Xingxing Zhang

The dependence of the interlayer interaction on the stacking order of MoS 2 is investigated by Raman spectroscopy using three excitation energies of 2.81, 2.41, and 1.96 eV. The low-frequency Raman spectra show distinct correlation with the stacking type. The A-like intra-layer vibration mode at 405 cm −1 exhibit Davydov splitting due to interlayer interactions when the 1.96 eV excitation is used. The positions and the relative intensities of the Davydov-split peaks depend on the stacking type as well. By using the linear-chain model, the interlayer force constants are extracted and compared for 3-layer samples with different stacking types: the 3R-types have a larger force constant than the 2H-type. For the 2H-type, Davydov splitting is analyzed as a function of the number of layers. The force constants for the second-nearest-neighbor interaction are obtained and compared with other transition metal dichalcogenides.

We report the first observation of the non-magnetic Barkhausen effect in van der Waals layered crystals, specifically, during transitions between the T d and 1 T ′ phases in type-II Weyl semimetal MoTe 2 . Thinning down the MoTe 2 crystal from bulk material to about 25 nm results in a drastic strengthening of the hysteresis in the phase transition, with the difference in critical temperature increasing from ~40 K to more than 300 K. The Barkhausen effect appears for thin samples and the temperature range of the Barkhausen zone grows approximately linearly with reducing sample thickness, pointing to a surface origin of the phase pinning effects. The distribution of the Barkhausen jumps shows a power law behavior, with its critical exponent α   =  1.27, in good agreement with existing scaling theory. Temperature-dependent Raman spectroscopy on MoTe 2 crystals of various thicknesses shows results consistent with our transpor...

Chlorine-doped tungsten disulfide monolayer (1L-WS 2 ) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal’s electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this indicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning auger microscopy (SAM) and x-ray photoelectron s...

Growth of 2D materials is edge dependent and affected by a number of factors, such as substrate, H₂ and O₂ in the growth atmosphere, defects, coalescence of grains, etc. Mechanisms underlying the growth kinetics of 2D materials are systematically discussed, and the effects of various factors are summarized. In addition, progress on 2D materials' etching is also presented.

Abstract

During the last 10 years, remarkable achievements on the chemical vapor deposition (CVD) growth of 2D materials have been made, but the understanding of the underlying mechanisms is still relatively limited. Here, the current progress on the understanding of the growth kinetics of 2D materials, especially for their CVD synthesis, is reviewed. In order to present a complete picture of 2D materials' growth kinetics, the following factors are discussed: i) two types of growth modes, namely attachment‐limited growth and diffusion‐limited growth; ii) the etching of 2D materials, which offers an additional degree of freedom for growth control; iii) a number of experimental factors in graphene CVD synthesis, such as structure of the substrate, pressure of hydrogen or oxygen, temperature, etc., which are found to have profound effects on the growth kinetics; iv) double‐layer and few‐layer 2D materials' growth, which has distinct features different from the growth of single‐layer 2D materials; and v) the growth of polycrystalline 2D materials by the coalescence of a few single crystalline domains. Finally, the current challenges and opportunities in future 2D materials' synthesis are summarized.

Advanced Energy Materials, Volume 8, Issue 29, October 15, 2018.

Incorporating 2D materials into lithium/sodium metal anodes is an effective way to improve their cycle stability and safety performance. 2D materials can be used in components including composite anode preparation, separator modification, artificial solid electrolyte interphase, and solid‐state electrolyte fabrication.

Abstract

In view of their high theoretical specific capacity and low electrochemical potential, lithium/sodium metal anodes (MAs) have been revisited in recent years in the context of high‐energy‐density storage systems. However, the infinite volume change and the uncontrollable dendrite growth of MAs during cycles are obstructing the development of commercialization. Numerous strategies have been explored to surmount the barriers, including composite anode preparation, separator modification, artificial solid electrolyte interphase, and solid‐state electrolyte fabrication. With atomic thickness, two‐dimensional (2D) materials possess ultrahigh specific surface area, abundant surface chemistry, and high mechanical strength along with facile processibility. These properties render 2D materials useful in all components of lithium/sodium metal batteries, improving their cycle stability and safety performance of the whole system. Herein, current progress of 2D materials for MAs is summarized. Their limitations are also discussed. New perspectives and future directions for this area are also provided. With numerous new 2D materials discovered or underexplored, it is expected that 2D materials will find great opportunities in high‐energy‐density lithium/sodium metal batteries.

WS 2 -based photodetectors were fabricated by sputtering and electron beam irradiation (EBI), and the effect of EBI on the crystallization of WS 2 films was investigated. EBI at 1 kV energy for 1 min transformed the as-deposited amorphous structure of WS 2 film into a two-dimensional (2D) layered crystalline structure with high uniformity over a 50.8 mm diameter wafer. Additionally, EBI enhanced the photoelectrical properties of WS 2 -based photodetectors. The as-deposited WS 2 film yielded a responsivity of 0.10 mA · W −1 under 450 nm laser irradiation, but showed no response under 532 and 635 nm laser wavelengths. However, after 1 kV and 3 kV EBI of the WS 2 films, the responsivities under laser irradiation at 450, 532, and 635 nm were 0.36, 1.37, and 0.19 mA · W −1 , and 1.68, 2.45, and 1.09 mA · W −1 , respectively. The substrate temperatures after 1 min of 1 kV and 3 kV EBI were 102 °C and...

We present a comprehensive optical study of the excitonic Zeeman effects in transition metal dichalcogenide monolayers, which are discussed comparatively for selected materials: MoSe 2 , WSe 2 and WS 2 . We introduce a simple semi-phenomenological description of the magnetic field evolution of individual electronic states in fundamental sub-bands by considering three additive components: valley, spin and orbital terms. We corroborate the validity of the proposed description by inspecting the Zeeman-like splitting of neutral and charged excitonic resonances in absorption-type spectra. The values of all three terms are estimated based on the experimental data, demonstrating the significance of the valley term for a consistent description of magnetic field evolution of optical resonances, particularly those corresponding to charged states. The established model is further exploited for discussion of magneto-luminescence data. We propose an interpretation...

Advanced Materials, EarlyView.

Excitonic van der Waals semiconductors are an attractive building block for constructing on‐chip nanophotonic devices with unconventional functionalities. The recent advances in the understanding and fabrication of excitonic electroluminescent devices based on atomically thin transition metal dichalcogenides such as monolayer MoS₂ and WSe₂ are reviewed. Various strategies for realizing efficient excitonic emission are discussed.

Abstract

Ultrathin layers of van der Waals inorganic semiconductors represent a new class of excitonic materials with attractive light‐emitting properties. Recent observation of valley polarization, optically pumped lasing, exciton–polaritons, and single‐photon emission highlights the exciting prospects for two‐dimensional (2D) semiconductors for applications in novel photonic devices. Development of efficient and reliable light sources based on excitonic electroluminescence in 2D semiconductors is of fundamental importance toward the practical implementation of photonic devices. Achieving electroluminescence in these atomically thin layers requires unconventional device designs and in‐depth understanding of the carrier injection and transport mechanisms. Herein, various strategies for electrically generating excitons in 2D semiconducting transition metal dichalcogenides such as monolayer MoS₂ are reviewed and challenges and opportunities are outlined. Furthermore, novel device concepts such as tunable chiral emission, electrically driven quantum emission, and high‐frequency modulation are highlighted.

The transformation from semiconducting to metallic phase, accompanied by a structural transition in fluorinated WS₂ (FWS₂), is described. Fluorination enhances the stability of 1T FWS₂ and makes it energetically favorable. The arena beyond conventional methods is opened for the synthesis of the 1T phase of transition metal dichalcogenides, proven to be superior in many applications.

Abstract

The transformation from semiconducting to metallic phase, accompanied by a structural transition in 2D transition metal dichalcogenides has attracted the attention of the researchers worldwide. The unconventional structural transformation of fluorinated WS₂ (FWS₂) into the 1T phase is described. The energy difference between the two phases debugs this transition, as fluorination enhances the stability of 1T FWS₂ and makes it energetically favorable at higher F concentration. Investigation of the electronic and optical nature of FWS₂ is supplemented by possible band structures and bandgap calculations. Magnetic centers in the 1T phase appear in FWS₂ possibly due to the introduction of defect sites. A direct consequence of the phase transition and associated increase in interlayer spacing is a change in friction behavior. Friction force microscopy is used to determine this effect of functionalization accompanied phase transformation.

Transition metal ions (Mo, Co, W) self‐organize within a gelatin template into a lamellar‐nanostructured soft material (metallohydrogel). Subsequent carbonization at moderate temperatures in a reducing atmosphere (600 °C) yields ultrathin (≈10 nm) and large (≈100 μm) 2D transition metal carbide sheets with high conductivity and rich active sites, which are ideal for the hydrogen evolution reaction (HER).

Abstract

Low‐dimensional (0/1/2 dimension) transition metal carbides (TMCs) possess intriguing electrical, mechanical, and electrochemical properties, and they serve as convenient supports for transition metal catalysts. Large‐area single‐crystalline 2D TMC sheets are generally prepared by exfoliating MXene sheets from MAX phases. Here, a versatile bottom‐up method is reported for preparing ultrathin TMC sheets (≈10 nm in thickness and >100 μm in lateral size) with metal nanoparticle decoration. A gelatin hydrogel is employed as a scaffold to coordinate metal ions (Mo⁵⁺, W⁶⁺, Co²⁺), resulting in ultrathin‐film morphologies of diverse TMC sheets. Carbonization of the scaffold at 600 °C presents a facile route to the corresponding MoC _x , WC _x , CoC _x , and to metal‐rich hybrids (Mo_{2− x} W _x C and W/Mo₂C–Co). Among these materials, the Mo₂C–Co hybrid provides excellent hydrogen evolution reaction (HER) efficiency (Tafel slope of 39 mV dec⁻¹ and 48 mVj = 10 mA cm‐2 in overpotential in 0.5 m H₂SO₄). Such performance makes Mo₂C–Co a viable noble‐metal‐free catalyst for the HER, and is competitive with the standard platinum on carbon support. This template‐assisted, self‐assembling, scalable, and low‐cost manufacturing process presents a new tactic to construct low‐dimensional TMCs with applications in various clean‐energy‐related fields.

A MoS₂–upconversion‐nanoparticle (UCNP) nanocomposite is used to fabricate near‐infrared (NIR) modulated photonic resistive‐switching memory. The heterostructure between the MoS₂ and the UCNPs acting as excitons generation/separation centers remarkably improves the NIR‐light‐controlled memory performance. Meanwhile, the as‐fabricated photonic memory array also displays high integration with photodetectors, and can thus be used to make a core component of bioinspired vision systems.

Abstract

Photonic memories as an emerging optoelectronic technology have attracted tremendous attention in the past few years due to their great potential to overcome the von Neumann bottleneck and to improve the performance of serial computers. Nowadays, the decryption technology for visible light is mature in photonic memories. Nevertheless, near‐infrared (NIR) photonic memristors are less progressed. Herein, an NIR photonic memristor based on MoS₂–NaYF₄:Yb³⁺, Er³⁺ upconversion nanoparticles (UCNPs) nanocomposites is designed. Under excitation by 980 nm NIR light, the UCNPs show emissions well overlapping with the absorption band of the MoS₂ nanosheets. The heterostructure between the MoS₂ and the UCNPs acting as excitons generation/separation centers remarkably improves the NIR‐light‐controlled memristor performance. Furthermore, in situ conductive atomic force microscopy is employed to elucidate the photo‐modulated memristor mechanism. This work provides novel opportunities for NIR photonic memory that holds promise in future multifunctional robotics and electronic eyes.