Negligible contribution of inter-dot coherent modes to heat conduction in quantum-dot superlattice

Abstract

Colloidal quantum dots (QDs) superlattice, which is made of inorganic cores and can self-assemble into various types of lattice structures, finds promising applications in optical, electrical, and optoelectronic devices. Recent inelastic neutron scattering measurement [NAT. COMMUN. 10:4236 (2019)] showed that the inter-quantum-dot vibrational frequencies can be tuned by varying the QDs shapes and ligand types, suggesting that the QDs superlattices can be a platform for phonon engineering. In this work, we quantify the impact of the second periodicity on thermal transport through full-scale molecular dynamics simulations of PbS QDs superlattice with realistic QD size and ligand morphology. The vibrational pattern analysis reveals that the vibrations can be classified into the inter-QDs coherent modes and the spatially localized modes arising from the geometry confinement. The spectral analysis indicates that spatially localized modes in the frequency range of 0.8–5 THz dominate the thermal transport and lead to an amorphous-like temperature dependence between 200 and 400 K. On the other hand, the inter-QDs coherent modes, albeit have an averaged relaxation of 10 ps, have a limited thermal conductivity value of 0.01 W/mK at room temperature due to the scarce of the vibrational states. We demonstrate that controlling the ligand morphology is more efficient than tuning the second periodicity in engineering the thermal conductivity of QDs superlattice.