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Metastable surface structureof the W(110) face
SURFACE
SCIENCE
METASTABLE
10 (1968) 392-398 0 North-Holland
SURFACE
STRUCTURE
R. BAUDOING
Publishing Co., Amsterdam
OF THE
W(110)
FACE*
**
Laboratoire de Spectrom&ie Physique, Faculte’ des Sciences de Grenoble, France
R. M. STERN Department of Physics, Polytechnic Institute of Brooklyn, Brooklyn, New York 11201, U.S.A.
Received 27 October 1967; revised manuscript
received 26 December
1967
The observation of a new metastable phase on the (110) tungsten surface contaminated with carbon is reported with an explanation based on a model of surface deformation in the (110) plane. The transition from the stable Cl structure to the metastable C2 structure is reversible and this cycle is reproducilbe.
1. Introduction The thermodynamics of surfaces has become subject to considerable interest as a result of the large number of observations of surface structures in low energy electron diffraction. The stability of each of the many atomic models proposed to explain the diffraction features observed in these cases can only be determined if the thermodynamics of each surface is well understood. In the absence of a reliable guide for constructing such surface models, there is much disagreement regarding the rules for surface adsorption and rearrangement. One of the most stable surfaces is found to be the (110) face of tungsten. It shows no tendency to produce facets even when maintained at temperatures near the melting point in vacuum, although the introduction of oxygen pressures greater then 10m6 Torr at temperatures near 2500°C produces facets which disappear after further heating in vacuum. Other surfaces tend, upon heating, to become covered ‘) with (110) facets which prove to be quite stable. * Supported by U.S.A.F. Contract A.F. 49 (638)-1369 ** Submitted in partial fulfillment of the requirements troisieme cycle, University of Grenoble. 392
of the degree of Doctorat
de
METASTABLE
SURFACE
STRUCTURE
OF THE W (110)
FACE
393
The study of a new metastable phase on the (110) tungsten surface contaminated with carbon might permit the identification of the parameters responsible for its stability. Such a state will be described, and an explanation of its stability proposed based on a model of surface deformation in the (110) plane. 2. Low temperature carbon structure The (110) surface of tungsten is observed to exhibit a very stable and reproducible long range structure which has been identified as originating in the carbon impurity’). This contamination, apparently stable to the melting point, can only be removed by chemically combining at high temperatures the carbon atoms dissolved in the crystal with gas adsorbing on the surface. It is independent of the carbon concentration in the bulk, since the continuous decrease of the bulk concentration by reduction with oxygen results only in the eventual decrease in the intensity of the pattern while its geometry does not change at all. This same carbon structure can be obtained by reducing a monolayer of carbon monoxide which has been adsorbed on the surface, either by heating the crystal in the presence of hydrogen, or under long term bombardment of the primary electron beam. In this case the structure obtained can simply be removed by heating. The unit cell of a single domain of the stable carbon superstructure in reciprocal lattice is shown in fig. tB, and in rea1 space as the smaller of the two unit cehs in fig. 1A. It should be noted that two domains are observed which interact incoherently. The diffraction pattern can be observed down to the diffraction limit of the most central spots, about 7 eV. 3. High temperature carbon structure If the crystal showing strong carbon structure is heated to near 2000°C and the heating current abruptly interrupted, a new pattern appears which is due to a unit cell having one dimension four times larger than that of the first carbon structure. If the crystal is then heated to 1600°C and returned slowly to room temperature, the original structure is found to reappear (this cycle is perfectly reproducible). A single domain of the diffraction pattern of the new metastable phase is shown schematically in fig. 1C. The unit cell on the crystal surface responsible for this pattern is shown in fig. lA, and can be seen to contain four unit cells of the stable structure. The diffraction pattern, which consists of the superposition of two mirror domain orientation is shown schematically in fig. 2A. A photograph of the diffraction pattern is shown in fig. 2C.
394
R. BALIDOiNG
AND
R. M. STERN
A
Fig. 1. (A) (110) plane showing the two carbon structures. The corresponding unit cells are superimposed on the tungsten lattice. Structure Cl has the smaller mesh, structure C2 has a mesh four times larger along the (112) direction. (B) Schematic diffraction pattern for one domain of the stable Cl structure shown in fig. 1A. (C) Schematic diffraction pattern for one domain of the metastable C2 structure shown in fig. 1A.
A photograph of the diffraction fig. 2B for comparison.
pattern
of the stable phase
is shown
in
Unfortunately the luminous intensity of the tungsten crystal prevents the observation of the annealing of the metstable state, so that no measurement of the energy difference associated with its formation can be made. It is possible, however, to speculate as to the origin of the state based on its geometry. Two strong possibilities for the origin of the carbon patterns are the existence of a regular structure of appropriate unit cell, or the presence of surface deformation. It is hard to conceive of the mechanism which provides for the extremely large dimensions of the unit cell necessary for the diffrac-
M~TASTAB~E
SURFACE
STRUCTURE
OF THE W (110)
FACE
395
Fig. 2. (A) Schematic representation of the diffraction pattern for the metastable carbon structure. The large spots represent the clean tungsten unit cell. The smaller spots represent the two domains of stable carbon structure (Cl) and the complete pattern including the smallest spots as well as the intermediate spots represents the C2 structure. The relative size of the diffraction features is shown by the size of the spot. (B) Photograph of the diffraction pattern from the Cl structure at 15 V. (C) Photograph of the diffraction pattern from the C2 structure at 15 V.
396
K.
BAUDOfNG
AND K. M. STERN
tion pattern based on the crystallography of a regular surface structure, although the existence of very dilute crystalline compounds is known. Even admitting this possibility, however, it is extremely difficult to propose the same argument for the second structure, which must have the same origin as the first. 4. Evidence for a periodic surface deformation origin The clean tungsten surface has been shown to exhibit diffraction features in the range of 500-800 eV 3, which are identified as being due to anisotropic thermal diffuse scattering, and which indicate the strong presence of surface localized vibrational modes. The presence of the carbon impurity almost completely suppresses these diffraction features, which means a significant decrease of the surface vibrations. The additional diffraction features are also smaller and much less diffuse then the normal tungsten diffraction spots. This indicates that the diameter of the bulk diffraction features is not limited by the electron beam diameter, but is broadened by the bulk thermal diffuse scattering, which is known to be isotropic. Both of these observations lead to the conclusion that the clean tungsten diffraction pattern contains strong thermal diffuse scattering and that the presence of carbon strongly suppresses the vibrational states of the surface. Measurement of the increased effective Debye temperature for the surface contaminated by carbon compared to the clean surface confirm thissy*). These observations suggest that the vibrational mode of the clean (110) tungsten surface responsible for the strong thermal diffuse scattering is made unstable by the presence of the carbon impurity. The carbon in effect triggers a periodic deformation which results in a surface which no longer has potentially unstable modes and hence exhibits little thermal diffuse scattering. The pattern observed for the carbon contaminated surface results from diffraction by the new two dimensional periodicity. It should be noted that for both structures, the vectors of the surface mesh are along the (ill) and the (i12)directions). The metastable state has an effective period which is four times larger than that of the stable state, along the direction normal to the (111) rows. This means that the new deformation results in a movement in which the most dense rows of atoms remain intact and move as a whole. This geometry seems most satisfying thermodynamically and elastically as the surface retains its order in this process. When a surface is deformed, it is expected that the successive lattice planes are deformed as well, but that the amplitude of the deformation decreases with increasing depth of the planes. The larger the amplitude of deformation, the further the deformation is expected to extend into the bulk.
METASTABLE
SURFACE
STRUCTURE
OF THE W (110)
FACE
397
On the other hand, the relationship between the period of the deformation and the degree of deformation can only be determined by detailed calculation of the surface energy, and the balance attained at equilibirum between the various terms which contribute to the total energy. It is found that the exlra diffraction features observed for the metastable structure become indistinguishable from the background intensity at a lower voltage (300 eV) than do those of the normally stable structure (900 eV). This would indicate that the structure with the longer period has the smaller deformation, and hence the bulk lattice is perturbed to a lesser depth. Since the carbon structure is independent of the bulk carbon concentration, and the vibrational states of the surface are strongly perturbed by its presence, it is possible to conclude that the carbon concentration of the surface is high, and is limited to the superficial plane. The intensity of the carbon pattern is almost the same when the crystal contains its original concentration, as when the pattern has been made by the reduction of a monolayer of adsorbed carbon monoxide. It is assumed that the reduction in intensity of the pattern, as the last trace of carbon is removed, is due to the decrease of size of the saturated surface domains. This model satisfactorily explains the existence of only one stable carbon pattern, independent of the bulk carbon concentration, and due to a deformation in the superficial plane caused by the saturation of that plane with carbon. The metastable pattern can then be explained on the basis of a second concentration (and/or deformation) which is stable at high temperatures, but which of course, may be associated with other than the first atomic plane. The existence of such structure has been predicted by Feuchtwangs). The conclusion to be made from these observations is that surface deformation can be considered as a strong possibility for the process by which surface atoms relax to minimize the surface energysrs). The new configuration is always the result of the energy balance determined by all the processes which contribute to the total energy of the surface. It is also expected that these deformations are related simply to the surface geometry, and that the presence of high concentrations in the surface plane of impurity atoms easily accommodated features observed.
by the lattice can lead to the very high order diffraction
It should be noted that although the periodicity in each dimension of the stable carbon structure can be described by a single Fourier component which is independent of the surface impurity concentration, this should not necessarily be the case in general. There have been several observations of systems (sulphur on copper?), oxygen on tungsten*), among others) where the apparent order of the extra diffraction features is a continuous function of the surface impurity concentration over certain limits. The concept of a
398
surface provides
R.BAUDOING
deformation an appealing
AND
whose periodicity explanation
R. M. STERN
depends
on impurity
concentration
for these observations.
Acknowledgments The authors wish to express their appreciation for the hospitality shown to them during the course of this work: R. B. to the Department of Physics, Polytechnic Institute of Brooklyn, where the experimental work was performed, and R.M.S. to the Laboratoire de Spectrometrie Physique, Universite de Grenoble, where this manuscript was prepared. References 1) 2) 3) 4) 5) 6) 7) 8)
J. Anderson and W.E. Danforth, J. Franklin Inst. 279 (1965) 160. R. M. Stern, Appl. Phys. Letters 5 (1964) 218. J. Aldag and R. M. Stern, Phys. Rev. Letters 14 (1965) 758. R. Baudoing and R. M. Stern, to be published. T.E. Feuchtwang, Phys. Rev. 155 (1967) 715. P. Ducros, Surface Sci. 10 (1968) 295. J.L. Domange and J. Oudar, to be published. R. M. Stern, unpublished results.
SCIENCE
METASTABLE
10 (1968) 392-398 0 North-Holland
SURFACE
STRUCTURE
R. BAUDOING
Publishing Co., Amsterdam
OF THE
W(110)
FACE*
**
Laboratoire de Spectrom&ie Physique, Faculte’ des Sciences de Grenoble, France
R. M. STERN Department of Physics, Polytechnic Institute of Brooklyn, Brooklyn, New York 11201, U.S.A.
Received 27 October 1967; revised manuscript
received 26 December
1967
The observation of a new metastable phase on the (110) tungsten surface contaminated with carbon is reported with an explanation based on a model of surface deformation in the (110) plane. The transition from the stable Cl structure to the metastable C2 structure is reversible and this cycle is reproducilbe.
1. Introduction The thermodynamics of surfaces has become subject to considerable interest as a result of the large number of observations of surface structures in low energy electron diffraction. The stability of each of the many atomic models proposed to explain the diffraction features observed in these cases can only be determined if the thermodynamics of each surface is well understood. In the absence of a reliable guide for constructing such surface models, there is much disagreement regarding the rules for surface adsorption and rearrangement. One of the most stable surfaces is found to be the (110) face of tungsten. It shows no tendency to produce facets even when maintained at temperatures near the melting point in vacuum, although the introduction of oxygen pressures greater then 10m6 Torr at temperatures near 2500°C produces facets which disappear after further heating in vacuum. Other surfaces tend, upon heating, to become covered ‘) with (110) facets which prove to be quite stable. * Supported by U.S.A.F. Contract A.F. 49 (638)-1369 ** Submitted in partial fulfillment of the requirements troisieme cycle, University of Grenoble. 392
of the degree of Doctorat
de
METASTABLE
SURFACE
STRUCTURE
OF THE W (110)
FACE
393
The study of a new metastable phase on the (110) tungsten surface contaminated with carbon might permit the identification of the parameters responsible for its stability. Such a state will be described, and an explanation of its stability proposed based on a model of surface deformation in the (110) plane. 2. Low temperature carbon structure The (110) surface of tungsten is observed to exhibit a very stable and reproducible long range structure which has been identified as originating in the carbon impurity’). This contamination, apparently stable to the melting point, can only be removed by chemically combining at high temperatures the carbon atoms dissolved in the crystal with gas adsorbing on the surface. It is independent of the carbon concentration in the bulk, since the continuous decrease of the bulk concentration by reduction with oxygen results only in the eventual decrease in the intensity of the pattern while its geometry does not change at all. This same carbon structure can be obtained by reducing a monolayer of carbon monoxide which has been adsorbed on the surface, either by heating the crystal in the presence of hydrogen, or under long term bombardment of the primary electron beam. In this case the structure obtained can simply be removed by heating. The unit cell of a single domain of the stable carbon superstructure in reciprocal lattice is shown in fig. tB, and in rea1 space as the smaller of the two unit cehs in fig. 1A. It should be noted that two domains are observed which interact incoherently. The diffraction pattern can be observed down to the diffraction limit of the most central spots, about 7 eV. 3. High temperature carbon structure If the crystal showing strong carbon structure is heated to near 2000°C and the heating current abruptly interrupted, a new pattern appears which is due to a unit cell having one dimension four times larger than that of the first carbon structure. If the crystal is then heated to 1600°C and returned slowly to room temperature, the original structure is found to reappear (this cycle is perfectly reproducible). A single domain of the diffraction pattern of the new metastable phase is shown schematically in fig. 1C. The unit cell on the crystal surface responsible for this pattern is shown in fig. lA, and can be seen to contain four unit cells of the stable structure. The diffraction pattern, which consists of the superposition of two mirror domain orientation is shown schematically in fig. 2A. A photograph of the diffraction pattern is shown in fig. 2C.
394
R. BALIDOiNG
AND
R. M. STERN
A
Fig. 1. (A) (110) plane showing the two carbon structures. The corresponding unit cells are superimposed on the tungsten lattice. Structure Cl has the smaller mesh, structure C2 has a mesh four times larger along the (112) direction. (B) Schematic diffraction pattern for one domain of the stable Cl structure shown in fig. 1A. (C) Schematic diffraction pattern for one domain of the metastable C2 structure shown in fig. 1A.
A photograph of the diffraction fig. 2B for comparison.
pattern
of the stable phase
is shown
in
Unfortunately the luminous intensity of the tungsten crystal prevents the observation of the annealing of the metstable state, so that no measurement of the energy difference associated with its formation can be made. It is possible, however, to speculate as to the origin of the state based on its geometry. Two strong possibilities for the origin of the carbon patterns are the existence of a regular structure of appropriate unit cell, or the presence of surface deformation. It is hard to conceive of the mechanism which provides for the extremely large dimensions of the unit cell necessary for the diffrac-
M~TASTAB~E
SURFACE
STRUCTURE
OF THE W (110)
FACE
395
Fig. 2. (A) Schematic representation of the diffraction pattern for the metastable carbon structure. The large spots represent the clean tungsten unit cell. The smaller spots represent the two domains of stable carbon structure (Cl) and the complete pattern including the smallest spots as well as the intermediate spots represents the C2 structure. The relative size of the diffraction features is shown by the size of the spot. (B) Photograph of the diffraction pattern from the Cl structure at 15 V. (C) Photograph of the diffraction pattern from the C2 structure at 15 V.
396
K.
BAUDOfNG
AND K. M. STERN
tion pattern based on the crystallography of a regular surface structure, although the existence of very dilute crystalline compounds is known. Even admitting this possibility, however, it is extremely difficult to propose the same argument for the second structure, which must have the same origin as the first. 4. Evidence for a periodic surface deformation origin The clean tungsten surface has been shown to exhibit diffraction features in the range of 500-800 eV 3, which are identified as being due to anisotropic thermal diffuse scattering, and which indicate the strong presence of surface localized vibrational modes. The presence of the carbon impurity almost completely suppresses these diffraction features, which means a significant decrease of the surface vibrations. The additional diffraction features are also smaller and much less diffuse then the normal tungsten diffraction spots. This indicates that the diameter of the bulk diffraction features is not limited by the electron beam diameter, but is broadened by the bulk thermal diffuse scattering, which is known to be isotropic. Both of these observations lead to the conclusion that the clean tungsten diffraction pattern contains strong thermal diffuse scattering and that the presence of carbon strongly suppresses the vibrational states of the surface. Measurement of the increased effective Debye temperature for the surface contaminated by carbon compared to the clean surface confirm thissy*). These observations suggest that the vibrational mode of the clean (110) tungsten surface responsible for the strong thermal diffuse scattering is made unstable by the presence of the carbon impurity. The carbon in effect triggers a periodic deformation which results in a surface which no longer has potentially unstable modes and hence exhibits little thermal diffuse scattering. The pattern observed for the carbon contaminated surface results from diffraction by the new two dimensional periodicity. It should be noted that for both structures, the vectors of the surface mesh are along the (ill) and the (i12)directions). The metastable state has an effective period which is four times larger than that of the stable state, along the direction normal to the (111) rows. This means that the new deformation results in a movement in which the most dense rows of atoms remain intact and move as a whole. This geometry seems most satisfying thermodynamically and elastically as the surface retains its order in this process. When a surface is deformed, it is expected that the successive lattice planes are deformed as well, but that the amplitude of the deformation decreases with increasing depth of the planes. The larger the amplitude of deformation, the further the deformation is expected to extend into the bulk.
METASTABLE
SURFACE
STRUCTURE
OF THE W (110)
FACE
397
On the other hand, the relationship between the period of the deformation and the degree of deformation can only be determined by detailed calculation of the surface energy, and the balance attained at equilibirum between the various terms which contribute to the total energy. It is found that the exlra diffraction features observed for the metastable structure become indistinguishable from the background intensity at a lower voltage (300 eV) than do those of the normally stable structure (900 eV). This would indicate that the structure with the longer period has the smaller deformation, and hence the bulk lattice is perturbed to a lesser depth. Since the carbon structure is independent of the bulk carbon concentration, and the vibrational states of the surface are strongly perturbed by its presence, it is possible to conclude that the carbon concentration of the surface is high, and is limited to the superficial plane. The intensity of the carbon pattern is almost the same when the crystal contains its original concentration, as when the pattern has been made by the reduction of a monolayer of adsorbed carbon monoxide. It is assumed that the reduction in intensity of the pattern, as the last trace of carbon is removed, is due to the decrease of size of the saturated surface domains. This model satisfactorily explains the existence of only one stable carbon pattern, independent of the bulk carbon concentration, and due to a deformation in the superficial plane caused by the saturation of that plane with carbon. The metastable pattern can then be explained on the basis of a second concentration (and/or deformation) which is stable at high temperatures, but which of course, may be associated with other than the first atomic plane. The existence of such structure has been predicted by Feuchtwangs). The conclusion to be made from these observations is that surface deformation can be considered as a strong possibility for the process by which surface atoms relax to minimize the surface energysrs). The new configuration is always the result of the energy balance determined by all the processes which contribute to the total energy of the surface. It is also expected that these deformations are related simply to the surface geometry, and that the presence of high concentrations in the surface plane of impurity atoms easily accommodated features observed.
by the lattice can lead to the very high order diffraction
It should be noted that although the periodicity in each dimension of the stable carbon structure can be described by a single Fourier component which is independent of the surface impurity concentration, this should not necessarily be the case in general. There have been several observations of systems (sulphur on copper?), oxygen on tungsten*), among others) where the apparent order of the extra diffraction features is a continuous function of the surface impurity concentration over certain limits. The concept of a
398
surface provides
R.BAUDOING
deformation an appealing
AND
whose periodicity explanation
R. M. STERN
depends
on impurity
concentration
for these observations.
Acknowledgments The authors wish to express their appreciation for the hospitality shown to them during the course of this work: R. B. to the Department of Physics, Polytechnic Institute of Brooklyn, where the experimental work was performed, and R.M.S. to the Laboratoire de Spectrometrie Physique, Universite de Grenoble, where this manuscript was prepared. References 1) 2) 3) 4) 5) 6) 7) 8)
J. Anderson and W.E. Danforth, J. Franklin Inst. 279 (1965) 160. R. M. Stern, Appl. Phys. Letters 5 (1964) 218. J. Aldag and R. M. Stern, Phys. Rev. Letters 14 (1965) 758. R. Baudoing and R. M. Stern, to be published. T.E. Feuchtwang, Phys. Rev. 155 (1967) 715. P. Ducros, Surface Sci. 10 (1968) 295. J.L. Domange and J. Oudar, to be published. R. M. Stern, unpublished results.