Growing perfect quintuple-layer nanowires using TiO₂

Bi₂Te₃ nanowires have great potential use for thermoelectric applications: however, the traditional Au-catalyzed method can lead to contamination and crystal defects. In APL Materials, researchers at Oxford University, the MPI-IS in Stuttgart, and the Humboldt University in Berlin share their research comparing nanowires grown using this traditional method to those grown with an alternative method using TiO₂.

One obstacle to efficient thermoelectric Bi₂Te₃ nanowires is high intrinsic doping by antisite defects. For this paper, physicist Thorsten Hesjedal and his collaborators demonstrated how the crystal structure of Bi₂Te₃ nanowires can be controlled through the choice of catalyst, and analyzed measurements of the basic thermoelectric properties, revealing some surprising results.

First, the researchers grew the Bi₂Te₃ nanostructures using Si (100) substrates coated with poly-L-lysine. Then catalyst nanoparticles were placed on the substrate, using Au nanoparticles with 5 nm diameter for one sample and 20 nm diameter TiO₂ nanoparticles for the other. Both samples were then grown via vapor transport in a horizontal tube furnace and were analyzed using electron microscopy.

Hesjedal sought to fully characterize selected individual nanowires using complementary techniques. By combining atomically resolved electron microscopy with focused ion beam milling, they were able to study the nanostructures in cross-section, and identify the septuple layers in Au-catalyzed nanowires. While defects in these nanowires increased conductivity as compared to the TiO₂-catalyzed samples, Hesjedal says the researchers were surprised that the influence of these defects on the thermoelectric properties was marginal.

According to Hesjedal, the crucial finding is that crystal perfection has no measurable effect on the thermoelectric power factor, signifying that one can work with less stringent fabrication or synthesis limitations, ultimately lowering costs of nanowire-based thermoelectric materials.

Source: “Perfect quintuple layer Bi₂Te₃ nanowires: Growth and thermoelectric properties,” by P. Schönherr, D. Kojda, V. Srot, S. F. Fischer, P. A. van Aken, and T. Hesjedal, APL Materials (2017). The article can be accessed at https://doi.org/10.1063/1.4986524 .