Recent Progress in the Fabrication, Properties, and Devices of Heterostructures Based on 2D Materials

Intrinsic Chiroptical Activity and Excitonic Properties of Group II-VI Magic-Size Clusters

Semiconductor magic-size clusters (MSCs), lying in the local minima of the potential landscape, are important intermediates that emerge during the synthesis of colloidal quantum dots. They have definite geometrical and electronic structures, thus serving as atomically precise building blocks for assembling supramolecular structures and devices with unprecedented functionalities. Here we report the intrinsic chiroptical activity in the magic-size cadmium and zinc chalcogenide clusters with magic numbers of 13, 33, and 34 possessing unique core-shell structures. They are responsive to circularly polarized light from the ultraviolet to visible region, with size-tunable energy gap, absorption wavelength, and excitonic characteristics. The origin of the chiroptical activity and the evolution of excitonic states with magic size are disclosed by time-dependent density functional theory calculations within a correlated electron-hole picture. This molecular-level understanding of the photophysical properties of group II-VI MSCs provides essential guidelines for utilizing them for chiral optoelectronics and photonics.

A resilient type-III broken gap Ga2O3/SiC van der Waals heterogeneous bilayer with band-to-band tunneling effect and tunable electronic property

The potential of van der Waals (vdW) heterostructure to incorporate the outstanding features of stacked materials to meet a variety of application requirements has drawn considerable attention. Due to the unique quantum tunneling mechanisms, a type-III broken-gap obtained from vdW heterostructure is a promising design strategy for tunneling field-effect transistors. Herein, a unique Ga2O3/SiC vdW bilayer heterostructure with inherent type-III broken gap band alignment has been revealed through first-principles calculation. The underlying physical mechanism to form the broken gap band alignment is thoroughly studied. Due to the overlapping band structures, a tunneling window of 0.609 eV has been created, which enables the charges to tunnel from the VBM of the SiC layer to the CBM of the Ga2O3 layer and fulfills the required condition for band-to-band tunneling. External electric field and strain can be applied to tailor the electronic behavior of the bilayer heterostructure. Positive external electric field and compressive vertical strain enlarge the tunneling window and enhance the band-to-band tunneling (BTBT) scheme while negative electric field and tensile vertical strain shorten the BTBT window. Under external electric field as well as vertical and biaxial strain, the Ga2O3/SiC vdW hetero-bilayer maintains the type-III band alignment, revealing its capability to tolerate the external electric field and strain with resilience. All these results provide a compelling platform of the Ga2O3/SiC vdW bilayer to design high performance tunneling field effect transistor.

Tunable Photocarrier Dynamics in CuS Nanoflakes under Pressure Modulation

Two-dimensional materials with a unique layered structure have attracted intense attention all around the world due to their extraordinary physical properties. Most importantly, the internal Coulomb coupling can be regulated, and thus electronic transition can be realized by manipulating the interlayer interaction effectively through adding external fields. At present, the properties of two-dimensional materials can be tuned through a variety of methods, such as adding pressure, strain, and electromagnetic fields. For optoelectronic applications, the lifetime of the photogenerated carriers is one of the most crucial parameters for the materials. Here, we demonstrate effective modulation of the optical band gap structure and photocarrier dynamics in CuS nanoflakes by applying hydrostatic pressure via a diamond anvil cell. The peak differential reflection signal shows a linear blueshift with the pressure, suggesting effective tuning of interlayer interaction inside CuS by pressure engineering. The results of transient absorption show that the photocarrier lifetime decreases significantly with pressure, suggesting that the dissociation process of the photogenerated carriers accelerates. It could be contributed to the phase transition or the decrease of the phonon vibration frequency caused by the pressure. Further, Raman spectra reveal the change of Cu-S and S-S bonds after adding pressure, indicating the possible occurrence of structural phase transition. Interestingly, all of the variation modes are reversible after releasing pressure. This work has provided an excellent sight to show the regulation of pressure on the photoelectric properties of CuS, exploring CuS to wider applications that can lead toward the realization of future excitonic and photoelectric devices modulated by high pressure.

WSe2/BN heterostructure as saturable absorber for a diode-pumped passively Q-switched 2 µm solid-state laser

We have successfully achieved the synthesis of heterojunction consisting of WSe2 and BN, by using a liquid phase exfoliation method, and characterization of the prepared materials under the microstructure. The WSe2/BN heterojunction was used as a saturable absorber in the Tm:YAP laser for passively Q-switched operation, and a pulsed laser with an output wavelength around 2 µm range was successfully obtained. After comparing the effects of resonators composed of different cavity mirrors, it is concluded that when the curvature radius of the input mirror is 250 mm and the transmittance of the output coupler is 2.5%, the best output performance was obtained. The maximum average output power of 834 mW was achieved, with a pulsed repetition frequency of 43.51 kHz and a minimum pulse duration of 1.28 µs, corresponding to a peak power of 14.97 W and a maximum single pulse energy of 19.17 µJ.

A mechanism for thickness-controllable single crystalline 2D materials growth

Recent efforts in growing two-dimensional (2D) multilayers have enabled the synthesis of single crystalline 2D multilayers in a wafer scale through the seamless stitching of multiple epitaxial 2D islands. Unlike previously observed wedding-cake or inverted-wedding-cake structures, these multilayer islands have the same size and shape in each layer with aligned edges. In this study, we investigated the underlying growth mechanisms of synchronic 2D multilayers growth and have showed that a heterogenous layer on a crystalline substrate is critical for maintaining the synchronic growth of 2D multilayers. During growth, the heterogenous layer passivates the edges of multilayer 2D island and thus prevents the coalescence of these active edges, while the high interfacial energy between the heterogenous surface layer and the substrate stabilizes the synchronic structure. Based on this model, we have successfully explained the previously observed synchronic growth of graphene and hexagonal boron nitride multilayers (Nat Nanotech 2020, 15: 861; Nature 2022, 606: 88). The deep understanding on the mechanism paves a way towards the synthesis of wafer-scale single-crystal 2D multilayers with a uniform thickness.

Substitutional doping of MoTe2/ZrS2 heterostructures for sustainable energy related applications

Stacking and/or substitutional doping are effective strategies to tune two-dimensional materials with desired properties, greatly extending the applications of the pristine materials. Here, by employing first-principles calculations, we propose that a pristine MoTe2/ZrS2 heterostructure is a distinguished lithium-ion battery anode material with a low Li diffusion barrier (∼0.26 eV), and it has a high maximum Li storage capacity (476.36 mA h g-1) and a relatively low open-circuit voltage (0.16 V) at Li4/MoTe2/Li/ZrS2/Li4. The other heterostructures with different types can be achieved by substitutional doping and their potential applications in sustainable energy related areas are further unraveled. For instance, a type-II TeMoSe/ZrS2 heterostructure could be a potential direct Z-scheme photocatalyst for water splitting with a high solar-to-hydrogen conversion efficiency of 17.62%. The TeMoSe/SZrO heterostructure is predicted to be a potential candidate for application in highly efficient solar cells. Its maximum power conversion efficiency can be as high as 19.21%, which is quite promising for commercial applications. The present results will shed light on the sustainable energy applications of pristine or doped MoTe2/ZrS2 heterostructures in the future.

Exploring the Synergies of Single-Molecule Fluorescence and 2D Materials Coupled by DNA

The world of 2D materials is steadily growing, with numerous researchers attempting to discover, elucidate, and exploit their properties. Approaches relying on the detection of single fluorescent molecules offer a set of advantages, for instance, high sensitivity and specificity, that allow the drawing of conclusions with unprecedented precision. Herein, it is argued how the study of 2D materials benefits from fluorescence-based single-molecule modalities, and vice versa. A special focus is placed on DNA, serving as a versatile adaptor when anchoring single dye molecules to 2D materials. The existing literature on the fruitful combination of the two fields is reviewed, and an outlook on the additional synergies that can be created between them provided.

Electronic and photophysical properties of an atomically thin bowl-shaped beryllene encapsulated inside the cavity of [6]cycloparaphenylene (Ben@[6]CPP)

Exotic metallic nanostructures are being intensely pursued for a myriad of applications, with ultrathin membranes currently at the heart of several investigations. The objective of the present study was to systematically assess the atom-by-atom encapsulation of Be in the molecular nanoring of [6]cycloparaphenylene ([6]CPP). Further, the study aimed to scrutinize the structure, stability, and properties of the encapsulated Ben@[6]CPP systems. The outcomes clearly revealed that [6]CPP enabled the cooperative confinement of atomically thin bowl-shaped beryllene inside its circular cavity. The confinement of Be in [6]CPP generated topologically anisotropic surfaces with distinct interior and exterior charge distributions. The Ben@[6]CPP complexes could render a cationic or anionic nature to Be depending on its neighbouring environment. Thus, the systems may offer a promising opportunity for the synergistic co-adsorption of multiple reactants that are involved in multicomponent reactions. Energy decomposition analysis (EDA) elucidated that the bonding between Be and [6]CPP was partially ionic and covalent in character. The progressive encapsulation of Be atoms inside the cavity of [6]CPP led to a red-shift of the excitation wavelength to the visible region. The calculated optical absorption coefficient was higher than 104 L mol-1 cm-1, which shows promise for diverse optoelectronic applications.

The Schottky barrier transistor in emerging electronic devices

This paper explores how the Schottky barrier (SB) transistor is used in a variety of applications and material systems. A discussion of SB formation, current transport processes, and an overview of modeling are first considered. Three discussions follow, which detail the role of SB transistors in high performance, ubiquitous and cryogenic electronics. For high performance computing, the SB typically needs to be minimized to achieve optimal performance and we explore the methods adopted in carbon nanotube technology and two-dimensional electronics. On the contrary for ubiquitous electronics, the SB can be used advantageously in source-gated transistors and reconfigurable field-effect transistors (FETs) for sensors, neuromorphic hardware and security applications. Similarly, judicious use of an SB can be an asset for applications involving Josephson junction FETs.

Identifying, Resolving, and Quantifying Anisotropy in ReS2 Nanomechanical Resonators

As an emerging two-dimensional semiconductor, rhenium disulfide (ReS₂ ) is renowned for its strong in-plane anisotropy in electrical, optical, and thermal properties. In contrast to the electrical, optical, optoelectrical, and thermal anisotropies that are extensively studied in ReS₂ , experimental characterization of mechanical properties has largely remained elusive. Here, it is demonstrated that the dynamic response in ReS₂ nanomechanical resonators can be leveraged to unambiguously resolve such disputes. Using anisotropic modal analysis, the parameter space for ReS₂ resonators in which mechanical anisotropy is best manifested in resonant responses is determined. By measuring their dynamic response in both spectral and spatial domains using resonant nanomechanical spectromicroscopy, it is clearly shown that ReS₂ crystal is mechanically anisotropic. Through fitting numerical models to experimental results, it is quantitatively determined that the in-plane Young's moduli are 127 and 201 GPa along the two orthogonal mechanical axes. In combination with polarized reflectance measurements, it is shown that the mechanical soft axis aligns with the Re-Re chain in the ReS₂ crystal. These results demonstrate that dynamic responses in nanomechanical devices can offer important insights into intrinsic properties in 2D crystals and provide design guidelines for future nanodevices with anisotropic resonant responses.

Two-Dimensional Magnetic Semiconducting Heterostructures of Single-Layer CrI3-CrI2

Single-layer heterostructures of magnetic materials are unique platforms for studying spin-related phenomena in two dimensions (2D) and have promising applications in spintronics and magnonics. Here, we report the fabrication of 2D magnetic lateral heterostructures consisting of single-layer chromium triiodide (CrI₃) and chromium diiodide (CrI₂). By carefully adjusting the abundance of iodine based on molecular beam epitaxy, single-layer CrI₃-CrI₂ heterostructures were grown on Au(111) surfaces with nearly atomic-level seamless boundaries. Two distinct types of interfaces, i.e., zigzag and armchair interfaces, have been identified by means of scanning tunneling microscopy. Our scanning tunneling spectroscopy study combined with density functional theory calculations indicates the existence of spin-polarized ground states below and above the Fermi energy localized at the boundary. Both the armchair and zigzag interfaces exhibit semiconducting nanowire behaviors with different spatial distributions of density of states. Our work presents a novel low-dimensional magnetic system for studying spin-related physics with reduced dimensions and designing advanced spintronic devices.

The effects of intercalated environmental gas molecules on carrier dynamics in WSe2/WS2 heterostructures

Effective tuning of carrier dynamics in two-dimensional (2D) materials is significant for multi-scene device applications. Using first-principles and ab initio nonadiabatic molecular dynamics calculations, the kinetics of O₂, H₂O, and N₂ intercalation into 2D WSe₂/WS₂ van der Waals heterostructures and its effect on carrier dynamics have been comprehensively explored. It is found that the O₂ molecule prefers to dissociate into atomic O atoms spontaneously after intercalation of WSe₂/WS₂ heterostructures, whereas H₂O and N₂ molecules remain intact. O₂ intercalation significantly speeds up the electron separation process, while H₂O intercalation largely speeds up the hole separation process. The lifetime of excited carriers can be prolonged by O₂ or H₂O or N₂ intercalations. These intriguing phenomena can be attributed to the effect of interlayer coupling, and the underlying physical mechanism for tuning the carrier dynamics is fully discussed. Our results provide useful guidance for the experimental design of 2D heterostructures for optoelectronic applications in photocatalysts and solar energy cells.

Controllable Synthesis of 2D Materials by Electrochemical Exfoliation for Energy Storage and Conversion Application

2D materials have captured much recent research interest in a broad range of areas, including electronics, biology, sensors, energy storage, and others. In particular, preparing 2D nanosheets with high quality and high yield is crucial for the important applications in energy storage and conversion. Compared with other prevailing synthetic strategies, the electrochemical exfoliation of layered starting materials is regarded as one of the most promising and convenient methods for the large-scale production of uniform 2D nanosheets. Here, recent developments in electrochemical delamination are reviewed, including protocols, categories, principles, and operating conditions. State-of-the-art methods for obtaining 2D materials with small numbers of layers-including graphene, black phosphorene, transition metal dichalcogenides and MXene-are also summarized and discussed in detail. The applications of electrochemically exfoliated 2D materials in energy storage and conversion are systematically reviewed. Drawing upon current progress, perspectives on emerging trends, existing challenges, and future research directions of electrochemical delamination are also offered.

Multifunctional MoSe2 @MXene Heterostructure-Decorated Cellulose Fabric for Wearable Thermal Therapy

A booming demand for wearable electronic devices urges the development of multifunctional smart fabrics. However, it is still facing a challenge to fabricate multifunctional smart fabrics with satisfactory mechanical property, excellent Joule heating performance, highly efficient photothermal conversion, outstanding electromagnetic shielding effectiveness, and superior anti-bacterial capability. Here, a MoSe₂ @MXene heterostructure-based multifunctional cellulose fabric is fabricated by depositing MXene nanosheets onto cellulose fabric followed by a facile hydrothermal method to grow MoSe₂ nanoflakes on MXene layers. A low-voltage Joule heating therapy platform with rapid Joule heating response (up to 230 °C in 25 s at a supplied voltage of 4 V) and stable performance under repeated bending cycles (up to 1000 cycles) is realized. Besides, the multifunctional fabric also exhibits excellent photothermal performance (up to 130 °C upon irradiation for 25 s with a light intensity of 400 mW cm^-2 ), outstanding electromagnetic interference shielding effectiveness (37 dB), and excellent antibacterial performances (>90% anti-bacterial rate toward Escherichia coli, Bacillus subtilis, and Staphylococcus aureus). This work offers an efficient avenue to fabricate multifunctional wearable thermal therapy devices for mobile healthcare and personal thermal management.

Electronic structure characteristics of two-dimensional ferroelectric heterostructuresα-In2Se3/ZnSe

At present, chips urgently need breakthrough development in the power consumption and integration. The chip integrates billions or even tens of billions of electronic components, such as field effect transistor, diode and so on. Therefore, the research and development of new low-power electronic components with smaller size is an effective method to reduce chip power consumption and improve chip integration. In this paper, the ferroelectric field effect transistor (Fe-FET) based on two-dimensional heterostructuresα-In₂Se₃/ZnSe is proposed. Based on the first principle, the program will analyze the stability and band structure ofα-In₂Se₃/ZnSe under different stacking modes. In the heterojunction, the microphysical mechanism of ferroelectric polarization affecting the electronic structure is revealed from the aspects of charge transfer at the interface and the asymmetric surfaces with different work function. Combined with the non-equilibrium Green's function transport theory, the transport properties of Fe-FET based on theirα-In₂Se₃/ZnSe will be studied. The application will provide sufficient theoretical support for research and development of the device based onα-In₂Se₃/ZnSe structure.

Design and Simulation of a Ratiometric SPR Sensor Based on a 2D van der Waals Heterojunction for Refractive Index Measurement

Surface plasmon resonance (SPR) sensors have been widely applied in many fields because of their advantages of working in real time and high sensitivity. However, because the spectrum of an SPR sensor is easily affected by the smoothness of the metal surface, this type of sensor has obvious disadvantages in the application of quantitative detection. We designed an SPR refractive index sensor for molecular detection that has the advantage of quantifiability. A ratio spectral quantitative analysis method was established based on the two coherent dips of the SPR spectrum formed by the strong coupling effect between the surface plasmon polaritons and the excitons of the J-aggregate molecule 5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)-2-benzimidazoline subunit] propenyl]-3-ethyl-1-(4-sulfobutyl) benzimidazole hydroxide inner salt (TDBC). The introduced MoS₂/graphene van der Waals heterojunction produced an effective charge transfer to the Ag film, resulting in significant electric field enhancement at the sensing interface and further improving the detection sensitivity of the sensor. The simulation results showed that for 43 nm Ag film, for example, the ratiometric SPR sensor with the Ag film structure can obtain 16.12 RIU^-1 sensing sensitivity, applied to the detection of gas molecules, while the SPR sensor with single-layer graphene and three layers of MoS₂ heterostructures can obtain 50.68 RIU^-1 sensing sensitivity. The addition of van der Waals heterostructures can significantly improve sensing performance by 215%.

Recent advances of graphene-biomacromolecule nanocomposites in medical applications

In recent years, graphene-based composites have received increasing attention due to their high biocompatibility, large specific surface area, high electrical conductivity and unique mechanical properties. The combination of biomacromolecules and graphene provides a promising route for the preparation of novel graphene-based nanocomposites. Novel graphene-based nanocomposites with unique functions could be applied to medicine, biology, biosensors, environmental science, energy storage and other fields. Graphene-biomacromolecule nanocomposites have excellent biocompatibility, outstanding biofunctionality and low cytotoxicity, and have more advantages and development prospects than other traditional graphene-based materials in biological and biomedical fields. In this work, we summarize the research on the covalent and non-covalent interactions between different biomacromolecules (peptides, DNA/RNA, proteins and enzymes) and graphene, as well as the synthesis methods of novel functionalized graphene-biomacromolecule composites in recent years. We mainly introduce the recent advances (last 5 years) of graphene-biomacromolecule nanocomposites in medical applications, such as medical detection and disease treatment. We hope that this review will help readers to understand the methods and mechanisms of biomolecules modifying the surface of graphene, as well as the synthesis and application of graphene-based nanocomposites, which will promote the future developments of graphene-biomolecule composites in biomedicine, tissue engineering, materials engineering, and so on.

Epitaxial Atomic Substitution for MoS2-MoN Heterostructure Synthesis

Integrating different two-dimensional (2D) crystals is highly demanded for advancing their application in next-generation electronics. 2D transition metal carbides, nitrides, and carbonitrides (MXenes), as new members in the 2D family, are promising candidates for 2D electrodes because of their high conductivity and stability. However, integrating MXenes with other 2D semiconductors has been underdeveloped due to the limitation of top-down etching synthesis of MXenes. Our recent development of atomic substitution synthesis achieved ultrathin non-van der Waals (non-vdW) transition metal nitrides (TMNs) through the conversion of vdW transition metal dichalcogenides (TMDs), opening opportunities of combining TMDs with TMNs via controllable partial conversion. Here, we perform an in-depth study of the atomic substitution process from semiconducting MoS₂ to metallic MoN and realize both lateral and vertical MoN-MoS₂ heterostructures via edge and surface epitaxial conversion, respectively. The structural evolution investigation from MoS₂ to MoN using high-resolution transmission electron microscopy suggests atomically bonded interface for lateral heterostructures and moiré pattern in vertical heterostructures. Moreover, mask-assisted atomic substitution is applied to create patterned MoN-MoS₂-MoN lateral heterostructures. Electrical measurements reveal a Schottky barrier height of meV for a three-layer MoS₂-MoN interface, showcasing the potential of atomically bonded lateral heterostructures for MoS₂ electronics with MoN as contact electrodes.

Designing a Built-In Electric Field for Efficient Energy Electrocatalysis

To utilize intermittent renewable energy as well as achieve the goals of peak carbon dioxide emissions and carbon neutrality, various electrocatalytic devices have been developed. However, the electrocatalytic reactions, e.g., hydrogen evolution reaction/oxygen evolution reaction in overall water splitting, polysulfide conversion in lithium-sulfur batteries, formation/decomposition of lithium peroxide in lithium-oxygen batteries, and nitrate reduction reaction to degrade sewage, suffer from sluggish kinetics caused by multielectron transfer processes. Owing to the merits of accelerated charge transport, optimized adsorption/desorption of intermediates, raised conductivity, regulation of the reaction microenvironment, as well as ease to combine with geometric characteristics, the built-in electric field (BIEF) is expected to overcome the above problems. Here, we give a Review about the very recent progress of BIEF for efficient energy electrocatalysis. First, the construction strategies and the characterization methods (qualitative and quantitative analysis) of BIEF are summarized. Then, the up-to-date overviews of BIEF engineering in electrocatalysis, with attention on the electron structure optimization and reaction microenvironment modulation, are analyzed and discussed in detail. In the end, the challenges and perspectives of BIEF engineering are proposed. This Review gives a deep understanding on the design of electrocatalysts with BIEF for next-generation energy storage and electrocatalytic devices.

A Brief Review: The Use of L-Ascorbic Acid as a Green Reducing Agent of Graphene Oxide

The reduced form of graphene oxide (r-GO) represents a versatile precursor to obtain graphene derivatives. Graphene oxide (GO) consists of a layered material based on a carbon skeleton functionalized by different oxygen-containing groups, while r-GO is obtained by the almost complete removal of these oxygen-containing functional groups. The r-GO has mechanical, electrical, and optical properties quite similar to graphene, thus, it proves to be a convenient 2D material useful for many technological applications. Nowadays, the most important aspects to consider in producing r-GO are: (i) the possibility of obtaining the highest reduction grade; (ii) the possibility of improving the dispersion stability of the resulting graphene using surfactants; (iii) the use of environmentally friendly and inexpensive reducing agents. Consequently, the availability of effective soft-chemistry approaches based on a green reducing agent for converting GO to r-GO are strongly needed. Among the green reductants, the most suitable is L-ascorbic acid (L-aa). Different studies have revealed that L-aa can achieve C/O ratio and conductivity values comparable to those obtained by hydrazine, a typical reducing agent. These aspects could promote an effective application strategy, and for this reason, this review summarizes and analyzes, in some detail, the up-to date literature on the reduction of GO by L-aa. The results are organized according to the two most important approaches, which are the reduction in liquid-phase, and the reduction in gel-phase. Reaction mechanisms and different experimental parameters affecting the processes were also compared.

Role of Surface Adsorbates on the Photoresponse of (MO)CVD-Grown Graphene-MoS2 Heterostructure Photodetectors

A promising strategy toward ultrathin, sensitive photodetectors is the combination of a photoactive semiconducting transition-metal dichalcogenide (TMDC) monolayer like MoS₂ with highly conductive graphene. Such devices often exhibit a complex and contradictory photoresponse as incident light can trigger both photoconductivity and photoinduced desorption of molecules from the surface. Here, we use metal-organic chemical vapor deposition (MOCVD) to directly grow MoS₂ on top of graphene that is deposited on a sapphire wafer via chemical vapor deposition (CVD) for realizing graphene-MoS₂ photodetectors. Two-color optical pump-electrical probe experiments allow for separation of light-induced carrier transfer across the graphene-MoS₂ heterointerface from adsorbate-induced effects. We demonstrate that adsorbates strongly modify both magnitude and sign of the photoconductivity. This is attributed to a change of the graphene doping from p- to n-type in case adsorbates are being desorbed, while in either case, photogenerated electrons are transferred from MoS₂ to graphene. This nondestructive probing method sheds light on the charge carrier transfer mechanisms and the role of adsorbates in two-dimensional (2D) heterostructure photodetectors.

Recent Progress in Fabrication and Physical Properties of 2D TMDC-Based Multilayered Vertical Heterostructures

Two-dimensional (2D) vertical heterojunctions (HSs), which are usually fabricated by vertically stacking two layers of transition metal dichalcogenide (TMDC), have been intensively researched during the past years. However, it is still an enormous challenge to achieve controllable preparation of the TMDC trilayer or multilayered van der Waals (vdWs) HSs, which have important effects on physical properties and device performance. In this review, we will introduce fundamental features and various fabrication methods of diverse TMDC-based multilayered vdWs HSs. This review focuses on four fabrication methods of TMDC-based multilayered vdWs HSs, such as exfoliation, chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), and pulsed laser deposition (PLD). The latest progress in vdWs HS-related novel physical phenomena are summarized, including interlayer excitons, long photocarrier lifetimes, upconversion photoluminescence, and improved photoelectrochemical catalysis. At last, current challenges and prospects in this research field are provided.

Controllable Valley Polarization and Strain Modulation in 2D 2H–VS2/CuInP2Se6 Heterostructures

Two–dimensional (2D) transition metal dichalcogenides endow individually addressable valleys in momentum space at the K and K’ points in the first Brillouin zone due to the breaking of inversion symmetry and the effect of spin–orbit coupling. However, the application of 2H–VS₂ monolayer in valleytronics is limited due to the valence band maximum (VBM) located at the Γ point. Here, by involving the 2D ferroelectric (FE) CuInP₂Se₆ (CIPSe), the ferrovalley polarization, electronic structure, and magnetic properties of 2D 2H–VS₂/CIPSe heterostructures with different stacking patterns and FE polarizations have been investigated by using first–principles calculations. It is found that, for the energetically favorable AB–stacking pattern, the valley polarization is preserved when the FE polarization of CIPSe is upwards (CIPSe↑) or downwards (CIPSe↓) with the splitting energies slightly larger or smaller compared with that of the pure 2H–VS₂. It is intriguing that, for the FE CIPSe↑ case, the VBM is expected to pass through the Fermi energy level, which can be eventually achieved by applying biaxial strain and thus the valleytronic nature is turned off; however, for the CIPSe↓ situation, the heterostructure basically remains semiconducting even under biaxial strains. Therefore, with the influence of proper strains, the FE polar reversal of CIPSe can be used as a switchable on/off to regulate the valley polarization in VS₂. These results not only demonstrate that 2H–VS₂/CIPSe heterostructures are promising potential candidates in valleytronics, but also shed some light on developing practical applications of valleytronic technology.

Mid-Infrared Optoelectronic Devices Based on Two-Dimensional Materials beyond Graphene: Status and Trends

Since atomically thin two-dimensional (2D) graphene was successfully synthesized in 2004, it has garnered considerable interest due to its advanced properties. However, the weak optical absorption and zero bandgap strictly limit its further development in optoelectronic applications. In this regard, other 2D materials, including black phosphorus (BP), transition metal dichalcogenides (TMDCs), 2D Te nanoflakes, and so forth, possess advantage properties, such as tunable bandgap, high carrier mobility, ultra-broadband optical absorption, and response, enable 2D materials to hold great potential for next-generation optoelectronic devices, in particular, mid-infrared (MIR) band, which has attracted much attention due to its intensive applications, such as target acquisition, remote sensing, optical communication, and night vision. Motivated by this, this article will focus on the recent progress of semiconducting 2D materials in MIR optoelectronic devices that present a suitable category of 2D materials for light emission devices, modulators, and photodetectors in the MIR band. The challenges encountered and prospects are summarized at the end. We believe that milestone investigations of 2D materials beyond graphene-based MIR optoelectronic devices will emerge soon, and their positive contribution to the nano device commercialization is highly expected.

Thin In-Plane In2 O3 /ZnIn2 S4 Heterostructure Formed by Topological-Atom-Extraction: Optimal Distance and Charge Transfer for Effective CO2 Photoreduction

Exploitation of atomic-level principles to optimize the charge transfer on ultrathin 2D heterostructures is an emerging frontier in relieving the energy and environmental crisis. Herein, a facile "topological-atom-extraction" protocol is disclosed, i.e., selective extraction of Zn from ultrathin half-unit-cell ZnIn₂ S₄ (HZIS) can embed thin In₂ O₃ domain into 1.60 nm thick HZIS layer to create an atomically thin in-plane In₂ O₃ /HZIS heterostructure. Thanks to the optimal distance and capability of charge separation, the in-plane In₂ O₃ /HZIS heterostructure is among the best ZnIn₂ S₄ -based CO₂ reduction reaction (CRR) photocatalysts, and indeed demonstrates a significant increase (from 6.8- to 128-fold) in CO production rate compared with those of out-plane ZIS@In₂ O₃ and out-plane In₂ O₃ -HZIS_calcined heterostructures. Density Functional Theory simulation reveals that whereas the out-plane heterostructure has a much smaller ∆q of 0.2-0.25 e, the in-plane heterostructure with "zero distance contact" has an optimal ∆q of 1.05 e between In₂ O₃ and HZIS that induces remarkable charge redistribution on the in-plane heterojunction interface and creates local electric field confined within the ultrathin layer. The charge redistribution efficiently directs the charge-carrier separation in S-scheme photocatalytic system and endows long-lifetime carrier to CRR active HZIS. The findings demonstrate the strong versatility of engineering atomic-level heterojunctions for efficient catalysts design.

Synthesis of AAB-Stacked Single-Crystal Graphene/hBN/Graphene Trilayer van der Waals Heterostructures by In Situ CVD

van der Waals heterostructures based on graphene and hBN layers with different stacking modes are receiving considerable attention because of their potential application in fundamental physics. However, conventional exfoliation fabrication methods and layer-by-layer transfer techniques have various limitations. The CVD synthesis of high-quality large-area graphene and hBN multilayer heterostructures is essential for the advancement of new physics. Herein, the authors propose an in situ CVD growth strategy for synthesizing wafer-scale AAB-stacked single-crystal graphene/hBN/graphene trilayer van der Waals heterostructures. Single-crystal CuNi(111) alloys are prepared on sapphire, followed by the pre-dissolution of carbon atoms. Single-crystal monolayer hBN is synthesized on a plasma-cleaned CuNi(111) surface. Then, a single-crystal monolayer graphene is epitaxially grown onto the hBN surface to form graphene/hBN bilayer heterostructures. A controlled decrease in the growth temperature allows the carbon atoms to precipitate out of the CuNi(111) alloy to form single-crystal graphene at the interface between hBN and CuNi(111), thereby producing graphene/hBN/graphene trilayer van der Waals heterostructures. The stacking modes between as-grown 2D layers are investigated through Raman spectroscopy and transmission electron microscopy. This study provides an in situ CVD approach to directly synthesize large-scale single-crystal low-dimensional van der Waals heterostructures and facilitates their application in future 2D-material-based integrated circuits.

Twist Angle Tuning of Moiré Exciton Polaritons in van der Waals Heterostructures

Twisted atomically thin semiconductors are characterized by moiré excitons. Their optical signatures and selection rules are well understood. However, their hybridization with photons in the strong coupling regime for heterostructures integrated in an optical cavity has not been the focus of research yet. Here, we combine an excitonic density matrix formalism with a Hopfield approach to provide microscopic insights into moiré exciton polaritons. In particular, we show that exciton-light coupling, polariton energy, and even the number of polariton branches can be controlled via the twist angle. We find that these new hybrid light-exciton states become delocalized relative to the constituent excitons due to the mixing with light and higher-energy excitons. The system can be interpreted as a natural quantum metamaterial with a periodicity that can be engineered via the twist angle. Our study presents a significant advance in microscopic understanding and control of moiré exciton polaritons in twisted atomically thin semiconductors.

Recent Research Progress in the Structure, Fabrication, and Application of MXene-Based Heterostructures

Two-dimensional (2D) materials have received increasing attention in the scientific research community owing to their unique structure, which has endowed them with unparalleled properties and significant application potential. However, the expansion of the applications of an individual 2D material is often limited by some inherent drawbacks. Therefore, many researchers are now turning their attention to combine different 2D materials, making the so-called 2D heterostructures. Heterostructures can integrate the merits of each component and achieve a complementary performance far beyond a single part. MXene, as an emerging family of 2D nanomaterials, exhibits excellent electrochemical, electronic, optical, and mechanical properties. MXene-based heterostructures have already been demonstrated in applications such as supercapacitors, sensors, batteries, and photocatalysts. Nowadays, increasing research attention is attracted onto MXene-based heterostructures, while there is less effort spent to summarize the current research status. In this paper, the recent research progress of MXene-based heterostructures is reviewed, focusing on the structure, common preparation methods, and applications in supercapacitors, sensors, batteries, and photocatalysts. The main challenges and future prospects of MXene-based heterostructures are also discussed to provide valuable information for the researchers involved in the field.

Visualization of Band Shifting and Interlayer Coupling in WxMo1-xS2 Alloys Using Near-Field Broadband Absorption Microscopy

Beyond-diffraction-limit optical absorption spectroscopy provides in-depth information on the graded band structures of composition-spread and stacked two-dimensional materials, in which direct/indirect bandgap, interlayer coupling, and defects significantly modify their optoelectronic functionalities such as photoluminescence efficiency. We here visualize the spatially varying band structure of monolayer and bilayer transition metal dichalcogenide alloys by using near-field broadband absorption microscopy. The near-field spectral and spatial information manifests the excitonic band shift that results from the interplay of composition spreading and interlayer coupling. These results enable us to identify, notably, the top layer of the bilayer alloy as pure WS₂. We also use the aberration-free near-field transmission images to demarcate the exact boundaries of alloyed and pure transition metal dichalcogenides. This technology can offer valuable insights on various layered structures in the era of "stacking science" in the quest of quantum optoelectronic devices.

Synthesis of transition metal dichalcogenide van der Waals heterostructures through chemical vapor deposition

Transition metal dichalcogenide (TMD) van der Waals (vdW) heterostructures show great potential in the exploration of novel physical phenomena and practical applications. Compared to the traditional mechanical stacking techniques, chemical vapor deposition (CVD) method exhibits more advantages in preparing TMD vdW heterostructures. CVD enables the large-scale production of high-quality materials with clean interfaces in the future. Herein, CVD methods for the synthesis of TMD vdW heterostructures are summarized. These methods are categorized in two major strategies, multi-step process and one-step process. The effects of various factors are demonstrated, including the temperature, nucleation, and precursors. Finally, the remaining challenges are discussed.

Opportunities and challenges for 2D heterostructures in battery applications: a computational perspective

With an increasing demand for large-scale energy storage systems, there is a need for novel electrode materials to store energy in batteries efficiently. 2D materials are promising as electrode materials for battery applications. Despite their excellent properties, none of the available single-phase 2D materials offers a combination of properties required for maximizing energy density, power density, and cycle life. This article discusses how stacking distinct 2D materials into a 2D heterostructure may open up new possibilities for battery electrodes, combining favourable characteristics and overcoming the drawbacks of constituent 2D layers. Computational studies are crucial to advancing this field rapidly with first-principles simulations of various 2D heterostructures forming the basis for such investigations that offer insights into processes that are hard to determine otherwise. We present a perspective on the current methodology, along with a review of the known 2D heterostructures as anodes and their potential for Li and Na-ion battery applications. 2D heterostructures showcase excellent tunability with different compositions. However, each of them has distinct properties, with its own set of challenges and opportunities for application in batteries. We highlight the current status and prospects to stimulate research into designing new 2D heterostructures for battery applications.

Recent Advances on Tuning the Interlayer Coupling and Properties in van der Waals Heterostructures

2D van der Waals (vdW) heterostructures are receiving increasing research attention due to the theoretically amazing properties and unprecedented application potential. However, the as-synthesized heterostructures are generally underperforming due to the weak interlayer coupling, which inspires the researchers to find ways to modulate the interlayer coupling and properties, realizing the tailored performance for actual applications. There have been a lot of publications regarding the controllable regulation of the structures and properties of 2D vdW heterostructures in the past few years, while a review work summarizing the current advances is not yet available, though it is significant. This paper conducts a state-of-the-art review regarding the current research progress of performance modulation of vdW heterostructures by different techniques. First, the general synthesis methods of vdW heterostructures are summarized. Then, different performance modulation techniques, that is, mechanical-based, external fields-assisted, and particle beam irradiation-based methods, are discussed and compared in detail. Some of the newly proposed concepts are described. Thereafter, applications of vdW heterostructures with tailored properties are reviewed for the application prospects of the topic around this area. Moreover, the future research challenges and prospects are discussed, aiming at triggering more research interest and device applications around this topic.

Recent progress of heterostructures based on two dimensional materials and wide bandgap semiconductors

Recent progress in the synthesis and assembly of two-dimensional (2D) materials has laid the foundation for various applications of atomically thin layer films. These 2D materials possess rich and diverse properties such as layer-dependent band gaps, interesting spin degrees of freedom, and variable crystal structures. They exhibit broad application prospects in micro-nano devices. In the meantime, the wide bandgap semiconductors (WBS) with an elevated breakdown voltage, high mobility, and high thermal conductivity have shown important applications in high-frequency microwave devices, high-temperature and high-power electronic devices. Beyond the study on single 2D materials or WBS materials, the multi-functional 2D/WBS heterostructures can promote the carrier transport at the interface, potentially providing novel physical phenomena and applications, and improving the performance of electronic and optoelectronic devices. In this review, we overview the advantages of the heterostructures of 2D materials and WBS materials, and introduce the construction methods of 2D/WBS heterostructures. Then, we present the diversity and recent progress in the applications of 2D/WBS heterostructures, including photodetectors, photocatalysis, sensors, and energy related devices. Finally, we put forward the current challenges of 2D/WBS heterostructures and propose the promising research directions in the future.

Selenium-Doped Amorphous Black Phosphorus@TiO2/C Heterostructures for High-Performance Li/Na/K Ion Batteries

Heterostructures have been confirmed to demonstrate better electrochemical performance than their individual building blocks, which is not only attributed to the complementary advantages of diverse materials but also to various synergistic effects, such as increased active sites at the heterointerfaces, enhanced kinetics from a built-in electric field, stable structure due to physical or chemical bonding, etc. However, constructing a desired heterostructure remains greatly challenging owing to the mismatch of crystal structures, atomic spacings, and reaction mechanisms between different electrode materials. In this study, an amorphous heterostructure composed of Se-doped black phosphorus and metal-organic framework (MOF)-derived TiO₂/C (Se-BP@TiO₂/C) was successfully fabricated using a simple Se-assisted ball-milling method. In addition to the inherent advantages of heterostructures, the novel material also had considerable free volume in the amorphous domains, which not only buffered the volume change of active materials during cycles but also provided space and interconnected channels for ion diffusion. When used as anode materials for Li/Na/K ion batteries, the Se-BP@TiO₂/C achieved high specific capacities, good cyclability, and fast rate capability. This work opens up a new route to design amorphous heterostructure electrodes for high-performance battery systems.

Recent progress on transition metal diselenides from formation and modification to applications

The development of graphene promotes the research of similar two-dimensional (2D) materials, especially 2D transition metal dichalcogenides (TMDCs) with semiconductor properties. Monolayer or few-layer TMDCs have several advantages, such as direct band gap, weak interlayer van der Waals force, large interlayer spacing, and abundant marginal active sites, which make them widely used in catalysis, optoelectronics, as well as energy conversion and storage devices. In addition, transition metal diselenides (TMDSs) also possess many intriguing characteristics. For instance, transition metal diselenides (e.g., MoSe₂) have a more stable 1T phase, larger interlayer spacing, smaller band gap, and more obvious metallic property of Se than TMDCs (e.g., MoS₂). Thus, it has become one of the most attractive research topics branching out from TMDCs. Herein, this review unveils the structures, synthesis, properties, modifications, applications, and perspectives for TMDSs.

Electrostatic Field Enhanced Photocatalytic CO2 Conversion on BiVO4 Nanowires

The recombination loss of photo-carriers in photocatalytic systems fatally determines the energy conversion efficiency of photocatalysts. In this work, an electrostatic field was used to inhibit the recombination of photo-carriers in photocatalysts by separating photo-holes and photo-electrons in space. As a model structure, (010) facet-exposed BiVO₄ nanowires were grown on PDMS-insulated piezo-substrate of piezoelectric transducer (PZT). The PZT substrate will generate an electrostatic field under a certain stress, and the photocatalytic behavior of BiVO₄ nanowires is influenced by the electrostatic field. Our results showed that the photocatalytic performance of the BiVO₄ nanowires in CO₂ reduction in the negative electrostatic field is enhanced to 5.5-fold of that without electrostatic field. Moreover, the concentration of methane in the products was raised from 29% to 64%. The enhanced CO₂ reduction efficiency is mainly attributed to the inhibited recombination loss of photo-carriers in the BiVO₄ nanowires. The increased energy of photo-carriers and the enhanced surface absorption to polar molecules, which are CO in this case, were also play important roles in improving the photocatalytic activity of the photocatalyst and product selectivity. This work proposed an effective strategy to improve photo-carriers separation/transfer dynamics in the photocatalytic systems, which will also be a favorable reference for photovoltaic and photodetecting devices.

Modeling of High-Performance SPR Refractive Index Sensor Employing Novel 2D Materials for detection of Malaria Pathogens

The present work exhibits a novel design of surface plasmon resonance (SPR) biosensor, which comprises CaF2 prism, TiO2, metal (Ag/Au), PtSe2, 2D materials (graphene/ transition metal dichalcogenides (MoS2/WS2)) and sensing medium, for point-of-care detection of various stages of malaria diseases. The transfer matrix method (TMM) is employed to examine the angular reflectivity of the proposed structure after judiciously optimizing the layer thicknesses and layer numbers. Phase interrogation technique is utilised to validate the position of occurrence of resonance angles. Additionally, the proposed SPR structure is designed using COMSOL Multiphysics, to assay the electric field intensity and electric field enhancement factor near the edge of 2D material-sensing layer interface. Simulation upshots revealed that the use of new class of 2D materials catapult the sensor performance to a new height compared to the traditional SPR configuration. A maximum sensitivity of 240.10°/RIU, quality factor of 78.46 RIU-1 and detection accuracy of 1.99 is attained for Ag-based SPR configuration with bilayer of WS2. Sensing parameters are compared with previously reported works to prove the superiority of the present research. Moreover, the real-time and label-free detection of malaria diseases makes the suggested sensor worth to fabricate as a SPR chip with the recent nanofabrication technologies.

Geometric Frustration and Long-Range Ordering Induced by Surface Pressure Oscillation in a Langmuir-Blodgett Monolayer of Magnetic Soft Spheres

As a step toward the bottom-up construction of magnonic systems, this paper demonstrates the use of a large-amplitude surface-pressure annealing technique to generate 2-D order in a Langmuir-Blodgett monolayer of magnetic soft spheres comprising a surfactant-encapsulated polyoxometalate. The films show a distorted square lattice interpreted as due to geometric frustration caused by 2-D confinement between soft walls, one being the air interface and the other the aqueous subphase. Hysteresis and relaxation phenomena in the 2-D layers are suggested to be due to folding and time-dependent interpenetration of surfactant chains.

Criteria for Assessing the Interlayer Coupling of van der Waals Heterostructures Using Ultrafast Pump-Probe Photoluminescence Spectroscopy

van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDCs) provide an excellent paradigm for next-generation electronic and optoelectronic applications. However, the reproducible fabrications of vdW heterostructure devices and the boosting of practical applications are severely hindered by their unstable performance, due to the lack of criteria to assess the interlayer coupling in heterostructures. Here we propose a physical model involving ultrafast electron transfer in the heterostructures and provide two criteria, η (the ratio of the transferred electrons to the total excited electrons) and ζ (the relative photoluminescence variation), to evaluate the interlayer coupling by considering the electron transfer in TMDC heterostructures and numerically simulating the corresponding rate equations. We have proved the effectiveness and robustness of two criteria by measuring the pump-probe photoluminescence intensity of monolayer WS₂ in the WS₂/WSe₂ heterostructures. During thermal annealing of WS₂/WSe₂, ζ varies from negative to positive values and η changes between 0 and 4.5 × 10^-3 as the coupling strength enhanced; both of them can well characterize the tuning of interlayer coupling. We also design a scheme to image the interlayer coupling by performing PL imaging at two time delays. Our scheme offers powerful criteria to assess the interlayer coupling in TMDC heterostructures, offering opportunities for the implementation of vdW heterostructures for broadband and high-performance electronic and optoelectronic applications.

Facile Preparation of MoS2 Nanocomposites for Efficient Potassium-Ion Batteries by Grinding-Promoted Intercalation Exfoliation

Efficient exfoliations of bulk molybdenum disulfide (MoS₂ ) into few-layered nanosheets in pure phase are highly attractive because of the promising applications of the resulted 2D materials in diversified optoelectronic devices. Here, a new exfoliation method is presented to prepare semiconductive 2D hexagonal phase (2H phase) MoS₂ -cellulose nanocrystal (CNC) nanocomposites using grinding-promoted intercalation exfoliation (GPIE). This method with facile grinding of the bulk MoS₂ and CNC powder followed by conventional liquid-phase exfoliation in water can not only efficiently exfoliate 2H-MoS₂ nanosheets, but also produce the 2H-MoS₂ /CNC 2D nanocomposites simultaneously. Interestingly, the intercalated CNC sandwiched in MoS₂ nanosheets increases the interlayer spacing of 2H-MoS₂ , providing perfect conditions to accommodate the large-sized ions. Therefore, these nanocomposites are good anode materials of potassium-ion batteries (KIBs), showing a high reversible capacity of 203 mAh g^-1 at 200 mA g^-1 after 300 cycles, a good reversible capacity of 114 mAh g^-1 at 500 mA g^-1 , and a low decay of 0.02% per cycle over 1500 cycles. With these impressive KIB performances, this efficient GPIE method will open up a new avenue to prepare pure-phase MoS₂ and promising 2D nanocomposites for high-performance device applications.

The Photodetectors Based on Lateral Monolayer MoS2/WS2 Heterojunctions

Monolayer transition metal dichalcogenides (TMDs) show promising potential for next-generation optoelectronics due to excellent light capturing and photodetection capabilities. Photodetectors, as important components of sensing, imaging and communication systems, are able to perceive and convert optical signals to electrical signals. Herein, the large-area and high-quality lateral monolayer MoS2/WS2 heterojunctions were synthesized via the one-step liquid-phase chemical vapor deposition approach. Systematic characterization measurements have verified good uniformity and sharp interfaces of the channel materials. As a result, the photodetectors enhanced by the photogating effect can deliver competitive performance, including responsivity of ~ 567.6 A/W and detectivity of ~ 7.17 × 1011 Jones. In addition, the 1/f noise obtained from the current power spectrum is not conductive to the development of photodetectors, which is considered as originating from charge carrier trapping/detrapping. Therefore, this work may contribute to efficient optoelectronic devices based on lateral monolayer TMD heterostructures.

Emerging of Heterostructure Materials in Energy Storage: A Review

With the ever-increasing adaption of large-scale energy storage systems and electric devices, the energy storage capability of batteries and supercapacitors has faced increased demand and challenges. The electrodes of these devices have experienced radical change with the introduction of nano-scale materials. As new generation materials, heterostructure materials have attracted increasing attention due to their unique interfaces, robust architectures, and synergistic effects, and thus, the ability to enhance the energy/power outputs as well as the lifespan of batteries. In this review, the recent progress in heterostructure from energy storage fields is summarized. Specifically, the fundamental natures of heterostructures, including charge redistribution, built-in electric field, and associated energy storage mechanisms, are summarized and discussed in detail. Furthermore, various synthesis routes for heterostructures in energy storage fields are roundly reviewed, and their advantages and drawbacks are analyzed. The superiorities and current achievements of heterostructure materials in lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), lithium-sulfur batteries (Li-S batteries), supercapacitors, and other energy storage devices are discussed. Finally, the authors conclude with the current challenges and perspectives of the heterostructure materials for the fields of energy storage.

Moiré superlattices and related moiré excitons in twisted van der Waals heterostructures

Recent advances in moiré superlattices and moiré excitons, such as quantum emission arrays, low-energy flat bands, and Mott insulators, have rapidly attracted attention in the fields of optoelectronics, materials, and energy research. The interlayer twist turns into a degree of freedom that alters the properties of the systems of materials, and the realization of moiré excitons also offers the feasibility of making artificial exciton crystals. Moreover, moiré excitons exhibit many exciting properties under the regulation of various external conditions, including spatial polarisation, alternating dipolar to alternating dipolar moments and gate-dependence to gate voltage dependence; all are pertinent to their applications in nano-photonics and quantum information. But the lag in theoretical development and the low-efficiency of processing technologies significantly limit the potential of moiré superlattice applications. In this review, we systematically summarise and discuss the recent progress in moiré superlattices and moiré excitons, and analyze the current challenges, and put forward relevant recommendations. There is no doubt that further research will lead to breakthroughs in their application and promote reforms and innovations in traditional solid-state physics and materials science.

2D Semiconductor Nanomaterials and Heterostructures: Controlled Synthesis and Functional Applications

Two-dimensional (2D) semiconductors beyond graphene represent the thinnest stable known nanomaterials. Rapid growth of their family and applications during the last decade of the twenty-first century have brought unprecedented opportunities to the advanced nano- and opto-electronic technologies. In this article, we review the latest progress in findings on the developed 2D nanomaterials. Advanced synthesis techniques of these 2D nanomaterials and heterostructures were summarized and their novel applications were discussed. The fabrication techniques include the state-of-the-art developments of the vapor-phase-based deposition methods and novel van der Waals (vdW) exfoliation approaches for fabrication both amorphous and crystalline 2D nanomaterials with a particular focus on the chemical vapor deposition (CVD), atomic layer deposition (ALD) of 2D semiconductors and their heterostructures as well as on vdW exfoliation of 2D surface oxide films of liquid metals.

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.

Evolution of low-dimensional material-based field-effect transistors

Field-effect transistors (FETs) have tremendous applications in the electronics industry due to their outstanding features such as small size, easy fabrication, compatibility with integrated electronics, high sensitivity, rapid detection and easy measuring procedures. However, to meet the increasing demand of the electronics industry, efficient FETs with controlled short channel effects, enhanced surface stability, reduced size, and superior performances based on low-dimensional materials are desirable. In this review, we present the developmental roadmap of FETs from conventional to miniaturized devices and highlight their prospective applications in the field of optoelectronic devices. Initially, a detailed study of the general importance of bulk and low-dimensional materials is presented. Then, recent advances in low-dimensional material heterostructures, classification of FETs, and the applications of low-dimensional materials in field-effect transistors and photodetectors are presented in detail. In addition, we also describe current issues in low-dimensional material-based FETs and propose potential approaches to address these issues, which are crucial for developing electronic and optoelectronic devices. This review will provide guidelines for low-dimensional material-based FETs with high performance and advanced applications in the future.

Interface chemistry of two-dimensional heterostructures - fundamentals to applications

Two-dimensional heterostructures (2D HSs) have emerged as a new class of materials where dissimilar 2D materials are combined to synergise their advantages and alleviate shortcomings. Such a combination of dissimilar components into 2D HSs offers fascinating properties and intriguing functionalities attributed to the newly formed heterointerface of constituent components. Understanding the nature of the surface and the complex heterointerface of HSs at the atomic level is crucial for realising the desired properties, designing innovative 2D HSs, and ultimately unlocking their full potential for practical applications. Therefore, this review provides the recent progress in the field of 2D HSs with a focus on the discussion of the fundamentals and the chemistry of heterointerfaces based on van der Waals (vdW) and covalent interactions. It also explains the challenges associated with the scalable synthesis and introduces possible methodologies to produce large quantities with good control over the heterointerface. Subsequently, it highlights the specialised characterisation techniques to reveal the heterointerface formation, chemistry and nature. Afterwards, we give an overview of the role of 2D HSs in various emerging applications, particularly in high-power batteries, bifunctional catalysts, electronics, and sensors. In the end, we present conclusions with the possible solutions to the associated challenges with the heterointerfaces and potential opportunities that can be adopted for innovative applications.