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  • 1.
    Knight, S.
    et al.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Schoeche, S.
    JA Woollam Co Inc, NE 68588 USA.
    Darakchieva, Vanya
    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.
    Carlin, J. -F.
    Ecole Polytech Federal Lausanne, Switzerland.
    Grandjean, N.
    Ecole Polytech Federal Lausanne, Switzerland.
    Herzinger, C. M.
    JA Woollam Co Inc, NE 68588 USA.
    Schubert, M.
    University of Nebraska, NE 68588 USA; 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.
    Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies2015In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 12, p. 2688-2691Article in journal (Refereed)
    Abstract [en]

    The effect of a tunable, externally coupled Fabry-Perot cavity to resonantly enhance the optical Hall effect signatures at terahertz frequencies produced by a traditional Drude-like two-dimensional electron gas is shown and discussed in this Letter. As a result, the detection of optical Hall effect signatures at conveniently obtainable magnetic fields, for example, by neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high-electron mobility transistor structure grown on a sapphire substrate is used for the experiment. The optical Hall effect signatures and their dispersions, which are governed by the frequency and the reflectance minima and maxima of the externally coupled Fabry-Perot cavity, are presented and discussed. Tuning the externally coupled Fabry-Perot cavity strongly modifies the optical Hall effect signatures, which provides a new degree of freedom for optical Hall effect experiments in addition to frequency, angle of incidence, and magnetic field direction and strength. (C) 2015 Optical Society of America

  • 2.
    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; 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.

  • 3.
    Schubert, M.
    et al.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA; Leibniz Institute Polymer Research Dresden, Germany.
    Korlacki, R.
    University of Nebraska, NE 68588 USA; University of Nebraska, NE 68588 USA.
    Knight, S.
    University of Nebraska, NE 68588 USA; 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.
    Schoeche, S.
    JA Woollam Corp Inc, Japan.
    Darakchieva, Vanya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Janzén, Erik
    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.
    Gogova, D.
    Bulgarian Academic Science, Bulgaria; Leibniz Institute Crystal Growth, Germany.
    Thieu, Q. -T.
    Tokyo University of Agriculture and Technology, Japan; Tokyo University of Agriculture and Technology, Japan.
    Togashi, R.
    Tokyo University of Agriculture and Technology, Japan.
    Murakami, H.
    Tokyo University of Agriculture and Technology, Japan.
    Kumagai, Y.
    Tokyo University of Agriculture and Technology, Japan.
    Goto, K.
    Tokyo University of Agriculture and Technology, Japan; Tamura Corp, Japan.
    Kuramata, A.
    Tamura Corp, Japan.
    Yamakoshi, S.
    Tamura Corp, Japan.
    Higashiwaki, M.
    National Institute Informat and Communicat Technology, Japan.
    Anisotropy, phonon modes, and free charge carrier parameters in monoclinic beta-gallium oxide single crystals2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 12, p. 125209-Article in journal (Refereed)
    Abstract [en]

    We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic beta-Ga2O3 single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and ((2) over bar 01), are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent beta-Ga2O3 Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with A(u) and B-u symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in beta-Ga2O3. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in beta-Ga2O3 and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for beta-Ga2O3.

  • 4.
    Wilson, Peter M.
    et al.
    University of Nebraska, USA.
    Zobel, Adam
    University of Nebraska, USA.
    Zaitouna, Anita J.
    University of Nebraska, USA.
    Lipatov, Alexey
    University of Nebraska, USA.
    Schubert, Eva
    University of Nebraska, USA.
    Hofmann, Tino
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering. University of Nebraska, USA.
    Schubert, Mathias
    University of Nebraska, USA.
    Lai, Rebecca
    University of Nebraska, USA.
    Sinitskii, Alexander
    University of Nebraska, USA; National University of Science and Technology MISIS, Russia.
    Solution-stable anisotropic carbon nanotube/graphene hybrids based on slanted columnar thin films for chemical sensing2016In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 6, no 68, p. 63235-63240Article in journal (Refereed)
    Abstract [en]

    Slanted columnar thin films (SCTFs) are promising anisotropic nano-structures for applications in optical sensing and chemical separation. However, the wide use of SCTFs is significantly limited by their poor mechanical properties and structural stability, especially in liquid media. In this work, we demonstrate the fabrication of solution-stable carbon nanotube (CNT)/graphene hybrid structures based on cobalt SCTFs. The CNT/graphene hybrid structures were synthesized through the use of a titanium underlayer for Co slanted nanopillars as a chemical vapor deposition catalyst, which allows simultaneous growth of CNTs at the Co/Ti interface and three-dimensional graphene over the surface of cobalt. Importantly, the CNT/graphene hybrid structures retain the anisotropy of the parent Co SCTFs and thus remain suitable for optical sensing. Graphene/CNT modification of Co SCTFs not only improves their stability in solutions but also enables their functionalization with pyrene-modified DNA probes, which can be monitored in real time by in situ ellipsometry measurements. In turn, the solution-stable DNA-modified SCTFs may find a wide range of applications in biosensing. The described synthetic approach that allows simultaneous growth of CNTs and graphene by engineering Co/Ti interfaces may also be applied to the fabrication of other kinds of complex CNT/graphene hybrid materials.

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