Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film

Abstract

Porous tungsten oxide films were deposited onto a sensor substrate with a Si bulk-micromachined hotplate, by drop-coating isopropyl alcohol solution of highly crystalline tungsten oxide $\left(\mathrm{W}{\mathrm{O}}_{2.72}\right)$ nanorods with average $75\mathrm{nm}$ length and $4\mathrm{nm}$ diameter. The temperature-dependent gas sensing characteristics of the films have been investigated over the mild temperature range from $20\text{to}250°\mathrm{C}$ . While the sensing responses for ammonia vapor showed increase in electrical conductivity at temperatures above $150°\mathrm{C}$ as expected for $n$ -type metal oxide sensors, they exhibited the opposite behavior of unusual conductivity decrease below $100°\mathrm{C}$ . Superb sensing ability of the sensors at room temperature in conjunction with their anomalous conductivity behavior might be attributed to unique nanostructural features of very thin, nonstoichiometric $\mathrm{W}{\mathrm{O}}_{2.72}$ .