Atom-Thin SnS2-xSex with Adjustable Compositions by Direct Liquid Exfoliation from Single Crystals

Atomically Substitutional Engineering of Transition Metal Dichalcogenide Layers for Enhancing Tailored Properties and Superior Applications

Transition metal dichalcogenides (TMDs) are a promising class of layered materials in the post-graphene era, with extensive research attention due to their diverse alternative elements and fascinating semiconductor behavior. Binary MX2 layers with different metal and/or chalcogen elements have similar structural parameters but varied optoelectronic properties, providing opportunities for atomically substitutional engineering via partial alteration of metal or/and chalcogenide atoms to produce ternary or quaternary TMDs. The resulting multinary TMD layers still maintain structural integrity and homogeneity while achieving tunable (opto)electronic properties across a full range of composition with arbitrary ratios of introduced metal or chalcogen to original counterparts (0-100%). Atomic substitution in TMD layers offers new adjustable degrees of freedom for tailoring crystal phase, band alignment/structure, carrier density, and surface reactive activity, enabling novel and promising applications. This review comprehensively elaborates on atomically substitutional engineering in TMD layers, including theoretical foundations, synthetic strategies, tailored properties, and superior applications. The emerging type of ternary TMDs, Janus TMDs, is presented specifically to highlight their typical compounds, fabrication methods, and potential applications. Finally, opportunities and challenges for further development of multinary TMDs are envisioned to expedite the evolution of this pivotal field.

Recent advances in solution-processed photodetectors based on inorganic and hybrid photo-active materials

Due to their excellent and tailorable optoelectronic performance, low cost, facile fabrication, and compatibility with flexible substrates, solution-processed inorganic and hybrid photo-active materials have attracted extensive interest for next-generation photodetector applications. This review gives a comprehensive compilation of solution-processed photodetectors. The basic structures of the device and important parameters of photodetectors will be firstly summarized. Then the development of various solution processing technologies containing solution synthesis and liquid phase film-forming processes for the preparation of semiconductor films is described. From the materials science point of view, we give a comprehensive overview about the current status of solution processed semiconductor materials including inorganic and hybrid photo-active materials for the application of photodetectors. Moreover, challenges and future trends in the field of solution-processed photodetectors are proposed.

Electronegativity, phase transition, and ferroelectricity of TeSe2 few-layers

We use first-principles calculations to demonstrate that γ TeSe₂ few-layers (FLs) are significantly more stable than α and β FLs due to the difference in the electronegativity of two kinds of atoms, while γ Te FLs are not due to the unfavorable multivalency of Te atoms. The quasiparticle single-shot G₀W₀ band gaps are 1.13 and 2.30 eV for γ and β monolayers (MLs), respectively. Therefore, they will be useful for optoelectronics operating at room temperature, which is further supported by their dynamic and thermal stability. The γ ML and bilayer (BL) are expected to undergo phase transitions to β ML and α BL under hole doping. Furthermore, the ionicity brings about spontaneous electric polarization in the α BL that is approximately 60% larger than that in the α Te BL. Its ferroelectricity (FE) is comparable to that of SnTe ML, the only 2D FE material experimentally identified up to now. The polarization can be further enhanced by more than 75% under uniaxial tensile strain.

Photo-enhanced gas sensing of SnS2 with nanoscale defects

Recently a SnS₂ based NO₂ gas sensor with a 30 ppb detection limit was demonstrated but this required high operation temperatures. Concurrently, SnS₂ grown by chemical vapor deposition is known to naturally contain nanoscale defects, which could be exploited. Here, we significantly enhance the performance of a NO₂ gas sensor based on SnS₂ with nanoscale defects by photon illumination, and a detection limit of 2.5 ppb is achieved at room temperature. Using a classical Langmuir model and density functional theory simulations, we show S vacancies work as additional adsorption sites with fast adsorption times, higher adsorption energies, and an order of magnitude higher resistance change compared with pristine SnS₂. More interestingly, when electron–hole pairs are excited by photon illumination, the average adsorption time first increases and then decreases with NO₂ concentration, while the average desorption time always decreases with NO₂ concentration. Our results give a deep understanding of photo-enhanced gas sensing of SnS₂ with nanoscale defects, and thus open an interesting window for the design of high performance gas sensing devices based on 2D materials.

A photon assisted SnS₂-based gas sensor with an ultra-high sensitivity of 3 ppb NO₂ has been achieved at room temperature.

Novel structured transition metal dichalcogenide nanosheets

Ultrathin two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their unique properties and great potential in a wide range of applications. Great efforts have been devoted to the preparation of novel-structured TMD nanosheets by engineering their intrinsic structures at the atomic scale. Until now, a lot of new-structured TMD nanosheets, such as vacancy-containing TMDs, heteroatom-doped TMDs, TMD alloys, 1T'/1T phase and in-plane TMD crystal-phase heterostructures, TMD heterostructures and Janus TMD nanosheets, have been prepared. These materials exhibit unique properties and hold great promise in various applications, including electronics/optoelectronics, thermoelectrics, catalysis, energy storage and conversion and biomedicine. This review focuses on the most recent important discoveries in the preparation, characterization and application of these new-structured ultrathin 2D layered TMDs.

Touching the theoretical capacity: synthesizing cubic LiTi2(PO4)3/C nanocomposites for high-performance lithium-ion battery

A cubic LiTi2(PO4)3/C composite is successfully prepared via a simple solvothermal method and further glucose-pyrolysis treatment. The as-fabricated LTP/C material delivers an ultra-high reversible capacity of 144 mA h g-1 at 0.2C rate, which is the highest ever reported, and shows considerable performance improvement compared with before. Combining this with the stable cycling performance and high rate capability, such material has a promising future in practical application.

Scalable exfoliation and dispersion of two-dimensional materials - an update

The preparation of dispersions of single- and few-sheet 2D materials in various solvents, as well as the characterization methods applied to such dispersions, is critically reviewed. Motivating factors for producing single- and few-sheet dispersions of 2D materials in liquids are briefly discussed. Many practical applications are expected for such materials that do not require high purity formulations and tight control of donor and acceptor concentrations, as required in conventional Fab processing of semiconductor chips. Approaches and challenges encountered in exfoliating 2D materials in liquids are reviewed. Ultrasonication, mechanical shearing, and electrochemical processing approaches are discussed, and their respective limitations and promising features are critiqued. Supercritical and more conventional liquid and solvent processing are then discussed in detail. The effects of various types of stabilizers, including surfactants and other amphiphiles, as well as polymers, including homopolymeric electrolytes, nonionic polymers, and nanolatexes, are discussed. Consideration of apparent successes of stabilizer-free dispersions indicates that extensive exfoliation in the absence of dispersing aids results from processing-induced surface modifications that promote stabilization of 2D material/solvent interactions. Also apparent paradoxes in "pristineness" and optical extinctions in dispersions suggest that there is much we do not yet quantitatively understand about the surface chemistry of these materials. Another paradox, emanating from modeling dilute solvent-only exfoliation by sonication using polar components of solubility parameters and surface tension for pristine graphene with no polar structural component, is addressed. This apparent paradox appears to be resolved by realizing that the reactivity of graphene to addition reactions of solvent radicals produced by sonolysis is accompanied by unintended polar surface modifications that promote attractive interactions with solvent. This hypothesis serves to define important theoretical and experimental studies that are needed. We conclude that the greatest promise for high volume and high concentration processing lies in applying methods that have not yet been extensively reported, particularly wet comminution processing using small grinding media of various types.

An open holey structure enhanced rate capability in a NaTi2(PO4)3/C nanocomposite and provided ultralong-life sodium-ion storage

Sodium-ion battery (SIB) technology is competitive in the fields of transportation and grid storage, which require electrode materials showing rapid energy conversion (high rate capability) and long cycle life. In this work, a NaTi₂(PO₄)₃/C (NTP/C) nanocomposite with an open holey-structured framework was successfully prepared for the first time using a solvothermal reaction followed by pyrolysis. The nanocomposite realized fast sodium-ion transport and thus preferable battery performances. Within the wide rate range of 0.5-50C, only a very small decrease in capacity from 124 to 120 mA h g^-1 was observed. A high discharge capacity of 103 mA h g^-1 (88.3% retention of the 1st cycle) was delivered even after 10 000 cycles at an ultrahigh rate of 50C without any obvious morphological change and without structural pulverization. Forming open channels for ion transport proved to contribute to such performance enhancement and therefore has the potential to become a universal model for the development of sustainable electrode materials in SIBs and other battery systems.

Auto-generated iron chalcogenide microcapsules ensure high-rate and high-capacity sodium-ion storage

Sodium-ion batteries (SIBs) are regarded as promising alternative energy-storage devices to lithium-ion batteries (LIBs). However, the trade-off of between energy density and power density under high mass-loading conditions restricts the application of SIBs. Herein, we synthesized an FeSe@FeS material via a facile solid-state reaction. A microcapsule architecture was spontaneously achieved in this process, which facilitated electron transport and provided stable diffusion paths for Na ions. The FeSe@FeS material exhibits a high capacity retention (485 mA h g^-1 at 3 A g^-1 after 1400 cycles) and superior rate capability (230 mA h g^-1 at 10 A g^-1 after 1600 cycles) in the half-cell test. Furthermore, superior cycling stability is achieved in the full-cell test. The high mass-loaded FeSe@FeS electrodes (8 mg cm^-2) realize a high areal capacity retention of 2.8 mA h cm^-2 and high thermal stability.

Unprecedented sensitivity towards pressure enabled by graphene foam

Reduced graphene oxide foam (RGOF)-based pressure sensors have been fabricated through the combination of ultrasonic dispersion and freeze-drying methods. Due to the maintenance of the highly disordered structure of the ultrasonic dispersed graphene oxides before the freezing process, the RGOF sensors demonstrated an ultra-high sensitivity of 22.8 kPa^-1, an ultra-low detection limit of around 0.1 Pa, and a superior separation of 0.2-Pascal-scale difference.

Unique Reversible Conversion-Type Mechanism Enhanced Cathode Performance in Amorphous Molybdenum Polysulfide

A unique reversible conversion-type mechanism is reported in the amorphous molybdenum polysulfide (a-MoS_5.7) cathode material. The lithiation products of metallic Mo and Li₂S₂ rather than Mo and Li₂S species have been detected. This process could yield a high discharge capacity of 746 mAh g^-1. Characterizations of the recovered molybdenum polysulfide after the delithiaiton process manifests the high reversibility of the unique conversion reaction, in contrast with the general irreversibility of the conventional conversion-type mechanism. As a result, the a-MoS_5.7 electrodes deliver high cycling stability with an energy-density retention of 1166 Wh kg^-1 after 100 cycles. These results provide a novel model for the design of high-capacity and long-life electrode materials.

Novel Metal Chalcogenide SnSSe as a High-Capacity Anode for Sodium-Ion Batteries

A novel layered SnSSe material is designed as a high-performance anode for sodium-ion batteries with characteristics of high capacity, superior cyclability, facile synthetic method, and large-scale production ability. The transformation from bulk SnSSe particles into closely packed nanoplate aggregates with greater resistance to structure pulverization and the partial pseudocapacitive capacity contribution may engender excellent cycling performance and rate capability.