Since the isolation of graphene in 2004, atomically-thin quasi-two-dimensional (quasi-2D) materials have proven to be an exciting platform for both applications in novel devices and exploring fundamental phenomena arising in low dimensions. This interesting low-dimensional behavior is a consequence of the combined effects of quantum confinement and stronger electron-electron correlations due to reduced screening. In this talk, I will discuss how the optical excitations (excitons) in quasi-2D materials, such as monolayer transition metal dichalcogenides and few-layer black phosphorus, differ from typical bulk materials. In particular, quasi-2D materials are host to a wide-variety of strongly-bound excitons with unusual excitation spectra and massless dispersion. The presence of these excitons can greatly enhance both linear and nonlinear response compared to bulk materials, making them ideal candidates for optoelectronics and energy applications. Moreover, due to enhanced correlations and environmental sensitivity, the electronic and optical properties of these materials can be easily tuned. I will discuss how substrate engineering, stacking of different layers, and the introduction or removal of defects can be used to tune the band gaps and optical selection rules in quasi-2D materials.

Diana Qiu is a postdoc in the Materials Science Division at Lawrence Berkeley National Lab and at the DOE Center for the Computational Study of Excited State Phenomena in Energy Materials (C2SEPEM). She received her Ph.D. in physics from UC Berkeley in 2017, where she was a recipient of the Berkeley Chancellor’s Fellowship, NSF Graduate Research Fellowship, and the Jackson C. Koo award in Condensed Matter Physics. Her research interests focus on the development and application of ab initio many-body perturbation theory methods to predict the excited-state properties of novel quantum materials, most notably excitons in quasi-2D materials.