Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

Black phosphorus field-effect transistors

We introduce the 2D counterpart of layered black phosphorus, which we call phosphorene, as an unexplored p-type semiconducting material. Same as graphene and MoS2, single-layer phosphorene is flexible and can be mechanically exfoliated. We find phosphorene to be stable and, unlike graphene, to have an inherent, direct, and appreciable band gap. Our ab initio calculations indicate that the band gap is direct, depends on the number of layers and the in-layer strain, and is significantly larger than the bulk value of 0.31–0.36 eV. The observed photoluminescence peak of single-layer phosphorene in the visible optical range confirms that the band gap is larger than that of the bulk system. Our transport studies indicate a hole mobility that reflects the structural anisotropy of phosphorene and complements n-type MoS2. At room temperature, our few-layer phosphorene field-effect transistors with 1.0 μm channel length display a high on-current of 194 mA/mm, a high hole field-effect mobility of 286 cm2/V·s, and an...

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Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility

Preceding the current interest in layered materials for electronic applications, research in the 1960's found that black phosphorus combines high carrier mobility with a fundamental band gap. We introduce its counterpart, dubbed few-layer phosphorene, as a new 2D p-type material. Same as graphene and MoS2, phosphorene is flexible and can be mechanically exfoliated. We find phosphorene to be stable and, unlike graphene, to have an inherent, direct and appreciable band-gap that depends on the number of layers. Our transport studies indicate a carrier mobility that reflects its structural anisotropy and is superior to MoS2. At room temperature, our phosphorene field-effect transistors with 1.0 um channel length display a high on-current of 194 mA/mm, a high hole field-effect mobility of 286 cm2/Vs, and an on/off ratio up to 1E4. We demonstrate the possibility of phosphorene integration by constructing the first 2D CMOS inverter of phosphorene PMOS and MoS2 NMOS transistors.

Phosphorene: A New 2D Material with High Carrier Mobility

We introduce the 2D counterpart of layered black phosphorus, which we call phosphorene, as an unexplored p-type semiconducting material. Same as graphene and MoS2, single-layer phosphorene is flexible and can be mechanically exfoliated. We find phosphorenetobestableand,unlikegraphene,tohaveaninherent, direct, and appreciable band gap. Our ab initio calculations indicate thatthebandgapisdirect,dependsonthenumberoflayersandthe in-layer strain, and is significantly larger than the bulk value of 0.31� 0.36 eV. The observed photoluminescence peak of single-layer phosphorene in the visible optical range confirms that the band gap is larger than that of the bulk system. Our transport studies indicate a hole mobility that reflects the structuralanisotropyofphosphoreneandcomplementsn-typeMoS2.Atroomtemperature,ourfew-layerphosphorene field-effecttransistorswith1.0 μm channellengthdisplayahighon-currentof194mA/mm,ahighhole field-effectmobilityof286cm 2 /V 3 s,andanon/offratioofupto10 4 .Wedemonstrate the possibility of phosphorene integration by constructing a 2D CMOS inverter consisting of phosphorene PMOS and MoS2 NMOS transistors.

Phosphorene: An Unexplored 2D Semiconductor with a High Hole

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Anomalous superconductivity in black phosphorus under high pressures

A low‐pressure plasma spraying technique for depositing high Tc Y‐Ba‐Cu‐O thick films has been developed. Films with a thickness range of 20–100 μm have been prepared by using Y0.3Ba0.7CuOx powders. After post‐annealing in oxygen for 1 h at 950 °C, the films, which were deposited on a nimonic alloy substrate heated at 650 °C during spraying, exhibited a zero resistance temperature of 90.6 K with a transition width (90%–10%) of 2 K and a critical current density (77 K, 0 T) of 690 A/cm2.

High Tc superconducting films of Y-Ba-Cu oxide prepared by low-pressure plasma spraying

The effects of the addition of a small amount of magnesium to the CuSn matrix in the composite-processed (bronze-processed) Nb 3 Sn super-conductor were investigated. It was found that the magnesium addition significantly suppresses the Nb 3 Sn grain coarsening. The fine Nb 3 Sn grain structure produced in the Nb/CuSnMg composite enhances the grain boundary diffusion of tin and consequently makes the Nb 3 Sn growth rate nearly twice as much as that of the Nb/CuSn composite. The significant increase in superconducting critical current attained in the Nb/CuSnMg composite shows that the magnesium addition is promising for the improvement of the current-carrying capacities of multifilamentary Nb 3 Sn superconductors.

Effects of magnesium addition to the CuSn matrix in the composite-processed Nb3Sn superconductor

We applied hot pressing to in situ powder-in-tube-processed (PIT-processed) MgB2 tapes. We enhanced the superconducting properties by adding nanometer-sized SiC powder and ethyltoluene (C9H12) to the mixed powder. Hot pressing was performed at 100 MPa and 630 °C in an Ar gas atmosphere for 2–10 h. Hot pressing reduced the cross-sectional area of the MgB2 tapes from ~ 0.55 mm2 for conventionally heat-treated tapes to ~ 0.44 mm2. This increased the MgB2 core density from 50% to 70%. Undoped MgB2 tapes hot pressed for 5 h exhibited a transport Jc of 90 A mm − 2 at 4.2 K and 10 T, which is about three times greater than that for a tape heat treated without hot pressing. The value of Jc for codoped tape hot pressed for 5 h was 450 A mm − 2 at 10 T, which is larger than that for codoped tape heat treated without hot pressing (280 A mm − 2). These results clearly demonstrate that hot pressing is effective in increasing the MgB2 core density, and hence in enhancing the Jc values of PIT-processed MgB2 tapes.

Kyoji Tachikawa

Papers

Anomalous superconductivity in black phosphorus under high pressures

High Tc superconducting films of Y-Ba-Cu oxide prepared by low-pressure plasma spraying

Effects of magnesium addition to the CuSn matrix in the composite-processed Nb3Sn superconductor

Improvement of the critical current properties of in situ powder-in-tube-processed MgB2 tapes by hot pressing

Anomalous superconductivity and pressure induced phase transitions in black phosphorus