Quantifying anisotropic thermal transport in two-dimensional perovskite (PEA2PbI4) through cross-sectional scanning thermal microscopy

Abstract

In this work, we investigated the anisotropic thermal transport in two-dimensional (2D) perovskite (phenethylammonium lead iodide) nanolayers through a novel measurement technique called crosssectional scanning thermal microscopy (xSThM). In this method, a target perovskite layer on a substrate was oblique polished with an Ar ion beam to create a low-angle wedge with nanoscale roughness that is followed by high vacuum SThM to obtain the thermal conductance map as a function of local thickness.The experimentally obtained data were processed with an analytical model and validated by the finite elemental analysis simulation to quantify the in-plane (𝑘 𝑙,𝑥𝑦 ) and cross-plane thermal conductivities (𝑘 𝑙,𝑧 ) of the 2D perovskite from a single set of measurements with nanoscale resolution. We obtained ultra-low thermal conductivity (𝑘 𝑙 = 0.25 ± 0.05 Wm -1 K -1 ) for the 2D perovskite along with an anisotropy (𝑘 𝑙,𝑥𝑦 = 0.45 ± 0.05 Wm -1 K -1 and 𝑘 𝑙,𝑧 = 0.13 ± 0.05 Wm -1 K -1 ) linked to the unique structure of the perovskite and different phonon lifetimes and group velocities for in-plane and out-of-plane directions. The results that are available for the first time, are essential for the thermal management of 2D perovskite-based optoelectronic devices, and potential thermoelectric applications of these materials.

Exciton Fine Structure in 2D Perovskites: The Out‐of‐Plane Excitonic State

“…We suppose this behavior results from the inherent anisotropy of 2DP, whose properties are very different in-and out-of-plane directions. [50][51][52] Assuming based on previous works that the broad sideband visible for in-plane exciton states is a result of phonon-replicas [46,47,49] we conclude that exciton-phonon coupling also exhibits strong anisotropy and vanishes for excitons with the out-of-plane dipole orientation. Further detailed analysis of the broad, high-energy part of the spectrum is beyond the scope of this work, but it certainly characterizes states with an in-plane dipole moment''.…”

Direct Measurements of Anisotropic Thermal Transport in γ‐InSe Nanolayers via Cross‐Sectional Scanning Thermal Microscopy

“…The measurements of the anisotropic thermal conductivity of InSe are directly comparable with similar studies done on other layered materials like Sb 2 Te 3 /MoS 2 superlattices [67] and perovskites. [81] These provided a similar range of heat para meters with a ratio of in-plane to cross-plane thermal conductivities ranging from 1.0 to 3.0, as well as other layered materials with a higher range of thermal conductivities such as a SiGe composition gradient alloy, [49] furtherly validating our approach.…”

“…For instance, transition metal dichalcogenides – bulk materials heterostructures and perovskites nanolayers were effectively studied similarly with the appropriate choice of substrate. [ 67,81 ]…”

“…For instance, transition metal dichalcogenides -bulk materials heterostructures and perovskites nanolayers were effectively studied similarly with the appropriate choice of substrate. [67,81] While the InSe polytype studied here is the γ-InSe, which has a rhombohedral crystal structure, InSe can also exist in two other polytypes (ε and β), with hexagonal symmetry. This could result in a difference in the thermal transport direction, as the main anisotropy is due to the difference in covalent intralayer bonds versus the weaker vdW interlayer links, as observed in our measurements.…”