关键词： Density functional theory   B2CX Monolayer   Electrical properties   Optical properties   Water splitting  

摘要： The structural and electronic stabilities as well as photocatalytic properties of two-dimensional (2D) Janus B2CX (X = O, S, Se) monolayers are theoretically assessed. First, with the help of cohesive energy computations and phonon dispersion simulations, the energetically and kinetically stabilities of the proposed 2D B2CX (X = O, S, Se) monolayers are confirmed. Next, electronic properties computations using hybrid density functional (HSE06) indicates that 2D B2CO, B2CS, B2CSe monolayers show semiconducting properties with moderate bandgaps of about 2.81, 2.63, 1.57 eV respectively. Finally, comparing the positions of band edge of the proposed monolayers with the redox potentials of water shows that 2D B2CX (X = O, S, Se) monolayers are promising photocatalysts materials for water splitting applications.

关键词： Monolayers  

摘要： Two-dimensional (2D) ferromagnetic (FM) semiconductors are highly desirable in next generation spintronic devices. Herein, we report an intrinsic FM semiconductor, an FeI3 monolayer which can be exfoliated from its bulk crystal owing to the small cleavage energy and the high in-plane stiffness. The FeI3 monolayer is dynamically and mechanically stable. Additionally, the FeI3 monolayer has sizable magneto-crystalline anisotropy energy (MAE) and the Curie temperature is higher than the liquid-nitrogen temperature (77 K), and there is a big discrepancy (Delta(cbm)) between the conduction band minimum of the two spin-channels and negligible thermally induced hop. Carrier doping less than 0.1 e per unit cell further improves the relevant properties by modulating the MAE, Curie temperature, and Delta(cbm). Moreover, the isoelectronic analogue FeI1.5Cl1.5 monolayer is a bipolar FM semiconductor with a high Curie temperature (260 K). Our results demonstrate promising applications of the FeI3 monolayer in next-generation spintronic devices owing to its robust intrinsic ferromagnetism and novel semiconducting properties.

关键词： First principles prediction   MgS monolayer   Electrical properties   Optical properties  

摘要： In the recent decade, extensive efforts have been made to find new two-dimensional (2D) materials. Theatrical studies can offer promising stable candidates for experimental peers to explore. In this study, the structural stability, electrical and optical properties of 2D tetragonal MgX (X = S, Se) monolayers were systematically investigated by using density functional theory (DFT). Based on our computation, 2D tetragonal MgS and MgSe monolayers indicate very good thermodynamic and kinetic stabilities which suggest their promising to be experimentally realized. The results demonstrated that 2D MgS and MgSe are indirect semiconductors showing strain tunable wide band gaps (WBG). Furthermore, analyzing the optical aspects of these 2D materials suggested their potential applicable characteristics for using in UV applications. As WBG semiconductors, 2D monolayer MgS and MgSe may have promising applications in electm-optical devices in nano-scale.

关键词： Monolayers  

摘要： Two-dimensional materials play a vital role in next-generation microelectronics, optoelectronics and flexible electronics due to their novel physical properties caused by quantum-confinement effects. In this work, we investigate the stability and the possibility of exfoliation of monolayer Bi2Se3-xTex (x = 0, 1, 2) using first-principles calculations. Our calculations show that these materials are indirect bandgap semiconductors, and the elastic modulus is smaller than other conventional materials, which indicates better flexibility. We find that the electron mobility of monolayer Bi2SeTe2 along the armchair direction is higher than that of black phosphorene, reaching 2708 cm(2) V-1 s(-1), and the electron mobility of monolayer Bi2Se3 along the zigzag direction is about 24 times larger than the hole mobility. The remarkable electron mobilities and highly anisotropic properties of these new monolayers pave the way for future applications in high-speed (opto)electronic devices.

关键词： nonlinear optics   sum frequency generation   exciton   two-dimensional semiconductors   van der Waals heterostructures   confocal microscopy   valleytronics  

摘要： We report continuous-wave second harmonic and sum frequency generation from two-dimensional transition metal dichalcogenide monolayers and their heterostructures with pump irradiances several orders of magnitude lower than those of conventional pulsed experiments. The high nonlinear efficiency originates from above-gap excitons in the band nesting regions, as revealed by wavelength-dependent second order optical susceptibilities quantified in four common monolayer transition metal dichalcogenides. Using sum frequency excitation spectroscopy and imaging, we identify and distinguish one- and two-photon resonances in both monolayers and heterobilayers. Data for heterostructures reveal responses from constituent layers accompanied by nonlinear signal correlated with interlayer transitions. We demonstrate spatial mapping of heterogeneous interlayer coupling by sum frequency and second harmonic confocal microscopy on heterobilayer MoSe2/WSe2.

关键词： Band structure   Layers   Monolayers   Magnetic properties   Heterostructures  

摘要： The integration of ferromagnetic and semiconducting properties in a single two-dimensional (2D) material has been recognized as a fertile ground for fundamental science as well as for practical applications in information processing and storage. CrI3 monolayer has recently drawn much attention due to its 2D long-range ferromagnetic (FM) order. However, its Curie temperature (T-C) is too low (similar to 45 K) for practical spintronic applications. Here, we show that the in -plane FM coupling of CrI3 can be remarkably enhanced by constructing a 2D heterostructure where CrI3 monolayer is supported on a nonmagnetic normal semiconductor/insulator substrate. Choosing MoTe2 monolayer as a substrate, we find that the CrI3/MoTe2 2D heterostructure is an intrinsic semiconducting ferromagnet with T-C of similar to 60 K. The Tc can be further increased to similar to 85 K by applying an out -of -plane pressure of similar to 4.2 GPa. The doubling of the T-C in this 2D heterostructure comes from the introduction of extra spin superexchange (Cr-Te-Cr) paths.

关键词： 2D semiconductors   WSe2 monolayer   field-effect transistor   high-k gate dielectric   CMOS  

摘要： A critical analysis of charge carrier statistics influenced by quantum capacitance is carried out in order to predict the electrical performance of a nanoscale metal-oxide-semiconductor field-effect transistor (MOSFET) with a channel made of a monolayer tungsten diselenide (WSe2) two-dimensional (2D) crystal semiconductor. Since quantum capacitance originating from two-dimensional electron gas in a quantum well or an inversion layer does not completely screen the quasistatic electric field during applied gate voltage, the partial penetration of an external electric field through the 2D semiconductor channel will generate excess charge carriers;thus quantum capacitance will play an important role in determining the overall charge carrier density in the channel. Therefore, common methods used to extract charge carrier density in the channel for three-dimensional (3D) crystal semiconductors will yield inaccurate results when used for 2D crystal semiconductors. To address this issue, this study proposes a modified approach for extracting charge carrier density in WSe2-based 2D semiconductors by combining the appropriate carrier statistics with consideration of quantum capacitance. In addition, the study investigates the effect of interface traps on overall capacitance, which may influence the electrical performance of a nanoscale MOSFET with monolayer WSe2 as a channel material.

关键词： Monolayers   graphene   Semiconducting antimony   antimony trioxide  

摘要： The first successful synthesis and characterization of single-crystalline two-dimensional (2D) semiconducting antimony tri oxide (Sb2O3) by direct chemical vapor deposition (CVD) growth on monolayer graphene is presented herein. The Sb2O3 flakes on graphene are regular triangles with smooth edges and a thickness of similar to 1.45 nm. The thickness dependence of the Raman spectra and scanning Kelvin probe microscopy (SKPM) were systematically studied.

关键词： 2D materials   density functional theory   MXenes   Ti2C   electronic and magnetic properties   surface functionalization  

摘要： Recently, two-dimensional (2D) transition metal carbides and nitrides known as MXenes, have gained a lot of attention because of their tunable electronic and magnetic properties depending on surface functionalization. In the present work, the structural, electronic, and magnetic properties of both T and H phases of bare Ti-2 C and fully surface terminated Ti-2 CT2 (T = -F, = O, -OH) are calculated using a set of first principles calculations. The ground state structures of Ti-2 CT2 are computed in two and four different configurations for both H and T phases, respectively. We demonstrate that while H phase of Ti-2 C exhibits half-metallic behavior with magnetic moments of 2 mu B per formula unit, it displays metallic behavior with magnetic moments of 1.27 mu B , 0.25 mu B per formula unit, and semiconductor behavior with 0.35 eV band gap in -F, -OH, and =O surface functionalization, respectively. We also show that while T phase of Ti-2 C exhibits metallic behavior with magnetic moment of 1.89 VB per formula unit, it stays in metallic nonmagnetic behavior in both -F and -OH. Meanwhile, it displays semiconductor behavior with 0.25 eV band gap in -O surface functionalization. We expect that our results can advance the future applications of MXenes from energy storage to spintronic.

关键词： 二维材料   应力   隧穿场效应晶体管   第一性原理计算  

摘要： 随着科学技术的飞速发展,晶体管沟道材料的长度越来越小,短沟道效应变得不容忽视,造成栅极对器件的控制能力减弱,从而导致器件性能降低和功耗变大等一系列问题.二维材料代替块体材料作为隧穿场效应晶体管(TFET)的沟道对于解决上述问题具有明显的优势,所以寻找一种电子性能优异的二维材料亟不可待.本文通过密度泛函理论计算了二维单层β-tellurene和GeP的电子性质,并在此基础上结合非平衡格林函数的方法,研究了二维单层β-tellurene和GeP分别作为沟道材料的β-tellurene TFET和GeP TFET的性能以及应力对β-tellurene TFEF性能的影响.计算结果显示,对于β-tellurene TFET,其开关比可达到10～5,并且SS低于60mV/dec.当对器件施加一定的应力,可以改善器件的性能.对于GeP TFET,由于其载流子迁移率具有各向异性,使器件在x方向的输运性能要优于y方向.并且其开关比可达到10～9,可以很好的实现器件的开与关.