Smooth TiO 2 Thin Films Grown by Aqueous Spray Deposition for Long-Wave Infrared Applications
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Smooth TiO2 Thin Films Grown by Aqueous Spray Deposition for Long-Wave Infrared Applications Sarmad Fawzi Hamza Alhasan,1,2 Seth R. Calhoun,3 Hussain Abouelkhair,3 Vanessa C. Lowry,3 Robert E. Peale,3 Imen Rezadad,4 Evan M. Smith5,6 Justin W. Cleary,6 Isaiah O. Oladeji7 1
Electrical and Computer Engineering, University of Central Florida, Orlando, FL 32816-2362, USA
2
Laser and Optoelectronics Engineering, University of Technology, Baghdad IRAQ.
3
Physics, University of Central Florida, Orlando FL 32816-2385 USA
4
NanoSpective Inc, 12565 Research Parkway, Ste. 390, Orlando FL 32826 USA
5
KBRWyle, Beavercreek OH 45431 USA.
6
Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB OH 45433, USA
7
SISOM Thin Films LLC, 1209 West Gore Street, Orlando, FL 32805, USA. [email protected]
ABSTRACT Self-assembled TiO2 films deposited by aqueous-spray deposition were investigated to evaluate morphology, crystalline phase, and infrared optical constants. The Anatase nanocrystalline film had ~10 nm characteristic surface roughness sparsely punctuated by defects of not more than 200 nm amplitude. The film is highly transparent throughout the visible to wavelengths of 12 m. The indirect band gap was determined to be 3.2 eV. Important for long-wave infrared applications is that dispersion in this region is weak compared with the more commonly used dielectric SiO2 for planar structures. An example application to a metal-insulator-metal resonant absorber is presented. The low-cost, large-area, atmosphericpressure, chemical spray deposition method allows conformal fabrication on flexible substrates for long-wave infrared photonics.
INTRODUCTION Titanium dioxide is a very well-studied material with wide ranging applications, as discussed in [1]. TiO2 films have been deposited by a wide variety of methods, representatively summarized in [1]. This paper considers TiO2 as a potentially
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useful material for long-wave infrared applications motivated by the relatively low dispersion for TiO2 in the 8 to 12 m wavelength range, in comparison to commonlyused SiO2. This work presents a potentially advantageous aqueous deposition method that provides smooth conformal coatings. We designate the film considered here as “smooth SPEED” TiO2 to distinguish it from a highly structured film grown by a different SPEED recipe reported earlier [1], which was designated “Ropy SPEED” according to its observed morphology. The complex refractive index spectra have been reported out to far infrared wavelengths (125 m) for TiO2 thin films prepared by vacuum-based physical deposition [2,3]. Our atmospheric-pressure aqueous inhomogeneous-chemical-reaction method has unique potential for low cost, large area, perfectly conformal [1, 4] roll-to-roll manufacturing. Physical and optical properties of s
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