  Atomically thin transition metal dichalcogenides (TMDs) hold promising
potential for applications in optoelectronics. Due to their direct band gap and
the extraordinarily strong Coulomb interaction, TMDs exhibit efficient
light-matter coupling and tightly bound excitons. Moreover, large spin orbit
coupling in combination with circular dichroism allows for spin and valley
selective optical excitation. As atomically thin materials, they are very
sensitive to changes in the surrounding environment. This motivates a
functionalization approach, where external molecules are adsorbed to the
materials surface to tailor its optical properties. Here, we apply the density
matrix theory to investigate the potential of non-covalently functionalized
TMDs. Considering exemplary spiropyran molecules with a strong dipole moment,
we predict spectral redshifts and the appearance of an additional side peak in
the absorption spectrum of functionalized TMDs. We show that the molecular
characteristics, e.g. coverage, orientation and dipole moment, crucially
influence the optical properties of TMDs, leaving a unique optical fingerprint
in the absorption spectrum. Furthermore, we find that the molecular dipole
moments open a channel for coherent intervalley coupling between the
high-symmetry K and K' points which may open new possibilities for
spin-valleytronics application.
