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The interface between thin films of metal and polymer materials play a significant role in modern flexible microelectronics viz., metal contacts on polymer substrates, printed electronics and prosthetic devices. The major emphasis in metal polymer interface is on studying how the externally applied stress in the polymer substrate leads to the deformation and cracks in metal film and vice versa. Usually, the deformation process involves strains varying over large lateral dimensions because of excessive stress at local imperfections. Here we show that the seemingly random phenomena at macroscopic scales can be rendered rather controllable at submicrometer length scales. Recently, we have created a metal polymer interface system with strains varying over periods of several hundred nanometers. This was achieved by exploiting the formation of surface relief grating (SRG) within the azobenzene containing photosensitive polymer film upon irradiation with light interference pattern. Up to a thickness of 60 nm, the adsorbed metal film adapts neatly to the forming relief, until it ultimately ruptures into an array of stripes by formation of highly regular and uniform cracks along the maxima and minima of the polymer topography. This surprising phenomenon has far-reaching implications. This is the first time a direct probe is available to estimate the forces emerging in SRG formation in glassy polymers. Furthermore, crack formation in thin metal films can be studied literally in slow motion, which could lead to substantial improvements in the design process of flexible electronics. Finally, cracks are produced uniformly and at high density, contrary to common sense. This could offer new strategies for precise nanofabrication procedures mechanical in character.
In this paper two groups supporting different views on the mechanism of light induced polymer deformation argue about the respective underlying theoretical conceptions, in order to bring this interesting debate to the attention of the scientific community. The group of Prof. Nicolae Hurduc supports the model claiming that the cyclic isomerization of azobenzenes may cause an athermal transition of the glassy azobenzene containing polymer into a fluid state, the so-called photo-fluidization concept. This concept is quite convenient for an intuitive understanding of the deformation process as an anisotropic flow of the polymer material. The group of Prof. Svetlana Santer supports the re-orientational model where the mass-transport of the polymer material accomplished during polymer deformation is stated to be generated by the light-induced re-orientation of the azobenzene side chains and as a consequence of the polymer backbone that in turn results in local mechanical stress, which is enough to irreversibly deform an azobenzene containing material even in the glassy state. For the debate we chose three polymers differing in the glass transition temperature, 32 degrees C, 87 degrees C and 95 degrees C, representing extreme cases of flexible and rigid materials. Polymer film deformation occurring during irradiation with different interference patterns is recorded using a homemade set-up combining an optical part for the generation of interference patterns and an atomic force microscope for acquiring the kinetics of film deformation. We also demonstrated the unique behaviour of azobenzene containing polymeric films to switch the topography in situ and reversibly by changing the irradiation conditions. We discuss the results of reversible deformation of three polymers induced by irradiation with intensity (IIP) and polarization (PIP) interference patterns, and the light of homogeneous intensity in terms of two approaches: the re-orientational and the photo-fluidization concepts. Both agree in that the formation of opto-mechanically induced stresses is a necessary prerequisite for the process of deformation. Using this argument, the deformation process can be characterized either as a flow or mass transport.
When photosensitive azobenzene-containing polymer films are irradiated with light interference patterns, topographic variations in the film develop that follow the local distribution of the electric field vector. The exact correspondence of e.g., the vector orientation in relation to the presence of local topographic minima or maxima is in general difficult to determine. Here, we report on a systematic procedure how this can be accomplished. For this, we devise a new set-up combining an atomic force microscope and two-beam interferometry. With this set-up, it is possible to track the topography change in-situ, while at the same time changing polarization and phase of the impinging interference pattern. This is the first time that an absolute correspondence between the local distribution of electric field vectors and the local topography of the relief grating could be established exhaustively. Our setup does not require a complex mathematical post-processing and its simplicity renders it interesting for characterizing photosensitive polymer films in general.
In this paper, we report on in-situ atomic force microscopy (AFM) studies of topographical changes in azobenzene-containing photosensitive polymer films that are irradiated with light interference patterns. We have developed an experimental setup consisting of an AFM combined with two-beam interferometry that permits us to switch between different polarization states of the two interfering beams while scanning the illuminated area of the polymer film, acquiring corresponding changes in topography in-situ. This way, we are able to analyze how the change in topography is related to the variation of the electrical field vector within the interference pattern. It is for the first time that with a rather simple experimental approach a rigorous assignment can be achieved. By performing in-situ measurements we found that for a certain polarization combination of two interfering beams [namely for the SP (a dagger center dot, a dagger") polarization pattern] the topography forms surface relief grating with only half the period of the interference patterns. Exploiting this phenomenon we are able to fabricate surface relief structures with characteristic features measuring only 140 nm, by using far field optics with a wavelength of 491 nm. We believe that this relatively simple method could be extremely valuable to, for instance, produce structural features below the diffraction limit at high-throughput, and this could significantly contribute to the search of new fabrication strategies in electronics and photonics industry.
In this paper, we report on differences in the response of photosensitive azobenzene containing films upon irradiation with the intensity or polarization interference patterns. Two materials are studied differing in the molecular weight: an azobenzene-containing polymer and a molecular glass formed from a much smaller molecule consisting of three connected azobenzene units. Topography changes occurring along with the changes in irradiation conditions are recorded using a homemade set-up combining an optical part for generation and shaping of interference patterns and an atomic force microscope for acquiring the kinetics of film deformation. In this way, we could reveal the unique behavior of photosensitive materials during the first few minutes of irradiation: the change in topography is initially driven by an increase in the azobenzene free volume along with the transcis isomerization, followed by the mass transport finally resulting in the surface relief grating. This study demonstrates the great potential of our setup to experimentally highlight puzzling processes governing the formation of surface relief gratings. (C) 2014 AIP Publishing LLC.
The geomagnetic activity index Kp is widely used but is restricted by low time resolution (3-hourly) and an upper limit. To address this, new geomagnetic activity indices, Hpo, are introduced. Similar to Kp, Hpo expresses the level of planetary geomagnetic activity in units of thirds (0o, 0+, 1-, 1o, 1+, 2-, horizontal ellipsis ) based on the magnitude of geomagnetic disturbances observed at subauroral observatories. Hpo has a higher time resolution than Kp. 30-min (Hp30) and 60-min (Hp60) indices are produced. The frequency distribution of Hpo is designed to be similar to that of Kp so that Hpo may be used as a higher time-resolution alternative to Kp. Unlike Kp, which is capped at 9o, Hpo is an open-ended index and thus can characterize severe geomagnetic storms more accurately. Hp30, Hp60 and corresponding linearly scaled ap30 and ap60 are available, in near real time, at the GFZ website (https://www.gfz-potsdam.de/en/hpo-index).
The atmospheric lunar tide is one known source of ionospheric variability. The subject received renewed attention as recent studies found a link between stratospheric sudden warmings and amplified lunar tidal perturbations in the equatorial ionosphere. There is increasing evidence from ground observations that the lunar tidal influence on the ionosphere depends on longitude. We use magnetic field measurements from the CHAMP satellite during July 2000 to September 2010 and from the two Swarm satellites during November 2013 to February 2017 to determine, for the first time, the complete seasonal- longitudinal climatology of the semidiurnal lunar tidal variation in the equatorial electrojet intensity. Significant longitudinal variability is found in the amplitude of the lunar tidal variation, while the longitudinal variability in the phase is small. The amplitude peaks in the Peruvian sector (similar to 285 degrees E) during the Northern Hemisphere winter and equinoxes, and in the Brazilian sector (similar to 325 degrees E) during the Northern Hemisphere summer. There are also local amplitude maxima at similar to 55 degrees E and similar to 120 degrees E. The longitudinal variation is partly due to the modulation of ionospheric conductivities by the inhomogeneous geomagnetic field. Another possible cause of the longitudinal variability is neutral wind forcing by nonmigrating lunar tides. A tidal spectrum analysis of the semidiurnal lunar tidal variation in the equatorial electrojet reveals the dominance of the westward propagating mode with zonal wave number 2 (SW2), with secondary contributions by westward propagating modes with zonal wave numbers 3 (SW3) and 4 (SW4). Eastward propagating waves are largely absent from the tidal spectrum. Further study will be required for the relative importance of ionospheric conductivities and nonmigrating lunar tides.
Magnetic reconnection is a multi-faceted process of energy conversion in astrophysical, space and laboratory plasmas that operates at microscopic scales but has macroscopic drivers and consequences.
Solar flares present a key laboratory for its study, leaving imprints of the microscopic physics in radiation spectra and allowing the macroscopic evolution to be imaged, yet a full observational characterization remains elusive.
Here we combine high resolution imaging and spectral observations of a confined solar flare at multiple wavelengths with data-constrained magnetohydrodynamic modeling to study the dynamics of the flare plasma from the current sheet to the plasmoid scale. The analysis suggests that the flare resulted from the interaction of a twisted magnetic flux rope surrounding a filament with nearby magnetic loops whose feet are anchored in chromospheric fibrils. Bright cusp-shaped structures represent the region around a reconnecting separator or quasi-separator (hyperbolic flux tube).
The fast reconnection, which is relevant for other astrophysical environments, revealed plasmoids in the current sheet and separatrices and associated unresolved turbulent motions.
Solar flares provide wide range of observational details about fundamental processes involved. Here, the authors show evidence for magnetic reconnection in a strong confined solar flare displaying all four reconnection flows with plasmoids in the current sheet and the separatrices.
It has been found in recent measurements that the singlet-to-triplet exciton ratio in organic light-emitting diodes (OLEDs) is larger than expected from spin degeneracy, and that singlet excitons form at a larger rate than triplets. We employed the technique of optically detected magnetic resonance to measure the spin-dependent exciton formation rates in films of a polymer and corresponding monomer, and explore the relation between the formation rates and the actual singlet-to-triplet ratio measured previously in OLEDs. We found that the spin-dependent exciton formation rates can indeed quantitatively explain the observed exciton yields, and that singlet formation rates and yields are significantly enhanced only in polymer OLEDs, but not in OLEDs made from the corresponding monomer
Entanglement combing
(2009)
We show that all multipartite pure states can, under local operations, be transformed into bipartite pairwise entangled states in a "lossless fashion": An arbitrary distinguished party will keep pairwise entanglement with all other parties after the asymptotic protocol-decorrelating all other parties from each other-in a way that the degree of entanglement of this party with respect to the rest will remain entirely unchanged. The set of possible entanglement distributions of bipartite pairs is also classified. Finally, we point out several applications of this protocol as a useful primitive in quantum information theory.
Observing the motion of the nuclear wave packets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wave packet in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 angstrom and temporal resolution of 230 fs full width at half maximum. The method is not only sensitive to the position but also the shape of the nuclear wave packet.
Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angstrom spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 angstrom) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.
We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.
Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction
(2018)
Conical intersections play a critical role in excited-state dynamics of polyatomic molecules because they govern the reaction pathways of many nonadiabatic processes. However, ultrafast probes have lacked sufficient spatial resolution to image wave-packet trajectories through these intersections directly. Here, we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas-phase electron diffraction. In the two-photon channel, we have mapped out the real-space trajectories of a coherent nuclear wave packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In the one-photon channel, we have resolved excitation of both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions. These findings benchmark and validate ab initio nonadiabatic dynamics calculations.
We report that the performances of blue polymer electrophosphorescent devices are crucially depending on the choice of the electron transporting material incorporated into the emissive layer. Devices with 1,3-bis[(4-tert- butylphenyl)-1,3,4-oxidiazolyl]phenylene (OXD-7) doped at similar to 40 wt% into a poly(vinylcarbazole) matrix exhibited significantly higher efficiencies than those with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), yielding maximum luminous and power efficiency values of 18.2 Cd/A and 8.8 lm/W, respectively. Time resolved photoluminescence measurements revealed a long lifetime phosphorescence component in layers with PBD, which we assign to significant triplet harvesting by this electron-transporting component. (c) 2006 American Institute of Physics
We demonstrate efficient single-layer polymer phosphorescent light-emitting devices based on a green-emitting iridium complex and a polymer host co-doped with electron-transporting and hole-transporting molecules. These devices can be operated at relatively low voltages, resulting in a power conversion efficiency of up to 24 lm/W at luminous efficiencies exceeding 30 cd/A. The overall performances of these devices suggest that efficient electrophosphorescent devices with acceptable operating voltages can be achieved in very simple device structures fabricated by spin coating. (C) 2004 American Institute of Physics
A commercially available Ir complex has been employed for the preparation of highly efficient (see Figure) single-layer phosphorescent polymer light,emitting diodes by use of appropriate thermal treatment and proper adjustment of the layer composition. These devices exhibit essentially no dependence of the driving field on the concentration of the Ir complex, suggesting that the build-up of space-charge in the layer is insignificant
Suppression of the keto-emission in polyfluorene light-emitting diodes : Experiments and models
(2004)
The spectral characteristics of polyfluorene (PF)-based light-emitting diodes (LEDs) containing a defined low concentration of either keto-defects or of the polymer poly(9.9-octylfuorene-co-benzothiadiazole) (F8BT) are preseneted. Both types of blend layers were tested in different device configurations with respect to the relative and absolute intensities of green blue emission components. It is shown that blending hole-transporting molecules into the emission layer at low concentration or incorporation of a suitable hole-transport layer reduces the green emission contribution in the electroluminescence (EL) spectrum of the PF:F8BT blend, which is similar to what is observed for the keto- containing PF layer. We conclude that the keto-defects in PF homopolymer layers mainly constitute weakly emissive electron traps, in agreement with the results of quantum-mechanical calculations
It is shown that several polymers can form insoluble interfacial layers on a poly (ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) layer after annealing of the double-layer structure. The thickness of the interlayer is dependent on the characteristics of the underlying PEDOT.PSS and the molecular weight of the polymers. It is further shown that the electronic structures of the interlayer polymers have a significant effect on the properties of red-light-emitting polymer-based electrophosphorescent devices. Upon increasing the highest occupied molecular orbital and lowest unoccupied molecular orbital positions, a significant increase in current density and device efficiency is observed. This is attributed to efficient blocking of electrons in combination with direct injection of holes from the interlayer to the phosphorescent dye. Upon proper choice of the interlayer polymer, efficient red, polymer-based electrophosphorescent devices with a peak luminance efficiency of 5.5 cd A(-1) (external quantum efficiency = 6 %) and a maximum power-conversion efficiency of 5 Im W-1 can be realized.
Polymeric electrophosphorescent LEDs with internal quantum efficiencies approaching unity have been fabricated. Such performance levels are previously unknown for OLEDs. The key to this success is redox chemically doped oxetane- crosslinkable hole-transporting layers with multilayer capability (see figure). They improve hole injection and act as electron-blocking layers, without the need to include exciton-or hole-blocking layers