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Valence state of low-dimensional thulium structures grown on molybdenum (110)
~ii~S~i~i~i~i~i~i~ii~i~i~i~i~iii~i~ii~i~i~i~i~i~i~ surface science ELSEVIER
Surface Science 307-309 (1994) 858-862
~/.~iiii!i~i~i~i~iii~! ~: ~:i~~i:i~i i~i ;~i~J~ii
Valence state of low-dimensional thulium structures grown on molybdenum (110) C.L. Nicklin .,a, C. Binns a, C. Norris a, E. Alleno b, M.-G. Barth6s-Labrousse b a
Department of Physics and Astronomy, University of Leicester, UK o CECM, CNRS, Vitry Sur Seine, Paris, France
(Received 20 August 1993)
Abstract
Valence band photoemission spectra of well-defined structures of the rare earth Tm deposited onto a Mo(ll0) surface have been measured using synchrotron radiation. The results correlate with the overlayer structure; mixed valence is observed at low coverages, in areas of island formation and beyond one monolayer. The presence of divalent features is associated with low coordinated sites, which provide insufficient transfer of energy to promote a 4f electron into the valence 5d6s states. This is consistent with theoretical predictions.
1. Introduction
pressure, temperature and chemical interaction
The rare earth (RE) metals and alloys have been the focus of considerable interest in recent years due to their unusual electronic and structural behaviour. This is as a consequence of the partially filled and highly localised 4f states which overlap with the extended 5d6s valence states. One effect is valence instability, which occurs whenever the energies of the [Xe] 4fn(5d6s) z and [Xe] 4fn-a(5d6s)3 configurations are sufficiently close that fluctuations between the two can occur. The relative, energies of these states depends on the number, distance and nature of the neighbours of the R E atoms and can be changed by
Johansson and Rosengren [5,6] and Liibke et al. [7] have estimated the energy difference between the divalent and trivalent states of the REs and related it to the coordination energy. They predict that, whereas Eu and Yb should be divalent in all configurations including the bulk with a coordination of 12, Sm should be trivalent in the bulk but divalent in coordinations of 10 or less, including the surface (9). This has been confirmed experimentally [8,9]. Tm, the heavy rare earth analogue of Sm (the 4f shell contains 7 more electrons than Sin), is the R E next most likely to show mixed valence behaviour. The energy difference between trivalent and divalent states (18 k c a l / m o l ) is much larger than for Sm (6 k c a l / m o l ) implying that atoms in a flat surface
* Corresponding author.
[1-4].
0039-6028/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0039-6028(93)E0786-T
C.L. Nicklin et aL / Surface Science 307-309 (1994) 858-862
of T m remain trivalent. This is supported by photoemission measurements of Tm metal which do not show divalent features [10-13]. If, however, the coordination is further reduced, as at an edge or a corner of an island, the divalent configuration may be favoured. Epitaxial growth on to an ordered non alloying substrate allows the atomic coordination and therefore the electronic state to be changed in a controlled manner. We have previously characterised the growth mode of T m on M o ( l l 0 ) [14]. Initially, at low coverages, T m is randomly distributed on the surface. As the coverage 0 increases, the overlayer starts to order into two-dimensional n × 2 structures, culminating in a 10 x 2 phase. At a critical coverage, and before the 10 x 2 structure entirely covers the surface, the atoms collapse into islands of a c(8 × 4) close packed pseudo hexagonal structure, which then grows to form a complete monolayer. In the second layer there is evidence of progressively rough growth. We define 1 monolayer (ML) as the saturation coverage 0sa t of the c(8 × 4) phase. The destruction of the 10 x 2 phase (0sat = 0.61 ML) occurs for 0 > 0.49 ML. We can thus identify several coverages at which Tm atoms will be found in a low coordinated sites and should therefore be divalent. These are: • very low coverages, • the collapse into c(8 × 4) islands with a significant number of atoms at corner and edge sites and , • above the monolayer where rough growth occurs.
2. Experimental The measurements were carried out on the SA73 beamline of the S U P E R - A C O storage ring at the Laboratoire pour l'Utilisation du Rayonnemerft Electromagnetique ( L U R E ) at the Universit6 Paris Sud. A blazed grating T G M monochromator produced usable radiation in the energy range 30-130 eV. Beam resolution was limited to by slits both before and after the monochromator. The measurement chamber had a base pressure of 8 × 10 -la mbar and was
859
equipped with rear view L E E D optics (VG) and a single pass CMA (Riber, DPC 103) for Auger electron spectroscopy (AES) analysis, together with an angularly resolving hemispherical analyser of 50 mm radius. The Mo crystal was cut and polished to within 0.1 ° of the (110) surface and cleaned as previously described [14]; surface cleanliness was checked using AES and LEED. T m was deposited from a tantalum crucible in a Knudsen cell which was surrounded by a water cooled shroud. No detectable pressure rise was seen during evaporation. For each coverage, a new film was prepared after flashing the Mo crystal to 2100 K. Coverages were monitored during deposition using secondary electron emission crystal current (SEECC) measurements, Auger signal versus time ( A S - t ) and work function change (A~b) measurements [14]. The curves were reproducible, and the evaporation rate was constant throughout, leading to small errors on the coverages indicated (approximately + 5%). Photoemission measurements were made using a photon energy of 90 eV. At this energy the cross-section for photoexcitation of the M o 4 d state is at a Cooper minimum whereas that for the Tm 4f level is close to its maximum and several orders of magnitude greater than that for the 5d6s valence states of Tm. The photon beam was incident on the sample at an angle of 45 ° and normally emitted electrons were recorded with the analyser in "Fixed Analyser Transmission" mode. The total resolution, as measured at the Fermi edge, was 500 meV. This allowed a suitable number of counts within the counting time limit of 40 min imposed by the high reactivity of the Tm layer.
3. Results Fig. 1 shows photoelectron spectra, normalised to the average beam current, for different Tm coverages. Also shown is the multiplet splitting for trivalent T m as calculated by Gerken's intermediate coupling scheme [15]; the origin of the main peaks are shown in spectroscopic notation. No contamination peaks are apparent in the
C.L. Nicklin et al. / Surface Science 307-309 (1994) 858-862
860
spectra and this was confirmed by AES measurements made afterwards which showed only minute traces (just above noise) of carbon and oxygen. A group of peaks between 4 and 11 eV binding energy (BE) grow in magnitude as the coverage is increased. These are assigned to the trivalent Tm configuration; they fit well with the theoretical multiplet splitting model and with previously recorded trivalent Tm spectra [10-13,15]. In the 0-4 eV region, where a divalent signal should appear, there is a change in the shape of the peak as the coverage is increased. This is most evident when comparing the 0.12 and 0.15 ML coverages (see Fig. 1 inset). The 1.48 ML coverage also shows quite a large difference in this region, compared to its neighbouring spectra.
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Fig. 2 shows the spectra after the Mo background (corrected for the attenuation of the overlayer) has been removed. The divalent feature in the energy region 0-4 eV is dearly revealed. Its intensity is plotted as a function of the coverage in Fig. 3. The divalent feature is most intense at the lowest coverages and weakens rapidly as the coverage is increased, reaching a minimum at 0.5 ML. Further deposition yields higher intensities at approximately 0.7 and 1.5 ML. The movement of the trivalent peaks with increasing coverage, as indicated by the lines marked on Fig. 1, is attributed to electronic rearrangement at the surface. Surface shifts of trivalent peaks in photoemission spectra from thick Tm films have been previously identified [17].
4. Discussion
~2
(e~O
Fig. 1. Photoemission spectra recorded for diferent coverages of Tm on Mo(110). Also shown is the calculated multiplet structure for trivalent Tm [15]• The inset shows a detailed view of the 0-4 eV binding energy region for 0.12 ML (dashed line) and 0.15 ML (full line) coverages• Shifts of the large trivalent peak are also indicated.
The appearance and change in intensity of the divalent feature can be attributed to the structural changes of the Tm overlayer, specifically the local atomic coordination. This is illustrated in Fig. 3 which shows the coverage span of the 10 × 2 and c(8 X 4) phases and change of the
C.L. Nicklin et al. / Surface Science 307-309 (1994) 858-862 k
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Fig. 3. A plot of the divalent signal intensity as a function of coverage. The work function change is reproduced from Ref. [14]. Also shown in (i) and (ii) are the proposed structures for the 10 x 2 overlayer and c(8 × 4) islands.
work function Ath as reported in the previous study [14]. A4) is sensitive to the local electronic structure and, therefore, indirectly to the atomic geometry. At low coverage (0 <0.15 ML) the absence of a LEED pattern and the rapid fall of A~b were explained by the dominance of isolated, low coordinated, Tm atoms randomly distributed on the Mo surface. Correspondingly, the divalent feature is most intense at this coverage. As more Tm atoms are adsorbed and the two-dimensionally ordered n x 2 structures develop, there is a change of slope in the A~ plot and a rapid fall in the divalent intensity. The weakest divalent signal at 0.5 ML coincides with the maximum brightness of the 10 x 2 LEED pattern [14]. Between 0.5 and 1.0 ML, the transition from the 10 x 2 to the c(8 x 4) phase exposes areas of clean substrate. The minimum in A~b and the increase in the divalent intensity at 0.75 ML can be associated with the presence of atoms at edge sites of the
861
c(8 x 4) islands and isolated Tm atoms in the regions between the islands. At 1 ML the smoothness of the c(8 x 4) layer should cause the divalent intensity to fall again. The photoemission spectrum recorded at 1.20 ML does show a slightly reduced divalent signal. As the second layer develops we would expect, in the simplest model, the maximum roughness to occur when the layer is half complete, that is at 1.5 ML, and to reduce again at 2.0 ML. The evidence from the earlier study is that rough growth persists to beyond 3 ML. Nevertheless, we note the increase in the divalent intensity at 1.48 ML, consistent with the occurrence of low coordinated Tm atoms in the upper layer. The correlation of the increase in the divalent intensity with incomplete overlayer structures confirms the role of coordination in promoting the mixed valent state in Tin. Mixed valent behaviour has been observed in other low coordinated Tm structures. These include a rough 20 ,~ thick layer grown on GaAs [18,19] and a thick "porous" layer grown on a copper substrate at low temperature [16]. The salient feature of the present study is that the mixed valence behaviour is seen to correlate with the more precisely defined structure of the overlayer at the initial stages of growth. Photoemission measurements have now been reported for sub-monolayer coverages of Yb, Sm and Tm on the Mo(ll0) surface. All three systems exhibit the same basic growth pattern, that is, the development of a series of n x 2 structures followed by a pseudo-hexagonal monolayer. The photoemission results are broadly consistent with the theory for unsupported structures if the interaction with the substrate atoms is taken into account. Thus for Yb, the divalent state is observed for all configurations, from the lowest coverage to the thick layer, as expected [20]. For Sm and Tin, on the other hand, the pseudohexagonal monolayer, which has a planar coordination of 6 or between 8 and 10 if the substrate is included, appears to be trivalent. This compares with the prediction of Lfibke et al. [7] that the trivalent state will be favoured for coordinations greater than 10 (Sm) and 7 (Tm). At low coverages, Sm is divalent [8] which contrasts with the
862
C.L. Nicklin et al. / Surface Science 307-309 (1994) 858-862
case of Tm, reported here, which always exhibits a strong trivalent feature. This is consistent with the greater energy difference between the two valence states and consequently the requirement of a lower coordination to support divalency in Tm than in Sm. Even the limited intralayer interaction at the start of adsorbate ordering, into n x 2 structures [14], is enough to encourage trivalency in Tm, in contrast to the case of Sm. We conclude that the valence behaviour of Tm is consistent with the trends predicted by theory and shown by Sm and Yb. However, even with a non-alloying substrate, such as Mo(ll0), it is clear that the adsorbate-substrate interaction is a significant factor in determining the stable valence state of the ultrathin, sub-monolayer RE films. Such structures cannot be regarded simply as unsupported two-dimensional layers in which only the intralayer interaction is important.
5. Acknowledgements The authors gratefully acknowledge the technical support of Lea Minel, Steve Taylor and Stuart Thornton. We would also like to thank Dr. N. Barrett, Dr. C. Guillot and the staff of LURE for their assistance. This work was supported by the Science and Engineering Research Council, Grant No. G R / F 17667.
6. References [1] B. Johansson and A. Rosefigren, Phys. Rev.. B 14 (1976) 361.
[2] K. Yoshimura, T. Nitta, M. Mekata, T. Shimizu, T. Sakakibara, T. Goto and G. Kido, Phys. Rev. Lett. 60 (1988) 851. [3] G.K. Wertheim, W. Eib, E. Kaldis and M. Campagna, Phys. Rev. B 22 (1980) 6240. [4] G. Kaindl, C. Laubschat, B. Reihl, R.A. Pollak, N. M~rtensson, F. Holtzberg and D.E. Eastman, Phys. Rev. B 26 (1982) 1713. [5] B. Johansson, Phys. Rev. B 19 (1979) 6615. [6] A. Rosengren and B. Johansson, Phys. Rev. B 26 (1982) 3068. [7] M. Liibke, B. Sonntag, W. Niemann and P. Rabe, Phys. Rev. B 34 (1986) 5184. [8] A. Stenborg, O. Bj6rneholm, A. Nilsson, N. M]rtensson, J.N. Andersen and C. Wigren, Phys. Rev. B 40 (1989) 5916. [9] A. F~ildt and H.P. Myers, Phys. Rev. B 34 (1986) 6675. [10] C.L. Nicklin, P.J. McCluskey, C. Binns, C. Norris, E. Alleno, M.-G. Barth~s-Labrousse and G. van der Laan, Resonant Photoemission Study of Tm, in preparation. [11] Y. Baer and G. Busch, J. Electron Spectrosc. Relat. Phenom. 5 (1974) 611. [12] L.1. Johansson, J.W. Allen and I. Lindau, Phys. Lett. 86 A (1981) 442. [13] F. Gerken, PhD Thesis, DESY Report No F41, HASYLAB 83-03 (Hamburg, Germany, 1982). [14] C.L. Nicklin, C. Binns, C. Norris, P. McCluskey and M.-G. Barth6s-Labrousse, Surf. Sci. 269/270 (1992) 700. [15] F. Gerken, J. Phys. F: Met. Phys. 13 (1983) 703. [16] M. Domke, C. Laubschat, M. Prietsch, T. Mandel, G. Kaindl and W.D. Schneider, Phys. Rev. Lett. 56 (1986) 1287. [17] F. Gerken, A.S. Flodstr6m, J. Barth, L.I. Johansson and C. Kunz, DESY Report No. SR 84-31 (1984). [18] M. Prietsch, M. Domke, C. Laubschat and G. Kaindl, Phys. Rev. Lett. 60 (1988) 436. [19] M. Prietsch, M. Domke, C. Laubschat and G. Kaindl, Phys. Rev. B 38 (1988) 10655. [20] A. Stenborg, O. Bj6rneholm, A. Nilsson, N. M~rtensson, J.N. Andersen and C. Wigren, Surf. Sci. 211/212 (1989) 470. [21] A. Stenborg and E. Bauer, Surf, Sci. 189/190 (1987) 570.
Surface Science 307-309 (1994) 858-862
~/.~iiii!i~i~i~i~iii~! ~: ~:i~~i:i~i i~i ;~i~J~ii
Valence state of low-dimensional thulium structures grown on molybdenum (110) C.L. Nicklin .,a, C. Binns a, C. Norris a, E. Alleno b, M.-G. Barth6s-Labrousse b a
Department of Physics and Astronomy, University of Leicester, UK o CECM, CNRS, Vitry Sur Seine, Paris, France
(Received 20 August 1993)
Abstract
Valence band photoemission spectra of well-defined structures of the rare earth Tm deposited onto a Mo(ll0) surface have been measured using synchrotron radiation. The results correlate with the overlayer structure; mixed valence is observed at low coverages, in areas of island formation and beyond one monolayer. The presence of divalent features is associated with low coordinated sites, which provide insufficient transfer of energy to promote a 4f electron into the valence 5d6s states. This is consistent with theoretical predictions.
1. Introduction
pressure, temperature and chemical interaction
The rare earth (RE) metals and alloys have been the focus of considerable interest in recent years due to their unusual electronic and structural behaviour. This is as a consequence of the partially filled and highly localised 4f states which overlap with the extended 5d6s valence states. One effect is valence instability, which occurs whenever the energies of the [Xe] 4fn(5d6s) z and [Xe] 4fn-a(5d6s)3 configurations are sufficiently close that fluctuations between the two can occur. The relative, energies of these states depends on the number, distance and nature of the neighbours of the R E atoms and can be changed by
Johansson and Rosengren [5,6] and Liibke et al. [7] have estimated the energy difference between the divalent and trivalent states of the REs and related it to the coordination energy. They predict that, whereas Eu and Yb should be divalent in all configurations including the bulk with a coordination of 12, Sm should be trivalent in the bulk but divalent in coordinations of 10 or less, including the surface (9). This has been confirmed experimentally [8,9]. Tm, the heavy rare earth analogue of Sm (the 4f shell contains 7 more electrons than Sin), is the R E next most likely to show mixed valence behaviour. The energy difference between trivalent and divalent states (18 k c a l / m o l ) is much larger than for Sm (6 k c a l / m o l ) implying that atoms in a flat surface
* Corresponding author.
[1-4].
0039-6028/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0039-6028(93)E0786-T
C.L. Nicklin et aL / Surface Science 307-309 (1994) 858-862
of T m remain trivalent. This is supported by photoemission measurements of Tm metal which do not show divalent features [10-13]. If, however, the coordination is further reduced, as at an edge or a corner of an island, the divalent configuration may be favoured. Epitaxial growth on to an ordered non alloying substrate allows the atomic coordination and therefore the electronic state to be changed in a controlled manner. We have previously characterised the growth mode of T m on M o ( l l 0 ) [14]. Initially, at low coverages, T m is randomly distributed on the surface. As the coverage 0 increases, the overlayer starts to order into two-dimensional n × 2 structures, culminating in a 10 x 2 phase. At a critical coverage, and before the 10 x 2 structure entirely covers the surface, the atoms collapse into islands of a c(8 × 4) close packed pseudo hexagonal structure, which then grows to form a complete monolayer. In the second layer there is evidence of progressively rough growth. We define 1 monolayer (ML) as the saturation coverage 0sa t of the c(8 × 4) phase. The destruction of the 10 x 2 phase (0sat = 0.61 ML) occurs for 0 > 0.49 ML. We can thus identify several coverages at which Tm atoms will be found in a low coordinated sites and should therefore be divalent. These are: • very low coverages, • the collapse into c(8 × 4) islands with a significant number of atoms at corner and edge sites and , • above the monolayer where rough growth occurs.
2. Experimental The measurements were carried out on the SA73 beamline of the S U P E R - A C O storage ring at the Laboratoire pour l'Utilisation du Rayonnemerft Electromagnetique ( L U R E ) at the Universit6 Paris Sud. A blazed grating T G M monochromator produced usable radiation in the energy range 30-130 eV. Beam resolution was limited to by slits both before and after the monochromator. The measurement chamber had a base pressure of 8 × 10 -la mbar and was
859
equipped with rear view L E E D optics (VG) and a single pass CMA (Riber, DPC 103) for Auger electron spectroscopy (AES) analysis, together with an angularly resolving hemispherical analyser of 50 mm radius. The Mo crystal was cut and polished to within 0.1 ° of the (110) surface and cleaned as previously described [14]; surface cleanliness was checked using AES and LEED. T m was deposited from a tantalum crucible in a Knudsen cell which was surrounded by a water cooled shroud. No detectable pressure rise was seen during evaporation. For each coverage, a new film was prepared after flashing the Mo crystal to 2100 K. Coverages were monitored during deposition using secondary electron emission crystal current (SEECC) measurements, Auger signal versus time ( A S - t ) and work function change (A~b) measurements [14]. The curves were reproducible, and the evaporation rate was constant throughout, leading to small errors on the coverages indicated (approximately + 5%). Photoemission measurements were made using a photon energy of 90 eV. At this energy the cross-section for photoexcitation of the M o 4 d state is at a Cooper minimum whereas that for the Tm 4f level is close to its maximum and several orders of magnitude greater than that for the 5d6s valence states of Tm. The photon beam was incident on the sample at an angle of 45 ° and normally emitted electrons were recorded with the analyser in "Fixed Analyser Transmission" mode. The total resolution, as measured at the Fermi edge, was 500 meV. This allowed a suitable number of counts within the counting time limit of 40 min imposed by the high reactivity of the Tm layer.
3. Results Fig. 1 shows photoelectron spectra, normalised to the average beam current, for different Tm coverages. Also shown is the multiplet splitting for trivalent T m as calculated by Gerken's intermediate coupling scheme [15]; the origin of the main peaks are shown in spectroscopic notation. No contamination peaks are apparent in the
C.L. Nicklin et al. / Surface Science 307-309 (1994) 858-862
860
spectra and this was confirmed by AES measurements made afterwards which showed only minute traces (just above noise) of carbon and oxygen. A group of peaks between 4 and 11 eV binding energy (BE) grow in magnitude as the coverage is increased. These are assigned to the trivalent Tm configuration; they fit well with the theoretical multiplet splitting model and with previously recorded trivalent Tm spectra [10-13,15]. In the 0-4 eV region, where a divalent signal should appear, there is a change in the shape of the peak as the coverage is increased. This is most evident when comparing the 0.12 and 0.15 ML coverages (see Fig. 1 inset). The 1.48 ML coverage also shows quite a large difference in this region, compared to its neighbouring spectra.
/:>+" •
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j
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] 10 Slale
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Fig. 2. Photoemission spectra with the contribution of the Mo substrate subtracted• +l
2K
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b'~
"
t ll . I
++
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//
"
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_~=A.. : ..... ,~
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o
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ctean +
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14
Energy
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20
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Fig. 2 shows the spectra after the Mo background (corrected for the attenuation of the overlayer) has been removed. The divalent feature in the energy region 0-4 eV is dearly revealed. Its intensity is plotted as a function of the coverage in Fig. 3. The divalent feature is most intense at the lowest coverages and weakens rapidly as the coverage is increased, reaching a minimum at 0.5 ML. Further deposition yields higher intensities at approximately 0.7 and 1.5 ML. The movement of the trivalent peaks with increasing coverage, as indicated by the lines marked on Fig. 1, is attributed to electronic rearrangement at the surface. Surface shifts of trivalent peaks in photoemission spectra from thick Tm films have been previously identified [17].
4. Discussion
~2
(e~O
Fig. 1. Photoemission spectra recorded for diferent coverages of Tm on Mo(110). Also shown is the calculated multiplet structure for trivalent Tm [15]• The inset shows a detailed view of the 0-4 eV binding energy region for 0.12 ML (dashed line) and 0.15 ML (full line) coverages• Shifts of the large trivalent peak are also indicated.
The appearance and change in intensity of the divalent feature can be attributed to the structural changes of the Tm overlayer, specifically the local atomic coordination. This is illustrated in Fig. 3 which shows the coverage span of the 10 × 2 and c(8 X 4) phases and change of the
C.L. Nicklin et al. / Surface Science 307-309 (1994) 858-862 k
I
i
I TLLT~TOTQT~I
~-
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Fig. 3. A plot of the divalent signal intensity as a function of coverage. The work function change is reproduced from Ref. [14]. Also shown in (i) and (ii) are the proposed structures for the 10 x 2 overlayer and c(8 × 4) islands.
work function Ath as reported in the previous study [14]. A4) is sensitive to the local electronic structure and, therefore, indirectly to the atomic geometry. At low coverage (0 <0.15 ML) the absence of a LEED pattern and the rapid fall of A~b were explained by the dominance of isolated, low coordinated, Tm atoms randomly distributed on the Mo surface. Correspondingly, the divalent feature is most intense at this coverage. As more Tm atoms are adsorbed and the two-dimensionally ordered n x 2 structures develop, there is a change of slope in the A~ plot and a rapid fall in the divalent intensity. The weakest divalent signal at 0.5 ML coincides with the maximum brightness of the 10 x 2 LEED pattern [14]. Between 0.5 and 1.0 ML, the transition from the 10 x 2 to the c(8 x 4) phase exposes areas of clean substrate. The minimum in A~b and the increase in the divalent intensity at 0.75 ML can be associated with the presence of atoms at edge sites of the
861
c(8 x 4) islands and isolated Tm atoms in the regions between the islands. At 1 ML the smoothness of the c(8 x 4) layer should cause the divalent intensity to fall again. The photoemission spectrum recorded at 1.20 ML does show a slightly reduced divalent signal. As the second layer develops we would expect, in the simplest model, the maximum roughness to occur when the layer is half complete, that is at 1.5 ML, and to reduce again at 2.0 ML. The evidence from the earlier study is that rough growth persists to beyond 3 ML. Nevertheless, we note the increase in the divalent intensity at 1.48 ML, consistent with the occurrence of low coordinated Tm atoms in the upper layer. The correlation of the increase in the divalent intensity with incomplete overlayer structures confirms the role of coordination in promoting the mixed valent state in Tin. Mixed valent behaviour has been observed in other low coordinated Tm structures. These include a rough 20 ,~ thick layer grown on GaAs [18,19] and a thick "porous" layer grown on a copper substrate at low temperature [16]. The salient feature of the present study is that the mixed valence behaviour is seen to correlate with the more precisely defined structure of the overlayer at the initial stages of growth. Photoemission measurements have now been reported for sub-monolayer coverages of Yb, Sm and Tm on the Mo(ll0) surface. All three systems exhibit the same basic growth pattern, that is, the development of a series of n x 2 structures followed by a pseudo-hexagonal monolayer. The photoemission results are broadly consistent with the theory for unsupported structures if the interaction with the substrate atoms is taken into account. Thus for Yb, the divalent state is observed for all configurations, from the lowest coverage to the thick layer, as expected [20]. For Sm and Tin, on the other hand, the pseudohexagonal monolayer, which has a planar coordination of 6 or between 8 and 10 if the substrate is included, appears to be trivalent. This compares with the prediction of Lfibke et al. [7] that the trivalent state will be favoured for coordinations greater than 10 (Sm) and 7 (Tm). At low coverages, Sm is divalent [8] which contrasts with the
862
C.L. Nicklin et al. / Surface Science 307-309 (1994) 858-862
case of Tm, reported here, which always exhibits a strong trivalent feature. This is consistent with the greater energy difference between the two valence states and consequently the requirement of a lower coordination to support divalency in Tm than in Sm. Even the limited intralayer interaction at the start of adsorbate ordering, into n x 2 structures [14], is enough to encourage trivalency in Tm, in contrast to the case of Sm. We conclude that the valence behaviour of Tm is consistent with the trends predicted by theory and shown by Sm and Yb. However, even with a non-alloying substrate, such as Mo(ll0), it is clear that the adsorbate-substrate interaction is a significant factor in determining the stable valence state of the ultrathin, sub-monolayer RE films. Such structures cannot be regarded simply as unsupported two-dimensional layers in which only the intralayer interaction is important.
5. Acknowledgements The authors gratefully acknowledge the technical support of Lea Minel, Steve Taylor and Stuart Thornton. We would also like to thank Dr. N. Barrett, Dr. C. Guillot and the staff of LURE for their assistance. This work was supported by the Science and Engineering Research Council, Grant No. G R / F 17667.
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