Efficient interfacial carrier generation in van der Waals heterostructures is critical for their electronic and optoelectronic applications. We demonstrate broadband photocarrier generation in WS2-graphene heterostructures by imaging interlayer coupling-dependent charge generation using ultrafast transient absorption microscopy. Interlayer charge-transfer (CT) transitions and hot carrier injection from graphene allow carrier generation by excitation as low as 0.8 eV below the WS2 bandgap. The experimentally determined interlayer CT transition energies are consistent with those predicted from the first-principles band structure calculation. CT interactions also lead to additional carrier generation in the visible spectral range in the heterostructures compared to that in the single-layer WS2 alone. The lifetime of the charge-separated states is measured to be ~1 ps. These results suggest that interlayer interactions make graphene-two-dimensional semiconductor heterostructures very attractive for photovoltaic and photodetector applications because of the combined benefits of high carrier mobility and enhanced broadband photocarrier generation.

https://advances.sciencemag.org/content/advances/4/2/e1700324.full.pdf

Photocarrier generation from interlayer charge-transfer transitions in WS2-graphene heterostructures

Electronic chirality near the Dirac point is a key property of graphene systems, which is revealed by the spectral intensity patterns as measured by angle-resolved photoemission spectroscopy under various polarization conditions. Specifically, the strongly modulated circular patterns for monolayer (bilayer) graphene rotate by ±90° (±45°) in changing from linearly to circularly polarized light; these angles are directly related to the phases of the wave functions and thus visually confirm the Berry's phase of π (2π) around the Dirac point. The details are verified by calculations.

https://absimage.aps.org/image/MAR12/MWS_MAR12-2011-001369.pdf

Visualizing Electronic Chirality and Berry's Phases in Graphene Systems Using Photoemission with Circularly Polarized Light

The investigation of van der Waals (vdW) heterostructures has been becoming an attractive research topic due to their unique electrical, optical and magnetic properties. The vdW heterostructures are generally constructed from stacks of atomically thin two-dimensional (2D) materials and their performance is closely dependent on the constituent 2D materials and the physical interactions between these layers. Importantly, a lot of interesting progress of vdW heterostructures in the electrical, optical and magnetic properties has been achieved recently, which lays a solid foundation for the fundamental research of this field and their future applications. In this review, a series of preparation methods of vdW heterostructures are introduced and the research on ultrafast exciton dynamics in vdW heterostructures is summarized and discussed. Furthermore, the nonlinear optical properties of vdW heterostructures and their applications in ultrafast lasers are also reviewed together with the challenges and future opportunities in this research field. This review aims to present a thorough summarization of the research progress of vdW heterostructures in ultrafast photonics, in order to promote the development in this field.

2D van der Waals heterostructures: processing, optical properties and applications in ultrafast photonics

Photodetectors are optoelectronic devices with excellent photoelectric conversion abilities. They have especially important applications in many fields, such as image sensing, video imaging, optical communications, aerospace, environmental detection, and military safety. Since the advent of graphene in 2004, scientists have been keen on the study of 2D materials. 2D materials have excellent physical, chemical, thermal, magnetic, electronic, and optical properties, which allow them to be widely used in the field of photodetectors. Recently, due to their many excellent properties, topological insulators (TIs), a class of novel 2D quantum material, have attracted much attention from scientists. TIs are promising layered materials with quantum properties. In bulk form, they have insulating band gaps and gapless conductive edge states or surface states, protected by time-reversal symmetry caused by robust spin–orbit coupling effects. This special energy band structure enables a weakening of the backscattering effect during electron transport, and thus they have extremely high carrier mobilities. At the same time, TIs have gapless edge states or surface states, linear band dispersion relationships, and good flexibility and mechanical properties, and they have especially important application prospects in high-performance broadband photodetectors with high sensitivity and in wearable flexible devices. In addition, TIs have excellent transparency, electrical conductivity, and environmental stability, and have great potential for use in the preparation of electrodes for photodetectors. Examining novel TI materials, this paper reviews the latest research progress relating to TI photodetectors. Firstly, the types and properties of TIs are introduced in detail. Secondly, common TI preparation methods are elaborated on, and then the applications of TIs in photodetectors are emphasized. Finally, the present challenges relating to TIs are summarized, and future development directions are given, providing some feasible guidance for the preparation of novel high-performance TI photodetectors.

Recent development and advances in Photodetectors based on two-dimensional topological insulators

https://iopscience.iop.org/article/10.1088/1674-1056/28/6/067403/pdf

Topological superconductivity in a Bi2Te3/NbSe2 heterostructure: A review*

Graphene and topological insulators (TI) possess two-dimensional (2D) Dirac fermions with distinct physical properties. Integrating these two Dirac materials in a single device creates interesting opportunities for exploring new physics of interacting massless Dirac fermions. Here we report on a practical route to experimental fabrication of graphene–Sb2Te3 heterostructure. The graphene–TI heterostructures are prepared by using a dry transfer of chemical-vapor-deposition grown graphene film. ARPES measurements confirm the coexistence of topological surface states of Sb2Te3 and Dirac π bands of graphene, and identify the twist angle in the graphene–TI heterostructure. The results suggest a potential tunable electronic platform in which two different Dirac low-energy states dominate the transport behavior.

/pdf/experimental-observation-of-two-massless-dirac-fermion-gases-1vkc62u13a.pdf

Experimental observation of two massless Dirac-fermion gases in graphene-topological insulator heterostructure

A thermoelectric structure including a thermoelectric material having a thickness less than 50 nm and a semi-insulating material in electrical contact with the thermoelectric material. The thermoelectric material and the semi-insulating materials have an equilibrium Fermi level, across a junction between the thermoelectric material and the semi-insulating material, which exists in a conduction band or a valence band of the thermoelectric material. The thermoelectric structure is for thermoelectric cooling and thermoelectric power generation.

Nanoscale, ultra-thin films for excellent thermoelectric figure of merit

Graphene and topological insulators (TI) possess two-dimensional Dirac fermions with distinct physical properties. Integrating these two Dirac materials in a single device creates interesting opportunities for exploring new physics of interacting massless Dirac fermions. Here we report on a practical route to experimental fabrication of graphene-Sb2Te3 heterostructure. The graphene-TI heterostructures are prepared by using a dry transfer of chemical-vapor-deposition grown graphene film. ARPES measurements confirm the coexistence of topological surface states of Sb2Te3 and Dirac {\pi} bands of graphene, and identify the twist angle in the graphene-TI heterostructure. The results suggest a potential tunable electronic platform in which two different Dirac low-energy states dominate the transport behavior.

/pdf/experimental-observation-of-two-massless-dirac-fermion-gases-5a3fw7rfzy.pdf

Experimental Observation of Two Massless Dirac-Fermion Gases in Graphene-Topological Insulator Heterostructure

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Experimental observation of two massless Dirac-fermion gases in graphene-topological insulator heterostructure

Nanoscale, ultra-thin films for excellent thermoelectric figure of merit

Experimental Observation of Two Massless Dirac-Fermion Gases in Graphene-Topological Insulator Heterostructure