Design of Novel Low-Dimensional Heterostructures via In Situ Atomic-Scale Observation

Control of material processes at the level of atoms and electrons is a ‘grand challenge’ of materials design. With the rise of quantum materials and increasing security, resource scarcity, and sustainability considerations, the need for alternative methods of manufacturing at the atomic scale is paramount. Direct visualization of atomic-scale mechanisms allows precise tailoring of nanostructures from the bottom up. <i>In situ </i>transmission electron microscopy (TEM) is powerful in achieving this goal due to its high spatial and temporal resolution, obtaining atomic-scale movies while the sample undergoes functional changes, for example during nucleation, phase transformation, or current-biasing. Such direct observation of atomic motion can uncover kinetic models of nucleation and growth, as well as allow direct comparison with <i>ab-initio</i> or molecular dynamics simulations.<br/>Here, I will explore the atomic-scale structure and <i>in situ </i>growth of an emerging class of van der Waals bonded materials termed ‘mixed dimensional’ heterostructures, which consist of two-dimensional (2D) + nD (where n is 0, 1 or 3) materials adhered primarily though non-covalent interactions.¹ The weak quasi-van der Waals bonding in certain 2D/3D heterostructures (exemplified by Au/MoS₂) results in reproducible moiré patterns that modify the electronic structure at the interface.² In contrast, more strongly bound heterostructures, such as Ti/Gr, exhibit ordered arrays of dislocation networks that are strongly modulated by the 2D material layer number and compliance. We introduce a criterion for dislocation formation in such suspended systems and tailor the thin non-dislocated structures towards ultra-thin heterostructure stacks, of application in quantum sensors.³ We demonstrate the feasibility of forming epitaxial and single crystalline metal/2D/metal (3D/2D/3D) heterostructures using suspended 2D materials, with implications for next-generation Josephson junctions. Lastly, we explore the nucleation and growth of lower symmetry structures, such as 1D nanowires and nanoribbons, examining the influence of symmetry on nanostructure morphology. Such understanding of growth kinetics allows versatile design of heterostructures for next generation nanoscale devices and showcases the powerful role of <i>in situ </i>TEM in unraveling the intricacies of quantum materials at the atomic scale.<br/>1. Jariwala, D., Marks, T. J. & Hersam, M. C. Mixed-dimensional van der Waals heterostructures. <i>Nat. Mater.</i> <b>16</b>, 170–181 (2017).<br/>2. Reidy, K.*, Varnavides, G.*, Dahl Thomsen, J., Kumar, A., Pham, T., Blackburn, A. M., Anikeeva, P., Narang, P., Lebeau, J. M. & Ross, F. M. Direct imaging and electronic structure modulation of moiré superlattices at the 2D/3D interface. <i>Nat. Commun.</i> <b>12</b>, 1290 (2021).<br/>3. Monticone, E., Castellino, M., Rocci, R. & Rajteri, M. Ti/Au Ultrathin Films for TES Application. <i>IEEE Trans. Appl. Supercond.</i> <b>8223</b>, 1–5 (2017).