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Synthesis of 3D periodic nanostructures in an interference field of UV laser light

Updated on November 30, 2009
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Synthesis of 3D periodic nanostructures in an interference field of UV laser light Students : A-L. Joudrier, M. Salaün Objectives  In 2000, Campbell et al. (Nature, 404, (2000) 53) demonstrated a fabrication of 3D photonic crystals through 3-dimensional holographic lithography (3D HL). Thick layers of a commercial epoxy photo-resist have been patterned within the duration of single optical pulses of 10 ns by the intensity modulations that arise from the interference of 4 non-coplanar UV laser beams. Both face- and body-centred cubic structures with sub-micron lattice periodicity required for band gaps at visible wavelengths have been created. From such results we have envisaged a possibility to obtain a direct growth of matter of pseudo-fcc organization (a = 922 nm) by chemical vapour decomposition within a similar 3D interference field of UV laser light. An experimental set-up, as reported in Duneau, Delyon and Audier (J. Appl. Phys., 96 (2004) 2428) has been designed on the basis of a theoretical study in order to optimize the geometry and the contrast of the interference field (i.e. with energy minima equal to zero). An interferometer adapted to a 10 Hz pulsed UV laser source at 355 nm has been realized and connected to a CVD reactor. Each laser beam has a diameter of 8 mm. A major difference with the single laser pulse irradiation of photo-resists is however that a CVD growth requires series of laser pulses. Therefore, preliminary studies on the interference stability and on its accuracy have been carried out both by video camera and by irradiation of organic-inorganic hybrids. For instance, a SEM image of 3D nanostructuration in an hybrid composite obtained after an irradiation of 50 laser pulses is shown in Fig.1.18. Then, the growth of 3D photonic crystals by photolysis of CrO2Cl2 has been studied.  Figure 1.18 - SEM image of a nanostructured organic-inorganic hybrid layer obtained through an irradiation of 50 laser pulses. Main results Either amorphous or crystalline chromium oxides were obtained by photolysis of chromyl chloride CrO2Cl2 for different experimental conditions. The most interesting products were obtained for a photolysis of CrO2Cl2 at low pressure, on cooled TiO2 single-crystal substrates and for a higher beam energy. The growth begins with the formation of epitaxial CrO2 phase which is partly transformed into Cr2O3 under UV irradiation. Due to crystallographic orientational relationships between CrO2 and Cr2O3, the growth of an organized 3D photonic crystal of Cr2O3, phase goes on according to the 3D periodic modulations of electromagnetic energy of the interference field. In the present case, the Cr2O3, phase exhibits 4 sets of equivalent crystallographic orientations with respect to the single-crystal substrate. Figure 1.19 - SEM images of the deposit obtained at 60 mJ.cm-2 per pulse onto a rutile (001) TiO2 substrate.  The Fig.1.19 (a) shows a SEM image corresponding to the centre of such a deposit obtained after about 12000 UV laser pulses. The deposit of grey contrast appears to be constituted of grains of equal size periodically organized. Many holes of dark grey contrast reveal a triperiodic arrangement in-depth. The particles are jointed between them. One also observes a dispersion of particles of clear contrast but which the size seems to correspond to those of grey contrast. The organized particles exhibit a surface more or less flat and perpendicular to the 3-fold axis of symmetry of the 3D    interference field. In Fig. 1.19 (b), the shift between hexagonal layers situated at the hole bottom and on the upper surface are analyzed. From both hexagonal lattices superimposed to particle centres the shift appears to be in agreement with the one between two successive (111) atomic layers of a fcc structure. Collaborations CPHT Ecole Polytechnique, Palaiseau. Fundings ACI NanoSciences-Nanotechnologies 2000, ACI Jeune Chercheur, LMGP and SIMAP.
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Date of update November 30, 2009

Univ. Grenoble Alpes