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1. In situ observation of coalescence of nuclei in colloidal crystal-crystal transitions

   https://doi.org/10.1038/s41467-023-40627-w  

   Peng, Yi; Li, Wei; Still, Tim; Yodh, Arjun G; Han, Yilong (December 2023, Nature Communications)

   Abstract Coalescence of nuclei in phase transitions significantly influences the transition rate and the properties of product materials, but these processes occur rapidly and are difficult to observe at the microscopic scale. Here, we directly image the coalescence of nuclei with single particle resolution during the crystal-crystal transition from a multilayer square to triangular lattices. The coalescence process exhibits three similar stages across a variety of scenarios: coupled growth of two nuclei, their attachment, and relaxation of the coalesced nucleus. The kinetics vary with nucleus size, interface, and lattice orientation; the kinetics include acceleration of nucleus growth, small nucleus liquefaction, and generation/annihilation of defects. Related mechanisms, such as strain induced by nucleus growth and the lower energy of liquid-crystal versus crystal-crystal interfaces, appear to be common to both atomic and colloidal crystals. 

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2. Structural flexibility of halogen bonds showed in a single-crystal-to-single-crystal [2+2] photodimerization

   https://doi.org/10.1107/S2052252518007583  

   Sinnwell, Michael A.; Blad, Jared N.; Thomas, Logan R.; MacGillivray, Leonard R. (July 2018, IUCrJ)

   Halogen bonds have emerged as noncovalent forces that govern the assembly of molecules in organic solids with a degree of reliability akin to hydrogen bonds. Although the structure-directing roles of halogen bonds are often compared to hydrogen bonds, general knowledge concerning the fundamental structural behavior of halogen bonds has had limited opportunity to develop. Following an investigation of solid-state reactions involving organic syntheses and the development of photoresponsive materials, this work demonstrates the ability of the components of intermolecular N...I halogen bonding – a `workhorse' interaction for the crystal engineer – to support a single-crystal-to-single-crystal [2+2] photodimerization. A comparison is provided of the geometric changes experienced by the halogen-bonded components in the single-crystal reaction to the current crystal landscape of N...I halogen bonds, as derived from the Cambridge Structural Database. Specifically, a linear-to-bent type of deformation of the halogen-bonded components was observed, which is expected to support the development of functional halogen-bonded materials containing molecules that can undergo movements in close-packed crystal environments. 

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3. Defect enhancement of Er emission in single crystal Er2O3

   https://doi.org/10.1063/5.0264573  

   Wu, H; Dodson, A; al-Wahish, A; O'Hara, A; Rai, A; Posadas, A B; Wang, Y Q; Lawrie, B; Titze, M; Davidson, J; et al (June 2025, Applied Physics Letters)

   We report observations of crystal defect induced modifications in the laser-excited Er3+ photoluminescence spectra of single-crystal Er2O3 thin films. These include marked enhancement of transition intensities known to be weak or nonexistent in as-grown samples. In this Letter, we examine the temperature and defect density dependence of the measurements and suggest photophysical processes that may account for these unexpected results. 

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4. Super‐ and Ferroelastic Organic Semiconductors for Ultraflexible Single‐Crystal Electronics

   https://doi.org/10.1002/anie.202004083  

   Park, Sang Kyu; Sun, Hong; Chung, Hyunjoong; Patel, Bijal B.; Zhang, Fengjiao; Davies, Daniel W.; Woods, Toby J.; Zhao, Kejie; Diao, Ying (May 2020, Angewandte Chemie International Edition)

   Abstract Like silicon, single crystals of organic semiconductors are pursued to attain intrinsic charge transport properties. However, they are intolerant to mechanical deformation, impeding their application in flexible electronic devices. Such contradictory properties, namely exceptional molecular ordering and mechanical flexibility, are unified in this work. We found that bis(triisopropylsilylethynyl)pentacene (TIPS‐P) crystals can undergo mechanically induced structural transitions to exhibit superelasticity and ferroelasticity. These properties arise from cooperative and correlated molecular displacements and rotations in response to mechanical stress. By utilizing a bending‐induced ferroelastic transition of TIPS‐P, flexible single‐crystal electronic devices were obtained that can tolerate strains (ϵ) of more than 13 % while maintaining the charge carrier mobility of unstrained crystals (μ>0.7 μ₀). Our work will pave the way for high‐performance ultraflexible single‐crystal organic electronics for sensors, memories, and robotic applications. 

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5. Single-crystal-to-single-crystal guest exchange in columnar assembled brominated triphenylamine bis-urea macrocycles

   https://doi.org/10.1039/c9cc01725a  

   Sindt, Ammon J.; Smith, Mark D.; Berens, Samuel; Vasenkov, Sergey; Bowers, Clifford R.; Shimizu, Linda S. (May 2019, Chemical Communications)

   Self-assembly of brominated triphenylamine bis-urea macrocycles affords robust porous materials. Urea hydrogen bonds organize these building blocks into 1-dimensional columns, which pack via halogen–aryl interactions. The crystals are stable when emptied, present two distinct absorption sites for Xe with restricted Xe diffusion, and exhibit single-crystal-to-single-crystal guest exchange. 

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