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.

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.

Synthesis and Modulation of Low-Dimensional Transition Metal Chalcogenide Materials via Atomic Substitution

In recent years,low-dimensional transition metal chalcogenide(TMC)materials have garnered growing research attention due to their superior electronic,optical,and catalytic properties compared to their bulk counterparts.The controllable synthesis and manipulation of these materials are crucial for tailoring their properties and unlocking their full potential in various applications.In this context,the atomic substitution method has emerged as a favorable approach.It involves the replacement of specific atoms within TMC structures with other elements and possesses the capability to regulate the compositions finely,crystal structures,and inherent properties of the resulting materials.In this review,we present a comprehensive overview on various strategies of atomic substitution employed in the synthesis of zero-dimensional,one-dimensional and two-dimensional TMC materials.The effects of substituting elements,substitution ratios,and substitution positions on the structures and morphologies of resulting material are discussed.The enhanced electrocatalytic performance and photovoltaic properties of the obtained materials are also provided,emphasizing the role of atomic substitution in achieving these advancements.Finally,challenges and future prospects in the field of atomic substitution for fabricating low-dimensional TMC materials are summarized.

Recent advances in optoelectronic properties and applications of two-dimensional metal chalcogenides

Since two-dimensional （2D） graphene was fabricated successfully, many kinds of graphene-like 2D materials have attracted extensive attention. Among them, the studies of 2D metal chalcogenides have become the focus of intense research due to their unique physical properties and promising applications. Here, we review significant recent advances in optoelectronic properties and applications of 2D metal chalcogenides. This review highlights the recent progress of synthesis, characterization and isolation of single and few layer metal chalco- genides nanosheets. Moreover, we also focus on the recent important progress of electronic, optical properties and optoelectronic devices of 2D metal chalcogenides. Additionally, the theoretical model and understanding on the band structures, optical properties and related physical mechanism are also reviewed. Finally, we give some per- sonal perspectives on potential research problems in the optoelectronic characteristics of 2D metal chalcogenides and related device applications.

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.

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.

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.

Dual-metal precursors for the universal growth of non-layered 2D transition metal chalcogenides with ordered cation vacancies

Two-dimensional(2D)transition metal chalcogenides(TMCs)are promising for nanoelectronics and energy applications.Among them,the emerging non-layered TMCs are unique due to their unsaturated dangling bonds on the surface and strong intralayer and interlayer bonding.However,the synthesis of non-layered 2D TMCs is challenging and this has made it difficult to study their structures and properties at thin thickness limit.Here,we develop a universal dual-metal precursors method to grow non-layered TMCs in which a mixture of a metal and its chloride serves as the metal source.Taking hexagonal Fe_(1-x)S as an example,the thickness of the Fe_(1-x)S flakes is down to 3 nm with a lateral size of over 100 μm.Importantly,we find ordered cation Fe vacancies in Fe_(1-x)S,which is distinct from layered TMCs like MoS_(2) where anion vacancies are commonly observed.Low-temperature transport measurements and theoretical calculations show that 2D Fe_(1-x)S is a stable semiconductor with a narrow bandgap of60 meV.In addition to Fe_(1-x)S,the method is universal in growing various non-layered 2D TMCs containing ordered cation vacancies,including Fe_(1-x)Se,Co_(1-x)S,Cr_(1-x)S,and V_(1-x)S.This work paves the way to grow and exploit properties of non-layered materials at 2D thickness limit.

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.

Novel two-dimensional transition metal chalcogenides created by epitaxial growth

Two-dimensional(2D) materials have been a very important field in condensed matter physics, materials science, chemistry, and electronics. In a variety of 2D materials, transition metal chalcogenides are of particular interest due to their unique structures and rich properties. In this review, we introduce a series of 2D transition metal chalcogenides prepared by epitaxial growth. We show that not only 2D transition metal dichalcogenides can be grown, but also the transition metal chalcogenides that do not have bulk counterparts, and even patterned transition metal chalcogenides can be fabricated. We discuss the formation mechanisms of the novel structures, their interesting properties, and potential applications of these 2D transition metal chalcogenides. Finally, we give a summary and some perspectives on future studies.

Role of binary metal chalcogenides in extending the limits of energy storage systems:Challenges and possible solutions

Binary metal chalcogenides(BMCs)have shown better electrochemical performance compared with their mono metal counterparts owing to their abundant phase interfaces,higher active sites,faster electrochemical kinetics and higher electronic conductivity.Nevertheless,their performance still undergoes adverse decline during electrochemical processes mainly due to poor intrinsic ionic conductivities,large volume expansions,and structural agglomeration and fracture.To tackle these problems,various strategies have been applied to engineer the BMC nanostructures to obtain optimized electrode materials.However,the lack of understanding of the electrochemical response of BMCs still hinders their large-scale application.This review not only highlights the recent progress and development in the preparation of BMC-based electrode materials but also explains the kinetics to further understand the relation between structure and performance.It will also explain the engineering of BMCs through nanostructuring and formation of their hybrid structures with various carbonaceous materials and three-dimensional(3 D)templates.The review will discuss the detailed working mechanism of BMC-based nanostructures in various electrochemical energy storage(EES)systems including supercapacitors,metal-ion batteries,metal-air batteries,and alkaline batteries.In the end,major challenges and prospective solutions for the development of BMCs in EES devices are also outlined.We believe that the current review will provide a guideline for tailoring BMCs for better electrochemical devices.

Two-dimensional Organic Metal Chalcogenides

Inorganic two-dimensional(2 D)materials have attracted tremendous interests recently.Controlled functionalization of 2 D materials can achieve additional functions and properties,but usually suffers from less modification ratio,poor controllability,defects and so on.2 D organic metal chalcogenide(OMC)materials with periodically arranged organic functional group between the inorganic analogues layers offer opportunities to develop adjustable electrical properties and extended applications.In this mini-review,we will provide an overview of the composition and preparation,band gap engineering,and conductivity modulation of the serial OMC materials and illustrate the application investigation such as biomimetic catalysis,photodetecting and chemiresistive gas sensing.

Design and Exploratory Synthesis of Infrared Nonlinear Optical Metal Chalcogenides: Recent Advances and Future Perspectives

Design and exploratory synthesis of novel infrared nonlinear optical chalcogenides have drawn extensive concerns owing to their excellent overall performance and important role in laser industry.During the past decades,a large number of infrared nonlinear optical chalcogenides have been developed and many effective design strategies have been summarized,which illuminates the path of future explorations.In this perspective,we discuss the feasibility and effectiveness of the representative design ideas.Moreover,we point out some topics to be investigated and discuss the future research directions.

Metal chalcogenide-based photoelectrodes for photoelectrochemical water splitting

Photoelectrochemical water splitting(PEC-WS)is a promising technique for transforming solar energy into storable and environmentally friendly chemical energy.Designing semiconductor photoelectrodes with high light absorption capability,rapid e-/h+separation and transfer,and sufficient chemical stability is vital for developing an efficient PEC-WS system.Metal chalcogenides(MCs)have emerged as promising candidates for light absorbers because of their unique electrical and optical characteristics.In this review,we present recent developments in hydrogen generation via PEC-WS using MC-based photoelectrodes.First,we present a simple illustration of PEC-WS fundamentals.Second,the current performance of various metal(mono-,di-,and tri-)chalcogenide/semiconductor photoelectrodes in PEC-WS is summarized.Then,the charge transfer mechanism at the MC/semiconductor interface and the PEC-WS mechanism is thoroughly explained.Finally,we discuss future research perspectives toward developing efficient and stable MC/semiconductor photoelectrodes.

Strongly Coupled 2D Transition Metal Chalcogenide-MXene-Carbonaceous Nanoribbon Heterostructures with Ultrafast Ion Transport for Boosting Sodium/Potassium Ions Storage

Combining with the advantages of two-dimensional(2D)nanomaterials,MXenes have shown great potential in next generation rechargeable batteries.Similar with other 2D materials,MXenes generally suffer severe self-agglomeration,low capacity,and unsatisfied durability,particularly for larger sodium/potassium ions,compromising their practical values.In this work,a novel ternary heterostructure self-assembled from transition metal selenides(MSe,M=Cu,Ni,and Co),MXene nanosheets and N-rich carbonaceous nanoribbons(CNRibs)with ultrafast ion transport properties is designed for sluggish sodium-ion(SIB)and potassium-ion(PIB)batteries.Benefiting from the diverse chemical characteristics,the positively charged MSe anchored onto the electronegative hydroxy(-OH)functionalized MXene surfaces through electrostatic adsorption,while the fungal-derived CNRibs bonded with the other side of MXene through amino bridging and hydrogen bonds.This unique MXene-based heterostructure prevents the restacking of 2D materials,increases the intrinsic conductivity,and most importantly,provides ultrafast interfacial ion transport pathways and extra surficial and interfacial storage sites,and thus,boosts the high-rate storage performances in SIB and PIB applications.Both the quantitatively kinetic analysis and the density functional theory(DFT)calculations revealed that the interfacial ion transport is several orders higher than that of the pristine MXenes,which delivered much enhanced Na+(536.3 mAh g^(−1)@0.1 A g^(−1))and K^(+)(305.6 mAh g^(−1)@1.0 A g^(−1))storage capabilities and excel-lent long-term cycling stability.Therefore,this work provides new insights into 2D materials engineering and low-cost,but kinetically sluggish post-Li batteries.

Recent advances in the electrochemistry of layered post-transition metal chalcogenide nanomaterials for hydrogen evolution reaction

Layered two-dimensional(2 D)materials have received tremendous attention due to their unique physical and chemical properties when downsized to single or few layers.Several types of layered materials,especially transition metal dichalcogenides(TMDs)have been demonstrated to be good electrode materials due to their interesting physical and chemical properties.Apart from TMDs,post-transition metal chalcogenides(PTMCs)recently have emerged as a family of important semiconducting materials for electrochemical studies.PTMCs are layered materials which are composed of post-transition metals raging from main group IIIA to group VA(Ga,In,Ge,Sn,Sb and Bi)and group VI chalcogen atoms(S,selenium(Se)and tellurium(Te)).Although a large number of literatures have reviewed the electrochemical and electrocatalytic applications of TMDs,less attention has been focused on PTMCs.In this review,we focus our attention on PTMCs with the aim to provide a summary to describe their fundamental electrochemical properties and electrocatalytic activity towards hydrogen evolution reaction(HER).The characteristic chemical compositions and crystal structures of PTMCs are firstly discussed,which are different from TMDs.Then,inherent electrochemistry of PTMCs is discussed to unveil the well-defined redox behaviors of PTMCs,which could potentially affect their efficiency when applied as electrode materials.Following,we focus our attention on electrocatalytic activity of PTMCs towards HER including novel synthetic strategies developed for the optimization of their HER activity.This review ends with the perspectives for the future research direction in the field of PTMC based electrocatalysts.

Recent advances in p-block metal chalcogenide electrocatalysts for high-efficiency CO_(2) reduction

Electrocatalytic CO_(2) reduction(ECR)to high-value fuels and chemicals offers a promising conversion technology for achieving sustainable carbon cycles.In recent years,although great efforts have been made to develop highefficiency ECR catalysts,challenges remain in achieving high activity and long durability simultaneously.Taking advantage of the adjustable structure,tunable component,and the M–Ch(M¼Sn,In,Bi,etc.,Ch¼S,Se,Te)covalent bonds stabilized metal centers,the p-block metal chalcogenides(PMC)based electrocatalysts have shown great potential in converting CO_(2) into CO or formates.In addition,the unique p-block electron structure can suppress the competitive hydrogen evolution reaction and enhance the adsorption of ECR intermediates.Seeking to systematically understand the structure–activity relationship of PMC-based ECR catalysts,this review summarizes the recent advances in designing PMC electrocatalysts for CO_(2) reduction based on the fundamental aspects of heterogeneous ECR process,including advanced strategies for optimizing the intrinsic activity and improving the loading density of catalytic sites,constructing highly stable catalysts,and tuning product.

Engineering two-dimensional metal oxides and chalcogenides for enhanced electro-and photocatalysis

Two-dimensional(2D)metal oxides and chalcogenides(MOs&MCs)have been regarded as a new class of promising electro-and photocatalysts for many important chemical reactions such as hydrogen evolution reaction,CO_(2) reduction reaction and N2 reduction reaction in virtue of their outstanding physicochemical properties.However,pristine 2D MOs&MCs generally show the relatively poor catalytic performances due to the low electrical conductivity,few active sites and fast charge recombination.Therefore,considerable efforts have been devoted to engineering 2D MOs&MCs by rational structural design and chemical modification to further improve the catalytic activities.Herein,we comprehensively review the recent advances for engineering technologies of 2D MOs&MCs,which are mainly focused on the intercalation,doping,defects creation,facet design and compositing with functional materials.Meanwhile,the relationship between morphological,physicochemical,electronic,and optical properties of 2D MOs&MCs and their electro-and photocatalytic performances is also systematically discussed.Finally,we further give the prospect and challenge of the field and possible future research directions,aiming to inspire more research for achieving high-performance 2D MOs&MCs catalysts in energy storage and conversion fields.

Recent progress and prospective on layered anode materials for potassium-ion batteries

Potassium-ion batteries(PIBs),also known as“novel post-lithium-ion batteries,”have promising energy storage and utilization prospects due to their abundant and inexpensive raw materials.Appropriate anode materials are critical for realizing high-performance PIBs because they are an important component determining the energy and power densities.Two-dimensional(2D)layered anode materials with increased interlayer distances,specific surface areas,and more active sites are promising candidates for PIBs,which have a high reversible capacity in the energetic pathway.In this review,we briefly summarize K+storage behaviors in 2D layered carbon,transition metal chalcogenides,and MXene materials and provide some suggestions on how to select and optimize appropriate 2D anode materials to achieve ideal electrochemical performance.

Biotoxicity of CdS/CdSe Core-Shell Nano-Structures

The cytotoxicity of hydrophobic QDs CdS/CdSe was tested assigning MTT assay on Human Embryonic Kidney cells (HEK-293), breast cancer cells (MCF-7) and Enrichlish Ascitices Cells (EAC). Approximately 65% bio-toxicity was observed in MCF-7 for the core-shell QDs. These QDs may also find effective applications in other optoelectronic devices. CdS/CdSe core-shell hetrostructure quantum dots (QDs) were generated by chemical reaction between the respective chalcogens and cadmium metal salt. Sulphur powder was utilized for CdS core preparation while selenium was extracted from an organoselenium compound to impart CdSe shell layer at a temperature between 150℃ - 200℃. So-prepared core-shell QDs showed good optical properties. The particle size was found to be in the range of 3 - 4 nm with spherical morphology and cubic crystal structure.