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Ba Adsorption on an Oxygen Chemisorbed O(2×1)/Ni(110) Surface
D. Vlachos, S.D. Foulias and M. Kamaratos
Department of Physics, University of Ioannina, PO Box 1186, GR-451 10, Ioannina, Greece
d.vlachos@cc.uoi.gr
Abstract
In this work we study Ba adsorption at submonolayer and monolayer coverages on an oxygen chemisorbed
O(2×1)/Ni(110) surface, by means of Auger electron spectroscopy, low energy electron diffraction and work function
measurements. Ba adsorption results initially in an incomplete two dimensional barium oxide layer, BaO, due to the
interaction of the Ba adatoms and the chemisorbed O atoms. A fraction of the first layer Ba atoms seems to be chemisorbed
reacting directly with Ni atoms. At higher coverages, Ba approaches the metallic phase. The observed energy shifts of the Ba
and BaO related low energy Auger transmission lines are attributed to both initial and final state effects, which are strongly
dependent on the substrate where the Ba-O interaction takes place.
1. Introduction
Barium oxide, BaO, is quite an important material in modern technology mainly because of its low work function, electron
emissivity and catalytic properties. Of special interest is the formation of ultra-thin films BaO on surfaces, where significant
changes of the electronic and the physicochemical properties of the substrate are observed. Ultrathin films of BaO on metallic
substrates of W and Pt, have given significant technological applications, in the fabrication of high current density cathodes
[1,2], and in the design of modern catalysts for NO x compounds storage in exhaust emission from motor vehicles [3,4].
To better understand the electronic and physicochemical properties of the BaO/substrate, it is important to study how the
BaO overlayer starts developing on the surface from sub-monolayer to monolayer. Also, it is quite crucial to percieve the
nature of the bonding between Ba and O atoms in BaO compound on surface. This is a controversial issue in the literature
regarding the degree of the ionicity. In this work, we study the Ba-O interaction on a metallic surface by the adsorption of Ba
on the oxygen chemisorbed O(2x1)/Ni(110) surface. Our intention is to investigate the Ba-O interaction on the surface from
the very first stage of Ba adsorption and to study how the substrate affects this interaction.
2. Experimental part
The experiments were performed in an ultra high vacuum system (UHV) at base pressure of 10 -10 Torr. The system was
equipped with Auger electron spectroscopy (AES), low energy electron diffraction (LEED), a quadrupole mass spectrometer
(QMS) and a Kelvin probe for work function measurements (WF). The sample was a Ni(110) single crystal with dimensions
1cm× 0.5cm× 0.1cm. Barium deposition was carried out by means of a commercial evaporation SAES Getters source at
constant current 6.5 A in steps of 2 min. Assuming the sticking coefficient of Ba on the oxygen predeposited nickel to be
constant and equal to that on the clean surface, the coverage could be estimated in monolayers (~0.1ML/2min) by combining
our results with previous AES, LEED, TDS and WF results [5]. The oxygen adsorption took place by supplying molecular
oxygen of purity 99.998 vol% into the experimental chamber through a leak valve. The oxygen exposure is counted in
Langmuirs, (L) where 1L=1× 10 -6 Torr.s.
AP-PH (arb. units)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Ba/O(2x1)/Ni(110) Ni(848eV)
Ba(590eV)x2
ΔΦ (eV)
0.0
-4.0
0 5 10 15 20 25 30 35 40 45
Ba deposition (min)
Figure 1 The AP-PHs of the Ba(590 eV) and Ni(848
eV) Auger transition lines and the WF change, ΔΦ,
for Ba deposition on the oxygen chemisorbed phase
O(2×1)/Ni(110).
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
ΔΦ (eV)
3. Results and discussion
The first step was to form the oxygen chemisorbed phase
O(2×1)/Ni(110) surface, by exposing the nickel surface to 2 L of
oxygen. This is the initial adsorption stage of oxygen on the
Ni(110) surface [6]. We then started the Ba deposition on the
oxygenated nickel surface. Figure 1 shows the measured Auger
signal from peak to peak height (AP-PH) for the Ba(590 eV) and
Ni(848 eV) Auger transition lines as well as the change of the
WF, ΔΦ, induced by Ba deposition. A linear increase of the Ba
AES signal is clear, with a change of the slope at periodic
deposition time intervals (14 and 28 min respectively). In parallel,
a linear decrease of the Ni AES signal is observed, with changes
of slope at about the same deposition times. This manner of the
adsorbate and substrate AES signal variation is characteristic of
the layer by layer growth mode. The same growth mode has been
documented for the Ba adsorption on the clean Ni(110) surface
[5]. The first break is attributed to the first physical layer of Ba on
the nickel surface at coverage 0.75 ML, while the second one to the second layer at 1.5 ML. However, a careful look at the
substrate signal decrease at the beginning of the Ba adsorption, shows a small deviation from linearity. This delay of the
Ni(848 eV) signal reduction is probably related to the (2×1) phase, where the oxygen atoms reconstruct the Ni(110) surface.
We believe that the Ba deposition on the O(2×1) phase, lifts the reconstruction of the nickel surface due to the Ba-O
interaction. Indeed, LEED observations showed that the (2×1) symmetry was disappearing even at the very low Ba coverage
(0.1 ML). The WF change, ΔΦ, shown also in Fig. 1, is quite similar to that of the Ba growth on the clean nickel surface
[5,7]. This indicates that Ba adatoms at low coverages are strongly positively polarized due to the reaction with the oxygen
and/or the nickel atoms, while at higher coverages (>0.75 ML) Ba approaches the metallic state.
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