Tilman Butz

Raman scattering in ZnO thin films doped with Fe, Sb, al, Ga, and Li

Polarized micro-Raman measurements were performed to study the phonon modes of Fe, Sb, Al, Ga, and Li doped ZnO thin films, grown by pulsed-laser deposition on c-plane sapphire substrates. Additional modes at about 277, 511, 583, and 644 cm−1, recently assigned to N incorporation [A. Kaschner et al., Appl. Phys. Lett. 80, 1909 (2002)], were observed for Fe, Sb, and Al doped films, intentionally grown without N. The mode at 277 cm−1 occurs also for Ga doped films. These modes thus cannot be related directly to N incorporation. Instead, we suggest host lattice defects as their origin. Further additional modes at 531, 631, and 720 cm−1 seem specific for the Sb, Ga, and Fe dopants, respectively. Li doped ZnO did not reveal additional modes.

Ferromagnetism in oriented graphite samples

We have studied the magnetization of various well characterized samples of highly oriented pyrolitic graphite (HOPG), Kish graphite, and natural graphite to investigate the recently reported ferromagneticlike signal and its possible relation to ferromagnetic impurities. The magnetization results obtained for HOPG samples for applied fields parallel to the graphene layers---to minimize the diamagnetic background---show no correlation with the magnetic impurity concentration. Our overall results suggest an intrinsic origin for the ferromagnetism found in graphite. We discuss possible origins of the ferromagnetic signal.

Dielectric functions (1 to 5 eV) of wurtzite MgxZn1−xO (x⩽0.29) thin films

The optical dielectric functions for polarization perpendicular and parallel to the c-axis (optical axis) of pulsed-laser-deposition grown wurtzite MgxZn1−xO (0⩽x⩽0.29) thin films have been determined at room temperature using ellipsometry for photon energies from 1 to 5 eV. The dielectric functions reveal strong excitonic contributions for all Mg concentrations x. The band gap energies (E0A=3.369 eV for ZnO to 4.101 eV for x=0.29) show a remarkable blueshift. The exciton binding energy (61 meV for ZnO) decreases to approximately 50 meV for x≈0.17 and increases to approximately 58 meV for x=0.29. In contrast to ZnO, the MgxZn1−xO alloys are found uniaxial negative below the band gap energy, opposite to previously reported results.

Is there penetration of titania nanoparticles in sunscreens through skin? A comparative electron and ion microscopy study

We report on a comparative study by Transmission Electron Microscopy (HRTEM) and Scanning Transmission Ion Microscopy (STIM) combined with Rutherford Backscattering Spectrometry (RBS) and Particle Induced X-Ray Emission (PIXE) on ultra-thin and thin cross-sections, respectively, of various skin samples (porcine skin, healthy human skin, human skin grafted on a severe combined immuno-deficient mouse model) to which we applied topically various formulations containing titanium dioxide (TiO2) nanoparticles with primary particle sizes in the range from 20–100 nm. Whereas the HRTEM and STIM/PIXE images reveal clear differences – mainly related to the different thickness of the cross-sections – they unambiguously show that penetration of TiO2 nanoparticles is restricted to the topmost 3–5 corneocyte layers of the stratum corneum (SC).

Observation of intrinsic magnetic domains in C60 polymer

A C60 polymer has been characterized for the first time with respect to impurity content and ferromagnetic properties by laterally resolved particle induced X-ray emission (PIXE), superconducting quantum interference device (SQUID) and magnetic force microscopy (MFM) in order to detect intrinsic ferromagnetic domains. In parts of the pure regions (concentration of magnetic impurities <1 μg/g), we found stripe-domain magnetic images with different orientations of domain magnetization. The size of the regions where magnetic domains were observed is ∼30% of the pure region. All these results reveal that the polymerized C60 sample is a mixture of magnetic and non-magnetic parts and only a fraction of the sample contributes to the ferromagnetism.

Radiation-Induced Bystander Effects

Background: The bystander effect is a relatively new area of radiobiological research, which is aimed at studying post-radiation changes in neighboring non-hit cells or tissues. The bystander effect of ionizing irradiation is important after low-dose irradiation in the range of up to 0.2 Gy, where a higher incidence of stochastic damage was observed than was expected from a linear-quadratic model. It is also important when the irradiation of a cell population is highly non-uniform. Objective: This review summarizes most of the important results and proposed bystander effect mechanisms as well as their impact on theory and clinical practice. The literature, in parts contradictory, is collected, the main topics are outlined, and some basic papers are described in more detail. In order to illustrate the microbeam technique, which is considered relevant for the bystander effect research, the state of the Leipzig LIPSION nanoprobe facility is described. Results: The resistence of a radiation-induced bystander effect is now generally accepted. The current state of knowledge on it is summarized here. Several groups worldwide are working on understanding its different aspects and its impact on radiobiology and radiation protection. Conclusion: The observation of a bystander effect has posed many questions, and answering them is a challenging topic for radiobiology in the future.Hintergrund: Der strahleninduzierte Bystander-Effekt ist ein relativ neues Feld der strahlenbiologischen Forschung. Im Zentrum steht die Reaktion nicht direkt selbst getroffener Zellen oder Gewebe auf ionisierende Strahlung. Der strahleninduzierte Bystander-Effekt spielt eine wichtige Rolle vor allem im Niedrigdosisbereich ≤ 0,2 Gy, wo die Inzidenz des stochastischen Zellschadens in Experimenten größer ist als die Berechnung nach dem linearquadratischen Modell. Eine weitere wichtige Rolle spielt der Bystander-Effekt, wenn die Bestrahlung einer Zellpopulation hochgradig nichtuniform ist. Ziel: Dieser Übersichtsartikel fasst die wichtigsten Ergebnisse der Bystander-Effekt-Forschung zusammen und gibt einen Ausblick auf mögliche Mechanismen sowie auf die Bedeutung für Theorie und klinische Praxis. Die teilweise widersprüchliche Literatur zum Bystander-Effekt wird zusammengefasst und nach den Hauptthemen unterteilt, und einige grundlegende Artikel werden ausführlicher dargestellt. Zur Veranschaulichung der für den Bystander-Effekt relevanten Mikrobeam-Technik stellen wir den aktuellen Stand an der Leipziger Nanosonde LIPSION vor. Ergebnisse: Die Existenz eines radiogenen Bystander-Effekts ist inzwischen allgemein anerkannt. Der Stand des Wissens wird hier zusammengefasst. Weltweit befassen sich mehrere Arbeitsgruppen mit der Vertiefung der Erkenntnisse seiner verschiedenen Aspekte und den Folgen für Radiobiologie und Radioprotektion. Schlussfolgerung: Der Bystander-Effekt hat viele Fragen aufgeworfen, deren Beantwortung eine Herausforderung der Radiobiologie der Zukunft ist.

The Leipzig high-energy ion nanoprobe: A report on first results

Abstract The high-energy ion nanoprobe LIPSION at the University of Leipzig has been operational since October 1998. Its magnetic quadrupole lens system, arranged as a separated Russian quadruplet, has been developed by the Microanalytical Research Centre (MARC), Melbourne. The ultrastable single-ended 3.5 MV SINGLETRON™ accelerator (High Voltage Engineering Europa) supplies H+ and He+ ion beams with a beam brightness in the range of 10–20 A rad −2 m −2 eV −1 [D.J.W. Mous, R.G. Haitsma, T. Butz, R.-H. Flagmeyer, D. Lehmann, J. Vogt, Nucl. Instr. and Meth. B 130 (1997) 31]. Due to this high brightness, the excellent optical properties of the focusing system of the nanoprobe and the suppression of mechanical vibrations, lateral resolutions of 100 nm for the low current mode (STIM) and 340 nm at a current of 10 pA (PIXE, RBS, SEI modes) were achieved. Further improvements are expected.

Radiation-induced bystander effects. Mechanisms, biological implications, and current investigations at the Leipzig LIPSION facility.

Background: The bystander effect is a relatively new area of radiobiological research, which is aimed at studying post-radiation changes in neighboring non-hit cells or tissues. The bystander effect of ionizing irradiation is important after low-dose irradiation in the range of up to 0.2 Gy, where a higher incidence of stochastic damage was observed than was expected from a linear-quadratic model. It is also important when the irradiation of a cell population is highly non-uniform. Objective: This review summarizes most of the important results and proposed bystander effect mechanisms as well as their impact on theory and clinical practice. The literature, in parts contradictory, is collected, the main topics are outlined, and some basic papers are described in more detail. In order to illustrate the microbeam technique, which is considered relevant for the bystander effect research, the state of the Leipzig LIPSION nanoprobe facility is described. Results: The resistence of a radiation-induced bystander effect is now generally accepted. The current state of knowledge on it is summarized here. Several groups worldwide are working on understanding its different aspects and its impact on radiobiology and radiation protection. Conclusion: The observation of a bystander effect has posed many questions, and answering them is a challenging topic for radiobiology in the future.Hintergrund: Der strahleninduzierte Bystander-Effekt ist ein relativ neues Feld der strahlenbiologischen Forschung. Im Zentrum steht die Reaktion nicht direkt selbst getroffener Zellen oder Gewebe auf ionisierende Strahlung. Der strahleninduzierte Bystander-Effekt spielt eine wichtige Rolle vor allem im Niedrigdosisbereich ≤ 0,2 Gy, wo die Inzidenz des stochastischen Zellschadens in Experimenten größer ist als die Berechnung nach dem linearquadratischen Modell. Eine weitere wichtige Rolle spielt der Bystander-Effekt, wenn die Bestrahlung einer Zellpopulation hochgradig nichtuniform ist. Ziel: Dieser Übersichtsartikel fasst die wichtigsten Ergebnisse der Bystander-Effekt-Forschung zusammen und gibt einen Ausblick auf mögliche Mechanismen sowie auf die Bedeutung für Theorie und klinische Praxis. Die teilweise widersprüchliche Literatur zum Bystander-Effekt wird zusammengefasst und nach den Hauptthemen unterteilt, und einige grundlegende Artikel werden ausführlicher dargestellt. Zur Veranschaulichung der für den Bystander-Effekt relevanten Mikrobeam-Technik stellen wir den aktuellen Stand an der Leipziger Nanosonde LIPSION vor. Ergebnisse: Die Existenz eines radiogenen Bystander-Effekts ist inzwischen allgemein anerkannt. Der Stand des Wissens wird hier zusammengefasst. Weltweit befassen sich mehrere Arbeitsgruppen mit der Vertiefung der Erkenntnisse seiner verschiedenen Aspekte und den Folgen für Radiobiologie und Radioprotektion. Schlussfolgerung: Der Bystander-Effekt hat viele Fragen aufgeworfen, deren Beantwortung eine Herausforderung der Radiobiologie der Zukunft ist.

High electron mobility of phosphorous-doped homoepitaxial ZnO thin films grown by pulsed-laser deposition

The transport properties of phosphorous-doped ZnO thin films, grown by pulsed-laser deposition on thermally pretreated hydrothermally grown ZnO single-crystal substrates, are reported. The ZnO:P thin films show very good morphological and structural properties as confirmed by atomic force microscopy (AFM), high resolution x-ray diffraction, and Rutherford backscattering (RBS) channeling. Steps of height c/2 are visible in AFM investigations for all samples. For an oxygen partial pressure of 0.1 mbar, two-dimensional growth was found. RBS channeling of a ZnO:P film shows a minimum yield of 0.034 which is comparable to that of an annealed substrate (0.033). Hall effect measurements revealed that all films are n-type for the present growth conditions. Peak mobilities of 800 cm2/Vs have been observed around 70 K, in line with the high structural quality of the samples. Room-temperature mobility in ZnO:P is up to 170 cm2/Vs.

Optical and TDPAC spectroscopy of Hg(II)-rubredoxin: model for a mononuclear tetrahedral [Hg(CysS)4]2− center

Rubredoxins possess a well-defined mononuclear tetrahedral tetrathiolate metal binding site, a feature exploited by several investigations to study the spectroscopic characteristics and the coordination chemistry of different metal ions at this binding site. In the present work, Hg(II)-substituted rubredoxin (Rd) from Desulfovibrio gigas has been studied by electronic absorption, circular dichroism (CD), magnetic circular dichroism (MCD), and time differential perturbed angular correlation of γ-rays (TDPAC) spectroscopies. The TDPAC spectrum of 199mHg-Rd at pH 8 exhibits a prevailing nuclear quadrupole interaction (NQI) with a precession frequency of ω1=0.09 Grad/s and an asymmetry parameter η=0, features characteristic of a slightly distorted tetrahedral tetrathiolate metal coordination, i.e, a HgCysS4 center. In addition, three minor populated NQIs have also been detected. They may represent a trigonal HgS3 (ω1=1.13 Grad/s, η=0.21), a digonal HgS2 (ω1=1.34 Grad/s, η=0.20), and a digonal Hg(II) coordination (ω1=1.58 Grad/s, η= 0.18) with unidentified ligands. Since similar studies at pH 2.5 revealed a time-dependent increase of the HgCysS4 population, the low populated sites may represent intermediate Hg(II) complexes formed prior to the generation of the thermodynamically stable structure. The metal-induced absorption envelope of Hg-Rd reveals three distinct transitions with Gaussian-resolved maxima located at 230, 257, and 284 nm, which are paralleled by dichroic features in the corresponding difference CD spectrum of Hg(II)-Rd versus apo-Rd. Based on the optical electronegativity theory of Jørgensen, the lowest energy transition has been attributed to a CysS-Hg(II) charge-transfer excitation. The Td type of metal coordination in Hg-Rd is supported by the presence of an unresolved A-term with a negative lobe at 295 nm in the difference MCD spectrum. These results point to the usefulness of optical and TDPAC spectroscopies for studying Hg(II) sites in other proteins.