zemin zheng

Density functional theory based calculations and experimental analysis on a limited number of real samples are performed to study how the presence of silver intercalated in the van der Waals gap of few-layer MoS 2 affects the low-frequency Raman active modes of this material. Silver is found to predominantly affect the breathing-like and shear-like vibrational modes of MoS 2 . These modes correspond to quasi-rigid movements of each individual layer with a restoring force (and, in turn, frequency) that is determined by modulations in the weak interlayer interactions. Noticeable red-shifts with increasing Ag concentration are found for all low-frequency modes. This finding indicates the potential for low-frequency vibrations as useful gauges for practical determination of silver concentration using low-frequency Raman spectroscopy. This work also describes a semi-classical linear chain model that allows to extrapolate results to a large number of layers. Further...

Twistronic van der Waals heterostrutures offer exciting opportunities for engineering optoelectronic properties of nanomaterials, in particular, due to the formation of moiré superlattice structures. In twisted bilayers of transition metal dichalcogenides moiré superlattice effects are additionally enriched by the lack of inversion symmetry in each monolayer unit cell. Here, we use multiscale modelling to establish a rich variety of confinement conditions for electrons, holes and layer-indirect excitons in twistronic WX 2 /MoX 2 bilayers (X = S,Se). Such trapping of charge carriers and excitons is caused by ferroelectric (interlayer) polarisation and piezoelectric effects generated by the reconstruction of twistronic bilayers into preferential stacking domains separated by domain wall networks. For almost aligned bilayers with anti-parallel (AP) orientation of WX 2 and MoX 2 unit cells, we find that upon lattice relaxation piezoelectric pot...

Next-generation spintronic devices will benefit from low-dimensionality, ferromagnetism, and half–metallicity, possibly controlled by electric fields. We find these technologically–appealing features to be combined with an exotic microscopic origin of magnetism in doped CdOHCl, a van der Waals material from which 2D layers may be exfoliated. By means of first principles simulations, we predict homogeneous hole–doping to give rise to p -band magnetism in both the bulk and monolayer phases and interpret our findings in terms of Stoner instability: as the Fermi level is tuned via hole–doping through singularities in the 2D-like density of states, ferromagnetism develops with large saturation magnetization of 1 µ B per hole, leading to a half-metallic behaviour for layer carrier densities of the order of 10 14 cm −2 . Furthermore, we put forward electrostatic doping as an additional handle to induce magnetism in monolayers and bila...

Fabricating van der Waals bilayer heterostructures (BL-HS) by stacking the same or different two-dimensional layers, offers a unique physical system with rich electronic and optical properties. Twist-angle between component layers has emerged as a remarkable parameter that can control the period of lateral confinement, and nature of the exciton (Coulomb bound electron–hole pair) in reciprocal space thus creating exotic physical states including moiré excitons (MXs). In this review article, we focus on opto-electronic properties of excitons in transition metal dichalcogenide semiconductor twisted BL-HS. We look at existing evidence of MXs in localized and strongly correlated states, and at nanoscale mapping of moiré superlattice and lattice-reconstruction. This review will be helpful in guiding the community as well as motivating work in areas such as near-field optical measurements and controlling the creation of novel physical states.

The rise of human–machine interaction and the internet of things technology requires the development of functionalized sensors that are mechanically flexible and fabulously wearable. Therefore, the emergence of new materials and devices is particularly important for technology design and development. Graphene has the atomically level thickness, mechanical flexibility, lightweights, and high conductivity and transparency. Especially, the large specific area of graphene enables the perception of external stimuli with high sensitivity, which is expected to be used in flexible sensor technology. In this review, we will introduce the research progress of graphene in flexible physical signal sensors, including the device structure design and the applications of these devices in wearable technology. We will overview the development of new directions of sensors, such as miniaturization, intelligence, and multi-modal. We will also focus on the latest technical progress of related sensing...

We study the stability and electronic structure of magic-angle twisted bilayer graphene on the hexagonal boron nitride (TBG/BN). Structural relaxation has been performed for commensurate supercells of the heterostructures with different twist angles ( ##IMG## [http://ej.iop.org/images/2053-1583/8/2/025025/tdmabddcbieqn1.gif] {$\theta^{^{\prime}}$} ) and stackings between TBG and BN. We find that the slightly misaligned configuration with ##IMG## [http://ej.iop.org/images/2053-1583/8/2/025025/tdmabddcbieqn2.gif] {$\theta^{^{\prime}} = 0.54^\circ$} and the AA/AA stacking has the globally lowest total energy due to the constructive interference of the moiré interlayer potentials and thus the greatly enhanced relaxation in its 1 × 1 commensurate supercell. Gaps are opened at the Fermi level ( E F ) for small supercells with the stackings that enable strong breaking of the C 2 symmetry in the atomic ...

Nature Communications, Published online: 02 February 2021; doi:10.1038/s41467-021-21042-5

A deep understanding of low-temperature transport properties of GeTe material remains a challenge. Here, the authors investigate phase-coherent phenomena in GeTe nanowire structures where the occurrence of magnetic flux-periodic oscillations come from the formation of a tubular hole accumulation layer.

Bilayer graphene (BLG) with 30°-twist (30°-tBLG) has been proven to possess a quasicrystal structure potentially providing novel applications. Despite the growth of BLG, especially the AB-stacking bilayer, has gained great attention, the growth of 30°-tBLG has been rarely achieved. Herein, for the first time, the decaborane-assisted synchronous growth of millimeter-sized single-crystalline 30°-tBLG was achieved on Cu foil by controlling the nucleation density and growth kinetics of graphene during chemical vapor deposition using diluted methane gas as the carbon source. The synchronous growth kinetics and decaborane-assisted co-catalysis mechanism are revealed by monitoring the growth process from the initial stage of graphene seeds to the millimeter-size scale. A 30°-tBLG based field effect transistor was fabricated and was found to possess a field-effect carrier mobility as high as 3671.3 cm 2 V −1 s −1 at room temperature. Thus, this work provide...

Understanding the fundamental steps of adsorption and controlled release of hydrogen in two-dimensional (2D) materials is of relevance for applications in nanoelectronics requiring tuning the physical properties or functionalization of the material, hydrogen storage and environmental sensors. Most applications demand that hydrogen adsorption and desorption can be controlled at room temperature. Here we report an element-specific study on the hydrogenation and dehydrogenation, in a low coverage regime, of a quasi-free standing 2D heterostructure (h-BNG) in the form of coexisting lateral domains of isostructural hexagonal boron nitride (h-BN) and graphene (Gr) on Pt(111). At very low hydrogen coverage a selective and partial hydrogenation of the Gr domains is observed in h-BNG. At the same time no changes are detected in the h-BN domains, indicating a preferential hydrogenation of Gr rather than h-BN domains. At higher coverage, hydrogenation of both Gr and h-BN domains is detecte...

We analyze the lineshape of the quasiparticle photoluminescence of monolayer (ML) and bilayer (BL) molybdenum ditelluride in temperature- and excitation intensity-dependent experiments. We confirm the existence of a negatively charged trion in the BL based on its emission characteristics and find hints for a coexistence of intra- and interlayer trions with a few meV splitting in energy. From the lineshape analysis of exciton and trion emission we extract values for exciton and trion deformation potentials as well as acoustical and optical phonon-limited mobilities in MoTe 2 . We estimate an acoustical phonon limited mobility of 6000 and 4300 cm 2  Vs −1 for the exciton at low temperature for ML and BL, respectively, which corresponds to an electron mobility of 10 5 cm 2  Vs −1 . At higher temperatures, the optical phonons limit the mobility to 1100 and 250 cm 2  Vs −1 for ML and BL.

Precise control of phase transitions in polymorphic 2D transition metal dichalcogenides (TMDs) is expected to play a key role in modern intelligent devices. However, an atomic-scale understanding and thus control of the phase transitions in the atomically-thin TMDs have not been reached, especially in some metastable phases. Here, in metastable monolayer 1T′ WS 2 , we demonstrate the dynamics of a phase transition nucleated from atomic defects by the means of time-resolved annular dark-field imaging and atomic-resolution electron energy-loss spectroscopy. It is found that the atomic and electronic structure of the 1T′ phase is inhomogeneous, which is decided by zone-dependent W–S bond strengths due to a Peierls-like structure distortion. Meanwhile, the W–S bonding is flexible to allow large nonequilibrium atom shifts for phase transition. Thus, just a few atomic defects can stabilize the atomic-scale nucleus of the new phase to initialize the phase transition from 1T′ t...

The two-dimensional layered material MoTe 2 has aroused extensive research interests in its rich optoelectronic properties in various phases. One property of particular interest is the circular photogalvanic effect (CPGE): a conventional second order nonlinear optical effect that is related to the chirality of materials. It has been demonstrated in T d -MoTe 2 , a type-II topological Weyl semimetal candidate, while it has been unclear so far whether it exists in the semimetallic 1T’ phase, another interesting phase that hosts a quantum spin hall state. In this article, we report a clear experimental observation of in-plane CPGE in 1T’-MoTe 2 . The observation is confirmed under various experimental designs with excitation by normally incident mid-infrared laser, and we find it to be related to an in-plane internal DC electric field. We attribute the circular photogalvanic response to a third-order nonlinear optical effect involving this DC el...

Nature Materials, Published online: 18 January 2021; doi:10.1038/s41563-021-00925-4

Author Correction: A general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte

Nature Communications, Published online: 27 January 2021; doi:10.1038/s41467-021-20919-9

Ubiquitous shadows cast from moving objects in hybrid energy-harvesting systems are undesirable as they degrade the performance of the photovoltaic cells. Here the authors report the shadow of the moving object in a hybrid energy-harvesting system shortens charging time to charge a self-charging power system.

Nature Communications, Published online: 15 January 2021; doi:10.1038/s41467-020-20621-2

Despite studies in FeSe1−xSx, it is yet unconfirmed whether nematic fluctuation can induce superconductivity. Here, the authors study single crystals of FeSe1−xTex showing enhanced superconductivity upon suppression of nematicity.

Atomic layers of black phosphorus (BP) present unique opto-electronic properties dominated by a direct tunable bandgap in a wide spectral range from visible to mid-infrared (IR). In this work, we investigate the IR photoluminescence (PL) of BP single crystals at very low temperature. Near-band-edge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier–Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons and free excitons in BP. As a result, the value of bulk BP bandgap is refined to 0.287 eV at 2 K.

Nature Communications, Published online: 08 January 2021; doi:10.1038/s41467-020-20497-2

Merons are a type of topological spin texture, with relevance for both fundamental and technological problems. In this theoretical work, Augustin et al. show that the van der Waals ferromagnetic CrCl3 can host merons and anti-merons, and explore the dynamics and interactions of these quasi-particles.