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  • 1
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    Band alignment of MAPb(I1–xBrx)3 thin films by vacuum deposition (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-09
    Beschreibung: In this work, the mixed bromide iodide lead perovskites CH 3 NH 3 Pb(I 1– x Br x ) 3 (0 ≤  x  ≤ 0.67) thin films were prepared by co-evaporation of CH 3 NH 3 I, PbI 2 , and PbBr 2 . The electronic properties of CH 3 NH 3 Pb(I 1– x Br x ) 3 thin films were investigated by X-ray and ultraviolet photoelectron spectroscopy in-situ . The results of core level binding energy show that there is no chemical shift of the C1s, N1s, Br3d5, and I3d5 when the Br composition changes, while there is an approximately linear chemical shift of Pb4f7 to higher binding energy as the Br composition increases. The density functional theory calculation reveals that there is more charge transfer from Pb to Br than I, which results in the chemical shift of Pb4f states. On the other hand, the valence band maximum increases as the Br composition increases, while the work function shows no obvious change, because the conduction band is dominated by Pb 6 p orbitals while the valence band is dominated by halide p orbitals. Our work demonstrates the adjustability of the energy level alignment of MAPb(I 1– x Br x ) 3 by the Br composition.
    Print ISSN: 0003-6951
    Digitale ISSN: 1077-3118
    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 2
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    Universality in freezing of an asymmetric drop (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-09
    Beschreibung: We present the evidence of universality in conical tip formation during the freezing of arbitrary-shaped sessile droplets. The focus is to demonstrate the relationship between this universality and the liquid drop shape. We observe that, in the case of asymmetric drops, this universal shape is achieved when the tip reconfigures by changing its location, which subsequently alters the frozen drop shape. The proposed “two-triangle” model quantifies the change in the tip configuration as a function of the asymmetry of the drop that shows a good agreement with the experimental evidence. Finally, based on the experimental and theoretical exercise, we propose the scaling dependence between the variations in the tip configuration and the asymmetry of the drop.
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    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 3
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    Gain spectroscopy of a type-II VECSEL chip (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-09
    Beschreibung: Using optical pump–white light probe spectroscopy, the gain dynamics is investigated for a vertical-external-cavity surface-emitting laser chip, which is based on a type-II heterostructure. The active region of the chip consists of a GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs multiple quantum well. For this structure, a fully microscopic theory predicts a modal room temperature gain at a wavelength of 1170 nm, which is confirmed by the experimental spectra. The results show a gain buildup on the type-II chip that is delayed relative to that of a type-I chip. This slower gain dynamics is attributed to a diminished cooling rate arising from the reduced electron–hole scattering.
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    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 4
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    Reconfigurable heat-induced spin wave lenses (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-08
    Beschreibung: We study the control and manipulation of propagating spin waves in yttrium iron garnet films using a local laser-induced heating. We show that, due to the refraction of spin waves in the thermal gradients, the heated region acts as a defocusing lens for Damon-Eshbach spin waves and as a focusing lens for backward volume waves enabling collimation of spin-wave beams in the latter case. In addition to the focusing/defocusing functionality, the local heating allows one to manipulate the propagation direction of the spin-wave beams and to efficiently suppress their diffraction spreading by utilizing caustic effects.
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    Digitale ISSN: 1077-3118
    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 5
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    Combined optical sizing and acoustical characterization of single freely-floating microbubbles (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-08
    Beschreibung: In this study we present a combined optical sizing and acoustical characterization technique for the study of the dynamics of single freely-floating ultrasound contrast agent microbubbles exposed to long burst ultrasound excitations up to the milliseconds range. A co-axial flow device was used to position individual microbubbles on a streamline within the confocal region of three ultrasound transducers and a high-resolution microscope objective. Bright-field images of microbubbles passing through the confocal region were captured using a high-speed camera synchronized to the acoustical data acquisition to assess the microbubble response to a 1-MHz ultrasound burst. Nonlinear bubble vibrations were identified at a driving pressure as low as 50 kPa. The results demonstrate good agreement with numerical simulations based on the shell-buckling model proposed by Marmottant et al . [J. Acoust. Soc. Am. 118 , 3499–3505 (2005)]. The system demonstrates the potential for a high-throughput in vitro characterization of individual microbubbles.
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    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 6
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    Mechanisms for light induced degradation in MAPbI3 perovskite thin films and solar cells (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-08
    Beschreibung: Organometal halide perovskites are highly promising materials for photovoltaic applications, yet their rapid degradation remains a significant challenge. Here, the light-induced structural degradation mechanism of methylammonium lead iodide (MAPbI 3 ) perovskite films and devices is studied in low humidity environment using X-Ray Diffraction, Ultraviolet-Visible (UV-Vis) absorption spectroscopy, Extended X-ray Absorption Fine Structure spectroscopy, Fourier Transform Infrared spectroscopy, and device measurements. Under dry conditions, the perovskite film degrades only in the presence of both light and oxygen, which together induce the formation of halide anions through donation of electrons to the surrounding oxygen. The halide anions generate free radicals that deprotonate the methylammonium cation and form the highly volatile CH 3 NH 2 molecules that escape and leave pure PbI 2 behind. The device findings show that changes in the local structure at the TiO 2 mesoporous layer occur with light, even in the absence of oxygen, and yet such changes can be prevented by the application of UV blocking layer on the cells. Our results indicate that the stability of mp-TiO2-MAPbI 3 photovoltaics can be dramatically improved with effective encapsulation that protects the device from UV light, oxygen, and moisture.
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    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 7
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    Tunnel electroresistance in BiFeO3 junctions: size does matter (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-08
    Beschreibung: In ferroelectric tunnel junctions, the tunnel resistance depends on the polarization orientation of the ferroelectric tunnel barrier, giving rise to tunnel electroresistance. These devices are promising to be used as memristors in neuromorphic architectures and as non-volatile memory elements. For both applications, device scalability is essential, which requires a clear understanding of the relationship between polarization reversal and resistance change as the junction size shrinks. Here we show a robust tunnel electroresistance in BiFeO 3 -based junctions with diameters ranging from 1200 to 180 nm. We demonstrate that the tunnel electroresistance and the corresponding fraction of reversed ferroelectric domains change drastically with the junction diameter: while the micron-size junctions display a reversal in less than 10% of the area, the smallest junctions show an almost complete polarization reversal. Modeling the electric-field distribution, we highlight the critical role of the bottom electrode resistance which significantly diminishes the actual electric field applied to the ferroelectric barrier in the mixed polarization state. A polarization-dependent critical electric field below which further reversal is prohibited is found to explain the large differences between the ferroelectric switchability of nano- and micron-size junctions. Our results indicate that ferroelectric junctions are downscalable and suggest that specific junction shapes facilitate complete polarization reversal.
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    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 8
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    Tailoring capping layers to reduce stress gradients in copper metallization (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-08
    Beschreibung: Capping layers for back-end-of-line metallization, which primarily serve as diffusion barriers to prevent contamination, also play a role in mitigating electromigration in the underlying conductive material. Stress gradients can be generated in copper metallization due to the conditions associated with the capping process. To study the effects of deposition and subsequent annealing on the mechanical response of copper films with various capping schemes, we employed a combination of conventional and glancing incidence X-ray diffraction techniques to quantify the stress gradient maxima. The Cu films with dielectric caps, such as silicon nitride, can exhibit large gradients that decrease slightly with thermal cycling. However, Co and TaN-based metallic capping layers create significantly lower stress gradient maxima in copper features both before and after annealing. The different evolution of stress gradients in Cu films with dielectric and metallic caps due to thermal cycling reveals the interaction of dislocation-mediated, plastic deformation with the cap/Cu interface.
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    Digitale ISSN: 1077-3118
    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 9
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    Decreasing luminescence lifetime of evaporating phosphorescent droplets (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-08
    Beschreibung: Laser-induced phosphorescence has been used extensively to study spray dynamics. It is important to understand the influence of droplet evaporation in the interpretation of such measurements, as it increases luminescence quenching. By suspending a single evaporating n-heptane droplet in an acoustic levitator, the properties of lanthanide-complex europium-thenoyltrifluoroacetone-trioctylphosphine oxide (Eu-TTA-TOPO) phosphorescence are determined through high-speed imaging. A decrease was found in the measured phosphorescence decay coefficient (780 → 200  μ s) with decreasing droplet volumes (10 −9  → 10 –11 m 3 ) corresponding to increasing concentrations (10 −4  → 10 −2 M). This decrease continues up to the point of shell-formation at supersaturated concentrations. The diminished luminescence is shown not to be attributable to triplet-triplet annihilation, quenching between excited triplet-state molecules. Instead, the pure exponential decays found in the measurements show that a non-phosphorescent quencher, such as free TTA/TOPO, can be attributable to this decay. The concentration dependence of the phosphorescence lifetime can therefore be used as a diagnostic of evaporation in sprays.
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    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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  • 10
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    Interface magnetization transition via minority spin injection (2016)
    American Institute of Physics (AIP)
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    Publikationsdatum: 2016-12-08
    Beschreibung: The interface magnetization of n-type BaTiO 3 /La 0.7 Sr 0.3 MnO 3 heterojunction is selectively probed by magnetic second-harmonic generation at 80 K. The injection of minority spins at the interface causes a sudden, reversible transition of the spin alignment of interfacial Mn ions from ferromagnetic to antiferromagnetic exchange coupled, while the bulk magnetization remains unchanged. We attribute the emergent interfacial antiferromagnetic interactions to weakening of the double-exchange mechanism caused by the strong Hund's rule coupling between injected minority spins and local magnetic moments. The effect is robust and may serve as a viable route for electronic and spintronic applications.
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    Thema: Physik
    Publiziert von American Institute of Physics (AIP)
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