Sapphire has found many device applications in optoelectronics and protective optics that require mechanical and temperature stability. However, the refraction index of sapphire (1.75-1.78 in the visible spectrum) is much larger than that of air, leading to strong reflection at the interface between sapphire and air. Such a drawback limits its application in devices that require high transmittance. Thin film coatings with intermediate or gradient refractive indexes are usually employed to suppress undesired reflection. However, the adhesiveness and thermal mismatch between the film and the sapphire may cause the stability problems. Surface-relief arrays with dimension smaller than the wavelength of incident light, also known as moth-eye structures, have been considered an alternative to the thin film coatings. Such kind of structures is more stable and durable than thin film coatings since only one material is involved. Furthermore, the moth-eye structure enables the reduction of optical reflection in the wider ranges of both spectrum and incident angles of light on condition that the periodic nanostructure is sufficiently smaller than the wavelength of antireflection spectrum. In the last years, various techniques for producing biomimetic moth-eye structures on sapphire have been reported, such as electron-beam lithography, laser interference lithography, and nanoimprint lithography. However, these techniques are restricted from practical application because of their expensive equipment and complicated procedures. Nanosphere lithography is a high-throughput and low-cost method to fabricate nanostructures with high uniformity over large areas, which can be used to pattern sapphire substrates. In this paper, corrugated sapphire nanocone arrays are fabricated on double-side polished sapphire substrates in order to increase the transmission over broadband spectra. The corrugated nanostructure is patterned by the combination of nanosphere lithography and inductively coupled plasma (ICP) dry etching. The etching process involves two steps, which utilizes two kinds of silica monolayer colloidal crystals in different sizes as masks, respectively. Firstly, the suspension consisting of silica colloidal spheres and 1-butanol was dropped on the water reservoir in a Petri dish. Then a monolayer of colloidal silica spheres was assembled on the air-water interface. Subsequently, the silica monolayer was transferred to a sapphire substrate and acted as a shadow mask for the first ICP etching. The residual silica spheres were removed after ICP etching by immersing in HF solution, resulting in a blunt cone array. A monolayer of smaller silica spheres were transferred to the surface of the resulting arrays of blunt cones. The corrugated sapphire nanocone arrays were obtained after the second ICP etching process using the smaller silica spheres as a mask. The optical reflection properties of the corrugated sapphire nanocone arrays are optimized by manipulating the size ratio of the two silica spheres and controlling the etching conditions. These surface-relief arrays are expected to improve the performance of optoelectronic devices such as light-emitting and photodetection devices due to their low reflection characteristics over a wide wavelength range.

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