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The structure of overlayers
SURFACE
SCIENCE 40 (1973) 439444
THE
STRUCTURE
0 North-Holland
Publishing Co.
OF OVERLAYERS
I. Se ON Ag {OOl} Received 16 July 1973
As part of a long-range program aimed at the elucidation of the atomic and electronic structure of over-layers on solid crystalline surfaces, we have carried out experimental and theoretical investigations of overlayers of sulfur, selenium and tellurium on Ag (001) l). We report here some results for the Se/Ag system. . The experiments were performed in a standard Varian/360 LEED-Auger system equipped with a source of selenium consisting of an appropriately oriented pre-evacuated Pyrex vial with break-off tip. The Ag (001) surface was cleaned with alternate treatments of argon-ion bombardments and anneals as described elsewheres). Selenium was deposited onto the clean Ag(001) surface from a heated Pyrex vial at rates of approximately 2 to 5 Ajmin and chamber pressures ranging between 0.5 and 1 x 1O-1o torr. Selenium coverages equivalent to approximately 1 monolayer with the Ag (001) substrate at room temperature produced LEED patterns of a poorly ordered P(2 x 2) superstructure. Higher coverages equivalent to approximately 2 atomic layers gave rise to LEED patterns of a well-ordered C (2 x 2) superstructure. It is this C(2 x 2) structure that constitutes the object of the investigations reported in the present note. The C (2 x 2) is one of the simplest among the LEED patterns observed for overlayer structures in a variety of systemss), because it gives rise to only one “extra” beam (also called fractional-order beam) in the unit mesh of the LEED pattern of the clean substrate surface. We have collected intensity-versus-energy spectra for a number of “normal” (or integral-order) as well as “fractional-order” beams of the Ag (001) C (2 x 2)-Se pattern. The primary electron energy was varied from approximately 10 eV to 300 eV and the angle of incidence from 0” to 20”. The simplest model that is consistent with the geometry and the symmetry of the observed C (2 x 2) diffraction pattern is that of a centered square overlayer of Se atoms distributed over the Ag{OOl) surface with the size of the centered unit mesh of the Se net twice as large as that of the primitive Ag (001) net. Fig. 1 depicts such a model. It is clear, however, that there are several possible positions of the Se overlayer relative to the substrate surface. Fig. 1 shows three such relative positions, corresponding to the Se atoms 439
440
A.IGNATIEV
ET AL.
sitting on high-symmetry adsorption sites. In the model labelled “4-fold sites”, the Se atoms sit on sites with four-fold symmetry and have four substrate atoms as nearest neighbors; in the model labelled “bridge sites”, the Se atoms sit across two substrate atoms in a bridge-like position; in the model labelled “top-atom sites”, the Se atoms sit on top of single substrate atoms. We have examined the relative validity of these three structural models by
4-FOLD SITES
BRIDGE SITES
TOP ATOM SITES
Fig. 1. Models for the C (2 x 2) structure of Se overlayers on Ag (001). Empty circles represent substrate (Ag) atoms, hatched circles represent adsorbed Se atoms. On the left, the Se atoms sit across four substrate atoms in four-fold positions; in the center, the Se atoms sit on bridge positions across two substrate atoms; on the right, each Se atom sits on top of a substrate atom. comparing with one another and with the experiment the intensity-versusenergy spectra calculated for the relative atomic positions pertinent to each of the three models. The calculations were carried out by the layer-KKR procedure, described elsewhere, that has been proven satisfactory for a number of surfaces of fee metals4) and, in particular, for the clean Ag{OOl} surfaces). For the silver substrate, the same band-structure potential that has provided satisfactory agreement with the experimental LEED data of the clean Ag (001) surface was useds). For the selenium overlayer, a potential obtained from the superposition of atomic charge densities was used. Eight phase shifts and 56 beams were used throughout. Absorption was taken into account by supplementing the band-structure potential with an energy-depending imaginary part that was determined from experimental data, as described elsewheres). For each of the three models depicted in fig. 1 calculations were carried out for a large number of values of the distance d, between the plane of the Se atoms and the plane of the first layer of atoms in the substrate. In the calculations presented in this note the overlayer Debye temperature was 150°K and the substrate Debye temperature was 215°K. Fig. 2 shows a comparison between the experimental 00 beam spectrum (top curve) measured for an incidence angle 0 of 5” and the corresponding spectra calculated for the three structural models depicted in fig. 1. It should be noted that the bridge-site model has only two-fold symmetry around the normal to the surface. Therefore, a change in azimuth angle from 4=0” to
STRUCTURE
OF OVERLAYERS.
441
I
6,= 90"is expected to cause changes in the LEED spectra. If it were assumed that the two possible domains of the bridge-site model were represented equally on the surface, the measured intensities of diffracted beams would be averages of those from each domain. This assumption was in fact made and therefore two calculations were carried out, one for r$=O” and the other for (p= 90”. The resulting curves were then averaged. It is these averaged curves
c-
1
Ag {OOl)c12x2)-
THEOR. Clop atom
Se
1
sites 1
0
0
20
40
60
_
60
ENERGY
100
120
140
1
160
(eV)
Fig. 2. Ag {OOl} C (2 x 2)-Se: 00 spectrum for 8= 5” and 4 = 0”. The top curve is experimental. The other curves were calculated for selenium atom positions corresponding to the three models of fig. 1, as IabeIled.
that are plotted in the figures with the label “bridge-sites”. Examination of fig. 2 shows that the 4-fold-site model provides the best agreement with experiment. Each of the theoretical spectra presented in fig. 2 corresponds to the choice of the distance d, between Se overlayer and substrate surface which gives the best agreement with experiment. This distance d, was varied from 1.1 A to 2.6 A to obtain the optimal values. Fig. 3 shows that the 00 beam spectrum (as well as others not shown here) depends rather sensitively upon the parameter d,. Fig. 3 applies to the 4-fold-site model but similar sensitivity was found for the other two models as well. Best agreement with experiment for Se atoms on 4-fold sites is found for the value d, = 1.91 A corresponding to a Se-Ag bond distance of 2.80 A*. A change as small as kO.09 A in d,
* This is to be compared with typical bond lengths of 3.21 _&(ionic), 2.69 A (covalent), 2.60 A (metallic) as given by Pauling7).
442
A. IGNATIEV
ET AL.
worsens the agreement quite noticeably. Fig. 4 compares the results of calculations of the 01 beam with experiment for the same three models as in fig. 2. Again, it appears that the model based on the 4-fold adsorption site fits the experiment much better than the other two models. The same conclusion is reached from similar comparisons involving the TO, 10, 11, and ii beams. The agreement between theory and experiment is not as good, however, for the fractional-order beams. Fig. 5 depicts the case of the -$-$beam showing that for none of the three models is the agreement between theory Ag{OOi)c(2x2)-Se
8=5”
00
0
20
40
60
a0 ENERGY
BEAM
100 (eV
+=CP
I20
140
160
1
Fig. 3. Ag {OOl1 C (2 x 2)Se: Dependence of the 00 spectrum (0 = 5”, $= 0”) upon the distance d, of the Se layer over the substrate surface for the “Cfold” model depicted in fig, 1. The top curve is experimental. The second curve (calculated with dz==1.91 A) represents the best agreement between theory and experiment. The sensitivity of the spectrum on d, is shown in the two bottom curves, calcutated with dz= 2.09 A and d,- 1.73 A, respectively.
and experiment really satisfactory. This also holds true for the s+ beam (not shown). The conclusion that we can draw at this stage is the following. Among the three structural models depicted in fig. 1 the one labelled “4-fold-sites” seems to provide the best agreement of calculated intensitites with experiment. The overall agreement, however, particularly that concerning the fractional-order beams, is not as good as has been obtained for ordered overlayers of chalcogens on Ni {OOt1 with the same calculation procedure6). It is
STRUCTURE
OF OVERLAYERS.
(top atom sites 0
0
20
40
60
443
I
1
60
100
ENERGY
(eV
120
140
160
1
Fig. 4. Ag (001) C (2 x 2)-Se: 01 spectrum for 0 = 5” and #=O”. The experimental curve is on top, The remaining curves were calculated for the three models shown in fig. 1, as labelled.
THEOR.
(top olom sites)
0
20
40
60
80
ENERGY
100
120
140
160
(eV)
Fig. 5. Ag (001) C (2 x 2)-Se: 3 3 spectrum for 0 = 5” and 4 = 0”. The experimental curve is on top. The remaining curves were calculated for the three models shown in fig. 1, as labelled.
444
A.ICiNATIEV
ET AL.
possible that the simple model that works so well for the chal~ogens~nicke1 system may not be correct for the Se/Ag system. For example, atomic placeexchange on the surface may lead to the formation of a Se-Ag compound, or alloy, which has the symmetry and periodicity observed in the LEED patterns but a structure involving mixed layers, i.e., layers containing both Se and Ag atoms. Calculations of the LEED spectra expected from such a model of the overlayer are presently in progress and will be reported later. The present study, however, shows that the agreement between experiment and calculations based on the 4fold-site model, while not perfect, is better than accidental. This indicates that some of the interatomic distances selected for this model are close to being correct. A. IGNATIEV * and F. JONA* Department of Materials Science, State University of New York, Stony Brook, New York I1 790, U.S.A.
and D. IV. JEPSENand P. M. MARCUS IBM Research Center, Yorktown Heights, New York 10598, U.S.A.
References I) A. Ignatiev, F. Jona, D. W. Jepsen and P. M. Marcus, Bull. Am. Phys. Sot. [2] 18 2) 3) 4) 5) 6) 7)
(1973) 384. E. W. Hu, R. M. Goodman and F. Jona, submitted to Phys. Rev. (1973). See, e.g., G. A. Somorjai and H. H. Farrell, Advan. Chem. Phys. 20 (1971) 215. D. W. Jepsen, P. M. Marcus and F. Jona, Phys. Rev. BS (1972) 3933. D. W. Jepsen, P. M. Marcus and F. Jona, submitted to Phys. Rev. (1973). J. E. Demuth, D. W. Jepsen and P. M. Marcus, to be published. L. Pauling, The Natttre ofthe C~e~~c~Z Bond, 3rd ed. (Cornell University Press, 1970).
* The work of these authors was sponsored in part by the Air Force Office of Scientific Research, Air Force Systems Command, under Grant No. AFOSR-72-2151.
SCIENCE 40 (1973) 439444
THE
STRUCTURE
0 North-Holland
Publishing Co.
OF OVERLAYERS
I. Se ON Ag {OOl} Received 16 July 1973
As part of a long-range program aimed at the elucidation of the atomic and electronic structure of over-layers on solid crystalline surfaces, we have carried out experimental and theoretical investigations of overlayers of sulfur, selenium and tellurium on Ag (001) l). We report here some results for the Se/Ag system. . The experiments were performed in a standard Varian/360 LEED-Auger system equipped with a source of selenium consisting of an appropriately oriented pre-evacuated Pyrex vial with break-off tip. The Ag (001) surface was cleaned with alternate treatments of argon-ion bombardments and anneals as described elsewheres). Selenium was deposited onto the clean Ag(001) surface from a heated Pyrex vial at rates of approximately 2 to 5 Ajmin and chamber pressures ranging between 0.5 and 1 x 1O-1o torr. Selenium coverages equivalent to approximately 1 monolayer with the Ag (001) substrate at room temperature produced LEED patterns of a poorly ordered P(2 x 2) superstructure. Higher coverages equivalent to approximately 2 atomic layers gave rise to LEED patterns of a well-ordered C (2 x 2) superstructure. It is this C(2 x 2) structure that constitutes the object of the investigations reported in the present note. The C (2 x 2) is one of the simplest among the LEED patterns observed for overlayer structures in a variety of systemss), because it gives rise to only one “extra” beam (also called fractional-order beam) in the unit mesh of the LEED pattern of the clean substrate surface. We have collected intensity-versus-energy spectra for a number of “normal” (or integral-order) as well as “fractional-order” beams of the Ag (001) C (2 x 2)-Se pattern. The primary electron energy was varied from approximately 10 eV to 300 eV and the angle of incidence from 0” to 20”. The simplest model that is consistent with the geometry and the symmetry of the observed C (2 x 2) diffraction pattern is that of a centered square overlayer of Se atoms distributed over the Ag{OOl) surface with the size of the centered unit mesh of the Se net twice as large as that of the primitive Ag (001) net. Fig. 1 depicts such a model. It is clear, however, that there are several possible positions of the Se overlayer relative to the substrate surface. Fig. 1 shows three such relative positions, corresponding to the Se atoms 439
440
A.IGNATIEV
ET AL.
sitting on high-symmetry adsorption sites. In the model labelled “4-fold sites”, the Se atoms sit on sites with four-fold symmetry and have four substrate atoms as nearest neighbors; in the model labelled “bridge sites”, the Se atoms sit across two substrate atoms in a bridge-like position; in the model labelled “top-atom sites”, the Se atoms sit on top of single substrate atoms. We have examined the relative validity of these three structural models by
4-FOLD SITES
BRIDGE SITES
TOP ATOM SITES
Fig. 1. Models for the C (2 x 2) structure of Se overlayers on Ag (001). Empty circles represent substrate (Ag) atoms, hatched circles represent adsorbed Se atoms. On the left, the Se atoms sit across four substrate atoms in four-fold positions; in the center, the Se atoms sit on bridge positions across two substrate atoms; on the right, each Se atom sits on top of a substrate atom. comparing with one another and with the experiment the intensity-versusenergy spectra calculated for the relative atomic positions pertinent to each of the three models. The calculations were carried out by the layer-KKR procedure, described elsewhere, that has been proven satisfactory for a number of surfaces of fee metals4) and, in particular, for the clean Ag{OOl} surfaces). For the silver substrate, the same band-structure potential that has provided satisfactory agreement with the experimental LEED data of the clean Ag (001) surface was useds). For the selenium overlayer, a potential obtained from the superposition of atomic charge densities was used. Eight phase shifts and 56 beams were used throughout. Absorption was taken into account by supplementing the band-structure potential with an energy-depending imaginary part that was determined from experimental data, as described elsewheres). For each of the three models depicted in fig. 1 calculations were carried out for a large number of values of the distance d, between the plane of the Se atoms and the plane of the first layer of atoms in the substrate. In the calculations presented in this note the overlayer Debye temperature was 150°K and the substrate Debye temperature was 215°K. Fig. 2 shows a comparison between the experimental 00 beam spectrum (top curve) measured for an incidence angle 0 of 5” and the corresponding spectra calculated for the three structural models depicted in fig. 1. It should be noted that the bridge-site model has only two-fold symmetry around the normal to the surface. Therefore, a change in azimuth angle from 4=0” to
STRUCTURE
OF OVERLAYERS.
441
I
6,= 90"is expected to cause changes in the LEED spectra. If it were assumed that the two possible domains of the bridge-site model were represented equally on the surface, the measured intensities of diffracted beams would be averages of those from each domain. This assumption was in fact made and therefore two calculations were carried out, one for r$=O” and the other for (p= 90”. The resulting curves were then averaged. It is these averaged curves
c-
1
Ag {OOl)c12x2)-
THEOR. Clop atom
Se
1
sites 1
0
0
20
40
60
_
60
ENERGY
100
120
140
1
160
(eV)
Fig. 2. Ag {OOl} C (2 x 2)-Se: 00 spectrum for 8= 5” and 4 = 0”. The top curve is experimental. The other curves were calculated for selenium atom positions corresponding to the three models of fig. 1, as IabeIled.
that are plotted in the figures with the label “bridge-sites”. Examination of fig. 2 shows that the 4-fold-site model provides the best agreement with experiment. Each of the theoretical spectra presented in fig. 2 corresponds to the choice of the distance d, between Se overlayer and substrate surface which gives the best agreement with experiment. This distance d, was varied from 1.1 A to 2.6 A to obtain the optimal values. Fig. 3 shows that the 00 beam spectrum (as well as others not shown here) depends rather sensitively upon the parameter d,. Fig. 3 applies to the 4-fold-site model but similar sensitivity was found for the other two models as well. Best agreement with experiment for Se atoms on 4-fold sites is found for the value d, = 1.91 A corresponding to a Se-Ag bond distance of 2.80 A*. A change as small as kO.09 A in d,
* This is to be compared with typical bond lengths of 3.21 _&(ionic), 2.69 A (covalent), 2.60 A (metallic) as given by Pauling7).
442
A. IGNATIEV
ET AL.
worsens the agreement quite noticeably. Fig. 4 compares the results of calculations of the 01 beam with experiment for the same three models as in fig. 2. Again, it appears that the model based on the 4-fold adsorption site fits the experiment much better than the other two models. The same conclusion is reached from similar comparisons involving the TO, 10, 11, and ii beams. The agreement between theory and experiment is not as good, however, for the fractional-order beams. Fig. 5 depicts the case of the -$-$beam showing that for none of the three models is the agreement between theory Ag{OOi)c(2x2)-Se
8=5”
00
0
20
40
60
a0 ENERGY
BEAM
100 (eV
+=CP
I20
140
160
1
Fig. 3. Ag {OOl1 C (2 x 2)Se: Dependence of the 00 spectrum (0 = 5”, $= 0”) upon the distance d, of the Se layer over the substrate surface for the “Cfold” model depicted in fig, 1. The top curve is experimental. The second curve (calculated with dz==1.91 A) represents the best agreement between theory and experiment. The sensitivity of the spectrum on d, is shown in the two bottom curves, calcutated with dz= 2.09 A and d,- 1.73 A, respectively.
and experiment really satisfactory. This also holds true for the s+ beam (not shown). The conclusion that we can draw at this stage is the following. Among the three structural models depicted in fig. 1 the one labelled “4-fold-sites” seems to provide the best agreement of calculated intensitites with experiment. The overall agreement, however, particularly that concerning the fractional-order beams, is not as good as has been obtained for ordered overlayers of chalcogens on Ni {OOt1 with the same calculation procedure6). It is
STRUCTURE
OF OVERLAYERS.
(top atom sites 0
0
20
40
60
443
I
1
60
100
ENERGY
(eV
120
140
160
1
Fig. 4. Ag (001) C (2 x 2)-Se: 01 spectrum for 0 = 5” and #=O”. The experimental curve is on top, The remaining curves were calculated for the three models shown in fig. 1, as labelled.
THEOR.
(top olom sites)
0
20
40
60
80
ENERGY
100
120
140
160
(eV)
Fig. 5. Ag (001) C (2 x 2)-Se: 3 3 spectrum for 0 = 5” and 4 = 0”. The experimental curve is on top. The remaining curves were calculated for the three models shown in fig. 1, as labelled.
444
A.ICiNATIEV
ET AL.
possible that the simple model that works so well for the chal~ogens~nicke1 system may not be correct for the Se/Ag system. For example, atomic placeexchange on the surface may lead to the formation of a Se-Ag compound, or alloy, which has the symmetry and periodicity observed in the LEED patterns but a structure involving mixed layers, i.e., layers containing both Se and Ag atoms. Calculations of the LEED spectra expected from such a model of the overlayer are presently in progress and will be reported later. The present study, however, shows that the agreement between experiment and calculations based on the 4fold-site model, while not perfect, is better than accidental. This indicates that some of the interatomic distances selected for this model are close to being correct. A. IGNATIEV * and F. JONA* Department of Materials Science, State University of New York, Stony Brook, New York I1 790, U.S.A.
and D. IV. JEPSENand P. M. MARCUS IBM Research Center, Yorktown Heights, New York 10598, U.S.A.
References I) A. Ignatiev, F. Jona, D. W. Jepsen and P. M. Marcus, Bull. Am. Phys. Sot. [2] 18 2) 3) 4) 5) 6) 7)
(1973) 384. E. W. Hu, R. M. Goodman and F. Jona, submitted to Phys. Rev. (1973). See, e.g., G. A. Somorjai and H. H. Farrell, Advan. Chem. Phys. 20 (1971) 215. D. W. Jepsen, P. M. Marcus and F. Jona, Phys. Rev. BS (1972) 3933. D. W. Jepsen, P. M. Marcus and F. Jona, submitted to Phys. Rev. (1973). J. E. Demuth, D. W. Jepsen and P. M. Marcus, to be published. L. Pauling, The Natttre ofthe C~e~~c~Z Bond, 3rd ed. (Cornell University Press, 1970).
* The work of these authors was sponsored in part by the Air Force Office of Scientific Research, Air Force Systems Command, under Grant No. AFOSR-72-2151.