Researchers at the US Department of Energy's Ames Laboratory have discovered that when adding tin to the core of germanium nanoparticles, the core’s lattice structure better matches the lattice structure of the cadmium-sulfide coating, which significantly improves the germanium-tin nanoparticles’ photoluminescence.
The group used a wet chemistry method to synthesize the core and the shell nanocrystals. Ranging from 6 to 13 nm in diameter, the nanocrystals exhibit a photoluminescence up to 15 times higher than that of germanium/cadmium-sulfide nanocrystals.
“While others have reported on germanium/cadmium sulfide core/shell nanocrystals and germanium-tin alloy nanocrystals, to the best of our knowledge, this research is the first report of combining these ideas into germanium-tin alloy/cadmium sulfide core/shell nanocrystals,” says Ames Laboratory scientist Emily Smith, adding that these newly created nanomaterials also exhibit higher luminescence than comparable germanium/cadmium-sulfide nanocrystals without the tin in the core.
The expert explains that the presence of the tin in the core allowed for improved epitaxy between the mixed germanium-tin core and the cadmium-sulfide shell, leading to improved surface passivation and, thus, increased luminescence. “These nanocrystals are not necessarily better light absorbers,” Smith explains. “It’s that they exhibit increased luminescence because of improved surface passivation. Without effective passivation, germanium nanocrystals are prone to oxidation that greatly reduces luminescence.”
Enabling novel biosensors
The pioneering tin-spiked germanium nanoparticles with improved photoluminescence could enable new kinds of applications that had not been feasible before. “Coupled with their small size and reported strategies for biocompatibilty, the increased luminescence of these nanocrystals makes them good candidates for biosensors,” Smith says. According to the expert, these nanocrystals exhibit luminescence at approximately 950 nm, a range well-suited for biological tissues and for applications where fluorescent dyes are less common.
The Ames Lab team used a method called successive ion layer adsorption and reaction (SILAR) to grow the intricate shells on the preformed germanium-tin alloy nanocrystals by adding small aliquots of sulfur and cadmium precursor solutions alternately to the reaction vessel.
The group used transmission electron microscopy imaging and powder X-ray diffraction to study the structural characteristics of the nanoparticles and Raman and photoluminescence spectroscopies to quantify the lattice strain and photoluminescence behavior. Their finding? There is a correlation between the amount of tin in the core and how well the core's lattice matched that of the cadmium-sulfide outer shell. And assimilating the core and shell’s atomic structure enhances the nanoparticle’s ability to absorb light. The germanium-tin particles’ characteristics improved once the spacing of the atoms better matched the spacing of the atoms in the cadmium-sulfide coating. The result was a shell that is less likely to cause chemical changes to the surface of the nanoparticle core.
Improved epitaxy is key
Smith notes that the experiments with the pure germanium compared with the germanium-tin alloy nanocrystals showed no measurable luminescence increase due to the tin incorporation alone. Rather, it is the tin incorporation that results in a lower lattice mismatch between the core and shell materials — which means improved epitaxy and, therefore, the increased luminescence. “It was surprising that such a small change in the lattice parameter affected the luminescence so strongly,” Smith shares.
“Our work in understanding the properties of these interesting luminescent nanoparticles is just beginning,” says Smith, speaking about what is next for her and her Ames Laboratory colleagues in this research endeavor. Furthermore, the scientist reveals that her team is also currently studying other nanomaterials with good potential for use in next-generation energy capture and storage applications.
The work is detailed in the paper "Germanium-Tin/Cadmium Sulfide Core/Shell Nanocrystals with Enhanced Near-Infrared Photoluminescence," published in the American Chemical Society's journal Chemistry of Materials.
Written by Sandra Henderson, Research Editor, Solar Novus Today