombohedral cell. The active vibrational modes are found to be affected only by the temperature of substrates during the preparation.