高压下过渡金属二硫属化物的超导电性

过渡金属二硫属化物是一类典型的二维类石墨烯层状结构的材料,相比于石墨烯的全碳元素组成以及无带隙的电子结构特点,具有更丰富的元素组成、多样的微观结构和奇异的物理性质。过渡金属二硫属化物强烈的各向异性以及在催化、光伏器件和储能材料等领域的优异表现,引起了科学家们浓厚的研究兴趣。它们的层间范德瓦耳斯间隙、层间范德瓦耳斯相互作用、层间堆垛次序对压力非常敏感,易于通过压力调控其晶体结构和电子能带结构,进而发生电子基态的变化。过渡金属二硫属化物的电子基态可以是莫特绝缘体、激子绝缘体、电荷密度波、半导体、(拓扑)半金属、金属,甚至是超导体。在常压条件下,部分过渡金属二硫属化物具有超导电性。实验表明,压力可以诱导过渡金属二硫属化物非超导母体发生超导转变,或者提高超导母体的超导转变温度。文章以典型的过渡金属二硫属化物为例,概述了其在高压调控下超导电性的响应,并简要讨论产生超导电性的物理机制。     

二维过渡金属二硫化物中自旋能谷耦合的谷电子学

电子的电荷自由度与自旋自由度是现代电子器件的基础核心之一。随着二维材料,尤其是二维过渡族硫化物(TMDCs)的研究深入,另一个自由度——能谷——也引起了人们极大的研究兴趣。由于TMDCs中自旋与能谷的强耦合,自旋(能谷)可以通过能谷(自旋)方便地进行调控和探测,为电子自旋和能谷的相关领域提供了新的手段和方法。文章首先对能谷自由度以及TMDCs中自旋与能谷的强耦合进行了介绍,然后介绍基于圆偏振光激发和自旋注入两种方式进行的自旋调控和探测的理论和实验工作,最后对基于能谷的自旋调控进行了总结和展望。     

Electronic and optical properties of van der Waals vertical heterostructures based on two-dimensional transition metal dichalcogenides: First-principles calculations

Four vertical heterostructures based on two-dimensional transition-metal dichalcogenides (TMDs) – MoS₂/GeC, MoSe₂/GeC, WS₂/GeC, and WSe₂/GeC, were studied by density functional theory calculations to investigate their structure, electronic characteristics, principle of photogenerated electron–hole separation, and optical-absorption capability. The optimized heterostructures were formed by van der Waals (vdW) forces and without covalent bonding. Their most stable geometric configurations and band structures display type-II band alignment, which allows them to spontaneously separate photogenerated electrons and holes. The charge difference and built-in electric field across the interface of these vdW heterostructures also contribute to preventing the photogenerated electron–hole recombination. Finally, the high optical absorption of the four TMD-based vdW heterostructures in the visible and near-infrared regions indicates their suitability for photocatalytic, photovoltaic, and optical devices.     

Photodetectors based on junctions of two-dimensional transition metal dichalcogenides

Transition metal dichalcogenides(TMDCs) have gained considerable attention because of their novel properties and great potential applications. The flakes of TMDCs not only have great light absorption from visible to near infrared, but also can be stacked together regardless of lattice mismatch like other two-dimensional(2D) materials. Along with the studies on intrinsic properties of TMDCs, the junctions based on TMDCs become more and more important in applications of photodetection. The junctions have shown many exciting possibilities to fully combine the advantages of TMDCs, other2 D materials, conventional and organic semiconductors together. Early studies have greatly enriched the application of TMDCs in photodetection. In this review, we investigate the efforts in photodetectors based on the junctions of TMDCs and analyze the properties of those photodetectors. Homojunctions based on TMDCs can be made by surface chemical doping,elemental doping and electrostatic gating. Heterojunction formed between TMDCs/2D materials, TMDCs/conventional semiconductors and TMDCs/organic semiconductor also deserve more attentions. We also compare the advantages and disadvantages of different junctions, and then give the prospects for the development of junctions based on TMDCs.     

氢化二维过渡金属硫化物的稳定性和电子特性:第一性原理研究

用氢对单层二维过渡金属硫化物(TMDs)进行功能化是调节单层TMDs电子性质的既有效又经济的方法.采用密度泛函理论,对单层TMDs (MX_2 (M=Mo, W; X=S, Se, Te))的稳定性和电子性质进行理论研究,发现在单层MX_2 的层间有一个比其表面更稳定的氢吸附位点.当同阳离子时,随着阴离子原子序数的增加, H原子与MX_2 层的结合越强,氢化单层MX_2 结构越稳定;相反,同阴离子时,随着阳离子原子序数的增加, H原子与MX_2 层的结合越弱.氢原子从MoS_2的表面经层间穿越到另一表面的扩散势垒约为0.9 eV.氢化对单层MX_2 的电子特性也会产生极大的影响,主要表现在氢化实现了MX_2 体系从无磁性到磁性体系的过渡.表面氢化会使MX_2 层的带隙急剧减小,而层间氢化使MX_2 的电子结构从半导体转变为金属能带.     

First principles study on geometric and electronic properties of two-dimensional Nb2CTx MXenes

MXenes are a new type of two-dimensional carbides with rich physical and chemical properties. The physics of MXenes, and thus the applications, are dominated by surface functional groups. Herein, the effects of different terminations (O, S, Se, Te) on the geometric and electronic properties of Nb₂C MXenes were studied via density functional theory (DFT) calculations. Three adsorption sites were examined to determine the most stable configurations. The results showed that both the types and the positions of surface functional groups influence the geometric stability and physical characters of Nb₂C. The S and Se terminations make the Nb₂C MXenes to be semiconductor, while Nb₂C MXenes with other terminations (O, Te) are conductor. The electron location function, density of states, Bader charge distribution, and the projected crystal orbital Hamilton population were conducted to explain the origin of adsorption stability and electronic nature difference. Our results provide a fundamental understanding about the effects of surface terminations on the intrinsic stability and electronic properties of Nb₂C MXenes.     

Structural,electronic,and magnetic properties of transition-metal atom adsorbed two-dimensional GaAs nanosheet

By using first-principles calculations within the framework of density functional theory,the electronic and magnetic properties of 3d transitional metal(TM) atoms(from Sc to Zn) adsorbed monolayer Ga As nanosheets(Ga As NSs) are systematically investigated.Upon TM atom adsorption,Ga As NS,which is a nonmagnetic semiconductor,can be tuned into a magnetic semiconductor(Sc,V,and Fe adsorption),a half-metal(Mn adsorption),or a metal(Co and Cu adsorption).Our calculations show that the strong p–d hybridization between the 3d orbit of TM atoms and the 4p orbit of neighboring As atoms is responsible for the formation of chemical bonds and the origin of magnetism in the Ga As NSs with Sc,V,and Fe adsorption.However,the Mn 3d orbit with more unpaired electrons hybridizes not only with the As 4p orbit but also with the Ga 4p orbit,resulting in a stronger exchange interaction.Our results may be useful for electronic and magnetic applications of Ga As NS-based materials.     

Review of ultrafast spectroscopy studies of valley carrier dynamics in two-dimensional semiconducting transition metal dichalcogenides

The two-dimensional layered transition metal dichalcogenides provide new opportunities in future valley-based information processing and also provide an ideal platform to study excitonic effects. At the center of various device physics toward their possible electronic and optoelectronic applications is understanding the dynamical evolution of various manyparticle electronic states, especially exciton which dominates the optoelectronic response of TMDs, under the novel context of valley degree of freedom. Here, we provide a brief review of experimental advances in using helicity-resolved ultrafast spectroscopy, especially ultrafast pump–probe spectroscopy, to study the dynamical evolution of valley-related many-particle electronic states in semiconducting monolayer transitional metal dichalcogenides.     

Electrical conductivity dependence of thin metallic films of Au and Pd as a top electrode in capacitor applications

Electrical conductivity dependence of thin metallic films of Au and Pd over the different perovskites was investigated. It is found from electrical properties that crystallographic growth orientation of Au and Pd thin layers attained from X-ray diffraction results indicate the slop of current (I)-voltage (V) plots. Besides, surface morphology and topography was considered using Field Emission Scanning Electron Microscopy and Atomic Force Microscopy, respectively. Obtained results showed the Stranski-Krastanov growth of the Pd and Au. Indeed, diminishing of the root-mean-square roughness of Pd/BiMnO₃/SrTiO₃ following by Au deposition should be concerned due to growth of Au onto the crack-like parts of the substrate. These crack-like parts appeared due to parasitic phases of the Bi-Mn-O system mainly Mn₃O₄ (l 0 l) and Mn₃O₄ (0 0 4 l).The different response in the electrical properties of heterostructures suggests that electrical conductance of the Au and Pd thin metallic films have the crystallographic orientation dependence. Furthermore, polycrystallinity of the thin metallic films are desired in electrode applications due to increase the conductivity of the metallic layers.     

Improving the device performances of two-dimensional semiconducting transition metal dichalcogenides: Three strategies

Two-dimensional (2D) semiconductors are emerging as promising candidates for the next-generation nanoelectronics. As a type of unique channel materials, 2D semiconducting transition metal dichalcogenides (TMDCs), such as MoS₂ and WS₂, exhibit great potential for the state-of-the-art field-effect transistors owing to their atomically thin thicknesses, dangling-band free surfaces, and abundant band structures. Even so, the device performances of 2D semiconducting TMDCs are still failing to reach the theoretical values so far, which is attributed to the intrinsic defects, excessive doping, and daunting contacts between electrodes and channels. In this article, we review the up-to-date three strategies for improving the device performances of 2D semiconducting TMDCs: (i) the controllable synthesis of wafer-scale 2D semiconducting TMDCs single crystals to reduce the evolution of grain boundaries, (ii) the ingenious doping of 2D semiconducting TMDCs to modulate the band structures and suppress the impurity scatterings, and (iii) the optimization design of interfacial contacts between electrodes and channels to reduce the Schottky barrier heights and contact resistances. In the end, the challenges regarding the improvement of device performances of 2D semiconducting TMDCs are highlighted, and the further research directions are also proposed. We believe that this review is comprehensive and insightful for downscaling the electronic devices and extending the Moore’s law.     

Extraordinary resonance transmission of two-dimensional terahertz metallic photonic crystals without defect

In this paper, we have demonstrated a kind of extraordinary transmission in the photonic first pass band of two-dimensional terahertz metallic photonic crystals: in the regime of low-middle metal filling radio, resonances among the metallic cylinders lead to sharp and high resonance peaks of Lorentz type, layer dependency of the peak numbers. The effect of positional disorders of metallic cylinders has been investigated as well. Based on a simple model proposed in this paper, we can intuitively understand the extraordinary transmission and roughly estimate resonance frequencies. Our results agree qualitatively with the experimental data reported in Appl. Phys. Lett. 65, 645.     

Topological nature of in‐gap bound states in disordered large‐gap monolayer transition metal dichalcogenides

We propose a physical model based on disordered (a hole punched inside a material) monolayer transition metal dichalcogenides (TMDs) to demonstrate a large‐gap quantum valley Hall insulator. We find an emergence of bound states lying inside the bulk gap of the TMDs. They are strongly affected by spin–valley coupling, rest‐ and kinetic‐mass terms and the hole size. In addition, in the whole range of the hole size, at least two in‐gap bound states with opposite angular momentum, circulating around the edge of the hole, exist.Their topological insulator (TI) feature is analyzed by the Chern number, characterized by spacial distribution of their probabilities and confirmed by energy dispersion curves (energy vs. angular momentum). It not only sheds light on overcoming low‐temperature operating limitation of existing narrow‐gap TIs, but also opens an opportunity to realize valley‐ and spin‐qubits. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

A First-Principles Study on the Vibrational and Electronic Properties of Zr-C MXenes

Within the framework of density functional theory calculations,the structural,vibrational,and electronic properties of Zr_(n )C_(n-1)(n=2,3,and 4)and their functionalized MXenes have been investigated.We find that the most stable configurations for Zr-C MXene are the ones that the terminal groups F,O,and OH locate on the common hollow site of the superficial Zr layer and its adjacent C layer.F and OH-terminated Zr_3C_(2 )and Zr_4C_(3 )have small imaginary acoustic phonon branches aroundΓpoint while the others have no negative phonon modes.The pristine MXenes(Zr_2C,Zr_3C_(2 )and Zr_4C_3)are all metallic with large DOS contributed by the Zr atom at the Fermi energy.When functionalized by F,O and OH,new hybridization states appear and the DOS at the Fermi level are reduced.Moreover,we find that their metallic characteristic increases with an increase in n.For(Zr_(n )C_(n-1))O_2,Zr_2CO_(2 )is a semiconductor,Zr_3C_2O_(2 )is a semimetal,and Zr_4C_3O_(2 )becomes a metal.     

The effect of S-functionalized and vacancies on V2C MXenes as anode materials for Na-ion and Li-ion batteries

The electrochemical properties of V₂C and V₂CT₂ (T = O, S) MXenes with and without vacancy as anode materials for Na-ion and Li-ion batteries, have been studied using first-principles calculation. The present results indicate that the adsorption strength of Li-ion and Na-ion on V₂CS₂ are less than that of O-functionalized, together with a lower diffusion barrier. Simultaneously, V₂CS₂ monolayer exhibits lower open-circuit voltage (OCV) values of 0.72 and 0.49 V for Li- and Na-ion, respectively. Interestingly, the presence of atomic vanadium vacancy on V₂CS₂ monolayer exerts more prominent effects on enhancing adsorption strength than that of carbon vacancy for Li-ion and Na-ion, but with an exception for the diffusion of Li-ion and Na-ion on V₂CS₂ monolayer. The finding suggests that the V₂CS₂ monolayer is expected to be a potential candidate as anode material for Li-ion and Na-ion battery due to its lower open-circuit voltages and diffusion barriers.     

Structural symmetry of two-dimensional metallic arrays: Implications for surface plasmon excitations

We report our recent investigation of the (0, ±1)-type SPP excitation of a gold two-dimensional nano-cavity array using finite-difference time-domain methodology. Our particular focus is on the symmetry properties of (0, ±1)-SPPs excited by different polarizations of light. Based on a group theory approach, we have shown that these (0, ±1) modes are originated from different symmetry modes at the Γ-point, and the B-(0, ±1) dispersion curve converges to the A-(−1, 0) dispersion curve at the Γ-point. This indicates these (0, ±1) modes are very different in their symmetry properties. As a result, the control of polarization may provide important insights into the manipulation of SPPs.     

Growth,characterization, and Raman spectra of the 1T phases of TiTe2, TiSe2, and TiS2

High-quality large 1$T$ phase of Ti$X_2$ ($X ={\rm Te}$, Se, and S) single crystals have been grown by chemical vapor transport using iodine as a transport agent. The samples are characterized by compositional and structural analyses, and their properties are investigated by Raman spectroscopy. Several phonon modes have been observed, including the widely reported $A_{1g}$ and $E_g$ modes, the rarely reported $E_u$ mode ($\sim$183 cm$^{-1}$ for TiTe$_2$, and $\sim$185 cm$^{-1}$ for TiS$_2$), and even the unexpected $K$ mode ($\sim$85 cm$^{-1}$) of TiTe$_2$. Most phonons harden with the decrease of temperature, except that the $K$ mode of TiTe$_2$ and the $E_u$ and "$A_{2u}$/Sh" modes of TiS$_2$ soften with the decrease of temperature. In addition, we also found phonon changes in TiSe$_2$ that may be related to charge density wave phase transition. Our results on Ti$X_2$ phonons will help to understand their charge density wave and superconductivity.     

Decay of a quantum dot in two-dimensional metallic photonic crystals

In this paper we have developed a theory for the decay of a quantum dot doped in a two-dimensional metallic photonic crystal consisting of two different metallic pillars in an air background medium. This crystal structure forms a full two-dimensional photonic band gap when the appropriate pillar sizes are chosen. The advantage of using two metals is that one can easily control the density of states and optical properties of these photonic crystals by changing the plasma energies of two metals rather than one. Using the Schrödinger equation method and the photonic density of states, we calculated the linewidth broadening and the spectral function of radiation due to spontaneous emission for two-level quantum dots doped in the system. Our results show that by changing the plasma energies one can control spontaneous emission of quantum dots doped in the metallic photonic crystal.     

Symmetrical metallic and magnetic edge states of nanoribbon from semiconductive monolayer PtS2

Transition metal dichalcogenides (TMD) MoS₂ or graphene could be designed to metallic nanoribbons, which always have only one edge show metallic properties due to symmetric protection. In present work, a nanoribbon with two parallel metallic and magnetic edges was designed from a noble TMD PtS₂ by employing first-principles calculations based on density functional theory (DFT). Edge energy, bonding charge density, band structure, density of states (DOS) and simulated scanning tunneling microscopy (STM) of four possible edge states of monolayer semiconductive PtS₂ were systematically studied. Detailed calculations show that only Pt-terminated edge state among four edge states was relatively stable, metallic and magnetic. Those metallic and magnetic properties mainly contributed from 5d orbits of Pt atoms located at edges. What's more, two of those central symmetric edges coexist in one zigzag nanoribbon, which providing two atomic metallic wires thus may have promising application for the realization of quantum effects, such as Aharanov–Bohm effect and atomic power transmission lines in single nanoribbon.