Transition metal dichalcogenides (TMDs) have attracted much interest in recent years due to their emerging material properties. In monolayer TMDs, such as MoS2, extreme quantum confinement is achieved in the monolayer limit. Although monolayer TMDs represent an ideal platform to explore excitonic physics using ultrafast spectroscopy, this exploration is currently limited by confusion regarding the origin of certain spectral features, including the below-bandgap PIA feature observed in pump-probe experiments. In this work, we document an absence of PIA features immediately after photoexcitation, indicating a lack of strong optically-induced biexciton formation. Below-bandgap PIA features are observed to grow in with a time constant of 110 ± 10 fs, indicative of other factors responsible for their origin. These results indicate that optically-induced biexciton formation is most likely not responsible for the previously observed PIA features in MoS2 monolayers.
Stacking and twisting 2D van der Walls (vdW) materials can create unique electronic properties that are not accessible in a single sheet of material. When two sheets of van der Waals material such as graphene are stacked in an off-axis angle, in a twisted bilayer graphene (tBLG) configuration, electronic properties are modified from interlayer orbital hybridization effects. For instance, in tBLG we can access both massless and massive chiral quasiparticles characteristics of graphene and bilayer graphene, as well as angle tunable optical resonances that are not present in graphene or bilayer graphene. In addition, first principle simulation predicts that upon optical resonant excitation of tBLG, bound exciton formation is a possibility due to cancelation of exciton-continuum coupling from anti-symmetric superposition of degenerate resonant transitions. In order to study possible bound exciton formation, we map out the electronic structure of single grain tBLG using multi-photon transient absorption microscopy. Surprisingly, upon resonant optical excitations, tBLG shows enhanced transient response with longer carrier compared to AB stacked bilayer graphene. Further, we find that the origin of this unexpected optical response can be best explained by the presence of a lower lying bound exciton state predicted by recent theoretical simulations. This suggests that tBLG is a novel 2D hybrid material that enables the creation of both strongly-bound excitons along-side highly-conductive continuum states. Recently, the family of 2D vdW materials has grown appreciably. As such, there are countless possibilities for stacking and twisting 2D vDw materials to produce similar interlayer electronic states for next generation optoelectronics.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.