Memristive Devices Based on Two-Dimensional Transition Metal Chalcogenides for Neuromorphic Computing

Two-dimensional(2D)transition metal chalcogenides(TMC)and their heterostructures are appealing as building blocks in a wide range of electronic and optoelectronic devices,particularly futuristic memristive and synaptic devices for brain-inspired neuromorphic computing systems.The distinct properties such as high durability,electrical and optical tunability,clean surface,flexibility,and LEGO-staking capability enable simple fabrication with high integration density,energy-efficient operation,and high scalability.This review provides a thorough examination of high-performance memristors based on 2D TMCs for neuromorphic computing applications,including the promise of 2D TMC materials and heterostructures,as well as the state-of-the-art demonstration of memristive devices.The challenges and future prospects for the development of these emerging materials and devices are also discussed.The purpose of this review is to provide an outlook on the fabrication and characterization of neuromorphic memristors based on 2D TMCs.

Flexible and stretchable photodetectors and gas sensors for wearable healthcare based on solution-processable metal chalcogenides

Wearable smart sensors are considered to be the new generation of personal portable devices for health monitoring.By attaching to the skin surface,these sensors are closely related to body signals(such as heart rate,blood oxygen saturation,breath markers,etc.)and ambient signals(such as ultraviolet radiation,inflammable and explosive,toxic and harmful gases),thus providing new opportunities for human activity monitoring and personal telemedicine care.Here we focus on photodetectors and gas sensors built from metal chalcogenide,which have made great progress in recent years.Firstly,we present an overview of healthcare applications based on photodetectors and gas sensors,and discuss the requirement associated with these applications in detail.We then discuss advantages and properties of solution-processable metal chalcogenides,followed by some recent achievements in health monitoring with photodetectors and gas sensors based on metal chalcogenides.Last we present further research directions and challenges to develop an integrated wearable platform for monitoring human activity and personal healthcare.

Growth and Application of Chalcogenides of Lead and Related Compounds

Large grain, low-dislocation, high-quality single crystals of various Pb-salt compounds have been grownreproducibly by the Horizontal Unseeded Vapor Growth (HUVG) technique. The Tunable Diode Lasers withbetter performance have been made with such crystals. The annealing feature, dislocations and diffusion in thecrystals have also been investigated.

Three Two-dimensional Heterometallic Chalcogenides[TM(tren)][InSbSe_3S](TM = Fe, Co, Mn): Syntheses,Crystal Structures, and Properties

Three two-dimensional(2-D) heterometallic chalcogenides [TM(tren)][InSbSe_3S](TM = Fe(1), Co(2), Mn(3); tren = tris(2-aminoethyl)-amine) have been solvothermally synthesized in this paper. Single-crystal X-ray analysis indicates that they are isostructural to each other except the different transition metal ions. These compounds contain an unsaturated complex cation [TM(tren)]^(2+), which can be further coordinated by the 2-D [InSbSSe_3]_n^(2n-) anion, resulting in a new neutral organic-decorated heterometallic chalcogenide. These compounds crystallize in monoclinic space group P21/c, with a = 11.768(12), b = 13.884(14), c = 11.095(11) ?, Z = 4, D_c =2.910 Mg·m^(-3), F(000) = 1304, M_r = 707.60 for 1; a = 11.843(9), b = 14.064(10), c = 10.979(8) ?, Z= 4, Dc = 2.875 Mg·m^(-3), F(000) = 1308, M_r = 710.68 for 2; a = 11.969(10), b = 14.191(11), c =11.112(9) ?, Z = 4, D_c = 2.779 Mg·m^(-3), F(000) = 1300, Mr = 706.69 for 3. The maximum and minimum peaks of compounds 1～3 are 6.996 and –2.880 e·?^(-3), 2.242 and –3.066 e·?^(-3), 3.655 and –3.569 e·?^(-3), respectively. These compounds were structurally characterized by powder X-ray diffraction measurement, thermal analysis, infrared spectroscopy and UV-Vis diffuse reflectance spectroscopy. A solid-state UV/Vis reflectance spectroscopy measurement on 1, 2 and 3 confirmed that these compounds are semiconductor materials.

Electronic structure and spin–orbit coupling in ternary transition metal chalcogenides Cu_(2)TlX_(2)(X = Se, Te)

Ternary transition metal chalcogenides provide a rich platform to search and study intriguing electronic properties. Using angle-resolved photoemission spectroscopy and ab initio calculation, we investigate the electronic structure of Cu_(2)TlX_(2)(X = Se, Te), ternary transition metal chalcogenides with quasi-two-dimensional crystal structure. The band dispersions near the Fermi level are mainly contributed by the Te/Se p orbitals. According to our ab-initio calculation, the electronic structure changes from a semiconductor with indirect band gap in Cu_(2)TlSe_(2) to a semimetal in Cu_(2)TlTe_(2), suggesting a band-gap tunability with the composition of Se and Te. By comparing ARPES experimental data with the calculated results, we identify strong modulation of the band structure by spin–orbit coupling in the compounds. Our results provide a ternary platform to study and engineer the electronic properties of transition metal chalcogenides related to large spin–orbit coupling.

Physicochemical Properties of Thin Rhenium Chalcogenides Coatings

Some physicochemical properties of thin layers of rhenium chalcogenides alloys such as micro-hardness and inner tension have been investigated. An influence of electrolyte composition and content of rhenium in the alloy on inner tension and microhardness of rhenium chalcogenides alloy was studied. It is established that the electrolytic alloys of rhenium chalcogenides obtained from different electrolytes possess inner tension of stretching. The most favourable condition for reaching the uniform coating of rhenium chalcogenides from sulphate and chloride-sulphate electrolyte using dispersive capacity (DC) of electrolyte has been found out. The best DC in chloride-sulphate and sulphate electrolyte takes place at current density of 0.8 A/dm2 at 75℃.

Structural and electronic properties of transition-metal chalcogenides Mo5S4 nanowires

Transition-metal chalcogenide nanowires(TMCN) as a viable candidate for nanoscale applications have been attracting much attention for the last few decades. Starting from the rigid building block of M6 octahedra(M = transition metal),depending on the way of connection between M6 and decoration by chalcogenide atoms, multiple types of extended TMCN nanowires can be constructed based on some basic rules of backbone construction proposed here. Note that the well-known Chevrel-phase based M6X6 and M6X9(X = chalcogenide atom) nanowires, which are among our proposed structures, have been successfully synthesized by experiment and well studied. More interestingly, based on the construction principles, we predict three new structural phases(the cap, edge, and C&E phases) of Mo5S4, one of which(the edge phase) has been obtained by top-down electron beam lithography on two-dimensional MoS2, and the C&E phase is yet to be synthesized but appears more stable than the edge phase. The stability of the new phases of Mo5S4 is further substantiated by crystal orbital overlapping population(COOP), phonon dispersion relation, and thermodynamic calculation. The barrier of the structural transition between different phases of Mo5S4 shows that it is very likely to realize an conversion from the experimentally achieved structure to the most stable C&E phase. The calculated electronic structure shows an interesting band nesting between valence and conduction bands of the C&E Mo5S4 phase, suggesting that such a nanowire structure can be well suitable for optoelectronic sensor applications.

Superconductivity in CuIr_(2-x)Al_(x)Te_(4) telluride chalcogenides

The relationship between charge-density-wave(CDW) and superconductivity(SC), two vital physical phases in condensed matter physics, has always been the focus of scientists’ research over the past decades. Motivated by this research hotspot, we systematically studied the physical properties of the layered telluride chalcogenide superconductors CuIr_(2-x)Al_(x)Te_(4)(0 ≤x≤ 0.2). Through the resistance and magnetization measurements, we found that the CDW order was destroyed by a small amount of Al doping. Meanwhile, the superconducting transition temperature(T_(c)) kept changing with the change of doping amount and rose towards the maximum value of 2.75 K when x = 0.075. The value of normalized specific heat jump(△C/γT_(c)) for the highest T_(c) sample CuIr_(2-x)Al_(x)Te_(4)was 1.53, which was larger than the BCS value of 1.43 and showed the bulk superconducting nature. In order to clearly show the relationship between SC and CDW states,we propose a phase diagram of T_(c) vs. doping content.

Tungsten chalcogenides as anodes for potassium-ion batteries

Potassium-ion batteries(PIBs)by virtue of their strong cost competitiveness and similar electrochemical properties to lithiumion batteries have been deemed to be a promising electrochemical energy storage technology.To promote the application in the commercial market,developing electrode materials with high specifi c capacities,superior cycling stability,and reliable safety is of great importance.Anode materials as an important component of PIBs play a decisive role,among which twodimensional transition metal chalcogenides(2D TMCs)have attracted wide attention owing to their unique material and electrochemical properties.In the 2D TMCs’family,molybdenum chalcogenides as fl agship are the most studied materials and demonstrated the potential as anodes.With the deepening of research on 2D TMCs,another shining member that possesses similar properties to molybdenum chalcogenides,tungsten chalcogenides(WS 2,WSe 2,and WTe 2),has aroused tremendous attention.Despite many inspiring results,various challenges remain to be further addressed;meanwhile,some results are still unclear and disputed.Herein,this review fi rst introduces their material properties and electrochemical storage mechanisms.Then,we systematically overview the research progress and put forward promoting improvement strategies.Finally,challenges and opportunities that would be future research directions are discussed.

Self-supported transition metal chalcogenides for oxygen evolution

Owing to stable spatial framework and large electrochemical interface,self-supported transition metal chalcogenides have been actively explored in renewable energy fields,especially in oxygen evolution reaction(OER).Here,we review the research progress of self-supported transition metal chalcogenides(including sulfides,selenides,and tellurides)for the OER in recent years.The basic principle and evaluation parameters of OER are first introduced,and then the preparation methods of transition metal chalcogenides on various self-supporting substrates(including Ni foam(NF),carbon cloth(CC),carbon fiber paper(CFP),metal mesh/plate,etc.)are systematically summarized.Subsequently,advanced optimization strategies(including interface and defect engineering,heteroatom doping,edge engineering,surface morphology engineering,and construction of heterostructure)are introduced in detail to improve the inherent catalytic activity of self-supported electrocatalysts.Finally,the challenges and prospects of developing more promising self-supported chalcogenide electrocatalysts are proposed.

Salt-inclusion chalcogenides:Double functional moieties design strategy toward excellent nonlinear optical materials

Nonlinear optical(NLO)material are of great importance in converting the frequency to extend the laser spectrum in mid-and far-infrared(IR)regions where lasers operate poorly or are unavailable.However,the contradiction between large second-harmonic generation(SHG)intensity and wide band gap(E_(g))is a long-standing problem for IR NLO materials.To address the issue,metal chalcogenides have been successfully synthesized and developed excellent optical properties in the past few decades.As a newly discovered category of chalcogenides,salt-inclusion chalcogenides with distinctive structures and brilliant performances have been placed great expecta-tions to be a novel problem-solving.In this review,52 compounds in this thriving family are categorized via dimensions of host and guest parts,further discussing the relationships between their structures and NLO properties,as well as future perspectives of this family.

Metal chalcogenides for potassium storage

Potassium-based energy storage technologies,especially potassium ion batteries(PIBs),have received great interest over the past decade.A pivotal challenge facing high-performance PIBs is to identify advanced electrode materials that can store the large-radius K+ions,as well as to tailor the various thermodynamic parameters.Metal chalcogenides are one of the most promising anode materials,having a high theoretical specific capacity,high in-plane electrical conductivity,and relatively small volume change on charge/discharge.However,the development of metal chalcogenides for PIBs is still in its infancy because of the limited choice of high-performance electrode materials.However,numerous efforts have been made to conquer this challenge.In this article,we overview potassium storage mechanisms,the technical hurdles,and the optimization strategies for metal chalcogenides and highlight how the adjustment of the crystalline structure and choice of the electrolyte affect the electrochemical performance of metal-chalcogenide-based electrode materials.Other potential potassium-based energy storage systems to which metal chalcogenides can be applied are also discussed.Finally,future research directions focusing on metal chalcogenides for potassium storage are proposed.

Enhancing stability by tuning element ratio in 2D transition metal chalcogenides

Two-dimensional(2D)transition metal chalcogenides(TMCs)are known to be susceptible to the atmosphere,which greatly obscures the intrinsic physical and chemical properties.The quantitative origin of the instability on the atomic scale has not been well investigated due to the lack of environmentally stable TMCs sample.Here,we find the stability of the grown TMCs is strongly relevant to their initial element ratios,and thus the stoichiometric bonded TMCs have favorable stability,benefitted from the TMCs with controllable chalcogenisation.In this study,the degree of structural degradation has been quantitatively defined by the reduced element ratio of chalcogen to metal through the time-dependent characterizations,and the non-stoichiometric ratios in TMCs reveal the atomic lattices with point defects like additive bonds or vacancies inside.This study not only provides a potential view to fabricate environmentally stable TMCs based devices,but also will bring an effective feasibility of stacking stable vertical heterostructures.

Bimetallic chalcogenides for electrocatalytic CO_(2)reduction

Electrocatalytic CO_(2)reduction reaction(CO_(2)RR)converts CO_(2)into valuable chemical fuels,which can effectively alleviate global warming and energy crisis.However,limited by its slow reaction rate and low product selectivity,it is urgent to design efficient,cheap,safe,and highly selective CO_(2)RR electrocatalysts.Owing to the advantages of adjustable electronic structure,abundant active sites,low cost,environmental friendliness and excellent electrochemical performance,bimetallic chalcogenides have aroused great interest.Here,we briefly summarized different bimetallic oxides and sulfides for electrocatalytic CO_(2)RR in the past five years.In addition,different hybridizations formed between metal atoms,including intermetallic compounds,heterostructures and metal doping,were generalized.Their positive effects on CO_(2)RR catalytic selectivity and activity were deeply uncovered.Besides,we also put forward some views about the future research directions and perspectives in CO_(2)RR field.This review aims to provide a reference for the rational design of bimetallic chalcogenides towards electrocatalytic CO_(2)reduction.

Two-dimensional magnetic transition metal chalcogenides

The field of two-dimensional(2D)magnets has expanded rapidly during the past several years since the first demonstration of intrinsic 2D magnetism in atomically thin CrI_(3) and Cr_(2)Ge_(2)Te_(6) in 2017.2D transition metal chalcogenides(TMCs),a class of strongly correlated materials,have exhibited a wide variety of novel electronic and optical properties,and more recently magnetism.Here,we review recent experimental progress achieved in the growth of 2D magnetic TMC materials using chemical vapor deposition and molecular beam epitaxy methods.Outstanding examples include the demonstration of room temperature intrinsic and extrinsic ferromagnetism in monolayer VSe_(2),MnSe_(2),Cr_(3)Te_(4),V-doped WSe_(2),and so on.A brief discussion on the origin of the exotic magnetic properties and emergent phenomena is also presented.Finally,we summarize the remaining challenges and future perspective on the development of 2D magnetic materials for next-generation spintronic devices.

Chalcogenide Ovonic Threshold Switching Selector

Today’s explosion of data urgently requires memory technologies capable of storing large volumes of data in shorter time frames,a feat unattain-able with Flash or DRAM.Intel Optane,commonly referred to as three-dimensional phase change memory,stands out as one of the most promising candidates.The Optane with cross-point architecture is constructed through layering a storage element and a selector known as the ovonic threshold switch(OTS).The OTS device,which employs chalcogenide film,has thereby gathered increased attention in recent years.In this paper,we begin by providing a brief introduction to the discovery process of the OTS phenomenon.Subsequently,we summarize the key elec-trical parameters of OTS devices and delve into recent explorations of OTS materials,which are categorized as Se-based,Te-based,and S-based material systems.Furthermore,we discuss various models for the OTS switching mechanism,including field-induced nucleation model,as well as several carrier injection models.Additionally,we review the progress and innovations in OTS mechanism research.Finally,we highlight the successful application of OTS devices in three-dimensional high-density memory and offer insights into their promising performance and extensive prospects in emerging applications,such as self-selecting memory and neuromorphic computing.

Non-volatile dynamically switchable color display via chalcogenide stepwise cavity resonators

High-resolution multi-color printing relies upon pixelated optical nanostructures,which is crucial to promote color display by producing nonbleaching colors,yet requires simplicity in fabrication and dynamic switching.Antimony trisulfide(Sb_(2)S_(3))is a newly rising chalcogenide material that possesses prompt and significant transition of its optical characteristics in the visible region between amorphous and crystalline phases,which holds the key to color-varying devices.Herein,we proposed a dynamically switchable color printing method using Sb_(2)S_(3)-based stepwise pixelated Fabry-Pérot(FP)cavities with various cavity lengths.The device was fabricated by employing a direct laser patterning that is a less timeconsuming,more approachable,and low-cost technique.As switching the state of Sb_(2)S_(3) between amorphous and crystalline,the multi-color of stepwise pixelated FP cavities can be actively changed.The color variation is due to the profound change in the refractive index of Sb_(2)S_(3) over the visible spectrum during its phase transition.Moreover,we directly fabricated sub-50 nm nano-grating on ultrathin Sb_(2)S_(3) laminate via microsphere 800-nm femtosecond laser irradiation in far field.The minimum feature size can be further decreased down to~45 nm(λ/17)by varying the thickness of Sb_(2)S_(3) film.Ultrafast switchable Sb_(2)S_(3) photonic devices can take one step toward the next generation of inkless erasable papers or displays and enable information encryption,camouflaging surfaces,anticounterfeiting,etc.Importantly,our work explores the prospects of rapid and rewritable fabrication of periodic structures with nano-scale resolution and can serve as a guideline for further development of chalcogenide-based photonics components.

Predicting diamond-like Co-based chalcogenides as unconventional high temperature superconductors

We predict Co-based chalcogenides with a diamond-like structure can host unconventional high temperature superconductivity(high-Tc). The essential electronic physics in these materials stems from the Co layers with each layer being formed by vertex-shared CoA_4(A=S, Se, Te) tetrahedra complexes, a material genome proposed recently by us to host potential unconventional high-Tcclose to a d7 filling configuration in 3 d transition metal compounds. We calculate the magnetic ground states of different transition metal compounds with this structure. It is found that(Mn, Fe, Co)-based compounds all have a G-type antiferromagnetic(AFM) insulating ground state while Ni-based compounds are paramagnetic metal.The AFM interaction is the largest in the Co-based compounds as the three t2 gorbitals all strongly participate in AFM superexchange interactions. The abrupt quenching of the magnetism from the Co to Ni-based compounds is very similar to those from Fe to Co-based pnictides in which a C-type AFM state appears in the Fe-based ones but vanishes in the Co-based ones. This behavior can be considered as an electronic signature of the high-Tcgene. Upon doping, as we predicted before, this family of Co-based compounds favor a strong d-wave pairing superconducting state.

Recent advances in transition metal chalcogenides for lithium-ion capacitors

Transition metal chalcogenides(TMCs)and TMCs-based nanocomposites have attracted extensive attention due to their versatile material species,low cost,and rich physical and chemical characteristics.As anode materials of lithium-ion capacitors(LICs),TMCs have exhibited high theoretical capacities and pseudocapacitance storage mechanism.However,there are many intrinsic challenges,such as low electrical conductivity,repeatedly high-volume changes and sluggish ionic diffusion kinetics.Hence,many traditional and unconventional techniques have been reported to solve these critical problems,and many innovative strategies are also used to prepare high quality anode materials for LICs.In this mini review,a detailed family member list and comparison of TMCs in the field of lithium-ion capacitors have been summarized firstly.Then,many rectification stratagems and recent researches of TMCs have been exhibited and discussed.In the end,as an outcome of these discussions,some further challenges and perspectives are envisioned to promote the application of TMCs materials for lithium-ion c apacitors.

Thermoelectric transport properties in chalcogenides ZnX (X=S, Se): From the role of electron-phonon couplings

Electron-phonon coupling(EPC)is a key factor for thermoelectric properties of materials.In this paper,the thermoelectric properties of zinc-blende chalcogenides(p-type)ZnS and ZnSe have been studied through full evaluation of EPC from first-principles,including the influences on both electrical and thermal transport.We find that the polar longitudinal optical phonon scattering is the dominant mechanism for electrical transport.Due to the triple degeneracy near the valence band maximum,the inter-band scattering also has detrimental contributions to the electrical conductivities.For phonon transport,it shows that the lattice thermal conductivity can be reduced by the electron-phonon scattering significantly at high carrier concentrations(e.g.,at 300 K with 10^(21) cm^(3) of hole,the reduction is-24.9%for ZnS and-28.4%for ZnSe,respectively).Finally,the p-type thermoelectric figure of merit(ZT)of two systems have been obtained,which are 0.129 for ZnS and 0.141 for ZnSe,at 700 K with their respective optimal hole concentrations.Our work provides a complete and in-depth study of thermoelectric properties in chalcogenides ZnX from the role of EPC.The results suggest EPC plays an important role on the thermoelectric properties and thus full evaluation of EPC is necessary especially for polar materials.