Welcome to one of our first guest columns, where active researchers can share their views on topics relevant to materials science. To start us off, we invited Professor Geoffrey Ozin of the University of Toronto to share his honest opinion on the current state of nanoscience research. He took up the challenge and provided us with two inaugural articles. Here, he shares his views on the future of the promising nanomaterials, while in the other article he discusses the immense amount of reported nanomaterials.
After a century or more of traditional materials research, which in one form or another ended up in products and processes that influence our everyday lives, there has for the past twenty-five years been an intense global effort to remodel these materials through chemistry from the macroscopic scale where they display conventional bulk properties to the nanoscale where materials properties, chemical and physical, are often defined by quantum mechanical scaling laws and show anything but conventional behaviour.
This intermediate state of spatially confined nanomatter lies in the fuzzy regime straddled on one side by molecules with their chemistry and molecular orbitals and on the other side by materials with their physics and electronic bands. It is still not very clear how to demarcate these three regimes!
All of today’s effort in nano is directed towards a search, understanding and exploitation of chemical and physical phenomenon that are unique to this small scale, a key phase in the drive to re-invent the world of materials, products and processes, a means to gain a competitive edge in a knowledge-based, high-technology, rapidly expanding, global marketplace.
These days, all sorts of nanomaterials are being synthesized and self-assembled into new structures over multiple length scales in much the same way atoms and molecules have been coerced through chemistry to form new compounds for over a century. And just as the library of known molecules today has reached countless millions it looks like nanoscale materials are well on the way to match their molecular-scale brethren in proportion. But it is not quite as simple as that.
While molecules are atom precise and structure perfect and offer predictable properties and function, nothing can be further from the truth for most nanomaterials.
Yet even imperfect nanomaterials with respect to inhomogeneity of size, shape and surface, offer more than just a length-scale advantage over bulk analogues. We tend to be impressed with the effects of quantum and spatial confinement on the electronic, optical, magnetic and mechanical properties of nanomaterials yet surface physicochemical phenomenon can be where the real action in synthesis and structure, property, function and utility lies. The surface advantage of nanomaterials lies in their disproportionately large surface area compared to its volume (S/V). The surface challenge is to control, understand and exploit structure and chemistry, defects and reconstruction of nanomaterials surfaces at the atomic and molecular scale.
Beneficial surface effects at the nanoscale can for example be seen in enhanced electron and phonon scattering in nanothermoelectrics, conduction electron resonances in nanoplasmonics, fast-ion transport in nanoionics, chemical activity and selectivity in nanocatalysts, molecule recognition and detection in nanosensors, ion and molecule diffusion in nanofluidics, mechanics of nanometals and nanoalloys, electron–hole separation and charge transport in nanophotovoltaics, and electroluminescent quantum yield in nano-optoelectronics.
The surface advantage of nanomaterials also provides notable benefits when used as precursors in a solid-state materials synthesis through larger contact area and smaller diffusion lengths, which synergistically reduce reaction temperature, increase reaction rate and boost product yield and purity. In a heterogeneous materials synthesis involving solid nanomaterial and solution-phase precursors the surface advantage is manifest as a slow and steady release of reactants, which can facilitate controlled nucleation and growth of a product nanomaterial. The surface advantage is also appreciated in novel ion-exchange, galvanostatic and Kirkendall reactions used to tailor the composition, modify the structure and make hollow capsules for storage and release of pharmaceuticals.
But as we walk the nano path into the future it is important to ask, where is it taking us? Is our present mindset stifling the true potential of nano? Can we still take what we have and perfect and utilize it to develop nanotechnology that works for the greater good of science and society? Or is the field about to self-destruct?
I think the gradual transition of century-old colloid chemistry to today’s nanochemistry has provided us with better and better size and shape control of metal, semiconductor and insulator-based colloidal particles. It has provided us with the know-how to modify their surfaces and create stable dispersions of these colloids, so vital for the reliable and reproducible production of colloid morphology exemplified by colloidal films, multilayers and patterns as well as other colloid forms like sheets and spheres, rods and wires, made from these colloids. These are key milestones in the quest for cost-effective and safe manufacturing solutions through the processing of colloids.
But we are not there yet with respect to the degree of perfection of these colloids (e.g., size- and shape-specific synthesis and size- and shape-selective separation), single-crystal X-ray structures of archetype colloids (e.g., nanocrystals or superlattices), the quality of colloid organization (e.g., self-assembly, co-assembly or directed assembly) into hierarchical constructs by design, and the monumental task of scaling well-defined colloids (e.g., hundreds of kilograms) for industrial manufacturing of colloid-based good nano stuff.
These are important challenges for bottom-up nanomaterials, for nanochemistry and for nanoscience, because the performance of a wide range of solid-state products will depend on how well we are able to manipulate and control, electrons and holes, photons and excitons, phonons and plasmons, and electron and nuclear spins in periodic or aperiodic assemblies of colloidal nanocrystals, at least as well as it is done in the parent solid-state bulk materials.
And the success of this endeavour will be predicated not just upon how perfect we can make our nanocrystals and how well we can organize them into predetermined forms, but also on how well we can chemically command that teeny-weeny space between nanocrystals, which controls their collective interactions and their translation into the electrical, optical, magnetic and mechanical properties so central to the function and utility of any nanocrystal-based product exemplified by nanostructured solar cells and batteries, light-emitting diodes and lasers, photodetectors and sensors.
|One of Prof. Ozin’s textbooks, Concepts of Nanochemistry, co-authored with L. Cademartiri, and published by Wiley-VCH. Along with A. C. Arsenault, they also wrote Nanochemistry – A Chemical Approach to Nanomaterials (published by the Royal Society of Chemistry).|
A closing thought to express my optimism about recent developments in the field of Nanochemistry. I have said in my paper, Nanochemistry: Synthesis in Diminishing Dimensions, and expressed in two recent graduate and undergraduate textbooks on the subject of Nanochemistry, that synthetic chemists pride themselves on being able to synthesize perfect objects having nanometre-scale dimensions. They have worked hard for over a hundred years to hone their skills at making incredibly beautiful and important atom and structure perfect molecules, clusters and polymers.
And just to put things in perspective, they have been working equally hard for just the past twenty years or so to match these molecular-scale accomplishments at the nanoscale but now learning how to achieve comparable fidelity over the size, shape and surface, and self-assembly of myriad new nanomaterials.
Making nanomaterials through nanochemistry is a large and important field, there is still much to do and we have taken the first step!
Geoffrey A. Ozin
Materials Chemistry and Nanochemistry Research Group
Center for Inorganic and Polymeric Nanomaterials, Chemistry Department
80 St. George Street, University of Toronto, Toronto, ON, M5S 3H6, Canada
E-mail: [email protected]
The cover image of this article shows the co-assembly of dendrimer silica and structure-directing organic templates to form periodic mesoporous dendrisilicas. Created by Ludovico Cademartiri.