Isotope Effect in Bilayer WSe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• Isotopes, atoms of the same element with different neutron numbers, influence material properties.
• Transition metal dichalcogenides (TMDs) like MX2 exhibit thickness-dependent characteristics.
• The isotopic effect in atomically thin TMDs, particularly for phonon-assisted indirect excitonic transitions, remains poorly understood.

Purpose of the Study:

• To investigate the isotope effect on the electronic and vibrational properties of transition metal dichalcogenides (TMDs).
• To explore the influence of isotopic composition on phonon-assisted indirect excitonic transitions in bilayer tungsten diselenide (WSe2).

Main Methods:

• Utilized naturally abundant WSe2 and isotopically pure 186W80Se2 bilayer single crystals.
• Conducted experiments over a temperature range of 4.4–300 K.
• Analyzed electronic and vibrational properties, including optical band gap energy and phonon lifetimes.

Main Results:

• Demonstrated a higher optical band gap energy in isotopically pure 186W80Se2 compared to naturally abundant WSe2 (3.9 ± 0.7 meV).
• Observed decreased phonon energies in the isotopically pure crystal due to atomic mass dependence.
• Reported longer E2g and A21g phonon lifetimes in the isotopically pure sample.

Conclusions:

• The isotopic composition significantly impacts the electronic and vibrational properties of WSe2.
• The observed changes in band gap energy are attributed to electronic band gap renormalization.
• This study provides the first observation of the isotope effect in TMDs, offering insights into their fundamental properties.