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 cm(2)/V·s, and an on/off ratio of up to 10(4). We demonstrate the possibility of phosphorene integration by constructing a 2D CMOS inverter consisting of phosphorene PMOS and MoS2 NMOS transistors.

Phosphorene, an elemental 2D material, which is the monolayer of black phosphorus, has been mechanically exfoliated recently. In its bulk form, black phosphorus shows high carrier mobility (~10000 cm 2 /V· s) and a ~0.3 eV direct bandgap. Well-behaved p-type field-effect transistors with mobilities of up to 1000 cm 2 /V· s, as well as phototransistors, have been demonstrated on few-layer black phosphorus, showing its promise for electronics and optoelectronics applications due to its high hole mobility and thickness-dependence direct bandgap. However, p-n junctions, the basic building blocks of modern electronic and optoelectronic devices, have not yet been realized based on black phosphorus. In this paper, we demonstrate a gate tunable p-n diode based on a p-type black phosphorus/n-type monolayer MoS2 van der Waals p-n heterojunction. Upon illumination, these ultra-thin p-n diodes show a maximum photodetection responsivity of 418 mA/W at the wavelength of 633 nm, and photovoltaic energy conversion with an external quantum efficiency of 0.3%. These p-n diodes show promise for broadband photodetection and solar energy harvesting.Key words: black phosphorus, phosphorene, MoS2, p-n diode, van der Waals heterojunction, photodetection, solar cell 3 The successful isolation of graphene from graphite has led to its extensive study in physics, materials, and nano-engineering due to its extraordinary electrical and mechanical properties. [1][2][3][4] However, a lack of a bandgap limits its potential for electronic device applications, and has inspired the exploration of other 2D layered materials. [5][6][7] Among them, transition metal dichalcogenides (TMDCs), such as MoS2, are the most studied materials. [8][9][10][11] Recently, phosphorene, the monolayer form of black phosphorus, has been successfully isolated. 12 Analogous to graphite and graphene, black phosphorus is a stack of phosphorene monolayers, bound together by van der Waals interactions. 12,13 Bulk black phosphorus shows a ~0.3 eV direct bandgap and a mobility of up to ~10000 cm 2 /V· s. 14-17 Its bandgap increases as its thickness decreases, and is predicted to have a >1 eV direct bandgap in its monolayer form. 12,13 Well-behaved p-type field-effect transistors with mobilities of up to 1000 cm 2 /V· s, as well as inverters, have been demonstrated on few-layer black phosphorus. 12,13,[18][19][20] Based on its direct bandgap, few-layer black phosphorus phototransistors have been demonstrated with a responsivity of 4.8 mA/W. 19 These results indicate that black phosphorus is a promising candidate for both high performance electronics and optoelectronics applications due to its ultra-thin 2D nature, high hole mobility, and narrower direct bandgap compared to most of TMDCs. P-N junctions are the basic building blocks of modern semiconductor devices, including diodes, bipolar transistors, photodiodes, light-emitting diodes, and solar cells. In the conventional p-n homo-junction, the p-and n-type regions are formed by 4 chemically doping a bulk semiconduct...

The reliable production of two-dimensional crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with a process-tunable thickness, from monolayer to tens of nanometres, and with lateral sizes of up to 100 µm. The chiral-chain van der Waals structure of tellurene gives rise to strong in-plane anisotropic properties and large thicknessdependent shifts in Raman vibrational modes, which is not observed in other 2D layered materials. We also fabricate tellurene field-effect transistors, which exhibit air-stable performance at room temperature for over two months, on/off ratios on the order of 10 6 and field-effect mobilities of around 700 cm 2 /Vs. Furthermore, by scaling down the channel length and integrating with high-k dielectrics, transistors with a significant on-state current density of 1 A/mm are demonstrated. MainThe continuing development of two-dimensional materials, be it the exploration of new science 1-3 or the implementation of new technologies 4-8 , requires reliable methods of synthesising 2D crystals. Whether current approaches can be scaled up though remains uncertain 9,10 and are restricted by factors such as growth substrates and conditions 11-13 , small crystal sizes 14 and the instability of the synthesized materials 11,15,16 .

Black phosphorus has been revisited recently as a new two-dimensional material showing potential applications in electronics and optoelectronics. Here we report the anisotropic in-plane thermal conductivity of suspended few-layer black phosphorus measured by micro-Raman spectroscopy. The armchair and zigzag thermal conductivities are ∼20 and ∼40 W m−1 K−1 for black phosphorus films thicker than 15 nm, respectively, and decrease to ∼10 and ∼20 W m−1 K−1 as the film thickness is reduced, exhibiting significant anisotropy. The thermal conductivity anisotropic ratio is found to be ∼2 for thick black phosphorus films and drops to ∼1.5 for the thinnest 9.5-nm-thick film. Theoretical modelling reveals that the observed anisotropy is primarily related to the anisotropic phonon dispersion, whereas the intrinsic phonon scattering rates are found to be similar along the armchair and zigzag directions. Surface scattering in the black phosphorus films is shown to strongly suppress the contribution of long mean-free-path acoustic phonons.

Thermal conductivity of nanocrystalline silicon by direct molecular dynamics simulation J. Appl. Phys. 112, 064305 (2012) Electrical and heat conduction mechanisms of GeTe alloy for phase change memory application J. Appl. Phys. 112, 053712 (2012) Thermal rectification and phonon scattering in silicon nanofilm with cone cavity J. Appl. Phys. 112, 054312 (2012) Analysis of the "3-Omega" method for substrates and thick films of anisotropic thermal conductivity This work describes an experimental study of thermal conductance across multiwalled carbon nanotube ͑CNT͒ array interfaces, one sided ͑Si-CNT-Ag͒ and two sided ͑Si-CNT-CNT-Cu͒, using a photoacoustic technique ͑PA͒. Well-anchored, dense, and vertically oriented multiwalled CNT arrays have been directly synthesized on Si wafers and pure Cu sheets using plasma-enhanced chemical vapor deposition. With the PA technique, the small interface resistances of the highly conductive CNT interfaces can be measured with accuracy and precision. In addition, the PA technique can resolve the one-sided CNT interface component resistances ͑Si-CNT and CNT-Ag͒ and the two-sided CNT interface component resistances ͑Si-CNT, CNT-CNT, and CNT-Cu͒ and can estimate the thermal diffusivity of the CNT layers. The thermal contact resistances of the one-and two-sided CNT interfaces measured using the PA technique are 15.8± 0.9 and 4.0± 0.4 mm 2 K/W, respectively, at moderate pressure. These results compare favorably with those obtained using a steady state, one-dimensional reference bar method; however, the uncertainty range is much narrower. The one-sided CNT thermal interface resistance is dominated by the resistance between the free CNT array tips and their opposing substrate ͑CNT-Ag͒, which is measured to be 14.0± 0.9 mm 2 K / W. The two-sided CNT thermal interface resistance is dominated by the resistance between the free tips of the mating CNT arrays ͑CNT-CNT͒, which is estimated to be 2.1± 0.4 mm 2 K/W.