VOLUME 8, NUMBER 8
AUGUST 1996
© Copyright 1996 by the American Chemical Society
Editorial Preface to the Special Issue Small is beautiful. Ultimately, all materials and living organisms have their origins in the collective assembly of a small number of atoms or molecules. Since the time of the discovery of the optical microscope in the sixteenth century, humans have been fascinated by the world within, the secrets of the cell, and the building blocks of matter. Microscopes and other tools gave more and more power to see and understand that world, but it turned out to be much more difficult to change, manipulate, and sculpt matter at nanoscopic dimensions for certain purposes. Following Feynman’s challenge that “there is plenty of room at the bottom” [see: Mehra, J. The Beat of a Different Drum. The Life and Science of Richard Feynman; Clarendon Press: Oxford, 1994; p 441], the semiconductor industry became a master in sculpting silicon and other materials with extraordinary precision and efficiency. This feat is achieved by “chiseling” structures at submicron resolution from blocks of raw material using the tools of lithography and electron beam writing. But how small are the things the chiselers can sculpt? In a very different realm, the disciples of the alchemists have learned how to transform one kind of matter into another, at the molecular level, leading to the vast enterprise of chemical industry. But how large are the things chemists can sculpt? To put it in the words of Jean-Marie Lehn, “beyond molecular chemistry, based on the covalent bond, lies the field of supramolecular chemistry, the aim of which is to gain control over the intermolecular bond.” [Lehn, J.-M. Foreword, Comprehensive Supramolecular Chemistry; Atwood, J. L., et al., Eds.; Pergamon: Oxford, 1996]. We are witnessing an exciting period, where the two worlds are beginning to move together, where chemists begin to control the S0897-4756(96)00902-7 CCC: $12.00
noncovalent bonds, to build larger and more complex objects from the bottom up, to connect atomistic and continuum scales. This Special Issue of Chemistry of Materials is dedicated to the subject of Nanostructured Materials. Judging from the explosion of symposia and publications and from the enthusiastic response to this issue, an enormous interest is developing in this subject. Noncovalent forces are utilized to assemble an astounding variety of molecules and atoms into more complex structures, ranging from the construction of self-assembled monoand multilayers with various functionalities (using dispersion forces) over multilayers of charged components such as polyelectrolytes (using also electrostatic forces), intercalation in preassembled layered hosts (sometimes using electron transfer), construction of chelate complexes in nanoscale zeolite cages, to the elegant structural control afforded by the liquid-crystalline assemblies used in the synthesis of metal oxide mesostructures. This issue offers a timely overview on many of the topics pursued in the quest for greater structural control at the nanometer level. The subject areas are organized, within the usual categories of communications, reviews, and articles, by increasing complexity of the structures involved. Thus, two-dimensional structures are covered first, followed by organic, amorphous/hybrid and cluster materials, intercalation compounds, and inorganic three-dimensional networks. The physical properties and specific functions of nanostructured materials are also addressed. What are the main strategies that have been developed so far to control structure in extended systems? © 1996 American Chemical Society
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A delicate balance of dispersive forces between longchain hydrocarbons and binding forces of surface attachment groups such as thiols on gold is being exploited to form self-assembled mono- and multilayers with many different functionalities and building blocks. In an extension of this approach, electrostatic interactions assist in the assembly of multilayers from polyelectrolytes and particulates or sheet structures. Films of inorganic systems such as zeolites can be prepared according to different mechanisms involving covalent bonding, including growth on precursor zeolite/alumina films, or on organic monolayers presenting functional groups at the interface. These strategies are inspired by biomineralization processes. Total control of atomic layering can be achieved by sequential deposition of reactants from the gas phase; the resulting structures can be used to study nucleation and subsequent reaction pathways and to generate new compounds and heterostructure films. Hydrogen bonding is one of the most ubiquitous “noncovalent” forces leading to intricate assemblies of organic molecules. Structural control comes from the ability to arrange hydrogen-bond donors and acceptors on the building block molecules with the powerful synthetic methods of organic chemistry. Complex but amorphous hybrid materials can be assembled by linking clusters such as silsesquioxanes with organic bridges, via either organic or inorganic (sol-gel) linkages. Hybrid systems can also be formed by mineralization in polymers. Entrapment in polar polymers permits the stabilization of nanosize metal or metal oxide particles, a method related to arrested precipitation of semiconductor clusters by means of strong ligands. The use of preexisting host structures to intercalate (functional) guests is one of the most popular means to
create new nanoscale composites, for instance with polymers showing ionic and electronic conductivity. An important means of forming complex solids is the “template” approach. Usually an organic molecule is used to impart certain structural features to the (porous) solid under construction. In some cases, the noncovalent interactions can exert exquisite control over the structure of the emerging host. A key example is the new family of mesoporous materials that is formed in a cooperative assembly process from organic surfactants and inorganic building blocks that cross-link during synthesis. The template concept can also be turned around to use the hollow host as a template to structure guests such as metal wires, etc. Other tools for structure control include the use of local bonding geometry when assembling building blocks in three dimensions, for instance in coordination polymers linking transitionmetal sites with appropriate ligands. It is obvious that the exploration of nanostructured materials is just at the beginning of an exciting journey. Numerous physical properties and potential applications are being studied with nanostructured materials, including structural, nonlinear optical, magnetic, and electronic effects, as well as catalytic reactions. We hope that this special issue will contribute to the growing interest in this fascinating field. Thomas Bein Purdue University Lafayette, Indiana Galen D. Stucky University of California Santa Barbara CM960902S
