C.Z. Gu

Optical properties of boron-doped diamond

We report optical reflectivity study on pure and boron-doped diamond films grown by a hot-filament chemical vapor deposition method. The study reveals the formation of an impurity band close to the top of the valence band upon boron doping. A schematic picture for the evolution of the electronic structure with boron doping was drawn based on the experimental observation. The study also reveals that the boron doping induces local lattice distortion, which brings an infrared-forbidden phonon mode at 1330 cm(-1) activated in the doped sample. The antiresonance characteristic of the mode in conductivity spectrum evidences the very strong coupling between electrons and this phonon mode.

Chemical gases sensing properties of diamond nanocone arrays formed by plasma etching

A uniform diamond nanocone array was formed by plasma etching of diamond film in a hot filament chemical vapor deposition (HFCVD) system. A surface amorphous carbon coating layer, which is formed during CH4/H2 plasma-etching process, was removed by Ar plasma in a reactive ion etching system. The hydrogenation of diamond nanocones was performed in H2 ambience by using the same HFCVD system. The air-diluted NH3 and NO2 gases sensing properties of the diamond cone arrays had been studied by using electric current versus measurement time characteristics at room temperature. The repeatable chemical sensing properties of the hydrogenated diamond cone array sensor are enhanced, in comparison with as-formed diamond film. Surface two-dimensional hole gas structure and greatly increased surface-to-volume ratio both play a key role for the excellent detection performance. As-formed diamond nanocone arrays show a promising prospect for applications as chemical sensor for both reducing (NH3) and oxidizing (NO2) gases.

Improving mechanical properties of amorphous carbon nitride films by titanium doping

Titanium doped amorphous carbon nitride (a-CNx) films with a nitrogen content of ∼24at.% were synthesized by radio frequency magnetron sputtering method. The effects of incorporating Ti on the mechanical properties of a-CNx films were investigated by nanoindentation, scanning electron microscope, x-ray diffraction spectra, and x-ray photoelectron spectroscopy, respectively. It was found that nanometer sized TiN crystallites were formed and embedded in the a-CNx matrix, causing an enhancement of hardness from ∼28to∼40GPa in the a-CNx films. The improved mechanical properties with addition of Ti are attributed to the densified microstructure due to the development of fine grain size of TiN and sp3 C–N bonds. These TiN nanocrystal grains are separated by an amorphous phase, preventing dislocation movement and hence enhancing the hardness of the film. The increased sp3 C–N bond fraction induced by incorporation of Ti also plays an important role in the enhancement of hardness.

Diamond cone arrays with controlled morphologies formed by self-organized selective ions sputtering

Controlled preparation of nanoscale materials and the underlying mechanisms are essential issues nowadays. Here, we report a significant subtractive formation process of large-area diamond conical nanostructure arrays using a hot filament chemical vapor deposition (HFCVD) system with negative biasing of the substrates, and the etching effect of energetic ions on the formation of diamond cone arrays with controlled morphology has been studied in detail. It shows that methylic ions dominantly contribute to diamond cone formation based on a neutral-ion charge exchange collision model. The self-organized selective sputtering process of as-formed hillock bottoms on a roughened surface by low energetic ions plays a key role for the formation and development of diamond cones. The cone morphologies under various experimental parameters are systematically studied, and they nicely confirm and supplement the as-established cone formation mechanism.

Nickel silicide nanowires formed in pre-patterned SiO2 trenches and their electrical transport properties

Nickel silicide (NiSi) nanowires with different linewidth (from 1000 to 32 nm) are formed in pre-patterned SiO2 trenches on a silicon substrate. SiO2 trenches are milled by focused ion beam (FIB) etching, and an electrical endpoint detection technique is used to control the FIB milling depth to just reveal the silicon surface. The formation is based on Ni thin film deposition and the subsequent annealing at 550 °C. Compared with the previously reported self-aligned formation process of NiSi nanowires, the present technique can form NiSi nanowires with controlled length and linewidth because of the pre-patterning of the SiO2 trenches. The as-formed NiSi nanowires show metallic transport characteristics with a rather low resistivity of about 15 µΩ cm, which exhibits very weak dependence on linewidth above 50 nm. The lowest resistivity of the nanowire is observed at a linewidth of about 100 nm. The low resistivity is thought to be due to the good crystalline structure of our as-formed NiSi nanowires. When the linewidth of NiSi nanowires is decreased to 32 nm, the resistivity increased abruptly to 22.7 µΩ cm, which can be explained by considering the short electron mean free path (about 5 nm), as well as the obvious effect of grain boundary on electron transport properties of NiSi.

Effect of grain boundary on local surface conductivity of diamond film

In this article, the direct experimental evidences to determine the effect of grain boundary on local surface conductivity (SC) of diamond films were provided by the measurement using double probe scanning electron microscopy (SEM) technology. Undoped diamond films with (001) orientation were first grown by microwave plasma enhanced chemical vapor deposition and were then hydrogenated at different conditions for SC measurement. In the SEM system, double probes with tiny tip radius severed as two leads were moved along and contacted with the diamond film surface to directly test the local SC of diamond film. The surface electrical property results indicate that for the same distance between the two probes, the local SC of the area across grain boundary is much higher than that of area without grain boundary for the same duration of hydrogenation degrees. In addition, local SC of the area between the two probes increases with the number of grain boundaries in this area, which demonstrates that the grain bou...

Pressure-induced iso-structural phase transition and metallization in WSe2

We present in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman spectroscopy study, and electrical transport measurement of single crystal WSe2 in diamond anvil cells with pressures up to 54.0–62.8 GPa. The XRD and Raman results show that the phase undergoes a pressure-induced iso-structural transition via layer sliding, beginning at 28.5 GPa and not being completed up to around 60 GPa. The Raman data also reveals a dominant role of the in-plane strain over the out-of plane compression in helping achieve the transition. Consistently, the electrical transport experiments down to 1.8 K reveals a pressure-induced metallization for WSe2 through a broad pressure range of 28.2–61.7 GPa, where a mixed semiconducting and metallic feature is observed due to the coexisting low- and high-pressure structures.

Superconductivity in HfTe 5 across weak to strong topological insulator transition induced via pressures

Recently, theoretical studies show that layered HfTe5 is at the boundary of weak & strong topological insulator (TI) and might crossover to a Dirac semimetal state by changing lattice parameters. The topological properties of 3D stacked HfTe5 are expected hence to be sensitive to pressures tuning. Here, we report pressure induced phase evolution in both electronic & crystal structures for HfTe5 with a culmination of pressure induced superconductivity. Our experiments indicated that the temperature for anomaly resistance peak (Tp) due to Lifshitz transition decreases first before climbs up to a maximum with pressure while the Tp minimum corresponds to the transition from a weak TI to strong TI. The HfTe5 crystal becomes superconductive above ~5.5 GPa where the Tp reaches maximum. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. Crystal structure studies indicate that HfTe5 transforms from a Cmcm phase across a monoclinic C2/m phase then to a P-1 phase with increasing pressure. Based on transport, structure studies a comprehensive phase diagram of HfTe5 is constructed as function of pressure. The work provides valuable experimental insights into the evolution on how to proceed from a weak TI precursor across a strong TI to superconductors.

Domain wall scattering in the nanocontacts of ferromagnetic metals with different coercive forces

The electrical transport properties of nanocontact nanostructures of ferromagnetic metals with different coercive forces have been studied by the I-V measurements at room temperature without applied magnetic field. The result indicates that the nanocontact structure within the critical contact width for NiFe, Fe and Ni can pin a single domain wall at the nanocontact position. A sharp resistance drop will happen when the domain wall is depinned from the contact position by injecting the spin polarized current. Furthermore, we have found that the critical current density for depinning is constant for each ferromagnetic metal. They are 1.8 x 10(7) A cm(-2) for NiFe, 3.2 x 10(7) A cm(-2) for Ni and 3.8 x 10(7) A cm(-2) for Fe, respectively, which increases with the increase of the coercive force of ferromagnetic metal. Micromagnetic simulation results provided a detailed understanding on the magnetic configuration for ferromagnetic metals with different coercive forces.

Large magnetoresistance in La2/3Ca1/3MnO3 thin films induced by metal masked ion damage technique

We have developed a simple process to obtain large magnetoresistance (MR) in perovskite manganite thin films by a combination of focused ion beam milling and 120keV H2+ ion implantation. Metal slits about 70nm in width were printed by 30kV focused Ga ion beam nanolithography on a 4μm track, and the materials in these slits are then irradiated by the accelerated H2+ ions. Using this method, in a magnetic field of 5T we can get a MR>60% over a 230K temperature scope, with a maximum value of 95% at around 70K. This technique is very promising in terms of its simplicity and flexibility of fabrication and has potential for high-density integration.