Si₂₄: An Efficient Solar Cell Material

Si₂₄, a recently synthesized allotrope of silicon, has received much attention due to its quasi-direct band gap around 1.3 eV. To explore its potential application as a solar cell device, we investigated the doping effect on the electronic properties of Si₂₄ using first-principles calculations. It is found that Si₂₄ can be easily doped as both <i>p</i>- and <i>n</i>-type, and the dopants are readily ionized. Furthermore, the incorporation of these dopants only reduces the band gap of Si₂₄ slightly, which remains in the ideal region for solar cells. Boron and phosphorus are identified as the most promising elements for the <i>p</i>-type and <i>n</i>-type doping in Si₂₄, respectively, due to their low formation energies, small ionization energies, and small reductions in the band gap. These properties suggest great potential in constructing a novel Si₂₄-based <i>p-n</i> junction which is highly desired for future industrial application of photovoltaic devices

Si₂₄: An Efficient Solar Cell Material

Si₂₄, a recently synthesized allotrope of silicon, has received much attention due to its quasi-direct band gap around 1.3 eV. To explore its potential application as a solar cell device, we investigated the doping effect on the electronic properties of Si₂₄ using first-principles calculations. It is found that Si₂₄ can be easily doped as both <i>p</i>- and <i>n</i>-type, and the dopants are readily ionized. Furthermore, the incorporation of these dopants only reduces the band gap of Si₂₄ slightly, which remains in the ideal region for solar cells. Boron and phosphorus are identified as the most promising elements for the <i>p</i>-type and <i>n</i>-type doping in Si₂₄, respectively, due to their low formation energies, small ionization energies, and small reductions in the band gap. These properties suggest great potential in constructing a novel Si₂₄-based <i>p-n</i> junction which is highly desired for future industrial application of photovoltaic devices

NIR Schottky Photodetectors Based on Individual Single-Crystalline GeSe Nanosheet

We have synthesized high-quality, micrometer-sized, single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the <i>I</i>–<i>V</i> curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ∼5.3 × 10² % at 4 V) at a wavelength of 808 nm. The significant photoresponse and good responsitivity (∼3.5 A W^–1) suggests its potential applications as photodetectors

Manipulation of the Topological Ferromagnetic State in a Weyl Semimetal by Spin–Orbit Torque

Magnetic Weyl semimetals (MWSMs) exhibit unconventional transport phenomena, such as large anomalous Hall (and Nernst) effects, which are absent in spatial inversion asymmetry WSMs. Compared with its nonmagnetic counterpart, the magnetic state of a MWSM provides an alternative way for the modulation of topology. Spin–orbit torque (SOT), as an effective means of electrically controlling the magnetic states of ferromagnets, may be used to manipulate the topological magnetic states of MWSMs. Here we confirm the MWSM state of high-quality Co2MnGa film by systematically investigating the transport measurements and demonstrating that the magnetization and topology of Co2MnGa can be electrically manipulated. The electrical and magnetic optical measurements further reveal that the current-induced SOT switches the topological magnetic state in a 180-degree manner by applying positive/negative current pulses and in a 90-degree manner by alternately applying two orthogonal current pulses. This work opens up more opportunities for spintronic applications based on topological materials

Hydrogen Evolution Catalyzed by a Molybdenum Sulfide Two-Dimensional Structure with Active Basal Planes

Molybdenum disulfide has been demonstrated as a promising catalyst for hydrogen evolution reaction (HER). However, its performance is limited by fractional active edge sites and the strong dependence on hydrogen coverage. In this study, we find an enhanced HER performance in a two-dimensional substoichiometric molybdenum sulfide. Both first-principles calculations and experimental results demonstrate that the basal plane is catalytically active toward HER, as evidenced by an optimum Gibbs free energy and a low reaction overpotential. More interestingly, the HER performance is insensitive to hydrogen coverage and can be improved under compressive in-plane biaxial strains. Our results suggest not only an improved HER performance of substoichiometric molybdenum sulfide due to its chemical reactive basal plane but also a way to tune the performance

Defect Evolution Enhanced Visible-Light Photocatalytic Activity in Nitrogen-Doped Anatase TiO₂ Thin Films

Doping nitrogen (N) into TiO₂ is one of the promising ways to extend the photocatalytic activity into the visible-light range, enabling to harvest more solar energy. In this study, we realize a high concentration of N incorporated into the anatase TiO₂ films on indium tin oxide substrates. The band gap of TiO₂ with a high N substitutional doping is reduced to 1.91 eV, showing a much improved photocatalytic reactivity, as supported by the degrading methyl orange solution radiated with visible light. First-principles calculations further suggest that the form of dominant defects evolves from the substitution of N (N_O) to the coexistence of N_O and oxygen vacancies (O_V) when the N-doping concentration is increased, which leads to the reduction of band gap in the visible-light range and more delocalized charge distribution. Our results demonstrate a novel synthesis route that can realize a high concentration of N substitutional doping in TiO₂ films and provide an improved understanding of enhanced visible-light photocatalytic performance of N-doped TiO₂

Phonon-Mediated Colossal Magnetoresistance in Graphene/Black Phosphorus Heterostructures

There is a huge demand for magnetoresistance (MR) sensors with high sensitivity, low energy consumption, and room temperature operation. It is well-known that spatial charge inhomogeneity due to impurities or defects introduces mobility fluctuations in monolayer graphene and gives rise to MR in the presence of an externally applied magnetic field. However, to realize a MR sensor based on this effect is hampered by the difficulty in controlling the spatial distribution of impurities and the weak magnetoresistance effect at the monolayer regime. Here, we fabricate a highly stable monolayer graphene-on-black phosphorus (G/BP) heterostructure device that exhibits a giant MR of 775% at 9 T magnetic field and 300 K, exceeding by far the MR effects from devices made from either monolayer graphene or few-layer BP alone. The positive MR of the G/BP device decreases when the temperature is lowered, indicating a phonon-mediated process in addition to scattering by charge impurities. Moreover, a nonlocal MR of >10 000% is achieved for the G/BP device at room temperature due to an enhanced flavor Hall effect induced by the BP channel. Our results show that electron–phonon coupling between 2D material and a suitable substrate can be exploited to create giant MR effects in Dirac semimetals

Substoichiometric Molybdenum Sulfide Phases with Catalytically Active Basal Planes

Molybdenum sulfide (MoS₂) is widely recognized for its catalytic activities where the edges of the crystals turn over reactions. Generating sulfur defects on the basal plane of MoS₂ can improve its catalytic activity, but generally, there is a lack of model systems for understanding metal-centered catalysis on the basal planes. Here, we synthesized a new phase of substoichiometric molybdenum sulfide (s-MoS_<i>x</i>) on a sulfur-enriched copper substrate. The basal plane of s-MoS_<i>x</i> contains chemically reactive Mo-rich sites that can undergo dynamic dissociative adsorption/desorption processes with molecular hydrogen, thus demonstrating its usefulness for hydrogen-transfer catalysis. In addition, scanning tunneling microscopy was used to monitor surface-directed Ullmann coupling of 2,8-dibromo-dibenzothiophene molecules on s-MoS_<i>x</i> nanosheets, where the 4-fold symmetric surface sites on s-MoS_<i>x</i> direct C–C coupling to form cyclic tetramers with high selectivity

Phase Selection Enabled Formation of Abrupt Axial Heterojunctions in Branched Oxide Nanowires

Rational synthesis of nanowires via the vapor–liquid–solid (VLS) mechanism with compositional and structural controls is vitally important for fabricating functional nanodevices from bottom up. Here, we show that branched indium tin oxide nanowires can be in situ seeded in vapor transport growth using tailored Au–Cu alloys as catalyst. Furthermore, we demonstrate that VLS synthesis gives unprecedented freedom to navigate the ternary In–Sn–O phase diagram, and a rare and bulk-unstable cubic phase can be selectively stabilized in nanowires. The stabilized cubic fluorite phase possesses an unusual almost equimolar concentration of In and Sn, forming a defect-free epitaxial interface with the conventional bixbyite phase of tin-doped indium oxide that is the most employed transparent conducting oxide. This rational methodology of selecting phases and making abrupt axial heterojunctions in nanowires presents advantages over the conventional synthesis routes, promising novel composition-modulated nanomaterials

Efficient Spin–Orbit Torque Switching in a Perpendicularly Magnetized Heusler Alloy MnPtGe Single Layer

Electrically manipulating magnetic moments by spin–orbit torque (SOT) has great potential applications in magnetic memories and logic devices. Although there have been rich SOT studies on magnetic heterostructures, low interfacial thermal stability and high switching current density still remain an issue. Here, highly textured, polycrystalline Heusler alloy MnxPtyGe (MPG) films with various thicknesses are directly deposited onto thermally oxidized silicon wafers. The perpendicular magnetization of the MPG single layer can be reversibly switched by electrical current pulses with a magnitude as low as 4.1 × 1010Am–2, as evidenced by both the electrical transport and the magnetic optical measurements. The switching is shown to arise from inversion symmetry breaking due to the vertical composition gradient of the films after sample annealing. The SOT effective fields of the samples are analyzed systematically. It is found that the SOT efficiency increases with the film thickness, suggesting a robust bulk-like behavior in the single magnetic layer. Furthermore, a memristive characteristic has been observed due to a multidomain switching property in the single-layer MPG device. Additionally, deterministic field-free switching of magnetization is observed when the electric current flows orthogonal to the direction of the in-plane compositional gradient due to the in-plane symmetry breaking. This work proves that the MPG is a good candidate to be utilized in high-density and efficient magnetoresistive random access memory devices and other spintronic applications