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Recent Publications

2021/03/01

Our publications in recent months include an experimental-quantum chemical combined study of small cationic CdSe nanoclusters, where besides structure elucidation also the optical behaviour by photoabsorption was investigated. Larger species of these nanoclusters (<27 atoms) were illuminated in a purely theoretical work, which mainly highlights differences to the optical behaviour of colloidal CdSe quantum dots. To elucidate catalytic properties of clusters on surfaces, a radiation heater for high-temperature ultra-high vacuum synthesis was constructed and designed using finite element simulations for the preparation of defined surfaces.

In the past decades, the exploration of smaller and smaller semiconductor materials has drawn significant attention in industry and fundamental research. Especially so‐called quantum dots (QDs) consisting of II–VI semiconductor materials as CdSe exhibit intriguing optoelectronic properties, because its photoluminescence can be tuned by varying the particle size. In most experimental studies, colloidal CdSe QDs were examined. It is nowadays not only possible to synthesize and isolate colloidal QD samples with a well defined size, but also feasible to control the surface composition. The optical absorption of these QDs shows for example a red‐shift with increasing Se content. In order to get a fundamental understanding of how the optical behaviour of semiconductor nanoparticles changes with their size and chemical composition in detail, the investigation of isolated semiconductor species is necessary, because by this ligand and solvent effects are ruled out.

Therefore, we have studied the influence of the stoichiometry on the geometric and electronic structure of isolated CdSe clusters, in order to peel out the intrinsic properties of the system. To work out the impact of stoichiometry, isolated clusters with an excess of Se or Cd are compared to the stoichiometric species. Comparing the experimentally measured absorption spectra with quantum‐chemical calculations results in a detailed insight of the chemical bonding in CdSe nanoclusters. Single positively charged species have been investigated because the defect electron in the clusters acts as a sensitive probe for the electronic structure.

We found that Se‐rich nanoclusters contain a Se subunits which are responsible for the enhanced light absorption in the visible spectral range. In contrast, the radical electron of the Cd‐rich species is localized at a Cd centre. The excess Cd atom is partly responsible for an intense optical absorption in the near ultraviolet. The variation of the geometry and the optical behaviour in dependence of the stoichiometry is discussed with respect to the photocatalytic properties of CdSe nanoparticles.

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In cluster catalysis, the surface of the catalyst support plays just as important a role as the catalyst itself. A new reactor concept was developed for the preparation of this surface. With the help of COMSOL Multiphysics, a radiation heater was designed for high-temperature ultra-high vacuum synthesis of defined surfaces.

The radiation is generated by an electrically heated pyrolitic boron nitride ceramic and directed through an additive manufactured (“3D printed”) niobium heat shield to over 90% of the sample. The design, which easily reaches temperatures of 1300 K at pressures of 10-8 mbar, was then realised and experimentally tested.

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A fundamental understanding of how physicochemical properties evolves with particle size and composition is one of the central issues in the research of new materials. The driving force to investigate clusters is the endeavor to close the knowledge gap and to understand how material properties develop with respect to various parameters such as particle size and composition. In the present work, it is systematically shown how optoelectronic properties and geometries of the smallest isolated, cationic CdSe clusters develop with system size and composition. Surprisingly, their optical behavior is almost opposite to CdSe quantum dots (QDs).

Global energy minimum structures and optoelectronic properties are presented for isolated, cationic CdSe clusters with less than 27 atoms. The compositional‐ and size‐dependent variation of optical, electronic and geometric properties is systematically studied within the framework of ground state and time‐dependent density functional theory. The applied methods are justified by benchmarks with experimental data. It is shown that the optical gap can be tuned by more than 2 eV by only changing the composition for a fixed number of atoms. The stoichiometric species reveal an unexpected size‐dependent behavior in comparison to larger colloidal CdSe QDs, that is, a redshift of the optical gap was observed with decreasing cluster size in contrast to predictions by quantum‐size effects. This unexpected result is discussed in detail taking the positive charge of the clusters into account.

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