Alexander Mozalev

Multifrequency interrogation of nanostructured gas sensor arrays : a tool for analyzing response kinetics

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.

Formation–structure–properties of niobium-oxide nanocolumn arrays via self-organized anodization of sputter-deposited aluminum-on-niobium layers

Nanostructured niobium oxide (NO) semiconductors are gaining increasing attention as electronic, optical, and electro-optic materials. However, the preparation of stable NO nanofilms with reproducible morphology and behavior remains a challenge. Here we show a rapid, well-controlled, and efficient way to synthesize NO films with 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 anodization of the Nb underlayer. The columns may grow 30–150 nm wide, up to 900 nm long, with an aspect ratio of 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 Nb2O5 mixed with Al2O3, Si-, and P-containing species in the surface region to NbO2 in the lower film layer. The post-anodization treatments result in the controlled formation of Nb2O5, NbO2, 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 layers for semiconductor gas microsensors with the sensitivity to ethanol and the response to hydrogen being among best ever reported.

Metal-substrate-supported tungsten-oxide nanoarrays via porous-alumina-assisted anodization: from nanocolumns to nanocapsules and nanotubes

An array of highly aligned tungsten-oxide (TO) nanorods, ∼80 nm wide, up to 900 nm long, spatially separated at their bottoms by tungsten metal on a substrate is synthesized via the self-localized anodization of aluminum followed by the porous-alumina-assisted re-anodization of tungsten in a sputter-deposited Al/W bilayer. Moreover, the pore-directed TO nanocapsules may grow, which can be electrochemically top-opened in alumina nanopores and transformed to TO nanotubes, representing unique architectures built up on tungsten substrates to date. The as-grown nanorods are composed of amorphous WO3 mixed with minor amounts of WO2 and Al2O3 in the outer layer and oxide–hydroxide compound (WO3·nH2O) with aluminum tungstate (2Al2O3·5WO3), mainly present inside the rods. Once the growing oxide fills up the pores, it comes out as an array of exotic protuberances of highly hydrated TO, with no analogues among the other valve-metal oxides. Vacuum or air annealing at 550 °C increases the portion of non-stoichiometric oxides ‘doped’ with OH-groups and gives monoclinic WO2.9 or a mixture of WO3 and WO2.9 nanocrystalline phases, respectively. The nanorods show n-type semiconductor behavior when examined by Mott–Schottky analysis, with a high carrier density of 7 × 1019 or 3 × 1019 cm−3 for the air- or vacuum-annealed samples, associated with a charge depletion layer of about 8 or 10 nm, respectively. A model for the growth of the metal-substrate-separated TO nanocapsules and tubes is proposed and experimentally justified. The findings suggest that the new TO nanoarrays with well-defined nano-channels for carriers may form the basic elements for photoanodes or emerging 3-D micro- and nano-sensors.

Selective hydrogen detection with TiO 2 nanofilm via the porous-alumina-assisted anodizing of titanium layers

Nanostructured metal oxides grown via porous-alumina (PA)-assisted anodization of sputtered refractory metals are of increasing interest as active layers for gas microsensors. Although nanostructured titanium oxide is particularly attractive, PA-assisted anodic formation of titanium oxide nanostructures remains a challenge because of their chemical instability caused by substantially mixing titania with alumina during film growth. Here we advance the approach and employ a unique combination of electrical and electrolytic conditions to achieve the PA-assisted formation of a stable amorphous TiO2 nanofilm on dielectric substrates, followed by high-temperature annealing to attain crystallization and enhance semiconductor properties of the film. A laboratory-scale microsensor employing the TiO2 nanofilm as active layer shows good sensitivity and selectivity to hydrogen against ethanol and carbon monoxide, which is in demand for proton-exchange fuel-cell applications.

Resistive switching in TiO2 nanocolumn arrays electrochemically grown

Resistive switching in metal oxides, especially in TiO2, has been intensively investigated for potential application in non-volatile memory microdevices. As one of the working mechanisms, a conducting filament consisting of a substoichiometric oxide phase is created within the oxide layer. With the aim of investigating the filament formation in spatially confined elements, we fabricate arrays of self-ordered TiO2 nanocolumns by porous-anodic-alumina (PAA)-assisted anodizing, incorporate them into solid-state microdevices, study their electron transport properties, and reveal that this anodizing approach is suitable for growing TiO2 nanostructures exhibiting resistive switching. The electrical properties and resistive switching behavior are both dependent on the electrolytic formation conditions, influencing the concentration and distribution of oxygen vacancies in the nanocolumn material during the film growth. Therefore, the PAA-assisted TiO2 nanocolumn arrays can be considered as a platform for investigating various phenomena related to resistive switching in valve metal oxides at the nanoscale.

3-D nanostructured tungsten-oxide gas-sensing film via anodizing sputter-deposited Al/W metal layers

The making of 3-D nanostructured metal oxide films is an active and competitive area of research, aiming at novel materials with enhanced properties and sensing devices with improved performances. Here we present the preparation procedure and gas sensing behavior of a novel self-assembled 3-D WO3 nanofilm that effectively combines the advantages of inorganic materials with the simplicity and universality offered by electrochemistry-based formation techniques. The film is formed mainly by electrochemical anodizing and is composed of an array of spatially-ordered upright-standing WO3 nanorods, assembled between the two noble metal patterned electrodes, which serve as direct semiconducting pathways for chemisorption reactions in a gas atmosphere. A test microsensor employing the nanofilm and assembled on a standard TO-8 Metal Can Package showed the fast and intensive response to H2, leaving more opportunities for further improvement of the active film configuration and sensor performance based on the computer-aided modelling and simulation results.