Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films or requires high voltages. Although single layers of MoS(2) have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5-3 cm(2) V(-1) s(-1) range are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS(2) mobility of at least 200 cm(2) V(-1) s(-1), similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 10(8) and ultralow standby power dissipation. Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS(2) could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.

/pdf/single-layer-mos2-transistors-6go64w4t3p.pdf

Single-layer MoS2 transistors

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

/pdf/dye-sensitized-solar-cells-50b0w2bhsr.pdf

Dye-Sensitized Solar Cells

Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.

Emerging Photoluminescence in Monolayer MoS2

TiO2 photocatalysis and related surface phenomena

WS2 nanoropes merged from nanotube coalescence present a new case of inorganic fiber composites with several nanotubes grown side by side up to a few millimeters in length. The extremities of external and inner spirally rolled-up molecular layer building the tubes are at the origin of the nanotube nucleation leading to the syntactic growth of nanotubes. The model of undulation of thin crystal flakes based on stacking mismatch is discussed. The nanoropes grow also by self-assembly or by coalescence of single tubes at low angles of incidence. An attractive long-range force leading to the nanotube approach is proposed.

Syntactic coalescence of WS2 nanotubes

Reference CIME-ARTICLE-1986-001View record in Web of Science Record created on 2007-02-15, modified on 2017-05-12

Correlation between process conditions, chemical composition and morphology of MoS2 films prepared by RF planar magnetron sputtering

The electrical properties of thin-film metal-oxide-metal Nb 2 O 5 oxygen sensors have been investigated in the temperature range 400–600°C. They are shown to be strongly dependent on the electrode material. With Nb electrodes, the cathode is a good electron injector. At low bias, the oxygen pressure dependence of the conductivity follows P (O 2 ) −1/ n with n equal 1.0 ± 0.2 over a pressure range extending at least from 10 5 to 10 Pa. A model is proposed explaining this behaviour by the presence of surface electron traps associated with chemisorbed oxygen. The films present a very large concentration of chemisorption sites as atmospheric oxygen is able to diffuse into the bulk through channels running between the Nb 2 O 5 chains of the crystal structure. Above a certain threshold voltage, a non-linear regime where the current density is proportional to the square of the applied voltage is observed. This regime is explained by the anodic oxidation of the Nb anode. Cr and Pt electrodes behave like valve contacts: they are blocking at low voltage and injecting at high applied voltage. Structures shorter than 50 μm are shown to be depleted of their oxygen vacancies for electric fields in the range 10 3 –10 4 V cm −1 . The electrical admittance presents a steep non-linear double-injection regime above a certain threshol voltage. In this bias range the current density varies like V n with n ≥ 7. The threshold voltage is sensitive to the ambient and decreases when the sensor is exposed to an atmosphere with low oxygen concentration. It is shown that the migration of ionized oxygen vacancies induced by the electric field actually controls the electronic transport through the metal-oxide-metal structure. These results provide new insights into the capabilities and limitations of Nb 2 O 5 sensors.

Electrical transport properties of thin-film metal-oxide-metal Nb2O5 oxygen sensors

Amorphous Nb2O5 thin films of three different thicknesses (10, 100, 400 nm) were deposited onto SiO2/Si substrates by reactive sputtering in an Ar–O2 plasma. Thermal treatments were performed at different temperatures between 500 and 1100 °C. The structural and morphological evolution with temperature is shown to be dependent on the film thickness. At 600 °C, the films essentially crystallize in the TT phase. On the thickest films, the T phase also appears. Annealing at higher temperature progressively increases the concentration of the T phase. The films show large flat grains extending over the whole film thickness. In addition, a large number of polyhedral bubbles is present in the 100 and 400 nm films due to Ar atoms trapped during sputtering. After annealing at 1100 °C the Ar bubbles are no longer present and partial diffusion of the films into the substrate is observed. The modification at high temperature, explained either by the M or the H phase, is favored on the thickest films and leads to plate...

Francis Lévy

Papers

Syntactic coalescence of WS2 nanotubes

Correlation between process conditions, chemical composition and morphology of MoS2 films prepared by RF planar magnetron sputtering

Electrical transport properties of thin-film metal-oxide-metal Nb2O5 oxygen sensors

Structural and morphological characterization of Nb2O5 thin films deposited by reactive sputtering

High rate and process control of reactive sputtering by gas pulsing: the Ti O system