Surface Science 488 (2001) 23±31
www.elsevier.com/locate/susc
Adsorption and decomposition of C60 on Ni(1 1 0) surfaces
V. Saltas, C.A. Papageorgopoulos *
Department of Physics, University of Ioannina, P.O. Box 1186, GR-451 10 Ioannina, Greece
Received 12 May 2000; accepted for publication 12 April 2001
Abstract
In the present paper we study the adsorption of C60 on Ni(1 1 0) surfaces at RT and 650 K, as well as the desorption/
decomposition process. The investigation took place in UHV by means of Auger electron spectroscopy, low energy
electron di€raction, thermal desorption spectroscopy and work function (WF) measurements. The observed overlayer
structures during C60 deposition at 650 K are in complete agreement with reported results. The sticking coecient is
nearly the same for both RT and 650 K. Heating of the 1 ML C60 -covered Ni(1 1 0) surface, however, at 750 K, causes a
partial fragmentation of C60 to a 2D graphite-like layer, while the remaining C60 molecules are rearranged in the …5 3†
structure, in contrast to reported results. Further heating to 800 K results in the complete fragmentation of C60 accompanied by carbon desorption, possibly in molecular Cn …n > 10† form. Above 800 K, the C of the graphite-like layer
is desorbed in both atomic and molecular form. The WF at saturation coverage was found to be 5.61 eV for both
deposition at RT and 650 K. This value is substantially greater than the average value of 5 eV, which has been suggested
to hold for C60 overlayer systems, regardless of the metal substrate. Ó 2001 Elsevier Science B.V. All rights reserved.
Keywords: Auger electron spectroscopy; Low energy electron di€raction (LEED); Thermal desorption spectroscopy; Work function
measurements; Adsorption kinetics; Carbon
1. Introduction
Since the discovery of a method for producing
macroscopic quantities of C60 [1], the interaction
of C60 with metal surfaces has been studied intensively, by using a variety of surface analysis
techniques [2±15]. This great interest arises mainly
from the signi®cant physical properties of C60
based compounds, such as superconductivity, and
possible applications of C60 in microelectronics
[16±19]. In all cases, it is important to determine
*
Corresponding author. Tel.: +30-651-98570; fax: +30-65145381.
E-mail address: [email protected] (C.A. Papageorgopoulos).
the charge state of the C60 molecules. It is by now
well established that C60 (with a coverage up to
one physical monolayer) forms a chemical bond
with metal surfaces and the electronic charge is
transferred from the substrate to the overlayer C60
molecules [3,20±23]. The type of interaction and
the charge transfer (CT) is strongly dependent
upon the kind of the metal substrate [13]. The
transferred charge has been found to vary from
less than one electron up to six electrons per C60
molecule. The latter case appears when the metal
substrates are covered by alkali [3,24]. When the
alkali, however, are intermixed with fullerenes
(superfullerides) up to 11 electrons are donated to
each C60 molecule [25±27]. According to Maxwell
et al., the bonding of C60 with metal substrates,
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 1 0 5 - 0
24
V. Saltas, C.A. Papageorgopoulos / Surface Science 488 (2001) 23±31
can be intermediate up to strong covalent or
intermediate ionic [28]. A strong predominately covalent bonding has been reported for one monolayer of C60 on Ni(1 1 0) surfaces and the CT has
been found to be 2 1 electrons per molecule,
leading to a metallic C60 overlayer [29]. In the
latter case, the metal surface acts as catalyst for
the thermal decomposition process of the deposited C60 molecules [23,30]. More recent reported
photoemission studies suggest that 1 ML of C60
adsorbed on various metal substrates appears to
have a work function (WF) value close to 5 eV,
regardless of the metal substrate [28,31]. However,
the existing WF measurements of the metallic C60
overlayer on di€erent substrates are spread from
4.8 to 5.4 eV. In all the above cases the measured
WF values arise from the cut o€ of the UPS spectra
and there are no detailed WF studies during C60
deposition on metal substrates at room and elevated temperatures.
In the present work we study the C60 adsorption
on Ni(1 1 0) at RT and 650 K, using low energy
electron di€raction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy
(TDS) and WF measurements. The thermal stability and the decomposition mechanism of the
ordered C60 monolayer have been investigated as
compared to the existing literature [23,30].
2. Experiment
The experiments were carried out in an ultrahigh vacuum chamber (base pressure 1 10 10
mbar) equipped with a cylindrical mirror analyzer
for AES measurements, a quadrupole mass spectrometer for TDS measurements, a Kelvin probe
for WF measurements and a LEED system for
structural studies.
C60 of 99.95% purity was evaporated onto the
sample from a Ta crucible at 700 K. The C60 source
was carefully outgassed for several hours, so the
pressure during evaporation never exceed 1 10 9
mbar. A shutter between the source and the sample
was used to control the deposition time.
The Ni(1 1 0) sample was cleaned by Ar‡ ion
sputtering (1 KV, 10 lA) at 5 10 5 mbar for 10
min and subsequent annealing at 1000 K. This
cycle was repeated several times until the AES
peak heights of the main impurities (sulfur and
carbon) had vanished and a well-de®ned LEED
pattern of the …1 1† structure was observed. The
temperature of the sample was measured by a
chromel±alumel thermocouple inserted between
the sample and its Ta foil case. The linear heating
rate in the TD experiments was 15 K/s. The AES,
WF and LEED measurements at the annealing
procedure were taken after the sample was cooled
down at RT.
3. Experimental results and discussion
As mentioned in the introduction, there has already been substantial research on the adsorption
of C60 on Ni(1 1 0). Well-ordered C60 structures
have been observed only for growth temperatures
above 620 K [29], while STM studies suggest that
C60 induces a restructuring of the Ni(1 1 0) surface
[12]. In this work the LEED observations have
been repeated, in order to correlate them with the
AES, WF and TDS measurements.
Fig. 1 shows the Auger peak to peak heights
(Ap-pH) of Ni-61 eV and C-272 eV during C60
deposition on Ni(1 1 0) at 650 K and the subsequent temperature increase of the C60 -covered
Ni(1 1 0) surface, up to 900 K, in correlation with
the LEED patterns. During the annealing procedure, the measurements were taken after cooling
the sample at RT. As it is seen in Fig. 1, the peak
height of C increases almost linearly up to 50 min
and levels o€ for higher deposition time, while the
Ni-61 eV signal decreases analogously. It is well
known that deposited C60 on Ni(1 1 0) surfaces at
650 K saturates at the completion of 1 ML
(physical monolayer) [29], that is, after 50 min of
deposition one monolayer of C60 has been formed
on the surface. At the third dose of deposition, a
…5 3† LEED pattern appears, which maintains
with increasing intensity up to the 13th dose. With
more deposition, the pattern changes gradually to
a hexagonal con®guration (hcp), and its intensity
is maximized at the completion of 1 ML. The
coverage of a complete …5 3† structure corresponds to 0.8 of a physical C60 monolayer. The
V. Saltas, C.A. Papageorgopoulos / Surface Science 488 (2001) 23±31
25
Fig. 1. Auger peak to peak heights (Ap-pH) of Ni-61 eV and C-272 eV during C60 deposition on Ni(1 1 0) at 650 K and subsequent
heating of the C60 -saturated Ni(1 1 0) surface up to 900 K. The observed LEED patterns are also indicated.
fact, however, that this pattern appears at 0.19 of
the physical monolayer, indicates that C60 initially
forms islands, in agreement with reported observations [29].
Subsequent heating of the C60 -covered Ni(1 1 0)
surface causes a gradual decrease of the C-272 eV
peak, while the Ni-61 eV peak height increases
analogously up to 900 K where C has been totally
removed from the surface. At 700 K, the hcp
pattern becomes streaky along the ‰1 1 0Š direction
of the substrate. As the temperature increases to
750 K, a streaky …5 3† pattern appears at Ep ˆ
33:5 eV, coexistent with rings visible at Ep ˆ
59 eV. At 800 K, a well-resolved ring dominates in
the LEED pattern. This ring is formed by 12 diffused spots, and is superimposed with the …1 1†
pattern of the substrate. The above LEED patterns and the corresponding sketches are shown in
Fig. 2. Most likely, the observed ring at Fig. 2b
consists of two similar hexagons rotated by 25°
with respect to each other. Each hexagon corre which
sponds to a lattice constant of 2:1 0:1 A,
is 13% smaller than that of graphite (2.46 A).
Cepek et al. observed a …4 5† pattern at 760 K,
and a graphitic ring superimposed on the …4 5†
structure at 800 K which, however, are not suciently clear in the published ®gure [23]. They attributed this …4 5† structure to atomic carbidic
carbon formed on the surface after the fragmentation of the C60 molecules at 760 K. According to
the same authors, the rings are due to graphitic
domains formed on the Ni surface above 800 K.
Despite our e€ort we did not observe any …4 5†
pattern. Instead, heating at 750 K produced the
…5 3† pattern coexisting with rings. Most likely,
heating up to 750 K causes a partial fragmentation
of C60 molecules, while the remaining C60 molecules are rearranged in a …5 3† structure. The
fragments, lying on the Ni substrate, are structurally similar to a 2D graphite reduced in size by
13%. The divergence in the lattice constant from
that of bulk graphite, can be explained by the interaction of the graphite overlayer with the Ni
substrate, which causes a reduction in the C±C
bond distance due to their CT to the substrate.
This CT is consistent with the ®nding of Zandlberg
et al. that graphite islands on metal substrates reduce the WF [32]. Schouten et al. observed a
…4 5† carbon superstructure, which is formed on
Ni(1 1 0) surface at the temperature range 620±670
K, after the decomposition of methane [33].
However, prolonged heating in vacuum at T >
600 K caused the disappearance of the …4 5†
pattern due to the di€usion of carbon into the
bulk, while graphite rings were only observed in
crystals that had been heavily doped with carbon.
26
V. Saltas, C.A. Papageorgopoulos / Surface Science 488 (2001) 23±31
Fig. 2. LEED patterns during heating of the C60 -saturated at 650 K Ni(1 1 0) surface. (a) Streaky …5 3† structure observed after
heating at 750 K with Ep ˆ 34:5 eV. (b) Hexagonal rings superimposed to the …1 1† pattern of the substrate after heating at 800 K
(Ep ˆ 59 eV). The corresponding sketches of the above LEED patterns are also shown. Full circles denote the integer order spots of the
Ni substrate.
This ®nding suggests that annealing the C-…4 5†
structure to higher temperature should not result
in the observation of graphite rings, as seen by
Cepek et al., unless the Ni substrate is heavily
doped with carbon [23]. We also note that the
HREEL spectra of a monolayer of C60 on Ni(1 1 0)
measured by Cepek et al. at 760 K [23] show a
strong C60 contribution, indicating that a signi®-
cant amount of C60 remains on the Ni surface at
760 K. If this was ordered, it would contribute to
the observed …5 3† LEED pattern of C60 . The
decrease of C-272 eV peak by heating from 650 to
750 K (Fig. 1) may indicate that part of C60 fragments are desorbed from the surface. With increasing temperature up to 800 K, almost all of the
C60 molecules undergo fragmentation. As it is seen
V. Saltas, C.A. Papageorgopoulos / Surface Science 488 (2001) 23±31
27
Fig. 4. Total area of TD spectra of Fig. 3 versus C60 coverage.
Fig. 3. A series of TD spectra of atomic carbon for various
coverages of C60 deposited on Ni(1 1 0) at 650 K. The linear
heating rate is 15 K/s.
in Fig. 2b, the graphite-like rings coexist with intense integer order spots of the substrate. A complete fragmentation of a physical ML of C60 would
produce 5 MLs of atomic carbon, if all the fragments had remained on the surface. In this case,
we would expect the integer order beams of the
substrate to be very weak, in contrast to our observation. This argument, in correlation with the
drastic decrease of C-272 eV peak from 750 to 850
K, indicates that a substantial amount of carbon
has been removed from the surface in this temperature range. The mechanisms of C60 desorption
and/or decomposition during heating will be further discussed in correlation with the following
TDS and WF measurements.
Fig. 3 shows a series of TD spectra of atomic
carbon for di€erent coverages of C60 deposited on
Ni(1 1 0) at 650 K. For coverages up to 0.4 ML the
spectrum shows one single broad peak with maximum intensity at 875 K. Above 0.4 ML this peak
is reduced and a new peak appears at 900 K. With
increasing coverage the latter peak dominates and
shifts slightly to higher binding energy (BE), while
the ®rst peak disappears. We believe that at low
coverages the broad TD peak at 875 K is due
to the C±Ni binding, while at higher coverages the
C±C interaction dominates resulting the higher
energy peak. The binding energies of the above
peaks can be calculated by using an approximation
formula suggested by Seebauer [34]:
1
ln Y ln Y
Eb ˆ RT
…2
ln
Y
†
‡ Y ln Y ‡
2
Y
2Y 2
…1†
where Y ˆ ln…m0 T =b†. In the above expression we
assume a constant pre-exponential factor m0 1013
s 1 . We found that the BE corresponding to 875
and 920 K are 2.34 and 2.47 eV/atom, respectively.
These values are relatively small as compared to
those of carbides. This is consistent with our observation that the Auger lineshape of C-272 eV did
not show the characteristic shape of carbides at
temperatures up to 850 K. The small Auger C
signal, however, does not allow exclusion of the
28
V. Saltas, C.A. Papageorgopoulos / Surface Science 488 (2001) 23±31
possibility of C di€usion into the bulk or carbide
formation at higher temperatures. The TD spectra
of Fig. 3, suggest that there is no desorption of
atomic carbon below 750 K. Most likely, molecular desorption takes place, mainly, in the form
of Cn molecules with n > 10, since we did not
observe any TD spectra of Cn molecules with
n < 10. The desorption of elemental C above 800
K is due to the decomposition of the 2D graphitelike structure. The total area under the TD peaks
versus C60 coverage is shown separately in Fig. 4.
According to this ®gure, the area increases up to
0.4 MLs and subsequently decreases, i.e. the
amount of atomic carbon, which is desorbed from
the surface decreases substantially as the quantity
of C60 approaches the coverage of one saturated
layer. It seems that the desorption of C in the form
of large molecules (clusters) is more probable at
high coverages.
Fig. 5 shows the WF change during C60 deposition on Ni(1 1 0) surface at RT, 650 K and following heating of 1ML C60 -covered Ni(1 1 0) at
650 K. As it is seen in this ®gure, the WF at the
completion of 1 ML of C60 at 650 K and RT is 5.61
eV, considering that the WF of clean Ni(1 1 0) is
5.04 eV [35]. This value is substantially greater
than the WF of the bulk C60 , 4.7 eV [36]. The WF
initially increases almost linearly, then deviates
from the linearity due to the depolarization e€ect,
and reaches a maximum value. After the maximum value, the WF decreases slightly and levels
o€ at the value of 5.61 eV. The ®nal decrease and
leveling of the WF at RT may be attributed to a
metalization of C60 at the completion of 1 ML on
Ni(1 1 0), in agreement with the literature [29]. As
it is seen in Fig. 5, the WF curve at 650 K does not
reach the maximum value, and instead goes directly to the ®nal value of 5.61 eV. The fact that
the value of 5.61 eV is substantially greater than
4.7 eV of the bulk C60 WF may be due to the CT
from the Ni substrate to the C60 overlayer. The
di€erence between the WF curves at RT and 650 K
(Fig. 5) may be explained by the di€erent growth
mechanisms. At RT the molecules are randomly
distributed on the surface, while at 650 K, the
LEED patterns suggest that the C60 molecules
form ordered islands from the early stages of deposition and a coalescence of the islands leads to
the ®nal hcp structure. A possible greater sticking coecient at RT than at 650 K is discounted,
Fig. 5. WF change during C60 deposition on Ni(1 1 0) at 650 K and RT. The WF temperature dependence of the C60 -saturated at 650
K Ni(1 1 0) surface is also shown.
V. Saltas, C.A. Papageorgopoulos / Surface Science 488 (2001) 23±31
29
Fig. 6. Ap-pH of Ni-61 eV and C-272 eV during C60 deposition on Ni(1 1 0) at RT. The corresponding Ap-pHs at 650 K are also
shown for comparison.
because according to Fig. 6, the Auger variations
of C-272 eV and Ni-61 eV during deposition at RT
are quite similar to those at 650 K.
By heating the saturate monolayer of C60 , which
has been deposited at 650 K on Ni(1 1 0) surface,
the WF decreases initially by about 0.06 eV. This
decrease is probably due to the C60 fragmentation
and desorption of Cn molecules from the surface.
This is in agreement with the AES and LEED
measurements, since the Ap-pH decreases and the
LEED pattern goes back to the …5 3†. The latter
WF value remains nearly constant within the experimental error, up to 800 K, where the hexagonal ring dominates in the LEED pattern. Above
800 K the WF decreases drastically up to 900 K,
due to the desorption of C in molecular and/or
atomic form from the graphite-like layer bound
directly to the surface, in agreement with the TDS,
AES and LEED measurements.
It has been already reported that the electronic
structure of a C60 monolayer adsorbed on metal
surfaces is clearly metallic [29]. Maxwell et al.
concluded, from all cases studied thus far, that the
C60 adsorbed on metal surfaces appears to have a
WF close to 5 eV [28]. The authors, however, do
not have conclusive explanations for these results.
Tsuei et al., on the other hand, based on the previous report [28], suggest that the measured WFs
are a property of the metallic C60 overlayers that
give similar values (5 eV) regardless of the metal
substrate [31]. If we consider all the cases, which
have been studied thus far, however, the measured
WF values of 1 ML of C60 on di€erent metal substrates cover a wide range. As seen in Table 1 the
WF values of C60 -covered metal surfaces vary
from 4.82 to 5.61 eV. Based on this observation,
the deviations of the measured values from 5 eV
are large and cannot be considered as an error.
Probably, a redistribution of the charge between
C60 and the metal substrate re¯ects a more complex binding between them. This is responsible for
the electronic behavior and thus the WF of the C60
overlayer in contact to the metal substrate.
The question which arises from our measurements is why after the deposition of a C60 monolayer on Ni(1 1 0) surface the WF increases from
5.04 to 5.61 eV and does not remain near 5 eV
which has been assumed to be the value of C60
covered metal surfaces. According to Table 1 the
WF of C60 /metals, increases grossly for substrates
30
V. Saltas, C.A. Papageorgopoulos / Surface Science 488 (2001) 23±31
Table 1
WFs of clean metallic substrates and after the adsorption of
1 ML C60
Substrate
Us of clean
substrate (eV)
UC60 of 1 ML
C60 on metals
DU (eV)
Al(1 1 1)
Al(1 1 0)
Ta(1 1 0)
Ag(1 1 0)
Ni(1 1 0)
Cu(1 1 1)
Ni(1 1 1)
Rh(1 1 1)
Au(1 1 0)
4.25 0.05
4.35 0.05
4.80
4.52
5.04
4.94
5.36
5.40
5.37
5.15 0.05
5.25 0.05
5.4
4.9
5.61
4.86
4.93
4.95
4.82 0.05
‡0.95 0.07
‡0.95 0.07
‡0.6
‡0.38
‡0.57
0.08
0.43
0.45
0.45 0.05
The values have been taken from Refs. [4,6,26,28,37] and present work.
with low WF, whereas, for substrates with high
values …Us > 5 eV†, the WF changes are negative.
In the last cases the CT should be rather limited
and the WF values tend to approach the WF value
of bulk C60 , 4.7 eV. It is possible that, besides the
WF of the substrate, the nature of binding between
C60 molecules and the metal substrate plays some
role to the CT.
4. Conclusions
In this paper, we study the adsorption and desorption process of C60 on Ni(1 1 0) surfaces. The
investigation took place in UHV with the use of
LEED, AES, TDS and WF measurements. According to the LEED observations, deposited C60
at 650 K forms initially islands with …5 3† structure, which at the completion of one physical ML
changes to a hcp structure, in agreement with reported results. When C60 is deposited at RT it does
not form any ordered structures, while the sticking
coecient is nearly the same as at 650 K. Heating
of 1ML C60 -covered Ni(1 1 0) surface at 750 K,
causes a partial fragmentation of C60 molecules to
a 2D graphite-like layer, while the remained C60
molecules are rearranged to the …5 3† structure.
With increasing temperature to 800 K, the fragmentation of all C60 has been completed. Part of
the fragments has been desorbed as large molecules and the remained fragments participate the
2D graphite-like layer, which appears at two dif-
ferent orientations. With further heating, C is desorbed from the graphite-like layer bound directly
to the surface in molecular and/or atomic state.
There is no indication of atomic carbon on the
surface below 750 K. Our results suggest that the
decomposition/desorption mechanism is quite different than that reported by Cepek et al. The WF
value at saturation coverage was found to be 5.61
eV, for both deposition at RT and 650 K. The fact
that 1 ML of C60 on Ni(1 1 0) increases the WF to
5.61 eV, suggests that the WF of a C60 overlayer
on di€erent metal substrates is in¯uenced to some
extend by the WF of the latter.
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