Dejun Fu

Magnetic Characteristic of Mn+ Ion Implanted GaN Epilayer

Nanoscale ferromagnets (GaMn) with the implantation of Mn+ ions have been incorporated into unintentionally doped (n-type) GaN epilayers grown on sapphire substrate by molecular beam epitaxy (MBE). Energy dispersive X-ray (EDX) spectrometry, atomic force and magnetic force microscopy (AFM and MFM) are used to characterize the GaMn precipitates which form within the GaN epilayer. MFM images reveal nanoscale ferromagnets (GaMn), and a small magnetic hysteresis loop indicates that there are ferromagnetic particles in our GaN:Mn layer involving the paramagnetic property and is measured by superconducting quantum interference device (SQUID) magnetometer.

Optical characteristics of Mn+-ion-implanted GaN epilayers

Abstract Low-temperature (13xa0K) photoluminescence (PL) measurements of Mn + -ion-implanted GaN epilayers show distinctive features. For the thermally annealed GaN:Mn epilayers subsequent to the implantation of Mn + ions, the typical (D 0 ,xa0X) transition at 3.47xa0eV disappeared and two new strong excitonic transitions emerge at 3.31 and 3.37xa0eV. This is due to the fact that the incorporation of Mn + forms cubic lattice structure and the interface between the cubic and wurzite phases is responsible for the new PL doublet structures. In addition, three more transitions believed to be associated with the Mn + ions are resolved. Such results demonstrate the change of lattice structures due to the transformation from binary compound of GaN into ternary compound of GaN:Mn.

Spin cycloid destruction in Pr doped BiFeO3 films studied by conversion-electron Mossbauer spectroscopy

Crystalline Bi1−xPrxFeO3 films were fabricated on Si (100) substrates by sol-gel technique. X-ray diffraction revealed a decrease of lattice constants of the doped samples evidencing substitution of Bi by Pr ions. Ferromagnetism with low saturation magnetization Msu2009=u20091–3u2009emu/cm3 of the films were observed by Vibrating sample magnetometry. Systematic change of Mossbauer parameters of the films with Pr doped concentration increase were obtained by Conversion-electron Mossbauer spectroscopy (CEMS). The results of both one- and two-sextet fittings of the CEMS spectra provide evidence for destruction of the spin cycloid in Pr-doped BiFeO3.

MoS2 memristor with photoresistive switching.

A MoS2 nanosphere memristor with lateral gold electrodes was found to show photoresistive switching. The new device can be controlled by the polarization of nanospheres, which causes resistance switching in an electric field in the dark or under white light illumination. The polarization charge allows to change the switching voltage of the photomemristor, providing its multi-level operation. The device, polarized at a voltage 6u2009V, switches abruptly from a high resistance state (HRSL6) to a low resistance state (LRSL6) with the On/Off resistance ratio of about 10 under white light and smooth in the dark. Analysis of device conductivity in different resistive states indicates that its resistive state could be changed by the modulation of the charge in an electric field in the dark or under light, resulting in the formation/disruption of filaments with high conductivity. A MoS2 photomemristor has great potential as a multifunctional device designed by using cost-effective fabrication techniques.

Ti-containing amorphous carbon nanocomposite coatings prepared by means of eight-target arc-assisted middle frequency magnetron sputtering

Ti-containing amorphous carbon nanocomposite coatings were deposited by using a modified closed field twin unbalanced magnetron sputtering system which is arc assisted and consists of two circles of targets, in an Ar ambient and at a substrate temperature of 100°C High-resolution transmission electron microscopy shows that the coatings contain Ti nanocrystals embedded in the amorphous carbon matrix. Mechanical and tribological properties of the coatings are influenced by the substrate bias voltage and Ti pair-target current. For coatings prepared at a bias of 100 V, the hardness increases from 12 GPa at a Ti-target current of 4 A to 27 GPa at 15 A. The coating at 4 A exhibits very low friction coefficient (~0.07) and excellent tribological performance with a wear rate of about 1.4×10-16 m3 N-1 m-1.

Formation of β-C3N4 Nanocrystals in Ti-Doped Carbon Nitride Films Prepared by Cathode Arc-Assisted Middle-Frequency Magnetron Sputtering

Ti-doped carbon nitride thin films have been deposited by cosputtering Ti and graphite targets in a mixed plasma of Ar and N. Transmission electron microscopy revealed that the nucleation of β-C3N4 could be promoted by Ti incorporation. The N content of the films was found to strongly depend on Ti concentration, even a small amount of Ti incorporation resulted in a drastic increase in N content. An evolution from amorphous layers with fine mixtures of C, N, and Ti to matrices embedded with β-C3N4 nanocrystals was observed with increasing bias voltage.

A Novel Way to Fabricate Superhydrophilic and Antibacterial TiO2 Nanofilms on Glass by Ion Implantation and Subsequent Annealing

We report a simple and novel method to fabricate high quality titanium dioxide (TiO2) nanofilms on soda lime glass by a solid phase growth process of Ti implantation and subsequent annealling at 500 °C in oxygen atmosphere. The formation of the TiO2 nanofilms is resulted from the slow out-diffusion of implanted Ti ions from the substrates and being oxidized at the surfaces. The formed TiO2 nanofilms show superhydrophilicity and good antibacterial property with good adhesion to substrate and stability, suggesting that the TiO2 nanofilms formed by this method have great potential applications such as antibacterial, anti-fog and self-cleaning transparent glass.

RBS Depth Profiling Analysis of (Ti, Al)N/MoN and CrN/MoN Multilayers

Abstract(Ti, Al)N/MoN and CrN/MoN multilayered films were synthesized on Si (100) surface by multi-cathodic arc ion plating system with various bilayer periods. The elemental composition and depth profiling of the films were investigated by Rutherford backscattering spectroscopy (RBS) using 2.42 and 1.52xa0MeV Li2+ ion beams and different incident angles (0°, 15°, 37°, and 53°). The microstructures of (Ti, Al)N/MoN multilayered films were evaluated by X-ray diffraction. The multilayer periods and thickness of the multilayered films were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) and then compared with RBS results.

Fabrication of nitrogen-containing coatings in reed switches by pulsed ion-plasma treatment

We focus on the detailed characterization of the coatings produced by pulsed ion-plasma treatment on contact surfaces of permalloy (iron-nickel) blades directly in nitrogen-filled reed switches. The measurements of electrical resistance, breakdown voltage and return factor of reed switches were carried out in situ prior to and after ion-plasma treatment, and the morphology and elemental (chemical) composition of the coatings were monitored ex situ by means of scanning electron microscopy and energy dispersive X-ray microanalysis. The main processes occurring on the contact surfaces in the course of ion-plasma treatment were discussed in the frame of anomalous glow and arc discharges theories by Mesyats. It was shown that ionplasma treatment allows fabrication of erosion- and corrosion-resistant nitrogenated iron-nickel coatings with the electrical resistance of 0.1 Ohm. In the optimal symmetrical mode with the processing time of ca. 7 min the nitrogen concentration in the coatings was estimated of 20 at %. Higher and lower nitrogen content and the presence of oxygen in the coatings showed negative influence on the electrical resistance, which determines the quality of reed switches. Prototype models of the reed switches with nitrogenated coatings successfully passed switchgear reliability tests and demonstrated the competitive capacity as regards the standard reed switches with electroplating coatings based on the noble metals.

Formation of Hexagonal GaN Pyramids by Photo Assisted Electroless Chemical Etching

Hexagonal GaN pyramids were formed by the photo enhanced chemical etching of GaN epilayers grown on sapphire by molecular-beam epitaxy. Defective areas of the epilayers were selectively etched in a mixed solution of KOH and K2S2O8 under ultraviolet illumination, producing submicron-size pyramids nonuniformly distributed on the substrate. The cathodoluminescence of the pyramids was redshifted compared with that of the sample before etching. Atomic force microscopy suggested the facets of the pyramids to be (1012) oriented. High-resolution X-ray diffraction showed that the width of the (105) plane of the etched sample is smaller than that of the unetched sample. A larger quantity of K2S2O8 added to the KOH solution led to a high etching rate and the etched sample exhibited a strong cathodoluminescence.