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  • 1.
    Adam, Stefan
    et al.
    Leibniz Institute Polymerforsch eV, Germany; Technical University of Dresden, Germany.
    Koenig, Meike
    Leibniz Institute Polymerforsch eV, Germany; Technical University of Dresden, Germany; Karlsruhe Institute Technology, Germany.
    Rodenhausen, Keith Brian
    University of Nebraska, NE 68588 USA; Biolin Science Inc, NJ 07652 USA.
    Eichhorn, Klaus-Jochen
    Leibniz Institute Polymerforsch eV, Germany.
    Oertel, Ulrich
    Leibniz Institute Polymerforsch eV, Germany.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Leibniz Institute Polymerforsch eV, Germany; University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Stamm, Manfred
    Leibniz Institute Polymerforsch eV, Germany; Technical University of Dresden, Germany.
    Uhlmann, Petra
    Leibniz Institute Polymerforsch eV, Germany; University of Nebraska, NE 68588 USA.
    Quartz crystal microbalance with coupled Spectroscopic Ellipsometry-study of temperature-responsive polymer brush systems2017In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, p. 843-851Article in journal (Refereed)
    Abstract [en]

    Using a combined setup of quartz crystal microbalance with dissipation monitoring together with spectroscopic ellipsometry, the thermo-responsive behavior of two different brush systems (poly(N-isopropyl acrylamide) and poly(2-oxazoline)s) was investigated and compared to the behavior of the free polymer in solution. Poly(2-oxazoline)s with three different hydrophilicities were prepared by changing the content of a hydrophilic comonomer. While both polymer types exhibit a sharp, discontinuous thermal transition in solution, in the brush state the transition gets broader in the case of poly(N-isopropyl acrylamide) and is transformed into a continuous transition for poly(2-oxazoline)s. The position of the transition in solution is influenced by the degree of hydrophilicity of the poly(2-oxazoline). The difference in areal mass detected by quartz crystal microbalance and by spectroscopic ellipsometry, has been attributed to the chain segment density profile of the polymer brushes. Applying this density profile information, for poly(N-isopropyl acrylamide) two different swelling stages could be identified, while for poly(2-oxazoline) the transition between a parabolic and more step-wise profile is found continuous. The different swelling characteristics were attributed to the different miscibility behavior types, with the brush state acting similar to a crosslinked system. (C) 2017 Elsevier B.V. All rights reserved.

  • 2.
    Armakavicius, Nerijus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Bouhafs, Chamseddine
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kühne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Knight, Sean
    Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, USA.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, USA / Department of Physics and Optical Science, University of North Carolina at Charlotte, USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies2017In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, p. 357-360Article in journal (Refereed)
    Abstract [en]

    Cavity-enhanced optical Hall effect at terahertz (THz) frequencies is employed to determine the free charge carrier properties in epitaxial graphene (EG) with different number of layers grown by high-temperature sublimation on 4H-SiC(0001). We find that one monolayer (ML) EG possesses p-type conductivity with a free hole concentration in the low 1012 cmᅵᅵᅵ2 range and a free hole mobility parameter as high as 1550 cm2/Vs. We also find that 6 ML EG shows n-type doping behavior with a much lower free electron mobility parameter of 470 cm2/Vs and an order of magnitude higher free electron density in the low 1013 cmᅵᅵᅵ2 range. The observed differences are discussed. The cavity-enhanced THz optical Hall effect is demonstrated to be an excellent tool for contactless access to the type of free charge carriers and their properties in two-dimensional materials such as EG.

  • 3.
    Armakavicius, Nerijus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Knight, Sean Robert
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stokey, Megan
    Univ Nebraska Lincoln, NE 68588 USA.
    Sorensen, Preston
    Univ Nebraska Lincoln, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska Lincoln, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden; Univ Nebraska Lincoln, NE 68588 USA.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Electron effective mass in GaN revisited: New insights from terahertz and mid-infrared optical Hall effect2024In: APL Materials, E-ISSN 2166-532X, Vol. 12, no 2, article id 021114Article in journal (Refereed)
    Abstract [en]

    Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38-340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 +/- 0.02) m(0) to (0.34 +/- 0.01) m(0) at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 +/- 0.002) m(0). A possible explanation for the different findings from THz OHE and MIR OHE is proposed. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

  • 4.
    Armakavicius, Nerijus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Eriksson, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Sensor and Actuator Systems. Linköping University, Faculty of Science & Engineering.
    Bouhafs, Chamseddine
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Ist Italiano Tecnol, Italy.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Zakharov, Alexei A.
    Lund Univ, Sweden.
    Al-Temimy, Ameer
    Ist Italiano Tecnol, Italy.
    Coletti, Camilla
    Ist Italiano Tecnol, Italy; Ist Italiano Tecnol, Italy.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68508 USA; Leibniz Inst Polymerforsch eV, Germany.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect2021In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 172, p. 248-259Article in journal (Refereed)
    Abstract [en]

    In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-freestanding-bilayer epitaxial graphene on 4H-SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasifree-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene. (C) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license.

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  • 5.
    Armakavicius, Nerijus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Knight, Sean
    Univ Nebraska, NE 68588 USA.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Electron effective mass in In0.33Ga0.67N determined by mid-infrared optical Hall effect2018In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 8, article id 082103Article in journal (Refereed)
    Abstract [en]

    Mid-infrared optical Hall effect measurements are used to determine the free charge carrier parameters of an unintentionally doped wurtzite-structure c-plane oriented In0.33Ga0.67N epitaxial layer. Room temperature electron effective mass parameters of m(perpendicular to)* = (0.205 +/- 0.013) m(0) and m(parallel to)* = (0.204 +/- 0.016) m(0) for polarization perpendicular and parallel to the c-axis, respectively, were determined. The free electron concentration was obtained as (1.7 +/- 0.2) x 10(19) cm(-3). Within our uncertainty limits, we detect no anisotropy for the electron effective mass parameter and we estimate the upper limit of the possible effective mass anisotropy as 7%. We discuss the influence of conduction band nonparabolicity on the electron effective mass parameter as a function of In content. The effective mass parameter is consistent with a linear interpolation scheme between the conduction band mass parameters in GaN and InN when the strong nonparabolicity in InN is included. The In0.33Ga0.67N electron mobility parameter was found to be anisotropic, supporting previous experimental findings for wurtzite-structure GaN, InN, and AlxGa1-xN epitaxial layers with c-plane growth orientation. Published by AIP Publishing.

  • 6.
    Bouhafs, Chamseddine
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Zakharov, A. A.
    Lund University, Sweden.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, USA.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Iakimov, Tihomir
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering. University of Nebraska, USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Decoupling and ordering of multilayer graphene on C-face 3C-SiC(111)2016In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 109, no 20, article id 203102Article in journal (Refereed)
    Abstract [en]

    We show experimentally that few layer graphene (FLG) grown on the carbon terminated surface (C-face) of 3C-SiC(111) is composed of decoupled graphene sheets. Landau level spectroscopy on FLG graphene is performed using the infrared optical Hall effect. We find that Landau level transitions in the FLG exhibit polarization preserving selection rules and the transition energies obey a square-root dependence on the magnetic field strength. These results show that FLG on C-face 3C-SiC(111) behave effectively as a single layer graphene with linearly dispersing bands (Dirac cones) at the graphene K point. We estimate from the Landau level spectroscopy an upper limit of the Fermi energy of about 60 meV in the FLG, which corresponds to a carrier density below 2.5 x 10(11) cm(-2). Low-energy electron diffraction mu-LEED) reveals the presence of azimuthally rotated graphene domains with a typical size of amp;lt;= 200 nm.mu-LEED mapping suggests that the azimuth rotation occurs between adjacent domains within the same sheet rather than vertically in the stack. Published by AIP Publishing.

  • 7.
    Bouhafs, Chamseddine
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Zakharov, A. A.
    Lund University, Sweden.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Giannazzo, F.
    CNR IMM, Italy.
    Eriksson, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Applied Sensor Science. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Iakimov, Tihomir
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska Lincoln, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska Lincoln, NE 68588 USA.
    Roccaforte, F.
    CNR IMM, Italy.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Multi-scale investigation of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC2017In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 116, p. 722-732Article in journal (Refereed)
    Abstract [en]

    In this work, we report a multi-scale investigation using several nano-, micro and macro-scale techniques of few layer graphene (FLG) sample consisting of large monolayer (ML) and bilayer (BL) areas grown on C-face 4H-SiC (000-1) by high-temperature sublimation. Single 1 x 1 diffraction patterns are observed by micro-low-energy electron diffraction for ML, BL and trilayer graphene with no indication of out-of-plane rotational disorder. A SiOx layer is identified between graphene and SiC by X-ray photoelectron emission spectroscopy and reflectance measurements. The chemical composition of the interface layer changes towards SiO2 and its thickness increases with aging in normal ambient conditions. The formation mechanism of the interface layer is discussed. It is shown by torsion resonance conductive atomic force microscopy that the interface layer causes the formation of non-ideal Schottky contact between ML graphene and SiC. This is attributed to the presence of a large density of interface states. Mid-infrared optical Hall effect measurements revealed Landau-level transitions in FLG that have a square-root dependence on magnetic field, which evidences a stack of decoupled graphene sheets. Contrary to previous works on decoupled C-face graphene, our BL and FLG are composed of ordered decoupled graphene layers without out-of-plane rotation. (C) 2017 Elsevier Ltd. All rights reserved.

  • 8.
    Briley, Chad
    et al.
    University of Nebraska, NE 68588 USA.
    Mock, Alyssa
    University of Nebraska, NE 68588 USA.
    Korlacki, Rafal
    University of Nebraska, NE 68588 USA.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE 68588 USA.
    Schubert, Eva
    University of Nebraska, NE 68588 USA.
    Schubert, Mathias
    University of Nebraska, NE 68588 USA.
    Effects of annealing and conformal alumina passivation on anisotropy and hysteresis of magneto-optical properties of cobalt slanted columnar thin films2017In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, p. 320-324Article in journal (Refereed)
    Abstract [en]

    We present magneto-optical dielectric tensor data of cobalt and cobalt oxide slanted columnar thin films obtained by vector magneto-optical generalized ellipsometry. Room-temperature hysteresis magnetization measurements were performed in longitudinal and polar Kerr geometries on samples prior to and after a heat treatment process with and without a conformal Al2O3 passivation coating. The samples have been characterized by generalized ellipsometry, scanning electron microscopy, and Raman spectroscopy in conjuncture with density functional theory. We observe strongly anisotropic hysteresis behaviors, which depend on the nanocolumn and magnetizing field orientations. We find that deposited cobalt films that have been exposed to heat treatment and subsequent atmospheric oxidation into Co3O4, when not conformally passivated, reveal no measurable magneto-optical properties while cobalt films with passivation coatings retain highly anisotropic magneto-optical properties (C) 2016 Published by Elsevier B.V.

  • 9.
    Chae, Soosang
    et al.
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Choi, Won Jin
    Univ Michigan, MI 48109 USA.
    Fotev, Ivan
    Helmholtz Zentrum Dresden Rossendorf eV, Germany; Tech Univ Dresden, Germany.
    Bittrich, Eva
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Uhlmann, Petra
    Leibniz Inst Polymerforsch Dresden eV, Germany; Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Leibniz Inst Polymerforsch Dresden eV, Germany; Univ Nebraska, NE 68588 USA.
    Makarov, Denys
    Helmholtz Zentrum Dresden Rossendorf eV, Germany.
    Wagner, Jens
    Tech Univ Dresden, Germany; Tech Univ Dresden, Germany.
    Pashkin, Alexej
    Helmholtz Zentrum Dresden Rossendorf eV, Germany.
    Fery, Andreas
    Leibniz Inst Polymerforsch Dresden eV, Germany; Tech Univ Dresden, Germany.
    Stretchable Thin Film Mechanical-Strain-Gated Switches and Logic Gate Functions Based on a Soft Tunneling Barrier2021In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 33, no 41, article id 2104769Article in journal (Refereed)
    Abstract [en]

    Mechanical-strain-gated switches are cornerstone components of material-embedded circuits that perform logic operations without using conventional electronics. This technology requires a single material system to exhibit three distinct functionalities: strain-invariant conductivity and an increase or decrease of conductivity upon mechanical deformation. Herein, mechanical-strain-gated electric switches based on a thin-film architecture that features an insulator-to-conductor transition when mechanically stretched are demonstrated. The conductivity changes by nine orders of magnitude over a wide range of tunable working strains (as high as 130%). The approach relies on a nanometer-scale sandwiched bilayer Au thin film with an ultrathin poly(dimethylsiloxane) elastomeric barrier layer; applied strain alters the electron tunneling currents through the barrier. Mechanical-force-controlled electric logic circuits are achieved by realizing strain-controlled basic (AND and OR) and universal (NAND and NOR) logic gates in a single system. The proposed material system can be used to fabricate material-embedded logics of arbitrary complexity for a wide range of applications including soft robotics, wearable/implantable electronics, human-machine interfaces, and Internet of Things.

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  • 10.
    Chen, Shangzhi
    et al.
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Knight, Sean
    Univ Nebraska, NE 68588 USA.
    Brooke, Robert
    RISE Acreo, Sweden.
    Petsagkourakis, Ioannis
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Crispin, Xavier
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymerforsch Dresden eV, Germany.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Jonsson, Magnus
    Linköping University, Department of Science and Technology, Laboratory of Organic Electronics. Linköping University, Faculty of Science & Engineering.
    On the anomalous optical conductivity dispersion of electrically conducting polymers: ultra-wide spectral range ellipsometry combined with a Drude-Lorentz model2019In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 7, no 15, p. 4350-4362Article in journal (Refereed)
    Abstract [en]

    Electrically conducting polymers (ECPs) are becoming increasingly important in areas such as optoelectronics, biomedical devices, and energy systems. Still, their detailed charge transport properties produce an anomalous optical conductivity dispersion that is not yet fully understood in terms of physical model equations for the broad range optical response. Several modifications to the classical Drude model have been proposed to account for a strong non-Drude behavior from terahertz (THz) to infrared (IR) ranges, typically by implementing negative amplitude oscillator functions to the model dielectric function that effectively reduce the conductivity in those ranges. Here we present an alternative description that modifies the Drude model via addition of positive-amplitude Lorentz oscillator functions. We evaluate this so-called Drude-Lorentz (DL) model based on the first ultra-wide spectral range ellipsometry study of ECPs, spanning over four orders of magnitude: from 0.41 meV in the THz range to 5.90 eV in the ultraviolet range, using thin films of poly(3,4-ethylenedioxythiophene): tosylate (PEDOT: Tos) as a model system. The model could accurately fit the experimental data in the whole ultrawide spectral range and provide the complex anisotropic optical conductivity of the material. Examining the resonance frequencies and widths of the Lorentz oscillators reveals that both spectrally narrow vibrational resonances and broader resonances due to localization processes contribute significantly to the deviation from the Drude optical conductivity dispersion. As verified by independent electrical measurements, the DL model accurately determines the electrical properties of the thin film, including DC conductivity, charge density, and (anisotropic) mobility. The ellipsometric method combined with the DL model may thereby become an effective and reliable tool in determining both optical and electrical properties of ECPs, indicating its future potential as a contact-free alternative to traditional electrical characterization.

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  • 11.
    Gogova, Daniela
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Ghezellou, Misagh
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat Q.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, P. O. Box 118, 221 00 Lund, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    ul-Hassan, Jawad
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Axel R.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Persson, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Kordina, Olof
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Hilfiker, Matthew
    Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
    Paskov, Plamen P.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, P. O. Box 118, 221 00 Lund, Sweden.
    Epitaxial growth of β-Ga2O3 by hot-wall MOCVD2022In: AIP Advances, E-ISSN 2158-3226, Vol. 12, no 5, article id 055022Article in journal (Refereed)
    Abstract [en]

    The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of beta-Ga2O3. Epitaxial beta-Ga2O3 layers at high growth rates (above 1 mu m/h), at low reagent flows, and at reduced growth temperatures (740 degrees C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial beta-Ga2O3 layers are demonstrated with a 201 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (201) beta-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of beta-Ga2O3.

  • 12.
    Gopalan, Prashanth
    et al.
    Univ Utah, UT 84112 USA.
    Knight, Sean Robert
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Chanana, Ashish
    Univ Utah, UT 84112 USA.
    Stokey, Megan
    Univ Nebraska Lincoln, NE 68588 USA.
    Ranga, Praneeth
    Univ Utah, UT 84112 USA.
    Scarpulla, Michael A.
    Univ Utah, UT 84112 USA.
    Krishnamoorthy, Sriram
    Univ Utah, UT 84112 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Galazka, Zbigniew
    Leibniz Inst Kristallzuchtung Berlin eV, Germany.
    Irmscher, Klaus
    Leibniz Inst Kristallzuchtung Berlin eV, Germany.
    Fiedler, Andreas
    Leibniz Inst Kristallzuchtung Berlin eV, Germany.
    Blair, Steve
    Univ Utah, UT 84112 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Sensale-Rodriguez, Berardi
    Univ Utah, UT 84112 USA.
    The anisotropic quasi-static permittivity of single-crystal <bold>beta</bold>-Ga2O3 measured by terahertz spectroscopy2020In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 117, no 25, article id 252103Article in journal (Refereed)
    Abstract [en]

    The quasi-static anisotropic permittivity parameters of electrically insulating beta gallium oxide (beta -Ga2O3) were determined by terahertz spectroscopy. Polarization-resolved frequency domain spectroscopy in the spectral range from 200GHz to 1THz was carried out on bulk crystals along different orientations. Principal directions for permittivity were determined along crystallographic axes c and b and reciprocal lattice direction a *. No significant frequency dispersion in the real part of dielectric permittivity was observed in the measured spectral range. Our results are in excellent agreement with recent radio frequency capacitance measurements as well as with extrapolations from recent infrared measurements of phonon mode and high-frequency contributions and close the knowledge gap for these parameters in the terahertz spectral range. Our results are important for applications of beta -Ga2O3 in high-frequency electronic devices.

  • 13.
    Hilfiker, Matthew
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska Lincoln, NE 68588 USA.
    Mock, Alyssa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Knight, Sean
    Univ Nebraska Lincoln, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska Lincoln, NE 68588 USA.
    Mauze, Akhil
    Univ Calif Santa Barbara, CA 93106 USA.
    Zhang, Yuewei
    Univ Calif Santa Barbara, CA 93106 USA.
    Speck, James
    Univ Calif Santa Barbara, CA 93106 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA; Leibniz Inst Polymerforsch eV, Germany.
    Dielectric function tensor (1.5 eV to 9.0 eV), anisotropy, and band to band transitions of monoclinic -(AlxGa1-x)(2)O-3 (x 0.21) films2019In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 114, no 23, article id 231901Article in journal (Refereed)
    Abstract [en]

    A set of monoclinic -(AlxGa1-x)(2)O-3 films coherently grown by plasma-assisted molecular beam epitaxy onto (010)-oriented -Ga2O3 substrates for compositions x0.21 is investigated by generalized spectroscopic ellipsometry at room temperature in the spectral range of 1.5eV-9.0eV. We present the composition dependence of the excitonic and band to band transition energy parameters using a previously described eigendielectric summation approach for -Ga2O3 from the study by Mock et al. All energies shift to a shorter wavelength with the increasing Al content in accordance with the much larger fundamental band to band transition energies of Al2O3 regardless of crystal symmetry. The observed increase in the lowest band to band transition energy is in excellent agreement with recent theoretical predictions. The most important observation is that charge confinement in heterostructures will strongly depend on the growth condition due to the strongly anisotropic properties of the band to band transitions.

  • 14.
    Hilfiker, Matthew
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska Lincoln, NE 68588 USA.
    Stokey, Megan
    Univ Nebraska Lincoln, NE 68588 USA.
    Jinno, Riena
    Cornell Univ, NY 14853 USA; Kyoto Univ, Japan.
    Cho, Yongjin
    Cornell Univ, NY 14853 USA.
    Xing, Huili Grace
    Cornell Univ, NY 14853 USA; Cornell Univ, NY 14853 USA.
    Jena, Debdeep
    Cornell Univ, NY 14853 USA; Cornell Univ, NY 14853 USA.
    Korlacki, Rafal
    Univ Nebraska Lincoln, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in α-(AlxGa1−x)2O3 (0≤𝑥≤10≤x≤1)2022In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 121, no 5, article id 052101Article in journal (Refereed)
    Abstract [en]

    Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure alpha-(AlxGa1-x)(2)O-3 thin films (0 &lt;= x &lt;= 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular (b(Eg,⊥) = 1.31 eV) and parallel (b(Eg,||) = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between alpha-Ga2O3 and alpha-Al2O3, respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in alpha-Ga2O3 whereas for alpha-Al2O3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M-1 type in alpha-Ga2O3 to M-0 type van Hove singularity in alpha-Al2O3.

  • 15.
    Hilfiker, Matthew
    et al.
    Univ Nebraska, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska, NE 68588 USA.
    Stokey, Megan
    Univ Nebraska, NE 68588 USA.
    Jinno, Riena
    Cornell Univ, NY 14853 USA; Kyoto Univ, Japan.
    Cho, Yongjin
    Cornell Univ, NY 14853 USA.
    Xing, Huili Grace
    Cornell Univ, NY 14853 USA; Cornell Univ, NY 14853 USA.
    Jena, Debdeep
    Cornell Univ, NY 14853 USA; Cornell Univ, NY 14853 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymerforsch eV, Germany.
    High-frequency and below bandgap anisotropic dielectric constants in alpha-(AlxGa1-x)(2)O-3 (0 <= x <= 1)2021In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 119, no 9, article id 092103Article in journal (Refereed)
    Abstract [en]

    A Mueller matrix spectroscopic ellipsometry approach was used to investigate the anisotropic dielectric constants of corundum alpha-(AlxGa1-x)(2)O-3 thin films in their below bandgap spectral regions. The sample set was epitaxially grown using plasma-assisted molecular beam epitaxy on m-plane sapphire. The spectroscopic ellipsometry measurements were performed at multiple azimuthal angles to resolve the uniaxial dielectric properties. A Cauchy dispersion model was applied, and high-frequency dielectric constants are determined for polarization perpendicular (epsilon(infinity,perpendicular to)) and parallel (epsilon(infinity,parallel to)) to the thin film c-axis. The optical birefringence is negative throughout the composition range, and the overall index of refraction substantially decreases upon incorporation of Al. We find small bowing parameters of the highfrequency dielectric constants with b(perpendicular to) = 0:386 and b(parallel to) = 0:307. Published under an exclusive license by AIP Publishing.

  • 16.
    Hilfiker, Matthew
    et al.
    Univ Nebraska, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Jinno, Riena
    Cornell Univ, NY 14853 USA; Kyoto Univ, Japan.
    Cho, Yongjin
    Cornell Univ, NY 14853 USA.
    Xing, Huili Grace
    Cornell Univ, NY 14853 USA.
    Jena, Debdeep
    Cornell Univ, NY 14853 USA.
    Kilic, Ufuk
    Univ Nebraska, NE 68588 USA.
    Stokey, Megan
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymerforsch eV, Germany.
    Anisotropic dielectric functions, band-to-band transitions, and critical points in <bold>alpha</bold>-Ga2O32021In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 118, no 6, article id 062103Article in journal (Refereed)
    Abstract [en]

    We use a combined generalized spectroscopic ellipsometry and density functional theory approach to determine and analyze the anisotropic dielectric functions of an alpha -Ga2O3 thin film. The sample is grown epitaxially by plasma-assisted molecular beam epitaxy on m-plane sapphire. Generalized spectroscopic ellipsometry data from multiple sample azimuths in the spectral range from 0.73eV to 8.75eV are simultaneously analyzed. Density functional theory is used to calculate the valence and conduction band structure. We identify, for the indirect-bandgap material, two direct band-to-band transitions with M-0-type van Hove singularities for polarization perpendicular to the c axis, E 0 , perpendicular to = 5.46 ( 6 ) eV and E 0 , perpendicular to = 6.04 ( 1 ) eV, and one direct band-to-band transition with M-1-type van Hove singularity for polarization parallel to E 0 , | | = 5.44 ( 2 ) eV. We further identify excitonic contributions with a small binding energy of 7meV associated with the lowest ordinary transition and a hyperbolic exciton at the M-1-type critical point with a large binding energy of 178meV.

  • 17.
    Hilfiker, Matthew
    et al.
    Univ Nebraska, NE 68588 USA.
    Stokey, Megan
    Univ Nebraska, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska, NE 68588 USA.
    Galazka, Zbigniew
    Leibniz Inst Kristallzuchtung, Germany.
    Irmscher, Klaus
    Leibniz Inst Kristallzuchtung, Germany.
    Zollner, Stefan
    New Mexico State Univ, NM 88003 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymerforsch eV, Germany.
    Zinc gallate spinel dielectric function, band-to-band transitions, and Gamma-point effective mass parameters2021In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 118, no 13, article id 132102Article in journal (Refereed)
    Abstract [en]

    We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75eV) into the vacuum ultraviolet (8.5eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone Gamma-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24m(e)) at the bottom of the Gamma-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24m(e)). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the Gamma-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M-0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier-Mott type excitons with an effective Rydberg energy of 14.8meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements (root epsilon(infinity)= 1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)]. Published under license by AIP Publishing.

  • 18.
    Hilfiker, Matthew
    et al.
    Univ Nebraska, NE 68588 USA.
    Williams, Emma
    Univ Nebraska, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska, NE 68588 USA.
    Traouli, Yousra
    Univ Nebraska, NE 68588 USA; Univ Angers, France.
    Koeppe, Nate
    Univ Nebraska, NE 68588 USA.
    Rivera, Jose
    Univ Nebraska, NE 68588 USA; Univ Puerto Rico Mayaguez, PR 00682 USA.
    Abakar, Assya
    Univ Nebraska, NE 68588 USA; Univ Angers, France.
    Stokey, Megan
    Univ Nebraska, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Galazka, Zbigniew
    Leibniz Inst Kristallzuchtung, Germany.
    Irmscher, Klaus
    Leibniz Inst Kristallzuchtung, Germany.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA.
    Elevated temperature spectroscopic ellipsometry analysis of the dielectric function, exciton, band-to-band transition, and high-frequency dielectric constant properties for single-crystal ZnGa2O42022In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 120, no 13, article id 132105Article in journal (Refereed)
    Abstract [en]

    We report the elevated temperature (22 degrees C &lt;= T &lt;= 600 degrees C) dielectric function properties of melt grown single crystal ZnGa2O4 using a spectroscopic ellipsometry approach. A temperature dependent Cauchy dispersion analysis was applied across the transparent spectrum to determine the high-frequency index of refraction yielding a temperature dependent slope of 3.885(2) x 10(-5)K(-1). A model dielectric function critical point analysis was applied to examine the dielectric function and critical point transitions for each temperature. The lowest energy M-0-type critical point associated with the direct bandgap transition in ZnGa2O4 is shown to red-shift linearly as the temperature is increased with a subsequent slope of -0.72(4) meV K-1. Furthermore, increasing the temperature results in a reduction of the excitonic amplitude and increase in the exciton broadening akin to exciton evaporation and lifetime shortening. This matches current theoretical understanding of excitonic behavior and critically provides justification for an anharmonic broadened Lorentz oscillator to be applied for model analysis of excitonic contributions.

  • 19.
    Hofmann, Tino
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of North Carolina Charlotte, NC 28223 USA; University of Nebraska, NE USA.
    Knight, S.
    University of Nebraska, NE USA.
    Sekora, D.
    University of Nebraska, NE USA.
    Schmidt, D.
    University of Nebraska, NE USA.
    Herzinger, C. M.
    JA Woollam Co Inc, NE 68508 USA.
    Woollam, J. A.
    JA Woollam Co Inc, NE 68508 USA.
    Schubert, E.
    University of Nebraska, NE USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE USA.
    Screening effects in metal sculptured thin films studied with terahertz Mueller matrix ellipsometry2017In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, p. 513-517Article in journal (Refereed)
    Abstract [en]

    The anisotropic optical dielectric functions of a metal (cobalt) slanted columnar thin film deposited by electronbeam glancing angle deposition are reported for the terahertz (THz) frequency domain before and after the slanted columnar thin film was passivated by a conformal alumina coating. A simple effective medium dielectric function homogenization approach which describes isolated, electrically conductive columns rendering the thin film biaxial (orthorhombic) is used to model the observed optical responses. Upon passivating the slanted columnar thin film with a 3 nm thick alumina film an increase of both the real and the imaginary part of the dielectric function for all major polarizability directions is found and attributed to screening effects within the spatially coherent metal nanocolumns. (C) 2016 Elsevier B.V. All rights reserved.

  • 20.
    Kananizadeh, Negin
    et al.
    Univ Nebraska, NE 68588 USA; Clemson Univ, SC 29625 USA.
    Lee, Jaewoong
    Natl Inst Environm Res, South Korea.
    Mousavi, Ehsan S.
    Clemson Univ, SC 29634 USA.
    Rodenhausen, Keith B.
    Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Sekora, Derek
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res IPF Dresden, Germany.
    Bartelt-Hunt, Shannon
    Univ Nebraska, NE 68588 USA.
    Schubert, Eva
    Univ Nebraska, NE 68588 USA.
    Zhangh, Jianmin
    Sichuan Univ, Peoples R China.
    Li, Yusong
    Univ Nebraska, NE 68588 USA.
    Deposition of titanium dioxide nanoparticles onto engineered rough surfaces with controlled heights and properties2019In: Colloids and Surfaces A: Physicochemical and Engineering Aspects, ISSN 0927-7757, E-ISSN 1873-4359, Vol. 571, p. 125-133Article in journal (Refereed)
    Abstract [en]

    Understanding the influence of surface roughness on the deposition of nanoparticles is important to a variety of environmental and industrial processes. In this work, slanted columnar thin films (SCTFs) were engineered to serve as an analogue for rough surfaces with controlled height and surface properties. The deposition of titanium dioxide nanoparticles (TiO(2)NPs) onto alumina-or silica-coated SCTFs (Al2O3-Si-SCTF, SiO2-Si-SCTF) with varying heights (50 nm, 100 nm, and 200 nm) was measured using a combined quartz crystal microbalance with dissipation monitoring (QCM-D) and generalized ellipsometry (GE) technique. No TiO2NP deposition was observed on flat, silica-coated QCM-D sensors or rough, 100 nm thick SiO2-Si-SCTF. TiO2NP deposition onto Al2O3-Si-SCTFs in ultra-pure water was significantly higher than on the flat alumina-coated QCM-D sensor, and deposition increased as the roughness height increased. The nanoparticle attachment was sensitive to the local flow field and the interaction energy between nanoparticles and the QCM-D sensor. At a higher ionic strength condition (100 mM NaCl), TiO2NP aggregates with varying sizes formed a rigid layer on top of SCTFs. For the first time, deposition of nanoparticles was measured as a function of roughness height, and the impact of roughness on the properties of the attached nanoparticle layers was revealed. This finding indicates that key parameters describing surface roughness should be explicitly included into models to accurately predict the transport of nanoparticles in the subsurface.

  • 21.
    Kananizadeh, Negin
    et al.
    Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Peev, Darin
    Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Delon, Thompson
    Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Schubert, Eva
    Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Bartelt-Hunt, Shannon
    Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res IPF Dresden, Germany.
    Zhang, Jianming
    Sichuan Univ, Peoples R China.
    Uhlmann, Petra
    Leibniz Inst Polymer Res IPF Dresden, Germany.
    Lederer, Albena
    Leibniz Inst Polymer Res IPF Dresden, Germany.
    Li, Yusong
    Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Visualization of label-free titanium dioxide nanoparticle deposition on surfaces with nanoscale roughness2019In: ENVIRONMENTAL SCIENCE-NANO, ISSN 2051-8153, Vol. 6, no 1, p. 248-260Article in journal (Refereed)
    Abstract [en]

    Understanding the transport of engineered nanoparticles (ENPs) and their interaction with environmental surfaces is critical to predicting their fate and environmental implications. Here, we report an innovative technique to visualize and quantify the deposition of titanium dioxide nanoparticles (TiO2NP) on engineered surfaces with nanoscale roughness, i.e., slanted columnar thin films (SCTFs). The attachment of TiO2NP on the surface of SCTFs changed the optical property of SCTFs, which was detected using a generalized ellipsometry (GE)-based instrument, an anisotropy contrast optical microscope (ACOM). An anisotropic effective medium model was applied to quantitatively analyze ACOM images of SCTF surfaces, which provided the mass distribution of TiO2NP. The TiO2NP mass measured by ACOM was in good agreement with the known amount of TiO2NP mass dispersed by controlled piezoelectric plotting. The detection of a few picograms of nanoparticle mass by an individual pixel measuring 7 x 7-micrometer squared was demonstrated. Further, a glass microfluidic channel with SCTF embedded was developed. The areal mass density of attached TiO2NP on SCTF surfaces as they flow through the channel under various flow rates was quantitatively measured in situ. At the end of the experiment, the distribution of the attached TiO2NP on the SCTF surface was visualized. The averaged mass density estimated by integrating the distribution map was in close agreement with the estimation from dynamic measurements and between repeating experiments. The capability of this novel technique to sense, quantify and visualize the mass distribution of TiO2NP provides a valuable approach to investigate the behavior of nanoparticles at the interface of flow and rough surfaces.

  • 22.
    Kilic, Ufuk
    et al.
    Univ Nebraska, NE 68588 USA.
    Hilfiker, Matthew
    Univ Nebraska, NE 68588 USA.
    Ruder, Alexander
    Univ Nebraska, NE 68588 USA.
    Feder, Rene
    Fraunhofer Inst Microstruct Mat & Syst IMWS, Germany.
    Schubert, Eva
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Argyropoulos, Christos
    Univ Nebraska, NE 68588 USA.
    Broadband Enhanced Chirality with Tunable Response in Hybrid Plasmonic Helical Metamaterials2021In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 31, no 20, article id 2010329Article in journal (Refereed)
    Abstract [en]

    Designing broadband enhanced chirality is of strong interest to the emerging fields of chiral chemistry and sensing, or to control the spin orbital momentum of photons in recently introduced nanophotonic chiral quantum and classical optical applications. However, chiral light-matter interactions have an extremely weak nature, are difficult to control and enhance, and cannot be made tunable or broadband. In addition, planar ultrathin nanophotonic structures to achieve strong, broadband, and tunable chirality at the technologically important visible to ultraviolet spectrum still remain elusive. Here, these important problems are tackled by experimentally demonstrating and theoretically verifying spectrally tunable, extremely large, and broadband chiroptical response by nanohelical metamaterials. The reported new designs of all-dielectric and dielectric-metallic (hybrid) plasmonic metamaterials permit the largest and broadest ever measured chiral Kuhns dissymmetry factor achieved by a large-scale nanophotonic structure. In addition, the strong circular dichroism of the presented bottom-up fabricated optical metamaterials can be tuned by varying their dimensions and proportions between their dielectric and plasmonic helical subsections. The currently demonstrated ultrathin optical metamaterials are expected to provide a substantial boost to the developing field of chiroptics leading to significantly enhanced and broadband chiral light-matter interactions at the nanoscale.

  • 23.
    Kilic, Ufuk
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Mock, Alyssa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Feder, Rene
    Fraunhofer Inst Microstruct Mat and Syst IMWS, Germany.
    Sekora, Derek
    Univ Nebraska Lincoln, NE 68588 USA.
    Hilfiker, Matthew
    Univ Nebraska Lincoln, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska Lincoln, NE 68588 USA.
    Schubert, Eva
    Univ Nebraska Lincoln, NE 68588 USA.
    Argyropoulos, Christos
    Univ Nebraska Lincoln, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Tunable plasmonic resonances in Si-Au slanted columnar heterostructure thin films2019In: Scientific Reports, E-ISSN 2045-2322, Vol. 9, article id 71Article in journal (Refereed)
    Abstract [en]

    We report on fabrication of spatially-coherent columnar plasmonic nanostructure superlattice-type thin films with high porosity and strong optical anisotropy using glancing angle deposition. Subsequent and repeated depositions of silicon and gold lead to nanometer-dimension subcolumns with controlled lengths. We perform generalized spectroscopic ellipsometry measurements and finite element method computations to elucidate the strongly anisotropic optical properties of the highly-porous Si-Au slanted columnar heterostructures. The occurrence of a strongly localized plasmonic mode with displacement pattern reminiscent of a dark quadrupole mode is observed in the vicinity of the gold subcolumns. We demonstrate tuning of this quadrupole-like mode frequency within the near-infrared spectral range by varying the geometry of Si-Au slanted columnar heterostructures. In addition, coupled-plasmon-like and inter-band transition-like modes occur in the visible and ultra-violet spectral regions, respectively. We elucidate an example for the potential use of Si-Au slanted columnar heterostructures as a highly porous plasmonic sensor with optical read out sensitivity to few parts-per-million solvent levels in water.

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  • 24.
    Kilic, Ufuk
    et al.
    Univ Nebraska, NE 68588 USA.
    Mock, Alyssa
    Univ Nebraska, NE 68588 USA.
    Sekora, Derek
    Univ Nebraska, NE 68588 USA.
    Gilbert, Simeon
    Univ Nebraska, NE 68588 USA.
    Valloppilly, Shah
    Univ Nebraska, NE 68588 USA.
    Ianno, Natale
    Univ Nebraska, NE 68588 USA.
    Langell, Marjorie
    Univ Nebraska, NE 68588 USA.
    Schubert, Eva
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymerforsch Dresden eV, Germany.
    Precursor-surface interactions revealed during plasma-enhanced atomic layer deposition of metal oxide thin films by in-situ spectroscopic ellipsometry2020In: Scientific Reports, E-ISSN 2045-2322, Vol. 10, no 1Article in journal (Refereed)
    Abstract [en]

    We find that a five-phase (substrate, mixed native oxide and roughness interface layer, metal oxide thin film layer, surface ligand layer, ambient) model with two-dynamic (metal oxide thin film layer thickness and surface ligand layer void fraction) parameters (dynamic dual box model) is sufficient to explain in-situ spectroscopic ellipsometry data measured within and across multiple cycles during plasma-enhanced atomic layer deposition of metal oxide thin films. We demonstrate our dynamic dual box model for analysis of in-situ spectroscopic ellipsometry data in the photon energy range of 0.7-3.4eV measured with time resolution of few seconds over large numbers of cycles during the growth of titanium oxide (TiO2) and tungsten oxide (WO3) thin films, as examples. We observe cyclic surface roughening with fast kinetics and subsequent roughness reduction with slow kinetics, upon cyclic exposure to precursor materials, leading to oscillations of the metal thin film thickness with small but positive growth per cycle. We explain the cyclic surface roughening by precursor-surface interactions leading to defect creation, and subsequent surface restructuring. Atomic force microscopic images before and after growth, x-ray photoelectron spectroscopy, and x-ray diffraction investigations confirm structural and chemical properties of our thin films. Our proposed dynamic dual box model may be generally applicable to monitor and control metal oxide growth in atomic layer deposition, and we include data for SiO2 and Al2O3 as further examples.

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  • 25.
    Kilic, Ufuk
    et al.
    Univ Nebraska, NE 68588 USA.
    Sekora, Derek
    Univ Nebraska, NE 68588 USA.
    Mock, Alyssa
    Univ Nebraska, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Valloppilly, Shah
    Univ Nebraska, NE 68588 USA.
    Echeverria, Elena M.
    Univ Nebraska, NE 68588 USA.
    Ianno, Natale
    Univ Nebraska, NE 68588 USA.
    Schubert, Eva
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Critical-point model dielectric function analysis of WO3 thin films deposited by atomic layer deposition techniques2018In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 124, no 11, article id 115302Article in journal (Refereed)
    Abstract [en]

    WO3 thin films were grown by atomic layer deposition and spectroscopic ellipsometry data gathered in the photon energy range of 0.72-8.5 eV, and from multiple samples were utilized to determine the frequency dependent complex-valued isotropic dielectric function for WO3. We employ a critical-point model dielectric function analysis and determine a parameterized set of oscillators and compare the observed critical-point contributions with the vertical transition energy distribution found within the band structure of WO3 calculated by the density functional theory. The surface roughness was investigated using atomic force microscopy, and compared with the effective roughness as seen by the spectroscopic ellipsometry. Published by AIP Publishing.

  • 26.
    Kimel, Alexey
    et al.
    Radboud Univ Nijmegen, Netherlands.
    Zvezdin, Anatoly
    Russian Acad Sci, Russia.
    Sharma, Sangeeta
    Max Born Inst Nichtlineare Opt & Kurzzeitspektros, Germany.
    Shallcross, Samuel
    Max Born Inst Nichtlineare Opt & Kurzzeitspektros, Germany.
    de Sousa, Nuno
    Donostia Int Phys Ctr DIPC, Spain.
    Garcia-Martin, Antonio
    CEI UAM CSIC, Spain.
    Salvan, Georgeta
    Tech Univ Chemnitz, Germany.
    Hamrle, Jaroslav
    Charles Univ Prague, Czech Republic.
    Stejskal, Ondrej
    Charles Univ Prague, Czech Republic.
    McCord, Jeffrey
    Univ Kiel, Germany.
    Tacchi, Silvia
    Univ Perugia, Italy.
    Carlotti, Giovanni
    Univ Perugia, Italy.
    Gambardella, Pietro
    Swiss Fed Inst Technol, Switzerland.
    Salis, Gian
    IBM Res Zurich, Switzerland.
    Muenzenberg, Markus
    Univ Greifswald, Germany.
    Schultze, Martin
    Graz Univ Technol, Austria.
    Temnov, Vasily
    CNRS, France.
    Bychkov, Igor V
    Chelyabinsk State Univ, Russia.
    Kotov, Leonid N.
    Syktyvkar State Univ, Russia.
    Maccaferri, Nicolo
    Umea Univ, Sweden; Univ Luxembourg, Luxembourg.
    Ignatyeva, Daria
    Russian Quantum Ctr, Russia; VI Vernadsky Crimean Fed Univ, Russia; Lomonosov Moscow State Univ, Russia.
    Belotelov, Vladimir
    Russian Quantum Ctr, Russia; VI Vernadsky Crimean Fed Univ, Russia; Lomonosov Moscow State Univ, Russia.
    Donnelly, Claire
    Max Planck Inst Chem Phys Solids, Germany.
    Rodriguez, Aurelio Hierro
    Univ Oviedo, Spain; Univ Oviedo, Spain.
    Matsuda, Iwao
    Univ Tokyo, Japan.
    Ruchon, Thierry
    Univ Paris Saclay, France.
    Fanciulli, Mauro
    CY Cergy Paris Univ, France; Sorbonne Univ, France.
    Sacchi, Maurizio
    Synchrotron SOLEIL, France.
    Du, Chunhui Rita
    Univ Calif San Diego, CA 92093 USA; Univ Calif San Diego, CA 92093 USA.
    Wang, Hailong
    Univ Calif San Diego, CA 92093 USA.
    Armitage, N. Peter
    Johns Hopkins Univ, MD 21210 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Liu, Bilu
    Tsinghua Univ, Peoples R China; Tsinghua Univ, Peoples R China.
    Huang, Ziyang
    Tsinghua Univ, Peoples R China; Tsinghua Univ, Peoples R China.
    Ding, Baofu
    Tsinghua Univ, Peoples R China; Tsinghua Univ, Peoples R China; Chinese Acad Sci, Peoples R China.
    Berger, Andreas
    CIC nanoGUNE BRTA, Spain.
    Vavassori, Paolo
    CIC nanoGUNE BRTA, Spain; IKERBASQUE, Spain.
    The 2022 magneto-optics roadmap2022In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 55, no 46, article id 463003Article, review/survey (Refereed)
    Abstract [en]

    Magneto-optical (MO) effects, viz. magnetically induced changes in light intensity or polarization upon reflection from or transmission through a magnetic sample, were discovered over a century and a half ago. Initially they played a crucially relevant role in unveiling the fundamentals of electromagnetism and quantum mechanics. A more broad-based relevance and wide-spread use of MO methods, however, remained quite limited until the 1960s due to a lack of suitable, reliable and easy-to-operate light sources. The advent of Laser technology and the availability of other novel light sources led to an enormous expansion of MO measurement techniques and applications that continues to this day (see section 1). The here-assembled roadmap article is intended to provide a meaningful survey over many of the most relevant recent developments, advances, and emerging research directions in a rather condensed form, so that readers can easily access a significant overview about this very dynamic research field. While light source technology and other experimental developments were crucial in the establishment of todays magneto-optics, progress also relies on an ever-increasing theoretical understanding of MO effects from a quantum mechanical perspective (see section 2), as well as using electromagnetic theory and modelling approaches (see section 3) to enable quantitatively reliable predictions for ever more complex materials, metamaterials, and device geometries. The latest advances in established MO methodologies and especially the utilization of the MO Kerr effect (MOKE) are presented in sections 4 (MOKE spectroscopy), 5 (higher order MOKE effects), 6 (MOKE microscopy), 8 (high sensitivity MOKE), 9 (generalized MO ellipsometry), and 20 (Cotton-Mouton effect in two-dimensional materials). In addition, MO effects are now being investigated and utilized in spectral ranges, to which they originally seemed completely foreign, as those of synchrotron radiation x-rays (see section 14 on three-dimensional magnetic characterization and section 16 on light beams carrying orbital angular momentum) and, very recently, the terahertz (THz) regime (see section 18 on THz MOKE and section 19 on THz ellipsometry for electron paramagnetic resonance detection). Magneto-optics also demonstrates its strength in a unique way when combined with femtosecond laser pulses (see section 10 on ultrafast MOKE and section 15 on magneto-optics using x-ray free electron lasers), facilitating the very active field of time-resolved MO spectroscopy that enables investigations of phenomena like spin relaxation of non-equilibrium photoexcited carriers, transient modifications of ferromagnetic order, and photo-induced dynamic phase transitions, to name a few. Recent progress in nanoscience and nanotechnology, which is intimately linked to the achieved impressive ability to reliably fabricate materials and functional structures at the nanoscale, now enables the exploitation of strongly enhanced MO effects induced by light-matter interaction at the nanoscale (see section 12 on magnetoplasmonics and section 13 on MO metasurfaces). MO effects are also at the very heart of powerful magnetic characterization techniques like Brillouin light scattering and time-resolved pump-probe measurements for the study of spin waves (see section 7), their interactions with acoustic waves (see section 11), and ultra-sensitive magnetic field sensing applications based on nitrogen-vacancy centres in diamond (see section 17). Despite our best attempt to represent the field of magneto-optics accurately and do justice to all its novel developments and its diversity, the research area is so extensive and active that there remains great latitude in deciding what to include in an article of this sort, which in turn means that some areas might not be adequately represented here. However, we feel that the 20 sections that form this 2022 magneto-optics roadmap article, each written by experts in the field and addressing a specific subject on only two pages, provide an accurate snapshot of where this research field stands today. Correspondingly, it should act as a valuable reference point and guideline for emerging research directions in modern magneto-optics, as well as illustrate the directions this research field might take in the foreseeable future.

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  • 27.
    Knight, Sean
    et al.
    University of Nebraska, NE 68588 USA.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE 68588 USA; University of N Carolina, NC 28223 USA.
    Bouhafs, Chamseddine
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Wimer, Shawn
    University of Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE 68588 USA; Leibniz Institute Polymerforsch Dresden eV, Germany.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene2017In: Scientific Reports, E-ISSN 2045-2322, Vol. 7, article id 5151Article in journal (Refereed)
    Abstract [en]

    Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.

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  • 28.
    Knight, Sean
    et al.
    Univ Nebraska, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Dugan, Christina
    US Air Force, OH 45433 USA.
    Petrosky, James C.
    US Air Force, OH 45433 USA.
    Mock, Alyssa
    Univ Nebraska, NE 68588 USA.
    Dowben, Peter A.
    Univ Nebraska, NE 68588 USA.
    Matthew Mann, J.
    US Air Force, OH 45433 USA.
    Kimani, Martin M.
    US Air Force, OH 45433 USA; KBRwyle, OH 45431 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Infrared-active phonon modes in single-crystal thorium dioxide and uranium dioxide2020In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 127, no 12, article id 125103Article in journal (Refereed)
    Abstract [en]

    The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO2) and uranium dioxide (UO2), were investigated using spectroscopic ellipsometry and compared with density functional theory. Both ThO2 and UO2 are found to have one infrared-active phonon mode pair [consisting of one transverse optic (TO) and one associated longitudinal optic (LO) mode], which is responsible for the dominant features in the ellipsometric data. At room temperature, our results for the mode pairs resonant frequencies and broadening parameters are comparable with previous reflectance spectroscopy characterizations and density functional theory predictions. For ThO2, our ellipsometry and density function theory results both show that the LO mode broadening parameter is larger than the TO mode broadening. This signifies mode anharmonicity, which can be attributed to the intrinsic phonon-phonon interaction. In addition to the main mode pair, a broad low-amplitude impurity-like vibrational mode pair is detected within the reststrahlen band for both ThO2 and UO2. Elevated temperature measurements were performed for ThO2 in order to study the mechanisms by which the phonon parameters evolve with increased heat. The observed change in the TO resonant frequency is in excellent agreement with previous density functional calculations, which only consider volume expansion of the crystal lattice. This suggests that the temperature-dependent change in the TO frequency is primarily due to volume expansion. The change in the main mode pairs broadening parameters is nearly linear within the temperature range of this study, which indicates the intrinsic anharmonic scattering (via cubic anharmonicities) as the main decay mechanism. Published under license by AIP Publishing.

  • 29.
    Knight, Sean
    et al.
    University of Nebraska, NE 68588 USA.
    Mock, Alyssa
    University of Nebraska, NE 68588 USA.
    Korlacki, Rafal
    University of Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Tokyo University of Agriculture and Technology, Japan.
    Kumagai, Yoshinao
    Tokyo University of Agriculture and Technology, Japan; Tokyo University of Agriculture and Technology, Japan.
    Goto, Ken
    Tokyo University of Agriculture and Technology, Japan; Tamura Corp, Japan.
    Higashiwaki, Masataka
    National Institute Informat and Commun Technology, Japan.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE 68588 USA; Leibniz Institute Polymer Research Dresden, Germany.
    Electron effective mass in Sn-doped monoclinic single crystal beta-gallium oxide determined by mid-infrared optical Hall effect2018In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 1, article id 012103Article in journal (Refereed)
    Abstract [en]

    The isotropic average conduction band minimum electron effective mass in Sn-doped monoclinic single crystal beta-Ga2O3 is experimentally determined by the mid-infrared optical Hall effect to be (0.2846 +/- 0.013)m(0) combining investigations on (010) and ((2) over bar 01) surface cuts. This result falls within the broad range of values predicted by theoretical calculations for undoped beta-Ga2O3. The result is also comparable to recent density functional calculations using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional, which predict an average effective mass of 0.267m(0). Within our uncertainty limits, we detect no anisotropy for the electron effective mass, which is consistent with most previous theoretical calculations. We discuss upper limits for possible anisotropy of the electron effective mass parameter from our experimental uncertainty limits, and we compare our findings with recent theoretical results. Published by AIP Publishing.

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  • 30.
    Knight, Sean
    et al.
    Univ Nebraska, NE 68588 USA.
    Prabhakaran, Dharmalingam
    Univ Oxford, England.
    Binek, Christian
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymerforsch Dresden eV, Germany.
    Electromagnon excitation in cupric oxide measured by Fabry-Perot enhanced terahertz Mueller matrix ellipsometry2019In: Scientific Reports, E-ISSN 2045-2322, Vol. 9, article id 1353Article in journal (Refereed)
    Abstract [en]

    Here we present the use of Fabry-Perot enhanced terahertz (THz) Mueller matrix ellipsometry to measure an electromagnon excitation in monoclinic cupric oxide (CuO). As a magnetically induced ferroelectric multiferroic, CuO exhibits coupling between electric and magnetic order. This gives rise to special quasiparticle excitations at THz frequencies called electromagnons. In order to measure the electromagnons in CuO, we exploit single-crystal CuO as a THz Fabry-Perot cavity to resonantly enhance the excitations signature. This enhancement technique enables the complex index of refraction to be extracted. We observe a peak in the absorption coefficient near 0.705 THz and 215 K, which corresponds to the electromagnon excitation. This absorption peak is observed along only one major polarizability axis in the monoclinic a-c plane. We show the excitation can be represented using the Lorentz oscillator model, and discuss how these Lorentz parameters evolve with temperature. Our findings are in excellent agreement with previous characterizations by THz time-domain spectroscopy (THz-TDS), which demonstrates the validity of this enhancement technique.

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  • 31.
    Knight, Sean Robert
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Richter, Steffen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Guo, Shiqi
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Persson, Axel R.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Rindert, Viktor
    Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Persson, Per O. Å.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Paskov, Plamen P.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden; Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, 68588, NE, United States.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Solid State Physics and NanoLund, Lund University, Lund, 22100, Sweden.
    Room temperature two-dimensional electron gas scattering time, effective mass, and mobility parameters in AlxGa1−xN/GaN heterostructures (0.07 ≤ x ≤ 0.42)2023In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 134, no 18, article id 185701Article in journal (Refereed)
    Abstract [en]

    Al xGa 1−xN/GaN high-electron-mobility transistor (HEMT) structures are key components in electronic devices operating at gigahertz or higher frequencies. In order to optimize such HEMT structures, understanding their electronic response at high frequencies and room temperature is required. Here, we present a study of the room temperature free charge carrier properties of the two-dimensional electron gas (2DEG) in HEMT structures with varying Al content in the Al xGa 1−xN barrier layers between x=0.07 and x=0.42⁠. We discuss and compare 2DEG sheet density, mobility, effective mass, sheet resistance, and scattering times, which are determined by theoretical calculations, contactless Hall effect, capacitance-voltage, Eddy current, and cavity-enhanced terahertz optical Hall effect (THz-OHE) measurements using a low-field permanent magnet (0.6 T). From our THz-OHE results, we observe that the measured mobility reduction from x=0.13 to x=0.42 is driven by the decrease in 2DEG scattering time, and not the change in effective mass. For x<0.42⁠, the 2DEG effective mass is found to be larger than for electrons in bulk GaN, which in turn, contributes to a decrease in the principally achievable mobility. From our theoretical calculations, we find that values close to 0.3 m0 can be explained by the combined effects of conduction band nonparabolicity, polarons, and hybridization of the electron wavefunction through penetration into the barrier layer.

  • 32.
    Knight, Sean
    et al.
    Univ Nebraska, NE 68588 USA.
    Schoche, Stefan
    JA Woollam Co Inc, NE 68508 USA.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Hofmann, Tino
    Univ N Carolina, NC 28223 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Tunable cavity-enhanced terahertz frequency-domain optical Hall effect2020In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 91, no 8, article id 083903Article in journal (Refereed)
    Abstract [en]

    Presented here is the development and demonstration of a tunable cavity-enhanced terahertz (THz) frequency-domain optical Hall effect (OHE) technique. The cavity consists of at least one fixed and one tunable Fabry-Perot resonator. The approach is suitable for the enhancement of the optical signatures produced by the OHE in semi-transparent conductive layer structures with plane parallel interfaces. Tuning one of the cavity parameters, such as the external cavity thickness, permits shifting of the frequencies of the constructive interference and provides substantial enhancement of the optical signatures produced by the OHE. A cavity-tuning optical stage and gas flow cell are used as examples of instruments that exploit tuning an external cavity to enhance polarization changes in a reflected THz beam. Permanent magnets are used to provide the necessary external magnetic field. Conveniently, the highly reflective surface of a permanent magnet can be used to create the tunable external cavity. The signal enhancement allows the extraction of the free charge carrier properties of thin films and can eliminate the need for expensive superconducting magnets. Furthermore, the thickness of the external cavity establishes an additional independent measurement condition, similar to, for example, the magnetic field strength, THz frequency, and angle of incidence. A high electron mobility transistor (HEMT) structure and epitaxial graphene are studied as examples. The tunable cavity-enhancement effect provides a maximum increase of more than one order of magnitude in the change of certain polarization components for both the HEMT structure and epitaxial graphene at particular frequencies and external cavity sizes.

  • 33.
    Koenig, Meike
    et al.
    Leibniz Inst Polymerforsch Dresden eV, Germany; Karlsruhe Inst Technol, Germany.
    Rodenhausen, Keith Brian
    Univ Nebraska Lincoln, NE 68588 USA; Biolin Sci Inc, NJ 07652 USA.
    Rauch, Sebastian
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Bittrich, Eva
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Eichhorn, Klaus-Jochen
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Stamm, Manfred
    Leibniz Inst Polymerforsch Dresden eV, Germany; Tech Univ Dresden, Germany.
    Uhlmann, Petra
    Leibniz Inst Polymerforsch Dresden eV, Germany; Univ Nebraska Lincoln, NE 68588 USA.
    Salt Sensitivity of the Thermoresponsive Behavior of PNIPAAm Brushes2018In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 34, no 7, p. 2448-2454Article in journal (Refereed)
    Abstract [en]

    We report investigations on the salt sensitivity of the thermoresponsive behavior of PNIPAAm brushes applying the quartz crystal microbalance coupled with spectroscopic ellipsometry technique. This approach enables a detailed study of the optical and mechanical behavior of the polymer coatings. Additional conclusions can be drawn from the difference between both techniques due to a difference in the contrast mechanism of both methods. A linear shift of the phase transition temperature to lower temperatures with the addition of sodium chloride was found, similar to the behavior of free polymer chains in solution. The thermal hysteresis was found to be decreased by the addition of sodium chloride to the solution, hinting to the interaction of the ions with the amide groups of the polymer, whereby the formation of hydrogen bonds is hindered. The results of this study are of relevance to the application of PNIPAAm brushes in biological fluids and demonstrate the additional potential of the ion sensitivity besides the better known thermosensitivity.

  • 34.
    Korlacki, R.
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Knudtson, J.
    Univ Nebraska Lincoln, NE 68588 USA.
    Stokey, M.
    Univ Nebraska Lincoln, NE 68588 USA.
    Hilfiker, M.
    Univ Nebraska Lincoln, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Linear strain and stress potential parameters for the three fundamental band to band transitions in beta-Ga2O32022In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 120, no 4, article id 042103Article in journal (Refereed)
    Abstract [en]

    We report the strain and stress relationships for the three lowest energy direct band to band transitions at the Brillouin zone center in monoclinic beta-Ga2O3. These relationships augment four linear perturbation parameters for situations, which maintain the monoclinic symmetry, which are reported here as numerical values obtained from density functional theory calculations. With knowledge of these perturbation parameters, the shift of each of the three lowest band to band transition energies can be predicted from the knowledge of the specific state of strain or stress, thus providing a useful tool for modeling performance of power electronic devices and rational strain engineering in heteroepitaxy.

  • 35.
    Korlacki, R.
    et al.
    Univ Nebraska, NE 68588 USA.
    Stokey, M.
    Univ Nebraska, NE 68588 USA.
    Mock, A.
    NRC Res Associateship Programs, DC 20001 USA.
    Knight, S.
    Univ Nebraska, NE 68588 USA.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Strain and stress relationships for optical phonon modes in monoclinic crystals with beta-Ga2O3 as an example2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 18, article id 180101Article in journal (Refereed)
    Abstract [en]

    Strain-stress relationships for physical properties are of interest for heteroepitaxial material systems, where strain and stress are inherent due to thermal expansion and lattice mismatch. We report linear perturbation theory strain and stress relationships for optical phonon modes in monoclinic crystals for strain and stress situations which maintain the monoclinic symmetry of the crystal. By using symmetry group analysis and phonon frequencies obtained under various deformation scenarios from density-functional perturbation theory calculations on beta-Ga2O3, we obtain four strain and four stress potential parameters for each phonon mode. We demonstrate that these parameters are sufficient to describe the frequency shift of the modes regardless of the stress or strain pattern which maintain the monoclinic symmetry of the crystal. The deformation potentials can be used together with experimentally determined phonon frequency parameters from Raman or infrared spectroscopy to evaluate the state of strain or stress of beta-Ga2O3, for example, in epitaxial heterostructures.

  • 36.
    Korlacki, Rafal
    et al.
    Univ Nebraska Lincoln, NE 68588 USA.
    Hilfiker, Matthew
    Univ Nebraska Lincoln, NE 68588 USA.
    Knudtson, Jenna
    Univ Nebraska Lincoln, NE 68588 USA.
    Stokey, Megan
    Univ Nebraska Lincoln, NE 68588 USA.
    Kilic, Ufuk
    Univ Nebraska Lincoln, NE 68588 USA.
    Mauze, Akhil
    Univ Calif Santa Barbara, CA 93106 USA.
    Zhang, Yuewei
    Univ Calif Santa Barbara, CA 93106 USA.
    Speck, James
    Univ Nebraska Lincoln, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Strain and Composition Dependencies of the Near-Band-Gap Optical Transitions in Monoclinic (AlxGa1-x)2O3 Alloys with Coherent Biaxial In-Plane Strain on Ga2O3(010)2022In: Physical Review Applied, E-ISSN 2331-7019, Vol. 22, no 6, article id 064019Article in journal (Refereed)
    Abstract [en]

    The bowing of the energy of the three lowest band-to-band transitions in beta-(AlXGa1-X)2O3 alloys is resolved using a combined density-functional theory (DFT) and generalized spectroscopic ellipsometry approach. The DFT calculations of the electronic band structure of both beta-Ga2O3 and theta-Al2O3 allow the linear portion of the energy shift in the alloys to be extracted, and provide a method for quantifying the role of coherent strain present in the beta-(AlXGa1-X)2O3 thin films on (010) beta-Ga2O3 substrates. The energies of band-to-band transitions are obtained using the spectroscopic ellipsometry eigenpolarization model approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)]. After subtracting the effects of strain, which also induces additional bowing and after subtraction of the linear portion of the energy shift due to alloying, the bowing parameters associated with the three lowest band-to-band transitions in monoclinic beta-(AlXGa1-X)2O3 are found.

  • 37.
    Korlacki, Rafal
    et al.
    Univ Nebraska, Canada.
    Mock, Alyssa
    US Navy, DC 20375 USA.
    Briley, Chad
    Univ Nebraska, Canada.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Tokyo Univ Agr and Technol, Japan.
    Kumagai, Yoshinao
    Tokyo Univ Agr and Technol, Japan.
    Goto, Ken
    Tokyo Univ Agr and Technol, Japan; Tamura Corp, Japan.
    Higashiwaki, Masataka
    Natl Inst Informat and Commun Technol, Japan.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, Canada; Leibniz Inst Polymer Res Dresden, Germany.
    Comment on "Characteristics of Multi-photon Absorption in a beta-Ga2O3 Single Crystal" [J. Phys. Soc. Jpn. 88, 113701 (2019)]2020In: Journal of the Physical Society of Japan, ISSN 0031-9015, E-ISSN 1347-4073, Vol. 89, no 3, article id 036001Article in journal (Other academic)
    Abstract [en]

    n/a

  • 38.
    Kuhne, Philipp
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Papamichail, Alexis
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Tran, Dat
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska Lincoln, NE 68588 USA.
    Paskov, Plamen
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Lund Univ, Sweden.
    Enhancement of 2DEG effective mass in AlN/Al0.78Ga0.22N high electron mobility transistor structure determined by THz optical Hall effect2022In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 120, no 25, article id 253102Article in journal (Refereed)
    Abstract [en]

    We report on the free charge carrier properties of a two-dimensional electron gas (2DEG) in an AlN/AlxGa1-xN high electron mobility transistor structure with a high aluminum content (x = 0.78). The 2DEG sheet density N s = ( 7.3 +/- 0.7 ) x 10 12 cm(-2), sheet mobility mu s = ( 270 +/- 40 ) cm(2)/(Vs), sheet resistance R- s = ( 3200 +/- 500 ) omega/ ?, and effective mass m( eff) = ( 0.63 +/- 0.04 ) m( 0) at low temperatures ( T = 5 K ) are determined by terahertz (THz) optical Hall effect measurements. The experimental 2DEG mobility in the channel is found within the expected range, and the sheet carrier density is in good agreement with self-consistent Poisson-Schrodinger calculations. However, a significant increase in the effective mass of 2DEG electrons at low temperatures is found in comparison with the respective value in bulk Al0.78Ga22N ( m( eff) = 0.334 m( 0)). Possible mechanisms for the enhanced 2DEG effective mass parameter are discussed and quantified using self-consistent Poisson-Schrodinger calculations .Published under an exclusive license by AIP Publishing.

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  • 39.
    Kuhne, Philipp
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Herzinger, Craig M,
    J. A. Woollam Company, Inc., Lincoln, NE, USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications2018In: IEEE Transactions on Terahertz Science and Technology, ISSN 2156-342X, Vol. 8, no 3, p. 257-270Article in journal (Refereed)
    Abstract [en]

    We present a terahertz (THz) frequency-domain spectroscopic ellipsometer design that suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements geometry, coating, and modulation. The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, in situ gas cells, or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit operation in the spectral range of 0.1–1 THz (3.3–33 cm−1 or 0.4–4 meV). The THz frequency-domain ellipsometer allows for standard and generalized ellipsometry at variable angles of incidence in both reflection and transmission configurations. The methods used to suppress standing waves and strategies for an accurate frequency calibration are presented. Experimental results from dielectric constant determination in anisotropic materials, and free charge carrier determination in optical Hall effect (OHE), resonant-cavity enhanced OHE, and in situ OHE experiments are discussed. Examples include silicon and sapphire optical constants, free charge carrier properties of two-dimensional electron gas in a group III nitride high electron mobility transistor structure, and ambient effects on free electron mobility and density in epitaxial graphene.

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  • 40.
    Mantz, Amy
    et al.
    Univ Nebraska, NE 68588 USA.
    Rosenthal, Alice
    Leibniz Inst Polymerforsch Dresden eV, Germany; Tech Univ Dresden, Germany.
    Farris, Eric
    Univ Nebraska, NE 68588 USA.
    Kozisek, Tyler
    Univ Nebraska, NE 68588 USA.
    Bittrich, Eva
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Nazari, Saghar
    Leibniz Inst Polymerforsch Dresden eV, Germany.
    Schubert, Eva
    Univ Nebraska, NE USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymerforsch Dresden eV, Germany; Univ Nebraska, NE USA.
    Stamm, Manfred
    Leibniz Inst Polymerforsch Dresden eV, Germany; Tech Univ Dresden, Germany.
    Uhlmann, Petra
    Leibniz Inst Polymerforsch Dresden eV, Germany; Univ Nebraska, NE 68588 USA.
    Pannier, Angela K.
    Univ Nebraska, NE 68588 USA.
    Free Polyethylenimine Enhances Substrate-Mediated Gene Delivery on Titanium Substrates Modified With RGD-Functionalized Poly(acrylic acid) Brushes2019In: Frontiers in Chemistry, E-ISSN 2296-2646, Vol. 7, article id 51Article in journal (Refereed)
    Abstract [en]

    Substrate mediated gene delivery (SMD) is a method of immobilizing DNA complexes to a substrate via covalent attachment or nonspecific adsorption, which allows for increased transgene expression with less DNA compared to traditional bolus delivery. It may also increase cells receptivity to transfection via cell-material interactions. Substrate modifications with poly(acrylic) acid (PM) brushes may improve SMD by enhancing substrate interactions with DNA complexes via tailored surface chemistry and increasing cellular adhesion via moieties covalently bound to the brushes. Previously, we described a simple method to graft PM brushes to Ti and further demonstrated conjugation of cell adhesion peptides (i.e., RGD) to the PM brushes to improve biocompatibility. The objective of this work was to investigate the ability of Ti substrates modified with PM-RGD brushes (PM-RGD) to immobilize complexes composed of branched polyethyleneimine and DNA plasmids (bPEI-DNA) and support SMD in NIH/3T3 fibroblasts. Transfection in NIH/3T3 cells cultured on bPEI-DNA complexes immobilized onto PM-RGD substrates was measured and compared to transfection in cells cultured on control surfaces with immobilized complexes including Flat Ti, PM brushes modified with a control peptide (RGE), and unmodified PM. Transfection was two-fold higher in cells cultured on PM-RGD compared to those cultured on all control substrates. While DNA immobilization measured with radiolabeled DNA indicated that all substrates (PM-RGD, unmodified PM, Flat Ti) contained nearly equivalent amounts of loaded DNA, ellipsometric measurements showed that more total mass (i.e., DNA and bPEI, both complexed and free) was immobilized to PM and PM-RGD compared to Flat Ti. The increase in adsorbed mass may be attributed to free bPEI, which has been shown to improve transfection. Further transfection investigations showed that removing free bPEI from the immobilized complexes decreased SMD transfection and negated any differences in transfection success between cells cultured on PM-RGD and on control substrates, suggesting that free bPEI may be beneficial for SMD in cells cultured on bPEI-DNA complexes immobilized on PM-RGD grafted to Ti. This work demonstrates that substrate modification with PM-RGD is a feasible method to enhance SMD outcomes on Ti and may be used for future applications such as tissue engineering, gene therapy, and diagnostics.

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  • 41.
    Mock, A.
    et al.
    Univ Nebraska, NE 68588 USA.
    Korlacki, R.
    Univ Nebraska, NE 68588 USA.
    Knight, S.
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Anisotropy and phonon modes from analysis of the dielectric function tensor and the inverse dielectric function tensor of monoclinic yttrium orthosilicate2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 16, article id 165203Article in journal (Refereed)
    Abstract [en]

    We determine the frequency dependence of the four independent Cartesian tensor elements of the dielectric function for monoclinic symmetry Y2SiO5 using generalized spectroscopic ellipsometry from 40-1200 cm(-1). Three different crystal cuts, each perpendicular to a principle axis, are investigated. We apply our recently described augmentation of lattice anharmonicity onto the eigendielectric displacement vector summation approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)], and we present and demonstrate the application of an eigendielectric displacement loss vector summation approach with anharmonic broadening. We obtain an excellent match between all measured and model-calculated dielectric function tensor elements and all dielectric loss function tensor elements. We obtain 23 A(u) and 22 B-u symmetry long-wavelength active transverse and longitudinal optical mode parameters including their eigenvector orientation within the monoclinic lattice. We perform density functional theory calculations and obtain 23 A(u) symmetry and 22 B-u transverse and longitudinal optical mode parameters and their orientation within the monoclinic lattice. We compare our results from ellipsometry and density functional theory and find excellent agreement. We also determine the static and above reststrahlen spectral range dielectric tensor values and find a recently derived generalization of the Lyddane-Sachs-Teller relation for polar phonons in monoclinic symmetry materials satisfied.

  • 42.
    Mock, A.
    et al.
    University of Nebraska Lincoln, NE 68588 USA.
    Korlacki, R.
    University of Nebraska Lincoln, NE 68588 USA.
    Knight, S.
    University of Nebraska Lincoln, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska Lincoln, NE 68588 USA; Leibniz Institute Polymer Research Dresden, Germany.
    Anisotropy, phonon modes, and lattice anharmonicity from dielectric function tensor analysis of monoclinic cadmium tungstate2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 16, article id 165202Article in journal (Refereed)
    Abstract [en]

    We determine the frequency dependence of four independent Cartesian tensor elements of the dielectric function for CdWO4 using generalized spectroscopic ellipsometry within mid-infrared and far-infrared spectral regions. Different single crystal cuts, (010) and (001), are investigated. From the spectral dependencies of the dielectric function tensor and its inverse we determine all long-wavelength active transverse and longitudinal optic phonon modes with A(u) and B-u symmetry as well as their eigenvectors within the monoclinic lattice. We thereby demonstrate that such information can be obtained completely without physical model line-shape analysis in materials with monoclinic symmetry. We then augment the effect of lattice anharmonicity onto our recently described dielectric function tensor model approach formaterials with monoclinic and triclinic crystal symmetries [ M. Schubert et al., Phys. Rev. B 93, 125209 (2016)], and we obtain an excellent match between all measured and modeled dielectric function tensor elements. All phonon mode frequency and broadening parameters are determined in our model approach. We also perform density functional theory phonon mode calculations, and we compare our results obtained from theory, from direct dielectric function tensor analysis, and from model line- shape analysis, and we find excellent agreement between all approaches. We also discuss and present static and above reststrahlen spectral range dielectric constants. Our data for CdWO4 are in excellent agreement with a recently proposed generalization of the Lyddane-Sachs-Teller relation for materials with low crystal symmetry [ M. Schubert, Phys. Rev. Lett. 117, 215502 (2016)].

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  • 43.
    Mock, A.
    et al.
    Univ Nebraska, USA.
    VanDerslice, J.
    JA Woollam Co Inc, USA.
    Korlacki, R.
    Univ Nebraska, USA.
    Woollam, J. A.
    Univ Nebraska, USA; JA Woollam Co Inc, USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, USA; Leibniz Inst Polymer Res Dresden, Germany.
    Elevated temperature dependence of the anisotropic visible-to-ultraviolet dielectric function of monoclinic beta-Ga2O32018In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 4, article id 041905Article in journal (Refereed)
    Abstract [en]

    We report on the temperature dependence of the dielectric tensor elements of n-type conductive beta-Ga2O3 from 22 degrees C to 550 degrees C in the spectral range of 1.5 eV-6.4 eV. We present the temperature dependence of the excitonic and band-to-band transition energy parameters using a previously described eigendielectric summation approach [A. Mock et al., Phys. Rev. B 96, 245205 (2017)]. We utilize a Bose-Einstein analysis of the temperature dependence of the observed transition energies and reveal electron coupling with average phonon temperature in excellent agreement with the average over all longitudinal phonon plasmon coupled modes reported previously [M. Schubert et al., Phys. Rev. B 93, 125209 (2016)]. We also report a linear temperature dependence of the wavelength independent Cauchy expansion coefficient for the anisotropic below-band-gap monoclinic dielectric tensor elements. Published by AIP Publishing.

  • 44.
    Mock, Alyssa
    et al.
    University of Nebraska Lincoln, NE USA.
    Carlson, Timothy
    University of Nebraska Lincoln, NE USA.
    VanDerslice, Jeremy
    University of Nebraska Lincoln, NE USA; JA Woollam Co Inc, NE USA.
    Mohrmann, Joel
    JA Woollam Co Inc, NE USA.
    Woollam, John A.
    University of Nebraska Lincoln, NE 68588 USA; JA Woollam Co Inc, NE USA.
    Schubert, Eva
    University of Nebraska Lincoln, NE USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska Lincoln, NE USA; Leibniz Institute Polymer Research Dresden, Germany.
    Multiple-layered effective medium approximation approach to modeling environmental effects on alumina passivated highly porous silicon nanostructured thin films measured by in-situ Mueller matrix ellipsometry2017In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 421, p. 663-666Article in journal (Refereed)
    Abstract [en]

    Optical changes in alumina passivated highly porous silicon slanted columnar thin films during controlled exposure to toluene vapor are reported. Electron-beam evaporation glancing angle deposition and subsequent atomic layer deposition are utilized to deposit alumina passivated nanostructured porous silicon thin films. In-situ Mueller matrix generalized spectroscopic ellipsometry in an environmental cell is then used to determine changes in optical properties of the nanostructured thin films by inspection of individual Mueller matrix elements, each of which exhibit sensitivity to adsorption. The use of a multiple-layered effective medium approximation model allows for accurate description of the inhomogeneous nature of toluene adsorption onto alumina passivated highly porous silicon slanted columnar thin films. (C) 2016 Elsevier B.V. All rights reserved.

  • 45.
    Mock, Alyssa
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA.
    Dugan, Christina
    US Air Force, OH 45433 USA.
    Knight, Sean
    Univ Nebraska, NE 68588 USA.
    Korlacki, Rafal
    Univ Nebraska, NE 68588 USA.
    Mann, J. Matthew
    US Air Force, OH 45433 USA.
    Kimani, Martin M.
    US Air Force, OH 45433 USA; KBRwyle, OH 45431 USA.
    Petrosky, James C.
    US Air Force, OH 45433 USA.
    Dowben, Peter A.
    Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Band-to-band transitions and critical points in the near-infrared to vacuum ultraviolet dielectric functions of single crystal urania and thoria2019In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 114, no 21, article id 211901Article in journal (Refereed)
    Abstract [en]

    Band-to-band transition energy parameters for single-crystal actinide samples of uranium oxide and thorium oxide were determined and compared using spectroscopic ellipsometry and critical-point dielectric function analyses. Spectroscopic ellipsometry measurements from the near-infrared to the vacuum ultraviolet spectral region were used to determine the dielectric functions of uranium oxide and thorium oxide. The critical-point structure is similar between UO2 and ThO2 but strongly blue shifted for ThO2. We find bandgap energies of 2.1eV and 5.4eV for UO2 and ThO2, respectively.

  • 46.
    Mock, Alyssa
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA.
    Korlacki, R.
    Univ Nebraska, NE 68588 USA.
    Knight, S.
    Univ Nebraska, NE 68588 USA.
    Stokey, M.
    Univ Nebraska, NE 68588 USA.
    Fritz, A.
    Univ Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Leibniz Inst Polymer Res Dresden, Germany.
    Lattice dynamics of orthorhombic NdGaO32019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 18, article id 184302Article in journal (Refereed)
    Abstract [en]

    A complete set of infrared-active and Raman-active lattice modes is obtained from density functional theory calculations for single-crystalline centrosymmetric orthorhombic neodymium gallate. The results for infrared-active modes are compared with an analysis of the anisotropic long-wavelength properties using generalized spectroscopic ellipsometry. The frequency-dependent dielectric function tensor and dielectric loss function tensor of orthorhombic neodymium gallium oxide are reported in the spectral range of 80-1200 cm(-1). A combined eigendielectric displacement vector summation and dielectric displacement loss vector summation approach augmented by considerations of lattice anharmonicity is utilized to describe the experimentally determined tensor elements. All infrared-active transverse and longitudinal optical mode pairs obtained from density functional theory calculations are identified by our generalized spectroscopic ellipsometry investigation. The results for Raman-active modes are compared to previously published experimental observations. Static and high-frequency dielectric constants from theory as well as experiment are presented and discussed in comparison with values reported previously in the literature.

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  • 47.
    Mock, Alyssa
    et al.
    University of Nebraska, NE 68588 USA.
    Korlacki, Rafal
    University of Nebraska, NE 68588 USA.
    Briley, Chad
    University of Nebraska, NE 68588 USA.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Tokyo University of Agriculture and Technology, Japan.
    Kumagai, Yoshinao
    Tokyo University of Agriculture and Technology, Japan; Tokyo University of Agriculture and Technology, Japan.
    Goto, Ken
    Tokyo University of Agriculture and Technology, Japan; Tamura Corp, Japan.
    Higashiwaki, Masataka
    National Institute Informat and Commun Technology, Japan.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA; Leibniz Institute Polymer Research Dresden, Germany.
    Band-to-band transitions, selection rules, effective mass, and excitonic contributions in monoclinic beta-Ga2O32017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 245205Article in journal (Refereed)
    Abstract [en]

    We employ an eigenpolarization model including the description of direction dependent excitonic effects for rendering critical point structures within the dielectric function tensor of monoclinic beta-Ga2O3 yielding a comprehensive analysis of generalized ellipsometry data obtained from 0.75-9 eV. The eigenpolarization model permits complete description of the dielectric response. We obtain, for single-electron and excitonic band-to-band transitions, anisotropic critical point model parameters including their polarization vectors within the monoclinic lattice. We compare our experimental analysis with results from density functional theory calculations performed using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional. We present and discuss the order of the fundamental direct band-to-band transitions and their polarization selection rules, the electron and hole effective mass parameters for the three lowest band-to-band transitions, and their excitonic contributions. We find that the effective masses for holes are highly anisotropic and correlate with the selection rules for the fundamental band-to-band transitions. The observed transitions are polarized close to the direction of the lowest hole effective mass for the valence band participating in the transition.

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  • 48.
    Mock, Alyssa
    et al.
    University of Nebraska, NE 68588 USA.
    Korlacki, Rafal
    University of Nebraska, NE 68588 USA.
    Briley, Chad
    University of Nebraska, NE 68588 USA.
    Sekora, Derek
    University of Nebraska, NE 68588 USA.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Wilson, Peter
    University of Nebraska, NE 68588 USA.
    Sinitskii, Alexander
    University of Nebraska, NE 68588 USA.
    Schubert, Eva
    University of Nebraska, NE 68588 USA.
    Schubert, Mathias
    University of Nebraska, NE 68588 USA.
    Anisotropy, band-to-band transitions, phonon modes, and oxidation properties of cobalt-oxide core-shell slanted columnar thin films2016In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 108, no 5, p. 051905-Article in journal (Refereed)
    Abstract [en]

    Highly ordered and spatially coherent cobalt slanted columnar thin films (SCTFs) were deposited by glancing angle deposition onto silicon substrates, and subsequently oxidized by annealing at 475 degrees C. Scanning electron microscopy, Raman scattering, generalized ellipsometry, and density functional theory investigations reveal shape-invariant transformation of the slanted nanocolumns from metallic to transparent metal-oxide core-shell structures with properties characteristic of spinel cobalt oxide. We find passivation of Co-SCTFs yielding Co-Al2O3 core-shell structures produced by conformal deposition of a few nanometers of alumina using atomic layer deposition fully prevents cobalt oxidation in ambient and from annealing up to 475 degrees C. (C) 2016 AIP Publishing LLC.

  • 49.
    Peev, D.
    et al.
    University of Nebraska Lincoln, NE 68588 USA.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska Lincoln, NE 68588 USA; University of North Carolina Charlotte, NC 28223 USA.
    Kananizadeh, N.
    University of Nebraska Lincoln, NE 68588 USA.
    Beeram, S.
    University of Nebraska Lincoln, NE 68588 USA.
    Rodriguez, E.
    University of Nebraska Lincoln, NE 68588 USA.
    Wimer, S.
    University of Nebraska Lincoln, NE 68588 USA.
    Rodenhausen, K. B.
    Biolin Science Inc, NJ 07652 USA.
    Herzinger, C. M.
    JA Woollam Co Inc, NE 68508 USA.
    Kasputis, T.
    University of Michigan, MI 48109 USA.
    Pfaunmiller, E.
    Celerion Inc, NE 68502 USA.
    Nguyen, A.
    University of Nebraska Lincoln, NE 68583 USA.
    Korlacki, R.
    University of Nebraska Lincoln, NE 68588 USA.
    Pannier, A.
    University of Nebraska Lincoln, NE 68588 USA.
    Li, Y.
    University of Nebraska Lincoln, NE 68588 USA.
    Schubert, E.
    University of Nebraska Lincoln, NE 68588 USA.
    Hage, D.
    University of Nebraska Lincoln, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska Lincoln, NE 68588 USA; Leibniz Institute Polymer Research IPF Dresden, Germany.
    Anisotropic contrast optical microscope2016In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 87, no 11, article id 113701Article in journal (Refereed)
    Abstract [en]

    An optical microscope is described that reveals contrast in the Mueller matrix images of a thin, transparent, or semi-transparent specimen located within an anisotropic object plane (anisotropic filter). The specimen changes the anisotropy of the filter and thereby produces contrast within the Mueller matrix images. Here we use an anisotropic filter composed of a semi-transparent, nanostructured thin film with sub-wavelength thickness placed within the object plane. The sample is illuminated as in common optical microscopy but the light is modulated in its polarization using combinations of linear polarizers and phase plate (compensator) to control and analyze the state of polarization. Direct generalized ellipsometry data analysis approaches permit extraction of fundamental Mueller matrix object plane images dispensing with the need of Fourier expansion methods. Generalized ellipsometry model approaches are used for quantitative image analyses. These images are obtained from sets of multiple images obtained under various polarizer, analyzer, and compensator settings. Up to 16 independent Mueller matrix images can be obtained, while our current setup is limited to 11 images normalized by the unpolarized intensity. We demonstrate the anisotropic contrast optical microscope by measuring lithographically defined micro-patterned anisotropic filters, and we quantify the adsorption of an organic self-assembled monolayer film onto the anisotropic filter. Comparison with an isotropic glass slide demonstrates the image enhancement obtained by our method over microscopy without the use of an anisotropic filter. In our current instrument, we estimate the limit of detection for organic volumetric mass within the object plane of approximate to 49 fg within approximate to 7 x 7 mu m(2) object surface area. Compared to a quartz crystal microbalance with dissipation instrumentation, where contemporary limits require a total load of approximate to 500 pg for detection, the instrumentation demonstrated here improves sensitivity to a total mass required for detection by 4 orders of magnitude. We detail the design and operation principles of the anisotropic contrast optical microscope, and we present further applications to the detection of nanoparticles, to novel approaches for imaging chromatography and to new contrast modalities for observations on living cells. Published by AIP Publishing.

  • 50.
    Persson, Ingemar
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Armakavicius, Nerijus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Bouhafs, Chamseddine
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Inst Italiano Tecnol, Italy.
    Stanishev, Vallery
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Kuhne, Philipp
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Schubert, Mathias
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. Univ Nebraska, NE 68588 USA; Univ Nebraska, NE 68588 USA.
    Rosén, Johanna
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Persson, Per O A
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Origin of layer decoupling in ordered multilayer graphene grown by high-temperature sublimation on C-face 4H-SiC2020In: APL Materials, E-ISSN 2166-532X, Vol. 8, no 1, article id 011104Article in journal (Refereed)
    Abstract [en]

    We study the origin of layer decoupling in ordered multilayer graphene grown by high temperature sublimation on C-face 4H-SiC. The mid-infrared optical Hall effect technique is used to determine the magnetic field dependence of the inter-Landau level transition energies and their optical polarization selection rules, which unambiguously show that the multilayer graphene consists of electronically decoupled layers. Transmission electron microscopy reveals no out-of-plane rotational disorder between layers in the stack, which is in contrast to what is typically observed for C-face graphene grown by low temperature sublimation. It is found that the multilayer graphene maintains AB-stacking order with increased interlayer spacing by 2.4%-8.4% as compared to highly oriented pyrolytic graphite. Electron energy loss spectroscopy mapping reveals Si atoms trapped in between layers, which are proposed to be the cause for the observed increased interlayer spacing leading to layer decoupling. Based on our results, we propose a defect-driven growth evolution mechanism for multilayer graphene on C-face SiC via high temperature sublimation.

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