LEED-Auger study of the interaction of ethylene and hydrogen with Al(100)

Applications of Surface Science 6 (1980) 297—308 © North-Holland Publishing Company

LEED—AUGER STUDY OF THE INTERACTION OF ETHYLENE AND HYDROGEN WITH Al(100) K. KHONDE, J. DARVILLE, S.E. DONNELLY * and J.M. GILLES Institute for Research in Interface Sciences, L.A.S.M. 0.5., Département de Physique, Facultés Universitaires Notre-Dame de Ia Paix, B-5000 Namur, Belgium Received 8 May 1980 Revised manuscript received 25 July 1980

AES, LEED, ELS and work function determination were used in order to study H 2 and C2H4 adsorption on Al(100). Results indicate that C2H4 and H2 adsorb dissociatively at room temperature. The adsorbed species are incorporated into ordered islands at intermediate exposures. At high exposures the adsorbed species are probably deposited on the surface. Present results indicate that incorporation is randomlike for C2H4 adsorption at —80°C.Conversion to an ordered structure results from heating at —30°C.At high current density electron beam stimulateshydrocarbon desorption upon C2H4 adsorption and leads to a random incorporation of hydrogen, probably in the form of an amorphous hydride.

1. Introduction Adsorption and chemisorption of H2 and C2H4 have been extensively studied, mostly experimentally, for the case of transition metals [1—8].It is generally observed that, initially, ethylene dissociates and forms a carbonaceous adsorbed layer. Theoretical calculations, however, have been mostly made for the case of simple metals. In particular, atomic adsorption has been studied recently with the help of the jellium model [9—13]. It is concluded that hydrogen adsorption is marginally possible at low temperatures [12,13]. This agrees with the results of early experiments [14,15]. It has also been reported that atomic hydrogen adsorbs at 77K on aluminium films with an increase of the work function, whereas hydrogen molecules are inactive [16]. Essentially no H2 nor H adsorption has been seen with UPS up to 100 L on aluminium films at room temperature [17]. As far as ethylene is concerned, a slow adsorption has been found [14,15]. More recently a positive change of the work function has been observed at the beginning of the exposure to C2H4 of aluminium films at room temperature [18]. This change has been attributed to dissociative adsorption of C2H4 so that it would largely be *

IRIS postdoctoral fellow.

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due to hydrogen adsorption. At high exposures, the work function decreases, due to the formation of a bound hydrocarbon. In this paper we present the results ofa number ofmeasurements using AES, ELS, LEED and work function determination concerning the problem of the interaction of C2H4 and H2 with Al(l00).

2. Experimental The aluminium sample was a 12 mm diameter single crystal disc provided by Dr. Welter (KFA Jülich) with orientation (100). Standard mechanical polishing was conducted down to a final grit size of 0.05 pm magnesia. This was followed by electrochemical polishing for 40 mm in a solution of 93 vol.% of ethanol and 7 vol.% of perchloric acid cooled to 2. —15°Cwith a voltage ofthe 5 Vsurface, and a current density beAfter this treatment, examined by scantweenelectron 0.17 and 0.20 A cmwas optically flat without observable blemish. Specimen ning microscopy, preparation was completed in our Riber UHV system by a series of sequential 30 miii sputtering and annealing cycles. Ar+ ion beam parameters were 500 eV, 0.5 pA cm2 and 300 incidence angle w.r.t. the surface. Annealingwas performed at 350°C. After this treatment, the contamination carbon and oxygen AES signals were less than 1% of the Al peak and the (1 X 1) A1(100) LEED pattern with sharp spots was observed. The AES—LEED system was of the four grids type. The angle ofincidence for AES and ELS was 17°.For all the measurements the incident current densities were typically either 0.02 pA mm2 (“low”) or 0.5 pA mm2 (“high”). Primary energies were 400 and 2500 eV for ELS and AES, respectively. The Auger currents were normalized with respect to the elastically “backscattered” electron current. Exposures for LEED, Auger and ELS experiments were performed with i0~ Torr of ethylene and 10—8 Torr ofhydrogen up to 520 Land with 10—6 Torr above 520 L. Pressures indicated by a Varian VT 71-0015 ionization gauge were corrected by assuming sensitivities relative to N 2 for C2H4 and H2 of 2.14 and 0.44, respectively [19]. Work function measurements were was conducted with a Kelvin probe at a 7 Torr. Each experiment repeated several times. The [20] reported pressureareofaverages. i0 values Maximum deviations from the mean are typically 2%. Adsorptions were cumulative and were done on the non irradiated sample. For each exposure the LEED observations were made on a spot just studied by AES. Separate AES and LEED measurements were also done for C 2H4 at low current

densities. The results were identical with those obtained when both measurements were conducted concurrently. This means that all electronic effects due to the beam must be the same with both techniques in the low current case. The expected error on the sample temperature is about 5°C.

K. Khonde eta!. I Interaction of ethylene and hydrogen with A!(100)

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3. Results 3.1. Results on C2H4 adsorption In the following, an intensity change from I~to ‘2 is reported in relative value as 100 (~2 ‘~)/‘~~ Depending on the incident current density value, two different behaviours are observed at room temperature. At high current densities the following observations have been made. The intensity first decreases by 12% at I L, then increases by 17% at 2 L (fig. 1). This oscillation is followed by 1) an exponential decrease 17% of the starting value, atof70theL and no change upon to further exposure. No increase (slope X l02 L over the residual C signal could be detected, even at 350 L. C Auger1.9 peak (272 eV) In the corresponding LEED photographs the background intensity increases and, at 60 L, the contrast between the spots and the background has completely disappeared. The size of the unit cell remains constant and at no stage does any superstructure appear. Upon heating the sample for 30 s to 450°Cafter a 350 L exposure, the clean Al(l00) LEED pattern is recovered and the 67 eV Al peak grows back to at least 92% of its initial value. At low current densities, very different results are obtained although the 67 eV —

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K. Khonde eta!. / Interaction of ethylene and hydrogen with Al(1 00)

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EXPOSURE(10~L) Fig. 2. Variation of the normalized aluminium Auger peak (67 eV) •, and carbon peak (272 eV) •, with C 2H4 dose for the case of low incident current density at room temperature. 0 indicates low dose behaviour for aluminium and o indicates low dose behaviour for carbon on an extended

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Auger Al peak still does not shift and still shows an initial oscillation around 1 L (fig. 2). Its intensity variation is negligible up to 350 L. Then comes a first exponential decrease by 75% at 1400 L (slope: 1.4 X l0~ a second one 3 L~)by whichL~)followed the Al peak is by further reduced starting at 1600 L (slope :3.1 X iO by one order of magnitude. The residual 272 eV C peak is unchanged up to 1 L but suddenly increases at 1 .5 L, together with the increase of the Al peak. The C signal has been multiplied by a factor of 15 around 1500 L where it seems to saturate. This part of the variation can be represented by the expression: 1

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K. Khonde eta!. /Interaction of ethylene and hydrogen with Al(100)

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Preliminary ELS measurements at room temperature show a strong surface plasmon at 10.1 (±0.2)eV and a weaker bulk plasmon at 15.1 (±0.2)eV which do not shift with exposure. Their intensities show also an initial oscillation (fig. 3). Then the bulk plasmon intensity decreases exponentially with exposure to about 50% of its initial value, at 350 L (slope: 1.9 X l0~ L~).A second region of slower exponential decrease extends up to 3000 L (slope: 1.7 X iü~ L~).The surface plasmon intensity first diminishes by 25% at 200 L, then suffers a slow exponential decrease to 1100 L (slope: 1.7 X l0~ L~),followed by a fast exponential decrease upto3000L(slope:l.1 X i0~ L~). The work function under stationary pressure of l0~ Torr increases linearly by 0.24 eV at 1 L. Then it undergoes a rapid exponential decrease of 0.40 eV at 3 L. A second much slower exponential decrease by a further 0.25 eV is observed up to 2800 L. Adsorption of ethylene was also performed at low temperature (—80°C). The Auger Al peak shows again an oscillatory behaviour but, now, at a very low exposure (fig. 4). The intensity decreases by 12% at 0.04 L! It then stays essentially constant up to 20 L at which point a rapid decrease begins until 65 L when it has dropped by 50% (exponential slope:1.5 X 10—2 L~).After a plateau to 160 L, the intensity decreases again to about 12% of the original level at 360 L. The initial exponen02

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1 and there is an indication of a step at 290 L. The retial slope is 8.5 X 10~ L due to hydrocarbon trapping. The C signal shows a slight sidual C level is significant, increase at about 1 L followed by an increase in two steps at 10 and 40 L, concomitant with the fast decrease in the Al peak, then a slow but steady exponential increase to 370 L, the highest exposure used for this experiment (slope:2.2 X i03 L1). Upon adsorption at —80°Cthe background of the LEED pattern increases so that contrast with the spots disappears at about 250 L. However, upon heating so —30°C or above, after 250 L exposure, the (I X 1) Al pattern is recovered while the Auger spectrum is not modified. 3.2. Results on H 2 adsorption There is no noticeable influence of the current density on the hydrogen adsorption results. As in the case of C2H4 adsorption, the Al Auger peak neither shifts nor broadens. At room temperature, the Al Auger signal oscillates between —21% at 2.5 L to +53% at 6 L (fig. 5) then drops by 23% to a constant level from 10 to 40 L. Three regions of approximate are then observed from 40 to to 7000 100 L 3 L—’),exponential 100 to 900 decrease L (slope:7.7 X i0~ L1) and 1000 L (slope :3.9 XX i0 (slope:2.8 iO~ L1) where the final level of the Al Auger peakat 40000 Lis

K. Khonde eta!. /Interaction of ethylene and hydrogen with A!(100)

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almost reached. This level corresponds to 34% of the initial clean value. By heating the sample for 30 s to 400°C after a 40000 L exposure, the Al peak grows back to 97% of its initial value. In the corresponding LEED photographs, the (1 X I) AI(100) pattern is maintained up to 40 000 L but a small decrease of the contrast is observed. The equilibrium work function increases linearly between 0 and 2 L with a final L~I = 0.235 eV. From 2 to 200 L, the slope is much smaller and, at 200 L, the total work function change is only 0.250 eV. This value is unchanged up to 3300 L. AES measurements have also been performed at low temperature (—80°C).The Al peak oscillates between —21% at 2 L to +145% at 12 L (fig. 6), and then suffers a decrease which, up to 300 L, is similar to that observed at room temperature. Preliminary ELS measurements at —80°Cshow again no shift of the surface or of the bulk plasmon. The “usual” oscillatory behaviour is observed at low exposure for both plasmons at about 5 L (fig. 7). From 15 to 300 L, the surface plasmon intensity slowly decreases (exponential slope:1.2 X l0~ L~) whereas the bulk plasmon intensity rapidly decreases by 55% from 10 to 100 L, then remains essentially constant up to 300 L.

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K. Khonde eta!. / Interaction of ethylene and hydrogen with Al(100)

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4. Discussion In this section, we give a tentative explanation of our observations. A more quantitative interpretation must await complementary observations. The initial work function increase measured up to I L for C2H4 and up to 2 L for H2 is in agreement with the work of Franken and Ponec [18]. Since no carbon increase is detected in this range of C2H4 exposures, this implies that C2H4 spontaneously dehydrogenates at room temperature on AI(100) and that the resulting hydrocarbon is readily desorbing. This initial desorption is not due to the primary electron beam since exposure to H2 and to C2H4 give a similar increase in the work function. This increase indicates that the hydrogen atoms are bound to the surface with a negative dipole moment. Theoretical calculations give indeed for this dipole moment:0.13 au. for the on-top position [11—13]and practicallyzerofortheenergetically favoured bridge position. We thus suggest that adsorption of H2 is occurring first “on-top” in sites that can, for example, be favourably exposed positions on edges of terraces. As exposure proceeds, the number of bridge positions occupied increases so that the potential drop created by the adsorbed layer increases more slowly. This is indeed what we observe beyond 2.5 L for the work function change upon H2 exposure at room temperature. The initial oscillation of the AES signal and of the plasmon intensity is more difficult to explain. The initial decrease occurs when only a few hydrogen atoms are present on the surface : at —80°C,the minimum value is already reached for 0.04 L of C2 H4. It is thus extremely unlikely that the initial decrease of the Al Auger signal be entirely due to absorption of the glancing electron beam and of the Auger electrons by the on-top adsorbed hydrogen atoms. We then suggest that part of the effect, for the Auger spectrum, is due to a change in the Al transition matrix elements due to the local fields created by the adsorption. Pellerin and collaborators [211 have also observed a peculiar decrease of the Auger signal of clean aluminium 2) but only theour primary than thosewhen used in study. beam current density is much higher (10—2 A cm AES and work function results indicate that hydrocarbon desorption stops when C 2H4 exposure increases beyond 1 L unless it is stimulated by a high current density. Furthermore the hydrocarbon must undergo a further reaction, possibly dehydrogenation and cyclization or polymerization, that binds it in a stable manner to the surface so that it cannot be released, even by heating at 450°C.Since carbon and hydrogen have comparable electronegativities, one can conjecture that the carbon atoms produce a positive surface dipole moment if they are incorporated which explains the drop of the work function beyond I L. The decrease of the Auger intensity beyond a few langrnuirs both for the H2 and C2H4 adsorptions can be interpreted in the light ofthe ELS and LEED results. During low current density C2H4 or H2 room temperature adsorptions, the adatoms must be distributed in such a way that they preserve the symmetry of the 2-dimensional

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K. Khonde eta!. /Interaction of ethylene and hydrogen with Al(1 00,)

Al(100) lattice. In contrast, low temperature C2H4 adsorption leads to a random distribution but heating to —30°Ctransforms this distribution into an ordered one. On the other hand the disordered phase formed at high current density is presumably amorphous aluminium hydride. This compound is known to decompose at 150°C, which explains why hydrogen is removed and the aluminium lattice reconstructed by heating at 450°C. The decrease in aluminium c~ensityon going from Al to AlH3 is responsible for a 50% decrease in the Al Auger peak intensity. But since the final Al signal is still lower by a factor of 3, either the hydrogen concentration in the upper layers is much higher than in A1H3 or the Auger sensitivity factor for Al is decreased in A1H3. In the case of low current density C2H4 or H2 exposure at room temperature, the variations of the Auger peaks and of the plasmon signals can be interpreted by assuming that incorporation of carbon and hydrogen in the upper layer is occurring in the form of ordered islands. In a study on the oxydation of magnesium (0001) face, Namba et al. [22] have found that random adsorption was pinning electrons that would otherwise participate in the collective plasmon oscillation. The latter is thus not excited in the vicinity of an isolated adatom and the measured surface plasmon intensity is rapidly decreasing. The bulk plasmon intensity is also decreasing since the excitation region lies deeper into the bulk due to the presence of an incorporated layer of oxygen ions. Similarly, during the first 200 L of C2H4 exposure, where the surface plasmon intensity is slightly decreasing, pinning is probably occurring but it ceases from 200 L up to 1100 L, since the surface plasmon intensity is hardly varying at all. We thus anticipate that the nuclei of the ordered islands are formed around 100—200 L. Furthermore, since the Auger peak is almost unchanged in this range, it is probable that the carbon and hydrogen species are incorporated below the first aluminium plane in agreement with the work function decrease. This is also confirmed by the fast initial decrease of the bulk plasmon intensity. This comes from the progressive migration into the bulk of the excitation region of the bulk plasmon, due initially to pinning and, beyond 100—200 L, to the presence of the ordered islands. Since the bulk plasmon ceases to decrease and the Auger Al peak begins to decrease around 350 L, we suggest that this exposure corresponds to the merging of the islands which creates at the surface a layer of a solid solution of carbon and hydrogen into aluminium. Upon further exposure both plasmon intensities decrease only very slowly. We infer that the thickness of the solution layer is essentially constant and that the supplementary incorporation of carbon and hydrogen is ordered and does not modify the surface plasmon energy. However, surface aluminium atoms must either be displaced or covered by carbon atoms in order to explain the simple relation between the decrease of the Al peak and the increase of the carbon signal. Since the latter seems to level off where the former is stationary, we assume that the solution layer is saturated at 1500 L. Merging of the islands occurs then at half the saturation carbon concentration. At —80°C,carbon incorporation seems to follow the same sequence but is faster by a factor of ten. The solution layer is saturated at 70 L.

K. Khonde eta!. /Interaction of ethylene and hydrogen with A!(100)

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The structure of the layer is still unknown but it could be similar to that of NaC1 since the Al—C distance is close to half the lattice constant ofaluminium [23]. Beyond 1600 L at room temperature and 160 L at —80°C,aluminium atoms are masked by anew carbon layer which efficiently inhibits the room temperature surface plasmon. To what extent hydrogen incorporates beyond 1 L of C2H4 is still an open question since its presence cannot be directly detected by AES. However, H2 exposure leads to observations that resemble those made with C2 H4. In particular, the LEED pattern is conserved at room temperature even when the Al peak decreases. At —80°C, the bulk plasmon decreases rapidly while up to 60 L at least, the Al peak and the surface plasmon are nearly constant. It is thus probable that ordered incorporation is possible for hydrogen as well but complementary observations are needed in order to reach a more satisfactory picture. In particular, we intend to perform mass spectrometric and infrared measurements in order to identify the species that are present at or desorbing from the surface. —

—

Acknowledgement We would like to thank Dr. Welter (KFA Julich) for the generous gift ofa number of single crystals of aluminium. We are grateful to Prof. A. Lucas for many helpful discussions, to Prof. R. Caudano for his kind interest and special consideration is given to M. Renier for his very skilful technical help. One of us (K. K.) wants to thank the Administration pour Ia Cooperation et le Développement for a grant. This research was performed under the auspices of the Belgian Ministry for Science Policy.

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[17] S.A. Flodström, L.G. Petersson and S.B.M. Hagstrom, J. Vacuum Sci. Technol. 13 (1976) 280. [18] P.E.C. Franken and V. Ponec, Surface Sci. 53 (1975) 341. [19] F. Nakao, Vacuum 25 (1975) 431. [20] K. Besocke and S. Berger, Rev. Sci. Instrum. 47 (1976) 840. [21] F. Pellerin, C. le Gressus and D. Massignon, private communication. [22] H. Namba, J. Darville and J.M. Gilles, Solid State Commun. 34 (1980) 287. [23] GA. Jeffrey and V.Y. Wu, Acta Cryst. 16 (1963) 559.