@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{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{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{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{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{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{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{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} }