Zijie Yan

Impact of annealing on morphology and ferromagnetism of ZnO nanorods

Room temperature ferromagnetism has been observed in ZnO nanorods prepared by hydrothermal method. Saturation magnetization of ∼0.004emu∕g was measured in the nanorods with diameters of ∼10nm and lengths of below 100nm, and the magnetization reduced with the increase of the size. Annealing of the samples at 900°C in air completely transformed the nanorods into twinning structures and weakened the magnetizations. The mechanism of morphology transformation was discussed. Analysis indicates that the interstitial zinc at the surface may contribute to the ferromagnetism in ZnO nanorods.

Three-dimensional optical trapping and manipulation of single silver nanowires.

We report the first experimental realization of all-optical trapping and manipulation of plasmonic nanowires in three dimensions. The optical beam used for trapping is the Fourier transform of a linearly polarized Bessel beam (termed FT-Bessel). The extended depth of focus of this beam enables the use of a retroreflection geometry to cancel radiation pressure in the beam propagation direction, making it possible to trap highly scattering and absorbing silver nanowires. Individual silver nanowires with lengths of several micrometers can be positioned by the trapping beam with a precision better than 100 nm and are oriented by the polarization of the trapping light with a precision of approximately 1°. Multiple nanowires can be trapped simultaneously in spatially separated maxima of the trapping field. Since trapping in the interferometric FT-Bessel potential is robust in bulk solution and near surfaces, it will enable the controlled assembly of metal nanowires into plasmonic nanostructures.

Guiding Spatial Arrangements of Silver Nanoparticles by Optical Binding Interactions in Shaped Light Fields

We demonstrate assembly of spheroidal Ag nanoparticle clusters, chains and arrays induced by optical binding. Particles with diameters of 40 nm formed ordered clusters and chains in aqueous solution when illuminated by shaped optical fields with a wavelength of 800 nm; specifically, close-packed clusters were formed in cylindrically symmetric optical traps, and linear chains were formed in line traps. We developed a coupled-dipole model to calculate the optical forces between an arbitrary number of particles and successfully predicted the experimentally observed particle separations and arrangements as well as their dependence on the polarization of the incident light. This demonstrates that the interaction between these small Ag particles and light is well described by approximating the particles as point dipoles, showing that these experiments extend optical binding into the Rayleigh regime. For larger Ag nanoparticles, with diameters of approximately 100 nm, the optical-binding forces become comparable to the largest gradient forces in the optical trap, and the particles can arrange themselves into regular arrays or synthetic photonic lattices. Finally, we discuss the differences between our experimental observations and the point dipole theory and suggest factors that prevent the Ag nanoparticles from aggregating as expected from the theory.

Excimer laser ablation of a Pt target in water: the observation of hollow particles

Micro/nanoparticles were fabricated by pulsed-excimer-laser ablation of a Pt target in water. Three kinds of hollow Pt particles (coalesced by micrograins, assembled by nanocrystals or with smooth shells) were observed together with solid particles using different laser fluences (2.3-6.8 J cm(-2)) and after 6000 laser shots. We propose that the hollow particles were formed on laser-produced bubbles which provided thermodynamically preferred nucleation sites and diffusion sinks for the laser-fabricated Pt clusters or particles. Although the hollow particles are a small proportion, the results have extended the scope of particles that pulsed-laser ablation in liquid can fabricate, and have enriched the mechanistic scenario of laser ablation and nanostructure formation in liquid.

Enhanced ferromagnetism and ferroelectricity in multiferroic CuCr1-xNixO2

Polycrystalline CuCr1−xNixO2 is synthesized and its multiferrocity is characterized in order to enhance the ferromagnetism and ferroelectricity of CuCrO2-based multiferroics. At the optimized doping level x=0.05, we observe not only an enhancement of one order of magnitude in magnetization but also a remarkable increasing of polarization up to ∼50 μC/m2 from ∼35 μC/m2 of polycrystalline CuCrO2. It is argued that the Ni-doping may modulate the antiferromagnetic interactions between Cr3+ ions and probably induce the conical-like spin component responsible for the enhanced ferromagnetism.

Unipolar resistive switching effect in YMn1−δO3 thin films

Steady unipolar resistive switching of Pt/YMn(1-delta)O(3)/Pt MIM structure is investigated. High resistance ratio (>10(4)) of high resistance state (HRS) over low resistance state (LRS) and long retention (>10(5) s) are achieved. It is suggested that the Joule heating and Poole-Frenkel effect dominate respectively the conduction of the LRS and HRS in high electric field region. The resistive switching is explained by the rupture and formation of conductive filaments in association with the local Joule-heat-induced redox inside YMn(1-delta)O(3).

Hollow nanoparticle generation on laser-induced cavitation bubbles via bubble interface pinning

We report the self-assembly of ZnOx (0≤x≤1) (and permalloy) nanoclusters into hollow nanoparticles using pulsed laser ablation of bulk Zn (or permalloy) in ethanol–water binary mixture. The self-assembly is due to the trapping of laser-produced nanoclusters by the interfaces of cavitation bubbles and the bonding of the nanoclusters by capillary attraction. It was found that the bubbles generated in the mixture have significantly longer lifetimes compared to water alone, which provide an increasing chance to absorb diffusive nanoclusters. The mixture could be adjusted by adding viscous surfactant that makes the pulsed laser ablation in liquid a promising method for the discovery and fabrication of other hollow geometries.

Generation of Ag2O Micro-/Nanostructures by Pulsed Excimer Laser Ablation of Ag in Aqueous Solutions of Polysorbate 80

A new route to synthesis of Ag(2)O micro-/nanostructures, including a mixture of cubes, pyramids, triangular plates, pentagonal rods, and bars, has been developed by pulsed excimer laser ablation of bulk silver in water using polysorbate 80 as surfactant. The polysorbate 80 played an important role in the formation of the Ag(2)O structures, and similar structures could be obtained in polysorbates 20 and 40 aqueous solutions. We have proposed a mechanism to explain the formation of Ag(2)O structures. This laser ablation method provides a unique approach to discover and fabricate new Ag(2)O morphologies.

Why Single-Beam Optical Tweezers Trap Gold Nanowires in Three Dimensions

Understanding whether noble-metal nanostructures can be trapped optically and under what conditions will enable a range of applications that exploit their plasmonic properties. However, there are several nontrivial issues that first need to be resolved. A major one is that metal particles experience strong radiation pressure in optical beams, while stable optical trapping requires an attractive force greater than this radiation pressure. Therefore, it has generally been considered impossible to obtain sufficiently strong gradient forces using single-beam optical tweezers to trap relatively large metal nanostructures in three dimensions. Here we demonstrate that a single, tightly focused laser beam with a wavelength of 800 nm can achieve three-dimensional optical trapping of individual gold (Au) nanowires with lengths over 2 μm. Nanowires can be trapped by the beam at one of their ends, in which case they undergo significant angular fluctuations due to Brownian motion of the untrapped end. They can also be trapped close to their midpoints, in which case they are oriented approximately perpendicular to the light polarization direction. The behavior is markedly different from that of Ag nanowires with similar length and diameter, which cannot be trapped in three dimensions by a single focused Gaussian beam. Our results, including electrodynamics simulations that help to explain our experimental findings, suggest that the conventional wisdom, which holds that larger metal particles cannot be trapped, needs to be replaced with an understanding based on the details of plasmon resonances in the particles.

Laser direct-write of single microbeads into spatially-ordered patterns

Fabrication of heterogeneous microbead patterns on a bead-by-bead basis promotes new opportunities for sensors, lab-on-a-chip technology and cell-culturing systems within the context of customizable constructs. Laser direct-write (LDW) was utilized to target and deposit solid polystyrene and stem cell-laden alginate hydrogel beads into computer-programmed patterns. We successfully demonstrated single-bead printing resolution and fabricated spatially-ordered patterns of microbeads. The probability of successful microbead transfer from the ribbon surface increased from 0 to 80% with decreasing diameter of 600 to 45 µm, respectively. Direct-written microbeads retained spatial pattern registry, even after 10 min of ultrasonication treatment. SEM imaging confirmed immobilization of microbeads. Viability of cells encapsulated in transferred hydrogel microbeads achieved 37 ± 11% immediately after the transfer process, whereas randomly-patterned pipetted control beads achieved a viability of 51 ± 25%. Individual placement of >10 µm diameter microbeads onto planar surfaces has previously been unattainable. We have demonstrated LDW as a valuable tool for the patterning of single, micrometer-diameter beads into spatially-ordered patterns.