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Posted: Nov 12, 2007
Nanotechnology fabrication techniques move towards multifunctional architectures
(Nanowerk Spotlight) Some pundits writing about nanotechnology get carried away by their own hype and talk about self-assembly as if bottom-up fabrication technologies, where molecules get assembled into everyday products, are just around the corner. We took a swing at this in our Spotlight from a few days ago (Nanotechnology 'pencil sharpeners' add to researchers' nanofabrication toolbox).
Today we bring you another example from the cold reality of the labs that makes clear how early stages this whole field of self-assembly really is. Today, when researchers - with both feet firmly on the ground - talk about self-assembly they mostly talk about template-assisted nanocrystal superlattices in the form of planar thin films.
Bottomline is that even the controllable fabrication of highly ordered homogeneous nanostructures on surfaces remains a difficult challenge. And IBM's much touted 'self-assembling nanotechnology' (see: IBM applies self-assembling nanotechnology to conventional chip manufacturing) is nothing more than a patterning process that creates a film with trillions of holes around the on-chip wiring. Moving from a planar geometry of self-assembled nanoscale building blocks such as nanocrystals or nanotubes to a free-standing, three-dimensional multifunctional architecture is not a trivial undertaking. Researchers are just about to make the first steps to such multifunctional (still nanoscale) hierarchical architectures that both retain the properties of the nanocrystals and offer multifunctionality.
"The extension of nanocrystal assemblies to arbitrary geometries requires the development of programmed interparticle interactions or the development of a robust template- assisted self-assembly strategy" Dr. Geoffrey Ozin explains to Nanowerk. "In our recent work, we address this problem by demonstrating with one-dimensional and three-dimensional templates how evaporation-induced self-assembly and nanocrystal plasma polymerization allow one to overcome the limitations of all previous methods and obtain free-standing, mechanically stable, multifunctional architectures entirely composed of nanocrystals of choice."
Ozin, a professor in the Department of Chemistry at the University of Toronto, and his group have just reported promising results of a new
casting technique, which allows for the preparation of virtually arbitrary architectures entirely made of nanocrystals in a way that is compatible with flexible substrates. Their findings have been published in a paper in the November 2, 2007 online edition of Nano Letters ("Plasma within Templates: Molding Flexible Nanocrystal Solids into Multifunctional Architectures").
"We found that it is possible to create self-supporting arbitrary architectures entirely made of nanocrystals that still possess all the distinct properties of nanocrystals" Ludovico Cademartiri, a PhD student in Ozin's group, and a co-author of the above paper, tells Nanowerk. "Such architectures also display functionality originating from their geometry. It was known that nanocrystal superlattices could be formed on flat substrates but very rarely any real multifunctionality has been shown. And this is the first case to our knowledge in which such multifunctionality is embedded in functional architectures like photonic crystals."
In addition, the specific technique used by the UToronto scientists allows the removal of the organic fraction from the colloidal nanocrystals thus resulting in mostly inorganic solids.
The researchers point out that their methodology is very simple: "the template (a and e in the image below) is immersed in the nanocrystal dispersion (b,f); the drying of the solvent leads to a complete infiltration of the template (Figure c,g); the nanocrystals are then consolidated within the template by using nanocrystal plasma polymerization and then the template is dissolved (d). In the case of polymeric templates, the plasma processing consolidates the nanocrystals and removes the template at the same time.
Template-assisted nanocrystal self-assembly: (a-d) these panels show infiltration of an alumina template with colloidal nanocrystals. The template (a) is attached to a glass tube shown in (b). The tube is sealed using a rubber septum to restrict solvent evaporation to that which takes place through the template nanochannels, and then the colloidal nanocrystal dispersion is injected into the tube. As the solvent evaporates, the nanocrystals are pulled into and pack inside the template channels yielding the infiltrated template shown in (c). Air plasma nanocrystal polymerization and selective removal of the template yield free-standing nanocrystal nanorods (d). (e-h) These panels show infiltration of an opal with colloidal nanocrystals. First, the opal is deposited onto a substrate (e) and subsequently submerged in a dispersion of colloidal nanocrystals (f). As the solvent evaporates, the nanocrystals pack inside the voids within the opal as shown in (g). Selective removal of the opal template yields a freestanding nanocrystal inverse opal (h). (Reprinted with permission from American Chemical Society)
Cademartiri says that these findings are a relevant step towards a new class of materials entirely composed of nanoscale building blocks, where the macroscopic properties can be designed ahead: "Nanocomposites with unprecedented combinations of properties can be envisioned, as we tried to show in our work."
Ozin's group is contemplating the development of more complex architectures with a more strict control on some geometrical parameters.
"For example" says Cademartiri, "we showed nanocrystal superlattice nanorods: now it would be interesting to give them a core-shell architecture or even a bar-coded architecture."
He notes that the groups of Thomas Mallouk at PennState and Chad Mirkin at Northwestern University have done some amazing work on structures of that kind, demonstrating their potential for bioassays, for example.
Cademartiri concludes that the main challenges mostly lies in the study and the development of a robust scientific framework with
which to predict and control the deposition of nanocrystals on the template. "Once the nanocrystals are were they are supposed to be, the plasma will do the rest."