Antireflective structures (ARSs) reduce Fresnel reflection to boost light transmission or absorption and improve the performance of optical devices over a wide range of wavelengths. Researchers at Shanghai Jiao Tong University, China, have fabricated biomorphic SiO2 with ARSs that are inspired from cicada wings using a simple and inexpensive sol-gel ultrasonic method combined with calcination.
Black cicada (Cryptotympana atrata Fabricius) wings were chosen as the biological prototype due to the nano-nipple array structure on their wings. The cicada wings were cleaned with absolute ethanol followed by deionized water, dried in air and pretreated with 8% NaOH. Ethanol/water/TEOS/HCl mixture at a molar ratio of 3:12:1:0.03 modified with Triton X-100 was used as a precursor sol for SiO2. The pretreated cicada wings were immersed in the precursor sol and sonicated using high-intensity ultrasonic irradiation (20 kHz; 100W/cm2) at room temperature for 3 h. Subsequently, the cicada wings were kept in the precursor sol for 12 h for solidification, cleaned with ethanol and dried at 60 °C under vacuum. The SiO2 coated wings were calcined in vacuum at 500 °C for 2 h to remove the organic template, leaving behind SiO2 with the surface structure of the cicada wing (biomorphic SiO2).
Fig. 1 Schematic of the synthesis process for the fabrication of biomorphic SiO2
The morphological features of the biomorphic SiO2 indicate replication of the nano-nipple array structures (Figs. 2(a) and 2(b)) similar to that of the cicada wing (inset of Fig. 2(a)). The nano-nipple arrays increased the surface roughness and decreased the water contact angle to 16° (inset of Fig. 2(b)), thus imparting hydrophilic properties for the biomorphic SiO2. The reflectance spectra of biomorphic SiO2 gradually changed from 0.3% to 3.3% as the angle of incidence is changed from 10° to 60° (Figs. 2(c) and 2(d)). The excellent antireflection property is due to the formation of ARS on the surface of biomorphic SiO2. The gradation in the refractive index between air and SiO2 introduced by the ARS causes a dramatic reduction in the reflectance in the visible wavelength range (450–800 nm) over a wide range of incident angles.
Fig. 2 SEM images: (a) top view; (b) side view; and (c) angle dependent; and (d) counter map angle dependent antireflection properties of biomorphic SiO2
The antireflective properties of biomorphic SiO2 are promising and suitable for applications in photovoltaic devices and solar cells. Similarly, the hydrophilic properties of biomorphic SiO2 will be useful for the development of self-cleaning and antifogging optical materials.
T.S.N. Sankara Narayanan
For further information, the reader may kindly refer to: Imran Zada et al., Multifunctional, angle dependent antireflection, and hydrophilic properties of SiO2 inspired by nano-scale structures of cicada wings, Appl. Phys. Lett. 111, 153701 (2017); doi: 10.1063/1.4986133
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