Xiaomeng Ma

Comprehensive study of the resistance switching in SrTiO3 and Nb-doped SrTiO3

We have demonstrated that the resistance switching (RS) effect can be controlled by the modification of the electrode configurations and the carrier densities in the Ag/SrTiO3 and Ag/Nb-doped SrTiO3(Nb:STO) structures. The elimination of the Schottky junction in the metal/Nb:STO completely destroys the RS effect, which suggests that the RS effect originates from the modification of Schottky-like barrier formed at the interface of metal/Nb:STO. The rectifying I-V curves revealed that the change in resistance was attributed to the trapping or detrapping carriers at the interface. The carrier density plays an important role in the determination of RS effect. The presence of the RS in SrTiO3 requires an appropriate doping level to provide conditions for trapping carriers at the interface.

Anisotropic nanocrystalline MnBi with high coercivity at high temperature

Magnetic hard nanocrystalline MnBi has been prepared by melt spinning and subsequent low temperature annealing. A coercivity of 2.5 T can be achieved at 540 K for MnBi with an average grain size of about 20-30 nm. The coercivity iHc, mainly controlled by the coherent magnetization rotation, shows a strong dependence on the time of grinding and exhibits a positive temperature coefficient from 100 up to 540 K. The unique temperature dependent behavior of the coercivity (magnetocrystalline anisotropy) has a relationship with the variations in the crystal lattice ratio of c/a with temperatures. In addition, discontinuity can not be found in the lattice parameters of a, c, and c/a ratio at the magnetostructural transition temperature. The nanocrystalline MnBi powder fixed in an epoxy resin and under an applied magnetic field of 24 kOe shows a maximum energy product of 7.1 MGOe at room temperature and shows anisotropic characteristics with high Mr/Ms ratio up to 560 K.

Probing the anisotropic behaviors of black phosphorus by transmission electron microscopy, angular-dependent Raman spectra, and electronic transport measurements

In this study, we correlated the angular dependence of the Raman response of black phosphorus to its crystallographic orientation by using transmission electron microscopy and Raman spectroscopy. It was found that the intensity of the Ag2 mode reached a maximum when the polarization direction of the incident light was parallel to the zigzag crystallographic orientation. Notably, it was further confirmed that the zigzag crystallographic direction exhibited superior conductance and carrier mobility. Because of the lattice extension along the armchair direction, an intensification of the anisotropic Raman response was observed. This work provides direct evidence of the correlation between anisotropic properties and crystallographic direction and represents a turning point in the discussion of the angular-dependent electronic properties of black phosphorus.

Graphene/Si CMOS Hybrid Hall Integrated Circuits

Graphene/silicon CMOS hybrid integrated circuits (ICs) should provide powerful functions which combines the ultra-high carrier mobility of graphene and the sophisticated functions of silicon CMOS ICs. But it is difficult to integrate these two kinds of heterogeneous devices on a single chip. In this work a low temperature process is developed for integrating graphene devices onto silicon CMOS ICs for the first time, and a high performance graphene/CMOS hybrid Hall IC is demonstrated. Signal amplifying/process ICs are manufactured via commercial 0.18 um silicon CMOS technology, and graphene Hall elements (GHEs) are fabricated on top of the passivation layer of the CMOS chip via a low-temperature micro-fabrication process. The sensitivity of the GHE on CMOS chip is further improved by integrating the GHE with the CMOS amplifier on the Si chip. This work not only paves the way to fabricate graphene/Si CMOS Hall ICs with much higher performance than that of conventional Hall ICs, but also provides a general method for scalable integration of graphene devices with silicon CMOS ICs via a low-temperature process.

Ultra-sensitive graphene Hall elements

Hall elements were fabricated based on high quality chemical vapor deposition grown graphene, and their performance limit was explored. The as-fabricated graphene Hall element exhibits current-related sensitivity of up to 2093 V/AT under 200 μA, and magnetic resolution of around 1 mG/Hz0.5 at 3 kHz. This ultrahigh sensitivity and resolution stem from high carrier mobility, small Dirac point voltage of 3 V, and low carrier density of about 3 × 1011 cm−2 in graphene device. The current sensitivity is found to decrease with increasing current bias at large bias, and this phenomenon is attributed to the drain induced Dirac point shift effect in graphene channel.

Multifunctional graphene sensors for magnetic and hydrogen detection.

Multifunctional graphene magnetic/hydrogen sensors are constructed for the first time through a simple microfabrication process. The as-fabricated graphene sensor may act as excellent Hall magnetic detector, demonstrating small linearity error within 2% and high magnetic resolution up to 7 mG/Hz(0.5). Meanwhile the same graphene sensor is also demonstrated as high-performance hydrogen sensor with high gas response, excellent linearity, and great repeatability and selectivity. In particular, the graphene sensor exhibits high hydrogen response up to 32.5% when exposed to 1000 ppm hydrogen, outperforming most graphene-based hydrogen sensors. In addition the hydrogen-sensing mechanism of Pd-decorated graphene is systematically explored through investigating its transfer characteristics during gas detection. Our work demonstrates that graphene is a terrific material for multifunctional sensing, which may in principle reduce the complexity of manufacturing process, lower the number of sensors required in the sensing systems, and potentially derive new and more powerful functions.

Performance change of few layer black phosphorus transistors in ambient

Transistors were fabricated based on mechanical exfoliated few layer black phosphorus (BP) flakes, and performance change of these devices exposed to air was explored systematically. BP devices were found to suffer severe performance degradation in ambient conditions, and the field effect mobility drops to less than 1/10 of the original in no more than 120 hours after fabrication. However the current on/off ratio shows completely different time dependent behavior to the published result, i.e. increases with exposure time in air, since the minimum current decreases with exposure time to air, which is probably originated from the decrease of layer number in BP. A model is developed to estimate the bandgap change of BP according to the time dependent minimum current of the BP device.