Chaoping Liu

Electrical properties of zinc oxide nanowires and intramolecular p–n junctions

Electrical properties of ZnO nanowires and intramolecular p–n junctions were characterized by I–V measurements. These nanowires were grown embedded in anodic aluminum oxide (AAO) templates by vapor-phase-transport growth method. The nanowires were dense, continuous, and uniform in diameter along the length of the wires. I–V measurements showed the average resistivity of the ZnO nanowires in AAO templates was about one order of magnitude higher than that of the naked single ZnO nanowire. The p–n junctions in ZnO nanowires were fabricated by a two-step growth of ZnO with and without dopant of boron (∼1 wt %) in the source. I–V results suggested that p–n junctions in ZnO nanowires were formed by the two-step method.

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 ∼60 nm 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%.

Tunable p-type conductivity and transport properties of AlN nanowires via Mg doping.

Arrays of well-aligned AlN nanowires (NWs) with tunable p-type conductivity were synthesized on Si(111) substrates using bis(cyclopentadienyl)magnesium (Cp(2)Mg) vapor as a doping source by chemical vapor deposition. The Mg-doped AlN NWs are single-crystalline and grow along the [001] direction. Gate-voltage-dependent transport measurements on field-effect transistors constructed from individual NWs revealed the transition from n-type conductivity in the undoped AlN NWs to p-type conductivity in the Mg-doped NWs. By adjusting the doping gas flow rate (0-10 sccm), the conductivity of AlN NWs can be tuned over 7 orders of magnitude from (3.8-8.5) × 10(-6) Ω(-1) cm(-1) for the undoped sample to 15.6-24.4 Ω(-1) cm(-1) for the Mg-doped AlN NWs. Hole concentration as high as 4.7 × 10(19) cm(-3) was achieved for the heaviest doping. In addition, the maximum hole mobility (∼6.4 cm(2)/V s) in p-type AlN NWs is much higher than that of Mg-doped AlN films (∼1.0 cm(2)/V s). (2) The realization of p-type AlN NWs with tunable electrical transport properties may open great potential in developing practical nanodevices such as deep-UV light-emitting diodes and photodetectors.

Coherent nanoscale cobalt/cobalt oxide heterostructures embedded in porous carbon for the oxygen reduction reaction

Cost-effective and efficient electrocatalysts for the oxygen reduction reaction (ORR) are crucial for fuel cells and metal–air batteries. Herein, we report the facile synthesis of a Co/CoO/Co3O4 heterostructure embedded in a porous carbon matrix by refluxing and annealing. This composite exhibits several structural merits for catalyzing the ORR: (1) the existence of metallic Co and graphitic carbon enhanced the electrical conduction; (2) the porous, loose carbon network facilitated the electrolyte permeation and mass transport; (3) more importantly, the nanosized coherent CoO/Co3O4 heterojunctions with structural defects and oxygen vacancies enhanced the charge transport/separation at the interface and adsorption affinity to O2, thus promoting the ORR kinetics and lowering the reaction barrier. Consequently, the composite electrode manifests high electrocatalytic activity, attaining a current density of 6.7 mA cm−2 at −0.8 V (vs. Ag/AgCl), which is superior to pure CoO nanoparticles (4.7 mA cm−2), and has good methanol tolerance. The present strategy based on heterostructure and vacancy engineering may pave the way for the exploration of more advanced, low-cost electrocatalysts for electrochemical reduction and evolution processes.

Transparent conducting amorphous CdO-Ga 2 O 3 films synthesized by room temperature sputtering

CdO-Ga₂O₃ alloy films (Cd_1-xGa_xO_1+δ) over the whole composition range (x=0 to 1) were synthesized by room temperature radio frequency magnetron sputtering. We found that the intrinsic band gap of these alloys can be tuned in a wide range from 2.2 to 4.8 eV. As the Ga content increases to x>0.3 the alloy becomes entirely amorphous and the resistivity increases from 10^-3 to 10^-1 ohm-cm while the mobility decreases gradually from 15 (x=0.34) to 10 cm²/Vs (x=0.6). The resistivity of amorphous Cd_1-xGa_xO_1+δ films can be further controlled from 10^-3 to 10² ohm-cm by oxygen doping. Moreover, alloy films grown on plastic (PEN) substrate exhibit similar electrical and optical properties. These results suggest that amorphous Cd_1-xGa_xO_1+δ alloy films can potentially be used as highly conducting transparent conductors on flexible solar cells as well as gate electrodes with a wide bandgap tunabilty for transparent/flexible devices.