The isolation of graphene -a single layer of carbon atoms- in 2004 opened the door to a flourishing field of research and nowadays a plethora of other two-dimensional (2D) crystals have been discovered. These materials are characterised by a layered structure in the bulk form, where the different layers are kept together via weak van der Waals (vdW) forces. Due to these weak bonds, it is possible to isolate single layers of the material and unique phenomena arise, stemming from the lowered dielectric screening and enhanced quantum effects. These 2D crystals are all incredibly flexible and robust, and feature diverse electronic properties, including insulating (e.g., boron nitride), semiconducting (e.g., WS2, MoS2, WSe2, MoSe2, MoTe2), semi-metallic (e.g., graphene, WTe2) and superconducting (e.g., NbSe2, ReS2) materials. The great interest attracted by these crystals in the past decade is now moving a step forward, towards the creation of 2D heterostructures. While the fabrication of conventional heterostructures suffers from lattice mismatch constraints, 2D crystals stack together via vdW interactions, that allows an unprecedented tunability. The exploration of this wide field has just commenced, and novel heterostructures with alluring characteristics could be created by assembling crystals with different electronic properties. This project aims at the fabrication and study of a variety of 2D heterostructures, and the effect of hydrogen-irradiation in these systems will also be addressed. The heterostructures will be characterised by Raman, photoluminescence, non-linear optics studies and carriers' lifetime studies. Hydrogen-irradiation treatments in bulk vdW crystals led to the formation of hydrogen-filled bubbles on the sample surface and provided a means to strain these materials. Here, we propose to exploit irradiation processes to develop strain-engineering protocols in 2D heterostructures and to study how strain affects their opto-electronic properties.
The great attention received by van der Waals (vdW) crystals in the past decade is due, among other factors, to their exceptional flexibility, which makes them particularly appealing for the realisation of flexible electronic and optoelectronic devices [Aja,16]. On the other hand, previous studies have shown how the electronic properties of these materials are sensitive to mechanical deformations [Cha,13][Blu,20,prr], which might affect the operation of the flexible devices. It is thus essential to acquire thorough knowledge of how mechanical deformations modify the inherent properties of these materials. Extensive studies have been carried out in this respect for isolated two-dimensional (2D) materials, while very little is known in the relatively new field of vdW heterostructures.
The project LEGO GAME aims on the on side at the fabrication and characterisation of new heterostructures; on the other side, we will undertake studies to address the issue of the effect of mechanical deformations, and hence of strain, on these systems. The focus will be chiefly on heterostructures made of semiconducting materials, which are the most promising in the field of optoelectronics due to the excellent optical properties of most 2D semiconductors (belonging to the family of transition-metal dichalcogenides, TMDs). As described in the previous section, intriguing phenomena arise when coupling different 2D semiconductors, such as moiré excitons and interlayer excitons (see Fig. 3). Here, we propose to exploit the methodology discussed in Fig. 6 to investigate the possibility to activate the formation of these excitons by realising non-trivial strain fields.
So far, moiré excitons have been observed in periodic potentials that spontaneously arise when stacking two different crystals with specific twist angles (see Fig. 3a). Here, we propose to generate periodic potentials via the realisation of ordered arrays of domes, such as in Fig. 5b. In our previous studies, we demonstrated that the formation of domes locally subject the 2D membrane to non-uniform strains that seamlessly increase while going from the edge towards the centre of the dome [Blu,20,prr]. In TMDs, strain leads to a reduction of the bandgap [Cha,13]. Therefore, whenever arrays of domes are created, we achieve local seamless bandgap reductions that are expected to act as a periodic potential, which would trap charged carriers at the potential minima on top of the domes. Here, we propose to study possible mechanisms (Fig. 6) towards the formation of ordered arrays of heterostructured domes, where to induce the formation of moiré excitons. While the common method to create moiré excitons is characterised by a low controllability (the only degree of freedom is the twist angle), the method we propose would enable the formation of moiré excitons in a controlled manner. In fact, given the tunability of our lithographic approach [Blu,20,admi], the moiré exciton properties could be varied by modifying the array periodicity, its geometry, and the dimensions of the domes.
In addition, the seamless strain variation could activate the formation of unprecedented interlayer excitons. For instance, interlayer excitons have been observed in MoSe2/WSe2 heterostructures, which have a nearly resonant valence band maximum (VBM) [Tra,19], but not in MoSe2/WS2 heterostructures, since in this case neither the VBM nor the conduction band minimum (CBM) are close in energy. However, the CBM of WS2 features a remarkable shift with strain, so that for strains in between 2% and 4% the CBM of WS2 and MoSe2 would become resonant [Cha,13] and interlayer excitons could be possibly be observed.
The methodology proposed in this project is aimed at defining a protocol to induce non uniform strains in vdW heterostructures. This would allow us to gain control over the formation of excitonic states in these materials and widen the state-of-the-art knowledge in this field. The proposed research is also expected to have impact in the field of quantum optics, since spatially confined moiré or interlayer excitons in a periodic ordering might serve as a platform towards the generation of entangled single photons.