G. H. Wang

Electric-field-induced magnetization in Pb(Zr,Ti)O3/Terfenol-D composite structures

We report the electric-field-induced magnetization (EIM) in the composite structures made by combining Pb(Zr0.52Ti0.48)O3 (PZT) and Tb0.30Dy0.7Fe2 (Terfenol-D). The results showed that the EIM could be generated in the composite structures due to the efficient stress-mediated electromagnetic coupling interaction between the piezoelectric PZT and magnetostrictive Terfenol-D. This EIM effect depended significantly on the driving electric field frequency and was highly sensitive to the dc magnetic bias, which exhibited a promising potential in the low-level dc magnetic field detecting application.

Solvothermal Synthesis of Lateral Heterojunction Sb2Te3/Bi2Te3 Nanoplates

A lateral heterojunction of topological insulator Sb2Te3/Bi2Te3 was successfully synthesized using a two-step solvothermal method. The two crystalline components were separated well by a sharp lattice-matched interface when the optimized procedure was used. Inspecting the heterojunction using high-resolution transmission electron microscopy showed that epitaxial growth occurred along the horizontal plane. The semiconducting temperature-resistance curve and crossjunction rectification were observed, which reveal a staggered-gap lateral heterojunction with a small junction voltage. Quantum correction from the weak antilocalization reveals the well-maintained transport of the topological surface state. This is appealing for a platform for spin filters and one-dimensional topological interface states.

Free-standing graphene by scanning transmission electron microscopy

Free-standing graphene sheets have been imaged by scanning transmission electron microscopy (STEM). We show that the discrete numbers of graphene layers enable an accurate calibration of STEM intensity to be performed over an extended thickness and with single atomic layer sensitivity. We have applied this calibration to carbon nanoparticles with complex structures. This leads to the direct and accurate measurement of the electron mean free path. Here, we demonstrate potentials using graphene sheets as a novel mass standard in STEM-based mass spectrometry.

Enhanced thermal stability of monodispersed silver cluster arrays assembled on block copolymer scaffolds.

Triblock copolymer poly(styrene-b-butadiene-b-styrene) (SBS) films with long-range ordered self-assembled nanopatterns are used as templates to selectively adsorb soft-landing silver clusters. Closely spaced cluster arrays with high monodispersity are formed through the confinement of the block copolymer scaffolds, and show a much enhanced thermal stability as compared with the cluster assemblies on the surfaces of covalent amorphous solids, or even on the disordered SBS films. Their morphologies are barely influenced by long time thermal annealing at a temperature as high as 180 degrees C, while in the latter case intense aggregations and coalescences of silver clusters are commonly observed upon annealing. The different thermal stabilities of the cluster assemblies also induce different evolutions of their optical extinction spectra under annealing. This promises a simple way to control the monodispersity and thermal stability of metal cluster assembly via self-assembled block copolymer template.

The local electronic and magnetic properties of Fe impurity in Al clusters

The local magnetic property,d electronic structure and the charge transfer effect of Fe impurity in Al clusters have been studied by using a tight-binding model Hamiltonian in the unrestricted Hartree-Fock approximation, which includes intra-atomic and interatomic Coulomb interactions. We have obtained that local magnetic moment of Fe impurity in FeAlN clusters decreases with increasing cluster size and convergences to zero (that of bulk given by Anderson) withN larger than 12, meanwhile, the local magnetic moment for smaller clusters depends on the clusters size and it is a monotonous descent function of cluster size. We have also found that the spin splitting of the localizedd states decreases as the cluster size increases, which mainly results from the interaction between the localized electrons of Fe atom and the delocalized electrons of Al atoms.

Magnetoelectric CoFe2O4-Pb(Zr0.52Ti0.48)O-3 composite films prepared by pulsed-laser deposit method

ABSTRACT We report the magnetoelectric CoFe2O4-Pb(Zr0.52T0.48)O3 composite nanofilms prepared by pulse laser deposition method. X-ray diffraction and scanning electron microscopy were used to characterize the phase structures and morphologies of the films. Superconducting quantum interference device magnetometer, impedance analyzer, ferroelectric test unit and magnetoelectric measuring device were used to measure the magnetic, dielectric and ferroelectric properties, as well as the magnetoelectric effect of the films. It is shown that the films possess good dielectric, ferroelectric and magnetic properties. The films exhibit magnetoelectric coupling properties similar to the bulk composites. The magnetoelectric effect of the films were studied in detail.

Response Characteristics of Hydrogen Sensors Based on PMMA-Membrane-Coated Palladium Nanoparticle Films

Coating a polymeric membrane for gas separation is a feasible approach to fabricate gas sensors with selectivity. In this study, poly(methyl methacrylate)-(PMMA-)membrane-coated palladium (Pd) nanoparticle (NP) films were fabricated for high-performance hydrogen (H2) gas sensing by carrying out gas-phase cluster deposition and PMMA spin coating. No changes were induced by the PMMA spin coating in the electrical transport and H2-sensing mechanisms of the Pd NP films. Measurements of H2 sensing demonstrated that the devices were capable of detecting H2 gas within the concentration range 0-10% at room temperature and showed high selectivity to H2 due to the filtration effect of the PMMA membrane layer. Despite the presence of the PMMA matrix, the lower detection limit of the sensor is less than 50 ppm. A series of PMMA membrane layers with different thicknesses were spin coated onto the surface of Pd NP films for the selective filtration of H2. It was found that the device sensing kinetics were strongly affected by the thickness of the PMMA layer, with the devices with thicker PMMA membrane layers showing a slower response to H2 gas. Three mechanisms slowing down the sensing kinetics of the devices were demonstrated to be present: diffusion of H2 gas in the PMMA matrix, nucleation and growth of the β phase in the α phase matrix of Pd hydride, and stress relaxation at the interface between Pd NPs and the PMMA matrix. The retardation effect caused by these three mechanisms on the sensing kinetics relied on the phase region of Pd hydride during the sensing reaction. Two simple strategies, minimizing the thickness of the PMMA membrane layer and reducing the size of the Pd NPs, were proposed to compensate for retardation of the sensing response.

Z-contrast atom counting using the few-layer graphene sheets

Upon the development of the quantitative scanning transmission electron microscopy (QSTEM), individually counting the gold atom has been achieved. The correlation of the signals to the atom quantity (N), i.e. calibration, is its crucial work. One has to seek the ultra-fine mass standards, where the size-selected gold clusters rank the best recently. However, practically this is difficult for light elements, since STEM intensity is proportional to Zα, where Z is atomic number of the elements and α is in the range of 1.5-1.9 depending on the detector collection angle, sample thickness, and the Debye-Waller factor of the atomic species. As a result, the light elements would have a much weaker image contrast than that of heavy elements. This could be even more difficult for the carbon particles supported by the Formvar films. Graphene layers are featured by large scale uniformity and discrete layer steps, which shed light on precision calibration of the carbon atomic scales. To establish direct correlation between the STEM intensity with the number of layers in graphene sheets, we apply an independent layer counting method by utilizing dark lines in bright field TEM images at the edge of graphene sheets [2]. We search over a large area of the sample for the freely suspending graphene sheets (for example, the area indicated by the arrows in Fig. 1a)[3]. Fig. 1b displays a typical example of such an edge, where the layer number can be counted in the high-resolution bright field image and the STEM intensity is also obtained. The calibration in the few-layer range gives Fig. 1c, where a linear relationship is apparent. The calibration performed over an extended thickness presents a precise exponential growth. The behaviour can be described by the incoherent scattering picture of single scattering. This leads to accurate measurement of the electron mean free path (The error is reduced to 5 percent). The calibration relation works from individual up to millions of carbon atoms.

Study the electronic responses of individual nanoparticles by electron energy loss spectroscopy in a transmission electron microscope

Plasmon coupling of dimers of Ag nanoparticles is studied by the EELS as shown in Fig 1a [1]. The spectroscopic images in the above Fig. 1a are extracted from 2.8eV, 3.4eV and 3.6eV respectively, where one can see the preferable exciting points of the features around the Ag nanoparticles. With the aid of DDA calculation(Fig. 1b), the three features are identified as the in-phase plasmon coupling, non-coupling and anti-phase coupling modes. The in-phase mode is able to conduct the light transportation due to its dipolar nature, which redshifts from the 3.4eV at large spacings to 2.7eV near touching (Fig. 1c). Such behaviour is also observed in the chain of 3 and 4 nanoparticles. The experiment provides clear evidence on the plasmon coupling of Ag nanoparticles. Extracting the dopant states in the Co-doped ZnO nanoparticles [2]. In order to assign the dopant-introduced states in the low-loss EELS, we employ both the core-loss EELS and the valence EELS on the same Co-doped ZnO nanoparticles by spatially mapping the dopant concentration distribution via the core-loss EELS (Fig 2a 2b 2c) and simultaneously by identifying the dopant features in the accumulated valence EELS via the measured dopant richness (Fig 2d). Three Coincidentally-growing Co dopant states are successfully determined in Fig. 2e. The experimental inputs feed the first-principles calculation, which generates full electronic structure of the doped ZnO nanoparticles. The experiment supports the carrier mediated room-temperature magnetism, which is a main confusion with current dilute magnetic semiconductors.

Scaling the dynamic electron scattering in HAADF-imaging the graphene sheets

Free-standing graphene sheets (GS) with several to over a hundred layers are prepared by splitting the expandable graphite. The raw material is firstly transferred to a heating chamber for the thermal flashing at 1500 οC in a hydrogen chamber. The expanded powder (1 mg) is then dispersed in 10 ml of the PmPV/DCE suspension for high-power sonication and further centrifugation. After drop-cast onto a holey Formvar film, the GSs are suspended on the grid. The electron scattering is carried out in a Tecnai F20 TEM/STEM with a field emission gun. A high-angle annular dark field (HAADF) detector is fitted to collect the scattered electrons with a tunable collecting angle. The quantitative STEM image simulation is carried out using amultislice algorithm [4].