Team
Synthesis and Characterisation of functional assembled Nanostructures
Colloidal nanoparticles of noble metal, semiconducting, and magnetic materials have a broad spectrum of applications, from sensors and optics to biomedical imaging and treatment techniques, to heterogeneous catalysis and photocatalysis. Some of these applications require the immobilization of nanoparticles in so-called assemblies. Depending on the desired material function (mechanical and chemical stability, arrangement geometry and, in particular, interparticle interactions), assemblies can be template-supported or template-free, ordered or disordered and macroscopic or microscopic. For this purpose, control of particle arrangement in the respective architecture must be achieved. Special assemblies also often have interesting new characteristics.
Nanocrystals: Building blocks for Functional Nanostructures
Shape, size and property control of colloidal nanocrystals
Our research focuses on the controlled assembly of nanoparticles, as described in the other chapters below. In order to achieve such nanostructures with the highest functionality, the control over the properties of the "building blocks", namely the nanoparticles, is unavoidable. In particular, it is about a precise control of the following nanoparticle properties:
- Quantum size effect in semiconductor nanoparticles
- Superparamagnetism in nanoparticles of magnetic matter
- Localized surface plasmon resonance in noble metal nanoparticles
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Sven Getschmann
Synthesis strategies for tailored nanocrystals
These properties must at first be precisely controlled in the nanoparticles in order to achieve the desired material functions in the subsequent assembly. For this purpose, the nanoparticles must be produced in size, shape and material composition highly similar. For the synthesis of such tailored nanocrystals we use modern wet chemical methods such as
- Synthesis in high-boiling solvents
- "Hot Injection"-Method
- Seed-induced Growth
Subsequent surface modification with polymer shells or by so-called ligand exchange reactions are possible as well.
Macroscopic assemblies of tailored nanocrystals
Aerogels und hydrogels of colloidal nanocrystals
Aerogels and hydrogels are ultralight macroscopic materials made from tailored nanoparticles. They are composed of controlled cross-linked nanoparticle building blocks and result in a monolithic and nanoporous structure with a large specific surface. Our research focus is to synthesize such gel structures with the functions of the nanoparticle building blocks and thus to make the nanoscopic world with its special physical properties accessible to the "everyday" macroscopic world. Under special conditions, it is even possible to synthesize aerogels whose physical properties are new: for example, we have been able to detect charge carrier transport and increased exciton lifetime in the nanoparticle networks.
Cryoaerogels of colloidal Nanocrystals
A special synthesis strategy developed by us for the preparation of gel structures from colloidal nanoparticles is cryoaerogelation. Here, the effect is used that during rapid freezing of aqueous nanoparticle solutions they are not incorporated into the ice crystals but accumulate between the ice crystal spaces. By using sufficiently high nanoparticle concentrations, continuous networks of nanoparticles form in the ice-crystal boundaries, which remain even after subsequent lyophilization. This type of aerogels has the advantage that it does not depend on the surface chemistry or type of nanoparticles and is thus extremely versatile.
Microscopic assemblies of tailored nanocrystals
EN [Bild: Mikroskopische Assemblierung maßgeschneiderter Nanokristalle]
Microscopic Assemblies of superparamagnetic Nanocrystals
Superparamagnetic nanoparticles can be used to produce microscopically sized assemblies that are colloidally soluble again. Such structures can be further modified by polymers or by other nanoparticles to multifunctional colloidal objects. Packing density, interparticle distance, nanoparticle building block sizes, type of polymer, etc. play important roles in the expected physicochemical properties. Applications of such colloidal assemblies exist for example in magnetic resonance imaging as a contrast agent, for magnetically-guided drug delivery as well as for hyperthermic therapies in nanobiomedicine.
Microscopic assemblies of semiconductor nanocrystals
Semiconductor nanoparticles can also be used to synthesize microscopically sized assemblies. In particular, the shape of the nanoparticles used have an influence on the resulting assembly geometry. For example, we are interested in how or in such materials charge carriers or excitons can migrate from particle to particle within an object. For this we use spectroelectrochemical methods.
Structure-property correlation of functional nanostructures
Spectroelectrochemistry and electrochemistry
In the field of spectroelectroscopy, we work with semiconductor nanoparticles and their assemblies. On the one hand, spectroelectrochemistry provides us with information about the electronic states and charge carrier transfer processes. On the other hand, we are also working to represent functional electrodes that have long-term potential in the field of photoelectrochemical sensor technology through the targeted construction of nanoparticle assemblies.
Because noble metal nanoparticles and especially their aerogel-like assemblies are used in applications in catalysis and electrocatalysis, some of our research is focused on researching the electrochemical properties of these functional nanostructures.
Absorption, extinction and emission spectroscopy
For a detailed structure-property correlation of the functional nanostructures a spectroscopic investigation is necessary. We are interested in the optical properties in the UV-Vis-NIR range of the electromagnetic spectrum. Measurements in transmission as well as diffuse reflection are possible to obtain the absorption, extinction and emission spectra. In addition, emission lifetime measurements in the μs to ps range are often of interest in order to be able to differentiate between radiative and nonradiative charge carrier recombination processes, as well as spectroscopic analyses at cryogenic temperatures.
Structural characterization
In addition to the characterization methods described above, transmission and scanning electron microscopy as well as X-ray diffraction are used as standard methods for the structural characterization of our functional nanostructures.
