Jiuxiang Dai

High-entropy atomic layers of transition-metal carbide (HE-MXene) are produced via selectively etching a high-entropy MAX phase, in which five transition-metal species are homogeneously dispersed into one MX slab, giving rise to stable transition-metal carbide in the atomic layers. The resultant HE-MXene has distinct lattice distortions and strains, efficiently guiding the nucleation and uniform growth of lithium.

Abstract

High-entropy materials (HEMs) have great potential for energy storage and conversion due to their diverse compositions, and unexpected physical and chemical features. However, high-entropy atomic layers with fully exposed active sites are difficult to synthesize since their phases are easily segregated. Here, it is demonstrated that high-entropy atomic layers of transition-metal carbide (HE-MXene) can be produced via the selective etching of novel high-entropy MAX (also termed M _{n +1}AX _n (n = 1, 2, 3), where M represents an early transition-metal element, A is an element mainly from groups 13–16, and X stands for C and/or N) phase (HE-MAX) (Ti_1/5V_1/5Zr_1/5Nb_1/5Ta_1/5)₂AlC, in which the five transition-metal species are homogeneously dispersed into one MX slab due to their solid-solution feature, giving rise to a stable transition-metal carbide in the atomic layers owing to the high molar configurational entropy and correspondingly low Gibbs free energy. Additionally, the resultant high-entropy MXene with distinct lattice distortions leads to high mechanical strain into the atomic layers. Moreover, the mechanical strain can efficiently guide the nucleation and uniform growth of dendrite-free lithium on HE-MXene, achieving a long cycling stability of up to 1200 h and good deep stripping–plating levels of up to 20 mAh cm⁻².

Ultra-small Pt particles are generated in situ on the surface of PtSe₂ nanosheet with Se vacancies for enhanced hydrogen evolution reaction.

Abstract

PtSe₂ is a typical noble metal dichalcogenide (NMD) that holds promising possibility for next-generation electronics and photonics. However, when applied in hydrogen evolution reaction (HER), it exhibits sluggish kinetics due to the insufficient capability of absorbing active species. Here, we construct PtSe₂/Pt heterointerface to boost the reaction dynamics of PtSe₂, enabled by an in situ electrochemical method. It is found that Se vacancies are induced around the heterointerface, reducing the coordination environment. Correspondingly, the exposed Pt atoms at the very vicinity of Se vacancies are activated, with enhanced overlap with H 1s orbital. The adsorption of H^. intermediate is thus strengthened, achieving near thermoneutral free energy change. Consequently, the as-prepared PtSe₂/Pt exhibits extraordinary HER activity even superior to Pt/C, with an overpotential of 42 mV at 10 mA cm⁻² and a Tafel slope of 53 mV dec⁻¹. This work raises attention on NMDs toward HER and provides insights for the rational construction of novel heterointerfaces.

The electrode material used in liquid cell transmission electron microscopy plays an important role in determining both electrochemical performance and image resolution. Through modeling and metal deposition experiments, it is shown that multilayer graphene acts as a suitable electrode for electrochemical liquid cell microscopy that enables quantitative and higher resolution electrochemical experiments.

Abstract

The combination of imaging with electrochemical quantification in liquid cell transmission electron microscopy (TEM) provides opportunities for visualizing material processes in liquid with good spatial and temporal resolution in a way that is inaccessible in bench-top electrochemical experiments. The electrode material used in liquid cell TEM determines the reliability and consistency of the electrochemical measurements and also influences the resolution when imaging processes on the electrode. Here, the opportunities arising from the use of 2D materials in liquid cell electrochemistry are explored. Through electrochemical imaging and modeling, it is demonstrated that the use of graphene electrodes enables quantitative study of electrochemical metal deposition and it is suggested that the minimal electron scattering and electric-field enhanced wettability are advantageous in obtaining interpretable and higher resolution data. It is anticipated that incorporation of 2D materials into electrode design will present new opportunities for investigating problems in crystal growth, energy storage, and electrocatalysis.

The rational design of wrinkle-less transfer of transition metal dichalcogenide monolayer via an adjustable wettability-assisted transfer (AWAT) method is demonstrated. With AWAT, the density of wrinkles can be decreased by ≈15–20% and carrier mobility can be enhanced 30 times compared with the conventional transfer method by the pure DI water.

Abstract

Transfer-induced wrinkles are universal issues when transferring transition metal dichalcogenide (TMDC) monolayer from an as-grown substrate to a target substrate. The undesired transfer-induced wrinkles can mainly be attributed to wettability, which refers to the ability of a liquid to come in contact with a solid surface. Herein, an adjustable wettability-assisted transfer (AWAT) method with different mixtures of transfer media to reduce the density of wrinkles is developed. By manipulating the wettability of the transfer medium with different ratios of alcohol and de-ionized (DI) water, the TMDC monolayer is smoothly attached to the target substrate, thus achieving a wrinkle-less transferred TMDC monolayer. With this method, the density of wrinkles can be decreased by ≈15–20% compared with the conventional transfer method by pure DI water. The transferred MoS₂ monolayer with the AWAT method can achieve enhanced carrier mobility from ≈20 to ≈35 cm² V⁻¹ s⁻¹ in average, which is 30 times larger than that transferred by pure DI water. The AWAT method applied to a WS₂ monolayer onto a SiO₂/p⁺-Si substrate and a MoS₂ monolayer onto a HfO₂/p⁺-Si substrate are demonstrated, which is beneficial in research and applications involving the transfer of TMDC monolayer.

This work synthesizes 2D Ge-TCNQ complex crystals with a thickness of 13.4 nm by a facile in-air sublimation growth. The 2D Ge-TCNQ crystals impressively depict decent PL characteristics, which can be further employed as active optical waveguides with a low optical loss coefficient of 0.078 dB µm⁻¹.

Abstract

2D emissive crystals have attracted tremendous research interests due to their outstanding compatibility with an integrated planar photonic system, which ideally meet the requirement for versatile photonic device applications, such as lasers, light-emitting devices, and optical waveguides. Among 2D emissive materials, metal-organic complex crystals are highly desirable because of their strong charge-transfer interactions and enhanced optical absorption that benefit superior luminescence efficiency. Here, the in-air sublimation method is adopted to synthesize 2D Ge-TCNQ microplate crystals (TCNQ, 7,7,8,8-tetracyanoquinodimethane) with thickness down to 13.4 nm. Such ultrathin Ge-TCNQ crystals exhibit an exciton binding energy of 40.7 meV and a decent photoluminescence (PL) quantum efficiency of 5.53%. The fitting slope of excitation power-dependent PL intensity displays a transition from linear to superlinear characteristics that reveals both trap-assisted recombination and free carrier recombination are present in the luminescence process. The energy band structure of Ge-TCNQ is examined by ultraviolet photoelectron spectroscopy and UV–vis spectroscopy to elucidate the physical mechanism of photo excitation and fluorescence processes. The emissive Ge-TCNQ crystals are further employed as high-performance optical waveguides with a small optical loss coefficient of 0.078 dB µm⁻¹. This work opens new avenues using 2D metal-organic complex crystals to develop advanced optoelectronic devices.

Assembling Ti₃C₂T _x nanosheets into films with well-defined microstructures and unique surface properties creates attractive physicochemical properties favorable for device design, which has emerged as a prevailing paradigm and expanded the scopes in functional film materials. This review provides a series of optimizing strategies about the competitive features of Ti₃C₂T _x -based films and its applications. In addition, prospects on the future development of Ti₃C₂T _x -based films are provided.

Abstract

2D titanium carbide (Ti₃C₂T _x ) MXene films, with their well-defined microstructures and chemical functionality, provide a macroscale use of nano-sized Ti₃C₂T _x flakes. Ti₃C₂T _x films have attractive physicochemical properties favorable for device design, such as high electrical conductivity (up to 20 000 S cm^–1), impressive volumetric capacitance (1500 F cm^–3), strong in-plane mechanical strength (up to 570 MPa), and a high degree of flexibility. Here, the appealing features of Ti₃C₂T _x -based films enabled by the layer-to-layer arrangement of nanosheets are reviewed. We devote attention to the key strategies for actualizing desirable characteristics in Ti₃C₂T _x -based functional films, such as high and tunable electrical conductivity, outstanding mechanical properties, enhanced oxidation-resistance and shelf life, hydrophilicity/hydrophobicity, adjustable porosity, and convenient processability. This review further discusses fundamental aspects and advances in the applications of Ti₃C₂T _x -based films with a focus on illuminating the relationship between the structural features and the resulting performances for target applications. Finally, the challenges and opportunities in terms of future research, development, and applications of Ti₃C₂T _x -based films are suggested. A comprehensive understanding of these competitive features and challenges shall provide guidelines and inspiration for the further development of Ti₃C₂T _x -based functional films, and contribute to the advances in MXene technology.

We study the electronic properties of the heterobilayer of vanadium and iron oxychlorides, VOCl and FeOCl, two layered air stable van der Waals insulating oxides with different types of antiferromagnetic order in bulk: VOCl monolayers are ferromagnetic (FM) whereas the FeOCl monolayers are antiferromagnetic (AF). We use density functional theory calculations, with Hubbard correction that is found to be needed to describe correctly the insulating nature of these compounds. We compute the magnetic anisotropy and propose a spin model Hamiltonian. Our calculations show that interlayer coupling is weak and hence the magnetic order of each monolayers is preserved in the heterobilayer. Thus, the heterobilayer combines antiferromagnetic and ferromagnetic orders. Interlayer exchange should lead both to exchange bias and to the emergence of hybrid collective modes that combine FM and AF magnons. The energy band of the heterobilayer show a type II band alignment, and feature spin-splitting...

Hexagonal boron nitride (h-BN) was synthesized by molecular beam epitaxy on polycrystalline Ni foils using borazine (B 3 N 3 H 6 ) as precursor. Our photoemission analysis shows that several components of boron and nitrogen are detected, suggesting the complex nature of the bonds noticeably at the h-BN/Ni interface. The BN thickness was estimated by photoemission and the BN distribution by time-of-flight secondary ion mass spectroscopy. Due to the catalytic effect of the Ni substrate, this thickness is self-limited in the range 1–2 layers regardless of the borazine dose. A spatially resolved photoemission study was carried out before and after transfer of the h-BN on a Si substrate. It shows that a strong electronic coupling exists at the interface between h-BN and polycrystalline Ni, not only for (111) grains, which disappears after transfer on Si. In addition, we highlight the importance of detecting π plasmons in the photoemission spectra to ...

In this paper, tin oxidation (SnO x )/tin-sulfide (SnS) heterostructures are synthesized by the post-oxidation of liquid-phase exfoliated SnS nanosheets in air. We comparatively analyzed the NO 2 gas response of samples with different oxidation levels to study the gas sensing mechanisms. The results show that the samples oxidized at 325 °C are the most sensitive to NO 2 gas molecules, followed by the samples oxidated at 350 °C, 400 °C and 450 °C. The repeatabilities of 350 °C samples are better than that of 325 °C, and there is almost no shift in the baseline. Thus this work systematically analyzed the gas sensing performance of SnO x /SnS-based sensor oxidized at 350 °C. It exhibits a high response of 171% towards 1 ppb NO 2 , a wide detecting range (from 1 ppb to 1 ppm), and an ultra-low theoretical detection limit of 5 ppt, and excellent repeatability at room temperature. The sensor also shows superior gas selec...

The transformation of the stripe domain structure of spontaneously-formed epitaxial silicene on ZrB 2 thin films into a single-domain driven by the adsorption of a fraction of a monolayer of silicon was used to investigate how dislocations react and eventually annihilate in a two-dimensional honeycomb structure. The in-situ real time scanning tunneling microscopy monitoring of the evolution of the domain structure after Si deposition revealed the mechanisms leading to the nucleation of a single-domain island into a domain structure through a stepwise reaction of partial dislocations. After its nucleation, the single-domain island extends by the propagation of edge dislocations at its frontiers. The identification of this particular nucleation-propagation formation of dislocation-free silicene sheet provides insights into how crystallographic defects can be healed in two-dimensional materials.

Twisted bilayers of two-dimensional materials, such as twisted bilayer graphene, often feature flat electronic bands that enable the observation of electron correlation effects. In this work, we study the electronic structure of twisted transition metal dichalcogenide homo- and heterobilayers that are obtained by combining MoS 2 , WS 2 , MoSe 2 and WSe 2 monolayers, and show how flat band properties depend on the chemical composition of the bilayer as well as its twist angle. We determine the relaxed atomic structure of the twisted bilayers using classical force fields and calculate the electronic band structure using a tight-binding model parametrized from first-principles density-functional theory. We find that the highest valence bands in these systems can derive either from Γ-point or K/ ##IMG## [http://ej.iop.org/images/2053-1583/8/4/045010/tdmac15d9ieqn1.gif] {$K^{^{\prime}}$} -point states of the constituent monola...

npj 2D Materials and Applications, Published online: 06 August 2021; doi:10.1038/s41699-021-00253-w

Dipole-induced Ohmic contacts between monolayer Janus MoSSe and bulk metals