Germanium-tin nanoparticles: Correlated with Raman spectroscopy

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  • Published: Dec 1, 2017
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Germanium-tin nanoparticles: Correlated with Raman spectroscopy

Particular photovoltaics

Raman and photoluminescence spectroscopies have been used to quantify lattice strain and photoluminescence behaviour in tin nanoparticles. The investigation reveals a correlation between the amount of tin in the nanoparticle core and how well the core's lattice matched that of its cadmium sulfide outer shell. (Credit: Ames Laboratory)

Raman and photoluminescence spectroscopies have been used to quantify lattice strain and photoluminescence behaviour in tin nanoparticles. The investigation reveals a correlation between the amount of tin in the nanoparticle core and how well the core's lattice matched that of its cadmium sulfide outer shell.

Scientists at Ames Laboratory, a US Department of Energy lab, have developed germanium nanoparticles that have improved photoluminescence, making them potentially better materials for solar cells and probes, or labels, for protein imaging applications. The team has now demonstrated that by adding tin to the germanium core of the nanoparticle, they can better match lattice structure to the overall lattice structure of the cadmium sulfide coating. This results in a greater absorption of light.

"For a photovoltaic material, obviously absorbing light is the first part and converting that solar energy into electrical energy is the second part," explains team member Emily Smith. "So you want a material that does both efficiently. Germanium has some desirable characteristics for photovoltaic materials, but unfortunately it doesn’t absorb light well."

Protection racket

Unfortunately, for most germanium nanoparticles their outer surface changes over time primarily through oxidation. In earlier work at Ames Lab, Javier Vela's group had shown that coating nanoparticles, a process known as surface passivation, would improve them in some ways and bolster their ability to absorb light. "We aren't actually measuring absorption," Smith explains, "We measure the luminescence, the amount light given off after a photon is absorbed." Germanium does not intrinsically absorb light well as it is an indirect band gap material. The team's efforts are thus focused on making a more direct band gap material. It was already known that doping germanium materials with tin would their light-absorption properties. However, the Ames researchers have found that simply adding tin is not enough. The nanoparticles formulated in that manner still need surface passivation. Moreover, the team also found that the relationship between the atomic structure of the passivation material and the nanoparticle core could itself lead to an improved light absorption. Thus the team employed successive ion layer adsorption and reaction (SILAR). That technique had been developed several years ago for group IV colloidal materials.

Devloping expertise

"We have been developing the expertise required to grow intricate core/shell and other well-defined nanoparticles for many years," Vela adds, "Through our collaboration with Emily Smith's group, we hope to continue making inroads in our ability to manipulate and direct energy flows at the nanoscale."

The team used transmission electron microscopy (TEM) imaging and powder X-ray diffraction to study the structural characteristics of their nanoparticles and bolstered the data with Raman and photoluminescence spectroscopies to quantify lattice strain and photoluminescence behaviour.

"The atoms are in a very specific location within the nanocrystal core and when you apply the shell around the nanocrystal, the atoms of the shell may not match perfectly with the atoms of the core," Smith explains. "With the germanium only material used previously, the core and shell didn't match perfectly." The researchers suspect that for germanium-tin particles, functionality is improved through better spacing of the atoms that matches the spacing of the atoms in the passivation layer. "By doing that, you're getting a more perfect shell that's less likely to cause chemical changes to the surface of the nanoparticle core," Smith explains.

Related Links

Chem Mater 2017, 29, 6012–6021: "Germanium-Tin/Cadmium Sulfide Core/Shell Nanocrystals with Enhanced Near-Infrared Photoluminescence,”"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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