Numerous studies of surface nanostructures have been conducted to investigate enhancement of the properties of bulk materials to improve their selectivity, applicability, and effectiveness. During the past decades, the technological basis for nanofabrication has been developed by vigorous efforts to develop next-generation lithography for highly resolved patterns up to the industrial level of semiconductor production. Nanofabrication techniques using transparent materials such as quartz comprise one of the most attractive approaches to optical and opto-electrical studies as well as to highly sensitive biosensor fields since quartz is commonly employed in these fields [1–4].
Various methods are used to achieve antireflective property in quartz or glass to increase light transmission. Single- or multilayered thin film coatings, porous coatings, and fabrication of sub-wavelength nanostructures on surfaces using conventional lithography have been the focus of many studies [5–7]. However, the aforementioned conventional methods have some drawbacks. For example, it is difficult to maintain long-term stability of multilayer polymeric coatings because multilayers are unstable in humid environments and temperature changes, and most polymeric materials have strong absorption in the UV region . Moreover, single- or multilayer fabrication methods are only effective for a narrow spectral range. Lithography techniques have the shortcomings of being time consuming, expensive, and restricted to small areas.
In comparison with thin-film-coated surfaces, directly patterned surfaces usually guarantee good mechanical stability because they are free from adhesion problems and tensile stress. Recently, a few techniques for the direct fabrication of sub-wavelength nanostructures on quartz or glass surfaces have been attempted by several groups. Lohmüller et al. and Christopher et al. used block copolymer micelle lithography with reactive ion etching (RIE) and reported that array pattern of a quartz nanostructure showed excellent antireflectivity and anti-fogging in UV and deep-UV region [1, 9]. Li et al. applied nanosphere lithography using PS microspheres 210 nm in diameter and reported broad spectrum antireflectivity from 300 to 800 nm and anti-fogging . If a simpler and faster technique is available to obtain appropriate nanostructures, it is highly desirable. Hein et al. reported an innovatively simple and fast method of nanostructure fabrication on glass surfaces by performing RIE after deposition of an approximately 10-nm-thin lithographically unstructured metallic layer onto the surface . We also reported a simple and fast mask-free approach to fabricate nanostructures, which uses two-step RIE (O2 and CF4 RIE) of polymer-coated quartz in our previous studies [3, 4].
Superhydrophobic behavior of a surface can also be achieved by introducing micro- and/or nanostructures at the surface. A superhydrophobic surface is usually defined as having a contact angle greater than 150°. The self-cleaning effect refers to cases in which contaminant particles adhered to a surface are easily washed off with rolling of water droplets. To have a self-cleaning effect, the surface should possess minimized adhesion properties as well as superhydrophobicity [12–14].
The mimetic fabrication of a superhydrophobic surface was primarily inspired by the self-cleanable leaves of the lotus, which have an array of protrusions on the surface [15, 16]. The fabrication of self-cleanable surfaces has received a great deal of attention in various novel applications, such as for easy removal of undesirable contaminants from the surface of semiconductors and solar cells, prevention of water corrosion on the exterior skins of automobiles and building units, biomaterials used in clinical therapies that require minimal contamination, no-mass-loss transport of water droplet systems, and microarrays that require specific wetting properties of the substrates for precise spotting . Especially in dry condition, which is common in optical or opto-electric applications, the superhydrophobic surface is expected to decrease the adhesion of dusts because the surface energy in surface-air interface decreases as the surface becomes superhydrophobic.
We previously reported a systematic approach to obtain a superhydrophobic surface with tunable adhesiveness and suggested a useful nanofabrication strategy for achieving self-cleanability that involved fabrication of pillar arrays without dead-end nanopores covered with low-surface-energy materials [12, 14]. In this study, we demonstrate both the remarkable broad spectrum antireflectivity including deep-UV region and self-cleanability in nanostructured quartz surfaces using our mask-free fabrication method.