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The charge density wave (CDW) state in van der Waals systems shows interesting scaling phenomena as the number of layers can significantly affect the CDW transition temperature, $T_{CDW}$. However, it is often difficult to use conventional methods to study the phase transition in these systems due to their small size and sensitivity to degradation. Degradation is an important parameter which has been shown to greatly influence the superconductivity in layered systems. Since the CDW state competes with the onset of superconductivity, it is expected that $T_{CDW}$ will also be affected by the degradation. Here, we probe the CDW phase transition by the mechanical resonances of suspended 2H-TaS2 and 2H-TaSe2 membranes and study the effect of disorder on the CDW state. Pristine flakes show the transition near the reported values of 75 K and 122 K respectively. We then study the effect of degradation on 2H-TaS2 which displays an enhancement of $T_{CDW}$ up to 129 K after degradation in ambient air. Finally, we study a sample with local degradation and observe that multiple phase transitions occur at 87 K, 103 K and 118 K with a hysteresis in temperature in the same membrane. The observed spatial variations in the Raman spectra suggest that variations in crystal structure cause domains with different transition temperatures which could result in the hysteresis. This work shows the potential of using nanomechanical resonance to characterize the CDW in suspended 2D materials and demonstrate that degradation can have a large effect on transition temperatures., The following article has been accepted by Applied Physics Letters. After it is published, it will be found at [https://doi.org/10.1063/5.0051112]

Materials with a negative Poisson's ratio, also known as auxetics, have attracted a lot attention as they have shown innovation potential in applications for energy damping, modern fasteners, biomedical implants, piezoelectric sensors, and soft actuators. In this study, we introduce different patterns of graded distribution of unit cells with positive and negative Poisson's ratio in tubular configurations. Seven types of patterns are programmed into the fabric of the metamaterial to create desired shape changes upon applying far-field external loads. Two of the patterns demonstrate near-zero Poisson's ratio even at axial strains as high as 44%. Other pattern distributions convert the initially cylindrical shape of the tubes to vase, barrel, hourglass, nonsymmetrical vase, and nonsymmetrical hourglass geometries. The experimental Poisson's ratio values for the linear negative-to-positive-to-negative gradient (resulting in hourglass shape) and linear positive-to-negative-to-positive gradient (resulting in barrel shape) cases are +0.53 and -0.47, respectively. The measured Poisson's ratio values at tube level are in good accordance with the analytical values of +0.5 and -0.5. Benefits of the proposed designs in applications such as action-at-a-distance actuators and wrinkle-free jointless hinges in both 3D and 2D configurations are demonstrated.

We report on the crystal morphology and Raman scattering features of high structural quality GaSb1-xBix alloys grown by molecular beam epitaxy with a high Bi content (x up to ∼0.10). The Raman spectra were measured at room temperature with different laser excitation wavelengths of 532 nm, 633 nm, and 785 nm. We observed well-defined Bi-induced Raman peaks associated with atomic Bin clusters and GaBi vibrational modes. Remarkably, some Bi-induced Raman modes were strongly enhanced when the laser energy was selected near an optical transition for the 5.8%Bi sample. This effect was attributed to a Raman resonant effect near an excited optical transition of the GaSbBi layer and has been used to identify the nature of the observed Raman peaks. acceptedVersion

As one kind of special beams, a terahertz (THz) cylindrical vector Bessel beam processes centrosymmetric polarization distribution and nondiffractive propagation properties. Such a vector Bessel beam is potential in various practical applications ranging from THz communication and electron acceleration to sensing and imaging. Here, we propose and experimentally demonstrate a method to directly emit a broadband THz cylindrical vector Bessel (CVB) beam using nonlinear effects. By photo-exciting an indium tin oxide film coated axicon with a circularly polarized femtosecond laser pulse, a high-quality CVB beam is obtained in a broadband THz frequency range. The proposed method is universal and low-cost, opening a simple avenue for the emission of broadband THz specialty beams.

The newly two-dimensional layered materials MA2Z4 (M = Ti/Cr/Mo, A = Si/Ge, Z = N/P/As) possess excellent stability, strong valley-dependent properties, and novel electronic structures. Motivated by recent experimental advances, we have constructed a van der Waals heterostructure consisting of MoSi2N4 and ferromagnetic CrCl3 monolayer and performed first principles calculations to investigate the spin-valley coupling and valley splitting properties. The MoSi2N4/CrCl3 heterostructure is thermally stable at room temperature. Our calculations have shown an enhancement of both valley splitting and valley-contrasting transport properties due to the magnetic proximity effect. Most importantly, the valley-related properties of the MoSi2N4 monolayer are predicated to be rather robust resulting from the protection of the outermost N–Si sublayers. These findings could be helpful to the understanding of the ferromagnetic substrate effect on the valley-dependent properties of MA2Z4-based heterostructures.

Metal–semiconductor junctions are at the heart of modern electronics. Various two-dimensional materials (2D) based electronic devices have been reported. However, Ohmic contacts between evaporated metal contacts and semiconducting 2D materials are still hard to achieve. Thus, design and fabrication of Schottky diodes based on layered WSe2 remain challenging. Here, we report a doping-free strategy to achieve Ohmic contacts in WSe2 diodes via van der Waals (vdW) contacts. We designed and fabricated vertical WSe2/Pd Schottky diodes, in which Ohmic and Schottky junctions can be realized simultaneously via vdW contacts and evaporated contacts, respectively. The specific contact resistance of the vdW contact is about 74.5 kΩ·μm2. The Schottky diodes exhibit strong rectification behavior with rectification ratio up to 105.

Quasi-one-dimensional layered TiS3 nanosheets possess highly anisotropic physical properties. Herein, we reported the anisotropic strain response of Raman and the piezoresistance effect in layered TiS3 nanosheets. An attractive Gruneisen parameter ( γm) of 5.82 was achieved for AgIII mode in the b-axis strained TiS3 nanosheet, while a negligible value of γm was obtained when the strain is applied along the a-axis direction. We also revealed the opposite piezoresistive effect with strains applied along the two principal axes, demonstrating a gauge factor ratio of approximately −1:3.2. The giant anisotropy is attributed to the strain modulated bandgap, which was further confirmed by density functional theory calculations.

The structural and electronic properties of two-dimensional gallium nitrides (2D GaNs) on a van der Waals (vdW) epitaxial substrate are investigated using first-principles calculations. We show that the structures and electronic properties of mono- and bilayer GaN are hardly affected when deposited on the vdW substrate comprising a graphene sheet placed on the GaN(0001) bulk surface. A weak attractive interaction is found to work between the 2D GaNs and vdW substrate, which is still sufficient to maintain the 2D GaNs on the substrate and could also be exploited to control their stability. The present findings demonstrate that the vdW substrate is propitious to grow and hold the 2D GaNs without altering their intrinsic properties, in contrast to previously examined metal substrates.

A divergent microstructure was fabricated by CMOS compatible processes on the central region of a Ge p i-n photodetector to enhance the residual tensile strain. Tunable biaxial tensile strain of ~0.22-1.01% was achieved by varying the geometrical factors, and it was confirmed by Raman measurements and finite element method (FEM) simulations. The suspended germanium membranes enhance the absorption across the C- and L-band (1,528–1,560 nm and 1,561–1,620 nm) and extend the cutoff wavelength to ~ 1,700-1,937 nm. The Ge absorption coefficient is enhanced by ∼4.2× to 2,951 cm−1 at 1,630 nm which is comparable with that of In0.53Ga0.47As. Furthermore, due to the varying strain distribution on the Ge mesa, each photodetector presents the location-determined wavelength-selective photoresponsivity characteristics. This work offers a promising approach for adjusting the absorption spectra of the photodetector by harnessing geometrically amplified biaxial strain. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version The work was financially supported by Singapore National Research Foundation under the Competitive Research Program (NRF–CRP19–2017– 01) and Ministry of Education (AcRF Tier 1 2019-T1- 002-040 (RG147/19 (S))).

Hyperbolic crystals like {\alpha}-MoO$_3$ can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Raman scattering from flakes of {\alpha}-MoO$_3$. Both helicity-preserving and helicity-reversing Raman scattering are observed. We observe that helical selectivity is largely governed by the underlying crystal symmetry. This study shed light on the chiral character of the high symmetry phonons in these hyperbolic crystals. It paves the way for exploiting proposed schemes of coupling chiral phonon modes into propagating surface plasmon polaritons and for compact photonic circuits based on helical polarized light., Comment: The following article has been submitted to Applied Physics Letters

Due to its high chemical stability and high power conversion efficiency as a solar cell absorber, the inorganic halide perovskite, CsPbI3, is considered one of the most promising competitors to its hybrid organic-inorganic counterpart, CH3NH3PbI3. However, the phase transition from the photoactive black phase to the inactive yellow phase is a remarkable limitation that harms long-term phase stability. In particular, the phase transitions follow different pathways as the temperature increases and/or decreases, a phenomenon that is anomalous and remains poorly understood. In this study, we systematically calculated the temperature-dependent free energy of CsPbI3 in different crystal phases (α, β, γ, δ) by considering the phonon contribution to the Gibbs free energy. It is found that the free energy results from calculations that include harmonic phonons cannot reproduce experimental observations. Alternatively, we utilized the renormalized phonon quasiparticle approach to derive the free energies of different CsPbI3 phases at finite temperatures. Based on these calculated free energies, whose derivations included the anharmonic effect, we observed phase-transition processes consistent with experimental results. The analysis of the temperature effect on the phonon frequencies further demonstrated that anharmonic effects in the CsPbI3 had a significant influence on its phase transitions.

Diamond has the advantages of excellent thermal conductivity, and it is the material with the largest thermal conductivity currently known, which makes it the ideal heat sink. However, in the process of processing or as a heat sink for electronic devices, diamond is often exposed to a high-frequency, high-voltage, high-temperature, and high-energy environment, which has greatly affected the performance of diamonds. Therefore, in this paper, it systematically explores the influence of near threshold energy electron irradiation and annealing at different temperatures on the thermal conductivity of IIa diamond by the method of laser excited Raman spectroscopy. The results show that before 900 °C annealing, the main factor affecting thermal conductivity in IIa diamond is the large number of vacancy defects caused by near threshold energy electron irradiation. The irradiation and annealing both affect the thermal conductivity by controlling the vacancy defects concentration.

MnNiSi-based compounds exhibit giant isothermal magnetic entropy change (ΔSM) across their induced first-order coupled magnetostructural transition (MST) in the vicinity of room temperature, though in most of the cases, the use of Maxwell relation from a very frequently used but incorrect measurement protocol provides a nonphysical spike to the calculated ΔSM. Herein, to realize the accurate measurement protocol, we explore magnetocaloric properties of a (FeCoGa)x doped (MnNiSi)1−x compound with x = 0.165 rigorously. Several methods, including the estimation of ΔSM using Maxwell relation, Clausius-Clapeyron equation, and also from the calorimetry measurement, are discussed explicitly. The studied material is observed to show a MST at 265 K and a giant ΔSM as large as about −29.3 J kg−1 K−1 due to a magnetic field change of 5 T following the Maxwell equation during discontinuous cooling and field increasing mode, which enables the material as a promising candidate for magnetic refrigeration.

Unidirectional micro-nano-light sources have been widely applied in integrated photonics and optoelectronic devices. In this study, on-chip all-inorganic cesium lead halide perovskite nanowires are employed and excited as a bright light source to generate unidirectional light. Here, we directly image the angular emission patterns of perovskite nanowires by using fluorescence Fourier microscopy. The extinction ratio for the unidirectional emission is achieved up to 11 dB, and the detected minimum polar divergence angle reaches 8.6° in experiments. The emission direction also can be tuned flexibly within the range of 48° by changing the upper surrounding medium. Detailed calculations based on multi-mode expansion theory are used to reveal the inner physical mechanisms. Further analysis of the far-field emission patterns corresponds well with the experiment results. Our works investigating the perovskite nanowire light source emission from the perspective of Fourier space have potential applications in photon-detection and integrated photonics chips.

Topological protection against localization causes electrical transport phenomena in disordered topological materials to differ from those in topologically trivial systems. For example, a transition between a regime of weak localization to one of weak antilocalization can occur in systems such as topological insulators and topological semimetals when an external potential is applied across the system. Here, we report on the transverse magnetoresistance of thin films of cadmium arsenide, a topologically nontrivial, as we tune the electronic states and the Fermi level. We show that the appearance of weak localization and weak antilocalization sensitively reflects the relative contributions of multiple transport channels involving both gapless (massless) and gapped (massive) Dirac fermion states present in these films. The data are consistent with expectations of the different topological states of these films. Weak (anti-)localization phenomena can, therefore, serve as a probe of the types of Dirac fermions present in topological semimetals.

We present single-shot quantitative phase imaging with polarization differential interference contrast (PDIC) for a differential interference contrast microscope which records the unfiltered Stokes vector of the differential interference pattern with a polarization camera. PDIC enables high spatial resolution phase imaging in real-time, applicable to either absorptive or transparent samples and integrates simply with epi-fluorescence imaging. An algorithm with total variation regularization is further introduced to solve the quantitative phase map from the partial derivative along one single axis, improving the accuracy and the image quality from the Fourier transform solution. After quantifying the accuracy of PDIC phase imaging with numerical simulations and phantom measurements, we demonstrate the biomedical applications by imaging the quantitative phase of both stained and unstained histological tissue sections and visualizing the fission yeast Schizosaccharomyces pombe's cytokinesis.

As an emerging class of two-dimensional (2D) materials, van der Waals (vdW) magnets have attracted a lot of research attention since they can give access to fundamental physics and potential spintronic device applications. Among these 2D vdW magnets, CrSiTe3, as an intrinsic ferromagnetic semiconductor, exhibits great potentials in low-dimensional spintronics. Of particular interest in this 2D vdW magnet is the electronic and magnetic properties at the atomic-scale, which has yet been fully explored so far. Here, combing angle-resolved photoemission spectroscopy, bulk magnetic measurements, and synchrotron-based x-ray techniques, an unambiguous picture of the electronic and magnetic states of CrSiTe3 is presented. Hybridization of Cr-3d and Te-5p orbitals and the semiconducting behavior are confirmed by the band structure detection. Intrinsic ferromagnetism with a magnetic anisotropy constant of 1.56 × 105 erg/cm3 is attributed to the superexchange interaction of the Cr3+ ions. In addition, temperature-dependent spin and orbital moments are determined, and a fitted critical exponent of 0.169 implies that CrSiTe3 is in good agreement with the 2D Ising model. More remarkably, unquenched orbital moments are experimentally evidenced, bringing CrSiTe3 with orbital-dependent intriguing effects and great potentials toward the spintronic devices.

Two-dimensional van der Waals magnetic crystals have been attracting significant research interest in recent years, and the manipulation of their magnetism is important for understanding their physical property and achieving their actual applications. Here, we systematically studied the manipulation of magnetic properties of a CrTe2 bilayer through in-plane strain and self-intercalation. We found that the magnetic ground state of the CrTe2 bilayer varies from intralayer antiferromagnetic coupling to ferromagnetic coupling and then to interlayer antiferromagnetic coupling when the strain changes from −6% to 4%, which should result from the coupling between intralayer Cr atoms tuned from direct Cr–Cr exchange to indirect Cr–Te–Cr superexchange. The magnetic easy axis of the CrTe2 bilayer varies from the in-plane to the out-of-plane owing to the change of pz orbital occupation from Te atoms near the Fermi level. Moreover, the magnetic ground states of different Cr-intercalated concentrations for the CrTe2 bilayer are all ferromagnetic, and the magnetic easy axis is in-plane, which are different from the intrinsic one. Our results indicate that the magnetic property of the CrTe2 bilayer is sensitive to the in-plane strain and self-intercalation, which provides important guidance for the further magnetic manipulation of the CrTe2 bilayer in theoretical research and application of magnetic strain sensors and spin transistors.

Cs2AgBiBr6 double perovskite nanocrystals (NCs) are materials within the halide perovskite family with promising application potential, owing to their nontoxicity and excellent stability. Herein, Cs2AgBiBr6 NCs were synthesized and their photo-physical properties were investigated in detail. The temperature-dependent photoluminescence (PL) test confirms that their PL mechanism is dependent on the competition between the PL originating from carrier recombination in the conduction band and that in the self-trapping states. Their carrier recombination constants (involving the monomolecular, bimolecular, and Auger recombination rate constants) were obtained from transient absorption data and indicated that the monomolecular recombination decreases with the lowering of temperature, while the bimolecular and Auger recombination show opposite trends. The nonlinear optical properties were studied based on the Z-scan technique, with a two-photon absorption cross section of approximately 1906 GM. Our results suggest that Cs2AgBiBr6 NCs can be extensively applied in the field of optoelectronics, and conventional electronic switches will benefit from the excellent carrier dynamics and nonlinear optical properties.

Designing efficient oxygen evolution reaction (OER) electrocatalysts is essential for numerous sustainable energy conversion technologies. An obstacle that impedes the development of OER electrocatalysts is the insufficient emphasis on the spin attribution of electrons. Recently, the different spin configuration of reactants and products in the OER has been recognized as the factor that slows down the reaction kinetics. In this work, Mn substitution was introduced to LaCoO3, which brought about lattice expansion and reduced crystalline field splitting energy. This led to the increase in the effective magnetic moment, which triggers the transfer of Co3+ from low to higher spin states. Thus, the hybridization of Co eg and O 2p states across the Fermi level was strengthened. Specifically, with 25 at. % Mn substitution, LaCoO3 transits from a semiconductor to a half-metal, which benefits the spin-oriented electronic transport and resultantly promotes the OER. This method paves the way for the construction of spin pathways in catalysts.

The paucity of research on hydrogen evolution reaction (HER) under neutral conditions, which is a more sustainable way to produce H-2 compared to acidic and alkaline HER, encourages the development of efficient catalytic materials and devices and deeper investigation of the mechanisms behind neutral HER. We present an electrode concept for facilitating HER under neutral conditions. The concept entails the use of annealing-reshaped silver (Ag) nanoparticle array on monolayer epitaxial graphene (MEG) on 4H-SiC. Measurements of HER performance show more positive onset potential of the cathodic HER for Ag-decorated MEG compared to that for pristine MEG, indicating improved water dissociation at Ag/MEG electrodes. Complementary morphological characterization, absorption measurements, and Raman mapping analysis enable us to ascribe the enhanced catalytic performance of electrodes decorated with 2 nm thick annealed Ag on the synergetic effect originating from simultaneous water reduction on circular Ag nanoparticles of 31 nm in diameter and on compressively strained Ag-free graphene regions. The overall results pave the way toward development of stable van der Waals heterostructure electrodes with a tunable metal-carbon interaction for fast HER under neutral conditions. (C) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Funding Agencies|Angpanneforeningens Forskningsstiftelse [16-541, 21-112]; VRSwedish Research Council [2018-04962, 2016-05362]; Swedish Research Council (Vetenskapsradet)Swedish Research Council [2018-04670, 2020-04400]; AForsk foundation [19-311]; FCT/MEC [UIDB/50025/2020, UIDP/50025/2020]

In-plane propagation of excited light in layered materials boosts the observation of novel phenomena, which differ from out-of-plane propagation. In this work, we perform the Raman study with light propagation parallel to the plane of layered MoS2. The Raman signal is unveiled at least an order of magnitude larger than that perpendicular to the plane when the excitation photon energy at ∼2.81 eV. We attribute this high-yield Raman spectra to the stronger photon–exciton coupling with in-plane propagation of light. Furthermore, we show that the exciton-mediated phonon excitation in the first-order Raman process is dominant with consideration of angular momentum transfer between phonons and photons through the measurements of the circularly polarized Raman spectra. This experimental setup configuration paves a way with high efficiency to investigate the phonon information in layered materials.

We experimentally investigated the anomalous Nernst effect (ANE) in an Fe3Si film, whose ANE was predicted to be large, based on the topological property and the Berry curvature, and systematically compared it with other compositions of iron-silicide, viz., Fe2Si, FeSi, and FeSi2 films. Although both the ferromagnetic Fe3Si and Fe2Si films showed an ANE voltage, the highest ANE coefficient SANE = 1.0 μV K−1 was obtained for Fe3Si, which is larger than that for Fe. We also measured the Seebeck and anomalous Hall effects to estimate the transverse thermoelectric conductivity αyx, suggesting that the contribution of αyx was dominant in the Fe3Si ANE enhancement.

Phonon-mediated motion transfer plays an essential role in scalable phononic devices. We present geometric motion transfer between two mechanical resonators, which are indirectly coupled by parametrically coupling to the same intermediate mechanical mode. The phonon-mediated coupling is controlled so that the intermediate mode remains unoccupied after the system undergoes a cyclic evolution. Pure geometric motion transfer between the indirectly coupled mechanical resonators is achieved experimentally through a controlling motion evolution path in the Hilbert space. Because the geometric phase is robust against local fluctuation of the evolution path, our research provides a prospective scenario for noise-resilient phonon-based information processing.

For zinc blende semiconductors, such as InP, the Raman selection rules for a backscattering configuration from the (001) surface forbid the transversal optical (TO) phonon mode, whereas the longitudinal optical mode is allowed. However, when InP is highly doped with Si atoms, InP-Si clusters with the reduced C3v symmetry allow TO modes in the Raman spectrum with the backscattering configuration. Here, we demonstrate that the amplitude of the TO modes can be modulated spatially by using a highly doped InP grating. By exciting the sample with a laser linearly polarized parallel and perpendicular to the grating grooves, we observe a change in amplitude of the phonon optical response for the TO mode.

In this report, the thermal performance of a hydrogen (H)-terminated diamond field-effect transistor (FET) is investigated using Raman spectroscopy and electrothermal device modeling. First, the thermal conductivity (κdiamond) of the active diamond channel was determined by measuring the temperature rise of transmission line measurement structures under various heat flux conditions using nanoparticle-assisted Raman thermometry. Using this approach, κdiamond was estimated to be 1860 W/m K with a 95% confidence interval ranging from 1610 to 2120 W/m K. In conjunction with measured electrical output characteristics, this κ was used as an input parameter for an electrothermal device model of an H-terminated diamond FET. The simulated thermal response showed good agreement with surface temperature measurements acquired using nanoparticle-assisted Raman thermometry. These diamond-based structures were highly efficient at dissipating heat from the active device channel with measured device thermal resistances as low as ∼1 mm K/W. Using the calibrated electrothermal device model, the diamond FET was able to operate at a very high power density of 40 W/mm with a simulated temperature rise of ∼33 K. Finally, the thermal resistance of these lateral diamond FETs was compared to lateral transistor structures based on other ultrawide bandgap materials (Al0.70Ga0.30N, β-Ga2O3) and wide bandgap GaN for benchmarking. These results indicate that the thermal resistance of diamond-based lateral transistors can be up to ∼10× lower than GaN-based devices and ∼50× lower than other UWBG devices.

We advance and experimentally implement a protocol to generate optical coherence lattices (OCLs) that are not modulated by an envelope field. Here, we dub them perfect OCLs. Structuring the amplitude and phase of an input partially coherent beam in a Fourier plane of an imaging system lies at the heart of our protocol. In the proposed approach, the OCL node profile depends solely on the degree of coherence of the input beam such that, in principle, any lattice structure can be attained via proper manipulations in the Fourier plane. Moreover, any genuine partially coherent source can serve as an input to our lattice generating imaging system. Our results are anticipated to find applications to optical field engineering and multi-target probing among others.

The surface plasmon resonance (SPR) effects of metal nanoparticles prove versatile in terms of equipping wideband-gap photocatalysts, such as TiO2, with visible-light responsiveness. In this regard, Au is the most frequently used material for its SPR, but alternative materials are also being actively developed to effectively utilize solar light from the visible to the near-infrared region. In this work, we found that Pt particles loaded on TiO2 were also active in effecting SPR-induced photocatalysis. Time-resolved absorption measurements confirmed that visible-light irradiation induces electron transfer from Pt to TiO2, and the efficiency of this electron injection increases as the wavelength of the incident light decreases from 660 to 450 nm. Observations of the peak shift of the vibrational frequency of adsorbed CO on Pt confirmed that the Fermi level of Pt decreases as the electron transfer from Pt to TiO2 proceeds. These results imply that Pt nanoparticles can act as sensitizers to induce electron transfer from Pt to TiO2, although Pt is a well-known cocatalyst that enhances H2 evolution by collecting electrons from TiO2. However, as the intensity of the irradiated light increased beyond 50 mW cm−2, a portion of the Pt particles started to capture the injected electrons from TiO2, suggesting that the electron transfer from one Pt particle to the other Pt particles via the conduction band of TiO2 proceeds under visible light illumination. These opposing roles may be ascribed to the variation in the size of Pt particles as well as density of electrons injected into TiO2.

Scientists have been seeking for suitable materials with nonvolatile resistive switching (RS) performance for memristive applications. Recently, nonvolatile RS behaviors have been achieved in an increasing number of two-dimensional (2D) materials. However, 2D InSe layers have not been reported to demonstrate such nonvolatile RS behaviors. Herein, we experimentally observe nonvolatile bipolar RS behaviors in 2D InSe nanosheets through controllable oxidation. In our experiments, the exfoliated InSe nanosheets annealed at the temperature of 350 °C for 2 h show typical nonvolatile bipolar RS performance with a low SET voltage of ∼0.3 V and a high ON/OFF ratio of 4.5 × 103 at the read voltage of 0.1 V. Raman and x-ray photoelectron spectroscopy characterizations confirm the partial oxidation in InSe nanosheets after annealing. The observed nonvolatile RS behaviors are owing to the formation of In2O3 and the increased insulating characteristic in the annealed InSe nanosheets. Furthermore, the fabricated memristor exhibits good retention property and endurance performance. Such annealed InSe nanosheets not only demonstrate decent RS performance but also enrich the family of 2D materials to fabricate memristors for applications in next-generation nonvolatile memory.

The classical Raman tensor approach does not work for optically anisotropic materials in general. For the transparency regime, this can be circumvented by using an effective Raman tensor that considers the material's birefringence. In the specific case of an uniaxial crystal with in-plane orientation of principal axis, this effective Raman tensor is similar to the classical one, except for an additional phase factor for each tensor element. This phase is dependent on the sample thickness, which is demonstrated here experimentally via polarization resolved Raman scattering of a-plane GaN thin films. The experimental observations coincide very well with our model predictions.

Transition metal-doped two-dimension carbon matrices have attracted particular interest as oxygen reduction reaction (ORR) catalysts because of their low-cost, good conductivity of electricity, and promising applications in fuel cells and metal-air batteries. Herein, a density functional theory study is performed on the CoNxC4-x (x = 0–4) embedded graphene to investigate the effect of N atoms doping number and doping configurations. The calculated formation energy and average bond length of Co–C/N drop off with the increase in N atoms of the CoNxC4-x graphene system. The most stable adsorption configurations and the relevant adsorption free energies of key ORR intermediates on Co–N sites toward the CoNxC4-x graphene system are obtained, indicating that N doping levels and doping configurations have a regular influence on this system. On this basis, scaling relations can be obtained among the adsorption free energies of *OH, *OOH, and *O. The volcano plot of ORR theoretical overpotential (ηth) using ΔG*OH−ΔG*O as a descriptor was further established, which revealed that ηth is influenced by the adsorption mode and the free energy change in the active site. For all studied systems, the ORR substeps are all downhill at zero potential from the plotted free energy diagrams. The density of states is employed to further illustrate that the hybridization between the Co atom and the O atom is a deterring factor on electrocatalyst activity. These calculations reveal the influence of nitrogen atom doping in Co–N-graphene catalysts and afterward point a direction for designing high-performance non-precious metal ORR electrocatalysts.

Full control of the ellipticity of broadband pulses of THz radiation, from linear to left- or right-handed circular polarization, was demonstrated via a 4-pixel photoconductive emitter with an integrated achromatic waveplate. Excellent polarization purity and accuracy was achieved, with Stokes parameters exceeding 97% for linear and circular polarization, via a robust scheme that corrected electrically for polarization changes caused by imperfect optical elements. Further, to assess the speed and precision of measurements of the THz polarization we introduced a new figure of merit, the standard error after one second of measurement, found to be 0.047$^{\circ}$ for the polarization angle., Comment: 5 pages, 4 figures, 1 table

We investigate the Hall resistance of metallic multilayers Ta/Cr/CoTb/Ta/Si. In addition to the anomalous Hall effect originating from the ferrimagnetic CoTb layer, the unconventional Hall effect (UHE) is observed in our multilayer samples. The UHE depends not only on the current and magnetic fields but also on the device geometry and temperature in a unique way. Our results suggest that the UHE does not originate from the spin–orbit torque driven magnetization tilting but occurs possibly due to thermionic emission and Lorentz force at the metal/Si interface, where the Schottky barrier is formed. We also find that the space-charge effect causes geometric dependence of the Hall resistance. The magnitude of UHE is sizable and linearly proportional to the longitudinal magnetic field, suggesting that the observed UHE is attractive to the magnetic-field sensing industry.

Strontium titanate (SrTiO3) is one of the most promising photocatalysts for overall water splitting (OWS). Strains can be commonly introduced in SrTiO3 during the synthetic processes, for example, in pulsed laser deposition. However, the effect of the strains on the catalytic performance of OWS is still unclear to date. Herein, first-principles calculations were performed to evaluate the impact of biaxial strains applied in SrTiO3 (001) surface on the oxygen evolution reaction (OER), the bottleneck of OWS. It was found that the applied strains, especially the compressive strains, significantly affect the free energy changes for H2O dissociation and *OOH deprotonation of OER on SrTiO3 (001). More interestingly, the tensile strains reduce the surface energy and lower the OER overpotential through reducing the free energy change for the rate-determining step, i.e., the second step of OER (*OH deprotonation), while the compressive strains increase the surface energy and the OER overpotential. Consequently, it is probable to introduce the tensile strains in the synthetic process to enhance the OER performance on SrTiO3 (001) surface.

In this paper, we demonstrate a frequency modulation technique to accurately measure the probe Stark shift of optical clocks. The effective coupling strength of the probe laser to the clock transition is precisely controlled by this frequency modulation technique, which is different from the traditional method by controlling the effective coupling strength via changing the probe light power. In an 87Sr optical lattice clock, the probe Stark shifts are measured as a function of the square of the bare state coupling strength. The probe Stark shift coefficient of 87Sr is experimentally determined as −6.0(5)×10−7/Hz using this frequency modulation technique. With the help of frequency modulation, the probe light power can be as large as possible without decreasing the measurement precision and the achieved measurement lever arm is improved by more than one order of magnitude compared with previous results.

Two-dimensional (2D) ferromagnetic materials with high Curie temperatures (TC) and tunable physical properties are crucial to the development of nanoscale spintronics devices. Here, we investigate the newly synthesized iodinene using first-principles calculations. Our results show that doping carriers in monolayer and bilayer iodinene can easily introduce itinerant ferromagnetism due to a flatband structure near the Fermi level, and that the associated TC is higher than room temperature. Moreover, we find that a structural phase transition can be achieved through the application of moderate tensile strain for both monolayer and bilayer iodinene. The magnetic moment, Curie temperature, band structure, carrier effective mass, and optical absorption can be changed significantly through this phase transition, and the transition can also lead to a magnetic phase transition with an appropriate doping concentration. Our work provides a feasible approach for designing 2D magnetic materials with potential for application in microelectronics devices.

Non-diffracting Bessel acoustic beam that propagates in three dimensions has the properties of self-healing, self-bending, and self-acceleration, giving them many potential applications. Here, we report a zero-order Bessel-like acoustic beam that propagates along a curved trajectory with a phase-modulating acoustic holographic lens. Its pressure distribution on the transverse section approximately conforms to the zero-order Bessel function, and the self-healing property is found. In addition, the maximum point of acoustic intensity on the beam mainlobe can be adjusted by the change in the acoustic frequency. Microbubbles can be forced toward the maximum point along the predefined curve. Therefore, such a beam can be used for the directional transportation of microbubbles with a predefined curve and an adjustable destination. We have numerically calculated the primary Bjerknes force, including the oscillation of the microbubbles with the shell and experimentally proved the curved trajectory and the adjustable destination of the transportation of microbubbles. The proposed method uses the conservative part of the radiation force, which controls the trajectory of microbubbles by the amplitude gradient. The relatively free design of the transport trajectory and destination makes the approach suitable in vivo.

Focusing to a small spot along a large depth of focus (DOF) beyond the limits imposed by diffraction has attracted extensive theoretical as well as practical interest. The aspect ratio, defined as the DOF divided by the diameter of spot, has been extended by several schemes with reasonable success, but there is no report that enables one to modulate the aspect ratio as long as possible with sidelobe suppression. In this Letter, we propose a method to generate ultrahigh-aspect-ratio beams (UARBs) by introducing an analytic procedure using Zernike polynomials in the modified iterative Fourier-transform algorithm, and a super-resolution spot is also realized to further extend the aspect ratio. We design the diffractive optical elements that generate UARBs with sidelobe suppression and further experimentally demonstrate a stretched 1602:1 aspect ratio of UARBs with a lateral super-resolution spot, suggesting potential applications for further super-resolution imaging or precision fabrication.

Recently discovered massive Dirac fermions and room temperature skyrmions in the ferromagnetic kagome metal Fe3Sn2 have attracted extensive attention due to the potential applications in topological and spintronic devices. Because of its centrosymmetric lattice, the topological spin texture in Fe3Sn2 arises primarily from the competition between anisotropy and exchange interaction. In this Letter, we have identified the spin-reorientation at around 80 K by the magnetization and AC-susceptibility measurements. The evolution of the anisotropy is revealed by neutron powder diffraction data from 18 to 700 K, where the “breathing”-like kagome structure evolves into an ideal kagome lattice at higher temperature. Meanwhile, the deviation between the experimental and calculated results on heat capacity at high temperature and the softening of optical phonon modes in Raman spectra suggest the presence of anharmonic phonons in Fe3Sn2, which is responsible for the degeneracy of lattice thermal conductivity at high temperature. Our study indicates that Fe3Sn2 possesses a promising future in the design and development of topological, spintronic as well as thermoelectric devices.

The cubic boron nitride is the second most important superhard materials. To date, the effect of crystallographic anisotropy of the {111} plane on the Knoop microhardness of cubic boron nitride single crystals has not been experimentally obtained. Herein, we measure the detailed Knoop hardness and Vickers hardness for the {111} plane of the amber cubic boron nitride. We find that the crystal orientation dependence of Knoop hardness for amber samples is remarkable and has an obvious regular pattern. The Knoop hardness for amber samples shows a minima about 5° from ⟨110⟩ directions, which is different from the traditional viewpoint that the minimum hardness is on ⟨110⟩ directions. Raman spectra of cubic boron nitrides are measured and help us to explain the hardening of samples.

Realizing and controlling topological spin textures are widely attractive for their promising applications in information and communication technology. Here, using first-principles calculations and micromagnetic simulations, a stable magnetic Janus material MnBi2(Se, Te)4 with an inherent Dzyaloshinskii–Moriya interaction is proposed to stabilize a variety of topological magnetic structures under temperature and magnetic fields. Furthermore, a sub-10 nm skyrmion can spontaneously form in the absence of magnetic fields in MnBi2(Se, Te)4 by using van der Waals heterostructure engineering. These results provide a promising route for the generation and modulation of topological spin textures in two-dimensional spintronic devices.

Stacking two-dimensional materials vertically by van der Waals interaction is a promising approach to realize band structure engineering. Recently, layered two-dimensional materials with adjusted twist angle provide a platform for various novel phenomena. By means of density functional theory and tight-binding modeling, we investigate the bilayer polyaniline (PANI) as a prototype to explore the bandgap tuning mechanism by interlayer twist. A wider and continuous bandgap tuning is found in twisted bilayer PANI compared with the tuning by interlayer translation. The unique bandgap tuning by twist angle can be understood by fast exponential decaying of interlayer coupling strength, twist angle dependent interlayer coupling patterns, and charge transfer effect. These findings might provide an efficient bandgap tuning method by twist angle in 2D layered materials, which is the key to applications in electronic and optical devices.

Using electron emission spectroscopy, measurement and analysis were conducted on the energy distribution of vacuum emitted electrons from an electrically driven InGaN/GaN commercial blue c-plane (peak wavelengths λ≈465 nm) light emitting diode (LED) with 60 nm of p-GaN on top of the active region. The signal-to-noise ratio of semiconductor peaks is improved on the thin p-GaN LED compared to previously published data on thicker p-GaN samples and is attributed to reduced loss of electrons en route to emission into vacuum during transit through the p-GaN. This further proves that hot electrons are generated in the bulk region and not by light or other hot electron generation mechanisms at the surface. Using square root of the light output power as a proxy for the active region carrier density, n, the hot electron integrated peak intensity is shown to be proportional to n3 and, thus, is directly attributed to a 3-body Auger process. Since there are significant Auger recombination currents even at low injection current densities, it is expected that Auger recombination current will dominate over radiation recombination and Shockley–Read–Hall (SRH) currents at higher current densities. This identifies Auger recombination as the dominant cause of efficiency droop.

Crystal-orientation-dependent properties in low symmetric two-dimensional material ReS2 have inspired its novel applications in photonics, electronics, and optoelectronics. In this work, Ar+ ion bombardment is used to treat ReS2 with different thicknesses, and different amounts of defect densities are introduced in ReS2 flakes. Angular dependent polarized Raman spectra is used to probe the change in anisotropy. The intensities of mode V exhibit maximum and minimum intensities with incident polarization along the b-axis and perpendicular. The intensity ratio Imin/Imax is used as a quantity to detect the change in anisotropy. It is found that with the same defect density, the change in anisotropy in 1-layer (L) ReS2 is the highest among ReS2 flakes; thus, 1L ReS2 is susceptible to ion bombardment. With an increase in the defect density in 1L ReS2, an increase in the Imin/Imax value is observed. A phenomenological model is established to understand the quantitative relationship between defect density and anisotropy, and the isotropic scattering range associated with defects is obtained. The results here provide ways of quantitatively understanding anisotropy by angular polarized Raman spectroscopy as well as ways of tuning anisotropy in two-dimensional anisotropic materials by ion bombardments.

In this paper, negative transconductance (NTC) behavior in molybdenum disulfides (MoS2) field effect transistors (FETs) is investigated. Combining experimental observation and numerical analysis, we demonstrate that positive shift in the device transfer curves results from the electron trapping/de-trapping processes, where the defect densities at the MoS2/SiO2 interface are reduced when the temperature T decreases from 300 to 200 K. Moreover, the main types of defects that affect the device electrical performance are the interface defect and bulk sulfur vacancy VS in which VS induces the p-type doping effect. While decreasing T below 100 K, NTC occurs when their active layer thickness t (=41 and 35 nm) is larger than the Debye length λ (28 nm). Considering the n-type doping effect induced by the interface defects and the p-type doping caused by the bulk S vacancies, these two opposite doping regions are carefully implemented in simulation at T = 70 K. A vertical barrier induced by the inhomogeneous electron distribution enlarges with the increased gate bias VGS and, thereafter, leads to the unconventional increase in the contact and total resistances with t > λ. While t ≦ λ, the barrier and NTC behavior disappear. The current IDS and transconductance g obtained from the simulation confirm the low-temperature NTC mechanism related to the defects as discussed above. The material defects and physical origin of NTC discussed in the multilayer MoS2 transistors provide the theoretical foundation for designing and realizing novel structures of functional devices via defect engineering in the two-dimensional FET.

The heart of current high-throughput computational design for two-dimensional (2D) van der Waals (vdW) functional heterostructures is Anderson's rule (AR). This non-interacting model, however, inevitably introduces error and uncertainty in design results, problems which remain neglected. We report that, even for a non-magnetic system, the current high-throughput design framework commonly omits potential candidates in specific fields, such as heterostructure solar cell research. Therefore, this framework is fragile to extend to all subfields of functional vdW heterostructures. Through the analysis of several factors, such fragility is found to arise mainly from interlayer orbital coupling. This effect is omitted by AR and causes statistical deviation in the prediction of electronic properties. In the absence of a more advanced and universal physical model for describing interlayer orbital coupling, we propose a robust high-throughput design framework to reduce the omission of potential candidates. This work deepens the understanding of high-throughput design for 2D vdW functional heterostructures and provides insight into their development.

Currently, intercalation of foreign guest atoms into two-dimensional (2D) layered van der Waals materials is an active research area motivated in part by the development of next-generation energy-storage technologies and optoelectronic devices. One such extensively studied 2D material is the graphene-on-SiC system. To realize and control the desired intercalated structures, it is fundamentally important to understand the kinetic process of intercalation. For the intercalation of a guest atom into graphene layers on SiC substrate, a critical kinetic parameter is the energy barrier of a guest atom penetrating the perfect graphene top layer into the gallery under it. However, accurate theoretical calculations for such penetration barriers are unavailable in literature. From our first-principles density functional theory calculations, we obtain the global energy barriers of 3.47 and 1.80 eV for single Dy and H atoms penetrating the graphene top layer on a graphene buffer layer supported by a Si-terminated 6H-SiC(0001) substrate, respectively. For comparison as well as for examining the lateral strain effects, we also obtain the global barriers of 5.05 and 1.50 eV for single Dy and H atoms penetrating freestanding bilayer graphene with a tensile strain of about 8.8% to match our model for supported graphene, as well as the global barriers of 7.21 and 4.18 eV for penetrating unstrained freestanding bilayer graphene, respectively. From corresponding minimum energy paths with multiple energy minima and saddle points, we can also obtain various local energy barriers and the global backward barrier from the graphene gallery back to the top surface.

Electron spins in semiconductor quantum dots are a promising platform for quantum-information processing applications because their quantum phase coherence can persist for extremely long times. Nearest-neighbor electron spins naturally interact with each other via Heisenberg exchange coupling. Heisenberg exchange coupling results from the interplay of the electrostatic confinement potential together with the Pauli exclusion principle, which requires that no two electrons can have the same quantum number. Exchange coupling enables a host of useful capabilities, including the generation of different types of qubits, multi-qubit gates, ways to increase connectivity in systems of quantum-dot spin qubits, and routes to explore intriguing many-body phenomena.

The interactive control between electron charge and spin is essential to spintronics. Two-dimensional (2D) ferromagnetic semiconductors (FMSs) are a kind of unique candidate materials that could lead to flexible magnetoelectric and magneto-optic applications. In this Letter, we construct a series of van der Waals heterostructures by 2D magnetic CrXh (X = S/Se, h = Cl/Br/I) monolayers. Based on first-principles calculations, both monolayers and heterostructures have been demonstrated as intrinsic FMSs with large magnetic moments, suitable bandgaps, good carrier mobility, and high Curie temperatures. Taking advantage of the heterojunction effect, one can precisely control the charge transport behavior by forming three types of band alignments. Under an external alternating magnetic field, CrSeBr/CrSeCl and CrSBr/CrSI heterostructures can reversibly reconfigure between type-II and type-I band alignments upon switching of the spin direction. Combining the robust magnetic ordering and distinctive spin-polarized band alignment, our designed CrXh based magnetic heterostructures are ideal candidates for innovative magnetic-field-modulated photoelectric devices for realizing ultrarapid and reversible “write-read” processes.