Ultrathin two-dimensional inorganic materials: new opportunities for solid state nanochemistry.

Review: 44 refs.

Ultrathin two-dimensional materials: New opportunities and challenges in ultra-sensitive gas sensing

Ultrathin two-dimensional (2D) materials exhibit broad application prospects in many fields due to the enhanced specific surface area to volume ratio and quantum confinement effect. Because of the atomic thickness and various orientations, ultrathin 2D materials exposing specific facets have drawn great attention for various applications in catalysis, batteries, optoelectronics, magnetism, epitaxial template for material growth, etc. Though maintaining the atomic thickness of 2D materials while controlling crystal facets is an enormous challenge, breakthroughs are being made. This review provides a comprehensive overview of the recent advances in the facet engineering of 2D materials, ranging from a basic understanding of facets and the corresponding approaches and the significance of facet engineering. We also propose current challenges and forecast future development directions including the establishment of a facet database, the fabrication of new 2D materials, the design of specific substrates, and the introduction of theoretical calculations and in situ characterization techniques. This review can guide researchers to design ultrathin 2D materials with unique and distinct facets and provide an insight into the applications of energy, magnetism, optics, biomedicine, and other fields.

Defects in 2D materials are becoming prominent candidates for quantum emitters and scalable optoelectronic applications. However, several physical properties that characterize their behavior, such as charged defect ionization energies, are difficult to simulate with conventional first-principles methods, mainly because of the weak and anisotropic dielectric screening caused by the reduced dimensionality. We establish fundamental principles for accurate and efficient calculations of charged defect ionization energies and electronic structure in ultrathin 2D materials. We propose to use the vacuum level as the reference for defect charge transition levels (CTLs) because it gives robust results insensitive to the level of theory, unlike commonly used band edge positions. Furthermore, we determine the fraction of Fock exchange in hybrid functionals for accurate band gaps and band edge positions of 2D materials by enforcing the generalized Koopmans' condition of localized defect states. We found the obtained fractions of Fock exchange vary significantly from 0.2 for bulk $h$-BN to 0.4 for monolayer $h$-BN, whose band gaps are also in good agreement with experimental results and calculated GW results. The combination of these methods allows for reliable and efficient prediction of defect ionization energies (difference between CTLs and band edge positions). We motivate and generalize these findings with several examples including different defects in monolayer to few-layer hexagonal boron nitride ($h$-BN), monolayer MoS$_2$ and graphane. Finally, we show that increasing the number of layers of $h$-BN systematically lowers defect ionization energies, mainly through CTLs shifting towards vacuum, with conduction band minima kept almost unchanged.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Report from the third workshop on future directions of solid-state chemistry: The status of solid-state chemistry and its impact in the physical sciences

Ultrathin two-dimensional materials for photo- and electrocatalytic hydrogen evolution

Advances in ultrathin borophene materials

Ultra-wide bandgap (UWBG) semiconductors and ultra-thin two-dimensional materials (2D) are at the very frontier of the electronics for energy management or <i>energy electronics</i>. A new generation of UWBG semiconductors will open new territories for higher power rated power electronics and deeper ultraviolet optoelectronics. Gallium oxide - Ga₂O₃ (4.5-4.9 eV), has recently emerged as a suitable platform for extending the limits which are set by conventional (~3 eV) WBG e.g. SiC and GaN and transparent conductive oxides (TCO) e.g. In₂O₃, ZnO, SnO₂. Besides, Ga₂O₃, the first efficient oxide semiconductor for energy electronics, is opening the door to many more semiconductor oxides (indeed, the largest family of UWBGs) to be investigated. Among these new power electronic materials, ZnGa₂O₄ (~5 eV) enables bipolar energy electronics, based on a spinel chemistry, for the first time. In the lower power rating end, power consumption also is also a main issue for modern computers and supercomputers. With the predicted end of the Moore’s law, the memory wall and the heat wall, new electronics materials and new computing paradigms are required to balance the big data (information) and energy requirements, just as the human brain does. Atomically thin 2D-materials, and the rich associated material systems (e.g. graphene (metal), MoS₂ (semiconductor) and h-BN (insulator)), have also attracted a lot of attention recently for <i>beyond-silicon</i> neuromorphic computing with record ultra-low power consumption. Thus, energy nanoelectronics based on UWBG and 2D materials are simultaneously extending the current frontiers of electronics and addressing the issue of electricity consumption, a central theme in the actions against climate change.

Graphene and graphene-like two-dimensional (2D) nanomaterials, such as black phosphorus (BP), transition metal carbides/carbonitrides (MXene) and transition metal dichalcogenides (TMD), have been extensively studied in recent years due to their unique physical and chemical properties. With atomic-scale thickness, these 2D materials and their derivatives can react with ROS and even scavenge ROS in the dark. With excellent biocompatibility and biosafety, they show great application potential in the antioxidant field and ROS detection for diagnosis. They can also generate ROS under light and be applied in antibacterial, photodynamic therapy (PDT), and other biomedical fields. Understanding the degradation mechanism of 2D nanomaterials by ROS generated under ambient conditions is crucial to developing air stable devices and expanding their application ranges. In this review, we summarize recent advances in 2D materials with a focus on the relationship between their intrinsic structure and the ROS scavenging or generating ability. We have also highlighted important guidelines for the design and synthesis of highly efficient ROS scavenging or generating 2D materials along with their biomedical applications.

The miniaturization of electronic devices is increasingly requiring some low-dimensional magnetic materials with excellent properties, so ultra-thin two-dimensional magnetic materials have attracted extensive attention. However, most two-dimensional materials exfoliated from bulk either lack intrinsic magnetism or have low magnetic transition temperatures, which greatly limits their practical applications. Here, using magnetic superatom TM@Sn12 (TM = Sc, Ti, V, Cr, Mn, Fe) clusters as building blocks, a series of two-dimensional materials are designed and the underlying mechanism for magnetic order and stability are explained by direct exchange of outer superatom orbitals (1G, 2P and 2D). The honeycomb lattice of TM@Sn12 (TM = V, Cr, Fe) and the square lattice of Ti@Sn12 are ferromagnetic. The Cr@Sn12 honeycomb lattice has a large out-of-plane magnetic anisotropic energy of 2.21 meV and its Curie temperature reaches 162 K, while the Fe@Sn12 honeycomb lattice has a large in-plane magnetic anisotropic energy of 3.58 meV. This research provides a new avenue for developing novel magnetic materials with excellent properties.

Improving electrochemical activity of graphene is crucial for its various applications, which requires delicate control over its geometric and electronic structures. We demonstrate that precise control of the density of vacancy defects, introduced by Ar(+) irradiation, can improve and finely tune the heterogeneous electron transfer (HET) rate of graphene. For reliable comparisons, we made patterns with different defect densities on a same single layer graphene sheet, which allows us to correlate defect density (via Raman spectroscopy) with HET rate (via scanning electrochemical microscopy) of graphene quantitatively, under exactly the same experimental conditions. By balancing the defect induced increase of density of states (DOS) and decrease of conductivity, the optimal HET rate is attained at a moderate defect density, which is in a critical state; that is, the whole graphene sheet becomes electronically activated and, meanwhile, maintains structural integrity. The improved electrochemical activity can be understood by a high DOS near the Fermi level of defective graphene, as revealed by ab initio simulation, which enlarges the overlap between the electronic states of graphene and the redox couple. The results are valuable to promote the performance of graphene-based electrochemical devices. Furthermore, our findings may serve as a guide to tailor the structure and properties of graphene and other ultrathin two-dimensional materials through defect density engineering.

Organic framework materials, including MOFs and HOFs, are widely used in multiple fields. MOFs are formed by the self-assembly of inorganic metal centers and organic ligands, and there are various types such as IRMOFs and ZIFs. Post-synthetic modification (PSM) can expand their functional groups. HOFs connect building units through hydrogen bonds. They have advantages like mild preparation conditions and good solution-processing performance, but the characteristics of hydrogen bonds also limit their development. Two-dimensional MOFs combine the advantages of MOFs and ultrathin two-dimensional materials. There are two preparation strategies: "top-down" and "bottom - up". The "top-down" method, including physical and chemical exfoliation methods, can exfoliate bulk MOFs into nanosheets, but there are problems such as uneven product thickness and low yield. The "bottom-up" methods, such as the solvothermal method, interface synthesis method, and auxiliary synthesis method, can prepare nanosheets with uniform thickness, but each has its pros and cons. Overall, organic framework materials have broad prospects, but they still face challenges in synthesis, performance optimization, etc., and further research and improvement are needed.

ConspectusPhotocatalysis technology has gained extensive attention in the past few decades due to its potential to alleviate or solve energy and environmental contamination problems. The development and design of new photocatalytic semiconductor materials with high catalytic activity has become a research hotspot in this field. In recent years, inorganic ultrathin two-dimensional (2D) semiconductor photocatalysts have shown excellent performance in photocatalytic applications due to their high specific surface area, clear atomic structure, unique electronic structure, intrinsic quantum confined electrons, and high atomic exposure ratio. When the thickness of the bulk semiconductor is decreased to the atomic level, its local atomic structure changes prominently. This is a major reason why the atomic thin 2D materials could show improved inherent properties and produce new properties that are not available in the corresponding bulk semiconductors. Furthermore, compared with the bulk photocatalysts, the surface electronic structure of inorganic ultrathin 2D materials is more sensitive and thus could be regulated more easily by surface and interfacial modification methods, leading to great optimization of photocatalytic properties. Therefore, inorganic ultrathin 2D materials not only provide an ideal reaction model for clearly revealing the relationships between surface/interface structure characteristics and photocatalytic performance but also bring new opportunities for the development of efficient catalysts to resolve energy crises and environmental problems.In this Account, we summarize our previous work on the applications of inorganic ultrathin 2D nanomaterials in the field of photocatalysis. First, we briefly introduce the classification and controlled fabrication of inorganic ultrathin 2D nanomaterials, including metal chalcogenides, bismuth-based photocatalysts, metal oxides, and metal-free materials. Then, we focus on the surface and interfacial modification strategies for effectively engineering the photocatalytic activity of 2D photocatalysts, including defect engineering, metal doping, single atom loading, and heterojunction construction. These strategies can effectively adjust the inherent physical and chemical properties of inorganic ultrathin 2D nanomaterials, change their electronic structure to improve the light absorption ability, promote the separation and migration of photogenerated electrons and holes, optimize the catalytic active sites, and even change the reaction energy barrier of the photocatalytic reaction. Additionally, we discuss the research progress of inorganic ultrathin 2D photocatalysts in hydrogen evolution, contaminant removal, and sterilization. Finally, we put forward the prospects, challenges, and novel viewpoints of feasible solutions of inorganic ultrathin 2D materials in photocatalysis applications. Overall, this Account provides in-depth insight into modulation strategies and the structure–activity relationship of inorganic ultrathin 2D materials for researchers in photocatalytic fields, which is beneficial to accelerate the practical applications of inorganic ultrathin 2D materials in the field of environment and energy.

A promising strategy for further miniaturizing metal-oxide-semiconductor field-effect transistors is the use of ultrathin two-dimensional channel materials. However, achieving robust dielectric integration with a sub-1-nm capacitance equivalent thickness (CET) remains challenging. Here we present a wafer-scale monolayer MoO3, transformed from MoS2, which can be seamlessly integrated with atomically thin semiconductors. Its atomically flat surface and the strong electronegativity of Mo6+ further enable the uniform deposition of high-κ dielectrics. Utilizing the 0.96-nm-CET MoO3/HfO2 as the dielectric, the top-gated p-type (n-type) two-dimensional transistors show a high ON/OFF ratio of 6.5 × 106 (3.2 × 108) and a steep subthreshold swing of 60.8 (63.1) mV dec-1. Statistical analysis of a 1,024-device array achieves a high yield of 92.2%. Furthermore, when monolayer MoO3 is used as the top-gated dielectric with an ultimately scaled CET of 0.64 nm, the gate leakage current meets the low-power limit standard (1.5 × 10-2 A cm-2) over the entire bias range. Our study provides a scalable approach for the integration of ultralow-CET dielectrics on two-dimensional materials, marking a critical step towards their future industrial deployment.