FIELD ION MICROSCOPE STUDIES OF SURFACE RECONSTRUCTIONS

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FIELD ION MICROSCOPE STUDIES OF SURFACE RECONSTRUCTIONS

G. Kellogg

To cite this version:

G. Kellogg. FIELD ION MICROSCOPE STUDIES OF SURFACE RECONSTRUCTIONS. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-59-C6-63. �10.1051/jphyscol:1987610�. �jpa-00226813�

Colloque C6, suppldment au n O 1 l , Tome 48, novembre 1987

FIELD ION MICROSCOPE STUDIES OF SURFACE RECONSTRUCTIONS

G . L. Kellogg

Sandia National Laboratories, Albuquerque, NM 87185, U.S.A.

Abstract.- The reconstruction of various metal surfaces has been investigated in atomic detail with the field ion microscope. Unreconstructed (bulk- terminated) surfaces were produced by low-temperature field evaporation and induced to reconstruct either by heating alone or heating in the presence of deuterium. Field ion microscope images recorded before and after the field- free heating interval were used to determine the temperature at which restructuring takes place and the structure of the reconstructed surface. The results clearly showed that the (110) planes of Pt and Ir reconstruct to missing-row structures. The temperature required for restructuring was approximately 300 K for Pt and 500 K for Ir. The reconstructions were observed for (110) planes as large as 40-50 A diameter and for clusters as small as five atoms. The direct observation of missing-row structures for these small clusters provides convincing evidence that the reconstructions are driven by short-ranged atomic interactions. Missing-row structures were also produced on higher index planes of Pt such as (311), (211), and (511) by heating to temperatures above 400 K. Small clusters of atoms on the (110) plane of Ni did not reconstruct thermally, but did reconszguct to a missing-row structure when heated to 150-170 K in the presence of 10 Torr deuterium.

I - Introduction

The atomic structure of a single-crystal surface may differ from a simple termination of the bulk structure. When this occurs the surface is said to be reconstructed /I/. For some crystal surfaces, such as the (110) planes of Pt, Ir, and Au, reconstructions occur on the clean surface, whereas for others, such as the (110) planes of Ni and Cu, reconstructions are induced by surface adsorbates /I/.

Reconstructions of both types are important in a variety of scientific areas including crystal growth and epitaxy, surface chemical reactions, surface phase transitions, and calculations of surface electronic structure. The most widely used technique to study surface reconstructions is low energy electron diffraction (LEED). Although the periodicity of the reconstructed surface atoms is easily deduced from the LEED patterns, a determination of the actual atomic positions is not straightforward. This difficulty has provided a motivation to study reconstructed surfaces with more direct methods.

The field ion microscope (FIM), which has the ability to image individual atoms on a solid surface, provides a direct means to investigate surface reconstructions.

Moreover, field evaporation at low temperatures typically produces unreconstructed surfaces, so that subsequent heating permits one to determine the temperature at which the reconstruction occurs and examine the atomic details of the reconstruction process. In recent studies the field ion microscope has been used to investigate the reconstruction of various low-index planes of Pt and Ir /2-9/. In this paper our work related to the reconstruction of the (110) and (100) planes of Pt and Ir

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987610

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and some higher index planes of Pt is reviewed. The results of preliminary FIM studies of the deuterium-induced reconstruction of the Ni(ll0) plane are also reported /lo/.

11 - Ex~erimental ADDaratuS and Procedures

With the exception of the Ni(ll0) study, the apparatus used to investigate surface reconstructions was an all-metal, u1tra:Pfgh vacuum field ion microscope with background pressures in the mid to upper 10 Torr range. Purified neon and helium were used to image Pt and Ir, respectively. An internal channel plate was used for image intensification and field ion images were recorded on Kodak Tri-X film with a 35 mm camera. The apparatus used in the study of the deuterium-induced reconstruc- tion of Ni(ll0) was an ultra-high vacuum imaging atom-probe 16double channel-plate detector assembly) with a background pressure of 1-2 x 10- Torr. Spectroscopic- grade neon without further purification was used as the imaging gas. The images were recorded on Polaroid Type 55 film by direct contact with the fiberoptics faceplate.

The experimental procedure used in the investigation involved several steps. The field emitter surface was first cleaned by a combination of thermal annealing, neon ion bombardment, and field evaporation. The plane of interest was field evaporated at 77 K to produce an unreconstructed surface. Typically, the topmost plane left by field evaporation was a small cluster of 5-20 atoms. A field ion image of the unreconstructed surface was recorded. The tips were then heated under field-free conditions to induce the restructuring of the surface ttoms. In the study of Ni(ll0) deuterium was introduced at a pressure of 1 x 10- during the heating interval. The method of this introduction is reported in a separate article /lo/.

The sample was then cooled to 77 K (in the Ni(ll0) study the deuterium was removed at this point) and a field ion image of the reconstructed surface was recorded. A comparison of the before and after field ion images was then used to determine the structure of the reconstructed surface.

111 - Results and Discussion

Field ion microscope images indicating the reconstruction of the (110) surfaces of Pt and Ir are shown in Figs. 1 and 2, respectively. In both cases the surface

Fig. 1 - Field ion microscope images showing the reconstruction of the (110) plane of Pt. The surface in (a) was produced by field evaporation at 77 K. The topmost layer of atoms consists of five short chains in in adjacent channels of the underlying plane. The surface in (b) was produced by heating the surface in (a) to 310 K for 1 min. The topmost layer of atoms now consists of three chains in next-adjacent channels providing direct evidence for the missing-row structure. The arrows point to a row of atoms in (a) which is missing in (b).

conclusively that the (1x2) diffraction pattern observed in LEED arose from a missing-row structure and not a "sawtooth" or some other structure proposed previously /2/. Also, the fact tha* the transformations occurred for very small clusters of atoms strongly suggested that the reconstruction of Pt(ll0) involves very short-ranged, atomic interactions /2/. Subsequent FIM studies in which the reconstruction was induced by laser pulses actually identified the atomic processes involved in the restructuring /3/. Transformations to the missing-row structure were very reproducible for both Pt(ll0) and Ir(ll0). The only difference between the Pt(ll0) and Ir(ll0) reconstructions was that the temperature required to initiate the transformation to the missing rows was approximately 300 K for Pt and 500 K for I 6 This difference can be attributed to the higher temperature required to induce mobility of the Ir surface atoms.

Fig. 2 - Field ion microscope images showing the reconstruction of the (110) plane of Ir. The three short chains of atoms in adjacent channels shown in (a) reconstruct to the two chains with a missing row shown in (b). Between the photographs the sample was heated to 500 K for 1 min.

The arrows point to the row of atoms in (a) which is missing in (b).

The same type of missing-row structure observed for Pt(ll0) and Ir(ll0) wes also observed for higher index planes of Pt such as (31l), (211), and (511) /6/. These reconstructions required higher temperatures than Pt(ll0) (typically above 400 K) and were not as reproducible. Interestingly, detailed studies of the interaction between Pt atoms on Pt(311) indicate an attractive interaction between two atoms in the same [lTO] surface channel and repulsive interactions between two atoms in adjacent channels at their closest separation /11/. These types of interactions are precisely the type which one would expect to lead to the missing-row structure.

Although reconstructions of these higher-index channeled surfaces of Pt have not been investigated by LEED or other surface techniques, the field ion microscope results strongly suggest that missing-row structure is the preferred structure for the channeled surfaces of Pt and Ir.

The field ion microscope has also been used to investigate the reconstruction of the (100) surfaces of Pt and Ir /4,5,7-9/. In our studies surface rearrangement was detected at temperatures of 270 and 470 K for Pt and Ir, respectively /4/. There is evidence that the changes in surface atom configuration under these conditions is due to some kind of a surface reconstruction / 4 / . However, subsequent FIM investigations /5,7-9/ have shown that in order to obtain the Ir(lx5) or Pt(5x20) reconstructions observed in LEED, higher temperatures are required. At these higher temperatures, the FIM images directly show the buckling of atomic rows associated with the Ir(lx5) reconstruction /5,7-9/, and field evaporation of these recon- structed surfaces has revealed the expected hexagonal arrangement of atoms in the topmost layer /8,9/.

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In all of the above studies the reconstruction of the surface was induced by heating alone. It has been well-established by investigations with LEED and various forms of ion scattering that some surfaces do not reconstruct thermally, but do reconstruct if an adsorbate is present on the surface /I/. An example is the hydrogen- (or deuterium-) induced recon$truction of Ni(ll0). LEED studies have shown conclusively that, although Ni(ll0) does not reconstruct thermally, within a certain range of temperature and adsorbate coverage this surface undergoes a reconstruction to a (1x2) structure /12/. However, there has been considerable debate as to whether the (1x2) reconstruction is due to a missing-row structure of the type discussed above or to a structure involving the pairing of rows of surface atoms. Most recent studies favor the "paired-row" model /l2/. Our study of the deuterium-induced reconstruction of Ni(110) was pr5marily directed towards distinguishing between the two proposed models.

Field ion microscope images showing a Ni(ll0) surface before and after heating to 165 K in 10- Torr deuterium are shown in Fig. 3. In Fig. 3(a) the (110) plane was field evaporated such that nine atoms remained in the topmost surface layer. These atoms are in three short rows in adjacent channels of the underlying surface. It should be noted that heating similar surfaces in the absence of deuterium produced no changes in the atomic positions. In Fig. 3(b) it is evident that heating the surface in the p,resence of deuterium has caused the surface atoms to rearrange.

Analysis of the field ion image indicate that the atoms had rearranged to a missing- row structure of the type reported above for the (110) planes of Pt and Ir. As in the case of the thermal reconstruction of Pt(ll0) and Ir(llO), this result was very reproducible. Thus, our results indicate that, at least for small clusters of atoms, the missing-row structure is formed on Ni(ll0) in the presence of deuterium.

The relationship of this structure to the proposed "paired-row" structure which apparently forms on macroscopic surfaces is discussed in a separate article /lo/.

Fig. 3

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IV

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The atomic resolution offered by t!le field ion microscope has been shown to be very useful for the study of reconstructed surfaces. Not only is it possible to identify surface structures in atomic detail, but the temperature at which restructuring takes place and the interactions which cause the reconstructions can also be investigated. The FIM has been shown to be particularly suited to study reconstructions which involve significant rearrangement of the surface atoms such as the missing-row reconstructions of channeled surfaces of Pt and Ir. It has now also

This work was supported hy the U. S. Department of Energy under contract #DE-AC04- 76DP00789.

References

/1/ P. J. Estrup, in Chemistry and Physics of Solid Surfaces, edited by R.

Vanselow and R. Howe (Springer-Verlag, Berlin, 1984) vol. 5, p. 205.

/2/ G. L. Kellogg, Phys. Rev. Lett. 55, 2168 (1985).

/3/ Q. Gao and T. T. Tsong, Phys. Rev. Lett.

x,

/4/ G. L. Kellogg, Surface Sci. 177, L1021 (1986).

/5/ J. Witt and K. Muller, Phys. Rev. Lett.

z,

/6/ G. L. Kellogg, J. Vac. Sci. Technol. A 5 , 747 (1987).

/7/ K. Muller, J. Witt, and 0. Schutz, J. Vac. Sci. Technol. A 5, 757 (1987).

/8/ Q. J. Gao and T. T. Tsong, J. Vac. Sci. Technol. A 5 , 761 (1987).

/9/ T. T. Tsong and Q. Gao, Phys. Rev. B15 (in press).

/lo/ G. L. Kellogg, submitted to Phys. Rev. Lett.

/11/ G. L. Kellogg, J. de Physique (Paris), Colloque C2, 47, 331 (1986).

/12/ See, for example: G. Kleinle, V. Penka, R. J. Behm, G. Ertl, and W. Moritz, Phys. Rev. Lett. 58, 148 (1987) and references therein.