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.

Real-time, high resolution, simultaneous measurement of multiple ionic species is challenging with existing chromatographic, spectrophotometric and potentiometric techniques. Potentiometric ion sensors exhibit limitations in both resolution and selectivity. Herein, we develop wafer scale graphene transistor technology for overcoming these limitations. Large area graphene is an ideal material for high resolution ion sensitive field effect transistors (ISFETs), while simultaneously enabling facile fabrication as compared to conventional semiconductors. We develop the ISFETs into an array and apply Nikolskii–Eisenman analysis to account for cross-sensitivity and thereby achieve high selectivity. We experimentally demonstrate real-time, simultaneous concentration measurement of K+, Na+, $${{\rm{NH}}}_{4}^{+}$$NH4+, $${{\rm{NO}}}_{3}^{-}$$NO3−, $${{\rm{SO}}}_{4}^{2-}$$SO42−, $${{\rm{HPO}}}_{4}^{2-}$$HPO42− and Cl− with a resolution of $$\sim\! 2\times 1{0}^{-3}\,{\mathrm{log}}\,$$~2×10−3log concentration units. The array achieves an accuracy of  ±0.05 log concentration. Finally, we demonstrate real-time ion concentration measurement in an aquarium with lemnoideae lemna over three weeks, where mineral uptake by aquatic organisms can be observed during their growth.

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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]

The environmental stability of the layered semiconductor black phosphorus (bP) remains a challenge. Passivation of the bP surface with phosphorus oxide, PO x , grown by a reactive ion etch with oxygen plasma is known to improve photoluminescence efficiency of exfoliated bP flakes. We apply phosphorus oxide passivation in the fabrication of bP field effect transistors using a gate stack consisting of a PO x layer grown by reactive ion etching followed by atomic layer deposition of Al 2 O 3 . We observe room temperature top-gate mobilities of 115 cm 2 V −1 s −1 in ambient conditions, which we attribute to the low defect density of the bP/PO x interface.Black phosphorus (bP) is a direct band gap (E g = 0.3 eV) semiconductor with a puckered honeycomb layer structure characterized by van der Waals interlayer bonding 1,2 . The most thermodynamically stable allotrope of phosphorus, bP exhibits ambipolar conduction, anisotropic conductivity, and can be exfoliated down to the atomic monolayer limit 3-6 . Exfoliation of bP in a nitrogen environment followed by encapsulation with hexagonal boron nitride in a vacuum environment has led to the observation of ∼ 45, 000 cm 2 V −1 s −1 hole mobility at cryogenic temperatures 7 . Importantly, bP is subject to degradation by photooxidation with a reaction rate that increases as bP thickness decreases 9 . A number of passivation techniques have been developed with varying degrees of success, including encapsulation with Al 2 O 3 10-13 , hexagonal boron nitride (h-BN) 7,14,15,17 , polymer layers 6,8 and functionalization with nickel nanoparticles 16 . More recently, it has been demonstrated that the formation of a dense phosphorus oxide, PO x , layer by oxygen plasma dry etching followed by Al 2 O 3 deposition results in stable encapsulation of bP without compromising photoluminescence (PL) efficiency 18 . The preservation of PL efficiency indicates that the interface between bP and PO x does not measurably increase non-radiative recombination rates and is thus an effective surface passivation strategy.In this work, we apply the PO x passivation approach of Pei et al. 18 to fabricate top-gated bP field effect transistors (FETs). The use of a native oxide for passivation and gate stack formation in bP FETs is appealing as a direct analogue to the use of silicon oxide in silicon FET technology. Various phases of PO x are known, including a rhombohedral crystal of molecular P 4 O 10 19 , and the most thermodynamically stable form of P 2 O 5 which is itself a layered material composed of a hexagonal network of edge connected (PO) 3− 4 tetrahedra 20 .The PO x layer passivates the bP surface, and acts as a seeding layer for subsequent atomic layer deposition of high-quality gate dielectrics such as Al 2 O 3 . We have fabricated dual-gate bP FETs, a bottom gate formed by a heavily doped, oxidized silicon substrate, and a top gate structure with a PO x /Al 2 O 3 dielectric stack. Room temperature top gate field effect mobilities of up to 115 cm 2 V −1 s −1 are achieved.In our experiments, w...