Search

Molybdenum disulfide (MoS2) nanosheet, one of two-dimensional (2D) semiconductors, has recently been regarded as a promising material to break through the limit of present semiconductors. With an apparent energy band gap, it certainly provides a high carrier mobility, superior subthreshold swing, and ON/OFF ratio in field-effect transistors (FETs). However, its potential in carrier mobility has still been depreciated since the field-effect mobilities have only been measured from metal–insulator–semiconductor (MIS) FETs, where the transport behavior of conducting carriers located at the insulator/MoS2 interface is unavoidably interfered by the interface traps and gate voltage. Moreover, thin MoS2 MISFETs have always shown large hysteresis with unpredictable negative threshold voltages. Here, we for the first time report MoS2-based metal semiconductor field-effect transistors (MESFETs) using NiOx Schottky electrode which makes van der Waals interface with MoS2. We thus expect that the maximum mobilities or ...

We report the realization of novel symmetry-protected Dirac fermions in a surface-doped two-dimensional (2D) semiconductor, black phosphorus. The widely tunable band gap of black phosphorus by the surface Stark effect is employed to achieve a surprisingly large band inversion up to ∼0.6 eV. High-resolution angle-resolved photoemission spectra directly reveal the pair creation of Dirac points and their movement along the axis of the glide-mirror symmetry. Unlike graphene, the Dirac point of black phosphorus is stable, as protected by space-time inversion symmetry, even in the presence of spin-orbit coupling. Our results establish black phosphorus in the inverted regime as a simple model system of 2D symmetry-protected (topological) Dirac semimetals, offering an unprecedented opportunity for the discovery of 2D Weyl semimetals.

Based on the first principles calculation, we investigate the electronic band structures of graphene-MoS2 and Ti-MoS2 heterojunctions under gate-voltages. By simultaneous control of external electric fields and carrier charging concentrations, we show that the graphene’s Dirac point position inside the MoS2 bandgap is easily modulated with respect to the co-varying Fermi level, while keeping the graphene’s linear band structure around the Dirac point. The easy modulation of graphene bands is not confined to the special cases where the conduction-band-minimum point of MoS2 and the Dirac point of graphene are matched up in reciprocal space, but is generalized to their dislocated cases. This flexibility caused by the strong decoupling between graphene and MoS2 bands enhances the gate-controlled switching performance in MoS2-graphene hybrid stacking-device.

Based on the first principles calculation, we investigate the electronic band structures of graphene-MoS2 and Ti-MoS2 heterojunctions under gate-voltages. By simultaneous control of external electric fields and carrier charging concentrations, we show that the graphene’s Dirac point position inside the MoS2 bandgap is easily modulated with respect to the co-varying Fermi level, while keeping the graphene’s linear band structure around the Dirac point. The easy modulation of graphene bands is not confined to the special cases where the conduction-band-minimum point of MoS2 and the Dirac point of graphene are matched up in reciprocal space, but is generalized to their dislocated cases. This flexibility caused by the strong decoupling between graphene and MoS2 bands enhances the gate-controlled switching performance in MoS2-graphene hybrid stacking-device.

A widely tunable band gap in few-layer K-doped black phosphorus is realized by in situ deposition of potassium atoms.

Black phosphorus consists of stacked layers of phosphorene, a two-dimensional semiconductor with promising device characteristics. We report the realization of a widely tunable band gap in few-layer black phosphorus doped with potassium using an in situ surface doping technique. Through band structure measurements and calculations, we demonstrate that a vertical electric field from dopants modulates the band gap, owing to the giant Stark effect, and tunes the material from a moderate-gap semiconductor to a band-inverted semimetal. At the critical field of this band inversion, the material becomes a Dirac semimetal with anisotropic dispersion, linear in armchair and quadratic in zigzag directions. The tunable band structure of black phosphorus may allow great flexibility in design and optimization of electronic and optoelectronic devices.

Thin flakes of black phosphorus (BP) are a two-dimensional (2D) semiconductor whose energy gap is predicted being sensitive to the number of layers and external perturbations. Very recently, it was found that a simple method of potassium (K) doping on the surface of BP closes its band gap completely, producing a Dirac semimetal state with a linear band dispersion in the armchair direction and a quadratic one in the zigzag direction. Here, based on first-principles density functional calculations, we predict that, beyond the critical K density of the gap closure, 2D massless Dirac Fermions (i.e., Dirac cones) emerge in K-doped few-layer BP, with linear band dispersions in all momentum directions, and the electronic states around Dirac points have chiral pseudospins and Berry's phase. These features are robust with respect to the spin-orbit interaction and may lead to graphene-like electronic transport properties with greater flexibility for potential device applications.

By investigating the magnetization variation of Fe-rich multicomponent alloys with solute concentration, the magnetic flux density of Si electrical steel is optimized with average valency. For binary alloys of $\mathrm{Fe}$-$X$ [$X=d$--$5d$ transition-metal (TM) and $3sp$--$6sp$ elements], the usual mountain-shape behavior of the Slater-Pauling curve is produced even for late $4d$--$5d$ TM, and the monotonically decreasing behavior for $3sp$--$5sp$ elements. Anomalously, a rise-and-fall pattern is found for the $6sp$ element of $X=\mathrm{Bi}$. For ternary alloys of $\mathrm{Fe}$-Si-$X$ ($X=3d$--$5d$ TM and $3sp$--$6sp$ elements), the role of Si is shown to shift the starting point of moment variation and the magnetic flux density of Fe-Si-$X$ alloys is found to repeat the Si-absent binary pattern at the shifted reference moment. On the basis of the calculated magnetic moments, Fe-Si-$X$-$Y$ ($X=\mathrm{Co}$, Pd, Pt and $Y=\mathrm{Al}$, Sb, Bi) is proposed as a viable candidate for the optimum products of electrical steels.

Formation-energy formalism for a general interface is developed and applied to $\ensuremath{\alpha}\text{-Fe}$ host with boron (B) impurities. In bulk $\ensuremath{\alpha}\text{-Fe}$, B impurities prefer to be located at substitutional position rather than at interstitial. The estimated formation energy of substitutional B is lower than that of interstitial B by 0.10 eV in the dilute impurity limit. At the surface, however, the interstitial site is found to be preferred, and B impurities on top of the surface are the most stable as compared to those in the subsurface positions. The stability of B impurities increases as they get close to the free surface, indicating that B impurities tend to segregate toward the free surface. This surface segregation of B impurities is found to be the direct reflection of the surface-energy minimization of Fe-B system. Based on the first-principles band calculations, it is deduced that the dominant B diffusion behavior near the free surface is described by the interstitial-to-interstitial diffusion mechanism.

We have studied the electronic structure of C and N co-doped TiO2 using hard x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Our results reveal overlap of the 2p states of O, N, and C in the system which shifts the valence band maximum towards the Fermi level. Combined with optical data we show that co-doping is an effective route for band gap reduction in TiO2. Comparison of the measured valence band with theoretical photoemission density of states reveals the possibility of C on Ti and N on O site.

Electronic structures of Cr-based chalcogenide magnetic semiconductors FeCr2Se4 and Fe0.5Cu0.5Cr2Se4 are investigated by using the full-potential augmented plane wave (FLAPW) band method in both the generalized gradient approximation (GGA) and the GGA+U (GGA incorporating the on-site Coulomb interaction). The GGA band calculation for the Jahn–Teller distorted FeCr2Se4 with monoclinic structure yields an antiferromagnetic (AF) metallic electronic structure. The GGA+U band calculation yields an insulating electronic structure for AF FeCr2Se4 in agreement with experiments. The orbital ordering in FeCr2Se4 driven by the Jahn–Teller and on-site Coulomb interactions is demonstrated based on the GGA+U electronic structures. For ferrimagnetic Fe0.5Cu0.5Cr2Se4 with a cubic spinel structure, the GGA and the GGA+U yield nearly insulating and insulating electronic structures, respectively. We have also studied the experimental electronic structure of FeCr2Se4 by employing soft x-ray absorption spectroscopy, soft x-ray magnetic circular dichroism and valence-band photoemission spectroscopy. Both calculational and experimental electronic structure studies indicate that the valence states of Fe and Cr ions in FeCr2Se4 are nearly divalent (Fe2+) and trivalent (Cr3+), respectively.