Progress on two-dimensional ferrovalley materials

The electron's charge and spin degrees of freedom are at the core of modern electronic devices. With the in-depth investigation of two-dimensional materials, another degree of freedom, valley, has also attracted tremendous research interest. The intrinsic spontaneous valley polarization in two-dimensional magnetic systems, ferrovalley material, provides convenience for detecting and modulating the valley. In this review, we first introduce the development of valleytronics.Then, the valley polarization forms by the p-, d-, and f-orbit that are discussed. Following, we discuss the investigation progress of modulating the valley polarization of two-dimensional ferrovalley materials by multiple physical fields, such as electric, stacking mode, strain, and interface. Finally, we look forward to the future developments of valleytronics.

Families of magnetic semiconductors——an overview

The interplay of magnetic and semiconducting properties has been in the focus for more than a half of the century. In this introductory article we briefly review the key properties and functionalities of various magnetic semiconductor families, including europium chalcogenides, chromium spinels, dilute magnetic semiconductors, dilute ferromagnetic semiconductors and insulators, mentioning also sources of non-uniformities in the magnetization distribution, accounting for an apparent high Curie temperature ferromagnetism in many systems. Our survey is carried out from today's perspective of ferromagnetic and antiferromagnetic spintronics as well as of the emerging fields of magnetic topological materials and atomically thin 2D layers.

Narrow Waveguide Based on Ferroelectric Domain Wall

Ferroelectric materials are spontaneous symmetry breaking systems that are characterized by ordered electric polarizations.Similar to its ferromagnetic counterpart,a ferroelectric domain wall can be regarded as a soft interface separating two different ferroelectric domains.Here we show that two bound state excitations of electric polarization(polar wave),or the vibration and breathing modes,can be hosted and propagate within the ferroelectric domain wall.In particular,the vibration polar wave has zero frequency gap,thus is constricted deeply inside ferroelectric domain wall,and can even propagate in the presence of local pinnings.The ferroelectric domain wall waveguide as demonstrated here offers a new paradigm in developing ferroelectric information processing units.

Multifunctional solar-blind ultraviolet photodetectors based on p-PCDTBT/n-Ga_(2)O_(3)heterojunction with high photoresponse

Solar-blind ultraviolet(UV)photodetectors based on p-organic/n-Ga_(2)O_(3) hybrid heterojunctions have attracted extensive attention recently.Herein,the multifunctional solar-blind photodetector based on p-type poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)](PCDTBT)/n-type amorphous Ga_(2)O_(3)(a-Ga_(2)O_(3))is fabricated and investigated,which can work in the phototransistor mode coupling with self-powered mode.With the introduction of PCDTBT,the dark current of such the a-Ga_(2)O_(3)-based photodetector is decreased to 0.48 pA.Meanwhile,the photoresponse parameters of the a-Ga_(2)O_(3)-based photodetector in the phototransistor mode to solar-blind UV light are further increased,that is,responsivity(R),photo-detectivity(D*),and external quantum efficiency(EQE)enhanced to 187 A W^(-1),1.3×10^(16) Jones and 9.1×10^(4)% under the weak light intensity of 11μW cm^(-2),respectively.Thanks to the formation of the built-in field in the p-PCDTBT/n-Ga_(2)O_(3) type-Ⅱ heterojunction,the PCDTBT/Ga_(2)O_(3) multifunctional photodetector shows self-powered behavior.The responsivity of p-PCDTBT/n-Ga_(2)O_(3) multifunctional photodetector is 57.5 mA W^(-1) at zero bias.Such multifunctional p-n hybrid heterojunction-based photodetectors set the stage for realizing high-performance amorphous Ga_(2)O_(3) heterojunction-based photodetectors.

Artificial optoelectronic synapse based on spatiotemporal irradiation to source-sharing circuitry of synaptic phototransistors

To overcome the intrinsic inefficiency of the von Neumann architecture,neuromorphic devices that perform analog vector–matrix multiplication have been highlighted for achieving power-and time-efficient data processing.In particular,artificial synapses,of which conductance should be programmed to represent the synaptic weights of the artificial neural network,have been intensively researched to realize neuromorphic devices.Here,inspired by excitatory and inhibitory synapses,we develop an artificial optoelectronic synapse that shows both potentiation and depression characteristics triggered only by optical inputs.The design of the artificial optoelectronic synapse,in which excitatory and inhibitory synaptic phototransistors are serially connected,enables these characteristics by spatiotemporally irradiating the phototransistor channels with optical pulses.Furthermore,a negative synaptic weight can be realized without the need for electronic components such as comparators.With such attributes,the artificial optoelectronic synapse is demonstrated to classify three digits with a high recognition rate(98.3%)and perform image preprocessing via analog vector-matrix multiplication.

Fine-Tuning Contact via Complexation for High-Performance Organic Solar Cells

For organic optoelectronic devices,precise tuning of the electrical property of both active layers and interfaces is crucial to achieve enhanced device performance.Herein,we developed a facile method using complexation to modify the work function and energy levels of cathode contact layers in organic solar cells(OSCs)to achieve suitable work function and energy levels while retaining relatively good conductivity.Compared with the control devices with neat(N,Ndimethyl-ammonium N-oxide)propyl perylene diimide(PDINO)contacts,the tris(pentafluorophenyl)borane(BCF)-complexed PDINO cathode contacts showed enhanced power conversion efficiencies(PCEs),which is independent of the composition of the active layer.More specifically,single-junction OSCs employing PDINO cathode contact with 2 wt%BCF-additive achieved an average PCE of 17.7%.Based on experimental data and theoretical modeling,we found that the boron cores of BCF coordinate with the amino N-oxide terminal substituent of PDINO after generating BCF–H2O/methanol complexes.BCF segments with a strong electron-withdrawing property can effectively reduce the energy levels of the PDINO–BCF complex,and thus enhance device PCE when it is used as a cathode contact in OSCs.This strategy can be extended to other types of photovoltaic devices,photodetectors,and light-emitting diodes.

Variation of contact resonance frequency during domain switching in PFM measurements for ferroelectric materials

Piezoresponse Force Spectroscopy(PFS)is a powerful technique widely used for measuring the nanoscale electromechanical coupling of the ferro-/piezo-electric materials.However,it is found that certain nonferroelectric materials can also generate the“hysteresis-loop-like”responses from the PFS measurements due to many other factors such as electrostatic effects.This work therefore studies the signal of the contact resonance frequency during the PFS measurements.By comparing the results from ferroelectric and non-ferroelectric materials,it is found there are distinct differences between these two types of materials in the variation of the contact resonance frequency during the PFS measurements.A momentary and sharp increase of the contact resonance frequency occurs when the domain is switched by applying the DC bias,which can be regarded as a unique characteristic for the ferroelectric materials.After analyzing the reliability and mechanism of this method,it is proposed that the contact resonance frequency variation at the coercive bias is capable to differentiate the electromechanical responses of the ferroelectric and non-ferroelectric materials during the PFS measurements.

Pseudogap,Fermi arc,and Peierls-insulating phase induced by 3D-2D crossover in monolayer VSe2

One of important challenges in condensed-matter physics is to realize new quantum states of matter by manipulating the dimensionality of materials,as represented by the discovery of high-temperature superconductivity in atomic-layer pnictides and room-temperature quantum Hall effect in graphene.Tran sition-metal dichalcogenides(TMDs)provide a fertile platform for exploring novel quantum phenomena accompanied by the dimensionality change,since they exhibit a variety of electronic/magnetic states owing to quantum confinement.Here we report an anomalous metal-i nsulator transition in duced by three-dimensional(3D)-two-dimensional(2D)crossover in mono layer 1T-VSe2 grown on bilayer graphene.We observed a complete insulating state with a finite energy gap on the entire Fermi surface in monolayer 1T-VSe2 at low temperatures,in sharp contrast to metallic nature of bulk.More surprisingly,monolayer 1T-VSe2 exhibits a pseudogap with Fermi arc at temperatures above the charge-density-wave temperature,showing a close resemblanee to high-temperature cuprates.This similarity suggests a common underlying physics between two apparently different systems,pointing to the importance of charge/spin fluctuations to create the novel electronic states,such as pseudogap and Fermi arc,in these materials.

Improving solar control of magnetism in ternary organic photovoltaic system with enhanced photo-induced electrons doping

§The growing demand for storage space has promoted in-depth research on magnetic performance regulation in an energy-saving way.Recently,we developed a solar control of magnetism,allowing the magnetic moment to be manipulated by sunlight instead of the magnetic field,current,or laser.Here,binary and ternary photoactive systems with different photon-to-electron conversions are proposed.The photovoltaic/magnetic heterostructures with a ternary system induce larger magnetic changes due to higher short current density(J SC)(20.92 mA·cm^(−2))compared with the binary system(11.94 mA·cm^(−2)).During the sunlight illumination,ferromagnetic resonance(FMR)shift increases by 80%(from 169.52 to 305.48 Oe)attributed to enhanced photo-induced electrons doping,and the variation of saturation magnetization(M S)is also amplified by 14%(from 9.9%to 11.3%).Furthermore,photovoltaic performance analysis and the transient absorption(TA)spectra indicate that the current density plays a major role in visible light manipulating magnetism.These findings clarify the laws of sunlight control of magnetism and lay the foundation for the next generation solar-driven magneto-optical memory applications.

自旋电子学

本书是关于自旋电子学的新著，内容涉及磁纳米结构的材料、制造和特性，磁纳米结构中的磁性和自旋控制，半导体中的磁光性质、磁电子学及器件。

Piezo-/ferroelectric phenomena in biomaterials: A brief review of recent progress and perspectives

There have been overwhelming observations of piezo-/ferroelectric phenomena in many biological tissues and macromolecules,boosting the development of bio-based smart devices and the applications using electromechanical coupling phenomena in biological systems.The electromechanical coupling is believed to be responsible for various biophysical behaviors and remarkable biomaterial properties.Despite the abundant phenomenal observations,the fundamental understanding of the piezo-/ferroelectric effect in biomaterials/systems and the rational design of biobased macroscopic materials with desired piezoelectric responses are still scarce.In this review,we firstly present remarkable historical events on the development of piezo-/ferroelectricity in biomaterials,followed by a brief overview of the fundamental physics of piezo-/ferroelectricity.The developments of biopiezo-/bioferroelectricity in protein-based biomaterials and their implications are highlighted subsequently.In experimental studies,to identify the intrinsic piezo-/ferroelectric properties from other effects or artifacts is usually elusive.This issue is also addressed and discussed in detail,especially using piezoelectric force microscopy(PFM)and spectroscopy techniques to investigate the local piezo-/ferroelectric phenomena in nanostructured materials are highlighted emphatically.

Defects inducing anomalous exciton kinetics in monolayer WS_(2)

Two-dimensional(2D)transition metal dichalcogenide(TMD)has emerged as an effective optoelectronics material due to its novel optical properties.Understanding the role of defects in exciton kinetics is crucial for achieving high-efficiency TMD devices.Here,we observe defects induced anomalous power dependence exciton dynamics and spatial distribution in hexagonal heterogeneous WS_(2).With transient absorption microscopy study,we illustrate that these phenomena originate from the competition between radiative and defect-related non-radiative decays.To understand the physics behind this,a decay model is introduced with two defect-related channels,which demonstrates that more excitons decay through non-radiative channels in the dark region than the bright region.Our work reveals the mechanisms of anomalous exciton kinetics by defects and is instrumental for understanding and exploiting excitonic states in emerging 2D semiconductors.

Intriguing magnetoelectric effect in two-dimensional ferromagnetic/perovskite oxide ferroelectric heterostructure

Two-dimensional (2D) magnets have broad application prospects in the spintronics,but how to effectively control them with a small electric field is still an issue.Here we propose that 2D magnets can be efficiently controlled in a multiferroic heterostructure composed of 2D magnetic material and perovskite oxide ferroelectric (POF) whose dielectric polarization is easily flipped under a small electric field.We illustrate the feasibility of such strategy in the bilayer CrI_(3)/BiFeO_(3)(001) heterostructure by using the first-principles calculations.Different from the traditional POF multiferroic heterostructures which have strong interface interactions,we find that the interface interaction between CrI_(3) and BiFeO_(3)(001) is van der Waals type.Whereas,the heterostructure has particular strong magnetoelectric coupling where the bilayer CrI_(3) can be efficiently switched between ferromagnetic and antiferromagnetic types by the polarized states P↑ and P↓ of BiFeO_(3)(001).We also discover the competing effect between electron doping and the additional electric field on the interlayer exchange coupling interaction of CrI_(3),which is responsible to the magnetic phase transition.Our results provide a avenue for the tuning of 2D magnets with a small electric field.