Suspended Excitons in 2D Materials

Excitons in atomically thin transition metal dichalcogenides (TMDCs, 2D materials) are strongly influenced by their environment. To eliminate that influence, we prepare suspended monolayer (1L) WSe₂ samples, enabling suspended excitons. We perform reflectance measurements and calculate the excitonic binding energies from the experimental observables: the energy differences between the 1s, 2s, and 3s states. We employ the recently developed quantum electrostatic heterostructure model for the calculations. We see that the binding energy of the ground state A exciton increases from about 0.3 eV (on substrate) to above 0.4 eV (suspended). A more striking feature of the suspended excitons is the ability to tune them even further by applying mechanical strain. By applying air pressure, we obtain reversible 0.15 eV redshift in the exciton resonance of a suspended 1L sample on a circular hole of 8 μm diameter under a pressure of 40 psi. Interestingly, the linewidth of the A exciton decreases more than half from about 45-50 meV to 20 meV at room temperature, due to the suppression of the intervalley exciton-phonon scattering. Our results show that suspended 2D materials are novel systems for fundamental studies as well as strong and dynamic tuning of the excitons.