X-ray photoelectron spectroscopy investigation of magnetron sputtered MgeTieH thin films I.J.T. Jensen a, *, A. Thøgersen b , O.M. Løvvik b , H. Schreuders d , B. Dam d , S. Diplas b,c a Department of Physics, University of Oslo, P/O Box 1048 Blindern, 0316 Oslo, Norway b SINTEF Materials and Chemistry, P/O Box 124 Blindern, 0314 Oslo, Norway c Center for Materials Science and Nanotechnology, P/O Box 1126 Blindern, 0318 Oslo, Norway d Materials for Energy Conversion or Storage (MECS), DelftChemTech, Faculty of Applied Science, Technical University Delft, P/O Box 5045, NL-2600 GA Delft, The Netherlands article info Article history: Received 22 January 2013 Received in revised form 22 May 2013 Accepted 26 May 2013 Available online 8 July 2013 Keywords: Thin films Magnesium Titanium Immiscible elements Interface XPS abstract Thin film samples of Mg 80 Ti 20 (MgeTi) and Mg, both with and without H, were investigated in a series of X-ray photoelectron spectroscopy (XPS) measurements. The samples were covered with a thin protective layer of Pd, which was removed by Ar þ sputtering prior to data acquisition. This sputtering was found to reduce both oxides and hydrides. A distinct, previously unknown peak was revealed in the Mg KLL spectrum of the MgeTieH samples, located between the metallic and the MgO component. This peak was attributed to trap- ping of H in very stable interstitial sites at the interface between Ti nano-clusters and the Mg matrix, based on earlier density functional theory calculations and supported by so-called Bader analysis. The latter was performed in order to study the theoretical charge distribution between Mg, Ti and H, establishing a link between the position of the previ- ously unknown peak and the effect of H on the valence state of Mg. The composition of the samples was studied both by energy dispersive spectroscopy using transmission electron microscopy and by quantitative XPS analysis. Final state Auger parameters (AP) were obtained for metallic Mg, MgO and MgH 2 , as well as Mg affected by trapped H. No difference between the AP values from the metallic components was found between the Mg and the MgeTi samples. The AP values for MgO and MgH 2 were consistent with previous reports in literature; several eV lower than the metallic value. Mg in the vicinity of trapped hydrogen, on the other hand, showed a more metallic character, with its corresponding AP value less than 1 eV below the AP for pure Mg. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction The discovery of so-called switchable mirrors of Y and La hydrides [1] in 1996 sparked an interest in thin film hydrides, leading to the discoveries of several hydride systems with tunable electrical and optical properties [2e6]. MgeTi thin films stand out among them, as they appear black rather than transparent in the fully hydrogenated state [6]. Possible applications for MgeTieH and other switchable hydride films range from coatings on solar collectors [6] and smart windows * Corresponding author. Present address: SINTEF Materials and Chemistry, P/O Box 124 Blindern, 0314 Oslo, Norway. Tel.: þ47 98230513. E-mail address: [email protected](I.J.T. Jensen). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 38 (2013) 10704 e10715 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.05.142
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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 5
Available online at w
journal homepage: www.elsevier .com/locate/he
X-ray photoelectron spectroscopy investigation ofmagnetron sputtered MgeTieH thin films
I.J.T. Jensen a,*, A. Thøgersen b, O.M. Løvvik b, H. Schreuders d, B. Damd,S. Diplas b,c
aDepartment of Physics, University of Oslo, P/O Box 1048 Blindern, 0316 Oslo, Norwayb SINTEF Materials and Chemistry, P/O Box 124 Blindern, 0314 Oslo, NorwaycCenter for Materials Science and Nanotechnology, P/O Box 1126 Blindern, 0318 Oslo, NorwaydMaterials for Energy Conversion or Storage (MECS), DelftChemTech, Faculty of Applied Science,
Technical University Delft, P/O Box 5045, NL-2600 GA Delft, The Netherlands
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 510706
In the over-night exposure (ON) experiments the measured
sample was left in position in the XPS analysis chamber for
about 24 h before the same measurement was repeated.
Magnesium is highly reactive to oxygen, which means that it
will oxidize even in UHV. Thus the results from the ON
experiment show the changes in spectra upon gentle oxida-
tion, comprising data fromMg,MgwithH andMgeTiwithH. A
comparative depth profile (DP) experiment was performed for
MgeTi with and without H. For each sample measurements
where done at intervals of 5 min of Arþ sputtering at 0.5 kV,
starting at 5 min and ending at 30 min total sputtering time.
The sputtering time was not converted to sample depth as the
craters were too shallow to be measured by available
methods, e.g., stylus profilometry or atomic force microscopy.
Details of the measurements are listed in Table 1. The Mg 2p,
Ti 2p and O 1s photoelectron peaks and theMg KLLAuger peak
were acquired. In some of the measurements the Mg 1s peak
was also obtained. Data processing was done using CasaXPS
[24]. All spectra were fitted with a standard Shirley back-
ground [25] and calibrated setting the Mg 2pmetal component
to 49.4 � 0.05 eV [26], due to the absence of adventitious car-
bon. In quantifications Scofield cross-sections were used for
the relative sensitivity factors [27].
Cross-sectional transmission electron microscopy (TEM)
samples were prepared by ion-milling using a Gatan precision
Table 1 e List of measurements, labeled bymeasurementnumbers (M no.). The experiments are categorized asinitial, depth profile (DP) or over-night exposure (ON). Thecompositions refer to Mg (Mg) and Mg80Ti20 (MgeTi) withand without H. Information about sample batch numberandArD sputtering parameters are also given. See text fordetails.
M no. Experiment Composition Batch no. Arþ sputtering
Volt.[kV]
Time[min]
M1 Initial Mg 1 2 3.5
M2 Initial MgeTi 1 2 3.5
M3 DP MgeTi 2 0.5 5
M4 DP MgeTi 2 0.5 10
M5 DP MgeTi 2 0.5 15
M6 DP MgeTi 2 0.5 20
M7 DP MgeTi 2 0.5 25
M8 DP MgeTi 2 0.5 30
M9 DP MgeTieH 2 0.5 5
M10 DP MgeTieH 2 0.5 10
M11 DP MgeTieH 2 0.5 15
M12 DP MgeTieH 2 0.5 20
M13 DP MgeTieH 2 0.5 25
M14 DP MgeTieH 2 0.5 30
M15 ON MgeTieH 1 2 10
M16 ON MgeTieH 1 2 a
M17 ON MgeTieH 1 2 b
M18 ON MgeH 1 2 5
M19 ON MgeH 1 2 b
M20 ON Mg 2 2 40
M21 ON Mg 2 2 b
a Same as above, measured w2 h later.
b Same as above, next day measurement.
ion polishing system with 5 kV gun voltage. The sample was
analyzed in a 200 keV JEOL 2010F microscope with a Gatan
imaging filter and detector, and a NORAN Vantage DIþ energy
dispersive spectroscopy (EDS) system. A low intensity beam
and minimal exposure were used in order to reduce the
knock-on damage by the electron beam.
DFT calculations at the PBE-GGA level [28] were performed
for a Mg81.25Ti18.75 composition using the Vienna ab-initio
simulation package (VASP) [29,30]. A 4 � 4 � 2 Mg unit cell
(64 atoms) was used as a starting point, with Ti arranged in
nano-clusters. Hydrogen was introduced to the system grad-
ually. In this structure the sites available for hydrogen will
have the amount of Ti in the first coordination sphere (nTi)
varying from 6 to 0. The sites where filled according to their
stability, in the following order: nTi ¼ 3, 6, 2, 1 and 0. A detailed
description of the calculations can be found in Ref. [20]. In the
present work Bader analysis [31] was employed to investigate
the charge transfer upon hydrogenation. This is a method to
determine which electrons belong to which atoms in the DFT
unit cell, where a surface is drawn around each atom
perpendicular to minima in the charge density.
3. Results and discussion
3.1. Microstructure and composition
The TEM image in Fig. 1 shows a 200 nm thick MgeTieH film
from sample batch 2, on a Si substrate. A 3 nm thick SiOx film
is visible at the MgeTieH/Si interface, and the Pd capping
layer at the surface is estimated to be about 10 nm thick.
However, the surface is rough and the thickness of the Pd
layermay therefore be overestimated. The electron diffraction
pattern presented in Fig. 1 shows theMg [100] and Si [110] zone
axis. The Mg grains grow perpendicular to the substrate sur-
face, in the [002] direction. The lattice parameters measured
on the diffraction pattern of Mg (P63/mmc) was a ¼ b ¼ 3.11 �A
Fig. 1 e Cross-sectional TEM image of the film, with an
electron diffraction pattern as an inset for the Mg [100] and
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 7 0 4e1 0 7 1 510714
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