Bibo Yan

Multifunctional CuO nanowire devices: p-type field effect transistors and CO gas sensors

We report the properties of a field effect transistor (FET) and a gas sensor based on CuO nanowires. CuO nanowire FETs exhibit p-type behavior. Large-scale p-type CuO nanowire thin-film transistors (10(4) devices in a 25 mm(2) area) are fabricated and we effectively demonstrate their enhanced performance. Furthermore, CuO nanowire exhibits high and fast response to CO gas at 200 degrees C, which makes it a promising candidate for a poisonous gas sensing nanodevice.

P-type electrical, photoconductive, and anomalous ferromagnetic properties of Cu2O nanowires

Cu2O nanowires are synthesized by reduction of CuO nanowires with hydrogen gas. Strong green photoluminescence dominated by band-edge emission is observed. Field effect transistors fabricated from individual Cu2O nanowires present high on-off ratio (>106) and high mobility (>95 cm2/V s). Furthermore, the device demonstrates a fast photoelectric response to blue illumination in air at room temperature. In addition, anomalous ferromagnetism appears in Cu2O nanowires, which may originate from the defects in Cu2O nanowires. This work shows the application potentials of the Cu2O nanowires, especially in an electrical and photonic device.

Effects of pre-treatments and interlayers on the nucleation and growth of diamond coatings on titanium substrates

During diamond deposition on titanium substrates, two processes exist: (1) diffusion of hydrogen into a titanium substrate and the formation of hydride thereby degrading the mechanical properties of the substrate; and (2) competition among the rapid diffusion of carbon atoms into substrates, the formation of carbide and the nucleation of diamond crystals (thereby affecting the nucleation and growth rate of the diamond coating). To increase the diamond nucleation rate and prevent the rapid diffusion of hydrogen and carbon into the substrate, different surface treatments and interlayers were studied in this paper. Results showed that polishing with diamond pastes and ultrasonic pre-treatment in diamond suspensions will significantly increase the nuclei density of diamond crystals. However, the diffusion of hydrogen into the substrate could not be prevented. Pre-etching of the titanium substrate using hydrogen plasma for a short time significantly increased the nuclei density of diamond crystals. Results showed that on a TiN interlayer, there was no significant improvement in diamond nucleation and growth, and the deposited diamond coatings showed poor adhesion. New diamond crystals were formed on the DLC interlayer in which DLC acted as the precursor for diamond nucleation. However, the so-formed diamond coating showed spallation. The plasma nitrided layer could prevent the rapid diffusion of hydrogen and carbon into the titanium substrate, but results showed a relatively low nucleation density of diamond crystals and poor adhesion. A graded interlayer combining plasma nitriding followed by plasma carbonitriding was effective in preventing the rapid diffusion of hydrogen and carbon into the substrate and improving the nucleation rate and adhesion of diamond coating.

Deposition of diamond coating on pure titanium using micro-wave plasma assisted chemical vapor deposition

The nucleation and growth of diamond coatings on pure Ti substrate were investigated using microwave plasma assisted chemical vapor deposition (MW-PACVD) method. The effects of hydrogen plasma, plasma power, gas pressure and gas ratio of CH4 and H2 on the microstructure and mechanical properties of the deposited diamond coatings were evaluated. Results indicated that the nucleation and growth of diamond crystals on Ti substrate could be separated into different stages: (1) surface etching by hydrogen plasma and the formation of hydride; (2) competition between the formation of carbide, diffusion of carbon atoms and diamond nucleation; (3) growth of diamond crystals and coatings on TiC layer. During the deposition of diamond coatings, hydrogen diffused into Ti substrate forming titanium hydride and led to a profound microstructure change and a severe loss in impact strength. Results also showed that pre-etching of titanium substrate with hydrogen plasma for a short time significantly increased the nuclei density of diamond crystals. Plasma power had a significant effect on the surface morphology and the mechanical properties of the deposited diamond coatings. The effects of gas pressure and gas ratio of CH4 and H2 on the nucleation, growth and properties of diamond coatings were also studied. A higher ratio of CH4 during deposition increased the nuclei density of diamond crystals but resulted in a poor and cauliflower coating morphology. A lower ratio of CH4 in the gas mixture produced a high quality diamond crystals, however, the nuclei density and the growth rate decreased dramatically.

Friction and wear behaviour of carbon nitride films deposited on plasma nitrided Ti–6Al–4V

Amorphous carbon nitride (CNx) films were deposited on plasma nitrided Ti–6Al–4V substrate in order to improve the adhesion strength and tribological behaviour. Scratch and ball-on-disk wear tests were performed to evaluate the load bearing capacity, wear and friction characteristics of the duplex-treated coatings. Compared with a CNx film deposited on Ti–6Al–4V substrate, the load bearing capacity of a CNx film deposited on plasma nitrided layer was improved dramatically. Results showed that under dry sliding condition, the duplex-treated system was more effective in maintaining a favourable low and stable coefficient of friction and improving the wear resistance than both individual plasma nitriding and CNx film on Ti–6Al–4V substrate. The reasons for this significant improvement in tribological behaviour with the application of duplex treatment can be attributed to the combined benefits from both plasma nitriding and CNx films. (1) Plasma nitriding of Ti–6Al–4V produces a graded hardened case which serves as an excellent supporting and load bearing layer for hard CNx films. (2) The CNx film deposited at low temperature can produce a wear resistant and low-friction surface without impairing the beneficial effects from plasma nitriding treatment, and the smooth CNx films could effectively reduce both the interfacial stresses and the stresses near the surface thus providing a good tribological behaviour. (3) The graphitisation of the wear debris during dry sliding condition can help to decrease the coefficient of friction and improve the wear resistance.

Characterization and tribological evaluation of MW-PACVD diamond coatings deposited on pure titanium

Titanium alloys are widely used in aerospace and biomedical conditions, however, they are notorious for the poor tribological properties. The deposition of a well adherent diamond coating is a promising way to solve this problem. In this study, diamond coatings were deposited on pure titanium using microwave plasma assisted chemical vapour deposition (MW-PACVD). Characterisation of diamond coatings was performed using scanning electron microscopy (SEM), laser profilometry, Raman spectroscopy, grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM). Tribological properties of diamond coatings were evaluated using a ball-on-disk wear tester (sliding with Al2O3 balls) and a scratch tester (sliding with diamond pin). Results showed that the friction and wear properties of polycrystalline diamond coatings as well as the wear of the counterface were dependent significantly on the surface roughness, the morphology and crystalline structure of diamond coatings as well as the counterface materials. For (111)-textured diamond coatings with rough surface and sharp asperities sliding with Al2O3 balls, the coefficient of friction was much higher than that of (100)-textured coatings, and the wear of the counterface material was quite high. After polishing the diamond coating, the surface roughness, coefficient of friction and wear of counterface decreased significantly. If sliding with diamond pins, the coefficient of friction of diamond coating shows a quite low and stable value. To improve the tribological properties, a three-step deposition method was proposed to obtain a smooth and nano-crystalline diamond layer on bulk diamond coatings. The so-formed diamond coating showed the highest load bearing capacity, the lowest coefficient of friction and the lowest wear of the counterface.

Improving diamond–metal adhesion with graded TiCN interlayers

An approach improving diamond–metal adhesion has been developed based on modeling predictions and experimental verifications on the interfacial stresses modified by catalytic reaction. It is found that N-plasma irradiating onto Ti and tungsten-carbide substrates generates tensile surface stresses while C-plasma irradiation creates strongly compressive stress at the surfaces, both of which deteriorate the diamond–metal adhesion. It is also found that surface oxidation prevents diamondnucleation. Therefore, we applied a graded TiCN interlayer with carefully adjusting the ratio of C and N in the gas mixture to neutralize the interfacial stress and, hence, we have improved the diamond–metal adhesion substantially.

From diamond to crystalline silicon carbonitride: effect of introduction of nitrogen in CH4/H2 gas mixture using MW-PECVD

Microwave plasma enhanced chemical vapor deposition (MW-PECVD) is considered as one of the most successful growth techniques in recent diamond and crystalline carbon nitride investigations. In this study, we tried to synthesize crystalline carbon nitride film using MW-PECVD by gradually increasing the content of nitrogen into H2/CH4 gas mixture. Well-faceted crystalline diamond films could be synthesized with a H2/CH4 gas ratio of 198:2. With the gradual increase of nitrogen content up to 3% in the gas mixture diamond film quality deteriorates seriously, and the morphological crystal size and growth rate of diamond coatings decreased significantly. With the nitrogen gas content increased to approximately 6–22%, a lot of separated round diamond or diamond-like carbon particles formed on the surface rather than a continuous film. Only with the nitrogen content increased above 72%, could some tiny crystals with a type of hexagonal facet form on the silicon surface, together with many large, round diamond particles. With the further increase of nitrogen gas content above 90%, many large, well-faceted hexagonal crystals formed on Si surface. However, XRD, energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy and nano-indentation analysis indicated that these crystals were actually silicon carbonitride (Si–C–N) with a crystalline structure of Si3N4 modified with the introduction of carbon atoms, rather than carbonitride as expected and regarded.

Tunable transport properties of n-type ZnO nanowires by Ti plasma immersion ion implantation

Single-crystalline, transparent conducting ZnO nanowires were obtained simply by Ti plasma immersion ion implantation (PIII). Electrical transport characterizations demonstrate that the n-type conduction of ZnO nanowire could be tuned by appropriate Ti-PIII. When the energy of PIII is increased, the resistivity of ZnO decreases from 4×102 to 3.3×10−3 Ω cm, indicating a semiconductor-metal transition. The failure-current densities of the metallic ZnO could be up to 2.75×107 A/cm2. Therefore, this facile method may provide an inexpensive alternative to tin doped indium oxide as transparent conducting oxide materials.

Crystalline carbonitride forms harder than the hexagonal Si-carbonitride crystallite

It is shown that large and well faceted hexagonal crystallites can grow on Si and Ti substrates under higher nitrogen gas flow in the gaseous mixture of CH4 and H2 in the normal process of diamond deposition using microwave plasma chemical vapour deposition (MP-CVD). Grazing incidence x-ray diffraction (GIXRD), energy dispersive x-ray spectrometry (EDX), wavelength dispersive x-ray spectroscopy (WDX) and Raman analysis revealed that these crystals are silicon carbonitride rather than carbonitride as we expected and usually regarded. This indicates that the hypothetical covalent carbonitride is not as easily formed as the crystallite with Si substitution. Comparing films deposited on Ti alloy substrates, with and without Si chips present during the deposition, we found that the Si chips activated by the plasma sputtering provides Si sources for crystallite SiCN formation on Ti substrate. The hexagonal SiCN structure and its lower hardness are attributed to the C3v-symmetric quasi-tetrahedral nitride with a nonbonding lone pair. The involvement of the less electronegative Si specimen may make the SiCN form easier than the covalent carbonitride.