Bias-dependent photoresponse of Td-WTe2grown by chemical vapor deposition.

The type-II Weyl semimetal Td-WTe2is one of the wonder materials for high-performance optoelectronic devices. We report the self-powered Td-WTe2photodetectors and their bias-dependent photoresponse in the visible region (405, 520, 638 nm) driven by the bulk photovoltaic effect. The device shows the responsivity of 15.8 mAW-1and detectivity of 5.2 × 109Jones at 520 nm. Besides, the response time of the WTe2photodetector shows the strong bias-voltage dependent property. This work offers a physical reference for understanding the photoresponse process of Td-WTe2photodetectors.

Tunable Negative and Positive Photoconductance in Van Der Waals Heterostructure for Image Preprocessing.

The processing of visual information occurs mainly in the retina, and the retinal preprocessing function greatly improves the transmission quality and efficiency of visual information. The artificial retina system provides a promising path to efficient image processing. Here, graphene/InSe/h-BN heterogeneous structure is proposed, which exhibits negative and positive photoconductance (NPC and PPC) effects by altering the strength of a single wavelength laser. Moreover, a modified theoretical model is presented based on the power-dependent photoconductivity effect of laser: I ph = - mP α 1 + nP α 2 ${\rm I}_{\rm ph}\,=\,-{\rm mP}^{\alpha _{1}} + {\rm nP}^{\alpha _{2}}$ , which can reveal the internal physical mechanism of negative/positive photoconductance effects. The present 2D structure design allows the field effect transistor (FET) to exhibit excellent photoelectric performance (RNPC = 1.1× 104 AW-1, RPPC = 13 AW-1) and performance stability. Especially, the retinal pretreatment process is successfully simulated based on the negative and positive photoconductive effects. Moreover, the pulse signal input improves the device responsivity by 167%, and the transmission quality and efficiency of the visual signal can also be enhanced. This work provides a new design idea and direction for the construction of artificial vision, and lay a foundation for the integration of the next generation of optoelectronic devices.

Phase transition in WSe2-xTex monolayers driven by charge injection and pressure: a first-principles study.

Alloying strategies permit new probes for governing structural stability and semiconductor-semimetal phase transition of transition metal dichalcogenides (TMDs). However, the possible structure and phase transition mechanism of the alloy TMDs, and the effect of an external field, have been still unclear. Here, the enrichment of the Te content in WSe2-xTex monolayers allows for coherent structural transition from the H phase to the T' phase. The crystal orbital Hamiltonian population (COHP) uncovers that the bonding state of the H phase approaches the high-energy domain near the Fermi level as the Te concentration increases, posing a source of structural instability followed by a weakened energy barrier for the phase transition. In addition, the structural phase transition driven by charge injection opens up new possibilities for the development of phase-change devices based on atomic thin films. For WSe2-xTex monolayers with the H phase as the stable phase, the critical value of electron concentration triggering the phase transition decreases with an increase in the x value. Furthermore, the energy barrier from the H phase to the T' phase can be effectively reduced by applying tensile strain, which could favor the phase switching in electronic devices. This work provides a critical reference for controllable modulation of phase-sensitive devices from alloy materials with rich phase characteristics.

Optically Induced Multistage Phase Transition in Coherent Phonon-Dominated a-GeTe.

Ultrafast photoexcitation can decouple the multilevel nonequilibrium dynamics of electron-lattice interactions, providing an ideal probe for dissecting photoinduced phase transition in solids. Here, real-time time-dependent density functional theory simulations combined with occupation-constrained DFT methods are employed to explore the nonadiabatic paths of optically excited a-GeTe. Results show that the short-wavelength ultrafast laser is capable of generating full-domain carrier excitation and repopulation, whereas the long-wavelength ultrafast laser favors the excitation of lone pair electrons in the antibonded state. Photodoping makes the double-valley potential energy surface shallower and allows the insertion of A1g coherent forces in the atomic pairs, by which the phase reversal of Ge and Te atoms in the ⟨001⟩ direction is activated with ultrafast suppression of the Peierls distortion. These findings have far-reaching implications regarding nonequilibrium phase engineering strategies based on phase-change materials.

Tunable lattice dynamics and dielectric functions of two-dimensional Bi2O2Se: striking layer and temperature dependent effects.

Two-dimensional (2D) Bi2O2Se semiconductors with a narrow band gap and ultrahigh mobility have been regarded as an emerging candidate for optoelectronic devices, whereas the ambiguous phonon characteristics and optical properties still limit their future applications. Herein, high-quality centimeter-scale 2D Bi2O2Se films are successfully synthesized to disclose the lattice dynamics and dielectric functions under the control of thickness and temperature. It has been demonstrated that the stronger electrostatic Bi-Se interactions result in a stiffened phonon vibration of thicker Bi2O2Se layers. Three excitons (Ea, Eb, and Ec) exhibit significant red shifts with layer stacking. Interestingly, the dielectric properties in the visible-near infrared region (Ea and Eb) are dominated by the combined effect of the joint density of states and mass density, whereas the dielectric properties in the ultraviolet region (Ec) are dominated by the exciton effect. Furthermore, the temperature-sensitivity of the phonon frequency and exciton transition energies is revealed to be layer-dependent. In particular, the optical response of Eb excitons exhibits a prominent dependence on temperature, which indicates a promising optical modulation by temperature in the visible spectrum. This study enriches the knowledge about phonon dynamics and dielectric properties for 2D Bi2O2Se, which provides an essential reference for high-performance related optoelectronic devices.

Directly measuring flexoelectric coefficients µ11 of the van der Waals materials.

Flexoelectricity originates from the electromechanical coupling interaction between strain gradient and polarization, broadly applied in developing electromechanical and energy devices. However, the study of quantifying the longitudinal flexoelectric coefficient (µ11) which is important for the application of atomic-scale two-dimensional (2D) materials is still in a slow-moving stage, owing to the technical challenges. Based on the free-standing suspension structure, this paper proposes a widely applicable method and a mensurable formula for determining the µ11 constant of layer-dependent 2D materials with high precision. A combination of in situ micro-Raman spectroscopy and piezoresponse force microscopy (PFM) imaging was used to quantify the strain distribution and effective out-of-plane electromechanical coupling, respectively, for µ11 constant calculation. The µ11 constants and their physical correlation with the variable mechanical conditions of naturally bent structures have been obtained extensively for the representative mono-to-few layered MX2 family (M = W and Mo; X = S and Se), and the result is perfectly consistent with the estimated order-of-magnitude of the µ11 value (about 0.065) of monolayer MoS2. The quantification of the flexoelectric constant in this work not only promotes the understanding of mechanical and electromechanical properties in van der Waals materials, but also paves the way for developing novel 2D nano-energy devices and mechanical transducers based on flexoelectric effects.

Mid-infrared photoluminescence revealing internal quantum efficiency enhancement of type-I and type-II InAs core/shell nanowires.

Internal quantum efficiency (IQE) is an important figure of merit for photoelectric applications. While the InAs core/shell (c/s) nanowire (NW) is a promising solution for efficient quantum emission, the relationship between the IQE and shell coating remains unclear. This Letter reports mid-infrared PL measurements on InAs/InGaAs, InAs/AlSb, and InAs/GaSb c/s NWs, together with bare InAs NWs as a reference. Analyses show that the IQE is depressed by a shell coating at 9 K but gets improved by up to approximately 50% for the InGaAs shell coating at 40 -140 K and up to approximately 20% beyond 110 K for the AlSb shell. The effect is ascribed not only to the crystal quality but more importantly to the radial band alignment. The result indicates the high-temperature IQE improvement of the type-I and type-II c/s NWs and the appropriateness of the mid-infrared PL analyses for narrow-gap NW evaluation.

Ultra-Stable, Endurable, and Flexible Sb2TexSe3-x Phase Change Devices for Memory Application and Wearable Electronics.

Flexible memory and wearable electronics represent an emerging technology, thanks to their reliability, compatibility, and superior performance. Here, an Sb2TexSe3-x (STSe) phase change material was grown on flexible mica, which not only exhibited superior nature in thermal stability for phase change memory application but also revealed novel function performance in wearable electronics, thanks to its excellent mechanical reliability and endurance. The thermal stability of Sb2Te3 was improved obviously with the crystallization temperature elevated 60 K after Se doping, for the enhanced charge localization and stronger bonding energy, which was validated by the Vienna ab initio simulation package calculations. Based on the ultra-stability of STSe, the STSe-based phase change memory shows 65 000 reversible phase change ability. Moreover, the assembled flexible device can show real-time monitoring and recoverability response in sensing human activities in different parts of the body, which proves its effective reusability and potential as wearable electronics. Most importantly, the STSe device presents remarkable working reliability, reflected by excellent endurance over 100 s and long retention over 100 h. These results paved a novel way to utilize STSe phase change materials for flexible memory and wearable electronics with extreme thermal and mechanical stability and brilliant performance.

Designing Monoclinic Heterophase Coexistence for the Enhanced Piezoelectric Performance in Ternary Lead-Based Relaxor Ferroelectrics.

Enhanced piezoelectric, dielectric properties and thermal stability in ternary relaxor-PbTiO3 based ferroelectric crystals are expected to develop the next-generation of electromechanical devices. However, due to their increased disorder compared to other ferroelectrics, designing a controllable phase boundary structure and engineered domain remains a challenging task. Here, we construct a monoclinic heterophase coexisting in a ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal with optimized composition and an ultrahigh piezoelectric coefficient of 1400 pC N-1, to quantify the correlation between spontaneous nanopolarity and phase heterogeneity, in an attempt to understand the origin of the exceptional functionalities. By designing an in situ high-resolution spectroscopic-microscopic technique, we have observed Ma and Mc heterophase mixtures spatially separated by the monoclinic heterophase boundary (MHB), which are responsible for the ferroelectric-dominated and relaxor-ferroelectric-dominated nanodomain structure, respectively. Internal energy mapping from optical soft mode dynamics reveals the inhomogeneous polarization and local symmetry on both sides of the MHB. Various molecular polarizabilities and localized octahedral distortions correlate directly with monoclinic regions and electromechanical contribution. This work clarifies the heterogeneity between structure, energy, and polar order and provides a new design freedom for advanced relaxor ferroelectrics.

Mid-infrared photoluminescence revealing internal quantum efficiency enhancement of type-I and type-II InAs core/shell nanowires: publisher's note.

This publisher's note contains corrections to Opt. Lett.47, 5208 (2022)10.1364/OL.473154.

Flexo-photoelectronic effect in n-type/p-type two-dimensional semiconductors and a deriving light-stimulated artificial synapse.

Flexoelectricity and photoelectricity with their coupled effect (the so-called flexo-photoelectronic effect), are of increasing interest in the study of electronics and optoelectronics in van der Waals layered semiconductors. However, the related device design is severely restricted owing to the ambiguous underlying physical nature of flexo-photoelectronic effects originating from the co-manipulation of light and strain-gradients. Here, flexoelectric polarization and the flexo-photoelectronic effect of few-layered semiconductors have been multi-dimensionally investigated from high-resolution microscopic characterization on the nanoscale, physics analysis, and deriving a device design. We found that two back-to-back built-in electric fields form in bent InSe and WSe2, and greatly modulate the transport behaviors of photogenerated carriers, further facilitating the separation of photogenerated electron-hole pairs and trapping the holes/electrons in InSe or WSe2 channels, recorded in realtime by a home-made technique of lighting Kelvin probe force microscopy (KPFM). The slow release of trapped carriers contributes to the photoconductance relaxation after illumination. Utilizing the photoconductance relaxation, a light-stimulated artificial synapse based on the flexo-photoelectronic effect of bent InSe has been achieved. Significantly, all the pair-pulse facilitation (PPF) behavior, spike frequency-dependent excitatory post-synaptic current (EPSC) and the transition from short-term memory (STM) to long-term memory (LTM) have been successfully realized in this artificial synapse. This work adds to the investigation of flexo-photoelectronic effects on 2D optoelectronics, and moves towards the development of 2D neuromorphic electronics.

Thermal Conductivity of Large-Area Polycrystalline MoSe2 Films Grown by Chemical Vapor Deposition.

It is of great importance to understand the thermal properties of MoSe2 films for electronic and optoelectronic applications. In this work, large-area polycrystalline MoSe2 films are prepared using a low-cost, controllable, large-scale, and repeatable chemical vapor deposition method, which facilitates direct device fabrication. Raman spectra and X-ray diffraction patterns indicate a hexagonal (2H) crystal structure of the MoSe2 film. Ellipsometric spectra analysis indicates that the optical band gap of the MoSe2 film is estimated to be ∼1.23 eV. From the analysis of the temperature-dependent and laser-power-dependent Raman spectra, the thermal conductivity of the suspended MoSe2 films is found to be ∼28.48 W/(m·K) at room temperature. The results can provide useful guidance for an effective thermal management of large-area polycrystalline MoSe2-based electronic and optoelectronic devices.

Effects of SF6 decomposition components and concentrations on the discharge faults and insulation defects in GIS equipment.

Gas-insulated switchgear (GIS) is widely used across multiple electric stages and different power grid levels. However, the threat from several inevitable faults in the GIS system surrounds us for the safety of electricity use. In order to improve the evaluation ability of GIS system safety, we propose an efficient strategy by using machine learning to conduct SF6 decomposed components analysis (DCA) for further diagnosing discharge fault types in GIS. Note that the empirical probability function of different faults fitted by the Arrhenius chemical reaction model has been investigated into the robust feature engineering for machine learning based GIS diagnosing model. Six machine learning algorithms were used to establish models for the severity of discharge fault and main insulation defects, where identification algorithms were trained by learning the collection dataset composing the concentration of the different gas types (SO2, SOF2, SO2F2, CF4, and CO2, etc.) in the system and their ratios. Notably, multiple discharge fault types coexisting in GIS can be effectively identified based on a probability model. This work would provide a great insight into the development of evaluation and optimization on solving discharge fault in GIS.

Ferroelectric-Modulated MoS2 Field-Effect Transistors as Multilevel Nonvolatile Memory.

Ferroelectric field-effect transistors (FeFETs) with semiconductors as the channel material and ferroelectrics as the gate insulator are attractive and/or promising devices for application in nonvolatile memory. In FeFETs, the conductivity states of the semiconductor are utilized to explore the polarization directions of the ferroelectric material. Herein, we report FeFETs based on a few layers of MoS2 on a 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystal with switchable multilevel states. It was found that the On-Off ratios can reach as high as 106. We prove that the interaction effect of ferroelectric polarization and interface charge traps has a great influence on the transport behaviors and nonvolatile memory characteristics of MoS2/PMN-PT FeFETs. In order to further study the underlying physical mechanism, we have researched the time-dependent electrical properties in the temperature range from 300 to 500 K. The separation of effects from ferroelectric polarization and interfacial traps on electrical behaviors of FeFETs provides us with an opportunity to better understand the operation mechanism, which suggests a fantastic way for multilevel, low-power consumption, and high-density nonvolatile memory devices.

Ferroelectric and dipole control of band alignment in the two dimensional InTe/In2Se3 heterostructure.

Two dimensional (2D) ferroelectric materials are gaining growing attention due to their nontrival ferroelectricity, and the 2D ferroelectric heterostructures with tunable electronic, optoelectronic, or even magnetic properties, show many novel properties that do not exist in their constituents. In this work, by using the first-principles calculations, we investigate the ferroelectric and dipole control of electronic structures of the 2D ferroelectric heterostructure InTe/In2Se3. It is found that band alignment is closely dependent on the ferroelectric polarization of In2Se3. By switching the polarization of In2Se3, the band alignment of InTe/In2Se3 switches from a staggered (type II) to a straddling type (type I), and the band gap changes from indirect gap 0.76 eV to direct gap 0.15 eV. When the ferroelectric field of In2Se3 is reversed, the band alignment of InTe/In2Se3 switches from type-I to type-II, and the band gap changes from indirect gap 0.76 eV to direct gap 0.15 eV. In addition, we find that the interlayer dipole can also effectively modulate the band structure and induce the type-I to type-II band alignment transition. Our present results indicate that the 2D ferroelectric heterostructure with the tunable band alignment and band gap can be of great significance in the optoelectronic devices.

Effects of composition and temperature on the exciton emission behaviors of Mo(S x Se1-x )2 monolayer: experiment and theory.

Exploring the excitonic behavior of two-dimensional (2D) alloys is of great significance, which not only could promote the understanding of fundamental photophysics in optoelectric devices, but could also guide the functional improvement of future applications. Here, we demonstrate the controllable synthesis of monolayer Mo(S x Se1-x )2 nanosheets using a one-step chemical vapor deposition method and systematical investigation on the exciton emission characteristics based on the temperature-dependent photoluminescence spectroscopy (PL) experiments. As a result, the tunable bandgap of Mo(S x Se1-x )2 alloys between 1.52 and 1.85 eV can be achieved, which is consistent with the theoretical results calculated by the ab initio density function theory. Besides, both the exciton and trion behaviors in Mo(S x Se1-x )2 are observed from the PL spectra at T = 80 K. More intriguingly, the differences between the emission energy of exciton and trion (ΔE), known as the dissociation energy of the trion, are positively correlated to the concentrations of the sulfur (S) elements, which is also proved by the theoretical calculation. Combining the experimental and theoretical results, the phenomena can be explained by the reduced dielectric screening effect and the increasing Fermi energy (E F) along with the increasing of sulfur in Mo(S x Se1-x )2 nanosheets, jointly leading to the increase of ΔE. Furthermore, the evolutions of ΔE in Mo(S x Se1-x )2 alloys as a function of temperature have been also discovered, which lay the foundation for the potential uses of 2D alloys in optoelectronic devices.

Spatially resolved and two-dimensional mapping modulated infrared photoluminescence spectroscopy with functional wavelength up to 20 µm.

The pixel-scale nonuniformity of the photoelectric response may be due either to the in-plane electronic inhomogeneity of the narrow-gap semiconductor or to the craft fluctuation during the fabrication process, which limits the imaging performance of the infrared focal plane array (FPA) photodetector. Accordingly, a nondestructive technique is most desirable for examining the spatial uniformity of the optoelectronic properties of the narrow-gap semiconductor to identify the origin of the FPA response nonuniformity. This article introduces a spatially resolved and two-dimensional mapping infrared photoluminescence (PL) technique, especially suitable for characterizing FPA narrow-gap semiconductors, based on the modulated PL method with a step-scan Fourier transform infrared spectrometer. The experimental configuration is described, and typical applications are presented as examples to a 960 × 640 µm2 area of an InAsSbP-on-InAs layer in the medium-wave infrared range and a 960 × 960 µm2 area of a HgTe/HgCdTe superlattice (SL) in the long-wave infrared range. The results indicate that, within a measurement duration of about 30 s/spectrum, a sufficiently high signal-to-noise ratio (SNR) of over 50 is achieved with a spectral resolution of 16 cm-1 for the InAsSbP-on-InAs layer and a SNR over 30 is achieved with a spectral resolution of 12 cm-1 for the HgTe/HgCdTe SL, which warrants reliable identification of the subtle differences among the spatially resolved and two-dimensional mapping PL spectra. The imaging of the in-plane distribution of PL energy, intensity, and linewidth is realized quantitatively. The results indicate the feasibility and functionality of the spatially resolved and two-dimensional mapping PL spectroscopy for the narrow-gap semiconductors in a wide infrared range.

Electric manipulation of magnetism in bilayer van der Waals magnets.

The ferromagnetism of the two dimensional (2D) Cr2Ge2Te6 atomic layers with the perpendicular magnetic anisotropy and the Curie temperature 30-50 K has recently been experimentally confirmed. By performing the density-functional theory calculations, we demonstrate that the magnetic properties of bilayer Cr2Ge2Te6 can be flexibly tailored, due to the effective band structure tuning by the external electric field. The electric field induces the semiconductor-metal transition and redistributes charge and spin between the two layers. Furthermore, the magnetic anisotropy energy of the bilayer Cr2Ge2Te6 can be obviously enhanced by the electric field, which is helpful to stabilize the long-range ferromagnetic order. Our study about the electric manipulation of magnetism based on the band structure engineering generally exists in 2D magnetic systems and will be of great significance in low-dimensional all-electric spintronics.

Photoluminescence spectra of the Mn2+d-d multiplets transitions of zinc-blende MnTe epitaxial films: laser and deuterium lamp excitation study.

The luminescences of zinc-blende MnTe epitaxial films are respectively excited by a laser and deuterium lamp to study Mn2+d-d multiplets transitions. Besides the inclusion of the band gap, all other excited states related to Mn2+d-d transitions including T14(G4), T24(G4), A14[E4(G4)], T24(D4), E4(D4), and T22(I2) are observed. The shift and broadening of the T14(G4) and T24(G4) lines with increasing temperature are described by the electron-phonon coupling. Step-like energy and intensity shifts for the A14[E4(G4)], T24(D4), E4(D4), and T22(I2) transitions occur in the vicinity of the Néel point, which can be ascribed to the different spin-ordering-induced energy relaxation in ground and excited states of Mn2+d-d multiplets, and these transitions show temperature independence and weak quenching.

Midinfrared Photoluminescence up to 290 K Reveals Radiative Mechanisms and Substrate Doping-Type Effects of InAs Nanowires.

Photoluminescence (PL) as a conventional yet powerful optical spectroscopy may provide crucial insight into the mechanism of carrier recombination and bandedge structure in semiconductors. In this study, mid-infrared PL measurements on vertically aligned InAs nanowires (NWs) are realized for the first time in a wide temperature range of up to 290 K, by which the radiative recombinations are clarified in the NWs grown on n- and p-type Si substrates, respectively. A dominant PL feature is identified to be from the type-II optical transition across the interfaces between the zinc-blend (ZB) and the wurtzite (WZ) InAs, a lower-energy feature at low temperatures is ascribed to impurity-related transition, and a higher-energy feature at high temperatures originates in the interband transition of the WZ InAs being activated by thermal-induced electron transfer. The optical properties of the ZB-on-WZ and WZ-on-ZB interfaces are asymmetric, and stronger nonradiative recombination and weaker carrier-phonon interaction show up in the NWs on p-type substrate in which built-in electric field forms and leads to carrier assembling around the WZ-on-ZB interface. The results indicate that wide temperature-range infrared PL analysis can serve as efficient vehicle for clarifying optical properties and bandedge processes of the crystal-phase interfaces in vertically aligned InAs NWs.