Atomic engineering of two-dimensional materials via liquid metals.

Two-dimensional (2D) materials, known for their distinctive electronic, mechanical, and thermal properties, have attracted considerable attention. The precise atomic-scale synthesis of 2D materials opens up new frontiers in nanotechnology, presenting novel opportunities for material design and property control but remains challenging due to the high expense of single-crystal solid metal catalysts. Liquid metals, with their fluidity, ductility, dynamic surface, and isotropy, have significantly enhanced the catalytic processes crucial for synthesizing 2D materials, including decomposition, diffusion, and nucleation, thus presenting an unprecedented precise control over material structures and properties. Besides, the emergence of liquid alloy makes the creation of diverse heterostructures possible, offering a new dimension for atomic engineering. Significant achievements have been made in this field encompassing defect-free preparation, large-area self-aligned array, phase engineering, heterostructures, etc. This review systematically summarizes these contributions from the aspects of fundamental synthesis methods, liquid catalyst selection, resulting 2D materials, and atomic engineering. Moreover, the review sheds light on the outlook and challenges in this evolving field, providing a valuable resource for deeply understanding this field. The emergence of liquid metals has undoubtedly revolutionized the traditional nanotechnology for preparing 2D materials on solid metal catalysts, offering flexible possibilities for the advancement of next-generation electronics.

Phase and Composition Engineering of Self-Intercalated 2D Metallic Tantalum Sulfide for Second-Harmonic Generation.

Self-intercalation in two-dimensional (2D) materials is significant, as it offers a versatile approach to modify material properties, enabling the creation of interesting functional materials, which is essential in advancing applications across various fields. Here, we define ic-2D materials as covalently bonded compounds that result from the self-intercalation of a metal into layered 2D compounds. However, precisely growing ic-2D materials with controllable phases and self-intercalation concentrations to fully exploit the applications in the ic-2D family remains a great challenge. Herein, we demonstrated the controlled synthesis of self-intercalated H-phase and T-phase Ta1+xS2 via a temperature-driven chemical vapor deposition (CVD) approach with a viable intercalation concentration spanning from 10% to 58%. Atomic-resolution scanning transmission electron microscopy-annular dark field imaging demonstrated that the self-intercalated Ta atoms occupy the octahedral vacancies located at the van der Waals gap. The nonperiodic Ta atoms break the centrosymmetry structure and Fermi surface properties of intrinsic TaS2. Therefore, ic-2D T-phase Ta1+xS2 consistently exhibit a spontaneous nonlinear optical (NLO) effect regardless of the sample thickness and self-intercalation concentrations. Our results propose an approach to activate the NLO response of centrosymmetric 2D materials, achieving the modulation of a wide range of optoelectronic properties via nonperiodic self-intercalation in the ic-2D family.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions.

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

Liquid metal catalyzed chemical vapor deposition towards morphology engineering of 2D epitaxial heterostructures.

The past decades have witnessed significant advancements in the growth of two-dimensional (2D) materials, offering a wide range of potential applications in the fields of electronics, optoelectronics, energy storage, sensors, catalysis, and biomedical treatments. Epitaxial heterostructures based on 2D materials, including vertical heterostructures, lateral structures, and superlattices, have emerged as novel material systems to manipulate the intrinsic properties and unlock new functionalities. Therefore, the development of controllable preparation methods for tailored epitaxial heterostructures serves as a fundamental basis for extensive property investigation and further application exploration. However, this pursuit presents formidable challenges due to the incomplete understanding of growth mechanisms and limited designable strategies. Chemical vapor deposition (CVD) is deemed as a promising and versatile platform for the controlled synthesis of 2D materials, especially with regard to achieving lattice matching, a critical factor in epitaxial growth. Consequently, CVD holds potential to overcome these hurdles. In this Feature Article, we present our recent breakthroughs in the controllable preparation of 2D epitaxial heterostructures using CVD. Our focus revolves around the processes of morphology engineering, interface engineering, size and density engineering, and striking the delicate balance between growth and etching. Using molten metals or alloys as primary catalysts, we have achieved remarkable control over the fabrication of graphene/hexagonal boron nitride (hBN) super-ordered arrays, enabled multistage etching of graphene/hBN heterostructures, and successfully realized the construction of graphene/MXene heterostructures. Furthermore, our research endeavors encompass both bottom-up and top-down fabrication methods, offering a novel perspective on the synthesis of 2D epitaxial heterostructures. The resulting products hold immense potential for enhancing the efficiency of critical reactions such as oxygen reduction, CO2 reduction, and hydrogen evolution reactions. By presenting our methodologies for obtaining 2D epitaxial heterostructures through CVD, we aspire to inspire fellow researchers in this field to devise more feasible and controllable fabrication techniques while also fostering the exploration of diverse heterostructure configurations. Together, these advancements will undoubtedly pave the way for further breakthroughs in atomic manufacturing and novel applications.

Ultralow-Power Vertical Transistors for Multilevel Decoding Modes.

Organic field-effect transistors with parallel transmission and learning functions are of interest in the development of brain-inspired neuromorphic computing. However, the poor performance and high power consumption are the two main issues limiting their practical applications. Herein, an ultralow-power vertical transistor is demonstrated based on transition-metal carbides/nitrides (MXene) and organic single crystal. The transistor exhibits a high JON of 16.6 mA cm-2 and a high JON /JOFF ratio of 9.12 × 105 under an ultralow working voltage of -1 mV. Furthermore, it can successfully simulate the functions of biological synapse under electrical modulation along with consuming only 8.7 aJ of power per spike. It also permits multilevel information decoding modes with a significant gap between the readable time of professionals and nonprofessionals, producing a high signal-to-noise ratio up to 114.15 dB. This work encourages the use of vertical transistors and organic single crystal in decoding information and advances the development of low-power neuromorphic systems.

Additive-Assisted Growth of Scaled and Quality 2D Materials.

2D materials are increasingly becoming key components in modern electronics because of their prominent electronic and optoelectronic properties. The central and premise to the entire discipline of 2D materials lie in the high-quality and scaled preparations. The chemical vapor deposition (CVD) method offers compelling benefits in terms of scalability and controllability in shaping large-area and high-quality 2D materials. The past few years have witnessed development of numerous CVD growth strategies, with the use of additives attracting substantial attention in the production of scaled 2D crystals. This review provides an overview of different additives used in CVD growth of 2D materials, as well as a methodical demonstration of their vital roles. In addition, the intrinsic mechanisms of the production of scaled 2D crystals with additives are also discussed. Lastly, reliable guidance on the future design of optimal CVD synthesis routes is provided by analyzing the accessibility, pricing, by-products, controllability, universality, and commercialization of various additives.

A minireview on chemical vapor deposition growth of wafer-scale monolayer h-BN single crystals.

Hexagonal boron nitride (h-BN), with its excellent stability, flat surface, and large bandgap, plays a role in a variety of fundamental science and technology fields. The past few years have witnessed significant development in the scaled growth of h-BN single crystals. Currently, the size of h-BN crystal can be reached up to wafer-scale, paving the way towards industrial production and commercial applications. In this minireview, recent academic breakthroughs regarding the controlled growth of large-sized h-BN single crystals via chemical vapor deposition (CVD) are presented. The as-developed technique in terms of growth parameters, choice of catalysts, and the mechanism is fully emphasized, offering a guideline in enhancing the size and quality of h-BN. Several typical metal catalysts have been used in shaping scaled h-BN single crystals, of which the metal Cu substrate has drawn the most intensive attention. The significant advances in expanding the size of h-BN single crystals will largely push forward the way to h-BN industrialization and commercialization. The past few years have witnessed significant development in the scaled growth of h-BN single crystals. Currently, the size of h-BN crystal can be reached up to wafer-scale, paving the way towards industrial production and commercial applications. In this minireview, recent academic breakthroughs regarding controlled growth of large-sized h-BN single crystals via chemical vapor deposition (CVD) are present. The as-developed technique in terms of growth parameters, choice of catalysts and mechanism is fully emphasized, offering a guideline in enhancing size and quality of h-BN. Several typical metal catalysts are exhibited in shaping scaled h-BN single crystals, of which the metal Cu substrate has drawn the most intensive attentions.

When graphene meets white graphene - recent advances in the construction of graphene and h-BN heterostructures.

2D heterostructures have very recently witnessed a boom in scientific and technological activities owing to the customized spatial orientation and tailored physical properties. A large amount of 2D heterostructures have been constructed on the basis of the combination of mechanical exfoliation and located transfer method, opening wide possibilities for designing novel hybrid systems with tuned structures, properties, and applications. Among the as-developed 2D heterostructures, in-plane graphene and h-BN heterostructures have drawn the most attention in the past few decades. The controllable synthesis, the investigation of properties, and the expansion of applications have been widely explored. Herein, the fabrication of graphene and h-BN heterostructures is mainly focused on. Then, the spatial configurations for the heterostructures are systematically probed to identify the highly related unique features. Moreover, as a most promising approach for the scaled production of 2D materials, the in situ CVD fabrication of the heterostructures is summarized, demonstrating a significant potential in the controllability of size, morphology, and quality. Further, the recent applications of the 2D heterostructures are discussed. Finally, the concerns and challenges are fully elucidated and a bright future has been envisioned.

Continuous orientated growth of scaled single-crystal 2D monolayer films.

Single-crystal 2D materials have attracted a boom of scientific and technological activities. Recently, chemical vapor deposition (CVD) shows great promise for the synthesis of high-quality 2D materials owing to high controllability, high scalability and ultra-low cost. Two types of strategies have been developed: one is single-seed method, which focuses on the ultimate control of the density of nucleation into only one nucleus and the other is a multi-seed approach, which concentrates on the precise engineering of orientation of nuclei into a uniform alignment. Currently, the latter is recognized as a more effective method to meet the demand of industrial production, whereas the oriented domains can seamlessly merge into a continuous single-crystal film in a short time. In this review, we present the detailed cases of growing the representative monocrystalline 2D materials via the single-seed CVD method as well as show its advantages and disadvantages in shaping 2D materials. Then, other typical 2D materials (including graphene, h-BN, and TMDs) are given in terms of the unique feature under the guideline of the multi-seed growth approach. Furthermore, the growth mechanism for the 2D single crystals is presented and the following application in electronics, optics and antioxidation coatings are also discussed. Finally, we outline the current challenges, and a bright development in the future of the continuous orientated growth of scaled 2D crystals should be envisioned.

Controlled growth of 2D ultrathin Ga2O3 crystals on liquid metal.

2D metal oxides (2DMOs) have drawn intensive interest in the past few years owing to their rich surface chemistry and unique electronic structures. Striving for large-scale and high-quality novel 2DMOs is of great significance for developing future nano-enabled technologies. In this work, we demonstrate for the first time controllable growth of highly crystalline 2D ultrathin Ga2O3 single crystals on liquid Ga by the chemical vapor deposition approach. With the introduction of oxygen into the growth process, large-area hexagonal α-Ga2O3 crystals with a uniform size distribution have been produced. At high temperature, fast diffusion of oxygen atoms onto the liquid surface facilitates reaction with Ga and thus leads to in situ formation of 2D ultrathin crystals. By precisely controlling the amount of oxygen, the vertical growth of the Ga2O3 single crystal has been realized. Furthermore, phase engineering can be achieved and thus 2D ß-Ga2O3 crystals were also prepared by precisely tuning the growth temperature. The controlled growth of 2D Ga2O3 crystals offers an applicable avenue for fabrication of other 2D metal oxides and can further open up possibilities for future electronics.

Recent Advances in Growth of Large-Sized 2D Single Crystals on Cu Substrates.

Large-scale and high-quality 2D materials have been an emerging and promising choice for use in modern chemistry and physics owing to their fascinating property profile. The past few years have witnessed inspiringly progressing development in controlled fabrication of large-sized and single-crystal 2D materials. Among those production methods, chemical vapor deposition (CVD) has drawn the most attention because of its fine control over size and quality of 2D materials by modulating the growth conditions. Meanwhile, Cu has been widely accepted as the most popular catalyst due to its significant merit in growing monolayer 2D materials in the CVD process. Herein, very recent advances in preparing large-sized 2D single crystals on Cu substrates by CVD are presented. First, the unique features of Cu will be given in terms of ultralow precursor solubility and feasible surface engineering. Then, scaled growth of graphene and hexagonal boron nitride (h-BN) crystals on Cu substrates is demonstrated, wherein different kinds of Cu surfaces have been employed. Furthermore, the growth mechanism for the growth of 2D single crystals is exhibited, offering a guideline to elucidate the in-depth growth dynamics and kinetics. Finally, relevant issues for industrial-scale mass production of 2D single crystals are discussed and a promising future is expected.

Graphene-Induced in Situ Growth of Monolayer and Bilayer 2D SiC Crystals Toward High-Temperature Electronics.

A reproducible graphene-induced in situ process is demonstrated for the first time for growing large-scale monolayer and bilayer cubic silicon carbide (SiC) crystals on a liquid Cu surface by chemical vapor deposition (CVD) method. Precise control over the morphology of SiC crystals is further realized by modulating growth conditions, thus leading to the formation of several shaped SiC crystals ranging from triangular, rectangular, pentagonal, and even to hexagonal kind. Simulations based on density functional theory are carried out to elucidate the growth mechanism of SiC flakes with various morphologies, which are in striking consistency with experimental observations. In the liquid Cu-assisted CVD system, growth temperature (∼1100 °C) enables sublimation and deposition of silicon oxide (SiO2) derived from quartz tube, while liquid Cu facilitates preformation of graphene originated from methane. The SiO2 and graphene, grown and reacted in situ in the CVD process, are served as the silicon and carbon source for the cubic SiC crystals, respectively. Moreover, the gradual transformation process from SiO2 particles to SiC flakes is directly observed, with several middle stages clearly displayed. The direct in situ growth of SiC crystals offers a novel method for scaled production of SiC crystals and is beneficial to understand its growth mechanism, and thus push forward the way to develop high-temperature and high-frequency electronic devices.

High-Concentration Niobium-Substituted WS2 Basal Domains with Reconfigured Electronic Band Structure for Hydrogen Evolution Reaction.

Extrinsically controlling the intrinsic activity and stability of two-dimensional (2D) semiconducting materials by substitutional doping is crucial for energy-related applications. However, an in situ transition-metal doping strategy for uniform and large-area chemical vapor deposited 2D semiconductors remains a formidable challenge. Here, we successfully synthesize highly uniform niobium-substituted tungsten disulfide (Nb-WS2) monolayers, with a doping concentration of nearly 7% and sizes reaching 100 µm, through a metal dopant precursor route, using salt-catalyzed chemical vapor deposition (CVD). Our results reveal unusual effects in the structural, optical, electronic, and electrocatalysis characteristics of the Nb-WS2 monolayer. The Nb dopants readily induce a band restructuring effect, providing the most active site with a hydrogen adsorption energy of 0.175 eV and hence greatly improving its hydrogen evolution activity. The combined advantages of the unusual physics and chemistry by in situ CVD doping technique open the possibility in designing 2D-material-based electronics and catalysts of novel functionalities.

Primary Nucleation-Dominated Chemical Vapor Deposition Growth for Uniform Graphene Monolayers on Dielectric Substrate.

Direct chemical vapor deposition growth of high quality graphene on dielectric substrates holds great promise for practical applications in electronics and optoelectronics. However, graphene growth on dielectrics always suffers from the issues of inhomogeneity and/or poor quality. Here, we first reveal that a novel precursor-modification strategy can successfully suppress the secondary nucleation of graphene to evolve ultrauniform graphene monolayer film on dielectric substrates. A mechanistic study indicates that the hydroxylation of silica substrate weakens the binding between graphene edges and substrate, thus realizing the primary nucleation-dominated growth. Field-effect transistors based on the graphene films show exceptional electrical performance with the charge carrier mobility up to 3800 cm2 V-1 s-1 in air, which is much higher than those reported results of graphene films grown on dielectrics.

Formation of Twinned Graphene Polycrystals.

Liquid metals have been widely used as substrates to grow graphene and other 2D materials. On a homogeneous and isotropic liquid surface, a polycrystalline 2D material is formed by coalescence of many randomly nucleated single-crystal islands, and as a result, the domains in a polycrystal are expected to be randomly aligned. Here, we report the unexpected finding that only 30°-twinned graphene polycrystals are grown on a liquid Cu surface. Atomic simulations confirm that the unique domain alignment in graphene polycrystals is due to the free rotation of graphene islands on the liquid Cu surface and the highly stable 30°-grain boundaries in graphene. In-depth analysis predicts 30 types of possible 30°-twinned graphene polycrystals and 27 of them are observed. The revealed formation mechanism of graphene polycrystals on a liquid Cu surface deepens our fundamental understanding on polycrystal growth and could serve as a guideline for the controlled synthesis of 2D materials.

Thermal-Assisted Vertical Electron Injections in Few-Layer Pyramidal-Structured MoS2 Crystals.

The interlayer screening effects and charge conduction mechanisms in atomically thin two-dimensional (2D) materials are crucial for electronics and optoelectronics applications. However, such effects remain largely unexplored in chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS2) crystals. Here, we report a controllable CVD-grown monolayer MoS2 and layer-by-layer pyramidal-structured MoS2 crystals with an oxidized Mo foil precursor. The interlayer screening effects and charge conduction mechanisms in the pyramidal-structured MoS2 crystals are studied. Although the Fowler-Nordheim (FN) tunneling model is widely adopted to describe the vertical charge transport mechanism at the 2D semiconductor/bulk metal interface, we found that such a mechanism cannot satisfactorily explain the electrical measurement obtained from our CVD-grown MoS2 samples. Instead, our analysis reveals that Richardson-Schottky (RS) emission is the dominant transport mechanism when Vbias < 1 V. Our findings provide a fundamental understanding of the charge conduction mechanism in CVD-grown MoS2 crystals, which is crucial for development of MoS2 electronics and optoelectronics devices.

Location-selective growth of two-dimensional metallic/semiconducting transition metal dichalcogenide heterostructures.

An electrical contact between metallic electrodes and semiconductors is critical for the performance of electronic and optoelectronic devices. Two-dimensional (2D) transition metal dichalcogenides (TMDs) contain semiconducting, metallic and insulating material members, which enables the fabrication of highly integrated electronic devices fully based on 2D TMDs. However, location-selective synthesis of metallic/semiconducting heterostructures by a chemical vapor deposition (CVD) method has rarely been reported. In this study, a two-step CVD method was applied to fabricate 2D metallic/semiconducting heterostructures. Semiconducting WS2 was first synthesized and served as the template for the following CVD growth of metallic NbS2. In the growth process, NbS2 flakes selectively nucleate at the edges of WS2 monolayers, thus resulting in the formation of NbS2 islands circling around the WS2 monolayers. The as-grown NbS2/WS2 heterostructure was further systematically characterized by Raman spectroscopy, atomic force microscopy (AFM) and scanning transition electron microscopy (STEM). The NbS2 layers epitaxially grown on the WS2 monolayers exhibit a 3R phase and there was no discernible lattice strain in the NbS2/WS2 van der Waals (vdW) heterostructure. The growth of the metallic/semiconducting 2D heterostructures could benefit the nanoelectronic device fabrication and provide a platform for the 2D contact resistance study.

Edge Segregated Polymorphism in 2D Molybdenum Carbide.

Molybdenum carbide (Mo2 C), a class of unterminated MXene, is endowed with rich polymorph chemistry, but the growth conditions of the various polymorphs are not understood. Other than the most commonly observed T-phase Mo2 C, little is known about other phases. Here, Mo2 C crystals are successfully grown consisting of mixed polymorphs and polytypes via a diffusion-mediated mechanism, using liquid copper as the diffusion barrier between the elemental precursors of Mo and C. By controlling the thickness of the copper diffusion barrier layer, the crystal growth can be controlled between a highly uniform AA-stacked T-phase Mo2 C and a "wedding cake" like Mo2 C crystal with spatially delineated zone in which the Bernal-stacked Mo2 C predominate. The atomic structures, as well as the transformations between distinct stackings, are simulated and analyzed using density functional theory (DFT)-based calculations. Bernal-stacked Mo2 C has a d band closer to the Fermi energy, leading to a promising performance in catalysis as verified in hydrogen evolution reaction (HER).

Effects of precursor pre-treatment on the vapor deposition of WS2 monolayers.

Transition metal oxide powders have been widely used as the growth precursors for monolayer transition metal dichalcogenides (TMDCs) in chemical vapor deposition (CVD). It has been proposed that metal oxide precursors in the gas phase undergo a two-step reaction during CVD growth, where transition metal sub-oxides are likely formed first and then the sulfurization of these sub-oxides leads to the formation of TMDCs. However, the effects of stoichiometry of transition metal oxide precursors on the growth of TMDC monolayers have not been studied yet. In this contribution, we report the critical role of the WO3 precursor pre-annealing process on the growth of WS2 monolayers. Besides, several WO3 precursors with different types of oxygen vacancies have also been prepared and investigated by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and density functional theory calculation. Among all the non-stoichiometric WO3 precursors, thermally annealed WO3 powder exhibits the highest oxygen vacancy concentration and produces WS2 monolayers with significantly improved quality in terms of lateral size, density, and crystallinity. Our comprehensive study suggests that the chemical composition of transition metal oxide precursors would be fundamentally critical for the growth of large-area and high-quality WS2 monolayers, which further pave the way for revealing their intrinsic properties and unique applications.

Recent Advances in Growth of Novel 2D Materials: Beyond Graphene and Transition Metal Dichalcogenides.

Since the discovery of graphene just over a decade ago, 2D materials have been a central focus of materials research and engineering because of their unique properties and potential of revealing intriguing new phenomena. In the past few years, transition metal dichalcogenides (TMDs) have also attracted considerable attention because of the intrinsically opened bandgap. The exceptional properties and potential applications of graphene and TMDs have inspired explosive efforts to discover novel 2D materials. Here, emerging novel 2D materials are summarized and recent progress in the preparation, characterization, and application of 2D materials is highlighted. The experimental realization methods for these materials are emphasized, while the large-area growth and controlled patterning for industrial productions are discussed. Finally, the remaining challenges and potential applications of 2D materials are outlined.