zemin zheng

Rechargeable batteries can effectively mitigate the increasing crisis associated with clean energy storage technologies. The alkali metal-ion based rechargeable batteries require a low diffusion barrier, a low average open-circuit voltage (OCV), and a high storage capacity for their superior performance. Using comprehensive first-principle calculations, we demonstrate that calcium carbide monolayer (Ca 2 C-ML) MXene meets all the aforementioned criteria and is a superior anode material for lithium (Li), sodium (Na), and potassium (K) metal-ion batteries. By first-principles calculations, the structural and electronic properties of Ca 2 C-ML and its extensive ion battery applications are studied. The adsorption properties of Li, Na, and K alkali ions on the Ca 2 C-ML sheet confirm excellent charge transfer and electrical conductivity. The ultra-low diffusion barriers of 0.027, 0.059, and 0.028 eV for Li, Na, and K alkali ions, respectively, indicate the...

Several Ising-type magnetic van der Waals (vdW) materials exhibit stable magnetic ground states. Despite these clear experimental demonstrations, a complete theoretical and microscopic understanding of their magnetic anisotropy is still lacking. In particular, the validity limit of identifying their one-dimensional Ising nature has remained uninvestigated in a quantitative way. Here we performed the complete mapping of magnetic anisotropy for a prototypical Ising vdW magnet FePS 3 for the first time. Combining torque magnetometry measurements with their magnetostatic model analysis and the relativistic density functional total energy calculations, we successfully constructed the three-dimensional mappings of the magnetic anisotropy in terms of magnetic torque and energy. The results not only quantitatively confirm that the easy axis is perpendicular to the ab plane, but also reveal the anisotropies within the ab, ac , and bc planes. Our approach can...

Semiconducting transition metal dichalcogenides have significant nonlinear optical effects. In this work we have used second-harmonic generation and the four-wave mixing spectroscopy in resonance with the excitons in MoS 2 , MoSe 2 , and WS 2 monolayers to characterize the nonlinear optical properties of these materials. We show that trions and excitons are responsible for enhancing the nonlinear optical response and determine the exciton and trion energies by comparing with the photoluminescence spectra. Moreover, we extract the second- and third-order optical sheet susceptibility ( χ (2) and χ (3) ) across exciton energies and compare with values found in the literature. We also demonstrate the ability to generate different nonlinear effects in a wide spectral range in the visible region for monolayer MoS 2 , opening the possibility of using two-dimensional materials for nonlinear optoelectronic and photoni...

Bismuth-based binary compounds, including Bi 2 Se 3 and Bi 2 Te 3 , have attracted increasing attention as well-known topological insulators. On the other hand, bismuth-based ternary compounds exhibit diverse properties, such as, ultrahigh carrier mobility, and strong Rashba spin splitting. Moreover, they boast of superior photocatalytic properties, implying great potential to be used in a wide range of applications. The unique structure and properties of two-dimensional (2D) materials, especially the extraordinary electronic and optical properties of 2D Bi 2 O 2 Se, have given rise to significant research interests for the exploration of 2D bismuth-based ternary compounds. In this review, we will comprehensively discuss the properties of three important families of bismuth-based ternary compounds, including Bi 2 O 2 X (X = S, Se, Te), BiTeX (X = Cl, Br, I), and BiOX (X = Cl, Br, I). In particular, we have p...

Nature Communications, Published online: 30 March 2021; doi:10.1038/s41467-021-22233-w

The relationship between Mott state and charge density wave state in two dimensional materials remains unclear. Here, Liu et al. reveal spatial distribution of a Mott-Hubbard band in monolayer 1T-NbSe2 forming a new periodic pattern in addition to the well-known CDW pattern.

Hybridisation of electronic bands of two-dimensional materials, assembled into twistronic heterostructures, enables one to tune their optoelectronic properties by selecting conditions for resonant interlayer hybridisation. Resonant interlayer hybridisation qualitatively modifies the excitons in such heterostructures, transforming these optically active modes into superposition states of interlayer and intralayer excitons. For MoSe 2 /WSe 2 heterostructures, strong hybridization of both single particle and excitonic states can occur via single particle tunnelling. Here we use resonance Raman scattering to provide direct evidence for the hybridisation of excitons in twistronic MoSe 2 /WSe 2 structures, by observing scattering of specific excitons by phonons in both WSe 2 and MoSe 2 . We also demonstrate that resonance Raman scattering spectroscopy opens up a wide range of possibilities for quantifying the layer composition o...

Monolayers of hexagonal boron nitride (hBN) are grown on graphite substrates using high-temperature molecular beam epitaxy (HT-MBE). The hBN monolayers are observed to grow predominantly from step edges on the graphite surface and exhibit a strong dependence of the morphology, including the dominant crystallographic edge, of the hBN monolayers, on the growth temperature, as well as systematic variations in growth rate and coverage, and significant differences in the growth at monolayer and multilayer graphite steps. At graphite monolayer steps hBN grows laterally across the surface on the lower terrace, but hBN growth on the upper side of the graphite step is more limited and is nucleated by three-dimensional clusters. Multilayer graphite steps exhibit a much higher density of non-planar hBN aggregates and growth on both the upper and lower terraces occurs. The results show that the hBN monolayer growth edge type, hBN island shape and the presence of hBN aggregates can be contro...

Nature Communications, Published online: 12 March 2021; doi:10.1038/s41467-021-21792-2

Here, the authors use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at domain walls separating micron-scale AB and BA stacking regions in twisted bilayer graphene, and observe hyperbolic enhancement of the photocurrent pattern.

Nature Communications, Published online: 12 March 2021; doi:10.1038/s41467-021-21013-w

The valley Hall effect in 2D materials is a promising approach for future valleytronic applications, but it is usually based on excitons with short lifetimes. Here, spin polarized electrons are injected from WTe2 into MoS2, leading to a unipolar valley Hall effect with enhanced lifetimes and mobility.

Nature Communications, Published online: 12 March 2021; doi:10.1038/s41467-021-21862-5

Here, the authors use a nanoscale probe to study the photoresponse within a single moiré unit cell of minimally twisted bilayer graphene, and observe an intricate photo-thermoelectric response attributed to the Seebeck coefficient variation at AB-BA domain boundaries.

We report on van der Waals epitaxy of two-dimensional (2D) molybdenum trioxide (MoO 3− x ) with monolayer thickness directly grown on highly oriented pyrolytic graphite by thermal evaporation under ultrahigh vacuum. The chemical composition, electronic and crystalline lattice structures of the mono-and few-layer MoO 3− x sheets are analysed. Using scanning tunnelling microscopy and spectroscopy, we investigate the electronic properties of MoO 3− x as a function of the number of layers and measure the apparent energy gap to be 0.4 eV for the first three layers of MoO 3− x on graphite. We carried out density functional theory calculations to shed light on the mechanism underlying the observed narrow bandgap with oxygen deficiency. Moreover, the air exposure effect on monolayer MoO 3− x is investigated confirming that the apparent bandgap closes, and additionally we show the reduction of the work function...

Accessing the metastable phases in a controlled fashion can further expand the applications of atomically thin transition metal dichalcogenides (TMDs). Although top-down approaches based on ion intercalation exfoliation have shown to be an effective route to transform 2H phase into 1T and/or 1T′ polytype phases, a bottom-up growth strategy could be more suitable for device integration. Herein, we show that by assisting the atmospheric pressure chemical vapor deposition (APCVD) growth with a specific alkali metal halide (AMH), it possible to induce the direct synthesis of 1T phase domains coexisting with 2H phase structure in micrometer-sized MoS 2 monolayer flakes. The photoluminescence emission and structural properties of three different AMH (NaCl, KBr and KCl) MoS 2 crystals are compared. Both NaCl and KBr assisted MoS 2 monolayers displayed the semiconducting 2H-phase. On the other hand, we demonstrate that KCl promotes the formation of a 1T–2H ...