Magnetotransport across the metal-graphene hybrid interface and its modulation by gate voltage.

The graphene-metal contact is very important for optimizing the performance of graphene based electronic devices. However, it is difficult to probe the properties of the graphene/metal interface directly via transport measurements in traditional graphene lateral devices, because the dominated transport channel is graphene, not the interface. Here, we employ the Au/graphene/Au vertical and lateral hybrid structure to unveil the metal-graphene interface properties, where the transport is dominated by the charge carriers across the interface. The magnetoresistance (MR) of Au/monolayer graphene/Au and Au/stacked two-layered graphene/Au devices is measured and modulated by gate voltage, demonstrating that the interface is a device. The gate-tunable MR is identified from the graphene lying on the SiO2 substrate and underneath the top metal electrode. Our unique structures couple the in-plane and out-of-plane transport and display linear MR with small amplitude oscillations at low temperatures. Under a magnetic field, the electronic coupling between the graphene edge states and the electrode leads to the appearance of quantum oscillations. Our results not only provide a new pathway to explore the intrinsic transport mechanism at the graphene/metal interface but also open up new vistas of magnetoelectronics.

Layer-by-layer assembly of vertically conducting graphene devices.

Graphene has various potential applications owing to its unique electronic, optical, mechanical and chemical properties, which are primarily based on its two-dimensional nature. Graphene-based vertical devices can extend the investigations and potential applications range to three dimensions, while interfacial properties are crucial for the function and performance of such graphene vertical devices. Here we report a general method to construct graphene vertical devices with controllable functions via choosing different interfaces between graphene and other materials. Two types of vertically conducting devices are demonstrated: graphene stacks sandwiched between two Au micro-strips, and between two Co layers. The Au|graphene|Au junctions exhibit large magnetoresistance with ratios up to 400% at room temperature, which have potential applications in magnetic field sensors. The Co|graphene|Co junctions display a robust spin valve effect at room temperature. The layer-by-layer assembly of graphene offers a new route for graphene vertical structures.

Synthesis and quantum transport properties of Bi2Se3 topological insulator nanostructures.

Bi2Se3 nanocrystals with various morphologies, including nanotower, nanoplate, nanoflake, nanobeam and nanowire, have been synthesized. Well-distinguished Shubnikov-de Haas (SdH) oscillations were observed in Bi2Se3 nanoplates and nanobeams. Careful analysis of the SdH oscillations suggests the existence of Berry's phase π, which confirms the quantum transport of the surface Dirac fermions in both Bi2Se3 nanoplates and nanobeams without intended doping. The observation of the singular quantum transport of the topological surface states implies that the high-quality Bi2Se3 nanostructures have superiorities for investigating the novel physical properties and developing the potential applications.

Stretch-induced stiffness enhancement of graphene grown by chemical vapor deposition.

The mechanical properties of ultrathin membranes have attracted considerable attention recently. Nanoindentation based on atomic force microscopy is commonly employed to study mechanical properties. We find that the data processing procedures in previous studies are nice approximations, but it is difficult for them to illustrate the mechanical properties precisely. Accordingly, we develop a revised numerical method to describe the force curve properly, by which the intrinsic mechanical properties of these membranes can be acquired. Combining the nanoindentation measurements with the revised numerical method, we demonstrate that loading-unloading cycles under large load can lead to a pronounced improvement in stiffness of graphene grown by chemical vapor deposition (CVD). The Young's moduli of the stretched CVD graphene membranes can be improved to ∼1 TPa, closing to the value of the pristine graphene. Our findings demonstrate a possible way to recover the exceptional elastic properties of CVD graphene from the softened stiffness caused by wrinkles.

Large magnetoresistance in few layer graphene stacks with current perpendicular to plane geometry.

A large magnetoresistance (MR) effect of few-layers graphene between two non-magnetic metal electrodes with current perpendicular to graphene plane is studied. A non-saturation and anisotropic MR with the value over 60% at 14 T is observed in a two-layer graphene stack at room temperature. The resistance of the device is only tens of ohms, having the advantage of low power consumption for magnetic device applications.

Improved performance of ZnO nanowire field-effect transistors via focused ion beam treatment.

A seven orders of magnitude increase in the current on/off ratio of ZnO nanowire field-effect transistors (FETs) after Ga( + ) irradiation was observed. Transmission electron microscopy characterization revealed that the surface crystal quality of the ZnO nanowire was improved via the Ga( + ) treatment. The Ga( + ) irradiation efficiently reduces chemisorption effects and decreases oxygen vacancies in the surface layer. The enhanced performance of the nanowire FET was attributed to the decrease of surface trapped electrons and the decrease in carrier concentration.

Self-powered, ultrafast, visible-blind UV detection and optical logical operation based on ZnO/GaN nanoscale p-n junctions.

Ultrafast-response (20 µs) UV detectors, which are visible-blind and self-powered, in devices where an n-type ZnO nanowire partially lies on a p-type GaN film, are demonstrated. Moreover, a CdSe-nanowire red-light detector powered by a nanoscale ZnO/GaN photovoltaic cell is also demonstrated, which extends the device function to a selective multiwavelength photodetector and shows the function of an optical logical AND gate.

Ion irradiation induced structural and electrical transition in graphene.

The relationship between the electrical properties and structure evolution of single layer graphene was studied by gradually introducing the gallium ion irradiation. Raman spectrums show a structural transition from nano-crystalline graphene to amorphous carbon as escalating the degree of disorder of the graphene sample, which is in correspondence with the electrical transition from a Boltzmann diffusion transport to a carrier hopping transport. The results show a controllable method to tune the properties of graphene.

Hysteresis reversion in graphene field-effect transistors.

To enhance performances of graphene/SiO(2) based field-effect transistors (FETs), understanding of the transfer of carriers through the graphene/SiO(2) interface is crucial. In this paper, we have studied the temperature dependent transfer characters of graphene FETs. Hysteresis loop is shown to be dominated by trapping/detrapping carriers through the graphene/SiO(2) interface.

Effect of contact barrier on electron transport in graphene.

The influence of the barrier between metal electrodes and graphene on the electrical properties was studied on a two-electrode device. A classical barrier model was used to analyze the current-voltage characteristics. Primary parameters including barrier height and effective resistance were achieved. The electron transport properties under magnetic field were further investigated. An abnormal peak-valley-peak shape of voltage-magnetoresistance curve was observed. The underlying mechanisms were discussed under the consideration of the important influence of the contact barrier. Our results indicate electrical properties of graphene based devices are sensitive to the contact interface.

Influence of temperature and illumination on surface barrier of individual ZnO nanowires.

The current-voltage (I-V) characteristics of single ZnO nanowires were measured varying with temperature and illumination. A model of the ZnO nanowire sandwiched by back-to-back diodes was utilized to explain the experimental data. Simulations of the I-V curves exhibited that the surface barrier height was independent of temperature from 180 to 290 K. This work also shows that the larger the incident laser power is, the smaller the contact surface barrier height will be. The photon induced reduction in the surface barrier height is attributed to the photogenerated holes, which result in a shielding effect on the surface trapped electrons.

The effect of adsorbates on the space-charge-limited current in single ZnO nanowires.

We studied the influence of adsorbates on the space-charge-limited current (SCLC) in individual ZnO nanowires through varying the bias voltage, laser illumination, and ambient pressure. In dark and air conditions, the free carriers were depleted by the surface adsorbates, and electrons injected from the electrode to the nanowire dominated the electron transport properties. Under laser illumination, the current-voltage characteristic was linear at low voltage and superlinear at high voltage, and the SCLC regime occurred at high voltages due to the surface desorption. The time response of photoconductivity further revealed the dynamic process of elimination of SCLC by desorption of oxygen molecules at the ZnO nanowire surface.