Combining first-principles calculations with Landauer-Büttiker formalism, ballistic thermoelectric transport properties of semiconducting two-dimensional transition metal dichalcogenides (TMDs) and ...oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are investigated in their 2H and 1T phases. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we report the thermoelectric properties of the semiconducting ones. We find that 2H phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T phases. The maximum room temperature p-type thermoelectric figure of merit (ZT) of 1.57 is obtained for 2H−HfSe2, which can be as high as 3.30 at T=800 K. Additionally, 2H−ZrSe2, 2H−ZrTe2, and 2H−HfS2 have considerable ZT values (both n- and p-type), that are above 1 at room temperature. The 1T phases of Zr and Hf-based oxides possess relatively high power factors, however their high lattice thermal conductance values limit their ZT values to below 1 at room temperature.
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 − Hf Se2 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 Hf Se2 are enhanced in NRs. We report that the p-type ZT value of Hf Se2 NRs at 300 and 800 K are enhanced by factors of 4 and 3, respectively.
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