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7 September 2018 Ultrafast revealing of invisible patterns encrypted in colloidal photonic crystals
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We describe the necessary steps towards the realization of ultrafast revealing of invisible patterns encrypted in colloidal photonic crystals. These include the development of hollow air-core – dense-silica-shell core-shell monodisperse and spherical nanoparticles; introducing of a pattern of hydrophilic regions in a hydrophobic surrounding; and the combination of these two approaches by selective oxygen plasma etching of hollow core-shell nanospheres. The pattern imprinted by the difference in only surface property remains invisible in normal conditions of static environmental humidity. The hydrophilic regions in the patterns are reversible and immediately unveiled by dynamic humid flow. The specific properties of a human breath in terms of relative humidity and vapor flow are ideal for optimal revealing in terms of the spectral shift of the photonic bandgap of the colloidal crystal. The revealing of the pattern is determined by the surface tension of the vapor, while the color of the imprinted pattern is independently determined by its refractive index.
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Kuo Zhong, Kai Song, and Koen Clays "Ultrafast revealing of invisible patterns encrypted in colloidal photonic crystals", Proc. SPIE 10724, Physical Chemistry of Semiconductor Materials and Interfaces XVII, 107241D (7 September 2018);


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