Prof. Dr. Norbert Esser

associated member

Grenzflächenanalytik
Leibniz-Institut für Analytische Wissenschaften – ISAS – e.V.

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  • Controlled growth of ordered monolayers of N-heterocyclic carbenes on silicon
    Franz, M. and Chandola, S. and Koy, M. and Zielinski, R. and Aldahhak, H. and Das, M. and Freitag, M. and Gerstmann, U. and Liebig, D. and Hoffmann, A.K. and Rosin, M. and Schmidt, W.G. and Hogan, C. and Glorius, F. and Esser, N. and Dähne, M.
    Nature Chemistry (2021)
    N-Heterocyclic carbenes (NHCs) are promising modifiers and anchors for surface functionalization and offer some advantages over thiol-based systems. Because of their strong binding affinity and high electron donation, NHCs can dramatically change the properties of the surfaces to which they are bonded. Highly ordered NHC monolayers have so far been limited to metal surfaces. Silicon, however, remains the element of choice in semiconductor devices and its modification is therefore of utmost importance for electronic industries. Here, a comprehensive study on the adsorption of NHCs on silicon is presented. We find covalently bound NHC molecules in an upright adsorption geometry and demonstrate the formation of highly ordered monolayers exhibiting good thermal stability and strong work function reductions. The structure and ordering of the monolayers is controlled by the substrate geometry and reactivity and in particular by the NHC side groups. These findings pave the way towards a tailor-made organic functionalization of silicon surfaces and, thanks to the high modularity of NHCs, new electronic and optoelectronic applications. [Figure not available: see fulltext.] © 2021, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstract10.1038/s41557-021-00721-2
  • Determination of residual dimethyl sulfoxide by high-resolution continuum source graphite furnace molecular absorption spectrometry
    Huang, M.D. and Esser, N. and Hinrichs, K. and Tan, Q. and Rappich, J. and Nickel, N.H. and Dittrich, T.
    Spectrochimica Acta - Part B Atomic Spectroscopy 177 (2021)
    High-resolution continuum source molecular absorption spectrometry (HR-CSMAS) has been extended to the determination of sulfur-containing organic molecule, on the example of dimethyl sulfoxide (DMSO) as a residual solvent in CH3NH3PbI3 perovskite thin films. For this purpose, DMSO molecules were converted into CS molecules by pyrolysis in a graphite furnace at low temperature and the sulfur content was determined by measuring molecular absorption of CS at 258.055 nm. An aqueous solution of DMSO was used for calibration. A characteristic mass of 17 ng was achieved for S after pyrolysis at 160 °C by using Pd as a chemical modifier. Furthermore, the content of DMSO was normalized to that of Pb whereas the content of Pb was determined by reducing the Pb in the perovskite to metallic Pb with Zn powder and measuring absorption of the weak Pb line at 261.418 nm by high-resolution continuum source atomic absorption spectrometry (HR-CSAAS). The Pb content remained constant whereas the S/Pb molar ratio decreased with increasing annealing time. Our results open new opportunities for the characterization of residual DMSO in wide classes of materials. © 2020 Elsevier B.V.
    view abstract10.1016/j.sab.2020.106050
  • pH-Dependent Flavin Adenine Dinucleotide and Nicotinamide Adenine Dinucleotide Ultraviolet Resonance Raman (UVRR) Spectra at Intracellular Concentration
    Merk, V. and Speiser, E. and Werncke, W. and Esser, N. and Kneipp, J.
    Applied Spectroscopy (2021)
    The ultraviolet resonance Raman spectra of the adenine-containing enzymatic redox cofactors nicotinamide adenine dinucleotide and flavin adenine dinucleotide in aqueous solution of physiological concentration are compared with the aim of distinguishing between them and their building block adenine in potential co-occurrence in biological materials. At an excitation wavelength of 266 nm, the spectra are dominated by the strong resonant contribution from adenine; nevertheless, bands assigned to vibrational modes of the nicotinamide and the flavin unit are found to appear at similar signal strength. Comparison of spectra measured at pH 7 with data obtained pH 10 and pH 3 shows characteristic changes when pH is increased or lowered, mainly due to deprotonation of the flavin and nicotinamide moieties, and protonation of the adenine, respectively. © The Author(s) 2021.
    view abstract10.1177/00037028211025575
  • Surface localized phonon modes at the Si(553)-Au nanowire system
    Plaickner, J. and Speiser, E. and Braun, C. and Schmidt, W.G. and Esser, N. and Sanna, S.
    Physical Review B 103 (2021)
    The vibrational properties of the Si(553)-Au surface are studied by Raman spectroscopy and ab initio calculations. A multitude of surface localized phonon modes with wave number below 200 cm-1 is experimentally observed, along with two modes at about 400 cm-1. Atomistic models within density functional theory allow to assign the low-energy spectral features to vibrations within the Au chain, while the Raman signatures at higher energies are mostly localized at the Si step edge. The Raman activity of nominally silent modes associated with a large charge transfer between Au chain and Si step edge states is explained by scattering at charge density fluctuations. Temperature-dependent measurements reveal specific mode shifts that are discussed in terms of a recently proposed order-disorder phase transition. The presence of model-specific displacement patterns allows us to identify the structural models compatible with the measured spectra at low and at room temperature. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.103.115441
  • Adsorption of toluene-3,4-dithiol on silver islands investigated by surface-enhanced Raman spectroscopy
    Plaickner, J. and Speiser, E. and Chandola, S. and Esser, N. and Singh, D.K.
    Journal of Raman Spectroscopy 51 (2020)
    The adsorbtion of toluene-3,4-dithiol (TDT) molecules on Ag islands was investigated with surface-enhanced Raman spectroscopy (SERS). The SERS spectrum was compared with the ordinary Raman spectrum of TDT, and differences in spectral features were discussed with the support of density functional theory calculations. The disappearance of all the S–H-related bands in the SERS spectrum indicates that TDT adsorbs on Ag via both thiol groups, as supported by the emergence of a weak band at 223 cm−1 attributed to Ag–S stretching. Signatures of Ag–S–C complexes suggest that the adsorption is partially disordered. The observation of all the vibrational modes involving the methyl group indicates that the methyl group is exposed, offering new functionalization possibilities of the surface. © 2020 John Wiley & Sons, Ltd.
    view abstract10.1002/jrs.5843
  • Vibration-Driven Self-Doping of Dangling-Bond Wires on Si(553)-Au Surfaces
    Braun, C. and Neufeld, S. and Gerstmann, U. and Sanna, S. and Plaickner, J. and Speiser, E. and Esser, N. and Schmidt, W.G.
    Physical Review Letters 124 (2020)
    Density-functional theory is used to explore the Si(553)-Au surface dynamics. Our study (i) reveals a complex two-stage order-disorder phase transition where with rising temperature first the ×3 order along the Si step edges and, subsequently, the ×2 order of the Au chains is lost, (ii) identifies the transient modification of the electron chemical potential during soft Au chain vibrations as instrumental for disorder at the step edge, and (iii) shows that the transition leads to a self-doping of the Si dangling-bond wire at the step edge. The calculations are corroborated by Raman measurements of surface phonon modes and explain previous electron diffraction, scanning tunneling microscopy, and surface transport data. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/" Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevLett.124.146802
  • Vibrational Raman spectroscopy on adsorbate-induced low-dimensional surface structures
    Speiser, E. and Esser, N. and Halbig, B. and Geurts, J. and Schmidt, W.G. and Sanna, S.
    Surface Science Reports 75 (2020)
    Low-dimensional self-organized surface structures, induced by (sub)monolayer metal adsorbates on semiconductor surfaces may give rise not only to a variety of emergent electronic properties, but also to a multitude of specific localized vibronic features. The focus of this review is on the analysis of these novel surface vibration eigenmodes. The application of in situ surface Raman spectroscopy under UHV conditions on clean semiconductor surfaces and those with self-ordered adsorbates, in close conjunction with the calculations of Raman spectra, based on the first-principles determination of the structural, electronic and vibronic properties, allows a consistent determination of the vibration eigenfrequencies, symmetry properties, and elongation patterns of the systems of interest. The localized nature of the surface eigenmodes determines the surface sensitivity, independent of the large penetration depth of light. The surface contribution can be selectively enhanced by employing resonance conditions to surface electronic transitions. Moreover, surface and bulk contributions can be separated by taking difference spectra between various stages of surface preparation. The relevant surfaces are Ge and especially Si with different orientations ((111) and vicinal (hhk)), on which the adsorption of various metals (Au, Sn, Pb, or In) gives rise to two- and quasi-one-dimensional structures (e.g. Au-(5 × 2)/Si(111)) with a variety of vibration modes. The Raman analysis of these modes not only enables the distinction between different proposed structural models (e.g. for Au-(3×3)/Si(111)), but also gives access to the role of electron-phonon coupling in structural phase transitions (e.g. for In-(8 × 2)–(4 × 1)/Si(111)). © 2020 The Authors
    view abstract10.1016/j.surfrep.2020.100480
  • Electric Field Induced Raman Scattering at the Sb–InP(110) Interface: The Surface Dipole Contribution
    Esser, N. and Schmidt, W.-G.
    Physica Status Solidi (B) Basic Research 256 (2019)
    Using the prototypical interface of Sb on InP(110), it has been shown that strong local electric dipole fields may arise at semiconductor surfaces which need to be considered in addition to band bending-induced electric fields for explaining electro-optic effects. Symmetry forbidden LO phonon Raman scattering, which has been often used in the past to analyze internal electric fields related to band bending, is a suitable optical probe. The surface local dipole field, related to reconstruction or relaxation-dependent atomic displacements in the outermost atomic planes, delivers a significant contribution to the forbidden LO phonon scattering, in addition to the band bending-related Schottky field. Taking this into account, apparent inconsistencies of the electric field-induced Raman scattering (EFIRS) and photoemission spectroscopy from the previous work can be resolved. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201800314
  • Gradient metal nanoislands as a unified surface enhanced Raman scattering and surface enhanced infrared absorption platform for analytics
    Gkogkou, D. and Shaykhutdinov, T. and Kratz, C. and Oates, T.W.H. and Hildebrandt, P. and Weidinger, I.M. and Ly, K.H. and Esser, N. and Hinrichs, K.
    Analyst 144 (2019)
    In the last few decades, the use of plasmonics in vibrational spectroscopy has expanded the scope of (bio)analytical investigations. Nevertheless, there is a demand for a combined platform that can be simultaneously efficient for Surface Enhanced Raman Scattering (SERS) and Surface Enhanced Infrared Absorption (SEIRA). Here, we present a solution on the basis of a plasmonic Ag nanoparticle layer with a thickness gradient. The optical resonance along the layer varies from the visible to the infrared range offering optimal and intermediate sites for SERS and SEIRA of the analyte molecule (mercaptobenzonitrile). Enhancement factors for the same mode were determined to be ca. 104 and 170 for SERS and SEIRA, respectively. We present a full optical and vibrational characterization and demonstrate further tunability. The platform resolves reproducibility and comparability issues by a combination of the two methods. It also offers individualized solutions for different investigation conditions, i.e. a choice between excitation wavelengths and resonant Raman molecules. The multiple applicabilities of the presented unifying substrate can contribute to the expansion of the vibrational spectroscopic field and to analytics. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9an00839j
  • Organic Molecule Adsorption on Stepped Si–Au Surfaces: Role of Functional Group on Geometry and Electronic Structure
    Hogan, C. and Suchkova, S. and Bechstedt, F. and Speiser, E. and Chandola, S. and Esser, N.
    Physica Status Solidi (B) Basic Research 256 (2019)
    Stepped gold-stabilized Si(111) surfaces offer much potential for self-organized assembly of one- and two-dimensional nanostructured arrays of organic molecules due to the presence of regular atomic chains of Au and Si on the surface, parallel to the step edges. In this theoretical study, the adsorption of various simple organic molecules with different functional groups on the Si(553)–Au surface has been investigated. It has been found that adsorption at the step edge site is favored by all species. If the step edge is first passivated by hydrogen, adsorption occurs on Si atoms on the terraces. Instead, adsorption on the gold chains is completely unfavored. The influence of organic molecule adsorption on the electronic band structure and surface metallicity is investigated. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201800653
  • Raman Spectroscopy on Surface Phonons of Si(hhk) Surfaces Modified by Au Submonolayers
    Speiser, E. and Plaickner, J. and Chandola, S. and Esser, N. and Halbig, B. and Geurts, J.
    Physica Status Solidi (B) Basic Research 256 (2019)
    Raman spectroscopy has evolved to a sensitive tool for analyzing vibration modes of ordered (sub)monolayers of metal adatoms on semiconductor surfaces, which are typical systems for electron correlation effects. We report on the in situ UHV Raman analysis of self-organized Au submonolayers on nominal Si(111) surfaces, as well as on vicinal Si(553), Si(775), and Si(557). For the two-dimensional Au-(√3×√3 and the one-dimensional Au-(5 × 2) reconstruction several specific Raman peaks in the spectral range below 200 cm −1 are assigned to Au-dominated vibration modes, while Si-step-edge vibration modes around 400 cm −1 are fingerprints of the substrate orientation. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201800341
  • A new concept of efficient therapeutic drug monitoring using the high-resolution continuum source absorption spectrometry and the surface enhanced Raman spectroscopy
    Xing, Y. and Fuss, H. and Lademann, J. and Huang, M.D. and Becker-Ross, H. and Florek, S. and Patzelt, A. and Meinke, M.C. and Jung, S. and Esser, N.
    Spectrochimica Acta - Part B Atomic Spectroscopy 142 (2018)
    In this study, a new therapeutic drug monitoring approach has been tested based on the combination of CaF molecular absorption using high-resolution continuum source absorption spectrometry (HR-CSAS) and surface enhanced Raman spectroscopy (SERS). HR-CSAS with mini graphite tube was successfully tested for clinical therapeutic drug monitoring of the fluorine-containing drug capecitabine in sweat samples of cancer patients: It showed advantageous features of high selectivity (no interference from Cl), high sensitivity (characteristic mass of 0.1 ng at CaF 583.069 nm), low sample consumption (down to 30 nL) and fast measurement (no sample pretreatment and less than 1 min of responding time) in tracing the fluorine signal out of capecitabine. However, this technique has the disadvantage of the total loss of the drug's structure information after burning the sample at very high temperature. Therefore, a new concept of combining HR-CSAS with a non-destructive spectroscopic method (SERS) was proposed for the sensitive sensing and specific identification of capecitabine. We tested and succeed in obtaining the molecular characteristics of the metabolite of capecitabine (named 5-fluorouracil) by the non-destructive SERS technique. With the results shown in this work, it is demonstrated that the combined spectroscopic technique of HR-CSAS and SERS will be very useful in efficient therapeutic drug monitoring in the future. © 2018
    view abstract10.1016/j.sab.2018.02.007
  • An in situ XPS study of L-cysteine co-adsorbed with water on polycrystalline copper and gold
    Jürgensen, A. and Raschke, H. and Esser, N. and Hergenröder, R.
    Applied Surface Science 435 (2018)
    The interactions of biomolecules with metal surfaces are important because an adsorbed layer of such molecules introduces complex reactive functionality to the substrate. However, studying these interactions is challenging: they usually take place in an aqueous environment, and the structure of the first few monolayers on the surface is of particular interest, as these layers determine most interfacial properties. Ideally, this requires surface sensitive analysis methods that are operated under ambient conditions, for example ambient pressure x-ray photoelectron spectroscopy (AP-XPS). This paper focuses on an AP-XPS study of the interaction of water vapour and l-Cysteine on polycrystalline copper and gold surfaces. Thin films of l-Cysteine were characterized with XPS in UHV and in a water vapour atmosphere (P ≤ 1 mbar): the structure of the adsorbed l-Cysteine layer depended on substrate material and deposition method, and exposure of the surface to water vapour led to the formation of hydrogen bonds between H2O molecules and the COO− and NH2 groups of adsorbed l-Cysteine zwitterions and neutral molecules, respectively. This study also proved that it is possible to investigate monolayers of biomolecules in a gas atmosphere with AP-XPS using a conventional laboratory Al-Kα x-ray source. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.11.150
  • Controlling the Local Electronic Properties of Si(553)-Au through Hydrogen Doping
    Hogan, C. and Speiser, E. and Chandola, S. and Suchkova, S. and Aulbach, J. and Schäfer, J. and Meyer, S. and Claessen, R. and Esser, N.
    Physical Review Letters 120 (2018)
    We propose a quantitative and reversible method for tuning the charge localization of Au-stabilized stepped Si surfaces by site-specific hydrogenation. This is demonstrated for Si(553)-Au as a model system by combining density functional theory simulations and reflectance anisotropy spectroscopy experiments. We find that controlled H passivation is a two-step process: step-edge adsorption drives excess charge into the conducting metal chain "reservoir" and renders it insulating, while surplus H recovers metallic behavior. Our approach illustrates a route towards microscopic manipulation of the local surface charge distribution and establishes a reversible switch of site-specific chemical reactivity and magnetic properties on vicinal surfaces. © 2018 American Physical Society..
    view abstract10.1103/PhysRevLett.120.166801
  • Design and synthesis of a series of bioavailable fatty acid synthase (FASN) KR domain inhibitors for cancer therapy
    Lu, T. and Schubert, C. and Cummings, M.D. and Bignan, G. and Connolly, P.J. and Smans, K. and Ludovici, D. and Parker, M.H. and Meyer, C. and Rocaboy, C. and Alexander, R. and Grasberger, B. and De Breucker, S. and Esser, N. and Fraiponts, E. and Gilissen, R. and Janssens, B. and Peeters, D. and Van Nuffel, L. and Vermeulen, P. and Bischoff, J. and Meerpoel, L.
    Bioorganic and Medicinal Chemistry Letters 28 (2018)
    We designed and synthesized a new series of fatty acid synthase (FASN) inhibitors with potential utility for the treatment of cancer. Extensive SAR studies led to highly active FASN inhibitors with good cellular activity and oral bioavailability, exemplified by compound 34. Compound 34 is a potent inhibitor of human FASN (IC50 = 28 nM) that effectively inhibits proliferation of A2780 ovarian cells (IC50 = 13 nM) in lipid-reduced serum (LRS). This cellular activity can be rescued by addition of palmitate, consistent with an on-target effect. Compound 34 is also active in many other cell types, including PC3M (IC50 = 25 nM) and LnCaP-Vancouver prostate cells (IC50 = 66 nM), and is highly bioavailable (F 61%) with good exposure after oral administration. In a pharmacodynamics study in H460 lung xenograft-bearing mice, oral treatment with compound 34 results in elevated tumor levels of malonyl-CoA and decreased tumor levels of palmitate, fully consistent with the desired target engagement. © 2018 Elsevier Ltd
    view abstract10.1016/j.bmcl.2018.05.014
  • Electronic Properties of Ag Reconstructions on Si(111): Coulomb Blockade Behavior at Room Temperature
    Chandola, S. and McGilp, J. and Esser, N.
    Physica Status Solidi (B) Basic Research 255 (2018)
    The electronic properties and optical response of Si(111)–Ag reconstructions, with distinct interface structures, are studied with scanning tunneling microscopy (STM), reflectance anisotropy spectroscopy (RAS), and infrared spectroscopic ellipsometry (IRSE). The results are compared to previous studies on the Si(111)-(3 × 1)–Ag surface. Depending on the preparation conditions, STM shows hybrid surfaces comprising (√3 × √3) areas with semiconducting (3 × 1)–Ag regions or metallic Ag islands. RAS of the (√3 × √3)–Ag reconstruction mostly shows a flat response, confirming the isotropic nature of the surface while Ag islands on this surface produce a steep increase in the infrared as well as a strong optical anisotropy at 3.5 eV. Scanning tunneling spectroscopy (STS) of the islands shows a high density of states with almost no structure. STS of Ag nanodots on the (3 × 1)–Ag surface shows distinct Coulomb resonances within the Si bandgap while the Si(111)-(3 × 1)–Ag surface reveals an expected semiconducting behavior with a band gap and no sign of resonances. Ag islands on the (3 × 1)–Ag surface also show weak Coulombic oscillations indicating that local transport depends on the electronic properties of the interface. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201700494
  • Influence of anisotropic Si(111)-(4 × 1)-In surface on growth of nanoscale in islands
    Chandola, S. and Esser, N.
    Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 36 (2018)
    Reflectance anisotropy spectroscopy (RAS) and scanning tunneling microscopy (STM) are used to study the growth of indium (In) on the anisotropic Si(111)-(4 × 1)-In surface at room temperature. RAS shows that epitaxial growth of In is accompanied by the disappearance of the surface optical anisotropy at 1.9 eV which is the fingerprint for the (4 × 1)-In surface reconstruction and the appearance of a large minimum at 1.4 eV which is at the same energy as interband transitions observed on bulk crystalline In. Subsequent spectra taken on the surface, over 3 h, show that this minimum decreases and eventually disappears along with the reappearance of the original RAS signature of the (4 × 1)-In surface. STM of this surface shows elongated, anisotropic In crystal islands on top of a (4 × 1)-In reconstructed surface. Upon annealing the surface to 720 K, the surface reconstruction changes with STM showing regions covered with a phase that resembles the (√7 × √3)-In reconstruction and RAS showing a large positive anisotropy at about 1.5 eV. The epitaxial In islands now show a hexagonal shape, unlike on the (4 × 1) surface. Thus, the growth morphology of the islands is shown to be dependent on the initial surface reconstruction. The authors attribute these findings to Ostwald ripening of the In islands mediated by diffusion, which is dependent on the structurally different In wetting layers on the Si substrate. © 2018 Author(s).
    view abstract10.1116/1.5031228
  • Optical properties of In2O3 from experiment and first-principles theory: Influence of lattice screening
    Schleife, A. and Neumann, M.D. and Esser, N. and Galazka, Z. and Gottwald, A. and Nixdorf, J. and Goldhahn, R. and Feneberg, M.
    New Journal of Physics 20 (2018)
    The framework of many-body perturbation theory led to deep insight into electronic structure and optical properties of diverse systems and, in particular, many semiconductors. It relies on an accurate approximation of the screened Coulomb electron-electron interaction W, that in current implementations is usually achieved by describing electronic interband transitions. However, our results for several oxide semiconductors indicate that for polar materials it is necessary to also account for lattice contributions to dielectric screening. To clarify this question in this work, we combine highly accurate experimentation and cutting-edge theoretical spectroscopy to elucidate the interplay of quasiparticle and excitonic effects for cubic bixbyite In2O3 across an unprecedentedly large photon energy range. We then show that the agreement between experiment and theory is excellent and, thus, validate that the physics of quasiparticle and excitonic effects is described accurately by these first-principles techniques, except for the immediate vicinity of the absorption onset. Finally, our combination of experimental and computational data clearly establishes the need for including a lattice contribution to dielectric screening in the screened electron-electron interaction, in order to improve the description of excitonic effects near the absorption edge. © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/aabeb0
  • Ordinary dielectric function of corundumlike α-Ga2 O3 from 40 meV to 20 eV ORDINARY DIELECTRIC FUNCTION of CORUNDUMLIKE ... MARTIN FENEBERG et al.
    Feneberg, M. and Nixdorf, J. and Neumann, M.D. and Esser, N. and Artús, L. and Cuscó, R. and Yamaguchi, T. and Goldhahn, R.
    Physical Review Materials 2 (2018)
    The linear optical response of metastable α-Ga2O3 is investigated by spectroscopic ellipsometry. We determine the ordinary dielectric function from lattice vibrations up to the vacuum ultraviolet spectral range at room temperature for a sample with a (0001) surface. Three out of four Eu infrared-active phonon modes are unambiguously determined, and their frequencies are in good agreement with density functional theory calculations. The dispersion of the refractive index in the visible and ultraviolet part of the spectrum is determined. High-energy interband transitions are characterized up to 20eV. By comparison with the optical response of α-Al2O3 and with theoretical results, a tentative assignment of interband transitions is proposed. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.044601
  • Selective adsorption of toluene-3,4-dithiol on Si(553)-Au surfaces
    Suchkova, S. and Hogan, C. and Bechstedt, F. and Speiser, E. and Esser, N.
    Physical Review B 97 (2018)
    The adsorption of small organic molecules onto vicinal Au-stabilized Si(111) surfaces is shown to be a versatile route towards controlled growth of ordered organic-metal hybrid one-dimensional nanostructures. Density functional theory is used to investigate the site-specific adsorption of toluene-3,4-dithiol (TDT) molecules onto the clean Si(553)-Au surface and onto a co-doped surface whose steps are passivated by hydrogen. We find that the most reactive sites involve bonding to silicon at the step edge or on the terraces, while gold sites are relatively unfavored. H passivation and TDT adsorption both induce a controlled charge redistribution within the surface layer, causing the surface metallicity, electronic structure, and chemical reactivity of individual adsorption sites to be substantially altered. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.045417
  • Surface vibrations in the T4 and H3 Pb phases on Si(111)
    Speiser, E. and Baumann, A. and Chandola, S. and Esser, N. and Zahedifar, M. and Kratzer, P. and Tegenkamp, C.
    Physical Review B 98 (2018)
    We present here a combined experimental Raman spectroscopy and ab initio theoretical study of the vibrational modes of the (3×3) reconstructed SIC phase of Pb on Si(111) and discuss their relation to the atomic surface structure. The Raman response of the surface localized vibrational modes, in particular, is identified in the low-frequency spectral range (down to 15cm-1). We demonstrate that Raman spectroscopy is a very powerful approach to test atomic structures of surfaces and a valuable complement to standard surface analytics. While the calculated spectra of H3 and T4 are too similar to allow a discrimination of these phases, the good overall agreement to the measured Raman spectra enables a classification of the observed vibrational modes. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.195427
  • Vibrational properties of the Au-(3 × 3)/Si(111) surface reconstruction
    Halbig, B. and Liebhaber, M. and Bass, U. and Geurts, J. and Speiser, E. and Räthel, J. and Chandola, S. and Esser, N. and Krenz, M. and Neufeld, S. and Schmidt, W.G. and Sanna, S.
    Physical Review B 97 (2018)
    The vibrational properties of the Au-induced (3×3)R30 reconstruction of the Si(111) surface are investigated by polarized surface Raman spectroscopy and density-functional theory. The Raman measurements are performed in situ at room temperature as well as 20 K, and they reveal the presence of vibrational eigenmodes in the spectral range from 20 to 450 cm-1. In particular, two peaks of E symmetry at 75 and 183 cm-1 dominate the spectra. No substantial difference between room- and low-temperature spectra is observed, suggesting that the system does not undergo a phase transition down to 20 K. First-principles calculations are performed based on the structural models discussed in the literature. The thermodynamically stable conjugate honeycomb-chained-trimer model (CHCT) [Surf. Sci. 275, L691 (1992)SUSCAS0039-602810.1016/0039-6028(92)90785-5] leads to phonon eigenvalues compatible with the experimental observations in the investigated spectral range. On the basis of the phonon eigenfrequencies, symmetries, and Raman intensities, we assign the measured spectral features to the calculated phonon modes. The good agreement between measured and calculated modes provides a strong argument in favor of the CHCT model. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.035412
  • Bi-Axial Growth Mode of Au-TTF Nanowires Induced by Tilted Molecular Column Stacking
    Xing, Y. and Speiser, E. and Singh, D.K. and Dittrich, P.S. and Esser, N.
    Journal of Physical Chemistry C 121 (2017)
    In this study, to understand the molecular self-organization in metal-organic charge-transfer nanowires, single gold-tetrathiafulvalene (Au-TTF) nanowires were analyzed using polarized Raman spectroscopy, combined with density functional theory (DFT) calculations. To verify the methodology, an investigation was done for neutral tetrathiafulvalene (TTF) bulk crystals with well-known structure. On the basis of the DFT calculation of the molecular Raman tensor and simulation of the angular-dependent depolarization ratio, the molecular orientation in single TTF crystals was verified. Thereon, the combined experimental and ab initio-simulation method was applied to study single Au-TTF nanowires. Our results clearly demonstrate, in contrast to the commonly accepted parallel molecular stacking model, that at least two molecules with different orientations are located in the unit cell of the nanowire's crystal structure. The new tilted molecular column stacking wire model explains also the axial and radial growth mechanism of Au-TTF wires. © 2017 American Chemical Society.
    view abstract10.1021/acs.jpcc.7b05924
  • Characterization of anisotropically shaped silver nanoparticle arrays via spectroscopic ellipsometry supported by numerical optical modeling
    Gkogkou, D. and Shaykhutdinov, T. and Oates, T.W.H. and Gernert, U. and Schreiber, B. and Facsko, S. and Hildebrandt, P. and Weidinger, I.M. and Esser, N. and Hinrichs, K.
    Applied Surface Science 421 (2017)
    The present investigation aims to study the optical response of anisotropic Ag nanoparticle arrays deposited on rippled silicon substrates by performing a qualitative comparison between experimental and theoretical results. Spectroscopic ellipsometry was used along with numerical calculations using finite-difference time-domain (FDTD) method and rigorous coupled wave analysis (RCWA) to reveal trends in the optical and geometrical properties of the nanoparticle array. Ellipsometric data show two resonances, in the orthogonal x and y directions, that originate from localized plasmon resonances as demonstrated by the calculated near-fields from FDTD calculations. The far-field calculations by RCWA point to decoupled resonances in x direction and possible coupling effects in y direction, corresponding to the short and long axis of the anisotropic nanoparticles, respectively. © 2016 Elsevier B.V.
    view abstract10.1016/j.apsusc.2016.10.105
  • Dependence of the optical constants and the performance in the SPREE gas measurement on the thickness of doped tin oxide over coatings
    Fischer, D. and Hertwig, A. and Beck, U. and Negendank, D. and Lohse, V. and Kormunda, M. and Esser, N.
    Applied Surface Science 421 (2017)
    In this study, thickness related changes of the optical properties of doped tin oxide were studied. Two different sets of samples were prepared. The first set was doped with iron or nickel on silicon substrate with thicknesses of 29-56. nm, the second was iron doped on gold/glass substrate with 1.6-6.3. nm. The optical constants were determined by using spectral ellipsometry (SE) followed by modelling of the dielectric function with an oscillator model using Gaussian peaks. The analysis of the optical constants shows a dependence of the refraction and the absorption on the thickness of the doped tin oxide coating. In addition to the tin oxide absorption in the UV, one additional absorption peak was found in the near-IR/red which is related to plasmonic effects due to the doping. This peak shifts from the near-IR to the red part of the visible spectrum and becomes stronger by reducing the thickness, probably due to the formation of metal nanoparticles in this layer. These results were found for two different sets of samples by using the same optical model. Afterwards the second sample set was tested in the Surface Plasmon Resonance Enhanced Ellipsometric (SPREE) gas measurement with CO gas. It was found that the thickness has significant influence on the sensitivity and thus the adsorption of the CO gas. By increasing the thickness from 1.6. nm to 5.1. nm, the sensing ability is enhanced due to a higher coverage of the surface with the over coating. This is explained by the high affinity of CO molecules to the incorporated Fe-nanoparticles in the tin oxide coating. By increasing the thickness further to 6.3. nm, the sensing ability drops because the layer disturbs the SPR sensing effect too much. © 2016.
    view abstract10.1016/j.apsusc.2016.11.188
  • Localized Synthesis of Conductive Copper-Tetracyanoquinodimethane Nanostructures in Ultrasmall Microchambers for Nanoelectronics
    Xing, Y. and Sun, G. and Speiser, E. and Esser, N. and Dittrich, P.S.
    ACS Applied Materials and Interfaces 9 (2017)
    In this work, the microfluidic-assisted synthesis of copper-tetracyanoquinodimethane (Cu-TCNQ) nanostructures in an ambient environment is reported for the first time. A two-layer microfluidic device comprising parallel actuated microchambers was used for the synthesis and enabled excellent fluid handling for the continuous and multiple chemical reactions in confined ultrasmall chambers. Different precautions were applied to ensure the reduction state of copper (Cu) for the synthesis of Cu-TCNQ charge-transfer compounds. The localized synthesis of Cu and in situ transformation to Cu-TCNQ complexes in solution were achieved by applying different gas pressures in the control layer. Additionally, various diameters of the Cu-TCNQ nano/microstructures were obtained by adjusting the concentration of the precursors and reaction time. After the synthesis, platinum (Pt) microelectrode arrays, which were aligned at the microchambers, could enable the in situ measurements of the electronic properties of the synthesized nanostructures without further manipulation. The as-prepared Cu-TCNQ wire bundles showed good conductivity and a reversible hysteretic switching effect, which proved the possibility in using them to build advanced nanoelectronics. © 2017 American Chemical Society.
    view abstract10.1021/acsami.7b01664
  • Optical anisotropy of quasi-1D rare-earth silicide nanostructures on Si(001)
    Chandola, S. and Speiser, E. and Esser, N. and Appelfeller, S. and Franz, M. and Dähne, M.
    Applied Surface Science 399 (2017)
    Rare earth metals are known to interact strongly with Si(001) surfaces to form different types of silicide nanostructures. Using STM to structurally characterize Dy and Tb silicide nanostructures on vicinal Si(001), it will be shown that reflectance anisotropy spectroscopy (RAS) can be used as an optical fingerprint technique to clearly distinguish between the formation of a semiconducting two-dimensional wetting layer and the metallic one-dimensional nanowires. Moreover, the distinctive spectral features can be related to structural units of the nanostructures. RAS spectra of Tb and Dy nanostructures are found to show similar features. © 2016 Elsevier B.V.
    view abstract10.1016/j.apsusc.2016.12.044
  • Thin SnOx films for surface plasmon resonance enhanced ellipsometric gas sensing (SPREE)
    Fischer, D. and Hertwig, A. and Beck, U. and Lohse, V. and Negendank, D. and Kormunda, M. and Esser, N.
    Beilstein Journal of Nanotechnology 8 (2017)
    Background: Gas sensors are very important in several fields like gas monitoring, safety and environmental applications. In this approach, a new gas sensing concept is investigated which combines the powerful adsorption probability of metal oxide conductive sensors (MOS) with an optical ellipsometric readout. This concept shows promising results to solve the problems of cross sensitivity of the MOS concept.Results: Undoped tin oxide (SnOx) and iron doped tin oxide (Fe:SnOx) thin add-on films were prepared by magnetron sputtering on the top of the actual surface plasmon resonance (SPR) sensing gold layer. The films were tested for their sensitivity to several gas species in the surface plasmon resonance enhanced (SPREE) gas measurement. It was found that the undoped tin oxide (SnOx) shows higher sensitivities to propane (C3H8) then to carbon monoxide (CO). By using Fe:SnOx, this relation is inverted. This behavior was explained by a change of the amount of binding sites for CO in the layer due to this iron doping. For hydrogen (H2) no such relation was found but the sensing ability was identical for both layer materials. This observation was related to a different sensing mechanism for H2 which is driven by the diffusion into the layer instead of adsorption on the surface. Conclusion: The gas sensing selectivity can be enhanced by tuning the properties of the thin film overcoating. A relation of the binding sites in the doped and undoped SnOx films and the gas sensing abilities for CO and C3H8 was found. This could open the path for optimized gas sensing devices with different coated SPREE sensors. © 2017 Fischer et al.; licensee Beilstein-Institut.
    view abstract10.3762/bjnano.8.56
  • Conductive single nanowires formed and analysed on microfluidic devices
    Xing, Y. and Esser, N. and Dittrich, P.S.
    Journal of Materials Chemistry C 4 (2016)
    In this work, we studied the formation of fibres and particles made of metal salts and derivatives of tetrathiafulvalene (TTF) on a microfluidic device and in a conventional reaction flask, and characterized their morphologies, optical properties and electrical conductivities. A series of uniform one-dimensional (1D) structures were successfully formed via charge transfer interactions at the interface of two laminar streams on the microdevice. In general, these structures were significantly thinner and longer than those obtained in the standard bulk approach. The reaction between metal ions and different TTF derivatives indicated that the high planarity and strong molecular interaction of TTF derivatives are beneficial for nanowire formation. The conductivities of these metal-tetrathiafulvalene (M-TTF) and metal-formyl-tetrathiafulvalene (M-FTTF) nanowires were in the range of 10-1 to 101 S cm-1 at room temperature, being one to two orders of magnitude greater than those of metal-bis(ethylenedithio)tetrathiafulvalene (M-BEDT-TTF) (4.8 × 10-3 to 1.2 × 10-2 S cm-1 at room temperature). However, not all combinations of metal salts and TTF derivatives yielded fibres. In many cases, we obtained particles or dendritic structures both in bulk reactions and on the microdevice. Hence, our study provides a comprehensive overview of the morphologies of the products obtained from reactions of metal salts and TTF including different commercially available derivatives. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6tc02791a
  • Grand canonical Peierls transition in In/Si(111)
    Jeckelmann, E. and Sanna, S. and Schmidt, W.G. and Speiser, E. and Esser, N.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    Starting from a Su-Schrieffer-Heeger-like model inferred from first-principles simulations, we show that the metal-insulator transition in In/Si(111) is a first-order grand canonical Peierls transition in which the substrate acts as an electron reservoir for the wires. This model explains naturally the existence of a metastable metallic phase over a wide temperature range below the critical temperature and the sensitivity of the transition to doping. Raman scattering experiments corroborate the softening of the two Peierls deformation modes close to the transition. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.241407
  • Inversion of absorption anisotropy and bowing of crystal field splitting in wurtzite MgZnO
    Neumann, M.D. and Esser, N. and Chauveau, J.-M. and Goldhahn, R. and Feneberg, M.
    Applied Physics Letters 108 (2016)
    The anisotropic optical properties of wurtzite MgxZn1-xO thin films (0≤x≤0.45) grown on m-plane ZnO substrates by plasma assisted molecular beam epitaxy are studied using spectroscopic ellipsometry at room temperature. The data analysis provides the dielectric functions for electric field polarizations perpendicular and parallel to the optical axis. The splitting between the absorption edges of the two polarization directions decreases between x = 0 and x = 0.24, while an inverted absorption anisotropy is found at higher Mg content, indicating a sign change of the crystal field splitting Δcr as for the spin orbit parameter. The characteristic energies such as exciton binding energies and band gaps are determined from the analysis of the imaginary parts of the dielectric functions. In particular, these data reveal a bowing parameter of b=-283 meV for describing the compositional dependence of the crystal field splitting and indicate Δcr=-327 meV for wurtzite MgO. The inverted valence band ordering of ZnO (Γ7-Γ9-Γ7) is found to be preserved with increasing Mg content, while the optical selection rules interchange. © 2016 Author(s).
    view abstract10.1063/1.4953159
  • Polarization- and Wavelength-Dependent Surface-Enhanced Raman Spectroscopy Using Optically Anisotropic Rippled Substrates for Sensing
    Gkogkou, D. and Schreiber, B. and Shaykhutdinov, T. and Ly, H. K. and Kuhlmann, U. and Gernert, U. and Facsko, S. and Hildebrandt, P. and Esser, N. and Hinrichs, K. and Weidinger, I. M. and Oates, T. W. H.
    Acs Sensors 1 (2016)
    Anisotropic Ag nanoparticle arrays were created by metal evaporation on rippled silicon templates for sensing of molecules with surface-enhanced Raman spectroscopy. Our results show that these substrates can be used for analysis of complex molecular mixtures and discrimination of solvent molecules. These properties are due to their polarization and wavelength dependency that provide enhancement in a wide spectral range. The dielectric function parallel and perpendicular to the long axis of the nanostructures was determined via ellipsometry yielding two different plasmonic resonances. Polarized surface-enhanced raman scattering (SERS) was subsequently measured as a function of the polarization angle theta for a 4-mercaptobenzonitrile self-assembled monolayer covalently attached to the Ag surface. For 514 nm excitation a cos(2)theta-dependence and for 647 nm excitation a sine theta-dependency were found, with the maxima expressing the resonances perpendicular and parallel to the ripples, respectively. Those results open the path for using such a substrate as a chemical sensor providing strong enhancement in a broad range of laser wavelengths on only one sensing surface and increasing the specificity by matching resonant Raman conditions.
    view abstract10.1021/acssensors.5b00176
  • Preparation and structure of ultra-thin GaN (0001) layers on In0.11Ga0.89N-single quantum wells
    Alamé, S. and Quezada, A.N. and Skuridina, D. and Reich, C. and Henning, D. and Frentrup, M. and Wernicke, T. and Koslow, I. and Kneissl, M. and Esser, N. and Vogt, P.
    Materials Science in Semiconductor Processing 55 (2016)
    Multiple surface reconstructions have been observed on ultra-thin GaN (0001) layers of 1-10nm thickness, covering a 3nm thick In0.11Ga0.89N single quantum well in a GaN matrix. Low energy electron diffraction patterns show (2×2) and (√3×√3)-R30° symmetries for samples annealed in nitrogen plasma, and (2×2), (3×3), (4×4), and (6×6) symmetries for samples overgrown with an additional monolayer-thin GaN film by molecular beam epitaxy under Ga-rich growth conditions. Photoelectron spectroscopy shows that the InGaN quantum wells and capping layers are stable for growth temperatures up to 760°C, and do not show formation of indium or gallium droplets on the surface. The photoluminescence emission from the buried InGaN SQWs remains unchanged by the preparation process, demonstrating that the SQWs do not undergo any significant modification. © 2016.
    view abstract10.1016/j.mssp.2016.02.013
  • Preparation of pulsed DC magnetron deposited Fe-doped SnO2 coatings
    Kormunda, M. and Fischer, D. and Hertwig, A. and Beck, U. and Sebik, M. and Esser, N.
    Physica Status Solidi (A) Applications and Materials Science 213 (2016)
    Iron-doped SnO2 coatings were deposited in a 50 kHz DC-pulsed magnetron sputtering discharge. The pulses had a duration of 4 μs in selected gas mixtures from pure argon up to 60% of oxygen at a constant total pressure of 0.2 Pa. A single target of SnO2 with Fe inset was used. The mass spectrometry study detected the gas-related ions Ar+, O2 + and O+, where the last one becomes the dominant positive ion at higher oxygen contents. Atomic oxygen ions had a higher energy as it resulted from the collision-caused dissociation on the target surface. The tin-related species were detected as Sn+ and SnO+. SnO2 + species were not detected. The deposition rate decreased by using gas mixtures with oxygen as well as the corresponding amount of Sn-related species in the plasma. The increase of oxygen also increased significantly the sheet resistance of the films. The XPS study showed that the iron concentration decreased by using additional oxygen. But the O/Sn ratio in the coatings was constant, contrary to the increased FeO/Fe ratio in the films. An additional analysis of the coatings by spectroscopic ellipsometry has shown a dependence of the polarizability and the permittivity on the amount of oxygen used during the deposition. In contrast, the study has found no such dependence for the absorption of the layers. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssa.201532882
  • Surface vibrational Raman modes of In:Si(111) (4×1) and (8×2) nanowires
    Speiser, E. and Esser, N. and Wippermann, S. and Schmidt, W.G.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    High-resolution Raman spectroscopy at low frequencies (<100cm-1) is used to reinvestigate and identify the surface phonons localized in the first atomic layers of In:Si(111)(4×1) and (8×2). The frequency and symmetry of low-energy surface phonons are strongly related to surface structure. The measured phonons are assigned to characteristic modes of the quasi-one-dimensional indium nanowires on Si(111) by means of density-functional theory calculations and symmetry considerations. It is found that the low-temperature (8×2) and the room-temperature (4×1) phases of the In:Si(111) surface are characterized by distinct sets of phonon modes. Strong modifications in Raman spectra upon the phase transition are, on the one hand, related to backfolding induced by symmetric quadruplication of the surface unit cell and, on the other hand, to structural changes within the surface unit cell. These are indications for two distinct structural phases, supporting the first-order transition scenario. The characteristic structural changes are thus verified in detail. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.075417
  • Vibration eigenmodes of the Au- (5×2) /Si(111) surface studied by Raman spectroscopy and first-principles calculations
    Liebhaber, M. and Halbig, B. and Bass, U. and Geurts, J. and Neufeld, S. and Sanna, S. and Schmidt, W.G. and Speiser, E. and Räthel, J. and Chandola, S. and Esser, N.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    Ordered submonolayers of adsorbate atoms on semiconductor surfaces constitute a playground for electronic correlation effects, which are tightly connected to the local atomic arrangement and the corresponding vibration eigenmodes. We report on a study of the vibration eigenmodes of Au-covered Si(111) surfaces with (5×2) reconstruction using polarized Raman spectroscopy and first-principles calculations. Upon Au coverage, the vibration eigenmodes of the clean reconstructed Si(111)-(7×7) surface are quenched and replaced by new eigenmodes, determined by the Au-(5×2) reconstruction. Several polarization-dependent surface eigenmodes emerge in the spectral range from 25 to 120cm-1, with the strongest ones at 29, 51, and 106cm-1. In our first-principles calculations we have determined the vibration frequencies, the corresponding elongation patterns, and the Raman intensities for two different structure models currently discussed in the literature. The best agreement with the experimental results is achieved for a model with 0.7 monolayer coverage and seven Au atoms per unit cell, proposed by S. G. Kwon and M. H. Kang [Phys. Rev. Lett. 113, 086101 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.086101]. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.235304
  • Deposition and characterization of single magnetron deposited Fe:SnOx coatings
    Kormunda, M. and Fischer, D. and Hertwig, A. and Beck, U. and Sebik, M. and Pavlik, J. and Esser, N.
    Thin Solid Films 595 (2015)
    Coatings deposited bymagnetron co-sputtering froma single RF magnetronwith a ceramic SnO2 target with iron inset in argon plasma were studied. Themass spectra of the process identified Sn+ and SnO+ species as the dominant species sputtered fromthe target, but no SnO2 + specieswere detected. The dominant positive ions in argon plasma are Ar+ species. The only detected negative ions were O-. Sputtered neutral tin related species were not detected. Iron related species were also not detected because their concentration is below the detection limit. The concentration of iron dopant in the tin oxide coatings was controlled by the RF bias applied on the substrate holder while the discharge pressure also has some influence. The iron concentration was in the range from 0.9 at.% up to 19 at.% increasing with the substrate bias while the sheet resistivity decreases. The stoichiometry ratio of O/(Sn+Fe) in the coatings increased from 1.7 up to 2 in dependence on the substrate bias from floating bias (-5 V) up to-120 V of RF self-bias, respectively. The tin in the coatings was mainly bonded in Sn4+ state and iron was mainly in Fe2+ state when other tin bonding states were detected only in a small amounts. Iron bonding states in contrary to elemental compositions of the coatings were not influenced by the RF bias applied on the substrate. The coatings showed high transparency in the visible spectral range. However, an increased metallic behavior could be detected by using a higher RF bias for the deposition. The X-ray diffraction patterns and electron microscopy pictures made on the coatings confirmed the presence of an amorphous phase. © 2015 Published by Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2015.11.009
  • GaN m -plane: Atomic structure, surface bands, and optical response
    Landmann, M. and Rauls, E. and Schmidt, W.G. and Neumann, M.D. and Speiser, E. and Esser, N.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    Density-functional-theory calculations are combined with many-body perturbation theory in order to elucidate the geometry, electronic, and optical properties of the wz-GaN(11¯00) surface, i.e., the so-called m-plane. The optical absorption and reflection anisotropy related to electronic transitions between surface states are identified by comparison with measured data covering transition energies from 2.4 up to 5.4 eV. Our results show a surface relaxation mechanism consistent with the electron counting rule that causes a moderate buckling of the GaN surface dimers and gives rise to two distinct surface states: The doubly occupied N dangling bonds form a surface band that is resonant with the GaN valence-band edge at the center of the Brillouin zone, whereas the empty Ga dangling bonds occur within the GaN band gap closely following the dispersion of the conduction-band edge. These two states contribute strongly to the formation of surface excitons that redshift the optical absorption with respect to the bulk optical response. The surface optical absorption i.e., the excitonic onset below the bulk band gap followed by a broad absorption band at higher energies related to the dispersion of the surface band structure, is calculated in agreement with the experimental data. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.91.035302
  • Label-free biosensors based on in situ formed and functionalized microwires in microfluidic devices
    Xing, Y. and Wyss, A. and Esser, N. and Dittrich, P.S.
    Analyst 140 (2015)
    Label-free biosensors based on in situ formed and functionalized TTF-Au wires were developed using an integrated microfluidic system. By applying different modification protocols, TTF-Au wires were successfully used for sensitive label-free detection of catecholamines and human IgG by Raman spectroscopy. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5an01240f
  • Metal-to-Insulator Transition in Au Chains on Si(111)-5×2-Au by Band Filling: Infrared Plasmonic Signal and Ab Initio Band Structure Calculation
    Hötzel, F. and Seino, K. and Chandola, S. and Speiser, E. and Esser, N. and Bechstedt, F. and Pucci, A.
    Journal of Physical Chemistry Letters 6 (2015)
    The Si(111)-5×2-Au surface is increasingly of interest because it is one of the rare atomic chain systems with quasi-one-dimensional properties. For the deposition of 0.7 monolayers of Au, these chains are metallic. Upon the evaporation of an additional submonolayer amount of gold, the surface becomes insulating but keeps the 5×2 symmetry. This metal-to-insulator transition was in situ monitored based on the infrared plasmonic signal change with coverage. The phase transition is theoretically explained by total-energy and band-structure calculations. Accordingly, it can be understood in terms of the occupation of the originally half-filled one-dimensional band at the Fermi level. By annealing the system, the additional gold is removed from the surface and the plasmonic signal is recovered, which underlines the stability of the metallic structure. So, recent results on the infrared plasmonic signals of the Si(111)-5 × 2-Au surface are supported. The understanding of potential one-dimensional electrical interconnects is improved. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpclett.5b01530
  • Near valence-band electronic properties of semiconducting β-Ga2 O3 (100) single crystals
    Navarro-Quezada, A. and Alamé, S. and Esser, N. and Furthmüller, J. and Bechstedt, F. and Galazka, Z. and Skuridina, D. and Vogt, P.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    β-Ga2O3 is a transparent wide-band-gap semiconductor that has attracted considerable interest in recent years due to its suitable electrical conductivity and transparency in the ultraviolet spectral region. In this work we investigate the electronic properties of the near valence-band-edge region for semiconducting β-Ga2O3 (100) bulk single crystals using core-level photoelectron spectroscopy and ab initio theory within the framework of density functional theory and the GW approach. We find good agreement between the experimental results and the theoretical calculations. This is explained by the hybridization of the Ga 3d and O 2s states, similar as for In2O3. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.195306
  • Spectrometer system using a modular echelle spectrograph and a laser-driven continuum source for simultaneous multi-element determination by graphite furnace absorption spectrometry
    Geisler, S. and Okruss, M. and Becker-Ross, H. and Huang, M.D. and Esser, N. and Florek, S.
    Spectrochimica Acta - Part B Atomic Spectroscopy 107 (2015)
    A multi-element absorption spectrometer system has been developed based on a laser-driven xenon continuum source and a modular simultaneous echelle spectrograph (MOSES), which is characterized by a minimized number of optical components resulting in high optical throughput, high transmittance and high image quality. The main feature of the new optical design is the multifunction usage of a Littrow prism, which is attached on a rotation stage. It operates as an order-sorter for the echelle grating in a double-pass mode, as a fine positioning device moving the echelle spectrum on the detector, and as a forwarder to address different optical components, e.g., echelle gratings, in the setup. Using different prisms, which are mounted back to back on the rotation stage, a multitude of different spectroscopic modes like broad-range panorama observations, specific UV-VIS and NIR studies or high resolution zoom investigations of variable spectral channels can be realized. In the UV panorama mode applied in this work, MOSES has simultaneously detectable wavelength coverage from 193 nm to 390 nm with a spectral resolution λ/Δλ of 55,000 (3-pixel criterion). In the zoom mode the latter can be further increased by a factor of about two for a selectable section of the full wavelength range. The applicability and the analytical performance of the system were tested by simultaneous element determination in a graphite furnace, using eight different elements. Compared to an instrument operating in the optimized single line mode, the achieved analytical sensitivity using the panorama mode was typically a factor of two lower. Using the zoom mode for selected elements, comparable sensitivities were obtained. The results confirm the influence of the different spectral resolutions. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.sab.2015.02.006
  • Surface properties of annealed semiconducting β-Ga2O3 (1 0 0) single crystals for epitaxy
    Navarro-Quezada, A. and Galazka, Z. and Alamé, S. and Skuridina, D. and Vogt, P. and Esser, N.
    Applied Surface Science 349 (2015)
    We present a detailed study on the surface properties of conductive β-Ga<inf>2</inf>O<inf>3</inf> (1 0 0) single-crystal epiready substrates by means of photoelectron emission spectroscopy. The surface properties are studied prior and after annealing in ultra-high vacuum (UHV). We find that untreated substrates contain a significant amount of adsorbed carbon contaminations at the surface, which can be partly removed by annealing at 800 °C in UHV. Valence band photoemission evidences an upward band bending of about 0.5 eV that increases with annealing, revealing the presence of an electron depletion layer at the near-surface region responsible for the insulating behavior commonly observed for semiconductive β-Ga<inf>2</inf>O<inf>3</inf> single crystals. Our findings become crucial for epitaxial growth, as it is known that carbon modifies the electrical and structural properties of subsequent epitaxial layers. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apsusc.2015.04.225
  • Vibrational Raman scattering from surfaces of III-V semiconductors: Microscopic and macroscopic surface modes
    Speiser, E. and Hinrichs, K. and Prete, P. and Lovergine, N. and Esser, N.
    Physica Status Solidi (B) Basic Research 252 (2015)
    With ongoing instrumental improvements Raman spectroscopy (RS) is advancing into the study of surface vibrational modes of semiconductors. On compound semiconductors, like the III-V's, two distinct types of surface modes occur, microscopic modes being vibrations confined to the surface and macroscopic modes penetrating much deeper into the bulk, dependent on the electromagnetic boundary conditions. Both mode types depend on surface properties, the microscopic ones on the atomic scale, and the macroscopic ones on the scale of the surface polariton wavelength. While the former one delivers surface atomic structure information, the latter one may be useful for instance to study the morphology of surfaces, such as the shape of semiconductor nanowires. We discuss Raman spectra obtained on the atomically well-defined III-V(110) model surfaces and recent results obtained on isolated III-V nanowires. The comparison of both gives insight in the capabilities of Raman scattering from surface phonons: The contributions of surface phonons, surface resonances, and macroscopic modes (Fuchs-Kliewer modes, surface phonon polaritons) to the Raman spectra become evident. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201451355
  • A synchrotron-radiation-based variable angle ellipsometer for the visible to vacuum ultraviolet spectral range
    Neumann, M.D. and Cobet, C. and Kaser, H. and Kolbe, M. and Gottwald, A. and Richter, M. and Esser, N.
    Review of Scientific Instruments 85 (2014)
    A rotating analyzer spectroscopic polarimeter and ellipsometer with a wide-range θ-2θ goniometer installed at the Insertion Device Beamline of the Metrology Light Source in Berlin is presented. With a combination of transmission- and reflection-based polarizing elements and the inherent degree of polarization of the undulator radiation, this ellipsometer is able to cover photon energies from about 2 eV up to 40 eV. Additionally, a new compensator design based on a CaF2 Fresnel rhomb is presented. This compensator allows ellipsometric measurements with circular polarization in the vacuum ultraviolet spectral range and thus, for example, the characterization of depolarizing samples. The new instrument was initially used for the characterization of the polarization of the beamline. The technical capabilities of the ellipsometer are demonstrated by a cohesive wide-range measurement of the dielectric function of epitaxially grown ZnO. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4878919
  • Band gap renormalization and Burstein-Moss effect in silicon- and germanium-doped wurtzite GaN up to 1020 cm-3
    Feneberg, M. and Osterburg, S. and Lange, K. and Lidig, C. and Garke, B. and Goldhahn, R. and Richter, E. and Netzel, C. and Neumann, M.D. and Esser, N. and Fritze, S. and Witte, H. and Bläsing, J. and Dadgar, A. and Krost, A.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    The interplay between band gap renormalization and band filling (Burstein-Moss effect) in n-type wurtzite GaN is investigated. For a wide range of electron concentrations up to 1.6×1020cm-3 spectroscopic ellipsometry and photoluminescence were used to determine the dependence of the band gap energy and the Fermi edge on electron density. The band gap renormalization is the dominating effect up to an electron density of about 9×1018cm-3; at higher values the Burstein-Moss effect is stronger. Exciton screening, the Mott transition, and formation of Mahan excitons are discussed. A quantitative understanding of the near gap transition energies on electron density is obtained. Higher energy features in the dielectric functions up to 10eV are not influenced by band gap renormalization. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.075203
  • Confirmation of intrinsic electron gap states at nonpolar GaN(1-100) surfaces combining photoelectron and surface optical spectroscopy
    Himmerlich, M. and Eisenhardt, A. and Shokhovets, S. and Krischok, S. and Speiser, E. and Neumann, M.D. and Navarro-Quezada, A. and Esser, N.
    Applied Physics Letters 104 (2014)
    The electronic structure of GaN(1-100) surfaces is investigated in-situ by photoelectron spectroscopy (PES) and reflection anisotropy spectroscopy (RAS). Occupied surface states 3.1eV below the Fermi energy are observed by PES, accompanied by surface optical transitions found in RAS around 3.3eV, i.e., below the bulk band gap. These results indicate that the GaN(1-100) surface band gap is smaller than the bulk one due to the existence of intra-gap states, in agreement with density functional theory calculations. Furthermore, the experiments demonstrate that RAS can be applied for optical surface studies of anisotropic crystals. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4873376
  • Correlation of band gap position with composition in high-k films
    Ding, L. and Friedrich, M. and Fronk, M. and Gordan, O.D. and Zahn, D.R.T. and Chen, L. and Zhang, D.W. and Cobet, C. and Esser, N.
    Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures 32 (2014)
    Two series of ultrathin high-k samples (mixed layers Hf<inf>x</inf>Al <inf>y</inf>O<inf>z</inf> and bilayers HfO<inf>2</inf> on Al<inf>2</inf>O <inf>3</inf>) prepared by atomic layer deposition were investigated using spectroscopic ellipsometry in the energy range of 0.7-10.0 eV. The (effective) optical gap of both mixed layer and bilayer structures can be tuned by the film composition. The optical gap of mixed layers is linearly dependent on the Hf fraction from 5.77 (±0.02) eV for pure HfO<inf>2</inf> to 6.71 (±0.02) eV for pure Al<inf>2</inf>O<inf>3</inf>. The effective absorption gap of bilayers measured in reflection geometry is lower than that of mixed layers with a comparable Hf fraction due to the dominant effect of the top layer. An increase in film thickness as well as a decrease in refractive index and gap energy was observed after sample storage for two months in atmosphere. The aging effect is likely due to further oxidation of the oxygen deficient high-k films caused by the oxygen diffusion from air into the films. © 2014 American Vacuum Society.
    view abstract10.1116/1.4866399
  • DNA Structures on Silicon and Diamond
    Pop, S.D. and Hinrichs, K. and Wenmackers, S. and Cobet, C. and Esser, N. and Zahn, D.R.T.
    Springer Series in Surface Sciences 52 (2014)
    In the design of DNA-based hybrid devices, it is essential to have knowledge of the structural, electronic and optical properties of these biomolecular films. Spectroscopic ellipsometry is a powerful technique to probe and asses these properties. In this chapter, we review its application to biomolecular films of single DNA bases and molecules on silicon and diamond surfaces characterized in the spectral range from the near-infrared (NIR) through the visible (Vis) and toward the vacuum ultraviolet (VUV). The reported optical constants of various DNA structures are of great interest, particularly in the development of biosensors. © Springer-Verlag Berlin Heidelberg 2014.
    view abstract10.1007/978-3-642-40128-2_3
  • Optical properties of magnesium doped AlxGa1-xMN (0.61≤ x ≤0.73)
    Feneberg, M. and Osterburg, S. and Romero, M.F. and Garke, B. and Goldhahn, R. and Neumann, M.D. and Esser, N. and Yan, J. and Zeng, J. and Wang, J. and Li, J.
    Journal of Applied Physics 116 (2014)
    We investigate the optical properties of AlxGa1-xN:Mg with aluminum content of 0.61≤ x≤ 0.733 in comparison to undoped and silicon doped reference samples. The ordinary dielectric functions, excitation, and emission spectra are reported at different temperatures. A comprehensive analysis yields quantitative data on the valence band structure of the ternary alloy, i.e., splitting and order of valence bands with different symmetries. Finally, the near band gap emission in AlGaN:Mg is found to be most probably dominated by donor to free-hole recombination. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4897449
  • Ordinary and extraordinary dielectric functions of rutile SnO2 up to 20 eV
    Feneberg, M. and Lidig, C. and Lange, K. and Goldhahn, R. and Neumann, M.D. and Esser, N. and Bierwagen, O. and White, M.E. and Tsai, M.Y. and Speck, J.S.
    Applied Physics Letters 104 (2014)
    view abstract10.1063/1.4882237
  • Raman spectroscopy study of silicon nanoribbons on Ag(110)
    Speiser, E. and Buick, B. and Esser, N. and Richter, W. and Colonna, S. and Cricenti, A. and Ronci, F.
    Applied Physics Letters 104 (2014)
    The grating of self-assembled Si nanoribbons at the Ag(110) surface has been studied by Raman spectroscopy. The study, conducted in situ with uncapped samples, resulted in phonon frequencies in disagreement with the results of theoretical calculations reported in literature for freestanding silicene sheets and nanoribbons. These results suggest that the structure of these silicon nanoribbons is very different from the planar and lightly buckled structural models and that the influence of the substrate might be underestimated in the discussed structural models. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4872460
  • Reflectance difference spectroscopy of water on Cu(110)
    Denk, M. and Hohage, M. and Suna, L.D. and Zeppenfeld, P. and Esser, N. and Cobet, C.
    Surface Science 627 (2014)
    The adsorption of H2O on Cu(110)was probed bymeans of reflectance difference spectroscopy (RDS) in the energy range between 1.5 and 9.3 eV and by scanning tunnelingmicroscopy (STM). The adsorption ofwater on the pristine Cu(110) substrate mainly induces changes in the Cu surface state related optical transitions. Furthermore, the H2O adsorbate modifies the Cu bulk transitions. In particular, our investigations demonstrate that the coverage-dependent phase transition from 1D pentamer chains to a (7 × 8) superstructure can bemonitored by means of RDS. In the vacuum-UV range, new RD features assigned to Cu bulk transitions were detected. Adsorption on themetal surface stronglymodifies or quenches theH2OHOMO-LUMOtransition, whereby a distinct RD feature of the water molecules themselves in the vacuum-UV range is absent. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.susc.2014.04.009
  • Surface phonons of the Si(111)-(7 × 7) reconstruction observed by Raman spectroscopy
    Liebhaber, M. and Bass, U. and Bayersdorfer, P. and Geurts, J. and Speiser, E. and Räthel, J. and Baumann, A. and Chandola, S. and Esser, N.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    We have studied the surface phonon modes of the reconstructed Si(111)-(7×7) surface by polarized Raman spectroscopy. Six surface vibration modes are observed in the frequency range between 62.5 and 420.0 cm-1. The mode frequencies agree very well with reported calculation results. This enables their attribution to calculated eigenmodes, whose elongation patterns are dominated by specific atomic sites: the two most characteristic novel fingerprints of the (7×7) reconstruction are sharp Raman peaks from localized adatom vibrations, located at 250.9 cm-1, and collective vibrations of the adatoms and first- and second-layer atoms, located at 420.0 cm-1. While the sharp localized adatom vibration peak is a substantial refinement of an earlier broad spectral structure from electron energy-loss spectroscopy, no spectroscopic features were reported before in the collective-vibration frequency region. Furthermore, we observe in-plane wagging vibrations in the range from 110 to 140 cm-1, and finally the backfolded acoustic Rayleigh wave at 62.5 cm-1, which coincides with helium atom scattering data. Moreover, the Raman peak intensities of the surface phonons show a mode-specific dependence on the polarization directions of incident and scattered light. From this polarization dependence the relevant symmetry components in the Raman scattering process (A1 and/or E symmetry) are deduced for each mode. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.045313
  • Synchrotron-based VUV spectroscopic ellipsometry system in application to optical properties studies of wide-bandgap materials for optoelectronics
    Dorywalski, K. and Patryn, A. and Andriyevsky, B. and Cobet, C. and Esser, N.
    Przeglad Elektrotechniczny 90 (2014)
    The paper presents the implementation of spectroscopic ellipsometry measurements in the vacuum-ultraviolet spectral range with use of synchrotron radiation as a light source. Current status of VUV ellipsometer and the principle of its operation is described. The procedure of measurements and ellipsometric data evaluation taking into account surface roughness of the measured specimen of optically uniaxial widebandgap single crystal is presented through the example of Sr61Ba39Nb2O6 ellipsometric characterization. Complex dielectric function ε(E) = ε1(E) + iε2(E) for Sr61Ba39Nb2O6 was obtained in the spectral photon energy range 2-25 eV for polarization direction along b and c crystallographic axes.
    view abstract10.12915/pe.2014.03.56
  • Temperature dependent dielectric function and reflectivity spectra of nonpolar wurtzite AlN
    Feneberg, M. and Romero, M.F. and Neuschl, B. and Thonke, K. and Röppischer, M. and Cobet, C. and Esser, N. and Bickermann, M. and Goldhahn, R.
    Thin Solid Films 571 (2014)
    Exciton resonances are observed in the dielectric functions of wurtzite AlN by spectroscopic ellipsometry for the electric field vector parallel and perpendicular to the optical axis. The temperature dependence of these excitons is analyzed in detail. From the dielectric functions, reflectivity spectra are calculated and compared to experimentally obtained reflectivity, perfect agreement is found. The relative oscillator strengths of excitons showing different symmetries are discussed yielding an unambiguous assignment of free excitons formed by holes from the highest valence band. As a consequence, a negative exciton spin-exchange coupling constant of j = - 4 meV is derived. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2013.10.092
  • Aging-induced optical anisotropy in thermally grown thin ZnTPP films on Si
    Kate, S.P. and Pop, S. and Esser, N. and Rappich, J. and Zhang, X. and Hinrichs, K.
    Physica Status Solidi (B) Basic Research 250 (2013)
    We present reflection anisotropy spectroscopy (RAS) as a powerful optical technique for degradation studies of structurally changing thin organic films, here the case of porphyrin films. The measured optical anisotropy of a thin (d=16nm) ZnTPP (Zinc tetraphenylporphyrin) film grown by thermal evaporation on silicon using reflectance anisotropy spectroscopy was studied before and after aging in ambient conditions. The observed absorption features, also found by standard transmission spectroscopy, are correlated with the π-π*electronic transitions of the Q and Soret bands of the molecule. Simulation of RAS spectra revealed blue-shifted resonances, which are in agreement with a preferential face-to-face (stacking) arrangement of the ZnTPP molecules. The findings are in agreement with atomic force microscopy studies of the surface morphology showing with aging a transition from a closed film to separated aggregates. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201248538
  • Anisotropic absorption and emission of bulk (11̄00) AlN
    Feneberg, M. and Romero, M.F. and Röppischer, M. and Cobet, C. and Esser, N. and Neuschl, B. and Thonke, K. and Bickermann, M. and Goldhahn, R.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    The intrinsic anisotropic optical properties of wurtzite AlN are investigated in absorption and emission. Full access to the anisotropy of the optical response of the hexagonal material is obtained by investigating the (11̄00) plane of a high-quality bulk crystal allowing electric field E polarization perpendicular (E&perp;c) and parallel (E II c) to the optical axis c. Spectroscopic ellipsometry yields the ordinary (E&perp;) and extraordinary (EII) dielectric functions (DFs) from 0.58 up to 20 eV. The comparison of the experimental data with recently calculated DFs demonstrates that Coulomb interaction has a strong impact not only on the spectral dependence around the fundamental absorption edge but also on the high-energy features usually discussed in terms of van Hove singularities. The fits of the second-order derivatives of EII and E&perp; provide the transition energies for the main features in this range. The DFs close to the fundamental absorption edge, dominated by free excitons, exciton-phonon complexes, and the exciton continuum, are independently confirmed by reflectivity and synchrotron-based photoluminescence excitation studies. Values for the band gaps, the crystal field (Δcf=-221±2 meV), and spin-orbit splittings (Δso=13±2 meV) are obtained. Furthermore, we obtain accurate values for the static dielectric constants of ES &perp;=7.65 and ES II =9.21, entering, e.g., the calculations of exciton binding energies. Photoluminescence is used to investigate the emission properties of the same sample. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.235209
  • Ellipsometric study of near band gap optical properties of Sr xBa1-xNb2O6 crystals
    Dorywalski, K. and Andriyevsky, B. and Cobet, C. and Piasecki, M. and Kityk, I.V. and Esser, N. and Łukasiewicz, T. and Patryn, A.
    Optical Materials 35 (2013)
    Complex refractive indices N = n + ik for a-cut SrxBa 1-xNb2O6 single crystals with four nominal values of composition x = 0.40; 0.50; 0.61; 0.75 were studied by spectroscopic ellipsometry in the photon energy range 2-5 eV of crystal transparency and fundamental electronic absorption. The ellipsometric data were evaluated taking into account an uniaxial anisotropy and an unintentional surface roughness. We use a global composition dependent Sellmeier fit as a reference for monitoring the crystals composition. The optical model yields indirect and direct optical band gap energies Eg in the spectral range 3.77-4.03 eV and 4.28-4.55 eV, respectively, depending on the Sr/(Ba + Sr) content ratio x and light polarization. The congruently melting composition Sr0.61Ba 0.39Nb2O6 shows distinct dispersion features in the imaginary part of the UV complex refractive index with respect to other compositions. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.optmat.2012.10.050
  • Negative spin-exchange splitting in the exciton fine structure of AlN
    Feneberg, M. and Fátima Romero, M. and Neuschl, B. and Thonke, K. and Röppischer, M. and Cobet, C. and Esser, N. and Bickermann, M. and Goldhahn, R.
    Applied Physics Letters 102 (2013)
    The exact energy position of the free exciton transition and thus the lowest band gap in bulk wurtzite AlN are still under discussion. By combined high resolution optical emission and absorption experiments on a sample with (1 1 ̄ 00) surface, we resolve the fine structure of the lowest energy free exciton and determine an electron-hole spin-exchange interaction constant of j = - 4 meV. This results in a low energy Γ1 exciton at 6.032 eV and a high energy Γ5 component at 6.040 eV. Only the latter one is observable for (0001) oriented AlN films due to selection rules. © 2013 American Institute of Physics.
    view abstract10.1063/1.4790645
  • Specific features of Yb3+ ions in electronic band energy structure and optical functions of RbNd(WO4)2 crystals: Synchrotron ellipsometry measurements and DFT simulations
    Andriyevsky, B. and Piasecki, M. and Dorywalski, K. and Cobet, C. and Esser, N. and Świrkowicz, M. and Majchrowski, A. and Jaroszewicz, L.R. and Kityk, I.V.
    Journal of Alloys and Compounds 577 (2013)
    The influence of 5% and 10% (in molar units) of ytterbium doping of RbNd(WO4)2 single crystals on the dielectric permittivity e in the spectral range of electronic excitations was studied. The corresponding differences of the pseudo-dielectric functions ≪ε(E)≫ for pure and Yb-doped RbNd(WO4)2 were studied by spectroscopic ellipsometry method including VUV spectral range using synchrotron radiation light source. This method opens an opportunity for spectroscopic diagnostics of the rare earth dopants in crystals using information concerning the electronic inter-band optical transitions. To better understand the experimental results, for the first time the ab initio calculations of band structure, related properties and optical spectra of the centrosymmetric RbNd(WO4) 2 single crystal were performed using the VASP code. The calculated real and imaginary parts of dielectric function dispersions ε 1(E) and ε2(E) agree satisfactorily with our experimental results. It was established that differences of optical functions for pure and doped crystals may serve as a powerful tool for spectroscopic diagnostic of localized rare earth ions with respect to the strong inter-band transitions in the wide band dielectrics. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2013.04.066
  • Spectral ellipsometry study in the range of electronic excitations and band structure of [(CH3)2CHNH3]4Cd 3Cl10 crystals
    Andriyevsky, B. and Dorywalski, K. and Jaskólski, M. and Czapla, Z. and Patryn, A. and Esser, N.
    Materials Chemistry and Physics 139 (2013)
    Optical dielectric functions ε(E) of the (IPA)4Cd 3Cl10 crystal were measured in the spectral range of fundamental electronic excitations 3.5-10 eV and in the temperature range of 310-400 K containing the phase transition point between the orthorhombic phases Cmce and Pbca. Measurements were performed by spectroscopic ellipsometry with using of synchrotron radiation. Electronic band structure, density of states and dielectric functions ε(E) of (IPA)4Cd3Cl 10 were calculated and analyzed on the basis of the density functional theory. Top valence and bottom conduction bands were found to be formed mainly by the cadmium-chlorine complexes of the crystals. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.matchemphys.2013.02.030
  • STM analysis of defects at the GaAs(001)-c(4 × 4) surface
    Bruhn, T. and Fimland, B.-O. and Esser, N. and Vogt, P.
    Surface Science 617 (2013)
    Atomic structure models of semiconductor surfaces consider usually ideal models based on density functional theory calculations. In reality, however, semiconductor surfaces exhibit a variety of defects which deviate from this ideal surface structure. Depending on their specific nature and amount, these defects can contribute significantly to the total energy of the surface. Furthermore, the electronic properties and consequently their specific reactivity towards adsorption processes can be modified significantly due to the existence of surface defects. Here, we present an analysis of different kinds of defects at the GaAs(001)-c(4 × 4) surface reconstruction. The surfaces were prepared by thermal decapping of GaAs(001) epilayers grown by molecular beam epitaxy and capped by an amorphous As cap. High resolution measurements with scanning tunneling microscopy were performed at room temperature and allowed the identification and atomic analysis of several different kinds of surface defects. Apart from other defects we found indications that approximately 3% of the surface dimers are incomplete, consisting only of one As atom instead of two. © 2013 Elsevier B.V.
    view abstract10.1016/j.susc.2013.07.012
  • Structural phase transitions in ferroelectric crystals and thin films studied by VUV spectroscopic ellipsometry with synchrotron radiation
    Dorywalski, K. and Andriyevsky, B. and Piasecki, M. and Kityk, I. and Cobet, C. and Esser, N. and Patryn, A.
    Phase Transitions 86 (2013)
    Results of thermo-optical investigations for lead germanate (Pb5Ge3O11), potassium dihydrogen phosphate (KH2PO4) ferroelectric crystals, and lead titanate (PbTiO3) thin films expitaxially deposited on (001) SrTiO3 substrate are presented in this article. The measurements were performed using VUV spectroscopic ellipsometry with synchrotron radiation in the temperature range comprising structural phase transitions (PTs) in the materials under study. Distinct anomalies in the temperature dependencies of complex dielectric permittivity and the intensity of reflected synchrotron light are found in the vicinity of ferroelectric-to-paraelectric PT point and discussed. Different temperature points of the characteristic anomalies at the PT of the real and imaginary components of the pseudo-dielectric function are observed for Pb5Ge3O11 and KH2PO4. It is crucial that anomalies in the measured temperature dependences for PbTiO3 thin films on SrTiO3 substrates are observed at the temperature near 105 K, corresponding to the PT in the SrTiO3 single crystal. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/01411594.2012.745540
  • Transition energies and direct-indirect band gap crossing in zinc-blende AlxGa1-xN
    Landmann, M. and Rauls, E. and Schmidt, W.G. and Röppischer, M. and Cobet, C. and Esser, N. and Schupp, T. and As, D.J. and Feneberg, M. and Goldhahn, R.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    The electronic and optical properties of zinc-blende (zb) Al xGa1-xN over the whole alloy composition range are presented in a joint theoretical and experimental study. Because zb-GaN is a direct (Γv→Γc) semiconductor and zb-AlN shows an indirect (Γv→Xc) fundamental band gap, the ternary alloy exhibits a concentration-dependent direct-indirect band gap crossing point the position of which is highly controversial. The dielectric functions of zb-AlxGa1-xN alloys are measured employing synchrotron-based ellipsometry in an energy range between 1 and 20 eV. The experimentally determined fundamental energy transitions originating from the Γ, X, and L points are identified by comparison to theoretical band-to-band transition energies. In order to determine the direct-indirect band gap crossing point, the measured transition energies at the X point have to be aligned by the calculated position of the highest valence state. Thereby density-functional theory (DFT) based approaches to the electronic structure, ranging from the standard (semi)local generalized gradient approximation (GGA), self-energy corrected local density approximation (LDA-1/2), and meta-GGA DFT (TB-mBJLDA) to hybrid functional DFT and many-body perturbation theory in the GW approximation, are applied to random and special quasirandom structure models of zb-AlxGa1-xN. This study provides interesting insights into the accuracy of the various numerical approaches and contains reliable ab initio data on the electronic structure and fundamental alloy band gaps of zb-AlxGa1-xN. Nonlocal Heyd-Scuseria-Ernzerhof-type hybrid-functional DFT calculations or, alternatively, GW quasiparticle calculations are required to reproduce prominent features of the electronic structure. The direct-indirect band gap crossing point of zb-Al xGa1-xN is found in the Al rich composition range at an Al content between x=0.64 and 0.69 in hybrid functional DFT, which is in good agreement with x=0.71 determined from the aligned experimental transition energies. Thus our study solves the long-standing debate on the nature of the fundamental zb-AlxGa1-xN alloy band gap. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.195210
  • Ultraviolet vacuum ultraviolet optical functions for SrTiO3 and NdGaO3 crystals determined by spectroscopic ellipsometry
    Dorywalski, K. and Andriyevsky, B. and Piasecki, M. and Lemee, N. and Patryn, A. and Cobet, C. and Esser, N.
    Journal of Applied Physics 114 (2013)
    Complex dielectric functions ε(E) = ε1(E) + ε2(E) were experimentally evaluated within the spectral range E = 2-25 eV and E = 2-20 eV for SrTiO3 and NdGaO3 single crystals, respectively, using synchrotron-based spectroscopic ellipsometry measurements. The ellipsometric spectra were evaluated within a framework of optical layer model taking into account sample surface roughness and anisotropy of NdGaO3. The parameters of Herzinger-Johs oscillator model were fitted to reproduce sufficiently all features of the optical spectra within the spectral range 2-10 eV. Only slight differences were revealed for spectra polarized along b and c crystallographic axes of the NdGaO3, which can confirm weak optical anisotropy. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4816624
  • Electronic and optical properties of strontium barium niobate single crystals
    Andriyevsky, B. and Patryn, A. and Dorywalski, K. and Cobet, C. and Piasecki, M. and Kityk, I. and Esser, N. and Ukasiewicz, T. and Dec, J.
    Ferroelectrics 426 (2012)
    Electronic structure of Sr x Ba 1-x Nb 2O 6 crystals in the energy range involving the valence and conduction bands has been studied theoretically and experimentally using the DFT-based ab initio VASP code and the spectroscopic ellipsometry method with synchrotron radiation. The Sr 0.61Ba 0.39Nb 2O 6 sample of the congruently melting composition is characterized by the greatest difference of the experimental dielectric function (E) compared to the non congruently melting compositions of Sr x Ba 1-x Nb 2O 6 (x = 0.40, 0.55, 0.65, 0.75). Electronic band structures of Sr 0.60Ba 0.40Nb 2O 6 and LiNbO 3 crystals in the energy range of 6-8 eV and the corresponding dielectric functions (E) in the photon energy range of 3-10 eV have been found to be similar, what is caused by similar octahedral NbO 6 complexes in the crystals. © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2012.671735
  • Near ambient pressure XPS with a conventional X-ray source
    Jürgensen, A. and Esser, N. and Hergenröder, R.
    Surface and Interface Analysis 44 (2012)
    The Leibniz-Institut für Analytische Wissenschaften-ISAS-e.V. recently acquired a new XPS system from SPECS for measurements at near atmospheric pressures and using a conventional laboratory X-ray source (Al Kα and Mg Kα). This allows the in situ analysis of gas-surface interactions under reaction conditions to study, for example, heterogeneous catalysis and corrosion. Furthermore, with operating pressures of up to approximately 0.5 mbar, it is possible to set up a water atmosphere that stabilises biological molecules, for example, lipid bilayer films on metal surfaces. By using a conventional laboratory X-ray source, the radiation damage to these molecules can be minimised. Measurements to establish optimal parameters for operation of the instrument have been completed, and the results are discussed. Future modifications to improve photon and electron flux are presented. The first experimental data obtained with this instrument are also shown. Copyright © 2012 John Wiley & Sons, Ltd. Copyright © 2012 John Wiley & Sons, Ltd.
    view abstract10.1002/sia.4826
  • Optical properties of cubic GaN from 1 to 20 eV
    Feneberg, M. and Röppischer, M. and Cobet, C. and Esser, N. and Schörmann, J. and Schupp, T. and As, D.J. and Hörich, F. and Bläsing, J. and Krost, A. and Goldhahn, R.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    We present a comprehensive overview of the optical properties of zinc-blende GaN. By a variety of different methods, such as temperature- dependent photoluminescence, photoluminescence excitation spectroscopy, photoreflectance, and ellipsometry, we investigate its emission and absorption related characteristics. The sample under study is a nearly strain-free epitaxial layer grown on freestanding cubic SiC. The light-hole/heavy-hole exciton was found at 3.271eV at 5K, shifting to 3.208eV at 295K. The split-off exciton transition was detected to be 21meV higher in energy. Taking the difference in the exciton binding energies into account, this yields a spin-orbit energy of Δ so=15meV. Donor and acceptor binding energies could be estimated by photoluminescence to be 30 and 130meV, respectively. By synchrotron-based spectroscopic ellipsometry the complex dielectric function up to an energy of 20eV could be determined. Comparison with ab initio calculations allows an assignment of high-energy features to the peculiarities of the band structure. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.155207
  • Preparation and atomic structure of reconstructed (0001) InGaN surfaces
    Friedrich, C. and Biermann, A. and Hoffmann, V. and Kneissl, M. and Esser, N. and Vogt, P.
    Journal of Applied Physics 112 (2012)
    The preparation and surface structure of high quality group-III-polar (0001) InGaN layers grown by metal-organic vapor phase epitaxy have been investigated. In order to obtain a clean and well-ordered surface we studied the preparation by annealing at various temperatures under ultra high vacuum and nitrogen-rich conditions in nitrogen-plasma. We show that different InGaN surface reconstructions such as (1×1), (1+1/6), (2×2), and (√3 × √3) R 30°can be obtained as observed by low energy electron diffraction. Dependent on the annealing temperature and nitrogen supply these surfaces exhibit significant differences in stoichiometry and morphology as determined by Auger electron spectroscopy and atomic force microscopy measurements. The (1×1), (2×2), and (√3 × √3) R 30°superstructures are terminated by single group-III-adatoms, whereas the (1+1/6) exhibits a incommensurate overlayer of group-III-atoms. We show that the (2×2) and (√3 × √3) R 30°an In depletion in the first group-III layer and In or Ga adatoms in ontop position. Strain-relaxation is suggested to explain this structure formation. © 2012 American Institute of Physics.
    view abstract10.1063/1.4743000
  • Pyrrole adsorption on GaAs(001)-c(4×4): The role of surface defects
    Bruhn, T. and Fimland, B.-O. and Esser, N. and Vogt, P.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    We report on the adsorption mechanism of pyrrole on the As-rich GaAs(001)-c(4×4) surface. We present measurements with reflectance anisotropy spectroscopy, x-ray photoelectron spectroscopy, and high-resolution scanning tunneling microscopy and spectroscopy (STS) and our experiments show that the surface defects play a crucial role for the adsorption configuration. We found that pyrrole physisorbs on the ideal c(4×4) surface and does not form any covalent bonds to the surface atoms. At submonolayer coverage, however, we found evidence for single molecules that are chemisorbed at surface defects. We could identify the molecular electronic states with single molecule STS and could distinguish the chemisorbed and physisorbed molecular species in the STS spectra. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.075322
  • Surface phonons of Ge(001) and their correlation with the p(2×1) and c(4×2) reconstruction as shown by Raman spectroscopy
    Räthel, J. and Speiser, E. and Esser, N. and Bass, U. and Meyer, S. and Schäfer, J. and Geurts, J.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    The p(2×1)/c(4×2) and the c(4×2) reconstruction of the Ge(001) surface have been studied by polarized Raman spectroscopy at 300 and 40 K, respectively. Raman spectra show several well-defined surface phonon modes related to the atomic structure of the Ge surface. Four modes are detected in the range between 5.70 and 28.15 meV. Their eigenenergies and polarization dependence agree with reported calculation results from the adiabatic bond charge model and from density-functional perturbation theory. The temperature-induced phase transition between both reconstructions is reflected in the symmetry selection rules. Moreover, our results reveal that in Raman scattering the impact of the well-known flipping of the buckled dimers in the p(2×1) reconstruction is fundamentally different from the time averaging, which occurs for low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Since the phonon time scale is several orders of magnitude faster than the dimer flipping, the phonon oscillations and their Raman scattering are described consistently within the framework of quasistatic buckled dimers with a short-range antiferromagnetic in-row buckling order. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.035312
  • Adsorbate-induced modification of the surface electronic structure at GaAs(001) surfaces
    Bruhn, T. and Fimland, B.-O. and Kneissl, M. and Esser, N. and Vogt, P.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    We report on the influence of hydrocarbon ring molecule adsorption on the surface electronic properties of GaAs(001)-(2×4) and c(4×4) surfaces. The adsorption geometry has been published previously, and it has been shown that there are strong indications that the surface As dimer bond is cleaved upon the chemisorption of the molecules at the c(4×4) reconstruction whereas the As dimers remain intact at the (2×4) reconstruction. At these different interfaces we have studied the adsorption-induced modification of the surface electronic properties. The surface band bending was investigated by synchrotron-based x-ray photoelectron spectroscopy, and the surface electric field was determined optically by reflectance anisotropy spectroscopy via the linear electro-optic effect. While the band bending at the GaAs(001)-(2×4) surface remains unaffected upon molecule adsorption, the surface electronic properties of the c(4×4) reconstructed surface undergo substantial changes. The surface band bending is reduced by 300 meV while the surface electric field increases significantly. These observations are explained within a model for the band bending at GaAs(001) surfaces that was recently suggested by Lastras-Martínez [Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.96.047402 96, 047402 (2006); Phys. Rev. BPRLTAO1098-012110.1103/PhysRevB.75.235315 75, 235315 (2007)], and the results underline the important role of dimer-induced strain for the surface band structure at the GaAs(001)-c(4×4) surface. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.045307
  • Electronic properties of KDP and DKDP crystals: Ab-initio calculations and spectral ellipsometry experiment
    Andriyevsky, B. and Patryn, A. and Cobet, C. and Przesławski, J. and Kosturek, B. and Esser, N. and Dorywalski, K.
    Ferroelectrics 417 (2011)
    Electronic band structure, density of states and complex dielectric function ε(E) of KDP (KH 2PO 4) crystal at Fdd2 and F4d2 space groups of symmetry corresponding to the ferroelectric and paraelectric phases, have been calculated within the density functional theory using the VASP code. The experimental dielectric function ε(E) of KDP and DKDP (KD 2PO 4) crystals have been studied by the spectroscopic ellipsometry method in the photon energy range of 5-18 eV using synchrotron radiation. Temperature dependences of the dielectric function ε(T) and the intensity of reflected light I R(T) of KDP and DKDP crystals have been measured and discussed. Copyright © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2011.578461
  • In-situ optical spectroscopy and electronic properties of pyrrole sub-monolayers on Ga-rich GaAs(001)
    Bruhn, T. and Ewald, M. and Fimland, B.-O. and Kneissl, M. and Esser, N. and Vogt, P.
    Journal of Nanoparticle Research 13 (2011)
    We report on the characterization of submonolayers of pyrrole adsorbed on Ga-rich GaAs(001) surfaces. The interfaces were characterized by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and reflectance anisotropy spectroscopy (RAS) in a spectral range between 1.5 and 8 eV. The adsorption of pyrrole on Ga-rich GaAs(001) modifies the RAS spectrum of the clean GaAs surface significantly at the surface transitions at 2.2 and 3.5 eV indicating a chemisorption of the molecules. By the help of transients at these surface transitions during the adsorption process, we were able to prepare different molecular coverages from a sub-monolayer up to a complete molecular layer. The different coverages of pyrrole were imaged by STM and electronically characterized by STS. The measurements reveal that the adsorbed molecules electronically insulate the surface and indicate the formation of new interface states around-3.5 and +4.2 eV. The RAS measurements in the UV region show new anisotropies in the spectral range of the optical transitions of the adsorbed pyrrole molecules. Our measurements demonstrate the potential of optical and electronic spectroscopy methods for the characterization of atomically thin molecular layers on semiconductor surfaces allowing a direct access to the properties of single adsorbed molecules. © Springer Science+Business Media B.V. 2011.
    view abstract10.1007/s11051-011-0340-0
  • Influence of exciton-phonon coupling and strain on the anisotropic optical response of wurtzite AlN around the band edge
    Rossbach, G. and Feneberg, M. and Röppischer, M. and Werner, C. and Esser, N. and Cobet, C. and Meisch, T. and Thonke, K. and Dadgar, A. and Bläsing, J. and Krost, A. and Goldhahn, R.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    The optical properties around the absorption edge of high-quality wurtzite c-plane AlN layers are investigated by spectroscopic ellipsometry focusing on the anisotropy of the optical response. The spectral dependence of dielectric function shows a strong contribution of exciton-phonon coupling superimposed to the exciton continuum. Crystal field splitting and spin-orbit coupling energies are found to be Δcf=-212 meV and Δso=16 meV, respectively. These values are accessible because our data allow extraction of the transition energies of excitons with holes from all three highest valence bands. Energy positions are cross-checked by photoluminescence measurements. As the samples are grown on different substrates and exhibit varying biaxial strain determined by high resolution x-ray diffraction, we are also able to determine the deformation potentials a-D1=-6.9 eV, a-D2=-15.2 eV, D3=8.3 eV, and D4=-4.15 eV for AlN. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.195202
  • Infrared spectroscopic ellipsometry (IRSE) and X-ray photoelectron spectroscopy (XPS) monitoring the preparation of maleimide-functionalized surfaces: From Au towards Si (111)
    Sun, G. and Hovestädt, M. and Zhang, X. and Hinrichs, K. and Rosu, D.M. and Lauermann, I. and Zielke, C. and Vollmer, A. and Löchel, H. and Ay, B. and Holzhütter, H.-G. and Schade, U. and Esser, N. and Volkmer, R. and Rappich, J.
    Surface and Interface Analysis 43 (2011)
    The IR ellipsometric technique was used to identify the surface species and to control the preparation of maleimide-terminated surfaces. Because of higher s/n ratios for metallic substrates, the protocol was initially developed on Au surfaces, was later successfully transferred to technologically more relevant Si (111) substrates. The functionalized surfaces were achieved by electrochemical deposition of diazonium linker films and following chemical adsorption steps. Complementary XPS was also employed to detect the surface species in the process of preparation. The immobilization of different functional molecules was proven by interpreting the specific vibrational bands in IR spectra and additionally confirmed by XPS experiments. The surface homogeneity was investigated by FT-IR synchrotron mapping ellipsometry. This work shows that the proposed protocol is an effective pathway to achieve the desired functionalized surfaces. Copyright © 2010 John Wiley & Sons, Ltd.
    view abstract10.1002/sia.3699
  • Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes
    Neumann, M.D. and Cobet, C. and Esser, N. and Laumer, B. and Wassner, T.A. and Eickhoff, M. and Feneberg, M. and Goldhahn, R.
    Journal of Applied Physics 110 (2011)
    The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite MgxZn1- xO grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x=0 and x=0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model. © 2011 American Institute of Physics.
    view abstract10.1063/1.3606414
  • Reconstruction dependent growth of lead phthalocyanine layers on GaAs(001) surfaces
    Riele, L. and Bruhn, T. and Rackwitz, V. and Passmann, R. and Fimland, B.-O. and Esser, N. and Vogt, P.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We have investigated the adsorption of lead phthalocyanine (PbPc) layers on GaAs(001)-c(4×4) and (2×4) reconstructed surfaces. Samples with different PbPc coverages from submonolayers up to ≈20-nm-thick layers were prepared under ultra-high vacuum conditions and investigated by scanning tunneling microscopy (STM) and reflectance anisotropy spectroscopy (RAS). The STM measurements showed different adsorption geometries of the PbPc molecules on the different reconstructions. The RAS results revealed that these different adsorption geometries in the first monolayer induce different molecular arrangements within thicker adsorbed layers on the two different substrates. These results give strong evidence for an epitaxial-like growth mode of PbPc molecules on GaAs(001) surfaces driven by the atomic arrangement of the GaAs surface. We could also demonstrate RAS as a powerful tool to analyze the growth behavior of thin organic layers. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.205317
  • Spectral ellipsometry study of SBN single crystals in visible and ultraviolet region
    Andriyevsky, B. and Dorywalski, K. and Kityk, I. and Piasecki, M. and Łukasiewicz, T. and Świrkowicz, M. and Patryn, A. and Dec, J. and Esser, N. and Cobet, C.
    Ferroelectrics 417 (2011)
    The pseudo-dielectric function < ε&gt;(E) = < ε 1&gt;(E) + i< ε 2&gt;(E) of strontium-barium niobate single crystals, Sr xBa 1-xNb 2O 6 (SBN), was studied at room temperature for five nominal compounds indices x (x = 0.40, 0.55, 0.61, 0.65, 0.75) by spectroscopic ellipsometry using the synchrotron light ellipsometer in the photon energy range of 1.5-10 eV. The spectra of < ε 2&gt;(E) obtained appear to be similar to the analogous spectrum of LiNbO 3 crystal, where the NbO 6 octahedral groups are responsible for the features of < ε 2&gt; (E). Differences in the dielectric function of SBN with various x have been found and analyzed. The nonlinear dependences of optical values on the compound index x have been found in SBN. Copyright © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2011.578460
  • Synchrotron-based photoluminescence excitation spectroscopy applied to investigate the valence band splittings in AlN and Al 0.94Ga 0.06N
    Feneberg, M. and Rppischer, M. and Esser, N. and Cobet, C. and Neuschl, B. and Meisch, T. and Thonke, K. and Goldhahn, R.
    Applied Physics Letters 99 (2011)
    We demonstrate that synchrotron-based photoluminescence excitation (PLE) spectroscopy is a versatile tool for determining valence band splittings of AlN and high aluminum content AlGaN. PLE results are independently confirmed by synchrotron-based spectroscopic ellipsometry. The splittings between the ordinary and the extraordinary absorption edges are found to be -240 meV and -170 meV for AlN and Al 0.94Ga 0.06N, respectively. These values differ from the crystal field energy due to residual strain. © 2011 American Institute of Physics.
    view abstract10.1063/1.3610469
  • A new strategy for the preparation of maleimide-functionalised gold surfaces
    Zhang, X. and Sun, G. and Hovestädt, M. and Syritski, V. and Esser, N. and Volkmer, R. and Janietz, S. and Rappich, J. and Hinrichs, K.
    Electrochemistry Communications 12 (2010)
    We have developed and investigated a new route to functionalise Au surfaces using maleimide groups. This functionalisation has been performed by grafting aminophenyl (AP) via an electrochemical reduction of 4-aminophenyldiazonium salt in acetonitrile solution and the subsequent chemical binding of N-(2-carboxyethyl) maleimide (NCEM). The resulting maleimide functionalised surface was interacted with a cysteine-modified peptide. The grafting of AP was monitored by the occurrence of NH2 and aryl ring vibrations, whereas the binding of the NCEM led to a strong and sharp peak because of the C O stretching mode. The immobilisation of the peptide was identified by the appearance of the amide I band. Half of the maleimide surface groups reacted with the peptide because of steric hindrance. The charge efficiency for the AP layer formation was about 45% at a thickness of about 6-8 nm. © 2010 Elsevier B.V.
    view abstract10.1016/j.elecom.2010.07.032
  • Adsorbate-induced modification of the surface electric field of GaAs (001)-c(4T4) measured via the linear electro-optic effect
    Bruhn, T. and Passmann, R. and Fimland, B.O. and Kneissl, M. and Esser, N. and Vogt, P.
    Physica Status Solidi (B) Basic Research 247 (2010)
    We have investigated the modification of the surface electric field (SEF) of the GaAs (001)-c(4×4) surface during the adsorption of cyclopentene and 1,4-cyclohexadiene molecules by reflectance anisotropy spectroscopy (RAS) in the spectral range from 1.5 to 5.0 eV. At around 3 eV the RAS line shape originates from the so-called linear electro-optic effect (LEO). The amplitude of the LEO oscillation scales linearly with the SEF within the RAS penetration depth and is thus a measure for it. For the separation of the LEO from the RAS spectra several different methods are described in the literature. Here, we present a modified method which allows the LEO separation in particular for interfaces between GaAs and organic molecules. The results obtained this way show a significant increase of the LEO effect upon molecule adsorption indicating a higher SEF. The observed changes of the surface electronic properties are probably related to a modification of surface strain upon molecule adsorption. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.200983953
  • Combined ellipsometry and X-ray related techniques for studies of ultrathin organic nanocomposite films
    Krämer, M. and Roodenko, K. and Pollakowski, B. and Hinrichs, K. and Rappich, J. and Esser, N. and Von Bohlen, A. and Hergenröder, R.
    Thin Solid Films 518 (2010)
    Ultrathin nanocomposite films of nitrobenzene on silicon were analyzed by Infrared Spectroscopic Ellipsometry (IRSE), X-ray reflectivity (XRR) and X-ray standing waves (XSW) before and after evaporation of gold. Infrared Spectroscopic Ellipsometry measurements were performed for identification of adsorbates and for investigation of the molecular orientation. Results for film thickness were correlated with XRR measurements. Further, XSW measurements of elements incorporated in nitrobenzene (C, N, and O) were performed with soft X-rays. The combination of the different methods allowed to confirm a model for the electrochemically deposited nitrobenzene films before and after gold evaporation. The characterization by XRR and XSW scans using hard X-rays showed that gold had penetrated into the nitrobenzene film and thus changed density and optical properties of this layer significantly. A depth profile correlated to the electron density is deduced from the XRR measurements. This profile allows to localize-in vertical direction-gold islands within the composite film. © 2010 Elsevier B.V.
    view abstract10.1016/j.tsf.2010.04.033
  • Detection of Yb impurities in the VUV spectral range of NdGaO3 crystals
    Piasecki, M.M. and Andriyevsky, B.B. and Cobet, C.C. and Esser, N.N. and Kityk, I.V. and Świrkowicz, M.M. and Majchrowski, A.A.
    Optics Communications 283 (2010)
    Substantial influence of the 1 at.% Yb doping of NdGaO3 single crystals on the optical functions ε1(E) and ε 2(E) in the spectral range of electronic excitations is established. The corresponding differences of the optical functions for pure and doped NdGaO3 have been monitored using spectroscopic ellipsometry method and synchrotron radiation light source. This opens an opportunity for the spectroscopic diagnostics of the rare earth dopants in crystals using the spectroscopic information concerning the electronic inter-band optical transitions. To understand better the experimental results obtained, the ab-initio calculations of band structure and optical spectra of the centrosymmetric single crystal NdGaO3 have been performed using the VASP code (Vienna Ab-initio Simulation Package). The calculated dielectric functions ε1(E) and ε2(E) agree satisfactorily with our experimental results obtained with using the spectroscopic ellipsometry method and synchrotron radiation. It was demonstrated that differences of the optical functions for pure and doped crystals using stable synchrotron source may serve as a powerful tool for spectroscopic diagnostic of localised rare earth ions with respect to the strong inter-band transitions. © 2010 Elsevier B.V.
    view abstract10.1016/j.optcom.2010.06.004
  • Electronic excitations in B12AS2 and their temperature dependence by vacuum ultraviolet ellipsometry
    Bakalova, S. and Gong, Y. and Cobet, C. and Esser, N. and Zhang, Y. and Edgar, J.H. and Zhang, Y. and Dudley, M. and Kuball, M.
    Journal of Physics Condensed Matter 22 (2010)
    The dielectric response function of epitaxial B12As2 films on 4H-SiC was determined at room temperature and at 10 K in the spectral region of 3.6-9.8 eV, i.e., in the vacuum ultraviolet (VUV) spectral region, by synchrotron ellipsometry. The experimental dielectric function was simulated with the critical point parabolic band model. The parameters of the dispersive structures were derived by numerical fitting of the experimental data to the proposed model. New high energy optical transitions are resolved at 5.95, 7.8 and 8.82 eV and their lineshape and origin are discussed. The temperature dependence of the critical point energies and transition strengths was determined, and the excitonic effect is considered. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/22/39/395801
  • Energy band structure and optical response function of icosahedral B12 As2: A spectroscopic ellipsometry and first-principles calculational study
    Bakalova, S. and Gong, Y. and Cobet, C. and Esser, N. and Zhang, Y. and Edgar, J.H. and Zhang, Y. and Dudley, M. and Kuball, M.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    An experimental and theoretical study on the dielectric-response function of icosahedral B12 As2 in the spectral region between 1.24 and 9.8 eV is presented. Comprehensive experimental information on the energy band structure from the analysis of features in the optical dispersion was complemented by spin-orbit first-principles calculations. The lowest indirect band gap width is 3.2 eV; the two lowest direct interband transitions are at 3.46 and 3.9 eV. High-energy critical points are assigned to specific electron transitions in the Brillouin zone and their dimensionality was determined. The static dielectric constant of B12 As2 is uniaxially anisotropic with values of 7.84 and 9.02 for polarization perpendicular and parallel to the trigonal axis. Hole and electron effective masses are derived from the band dispersions. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.075114
  • In situ studies on the switching behavior of ultrathin poly(acrylic acid) polyelectrolyte brushes in different aqueous environments
    Aulich, D. and Hoy, O. and Luzinov, I. and Brücher, M. and Hergenröder, R. and Bittrich, E. and Eichhorn, K.-J. and Uhlmann, P. and Stamm, M. and Esser, N. and Hinrichs, K.
    Langmuir 26 (2010)
    The pH-dependent switching of a poly(acrylic acid) (PAA) polyelectrolyte brush was investigated in situ using infrared spectroscopic ellipsometry (IRSE). The brush was synthesized by a "grafting to" procedure on silicon substrate with a native oxide layer. The overall thickness of the PAA brush in the dry state was ∼5 nm. Reversible switching of the polymer brush was studied at titration from pH 2 to 10 and back in steps of 1 pH unit. The switching process was observed by monitoring the characteristic vibrational bands of the carboxylic groups of the PAA molecules. Decreasing of the C=O vibrational band amplitude and arising of a COO- vibrational band proved the chemical changes in the molecular structure of the brushes due to changes of the pH value in the aqueous solution. Due to the strong absorption of these bands in the IR region, the switching process could be monitored clearly. Switching the brush in several cycles with increasing and decreasing pH value showed a hysteresis-like behavior. For the first time, such hysteresis is observed in titration experiments of polyelectrolyte brushes. This behavior is attributed to the complex mechanisms of the ions mobility in the brush layer which is explained with a suggested simplifying model describing the influence of ions inside the brush layer. In addition to the IRSE measurements, X-ray standing waves (XSW), in situ visible ellipsometry, and contact angle measurements have been performed and were in good agreement with the results from IRSE. Repetition of the in situ measurement cycles proved the good reversibility of the switching process which is highly important for practical applications of polymer brushes. © 2010 American Chemical Society.
    view abstract10.1021/la101762f
  • In-situ IR synchrotron mapping ellipsometry on stimuli-responsive PAA-b-PS/PEG mixed polymer brushes
    Aulich, D. and Hoy, O. and Luzinov, I. and Eichhorn, K.-J. and Stamm, M. and Gensch, M. and Schade, U. and Esser, N. and Hinrichs, K.
    Physica Status Solidi (C) Current Topics in Solid State Physics 7 (2010)
    A binary polymer brush consisting of weak polyelectrolytes was investigated with infrared synchrotron mapping ellipsometry in-situ under the influence of different aqueous solutions. Thickness of the brush layer in dry state was ∼15 nm. The brush, consisting of poly(ethylene glycol) and poly(acrylic acid)-b-poly(styrene) in a 50/50 composition was switched between two different states by changing the pH of the solution. An IR mapping ellipsometer at the IRIS beamline located at the BESSY II synchrotron facility in Berlin, Germany, was used for high lateral resolution in-situ measurements. The results show strong chemical changes in the brush layer due to COOH - COO- conversion of the PAA's carboxylic groups. Measurements with spot sizes of ∼1 mm on different positions on the samples proved good homogeneity of the brush layer and the qualification of this method for investigation of ultrathin organic films in aqueous solutions in-situ with IR ellipsometry. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.200982492
  • MBE growth of cubic AlN on 3C-SiC substrate
    Schupp, T. and Rossbach, G. and Schley, P. and Goldhahn, R. and Rp̈ppischer, M. and Esser, N. and Cobet, C. and Lischka, K. and As, D.J.
    Physica Status Solidi (A) Applications and Materials Science 207 (2010)
    We present our recent results on the growth of cubic AlN (001) layers by plasma assisted molecular beam epitaxy (PAMBE) using freestanding 3C-SiC (001) substrate. For high-quality c-AlN layers reflection high-electron energy diffraction (RHEED) patterns in all azimuths show RHEED patterns of the cubic lattice, hexagonal reflections are absent. Highresolution X-ray diffraction (HRXRD) measurements confirm the cubic structure of the c-AlN layers with a lattice parameter of 4.373Å. Atomic force microscopy (AFM) scans show an atomically smooth surface with a roughness of 0.2nm RMS. Ellipsometry studies yield the dielectric function (DF) of c-AlN from 1 to 10eV. The direct gap is determined with 5.93eV at room temperature, while the indirect one is below 5.3 eV (onset of adsorption). The high-energy part of the DF is dominated by two transitions at 7.20 and 7.95 eV attributed to critical points of the band structure. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.200983437
  • Metal-insulator transition in Si(111)-(4 × 1)/(8 × 2)-In studied by optical spectroscopy
    Speiser, E. and Chandola, S. and Hinrichs, K. and Gensch, M. and Cobet, C. and Wippermann, S. and Schmidt, W.G. and Bechstedt, F. and Richter, W. and Fleischer, K. and McGilp, J.F. and Esser, N.
    Physica Status Solidi (B) Basic Research 247 (2010)
    Measurements of the surface vibrational modes and optical response of Si(111)-(4 × 1)/(8 × 2)-In are compiled and a comparison to ab initio calculations performed within DFTLDA formalism is given. Surface resonant Raman spectroscopy allows identifying a number of surface phonons with high spectral precision. The phase transition of the (4 × 1)-(8 × 2) surface structure is found to be accompanied by characteristic changes of the surface phonons, which are discussed with respect to various structural models suggested. The optical anisotropy of the (8×2) phase shows that the anisotropic Drude tail of the (4 × 1) phase is replaced by two peaks at 0.50 and 0.72 eV. The spectroscopic signatures of the (4 × 1) and (8×2) phases agree with a metal-insulator transition. The mid-IR-anisotropic optical response of the insulating (8 × 2) phase is interpreted in terms of electronic single particle excitations between surface electronic bands related to the In-nanowire surface. Comparison of the measured optical transitions with DFT ab initio calculations for the hexagon model and the trimer model of the (8 × 2) structure shows evidence for the existence of the hexagon structure. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.200983961
  • Optical anisotropy of Si(111)- (4×1)/(8×2)-In nanowires calculated from first-principles
    Wippermann, S. and Schmidt, W.G. and Bechstedt, F. and Chandola, S. and Hinrichs, K. and Gensch, M. and Esser, N. and Fleischer, K. and McGilp, J.F.
    Physica Status Solidi (C) Current Topics in Solid State Physics 7 (2010)
    First-principles calculations of the anisotropic optical response of Si(111)-(4×1)(8×2)-In from the mid-infrared to the visible region are compared with recent experimental data. The experimental data show that the anisotropic Drude tail of the (4×1) room-temperature phase is replaced by two peaks at 0.50eV and 0.72eV after the phase transition to the low-temperature (8×2) structure took place. The spectrum calculated from both intraband and interband transitions for the metallic zig-zag chain model of the (4×1) phase accounts well for the room-temperature data. The low-temperature data are well explained on the basis of the semiconducting hexagon (8×2) model by interband transitions mainly from a region close the XM high-symmetry line of the Brillouin zone. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.200982413
  • Optical characterisation of BiFeO3 epitaxial thin films grown by pulsed-laser deposition
    Himcinschi, C. and Vrejoiu, I. and Friedrich, M. and Ding, L. and Cobet, C. and Esser, N. and Alexe, M. and Zahn, D.R.T.
    Physica Status Solidi (C) Current Topics in Solid State Physics 7 (2010)
    Epitaxial thin films of bismuth ferrite, BiFeO3, were deposited by pulsed laser deposition (PLD) on SrTiO3 (100), Nbdoped SrTiO 3 (100) and DyScO3 (110) substrates. Ellipsometric spectra are obtained in the energy range 0.73-9.5 eV by combining Variable Angle Spectroscopic Ellipsometry (VASE) and vacuum ultraviolet (VUV) ellipsometry with synchrotron radiation. The optical constants of BiFeO3 films were determined by analysing the ellipsometric spectra with a model that describes the optical response of a system consisting of air, film and substrate. The shift towards higher energies of the refractive index and extinction coefficient of the film deposited onto Nb-doped SrTiO3 as compared with that deposited onto DyScO3 was attributed to a more compressive in-plane epitaxial strain. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.200982414
  • Optical spectra of ZnO in the far ultraviolet: First-principles calculations and ellipsometric measurements
    Gori, P. and Rakel, M. and Cobet, C. and Richter, W. and Esser, N. and Hoffmann, A. and Del Sole, R. and Cricenti, A. and Pulci, O.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    We present ellipsometry data of the dielectric function of wurtzite ZnO in a wide energy range (2.5-32 eV). The ordinary and extraordinary components show a strong anisotropy above 10 eV, a feature for which ZnO deviates from the other II-VI wurtzite compounds. With the aid of ab initio calculations, performed within many-body perturbation theory (MBPT) and within time-dependent density-functional theory (TDDFT), we analyze the origin of the measured optical structures. TDDFT, with the use of a static long-range exchange-correlation kernel, proves to be a cheaper computational tool than MBPT to yield a good description of the whole spectrum. Theoretical results for the zinc-blende phase are also presented. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.125207
  • Passivation of Si(111) surfaces with electrochemically grafted thin organic films
    Roodenko, K. and Yang, F. and Hunger, R. and Esser, N. and Hinrichs, K. and Rappich, J.
    Surface Science 604 (2010)
    Ultra thin organic films (about 5 nm thick) of nitrobenzene and 4-methoxydiphenylamine were deposited electrochemically on p-Si(111) surfaces from benzene diazonium compounds. Studies based on atomic force microscopy, infrared spectroscopic ellipsometry and x-ray photoelectron spectroscopy showed that upon exposure to atmospheric conditions the oxidation of the silicon interface proceed slower on organically modified surfaces than on unmodified hydrogen passivated p-Si(111) surfaces. Effects of HF treatment on the oxidized organic/Si interface and on the organic layer itself are discussed. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.susc.2010.06.005
  • Polariton effects in the dielectric function of ZnO excitons obtained by ellipsometry
    Cobet, M. and Cobet, C. and Wagner, M.R. and Esser, N. and Thomsen, C. and Hoffmann, A.
    Applied Physics Letters 96 (2010)
    The complex dielectric tensor of ZnO in the regime of the excitonic transitions is determined with ellipsometry and analyzed concerning the quantization of the electromagnetic field in terms of coupled polariton-eigenmodes. Negative sections in the real part indicate the significant formation of polaritons for the dipole-allowed excitons of the three upper valence-bands γ7, γ9, γ7. The transverse-longitudinal splittings which separate the upper polariton branch from the lower branch, corresponding to the k -vector of the used light, are deduced precisely for each subband. Mainly for Ε∥c, additional absorption peaks are observed at the longitudinal B-exciton and closely above. One is considered to be a mixed-mode and the other is seen as a consequence of interference effects in an exciton free surface layer which is also visible in reflectance anisotropy spectroscopy. © 2010 American Institute of Physics.
    view abstract10.1063/1.3284656
  • Substrate influence on the optical and structural properties of pulsed laser deposited BiFeO3 epitaxial films
    Himcinschi, C. and Vrejoiu, I. and Friedrich, M. and Nikulina, E. and Ding, L. and Cobet, C. and Esser, N. and Alexe, M. and Rafaja, D. and Zahn, D.R.T.
    Journal of Applied Physics 107 (2010)
    Epitaxial BiFeO3 films pulsed laser deposited on SrTiO 3, Nb:doped SrTiO3, and DyScO3 were studied using variable angle spectroscopic ellipsometry, vacuum ultraviolet ellipsometry, micro-Raman spectroscopy, and x-ray diffraction. The energy band gap of the film deposited on DyScO3 is 2.75 eV, while the one for the film deposited on Nb:doped SrTiO3 is larger by 50 meV. The blueshift in the dielectric function of the BiFeO3 films deposited on Nb:doped SrTiO3 compared to the films deposited on DyScO3, indicates a larger compressive strain in the films deposited on Nb:doped SrTiO3. This is confirmed by Raman spectroscopy and by high resolution x-ray diffraction investigations. © 2010 American Institute of Physics.
    view abstract10.1063/1.3437059
  • Synchrotron infrared spectroscopic ellipsometry for characterization of biofunctional surfaces
    Sun, G. and Rosu, D.M. and Zhang, X. and Hovestädt, M. and Pop, S. and Schade, U. and Aulich, D. and Gensch, M. and Ay, B. and Holzhütter, H.-G. and Zahn, D.R.T. and Esser, N. and Volkmer, R. and Rappich, J. and Hinrichs, K.
    Physica Status Solidi (B) Basic Research 247 (2010)
    The structural and thickness homogeneity of different biofunctional surfaces after coating an Au substrate was studied by infrared spectroscopic ellipsometry (IRSE). Monolayer coverage was achieved by washing the samples in aqueous solution. A selected cysteine-modified CHL peptide and an antiglutathione S-transfer (GST) antibody produced a characteristic change in the IR ellipsometric spectra after peptide and antibody adsorption, confirming the biosensing ability of this technique. IR ellipsometric mapping at the synchrotron storage ring BESSY II in Berlin allowed laterally resolved characterization of the protein films. The IR ellipsometric tanψ and δ maps revealed that the protein films are inhomogeneous in structure and thickness after adsorption of the peptide and antibody. The absolute thickness of the protein films varied considerably by up to 3 to 7 nm. We also studied cytosine films with different thicknesses (d = 58 nm and 125 nm) and a guanine film (d = 84 nm). For the two cytosine films the synchrotron spectra revealed that the thicker film in particular was rather homogeneous in thickness but inhomogeneous in structure. For the guanine film the shape of vibrational bands in the ellipsometric spectra correlated with anisotropic molecular orientations. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.200983945
  • Valence-band splitting and optical anisotropy of AlN
    Rossbach, G. and Röppischer, M. and Schley, P. and Gobsch, G. and Werner, C. and Cobet, C. and Esser, N. and Dadgar, A. and Wieneke, M. and Krost, A. and Goldhahn, R.
    Physica Status Solidi (B) Basic Research 247 (2010)
    The dielectric function (DF) of hexagonal AlN on Si(111) is determined in the range between 1 and 9.8 eV by spectroscopic ellipsometry (SE). Due to its large negative crytal-field splitting wurtzite AlN features large dichroism. Showing that SE is sensitive to both components of the DF around the absorption edge, a uniaxial model is applied which yields transition energies for the free excitonic state. The in-plane tensile stress leads to a red-shift of these transitions and to an enlarged splitting. The experimental data are compared to the results of band-structure calculations demonstrating excellent overall agreement. In addition, two high-energy critical points in the ordinary DF were determined at energies of about 7.75 and 8.85 eV. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.200983677

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