Challenges in determining the thermal conductivity of core-shell nanowires by atomistic simulation

In the present work, we investigate the thermal conductivity (κ) of different core-shell nanowires using molecular dynamics simulation and Green-Kubo (EMD), imposing a temperature gradient (NEMD) and Müller-Plathe (rNEMD) approaches. We show that in GaAs@InAs nanowires, the interface effect becomes more significant than the nanowire cross-sectional geometry. In particular, κ decreases as the interface area increases, reaching a minimum, and then increases when the interface strain relaxes. This is particularly important for thermoelectric applications, where minimization of κ is desired. In particular, the different methods can predict minima at different core diameters without special considerations. In addition, the NEMD approach and, to a lesser extent, rNEMD tend to overestimate the κ values, which cannot be corrected with the methods available in the literature. By analyzing the temperature and length dependence, (I) we show that interfacial scattering primarily involves phonon-phonon interactions, which mainly affect low-energy modes, a mechanism that effectively reduces κ at low temperatures. (II) The Langevin thermostat tends to pump low-energy modes in the NEMD approach, but this effect decreases with longer nanowires. (III) Energy exchanges in rNEMD stimulate high-energy phonons, derived from the saturation of κ at a much shorter nanowire length than NEMD. These findings highlight the challenges of accurately determining κ of ultrathin core-shell nanowires, where only the EMD approach provides precise results. With the recognition of non-equilibrium contributions to the overestimation of κ by NEMD and rNEMD, these methods can still provide valuable insights for a comprehensive understanding of the underlying thermal transport mechanisms.