Black phosphorus (bP) is the second known elemental allotrope with a layered crystal structure that can be mechanically exfoliated to atomic layer thickness. Unlike metallic graphite and semi-metallic graphene, bP is a semiconductor in both bulk and few-layer form. Here we fabricate bP-naked quantum wells in a back-gated field effect transistor geometry with bP thicknesses ranging from 6±1 nm to 47±1 nm. Using a polymer encapsulant, we suppress bP oxidation and observe field effect mobilities up to 900 cm2 V−1 s−1 and on/off current ratios exceeding 105. Shubnikov-de Haas oscillations observed in magnetic fields up to 35 T reveal a 2D hole gas with Schrödinger fermion character in a surface accumulation layer. Our work demonstrates that 2D electronic structure and 2D atomic structure are independent. 2D carrier confinement can be achieved without approaching atomic layer thickness, advantageous for materials that become increasingly reactive in the few-layer limit such as bP.

Stannous selenide is a layered semiconductor that is a polar analogue of black phosphorus, and of great interest as a thermoelectric material. Unusually, hole doped SnSe supports a large Seebeck coefficient at high conductivity, which has not been explained to date. Angle resolved photo-emission spectroscopy, optical reflection spectroscopy and magnetotransport measurements reveal a multiplevalley valence band structure and a quasi two-dimensional dispersion, realizing a Hicks-Dresselhaus thermoelectric contributing to the high Seebeck coefficient at high carrier density. We further demonstrate that the hole accumulation layer in exfoliated SnSe transistors exhibits a field effect mobility of up to 250 cm 2 /Vs at T = 1.3 K. SnSe is thus found to be a high quality, quasi twodimensional semiconductor ideal for thermoelectric applications. arXiv:1802.08069v1 [cond-mat.mtrl-sci]

Weak localization was observed in a black phosphorus field-effect transistor 65 nm thick. The weak localization behavior was found to be in excellent agreement with the Hikami-Larkin-Nagaoka model for fields up to 1 T, from which characteristic scattering lengths could be inferred. The temperature dependence of the phase coherence length L ϕ was investigated, and above 1 K, it was found to decrease weaker than the L ϕ ∝ T −1/2 dependence characteristic of electron-electron scattering in the presence of elastic scattering in two dimensions. Rather, the observed power law was found to be close to that observed previously in quasi-one-dimensional systems such as metallic nanowires and carbon nanotubes.

The layered semiconductor black phosphorus has attracted attention as a 2D atomic crystal that can be prepared in ultra-thin layers for operation as field effect transistors [1][2][3]. Despite the susceptibility of black phosphorus to photo-oxidation [4], improvements to the electronic quality of black phosphorus devices has culminated in the observation of the quantum Hall effect [5]. In this work, we demonstrate the room temperature operation of a dual gated black phosphorus transistor operating as a velocity modulated transistor [6], whereby modification of hole density distribution within a black phosphorus quantum well leads to a two-fold modulation of hole mobility. Simultaneous modulation of Schottky barrier resistance leads to a four-fold modulation of transconductance at a fixed hole density. Our work explicitly demonstrates the critical role of charge density distribution upon charge carrier transport within 2D atomic crystals.Black phosphorus (bP) is an elemental allotrope and a direct bandgap semiconductor with a puckered, honeycomb layer structure [7,8] that can be exfoliated down to atomic few-layer thickness [1-4, 9, 10]. Although bP is the most thermodynamically stable allotrope of phosphorus, photo-oxidation in the presence of water, oxygen and visible light is known to degrade bP with a reaction rate that increases as bP layer thickness decreases [4]. Several materials have been used to encapsulate bP in order to protect it against photo-oxidation, including hexagonal boron-nitride [11][12][13], aluminum oxide [14], parylene [4], and poly-methylmethacrylate [15]. Recent works have also shown that 2D hole transport can be achieved in a single 2D sub-band within an accumulation layer of many-layer bP [11,13,15], effectively combining 2D transport characteristics with the increased chemical stability of many-layer bP. These advances have culminated in the observation of the quantum Hall effect in bP [5]. Nonetheless, further understanding and control of transconductance, carrier mobility and contact resistance in bP field effect transistors (FETs) is desired.We report here an experimental investigation of the transport characteristics of bP FETs with an asymmetric dual gate geometry consisting of top and bottom gate electrodes. The top gate is found to be effective in modulating the back gate FET transfer characteristics, including both field effect mobility and Schottky barrier contact resistance. The mobility modulation effect enables operation of the dual gate bP FET as a velocity modulated transistor (VMT), first proposed by Sakaki [6] [18]. Asymmetric dual-gate bP FETs exhibit a two-fold mobility modulation at room temperature, and the underlying mechanism is modulation of hole density distribution with the naked bP quantum well channel of the bP FET, and a resultant modulation of scattering by charged impurities within the gate oxide, surface roughness, and other spatially dependent scattering mechanisms. Simultaneously, bP FETs exhibit strong Schottky barrier modulation. First conclusive...

Free-standing films of reduced graphene oxide were prepared by evaporative drying of drop-cast graphene oxide followed by thermal reduction. The electrical resistance of reduced graphene oxide films showed a strong temperature dependence, reaching a temperature coefficient of resistance of 44×103 Ω/K at 60 K. The bolometric response under black body illumination was measured from 50 K to 300 K, reaching a voltage responsivity of up to 82 × 103 V/W at 50 K.