Quasi-Ballistic Thermal Transport Across MoS 2 Thin Films

Aditya Sood, Feng Xiong, Shunda Chen, Ramez Cheaito, Feifei Lian, Mehdi Asheghi, Yi Cui, Davide Donadio, Kenneth E. Goodson, Eric Pop

Research output: Contribution to journalArticlepeer-review

63 Scopus citations


Layered two-dimensional (2D) materials have highly anisotropic thermal properties between the in-plane and cross-plane directions. Conventionally, it is thought that cross-plane thermal conductivities (κ z ) are low, and therefore c-axis phonon mean free paths (MFPs) are small. Here, we measure κ z across MoS 2 films of varying thickness (20-240 nm) and uncover evidence of very long c-axis phonon MFPs at room temperature in these layered semiconductors. Experimental data obtained using time-domain thermoreflectance (TDTR) are in good agreement with first-principles density functional theory (DFT). These calculations suggest that ∼50% of the heat is carried by phonons with MFP > 200 nm, exceeding kinetic theory estimates by nearly 2 orders of magnitude. Because of quasi-ballistic effects, the κ z of nanometer-thin films of MoS 2 scales with their thickness and the volumetric thermal resistance asymptotes to a nonzero value, ∼10 m 2 K GW -1 . This contributes as much as 30% to the total thermal resistance of a 20 nm thick film, the rest being limited by thermal interface resistance with the SiO 2 substrate and top-side aluminum transducer. These findings are essential for understanding heat flow across nanometer-thin films of MoS 2 for optoelectronic and thermoelectric applications.

Original languageEnglish (US)
Pages (from-to)2434-2442
Number of pages9
JournalNano Letters
Issue number4
StatePublished - Apr 10 2019
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • General Chemistry
  • Condensed Matter Physics
  • Mechanical Engineering
  • Bioengineering
  • General Materials Science


  • MoS
  • Phonon
  • cross-plane
  • mean free path
  • thermal conductivity
  • time-domain thermoreflectance


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