2D Organic Materials for Optoelectronic Applications

The remarkable merits of 2D materials with atomically thin structures and optoelectronic attributes have inspired great interest in integrating 2D materials into electronics and optoelectronics. Moreover, as an emerging field in the 2D‐materials family, assembly of organic nanostructures into 2D forms offers the advantages of molecular diversity, intrinsic flexibility, ease of processing, light weight, and so on, providing an exciting prospect for optoelectronic applications. Herein, the applications of organic 2D materials for optoelectronic devices are a main focus. Material examples include 2D, organic, crystalline, small molecules, polymers, self‐assembly monolayers, and covalent organic frameworks. The protocols for 2D‐organic‐crystal‐fabrication and ‐patterning techniques are briefly discussed, then applications in optoelectronic devices are introduced in detail. Overall, an introduction to what is known and suggestions for the potential of many exciting developments are presented.     

Thermomechanical Nanocutting of 2D Materials

Atomically thin materials, such as graphene and transition metal dichalcogenides, are promising candidates for future applications in micro/nanodevices and systems. For most applications, functional nanostructures have to be patterned by lithography. Developing lithography techniques for 2D materials is essential for system integration and wafer-scale manufacturing. Here, a thermomechanical indentation technique is demonstrated, which allows for the direct cutting of 2D materials using a heated scanning nanotip. Arbitrarily shaped cuts with a resolution of 20 nm are obtained in monolayer 2D materials, i.e., molybdenum ditelluride (MoTe₂), molybdenum disulfide (MoS₂), and molybdenum diselenide (MoSe₂), by thermomechanically cleaving the chemical bonds and by rapid sublimation of the polymer layer underneath the 2D material layer. Several micro/nanoribbon structures are fabricated and electrically characterized to demonstrate the process for device fabrication. The proposed direct nanocutting technique allows for precisely tailoring nanostructures of 2D materials with foreseen applications in the fabrication of electronic and photonic nanodevices.     

High‐Quality‐Factor Mid‐Infrared Toroidal Excitation in Folded 3D Metamaterials

With unusual electromagnetic radiation properties and great application potentials, optical toroidal moments have received increasing interest in recent years. 3D metamaterials composed of split ring resonators with specific orientations in micro‐/nanoscale are a perfect choice for toroidal moment realization in optical frequency considering the excellent magnetic confinement and quality factor, which, unfortunately, are currently beyond the reach of existing micro‐/nanofabrication techniques. Here, a 3D toroidal metamaterial operating in mid‐infrared region constructed by metal patterns and dielectric frameworks is designed, by which high‐quality‐factor toroidal resonance is observed experimentally. The toroidal dipole excitation is confirmed numerically and further demonstrated by phase analysis. Furthermore, the far‐field radiation intensity of the excited toroidal dipoles can be adjusted to be predominant among other multipoles by just tuning the incident angle. The related processing method expands the capability of focused ion beam folding technologies greatly, especially in 3D metamaterial fabrication, showing great flexibility and nanoscale controllability on structure size, position, and orientation.     

Micro/Nanoscale 3D Assembly by Rolling,Folding, Curving,and Buckling Approaches

The miniaturization of electronics has been an important topic of study for several decades. The established roadmaps following Moore's Law have encountered bottlenecks in recent years, as planar processing techniques are already close to their physical limits. To bypass some of the intrinsic challenges of planar technologies, more and more efforts have been devoted to the development of 3D electronics, through either direct 3D fabrication or indirect 3D assembly. Recent research efforts into direct 3D fabrication have focused on the development of 3D transistor technologies and 3D heterogeneous integration schemes, but these technologies are typically constrained by the accessible range of sophisticated 3D geometries and the complexity of the fabrication processes. As an alternative route, 3D assembly methods make full use of mature planar technologies to form predefined 2D precursor structures in the desired materials and sizes, which are then transformed into targeted 3D mesostructures by mechanical deformation. The latest progress in the area of micro/nanoscale 3D assembly, covering the various classes of methods through rolling, folding, curving, and buckling assembly, is discussed, focusing on the design concepts, principles, and applications of different methods, followed by an outlook on the remaining challenges and open opportunities.     

Small‐Scale Biological and Artificial Multidimensional Sensors for 3D Sensing

A vast majority of existing sub‐millimeter‐scale sensors have a planar, 2D geometry as a result of conventional top‐down lithographic procedures. However, 2D sensors often suffer from restricted sensing capability, allowing only partial measurements of 3D quantities. Here, nano/microscale sensors with different geometric (1D, 2D, and 3D) configurations are reviewed to introduce their advantages and limitations when sensing changes in quantities in 3D space. This Review categorizes sensors based on their geometric configuration and sensing capabilities. Among the sensors reviewed here, the 3D configuration sensors defined on polyhedral structures are especially advantageous when sensing spatially distributed 3D quantities. The nano‐ and microscale vertex configuration forming polyhedral structures enable full 3D spatial sensing due to orthogonally aligned sensing elements. Particularly, the cubic configuration leveraged in 3D sensors offers an array of diverse applications in the field of biosensing for micro‐organisms and proteins, optical metamaterials for invisibility cloaking, 3D imaging, and low‐power remote sensing of position and angular momentum for use in microbots. Here, various 3D sensors are compared to assess the advantages of their geometry and its impact on sensing mechanisms. 3D biosensors in nature are also explored to provide vital clues for the development of novel 3D sensors.     

Regular Aligned 1D Single‐Crystalline Supramolecular Arrays for Photodetectors

Solution‐processed semiconductor single‐crystal patterns possess unique advantages of large scale and low cost, leading to potential applications toward high‐performance optoelectronic devices. To integrate organic semiconductor micro/nanostructures into devices, various patterning techniques have been developed. However, previous patterning techniques suffer from trade‐offs between precision, scalability, crystallinity, and orientation. Herein, a patterning method is reported based on an asymmetric‐wettability micropillar‐structured template. Large‐scale 1D single‐crystalline supramolecular arrays with strict alignment, pure crystallographic orientation, and precise position can be obtained. The wettability difference between tops and sidewalls of micropillars gives rise to the confinement of organic solutions in discrete capillary tubes followed by dewetting and formation of capillary trailing. The capillary trailing enables unidirectional dewetting, regulated mass transport, and confined crystal growth. Owing to the high crystallinity and pure crystallographic orientation with Pt atomic chains parallel to the substrate, the photodetectors based on the 1D arrays exhibit improved responsivity. The work not only provides fundamental understanding on the patterning and crystallization of supramolecular structures but also develops a large‐scale assembly technique for patterning single‐crystalline micro/nanostructures.     

4D Biofabrication Using Shape‐Morphing Hydrogels

Despite the tremendous potential of bioprinting techniques toward the fabrication of highly complex biological structures and the flourishing progress in 3D bioprinting, the most critical challenge of the current approaches is the printing of hollow tubular structures. In this work, an advanced 4D biofabrication approach, based on printing of shape‐morphing biopolymer hydrogels, is developed for the fabrication of hollow self‐folding tubes with unprecedented control over their diameters and architectures at high resolution. The versatility of the approach is demonstrated by employing two different biopolymers (alginate and hyaluronic acid) and mouse bone marrow stromal cells. Harnessing the printing and postprinting parameters allows attaining average internal tube diameters as low as 20 µm, which is not yet achievable by other existing bioprinting/biofabrication approaches and is comparable to the diameters of the smallest blood vessels. The proposed 4D biofabrication process does not pose any negative effect on the viability of the printed cells, and the self‐folded hydrogel‐based tubes support cell survival for at least 7 d without any decrease in cell viability. Consequently, the presented 4D biofabrication strategy allows the production of dynamically reconfigurable architectures with tunable functionality and responsiveness, governed by the selection of suitable materials and cells.     

A 3D Self‐Shaping Strategy for Nanoresolution Multicomponent Architectures

3D printing or fabrication pursues the essential surface behavior manipulation of droplets or a liquid for rapidly and precisely constructing 3D multimaterial architectures. Further development of 3D fabrication desires a self‐shaping strategy that can heterogeneously integrate functional materials with disparate electrical or optical properties. Here, a 3D liquid self‐shaping strategy is reported for rapidly patterning materials over a series of compositions and accurately achieving micro‐ and nanoscale structures. The predesigned template selectively pins the droplet, and the surface energy minimization drives the self‐shaping processing. The as‐prepared 3D circuits assembled by silver nanoparticles carry a current of 208–448 µA at 0.01 V impressed voltage, while the 3D architectures achieved by two different quantum dots show noninterfering optical properties with feature resolution below 3 µm. This strategy can facilely fabricate micro‐nanogeometric patterns without a modeling program, which will be of great significance for the development of 3D functional devices.     

Stimulus Responsive 3D Assembly for Spatially Resolved Bifunctional Sensors

3D electronic/optoelectronic devices have shown great potentials for various applications due to their unique properties inherited not only from functional materials, but also from 3D architectures. Although a variety of fabrication methods including mechanically guided assembly have been reported, the resulting 3D devices show no stimuli‐responsive functions or are not free standing, thereby limiting their applications. Herein, the stimulus responsive assembly of complex 3D structures driven by temperature‐responsive hydrogels is demonstrated for applications in 3D multifunctional sensors. The assembly driving force, compressive buckling, arises from the volume shrinkage of the responsive hydrogel substrates when they are heated above the lower critical solution temperature. Driven by the compressive buckling force, the 2D‐formed membrane materials, which are pre‐defined and selectively bonded to the substrates, are then assembled to 3D structures. They include “tent,” “tower,” “two‐floor pavilion,” “dome,” “basket,” and “nested‐cages” with delicate geometries. Moreover, the demonstrated 3D bifunctional sensors based on laser induced graphene show capability of spatially resolved tactile sensing and temperature sensing. These multifunctional 3D sensors would open new applications in soft robotics, bioelectronics, micro‐electromechanical systems, and others.     

Development of Bioimplants with 2D, 3D,and 4D Additive Manufacturing Materials

Over the past 30 years, additive manufacturing (AM) has developed rapidly and has demonstrated great potential in biomedical applications. AM is a materials-oriented manufacturing technology, since the solidification mechanism, architecture resolution, post-treatment process, and functional application are based on the materials to be printed. However, 3D printable materials are still quite limited for the fabrication of bioimplants. In this work, 2D/3D AM materials for bioimplants are reviewed. Furthermore, inspired by Tai Chi, a simple yet novel soft/rigid hybrid 4D AM concept is advanced to develop complex and dynamic biological structures in the human body based on 4D printing hybrid ceramic precursor/ceramic materials that were previously developed by our group. With the development of multi-material printing technology, the development of bioimplants and soft/rigid hybrid biological structures with 2D/3D/4D AM materials can be anticipated.     

Fabrication and Applications of 3D Nanoarchitectures for Advanced Electrocatalysts and Sensors

For the last few decades, nanoscale materials and structures have been extensively studied and developed, making a huge impact on human sustainability. For example, the introduction of nanostructures has brought substantial development in electrocatalysts and optical sensing applications. However, there are still remaining challenges that need to be resolved to further improve their performance, reliability, and cost-effectiveness. Herein, long-range ordered 3D nanostructures and their design principles are introduced with an emphasis on electrocatalysts for energy conversion and plasmonic nanostructures for optical sensing. Among the various fabrication techniques, sequential solvent-injection-assisted nanotransfer printing is suggested as a practical fabrication platform for tunable long-range ordered 3D nanostructures composed of ultrahigh-resolution building blocks. Furthermore, the importance of understanding and controlling the 3D design parameters is discussed to realize more efficient energy conversion as well as effective surface-enhanced Raman spectroscopy analyses, suggesting new solutions for clean energy and healthcare issues.     

Parallel,Multi‐Material Electrohydrodynamic 3D Nanoprinting

Direct mass‐transfer via liquid nanodroplets is one of the most powerful approaches for additive micro/nanofabrication. Electrohydrodynamic (EHD) dispensing has made the delivery of nanosized droplets containing diverse materials a practical reality; however, in its serial form it has insufficient throughput for large‐area processing. Here, a parallel, nanoscale EHD method is developed that offers both improved productivity and material diversity in 3D nanoprinting. The method exploits a double‐barreled glass nanopipette filled with material inks to parallelize nanodripping ejections, enabling a dual 3D nanoprinting process. It is discovered that an unusual electric field distribution created by cross talk of neighboring pipette apertures can be used to steer the microscopic ejection paths of the ink at will, enabling on‐demand control over shape, placement, and material mixing in 3D printed nanostructures. After thorough characterizations of the printing conditions, the parallel fabrication of nanomeshes and nanowalls of silver, CdSe/ZnS quantum dots, and their composites, with programmed designs is demonstrated. This method is expected to advance productivity in the heterogeneous integration of functional 3D nanodevices in a facile manner.     

Microfluidic Platforms toward Rational Material Fabrication for Biomedical Applications

The emergence of micro/nanomaterials in recent decades has brought promising alternative approaches in various biomedicine‐related fields such as pharmaceutics, diagnostics, and therapeutics. These micro/nanomaterials for specific biomedical applications shall possess tailored properties and functionalities that are closely correlated to their geometries, structures, and compositions, therefore placing extremely high demands for manufacturing techniques. Owing to the superior capabilities in manipulating fluids and droplets at microscale, microfluidics has offered robust and versatile platform technologies enabling rational design and fabrication of micro/nanomaterials with precisely controlled geometries, structures and compositions in high throughput manners, making them excellent candidates for a variety of biomedical applications. This review briefly summarizes the progress of microfluidics in the fabrication of various micro/nanomaterials ranging from 0D (particles), 1D (fibers) to 2D/3D (film and bulk materials) materials with controllable geometries, structures, and compositions. The applications of these microfluidic‐based materials in the fields of diagnostics, drug delivery, organs‐on‐chips, tissue engineering, and stimuli‐responsive biodevices are introduced. Finally, an outlook is discussed on the future direction of microfluidic platforms for generating materials with superior properties and on‐demand functionalities. The integration of new materials and techniques with microfluidics will pave new avenues for preparing advanced micro/nanomaterials with enhanced performance for biomedical applications.     

Atomic Layer Deposition for Membranes,Metamaterials, and Mechanisms

Bending and folding techniques such as origami and kirigami enable the scale‐invariant design of 3D structures, metamaterials, and robots from 2D starting materials. These design principles are especially valuable for small systems because most micro‐ and nanofabrication involves lithographic patterning of planar materials. Ultrathin films of inorganic materials serve as an ideal substrate for the fabrication of flexible microsystems because they possess high intrinsic strength, are not susceptible to plasticity, and are easily integrated into microfabrication processes. Here, atomic layer deposition (ALD) is employed to synthesize films down to 2 nm thickness to create membranes, metamaterials, and machines with micrometer‐scale dimensions. Two materials are studied as model systems: ultrathin SiO₂ and Pt. In this thickness limit, ALD films of these materials behave elastically and can be fabricated with fJ‐scale bending stiffnesses. Further, ALD membranes are utilized to design micrometer‐scale mechanical metamaterials and magnetically actuated 3D devices. These results establish thin ALD films as a scalable basis for micrometer‐scale actuators and robotics.     

Directional Photofluidization Lithography: Micro/Nanostructural Evolution by Photofluidic Motions of Azobenzene Materials

This review demonstrates directional photofluidization lithography (DPL), which makes it possible to fabricate a generic and sophisticated micro/nanoarchitecture that would be difficult or impossible to attain with other methods. In particular, DPL differs from many of the existing micro/nanofabrication methods in that the post‐treatment (i.e., photofluidization), after the preliminary fabrication process of the original micro/nanostructures, plays a pivotal role in the various micro/nanostructural evolutions including the deterministic reshaping of architectures, the reduction of structural roughness, and the dramatic enhancement of pattern resolution. Also, DPL techniques are directly compatible with a parallel and scalable micro/nanofabrication. Thus, DPL with such extraordinary advantages in micro/nanofabrication could provide compelling opportunities for basic micro/nanoscale science as well as for general technology applications.     

Design,Fabrication, and Mechanics of 3D Micro‐/Nanolattices

Over the past several decades, lattice materials have been developed and used as engineering materials for lightweight and stiff industrial structures. Recent advances in additive manufacturing techniques have prompted the emergence of architected materials with minimum characteristic sizes ranging from several micrometers to hundreds of nanometers. Taking advantage of the topological design, structural optimization, and size effects of nanomaterials, various 3D micro‐/nanolattice materials composed of different materials exhibit combinations of superior mechanical properties, such as low density, high strength (even approaching the theoretical limits), large deformability, good recoverability, and flaw tolerance. As a result, some micro‐/nanolattices occupy an unprecedented area in Ashby charts with a combination of different material properties. Here, recent advances in the fabrication and mechanics of micro‐/nanolattices are described. First, various design principles and advanced techniques used for the fabrication of micro‐/nanolattices are summarized. Then, the mechanical behaviors and properties of micro‐/nanolattices are further described, including the compressive Young's modulus, strength, energy absorption, recoverability, and tensile behavior, with an emphasis on mechanistic insights and origins. Finally, the main challenges in the fabrication and mechanics of micro‐/nanolattices are addressed and an outlook for further investigations and potential applications of micro‐/nanolattices in the future is provided.     

Progression in the Fountain Pen Approach: From 2D Writing to 3D Free‐Form Micro/Nanofabrication

The fountain pen approach, as a means for transferring materials to substrates, has shown numerous incarnations in recent years for creating 2D micro/nanopatterns and even generating 3D free‐form nanostructures using a variety of material “inks”. While the idea of filled reservoirs used to deliver material to a substrate via a capillary remains unchanged since antiquity, the advent of precise micromanipulation systems and functional material “inks” allows the extension of this mechanism to more high‐tech applications. Herein, the recent growth in meniscus guided fountain pen approaches for benchtop micro/nanofabrication, which has occurred in the last decade, is discussed. Particular attention is given to the theory, equipment, and experimentation encompassing this unique direct writing approach. A detailed exploration of the diverse ink systems and functional device applications borne from this strategy is put forth to reveal its rapid expansion to a broad range of scientific and engineering disciplines. As such, this informative review is provided for researchers considering adoption of this recent advancement of a familiar technology.     

Direct Imprinting of Quasi-3D Nanophotonic Structures into Colloidal Quantum-Dot Devices

Three-dimensional (3D) subwavelength nanostructures have emerged and triggered tremendous excitement because of their advantages over the two-dimensional (2D) counterparts in fields of plasmonics, photonic crystals, and metamaterials. However, the fabrication and integration of 3D nanophotonic structures with colloidal quantum dots (CQDs) faces several technological obstacles, as conventional lithographic and etching techniques may affect the surface chemistry of colloidal nanomaterials. Here, the direct fabrication of functional quasi-3D nanophotonic structures into CQD films is demonstrated by one-step imprinting with well-controlled precision in both vertical and lateral directions. To showcase the potential of this technique, diffraction gratings, bilayer wire-grid polarizers, and resonant metal mesh long-pass filters are imprinted on CQD films without degrading the optical and electrical properties of CQD. Furthermore, a dual-diode CQD detector into an unprecedented mid-wave infrared two-channel polarization detector is functionalized by embedding an imprinted bilayer wire-grid polarizer within the CQDs. The results show that this approach offers a feasible pathway to combine quasi-3D nanostructures with colloidal materials-based optoelectronics and access a new level of light manipulation.     

Assembly and Self‐Assembly of Nanomembrane Materials—From 2D to 3D

Nanoscience and nanotechnology offer great opportunities and challenges in both fundamental research and practical applications, which require precise control of building blocks with micro/nanoscale resolution in both individual and mass‐production ways. The recent and intensive nanotechnology development gives birth to a new focus on nanomembrane materials, which are defined as structures with thickness limited to about one to several hundred nanometers and with much larger (typically at least two orders of magnitude larger, or even macroscopic scale) lateral dimensions. Nanomembranes can be readily processed in an accurate manner and integrated into functional devices and systems. In this Review, a nanotechnology perspective of nanomembranes is provided, with examples of science and applications in semiconductor, metal, insulator, polymer, and composite materials. Assisted assembly of nanomembranes leads to wrinkled/buckled geometries for flexible electronics and stacked structures for applications in photonics and thermoelectrics. Inspired by kirigami/origami, self‐assembled 3D structures are constructed via strain engineering. Many advanced materials have begun to be explored in the format of nanomembranes and extend to biomimetic and 2D materials for various applications. Nanomembranes, as a new type of nanomaterials, allow nanotechnology in a controllable and precise way for practical applications and promise great potential for future nanorelated products.     

Freestanding 3D Mesostructures,Functional Devices,and Shape‐Programmable Systems Based on Mechanically Induced Assembly with Shape Memory Polymers

Capabilities for controlled formation of sophisticated 3D micro/nanostructures in advanced materials have foundational implications across a broad range of fields. Recently developed methods use stress release in prestrained elastomeric substrates as a driving force for assembling 3D structures and functional microdevices from 2D precursors. A limitation of this approach is that releasing these structures from their substrate returns them to their original 2D layouts due to the elastic recovery of the constituent materials. Here, a concept in which shape memory polymers serve as a means to achieve freestanding 3D architectures from the same basic approach is introduced, with demonstrated ability to realize lateral dimensions, characteristic feature sizes, and thicknesses as small as ≈500, 10, and 5 µm simultaneously, and the potential to scale to much larger or smaller dimensions. Wireless electronic devices illustrate the capacity to integrate other materials and functional components into these 3D frameworks. Quantitative mechanics modeling and experimental measurements illustrate not only shape fixation but also capabilities that allow for structure recovery and shape programmability, as a form of 4D structural control. These ideas provide opportunities in fields ranging from micro‐electromechanical systems and microrobotics, to smart intravascular stents, tissue scaffolds, and many others.