Kinetics of Linear Defect Formation in Gallia-Doped Rutile
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Kinetics of Linear Defect Formation in Gallia-Doped Rutile. Nathan Empie and Doreen Edwards Alfred University School of Engineering New York State College of Ceramics Alfred, NY, 14802 ABSTRACT The diffusion of Ga2O3 into the surface of single crystal [001] rutile leads to the insertion of ß-gallia subunits along {210} planes of the parent rutile structure. These linear defects introduce hexagonally shaped tunnels, approximately 2.5 Å in diameter, normal to the [001] surface. Because these tunnels may serve as highly reactive sites for the attachment of macromolecules, we are exploring the application of these linear defects for creating nanostructures. The current work investigates the kinetics of defect formation and the factors that affect defect periodicity and orientation. Gallium oxide was applied to the surfaces of [001]oriented TiO2 single-crystal substrates via a sol-gel spin-coating process using a galliumcontaining precursor. Thermal treatments were systematically varied to obtain different defect surface structures. Defect orientation and the surface concentration of rows of defects were characterized via tapping mode atomic force microscopy. INTRODUCTION Beta-gallia-rutile intergrowths form according to Ga4Tin-4O2n-2 (where n is 9-51 and odd).1-4 Beta-gallia subunits occur along {210} planes of the parent rutile structure (Figure 1).1,2,4-7 The ratio of Ga to Ti dictates the distance between gallia-rich regions, i.e. increasing n increases the distance between adjacent (210) defect boundaries.4,8 The incorporation of betagallia subunits into rutile forms ß-Gallia hexagonally shaped tunnels ~2.5 Å in subunit diameter, which are adequate for small to mid-sized cation insertion. Mica has been used as a substrate for DNA attachment, where the two negatively charged species are bound by cations acting as an electrostatic bridge.9-11 We speculate that cations inserted throughout ß-gallia-rutile intergrowth tunnel sites along {210}r defect boundaries could perform similarly, binding macromolecules to the surface. The periodicity of tunnel sites in conjunction with the tailor-able (210) (210)r defect boundary separation could be exploited to design Figure 1. Projection of the ß-gallia rutile patterned substrates to aid the assembly of structure along [001] of parent rutile structure.4 organized nano- and micro-constructs.
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EXPERIMENTAL PROCEDURE The Ga2O3 coatings were produced from a gallium isopropoxide solution precursor, using techniques developed by Li et.al.12 The isopropoxide was diluted to a concentration of 0.15 M in isopropanol. The sol was sonicated for one-hour, and let stand for 24 hours prior to coating. The sol was applied to the rutile substrate ((001) oriented, 5x5x0.5mm, MTI corp.) via spin coating for 25 seconds at 2500 rpm. The samples were allowed to dry at room temperature for 24 hrs, followed by separate firing cycles for the two samples. Sample A was fired at 900 oC, 1000 oC, 1100 oC, 1200 oC, and 1300 oC for 24 hrs at each temperature before cycles of 96, 192, 288,
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