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1. Shear-assisted grain coarsening in colloidal polycrystals

   https://doi.org/10.1073/pnas.2013456117  

   Li, Wei; Peng, Yi; Zhang, Yongjun; Still, Tim; Yodh, A. G.; Han, Yilong (September 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Grain growth under shear annealing is crucial for controlling the properties of polycrystalline materials. However, their microscopic kinetics are not well understood because individual atomic trajectories are difficult to track. Here, we study grain growth with single-particle kinetics in colloidal polycrystals using video microscopy. Rich grain-growth phenomena are revealed in three shear regimes, including the normal grain growth (NGG) in weak shear melting–recrystallization process in strong shear. For intermediate shear, early stage NGG is arrested by built-up stress and eventually gives way to dynamic abnormal grain growth (DAGG). We find that DAGG occurs via a melting–recrystallization process, which naturally explains the puzzling stress drop at the onset of DAGG in metals. Moreover, we visualize that grain boundary (GB) migration is coupled with shear via disconnection gliding. The disconnection-gliding dynamics and the collective motions of ambient particles are resolved. We also observed that grain rotation can violate the conventional relation R × θ = c o n s t a n t (R is the grain radius, and θ is the misorientation angle between two grains) by emission and annihilation of dislocations across the grain, resulting in a step-by-step rotation. Besides grain growth, we discover a result in shear-induced melting: The melting volume fraction varies sinusoidally on the angle mismatch between the triangular lattice orientation of the grain and the shear direction. These discoveries hold potential to inform microstructure engineering of polycrystalline materials. 

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2. Effect of growth temperature on the microstructure and properties of epitaxial MoS2 monolayers grown by metalorganic chemical vapor deposition

   https://doi.org/10.1116/6.0003296  

   Chen, Chen; Trainor, Nicholas; Kumari, Shalini; Myja, Henrik; Kümmell, Tilmar; Zhang, Zhiyu; Zhang, Yuxi; Bisht, Anuj; Sadaf, Muhtasim Ul; Sakib, Najam U.; et al (March 2024, Journal of Vacuum Science & Technology A)

   Metalorganic chemical vapor deposition (MOCVD) is a promising technique for wafer-scale synthesis of MoS2 monolayers for 2D field-effect transistors (2D-FETs) and related devices. Epitaxial growth of MoS2 on sapphire provides films that are crystallographically well-oriented but typically contain low-angle grain boundaries (e.g., mirror twins), voids, and other defects depending on growth conditions and substrate characteristics. In this study, we investigate microstructure, optical properties, and field-effect characteristics of wafer-scale MoS2 monolayers grown by MOCVD on c-plane sapphire over a narrow window of growth temperatures (900–1000 °C). The density of low-angle grain boundaries in the MoS2 monolayer was found to decrease dramatically from 50% areal coverage for films grown at 900 °C to 5% at 1000 °C. This decrease in low-angle grain boundary density is correlated with an increase in the room-temperature photoluminescence intensity of A excitons and a decrease in the full-width-half maximum (FWHM) of the Raman A1g peak, which are typically indicative of a general reduction in defects in MoS2. However, the best transport properties (e.g., mean field-effect mobility mFE = 17.3 cm2/V s) were obtained in MoS2 monolayers grown at an intermediate temperature of 950 °C. It was found that as the growth temperature increased, small regions bound by high-angle boundaries begin to appear within the monolayer and increase in areal coverage, from ∼2% at 900 °C to ∼5% at 950 °C to ∼10% at 1000 °C. The growth temperature of 950 °C, therefore, provides an intermediate condition where the combined effects of low-angle and high-angle boundaries are minimized. The results of this study provide guidance on MOCVD growth and characterization that can be used to further optimize the performance of MoS2 2D-FETs. 

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3. Experimental evidence and first-principles verification of deformation of basal twist grain boundaries in Ti

   https://doi.org/10.1016/j.actamat.2025.120878  

   Yang, Biaobiao; Hémery, Samuel; Shao, Wei; Tucker, Victoria; Titus, Michael S; Monclús, Miguel A; LLorca, Javier (May 2025, Acta Materialia)

   Recent experimental evidence indicates that Basal Twist Grain Boundaries (BTGBs) are responsible for the nucleation of fatigue cracks in Ti alloys. They are composed of two hexagonal close-packed α crystals rotated about the [0001] axis separated by a grain boundary parallel to the (0001) plane. To clarify the origin of this critical behavior, the mechanical response of bicrystal micropillars containing BTGBs was characterized by means of micropillar compression tests. Afterwards, the deformation mechanisms around the BTGB were analyzed by means of high-resolution scanning and transmission electron microscopy. It was found that shear deformation occurred by the glide of one basal grain over the other basal grain at very low values of the critical resolved shear stress (45 - 205 MPa), much lower than that of basal slip (400 - 600 MPa). To understand this mechanism, the Grain Boundary Energies (GBEs) of coincident site lattice BTGBs with different twist angles were determined by means of first-principles calculations of supercells. It was found that the GBE variations with respect to twist angle and in-plane translation were negligible for all coincident site lattice BTGBs investigated, leading to a very low shear resistance. 

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4. Aging near rough and smooth boundaries in colloidal glasses

   https://doi.org/10.1063/1.5000445  

   Cao, Cong; Huang, Xinru; Roth, Connie B; Weeks, Eric R (December 2017, The Journal of Chemical Physics)

   We use a confocal microscope to study the aging of a bidisperse colloidal glass near rough and smooth boundaries. Near smooth boundaries, the particles form layers, and particle motion is dramatically slower near the boundary as compared to the bulk. Near rough boundaries, the layers nearly vanish, and particle motion is nearly identical to that of the bulk. The gradient in dynamics near the boundaries is demonstrated to be a function of the gradient in structure for both types of boundaries. Our observations show that wall-induced layer structures strongly influence aging. 

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5. Grain rotation mechanisms in nanocrystalline materials: Multiscale observations in Pt thin films

   https://doi.org/10.1126/science.adk6384  

   Tian, Yuan; Gong, Xiaoguo; Xu, Mingjie; Qiu, Caihao; Han, Ying; Bi, Yutong; Estrada, Leonardo Velasco; Boltynjuk, Evgeniy; Hahn, Horst; Han, Jian; et al (October 2024, Science)

   Near-rigid-body grain rotation is commonly observed during grain growth, recrystallization, and plastic deformation in nanocrystalline materials. Despite decades of research, the dominant mechanisms underlying grain rotation remain enigmatic. We present direct evidence that grain rotation occurs through the motion of disconnections (line defects with step and dislocation character) along grain boundaries in platinum thin films. State-of-the-art in situ four-dimensional scanning transmission electron microscopy (4D-STEM) observations reveal the statistical correlation between grain rotation and grain growth or shrinkage. This correlation arises from shear-coupled grain boundary migration, which occurs through the motion of disconnections, as demonstrated by in situ high-angle annular dark-field STEM observations and the atomistic simulation–aided analysis. These findings provide quantitative insights into the structural dynamics of nanocrystalline materials. 

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