Optoelectronic properties of functional defects in 2D semiconductors

Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides, and hexagonal boron nitride, have garnered immense interest due to their unique electronic, optical, and mechanical properties. Thus far, a large variety of two dimensional materials have successfully been isolated, creating different families of materials, sorted by their intrinsic properties. By combining two-dimensional materials of different types, e. g. conducting, semiconducting and insulating materials, structures with superior properties to those of the single layered materials can be created. These van der Waals structures often preserve the materials intrinsic properties but can also induce new physical phenomena and new device functionalities.
Within this research field, defect engineering has emerged as a powerful tool to further tailor these materials, also embedded within different heterostructures, for advanced applications, particularly in the fields of spintronics and quantum information processing. This thesis explores the electronic and optical properties of atomic-scale defects in two-dimensional semiconductors, specificially vanadium impurities in WSe2 and vacancy defects in MoS2, towards their utilization in novel device concepts, such as 2D dilute magnetism and 2D integrated single-photon sources.
In the first part of this thesis, we attempt to create and control magnetic defects in 2D materials. We characterize WSe2 monolayers that are substitutionally doped with vanadium atoms by means of optoelectronic spectroscopy and low-temperature luminescence measurements. We observe a p-type doping and a new luminescence peak that both scale with the nominal percentage of vanadium. Magneto-spectroscopy reveals a non-linear exciton g-factor. Optoelectronic measurements under an externally applied magnetic field further uncover a hysteretic transport behavior in vanadium doped WSe2. Together, these results are experimental indications of a magnetic order in V-doped WSe2. In the second part of the thesis, we study vacancy defects in MoS2, which have previously been shown to act as single-photon emitters, by low-temperature scanning tunneling spectroscopy and room-temperature conductive atomic force microscopy. Towards their implementation in electronic devices, we focus on defects in MoS2, specifically within a graphene/hexagonal boron nitride/MoS2 heterostructure, where we probe the interplay between vertical tunneling and lateral transport phenomena. While thick insulating barriers suppress tunneling currents into the conductive substrate, a side-contact still enables addressing the defect states via lateral current flow. In the last part, we take steps towards the realization of electrically driven single-photon emitters based on these defects, which hold promise for applications in quantum optics and secure communication systems. We precisely implement optically active defect sites in MoS2 through helium ion treatment. The defects are embedded such that the defects can be electrically addressed. We reveal enhanced electroluminescence in defect-rich regions that we attribute to localized states within the MoS2 bandgap. Overall, the findings presented in this thesis highlight the pivotal role of defect engineering in unlocking new functionalities in two-dimensional materials.