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A LEED and AES study of the adsorption of iodine on W(100) at room temperature
Surface Science 95 (1980) 245-256 0 North-Holland Publishing Company
A LEED AND AES STUDY OF THE ADSORPTION OF IODINE ON W( 100) AT ROOM TEMPERATURE K.J. RAWLINGS, G.G. PRICE * and B.J. HOPKINS Surface Physics, University of Southampton,
Southampton
SO9 SNH, England
Received 25 October 1979; accepted for publication 17 January 1980
The adsorption of iodine onto W(100) at room temperature has been characterized structurally by LEED with the aid of relative coverages from AES. Iodine adsorption is dissociative up to saturation, occurring with high sticking coefficient via a precursor state. There is an attractive interaction between neighbouring adatoms which order into c(2 X 2) structured islands at sufficiently low coverage. Adsorption can be increased beyond 5 X lo’* adatoms m-’ by compression of the c(2 X 2) structure in a particular (1,O) direction whereby a series of pseudo hexagonal close packed arrays result. Saturation occurs at 8 X lo’* adatoms rn-?, being so limited by the size of the adatom. The surface structures observed were a function of iodine coverage alone, annealing merely changing this by thermal desorption.
1. Introduction There have been relatively few studies of halogen gas adsorption onto tungsten single crystal surface [l-9] and two early studies [ IO,1 I] on polycrystalline tungsten. This paper is the second of three LEED/Auger studies of halogen gas adsorption onto a W(100) surface at room temperature. In degree of complexity of the surface structures formed, iodine comes second to chlorine [8]. The iodine-W(100) system has been investigated via work function measurements [2,4,9], LEED [3], and UPS [9]. The present LEED/Auger study combines a surface structure analysis with relative coverage measurements to give an internally consistent and plausible explanation of the LEED patterns observed. A similar series of halogen adsorption experiments on Fe(lOO) has just been reported [ 12-141. These will be discussed in detail in the final paper on bromine adsorption [ 151. 2. Experimental The UHV chamber, the W( 100) crystal and its preparation have been described before [ 81. The iodine was prepared in Pyrex ampoules by multiple distillation of * Present address: Department of Metallurgy and Material Science, Pembroke Street, University of Cambridge, Cambridge, CB2 3QZ England. 245
246
K.J. Rw&gs
et al. /Adsorption
of iodine on W(lOO)
Analar grade iodine using an all glass mercury diffusion pumped system. These ampoules were subsequently sealed onto the all glass gas handling line of the stainless steel experimental unit. Prior to experiments the crystal was cleaned by heating to 1500 K in 5 X low8 Torr of oxygen and then to 2000 K for a few seconds in 3 X lo-” Torr to flash off the oxide layer. Adsorption experiments commenced after the specimen temperature had fallen below 400 K (about 2 mm). The energy scale in AES is referenced to the system Fermi level by using the tungsten emissions around 170 and 350 eV as standards [16]. The iodine gas exposures are expressed in L and are uncorrected by a gauge factor. Two estimates of this factor are 3.0 [ 171 and 5.4 [ 181. No electron beam effects were observed for the iodine adlayer.
3. Results 3.1. Auger electron spectroscopy,massspectrometry An Auger spectrum of the iodine saturated W(100) surface is shown in fig. 1. The tungsten and iodine features are marked as well as the region where the carbon KLL emissions would occur. By combining Auger line energy calculations [19] with transition rate calculations [20] it was possible to identify the observed iodine Auger emissions. The largest iodine features have negative excursions at 515 eV
N,N X 173 NSN X
200
300
Fig. 1. AES of W(100) after room temperature vm = 7 V p-p, 7 = 1 s, scan rate 1 eV s-l).
400
500
E(cv) from E,
iodine saturation
(Ip = 7.7 4,
Ep = 2.5 kV,
247
K.J. Rawlingset al. /Adsorption of iodine on W(lO0)
(MsN4,sN4,s) and 524 eV (M4N4,5N4,5). The peak to peak height (ptph) of the 5 15 eV feature was used for measurement of relative iodine coverages on W(100) and reached a value of 0.24 (+ 0.02) of the ptph of the 173 eV feature for the clean substrate. Two smaller overlapping doublets due to iodine Auger emission occur at 441, 450 eV (Ms and M4N2,sN4,s) and 376,385 eV (M, and M4N1N4,s). In the same region there are substrate Auger emission peaks marked at 354,392 and 418 eV. The room temperature adsorption profile for iodine on W(100) is shown in fig. 2. The relative coverage axis has been converted to one of absolute coverage for reasons which will become apparent in the LEED discussion. Information pertinent to the LEED results is also marked in fig. 2. The iodine saturation exposure was well defined as 2.6 (? 0.3)L. With unit sticking coefffrcient and a gauge factor of 3.0 this would correspond to 0 = 0.22 (? 0.03) for iodine atoms on W(100) where 8 is the overlayer coverage relative to the surface tungsten atom density (1 X 1019 mm2). At saturation the substrate Auger signal attenuation at 170 eV was 39 (’ 2)%. Desorption of iodine from the room temperature saturated surface was followed by measuring the Auger ptph at 5 15 eV after annealing to successively higher tem-
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8 EXPOSURE
9
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Fig. 2. Iodine adsorption profde on room temperature W(100) via the iodine MsN4,5N4,5 ptph. See the LEED discussion about the coverage quantification.
248
K. J. Rawlings et al, / Adsorption
of iodine on W(I 00)
peratures. Fig. 3 shows the results of these experiments for a 150 s heating period at each temperature. Once again the relative coverage measurements have been put on an absolute scale and also information relating to the LEED results to follow is marked. Fig. 3 shows that no iodine remained on the W(100) surface after heating to 1150 (’ 50)K. For flash desorption with a quadrupole mass spectrometer having line of sight with the W(100) surface atomic desorption peaks were observed at 400 and 900 K where the first pressure burst was an order of magnitude down on the second. A molecular desorption peak was also detected at 900 K and was an order of magnitude smaller than the corresponding atomic one. The peak resolution was poor (*lo0 K) owing to the slow pumping action of the ion pump to iodine. The heating rate was 100 K s-r at 400 K and 500 K s-l at 900 K. 3.2. LEED Room temperature adsorption of iodine onto W(100) gave rise to a sequence of LEED patterns. These are shown in fig. 4 together with schematics to aid interpretation. Fig. 4(i) shows that the first extra features to appear were blurred ones in {i, :} positions. As the iodine exposure increased these half order spots showed splitting behaviour in (1, 1) directions, fig. 4(ii). By 1.5 (+ O.l)L exposure the split-
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400
500
GO0
700
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900
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1100 (K)
Fig. 3. Thermal desorption of iodine from W(100) by AES.
1200
K.J. Rawlingset al. /Adsorption of iodine on W(I00)
249
ting had gone leaving an intense c(2 X 2) pattern, fig. 4(iii). Further exposure caused splitting of the 4, a} features in (1,O) directions and satellites to move out from the substrate reflections in (1,O) directions, the spot movement appearing to be continuous with increasing iodine dosage. Two patterns from this sequence are shown in figs. 4(iv) and 4(v). Further exposure led to the pattern of fig. 4(vi) which changes on increased dosage to the final pattern of fig. 4(iii) at an exposure of 2.6 (’ O.l)L. Heating of the room temperature saturated specimen produced the same sequence of LEED patterns in reverse order. Consequently the patterns can be characterized either by an iodine exposure to the clean surface at room temperature or by an annealing temperature of the room temperature saturated W(100). The patterns in fig. 4 have been marked in this way but it was difficult to assign temperatures to patterns such as figs. 4(iv) and 4(v) because the extra features were in continuous movement. The iodine exposures from the room temperature adsorption LEED experiments have been marked on fig. 2. Likewise the annealing temperatures of the room temperature saturated W(100) producing the same sequence of LEED patterns have been marked on fig. 3.
4. Discussion of results The adsorption profile of fig. 2 is indicative of a high sticking coefficient of iodine to the W(100) surface. Though there is some scatter in these data it looks like surface coverage is directly proportional to iodine exposure right up to saturation. This implies adsorption to occur via a precursor state on W(100) at room temperature. The saturation coverage estimated as 22% of the surface tungsten atom density seems too low. The large attenuation (39%) of the substrate Auger signal would suggest something closer to monolayer coverage. A monolayer of chemisorbed carbon causes -20% attenuation [21] for example. The LEED pattern in fig. 4(iii) also suggests one half monolayer coverage at this subsaturation coverage (see later). The saturation coverage estimated from fig. 2 may be inaccurate through use of an inappropriate gauge factor and also the pumping action of ion gauges [22] coupled to the main chamber via tubing may be another source of error. The desorption profile of fig. 3 shows that iodine desorbed from the room temperature saturated surface over a wide temperature range, the 90% and 10% coverage points occurring for approximately 600 and 1100 K respectively. The activation energy for desorption is thus a function of iodine concentration, being higher for lower coverages. This point is expanded by Price [23] and has also been discussed in the similar study on Fe(lOO) [ 13,141. Fig. 3 also shows that after 150 s at 1150 K no iodine remains on W(100). Early flash desorption experiments on tungsten ribbon [lo] of predominantly (100) orientation gave a peak at 1500 K. Our thermal desorption work corresponds to a heating rate of about 0.3 K s-r
250
K.J. Rawlings et al. /Adsorption
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a Fig. 4. LEED pattern sequence with explanatory schematics for room temperature iodine adsorption on W(100) (or annealing of the room temperature saturated surface): (i) 0.4 L (or 1050 K); (ii) 1.0 L (or 950 K); (iii) 1.5 L (or 800 K); (iv) 1.8 L (or about 760 K);(v) 2.1 L (or about 660 K); (vi) 2.3 L (or 580 K); (vii) 2.6 L (300 K). Ep = 70 eV for patterns (i) to (v) and 60 eV for (vi) and (vii).
giving a broad desorption peak at about 900 K. A heating rate of 2500 K s-l as used in the early work [lo] would shift our peak to about 1200 K. This large discrepancy may be caused by faces other than (100) on the tungsten ribbon affect-
252
K. J. Rawlings et al. /Adsorption
of iodine on W(lO0)
ing desorption or perhaps desorption from the crystal supports was being detected by the mass spectrometer. The UPS study on W(100) [9] is in closer agreement with the present work. An approximate desorption temperature of 1300 K for an unspecified heating rate is quoted [9]. In the flash desorption experiments a small atomic desorption peak was observed at 400 (& 100)K which may or may not have been an artefact. A molecular desorption peak was observed around this temperature in the early work [IO] but the absence of any sign of coverage change in fig. 3 below 500 K means that it was either due to desorption from the specimen supports or to a weakly bound species that was desorbed rapidly under the electron beam in AES. Since the pressure burst was atomic rather than molecular we favour the artefact interpretation. In the previous flash desorption study [lo] the molecular species desorbing at 500 K is presumably coming from areas other than (100) orientation on the ribbon’s surface. The LEED investigation shows that room temperature adsorption of iodine gives an ordered surface species. We are largely in agreement with the previous study [3 ] regarding the sequence of patterns obtained. The only serious discrepancy is that we failed to observe their first pattern which according to the estimated coverage should fall between our figs. 4(i) and 4(ii). For the rest of the adsorption sequence we find the same series of LEED patterns but there is an important difference in interpretation of the penultimate pattern, fig. 4(vi). The use of AES in the present work rules out the previous interpretation [3]. The current explanation of fig. 4(vi) agrees with the relative coverage measurements from AES and is intuitively more ~tisfactory than the previous model as will be shown. The adsorption sequence in fig. 4 is explained as follows. At low exposures adsorbed iodine orders itself into small islands of c(2 X 2) structure (fig. 4(i)). As adsorption proceeds these islands grow until neighbouring islands are sufficiently close that they scatter electron waves coherently. The spot splitting in fig. 4(ii) is thus attributed to antiphase domains of the c(2 X 2) structure with the domain boundaries in (1, 1 > directions. As adsorption increases the antiphase ~(2 X 2) domains are eliminated to allow more efficient packing in the overlayer (fig. 4(iii)). Further adsorption causes compression of the c(2 X 2) structure along one of the (1,O) directions. The continuous movement of the extra diffraction features for exposures between 1.5 and 2.3 L of iodine suggests a continuous compression of the c(2 X 2) which would now be better described as hexagonal or pseudo hexagonal close packed. Figs. 4(iv) and 4(v) therefore correspond to diffraction from domains of an hexagonal overlayer which matches the substrate spacing in one (1 , 0 > but is incommensurate in the other (1,O) direction. All of the diffraction beams can be accounted for by assuming two such domain types at 90” rotation to each other and including double diffraction. With further adsorption fig. 4(vi) results which is a special case of the c(2 X 2) compression invoked to explain figs. 4(iv) and 4(v). In Ag. 4(vi) the compression in the overlayer has reached a stage where there is once again a high degree of matching between the overlayer and the substrate in both (1,O) directions. Fig. 5(a) shows the analysis for one domain in
253
K.J. Rawlingset al. /Adsorption of iodine on W(lO0)
reciprocal space, including double diffraction, and fig. 5(b) depicts the corresponding real space array together with the c(2 X 2) structure for comparison. The final LEED pattern in fig. 4(vii) arises from further compression in the overlayer to accomodate more adsorbate and also corresponds to a high degree of overlayersubstrate matching. It is not clear from the results whether there was continuous spot movement from pattern 4(vi) to 4(vii), i.e. whether incommensurate structures existed at coverages intermediate to those of figs. 4(vi) and 4(vii). Saturation occurring at fig. 4(vii) is probably related to the size of the iodine atom. The final structure corresponds to an overlayer density, 0 = 0.8. Since the iodine atom is larger than tungsten a coverage increase beyond the final hexagonal overlayer with 8 = 0.8 would result in considerable repulsion between neighbouring iodine atoms before the next commensurate overlayer structure was reached. According to the adsorption profile of fig. 2 the coverage at fig. 4(vi) was f3 = 0.72 f 0.04 if the final pattern corresponds to ~9= 0.8. The previous explanation of this LEED pattern [3] therefore doesn’t fit (0 = 0.56). The present explanation which fits in well with the general interpretation has a maximum coverage 0 = 0.75 which can be calculated from fig. 5. The previous analysis [3] applied the concept of transitional disordered states [24] to explain the LEED sequence of figs. 4(iii) to 4(vi) but we have shown with the aid of AES that the whole sequence from fig. 4(iii) actually arises by compression of a c(2 X 2) overlayer structure. The same phenomenon occurs at some stages of iodine [ 131 and bromine [ 141 adsorption on Fe(lOO) and indeed was suggested to occur for iodine on W(100) by Jones and Perry [ 141 in their closing review. The sequence of LEED patterns obtained by annealing the room temperature saturated W(100) was a reverse of the adsorption sequence, as was found in the
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Fig. 5. Analysis of one domain of the fii. 4(vi) LEED pattern, the other domain being a 90” space; (b) the corresponding real space structure with the ~$2 X 2) for
rotation; (a) reciprocal comparison.
254
K.J. Rawlings
et al. /Adsorption
of iodine
on W(lO0)
previous LEED study [3]. Once again by assuming the saturation coverage structure to correspond to 80% coverage on the surface tungsten atom density, fig. 4(vi) corresponds to a well defined annealing temperature and a coverage of 74 (? 3)% in good agreement with the theoretical maximum of 75%. Also fig. 4(iii) has a well defined “temperature” and the desorption work gave a coverage of 54 (? 3)0/o for this stage compared to 50% expected from theory. Figs. 4(iv) and 4(v) are assigned approximate “temperatures” because the extra diffraction beams moved continuously in this coverage range intermediate to 50% and 75%. It is interesting to note that the commensurate structures of figs. 4(G), 4(vi) and 4(vii) had well defined annealing temperatures suggesting that they were more stable thermodynamically than the incommensurate ones associated with figs. 4(iv) and 4(v). Calculations [25] have shown that interfacial energy minima should occur when the two surface structures on either side of an interface have a high degree of coincidence. The low coverage LEED interpretation, i.e. islands of c(2 X 2) structure suggests an attractive interaction between adsorbate atoms. Beyond 0 = 0.5 the overlayer structure is compressed. This should result in a less attractive interaction between adatoms, or repulsion relative to the c(2 X 2) state and should be manifested as a decrease in binding energy of the adsorbate. Fig. 3 does support this prediction as mentioned at the beginning of the discussion. The thermal desorption work coupled with the LEED observations show that the transition from a compressed overlayer to the c(2 X 2) state occurs on heating the room temperature saturated W(100) to 800 K or above. This seems to correspond quite closely to the transition temperature of 850 K obtained in the UPS study [9] for a marked change in the adsorbate electronic structure. State I in the UPS work [9] is thus inferred to represent the c(2 X 2) iodine overlayer on W( loo), i.e. for 19< 0.5, and state II is an oversimplification of the series of pseudo hexagonal close packed structures in the coverage range 19= 0.5 to 0.8 produced by compression of the c(2 X 2) structure along one (0, 1) direction. It thus appears from the UPS result [9] that the surface electronic structure was similar for all the adlayer configurations beyond c(2 X 2) suggesting the overlayer symmetry rather than its compression to be the primary determining factor for electronic structure. The c(2 X 2) mesh has four fold symmetry whereas the compression structures are all two fold symmetric. The previous LEED investigation [3] reported that the saturation coverage structure was stable with heating up to about 1070 K. This is at variance with the present observations (580 K) and is also inconsistent with the UPS study [9]. The discrepancy may be due to the high iodine pressure in the chamber (lo-’ to 10-a Torr) during the heating [3]. The question of dissociation has so far been ignored. In fact the LEED analysis implicitly assumes adsorbed iodine to be atomic. If the LEED interpretation is correct then it is reasonable to conclude that iodine adsorption onto W(100) at room temperature is dissociative up to saturation coverage. The interscattering unit spacings in the models for 8 > 0.5 would not be large enough to accommodate iodine molecules.
K.J. Rawiings et al. /Adsorption
of”iodine on W(lO0)
255
5. Conclusion
iodine adsorption onto W(100) at room temperature occurs with high sticking coefficient via a precursor state. Adsorption is dissociative with an attractive interaction between nei~bouring adatoms. Initial adsorption occurs into a c(2 X 2) overlayer structure which is followed by a series of pseudo hexagonal close packed structures formed by compression of the c(2 X 2) array in particular (1,O) directions to accomodate extra adsorbate. Saturation coverage is limited by the size of the iodine adatom and occurs at 80% coverage on the surface tungsten atom density (10’ 9 mm*). The final structure exhibits a high degree of registry with the W(100) substrate. Such coincidence overlayer structures were observed to be thermodynamically more stable than the less coincident ones obtained at intermediate coverages. Annealing of the room temperature saturated surface showed that the overlayer structures were a function of coverage only. ~nea~ng merely allowed the coverage to be changed by thermal desorption from the adlayer.
Acknowledgements
K.J. Rawlings and G.G. Price gratefully acknowledge financial support from the SRC during the course of this work.
References [I] P.J. Es&up and J. Anderson, Surface Sci. 8 (1967) 104. [ 21 D.L. Fehrs and R.E. Stickney, Surface Sci. 17 (1969) 298. [ 31 J.J. Lander and J. Morrison, Surface Sci. 17 (1969) 469. [4] C.W. Jowett and B.J. Hopkins, Surface Sci. 22 (1970) 392. [S] P.J. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1. [6] N.R. Avery, Surface Sci. 43 (1974) 101. [7] K. Faulian and E. Bauer, Phys. Letters A54 (1975) 313. 18) G.G. Price, K.J. Rawlings and B.J. Hopkins, Surface Sci. 85 (1979) 379. [9] A.K. Bhattacharya, J.Q. Broughton and D.L. Perry, Faraday Trans. I, 75 (1979) 850. [ 10) B. McCarrof, J. Chem. Phys. 4’7 (1967) 5077. [ 111 B. McCarrol, J. Appl. Phys. 40 (1969) 1. [ 121 P.A. Dowben and R.G. Jones, Surface Sci. 84 (1979) 449. [13] R.G. Jones and D.L. Perry, Surface Sci. 88 (1979) 331. [ 141 P.A. Dowben and R.G. Jones, Surface Sci. 88 (1979) 348. [ 151 K.J. Rawlings, G.G. Price and B.J. Hopkins, in preparation. [ 161 K.J. Rawlings, B.J. Hopkins and SD. Foulias, J. Electron Spectrosc. Related Phenomena 18 (1980) 213. [17] J.J. Lander and J. Morrison, J. Chem. Phys. 37 (1962) 729. [18] L. Holland, W. Steckelmacher and J. Yarwood, Vacuum Manual (Spar, London, 1974) pp. 52-54.
256 [19] [20] [21] [22]
K.J. Rawlings et al. /Adsorption
of iodine on W(I 00)
F.P. Larkins, Atomic Data and Nuclear Data Tables 20 (1977) pp. 311-387. E.J. McGuire, Phys. Rev. A5 (1972) 1052. K.J. Rawlings, B.J. Hopkins and S.D. Foulias, Surface Sci. 77 (1978) 561. D. Lee, H. Tomaschke and D. Alpert, in: Trans. 8th National Vacuum Symp. (1961) pp. 115-159. [23] G.G. Price, Ph.D. Thesis, University of Southampton, 1978. [24] L.H. Germer, J.W. May and R.J. Szostak, Surface Sci. 7 (1967) 430. [25] N.H. Fletcher, J. Appl. Phys. 35 (1964) 234.
A LEED AND AES STUDY OF THE ADSORPTION OF IODINE ON W( 100) AT ROOM TEMPERATURE K.J. RAWLINGS, G.G. PRICE * and B.J. HOPKINS Surface Physics, University of Southampton,
Southampton
SO9 SNH, England
Received 25 October 1979; accepted for publication 17 January 1980
The adsorption of iodine onto W(100) at room temperature has been characterized structurally by LEED with the aid of relative coverages from AES. Iodine adsorption is dissociative up to saturation, occurring with high sticking coefficient via a precursor state. There is an attractive interaction between neighbouring adatoms which order into c(2 X 2) structured islands at sufficiently low coverage. Adsorption can be increased beyond 5 X lo’* adatoms m-’ by compression of the c(2 X 2) structure in a particular (1,O) direction whereby a series of pseudo hexagonal close packed arrays result. Saturation occurs at 8 X lo’* adatoms rn-?, being so limited by the size of the adatom. The surface structures observed were a function of iodine coverage alone, annealing merely changing this by thermal desorption.
1. Introduction There have been relatively few studies of halogen gas adsorption onto tungsten single crystal surface [l-9] and two early studies [ IO,1 I] on polycrystalline tungsten. This paper is the second of three LEED/Auger studies of halogen gas adsorption onto a W(100) surface at room temperature. In degree of complexity of the surface structures formed, iodine comes second to chlorine [8]. The iodine-W(100) system has been investigated via work function measurements [2,4,9], LEED [3], and UPS [9]. The present LEED/Auger study combines a surface structure analysis with relative coverage measurements to give an internally consistent and plausible explanation of the LEED patterns observed. A similar series of halogen adsorption experiments on Fe(lOO) has just been reported [ 12-141. These will be discussed in detail in the final paper on bromine adsorption [ 151. 2. Experimental The UHV chamber, the W( 100) crystal and its preparation have been described before [ 81. The iodine was prepared in Pyrex ampoules by multiple distillation of * Present address: Department of Metallurgy and Material Science, Pembroke Street, University of Cambridge, Cambridge, CB2 3QZ England. 245
246
K.J. Rw&gs
et al. /Adsorption
of iodine on W(lOO)
Analar grade iodine using an all glass mercury diffusion pumped system. These ampoules were subsequently sealed onto the all glass gas handling line of the stainless steel experimental unit. Prior to experiments the crystal was cleaned by heating to 1500 K in 5 X low8 Torr of oxygen and then to 2000 K for a few seconds in 3 X lo-” Torr to flash off the oxide layer. Adsorption experiments commenced after the specimen temperature had fallen below 400 K (about 2 mm). The energy scale in AES is referenced to the system Fermi level by using the tungsten emissions around 170 and 350 eV as standards [16]. The iodine gas exposures are expressed in L and are uncorrected by a gauge factor. Two estimates of this factor are 3.0 [ 171 and 5.4 [ 181. No electron beam effects were observed for the iodine adlayer.
3. Results 3.1. Auger electron spectroscopy,massspectrometry An Auger spectrum of the iodine saturated W(100) surface is shown in fig. 1. The tungsten and iodine features are marked as well as the region where the carbon KLL emissions would occur. By combining Auger line energy calculations [19] with transition rate calculations [20] it was possible to identify the observed iodine Auger emissions. The largest iodine features have negative excursions at 515 eV
N,N X 173 NSN X
200
300
Fig. 1. AES of W(100) after room temperature vm = 7 V p-p, 7 = 1 s, scan rate 1 eV s-l).
400
500
E(cv) from E,
iodine saturation
(Ip = 7.7 4,
Ep = 2.5 kV,
247
K.J. Rawlingset al. /Adsorption of iodine on W(lO0)
(MsN4,sN4,s) and 524 eV (M4N4,5N4,5). The peak to peak height (ptph) of the 5 15 eV feature was used for measurement of relative iodine coverages on W(100) and reached a value of 0.24 (+ 0.02) of the ptph of the 173 eV feature for the clean substrate. Two smaller overlapping doublets due to iodine Auger emission occur at 441, 450 eV (Ms and M4N2,sN4,s) and 376,385 eV (M, and M4N1N4,s). In the same region there are substrate Auger emission peaks marked at 354,392 and 418 eV. The room temperature adsorption profile for iodine on W(100) is shown in fig. 2. The relative coverage axis has been converted to one of absolute coverage for reasons which will become apparent in the LEED discussion. Information pertinent to the LEED results is also marked in fig. 2. The iodine saturation exposure was well defined as 2.6 (? 0.3)L. With unit sticking coefffrcient and a gauge factor of 3.0 this would correspond to 0 = 0.22 (? 0.03) for iodine atoms on W(100) where 8 is the overlayer coverage relative to the surface tungsten atom density (1 X 1019 mm2). At saturation the substrate Auger signal attenuation at 170 eV was 39 (’ 2)%. Desorption of iodine from the room temperature saturated surface was followed by measuring the Auger ptph at 5 15 eV after annealing to successively higher tem-
4
Ex~rrimentol
VU-S&
10 raturotion cx)os”rcs C omp*e,sron of ~(2x2)
4lii)
OL[ij
in LEtD
c(2r7)
/ &_f-_(‘! 1
41;). 2
3
4
-i 5
6
7
8 EXPOSURE
9
10
(~.~j~d
Fig. 2. Iodine adsorption profde on room temperature W(100) via the iodine MsN4,5N4,5 ptph. See the LEED discussion about the coverage quantification.
248
K. J. Rawlings et al, / Adsorption
of iodine on W(I 00)
peratures. Fig. 3 shows the results of these experiments for a 150 s heating period at each temperature. Once again the relative coverage measurements have been put on an absolute scale and also information relating to the LEED results to follow is marked. Fig. 3 shows that no iodine remained on the W(100) surface after heating to 1150 (’ 50)K. For flash desorption with a quadrupole mass spectrometer having line of sight with the W(100) surface atomic desorption peaks were observed at 400 and 900 K where the first pressure burst was an order of magnitude down on the second. A molecular desorption peak was also detected at 900 K and was an order of magnitude smaller than the corresponding atomic one. The peak resolution was poor (*lo0 K) owing to the slow pumping action of the ion pump to iodine. The heating rate was 100 K s-r at 400 K and 500 K s-l at 900 K. 3.2. LEED Room temperature adsorption of iodine onto W(100) gave rise to a sequence of LEED patterns. These are shown in fig. 4 together with schematics to aid interpretation. Fig. 4(i) shows that the first extra features to appear were blurred ones in {i, :} positions. As the iodine exposure increased these half order spots showed splitting behaviour in (1, 1) directions, fig. 4(ii). By 1.5 (+ O.l)L exposure the split-
v5. 150
seconds
ot
each
T
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400
500
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700
000
900
1000 TEMP.
1100 (K)
Fig. 3. Thermal desorption of iodine from W(100) by AES.
1200
K.J. Rawlingset al. /Adsorption of iodine on W(I00)
249
ting had gone leaving an intense c(2 X 2) pattern, fig. 4(iii). Further exposure caused splitting of the 4, a} features in (1,O) directions and satellites to move out from the substrate reflections in (1,O) directions, the spot movement appearing to be continuous with increasing iodine dosage. Two patterns from this sequence are shown in figs. 4(iv) and 4(v). Further exposure led to the pattern of fig. 4(vi) which changes on increased dosage to the final pattern of fig. 4(iii) at an exposure of 2.6 (’ O.l)L. Heating of the room temperature saturated specimen produced the same sequence of LEED patterns in reverse order. Consequently the patterns can be characterized either by an iodine exposure to the clean surface at room temperature or by an annealing temperature of the room temperature saturated W(100). The patterns in fig. 4 have been marked in this way but it was difficult to assign temperatures to patterns such as figs. 4(iv) and 4(v) because the extra features were in continuous movement. The iodine exposures from the room temperature adsorption LEED experiments have been marked on fig. 2. Likewise the annealing temperatures of the room temperature saturated W(100) producing the same sequence of LEED patterns have been marked on fig. 3.
4. Discussion of results The adsorption profile of fig. 2 is indicative of a high sticking coefficient of iodine to the W(100) surface. Though there is some scatter in these data it looks like surface coverage is directly proportional to iodine exposure right up to saturation. This implies adsorption to occur via a precursor state on W(100) at room temperature. The saturation coverage estimated as 22% of the surface tungsten atom density seems too low. The large attenuation (39%) of the substrate Auger signal would suggest something closer to monolayer coverage. A monolayer of chemisorbed carbon causes -20% attenuation [21] for example. The LEED pattern in fig. 4(iii) also suggests one half monolayer coverage at this subsaturation coverage (see later). The saturation coverage estimated from fig. 2 may be inaccurate through use of an inappropriate gauge factor and also the pumping action of ion gauges [22] coupled to the main chamber via tubing may be another source of error. The desorption profile of fig. 3 shows that iodine desorbed from the room temperature saturated surface over a wide temperature range, the 90% and 10% coverage points occurring for approximately 600 and 1100 K respectively. The activation energy for desorption is thus a function of iodine concentration, being higher for lower coverages. This point is expanded by Price [23] and has also been discussed in the similar study on Fe(lOO) [ 13,141. Fig. 3 also shows that after 150 s at 1150 K no iodine remains on W(100). Early flash desorption experiments on tungsten ribbon [lo] of predominantly (100) orientation gave a peak at 1500 K. Our thermal desorption work corresponds to a heating rate of about 0.3 K s-r
250
K.J. Rawlings et al. /Adsorption
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a Fig. 4. LEED pattern sequence with explanatory schematics for room temperature iodine adsorption on W(100) (or annealing of the room temperature saturated surface): (i) 0.4 L (or 1050 K); (ii) 1.0 L (or 950 K); (iii) 1.5 L (or 800 K); (iv) 1.8 L (or about 760 K);(v) 2.1 L (or about 660 K); (vi) 2.3 L (or 580 K); (vii) 2.6 L (300 K). Ep = 70 eV for patterns (i) to (v) and 60 eV for (vi) and (vii).
giving a broad desorption peak at about 900 K. A heating rate of 2500 K s-l as used in the early work [lo] would shift our peak to about 1200 K. This large discrepancy may be caused by faces other than (100) on the tungsten ribbon affect-
252
K. J. Rawlings et al. /Adsorption
of iodine on W(lO0)
ing desorption or perhaps desorption from the crystal supports was being detected by the mass spectrometer. The UPS study on W(100) [9] is in closer agreement with the present work. An approximate desorption temperature of 1300 K for an unspecified heating rate is quoted [9]. In the flash desorption experiments a small atomic desorption peak was observed at 400 (& 100)K which may or may not have been an artefact. A molecular desorption peak was observed around this temperature in the early work [IO] but the absence of any sign of coverage change in fig. 3 below 500 K means that it was either due to desorption from the specimen supports or to a weakly bound species that was desorbed rapidly under the electron beam in AES. Since the pressure burst was atomic rather than molecular we favour the artefact interpretation. In the previous flash desorption study [lo] the molecular species desorbing at 500 K is presumably coming from areas other than (100) orientation on the ribbon’s surface. The LEED investigation shows that room temperature adsorption of iodine gives an ordered surface species. We are largely in agreement with the previous study [3 ] regarding the sequence of patterns obtained. The only serious discrepancy is that we failed to observe their first pattern which according to the estimated coverage should fall between our figs. 4(i) and 4(ii). For the rest of the adsorption sequence we find the same series of LEED patterns but there is an important difference in interpretation of the penultimate pattern, fig. 4(vi). The use of AES in the present work rules out the previous interpretation [3]. The current explanation of fig. 4(vi) agrees with the relative coverage measurements from AES and is intuitively more ~tisfactory than the previous model as will be shown. The adsorption sequence in fig. 4 is explained as follows. At low exposures adsorbed iodine orders itself into small islands of c(2 X 2) structure (fig. 4(i)). As adsorption proceeds these islands grow until neighbouring islands are sufficiently close that they scatter electron waves coherently. The spot splitting in fig. 4(ii) is thus attributed to antiphase domains of the c(2 X 2) structure with the domain boundaries in (1, 1 > directions. As adsorption increases the antiphase ~(2 X 2) domains are eliminated to allow more efficient packing in the overlayer (fig. 4(iii)). Further adsorption causes compression of the c(2 X 2) structure along one of the (1,O) directions. The continuous movement of the extra diffraction features for exposures between 1.5 and 2.3 L of iodine suggests a continuous compression of the c(2 X 2) which would now be better described as hexagonal or pseudo hexagonal close packed. Figs. 4(iv) and 4(v) therefore correspond to diffraction from domains of an hexagonal overlayer which matches the substrate spacing in one (1 , 0 > but is incommensurate in the other (1,O) direction. All of the diffraction beams can be accounted for by assuming two such domain types at 90” rotation to each other and including double diffraction. With further adsorption fig. 4(vi) results which is a special case of the c(2 X 2) compression invoked to explain figs. 4(iv) and 4(v). In Ag. 4(vi) the compression in the overlayer has reached a stage where there is once again a high degree of matching between the overlayer and the substrate in both (1,O) directions. Fig. 5(a) shows the analysis for one domain in
253
K.J. Rawlingset al. /Adsorption of iodine on W(lO0)
reciprocal space, including double diffraction, and fig. 5(b) depicts the corresponding real space array together with the c(2 X 2) structure for comparison. The final LEED pattern in fig. 4(vii) arises from further compression in the overlayer to accomodate more adsorbate and also corresponds to a high degree of overlayersubstrate matching. It is not clear from the results whether there was continuous spot movement from pattern 4(vi) to 4(vii), i.e. whether incommensurate structures existed at coverages intermediate to those of figs. 4(vi) and 4(vii). Saturation occurring at fig. 4(vii) is probably related to the size of the iodine atom. The final structure corresponds to an overlayer density, 0 = 0.8. Since the iodine atom is larger than tungsten a coverage increase beyond the final hexagonal overlayer with 8 = 0.8 would result in considerable repulsion between neighbouring iodine atoms before the next commensurate overlayer structure was reached. According to the adsorption profile of fig. 2 the coverage at fig. 4(vi) was f3 = 0.72 f 0.04 if the final pattern corresponds to ~9= 0.8. The previous explanation of this LEED pattern [3] therefore doesn’t fit (0 = 0.56). The present explanation which fits in well with the general interpretation has a maximum coverage 0 = 0.75 which can be calculated from fig. 5. The previous analysis [3] applied the concept of transitional disordered states [24] to explain the LEED sequence of figs. 4(iii) to 4(vi) but we have shown with the aid of AES that the whole sequence from fig. 4(iii) actually arises by compression of a c(2 X 2) overlayer structure. The same phenomenon occurs at some stages of iodine [ 131 and bromine [ 141 adsorption on Fe(lOO) and indeed was suggested to occur for iodine on W(100) by Jones and Perry [ 141 in their closing review. The sequence of LEED patterns obtained by annealing the room temperature saturated W(100) was a reverse of the adsorption sequence, as was found in the
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254
K.J. Rawlings
et al. /Adsorption
of iodine
on W(lO0)
previous LEED study [3]. Once again by assuming the saturation coverage structure to correspond to 80% coverage on the surface tungsten atom density, fig. 4(vi) corresponds to a well defined annealing temperature and a coverage of 74 (? 3)% in good agreement with the theoretical maximum of 75%. Also fig. 4(iii) has a well defined “temperature” and the desorption work gave a coverage of 54 (? 3)0/o for this stage compared to 50% expected from theory. Figs. 4(iv) and 4(v) are assigned approximate “temperatures” because the extra diffraction beams moved continuously in this coverage range intermediate to 50% and 75%. It is interesting to note that the commensurate structures of figs. 4(G), 4(vi) and 4(vii) had well defined annealing temperatures suggesting that they were more stable thermodynamically than the incommensurate ones associated with figs. 4(iv) and 4(v). Calculations [25] have shown that interfacial energy minima should occur when the two surface structures on either side of an interface have a high degree of coincidence. The low coverage LEED interpretation, i.e. islands of c(2 X 2) structure suggests an attractive interaction between adsorbate atoms. Beyond 0 = 0.5 the overlayer structure is compressed. This should result in a less attractive interaction between adatoms, or repulsion relative to the c(2 X 2) state and should be manifested as a decrease in binding energy of the adsorbate. Fig. 3 does support this prediction as mentioned at the beginning of the discussion. The thermal desorption work coupled with the LEED observations show that the transition from a compressed overlayer to the c(2 X 2) state occurs on heating the room temperature saturated W(100) to 800 K or above. This seems to correspond quite closely to the transition temperature of 850 K obtained in the UPS study [9] for a marked change in the adsorbate electronic structure. State I in the UPS work [9] is thus inferred to represent the c(2 X 2) iodine overlayer on W( loo), i.e. for 19< 0.5, and state II is an oversimplification of the series of pseudo hexagonal close packed structures in the coverage range 19= 0.5 to 0.8 produced by compression of the c(2 X 2) structure along one (0, 1) direction. It thus appears from the UPS result [9] that the surface electronic structure was similar for all the adlayer configurations beyond c(2 X 2) suggesting the overlayer symmetry rather than its compression to be the primary determining factor for electronic structure. The c(2 X 2) mesh has four fold symmetry whereas the compression structures are all two fold symmetric. The previous LEED investigation [3] reported that the saturation coverage structure was stable with heating up to about 1070 K. This is at variance with the present observations (580 K) and is also inconsistent with the UPS study [9]. The discrepancy may be due to the high iodine pressure in the chamber (lo-’ to 10-a Torr) during the heating [3]. The question of dissociation has so far been ignored. In fact the LEED analysis implicitly assumes adsorbed iodine to be atomic. If the LEED interpretation is correct then it is reasonable to conclude that iodine adsorption onto W(100) at room temperature is dissociative up to saturation coverage. The interscattering unit spacings in the models for 8 > 0.5 would not be large enough to accommodate iodine molecules.
K.J. Rawiings et al. /Adsorption
of”iodine on W(lO0)
255
5. Conclusion
iodine adsorption onto W(100) at room temperature occurs with high sticking coefficient via a precursor state. Adsorption is dissociative with an attractive interaction between nei~bouring adatoms. Initial adsorption occurs into a c(2 X 2) overlayer structure which is followed by a series of pseudo hexagonal close packed structures formed by compression of the c(2 X 2) array in particular (1,O) directions to accomodate extra adsorbate. Saturation coverage is limited by the size of the iodine adatom and occurs at 80% coverage on the surface tungsten atom density (10’ 9 mm*). The final structure exhibits a high degree of registry with the W(100) substrate. Such coincidence overlayer structures were observed to be thermodynamically more stable than the less coincident ones obtained at intermediate coverages. Annealing of the room temperature saturated surface showed that the overlayer structures were a function of coverage only. ~nea~ng merely allowed the coverage to be changed by thermal desorption from the adlayer.
Acknowledgements
K.J. Rawlings and G.G. Price gratefully acknowledge financial support from the SRC during the course of this work.
References [I] P.J. Es&up and J. Anderson, Surface Sci. 8 (1967) 104. [ 21 D.L. Fehrs and R.E. Stickney, Surface Sci. 17 (1969) 298. [ 31 J.J. Lander and J. Morrison, Surface Sci. 17 (1969) 469. [4] C.W. Jowett and B.J. Hopkins, Surface Sci. 22 (1970) 392. [S] P.J. Estrup and E.G. McRae, Surface Sci. 25 (1971) 1. [6] N.R. Avery, Surface Sci. 43 (1974) 101. [7] K. Faulian and E. Bauer, Phys. Letters A54 (1975) 313. 18) G.G. Price, K.J. Rawlings and B.J. Hopkins, Surface Sci. 85 (1979) 379. [9] A.K. Bhattacharya, J.Q. Broughton and D.L. Perry, Faraday Trans. I, 75 (1979) 850. [ 10) B. McCarrof, J. Chem. Phys. 4’7 (1967) 5077. [ 111 B. McCarrol, J. Appl. Phys. 40 (1969) 1. [ 121 P.A. Dowben and R.G. Jones, Surface Sci. 84 (1979) 449. [13] R.G. Jones and D.L. Perry, Surface Sci. 88 (1979) 331. [ 141 P.A. Dowben and R.G. Jones, Surface Sci. 88 (1979) 348. [ 151 K.J. Rawlings, G.G. Price and B.J. Hopkins, in preparation. [ 161 K.J. Rawlings, B.J. Hopkins and SD. Foulias, J. Electron Spectrosc. Related Phenomena 18 (1980) 213. [17] J.J. Lander and J. Morrison, J. Chem. Phys. 37 (1962) 729. [18] L. Holland, W. Steckelmacher and J. Yarwood, Vacuum Manual (Spar, London, 1974) pp. 52-54.
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K.J. Rawlings et al. /Adsorption
of iodine on W(I 00)
F.P. Larkins, Atomic Data and Nuclear Data Tables 20 (1977) pp. 311-387. E.J. McGuire, Phys. Rev. A5 (1972) 1052. K.J. Rawlings, B.J. Hopkins and S.D. Foulias, Surface Sci. 77 (1978) 561. D. Lee, H. Tomaschke and D. Alpert, in: Trans. 8th National Vacuum Symp. (1961) pp. 115-159. [23] G.G. Price, Ph.D. Thesis, University of Southampton, 1978. [24] L.H. Germer, J.W. May and R.J. Szostak, Surface Sci. 7 (1967) 430. [25] N.H. Fletcher, J. Appl. Phys. 35 (1964) 234.