Nanostructured niobium oxide (NO) semiconductor is gaining increasing attention as electronic, optical, and electro-optic material. However, the preparation of stable NO nanofilms with reproducible morphology and behavior remains a challenge. 10Here we show a rapid, well-controlled, and efficient way to synthesize NO films with the self-organized columnlike nanostructured morphologies and advanced functional properties. The films are developed via the growth of a nanoporous anodic alumina layer, followed by the pore-directed 15 anodizing of the Nb underlayer. The columns may grow 30-150 nm wide, up to 900 nm long, with the aspect ratio up to 20, being anchored to a thin continuous oxide layer that separates the columns from the substrate. The as anodized films have a graded chemical composition changing from amorphous Nb 2 O 5 mixed with Al 2 O 3 , Si-, and P-containing species in the surface region to NbO 2 in the lower film layer. The post-anodizing treatments result in the controlled 20 formation of Nb 2 O 5 , NbO 2 , and NbO crystal phases, accompanied by transformation from nearly perfect dielectric to n-type semiconductor behavior of the films. The approach allows for the smooth film growth without early dielectric breakdown, stress-generated defects, or destructive dissolution at the respective interfaces, which is a unique situation in the oxide films on niobium. The functional properties of the NO films, revealed to date, allow for potential applications as nanocomposite capacitor dielectrics and active 25 layers for semiconductor gas microsensors with the sensitivity to ethanol and the response to hydrogen being among best ever reported.

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This paper presents a unique perspective on enhancing the physicochemical mechanisms of two distinct highly sensitive nanostructured metal oxide micro hot plate gas sensors by utilizing an innovative multifrequency interrogation method. The two types of sensors evaluated here employ an identical silicon transducer geometry but with a different morphological structure of the sensitive film. While the first sensing film consists of self-ordered tungsten oxide nanodots, limiting the response kinetics of the sensor-chemical species pair only to the reaction phenomena occurring at the sensitive film surface, the second modality is a three-dimensional array of tungsten oxide nanotubes, which in turn involves both the diffusion and adsorption of the gas during its reaction kinetics with the sensitive film itself. By utilizing the proposed multifrequency interrogation methodology, we demonstrate that the optimal temperature modulation frequencies employed for the nanotubes-based sensors to selectively detect hydrogen, carbon monoxide, ethanol, and dimethyl methyl phosphonate (DMMP) are significantly higher than those utilized for the nanodot-based sensors. This finding helps understand better the amelioration in selectivity that temperature modulation of metal oxides brings about, and, most importantly, it sets the grounds for the nanoengineering of gas-sensitive films to better exploit their practical usage.

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