Applications and mechanisms of photoluminescence on biomaterials, semiconductors, and metals

Abstract

We discuss the metal-semiconductor heterojunction-assisted photoluminescence (PL) emission mechanism for Bi/Bi₂O₃ heterojunction nanoparticles. These heterojunction nanoparticles are comprised of metallic Bi nanoparticles with Bi₂O₃ surface layers. The PL emissions that come from the Bi₂O₃ surface layers can be attributed to electron transitions from the conduction band, Fermi level (EF), and donor state (ED), respectively, to the valence bands in the Bi₂O₃ surface layers. These Bi nanoparticles are helpful in the formation of PL emissions, because they provide plenty of free Fermi-level electrons to theBi₂O₃ layers across the heterojunction. The Bi/Bi₂O₃ heterojunction nanoparticles were synthesized using the lower-energy pulsed laser deposition technique. As the incident laser supplies thermal energy to the free electrons of the metallic Bi nanoparticles, they easily overcome the Ohmic barrier and move to the Bi₂O₃ layers. This is because the as-synthesized Bi₂O₃ layers are n-type semiconductors, so that Ohmic contacts, which are one of the metal-semiconductor heterojunctions, are formed at the interfaces between the Bi nanoparticles and Bi₂O₃ layers. These Bi/Bi₂O₃ heterojunction nanoparticles are potential candidates for use in optoelectronic nanodevices because they have good and effective visible light emissions at room temperature.
In addition, the female damselfly Ischnura senegalensis exhibits color dimorphism. The andromorph helps to reduce intrasexual and male harassment and allows them to gain mature mating. The andromorphic colors may be used as signals to improve sexual selection, because they provide good visual contrast for conspecific identity in the ambient light of a varying environmental background. The capability of assuming conspecific identity provides not only a better chance for sexual selection, but also an advantage in territorial defense. The PL emissions from the cuticles of various colored-body portions of the male damselfly were observed and verified in PL spectromicroscopy and confocal fluorescence microscopy. The intensity of the PL emissions embodies a color sign of health and strength, which other damselflies can observe. The males will choose not to intrude on its territorial airspace, but the females will prefer to mate with this individual. Hence, the PL emissions play a key role in the evolution of damselflies. They can strongly help to enhance or modify the colors or colorations of the various colored-body portions, as biological pigments and multilayer structures. The PL emissions from the cuticle of the various colored-body portions of the male damselfly Ischnura senegalensis  range between 450 and 550 nm, and the maximum intensities reach ~1.0 x 10⁴ counts per second. The PL and micro-PL confocal images verify that the PL emissions can strongly influence the surface colors of the cuticle, and show why the damselfly Ischnura senegalensis is called a bluetail.