Semimetal to semiconductor transition in $\text{Bi}/{\text{TiO}}_2$ core/shell nanowires

Abstract

We demonstrate the full thermoelectric and structural characterization of individual bismuth-based (Bi-based) core/shell nanowires. The influence of strain on the temperature dependence of the electrical conductivity, the absolute Seebeck coefficient and the thermal conductivity of bismuth/titanium dioxide ($\text{Bi}/{\text{TiO}}_2$) nanowires with different diameters is investigated and compared to bismuth (Bi) and bismuth/tellurium (Bi/Te) nanowires and bismuth bulk. Scattering at surfaces, crystal defects and interfaces between the core and the shell reduces the electrical conductivity to less than $5\%$ and the thermal conductivity to less than $25\%$ to $50\%$ of the bulk value at room temperature. On behalf of a compressive strain, $\text{Bi}/{\text{TiO}}_2$ core/shell nanowires show a decreasing electrical conductivity with decreasing temperature opposed to that of Bi and Bi/Te nanowires. We find that the compressive strain induced by the ${\text{TiO}}_2$ shell can lead to a band opening of bismuth increasing the absolute Seebeck coefficient by $10\%$ to $30\%$ compared to bulk at room temperature. In the semiconducting state, the activation energy is determined to $\left|41.3\pm 0.2\right|\text{meV}$. We show that if the strain exceeds the elastic limit the semimetallic state is recovered due to the lattice relaxation.