390246 Functionalization of Carbon Nanotube Thin-Film Transistors Fabricated By Material Recognition Property of Peptide Aptamer
Carbon nanotubes (CNTs) are an attractive material possessing unique electrical, mechanical and optical properties which are the capability of metal or semiconductor derived from its structure difference, high electrical and thermal conductivity, and high-tensile strength and flexibility. Those properties hold great promise for development of micro/nano-electronic components, such as transparent and flexible electrodes, wirings, field-effect transistors, sensors, and so on. Therefore, handling techniques of the CNTs have been becoming important. Especially, isolation of conducting or semiconducting CNTs, site-selective alignment of the each CNT, and surface modification of the CNTs are key techniques for creating devices with high energy efficiency or sensors with high-sensitivity and selectivity. Actually, the electron field-effect mobility in excess of 106 cm2 /V∙s was achieved on the transistor with applying an individual semiconducting CNT, and various sensors complexed with nanoparticles or biomolecules were reported as the application of detecting against a specific gas or cancer, respectively. However, many complicated processes were required for constructing those device structures. Biomolecules with recognition against specific material surfaces, known as aptamer, have a potential to achieve them by simple process. In this study, we demonstrated autonomous immobilization of CNTs on a solid surface by utilizing the property of the peptide aptamer, and fabricated the CNT thin-film transistors (CNT-TFTs) with applicative field-effect mobility. Furthermore, we examined the functionalization process of the CNT surface by metal nanoparticles toward the construction of the sensors.
The CNT-TFT was fabricated by following procedure. Au as source and drain electrodes were patterned on the silicon substrate as a back gate (phosphorus-doped, 0.001 Ω∙cm resistivity) with thermal oxide film with a thickness of 100 nm. For immobilizing the CNTs as a channel, the peptide aptamer (CNTBP) of amino-acid sequence with [HMGLTKIHYSAL] which was previously identified for single-walled CNTs (SWCNTs) were introduced via silane coupling reagent (3-Aminopropyl triethoxysilane). Subsequently, the droplet of the SWCNT distributed with the single-stranded DNA was applied between the electrodes, and the substrate was washed with enough deionized water. To functionalize the CNT surface by Pd nanoparticles, sodium tetrachloropalladate (II) and the following reducing agent, sodium borohydride or L(+)-ascorbic acid, was applied. Morphological observation was investigated of the CNT surface was investigated through a scanning electron microscope (SEM) or transmission electron microscope (TEM), and the component analysis was performed by energy dispersive X-ray spectroscopy (EDX). Electrical characteristics of the CNT-TFT device were evaluated through a semiconductor parameter analyzer.
The SWCNTs immobilized surface was observed through optical microscopy and SEM. Although the deposited CNTs were invisible through optical microscopy, high-density and thin layered CNTs network was found only in the case with the CNTBP through SEM observation. Furthermore, the drain current-voltage characteristics of the CNT-TFT were investigated. In the case of using semiconducting CNTs, the drain current was increased in response to the gate voltage and the field-effect mobility reached 0.23～6.7×10−1 cm2 /V∙s which value was not ideal but applicative. In contrast, ohmic properties were observed in the case with conducting CNTs. These results showed transparent CNTs electronic components could be formed by utilizing the peptide aptamer. Furthermore, Pd nanoparticles deposited on the CNT surface was evaluated through TEM and EDX. In the preparation using ascorbic acid as a reducing agent, the particles with an average diameter of 5 nm were found on the CNT networks successfully. In the case using sodium borohydride, aggregated particles were observed relatively. In the case of both, the depositing particles of Pd were confirmed. As evidenced here, this simple process has a widespread potential for the fabrication of sensors with high-sensitivity and selectivity using CNT-based nanostructures.