Researchers Synthesize Nanosize Semiconductors in a Plasmonic Nanoantenna

Researchers at Hokkaido University have developed an approach to placing nanoscale semiconductors on metallic particles that is precise and cost-effective. Heat is localized on a gold nanoparticle within a butterfly-shaped nanostructure. The heat causes hydrothermal synthesis, which in turn causes semiconducting zinc oxide to crystallize on the gold nanoparticle. The Hokkaido team’s approach could open a new route to making nanosize semiconductors for nanolasing, nanolithography, and other applications.

First, the team conducted simulations to determine the optimal conditions for controlling the generation of heat in nanostructures. They used surface plasmon resonance, a process that partially converts light to heat in metallic materials. The excitation of localized surface plasmon resonances in metal nanostructures enables subwavelength photon localization and large electric field enhancement, which can then be used to enhance light-matter interactions at the nanoscale.

Calculated localized field (left) and temperature distribution (right) of the nanobutterfly structure. Courtesy of Fujiwara H. et al, Nano Letters, Dec. 23, 2019.

The simulations revealed that a butterfly-shaped nanostructure consisting of two rhombus gold particles (the wings of the butterfly) placed on either side of a gold nanorod (the butterfly’s body) could provide optimal conditions for controlled heat generation.

The team surmised that the nanorod, or butterfly body, would work as a nanoheater using polarized light. It further surmised that after the polarized light was rotated 90°, the rhombus particles, or the wings of the butterfly, would work as an antenna to gather light at subwavelength spots in the butterfly’s semiconductor “skin.”

To test their theory, the researchers fabricated the gold butterfly and placed it in water inside a glass chamber. A solution made from equal parts zinc nitrate hexahydrate and hexamethylene tetramine was added to the chamber, which was then sealed and placed on a microscopic stage. When the laser light was shone on the system inside the glass chamber, the butterfly’s nanorod body heated up and semiconducting zinc oxide particles crystallized along its surface, meeting the expectations of the team.

Scanning electron microscope (SEM) images of the nanobutterfly structure before (left) and after (right) laser irradiation. Semiconductor zinc oxide has crystallized on the surface of the gold nanorod. Courtesy of Fujiwara H. et al., Nano Letters, Dec. 23, 2019.

The butterfly-shaped gold nanoantennas were able to provide precise control over where plasmon-assisted hydrothermal synthesis occurred in the system and could thus enable the localized formation of nanosize semiconductors.

“Further research is expected to lead to the development of powerful nanosize light sources, highly efficient photoelectric conversion devices, and photocatalysts,” professor Keiji Sasaki said. “It could also lead to applications in semiconductor electronics and optical quantum information processing.”

The research was published in Nano Letters (www.doi.org/10.1021/acs.nanolett.9b04073).