Control of Schottky Barrier between Carbon Nanotube Network and Metal Electrode for Electronic and Biosensor Application

Abstract

Since the discovery of carbon nanotubes (CNTs), CNTs have been one of the most attractive materials in nanotechnology. Especially, semiconducting CNTs have several properties that are advantageous for field-effect-transistors (FET) such as high mobility and capability to withstand high current densities. In some applications, they exhibit capabilities superior to conventional silicon-based electronic devices. The CNT-FET transistors operate as unconventional Schottky barrier transistors, in which transistor action occurs primarily by varying the contact resistance rather than the channel conductance. Therefore, to properly design CNT devices for electronic and biosensor applications, it is important to understand how Schottky barriers in CNT-metal contacts are formed and what factors influence their width and height. In this dissertation, based on understanding of the Schottky barrier, I and co-workers have explored the mechanisms and limitations of CNT-FET applications. In addition, we will propose the new strategies to overcome these limitations of CNT-FET applications. First, we will discuss the floating electrode-based thin-film transistors (F-TFTs) based on purified semiconducting single-walled CNTs (swCNTs) network for a high source-drain voltage operation. At a high source drain voltage, a conventional swCNT-TFT exhibited poor transistor performances with a small on-off ratio, which was attributed to the reduced Schottky barrier width modulation at a large bias. In the F-TFT device, a swCNT network channel was separated into a number of channels connected by floating electrodes. The applied voltage was divided into several Schottky barriers on divided channels. Therefore the F-TFTs exhibited a much higher on-off ratio than a conventional swCNT-TFT with a single channel. Next, the aptamer sandwich-based CNT sensor strategy for small molecular detection will be discussed. The sensing mechanisms of CNT-based sensors are electrostatic gating effect and Schottky barrier height modulation. Therefore, it is difficult to detect non-polar small molecules using the conventional CNT-based sensor. The aptamers were utilized to capture target molecules as well as to enhance the Schottky barrier height change. We successfully demonstrated the detection of non-polar bisphenol A (BPA) molecules with an 1 pM sensitivity. Significantly, our sensors were able to distinguish between similar small molecular species with single-carbon-atomic resolution. Furthermore, using the additional biotin modification on labeling aptamer, we enhanced the detection limit of our sensors down to 10 fM. This strategy allowed us to detect non-polar small molecular species using carbon nanotube transistors, thus overcoming the fundamental limitation of transistor-based sensors. These works should provide an important insight regarding the characteristics of a CNT-FET with a Schottky barrier modulation mechanism and can be an important guideline for the future electronic and biosensor applications.