Hong-En Wang

Tunable Band Gaps and p-Type Transport Properties of Boron-Doped Graphenes by Controllable Ion Doping Using Reactive Microwave Plasma

We report tunable band gaps and transport properties of B-doped graphenes that were achieved via controllable doping through reaction with the ion atmosphere of trimethylboron decomposed by microwave plasma. Both electron energy loss spectroscopy and X-ray photoemission spectroscopy analyses of the graphene reacted with ion atmosphere showed that B atoms are substitutionally incorporated into graphenes without segregation of B domains. The B content was adjusted over a range of 0-13.85 atom % by controlling the ion reaction time, from which the doping effects on transport properties were quantitatively evaluated. Electrical measurements from graphene field-effect transistors show that the B-doped graphenes have a distinct p-type conductivity with a current on/off ratio higher than 10(2). Especially, the band gap of graphenes is tuned from 0 to ~0.54 eV with increasing B content, leading to a series of modulated transport properties. We believe the controllable doping for graphenes with predictable transport properties may pave a way for the development of graphene-based devices.

Hydrothermal synthesis of hierarchical SnO2 microspheres for gas sensing and lithium-ion batteries applications: Fluoride-mediated formation of solid and hollow structures

Hierarchical solid and hollow microspheres composed of oriented aligned cone-like SnO2 nanoparticles are prepared by a hydrothermal route using either NH4F or NaF, as morphology controlling agents. Their structures and morphology evolution are comprehensively characterized by TEM, SEM, XRD, XPS and the Brunauer–Emmett–Teller (BET) method, and a formation mechanism is proposed. Both solid and hollow SnO2 microspheres are formed via an Ostwald ripening process undergoing different reorganization paths in the presence of either NH4F or NaF. The solid spheres preferentially recrystallize starting from the cores and grow by consuming adjacent smaller particles, while the hollow spheres preferentially recrystallize starting from outer shells and grow by consuming the entrapped core materials via the mechanism of solid evacuation. As gas sensing materials, both solid and hollow SnO2 microspheres demonstrate sensitive and selective response to several hazardous gases, such as formaldehyde, ammonia, benzene, acetone, and methanol. As lithium storage materials, the hierarchical SnO2 hollow spheres show a higher charge/discharge capacity and better cyclic performance than the hierarchical SnO2 solid spheres. The discharge capacity of the hierarchical SnO2 hollow spheres is 187 mAh g−1 higher than the solid spheres for up to 50 discharge/charge cycles.

Hydrothermal synthesis of ordered single-crystalline rutile TiO2 nanorod arrays on different substrates

We report the mild hydrothermal synthesis of single-crystalline rutile TiO2 nanorod arrays (NRAs). The method reported here shows great versatility and can be used to grow TiO2 NRAs on a large diversity of substrates including Si, Si/SiO2, sapphire, Si pillars, and fluorine doped tin oxide (FTO)-covered glass. The average diameter and length of the nanorods prepared at typical conditions are ∼60u2002nm and 400 nm, respectively. Dye-sensitized solar cells assembled with the TiO2 NRAs grown on the FTO-covered glass as photoanode were prepared with a photoconversion efficiency of ∼1.10%.

Applications of silicon nanowires functionalized with palladium nanoparticles in hydrogen sensors

Silicon nanowires (SiNWs) modified by palladium (Pd) nanoparticles were investigated for hydrogen detection. SiNWs were fabricated via a thermal evaporation method using tin (Sn) as the catalyst. The as-grown SiNWs were chemically treated to remove surface oxide and then coated with a thin layer of Pd nanoparticles. A gas sensor device was fabricated with the Pd-functionalized SiNWs. The sensor showed better sensitivity to hydrogen and faster responding time than the macroscopic Pd wire hydrogen sensor.

Facile solution growth of vertically aligned ZnO nanorods sensitized with aqueous CdS and CdSe quantum dots for photovoltaic applications

Vertically aligned single crystalline ZnO nanorod arrays, approximately 3 μm in length and 50-450 nm in diameter are grown by a simple solution approach on a Zn foil substrate. CdS and CdSe colloidal quantum dots are assembled onto ZnO nanorods array using water-soluble nanocrystals capped as-synthesized with a short-chain bifuncional linker thioglycolic acid. The solar cells co-sensitized with both CdS and CdSe quantum dots demonstrate superior efficiency compared with the cells using only one type of quantum dots. A thin Al2O3 layer deposited prior to quantum dot anchoring successfully acts as a barrier inhibiting electron recombination at the Zn/ZnO/electrolyte interface, resulting in power conversion efficiency of approximately 1% with an improved fill factor of 0.55. The in situ growth of ZnO nanorod arrays in a solution containing CdSe quantum dots provides better contact between two materials resulting in enhanced open circuit voltage.

Influence of the donor/acceptor interface on the open-circuit voltage in organic solar cells

The donor/acceptor interface in a standard CuPc/C60 organic solar cell was modified by insertion of a thin layer of molybdenum trioxide (MoO3). An ultrathin layer of MoO3 between the donor and acceptor increased the open-circuit voltage (VOC) from 0.45 to 0.85 V. The enhancement in VOC is explained by the increase in the energy level offset between the lowest unoccupied molecular orbital of the acceptor and the highest occupied molecular orbital of the donor (EDHOMO-EALUMO). The explanation is supported by the energy level analysis of the donor/acceptor interface by ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy.

Heteroepitaxial growth and optical properties of ZnS nanowire arrays on CdS nanoribbons

The authors present the results of heteroepitaxial growth of single-crystalline ZnS nanowire arrays on CdS nanoribbon substrates by the metal-catalyzed vapor-liquid-solid growth method. ZnS nanowire arrays were vertically or crosswise aligned to the surface of CdS nanoribbon substrates. Room-temperature lasing from ZnS nanowire arrays was demonstrated. The present synthesis provides a new approach to the rational design of building blocks for nanodevices.

p-type conductivity in silicon nanowires induced by heterojunction interface charge transfer

p-type conductivity in intrinsic silicon nanowires (SiNWs) synthesized by an etching method was achieved via surface coating of MoO3 and tetrafluoro-tetracyanoquinodimethane thin layers. Characterization of field-effect transistors fabricated from single SiNW revealed a decrease in resistivity by six orders of magnitude and an increase in hole concentration by eight orders of magnitude with respect to the original silicon wafers. The enhancement of p-type conduction was demonstrated to originate from the interface charge transfer between inorganic/inorganic and organic/inorganic heterojunctions and the enrichment of hole concentration in SiNW surfaces based on band energy alignment and x-ray photoelectron spectroscopic analysis.

Crossbar heterojunction field effect transistors of CdSe:In nanowires and Si nanoribbons

Crossbar heterojunction arrays of n-CdSe:In nanowires (NWs) and p-Si nanoribbons (NRs) were built by ac electrical field-directed assembly. The junctions showed significant rectifying characteristics. With the p-Si NRs as gates, the junctions function as junction field effect transistors (JFETs) with high stability and reproducibility in performance. A small variation in gate voltage from −2 to −1 V was demonstrated to manipulate the current in n-CdSe:In NW channels by a factor near 103. The JFETs also showed a subthreshold swing value (67 mV/dec) approaching the theoretical limit (60 mV/dec), suggesting the great application potentials of the junctions in integrated nanoelectronics.