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.

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.

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.

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.

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.

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 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.

Controlled synthesis of ferromagnetic MnSe_x particles

The MnSe_x(x = 1,2) nanoparticles were synthesized under hydrothermal condition,by reaction of the reduced selenium and Mn^(2+) ion in the presence of hydrazine and acetic acid.By precisely controlling the pH value of the solution,a series of MnSe_x particles were synthesized.The structure and morphology of as-prepared particles were examined with x-ray diffractometer(XRD),transmission electron microscopy(TEM),and scanning electron microscopy(SEM).The average sizes of as-prepared particles varied from nanoscale to microscale with pH value increase.Furthermore,the nucleation and growth mechanism associated with pH values were discussed,which can be applied to the hydrothermal synthesis of metal chalcogenide in general.Finally,the optical and magnetic properties of as-prepared particles were measured.All as-made particles exhibit a ferromagnetic behavior with low coercivity and remanence at room temperature.

A review of transition metal chalcogenide/graphene nanocomposites for energy storage and conversion

To meet the ever-increasing energy demands, advanced electrode materials are strongly requested for the exploration of advanced energy storage and conversion technologies, such as Li-ion batteries, Li-S batteries, Li-/Zn-air batteries, supercapacitors, dye-sensitized solar cells, and other electrocatalysis process(e.g., oxygen reduction/evolution reaction, hydrogen evolution reaction). Transition metal chalcogenides(TMCs, i.e., sulfides and selenides) are forcefully considered as an emerging candidate,owing to their unique physical and chemical properties. Moreover, the integration of TMCs with conductive graphene host has enabled the significant improvement of electrochemical performance of devices. In this review, the recent research progress on TMC/graphene composites for applications in energy storage and conversion devices is summarized. The preparation process of TMC/graphene nanocomposites is also included. In order to promote an in-depth understanding of performance improvement for TMC/graphene materials, the operating principle of various devices and technologies are briefly presented. Finally, the perspectives are given on the design and construction of advanced electrode materials.

Two-dimensional transition metal chalcogenide nanomaterials for cancer diagnosis and treatment

For more than a decade,the exfoliation of graphene and other layered materials has led to a tremendous amount of research in two-dimensional(2D)materials,among which 2D transition metal chalcogenides(TMCs)nanomaterials have attracted much attention in a wide range of applications including photoelectric devices,lithium-ion batteries,catalysis,and energy conversion and storage owing to their unique photoelectric physical properties.With such large specific surface area,strong near-infrared(NIR)absorption and abundant chemical element composition,2D TMCs nanomaterials have become good candidates in biomedical imaging and cancer treatment.This review systematically summarizes recent progress on 2D TMCs nanomaterials,which includes their synthesis methods and applications in cancer treatment.At the end of this review,we also highlight the future prospects and challenges of 2D TMCs nanomaterials.It is expected that this work can provide the readers with a detailed overview of the synthesis of 2D TMCs and inspire more novel functional biomaterials based on 2D TMCs for cancer treatment in the future.

Construction of high-efficiency CoS@Nb_(2)O_(5) heterojunctions accelerating charge transfer for boosting photocatalytic hydrogen evolution

The random movement and easy recombination of photoinduced charges lead to a low conversion efficiency for photocatalytic hydrogen evolution.The cocatalyst design is a promising route to address such problem through introducing an appropriate cocatalyst on the semiconductor photocatalysts to construct the high-efficiency heterojunctions.Herein,novel CoS/Nb_(2)O_(5) heterojunctions were constructed via in-situ loading CoS cocatalyst on the surface of Nb_(2)O_(5) nanosheets.Through the femtosecond-resolved transient absorption spectroscopy,the average lifetime of charge carriers for 10 wt% CoS/Nb_(2)O_(5)(159.6 ps)is drastically shortened by contrast with that of Nb_(2)O_(5)(5531.9 ps),strongly suggesting the rapid charge transfer from Nb_(2)O_(5) to CoS.The significantly improved charge-transfer capacity contributes to a high photocatalytic hydrogen evolution rate of 355µmol/h,up to 17.5 times compared with pristine Nb_(2)O_(5).This work would provide a new design platform in the construction of photocatalytic heterojunctions with high charge-transfer efficiency.

Co-Construction of Sulfur Vacancies and Heterogeneous Interface into Ni_(3)S_(2)/MoS_(2) Catalysts to Achieve Highly Efficient Overall Water Splitting

Integrating the advantages of anion vacancies and heterostructures into the catalytic materials may increase the binding affinities to intermediates, provide more active sites, and significantly promote the activity of overall water splitting. However, the successful assembly of anion vacancies and heterostructures for high-efficiency water splitting performance is still challenging. In this work, we ingeniously present the co-construction of sulfur vacancies and heterogeneous interface into Ni_(3)S_(2)/MoS_(2) catalysts on nickel foam(NF). The introduction of sulfur vacancies and Ni_(3)S_(2)/MoS_(2) heterostructures can significantly improve electron and ion transport, effectively improve structural stability, and enhance overall water splitting activity. The obtained VSNi_(3)S_(2)/MoS_(2) catalysts(VS stands for sulfur vacancies) exhibit superior OER and HER activities,and the overpotentials for OER and HER are 180 and 71 mV at 10 mA·cm^(-2), respectively. Furthermore, a low water splitting voltage of 1.46 V is required at 10 mA·cm^(-2) for the VS-Ni_(3)S_(2)/MoS_(2) catalysts, which is considerably lower than most that of water splitting electrocatalysts currently reported. This work offers an effective mean for the preparation of catalysts with both anion vacancies and heterostructures for achieving high-performance alkaline overall water splitting.

Recent development of nanomaterials for carbon dioxide electroreduction

Electrochemical CO_(2) reduction reaction(CO_(2)RR)to produce value‐added products has received tremendous research attention in recent years.With research efforts across the globe,remarkable advancement has been achieved,including the improvement of selectivity for the reduction products,the realization of efficient reduction beyond two electrons,and the delivery of industrially relevant current densities.In this review,we introduce the recent development of nanomaterials for CO_(2)RR,including the zero‐dimensional graphene quantum dots,two‐dimensional materials such as metal chalcogenides and metal/covalent organic framework,singleatom catalysts,and nanostructured metal catalysts.The engineering of materials into three‐dimensional structure will also be discussed.Finally,we will provide a summary of the catalytic performance and perspectives on future development.