Local optical spectroscopy of opaline photonic crystal films
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MATERIALS AND CERAMICS
Local Optical Spectroscopy of Opaline Photonic Crystal Films T. Bakhiaa, A. E. Baranchikovb, V. S. Gorelikc, and S. O. Klimonskya,* a
b
Moscow State University, Moscow, 119991 Russia Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia c Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia *e-mail: [email protected] Received October 18, 2016
Abstract—The homogeneity of opaline films obtained by vertical deposition of colloidal SiO2 microparticles has been studied by scanning electron microscopy (SEM) and local optical spectroscopy. It was found that the particle size distribution is narrowed during the deposition, the microstructure of the films improves, and the reflection peak in the first photonic stop band increases and narrows. These changes may be due to the fact that large microparticles, whose mass significantly exceeds the average mass, leave the solution in the course of time, falling on the bottom of the vessel under gravity. It is established that the microstructure of opaline films is improved with a decrease in thickness. DOI: 10.1134/S1063774517050029
INTRODUCTION Starting from the pioneering work [1], synthetic opals consisting of close-packed microspheres attract much researchers’ attention, because they exhibit properties of photonic crystals [2]. Bulk opals are usually obtained by sedimentation of colloidal microparticles in the field of gravity. Their major defect is the partition into many small domains with different crystallographic orientations [3]. Thin films may have a better structure. The regions of films characterized by different structure periods or crystallographic orientations will be called domains as before. They can be separated by thin cracks. According to [4], the mutual disorientation of domains in synthetic opal films of obtained by vertical deposition of colloidal SiO2 microparticles lies within ±1.6°. It should be noted that the most important factor determining the structural quality of synthetic opals is the dispersion of microparticle sizes [5]. An ordered structure cannot be formed if the relative standard deviation of microparticle diameters from the mean value exceeds 8% [4]. One would expect the differences in the crystal structure period and the orientation of different domains to increase with an increase in dispersion. In this paper, we report the results of a systematic study of the relationship between the dispersion of microparticles, the microstructure of film, and its optical properties; the analysis was performed on a sample whose microstructure changed during deposition.
EXPERIMENTAL Microparticles of amorphous SiO2 were obtained by the method of seeded growth [6]. Initial nuclei ~30 nm in diameter were synthesized by the Stöber method [7]. After the growth, the diameter of microparticles exceeded 260 nm and the dispersion of their sizes decreased; however, anomalously large twin particles arose in small numbers. The resulting alcoholic colloidal s
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