Self-Powered Infrared Detection in Low-Dimensional Carbon Assemblies

Abstract

Room-temperature mid-infrared photodetection meet upcoming demands including real-time health condition monitoring, low-cost industrial inspection and distributive sensing for Internet-of-Things. Photo-thermoelectric (PTE) effect is a bandgap limitless photodetection mechanism which utilizes photons induced thermoelectric effect at material interfaces. The 1/f noise and shot noise in dark current can be significantly reduced in a zero-biased PTE detector. Carbon nanotubes (CNTs) and graphene are emerging low-dimensional materials with excellent PTE properties. Besides the strong and broadband light-matter interaction, their increased electrical to thermal conductivity ratio and electron density-of-states dependence on energy also lead to enhanced thermoelectric conversion efficiency. In this thesis, we present two self-powered PTE detection architectures. In the first one, vertical photo-thermoelectric effect of an anti-reflecting carbon nanotube forest (CNTF) is employed in a broadband mid-infrared detector. 99.4% average reflection suppression in the CNTF at 2.5~25 µm spectral range enables responsivity of 6 V W-1 and detectivity of 2.2×107 cm Hz1/2 W-1 under very weak illumination power, rendering sensitive weak infrared photodetection in real life. Top-electrode material, thickness and patterns are systematically studied related to the PTE response, and further improvement is possible by increasing the CNTF height and reducing the photosensitive area. In the second architecture, CNTs/Poly vinyl alcohol (PVA) composite based planar photodetector with asymmetric metallic electrodes is investigated. PTE voltage response is optimized via mixing 25 wt.% CNTs into PVA matrix attributed to the enhanced phonon scattering at CNTs/PVA interfaces. Moreover, crystallization of PVA around CNTs networks contributes to a rather stable photoresponse (variation < 4 %) under significant bending down to a 3.5 mm radius. This flexible, wearable photodetector also proves preliminary passive imaging of human body radiation. Finally, a unique and facile fabrication technique is demonstrated for the integration of a flexible, semi-transparent photodetector based on graphene nanoplatelets/PEDOT: PSS composite. This photodetector exhibits enhanced PTE response, high flexibility, and good optical transparency at a low loading of graphene.