Crystal formation in tetracyanoquinodimethane on the nanoscale: polymorphism and progression of self-assembly
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Crystal formation in tetracyanoquinodimethane on the nanoscale: polymorphism and progression of self-assembly Maki Nishida and Edward R. Van Keuren, Department of Physics, Georgetown University, Washington, District of Columbia 20057 Address all correspondence to Edward R. Van Keuren at [email protected] (Received 27 June 2011; accepted 8 July 2011)
Abstract Molecules self-assembling in solution may pass through multiple phases and morphologies before reaching a thermodynamically stable state. Here we demonstrated this effect in tetracyanoquinodimethane (TCNQ), an organic molecule often used as an electron acceptor in charge transfer complex compounds. We showed that it self-assembles in a wide range of crystal habits, from nanocoils to polyhedral crystals. Scanning electron microscopy imaging on freeze-dried samples revealed the crystal growth of TCNQ starting from seed-shaped nucleation sites, progressing through flower-like structures and finally forming polyhedral micro-crystals. These results are supplemented by absorption spectroscopy as well as x-ray powder diffraction (XRPD) characterization of a powder sample.
Introduction Nucleation and growth are underlying phenomena responsible for the formation of many materials. Processes such as solution crystallization, self-assembly, protein aggregation, biomineralization and many types of nanoparticle formation all begin with the initial dimerization of molecules to form nucleation sites, followed by the addition of monomers. Both the kinetics and the thermodynamics are important for understanding the properties of the resulting materials, and the details of intermediate processes can have a critical effect on the characteristics of the final materials. From the classical point of view, crystallization is described as a single-step process with a parent phase directly transforming to the thermodynamically stable crystalline phase. However, crystal morphology (phase and habit) can pass through various metastable transient regions before reaching the stable one. For the phase, Ostwald’s rule of stages describes such a process by predicting that the metastable phase that requires the smallest free energy change will form before the most thermodynamically stable form is reached.[1] Although Ostwald’s rule is not universal,[2] some experimental evidence exists for this rule; for example, Chung et al. have demonstrated a progression of phases in a metal phosphate nanocrystal using in situ highresolution electron microscopy.[3] Numerous studies of the kinetics of the transition process in crystal nucleation have been used to establish analytical models that confirm the existence of the metastable phase.[4–6] Likewise, crystal habit may change during the crystallization as more stable forms emerge and as the local level of supersaturation changes.[7] Here we report the observation of a metastable transition process in 7,7,8,8-tetracyanoquinodimethane (TCNQ) crystals. TCNQ is an organic electron-acceptor compound, which
is often used in charge
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