Shijun Yuan

Hopping transport through defect-induced localized states in molybdenum disulphide

Molybdenum disulphide is a novel two-dimensional semiconductor with potential applications in electronic and optoelectronic devices. However, the nature of charge transport in back-gated devices still remains elusive as they show much lower mobility than theoretical calculations and native n-type doping. Here we report a study of transport in few-layer molybdenum disulphide, together with transmission electron microscopy and density functional theory. We provide direct evidence that sulphur vacancies exist in molybdenum disulphide, introducing localized donor states inside the bandgap. Under low carrier densities, the transport exhibits nearest-neighbour hopping at high temperatures and variable-range hopping at low temperatures, which can be well explained under Mott formalism. We suggest that the low-carrier-density transport is dominated by hopping via these localized gap states. Our study reveals the important role of short-range surface defects in tailoring the properties and device applications of molybdenum disulphide.

Two-dimensional quasi-freestanding molecular crystals for high-performance organic field-effect transistors

Two-dimensional atomic crystals are extensively studied in recent years due to their exciting physics and device applications. However, a molecular counterpart, with scalable processability and competitive device performance, is still challenging. Here, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. The crystalline layers are atomically smooth and effectively decoupled from the substrate due to weak van der Waals interactions, affording a pristine interface for high-performance organic transistors. As a result, monolayer dioctylbenzothienobenzothiophene molecular crystal field-effect transistors on boron nitride show record-high carrier mobility up to 10 cm(2) V(-1) s(-1) and aggressively scaled saturation voltage ~1 V. Our work unveils an exciting new class of two-dimensional molecular materials for electronic and optoelectronic applications.

Te‐Doped Black Phosphorus Field‐Effect Transistors

Element doping allows manipulation of the electronic properties of 2D materials. Enhanced transport performances and ambient stability of black-phosphorus devices by Te doping are presented. This provides a facile route for achieving airstable black-phosphorus devices.

Uniformly Wetting Deposition of Co Atoms on MoS2 Monolayer: A Promising Two-Dimensional Robust Half-Metallic Ferromagnet

Synthesis of two-dimensional (2D) metal chalcogenide based half-metallic nanosheets is in high demand for modern electronics and spintronics applications. Herein, we predict from first-principles calculations that the 2D heterostructure Co/MoS2, consisting of a monolayer of Co atoms deposited on a single MoS2 sheet, possesses robust ferromagnetic and half-metallic features and exhibits 100% spin-filter efficiency within a broad bias range. Its ferromagnetic and half-metallic nature persists even when overlaid with a graphene sheet. Because of the relatively strong surface binding energy and low clustering ratio of Co atoms on the MoS2 surface, we predict that the heterostructure is synthesizable via wetting deposition of Co on MoS2 by electron-beam evaporation technique. Our work strongly suggests Co/MoS2 as a compelling and feasible candidate for highly effective information and high-density memory devices.

Towards a Comprehensive Understanding of the Reaction Mechanisms Between Defective MoS2 and Thiol Molecules

Sulfur vacancies (SVs) inherent in MoS2 are generally detrimental for carrier mobility and optical properties. Thiol chemistry has been explored for SV repair and surface functionalization. However, the resultant products and reaction mechanisms are still controversial. Herein, a comprehensive understanding on the reactions is provided by tracking potential energy surfaces and kinetic studies. The reactions are dominated by two competitive mechanisms that lead to either functionalization products or repair SVs, and the polarization effect from decorating thiol molecules and thermal effect are two determining factors. Electron-donating groups are conducive to the repairing reaction whereas electron-withdrawing groups facilitate the functionalization process. Moreover, the predominant reaction mechanism can be switched by increasing the temperature. This study fosters a way of precisely tailoring the electronic and optical properties of MoS2 by means of thiol chemistry approaches.

Surface Vacancy-Induced Switchable Electric Polarization and Enhanced Ferromagnetism in Monolayer Metal Trihalides

Monolayer chromium triiodide (CrI3), as the thinnest ferromagnetic material demonstrated in experiment [ Huang et al. Nature 2017 , 546 , 270 ], opens up new opportunities for the application of two-dimensional (2D) materials in spintronic nanodevices. Atom-thick 2D materials with switchable electric polarization are now urgently needed for their rarity and important roles in nanoelectronics. Herein, we unveil that surface I vacancies not only enhance the intrinsic ferromagnetism of monolayer CrI3 but also induce switchable electric polarization. I vacancies bring about an out-of-plane polarization without breaking the nonmetallic nature of CrI3. Meanwhile, the induced polarization can be reversed in a moderate energy barrier, arising from the unique porosity of CrI3 that contributes to the switch of I vacancies between top and bottom surfaces. Engineering 2D switchable polarization through surface vacancies is also applicable to many other metal trihalides, which opens up a new and general way toward pursuing low-dimensional multifunctional nanodevices.

Magnetic anisotropies in epitaxial Fe3O4/GaAs(100) patterned structures

Previous studies on epitaxial Fe3O4 rings in the context of spin-transfer torque effect have revealed complicated and undesirable domain structures, attributed to the intrinsic fourfold magnetocrystalline anisotropy in the ferrite. In this Letter, we report a viable solution to this problem, utilizing a 6-nm-thick epitaxial Fe3O4 thin film on GaAs(100), where the fourfold magnetocrystalline anisotropy is negligible. We demonstrate that in the Fe3O4 planar wires patterned from our thin film, such a unique magnetic anisotropy system has been preserved, and relatively simple magnetic domain configurations compared to those previous reports can be obtained.

Half-metallicity and enhanced ferromagnetism in Li-adsorbed ultrathin chromium triiodide

Two-dimensional materials with robust magnetism have been a long-sought goal because of their potential applications in information technology. Very recently, intrinsic ferromagnetic chromium triiodide (CrI3) was reported down to monolayer experimentally (B. Huang, et al., Nature, 2017, 546, 270). Herein, based on first-principles calculations, we show that the CrI3 monolayer can be switched from semiconducting to half-metallicity by lithium atom adsorption. The half-metallicity can be well preserved regardless of the Li concentration or the thickness of the CrI3 sheet. Moreover, the adsorption of Li atoms can further enhance the ferromagnetism of CrI3 sheets by increasing both the magnetic moment and the Curie temperature. The robust half-metallicity together with improved ferromagnetism may open up new promising applications of CrI3-based materials in spintronics.

Prediction of a room-temperature eight-coordinate two-dimensional topological insulator: penta-RuS 4 monolayer

We predict a stable eight-coordinate two-dimensional RuS4 monolayer, with a trilayer S–Ru–S and pentagonal rings tiling configuration by first-principles calculations. This monolayer exhibits unique anisotropic quadratic energy dispersion with two Dirac points emerging at the high-symmetric Γ point. When the spin-orbit coupling is included, a large nontrivial energy gap (70 meV) appears near the Fermi level. The topological nature of RuS4 monolayer is also confirmed by the nontrivial Z2 invariant and gapless edge states. A four-band tight-binding model is further proposed, which reveals that the topological states arise from the indirect interaction of the Ru-dxz and dyz orbitals.Computational materials: Topological insulator of 2D penta-RuS 4First-principles calculations predict a stable, atomically thin RuS4 crystal with eight-coordinate pentagonal structure. A team led by Jinlan Wang at Southeast University used ab-initio simulations based on density functional theory to design a RuS4 crystal consisting of a Ru layer sandwiched between two S layers, with eight-coordinate Ru atoms sitting at the center of square prisms of S atoms. Penta-RuS4 monolayers are energetically and dynamically stable, and exhibit anisotropic quadratic energy bands with two Dirac cones merging near the Fermi level. When the spin-orbit coupling effect is included in the band structure calculations owing to the fact that Ru is a heavy atom, a nontrivial topology emerges in the electronic structure, resulting in a bandgap opening of 70 meV. RuS4 enriches the family of atomically thin topological insulators with a new eight-coordinate compound.

Investigation on transformation of spindle-like Fe3O4 nanoparticles from self-assembling α-Fe2O3

Porous monodisperse spindle-like α-Fe2O3 nanomaterials are first synthesized successfully by a hydrothermal method, and then the as-prepared nanoparticles are annealed at different temperatures under various atmospheres to achieve the spindle-like Fe3O4 nanoparticles. The evolution of the features of nanoparticles, including the changes of the structures and microstructures as well as the magnetic properties, during the reduction process has been investigated by using the Raman spectrum and Mossbauer spectrum. Our research reveals that the α-Fe2O3 nanoparticles annealed by covering of the C powder become a mixture of α-Fe2O3 and Fe3O4 in the range of annealing temperature from 300 °C to 800 °C. With reduced atmospheric H2, spindle-like α-Fe2O3 nanoparticles are transferred to mixture of α-Fe2O3, Fe3O4 and Fe as temperature increases. They are also converted from a typical rhombohedral structure to a cubic α-Fe phase at 500 °C. Finally, with the atmosphere of H2/Ar (5%/95%), a pure Fe3O4 phase, and its exc...