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Adsorption and desorption of lead on low-index and stepped copper surfaces
Surface Science 80 (1979) 421-429 0 North-Holland Publishing Company
ADSORPTION AND DESORPTION OF LEAD ON LOW-INDEX AND STEPPED COPPER SURFACES Marie-Genevieve BARTH& and Gordon E. RHEAD Laboratoire de Physico-Chimiedes Surfaces, ENSCP, UniversitiPierre et Marie Curie, II, rue Pierre et Marie Curie, F-75005 Paris, France Results from LEED, AES and isothermal desorption experiments are reported for monolayers of lead on low-index and stepped copper surfaces. Data for the (loo), (711), (511), (311), (211) and (111) substrate orientations are discussed. Plots were made of the Auger peak-topeak heights as a function of time during adsorption. These plots show breaks corresponding to the formation of dense monolayers and also earlier breaks due to changes in the sticking probability at the completion of submonolayer structures. The breaks permit a calibration of the spectrometer. For all substrate orientations LEED patterns were obtained with four-fold coincidence periodicities which are interpreted in terms of dense rows of lead atoms. For the stepped surfaces these rows are always parallel to the step directions. The desorption data (peak-to-peak heights as a function of time) show zeroorder kinetics with initially a fast reaction and then a slow reaction that sets in at a coverage that depends on the orientation. The dominance of the fast reaction for (111) is related to the subsistence of islands of the compact monolayer arrangement down to low coverages.
1. Introduction This paper reports results obtained by LEED, Auger electron spectroscopy @ES) and isothermal desorption experiments on lead monolayers adsorbed onto several low-index and stepped copper surfaces. Two previous papers [1,23 have described certain features of the adsorption of lead on copper. The first study [l] dealt with low-index surfaces, examined by LEED alone, and was especially concerned with the melting behaviour of the complete monolayers. In the second paper
Table 1 Type of experiment reported in different papers Substrate orientation
LEED AES Desorption
(100)
(711)
(511)
(311)
(211)
(111)
111 121
VI 121
VI 121
BR
BR
BR BR BR
BR BR BR
BR BR
421
111
422
M-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
[2] the emphasis was on data from AES and on the effect of atomic steps on the growth modes and structures of the adsorbed layers. Table 1 shows how the various studies on different faces are reported in different papers. BR refers to the work reported here. We have completed the data with results from Auger spectroscopy for the (111) face and from LEED and AES on two stepped orientations: (3 11) and (211). In addition we have performed some desorption experiments on two lowindex and three stepped surfaces. We set out to answer the following questions: (i) In what way do atomic steps affect the sticking probabilities of an impinging vapour, the structure of the adsorbed layer and the early stages of growth of ultrathin iilms? (ii) Can decoration of steps coupled with detection by AES lead to a practical method for the detection of steps? (iii) How do steps affect desorption processes? (iv) Can desorption measurements provide a means for monitoring steps concentrations? (Questions (i) and (ii) were also examined in ref. [2] .) The basic strategy of using an adsorbed vapour beam to probe surface properties has been discussed elsewhere [3]. An advantage of using a metal adsorbate is that in comparison with more reactive elements it is less likely to provoke changes in the substrate structure.
2. Experimental The general experimental arrangement and procedures were the same as those described previously [2,4] with the exception that the Auger spectra were obtained with cylindrical mirror analyser (Riber). A constant flux of lead vapour, equivalent to about 1 monolayer in 10 min, was obtained from a crucible maintained at a stabilized temperature of about 6OO’C. The vapour beam could be interrupted by means of a shutter. All the adsorption experiments were performed with the specimen at ambient temperature. In a typical desorption experiment the substrate, covered with about one monolayer of lead, was heated as quickly as possible to about 48O’C (very slow desorption started at about 3OO’C). The Auger peaks for lead and copper were then recorded at regular intervals.
3, Results 3.1. Results from LEED and AES Fig. 1 shows the AES results obtained for adsorption on the (111) substrate. Peak-to-peak heights in the differentiated Auger spectra for the substrate and the adsorbate are plotted (a) against deposition time, (b) against each other. The form of fig. la corresponds to the growth of a first monolayer (linear variations up to 8 min) followed by a second layer (change of slope). The break at the first mono-
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
423
200.
150.
ioo-
50.
1
0 a
50
40
KF b
a
b
Fig. 1. AES data for the adsorption of PB on Cu(ll1). (a) Substrate and adsorbate signals as a function of deposition time. (b) Substrate versus adsorbate signal.
layer is slightly rounded. This type of behaviour (Frank-van der Merwe mode [3]) is quite different from that observed for the other faces examined - all of which exhibit more or less flat plateaux after the “monolayer break” (Stranski-Krastanov mode, i.e. a monolayer followed by small crystallites). The only LEED pattern observed from the adsorbed layer was a p(4 X4) coincidence pattern as reported previously [ 11. This has been ascribed to a dense pseudo-hexagonal arrangement [ 11. (Four-fold coincidences are common for this adsorbate-substrate system because the ratio of the atomic diameters, dPb/dCu = 1.37, is close to 4/3.) The possibility of second layer growth on the (111) orientation was already indicated in ref. [ 11. No new diffraction spots are observed for the second layer but the {3/4 0) type spots that correspond to the lead crystalline parameter gradually become enhanced at the expense of the other spots [ 11. The Auger plot of fig. lb for the (111) face shows, however, an anomaly in the second monolayer region. A simple analysis [5] shows that this plot should be in the form of two straight lines with a change of slope at the completion of the first monolayer. This change should depend only on the relative attenuation coefficients for the copper (63 eV) and the lead (93 eV) Auger electrons through lead. The observed variation suggests that there may be continuous changes in the effective
424
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
attenuation coefficients, possibly related to electronic changes in the adsorbed layers. Other evidence has been found for anomalous AES response from double layers [6] : further results (for lead on stepped gold surfaces) will be presented elsewhere
151. The AES plot for adsorption on the (311) face is shown in fig. 2. The break rjt q corresponds to the completion of a dense monolayer. The slow variations after q are probably due to the formation of crystallites (Stranski-Krastanov modi) rather than to a second layer. An “early break” at p precedes the “monolay& break”. Similar early breaks have been observed for different systems [2,6]. They can be ascribed to sudden changes in the sticking probability that can occur when an ad-layer structure in the submonolayer range reaches its maximum density. From the decreases in the substrate signal and the increases in the adsorbate signal (both assumed to be proportional to the coverage) it is found that the break at p corresponds to a coverage of 0.71 times the dense monolayer. LEED from the clean (3 11) substrate shows the normal, unreconstructed, mesh. Two different LEED patterns are observed during adsorption: the corresponding meshes are shown in fig. 3. The first structure (fig. 3a) can be described by adsorp-
Pb (93ev)
5 Fig. 2.
10
15
20
t (mln) 25
AES data for the adsorption of Pb on Cu(311).
-
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
425
Fig. 3. Structuralinterpretationsfor the LEEDpatternsobservedfor the (311) face.
tion of dense rows parallel to the steps while the second (fig. 3b) can be interpreted as a pseudo-hexagonal arrangement. The ad-atoms are somewhat compressed in the first structure but the same five-fold coincidence with ad-atom chains was also found for the (110) substrate [ 11. (In all the structures proposed the exact registry between adsorbate and substrate is of course unknown. For the highly stepped (3 11) and (211) faces the notion of “step site” loses some meaning: we have chosen registries that place some ad-atoms in sites of four-fold coordination.) The first ad-layer model for the (3 11) face has a density 0.7 1 times the second - which agrees with results from AES and so supports the plausibility of these interpretations. For the (211) face the AES plots exhibit a single sharp dense monolayer break followed by plateaux. LEED from the clean surface shows a mesh characteristic of the normali unreconstructed, substrate. On adsorption of lead parallel streaks are observed with spacings which correspond to a four-fold parameter parallel to the step direction (i.e. ascribable to linear chains of lead atoms parallel to the steps). Near the break in the Auger plot the pattern corresponds to the mesh shown in fig. 4. A pseudo-hexagonal overlayer could fill this mesh as shown. The pattern is, however,
Fig. 4.
structuoral
:xpretation for the LEED pattern obsex,ved for the
(211) face.
426
M.-G.Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
rather blurred and there could occur mixtures of this structure and a p(4 X 1) structure which in turn could be due to a different pseudohexagonal arrangement related to that shown in fig. 4 by a shear displacement parallel to the steps. Thus for all the stepped substrates, including those reported previously [2], there is evidence for compact arrangements in which dense rows of adsorbed lead atoms are oriented parallel to the atomic steps. 3.2. Isothermal desorption measurements Several recent studies [7-IO] have investigated the desorption of metal monolayers under isothermal conditions by using AES to monitor the quantity of adsorbate remaining on the surface. We have adopted the same general method: the specimen was heated to a chosen, stabilized, temperature and the Auger peaks for the adsorbate and the substrate were recorded at regular intervals, typically every two minutes. The experiments could be performed satisfactorily only over a narrow temperature range: about 460 to 520°C. The rate was too low at lower temperatures and too rapid for the recording of the spectra at higher temperatures. Fig. 5 shows the results obtained for the different orientations at 485 * 10°C. The various curves were obtained from the desorption Auger plots (peak-to-peak heights as a function of time) calibrated in terms of the dense monolayer by means of the Auger plots obtained for adsorption. There are marked differences between
Fig. 5. Isothermal desorption plots at 485 f 10°C for lead on different substrates. Coverage, normalized to the dense monolayer (0 = l.O), as a function of time t (origin displaced).
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
427
the different orientations. For the (111) face there occurs a rapid desorption, at a constant rate, until about 0.7 of a monolayer has been desorbed - thereafter the rate falls to about 7% of the initial value. For the other orientations the slow rate (approximately the same for each face) sets in at much higher coverages. For the rapid desorption from the (111) face it was possible to make measurements at several temperatures and to obtain a very approximate value for the apparent activation energy: 35 + 10 kcal mole’ (The enthalpy of sublimation of lead is 46 kcal mole-r). The form of the curves shows that more than one type of desorption process must be taking place. To fit the rapidly varying curves to a single reaction would require an order of reaction greater than 5 - which is physically unreasonable. One plausible rationalization of the data would be to represent the plots by two straight lines, i.e. two zeroorder desorption reactions with, for each substrate, an overlap of the two reactions over a coverage range of 7 to 10% of a monolayer. (The slight differences in the fast rate for the different orientations is probably not significant in view of the difficulties in reproducing the temperature precisely.) An interpretation in terms of zeroorder reactions would agree with recent work, as discussed below.
4. Discussion The results from LEED and AES for the stepped surfaces can be compared with those from the previous study [2]. The final monolayer structures can all be interpreted as dense pseudo-hexagonal arrangements with dense rows parallel to the step directions. However, there is no evidence that the (3 11) and (211) stepped surfaces give rise to disordered adsorbed layers at lower coverages as was the case for the (711) and (5 11) faces. Once again (for (3 11)) we find evidence of a “premonolayer break” due to a sudden change in sticking probability at a particular coverage that coincides with the completion of chains of atoms parallel to the steps. No “premonolayer break” was observed for (211). It is significant that out of six substrate orientations examined only the (111) face - the most atomically smooth - shows clear evidence of growth of a second adsorbate layer. An important feature of the isothermal desorption plots (fig. 5) is that the (100) substrate shows the same kind of desorption behaviour as the stepped surfaces, i.e. a dominance of the slow desorption rate that sets in at a high coverage. Thus the special behaviour of the (111) face in comparison with the other substrates must arise from some factor other than the absence of steps. We believe this special behaviour is due to the tendency for the dense compact monolayer structure to subsist even at quite low coverages (cf. fig. la). We suggest that the rapid desorption mode is through activation of atoms on top of the first layer while the slow mode can be ascribed to desorption from a more dispersed adsorbate phase in a first-layer structure.
428
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
This interpretation would agree with the explanations put forward for zero-order kinetics in desorption by Venables and Bienfait [ll]. These authors have shown that zero-order kinetics can occur simply through the simultaneous presence of two adsorbed surfaces phases, e.g. a condensed crystalline phase and a more dispersed phase - which may be either another crystalline structure with a lower density or a gas-like phase either on the substrate or on top of the dense first layer. According to this model [ 1l] desorption occurs from the lowdensity phase in conditions of rapid exchange between phases. Thus starting from a complete monolayer the initial desorption is most likely to be via activation of atoms into a mobile gas-like layer on top of the first layer. This mechanism must stop as soon as the ad-layer density cannot maintain a dense compact structure necessary to support a second layer. Thereafter desorption would be via a dispersed submonolayer phase on the substrate and in general it would occur at a different rate. Thus the change in the desorption rate would mark the disappearance of the dense monolayer structure. Apparently because of strong lateral cohesion the p(4 X4) structure on (111) is still present down to a coverages less than half a monolayer. (We note that as well as the subsistence of the monolayer structure an additional factor that may influence the desorption kinetics from this face is the ease in which a second layer can be formed.) To pursue the argument quantitatively for the other substrates is difficult because the effect of temperature on the ad-layer structures is not known. This makes it difficult to say at what coverage the dense monolayer structure should disappear. For the (100) face the situation is complicated by the observation [l] of a sharp melting-like behaviour at only 220°C. (For the (111) substrate the disordering of the monolayer occurred at a higher temperature, was much less sharp, and there was evidence for the persistence of some ordered regions even at high temperature [l] .) A further problem is the rounding of plots in fig. 5. (In other work sharp breaks have been observed [7,8,10].) This rounding may be related to the essentially nonequilibrium conditions - a sharp change requires rapid equilibrium between surface phases [ 111. Alternatively, it may represent a range over which the reaction order is 1; such transitional regions are also envisaged theoretically [ 1l] and in the present case would correspond to conditions in which the “gas” on the substrate is relatively dense. In spite of these difficulties of interpretation it is clear from fig. 5, in which there is no simple correlation with step densities, that in these experiments the substrate structure influences the desorption processes not so much directly as indirectly via the ad-layer structure itself and the way it changes during desorption.
5. Conclusion In response to the questions posed in the introduction it has been found that: (i) Sticking probabilities can decrease suddenly at the completion of a submonolayer
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
429
structure and such structures may be related to adsorption at steps. Compact pseudo-hexagonal layers can form equally well on stepped surfaces as on low-index surfaces; in general they form with dense rows parallel to the (dense) step direction. The stepped surfaces examined all exhibited the Stranski-Krastanov growth mode. (ii) Step decoration and detection by AES appears to be a difficult and not very sensitive procedure for monitoring step concentrations. (iii) and (iv) Certain features of the desorption processes may be affected by the presence of steps but the structure of the adsorbate itself may be more important than the direct influence of the substrate structure.
References
[l] J. Henrionand G.E. Rhead,SurfaceSci 29 (1972) 20. [ 21 A. Sepulvedaand GE. Rhead,SurfaceSei. 66 (1977) 436. [3] G.E. Rbead,J. VacuumSci. Technol. 13 (1976) 603. [4] C. Argileand GE. Rhead,SurfaceSci. 53 (1975) 659. [S] M.G.Barth&sand G.E. Rhead, to be published. [6] C. Argile and G.E. Rhead, Surface Sci. 78 (1978) 115,125. [7] G. Le Lay, Manneville and R. Kern, Surface Sci. 65 (1977) 261. [ 81 R. Kern and G. Le Lay, J. Phys. (Paris) (Colloque C4 Suppl. au No. 10) 38 (1977) C4-99. [9] R.G. Jones and D.L. Perry, Surface Sci. 71 (1978) 59. [lo] G. Le Lay, M. Manneville and R. Kern, Surface Sci. 72 (1978) 405. [ 111 J.A. Venables and M. Bienfait, Surface Sci. 61 (1976) 667.
ADSORPTION AND DESORPTION OF LEAD ON LOW-INDEX AND STEPPED COPPER SURFACES Marie-Genevieve BARTH& and Gordon E. RHEAD Laboratoire de Physico-Chimiedes Surfaces, ENSCP, UniversitiPierre et Marie Curie, II, rue Pierre et Marie Curie, F-75005 Paris, France Results from LEED, AES and isothermal desorption experiments are reported for monolayers of lead on low-index and stepped copper surfaces. Data for the (loo), (711), (511), (311), (211) and (111) substrate orientations are discussed. Plots were made of the Auger peak-topeak heights as a function of time during adsorption. These plots show breaks corresponding to the formation of dense monolayers and also earlier breaks due to changes in the sticking probability at the completion of submonolayer structures. The breaks permit a calibration of the spectrometer. For all substrate orientations LEED patterns were obtained with four-fold coincidence periodicities which are interpreted in terms of dense rows of lead atoms. For the stepped surfaces these rows are always parallel to the step directions. The desorption data (peak-to-peak heights as a function of time) show zeroorder kinetics with initially a fast reaction and then a slow reaction that sets in at a coverage that depends on the orientation. The dominance of the fast reaction for (111) is related to the subsistence of islands of the compact monolayer arrangement down to low coverages.
1. Introduction This paper reports results obtained by LEED, Auger electron spectroscopy @ES) and isothermal desorption experiments on lead monolayers adsorbed onto several low-index and stepped copper surfaces. Two previous papers [1,23 have described certain features of the adsorption of lead on copper. The first study [l] dealt with low-index surfaces, examined by LEED alone, and was especially concerned with the melting behaviour of the complete monolayers. In the second paper
Table 1 Type of experiment reported in different papers Substrate orientation
LEED AES Desorption
(100)
(711)
(511)
(311)
(211)
(111)
111 121
VI 121
VI 121
BR
BR
BR BR BR
BR BR BR
BR BR
421
111
422
M-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
[2] the emphasis was on data from AES and on the effect of atomic steps on the growth modes and structures of the adsorbed layers. Table 1 shows how the various studies on different faces are reported in different papers. BR refers to the work reported here. We have completed the data with results from Auger spectroscopy for the (111) face and from LEED and AES on two stepped orientations: (3 11) and (211). In addition we have performed some desorption experiments on two lowindex and three stepped surfaces. We set out to answer the following questions: (i) In what way do atomic steps affect the sticking probabilities of an impinging vapour, the structure of the adsorbed layer and the early stages of growth of ultrathin iilms? (ii) Can decoration of steps coupled with detection by AES lead to a practical method for the detection of steps? (iii) How do steps affect desorption processes? (iv) Can desorption measurements provide a means for monitoring steps concentrations? (Questions (i) and (ii) were also examined in ref. [2] .) The basic strategy of using an adsorbed vapour beam to probe surface properties has been discussed elsewhere [3]. An advantage of using a metal adsorbate is that in comparison with more reactive elements it is less likely to provoke changes in the substrate structure.
2. Experimental The general experimental arrangement and procedures were the same as those described previously [2,4] with the exception that the Auger spectra were obtained with cylindrical mirror analyser (Riber). A constant flux of lead vapour, equivalent to about 1 monolayer in 10 min, was obtained from a crucible maintained at a stabilized temperature of about 6OO’C. The vapour beam could be interrupted by means of a shutter. All the adsorption experiments were performed with the specimen at ambient temperature. In a typical desorption experiment the substrate, covered with about one monolayer of lead, was heated as quickly as possible to about 48O’C (very slow desorption started at about 3OO’C). The Auger peaks for lead and copper were then recorded at regular intervals.
3, Results 3.1. Results from LEED and AES Fig. 1 shows the AES results obtained for adsorption on the (111) substrate. Peak-to-peak heights in the differentiated Auger spectra for the substrate and the adsorbate are plotted (a) against deposition time, (b) against each other. The form of fig. la corresponds to the growth of a first monolayer (linear variations up to 8 min) followed by a second layer (change of slope). The break at the first mono-
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
423
200.
150.
ioo-
50.
1
0 a
50
40
KF b
a
b
Fig. 1. AES data for the adsorption of PB on Cu(ll1). (a) Substrate and adsorbate signals as a function of deposition time. (b) Substrate versus adsorbate signal.
layer is slightly rounded. This type of behaviour (Frank-van der Merwe mode [3]) is quite different from that observed for the other faces examined - all of which exhibit more or less flat plateaux after the “monolayer break” (Stranski-Krastanov mode, i.e. a monolayer followed by small crystallites). The only LEED pattern observed from the adsorbed layer was a p(4 X4) coincidence pattern as reported previously [ 11. This has been ascribed to a dense pseudo-hexagonal arrangement [ 11. (Four-fold coincidences are common for this adsorbate-substrate system because the ratio of the atomic diameters, dPb/dCu = 1.37, is close to 4/3.) The possibility of second layer growth on the (111) orientation was already indicated in ref. [ 11. No new diffraction spots are observed for the second layer but the {3/4 0) type spots that correspond to the lead crystalline parameter gradually become enhanced at the expense of the other spots [ 11. The Auger plot of fig. lb for the (111) face shows, however, an anomaly in the second monolayer region. A simple analysis [5] shows that this plot should be in the form of two straight lines with a change of slope at the completion of the first monolayer. This change should depend only on the relative attenuation coefficients for the copper (63 eV) and the lead (93 eV) Auger electrons through lead. The observed variation suggests that there may be continuous changes in the effective
424
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
attenuation coefficients, possibly related to electronic changes in the adsorbed layers. Other evidence has been found for anomalous AES response from double layers [6] : further results (for lead on stepped gold surfaces) will be presented elsewhere
151. The AES plot for adsorption on the (311) face is shown in fig. 2. The break rjt q corresponds to the completion of a dense monolayer. The slow variations after q are probably due to the formation of crystallites (Stranski-Krastanov modi) rather than to a second layer. An “early break” at p precedes the “monolay& break”. Similar early breaks have been observed for different systems [2,6]. They can be ascribed to sudden changes in the sticking probability that can occur when an ad-layer structure in the submonolayer range reaches its maximum density. From the decreases in the substrate signal and the increases in the adsorbate signal (both assumed to be proportional to the coverage) it is found that the break at p corresponds to a coverage of 0.71 times the dense monolayer. LEED from the clean (3 11) substrate shows the normal, unreconstructed, mesh. Two different LEED patterns are observed during adsorption: the corresponding meshes are shown in fig. 3. The first structure (fig. 3a) can be described by adsorp-
Pb (93ev)
5 Fig. 2.
10
15
20
t (mln) 25
AES data for the adsorption of Pb on Cu(311).
-
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
425
Fig. 3. Structuralinterpretationsfor the LEEDpatternsobservedfor the (311) face.
tion of dense rows parallel to the steps while the second (fig. 3b) can be interpreted as a pseudo-hexagonal arrangement. The ad-atoms are somewhat compressed in the first structure but the same five-fold coincidence with ad-atom chains was also found for the (110) substrate [ 11. (In all the structures proposed the exact registry between adsorbate and substrate is of course unknown. For the highly stepped (3 11) and (211) faces the notion of “step site” loses some meaning: we have chosen registries that place some ad-atoms in sites of four-fold coordination.) The first ad-layer model for the (3 11) face has a density 0.7 1 times the second - which agrees with results from AES and so supports the plausibility of these interpretations. For the (211) face the AES plots exhibit a single sharp dense monolayer break followed by plateaux. LEED from the clean surface shows a mesh characteristic of the normali unreconstructed, substrate. On adsorption of lead parallel streaks are observed with spacings which correspond to a four-fold parameter parallel to the step direction (i.e. ascribable to linear chains of lead atoms parallel to the steps). Near the break in the Auger plot the pattern corresponds to the mesh shown in fig. 4. A pseudo-hexagonal overlayer could fill this mesh as shown. The pattern is, however,
Fig. 4.
structuoral
:xpretation for the LEED pattern obsex,ved for the
(211) face.
426
M.-G.Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
rather blurred and there could occur mixtures of this structure and a p(4 X 1) structure which in turn could be due to a different pseudohexagonal arrangement related to that shown in fig. 4 by a shear displacement parallel to the steps. Thus for all the stepped substrates, including those reported previously [2], there is evidence for compact arrangements in which dense rows of adsorbed lead atoms are oriented parallel to the atomic steps. 3.2. Isothermal desorption measurements Several recent studies [7-IO] have investigated the desorption of metal monolayers under isothermal conditions by using AES to monitor the quantity of adsorbate remaining on the surface. We have adopted the same general method: the specimen was heated to a chosen, stabilized, temperature and the Auger peaks for the adsorbate and the substrate were recorded at regular intervals, typically every two minutes. The experiments could be performed satisfactorily only over a narrow temperature range: about 460 to 520°C. The rate was too low at lower temperatures and too rapid for the recording of the spectra at higher temperatures. Fig. 5 shows the results obtained for the different orientations at 485 * 10°C. The various curves were obtained from the desorption Auger plots (peak-to-peak heights as a function of time) calibrated in terms of the dense monolayer by means of the Auger plots obtained for adsorption. There are marked differences between
Fig. 5. Isothermal desorption plots at 485 f 10°C for lead on different substrates. Coverage, normalized to the dense monolayer (0 = l.O), as a function of time t (origin displaced).
M.-G. Barthes, G.E. Rhead /Adsorption and desorption of lead on copper
427
the different orientations. For the (111) face there occurs a rapid desorption, at a constant rate, until about 0.7 of a monolayer has been desorbed - thereafter the rate falls to about 7% of the initial value. For the other orientations the slow rate (approximately the same for each face) sets in at much higher coverages. For the rapid desorption from the (111) face it was possible to make measurements at several temperatures and to obtain a very approximate value for the apparent activation energy: 35 + 10 kcal mole’ (The enthalpy of sublimation of lead is 46 kcal mole-r). The form of the curves shows that more than one type of desorption process must be taking place. To fit the rapidly varying curves to a single reaction would require an order of reaction greater than 5 - which is physically unreasonable. One plausible rationalization of the data would be to represent the plots by two straight lines, i.e. two zeroorder desorption reactions with, for each substrate, an overlap of the two reactions over a coverage range of 7 to 10% of a monolayer. (The slight differences in the fast rate for the different orientations is probably not significant in view of the difficulties in reproducing the temperature precisely.) An interpretation in terms of zeroorder reactions would agree with recent work, as discussed below.
4. Discussion The results from LEED and AES for the stepped surfaces can be compared with those from the previous study [2]. The final monolayer structures can all be interpreted as dense pseudo-hexagonal arrangements with dense rows parallel to the step directions. However, there is no evidence that the (3 11) and (211) stepped surfaces give rise to disordered adsorbed layers at lower coverages as was the case for the (711) and (5 11) faces. Once again (for (3 11)) we find evidence of a “premonolayer break” due to a sudden change in sticking probability at a particular coverage that coincides with the completion of chains of atoms parallel to the steps. No “premonolayer break” was observed for (211). It is significant that out of six substrate orientations examined only the (111) face - the most atomically smooth - shows clear evidence of growth of a second adsorbate layer. An important feature of the isothermal desorption plots (fig. 5) is that the (100) substrate shows the same kind of desorption behaviour as the stepped surfaces, i.e. a dominance of the slow desorption rate that sets in at a high coverage. Thus the special behaviour of the (111) face in comparison with the other substrates must arise from some factor other than the absence of steps. We believe this special behaviour is due to the tendency for the dense compact monolayer structure to subsist even at quite low coverages (cf. fig. la). We suggest that the rapid desorption mode is through activation of atoms on top of the first layer while the slow mode can be ascribed to desorption from a more dispersed adsorbate phase in a first-layer structure.
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This interpretation would agree with the explanations put forward for zero-order kinetics in desorption by Venables and Bienfait [ll]. These authors have shown that zero-order kinetics can occur simply through the simultaneous presence of two adsorbed surfaces phases, e.g. a condensed crystalline phase and a more dispersed phase - which may be either another crystalline structure with a lower density or a gas-like phase either on the substrate or on top of the dense first layer. According to this model [ 1l] desorption occurs from the lowdensity phase in conditions of rapid exchange between phases. Thus starting from a complete monolayer the initial desorption is most likely to be via activation of atoms into a mobile gas-like layer on top of the first layer. This mechanism must stop as soon as the ad-layer density cannot maintain a dense compact structure necessary to support a second layer. Thereafter desorption would be via a dispersed submonolayer phase on the substrate and in general it would occur at a different rate. Thus the change in the desorption rate would mark the disappearance of the dense monolayer structure. Apparently because of strong lateral cohesion the p(4 X4) structure on (111) is still present down to a coverages less than half a monolayer. (We note that as well as the subsistence of the monolayer structure an additional factor that may influence the desorption kinetics from this face is the ease in which a second layer can be formed.) To pursue the argument quantitatively for the other substrates is difficult because the effect of temperature on the ad-layer structures is not known. This makes it difficult to say at what coverage the dense monolayer structure should disappear. For the (100) face the situation is complicated by the observation [l] of a sharp melting-like behaviour at only 220°C. (For the (111) substrate the disordering of the monolayer occurred at a higher temperature, was much less sharp, and there was evidence for the persistence of some ordered regions even at high temperature [l] .) A further problem is the rounding of plots in fig. 5. (In other work sharp breaks have been observed [7,8,10].) This rounding may be related to the essentially nonequilibrium conditions - a sharp change requires rapid equilibrium between surface phases [ 111. Alternatively, it may represent a range over which the reaction order is 1; such transitional regions are also envisaged theoretically [ 1l] and in the present case would correspond to conditions in which the “gas” on the substrate is relatively dense. In spite of these difficulties of interpretation it is clear from fig. 5, in which there is no simple correlation with step densities, that in these experiments the substrate structure influences the desorption processes not so much directly as indirectly via the ad-layer structure itself and the way it changes during desorption.
5. Conclusion In response to the questions posed in the introduction it has been found that: (i) Sticking probabilities can decrease suddenly at the completion of a submonolayer
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structure and such structures may be related to adsorption at steps. Compact pseudo-hexagonal layers can form equally well on stepped surfaces as on low-index surfaces; in general they form with dense rows parallel to the (dense) step direction. The stepped surfaces examined all exhibited the Stranski-Krastanov growth mode. (ii) Step decoration and detection by AES appears to be a difficult and not very sensitive procedure for monitoring step concentrations. (iii) and (iv) Certain features of the desorption processes may be affected by the presence of steps but the structure of the adsorbate itself may be more important than the direct influence of the substrate structure.
References
[l] J. Henrionand G.E. Rhead,SurfaceSci 29 (1972) 20. [ 21 A. Sepulvedaand GE. Rhead,SurfaceSei. 66 (1977) 436. [3] G.E. Rbead,J. VacuumSci. Technol. 13 (1976) 603. [4] C. Argileand GE. Rhead,SurfaceSci. 53 (1975) 659. [S] M.G.Barth&sand G.E. Rhead, to be published. [6] C. Argile and G.E. Rhead, Surface Sci. 78 (1978) 115,125. [7] G. Le Lay, Manneville and R. Kern, Surface Sci. 65 (1977) 261. [ 81 R. Kern and G. Le Lay, J. Phys. (Paris) (Colloque C4 Suppl. au No. 10) 38 (1977) C4-99. [9] R.G. Jones and D.L. Perry, Surface Sci. 71 (1978) 59. [lo] G. Le Lay, M. Manneville and R. Kern, Surface Sci. 72 (1978) 405. [ 111 J.A. Venables and M. Bienfait, Surface Sci. 61 (1976) 667.