Silicon Nanocone Arrays via Pattern Transfer of Mushroomlike SiO2 Nanopillars for Broadband Antireflective Surfaces

Broadband antireflection is crucial to improving the performance of various optical, photofunctional, and optoelectronic devices. The present study demonstrates an unconventional etching process employing pillar-shaped SiO2 masks that allows the synthesis of black silicon exhibiting ultralow reflectivity. In this process, the morphologies of mushroomlike SiO2 nanopillar arrays fabricated from self-assembled block copolymers and having a high aspect ratio are drastically changed during etching. This effect results in multistage etching of an underlying silicon substrate to produce a silicon nanocone array with an average height of 800 nm and multiple nanoscale tips. This silicon nanostructure exhibits a low reflectance of less than 0.06% when exposed to light at a normal angle of incidence over a wide range of wavelengths from 300 to 900 nm. In addition, the reflectance remains as low as 1.6% or less for incident angles of 0 to 60 degrees, and this omnidirectional antireflection behavior appears in both s- and p-polarized light. A silicon nanocone array fabricated in this manner effectively absorbs UV laser light and was applied as an inorganic substrate for matrix-free laser desorption/ionization mass spectrometry. The morphology of this material facilitated the desorption of various analyte molecules, including drugs, peptides, and proteins with masses up to approximately 17 kDa. This work demonstrates an alternative approach to obtaining black silicon that has considerable potential for applications in various fields, such as optics, optoelectronics, and chemical analysis.