Researchers have developed a triboelectric nanogenerator (TENG) wherein flexible single crystals of an organic compound was used as the main component for the fabrication of a self-powered tactile sensor which was used to monitor finger joint movements. This has significant importance particularly in biomedical and robotic system development and could pave the way for their utilization as biomedical wearable devices.

Organic materials are gaining importance for optoelectronic applications because of their lower cost and environmental footprint, easy fabrication, and practical feasibility. Among various organic materials, single crystals are better suited for device fabrication because of their well-ordered packing and precise spatial arrangements. They also possess intrinsic long range structural order and anisotropy and tuneable optical and electronic properties, thereby resulting in superior device performance. Additionally, the ease of synthesis, crystallization, and device fabrication position organic single crystals as promising candidates to meet the evolving demand for next generation technologies.

Scientists from Institute of Nano Science and Technology (INST), Mohali, an autonomous institute of the Department of Science and Technology (DST) demonstrated the first ever fabrication of TENG incorporating flexible single crystals of small organic molecules. The research involved has been published in the Journal of the American Chemical Society.

Triboelectrification was attained in single crystals as a result of surface functionalization with positively and negatively charged moieties, viz. Zn²⁺ and F⁻, respectively, which resulted in different surface potentials thereby leading to reversible adhesion through electrostatic interaction and induction phenomena.

The TENG incorporating the single crystals was capable of charging commercial capacitors thereby ensuring its ability to be used as a self-powered touch sensor. Capitalizing on these features, a self-powered tactile sensor was fabricated to demonstrate limb movements.

The protocol adopted to vary the surface charge on single crystals through surface functionalization techniques was simple and scalable. Moreover, the TENG developed works in the non-contact mode and promises outstanding endurance properties.

The excellent mechano-electric sensitivity (∼102 mV/kPa until 6 kPa range) and response time (∼38 ms) establish the viability of flexible organic single crystals for mechanical energy harvesting and biosensing applications.

Biomechanical signal sensing using TENG. Photographs of TENG attached to a finger along with the measured output voltage response at (a) 30°, (b) 60°, and (c) 90° bending angles.

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