Tianzhong Yang

Controlled Synthesis of Large‐Scale, Uniform, Vertically Standing Graphene for High‐Performance Field Emitters

Large-scale, uniform, vertically standing graphene with atomically thin edges are controllably synthesized on copper foil using a microwave-plasma chemical vapor deposition system. A growth mechanism for this system is proposed. This film shows excellent field-emission properties, with low turn-on field of 1.3 V μm(-1) , low threshold field of 3.0 V μm(-1) and a large field-enhancement factor more than 10 000.

Control of Superhydrophilic and Superhydrophobic Graphene Interface

Superhydrophobic and superhydrophilic properties of chemically-modified graphene have been achieved in larger-area vertically aligned few-layer graphene nanosheets (FLGs), prepared on Si (111) substrate by microwave plasma chemical vapor deposition (MPCVD). Furthermore, in order to enhance wettability, silicon wafers with microstructures were fabricated, on which graphene nanosheets were grown and modified by a chemical method to form hydrophilic and hydrophobic structures. A superhydrophilic graphene surface (contact angle 0°) and a superhydrophobic graphene surface (contact angle 152.0°) were obtained. The results indicate that the microstructured silicon enhances the hydrophilic and hydrophobic wettabilities significantly.

Intrinsic current-voltage properties of nanowires with four-probe scanning tunneling microscopy : A conductance transition of ZnO nanowire

We report intrinsic current-voltage properties of ZnO nanowire measured by a four-tip scanning tunneling microscopy (F-STM). It is found that after bending the nanowire with the F-STM the conductance is reduced by about five orders of magnitude. The cathodoluminescent spectra indicate that the ZnO nanowires contain a sizable amount of defects in the surface region, responsible for their conduction. It is suggested that the observed huge conductance changes are caused by the shifting of the surface defect states in the ZnO nanowires in response to the applied surface strain.

Wet chemical synthesis of gold nanoparticles using silver seeds: a shape control from nanorods to hollow spherical nanoparticles

A seed-mediated method was employed here for CTAB-assisted gold nanoparticle growth. 3–4 nm silver aqueous colloid was stabilized by sodium citrate and used as seed solution to initial gold particle growth. The concentration of seed solution was calculated based on its relationship with silver atom concentration and seed particle statistical mean volume. It was found that there is a maximum seed concentration of 8.57 × 10 −12 M( ∼25 μ l0 .343 × 10 −8 M seed solution added) in 10 ml 2.5 × 10 −4 M HAuCl4 growth solution for growth of rodlike particles. Below this seed amount, the aspect ratio of nanorods could be controlled by varying the silver seed amount, i.e. nanorods with aspect ratio ∼18.9 were obtained when the seed concentration in the growth solution was 0.343 × 10 −12 M by adding 1 μ l0 .343 × 10 −8 M silver seed solution and nanorods with aspect ratio ∼9.69 were obtained when the seed concentration in the growth solution was 1.715 × 10 −12 M by adding 5 μ l0 .343 × 10 −8 M silver seed solution. As the seed concentration in the growth solution was more than 8.58 × 10 −12 M( 25μ l0 .343 × 10 −8 M silver seed solution was added), there were no rodlike particles formed but spherical ones instead. These spheres were further studied by TEM and found to all be hollow structures. It was suggested that there were probably two different nucleation processes for growth of nanorods and spheres. For hollow spheres, the reaction between Ag seeds and Au ions formed hollow structures based on the Ag particle template effect. Then further growth of Au on these hollow structures produced hollow gold nanospheres. For nanorods, due to the very low concentration of silver seed (molar ratio of Ag seed: Au = 3.426 × 10 −8 ), the growth process here probably was started by silver-induced Au nucleation, in which reduction of gold ions by silver resulted in small gold clusters. These gold clusters further grew up into nanoparticles and nanorods in the presence of CTAB. S Supplementary data are available from stacks.iop.org/Nano/18/115608

Large scale SiC∕SiOx nanocables: Synthesis, photoluminescence, and field emission properties

Large scale, high yield SiC∕SiOx nanocables have been synthesized by thermal evaporation of carbon powders and silicon powders in the presence of Fe3O4 nanoparticle catalysts. Transmission electron microscopy and high-resolution transmission electron microscopy show that the nanocables consist of a 50–300nm single-crystalline β‐SiC core wrapped with a 10–20nm amorphous SiOx shell. The nanocables have two broad photoluminescence peaks located around 390 and 460nm when the 250nm ultraviolet fluorescent light excitation is applied at room temperature. The results of field emission measurement of SiC∕SiOx nanocables indicate the low turn-on and threshold electric fields of 3.2 and 5.3V∕μm at the vacuum gap of 200μm, respectively. When the vacuum gap was increased to 1000μm, the turn-on and threshold electric fields were decreased to 1.1 and 2.3V∕μm, respectively. The SiC∕SiOx nanocables with good photoluminescence and field emission properties are promising candidates for ultraviolet-blue emitting devices, fl...

Temperature-dependent modulation characteristics for 1.3 mu m InAs/GaAs quantum dot lasers

Temperature-dependent modulation characteristics of 1.3 mu m InAs/GaAs quantum dot (QD) lasers under small signals have been carefully studied at various bias currents. Based on experimental observations, it is found that the modulation bandwidth significantly increases when excited state (ES) lasing emerges at high temperature. This is attributed to additional photons emitted by ES lasing which contribute to the modulation response. A rate equation model including two discrete electron energy levels and the level of wetting layer has been used to investigate the temperature-dependent dynamic behavior of the QD lasers. Numerical investigations confirm that the significant jump for the small signal modulation response is indeed caused by ES photons. Furthermore, we identify how the electron occupation probabilities of the two discrete energy levels can influence the photon density of different states and finally the modulation rate. Both experiments and numerical analysis show that the modulation bandwidth of QD lasers at high temperature can be increased by injecting more carriers into the ES that has larger electron state degeneracy and faster carriers relaxation time than the ground state.

Facile synthesis of hollow nano-spheres and hemispheres of cobalt by polyol reduction

The hydrophilic hollow nano-spheres and hemispheres of Co are synthesized via ethylene glycol reduction of cobalt acetate in the presence of PVP and Pd nano-particle seeds. The dimensions of the hollow core can be tuned from 100 to 300 nm by controlling the amount of Pd nano-particle seeds. The morphology of the hollow materials strongly depends on the molar ratio of the amide unit in PVP over Co and the M(w) of PVP. The hollow structure is formed when the ratio falls in the range 1-1.5 and the M(w) is over 40,000. Based on the experimental data, a possible formation mechanism of Co hollow spheres is proposed.

Optically Unraveling the Edge Chirality-Dependent Band Structure and Plasmon Damping in Graphene Edges

The nontrivial topological origin and pseudospinorial character of electron wavefunctions make edge states possess unusual electronic properties. Twenty years ago, the tight-binding model calculation predicted that zigzag termination of 2D sheets of carbon atoms have peculiar edge states, which show potential application in spintronics and modern information technologies. Although scanning probe microscopy is employed to capture this phenomenon, the experimental demonstration of its optical response remains challenging. Here, the propagating graphene plasmon provides an edge-selective polaritonic probe to directly detect and control the electronic edge state at ambient condition. Compared with armchair, the edge-band structure in the bandgap gives rise to additional optical absorption and strongly absorbed rim at zigzag edge. Furthermore, the optical conductivity is reconstructed and the anisotropic plasmon damping in graphene systems is revealed. The reported approach paves the way for detecting edge-specific phenomena in other van der Waals materials and topological insulators.

High-quality graphene grown on polycrystalline PtRh20 alloy foils by low pressure chemical vapor deposition and its electrical transport properties

High-quality continuous uniform monolayer graphene was grown on polycrystalline PtRh20 alloy foils by low pressure chemical vapor deposition. The morphology of graphene was investigated by Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. Analysis results confirm that high quality single-layer graphene was fabricated on PtRh20 foil at 1050 °C using a lower flux of methane under low pressure. Graphene films were transferred onto the SiO2/Si substrate by the bubbling transfer method. The mobility of a test field effect transistor made of the graphene grown on PtRh20 was measured and reckoned at room temperature, showing that the carrier mobility was about 4000 cm2 V−1 s−1. The results indicate that desired quality of single-layer graphene grown on PtRh20 foils can be obtained by tuning reaction conditions.

Preparation of few-layer graphene-capped boron nanowires and their field emission properties*

Large-area boron nanowire (BNW) films were fabricated on the Si(111) substrate by chemical vapor deposition (CVD). The average diameter of the BNWs is about 20 nm, with lengths of 5–10 μm. Then, graphene-capped boron nanowires (GC-BNWs) were obtained by microwave plasma chemical vapor deposition (MPCVD). Characterization by scanning electron microscopy indicates that few-layer graphene covers the surface of the boron nanowires. Field emission measurements of the BNWs and GC-BNW films show that the GC-BNW films have a lower turn-on electric field than the BNW films.