TEM and STEM Studies on the Cross-sectional Morphologies of Dual-/Tri-layer Broadband SiO2 Antireflective Films
© The Author(s). 2018
Received: 6 December 2017
Accepted: 9 January 2018
Published: 12 February 2018
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.
where np and ns refer to the refractive index of the porous material and solid material, respectively, and p is the porosity of a porous material. However, it is difficult to measure the pore size, grain size, and porosity of the film as the thickness is only tens to a hundred of nanometers. Most of the reported porosity measurement methods are calculated or analogical. For example, Orignac et al.  reported the porosity volume fraction Vp is estimated as the ratio between the sum of the areas of the pores measured in the SEM image and the total imaging area of the sample. Xiao et al.  measured the reflective index based on the relationship between the reflective index and acid- or base-catalyzed sol ratio. They found the refractive index of the mixed AR films is proportional to the acid- to base-catalyzed sol ratio. With an acidic catalyst, the growth of silica sol tends to form linear chains, giving the acid-catalyzed AR film a refractive index of 1.44. By mixing the base-catalyzed and acid-catalyzed silica sols together, AR film with refractive index varying from 1.22 to 1.44 can be prepared. Ye et al. [24, 25] reported another method to measure the porosity of the films based on Brunauer–Emmett–Teller’s (BET) surface area test method. 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. Although 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.
In this work, the cross-sectional morphologies of the dual-/tri-layer films were characterized by SEM and TEM. The sizes of pores and silica grains of each two layers were analyzed. The results show that the sizes of pores as well as the silica grains were increased from the bottom to the top layer. In addition, there is an apparent interface between two layers. 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 density ratio is 1.69:1 and 2.1:1.7:1 for dual-layer and tri-layer films, respectively. Firstly, the dual-layer and tri-layer broadband AR films were prepared by a sol–gel process via pulling method. The bottom layer was prepared by mixing the acid-catalyzed and base-catalyzed silica sols, and the top layer was prepared from PO modified silica sol according to literature reports . The sols were characterized by TEM, FTIR spectrum, and NMR spectrum. The results revealed that the 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. The dual-layer silica film showed a simultaneously high transmittance at wavelengths of 351 nm laser and 1053 nm laser. Moreover, the film showed good stability. After 63 days, there was no obvious difference compared with the initial spectrum.
2.1 Preparation of Silica Sol
The process of the preparation of different sols are based on the literature reports , following below:
2.1.1 Preparation of Base-Catalyzed Silica Sol (Sol A)
Tetraethyl silicate (164 g) was mixed with anhydrous ethanol (1385 g), ammonia water (25–28%) 8.7 g, and deionized water (10 g). The solution was set in a closed glass container and stirred at 30 °C for 2 h and then aged at 25 °C for 7 days. It was then refluxed for more than 24 h to remove ammonia. This yielded a 3% by weight base-catalyzed sol of silica in ethanol, and this was finally filtered through a 0.22-lm PVDF membrane filter prior to use.
2.1.2 Preparation of Acid-Catalyzed Silica Sol (Sol B)
Tetraethyl silicate (104 g) was mixed with anhydrous ethanol (860 g) and water (36 g) that contained concentrated hydrochloric acid (0.2 g). The solution was left in a closed glass container and stirred at 30 °C for 2 h and then aged at 25 °C for 7 days. This yielded a sol of acid-catalyzed silica in ethanol with an equivalent silica concentration of 3%. It was also filtered through a 0.22-μm PVDF membrane filter prior to use.
2.1.3 Preparation of Base-/Acid-Catalyzed Mixed Sol (Sol C)
The 3% based-catalyzed silica sol and the 3% acid-catalyzed silica sol were mixed in proportions to prepare acid-catalyzed silica in total silica of 0–80% and stirred at 30 °C for 2 h.
2.1.4 Preparation of PO Modified Silica Sols (Sol D)
Tetraethyl silicate (164 g) was mixed with anhydrous ethanol (1385 g), ammonia water (25–28%) 8.7 g, and deionized water (10 g), and then, 0.92, 1.84, 2.76, 3.64, 4.6, 7.36, and 9.2 g PO were also added into the mixed solution to give PO weight ratio to silica of 2–20%, respectively. The final solution was left in a closed glass container and stirred at 30 °C for 2 h and then aged at 25 °C for 14 days.
2.2 Preparation of AR Film
The fused silica substrates were ultra-sonicated in acetone for 10 min and wiped carefully using clean room wipers. For dual-layer silica AR film, Sol C and Sol D were deposited on well-cleaned fused silica substrates by dip film, respectively. The thickness of each film was finely tailored by tuning the withdraw rates. The films were heat-treated at 160 °C for 8 h under ambient atmosphere. The tri-layer silica AR films were prepared according to the reports by Ye et al.  briefly. The PVDF-modified base-catalyzed silica sol was used for the middle layer of the three-layer film. The mixture of PVDF-modified base- and acid-catalyzed was used for the bottom layer. The final ORMOSIL sol was named as Sol E, which was used for the top layer of the three-layer film.
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, JEM-2010FEF). Selected area electron diffraction (SAED) was also recorded using the same equipment.
Results and Discussion
3.1 Characterizations of Silica Sols
3.2 SEM and TEM Characterizations of Dual-layer and Tri-layer Films
3.3 Optical Performance of Dual-layer Films
Dual-/tri-layer broadband AR films were prepared by a sol–gel process. The sols and films were characterized by FTIR, NMR, and TEM. FTIR spectrum indicates that the PO molecules were covalently bonded to the silica particles. The bridge structure existing in PO modified sol contributes to larger silica particles in the layer with low density. Both pore size and grain size demonstrate an increasing trend from bottom layer to top layer. An apparent interface can be observed between each two layers. The density ratios between different layers are measured by cross-sectional STEM. For the dual-layer film, the density ratio of bottom layer and top layer is 1.69:1; for the tri-layer film, the density ratio of bottom layer, middle layer, and top layer is 2.1:1.7:1. The dual-layer AR film shows a good transmittance simultaneously in the wavelengths of 351 and 1053 nm, while the maximum transmittance for tri-layer appeared at 527 and 1053 nm, nearly 100%. Besides, there is no distinctive difference on transmittance after 63 days in terms of the dual-layer AR film.
We would like to thank the testers in Analytical and Testing Center, Sichuan University, for their help in the SEM observation.
This study was supported financially by the NSAF Joint Foundation of China (U1630126).
SY proposed the research work and wrote the paper. HW and XD carried out the major work of the sample preparation. DJ, SB, and WL prepared the TEM, FTIR, and NMR data. QL and XX helped to correct and polish the manuscript. XT provided the funding support. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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