where Ry is the Rydberg energy of the hydrogen atom, µ the reduced carrier mass, m0 the free electron mass, r the relative permittivity and a the radius of the quantum dot. The calculated energy gap agrees very well with the measured PL peaks for the native CsPbBr3 as well as CsPbBr2Cl NCs. However, it becomes evident that the measured blue-shift of 100 meV (22 nm) in case of CsPbBr3 NCs and 120 meV (19 nm) in case of CsPbBr2Cl NCs cannot arise from dielectric screening, that is, an increase in the permittivity because the exciton energy is too small to account for these shifts. Apparently, the only term that shows the potential to induce such strong blue-shifts is E confinement . However, as the particle size itself does not decrease, the only variable that is left is the reduced mass. (𝜇 ≠ 0 ) (8.5)