Browsing by Author "Heinze, Stefan"

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Open Access
Controlled formation of porous 2D lattices from C3‐symmetric Ph6-Me‐tribenzotriquinacene‐OAc3
(2023) Toksabay, Sinem; Leisegang, Markus; Christ, Andreas; Härtl, Patrick; Krebs, Johannes; Marder, Todd B.; Haldar, Soumyajyoti; Heinze, Stefan; Bode, Matthias; Krueger, Anke
The on‐surface self‐assembly of molecules to form holey nanographenes is a promising approach to control the properties of the resulting 2D lattice. Usually, planar molecules are utilized to prepare flat, structurally confined molecular layers, with only a few recent examples of warped precursors. However, control of the superstructures is limited thus far. Herein, we report the temperature‐controlled self‐assembly of a bowl‐shaped, acetylated C3‐symmetric hexaphenyltribenzotriquinacene derivative on Cu(111). Combining scanning tunneling microscopy (STM) and density functional theory (DFT) confirms the formation of highly differing arrangements starting with π‐stacked bowl‐to‐bowl dimers at low coverage at room temperature via chiral honeycomb structures, an intermediate trigonal superstructure, followed by a fully carbon‐based, flattened hexagonal superstructure formed by on‐surface deacetylation, which is proposed as a precursor for holey graphene networks with unique defect structures.
Open Access
Thermoelectric properties of oxide heterostructures
(2013) Heinze, Stefan; Keimer, Bernhard (Prof. Dr.)
The thermoelectric properties and the electrical conductivity of thin films of the high temperature superconductor YBa2Cu3O7-d and the ferromagnet/paramagnet La2/3Ca1/3MnO3 as well as heterostructures and superlattices of both materials were investigated in this thesis. Since the properties of YBa2Cu3O7-d and La2/3Ca1/3MnO3 are antagonistic, the simultaneous occurrence of both properties in heterostructures and superlattices and the mutual interactions between both materials are in the focus of interest. The investigations should gain insight into the nature of these interactions and can be seen complementary to the investigations using Hall coefficient and normal resistivity measurements. Also, the studies should lead to information about the properties of the interface between both materials. The thin films used in this thesis exhibit mainly a total thickness of 2000 Å. They were grown on different substrates (SrTiO3, (La0.27Sr0.73)(Al0.65Ta0.35)O3, NdGaO3) by pulsed laser deposition (PLD) techniques and studied in different temperature ranges (20 K to 390 K or 320 K to 750 K). The concept was to investigate at first the physical properties of single layer YBa2Cu3O7-d and La2/3Ca1/3MnO3. In a second step, heterostructures and superlattices of both materials, also with a total thickness of 2000 Å, were fabricated and studied. Using systems with constant total thickness, but with different thicknesses of the single layer materials in the structures and thus varying number of interfaces should lead to knowledge about the mutual influences between both materials and the properties of the interface between both materials. The investigations of single layer YBa2Cu3O7-d and La2/3Ca1/3MnO3 thin films lead to findings which were in a good agreement with the results published by different groups before. The measured Seebeck-coefficients of optimally doped YBa2Cu3O7 d (d = 0.05) are zero in the superconducting state and nearly zero (S(T) = ± 2 µV/K) up to temperatures of 400 K. The Seebeck-coefficients determined for La2/3Ca1/3MnO3 are nearly zero (S(T) = ± 2 µV/K) in the ferromagnetic regime and negative (S(T) = 10 µV/K to 17 µV/K) in the paramagnetic state. Both materials were combined in heterostructures and superlattices. The investigated Seebeck-coefficients result always in positive values between 20 K and 390 K. The findings are completely unexpected since they are strongly deviating from a theoretical combination of the physical properties of the single layer materials according to Kirchhoff’s law. The calculated values are nearly matching with the Seebeck-coefficients investigated for YBCO. Finally, the findings were addressed to long range interactions nucleated at an electronic and magnetic reconstructed interface between YBa2Cu3O7-d and La2/3Ca1/3MnO3. Information about the physical properties of the interface could not be gathered directly, because of its high resistivity.