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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. In‐Plane Direct‐Write Assembly of Iridescent Colloidal Crystals

   https://doi.org/10.1002/smll.201905519  

   Tan, Alvin T. L. ; Nagelberg, Sara ; Chang‐Davidson, Elizabeth ; Tan, Joel ; Yang, Joel K. W. ; Kolle, Mathias ; Hart, A. John ( December 2019 , Small)

   Materials made by directed self‐assembly of colloids can exhibit a rich spectrum of optical phenomena, including photonic bandgaps, coherent scattering, collective plasmonic resonance, and wave guiding. The assembly of colloidal particles with spatial selectivity is critical for studying these phenomena and for practical device fabrication. While there are well‐established techniques for patterning colloidal crystals, these often require multiple steps including the fabrication of a physical template for masking, etching, stamping, or directing dewetting. Here, the direct‐writing of colloidal suspensions is presented as a technique for fabrication of iridescent colloidal crystals in arbitrary 2D patterns. Leveraging the principles of convective assembly, the process can be optimized for high writing speeds (≈600 µm s⁻¹) at mild process temperature (30 °C) while maintaining long‐range (cm‐scale) order in the colloidal crystals. The crystals exhibit structural color by grating diffraction, and analysis of diffraction allows particle size, relative grain size, and grain orientation to be deduced. The effect of write trajectory on particle ordering is discussed and insights for developing 3D printing techniques for colloidal crystals via layer‐wise printing and sintering are provided.

    

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3. Strengthening boron carbide by doping Si into grain boundaries

   https://doi.org/10.1111/jace.18028  

   Shen, Yidi ; Yang, Moon Young ; Goddard, III, William A. ; An, Qi ( July 2021 , Journal of the American Ceramic Society)

   Grain boundaries, ubiquitous in real materials, play an important role in the mechanical properties of ceramics. Using boron carbide as a typical superhard but brittle material under hypervelocity impact, we report atomistic reactive molecular dynamics simulations using the ReaxFF reactive force field fitted to quantum mechanics to examine grain‐boundary engineering strategies aimed at improving the mechanical properties. In particular, we examine the dynamical mechanical response of two grain‐boundary models with or without doped Si as a function of finite shear deformation. Our simulations show that doping Si into the grain boundary significantly increases the shear strength and stress threshold for amorphization and failure for both grain‐boundary structures. These results provide validation of our suggestions that Si doping provides a promising approach to mitigate amorphous band formation and failure in superhard boron carbide.

    

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4. 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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5. Shear‐Induced Assembly of Liquid Colloidal Crystals for Large‐Scale Structural Coloration of Textiles

   https://doi.org/10.1002/adfm.202010746  

   Li, Yichen ; Fan, Qingsong ; Wang, Xiaohui ; Liu, Guojin ; Chai, Liqin ; Zhou, Lan ; Shao, Jianzhong ; Yin, Yadong ( February 2021 , Advanced Functional Materials)

   Photonic crystals (PCs) constructed from colloidal building blocks have attracted increasing attention because their brilliant structural colors may find broad applications in paints, sensors, displays, and security devices. However, producing high‐quality structural colors on flexible substrates such as textiles in an efficient and scalable manner remains a challenge. Here a robust and ultrafast approach to produce industrial‐scale colloidal PCs by the shear‐induced assembly of liquid colloidal crystals of polystyrene beads pre‐formed spontaneously over a critical volume fraction is demonstrated. The pre‐crystallization of colloidal crystals allows their efficient assembly into large‐scale PCs on flexible fabric substrates under shear force. Further, by programming the wettability of the fabric substrate with hydrophilic–hydrophobic regions, this shear‐based assembly strategy can conveniently generate pre‐designed patterns of complex structural colors. This assembly strategy brings structural coloration to flexible fabrics at a scale suitable for commercial applications; therefore, it holds the potential to revolutionize the coloration technology in the textile industry.

    

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