分子の電気識別に向けた単結晶酸化亜鉛ナノワイヤデバイスの無機・有機界面材料設計

概要

Electrical recognition of chemical species using sensor devices is attracting increasing interest in applications in the fields of healthcare, industrial production, and environmental monitoring. As promising chemical sensing material, single crystalline metal oxide nanostructures modified by self-assembled monolayer (SAM) have been intensively studied because of their molecular selectivity, large surface-to-volume ratio, and semiconducting electrical property. Previous studies on SAM-modified metal oxide nanostructure catalysts reported the effective catalytic properties by controlling surface chemical reactions using SAM, indicating possible improvements of sensor property using SAM. However, previous studies on SAM-modified metal oxide sensors mainly focused on interactions between SAM and molecules without considering surface reaction on a metal oxide sensor channel. This thesis focuses on the interface material design of single crystalline zinc oxide nanowire sensors. First, a strategy for achieving atmospheric electrical stability of zinc oxide nanowire devices is demonstrated. It is found that atmospheric CO2 degrades the electrical property of hydrothermally grown single crystalline zinc oxide nanowires by forming zinc carbonate on the nanowire surface. Based on analysis of the surface carbonation mechanism, a strategy to suppress the detrimental surface reaction, which is based on (1) reducing the density of surface hydroxyl groups and (2) improving the nanowire crystallinity by thermal pretreatment, is proposed. Second, SAM formation mechanism on single crystalline zinc oxide nanowires is investigated. It was clarified that in methanol, which is generally used for SAM modification, a zinc phosphonate layered substance is formed by dissolving the surface of zinc oxide nanowires. The surface dissolution is successfully suppressed by lowering the polarity of the SAM-modifying solvent. Third, C9 aldehyde (nonanal) is electrically detected using a SAM-modified zinc oxide nanowire sensor. It was found that the recovery time of sensor response for SAM-modified zinc oxide nanowire sensor is significantly reduced without sacrificing sensor response value. Comprehensive analysis of surface reaction and molecular adsorption on zinc oxide nanowires reveals that alkyl chains of SAM change the conformation of nonanal on zinc oxide surface, resulting in effective oxidation of nonanal and fast desorption of nanonoic acid from the nanowire surface. The surface reaction control using SAM provides an ideal sensor design strategy that realizes a high sensor response with a quick recovery.