@article{PietschGeueHennebergetal.2003,
  author    = {Pietsch, Ullrich and Geue, Thomas and Henneberg, Oliver and Saphiannikova, Marina},
  title     = {X-ray investigations of formation efficiency of buried azobenzene polymer density gratings},
  doi       = {10.1063/1.1554753},
  year      = {2003},
  language  = {en}
}
@article{HennebergRochonPanzneretal.2004,
  author    = {Henneberg, Oliver and Rochon, Paul and Panzner, Tobias and Finkelstein, Kenneth D. and Geue, Thomas and Saphiannikova, Marina and Pietsch, Ullrich},
  title     = {In-situ Investigation of Surface Relief Grating Formation in Photosensitive Polymers},
  year      = {2004},
  language  = {en}
}
@book{PietschHolyBaumbach2004,
  author    = {Pietsch, Ullrich and Hol{\´y}, V{\´a}clav and Baumbach, Tilo},
  title     = {High resolution X-ray scattering from thin films and lateral nanostructures},
  series = {Advanced texts in physics : physics and astronomy online library},
  journal   = {Advanced texts in physics : physics and astronomy online library},
  edition   = {2. ed.},
  publisher = {Springer},
  address   = {New York},
  isbn      = {0-387-40092-3},
  pages     = {XVI, 408 S. : graph. Darst.},
  year      = {2004},
  language  = {en}
}
@article{PietschSaphiannikovaHennebergetal.2004,
  author    = {Pietsch, Ullrich and Saphiannikova, Marina and Henneberg, Oliver and Geue, Thomas},
  title     = {Non-linear effects during inscription of azobenzene surface relief gratings},
  year      = {2004},
  language  = {en}
}
@article{SaphiannikovaHennebergGeneetal.2004,
  author    = {Saphiannikova, Marina and Henneberg, Oliver and Gene, T. M. and Pietsch, Ullrich and Rochon, Paul},
  title     = {Nonlinear effects during inscription of azobenzene surface relief gratings},
  issn      = {1520-6106},
  year      = {2004},
  abstract  = {Surface relief gratings were inscribed on azobenzene polymer films using a pulselike exposure of an Ar+ laser. The inscription process was initiated by a sequence of short pulses followed by much longer relaxation pauses. The development of the surface relief grating was probed by a He-Ne laser measuring the scattering intensity of the first- order grating peak. The growth time of the surface relief grating was found to be larger than the length of the pulses used. This unusual behavior can be considered as a nonlinear material response associated with the trans-cis isomerization of azobenzene moieties. In this study the polymer stress was assumed to be proportional to the number of cis-isomers. One-dimensional viscoelastic analysis was used to derive the polymer deformation. The rate of trans-cis isomerization increases with the intensity of the inscribing light; in the dark it is equal to the rate of thermal cis- trans isomerization. The respective relaxation times were estimated by fitting theoretical deformation curves to experimental data},
  language  = {en}
}
@article{PietschGuptaGuptaetal.2004,
  author    = {Pietsch, Ullrich and Gupta, Ajay and Gupta, Mukul and Ayachit, S. and Rajagopalan, S. and Balamurgan, A. K. and Tyagi, A. K.},
  title     = {Iron self-diffusion in nanocrystalline FeCr thin films},
  year      = {2004},
  abstract  = {Thin films of amorphous Fe85Zr15 alloy were deposited by ion-beam sputtering of a composite target. Analogous to the melt-spun amorphous alloys of similar composition, the crystallization of the amorphous film occurs in two steps, however, with a substantially reduced thermal stability. After completion of the first crystallization step which starts at 473 K, the microstructure consists of 12 nm nanocrystals of bcc-Fe embedded in a grain boundary region of the remaining amorphous phase. At 673 K, the remaining amorphous phase transforms into the Fe2Zr alloy. The self-diffusion measurements of iron in the nanocrystalline state and in the parent amorphous state has been carried out using secondary ion mass spectroscopy (SIMS) depth profiling and neutron reflectivity techniques. In contrast to the case of finemet Fe73.5Si13.5B9Nb3Cu1 alloy, where a significant enhancement of diffusivity takes place in the nanocrystalline state, in the present case the diffusivity in the nanocrystalline state is similar to that in the parent amorphous state. It is suggested that in this system the atomic diffusion occurs mainly via the grain boundary regions. The calculated values of the pre-exponential factor and the activation energy for the diffusion are D-0 = 1 x 10(-14+/-1) m(2)/s and E = (0.7 +/- 0.1) eV respectively. (C) 2004 Published by Elsevier B.V.},
  language  = {en}
}
@article{PietschGrenzerGrigorianetal.2004,
  author    = {Pietsch, Ullrich and Grenzer, J{\"o}rg and Grigorian, Souren A. and Weyers, Markus and Zeimer, Ute and Feranchuk, S. and Fricke, J. and Kissel, H. and Knauer, A. and Tr{\"a}nkle, G.},
  title     = {Nanoengineering of lateral strain-modulation in quantum well heterostructures},
  year      = {2004},
  abstract  = {We have developed a method to design a lateral band-gap modulation in a quantum well heterostructure. The lateral strain variation is induced by patterning of a stressor layer grown on top of a single quantum well which itself is not patterned. The three-dimensional (3D) strain distribution within the lateral nanostructure is calculated using linear elasticity theory applying a finite element technique. Based on the deformation potential approach the calculated strain distribution is translated into a local variation of the band-gap energy. Using a given vertical layer structure we are able to optimize the geometrical parameters to provide a nanostructure with maximum lateral band-gap variation. Experimentally such a structure was realized by etching a surface grating into a tensile-strained InGaP stressor layer grown on top of a compressively strained InGaAs-single quantum well. The achieved 3D strain distribution and the induced band-gap variation are successfully probed by x-ray grazing incidence diffraction and low-temperature photoluminescence measurements, respectively},
  language  = {en}
}
@article{PietschHazraChinietal.2004,
  author    = {Pietsch, Ullrich and Hazra, S. and Chini, T. K. and Sanyal, M. K. and Grenzer, J{\"o}rg},
  title     = {Ripple structure of crystalline layers in ion beam induced Si wafers},
  year      = {2004},
  abstract  = {Ion-beam-induced ripple formation in Si wafers was studied by two complementary surface sensitive techniques, namely atomic force microscopy (AFM) and depth-resolved x-ray grazing incidence diffraction (GID). The formation of ripple structure at high doses (similar to7x10(17) ions/cm(2)), starting from initiation at low doses (similar to1x10(17) ions/cm(2)) of ion beam, is evident from AFM, while that in the buried crystalline region below a partially crystalline top layer is evident from GID study. Such ripple structure of crystalline layers in a large area formed in the subsurface region of Si wafers is probed through a nondestructive technique. The GID technique reveals that these periodically modulated wavelike buried crystalline features become highly regular and strongly correlated as one increases the Ar ion-beam energy from 60 to 100 keV. The vertical density profile obtained from the analysis of a Vineyard profile shows that the density in the upper top part of ripples is decreased to about 15\% of the crystalline density. The partially crystalline top layer at low dose transforms to a completely amorphous layer for high doses, and the top morphology was found to be conformal with the underlying crystalline ripple},
  language  = {en}
}
@article{PietschGuptaPauletal.2004,
  author    = {Pietsch, Ullrich and Gupta, Amod and Paul, A. and Meneghini, C. and Mibu, K. and Maddalena, S. and Dal Toe, S. and Principi, G.},
  title     = {Structural characterization of epitaxial Fe/Cr multilayers using anomalous x-ray scattering and neutron reflectivity},
  year      = {2004},
  abstract  = {The interface structure of epitaxial Fe/Cr multilayers was studied using anomalous X-ray and neutron reflectivity. The analysis of X-ray reflectivity at three different energies provided a reliable information about the interface roughnesses. It is found that the Cr-on-Fc interface is more diffused as compared to the Fe-on-Cr interface and that the roughness exhibits a significant increase with increasing depth. The magnetic roughness, as determined from neutron reflectivity, is lower than the geometrical roughness, in conformity with the behavior of a number of magnetic thin films and multilayers. (C) 2004 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{HennebergGeuePietschetal.2004,
  author    = {Henneberg, Oliver and Geue, Thomas and Pietsch, Ullrich and Winter, Bernd},
  title     = {Investigation of azobenzene side group orientation in polymer surface relief gratings by means of photoelectron spectroscopy},
  year      = {2004},
  abstract  = {The molecular orientation of azobenzene side groups in polymer films before (nonpatterned) and after (patterned) development of a surface relief grating has been investigated by photoelectron spectroscopy using synchrotron radiation. The photoemission spectra obtained for 60-100 eV photons of a patterned and a nonpatterned surface are similar when the polarization vector of the synchrotron light is parallel to the grating vector. However, for perpendicular excitation, considerable spectral intensity differences can be observed for 9-14 eV electron binding energy. The observed changes are attributed to the formation of well-oriented azobenzenes at the surface. (C) 2004 American Institute of Physics},
  language  = {en}
}
@article{PietschJarreSaldittetal.2004,
  author    = {Pietsch, Ullrich and Jarre, A. and Salditt, T. and Panzner, Tobias and Pfeiffer, F.},
  title     = {White beam x-ray waveguide optics},
  year      = {2004},
  abstract  = {We report a white beam x-ray waveguide (WG) experiment. A resonant beam coupler x-ray waveguide (RBC) is used simultaneously as a broad bandpass (or multibandpass) monochromator and as a beam compressor. We show that, depending on the geometrical properties of the WG, the exiting beam consists of a defined number of wavelengths which can be shifted by changing the angle of incidence of the white x-ray synchrotron beam. The characteristic far-field pattern is recorded as a function of exit angle and energy. This x-ray optical setup may be used to enhance the intensity of coherent x-ray WG beams since the full energetic acceptance of the WG mode is transmitted. (C) 2004 American Institute of Physics},
  language  = {en}
}
@article{PietschMukhopadhyaySanyaletal.2004,
  author    = {Pietsch, Ullrich and Mukhopadhyay, M. K. and Sanyal, M. K. and Datta, A. and Mukherjee, M. and Geue, Thomas and Grenzer, J{\"o}rg},
  title     = {Transition from two-dimensional to three-dimensional melting in Langmuir-Blodgett films},
  year      = {2004},
  abstract  = {Results of energy-dispersive x-ray reflectivity and grazing incidence diffraction studies of Langmuir-Blodgett films exhibited evolution of conventional three-dimensional melting from continuous melting, characteristic of two- dimensional systems, as a function of deposited monolayers. Continuous expansion followed by a sharp phase transition of the in-plane lattice was observed before the melting point and found to be independent of number of deposited layers. Evolution of conventional melting with an increase in the number of monolayers could be quantified by measuring stiffness against tilting of the vertical stack of molecules, which are kept together by an internal field. The internal field as defined in this model reduces as the in-plane lattice expands and the sample temperature approaches melting point. The sharpness of the melting transition, which has been approximated by a Langevin function, increases with the number of deposited monolayers},
  language  = {en}
}
@article{PietschDavaasambuuKochinetal.2004,
  author    = {Pietsch, Ullrich and Davaasambuu, Jav and Kochin, V. and Schwarz, K. H. and Blaha, Pawel},
  title     = {The atomistic origin of the inverse piezoelectric effect in a-quartz},
  year      = {2004},
  abstract  = {Ab initio calculations have been carried out using the FP-APW+lo method in order to understand the atomic origin of the inverse piezoelectric effect in x-quartz. The external electric field was modelled by a saw-like potential V-ext in order to achieve translational symmetry within a supercell (SC) containing 72 atoms. The original trigonal quartz structure was repeated along the [110] direction, which corresponds to the direction of the external field. An electric field with 550 kV/mm was applied and the atomic positions of the SC were relaxed until the forces acting on the atoms vanished. In parts of the SC, V-ext changes almost linearly and thus the relaxed atomic positions can be used to determine the structural response due to the external electric field. The calculations provide the piezoelectric modulus of the correct order of magnitude. In contrast to previous models and in agreement with recent experimental results, the atomic origin of the piezoelectric effect can be described by a rotation of slightly deformed SiO4 tetrahedra against each other. The change of the Si-O bond lengths and the tetrahedral O-Si-O angles is one order of magnitude smaller than that of the Si-O-Si angles between neighbouring tetrahedra. The calculated changes of X-ray structure factors are in agreement with experiment when the theoretical data are extrapolated down to the much smaller field strength that is applied in the experiment (E < 10 kV/mm). (C) 2004 Elsevier Ltd. All rights reserved},
  language  = {en}
}
@article{SaphiannikovaGeueHennebergetal.2004,
  author    = {Saphiannikova, Marina and Geue, Thomas and Henneberg, Oliver and Morawetz, Knut and Pietsch, Ullrich},
  title     = {Linear viscoelastic analysis of formation and relaxation of azobenzene polymer gratings},
  doi       = {10.1063/1.1642606},
  year      = {2004},
  abstract  = {Surface relief gratings on azobenzene containing polymer films were prepared under irradiation by actinic light. Finite element modeling of the inscription process was carried out using linear viscoelastic analysis. It was assumed that under illumination the polymer film undergoes considerable plastification, which reduces its original Young's modulus by at least three orders of magnitude. Force densities of about 10(11) N/m(3) were necessary to reproduce the growth of the surface relief grating. It was shown that at large deformations the force of surface tension becomes comparable to the inscription force and therefore plays an essential role in the retardation of the inscription process. In addition to surface profiling the gradual development of an accompanying density grating was predicted for the regime of continuous exposure. Surface grating development under pulselike exposure cannot be explained in the frame of an incompressible fluid model. However, it was easily reproduced using the viscoelastic model with finite compressibility. (C) 2004 American Institute of Physics},
  language  = {en}
}
@article{HennebergPanznerPietschetal.2004,
  author    = {Henneberg, Oliver and Panzner, Tobias and Pietsch, Ullrich and Geue, Thomas and Saphiannikova, Marina and Rochon, Paul and Finkelstein, Kenneth D.},
  title     = {X-ray and VIS light scattering from light-induced polymer gratings},
  issn      = {0044-2968},
  year      = {2004},
  abstract  = {Sinusoidally shaped surface relief gratings made of polymer films containing, azobenzene moieties can be created by holographic illumination with laser light of about lambda approximate to 500 nm. The remarkable material transport takes place at temperatures far (100 K) below the glass transition temperature of the material. As probed by visible light scattering the efficiency of grating formation crucially depends on the polarization state of the laser light and is maximal when circular polarization is used. In contrast to VIS light scattering X-ray diffraction is most sensitive for periodic surface undulations with amplitudes below 10 nm. Thus, combined in-situ X-ray and visible light scattering at CHESS were used to investigate the dynamics of surface relief grating formations upon laser illumination. The time development of grating peaks up to 9th order at laser power of P = 20 mW/cm(2) could be investigated, even the onset of grating formation as a function of light polarization. A linear growth of grating amplitude was observed for all polarizations. The growth velocity is maximal using circularly polarized light but very small for s-polarized light},
  language  = {en}
}
@article{PietschBodenthinGrenzeretal.2005,
  author    = {Pietsch, Ullrich and Bodenthin, Yves and Grenzer, J{\"o}rg and Geue, Thomas and M{\"o}hwald, Helmuth and Kurth, Dirk G.},
  title     = {Structure and temperature behavior of metallo-supramolecular assemblies},
  year      = {2005},
  abstract  = {A detailed structural analysis of a Langmuir-Blodgett (LB) multilayer composed of a polyelectrolyte-amphiphile complex (PAC) is presented. The PAC is self-assembled from metal ions, ditopic bis-terpyridines, and amphiphiles. The vertical structure of the LB multilayer is investigated by X-ray reflectometry. The multilayer has a periodicity of 57 A, which corresponds to an architecture of flat lying metallo-supramolecular coordination polyelectrolyte (MEPE) rods and upright-standing amphiphiles (dihexadecyl phosphate, DHP). In-plane diffraction reveals hexagonal packing of the DHP molecules. Using extended X-ray absorption fine structure (EXAFS) experiments, we prove that the central metal ion is coordinated to the terpyridine moieties in a pseudo-octahedral coordination environment. The Fe-N bond distances are 1.82 and 2.0 angstrom, respectively. Temperature resolved measurements indicate a reversible phase transition in a temperature range up to 55 degrees C. EXAFS measurements indicate a lengthening of the average Fe-N bond distance from 1.91 to 1.95 angstrom. The widening of the coordination cage upon heating is expected to lower the ligand field stabilization, thus giving rise to spin transitions in these composite materials},
  language  = {en}
}
@article{PietschBhattacharyaSanyaletal.2005,
  author    = {Pietsch, Ullrich and Bhattacharya, M. K. and Sanyal, M. K. and Geue, Thomas},
  title     = {Glass transition in Ultrathin Polymer Films : a Thermal Expansion Study},
  year      = {2005},
  abstract  = {The glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of the glass transition temperature (T-g) as the thickness of an ultrathin polymer film reduces below the radius of gyration but the change in the average T-g was found to be very small. The existence of reversible negative and positive thermal expansion below and above T-g increased the sensitivity of our thickness measurements performed using energy-dispersive x-ray reflectivity. A simple model of the T-g variation as a function of depth expected from sliding motion could explain the results},
  language  = {en}
}
@article{PietschGrenzerBischoff2005,
  author    = {Pietsch, Ullrich and Grenzer, J{\"o}rg and Bischoff, Lothar},
  title     = {Grazing-incidence diffraction strain analysis of a laterally patterned Si wafer treated by focused Ge and An ion beam implantation},
  year      = {2005},
  abstract  = {Strain analysis of a laterally patterned Si-wafer was carried out utilizing X-ray grazing-incidence diffraction with synchrotron radiation. The lateral patterning was done by focused ion beam implantation using an ion source of liquid AuGeSi alloy. Samples were prepared by either 35 keV Au+ ions (dose: 0.2, 2 x 10(14) cm(-2)) or 70 keV Ge++ ions (dose: 8 X 10(14) cm(-1)). It was shown that due to implantation a periodical defect structure is created consisting of both implanted and not implanted stripes. The evaluated depth distribution of defects within the implanted stripes corresponds to that obtained by TRIM calculation. The induced strain distribution, however, shows no periodicity. This can be explained by an overlap of the strain fields created in each implanted stripe. (c) 2005 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
  language  = {en}
}
@article{PietschBodenthinMoehwaldetal.2005,
  author    = {Pietsch, Ullrich and Bodenthin, Yves and M{\"o}hwald, Helmuth and Kurth, Dirk G.},
  title     = {Inducing spin crossover in metallo-supramolecular polyelectrolytes through an amphiphilic phase transition},
  year      = {2005},
  abstract  = {A phase transition in an amphiphilic mesophase is explored to deliberately induce mechanical strain in an assembly of tightly coupled metal ion coordination centers. Melting of the alkyl chains in the amphiphilic mesophase causes distortion of the coordination geometry around the central transition metal ion. As a result, the crystal field splitting of the d-orbital subsets decreases resulting in a spin transition from a low-spin to a high-spin state. The diamagnetic-paramagnetic transition is reversible. This concept is demonstrated in a metallo-supramolecular coordination polyelectrolyte-amphiphile complex self-assembled from ditopic bis-terpyridines, Fe(II) as central transition metal, and dialkyl phosphates as amphiphiles. The magnetic properties are studied in a Langmuir-Blodgett multilayer. The modularity of this concept provides extensive control of structure and function from molecular to macroscopic length scales and gives access to a wide range of new molecular magnetic architectures such as nanostructures, thin films, and liquid crystals},
  language  = {en}
}
@article{GorfmanTsirelsonPietsch2005,
  author    = {Gorfman, S. V. and Tsirelson, Vladimir G. and Pietsch, Ullrich},
  title     = {X-ray diffraction by a crystal in a permanent external electric field : general considerations},
  issn      = {0108-7673},
  year      = {2005},
  abstract  = {The variations of X-ray diffraction intensities from a crystal in the presence of a permanent external electric field is modeled analytically using a first-order stationary perturbation theory. The change in a crystal, induced by an external electric field, is separated into two contributions. The first one is related to a pure polarization of an electron subsystem, while the second contribution can be reduced to the displacements of the rigid pseudoatoms from their equilibrium positions. It is shown that a change of the X-ray diffraction intensities mainly originates from the second contribution, while the influence of the pure polarization of a crystal electron subsystem is negligibly small. The quantities restored from an X-ray diffraction experiment in the presence of an external electric field were analyzed in detail in terms of a rigid pseudoatomic model of electron density and harmonic approximation for the atomic thermal motion. Explicit relationships are derived that link the properties of phonon spectra with E-field-induced variations of a structure factor, pseudoatomic displacements and piezoelectric strains. The displacements can be numerically estimated using a model of independent atomic motion if the Debye - Waller factors and pseudoatomic charges are known either from a previous single-crystal X-ray diffraction study or from density functional theory calculations. The above estimations can be used to develop an optimum strategy for a data collection that avoids the measurements of reflections insensitive to the electric-field-induced variations},
  language  = {en}
}
@article{PietschPanznerLeitenbergeretal.2005,
  author    = {Pietsch, Ullrich and Panzner, Tobias and Leitenberger, Wolfram and Vartanyants, Ivan A.},
  title     = {Coherence experiments using white synchrotron radiation},
  year      = {2005},
  abstract  = {Experiments at the bending magnet beamline at BESSY II (EDR beamline) profit from the excellent coherence properties of third generation synchrotron sources. Considering the exponentially decaying incident spectrum, and because no optical elements are installed except slits and vacuum windows, coherence experiments can be performed between 5 keV < E < 15 keV. First, the energy dependence of spatial coherence properties were determined measuring diffraction at single and double pinholes. Next, the coherent white radiation was used to probe the morphology of thin films in reflection geometry. The recorded intensity maps (reflectivity versus sample position) provide speckle patterns which reveal the locally varying sample morphology. Setting the incident angle, alpha(i), smaller or larger than the critical angle of total external reflection, alpha(c), one should be able to separate the surface height profile from the subsurface density modulation of a sample. The validity of this approach is verified at the example of reciprocal space maps taken from a polymer surface where we could reconstruct the lateral height profile from speckle data. (C) 2004 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{PietschPanznerLeitenbergeretal.2005,
  author    = {Pietsch, Ullrich and Panzner, Tobias and Leitenberger, Wolfram and Vartanyants, Ivan A.},
  title     = {Coherence experiments at the EDR-beamline of BESSY II},
  year      = {2005},
  language  = {en}
}
@article{TalalaevTommElsaesseretal.2005,
  author    = {Talalaev, V and Tomm, JW and Elsaesser, T and Zeimer, Ute and Fricke, J and Knauer, A and Kissel, H and Weyers, Markus and Tarasov, GG and Grenzer, J{\"o}rg and Pietsch, Ullrich},
  title     = {Carrier dynamics in laterally strain-modulated InGaAs quantum wells},
  year      = {2005},
  abstract  = {We investigate the transient recombination and transfer properties of nonequilibrium carriers in an In0.16Ga0.84As/GaAs quantum well (QW) with an additional lateral confinement implemented by a patterned stressor layer. The structure thus contains QW- and quantum-wire-like areas. At low excitation densities, photoluminescence (PL) transients from both areas are well described by a rate equation model for a three-level system with a saturable interlevel carrier transfer representing the lateral drift of carriers from the QW regions into the wires. Small-signal carrier lifetimes for QW, wires, and transfer time from QW to wire are 180, 190, and 28 ps, respectively. For high excitation densities the time constants of the observed transients increase, in agreement with the model. In addition, QW and wire PL lines merge indicating a smoothening of the potential difference, i.e., the effective carrier confinement caused by the stressor structure becomes weaker with increasing excitation. (c) 2005 American Institute of Physics},
  language  = {en}
}
@article{PietschPanznerPfeifferetal.2005,
  author    = {Pietsch, Ullrich and Panzner, Tobias and Pfeiffer, Franz and Robinson, Ian K.},
  title     = {Substrate morphology repetition in "thick" polymer films},
  year      = {2005},
  abstract  = {Using Grazing-incidence small-angle scattering (GISAXS) technique we investigated the surface morphology of polymer films spin-coated on different silicon substrates. As substrates we used either technologically smooth silicon wafers or the same silicon wafer coated with thin aluminium or gold films which show a granular structure at the surface. Although the polymer thickness exceeds 300 nm the GISAXS pattern of the film shows the same in-plane angle distribution Delta2theta as the underlying substrate. Annealing the polymer films at a temperature above its glass transition temperature Delta2theta changed from a broad to a narrow distribution as it is typically for films on pure silicon. The experiment can be interpreted by roughness replication and density fluctuation within the polymer film created while spin-coating at room temperature. Due to the low segment mobility there are density fluctuations which repeat the surface morphology of the substrate. Above the glass temperature the polymer density can be homogenized independently from the morphology of the substrate. (C) 2004 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{ZeimerPietschGrenzeretal.2005,
  author    = {Zeimer, Ute and Pietsch, Ullrich and Grenzer, Joerg and Fricke, J. and Knauer, A. and Weyers, Markus},
  title     = {Optimised two layer overgrowth of a lateral strain-modulated nanostructure},
  issn      = {0925-8388},
  year      = {2005},
  abstract  = {Recently it has been shown that lateral carrier confinement in an InGaAs quantum well (QW) embedded in GaAs can be achieved by using a laterally patterned InGaP stressor layer on top of the heterostructure. To exploit this effect in a device the structure has to be planarized by a second epitaxial step. It has been shown that the lateral strain modulation almost vanishes after overgrowth with GaAs, whereas overgrowth with a single ternary layer of opposite strain compared to the stressor layer suffers from strain induced decomposition. Here we show that the lateral carrier confinement of the initially free standing nanostructure can almost be maintained using a two step process for overgrowth, where a strained thin ternary layer is grown first followed by GaAs up to complete planarization of the patterned structure. Thickness and composition of the ternary layer are adjusted on the basis of finite element calculations of the strain distribution (FEM). The strain field achieved after overgrowth is probed by X-ray grazing- incidence diffraction (GID). (c) 2005 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{KarageorgievNeherSchulzetal.2005,
  author    = {Karageorgiev, Peter and Neher, Dieter and Schulz, Burkhard and Stiller, Burkhard and Pietsch, Ullrich and Giersig, Michael and Brehmer, Ludwig},
  title     = {From anisotropic photo-fluidity towards nanomanipulation in the optical near-field},
  issn      = {1476-1122},
  year      = {2005},
  abstract  = {An increase in random molecular vibrations of a solid owing to heating above the melting point leads to a decrease in its long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. Here we report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it shows considerable fluidity only in the direction parallel to the light field vector under linear polarized light. The fluidization phenomenon is related to photoinduced motion of azobenzene-functionalized molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also describe here how the photofluidization allows nanoscopic elements of matter to be precisely manipulated},
  language  = {en}
}
@article{HennebergPietschPanzneretal.2006,
  author    = {Henneberg, Oliver and Pietsch, Ullrich and Panzner, Tobias and Geue, Thomas and Finkelstein, Kenneth D.},
  title     = {Simultaneous X-ray and visible light diffraction for the investigation of surface relief and density grating formation in azobenzene containing polymer films},
  issn      = {1542-1406},
  doi       = {10.1080/15421400500383345},
  year      = {2006},
  abstract  = {The development of surface relief and density patterns in azobenzene polymer films was studied by diffraction at two different wavelengths. We used x-ray diffraction of synchrotron radiation at 0.124 nm in combination with visible light diffraction at a wavelength of 633 nm. In contrast to visible light scattering x-ray diffraction allows the separation of a surface relief and a density grating contribution due to the different functional dependence of the scattering power. Additionally, the x-ray probe is most sensitive for the onset of the surface grating formation},
  language  = {en}
}
@article{GorfmanTsirelsonPucheretal.2006,
  author    = {Gorfman, Semen and Tsirelson, Vladimir and Pucher, Andreas and Morgenroth, Wolfgang and Pietsch, Ullrich},
  title     = {X-ray diffraction by a crystal in a permanent external electric field : electric-field-induced structural response in alpha-GaPO4},
  issn      = {0108-7673},
  doi       = {10.1107/S0108767305036111},
  year      = {2006},
  abstract  = {For the first time, site-selective distortion has been investigated for two different structural units in the ternary compound alpha-GaPO4 under the influence of a permanent external electric field. Based on 54 measured reflection intensities, the electric-field-induced distortion of PO4 and GaO4 tetrahedra in alpha-GaPO4 crystals is evaluated using a model of pseudoatomic displacements introduced recently [Gorfman, Tsirelson \& Pietsch (2005). Acta Cryst. A61, 387- 396]. A stronger variation of the P-O bond lengths in the PO4 tetrahedron was found compared to the bonds in the GaO4 tetrahedron. The different distortions of the tetrahedra owing to the electric field were analysed in terms of the valence charge density of alpha-GaPO4 and its topological characteristics. The larger charge of the P pseudoatom compared to the Ga atom was recognized as the main reason for the higher sensitivity of the PO4 tetrahedron to a permanent external electric field},
  language  = {en}
}
@article{ZenSaphiannikovaNeheretal.2006,
  author    = {Zen, Achmad and Saphiannikova, Marina and Neher, Dieter and Grenzer, J{\"o}rg and Grigorian, Souren A. and Pietsch, Ullrich and Asawapirom, Udom and Janietz, Silvia and Scherf, Ullrich and Lieberwirth, Ingo and Wegner, Gerhard},
  title     = {Effect of molecular weight on the structure and crystallinity of poly(3-hexylthiophene)},
  doi       = {10.1021/Ma0521349},
  year      = {2006},
  abstract  = {Recently, two different groups have reported independently that the mobility of field-effect transistors made from regioregular poly(3-hexylthiophene) (P3HT) increases strongly with molecular weight. Two different models were presented: one proposing carrier trapping at grain boundaries and the second putting emphasis on the conformation and packing of the polymer chains in the thin layers for different molecular weights. Here, we present the results of detailed investigations of powders and thin films of deuterated P3HT fractions with different molecular weight. For powder samples, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate the structure and crystallization behavior of the polymers. The GPC investigations show that all weight fractions possess a rather broad molecular weight distribution. DSC measurements reveal a strong decrease of the crystallization temperature and, most important, a significant decrease of the degree of crystallinity with decreasing molecular weight. To study the structure of thin layers in lateral and vertical directions, both transmission electron microscopy (TEM) and X-ray grazing incidence diffraction (GID) were utilized. These methods show that thin layers of the low molecular weight fraction consist of well-defined crystalline domains embedded in a disordered matrix. We propose that the transport properties of layers prepared from fractions of poly(3-hexylthiophene) with different molecular weight are largely determined by the crystallinity of the samples and not by the perfection of the packing of the chains in the individual crystallites},
  language  = {en}
}
@article{LeitenbergerPietsch2007,
  author    = {Leitenberger, Wolfram and Pietsch, Ullrich},
  title     = {A monolithic Fresnel bimirror for hard X-rays and its application for coherence measurements},
  issn      = {0909-0495},
  doi       = {10.1107/S0909049507003846},
  year      = {2007},
  abstract  = {Experiments using a simple X-ray interferometer to measure the degree of spatial coherence of hard X-rays are reported. A monolithic Fresnel bimirror is used at small incidence angles to investigate synchrotron radiation in the energy interval 5-50 keV with monochromatic and white beam. The experimental setup was equivalent to a Young's double-slit experiment for hard X-rays with slit dimensions in the micrometre range. From the high-contrast interference pattern the degree of coherence was determined.},
  language  = {en}
}
@article{PanznerGleberSantetal.2007,
  author    = {Panzner, Tobias and Gleber, Gudrun and Sant, Tushar and Leitenberger, Wolfram and Pietsch, Ullrich},
  title     = {Coherence experiments at the white-beam beamline of BESSYI},
  doi       = {10.1016/j.tsf.2006.12.002},
  year      = {2007},
  abstract  = {The third-generation X-ray source BESSYII (Berlin, Germany) provides coherent X-ray radiation which can be used for static and dynamic speckle analysis. Recently we have demonstrated that one can perform experiments with coherent white radiation provided by a bending magnet (5 < E < 20 keV). In this paper we show that the diffraction figure of the initial pinhole must be considered for the interpretation of coherent experiments. The reflectivity spectrum of a sample results from the Fresnel diffraction of the incident pinhole deformed by the static speckle features of the sample surface. For dynamical experiments all speckle like features alter with time whereas the pure Fresnel fringes remain constant. (c) 2007 Published by Elsevier B.V.},
  language  = {en}
}
@article{BrajpuriyaTripathiSharmaetal.2007,
  author    = {Brajpuriya, Ranjeet and Tripathi, Sumit and Sharma, Abhishek and Chaudhari, S.M. and Phase, D.M. and Gupta, Ajay and Shripathi, Thoudinja and Leitenberger, Wolfram and Pietsch, Ullrich and Laxmi, N.},
  title     = {Temperature dependent energy-dispersive X-ray diffraction and magnetic study of Fe/Al interface},
  issn      = {0169-4332},
  doi       = {10.1016/j.apsusc.2007.04.069},
  year      = {2007},
  abstract  = {In situ temperature dependent energy-dispersive structural and magnetic study of electron beam evaporated Fe/Al multilayer sample (MLS) has been investigated. The structural studies show the formation of an intermixed FeAl transition layer of a few nanometers thick at the interface during deposition, which on annealing at 300 degrees C transforms to B2FeAl intermetallic phase. Magnetization decreases with increase in temperature and drops to minimum above 300 degrees C due to increase in anti-ferromagnetic interlayer coupling and formation of nonmagnetic FeAl phase at the interface. The Curie temperature (T-c) is found to be 288 degrees C and is much less than that of bulk bcc Fe.},
  language  = {en}
}
@article{LeitenbergerHartmannPietschetal.2008,
  author    = {Leitenberger, Wolfram and Hartmann, Robert and Pietsch, Ullrich and Andritschke, Robert and Starke, Ines and Str{\"u}der, Lothar},
  title     = {Application of a pnCCD in X-ray diffraction : a three-dimensional X-ray detector},
  doi       = {10.1107/S0909049508018931},
  year      = {2008},
  abstract  = {The first application of a pnCCD detector for X-ray scattering experiments using white synchrotron radiation at BESSY II is presented. A Cd arachidate multilayer was investigated in reflection geometry within the energy range 7 keV < E < 35 keV. At fixed angle of incidence the two-dimensional diffraction pattern containing several multilayer Bragg peaks and respective diffuse-resonant Bragg sheets were observed. Since every pixel of the detector is able to determine the energy of every incoming photon with a resolution Delta E/E similar or equal to 10(-2). a three-dimensional dataset is finally obtained. In order to achieve this energy resolution the detector was operated in the so-called single-photon- counting mode. A full dataset was evaluated taking into account all photons recorded within 10(5) detector frames at a readout rate of 200 Hz. By representing the data in reciprocal-space coordinates, it becomes obvious that this experiment with the pnCCD detector provides the same information as that obtained by combining a large number of monochromatic scattering experiments using conventional area detectors.},
  language  = {en}
}
@article{SendvonKozierowskiPanzneretal.2009,
  author    = {Send, Sebastian and von Kozierowski, Marc and Panzner, Tobias and Gorfman, Semen and Nurdan, Kivanc and Walenta, Albert H. and Pietsch, Ullrich and Leitenberger, Wolfram and Hartmann, Robert and Str{\"u}der, Lothar},
  title     = {Energy-dispersive Laue diffraction by means of a frame-store pnCCD},
  issn      = {0021-8898},
  doi       = {10.1107/S0021889809039867},
  year      = {2009},
  language  = {en}
}
@article{ReinholdGeueHuberetal.2009,
  author    = {Reinhold, Beate and Geue, Thomas and Huber, Patrick and Sant, Tushar and Pietsch, Ullrich and Sztucki, Michael},
  title     = {In situ and ex situ SAXS investigation of colloidal sedimentation onto laterally patterned support},
  issn      = {0743-7463},
  doi       = {10.1021/La803078b},
  year      = {2009},
  abstract  = {We report on in situ investigations of colloidal ordering during gravity sedimentation from a colloidal suspension onto a prepatterned support using a polymeric surface relief grating (SRG) as the support. The ordering of colloids with a diameter of 420 nm was investigated by means of grazing-incidence small-angle X-ray scattering (GISAXS) and transmission SAXS using a preparation cell guaranteeing stable temperature and humidity. GISAXS was used for in situ monitoring of the time evolution of colloidal ordering within the whole illuminated sample area. The onset of ordering was indicated by the increase of integrated intensity within a small time frame shortly before complete evaporation of the dispersant. Single domains of coated samples were investigated ex situ by SAXS in transmission geometry where the irradiated sample area was 200 x 200 mu m(2) only. Domains with the typical size of a few millimeters were observed varying in orientation and crystallographic structure for various positions at the sample. They were mainly oriented along the grooves of the grating, confirming the influence of the underlying grating on colloidal ordering.},
  language  = {en}
}
@article{ReddyGuptaGomeetal.2009,
  author    = {Reddy, Raghavendra V. and Gupta, Ajay and Gome, Anil and Leitenberger, Wolfram and Pietsch, Ullrich},
  title     = {In situ x-ray reflectivity and grazing incidence x-ray diffraction study of L1(0) ordering in Fe-57/Pt multilayers},
  issn      = {0953-8984},
  doi       = {10.1088/0953-8984/21/18/186002},
  year      = {2009},
  abstract  = {In situ high temperature x-ray reflectivity and grazing incidence x-ray diffraction measurements in the energy dispersive mode are used to study the ordered face-centered tetragonal (fct) L1(0) phase formation in [Fe(19 angstrom)/ Pt(25 angstrom)](x10) multilayers prepared by ion beam sputtering. With the in situ x-ray measurements it is observed that (i) the multilayer structure first transforms to a disordered FePt and subsequently to an ordered fct L1(0) phase, (ii) the ordered fct L1(0) FePt peaks start to appear at 320°C annealing, (iii) the activation energy of the interdiffusion is 0.8 eV and (iv) ordered fct FePt grains have preferential out-of-plane texture. The magneto-optical Kerr effect and conversion electron Mossbauer spectroscopies are used to study the magnetic properties of the as- deposited and 400°C annealed multilayers. The magnetic data for the 400°C annealed sample indicate that the magnetization is at an angle of ~50° from the plane of the film.},
  language  = {en}
}
@article{JoshiPingelGrigorianetal.2009,
  author    = {Joshi, Siddharth and Pingel, Patrick and Grigorian, Souren and Panzner, Tobias and Pietsch, Ullrich and Neher, Dieter and Forster, Michael and Scherf, Ullrich},
  title     = {Bimodal temperature behavior of structure and mobility in high molecular weight p3ht thin films},
  issn      = {0024-9297},
  doi       = {10.1021/Ma900021w},
  year      = {2009},
  abstract  = {We report a temperature dependent crystalline structure of spin-coated thin films of high molecular weight regioregular poly(3-hexylthiophene) (P3HT) (M-n similar to 30000 g/mol) and its correlation with charge carrier mobility. These investigations show a reversible change of the crystalline structure, where the interlayer lattice spacing (100)along the alkyl side chains continuously increases up to a temperature of about 220 degrees C; in contrast, the in-plane pi-pi distance reduces with increasing temperature. These changes in structure are reversible and can be repeated several times. The temperature-induced structural properties differ for thick and thin films, pointing to a surface/interface role in stabilization of the layer morphology. In contrast to the structural changes, the carrier mobility is rather constant in the temperature range from room temperature up to 100-120 degrees C, followed by a continuous decrease. For thick layers this drop is significant and the transistor performance almost vanishes at high temperature, however, it completely recovers upon cooling back to roorn temperature. The drop of the charge carrier mobility at higher temperatures is in contrast with expectations front the structural studies, considering the increase of crystalline fraction of the polycrystalline layer. our electrical measurements Underscore that the reduction of the macroscopic mobility is mostly caused by it pronounced decrease of the intergrain transport. The thermally induced crystallization along(100) direction and the creation of numerous small crystallites at the film-substrate interface reduce the number of long polymer chain, bridging crystalline domains, which ultimately limits the macroscopic charge transport.},
  language  = {en}
}
@article{SendAbboudLeitenbergeretal.2012,
  author    = {Send, Sebastian and Abboud, Ali and Leitenberger, Wolfram and Weiss, Manfred S. and Hartmann, Robert and Str{\"u}der, Lothar and Pietsch, Ullrich},
  title     = {Analysis of polycrystallinity in hen egg-white lysozyme using a pnCCD},
  series = {Journal of applied crystallography},
  volume    = {45},
  journal   = {Journal of applied crystallography},
  number    = {6},
  publisher = {Wiley-Blackwell},
  address   = {Hoboken},
  issn      = {0021-8898},
  doi       = {10.1107/S0021889812015038},
  pages     = {517 -- 522},
  year      = {2012},
  abstract  = {A crystal of hen egg-white lysozyme was analyzed by means of energy-dispersive X-ray Laue diffraction with white synchrotron radiation at 2.7 angstrom resolution using a pnCCD detector. From Laue spots measured in a single exposure of the arbitrarily oriented crystal, the lattice constants of the tetragonal unit cell could be extracted with an accuracy of about 2.5\%. Scanning across the sample surface, Laue images with split reflections were recorded at various positions. The corresponding diffraction patterns were generated by two crystalline domains with a tilt of about 1 degrees relative to each other. The obtained results demonstrate the potential of the pnCCD for fast X-ray screening of crystals of macromolecules or proteins prior to conventional X-ray structure analysis. The described experiment can be automatized to quantitatively characterize imperfect single crystals or polycrystals.},
  language  = {en}
}
@article{AbboudSendPashniaketal.2013,
  author    = {Abboud, Ali and Send, Sebastian and Pashniak, N. and Leitenberger, Wolfram and Ihle, Sebastian and Huth, M. and Hartmann, Robert and Str{\"u}der, Lothar and Pietsch, Ullrich},
  title     = {Sub-pixel resolution of a pnCCD for X-ray white beam applications},
  series = {Journal of instrumentation},
  volume    = {8},
  journal   = {Journal of instrumentation},
  number    = {3},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {1748-0221},
  doi       = {10.1088/1748-0221/8/05/P05005},
  pages     = {17},
  year      = {2013},
  abstract  = {A new approach to achieve sub-pixel spatial resolution in a pnCCD detector with 75 x 75 mu m(2) pixel size is proposed for X-ray applications in single photon counting mode. The approach considers the energy dependence of the charge cloud created by a single photon and its split probabilities between neighboring pixels of the detector based on a rectangular model for the charge cloud density. For cases where the charge of this cloud becomes distributed over three or four pixels the center position of photon impact can be reconstructed with a precision better than 2 mu m. The predicted charge cloud sizes are tested at selected X-ray fluorescence lines emitting energies between 6.4 keV and 17.4 keV and forming charge clouds with size (rms) varying between 8 mu m and 10 mu m respectively. The 2 mu m enhanced spatial resolution of the pnCCD is verified by means of an x-ray transmission experiment throughout an optical grating.},
  language  = {en}
}
@article{SendAbboudHartmannetal.2013,
  author    = {Send, Sebastian and Abboud, Ali and Hartmann, Robert and Huth, M. and Leitenberger, Wolfram and Pashniak, N. and Schmidt, J. and Str{\"u}der, Lothar and Pietsch, Ullrich},
  title     = {Characterization of a pnCCD for applications with synchrotron radiation},
  series = {Nuclear instruments \& methods in physics research : a journal on accelerators, instrumentation and techniques applied to research in nuclear and atomic physics, materials science and related fields in physics ; A, Accelerators, spectrometers, detectors and associated equipment},
  volume    = {711},
  journal   = {Nuclear instruments \& methods in physics research : a journal on accelerators, instrumentation and techniques applied to research in nuclear and atomic physics, materials science and related fields in physics ; A, Accelerators, spectrometers, detectors and associated equipment},
  number    = {5},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0168-9002},
  doi       = {10.1016/j.nima.2013.01.044},
  pages     = {132 -- 142},
  year      = {2013},
  abstract  = {In this work we study the response of a pnCCD by means of X-ray spectroscopy in the energy range between 6 key and 20 key and by Laue diffraction techniques. The analyses include measurements of characteristic detector parameters like energy resolution, count rate capability and effects of different gain settings. The limit of a single photon counting operation in white beam X-ray diffraction experiments is discussed with regard to the occurrence of pile-up events, for which the energy information about individual photons is lost. In case of monochromatic illumination the pnCCD can be used as a fast conventional CCD with a charge handling capacity (CHC) of about 300,000 electrons per pixel. If the CHC is exceeded, any surplus charge will spill to neighboring pixels perpendicular to the transfer direction due to electrostatic repulsion. The possibilities of increasing the number of storable electrons are investigated for different voltage settings by exposing a single pixel with X-rays generated by a microfocus X-ray source. The pixel binning mode is tested as an alternative approach that enables a pnCCD operation with significantly shorter readout times.},
  language  = {en}
}