For the latter half of the “Solid State 20th Century” materials science has been the engine that propelled technology. As we enter the “Materials 21st Century” it is abundantly clear that the insatiable demand for new materials for emerging technologies is driving materials synthesis and change. Materials chemistry will play a central role in this endeavor through the creation of materials with structures and properties able to meet the demands required by up-and-coming technologies.
In our recent research we have adopted a far-sighted and innovative strategy to the discovery of new materials. It takes solid-state chemistry beyond fifty years of thermodynamic phases and microscale structures, to a new era of self-assembly chemistry focused on metastable phases, mesoscale and macroscale structures, with accessible surfaces and well-defined interfaces that together determine function and utility. It is an interdisciplinary approach that combines synthesis, solid-state architecture and functional hierarchy to create an innovative strategy for materials chemistry in the new millennium.
The attractive feature of this approach is the ability to assemble complex structures rationally from designed modular components and integrate them into self-assembling constructions for a range of perceived applications. By creating a series of purposeful strategies it is believed that truly revolutionary advances in materials science and technology can result from this approach.
ROOM AT THE TOP AND BOTTOM
Not so long ago in materials chemistry it seemed that there was only “room at the bottom”. The trend was to synthesize and organize nanoscopic materials. As we enter the new millennium it is becoming abundantly clear that there is also “room at the top”. What has changed and why is this important?
We believe that the paradigmatic shift comes from the realization that self-assembly, templating, patterning, capping, layering and molding methods have expanded the materials chemist’s tool-box to include synthesis and organization of materials at “all’ length scales and in “all” three spatial dimensions. Hierarchy has been introduced into materials chemistry and purely synthetic integrated chemical systems that are designed to achieve a particular function are becoming a reality.
This augurs well for the development of materials, composites and systems with novel properties, new functions and perceived value in a range of applications in the biomedical, pharmaceutical, aerospace, automotive, construction, energy, electronics and photonics user sectors.
SELF-ASSEMBLING MATERIALS
Our research group’s story is about interfaces between organics and inorganics and how they can be controlled synthetically at the molecular level to produce composite materials in which structure is prescribed from angstrom to centimeter length scales. The construction kit that we use consists of complementary organics and inorganics that spontaneously assemble through lock-and-key intermolecular interactions. The driving forces for molecular recognition and spontaneous organization are quite varied and can be based upon ionic, covalent, hydrogen, non-covalent, metal-ligand, capillary, elastic and colloidal bonding interactions, which may result in structures and properties not found in the individual components.
In this context, self-assembly may be viewed in terms of a map of bonding forces that operate between building blocks and over different length scales. In a self-organizing system, basic construction-units spontaneously associate to form a particular structure, the architecture of which is solely determined by the bonding properties and shapes of the individual components. The system proceeds towards a state of lower free energy and greater structural stability. Self-assembly is usually entropically driven in an aqueous system, where association of modules is accompanied by exclusion of ordered water molecules.
MATERIALS COMPLEXITY AND HIERARCHY
A feature of self-assembly is hierarchy, where primary building blocks associate into more complex secondary structures that are integrated into the next size level in the hierarchy. This organizational scheme continues until the highest level in the hierarchy is reached. These hierarchical constructions may exhibit unique properties that are not found in the individual components. Complexity and hierarchy is a characteristic of many self-assembling biological structures and is beginning to emerge as a hallmark of supramolecular materials. Self-assembly is considered to be distinct from template-directed assembly, which involves structure-directing additives, often organics or polymers, in addition to the constituent building-units, which may be inorganics. The template can serve to fill space, balance charge and direct the formation of a specific structure. In this definition, template assembly is synonymous with co-assembly and distinct from self-assembly.
SELF-ASSEMBLING MATERIALS OVER MULTIPLE LENGTH SCALES
Simple, elegant and robust attributes of self-assembly are now being combined with the powerful methods of inorganic and solid state chemistry to create supramolecular materials with unprecedented structures, compositions and morphologies. The paradigmatic shift of utilizing organics for templating, capping, layering, wiring, patterning and molding inorganics is having a revolutionary effect on the field of materials research. This is because it enables, for the first time, self-assembly of most elemental compositions of the periodic table but without the usual restriction of the dimension of structural components. This is facilitating purely synthetic approaches to hierarchical systems with the construction pieces integrated over micro-, meso- and macroscopic length scales.
This kind of ‘panoscopic’ synthesis, so to speak, has up until very recently been unparalleled in the field of solid state and materials chemistry. By combining the methodologies of self-assembly and microfabrication it has become feasible to assume the challenge of self-organizing and interconnecting functional organic, bioorganic, inorganic, organometallic and polymeric chemical components into integrated electronic, photonic, mechanical, analytical and chemical systems for future ‘panoscale’ (panoscale: of any sizes, pano, L) devices.
Part of the motivation stems from the notion that the architecture of complex macrosystems in biology and engineering physics are generally based on hierarchical building principles, that is, smaller units are assembled into larger ones, which in turn are organized at a higher dimension. This construction process is continued until the highest level of structural complexity in the hierarchy has been attained.
The hallmark of an integrated chemical, physical or biological system is the assembly of components into a particular architecture that performs a certain function. In the cell of a higher green plant the photosynthetic chloroplast and oxidative phosphorylation mitochondrion machines are perhaps two of the most impressive examples of functional integrated biological systems.
Within the body of a computer the atoms are assembled into insulators, semiconductors and metals, dopants, junctions, metal leads and contacts. These are the building blocks that constitute the transistors, diodes and capacitors of the integrated circuits, which comprise circuit boards and sub-assemblies of an integrated microelectronic system.
Familiar integrated chemical systems include heterogeneous catalysts, photoelectrochemical cells, solid state lithium batteries, hydrogen- oxygen fuel cells, instant color photographic film, sensors and chromatographic stationary phases. The ability to self-assemble diverse kinds of materials over “all” length scales and spatial dimensions has taken synthetic chemistry to a new level of structural complexity that begins to match those found in biology and engineering physics. It is now feasible to devise strategies for organizing, patterning and linking chemical components into functional architectures that were not previously accessible purely through synthesis.
Representative examples taken from the recent literature include, layer-by-layer assembly of a thin film Zerner diode from conducting polymers and monodispersed capped semiconductor nanoclusters, a metal-insulator-metal nanocluster-insulator-metal (MINIM) single electron transistor (SET), a multicolor pixel voltage-controllable semiconductor cluster-luminescent polymer light emitting diode (LED) an all-plastic field effect transistor driven light emitting diode (FET-LED), and a high density rechargeable ultrathin graphite oxide nanoplatelet polyelectrolyte lithium ion battery.
These few examples serve to demonstrate the power and versatility of a self-assembly materials chemistry approach to “panostructured” integrated chemical systems that perform a specific function. “Panoscopic” synthesis can be viewed in terms of molecular level control of interfaces between organics and inorganics. This allows one to embrace molecular, supramolecular, macromolecular, and colloidal crystalline assemblies under the umbrella of organic template-based assembly, facilitating the synthesis of inorganic materials over multiple length scales, with structural features that may span angstroms to microns.
Through a creative fusion of organic templating, inorganic chemistry, soft lithographic patterning and micromolding methodologies, it is feasible to create functional hierarchical materials. Entirely through chemistry one can plan how to synthesize, self-organize and interconnect different kinds of materials over “all” length scales to create an integrated chemical system with single or multiple functions. Our recent research illustrates where “panoscale” synthesis may find “room at the bottom as well as the top” of the new materials world.
VALUE OF SELF-ASSEMBLING PANOSCOPIC MATERIALS
Ultimately, the scientific and technological impact of any new class of materials depends on the ability to control the size, morphology and aggregate structure of primary particles. Self-assembling “panoscopic” materials are no exception in this respect. They introduce hierarchy into materials chemistry and bode well for the development of materials and composites with novel properties, new functions and perceived utility in a range of applications in the biomedical, pharmaceutical, aerospace, automotive, construction, energy, electronics and photonics sectors.
Possibilities for “panostructured” materials include large molecule catalysis, membrane separation and sensing, battery electrolytes, low dielectric electronics packaging, chiral separation stationary phases, bone implants, chemical delivery vehicles, and toxic clean-up of water streams.
The future looks very bright for self-assembling “panoscopic materials” and our research group is actively working at the frontier of this new and exciting direction in materials chemistry .