Scanning tunneling microscopy and spectroscopy investigation of the atomic and electronic structure of CoO islands on Ag(0 0 1)

Article abstract of DOI:10.1016/S0039-6028(03)00192-4

Ultrathin CoO-films were deposited on Ag(001) by evaporation of Co in O₂-atmosphere. During subsequent heating (400-500 K) well-ordered island with (001) and (111) orientation are formed. Scanning tunneling microscopy (STM) images of CoO(001) islands show a corrugation with the atomic periodicity of the Ag(001) substrate for gap voltages in the range of -1.5 to +1.5 V. (111) oriented Co oxide islands reveal a moiré-like modulation with hexagonal structure. The heights of the different islands, as measured by STM, depend in a characteristic way on the bias voltage. Maps of the tunneling current as a function of bias voltage have been recorded using scanning tunneling spectroscopy (STS) at a temperature of 100 K. The evaluation of the multidimensional data set allows the characterization of the CoO(001) and (111) oriented Co oxide islands as well as the Ag(001) substrate, with respect to the electronic density of states and the influence on the imaging contrast in STM.

Full text of DOI:10.1016/S0039-6028(03)00192-4

Surface Science 532–535 (2003) 346–350
www.elsevier.com/locate/susc
Scanning tunneling microscopy and spectroscopy
investigation of the atomic and electronic structure
of CoO islands on Ag(001)
*
Ch. Hagendorf , R. Shantyr, K. Meinel, K.-M. Schindler, H. Neddermeyer
Fachbereich Physik, Martin-Luther-Universit€at Halle-Wittenberg, D-06099 Halle, Germany
Abstract
Ultrathin CoO-films were deposited on Ag(0 0 1) by evaporation of Co in O2-atmosphere. During subsequent
heating (400–500 K) well-ordered island with (0 0 1) and (1 1 1) orientation are formed. Scanning tunneling microscopy
(STM) images of CoO(0 0 1) islands show a corrugation with the atomic periodicity of the Ag(0 0 1) substrate for gap
ꢀ
voltages in the range of )1.5 to +1.5 V. (1 1 1) oriented Co oxide islands reveal a moire-like modulation with hexagonal
structure. The heights of the different islands, as measured by STM, depend in a characteristic way on the bias voltage.
Maps of the tunneling current as a function of bias voltage have been recorded using scanning tunneling spectroscopy
(STS) at a temperature of 100 K. The evaluation of the multidimensional data set allows the characterization of the
CoO(0 0 1) and (1 1 1) oriented Co oxide islands as well as the Ag(0 0 1) substrate, with respect to the electronic density
of states and the influence on the imaging contrast in STM.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Cobalt oxides; Growth; Insulating films; Scanning tunneling microscopy; Scanning tunneling spectroscopies
1. Introduction
monly used for the investigation of morphology
and atomic structure of thin films, scanning tun-
neling spectroscopy (STS) and voltage dependent
STM provide the possibility to study local elec-
tronic properties of oxide layers and the oxide
metal interface [3].
In previous STM studies it has been shown that
a Co/O precursor and CoO layers grow on
Ag(0 0 1) in (0 0 1) orientation depending on the
substrate temperature during deposition and sub-
sequent heating [4]. Additionally, a pronounced
bias voltage dependent STM contrast was ob-
served for CoO(0 0 1), that could be correlated to
the width of the optical band gap of CoO.
Thin transition metal oxide films are of major
interest for the engineering of catalytic systems [1]
and new magneto electric devices [2]. These ap-
plications require a thorough understanding of
the growth conditions and related structural and
electronic properties on an atomic scale. Whereas
scanning tunneling microscopy (STM) is com-
* Corresponding author. Address: Fachgruppe Oberflachen-
und Grenzflachenphysik, Fachbereich Physik, Friedmann-
Bach-Platz 6, Halle 06108, Germany. Tel.: +49-345-552-5566;
fax: +49-345-552-7160.
The present work deals with growth and atomic
structure of submonolayer CoO films on Ag(0 0 1)
E-mail address: hagendorf@physik.uni-halle.de (Ch. Hagen-
dorf).
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00192-4

Ch. Hagendorf et al. / Surface Science 532–535 (2003) 346–350
347
investigated by means of STM. Besides CoO(0 0 1)
islands, layers with (1 1 1) orientation were pre-
pared and studied with respect to their atomic
and electronic structure. Since electron tunneling
through layer systems is influenced by various
parameters (e.g., density of states in the electrodes,
width and height of the tunneling barrier), the
interpretation of STM images is not straightfor-
ward. For a deeper understanding I=V -STS ex-
periments have been implemented. The obtained
results will be discussed in the context of the
electronic properties of CoO(0 0 1), CoO(1 1 1) and
the Ag(0 0 1) substrate.
formation of a CoO oxide precursor. After sub-
sequent annealing at 400–500 K well ordered CoO
layers were obtained. STM and STS measurements
were performed at 100 K. Cooling of the entire
STM and its housing improved the stability of the
tunneling tips made of tungsten and reduced the
contamination of the sample due to residual gas
adsorption. The bias voltage was applied to the
sample. Accordingly electrons are tunneling into
unoccupied states of the sample at positive bias
voltage.
3. Results and discussion
2. Experimental
CoO(0 0 1) islands shown in Fig. 1 were grown
by deposition at room temperature in an O2 at-
mosphere of 2 Â 10À6 mbar and subsequent an-
nealing at 470 K in UHV for several minutes.
Single islands with an average diameter of 5 nm
can be identified in the constant current STM
images in Fig. 1. The analysis of a series of STM
images recorded at bias voltages ranging from +6
to )6 V yield a pronounced bias voltage dependent
STM contrast with a maximum height variation of
0.6 nm. Between )1.5 and +2.2 V the islands ap-
pear depressed (Fig. 1b), whereas above and below
these voltages they are protruding from the
Ag(0 0 1) substrate (Fig. 1a). This is consistent with
results of Sebastian and Neddermeyer [5] and has
been explained with the existence of an additional
tunneling barrier occurring due to the bandgap of
CoO(0 0 1) double-layers.
The experiments were performed in an ultra
high vacuum (UHV) chamber equipped with a low
temperature STM and facilities for sample prepa-
ration (Arþ sputtering, Co evaporation, O2 gas
inlet). The base pressure in the chamber was
<7 Â 10À11 mbar. The Ag(0 0 1) substrate was
cleaned by cycles of Arþ ion sputtering and an-
nealing. STM images showed a clean surface, free
from impurities or step pinning due to contami-
nation. CoO films were grown by deposition of Co
on Ag(0 0 1) (0.2 ML minÀ1) in an O2 atmosphere
(2 Â 10À6 to 5 Â 10À6 mbar). The thickness of the
films was less than 1 ML to allow an investigation
of the oxide layers in comparison to the metallic
Ag substrate. During the deposition the substrate
was kept at room temperature resulting in the
Fig. 1. (a), (b) CoO(0 0 1) islands on Ag(0 0 1) at different a sample bias of )2.5 V (a) and 1.5 V (b). Due to a slight drift the imaged
area is shifted (white arrow marks the same island). The area is 22 nm  22 nm, the tunneling current is 0.06 nA. (c) Encircled part of
Fig. 1b at higher resolution (10 nm  10 nm). The image has been processed to achieve a better contrast of the atomic corrugation
inside of the island. The sample bias is +1.5 V and the tunneling current 0.06 nA.

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Ch. Hagendorf et al. / Surface Science 532–535 (2003) 346–350
Furthermore, three different kinds of islands
UHV, the hexagonal atomic corrugation disap-
pears and a rearrangement of the Co oxide layer
takes place (Fig. 2). Regions with different mor-
phology (A, B, C and D) can be distinguished.
Region A has been identified as CoO(0 0 1) by
comparing the shape and the voltage dependent
STM contrast with previous measurements. In the
same way, region D has been assigned to the un-
covered Ag(0 0 1) substrate. Furthermore, areas are
observed, which exhibit irregularily shaped grooves
(B) and ordered structures with threefold symme-
try and a periodicity of 1.7 nm (C). The islands C
are around two times higher (0.5 nm) than B (0.23
nm) relative to the Ag(0 0 1) substrate. With regard
to the observed (1 1 1) oriented Co/O precursor, we
assume, that regions B and C consist of single- and
double-layers of (1 1 1) oriented Co oxide, respec-
tively. The ordered hexagonal structures on island
can be distinguished in Fig. 1a and b (A, B and C).
For each of them the same voltage dependent
height variation has been verified with a shift of 0
nm (A), 0.2 nm (B) and 0.4 nm (C), respectively.
Since these shifts in height correspond to the
monolayer height of the silver substrate, we as-
sume that the islands consist of CoO(0 0 1) double-
layers either floating on the Ag(0 0 1) surface (C),
being half (B) or completely (A) embedded into the
Ag(0 0 1) substrate. It should be noted that the
completely embedded island (A) has edges ori-
ented in the h110i directions of the substrate. Type
B and C appear more roundly, because short
segments of the step edges are aligned in the h100i
directions. This is related to the formation of
nonpolar CoO step edges upon annealing [4].
Fig. 1c shows a CoO island at higher resolution
and a tunneling voltage of +1.5 V. The image has
been differentiated to enhance small height varia-
tions at short distances. Inside of the island
structures with a square lattice periodicity of
around 0.3 nm are visible. This corresponds to the
next neighbor distance of the 1 Â 1 Ag(0 0 1) sur-
face (0.28 nm). However, the same periodicity is
adopted by the Co and the O sublattice of
pseudomorphically grown CoO(0 0 1). The corru-
gation amounts to 80 pm. Atomically resolved
images could also be obtained at small negative
voltages ()0.4 to )1.5 V). In principle, the atomic
species on the CoO(0 0 1) surface can be distin-
guished through the polarity of the bias voltage.
According to Castell et al. [6] tunneling occurs into
the empty Co 3d states at positive voltage, whereas
at negative voltage the filled states of the oxygen
sites are visible. However, at bias voltages within
the band gap region tunneling through the oxide
layer has to be taken into account [3]. Thus, a clear
assignment of the observed corrugation is difficult,
since it might be related to an interaction with the
CoO surface or to the local density of states at the
metal/oxide interface.
ꢀ
C indicate a moire-like reconstruction. The for-
mation of a hexagonal moire-pattern implies the
ꢀ
existence of at least two hexagonal lattices with a
ꢀ
slightly different periodicity. Two 30°-rotated moire-
patterns are observed due to the two equivalent
After deposition of Co at room temperature in
5 Â 10À6 mbar O2 atmosphere, islands with a hex-
agonal lattice periodicity of 0.3 nm are observed
(not shown here). This indicates the formation of a
Co/O precursor with (1 1 1) orientation. After re-
peated heating for several minutes at 470 K in
Fig. 2. Overview image (25 nm  25 nm) of Co oxide on
Ag(0 0 1). The marked regions correspond to CoO(0 0 1) (A),
single-layer of (1 1 1) oriented Co oxide (B), double-layer of
(1 1 1) oriented Co oxide (C) and Ag(0 0 1) (D). The contrast of
the step edges has been enhanced by image processing. The
sample bias is )5 V and the tunneling current 0.08 nA.

Ch. Hagendorf et al. / Surface Science 532–535 (2003) 346–350
349
relative orientations between a hexagonal and a
square lattice.
emental composition of the (1 1 1) oriented Co
oxide remains an open question.
It is known, that defects and the stoichiometry
of Co oxide surfaces depend on O2 partial pressure
and temperature [7]. The formation of Co3O4
layers on CoO(0 0 1) at 5 Â 10À7 mbar O2 partial
pressure and 625 K has been reported [8]. Since in
our experiments (1 1 1) oriented Co oxide has been
observed after Co deposition at a slightly increased
O2 partial pressure and also after subsequent
heating in >10À6 mbar O2, these structures can be
related to Co3O4. However, the formation of a
CoO(1 1 1) surface cannot be ruled out. In this case
a reconstruction should appear to compensate for
the polarity of the CoO(1 1 1) surface. This was not
observed in the experiments. In summary, the el-
Fig. 3a and b shows a constant current STM
image and a simultaneously recorded map of I=V -
spectra. The achieved spatial resolution is better
than 1 nm. In the STM image surface regions with
Ag(0 0 1), CoO(0 0 1) and (1 1 1) oriented Co oxide
can clearly be distinguished (compare Fig. 2). Each
pixel in the I=V -STS map represents the tunneling
current at 2.5 V sample bias voltage during re-
cording the I=V -spectrum at the given position.
The spectra have been acquired at 100 Â 100 points
on the area shown in the constant current image.
After stabilizing the tip at the given tunneling
conditions (Fig. 3a), the feedback loop is switched
off and the voltage is ramped at a given amplitude
Fig. 3. (a) Constant current image of CoO(0 0 1) and (1 1 1) oriented Co oxide on Ag(0 0 1) at a sample bias of )3 V and a tunneling
current of 0.08 nA. The area is 25 nm  25 nm. At a raster grid of 100  100 points I=V -STS spectra were recorded. After stabilizing the
tip sample distance, the sample bias was ramped from )3 to +3 V in 100 steps with feedback switched off. (b) Spatial distribution of the
tunneling current at a sample bias of +2.5 V taken from 100 Â 100 I=V -STS spectra recorded on the area displayed in Fig. 3a. (c) I=V -
STS spectra after averaging over regions with equivalent composition. (d) Normalized differential conductance ðdI=dV Þ=ðI=V Þ cal-
culated from I=V -STS spectra in (c).

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Ch. Hagendorf et al. / Surface Science 532–535 (2003) 346–350
and speed. At 2.5 V the (1 1 1) oriented Co oxide
islands appear dark, CoO(0 0 1) bright and
Ag(0 0 1) at a medium grey level (grey scale ad-
justed for best contrast). The large number of ac-
quired spectra allows to check for inhomogeneities
(e.g., defects, step edges). Additionally spectra
from the same surface region may be averaged to
reduce noice in the data (Fig. 3c). For further
analysis an approximate density of states has been
derived by calculating the normalized differential
conductance ðdI=dV Þ=ðI=V Þ [9]. The resulting
spectra for the three regions are plotted in Fig. 3d.
Experiments with different voltage amplitudes (ꢀ2
to ꢀ6 V) qualitatively yield the same structures.
Changing the polarity of the stabilization voltage
mainly affects the intensity of the features, but not
their general shape and their position on the
voltage scale.
The I=V - as well as the ðdI=dV Þ=ðI=V Þ-spec-
trum of Ag(0 0 1) appears relatively flat. We take
this curve as characteristic for a metal/vacuum/
metal tunneling junction. In comparison, the I=V -
spectrum of CoO(0 0 1) shows an increase in tun-
neling current at +2.2 V (unoccupied states). This
corresponds to a maximum in the ðdI=dV Þ=ðI=V Þ-
spectrum. A smaller maximum is visible at )1.6 V
(occupied states). The energetic positions of these
structures coincide with the voltage dependent
contrast reversal in the STM images of CoO(0 0 1).
According to Viernow et al. [10] pronounced peaks
in the normalized differential conductance can be
explained by a divergence of ðdI=dV Þ=ðI=V Þ at the
band edge. Inside the bandgap the tunneling cur-
rent stays finite due to tunneling into the Ag(0 0 1)
substrate. Thus the two maxima in the ðdI=
dV Þ=ðI=V Þ-spectra are assigned to the edges of
its bandgap. The shoulder at +1.8 V can be at-
tributed to electron tunneling through CoO(0 0 1)
into the underlying Ag(0 0 1) substrate. It should
be noted that the atomic periodicity shown in Fig.
1c was imaged with a bias voltage in the range of
+1.5 V. Therefore, tunneling into states at the
oxide/metal interface seems to be a possible ex-
planation for the observed corrugation. The I=V -
spectrum obtained from (1 1 1) oriented Co oxide
is similar to that of Ag(0 0 1). The ðdI=dV Þ=ðI=V Þ-
spectrum exhibits no strong maxima in the inves-
tigated voltage range. A weak maximum at +1 V
cannot be correlated to any spectroscopic feature
of the Ag(0 0 1) substrate. Furthermore, no voltage
dependent contrast can be observed in STM im-
ages. This leads to the conclusion, that (1 1 1)
oriented Co oxide layers exhibit no extended
bandgap in contrast to CoO(0 0 1) layers.
In summary, we have demonstrated that
CoO(0 0 1) and also (1 1 1) oriented Co oxide is
epitaxially grown on Ag(0 0 1) by deposition of Co
in an O2 atmosphere of >1 Â 10À6 mbar. A strong
bias voltage dependent contrast reversal occurs in
STM images of CoO(0 0 1) islands ()1.5 to +2.2
V). This behaviour can be explained with tunnel-
ing through a bandgap of the CoO(0 0 1) double-
layer into the underlying Ag(0 0 1). I=V -STS
studies confirm the existance of a bandgap. A
strong voltage dependent STM contrast is not
observed for layers of (1 1 1) oriented Co oxide. Its
electronic structure shows a finite density of states
close to the Fermi level as deduced from I=V -STS.
Acknowledgement
Financial support by the Deutsche For-
schungsgemeinschaft through the Research Group
‘‘Oxidic Interfaces’’ is gratefully acknowledged.
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