Low-temperature processing and control of structure and properties of TiO 2 / c -sapphire epitaxial heterostructures
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Shivani Joshi Amity Institute of Nanotechnology, Noida, Uttar Pradesh-201301, India
Roger Jay Narayan Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907; and Joint Department of Biomedical Engineering, UNC Chapel Hill and North Carolina State University, Raleigh, North Carolina 27695-7115
Jay Narayanb) Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907 (Received 8 November 2012; accepted 7 February 2013)
We have investigated the formation of the rutile and the anatase polymorphs of TiO2, with emphasis on epitaxial growth characteristics, and defect content as a function of laser and substrate variables. X-ray diffraction (XRD) studies revealed that the rutile phase is more stable at higher substrate temperatures and lower oxygen pressures; in contrast, decreasing the temperature and increasing the oxygen pressure gave rise to formation of anatase. Epitaxial rutile films with a ,100] orientation were obtained at 450 °C using the pressure of 5 104 Torr, and laser energy of 3.5–4.0 J/cm2. The epitaxial relationship, determined by 2hh and f scan of XRD and confirmed by transmission electron microscopy (TEM) diffraction patterns, was shown to be rutile(100)||sapphire(0001), rutile[001]||sapphire[1010] and rutile[010]||sapphire[1210]. An atomically sharp interface between the rutile epitaxial film and the sapphire substrate was observed in the scanning transmission electron microscopy (STEM) images. The films exhibited a transmittance of 75–90% over the visible region. The absorption edge was observed to shift toward longer wave lengths at higher deposition temperatures or lower pressures. X-ray photoelectron spectroscopy and photoluminescence results showed that concentration of lattice point defects, namely oxygen vacancies and titanium interstitials, increased at lower oxygen pressures. Formation of nanostructured films with a surface roughness of -1.5–13.1 nm was confirmed by atomic force microscopy investigations.
I. INTRODUCTION
Titanium dioxide (TiO2) is a wide-band-gap semiconductor with versatile applications such as photocatalysis,1–4 photovoltaics,5–7 antibacterial applications,8,9 hydrophilic surfaces and antifogging mirrors, 10–12 gas sensors,13–15 bio-devices, 16,17 microelectronic devices, 18–20 and pigments.21,22 As often described, TiO2 crystallizes in three different polymorphs: rutile (tetragonal, space group P42/mnm, a 5 b 5 0.45933 nm and c 5 0.29592 nm), anatase (tetragonal, space group I41/amd, a 5 b 5 0.37852 nm a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2013.42 J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013
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and c 5 0.95139 nm), and brooki
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