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Die Arbeitsgruppe verbindet in der Forschung elektrische Meßtechnik mit elektronenmikroskopischen und spektroskopischen Verfahren der Strukturaufkärung, um Struktur-Eigenschafts-Korrellationen funktioneller Materialien und Bauelemente zu untersuchen. Unsere langjährige Erfahrung und Kompetenz im Bereich der supraleitenden und thermoelektrischen Materialien haben wir erweitert und einen neuen Schwerpunkt bei Solarzellen (Photovoltaik) eingerichtet. Damit können an einer Probe sowohl die elektrischen als auch die strukturellen Eigenschaften bestimmt werden und so lässt sich der Zusammenhang, den der strukturelle Aufbau auf die elektrischen Eigenschaften hat, feststellen. Diese Verfahren wurden auch auf einzelne Bi2Te3 Nanodrähte und andere nanostrukturierte Materialien angewendet. Besonders vorteilhaft lassen sich die Messverfahren auf Untersuchungen von Metallkontakt-Grenzflächen, z.B. an Halbleitern, anwenden, Materialien der Energietechnik sind unser Schwerpunkt. Die in der Arbeitsgruppe etablierten Verfahren der analytischen Elektronenmikroskopie verbinden Abbildung und Spektroskopie miteinander und erlauben hochpräzise Messungen der chemischen Zusammensetzung von nanostrukturierten Materialien und Bauelementen. Diese Messverfahren wenden wir auch auf biologische Proben an, um die chemische Zusammensetzung von sub-zellulären Strukturen zu messen und alters- bzw. krankheitsbedingte Veränderungen im Gewebe der Retina und des Gehirns nachzuweisen. In der Speziallehre engagieren wir uns für elektrische Messtechnik, elektronenmikroskopische und spektroskopische Verfahren der Strukturaufklärung, sowie physikalische Grundlagen der Struktur-Eigenschafts-Korrelation.
Thermoelektrische Nanomaterialien - SPP1386:
Die im Rahmen des SPP erarbeiteten Ergebnisse zu den Bi2Te3-Nanomaterialien sind als Buch erschienen:
- Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials
- DyBa2Cu3O7-x superconducting coated conductors with critical currents exceeding 1000 A/cm
- Growth behavior of superconducting DyBa2Cu3O7-x thin films deposited by inclined substrate deposition for coated conductors
- Stoichiometry Controlled, Single-Crystalline Bi2Te3 Nanowires for Transport in the Basal Plane
- Switching of the Natural Nanostructure in Bi2Te3 Materials by Ion Irradiation
- Effect of HPT processing on the structure, thermoelectric and mechanical properties of Sr0.07Ba0.07Yb0.07Co4Sb12
- A Membrane Device for Substrate-Free Photovoltaic Characterization of Quantum Dot Based p-i-n Solar Cells
D. Giancoli - Physik
Für Studenten mit Nebenfach Physik (Experimentalphysik) empfehle ich die deutsche Ausgabe des Buches "Physik" von Douglas Giancoli, die ich gemeinsam mit Prof. Jörg Ihringer (IAP Tübingen) und Prof. Behn (Universität Leipzig) herausgegeben habe. Die amerikanische Originalausgabe ist langjährig in der Lehre erprobt und für ihre Verständlichkeit und die lebendige Präsentation des Stoffes bekannt. Dieses Buch bietet die komplette klassische und moderne Physik auf ca. 1500 Seiten.
Title: Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials (Journal of ELECTRONIC MATERIALS 41 (2012), p. 1792-1798)
Abstract: The effect of dimensionality and nanostructure on thermoelectric properties in Bi2Te3-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi2Te3 nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al2O3 matrix, yielding transport in the basal plane. Polycrystalline, textured Sb2Te3 and Bi2Te3 thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb2Te3 films revealed low charge carrier density of 2.6 9 10^19 cm^3, large thermopower of 130 µV/K, and large charge carrier mobility of 402 cm^2/(V s). Bi2(Te0.91Se0.09)3 and (Bi0.26Sb0.74)2Te3 nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50 nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi2Te3 nanomaterials are discussed.
Title: DyBa2Cu3O7-x superconducting coated conductors with critical currents exceeding 1000 A/cm (Superconductor Science and Technology 25 (2012), p. 105007).
Abstract: DyBa2Cu3O7-x (DyBCO) films with MgO buffer layers were grown on Hastelloy substrates by inclined substrate deposition (ISD). An almost linear increase of the critical current with the DyBCO film thickness was observed. A maximum critical current of 1018 A/cm was measured for a DyBCO film with 5.9 μm thickness, yielding a critical current density of 1.7 MA cm^2 at 77 K and self-field. Transmission electron microscopy (TEM) yielded highly biaxially textured DyBCO films at all thicknesses and, thus, no significant decrease of the critical current density occurs with the film thickness. ISD yields a non-zero component of the growth direction parallel to the DyBCO (a, b)-plane since the DyBCO grows on a faceted MgO surface and avoids a-axis growth. Therefore, the ISD technology offers a unique possibility to overcome thickness limitations in coated conductor technology.
Title: Growth behavior of superconducting DyBa2Cu3O7-x thin films deposited by inclined substrate deposition for coated conductors (Acta Materialia 60 (2012) p. 6592).
Abstract: Superconducting DyBa2Cu3O7-x (DyBCO) films were grown on biaxially textured MgO buffer layers deposited by inclined substrate deposition (ISD) on Hastelloy substrates. Despite the large lattice mismatch (8.5%) between DyBCO and MgO, the DyBCO grew epitaxially on the MgO buffer layer and the biaxial texture of the MgO was well transferred to the DyBCO. Typical critical current densities, jc, of the DyBCO film were 2.1 MA cm^-2 at 77 K in a self-field. Biaxial texturing is the key for reaching the high critical current densities and was investigated by transmission electron microscopy. DyBCO grains were found to be 130-500 nm in size, with faceted grain boundaries. The c-axis of the DyBCO grains was tilted away from the substrate normal by 29° such that it was perpendicular to the MgO (0 0 2) facets. A high dislocation density of 7.4 x 10^11 cm^-2 and stacking faults along the ab-planes were observed in the DyBCO film. Interface, grain boundary and volume energies of the DyBCO film were calculated and a growth model for the DyBCO film is discussed. ISD offers the potential for high-quality, biaxially textured MgO buffer layers suitable for long-length superconducting coated conductors.
Title: Stoichiometry Controlled, Single-Crystalline Bi2Te3 Nanowires for Transport in the Basal Plane (Advanced Functional Materials 22 (2012), p. 151–156)
Abstract: Thermoelectric Bi2Te3 based bulk materials are widely used for solid-state refrigeration and power-generation at room temperature. For low-dimensional and nanostructured thermoelectric materials an increase of the thermoelectric figure of merit ZT is predicted due to quantum confinement and phonon scattering at interfaces. Therefore, the fabrication of Bi2Te3 nanowires, thin films, and nano-structured bulk materials has become an important and active field of research. Stoichiometric Bi2Te3 nanowires with diameters of 50–80 nm and a length of 56 µm are grown by a potential-pulsed electrochemical deposition in a nano-structured Al2O3 matrix. By transmission electron microscopy (TEM), dark-field images together with electron diffraction reveal single-crystalline wires, no grain boundaries can be detected. The stoichiometry control of the wires by high-accuracy, quantitative enegy-dispersive X-ray spectroscopy (EDX) in the TEM instrument is of paramount importance for successfully implementing the growth technology. Combined electron diffraction and EDX spectroscopy in the TEM unambiguously prove the correct crystal structure and stoichiometry of the Bi2Te3 nanowires. X-ray and electron diffraction reveal growth along the  and  directions and the c axis of the Bi2Te3 structure lies perpendicular to the wire axis. For the first time single crystalline, stoichiometric Bi2Te3 nanowires are grown that allow transport in the basal plane without being affected by grain boundaries.
Title: Switching of the Natural Nanostructure in Bi2Te3 Materials by Ion Irradiation (Advanced Materials 24 (2012) p. 4605).
Abstract: In Bi2Te3 materials the natural nanostructure (nns) with a wavelength of 10 nm can be reproducibly switched ON and OFF by Ar+ ion irradiation at 1.5 and 1 keV. Controlled formation of the nns in Bi2Te3 materials has potential for reducing its thermal conductivity and could increase the thermoelrctric figure of merit.
Title: Effect of HPT processing on the structure, thermoelectric and mechanical properties of Sr0.07Ba0.07Yb0.07Co4Sb12 (Journal of Alloys and Compounds 537 (2012) p. 183-189)
Abstract: N-type skutterudite Sr0.07Ba0.07Yb0.07Co4Sb12with ZT = 1.4 at 800 K was processed by high pressure torsion (HPT), a technique of severe plastic deformation (SPD) to produce a nanocrystalline material with many deformation induced lattice defects like dislocations and vacancies. As already shown previously, after HPT processing ZT ˜ 1.8 was reached mainly due to a significantly reduced thermal conductivity (the lattice thermal conductivity reached almost the theoretical calculated minimum) although the electrical resistivity was higher. In this paper, the microstructural changes after HPT leading to such high ZT values were investigated. X-ray line profile analysis (XPA) before and after HPT was used to detect a smaller crystallite size and a high number of defects (dislocations and vacancies) resulting in an increase of the electrical resistivity but a significant decrease of the thermal conductivity after HPT processing. The decrease of the crystallite size could also be identified as the reason for enhanced microhardness, which means that Hall-Petch strengthening applies. In addition, for the first time, energy filtered transmission electron microscopy (TEM) was employed for the investigation of HPT processed skutterudites. Dislocations as well as grain boundaries of two types (polarised dipole walls and polarised tilt walls) could be directly observed, confirming what so far was assumed. Also for the first time thermal expansion was measured below and above room temperature and compared with the results before HPT revealing a slightly lower thermal expansion coefficient, the same Debye temperature but an Einstein temperature only half of that before HPT, the latter indicating lower frequencies of the filler atoms after HPT processing. Furthermore it could be shown that the decrease of the electrical resistivity after reaching a maximum runs parallel with a shrinking of the sample during thermal expansion measurements, proving that annealing out and closing of microcracks are responsible for this behaviour.
Title: A Membrane Device for Substrate-Free Photovoltaic Characterization of Quantum Dot Based p-i-n Solar Cells (Advanced Materials 24 (2012), p. 124–3129)
Abstract: A membrane based silicon nanocrystal p-i-n diode is presented that enables the photovoltaic characterization of silicon quantum dots produced by high-temperature routes. The membrane p-i-n diode decouples silicon nanocrystal formation from the formation of selective contacts and therefore enables extraction of the full photovoltaic potential. Open-circuit voltages of up to 370 mV are shown, which is encouraging for future tandem solar cells.