The photophysical properties of two-dimensional (2D) layered van der Waals (vdW) materials, and their heterostructures are manifestly distinct from crystalline bulk materials. Recently, the discovery of new 2D vdW materials and strongly-bound interlayer excitons in these materials has created a new branch in nanoscience. As such, there are a number of ways to stack 2D materials to produce novel heterostructures with ground-breaking interlayer electronic and photophysical properties. This dissertation explores the stacking and twisting degree of 2D vdW layers to engineer new interlayer electronic states that are not present in as-grown stacked materials. In particular, graphene is the prototypical highly conductive, metallic vdW material consisting of carbon atoms arranged in a hexagonal lattice. When two sheets of graphene are stacked at an off-axis angle, twisted bilayer graphene (tBLG) is formed with modified interlayer electronic properties from the orbital hybridization.This dissertation presents the first exploration and discovery of bound excitons in electronically hybridized interlayer states in tBLG. Using tBLG as a prototype, this research provides new methodologies to understand the time-domain light-matter interactions in a 2D heterostructure. In particular, we film the journey of electrons in tBLG from excitation to relaxation with space-time and energy resolution to study fundamental interlayer electronic properties and many-body electronic effects.When two graphene sheets stack in a tBLG configuration, angle-tunable optical absorption resonances are generated owing to the rehybridization of interlayer orbitals. Early characterization of tBLG graphene was limited to optical absorption and Raman scattering studies. We apply advanced nonlinear optical techniques to tBLG for the first time to understand the underlying Physics of interlayer electronic interactions in stacked and twisted graphene materials. To accomplish this, we developed a novel ultrafast multi- photon transient absorption (TA) microscopy technique to map relative electron population in a single grain of tBLG with the space-time and energy resolution. Even though graphene is a semimetal, upon resonant excitation of interlayer excitons, surprisingly, we observe a clear electronic relaxation bottleneck that is not present in either ‘non-twisted’ Bernal stacked bilayer graphene or monolayer graphene. This bottleneck can be best explained by the existence of a strongly bound, dark excitonic state. To further investigate the nature of the excitonic states and the exciton binding energy, we employ near-IR excited state absorption (ESA) microscopy approach. By measuring the excited state absorption manifold in tBLG, we found exciton binding energy of ~ 0.5 eV. Such a large exciton binding energy in tBLG is comparable to other 2D semiconductor vdW materials such as transition metal dichalcogenides (TMDs) and an order of magnitude larger than the reported value in metallic carbon nanotubes (CNTs).Lastly, we report light emission from a semimetal tBLG for the first time. We developed a novel 2-photon photoluminescence (PL) microscopy technique to detect emission. The detected PL energy is tunable with the absorption resonance energy. We find that upon resonant two-photon excitation, the relatively slow process of PL is possible owing to the strongly-bound excitons with large binding energy and the corresponding long-lived recombination times measured.Collectively, our results demonstrate that resonantly excited tBLG functions as a unique 2D-hybrid material where free-electron continuum states may co-exist alongside strongly-bound and stable exciton states. The discovery of these remarkable interlayer interactions opens up possible new avenues for excitonic applications with graphene- based light harvesting technologies and fast photosensor development.
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