pubs.acs.org/crystal Published on Web 02/24/2010 r 2010 American Chemical Society DOI: 10.1021/cg1000254 2010, Vol. 10 1923–1928 CrO 2 -to-Cr 2 O 3 Transformation in a Three-Dimensional Interference Field of Ultraviolet Laser Light Marc Audier,* ,† Mathieu Sala€ un, †,# Herv e Roussel, † Franc -ois Delyon, ‡ and Michel Duneau ‡ † Laboratoire des Mat eriaux et du G enie Physique, UMR CNRS 5628, Minatec-INP Grenoble, 3 parvis Louis N eel, BP 257, 38016 Grenoble Cedex 1, France, ‡ Centre de Physique Th eorique, Ecole Polytechnique, UMR CNRS 7644, F-91128 Palaiseau Cedex, France, and # Tyndall National Institute, University College Cork, “Lee Maltings”, Prospect Row, Cork, Ireland Received January 7, 2010; Revised Manuscript Received February 5, 2010 w n This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: In a previous work (J. Cryst. Growth 2009, 311, 2590), direct growth of a fcc architecture of Cr 2 O 3 phase onto (001)TiO 2 substrates has been obtained by photolysis of CrO 2 Cl 2 in a three-dimensional interference field of UV laser light. This growth includes a step of CrO 2 -to-Cr 2 O 3 transformation which is characterized in the present work from CrO 2 layers irradiated in UV interferences. Cr 2 O 3 is confirmed to form with specific crystallographic orientational relationships from results of X-ray diffraction, namely, [100] TiO 2 ) [100] CrO 2 ) [10 1] Cr 2 O 3 , [010] TiO 2 ) [010] CrO 2 ) [111] Cr 2 O 3 , [001] TiO 2 ) [001] CrO 2 ) [1 21] Cr 2 O 3 . According to these relationships, four orientations of Cr 2 O 3 domains are observed on (001)CrO 2 but only two on (100)CrO 2 . From transmission electron microscopy observations, thin layers of Cr 2 O 3 onto (100)CrO 2 exhibit a structure in domains with two orientations which can be assumed to be in epitaxy with habit planes {100}CrO 2 ) {10 1} Cr 2 O 3 . This epitaxy is justified from the similarities existing between the atomic arrangements of these planes. Finally, experimental conditions for further growth of a fcc architecture constituted of pure epitaxial CrO 2 phase are discussed. Introduction The growth of a three-dimensional (3D) periodic architec- ture of metal oxide of submicronic cell parameter obtained by decomposition of gas precursor onto a substrate surface exposed to 3D interference field of pulsed UV laser light (355 nm) has recently been proven 1-3 (Figure 1). The principle of this experiment 4 was derived from the method of Campbell et al. 5 who have demonstrated the fabrication of 3D photonic crystals through 3D holographic lithography. Chromyl chlor- ide (CrO 2 Cl 2 ) has been used as gas phase precursor as it could be decomposed by photolysis in an adsorbed state into CrO 2 , in a large range of photon energy including the UV-visible spectrum. 6,7 Because CrO 2 is however metastable and strongly absorbs light, it could be transformed by a thermal effect into Cr 2 O 3 under UV irradiation. 8 Quite well organized fcc architectures, as the one shown in Figure 1, have been obtained at relatively high UV power density (about 6  10 6 W 3 cm -2 per pulse of 10 ns) and low CrO 2 Cl 2 pressure on TiO 2 single crystal substrates cooled down to about 10 °C. In this case, the growth was beginning with an epitaxial formation of CrO 2 phase which partly transforms into Cr 2 O 3 under UV irradiation. Wih the Cr 2 O 3 phase exhibiting specific crystallographic orienta- tional relationships with respect to CrO 2 , its growth con- tinued with well-defined crystallographic orientations according to the 3D periodic modulations of electromag- netic energy of the interference field. From observations by transmission electron microscopy, it was found that indi- vidual particles forming the 3D periodic architecture are Cr 2 O 3 single crystal grains. In the present work, characteristics of the CrO 2 -to-Cr 2 O 3 transformation alone, obtained by irradiation of CrO 2 layers within the 3D interference field of UV laser light, were studied. One of our objectives was to estimate conditions for a future fabrication of a periodic architecture of the CrO 2 phase, without any transformation into Cr 2 O 3 . Furthermore, as many works have been devoted to the half metallic properties of the ferromagnetic CrO 2 phase and to the role of a very thin Cr 2 O 3 layer at its surface, 9-14 CrO 2 -Cr 2 O 3 interfaces were also studied. Experimental Section Epitaxial CrO 2 layers on (001) and (110) TiO 2 rutile substrates were prepared according to a method proposed by DeVries 15 and which, for instance, has been used by Ranno et al. 16 for a study of magnetic and electrical properties of CrO 2 . This method is based on the following two properties of CrO 2 : (i) CrO 2 becomes thermody- namically stable by increasing the oxygen pressure 17 and (ii) its structure is isomorphic of that of TiO 2 rutile. 18 For the preparation of each CrO 2 epitaxial layer, a TiO 2 single crystal was placed within a brass container of 3.5 cm 3 filled three-quarters with CrO 3 powder. The container was tightly closed with an aluminum joint and a brass cover. It was heated in a furnace up to 500 °C at a rate of 1 °C 3 min -1 , then maintained at 500 °C for 1 h and quickly cooled down by getting out the container from the furnace. Through this thermal treatment, the CrO 3 phase transforms into CrO 2 and oxygen. CrO 2 remains stable because the oxygen pressure goes up to about 1000 bar in the container. Opening the container, the powder initially dark-brown was black and the TiO 2 crystal, initially transparent and colorless, was covered by a shiny black layer of CrO 2 of about 2 μm. This layer was polished on abrasive diamond discs until a surface of mirror quality was obtained. Let us recall that 3D interferences are generated from a pulsed UV laser beam source entering an interferometer where it is divided into four beams converging at one point. According to the geometry *To whom correspondence should be addressed. E-mail: Marc.Audier@ grenoble-inp.fr.