Room-Temperature Seeded Growth of Tetraphenylphosphonium Bromide Single-Crystalline Fibers

Abstract

Growth of single-crystalline fibers (SCFs) has so far mainly depended on melt growth techniques, which have limitations and are not suitable for growth of thermally unstable or polymorphic SCFs. Here, a solution growth method for high-quality SCFs is reported. By seeded growth in solution at room temperature, tetraphenylphosphonium bromide (TppBr) SCFs with width of 1 to 10 μm and length of a few centimeters are produced successfully. The defect-free TppBr SCF seed grows rapidly along 1D, while its prismatic growth is completely suppressed. The steady 1D growth can be initiated in solution at 13.4% supersaturation and kept until the supersaturation rises up to 62.6%. AFM reveals that TppBr SCF obtained has a highly anisotropic surface micromorphology. We propose an enhanced anisotropic 2D nucleation mechanism to interpret the 1D growth of TppBr fiber seeds. The solution-seeded growth technique of SCFs will greatly expand the types of fiber crystals that can be grown for advanced device applications.

Numerical Investigation of the Stimulated Growth of Single-Crystal Fibers by Point-Effect-Induced Fluid Dynamics

“…The above discussions and numerical calculations show unequivocally that the underlying physics for prismatic facet growth arrest and fast pyramidal face growth is the combined action of self-shielding effect and fiber aspect-ratio-enabled mesoscopic self-channeling surface solute flow, not some suddenly appearing “ultrahigh” 2D nucleation energy barrier. , We note that these predictions have all been verified experimentally, a remarkable testimonial of the solid scientific foundation of the two-component surface chemical reaction-based crystal growth diffusion model presented in this work.…”

“…This is simply because at mesoscopic scales the observed one-dimensional growth processes are dominantly governed by mesoscopic physics, not by any microscopic growth suppression mechanism, which would have prevented any single-crystal seeds from growing to microscopic observable sizes in the first place. We stress that a "flat" prismatic facet without the usual growth steps 17,18 does not in any way support or validate a microscopic "2D ultrahigh nucleation barrier" that suddenly appears at the micrometer regime. Such featureless facets are merely the natural consequence of prismatic facet growth arrest, not the cause of it.…”

“…However, it does not entertain the concept of an energy barrier that can suddenly become "so high" at a few micrometer sizes. 17,18 In fact, the "2D ultrahigh nucleation barrier growth suppression mechanism" proposed in ref 17, except for "superficially" agreeing with AFM-observed flat prismatic facets, cannot explain any experimental observations listed above. This is simply because at mesoscopic scales the observed one-dimensional growth processes are dominantly governed by mesoscopic physics, not by any microscopic growth suppression mechanism, which would have prevented any single-crystal seeds from growing to microscopic observable sizes in the first place.…”

“…19,20 The aim of the present work is to explain, with extensive numerical simulations based on diffusion physics and fluid dynamics, the importance and decisive roles played by physical and chemical conditions at the mesoscopic level. We believe that it is the mesoscopic physics and geometry of the fiber seed, not any ultrahigh microscopic multidimensional nucleation barrier, 17,18 that governs the one-dimensional selfrestricted growth of single-crystal fibers reported. In Section 2, we first carry out density functional theoretical calculations and show a self-shielding effect arising from crystal surface selectivity.…”

“…That is, the growth of micrometer size fiber itself has already clearly rejected any microscopic nucleation-based growth suppression mechanisms. 17,18 Furthermore, the fact that only fibers with small widths and large initial aspect ratios can exhibit sustained fast one-dimensional growth with complete lateral growth suppression strongly suggests that within a few micrometer fiber diameter range the observed growth dynamics is strongly dependent upon seed fibers' mesoscopic dimensions and geometries. 19,20 The aim of the present work is to explain, with extensive numerical simulations based on diffusion physics and fluid dynamics, the importance and decisive roles played by physical and chemical conditions at the mesoscopic level.…”