N. Hemsworth

Two-dimensional magnetotransport in a black phosphorus naked quantum well

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.

Dual-Gate Velocity-Modulated Transistor Based on Black Phosphorus

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 [1–3]. 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 [11–13], 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] to overcome the limitation on transistor switching frequency imposed by the channel transit time of charge carriers. Mobility modulation has since been demonstrated in GaAs/AlGaAs heterojunctions [16], wide GaAs/AlGaAs quantum wells [17], silicon-on-insulator FETs [18] and the LaAlO3/SrTiO3 interface [19]. Room temperature VMT operation in silicon-on-insulator FETs has been demonstrated with up to 1.4-fold mobility modulation [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 conclusively observed in carbon nanotube FETs [20], Schottky barrier modulation has recently been shown to dominate off-state conductance of bP FETs [21]. The combined effects of mobility modulation and Schottky barrier modulation of dual-gate bP FETs enables four-fold transconductance modulation at a fixed carrier density of 4 × 1011cm−2.

Bolometric response of free-standing reduced graphene oxide films

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.

Measurement of topological Berry phase in highly disordered graphene

We have observed the quantum Hall effect (QHE) and Shubnikov-de Haas (SdH) oscillations in highly disordered graphene at magnetic fields up to 65 T. Disorder was introduced by hydrogenation of graphene up to a ratio H/C

Dephasing in strongly anisotropic black phosphorus

\approx 0.1\%

Quasi-two-dimensional thermoelectricity in SnSe

. The analysis of SdH oscillations and QHE indicates that the topological part of the Berry phase, proportional to the pseudo-spin winding number, is robust against introduction of disorder by hydrogenation in large scale graphene.

Measurement of electronic heat dissipation in highly disordered graphene

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

Non-Classical Longitudinal Magneto-Resistance in Anisotropic Black Phosphorus.

{L}_{\ensuremath{\varphi}}

Large magnetoresistance by Pauli blockade in hydrogenated graphene

was investigated, and above 1 K, it was found to decrease weaker than the

Giant magnetoresistance by Pauli blockade in hydrogenated graphene

{L}_{\ensuremath{\varphi}}\ensuremath{\propto}{T}^{\ensuremath{-}1/2}