Magnetotransport and photocurrent spectroscopy in 2D materials

Resumen

This thesis falls within the broad research field of 2D van der waals materials. The isolation of single-layer graphene and the subsequent discovery of several of these materials has led to the emergence of a new platform to study different physical phenomena, pushing the limits in condensed matter physics. In this thesis I present two research lines, both within this field. The first part focuses in the study of the quantum hall effect at high temperature in graphene encapsulated with h-BN. Prior to the isolation of graphene, the quantum Hall effect in 2D systems was typically observed only at low temperatures. However, the irruption of graphene allows for investigating the quantum Hall effect up to room temperature due to its large Landau level separation. In this study, through the analysis of thermally-activated transport at filling factor 2 up to room temperature in high quality graphene devices, we reveal a new transport regime where electron-phonon scattering is the main source of dissipation in the quantum hall phase. Furthermore, we establish a link between the activation of the quantum hall effect and the quality of our devices. Thanks to van der waals heterostructures of hBN and graphene we give a further notion of the quantum hall effect, even 40 years after its discovery. The second part centers on the study of excitons in transition metal dicalchogenides (TMDs) using low temperature photocurrent spectroscopy. 2D transition metal dichalcogenides shows strongly bound excitons that largely influences its optoelectronic response. The binding energy of excitons in these materials reaches values of two orders of magnitude larger than conventional semiconductors. Therefore, 2D-TMDs are a perfect system to study excitons. Before this dissertation the main part of the research of excitonic phenomena are commonly explored using photoluminescence spectroscopy, nevertheless, this technique may not reveal non-radiative excitons. Hence, we propose the use of low temperature photocurrent spectroscopy to study excitonic physics in 2D-TMDs based devices, concretely MoS2, MoSe2 and ReS2. Photocurrent spectroscopy is a powerful technique that allows us to obtain transition linewidths of roughly 10 meV, similar to previous photoluminescence studies. Furthermore, using this technique we reveal excitonic peaks not reported in previous literature, demonstrating the potential of this technique. With our investigation we provide new insights on the excitonic physics of 2D-TMDs and we provide an experimental tool with huge possibilites in the future. Finally, I present future research topics that can be devoloped using low-temperature photocurrent spectroscopy.