Two dimensional (2D) materials offer a fascinating playground to create new materials on demand. When stacking different 2D layered materials on top of each other, layered heterostructures are created. If the combination or stacking orientation of the individual layers is chosen well, the layered heterostructure properties extend the properties of the separate layers significantly due to the layers' interaction. When the layered heterostructure shows at one moment more than one active degree of freedom, it becomes a 'multifunctional' material. Magnetoelectric multiferroics are the benchmark of the class of multifunctional materials. Those materials show both ferroelectric and magnetic order, which couple via the magnetoelectric coupling. If the magnetoelectric coupling is strong enough, it allows to control/induce/switch the magnetic properties via an electric field and vice versa. Even despite exhaustive material exploration, only a handful of single-phase multiferroics (e.g. BiFeO3, BiCoO3, ScFeO3, LuFeO3) has been found that operate at room temperature.
Although there are no stacking limitations, a non-neglectable disadvantage of the vdWIs is the weak mechanical and electronic coupling between the layers, which is only enhanced for the so called magic angles - special orientations of the in-plane crystallographic axes. To combine the advantage of hybridisation-less stacking and inter-layer coupling, we explore van der Waals stackable heterostructures, where a coupling between the heterostructure layers is mediated by charge transfer and/or electron sharing. We call these heterostructures 2D sandwiches.
I think that there is a lack of feasible processing routes, such as exfoliation, chemical vapour deposition or MBE towards the creation of 2D sandwiches with (1) atomically clean interfaces, (2) the stacking of various layered materials classes with a different bonding mechanism at a defined place in specified order with defined thickness and (3) the ability to scale up the number of stacked layers, to create structures with ten or even more layers. The modulated elemental reactants method tackles the difficulty of the growth of 2D heterostructures encountered by the previous mentioned methods by reducing the crystal growth process's complexity. The growth of 2D crystals is simplified to two essential steps: the diffusion of the different reactants to the place of crystallisation and the crystallisation of these reactants to form the final product. The MER method reduces the reactants' diffusion to an absolute minimum by growing the whole final structure as amorphous layers of the pure chemical elements, with their thickness controlled at the Ångström scale, called the precursor. Subsequently, the precursor can be annealed at relatively low temperatures to kinetically self-assemble in the desired crystalline heterostructure. Due to the low thermal energy needed to create the final crystalline phase, thermodynamically metastable phases, such as layered structures, can be prepared.
The proposed approach in this project will allow us to make 2D multiferroic sandwiches, where a strong magnetoelectric coupling is present between the ferromagnetic and ferroelectric layers of the 2D sandwich.
The basic understanding of the design strategies obtained during the research can revolutionize the creation of artificial layered materials and provide new insights for the interbreeding of different classes of layered quantum materials, such as complex oxides, 2D layered crystals, cuprate superconductors or topological insulators or semimetals, which are, up to now, isolated islands.
