The angle-resolved photoemission study for ultrathin NiO and CoO thin films on Ag(1 0 0) surfaces

Article abstract of DOI:10.1016/j.physb.2007.10.184

We have performed a high resolution angle-resolved photoemission study on ultrathin NiO and CoO films on Ag(1 0 0) surfaces. The NiO and CoO thin films were grown on Ag(1 0 0) at room temperature and 180 ^{{ring operator}} C, respectively, for an O₂ pressure of 7.5 × 10^{- 7} Torr . The thickness of thin film was determined by a quartz monitor and RHEED. A clearest (2 × 1) low-energy electron diffraction (LEED) pattern was observed at a NiO coverage of 0.9 monolayer(ML), consistent with literature. LEED shows a clear (1 × 1) pattern indicating well-ordered domains for the 2 ML NiO and CoO thin films. Angler-integrated photoemission spectra (AIPES) show a finite density of states at the Fermi level for the 2 ML NiO thin film, suggesting a metallic character, while the 2 ML CoO thin film was insulating. Angle-resolved photoemission spectroscopy (ARPES) was used to detect the band dispersion of 2 ML NiO and CoO thin films. Very different band dispersions of the valence bands were observed. The insulating nature of bulk NiO and CoO is related to their antiferromagnetic order, but the unusual metallic behavior of ultra NiO thin film may affect its magnetic order. The detail comparison of electronic structure for 2 ML NiO and CoO thin films will be discussed.

Full text of DOI:10.1016/j.physb.2007.10.184

ARTICLE IN PRESS
Physica B 403 (2008) 1539–1541
www.elsevier.com/locate/physb
The angle-resolved photoemission study for ultrathin NiO and CoO
thin films on Ag(1 0 0) surfaces
a,
C.-M. Cheng , C.-C. Wang , H.-T. Jeng , C.S. Hsue , B.Y. Hsu ,
Ã
b
c,d
c
a
a,c
D.-J. Huang , K.-D. Tsuei
a,c
a
National Synchrotron Research Center, Hsinchu 300, Taiwan
Department of Physics, National Taiwan University, Taipei 106, Taiwan
Department of Physics, National Tsing-Hua University, Hsinchu 300, Taiwan
b
c
d
Insititute of Physics, Acdemia Sinica, Taipei 115, Taiwan
Abstract
We have performed a high resolution angle-resolved photoemission study on ultrathin NiO and CoO films on Ag(1 0 0) surfaces. The
ꢀ
ꢂ7
NiO and CoO thin films were grown on Ag(1 0 0) at room temperature and 180 C, respectively, for an O2 pressure of 7:5 ꢁ 10 Torr:
The thickness of thin film was determined by a quartz monitor and RHEED. A clearest ð2 ꢁ 1Þ low-energy electron diffraction (LEED)
pattern was observed at a NiO coverage of 0.9 monolayer(ML), consistent with literature. LEED shows a clear ð1 ꢁ 1Þ pattern indicating
well-ordered domains for the 2 ML NiO and CoO thin films. Angler-integrated photoemission spectra (AIPES) show a finite density of
states at the Fermi level for the 2 ML NiO thin film, suggesting a metallic character, while the 2 ML CoO thin film was insulating. Angle-
resolved photoemission spectroscopy (ARPES) was used to detect the band dispersion of 2 ML NiO and CoO thin films. Very different
band dispersions of the valence bands were observed. The insulating nature of bulk NiO and CoO is related to their antiferromagnetic
order, but the unusual metallic behavior of ultra NiO thin film may affect its magnetic order. The detail comparison of electronic
structure for 2 ML NiO and CoO thin films will be discussed.
r 2007 Elsevier B.V. All rights reserved.
PACS: 71.20.Be; 71.27.+a; 73.20.At; 79.60.ꢂi
Keywords: NiO; Electronic structure; Angle-resolved photoemission spectroscopy; Transition metal oxide
1
. Introduction
oxides on metal surfaces is significant to understand how to
tailor these interesting properties of ultrathin films of more
complex oxides and other strongly correlated electron
systems [1,2].
The electronic structure of transition metal oxides thin
films on metal substrates has been an important topic in
recent years. Much interest focused on the potential of NiO
thin films to be used in magnetic nanodevices technology.
Monoxides such as NiO and CoO are also prototype
compounds to study the strongly correlated electron
systems. The geometric structure, electronic properties
and magnetic order of these oxides with several atomic
layers on metal surfaces are expected to be very different
from the bulk oxides. The study of these relatively simple
2. Experiments and discussion
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 ꢃ8 collecting angle. An angular resolution
ꢀ
Ã
of 0:5 was chosen for the angle-resolved measurements.
All spectra are collected with 51 eV photons and
Corresponding author. Tel.: +886 3 5780281x7331;
fax: +886 3 5783813.
E-mail address: makalu@nsrrc.org.tw (C.-M. Cheng).
HeIIa(40.8 eV) at T ¼ 40 K and the overall energy
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2007.10.184

ARTICLE IN PRESS
C.-M. Cheng et al. / Physica B 403 (2008) 1539–1541
1540
resolution was 15 meV. The Ag(1 0 0) crystal was cleaned
by the standard sputtering and annealing process. Well-
ordered thin films were prepared by evaporating Ni and Co
in an oxygen atmosphere onto Ag(1 0 0) [3], where the O2
ꢂ7
pressure was set at 7:5 ꢁ 10 Torr [3]. The substrate
ꢀ
temperature was kept at 180 C and RT during growth for
CoO and NiO, respectively. The thickness of these thin
films was determined by a quartz thickness monitor and
RHEED. A clearest ð2 ꢁ 1Þ low-energy electron diffraction
1
.2 ML NiO/Ag(100)
RT growth
(
LEED) pattern for the 0.9 monolayer(ML) NiO thin film
was also used to calibrate the thickness. The post annealing
in an O2 atmosphere helped the oxidization of residual Ni
and Co and led to more ordered films as revealed by
LEED. A 10 ML thick film was also grown to compare to
the bulk spectra.
1
1
.2 ML NiO/Ag(100)
°
80 C growth
0
.9 ML Ni/Ag(100)
RTdeposition
As seen in Fig. 1 of Angler-integrated photoemission
spectra (AIPES), for 0.9 ML Ni deposited on Ag substrate,
the 3d band crosses the Fermi level. For the 1.2 ML NiO
X 0.7
clean Ag(100)
ꢀ
thin film grown at 180 C, the Ni induced peak shifts below
1
0
8
6
4
2
0
the Fermi level at about 0.7 eV but still has high intensity at
the Fermi level. On the contrary, for the 1.2 ML NiO film
grown at RT, the Fermi level intensity diminishes, similar
to bulk NiO, and the intensity of the Ag 4d band is much
Binding Energy (eV)
Fig. 1. AIPES of clean Ag(1 0 0), 0.9 ML NiO/Ag(1 0 0) as well as 1.2 ML
ꢀ
NiO/Ag(1 0 0) grown at 180 C and at RT with post annealing, all taken
spectra at normal emission using HeIIa at RT.
ꢀ
more attenuated than that of the film grown at 180 C. A
2
ML NiO/Ag(100)
2 ML NiO/Ag(100)
-
0.48 Å-1
-
0.48 Å-1
Γ
Γ
0.18 Å-1
0
.18 Å-1
4
3
2
1
0
0.5 0.4 0.3 0.2 0.1
0 -0.1
Binding Energy (eV)
Binding Energy (eV)
2
ML CoO/Ag(100)
2 ML CoO/Ag(100)
-
0.24 Å-1
-
0.24 Å-1
Γ
Γ
.42 Å-1
0.42 Å-1
0
4
3
2
1
0
0.5 0.4 0.3 0.2 0.1
0
-0.1
Binding Energy (eV)
Binding Energy (eV)
Fig. 2. ARPES spectra of (a) the valence band of 2 ML NiO/Ag(1 0 0) grown at RT also (b) the enlarged region near the Fermi level for (a), (c) valence
ꢀ
¯
¯
¯
band of 2 ML CoO/Ag(1 0 0) grown at 180 C and (d) the enlarged region near the Fermi level for (c). The measurement is along G–M direction, where M
is referred to the ð1 ꢁ 1Þ Ag surface Brillouin zone. All spectra were taken at 51 eV photo energy.

ARTICLE IN PRESS
C.-M. Cheng et al. / Physica B 403 (2008) 1539–1541
1541
combined STM and photoemission study on Ni/Ag(1 0 0)
showed that Ni clusters grow on Ag(1 0 0) surfaces and a
large fraction of Ni may segregate into the subsurface
region of Ag with increasing temperature of the substrate
Fermi level was observed on a 2 ML NiO thin film, which
indicated an metallic behavior, but there was no clear
features near the Fermi level for a 2 ML CoO thin film.
Because the insulating nature of bulk NiO and CoO is
related to the antiferromagnetic order, the metallic
character of the 2 ML NiO may affect the magnetic order.
The XMLD result shows the antiferromagnetic order and
strong linear dichroism effect for the 3 ML NiO thin film
[1], different from the bulk crystal. A theoretical calcula-
tion may help elucidate the band dispersion and the
metallic characters observed in our measurements, and
the possible variation of the magnetic ordering in these
thin films.
[
4]. The stronger intensity of Ag 4d band and high intensity
ꢀ
at the Fermi level for NiO film grown at 180 C in Fig. 1
suggest less NiO on the Ag surface and more Ni in the
subsurface with higher temperature of the substrate. We
thus conclude that growth of ultrathin NiO films on
Ag(1 0 0) at RT with less unreacted Ni is more suitable than
at high temperature. Due to a bilayer growth in the O2
atmosphere observed by STM [3,5], we study the electronic
structure of 2 ML films of NiO and CoO. We have made a
quantitative analysis on the contribution of the Fermi level
intensity of the AIPES spectra of different thickness. We
found that for 2 ML NiO thin film, there exist a finite
density of state at the Fermi level, which indicates that this
film contains a metallic character.
References
[1] S. Altieri, et al., Phys. Rev. Lett. 91 (2003) 137201.
[
[
[
[
[
2] M.W. Haverkort, et al., Phys. Rev. B 69 (2004) 020408.
3] C. Giovanardi, et al., Phys. Rev. B 69 (2004) 075418.
4] D.A. Hite, et al., J. Vaccum Sci. Tech. A 18 (2000) 1950.
5] Ch. Hagendorf, et al., Surface Sci. 532–535 (2003) 346.
6] Z.-X. Shen, et al., Phys. Rev. B 44 (1991) 3604.
Angle-resolved photoemission spectroscopy (ARPES)
was used to detect the electronic structure of the 2 ML
NiO and CoO thin films on Ag(1 0 0) (Fig. 2), as well as
10 ML thin films. The results of 10 ML are consistent with
bulk crystals [6,7]. A shallow band dispersive crossing the
[7] Z.-X. Shen, et al., Phys. Rev. B 42 (1990) 1817.