Self-assemblies of Inorganic and Organic Nanomaterials

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EPL Focus

 

Preface

Self-assembly is a generic term indicating organized structures at various scales. When the constitutive components are molecules the term is molecular assemblies, whereas with nanocrystals it is called “super”/“supra” structures. The thermodynamic stable self-organizations are non-covalent, spontaneous and reversible assemblies. Usually, weak interactions such as van der Waals interactions, hydrogen bonds, ionic and metallic bonds govern them as well as repulsive forces. Thermodynamic stability and weak interactions make 3D crystalline structures highly sensitive to external perturbations with a potential reversibility of the ordering. These assemblies have a mesoscopic dimension, whereas the building blocks exist at the nanoscale. The building blocks are molecules, polymers and nanomaterials. They differ by sizes, shapes, chemical compositions and functionalities. Molecular self-assembly underlies the construction of biologic mesoscopic assemblies in living organisms, and so is crucial to the function of cells. It is exhibited in the self-assembly of lipids that are considered as surfactants containing both hydrophobic and hydrophilic groups as tails and heads, respectively. They are characterized by a low surface tension between two immiscible liquids and/or between solid and liquid. The spontaneous assembly of uniformsized globular entities into ordered arrays is a universal phenomenon observed for objects with diameters spanning a broad range of length scales. These extend from the atomic scale (10−8 cm), through molecular and macromolecular scales with proteins, synthetic low polymers and colloidal crystals (∼ 10−6 cm), to the wavelength of visible light (∼ 10−5 cm). The associated concepts of sphere packing have had an influence in diverse fields ranging from pure geometrical analysis to architectural models or ideals. Self-assembly of atoms, molecules or nanocrystals into ordered functional superstructures is a universal process and prevalent topic in science. About five billion years ago in the early solar system, highly uniform magnetite particles of micrometers in size were assembled in 3D arrays. Thirty million years ago, silicate particles with sub-micrometer size were self-organized in the form of opal. Opal is colorless when composed of disordered silicate microparticles, whereas it shows specific reflectivity when particles order in arrays. Nowadays, nanocrystals, characterized by a narrow size distribution and coated with alkyl chains to maintain their integrity, self-assemble to form crystallographic orders called supracrystals. Nanocrystals and supracrystals are highly ordered arrangements of atoms and nanocrystals, respectively. The morphologies of nanocrystals, supracrystals and minerals are similar at various scales from nanometer to millimeter scale. Such suprastructures, allowing to design novel materials, are expected to become one of the main driving forces in material research for the 21st century. Assemblies of both organic molecules and nanocrystals become an easy and inexpensive way to fabricate them. They could be highly useful for future devices in solar energy, batteries, organic photovoltaic cells, microfluidic sensors as well as in medicine. In this Focus Issue we wanted to give to the reader a flavor on the various activities existing nowadays in the field of self-assemblies of organic and inorganic systems: Hence, hierarchical and complexity on self-assembly of molecules such as peptides are key parameter on developing new functional materials leading to a wide variety of applications. Here computation methods at the molecular levels are presented. For reliable applications of inorganic 3D superlattices we need to find the best conditions to produce highly stable nanocrystals. Here we revisit the criteria used to explain the experimental stability of noble nanocrystals. This could by confirmed by recent technologies that permit to describe in detail the crystalline structure of nanomaterials as well as the chemical composition of the matter. We know that the 3D superlattices at the mesoscopic scale growth processes depend of a large number of parameters (solvent, coating agent etc.). The formation of 3D superlattices at the mesoscopic scale is governed by the interplay of a range of thermodynamic and kinetic factors. The role of time-resolved X-ray scattering techniques combined with in situ sample environments to gain unique insights into the relevant processes is summarized. The 3D superlattices can be obtained with one building block. However, other types of supracrystals are obtained by using nanocrystals having two different sizes. A large number of structures are produced with the appearance of unexpected structures by ligand exchange. Below it is demonstrated that these ligand exchange could alter the physical properties. The deposition processes are also a key parameter in the control of the nanocrystals deposition. Here an overview on the various processes involved during the nanoparticle deposition is presented. As already mentioned, a rather large number of objects such as molecules, inorganic nanocrystals, and polymers can be used as building blocks. Here self-assemblies of charged micro- and nano-particles at oil/water interface offer great opportunities as model systems to investigate the structural and mechanical response of materials and as versatile patterning tools for surface nanostructuring. Similarly a new and easy method of cellulose coating on titania surface and free-standing hydrides is proposed for a wide range of photochemical devices such as films for optics, drug delivery systems and inks for printing of biologically relevant lab-on-chips. Combination of nanochemistry and self-assembly methods provide an alternative for the fabrication of 2D and 3D assemblies exhibiting meta-properties in visible light. The physical properties of such 3D superlattices are neither those of nanocrystals nor those of the bulk phase. They are due to the assemblies. Hence amphiphilic self-assembly of semiconductor nanocrystals with heterogeneous compositions are of importance from the viewpoint of band gap engineering to tune their optical and electronic properties. Rather few studies of electrical transport have be developed over these last years, here the state-of-the-art as well as possible challenges in the field of electrical transport through 2D and 3D inorganic nanomaterials are reviewed. Other properties such as optical and mechanical properties remain an open question. Here, new strategies are proposed to understand the optical properties of metallic nanoparticles assembled in 2D and 3D superlattices and mechanical properties in various supracrystal growth processes are proposed.

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Marie-Paule Pileni
Focus Issue Editor
Fondation de la Maison de la Chimie
28, rue Saint Dominique, 75008 Paris, France
E-mail: mppileni@orange.fr

Marie-Paule Pileni is a Distinguished Professor at University and senior scientist at “Commissariat atomique et environnemental” (CEA) She is a member (1999–present) and chair (2004–2010) of Institut Universitaire de France, IUF, which favors the development of high-quality research and interdisciplinary projects among French universities. She has published more than 450 articles with H index of 72 and an average citation per item of 55.8. Her major contributions are: i) Understanding the fundamentals of the kinetics and mechanisms in colloidal solutions, which led her to the creation of inorganic nanocrystals differing by size, distribution, crystalline structure shape as well as to the chemical modification of enzymes. ii) Building up thermodynamically stable states of self-assemblies, both for surfactant molecules (supra-aggregates) and inorganic nanocrystals (supracrystals). iii) Finding collective optical and magnetic properties induced by dipolar interactions and due to the nanocrystal arrangements in 1D, 2D and 3D superlattices. iv) Discovering chemical and physical intrinsic properties due to the crystalline structure of isolated nanocrystals. v) Discovering different physical (vibrational, magnetic, optical) properties of nanocrystal assemblies depending on the crystalline atomic structure of nanoparticles. vi) Developing conceptual analogies between supracrystals and atomic crystalline structures. vii) Solubilization of hydrophobic supracrystals in aqueous solution for biomedical (imaging and hyperthermia) applications Over her career, she received the Langmuir Award of the American Chemical Society (ACS), the lecture award of the Japanese Chemical Society, the research award of the Alexander von Humboldt Foundation in Germany, the Descartes-Huygens Prize jointly delivered by the Institute of France and the Royal Dutch Academy of Sciences; the ´Emilia Valori Prize of the French Academy of Sciences, the Catal´an-Sabatier Lectureship award from the Royal Society of Chemistry of Spain, the Life Achievement award from Journal of Colloid and Interface Science and the Chalmers Lecture award. She is also recipient of the Pierre Sue award of the French Chemical Society (SFC) and of the 2017-ACS-SFC award. In addition, she received the French citation award laureate from the Institute of Scientific Information for most quoted French scientist between 1981 and 1998. She is a member of the Royal Swedish Academy of Engineering Sciences, the European Academy of Science (presidium 2009–2010), the Academia Europaea-The Academy of Europe, a foreign member of The Royal Society of Arts and Sciences in Gothenburg, Fellow of the Royal Society of Chemistry and has a doctorate honoris causa from Chalmers University, Gothenburg, Sweden. She is Commander of the “Ordre National de la Légion d’Honneur”.

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