Stacking single layers of atoms on top of each other provides a fundamental way to achieve novel material systems and engineer their physical properties, which offers opportunities for exploring fundamental physics and realizing next-generation optoelectronic devices. Among the two-dimensional (2D)-stacked systems, transition metal dichalcogenide (TMDC) heterostructures are particularly attractive because they host tightly-bonded interlayer excitons which possess various novel and appealing properties. These interlayer excitons have drawn significant research attention and hold high potential for the application in unique optoelectronic devices, such as polarization- and wavelength-tunable single photon emitters, valley Hall transistors, and possible high-temperature superconductors. The development of these devices requires a comprehensive understanding of the fundamental properties of these interlayer excitons and the impact of electric fields on their behaviors. In this review, we summarize the recent advances on the understanding of interlayer exciton dynamics under electric fields in TMDC heterostructures. We put emphasis on the electrical modulation of interlayer excitons’ emission, the valley Hall transport of charge carriers after the separation of interlayer excitons by an electric field, and the correlation physics of interlayer excitons and charges under electrical doping and tuning. Challenges and perspectives are finally discussed for the application of TMDC heterostructures in future optoelectronics.

The discovery of two-dimensional (2D) semiconductor has opened up new avenues for the development of short-channel field-effect transistors (FETs) with desired electrical performance. Among them, orthorhombic tin-selenide (SnSe) has garnered increasing attention due to its potential applications in a variety of electronic, optoelectronic, and thermoelectric devices. However, the realization of high-performance SnSe FETs with low contact resistance (R_c) remains a challenge. Herein, we systematically investigate the contact of few-layer SnSe FETs through the modulation of native oxide on SnSe by using different metals. It is found that chromium (Cr)-contacted devices possess the best FET performance, such as electron mobility up to 606 cm²/(V·s) at 78 K, current on/off ratio exceeding 10¹⁰, and saturation current of ~ 550 μA/μm, where a negligible Schottky barrier (SB) of ~ 30 meV and a low contact resistance of ~ 425 Ω μm are achieved. X-ray photoelectron spectroscopy (XPS) and cross-sectional electron dispersive X-ray spectroscopy (EDX) results further reveal that the improved contact arises from the Cr-induced reduction of native oxide (SnO_x) to Sn, which thins the tunneling barrier for efficient electron injection. Our findings provide a deep insight into the 2D-metal contact of SnSe and pave the way for its applications in future nanoelectronics.

Photoelectric synaptic devices have been considered as one of the key components in artificial neuromorphic systems due to their excellent capability to emulate the functions of visual neurons, such as light perception and image processing. Herein, we demonstrate an optically-stimulated artificial synapse with a clear photoresponse from ultraviolet to visible light, which is established on a novel heterostructure consisting of monocrystalline Cs₂AgBiBr₆ perovskite and indium–gallium–zinc oxide (IGZO) thin film. As compared with pure IGZO, the heterostructure significantly enhances the photoresponse and corresponding synaptic plasticity of the devices, which originate from the superior visible absorption of single-crystal Cs₂AgBiBr₆ and effective interfacial charge transfer from Cs₂AgBiBr₆ to IGZO. A variety of synaptic behaviors are realized on the fabricated thin-film transistors, including excitatory postsynaptic current, paired pulse facilitation, short-term, and long-term plasticity. Furthermore, an artificial neural network is simulated based on the photonic potentiation and electrical depression effects of synaptic devices, and an accuracy rate up to 83.8% ± 1.2% for pattern recognition is achieved. This finding promises a simple and efficient way to construct photoelectric synaptic devices with tunable spectrum for future neuromorphic applications.

Broken-gap (type-III) two-dimensional (2D) van der Waals heterostructures (vdWHs) offer an ideal platform for interband tunneling devices due to their broken-gap band offset and sharp band edge. Here, we demonstrate an efficient control of energy band alignment in a typical type-III vdWH, which is composed of vertically-stacked molybdenum telluride (MoTe₂) and tin diselenide (SnSe₂), via both electrostatic and optical modulation. By a single electrostatic gating with hexagonal boron nitride (h-BN) as the dielectric, a variety of electrical transport characteristics including forward rectifying, Zener tunneling, and backward rectifying are realized on the same heterojunction at low gate voltages of ±1 V. In particular, the heterostructure can function as an Esaki tunnel diode with a room-temperature negative differential resistance. This great tunability originates from the atomically-flat and inert surface of h-BN that significantly suppresses the interfacial trap scattering and strain effects. Upon the illumination of an 885 nm laser, the band alignment of heterojunction can be further tuned to facilitate the direct tunneling of photogenerated charge carriers, which leads to a high photocurrent on/off ratio of > 10 ⁵ and a competitive photodetectivity of 1.03 × 10¹² Jones at zero bias. Moreover, the open-circuit voltage of irradiated heterojunction can be switched from positive to negative at opposite gate voltages, revealing a transition from accumulation mode to depletion mode. Our findings not only promise a simple strategy to tailor the bands of type-III vdWHs but also provide an in-depth understanding of interlayer tunneling for future low-power electronic and optoelectronic applications.

Doping of semiconductors, i.e., accurately modulating the charge carrier type and concentration in a controllable manner, is a key technology foundation for modern electronics and optoelectronics. However, the conventional doping technologies widely utilized in silicon industry, such as ion implantation and thermal diffusion, always fail when applied to two-dimensional (2D) materials with atomically-thin nature. Surface charge transfer doping (SCTD) is emerging as an effective and non-destructive doping technique to provide reliable doping capability for 2D materials, in particular 2D semiconductors. Herein, we summarize the recent advances and developments on the SCTD of 2D semiconductors and its application in electronic and optoelectronic devices. The underlying mechanism of STCD processes on 2D semiconductors is briefly introduced. Its impact on tuning the fundamental properties of various 2D systems is highlighted. We particularly emphasize on the SCTD-enabled high-performance 2D functional devices. Finally, the challenges and opportunities for the future development of SCTD are discussed.