S. Grafström

Technische Universität Dresden, Dresden, Saxony, Germany

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Publications (62)123.67 Total impact

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    ABSTRACT: Films of cerium-doped LaMnO$_3$, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La$_{0.7}$Ce$_{0.3}$MnO$_3$ (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2+/3+ state, independently of the film thickness and the degree of CeO$_2$ segregation. Charge injection from the photoconductive SrTiO$_3$ substrate into the Mn e$_g$ band with carrier lifetimes in the range of tens of seconds is discussed as the most probable source of the Mn valence shift and the subsequent electron doping.
    Journal of Physics Condensed Matter 01/2014; 26. DOI:10.1088/0953-8984/26/4/045502 · 2.22 Impact Factor
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    Dataset: 1310.4937v2
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    ABSTRACT: Surface photovoltage (SPV) spectroscopy, which is a versatile method to analyze the energetic distribution of electronic defect states at surfaces and interfaces of wide-bandgap semiconductor (hetero-)structures, is applied to comparatively investigate heterostructures made of 5-unit-cell-thick LaAlO$_3$ films grown either on TiO$_2$- or on SrO-terminated SrTiO$_3$. As shown in a number of experimental and theoretical investigations in the past, these two interfaces exhibit dramatically different properties with the first being conducting and the second insulating. Our present SPV investigation reveals clearly distinguishable interface defect state distributions for both configurations when interpreted within the framework of a classical semiconductor band scheme. Furthermore, bare SrTiO$_3$ crystals with TiO$_2$ or mixed SrO/TiO$_2$ terminations show similar SPV spectra and transients as do LaAlO$_3$-covered samples with the respective termination of the SrTiO$_3$ substrate. This is in accordance with a number of recent works that stress the decisive role of SrTiO$_3$ and the minor role of LaAlO$_3$ with respect to the electronic interface properties.
    Journal of Applied Physics 12/2013; 114(24):243709. DOI:10.1063/1.4858376 · 2.19 Impact Factor
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    ABSTRACT: The material built-up at sharp corners of a workpiece during electroplating, so-called dog-boning, was utilized for bottom-up creation of nanofences – free standing lines of high aspect ratio nanowires – on silicon oxide substrates. This was realized by electroplating gold into nanoporous aluminum oxide templates with underlying micropatterned gold pads, serving as working electrodes. Using the right parameter set, initial preferred nucleation at the gold pad rim site and the unique nanomatrix geometry led to the formation of lines of nanowires at the rim site separated by an adjacent depletion area from an extended bulk nanorod array region. It was found that the underlying mechanisms of this growth process are the increased electric field densities at the electrode edges and a field screening effect due to already grown neighboring wires, both identified by finite element electric field simulations, as well as the restricted diffusion dynamics of gold ions in the presence of nanopores. The obtained high aspect ratio nanofences might be applicable for enhanced catalysis, dielectric index sensing or in optical filters.
    Journal of The Electrochemical Society 11/2013; 161(1):D26-D30. DOI:10.1149/2.024401jes · 2.86 Impact Factor
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    ABSTRACT: Films of cerium-doped LaMnO$_3$, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La$_{0.7}$Ce$_{0.3}$MnO$_3$ (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2$+$/3$+$ state, independently of the film thickness and the degree of CeO$_2$ segregation. Charge injection from the photoconductive SrTiO$_3$ substrate into the Mn e$_g$ band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron-hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping.
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    ABSTRACT: In the past, surface photovoltage (SPV) analysis has been successfully applied to derive the electronic defect status of a number of wide-bandgap semiconductor surfaces. Here, the method is applied to the model perovskite strontium titanate, whose SPV phenomena are comprehensively studied over seven decades of excitation-light intensity. The SPV was recorded by a Kelvin probe setup as a function of wavelength in order to extract the energetic positions of electronic surface states within the bandgap. At selected wavelengths addressing distinct surface states, SPV transients were measured as a function of light intensity and temperature. Several models known from the literature were used to estimate and cross check surface state parameters such as surface state densities, capture cross sections for photons and electrons, and the surface band bending in the dark and under illumination. In contrast to other wide-bandgap materials, SPV transients of SrTiO3 exhibit highly complex shapes, i.e. they (i) show signatures of multiple carrier transitions, (ii) mixtures of surface and bulk contributions, as well as (iii) both ex- and intrinsic SPV processes.
    Surface Science 06/2013; 612:1–9. DOI:10.1016/j.susc.2013.01.022 · 1.87 Impact Factor
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    ABSTRACT: We demonstrate that simulating plasmonic nanostructures by means of curved elements (CEs) significantly increases the accuracy and computation speed not only in the linear but also in the nonlinear regime. We implemented CEs within the discontinuous Galerkin (DG) method and, as an example of a nonlinear effect, investigated second-harmonic generation (SHG) at a silver nanoparticle. The second-harmonic response of the material is simulated by an extended Lorentz model (ELM). In the linear regime the CEs are ≈ 9 times faster than ordinary elements for the same accuracy, provide a much better convergence and show fewer unphysical field artifacts. For DG-SHG calculations CEs are almost indispensable to obtain physically reasonable results at all. Additionally, their boundary approximation has to be of the same order as their polynomial degree to achieve artifact-free field distributions. In return, the use of such CEs with the DG method pays off more than evidently, since the additional computation time is only 1%.
    Optics Express 07/2011; 19(15):14426-36. DOI:10.1364/OE.19.014426 · 3.53 Impact Factor
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    ABSTRACT: The irradiation of the junction of an STM with laser light may give access to the study of resonant effects. Since high laser intensities are required, an active compensation setup, designed to suppress the dominating nonresonant effects is used. Mixed films of the dye perylene-tetracarboxylic-dianhydride (PTCDA) and the liquid crystal octylcyanobiphenyl (8CB) were prepared on graphite and MoS2. STM measurements show small isolated domains of PTCDA embedded in 8CB. The islands could be imaged with high resolution even under the illumination with intense laser light. First experiments investigating the laser-induced contrast observed on the dye islands under active compensation of the nonresonant effects are discussed.
    07/2011: pages 269-274;
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    ABSTRACT: The outcoupling of light from AlGaAs-based light-emitting diodes (LEDs) can be improved by means of metal nanoparticles deposited on the surface of the device: light that would otherwise remain trapped in the high- refractive-index material by total internal reflection is scattered to the outside. We present an experimental study on the emission enhancement produced by single isolated gold nanoparticles of various sizes ( 60 – 150 nm in diameter) and compare the results with numerical simulations. We find a clear enhancement as long as the dipole plasmon resonance of the particle is at a shorter wavelength than the LED emission. If the plasmon resonance coincides with the LED emission or is at a larger wavelength, the enhancement turns into damping. The simulations indicate that this latter effect is mainly caused by the particle quadrupole resonance producing absorption.
    Journal of the Optical Society of America B 03/2011; 28(4):851-855. DOI:10.1364/JOSAB.28.000851 · 1.81 Impact Factor
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    ABSTRACT: The energetic distribution of electronic defect states in oxide heterostructures made of ultrathin lanthanum manganite (La0.7Ca0.3MnO3, La0.7Ce0.3MnO3) films on SrTiO3 substrates is investigated by surface photovoltage (SPV) spectroscopy. Within a comparative evaluation of the SPV spectra of both the film/substrate structures and pure substrates at different temperatures we were able to elaborate a map of defect states across the SrTiO3-bandgap and we find that the defect state distribution is mainly affected by the substrate, i.e., no specific film-induced defect states were detected. Possible origins of the defect states are discussed within the framework of a semiconductor band scheme, taking into account complementary photoconductivity and SPV transient data.
    Materials Science and Engineering B 03/2011; 176(5):446-452. DOI:10.1016/j.mseb.2010.12.014 · 2.12 Impact Factor
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    ABSTRACT: Fluorescence microscopy is a widespread and elaborate method in imaging. However, in practice several disadvantages, most notably bleaching and blinking, can be observed. Second-harmonic generation (SHG) in noncentrosymmetric nanocrystals with diameters up to 100 nm seem to be a promising alternative. These particles circumvent the mentioned constraints as no real transition is invoked and thereby no phototoxic or dark state can be excited. Long-time stability as well as sufficient signal strength for standard microscopy techniques was obtained for barium titanate nanoparticles with tetragonal crystal structure. The SHG signal increases quadratically with the excitation intensity (typ. 10 9 W/cm2); these nanoparticles outshine all fluorescent markers at dye saturation. Moreover, the emitted second-harmonic radiation is spectrally sharp (∼6 nm) so that the signal-to-noise ratio can be augmented by means of narrow bandpass filters. We present the experimental results on the nonlinear optical properties of single BaTiO3 nanoparticles including the dependence on intensity, excitation wavelength and in-plane polarization as well as obtain the according crystal axis theoretically.
    DPG Spring Meeting of the Condensed Matter Section (SKM) and the Atomic, Molecular, Plasma Physics and Quantum Optics Section; 01/2011
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    ABSTRACT: Our work concentrates on triggering photochemical reactions on the nanometer length scale by means of nonlinear optical effects [1,2]. We use metallic nanostructures illuminated by a focused femtosecond laser beam at 800 nm wavelength in order to induce localized photochemical reactions in a specially synthesized polymer thin film. Due to strong electric field enhancements at sharp edges and tips, second-harmonic (SHG) light thus is produced at 400 nm. Since the nanostructures are embedded in a photoreactive polymer matrix, both absorption of the SHG light and multiphoton absorption of the fundamental wave can invoke photochemical reactions within the near-field of the metallic nanoantennas. Thus photochemical reactions can be induced on length scales below 100 nm. This research paves the way towards a new photochemical lithography technique on the nanometer length scale. Here we present first tests of the polymers used, especially in order to clarify if SHG or two-photon absorption is the dominant mechanism for these photo- chemical reactions
    DPG Spring Meeting of the Condensed Matter Section (SKM) and the Atomic, Molecular, Plasma Physics and Quantum Optics Section; 01/2011
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    ABSTRACT: We show that the optical properties of arrays of parallel-aligned metallic nanorods can be understood by means of a retarded dipolar interaction model. Exemplarily, arrays of gold nanorods having various lengths and diameters are investigated experimentally. A strong diameter dependence of the long-axis surface plasmon resonance (LSPR) as well as a lower energy limit of this mode for varying length was found. The model also shows that, for small nanorod distances ( d < λ / 2 ) , the optical properties are independent of the azimuthal angle of the incoming plane wave and of the detailed arrangement of the nanorods. Furthermore, the model was used to explain the dependence of the LSPR on the angle of incidence and to find the conditions for which negative and extraordinary positive refractions occur in these structures.
    Journal of the Optical Society of America B 08/2010; 27(9):1819-1827. DOI:10.1364/JOSAB.27.001819 · 1.81 Impact Factor
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    ABSTRACT: We report on novel approaches using scanning force methods [i.e. piezoresponse force microscopy (PFM), Kelvin probe force microscopy (KPFM) and pull-off force spectroscopy (PFS)] in order to deduce the local dielectric and polarization properties of PZT thin films both at outer and inner interfaces with a lateral resolution <50 nm. We show that the polarization profile into the depth of the PZT sample varies dramatically being built up at the bottom Pt electrode over a transition layer of more than 200 nm in thickness. The results are explained both in the view of negatively charged defects pinned at the PZT/Pt interface as well as the possible variation in the local dielectric properties across the film thickness. Investigating the latter made the quantitative deduction of values such as the effective dielectric polarization P z , the deposited charge density σ, and the surface dielectric constant ϵsurface in thin ferroelectric PZT films necessary. We illustrate that such measurements in fact are possible on the nanometer scale revealing quantitative data when combining PFM and PFS.
    Integrated Ferroelectrics 08/2010; 2004(Vol. 62):13-21. DOI:10.1080/10584580490460277 · 0.37 Impact Factor
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    ABSTRACT: We report on implementing curved elements (CEs) into the discontinuous Galerkin (DG) method in order to improve the nano-optical simulation of realistic nanostructures. Compared to straight meshes, CEs as realized here by triangles with one bent side allow for a faster, more accurate and robust computation. Clearly, CEs much better match real physical nanostructures, where surface energies, tension and adhesion give rise to rounded geometries rather than planar surfaces. The novel code was tested by calculating the spatial field distributions and scattering cross sections for two-dimensional (2D) nano-objects, namely a nanosphere and a V-groove. When using a fixed mesh size, we found that CEs much more accurately describe the curved geometries than ordinary linear elements, leading to significantly smaller errors. Moreover, CEs turned out to be between 2.5 and 37 times faster for the same error margin and to be more robust against unphysical behavior, such as hot spots at element vertices or unmotivated spectral features. Integrating CEs into the DG algorithm thus constitutes an excellent choice for numerical modeling of complex problems-especially in the context of plasmonics.
    Journal of Computational and Theoretical Nanoscience 07/2010; 7(8):1581-1586. DOI:10.1166/jctn.2010.1522 · 1.03 Impact Factor
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    ABSTRACT: The electrical resistance of stoichiometric and oxygen-deficient epitaxial 10 nm thick La(0.7)Ca(0.3)MnO(3) thin films on SrTiO(3) under photoexcitation covering the visible to the ultraviolet range has been investigated systematically as a function of illumination intensity, wavelength and temperature. In contrast to as-prepared films, the oxygen-deficient samples exhibit large photoconductivity of several orders of magnitude at low temperatures. By our detailed comparative analysis of the electrical conductivity of the film/substrate heterostructure and the bare substrate we are able to elucidate contributions of both carrier generation in the film and carrier injection from the substrate to the observed effect.
    Journal of Physics Condensed Matter 05/2010; 22(17):175506. DOI:10.1088/0953-8984/22/17/175506 · 2.22 Impact Factor
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    ABSTRACT: Tetravalent-ion-doped lanthanum manganite films typically suffer from overoxygenation in the as-prepared state, which in turn leads to an effective hole doping instead of the nominal electron doping. This problem can be overcome by post-deposition annealing in a reducing atmosphere, which, however, suppresses the phase transition from an insulating to a metallic phase at the magnetic ordering temperature so that the films are insulating in the whole temperature range. In the present work, reduced La0.7Ce0.3MnO3−δ thin films were investigated with respect to their transport characteristics under photoexcitation. While the films are insulating in the dark, even the exposure to diffuse daylight recovers the insulator-metal transition (IMT). Excitation with continuous visible laser light further decreases the resistance by up to seven orders of magnitude and shifts the IMT to higher temperatures. The spectral, temporal, intensity, and temperature dependences of the photoconductivity have been investigated. The results suggest that (i) the manganite film shows a light-induced IMT and large photoconductivity, (ii) the substrate has an influence on the photoconductivity (through carrier injection into the film and/or substrate photoconduction) that grows with decreasing wavelength of the light, and (iii) an electron-doped metal state might be present under photoexcitation.
    Physical Review B 08/2009; 80(7):075106. DOI:10.1103/PhysRevB.80.075106 · 3.66 Impact Factor
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    ABSTRACT: For two-dimensional (2D) arrays of metallic nanorods arranged perpendicular to a substrate several methods have been proposed to determine the electromagnetic near-field distribution and the surface plasmon resonances, but an analytical approach to explain all optical features on the nanometer length scale has been missing to date. To fill this gap, we demonstrate here that the field distribution in such arrays can be understood on the basis of surface plasmon polaritons (SPPs) that propagate along the nanorods and form standing waves. Notably, SPPs couple laterally through their optical near fields, giving rise to collective surface plasmon (CSP) effects. Using the dispersion relation of such CSPs, we deduce the condition of standing-wave formation, which enables us to successfully predict several features, such as eigenmodes and resonances. As one such property and potential application, we show both theoretically and in an experiment that CSP propagation allows for polarization conversion and optical filtering in 2D nanorod arrays. Hence, these arrays are promising candidates for manipulating the light polarization on the nanometer length scale.
    Optics Express 01/2009; 16(26):21671-81. DOI:10.1364/OE.16.021671 · 3.53 Impact Factor
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    ABSTRACT: We report on the implementation of metal nanoparticles as probes for scattering and apertureless near-field optical investigations in the mid-infrared (mid-IR) spectral regime. At these wavelengths, an efficient electric-field confinement is necessary and achieved here through a gold metal nanoparticle of 80 nm in diameter (Au80-MNP) acting as the optical antenna. The Au80-MNP is attached to a standard AFM cantilever used as the spatial manipulator. When approached to a sample surface while being illuminated with an infrared beam, the Au80-MNP produces a considerably improved spatial confinement of the electric field compared to an ordinary scattering AFM tip. We demonstrate here the confinement normal to the sample surface by making use of a sample-induced phonon polariton resonance in a ferroelectric lithium niobate sample. Our experimental findings are in very good agreement with the quasistatic dipole model and show improved optical resolution via well-selected antenna particles.
    Optics Express 09/2008; 16(16):12302-12. DOI:10.1364/OE.16.012302 · 3.53 Impact Factor
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    ABSTRACT: We demonstrate the imaging of ferroelectric domains in BaTiO3, using an infrared-emitting free-electron laser as a tunable optical source for scattering scanning near-field optical microscopy and spectroscopy. When the laser is tuned into the spectral vicinity of a phonon resonance, ferroelectric domains can be resolved due to the anisotropy of the dielectric properties of the material. Slight detuning of the wavelength gives rise to a contrast reversal clearly evidencing the resonant character of the excitation. The near-field domain contrast shows that the orientation of the dielectric tensor with respect to the sample surface has a clear influence on the near-field signal.
    Physical Review Letters 06/2008; 100(25):256403. DOI:10.1103/PhysRevLett.100.256403 · 7.73 Impact Factor

Publication Stats

964 Citations
123.67 Total Impact Points

Institutions

  • 2001–2014
    • Technische Universität Dresden
      • • Institut für Angewandte Photophysik
      • • Institute of Applied Computer Science
      Dresden, Saxony, Germany
  • 1988–2011
    • Universität Heidelberg
      • Institute of Physics
      Heidelburg, Baden-Württemberg, Germany
  • 2002
    • Leibniz Institute of Polymer Research Dresden
      Dresden, Saxony, Germany
  • 1993
    • GSI Helmholtzzentrum für Schwerionenforschung
      • Materials Research
      Darmstadt, Hesse, Germany