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Abstract
Layered van der Waals materials can be thinned down to their constituent two dimensionalmonolayers. These monolayer sheets, when stacked on top of eachother with a small rotational misalignment, result in a geometric interference patterncalled a moire superlattice. The moire superlattice unit cell which can be ordersof magnitude larger than the original. Spatially varying registration of the atomswithin the superlattice unit cell can result in significant lattice reconstruction of theconstituent monolayers and act as a periodic modulation of interlayer interactions. Theresulting heterostructures can have drastically different electronic properties than theconstituent monolayers. This novel degree of freedom- the interlayer-twist angle, hasgained tremendous attention in recent years. In this dissertation, I present scanningtunneling microscopy studies of two such heterostructures: twisted bilayer grapheneand twisted molybdenum disulfide. Additionally, I also discuss STM measurementson graphene/TaS2 heterostructures which do not rely on the interlayer twist angle.
Magic-angle twisted bilayer graphene has emerged as a powerful platformfor studying strongly correlated electron physics, owing to its almost dispersionlesslow-energy bands and the ability to tune the band filling by electrostatic gating.Techniques to control the twist angle between graphene layers have led to rapidexperimental progress but improving sample quality is essential for separating thedelicate correlated-electron physics from disorder effects. Owing to the 2D nature ofthe system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape.This potential disorder is distinct from the twist-angle variation which has been studiedelsewhere. My findings demonstrate that flat bands in twisted bilayer graphene canamplify small doping inhomogeneity that surprisingly leads to carrier confinement,which in graphene could previously only be realized in the presence of a strong magneticfield.
Similar experiments were also carried out on MoS2 bilayers in a wide rangeof twist angles near 0°. Topography scans show the twist angle-dependence of themoire pattern which is dominated by lattice reconstruction for small angles (< 2°)leading to large triangular domains with rhombohedral stacking. Local spectroscopymeasurements reveal a large moire-potential strength of 100-200 meV for angles < 3°.In reconstructed regions, a bias-dependent asymmetry between neighboring triangulardomains is observed which we relate to the vertical polarization which is intrinsic torhombohedral stacked TMDs. This viewpoint is further supported by spectroscopymaps and ambient Piezoresponse measurements. These results provide a microscopicperspective on this new class of interfacial ferroelectrics and can offer clues for designingnovel heterostructures which harness this effect.
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