Jingbiao Cui

Carbon nanotube memory devices of high charge storage stability

Molecular memory devices with semiconducting single-walled carbon nanotubes constituting a channel of 150 nm in length are described. Data storage is achieved by sweeping gate voltages in the range of 3 V, associated with a storage stability of more than 12 days at room temperature. By annealing in air or controlled oxygen plasma exposure, efficient switching devices could be obtained from thin nanotube bundles that originally showed only a small gate dependence of conductance.

Electrodeposition and room temperature ferromagnetic anisotropy of Co and Ni-doped ZnO nanowire arrays

Cobalt and nickel doped ZnO nanowire arrays were synthesized by an electrochemical process at a temperature of 90°C. Energy dispersive x-ray spectroscopy and x-ray diffraction show that the dopants are incorporated into the wurtzite-structure ZnO. Anisotropic ferromagnetism with an easy direction of magnetization either perpendicular or parallel to the wire axis, depending on the wire geometry and density, was observed in 1.7% Co and 2.2% Ni-doped ZnO nanowires at room temperature. The anisotropic magnetism was explained in terms of a competition between self-demagnetization and magnetostatic coupling among the nanowires.

Pure and stable metallic phase molybdenum disulfide nanosheets for hydrogen evolution reaction

Metallic-phase MoS2 (M-MoS2) is metastable and does not exist in nature. Pure and stable M-MoS2 has not been previously prepared by chemical synthesis, to the best of our knowledge. Here we report a hydrothermal process for synthesizing stable two-dimensional M-MoS2 nanosheets in water. The metal–metal Raman stretching mode at 146 cm−1 in the M-MoS2 structure, as predicted by theoretical calculations, is experimentally observed. The stability of the M-MoS2 is associated with the adsorption of a monolayer of water molecules on both sides of the nanosheets, which reduce restacking and prevent aggregation in water. The obtained M-MoS2 exhibits excellent stability in water and superior activity for the hydrogen evolution reaction, with a current density of 10 mA cm−2 at a low potential of −175 mV and a Tafel slope of 41 mV per decade.

Noncontact temperature measurements of diamond by Raman scattering spectroscopy

The possibility of determining the temperature of diamond by noncontact Raman spectroscopy is assessed critically. The intensity ratio of Stokes to anti-Stokes lines is shown to be ill suited for temperatures above ∼750 K. Employing the temperature coefficient of the Raman line position, on the other hand, turns out to be a straightforward and highly reliable means to measure diamond temperatures between 300 and 2000 K with an accuracy of ±10 K. A prerequisite for the application of this method is an empirically developed formula which describes the temperature coefficient of the Raman active phonon frequency with high accuracy. Examples of temperature measurements on single crystal diamond and diamond films grown by chemical vapor deposition are given. The application of this procedure to the temperature measurement of silicon and germanium is demonstrated.

Low-temperature growth and field emission of ZnO nanowire arrays

Structural, optical, and field-emission properties of ZnO nanowire arrays grown at 90°C are investigated. Single-crystalline ZnO nanowires with low level of oxygen vacancies are obtained at low temperatures. The nanowire growth is strongly dependent on the seeding method used but independent of the substrate materials, which enable large scale growth of ZnO arrays on all kinds of substrates including polymers. We have demonstrated stable electron emission at low-field strengths for nanowires grown on polystyrene and polyethylene foils, making them promising candidates for fabrication of flexible cold cathodes. Deposition of a few nanometers of gold on ZnO nanowires significantly lowers the field required for electron emission, which is explained in terms of additional field enhancement from Au islands on top of the ZnO nanowires.

Modulating the Interface Quality and Electrical Properties of HfTiO/InGaAs Gate Stack by Atomic-Layer-Deposition-Derived Al2O3 Passivation Layer

In current work, the effect of the growth cycles of atomic-layer-deposition (ALD) derived ultrathin Al2O3 interfacial passivation layer on the interface chemistry and electrical properties of MOS capacitors based on sputtering-derived HfTiO as gate dielectric on InGaAs substrate. Significant suppression of formation of Ga-O and As-O bond from InGaAs surface after deposition of ALD Al2O3 with growth cycles of 20 has been achieved. X-ray photoelectron spectroscopy (XPS) measurements have confirmed that suppressing the formation of interfacial layer at HfTiO/InGaAs interface can be achieved by introducing the Al2O3 interface passivation layer. Meanwhile, increased conduction band offset and reduced valence band offset have been observed for HfTiO/Al2O3/InGaAs gate stack. Electrical measurements of MOS capacitor with HfTiO/Al2O3/InGaAs gate stacks with dielectric thickness of ∼4 nm indicate improved electrical performance. A low interface-state density of (∼1.9) × 10(12) eV(-1) cm(-2) with low frequency dispersion ( ∼ 3.52%), small border trap density of 2.6 × 10(12) cm(-2), and low leakage current of 1.17 × 10(-5) A/cm(2) at applied gate voltage of 1 V have been obtained. The involved leakage current conduction mechanisms for metal-oxide-semiconductor (MOS) capacitor devices with and without Al2O3 interface control layer also have been discussed in detail.

Role of hydrogen on field emission from chemical vapor deposited diamond and nanocrystalline diamond powder

The field emission properties of diamond-graphite composites were investigated as a function of composition both for oxidized and hydrogen covered diamond. The composites consist of mixtures of nanocrystalline diamond and graphite particles. In this way their composition could be varied at will while the field enhancement factor of the individual crystallites remained unchanged. The measurements prove that graphite is the phase responsible for low threshold field emission. The apparent emission threshold is strongly influenced by the conductivity of the composites. Hydrogenation has two beneficial effects. It provides a conducting path to the emission sites via the hydrogen induced surface conductivity of diamond. It also lowers the effective emission threshold of graphite in contact with diamond that exhibits negative electron affinity after hydrogenation. The latter effect was experimentally verified by photoelectron yield spectroscopy.

Junction investigation of graphene/silicon Schottky diodes

Here we present a facile technique for the large-scale production of few-layer graphene flakes. The as-sonicated, supernatant, and sediment of the graphene product were respectively sprayed onto different types of silicon wafers. It was found that all devices exhibited current rectification properties, and the supernatant graphene devices have the best performance. Schottky junctions formed between graphene flakes and silicon n-type substrates exhibit good photovoltaic conversion efficiency while graphene/p-Si devices have poor light harvesting capability.

Synthesis and magnetic properties of Co-doped ZnO nanowires

An electrochemical approach to the synthesis of Co-doped ZnO nanowire arrays on conducting substrates at 90 °C is reported. The structure and properties of as-grown and annealed nanowires were studied by x-ray diffraction, scanning electron microscope, energy dispersive x-ray spectroscopy, photoluminescence, and vibrating-sample magnetometry. The Co-doped ZnO nanowires exhibit a broadened band-edge emission at 386nm with a strong emission band around 550nm. Room-temperature ferromagnetism was observed in the doped ZnO nanowires, which makes them potentially useful as building components for spintronics. Effects of Co concentration and annealing on the magnetic properties were investigated.

Surface Electronic Properties of Diamond

The influence of surface states, defects and adsorbates on the electronic properties of diamond surfaces are discussed. As far as surface states and reconstructions are concerned the principal crystallographic surfaces, (100), (111) and (110), are essentially understood in their adsorbate free form and also when terminated by hydrogen. The role of surface defects is addressed and the correlation between the position of the surface Fermi level and the concentration of surface defects is discussed quantitatively for p-type diamond. Hydrogen passivation leads to a negative electron affinity of diamond surfaces due to a dipole layer which is induced by the heteropolar carbon–hydrogen bonds of the surface atoms. This aspect is discussed quantitatively. Finally, an experiment in which photoelectron spectroscopy and in situ conductivity measurements were combined to elucidate the surface conductivity of diamond is described and analyzed.