Author : Saiphaneendra Bachu
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (138 download)
Book Synopsis Probing the Interfaces in Two-dimensional Heterostructures Using Advanced Transmission Electron Microscopy by : Saiphaneendra Bachu
Download or read book Probing the Interfaces in Two-dimensional Heterostructures Using Advanced Transmission Electron Microscopy written by Saiphaneendra Bachu and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Two-dimensional (2D) heterostructures made of graphene and transition metal dichalcogenides (TMDs) attract huge research interest because they offer wide variety of band structures and are suitable for multicomponent semiconductor device applications. However, the properties of these heterostructures are highly dependent on the characteristics of the interface formed between the constituent 2D layers. So, it is imperative to investigate the interface structure, defects and possible relaxation effects at different length scales and correlate them with the synthesis and the properties of the heterostructures. To that end, this dissertation investigates the interfaces in three heterostructure systems, namely WSe2/graphene vertical heterostructures, ReS2/MoS2 vertical and in-plane heterostructures and MoSe2/WSe2 nanodot/matrix in-plane heterostructures, using advanced transmission electron microscopy and spectroscopy techniques. The first chapter briefly introduces the 2D materials and the two types of heterostructure geometries. It also highlights the importance of probing the interfaces in 2D heterostructures and how that is crucial to understanding and controlling the properties of the 2D heterostructures. The second chapter describes various experimental procedures utilized in the dissertation such as the synthesis of heterostructures using chemical vapor deposition (CVD) processes, various scanning/transmission electron microscopy (S/TEM) techniques and the associated sample preparation methods. The third chapter presents a detailed investigation of how the microstructure of bilayer graphene can have a profound effect on the nucleation of WSe2 overlayers towards the formation of WSe2/graphene vertical heterostructures. Through the combination of S/TEM experiments and ReaxFF force field molecular dynamics simulations, we uncover a high density of interlayer dislocations in Bernal-staked bilayer graphene leading to a significantly high nucleation density of WSe2 islands. Surprisingly, we observe WSe2 nucleation to be heavily suppressed in twisted bilayer graphene due to the lack of interlayer dislocations. We show that strain relaxation driven by strong interlayer coupling in Bernal-stacked bilayer graphene promotes the formation of these interlayer dislocations and localized buckling. Our simulations further show that these localized buckles in graphene serve as thermodynamically favorable sites for binding WSex molecules and could drive the higher nucleation density of WSe2 on Bernal-stacked graphene. The fourth chapter reports the synthesis of ReS2/MoS2 vertical and in-plane heterostructures using a two-step CVD process, in which triclinic ReS2 is anisotropic and hexagonal MoS2 is isotropic. Using a combination of S/TEM characterization and density functional theory (DFT) calculations, we investigated the nature of structural relaxations in ReS2 at the interface. In the vertical heterostructures, we observed the ReS2 undergoes stacking stabilizations at the atomic scale, Re chain rotations at the nanometer scale and domain inversions at the sub-micron scale, towards exhibiting a mesoscale quasi-epitaxy with the MoS2 underneath. In the in-plane heterostructures, ReS2 attempts to adopt the isotropic hexagonal structure of the MoS2 close to the interface by distorting its chain-like structure. Away from the interface, the ReS2 retains its triclinic chain-like structure and consequently induces an overall rotational misorientation of 5-6° with respect to the MoS2 lattice. The fifth chapter utilizes cathodoluminescence (CL)-in-STEM to experimentally prove that it is possible to realize localized light emission from MoSe2/WSe2 nanodot/matrix in-plane heterostructures where the MoSe2 nanodots are embedded in the WSe2 matrix. Furthermore, it is observed that the light emission originates from the MoSe2 excitons in nanodots larger than 85 nm. However, at smaller nanodot sizes, the interface excitons, formed by the electrons from MoSe2 and the holes from the WSe2, dominate the light emission. And, when the nanodot size goes below a critical value, the electron energy levels in the nanodot become quantized and induces a blue shift to the light emission. The sixth chapter concludes the observations from all the earlier chapters and offers a few future perspectives. Overall, the work performed in this dissertation highlights how TEM can be a powerful technique to establish synthesis -- structure -- property correlations in 2D heterostructure systems at different length scales, ranging from micron to atomic level.
