Advanced photons in science

David Bradley is currently working with Argonne National Laboratory on a series of articles for the annual report of the ANL's Advanced Photon Source. For December's Elemental Discoveries, I will be offering a preview of the advanced science featured in the report.

Advanced Photon SourceElemental Discoveries shows how X-rays can reveal some of the inner secrets of the world around us as seen under the illumination of ANL's Advanced Photon Source:


Are films ferroelectric?
Discipline for gold nanocrystals
Photosynthetic systems
Dissecting the atom
Catalytic clues
SAXS and the water channel
Digging in the dirt
Folding Protein Sensors
X-ray movies


Discipline for gold nanocrystals

by David Bradley

Gold nanocrystals are not the most well-behaved of materials. Admittedly, simply mixing the right ingredients will induce them to self-assemble, but the microscopic and mesoscopic organization of the nanocrystal superlattices is not predictable.

Researchers would like to be able to control the formation of nanocrystal superlattices for a whole range of applications from novel types of catalysts for speeding up useful chemical reactions to building tiny components for applications in optoelectronics and nanotechnology. However, they face a serious problem in trying to tame the structure of the nanocrystal superlattices as they form. They simply cannot predict the effect on structure of the numerous different forces between the clusters of gold atoms and the chemical reagents they use to make the nanocrystals.

Suresh Narayanan and Jin Wang of the Advanced Photon Source and Xiao-Min Lin of Argonne's Materials Science Division and Chemistry Division hope to remedy this situation. They are using time-resolved small-angle x-ray scattering measurements on beamline XOR-1BM to take a closer look at the physical processes taking place as gold nanocrystal superlattices form. They hope that their insights into these complex processes will allow researchers to better control them with a view to making gold nanocrystal superlattices of precise design.

The researchers point out that, in conventional wisdom, gold nanocrystals can condensed on to a substrate surface from evaporating suspension of gold nanoparticles and a reactive organic molecule containing sulfur, a dodecanethiol ligand in organic solvent. As the liquid suspension dries, the nanoparticles coated with the ligands may self-assemble to produce so-called superlattices as the liqands act as a kind of spacer. Other researchers have found that using this approach to nanocrystal construction leads to a wide range of superlattice structures from ordered two-dimensional and three-dimensional patterns to fractal-like aggregates and even structures full of tunnels and channels.

One of the problems to making desirable two-dimensional nanocrystaluperlattices is a misunderstanding regarding the mechanism of their formation. Previous speculations had pointed to the idea that the self-assembly of nanocrystals occurs at the interface between the liquid and the substrate surface. At this interface the nanocrystal particles can move freely at the substrate surface and as the liquid evaporates and de-wets the surface, the superlattice structures are left behind.

Lin and his colleagues have used transmission electron microscopy to show that there is too much order for the de-wet idea to hold true. To prove the point, the team turned to the non-intrusive analytical technique of small-angle x-ray scattering (SAXS). Using this technique, the researchers can watch the formation of nanocrystal superlattices as the nanoparticle suspension droplets evaporates. Their results show that the self-assembly process does not occur at the interface between the liquid and the substrate but at the interface of the liquid surface and the surrounding air. They further demonstrated this to be the case by speeding up the rate of evaporation and showed that the same starting materials could produce either two-dimensional or three-dimensional lattices depending on how quickly the liquid evaporated. The faster rate of evaporation produces 2D structures whereas a slower rate of evaporation allows time for 3D structures to form. The self-assembly of 2D nanocrystals superlattices at the liquid-air interface opens up the possibility of annealling out the defects for allowing ordered 2D superlattices in mesoscopic scale to form, which has not been proven possible in the past.

Source: Suresh Narayanan, 1 Jin Wang, 1 and Xiao-Min Lin2 "Dynamical Self-Assembly of Nanocrystal Superlattices during Colloidal Droplet Evaporation by in situ Small Angle X-Ray Scattering," Phys. Rev. Lett. 93, 135503 (2004).

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