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.

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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 ...

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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

Ultrasonic Properties of Quasi One-Dimensional (TaSe4)2I at the Peierls Transition

Physical vapor deposition and chemical vapor deposition include many methods of material transport and film growth. They are applied to functional (protective, decorative, lubricating, electronic) coatings. Sputtering and thermal spraying are versatile techniques. Pyrolysis is efficient in particular for silicon. Sophisticated developments allow epitaxial growth of perfectly controlled thin films for electrical and optical devices. Molecular beam epitaxy and metal organic chemical vapor deposition are essential to semiconductor technology. Film growth from condensed phase concerns specific applications.

Film Growth and Epitaxy: Methods

Reflection spectra have been measured on the as-grown and fresh surfaces of strain-free single crystals of ${\mathrm{ZrS}}_{3}$ and ${\mathrm{ZrSe}}_{3}$ at room temperature and at 2 K. Absorption spectra are obtained by a Kramers-Kronig transformation. Sharp exciton absorption bands due to direct allowed transition are observed at 2.573 eV for ${\mathrm{ZrS}}_{3}$ and at 1.818 eV for ${\mathrm{ZrSe}}_{3}$ at 2 K only for the E\ensuremath{\parallel}b (chain) polarization. For the E\ensuremath{\parallel}b polarization, a small absorption band is observed at 2.441 eV for ${\mathrm{ZrS}}_{3}$ but a band edge is not clearly seen for ${\mathrm{ZrSe}}_{3}$. Some spectra are interpreted in terms of paired anion models and in comparison with calculated band structures.

Optical spectra near the band edge of ZrS3 and ZrSe3.

A cathodic sputtering operation using a target of a material containing titanium, aluminum, oxygen and nitrogen in an argon atmosphere is disclosed. This sputtering operation gives rise to a layer containing the same elements as the target. The resulting layer is opaque, has an attractive, deep and brilliant black color, adheres well to the substrate and is resistant to wear and corrosion.

Process for depositing a black-colored coating on a substrate and a black-colored coating obtained using this process

Spontaneous excitonic luminescence in GaSe is investigated from 80 to 300 K and at weak laser excitation intensity. Radiative recombinations of direct and indirect free excitons are clearly observed. The analysis of the emission lines as a function of the temperature, of the excitation intensity, and of laser energy permits to attribute two lines of the intrinsic luminecsence to the recombination of direct and indirect excitons bound to defect centres localized in the energy gap. On the basis of recent theories on exciton-impurity complexes, it is found that in the crystals the excitonic binding centres are deep neutral acceptors. From the dissociation energy of the bound excitons the ionization energy of the acceptor levels is estimated. Moreover, the measurements seem to indicate that thermal equilibrium between direct and indirect excitonic states is lacking below about 100 K.



Die Spontane Exzitonenlumineszenz in GaSe wird zwischen 80 und 300 K und mit schwacher Laseranregungsintensitat untersucht. Strahlende Rekombination von direkten und indirekten freien Exzitonen wird deutlich beobachtet. Die Analyse der Emissionslinien in Abhangigkeit von der Temperatur, der Anregungsintensitat und der Laserenergie erlaubt, zwei Linien der intrinsischen Lumineszenz der Rekombination von direkten und indirekten Exzitonen zuzuordnen, die an in der Energielucke lokalisierten Defektzentren gebunden sind. Auf der Grundlage neuerer Theorien zu Exziton-Storstellenkomplexen wird gefunden, das in den untersuchten Kristallen die Exzitonenbindungszentren tiefe neutrale Akzeptoren sind. Aus der Dissoziationsenergie gebundener Exzitonen last sich die Ionisierungsenergie der Akzeptorniveaus berechnen. Daruberhinaus scheinen die Mesergebnisse zu zeigen, das unterhalb etwa 100 K kein thermisches Gleichgewicht mehr besteht zwischen direkten und indirekten Exzitonenzustatnden.

Francis Lévy

Papers

Ultrasonic Properties of Quasi One-Dimensional (TaSe4)2I at the Peierls Transition

Film Growth and Epitaxy: Methods

Optical spectra near the band edge of ZrS3 and ZrSe3.

Process for depositing a black-colored coating on a substrate and a black-colored coating obtained using this process

Radiative Decay from Free and Bound Excitons in GaSe