Multiscale Graphene Topographies Programmed by Sequential Mechanical Deformation

Fully Integrated Multiplexed Wristwatch for Real-Time Monitoring of Electrolyte Ions in Sweat

Considerable progress has already been made in sweat sensors based on electrochemical methods to realize real-time monitoring of biomarkers. However, realizing long-term monitoring of multiple targets at the atomic level remains extremely challenging, in terms of designing stable solid contact (SC) interfaces and fully integrating multiple modules for large-scale applications of sweat sensors. Herein, a fully integrated wristwatch was designed using mass-manufactured sensor arrays based on hierarchical multilayer-pore cross-linked N-doped porous carbon coated by reduced graphene oxide (NPCs@rGO-950) microspheres with high hydrophobicity as core SC, and highly selective monitoring simultaneously for K+, Na+, and Ca2+ ions in human sweat was achieved, exhibiting near-Nernst responses almost without forming an interfacial water layer. Combined with computed tomography, solid-solid interface potential diffusion simulation results reveal extremely low interface diffusion potential and high interface capacitance (598 μF), ensuring the excellent potential stability, reversibility, repeatability, and selectivity of sensor arrays. The developed highly integrated-multiplexed wristwatch with multiple modules, including SC, sensor array, microfluidic chip, signal transduction, signal processing, and data visualization, achieved reliable real-time monitoring for K+, Na+, and Ca2+ ion concentrations in sweat. Ingenious material design, scalable sensor fabrication, and electrical integration of multimodule wearables lay the foundation for developing reliable sweat-sensing systems for health monitoring.

Delamination-Assisted Ultrafast Wrinkle Formation in a Freestanding Film

Freestanding films provide a versatile platform for materials engineering thanks to additional structural motifs not found in films with a substrate. A ubiquitous example is wrinkles, yet little is known about how they can develop over as fast as a few picoseconds due to a lack of experimental probes to visualize their dynamics in real time on the nanoscopic scale. Here, we use time-resolved electron diffraction to directly observe light-activated wrinkling formation in freestanding La2/3Ca1/3MnO3 films. Via a "lock-in" analysis of oscillations in the diffraction peak position, intensity, and width, we quantitatively reconstructed how wrinkles develop on the time scale of lattice vibration. Contrary to the common assumption of fixed boundary conditions, we found that wrinkle development is associated with ultrafast delamination at the film boundaries. Our work provides a generic protocol to quantify wrinkling dynamics in freestanding films and highlights the importance of the film-substrate interaction in determining the properties of freestanding structures.

Ferroelectric Domain and Switching Dynamics in Curved In2Se3: First-Principles and Deep Learning Molecular Dynamics Simulations

Despite its prevalence in experiments, the influence of complex strain on material properties remains understudied due to the lack of effective simulation methods. Here, the effects of bending, rippling, and bubbling on the ferroelectric domains are investigated in an In2Se3 monolayer by density functional theory and deep learning molecular dynamics simulations. Since the ferroelectric switching barrier can be increased (decreased) by tensile (compressive) strain, automatic polarization reversal occurs in α-In2Se3 with a strain gradient when it is subjected to bending, rippling, or bubbling deformations to create localized ferroelectric domains with varying sizes. The switching dynamics depends on the magnitude of curvature and temperature, following an Arrhenius-style relationship. This study not only provides a promising solution for cross-scale studies using deep learning but also reveals the potential to manipulate local polarization in ferroelectric materials through strain engineering.

Stretchable and Compliant Sensing of Strain, Pressure and Vibration of Soft Deformable Structures

Soft robotic and medical devices will greatly benefit from stretchable and compliant pressure sensors that can detect deformation and contact forces for control and task safety. In addition to traditional 2D buckling via planar substrates, 3D buckling via curved substrates has emerged as an alternative approach to generate tunable and highly convoluted hierarchical wrinkle morphologies. Such wrinkles may provide advantages in pressure sensing, such as increased sensitivity, ultra-stretchability, and detecting changing curvatures. In this work, we fabricated stretchable sensors using wrinkled MXene electrodes obtained from 3D buckling. We then characterized the sensors’ performance in detecting strain, pressure, and vibrations. The fabricated wrinkled MXene electrode exhibited high stretchability of up to 250% and has a strain sensitivity of 0.1 between 0 and 80%. The fabricated bilayer MXene pressure sensor exhibited a pressure sensitivity of 0.935 kPa−1 and 0.188 kPa−1 at the lower (<0.25 kPa) and higher-pressure regimes (0.25 kPa–2.0 kPa), respectively. The recovery and response timing of the wrinkled MXene pressure sensor was found to be 250 ms and 400 ms, respectively. The sensor was also capable of detecting changing curvatures upon mounting onto an inflating balloon.

Constructing Fast Transmembrane Pathways in a Layered Double Hydroxide Nanosheets/Nanoparticles Composite Film for an Inorganic Anion-Exchange Membrane

Anion-exchange membranes (AEMs) with high conductivity are crucial for realizing next-generation energy storage and conversion systems in an alkaline environment, promising a huge advantage in cost reduction without using precious platinum group metal catalysts. Layered double hydroxide (LDH) nanosheets, exhibiting a remarkably high hydroxide ion (OH^-) conductivity approaching 10^-1 S cm^-1 along the in-plane direction, may be regarded as an ideal candidate material for the fabrication of inorganic solid AEMs. However, two-dimensional anisotropy results in a substantially low conductivity of 10^-6 S cm^-1 along the cross-plane direction, which poses a hurdle to achieve fast ion conduction across the membrane comprising restacked nanosheets. In the present work, a composite membrane was prepared based on mixing/assembling micron-sized LDH nanosheets with nanosized LDH platelets (nanoparticles) via a facile vacuum filtration process. The hybridization with nanoparticles could alter the orientation of LDH nanosheets and reduce the restacking order, forming diversified fast ion-conducting pathways and networks in the composite membrane. As a result, the transmembrane conductivity significantly improved up to 1000-fold higher than that composed of restacked nanosheets only, achieving a high conductivity of 10^-2 to 10^-1 S cm^-1 in both in-plane and cross-plane directions.

Structural Integrity Preserving and Residue-Free Transfer of Large-Area Wrinkled Graphene onto Polymeric Substrates

Wrinkled graphene offers many advantageous features resulting from modifying the structural and physical properties as well as the chemical reactivity of graphene. However, its inadequate transferability to other substrates has limited its usability. This paper reports a roll-based clean transfer approach that enables the damage-free and contamination-free transfer of large-area wrinkled graphene onto polymeric substrates without compromising the integrity of wrinkle structures. The method implements the simultaneous imidazole-assisted etching and doping of chemical vapor-deposited graphene to fabricate multilayer graphene on a thermoplastic polystyrene (PS) substrate coated with a water-soluble poly(4-styrenesulfonic acid) (PSS) sacrificial layer via a roll-based transfer process. The compliant PSS layer affords the conformal contact between the PS substrate and graphene during the wrinkle formation process, enabling the controllable fabrication of graphene wrinkle structures on a large area. The water-soluble properties of PSS simplify the typically difficult separation of wrinkled graphene from the PS substrate after its transfer onto a target substrate. This improves the transferability of wrinkled graphene, rendering the transfer process solvent-free and residue-free. This work demonstrates the feasibility of the formulated method by transferring centimeter-scale wrinkled graphene onto currently used transparent flexible substrates (i.e., polyethylene terephthalate and polydimethylsiloxane). The results indicate that the transferred wrinkled graphene possesses the desirable combination of superior stretchability, optical transmittance, sheet resistance, and electromechanical stability, rendering its suitable application to transparent flexible and stretchable electronics.

Highly spatial imaging of electrochemical activity on the wrinkles of graphene using all-solid scanning electrochemical cell microscopy

Here, all-solid scanning electrochemical cell microscopy (SECCM) is first established by filling polyacrylamide (PAM) into nanocapillaries as a solid electrolyte. A solid PAM nanoball at the tip of a nanocapillary contacts graphene and behaves as an electrochemical cell for simultaneously measuring the morphology and electrochemical activity. Compared with liquid droplet-based SECCM, this solid nanoball is stable and does not leave any electrolyte at the contact regions, which permits accurate and continuous scanning of the surface without any intervals. Accordingly, the resolutions in the lateral (x-y) and vertical (z) directions are improved to ∼10 nm. The complete scanning of the wrinkles on graphene records low currents at the two sidewalls of the wrinkles and a relatively high current at the center of the wrinkles. The heterogeneity in the electrochemical activity of the wrinkle illustrates different electron transfer features on surfaces with varied curvatures, which is hardly observed by the current electrochemical or optical methods. The successful establishment of this high spatial electrochemical microscopy overcomes the current challenges in investigating the electrochemical activity of materials at the nanoscale, which is significant for a better understanding of electron transfer in materials.

Hierarchical Wrinkle-Structured Catalyst Layer/Membrane Interface for Ultralow Pt-Loading Polymer Electrolyte Membrane Fuel Cells (PEMFCs)

The optimal architecture of three-dimensional (3D) interface between a polymer electrolyte membrane (PEM) and catalyst layer (CL) is one of the most important issues to improve PEM fuel cells' (PEMFCs) performance. Here, we report the fabrication of hierarchical wrinkled PEM/CL interface over a large area. We fabricated the hierarchical wrinkles on a multiscale from nanometers to micrometers by bottom-up-based facile, scalable, and simple method. Notably, it allows one to go beyond the limit of the catalyst utilization by extremely enlarged interfacial area. The resulting hierarchical wrinkled PEM/CL displays a dramatically increased electrochemically active surface area (ECSA) and power performance by the enhancement factors of 89% and 67% compared with those of flat interface, which is one of the best enhancements compared to previous PEMFCs. We believe the scalability of hierarchical wrinkled interface can be exploited to design advanced 3D interfaces for high-performance PEMFCs even with ultralow Pt-loading.

High-speed graphene/InGaN heterojunction photodetectors for potential application in visible light communication

Due to the wavelength-selective absorption characteristic of indium gallium nitride (InGaN) ternary alloy, the InGaN-based photodetectors (PDs) show great potential as high signal-to-noise ratio (SNR) receivers in the visible light communication (VLC) system. However, the application of InGaN-based PDs with simple structure in the VLC system is limited by slow speed. Integration of graphene (Gr) with InGaN is an effective strategy for overcoming the limitation. Herein, we report on a high responsivity and fast response PDs based on Gr/InGaN heterojunctions. It finds that the three-layer Gr (T-Gr) can effectively improve the InGaN-based PDs photoelectric properties. The T-Gr/InGaN PDs show a high responsivity of 1.39 A/W@-3 V and a short rise/fall time of 60/200 µs, which are attributed to the combination of the high-quality InGaN epitaxial films and finite density of states of three-layer graphene. The fast response with high responsivity endows the T-Gr/InGaN PDs with great potential for selective detection of the VLC system.

Controlled Release of Molecular Intercalants from Two-Dimensional Nanosheet Films

Solution co-deposition of two-dimensional (2D) nanosheets with chemical solutes yields nanosheet-molecular heterostructures. A feature of these macroscopic layered hybrids is their ability to release the intercalated molecular agent to express chemical functionality on their surfaces or in their near surroundings. Systematic design methods are needed to control this molecular release to match the demand for rate and lifetime in specific applications. We hypothesize that release kinetics are controlled by transport processes within the layered solids, which primarily involve confined molecular diffusion through nanochannels formed by intersheet van der Waals gaps. Here a variety of graphene oxide (GO)/molecular hybrids are fabricated and subject to transient experiments to characterize release kinetics, locations, and mechanisms. The measured release rate profiles can be successfully described by a numerical model of internal transport processes, and the results used to extract effective Z-directional diffusion coefficients for various film types. The diffusion coefficients are found to be 8 orders of magnitude lower than those in free solution due to nanochannel confinement and serpentine path effects, and this retardation underlies the ability of 2D materials to control and extend release over useful time scales. In-plane texturing of the heterostructured films by compressive wrinkling or crumpling is shown to be a useful design tool to control the release rate for a given film type and molecular intercalant. The potential of this approach is demonstrated through case studies on the controlled release of chemical virucidal agents.

Ultrasensitive Detection of Dopamine, IL-6 and SARS-CoV-2 Proteins on Crumpled Graphene FET Biosensor

Universal platforms for biomolecular analysis using label-free sensing modalities can address important diagnostic challenges. Electrical field effect-sensors are an important class of devices that can enable point-of-care sensing by probing the charge in the biological entities. Use of crumpled graphene for this application is especially promising. It is previously reported that the limit of detection (LoD) on electrical field effect-based sensors using DNA molecules on the crumpled graphene FET (field-effect transistor) platform. Here, the crumpled graphene FET-based biosensing of important biomarkers including small molecules and proteins is reported. The performance of devices is systematically evaluated and optimized by studying the effect of the crumpling ratio on electrical double layer (EDL) formation and bandgap opening on the graphene. It is also shown that a small and electroneutral molecule dopamine can be captured by an aptamer and its conformation change induced electrical signal changes. Three kinds of proteins were captured with specific antibodies including interleukin-6 (IL-6) and two viral proteins. All tested biomarkers are detectable with the highest sensitivity reported on the electrical platform. Significantly, two COVID-19 related proteins, nucleocapsid (N-) and spike (S-) proteins antigens are successfully detected with extremely low LoDs. This electrical antigen tests can contribute to the challenge of rapid, point-of-care diagnostics.

Exceptional Elasticity of Microscale Constrained MoS2 Domes

The outstanding mechanical performances of two-dimensional (2D) materials make them appealing for the emerging fields of flextronics and straintronics. However, their manufacturing and integration in 2D crystal-based devices rely on a thorough knowledge of their hardness, elasticity, and interface mechanics. Here, we investigate the elasticity of highly strained monolayer-thick MoS₂ membranes, in the shape of micrometer-sized domes, by atomic force microscopy (AFM)-based nanoindentation experiments. A dome's crushing procedure is performed to induce a local re-adhesion of the dome's membrane to the bulk substrate under the AFM tip's load. It is worth noting that no breakage, damage, or variation in size and shape are recorded in 95% of the crushed domes upon unloading. Furthermore, such a procedure paves the way to address quantitatively the extent of the van der Waals interlayer interaction and adhesion of MoS₂ by studying pull-in instabilities and hysteresis of the loading-unloading cycles. The fundamental role and advantage of using a superimposed dome's constraint are also discussed.

Controlling Graphene Wrinkles through the Phase Transition of a Polymer with a Low Critical Solution Temperature

A novel method for controlling reduced graphene oxide (rGO) wrinkles through a phase transition in a solution using a low critical solution temperature (LCST) polymer dispersant has been developed. The polymer dispersant is designed by control of architecture and composition using reversible addition-fragmentation chain transfer polymerization. Synthesized poly(2-(dimethylaminoethyl) methacrylate-block-styrene) (PDbS) can be successfully functionalized on the rGO surface via noncovalent functionalization. PDbS-functionalized rGO (PDbS-rGO) exhibits good dispersibility in an aqueous phase at room temperature and forms wrinkles on the PDbS-rGO surface because of phase transition at the LCST of the polymer dispersant. The formation of PDbS-rGO wrinkles is controlled by varying the aggregation number of the polymer dispersant on the PDbS-rGO surface that strongly depends on temperature. This is confirmed by transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy (I_D' /I_G ratios are 0.560, 0.579, and 0.684, which correspond to 45, 70, and 95 °C, respectively). In addition, the mechanism of wrinkle control is proved by gold nanoparticles that are grown in polymer dispersant on the PDbS-rGO surface.

Micropatterns Fabricated by Photodimerization-Induced Diffusion

Pattern technology plays an important role in the generation of microstructures with different functionalities and morphologies. In this report, a straightforward and versatile strategy is presented for spatially regulating the growth of a microstructure on a surface by the photodimerization of maleimide (MI). Upon exposure of ultraviolet (UV) light, photodimerization of MI in a film comprising furan-grafted polymer and bismaleimide (BMI) produces a chemical gradient, which can drive the diffusion of BMI from the unexposed to the exposed region and from the bottom to the surface, resulting in the growth of micropatterns. Sequential crosslinking induced by the Diels-Alder reaction between MI and furan maintains the stability of pattern shape. Theoretical modeling with reaction-diffusion equations reveal that as photodimerization moves the system far from thermodynamic equilibrium, the formation of a chemical potential gradient requires the redistribution of matter, resulting in the formation of topographies. Directional molecular motion induced by UV light can generate complex morphology, and produce materials with unique optical functions, such as charming-ordered gratings. This straightforward method of fabricating micropatterns by photodimerization-induced diffusion is successfully applied to patterned curved surfaces, microfluidic channels and encapsulation of integrated light emitting diode chips.

Multi-Scale Structure-Mechanical Property Relations of Graphene-Based Layer Materials

Pristine graphene is one of the strongest materials known in the world, and may play important roles in structural and functional materials. In order to utilize the extraordinary mechanical properties in practical engineering structures, graphene should be assembled into macroscopic structures such as graphene-based papers, fibers, foams, etc. However, the mechanical properties of graphene-based materials such as Young’s modulus and strength are 1–2 orders lower than those of pristine monolayer graphene. Many efforts have been made to unveil the multi-scale structure–property relations of graphene-based materials with hierarchical structures spanning the nanoscale to macroscale, and significant achievements have been obtained to improve the mechanical performance of graphene-based materials through composition and structure optimization across multi-scale. This review aims at summarizing the currently theoretical, simulation, and experimental efforts devoted to the multi-scale structure–property relation of graphene-based layer materials including defective monolayer graphene, nacre-like and laminar nanostructures of multilayer graphene, graphene-based papers, fibers, aerogels, and graphene/polymer composites. The mechanisms of mechanical property degradation across the multi-scale are discussed, based on which some multi-scale optimization strategies are presented to further improve the mechanical properties of graphene-based layer materials. We expect that this review can provide useful insights into the continuous improvement of mechanical properties of graphene-based layer materials.

Controlling pore structure and conductivity in graphene nanosheet films through partial thermal exfoliation

Thermal exfoliation is an efficient and scalable method for the production of graphene nanosheets or nanoplatelets, which are typically re-assembled or blended to form new macroscopic "graphene-based materials". Thermal exfoliation can be applied to these macroscopic graphene-based materials after casting to create internal porosity, but this process variant has not been widely studied, and can easily lead to destruction of the physical form of the original cast body. Here we explore how the partial thermal exfoliation of graphene oxide (GO) multilayer nanosheet films can be used to control pore structure and electrical conductivity of planar, textured, and confined GO films. The GO films are shown to exfoliate explosively when the instrument-set heating rates are 100 K/min and above leading to complete destruction of the film geometry. Textured films with engineered micro-wrinkling and crumpling show similar thermal behavior to planar films. Here, we also demonstrate a novel method to produce fairly large size intact rGO films of high electrical conductivity and microporosity based on confinement. Sandwiching GO precursor films between inert plates during partial exfoliation at 250°C produces high conductivity and porosity material in the form of a flexible film that preserves the macroscopic structure of the original cast body.

An all-inorganic, fully dense, stretchable ceramic magnetic film

There is widespread interest in new materials-based approaches for introducing flexibility to electromagnetic devices, such as displays, human-machine interfaces, smart textiles, and biomedical implants. From fabrication to application, incorporating ceramic components is particularly challenging due to their extreme stiffness. Here, we introduce a new approach for designing flexible ceramic films and demonstrate it by fabricating fully dense, pre-wrinkled magnetic cobalt ferrite films composed of tiled nanoplatelets. The method relies on the colloidal engineering of metalized graphene nanosheets, which are cast and compressed into wrinkled composite films with accurate control of composition and morphology. Removal of the graphene template by thermal oxidation yields free-standing cobalt ferrite films that can be stretched up to 200% and bent to radii of 2.5 mm while maintaining their magnetic properties. Magnetization retention of 73% is documented after 150% linear mechanical stretching over 100 cycles. The significant stretchability and flexibility in this hard magnetic material is achieved at near full metal oxide crystal density without addition of significant void space or a polymeric elastomer matrix.

Controlling nanochannel orientation and dimensions in graphene-based nanofluidic membranes

There is great interest in exploiting van der Waals gaps in layered materials as nanofluidic channels. Graphene oxide (GO) nanosheets are known to spontaneously assemble into stacked planar membranes with transport properties that are highly selective to molecular structure. Use of conventional GO membranes in liquid-phase applications is often limited by low flux values, due to intersheet nanochannel alignment perpendicular to the desired Z-directional transport, which leads to circuitous fluid pathways that are orders of magnitude longer than the membrane thickness. Here we demonstrate an approach that uses compressive instability in Zr-doped GO thin films to create wrinkle patterns that rotate nanosheets to high angles. Capturing this structure in polymer matrices and thin sectioning produce fully dense membranes with arrays of near-vertically aligned nanochannels. These robust nanofluidic devices offer pronounced reduction in fluid path-length, while retaining the high selectivity for water over non-polar molecules characteristic of GO interlayer nanochannels.

Vertically stacked graphene oxide sheets are promising structures for molecular sieving technologies. By folding large planar sheets in an accordion-like manner, Liu et al. fabricate a thin robust filter with near-vertically aligned nanochannels geared towards commercial separation membranes.

Fundamental Insights into Graphene Strain Sensing

Graphene has been studied extensively for use in flexible electronics as ultrasensitive and wide-area strain sensors. Many sensors demonstrated so far rely on graphene networks, such that the spatial resolution is compromised, and they are unable to measure strain variations on a fine scale such as those resulting from substrate/interface failure. In this study, mono-/few-layer graphene are demonstrated to be good candidates for strain sensing with high spatial resolution to evaluate features <100 nm. The fundamentals of strain sensing-interaction with the target-have been discussed to shed light on the sensitivity and durability for future sensor fabrication. The proof-of-concept strain sensors have been shown to be able to monitor different states, e.g., the initiation and evolution, of crazes. The analysis also leads to the evaluation of interfacial energy and realization of high local strain in graphene that is applicable for other 2D materials for ultrasensitive strain sensing and bandgap opening applications.

Light-Induced Programmable 2D Ordered Patterns Based on a Hyperbranched Poly(ether amine) (hPEA)-Functionalized Graphene Film

Dynamic complex surface topography with ordered and tunable morphologies, which can provide on-demand control of surface properties to realize smart surfaces, is gaining much attention yet remains challenging in terms of fabrication. Here, a facile, robust, and controllable method is demonstrated to fabricate programmable two-dimensional (2D) ordered patterns with multiresponsive 2D ultrathin materials, comprised of anthracene-capped hyperbranched poly(ether amine) (hPEA-AN)-functionalized graphene (hPEA-AN@G). By combining the stimuli-responsiveness and UV sensitivity of hPEA-AN and excellent out-of-plane deformation and NIR-to-thermal conversion of graphene, the process of "writing/uploading" initial information is conducted through the initial exposure to 365 nm UV light to generate the 2D ordered pattern first; second, inducing swelling strain via moisture to create the hierarchical topographic pattern (orderly oriented pattern) is the process of "modification and erasable rewriting"; third, alternating NIR or 254 nm UV light blanket exposure are the two ways of erasing the information. Consequently, taking advantage of the multiresponsive dynamic wrinkling/ordered patterning, we can program globally 2D ordered surface patterns with diverse morphologies on demand and manipulate the resulted surface properties as desired.

Spontaneously Ordered Hierarchical Two-Dimensional Wrinkle Patterns in Two-Dimensional Materials

Achieving two-dimensionally (2D) ordered surface wrinkle patterns is still challenging not only for the atomic-thick 2D materials but also in general for all soft surfaces. Normally disordered 2D wrinkle patterns on isotropic surfaces can be rendered via biaxial straining. Here, we report that the 1D and 2D ordered wrinkle patterns in 2D materials can be produced by sequential wrinkling controlled by thermal straining and vertical spatial confinement. The various hierarchical patterns in 2D materials generated by our method are highly periodic, and the hexagonal crystal symmetry is obeyed. More interestingly, these patterns can be maintained in suspended monolayers after delamination from the underlying surfaces which shows the great application potentials. Our new approach can simplify the patterning processes on 2D layered materials and reduce the risk of damage compared to conventional lithography methods, and numerous engineering applications that require nanoscale ordered surface texturing could be empowered.

Synergistic Antimicrobial Titanium Carbide (MXene) Conjugated with Gold Nanoclusters

Bacterial resistance toward antibiotics is a world-wide problem, and one potential solution to fight against the resistance is to develop multi-mechanism antimicrobial agents to achieve synergistic performance. Titanium carbide (MXene) is an emerging 2D nanomaterial with antimicrobial ability to physically damage bacterial membrane and chemically induce oxidative stress, and it can be further conjugated with nanomaterials to improve its antibacterial performance. Herein, a synergistic antimicrobial agent is developed through conjugation of the ultra-small gold nanoclusters (AuNCs) on MXene nanosheets. The conjugated AuNCs are effectively delivered into bacteria after bacterial membrane damage caused by MXene, generating localized reactive oxygen species (ROS) of high concentration to effectively oxidize bacterial membrane lipid for enhanced membrane broken, as well as bacterial DNA for violent fragmentation. Thus, the synergistic physical (via MXene) and chemical (via MXene and AuNCs) antimicrobial mechanisms lead to eventual bacterial death of both Gram-positive and Gram-negative bacteria, with low IC₅₀ values of 11.7 µg mL^-1 of MXene and 0.04 µm of AuNCs. Moreover, the crumpled MXene-AuNCs structure is constructed to inhibit biofilm formation, which hold synergistic antibacterial ability of MXene-AuNCs conjugation, hydrophobic surface to prevent bacterial attachment, and large surface area containing higher density of bactericides.

3D Graphene Materials: From Understanding to Design and Synthesis Control

Carbon materials, with their diverse allotropes, have played significant roles in our daily life and the development of material science. Following 0D C₆₀ and 1D carbon nanotube, 2D graphene materials, with their distinctively fascinating properties, have been receiving tremendous attention since 2004. To fulfill the efficient utilization of 2D graphene sheets in applications such as energy storage and conversion, electrochemical catalysis, and environmental remediation, 3D structures constructed by graphene sheets have been attempted over the past decade, giving birth to a new generation of graphene materials called 3D graphene materials. This review starts with the definition, classifications, brief history, and basic synthesis chemistries of 3D graphene materials. Then a critical discussion on the design considerations of 3D graphene materials for diverse applications is provided. Subsequently, after emphasizing the importance of normalized property characterization for the 3D structures, approaches for 3D graphene material synthesis from three major types of carbon sources (GO, hydrocarbons and inorganic carbon compounds) based on GO chemistry, hydrocarbon chemistry, and new alkali-metal chemistry, respectively, are comprehensively reviewed with a focus on their synthesis mechanisms, controllable aspects, and scalability. At last, current challenges and future perspectives for the development of 3D graphene materials are addressed.

Recent Advances in Strain-Induced Piezoelectric and Piezoresistive Effect-Engineered 2D Semiconductors for Adaptive Electronics and Optoelectronics

The development of two-dimensional (2D) semiconductors has attracted widespread attentions in the scientific community and industry due to their ultra-thin thickness, unique structure, excellent optoelectronic properties and novel physics. The excellent flexibility and outstanding mechanical strength of 2D semiconductors provide opportunities for fabricated strain-sensitive devices and utilized strain tuning their electronic and optic-electric performance. The strain-engineered one-dimensional materials have been well investigated, while there is a long way to go for 2D semiconductors. In this review, starting with the fundamental theories of piezoelectric and piezoresistive effect resulted by strain, following we reviewed the recent simulation works of strain engineering in novel 2D semiconductors, such as Janus 2D and 2D-Xene structures. Moreover, recent advances in experimental observation of strain tuning PL spectra and transport behavior of 2D semiconductors are summarized. Furthermore, the applications of strain-engineered 2D semiconductors in sensors, photodetectors and nanogenerators are also highlighted. At last, we in-depth discussed future research directions of strain-engineered 2D semiconductor and related electronics and optoelectronics device applications.

Bioinspired Multiscale Wrinkling Patterns on Curved Substrates: An Overview

The surface wrinkling of biological tissues is ubiquitous in nature. Accumulating evidence suggests that the mechanical force plays a significant role in shaping the biological morphologies. Controlled wrinkling has been demonstrated to be able to spontaneously form rich multiscale patterns, on either planar or curved surfaces. The surface wrinkling on planar substrates has been investigated thoroughly during the past decades. However, most wrinkling morphologies in nature are based on the curved biological surfaces and the research of controllable patterning on curved substrates still remains weak. The study of wrinkling on curved substrates is critical for understanding the biological growth, developing three-dimensional (3D) or four-dimensional (4D) fabrication techniques, and creating novel topographic patterns. In this review, fundamental wrinkling mechanics and recent advances in both fabrications and applications of the wrinkling patterns on curved substrates are summarized. The mechanics behind the wrinkles is compared between the planar and the curved cases. Beyond the film thickness, modulus ratio, and mismatch strain, the substrate curvature is one more significant parameter controlling the surface wrinkling. Curved substrates can be both solid and hollow with various 3D geometries across multiple length scales. Up to date, the wrinkling morphologies on solid/hollow core-shell spheres and cylinders have been simulated and selectively produced. Emerging applications of the curved topographic patterns have been found in smart wetting surfaces, cell culture interfaces, healthcare materials, and actuators, which may accelerate the development of artificial organs, stimuli-responsive devices, and micro/nano fabrications with higher dimensions.

Mechanochemical engineering of 2D materials for multiscale biointerfaces

Atomically thin nanomaterials represent a unique paradigm for interfacing with biological systems due to their mechanical flexibility, exceptional interfacial area, and ease of chemical functionalization. In particular, these two-dimensional (2D) materials are able to bend, curve, and fold in response to biologically-generated forces or other external stimuli. Such origami-like folding of 2D materials into wrinkled or crumpled topographies allows them to withstand large deformations by accordion-like unfolding, with implications for stretchable and shape-changing devices. Here, we review how mechanically manipulated 2D materials can interact with biological systems across a multitude of length scales. We focus on recent work where wrinkling, crumpling, or bending of 2D materials permits new chemical and material properties, with four case studies: (i) programming biomolecular reactivity and enhanced sensing, (ii) directed adhesion and encapsulation of bacteria or mammalian cells, (iii) stimuli-responsive actuators and soft robotics, and (iv) stretchable barrier technologies and wearable human-scale sensors. Finally, we consider future directions for manufacturing, materials and systems integration, as well as biocompatibility. Taken together, these 2D materials may enable new avenues for ultrasensitive molecular detection, biomaterial scaffolds, soft machines, and wearable technologies.

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.

Microphase separation in oriented polymeric chains at the surface of nanomaterials during nanofiber formation

The presence of low-dimensional functional nanofillers during the formation of morphological phase boundaries in polymeric nanofibers by electrospinning was highlighted in this study. PAN and TPU were both selected with differential viscosities to understand the phase-segregated internal supramolecular structures on functional surfaces of different length scales. The low-dimensional carbon nanofillers displayed a significant role in the topological orientation of the polymeric chains in TPU due to the presence of hard and soft segments in the geometry of TPU. The nano-hybrid shish-kebab-type microphase separation was observed on 1D nanofillers, whereas the anisotropic hierarchical microdomains were formed in the presence of 0D nanofillers. The 2D functional surface produced highly folded nanoscale lamellae by molecular interactions with polymeric chains. By combining different dimensional nanofillers, the hybrid 1D-2D networks created multifaceted structural hierarchies with epitaxial growth on the planar surface and shish-kebab geometry on the 1D functional backbone. Our study has demonstrated the significance of the configuration of nanoscale functional surfaces on the texture of polymeric chain assemblies during electrospinning for controlled flexible scaffolds.

Bending of Multilayer van der Waals Materials

Out-of-plane deformation patterns, such as buckling, wrinkling, scrolling, and folding, formed by multilayer van der Waals materials have recently seen a surge of interest. One crucial parameter governing these deformations is bending rigidity, on which significant controversy still exists despite extensive research for more than a decade. Here, we report direct measurements of bending rigidity of multilayer graphene, molybdenum disulfide (MoS_{2}), and hexagonal boron nitride (hBN) based on pressurized bubbles. By controlling the sample thickness and bubbling deflection, we observe platelike responses of the multilayers and extract both their Young's modulus and bending rigidity following a nonlinear plate theory. The measured Young's moduli show good agreement with those reported in the literature (E_{graphene}>E_{hBN}>E_{MoS_{2}}), but the bending rigidity follows an opposite trend, D_{graphene}<D_{hBN}<D_{MoS_{2}} for multilayers with comparable thickness, in contrast to the classical plate theory, which is attributed to the interlayer shear effect in the van der Waals materials.

Buckled Structures: Fabrication and Applications in Wearable Electronics

Wearable electronics have attracted a tremendous amount of attention due to their many potential applications, such as personalized health monitoring, motion detection, and smart clothing, where electronic devices must conformably form contacts with curvilinear surfaces and undergo large deformations. Structural design and material selection have been the key factors for the development of wearable electronics in the recent decades. As one of the most widely used geometries, buckling structures endow high stretchability, high mechanical durability, and comfortable contact for human-machine interaction via wearable devices. In addition, buckling structures that are derived from natural biosurfaces have high potential for use in cost-effective and high-grade wearable electronics. This review provides fundamental insights into buckling fabrication and discusses recent advancements for practical applications of buckled electronics, such as interconnects, sensors, transistors, energy storage, and conversion devices. In addition to the incorporation of desired functions, the simple and consecutive manipulation and advanced structural design of the buckled structures are discussed, which are important for advancing the field of wearable electronics. The remaining challenges and future perspectives for buckled electronics are briefly discussed in the final section.

Mosaic pattern formation in exfoliated graphene by mechanical deformation

Graphene is susceptible to morphological instabilities such as wrinkles and folds, which result from the imposition of thermo-mechanical stresses upon cooling from high temperatures and/ or under biaxial loading. A particular pattern encountered in CVD graphene is that of mosaic formation. Although it is understood that this pattern results from the severe biaxial compression upon cooling from high temperatures, it has not been possible to create such a complex pattern at room temperature by mechanical loading. Herein, we have managed by means of lateral wrinkling induced by tension and Euler buckling resulting from uniaxial compression upon unloading, to create such patterns in exfoliated graphene. We also show that these patterns can be used as channels for trapping or administering fluids at interstitial space between graphene and its support. This opens a whole dearth of new applications in the area of nano-fluidics but also in photo-electronics and sensor technologies.

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.

Robust Superhydrophobic/Superoleophilic Wrinkled Microspherical MOF@rGO Composites for Efficient Oil-Water Separation

Graphene/MOF-based composite materials in three-dimensional (3D) architectures are promising for the treatment of oil-containing wastewater by absorption owing to their intrinsic properties of graphene and metal-organic frameworks (MOFs), such as high porosity, ultralow density, and facilely tailored superwettability. In this study, novel wrinkled 3D microspherical MOF@rGO composites with both superhydrophobic and superoleophilic properties were developed by embedding MOF nanoparticles between graphene oxide (GO) nanosheets, followed by high-temperature reduction self-assembly. The microspherical composites feature a unique micro/nano hierarchy consisting of crumpled reduced GO (rGO) nanosheets intercalated with well-dispersed MOF nanoparticles. Combined with the superwettability and abundant meso/microporosity, the peculiar architectures of wrinkled ZIF-8@rGO microspheres show very fast absorption rates and high sorption selectivity for organic solvents and oils from water.

Engineering Surface Patterns with Shape Memory Polymers: Multiple Design Dimensions for Diverse and Hierarchical Structures

Deterministic design of surface patterns has seen a surge of interests because of their wide applications in flexible and stretchable electronics, microfluidics, and optical devices. Recently, instability of bilayer systems has been extensively utilized by which micro-/nano-patterns of a film can be easily achieved through macroscopically deforming the underlying substrate. For a bilayer system with traditional thermostable substrates, the pattern morphology is only determined by initial strain mismatch of the two layers, and the realization of localized patterns appears to be particularly challenging because of the difficulties associated with manipulating inhomogeneous deformations. In this work, we exploit cross-linked polyethylene ( cPE), a shape memory polymer (SMP), as the flexible substrate for building micro-/nano-structures of sputtered gold films. We find that the shape memory effect can offer new dimensions for designing diverse and hierarchical surface structures by harnessing film thickness orheating time and by globally or locally controlling the thermal field. By combining those strategies, we further demonstrate versatile hierarchical, superimposed, and local surface patterns based on this cPE/gold (Au) system. Piezoresistive pressure sensors are assembled with the obtained patterned surface, which have high sensitivity, operational range, and cyclic stability. These results highlight the unique advantages of SMPs for building arbitrary surface patterns.

Hierarchical Reduced Graphene Oxide Ridges for Stretchable, Wearable, and Washable Strain Sensors

Recently, flexible and wearable devices are increasingly in demand and graphene has been widely used due to its exceptional chemical, mechanical and electrical properties. Building complex buckling patterns of graphene is an essential strategy to increase its flexible and stretchable properties. Herein, a facile dimensionally controlled four-dimensional (4D) shrinking method was proposed to generate hierarchical reduced graphene oxide (rGO) buckling patterns on curved substrates mimicking different parts of the uniforms. The reduced graphene oxide ridges (rGORs) generated on the spherical substrate seem isotropic, while those generated on the cylindrical substrate are obviously more hierarchical or oriented, especially when the cylindrical substrate are shrinking via two steps. The oriented rGORs are superhydrophobic and strain sensitive but obviously anisotropic along the axial and circumferential directions. The sensitivity of rGORs along the axial direction is much higher than those along the circumferential direction. In addition, the intrinsic solvent barrier property of graphene enables the crack-free rGORs an excellent chemical protective performance, withstanding DCM immersion for more than 2.5 h. The flexible rGORs-based strain sensors can be used to detect both large and subtle human motions and activities by achieving high sensitivity (maximum gauge factor up to 48), high unidirectional stretchability (300-530%), and ultrahigh areal stretchability (up to 2690%). Excellent durability was also demonstrated for human motion monitoring with resistance to hand rubbing, ultrasonic cleaning, machine washing, and chemical immersion.

Enhancing the adhesion of graphene to polymer substrates by controlled defect formation

The mechanical integrity of composite materials depends primarily on the interface strength and the defect density of the reinforcement which is the provider of enhanced strength and stiffness. In the case of graphene/polymer nanocomposites which are characterized by an extremely large interface region, any defects in the inclusion (such as folds, cracks, holes, etc) will have a detrimental effect to the internal strain distribution and the resulting mechanical performance. This conventional wisdom, however, can be challenged if the defect size is reduced beyond the critical size for crack formation to the level of atomic vacancies. In that case, there should be no practical effect on crack propagation and depending on the nature of the vacancies the interface strength may in fact increase. In this work we employed argon ion (Ar⁺) bombardment and subsequent exposure to hydrogen (H₂) to induce (as revealed by x-ray and ultraviolet photoelectron spectroscopy and Raman spectroscopy) passivated atomic single vacancies to CVD graphene. The modified graphene was subsequently transferred to PMMA bars and the morphology, wettability and the interface adhesion of the CVD graphene/PMMA system were investigated with atomic force microscopy technique and Raman analysis. The results obtained showed clearly an overall improved mechanical behavior of graphene/polymer interface, since an increase as well as a more uniform shift distribution with strain is observed. This paves the way for interface engineering in graphene/polymer systems which, in pristine condition, suffer from premature graphene slippage and subsequent failure.

Design and fabrication of biocompatible wrinkled hydrogel films with selective antibiofouling properties

In this article, we explored the selective antibiofouling capacity acquired by functional wrinkled hydrogel films via a fine tuning of their chemical structure through the gradual insertion of hydrophobic radical groups in their network. The hydrogel consists of three main components: hydroxyethyl methacrylate (HEMA, amphiphilic monomer), trifluoroethyl methacrylate (TFMA, hydrophobic monomer), and poly(ethylene glycol) diacrylate (PEGDA, hydrophilic crosslinking agent). Interestingly, the manipulation of the chemical composition affects both, surface morphology and physicochemical characteristics of the patterns, inducing transitions between different surface microstructures, i.e. from wrinkles to creases, to folds, and to crumples. Contact angle measurements show that the insertion of TFMA produces a slight decrease in surface wettability, remaining however highly hydrophilic. By using confocal Raman spectroscopy, important information about wrinkle formation mechanism could be obtained. The procedure presented in this article involves two consecutive thermal and photopolymerization steps, generating a "pseudo" two-layer system, which contracts at different extents when is exposed to external stimuli, leading to the formation of wrinkled surfaces. Finally, bacterial and cellular adhesion/proliferation studies were carried out, evidencing that the amount of TFMA included clearly reduce the bacterial adhesion while mammalian cells are able to still proliferate.

Multifunctionality and Mechanical Actuation of 2D Materials for Skin-Mimicking Capabilities

Human skin serves as a multifunctional organ with remarkable properties, such as sensation, protection, regulation, and mechanical stretchability. The mimicry of skin's multifunctionalities via various nanomaterials has become an emerging topic. 2D materials have attracted much interest in the field of skin mimicry due to unique physiochemical properties. Herein, recent developments of using various 2D materials to mimic skin's sensing, protecting, and regulating capabilities are summarized. Next, to endow high stretchability to 2D materials, the approaches for fabrication of stretchable bilayer structures by integrating higher dimensional 2D materials onto soft elastomeric substrates are introduced. Accordion-like 2D material structures can elongate with elastomers and undergo programmed folding/unfolding processes to mimic skin's stretchability. That stretchable 2D material devices can achieve effective tactile sensing and protecting capabilities under large deformation is then highlighted. Finally, multiple key directions and existing challenges for future development are discussed.

Controlled Crumpling of Two-Dimensional Titanium Carbide (MXene) for Highly Stretchable, Bendable, Efficient Supercapacitors

Two-dimensional MXene materials have demonstrated attractive electrical and electrochemical properties in energy storage applications. Adding stretchability to MXene remains challenging due to its high mechanical stiffness and weak intersheet interaction, so the assembling techniques for mechanically stable MXene architectures require further development. We report a simple fabrication by harnessing the interfacial instability to generate higher dimensional MXene nanocoatings capable of programmed crumpling/unfolding. A sequential patterning approach enabled the design of sequence-dependent MXene textures across multiple length scales, which were utilized for controllable wetting surfaces and high-areal-capacitance electrodes. We next transferred the crumpled MXene nanocoating onto an elastomer to fabricate an MXene/elastomer electrode with high stretchability. The accordion-like MXene can be reversibly folded/unfolded and still preserve efficient specific capacitances. We further fabricated asymmetric MXene supercapacitors, and the devices demonstrated efficient electrochemical performance and large deformability (180° bendability, 100% stretchability). Our texturing techniques can be applied to large MXene families for designing stretchable architectures in wearable electronics.

Crumpling and Unfolding of Montmorillonite Hybrid Nanocoatings as Stretchable Flame-Retardant Skin

Flame-retardant coatings are widely used in a variety of personnel or product protection, and many applications would benefit from film stretchability if suitable materials are available. It is challenging to develop flame-retardant coatings that are stretchable, eco-friendly, and capable of being integrated on mechanically dynamic devices. Here, a concept is reported that uses pretextured montmorillonite (MMT) hybrid nanocoatings that can undergo programed unfolding to mimic the stretchability of elastomeric materials. These textured MMT coatings can be transferred onto an elastomeric substrate to achieve an MMT/elastomer bilayer device with high stretchability (225% areal strain) and effective flame retardancy. The bilayer composite is utilized as flame-retardant skin for a soft robotic gripper, and it is demonstrated that the actuated response can manipulate and rescue irregularly shaped objects from a fire scene. Furthermore, by depositing the conformal MMT nanocoatings on nitrile gloves, the firefree gloves can endure direct flame contact without ignition.

Wrinkled Nitrogen-doped Carbon Belts

Graphene, carbon nanotubes, and fullerenes are nanomaterials with outstanding properties such as electrical, thermal, mechanical strength, flexibility, and high surface area. These nanomaterials are used as building blocks for the construction of novel and astonishing 3D-dimensional networks. In the present work, nitrogen-doped carbon belt (N-CB) structures containing wrinkled carbon fibres as building blocks were synthesized under unstable conditions in a chemical vapour deposition experiment. N-CB structures with 0.2-3.0 microns of wide and 350 nm thick were assembled from complex individual wrinkled carbon fibres grown on Co/Cu films. These complex structures have a tubular appearance, showing holed and wrinkled graphite layers. Sulphur and copper atoms drastically affect the catalytic role of cobalt, changing the conventional growth of carbon nanotubes. Chemical functional groups, N-doping, and carbons hybridizations involved in the winkled carbon fibres are investigated. These findings provides a novel material that can be used as an excellent oxygen-reduction reaction catalyst or nano-electronics component.

Ultrastretchable Graphene-Based Molecular Barriers for Chemical Protection, Detection, and Actuation

A wide range of technologies requires barrier films to impede molecular transport between the external environment and a desired internal microclimate. Adding stretchability to barrier films would enable the applications in packaging, textiles, and flexible devices, but classical barrier materials utilize dense, ordered molecular architectures that easily fracture under small tensile strain. Here, we show that textured graphene-based coatings can serve as ultrastretchable molecular barriers expandable to 1500% areal strain through programmed unfolding that mimics the elasticity of polymers. These coatings retain barrier function under large deformation and can be conformally applied to planar or curved surfaces, where they are washfast and mechanically robust to cycling. These graphene-polymer bilayer structures also function as sensors or actuators by transducing chemical stimuli into mechanical deformation and electrical resistance change through asymmetric polymer swelling. These results may enable multifunctional fabrics that integrate chemical protection, sensing, and actuation, with further applications as selective barriers, membranes, stretchable electronics, or soft robotics.