Anisotropy in the electronic structure of ${\text{V}}_{2}\text{GeC}$ investigated by soft x-ray emission spectroscopy and first-principles theory

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Anisotropy in the electronic structure of $V_{2} GeC$ investigated by soft x-ray emission spectroscopy and first-principles theory

Martin Magnuson, Ola Wilhelmsson, Maurizio Mattesini, Sa Li, Rajeev Ahuja, Olle Eriksson, Hans Högberg, Lars Hultman, and Ulf Jansson

Phys. Rev. B 78, 035117 – Published 15 July 2008

Abstract

The anisotropy of the electronic structure of ternary nanolaminate $V_{2} GeC$ is investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured polarization-dependent emission spectra of $V L_{2, 3}$ , $C K$ , $Ge M_{1}$ , and $Ge M_{2, 3}$ in $V_{2} GeC$ are compared with those from monocarbide VC and pure Ge. The experimental emission spectra are interpreted with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Different types of covalent chemical bond regions are revealed: $V 3 d -C 2 p$ bonding at $- 3.8 eV$ , $Ge 4 p -C 2 p$ bonding at $- 6 eV$ , and $Ge 4 p -C 2 s$ interaction mediated via the $V 3 d$ orbitals at $- 11 eV$ below the Fermi level. We find that the anisotropic effects are high for the $4 p$ valence states and the shallow $3 d$ core levels of Ge, while relatively small anisotropy is detected for the $V 3 d$ states. The macroscopic properties of the $V_{2} GeC$ nanolaminate result from the chemical bonds with the anisotropic pattern as shown in this work.

Received 10 March 2008

DOI:https://doi.org/10.1103/PhysRevB.78.035117

Authors & Affiliations

Martin Magnuson^1,2,*, Ola Wilhelmsson³, Maurizio Mattesini⁴, Sa Li^1,5, Rajeev Ahuja¹, Olle Eriksson¹, Hans Högberg², Lars Hultman², and Ulf Jansson³

¹Department of Physics, Uppsala University, P.O. Box 530, S-751 21 Uppsala, Sweden
²Thin Film Physics Division, Department of Physics, IFM, Linköping University, SE-58183 Linköping, Sweden
³Department of Materials Chemistry, The Ångström Laboratory, Uppsala University, P.O. Box 538, SE-75121 Uppsala, Sweden
⁴Departamento de Física de la Tierra, Astronomía y Astrofísica I, Universidad Complutense de Madrid, E-28040 Madrid, Spain
⁵Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284-2000, USA

^*Corresponding author: Martin.Magnuson@ifm.liu.se

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Vol. 78, Iss. 3 — 15 July 2008

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Figure 1
(Color online) The hexagonal crystal structure of the 211 phase $(V_{2} GeC)$ . There is one $A$ (Ge) layer for every second layer of $M$ (V) in $V_{2} GeC$ . The $M$ (V) atoms have chemical bonds to both $X$ (C) and $A$ (Ge). The lengths of the measured (calculated) $a$ and $c$ axes of the hexagonal unit cell of $V_{2} GeC$ are 2.99 $(3.01) Å$ and 12.28 $(12.18) Å$ , respectively. The length of the $a$ axis in cubic VC is 4.14 $(4.16) Å$ .Reuse & Permissions
Figure 2
XRD from the $V_{2} GeC (0001)$ and VC(111) thin-film samples grown at 700 and $500 ° C$ , respectively. The diffraction peaks at $2 θ = 18.4 °$ and $57.4 °$ indicated by the vertical dashed lines are due to a minor fraction of coherent ${VC}_{x}$ inclusions.Reuse & Permissions
Figure 3
(Color online) The top-right curves show $V 2 p_{3 / 2, 1 / 2}$ SXA spectra measured at normal incidence $(E ∥ a, b)$ , normalized to each other below and far above the absorption step edge. In the middle (top to bottom), a series of $V L_{2, 3}$ SXE spectra of $V_{2} GeC$ and VC are shown. The SXE spectra are excited with the polarization vector both parallel $(E ∥ a, b)$ and perpendicular $(E ∥ c)$ to the surface plane at 516.9, 522.1 (resonant) and 560.0 eV (nonresonant). For each excitation energy, the spectra were normalized to the time and incoming photon flux by the measured current from a gold mesh in the photon beam. The excitation energies of the resonant SXE spectra with $E ∥ a, b$ polarization are indicated by the strong elastic peaks determining the total resolution [full width at half maximum $(FWHM) = 0.6 eV$ ]. The spectra are plotted both on a photon energy scale (top) and a common energy scale relative to the $E_{F}$ (bottom) using the $2 p_{1 / 2}$ $(2 p_{3 / 2})$ core-level XPS binding energy of 519.9 eV (512.3 eV) for $V_{2} GeC$ . For the nonresonant spectra at the bottom, the calculated isotropic SXE spectra are shown by the dotted curves.Reuse & Permissions
Figure 4
(Color online) The top-right curves show $C 1 s$ SXA spectra measured at normal incidence $(E ∥ a, b)$ , normalized to each other below and far above the absorption step edge. In the middle (top to bottom), the $C K$ SXE spectra of $V_{2} GeC$ and VC are shown. The SXE spectra are excited with the polarization vector both parallel $(E ∥ a, b)$ and perpendicular $(E ∥ c)$ to the surface plane at 284.2 eV (resonant) and 310 eV (nonresonant). For each excitation energy, the spectra are normalized to the time and incoming photon flux from a gold mesh in the photon beam. The excitation energy of the resonant SXE spectra at $E ∥ a, b$ polarization (284.2 eV) is indicated by the strong elastic peaks determining the total resolution $(FWHM = 0.35 eV)$ . The spectra are plotted both on a photon energy scale (top) and a common energy scale with respect to the $E_{F}$ (bottom) using the measured $C 1 s$ XPS binding energy of 281.2 eV. For the nonresonant spectra at the bottom, the calculated isotropic SXE spectra are shown by the dotted curves.Reuse & Permissions
Figure 5
(Color online) Experimental and calculated Ge SXE spectra of $V_{2} GeC$ and single-crystal Ge (001). Top panel: $M_{1}$ spectra nonresonantly excited at 220 eV (40 eV above the $3 s$ threshold at 180.4 eV XPS binding energy) with the polarization vector parallel $(E ∥ a, b)$ and perpendicular $(E ∥ c)$ to the surface plane. A common energy scale relative to the $E_{F}$ is shown in the middle. Bottom panel: $M_{2, 3}$ spectra nonresonantly excited at 165 eV (42 eV above the $3 p_{1 / 2}$ threshold) with the polarization vector parallel $(E ∥ a, b)$ and perpendicular $(E ∥ c)$ to the surface plane. The spectra are plotted on a photon energy scale relative to the $E_{F}$ using the $Ge 3 p_{1 / 2}$ core-level XPS binding energy of 124.7 eV for $V_{2} GeC$ and 125.8 eV for Ge (001). The calculated isotropic SXE spectra are shown by the dotted curves. The calculated intensities for the shallow $3 d$ core levels were scaled down by a factor of 10 (dashed curves).Reuse & Permissions
Figure 6
(Color online) Calculated BCOOPs of $V_{2} GeC$ (top) and VC (bottom).Reuse & Permissions

Physical Review B

covering condensed matter and materials physics

Anisotropy in the electronic structure of $V_{2} GeC$ investigated by soft x-ray emission spectroscopy and first-principles theory

Martin Magnuson, Ola Wilhelmsson, Maurizio Mattesini, Sa Li, Rajeev Ahuja, Olle Eriksson, Hans Högberg, Lars Hultman, and Ulf Jansson

Phys. Rev. B 78, 035117 – Published 15 July 2008

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Physical Review B

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Anisotropy in the electronic structure of V2GeC investigated by soft x-ray emission spectroscopy and first-principles theory

Martin Magnuson, Ola Wilhelmsson, Maurizio Mattesini, Sa Li, Rajeev Ahuja, Olle Eriksson, Hans Högberg, Lars Hultman, and Ulf Jansson

Phys. Rev. B 78, 035117 – Published 15 July 2008

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Anisotropy in the electronic structure of $V_{2} GeC$ investigated by soft x-ray emission spectroscopy and first-principles theory