Ultraclean Layer Transfer and Atomic Reconstruction in Twisted TMD Lattices

Layer-by-layer assembly of van der Waals (vdW) heterostructures underpins new discoveries in solid state physics, material science and chemistry. Despite successes, all current assembly techniques use polymeric supports which limit their cleanliness, ultimate electronic performance, and potential for optoelectronic applications. In the first part of the talk, I will introduce a polymer-free platform for heterostructure assembly using re-usable flexible silicon nitride membranes. This approach enables production of heterostructures with interfaces free from interlayer contamination and correspondingly excellent (opto)electronic behaviour. In addition, eliminating polymeric supports allows new possibilities for vdW heterostructure fabrication: assembly at temperatures up to 600°C, and in different environments including ultra-high vacuum (UHV) and liquid submersion. We demonstrate UHV heterostructure assembly for the first time and show the reliable creation of moiré superlattices with over x10 improvement in homogeneity.<br/>In the second part of the talk, I will review our recent progress on twisted bilayer TMDs bilayers in the reconstruction regime. I will discuss their atomic structure observed in STEM, which is strikingly different for aligned (R) and anti-aligned (H) TMD monolayer orientations. This difference gives rise to correspondingly different optoelectronic properties, with ferroelectric domains arising in the former case and piezoelectric charge texturing in the latter. I will report our recent investigation of sliding ferroelectricity, and its switching behaviour observed in double gated structures using the electron channelling contrast SEM.