Fangyan Xie

Facile and environmentally friendly solution-processed aluminum oxide dielectric for low-temperature, high-performance oxide thin-film transistors.

We developed a facile and environmentally friendly solution-processed method for aluminum oxide (AlOx) dielectrics. The formation and properties of AlOx thin films under various annealing temperatures were intensively investigated by thermogravimetric analysis-differential scanning calorimetry (TGA-DSC), X-ray diffraction (XRD), spectroscopic ellipsometry, atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), impedance spectroscopy, and leakage current measurements. The sol-gel-derived AlOx thin film undergoes the decomposition of organic residuals and nitrate groups, as well as conversion of aluminum hydroxides to form aluminum oxide, as the annealing temperature increases. Finally, the AlOx film is used as gate dielectric for a variety of low-temperature solution-processed oxide TFTs. Above all, the In2O3 and InZnO TFTs exhibited high average mobilities of 57.2 cm(2) V(-1) s(-1) and 10.1 cm(2) V(-1) s(-1), as well as an on/off current ratio of ∼10(5) and low operating voltages of 4 V at a maximum processing temperature of 300 °C. Therefore, the solution-processable AlOx could be a promising candidate dielectric for low-cost, low-temperature, and high-performance oxide electronics.

Low-Voltage Organic Field-Effect Transistors (OFETs) with Solution-Processed Metal-Oxide as Gate Dielectric

In this study, low-voltage copper phthalocyanine (CuPc)-based organic field-effect transistors (OFETs) are demonstrated utilizing solution-processed bilayer high-k metal-oxide (Al(2)O(y)/TiO(x)) as gate dielectric. The high-k metal-oxide bilayer is fabricated at low temperatures (< 200 °C) by a simple spin-coating technology and can be controlled as thin as 45 nm. The bilayer system exhibits a low leakage current density of less than 10(-5) A/cm(2) under bias voltage of 2 V, a very smooth surface with RMS of about 0.22 nm and an equivalent k value of 13.3. The obtained low-voltage CuPc based OFETs show high electric performance with high hole mobility of 0.06 cm(2)/(V s), threshold voltage of -0.5 V, on/off ration of 2 × 10(3) and a very small subthreshold slope of 160 mV/dec when operated at -1.5 V. Our study demonstrates a simple and robust approach that could be used to achieve low-voltage operation with solution-processed technique.

Electrodeposited palladium nanostructure as novel anode for direct formic acid fuel cell

A novel membrane electrode assembly (MEA) structure for direct formic acid fuel cell (DFAFC) has been designed. The novelty is that Pd nanothorns are directly electrodeposited onto the carbon paper to form the anode catalyst layer. The Pd nanothorns are formed by a two step square wave electrodeposition. The dispersion and morphology of the Pd nanothorns on the carbon paper are observed by scanning electron microscopy (SEM). The crystal characteristics of the Pd nanothorns are studied by high resolution transmission electron microscopy (HRTEM) and the metallic property of the deposited Pd nanothorns is investigated by X-ray photoelectron spectroscopy (XPS). MEA is prepared with the Pd nanothorn covered carbon paper. The novel MEA provides 2.4 times higher peak power density than the conventional MEA. This increase in the performance is due to the improved mass transport of formic acid in the catalyst and diffusion layers, better Pd utilization and higher electroactivity of the Pd single crystal nanothorns.

Device lifetime improvement of polymer-based bulk heterojunction solar cells by incorporating copper oxide layer at Al cathode

Organic solar cells are commonly susceptible to degradation in air. We present that insertion of a thin layer of thermally evaporated copper oxide (CuOx) between the organic active layer and the Al cathode can greatly extend the lifetime of P3HT:PCBM based bulk heterojunction solar cells. The performance can be further improved by applying an interfacial bilayer of CuOx/LiF. Our results suggest that the CuOx functions not only as a charge transport layer but also as a protection layer, which prevents formation of thick organic-Al interdiffusion area. This leads to a more air-resistive cathode/organic interface.

Morphology controllable growth of Pt nanoparticles/nanowires on carbon powders and its application as novel electro-catalyst for methanol oxidation

Pt nanowires (PtNWs) have been controllably synthesized on carbon powders by the reduction of H(2)PtCl(6) with HCOOH. By adjusting the pH value of the solution, PtCl(6)(2-) can be controllable reduced into particles or nanowires. The Pt nanowires are single crystals growing along the <111> direction with a diameter of 3 nm and a length of 10 nm. The dispersion of Pt nanowires on the surface of carbon powders can be controlled by changing the loading of Pt. The PtNWs/C is evaluated as the catalyst for methanol oxidation. The PtNWs/C with 20 wt% Pt has a larger electrochemical active surface area and much higher mass activity for methanol oxidation than that of commercial Pt/C catalyst. The PtNWs/C catalyst shows significant improvement in the kinetics for methanol oxidation and mass transfer property due to the single crystal structure of the Pt nanowires. The PtNWs/C catalyst holds promising potential applications in energy converting devices and environmental protection.

Growth of vertically aligned MoS2 nanosheets on a Ti substrate through a self-supported bonding interface for high-performance lithium-ion batteries: a general approach

A promising new concept is to apply additive/binder-free electrodes for lithium-ion batteries by directly growing active materials on current collectors, however, they still suffer from the low quality electronic contact and poor mechanical stability due to the lattice mismatch between active materials and substrates. Here we present the direct growth of vertically aligned MoS2 nanosheets on a Ti substrate through a self-supported TiO2 bonding interface route, thereby preferentially exposing the edges on the film surface. In this unique structure, MoS2 nanosheets are interconnected with each other and create a strong adhesion to the Ti substrate through the TiO2 bonding interface, which would allow electrons to easily transport throughout the whole electrode. Furthermore, their edge-terminated structure can provide more sites for Li-ion intercalation. By using this additive/binder-free electrode, it can deliver a discharge capacity as high as 1189 mA h g−1 at a current density of 1000 mA g−1 after 600 cycles. Importantly, our synthetic approach, based on this metal surface dissolution–crystallization produced bonding interface, can provide a general strategy for the direct growth of other metal sulphides such as NiS and CoS nanosheets on the Ti substrate with great promise for various applications.

Nanoscale Insights into the Hydrogenation Process of Layered α-MoO3

The hydrogenation process of the layered α-MoO3 crystal was investigated on a nanoscale. At low hydrogen concentration, the hydrogenation can lead to formation of HxMoO3 without breaking the MoO3 atomic flat surface. For hydrogenation with high hydrogen concentration, hydrogen atoms accumulated along the <101> direction on the MoO3, which induced the formation of oxygen vacancy line defects. The injected hydrogen atoms acted as electron donors to increase electrical conductivity of the MoO3. Near-field optical measurements indicated that both of the HxMoO3 and oxygen vacancies were responsible for the coloration of the hydrogenated MoO3, with the latter contributing dominantly. On the other hand, diffusion of hydrogen atoms from the surface into the body of the MoO3 will encounter a surface diffusion energy barrier, which was for the first time measured to be around 80 meV. The energy barrier also sets an upper limit for the amount of hydrogen atoms that can be bound locally inside the MoO3 via hydrogenation. We believe that our findings has provided a clear picture of the hydrogenation mechanisms in layered transition-metal oxides, which will be helpful for control of their optoelectronic properties via hydrogenation.

Characteristics of a silicon nanowires/PEDOT:PSS heterojunction and its effect on the solar cell performance.

The interfacial energy-level alignment of a silicon nanowires (SiNWs)/PEDOT:PSS heterojunction is investigated using Kelvin probe force microscopy. The potential difference and electrical distribution in the junction are systematically revealed. When the PEDOT:PSS layer is covered at the bottom of the SiNW array, an abrupt junction is formed at the interface whose characteristics are mainly determined by the uniformly doped Si bulk. When the PEDOT:PSS layer is covered on the top, a hyperabrupt junction localized at the top of the SiNWs forms, and this characteristic depends on the surface properties of the SiNWs. Because the calculation shows that the absorption of light from the SiNWs and the Si bulk are equally important, the bottom-coverage structure leads to better position matching between the depletion and absorption area and therefore shows better photovoltaic performance. The dependence of JSC and VOC on the junction characteristic is discussed.

Low-temperature facile solution-processed gate dielectric for combustion derived oxide thin film transistors

In this work, acetylacetone assisted solution-processed In–Ga–Zn–O (IGZO) thin film transistors (TFTs) using Al2O3 as gate dielectrics were investigated. Normally, fully covered Al2O3 thin films are difficult to achieve by spin coating with conventional solvent, such as 2-methoxyethanol, due to the poor wettability of highly doped silicon. Here, a conventional aluminum nitrate solution with an additive was designed to spin coat robust continuous Al2O3 thin films, resulting from improved solution hydrophilic with a contact angle of 17°. For active layer fabrication, we utilized the previous reported combustion process to lower treatment temperature, which could be confined in the range from 220 °C to 300 °C, without losing the device performance. Results show that all the devices performed well. Especially, after 240 °C annealing of both Al2O3 (in thickness of around 45 nm) and IGZO thin films (in thickness of around 30 nm), we have obtained the following device parameters, namely a Al2O3 dielectric breakdown electric field at 7.8 MV cm−1, a current density of around 1 × 10−6 A cm−2 in the voltage range of −3 V to 3 V, a areal capacitance of 291 nF cm−2 at 100 Hz, a carrier mobility of 0.74 cm2 V−1 s−1, a threshold voltage of −0.4 V, a current on–off ratio of 6 × 103, a subthreshold swing of 375 mV per decade. Fabrication of combustion-processed active layers and our facile solution processed high-k dielectrics provides a feasible approach for low cost oxide flexible TFTs applications.

Synthesis and Raman spectroscopic study of W20O58 nanowires

Crystalline W20O58 nanowires were grown on silicon substrates by thermal evaporation of tungsten powders in a flow of argon gas without using any catalyst. Scanning electron microscopy shows that typical nanowires vary from 30 to 100?nm in diameter and around 8??m in length. X-ray diffraction and high-resolution transmission electron microscopy with electron diffraction study reveal that the nanostructures are crystalline. The laser power effect on the structure change of W20O58 nanowires has also been studied, which shows that W20O58 can be directly oxidized to WO3 very easily only by laser heating.