Realizing on-site carbon nanotube photo-thermoelectric imaging

Led by Assistant Professor Kou Li, a research group in Chuo University, Japan, has developed chemically enriched photo-thermoelectric (PTE) imagers using semiconducting carbon nanotube (CNT) films, resulting in the achievement of enhanced response intensity and noise reduction, that enables efficient remote and on-site inspections, according to a recent paper publication in Communications Materials. CNT film-based PTE imagers are crucial for multimodal non-destructive inspection, but conventional device design strategies have faced challenges in achieving high response intensity for wireless data logging.

CNT film-based PTE imagers enable functional electromagnetic-wave monitoring, potentially facilitating multimodal non-destructive inspection device usage. The CNT film compositions govern the fundamental device performance, and satisfying high PTE conversion efficiency (higher response and lower noise) is essential for sensitive operations. Although typical sensitive design focuses on minimising noise, the associated levelling-off response intensity (up to a few millivolts) induces technical limitations in device operations. These issues include mismatching for coupling with compact wireless circuits, which are indispensable for on-site inspection applications and require high-intensity responses at least a few millivolt orders. This work develops chemically enriched PTE imagers comprising semiconducting CNT (semi-CNT) films. While semi-CNTs provide greater intensity thermoelectric responses than semi-metal mixture compositions in the conventional PTE device, the presented imager employs p-/n-type chemical carrier doping to relax inherent significant bottlenecking noise. Such doping enhances material properties for PTE conversion with semi-CNTs up to 4,060 times. The imager satisfies similar performances to conventional CNT film devices, including ultrabroadband sensitive photo-detection (with minimum noise sensitivity of 5 pWHz−1/2) under repeatedly deformable configurations, and advantageously exhibits response signal intensity exceeding a few–tens of millivolts. These features enable remote on-site non-destructive PTE imaging inspection with palm-sized wireless circuits.

Research background

While quality testing techniques are essential in rapidly growing industrial mass-production and social distribution, optically broadband and mechanically deformable photo image sensors (photo-imagers) potentially play a leading role in non-destructive inspection by handling diverse target objects. However, such the existing photo-imagers still function with low (from tens to hundreds of microvolts) intensity responses, limiting choices of system setups. These crucial bottlenecks include mismatching for operating the above devices with compact wireless datalogger circuits (available in millivolt (mV) orders), which are essential in non-destructive inspection regarding on-site use. Furthermore, due to levelling-off response intensities, the conventional device design has mainly focused on minimising noise signals for sensitivity enhancements, which are off the point for satisfying technical requirements for the above remote operations. These situations hinder the broadband deformable photo-imagers from their versatile applications.

The importance and novelty of this work

To this end, this work made the following significant contributions.

1. Developing optically ultrabroadband, mechanically deformable, and wirelessly functional imager sheets based on an operating mechanism of the PTE effect with chemically enriched single-walled semiconductingCNT (semi-CNT) thin films.
2. Combining p- and n-type chemical carrier doping for the imager channel: pn junction-based semi-CNTs to improve the inherent bottlenecking giant noise and insufficient photo-absorption of the above material.
3. Collectively satisfying a comparable photo-detection sensitivity by the imager with that of the existing bulky solid-state sensors and ultrabroadband operations (from millimetre-wave to visible light) over them.
4. Achieving the fundamental wireless use as the PTE imager by synergising its inherent high thermoelectric conversion efficiency and enriched electrical/optical properties, into advantageous over ten-fold large photo-detection responses (up to tens of mV) than the conventional ultrabroadband deformable sensors.
5. Demonstrating on-site non-destructive testing of an aerial object with the remotely controlled semi-CNT film wireless PTE imager in an advantageous omni-directional and multi-wavelength monitoring manner.