Sheng-Bo Wang

Band Gap Engineering of Chemical Vapor Deposited Graphene by in-situ BN Doping

Band gap opening and engineering is one of the high priority goals in the development of graphene electronics. Here, we report on the opening and scaling of band gap in BN doped graphene (BNG) films grown by low-pressure chemical vapor deposition method. High resolution transmission electron microscopy is employed to resolve the graphene and h-BN domain formation in great detail. X-ray photoelectron, micro-Raman, and UV-vis spectroscopy studies revealed a distinct structural and phase evolution in BNG films at low BN concentration. Synchrotron radiation based XAS-XES measurements concluded a gap opening in BNG films, which is also confirmed by field effect transistor measurements. For the first time, a significant band gap as high as 600 meV is observed for low BN concentrations and is attributed to the opening of the π-π* band gap of graphene due to isoelectronic BN doping. As-grown films exhibit structural evolution from homogeneously dispersed small BN clusters to large sized BN domains with embedded diminutive graphene domains. The evolution is described in terms of competitive growth among h-BN and graphene domains with increasing BN concentration. The present results pave way for the development of band gap engineered BN doped graphene-based devices.

Plasmonic Ag@Ag3(PO4)1−x nanoparticle photosensitized ZnO nanorod-array photoanodes for water oxidation

We report the new design of a high-activity model for photocatalytic nanosystem comprising an Ag core covered with an approximately 2 nm thick nanoshell of Ag3(PO4)1–x (Ag@Ag3(PO4)1–x) on the ZnO NRs that are visible-light-sensitive photofunctional electrodes with strong photooxidative capabilities to evolve O2 from water. The maximum photoconversion efficiency that could be successfully achieved was 2%, with a significant photocurrent of 3.1 mA cm−2. Furthermore, in addition to achieving a maximum IPCE value of 90%, it should be noted that the IPCE of Ag@Ag3(PO4)1−x photosensitized ZnO photoanodes at the monochromatic wavelength of 400 nm is up to 60%. Our photoelectrochemical performances are comparable to those of many oxide-based photoanodes in recent reports. The improvement in photoactivity of PEC water-splitting may be attributed to the enhanced near-field amplitudes resulting from localized surface plasmon resonance (LSPR) of Ag–core and absorption edge of the Ag3(PO4)1−x nanoshell, which increase the rate of formation of electron–hole pairs at the nearby surface of Ag3(PO4)1−x nanoshell and ZnO nanorod, thus enlarging the amount of photogenerated charge contributing to photocatalysis. The capability of developing highly photoactive Ag@Ag3(PO4)1−x-photosensitized ZnO photoanodes opens up new opportunities in various photocatalytic areas, particularly solar-hydrogen fields.

ZnO Branched Nanowires and the p-CuO/n-ZnO Heterojunction Nanostructured Photodetector

The authors report the growth of ZnO branched nanowires on the CuO nanowires and the fabrication of p-CuO/n-ZnO heterojunction nanostructured photodetector (PD). It was found that the hydrothermally grown ZnO branched nanowires were reasonably uniform with an average length of 200 nm and an average diameter of 50 nm. Under forward bias, it was found that turn on voltage of the fabricated PD reduced from ~0.7 to ~0.2 V under ultraviolet (UV) illumination. It was also found that UV-to-visible rejection ratio of the fabricated device was larger than 100.

CuO Nanowire-Based Humidity Sensor

The authors report the growth of CuO nanowires on an oxidized Cu wire and the fabrication of a CuO nanowire humidity sensor. It was found that we could transform a Cu wire into CuO/Cu2O/Cu core-shell tri-layers covered with high density CuO nanowires by thermal annealing. It was also found that steady state currents of the sensor were about 2.44, 2.32, 2.23, and 2.15 μA , respectively, when measured with 20, 40, 60, and 80% relative humidity. Furthermore, it was found that sensing property of the fabricated device was stable and reproducible.

Growth of β-Ga2O3 and GaN nanowires on GaN for photoelectrochemical hydrogen generation

Enhanced photoelectrochemical (PEC) performances of Ga(2)O(3) and GaN nanowires (NWs) grown in situ from GaN were demonstrated. The PEC conversion efficiencies of Ga(2)O(3) and GaN NWs have been shown to be 0.906% and 1.09% respectively, in contrast to their 0.581% GaN thin film counterpart under similar experimental conditions. A low crystallinity buffer layer between the grown NWs and the substrate was found to be detrimental to the PEC performance, but the layer can be avoided at suitable growth conditions. A band bending at the surface of the GaN NWs generates an electric field that drives the photogenerated electrons and holes away from each other, preventing recombination, and was found to be responsible for the enhanced PEC performance. The enhanced PEC efficiency of the Ga(2)O(3) NWs is aided by the optical absorption through a defect band centered 3.3 eV above the valence band of Ga(2)O(3). These findings are believed to have opened up possibilities for enabling visible absorption, either by tailoring ion doping into wide bandgap Ga(2)O(3) NWs, or by incorporation of indium to form InGaN NWs.

Enhancing efficiency with fluorinated interlayers in small molecule organic solar cells

This study presents a simple approach to improve the performance of small molecule based organic solar cells (OSCs) by inserting a fluorinated buffer layer (e.g., PFAS) at the hetero-interface of bilayer devices. As demonstrated in this work, the PFAS modification reduces the surface energy of the conventional PEDOT : PSS photoanode and results in a significant improvement in the pentacene based OSC. The passivated PEDOT : PSS surface after PFAS modification has a lower interface energy with pentacene and facilitates 3D single crystalline (dendritic-like) phase pentacene growth. Concurrently, the accumulated negative charges of the fluorinated PFAS layer result in the development of interfacial dipole moments that in turn lead to an enhanced built-in potential across the devices, and consequently enhanced hole transport efficiency. Improved performance of the modified OSCs is evident from the ∼97% enhancement in efficiency from 0.88% to 1.73%, along with the open-circuit voltage improvement from 0.29 to 0.42 V. As well as improving the photovoltaic performance, the PFAS treatment also enhances the stability of the device under high temperature annealing, which is essential in the fabrication process.

Enhancement of exchange coupling between GaMnAs and IrMn with self-organized Mn(Ga)As at the interface

An atomically flat and uniform reaction layer of Mn(Ga)As was found to self-organize at the (Ga,Mn)As∕IrMn interface by postannealing. The Mn(Ga)As layer exhibits strong ferromagnetic characteristics up to the measured 300K. In particular, the manifested horizontal shift of field-cooled hysteresis loops shows a clear signature of exchange bias attributable to the exchange coupling between IrMn and Mn(Ga)As. Implication from composition analyses, exchange-bias effect, and thickness dependence of the Mn(Ga)As layer versus annealing conditions is also discussed.

High K Nanophase Zinc Oxide on Biomimetic Silicon Nanotip Array as Supercapacitors

A 3D trenched-structure metal-insulator-metal (MIM) nanocapacitor array with an ultrahigh equivalent planar capacitance (EPC) of ~300 μF cm(-2) is demonstrated. Zinc oxide (ZnO) and aluminum oxide (Al2O3) bilayer dielectric is deposited on 1 μm high biomimetic silicon nanotip (SiNT) substrate using the atomic layer deposition method. The large EPC is achieved by utilizing the large surface area of the densely packed SiNT (!5 × 10(10) cm(-2)) coated conformally with an ultrahigh dielectric constant of ZnO. The EPC value is 30 times higher than those previously reported in metal-insulator-metal or metal-insulator-semiconductor nanocapacitors using similar porosity dimensions of the support materials.

Gold nanoparticle-modulated conductivity in gold peapodded silica nanowires.

We report the enhanced electrical conductivity properties of single gold-peapodded amorphous silica nanowires synthesized using microwave plasma enhanced chemical vapor deposition. Dark conductivity of the gold-peapodded silica nanowires can be adjusted by controlling the number of incorporated metal nanoparticles. The temperature-dependent conductivity measurement reveals that the band tail hopping mechanism dominates the electron transport in the gold-peapodded silica nanowires. The high conductivity in the nano-peapodded nanowires with more embedded gold-nanoparticles can be explained by the higher density of hopping states and shorter hopping distance. These Au-embedded amorphous silica nanowires have provided a new approach to enhance not only the electron conduction, but also the chemical-sensor response/sensitivity.

Anisotropic surface plasmon excitation in Au/silica nanowire

The surface plasmon (SP) excitations of gold/silica nanowire, investigated by electron energy-loss spectroscopy in conjunction with scanning transmission electron microscopy, are found to be anisotropic with stronger SP intensities observed along the transverse direction of the nanowire. This indicates that the charge carriers generated near the surface of the nanowires by the decay of SP resonance play a significant role to the enhanced photoconductivity. This conclusion is reaffirmed by the polarization dependent photoconductivity measurement.