Ultrafast Pump-Probe Microscopy Studies on Multilayer MoSe₂

As materials in electronics and photonics continue to shrink, nano- and micro-scale structures begin to have an outsized impact on the electronic properties. Studies have found that depending on the material, the structure, and the fabrication process, the energetics and carrier dynamics of a material can change in unexpected ways. Despite all this, structural features are often comingled in experiments, obscuring their individual effects. In order to overcome this, we use ultrafast pump-probe microscopy to isolate and study individual structural features in materials. This presentation will focus on the effects of thickness in the multilayer transition metal dichalcogenide MoSe₂ which shows promise in optoelectronic and photocatalytic device applications. Although MoSe₂ has been extensity studied in the monolayer, we still do not fully understand how changes in thickness and local structure change its electronic and optical properties. Using the diffraction limited spot size of our setup, we isolate the dynamics at the center of MoSe₂ nanoflakes and determine how the dominant carrier relaxation processes change going from 20- to 100-layers in thickness. We also observe and characterize non-linear modulations in the bandgap energy upon photoexcitation and how they change with time and carrier concentration. Finally, we observe a thickness-dependent internal etalon effect in the material which we analyze via a computational dielectric model. Using this comprehensive understanding of the thickness dependence of the carrier dynamics at the center of MoSe₂ flakes, we can in the future compare these results to the dynamics at the edges of flakes and around other internal structures (buckles/tears). Overall, by better understanding the impact these structural features have on the carrier dynamics and optical properties of these materials, our work allows for future structural optimization of these materials when used in new or improved optoelectronics and photocatalytic device architectures.