A comparison between the optical properties of amorphous and crystalline monolayers of silica particles

We compared optical properties of amorphous and crystalline monolayers formed from silica particles on glass substrates. The crystalline monolayers were grown from water suspensions by forming suspension wetting film on the glass surface and controlling the rate of the receding glass-suspension-air three-phase contact line. To form the amorphous monolayers, the particle powder was pressed toward the substrate and smeared by using a silicon rubber piece specially prepared with flat and smooth surface. The field emission scanning electron microscopy observations showed particles of slightly varying diameters randomly distributed within the amorphous monolayer. The crystalline layers were built of differently sized domains of hexagonally packed particles. illuminated by daylight and observed by using a low power optical microscope or the naked eye, the amorphous monolayers of particles between 200 and 500 nm in diameter looked softly colored and matted. The crystalline monolayers of 500 and 1000 nm in diameter exhibited enhanced sharpness of color and brilliancy at some observation angles. The amorphous and crystalline 100 nm particle monolayers exhibited similar antireflective properties. The difference in color appearance between amorphous and crystalline monolayers was experimentally legitimated by the corresponding reflectivity spectra.