Xiangheng Xiao

Interface Engineering for High‐Performance Top‐Gated MoS2 Field‐Effect Transistors

In recent years, due to the intriguing electrical and optical characteristics, two dimensional layered transition metal dichalcogenides such as MoS2 have attracted tremendous research attention. In a distinct contrast to the bandgap issue of graphene, MoS2 is semiconducting with a satisfied thickness-dependent bandgap of 1.2 to 1.8 eV, which can enable lots of fascinating device applications. However, until now, majority of the efforts have been focused on the integration of MoS2 devices in the back- or dual-gated geometry due to the difficulty of compact and conformal top-gated dielectric deposition directly onto the 2-D channel for the realization of high-performance top-gated FETs. In this regard, interface or dielectric engineering is an important step towards the practical implementation of MoS2 devices with the optimized performance.

Controllable Synthesis, Magnetic Properties, and Enhanced Photocatalytic Activity of Spindlelike Mesoporous α-Fe2O3/ZnO Core–Shell Heterostructures

Mesoporous spindlelike iron oxide/ZnO core-shell heterostructures are successfully fabricated by a low-cost, surfactant-free, and environmentally friendly seed-mediate strategy with the help of postannealing treatment. The material composition and stoichiometry, as well as these magnetic and optical properties, have been examined and verified by means of high-resolution transmission electron microscopy and X-ray diffraction, the thickness of ZnO layer can be simply tailored by the concentration of zinc precursor. Considering that both α-Fe2O3 and ZnO are good photocatalytic materials, we have investigated the photodegradation performances of the core-shell heterostructures using organic dyes Rhodamin B (RhB). It is interesting to find that the as-obtained iron oxides/ZnO core-shell heterostructures exhibited enhanced visible light or UV photocatalytic abilities, remarkably superior to the as-used α-Fe2O3 seeds and commercial TiO2 products (P25), mainly owing to the synergistic effect between the narrow and wide bandgap semiconductors and effective electron-hole separation at the interfaces of iron oxides/ZnO.

Significantly Enhanced Visible Light Photoelectrochemical Activity in TiO2 Nanowire Arrays by Nitrogen Implantation

Titanium oxide (TiO2) represents one of most widely studied materials for photoelectrochemical (PEC) water splitting but is severely limited by its poor efficiency in the visible light range. Here, we report a significant enhancement of visible light photoactivity in nitrogen-implanted TiO2 (N-TiO2) nanowire arrays. Our systematic studies show that a post-implantation thermal annealing treatment can selectively enrich the substitutional nitrogen dopants, which is essential for activating the nitrogen implanted TiO2 to achieve greatly enhanced visible light photoactivity. An incident photon to electron conversion efficiency (IPCE) of ∼10% is achieved at 450 nm in N-TiO2 without any other cocatalyst, far exceeding that in pristine TiO2 nanowires (∼0.2%). The integration of oxygen evolution reaction (OER) cocatalyst with N-TiO2 can further increase the IPCE at 450 nm to ∼17% and deliver an unprecedented overall photocurrent density of 1.9 mA/cm(2), by integrating the IPCE spectrum with standard AM 1.5G solar spectrum. Systematic photoelectrochemical and electrochemical studies demonstrated that the enhanced PEC performance can be attributed to the significantly improved visible light absorption and more efficient charge separation. Our studies demonstrate the implantation approach can be used to reliably dope TiO2 to achieve the best performed N-TiO2 photoelectrodes to date and may be extended to fundamentally modify other semiconductor materials for PEC water splitting.

Synthesis and Magnetic Properties of Maghemite (γ-Fe2O3) Short-Nanotubes

We report a rational synthesis of maghemite (γ-Fe2O3) short-nanotubes (SNTs) by a convenient hydrothermal method and subsequent annealing process. The structure, shape, and magnetic properties of the SNTs were investigated. Room-temperature and low-temperature magnetic measurements show that the as-fabricated γ-Fe2O3 SNTs are ferromagnetic, and its coercivity is nonzero when the temperature above blocking temperature (TB). The hysteresis loop was operated to show that the magnetic properties of γ-Fe2O3 SNTs are strongly influenced by the morphology of the crystal. The unique magnetic behaviors were interpreted by the competition of the demagnetization energy of quasi-one-dimensional nanostructures and the magnetocrystalline anisotropy energy of particles in SNTs.

Rational design of amorphous indium zinc oxide/carbon nanotube hybrid film for unique performance transistors.

Here we report unique performance transistors based on sol-gel processed indium zinc oxide/single-walled carbon nanotube (SWNT) composite thin films. In the composite, SWNTs provide fast tracks for carrier transport to significantly improve the apparent field effect mobility. Specifically, the composite thin film transistors with SWNT weight concentrations in the range of 0-2 wt % have been investigated with the field effect mobility reaching as high as 140 cm(2)/V·s at 1 wt % SWNTs while maintaining a high on/off ratio ∼10(7). Furthermore, the introduction SWNTs into the composite thin film render excellent mechanical flexibility for flexible electronics. The dynamic loading test presents evidently superior mechanical stability with only 17% variation at a bending radius as small as 700 μm, and the repeated bending test shows only 8% normalized resistance variation after 300 cycles of folding and unfolding, demonstrating enormous improvement over the basic amorphous indium zinc oxide thin film. The results provide an important advance toward high-performance flexible electronics applications.

Mechanism of the enhancement and quenching of ZnO photoluminescence by ZnO-Ag coupling

New nanostructural composites consisting of Ag nanoparticles (NPs)-SiO2-ZnO films were fabricated by depositing ZnO films on silica substrates which had already been implanted by Ag ions at different energies and fluences. The photoluminescence (PL) emission of ZnO films from these nanostructural composites can be enhanced or quenched comparing to that of a ZnO film directly deposited on bare silica substrate. The enhancement of the band gap emission is ascribed to the local field enhancement induced by the resonant coupling between the excitons of ZnO and the surface plasmons (SPs) of Ag NPs, while the quenching is due to the electron transfer from ZnO to Ag NPs. Our results can be used to clarify the ambiguity in controlling the light emission enhancement and quenching of a semiconductor coupled with the SPs of metal NPs, which is very important for the design and applications of semiconductor and metal coupling to highly efficient optoelectronic devices, biosensor, etc.

Surface plasmon-enhanced light emission using silver nanoparticles embedded in ZnO

The Ag nanoparticles-SiO2–ZnO film sandwiched structure was fabricated by depositing ZnO films on silica substrates which had been implanted by Ag ions. Enhancement of emission of the sandwiched structure was observed. The enhancement emission is caused by the resonant coupling between the surface plasmons of Ag and the spontaneous emission of ZnO. The enhancement mechanism is confirmed by optical absorption spectra, transmission electron microscopy, and time-resolved photoluminescence. The key is to deposit ZnO on Ag nanoparticles covered with silica to prevent oxidation of Ag by direct contact with ZnO. This structure will be very useful for highly efficient optoelectronic devices.

A one-pot route to the synthesis of alloyed Cu/Ag bimetallic nanoparticles with different mass ratios for catalytic reduction of 4-nitrophenol

Copper-based alloy nanoparticles (NPs) have recently triggered much research interest for the development of low-cost and high-performance bimetallic catalysts that have industrial applications. Here, we present alloyed Cu/Ag bimetallic NPs synthesized by a facial one-pot reduction method. The catalytic properties of these prepared Cu/Ag bimetallic NPs with different ratios have been investigated by reducing 4-nitrophenol (Nip) into 4-aminophenol (Amp) in the presence of NaBH4. In comparison with pristine Ag monometallic NPs, alloyed Cu/Ag bimetallic NPs exhibit high catalytic performance on the reduction of Nip. The current synthesis method of Cu/Ag bimetallic NPs does not require rigorous conditions or toxic agents, and thus it is a rapid, efficient, and green approach for the fabrication of active bimetallic catalysts.

High Mobility MoS2 Transistor with Low Schottky Barrier Contact by Using Atomic Thick h-BN as a Tunneling Layer.

High-performance MoS2 transistors are developed using atomic hexagonal boron nitride as a tunneling layer to reduce the Schottky barrier and achieve low contact resistance between metal and MoS2 . Benefiting from the ultrathin tunneling layer within 0.6 nm, the Schottky barrier is significantly reduced from 158 to 31 meV with small tunneling resistance.

Tube-Like Ternary α-Fe2O3@SnO2@Cu2O Sandwich Heterostructures: Synthesis and Enhanced Photocatalytic Properties

Heterogeneous photocatalysis is of great interest for environmental remediation applications. However, fast recombination of photogenerated electron-hole pair and a low utilization rate of sunlight hinder the commercialization of currently available semiconductor photocatalysts. In this regard, we developed a unique ternary single core-double shell heterostructure that consists of α-Fe2O3@SnO2@Cu2O. This heterostructure exhibits a tube-like morphology possessing broad spectral response for the sunlight due to the combination of narrow bandgap and wide bandgap semiconductors forming a p-n heterojunction. To fabricate such a short nanotube (SNT), we used an anion-assisted hydrothermal route for deposition of α-Fe2O3, a seed-mediated deposition strategy for SnO2, and finally an aging process to deposit a Cu2O layer to complete the tube-like ternary α-Fe2O3@SnO2@Cu2O single core-double shell heterostructures. The morphology, composition, and photocatalytic properties of those ternary core-shell-shell heterostructures were characterized by various analytical techniques. These ternary heterostructures exhibited enhanced photocatalytic properties on the photodegradation of the organic dye of Rhodamine B (RhB) under simulated sunlight irradiation. The origin of enhanced photocatalytic activity is due to the synergistic effect of broad spectral response by combining narrow bandgap and wide bandgap semiconductors and, hence, an efficient charge separation of photogenerated electron-hole pairs facilitated through the p-n heterojunction. Furthermore, our unique structure provides an insight on the fabrication and controlled preparation of multilayer heterostructural photocatalysts that have intriguing properties.