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) WSe₂ flakes which are sandwiched by top metal layers of Al, Au, and Ti and the bottom Al₂O₃ 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 WSe₂ 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 WSe₂ thickness; when referenced to TBR for a system without WSe₂, 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 WSe₂ layer, but in return gives rise to a large mismatch in the phonon density of states between the first and second WSe₂ layers so that the WSe₂-WSe₂ 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 WSe₂.

SUBMITTER: Choi YG 

PROVIDER: S-EPMC6527611 | biostudies-literature | 2019 May

REPOSITORIES: biostudies-literature