Browsing by Author "Gupta, Nitant"

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    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.
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    Molecular Dynamics of Catalyst-Free Edge Elongation of Boron Nitride Nanotubes Coaxially Grown on Single-Walled Carbon Nanotubes
    (American Chemical Society, 2024) Hisama, Kaoru; Bets, Ksenia V.; Gupta, Nitant; Yoshikawa, Ryo; Zheng, Yongjia; Wang, Shuhui; Liu, Ming; Xiang, Rong; Otsuka, Keigo; Chiashi, Shohei; Yakobson, Boris I.; Maruyama, Shigeo
    Recent advances in low-dimensional materials have enabled the synthesis of single-walled carbon nanotubes encapsulated in hexagonal boron nitride (BN) nanotubes (SWCNT@BNNT), creating one-dimensional van der Waals (vdW) heterostructures. However, controlling the quality and crystallinity of BNNT on the surface of SWCNTs using chemical vapor deposition (CVD) remains a challenge. To better understand the growth mechanism of the BNNT in SWCNT@BNNT, we conducted molecular dynamics (MD) simulations using empirical potentials. The simulation results suggest that spontaneous BN nucleation is unlikely to occur on the outer surface of the SWCNT when we assume only vdW interaction between the BN and SWCNT layers. However, we observe the elongation of the BNNT when a short BNNT is provided as a seed nucleus on the SWCNT. This grown BNNT structure, with its sharply cut edges, aligns with experimental observations made using transmission electron microscopy (TEM). Moreover, the edge-reconstruction process favors zigzag B edges, which exhibit low edge energy according to the ReaxFF potential. Our simulation successfully provides insights into the catalyst-free growth process of this one-dimensional vdW heterostructure.
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    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.
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    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.
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    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.