The progressive miniaturisation of electronic and optoelectronic devices has heightened the interest for ultrathin and flexible materials. Research in this field received a tremendous boost with the discovery of graphene, a single, atomically-thin layer of carbon atoms.
Graphene can be isolated from graphite thanks to the weak van der Waals (vdW) forces which bind the different layers together. In addition to graphite, many other vdW crystals were discovered in the past decade, and many other atomically-thin crystals were isolated. The latter typically possess unique properties, due to the enhanced quantum effects related to their two-dimensional (2D) nature.
If vdW materials in their 2D form have already revealed extraordinary potentialities per se, new rich scenarios are envisaged by creating 2D heterostructures (HSs).
Conventional HSs such as Si/Ge, InGaP/GaP, etc., have been the essential elements in modern semiconductor research and industry but their growth is limited by lattice mismatch constraints. On the other side, 2D crystals bind together via weak vdW forces and 2D HSs can thus be assembled layer-by-layer with unprecedented tunability.
Within this context, HSs based on 2D semiconducting materials are particularly interesting since coupling phenomena between the different layers were shown to lead to a strong light emission, which is promising for optoelectronic applications.
Few material combination were however studied and the exploration of this wide field has just commenced.
Here, we propose to exploit novel atomically-thin semiconductors for the fabrication of new HSs. Besides the most well-known 2D semiconductors, we will exploit 2D alloys, which were recently synthesised and whose physical properties are still not well-studied. Optical studies will first allow us to characterise the electronic and spin properties of these materials. The fabrication of alloy-based HSs will then allow us to create light emitting sources with engineered properties.
In the past decade, 2D TMD crystals have received enormous attention because of their capability to emit light efficiently and their strong spin-orbit coupling, which makes them interesting for optoelectronics and spintronics [Aja16]. The potentiality of these materials has been greatly enhanced by the creation of TMD-based HSs, giving rise to the observation of novel phenomena, such as IXs characterised by high emission and absorption efficiencies and long carrier lifetimes. These peculiarities make TMD-based HSs excellent candidates for applications related to carrier absorption and harvesting, such as solar cells, and to applications related to light emission. The great potentiality of these HSs is however limited by the possible material combinations available, resulting in the observation of a few IXs emitting at some specific wavelengths. Methods to expand the 2D-HS phase space are expected to have prominent impact in the field and pave the way towards the real integration of these materials into devices.
The goal of this project is to exploit TMD alloys with tuneable metal or chalcogen concentrations to expand the potentiality of 2D materials and HSs and achieve light emission at specific wavelengths. More specifically, this project would lead to a series of novel results concerning (i) the optical, electronic and spin properties of TMD alloys, (ii) the quality of alloy-based HSs and the formation of novel interlayer exciton species over a wide range of wavelengths; (iii) the possibility to dynamically tune interlayer excitons.
The tuneability enabled by the use of alloys will be exploited to create, for the first time, 2D HSs emitting at telecom wavelengths. The achievement of this result would undoubtedly represent a breakthrough in the field, going towards the possibility to exploit 2D materials for everyday life applications.
Furthermore, the possibility to dynamically tune light emission in 2D HSs will be probed by introducing variable strain gradients in the system, by exploiting the formation of bubbles in TMDs [Blu20,prr][Blu20,admi]. While the use of alloys would enable a static tuning over a wide wavelength range of about 2 micrometres, strain would enable a fine tuning over a range of 50-100 nanometres. This result, coupled with the static tunability introduced by the use of alloys, will introduce a further degree of freedom with high potential for quantum light applications. In fact, IXs were demonstrated to derive from localised states, since the coupling between two different materials with different lattice parameter or twist angle leads to a super-potential known as moiré potential [Ale19][Jin19][Sey19][Tra19]. Carriers localise in the moiré potential minima, and such localised state were demonstrated to act as single photon sources [Bae20]. A dynamic tuning of the emission wavelength of these single photon emitters would allow us to obtain different sources of nearly identical photons, and 2D HSs might thus serve as unique platforms towards the generation of entangled single photons emitting at the desired wavelength. This project will test the feasibility of this major achievement by probing the dynamic tuning of the HSs.
This project is thus expected to be a TIGERISH project, whose strength and determination will bring the field of 2D materials a step forward.