Abstract
Dual-layer and tri-layer broadband antireflective (AR) films with excellent transmittance were successfully fabricated using base-/acid-catalyzed mixed sols and propylene oxide (PO) modified silica sols. The sols and films were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), transmission electron microscope (TEM), and scanning transmission electron microscope (STEM). FTIR and TEM results suggest that the PO molecules were covalently bonded to the silica particles and the bridge structure existing in PO modified silica sol is responsible for the low density of the top layer. The density ratio between different layers was measured by cross-sectional STEM, and the results are 1.69:1 and 2.1:1.7:1 from bottom-layer to top-layer for dual-layer and tri-layer films, respectively. The dual-layer film demonstrates good stability with 99.8% at the central wavelength of 351 nm and nearly 99.5% at the central wavelength of 1053 nm in laser system, and for the tri-layer AR film, the maximum transmittance reached nearly 100% at both the central wavelengths of 527 and 1053 nm.
Highlights
Broadband antireflective (AR) films have been widely used in optical devices such as automotive windows, solar cells, laser systems, and many energy-related applications to increase the availability of light [1–7]
The results revealed that the propylene oxide (PO) molecules were covalently bonded to the silica particles and the bridge structure existing in PO modified silica sol contributed to the low density of the top layer
Microstructures and morphologies of silica sols and AR films were characterized by microstructures and morphologies of silica sols, and AR films were characterized by Fourier transform infrared spectroscopy (FTIR, IRTracer100), nuclear magnetic resonance (NMR, EchoMRI-500), scanning electron microscope (SEM, JEOL JSM-7001F at 15 kV), and transmission electron microscope (TEM, JEM2010FEF)
Summary
Broadband antireflective (AR) films have been widely used in optical devices such as automotive windows, solar cells, laser systems, and many energy-related applications to increase the availability of light [1–7]. According to Eq 1, the key parameter for preparing dual- and tri-layer films is to adjust the refractive index of each layer. By mixing the basecatalyzed and acid-catalyzed silica sols together, AR film with refractive index varying from 1.22 to 1.44 can be prepared. In order to quantitatively demonstrate the porosity of the films, the xerogel powders were produced under a similar condition to the fabrication of films, so The BET data of these xerogel powders should be close to the actual properties of the corresponding films to some extent This method can be used to approximately calculate the porosity of the film, it is difficult to verify the data error between the film and the xerogel powders. The density ratio from the bottom to top film in dual-/tri-layer film was measured by a dark-filed STEM, according to the element signal intensity. The dual-layer and tri-layer broadband AR films were prepared by a sol–gel process via pulling method. After 63 days, there was no obvious difference compared with the initial spectrum
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