Ultrafast Carrier Dynamics in Two-Dimensional Materials

Abstract

The discovery of graphene led to an eruption of research into the expansive collection of two-dimensional materials. The ability to fabricate stacked heterostructures with van der Waals materials layer-by-layer has allowed the production of unique devices and has rapidly advanced research in electronics and optics. Understanding the dynamics of carrier and phonon interactions within these systems is crucial for the development of optoelectronic devices. This thesis explores the dynamics of photo-excited carriers in two-dimensional systems: the effects of substrate choice on carrier relaxation in graphene and phonon induced bandgap renormalization in monolayer tungsten disulfide at high carrier densities. Graphene-hBN (hexagonal boron nitride) heterostructures show promising use in electronics applications due to high carrier mobility. We first explore the effect of the hBN substrate on the relaxation rates of photo-excited carriers in these heterostructures using femtosecond pump-probe spectroscopy. Time dynamics of photo-excited carriers in graphene-hBN heterostructures show a cooling rate approximately four times faster on hBN substrates compared to silicon oxide substrates. We next study the effect of variation in isotopic concentration in hBN substrates on the relaxation rates of photo-excited carriers. We measure and compare the time dynamics of photo-excited carriers in graphene-hBN heterostructures using naturally occurring hBN (containing 20% 10B and 80% 11B) and isotopically pure hBN (containing 100% 10B or 100% 11B). We observed a carrier relaxation rate ~1.7 times faster for isotopically pure hBN substrate. Isotopically pure hBN substrates samples allow more efficient decay of optical phonons from graphene into acoustic phonons in the substrate, while the isotopic disorder in naturally occurring hBN causes isotope-phonon scattering. Monolayer transition metal dichalcogenides are another van der Waals material which have garnered a lot of interest due to their direct optical band gap and strongly bound excitonic states in the visible light range. We utilize non-degenerate femtosecond pump-probe spectroscopy to measure the differential reflectivity in monolayer WS2 to investigate the interactions between carriers, defects, and phonons in the high carrier density regime. We find photo-excited carriers are trapped by defect states, which act as non-radiative recombination sites and emit phonons, which cause a phonon-induced band gap renormalization up to 23 meV.