Figure shows oblique FESEM images of the fabricated nanopillar arrays obtained for each PMMA resist using the mask-free method. The widths of the quartz nanopillars with A2, A8, and A11 PMMA resists were estimated to be 33
5, and 73
nm, respectively, from the FESEM images. The heights of the pillars were 95
15, and 265
nm, respectively. The average pillar diameters, pillar heights, and inter-pillar distances were uniform all over the area (2.5
) in each sample. These results demonstrate that it is feasible to systematically control the dimensional features of the pillar pattern. Specifically, larger and higher pillars can be formed by controlling the thickness of the PMMA resist. The mechanism of the nanostructure formation was previously reported [4
]. Explaining briefly, pillar-like nanostructures can be fabricated by the O2
two-step RIE process because, by controlling the RIE conditions appropriately, CF4
polymeric mask is automatically and selectively deposited during the CF4
RIE process on the top of the dot-like nanostructures of the PMMA resist, which are formed during the preceding O2
SEM images of nanopillar array manufactured by new mask-free lithography with various PMMA resists. (a) PMMA A2, (b) PMMA A8, and (c) PMMA A11.
Table shows the static, advancing, and receding water contact angles of the HDFS-modified nanopillar quartz patterns. Photographs of the static water droplets on each patterned surface are also shown in Figure . The static contact angles were measured at more than three points in each specimen, and the average values were acquired. A quartz surface is known to be hydrophilic due to the existence of hydroxyl groups on the surface, which is the reason why a quartz surface is easily contaminated by dusts, because nature wants to decrease interfacial energy between the hydrophilic quartz and the most hydrophobic air. In order to make the quartz surface hydrophobic, so as to decrease the interfacial energy, a covalently immobilized monolayer of HDFS molecules was formed on its surface, and the hydrophobicity could be further increased to superhydrophobicity by the nanostructures. All of the HDFS-treated nanopatterns had dynamic contact angles (Table ) in the superhydrophobic range (greater than 150°) with low hysteresis of about 10°, which demonstrates the self-cleaning effect (Additional file 1
: Video S1). In contrast, the HDFS-modified surface of a plain quartz has an advancing angle of about 120° with large hysteresis (40.0°).
Contact angles of the nanostructured quartz
Contact angle measurements of the structured quartz surface. Prepared using (a) PMMA A2, (b) PMMA A8, and (c) PMMA A11.
In nature, some insects have sub-wavelength scale structure patterns with nipple-like or tapered profiles on the cornea that exhibit a gradient in the refractive index between the air and tissue interface. These characteristics play an important role in increasing light transmission. Theoretically, for a thin-film coating, overall reflectance can be a function of antireflection (AR) layer thickness d
and the wavelength λ
. For a graded-index transition, substantial antireflection can be obtained when the ratio d
is about 0.4 or higher [1
]. To enhance the transmission of light and suppress the reflection, the structural size has to be in the sub-wavelength range. In the spectral region from UV to visible light, the structural dimension has to be smaller than 200
Figure shows the transmission properties of the structured quartz manufactured using A2, A8, and A11 PMMA resists. Transmission data from unstructured quartz samples were used as a reference. All structured quartz prepared using A2, A8, and A11 showed improved transmission of about 2% to 3% over unstructured quartz in a broad spectral range from the UV to the infrared (IR) region (350 to 900
nm). The structured quartz manufactured using an A2 resist demonstrated transmission superior to the unstructured quartz even in the deep-UV range from 190 to 300
nm (Figure ). This deep-UV range is usually not covered by the conventional polymer AR-coating method. The antireflective property of the structured quartz using A2 varies from approximately 4.2% improvement at around 193
nm to 2.3% at around 340
nm, as indicated by the black arrow. The transmission of the structured surface using A8 and A11 is lower than that of the unstructured quartz below 300
nm, and this is partly the result of light scattering introduced during the fabrication process. In the region from the visible (350
nm) to IR (900
nm) range, the nanostructured quartz prepared using A8 exhibited a stable and uniform antireflective effect and a better optical performance above the 700-nm region than that obtained using the quartz prepared using A2. These experimental results are in good agreement with the aforementioned theories related to the height of nanopillars.
Transmission properties of structured quartz as determined by UV-visible spectrometry. Prepared using PMMA A2, PMMA A8, and PMMA A11 with unstructured quartz as a reference.
Transmission properties of structured quartz in the deep-UV region. Prepared using (a) PMMA A2, PMMA A8, PMMA A11, and unstructured quartz; (b) high antireflective performance of structured quartz prepared using PMMA A2.