Zhaohe Dai

Three-dimensional Sponges with Super Mechanical Stability: Harnessing True Elasticity of Individual Carbon Nanotubes in Macroscopic Architectures

Efficient assembly of carbon nanotube (CNT) based cellular solids with appropriate structure is the key to fully realize the potential of individual nanotubes in macroscopic architecture. In this work, the macroscopic CNT sponge consisting of randomly interconnected individual carbon nanotubes was grown by CVD, exhibiting a combination of super-elasticity, high strength to weight ratio, fatigue resistance, thermo-mechanical stability and electro-mechanical stability. To deeply understand such extraordinary mechanical performance compared to that of conventional cellular materials and other nanostructured cellular architectures, a thorough study on the response of this CNT-based spongy structure to compression is conducted based on classic elastic theory. The strong inter-tube bonding between neighboring nanotubes is examined, believed to play a critical role in the reversible deformation such as bending and buckling without structural collapse under compression. Based on in-situ scanning electron microscopy observation and nanotube deformation analysis, structural evolution (completely elastic bending-buckling transition) of the carbon nanotubes sponges to deformation is proposed to clarify their mechanical properties and nonlinear electromechanical coupling behavior.

Synergistic effect of a r-GO/PANI nanocomposite electrode based air working ionic actuator with a large actuation stroke and long-term durability

Actuators can directly convert various types of energy into mechanical motions. In this work, we constructed a novel air working ionic actuator by sandwiching sulfuric acid–poly(vinyl alcohol) (H2SO4–PVA) gel electrolyte between two pieces of reduced graphene oxide/polyaniline (r-GO/PANI) nanocomposite film based electrode, in which the PANI nanoparticles were uniformly decorated onto the r-GO sheet surfaces through an in situ polymerization method. A combination of the supramechanical properties, electrical conductivity, large surface area of the individual graphene sheets and excellent electrochemical properties of the PANI component results in the composite electrode having good mechanical properties and a high electrochemical capacitance. Later tests indicated that the actuator could be stimulated under a low driving voltage (≤0.5 V) without trade-off actuator strokes, and showed excellent long-term actuation durability. Under a 0.5 V operating voltage, the actuation strain of the r-GO/PANI actuator could reach 0.327%, corresponding to a 30 MPa generated stress. We attribute the excellent actuation performance to the synergistic effect of the r-GO sheets and the PANI component.

Measuring Interlayer Shear Stress in Bilayer Graphene

Monolayer two-dimensional (2D) crystals exhibit a host of intriguing properties, but the most exciting applications may come from stacking them into multilayer structures. Interlayer and interfacial shear interactions could play a crucial role in the performance and reliability of these applications, but little is known about the key parameters controlling shear deformation across the layers and interfaces between 2D materials. Herein, we report the first measurement of the interlayer shear stress of bilayer graphene based on pressurized microscale bubble loading devices. We demonstrate continuous growth of an interlayer shear zone outside the bubble edge and extract an interlayer shear stress of 40xa0kPa based on a membrane analysis for bilayer graphene bubbles. Meanwhile, a much higher interfacial shear stress of 1.64xa0MPa was determined for monolayer graphene on a silicon oxide substrate. Our results not only provide insights into the interfacial shear responses of the thinnest structures possible, but also establish an experimental method for characterizing the fundamental interlayer shear properties of the emerging 2D materials for potential applications in multilayer systems.

Tuning the Interfacial Mechanical Behaviors of Monolayer Graphene/PMMA Nanocomposites

The van der Waals (vdW) force dominated interface between graphene and polymer matrix creates weak points in the mechanical sense. Chemical functionalization was expected to be an effective approach in transfer of the outstanding performance of graphene across multiple length scales up to the macroscopic level, due to possible improvements in the interfacial adhesion. However, published works showed the contradiction that improvements, insensitivity, or even worsening of macro-mechanical performance have all been reported in graphene-based polymer nanocomposites. Particularly central cause of such discrepancy is the variations in graphene/polymer interfacial chemistry, which is critical in nanocomposites with vast interfacial area. Herein, O3/H2O gaseous mixture was utilized to oxidize monolayer graphene sheet with controlled functionalization degrees. Hydrogen bonds (H bonds) are expected to form between oxidized graphene sheet/poly(methyl methacrylate) (PMMA) at the interface. On the basis of in situ tensile-micro Raman spectroscopy, the impacts of bonding types (vdW and H-bonds) on both key interfacial parameters (such as interfacial shear strength and critical length) and failure modes of graphene/PMMA nanocomposite were clarified for the first time at the microscopic level. Our results show that owing to improved interfacial interaction via H bonds, the interface tends to be stiffening and strengthening. Moreover, the mechanical properties of the functionalized graphene/PMMA interface will be set by the competition between the enhanced interfacial adhesion and the degraded elastic modulus of graphene, which was caused by structural defects in the graphene sheet during the functionalization process and could lead to catastrophic failure of graphene sheets in our experimental observation. Our results will be helpful to design various nanofiller-based nanocomposites with high mechanical performance.

Extremely Vivid, Highly Transparent, and Ultrathin Quantum Dot Light‐Emitting Diodes

Displaying information on transparent screens offers new opportunities in next-generation electronics, such as augmented reality devices, smart surgical glasses, and smart windows. Outstanding luminance and transparency are essential for such see-through displays to show vivid images over clear background view. Here transparent quantum dot light-emitting diodes (Tr-QLEDs) are reported with high brightness (bottom: ≈43 000 cd m-2 , top: ≈30 000 cd m-2 , total: ≈73 000 cd m-2 at 9 V), excellent transmittance (90% at 550 nm, 84% over visible range), and an ultrathin form factor (≈2.7 µm thickness). These superb characteristics are accomplished by novel electron transport layers (ETLs) and engineered quantum dots (QDs). The ETLs, ZnO nanoparticle assemblies with ultrathin alumina overlayers, dramatically enhance durability of active layers, and balance electron/hole injection into QDs, which prevents nonradiative recombination processes. In addition, the QD structure is further optimized to fully exploit the device architecture. The ultrathin nature of Tr-QLEDs allows their conformal integration on various shaped objects. Finally, the high resolution patterning of red, green, and blue Tr-QLEDs (513 pixels in.-1 ) shows the potential of the full-color transparent display.

Multifunctional Polymer-Based Graphene Foams with Buckled Structure and Negative Poisson’s Ratio

In this study, we report the polymer-based graphene foams through combination of bottom-up assembly and simple triaxially buckled structure design. The resulting polymer-based graphene foams not only effectively transfer the functional properties of graphene, but also exhibit novel negative Poisson’s ratio (NPR) behaviors due to the presence of buckled structure. Our results show that after the introduction of buckled structure, improvement in stretchability, toughness, flexibility, energy absorbing ability, hydrophobicity, conductivity, piezoresistive sensitivity and crack resistance could be achieved simultaneously. The combination of mechanical properties, multifunctional performance and unusual deformation behavior would lead to the use of our polymer-based graphene foams for a variety of novel applications in future such as stretchable capacitors or conductors, sensors and oil/water separators and so on.

Low-cost, μm-thick, tape-free electronic tattoo sensors with minimized motion and sweat artifacts

Electronic tattoos (e-tattoos), also known as epidermal electronics, are ultra-thin and ultra-soft noninvasive but skin-conformable devices with capabilities including physiological sensing and transdermal stimulation and therapeutics. The fabrication of e-tattoos out of conventional inorganic electronic materials used to be tedious and expensive. Recently developed cut-and-paste method has significantly simplified the process and lowered the cost. However, existing cut-and-paste method entails a medical tape on which the electronic tattoo sensors should be pasted, which increases tattoo thickness and degrades its breathability. To address this problem, here we report a slightly modified cut-and-paste method to fabricate low-cost, open-mesh e-tattoos with a total thickness of just 1.5u2009μm. E-tattoos of such thinness can be directly pasted on human skin and conforms to natural skin texture. We demonstrate that this ultra-thin, tape-free e-tattoo can confidently measure electrocardiogram (ECG), skin temperature, and skin hydration. Heart rate and even respiratory rate can be extracted from the ECG signals. A special advantage of such ultra-thin e-tattoo is that it is capable of high-fidelity sensing with minimized motion artifacts under various body movements. Effects of perspiration are found to be insignificant due to the breathability of such e-tattoos.Flexible electronics: cutting plotters cut costs of electronic tattoosTattoos able to record heart rate and skin conditions have been fabricated with a low-cost cutting plotter. An international collaboration led by YongAn Huang and Nanshu Lu from the Huazhong University of Science and Technology in Wuhan, China, and the University of Texas at Austin, USA, have used this tool—a computer-controlled knife commonly employed to cut paper, vinyl and other materials in custom shapes—to define metallic serpentines on a plastic layer deposited on tattoo paper. The layer is so thin—about one thousandth of a millimeter—that the whole device is imperceptible when transferred on the skin, yet it adheres perfectly without blocking normal perspiration. Applied to human chest, these inexpensive metallic sensors monitor key health parameters, such as skin temperature and heart electric signal, without being affected by sweat and motion artefacts.

Multiscale Hierarchical Design of a Flexible Piezoresistive Pressure Sensor with High Sensitivity and Wide Linearity Range

Flexible piezoresistive pressure sensors have been attracting wide attention for applications in health monitoring and human-machine interfaces because of their simple device structure and easy-readout signals. For practical applications, flexible pressure sensors with both high sensitivity and wide linearity range are highly desirable. Herein, a simple and low-cost method for the fabrication of a flexible piezoresistive pressure sensor with a hierarchical structure over large areas is presented. The piezoresistive pressure sensor consists of arrays of microscale papillae with nanoscale roughness produced by replicating the lotus leafs surface and spray-coating of graphene ink. Finite element analysis (FEA) shows that the hierarchical structure governs the deformation behavior and pressure distribution at the contact interface, leading to a quick and steady increase in contact area with loads. As a result, the piezoresistive pressure sensor demonstrates a high sensitivity of 1.2 kPa-1 and a wide linearity range from 0 to 25 kPa. The flexible pressure sensor is applied for sensitive monitoring of small vibrations, including wrist pulse and acoustic waves. Moreover, a piezoresistive pressure sensor array is fabricated for mapping the spatial distribution of pressure. These results highlight the potential applications of the flexible piezoresistive pressure sensor for health monitoring and electronic skin.

Mechanics of spontaneously formed nanoblisters trapped by transferred 2D crystals

Significance Spontaneously formed nanoblisters are almost inevitable when transferring 2D crystals to a substrate. We hypothesize that those nanoblisters are filled with liquid according to our time-lapse atomic force microscopy scans and reasonable estimation of adhesion energy. We show that the liquid-filled nanoblisters form as a result of competition between the elastic energy of the deformed 2D crystal, the interfacial energy associated with van der Waals interactions, and surface tension of the liquid contents. Besides pointing to solutions for controlling their shape and internal pressure, our analysis provides a method to estimate the work of adhesion of 2D material interfaces by simply measuring the aspect ratios of the blisters, which is essential for the design and fabrication of 2D crystal-based applications. Layered systems of 2D crystals and heterostructures are widely explored for new physics and devices. In many cases, monolayer or few-layer 2D crystals are transferred to a target substrate including other 2D crystals, and nanometer-scale blisters form spontaneously between the 2D crystal and its substrate. Such nanoblisters are often recognized as an indicator of good adhesion, but there is no consensus on the contents inside the blisters. While gas-filled blisters have been modeled and measured by bulge tests, applying such models to spontaneously formed nanoblisters yielded unrealistically low adhesion energy values between the 2D crystal and its substrate. Typically, gas-filled blisters are fully deflated within hours or days. In contrast, we found that the height of the spontaneously formed nanoblisters dropped only by 20–30% after 3 mo, indicating that probably liquid instead of gas is trapped in them. We therefore developed a simple scaling law and a rigorous theoretical model for liquid-filled nanoblisters, which predicts that the interfacial work of adhesion is related to the fourth power of the aspect ratio of the nanoblister and depends on the surface tension of the liquid. Our model was verified by molecular dynamics simulations, and the adhesion energy values obtained for the measured nanoblisters are in good agreement with those reported in the literature. This model can be applied to estimate the pressure inside the nanoblisters and the work of adhesion for a variety of 2D interfaces, which provides important implications for the fabrication and deformability of 2D heterostructures and devices.