Browsing by Author "Gupta, Nitant"
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Item Fatigue in assemblies of indefatigable carbon nanotubes(AAAS, 2021) Gupta, Nitant; Penev, Evgeni S.; Yakobson, Boris I.Despite being one of the most consequential processes in the utilization of structural materials, fatigue at the nano- and mesoscale has been marginally explored or understood even for the most promising nanocarbon forms—nanotubes and graphene. By combining atomistic models with kinetic Monte Carlo simulations, we show that a pristine carbon nanotube under ambient working conditions is essentially indefatigable—accumulating no structural memory of prior load; over time, it probabilistically breaks, abruptly. In contrast, by using coarse-grained modeling, we demonstrate that any practical assemblies of nanotubes, e.g., bundles and fibers, display a clear gradual strength degradation in cyclic tensile loading due to recurrence and ratchet-up of slip at the tube-tube interfaces, not occurring under static load even of equal amplitude.Item Polycrystalline morphology and mechanical strength of nanotube fibers(Springer Nature, 2022) Gupta, Nitant; Penev, Evgeni S.; Yakobson, Boris I.Correlating mechanical performance with mesoscale structure is fundamental for the design and optimization of light and strong fibers (or any composites), most promising being those from carbon nanotubes. In all forms of nanotube fiber production strategies, due to tubes’ mutual affinity, some degree of bundling into liquid crystal-like domains can be expected, causing heterogeneous load transfer within and outside these domains, and having a direct impact on the fiber strength. By employing large-scale coarse-grained simulations, we demonstrate that the strength s of nanotube fibers with characteristic domain size D scales as s ~ 1/D, while the degree of longitudinal/axial disorder within the domains (akin to a smectic ↔ nematic phase transition) can substantially mitigate this dependence.Item Strain tolerance of two-dimensional crystal growth on curved surfaces(AAAS, 2019) Wang, Kai; Puretzky, Alexander A.; Hu, Zhili; Srijanto, Bernadeta R.; Li, Xufan; Gupta, Nitant; Yu, Henry; Tian, Mengkun; Mahjouri-Samani, Masoud; Gao, Xiang; Oyedele, Akinola; Rouleau, Christopher M.; Eres, Gyula; Yakobson, Boris I.; Yoon, Mina; Xiao, Kai; Geohegan, David B.Two-dimensional (2D) crystal growth over substrate features is fundamentally guided by the Gauss-Bonnet theorem, which mandates that rigid, planar crystals cannot conform to surfaces with nonzero Gaussian curvature. Here, we reveal how topographic curvature of lithographically designed substrate features govern the strain and growth dynamics of triangular WS2 monolayer single crystals. Single crystals grow conformally without strain over deep trenches and other features with zero Gaussian curvature; however, features with nonzero Gaussian curvature can easily impart sufficient strain to initiate grain boundaries and fractured growth in different directions. Within a strain-tolerant regime, however, triangular single crystals can accommodate considerable (<1.1%) localized strain exerted by surface features that shift the bandgap up to 150 meV. Within this regime, the crystal growth accelerates in specific directions, which we describe using a growth model. These results present a previously unexplored strategy to strain-engineer the growth directions and optoelectronic properties of 2D crystals.Item Theoretical and Computational Investigations into the Nanomechanics and Growth of Low Dimensional Materials(2021-11-30) Gupta, Nitant; Yakobson, Boris I.This thesis presents research that has been driven towards the exploration and exploitation of the fundamental inter-atomic interactions in low-dimensional crystalline materials like nanotubes and graphene. A special focus has been maintained on understanding their mechanical behavior which is directly related to the strength of these interactions as well as their structural arrangement. Thus, allowing one to explain not only their high strength but also their fatigue behavior. Another important aspect about these interactions is how they can be manipulated to synthesize materials with high crystallinity or with purposeful ordering of defects. This led to the investigation of strategies that, for example, allow growth of infinitely large single crystals of graphene, which are virtually defect free. On the other hand, if the growth substrate is allowed to curve controllably, the work showed how defects can be created in a deterministic way. Therefore, a key aspect of the research presented in this thesis has been to understand the structure-property correlations, which affect the nanomechanics and growth of these materials.