Isotope Effect in Bilayer WSe₂

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 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 ^NAW^NASe₂ 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 ^NAW^NASe₂ (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_2g and A²_1g 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.