Advanced Intercalation Strategies for Enhanced Thermoelectric Performance in TaS₂ and TaSe₂

Transition metal dichalcogenides (TMDCs) have recently gained significant attention in various research fields due to their versatile electronic properties, coupled with mechanical flexibility and optical sensitivity. These characteristics make TMDCs suitable for applications in semiconductor systems, lightweight wearables, and flexible technologies. Recently, TMDCs have also been proposed as potential thermoelectric materials because of their relatively high carrier mobility and tunable thermal conductivity. In this context, atomic-scale engineering represents a promising approach to further enhance their intrinsic thermoelectric performances, allowing a fine tuning of the three key quantities involved in determining the figure of merit ZT, i.e., thermal conductivity κ , electrical conductivity σ , and Seebeck coefficient S; thereby opening new opportunities for their practical implementation in thermoelectric devices.<br/>In this work, we explore two different strategies to enhance the thermoelectric figure of merit of two specific TMDCs, namely TaS₂ and TaSe₂ . First, we demonstrate through first-principles DFT calculations that the intercalation of specific functional groups, such as tert-butyl isocyanate, can dramatically decrease the lattice thermal conductivity of TaS₂ without affecting the corresponding thermoelectric power factor (S²σ) [1,2]. This effect can be attributed to two key mechanisms: <i>i)</i> the increase in inter-layer separation and <i>ii)</i> the presence of low-frequency molecular optical modes. The first mechanism inhibits specific Van der Waals quasi-acoustic inter-layer vibrational modes, which contribute approximately 55% of the lattice thermal conductivity. The second mechanism involves a significant decrease in phonon group velocities due to phonon-crossing phenomena between low-frequency molecular modes and acoustic modes, leading to a substantial reduction in all phonon lifetimes. These combined effects result in a strong reduction in thermal conductivity, thereby enhancing the thermoelectric performance of TaS2 by a factor ~40.<br/>In the second strategy, a significant increase in the power factor was achieved using a similar intercalation approach, but with metallic cations as spacers between TaSe2 layers. We demonstrated that the inclusion of metallic cations, similar to organic functionalization, impacts the low-frequency region of the phonon spectrum by reducing phonon group velocities. Additionally, the metallic cations induce non-negligible modifications in the electronic band structure, thereby affecting the power factor. The result of this second intercalation strategy is a substantial overall increase i the figure of merit, achieving an improvement by a factor of up to 10.<br/>[1] S. Wang, X. Yang, L. Hou, X. Cui, X. Zheng, J. Zheng, Nat. Commun. 2022, 13, 4401.<br/>[2] F. Siddi, A. Cappai, L. Colombo, C. Melis, Adv. Theory Simul. 2024, 7, 2400056.