- Email: [email protected]
Adsorption of hydrogen on thin fcc-iron films grown on Cu(100)
L271
Surface Science 215 (1989) L271-L278 North-Holland, Amsterdam
SURFACE SCIENCE LETTERS ADSORPTION ON Cu(100)
OF HYDROGEN ON THIN fee-IRON FILMS GROWN
Chikashi EGAWA, Elaine M. McCASH and Roy F. WILLIS * Cavendish Laboratory, Department Cambridge CB3 OHE, UK
of Physics, University of Cambridge,
Madingley
Road,
Received 19 December 1988; accepted for publication 3 February 1989
The desorption kinetics and surface structure of hydrogen adsorbed at 180 K on ultrathin (l-12 ML) iron films grown at 290 K on the (110) face of copper are reported. Below 3 ML, the results indicate that fihn growth is not epitaxial, and a desorption peak appears at 310 K accompanied by a low temperature shoulder. From 3-5 ML of iron, a (4x 1) LEED structure is observed, which persisted on hydrogen adsorption; the main desorption peak begins to shift down and the shoulder becomes more prominent. From 6-10 ML of iron, a p(2 X 2)-p4g superstructure is obtained which has systematic absences of the (h + l/2,0) and (0, k + l/2) spots, from the p(2~ 2), and is highly stabilised by the adsorption of hydrogen. This pattern appears in conjunction with the observation of only a single desorption peak at 290 K and is consistent with alternate displacements of adjacent iron atoms in a structure analogous to that of carbide formation on Ni(100). Above 10 ML of iron the surface structure begins to break down to form another structure. The heat of adsorption of hydrogen on the 8 ML film is 71 kJ mol-’ (+ 1) which is low compared to that on bee iron (88-109 kJ mol-‘).
The growth of ultrathin metal films on single crystal substrates has been an area of great interest for some time. Iron films are of particular relevance because of their unusual magnetic, electronic and in some cases, structural properties [l-6]. On Cu(100) it is found that it grows as a face centred cubic (fee) structure, dictated by the underlying copper surface, as opposed to the body centred cubic structure (bee) which occurs in, for example, iron single crystals [7-91. There exists considerable controversy over the explanation for the “composition” between fee and bee structures and the roles played by the electronic and magnetic properties of the films [l-12]. In order to gain further insight into the effects of these properties - which vary with film thickness - we have studied the adsorption of hydrogen as a function of iron coverage and compared the data to published work on bee iron crystals [13]. Our primary concern in this work is to study the chemistry * Current address: Department of Physics, Davy Laboratory, Pennsylvania State University, PA 16802, USA.
0039-6028/89/$03.50
(North-Holland
0 Elsevier Science Publishers B.V. Physics Publishing Division)
L212
C. Egawa et al. / Aakorption of hydrogen on fee-iron films grown on Cu(lO0)
of these films, in particular the way in which the strain between the epitaxial iron layers effects the chemical properties. We observe these to be quite different from the bulk bee iron and the difference persists even up to film thicknesses of several monolayers. Experiments were carried out in a stainless steel ultrahigh vacuum system equipped for low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work-function measurements. The chamber was pumped by an oil diffusion pump and titanium sublimation pump and operated at a base pressure of less than 1 X lo-*’ mbar. The copper crystal was prepared by mechanical polishing, followed by several cycles of argon ion bombardment and annealing at 450° C in UHV. Iron was deposited from a tungsten boat containing iron wire onto the Cu(100) crystal at 290 K. This substrate temperature was chosen in order to produce surfaces which were as flat as possible. Low substrate temperatures result in “rough” iron films, whereas higher substrate temperatures result in considerable interdiffusion of the iron into the copper. The pressure rise during deposition was to less than 2 X 10-i’ mbar. Surface cleanliness and iron coverage was monitored by AES. The iron uptake was obtained from the ratio of the normalized peak-to-peak Auger intensity of Fe(47 eV) and Cu(63 ev); breaks occurring on completion of each iron layer. (The coverages of iron were calibrated from films of greater than 3 ML.) This method of coverage calibration is quite inaccurate. We estimate an error of the order of half a monolayer in our case. The Fe deposition rate was controlled by monitoring the iron partial pressure during deposition - and this was found to lead to the formation of highly reproducible film thicknesses. Fig. 1 illustrates the LEED patterns obtained for various iron film thicknesses after iron deposition at 290 K and cooling to 180 K. Fig. 2 shows hydrogen thermal desorption spectra (TDS) as a function of iron coverage, after exposure of the iron films to 6 L of hydrogen at 180 K. The clean Cu(100) surface exhibited a p(1 X 1) LEED pattern with sharp substrate spots and a low background as shown in fig. la. On evaporation of up to 2 monolayers (ML) of iron, the p(1 X 1) pattern remained with a slight increase in background intensity. Adsorption of 6 L of hydrogen (corresponding to almost saturation coverages) led to no further change in the LEED pattern, and thermal desorption spectra which had main desorption peaks at 310 K with shoulders in the low temperature region. Thus these films appear to be highly stable. The intensity of the main desorption peak grew with increasing iron coverage up to 3 ML, with no change in position. Since hydrogen adsorption is a sensitive titration technique for measuring the number of surface iron atoms, as molecular hydrogen cannot be adsorbed on Cu(100) under these conditions, these results show that the copper substrate was not uniformly covered by iron atoms even at coverages as high as 2 ML.
C. Egawa et al. / Adsorption of hydrogen on fee-iron film grown on Cu(100)
L213
Fig. 1. LEED patterns for various iron film thicknesses on Cu(100) grown at 290 K. (a) clean Cu(100) - 103 eV; (b) Cu(100)/4ML Fe - 114 eV; (c) Cu(100)/4ML Fe after 6 L hydrogen adsorption at 180 K - 111 eV; (d) Cu(100)/8ML Fe - 114 eV; (e) Cu(lOO)/SML Fe after 6 L hydrogen adsorption at 180 K - 144 eV; (f) Cu(100)/12ML Fe - 112 eV.
L274
C. Egawa et al. / Adsorption of hydrogen on fee-iron films grown
250
300
on
Cu(IO0)
350 400
Temperature I K Fig. 2. Hydrogen thermal desorption spectra from iron films on Cu(100) as a function of iron coverage. Hydrogen exposure: 6 L. (a) 1 ML; (b) 2 ML; (c) 3 ML; (d) 4 ML; (e) 5 ML; (f) 8 ML; (g) 10 ML; (h) 12 ML iron thickness. The broken curve shows hydrogen desorption from an 8 ML film which had not been dosed with hydrogen.
These results are consisted with the formation of two-layer deep clusters in the “first” monolayer, or one-, two-, and three-atom deep islands, which agrees with recently reported work [8,14] using Auger and XPS forward scattering experiments. Another explanation is that there is interdiffusion of iron into the copper causing a lowered hydrogen sticking coefficient at iron coverages up to 2 ML. Above 3 ML iron coverages additional spots due to a (4 X 1) superstructure appeared in the LEED after deposition at 290 K. The LEED observations were obtained at both 290 K and also after cooling to 180 K. There was no apparent difference due to the temperature change. The LEED spots became sharper at 4 ML of iron (fig. lb), but streaky at 5 ML. Adsorption of hydrogen at 180 K or 300 K on these films slightly enhanced the intensities of the overlayer spots as shown in fig. lc. The change in LEED pattern was accompanied by corresponding changes in the thermal desorption spectra. There was an increase in prominence of the low temperature peak and a shift down to lower temperature of the main peak.
C. Egawa et al. / Adrovtion
of hydrogen on fee-iron films grown on Cu(lO0)
L215
In the range of iron film thickness from 6 to 10 ML a weak “(2 x 1)” LEED pattern was observed (fig. Id), after deposition at 290 K and cooling to 180 K. This became more intense on adsorption of hydrogen as shown in fig. le. We denote this pattern as “(2 x 1)” because systematic absences from the p(2 X 1) occurred, namely, the (h + l/2, 0) and (0, k + l/2) order spots were missing. This effect was most obvious at 8 ML iron coverages, where even the (l/2, l/2) order spots - which had previously been very weak - were quite clearly seen, but the systematic absences remained. From these observations the LEED structure obtained can be assigned to a p(2 X 2)-p4g structure. Deposition at 290 K and observation of the LEED pattern at this temperature gave a p(1 X l), although adsorption of hydrogen at this temperature, led to the extremely weak appearance of the “(2 X 1)“. Thus the reversible transition to the p(2 X 2)-p4g superstructure from the p(1 X 1) is associated with the adsorption and desorption of hydrogen under these conditions. These well-ordered LEED patterns are analogous to the (5 x l)/lML and (2 X 1)/2-3ML superstructures reported by Daum et al. [9], but for thicker films in our case. We interpret the difference in terms of the growth conditions, surface defects and roughness. We produced films at 290 K while Daum et al. grew theirs at 350 K. Recent work [15] has shown that significant interdiffusion can occur at high growth temperatures, but our thermal desorption peaks appeared at temperatures below those at which significant structural changes and interdiffusion takes place. The results obtained from repeated thermal desorption experiments on the same film depended on the temperature to which the film had been heated previously. If, for example, the TDS run terminated at 330 K, the results of the following experiment were similar, showing little change in surface structure. If the surface had previously been heated above this temperature and/or held at this temperature for a substantial amount of time, desorption results characteristic of thinner iron films were obtained. Daum et al. reported that the superstructure spot intensity increased with decreasing temperature, and so interpreted this as the transition taking place gradually over a large temperature range due to the repulsive stress between the iron atoms. Our data shows that the “(2 x 1)” superstructure spots were very weak, even after cooling to 180 K (fig. Id). However, they increased in intensity steadily with time, on exposure to hydrogen (fig. le). Furthermore, hydrogen desorption spectra taken from the surface observed in fig. Id showed evidence of the presence of small quantities of hydrogen on the surface (of the order of a few percent of saturation) presumably from adsorption of the hydrogen in the background gases while the surface was at low temperature. This is shown as a broken line in fig. 2. Clearly, the surface reconstruction to the p(2 x 2)-p4g structure is assisted by adsorbed hydrogen. The top-most iron atoms are alternatively displaced to form bonds with the hydrogen atoms in a structure analogous to that proposed for carbide formation on Ni(lOO) [16]. A
L276
C. Egawa et al. / A&sorption of hydrogen on fee-iron films grown on Cu(IO0)
model consistent with these observations is shown in fig. 3. The hydrogen atoms stabilise substrate distortions that lead to a bridge-bonded adsorption site for atomic hydrogen. The phonon softening observed by Daum et al. [9] adds support to the theory that the clean surface is incipiently unstable to lateral displacements of the surface atoms. The role of adsorbed hydrogen on the surface reconstruction is to stab&e the surface iron atoms. The reconstruction can be accounted for in terms of the metastable nature of the fee iron film, where lateral displacements are energetically possible and favoured by the adsorption of hydrogen [17]. In accordance with the development of the (4 X 1) superstructure above 3 ML of iron, the hydrogen desorption peak shifted gradually to lower temperature with iron coverage and finally gave a single desorption peak at 290 K for the 8 ML film as shown in fig. 2f. The desorption temperature and narrow half width of this peak at saturation clearly resembles those observed for the bee Fe(lOO)/H, system [13] where very high exposures (60-600 L of Hz) were used and a low temperature adsorption state was formed. At low exposures the observed hydrogen desorption peak on bee Fe(lOO) was at 400 K. The p(2 x 2)-p4g superstructure is not considered to correspond to adsorbate structure caused by the hydrogen; thus the absolute hydrogen coverage cannot be determined by this study. According to our surface model (shown in fig. 3) we postulate that the hydrogen coverage is 0.5. The desorp-
0
First layer Fe atoms
@ Second layer Fe atoms 0
Hydrogen atoms
Fig. 3. Surface structure model for the p(2 X 2)-p4g pattern of an 8 ML iron film on Cu(100) with adsorbed hydrogen. The dotted circles represent the unreconstructed (1 X 1) surface iron atoms.
C. Egawa et al. / Adsorption of hydrogen on fee-iron films grown on Cu(100)
L271
tion energy of hydrogen is calculated to be 71 kJ mole1 (+ 1) (using the Redhead method [18]) for the well defined 8 ML film. This is much lower than the corresponding values for the main desorption peaks reported for the low index faces of bee single crystals of iron (of the order 88-109 kJ mol-‘) [13]. This in turn suggests that the bonding of hydrogen is weaker on the fee iron surface and easily activated towards hydrogenation, compared to bee iron, which is important from the point of view of its catalytic properties. The iron films grown at coverages in excess of 10 ML, for example the 12 ML film shown in fig. If, gave LEED patterns which were p(3 X 1) with some streaking of the spots and a high background. Adsorption of hydrogen at 300 K caused little change in the LEED pattern (merely a slight additional streaking of the spots), but adsorption on these surfaces at 180 K, irreversibly gave even streakier spots and a higher background. The thermal desorption spectra began to show the formation of higher temperature thermal desorption extending up to 400 K, which is typical of the low indexed faces of bee iron. The streaking of the LEED spots and high background intensity indicates the onset of disorder due to possible formation of microfacets of bee iron, as the films revert to the more energetically favourable structure. The high backgrounds presumably result from the disordered fcc/bcc mixture of iron on the surface. However it is also possible that these observations for films of greater than 10 ML are due to other effects relating to disordering of the superlattice structure. The role of any change in the magnetic behaviour of these films has yet to be established [19]. The results obtained in this paper were critically dependent on the substrate temperature during deposition. This will be discussed fully in detail elsewhere
[201-
From this study we can see that growth of iron on Cu(100) is not uniform, at least up to formation of the first three monolayers. The surface reconstructions which occur above three monolayers of iron coverage are stabilised by the adsorption of hydrogen - particularly the p(2 x 2)-p4g superstructure which involves lateral shifts of the iron atoms and is stable between 6 and 10 monolayers. Above 10 ML, the films begin to disorder, with or without the presence of hydrogen, displaying desorption peaks characteristic of bee facets. The heat of adsorption of hydrogen on these fee iron films is considerably less (71 kJ mol-’ ( f 1)) than for bee iron (of the order 88-109 kJ mol-‘), which shows that the activation barrier to hydrogen adsorption is considerably lower on the fee surface. We would like to thank Miss Z. Li for technical assistance and Dr. W. Allison for useful comments. We thank SERC for a Postdoctoral Research Assistantship (C.E.) and a Postdoctoral Fellowship (E.McC.).
L278
C. Egawa et al. /Adsorption
of hydrogen on fee-ironfilms grown on Cu(IO0)
References [l] [2] [3] [4] [5] [6] [7] [S] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20]
J. Kubler, Phys. Letters A 81 (1981) 81. D. Bagayoko and J. Cahaway, Phys. Rev. B 28 (1983) 5419. V.L. Moruzzi, P.M. Marcus, K. Schwarz and P. Mohn, Phys. Rev. B 34 (1986) 1784. C.L. Fu and A.J. Freeman, Phys. Rev. B 35 (1987) 925. F.J. Pinski, J. Staunton, B.L. Gyorffy, D.D. Johnson and G.M. Stocks, Phys. Rev. Letters 56 (1986) 2096. R.F. Willis, J.A.C. Bland and W. Schwarzacher, J. Appl. Phys. 63 (1988) 4051. A. Clarke, P.J. Rous, M. Amott, G. Jennings and R.F. Willis, Surface Sci. 192 (1987) L843. S.A. Chambers, T.J. Wagener and J.H. Weaver, Phys. Rev. B 36 (1987) 8992. W. Daum, C. StuhImann and H. Ibach, Phys. Rev. Letters 60 (1988) 2741. P.A. Montano, G.W. Fernando, B.R. Cooper, E.R. Moog, H.M. Naik, S.D. Bader, Y.C. Lee, Y.N. Darici, H. Min and J. Marcano, Phys. Rev. Letters 59 (1987) 1041. C. Liu, E.R. Moog and SD. Bader, Phys. Rev. Letters 60 (1988) 2422. M. Stampanoni, A. Vaterlaus, M. Aeschlimann, F. Meier and D. Pescia, J. Appl. Phys. 64 (1988) 5231. F. Bozso, G. Ertl, M. Grunze and M. Weiss, Appl. Surface Sci. 1 (1977) 103. D.A. SteigenwaId, I. Jacob and W.F. Egelhoff, Jr., Surface Sci. 202 (1988) 472. M. Amott and W. Allison, Physica B, submitted. J.H. Onuferko and D.P. Woodruff, Surface Sci. 87 (1979) 357. A.M. Baro and W. Erley; Surface Sci. 112 (1981) L759. P.A. Redhead, Vacuum 12 (1962) 203. W.A.A. Macedo and H. Keune, Phys. Rev. Letters 61 (1988) 475. C. Egawa, E.M. McCash and R.F. Willis, to be published.
Surface Science 215 (1989) L271-L278 North-Holland, Amsterdam
SURFACE SCIENCE LETTERS ADSORPTION ON Cu(100)
OF HYDROGEN ON THIN fee-IRON FILMS GROWN
Chikashi EGAWA, Elaine M. McCASH and Roy F. WILLIS * Cavendish Laboratory, Department Cambridge CB3 OHE, UK
of Physics, University of Cambridge,
Madingley
Road,
Received 19 December 1988; accepted for publication 3 February 1989
The desorption kinetics and surface structure of hydrogen adsorbed at 180 K on ultrathin (l-12 ML) iron films grown at 290 K on the (110) face of copper are reported. Below 3 ML, the results indicate that fihn growth is not epitaxial, and a desorption peak appears at 310 K accompanied by a low temperature shoulder. From 3-5 ML of iron, a (4x 1) LEED structure is observed, which persisted on hydrogen adsorption; the main desorption peak begins to shift down and the shoulder becomes more prominent. From 6-10 ML of iron, a p(2 X 2)-p4g superstructure is obtained which has systematic absences of the (h + l/2,0) and (0, k + l/2) spots, from the p(2~ 2), and is highly stabilised by the adsorption of hydrogen. This pattern appears in conjunction with the observation of only a single desorption peak at 290 K and is consistent with alternate displacements of adjacent iron atoms in a structure analogous to that of carbide formation on Ni(100). Above 10 ML of iron the surface structure begins to break down to form another structure. The heat of adsorption of hydrogen on the 8 ML film is 71 kJ mol-’ (+ 1) which is low compared to that on bee iron (88-109 kJ mol-‘).
The growth of ultrathin metal films on single crystal substrates has been an area of great interest for some time. Iron films are of particular relevance because of their unusual magnetic, electronic and in some cases, structural properties [l-6]. On Cu(100) it is found that it grows as a face centred cubic (fee) structure, dictated by the underlying copper surface, as opposed to the body centred cubic structure (bee) which occurs in, for example, iron single crystals [7-91. There exists considerable controversy over the explanation for the “composition” between fee and bee structures and the roles played by the electronic and magnetic properties of the films [l-12]. In order to gain further insight into the effects of these properties - which vary with film thickness - we have studied the adsorption of hydrogen as a function of iron coverage and compared the data to published work on bee iron crystals [13]. Our primary concern in this work is to study the chemistry * Current address: Department of Physics, Davy Laboratory, Pennsylvania State University, PA 16802, USA.
0039-6028/89/$03.50
(North-Holland
0 Elsevier Science Publishers B.V. Physics Publishing Division)
L212
C. Egawa et al. / Aakorption of hydrogen on fee-iron films grown on Cu(lO0)
of these films, in particular the way in which the strain between the epitaxial iron layers effects the chemical properties. We observe these to be quite different from the bulk bee iron and the difference persists even up to film thicknesses of several monolayers. Experiments were carried out in a stainless steel ultrahigh vacuum system equipped for low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work-function measurements. The chamber was pumped by an oil diffusion pump and titanium sublimation pump and operated at a base pressure of less than 1 X lo-*’ mbar. The copper crystal was prepared by mechanical polishing, followed by several cycles of argon ion bombardment and annealing at 450° C in UHV. Iron was deposited from a tungsten boat containing iron wire onto the Cu(100) crystal at 290 K. This substrate temperature was chosen in order to produce surfaces which were as flat as possible. Low substrate temperatures result in “rough” iron films, whereas higher substrate temperatures result in considerable interdiffusion of the iron into the copper. The pressure rise during deposition was to less than 2 X 10-i’ mbar. Surface cleanliness and iron coverage was monitored by AES. The iron uptake was obtained from the ratio of the normalized peak-to-peak Auger intensity of Fe(47 eV) and Cu(63 ev); breaks occurring on completion of each iron layer. (The coverages of iron were calibrated from films of greater than 3 ML.) This method of coverage calibration is quite inaccurate. We estimate an error of the order of half a monolayer in our case. The Fe deposition rate was controlled by monitoring the iron partial pressure during deposition - and this was found to lead to the formation of highly reproducible film thicknesses. Fig. 1 illustrates the LEED patterns obtained for various iron film thicknesses after iron deposition at 290 K and cooling to 180 K. Fig. 2 shows hydrogen thermal desorption spectra (TDS) as a function of iron coverage, after exposure of the iron films to 6 L of hydrogen at 180 K. The clean Cu(100) surface exhibited a p(1 X 1) LEED pattern with sharp substrate spots and a low background as shown in fig. la. On evaporation of up to 2 monolayers (ML) of iron, the p(1 X 1) pattern remained with a slight increase in background intensity. Adsorption of 6 L of hydrogen (corresponding to almost saturation coverages) led to no further change in the LEED pattern, and thermal desorption spectra which had main desorption peaks at 310 K with shoulders in the low temperature region. Thus these films appear to be highly stable. The intensity of the main desorption peak grew with increasing iron coverage up to 3 ML, with no change in position. Since hydrogen adsorption is a sensitive titration technique for measuring the number of surface iron atoms, as molecular hydrogen cannot be adsorbed on Cu(100) under these conditions, these results show that the copper substrate was not uniformly covered by iron atoms even at coverages as high as 2 ML.
C. Egawa et al. / Adsorption of hydrogen on fee-iron film grown on Cu(100)
L213
Fig. 1. LEED patterns for various iron film thicknesses on Cu(100) grown at 290 K. (a) clean Cu(100) - 103 eV; (b) Cu(100)/4ML Fe - 114 eV; (c) Cu(100)/4ML Fe after 6 L hydrogen adsorption at 180 K - 111 eV; (d) Cu(100)/8ML Fe - 114 eV; (e) Cu(lOO)/SML Fe after 6 L hydrogen adsorption at 180 K - 144 eV; (f) Cu(100)/12ML Fe - 112 eV.
L274
C. Egawa et al. / Adsorption of hydrogen on fee-iron films grown
250
300
on
Cu(IO0)
350 400
Temperature I K Fig. 2. Hydrogen thermal desorption spectra from iron films on Cu(100) as a function of iron coverage. Hydrogen exposure: 6 L. (a) 1 ML; (b) 2 ML; (c) 3 ML; (d) 4 ML; (e) 5 ML; (f) 8 ML; (g) 10 ML; (h) 12 ML iron thickness. The broken curve shows hydrogen desorption from an 8 ML film which had not been dosed with hydrogen.
These results are consisted with the formation of two-layer deep clusters in the “first” monolayer, or one-, two-, and three-atom deep islands, which agrees with recently reported work [8,14] using Auger and XPS forward scattering experiments. Another explanation is that there is interdiffusion of iron into the copper causing a lowered hydrogen sticking coefficient at iron coverages up to 2 ML. Above 3 ML iron coverages additional spots due to a (4 X 1) superstructure appeared in the LEED after deposition at 290 K. The LEED observations were obtained at both 290 K and also after cooling to 180 K. There was no apparent difference due to the temperature change. The LEED spots became sharper at 4 ML of iron (fig. lb), but streaky at 5 ML. Adsorption of hydrogen at 180 K or 300 K on these films slightly enhanced the intensities of the overlayer spots as shown in fig. lc. The change in LEED pattern was accompanied by corresponding changes in the thermal desorption spectra. There was an increase in prominence of the low temperature peak and a shift down to lower temperature of the main peak.
C. Egawa et al. / Adrovtion
of hydrogen on fee-iron films grown on Cu(lO0)
L215
In the range of iron film thickness from 6 to 10 ML a weak “(2 x 1)” LEED pattern was observed (fig. Id), after deposition at 290 K and cooling to 180 K. This became more intense on adsorption of hydrogen as shown in fig. le. We denote this pattern as “(2 x 1)” because systematic absences from the p(2 X 1) occurred, namely, the (h + l/2, 0) and (0, k + l/2) order spots were missing. This effect was most obvious at 8 ML iron coverages, where even the (l/2, l/2) order spots - which had previously been very weak - were quite clearly seen, but the systematic absences remained. From these observations the LEED structure obtained can be assigned to a p(2 X 2)-p4g structure. Deposition at 290 K and observation of the LEED pattern at this temperature gave a p(1 X l), although adsorption of hydrogen at this temperature, led to the extremely weak appearance of the “(2 X 1)“. Thus the reversible transition to the p(2 X 2)-p4g superstructure from the p(1 X 1) is associated with the adsorption and desorption of hydrogen under these conditions. These well-ordered LEED patterns are analogous to the (5 x l)/lML and (2 X 1)/2-3ML superstructures reported by Daum et al. [9], but for thicker films in our case. We interpret the difference in terms of the growth conditions, surface defects and roughness. We produced films at 290 K while Daum et al. grew theirs at 350 K. Recent work [15] has shown that significant interdiffusion can occur at high growth temperatures, but our thermal desorption peaks appeared at temperatures below those at which significant structural changes and interdiffusion takes place. The results obtained from repeated thermal desorption experiments on the same film depended on the temperature to which the film had been heated previously. If, for example, the TDS run terminated at 330 K, the results of the following experiment were similar, showing little change in surface structure. If the surface had previously been heated above this temperature and/or held at this temperature for a substantial amount of time, desorption results characteristic of thinner iron films were obtained. Daum et al. reported that the superstructure spot intensity increased with decreasing temperature, and so interpreted this as the transition taking place gradually over a large temperature range due to the repulsive stress between the iron atoms. Our data shows that the “(2 x 1)” superstructure spots were very weak, even after cooling to 180 K (fig. Id). However, they increased in intensity steadily with time, on exposure to hydrogen (fig. le). Furthermore, hydrogen desorption spectra taken from the surface observed in fig. Id showed evidence of the presence of small quantities of hydrogen on the surface (of the order of a few percent of saturation) presumably from adsorption of the hydrogen in the background gases while the surface was at low temperature. This is shown as a broken line in fig. 2. Clearly, the surface reconstruction to the p(2 x 2)-p4g structure is assisted by adsorbed hydrogen. The top-most iron atoms are alternatively displaced to form bonds with the hydrogen atoms in a structure analogous to that proposed for carbide formation on Ni(lOO) [16]. A
L276
C. Egawa et al. / A&sorption of hydrogen on fee-iron films grown on Cu(IO0)
model consistent with these observations is shown in fig. 3. The hydrogen atoms stabilise substrate distortions that lead to a bridge-bonded adsorption site for atomic hydrogen. The phonon softening observed by Daum et al. [9] adds support to the theory that the clean surface is incipiently unstable to lateral displacements of the surface atoms. The role of adsorbed hydrogen on the surface reconstruction is to stab&e the surface iron atoms. The reconstruction can be accounted for in terms of the metastable nature of the fee iron film, where lateral displacements are energetically possible and favoured by the adsorption of hydrogen [17]. In accordance with the development of the (4 X 1) superstructure above 3 ML of iron, the hydrogen desorption peak shifted gradually to lower temperature with iron coverage and finally gave a single desorption peak at 290 K for the 8 ML film as shown in fig. 2f. The desorption temperature and narrow half width of this peak at saturation clearly resembles those observed for the bee Fe(lOO)/H, system [13] where very high exposures (60-600 L of Hz) were used and a low temperature adsorption state was formed. At low exposures the observed hydrogen desorption peak on bee Fe(lOO) was at 400 K. The p(2 x 2)-p4g superstructure is not considered to correspond to adsorbate structure caused by the hydrogen; thus the absolute hydrogen coverage cannot be determined by this study. According to our surface model (shown in fig. 3) we postulate that the hydrogen coverage is 0.5. The desorp-
0
First layer Fe atoms
@ Second layer Fe atoms 0
Hydrogen atoms
Fig. 3. Surface structure model for the p(2 X 2)-p4g pattern of an 8 ML iron film on Cu(100) with adsorbed hydrogen. The dotted circles represent the unreconstructed (1 X 1) surface iron atoms.
C. Egawa et al. / Adsorption of hydrogen on fee-iron films grown on Cu(100)
L271
tion energy of hydrogen is calculated to be 71 kJ mole1 (+ 1) (using the Redhead method [18]) for the well defined 8 ML film. This is much lower than the corresponding values for the main desorption peaks reported for the low index faces of bee single crystals of iron (of the order 88-109 kJ mol-‘) [13]. This in turn suggests that the bonding of hydrogen is weaker on the fee iron surface and easily activated towards hydrogenation, compared to bee iron, which is important from the point of view of its catalytic properties. The iron films grown at coverages in excess of 10 ML, for example the 12 ML film shown in fig. If, gave LEED patterns which were p(3 X 1) with some streaking of the spots and a high background. Adsorption of hydrogen at 300 K caused little change in the LEED pattern (merely a slight additional streaking of the spots), but adsorption on these surfaces at 180 K, irreversibly gave even streakier spots and a higher background. The thermal desorption spectra began to show the formation of higher temperature thermal desorption extending up to 400 K, which is typical of the low indexed faces of bee iron. The streaking of the LEED spots and high background intensity indicates the onset of disorder due to possible formation of microfacets of bee iron, as the films revert to the more energetically favourable structure. The high backgrounds presumably result from the disordered fcc/bcc mixture of iron on the surface. However it is also possible that these observations for films of greater than 10 ML are due to other effects relating to disordering of the superlattice structure. The role of any change in the magnetic behaviour of these films has yet to be established [19]. The results obtained in this paper were critically dependent on the substrate temperature during deposition. This will be discussed fully in detail elsewhere
[201-
From this study we can see that growth of iron on Cu(100) is not uniform, at least up to formation of the first three monolayers. The surface reconstructions which occur above three monolayers of iron coverage are stabilised by the adsorption of hydrogen - particularly the p(2 x 2)-p4g superstructure which involves lateral shifts of the iron atoms and is stable between 6 and 10 monolayers. Above 10 ML, the films begin to disorder, with or without the presence of hydrogen, displaying desorption peaks characteristic of bee facets. The heat of adsorption of hydrogen on these fee iron films is considerably less (71 kJ mol-’ ( f 1)) than for bee iron (of the order 88-109 kJ mol-‘), which shows that the activation barrier to hydrogen adsorption is considerably lower on the fee surface. We would like to thank Miss Z. Li for technical assistance and Dr. W. Allison for useful comments. We thank SERC for a Postdoctoral Research Assistantship (C.E.) and a Postdoctoral Fellowship (E.McC.).
L278
C. Egawa et al. /Adsorption
of hydrogen on fee-ironfilms grown on Cu(IO0)
References [l] [2] [3] [4] [5] [6] [7] [S] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20]
J. Kubler, Phys. Letters A 81 (1981) 81. D. Bagayoko and J. Cahaway, Phys. Rev. B 28 (1983) 5419. V.L. Moruzzi, P.M. Marcus, K. Schwarz and P. Mohn, Phys. Rev. B 34 (1986) 1784. C.L. Fu and A.J. Freeman, Phys. Rev. B 35 (1987) 925. F.J. Pinski, J. Staunton, B.L. Gyorffy, D.D. Johnson and G.M. Stocks, Phys. Rev. Letters 56 (1986) 2096. R.F. Willis, J.A.C. Bland and W. Schwarzacher, J. Appl. Phys. 63 (1988) 4051. A. Clarke, P.J. Rous, M. Amott, G. Jennings and R.F. Willis, Surface Sci. 192 (1987) L843. S.A. Chambers, T.J. Wagener and J.H. Weaver, Phys. Rev. B 36 (1987) 8992. W. Daum, C. StuhImann and H. Ibach, Phys. Rev. Letters 60 (1988) 2741. P.A. Montano, G.W. Fernando, B.R. Cooper, E.R. Moog, H.M. Naik, S.D. Bader, Y.C. Lee, Y.N. Darici, H. Min and J. Marcano, Phys. Rev. Letters 59 (1987) 1041. C. Liu, E.R. Moog and SD. Bader, Phys. Rev. Letters 60 (1988) 2422. M. Stampanoni, A. Vaterlaus, M. Aeschlimann, F. Meier and D. Pescia, J. Appl. Phys. 64 (1988) 5231. F. Bozso, G. Ertl, M. Grunze and M. Weiss, Appl. Surface Sci. 1 (1977) 103. D.A. SteigenwaId, I. Jacob and W.F. Egelhoff, Jr., Surface Sci. 202 (1988) 472. M. Amott and W. Allison, Physica B, submitted. J.H. Onuferko and D.P. Woodruff, Surface Sci. 87 (1979) 357. A.M. Baro and W. Erley; Surface Sci. 112 (1981) L759. P.A. Redhead, Vacuum 12 (1962) 203. W.A.A. Macedo and H. Keune, Phys. Rev. Letters 61 (1988) 475. C. Egawa, E.M. McCash and R.F. Willis, to be published.