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Isotope Effect in Bilayer WSe2
journal contribution
posted on 2019-02-12, 00:00 authored by Wei Wu, Mayra Daniela Morales-Acosta, Yongqiang Wang, Michael Thompson PettesIsotopes of an element
have the same electron number but differ
in neutron number and atomic mass. However, due to the thickness-dependent
properties in MX2 (M = Mo, W; X = S, Se, Te) transition
metal dichalcogenides (TMDs), the isotopic effect in atomically thin
TMDs still remains unclear especially for phonon-assisted indirect
excitonic transitions. Here, we report the first observation of the
isotope effect on the electronic and vibrational properties of a TMD
material, using naturally abundant NAWNASe2 and isotopically pure 186W80Se2 bilayer single crystals over a temperature range of
4.4–300 K. We demonstrate a higher optical band gap energy
in 186W80Se2 than in NAWNASe2 (3.9 ± 0.7 meV from 4.41 to 300
K), which is surprising as isotopes are neutral impurities. Phonon
energies decrease in the isotopically pure crystal due to the atomic
mass dependence of harmonic oscillations, with correspondingly longer
E2g and A21g phonon lifetimes than
in the naturally abundant sample. The change in electronic band gap
renormalization energy is postulated as being the dominant mechanism
responsible for the change in optical emission spectra.
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Keywords
thickness-dependent propertiestransition metal dichalcogenidesmass dependenceBilayer WSe 2 Isotopeselectron numberemission spectraE 2 gband gap renormalization energy2 1 g phonon lifetimesTMD materialtemperature rangeneutron number186 W 80 Se 2MX 2vibrational properties186 W 80 Se 2 bilayer single crystalsNA W NA Se 2isotopicallyIsotope Effectband gap energyexcitonic transitionsPhonon energies decreaseisotope effect
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