The electronic structure of ultrathin NiO films on Ag(1 0 0) surfaces studied by high resolution angle-resolved photoemission spectroscopy

Article abstract of DOI:10.1016/j.jmmm.2006.10.820

We have performed a high resolution angle-resolved photoemission study on ultrathin NiO films on Ag(1 0 0) surfaces. The NiO thin films were prepared on Ag(1 0 0) at room temperature by Ni deposition in O₂ atmosphere. The thickness of NiO thin films were determined by a quartz monitor and reflection high energy electron diffraction (RHEED). The low-energy electron diffraction (LEED) result shows a two-domain (2 × 1) pattern for the 0.9 monolayer (ML) NiO thin film at the optimum O₂ pressure of 7.5 × 10^{- 7} Torr at room temperature with subsequent annealing to 450 K. The (2 × 1) pattern evolves to a (1 × 1) pattern upon further annealing to 620 K in the presence of O₂ or upon increasing film thickness. Photoemission spectra show finite density of states at Fermi level for 0.9 ML (2 × 1) NiO and 2 ML (1 × 1) NiO on Ag(1 0 0). Angle-resolved photoemission spectroscopy was used to measure the band dispersion of the 0.9 and 2 ML NiO thin films on Ag(1 0 0), as well as of a 10 ML film. The band dispersion of the 10 ML thick film was similar to that of the bulk crystals. Weak dispersion of valence bands was observed on the (2 × 1) NiO thin film, and it was very different from that on the (1 × 1) for 0.9 ML and 2 ML NiO thin films. The interpretation of the electronic structure of these thin films is presented.

Full text of DOI:10.1016/j.jmmm.2006.10.820

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e764–e766
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The electronic structure of ultrathin NiO films on Ag(1 0 0) surfaces
studied by high resolution angle-resolved photoemission spectroscopy
Ã
C.-M. Chenga, , C. -C Wangb, J.-Y. Yuha, D.-J. Huanga,b, K.-D. Tsueia,b
aNational Synchrotron Radiation Center, Hsinchu, Taiwan, ROC
bDepartment of Physics, National Tsing-Hua University, Hsinchu, Taiwan, ROC
Available online 16 November 2006
Abstract
We have performed a high resolution angle-resolved photoemission study on ultrathin NiO films on Ag(1 0 0) surfaces. The
NiO thin films were prepared on Ag(1 0 0) at room temperature by Ni deposition in O2 atmosphere. The thickness of NiO thin
films were determined by a quartz monitor and reflection high energy electron diffraction (RHEED). The low-energy electron diffraction
(LEED) result shows a two-domain ð2 ꢀ 1Þ pattern for the 0.9 monolayer (ML) NiO thin film at the optimum O2 pressure of 7:5 ꢀ 10ꢁ7
Torr at room temperature with subsequent annealing to 450 K. The ð2 ꢀ 1Þ pattern evolves to a ð1 ꢀ 1Þ pattern upon further annealing to
620 K in the presence of O2 or upon increasing film thickness. Photoemission spectra show finite density of states at Fermi level for
0.9 ML ð2 ꢀ 1Þ NiO and 2 ML ð1 ꢀ 1Þ NiO on Ag(1 0 0). Angle-resolved photoemission spectroscopy was used to measure the band
dispersion of the 0.9 and 2 ML NiO thin films on Ag(1 0 0), as well as of a 10 ML film. The band dispersion of the 10 ML thick film was
similar to that of the bulk crystals. Weak dispersion of valence bands was observed on the ð2 ꢀ 1Þ NiO thin film, and it was very different
from that on the ð1 ꢀ 1Þ for 0.9 ML and 2 ML NiO thin films. The interpretation of the electronic structure of these thin films is
presented.
r 2006 Elsevier B.V. All rights reserved.
PACS: 73.20.At; 79.60.ꢁi
Keywords: Transition metal oxide; Electronic structure; Angle-resolved photoemission spectroscopy
1. Introduction
in order to understand fundamental aspects concerning the
structure and properties of ultrathin oxide films on metal
surfaces.
The investigation of the electronic structure of transition
metal oxides thin films on metal substrates has been an
important topic in recent years. NiO thin films have the
potential to be used in a number of applications, such as
magnetic nanodevices technology. The geometric structure,
electronic properties, and reactivity of oxide films with
several atomic layers thick deposited on metal surfaces are
expected to be different from bulk oxides due to the
nanometric thickness of the films and the interaction
with the substrate. The study of relatively simple
oxides such as NiO grown on metal surfaces is relevant
2. Experiments
The experiment was carried out at the National
Synchrotron Radiation Research Center in Hsinchu,
Taiwan using BL21B1 U9-CGM Spectroscopy Beamline.
The photoemission spectra were measured in a UHV
chamber equipped with a Scienta SES-200 hemispherical
analyzer with ꢂ8ꢃ collecting angle. The angular resolution
of 0:5ꢃ was chosen for the angle-resolved measurements.
All spectra are collected with 51 eV of photon energy and
40.8 eV of HeIIa discharge lamp source at T ¼ 40 K and
overall energy resolution was 15 meV. Well-ordered NiO
Ã
Corresponding author. Tel.: +886 3 5780281 7331;
fax: +886 3 5783813.
E-mail address: makalu@nsrrc.org.tw (C.-M. Cheng).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.10.820

ARTICLE IN PRESS
C.-M. Cheng et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e764–e766
e765
films were grown by evaporating Ni in an oxygen atmo-
sphere on Ag(1 0 0) [1]. The Ag(1 0 0) surface appears
to be an ideal substrate for NiO films due to the small
lattice mismatch and the scare reactivity of silver towards
O2. The thickness of NiO thin film was determined by
quartz thickness monitor and reflection high energy
electron diffraction (RHEED). The low-energy electron
diffraction (LEED) pattern shows a two-domain ð2 ꢀ 1Þ
pattern for the 0.9 monolayer (ML) NiO thin film at O2
pressure in the 5 ꢀ 10ꢁ7–8 ꢀ 10ꢁ7 Torr range at RT and
subsequently annealed to 450 K. The ð2 ꢀ 1Þ pattern
evolves to a ð1 ꢀ 1Þ pattern upon further annealing to
620 K in the presence of O2 or upon increasing film
thickness. A 10 ML thin film was also grown to compare to
the bulk result.
above 8 ꢀ 10ꢁ7 Torr. Moreover, with an O2 pressure of
7:5 ꢀ 10ꢁ7 Torr, a ð2 ꢀ 1Þ LEED pattern can be observed at
a NiO coverage of 0.7–1.2 ML, but the clearest ð2 ꢀ 1Þ
pattern was seen at a coverage of 0.9 ML. The post-
annealing at 450 K in an O2 atmosphere favors the
oxidization of residual Ni and led to more ordered
films as seen by LEED. Fig. 1 shows the angle-integrated
photoemission spectra near the Fermi level of the
clean Ag(1 0 0), Ni/Ag(1 0 0) and NiO/Ag(1 0 0) thin
films of different thickness using a helium discharge
lamp. These spectra were normalized to constant photon
flux. It is seen that the NiO bulk features appear
gradually with increasing thickness, and the Fermi level
intensity decrease down to zero above 3.2 ML. We have
performed a quantitative analysis and conclude that
the contribution of the Fermi level intensity for 0.9 ML
ð2 ꢀ 1Þ NiO/Ag(1 0 0) and 2 ML ð1 ꢀ 1Þ NiO/Ag(1 0 0)
film is predominantly from NiO films indicating that
these films contained metallic character. This metallic
character vanished above 3.2 ML. The insulating nature
of NiO bulk is related to the antiferromagnetic order.
The metallic character of ultra thin films may affect the
magnetic order.
3. Results and discussion
We monitored the LEED patterns of several thickness
NiO films grown on Ag(1 0 0) at RT growth for O2 pressure
between 5 ꢀ 10ꢁ7 and 8 ꢀ 10ꢁ7 Torr. We found that a
clearer ð2 ꢀ 1Þ LEED pattern can be seen at O2 pressure of
7:5 ꢀ 10ꢁ7 Torr. However, the ð2 ꢀ 1Þ LEED pattern
evolves to a clear ð1 ꢀ 1Þ LEED pattern for O2 pressure
Angle-resolved photoemission spectroscopy was used to
measure the band dispersion of the 0.9 ML ð2 ꢀ 1Þ NiO and
2 ML ð1 ꢀ 1Þ NiO thin films on Ag(1 0 0), as well as of a
10 ML film. The band dispersion of the 10 ML thick film
was similar to that of the bulk crystals [2]. Weak dispersion
10 ML
3.2 ML
2.5 ML
2.0 ML
0.9 ML (2x1) NiO/ Ag(100)
2 ML (1x1) NiO/ Ag(100)
1.8 ML
1.2 ML
-1.54
M
0.9ML (2x1)
NiO/Ag(100)
x 0.7
0.9ML Ni/Ag(100)
x 0.7
clean Ag(100)
Γ
0
10
8
6
4
2
0
4
3
2
1
0
4
3
2
1
0
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
Fig. 1. Photoemission spectra of clean Ag(1 0 0), 0.9 ML Ni/Ag(1 0 0) as
well as 0.9 ML, 1.2 ML ð2 ꢀ 1Þ NiO/Ag(1 0 0) and 1.8 ML, 2 ML, 2.5 ML,
3.2 ML , 10 ML ð1 ꢀ 1Þ NiO/Ag(1 0 0) thin films, all taken spectra at
normal emission using HeIIa at room temperature.
Fig. 2. Angle-resolved photoemission data for 0.9 ML ð2 ꢀ 1Þ NiO/
¯ ¯
Ag(1 0 0) and 2 ML ð1 ꢀ 1Þ NiO/Ag(1 0 0) thin films, along G–M direction,
¯
where M is referred to the ð1 ꢀ 1Þ Ag substrate direction, all taken spectra
at 51 eV of photon energy.

ARTICLE IN PRESS
C.-M. Cheng et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e764–e766
e766
of valence bands was observed on the ð2 ꢀ 1Þ NiO thin film,
which was very different from that on the ð1 ꢀ 1Þ for
0.9 ML and 2 ML NiO thin films. The difference of
structure between ð2 ꢀ 1Þ and ð1 ꢀ 1Þ NiO/Ag(1 0 0) [3] is
the reason for very different spectra profiles and band
dispersion. A tight-binding model can be used to explain
the dispersion (Fig. 2).
References
[1] C. Giovanardi, A. di Bona, S. Valeri, Phys. Rev. B 69 (2004) 075418.
[2] Z.-X. Shen, R.S. List, D.S. Dessau, B.O. Wells, O. Jepsen, A.J. Arko,
R. Barttlet, C.K. Shin, F. Parmigiani, Phys. Rev. B 44 (1991)
3604.
[3] M. Caffio, B. Cortigiani, G. Rovida, A. Atrei, C. Giovanardi, J. Phys.
Chem. B 108 (2004) 9925.