Monolayer transition metal dichalcogenide heterostructures with type II band alignment have generated wide interest in device physics at the two-dimensional limit. Here, Rivera et al. observe interlayer excitons in vertically stacked MoSe2–WSe2 heterostructures and demonstrate tunability of the energy and luminescence.

Observation of Long-Lived Interlayer Excitons in Monolayer MoSe2-WSe2 Heterostructures

High-quality two-dimensional atomic layered p–n heterostructures are essential for high-performance integrated optoelectronics. The studies to date have been largely limited to exfoliated and restacked flakes, and the controlled growth of such heterostructures remains a significant challenge. Here we report the direct van der Waals epitaxial growth of large-scale WSe2/SnS2 vertical bilayer p–n junctions on SiO2/Si substrates, with the lateral sizes reaching up to millimeter scale. Multi-electrode field-effect transistors have been integrated on a single heterostructure bilayer. Electrical transport measurements indicate that the field-effect transistors of the junction show an ultra-low off-state leakage current of 10−14 A and a highest on–off ratio of up to 107. Optoelectronic characterizations show prominent photoresponse, with a fast response time of 500 μs, faster than all the directly grown vertical 2D heterostructures. The direct growth of high-quality van der Waals junctions marks an important step toward high-performance integrated optoelectronic devices and systems. Growth of large area and defect-free two-dimensional semiconductor layers for high-performance p–n junction applications has been a great challenge. Yang et al. prepare millimeter-scaled WSe2/SnS2 vertical heterojunctions by two-step van der Waals epitaxy, which show excellent optoelectronic properties.

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Van der Waals epitaxial growth and optoelectronics of large-scale WSe 2 /SnS 2 vertical bilayer p–n junctions

Graphene and two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant interest due to their unique properties that cannot be obtained in their bulk counterparts. These atomically thin 2D materials have demonstrated strong light–matter interactions, tunable optical bandgap structures and unique structural and electrical properties, rendering possible the high conversion efficiency of solar energy with a minimal amount of active absorber material. The isolated 2D monolayer can be stacked into arbitrary van der Waals (vdWs) heterostructures without the need to consider lattice matching. Several combinations of 2D/3D and 2D/2D materials have been assembled to create vdWs heterojunctions for photovoltaic (PV) and photoelectrochemical (PEC) energy conversion. However, the complex, less-constrained, and more environmentally vulnerable interface in a vdWs heterojunction is different from that of a conventional, epitaxially grown heterojunction, engendering new challenges for surface and interface engineering. In this review, the physics of band alignment, the chemistry of surface modification and the behavior of photoexcited charge transfer at the interface during PV and PEC processes will be discussed. We will present a survey of the recent progress and challenges of 2D/3D and 2D/2D vdWs heterojunctions, with emphasis on their applicability to PV and PEC devices. Finally, we will discuss emerging issues yet to be explored for 2D materials to achieve high solar energy conversion efficiency and possible strategies to improve their performance.

Engineering graphene and TMDs based van der Waals heterostructures for photovoltaic and photoelectrochemical solar energy conversion

Recently there have been many research breakthroughs in two-dimensional (2D) materials including graphene, boron nitride (h-BN), black phosphors (BPs), and transition-metal dichalcogenides (TMDCs). The unique electrical, optical, and thermal properties in 2D materials are associated with their strictly defined low dimensionalities. These materials provide a wide range of basic building blocks for next-generation electronics. The chemical vapor deposition (CVD) technique has shown great promise to generate high-quality TMDC layers with scalable size, controllable thickness, and excellent electronic properties suitable for both technological applications and fundamental sciences. The capability to precisely engineer 2D materials by chemical approaches has also given rise to fascinating new physics, which could lead to exciting new applications. In this Review, we introduce the latest development of TMDC synthesis by CVD approaches and provide further insight for the controllable and reliable synthesis of at...

Epitaxial Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Growth Mechanism, Controllability, and Scalability.

Van der Waals heterostructures (vdWHs) based on 2D layered materials with selectable materials properties pave the way to integration at the atomic scale, which may give rise to fresh heterostructures exhibiting absolutely novel physics and versatility. This feature article reviews the state-of-the-art research activities that focus on the 2D vdWHs and their optoelectronic applications. First, the preparation methods such as mechanical transfer and chemical vapor deposition growth are comprehensively outlined. Then, unique energy band alignments generated in 2D vdWHs are introduced. Furthermore, this feature article focuses on the applications in light-emitting diodes, photodetectors, and optical modulators based on 2D vdWHs with novel constructions and mechanisms. The recently reported novel constructions of the devices are introduced in three primary aspects: light-emitting diodes (such as single defect light-emitting diodes, circularly polarized light emission arising from valley polarization), photodetectors (such as photo-thermionic, tunneling, electrolyte-gated, and broadband photodetectors), and optical modulators (such as graphene integrated with silicon technology and graphene/hexagonal boron nitride (hBN) heterostructure), which show promising applications in the nextgeneration optoelectronics. Finally, the article provides some conclusions and an outlook on the future development in the field.

2D Layered Material-Based van der Waals Heterostructures for Optoelectronics

2D ferroelectric material has emerged as an attractive building block for highdensity data storage nanodevices. Although monolayer van der Waals ferroelectrics have been theoretically predicted, a key experimental breakthrough for such calculations is still not realized. Here, hexagonally stacking α-In2Se3 nanoflake, a rarely studied van der Waals polymorph, is reported to exhibit out-of-plane (OOP) and in-plane (IP) ferroelectricity at room temperature. Ferroelectric multidomain states in a hexagonal α-In2Se3 nanoflake with uniform thickness can survive to 6 nm. Most strikingly, the electric-field-induced polarization switching and hysteresis loop are, respectively, observed down to the bilayer and monolayer (≈1.2 nm) thicknesses, which designates it as the thinnest layered ferroelectric and verifies the corresponding theoretical calculation. In addition, two types of ferroelectric nanodevices employing the OOP and IP polarizations in 2H α-In2Se3 are developed, which are applicable for nonvolatile memories and heterostructure-based nanoelectronics/ optoelectronics.

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Room-Temperature Ferroelectricity in Hexagonally Layered α-In2Se3 Nanoflakes down to the Monolayer Limit

It is critically important to characterize the band alignment in semiconductor heterojunctions (HJs) because it controls the electronic and optical properties. However, the well-known Anderson's model usually fails to predict the band alignment in bulk HJ systems due to the presence of charge transfer at the interfacial bonding. Atomically thin 2D transition metal dichalcogenide materials have attracted much attention recently since the ultrathin HJs and devices can be easily built and they are promising for future electronics. The vertical HJs based on 2D materials can be constructed via van der Waals stacking regardless of the lattice mismatch between two materials. Despite the defect-free characteristics of the junction interface, experimental evidence is still lacking on whether the simple Anderson rule can predict the band alignment of HJs. Here, the validity of Anderson's model is verified for the 2D heterojunction systems and the success of Anderson's model is attributed to the absence of dangling bonds (i.e., interface dipoles) at the van der Waal interface. The results from the work set a foundation allowing the use of powerful Anderson's rule to determine the band alignments of 2D HJs, which is beneficial to future electronic, photonic, and optoelectronic devices.

Band Alignment of 2D Transition Metal Dichalcogenide Heterojunctions

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large-area electronics and circuits strongly relies on wafer-scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal-guided selective growth (MGSG), is reported. The success of control over the transition-metal-precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p- and n-type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom-up complementary metal-oxide-semiconductor inverter based on p-type WSe2 and n-type MoSe2 is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.

Metal-Guided Selective Growth of 2D Materials: Demonstration of a Bottom-Up CMOS Inverter.

Transition-metal dichalcogenide (TMDC) homo- and heterostacks hold tantalizing prospects for being integrated as active components in future van der Waals (vdW) electronics and optoelectronics How

Hao Ling Tang

Papers

Room-Temperature Ferroelectricity in Hexagonally Layered α-In2Se3 Nanoflakes down to the Monolayer Limit

Band Alignment of 2D Transition Metal Dichalcogenide Heterojunctions

Metal-Guided Selective Growth of 2D Materials: Demonstration of a Bottom-Up CMOS Inverter.

Strain-Directed Layer-By-Layer Epitaxy Toward van der Waals Homo- and Heterostructures