• 2D materials
• chemical vapor deposition
• graphene
• tungsten disulfide
• van der Waals heterostructure

Graphene is undoubtedly emerging as one of the most promising nanomaterials because of its unique combination of superb properties, with high carrier mobility being the most notable. However, when graphene is put onto silicon oxide (SiO2), the substrate of choice in microfabrication of graphene for its availability and versatility, the electronic response becomes dominated by scattering from impurities, substrate surface roughness and SiO2 surface optical phonons. There is a growing need, therefore, to identify materials to be used as graphene encapsulants, that minimize extrinsic sources of scattering, coming from both interface with substrate and air. In this context, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDs), such as tungsten disulphide (WS2), have proved to be excellent candidates. In order to grow suitable graphene (and encapsulants) for large-scale device production, a precise control over flake size, layer number and morphology is necessary. To date, to move towards realistic applications, scalable graphene production techniques have been developed, with chemical vapor deposition (CVD) on transition metals being the most effective. However, suitable large area encapsulants capable of exploiting graphene’s remarkable electronic properties have still to be found. But while a scalable process for obtaining high quality h-BN is not already widely available, CVD processes for the synthesis of WS2 have been developed in the last few years. Therefore, it would be appropriate to understand if it is possible to grow WS2 directly on CVD graphene.
Besides being interesting as atomically flat encapsulant, TMDs assembled in van der Waals heterostructures (vdWHs) with graphene are also appealing for other physical properties. Although research on vdWHs is just at its beginning, such heterojunctions have been already applied to form devices, such as photodiodes, phototransistors, tunneling devices and memory devices. Concerning the WS2/graphene heterostructure, by combining WS2 optical absorbance with the outstanding transport properties of graphene, efficient photodetectors have been demonstrated. Developing a CVD process for the scalable synthesis of WS2/graphene heterostacks would mean the possibility to choose any desired substrate, paving the way for new large-scale applications.
This thesis work is focused on the synthesis of scalable graphene/WS2 heterostructures, the investigation of the properties of the two constituting materials and the assessment of electronic and optoelectronic performances of such a heterostack. By synthesizing both materials via CVD, with WS2 grown directly on graphene, I demonstrated that a fully and homogeneous coverage of graphene flakes with WS2 can be obtained. Moreover, the avoidance of WS2 transfer results in a cleaner and faster stacking process. However, besides the growth of monolayer WS2 flakes, three-dimensional nanocrystals are also present on the graphene surface. The nucleation of such crystals has proven to be due to the presence of PMMA residues from the transfer process of graphene. I used spectroscopic and microscopic techniques to outline the structural features of the grown material. Besides the importance of completely cover graphene with a flat and inert material for improving its quality, WS2 is interesting as active material. Therefore, a knowledge of its thickness is fundamental to figure out its optical properties, which in the case of TMDs are deeply connected with the number of layers. By performing Raman spectroscopy, I investigated the effects of the WS2 growth on the underlying graphene layer, which resulted strongly affected by compressive strain and hole doping. I demonstrated that the main reason of the high doping induced in graphene is its interaction with the silicon oxide substrate at high temperatures. Gate voltage-dependent electronic transport measurements were also performed after a GFET (graphene field-effect transistor) fabrication. Graphene carrier mobility was found substantially unchanged from that retrieved for pristine CVD graphene on SiO2 (i.e., without WS2 on top). This result is discussed in light of the heterostructure properties and possible improvements to obtain scalable graphene/WS2 heterostacks with improved electrical properties are suggested. Notably, the WS2/graphene heterostructure realized in this work shows significant interaction with light, with a photoresponsivity higher than the standing record of CVD WS2 on graphene.
In summary, the results presented in this thesis work are instrumental for the development of novel and scalable optoelectronic devices based on vertical van der Waals heterostacks.