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Self Assembly of Shape Controlled Colloidal Nanocrystals

Final Report Summary - SA-NANO (Self Assembly of Shape Controlled Colloidal Nanocrystals)

The aim of 'Self assembly of shape controlled colloidal nanocrystals' (SA-NANO) was to achieve control and understanding of self assembly of shape controlled colloidal nanocrystals (NCs). NCs have been widely developed during the last decade and already applications in diverse technological fields have been demonstrated, such as light-emitting diodes (LEDs), biological tagging, sensing, photovoltaics, electronics, and catalysis. There were several specific science and technology (S&T) objectives of the SA-NANO STREP:

- The synthesis of a new generation of shape- and composition- controlled nanocrystals with specific recognition elements and their surface functionalisation (Work package 1). Shape-controlled growth with topological control have been developed to grow nanorods and tetrapods with metal, semiconductor, and oxide tips that have served as anchor points. Many of these new hybrid nanocrystals have been functionalised with biomolecules for purposes of self assembly.
- The self assembly of shape-controlled nanocrystals in solution to form chains, propellers and three-dimensional (3D) structures of rods and tetrapods (Work package 2). The specific linking to Au tips on shape-controlled nanocrystals, in some cases using molecular and bio-molecular bonding, has been employed to generate chain-like assemblies of rods and 3D networks of tetrapods. Propeller structures have been realised by a nano-welding approach that involved the coalescence of Au domains on the tips of bullet-shaped nanocrystals.
- The preparation of substrates patterned with nanocrystal templates of several materials as anchoring points, and assembly of shape-controlled nanocrystals on such substrates (Workpackage 3). Patterned substrates have been fabricated with repeating motifs capable of selectively binding shaped nanocrystals. Nanocrystals have been assembled onto these patterned surfaces.
- The realisation of aligned assemblies of nanorods (Workpackage 4). Various methods to align nanorods have been developed including the use of external fields and microfluidics. Deposition of rods on patterned surfaces has been combined with the various alignment techniques.

The study of proximity effects on the electronic and optical properties of shape controlled NCs (Work package 5). Collective properties of nanocrystals in the assemblies have been studied by optical, magnetic and scanning probe techniques. In particular, the modification of the NC level structure due to proximity to neighbouring NCs and the onset of super-crystal effects has been addressed. The effect of alignment leading to anisotropic optical, magnetic and transport ensemble behaviour has been investigated. We have also attempted to study the electronic structure and conduction along rod chains. Theory and modelling of self assembly of shape-controlled nanocrystals (Work package 6). Various theoretical tools have been developed to model the self assembly processes of rods and tetrapods.

We have addressed issues related to the effects of shape, size, anisotropy, solvent and external perturbations on the resulting assemblies. Theoretical calculations have been performed in order to investigate the electronic and the optical properties of isolated and assembled nanocrystals.

End results, intentions for use and impact

The new assemblies of rods and tetrapods, created by self assembly and developed throughout the SA-NANO project, have exhibited novel properties stemming from the interactions between the nano-objects and from their collective behaviour. All these properties have never been investigated before in the case of shape-controlled nanocrystals. The development of controlled and aligned rod assemblies, for instance, has allowed us to examine for the first time the evolution of the level structure and single electron charging energy with the distance between neighbouring nanorods. In many cases we have found that the electronic coupling between NCs, resulting from wave function overlap between nearest neighbours, reduces the charging and confinement energies and modifies the level spectrum, in particular the band-gap, as compared to the isolated NCs. Moreover, in extreme cases such a coupling causes the formation of collective array mini-bands, as demonstrated for InAsNC arrays. Anisotropic conduction is also expected to be observed along versus perpendicular to the NC alignment direction.

These structures, once realised and once collective effects have been elucidated and rationalised, have clearly shown properties and performances that to some extent were predictable and controllable. One example comes from assemblies of semiconductor nanorods. Unlike ordered multilayers of spherical nanocrystals, in which the orientation of each individual nanocrystals is poorly defined, ordered arrays of nanorods have clearly shown coherent and unidirectional orientation of all nanorods along a given direction. This well defined geometrical arrangement, coupled with the anisotropic physical properties of the individual nanorods (i.e. linearly polarised absorption and emission) has been translated into a unique and predictable macroscopic property of the ensemble. This ensemble was, for instance, a flat surface showing highly polarised absorption and emission, but also strong directionality in the emission of light.

It is important to stress that materials and systems with predictable composition and structure will open the way to concrete applications. In the example cited above, one for instance can envisage novel optical detectors and emitters. Assemblies of magnetic nanorods are also expected to yield novel collective magnetic effects. As for what concerns tetrapods, ordered arrays on patterned substrates will facilitate the optical, transport and scanning probe investigation of their unique electronic structure. Tetrapods are being exploited as components in thin-film photovoltaic devices where they are incorporated in a host matrix made of a conductive polymer. Low-cost photovoltaics are today regarded as one of the promising applications of nanocrystals. Needless to say those photovoltaic devices have also tremendous advantages with respect to the environmental impact in energy conversion.

Ordered assemblies of shape-controlled nanocrystals will also be useful in catalysis. Shapecontrolled nanocrystals are grown such that certain crystallographic facets have much larger surface area than others. Such facets might have higher catalytic activity towards the photodegradation of some pollutants. The possibility of growing composite nanocrystals, such as the metal-tipped nanorods and tetrapods, will enhance this activity even further. In these materials separate redox processes will likely occur in different regions of the nanocrystal, thus vastly enhancing the catalytic activity, as has been demonstrated in metal-patched TiO2 nanocrystals. Networks of tetrapods (or of other 3D shaped nanocrystals), either free-standing or supported on a surface, could then serve as media for the rapid degradation of pollutants.

As can be clearly seen, the goals of SA-NANO aim to transform the versatility of the individual objects to large scale ensembles that hold a considerable potential for industrial applications.