Semiconductor nanowires and nanopillars have special optical properties thanks to their 1D structure. Engineering 3D arrays of such wires and pillars allows scientists to make optically tunable devices that can absorb a broad range of light wavelengths – something that is critical for efficient solar cells and photodetectors.

Ali Javey's team has made a new kind of nanopillar structure, based on germanium, with a small diameter tip and a large diameter base. This structure is optimized for broadband absorption of photons: the thin tip means reduced reflectance of light while the thick base allows a maximum number of incoming photons to be absorbed.

99% of photons absorbed
"Our structure allows an impressive 99% of photons over wavelengths between 300 to 900 nm to be absorbed," Javey told "And this for nanopillars with a height of only 2 µm, without having to rely on any anti-reflective coatings."

The researchers used a template-assisted vapour-liquid-solid (VLS) growth process to make the nanopillars. This first involved creating dual-diameter alumina templates using a two-step anodization technique. Once the templates had been made, a thin layer of gold was electrodeposited at the bottom of each pore, each of which was then used as a catalytic seed for subsequent VLS growth of the nanopillars.

Although germanium nanopillars were studied, the approach should work for other semiconductor materials too, say Javey and colleagues.

The team now plans to incorporate the dual-diameter nanopillar structures into real-world photonic devices.

The work was published in Nano Letters.