Semimetal to semiconductor transition in $\text{Bi}/\text{TiO}_{2}$ core/shell nanowires

TL;DR

This study thoroughly characterizes individual Bi/TiO2 core/shell nanowires, revealing how strain influences their thermoelectric properties and induces a transition from semimetal to semiconductor, with implications for nanoscale thermoelectric devices.

Contribution

It provides the first detailed thermoelectric and structural analysis of Bi/TiO2 nanowires, highlighting strain effects on electronic and thermal properties and demonstrating a semimetal to semiconductor transition.

Findings

Strain reduces electrical and thermal conductivities significantly.

Compressive strain increases the Seebeck coefficient by 10-30%.

Exceeding elastic strain limits reverts the nanowires to a semimetallic state.

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 $\left|41.3\pm0.2\right|\,\text{meV}$. We show that if the strain exceeds the elastic limit the semimetallic state is recovered due to the lattice relaxation.