zemin zheng

Boron nitride nanosheets (BNNS) are graphene-like materials with large bandgap and excellent thermal/chemical stability. Current BNNS synthesis methods show low yield and/or purity preventing effective implementation in real-life applications. This work reports two catalyst-free bottom-up approaches for BNNS synthesis using induction thermal plasma. High enthalpy and cooling rates of this plasma allow BNNS to form homogeneously when using solid ammonia borane (AB) as a precursor. In this case, clusters of B x N y H z nucleate to form particles of critical sizes on which BNNS propagate while releasing H 2 . Using boron powders instead of AB produces BNNS through a heterogeneous route. In this case, boron undergoes spheroidization while active nitrogen species diffuse on the liquid surface to form boron nitride nanowalls which propagate into BNNS. The operating pressure and nitrogen loading are shown to control BNNS n...

The miniaturization of ferroelectric devices offers prospects for non-volatile memories, low-power electrical switches and emerging technologies beyond existing Si-based integrated circuits. An emerging class of ferroelectrics is based on van der Waals (vdW) two-dimensional materials with potential for nano-ferroelectrics. Here, we report on ferroelectric semiconductor junctions (FSJs) in which the ferroelectric vdW semiconductor α -In 2 Se 3 is embedded between two single-layer graphene electrodes. In these two-terminal devices, the ferroelectric polarization of the nanometre-thick In 2 Se 3 layer modulates the transmission of electrons across the graphene/In 2 Se 3 interface, leading to memristive effects that are controlled by applied voltages and/or by light. The underlying mechanisms of conduction are examined over a range of temperatures and under light excitation revealing thermionic injection, tunnelling and ...

It becomes clear that, in two-dimensional (2D) materials-based devices, sheet resistances underneath electrodes change due to a metallic contact, leading to substantial errors in determining a transfer length. Thus, the extraction of transfer length and corresponding contact resistivity must be revisited to assess the performance of 2D devices. In this study, we present the three different approaches of determining the contact resistivity in 2D WSe 2 field effect transistors for the first time by theoretical analysis using the resistive network model as well as electrical measurements using the contact-end resistance and transfer length methods, based on the followings: (a) contact resistance multiplied by transfer length ( ##IMG## [http://ej.iop.org/images/2053-1583/8/4/045019/tdmac1adbieqn1.gif] {${R_{{\text{cf}}}}W \cdot {L_{{\text{Tk}}}}$} ), (b) integrated contact resistance ( ##IMG## {$\smallint\limits_0^L R \left( x \right) \cd...}

Nature Communications, Published online: 20 August 2021; doi:10.1038/s41467-021-25381-1

Expanding the available materials with moiré superlattices is interesting but also challenging. Here the authors use a one-step hydrothermal approach to synthesis WS2 moiré superlattices with high catalytic activity for hydrogen evolution reaction

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 ...

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...

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...

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...

The dynamics of suspended two-dimensional (2D) materials has received increasing attention during the last decade, yielding new techniques to study and interpret the physics that governs the motion of atomically thin layers. This has led to insights into the role of thermodynamic and nonlinear effects as well as the mechanisms that govern dissipation and stiffness in these resonators. In this review, we present the current state-of-the-art in the experimental study of the dynamics of 2D membranes. The focus will be both on the experimental measurement techniques and on the interpretation of the physical phenomena exhibited by atomically thin membranes in the linear and nonlinear regimes. We will show that resonant 2D membranes have emerged both as sensitive probes of condensed matter physics in ultrathin layers, and as sensitive elements to monitor small external forces or other changes in the environment. New directions for utilizing suspended 2D membranes for material characte...

Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe 2 monolayers (ML) encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS 2 ML at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening attributed to a much less pronounced halo. Spatially and spectrally resolved luminescence spectra reveal the buildup of a significant temperature gradient at high excitation power, that is attributed to the energy relaxation of photoinduced hot carriers. We show, via a numerical resolution of the transport equations for excitons and trions, that the halo can be interpreted as therma...

Nature Communications, Published online: 09 August 2021; doi:10.1038/s41467-021-25119-z

Lillianite materials usually have low thermoelectric efficiency due to the inherently inferior electrical properties. Here, the authors evaluate thermoelectric performances in Pb7Bi4Se13 based lillianites enabled by convergence of nested conduction bands.

Nature Communications, Published online: 10 August 2021; doi:10.1038/s41467-021-25164-8

Previous work has shown the existence of spin-orbit-entangled excitons and their coupling to antiferromagnetism in the correlated insulator NiPS3. Here the authors show that non-equilibrium driving of these excitons produces a transient metallic antiferromagnetic state that cannot be achieved by tuning the temperature in equilibrium.

Nature Communications, Published online: 16 August 2021; doi:10.1038/s41467-021-24925-9

Excitons in atomically thin crystals couple strongly with light. Here, the authors observe lattice polaritons in a tunable open optical cavity at room temperature, with an imprinted photonic lattice strongly coupled to excitons in a WS2 monolayer.

Nature Communications, Published online: 28 July 2021; doi:10.1038/s41467-021-24692-7

Ferromagnetic systems rarely display a large or non-saturating magnetoresistance, due to the low Fermi velocity of the predominant charge carrier. Here, the authors show that MnBi, a ferromagnet, bucks this trend, showing both large and non-saturating magnetoresistance, and high charge carrier motilities.

Scalable substitutional vanadium doping of 2D WSe₂ is achieved via MOCVD at FEOL and BEOL compatible temperatures. Detailed experimental and the first-principles calculations demonstrate that vanadium dopants introduce discrete acceptor levels that lie within the valance band. Transport measurements further confirm the p-type nature of the vanadium dopants where hole conduction is dominant with increasing the dopant concentration.

Abstract

Scalable substitutional doping of 2D transition metal dichalcogenides is a prerequisite to developing next-generation logic and memory devices based on 2D materials. To date, doping efforts are still nascent. Here, scalable growth and vanadium (V) doping of 2D WSe₂ at front-end-of-line and back-end-of-line compatible temperatures of 800 and 400 °C, respectively, is reported. A combination of experimental and theoretical studies confirm that vanadium atoms substitutionally replace tungsten in WSe₂, which results in p-type doping via the introduction of discrete defect levels that lie close to the valence band maxima. The p-type nature of the V dopants is further verified by constructed field-effect transistors, where hole conduction becomes dominant with increasing vanadium concentration. Hence, this study presents a method to precisely control the density of intentionally introduced impurities, which is indispensable in the production of electronic-grade wafer-scale extrinsic 2D semiconductors.

An on-surface reaction strategy is developed to integrate radicals with protective umbrellas on 2D layered material surfaces to construct stable and ordered radicals. For example, triethylamine reacts with dichloromethane to form quaternary ammonium salts with further Hofmann elimination to produce diethylmethyleneamine (DEMA) radicals, reacting with black phosphorus (BP) to produce P radicals with DEMA protective umbrellas integrated on BP surface.

Abstract

Radicals are closely related to human life and health and have been widely used in biology, chemistry, functional materials, etc. However, the high reactivity, disorder, and short half-lives limit their wide applications. Therefore, it remains a great challenge to prepare stable and ordered radicals. Herein, radicals are prepared with protective umbrellas (diethylmethyleneamine, DEMA) that are integrated on the surface of 2D layered materials to isolate water and oxygen and enhance the stability of radicals. Taking 2D black phosphorus (BP) as an example: triethylamine reacts with dichloromethane to form quaternary ammonium salts with further Hoffmann elimination to produce DEMA radicals that could react with one electron of a lone pair electrons in P on the surface of BP to produce P radicals, which shows a prolonged half-life of 21 days at room temperature. First-principle calculations and electron paramagnetic resonance fitting confirm that the steric hindrance constructed by dense DEMA passivation layer acts as a protective umbrella and the 2D coupling of P radicals and other P atoms in 2D BP plane to enhance the stability and strong superexchange interaction of P radicals. Furthermore, it is a general strategy to produce stable radicals integrated on the 2D plane.