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.
Highlights
With an advent of exfoliation technique for van der Waals materials, there have been tremendous progresses in 2D nanotechnology using graphene, transition metal dichalcogenides (TMDs), and so on[1,2,3,4]
We can consider the changes in electronic state and crystal symmetry of the top WSe2 layers as one of the key factors in understanding the observed significant modulation of the thermal boundary resistance in the metal/WSe2/sapphire structure
The metallization of the WSe2 layer occurs by a strong bonding between metal and WSe2 which is expected to modify the phonon density of states in the WSe2 layer
Summary
We can consider the changes in electronic state and crystal symmetry of the top WSe2 layers as one of the key factors in understanding the observed significant modulation of the thermal boundary resistance in the metal/WSe2/sapphire structure. As a light source for this pump-probe measurement, we used femtosecond laser pulses with a 785 nm wavelength and a repetition rate of 80 MHz. The beam was split for the pump and probe sources by a polarizing beam splitter, and the pump beam was modulated by an electro-optic modulator with a frequency 10 MHz. As schematically shown, the pump-induced temperature change of the Al thermal transducer was monitored by the reflectivity change of the probe beam of which arrival time at the sample position was adjusted by a 300-mm-long delay stage. The pump-induced temperature change of the Al thermal transducer was monitored by the reflectivity change of the probe beam of which arrival time at the sample position was adjusted by a 300-mm-long delay stage Both beams were focused by a 10× magnitude objective lens onto the sample where the beam size was about 5 μm of 1/e2 diameter. Polarization states of the fundamental and the second harmonic waves were controlled by using a half wave plate and polarizer, respectively
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