Covalent-bonding-induced strong phonon scattering in the atomically thin WSe2 layer

Abstract

In nano-device applications using two-dimensional (2D) van der Waals materials, a heat dissipation through nano-scale interfaces can be a critical issue for optimizing device performances. By using a time-domain thermoreflectance measurement technique, we examine a cross-plane thermal transport through mono-layered ( n = 1) and bi-layered ( n = 2) WSe 2 flakes which are sandwiched by top metal layers of Al, Au, and Ti and the bottom Al 2 O 3 substrate. In these nanoscale structures with hetero- and homo-junctions, we observe that the thermal boundary resistance (TBR) is significantly enhanced as the number of WSe 2 layers increases. In particular, as the metal is changed from Al, to Au, and to Ti, we find an interesting trend of TBR depending on the WSe 2 thickness; when referenced to TBR for a system without WSe 2 , TBR for n = 1 decreases, but that for n = 2 increases. This result clearly demonstrates that the stronger bonding for Ti leads to a better thermal conduction between the metal and the WSe 2 layer, but in return gives rise to a large mismatch in the phonon density of states between the first and second WSe 2 layers so that the WSe 2 -WSe 2 interface becomes a major thermal resistance for n = 2. By using photoemission spectroscopy and optical second harmonic generation technique, we confirm that the metallization induces a change in the valence state of W-ions, and also recovers a non-centrosymmetry for the bi-layered WSe 2 .

Thermionic transport across gold-graphene-WSe 2 van der Waals heterostructures

“…Thus, we cannot make direct comparisons between values determined in our work and those found in the literature. The contribution of electron-phonon coupling to the conductance of three- and four-layer WSe 2 is reported to be similar due to the ballistic nature of transport in these layers ( 16 ); however, the total thermal conductance across metal-WSe 2 -substrate interfaces has been shown to decrease following an increase in the number of WSe 2 layers ( 26 ). The trend in the latter is consistent with the ultralow thermal conductivity of WSe 2 ( 23 ), as the resistances compound following an increase in the number of layers.…”

Anisotropic thermal conductivity in two-dimensional van der Waals crystals

“…이 사용하는 optothermal 측정 방법에는 optothermal micro-Raman spectroscopy (ORS) [3] , time-domain thermo-reflectance (TDTR)과 frequency-domain thermo-reflectance (FDTR)을 포함하는 optical thermo-reflectance (OTR) [2,34] , light/laser flash technique (LFT) [35] 이 있고, electrothermal 측정 방 법에는 scanning thermal microscopy (SThM) [36] ,…”

Improving the Through-Thickness Thermal Conductivity of Carbon Fiber/Epoxy Laminates by Direct Growth of SiC/Graphene Heterostructures on Carbon Fibers

“…Experimental and theoretical studies have revealed that the nature of the interfacial bonding between layers is a significant factor for phonon transport and hence TC in the through-plane direction. Generally, the weak van der Waals (vdW) forces in layered materials hinder phonon transport when compared to covalent or ionic forces. In line with the above, TC in layered crystals, where layers are bonded by weak van del Waals forces, is considerably inferior in the through-plane direction ( K ⊥ ) compared to the in-plane direction, K // (e.g., K // ≈ 2200 W/mK vs K ⊥ ≈ 6 W/mK for graphite) …”