(355b) ZnO-Based Thin Films for Applications In Chemical Sensing

Zinc oxide (ZnO) is a wide band-gap
semiconductor with important applications in electronics, photonics, chemical
sensing and catalysis.  Sol-gel
chemistry provides a convenient route for low-cost fabrication of ZnO functional
coatings with controlled microstructure. Critical textural, chemical and
electronic properties can be further manipulated by doping ZnO with minor
components. In this work, we applied sol-gel wet-chemical techniques to
preparation of ZnO, Al-ZnO (Al:Zn = 1:10) and Cu-ZnO
(Cu:Zn = 1:10) thin film functional layers for chemiresistive sensors.

Cu and Al dopants influence the films'
surface morphology and their thermally induced chemical and structural
evolution. As prepared (room temperature) films exhibit the structure of
Layered Basic Zinc Acetate (LBZA), a lamellar ZnO precursor.  When annealed at temperatures through
700 ^oC, the films display similar chemical evolution
patterns—characterization by IR spectroscopy reveals that temperatures
above 500 ^oC are needed to completely desorb solvents and decompose
precursors. X-ray diffraction results show that Cu facilitates c-axis
orientation of the annealed film, while Al slows its crystallization. Details
of surface morphology, imaged by electron microscopy, also depend on the choice
of dopant. 

UV-vis spectra show that the Al dopant
slightly widens the band-gap of calcined film, while Cu increases absorption
broadly throughout the visible range. Chemiresistive sensors, fabricated by
coating thin film functional layers onto interdigitated electrode transducers,
were evaluated for their sensitivity to oxygen at operating temperatures
through 600 ^oC.  A
sensor coated with undoped ZnO displays good sensitivity for O₂ at
intermediate temperatures, ~400 ^oC, likely reflecting an optimal
balance between surface O₂ coverage and carrier availability.  Cu-ZnO, on the other hand, has a higher
base resistance (in nitrogen) and is less sensitive to O₂.