Thomas Heinzel

Coupling effects between self-assembled quantum dots in different layers separated by approximately 200 nm are studied by transient capacitance spectroscopy. This coupling manifests itself via interdependent time constants of the transients. Two different coupling mechanisms that act over such large spacings are identified. The first one is electrostatic coupling, caused by electrons in the first layer that modify the electric field at the site of the second layer and with it the lifetime of the discrete states in the quantum dots therein. The second mechanism is cascade coupling, where electrons emitted by the first layer are recaptured by the second layer. A rate equation model combined with self-consistent band structure calculations provides a quantification of the proposed mechanisms.

We introduce superparamagnetic biomembranes as a novel, actively tunable hybrid platform. The superparamagnetic biomembranes are supported lipid bilayers (SLBs) with defined lipid composition and site-specific coupling of proteins and superparamagnetic nanoparticles (MNPs). Application of magnetic field gradients induces rapid and fully reversible lateral transport of lipid-anchored MNPs with forces in the femtonewton range. Using single magnetic tips as well as multiple magnetic tips and line structures, we achieve parallelized and spatially defined MNP patterning. Transition of the SLB from the liquid-crystalline phase to the gel phase enables reversible immobilization of the patterned MNP distributions. Local MNP accumulation displaces lipids and membrane-associated proteins, including Neutravidin, mEGFP, and CD19, thereby generating inverse molecular gradients that are recognized by living cells. This biocompatible substrate with dynamically controllable nanoparticle and molecular organization provides a versatile framework for applications in bio- and nanotechnology.

We introduce superparamagnetic biomembranes as a novel, actively tunable hybrid platform. The superparamagnetic biomembranes are supported lipid bilayers (SLBs) with defined lipid composition and site-specific coupling of proteins and superparamagnetic nanoparticles (MNPs). 2 Application of magnetic field gradients induces rapid and fully reversible lateral transport of lipidanchored MNPs with forces in the femtonewton range. Using single magnetic tips as well as multiple magnetic tips and line structures, we achieve parallelized and spatially defined MNP patterning. Transition of the SLB from the liquid-crystalline to the gel phase enables reversible immobilization of the patterned MNP distributions. Local MNP accumulation displaces lipids and membrane-associated proteins, including Neutravidin, mEGFP, and CD19, thereby generating inverse molecular gradients that are recognized by living cells. This biocompatible substrate with dynamically controllable nanoparticle and molecular organization provides a versatile framework for applications in bio- and nanotechnology.

Nonlinearities in the classical Hall resistance of two-dimensional electron gases exposed to retroreflective obstacles at random positions are reported. The Hall slope develops a nonmonotonic component, which increases strongly in amplitude and shifts to larger magnetic fields as the obstacle density increases. At small magnetic fields, the relation between the longitudinal and transverse conductivity remains in qualitative agreement with the Drude-Boltzmann equations, although the characteristic timescale can no longer be interpreted as a momentum relaxation time. The corresponding Hall conductivity develops a marked minimum at a characteristic magnetic field outside the interval where the data agree qualitatively with the Drude-Boltzmann model. With the help of classical event-driven simulations, this minimum is attributed to successive retroreflections which lead to inverted transverse deflection.

Fluorescent carbon nanodots stain characteristic structures of brain organoids.

Hall field induced resistance oscillations (HIROs) are studied in two-dimensional electron gases with resistivities dominated by scattering at identical but randomly distributed circular obstacles with hard walls. For sufficiently high number densities, these obstacles dominate the HIROs, even though they remain sparse in comparison to the intrinsic scatterers. With increasing obstacle density, we observe a monotonous increase of the HIRO period up to a factor of 2. The results are interpreted in terms of a substantial localization of the Hall field around the designed obstacles, providing an alternative explanation for the HIRO scaling factor γ in pristine two-dimensional electron gases in terms of spatially varying Hall fields.

We report studies of the electron capture process in self-assembled quantum dots (SAQDs) far from equilibrium with their environment and at large distance to the reservoirs. Deep level transient spectroscopy is used to determine the capacitance transients in response to bias voltage pulses, from which the capture rates are obtained as a function of the temperature and the quantum dot occupancy. The observed activated character of the capture suggests that the dominant electron source is the back contact. A model is developed based on electrons diffusing from the reservoir across the flat band region and getting captured in the SAQDs after overcoming the barrier formed by the space charge region between its onset and the quantum dots. For small barriers, we identify a distinct tunneling contribution to the capture current.

Using multi-color flow cytometry analysis, we studied the immunophenotypical differences between leukemic cells from patients with AML/MDS and hematopoietic stem and progenitor cells (HSPCs) from patients in complete remission (CR) following their successful treatment. The panel of markers included CD34, CD38, CD45RA, CD123 as representatives for a hierarchical hematopoietic stem and progenitor cell (HSPC) classification as well as programmed death ligand 1 (PD-L1). Rather than restricting the evaluation on a 2- or 3-dimensional analysis, we applied a t-distributed stochastic neighbor embedding (t-SNE) approach to obtain deeper insight and segregation between leukemic cells and normal HPSCs. For that purpose, we created a t-SNE map, which resulted in the visualization of 27 cell clusters based on their similarity concerning the composition and intensity of antigen expression. Two of these clusters were “leukemia-related” containing a great proportion of CD34⁺/CD38⁻ hematopoietic stem cells (HSCs) or CD34⁺ cells with a strong co-expression of CD45RA/CD123, respectively. CD34⁺ cells within the latter cluster were also highly positive for PD-L1 reflecting their immunosuppressive capacity. Beyond this proof of principle study, the inclusion of additional markers will be helpful to refine the differentiation between normal HSPCs and leukemic cells, particularly in the context of minimal disease detection and antigen-targeted therapeutic interventions. Furthermore, we suggest a protocol for the assignment of new cell ensembles in quantitative terms, via a numerical value, the Pearson coefficient, based on a similarity comparison of the t-SNE pattern with a reference.