Twist Angle Dependent Phononic and Photoluminescence Properties of Twisted Bilayer Heterostructures

Abstract

This thesis presents research work on twisted bilayer graphene (t-BLG) and twisted tungsten diselenide (t-WSe2) using various measurement techniques. Since the isolation of graphene in 2004, two-dimensional (2D) materials have garnered significant attention due to their unique electronic, optical, and mechanical properties. Beyond graphene, transition metal dichalcogenides (TMDs) such as tungsten diselenide (WSe2) have attracted significant interest for their tunable bandgaps and strong light-matter interactions, making them promising candidates for advanced electronic and optoelectronic applications. While much research has focused on the electronic properties of twisted bilayer graphene (t-BLG) and twisted transition metal dichalcogenides (TMDs), the optical and vibrational properties—particularly those probed by Raman spectroscopy—remain less explored. In this thesis, we investigate the optical and phononic properties of graphene and WSe2, with a special emphasis on the effects of twist and moiré pattern formation. Utilizing Raman spectroscopy as a primary tool, we aim to understand their fundamental vibrational characteristics.

A central part of this study focused on twisted homobilayers of WSe2, where varying the twist angle from 0° to 7° systematically tunes the phonon modes. Non-invasive Raman measurements show that the nearly degenerate A1g/E2g phonon mode in WSe2/WSe2 splits into a doublet in twisted samples, with the maximum splitting occurring near 2°–3°. This splitting reflects strong phonon hybridization driven by interlayer coupling and atomic reconstruction induced by moiré potential. Our theoretical calculations qualitatively capture the observed angle-dependent splitting. Additionally, the experiments reveal enhanced anharmonic phonon-phonon interactions around 2°. Temperature-dependent measurements below 50 K display anomalous Raman frequency softening and linewidth broadening, suggesting a combined effect enhanced electron-phonon coupling and cubic anharmonic interactions.

Another area of investigation involved twisted bilayer graphene, where we systematically studied the evolution of G and 2D Raman modes across a range of twist angles from ~ 0.3° to 3°. We observed the spitting of G-mode near the magic angle (~1°) which was attributed to moiré potential-induced phonon hybridization due to lattice reconstruction. The linewidths of the low-frequency component of the G-mode and the main component of the 2D mode exhibit significant broadening near the magic angle, signalling enhanced electron-phonon coupling as flat electronic bands emerge. Moreover, temperature-dependent Raman measurements reveal an almost tenfold increase in phonon anharmonicity-induced temperature variations in t-BLG compared to Bernal-stacked bilayer graphene. These observations hint towards the role of phonon hybridization and moiré-induced phenomena in shaping phononic behaviour and potentially influencing the thermal management of graphene-based devices.

The influence of alignment on phonon-phonon and electron-phonon interactions is further explored in graphene/hBN heterostructures. Raman spectroscopy performed from 8K to 300K on various configurations—non-aligned, partially aligned, singly aligned, and doubly aligned—reveals distinct phononic behaviours. Non-aligned samples resemble pristine graphene, with minimal temperature-induced Raman frequency shifts and expected phonon-phonon interactions. In stark contrast, in doubly aligned samples, the Raman frequency decreases almost linearly with increasing temperature, and the linewidth decreases—opposite to the expected trend—indicating enhanced electron-phonon interactions. Such anomalous responses, which cannot be explained by standard intralayer phonon-phonon interaction models, point to strong interlayer phonon coupling and highlight the potential of layer alignment engineering in manipulating phononic properties.

Finally, photoluminescence (PL) measurements on twisted bilayer WSe2 spanning twist angles from 1° to 7° focus on neutral exciton. A pronounced extremum in exciton energy and linewidth occurs between approximately 2° and 4°, coinciding with the formation of flat electronic bands. While the detailed mechanisms behind these excitonic anomalies require further inquiry, the results suggest that moiré engineering can significantly influence exciton dynamics.

In summary, this thesis demonstrates how twist, alignment, and moiré pattern formation in graphene, WSe2, and related heterostructures reshape their phononic, electronic, and excitonic properties, enabling potential applications in advanced electronic, optoelectronic, and thermal technologies.