Abstract
Metal-assisted chemical etching (MACE) has been widely explored for developing silicon (Si)-based energy and optical devices with its benefits for low-cost and large-area fabrication of Si nanostructures of high aspect ratios. Surface structures and properties of Si nanostructures fabricated through MACE are significantly affected by experimental and environmental conditions of etchings. Herein, we showed that surfaces and interfacial energy states of fabricated Si nanowires can be critically affected by oxidants of MACE etching solutions. Surfaces of fabricated Si nanowires are porous and their tips are fully covered with lots of Si nano-sized grains. Strongly increased photoluminescence (PL) intensities, compared to that of the crystalline Si substrate, are observed for MACE-fabricated Si nanowires due to interfacial energy states of Si and SiOx of Si nano-sized grains. These Si grains can be completely removed from the nanowires by an additional etching process of the anisotropic chemical etching (ACE) of Si to taper the nanowires and enhance light trapping of the nanowires. Compared with the MACE-fabricated Si nanowires, ACE-fabricated tapered Si nanowires have similar Raman and PL spectra to those of the crystalline Si substrate, indicating the successful removal of Si grains from the nanowire surfaces by the ACE process.
Highlights
Metal-assisted chemical etching (MACE) is based on simple redox reactions those occur on the interfaces of catalytic metals and a Si substrate[1,2,11–13]
We showed that the Si nanowire surfaces are significantly affected by the concentration of H2O2 in the etching solution, which causes excessive injections of holes into the Si nanowires in its higher concentration, increasing the surface porosities of the nanowires with lots of Si nano-sized grains formed on the nanowire surfaces
These porous Si nanowires can be oxidized during and after the MACE process and their interfacial energy states are strongly affected by surface structure and oxidation of the nanowires, showing significant changes in the PL spectra with MACE etching conditions
Summary
MACE is based on simple redox reactions those occur on the interfaces of catalytic metals and a Si substrate[1,2,11–13]. Since the oxidant concentration in the etching solution is critical for hole generations on the catalytic metal surface, surface structures and morphologies of fabricated Si nanowires can be significantly affected by the concentration of H2O2 in the etching solution[1,13,20]. By this reason, the concentration of H2O2 needs to be optimized for fabricating Si nanowires through MACE. The formation and oxidization of the Si nano-sized grains on the nanowire surfaces are strongly affected by etching procedures and conditions[8,18,20–22], contributing in significant increases in PL intensities of the nanowires[9,17]. To better understand how surface oxidation of the Si nanowires during the MACE process will affect on crystallinities and interfacial energy states of the tapered Si nanowires of the ACE process, which is the post-etching process of the MACE process, changes in surface morphologies and optical properties of the tapered nanowires are compared with those of the MACE fabricated nanowires
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