Fabrication and characterization of transition metal dichalcogenide alloys and their heterostructures

Two-dimensional materials captured the attention of a huge number of researchers across the world. Their attractive properties caused an exponential increase of the investigation efforts, rapidly promoting them to one of the most researched platform for fundamental research and practical application. Also, the possibility to isolate and stack different 2D materials on top of each other open new prospectives and opportunities to fabricate atomically-thick devices, which are named van der Waals heterostructures. In 2D materials research, these represent the ultimate frontier for design and assembly of highly tailored nanometer-size devices. In this thesis, I report about the results of the optical investigations of such van der Waals heterostructures, focusing mostly on transition metal dichalcogenide heterobilayers. They form an atomically thin type-II heterojunction due to the materials band offset, which causes the rise of interlayer excitons, formed by electrostatically-bound carriers confined in different layers. In particular, I present the photoluminescence imaging technique by the mean of a modified optical microscope. With this imaging technique it is possible to unambiguously identify semiconductor monolayers on different substrates. Also, it is possible to monitor the electronic coupling of two vertically stacked transition metal dichalcogenide and the formation of interlayer excitons. In the second part, I present the results of band structure engineering of a series of WSe2/MoxW(1-x)Se2 heterobilayers. The effect of the band offset (controlled via chemical composition) on the PL emission of the interlayer excitons is observed. The gradual change from a hetero to homobilayer is studied and a general model is proposed. In the third part, I report on a study of MoxW(1-x)S2 alloys in their mono- bi- and trilayer from. The optical properties of these samples are probed to study the band structure dependence on the sample thickness and the material composition.