We describe herein a novel approach for visual indication of trace organic vapors. The sensor utilizes a microporous material within a visual thin film transducer to produce changes in color upon exposure to a very wide range of volatile organic compounds. Visual indication at 5 parts per million (ppm) is demonstrated, with optoelectronic detection achieved to below 50 parts per billion (ppb). Through a thoughtful design of the sensor, we are able to avoid interference from water vapor, a critical attribute needed for practical application.

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The organic red pigment N,N-di(3,5-xylyl)perylene-3,4:9,10bis(dicarboximide) is easily vacuum deposited at ambient substrate temperatures to form ordinary looking smooth films. When subsequently annealed in vacuo between ∼160 and 220 °C, an initially 1000 Å uniformly thick film transforms to an extremely dense, random array of discrete, crystalline whiskers, uniformly oriented with their long axes normal to the substrate. Whisker growth occurs at zero supersaturation ratio (no incident flux) and all film material is conserved. Whisker cross sectional dimensions are ∼27×55 nm, lengths 1–1.5 μm, and areal number densities ∼50 per square micron. The high-aspect-ratio whiskers grow purely by surface diffusion to minimum energy binding sites believed to be emergent screw dislocations on the whisker tips, while the uniform cross sectional shapes reflect the ratio of surface free energies of the bounding crystal faces. The transformation has been followed in situ with reflection absorption IR spectroscopy as a function of time and temperature to obtain a dimensionless growth parameter. The surface self-diffusion coefficient and surface entropy to free energy ratio are extracted. The pigment whiskers are readily sputter or vapor-coated to provide conformally coated nanostructure-sized features with metal and semiconductor properties. Vapor coating and annealing of the original pigment films can be done reproducibly in large areas and offer a novel alternative approach to plasma etching and photolithography for producing nanostructured surfaces with new properties.

The electrical characteristics of Au/n-TiO2 Schottky diodes have been studied using I-V and C-V measurements. TiO2 samples with working face perpendicular to the c axis are reduced in a vacuum of 10−6 Torr at 800 °C for about 5 h and then quenched. The resistivities are in the range 20–30 Ω cm. The barrier heights deduced from I-V characteristics in agreement with the thermionic emission theory are in the range 0.87–0.94 eV. C-V data yield lower barrier heights and show a frequency dependence attributed to relaxation phenomena occurring in a disturbed layer near the surface. Comparison with results relative to Au/n-SrTiO3 diodes shows that the barrier heights obey the Schottky model for these ionic semiconductors, confirming the role of the electron affinity in the band bending formation.

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