Processing–structure–property relationship in direct laser writing carbonization of polyimide

Thin Film of Poly-peri-naphthalene (PPN) Fabricated by Direct Laser Writing and Its Use for Humidity Sensing

Inspired by the versatility of the direct laser writing carbonization (DLWc) technique as well as the metal-ion-assisted coordination polymer of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), in the present study, we present a DLWc-enabled approach for cost-effectively fabricating a poly-peri-naphthalene (PPN) thin film with the advantages of patternability, environmental benignity, and scalability. Optical and scanning electron microscopy, as well as ultraviolet-visible-near-infrared, Fourier transform infrared, Raman, and X-ray photoelectron spectroscopies, were performed to confirm that the spray-coated thin film of the Mg-PTCDA coordination polymer can be in situ converted into a PPN thin film upon CO2 laser irradiation. The effects of the laser power and Mg2+ concentration on the structure and electrical properties of the laser-processed PPN thin films were investigated. Lastly, we demonstrated that the laser-processed PPN thin films can be used for humidity sensing with characteristics of a rapid response time and excellent hysteresis. It is expected that this new method for fabricating PPN thin films will lead to a wide range of applications of PPN in the fields of sensing, electronics, and energy storage.

Laser direct write of heteroatom-doped graphene on molecularly controlled polyimides for electrochemical biosensors with nanomolar sensitivity

Fabrication of heteroatom-doped graphene electrodes remains a challenging endeavor, especially on flexible substrates. Precise chemical and morphological control is even more challenging for patterned microelectrodes. We herein demonstrate a scalable process for directly generating micropatterns of heteroatom-doped porous graphene on polyimide with different backbones using a continuous-wave infrared laser. Conventional two-step polycondensation of 4,4'-oxydianiline with three different tetracarboxylic dianhydrides enabled the fabrication of fully aromatic polyimides with various internal linkages such as phenylene, trifluoromethyl or sulfone groups. Accordingly, we leverage this laser-induced polymer-to-doped-graphene conversion for fabricating electrically conductive microelectrodes with efficient utilization of heteroatoms (N-doped, F-doped, and S-doped). Tuning laser fluence enabled achieving electrical resistivity lower than ~13 Ω sq^-1 for F-doped and N-doped graphene. Finally, our microelectrodes exhibit superior performance for electrochemical sensing of dopamine, one of the important neurotransmitters in the brain. Compared with carbon fiber microelectrodes, the gold standard in electrochemical dopamine sensing, our F-doped high surface area graphene microelectrodes demonstrated 3 order of magnitude higher sensitivity per unit area, detecting dopamine concentrations as low as 10 nM with excellent reproducibility. Hence, our approach is promising for facile fabrication of microelectrodes with superior capabilities for various electrochemical and sensing applications including early diagnosis of neurological disorders.

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.