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Sushil Kumar

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Open access Jul 2026

Strain Relaxation in Layered Lateral Heterostructures: Insights From Molecular Simulations and Unconventional HAADF‐STEM Imaging

A unique relaxation mechanism in wet‐chemically synthesized Bi2Te3–Sb2Te3 lateral heterostructures is presented, combining insights from molecular statics/dynamics simulations with a novel characterization technique based on STEM imaging using a defocused probe. The molecular simulations indicate that these heterostructures undergo intrinsic structural relaxation due to lattice mismatch and elastic anisotropy, resulting in uniform bending into dome‐shaped geometries with radii of curvature on the micron scale. While conventional bright‐field TEM imaging experimentally shows symmetric bend contours due to uniform bending, we demonstrate that defocused HAADF‐STEM produces unique contrast patterns sensitive to the sign of the probe defocus. This allows for the acquisition of crystallographic information (both magnitude and sign of curvature) in real space over large fields of view. The underlying physical mechanism is elucidated through scattering theory, focusing on the matching of the incident electron wave phase front with the local crystal lattice orientation and is supported by comprehensive dynamical multislice simulations of electron channeling maps. These findings are further validated by electron tomography and momentum‐resolved 4D‐STEM experiments. These insights are not only relevant for deciphering the local crystallographic properties of the Bi2Te3–Sb2Te3 system, but also establish a STEM‐based approach to characterize crystal bending across different zone axes in 2D nanostructures. This methodology provides a robust, real‐space alternative for analyzing intrinsic or strain‐induced curvature over large fields of view using unconventional HAADF‐STEM.

Naveen Goyal, Koushik Jagadish, Ziria Herdegen et al. · Small Methods · 0 citations

Jul 2026

Phase-Field Modeling of Single Crystallization of NCM613 Cathodes

Single-Crystal (SC) layered oxides have been extensively studied as NCM cathode materials due to the enhanced mechanical durability, resistance to intergranular cracking, and improved interfacial stability with electrolyte. The difficulty in scaling up SC-LiNi 0.6 Co 0.1 Mn 0.3 O 2 (mid-nickel SC-NCM) synthesis stems from a lack of fundamental knowledge in understanding the phase transformation mechanism during high-temperature calcination and sintering process. Herein, we examine the single crystallization mechanism using a phase-field method to understand densification and grain growth during the high temperature process. The phase-field method employed allows for multiple simultaneous processes including diffusion along surface/grain boundary/bulk lattice, vapor transport, particle rigid body motions of translation and rotation, and grain growth through boundary migration. We find that the oxygen partial pressure plays an important role in governing grain growth kinetics. Specifically, the stabilization of grain boundaries under high oxygen partial pressure prevents localized coalescence, thereby ensuring a homogeneous microstructure despite lowering the sintering kinetics. In contrast, low-oxygen partial pressure facilitates rapid single-crystal growth by promoting diffusion, significantly reducing the time required for the sintering process. These findings elucidate the underlying mechanisms behind the experimental time-efficient sintering kinetics, offering a clear understanding of the balance between structural stability and processing time.

Hyeonho Park, Sushil Kumar, Youngjin Kim et al. · ECS Meeting Abstracts · 0 citations