Incoherent phonon transport dominates heat conduction across van der Waals superlattices

Abstract

Heat conduction mechanisms in superlattices could be different across different types of interfaces. Van der Waals superlattices are structures physically assembled through weak van der Waals interactions by design and may host properties beyond the traditional superlattices limited by lattice matching and processing compatibility, offering a different type of interface. In this work, natural van der Waals (SnS)1.17(NbS2)n superlattices are synthesized, and their thermal conductivities are measured by time-domain thermoreflectance as a function of interface density. Our results show that heat conduction of (SnS)1.17(NbS2)n superlattices is dominated by interface scattering when the coherent length of phonons is larger than the superlattice period, indicating that incoherent phonon transport dominates through-plane heat conduction in van der Waals superlattices even when the period is atomically thin and abrupt, in contrast to conventional superlattices. Our findings provide valuable insights into the understanding of the thermal behavior of van der Waals superlattices and devise approaches for effective thermal management of superlattices depending on the distinct types of interfaces.

Engineering Natural Layered Framework for Low and Anisotropic Thermal Conductivity

“…We can further identify trails for phonon localization behavior in this natural material with no vdW structure, which was only studied in vdW materials before when talking about natural materials. [27,30,61] Cahill has proposed that when scattering is strong enough to destroy authenticity of phonons and localized atomic vibrations are independent with interlocking coherence vanishing, which means coherent length l ≃ 𝜆 where 𝜆 is phonon wavelength, a crystal lattice reaches its minimum thermal conductivity. [62,63] 𝜅 l,min = ( 𝜋 6…”

“…The original (PbSe) 5 (Bi 2 Se 3 ) 3 compound is different from the misfit layered compounds like (SnS) 1.17 (NbS 2 ) n [ 27 ] with interlayer bonds and forced commensurability. [ 37,47,52 ] It's the special vdW bonding in the latter makes a layers amount of scatterings possible, [ 59 ] and causes incoherent phonon transport in an atomically sharp superlattice [ 27 ] . The vdW misfit compounds are however particular.…”

“…Researchers have started to study the coherent and incoherent phonon transport very recently. [25][26][27] The roughness of boundaries is found to determine the decoherence. To restore superlattice phenomena in natural layered substances, there're natural layered materials discovered with contrived layer structure restricting quasiparticles, [27][28][29][30] but they're particularities.…”

“…[25][26][27] The roughness of boundaries is found to determine the decoherence. To restore superlattice phenomena in natural layered substances, there're natural layered materials discovered with contrived layer structure restricting quasiparticles, [27][28][29][30] but they're particularities. Also there're natural intercalation superlattices similar to epitaxial ones, [25,[31][32][33] but they're drudgeries.…”

“…In this work, in attempt to build a quantum dots superlattice akin to PbSeTe/PbTe, we AgSb–codoped only the PbSe–layer part of the ‘natural superlattice’ Pb 5 Bi 6 Se 14 , similar to how researchers have AgSb–codoped bulk PbTe to get the nanodots–loaded AgPb m SbTe m + 2 (LAST–m) material. [ 22 ] We vindicated the existence of intentionally introduced scattering centers, and we found them break interlayer bonds to restore the structure of vdW layered compounds [ 27 ] that are true natural superlattices. The dopants conduce to the phonon localization and lead to further reducing of the already very low thermal conductivity down to near the Cahill's lower bound.…”