DOI: 10.1021/cg1000254
CrO2-to-Cr2O3 Transformation in a Three-Dimensional Interference Field of Ultraviolet Laser Light
2010, Vol. 10 1923–1928
Marc Audier,*,† Mathieu Sala€ un,†,# Herve Roussel,† Franc-ois Delyon,‡ and Michel Duneau‡ †
Laboratoire des Mat eriaux et du G enie Physique, UMR CNRS 5628, Minatec-INP Grenoble, 3 parvis eorique, Ecole Louis N eel, BP 257, 38016 Grenoble Cedex 1, France, ‡Centre de Physique Th 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 This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. n
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ABSTRACT: In a previous work (J. Cryst. Growth 2009, 311, 2590), direct growth of a fcc architecture of Cr2O3 phase onto (001)TiO2 substrates has been obtained by photolysis of CrO2Cl2 in a three-dimensional interference field of UV laser light. This growth includes a step of CrO2-to-Cr2O3 transformation which is characterized in the present work from CrO2 layers irradiated in UV interferences. Cr2O3 is confirmed to form with specific crystallographic orientational relationships from results of X-ray diffraction, namely, [100]TiO2 [100]CrO2 [101]Cr2O3, [010]TiO2 [010]CrO2 [111]Cr2O3, [001]TiO2 [001]CrO2 [121]Cr2O3. According to these relationships, four orientations of Cr2O3 domains are observed on (001)CrO2 but only two on (100)CrO2. From transmission electron microscopy observations, thin layers of Cr2O3 onto (100)CrO2 exhibit a structure in domains with two orientations which can be assumed to be in epitaxy with habit planes {100}CrO2 {101} Cr2O3. 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 CrO2 phase are discussed.
Introduction The growth of a three-dimensional (3D) periodic architecture 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 proven1-3 (Figure 1). The principle of this experiment4 was derived from the method of Campbell et al.5 who have demonstrated the fabrication of 3D photonic crystals through 3D holographic lithography. Chromyl chloride (CrO2Cl2) has been used as gas phase precursor as it could be decomposed by photolysis in an adsorbed state into CrO2, in a large range of photon energy including the UV-visible spectrum.6,7 Because CrO2 is however metastable and strongly absorbs light, it could be transformed by a thermal effect into Cr2O3 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 6 106 W 3 cm-2 per pulse of 10 ns) and low CrO2Cl2 pressure on TiO2 single crystal substrates cooled down to about 10 C. In this case, the growth was beginning with an epitaxial formation of CrO2 phase which partly transforms into Cr2O3 under UV irradiation. Wih the Cr2O3 phase exhibiting specific crystallographic orientational relationships with respect to CrO2, its growth continued with well-defined crystallographic orientations according to the 3D periodic modulations of electromagnetic energy of the interference field. From observations by transmission electron microscopy, it was found that individual particles forming the 3D periodic architecture are Cr2O3 single crystal grains.
In the present work, characteristics of the CrO2-to-Cr2O3 transformation alone, obtained by irradiation of CrO2 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 CrO2 phase, without any transformation into Cr2O3. Furthermore, as many works have been devoted to the half metallic properties of the ferromagnetic CrO2 phase and to the role of a very thin Cr2O3 layer at its surface,9-14 CrO2-Cr2O3 interfaces were also studied. Experimental Section
*To whom correspondence should be addressed. E-mail: Marc.Audier@ grenoble-inp.fr.
Epitaxial CrO2 layers on (001) and (110) TiO2 rutile substrates were prepared according to a method proposed by DeVries15 and which, for instance, has been used by Ranno et al.16 for a study of magnetic and electrical properties of CrO2. This method is based on the following two properties of CrO2: (i) CrO2 becomes thermodynamically stable by increasing the oxygen pressure17 and (ii) its structure is isomorphic of that of TiO2 rutile.18 For the preparation of each CrO2 epitaxial layer, a TiO2 single crystal was placed within a brass container of 3.5 cm3 filled three-quarters with CrO3 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 CrO3 phase transforms into CrO2 and oxygen. CrO2 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 TiO2 crystal, initially transparent and colorless, was covered by a shiny black layer of CrO2 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
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Figure 1. n w Three-dimensional periodic organization of the Cr2O3 deposit obtained on a (001) TiO2 substrate by photolysis of CrO2Cl2 in a 3D interference field of UV laser light (after ref 1). A Quicktime movie showing a simulated growth of a fcc architecture is available in the HTML version of this paper. between the four beams, their intensity ratios, and polarizations, a pseudo fcc interference pattern (trigonal R3c actually) of highest contrast, that is, with energy minima equal to zero, is obtained inside the overlapping volume between the four beams.4 Each laser beam has a diameter of 8 mm. CrO2 sample planes were placed perpendicularly (or nearly) to the 3-fold axis of symmetry of the interference field. UV irradiation was carried out by varying the power density (W) and the number of laser pulses (npulse). However, as the CrO2-to-Cr2O3 transformation occurs at about 400 C,15,17 a power density threshold was found to be in between 3 106 and 4 106 W 3 cm-2 per pulse of 10 ns. Above this threshold, results on the aspect of the Cr2O3 patterning, directly observed from visible light diffraction and after by optical microscopy were apparently found to depend on the number of photons (i.e., nph = npulseWtpulse/hν, where tpulse is the pulse duration and hν the photon energy at 355 nm): the amount of Cr2O3 phase increased with the total number of photons by increasing either npulse or W. Periodic patterning of these epitaxial CrO2 layers by UV irradiation was observed by optical microscopy (OM) using an oil immersion objective of magnification 160 and numerical aperture (N.A.) of 1.4. In this case, the smallest resolvable distance is about 200 nm from the Rayleigh criterion (i.e., d = 0.61λ/N.A.) and a visible wavelength of 450 nm (i.e., the shortest wavelength of visible light). This was sufficient since the smallest distance expected to be observed is a/61/2 = 376.4 nm for a 3D interference of pseudo fcc structure, of cell parameter a = 922 nm, projected along a 3-fold axis. Orientational crystallographic relationships between CrO2 and Cr2O3 were identified by X-ray diffraction using a texture goniometer, and interfaces between both these structures were characterized by electron diffraction and high resolution imaging on a transmission electron microscope (TEM Jeol 2011 UHR).
Results Periodic Cr2O3-CrO2 Patterning. As an example of results, Figure 2 shows two OM images of different magnifications of a patterning obtained on a (100) CrO2 layer for five pulses of irradiation at a power density of 1.2 107 W 3 cm-2 per pulse. A [110] direction of the pseudo-fcc interference pattern was set parallel to the c axis of the tetragonal CrO2 structure. Three insets show (i) the aspect of a nonirradiated
Figure 2. (a, b) Optical microscopy images of (100) CrO2 layer partly transformed into Cr2O3. Insets of (a) show, on the left, the nonirradiated CrO2 layer with dark traces along the [001] axis and, on the right, a diffraction of red to orange light of the irradiated area; the inset of (b) is a video-camera image of the interferences.
(100) CrO2 layer, exhibiting dark lines parallel to the c axis (also observed after irradiation on the image (a)), (ii) a diffraction of visible light extending nearly to the UV beam diameter, and (iii) a video camera image of the interference pattern which appears to be very similar to parts of the Cr2O3-CrO2 patterning. Large strips in the contrast variation observed on the image (a) are due to a slight disorientation between the normal to the sample plane and the 3-fold axis of symmetry of the interference pattern. Considering the conventional description of a ABC stacking of (111) fcc planes, the disorientation was determined to be of about 2.7 from the periodic distance between strips and the d111 spacing. Both Cr2O3 and CrO2 phases were found to exhibit respectively dark and clear contrasts from OM observations of nonirradiated and irradiated CrO2 layers more or less transformed. As the 3D interference is composed of two equivalent fcc lattices of hot and cold points related by a translation a/2 (i.e., like in a NaCl structure) it produces contrast inversions through a ABC stacking as observed on the image (a). Thus, the width ratio between dark and clear strips is directly proportional to the surface ratio of Cr2O3/ CrO2 phases. The majority phase on the image (a) appears to be CrO2. The dark and straight traces parallel to the c axis of
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observed at a polar angle χ = 90 due to a slight disorientation with respect to the [100] zone axis. In the case of a (001) TiO2 substrate, it was eight reflections for each phase in agreement with their corresponding [001]{211} pole figure (Figure 3b). The stereographic projections of both isomorphic CrO2 and TiO2 phases superimpose almost perfectly due to similar cell parameter ratios (a/c). • One scan on {112} reflections of Cr2O3 (2θ = 33.597) (corresponding to {104} reflections for the hexagonal cell choice). Cr2O3 is a trigonal structure isomorphic to sapphire Al2O3. This type of 112 reflection has a multiplicity of 6 and is the strongest reflection of Cr2O3. Eight large and intense reflections in mirror symmetry are observed in the case of (100) TiO2 substrate. There are four reflections at χ ≈ 32.5 and four at χ ≈ 90 (i.e., corresponding to hkl-hkl couples). In the case of a (001) TiO2 substrate, it is 12 reflections in 4-fold symmetry with four {112} reflections of Cr2O3 at χ ≈ 52 and eight at χ ≈ 72. For determining the different orientations of the Cr2O3 rhombohedral cell with respect to (100) CrO2, we have considered the four groups of equivalent crystallographic orientational relationships between Cr2O3 and (001) TiO2 (or (001)CrO2) previously found to be related by a 4-fold symmetry about the c axis of the tetragonal substrate1 and corresponding to )
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½100TiO2 ½100CrO2 ½101Cr2 O3
CrO2 observed before and after irradiation can be attributed to very thin Cr2O3 sandwiches in between elongated CrO2 domains, in agreement with Cr2O3 thin layers observed on CrO2 nanorods with a long axis corresponding to c.19 Similar results were obtained on (001) CrO2 layers. In this case, however, CrO2 domains of polygonal shape with dark line boundaries mainly along Æ100æ directions were observed on nonirradiated layers. Note that such engravings were barely identifiable by scanning electron microscopy because of a weak atomic number contrast between Cr2O3-CrO2 and a lack of variation in surface relief. An X-ray diffraction analysis of the Cr2O3 patterning of a (100) CrO2 layer has been performed on a texture goniometer (Figure 3a) in a similar way as those previously reported for CrO2-Cr2O3 deposits on (001) CrO2 substrates.1 The result is recalled in Figure 3b for a comparison. Both stereographic projections of experimental results shown in the upper part of the figure correspond to a superposition of three j-χ scans: • Two scans for Bragg’s angles corresponding to {211} reflections of TiO2 (2θ = 54.344) and CrO2 (2θ = 56.169). Four reflections were observed for each scan to be in agreement with a plot of their [100]{211} pole figure in the case of an epitaxy of CrO2 onto (100) TiO2 substrate. Couples of hkl-hkl reflections are not
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½001TiO2 ½001CrO2 ½121Cr2 O3 )
Figure 3. Comparison of X-ray diffraction pole figures for the growth of CrO2 and Cr2O3 phases on (a) (100) TiO2 and (b) (001) TiO2 substrates. As deduced from the plot of theoretical pole figures; two and four orientations of Cr2O3 rhombohedral cell are respectively observed with the same type of crystallographic orientational relationships (see text).
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½010TiO2 ½010CrO2 ½111Cr2 O3
From 90 rotation about the [010] axis of the tetragonal substrate the zone axis of orientation of the four rhombohedral cell becomes [111] and [101] (bottom part of Figure 3). But it appears that the Cr2O3 rhombohedral cell exhibits only two orientations of the [101] zone axis when comparing the plot of the corresponding pole figure (i.e., of [111]{112} and [101]{112} type) to experimental results. Let us note, however, that very weak intensities are observed for {112} reflections corresponding to both [111] orientations. This is probably due to very thin Cr2O3 sandwiches in between elongated CrO2 domains, previously observed by OM as dark lines parallel to the c axis. According to the above orientational relationships, such Cr2O3 layers should be oriented along a [111] zone axis parallel to the a axis of CrO2. TEM Characterizations. Samples for TEM observations were prepared by depositing fragments of irradiated or nonirradiated CrO2 layers on copper grids coated with holey carbon films. These fragments were obtained by scratching the layer surface with a diamond tip. With this simple method of preparation, a partial amorphization and reduction of the CrO2 phase, as we have observed by ion milling, could be avoided. Among the fragments observed, some were only of the Cr2O3 phase. They were always constituted of domains with two different orientations related by a mirror symmetry, as previously observed by X-ray diffraction. Figure 4 shows a high resolution image of the arrangement of these Cr2O3 domains with the corresponding selected area electron diffraction (SAED) pattern. From the interpretation of this pattern, it appears that domains oriented along a [101] zone
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Figure 4. High resolution TEM image of the Cr2O3 phase formed by UV irradiation of the (100) CrO2 layer and corresponding SAED pattern with its interpretation. Diffractograms obtained by FFT of the lattice image are respectively related to two different orientations of domain (A and B) and to the overall image (C). Moire effects results of domain boundaries on different surfaces, defined sometimes as a (111) twin plane (arrow).
axis are related between them by a mirror operation with (111) and (121) invariant planes. The diffracted beams come from domains of both orientations and of double diffractions effects between them. On the high resolution image, moire effects are due to superimposed domains with curved domain boundaries. Just a few domain boundaries were found to correspond to (111) and (121) twin planes (see arrows in Figure 4). The interpretation of the SAED pattern was confirmed by fast Fourier transform (FFT) on each domain orientation (A and B) and on the overall lattice image (C). The double diffraction effects are of strong intensity because of an important overlapping between domains. High resolution images and corresponding SAED patterns of nonirradiated and irradiated CrO2 phase are shown in Figures 5 and 6, respectively. The SAED pattern of Figure 5 exhibits reflections of strong intensity in accordance with those expected for the tetragonal structure of CrO2 oriented along a [100] zone axis but also satellite reflections of very weak intensity. The high resolution image shows that such satellite reflections are related to moire effects and therefore to a superimposition of a layer of different structure onto (100) CrO2. This layer is observed at the border of the fragment. It is constituted here of two domains which the structure corresponds to this of Cr2O3 observed along a [101]
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Figure 5. High resolution TEM image of a fragment of a nonirradiated (100) CrO2 layer and corresponding SAED pattern. The satellite reflections of very weak intensity are related to a thin Cr2O3 layer constituted of domains with two different orientations with respect to this of (100) CrO2, which are in agreement with the X-ray diffraction results shown in Figure 3a. The Cr2O3 structure is identified from the FFT taken on thin layer observed at the border of the fragment.
zone axis from the indexed FTT shown in inset and corresponding to the domain indicated by an arrow in figure 5. Both domains are related by a (121) twin plane and their crystallographic orientational relationships with the CrO2 structure correspond to those observed by X-ray diffraction. Therefore, satellite reflections on the SAED pattern can be interpreted as multiple diffractions between both Cr2O3 and CrO2 structures. Vectors of diffraction corresponding to 101, 020, 121 reflections of Cr2O3 are represented on the drawing of the SAED pattern in Figure 5. They have to be repeated around each reflection of CrO2 with mirror symmetries (e.g., through (111) Cr2O3) in order to generate all the satellites. Similar CrO2 fragments with a very thin Cr2O3 layer onto (100) CrO2 were also observed in UV irradiated samples. For a few fragments as the one shown in Figure 6 the structure of the Cr2O3 structure was also observed to be oriented along its [111] zone axis. The hexagonal symmetry of the lattice fringes of Cr2O3 along its ternary axis was verified by FFT. As there are two possibilities for orienting the rhombohedral cell along its ternary axis with respect to the 6-fold symmetry of the FFT pattern, domains with two different [111] orientations can be assumed to exist, in agreement with the crystallographic orientational relationships. On account of
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Figure 7. High resolution TEM image and corresponding SAED pattern of superimposed thick CrO2 and Cr2O3 layers. A comparison is made between patterns observed by SAED and those calculated from the cell parameters of CrO2 and Cr2O3.
Figure 6. High resolution TEM image of a fragment of an irradiated (100) CrO2 layer and corresponding SAED pattern where the Cr2O3 structure is observed along a [111] zone axis at the border of the fragment. An hexagon has been drawn on the FFT pattern in order to verify the 6-fold symmetry.
the X-ray diffraction results (Figure 3a), both these [111] orientations of Cr2O3 are not formed at the (100) surface of CrO2. They are likely related to the dark and straight traces parallel to the c axis of (100) CrO2 observed before and after irradiation (Figure 2a and inset on the left side) where they could preferentially be formed at a (010) boundary in between elongated domains of the CrO2 phase. From the cell parameter values of both phases CrO2 (a = 4.419 A˚, c = 2.9154 A˚, P42/mnm)18 and Cr2O3 (a = 5.36004 A˚, R = 55.1058, R3c),20 a misfit of about 1.8% should be observed on the SAED pattern between both the reflections, 002 of CrO2 (d002 = 1.4577 A˚) and 121 of Cr2O3 (d121 = 1.43157 A˚). However, any misfit is not observed between these reflections. There is a misfit of about 7.8% but between the 020 and 101 reflections of CrO2 and Cr2O3, respectively, which is close to the calculated value (i.e., 8.4%). Similarly, a diffraction pattern obtained on a thicker CrO2/Cr2O3 interface does not exhibit any misfit (Figure 7). The drawing of the superimposed diffraction patterns of [100] CrO2 and [101] Cr2O3 zone axes indicates what small intervals between reflections should be observed on account of the cell parameter values. Actually, the SAED pattern indicates that both lattices are commensurate with additional reflections resulting from multiple diffraction effects between (100) CrO2 and
both orientations of Cr2O3 of the [101] zone axis. Such reflections are related to moire fringes of larger periods on the high resolution image. The structures of CrO2 and Cr2O3 are therefore easily deformed by the strain developed at their interface. Discussion and Conclusions The present results show that Cr2O3 produced by UV pyrolysis of CrO2 always presents the same type of crystallographic orientational relationships with respect to the CrO2 structure. These relations have previously been determined by Zheng et al.19 by FFTs of a high resolution TEM image on a CrO2 nanorod oriented along a [100] zone axis. However, these authors have deduced such relations only from Cr2O3 oriented along a [111] zone axis parallel to the [100] zone axis of CrO2 but not from moire fringes, similar to those presently observed between (100) CrO2 and a thin (101) Cr2O3 layer (Figures 5 and 6). In accordance with the crystallographic orientational relationships, there are four possibilities to orient the rhombohedral cell of Cr2O3 with respect to the tetragonal cell of CrO2. They are indeed observed on a (001) CrO2 surface as due to the 4-fold symmetry of the tetragonal structure. But for {100} CrO2 surfaces, only two orientations corresponding to thin {101} Cr2O3 layers with a (111) mirror symmetry are observed. A reason is that both atomic structures of {100} CrO2 and {101} Cr2O3 layers exhibit similitudes (Figure 8). Their sublattices of oxygen atoms are constituted of the same number of O6 octahedra of similar orientation per unit area. O6 octahedra are either empty or centered on a Cr atom. As the ratios of O6 octahedra occupied by Cr are 1/2 for CrO2 and 2 /3 for Cr2O3, it implies that four layers of CrO2 transform into
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through the CrO2 layer resulting in a constant absorption of light and a decrease in the thermal diffusion based on an increase in the thickness of CrO2. The temperature of the CrO2 surface increases therefore with the layer thickness up to a critical value for which the CrO2-to-Cr2O3 transformation occurs. But as the CrO2-to-Cr2O3 transformation is endothermic,21 the transformation does not progress through the CrO2 layer. In the growth of a fcc architecture of epitaxial CrO2 phase on a TiO2 single crystal substrate by photolysis of CrO2Cl2 in UV interferences field, the CrO2-to-Cr2O3 transformation could therefore be avoided as long as the temperature of the CrO2 surface does not reach a critical value. This might be obtained either by cooling down the substrate or by lowering the UV energy. But lowering the UV energy is not, a priori, appropriate as the photolytic decomposition of adsorbed CrO2Cl2 molecules requires a certain number of photons at each laser pulse. The Cr2O3 deposit, shown in Figure 1, was obtained at 60 mJ 3 cm-2 per pulse and low pressure of CrO2Cl2 (0.15 Torr) on a (001) TiO2 substrate cooled down to 10 C.1 By X-ray diffraction, it was concluded that the CrO2 phase was first formed. Therefore, it seems that cooling down the substrate to a much lower temperature should allow one to perform the growth of the CrO2 phase only.
References Figure 8. Comparison between the atomic structures of (a) {100} CrO2 and (b) {101} Cr2O3 surface layers constituted of a same total number of CrO6 þ empty O6 octahedra. Edges of two empty O6 octahedra are drawn. Both these layers are related through 90 rotations on projections of [001] CrO2 and [111] Cr2O3 zone axes (bottom part of the figure) where it appears that both structures are respectively constituted of a periodic stacking of these layers.
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three layers of Cr2O3 in order to keep an invariant number of Cr atoms per unit area. Part of the Cr atoms have to jump between first neighboring O6 octahedra and between adjacent layers of O6 octahedra with a release of oxygen. The corresponding ratio of variation for oxygen atoms is 4/3 in agreement with the ratio of variation for the reaction 2CrO2 f Cr2O3 þ 1/2O2. According to this scheme, the minimal thickness of a {101} Cr2O3 would correspond to three layers of octahedra, that is, about 3/2aCrO2 = 6.63 A˚. Let us note that a similar description of the transformation can be proposed for the four orientations of the Cr2O3 rhombohedral cell observed along the 4-fold axis of CrO2. Corrugated layers of O6 octahedra however have to be considered in this case. Therefore, it can be concluded that Cr2O3 layers formed on {100} (and probably on {001}) CrO2 planes are in epitaxy with habit planes {100}CrO2 {101} Cr2O3 (and {001} CrO2 {121} Cr2O3). The CrO2-to-Cr2O3 transformation occurs by pyrolysis due to a strong UV absorption of the CrO2 phase. However, for the growth CrO2 layers on various substrates by photolysis of CrO2Cl2, it has been observed by Arnone et al.8 that the CrO2to-Cr2O3 transformation occurs for a critical thickness of CrO2 and is not complete. In their interpretation, these authors consider an increase of the temperature gradient
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