Strain engineering of two-dimensional materials

Graphene displays a range of remarkable properties that have catalyzed an impressive interest in the scientific community . Its unique electronic behavior stems from the hexagonal honeycomb structure of the one-atom thick carbon lattice, which forces low-energy conducting electrons to assume linear dispersions mimicking the one of massless relativistic fermions . After graphene, the past few years have been marked by the discovery and characterization of plenty of other two-dimensional (2D) materials and most of them share with graphene the hexagonal atomic network and have as many peculiar electrical and optical properties. Among this wide family, many of transition metal dichalcogenides (TMDs) have a gapped electronic structure and efficiently emit light, even at room temperature. Tungsten disulfide (WS2) belongs to the TMD family and has gained a preferential attention due its interesting features, including a strong, wide-band exciton photoluminescence (PL) around 630 nm when a monolayer is considered. Mechanical characterization of 2D materials revealed that they display an unusual good elasticity and high mechanical strength; for example, graphene crystals can sustain strain well beyond 10% without undergoing plastic deformation . In addition to that, it has been predicted , and in part experimentally demonstrated , that mechanical deformations in a 2D hexagonal system can significantly impact its physical properties. Random deformations are typically detrimental, but intriguing effects can emerge if proper strain fields are implemented in these systems . Strain gradients can be engineered to mimic the presence of a external magnetic field[18,19] applied perpendicularly to the 2D material plane. Such strain-induced field has been defined pseudo-magnetic field (PMF) since, at odd of the real magnetic field, it conserves the time reversal symmetry and has a pseudo-spin dependent orientation. The fact that 2D materials can sustain large strain, implies that large PMFs can be induced and used to dramatically change the material local energy spectrum. Strain can be also used to modulate the optoelectronic properties of TMDs by controlling the excitons PL wavelength with strain intensity and the excitons diffusion dynamics with strain gradients . However, these intriguing possibilities demand a control of strain that, while validated by artificial-lattice studies, to date remains elusive in the case of atomic lattices . Preliminary and remarkable results were obtained on self-assembled graphene nanobubbles and nano-wrinkles , using scanning probes or nano-patterned substrate , and in twisted bilayer systems . Micro-scale strain devices were implemented exploiting deformable substrates , uniform external loads , thin-film shrinkage or complex micro-actuation technologies, including in particular inorganic microelectromechanical systems (MEMS) . Nevertheless, in order to master the physical properties through deformations, a more accurate control and investigation of the 2D material strain field is required; the existing techniques lack of flexibility, they can be exploited on one flake at a time, and are in practice poorly suited to obtain arbitrary and reconfigurable strain fields. In this Thesis, I will demonstrate an innovative technique to strain 2D materials which deals with these limitations. The technique is based on polymeric micrometric artificial muscles (MAMs) which contract upon a high-dose electron beam exposure and can be used to induce a local deformation in the 2D material crystal. I will show that MAMs are patternable, therefore they are perfectly suited to create arbitrary strain patterns and the method will be first of all demonstrated on a set of suspended graphene devices. Moreover, I will show that by employing a clean and atomically flat substrate as a “low-friction” platform, the same technique can be applied to strain 2D materials while avoiding the critical steps of crystal transfer and suspension. To this end, I employed WS2 grown by chemical vapor deposition (CVD) directly on top of graphene obtained by thermal decomposition of silicon carbide (SiC). I investigated the WS2/graphene heterostructures combining a simple and scalable fabrication protocol with all the benefits of the MAMs-based strain engineering technique. I will demonstrate a local strain-controlled tuning of the WS2 photoluminescence wavelength as a proof of concept.