CoMoO₄ Nanoparticles Anchored on Reduced Graphene Oxide Nanocomposites as Anodes for Long-Life Lithium-Ion Batteries

A self-assembled CoMoO₄ nanoparticles/reduced graphene oxide (CoMoO₄NP/rGO), was prepared by a hydrothermal method to grow 3–5 nm sized CoMoO₄ particles on reduced graphene oxide sheets and used as an anode material for lithium-ion batteries. The specific capacity of CoMoO₄NP/rGO anode can reach up to 920 mAh g^–1 at a current rate of 74 mA g^–1 in the voltage range between 3.0 and 0.001 V, which is close to the theoretical capacity of CoMoO₄ (980 mAh g^–1). The fabricated half cells also show good rate capability and impressive cycling stability with 8.7% capacity loss after 600 cycles under a high current density of 740 mA g^–1. The superior electrochemical performance of the synthesized CoMoO₄NP/rGO is attributed to the synergetic chemical coupling effects between the conductive graphene networks and the high lithium-ion storage capability of CoMoO₄ nanoparticles

Experimental Determination of the Ionization Energies of MoSe₂, WS₂, and MoS₂ on SiO₂ Using Photoemission Electron Microscopy

The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe₂, WS₂, and MoS₂) on SiO₂ using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS₂, to WS₂, to MoSe₂, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron–hole separation in photovoltaics

Liquid Phase Exfoliation of Two-Dimensional Materials by Directly Probing and Matching Surface Tension Components

Exfoliation of two-dimensional (2D) materials into mono- or few layers is of significance for both fundamental studies and potential applications. In this report, for the first time surface tension components were directly probed and matched to predict solvents with effective liquid phase exfoliation (LPE) capability for 2D materials such as graphene, h-BN, WS₂, MoS₂, MoSe₂, Bi₂Se₃, TaS₂, and SnS₂. Exfoliation efficiency is enhanced when the ratios of the surface tension components of the applied solvent is close to that of the 2D material in question. We enlarged the library of low-toxic and common solvents for LPE. Our study provides distinctive insight into LPE and has pioneered a rational strategy for LPE of 2D materials with high yield

Chemical Vapor Deposition Growth of Crystalline Monolayer MoSe₂

Recently, two-dimensional layers of transition metal dichalcogenides, such as MoS₂, WS₂, MoSe₂, and WSe₂, have attracted much attention for their potential applications in electronic and optoelectronic devices. The selenide analogues of MoS₂ and WS₂ have smaller band gaps and higher electron mobilities, making them more appropriate for practical devices. However, reports on scalable growth of high quality transition metal diselenide layers and studies of their properties have been limited. Here, we demonstrate the chemical vapor deposition (CVD) growth of uniform MoSe₂ monolayers under ambient pressure, resulting in large single crystalline islands. The photoluminescence intensity and peak position indicates a direct band gap of 1.5 eV for the MoSe₂ monolayers. A back-gated field effect transistor based on MoSe₂ monolayer shows n-type channel behavior with average mobility of 50 cm² V^–1 s^–1, a value much higher than the 4–20 cm² V^–1 s^–1 reported for vapor phase grown MoS₂

Two-Step Growth of Two-Dimensional WSe₂/MoSe₂ Heterostructures

Two dimensional (2D) materials have attracted great attention due to their unique properties and atomic thickness. Although various 2D materials have been successfully synthesized with different optical and electrical properties, a strategy for fabricating 2D heterostructures must be developed in order to construct more complicated devices for practical applications. Here we demonstrate for the first time a two-step chemical vapor deposition (CVD) method for growing transition-metal dichalcogenide (TMD) heterostructures, where MoSe₂ was synthesized first and followed by an epitaxial growth of WSe₂ on the edge and on the top surface of MoSe₂. Compared to previously reported one-step growth methods, this two-step growth has the capability of spatial and size control of each 2D component, leading to much larger (up to 169 μm) heterostructure size, and cross-contamination can be effectively minimized. Furthermore, this two-step growth produces well-defined 2H and 3R stacking in the WSe₂/MoSe₂ bilayer regions and much sharper in-plane interfaces than the previously reported MoSe₂/WSe₂ heterojunctions obtained from one-step growth methods. The resultant heterostructures with WSe₂/MoSe₂ bilayer and the exposed MoSe₂ monolayer display rectification characteristics of a p–n junction, as revealed by optoelectronic tests, and an internal quantum efficiency of 91% when functioning as a photodetector. A photovoltaic effect without any external gates was observed, showing incident photon to converted electron (IPCE) efficiencies of approximately 0.12%, providing application potential in electronics and energy harvesting

3D Band Diagram and Photoexcitation of 2D–3D Semiconductor Heterojunctions

The emergence of a rich variety of two-dimensional (2D) layered semiconductor materials has enabled the creation of atomically thin heterojunction devices. Junctions between atomically thin 2D layers and 3D bulk semiconductors can lead to junctions that are fundamentally electronically different from the covalently bonded conventional semiconductor junctions. Here we propose a new 3D band diagram for the heterojunction formed between n-type monolayer MoS₂ and p-type Si, in which the conduction and valence band-edges of the MoS₂ monolayer are drawn for both stacked and in-plane directions. This new band diagram helps visualize the flow of charge carriers inside the device in a 3D manner. Our detailed wavelength-dependent photocurrent measurements fully support the diagrams and unambiguously show that the band alignment is type I for this 2D-3D heterojunction. Photogenerated electron–hole pairs in the atomically thin monolayer are separated and driven by an external bias and control the “on/off” states of the junction photodetector device. Two photoresponse regimes with fast and slow relaxation are also revealed in time-resolved photocurrent measurements, suggesting the important role played by charge trap states

Velcro-Inspired SiC Fuzzy Fibers for Aerospace Applications

The most recent and innovative silicon carbide (SiC) fiber ceramic matrix composites, used for lightweight high-heat engine parts in aerospace applications, are woven, layered, and then surrounded by a SiC ceramic matrix composite (CMC). To further improve both the mechanical properties and thermal and oxidative resistance abilities of this material, SiC nanotubes and nanowires (SiCNT/NWs) are grown on the surface of the SiC fiber via carbon nanotube conversion. This conversion utilizes the shape memory synthesis (SMS) method, starting with carbon nanotube (CNT) growth on the SiC fiber surface, to capitalize on the ease of dense surface morphology optimization and the ability to effectively engineer the CNT–SiC fiber interface to create a secure nanotube–fiber attachment. Then, by converting the CNTs to SiCNT/NWs, the relative morphology, advantageous mechanical properties, and secure connection of the initial CNT–SiC fiber architecture are retained, with the addition of high temperature and oxidation resistance. The resultant SiCNT/NW–SiC fiber can be used inside the SiC ceramic matrix composite for a high-heat turbo engine part with longer fatigue life and higher temperature resistance. The differing sides of the woven SiCNT/NWs act as the “hook and loop” mechanism of Velcro but in much smaller scale

Scalable Transfer of Suspended Two-Dimensional Single Crystals

Large-scale suspended architectures of various two-dimensional (2D) materials (MoS₂, MoSe₂, WS₂, and graphene) are demonstrated on nanoscale patterned substrates with different physical and chemical surface properties, such as flexible polymer substrates (polydimethylsiloxane), rigid Si substrates, and rigid metal substrates (Au/Ag). This transfer method represents a generic, fast, clean, and scalable technique to suspend 2D atomic layers. The underlying principle behind this approach, which employs a capillary-force-free wet-contact printing method, was studied by characterizing the nanoscale solid–liquid–vapor interface of 2D layers with respect to different substrates. As a proof-of-concept, a photodetector of suspended MoS₂ has been demonstrated with significantly improved photosensitivity. This strategy could be extended to several other 2D material systems and open the pathway toward better optoelectronic and nanoelectromechnical systems