NANO LETTERS
Fabrication of Well-Ordered High-Aspect-Ratio Nanopore Arrays in TiO2 Single Crystals
2006 Vol. 6, No. 5 1065-1068
Ruy Sanz,*,†,‡ Anders Johansson,† Marek Skupinski,† Jens Jensen,† Go1 ran Possnert,† Mats Boman,† Manuel Va´zquez,‡ and Klas Hjort† The Ångstro¨m Laboratory, Box 534, Uppsala UniVersity, SE-751 21 Uppsala, Sweden, and Instituto de Ciencia de Materiales de Madrid, Consejo Superior de InVestigaciones Cientı´ficas Cantoblanco, 28049 Madrid, Spain Received January 30, 2006; Revised Manuscript Received April 18, 2006
ABSTRACT In this work a successful method for producing high-aspect-ratio nanopatterned single-crystal TiO2 is presented. The method used is based on nanolithography involving swift heavy ion bombardment through a porous anodic alumina mask. Nanopatterning of large areas allows for fabrication of new devices, for example, photonic crystals and electrodes for energy storage and conversion. Crystalline TiO2 also presents optimal characteristics for optical and catalysis applications. Samples were irradiated by MeV Br7+ ions with fluencies ranging from 7.9 × 1013 to 1.2 × 1015 cm-2. The high-energy Br7+ ions induce latent tracks of amorphous material into the TiO2 crystal suitable for selective etching by hydrofluoric acid. High-aspect-ratio (16) nanopatterned areas, up to 4 mm2, were obtained in a single radiation spot onto single-crystalline TiO2.
Advances in technology call for improved low-cost methods for the production of high-aspect-ratio nanopatterns. Ordered arrays of pores in a variety of materials offer a palette of interesting potential applications, such as optical waveguides,1 catalytic supporting oxides,2 and the use of the pore array as a template for the synthesis of well-defined nanostructures.3-5 A method for electrochemical fabrication of almost perfect hexagonal arrays of pores in alumina has been known for a decade.6 However, this method is applicable only for the aluminum system. For production of ordered nanoporous arrays in other materials, a variety of techniques have been used, for example, laser ablation through focusing microspheres,7 chemical etching through anodic alumina membranes into TiO2,8 and nanopatterning using focused ion beam, FIB. Ion beam lithography through a porous alumina mask using low-energy (keV) ions has also been employed.9-12 In those studies the pattern from the alumina mask are transferred, but the aspect ratio was generally low. In addition, the lateral straggling of the induced features are high because of elastic scattering. Ion track lithography, using high-energy (MeV) ions, offers pattern transfer over large areas in a variety of materials at affordable costs and easy implementation. The basis of ion track lithography is the inelastic energy transfer from swift heavy ions to the atoms in the lattice of the * Corresponding author. E-mail:
[email protected]. † Uppsala University. ‡ Instituto de Ciencia de Materiales de Madrid. 10.1021/nl0602185 CCC: $33.50 Published on Web 04/27/2006
© 2006 American Chemical Society
bombarded materials via electronic ionization and excitation. The induced rapid heating and quenching (thermal spike) leads to creation of latent ion tracks of transformed material along the path of the ion passing through the material.13,14 Compared to low-energy ion beam lithography,12 where elastic scattering is a dominating factor, the spread of ions in ion track lithography is minute, yielding better defined lateral features. TiO2 has the highest known dielectric constant of the oxide materials. This property gives TiO2 suitable characteristics in electronic and optic applications; for example, it was suggested15 that TiO2 would be a good gate oxide for the next generation of MOSFET. This is due to its high dielectric constant that requires a low electric tunneling at nanometer scale, substituting SiO2-based devices with TiO2 devices. Additionally, crystalline TiO2 has interesting properties as a catalyst in some chemical reactions; for example, recent work demonstrates that the surface of crystalline nanoporous TiO2 improves the photocleavage of water.2 In this letter a method for fabrication of high-aspect-ratio nanopatterning of TiO2 is demonstrated. Our group has developed an aligning technique that makes use of the Rutherford Backscattered Spectroscopy (RBS) analysis,16 making it possible to obtain a good pattern transfer to SiO2 by ion track lithography.17 By applying the same method, we transferred the porous alumina pattern to TiO2 substrates of different crystalline orientations. The nanopatterned
Figure 1. Cross sections tilted 30°; the average pore depth is (a) 600 nm (13 MeV Br ions) and (b) 1100 nm (25 MeV Br ions).
surfaces obtained by the present method have a well-defined spatial porosity. With well-defined porosity of TiO2 it is possible to obtain a refraction index suitable for photonic crystal applications. There are studies that confirm that a photonic crystal made of silicon with the same kind of pattern can be used as a waveguide. By controlling the pore size and interpore diameter of alumina masks, it would be possible for one to develop photonic crystals suitable for optical and MHz-GHz band communications.18 A thin layer (
