Tuning the electrical and thermoelectric properties of bilayer graphene by band engineering

Abstract

Graphene and its two dimensional (2D) analogues have sustained the interests of the researchers for quite some time because of the rich physics they offer including many quantum phenomena and non-trivial topological properties. Bilayer graphene specifically offers far more tunability in its electronic properties because of the ability to break the inversion symmetry and open a band gap by an application of a transverse electric field. In this thesis. we study the electrical and thermoelectric properties of bilayer graphene under the influence of different types of external parameters namely transverse electric field, uniaxial strain and the superlattice potential due to the moiré lattice formation. While the opening of a band gap is a promising property for device applications, the presence of disorder and localized states within the gap implies that the electron transport still takes place via other mechanisms such as variable range hopping. In addition, bilayer graphene can still host robust edge states when the gap is open in the bulk because of its non-trivial marginal topology. The first part of the thesis attempts to establish the nature of transport in strongly localized bilayer graphene and bridge the two conflicting transport mechanisms that are predicted to occur namely the edge transport and the 2D hopping transport. Our results show possible evidence of one dimensional hopping transport occuring via the edges, possibly assisted by electron-electron interaction. Bilayer graphene has another interesting anomaly in its band structure near the band edge, i.e Lifshitz transition. While the energy scale associated with the trigonal warping induced Lifshitz transition is very small resulting in the masking of its effects on the transport properties, it can be enhanced with the help of external parameters such as strain and the electric field (D). We observe anamolous plateau like features in the thermopower measurements, close to the charge neutrality point in a ultrahigh mobility bilayer graphene. We have explained these unique features with the presence of van-Hove singularity near the Lifshitz transition which has been enhanced due to uniaxial strain and D. The observation of these effects in thermopower measurements with no corresponding features in the conductance measurements shows the extreme sensitivity of thermopower to the low energy features in density of states. Finally, we study the electrical and the thermoelectric properties of bilayer graphene aligned with two boron nitride layers resulting in the formation of a supermoiré lattice resulting in the modification of energy dispersion of BLG due to zone folding. The overlay of the two two-layer moiré superlattices results in a third superlattice, whose period can be larger than the maximum period (> 14 nm) in a graphene/hBN system. We observe multiple resistance ridges very close to the charge neutrality point which could possibly emerge due to higher order minibands in graphene-hBN moiré superlattices. The resistance at the secondary maxima seems to saturate to a quantized value of the resistance upon the application of an electric field possibly indicating a transport through one dimensional conducting modes. We also observe multiple sign changes in the thermopower measurements corresponding to the resistance ridges confirming these peaks as arising due to the miniband formation. This demonstration can pave way for constructing higher order moiré systems possibly resulting in the formation of ultra flat bands in graphene moiré superlattices.