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Atomic imaging of oxide surfaces
Surface
684
ATOMIC IMAGING II. Terbium oxide
OF OXIDE
Science 175 (1986) 6X4-692 North-Holland, Amsterdam
SURFACES
and
Received
6 January
1986; accepted
for publication
9 May 1986
The surface topography of anion-deficient. and nominally stoichinmetric. terbium oxides has been characterised at the atomic level by high resolution electron microscopy using the surface profile imaging technique. The predominant surface was found to be (111) which generally formed well-ordered terraces: other surfaces such as (100) and (022) were less prevalent and usually quite irregular. Surface modifications were ako otxerved in real-time which were face-dependent. and the rapidit) of the associated atomic arrangements appeared to correlate with the number of oxygen atom positions at the different surfaces.
I. Introduction
Knowledge of the atomic surface structure of materials is important for reaching an understanding of many of their macroscopic properties. Traditionally, LEED (low energy electron diffraction) and other diffraction and spectroscopic techniques have been used to characterise the surface structure but such methods cannot provide details of the local irregularities which can have such a marked effect on surface phenomena. On the other hand, real space information, for example about phase transitions. steps, superlattices and the emergence of dislocations and other extended defects can be obtained using electron microscopical methods. The various imaging methods. the materials 0039-6028/86/$03.50 1:) Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V
Z.C. Kong et al. / Oxide surfaces. II. Terbium oxide
685
studied, and the types of surface information available have recently been reviewed [l]. One technique that appears particularly promising for providing surface topographical information is profile imaging. In this operating mode, the surface profile of the sample is imaged at atomic resolution simply by utilising the normal microscope configuration for structure imaging. The technique was initiated by Sinclair et al. who studied topographical changes in CdTe [2]. Subsequent studies have been primarily of Au small particles and extended foils 131. Observations of compound materials have been restricted to the ceramic ZrO, [4] and the spine1 catalyst ZnFeCrO, [5,6]. In this paper, which is part of a series [7,X], we report on the surface structure of various fluorite-related terbium oxides.
2. Experimental
details
The terbium oxide (99.9999%) used in these observations was supplied in the form of the ignition product from a precipitated oxalate. This material was heated in a muffle furnace at 1000°C for 12 h. The furnace was turned off, allowing the sample to cool slowly in air to 200°C before removal to a desiccator. The powder was dispersed in acetone by ultrasonic vibration and a drop of this suspension was applied to a holey carbon film on a microscope grid. The samples were then examined immediately in either a JEM-200CX, or a JEM-4000EX, high resolution electron microscope (HREM). Both HREMs were equipped with a La$ electron source, a double-tilt top-entry specimen holder and an image pickup and viewing system. Videotape recordings of dynamic events occurring on these surfaces were made with these systems and these are described elsewhere [9].
3. The structure
of anion-deficient
fluorite-related
terbium oxide
The fluorite structure consists of a cubic-close-packed arrangement of metal atoms with non-metal atoms occupying all the tetrahedral sites but none in the octahedral sites: the composition is MO,. This structure is illustrated in fig. 1 where the circles represent the metal atoms and the triangles represent oxygen. Removal of oxygen from these fluorite-related oxides leaves tetrahedral sites unoccupied, and at low temperatures the high mobility of oxygen permits an ordered structure of low free energy to be formed rapidly [lo]. This results in an ordered distribution of tetrahedral voids which normally condense in pairs as corner-sharing double tetrahedra. These vacancy pairs are then ordered, or further condensed, depending on the composition. For the Tb,O,z, TbllOzo and Tb,,012 composition which we have primarily studied here,
686
one-seventh, the vacancy
one-eleventh and one-twelfth of the oxygen sites are vacant, and pairs are ordered along the fluorite (6X4) and (022) directions
n11. A projection of this anion-deficient fluorite structure in the (110) direction reflects the fact that the metal cations and the oxygen anions are aligned with the octahedral sites forming a large tunnel. When the crystal surface terminates with oxygen anions, there are different numbers of exposed vacancies depending on the particular composition and orientation of the surface. As noted below. these vacancies appear to have little to do with the surface effects reported here. The projected separation of metal atoms (or tunnels) is 0.301 nm along (111) and 0.522 nm along (100). The distance between the oxygen and metal atoms is 0.185 nm but because of the much lower scattering factor of oxygen relative to that of terbium it is not possible to distinguish the columns of oxygen atoms in either microscope. For observation in the [llO] zone. the surface structure, as defined by the cation column positions, should be revealed on the atomic scale [12].
4. Direct atomic
imaging
of the extended
oxide surface
In parallel with the program of experimental observations. and to determine the extent of apparent surface contractions on the (111) surface, image simulations were undertaken for the profile imaging mode and these will be reported elsewhere [12]. For the present purposes. it is sufficient to note that
687
Fig. 2. Pair of 400 kV high resolution electron micrographs of terbium oxide, recorded at (a) the optimum (black dot) focus and (b) reverse (white dot) focus. showing the remarkable cleanliness of the surface profile.
these calculations, which were for the [llO] projection, confirmed the existence of “black dot”/“ white dot” imaging conditions, as established previously for Au surfaces [13]. This meant that at the appropriate objective lens defocus, the terbium atomic columns at the surface appeared either as black or white spots, and that the surface morphology could be directly read off from the experimental images. A representative black/white pair of 400 kV micrographs of terbium oxide is shown in fig. 2 and these demonstrate one of the most striking features of the terbium oxide profile images, namely the exceptional cleanliness of the surface. In most materials, it is customary to observe a thin layer of surface contamination, generally supposed to consist of carbonaceous material, which is deposited from the atmosphere, impurities in the specimen, or from the residual gases in the microscope. Such overlayers were rarely observed in these terbium oxide crystals and certainly could not be linked with any particular surfaces.
5. Surface topography Based on the direct relationship established between the experimental micrographs and the atomic arrangements at the surface [12], it was relatively straightforward to characterise the topography of the various surface profiles observed in the [110] crystal projection.
688
Fig. 3. Surface profile
images of
the (111) surface of
terbium
oxide: (a) white dot focus; (b) black
dot focus.
5.1. (ill)
surfuce
Typical 400 kV images of the (111) surface are shown in figs. 3a and 3b; the former was taken at “white dot” focus, the latter at “black dot” focus. From many such images, it was concluded that the (111) surfaces generally dominated the crystal topography, forming well-ordered terraces with spacings corresponding to single-metal-atom planes. Step densities effectively then determined the surface profile. An example of increasing step density on the (111) surface is shown in fig. 4. and it is obvious that each of these steps is only one metal atomic plane in height. 5.2. (100) surface In general, as shown in fig. 5. the (100) surfaces are quite irregular. They often contain grooves and canyons and the black contrast at the atomic column positions also varies from place to place and with time. indicating
Fk!
4. Surface
profile
image dominated
by
(I 11)
terrace\, with
accommodate crvstal curvature.
increasing
step densirq
to
689
Fig. 5. Image showing
the typical
irregularity
of the terbium
oxide (100) surface
profile.
variations in the number of atoms in each column. Some images of the (100) surface show evidence for contractions - these are discussed elsewhere [12]. 5.3. (022) surface Termination of the bulk crystal at the (022) surface should give a regular sawtooth shape. Our observations, as represented by the 200 kV micrograph in fig. 6, show that the approximate (022) surface is usually very rough and irregular and can best be described as consisting of short (111) microfacets of highly variable length. Well-ordered (022) surfaces have not been seen. 5.4. High index (422) surface The high index (422) surface should consist basically of (111) planes with regular steps which should be 4(111) X (111). However, a lot of irregular steps are normally observed in the experimental images, as shown for example in fig. 7.
6. Surface modif ications It was observed considerably over
Fig. 6. 200 kV profile
that the contrast of atomic columns extended periods. sometimes even
Image of (022) terbium oxide surface the surface.
showing
at the surfaces varied disappearing and/or
the highly irregular
nature
of
690
Z. C. Kang
et al. /
Oxide
Fig. 7. Image of the (422) surface
surfaces.II. Terhm
recorded
at the optimum
oz.&
defocus
position
This often led to substantial alterations in the surface profile. In terms of decreasing stability (more activity), the crystal faces approximately rank in the order {ill}, high-index planes near {ill}, and then (100). and (022). Some interesting examples of the morphological changes can be seen in the three 200 kV images in fig. 8. which were taken at approximately 15 min intervals. Note that these were all recorded at white dot focus. In the case of the (111) surfaces, for example, it seems that a number of atomic columns at AB have been removed leaving a shorter terrace and the four columns on the (111) surface at D have also disappeared. At the same time, the basin at C has appearing.
recorded at the reverse (white Fig. 8. Series of 200 kV micrographs. modifications to the surface profile over a period of 30 min. Note the “loss” A, C and D and the rearrangements at B and E.
dot) focus. showing of atomic columns at
2. C. Kang et al. / Oxide surfaces. II. Terbium oxide
been extended, the steps between have tended to equalize in length.
B and C are wider whilst the terraces
691
at EF
7. Discussion The micrographs presented above are a small but representative selection from a considerable accumulation of surface profile images from terbium oxide crystals. From these, a number of general comments can be made. The exceptional cleanliness of the surfaces of these materials has already been noted. TbO,Y is an oxidation catalyst and could conceivably promote the oxidation of any carbonaceous contaminants to gaseous carbon oxides. In the absence of contamination, there is no doubt that the details of the surface morphology are more readily apparent. Moreover. any long-range driving forces responsible for any surface reconstruction will then not be adversely retarded by the presence of any surface impurities. It should also be noted that these oxides have been grown from precipitated oxalates with undisturbed rather than fractured faces. The {ill} closed-packed surface should have the lowest surface energy and hence it is not really surprising that this surface should be the predominant one. Similarly, it is interesting that the stability of the various crystal faces increases with the surface density of the metal atoms and hence with the number of oxygen atom positions at the surface. During extended periods of observation, electron-beam-induced reductions und/or oxidations of the crystal interiors were found to occur. Nevertheless, there was no substantive evidence that the different superstructures of the various oxide phases had any influence on either the surface morphology or the relative atomic mobilities. These basically depended only on the particular surface, so that any uncertainties in composition due either to natural heterogeneity or to electron-beam-induced effects were effectively not significant.
Acknowledgements This work has been supported by NSF Grant DMR-8108501 and used the facilities of the Center for High Resolution Electron Microscopy within the Center for Solid State Science at Arizona State University, supported by the Division of Materials Research. National Science Foundation, Grant DMR8308501.
References [I] D.J. Smith. in: Chemistry and Physics Howe (Springer. Berlin. 1986) ch. 15.
of Solid Surfaces,
Vol. 6. Eds. R. Vans&xv
and R.
[2] [3] [4] [5] [6] [7] [X] [9] [lo]
R. Sinclair, T. Yamashita and F.A. Ponce. Nature 290 (1981) 386. D.J. Smith and L.D. Marks, Ultramicroscopy 16 (1985) 101. C.E. Warble, Ultramicroscopy 15 (1984) 301. N.A. Briscoe and J.L. Hutchison. Inst. Phys. Conf. Ser. 68 (1984) 249. J.L. Hutchison and N.A. Briscoe. Ultramicroscopy 18 (1985) 435. D.J. Smith. L.A. Bursill and D.A. Jefferson. Surface Sci. 175 (1986) 673. D.J. Smith and L.A. Bursill. Surface Sci., submitted. Z.L. Kang, L. Eyrlng and D.J. Smith. Ultramicroscopy, in press. L. Eyring. in: Handbook on the Physics and Chemistry of the Rare Earths, Eds. K.A Gschneidner and L. Eyring (North-Holland, Amsterdam, 1979) ch. 27, pp. 337-339. [ll] R.T. Tuenge and L. Eyring, J. Solid State Chem. 41 (1982) 75. [12] Z.C. Kang, L.D. Marks, L. Eyring and D.J. Smith, in preparation. [13] L.D. Marks. Surface Sci. 139 (1984) 281.
684
ATOMIC IMAGING II. Terbium oxide
OF OXIDE
Science 175 (1986) 6X4-692 North-Holland, Amsterdam
SURFACES
and
Received
6 January
1986; accepted
for publication
9 May 1986
The surface topography of anion-deficient. and nominally stoichinmetric. terbium oxides has been characterised at the atomic level by high resolution electron microscopy using the surface profile imaging technique. The predominant surface was found to be (111) which generally formed well-ordered terraces: other surfaces such as (100) and (022) were less prevalent and usually quite irregular. Surface modifications were ako otxerved in real-time which were face-dependent. and the rapidit) of the associated atomic arrangements appeared to correlate with the number of oxygen atom positions at the different surfaces.
I. Introduction
Knowledge of the atomic surface structure of materials is important for reaching an understanding of many of their macroscopic properties. Traditionally, LEED (low energy electron diffraction) and other diffraction and spectroscopic techniques have been used to characterise the surface structure but such methods cannot provide details of the local irregularities which can have such a marked effect on surface phenomena. On the other hand, real space information, for example about phase transitions. steps, superlattices and the emergence of dislocations and other extended defects can be obtained using electron microscopical methods. The various imaging methods. the materials 0039-6028/86/$03.50 1:) Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V
Z.C. Kong et al. / Oxide surfaces. II. Terbium oxide
685
studied, and the types of surface information available have recently been reviewed [l]. One technique that appears particularly promising for providing surface topographical information is profile imaging. In this operating mode, the surface profile of the sample is imaged at atomic resolution simply by utilising the normal microscope configuration for structure imaging. The technique was initiated by Sinclair et al. who studied topographical changes in CdTe [2]. Subsequent studies have been primarily of Au small particles and extended foils 131. Observations of compound materials have been restricted to the ceramic ZrO, [4] and the spine1 catalyst ZnFeCrO, [5,6]. In this paper, which is part of a series [7,X], we report on the surface structure of various fluorite-related terbium oxides.
2. Experimental
details
The terbium oxide (99.9999%) used in these observations was supplied in the form of the ignition product from a precipitated oxalate. This material was heated in a muffle furnace at 1000°C for 12 h. The furnace was turned off, allowing the sample to cool slowly in air to 200°C before removal to a desiccator. The powder was dispersed in acetone by ultrasonic vibration and a drop of this suspension was applied to a holey carbon film on a microscope grid. The samples were then examined immediately in either a JEM-200CX, or a JEM-4000EX, high resolution electron microscope (HREM). Both HREMs were equipped with a La$ electron source, a double-tilt top-entry specimen holder and an image pickup and viewing system. Videotape recordings of dynamic events occurring on these surfaces were made with these systems and these are described elsewhere [9].
3. The structure
of anion-deficient
fluorite-related
terbium oxide
The fluorite structure consists of a cubic-close-packed arrangement of metal atoms with non-metal atoms occupying all the tetrahedral sites but none in the octahedral sites: the composition is MO,. This structure is illustrated in fig. 1 where the circles represent the metal atoms and the triangles represent oxygen. Removal of oxygen from these fluorite-related oxides leaves tetrahedral sites unoccupied, and at low temperatures the high mobility of oxygen permits an ordered structure of low free energy to be formed rapidly [lo]. This results in an ordered distribution of tetrahedral voids which normally condense in pairs as corner-sharing double tetrahedra. These vacancy pairs are then ordered, or further condensed, depending on the composition. For the Tb,O,z, TbllOzo and Tb,,012 composition which we have primarily studied here,
686
one-seventh, the vacancy
one-eleventh and one-twelfth of the oxygen sites are vacant, and pairs are ordered along the fluorite (6X4) and (022) directions
n11. A projection of this anion-deficient fluorite structure in the (110) direction reflects the fact that the metal cations and the oxygen anions are aligned with the octahedral sites forming a large tunnel. When the crystal surface terminates with oxygen anions, there are different numbers of exposed vacancies depending on the particular composition and orientation of the surface. As noted below. these vacancies appear to have little to do with the surface effects reported here. The projected separation of metal atoms (or tunnels) is 0.301 nm along (111) and 0.522 nm along (100). The distance between the oxygen and metal atoms is 0.185 nm but because of the much lower scattering factor of oxygen relative to that of terbium it is not possible to distinguish the columns of oxygen atoms in either microscope. For observation in the [llO] zone. the surface structure, as defined by the cation column positions, should be revealed on the atomic scale [12].
4. Direct atomic
imaging
of the extended
oxide surface
In parallel with the program of experimental observations. and to determine the extent of apparent surface contractions on the (111) surface, image simulations were undertaken for the profile imaging mode and these will be reported elsewhere [12]. For the present purposes. it is sufficient to note that
687
Fig. 2. Pair of 400 kV high resolution electron micrographs of terbium oxide, recorded at (a) the optimum (black dot) focus and (b) reverse (white dot) focus. showing the remarkable cleanliness of the surface profile.
these calculations, which were for the [llO] projection, confirmed the existence of “black dot”/“ white dot” imaging conditions, as established previously for Au surfaces [13]. This meant that at the appropriate objective lens defocus, the terbium atomic columns at the surface appeared either as black or white spots, and that the surface morphology could be directly read off from the experimental images. A representative black/white pair of 400 kV micrographs of terbium oxide is shown in fig. 2 and these demonstrate one of the most striking features of the terbium oxide profile images, namely the exceptional cleanliness of the surface. In most materials, it is customary to observe a thin layer of surface contamination, generally supposed to consist of carbonaceous material, which is deposited from the atmosphere, impurities in the specimen, or from the residual gases in the microscope. Such overlayers were rarely observed in these terbium oxide crystals and certainly could not be linked with any particular surfaces.
5. Surface topography Based on the direct relationship established between the experimental micrographs and the atomic arrangements at the surface [12], it was relatively straightforward to characterise the topography of the various surface profiles observed in the [110] crystal projection.
688
Fig. 3. Surface profile
images of
the (111) surface of
terbium
oxide: (a) white dot focus; (b) black
dot focus.
5.1. (ill)
surfuce
Typical 400 kV images of the (111) surface are shown in figs. 3a and 3b; the former was taken at “white dot” focus, the latter at “black dot” focus. From many such images, it was concluded that the (111) surfaces generally dominated the crystal topography, forming well-ordered terraces with spacings corresponding to single-metal-atom planes. Step densities effectively then determined the surface profile. An example of increasing step density on the (111) surface is shown in fig. 4. and it is obvious that each of these steps is only one metal atomic plane in height. 5.2. (100) surface In general, as shown in fig. 5. the (100) surfaces are quite irregular. They often contain grooves and canyons and the black contrast at the atomic column positions also varies from place to place and with time. indicating
Fk!
4. Surface
profile
image dominated
by
(I 11)
terrace\, with
accommodate crvstal curvature.
increasing
step densirq
to
689
Fig. 5. Image showing
the typical
irregularity
of the terbium
oxide (100) surface
profile.
variations in the number of atoms in each column. Some images of the (100) surface show evidence for contractions - these are discussed elsewhere [12]. 5.3. (022) surface Termination of the bulk crystal at the (022) surface should give a regular sawtooth shape. Our observations, as represented by the 200 kV micrograph in fig. 6, show that the approximate (022) surface is usually very rough and irregular and can best be described as consisting of short (111) microfacets of highly variable length. Well-ordered (022) surfaces have not been seen. 5.4. High index (422) surface The high index (422) surface should consist basically of (111) planes with regular steps which should be 4(111) X (111). However, a lot of irregular steps are normally observed in the experimental images, as shown for example in fig. 7.
6. Surface modif ications It was observed considerably over
Fig. 6. 200 kV profile
that the contrast of atomic columns extended periods. sometimes even
Image of (022) terbium oxide surface the surface.
showing
at the surfaces varied disappearing and/or
the highly irregular
nature
of
690
Z. C. Kang
et al. /
Oxide
Fig. 7. Image of the (422) surface
surfaces.II. Terhm
recorded
at the optimum
oz.&
defocus
position
This often led to substantial alterations in the surface profile. In terms of decreasing stability (more activity), the crystal faces approximately rank in the order {ill}, high-index planes near {ill}, and then (100). and (022). Some interesting examples of the morphological changes can be seen in the three 200 kV images in fig. 8. which were taken at approximately 15 min intervals. Note that these were all recorded at white dot focus. In the case of the (111) surfaces, for example, it seems that a number of atomic columns at AB have been removed leaving a shorter terrace and the four columns on the (111) surface at D have also disappeared. At the same time, the basin at C has appearing.
recorded at the reverse (white Fig. 8. Series of 200 kV micrographs. modifications to the surface profile over a period of 30 min. Note the “loss” A, C and D and the rearrangements at B and E.
dot) focus. showing of atomic columns at
2. C. Kang et al. / Oxide surfaces. II. Terbium oxide
been extended, the steps between have tended to equalize in length.
B and C are wider whilst the terraces
691
at EF
7. Discussion The micrographs presented above are a small but representative selection from a considerable accumulation of surface profile images from terbium oxide crystals. From these, a number of general comments can be made. The exceptional cleanliness of the surfaces of these materials has already been noted. TbO,Y is an oxidation catalyst and could conceivably promote the oxidation of any carbonaceous contaminants to gaseous carbon oxides. In the absence of contamination, there is no doubt that the details of the surface morphology are more readily apparent. Moreover. any long-range driving forces responsible for any surface reconstruction will then not be adversely retarded by the presence of any surface impurities. It should also be noted that these oxides have been grown from precipitated oxalates with undisturbed rather than fractured faces. The {ill} closed-packed surface should have the lowest surface energy and hence it is not really surprising that this surface should be the predominant one. Similarly, it is interesting that the stability of the various crystal faces increases with the surface density of the metal atoms and hence with the number of oxygen atom positions at the surface. During extended periods of observation, electron-beam-induced reductions und/or oxidations of the crystal interiors were found to occur. Nevertheless, there was no substantive evidence that the different superstructures of the various oxide phases had any influence on either the surface morphology or the relative atomic mobilities. These basically depended only on the particular surface, so that any uncertainties in composition due either to natural heterogeneity or to electron-beam-induced effects were effectively not significant.
Acknowledgements This work has been supported by NSF Grant DMR-8108501 and used the facilities of the Center for High Resolution Electron Microscopy within the Center for Solid State Science at Arizona State University, supported by the Division of Materials Research. National Science Foundation, Grant DMR8308501.
References [I] D.J. Smith. in: Chemistry and Physics Howe (Springer. Berlin. 1986) ch. 15.
of Solid Surfaces,
Vol. 6. Eds. R. Vans&xv
and R.
[2] [3] [4] [5] [6] [7] [X] [9] [lo]
R. Sinclair, T. Yamashita and F.A. Ponce. Nature 290 (1981) 386. D.J. Smith and L.D. Marks, Ultramicroscopy 16 (1985) 101. C.E. Warble, Ultramicroscopy 15 (1984) 301. N.A. Briscoe and J.L. Hutchison. Inst. Phys. Conf. Ser. 68 (1984) 249. J.L. Hutchison and N.A. Briscoe. Ultramicroscopy 18 (1985) 435. D.J. Smith. L.A. Bursill and D.A. Jefferson. Surface Sci. 175 (1986) 673. D.J. Smith and L.A. Bursill. Surface Sci., submitted. Z.L. Kang, L. Eyrlng and D.J. Smith. Ultramicroscopy, in press. L. Eyring. in: Handbook on the Physics and Chemistry of the Rare Earths, Eds. K.A Gschneidner and L. Eyring (North-Holland, Amsterdam, 1979) ch. 27, pp. 337-339. [ll] R.T. Tuenge and L. Eyring, J. Solid State Chem. 41 (1982) 75. [12] Z.C. Kang, L.D. Marks, L. Eyring and D.J. Smith, in preparation. [13] L.D. Marks. Surface Sci. 139 (1984) 281.