Tuning the Interlayer Exciton in Bilayer MoS₂ by Means of Self-Assembled Monolayers

The field of study surrounding Transition Metal Dichalcogenides (TMDs), a class of two dimensional materials that can be exfoliated down to a single molecular level, has recently seen large improvements and advancements. Whether in the of study of monolayer properties or in the various possibilities that stacking different materials as heterostructures allow for. However, one interesting application of these materials is in their natural homobilayer form. For example, in molybdenum disulfide (MoS₂), there exists an interlayer exciton that emerges purely out of the interlayer interaction inside the natural homobilayer.<br/><br/>This exciton is unique in its ability to couple to its environment due to having both strong in-plane and out-of-plane dipole sensitivity. This project attempts to leverage this sensitivity by utilizing self-assembled organic monolayers (SAMs), a method of dielectric tuning that is not commonly used for two dimensional materials. Our SAM is formed by a monolayer of non-conjugated organic molecules that spontaneously assemble and are covalently tethered to a substrate. By using SAMs with a strongly dipolar functional group, it is possible to create an effective electric field across the homobilayer to couple with the interlayer exciton without external electrical bias [1] or optical pumping.<br/><br/>In this project, we use SAMs with different dipole moments and sign. These molecules will form SAMs with a very strong electrostatic dipole (of opposite signs) in the vertical direction which makes them prime candidates for exerting a local electric field to the bilayer MoS₂. We then perform optical reflectance measurements in order to directly probe the interlayer exciton and the effects of the SAM interface.<br/><br/>We observed a clear reorganization of the excitonic properties of the MoS₂ and a splitting of the interlayer exciton due to the existence of an external electric field causing Stark splitting. We also observed a very strong dependence on annealing on our structures, where the existence of randomly oriented water dipoles completely screens the effect of the SAM dipole on the MoS₂ exciton.<br/><br/>Additionally, Angle Resolved Photoemission Spectroscopy (ARPES) is being performed in order to better understand the band structure of the coupled system. This method of interfacing organic molecules to TMDs is vastly unexplored and opens a new range of possibilities for materials by using two classes of materials both known for theirversatility. The development of bias free local tuning of TMDs that is easily deposited on a wide array of substrates offers a unique set of advantages compared to typical coupling methods such as depositing contacts, doping or coupling to optical cavities.<br/><br/>References:<br/>[1] Leisgang, N., Shree, S., Paradisanos, I. <i>et al.</i> Giant Stark splitting of an exciton in bilayer MoS₂. <i>Nat. Nanotechnol.</i> <b>15</b>, 901–907 (2020). https://doi.org/10.1038/s41565-020-0750-1