Enhancement of thermoelectric efficiency ofT−HfSe2via nanostructuring

Abstract

In this work, ab initio calculations based on density functional theory and the Landauer formalism are carried out to investigate ballistic thermoelectric properties of $T\text{\ensuremath{-}}\mathrm{Hf}{\mathrm{Se}}_{2}$ nanoribbons (NRs). The zigzag-edged NRs are metallic, and they are not included in this study. The armchair NRs possess two types of edge symmetries depending on the number of atoms present in a row; odd-numbered NRs have mirror symmetry, whereas the even-numbered NRs have glide reflection symmetry. The armchair-edged NRs are dynamically stable and show semiconducting properties with varying band gap values in the infrared and visible regions. Detailed transport analyses show that the $n$-type Seebeck coefficient and the power factor differ because of the structural symmetry, whereas the $p$-type thermoelectric coefficients are not significantly influenced. It is shown that the phonon thermal conductance is reduced to a third of its two-dimensional value via nanostructuring. The $p$-type Seebeck coefficient and the power factor for $T$-phase $\mathrm{Hf}{\mathrm{Se}}_{2}$ are enhanced in NRs. We report that the $p$-type $ZT$ value of $\mathrm{Hf}{\mathrm{Se}}_{2}$ NRs at 300 and 800 K are enhanced by factors of 4 and 3, respectively.