Semiconductor nanostructures such as CdSe quantum dots and colloidal nanoplatelets exhibit remarkable optical properties, making them interesting for applications in optoelectronics and photocatalysis. For both areas of application a detailed understanding of the electronic structure is essential to achieve highly efficient devices. The electronic structure can be probed using the fact that optical properties of semiconductor nanoparticles are found to be extremely sensitive to the presence of excess charges that can for instance be generated by means of an electrochemical charge transfer via an electrode. Here we present the use of EMAS as a versatile spectroelectrochemical method to obtain absolute band edge positions of CdSe nanostructures versus a well-defined reference electrode under ambient conditions. In this, the spectral properties of the nanoparticles are monitored with respect to an applied electrochemical potential. We developed a bleaching model that yields the lowest electronic state in the conduction band of the nanostructures. A change in the band edge positions caused by quantum confinement is shown both for CdSe quantum dots and for colloidal nanoplatelets. In the case of CdSe quantum dots these findings are in good agreement with tight binding calculations. The method presented is not limited to CdSe nanostructures but can be used as a universal tool. Hence, this technique allows the determination of absolute band edge positions of a large variety of materials used in various applications

In der vorgelegten Arbeit werden sowohl effiziente als auch einfache Modifikationsansätze zur funktionalen Polymerumhüllung von Gibbsit-Plättchen präsentiert. Die plättchen-förmige Morphologie bleibt dabei nach der Polymerumhüllung erhalten. Im ersten Teil wird ein einfacher Ansatz zur Synthese von anisotropen, plättchen-förmigen Gibbsit-Polydopamin (G-PDA) Partikeln vorgestellt. Au NPs von kontrollierbarer Größe wurden auf der G-PDA Partikeloberfläche gebildet. Diese zeigten katalytische Aktivität zur Reduktion von 4-Nitrophenol und Rhodamin B (RhB) mittels Borhydrid. Die Partikel können durch ihre große, plättchen-förmige Kontaktfläche und der stark adhäsiven Eigenschaften der PDA Hülle einfach mittels Spin-Coating auf Siliziumsubstrate aufgebracht werden. Der so präparierte Nanokatalysator kann nun einfach wiederaufbereitet werden und zeigt hervorragende Wiederverwendbarkeit. Im zweiten Teil wurden anisotrope, hybride Kern-Schale Mikrogele mit wohldefinierter Struktur synthetisiert. Dabei bilden die Gibbsit Nanoplättchen den Kern und vernetztes, thermosensitives Poly(N-isopropylacylamid) die Hülle. Depolarisierte dynamische Lichtstreuung zeigte, dass die hybriden Mikrogele im kollabierten Zustand durch die plättchen-förmigen Kerne eine anisotrope Form annehmen. Der dritte Teil der Arbeit befasst sich mit der Herstellung von hochdispergierbaren, mesoporösen und stickstoffhaltighohle Kohlenstoff-Nanoplättchen. Diese neuartige Kohlenstoff-Nanostruktur wurde mittels sogenannter Silika-Nanocasting Technik unter Veswendung von hexagonalen Gibbsit-Templat und Dopamin als Kohlenstoffquelle synthetisiert. Solche hohlen Kohlenstoff-Nanostrukturen weisen exzellente, kolloidale Stabilität in wässrigen Medien vor und können direkt als Elektrodenmaterial für Superkondensatoren verwendet werden. Außerdem können sie in polyionischen Flüssigkeiten hohe Kapazitäten erzielen, wobei gleichzeitig eine hervorragende elektrochemische Stabilität gewährleistet wird. / In the present thesis, efficient and simple modification approaches have been developed to coat gibbsite platelets with a controllable thickness of functional polymer shell, which preserves the plate-like morphology after the polymer coating. In the first part, a facile approach has been presented for the synthesis of anisotropic plate-like gibbsite-polydopamine (G-PDA) particles. Au NPs with tunable size have been formed on the G-PDA particle surface, which show efficient catalytic activity for the reduction of 4-nitrophenol and Rodamine B (RhB) in the presence of borohydride. Such nanocatalysts can be easily deposited on silicon substrate by spin coating due to the large contact area of the plate-like G-PDA particles and the strong adhesive behavior of the PDA layer. The substrate-deposited nanocatalyst can be easily recycled, which shows excellent reusability. Secondly, anisotropic hybrid core-shell microgels with well-defined structures have been synthesized using gibbsite nanoplate as core and crosslinked thermosensitive poly(N-isopropylacrylamide) as shell. The analysis by depolarized dynamic light scattering shows that the hybrid microgels have an anisotropic shape in the collapsed state, caused by the anisotropy of the plate-like core. In the third part, highly dispersible mesoporous nitrogen-doped hollow carbon nanoplates have been synthesized as a new carbon nanostructure via silica nanocasting technique using dopamine as carbon precursor and hexagonal-shaped gibbsite as template. Such hollow carbon nanoplates show excellent colloidal stability in aqueous media and can be directly applied as electrode materials in supercapacitors, which offer high capacitance and excellent electrochemical stability when using poly(ionic liquid) nanoparticles as binder.

A straightforward, rapid method to create colloidally stable and brightly luminescent core/shell CdSe-based nanoplatelets (NPLs) with fluorescence quantum yields (QYs) up to 50% is demonstrated. A layer-by-layer deposition technique based on a two-phase mixture ‒ consisting of a nonpolar phase which includes the NPLs, and a saturated ionic polar phase ‒ to separate the reagents and hinder the nucleation of the shell material is used. The deposition of the first sulfur layer leads to a significant red-shift (by more than 100 nm) of the optical absorption and emission of the NPLs. Hence, by varying either the sulfur precursor content or the reaction time one can precisely and continuously tune the absorption and emission maxima from 520 to 630 nm. This evolution of the absorption onset during the shell growth is explained quantitatively using density-functional theory and atomistic statistical simulations. The emission can be further enhanced by exposure of the NPL solution to ambient sunlight. Finally, it is demonstrated that the core/shell NPLs can be transferred from the organic solution to aqueous media with no reduction of their QY that opens the door to a broad range of practical applications.

We introduce new oxygen- and moisture-proof polymer matrixes based on polyisobutylene (PIB) and its block copolymer with styrene [poly(styrene-block-isobutylene-blockstyrene), PSt-b-PIB-b-PSt] for the encapsulation of colloidal semiconductor nanocrystals. In order to prepare transparent and processable composites, we developed a special procedure of nanocrystal surface engineering including ligand exchange of parental organic ligands to inorganic species followed by the attachment of specially designed short-chain PIB functionalized with an amino group. The latter provides excellent compatibility of the particles with the polymer matrixes. As colloidal nanocrystals, we chose CdSe nanoplatelets (NPLs) because they possess a large surface and thus are very sensitive to the environment, in particular in terms of their limited photostability. The encapsulation strategy is quite general and can be applied to a wide variety of semiconductor nanocrystals, as demonstrated on the example of PbS quantum dots. All obtained composites exhibited excellent photostability, being tested in a focus of a powerful white-light source, as well as exceptional chemical stability in a strongly acidic media. We compared these properties of the new composites with those of widely used polyacrylate-based materials, demonstrating the superiority of the former. The developed composites are of particular interest for application in optoelectronic devices, such as color-conversion light-emitting diodes, laser diodes, luminescent solar concentrators, etc.