Yoshihiro Kikuzawa

Improving the sensitivity and selectivity of alcohol sensors based on organic thin-film transistors by using chemically-modified dielectric interfaces

Organic thin-film transistors (OTFTs) with chemically-modified dielectric interfaces are proposed as exhaled-alcohol sensors. Although sensors based on OTFTs have high sensitivity to moisture and would therefore be unsuitable for use as exhaled-gas sensors, it was found that a chemically-modified dielectric interfaces play a significant role in the sensitivity and selectivity of ethanol/moisture. OTFTs fabricated on a surface with low surface free energy, such as (tridecafluoro-1,1,2,2,-tetrahydrooctyl) trimethoxysilane (FTS), octadecylmethoxysilane (OTS) and hexamethyldisilazen (HMDS)-treated surfaces, have low sensitivity to moisture. OTFTs fabricated on a surface with a high ratio of dispersive to polar components of the surface free energy, such as HMDS and OTS-treated surfaces, have high selectivity of ethanol to moisture.

Improving baseline stability of gas sensors based on organic field-effect transistors by monitoring carrier mobility

We have investigated the baseline stability of gas sensors based on organic field-effect transistors (OFETs). The time response of the drain current, carrier mobility, and threshold voltage to exposure to humidity were monitored. The extraction of these multiple parameters has so far been reported in order to enhance sensitivity and/or discrimination. We found that the time response of the carrier mobility possesses baseline stability, as opposed to the drain current and threshold voltage. The baseline drifts with time are mainly due to shifts in the threshold voltage that affect the drain current. Our results are expected to assist in making OFET-based gas sensors suitable for practical applications.

Transistor Performance and Film Structure of Hexabenzocoronene Derivatives

Hexabenzocoronene (HBC) derivatives that are designed to self-assemble into lamellar aggregates were synthesized. The derivatives were deposited as an active layer in an organic field-effect transistor (OFET) using vacuum sublimation. The dihexyl and tetrahexyl derivatives (2H-HBC, 4H-HBC) increased the field-effect mobilities and on/off ratios by a factor of 10 or more compared to unsubstituted HBC and hexahexyl-hexabenzocoronene (6H-HBC). The crystal and thin film structures were determined by powder x-ray diffraction and grazing incidence X-ray diffraction (GIXD). The data indicate that 2H-HBC and 4H-HBC self-assemble into lamellar aggregates. 2H-HBC forms layers of aromatic cores that are sandwiched by the layers of hexyl groups, which is a preferable crystal structure for carrier transport. The good OFET performance could be explained by the self-assembly in lamellar aggregates of 2H-HBC and 4H-HBC, in contrast to self-assembly in the columnar aggregate of 6H-HBC and the low self-assembling properties of unsubstituted HBC.