In this project we have made breakthrough development demonstrating UHV 2D crystal assembly for the first time. This required development of an entirely new approach to the building of 2D material heterostructures. We found that widely adopted technique for 2D heterostructure assembly using polymers is introducing major organic contamination and have developed a new technique that employs inorganic SiNx membranes to pick up and place individual layers, Nature Electronics 6, 981 (2023). We demonstrated that this allows fast and reproducible production of 2D heterostructures using both exfoliated and CVD-grown materials with perfect interfaces free from interlayer contamination and correspondingly excellent electronic behaviour, limited only by the size and intrinsic quality of the crystals used. Furthermore, removing the need for polymeric carriers allows new possibilities for vdW heterostructure fabrication: assembly at high temperatures up to 600°C, and in different environments including ultra-high vacuum (UHV) and when the materials are fully submerged in liquids.
This key development has allowed us to produce high quality heterostructures for this project, enabling observations of novel physical phenomena. One of such interesting discoveries is the demonstrations of a room temperature ferroelectric semiconductor that is assembled using mono- or few-layer transition mental dichalcogenides, Nature Nanotechnology 17, 390. These van der Waals heterostructures feature broken inversion symmetry, which, together with the asymmetry of atomic arrangement at the interface of two 2D crystals, enables ferroelectric domains with alternating out-of-plane polarization arranged into a twist-controlled network. The last can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. This has opened up several avenues for follow-up studies which we are currently pursuing.
One of the most interesting consequences of introducing twist in 2D materials is the emergence of flat bands, which give rise to diverse correlated electron phenomena. In twisted graphene layers this has led to observation of many exciting effects such as superconductivity and insulator-like behaviour, however consistent studies of the band structure have been missing. In this project we have conducted ARPES studies of twisted graphene layers, and the high cleanliness and uniformity of specimen we have achieved allowed us to map out the evolution of the flat bands with the twist angles, Nano Lett. 23, 5201 (2023). Moreover, we have performed exploration of thermopower in twisted graphene heterostructures, an alternative approach to study the electronic structure of 2D superlattices, Phys. Rev. B 108, 115418 (2023). This study shows that, the thermopower peaks around the neutrality point allow to probe the energy spectrum degeneracy, both in single and double aligned structures with features evidencing multiple cloned Dirac points caused by the differential super-moiré superlattice.
This project has also enabled fruitful collaborations with other groups, including University of Sheffield (Nature Comm. 14, 3818 (2023)), (RWTH Aachen University – Phys. Rev. B 107, 115426 (2023)) and several others ongoing studies.
