Hull structure nanoparticles improve performance of zinc oxide photodetectors

Improving the sensitivity of light sensors or the efficiency of solar cells requires a fine tuning of light capture. KAUST researchers have used complex geometry to develop tiny, shell-shaped coatings that can increase the efficiency and speed of photodetectors.

Many optical cavity designs have been studied to look for light efficiencies: either by trapping the electromagnetic wave, or by confining light to the active region of the device to increase absorption. Most use simple spheres at the micrometer or nanometer scale in which light travels in circles inside the surface, known as whisper mode.

Former KAUST scientist Der-Hsien Lien, now a postdoctoral fellow at the University of California at Berkeley, and his colleagues in China, Australia, and the United States demonstrate that a more complex geometry including convex nanoscale hulls improves the performance of photodetectors by increasing the speed. that they work and allowing them to detect light from all directions.

Surface effects play an important role in the operation of some devices, says Jr-Hau He, Principal Investigator at KAUST. Nanomaterials can improve performance because of their high surface area to volume ratio. "However, although nanomaterials are more sensitive to the detection of light in relation to the mass, the interactions between light and matter are weaker because they are finer," he explains. "To improve this, we design structures to trap light."

The researchers made their spherical multi-nanocells from semiconductor zinc oxide. They immersed the solid carbon spheres in a solution of zinc oxide salt, covering them with optical material. The heat treatment removed the carbon template and defined the geometry of the remaining zinc oxide nanostructures, including the number of shells and the spacing between them. Thus, Lien and his colleagues were able to design the interaction between the outer and inner shells to induce a whispering gallery mode and light absorption near the surface of the nanomaterial.

The team has incorporated their nanocells into a photodetector. The symmetry of the spherical nanocells meant that the whispering gallery mode could be excited with little dependence on the angle of incidence or the polarization of the incoming light.

A problem with earlier photodetectors based on metal oxide nanoparticles is their slow speed, devices taking several hundred seconds to respond. By using zinc oxide nanoshells, the photodetectors were able to respond in 0.8 milliseconds.

"This strategy can be applied to other work, such as solar cells and water splitting devices," he says. "In the future, we will be looking at different material systems and design structures that also improve device performance in these other applications."

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More information:
Der-Hsien Lien et al. Enhanced Resonance Absorption in Hollow Nanoshell Spheres with Omnidirectional Detection and High Reactivity and Speed, Advanced Materials (2018). DOI: 10.1002 / adma.201801972