An in-situ study of organic semiconductor thin films for gas sensing.

摘要

Organic semiconductors are an attractive platform for developing chemisensors, because of their customizable surface chemistry. An understanding of the sensing mechanism would help develop surface chemistry design for molecular recognition. We have studied the steric and chemical effects of acetone and ethanol on sublimated and spun organic films, which are used as chemisensor transducers.;We designed deposition and exposure systems to study the surface current and chemistry of rubrene, pentacene, and 5,5'-bis(4-hydroxyhexylphenyl)-2,2'-bithiophene (C6) in response to acetone and ethanol vapors. A rubrene crystal and spun film, as well as sublimated C6 and pentacene films and transistors were exposed under vacuum to saturated vapor pressure acetone gas. The surface current was recorded in real-time, while the infrared absorption (IRAS) signature of the acetone film effect was recorded in-situ.;Some chemical interaction between acetone and the organic substrate was observed, and led to the use of saturated alkyl perfluorinated trichlorosilane (FTS) monolayer coverage of rubrene to prevent acetone degradation and removal of an amorphous spun rubrene film. Acetone removed only 1.5% of the FTS film from saturated coverage, physisorbed multilayers of FTS on rubrene. Besides this small chemical effect, the main effect on the sensor current is due to the physisorption of acetone itself.;In the cases of pentacene and C6 sublimated films, the surface chemistry and surface current could both be measured on the same film. Acetone intercalation into pentacene and C6 sublimated films perturbed the out-of-plane C-H bending, possibly due to steric interaction. An irreversible reduction in surface current and carrier mobility was found. Acetone caused the thickest pentacene thin film tested (1500 A) to produce the highest intensity differential peaks, similar to the effect of cooling the film by 3°C--7°C. However, cooling did not fully account for transistor current reduction. Also, the surface chemistry response decreased upon reuse, and the surface current was not quantitatively reproducible. This behavior may be explained by the fact that the contact adhesive was soluble in acetone. Less surface chemical change occurred for ethanol than for acetone exposures, despite the fact that the C6 molecule has a more reactive hydroxy-hexanol termination.