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...

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.

The Poisson's ratio of a material characterizes its response to uniaxial strain. Materials normally possess a positive Poisson's ratio - they contract laterally when stretched, and expand laterally when compressed. A negative Poisson's ratio is theoretically permissible but has not, with few exceptions of man-made bulk structures, been experimentally observed in any natural materials. Here, we show that the negative Poisson's ratio exists in the low-dimensional natural material black phosphorus and that our experimental observations are consistent with first-principles simulations. Through applying uniaxial strain along armchair direction, we have succeeded in demonstrating a cross-plane interlayer negative Poisson's ratio on black phosphorus for the first time. Meanwhile, our results support the existence of a cross-plane intralayer negative Poisson's ratio in the constituent phosphorene layers under uniaxial deformation along the zigzag axis, which is in line with a previous theoretical prediction. The phenomenon originates from the puckered structure of its in-plane lattice, together with coupled hinge-like bonding configurations.

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980's. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the Γ point is explained by theoretical calculation. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with reduced thickness. Low-temperature Hall experiment sheds lights on more intrinsic anisotropic electrical transport, with hole mobility of 3,000 and 1,500 cm 2 /V· s along a-axis and c-axis respectively. Pronounced quantum oscillations in magnetoresistance are observed at low temperatures with highest electron mobility up to 44,000 cm 2 /V· s.Keywords: ZrTe5 Single crystal, 2D material, optical anisotropy, electrical anisotropy, quantum oscillations 3The discovery of graphene 1 began a new era of condensed-matter research because of its unique two-dimensional Dirac band structure, which hosts many profound physical phenomena such as the anomalous integer quantum Hall effect (IQHE) 2 . Since then great efforts have been made towards expanding the spectrum of topological materials and bringing many conceptual materials into reality. Transition metal pentatellurides such as ZrTe5 and HfTe5 have been widely studied in bulk form since early 1980's due to their anomalous resistivity peak and X-ray diffraction intensity peak at low temperature 3,4 , large thermoelectric power 5 , pressure-induced superconductivity 6,7 , absence of a structural phase transition corresponding to resistivity anomaly 8 , and chiral magnetic effect 9 . In recent years, ZrTe5 research has been revived because of its non-trivial topological properties. Some theoretical predictions and experimental results 10,11 indicate that it is a 3D Dirac semimetal, a mimic of graphene with linear energy dispersion in all three directions. On the other hand, its monolayer form is also claimed to be a candidate of quantum spin Hall insulator 12,13 , which is very rare among the natural compounds 14 . Shubnikov-de Haas oscillations 10,15,16 , Zeeman Splitting 17,18 , and fractional quantum Hall effect 19 were also observed in bulk ZrTe5.Meanwhile in recent years the 2D family has been expanded to a wide range of mat...

Topological insulators (TI) have attracted extensive research effort due to their insulating bulk states but conducting surface states. However, investigation and understanding of thermal transport in topological insulators, particularly the effect of surface states, are lacking. In this work, we studied thickness-dependent in-plane thermal and electrical conductivity of BiTeSe TI thin films. A large enhancement in both thermal and electrical conductivity was observed for films with thicknesses below 20 nm, which is attributed to the surface states and bulk-insulating nature of these films. Moreover, a surface Lorenz number much larger than the Sommerfeld value was found. Systematic transport measurements indicated that the Fermi surface is located near the charge neutrality point (CNP) when the film thickness is below 20 nm. Possible reasons for the large Lorenz number include electrical and thermal current decoupling in the surface state Dirac fluid, and bipolar diffusion transport. A simple computational model indicates that the surface states and bipolar diffusion indeed can lead to enhanced electrical and thermal transport and a large Lorenz number.

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The metal contacts on 2D black phosphorus field-effect transistor and photodetectors are studied. The metal work functions can significantly impact the Schottky barrier at the metal-semiconductor contact in black phosphorus devices. Higher metal work functions lead to larger output hole currents in p-type transistors, while ambipolar characteristics can be observed with lower work function metals. Photodetectors with record high photoresponsivity (223 mA/W) are demonstrated on black phosphorus through contact-engineering.