Study on the origin of amorphous carbon peaks on graphene films synthesized on nickel catalysts.

We present our investigation results on the origin of the morphological defects on graphene films synthesized by chemical vapor deposition method on nickel catalytic substrates. These defects are small-base-area (SBA) peaks with tens of nanometer heights, and they diminish the applicability of graphene films. From atomic force microscopy observations on the graphene films prepared in various ways, we found that significant portion of the SBA peaks is formed in the crevices on the nickel substrates. Our results may be useful for developing an efficient synthesis method to produce high-quality graphene films without the SBA peaks.

High-performance photoconductivity and electrical transport of ZnO/ZnS core/shell nanowires for multifunctional nanodevice applications.

We report the electrical and optical properties of ZnO/ZnS core/shell nanowire (NW) devices. The spatial separation of charge carriers due to their type II band structure together with passivation effect on ZnO/ZnS core/shell NWs not only enhanced their charge carrier transport characteristics by confining the electrons and reducing surface states in the ZnO channel but also increased the photocurrent under ultraviolet (UV) illumination by reducing the recombination probability of the photogenerated charge carriers. Here the efficacy of the type-II band structure and the passivation effect are demonstrated by showing the enhanced subthreshold swing (150 mV/decade) and mobility (17.2 cm2/(Vs)) of the electrical properties, as well as the high responsivity (4.4×10(6) A/W) in the optical properties of the ZnO/ZnS core/shell NWs, compared with the subthreshold swing (464 mV/decade), mobility (8.9 cm2/(Vs)) and responsivity (2.5×10(6) A/W) of ZnO NWs.

Investigation of threshold voltage instability induced by gate bias stress in ZnO nanowire field effect transistors.

We investigated the threshold voltage instability induced by gate bias (V(G)) stress in ZnO nanowire (NW) field effect transistors (FETs). By increasing the V(G) sweep ranges and repeatedly measuring the electrical characteristics of the ZnO NW FETs, the V(G) stress was produced in the dielectric layer underneath the ZnO NW. Consequently, the electrical conductance of the ZnO NW FETs decreased, and the threshold voltage shifted towards the positive V(G) direction. This threshold voltage instability induced by the V(G) stress is associated with the trapping of charges in the interface trap sites located in the ZnO NW-dielectric interface. Our study will be helpful for understanding the stability of ZnO NW FETs during repetitive operations.

Electrical and structural properties of antimony-doped p-type ZnO nanorods with self-corrugated surfaces.

We report on p-type conductivity in antimony (Sb)-doped ZnO (ZnO:Sb) nanorods which have self-corrugated surfaces. The p-ZnO:Sb/n-ZnO nanorod diode shows good rectification characteristics, confirming that a p-n homojunction is formed in the ZnO nanorod diode. The low-temperature photoluminescence (PL) spectra of the ZnO:Sb nanorods reveal that the p-type conductivity in p-ZnO:Sb is related to the Sb(Zn)-2V(Zn) complex acceptors. Transmission electron microscopy (TEM) analysis of the ZnO:Sb nanorods also shows that the p-type conductivity is attributed to the Sb(Zn)-2V(Zn) complex acceptors which can be easily formed near the self-corrugated surface regions of ZnO:Sb nanorods. These results suggest that the Sb(Zn)-2V(Zn) complex acceptors are mainly responsible for the p-type conductivity in ZnO:Sb nanorods which have corrugated surfaces.

A self-assembled Ag nanoparticle agglomeration process on graphene for enhanced light output in GaN-based LEDs.

We introduce Ag nanoparticles fabricated by a self-assembled agglomeration process in order to enhance the electrical properties, adhesive strength, and reliability of the graphene spreading layer in inorganic-based optoelectronic devices. Here, we fabricated InGaN/GaN multi-quantum-well (MQW) blue LEDs having various current spreading layers: graphene only, graphene with Ag nanoparticles covering the surface, and graphene with Ag nanoparticles only in selectively patterned micro-circles. Although the Ag nanoparticles were found to act as an additional current path that increases the current spreading, optical properties such as transmittance also need to be considered when the Ag nanoparticles are combined with graphene. As a result, LEDs having a graphene spreading layer with Ag nanoparticles formed in selectively patterned micro-circles displayed more uniform and stable light emission and 1.7 times higher light output power than graphene only LEDs.

The application of graphene as electrodes in electrical and optical devices.

Graphene is a promising next-generation conducting material with the potential to replace traditional electrode materials such as indium tin oxide in electrical and optical devices. It combines several advantageous characteristics including low sheet resistance, high optical transparency and excellent mechanical properties. Recent research has coincided with increased interest in the application of graphene as an electrode material in transistors, light-emitting diodes, solar cells and flexible devices. However, for more practical applications, the performance of devices should be further improved by the engineering of graphene films, such as through their synthesis, transfer and doping. This article reviews several applications of graphene films as electrodes in electrical and optical devices and discusses the essential requirements for applications of graphene films as electrodes.

Organic nonvolatile memory devices with charge trapping multilayer graphene film.

We fabricated an array-type organic nonvolatile memory device with multilayer graphene (MLG) film embedded in polyimide (PI) layers. The memory devices showed a high ON/OFF ratio (over 10(6)) and a long retention time (over 10(4) s). The switching of the Al/PI/MLG/PI/Al memory devices was due to the presence of the MLG film inserted into the PI layers. The double-log current-voltage characteristics could be explained by the space-charge-limited current conduction based on a charge-trap model. A conductive atomic force microscopy found that the conduction paths in the low-resistance ON state were distributed in a highly localized area, which was associated with a carbon-rich filamentary switching mechanism.

Thermal stability of multilayer graphene films synthesized by chemical vapor deposition and stained by metallic impurities.

Thermal stability is an important property of graphene that requires thorough investigation. This study reports the thermal stability of graphene films synthesized by chemical vapor deposition (CVD) on catalytic nickel substrates in a reducing atmosphere. Electron microscopies, atomic force microscopy, and Raman spectroscopy, as well as electronic measurements, were used to determine that CVD-grown graphene films are stable up to 700 °C. At 800 °C, however, graphene films were etched by catalytic metal nanoparticles, and at 1000 °C many tortuous tubular structures were formed in the film and carbon nanotubes were formed at the film edges and at catalytic metal-contaminated sites. Furthermore, we applied our pristine and thermally treated graphene films as active channels in field-effect transistors and characterized their electrical properties. Our research shows that remnant catalytic metal impurities play a critical role in damaging graphene films at high temperatures in a reducing atmosphere: this damage should be considered in the quality control of large-area graphene films for high temperature applications.

Unipolar bistable switching of organic non-volatile memory devices with poly(styrene-co-styrenesulfonic acid Na).

We demonstrated unipolar organic bistable memory devices with 8 x 8 cross-bar array type structure. The active material for the organic non-volatile memory devices is poly(styrene-co-styrenesulfonic acid Na) (PSSANa). From the electrical measurements of the PSSANa organic memory devices, we observed rewritable unipolar switching behaviors with a stable endurance and narrow cumulative probability. Also the PSSANa memory devices exhibited a uniform cell-to-cell switching with a high ON/OFF ratio of approximately 10(5) and good retention time of approximately 10(4) seconds without significant degradation.

Enhancement in the photodetection of ZnO nanowires by introducing surface-roughness-induced traps.

We investigated the enhanced photoresponse of ZnO nanowire transistors that was introduced with surface-roughness-induced traps by a simple chemical treatment with isopropyl alcohol (IPA). The enhanced photoresponse of IPA-treated ZnO nanowire devices is attributed to an increase in adsorbed oxygen on IPA-induced surface traps. The results of this study revealed that IPA-treated ZnO nanowire devices displayed higher photocurrent gains and faster photoswitching speed than transistors containing unmodified ZnO nanowires. Thus, chemical treatment with IPA can be a useful method for improving the photoresponse of ZnO nanowire devices.

A study of graphene films synthesized on nickel substrates: existence and origin of small-base-area peaks.

Large-area graphene films, synthesized by the chemical vapor deposition (CVD) method, have the potential to be used as electrodes. However, the electrical properties of CVD-synthesized graphene films fall short of the best results obtained for graphene films prepared by other methods. Therefore, it is important to understand the reason why these electrical properties are inferior to improve the applicability of CVD-grown graphene films. Here, we show that CVD-grown graphene films on nickel substrates contain many small-base-area (SBA) peaks that scatter conducting electrons, thereby decreasing the Hall mobility of charges in the films. These SBA peaks were induced by small peaks on the nickel surface and are likely composed of amorphous carbon. The formation of these SBA peaks on graphene films was successfully suppressed by controlling the surface morphology of the nickel substrate. These findings may be useful for the development of a CVD synthesis method that is capable of producing better quality graphene films with large areas.

Nonvolatile memory functionality of ZnO nanowire transistors controlled by mobile protons.

We demonstrated the nonvolatile memory functionality of ZnO nanowire field effect transistors (FETs) using mobile protons that are generated by high-pressure hydrogen annealing (HPHA) at relatively low temperature (400 °C). These ZnO nanowire devices exhibited reproducible hysteresis, reversible switching, and nonvolatile memory behaviors in comparison with those of the conventional FET devices. We show that the memory characteristics are attributed to the movement of protons between the Si/SiO(2) interface and the SiO(2)/ZnO nanowire interface by the applied gate electric field. The memory mechanism is explained in terms of the tuning of interface properties, such as effective electric field, surface charge density, and surface barrier potential due to the movement of protons in the SiO(2) layer, consistent with the UV photoresponse characteristics of nanowire memory devices. Our study will further provide a useful route of creating memory functionality and incorporating proton-based storage elements onto a modified CMOS platform for FET memory devices using nanomaterials.

Large-scale patterned multi-layer graphene films as transparent conducting electrodes for GaN light-emitting diodes.

This work demonstrates a large-scale batch fabrication of GaN light-emitting diodes (LEDs) with patterned multi-layer graphene (MLG) as transparent conducting electrodes. MLG films were synthesized using a chemical vapor deposition (CVD) technique on nickel films and showed typical CVD-synthesized MLG film properties, possessing a sheet resistance of [Formula: see text] with a transparency of more than 85% in the 400-800 nm wavelength range. The MLG was applied as the transparent conducting electrodes of GaN-based blue LEDs, and the light output performance was compared to that of conventional GaN LEDs with indium tin oxide electrodes. Our results present a potential development toward future practical application of graphene electrodes in optoelectronic devices.

Tuning of the electronic characteristics of ZnO nanowire field effect transistors by proton irradiation.

We demonstrated a controllable tuning of the electronic characteristics of ZnO nanowire field effect transistors (FETs) using a high-energy proton beam. After a short proton irradiation time, the threshold voltage shifted to the negative gate bias direction with an increase in the electrical conductance, whereas the threshold voltage shifted to the positive gate bias direction with a decrease in the electrical conductance after a long proton irradiation time. The electrical characteristics of two different types of ZnO nanowires FET device structures in which the ZnO nanowires are placed on the substrate or suspended above the substrate and photoluminescence (PL) studies of the ZnO nanowires provide substantial evidence that the experimental observations result from the irradiation-induced charges in the bulk SiO(2) and at the SiO(2)/ZnO nanowire interface, which can be explained by a surface-band-bending model in terms of gate electric field modulation. Our study on the proton-irradiation-mediated functionalization can be potentially interesting not only for understanding the proton irradiation effects on nanoscale devices, but also for creating the property-tailored nanoscale devices.

Tuning of operation mode of ZnO nanowire field effect transistors by solvent-driven surface treatment.

We report on the adjustment of the operation voltage in ZnO nanowire field effect transistors (FETs) by a simple solvent treatment. We have observed that by submerging ZnO nanowires in isopropyl alcohol (IPA), the surface of the ZnO nanowires is etched, generating surface roughness, and their defect emission peak becomes stronger. In particular, ZnO nanowire FETs before IPA treatment operate in the depletion-mode, but are converted to the enhancement-mode with a positive shift of threshold voltage after submersion in IPA. This solvent treatment can be a useful method for controlling the operation mode of ZnO nanowire FETs for wide applications of nanowire-based electronic devices and circuits.

The effect of excimer laser annealing on ZnO nanowires and their field effect transistors.

We have investigated the effect of excimer laser annealing on the chemical bonding, electrical, and optical properties of ZnO nanowires. We demonstrate that after laser annealing on the ZnO nanowire field effect transistors, the on-current increases and the threshold voltage shifts in the negative gate bias direction. These electrical results are attributed to the increase of oxygen vacancies as n-type dopants after laser annealing, consistent with the shifts towards higher binding energies of Zn 2p and O 1s in the x-ray photoelectron spectroscopy analysis of as-grown nanowires and laser-annealed ZnO nanowires.