Isotope Effect in Bilayer WSe<sub>2</sub> Wei Wu Mayra Daniela Morales-Acosta Yongqiang Wang Michael Thompson Pettes 10.1021/acs.nanolett.8b04269.s001 https://acs.figshare.com/articles/journal_contribution/Isotope_Effect_in_Bilayer_WSe_sub_2_sub_/7767017 Isotopes of an element have the same electron number but differ in neutron number and atomic mass. However, due to the thickness-dependent properties in MX<sub>2</sub> (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 <sup>NA</sup>W<sup>NA</sup>Se<sub>2</sub> and isotopically pure <sup>186</sup>W<sup>80</sup>Se<sub>2</sub> bilayer single crystals over a temperature range of 4.4–300 K. We demonstrate a higher optical band gap energy in <sup>186</sup>W<sup>80</sup>Se<sub>2</sub> than in <sup>NA</sup>W<sup>NA</sup>Se<sub>2</sub> (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 E<sub>2g</sub> and A<sup>2</sup><sub>1g</sub> 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. 2019-02-12 00:00:00 thickness-dependent properties transition metal dichalcogenides mass dependence Bilayer WSe 2 Isotopes electron number emission spectra E 2 g band gap renormalization energy 2 1 g phonon lifetimes TMD material temperature range neutron number 186 W 80 Se 2 MX 2 vibrational properties 186 W 80 Se 2 bilayer  single crystals NA W NA Se 2 isotopically Isotope Effect band gap energy excitonic transitions Phonon energies decrease isotope effect