Thermal transport by phonons is at the heart of materials research for energy harvesting applications, such as thermoelectricity, and thermal management in electronic and optoelectronic devices. The quest for more abundant and less toxic materials has turned to 2-dimensional (2D) materials and among them SnSe2 holds a promise with exceptional figures of merit ZT, eg., 2.95 at 800 K. To integrate thermoelectric generators in circuits, the challenge of controlling the heat directionality becomes increasingly important. Thus, the study of heat propagation in two-dimensional materials is highly relevant to assess their potential for applications that pose thermal management constraints.
We report a systematic study of the in-plane and cross-plane thermal conductivity of supported and suspended crystalline SnSe2 films of thickness from 16 to 190 nm, with state-of-the-art Raman thermometry and frequency domain thermoreflectance (see figure 1). The specific crystalline structure of materials determines their capacity of conducting heat along different directions, as in the case of SnSe2.
This work revealed that the thermal conductivity anisotropy ratio of the in-plane and cross-plane conductivities, is almost a factor of 10 and is independent of the SnSe2 film thickness in the range studied. This means that heat tends to propagate faster along the in-plane direction than along the cross-plane one. Moreover, the in-plane thermal conductivity drops when the ambient temperature increases, but this temperature dependence is weaker for thinner films (see figure 2). A full explanation is provided in terms of the phonon mean free path distribution for different thickness values and the role of surface phonon scattering.
This study yield useful information on heat transport in SnSe2, which can help design electronic labscale devices with improved thermal management when it is desirable to have heat dissipating mainly in one direction, while preserving thermal insulation in the other. This may be a way to avoid hot spots and improve device stability and performance.
