Ferroelectricity of pristine Hf_(0.5)Zr_(0.5)O_(2) films fabricated by atomic layer deposition

Hafnium-based ferroelectric films,remaining their ferroelectricity down to nanoscale thickness,present a promising application for low-power logic devices and nonvolatile memories.It has been appealing for researchers to reduce the required temperature to obtain the ferroelectric phase in hafnium-based ferroelectric films for applications such as flexible and wearable electronics.This work demonstrates that a remanent polarization(P_(r))value of>5μC/cm^(2)can be obtained in asdeposited Hf_(0.5)Zr_(0.5)O_(2)(HZO)films that are fabricated by thermal atomic layer deposition(TALD)under low temperature of 250℃.The ferroelectric orthorhombic phase(o-phase)in the as-deposited HZO films is detected by scanning transmission electron microscopy(STEM).This low fabrication temperature further extends the compatibility of ferroelectric HZO films to flexible electronics and avoids the cost imposed by following high-temperature annealing treatments.

High efficiency of broadband transmissive metasurface terahertz polarization converter

Terahertz wave is between microwave and infrared bands in the electromagnetic spectrum with the frequency range from 0.1 THz to 10 THz. Controlling and processing of the polarization state in terahertz wave are the focus due to its great influence on the characteristics. In this paper, a transmissive metasurface terahertz polarization converter is designed in 3D structure with an upper surface of ruler-like rectangular, an intermediate dielectric layer and a lower surface of metal grid wires. Numerical simulations of the converter show that the polarization conversion ratio(PCR) is above 99.9% at 0.288 THz–1.6 THz, the polarization rotation angle(PRA) is close to 90° at 0.06 THz–1.4 THz, and the ellipticity angle(EA) is close to 0° at 0.531 THz–1.49 THz. The origin of the efficient polarization conversion is explained by analyzing the electric field intensity, magnetic field intensity, surface current, electric field energy density, and magnetic field energy density distributions of the converter at 1.19 THz and 0.87 THz, which are consistent with the energy transmittance and transmittance coefficient. In addition, the effects of different thickness and material of intermediate layer, thickness of upper surface material, polarized wave incidence angle, and metasurface materials on the performance of the polarization converter are discussed, and how they affect the conversion performance of the polarization converter are also explained.Our results provide a strong theoretical basis and technical support to develop high performance transmission-type terahertz polarization converters, and play an important role to promote the development of terahertz science and technology.

Vein visualization enhancement by dual-wavelength phase-locked denoising technology

Visual near-infrared imaging equipment has broad applications in various fields such as venipuncture,facial injections,and safety verification due to its noncontact,compact,and portable design.Currently,most studies utilize near-infrared single-wavelength for image acquisition of veins.However,many substances in the skin,including water,protein,and melanin can create significant background noise,which hinders accurate detection.In this paper,we developed a dual-wavelength imaging system with phase-locked denoising technology to acquire vein image.The signals in the effective region are compared by using the absorption valley and peak of hemoglobin at 700nm and 940nm,respectively.The phase-locked denoising algorithm is applied to decrease the noise and interference of complex surroundings from the images.The imaging results of the vein are successfully extracted in complex noise environment.It is demonstrated that the denoising effect on hand veins imaging can be improved with 57.3%by using our dual-wavelength phase-locked denoising technology.Consequently,this work proposes a novel approach for venous imaging with dual-wavelengths and phase-locked denoising algorithm to extract venous imaging results in complex noisy environment better.

High performance infrared detectors compatible with CMOS-circuit process

A type of Si-based blocked impurity band photoelectric detector with a planar architecture is designed and demonstrated by a modified silicon semiconductor processing technique.In this route,multiple ion implantation is utilized to ensure the uniform distribution of the P elements in silicon,and rapid thermal annealing treatment is used to activate the P atoms and reduce damages caused by ion-implantation.The fabricated prototype device exhibits an excellent photoelectric response performance.With a direct current(DC)bias voltage of-2.3 V,the device detectivity to blackbody irradiation is as high as 5×10^(13)cm·Hz^(1/2)/W,which corresponds to a device responsivity of nearly 4.6 A/W,showing their potential applications in infrared detection,infrared astrophysics,and extraterrestrial life science.In particular,the developed device preparation process is compatible with that for the CMOS-circuit,which greatly reduces the manufacturing cost.

Anisotropic photoresponse of layered rhenium disulfide synaptic transistors

Layered ReS_(2) with direct bandgap and strong in-plane anisotropy shows great potential to develop high-performance angle-resolved photodetectors and optoelectronic devices.However,systematic characterizations of the angle-dependent photoresponse of ReS_(2) are still very limited.Here,we studied the anisotropic photoresponse of layered ReS_(2) phototransistors in depth.Angel-resolved Raman spectrum and field-effect mobility are tested to confirm the inconsistency between its electrical and optical anisotropies,which are along 120°and 90°,respectively.We further measured the angle-resolved photoresponse of a ReS_(2) transistor with 6 diagonally paired electrodes.The maximum photoresponsivity exceeds 0.515 A·W^(-1) along b-axis,which is around 3.8 times larger than that along the direction perpendicular to b axis,which is consistent with the optical anisotropic directions.The incident wavelength-and power-dependent photoresponse measurement along two anisotropic axes further demonstrates that b axis has stronger light-ReS_(2) interaction,which explains the anisotropic photoresponse.We also observed angle-dependent photoresistive switching behavior of the ReS_(2) transistor,which leads to the formation of angle-resolved phototransistor memory.It has simplified structure to create dynamic optoelectronic resistive random access memory controlled spatially through polarized light.This capability has great potential for real-time pattern recognition and photoconfiguration of artificial neural networks(ANN)in a wide spectral range of sensitivity provided by polarized light.

MBE Growth and Characterization of Strained Hg Te(111)Films on CdTe/GaAs

Strained Hg Te thin films are typical three-dimensional topological insulator materials.Most works have focused on Hg Te(100)films due to the topological properties resulting from uniaxial strain.In this study,strained Hg Te(111)thin films are grown on Ga As(100)substrates with Cd Te(111)buffer layers using molecular beam epitaxy(MBE).The optimal growth conditions for Hg Te films are determined to be a growth temperature of 160℃and an Hg/Te flux ratio of 200.The strains of Hg Te films with different thicknesses are investigated by highresolution x-ray diffraction,including reciprocal space mapping measurements.The critical thickness of Hg Te(111)film on Cd Te/Ga As is estimated to be approximately 284 nm by Matthews'equations,consistent with the experimental results.Reflection high-energy electron diffraction and high-resolution transmission electron microscopy investigations indicate that high-quality Hg Te films are obtained.This exploration of the MBE growth of Hg Te(111)films provides valuable information for further studies of Hg Te-based topological insulators.

High sensitivity of semimetal photodetection via Bose–Einstein condensation

The discovery of semiconductor has witnessed remarkable strides toward high performance of photodetectors attributed to its excellent carrier properties.However,semimetal,owning to the high carrier concentration and low carrier mobility compared to those of semiconductor,is generally considered unsuitable for photodetection.Herein,we demonstrate an outstanding photodetection in a layered semimetal titanium diselenide(TiSe_(2))in Bose-Einstein condensation(BEC)state.High sensitivity of semimetal photodetector is realized in the range of visible,infrared and terahertz bands.The noise equivalent power(NEP)has threefold improvement at the visible and infrared wavebands,and significant decrease by one order of magnitude in the terahertz frequencies via BEC phenomenon,attributed to the electrical parameter variation after condensation.The best NEP value in the terahertz frequency is comparable to that of commercial Si photodetector.Our results show another recipe to fabricate high performance of photodetection via semimetal except for semiconductor and pave the way to exploit macroscopic quantum phenomena for optoelectronics.

A gate-free MoS2 phototransistor assisted by ferroelectrics

During the past decades,transition metal dichalcogenides(TMDs) have received special focus for their unique properties in photoelectric detection.As one important member of TMDs,MoS2 has been made into photodetector purely or combined with other materials,such as graphene,ionic liquid,and ferroelectric materials.Here,we report a gate-free MoS2 phototransistor combined with organic ferroelectric material poly(vinylidene fluoride-trifluoroethylene)(P(VDF-TrFE)).In this device,the remnant polarization field in P(VDF-TrFE) is obtained from the piezoelectric force microscope(PFM) probe with a positive or negative bias,which can turn the dipoles from disorder to be the same direction.Then,the MoS2 channel can be maintained at an accumulated state with downward polarization field modulation and a depleted state with upward polarization field modulation.Moreover,the P(VDF-TrFE) segregates MoS2 from oxygen and water molecules around surroundings,which enables a cleaner surface state.As a photodetector,an ultra-low dark current of 10^–11 A,on/off ration of more than 10^4 and a fast photoresponse time of 120 μs are achieved.This work provides a new method to make high-performance phototransistors assisted by the ferroelectric domain which can operate without a gate electrode and demonstrates great potential for ultra-low power consumption applications.

A self-powered and sensitive terahertz photodetection based on PdSe_(2)

With the rapid development of terahertz technology,terahertz detectors are expected to play a key role in diverse areas such as homeland security and imaging,materials diagnostics,biology,medical sciences,and communication.Whereas self-powered,rapid response,and room temperature terahertz photodetectors are confronted with huge challenges.Here,we report a novel rapid response and self-powered terahertz photothermoelectronic(PTE)photodetector based on a lowdimensional material:palladium selenide(Pd Se_(2)).An order of magnitude performance enhancement was observed in photodetection based on PdSe_(2)/graphene heterojunction that resulted from the integration of graphene and enhanced the Seebeck effect.Under 0.1-THz and 0.3-THz irradiations,the device displays a stable and repeatable photoresponse at room temperature without bias.Furthermore,rapid rise(5.0μs)and decay(5.4μs)times are recorded under 0.1-THz irradiation.Our results demonstrate the promising prospect of the detector based on Pd Se2 in terms of air-stable,suitable sensitivity and speed,which may have great application in terahertz detection.

Stoichiometric effect on electrical and near-infrared photodetection properties of full-composition-range GaAs1−xSbx nanowires

As one of the most important narrow bandgap ternary semiconductors, GaAs1−xSbx nanowires (NWs) have attracted extensive attention recently, due to the superior hole mobility and the tunable bandgap, which covers the whole near-infrared (NIR) region, for technological applications in next-generation high-performance electronics and NIR photodetection. However, it is still a challenge to the synthesis of high-quality GaAs1−xSbx NWs across the entire range of composition, resulting in the lack of correlation investigation among stoichiometry, microstructure, electronics, and NIR photodetection. Here, we demonstrate the success growth of high-quality GaAs1−xSbx NWs with full composition range by adopting a simple and low-cost surfactant-assisted solid source chemical vapor deposition method. All of the as-prepared NWs are uniform, smooth, and straight, without any phase segregation in all stoichiometric compositions. The lattice constants of each NW composition have been well correlated with the chemical stoichiometry and confirmed by high-resolution transmission electron microscopy, X-ray diffraction, and Raman spectrum. Moreover, with the increase of Sb concentration, the hole mobility of the as-fabricated field-effect-transistors and the responsivity and detectivity of the as-fabricated NIR photodetectors increase accordingly. All the results suggest a careful stoichiometric design is required for achieving optimal NW device performances.

Macroscopic assembled graphene nanofilms based room temperature ultrafast mid-infrared photodetectors

Graphene with linear energy dispersion and weak electron-phonon interaction is highly anticipated to harvest hot electrons in a broad wavelength range.However,the limited absorption and serious backscattering of hot-electrons result in inadequate quantum yields,especially in the mid-infrared range.Here,we report a macroscopic assembled graphene(nMAG)nanofilm/silicon heterojunction for ultrafast mid-infrared photodetection.The assembled Schottky diode works in 1.5-4.0μm at room temperature with fast response(20-30 ns,rising time,4 mm2 window)and high detectivity(1.61011 to 1.9109 Jones from 1.5 to 4.0μm)under the pulsed laser,outperforming single-layer-graphene/silicon photodetectors by 2-8 orders.These performances are attributed to the greatly enhanced photo-thermionic effect of electrons in nMAG due to its high light absorption(~40%),long carrier relaxation time(~20 ps),low work function(4.52 eV),and suppressed carrier number fluctuation.The nMAG provides a long-range platform to understand the hot-carrier dynamics in bulk 2D materials,leading to broadband and ultrafast MIR active imaging devices at room temperature.

Ultrawide dynamic modulation of perfect absorption with a Friedrich–Wintgen BIC

Dynamical control of perfect absorption plays an indispensable role in optical switch and modulators.However,it always suffers from the limited modulation range,small depth,and susceptible absorption efficiencies.Here,we propose a new strategy based on Friedrich–Wintgen bound states in the continuum(F–W BICs)to realize a tunable perfect absorber with large dynamic modulation range.For proof of concept,we demonstrate a pentaband ultrahigh absorption system consisting of graphene gratings and graphene sheets through elaborately tuning F–W BIC.The nature of the F–W BIC arises from the destructive interference between Fabry–Perot resonance and guided mode resonance modes in the coherent phase-matching condition.The radiation channels are avoided from crossing.The BIC can be dynamically modulated by engineering the Fermi level of graphene gratings,which breaks the traditional modulation methods with an incidence angle.Remarkably,the perfect absorber with this F–W BIC approach achieves the largest modulation range of up to 3.5 THz.We believe that this work provides a new way to dynamically engineer perfect absorption and stimulates the development of multiband ultracompact devices.

Novel Biased Aggregation-Annihilation Model

We propose a novel two-species aggregation-annihilation model, in which irreversible aggregation reactions occur between any two aggregates of the same species and biased annihilations occur simultaneously between two different species. The kinetic scaling behavior of the model is then analytically investigated by means of the mean-field rate equation. For the system without the seff-aggregation of the un-annihilated species, the aggregate size distribution of the annihilated species always approaches a modified scaling form and vanishes finally; while for the system with the self-aggregation of the un-annihilated species, its scaling behavior depends crucially on the details of the rate kernels. Moreover, the results also exhibit that both species are conserved together in some cases, while only the un-annihilated species survives finally in other cases.

Configurable circular-polarization-dependent optoelectronic silent state for ultrahigh light ellipticity discrimination

Filterless light-ellipticity-sensitive optoelectronic response generally has low discrimination,thus severely hindering the development of monolithic polarization detectors.Here,we achieve a breakthrough based on a configurable circular-polarization-dependent optoelectronic silent state created by the superposition of two photoresponses with enantiomerically opposite ellipticity dependences.The zero photocurrent and the significantly suppressed noise of the optoelectronic silent state singularly enhance the circular polarization extinction ratio(CPER)and the sensitivity to light ellipticity perturbation.The CPER of our device approaches infinity by the traditional definition.The newly established CPER taking noise into account is 3-4 orders of magnitude higher than those of ordinary integrated circular polarization detectors,and it remains high in an expanded wavelength range.The noise equivalent light ellipticity difference goes below 0.009° Hz-1/2 at modulation frequencies above 1000 Hz by a light power of 281 uW.This scheme brings a leap in developing monolithic ultracompact circular polarization detectors.

Polychromatic full-polarization control in mid-infrared light

Objects with different shapes,materials and temperatures can emit distinct polarizations and spectral information in mid-infrared band,which provides a unique signature in the transparent window for object identification.However,the crosstalk among various polarization and wavelength channels prevents from accurate mid-infrared detections at high signal-to-noise ratio.Here,we report full-polarization metasurfaces to break the inherent eigen-polarization constraint over the wavelengths in mid-infrared.This recipe enables to select arbitrary orthogonal polarization basis at individual wavelength independently,therefore alleviating the crosstalk and efficiency degradation.A six-channel all-silicon metasurface is specifically presented to project focused mid-infrared light to distinct positions at three wavelengths,each with a pair of arbitrarily chosen orthogonal polarizations.An isolation ratio of 117 between neighboring polarization channels is experimentally recorded,exhibiting detection sensitivity one order of magnitude higher than existing infrared detectors.Remarkably,the high aspect ratio~30 of our meta-structures manufactured by deep silicon etching technology at temperature−150℃ guarantees the large and precise phase dispersion control over a broadband from 3 to 4.5μm.We believe our results would benefit the noise-immune mid-infrared detections in remote sensing and space-to-ground communications.

Two-Dimensional Electron Gas in MoSi_(2)N_(4)/VSi_(2)N_(4)Heterojunction by First Principles Calculation

Van der Waals(vdW)layered two-dimensional(2D)materials,which may have high carrier mobility,valley polarization,excellent mechanical properties and air stability,have been widely investigated before.We explore the possibility of producing a spin-polarized two-dimensional electron gas(2DEG)in the heterojunction composed of insulators MoSi_(2)N_(4)and VSi_(2)N_(4)by using first-principles calculations.Due to the charge transfer effect,the 2DEG at the interface of the MoSi_(2)N_(4)/VSi_(2)N_(4)heterojunction is found.Further,for different kinds of stacking of heterojunctions,lattice strain and electric fields can effectively tune the electronic structures and lead to metal-to-semiconductor transition.Under compressive strain or electric field parallel to c axis,the 2DEG disappears and band gap opening occurs.On the contrary,interlayer electron transfer enforces the system to become metallic under the condition of tensile strain or electric field anti-parallel to c axis.These changes are mainly attributed to electronic redistribution and orbitals’reconstruction.In addition,we reveal that MoSi_(2)N_(4)/VSi_(2)N_(4)lateral heterojunctions of armchair and zigzag edges exhibit different electronic properties,such as a large band gap semiconductor and a metallic state.Our findings provide insights into electronic band engineering of MoSi_(2)N_(4)/VSi_(2)N_(4)heterojunctions and pave the way for future spintronics applications.

Infrared avalanche photodiodes from bulk to 2D materials

Avalanche photodiodes (APDs) have drawn huge interest in recent years and have been extensively used in a range of fields including the most important one—optical communication systems due to their time responses and high sensitivities. This article shows the evolution and the recent development of A^(Ⅲ)B^(Ⅴ), A^(Ⅱ)B^(Ⅵ), and potential alternatives to formerly mentioned—“third wave” superlattices (SL) and two-dimensional (2D) materials infrared (IR) APDs. In the beginning, the APDs fundamental operating principle is demonstrated together with progress in architecture. It is shown that the APDs evolution has moved the device’s performance towards higher bandwidths, lower noise, and higher gain-bandwidth products. The material properties to reach both high gain and low excess noise for devices operating in different wavelength ranges were also considered showing the future progress and the research direction. More attention was paid to advances in A^(Ⅲ)B^(Ⅴ) APDs, such as AlInAsSb, which may be used in future optical communications, type-Ⅱ superlattice (T2SLs, “Ga-based” and “Ga-free”), and 2D materials-based IR APDs. The latter—atomically thin 2D materials exhibit huge potential in APDs and could be considered as an alternative material to the well-known, sophisticated, and developed A^(Ⅲ)B^(Ⅴ) APD technologies to include single-photon detection mode. That is related to the fact that conventional bulk materials APDs’ performance is restricted by reasonably high dark currents. One approach to resolve that problem seems to be implementing low-dimensional materials and structures as the APDs’ active regions. The Schottky barrier and atomic level thicknesses lead to the 2D APD dark current significant suppression. What is more, APDs can operate within visible (VIS), near-infrared (NIR)/mid-wavelength infrared range (MWIR), with a responsivity ~80 A/W, external quantum efficiency ~24.8%, gain ~10^(5) for MWIR [wavelength, λ = 4 μm, temperature, T = 10–180 K, Black Phosphorous (BP)/InSe APD]. It is believed that the 2D APD could prove themselves to be an alternative providing a viable method for device fabrication with simultaneous high-performance—sensitivity and low excess noise.

An Improved Room-Temperature Silicon Terahertz Photodetector on Sapphire Substrates

We design and fabricate a good performance silicon photoconductive terahertz detector on sapphire substrates at room temperature.The best voltage responsivity of the detector is 6679 V/W at frequency 300 GHz as well as low voltage noise of 3.8 nV/Hz1/2 for noise equivalent power 0.57 pW/Hz1/2.The measured response time of the device is about 9μs,demonstrating that the detector has a speed of>110 kHz.The achieved good performance,together with large detector size(acceptance area is 3μm×160μm),simple structure,easy manufacturing method,compatibility with mature silicon technology,and suitability for large-scale fabrication of imaging arrays provide a promising approach to the development of sensitive terahertz room-temperature detectors.

Study of the growth mechanism of a self‑assembled and ordered multi‑dimensional heterojunction at atomic resolution

Multi-dimensional heterojunction materials have attracted much attention due to their intriguing properties,such as high efciency,wide band gap regulation,low dimensional limitation,versatility and scalability.To further improve the performance of materials,researchers have combined materials with various dimensions using a wide variety of techniques.However,research on growth mechanism of such composite materials is still lacking.In this paper,the growth mechanism of multidimensional heterojunction composite material is studied using quasi-two-dimensional(quasi-2D)antimonene and quasione-dimensional(quasi-1D)antimony sulfde as examples.These are synthesized by a simple thermal injection method.It is observed that the consequent nanorods are oriented along six-fold symmetric directions on the nanoplate,forming ordered quasi-1D/quasi-2D heterostructures.Comprehensive transmission electron microscopy(TEM)characterizations confrm the chemical information and reveal orientational relationship between Sb2S3 nanorods and the Sb nanoplate as substrate.Further density functional theory calculations indicate that interfacial binding energy is the primary deciding factor for the self-assembly of ordered structures.These details may fll the gaps in the research on multi-dimensional composite materials with ordered structures,and promote their future versatile applications.

二维材料最新研究进展

Research on two-dimensional(2D) materials has been explosively increasing in last seventeen years in varying subjects including condensed matter physics, electronic engineering, materials science, and chemistry since the mechanical exfoliation of graphene in 2004. Starting from graphene, 2D materials now have become a big family with numerous members and diverse categories. The unique structural features and physicochemical properties of 2D materials make them one class of the most appealing candidates for a wide range of potential applications. In particular, we have seen some major breakthroughs made in the field of 2D materials in last five years not only in developing novel synthetic methods and exploring new structures/properties but also in identifying innovative applications and pushing forward commercialisation. In this review, we provide a critical summary on the recent progress made in the field of 2D materials with a particular focus on last five years. After a brief backgroundintroduction, we first discuss the major synthetic methods for 2D materials, including the mechanical exfoliation, liquid exfoliation, vapor phase deposition, and wet-chemical synthesis as well as phase engineering of 2D materials belonging to the field of phase engineering of nanomaterials(PEN). We then introduce the superconducting/optical/magnetic properties and chirality of 2D materials along with newly emerging magic angle 2D superlattices. Following that, the promising applications of 2D materials in electronics, optoelectronics, catalysis, energy storage, solar cells, biomedicine, sensors, environments, etc. are described sequentially. Thereafter, we present the theoretic calculations and simulations of 2D materials. Finally, after concluding the current progress, we provide some personal discussions on the existing challenges and future outlooks in this rapidly developing field.