Plasmonic nanostructures are playing an increasingly important role in photonics because of their intense interaction with light, and the use of metallic nanoparticles for improved photodetection and solar cell devices is of particular interest to researchers. A team from the National University of Singapore1 have now successfully demonstrated a novel photodetection concept by which light absorption is achieved using networks of metallic nanoparticles rather than conventional semiconductors.

The unique optical properties of gold nanoparticles arise from plasmonic resonances — collective oscillations of electrons on the particle surfaces. These plasmonic resonances interact strongly with incident light, leading to an efficient capture and absorption process. And unlike semiconductors, which only absorb light at specific band-gap energies, light absorption by metallic nanoparticles is not controlled by a band gap.

“This makes it possible to use metallic nanoparticles for photodetection in a broad wavelength range, as opposed to the specific wavelength range of inorganic semiconductors,” says Xian Ning Xie, one of the researchers on the team.

Fig. 1: Schematic diagram of a photoresponsive organic/gold nanoparticle network. Light (purple) is absorbed by the gold nanoparticles, which are connected via alkanethiol ligands.

The network of gold nanoparticles investigated by the researchers is held together by organic ligand molecules that bind to the nanoparticles (Fig. 1). The incoming light is absorbed by the gold nanoparticles and the charge carriers generated in this process are then carried away by the ligands. The on-off ratio of electrical current generated in the illuminated and dark states is about 10. While this represents a good value for a proof-of-principle demonstration, the researchers are confident that device performance can be improved further, such as through the use of organic linker molecules with electronic states that better match those of the gold nanoparticles.

Such improvements, says Xie, “are the prerequisite for the development of metallic nanoparticle-based photodetection devices, and ultimately may mean these networks may complement semiconductor-based devices.” In particular, the lack of any gap in the energy spectrum could become an advantage that could allow photodetection across a broad range of wavelengths in the visible region and beyond. “In our experiments, we already have observed the formation of photo-voltage in the gold nanoparticle network, and follow-up work is now in progress to investigate the feasibility for solar cell applications,” says Xie.