Pen‐on‐Paper Approach Toward the Design of Universal Surface Enhanced Raman Scattering Substrates

The translation of a technology from the laboratory into the real world should meet the demand of economic viability and operational simplicity. Inspired by recent advances in conductive ink pens for electronic devices on paper, we present a “pen‐on‐paper” approach for making surface enhanced Raman scattering (SERS) substrates. Through this approach, no professional training is required to create SERS arrays on paper using an ordinary fountain pen filled with plasmonic inks comprising metal nanoparticles of arbitrary shape and size. We demonstrate the use of plasmonic inks made of gold nanospheres, silver nanospheres and gold nanorods, to write SERS arrays that can be used with various excitation wavelengths. The strong SERS activity of these features allowed us to reach detection limits down to 10 attomoles of dye molecules in a sample volume of 10 μL, depending on the excitation wavelength, dye molecule and type of nanoparticles. Furthermore, such simple substrates were applied to pesticide detection down to 20 ppb. This universal approach offers portable, cost effective fabrication of efficient SERS substrates at the point of care. This approach should bring SERS closer to the real world through ink cartridges to be fixed to a pen to create plasmonic sensors at will.     

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.     

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.     

High‐Performance Materials for 3D Printing in Chemical Synthesis Applications

3D printing has emerged as an enabling technology for miniaturization. High‐precision printing techniques such as stereolithography are capable of printing microreactors and lab‐on‐a‐chip devices for efficient parallelization of biological and biochemical reactions under reduced uptake of reactants. In the world of chemistry, however, up until now, miniaturization has played a minor role. The chemical and thermal stability of regular 3D printing resins is insufficient for sustaining the harsh conditions of chemical reactions. Novel material formulations that produce highly stable 3D‐printed chips are highly sought for bringing chemistry up‐to‐date on the development of miniaturization. In this work, a brief review of recent developments in highly stable materials for 3D printing is given. This work focuses on three highly stable 3D‐printable material systems: transparent silicate glasses, ceramics, and fluorinated polymers. It is further demonstrated that 3D printing is also a versatile technique for surface structuring of polymers to enhance their wetting performance. Such micro/nanostructuring is key to selectively wetting surface patterns that are versatile for chemical arrays and droplet synthesis.     

Progress in 3D Printing of Carbon Materials for Energy‐Related Applications

The additive‐manufacturing (AM) technique, known as three‐dimensional (3D) printing, has attracted much attention in industry and academia in recent years. 3D printing has been developed for a variety of applications. Printable inks are the most important component for 3D printing, and are related to the materials, the printing method, and the structures of the final 3D‐printed products. Carbon materials, due to their good chemical stability and versatile nanostructure, have been widely used in 3D printing for different applications. Good inks are mainly based on volatile solutions having carbon materials as fillers such as graphene oxide (GO), carbon nanotubes (CNT), carbon blacks, and solvent, as well as polymers and other additives. Studies of carbon materials in 3D printing, especially GO‐based materials, have been extensively reported for energy‐related applications. In these circumstances, understanding the very recent developments of 3D‐printed carbon materials and their extended applications to address energy‐related challenges and bring new concepts for material designs are becoming urgent and important. Here, recent developments in 3D printing of emerging devices for energy‐related applications are reviewed, including energy‐storage applications, electronic circuits, and thermal‐energy applications at high temperature. To close, a conclusion and outlook are provided, pointing out future designs and developments of 3D‐printing technology based on carbon materials for energy‐related applications and beyond.     

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.     

3D Printed Polymer Photodetectors

Extrusion‐based 3D printing, an emerging technology, has been previously used in the comprehensive fabrication of light‐emitting diodes using various functional inks, without cleanrooms or conventional microfabrication techniques. Here, polymer‐based photodetectors exhibiting high performance are fully 3D printed and thoroughly characterized. A semiconducting polymer ink is printed and optimized for the active layer of the photodetector, achieving an external quantum efficiency of 25.3%, which is comparable to that of microfabricated counterparts and yet created solely via a one‐pot custom built 3D‐printing tool housed under ambient conditions. The devices are integrated into image sensing arrays with high sensitivity and wide field of view, by 3D printing interconnected photodetectors directly on flexible substrates and hemispherical surfaces. This approach is further extended to create integrated multifunctional devices consisting of optically coupled photodetectors and light‐emitting diodes, demonstrating for the first time the multifunctional integration of multiple semiconducting device types which are fully 3D printed on a single platform. The 3D‐printed optoelectronic devices are made without conventional microfabrication facilities, allowing for flexibility in the design and manufacturing of next‐generation wearable and 3D‐structured optoelectronics, and validating the potential of 3D printing to achieve high‐performance integrated active electronic materials and devices.     

Solution Mask Liquid Lithography (SMaLL) for One‐Step,Multimaterial 3D Printing

A novel methodology for printing 3D objects with spatially resolved mechanical and chemical properties is reported. Photochromic molecules are used to control polymerization through coherent bleaching fronts, providing large depths of cure and rapid build rates without the need for moving parts. The coupling of these photoswitches with resin mixtures containing orthogonal photo‐crosslinking systems allows simultaneous and selective curing of multiple networks, providing access to 3D objects with chemically and mechanically distinct domains. The power of this approach is showcased through the one‐step fabrication of bioinspired soft joints and mechanically reinforced “brick‐and‐mortar” structures.     

Comparison between 0D and 1D approaches for mechanical dissipation measurement during fatigue tests

Fatigue in materials is generally associated with the production of heat, leading to the “self‐heating” of the tested material. The associated heat power density, named mechanical dissipation or intrinsic dissipation, can be deduced from the temperature changes captured on the tested specimen's surface by infrared thermography. When mechanical dissipation is spatially homogeneous in the tested specimen, the processing can be performed using a macroscopic approach, also named zero‐dimensional (0D) approach. The latter uses an averaged temperature over the whole specimen's measurement zone. The present study aims to analyse the error generated by the 0D approach in the assessment of mechanical dissipation. This error was measured with respect to a one‐dimensional (1D) approach, which is applicable for longitudinal specimens subjected to uniaxial loading. Experimental tests were performed on pure copper and acrylic glass. A model was also developed to analyse the influence of the material and of the heat exchanges with the specimen's environment. The results obtained show that the error generated by the 0D approach in mechanical dissipation measurement may not be negligible and that attention should be paid to the choice of approach for fatigue analysis.     

Biodegradable Electronic Systems in 3D,Heterogeneously Integrated Formats

Biodegradable electronic systems represent an emerging class of technology with unique application possibilities, from temporary biomedical implants to “green” consumer gadgets. This paper introduces materials and processing methods for 3D, heterogeneously integrated devices of this type, with various functional examples in sophisticated forms of silicon‐based electronics. Specifically, techniques for performing multilayer assembly by transfer printing and for fabricating layer‐to‐layer vias and interconnects by lithographic procedures serve as routes to biodegradable, 3D integrated circuits composed of functional building blocks formed using specialized approaches or sourced from commercial semiconductor foundries. Demonstration examples range from logic gates and analog circuits that undergo functional transformation by transience to systems that integrate multilayer resistive sensors for in situ, continuous electrical monitoring of the processes of transience. The results significantly expand the scope of engineering options for biodegradable electronics and other types of transient microsystem technologies.     

3D Printing of Liquid Crystal Elastomeric Actuators with Spatially Programed Nematic Order

Liquid crystal elastomers (LCEs) are soft materials capable of large, reversible shape changes, which may find potential application as artificial muscles, soft robots, and dynamic functional architectures. Here, the design and additive manufacturing of LCE actuators (LCEAs) with spatially programed nematic order that exhibit large, reversible, and repeatable contraction with high specific work capacity are reported. First, a photopolymerizable, solvent‐free, main‐chain LCE ink is created via aza‐Michael addition with the appropriate viscoelastic properties for 3D printing. Next, high operating temperature direct ink writing of LCE inks is used to align their mesogen domains along the direction of the print path. To demonstrate the power of this additive manufacturing approach, shape‐morphing LCEA architectures are fabricated, which undergo reversible planar‐to‐3D and 3D‐to‐3D′ transformations on demand, that can lift significantly more weight than other LCEAs reported to date.     

Folding 2D Structures into 3D Configurations at the Micro/Nanoscale: Principles,Techniques, and Applications

Compared to their 2D counterparts, 3D micro/nanostructures show larger degrees of freedom and richer functionalities; thus, they have attracted increasing attention in the past decades. Moreover, extensive applications of 3D micro/nanostructures are demonstrated in the fields of mechanics, biomedicine, optics, etc., with great advantages. However, the mainstream micro/nanofabrication technologies are planar ones; therefore, they cannot be used directly for the construction of 3D micro/nanostructures, making 3D fabrication at the micro/nanoscale a great challenge. A promising strategy to overcome this is to combine the state‐of‐the‐art planar fabrication techniques with the folding method to produce 3D structures. In this strategy, 2D components can be easily produced by traditional planar techniques, and then, 3D structures are constructed by folding each 2D component to specific orientations. In this way, not only will the advantages of existing planar techniques, such as high precision, programmable patterning, and mass production, be preserved, but the fabrication capability will also be greatly expanded without complex and expensive equipment modification/development. The goal here is to highlight the recent progress of the folding method from the perspective of principles, techniques, and applications, as well as to discuss the existing challenges and future prospectives.     

Confinement Catalysis with 2D Materials for Energy Conversion

The unique electronic and structural properties of 2D materials have triggered wide research interest in catalysis. The lattice of 2D materials and the interface between 2D covers and other substrates provide intriguing confinement environments for active sites, which has stimulated a rising area of “confinement catalysis with 2D materials.” Fundamental understanding of confinement catalysis with 2D materials will favor the rational design of high‐performance 2D nanocatalysts. Confinement catalysis with 2D materials has found extensive applications in energy‐related reaction processes, especially in the conversion of small energy‐related molecules such as O₂, CH₄, CO, CO₂, H₂O, and CH₃OH. Two representative strategies, i.e., 2D lattice‐confined single atoms and 2D cover‐confined metals, have been applied to construct 2D confinement catalytic systems with superior catalytic activity and stability. Herein, the recent advances in the design, applications, and structure–performance analysis of two 2D confinement catalytic systems are summarized. The different routes for tuning the electronic states of 2D confinement catalysts are highlighted and perspectives on confinement catalysis with 2D materials toward energy conversion and utilization in the future are provided.     

Strain Engineering of 2D Materials: Issues and Opportunities at the Interface

Triggered by the growing needs of developing semiconductor devices at ever‐decreasing scales, strain engineering of 2D materials has recently seen a surge of interest. The goal of this principle is to exploit mechanical strain to tune the electronic and photonic performance of 2D materials and to ultimately achieve high‐performance 2D‐material‐based devices. Although strain engineering has been well studied for traditional semiconductor materials and is now routinely used in their manufacturing, recent experiments on strain engineering of 2D materials have shown new opportunities for fundamental physics and exciting applications, along with new challenges, due to the atomic nature of 2D materials. Here, recent advances in the application of mechanical strain into 2D materials are reviewed. These developments are categorized by the deformation modes of the 2D material–substrate system: in‐plane mode and out‐of‐plane mode. Recent state‐of‐the‐art characterization of the interface mechanics for these 2D material–substrate systems is also summarized. These advances highlight how the strain or strain‐coupled applications of 2D materials rely on the interfacial properties, essentially shear and adhesion, and finally offer direct guidelines for deterministic design of mechanical strains into 2D materials for ultrathin semiconductor applications.     

Multifunctional Nanocomposites with High Strength and Capacitance Using 2D MXene and 1D Nanocellulose

The family of two‐dimensional (2D) metal carbides and nitrides, known as MXenes, are among the most promising electrode materials for supercapacitors thanks to their high metal‐like electrical conductivity and surface‐functional‐group‐enabled pseudocapacitance. A major drawback of these materials is, however, the low mechanical strength, which prevents their applications in lightweight, flexible electronics. A strategy of assembling freestanding and mechanically robust MXene (Ti₃C₂T_x ) nanocomposites with one‐dimensional (1D) cellulose nanofibrils (CNFs) from their stable colloidal dispersions is reported. The high aspect ratio of CNF (width of ≈3.5 nm and length reaching tens of micrometers) and their special interactions with MXene enable nanocomposites with high mechanical strength without sacrificing electrochemical performance. CNF loading up to 20%, for example, shows a remarkably high mechanical strength of 341 MPa (an order of magnitude higher than pristine MXene films of 29 MPa) while still maintaining a high capacitance of 298 F g^?1 and a high conductivity of 295 S cm^?1. It is also demonstrated that MXene/CNF hybrid dispersions can be used as inks to print flexible micro‐supercapacitors with precise dimensions. This work paves the way for fabrication of robust multifunctional MXene nanocomposites for printed and lightweight structural devices.     

3D Printing of Living Responsive Materials and Devices

3D printing has been intensively explored to fabricate customized structures of responsive materials including hydrogels, liquid‐crystal elastomers, shape‐memory polymers, and aqueous droplets. Herein, a new method and material system capable of 3D‐printing hydrogel inks with programed bacterial cells as responsive components into large‐scale (3 cm), high‐resolution (30 μm) living materials, where the cells can communicate and process signals in a programmable manner, are reported. The design of 3D‐printed living materials is guided by quantitative models that account for the responses of programed cells in printed microstructures of hydrogels. Novel living devices are further demonstrated, enabled by 3D printing of programed cells, including logic gates, spatiotemporally responsive patterning, and wearable devices.     

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.     

The Role of Liquid Ink Transport in the Direct Placement of Quantum Dot Emitters onto Sub‐Micrometer Antennas by Dip‐Pen Nanolithography

Dip‐pen nanolithography (DPN) is used to precisely position core/thick‐shell (“giant”) quantum dots (gQDs; ≥10 nm in diameter) exclusively on top of silicon nanodisk antennas (≈500 nm diameter pillars with a height of ≈200 nm), resulting in periodic arrays of hybrid nanostructures and demonstrating a facile integration strategy toward next‐generation quantum light sources. A three‐step reading‐inking‐writing approach is employed, where atomic force microscopy (AFM) images of the pre‐patterned substrate topography are used as maps to direct accurate placement of nanocrystals. The DPN “ink” comprises gQDs suspended in a non‐aqueous carrier solvent, o‐dichlorobenzene. Systematic analyses of factors influencing deposition rate for this non‐conventional DPN ink are described for flat substrates and used to establish the conditions required to achieve small (sub‐500 nm) feature sizes, namely: dwell time, ink‐substrate contact angle and ink volume. Finally, it is shown that the rate of solvent transport controls the feature size in which gQDs are found on the substrate, but also that the number and consistency of nanocrystals deposited depends on the stability of the gQD suspension. Overall, the results lay the groundwork for expanded use of nanocrystal liquid inks and DPN for fabrication of multi‐component nanostructures that are challenging to create using traditional lithographic techniques.     

Synergetic Behavior in 2D Layered Material/Complex Oxide Heterostructures

The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO₃, graphene/LaAlO₃/SrTiO₃, graphene/ferroelectric oxide, MoS₂/SrTiO₃, and FeSe/SrTiO₃ heterostructures include voltage scaling in field‐effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next‐generation optoelectronic devices.