• transition metal dichalcogenides
• strain engineering
• polymeric actuator
• photoluminescence
• 2D material
• tungsten disulfide

The isolation of graphene by mechanical exfoliation in 2004 by Novoselov et al. (awarded with the Nobel prize in 2010) opened the door to a whole new field of research: two-dimensional materials. Nowadays beyond graphene there exist a host of two-dimensional materials that are expected to complement graphene in applications for which it does not possess the optimal properties.
Among these, the wide family of ”Transition metal dichalcogenides” (TMDs) combines the incredible thinness of graphene with exceptional semiconducting properties. Apart from their outstanding electrical and optical performances, atomically thin semiconductors have also shown very interesting mechanical properties. For example while silicon typically breaks at strain levels of 1.5%, TMDs not break until 10% of strain. This outstanding stretchability of 2D crystals can be used to realize devices with electronic properties that are tuned through the introduction of mechanical deformations thus opening new promising opportunities for the field of strain engineering.
This master thesis work has been focused on Tungsten Disulfide (WS2) that displays superior optical properties compared to other common TMDs such as MoS2 and, thanks to its large spin-orbit coupling, can have interesting applications in optoelectronics and spintronics. There are two main approaches to induce strain in two-dimensional crystals: the first is straining the substrate (directly with an applied force, with thermal dilation etc.) thus exploiting the superficial adhesion to strain the 2D-material. Unfortunately these methods cannot achieve high strain levels and are only suitable to induce uniform strain profiles. Another possible approach consist in removing the interaction with the substrate by working with suspended membranes and applying strain with pressure loads, indentation experiments or micro-actuators. These techniques can implement anisotropic strain profile and reach record strain levels but require challenging fabrication protocols, are mechanically fragile and are thus not appealing for large scale applications.
In this thesis work I demonstrate a new platform for strain engineering that consists of an heterostructure of bilayer graphene and tungsten disulfide and exploits Polymeric Micrometric Artificial Muscles (MAMs) as actuators. Bilayer graphene is grown via thermal decomposition of Silicon Carbide and the Tungsten Disulfide is grown directly on top of graphene via Chemical Vapour Deposition, avoiding transfer procedures that complicate the fabrication process.
2D-materials in heterostructure have recently shown a regime of lateral motion with ultra-low friction, called superlubricity, that allows the lateral sliding of the flakes. The idea is to exploit this phenomena with WS2 on top of graphene to create a non suspended set-up in which Tungsten Disulfide can be strained at will, responding to lateral forces like a free-standing membrane. The polymeric actuators are implemented spin coating the structure with Poly Methyl Methacrylate and exploiting its response to high-dose electron beam. In fact PMMA layer mechanically contracts when stimulated by electron radiation, via a phenomena call cross-linking, mimicking the response of a muscle to an electrical stimulus. In this process the PMMA pulls the tungsten disulfide which is free to slide on top of graphene. Since the process is done via electron beam lithography, is possible to implement custom strain profiles. Another benefit of this process is the possibility of in-situ direct imaging by scanning electron microscopy.
In order to demonstrate the action of the MAMs I have performed photoluminescence measurement on uniaxially strained Tungsten Disulfide, starting from the well established relation between the strain and the shift in the photoluminescence peak. The maximum strain achieved with this method has been  2.5%. Current results are limited by the crystalline quality of WS2 but it is expected that larger strains can be obtained using monocrystalline flakes.
In conclusion, in this work I have realized and characterized a playground for strain engineering of two-dimensional crystals in a completely new set-up, showing and exploiting superlubricity between Tungsten Disulfide and Graphene. This thesis can open the way to further study of strain engineering in particular to use this MAMs to implement anisotropic strain profile which could be appealing for generation of pseudomagnetic field and exciton funneling application.
Furthermore this work has shown a successful integration between polymers and 2D-materials in monolithic devices, paving the way to future research directions.