Derek R. Miller

Correlative STEM-Cathodoluminescence and Low-Loss EELS of Semiconducting Oxide Nano-Heterostructures for Resistive Gas-Sensing Applications

Recent advances in resistive-type oxide gas sensors have been made primarily by the combination of multiple materials into nano-heterostructures that often show enhanced or unique properties compared to each pure material constituent [1,2]. It has been shown to be especially useful to use highly crystalline one-dimensional nanorods and nanowires decorated with either discrete oxide particles or a continuous coating generating a core-shell structure [3]. The operating principle of these sensors is simply measuring a change in resistance of a film deposited between two or more electrodes, which varies with the number of charge carriers accepted or donated between the surface and the nearby gas molecules. Two different resistance-dominating mechanisms exist in these sensors when the nanowires are deposited as a random network film [1]. In both mechanisms a depletion layer is formed at the surface and a larger depletion layer creates a higher resistance. The first mechanism considers the potential energy barrier at the interface between two nanowires that an electron must overcome to move through the film. The second mechanism considers electrons moving along the axis of a nanowire and the constriction of the cross-sectional area that does not lie in the depletion region. This can be equated to pushing a current through a smaller diameter wire, which increases the resistance. The second axial mechanism can be engineered through decoration or coating of, for example, p-type Cr2O3 onto an n-type SnO2 nanowire. The p-n junctions created can affect this mechanism greatly, enhancing the sensor response and even making the material more selective toward specific gases and reducing cross-interference.