Ru(0001) by metastable quenching spectroscopy, infrared absorption spectroscopy and thermal desorption

Surface Science 205 (1988) 397-407 North-Holland, Amsterdam

397

AN EXPERIMENTAL STUDY OF Cu ADSORPTION ON CO/Ru(OOO1) BY METASTABLE QUENCHING SPECTROSCOPY, INFRARED ABSORPTION SPEClXOSCOPY AND THERMAL DESORPTION F.M. HOFFMANN Exxon Research and Engineering,

Clinton Township, Annandale,

Gerd ROCKER, Hiroshi TOCHIHARA, and Horia METIU Depariment

of Chemistry,

NJ 07036, USA

Richard M. MARTIN

University of California, Santa Barbara,

CA 93106, USA

Received 13 November 1987; accepted for publication 24 June 1988

Through combined use of metastable quenching spectroscopy, infrared reflection absorption spectroscopy, thermal desorption spectroscopy and work function measurements we show that exposure to Cu vapor of a Ru(OOO1) surface precovered by a monolayer of CO leads to Cu adsorption at 85 K. At that temperature the Cu atoms cover the CO layer. Annealing to 150 K causes CO penetration through the Cu layer and formation of a CO/[Cu/Ru] sandwich. Thermal desorption experiments show that CO diffusion through the Cu layer at 200 K is sufficiently rapid to give a large CO desorption flux.

1. Introduction Stimulated by Sinfelt’s work on bimetallic catalysts [l] many research groups have studied to what extent the chemisorption properties of a metal monolayer deposited on a solid substrate differ from those of’the bulk metal. A particular example, which is of interest here, is the Cu/Ru(OOOl) system which has been studied extensively with the help of many surface science techniques [2-161. These studies have found that Cu and Ru do not mix; that under certain conditions a Cu monolayer can be grown in registry with the Ru(OOO1) surface; and that the chemical properties of such a Cu monolayer are noticeably (but not spectacularly) different from those of a Cu single crystal. In this article we investigate how the properties of a Cu monolayer (or multilayer) are changed by depositing it on a Ru surface precovered by a CO layer. As far as we can tell, the properties of this miniscule metal-insulator-metal junction have not been studied previously. Such a study is interesting for two reasons. First, we want to know if the presence of an insulating layer between Cu and Ru will change substantially 0039-6028/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

398

F.M. Hoffmannet al. / Cu arlrorption on CO/ Ru(0001)

the properties of the Cu layer. Second, it is not at all sure that the Cu atoms deposited on a CO covered Ru(OOO1) surface will remain there. The system might lower its free energy if the Cu atoms and the CO molecules exchange places to create a more conventional CO/[Cu/Ru] sandwich. Or perhaps Cu does not “wet” the CO/Ru surface and prefers to cluster leaving most of the CO uncovered. The experimental metrology for making a choice among these possibilities is not yet fulIy developed. Many surface science methods do not have the surface specificity necessary to distinguish whether a Cu layer sits on top of a CO layer or vice-versa. Given the enormous interest in layered compounds, generated by microelectronics fabrication, the development of methodology for studying such layers is of interest even though no devices based on the Cu/[CO/Ru] system are contemplated. Our most important results are summarized below. We find that if we expose a CO/Ru(OOOl) surface to Cu vapor the Cu atoms stick on top of the CO layer. As the layer is heated CO diffuses through the Cu layer and desorbs. Several measurements have been used to support this conclusion. We have found that when a cold (i.e. 85 K) CO/Ru(~l) surface is exposed to Cu vapor the CO peaks disappear from the MQ spectrum and are replaced by features characteristic of Cu. This indicates that the Cu atoms sit on top of the CO layer. Deposition of Cu on the CO/Ru surface gives a CO thermal desorption spectrum which resembles that of CO/[Cu/Ru], except for the fact that the low temperature part is missing. This suggests that at low temperature Cu covers the CO monolayer and that CO diffuses through the Cu as the temperature is increased. With infrared absorption spectroscopy we have found that the deposition of two monolayers of Cu on a CO/Ru(OOOl) surface at 85 K completely eliminates the IR peak of CO/Ru, and leaves a small peak characteristic of CO/Cu having about 1% of the intensity of the CO/Ru peak. When the temperature is raised to 150 K, the CO/Cu peak increases with time, reaching a steady state after about 20 min. These data indicate again that at 85 K Cu covers the CO layer, and that the CO diffuses through the Cu layer at higher temperature. Admittedly any one of these experiments taken alone might be given a different interpretation. However together they make a strong case in favor of a model in which (1) the Cu atoms adsorbed at 85 K on CO/Ru(OOOl) cover the CO layer and (2) heating to 150-200 K causes CO diffusion through the Cu layer onto the Cu surface (i.e. a transition from Cu/[CO/Ru] to CO/[Cu/Ru]).

2. Ex~rimen~ The instrument in which the metastable quenching spectra were taken was described in previous work [17]. At Santa Barbara the Cu layers were de-

FM Hoffmann et cd. / Cu adsorption on CU/itu(ooOl)

399

posited on the Ru surface by Cu sublimation from a high purity Cu wire wrapped around a tungsten wire which can be resistively heated. At Exxon the Cu atoms were evaporated from a Cu ball melted to a tantalum wire [13]. In both laboratories the Ru surface was kept at 85 K during Cu deposition. After deposition on Ru the Cu layer was annealed to 540 K. By using AES and TDS [13,16] the Cu coverage was correlated with the evaporation time and the heating power. After calibration we used the latter parameters to obtain desired coverages. The change in work function was determined from the low energy cut-off for the secondary electron emission in the MQ spectrum. During these measurements the sample was biased at -4 V. For CO adsorption studies the high purity gas was introduced in the chamber through a leak valve. Reproducible exposures were obtained by mo~to~ng the back~ound pressure with an ion gauge. The Ru(OOO1)surface was cleaned by using a standard procedure @-IO].

3. Results and discussion To simplify ~rnrn~~ation we use notation such as 10 ML CO/(24 ML Cu/[lO L ~~/Ru(~~)]}. This must be read from right to left and it means that we have first exposed a Ru(OOO1)surface to 10 langmuir (L) CO; then the resulting surface was exposed to Cu vapor until the amount of Cu present on the surface is 2.4 times larger than the amount of Cu required to form a Cu monolayer on the Ru(~O1) surface; then the surface formed this way was exposed to 10 L CO. This notation only gives information regarding the method of sample preparation. It does not necessarily mean that the Cu is sitting on top of CO, even though throughout this paper the order of deposition appears to coincide with the order of the layers in the system. The boot of Cu on the sample is determined by thermal desorption. When the notation specifies 2.4 monolayers it does not specify the structure, e.g., it does not necessarily mean that a flat structure having two Cu monolayers covered by a 0.4 Cu submonolayer was formed.

Metastable quenching spectroscopy has been described in detail previously [17]. Here we make some brief remarks pertinent to the present work. In our MQS experiments, the surface is exposed to a thermal energy He beam containing about 90% He (2 ‘S) atoms with 20.6 eV electronic energy, and 10% He (2 3S) atoms with 19.8 eV energy. When the excited atoms come in contact with the surface they are quenched to the ground state which causes electron

ejection. The kinetic energy of these electrons is measured and the metastable quenching spectrum is the plot of the differentiated electron current versus electron kinetic energy. Metastable quenching takes place through two mechanisms. One is Penning ionization, which can be represented by the “reaction” S + He* = S+ + He + e. This is rather similar to photoelectron spectroscopy except that an excited atom, rather than a photon, is used as the energy source. The other mechanism has two steps: the approaching metastable is ionized by charge transfer to the surface (S + He* = He+ -t S-) and the ion is subsequently Auger neutralized (S- + He+ = e f S+ + He). The electrons ejected by the surface are produced in the process of ion neutr~~ation and the metastable quenching spectrum is identical to that obtained in ion neutralization spectroscopy (INS) fig]. The INS spectrum is less structured than the Penning spectrum, and is rarely useful in distinguishing between different surface species. Regardless of the quenching mechanism, one advantage of MQS is its surface specificity: the signal comes almost exclusively from the outermost surface layer. Specifically, if the Cu atoms stick on top of the adsorbed CO molecules then the MQ spectrum will consist only of a Cu signal. We have demonstrated this feature of the method previously by studying NH, [19] and H,O [20] adsorption on a CO covered Ru(O001) surface.

In fig. 1 we show the change in the MQ spectra caused by Gu deposition on a ~~/Ru(~l) surface (unless otherwise stated the CO exposure in all the experiments reported in this paper was 10 L). The deposition of a 0.25 monolayer of Cu causes the disappearance of the CO peaks. Further deposition, up to a monolayer of Cu, leads to a spectrum which has shoulders at the same energies as the peaks in the spectrum of a Cu monolayer deposited on Ru, as shown in fig. 2. The MQ spectrum of 1 ML Cu/[lO L CU,/Ruf in fig. 1 can alsa be compared with that of 10 L CO/[1 ML Cu/Ru] from our previous work [16]. The two spectra are similar in general shape, since both are predominantly INS-type spectra. However, small Penning ionization peaks of CO are observed in the 10 L CO/[1 ML Gu/Ru] spectrum at about 5 and 8 eV and the characteristic Cu peaks at about 10 and 12.5 eV are missing, which is opposite from the 1 ML Cu/[IO L CU/Ru] system. This indicates that the Cu atoms cover the CO layer. Since the characteristic CO and Cu peaks of the two systems are so small we consider this to be only tentative evidence in favor of this conclusion, and will present, in what follows, additional evidence. Several facts which are not pertinent to the main line of this article are of sufficient interest to be mentioned here. First, we note that the MQ spectrum of 1 ML Cu/[lO L CU/RuJ is quite different from that of 1 ML Cu/Ru, which implies that the EWQCu layers have different electronic properties. Secondly, it is curious that we do not see at all, at low Cu coverage, peaks

F.M.

~#f~nn

et al. / Cu adsorption on

CO/ RufOf_XX)

401

Fig. 1. MQ spectra of a clean Ru(~l) surface and Cu/[lOL CO/Ru] samples for &- = 0.25 monolayer (ML) and 1 ML. The spectra were obtained at 100 K.

MQS t-\

1’ : I

\ I ML Cu/[tO LCO/Ru]

‘\ \

: 1 ,

I

’

0

I

4

\

\

\

\

\.

I

I

I

8

I

I

12

16

1

E,&eV) -

Fig. 2. The MQ spectra of the surfaces 1 ML Cu/Ru (solid curve) and 1 ML Cu/[lO L COfRu] (dashed curve). The spectra were taken at 100 K.

402

F.&f. Hoffmann et al. / Cu adsorptionon CO/Ru(OOOl)

corresponding to isolated Cu atoms. This is probably due to the dominance of the INS mechanism with this system. It is also possible that even in the absence of lateral interactions between the adsorbed Cu atoms, their interaction with CO molecules and with the Ru surface (through the CO layer) distorts the atomic electronic structure of Cu beyond recognition. Another interesting feature is the rapid disappearance of the CO features in the MQ spectrum upon Cu deposition. The MQ spectrum of a CO/Ru surface on which we deposited 0.25 monolayer of Cu shows almost no CO signal (fig. 1). This indicates that even when Cu does not cover the CO, it is capable of shifting the MQ mechanism from Penning ionization of the CO to INS at the CO sites. This may occur by long range charge transfer from He* to Cu atoms. This mechanism would seem to require either polyatomic Cu structures or a strong interaction between Cu and Ru (through CO) to provide sufficient density of states to make the long range resonant ionization process efficient. Finally, we exposed the 1 ML Cu/[lO L CO/Ru(OOOl)] system at 85 K to CO and took the MQ spectrum of that system. We obtained an INS-type spectrum. 3.2. Thermal desorption In fig. 3 we show CO TD spectra obtained from the 10 L CO/Ru(OOOl) and 0.25 Cu/[lO L CO/Ru(OOOl)] systems. If we compare these spectra with the TD spectrum of CO/[1 ML Cu/Ru(OOOl)] in fig. 4, we see that the spectrum of the 0.25 ML Cu/[lO L CO/Ru(OOOl)] has a low temperature part which resembles CO desorption from a Cu layer and a high temperature part

200

400

300

500

600

T(K)-

Fig. 3. TD spectra of CO desorbed from Ru(OOO1)and 0.25 ML Cu/[lO L CO/Ru]. indicate CO desorption from the sample holder.

Dotted lines

FM. Hoffmann et al. / Cu aakotption on CO/Ru@Wl)

CO EXPOSURE:

100

200

300

400

403

10 L

500

T(K)-

Fig. 4. CO TD spectra of three different CO-Cu-Ru samples with 1 ML Cu, exposed to 10 L CO at 100 K. Dotted line as in fig. 3.

which resembles CO desorption from Ru. However, the low temperature threshold for the desorption of CO from the 0.25 ML Cu/[lO L CO/Ru(OOO1)] sample is higher than that of the CO/[1 ML Cu/Ru(OOOl)] sample. This effect is seen more clearly in fig. 4. The threshold for CO desorption from 10 L CO/[1 ML Cu/Ru(OOO1)] is 150 K while that for 1 ML Cu/[lO L CO/Ru(OOOl)] is 200 K. This suggests that CO desorption from the latter system is delayed by the fact that CO must go through the Cu layer before being able to desorb. The heating rate in this experiment is 10 K/s, and the fact that CO desorption is observed at 200 K does not mean that diffusion starts at that temperature and is absent at lower temperature. The experiment only indicates that at 200 K the rate of diffusion of CO to the surface becomes large enough to give a TD spectrum on the 10 K/s time scale. In fig. 4 we also show the TD spectrum of CO from the system 10 L CO/{ 1 ML Cu/[lO L CO/Ru(OOO1)]}. Note that the thermal desorption spectrum of the additional CO (adsorbed on Cu/[CO/Ru]) differs from that of CO from 1 ML Cu/Ru(OOO1). This indicates that CO has slightly different binding energies on the two Cu layers. Furthermore, the CO uptake by the Cu/[CO/Ru] system is lower. The penetration of CO through the Cu layer is further illustrated by the data shown in fig. 5. The full lines are the CO desorption spectra for 10 L CO/{ n ML Cu/[lO L CO/Ru(OOO1)]} for n equal to one, three and five. The dashed lines show the spectra for n ML Cu/[lO L CO/Ru(ooO1)]. The low

404

FM. Hoffmann et al. / Cu adsorptionon CO/ Ru(0001)

T (K)e Fig. 5. TD spectra of CO desorbed from 10 L CO/{Cu/[lO L CO/Ru]} samples with 0,” = 1, 3 and 5 ML, together with spectra of the corresponding surfaces without the final CO exposure (dashed lines). The TD spectrum of 10 L CO/[25 ML Cu/Ru] is also shown for comparison. Dotted line as in fig. 3.

temperature TD peak is missing for the latter systems, and we see that as the amount of Cu is increased the high temperature peak in the TD spectrum shifts toward higher temperatures. From fig. 5 we also see that as the Cu coverage on CO/{Cu/[CO/Ru]} increases from 1 to 5 monolayers, the uptake of CO on the Cu layer (indicated by the low temperature TD peak) also strongly increases. In addition, even with an exposure of 5 ML Cu the low temperature CO TDS spectrum is still very different from that of the 10 L CO/[25 ML Cu/Ru] system. We emphasize again that when we give the Cu coverage in monolayers we only indicate the total amount of Cu desorbed from the system. We have no evidence that the Cu forms uniform films with the specified layer thickness. 3.3. Infrared

spectra

Infrared absorption spectra monitoring the CO stretch are shown in fig. 6. These were obtained at Exxon Research Laboratories with a rapid scanning

F.h4. Hofjmann et al, / Cu adrorption on CO/Ru(0001) CO IR ABSORPTION

405

SPECTRA

2104’ co/cu 4 I 2000 2100 FREQUENCY (cm-‘) Fig. 6. The CO stretch absorption spectra of 4 L CO/Ru(OOO1) and 2 ML Cu/[4 L CO/Ru(GGO1)]. The Cu deposition and CO adsorption were made at 85 K. The four curves at the bottom show the change in the absorption spectrum upon annealing the 2 ML Cu/[4 L CO/R~~l)] surface at 150 K. The annealing time is indicated on the graphs.

Perlcin-Elmer 1800 FTIR instrument, using single reflection at 80” angle of incidence. The resolution is 2 cm- ‘. No data manipulation, except base line corrections, was performed. The top spectrum shows the CO absorption of the 4 L CO/Ru(OOOl) system, characterized by a single C-O stretch at 2058 cm-* [21]. Covering this surface with approximately two Cu monolayers at 85 K eliminates the IR absorption by CO/Ru and produces a very small CO/[Cu/Ru] adsorption peak [14] (about 1% of the intensity of the CO/Ru peak) indicating that most of the CO is underneath the Cu layer. There could be two reasons for the disappearance of the CO/Ru peak: the screening of CO from the electromagnetic radiation by the Cu layer, and/or the tilting of the CO molecule into a position nearly parallel to the surface which will tend to lower CO adsorption due to the dipole selection rules. In principle the screening effects can be modeled by solving Maxwell’s equation. However, since we do not know the Cu layer structure or its dielectric constant, a detailed modeling would be fairly pointless. We must note, however, that it is rather unlikely that such a thin metallic layer can screen so efficiently the incident radiation even in the infra red region. The temperature was increased from 85 to 150 K, and the CO/[Cu/Ru] IR peak was monitored as a function of time (fig. 6). The absorption signal

F.M. Hoffmann et al. / Cu adsorption on Co/ Ru(OOO1)

406

increased for about 20 mm, indicating that diffusion of CO through the Cu layer continues to occur during this period until a steady state CO concentration on the surface is reached. This is not inconsistent with the previously discussed onset of the CO TDS peak at about 200 K, when the sample is heated at a rate of 10 K/s. The lmeshape of the CO-Cu band is indicative of an essentially disordered copper overlayer. As pointed out in earlier work [Is], annealing temperatures of 250-300 K are required to form ordered copper layers on ruthenium. 3.4. Work function changes In table 1 we show the work function shifts caused by various adsorption processes. All the shifts are with respect to the clean Ru(OOO1) surface. As is well known from previous work, CO adsorption increases the work function on Ru and decreases that of Cu. The net electrostatic effect of bonding is that the negative charge on CO is increased when it chemisorbs on Ru and decreased upon chemisorption on Cu. Deposition of Cu on Ru(OOO1) lowers the work function by 0.7 eV while deposition of Cu on 10 L CO/Ru(~Ol) causes a work function lowering of 1.1 eV with respect to Ru and 1.7 eV with respect to 10 L CO/Ru(OOOl). This indicates that the properties of a Cu layer lying on top of CO/Ru differ from those of a layer deposited on Ru. We also note that CO adsorption on a Cu monolayer on Ru lowers the work function by 0.8 eV with respect to Ru, while CO adsorption on 1 ML Cu/[CO/Ru] causes a work function lowering of 1.45 eV with respect to Ru. Thus, the Cu/Ru and Cu/[CO/Ru] systems have rather different surface dipole layers and these layers are modified differently by CO adsorption. We conclude that the properties of a 0.1 layer on CO/Ru appear to be quite different from those of Cu/Ru. The MQ spectra are different (fig. 2), and the CO/{Cu/[CO/Ru]} TDS spectrum is different from that of CO/[Cu/Ru] at temperatures below 200 K, where the CO bound to Ru does Table 1 Work function

changes

caused

by Cu or CO deposition

on various

clean or layered

Surface

A+ (ev) a)

10 L CO/Ru{~l) 1 ML Cu/Ru(OOO1) 10 L CO/[1 ML Cu/Ru(OOOl)] 1 ML Cu/[lO L CO/Ru(OOOl)] 10 L CO/{ 1 ML Cu/[lO L CO/Ru(OCOl)]}

0.6 -0.7 -0.8 -1.1 - 1.45

2.4 ML Cu/Ru(OOOl) 10 L CO,‘[2.4 ML Cu/Ru(~l)] 2.4 ML Cu/[lO L CO/Ru(~l)]

- 0.75 -0.9 -1.0

a) A+ is the difference that of Ru(0001).

between

the work function

of the surface

shown

surfaces

in the first column

and

F. M. Hoffmann et al. / Cu adsorption on CO/ Ru(OOO1)

407

not contribute (fig. 4). (In addition, as shown in fig. 5 this low temperature TD peak grows rapidly as the Cu exposure in the CO/{Cu/[CO/Ru]} system is increased from one to five monolayers.) Finally, we note that CO adsorption on Cu/[CO/Ru] lowers the work function of the surface more than CO adsorption on Cu/Ru.

Acknowledgements

The Santa Barbara group is grateful to John Yates Jr. for donating the Ru(OOO1) sample and for teaching us how to work with the Cu/Ru(OOOl) system. We are also grateful to the Sandia Laboratory in Albuquerque, and in particular to Wayne Goodman, for helping us start our work on Cu/Ru. This work was supported by the National Science Foundation through grant CHE 86-12045.

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