Tianye Yang

Control of conductivity type in undoped ZnO thin films grown by metalorganic vapor phase epitaxy

The properties of the ZnO thin films prepared by metalorganic vapor phase epitaxy under various oxygen partial pressures were thoroughly studied. It was found that the conduction type in undoped ZnO epilayers could be controlled by adjusting the family VI precursor, oxygen partial pressure during growth. The films were characteristic of n-type conductivity under oxygen partial pressure lower than 45 Pa. With the increase of oxygen content, the crystallinity of the ZnO thin films was degraded to polycrystalline with additional (10–12) orientation and the intrinsic p-type ZnO was produced as the oxygen partial pressure was larger than 55 Pa. The hole concentration and mobility could reach to 1.59×1016 cm−3 and 9.23 cm2 V−1 s−1, and the resistivity was 42.7 Ω cm. The near-band-edge emission and the deep level emission in photoluminescence (PL) spectra at room temperature were influenced strongly by the oxygen partial pressure. Temperature-dependent PL spectra in n-type ZnO films showed a dominant neutral-don...

Enhanced HCHO gas sensing properties by Ag-loaded sunflower-like In2O3 hierarchical nanostructures

Nanoscale Ag-loaded sunflower-like In2O3 hierarchical nanostructures are developed for HCHO detection. Such unique architectures are synthesized by an ambient temperature and pressure hydrolysis reaction combined with a subsequent chemical reduction process. Morphology characterizations confirm that homodisperse nanochains assembled by nanoparticles along the same direction are radially linked to a center to construct sunflower-like hierarchical nanostructures. Novel highly porous and branched structure of the 3D hierarchical architectures and the chemical and electronic sensitization effect of Ag nanoparticles endow Ag-loaded In2O3 nanostructures-based sensors with enhanced gas sensing performances in terms of fast response time (0.9 s), recovery time (14 s), high sensitivity and good sensing selectivity for 20 ppm HCHO. A multistage reaction formation mechanism of the sunflower-like hierarchical nanostructures, and a morphology-dependent sensing mechanism are proposed.

Electrical properties and stability of p-type ZnO film enhanced by alloying with S and heavy doping of Cu

Single wurtzite p-type Zn1−yCuyO1−xSx alloy films with 0.081≤x≤0.186 and 0.09≤y≤0.159 were grown on quartz reproducibly by magnetron sputtering. The alloys show very stable p-type conductivity with a hole concentration of 4.31–5.78×1019 cm−3, a resistivity of 0.29–0.34 Ω cm and a mobility of 0.32–0.49 cm2 V−1 s−1. The p-type conductivity is attributed to substitution of Cu+1 for the Zn site, and the ionization energy of the Cu+1 acceptor is measured to be 53 meV, much less than that of Cu-doped ZnO reported previously. The small ionization energy is due to Cu heavy doping and increase in valence band maximum of ZnO induced by alloying with S.

Mechanism of p-type conductivity for phosphorus-doped ZnO thin film

A p-type phosphorus-doped ZnO film (ZnO : P) was grown on a quartz substrate by sputtering a ZnO target mixed with 2 wt% P2O5 using a mixture of Ar and O2 and then annealed rapidly at 750 °C for 5 min in air ambient. The lattice constant of the c-axis was 0.5176 nm, smaller than the value of 0.5211 nm of pure ZnO, implying substitutional P at a Zn antisite (PZn). The binding energy of P2p1/3 is 133.5 eV, which is different from that of the P–O bond in P2O5 and of the P–Zn bond in Zn3P2, but close to that of P–O–P and P–O–Zn bonds in zinc phosphate glass mainly composed of ZnO and P2O5. The 80 K photoluminescence spectrum shows neutral acceptor bound exciton emission at 3.34 eV. Based on the above experimental results, it is suggested that P substitutes for a Zn antisite in the ZnO : P and forms an acceptor complex with two Zn vacancies, and the acceptor complex is responsible for p-type conductivity of ZnO : P.

Structural and electrical characteristics of high quality (100) orientated-Zn3N2 thin films grown by radio-frequency magnetron sputtering

We report on highly crystalline zinc nitride (Zn3N2) thin films which were grown by rf magnetron sputtering on quartz substrates. The substrate temperature during growth is found to strongly affect the crystal quality of the thin films. The chemical bonding states were determined by x-ray photoelectron spectroscopy. Large chemical shifts in core-level N 1s peaks with binding energy of 396.4 eV were observed as compared to N 1s of free amine (398.8 eV), indicating Zn–N bond formation. Two N 1s states were found: one is N1 formed by Zn–N bonds and another is (N2) produced by substitution of N molecules at N ion sites, which leads to larger lattice constants, consistent with x-ray diffraction results. Temperature-dependent Hall effect measurements of our Zn3N2 films exhibited distinct conduction mechanisms at specific different temperature ranges.

Low-temperature solvothermal synthesis of hierarchical flower-like WO3 nanostructures and their sensing properties for H2S

In this work, hierarchical flower-like tungsten trioxide (WO3) nanostructures assembled by needle-like single-crystalline nanosheets were fabricated. These were synthesized via a facile and simple solvothermal method at a rather low temperature (100 °C) without any surfactants or templates. Time-dependent experiments were carried out to understand the formation process, which undergoes four stages: polymerizing, nucleating, assembling and growing from WO42− to the flower-like WO3. The as-prepared WO3 microflowers exhibit a good reversibility, fast response time (0.9 s) and recovery time (19 s) and good sensing selectivity at a relatively low working temperature (160 °C) after exposing to hydrogen sulfide (H2S). Such excellent performance can be attributed to the highly exposed surface area and the assembling of single-crystalline nanosheets. The sensing process is tentatively explained in terms of the adsorption-desorption mechanism and chemical kinetics theories are discussed in detail.

Facile fabrication and enhanced gas sensing properties of In2O3 nanoparticles

Nanoscale single crystalline In2O3 nanoparticles with sizes of 10–40 nm are prepared by annealing gas-liquid phase chemical deposition-synthesized In2S3 nanoparticles and are developed for the detection of acetone gas. The In2O3 nanoparticles are characterized by TEM, HRTEM, SAED, EDX and XRD. Moreover, the products are further studied by room temperature UV-absorption and photoluminescence (PL) spectroscopy. To demonstrate the usage of such nanoparticles, gas sensors based on the as-synthesized In2O3 nanoparticles are fabricated and exhibit good selectivity, high sensitivity, rapid response, a low concentration detection limit and better repeatability towards acetone gas at a relatively low operating temperature. Such excellent gas sensing performances are attributed to small crystal sizes and the existence of abundant oxygen vacancies. As demonstrated, the single crystalline In2O3 nanoparticles are highly promising for real-time monitoring gas sensor applications.

Fabrication and properties of B–N codoped p-type ZnO thin films

A p-type B–N codoped ZnO film was grown on quartz by magnetron sputtering and post-annealing techniques. It has room-temperature resistivity of 2.3 Ω cm, Hall mobility of 11 cm2 V−1 s−1 and carrier concentration of 1.2 × 1017 cm−3, better than the electrical properties of the N-doped p-type ZnO. The ZnO homojunction fabricated by deposition of an undoped n-type ZnO layer on the B–N codoped p-type ZnO layer showed clear p–n diode characteristics. Differing from the N-doped ZnO, the low-temperature photoluminescence spectrum of the codoped ZnO film consists of two dominant peaks located at 3.096 eV and 3.251 eV, respectively. The former is due to radiative electron transition from the conduction band to the Zn vacancy acceptor level, and the latter due to recombination of the donor–acceptor pair. The mechanism of p-type conductivity was discussed in this work.

Synthesis of ZnO nanosheet arrays with exposed (100) facets for gas sensing applications

ZnO nanosheet (NS) arrays have been synthesized by a facile ultrathin liquid layer electrodeposition method. The ion concentration and electrode potential play important roles in the formation of ZnO NS arrays. Studies on the structural information indicate that the NSs are exposed with (100) facets. The results of Raman and PL spectra indicate that there existed a large amount of oxygen vacancies in the NSs. The gas sensing performances of the ZnO NS arrays are investigated: the ZnO NS arrays exhibited high gas selectivity and quick response/recovery for detecting NO2 at a low working temperature. High binding energies between NO2 molecules and exposed ZnO(100) facets lead to large surface reconstructions, which is responsible for the intrinsic NO2 sensing properties. In addition, the highly exposed surface and a large amount of oxygen vacancies existing in the NSs also make a great contribution to the gas sensing performance.

Facile synthesis and enhanced gas sensing properties of In2O3 nanoparticle-decorated ZnO hierarchical architectures

In2O3 nanoparticle-decorated flowerlike ZnO hierarchical architectures are fabricated by a simple and facile two-step approach, including the room temperature synthesis of flowerlike ZnO and the subsequent decoration with In2O3 nanoparticles, and developed for HCHO gas detection. The SEM, TEM and HRTEM images indicate that the In2O3 nanoparticles have successfully grown and are closely attached to the surface of the ZnO. The special 3D hierarchical architectures, the appropriate decoration amount of In2O3 nanoparticles and the incorporation of interfaces between ZnO nanosheets and the In2O3 nanoparticles endow In2O3-decorated ZnO nanostructured sensors with enhanced HCHO gas sensing performances. A multistage reaction formation mechanism of flowerlike hierarchical nanostructures and a morphology-dependent sensing mechanism are proposed.