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Reaction and passivation of aluminum with C60
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ELSEVIER
Surface
Science
318 (1994) 368-378
Reaction and passivation of aluminum with C,, Alex V. Hamza *, John Dykes, W. David Mosley, Long Dinh, Mehdi Balooch Chemistry and Materials Science Department, University of California, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA Received
7 February
1994; accepted
for publication
7 June 1994
Abstract
The interaction of C,, with aluminum was investigated using Auger electron spectroscopy (AES), temperature programmed desorption (TPD), photoluminescence and soft X-ray photoelectron spectroscopy. The interaction of C,, with aluminum was also probed with soft X-ray photoelectron spectroscopy. The bonding of C,, and C,, to the aluminum surface is strong such that after multilayer desorption at 570 and 620 K, respectively, a monolayer coverage of the fullerene remains intact, as seen in the valence band spectra, on the surface to 700 K. Above surface temperatures of 700 K the continued presence of a C,, photoluminescence signal and the decline in the surface carbon concentration suggests that the C,, dissolves into the aluminum bulk. The chemical shift in the binding energy of the Al 2p electrons also indicates strong interaction between C,, and aluminum. Multilayer and monolayer coverages of C, on aluminum passivated the surface such that exposures of 340 L of water at room temperature led to no oxidation of the surface. Multilayer coverages of C,, on aluminum were protective against oxidation at ambient conditions.
1. Introduction
of the C,,-sapphire bond is weak (less than the C,,-C,, surface bond), bonding of C,, to silicon
Recent studies of fullerenes have shown that these carbon cluster materials are not as inert as
is quite strong (greater than the C,,-C,, surface bond). Bonding of C,, to metal surfaces is also
originally believed and, in fact, have quite a rich chemistry [l]. The control of this chemistry may lead to the formation of novel compounds and materials. In this paper we show how the interaction of C, and C,, with aluminum leads to the formation of such novel materials. In previous investigations we have examined the interaction of C,, with insulator [2,3] and semiconductor surfaces [3,4]. While the strength
known. Altman and Colton [5] have shown that annealing of Au(ll1) with a monolayer coverage of C,, to 773 K removes C,, from the terraces as observed by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). However, Weaver and co-workers [6] investigating C,,-metal interactions with X-ray and vacuum ultraviolet photoelectron spectroscopy and inverse photoemission suggested the interaction is a van der Waals interaction with some charge transfer from the metal to the C,,. The nature of the C,,-metal interaction is not yet well understood.
* Corresponding
author.
Fax: + 1 510 423 7040.
0039-6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(94)00378-M
Science
B.V. All rights reserved
A.V. Hamza et al. /Surface Science 318 (1994) 368-378
Recent determinations of the solubility of novel fullerene carbon clusters have shown them to be insoluble in pure water [71. These hydrophobic properties have been observed to drastically reduce the sticking probability of water to C,,coated sapphire [2]. These results suggest that C,, overlayers may form a protective barrier against water contamination on surfaces. This possibility motivates an examination into the potential protection for surfaces which are readily oxidized by hydration. We have chosen to investigate the passivation properties of C,, on aluminum since aluminum readily oxidizes when exposed to water vapor [8].
2. Experimental The ultrahigh vacuum chamber (UHV) (base pressure < 2 X lo-” Torr) is equipped with a cylindrical mirror analyzer for Auger electron spectroscopy and a differentially pumped quadrupole mass spectrometer (QMS). The sample was a 0.25 mm thick polycrystalline aluminum foil with an exposed surface area of approximately 1 cm x 1 cm. The sample was resistively heated by directly passing current through it. The temperature was measured by a k-type thermocouple attached to the front face of the sample by a high temperature adhesive. The heating rate of the sample was computer controlled at 3 K/s. The QMS was likewise computer controlled and could be multiplexed to monitor four mass-to-charge ratios simultaneously. The sensitivity of the QMS to C,, at mass-to-charge ratio of 360 was determined previously [21. Sample cleaning was accomplished by a series of AI-+ sputtering (5 keV) and annealing (800 K) cycles. Sample cleanliness was determined by AES. Before exposure of C,, to the aluminum surface the oxygen(503 eV)-toaluminum(68 eV> AES peak height ratio was less than 0.05. Pure C,, and C,, in the form of - 5 pm size powders were purchased from Texas Fullerenes Corporation, who used the contact-arc preparation procedure for the initial C,,/C,, mixture production. Pure C,, and C,, were obtained by column chromatography on neutral alumina with
369
hexane. The C,, or C,, powder was placed in a gold foil envelope fastened to a button heater. Many pinholes were made in the foil to allow uniform deposition. The distance between the button heater and the aluminum sample during deposition was - 3 cm. The powders were outgassed at 500 K for 2 h before deposition. Deposition was accomplished by heating the envelope to over 800 K allowing C,, or Go molecules to evaporate from the envelope and deposit on the aluminum. The synchrotron-radiation photoelectron spectroscopy measurements were done at Stanford Synchrotron Radiation Laboratory (SSRL) on beamline 8-l. The UHV chamber (base pressure - 2 x lop9 Torr) was equipped with a load-lock sample transfer mechanism. A C,, or C,, doser described above was available for in situ sample preparation. Due to the higher base pressure the residual oxygen-to-aluminum ratio was as high as 0.1 prior to C, or C,, dosing. Photoelectrons were collected with an angle integrating double pass cylindrical mirror analyzer. Overall resolution of the spectrometer was - 0.2 eV based on the lo%-90% rise of the aluminum Fermi edge. Photoluminescence from the C,,-aluminum system was measured by excitation with an argon ion laser operating at 488 nm and collecting the luminescence light with an Oriel l/8 meter monochromator and cooled (253 K> silicon diode array. A 550 nm cutoff filter was placed in the collection optics to remove any scattered excitation light. Excitation light and luminescence light was brought into and out of the UHV chamber through a fused silica window. As a check, no photoluminescence was observed from the clean aluminum substrate.
3. Results and discussion 3.1. Temperature programmed desorption and Auger electron spectroscopy of C,, on aluminum
The clean aluminum surface was exposed to C,, to produce multilayer coverages of C,,. An AES spectrum taken after multiple layers of C,, deposition showed only the characteristic 272 eV
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C,, (carbon) peak (i.e. no signal at 68 and 503 eV). Temperature programmed desorption (TPD), monitoring mass-to-charge ratio of 360 amu, was performed to 600 K revealing the desorption of the multilayer at 500 to 570 K. The peak shape and coverage dependence were indicative of zero-order kinetic behavior as seen previously for sapphire and silicon [2,3]. In addition, analysis of the TPD signal gives an activation energy for desorption of 33 f 2 kcal/mol in agreement with previous measurements [2,3]. The pre-exponential factor for the zero-order desorption is 9.3 X 1012 monolayers/s. The TPD signal was calibrated by experiments from Al,O, described previously [2]. AES of the surface following the TPD experiment to 600 K showed that a single layer C,, remained on the surface. The C,, is intact which is shown by the photoelectron spectroscopy discussed below. TPD experiments in which the temperature ramp was continued to 850 K revealed no desorption of C,, at surface temperatures between 600 and 850 K. Species with a mass-to-charge ratio greater than 200 (integral mode) were also monitored and not observed during TPD. A series of annealing experiments were performed monitoring the carbon(272 eV)-to-aluminum(68 eV> AES peak height ratio after desorption of the C,, multilayer at 570 K. Fig. 1 shows the results of such a series of anneals from 570 to 825 K. The carbon-to-aluminum ratio is constant
550
600
650
700
surface Anneal mnperatun
750
800
850
(K)
Fig. 1. Plot of the carbon(272 eV)-to-aluminum(68 eV) AES peak height ratio as a function of surface anneal temperature from 570 to 825 K.
Science 318 (I 994) 368-378
at - 3.0 from 570 to 650 K. This ratio is independent of the initial multilayer coverage (i.e. a single layer C,, is present). The carbon AES peak height is monitored before and after each TPD measurement. The decrease in the AES peak height is related to the amount of desorbed C,, in a nonlinear manner due to the escape depth of the Auger electrons. The escape depth effect is minimized by measuring the change of the AES carbon peak height at small coverages larger than a single layer, where the response is nearly linear. Thus, for example if 0.05 x 1014 C,, nanoclusters/cm2 desorbed and the peak height is changed by 5% then the remaining AES peak corresponds to - 1 x 1014 C,, nanoclusters/cm2. The change in the AES carbon peak height and the amount of desorbed multilayer determined from TPD give an estimate of 1.7 monolayer as the full C,, single layer coverage of the aluminum sample (one monolayer is 1.21 X lOi molecules/cm2). That full coverage is greater than one monolayer is attributed to surface roughness of the aluminum foil. From 650 to 775 K the carbon-toaluminum ratio decreases from - 3 to N 0.1 and is constant at - 0.1 from 775 to 825 K. The AES carbon peak shape is unchanged (i.e. graphitelike) as a function of anneal temperature, suggesting the chemical interaction of the remaining surface carbon is also unchanged (i.e. not carbidic). Since the C, does not thermally desorb in this temperature range, the decline in the carbon signal is due either to fragmentation and subsequent desorption or to dissolution of the C,, or carbon into the aluminum bulk. In this and subsequent sections of the present paper evidence will be presented that indicates the C,, dissolves intact into the aluminum between 650 and 775 K. The temperature dependence of the carbon/ aluminum peak ratio demonstrates strong single layer bonding of C,, to a clean aluminum substrate. Although desorption did not occur, the minimum bond strength was estimated as if desorption had occurred in the temperature range from 650 to 775 K, since annealing to surface temperatures below this range had no effect on the single layer C,,. Assuming first-order reaction kinetics and an exponential prefactor of 1013 s-l and using the data of Fig. 1, one estimates a
A. K Hamza et al. /Surface
lower bound to the binding energy to be 44 kcal/mol. This estimated minimum binding energy is limited by the assumed first-order prefactor of 1013 s-l. As a test of whether the material dissolves into the bulk or desorbs, an initially clean sample was held at 750 K and exposed to C,, for 30 min (an exposur: that at room temperature would lead to N 500 A thick C,, multilayer film). If the C,, fragments and desorbs then no change in the sample would be expected. However, a visible change was detected. The sample changed to a gold color on the sputter cleaned area of the sample. AES revealed a carbon-to-aluminum ratio of approximately 2. Fig. 2 compares the morphology of the sputter cleaned aluminum annealed to 750 K to that of the aluminum exposed to C,, at 750 K as seen by scanning electron microscopy. A very dramatic change in the surface texture is observed. Fig. 3 shows a comparison of an ex situ measure of the reflectivity of the cleaned aluminum surface and the C, exposed (at 750 K> sample. The lowest trace in Fig. 3 is a difference curve between the reflectivity of each sample. The decreased reflectivity of the C,, ex-
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posed sample manifests itself in the peak in the difference curve at 450 nm. This peak corresponds to a strong absorption feature for Cm. This experiment strongly indicates that the disappearance of carbon on the surface due to annealing above 650 K is due to dissolution into the aluminum bulk. Further, the reflectivity measurement and color change is suggestive that the C,, dissolves at least partially intact. The temperature range at which dissolution occurs is similar to the temperature range that aluminum surface thermal disordering occurs [9,101. True surface melting of the Al(110) surface does not occur until the surface temperature reaches > 800 K as measured by medium energy ion scattering [ll]. However, as shown by low energy electron diffraction, the mean square displacements of the aluminum atoms become anomalously large at surface temperatures greater than 620 K [lo]. A possible mechanism for the dissolution may involve large displacements of the surface allowing aluminum atoms to react with the C,, cage and subsequently leading to the aluminum surface engulfing the C,, molecule. The rough morphology seen in the scanning elec-
Fig. 2. Scanning electron micrograph of (a) cleaned aluminum surface annealed in vacuum to 750 K subsequently ambient conditions compared to (b) aluminum surface exposed to C,, at 750 K for 30 min also subsequently exposed conditions. Magnification is 30000 times and the 1 pm fiducial is shown by the dotted line.
exposed to to ambient
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tron micrograph (see Fig. 2) may be a result of this engulfing mechanism.
Fig. 4 shows the valence band photoelectron spectrum from “clean” aluminum collected with 75 eV photon energy. The valence band spectrum is very sensitive to the surface cleanliness. Any residual oxygen appears as a broad peak at 7.5 eV below the valence band m~mum (VBM). The amount of residual oxygen can be determined from the Al 2p photoelectron feature shown in Fig. 5. The signal with binding energy greater than 73.6 eV is due to Al atoms in AIO, oxidation state; the ratio of the area of signal with binding energy greater than 73.6 eV to the area of signal with binding energy at 72.5 eV is 0.09. Assuming x is near one gives an oxygen coverage of -b.O9 monolayer (where one monolayer is one oxygen atom per aluminum atom),
Energy
wn Fermi Level (ev)
Fig. 4. Valence band photoelectron spectra for multilayer C6a on aluminum (solid line), single layer C,, on aluminum (dotted line), and “clean” aluminum (dot-dashed line). The spectra are collected with a photon energy of 75 eV. Overall resolution of the system is determined by the lo%-90% rise of the aIuminum Fermi edge to be - 0.2 eV. Energy scale is in eV with respect to the Fermi energy.
Fig. 3. Reflectivity as a function of wavelength from 250 to 840 nm for cleaned aluminum subsequently exposed to ambient conditions (dashed line) and aluminum surface exposed to C, at 750 K for 30 min also subsequently exposed to ambient conditions (solid line). A difference curve is also plotted (dotted line).
since at 130 eV photon energy the measurement is surface sensitive. The residual oxygen could not be removed completely due to the poor vacuum, less than 3 x 10e9 Torr, and less than optimal sputtering geometry. Fig. 4 also shows the valence band spectra for multilayer and single layer C, coverage of aluminum (here single layer refers to the amount remaining after annealing the surface to 600 IQ. The multilayer spectrum is similar to that obtained by Weaver and co-workers 161 for C,, multilayers on GaAs. The two sharp features at 2.2 and 3.6 eV below the VBM are r-derived states and the features at 5.8, 8.2, and 10.3 eV are a-derived states of C,,. The highest occupied molecular orbital (HOMO) is at 2.2 eV below the Fermi level, just as in other multilayer films [6]. After heating the surface to 600 K, at which
A. K Hamza et al. /Surface
temperature the brownish color from the multilayer disappears, the single layer (1.7 monolayer, where one monolayer is 1.21 x 1014 molecules/ cm*> C,, film is present. The valence band spectrum clearly shows the broadened r-derived states which are shifted by 0.3 eV to higher binding energy. The a-derived states are still discernible but partially obscured by the broad oxygen feature from the residual surface oxygen. This result indicates the C, single layer on aluminum is made up of intact C,, cages. The shift to higher binding energy may be explained by a charge transfer from the C,, to the aluminum, which seems unlikely due to the high electron affinity of C,, [12]. Another possibility is that the screening of the final state is less effective for the single layer C,, on aluminum thus increasing the binding energy of the a-derived states. A comparison of the Al 2p photoelectron spectrum for the “clean” aluminum surface and the single layer C,, surface annealed to N 750 K is
40 . _ :
C, single layer annealed to -750 K “clean aluminum”
30 -
78
76
74
72
70
Binding Energy (eV)
Fig. 5. Al 2p photoelectron spectra for “clean” aluminum (dotted line) and single layer C, on aluminum annealed to - 750 K (solid line). Photon energy is 130 eV. Energy scale is binding energy in eV, where the aluminum metal peak is set at a binding energy of 72.5 eV.
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shown in Fig. 5. The multilayer and the single layer C, coverages on aluminum suppress the Al 2p peak such that the chemical shifts of the Al 2~‘s cannot be observed. However, annealing to 750 K causes dissolution of the surface C,, into the bulk (see Section 3.1 above). In the Al 2p photoelectron spectrum both the aluminum metal peak (binding energy of 72.5 eV) and a new peak at lower binding energy (71.7 eV) are observed. Assuming the C, dissolved in the bulk is intact, this observation could also be attributed to a charge transfer from the C, to the aluminum, which is consistent with the 0.3 eV shift in the rr-derived states to higher binding energy. However, since many other metals C, bonding systems involve charge transfer from the metal to C,, [6], it seems unlikely that the chemical shift observed here is due to charge transfer. Again, the chemical shift could also be ascribed to final state effects where the Al 2p electron binding energy for aluminum atoms bound to C,, would have a lower binding energy since screening of the final state hole would be easier. The C, molecule is polarizable as measured by its nonlinear optical properties. The polarizable C,, would then be able to screen core hole on aluminum atoms. The effect can be large and is observed in alkali halides where the halide ion is more polarizable than the alkali ion [13,14]. This lower binding energy for the Al 2~‘s is observable in this case because of the surface sensitivity using 130 eV photons and the intimate mixing of the C,, and aluminum in the dissolution layer. The behavior of C,, on aluminum is similar to that of C,,. Fig. 6 shows a comparison of the valence band photoelectron spectrum for the C,, multilayer to the C, multilayer. The shape and position of the C,, multilayer features agree with the measurements of Weaver and co-workers [15]. However, we discern only two features in each of the r-derived peaks for C,, as opposed to the three and two features of the r-derived peaks as seen by Jost et al. [15]. The splitting of the features is assigned to the reduced symmetry of the C,, relative to C,, removing some of the degeneracy in the molecular orbitals. Fig. 7 shows how the valence band photoelectron spectrum changes as the C,, exposed surface
A. % Hamza et al. /Surface
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is annealed. The multilayer C, is a purple color; heating the surface to 610 K causes the sample color to change to the original silver color of the “clean” aluminum sample. The photoelectron spectrum for this surface is labeled C, “monolayer”. Two points are to be made. First, the second T-derived feature is no longer discernible. Second, the first r-derived and the a-derived features are still present, suggesting the C, cage structure may still be intact. Thus, like C,, C,, is strongly bound to the aluminum surface. The disappearance of the second n--derived feature suggests that aluminum-C,, “monolayer” bond may involve bonding through these n-derived features. However, no discernible energy shift in the other features is observed, unlike the C, single layer, even though a similar chemical shift to lower binding energy for the aluminum 2p core level is seen (see below). Annealing the C,, monolayer to 650 K reduces the intensity of the C, “monolayer” fea-
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2.5x
0.0
4
16
a
’ 4 14
’ 12
’ 4
’
10
’
’ c’ 8
’
’b 6
’ *
’ 4
d’ ’ ’ 8 ’ ’ 2 0
Energy wn Fermi Level (eV)
Fig. 7. Valence band photoelectron spectra for (from the top) multilayer C,,, monolayer C,o, monolayer C, annealed to 650 K and “clean” aluminum. Photon energy is 75 eV. Enera; scale is in eV with respect to the Fermi energy.
energy
wt to Fermi Level WI
Fig. 6. Valence band phot~lectron spectra for multi~ayer C, (solid line) and multilayer C,, (dashed line). The spectra are collected with a photon energy of 65 eV. Energy scale is in eV with respect to the Fermi energy.
tures and allows the aluminum Fermi level to be observed. However, the position of the features remains unchanged. This is evidence of the beginning stage of dissolution of the C,, monolayer without changing the structure of the surface C,,. Because this is the early stage of dissolution only aluminum atoms interacting with the C,, monolayer can be observed in the photoelectron spectroscopy. Fig. 8 shows a comparison of the Al 2p photoelectron spectra for “clean” aluminum and the C,, “monolayer” annealed to 650 K. Since after annealing to 650 K the surface coverage of C,, is still high only a small Al 2p signal is observable. In order to make the comparison easier, the C,, annealed to 650 K trace was multiplied by seven. The binding energy of the Al 2~‘s for aluminum metal is set at 72.5 eV and the Al 2p peak for the C, layer heated to 650 K is observed at 71.6 eV, again exhibiting a similar chemical shift to lower binding energy as seen with C,,.
A.V. Hamza et al./Surface
3.3. Photoluminescence aluminum
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of C,, on and in
Fig. 9 shows the photoluminescence spectra from an aluminum-C,, sample prepared in the following manner. The aluminum surface was sputter cleaned and annealed to 800 K until the oxygen(503 eV)-to-aluminum(68 eV) AES peak height ratio was less than 0.04. Then the surface was heated to 750 K and exposed to C, for 30 min. After 30 min the surface was cooled to room temperature while dosing C,,. The dosing is continued until a C,, multilayer is visible. The sample is removed from UHV and transferred to UHV in the photoluminescence apparatus. The sample is heated to 400 K to thoroughly outgas the sample and holder. AES of the sample shows only the 272 eV carbon peak as expected. Photo-
6-
Monolayer Cm and
Multilayer C,
T 0 500
hk130eV
.
.
600 Wavelength (nm)
20 -
-
c,,
Fig. 9. Photoluminescence spectra for multilayer Cm (lowest trace), C,, monolayer on C,, dissolved in aluminum (middle trace) and for C, dissolved in aluminum (upper trace). See text for sample preparation. Excitation wavelength is 488 nm. Cutoff filter (550 nm) is part of detection optics.
monolayer
annealed to -650 K (x7) C,, monolayer -650 K . . . “clean aluminum”
annealed to
B B 2
I5 -
I s B 3 Y ‘1 9
78
76
74
72
70
Binding Energy (eV)
Fig. 8. Al 2p photoelectron spectra for “clean” aluminum (dashed line), “monolayer” C, annealed to 650 K (solid line), and “monolayer” C,, annealed to 650 K multiplied by 7 (solid line). Photon energy is 130 eV. Energy scale is binding energy in eV, where the aluminum metal peak is set at a binding energy of 72.5 eV.
luminescence of this sample is shown in Fig. 9 labeled multilayer C,,. The sample was then heated in UHV to N 650 K to remove the multilayer; photoluminescence of the heated sample is also shown in Fig. 9 labeled monolayer C,, and dissolved C,,. For comparison purposes this trace is multiplied by 250. Finally, the sample was heated to N 750 K to dissolve the monolayer into the aluminum-C, composite; photoluminescence of the N 750 K heated sample is the uppermost trace in Fig. 9 labeled C,, dissolved in Al. Again for comparison purposes this trace is multiplied by 500. The sample was then sputter cleaned. The clean aluminum surface exhibits no photoluminescence signal. From the sputtering time and the sputter removal rate of aluminum and C,,, the depth of dissolution was estimated to be N 250 A. The carbon concentration decreased monotonically; however, since the reac-
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tion of C,, with the surface makes the surface very rough, shadowing effects make the depth profile measurement uncertain. The photoluminescence data are suggestive that C, dissolves intact into aluminum, but not conclusive. The small shifts in the C, photoluminescence may be due to interaction of C,, with aluminum. However, that the shifts are due to C,, fragments dissolved in aluminum with similar photoluminescence spectrum to that of C, cannot be ruled out. Note: A single crystal aluminum (111) sample was exposed to C, and annealed to various surface temperatures up to 900 K. The AES and TPD behavior was the same as observed on the aluminum foil. Contamination of the sample surface with 0.1 monolayer oxygen had no effect on the C,,-aluminum reaction except to lower the available surface area for reaction. Apparently, the oxygen-aluminum interaction is so strong; once aluminum has reacted with oxygen, C,, does not have any effect on the aluminum-oxygen bond. 3.4. Passiuation
of aluminum
by C,,
In order to establish a foundation for comparison, we first oxidized a clean aluminum surface without the fullerene layers by exposing the surface to water vapor. A series of Auger spectra detailing this exposure are shown in Fig. 10. The aluminum sample was sputtered and annealed at 800 K to produce the clean surface. After this heating, the oxygen(503 eV)-to-aluminum(68 eV) AES peak height ratio was less than 0.04. Since the relative Auger sensitivity for oxygen is considerably higher than that of aluminum, these data indicate the presence of a pure aluminum surface. This sample was then exposed, at room temperature, to 340 L (one langmuir, 1 L, is an exposure of 10vh Torr * s) of water vapor (working base pressure: less than 10e9 Torr) emanating from a tube positioned Iess than 1 cm from the surface. The sample was again examined with the Auger spectrometer to determine the level of oxidation. The magnitude of the resulting aluminum peak was reduced and a significant oxygen presence was determined. The sample was
after340 L H,O
exposure
clean aluminum
Elecmn Energy (eV)
Fig. 10. A series of AE spectra detailing the oxidation of aluminum with water vapor. The lowest trace is from clean aluminum, sputtered and annealed to 800 K. The middle trace is from the clean surface exposed to 340 L of water vapor. The upper trace is from the water exposed surface heated to 800 K, heating rate is 3 K/s.
next heated to 800 K in a TPD cycle to determine desorption species. While a quantitative discussion of the desorbed species is beyond our present scope, we have identified QMS signals associated with hydrogen and water in this desorption. These results define the surface oxidation of unprotected aluminum for comparison to the C, protected surface when exposed to water vapor. The strong bonding of C, to the aluminum surface and the hydrophobic nature of C, begs for an investigation of the passivation of the aluminum surface against oxidation. In a typical experiment, this investigation was begun by cleaning the aluminum surface via the Ar+ sputtering and annealing to 800 IS (oxygen-to-aluminum AES peak height ratio less than 0.04) (see Fig. 11). Next, the C,, multilayers were adsorbed onto the surface. The deposition time during this typical
A. K Hamza et al. /Surface
afterheating to 600
after340
K
L H,O
clean aluminum
-0.5 -
-l.c..‘.‘.‘..‘“““““““‘““‘““” 100
200
300
400
500
600
Ekctron energy (ev)
Fig. 11. A series of AE spectra detailing the passivation of aluminum by multilayer and single layer coverage of Cm. The traces are displayed in chronological order form the bottom to the top. The lowest trace is for the sputter cleaned and annealed to 800 K sample. The next trace is for the clean surface exposed to C, for sufficient time to form a multilayer coverage. Each succeeding trace is for the sample receiving the labeled treatment and all previous treatment.
run was 12 min. An Auger spectrum, once again verified the existence of the multilayer coverage. The surface was then exposed to 340 L of water vapor (working pressure < 10m9 Tori-1 and the post-hydration Auger spectrum shows very little change from the pre-hydration. Most notably, there is no evidence of change in the intensity of the oxygen peak (503 eV> after exposure to water vapor. TPD measurements on the multilayer covered surface performed after hydration indicate desorption of the C,, multilayers (m/e = 360) upon heating to 600 K as was observed in the non-hydrated multilayer desorption described in Section 3.1. Also observed in this desorption were small amounts of hydrogen (m/e = 2) and water vapor
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(m/e = 181, however no signatures of oxygen desorption were present. In searching for any desorbed oxygen, various reasonable charge-to-mass ratios were considered in QMS monitoring (i.e. hydroxides, O,, C,O). After the TPD heating of the surface to 600 K, the presence of a C, monolayer was again verified using AES (Fig. 11). As indicated in the previous analysis of single layer coverages, the aluminum peak (68 eV) becomes apparent again due to the proximity of metallic aluminum to the surface. As expected, no significant oxygen peak is observed in this Auger spectrum. This single layer covering of fullerenes, formed by multilayer adsorption onto clean aluminum followed by sample heating past 600 K, was then exposed to the typical 340 L of water vapor. The post-hydration Auger spectrum for single layer coverage (Fig. 11) still indicates the same relative peak sizes of carbon to aluminum with no significant change in the oxygen peak. This Auger spectrum indicates a key result. Due to its hydrophobic nature and ease of uniformity in deposition, a single layer of C,, deposited on an aluminum surface will protect that surface from oxidation. Another temperature program desorption experiment, this time performed to 800 K, shows the usual background signal of hydrogen (m/e = 2) and a small water vapor background signal (m/e = 18) but indicates no desorption of C,, (m/e = 360) as expected. However, a subsequent Auger spectrum (Fig. 11) indicates that, while still present, the carbon peak (272 eV> is significantly reduced in intensity due to dissolution of the C,. Correspondingly, the intensity of the aluminum peak increases to almost the previous intensity levels for clean aluminum. As a final test of passivation, the aluminum and absorbed fullerene mixture was again exposed to 340 L of water vapor. While not presented here, this sample showed partial oxidation of the clean aluminum, albeit not to the levels of unprotected aluminum. We have also attempted to remove a C,, multilayer covered, sputter cleaned and annealed aluminum sample from ultrahigh vacuum (UHV) and replace it UHV to test the extent of C, passivation. The C, vacuum interface is free of any oxygen contamina-
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et al. /Surface Science 318 (1994) 368-378
tion after annealing to 400 K. Care must be taken to completely outgas the sample holder and supports after replacing the sample in UHV. Heating the sample to 600 K yields nearly a monolayer coverage of C,,. However, a small oxygen contamination is present due to the outgassing of the supports (1 X lo-* Torr) which expose the surface to oxygen containing gases while the sample is at elevated temperature. Perhaps some of the C,, reacts with these oxygen containing species at temperatures above 470 K leaving unprotected regions of the surface which can be oxidized by the outgassing. These results agree well with experiments by Hong et al. (16,171 where C, encapsulation was shown to protect surface reconstructions of Si(lll)-(7 x 7) and Si(lOO)-(2 x 1) to ambient exposure although without removal of the C, film. We have seen similar results with respect to C,, multilayer protection of $000) to ambient exposure by examining the interface by sputter depth profiling. In this case no outgassing occurs and no oxygen concentration is observable with Auger electron spectroscopy [18].
4. ~urnrna~ Three major observations were made in the course of this investigation. One, C,, dissolves into aluminum either intact or as C,, fragments at temperatures above 700 K. Two, C,, (C,,> is strongly bound to aluminum. The strength of the C,-aluminum interaction is also manifested in the aluminum Zp core level binding energy, which exhibits a chemical shift to lower binding energy. Three, the highly reactive aluminum surface may be passivated by a single layer C, coverage to oxidation by water vapor.
Acknowledgments
A.V.H. gratefully acknowledges Dr. L.J. Termine110 and Dr. G.D. Waddill for their help in
bringing the SSRL experiment on-line and operating and for their helpful discussions. A.V.H. also acknowledges R. Hill’s expert assistance in bringing the SSRL experiment on-line. This work was supported by the Division of Materials Science, Office of Basic Energy Sciences, U.S. Department of Energy, at Lawrence Livermore National Laboratory under contract No. W-7405 ENG-48. References
Ill R. Taylor and D.R.W. Walton, Nature 363 (1993) 685. I21A.V. Hamza and M. Balooch, Chem. Phys. Lett. 198 (1992) 603. 131A.V. Hamza and M. Balooch, Chem. Phys. Lett. 210 (1993) 404. [41 M. Balooch and A.V. Hamza, Appl. Phys. Lett. 63 (1993) 150. [51 E.I. Altman and R.J. Colton, Surf. Sci. 279 (1992) 49. 161T.R. Ohno, Y. Chen, SE. Harvey, G.H. Kroll, J.H. Weaver, R.E. Haufler and R.E. Smalley, Phys. Rev. B 44 (1991) 13747. I71 T. Andersson, K. Nilsson, M. Sundahl, G. Westman and 0. Wennerstrijm, J. Chem. Sot. Chem. Commun. 8 (1992) 604. 181 C.F. McConville, D.L. Seymour, D.P. Woodruff and S. Bao, Surf. Sci. 188 (1987) 1. [91 A. Pavlovska, M. Tikhov, Y. Gu and E. Bauer, Surf. Sci. 278 (1992) 303. llOl P. von Blanckenhagen, W. Schommers and V. Voegele, J. Vat. Sci. Technol. A5 (1987) 649. [111 A.W. Denier van der Gon, R.J. Smith, J.M. Gay, D.J. O’Conner, J.F. van der Veen, Surf. Sci. 227 (1990) 143. 1121 S.H. Yang, CL. Pettiette, J. Conceicao, 0. Chesnovsky and R.E. Smalley, Chem. Phys. Lett. 139 (1987) 233. t131 C.S. Fadley, S.B.M. Hagstrom, M.P. Klein and D.A. Shirley, J. Chem. Phys. 48 (1968) 3779. 1141P.H. Citrin and T.D. Thomas, J. Chem. Phys. 57 (1972) 4446. 1151 M.B. Jest, P.J. Benning, D.M. Poirier, J.H. Weaver, L.P.F. Chibante and R.E. Smalley, Chem. Phys. Lett. 184 (1991) 423. [I61 H. Hong, W.E. McMahon, P. Zschack, D.-S. Lin, R.D. Aburano, H. Chen and T.-C. Chiang, Appl. Phys. Len. 61 (1992) 3127. [171 H. Hong, R.D. Aburano, E.S. Hirshorn, P. Schack, H. Chen and T.-C. Chiang, Phys. Rev. B 47 (1993) 6450. USI M.A. Schildbach, M. Balooch and A.V. Hamza, unpublished results.
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ELSEVIER
Surface
Science
318 (1994) 368-378
Reaction and passivation of aluminum with C,, Alex V. Hamza *, John Dykes, W. David Mosley, Long Dinh, Mehdi Balooch Chemistry and Materials Science Department, University of California, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA Received
7 February
1994; accepted
for publication
7 June 1994
Abstract
The interaction of C,, with aluminum was investigated using Auger electron spectroscopy (AES), temperature programmed desorption (TPD), photoluminescence and soft X-ray photoelectron spectroscopy. The interaction of C,, with aluminum was also probed with soft X-ray photoelectron spectroscopy. The bonding of C,, and C,, to the aluminum surface is strong such that after multilayer desorption at 570 and 620 K, respectively, a monolayer coverage of the fullerene remains intact, as seen in the valence band spectra, on the surface to 700 K. Above surface temperatures of 700 K the continued presence of a C,, photoluminescence signal and the decline in the surface carbon concentration suggests that the C,, dissolves into the aluminum bulk. The chemical shift in the binding energy of the Al 2p electrons also indicates strong interaction between C,, and aluminum. Multilayer and monolayer coverages of C, on aluminum passivated the surface such that exposures of 340 L of water at room temperature led to no oxidation of the surface. Multilayer coverages of C,, on aluminum were protective against oxidation at ambient conditions.
1. Introduction
of the C,,-sapphire bond is weak (less than the C,,-C,, surface bond), bonding of C,, to silicon
Recent studies of fullerenes have shown that these carbon cluster materials are not as inert as
is quite strong (greater than the C,,-C,, surface bond). Bonding of C,, to metal surfaces is also
originally believed and, in fact, have quite a rich chemistry [l]. The control of this chemistry may lead to the formation of novel compounds and materials. In this paper we show how the interaction of C, and C,, with aluminum leads to the formation of such novel materials. In previous investigations we have examined the interaction of C,, with insulator [2,3] and semiconductor surfaces [3,4]. While the strength
known. Altman and Colton [5] have shown that annealing of Au(ll1) with a monolayer coverage of C,, to 773 K removes C,, from the terraces as observed by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). However, Weaver and co-workers [6] investigating C,,-metal interactions with X-ray and vacuum ultraviolet photoelectron spectroscopy and inverse photoemission suggested the interaction is a van der Waals interaction with some charge transfer from the metal to the C,,. The nature of the C,,-metal interaction is not yet well understood.
* Corresponding
author.
Fax: + 1 510 423 7040.
0039-6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(94)00378-M
Science
B.V. All rights reserved
A.V. Hamza et al. /Surface Science 318 (1994) 368-378
Recent determinations of the solubility of novel fullerene carbon clusters have shown them to be insoluble in pure water [71. These hydrophobic properties have been observed to drastically reduce the sticking probability of water to C,,coated sapphire [2]. These results suggest that C,, overlayers may form a protective barrier against water contamination on surfaces. This possibility motivates an examination into the potential protection for surfaces which are readily oxidized by hydration. We have chosen to investigate the passivation properties of C,, on aluminum since aluminum readily oxidizes when exposed to water vapor [8].
2. Experimental The ultrahigh vacuum chamber (UHV) (base pressure < 2 X lo-” Torr) is equipped with a cylindrical mirror analyzer for Auger electron spectroscopy and a differentially pumped quadrupole mass spectrometer (QMS). The sample was a 0.25 mm thick polycrystalline aluminum foil with an exposed surface area of approximately 1 cm x 1 cm. The sample was resistively heated by directly passing current through it. The temperature was measured by a k-type thermocouple attached to the front face of the sample by a high temperature adhesive. The heating rate of the sample was computer controlled at 3 K/s. The QMS was likewise computer controlled and could be multiplexed to monitor four mass-to-charge ratios simultaneously. The sensitivity of the QMS to C,, at mass-to-charge ratio of 360 was determined previously [21. Sample cleaning was accomplished by a series of AI-+ sputtering (5 keV) and annealing (800 K) cycles. Sample cleanliness was determined by AES. Before exposure of C,, to the aluminum surface the oxygen(503 eV)-toaluminum(68 eV> AES peak height ratio was less than 0.05. Pure C,, and C,, in the form of - 5 pm size powders were purchased from Texas Fullerenes Corporation, who used the contact-arc preparation procedure for the initial C,,/C,, mixture production. Pure C,, and C,, were obtained by column chromatography on neutral alumina with
369
hexane. The C,, or C,, powder was placed in a gold foil envelope fastened to a button heater. Many pinholes were made in the foil to allow uniform deposition. The distance between the button heater and the aluminum sample during deposition was - 3 cm. The powders were outgassed at 500 K for 2 h before deposition. Deposition was accomplished by heating the envelope to over 800 K allowing C,, or Go molecules to evaporate from the envelope and deposit on the aluminum. The synchrotron-radiation photoelectron spectroscopy measurements were done at Stanford Synchrotron Radiation Laboratory (SSRL) on beamline 8-l. The UHV chamber (base pressure - 2 x lop9 Torr) was equipped with a load-lock sample transfer mechanism. A C,, or C,, doser described above was available for in situ sample preparation. Due to the higher base pressure the residual oxygen-to-aluminum ratio was as high as 0.1 prior to C, or C,, dosing. Photoelectrons were collected with an angle integrating double pass cylindrical mirror analyzer. Overall resolution of the spectrometer was - 0.2 eV based on the lo%-90% rise of the aluminum Fermi edge. Photoluminescence from the C,,-aluminum system was measured by excitation with an argon ion laser operating at 488 nm and collecting the luminescence light with an Oriel l/8 meter monochromator and cooled (253 K> silicon diode array. A 550 nm cutoff filter was placed in the collection optics to remove any scattered excitation light. Excitation light and luminescence light was brought into and out of the UHV chamber through a fused silica window. As a check, no photoluminescence was observed from the clean aluminum substrate.
3. Results and discussion 3.1. Temperature programmed desorption and Auger electron spectroscopy of C,, on aluminum
The clean aluminum surface was exposed to C,, to produce multilayer coverages of C,,. An AES spectrum taken after multiple layers of C,, deposition showed only the characteristic 272 eV
A. V. Hamza et al. /Surface
370
C,, (carbon) peak (i.e. no signal at 68 and 503 eV). Temperature programmed desorption (TPD), monitoring mass-to-charge ratio of 360 amu, was performed to 600 K revealing the desorption of the multilayer at 500 to 570 K. The peak shape and coverage dependence were indicative of zero-order kinetic behavior as seen previously for sapphire and silicon [2,3]. In addition, analysis of the TPD signal gives an activation energy for desorption of 33 f 2 kcal/mol in agreement with previous measurements [2,3]. The pre-exponential factor for the zero-order desorption is 9.3 X 1012 monolayers/s. The TPD signal was calibrated by experiments from Al,O, described previously [2]. AES of the surface following the TPD experiment to 600 K showed that a single layer C,, remained on the surface. The C,, is intact which is shown by the photoelectron spectroscopy discussed below. TPD experiments in which the temperature ramp was continued to 850 K revealed no desorption of C,, at surface temperatures between 600 and 850 K. Species with a mass-to-charge ratio greater than 200 (integral mode) were also monitored and not observed during TPD. A series of annealing experiments were performed monitoring the carbon(272 eV)-to-aluminum(68 eV> AES peak height ratio after desorption of the C,, multilayer at 570 K. Fig. 1 shows the results of such a series of anneals from 570 to 825 K. The carbon-to-aluminum ratio is constant
550
600
650
700
surface Anneal mnperatun
750
800
850
(K)
Fig. 1. Plot of the carbon(272 eV)-to-aluminum(68 eV) AES peak height ratio as a function of surface anneal temperature from 570 to 825 K.
Science 318 (I 994) 368-378
at - 3.0 from 570 to 650 K. This ratio is independent of the initial multilayer coverage (i.e. a single layer C,, is present). The carbon AES peak height is monitored before and after each TPD measurement. The decrease in the AES peak height is related to the amount of desorbed C,, in a nonlinear manner due to the escape depth of the Auger electrons. The escape depth effect is minimized by measuring the change of the AES carbon peak height at small coverages larger than a single layer, where the response is nearly linear. Thus, for example if 0.05 x 1014 C,, nanoclusters/cm2 desorbed and the peak height is changed by 5% then the remaining AES peak corresponds to - 1 x 1014 C,, nanoclusters/cm2. The change in the AES carbon peak height and the amount of desorbed multilayer determined from TPD give an estimate of 1.7 monolayer as the full C,, single layer coverage of the aluminum sample (one monolayer is 1.21 X lOi molecules/cm2). That full coverage is greater than one monolayer is attributed to surface roughness of the aluminum foil. From 650 to 775 K the carbon-toaluminum ratio decreases from - 3 to N 0.1 and is constant at - 0.1 from 775 to 825 K. The AES carbon peak shape is unchanged (i.e. graphitelike) as a function of anneal temperature, suggesting the chemical interaction of the remaining surface carbon is also unchanged (i.e. not carbidic). Since the C, does not thermally desorb in this temperature range, the decline in the carbon signal is due either to fragmentation and subsequent desorption or to dissolution of the C,, or carbon into the aluminum bulk. In this and subsequent sections of the present paper evidence will be presented that indicates the C,, dissolves intact into the aluminum between 650 and 775 K. The temperature dependence of the carbon/ aluminum peak ratio demonstrates strong single layer bonding of C,, to a clean aluminum substrate. Although desorption did not occur, the minimum bond strength was estimated as if desorption had occurred in the temperature range from 650 to 775 K, since annealing to surface temperatures below this range had no effect on the single layer C,,. Assuming first-order reaction kinetics and an exponential prefactor of 1013 s-l and using the data of Fig. 1, one estimates a
A. K Hamza et al. /Surface
lower bound to the binding energy to be 44 kcal/mol. This estimated minimum binding energy is limited by the assumed first-order prefactor of 1013 s-l. As a test of whether the material dissolves into the bulk or desorbs, an initially clean sample was held at 750 K and exposed to C,, for 30 min (an exposur: that at room temperature would lead to N 500 A thick C,, multilayer film). If the C,, fragments and desorbs then no change in the sample would be expected. However, a visible change was detected. The sample changed to a gold color on the sputter cleaned area of the sample. AES revealed a carbon-to-aluminum ratio of approximately 2. Fig. 2 compares the morphology of the sputter cleaned aluminum annealed to 750 K to that of the aluminum exposed to C,, at 750 K as seen by scanning electron microscopy. A very dramatic change in the surface texture is observed. Fig. 3 shows a comparison of an ex situ measure of the reflectivity of the cleaned aluminum surface and the C, exposed (at 750 K> sample. The lowest trace in Fig. 3 is a difference curve between the reflectivity of each sample. The decreased reflectivity of the C,, ex-
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posed sample manifests itself in the peak in the difference curve at 450 nm. This peak corresponds to a strong absorption feature for Cm. This experiment strongly indicates that the disappearance of carbon on the surface due to annealing above 650 K is due to dissolution into the aluminum bulk. Further, the reflectivity measurement and color change is suggestive that the C,, dissolves at least partially intact. The temperature range at which dissolution occurs is similar to the temperature range that aluminum surface thermal disordering occurs [9,101. True surface melting of the Al(110) surface does not occur until the surface temperature reaches > 800 K as measured by medium energy ion scattering [ll]. However, as shown by low energy electron diffraction, the mean square displacements of the aluminum atoms become anomalously large at surface temperatures greater than 620 K [lo]. A possible mechanism for the dissolution may involve large displacements of the surface allowing aluminum atoms to react with the C,, cage and subsequently leading to the aluminum surface engulfing the C,, molecule. The rough morphology seen in the scanning elec-
Fig. 2. Scanning electron micrograph of (a) cleaned aluminum surface annealed in vacuum to 750 K subsequently ambient conditions compared to (b) aluminum surface exposed to C,, at 750 K for 30 min also subsequently exposed conditions. Magnification is 30000 times and the 1 pm fiducial is shown by the dotted line.
exposed to to ambient
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tron micrograph (see Fig. 2) may be a result of this engulfing mechanism.
Fig. 4 shows the valence band photoelectron spectrum from “clean” aluminum collected with 75 eV photon energy. The valence band spectrum is very sensitive to the surface cleanliness. Any residual oxygen appears as a broad peak at 7.5 eV below the valence band m~mum (VBM). The amount of residual oxygen can be determined from the Al 2p photoelectron feature shown in Fig. 5. The signal with binding energy greater than 73.6 eV is due to Al atoms in AIO, oxidation state; the ratio of the area of signal with binding energy greater than 73.6 eV to the area of signal with binding energy at 72.5 eV is 0.09. Assuming x is near one gives an oxygen coverage of -b.O9 monolayer (where one monolayer is one oxygen atom per aluminum atom),
Energy
wn Fermi Level (ev)
Fig. 4. Valence band photoelectron spectra for multilayer C6a on aluminum (solid line), single layer C,, on aluminum (dotted line), and “clean” aluminum (dot-dashed line). The spectra are collected with a photon energy of 75 eV. Overall resolution of the system is determined by the lo%-90% rise of the aIuminum Fermi edge to be - 0.2 eV. Energy scale is in eV with respect to the Fermi energy.
Fig. 3. Reflectivity as a function of wavelength from 250 to 840 nm for cleaned aluminum subsequently exposed to ambient conditions (dashed line) and aluminum surface exposed to C, at 750 K for 30 min also subsequently exposed to ambient conditions (solid line). A difference curve is also plotted (dotted line).
since at 130 eV photon energy the measurement is surface sensitive. The residual oxygen could not be removed completely due to the poor vacuum, less than 3 x 10e9 Torr, and less than optimal sputtering geometry. Fig. 4 also shows the valence band spectra for multilayer and single layer C, coverage of aluminum (here single layer refers to the amount remaining after annealing the surface to 600 IQ. The multilayer spectrum is similar to that obtained by Weaver and co-workers 161 for C,, multilayers on GaAs. The two sharp features at 2.2 and 3.6 eV below the VBM are r-derived states and the features at 5.8, 8.2, and 10.3 eV are a-derived states of C,,. The highest occupied molecular orbital (HOMO) is at 2.2 eV below the Fermi level, just as in other multilayer films [6]. After heating the surface to 600 K, at which
A. K Hamza et al. /Surface
temperature the brownish color from the multilayer disappears, the single layer (1.7 monolayer, where one monolayer is 1.21 x 1014 molecules/ cm*> C,, film is present. The valence band spectrum clearly shows the broadened r-derived states which are shifted by 0.3 eV to higher binding energy. The a-derived states are still discernible but partially obscured by the broad oxygen feature from the residual surface oxygen. This result indicates the C, single layer on aluminum is made up of intact C,, cages. The shift to higher binding energy may be explained by a charge transfer from the C,, to the aluminum, which seems unlikely due to the high electron affinity of C,, [12]. Another possibility is that the screening of the final state is less effective for the single layer C,, on aluminum thus increasing the binding energy of the a-derived states. A comparison of the Al 2p photoelectron spectrum for the “clean” aluminum surface and the single layer C,, surface annealed to N 750 K is
40 . _ :
C, single layer annealed to -750 K “clean aluminum”
30 -
78
76
74
72
70
Binding Energy (eV)
Fig. 5. Al 2p photoelectron spectra for “clean” aluminum (dotted line) and single layer C, on aluminum annealed to - 750 K (solid line). Photon energy is 130 eV. Energy scale is binding energy in eV, where the aluminum metal peak is set at a binding energy of 72.5 eV.
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shown in Fig. 5. The multilayer and the single layer C, coverages on aluminum suppress the Al 2p peak such that the chemical shifts of the Al 2~‘s cannot be observed. However, annealing to 750 K causes dissolution of the surface C,, into the bulk (see Section 3.1 above). In the Al 2p photoelectron spectrum both the aluminum metal peak (binding energy of 72.5 eV) and a new peak at lower binding energy (71.7 eV) are observed. Assuming the C, dissolved in the bulk is intact, this observation could also be attributed to a charge transfer from the C, to the aluminum, which is consistent with the 0.3 eV shift in the rr-derived states to higher binding energy. However, since many other metals C, bonding systems involve charge transfer from the metal to C,, [6], it seems unlikely that the chemical shift observed here is due to charge transfer. Again, the chemical shift could also be ascribed to final state effects where the Al 2p electron binding energy for aluminum atoms bound to C,, would have a lower binding energy since screening of the final state hole would be easier. The C, molecule is polarizable as measured by its nonlinear optical properties. The polarizable C,, would then be able to screen core hole on aluminum atoms. The effect can be large and is observed in alkali halides where the halide ion is more polarizable than the alkali ion [13,14]. This lower binding energy for the Al 2~‘s is observable in this case because of the surface sensitivity using 130 eV photons and the intimate mixing of the C,, and aluminum in the dissolution layer. The behavior of C,, on aluminum is similar to that of C,,. Fig. 6 shows a comparison of the valence band photoelectron spectrum for the C,, multilayer to the C, multilayer. The shape and position of the C,, multilayer features agree with the measurements of Weaver and co-workers [15]. However, we discern only two features in each of the r-derived peaks for C,, as opposed to the three and two features of the r-derived peaks as seen by Jost et al. [15]. The splitting of the features is assigned to the reduced symmetry of the C,, relative to C,, removing some of the degeneracy in the molecular orbitals. Fig. 7 shows how the valence band photoelectron spectrum changes as the C,, exposed surface
A. % Hamza et al. /Surface
374
is annealed. The multilayer C, is a purple color; heating the surface to 610 K causes the sample color to change to the original silver color of the “clean” aluminum sample. The photoelectron spectrum for this surface is labeled C, “monolayer”. Two points are to be made. First, the second T-derived feature is no longer discernible. Second, the first r-derived and the a-derived features are still present, suggesting the C, cage structure may still be intact. Thus, like C,, C,, is strongly bound to the aluminum surface. The disappearance of the second n--derived feature suggests that aluminum-C,, “monolayer” bond may involve bonding through these n-derived features. However, no discernible energy shift in the other features is observed, unlike the C, single layer, even though a similar chemical shift to lower binding energy for the aluminum 2p core level is seen (see below). Annealing the C,, monolayer to 650 K reduces the intensity of the C, “monolayer” fea-
Science 318 (1994) 368-378
2.5x
0.0
4
16
a
’ 4 14
’ 12
’ 4
’
10
’
’ c’ 8
’
’b 6
’ *
’ 4
d’ ’ ’ 8 ’ ’ 2 0
Energy wn Fermi Level (eV)
Fig. 7. Valence band photoelectron spectra for (from the top) multilayer C,,, monolayer C,o, monolayer C, annealed to 650 K and “clean” aluminum. Photon energy is 75 eV. Enera; scale is in eV with respect to the Fermi energy.
energy
wt to Fermi Level WI
Fig. 6. Valence band phot~lectron spectra for multi~ayer C, (solid line) and multilayer C,, (dashed line). The spectra are collected with a photon energy of 65 eV. Energy scale is in eV with respect to the Fermi energy.
tures and allows the aluminum Fermi level to be observed. However, the position of the features remains unchanged. This is evidence of the beginning stage of dissolution of the C,, monolayer without changing the structure of the surface C,,. Because this is the early stage of dissolution only aluminum atoms interacting with the C,, monolayer can be observed in the photoelectron spectroscopy. Fig. 8 shows a comparison of the Al 2p photoelectron spectra for “clean” aluminum and the C,, “monolayer” annealed to 650 K. Since after annealing to 650 K the surface coverage of C,, is still high only a small Al 2p signal is observable. In order to make the comparison easier, the C,, annealed to 650 K trace was multiplied by seven. The binding energy of the Al 2~‘s for aluminum metal is set at 72.5 eV and the Al 2p peak for the C, layer heated to 650 K is observed at 71.6 eV, again exhibiting a similar chemical shift to lower binding energy as seen with C,,.
A.V. Hamza et al./Surface
3.3. Photoluminescence aluminum
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of C,, on and in
Fig. 9 shows the photoluminescence spectra from an aluminum-C,, sample prepared in the following manner. The aluminum surface was sputter cleaned and annealed to 800 K until the oxygen(503 eV)-to-aluminum(68 eV) AES peak height ratio was less than 0.04. Then the surface was heated to 750 K and exposed to C, for 30 min. After 30 min the surface was cooled to room temperature while dosing C,,. The dosing is continued until a C,, multilayer is visible. The sample is removed from UHV and transferred to UHV in the photoluminescence apparatus. The sample is heated to 400 K to thoroughly outgas the sample and holder. AES of the sample shows only the 272 eV carbon peak as expected. Photo-
6-
Monolayer Cm and
Multilayer C,
T 0 500
hk130eV
.
.
600 Wavelength (nm)
20 -
-
c,,
Fig. 9. Photoluminescence spectra for multilayer Cm (lowest trace), C,, monolayer on C,, dissolved in aluminum (middle trace) and for C, dissolved in aluminum (upper trace). See text for sample preparation. Excitation wavelength is 488 nm. Cutoff filter (550 nm) is part of detection optics.
monolayer
annealed to -650 K (x7) C,, monolayer -650 K . . . “clean aluminum”
annealed to
B B 2
I5 -
I s B 3 Y ‘1 9
78
76
74
72
70
Binding Energy (eV)
Fig. 8. Al 2p photoelectron spectra for “clean” aluminum (dashed line), “monolayer” C, annealed to 650 K (solid line), and “monolayer” C,, annealed to 650 K multiplied by 7 (solid line). Photon energy is 130 eV. Energy scale is binding energy in eV, where the aluminum metal peak is set at a binding energy of 72.5 eV.
luminescence of this sample is shown in Fig. 9 labeled multilayer C,,. The sample was then heated in UHV to N 650 K to remove the multilayer; photoluminescence of the heated sample is also shown in Fig. 9 labeled monolayer C,, and dissolved C,,. For comparison purposes this trace is multiplied by 250. Finally, the sample was heated to N 750 K to dissolve the monolayer into the aluminum-C, composite; photoluminescence of the N 750 K heated sample is the uppermost trace in Fig. 9 labeled C,, dissolved in Al. Again for comparison purposes this trace is multiplied by 500. The sample was then sputter cleaned. The clean aluminum surface exhibits no photoluminescence signal. From the sputtering time and the sputter removal rate of aluminum and C,,, the depth of dissolution was estimated to be N 250 A. The carbon concentration decreased monotonically; however, since the reac-
A. V. Hamza et al. / Suface Science 318 (1994) 368-378
376
tion of C,, with the surface makes the surface very rough, shadowing effects make the depth profile measurement uncertain. The photoluminescence data are suggestive that C, dissolves intact into aluminum, but not conclusive. The small shifts in the C, photoluminescence may be due to interaction of C,, with aluminum. However, that the shifts are due to C,, fragments dissolved in aluminum with similar photoluminescence spectrum to that of C, cannot be ruled out. Note: A single crystal aluminum (111) sample was exposed to C, and annealed to various surface temperatures up to 900 K. The AES and TPD behavior was the same as observed on the aluminum foil. Contamination of the sample surface with 0.1 monolayer oxygen had no effect on the C,,-aluminum reaction except to lower the available surface area for reaction. Apparently, the oxygen-aluminum interaction is so strong; once aluminum has reacted with oxygen, C,, does not have any effect on the aluminum-oxygen bond. 3.4. Passiuation
of aluminum
by C,,
In order to establish a foundation for comparison, we first oxidized a clean aluminum surface without the fullerene layers by exposing the surface to water vapor. A series of Auger spectra detailing this exposure are shown in Fig. 10. The aluminum sample was sputtered and annealed at 800 K to produce the clean surface. After this heating, the oxygen(503 eV)-to-aluminum(68 eV) AES peak height ratio was less than 0.04. Since the relative Auger sensitivity for oxygen is considerably higher than that of aluminum, these data indicate the presence of a pure aluminum surface. This sample was then exposed, at room temperature, to 340 L (one langmuir, 1 L, is an exposure of 10vh Torr * s) of water vapor (working base pressure: less than 10e9 Torr) emanating from a tube positioned Iess than 1 cm from the surface. The sample was again examined with the Auger spectrometer to determine the level of oxidation. The magnitude of the resulting aluminum peak was reduced and a significant oxygen presence was determined. The sample was
after340 L H,O
exposure
clean aluminum
Elecmn Energy (eV)
Fig. 10. A series of AE spectra detailing the oxidation of aluminum with water vapor. The lowest trace is from clean aluminum, sputtered and annealed to 800 K. The middle trace is from the clean surface exposed to 340 L of water vapor. The upper trace is from the water exposed surface heated to 800 K, heating rate is 3 K/s.
next heated to 800 K in a TPD cycle to determine desorption species. While a quantitative discussion of the desorbed species is beyond our present scope, we have identified QMS signals associated with hydrogen and water in this desorption. These results define the surface oxidation of unprotected aluminum for comparison to the C, protected surface when exposed to water vapor. The strong bonding of C, to the aluminum surface and the hydrophobic nature of C, begs for an investigation of the passivation of the aluminum surface against oxidation. In a typical experiment, this investigation was begun by cleaning the aluminum surface via the Ar+ sputtering and annealing to 800 IS (oxygen-to-aluminum AES peak height ratio less than 0.04) (see Fig. 11). Next, the C,, multilayers were adsorbed onto the surface. The deposition time during this typical
A. K Hamza et al. /Surface
afterheating to 600
after340
K
L H,O
clean aluminum
-0.5 -
-l.c..‘.‘.‘..‘“““““““‘““‘““” 100
200
300
400
500
600
Ekctron energy (ev)
Fig. 11. A series of AE spectra detailing the passivation of aluminum by multilayer and single layer coverage of Cm. The traces are displayed in chronological order form the bottom to the top. The lowest trace is for the sputter cleaned and annealed to 800 K sample. The next trace is for the clean surface exposed to C, for sufficient time to form a multilayer coverage. Each succeeding trace is for the sample receiving the labeled treatment and all previous treatment.
run was 12 min. An Auger spectrum, once again verified the existence of the multilayer coverage. The surface was then exposed to 340 L of water vapor (working pressure < 10m9 Tori-1 and the post-hydration Auger spectrum shows very little change from the pre-hydration. Most notably, there is no evidence of change in the intensity of the oxygen peak (503 eV> after exposure to water vapor. TPD measurements on the multilayer covered surface performed after hydration indicate desorption of the C,, multilayers (m/e = 360) upon heating to 600 K as was observed in the non-hydrated multilayer desorption described in Section 3.1. Also observed in this desorption were small amounts of hydrogen (m/e = 2) and water vapor
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(m/e = 181, however no signatures of oxygen desorption were present. In searching for any desorbed oxygen, various reasonable charge-to-mass ratios were considered in QMS monitoring (i.e. hydroxides, O,, C,O). After the TPD heating of the surface to 600 K, the presence of a C, monolayer was again verified using AES (Fig. 11). As indicated in the previous analysis of single layer coverages, the aluminum peak (68 eV) becomes apparent again due to the proximity of metallic aluminum to the surface. As expected, no significant oxygen peak is observed in this Auger spectrum. This single layer covering of fullerenes, formed by multilayer adsorption onto clean aluminum followed by sample heating past 600 K, was then exposed to the typical 340 L of water vapor. The post-hydration Auger spectrum for single layer coverage (Fig. 11) still indicates the same relative peak sizes of carbon to aluminum with no significant change in the oxygen peak. This Auger spectrum indicates a key result. Due to its hydrophobic nature and ease of uniformity in deposition, a single layer of C,, deposited on an aluminum surface will protect that surface from oxidation. Another temperature program desorption experiment, this time performed to 800 K, shows the usual background signal of hydrogen (m/e = 2) and a small water vapor background signal (m/e = 18) but indicates no desorption of C,, (m/e = 360) as expected. However, a subsequent Auger spectrum (Fig. 11) indicates that, while still present, the carbon peak (272 eV> is significantly reduced in intensity due to dissolution of the C,. Correspondingly, the intensity of the aluminum peak increases to almost the previous intensity levels for clean aluminum. As a final test of passivation, the aluminum and absorbed fullerene mixture was again exposed to 340 L of water vapor. While not presented here, this sample showed partial oxidation of the clean aluminum, albeit not to the levels of unprotected aluminum. We have also attempted to remove a C,, multilayer covered, sputter cleaned and annealed aluminum sample from ultrahigh vacuum (UHV) and replace it UHV to test the extent of C, passivation. The C, vacuum interface is free of any oxygen contamina-
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tion after annealing to 400 K. Care must be taken to completely outgas the sample holder and supports after replacing the sample in UHV. Heating the sample to 600 K yields nearly a monolayer coverage of C,,. However, a small oxygen contamination is present due to the outgassing of the supports (1 X lo-* Torr) which expose the surface to oxygen containing gases while the sample is at elevated temperature. Perhaps some of the C,, reacts with these oxygen containing species at temperatures above 470 K leaving unprotected regions of the surface which can be oxidized by the outgassing. These results agree well with experiments by Hong et al. (16,171 where C, encapsulation was shown to protect surface reconstructions of Si(lll)-(7 x 7) and Si(lOO)-(2 x 1) to ambient exposure although without removal of the C, film. We have seen similar results with respect to C,, multilayer protection of $000) to ambient exposure by examining the interface by sputter depth profiling. In this case no outgassing occurs and no oxygen concentration is observable with Auger electron spectroscopy [18].
4. ~urnrna~ Three major observations were made in the course of this investigation. One, C,, dissolves into aluminum either intact or as C,, fragments at temperatures above 700 K. Two, C,, (C,,> is strongly bound to aluminum. The strength of the C,-aluminum interaction is also manifested in the aluminum Zp core level binding energy, which exhibits a chemical shift to lower binding energy. Three, the highly reactive aluminum surface may be passivated by a single layer C, coverage to oxidation by water vapor.
Acknowledgments
A.V.H. gratefully acknowledges Dr. L.J. Termine110 and Dr. G.D. Waddill for their help in
bringing the SSRL experiment on-line and operating and for their helpful discussions. A.V.H. also acknowledges R. Hill’s expert assistance in bringing the SSRL experiment on-line. This work was supported by the Division of Materials Science, Office of Basic Energy Sciences, U.S. Department of Energy, at Lawrence Livermore National Laboratory under contract No. W-7405 ENG-48. References
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