Two-dimensional (2D) materials have attracted increasing attention over the last decade owing to their remarkable mechanical, electrical and optical properties. Following the groundbreaking discovery of graphene, a plethora of other atomically-thin materials have emerged. They cover a broad spectrum of electronic character including semiconductors, metals and insulators. The functionalities of 2D materials can be further extended by stacking individual layers into vertical heterostructures. The resulting interlayer electrical coupling enables harnessing complementary properties of the constituent materials and exploring novel fundamental physics phenomena. Their versatility and intriguing optoelectronic properties make 2D materials promising alternatives and compliments to current semiconductor technologies which suffer from scale-down limitations. Among the 2D semiconductors, transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 stand out due to their direct band gap, strong spin-orbit coupling and valley degree of freedom. This class of materials could thus enable a new generation of 2D optoelectronic
devices in which information is processed through the electron spin or valley pseudospin, and accessed by optical means. Another promising 2D material is hexagonal boron nitride (h-BN), which is a transparent wide band gap insulator with excellent mechanical properties. Furthermore, the absence of dangling bonds and its strong dielectric screening capability make h-BN an ideal substrate for 2D materials. In general, crystalline defects such as vacancies, adatoms, and substitutional impurities have a significant impact on the properties of atomically-thin materials. Defects in TMDCs can trap free excitons, thus creating single-photon sources, while in h-BN they can introduce localized electronic states deep within its large band gap, resulting in stable quantum emission at room temperature. Defects can also be exploited to modulate the characteristics of the host 2D material, thus introducing functionalities that are absent in the pristine form. One option is the controlled introduction of substitutional impurities in TMDCs in order to induce tunable magnetic properties. The present thesis aims at exploring the optoelectronic properties of quantum emitters in h-BN and the magnetic properties arising from substitutional V doping in WSe2. Along these lines, a further task is to identify novel approaches to control the functionalities of the two different 2D semiconductors. In the first experimental part, the electrical tuning of single photon emitters in atomically-thin h-BN is investigated by applying a vertical or horizontal electric field. The large and robust Stark shift attained with both configurations testifies the efficiency of the electrical tuning to control the emission and underscores the ability of this method to address fundamental properties of the quantum emitters. The second experimental part describes the results of circular polarization-resolved PL and magnetotransport measurements performed with the aim of probing ferromagnetic order in V-WSe2. Although the transport measurements on few-layer flakes point toward a layered antiferromagnetic phase, the PL data gained on monolayer sheets suggest the presence of small local ferromagnetic domains. Taken together, these findings provide a valuable basis for further investigations on tailoring the properties of 2D materials via the controlled introduction of defects.
EPFL2020
