Abstract:

Physical properties of solids are inherently coupled to their structure and dimensionality. As such, the discovery of nascent physical phenomena and the realization of complex miniaturized devices in the solid state have incessantly relied upon the creation of stable low-dimensional crystals that approach the atomic limit. For example, the conceptual understanding of weak van der Waals (vdW) interactions in 2D materials comprised of single or few-atom-thick layers like graphene, has led to the demonstration of unusual structural landscapes and exotic physics from top-down exfoliation or bottom-up growth. However, while 2D vdW materials offer a direct means to create functional materials that approach the atomic regime, their intrinsic 2D bonding structure present longstanding synthetic challenges in accessing well-defined analogues with even lower dimensions and smaller form factors – in 1D. If accessed, these ultrathin 1D materials could revolutionize the way we think about directional, long-range, and dissipationless transport of charges, photons, magnetic spins, and even phonons. Towards this end, us in the Maxx X Lab are pioneering the discovery and chemical understanding of several classes of crystalline solid state materials comprising of sub-nanometer-thick inorganic chains that are held together by weak vdW or ionic interactions. Such 1D and quasi-1D structures could be thought of as freestanding “edge states” or “all-inorganic polymers” and could bridge the underexplored chemical and physical knowledge gap that exists between atomically precise 2D and 0D solids.

In this seminar, I will present our group’s efforts in elucidating the distinct chemical interactions which govern the structure, dimensionality, assembly, and physical properties of crystals comprised of weakly-bound inorganic chains. First, I will present a unique class of exfoliable III-VI-VII 1D vdW solids that crystallize as well-defined atomic-scale helical structures. I will describe how atomic level control of the local coordination environment along the INTRA-chain covalent bonding direction in these lattices induces helical aperiodicity that result into the emergence of a rare Boerdijk-Coxeter motif from a periodic tetrahelix. Owing to the intrinsic non-centrosymmetry of aperiodic helices, I will discuss how these tetrahelical wires exhibit visible range second harmonic generation in bulk and exfoliated nanoscale crystals. I will also demonstrate how compositional substitution in these modular phases could lead to a broad range of helical structures and accessible optical states. Second, I will talk about how we established the synthetic and materials design rules that enable access to dimensionally resolved optically active nanostructures ranging from 1D nanowires to quasi-2D nanoribbons and nanosheets, all based on the same quasi-1D vdW chain building block. I will show how precise and systematic bottom-up synthetic control over the INTER-chain interactions in these materials led to our observation of an emergent photoluminescence state which suggest the realization of the elusive indirect-to-direct band gap crossover in ultrathin nanowires of a model quasi-1D vdW phase, Sb2S3. I will conclude by illustrating how our findings generally translate towards directing the bottom-up assembly of a multitude of 1D and q-1D vdW solids with more intricate chemical structures and unique physical properties. Through these convergent efforts, we define the synthetic and materials design rules that dictate directed synthesis, complex atomic scale ordering, and anisotropic physical properties of these classes of 1D and quasi-1D vdW materials that are poised to become building blocks in next-generation quantum, energy, and sensing technologies.