Element-selective measure of atomic polarisation by XANES in external electric fields

09-03-2016

The electrical properties of functional electric materials can be studied using a new technique involving X-ray absorption spectroscopy. External electric fields were found to produce shifts of XANES spectra providing an element-selective measure of atomic polarisation originating from the permanent electric dipoles present in polar samples.

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Electrical functionality of matter plays an important role in today’s technology. Functional electric materials have a variety of applications such as piezoelectric actuators used as injectors in car engines and ferroelectric materials within FRAM computer memory. Most functional electric materials exhibit permanent electric polarisation which can be manipulated by electric fields. To improve these materials, electric polarisation needs to be tailored at the atomic scale, and a measurement technique is essential to gauge results.

Absorption spectroscopy in electric fields has a longstanding history. Classical examples are the Stark-effect [1], where atomic energy levels in (non-)polarised materials shift linearly (quadratic) with the applied electric field. Likewise, in semiconductors, the fundamental absorption was also found to shift with the electrical field – the so-called Franz-Keldysh effect [2]. These classical optical experiments have now been brought to the synchrotron and the first X-ray absorption spectroscopy measurements in an electric field have been performed at beamline ID12. These experiments have revealed the potential of the technique to become a versatile approach to study atomic electrical properties with element selectivity.

A range of prototypical samples was selected for this study, including the functional oxides cobalt-doped zincoxide (Co:ZnO) and gadolinium gallium garnet (Gd3Ga5O12; GGG) which are polar multi-constituent materials, i.e. the atoms in these oxides possess a permanent electric polarisation due to the low crystalline symmetry of the material. X-ray absorption near edge spectra (XANES) were recorded while applying an external voltage of up to 600 V along the polar direction of the material, which generates a strong electric field across the material (see feature image for the experimental geometry). The experiment shown in Figure 1a reveals that there is a small difference of less than 0.1% between the XANES recorded at +600 V and -600 V at the Zn K-edge of the Co:ZnO sample. This difference becomes less when the applied voltage is reduced. Figure 1b shows that the size of the difference depends linearly on the applied voltage; a first indication that the observed effect is the X-ray variant of the linear Stark effect. To demonstrate the element selectivity, the same experiment was repeated at the Co K-edge of the same Co:ZnO sample. For these samples, it is known that Co substitutes for Zn without changing the crystalline structure of the ZnO host [3]. Figure 2a shows the resulting difference signal at the Co K-edge for +600 V as well as the control experiment at 0 V. To shed further light on the origin of the difference signal, it was compared with the numerical derivative of the XANES and Figure 2b reveals that the two are a perfect match. It can be concluded, therefore, that the effect of the applied voltage is a shift of the (unoccupied) electronic states in energy which are probed in the XANES experiment. Consequently, the difference signal can be made to disappear or even be reverted by artificially shifting the XANES (-600 V) in photon energy with respect to the XANES (+600 V) as shown in Figure 2c. Obviously, the difference vanishes for a shift of 2.8 meV.

XANES at the Zn K-edge of a 20%Co:ZnO sample

Figure 1. a) XANES at the Zn K-edge of a 20%Co:ZnO sample recorded at +600 V and -600 V together with the resulting difference signal for various applied voltages. b) Dependence of the difference signal on applied voltage at the first negative peak in the difference spectrum reveals a linear relationship.

XANES and difference signal for +/-600 V as well as 0 V recorded at the Co K-edge of the same 20%Co:ZnO sample

Figure 2. a) XANES and difference signal for +/-600 V as well as 0 V recorded at the Co K-edge of the same 20%Co:ZnO sample as in Figure 1. b) Numerical derivative of the XANES in comparison with the difference signal. c) Analysis of the resulting energy shift: the difference between the XAS (+600 V) and XAS (-600 V) is shown for various shifts in photon energy of the XAS (-600 V).

Figure 3 is a compilation of the findings for all studied samples which reveal that the energy shift does not depend on the relative orientation of the synchrotron light and the applied voltage. Furthermore, the energy shift is the same at the Zn K-edge, no matter whether a Co:ZnO thin film or a zincselenide (ZnSe) single crystal is studied. In contrast, the shift is different for Co and Zn although both are contained in the identical Co:ZnO sample, which highlights the element specificity of the effect. Finally, it appears that the energetic shift depends linearly on the atomic number if all results for Co:ZnO, GGG(111) and ZnSe(111) are collected. The only exception is the Gd L3-edge which, however, probes electronic states of a different symmetry.

Summary of the magnitude of the energy shift for grazing and normal incidence

Figure 3. Summary of the magnitude of the energy shift for grazing (black squares) and normal (red circles) incidence for all studied samples versus atomic number of the studied element.

These experiments involved samples where the voltage generated an electric field parallel to the polar direction of the specimen. Further control experiments on non-polar strontium titanate (SrTiO3) and on highly conducting aluminium-doped ZnO (a known transparent conducting oxide) confirm that no difference signal can be recorded in cases where the material is non-polar or when mobile carriers screen the electric field. This finally confirms that the X-ray variant of the linear Stark effect has indeed been observed, permitting electrical polarisation to be studied with element selectivity. The electrical properties of a range of functional multi-constituent materials can now be studied with this new technique, taking advantage of the powerful characteristics of XANES.

 

Principal publication and authors
X-ray absorption spectroscopy in electrical fields: an element-selective probe of atomic polarization, V. Ney (a), F. Wilhelm (b), K. Ollefs (b), A. Rogalev (b), A. Ney (a) Phys. Rev. B 93, 035136 (2016); doi: 10.1103/PhysRevB.93.035136.
(a) Institut für Halbleiter- und Festkörperphysik, Johannes Kepler Universität, Linz (Austria)
(b) ESRF

 

References
[1] J. Stark, Annalen der Physik, 43, 965 (1914).
[2] W. Franz, Z. Naturforschg. 13a, 484 (1958); L.V. Keldysh, Soviet Phys. JETP 34, 788 (1958).
[3] A. Ney, V. Ney, M. Kieschnick, F. Wilhelm, K. Ollefs, and A. Rogalev, J. Appl. Phys. 115, 172603 (2014).

 

Top image: Difference in X-ray absorption of Co in Co:ZnO when a voltage is applied between the Au top- and the Cu back-contacts.