Excitonic binding energy. From these considerations, we infer an electrochemical bandgap of ~1.95 eV, which is less than 0.1 eV larger than the optical gap. Although this excitonic binding energy appears low for MoS2, it is in line with the strong dependence of the excitonic binding energy on the dielectric constant of the surrounding medium. In the present case, we may approximate the environment with pure acetonitrile, which exhibits a very large dielectric constant of ~35 and, thus, exerts strong dielectric screening for the electron-hole pair, which decreases the binding energy.