Two-dimensional inorganic molecular crystals

Ultrafast and Universal Synthetic Route for Nanostructured Transition Metal Oxides Directly Grown on Substrates

Nanostructured transition metal oxides (NTMOs) have consistently piqued scientific interest for several decades due to their remarkable versatility across various fields. More recently, they have gained significant attention as materials employed for energy storage/harvesting devices as well as electronic devices. However, mass production of high-quality NTMOs in a well-controlled manner still remains challenging. Here, a universal, ultrafast, and solvent-free method is presented for producing highly crystalline NTMOs directly onto target substrates. The findings reveal that the growth mechanism involves the solidification of condensed liquid-phase TMO microdroplets onto the substrate under an oxygen-rich ambient condition. This enables a continuous process under ambient air conditions, allowing for processing within just a few tens of seconds per sample. Finally, it is confirmed that the method can be extended to the synthesis of various NTMOs and their related compounds.

Dielectric Integrations and Advanced Interface Engineering for 2D Field-Effect Transistors

As silicon-based transistors approach their physical limits, the challenge of further increasing chip integration intensifies. 2D semiconductors, with their atomically thin thickness, ultraflat surfaces, and van der Waals (vdW) integration capability, are seen as a key candidate for sub-1 nm nodes in the post-Moore era. However, the low dielectric integration quality, including discontinuity and substantial leakage currents due to the lack of nucleation sites during deposition, interfacial states causing serious charge scattering, uncontrolled threshold shifts, and bad uniformity from dielectric doping and damage, have become critical barriers to their real applications. This review focuses on this challenge and the possible solutions. The functions of dielectric materials in transistors and their criteria for 2D devices are first elucidated. The methods for high-quality dielectric integration with 2D channels, such as surface pretreatment, using 2D materials with native oxides, buffer layer insertion, vdW dielectric transfer, and new dielectric materials, are then reviewed. Additionally, the dielectric integration for advanced 3D integration of 2D materials is also discussed. Finally, this paper is concluded with a comparative summary and outlook, highlighting the importance of interfacial state control, dielectric integration for 2D p-type channels, and compatibility with silicon processes.

High-pressure chemistry of functional materials

Functional materials, possessing specific properties and performing particular functions beyond their mechanical or structural roles, are the foundation of modern matter science including energy, environment, and quantum sciences. The atomic and electronic structures of these materials can be significantly altered by external stimuli such as pressure. High-pressure techniques have been extensively utilized to deepen our understanding of structure-property relationships of materials, while also enabling emergent or enhanced properties. In this feature article, we review the transformative impact of high pressure on the chemical and physical properties of functional materials, including perovskite materials, low-dimensional metal halides, metal chalcogenides, metal oxides, and inorganic molecular crystals. By analyzing recent advancements and methodological approaches in high-pressure research, we provide insights into the mechanisms driving structural and property changes in these materials. We also emphasize the significance of translating the knowledge gained from high pressure research to the design of new functional materials. Finally, we highlight the potential of high-pressure chemistry and nano-architectonics in advancing functional materials and discuss the future directions and challenges in this field.

A Single-Crystal Antimony Trioxide Dielectric for 2D Field-Effect Transistors

The remarkable potential of two-dimensional (2D) materials in sustaining Moore's law has sparked a research frenzy. Extensive efforts have been made in the research of utilizing 2D semiconductors as channel materials in field-effect transistors. However, the next generation of integrated devices requires the integration of gate dielectrics with wider bandgaps and higher dielectric constants. Here, insulating α-Sb2O3 single-crystal nanosheets are synthesized by one-step chemical vapor deposition method. Importantly, the α-Sb2O3 single-crystal dielectric exhibits a high dielectric constant of 11.8 and a wide bandgap of 3.78 eV. Besides, the atomically smooth interface between α-Sb2O3 and MoS2 enables the fabrication of dual-gated field-effect transistors with the top gate dielectric of α-Sb2O3 nanosheets. The field-effect transistors exhibit a switching ratio of exceeding 108, which achieves the manipulation of field-effect transistors by using 2D dielectric materials. These results hold significant implications for optimizing the performances of 2D devices and innovating microelectronics.

Voltage-Gated Switching of Moiré Patterns in Epitaxial Molecular Crystals

Studying molecular materials at the nanoscale allows us to gain a deeper understanding of supramolecular structure formation and serves as the basis for rationally controlling the resulting interfacial properties. Here, we describe the formation of extended Moiré patterns resulting from the assembly of dipolar π-conjugated molecules on highly oriented pyrolytic graphite at the liquid-solid interface as characterized by scanning tunneling microscopy (STM). By switching the bias of the sample and thus the orientation of the external electric field in the vicinity of the STM junction, structural reorganization of the molecular building blocks and the resulting organic 2D crystal is induced and can conveniently be monitored in situ by the appearance and disappearance of the Moiré patterns. Importantly, the formation and loss of the Moiré patterns are fully reversible, providing exquisite control over epitaxial molecular crystals. Our approach provides fundamental insights into the supramolecular organization and resulting superstructure formation of incommensurable 2D lattices upon applying an electric field and enables the rational tuning of Moiré patterns as a key step toward the potential integration of organic 2D crystals in molecular nanodevices.

Van der Waals epitaxial growth of single-crystal molecular film

Epitaxy is the cornerstone of semiconductor technology, enabling the fabrication of single-crystal film. Recent advancements in van der Waals (vdW) epitaxy have opened new avenues for producing wafer-scale single-crystal 2D atomic crystals. However, when it comes to molecular crystals, the overall weak vdW force means that it is a significant challenge for small molecules to form a well-ordered structure during epitaxy. Here we demonstrate that the vdW epitaxy of Sb2O3 molecular crystal, where the whole growth process is governed by vdW interactions, can be precisely controlled. The nucleation is deterministically modulated by epilayer-substrate interactions and unidirectional nuclei are realized through designing the lattice and symmetry matching between epilayer and substrate. Moreover, the growth and coalescence of nuclei as well as the layer-by-layer growth mode are kinetically realized via tackling the Schwoebel-Ehrlich barrier. Such precise control of vdW epitaxy enables the growth of single-crystal Sb2O3 molecular film with desirable thickness. Using the ultrathin highly oriented Sb2O3 film as a gate dielectric, we fabricated MoS2-based field-effect transistors that exhibit superior device performance. The results substantiate the viability of precisely managing molecule alignment in vdW epitaxy, paving the way for large-scale synthesis of single-crystal 2D molecular crystals.

Discovery and Manipulation of van der Waals Polarons in Sb2O3 Ultrathin Molecular Crystal

Manipulating single electrons at the atomic scale is vital for mastering complex surface processes governed by the transfer of individual electrons. Polarons, composed of electrons stabilized by electron-phonon coupling, offer a pivotal medium for such manipulation. Here, using scanning tunneling microscopy and spectroscopy (STM/STS) and density functional theory (DFT) calculations, we report the identification and manipulation of a new type of polaron, dubbed van der Waals (vdW) polaron, within mono- to trilayer ultrathin films composed of Sb2O3 molecules that are bonded via vdW attractions. The Sb2O3 films were grown on a graphene-covered SiC(0001) substrate via molecular beam epitaxy. Unlike prior molecular polarons, STM imaging observed polarons at the interstitial sites of the molecular film, presenting unique electronic states and localized band bending. DFT calculations revealed the lowest conduction band as an intermolecular bonding state, capable of ensnaring an extra electron through locally diminished intermolecular distances, thereby forming an intermolecular vdW polaron. We also demonstrated the ability to generate, move, and erase such vdW polarons using an STM tip. Our work uncovers a new type of polaron stabilized by coupling with intermolecular vibrations where vdW interactions dominate, paving the way for designing atomic-scale electron transfer processes and enabling precise tailoring of electron-related properties and functionalities.

Spheroid models to elaborate the broken symmetry and equivalent volume of molecules in crystalline phase

Dense packing of particles has provided powerful models to elaborate the important structural features of matter in various systems such as liquid, glassy, and crystalline phases. The simplest sphere packing models can represent and capture salient properties of the building blocks for covalent, metallic, and ionic crystals; it, however, becomes insufficient to reflect the broken symmetry of the commonly anisotropic molecules in molecular crystals. Here, we develop spheroid models with a minimal degree of anisotropy, which serve as a simple geometrical representation for a rich spectrum of molecules-including both isotropic and anisotropic, convex and concave ones-in crystalline phases. Our models are determined via an inverse packing approach: Given a molecular crystal, an optimal spheroid model is constructed using a contact diagram, which depicts the packing relationship between neighboring molecules within the crystal. The spheroid models are capable of accurately capturing the broken symmetry and characterizing the equivalent volume of molecules in the crystalline phases. Moreover, our model retrieves such molecular information from low-quality x-ray diffraction data with poorly resolved structures, and by using soft spheroids, it can also describe the packing behavior in cocrystals.

Van der Waals Epitaxially Grown Molecular Crystal Dielectric Sb2O3 for 2D Electronics

Two-dimensional (2D) semiconducting materials have attracted significant interest as promising candidates for channel materials owing to their high mobility and gate tunability at atomic-layer thickness. However, the development of 2D electronics is impeded due to the difficulty in formation of high-quality dielectrics with a clean and nondestructive interface. Here, we report the direct van der Waals epitaxial growth of a molecular crystal dielectric, Sb2O3, on 2D materials by physical vapor deposition. The grown Sb2O3 nanosheets showed epitaxial relations of 0 and 180° with the 2D template, maintaining high crystallinity and an ultrasharp vdW interface with the 2D materials. As a result, the Sb2O3 nanosheets exhibited a high breakdown field of 18.6 MV/cm for 2L Sb2O3 with a thickness of 1.3 nm and a very low leakage current of 2.47 × 10-7 A/cm2 for 3L Sb2O3 with a thickness of 1.96 nm. We also observed two types of grain boundaries (GBs) with misorientation angles of 0 and 60°. The 0°-GB with a well-stitched boundary showed higher electrical and thermal stabilities than those of the 60°-GB with a disordered boundary. Our work demonstrates a method to epitaxially grow molecular crystal dielectrics on 2D materials without causing any damage, a requirement for high-performance 2D electronics.

Two-dimensional material assisted-growth strategy: new insights and opportunities

The exploration and synthesis of novel materials are integral to scientific and technological progress. Since the prediction and synthesis of two-dimensional (2D) materials, it is expected to play an important role in the application of industrialization and the information age, resulting from its excellent physical and chemical properties. Currently, researchers have effectively utilized a range of material synthesis techniques, including mechanical exfoliation, redox reactions, chemical vapor deposition, and chemical vapor transport, to fabricate two-dimensional materials. However, despite their rapid development, the widespread industrial application of 2D materials faces challenges due to demanding synthesis requirements and high costs. To address these challenges, assisted growth techniques such as salt-assisted, gas-assisted, organic-assisted, and template-assisted growth have emerged as promising approaches. Herein, this study gives a summary of important developments in recent years in the assisted growth synthesis of 2D materials. Additionally, it highlights the current difficulties and possible benefits of the assisted-growth approach for 2D materials. It also highlights novel avenues of development and presents opportunities for new lines of investigation.

Efficient Carrier Transport in 2D Bi2O2Se/CsBi3I10 Perovskite Heterojunction Enables Highly-Sensitive Broadband Photodetection

2D Bi2O2Se has recently garnered significant attention in the electronics and optoelectronics fields due to its remarkable photosensitivity, broad spectral absorption, and excellent long-term environmental stability. However, the development of integrated Bi2O2Se photodetector with high performance and low-power consumption is limited by material synthesis method and the inherent high carrier concentration of Bi2O2Se. Here, a type-I heterojunction is presented, comprising 2D Bi2O2Se and lead-free bismuth perovskite CsBi3I10, for fast response and broadband detection. Through effective charge transfer and strong coupling effect at the interfaces of Bi2O2Se and CsBi3I10, the response time is accelerated to 4.1 µs, and the detection range is expanded from ultraviolet to near-infrared spectral regions (365-1500 nm). The as-fabricated photodetector exhibits a responsivity of 48.63 AW-1 and a detectivity of 1.22×1012 Jones at 808 nm. Moreover, efficient modulation of the dominant photocurrent generation mechanism from photoconductive to photogating effect leads to sensitive response exceeding 103 AW-1 for heterojunction-based photo field effect transistor (photo-FETs). Utilizing the large-scale growth of both Bi2O2Se and CsBi3I10, the as-fabricated integrated photodetector array demonstrates outstanding homogeneity and stability of photo-response performance. The proposed 2D Bi2O2Se/CsBi3I10 perovskite heterojunction holds promising prospects for the future-generation photodetector arrays and integrated optoelectronic systems.

Room-Temperature Magnetic-Induced Circularly Polarized Photoluminescence in Two-Dimensional Er2O2S

Two-dimensional (2D) materials with spin polarization have great potential for achieving next-generation spintronic applications. However, spin polarization of 2D materials is usually produced at a cryogenic temperature because of thermal fluctuations, which severely hinder their further applications. Here, we report room-temperature intrinsic magnetic-induced circularly polarized photoluminescence (PL) in 2D Er2O2S flakes. The geff factor of 2D Er2O2S stays at around -6.3 from the liquid He temperature limit to room temperature, which is independent of temperature. This anomalous phenomenon in Er2O2S is totally different from previous materials, which all have a decreasing Zeeman splitting with increasing temperature resulting from thermal fluctuations. The anomalous temperature-dependent magnetic-induced circularly polarized PL originates from the weak electron-phonon coupling in 2D Er2O2S, which has been proven by both the temperature-dependent Raman and theoretical calculations. This work sheds light on the understanding and manipulation of 2D materials for practical spintronic applications.

Emerging van der Waals Dielectrics of Inorganic Molecular Crystals for 2D Electronics

In the landscape of continuous downscaling metal-oxide-semiconductor field-effect transistors, two-dimensional (2D) semiconductors with atomic thinness emerge as promising channel materials for ultimate scaled devices. However, integrating compatible dielectrics with 2D semiconductors, particularly in a scalable way, remains a critical challenge that hinders the development of 2D devices. Recently, 2D inorganic molecular crystals (IMCs), which are free of dangling bonds and possess excellent dielectric properties and simplicity for scalable fabrication, have emerged as alternatives for gate dielectric integration in 2D devices. In this Perspective, we start with the introduction of structure and synthesis methods of IMCs and then discuss the explorations of using IMCs as the dielectrics, as well as some remaining relevant issues to be unraveled. Moreover, we look at the future opportunities of IMC dielectrics in 2D devices both for practical applications and fundamental research.

Two-Dimensional Oxide Crystals for Device Applications: Challenges and Opportunities

Atomically thin two-dimensional (2D) oxide crystals have garnered considerable attention because of their remarkable physical properties and potential for versatile applications. In recent years, significant advancements have been made in the design, preparation, and application of ultrathin 2D oxides, providing many opportunities for new-generation advanced technologies. This review focuses on the controllable preparation of 2D oxide crystals and their applications in electronic and optoelectronic devices. Based on their bonding nature, the various types of 2D oxide crystals are first summarized, including both layered and nonlayered crystals, as well as their current top-down and bottom-up synthetic approaches. Subsequently, in terms of the unique physical and electrical properties of 2D oxides, recent advances in device applications are emphasized, including photodetectors, field-effect transistors, dielectric layers, magnetic and ferroelectric devices, memories, and gas sensors. Finally, conclusions and future prospects of 2D oxide crystals are presented. It is hoped that this review will provide comprehensive and insightful guidance for the development of 2D oxide crystals and their device applications.

An Ultrathin Inorganic Molecular Crystal Interfacial Layer for Stable Zn Anode

Zn metal anode suffers from dendrite growth and side reactions during cycling, significantly deteriorating the lifespan of aqueous Zn metal batteries. Herein, we introduced an ultrathin and ultra-flat Sb2 O3 molecular crystal layer to stabilize Zn anode. The in situ optical and atomic force microscopes observations show that such a 10 nm Sb2 O3 thin layer could ensure uniform under-layer Zn deposition with suppressed tip growth effect, while the traditional WO3 layer undergoes an uncontrolled up-layer Zn deposition. The superior regulation capability is attributed to the good electronic-blocking ability and low Zn affinity of the molecular crystal layer, free of dangling bonds. Electrochemical tests exhibit Sb2 O3 layer can significantly improve the cycle life of Zn anode from 72 h to 2800 h, in contrast to the 900 h of much thicker WO3 even in 100 nm. This research opens up the application of inorganic molecular crystals as the interfacial layer of Zn anode.

On the Working Mechanisms of Molecules-Based Van der Waals Dielectrics

Sb2 O3 molecules offer unprecedented opportunities for the integration of a van der Waals (vdW) dielectric and a 2D vdW semiconductor. However, the working mechanisms underlying molecules-based vdW dielectrics remain unclear. Here, the working mechanisms of Sb2 O3 and two Sb2 O3 -like molecules (As2 O3 and Bi2 O3 ) as dielectrics are systematically investigated by combining first-principles calculations and gate leakage current theories. It is revealed that molecules-based vdW dielectrics have a considerable advantage over conventional dielectric materials: defects hardly affect their insulating properties. This shows that it is unnecessary to synthesize high-quality crystals in practical applications, which has been a long-standing challenge for conventional dielectric materials. Further analysis reveals that a large thermionic-emission current renders Sb2 O3 difficult to simultaneously satisfy the requirements of dielectric layers in p-MOS and n-MOS, which hinders its application for complementary metal-oxide-semiconductor (CMOS) devices. Remarkably, it is found that As2 O3 can serve as a dielectric for both p-MOS and n-MOS. This work not only lays a theoretical foundation for the application of molecules-based vdW dielectrics, but also offers an unprecedentedly competitive dielectric (i.e., As2 O3 ) for 2D vdW semiconductors-based CMOS devices, thus having profound implications for future semiconductor industry.

Identification of Material Dimensionality Based on Force Constant Analysis

Identification of low-dimensional structural units from the bulk atomic structure is a widely used approach for discovering new low-dimensional materials with new properties and applications. Such analysis is usually based solely on bond-length heuristics, whereas an analysis based on bond strengths would be physically more justified. Here, we study dimensionality classification based on the interatomic force constants of a structure with different approaches for selecting the bonded atoms. The implemented approaches are applied to the existing database of first-principles calculated force constants with a large variety of materials, and the results are analyzed by comparing them to those of several bond-length-based classification methods. Depending on the approach, they can either reproduce results from bond-length-based methods or provide complementary information. As an example of the latter, we managed to identify new non-van der Waals two-dimensional material candidates.

Tuning 2D Magnetism in Cobalt Monoxide Nanosheets Via In Situ Nickel-Doping

Element doping has become an effective strategy to engineer the magnetic properties of two-dimensional (2D) materials and is widely explored in van der Waals layered transition metal dichalcogenides. However, the high-concentration substitution doping of 2D nonlayered metal oxides, which can preserve the original crystal texture and guarantee the homogeneity of doping distribution, is still a critical challenge due to the isotropic bonding of closed-packed structures. In this work, the synthesis of high-quality 2D nonlayered nickel-doped cobalt monoxide nanosheets via in situ atmospheric pressure chemical vapor deposition method is reported. High-resolution transmission electron microscopy confirmed that nickel atoms are doped at the intrinsic cobalt atom sites. The nickel doping concentration is stable at ≈15%, superior to most magnetic dopants doping in 2D materials and metal oxides. Magnetic measurements showed that pristine cobalt monoxide is nonferromagnetic, whereas nickel-doped cobalt monoxide exhibits robust ferromagnetic behavior with a Curie temperature of ≈180 K. Density functional theory calculations reveal that nickel atoms can improve the internal ferromagnetic correlation, giving rise to significant ferromagnetic performance of cobalt monoxide nanosheets. These results provide a valuable case for tuning the competing correlated states and magnetic ordering by substitution doping in 2D nonlayered oxide semiconductors.

Emerging 2D Metal Oxides: From Synthesis to Device Integration

2D metal oxides have aroused increasing attention in the field of electronics and optoelectronics due to their intriguing physical properties. In this review, an overview of recent advances on synthesis of 2D metal oxides and their electronic applications is presented. First, the tunable physical properties of 2D metal oxides that relate to the structure (various oxidation-state forms, polymorphism, etc.), crystallinity and defects (anisotropy, point defects, and grain boundary), and thickness (quantum confinement effect, interfacial effect, etc.) are discussed. Then, advanced synthesis methods for 2D metal oxides besides mechanical exfoliation are introduced and classified into solution process, vapor-phase deposition, and native oxidation on a metal source. Later, the various roles of 2D metal oxides in widespread applications, i.e., transistors, inverters, photodetectors, piezotronics, memristors, and potential applications (solar cell, spintronics, and superconducting devices) are discussed. Finally, an outlook of existing challenges and future opportunities in 2D metal oxides is proposed.

Gate Dielectrics Integration for 2D Electronics: Challenges, Advances, and Outlook

2D semiconductors have emerged both as an ideal platform for fundamental studies and as promising channel materials in beyond-silicon field-effect-transistors due to their outstanding electrical properties and exceptional tunability via external field. However, the lack of proper dielectrics for 2D semiconductors has become a major roadblock for their further development toward practical applications. The prominent issues between conventional 3D dielectrics and 2D semiconductors arise from the integration and interface quality, where defect states and imperfections lead to dramatic deterioration of device performance. In this review article, the root causes of such issues are briefly analyzed and recent advances on some possible solutions, including various approaches of adapting conventional dielectrics to 2D semiconductors, and the development of novel dielectrics with van der Waals surface toward high-performance 2D electronics are summarized. Then, in the perspective, the requirements of ideal dielectrics for state-of-the-art 2D devices are outlined and an outlook for their future development is provided.

Giant Tunability of Charge Transport in 2D Inorganic Molecular Crystals by Pressure Engineering

The unique intermolecular van der Waals force in emerging two-dimensional inorganic molecular crystals (2DIMCs) endows them with highly tunable structures and properties upon applying external stimuli. Using high pressure to modulate the intermolecular bonding, here we reveal the highly tunable charge transport behavior in 2DIMCs for the first time, from an insulator to a semiconductor. As pressure increases, 2D α-Sb₂ O₃ molecular crystal undergoes three isostructural transitions, and the intermolecular bonding enhances gradually, which results in a considerably decreased band gap by 25 % and a greatly enhanced charge transport. Impressively, the in situ resistivity measurement of the α-Sb₂ O₃ flake shows a sharp drop by 5 orders of magnitude in 0-3.2 GPa. This work sheds new light on the manipulation of charge transport in 2DIMCs and is of great significance for promoting the fundamental understanding and potential applications of 2DIMCs in advanced modern technologies.

Antimonene: a tuneable post-graphene material for advanced applications in optoelectronics, catalysis, energy and biomedicine

The post-graphene era is undoubtedly marked by two-dimensional (2D) materials such as quasi-van der Waals antimonene. This emerging material has a fascinating structure, exhibits a pronounced chemical reactivity (in contrast to graphene), possesses outstanding electronic properties and has been postulated for a plethora of applications. However, chemistry and physics of antimonene remain in their infancy, but fortunately recent discoveries have shed light on its unmatched allotropy and rich chemical reactivity offering a myriad of unprecedented possibilities in terms of fundamental studies and applications. Indeed, antimonene can be considered as one of the most appealing post-graphene 2D materials reported to date, since its structure, properties and applications can be chemically engineered from the ground up (both using top-down and bottom-up approaches), offering an unprecedented level of control in the realm of 2D materials. In this review, we provide an in-depth analysis of the recent advances in the synthesis, characterization and applications of antimonene. First, we start with a general introduction to antimonene, and then we focus on its general chemistry, physical properties, characterization and synthetic strategies. We then perform a comprehensive study on the allotropy, the phase transition mechanisms, the oxidation behaviour and chemical functionalization. From a technological point of view, we further discuss the applications recently reported for antimonene in the fields of optoelectronics, catalysis, energy storage, cancer therapy and sensing. Finally, important aspects such as new scalable methodologies or the promising perspectives in biomedicine are discussed, pinpointing antimonene as a cutting-edge material of broad interest for researchers working in chemistry, physics, materials science and biomedicine.

Spatially Confined Face-Selective Growth of Large-Area 2D Organic Molecular Crystals in a Supramolecular Gel for Highly Efficient Flexible Photodetection

2D organic molecular crystals (2DOMCs) are promising materials for the fabrication of high-performance optoelectronic devices. However, the growth of organic molecules into 2DOMCs remains a challenge because of the difficulties in controlling their self-assembly with a preferential orientation in solution-process crystallization. Herein, fullerene is chosen as a model molecule to develop a supramolecular gel crystallization approach to grow large-area 2DOMCs by controlling the perfect arrangement on the {220} crystal plane with the assistance of a gelated solvent. In this case, the gel networks provide tuneable confined spaces to control the crystallization kinetics toward the growth of dominant crystal faces by their inhibiting motions of solvent or solute molecules to enable the growth of perfect crystals at appropriate nucleation rates. As a result, a large-area fullerene 2DOMC is produced successfully and its corresponding device on a flexible substrate exhibits excellent bendable properties and ultra-high weak light detection ability (2.9 × 1011 Jones) at a 10 V bias upon irradiation with 450 nm incident light. Moreover, its photoelectric properties remain unchanged after 200 cycles of bending at angles of 45, 90, and 180°. These results can be extended to the growth of other 2DOMCs for potentially fabricating advanced organic (opto)electronics.

Type-I Heterostructure Based on WS2/PtS2 for High-Performance Photodetectors

van der Waals (vdW) heterodiodes composed of two-dimensional (2D) layered materials led to a new prospect in photoelectron diodes and photovoltaic devices. Existing studies have shown that Type-I heterostructures have great potential to be used as photodetectors; however, the tunneling phenomena in Type-I heterostructures have not been fully revealed. Herein, a highly efficient nn⁺ WS₂/PtS₂ Type-I vdW heterostructure photodiode is constructed. The device shows an ultrahigh reverse rectification ratio of 10⁵ owing to the transmission barrier-induced low reverse current. A unilateral depletion region is formed on WS₂, which inhibits the recombination of carriers at the interface and makes the external quantum efficiency (EQE) of the device reach 67%. Due to the tunneling mechanism of the device, which allows the co-existence of a large photocurrent and a low dark current, this device achieves a light on/off ratio of over 10⁵. In addition, this band design allows the device to maintain a high detectivity of 4.53 × 10¹⁰ Jones. Our work provides some new ideas for exploring new high-efficiency photodiodes.

2D Ultrawide Bandgap Semiconductors: Odyssey and Challenges

2D ultrawide bandgap (UWBG) semiconductors have aroused increasing interest in the field of high-power transparent electronic devices, deep-ultraviolet photodetectors, flexible electronic skins, and energy-efficient displays, owing to their intriguing physical properties. Compared with dominant narrow bandgap semiconductor material families, 2D UWBG semiconductors are less investigated but stand out because of their propensity for high optical transparency, tunable electrical conductivity, high mobility, and ultrahigh gate dielectrics. At the current stage of research, the most intensively investigated 2D UWBG semiconductors are metal oxides, metal chalcogenides, metal halides, and metal nitrides. This paper provides an up-to-date review of recent research progress on new 2D UWBG semiconductor materials and novel physical properties. The widespread applications, i.e., transistors, photodetector, touch screen, and inverter are summarized, which employ 2D UWBG semiconductors as either a passive or active layer. Finally, the existing challenges and opportunities of the enticing class of 2D UWBG semiconductors are highlighted.

Emerging Two-Dimensional Inorganic Molecular Crystals: The Concept and Beyond

The concept of two-dimensional (2D) inorganic molecular crystals (IMCs) was first introduced by Zhai and coauthors in 2019. In contrast to the layered structures of graphene-like 2D materials, 2D IMCs consist of tiny inorganic molecules bonded together through all-around van der Waals (vdW) interactions. Their structural peculiarities lead to some special behaviors and appealing properties in their synthesis and applications. In this Perspective, we first introduce the concept of 2D IMCs and present the very first synthesis of 2D IMCs using a surface-passivated growth approach. The special intermolecular effects between the inorganic molecules are also summarized. In addition, because of its molecular structure, a vdW film of IMCs can be facilely fabricated, which exhibits appealing potential in integrated 2D devices. More importantly, we give a general outlook for the further development of 2D IMCs with the goal of attracting more attention to this emerging research frontier.

Scalable Van der Waals Encapsulation by Inorganic Molecular Crystals

Encapsulation is critical for devices to guarantee their stability and reliability. It becomes an even more essential requirement for devices based on 2D materials with atomic thinness and far inferior stability compared to their bulk counterparts. Here a general van der Waals (vdW) encapsulation method for 2D materials using Sb₂ O₃ layer of inorganic molecular crystal fabricated via thermal evaporation deposition is reported. It is demonstrated that such a scalable encapsulation method not only maintains the intrinsic properties of typical air-susceptible 2D materials due to their vdW interactions but also remarkably improves their environmental stability. Specifically, the encapsulated black phosphorus (BP) exhibits greatly enhanced structural stability of over 80 days and more sustaining-electrical properties of 19 days, while the bare BP undergoes degradation within hours. Moreover, the encapsulation layer can be facilely removed by sublimation in vacuum without damaging the underlying materials. This scalable encapsulation method shows a promising pathway to effectively enhance the environmental stability of 2D materials, which may further boost their practical application in novel (opto)electronic devices.

Modulation of the optical bandgap and photoluminescence quantum yield in pnictogen (Sb3+/Bi3+)-doped organic-inorganic tin(IV) perovskite single crystals and nanocrystals

Water-stable, lead-free zero-dimensional (0D) organic-inorganic hybrid colloidal tin(IV) perovskite, A₂SnX₆ (A is a monocationic organic ion and X is a halide) nanocrystals (NCs) with high photoluminescence (PL) quantum yield (QY) have rarely been explored. Herein, we report solution-processed colloidal NCs of blue light-emitting T₂SnCl₆ and orange light-emitting T₂Sn_1-xSb_xCl₆ [T⁺ = tetramethylammonium cation] from their corresponding single crystals (SCs). These colloidal NCs are well-dispersible in non-polar solvents, thereby maintaining their bright emission. This paves the way for fabricating homogeneous thin films of these NCs. Due to organic cation (T⁺)-controlled large spin-orbit coupling (SOC), the T₂Sn_1-xSb_xCl₆ NCs exhibit bright orange emission with an enhancement in PL QY of 41% compared to their bulk counterpart. Furthermore, we explore T₂Sn_1-xBi_xCl₆ and T₂Sn_1-x-yBi_xSb_yCl₆ SCs, which show blue and green emission, respectively; the latter is attributed to the newly formed Sb 5p and Sb 5 s orbital-driven band structures confirmed by applying density functional theory (DFT) calculations. The SCs and NCs exhibit excellent stability in water under ambient conditions because of the in-situ generation of a hydrophobic and oxygen-resistant passivating layer of oxychloride in the presence of water. Our findings open a pathway for designing lead-free perovskites materials for thin-film-based optoelectronic devices.

Effect of Strong Intermolecular Interaction in 2D Inorganic Molecular Crystals

Strong intermolecular interactions in 2D organic molecular crystals arising from π-π stacking have been widely explored to achieve high thermal stability, high carrier mobility, and novel physical properties, which have already produced phenomenal progress. However, strong intermolecular interactions in 2D inorganic molecular crystals (2DIMCs) have rarely been investigated, severely limiting both the fundamental research in molecular physics and the potential applications of 2DIMCs for optoelectronics. Here, the effect of strong intermolecular interactions induced by unique short intermolecular Se-Se and P-Se contacts in 2D α-P₄Se₃ nanoflakes is reported. On the basis of theoretical calculations of the charge density distribution and an analysis of the thermal expansion and plastic-crystal transition, the physical picture of strong intermolecular interactions can be elucidated as a higher charge density between adjacent P₄Se₃ molecules, arising from an orderly and close packing of P₄Se₃ molecules. More importantly, encouraged by the strong intermolecular coupling, the in-plane mobility of α-P₄Se₃ nanoflakes is first calculated with a quantum nuclear tunneling model, and a competitive hole mobility of 0.4 cm² V^-1 s^-1 is obtained. Our work sheds new light on the intermolecular interactions in 2D inorganic molecular crystals and is highly significant for promoting the development of molecular physics and optoelectronics.

2D Bi2Se3 materials for optoelectronics

2D layered materials with diverse exciting properties have recently attracted tremendous interest in the scientific community. Layered topological insulator Bi2Se3 comes into the spotlight as an exotic state of quantum matter with insulating bulk states and metallic Dirac-like surface states. Its unique crystal and electronic structure offer attractive features such as broadband optical absorption, thickness-dependent surface bandgap and polarization-sensitive photoresponse, which enable 2D Bi2Se3 to be a promising candidate for optoelectronic applications. Herein, we present a comprehensive summary on the recent advances of 2D Bi2Se3 materials. The structure and inherent properties of Bi2Se3 are firstly described and its preparation approaches (i.e., solution synthesis and van der Waals epitaxy growth) are then introduced. Moreover, the optoelectronic applications of 2D Bi2Se3 materials in visible-infrared detection, terahertz detection, and opto-spintronic device are discussed in detail. Finally, the challenges and prospects in this field are expounded on the basis of current development.

High-k 2D Sb2O3 Made Using a Substrate-Independent and Low-Temperature Liquid-Metal-Based Process

High dielectric constant (high-k) ultrathin films are required as insulating gate materials. The well-known high-k dielectrics, including HfO₂, ZrO₂, and SrTiO₃, feature three-dimensional lattice structures and are thus not easily obtained in the form of distinct ultrathin sheets. Therefore, their deposition as ultrathin layers still imposes challenges for electronic industries. Consequently, new high-k nanomaterials with k in the range of 40 to 100 and a band gap exceeding 4 eV are highly sought after. Antimony oxide nanosheets appear as a potential candidate that could fulfill these characteristics. Here, we report on the stoichiometric cubic polymorph of 2D antimony oxide (Sb₂O₃) as an ideal high-k dielectric sheet that can be synthesized via a low-temperature, substrate-independent, and silicon-industry-compatible liquid metal synthesis technique. A bismuth-antimony alloy was produced during the growth process. Preferential oxidation caused the surface of the melt to be dominated by α-Sb₂O₃. This ultrathin α-Sb₂O₃ was then deposited onto desired surfaces via a liquid metal print transfer. A tunable sheet thickness between ∼1.5 and ∼3 nm was achieved, while the lateral dimensions were within the millimeter range. The obtained α-Sb₂O₃ exhibited high crystallinity and a wide band gap of ∼4.4 eV. The relative permittivity assessment revealed a maximum k of 84, while a breakdown electric field of ∼10 MV/cm was observed. The isolated 2D α-Sb₂O₃ nanosheets were utilized in top-gated field-effect transistors that featured low leakage currents, highlighting that the obtained material is a promising gate oxide for conventional and van der Waals heterostructure-based electronics.

Elastic Properties of High-Symmetry Sb4O6 Cage-Molecular Crystal

The cubic-phase antimony trioxide (α-Sb₂O₃) is a room-temperature stable molecular crystal, composed of cage-like tetraantimony hexoxide (Sb₄O₆) molecules. Despite its versatile functionality, the van der Waals (vdW) bond-dominated nanomechanics is still unclear. Here, the bending plate-like linear behaviors of high-quality α-Sb₂O₃ nanoflakes were observed using the nanoindentation method. It is found that the cage-molecular crystal owns a very low in-plane Young's modulus of 14.9 ± 0.8 GPa and a remarkable maximum tensile strain of 6.0-8.8%, corresponding to a rupture strength of 0.89-1.31 GPa. Elucidated by the atomistic simulations, the compliant elastic modulus and the unexpectedly strong rupture strain are associated with the high-symmetry vdW bonding structure. The vdW nanomechanics is of fundamental and technological relevance to nanoelectronics.

Intercalation Strategy in 2D Materials for Electronics and Optoelectronics

Intercalation is an effective approach to tune the physical and chemical properties of 2D materials due to their abundant van der Waals gaps that can host high-density intercalated guest matters. This approach has been widely employed to modulate the optical, electrical, and photoelectrical properties of 2D materials for their applications in electronic and optoelectronic devices. Thus it is necessary to review the recent progress of the intercalation strategy in 2D materials and their applications in devices. Herein, various intercalation strategies and the novel properties of the intercalated 2D materials as well as their applications in electronics and optoelectronics are summarized. In the end, the development tendency of this promising approach for 2D materials is also outlined.

Phase-Selective Synthesis of Ultrathin FeTe Nanoplates by Controllable Fe/Te Atom Ratio in the Growth Atmosphere

Phase controllable synthesis of 2D materials is of significance for tuning related electrical, optical, and magnetic properties. Herein, the phase-controllable synthesis of tetragonal and hexagonal FeTe nanoplates has been realized by a rational control of the Fe/Te ratio in a chemical vapor deposition system. Using density functional theory calculations, it has been revealed that with the change of the Fe/Te ratio, the formation energy of active clusters changes, causing the phase-controllable synthesis of FeTe nanoplates. The thickness of the obtained FeTe nanoplates can be tuned down to the 2D limit (2.8 nm for tetragonal and 1.4 nm for hexagonal FeTe). X-ray diffraction pattern, transmission electron microscopy, and high resolution scanning transmission electron microscope analyses exhibit the high crystallinity of the as-grown FeTe nanoplates. The two kinds of FeTe nanoflakes show metallic behavior and good electrical conductivity, featuring 8.44 × 10⁴ S m^-1 for 9.8 nm-thick tetragonal FeTe and 5.45 × 10⁴ S m^-1 for 7.6 nm-thick hexagonal FeTe. The study provides an efficient and convenient route for tailoring the phases of FeTe nanoplates, which benefits to study phase-sensitive properties, and may pave the way for the synthesis of other multiphase 2D nanosheets with controllable phases.

Unconventional-Phase Crystalline Materials Constructed from Multiscale Building Blocks

Crystal phase, an intrinsic characteristic of crystalline materials, is one of the key parameters to determine their physicochemical properties. Recently, great progress has been made in the synthesis of nanomaterials with unconventional phases that are different from their thermodynamically stable bulk counterparts via various synthetic methods. A nanocrystalline material can also be viewed as an assembly of atoms with long-range order. When larger entities, such as nanoclusters, nanoparticles, and microparticles, are used as building blocks, supercrystalline materials with rich phases are obtained, some of which even have no analogues in the atomic and molecular crystals. The unconventional phases of nanocrystalline and supercrystalline materials endow them with distinctive properties as compared to their conventional counterparts. This Review highlights the state-of-the-art progress of nanocrystalline and supercrystalline materials with unconventional phases constructed from multiscale building blocks, including atoms, nanoclusters, spherical and anisotropic nanoparticles, and microparticles. Emerging strategies for engineering their crystal phases are introduced, with highlights on the governing parameters that are essential for the formation of unconventional phases. Phase-dependent properties and applications of nanocrystalline and supercrystalline materials are summarized. Finally, major challenges and opportunities in future research directions are proposed.

Broken-Gap PtS2/WSe2 van der Waals Heterojunction with Ultrahigh Reverse Rectification and Fast Photoresponse

Broken-gap van der Waals (vdW) heterojunctions based on 2D materials are promising structures to fabricate high-speed switching and low-power multifunctional devices thanks to its charge transport versus quantum tunneling mechanism. However, the tunneling current is usually generated under both positive and negative bias voltage, resulting in small rectification and photocurrent on/off ratio. In this paper, we report a broken-gap vdW heterojunction PtS₂/WSe₂ with a bilateral accumulation region design and a big band offset by utilizing thick PtS₂ as an effective carrier-selective contact, which exhibits an ultrahigh reverser rectification ratio approaching 10⁸ and on/off ratio over 10⁸ at room temperature. We also find excellent photodetection properties in such a heterodiode with a large photocurrent on/off ratio over 10⁵ due to its ultralow forward current and a comparable photodetectivity of 3.8 × 10¹⁰ Jones. In addition, the response time of such a photodetector reaches 8 μs owing to the photoinduced tunneling mechanism and reduced interface trapping effect. The proposed heterojunction not only demonstrates the high-performance broken-gap heterodiode but also provides in-depth understanding of the tunneling mechanism in the development of future electronic and optoelectronic applications.

Design and tailoring of two-dimensional Schottky, PN and tunnelling junctions for electronics and optoelectronics

Owing to their superior carrier mobility, strong light-matter interactions, and flexibility at the atomically thin thickness, two-dimensional (2D) materials are attracting wide interest for application in electronic and optoelectronic devices, including rectifying diodes, transistors, memory, photodetectors, and light-emitting diodes. At the heart of these devices, Schottky, PN, and tunneling junctions are playing an essential role in defining device function. Intriguingly, the ultrathin thickness and unique van der Waals (vdW) interlayer coupling in 2D materials has rendered enormous opportunities for the design and tailoring of various 2D junctions, e.g. using Lego-like hetero-stacking, surface decoration, and field-effect modulation methods. Such flexibility has led to marvelous breakthroughs during the exploration of 2D electronics and optoelectronic devices. To advance further, it is imperative to provide an overview of existing strategies for the engineering of various 2D junctions for their integration in the future. Thus, in this review, we provide a comprehensive survey of previous efforts toward 2D Schottky, PN, and tunneling junctions, and the functional devices built from them. Though these junctions exhibit similar configurations, distinct strategies have been developed for their optimal figures of merit based on their working principles and functional purposes.

Recent Advances in 2D Group VB Transition Metal Chalcogenides

2D materials have received considerable research interest owing to their abundant material systems and remarkable properties. Among them, 2D group VB transition metal chalcogenides (GVTMCs) stand out as emerging 2D metallic materials and significantly broaden the research scope of 2D materials. 2D GVTMCs have great advantages in electrical transport, 2D magnetism, charge density wave, sensing, catalysis, and charge storage, making them attractive in the fields of functional devices and energy chemistry. In this review, the recent progress of 2D GVTMCs is summarized systematically from fundamental properties, growth methodologies to potential applications. The challenges and prospects are also discussed for future research in this field.

Recent Advances in 2D Layered Phosphorous Compounds

2D layered phosphorous compounds (2D LPCs) have led to explosion of research interest in recent years. With the diversity of valence states of phosphorus, 2D LPCs exist in various material types and possess many novel physical and chemical properties. These properties, including widely adjustable range of bandgap, diverse electronic properties covering metal, semimetal, semiconductor and insulator, together with inherent magnetism and ferroelectricity at atomic level, render 2D LPCs greatly promising in the applications of electronics, spintronics, broad-spectrum optoelectronics, high-performance catalysts, and energy storage, etc. In this review, the recently research progress of 2D LPCs are presented in detail. First, the 2D LPCs are classified according to their elemental composition and the corresponding crystal structures are introduced, followed by their preparation methods. Then, the novel properties are summarized and the potential applications are discussed in detail. Finally, the conclusion and perspective of the promising 2D LPCs are discussed on the foundation of the latest research progress.

Antimony oxide nanostructures in the monolayer limit: self-assembly of van der Waals-bonded molecular building blocks

Antimony oxide nanostructures have been identified as candidates for a range of electronic and optoelectronic applications. Here we demonstrate the growth of 2-dimensional antimony oxide nanostructures on various substrates, including highly oriented pyrolytic graphite (HOPG), MoS₂ and α-Bi(110) nanoislands. Using scanning tunneling microscopy (STM) we show that the nanostructures formed are exclusively highly crystalline α-Sb₂O₃(111) monolayers with a lattice constant of 796 pm ± 7 pm. The nanostructures are triangular with lateral dimensions of up to ∼30 nm. Even though elemental antimony nanostructures are grown simultaneously mixed phases are not observed and both materials exhibit their own distinct growth modes. Moiré patterns are also observed and simulated, allowing confirmation of the atomic unit cell and an understanding of the orientation of the Sb₂O₃ structures with respect to the supporting materials. As in the bulk, the Sb₂O₃ nanostructures are formed from Sb₄O₆ molecules that are weakly interacting through van der Waals forces. This allows physical modification of the nanostructures with the STM tip. Scanning tunnelling spectroscopy reveals a wide band gap of at least 3.5 eV. Finally, we show that possible alternative structures that have unit cells comparable to those observed can be excluded based on our DFT calculations. The considered structures are a 2 × 2 reconstruction of β-Sb with one vacancy per unit cell and a van der Waals solid composed of Sb₄ clusters. Previous reports have predominantly demonstrated Sb₂O₃ structures with much larger thicknesses.

Inorganic Low k Cage-molecular Crystals

For the interlayer dielectric in microelectronics, light element compounds are preferably accepted due to less electronic polarization. Here, the nontrivial dielectric nature of the Sb₄O₆ cage-molecular crystal, known as α-antimony trioxide (α-Sb₂O₃), is reported. The gas-phase synthesized α-Sb₂O₃ nanoflakes are of high crystal quality, from which the abnormal local admittance responses were revealed by scanning microwave impedance microscopy (sMIM). The remarkably low dielectric constant (k), 2.0∼2.5, is corroborated by the analysis of the thickness-dependent sMIM-capacitance signal. In light of the theoretical calculations, the ultralow molecular density and the significantly suppressed ionic polarization are both crucial to the highly reduced k. Combining with the excellent optical band gap, thermal stability, and breakdown strength, α-Sb₂O₃ is a promising low k dielectric.

2D ternary vanadium phosphorous chalcogenide with strong in-plane optical anisotropy

2D ternary vanadium phosphorous chalcogenide V₂P₄S₁₃, with novel porous structure, ultra-low crystallographic symmetry, and highly optical anisotropy, was introduced as a new member of 2D anisotropic materials.

Structural order enhances charge carrier transport in self-assembled Au-nanoclusters

The collective properties of self-assembled nanoparticles with long-range order bear immense potential for customized electronic materials by design. However, to mitigate the shortcoming of the finite-size distribution of nanoparticles and thus, the inherent energetic disorder within assemblies, atomically precise nanoclusters are the most promising building blocks. We report an easy and broadly applicable method for the controlled self-assembly of atomically precise Au₃₂(ⁿBu₃P)₁₂Cl₈ nanoclusters into micro-crystals. This enables the determination of emergent optoelectronic properties which resulted from long-range order in such assemblies. Compared to the same nanoclusters in glassy, polycrystalline ensembles, we find a 100-fold increase in the electric conductivity and charge carrier mobility as well as additional optical transitions. We show that these effects are due to a vanishing energetic disorder and a drastically reduced activation energy to charge transport in the highly ordered assemblies. This first correlation of structure and electronic properties by comparing glassy and crystalline self-assembled superstructures of atomically precise gold nanoclusters paves the way towards functional materials with novel collective optoelectronic properties.

2D Inorganic Bimolecular Crystals with Strong In-Plane Anisotropy for Second-Order Nonlinear Optics

2D inorganic bimolecular crystals, consisting of two different inorganic molecules, are expected to possess novel physical and chemical properties due to the synergistic effect of the individual components. However, 2D inorganic bimolecular crystals remain unexploited because of the difficulties in preparation arising from non-typical layered structures and intricate intermolecular interactions. Here, the synthesis of 2D inorganic bimolecular crystal SbI₃ ·3S₈ nanobelts via a facile vertical microspacing sublimation strategy is reported. The as-synthesized SbI₃ ·3S₈ nanobelts exhibit strong in-plane anisotropy of phonon vibrations and intramolecular vibrations as well as show anisotropic light absorption with a high dichroism ratio of 3.9. Furthermore, it is revealed that the second harmonic generation intensity of SbI₃ ·3S₈ nanobelts is highly dependent on the excitation wavelength and crystallographic orientation. This work can inspire the growth of more 2D inorganic bimolecular crystals and excite potential applications for bimolecular optoelectronic devices.

Atomically Thin Oxyhalide Solar-Blind Photodetectors

2D wide-bandgap semiconductors demonstrate great potential in fabricating solar-blind ultraviolet (SBUV) photodetectors. However, the low responsivity of 2D solar-blind photodetectors still limits their practical applications. Here, high-responsivity solar-blind photodetectors are achieved based on 2D bismuth oxychloride (BiOCl) flakes. The 2D BiOCl photodetectors exhibit a responsivity up to 35.7 A W^-1 and a specific detectivity of 2.2 × 10¹⁰ Jones under 250 nm illumination with 17.8 µW cm^-2 power density. In particular, the enhanced photodetective performances are demonstrated in BiOCl photodetectors with increasing ambient temperature. Surprisingly, their responsivity can reach 2060 A W^-1 at 450 K under solar-blind light illumination, maybe owing to the formation of defective BiOCl grains evidenced by in situ transmission electron microscopy. The high responsivity throughout the solar-blind range indicates that 2D BiOCl is a promising candidate for SBUV detection.