Improvement of Photosensitivity in Carbon Nanotube-based Near-Infrared Phototransistor by Selective Wetting Pattern

Abstract

Recently, carbon nanotubes (CNT) based phototransistors (PT) have been gaining much attention due to the pronounced mechanical and electronic properties of CNTs, in addition to the ability of photodetection combined with the internal gain as in transistors. Near-infrared (NIR) detectors for medical applications such as health monitoring systems and industrial applications such as night vision and food inspection. For this, it is important to obtain devices the exhibit high photosensitivity for efficient and effective photoresponse to light exposure. To obtain high photosensitivity, photogenerated carrier recombination losses must be minimized. This is possible through the reduction of gate leakage current by controlling the vertical gate charge carriers to prevent them from passing through the gate insulator layer of the transistor device.

In this thesis, presented is an all solution processed NIR phototransistor consisting of a patterned network of single-walled carbon nanotubes as the photoactive layer of the transistor. The P-type lateral bottom-gate-top contact device was fabricated on a silicon substrate covered with a silicon dioxide insulator film and on a Glass substrate with patterned indium tin oxide (ITO) gate and a poly (4-vinyl phenol) (PVP) as the gate insulator. Applying a selective wetting method through a micro-patterning technique using a fluoropolymer-coated poly(dimethylsiloxane) (PDMS) stamp on the device insulator followed by deposition of carbon nanotubes (CNT) solution can leave a patterned film of a network of Single-walled CNTs (SWCNT). This assisted in confining the vertical flow of charge carriers by creating a hydrophically patterned barrier (PHB) and hence decreased the gate leakage current from order 〖10〗^(-8)~〖10〗^(-7) to an order of 〖10〗^(-9)~〖10〗^(-10). A photoresponse was only observable upon exposure to NIR (980 nm) laser following the improvement by reduction of gate leakage, while without the PHB layer there was no detectable response to light exposure. On current showed a steady value when both devices were compared, and a notable decrease in off current led to an increase in on/off ratio. Through these results, we prove that the patterning of PHB layer on the gate insulator assists in creating a barrier to improve device performance effectively by reducing the leakage current. With materials such as CNTs, that exhibit superior properties, this a step ahead towards fully solution processable carbon nanotube electronics, where cost-effective and minimum labor fabrication procedures are in high demand.