An Ab Initio Investigation of Ultra‐Low Thermal Conductivity in Organically Functionalized TaS2${\rm TaS}_2$

Abstract An ab initio characterization of the impact of tert‐Butyl isocyanate () functionalization on lattice thermal conductivity is presented. Such a system has been previously synthetized and experimentally charachterized, showing that the incorporation of covalently bonded on bulk leads to a dramatic in‐plane lattice thermal conductivity decrease by more than two orders of magnitudes. To elucidate these experimental findings, detailed calculations of the phonon dispersion relations and scattering rates in are performed. The analysis is addressed to discern the impact of inter‐layer covalently bonded molecules on these phonon properties, providing insights into the underlying mechanisms of the observed thermal conductivity decrease. Present calculations provide evidence that the observed lattice thermal conductivity reduction is attributed to two effects: the increase of inter‐layer separation and the presence of low‐frequency molecular optical modes. The first inhibits specific Van der Waals quasi‐acoustic inter‐layer vibrational modes contributing as much as in bulk . The second effect dramatically decreases the phonon group velocities as a consequence of phonon‐crossing phenomena among low‐frequency molecular modes and acoustic modes, eventually leading to a strong reduction of all phonon lifetimes.