Study on optical and electrical transport properties of twisted bilayer transition metal dichalcogenides
Van der Waals (vdW) heterostructures, where dissimilar atomically thin vdW crystals are vertically assembled, have initiated a new paradigm to create flexible multifunctional devices. Despite the weak nature of vdW interactions, unusually strong interlayer coupling and hybridization in these heterostructures lead to novel physical phenomena ranging from interfacial stress fields to modification of electronic band structure. In twisted van der Waals heterostructures (vdWHs), the angular mismatch between two similar lattices generates a large-scale interference pattern, known as the moiré pattern, which strongly impacts the electronic band structure of the superlattice. The moiré patterns in vdWHs create a periodic potential for electrons and excitons to yield many interesting phenomena such as Hofstadter butterfly spectrum, moiré excitons, tunable Mott insulator phases, unconventional superconductivity. In this thesis, we study the effects of moiré patterns on twisted TMDC bilayers by using Raman and PL measurements and try to probe the modified electronic properties in moiré superlattice through transport measurements. The relative rotation between the adjacent layers or the twist angle between them plays a crucial role in changing the electronic band structure of the superlattice. The first part of the thesis attempts to create such twisted TMDC bilayers with highly accurate twist angle. The assembly of multi-layers of precisely twisted two-dimensional layered materials requires knowledge of the atomic structure at the edge of the flake. Here, we demonstrate a simple and elegant transfer protocol using only optical microscope as an edge identifier tool, using which controlled transfer of twisted homobilayer and heterobilayer transition metal dichalcogenides is performed with close to 100 % yield. The fabricated twisted van der Waals heterostructures have been characterized by SHG, Raman spectroscopy, and photoluminescence spectroscopy, confirming the desired twist angle within 0.50 accuracy. The presented method is reliable, and quick, and prevents the use of invasive tools, which is desirable for reproducible device functionalities. Next, we study the phonon renormalization in twisted bilayer MoS2, which adds insight into the moiré physics. The interlayer coupling in these heterostructures is sensitive to twist angles (θ) and is key to controllably tuning several exotic properties. We demonstrate a systematic evolution of the interlayer coupling strength with twist angle in bilayer MoS2 using a combination of Raman spectroscopy and classical simulations. At zero doping, we show a monotonic increment of the separation between the A1g and E2g mode frequencies as θ decreases from 100 to 10, which saturates to that for a bilayer at small twist angles. Furthermore, we use doping-dependent Raman spectroscopy to reveal the θ-dependent softening and broadening of the A1g mode, whereas the E2g mode remains unaffected. Using first principles-based simulations, we demonstrate large (weak) electron-phonon coupling for the A1g (E2g) mode, explaining the observed trends. Our study provides a non-destructive way to characterize the twist angle and the interlayer coupling and establishes the manipulation of phonons in twisted bilayer MoS2 (twistnonics). Besides the closely aligned moiré lattice, intermediate misorientation (twist angles > 150) bilayers also offer a unique opportunity to tune excitonic behavior within these concurrent physical mechanisms but are seldom studied. To explore the light-matter interaction at an intermediate angle, we measure many-body excitonic complexes in monolayer (ML), natural bilayer (BL), and twisted bilayer (tBL) WSe2. Neutral biexciton (XX) is observed in tBL for the first time while being undetected in non-encapsulated ML and BL, demonstrating the unique effects of disorder screening in twisted bilayers. The XX, as well as charged biexciton (XX-), are robust to thermal dissociation and are controllable by electrostatic doping. Vanishing of momentum indirect interlayer excitons with increasing electron doping is demonstrated in tBL, resulting from the near-alignment of Q'-K and K-K valleys. Intermediate misorientation samples offer a high degree of control of excitonic complexes while offering possibilities for studying exciton-phonon coupling, band alignment, and screening. Finally, we investigate the electrical transport in Gr/tWSe2 heterostructure, using graphene as a sensing layer to probe the electronic effects of the underlying twisted TMDC structure on monolayer graphene. Unlike graphene, TMDC materials show a massive contact resistance. We tried to solve this issue by using different work function materials to reduce the Schottky barrier across the metal-semiconductor junction. However, getting an ohmic contact between the metal-semiconductor junction is a difficult technological challenge. We resolve this issue by using graphene as a sensing layer in monolayer graphene/twisted bilayer WSe2-based heterostructures. We observe the ferroelectricity in the sample, which can be understood by the presence of moiré ferroelectric domains in twisted TMDC lattice. We find that the polarization switching can be controlled through the vertical electric field. We also find a huge nonlocal signal in graphene at a zero magnetic field that can't be explained via classical contribution. Both the nonlocal and local resistance can be controlled through the electric field. We further explore the magnetotransport properties of the system and find that the magnetoresistance of the sample increases with an in-plane magnetic field.
- Physics (PHY)