Novel Top-Contact Monolayer Pentacene-Based Thin-Film Transistor for Ammonia Gas Detection

We report on the fabrication of an organic field-effect transistor (OFET) of a monolayer pentacene thin film with top-contact electrodes for the aim of ammonia (NH₃) gas detection by monitoring changes in its drain current. A top-contact configuration, in which source and drain electrodes on a flexible stamp [poly­(dimethylsiloxane)] were directly contacted with the monolayer pentacene film, was applied to maintain pentacene arrangement ordering and enhance the monolayer OFET detection performance. After exposure to NH₃ gas, the carrier mobility at the monolayer OFET channel decreased down to one-third of its original value, leading to a several orders of magnitude decrease in the drain current, which tremendously enhanced the gas detection sensitivity. This sensitivity enhancement to a limit of the 10 ppm level was attributed to an increase of charge trapping in the carrier channel, and the amount of trapped states was experimentally evaluated by the threshold voltage shift induced by the absorbed NH₃ molecular analyte. In contrast, a conventional device with a 50-nm-thick pentacene layer displayed much higher mobility but lower response to NH₃ gas, arising from the impediment of analyte penetrating into the conductive channel, owing to the thick pentacene film

Nanoseed Assisted PVT Growth of Ultrathin 2D Pentacene Molecular Crystal Directly onto SiO₂ Substrate

High order of molecular packing and perfect semiconductor/dielectric interface are two key factors to achieve high performance for organic field-effect transistors (OFET). Moreover, the thin crystal offers an improved efficiency of carrier injection for OFETs. To this aim, formation of thin and large single crystal directly on dielectrics is the basis to obtain the ideal crystal OFETs. Herein, we report the controlled growth of ultrathin 2D Pentacene (Pn) crystal via nanoseed assisted physical vapor transport (PVT) method grown directly on SiO₂. The size, thickness, and density of Pn crystals are systematically studied. Potentially effective parameters such as initially lowered Pn coverage and decreased supersaturation with the aid of carrier gas flow were optimized to grow large, ultrathin 2D Pn crystalline flakes efficient for the fabrication of crystal OFETs. The typical size and thickness of as-grown Pn crystalline flakes can be controlled to be large and thin enough. Device of ultrathin crystal with bottom gate and top contact configuration showed mobility as high as 5.6 cm² V^–1 s^–1, indicating that the proposed novel architecture of organic molecular crystals may pave the way toward the application of large-sized single crystals of Pn in organic electronics