International Journal of Extreme Manufacturing

Inspired by natural porous architectures, numerous attempts have been made to generate porous structures. Owing to the smooth surfaces, highly interconnected porous architectures, and mathematical controllable geometry features, triply periodic minimal surface (TPMS) is emerging as an outstanding solution to constructing porous structures in recent years. However, many advantages of TPMS are not fully utilized in current research. Critical problems of the process from design, manufacturing to applications need further systematic and integrated discussions. In this work, a comprehensive overview of TPMS porous structures is provided. In order to generate the digital models of TPMS, the geometry design algorithms and performance control strategies are introduced according to diverse requirements. Based on that, precise additive manufacturing methods are summarized for fabricating physical TPMS products. Furthermore, actual multidisciplinary applications are presented to clarify the advantages and further potential of TPMS porous structures. Eventually, the existing problems and further research outlooks are discussed.

The growing demand for electronic devices, smart devices, and the Internet of Things constitutes the primary driving force for marching down the path of decreased critical dimension and increased circuit intricacy of integrated circuits. However, as sub-10 nm high-volume manufacturing is becoming the mainstream, there is greater awareness that defects introduced by original equipment manufacturer components impact yield and manufacturing costs. The identification, positioning, and classification of these defects, including random particles and systematic defects, are becoming more and more challenging at the 10 nm node and beyond. Very recently, the combination of conventional optical defect inspection with emerging techniques such as nanophotonics, optical vortices, computational imaging, quantitative phase imaging, and deep learning is giving the field a new possibility. Hence, it is extremely necessary to make a thorough review for disclosing new perspectives and exciting trends, on the foundation of former great reviews in the field of defect inspection methods. In this article, we give a comprehensive review of the emerging topics in the past decade with a focus on three specific areas: (a) the defect detectability evaluation, (b) the diverse optical inspection systems, and (c) the post-processing algorithms. We hope, this work can be of importance to both new entrants in the field and people who are seeking to use it in interdisciplinary work.

Projection micro stereolithography (PμSL) is a high-resolution (up to 0.6 μm) 3D printing technology based on area projection triggered photopolymerization, and capable of fabricating complex 3D architectures covering multiple scales and with multiple materials. This paper reviews the recent development of the PμSL based 3D printing technologies, together with the related applications. It introduces the working principle, the commercialized products, and the recent multiscale, multimaterial printing capability of PμSL as well as some functional photopolymers that are suitable to PμSL. This review paper also summarizes a few typical applications of PμSL including mechanical metamaterials, optical components, 4D printing, bioinspired materials and biomedical applications, and offers perspectives on the directions of the further development of PμSL based 3D printing technology.

Achieving well-controlled directional steering of liquids is of great significance for both fundamental study and practical applications, such as microfluidics, biomedicine, and heat management. Recent advances allow liquids with different surface tensions to select their spreading directions on a same surface composed of macro ratchets with dual reentrant curvatures. Nevertheless, such intriguing directional steering function relies on 3D printed sophisticated structures and additional polishing process to eliminate the inevitable microgrooves-like surface deficiency generated from printing process, which increases the manufacturing complexity and severally hinders practical applications. Herein, we developed a simplified dual-scale structure that enables directional liquid steering via a straightforward 3D printing process without the need of any physical and chemical post-treatment. The dual-scale structure consists of macroscale tilt ratchet equipped with a reentrant tip and microscale grooves that decorated on the whole surface along a specific orientation. Distinct from conventional design requiring the elimination of microgrooves-like surface deficiency, we demonstrated that the microgrooves of dual-scale structure play a key role in delaying or promoting the local flow of liquids, tuning of which could even enable liquids select different spreading pathways. This study provides a new insight for developing surfaces with tunable multi-scale structures, and also advances our fundamental understanding of the interaction between liquid spreading dynamics and surface topography.

Highlights

• A new surface consists of periodically arranged dual-scale reentrant ratchets was developed to realize liquid directional steering, which was fabricated via a straightforward 3D printing process, eliminating the necessity of physical or chemical post-treatment in conventional design.

• Distinct from conventional design that requires the elimination of the microgrooves-like surface deficiency generated during 3D printing, we fabricated reentrant ratchets with tunable multi-scale topography by making use of layer-by-layer printing characteristics, which provide a new insight for developing versatile hierarchical structures.

• The microgrooves of dual-scale reentrant ratchets are found to promote or delay the local flow of liquids, tuning of which could even enable liquids select different pathways.

• The major synthetic strategies of van der Waals heterostructures based on 2D materials are introduced, including physical combination, gas-phase deposition, solvothermal synthesis and synchronous evolution.

• The fundamental principles, development trends, and strengths/weaknesses of each synthetic strategy are elaborated.

• The integration strategies of van der Waals heterostructures and their preferred applications are discussed, including electronics, photodetectors and energy-related applications.

• The potentials and challenges arising in the van der Waals heterostructures synthesis and applications are envisioned.

Abstract

Van der Waals heterostructures (vdWHs) are showing considerable potential in both fundamental exploration and practical applications. Built upon the synthetic successes of (two-dimensional) 2D materials, several synthetic strategies of vdWHs have been developed, allowing the convenient fabrication of diverse vdWHs with decent controllability, quality, and scalability. This review first summarizes the current state of the art in synthetic strategies of vdWHs, including physical combination, deposition, solvothermal synthesis, and synchronous evolution. Then three major applications and their representative vdWH devices have been reviewed, including electronics (tunneling field effect transistors and 2D contact), optoelectronics (photodetector), and energy conversion (electrocatalysts and metal ion batteries), to unveil the potentials of vdWHs in practical applications and provide the general design principles of functional vdWHs for different applications. Besides, moiré superlattices based on vdWHs are discussed to showcase the importance of vdWHs as a platform for novel condensed matter physics. Finally, the crucial challenges towards ideal vdWHs with high performance are discussed, and the outlook for future development is presented. By the systematical integration of synthetic strategies and applications, we hope this review can further light up the rational designs of vdWHs for emerging applications.

Additive manufacturing (AM) technology is considered one of the most promising manufacturing technologies in the aerospace and defense industries. However, AM components are known to have various internal defects, such as powder agglomeration, balling, porosity, internal cracks and thermal/internal stress, which can significantly affect the quality, mechanical properties and safety of final parts. Therefore, defect inspection methods are important for reducing manufactured defects and improving the surface quality and mechanical properties of AM components. This paper describes defect inspection technologies and their applications in AM processes. The architecture of defects in AM processes is reviewed. Traditional defect detection technology and the surface defect detection methods based on deep learning are summarized, and future aspects are suggested.

Reliable fabrication of micro/nanostructures with sub-10 nm features is of great significance for advancing nanoscience and nanotechnology. While the capability of current complementary metal-oxide semiconductor (CMOS) chip manufacturing can produce structures on the sub-10 nm scale, many emerging applications, such as nano-optics, biosensing, and quantum devices, also require ultrasmall features down to single digital nanometers. In these emerging applications, CMOS-based manufacturing methods are currently not feasible or appropriate due to the considerations of usage cost, material compatibility, and exotic features. Therefore, several specific methods have been developed in the past decades for different applications. In this review, we attempt to give a systematic summary on sub-10 nm fabrication methods and their related applications. In the first and second parts, we give a brief introduction of the background of this research topic and explain why sub-10 nm fabrication is interesting from both scientific and technological perspectives. In the third part, we comprehensively summarize the fabrication methods and classify them into three main approaches, including lithographic, mechanics-enabled, and post-trimming processes. The fourth part discusses the applications of these processes in quantum devices, nano-optics, and high-performance sensing. Finally, a perspective is given to discuss the challenges and opportunities associated with this research topic.

The exceptional physical properties and unique layered structure of two-dimensional (2D) materials have made this class of materials great candidates for applications in electronics, energy conversion/storage devices, nanocomposites, and multifunctional coatings, among others. At the center of this application space, mechanical properties play a vital role in materials design, manufacturing, integration and performance. The emergence of 2D materials has also sparked broad scientific inquiry, with new understanding of mechanical interactions between 2D structures and interfaces being of great interest to the community. Building on the dramatic expansion of recent research activities, here we review significant advances in the understanding of the elastic properties, in-plane failures, fatigue performance, interfacial shear/friction, and adhesion behavior of 2D materials. In this article, special emphasis is placed on some new 2D materials, novel characterization techniques and computational methods, as well as insights into deformation and failure mechanisms. A deep understanding of the intrinsic and extrinsic factors that govern 2D material mechanics is further provided, in the hopes that the community may draw design strategies for structural and interfacial engineering of 2D material systems. We end this review article with a discussion of our perspective on the state of the field and outlook on areas for future research directions.

Highlights

• The elastic properties, in-plane failures, fatigue performance, interfacial shear/friction, and adhesion behaviors of 2D materials are summarized.

• Novel characterization techniques and computational methods are discussed.

• Intrinsic and extrinsic factors that govern 2D material mechanics are further provided.

• The challenges and perspectives in the field of 2D materials mechanics are outlined.

Since the first report of amorphous In–Ga–Zn–O based thin film transistors, interest in oxide semiconductors has grown. They offer high mobility, low off-current, low process temperature, and wide flexibility for compositions and processes. Unfortunately, depositing oxide semiconductors using conventional processes like physical vapor deposition leads to problematic issues, especially for high-resolution displays and highly integrated memory devices. Conventional approaches have limited process flexibility and poor conformality on structured surfaces. Atomic layer deposition (ALD) is an advanced technique which can provide conformal, thickness-controlled, and high-quality thin film deposition. Accordingly, studies on ALD based oxide semiconductors have dramatically increased recently. Even so, the relationships between the film properties of ALD-oxide semiconductors and the main variables associated with deposition are still poorly understood, as are many issues related to applications. In this review, to introduce ALD-oxide semiconductors, we provide: (a) a brief summary of the history and importance of ALD-based oxide semiconductors in industry, (b) a discussion of the benefits of ALD for oxide semiconductor deposition (in-situ composition control in vertical distribution/vertical structure engineering/chemical reaction and film properties/insulator and interface engineering), and (c) an explanation of the challenging issues of scaling oxide semiconductors and ALD for industrial applications. This review provides valuable perspectives for researchers who have interest in semiconductor materials and electronic device applications, and the reasons ALD is important to applications of oxide semiconductors.

Highlights

• The history of oxide semiconductors and topics related to ALD-oxide semiconductors are reviewed.

• The benefits of ALD-oxide semiconductors for electronic devices applications are discussed.

• Challenging issues and the outlook for future development and scalability in industry are reviewed.

Freeform optics has become the most prominent element of the optics industry. Advanced freeform optical designs supplementary to ultra-precision manufacturing and metrology techniques have upgraded the lifestyle, thinking, and observing power of existing humans. Imaginations related to space explorations, portability, accessibility have also witnessed sensible in today's time with freeform optics. Present-day design methods and fabrications techniques applicable in the development of freeform optics and the market requirements are focussed and explained with the help of traditional and non-traditional optical applications. Over the years, significant research is performed in the emerging field of freeform optics, but no standards are established yet in terms of tolerances and definitions. We critically review the optical design methods for freeform optics considering the image forming and non-image forming applications. Numerous subtractive manufacturing technologies including figure correction methods and metrology have been developed to fabricate extreme modern freeform optics to satisfy the demands of various applications such as space, astronomy, earth science, defence, biomedical, material processing, surveillance, and many more. We described a variety of advanced technologies in manufacturing and metrology for novel freeform optics. Next, we also covered the manufacturing-oriented design scheme for advanced optics. We conclude this review with an outlook on the future of freeform optics design, manufacturing and metrology.

The exceptional physical properties and unique layered structure of two-dimensional (2D) materials have made this class of materials great candidates for applications in electronics, energy conversion/storage devices, nanocomposites, and multifunctional coatings, among others. At the center of this application space, mechanical properties play a vital role in materials design, manufacturing, integration and performance. The emergence of 2D materials has also sparked broad scientific inquiry, with new understanding of mechanical interactions between 2D structures and interfaces being of great interest to the community. Building on the dramatic expansion of recent research activities, here we review significant advances in the understanding of the elastic properties, in-plane failures, fatigue performance, interfacial shear/friction, and adhesion behavior of 2D materials. In this article, special emphasis is placed on some new 2D materials, novel characterization techniques and computational methods, as well as insights into deformation and failure mechanisms. A deep understanding of the intrinsic and extrinsic factors that govern 2D material mechanics is further provided, in the hopes that the community may draw design strategies for structural and interfacial engineering of 2D material systems. We end this review article with a discussion of our perspective on the state of the field and outlook on areas for future research directions.

Highlights

• The elastic properties, in-plane failures, fatigue performance, interfacial shear/friction, and adhesion behaviors of 2D materials are summarized.

• Novel characterization techniques and computational methods are discussed.

• Intrinsic and extrinsic factors that govern 2D material mechanics are further provided.

• The challenges and perspectives in the field of 2D materials mechanics are outlined.

• A novel NdFeB-Fe₃O₄ magnetic photosensitive resin comprising 20 wt.% solid loading of magnetic particles is developed to fabricate high-precision and ferromagnetic functional devices via micro-continuous liquid interface production (μCLIP) process.

• A penetration depth model is established to reveals the effect of particle size, solid loading, and absorbance on the curing characteristics of NdFeB-Fe₃O₄ magnetic photosensitive resin.

• Optimized resin (NdFeB:Fe₃O₄ = 1:1) is able to print centimeter-size samples with a sub-40 μm fine feature, reduced by 87% compared to existing hard magnetic photosensitive resin.

• Optimized resin (NdFeB:Fe₃O₄ = 1:1) exhibits significantly enhanced coercivity and remanence in comparison with existing soft magnetic photosensitive resins, showing by an increase of 24 times and 6 times, respectively.

Abstract

The development of projection-based stereolithography additive manufacturing techniques and magnetic photosensitive resins has provided a powerful approach to fabricate miniaturized magnetic functional devices with complex three-dimensional spatial structures. However, the present magnetic photosensitive resins face great challenges in the trade-off between high ferromagnetism and excellent printing quality. To address these challenges, we develop a novel NdFeB-Fe₃O₄ magnetic photosensitive resin comprising 20 wt.% solid loading of magnetic particles, which can be used to fabricate high-precision and ferromagnetic functional devices via micro-continuous liquid interface production process. This resin combining ferromagnetic NdFeB microparticles and strongly absorbing Fe₃O₄ nanoparticles is able to provide ferromagnetic capabilities and excellent printing quality simultaneously compared to both existing soft and hard magnetic photosensitive resins. The established penetration depth model reveals the effect of particle size, solid loading, and absorbance on the curing characteristics of magnetic photosensitive resin. A high-precision forming and ferromagnetic capability of the NdFeB-Fe₃O₄ magnetic photosensitive resin are comprehensively demonstrated. It is found that the photosensitive resin (NdFeB:Fe₃O₄ = 1:1) can print samples with sub-40 μm fine features, reduced by 87% compared to existing hard magnetic photosensitive resin, and exhibits significantly enhanced coercivity and remanence in comparison with existing soft magnetic photosensitive resins, showing by an increase of 24 times and 6 times, respectively. The reported NdFeB-Fe₃O₄ magnetic photosensitive resin is anticipated to provide a new functional material for the design and manufacture of next-generation micro-robotics, electromagnetic sensor, and magneto-thermal devices.

Direct ink writing (DIW) holds enormous potential in fabricating multiscale and multi-functional architectures by virtue of its wide range of printable materials, simple operation, and ease of rapid prototyping. Although it is well known that ink rheology and processing parameters have a direct impact on the resolution and shape of the printed objects, the underlying mechanisms of these key factors on the printability and quality of DIW technique remain poorly understood. To tackle this issue, we systematically analyzed the printability and quality through extrusion mechanism modeling and experimental validating. Hybrid non-Newtonian fluid inks were first prepared, and their rheological properties were measured. Then, finite element analysis of the whole DIW process was conducted to reveal the flow dynamics of these inks. The obtained optimal process parameters (ink rheology, applied pressure, printing speed, etc) were also validated by experiments where high-resolution (<100 μm) patterns were fabricated rapidly (>70 mm s⁻¹). Finally, as a process research demonstration, we printed a series of microstructures and circuit systems with hybrid inks and silver inks, showing the suitability of the printable process parameters. This study provides a strong quantitative illustration of the use of DIW for the high-speed preparation of high-resolution, high-precision samples.

Highlights

• A systematic approach was proposed to assess the printability of direct ink writing technology.

• The optimal printing parameters for direct ink writing were obtained based on the ink hydrodynamic analysis.

• Numerical calculations were conducted to further develop these concepts.

• The applicability of the printability parameters was verified through high-resolution printing experiments.

In order to mimic the natural heterogeneity of native tissue and provide a better microenvironment for cell culturing, multi-material bioprinting has become a common solution to construct tissue models in vitro. With the embedded printing method, complex 3D structure can be printed using soft biomaterials with reasonable shape fidelity. However, the current sequential multi-material embedded printing method faces a major challenge, which is the inevitable trade-off between the printed structural integrity and printing precision. Here, we propose a simultaneous multi-material embedded printing method. With this method, we can easily print firmly attached and high-precision multilayer structures. With multiple individually controlled nozzles, different biomaterials can be precisely deposited into a single crevasse, minimizing uncontrolled squeezing and guarantees no contamination of embedding medium within the structure. We analyse the dynamics of the extruded bioink in the embedding medium both analytically and experimentally, and quantitatively evaluate the effects of printing parameters including printing speed and rheology of embedding medium, on the 3D morphology of the printed filament. We demonstrate the printing of double-layer thin-walled structures, each layer less than 200 μm, as well as intestine and liver models with 5% gelatin methacryloyl that are crosslinked and extracted from the embedding medium without significant impairment or delamination. The peeling test further proves that the proposed method offers better structural integrity than conventional sequential printing methods. The proposed simultaneous multi-material embedded printing method can serve as a powerful tool to support the complex heterogeneous structure fabrication and open unique prospects for personalized medicine.

Highlights

• A simultaneous printing method is proposed for complex heterogeneous structures.

• Dynamics of the bioink within the embedding medium is analysed and justified.

• The fusion of filaments can be controlled without jeopardizing printing precision.

• Intestine and liver models are printed with 5% gelatin methacryloyl, and securely extracted.

• The printability, microstructural characteristics, phase transformation behaviors, and mechanical properties of LPBF-fabricated NiTi alloys are reviewed.

• Several common structures for LPBF-fabricated NiTi alloys are considered and their various fields of application are illustrated.

• The technical challenges, limitations, and development tendencies of LPBF-fabricated NiTi alloys are put forward.

Abstract

NiTi alloys have drawn significant attentions in biomedical and aerospace fields due to their unique shape memory effect (SME), superelasticity (SE), damping characteristics, high corrosion resistance, and good biocompatibility. Because of the unsatisfying processabilities and manufacturing requirements of complex NiTi components, additive manufacturing technology, especially laser powder bed fusion (LPBF), is appropriate for fabricating NiTi products. This paper comprehensively summarizes recent research on the NiTi alloys fabricated by LPBF, including printability, microstructural characteristics, phase transformation behaviors, lattice structures, and applications. Process parameters and microstructural features mainly influence the printability of LPBF-processed NiTi alloys. The phase transformation behaviors between austenite and martensite phases, phase transformation temperatures, and an overview of the influencing factors are summarized in this paper. This paper provides a comprehensive review of the mechanical properties with unique strain-stress responses, which comprise tensile mechanical properties, thermomechanical properties (e.g. critical stress to induce martensitic transformation, thermo-recoverable strain, and SE strain), damping properties and hardness. Moreover, several common structures (e.g. a negative Poisson's ratio structure and a diamond-like structure) are considered, and the corresponding studies are summarized. It illustrates the various fields of application, including biological scaffolds, shock absorbers, and driving devices. In the end, the paper concludes with the main achievements from the recent studies and puts forward the limitations and development tendencies in the future.

The exceptional physical properties and unique layered structure of two-dimensional (2D) materials have made this class of materials great candidates for applications in electronics, energy conversion/storage devices, nanocomposites, and multifunctional coatings, among others. At the center of this application space, mechanical properties play a vital role in materials design, manufacturing, integration and performance. The emergence of 2D materials has also sparked broad scientific inquiry, with new understanding of mechanical interactions between 2D structures and interfaces being of great interest to the community. Building on the dramatic expansion of recent research activities, here we review significant advances in the understanding of the elastic properties, in-plane failures, fatigue performance, interfacial shear/friction, and adhesion behavior of 2D materials. In this article, special emphasis is placed on some new 2D materials, novel characterization techniques and computational methods, as well as insights into deformation and failure mechanisms. A deep understanding of the intrinsic and extrinsic factors that govern 2D material mechanics is further provided, in the hopes that the community may draw design strategies for structural and interfacial engineering of 2D material systems. We end this review article with a discussion of our perspective on the state of the field and outlook on areas for future research directions.

Highlights

• The elastic properties, in-plane failures, fatigue performance, interfacial shear/friction, and adhesion behaviors of 2D materials are summarized.

• Novel characterization techniques and computational methods are discussed.

• Intrinsic and extrinsic factors that govern 2D material mechanics are further provided.

• The challenges and perspectives in the field of 2D materials mechanics are outlined.

• The printability, microstructural characteristics, phase transformation behaviors, and mechanical properties of LPBF-fabricated NiTi alloys are reviewed.

• Several common structures for LPBF-fabricated NiTi alloys are considered and their various fields of application are illustrated.

• The technical challenges, limitations, and development tendencies of LPBF-fabricated NiTi alloys are put forward.

Abstract

NiTi alloys have drawn significant attentions in biomedical and aerospace fields due to their unique shape memory effect (SME), superelasticity (SE), damping characteristics, high corrosion resistance, and good biocompatibility. Because of the unsatisfying processabilities and manufacturing requirements of complex NiTi components, additive manufacturing technology, especially laser powder bed fusion (LPBF), is appropriate for fabricating NiTi products. This paper comprehensively summarizes recent research on the NiTi alloys fabricated by LPBF, including printability, microstructural characteristics, phase transformation behaviors, lattice structures, and applications. Process parameters and microstructural features mainly influence the printability of LPBF-processed NiTi alloys. The phase transformation behaviors between austenite and martensite phases, phase transformation temperatures, and an overview of the influencing factors are summarized in this paper. This paper provides a comprehensive review of the mechanical properties with unique strain-stress responses, which comprise tensile mechanical properties, thermomechanical properties (e.g. critical stress to induce martensitic transformation, thermo-recoverable strain, and SE strain), damping properties and hardness. Moreover, several common structures (e.g. a negative Poisson's ratio structure and a diamond-like structure) are considered, and the corresponding studies are summarized. It illustrates the various fields of application, including biological scaffolds, shock absorbers, and driving devices. In the end, the paper concludes with the main achievements from the recent studies and puts forward the limitations and development tendencies in the future.

• Atomic layer deposition is versatile in precise design of heterostructure materials.

• Gas sensing performances of heterostructures engineered by ALD are reviewed.

• Future developments and challenges faced by ALD-designed materials are discussed.

Abstract

Atomic layer deposition (ALD) is a versatile technique to deposit metals and metal oxide sensing materials at the atomic scale to achieve improved sensor functions. This article reviews metals and metal oxide semiconductor (MOS) heterostructures for gas sensing applications in which at least one of the preparation steps is carried out by ALD. In particular, three types of MOS-based heterostructures synthesized by ALD are discussed, including ALD of metal catalysts on MOS, ALD of metal oxides on MOS and MOS core–shell (C–S) heterostructures. The gas sensing performances of these heterostructures are carefully analyzed and discussed. Finally, the further developments required and the challenges faced by ALD for the synthesis of MOS gas sensing materials are discussed.

Parts with high-quality freeform surfaces have been widely used in industries, which require strict quality control during the manufacturing process. Among all the industrial inspection methods, contact measurement with coordinate measuring machines or computer numerical control machine tool is a fundamental technique due to its high accuracy, robustness, and universality. In this paper, the existing research in the contact measurement field is systematically reviewed. First, different configurations of the measuring machines are introduced in detail, which may have influence on the corresponding sampling and inspection path generation criteria. Then, the entire inspection pipeline is divided into two stages, namely the pre-inspection and post-inspection stages. The typical methods of each sub-stage are systematically overviewed and classified, including sampling, accessibility analysis, inspection path generation, probe tip radius compensation, surface reconstruction, and uncertainty analysis. Apart from those classical research, the applications of the emerging deep learning technique in some specific tasks of measurement are introduced. Furthermore, some potential and promising trends are provided for future investigation.

Highlights

• A comprehensive review on the whole pipeline of contact measurement is provided.

• The application of deep learning technique in the contact measurement is presented.

• The potential research on free-form surface contact measurement is discussed.

• The major synthetic strategies of van der Waals heterostructures based on 2D materials are introduced, including physical combination, gas-phase deposition, solvothermal synthesis and synchronous evolution.

• The fundamental principles, development trends, and strengths/weaknesses of each synthetic strategy are elaborated.

• The integration strategies of van der Waals heterostructures and their preferred applications are discussed, including electronics, photodetectors and energy-related applications.

• The potentials and challenges arising in the van der Waals heterostructures synthesis and applications are envisioned.

Abstract

Van der Waals heterostructures (vdWHs) are showing considerable potential in both fundamental exploration and practical applications. Built upon the synthetic successes of (two-dimensional) 2D materials, several synthetic strategies of vdWHs have been developed, allowing the convenient fabrication of diverse vdWHs with decent controllability, quality, and scalability. This review first summarizes the current state of the art in synthetic strategies of vdWHs, including physical combination, deposition, solvothermal synthesis, and synchronous evolution. Then three major applications and their representative vdWH devices have been reviewed, including electronics (tunneling field effect transistors and 2D contact), optoelectronics (photodetector), and energy conversion (electrocatalysts and metal ion batteries), to unveil the potentials of vdWHs in practical applications and provide the general design principles of functional vdWHs for different applications. Besides, moiré superlattices based on vdWHs are discussed to showcase the importance of vdWHs as a platform for novel condensed matter physics. Finally, the crucial challenges towards ideal vdWHs with high performance are discussed, and the outlook for future development is presented. By the systematical integration of synthetic strategies and applications, we hope this review can further light up the rational designs of vdWHs for emerging applications.