Project Objectives

The broad goal of the project SA-NANO (Self Assembly of shape controlled NANOcrystals) is to achieve unprecedented control and understanding of self assembly of shape controlled colloidal nanocrystals (NCs). NCs are at the forefront of the present revolution of nanoscience and nanotechnology. NCs are highly versatile materials because their physico-chemical properties can be finely tuned by controlling the NCs size, shape and composition. NCs are chemically accessible and can be cheaply processed using self assembly concepts. As a consequence of these appealing properties NCs have been widely developed during the last decade and already applications in diverse technological fields have been demonstrated. This includes applications in light-emitting diodes (LEDs),(1) biological tagging,(2-7) sensing, (8-10) photovoltaics11, (12) electronics,(13) and catalysis.(14)

Recently, new synthetic methods were developed for growing NCs with elaborate shapes such as rods(15-17) and tetrapods (18,19) that represent the state of the art in nanocrystal research today. Rods and tetrapods, generally termed as 'shape controlled nanocrystals', with diameters in the range of 2-10 nm and lengths spanning 10-100 nm, are the focus of this project. The opportunity to tune the nanocrystal shape opens wide horizons for new properties stemming from the geometry of the artificial structures. It has already been shown that colloidal semiconductor nanorods, unlike spherical nanocrystals, emit linearly polarized light.(20,21) Rods also exhibit advantageous behavior compared to spherical nanocrystals in several devices, such as lower lasing threshold, (22) and improved power conversion efficiency in solar cells.(11) Tetrapods are another novel structure where four nanorods branch out at tetrahedral angles from a central 'branch point'. Tetrapods, yet hardly explored, prsent promising potential for building blocks in solar cells11, (12) and as field emitters.

For a breakthrough in the science and technology of NCs the understanding on how to organize them into complex, controllable geometries over large areas using bottom-up self assembly approaches, is of tremendous importance. This is an essential part of implementing shape-controlled nanocrystals in diverse applications where controlled assembly is required. It is very clear that only a self assembly approach is going to be applicable to create complex systems with millions or even billions of nano-components, and this is where the project will make a significant contribution. Such assemblies, which will be developed in this project, are interesting both in fundamental research, as they present a new platform on chemical and physical interactions of proximal nanocrystals, and for practical applications as they provide a transition path to the engineering of materials and the fabrication of functional devices.


(Figure above: Nanocrystals carrying anchoring points might be used to build complex nanoscale assemblies, using for instance biomolecules as glue. a) These assemblies could be realized directly in solution, such as oligomers and propeller-like structures. b) Anchoring of gold-tipped nanocrystals on patterned surfaces is yet another possibility to arrange nanocrystals in programmed locations and orientations.)

To this end we will borrow from concepts employed in self assembly of biological systems where highly complex structures are formed through specific chemical recognition mechanisms using elaborate building blocks.(23) A breakthrough innovation to be developed will be the creation of versatile hybrid metal/semiconductor rods and tetrapods with selective attachment properties analogous to the biological systems. Here the functionality of the nanostructure will be provided by one portion of the inorganic unit (e.g. the semiconductor part) and the recognition/assembly part will be afforded by another inorganic portion of the nanocrystal, located at specific positions of the nanocrystal unit, such as a metal tip (see Figure). Such tips provide natural anchor points for the self assembly of rods and tetrapods and open up new possibilities to create unique architectures that cannot be realized for dots or wires. A similar concept will be implemented for magnetic rods where, for example, Au tips will be grown onto Co nanorods.

The anchor tips will be used to self-assemble chains of nanorods as well as propeller structures from the combination of a tetrapod with rods. Three dimensional networks of tetrapods will also be targeted. Further, in analogy to the key-lock recognition of binding a ligand to a receptor such as substrate to an enzyme or an antibody to an antigen, we will develop unique surfaces with patterned anchor points tuned specifically to match precise rod or tetrapod dimensions. Tetrapods and rods with anchoring tips will then self assemble and lock onto the pre-defined and organized anchor points (see Figure). By tuning the spacing and packing of the anchor points on the surface, we will tune the geometry and distances between the deposited nanostructures, resulting in unprecedented controlled deposition. Additionally, new methods for aligning rods and chains of nanorods will be developed, which is a critical aspect for future device application with nanorods.

Theoretical modeling of self assembly of shape-controlled nanocrystals will be used throughout the implementation to feedback the experimental developments and to gain deeper understanding of the self assembly process. While significant theoretical work was successfully applied to spherical nanocrystals, (24,25) the treatment of self assembly of complex shapes such as rods and tetrapods is a great theoretical challenge. To meet this challenge, novel theoretical models and tools will be developed. We expect that theory will contribute to this project in much the same way that it has to the assembly of nanoparticles.(25)

Moreover theoretical methods will be developed and applied to study the electronic and optical properties of isolate and assembled nanocrystals. While the electronic structure of spheres and rods have been investigated (26-29), shape and size effects of tetrapods are not yet fully explored (30). Interactions of linked and closely packed nanocrystals can build up new collective properties that we will be investigated by experimental and theoretical methods.

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