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- ItemStructure-property relationships in nanoporous metallic glasses(Amsterdam [u.a.] : Elsevier Science, 2016) Şopu, D.; Soyarslan, C.; Sarac, B.; Bargmann, S.; Stoica, M.; Eckert, J.We investigate the influence of various critical structural aspects such as pore density, distribution, size and number on the deformation behavior of nanoporous Cu64 Zr36 glass. By using molecular dynamics and finite element simulations an effective strategy to control the strain localization in nanoporous heterostructures is provided. Depending on the pore distribution in the heterostructure, upon tensile loading the nanoporous glass showed a clear transition from a catastrophic fracture to localized deformation in one dominant shear band, and ultimately to homogeneous plastic flow mediated by a pattern of multiple shear bands. The change in the fracture mechanism from a shear band slip to necking-like homogeneous flow is quantitative interpreted by calculating the critical shear band length. Finally, we identify the most effective heterostructure with enhanced ductility as compared to the monolithic bulk metallic glass. The heterostructure with a fraction of pores of about 3% distributed in such a way that the pores do not align along the maximum shear stress direction shows higher plasticity while retaining almost the same strength as the monolithic glass. Our results provide clear evidence that the mechanical properties of nanoporous glassy materials can be tailored by carefully controlling the design parameters.
- ItemStructural and mechanical characterization of heterogeneities in a CuZr-based bulk metallic glass processed by high pressure torsion(Amsterdam [u.a.] : Elsevier Science, 2018) Ebner, Christian; Escher, Benjamin; Gammer, Christoph; Eckert, Jürgen; Pauly, Simon; Rentenberger, ChristianCu45Zr45Al5Ag5 bulk metallic glass samples, processed by high pressure torsion (HPT) under various conditions, were characterized using synchrotron X-ray diffraction, nanoindentation, differential scanning calorimetry, atomic force and transmission electron microscopy. The experimental results clearly show that HPT modifies the amorphous structure by increasing the mean atomic volume. The level of rejuvenation, correlated with the excess mean atomic volume, is enhanced at higher shear strains as inferred from relaxation enthalpies. By mapping of structural and mechanical quantities, the strain-induced rejuvenated state is characterized on cross-sectional HPT samples on a local scale. A clear correlation both between elastic and plastic softening and between softening and excess mean atomic volume is obtained. But also the heterogeneity of the HPT induced rejuvenation is revealed, resulting in the formation of highly strain-softened regions next to less-deformed ones. A hardness drop of up to 20% is associated with an estimated increase of the mean atomic volume of up to 0.75%. Based on synchrotron X-ray diffraction and nanoindentation measurements it is concluded that elastic fluctuations are enhanced in the rejuvenated material on different length scales down to atomic scale. Furthermore, the calculated flexibility volume and the corresponding average mean square atomic displacement is increased. The plastic response during nanoindentation indicates that HPT processing promotes a more homogeneous-like deformation.
- ItemFulleretic well-defined scaffolds: Donor–fullerene alignment through metal coordination and its effect on photophysics(Hoboken, NJ : Wiley, 2016) Williams, Derek E.; Dolgopolova, Ekaterina A.; Godfrey, Danielle C.; Ermolaeva, Evgeniya D.; Pellechia, Perry J.; Greytak, Andrew B.; Smith, Mark D.; Avdoshenko, Stanislav M.; Popov, Alexey A.; Shustova, Natalia B.Herein, we report the first example of a crystalline metal–donor–fullerene framework, in which control of the donor–fullerene mutual orientation was achieved through chemical bond formation, in particular, by metal coordination. The 13C cross‐polarization magic‐angle spinning NMR spectroscopy, X‐ray diffraction, and time‐resolved fluorescence spectroscopy were performed for comprehensive structural analysis and energy‐transfer (ET) studies of the fulleretic donor–acceptor scaffold. Furthermore, in combination with photoluminescence measurements, the theoretical calculations of the spectral overlap function, Förster radius, excitation energies, and band structure were employed to elucidate the photophysical and ET processes in the prepared fulleretic material. We envision that the well‐defined fulleretic donor–acceptor materials could contribute not only to the basic science of fullerene chemistry but would also be used towards effective development of organic photovoltaics and molecular electronics.
- ItemThe influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films(New York : American Institute of Physics, 2017) Yuan, Feifei; Iida, Kazumasa; Grinenko, Vadim; Chekhonin, Paul; Pukenas, Aurimas; Skrotzki, Werner; Sakoda, Masahito; Naito, Michio; Sala, Alberto; Putti, Marina; Yamashita, Aichi; Takano, Yoshihiko; Shi, Zhixiang; Nielsch, Kornelius; Hühne, RubenEpitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.
- ItemElectrically-Pumped Wavelength-Tunable GaAs Quantum Dots Interfaced with Rubidium Atoms(Washington, DC : ACS, 2017) Huang, Huiying; Trotta, Rinaldo; Huo, Yongheng; Lettner, Thomas; Wildmann, Johannes S.; Martín-Sánchez, Javier; Huber, Daniel; Reindl, Marcus; Zhang, Jiaxiang; Zallo, Eugenio; Schmidt, Oliver G.; Rastelli, ArmandoWe demonstrate the first wavelength-tunable electrically pumped source of nonclassical light that can emit photons with wavelength in resonance with the D2 transitions of 87Rb atoms. The device is fabricated by integrating a novel GaAs single-quantum-dot light-emitting diode (LED) onto a piezoelectric actuator. By feeding the emitted photons into a 75 mm long cell containing warm 87Rb vapor, we observe slow-light with a temporal delay of up to 3.4 ns. In view of the possibility of using 87Rb atomic vapors as quantum memories, this work makes an important step toward the realization of hybrid-quantum systems for future quantum networks.
- ItemUltrahigh Power Factor in Thermoelectric System Nb0.95M0.05FeSb (M = Hf, Zr, and Ti)(Chichester : John Wiley and Sons Ltd, 2018) Ren, W.; Zhu, H.; Zhu, Q.; Saparamadu, U.; He, R.; Liu, Z.; Mao, J.; Wang, C.; Nielsch, K.; Wang, Z.; Ren, Z.Conversion efficiency and output power are crucial parameters for thermoelectric power generation that highly rely on figure of merit ZT and power factor (PF), respectively. Therefore, the synergistic optimization of electrical and thermal properties is imperative instead of optimizing just ZT by thermal conductivity reduction or just PF by electron transport enhancement. Here, it is demonstrated that Nb0.95Hf0.05FeSb has not only ultrahigh PF over ≈100 µW cm−1 K−2 at room temperature but also the highest ZT in a material system Nb0.95M0.05FeSb (M = Hf, Zr, Ti). It is found that Hf dopant is capable to simultaneously supply carriers for mobility optimization and introduce atomic disorder for reducing lattice thermal conductivity. As a result, Nb0.95Hf0.05FeSb distinguishes itself from other outstanding NbFeSb-based materials in both the PF and ZT. Additionally, a large output power density of ≈21.6 W cm−2 is achieved based on a single-leg device under a temperature difference of ≈560 K, showing the realistic prospect of the ultrahigh PF for power generation.
- ItemControllable sliding transfer of wafer‐size graphene(Hoboken, NJ : Wiley, 2016) Lu, Wenjing; Zeng, Mengqi; Li, Xuesong; Wang, Jiao; Tan, Lifang; Shao, Miaomiao; Han, Jiangli; Wang, Sheng; Yue, Shuanglin; Zhang, Tao; Hu, Xuebo; Mendes, Rafael G.; Rümmeli, Mark H.; Peng, Lianmao; Liu, Zhongfan; Fu, LeiThe innovative design of sliding transfer based on a liquid substrate can succinctly transfer high‐quality, wafer‐size, and contamination‐free graphene within a few seconds. Moreover, it can be extended to transfer other 2D materials. The efficient sliding transfer approach can obtain high‐quality and large‐area graphene for fundamental research and industrial applications.
- ItemTuning the spin coherence time of Cu(II)−(bis)oxamato and Cu(II)−(bis)oxamidato complexes by advanced ESR pulse protocols(Frankfurt, M. : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2017-4-27) Zaripov, Ruslan; Vavilova, Evgeniya; Khairuzhdinov, Iskander; Salikhov, Kev; Voronkova, Violeta; Abdulmalic, Mohammad A.; Meva, Francois E.; Weheabby, Saddam; Rüffer, Tobias; Büchner, Bernd; Kataev, VladislavWe have investigated with the pulsed ESR technique at X- and Q-band frequencies the coherence and relaxation of Cu spins S = 1/2 in single crystals of diamagnetically diluted mononuclear [n-Bu4N]2[Cu(opba)] (1%) in the host lattice of [n-Bu4N]2[Ni(opba)] (99%, opba = o-phenylenebis(oxamato)) and of diamagnetically diluted mononuclear [n-Bu4N]2[Cu(opbon-Pr2)] (1%) in the host lattice of [n-Bu4N]2[Ni(opbon-Pr2)] (99%, opbon-Pr2 = o-phenylenebis(N(propyl)oxamidato)). For that we have measured the electron spin dephasing time Tm at different temperatures with the two-pulse primary echo and with the special Carr–Purcell–Meiboom–Gill (CPMG) multiple microwave pulse sequence. Application of the CPMG protocol has led to a substantial increase of the spin coherence lifetime in both complexes as compared to the primary echo results. It shows the efficiency of the suppression of the electron spin decoherence channel in the studied complexes arising due to spectral diffusion induced by a random modulation of the hyperfine interaction with the nuclear spins. We argue that this method can be used as a test for the relevance of the spectral diffusion for the electron spin decoherence. Our results have revealed a prominent role of the opba4– and opbon-Pr24– ligands for the dephasing of the Cu spins. The presence of additional 14N nuclei and protons in [Cu(opbon-Pr2)]2– as compared to [Cu(opba)]2– yields significantly shorter Tm times. Such a detrimental effect of the opbon-Pr24− ligands has to be considered when discussing a potential application of the Cu(II)−(bis)oxamato and Cu(II)−(bis)oxamidato complexes as building blocks of more complex molecular structures in prototype spintronic devices. Furthermore, in our work we propose an improved CPMG pulse protocol that enables elimination of unwanted echoes that inevitably appear in the case of inhomogeneously broadened ESR spectra due to the selective excitation of electron spins.
- ItemCurrent Advances in TiO2-Based Nanostructure Electrodes for High Performance Lithium Ion Batteries(Basel : MDPI, 2018-2-6) Madian, Mahmoud; Eychmüller, Alexander; Giebeler, LarsThe lithium ion battery (LIB) has proven to be a very reliably used system to store electrical energy, for either mobile or stationary applications. Among others, TiO2-based anodes are the most attractive candidates for building safe and durable lithium ion batteries with high energy density. A variety of TiO2 nanostructures has been thoroughly investigated as anodes in LIBs, e.g., nanoparticles, nanorods, nanoneedles, nanowires, and nanotubes discussed either in their pure form or in composites. In this review, we present the recent developments and breakthroughs demonstrated to synthesize safe, high power, and low cost nanostructured titania-based anodes. The reader is provided with an in-depth review of well-oriented TiO2-based nanotubes fabricated by anodic oxidation. Other strategies for modification of TiO2-based anodes with other elements or materials are also highlighted in this report.
- Item(Metallo)porphyrins for potential materials science applications(Frankfurt, M. : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2017-8-29) Smykalla, Lars; Mende, Carola; Fronk, Michael; Siles, Pablo F.; Hietschold, Michael; Salvan, Georgeta; Zahn, Dietrich R.T.; Schmidt, Oliver G.; Rüffer, Tobias; Lang, HeinrichThe bottom-up approach to replace existing devices by molecular-based systems is a subject that attracts permanently increasing interest. Molecular-based devices offer not only to miniaturize the device further, but also to benefit from advanced functionalities of deposited molecules. Furthermore, the molecules itself can be tailored to allow via their self-assembly the potential fabrication of devices with an application potential, which is still unforeseeable at this time. Herein, we review efforts to use discrete (metallo)porphyrins for the formation of (sub)monolayers by surface-confined polymerization, of monolayers formed by supramolecular recognition and of thin films formed by sublimation techniques. Selected physical properties of these systems are reported as well. The application potential of those ensembles of (metallo)porphyrins in materials science is discussed.