The electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties are reviewed. Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.

Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

The optical properties of graphene and emerging two-dimensional materials including transition metal dichalcogenides are reviewed with an emphasis on nanophotonic applications. Two-dimensional materials exhibit diverse electronic properties, ranging from insulating hexagonal boron nitride and semiconducting transition metal dichalcogenides such as molybdenum disulphide, to semimetallic graphene. In this Review, we first discuss the optical properties and applications of various two-dimensional materials, and then cover two different approaches for enhancing their interactions with light: through their integration with external photonic structures, and through intrinsic polaritonic resonances. Finally, we present a narrow-bandgap layered material — black phosphorus — that serendipitously bridges the energy gap between the zero-bandgap graphene and the relatively large-bandgap transition metal dichalcogenides. The plethora of two-dimensional materials and their heterostructures, together with the array of available approaches for enhancing the light–matter interaction, offers the promise of scientific discoveries and nanophotonics technologies across a wide range of the electromagnetic spectrum.

Two-dimensional material nanophotonics

Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.

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Emergent phenomena at oxide interfaces

Semiconductor technology is currently based on the manipulation of electronic charge; however, electrons have additional degrees of freedom, such as spin and valley, that can be used to encode and process information. Over the past several decades, there has been significant progress in manipulating electron spin for semiconductor spintronic devices, motivated by potential spin-based information processing and storage applications. However, experimental progress towards manipulating the valley degree of freedom for potential valleytronic devices has been limited until very recently. We review the latest advances in valleytronics, which have largely been enabled by the isolation of 2D materials (such as graphene and semiconducting transition metal dichalcogenides) that host an easily accessible electronic valley degree of freedom, allowing for dynamic control. The energy extrema of an electronic band are referred to as valleys. In 2D materials, two distinguishable valleys can be used to encode information and explore other valleytronic applications.

Valleytronics in 2D materials

In 1984, Bychkov and Rashba introduced a simple form of spin-orbit coupling to explain the peculiarities of electron spin resonance in two-dimensional semiconductors. Over the past 30 years, Rashba spin-orbit coupling has inspired a vast number of predictions, discoveries and innovative concepts far beyond semiconductors. The past decade has been particularly creative, with the realizations of manipulating spin orientation by moving electrons in space, controlling electron trajectories using spin as a steering wheel, and the discovery of new topological classes of materials. This progress has reinvigorated the interest of physicists and materials scientists in the development of inversion asymmetric structures, ranging from layered graphene-like materials to cold atoms. This Review discusses relevant recent and ongoing realizations of Rashba physics in condensed matter.

New perspectives for Rashba spin–orbit coupling

Tungsten diselenide (WSe2) and related transition metal dichalcogenides exhibit interesting optoelectronic properties owing to their peculiar band structures originating from the valley degree of freedom. Although the optical generation and detection of valley polarization has been demonstrated, it has been difficult to realize active valley-dependent functions suitable for device applications. We report an electrically switchable, circularly polarized light source based on the material's valley degree of freedom. Our WSe2-based ambipolar transistors emit circularly polarized electroluminescence from p-i-n junctions electrostatically formed in transistor channels. This phenomenon can be explained qualitatively by the electron-hole overlap controlled by the in-plane electric field. Our device demonstrates a route to exploit the valley degree of freedom and the possibility to develop a valley-optoelectronics technology.

http://science.sciencemag.org/content/sci/344/6185/725.full.pdf

Electrically Switchable Chiral Light-Emitting Transistor

Using a liquid gate has allowed electrically induced superconductivity in a solid specimen by means of carrier accumulation on the surface. But this phenomenon was limited to materials that became superconductors at low carrier density. It is now shown that superconductivity can be induced in a much wider range of materials by using an ionic liquid.

Liquid-gated interface superconductivity on an atomically flat film

Recent advances in atomic-scale preparation of ultrathin nano-sheets and efficient field-effect gating mediated by movement of ions have provided a prolific paradigm for creating exotic states at interfaces of a new-type of device called electric-double layer transistors (EDLTs). We present a short review on these liquid/solid interfaces formed on nano-sheets prepared by micro cleaving a bulk layered single crystal, which can be electrostatically doped to a high carrier density of ∼1014 cm−2. Atomically flat surfaces prepared on various layered materials allowed ideal transport when they acted as transistor channels after accumulating dense carriers. The unique system combining these two advantages enabled observations of novel transport phenomena showing quantum phase transition of charge and spin states controlled by electric field. Examples include gate-induced metal-insulator transition, opening of new transport channels, and field-induced interface superconductivity, which present a rapidly growing field with emerging opportunities for science and technology.

Interface transport properties in ion-gated nano-sheets

High carrier density part of many materials could be accessed by a variation of the field effect transistor technique: electric double layer transistor. Carrier density regime of n~1014 cm−2 can be easily accessed electrostatically realizing effective doping without chemical modification. In this study, we utilized micro-cleavage on a number of interesting layered materials. And realized high carrier density state and high performance transport on atomically flat surfaces.

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Gate-Induced Superconductivity in Layered-Material-Based Electric Double Layer Transistors

We present a study on the liquid/solid interface, which can be electrostatically doped to a high carrier density (n~1014 cm−2) by electric-double-layer gating. Using micro-cleavage technique on the layered materials: ZrNCl and graphene, atomically flat channel surfaces can be easily prepared. Intrinsic high carrier density transport regime is accessed at the channel interface of electric double-layer field effect transistor, where novel transport properties are unveiled as the field-induced superconductivity on the ZrNCl with high transition temperature at 15 K, and accessing a high carrier density up to 2×1014 cm−2 in graphene and its multi-layers.

Justin Ye

Papers

Electrically Switchable Chiral Light-Emitting Transistor

Liquid-gated interface superconductivity on an atomically flat film

Interface transport properties in ion-gated nano-sheets

Gate-Induced Superconductivity in Layered-Material-Based Electric Double Layer Transistors

Creating Novel Transport Properties in Electric Double Layer Field Effect Transistors Based on Layered Materials