Optical fingerprint of non-covalently functionalized transition metal dichalcogenides

TL;DR

This paper investigates how non-covalent functionalization with molecules like spiropyran alters the optical properties of transition metal dichalcogenides (TMDs), revealing spectral shifts and potential for spin-valleytronics applications.

Contribution

It introduces a density matrix theory approach to predict optical fingerprint changes in TMDs due to non-covalent molecular functionalization, highlighting the influence of molecular characteristics.

Findings

Spectral redshifts in absorption spectra due to functionalization

Appearance of an additional side peak in absorption spectra

Molecular dipoles enable coherent intervalley coupling

Abstract

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.