Surface Science 541 (2003) 197–206

www.elsevier.com/locate/susc

A photoemission study of CoO-films on Au(1 1 1)

S. Sindhu, M. Heiler, K.-M. Schindler *, H. Neddermeyer

Fachbereich Physik, Martin-Luther-Universit€aat, Halle-Wittenberg, D-06099 Halle, Germany

Received 30 April 2003; accepted for publication 2 July 2003

Abstract

Angle-resolved ultraviolet photoemission spectroscopy has been used to study thin layers of Co and CoO on an

Au(1 1 1) substrate. A bulk-like CoO layer was obtained for a nominal coverage of 7 ML Co, with Co evaporated in an

oxygen atmosphere of 10�4 Pa and the Au(1 1 1) substrate at 350 K. Low-energy electron diffraction of this layer ex-

hibits a (1 · 1) structure, indicating an unreconstructed CoO(1 1 1) surface. Considering the instability of a bulk-liketerminated CoO surface, we assume stabilization by OH termination. At low coverage, the CoO layer is oxygen de-

ficient and exhibits metallic character. For the bulk-like CoO layers we found band-like oxygen states and no detectable

dispersion for the states near the Fermi edge, which is in agreement with previous investigations of single-crystalline

CoO.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Cobalt oxides; Angle resolved photoemission; Gold; Surface structure, morphology, roughness, and topography

1. Introduction

The electronic structures of the late transition

metal oxides-like NiO and CoO is essential for the

understanding of their technical applications, suchas lasers, sensors and catalysts [1–4]. The nature of

their insulating character has been a highly con-

troversial topic and has recently been related to a

charge transfer process and strong correlation of

the d electrons [5]. Previous experimental investi-

gations on CoO were mainly performed on sur-

faces of single crystals and cover X-ray and UV

photoemission [6–13], electron energy loss studies

* Corresponding author. Tel.: +49-345-552-5563; fax: +49-

345-552-7160.

E-mail address: schindler@physik.uni-halle.de (K.-M.

Schindler).

0039-6028/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/S0039-6028(03)00917-8

of its electronic structure [14], and a low-energy

electron diffraction (LEED) analysis of the geo-

metric structure of the CoO(1 0 0) surface [15,16].

In most technical applications thin films of CoO

are used. Therefore, there is considerable interestin the properties of CoO thin films and how they

can be prepared. In addition, they have the ad-

vantage that sample charging during measure-

ments can be avoided and that polar surfaces such

as the (1 1 1) surface can be prepared and studied.

However, optimal preparation conditions such as

the amount of Co, the substrate temperature and

the oxygen pressure have to be determined.In order to study the electronic structure ex-

perimentally, photoelectron spectroscopy (PES) is

one of the most suited methods and has been used

successfully in testing the electronic structure

models of a wide variety of materials including

oxides [17]. Since the electron correlation is high in

ed.

198 S. Sindhu et al. / Surface Science 541 (2003) 197–206

CoO, readjustments occur after the emission of a

photoelectron and PES monitors an exited state of

the electron system. The configuration-interaction

(CI) model [18] can describe some features of the

photoemission spectra very accurately; in particu-

lar final state effects can properly be taken intoaccount. The most important result of the config-

uration-interaction model is that the first ioniza-

tion state of CoO involves ionization of O 2p

states. Consequently, the electronic structure of

CoO is considered a charge-transfer system [18].

Starting with an O 2p6 and Co 3d7 occupation in

the ground state, the electronic structure and the

photoemission spectra of CoO can be described asfollows: the Co 3d-band splits into localized sub-

bands through a repulsive two electron Coulomb

interaction associated with the energy U. The

lower band, situated below the Fermi edge, is fully

occupied and the upper band is above the Fermi

level and consequently empty. However, the mix-

ing and hybridization of the metal 3d-states with

the ligand (L) 2p-states has to be taken into ac-count as well and leads to the dnL notation. This

results in two final states in photoemission: (i) d6L

and (ii) d7L�1. The first one (d6L), the so-called

unscreened channel, has been assigned to the

�satellite� structure in the photoemission spectrumat about )10 eV by means of resonant photo-

emission spectroscopy. This technique uses the

phenomenon of enhancements or suppressions ofphotoemission features by opening a new emission

channel. Actually for CoO, this feature is reso-

nantly enhanced, when the photon energy reaches

the 3p to 3d absorption threshold (about 60 eV)

due to the following process: 3p63d7 + hm to 3p53d8

and further the decay 3p53d8 to 3p63d6 [19]. The

second channel (ii) involves the screening of the d-

band hole through a charge transfer from the li-gand to the localized metal ion, associated with the

charge transfer energy D. In experimental spectrathe resulting emission line is located near the

Fermi edge at )1.7 eV [19]. Consequently, the gapis formed between this d7L�1 state and the lowest

free state of the conduction band (d8). The mag-

nitude of the gap is given by the amount of the

charge transfer energy D and CoO should there-fore be considered as a charge transfer insula-

tor. Despite of this successful description of the

photoemission spectra, we have to keep in mind

that the configuration-interaction model is local-

ized in nature, hence it is not possible to describe

any kind of EðkÞ-dispersions of the bands, whichhas been found experimentally, in particular the

dispersion of the O 2p derived states.In this paper we have used angle-resolved

ultraviolet photoemission spectroscopy (ARUPS)

to investigate thin films of Co and CoO on an

Au(1 1 1) substrate following our work on NiO

layers on Au(1 1 1) [20,21]. In particular, the oxy-

gen partial pressure dependence while growth as

well as the influence of substrate temperature and

film thickness were studied. Finally, we present theangle-resolved UV photoemission spectra of a

bulk-like CoO(1 1 1) film.

2. Experimental

The angle-resolved ultraviolet photoemission

studies were performed using an ultra high vac-

uum (UHV) experimental setup consisting of two

chambers, one for sample preparation and the

other for photoemission experiments. The base

pressure was in the lower 10�8 Pa region. In orderto cut down the time for vacuum recovery after

elevated pressure conditions during preparation, a

valve allows to separate the chambers and a transfer

manipulator was used to move the sample between

the two chambers. The temperature of the sample

could be varied between room temperature (RT)

and 1000 K by radiative heating by a hot filament

close to the backside of the sample holder plate.The temperature of the sample was monitored by a

thermocouple. The Au(1 1 1) sample was cleaned

in-situ by Arþ ion bombardment and subsequent

heating up to 900 K. Its cleanliness was verified by

Auger electron spectroscopy and LEED. The

(23�p3) LEED pattern obtained is well docu-

mented for a clean Au(1 1 1) surface and stems

from the so called herringbone reconstruction [22].Co was evaporated from an effusion cell at a

rate of 1 ML/min. The amount deposited was

monitored using a quartz microbalance. All indi-

cated thicknesses in this paper correspond to the

nominal coverage of Co on Au(1 1 1), whereby we

S. Sindhu et al. / Surface Science 541 (2003) 197–206 199

assume that a monolayer (ML) corresponds to one

Co atom per one bulk Au atom. The measurement

chamber is equipped with an en electron gun for

Auger electron spectroscopy and a gas discharge

UV lamp for photoemission spectroscopy. Auger

and photoelectrons were analyzed using a toroidalanalyzer in angle-resolved mode. The photons

impinged on the sample with an angle of 35� withrespect to the surface normal. All spectroscopic

measurements were carried out at room tempera-

ture. The kinetic energy scale of the photoelectrons

was referenced against the Fermi edge (EF) of theclean Au(1 1 1) substrate.

3. Results and discussion

3.1. Co on Au(1 1 1)

Prior to cobalt oxide, the growth of metallic co-

balt on Au(1 1 1) was studied briefly for comparison

reasons. Fig. 1 shows angle-resolved photoemissionspectra withHe I excitation in normal emission. The

bottom spectrum is from the clean Au(1 1 1) sub-

strate and shows the characteristic pattern of va-

phot

oem

issi

on in

tens

ity (

arb.

units

)

-10 -8 -6 -4

initial energ

Co/Au(111)

hν = 21.22 eV; Θ = 0°

Fig. 1. He I photoemission-spectra (normal emission) of Co/Au(1 1

perature during the deposition. The film of the top spectrum was hea

lence band emission lines [23]. According to Ref.

[24] the peak at )0.5 eV stems from a surface stateand indicates the cleanliness of the surface. Fur-

thermore, the shoulder at )5.5 eV, the peak at )4.1eV and the shoulder at )2.7 eV have been related bythe (22�p

3) reconstruction of this surface [25]. Inaddition, the (22�p

3) LEED pattern has been

obtained.

The next spectra were taken after the deposition

of an increasing amount of Co as described in the

experimental section with the Au(1 1 1) substrate

at room temperature. With an increasing amount

of Co the Au valence band emission lines attenuate

and a broad line between the Fermi edge and )1eV is growing in, which is characteristic for the 3d-

bands of metallic Co. Although the Au related

lines attenuate, they do not completely vanish, but

are still present up to coverage of 5 ML Co. This

indicates that Co is growing in a 3D island mode

on Au(1 1 1) as was found with recent STM in-

vestigations [26]. The 3D growth mode is usually

attributed to the large lattice misfit (14%) betweenCo and Au(1 1 1). Finally, the photoemission

spectrum becomes Co-like at a Co coverage of 10

ML. The LEED patterns of this film exhibits

-2 0 2

y (eV)

0.5 ML(RT)

Au(111)

1 ML(RT)

5 ML(RT)

2 ML(RT)

10 ML(RT)

10ML(600K)

1) as a function of coverage. The substrate was at room tem-

ted up to 600 K after the deposition of 10 ML Co.

200 S. Sindhu et al. / Surface Science 541 (2003) 197–206

rather broad (1 · 1) spots of a Co(1 1 1) surfacewith an increased diffuse background indicating a

large degree of disorder.

The top spectrum is taken after the deposition

of 10 ML Co with the Au(1 1 1) substrate at 600 K.

This spectrum clearly resembles the bottom spec-trum of the clean Au(1 1 1) surface, including all

details of surface states and lines typical for the

(22�p3) reconstruction. Obviously, Co diffused

into the bulk of the Au substrate during deposi-

tion. This appears to be in contrast to Mamy et al.

[27], who found the growth of a closed overlay at

this substrate temperature. However, we relate this

to the following difference between their work andour: their Au substrate consisted of a thin Au film

of 10 nm thickness deposited onto an MoS2 sub-

strate, whereas we used a bulk single crystal.

Mamy et al. [27] reported indications for some

intermixing of Co and Au. Their amount of Co

might be enough to saturate the Au thin film and

enough excess Co would be left over to form the

film even at 600 K. In our case, however, thesubstrate crystal would act as a quasi-infinite sink

for Co, which cannot be saturated by the amount

phot

oem

issi

on in

tens

ity (

arb.

uni

ts)

-10 -8 -6 -4

initial ene

5 ML Co/Au(111) + O2

hν = 21.22eV; Θ = 0˚

Fig. 2. He I photoemission-spectra (normal emission) of 5 ML Co/A

room temperature during the deposition. The film of the top spectrum

for 20 min).

of Co deposited. As a result of this, all Co de-

posited would diffuse into the bulk and none can

be observed on the surface.

3.2. CoO on Au(1 1 1) (oxygen partial pressure

dependence)

In order to determine the oxygen partial pres-

sure, which is necessary to form a cobalt oxide

layer, 5 ML Co were evaporated at different oxy-

gen partial pressures. He I photoemission spectra

in normal emission have been recorded in order to

characterize the resulting film as shown in Fig. 2.

Again, the bottom spectrum is from the bareAu(1 1 1) substrate and serves as a reference for the

Au emission lines. The next three spectra are from

Co films, which were deposited onto the Au(1 1 1)

substrate at room temperature with increasing

oxygen partial pressures. The top spectrum is from

a Co film, which was oxidized after the deposition.

After deposition in 10�6 Pa oxygen the spectrum

(second from bottom in Fig. 2) is composed ofslightly modified Au lines and a new line between

the Fermi edge and )1.0 eV. Comparison with Fig.

-2 0 2

rgy (eV)

Au(111)

10-6

Pa

10-5

Pa

10-4

Pa

600L O2

400K, 20'

u(1 1 1) as a function of oxygen pressure. The substrate was at

was oxidized after the deposition of a Co film (600 L O2 at 400 K

S. Sindhu et al. / Surface Science 541 (2003) 197–206 201

1 reveals that the spectrum is actually identical to

the one where the same amount (5 ML) of metallic

Co was deposited. Obviously, an oxygen partial

pressure of 10�6 Pa is not sufficient to oxidize Co

to any noticeable extend. Deposition in 10�5 Pa

oxygen leads to quite a different spectrum (centerin Fig. 2). The emission lines of Au and metallic

Co are strongly attenuated and the spectrum is

close to that of bulk CoO with a prominent

shoulder at )1.7 eV. However, a more detailedinspection reveals the existence of states close to

the Fermi edge, which might be due to metallic Co

or oxygen defects in CoO. Both cases indicate in-

complete oxidation of the Co, which is actuallyconfirmed by the next spectrum (second from top

in Fig. 2) obtained after deposition in 10�4 Pa O2,

which is CoO-like together with the states at the

Fermi edge completely vanishing. In summary, the

three spectra monitor the transition from a layer of

metallic Co to one of insulating CoO. This shows

that an O2 partial pressure of 10�4 Pa during de-

position is required to completely oxidize a Colayer of 5 ML on Au(1 1 1).

Finally, a metallic Co layer of 5 ML on

Au(1 1 1) was exposed to 600 mbar �O2 and sub-sequently annealed at 400 K for 10 min (post-ox-

idation). Besides the characteristic CoO state at

)1.7 eV the He I photoemission spectrum of this

phot

oem

issi

on in

tens

ity (

arb.

uni

ts)

-10 -8 -6 -4

initial ene

hν = 21.22eV; Θ = 0˚

7 ML CoO/Au(111)

Fig. 3. He I photoemission spectra (normal emission) of 7 ML CoO/A

(measured at room temperature).

layer (top one in Fig. 1) also shows Au states at

)6.0 and )4.6 eV. This indicates that parts of theAu(1 1 1) substrate are covered by much less than 5

CoO layers and that 3D islands of CoO are

formed. This shows that the co-evaporation tech-

nique is better suited to obtain CoO films, whichcompletely cover the substrate, in analogy to the

findings with NiO [28]. The 3D islands, once

formed during the deposition of the metallic Co,

cannot be transformed into a smooth layer of CoO

by post oxidation. Finally it is worth mentioning

that no charging of the sample was observed.

3.3. CoO on Au(1 1 1) (substrate temperature

dependence)

Supposedly, the structure of the CoO film

does not reflect its thermodynamic equilibrium but

is strongly determined by the growth process.

Therefore, the dependence of the film structures on

the substrate temperature needs to be investigated.

In order to do so, CoO films were prepared with theAu(1 1 1) substrate at four different temperatures,

i.e. room temperature (RT), 350, 500 and 600 K.

Again, the Co was deposited in 10�4 Pa O2. Fig. 3

presents the corresponding series of He I spectra in

normal emission of 7 ML CoO/Au(1 1 1). The

spectrum obtained after deposition at 600 K is very

-2 0 2

rgy (eV)

RT

350K500K

600K

TAu(111)

u(1 1 1) as a function of substrate temperature during deposition

202 S. Sindhu et al. / Surface Science 541 (2003) 197–206

different from all the others. It actually resembles

the spectrum of the clean, (22�p3) reconstructed

Au(1 1 1) surface (see Fig. 2) remarkably well. All

the Au related emission lines are present including

the characteristic ones for the (22�p3) recon-

struction of the surface. Only the shape of the lineclose to the Fermi edge is different. It is less struc-

tured and compares quite well with the one of

metallic Co. This indicates that the surface mainly

consists of the clean, (22�p3) reconstructed

Au(1 1 1) surface and only a small amount of sup-

posedly metallic Co is present on the surface. This

finding is confirmed by Auger electron spectros-

copy. (The spectra are not shown here.) The AuNOO Auger line at 65 eV becomes clearly evident

and the Co MVV Auger line at 47 eV strongly at-

tenuates. Two models are in accordance with these

results: (a) clustering of CoO islands so that the

visible Au surface area becomes larger than the

surface of the CoO islands or (b) surface segrega-

tion of Au occurs. Although the latter is known

from the Co/Au(1 1 1) system [33], further investi-gations with other methods are needed to answer

this question beyond any doubt.

The spectra from the films deposited at RT, 350

and 500 K are quite similar to each other. A closer

phot

oem

issi

on in

tens

ity (

arb.

uni

ts)

-10 -8 -6 -4initial energy

CoO/Au(111)

hν = 21.22 eV; Θ = 0°

Fig. 4. CoO/Au(1 1 1), grown at 300 K substrate temperature in an oxy

emission) as a function of film thickness.

inspection reveals, however, that the film deposited

at 500 K has more intensity at the Fermi edge than

the others and there is a small but clearly visible

contribution of the Au line at )6 eV. This indicatesthat the pronounced changes of the 600 K film are

starting already at 500 K. The spectrum of the RTfilm has a stronger contribution of the Au line at

)6 eV than the 350 K film. This indicates that thisfilm is rougher than the film grown at 350 K be-

cause a higher roughness results in a smaller

overall attenuation of the substrate emission lines.

In summary, the film deposited at 350 K ap-

pears to have the most expressed CoO spectrum

with the smallest contribution from the Au sub-strate. Furthermore, the film shows a clear (1 · 1)LEED pattern of a (1 1 1) surface with little

background, indicating a well-ordered film.

3.4. CoO on Au(1 1 1) (thickness dependence)

In order to study the thickness dependence of

the film growth, a series of oxidized Co films wasevaporated onto the Au(1 1 1) substrate at room

temperature in an oxygen atmosphere of 10�4 Pa.

Fig. 4 shows the corresponding He I photoemis-

sion spectra in normal emission.The spectra shown

-2 0 2 (eV)

Au(111)

2 ML

1 ML

5 ML

7 ML

3 ML

gen atmosphere of 10�4 Pa: He I photoemission spectra (normal

5.5

5.0

4.5

4.0

3.5

wor

k fu

nctio

n (e

V)

1086420

Co coverage (ML)

CoO/Au(111)

ΦAu(111)

ΦCoO

Fig. 6. CoO/Au(1 1 1), grown at 300 K substrate temperature in

an oxygen atmosphere of 10�4 Pa: work function as a function

of Co coverage.

S. Sindhu et al. / Surface Science 541 (2003) 197–206 203

in the figure are normalized to the maximum in-

tensity. With an increasing amount of oxidized Co

deposited the Au emission lines are attenuated and

the CoO related emission lines at )1.7 and )5 eVincrease. However, an additional line close to the

Fermi edge is present at coverages of 1, 2 and 3ML. This suggests, that at least the Co atoms of

the first layer do not form an oxide with insulating

properties. This is confirmed by the coverage de-

pendence of the intensity ratio of the Co LMM

and O KLL Auger emission lines (Fig. 5). It is

clearly revealed that the oxide is O deficient at low

coverage. This indicates that the reactivity of the

first layer Co atoms towards O is reduced, possiblyby the interaction of the Co with the Au substrate

or even some kind of alloy formation. Higher Co

coverages are required for the formation of CoO

with bulk-like, insulating properties. These find-

ings are further supported by work function

measurement as shown in Fig. 6. The bare sub-

strate surface exhibits the expected work function

for Au of 5.31 eV [29]. At a coverage of 1 ML thework function is reduced to 4.9 eV, which corre-

sponds much better to that of metallic Co than

that of CoO (3.7 eV) [30]). At least 2, maybe even 3

ML of CoO are required to reduce the work

function to that value.

The fact that there is Co excess at low coverages

gives some insight about the structure of the

3.0

2.5

2.0

1.5

1.0

0.5

0.0

I (O

-KLL

, 510

eV

) / I

(C

o-LM

M, 7

75 e

V)

1086420

Co coverage (ML)

CoO/Au(111)

Fig. 5. CoO/Au(1 1 1), grown at 300 K substrate temperature in

an oxygen atmosphere of 10�4 Pa: intensity ratio of Co MNN

and O KLL Auger electron lines as a function of Co coverage.

Au/CoO interface. Assuming bulk truncation of

the Au substrate and the CoO, there are two

possibilities for the interface structure. Either theCoO film starts with a densely packed layer of O

or with one of Co. As a starting point one can

consider the energetics of bonds between Au and

Co or O. The formation of Au/Co alloys and the

weak adsorption of O on Au and the fact that Au

oxide easily decomposes indicate that the ener-

getics favor the bonds between Co and Au com-

pared to bonds between O and Au. Therefore, onewould expect that the CoO film starts with a layer

of Co. In line with this expectation, the ratio of the

Auger emission lines of Co and O show a corre-

sponding excess of Co at low coverages. However,

more direct evidence about the interface structure

and more details is clearly desired.

Another interesting finding is revealed by

comparing the spectra of the metallic Co and theoxidized Co at a coverage of 1 ML. Whereas the

spectrum with metallic Co still shows the Au

emission lines (Fig. 1), which are related to the

(22�p3) reconstruction of the Au(1 1 1) surface,

these lines are absent in the spectra of the oxidized

Co (Fig. 4). This indicates that the oxidized Co

grows in a layer-by-layer mode to a larger extend

than the metallic Co and covers more of theAu(1 1 1) surface, whereby lifting a larger area with

the (22�p3) reconstruction of the Au surface.

204 S. Sindhu et al. / Surface Science 541 (2003) 197–206

3.5. Angle-resolved photoemission spectroscopy of

CoO on Au(1 1 1)

Finally, we discuss the angle-resolved photo-

emission spectra and the assignment of thephotoemission lines of a CoO film with a thickness

of 10 ML and grown at a substrate temperature of

350 K (Fig. 7). Five different emission lines can

clearly be identified, labeled A, B, C, D and E. As

discussed in the introduction, the line at the Fermi

edge ()1.7 eV), labeled A, is assigned to the chargetransfer d7L�1 final state. Within the detection

limits of our experiment we find no dispersion withrespect to kk for this state. This is the expectedbehavior because the corresponding initial states

are supposed to be localized. The assignment of

emission line B ()3.65 eV) is more complex. Ex-perimentally we do not find any dispersion with

respect to kk similar to emission line A. This linewas also found in the spectra of [1 0 0] surfaces of

bulk CoO by several groups [19,31,32]. Hence, weconclude that the emission line B is not a feature

specific to the CoO(1 1 1) surface but is a general

feature of CoO. However, the charge transfer

phot

oem

issi

on in

tens

ity (

arb

units

)

-12 -10 -8 -6

initial energ

CoO(111)

DEhν = 21.22 eV; Γ L K

Fig. 7. Angle-resolved photoemission spectra of 10 ML CoO/Au(1 1

sphere of 10�4 Pa.

model fails to predict this feature. One proposal is

that line A and B both relate to the same charge

transfer mechanism with a d7L�1 final state and

that their separation is due to a crystal or ligand

field splitting similar to that of the d8L�1 state of

NiO. The next lines to discuss are the emissionlines C and D. Emission line C clearly shows dis-

persion with respect to kk. Emission line D can

only be clearly distinguished in a limited range of

high emission angles. Within this range no dis-

persion can be observed. For an assignment of the

lines we again compare our spectra with the ones

from CoO(1 0 0) surfaces. There, two dispersing

emission lines were found in this energy range andassigned to delocalized O 2p bands. This clearly

suggests that the emission line C in our spectra

corresponds to the upper O 2p band. Unfortu-

nately, the assignment of line D is less clear. We

suggest that it corresponds to the lower O 2p band

although we cannot detect the expected dispersion.

The reason for this might be that the intensity of

this band becomes to small to be detected at otherangles or that it merges in a very narrow range of

angle with emission line C. Finally, we assign the

-4 -2 0

y (eV)

90°

0°

45°

AB

C

1), grown at 350 K substrate temperature in an oxygen atmo-

S. Sindhu et al. / Surface Science 541 (2003) 197–206 205

emission line E at )10.7 eV, which does not showany dispersion, to the unscreened emission from

Co 3d states in accordance to the interpretation of

the spectra of the CoO(1 0 0) surface in the litera-

ture.

An important topic of (1 1 1) surfaces of fcctransition metal oxides is the surface termination.

In this plane the bulk crystal lattice can be termi-

nated with a close packed layer of either Co atoms

or O atoms. Both terminations are polar due to

their excess charge of opposite sign and are ex-

pected to have high surface energies. The NiO(1 1 1)

surface, for example, avoids such an unfavorable

polar termination by the formation of an octopolar(2· 2) reconstruction [33,34]. Our investigations donot show any direct evidence for the preferred ter-

mination. Another possibility is the termination of

the surface with OH� groups, which we cannot rule

out complete because they could be formed via re-

actions of excess oxygen left over from the deposi-

tion with components of the residual gas, for

example the dissociative adsorption of water orthe reaction with hydrogen. According to the liter-

ature the presence of OH� groups leads to two

emission lines with binding energies somewhere

between 5 and 10 eV and a separation of 3–4 eV

[31,35]. Although there is no direct evidence for

such additional emission lines, they might be

masked by other emission lines, in particular

emission line D.

4. Summary

We obtained bulk-like CoO(1 1 1) layers by the

evaporation of at least 7 ML of Co in an oxygen

atmosphere of 10�4 Pa onto an Au(1 1 1) substrate

at 350 K. At coverages lower than 3 ML the oxi-

dation of Co appears to be incomplete. In agree-

ment with STM measurements [26], we suggestthat the CoO/Au(1 1 1) interface is Co terminated.

The Co atoms of the first layer are likely to be

stabilized by the Au surface, so that each of them

has only 3 neighboring oxygen atoms as compared

to six of the bulk CoO structure. This might result

in Co derived states at the Fermi edge. Further-

more, the polar CoO(1 1 1) surface is probably

stabilized by an OH termination. At higher sub-

strate temperatures the ARUP spectra become

Au(1 1 1)-like, which indicates either segregation of

Au to the surface or clustering of CoO, resulting in

a larger surface contribution of Au.

Acknowledgements

The authors are grateful to thank I. Sebastian

and K. Meinel for supporting discussions about

their STM investigations and to the members of

our group for the stimulating atmosphere. Special

thanks go to Ch. Jeckstiess for her valuable tech-

nical support. This work was financially supportedthrough the Forschergruppe FG 404 ‘‘Oxidische

Grenzfl€aachen’’.

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