Thermoelectric and electrical transport properties of Bi${}_2$Te${}_3$ compacted nanoparticles bulk material

Three-dimensional (3D) topological insulators (TI) host surface carriers with extremely high mobility. However, their transport properties are typically dominated by bulk carriers that outnumber the surface carriers by orders of magnitude. We herein present a strategy to overcome the problem of bulk carrier domination by using 3D TI nanoparticles, which were compacted by hot-pressing to macroscopic nanograined bulk samples. Bi${}_2$Te${}_3$ nanoparticles well known for their excellent thermoelectric and 3D TI properties serve as our model system. As key enabler for this approach, we applied a specific synthesis that creates nanoparticles with a low level of impurities and surface contamination. The compacted nanograined bulk contains a high number of interfaces and grain boundaries. Here we show that these samples exhibit metallic-like electrical transport properties and a distinct weak antilocalization. A downwards trend in the electrical resistivity at temperatures below 5 K was attributed to an increase in the coherence length by applying the Hikami-Larkin-Nagaoka model. THz time-domain spectroscopy revealed a dominance of the surface transport at low frequencies with a mobility of above 10${}^3$ cm${}^2$ V${}^{-1}$ s${}^{-1}$ even at room temperature. These findings clearly demonstrate that nanograined bulk Bi${}_2$Te${}_3$ features surface carrier properties that are of importance for technical applications.