M-H. Choi, H. Noh, D. Velarde, Y. Yu, G. Min and S.B. Kaemmer
Park Systems,
United States
Keywords: low-angle twisted bilayer graphene (tBG), conductive atomic force microscopy (cAFM), moiré patterns, AA domain
Summary:
Manipulating the twist angle between layers serves as an effective means to adjust the electronic characteristics of two-dimensional (2D) van der Waals (vdW) materials. The magic angle of 1.1⁰ in twisted bilayer graphene (tBG) is renowned for its exceptional electronic properties, prompting extensive investigation into various twist angles to elucidate the underlying mechanisms. While prior research has explored the impact of interlayer twist on chemical reactivity, the focus has predominantly been on larger twist angles (≥7°). Examining the evolution of vertical conductivity in tBG with respect to twist angle at low angles (< a few degrees) reveals intriguing characteristics, including modulation of vibration modes, alterations in electronic structure, and electron-photon coupling behavior. At small twist angles, the competition among interlayer vdW interactions, atomic, and electronic reconstruction becomes particularly pronounced. The spacing between AA domains, where carbon atoms are directly aligned atop one another, proves highly relevant to the degree of twist, leading to the expansion or contraction of other domains. Conductive atomic force microscopy (cAFM) has emerged as a valuable tool for studying tBG due to its heightened sensitivity to local conductivity variations and superior spatial resolution. Previous investigations have unveiled the transition in tBG from incommensurate to commensurate domain structures, shedding light on unique electronic transport behaviors. Additionally, studies have showcased nonmonotonic vertical conductivity in tBG and explored symmetry breaking, alongside the identification of two metastable reconstruction states in small-angle twisted monolayer–multilayer graphene. In this study, we utilize cAFM to characterize the moiré pattern of low-twist angle tBG and explore its conductivity under both positive and negative potentials. The tBG sample under investigation is fabricated via graphene cutting using cAFM. Our measurements reveal current maps of moiré patterns at two distinct scales within the sample, accurately determining local twist angles and revealing deviations from intended values during fabrication. Employing moiré pattern simulations, we compare experimentally detected AA wavelengths and domain sizes, offering insight into the structural complexity of tBG. Further details will be presented on the poster.