Entropically Driven Self‐Assembly of Pear‐Shaped Nanoparticles
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UNIVERSITY The ambition to recreate highly complex and functional nanostructures found in living PRESS organisms marks one of the pillars of today‘s research in bio- and soft matter physics. Here, self-assembly has evolved into a prominent strategy in nanostructure formation and has proven to be a useful tool for many complex structures. However, it is still a challenge to design and realise particle properties such that they self-organise into a desired target configuration. One of the key design parameters is the shape of the constituent particles. This thesis focuses in particular on the shape sensitivity of liquid crystal phases by addressing the entropically driven colloidal self-assembly of tapered ellipsoids, reminiscent of „pear-shaped“ particles. Therefore, we analyse the formation of the gyroid and of the accompanying bilayer architecture, reported earlier in the so-called pear hard Gaussian overlap (PHGO) approximation, by applying various geometrical tools like Set-Voronoi tessellation and clustering algorithms. Using computational simulations, we also indicate a method to stabilise other bicontinuous structures like the diamond phase. Moreover, we investigate both computationally and theoretically FAU Forschungen, Reihe B, Medizin, Naturwissenschaft, Technik 32 (density functional theory) the influence of minor variations in shape on different pear- shaped particle systems, including the stability of the PHGO gyroid phase. We show that the formation of the gyroid is due to small non-additive properties of the PHGO potential. This phase does not form in pears with a „true“ hard pear-shaped potential. Philipp W. A. Schönhöfer Overall our results allow for a better general understanding of necessity and sufficiency of particle shape in regards to colloidal self-assembly processes. Furthermore, the pear-shaped particle system sheds light on a unique collective mechanism to generate bicontinuous phases. It suggests a new alternative pathway which might help us to solve still unknown characteristics and properties of naturally occurring Entropically driven self-assembly of gyroid-like nano- and microstructures. nanoparticles driven self-assembly of pear-shaped Entropically pear-shaped nanoparticles ISBN 978-3-96147-268-0 FAU UNIVERSITY PRESS 2012 FAU FAU UNIVERSITY PRESS 2019 FAU A. Schönhöfer W. P. Philipp W. A. Schönhöfer Entropically driven self-assembly of pear-shaped nanoparticles FAU Forschungen, Reihe B Medizin, Naturwissenschaft, Technik Band 32 Herausgeber der Reihe: Wissenschaftlicher Beirat der FAU University Press Philipp W. A. Schönhöfer Entropically driven self‐assembly of pear‐shaped nanoparticles Erlangen FAU University Press 2019 Bibliografische Information der Deutschen Nationalbibliothek: Die Deutsche Nationalbibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.d-nb.de abrufbar. Bitte zitieren als Schönhöfer, Philipp W. A. 2019. Entropically driven self‐assembly of pear‐shaped nanoparticles. FAU Forschungen, Reihe B, Medizin, Naturwissenschaft, Technik Band 32. Erlangen: FAU University Press, DOI: 10.25593/978-3-96147-269-7. Das Werk, einschließlich seiner Teile, ist urheberrechtlich geschützt. Die Rechte an allen Inhalten liegen bei ihren jeweiligen Autoren. Sie sind nutzbar unter der Creative Commons Lizenz BY. Der vollständige Inhalt des Buchs ist als PDF über den OPUS Server der Friedrich-Alexander-Universität Erlangen-Nürnberg abrufbar: https://opus4.kobv.de/opus4-fau/home Verlag und Auslieferung: FAU University Press, Universitätsstraße 4, 91054 Erlangen Druck: docupoint GmbH ISBN: 978-3-96147-268-0 (Druckausgabe) eISBN: 978-3-96147-269-7 (Online-Ausgabe) ISSN: 2198-8102 DOI: 10.25593/978-3-96147-269-7 Entropically driven self‐assembly of pear‐shaped nanoparticles Entropie‐dominierte Selbstorganisationsprozesse birnenförmiger Teilchensysteme Der Naturwissenschaftlichen Fakultät der Friedrich‐Alexander‐Universität Erlangen‐Nürnberg zur Erlangung des Doktorgrades Dr. rer. nat. vorgelegt von Philipp Wilhelm Albert Schönhöfer aus Erlangen Als Dissertation genehmigt von der Naturwissenschaftlichen Fakultät der Friedrich‐Alexander‐Universität Erlangen‐Nürnberg Tag der mündlichen Prüfung: 25. Juli 2019 Vorsitzender des Promotionsorgans: Prof. Dr. Georg Kreimer Gutachter: Prof. Dr. Klaus Mecke Gutachter: Prof. Dr. Hartmut Löwen Gutachter: Prof. Dr. Peter Harrowell Diese Arbeit ist Teil eines Cotutelle Verfahrens zwischen der Naturwissenschaftlichen Fakultät der Friedrich‐Alexander‐Universität Erlangen‐Nürnberg und der School of Engineering and Information Technology der Murdoch University, Perth blank! Abstract This thesis addresses the entropically driven colloidal self‐assembly of pear‐ shaped particle ensembles, including the formation of nanostructures based on triply periodic minimal surfaces, in particular of the Ia3d gyroid. One of the key results is that the formation of the Ia3d gyroid, reported earlier in the so‐called pear hard Gaussian overlap (PHGO) approximation and confirmed here, is due to a slight non‐additivity of that potential; this phase does not form in pears with true hard‐core potential. First, we computationally study the PHGO system and present the phase diagram of pears with an aspect ratio of 3 in terms of global density and particle shape (degree of taper), containing gyroid, isotropic, nematic and smectic phases. We confirm that it is adequate to interpret the gyroid as a warped smectic bilayer phase. The collective behaviour to arrange into interdigitated sheets with negative Gauss curvature, from which the gyroid results, is investigated through correlations of (Set‐)Voronoi cells and lo‐ cal curvature. This geometric arrangement within the bilayers suggests a fundamentally different stabilisation mechanism of the pear gyroid phase compared to those found in both lipid‐water and di‐block copolymer sys‐ tems forming the Ia3d gyroid. The PHGO model is only an approximation for hard‐core interactions, and we additionally investigate, by much slower simulations, pear‐assemblies with true hard‐core interactions (HPR). We find that HPR phase diagram only contains isotropic and nematic phases, but neither gyroid nor smectic phases. To understand this shape sensitivity more profoundly, the deple‐ tion interactions of both models are studied in two pear‐shaped colloids dis‐ solved in a hard sphere solvent. The HPR particles act as one would expect from a geometric analysis of the excluded‐volume minimisation, whereas the PHGO particles show deviations from this expectation. These differ‐ ences are attributed to the unusual angle dependency of the (non‐additive) contact function and, more so, to small overlaps induced by the approxima‐ tion. v For the PHGO model, we further demonstrate that the addition of a small concentration of hard spheres (“solvent”) drives the system towards a Pn3m diamond phase. This result is explained by the greater spatial heterogeneity of the diamond geometry compared to the gyroid where additional material is needed to relieve packing frustration. In contrast to copolymer systems, however, the solvent mostly aggregates near the diamond minimal surface, driven by the non‐additivity of the PHGO pears. At high solvent concentra‐ tions, the mixture phase separates into “inverse” micelle‐like structures with the blunt ends at the micellar centres and thin ends pointing outwards. The micelles themselves spontaneously cluster, indicative of a hierarchical self‐ assembly process for bicontinuous structures. Finally, we develop a density functional for hard solids of revolution (in‐ cluding pears) within the framework of fundamental measure theory. It is applied to low‐density ensembles of pear‐shaped particles, where we anal‐ yse their response near a hard substrate. A complex orientational ordering close to the wall is predicted, which is directly linked to the particle shape and gives insight into adsorption processes of asymmetric particles. This predicted behaviour and the differences between the PHGO and HPR model are confirmed by MC simulations. vi Zusammenfassung Diese Doktorarbeit befasst sich mit der Selbstorganisation entropisch ge‐ triebener, birnen‐förmiger Kolloidteilchen. Hierbei wird besonders auf die Bildung von Nanostrukturen eingegangen, die auf dreifach‐periodischen Minimalflächen und insbesondere auf der Ia3d Gyroid Minimalfläche fußen. Eines unserer wichtigsten Ergebnisse besteht aus der Erkenntnis, dass die spontane Bildung der Ia3d Gyroid Struktur, von welcher in früheren Studien über die pear hard Gaussian overlap (PHGO) Annäherung berichtet wurde und die hier bestätigt wurde, auf geringfügig nicht‐additive Eigenschaften dieses Potentials zurückzuführen ist; in Systemen mit Teilchen von perfekt harter Birnenform wird dieser dreifach‐periodische Flüssigkristallzustand nicht beobachtet. Zuerst untersuchen wir mit Hilfe von Computersimulationen das PHGO System, bei dem wir das Phasendiagram von Birnen‐Teilchen mit Aspektver‐ hältnis 3 präsentieren. Dieses ist in Abhängigkeit der globalen Dichte und der Teilchenform (Teilchenverjüngung) angegeben und beinhaltet sowohl gyroide, isotrope, nematische, als auch smektische Phasen. Wir bestäti‐ gen, dass die Gyroidphase als gekrümmte smektische Bilagenphase gedeutet werden kann. Das kollektive Verhalten der Birnenteilchen, sich in verzahn‐ te Schichten mit negativer Gausskrümmung, aus der der Gyroid resultiert, anzuordnen, wird erkundet, indem ein Zusammenhang zwischen den zu‐ grundeliegenden Mengen