Fucai Liu

High-sensitivity photodetectors based on multilayer GaTe flakes

Optoelectronic devices based on layered materials such as graphene have resulted in significant interest due to their unique properties and potential technological applications. The electric and optoelectronic properties of nano GaTe flakes as layered materials are described in this article. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of about 0.2 cm(2) V(-1) s(-1). The gate transistor exhibits a high photoresponsivity of 10(4) A/W, which is greatly better than that of graphene, MoS2, and other layered compounds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.

Chemical Vapor Deposition of High-Quality and Atomically Layered ReS2

Recently, anisotropic 2D materials, such as black phosphorus and rhenium disulfides (ReS2 ), have attracted a lot attention because of their unique applications on electronics and optoelectronics. In this work, the direct growth of high-quality ReS2 atomic layers and nanoribbons has been demonstrated by using chemical vapor deposition (CVD) method. A possible growth mechanism is proposed according to the controlled experiments. The CVD ReS2-based filed-effect transistors (FETs) show n-type semiconducting behavior with a current on/off ratio of ≈10(6) and a charge carrier mobility of ≈9.3 cm(2) Vs(-1). These results suggested that the quality of CVD grown ReS2 is comparable to mechanically exfoliated ReS2, which is also further supported by atomic force microscopy imaging, high-resolution transmission electron microscopy imaging and thickness-dependent Raman spectra. The study here indicates that CVD grown ReS2 may pave the way for the large-scale fabrication of ReS2-based high-performance optoelectronic devices, such as anisotropic FETs and polarization detection.

Controlled Synthesis of High-Quality Monolayered α-In2Se3 via Physical Vapor Deposition

In this work, we have demonstrated the synthesis of high-quality monolayered α-In2Se3 using physical vapor deposition method under atmospheric pressure. The quality of the In2Se3 atomic layers has been confirmed by complementary characterization technologies such as Raman/photoluminescence spectroscopies and atomic force microscope. The atomically resolved images have been obtained by the annular dark-field scanning transmission electron microscope. The field-effect transistors have been fabricated using the atomically layered In2Se3 and exhibit p-type semiconducting behaviors with the mobility up to 2.5 cm(2)/ Vs. The In2Se3 layers also show a good photoresponsivity of 340A/W, as well as 6 ms response time for the rise and 12 ms for the fall. These results make In2Se3 atomic layers a promising candidate for the optoelectronic and photosensitive device applications.

Monolayers of WxMo1−xS2 alloy heterostructure with in-plane composition variations

We report the fabrication of single-crystal monolayer WxMo1−xS2 alloy triangles using chemical vapor deposition method. Raman and photoluminescence property are investigated in correlation to the composition. In the monolayer triangles, the photoluminescence peak shifts continuously from 687.4 nm at the triangle center to 633.6 nm at the edge, corresponding to a switch from MoS2 to WS2 across the heterojunction. This composition-graded alloy may have interesting functions in broadband photodetection and multi-color light emission.

Large-Area and High-Quality 2D Transition Metal Telluride

Large-area and high-quality 2D transition metal tellurides are synthesized by the chemical vapor deposition method. The as-grown WTe2 maintains two different stacking sequences in the bilayer, where the atomic structure of the stacking boundary is revealed by scanning transmission electron microscopy. The low-temperature transport measurements reveal a novel semimetal-to-insulator transition in WTe2 layers and an enhanced superconductivity in few-layer MoTe2 .

Room-temperature ferroelectricity in CuInP2S6 ultrathin flakes

Two-dimensional (2D) materials have emerged as promising candidates for various optoelectronic applications based on their diverse electronic properties, ranging from insulating to superconducting. However, cooperative phenomena such as ferroelectricity in the 2D limit have not been well explored. Here, we report room-temperature ferroelectricity in 2D CuInP2S6 (CIPS) with a transition temperature of ∼320 K. Switchable polarization is observed in thin CIPS of ∼4 nm. To demonstrate the potential of this 2D ferroelectric material, we prepare a van der Waals (vdW) ferroelectric diode formed by CIPS/Si heterostructure, which shows good memory behaviour with on/off ratio of ∼100. The addition of ferroelectricity to the 2D family opens up possibilities for numerous novel applications, including sensors, actuators, non-volatile memory devices, and various vdW heterostructures based on 2D ferroelectricity.

Metal–Semiconductor Phase‐Transition in WSe2(1‐x)Te2x Monolayer

A metal-semiconductor phase transition in a ternary transition metal dichalcogenide (TMD) monolayer is achieved by alloying Te into WSe2 (WSe2(1-x) Te2x , where x = 0%-100%). The optical bandgaps of the WSe2(1-x) Te2x monolayer can be tuned from 1.67 to 1.44 eV (2H semiconductor) and drops to 0 eV (1Td metal), which opens up an exciting opportunity in functional electronic/optoelectronic devices.

High-quality monolayer superconductor NbSe 2 grown by chemical vapour deposition

The discovery of monolayer superconductors bears consequences for both fundamental physics and device applications. Currently, the growth of superconducting monolayers can only occur under ultrahigh vacuum and on specific lattice-matched or dangling bond-free substrates, to minimize environment- and substrate-induced disorders/defects. Such severe growth requirements limit the exploration of novel two-dimensional superconductivity and related nanodevices. Here we demonstrate the experimental realization of superconductivity in a chemical vapour deposition grown monolayer material—NbSe2. Atomic-resolution scanning transmission electron microscope imaging reveals the atomic structure of the intrinsic point defects and grain boundaries in monolayer NbSe2, and confirms the low defect concentration in our high-quality film, which is the key to two-dimensional superconductivity. By using monolayer chemical vapour deposited graphene as a protective capping layer, thickness-dependent superconducting properties are observed in as-grown NbSe2 with a transition temperature increasing from 1.0 K in monolayer to 4.56 K in 10-layer.Two-dimensional superconductors will likely have applications not only in devices, but also in the study of fundamental physics. Here, Wang et al. demonstrate the CVD growth of superconducting NbSe2 on a variety of substrates, making these novel materials increasingly accessible.

High Mobility 2D Palladium Diselenide Field‐Effect Transistors with Tunable Ambipolar Characteristics

Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe2 ), a noble-transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field-effect transistors (FETs) based on ultrathin PdSe2 show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe2 to exhibit electron-dominated transport with high mobility (µe (max) = 216 cm2 V-1 s-1 ) and on/off ratio up to 103 . Hole-dominated-transport PdSe2 can be obtained by molecular doping using F4 -TCNQ. This pioneer work on PdSe2 will spark interests in the less explored regime of noble-TMDCs.

3R MoS2 with Broken Inversion Symmetry: A Promising Ultrathin Nonlinear Optical Device

Nonlinear 2D layered crystals provide ideal platforms for applications and fundamental studies in ultrathin nonlinear optical (NLO) devices. However, the NLO frequency conversion efficiency constrained by lattice symmetry is still limited by layer numbers of 2D crystals. In this work, 3R MoS2 with broken inversion symmetry structure are grown and proved to be excellent NLO 2D crystals from monolayer (0.65 nm) toward bulk-like (300 nm) dimension. Thickness and wavelength-dependent second harmonic generation spectra offer the selection rules of appropriate working conditions. A model comprising of bulk nonlinear contribution and interface interaction is proposed to interpret the observed nonlinear behavior. Polarization enhancement with two petals along staggered stacking direction appears in 3R MoS2 is first observed and the robust polarization of 3R MoS2 crystal is caused by the retained broken inversion symmetry. The results provide a new arena for realizing ultrathin NLO devices for 2D layered materials.