Quasi-One-Dimensional Electronic Nature of Ta4SiTe4 Underlying the Giant Thermoelectric Performance

Ta4SiTe4 is a one-dimensional van der Waals material that exhibits an exceptionally large thermoelectric power factor below room temperature. However, since this material has been available only in the form of acicular microcrystals, experimental exploration of the electronic properties responsible for its giant thermoelectric performance has long been challenging. In this study, we quantitatively evaluated the one-dimensional electronic nature of Ta4SiTe4 by combining micro-spot angle-resolved photoemission spectroscopy and transport measurements on focused-ion-beam-processed samples. The angle-resolved photoemission spectroscopy measurements reveal anisotropic band dispersions along and perpendicular to the crystallographic c axis. Consistently, transport measurements demonstrate that the resistivity perpendicular to the c axis is approximately five times larger than that along the c axis at 200 K. These results provide direct experimental evidence for the quasi-one-dimensional electronic character of Ta4SiTe4, which underlies its giant thermoelectric response reported previously, and offer fundamental insights into the role of electronic dimensionality in enhancing thermoelectric performance.

💡 Research Summary

Ta₄SiTe₄ is a one‑dimensional (1D) van der Waals compound that exhibits an exceptionally large thermoelectric power factor (PF) below room temperature, a rare achievement among bulk thermoelectric materials. The authors address the long‑standing challenge of directly probing its electronic structure, which had been limited to indirect optical measurements because the material is only available as needle‑like whiskers a few hundred micrometres in diameter. By combining micro‑spot angle‑resolved photoemission spectroscopy (micro‑ARPES) with focused‑ion‑beam (FIB) fabricated micro‑devices, the study provides a comprehensive, quantitative picture of the quasi‑1D electronic nature that underlies the giant thermoelectric response.

Micro‑ARPES performed with a ~10 µm synchrotron beam at 20 K reveals highly anisotropic band dispersions. Along the crystallographic c‑axis (Γ–Z direction) the bands are steep, indicating a large carrier velocity and a small effective mass. Perpendicular to the c‑axis the bands are almost flat, producing a sharp energy dependence of the density of states near the Fermi level. This anisotropy directly explains how the material can simultaneously achieve a high electrical conductivity (thanks to the fast carriers along the chains) and a large Seebeck coefficient (due to the steep DOS variation across the chains). The valence‑band maximum is observed at ~0.18 eV below the Fermi level; the conduction band minimum is not directly accessible, but Hall‑effect measurements and temperature‑dependent carrier density analysis yield a narrow band gap of 0.13 ± 0.03 eV, consistent with density‑functional theory predictions (0.10–0.15 eV) and with the empirical rule that optimal thermoelectric performance occurs when E_g ≈ 10 k_B T (≈200 K for this gap).

FIB processing enabled the extraction of a 15 µm × 4 µm × 4.4 µm block from a single whisker, which was transferred onto a sapphire substrate and contacted with Pt‑deposited Au electrodes. Multi‑terminal measurements provided resistivity parallel (ρ_//) and perpendicular (ρ_⊥) to the c‑axis on the same crystal. At 200 K, ρ_⊥/ρ_// ≈ 5.3, decreasing to ≈ 7 at room temperature, confirming strong transport anisotropy. Hall measurements show a negative slope, indicating n‑type carriers (electrons) likely introduced by Te vacancies. The carrier concentration follows a thermally activated behavior, allowing extraction of the band gap as described above. Mobilities parallel and perpendicular to the chains follow μ ∝ T^p with p ≈ −1.6 in the 140–240 K range, and the similar temperature exponents demonstrate that scattering mechanisms are essentially isotropic; thus the resistivity anisotropy originates mainly from the band structure rather than from anisotropic scattering.

Quantitatively, at 200 K the electron density is n ≈ 2.3 × 10¹⁸ cm⁻³ and the mobility along the chains reaches μ_// ≈ 600 cm² V⁻¹ s⁻¹ (≈ 1700 cm² V⁻¹ s⁻¹ at 100 K). Compared with the benchmark thermoelectric Bi₂Te₃ (n ≈ 1.5 × 10¹⁹ cm⁻³, μ ≈ 200 cm² V⁻¹ s⁻¹ at 300 K), Ta₄SiTe₄ achieves comparable or lower resistivity despite a much lower carrier concentration, confirming that high mobility—stemming from the 1D orbital overlap of Ta and Te along the chains—is the key factor. This weakly 1D conduction is robust against disorder, as evidenced by the minimal impact of chemical substitution (e.g., Mo, Ti, Nb) on transport properties.

The combination of a steep, dispersive band along the chains (high conductivity) and a flat band across the chains (large Seebeck) partially lifts the conventional trade‑off between electrical conductivity and thermopower. Consequently, Ta₄SiTe₄ delivers a power factor of ≈ 80 µW cm⁻¹ K⁻² in the pristine material and up to ≈ 170 µW cm⁻¹ K⁻² with Mo substitution, surpassing the typical PF of Bi₂Te₃ (~30 µW cm⁻¹ K⁻²) at comparable temperatures. The work establishes a clear experimental link between quasi‑1D electronic structures, narrow band gaps, and enhanced low‑temperature thermoelectric performance, offering a design principle for future high‑performance thermoelectric materials: engineer weakly 1D conduction channels that provide high mobility while preserving a sharply varying density of states to boost the Seebeck coefficient.