Xuexia He

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.

Control of Radiative Exciton Recombination by Charge Transfer Induced Surface Dipoles in MoS2 and WS2 Monolayers.

Due to the two dimensional confinement of electrons in a monolayer of 2D materials, the properties of monolayer can be controlled by electrical field formed on the monolayer surface. F4TCNQ was evaporated on MoS2 and WS2 monolayer forming dipoles between strong acceptor, F4TCNQ, and monolayers of MoS2 or WS2. The strong acceptor attracts electrons (charge transfer) and decreases the number of the ionized excitons. Free excitons undergo radiative recombination in both MoS2 and WS2. Moreover, the photoluminescence enhancement is stronger in WS2 where the exciton-phonon coupling is weaker. The theoretical model indicates that the surface dipole controls the radiative exciton recombination and enhances photoluminescence radiation. Deposition of F4TCNQ on the 2D monolayers enables a convenient control of the radiative exciton recombination and leads to the applications of these materials in lasers or LEDs.

Highly Compressible Carbon Sponge Supercapacitor Electrode with Enhanced Performance by Growing Nickel–Cobalt Sulfide Nanosheets

The development of compressible supercapacitor highly relies on the innovative design of electrode materials with both superior compression property and high capacitive performance. This work reports a highly compressible supercapacitor electrode which is prepared by growing electroactive NiCo2S4 (NCS) nanosheets on the compressible carbon sponge (CS). The strong adhesion of the metallic conductive NCS nanosheets to the highly porous carbon scaffolds enable the CS-NCS composite electrode to exhibit an enhanced conductivity and ideal structural integrity during repeated compression-release cycles. Accordingly, the CS-NCS composite electrode delivers a specific capacitance of 1093 F g-1 at 0.5 A g-1 and remarkable rate performance with 91% capacitance retention in the range of 0.5-20 A g-1. Capacitance performance under the strain of 60% shows that the incorporation of NCS nanosheets in CS scaffolds leads to over five times enhancement in gravimetric capacitance and 17 times enhancement in volumetric capacitance. These performances enable the CS-NCS composite to be one of the promising candidates for potential applications in compressible electrochemical energy storage devices.

δ-MnO2 nanofiber/single-walled carbon nanotube hybrid film for all-solid-state flexible supercapacitors with high performance

δ-MnO2 ultralong nanofibres with smooth surface, good dispersion and a large length-to-width ratio were first controllably prepared by hydrothermally treating δ-MnO2 with low crystallinity at 150 °C for 20 h in 1 M NaOH. δ-MnO2 nanofiber/single-walled carbon nanotube (SWCNT) hybrid film electrodes with excellent flexibility and high capacitance were obtained between the δ-MnO2 nanofibre and SWCNTs via a simple vacuum filtration method. The formed regular fibrous network structure and a synergistic effect between the SWCNTs and δ-MnO2 nanofiber make the δ-MnO2/SWCNT-15 hybrid film electrode show a larger areal specific capacitance of 964 mF cm−2 at a current density of 1 mA cm−2 and excellent capacity retention of 81% when the current density was increased from 1 to 10 mA cm−2. By using δ-MnO2/SWCNT-15 film electrodes, a flexible all-solid-state δ-MnO2/SWCNT-15 hybrid supercapacitor was assembled by sandwiching the PVA–KOH solid-state electrolyte, which not only showed good flexibility and mechanical properties, but also gave a high energy density of 31.8 μW h cm−2 at a power density of 0.815 mW cm−2. δ-MnO2 nanofibre/SWCNTs hybrid film flexible supercapacitors have great potential in future portable and wearable electronic applications due to their excellent flexibility, high energy density, good cycle life and coulombic efficiency.

MoS2/Rubrene van der Waals Heterostructure: Toward Ambipolar Field‐Effect Transistors and Inverter Circuits

2D transition metal dichalcogenides are promising channel materials for the next-generation electronic device. Here, vertically 2D heterostructures, so called van der Waals solids, are constructed using inorganic molybdenum sulfide (MoS2 ) few layers and organic crystal - 5,6,11,12-tetraphenylnaphthacene (rubrene). In this work, ambipolar field-effect transistors are successfully achieved based on MoS2 and rubrene crystals with the well balanced electron and hole mobilities of 1.27 and 0.36 cm2 V-1 s-1 , respectively. The ambipolar behavior is explained based on the band alignment of MoS2 and rubrene. Furthermore, being a building block, the MoS2 /rubrene ambipolar transistors are used to fabricate CMOS (complementary metal oxide semiconductor) inverters that show good performance with a gain of 2.3 at a switching threshold voltage of -26 V. This work paves a way to the novel organic/inorganic ultrathin heterostructure based flexible electronics and optoelectronic devices.

Epitaxial growth of large-area and highly crystalline anisotropic ReSe2 atomic layer

The anisotropic two-dimensional (2D) layered material rhenium disulfide (ReSe2) has attracted considerable attention because of its unusual properties and promising applications in electronic and optoelectronic devices. However, because of its low lattice symmetry and interlayer decoupling, anisotropic growth and out-of-plane growth occur easily, yielding thick flakes, dendritic structure, or flower-like structure. In this study, we demonstrated a bottom-up method for the controlled and scalable synthesis of ReSe2 by van der Waals epitaxy. To achieve controllable growth, a micro-reactor with a confined reaction space was constructed by stacking two mica substrates in the chemical vapor deposition system. Within the confined reaction space, the nucleation density and growth rate of ReSe2 were significantly reduced, favoring the large-area synthesis of ReSe2 with a uniform monolayer thickness. The morphological evolution of ReSe2 with growth temperature indicated that the anisotropic growth was suppressed at a low growth temperature (<600 °C). Field-effect transistors employing the grown ReSe2 exhibited p-type conduction with a current ON/OFF ratio up to 105 and a hole carrier mobility of 0.98 cm2/(V·s). Furthermore, the ReSe2 device exhibited an outstanding photoresponse to near-infrared light, with responsivity up to 8.4 and 5.1 A/W for 850- and 940-nm light, respectively. This work not only promotes the large-scale application of ReSe2 in high-performance electronic devices but also clarifies the growth mechanism of low-lattice symmetry 2D materials.

Recent advances in ternary two-dimensional materials: synthesis, properties and applications

Two-dimensional (2D) materials have gained significant attention owing to their unique physical and chemical properties, which arise mainly from their high surface–bulk ratios and topological effects. Since the discovery of graphene in 2004, the family of 2D materials has expanded rapidly. Thus far, several single-element 2D materials (graphene, phosphorene, etc.) have been reported; the majority of them contain two (MoS2, WSe2, etc.) or more elements (Mo2CTx, CrPS4, Bi2Sr2CaCu2Ox, etc.). Of these, three-element 2D materials, also called ternary 2D materials, represent a rather attractive direction of recent years. Typical ternary 2D materials include metal phosphorous trichalcogenides (MPTs), ternary transition metal chalcogenides (TMDs), transition metal carbides and nitrides (MXenes) and 2D ternary oxides. Ternary 2D systems result in multiple degrees of freedom to tailor their physical properties via stoichiometric variation. Moreover, they exhibit some properties not characteristic of binary 2D systems, such as band gap tuning. In this paper, we have reviewed the recent progress in various ternary 2D materials on the basis of their classification (MPTs, ternary 2D MXenes, ternary TMDs, BCN and other ternary 2D materials). The synthesis methods, structures, key properties (such as band gap tuning, phase transition and topological phase), and their applications, are summarized. In addition, the strategies to tackle challenges, as well as the outlooks of this field, are presented.

CoNi2S4 Nanoparticle/Carbon Nanotube Sponge Cathode with Ultrahigh Capacitance for Highly Compressible Asymmetric Supercapacitor

Compared with other flexible energy-storage devices, the design and construction of the compressible energy-storage devices face more difficulty because they must accommodate large strain and shape deformations. In the present work, CoNi2 S4 nanoparticles/3D porous carbon nanotube (CNT) sponge cathode with highly compressible property and excellent capacitance is prepared by electrodepositing CoNi2 S4 on CNT sponge, in which CoNi2 S4 nanoparticles with size among 10-15 nm are uniformly anchored on CNT, causing the cathode to show a high compression property and gives high specific capacitance of 1530 F g-1 . Meanwhile, Fe2 O3 /CNT sponge anode with specific capacitance of 460 F g-1 in a prolonged voltage window is also prepared by electrodepositing Fe2 O3 nanosheets on CNT sponge. An asymmetric supercapacitor (CoNi2 S4 /CNT//Fe2 O3 /CNT) is assembled by using CoNi2 S4 /CNT sponge as positive electrode and Fe2 O3 /CNT sponge as negative electrode in 2 m KOH solution. It exhibits excellent energy density of up to 50 Wh kg-1 at a power density of 847 W kg-1 and excellent cycling stability at high compression. Even at a strain of 85%, about 75% of the initial capacitance is retained after 10 000 consecutive cycles. The CoNi2 S4 /CNT//Fe2 O3 /CNT device is a promising candidate for flexible energy devices due to its excellent compressibility and high energy density.