Nanoscale organic and polymeric field-effect transistors and their applications as chemical sensors

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Wang, Liang

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This work mainly focused on fabricating of nanoscale polycrystalline organic and conjugated polymeric thin-film field-effect transistors and investigating their scaling behaviors of electrical transport and chemical sensing properties. Devices with channel lengths systematically ranging from a few hundred microns down to sub 10 nm were successfully fabricated with the techniques such as stencil mask, photolithography, electron beam lithography, and break junction. The use of a novel four-terminal geometry ensures that the active area for charge transport and vapor sensing is truly nanoscale, and eliminates undesirable spreading currents traveling over the large area outside the defined channel to reduce the background signal level. It was discovered that upon scaling channel lengths from micron scale down to nanoscale, the dominating factors for charge transport and vapor sensing in organic thin-film transistors become different. At small dimensions, injection limited transport and field-dependent mobility are the dominant mechanisms for transport through the gate-modulated channel at low and high longitudinal fields respectively. Furthermore for sub 10 nm channels, tunneling effect plays an important role. In micron scale devices, the drain current usually decreases as a sensing response upon exposure of the polycrystalline organic/polymeric semiconductor layer to the analyte, mainly because of the transistor threshold shift caused by the immobile charges at grain boundaries trapped by the dipolar analyte molecules. The vapor sensing behavior of nanoscale organic transistors is markedly different (in an opposite direction of response) from that of large-scale devices for the same analyte-semiconductor combination, due to the fact that the electrical transport in a nanoscale OTFT depends on its morphological structure and interface properties (such as the injection barrier at the metal-organic semiconductor contacts) which could be modulated by the delivery of analyte.