Self-Sealing Carbon Patterns by One-Step Direct Laser Writing and Their Use in Multifunctional Wearable Sensors.

A combined photothermal simulation and experimental study leads to a novel internal reflection-assisted direct laser writing carbonization method (IR-DLWc), which enables in situ fabrication of carbon features/patterns that are self-sealed in the interior of a thin polyimide (PI) film in one step without additional packaging procedures. With this new method, carbon line patterns that are fully contained in a 50 µm PI film are fabricated, characterized, and evaluated for their electrical and piezoresistive performance. The self-sealing character of the carbon features created by IR-DLWc imparts them unprecedented mechanical stability/robustness as compared to those fabricated by the conventional DLWc method. Upon applying a double-writing scheme and strain-engineering treatment, the IR-DLWc-created carbon lines show significantly improved piezoresistive sensitivity with a gauge factor evaluated to be 428 in tension and 107 in compression. The high piezoresistive sensitivity, excellent dynamic response, reasonably good durability, self-sealing character, and compliant nature of the IR-DLWc generated carbon patterns make them suitable for a variety of wearable sensing applications. In this work, we demonstrated their use as a tactile sensor for sensing contact force; a functional bandage for monitoring physiological activities like swallowing, pulsing, and breathing; and a glove sensing system for finger gesture recognition.

Microcontact Printing with Laser Direct Writing Carbonization for Facile Fabrication of Carbon-Based Ultrathin Disk Arrays and Ordered Holey Films.

A nanometer-thick carbon film with a highly ordered pattern structure is very useful in a variety of applications. However, its large-scale, high-throughput, and low-cost fabrication is still a great challenge. Herein, microcontact printing (µCP) and direct laser writing carbonization (DLWc) are combined to develop a novel method that enables ease of fabrication of nanometer-thick and regularly patterned carbon disk arrays (CDAs) and holey carbon films (HCFs) from a pyromellitic dianhydride-oxydianiline-based polyamic acid (PAA) solution. The effect of PAA concentration and pillar lattice structure of the polydimethyl siloxane stamp are systematically studied for their influence on the geometrical parameter, surface morphology, and chemical structure of the finally achieved CDAs and HCFs. Within the PAA concentration being investigated, the averaged thickness of CDAs and HCFs can be tailored in a range from a few tens to a few hundred of nanometers. The µCP+DLWc-enabled electrically conductive CDAs and HCFs possess the characteristics of ease-of-fabrication, nanometer-thickness, highly regular and controlled patterns and structures, and the ability to form on both hard and soft substrates, which imparts usefulness in electronics, photonics, energy storage, catalysis, tissue engineering, as well as physical, chemical, and bio-sensing applications.

Direct Laser Writing-Assisted Method for Template-Free Fabrication of Biomass-Based Porous Carbon Platelets with Uniform Size and Arbitrarily Designed Shapes.

Because of a wide range of applications of porous carbon platelets (PCPs), a robust method for their facile synthesis/fabrication with controlled porous structure, size, and shape is constantly needed. Herein, we report a simple and scalable method for producing PCPs with uniform size and arbitrarily designed shapes. This approach relies on CO2 laser irradiation to induce carbonization of a biomass composite sheet formed by the infusion of sodium lignosulfonate into a cellulose paper to create porous carbon features with arbitrarily designed shapes. Upon subsequent water immersion treatment, the laser-written carbon features could spontaneously detach to form freestanding PCPs. The PCPs of different shapes were fabricated, characterized, and demonstrated for their potential applications in dye adsorption, as flexible sensors, and as miniaturized supercapacitors. Our method is expected to make great impacts in multiple fields, such as environment, energy storage, sensing, catalysis, and so forth.

A Route toward Ultrasensitive Layered Carbon Based Piezoresistive Sensors through Hierarchical Contact Design.

Ultrahigh sensitive piezoresistive sensors at small deformation are highly desired in many applications. Here, we propose a hierarchical contact design concept and implement it through a direct laser writing technique for fabricating layered carbon piezoresistive sensors with ultrahigh sensitivity. Sensors with unprecedented gauge factors (∼5000-10 000) at small deformation (ε < 0.1%) were successfully fabricated and demonstrated for their use in sensing both static and high-frequency (20-30 kHz) dynamic mechanical loads. A simple basic structure unit (BSU) contact network model was developed for understanding the importance of the BSU/BSU contact strength and network fractal dimension in dictating the piezoresistive sensitivity of the layered carbon piezoresistive sensors with designed hierarchical contact structures. The hierarchical contact design concept and the contact network model proposed in our work could open a general route for developing ultrasensitive piezoresistive sensors based on granular matter and composite materials.

Two-probe versus van der Pauw method in studying the piezoresistivity of single-wall carbon nanotube thin films.

The use of the van der Pauw (VDP) method for characterizing and evaluating the piezoresistive behavior of carbon nanomaterial enabled piezoresistive sensors have not been systematically studied. By using single-wall carbon nanotube (SWCNT) thin films as a model system, herein we report a coupled electrical-mechanical experimental study in conjunction with a multiphysics finite element simulation as well as an analytic analysis to compare the two-probe and VDP testing configuration in evaluating the piezoresistive behavior of carbon nanomaterial enabled piezoresistive sensors. The key features regarding the sample aspect ratio dependent piezoresistive sensitivity or gauge factor were identified for the VDP testing configuration. It was found that the VDP test configuration offers consistently higher piezoresistive sensitivity than the two-probe testing method.

Effect of Growth Temperature on Tailoring the Size and Aspect Ratio of Gold Nanorods.

The influence of temperature on the gold nanorod synthesis process and its effect on tailoring the size and aspect ratio have not been fully investigated and understood. A comprehensive study, involving SEM and TEM microscopy, Vis-NIR spectroscopy, quantitative data analysis and theoretical simulation, is performed to understand the effect of growth temperature on size, aspect ratio, and shape uniformity of gold nanorods that are synthesized by a recently developed binary-surfactant seed-mediated AuNR synthesis method. It has been demonstrated that the temperature can be used as a simple processing parameter to viably tailor the size and aspect ratio of AuNRs as well as the corresponding surface plasmon resonance behavior. The temperature coefficients for length, diameter, and aspect ratio have been, respectively, determined to be 3.5 nm/°C, 3.9 nm/°C, and -0.18/°C. With a combination of controlling temperature and formulation, the binary surfactant seed-mediated AuNR synthesis method expects to be a convenient way for producing gold nanorods with a large range of size and aspect ratio suitable for different applications.