Reversible solid-state phase transitions in confined two-layer colloidal crystals
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ORIGINAL CONTRIBUTION
Reversible solid-state phase transitions in confined two-layer colloidal crystals Zhuoqiang Jia 1 & Mena Youssef 2 & Alexandra Samper 1 & Stefano Sacanna 2 & Stephanie S. Lee 1 Received: 5 May 2020 / Revised: 28 July 2020 / Accepted: 10 September 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Using a combination of fluorescence and bright-field optical imaging, the solid-state packing structures of semi-confined twolayer spherical colloidal crystals were observed during modulation of an external AC electric field. Upon increasing field strength, the bottom layer of colloids (layer 1) transitioned from the entropically favored hexagonal packing structure with p6m symmetry to a square-packing structure with p4m symmetry. The packing structure of layer 2 was determined by the packing structure of layer 1, with layer 2 particles resting in, and moving in registry with, the low-energy interstitial sites of layer 1. Modulation of the field strength thus resulted in a reversible transition between a face-centered cubic crystal structure and a body-centered cubic crystal structure at low and high field strengths, respectively. These structures were found to be sensitive to the particle density in the wells, with vacancies and insertions leading to the formation of mixed crystal phases at high field strengths. Keywords Colloidal crystal . Solid-state phase transition . Negative dielectrophoresis . Phase diagram . Reversible assembly
Introduction In the absence of directional enthalpic interactions, the crystallization of both molecular and colloidal solids favors structures exhibiting the highest packing fraction to maximize entropy [1, 2]. In systems comprising spherical colloids, entropy-driven crystallization results in the adoption of the face-centered cubic (FCC) crystal structure, with a maximum packing fraction of φ = 0.74, under a wide range of deposition methods [3–6]. Achieving higher-energy packing structures has proven difficult, with colloidal crystals formed via direct sedimentation [7, 8], convective drying [9–12], and electrostatic deposition [13, 14], all adopting the FCC crystal structure [15, 16]. While other crystal structures in both two and Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00396-020-04752-y) contains supplementary material, which is available to authorized users. * Stephanie S. Lee [email protected] 1
Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, NJ 07030, USA
2
Department of Chemistry, New York University, New York, NY, USA
three dimensions are possible by tuning the particle geometry, these crystals still exhibit close-packing that maximizes the particle packing fraction [17–23]. Because the structure of colloidal crystals plays an important role in both scientific and industrial processes [24–29], strategies to achieve open packing structures are a critical challenge in the field of directed colloidal assembly. The fabric
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