zemin zheng

The Computational 2D Materials Database (C2DB) is a highly curated open database organising a wealth of computed properties for more than 4000 atomically thin two-dimensional (2D) materials. Here we report on new materials and properties that were added to the database since its first release in 2018. The set of new materials comprise several hundred monolayers exfoliated from experimentally known layered bulk materials, (homo)bilayers in various stacking configurations, native point defects in semiconducting monolayers, and chalcogen/halogen Janus monolayers. The new properties include exfoliation energies, Bader charges, spontaneous polarisations, Born charges, infrared polarisabilities, piezoelectric tensors, band topology invariants, exchange couplings, Raman spectra and second harmonic generation spectra. We also describe refinements of the employed material classification schemes, upgrades of the computational methodologies used for property evaluations, as well as signifi...

Nature Communications, Published online: 13 July 2021; doi:10.1038/s41467-021-24561-3

Magnetic Weyl semimetals in the 2D limit may behave like 2D Chern insulators and host the quantum anomalous Hall effect at high temperatures. Here, the authors report the observation of linearly dispersing topological states confined to the edges of the kagome Co3Sn terraces in the magnetic Weyl system Co3Sn2S2.

We demonstrate a method to induce tensile and compressive strain into two-dimensional transition metal dichalcogenide (TMDC) MoS 2 via the deposition of stressed thin films to encapsulate exfoliated flakes. With this technique we can directly engineer MoS 2 strain magnitude by changing deposited thin film stress, therefore allowing variable strain to be applied on a flake-to-flake level. These thin film stressors are analogous to SiN x based stressors implemented in industrial complementary metal-oxide-semiconductor (CMOS) processes to enhance Si mobility, suggesting that our concept is highly scalable and may be applied for large-scale integration of strain engineered TMDC devices. We choose optically transparent stressors to allow us to probe MoS 2 strain through Raman spectroscopy. Combining thickness dependent analyses of Raman peak shifts in MoS 2 with atomistic simulations, we can explore layer-by-layer strain tran...

We investigate, using a systematic computational approach, the possibility of the existence of two-dimensional quasicrystalline phases of binary metal-oxides. Our approach relies on the construction of the complete two-dimensional binary phase diagram through the use of unbiased global structural prediction methods. We then identify, in the low-energy periodic phases, structural elements that can be used to generate quasicrystalline phases through an inflation process. In this way we obtain chemically consistent two-dimensional quasicrystal approximants of both barium and titanium oxides. In the proposed structures, the metallic sites occupy the vertices of the aperiodic square-triangle tiling, while the oxygen atoms decorate the interior of the polygons. We then study the properties of the approximants, both free-standing and deposited on a metallic substrate. Finally, we discuss in which circumstances the formation of these phases seems to be favored.

In field-effect transistors with solid ionic conductors as the gate dielectric, the easy-axis of the ferromagnetism of Cr₂Ge₂Te₆ thin flakes can be uniformly tuned from the out-of-plane direction to the in-plane direction by an electric field, coinciding with a significant increase of the Curie temperature. The surface of the sample is fully exposed in this type of devices, making further heterostructure-engineering possible.

Abstract

The discovery of magnetism in 2D materials offers new opportunities for exploring novel quantum states and developing spintronic devices. In this work, using field-effect transistors with solid ion conductors as the gate dielectric (SIC-FETs), we have observed a significant enhancement of ferromagnetism associated with magnetic easy-axis switching in few-layered Cr₂Ge₂Te₆. The easy axis of the magnetization, inferred from the anisotropic magnetoresistance, can be uniformly tuned from the out-of-plane direction to an in-plane direction by electric field in the few-layered Cr₂Ge₂Te₆. Additionally, the Curie temperature, obtained from both the Hall resistance and magnetoresistance measurements, increases from 65 to 180 K in the few-layered sample by electric gating. Moreover, the surface of the sample is fully exposed in the SIC-FET device configuration, making further heterostructure-engineering possible. This work offers an excellent platform for realizing electrically controlled quantum phenomena in a single device.

The decoration of atomically thin metal flakes on two-dimensional (2D) material membranes to impart intriguing properties for applications such as sensors and catalysis has attracted tremendous interest. Here, we report the formation of atomically thin Mo nanoflakes on a molybdenum disulfide monolayer (ML-MoS 2 ) via a ‘self-feeding’ process using in situ transmission electron microscopy. Driven by energetic e-beam irradiation and thermal excitation, metallic Mo atoms preferentially segregate out and aggregate around mirror twin boundaries in the host MoS 2 ML, which then assemble into metallic nanoflakes: the associated dynamic process captured at the atomic scale. The Mo atoms constituting the nanoflakes tend to sit on the Mo-top and S-top sites if they are viewed as absorbed atoms with respect to the ML-MoS 2 substrate, which is further confirmed by theoretical calculations. Density functional theory calculations reveal that the entire syst...

The interface between two different semiconductors is crucial in determining the electronic properties at the heterojunction, therefore novel techniques that can probe these regions are of particular interest. Recently it has been shown that heterojunctions of two-dimensional transition metal dichalcogenides have sharp and epitaxial interfaces that can be used to the next generation of flexible and on chip optoelectronic devices. Here, we show that second harmonic generation (SHG) can be used as an optical tool to reveal these atomically sharp interfaces in different lateral heterostructures. We observed an enhancement of the SH intensity at the heterojunctions, and showed that is due to a coherent superposition of the SH emission from each material. This constructive interference pattern reveals a phase difference arising from the distinct second-order susceptibilities of both materials at the interface. Our results demonstrate that SHG microscopy is a sensitive characterizatio...

Among the large variety of two-dimensional (2D) materials discovered to date, elemental monolayers that host superconductivity are very rare. Using ab initio calculations we show that recently synthesized gallium monolayers, coined gallenene, are intrinsically superconducting through electron–phonon coupling. We reveal that Ga-100 gallenene, a planar monolayer isostructural with graphene, is the structurally simplest 2D superconductor to date, furthermore hosting topological edge states due to its honeycomb structure. Our anisotropic Eliashberg calculations show distinctly three-gap superconductivity in Ga-100, in contrast to the alternative buckled Ga-010 gallenene which presents a single anisotropic superconducting gap. Strikingly, the critical temperature ( T c ) of gallenene is in the range of 7–10 K, exceeding the T c of bulk gallium from which it is exfoliated. Finally we explore chemical functionalization of galle...

Nature Communications, Published online: 14 June 2021; doi:10.1038/s41467-021-23732-6

Heterobilayers of transition metal dichalcogenides host moiré superlattices that give rise to strong electron interactions. Here, the authors study the photoluminescence from interlayer excitons in a WS2/WSe2 heterobilayer to reveal the onset of various correlated insulating states.

Nature Communications, Published online: 18 June 2021; doi:10.1038/s41467-021-24019-6

Accurate control of the spatial location and the emission wavelength of single photon emitters (SPEs) in van der Waals materials is a crucial yet challenging endeavour. Here, the authors use an electron beam to generate SPE ensembles in high purity synthetic hBN with enhanced spatial accuracy and emission reproducibility.

Close to a magical angle, twisted bilayer graphene (TBLG) systems exhibit isolated flat electronic bands and, accordingly, strong electron localization. TBLGs have hence been ideal platforms to explore superconductivity, correlated insulating states, magnetism, and quantized anomalous Hall states in reduced dimension. Below a threshold twist angle (∼1.1 ∘ ), the TBLG superlattice undergoes lattice reconstruction, leading to a periodic moiré structure which presents a marked atomic corrugation. Using a tight-binding framework, this research demonstrates that superlattice reconstruction affects significantly the electronic structure of small-angle TBLGs. The first magic angle at ∼1.1 ∘ is found to be a critical case presenting globally maximized electron localization, thus separating reconstructed TBLGs into two classes with clearly distinct electronic properties. While low-energy Dirac fermions are still preserved at large twist angles ##IMG## {...}

The functional form of Coulomb interactions in the transition metal dichalcogenides (TDMs) and other van der Waals solids is critical to many of their unique properties, e.g. strongly-correlated electron states, superconductivity and emergent ferromagnetism. This paper presents measurements of key excitonic energy levels in MoSe 2 /WSe 2 heterostructures. These measurements are obtained from resonance Raman experiments on specific Raman peaks only observed at excited states of the excitons. This data is used to validate a model of the Coulomb potential in these structures which predicts the exciton energies to within ∼5 meV. This model is used to determine the effect of heterostructure formation on the single-particle band gaps of the layers and will have a wide applicability in designing the next generation of more complex TDM structures.