Arrays of semiconductor nanowires are of potential interest for applications including photovoltaic devices and IR detectors/imagers. While nominally uniform arrays have typically been studied, arrays containing nanowires with multiple diameters and/or random distributions of diameters could allow tailoring of the photonic properties of the arrays. In this Letter, we demonstrate the growth and optical properties of randomly branched InSb nanowire arrays. The structure mentioned can be approximated as three vertically stacked regions, with average diameters of 20, 100, and 150 nm within the respective layers. Reflectance and transmittance measurements on structures with different average nanowire lengths have been performed over the wavelength range of 300-2000 nm, and absorbance has been calculated from these measurements. The structures show low reflectance over the visible and IR regions and wavelength-dependent absorbance in the IR region. A model considering the diameter-dependent photonic coupling (at a given wavelength) and random distribution of nanowire diameters within the regions has been developed. The diameter-dependent photonic coupling results in a roll-off in the absorbance spectra at wavelengths well below the bulk cutoff of ∼7 μm, and randomness is observed to broaden the absorbance response. Varying the average diameters would allow tailoring of the wavelength dependent absorption within various layers, which could be employed in photovoltaic devices or wavelength-dependent IR imagers.

In this article, electrodeposition method is used to demonstrate growth of InSb nanowire (NW) arrays with hierarchical branched structures and complex morphology at room temperature using an all-solution, catalyst-free technique. A gold coated, porous anodic alumina membrane provided the template for the branched NWs. The NWs have a hierarchical branched structure, with three nominal regions: a “trunk” (average diameter of 150 nm), large branches (average diameter of 100 nm), and small branches (average diameter of sub-10 nm to sub-20 nm). The structural properties of the branched NWs were studied using scanning transmission electron microscopy, transmission electron microscopy, scanning electron microscopy, x-ray diffraction, energy dispersive x-ray spectroscopy, and Raman spectroscopy. In the as-grown state, the small branches of InSb NWs were crystalline, but the trunk regions were mostly nanocrystalline with an amorphous boundary. Post-annealing of NWs at 420 °C in argon produced single crystalline structures along ⟨311⟩ directions for the branches and along ⟨111⟩ for the trunks. Based on the high crystallinity and tailored structure in this branched NW array, the effective refractive index allows us to achieve excellent antireflection properties signifying its technological usefulness for photon management and energy harvesting.

Sharp metal tips have many uses, including in scanning probe microscopy. Silver is a particularly interesting metal for plasmonic enhancement, e.g., in tip-enhanced Raman spectroscopy, however few methods for the production of silver tips have been explored. A simple and reliable one step dc electrochemical method for the production of sharp silver tips in concentrated H2SO4 is presented. The optimal conditions are 10 V cell voltage and 99% sulfuric acid for tip radii below 100 nm. A LabView program was written to control the cut-off of the circuit to within a millisecond to avoid blunting the tips after drop-off.

Vertical arrays of high aspect ratio (>100) InSb nanowires with diameters of ~20 nm have been fabricated using a Porous Anodic Alumina (PAA) template that is supported on a Si substrate with a thin layer of titanium (Ti) sandwiched between them. The process described here uses a reverse anodization technique to penetrate the hemispherical pore bottom barrier oxide layer prior to the electrodeposition process, so as to form a direct electrical contact with the underlying Ti layer. Scanning electron microscopy results demonstrate that the InSb nanowires completely fill the channels of the PAA thereby acquiring a wire diameter of about 20 nm. Raman spectrum of the InSb nanowires indicates high crystal quality.

Fabrication of titanium tips via electrochemical etching is commonly hindered by the formation of a tough oxide layer. A new one-step approach based on dc etching in a non-aqueous electrolyte is presented. The electrochemical tip etching was carried out at 20 V in a benign etchant of 1M NaCl in ethylene glycol. Tip radii below 100 nm are demonstrated using current cut-off control and approaching 150 nm without cut-off control. The simplicity and efficiency of the approach eliminates pre and post electropolishing steps due to the electropolishing capability of a NaCl-ethylene glycol based electrolyte.