The Surface Structure of GoldTHE ORIGIN OF ABNORMALITIES IN THE OUTER ATOMIC LAYERS

P. J. DobsonPhysics Department, Imperial College, London

The anomalous nature of the first twoor three atomic layers of a gold surfacehas an important effect in a number oftechnical fields, including catalysis andthe bonding to semiconductors, andcould be responsible for its resistance tooxidation. This paper reviews ourpresent knowledge of the single crystalsurfaces of gold and attempts to recon-cile some conflicting ideas that have beenput forward on the origin of the anoma-lous structure observed on the cube face.

Gold, because of its relative inertness and resistanceto chemical attack, has been traditionally a popularchoice by research workers wishing to examine thebehaviour of a metallic surface unaffected by oxidesor corrosion products. However, the modern surfaceresearch worker with extreme criteria for cleanlinesson an atomic scale must question all assumptionsregarding the composition of any surface. In studiesof mechanical, electrical or chemical properties, themain questions to be answered about any cleansurface are: "How clean is it?"; "What is the atomicstructure and how does the surface deviate from thebulk material ?". The gold { 100} surface in particularhas produced a puzzle that, at present, is not com-pletely resolved.

Surface physics and chemistry received a boost inthe 1960's due to the ready availability of commercialultra-high vacuum apparatus. In particular, theavailability of equipment for low energy electrondiffraction (LEED) (1, 2, 3) and for reflection highenergy electron diffraction (RHEED) (4) permitted

Fig. 1 A typical 5 X 1 LEED pattern from a {100} goldsurface. This pattern may he explained by the presenceof a hexagonal overlayer on a cubic structure

determination of the atomic structure and spacingof the outermost layer of surfaces. LEED utiliseselectrons of energy in the range 30 to 1000 ev, atwhich energies the atomic scattering factors are large.Consequently the penetration of low energy electronsin a solid is very low (typically a few atomic spacings)and the diffracted beams give structural informationabout the outermost atomic layers. The mostcommonly used LEED apparatus is the direct displaytype in which the backscattered electron diffractionpattern is observed on a fluorescent screen. Theangles between the diffracted beams can be deter-mined from photographs of the diffraction patternand this gives directly the spacing of atoms in theplane of the surface. In principle, the spacing ofatoms normal to the surface can be obtained frommeasurements of the intensities of diffracted beamsas a function of primary beam energy. However, acomplete dynamical theory that takes into accountmultiple scattering of the diffracted beams is neededbefore such information can be extracted. Such atheory should be available within the next few years.

RHEED utilises electron beams of energy 20 keVto 100 keV incident at grazing incidence on surfaces.Even though penetration of high energy electrons isrelatively high, the use of grazing incidence (typicallyof the order of one degree) means that only the outer-most two or three atomic layers contribute to thediffraction pattern. Results from RHEED aregenerally complementary to those from LEED.

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Auger spectroscopy (5, 6) which may be con-veniently incorporated into LEED vacuum chamberswas also developed in recent years. This techniquepermits identification of atoms in the outermost twoor three atomic layers, and in combination withLEED forms a very powerful analytical tool.

Up to 1966 LEED studies had shown that thesurface of most metals was "normal", i.e., thespacing and arrangement of atoms in the surface wasthe same as expected from bulk crystallographicdata. However, in a systematic study of the gold{111} {110} and {100} surfaces, Fedak and Gjostein(7, 8) observed anomalous structures for the latterorientations. Since then, much attention has beenfucused on the {l00} surface. The LEED patternsobserved by Fedak and Gjostein and subsequentlyothers are summarised in the table. Most of these ex-hibited 1/5th order diffracted beams as shown in Figs.I and 2. These are known as 5 x I patterns after theterminology of surface crystallography (9). Thisimplies that there is a long range coincidence lattice

of spacing five times the spacing expected from abulk {100} plane. Later work (10) showed thatLEED patterns were not always simply 5><1 butmore likely 5 x 20, i.e., splitting of beams was ob-served under higher resolution, particularly at lowelectron beam energies (less than 50 eV). Thissplitting was 1/20th of the "normal" reciprocallattice vector. Fig. 2 shows a schematic diagramof such a pattern (11). The table summarises theresults of studies of the {100} gold surface. It shouldbe noted that the gold {110} surface exhibited 2 x 1patterns (8). Platinum (21) and iridium (22) {100}surfaces also exhibit 5x1 or 5x20 patterns. Themain question asked by surface research workersis whether these 5 x 1 or 5 x 20 LEED patterns arecharacteristic of an atomically clean gold {100}surface or are due to the presence of an impuritystructure on the surface. The answer to this questionshould be compatible with the fact that the othernoble metals, silver and copper, do not exhibit suchLEED patterns, i.e., they appear to be "normal".

Summary of Investigations on the {100} Gold Surface

Technique Structure Sample j Preparation Model

Fedak and Gjostein LEED 5x1 -^1 x1 Bulk Ion Impurity(7, 8, 10) 5x20 800°C crystal bombardment reconstructed

or ion bomb +anneal

Fedak et al (12) LEED Cl2 Bulk Ion Impurity5 x 20-^ 1 x 1 crystal bombardment reconstructed

+anneal

Mattera et al (13) LEED 5x1 Bulk Ion Surface6x6 crystal bombardment vacancy

+anneal

Palmberg and Rhodin LEED 5 x 1 Thin Epitaxial Clean(14, 15) 5x20 epitaxial growth reconstructed

film in u.h.v.

Bauer et al (16) RHEED 5x1 Thin Epitaxial Impurity:.^-;<<:_:_epitaxial growth reconstructedfilm in u.h.v.

Bauer (17) LEED 5x1 Gold Epitaxial Impurity5 x 20 crystal growth reconstructed

+Na of Na

Dennis and Dobson LEED 5x1 Thin Epitaxial Surface(18) (RHEED) 5x20 epitaxial growth twinning

film in u.h.v.

Gronlund and Hojlund- RHEED Hexagonal (similar to Bulk Heating toNielsen (19) overlayer 5x20) crystal 200°C at

10-8 torr

Rhead (11) LEED +trace S. Pb Bulk Ion CleanBiberian and Rhead 5x20-^1 x:1 crystal bombardment reconstructed

(20) +anneal

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Fig. 2 A schematic diagram of LEED patterns

(a) From an unreconstructed {100} facecentred cubic surface, i.e. a 1 X 1 pattern

(b) A 5 X 1 pattern, which implies a fivetimes repeat distance of the basic lattice

(c) A 5 X 20 pattern, implying a five timesrepeat distance in one direction and atwenty times repeat distance at rightangles. (After Rhead (11))

°°

• • •0

(a) e, (b)

:K.

I.. •

(C)

Models of the {100} Gold SurfaceIt is convenient to divide the models into three

main categories, namely the clean reconstructedsurface model; impurity stabilised reconstruction;surface defect model. For the first two models it isgenerally agreed that LEED and RHEED results arecompatible with the model of a hexagonal overlayerof atoms with interatomic spacing 5 per cent less thanbulk gold, superimposed on unreconstructed {100}gold planes (14, 15, 10). This is shown in Fig. 3.The 1/5th, 1/20th order features are then a result ofmultiple diffraction between "overlayer" and "sub-strate" beams (15). This model has been shown (11)to account for splitting of fractional order beams.

(a) Clean Reconstructed Surface Model

The basis of this model is that the constrainedhexagonal layer referred to above consists only ofgold atoms. This conjecture was supported byAuger spectroscopic evidence which, it was claimed,showed that the surface impurity concentration is atthe most, a few parts in 10 3. This statement canonly be justified if the Auger transitions due to animpurity do not coincide with gold Auger transitionsand the sensitivity of the technique is sufficient todetect particular contaminants. The justification ofthe model was based upon considerations of mini-misation of surface energy and enhancement of sur-face valence by contraction of interatomic spacing.From considerations of the electronic configuration,molecular dissociation energy and internuclear spac-ing in molecules and other reasons, Palmberg andRhodin (14, 15) concluded that gold was more likelythan copper and silver to exhibit spacing changes andhence reconstruction at the surface.

There are several points against this model. Theremust exist a fine balance of surface and interfacialfree energies if such a hexagonal layer is to be stablefor a pure system. Mass spectrometric observations(12) suggest that alkali metal impurities are presentat the surface and, in particular, the effect of chlorineadsorption (12) which irreversibly transforms the5 x 1 to 1 x 1 surface is difficult to explain by anyclean surface hypothesis. Finally, despite the con-vincing justification for occurrence of the anomalousstructure on gold {100} surfaces offered by Palmbergand Rhodin, no satisfactory account has been givenfor reconstruction of the {110} surface, or of simi-larities with platinum and iridium {100} surfaces.

(b) Impurity Stabilised Reconstruction

Fedak and Gjostein (10) suggested that it wasunlikely from an energetic point of view that ahexagonal layer of gold atoms should be stable on a{100} surface. Mass spectrometric results (12) suggest"alkali impurities and other studies have shown thatsodium structures, in particular, on gold yield 5 x 1and 5 x20 patterns (16, 17). The various reports oftransformation to 1 x 1 following adsorption ofeither chlorine (12), lead or sulphur (20) are morecompatible with this model than a "clean surface"model, the adsorbate possibly forming small nucleiof a compound (e.g. NaCl), undetected by LEED.The reversible change from 5 x 1 to 1 ><1 at hightemperatures (8, 10) is also compatible with impuritydesorption above 800°C, but re-appearance of theimpurity by diffusion from the bulk crystal as thetemperature is lowered.

Among the points against this model is the factthat different preparation and cleaning procedures

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Ali x j^

r + + + + + + + t + + +

+° +° +° - T t T 't °+ °+ °+ •+• • e ° ° ° • • • • • • °

1Sx

20x

(110) azimuth

Fig. 3 The 5 X 20 "cell" of the reconstructed {100} gold surface. Atomshave been omitted for clarity

+ Atom positions in bulk position in second layer

• Atom positions in outermost layer

X = ao%-/2 where ao =bulk lattice spacing

have resulted in similar 5 x 1 and 5 X 20 features,as will be seen from the table. It would at first sightseem unlikely that a particular impurity was alwayspresent. Also, there is the fact that Auger spectro-scopy (15) failed to reveal a contaminant. However,of the likely impurities, sodium has K shell Augertransitions at greater than 900 eV and a L,L,,Vtransition at –'17 eV, both outside of the rangecovered in early work. Potassium has a strong Augertransition at –252 eV, which almost coincides with agold Auger transition at —256 eV. Fedak et al. (12)did observe changes in the Auger spectrum afterchlorine adsorption. Therefore, it would seemnecessary to employ other techniques such as X-rayphotoelectron spectroscopy (23), secondary ion massspectroscopy (24) or ion reflection spectroscopy (25)to detect any possible impurities.

If, as seems likely, the level of impurity is low, therather drastic re-arrangement of the surface is sur-prising unless a well-defined compound is formed.If, as has been suggested (17), alkali plus noblemetal compounds are responsible, more data arerequired regarding the structure and chemistry ofthese compounds, particularly in order to explainthe non-reconstruction of copper and silver surfaces,and the reconstruction of the {110} gold surface.

(c) Surface Defect Models

Mattera et al. (13) suggested that several f.c.c.metal surfaces exhibited reconstruction due toordered arrays of vacancies which result from ionbombardment cleaning procedures. Some of thestructures observed have not been seen by otherworkers, e.g. 6x6 {100} gold. This model seems

difficult to justify on energetic grounds and does notexplain adsorption or thermal transformations.

Dennis and Dobson (18), on the basis of observa-tion of twinning by RHEED and electron micro-scopy in {100} gold films suggested that twins in thesurface region may be responsible for apparentreconstruction. Twinning in a f.c.c. lattice wouldexpose {221} planes parallel to the surface {100}.The {221 } plane is an open packed structure and ifatom positions from successive layers are projectedon to the surface, a pseudohexagonal structureresults that is almost identical with that proposedfor models (a) and (b). The low stacking fault andtwin boundary energies for gold (26) are points infavour of this model, in that twins are easily formed.However, this also underlines a weakness, in thatsilver with comparable twin boundary energy (26)does not show reconstruction and platinum withvery much higher twin boundary energy (26) doesshow reconstruction. Other points against thismodel have been published (27, 28), the difficulty ofexplaining adsorption effects being the principalreason for doubting the model.

Conclusions

The situation is still not completely resolved,although available evidence appears to be in favourof the impurity reconstruction model. It remains tobe seen whether this model will explain similarobservations for platinum (21, 29) and iridium (22)and the reconstruction of gold {110}. In addition tousing other detection techniques, it would appearto be worth while to re-examine LEED data, par-ticularly at low (<50eV) energies, when beam

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splitting is pronounced. Subtle differences existbetween published LEED patterns, and it wasobserved (10) that beam splitting had some beam

energy dependence.

Finally, it should be mentioned that careful X-ray

measurements (30) have indicated an outward

relaxation of the interplanar spacings of surface

layers of -4 per cent for gold {111}. These measure-

ments were performed on air contaminated surfaces,

and confirmation of such an effect must await LEEDor RHEED data (and adequate theory) from sur-

faces prepared in ultra high vacuum. There is

obviously a great deal of basic research yet to be

performed on "clean" gold surfaces.

References1 E. Bauer, "Techniques of Metals Research", Vol. 2,

Ch. 16, Ed. Bunshah, (1969) Wiley2 M. Prutton, Met. Rev., 1971, 16, 573 R. J. Estrup and E. G. McRae, Surface Sci., 1971,

25, 14 E. Bauer, "Techniques of Metals Research", Vol. 2,

Ch. 15, Ed. Bunshah, (1969) Wiley5 C. C. Chang, Surface Sci., 1971, 25, 536 P. W. Palmberg, "Electron Spectroscopy", North

Holland, Ed. Shirley, (1972), p. 8357 D. G. Fedak and N. A. Gjostein, Phys. Rev. Lett.,

1966, 16, 1718 D. G. Fedak and N. A. Gjostein, Acta Met., 1967, 15,

8279 E. A. Wood, y. Appl. Phys., 1964, 35, 1306

10 D. G. Fedak and N. A. Gjostein, Surface Sci., 1967, 8, 7711 G. E. Rhead,,. Phys. F. Metal Phys., 1973, 3, L5312 D. G. Fedak, J. V. Florio, W. D. Robertson, "The

Structure and Chemistry of Solid Surfaces", Ed. G. A.Somorjai (1969), Wiley, p. 74-1

13 A. M. Mattera, R. M. Goodman and G. A. Somorjai,Surface Sci., 1967, 7, 26

14 P. W. Palmberg and T. N. Rhodin, Phys. Rev., 1967,161, 586

15 P. W. Palmberg and T. N. Rhodin, Y. Chem. Phys.,1968, 49, 134

16 E. Bauer, A. K. Green and K. M. Kunz, Appl. Phys.Lett., 1966, 8, 248

17 E. Bauer, "Structure et Proprietes des Surfaces desSolides", Paris (1969), p. 111

18 P. N. J. Dennis and P. J. Dobson, Surface Sci., 1972,43, 3919

19 F. Grenlund and P. E. Hojlund-Nielsen, Y. Appl.Phys., 1972, 43, 3919

20 J. P. Biberian and G. E. Rhead, 7. Phys. F. MetalPhys., 1973, 3, 675

21 S. Hagstrom, H. B. Lyon and G. A. Somorjai, Phys.Rev. Lett., 1965, 15, 491

22 J. T. Grant, Surface Sci., 1969, 18, 22823 W. A. Fraser, J. V. Florio, W. N. Delgass and W. D.

Robertson, Surface Sci., 1973, 36, 66124 H. H. Brongersma and P. M. Mul, Surface Sci., 1973,

35, 39325 A. Benninghoven, Surface Sci., 1973, 35, 42726 L. E. Murr, Thin Solid Films, 1969, 4, 389 and Scripta

Met., 1972, 6, 20327 P. E. Hejlund-Nielsen, Surface Sci., 1973, 36, 77828 R. W. Joyner, submitted to Surface Sci.29 A. E. Morgan and G. A. Somorjai, Surface Sci., 1968,

12, 40530 R. W. Vook, S. Ouyang and M. A. Otooni, Surface Sci.,

1972, 29, 277

Ordering in Gold-Copper AlloysA STUDY BY THERMOPOWER MEASUREMENT

It is only comparatively recently that the parametersupon which depends the absolute thermopower of ametal have been fully understood and it is for thisreason that thermopower measurement has nothitherto been extensively utilised as a metallurgicaltool. R. D. Barnard and A. J. M. Chivers of the Univer-sity of Salford now suggest, however, that in certaincircumstances the measurement of thermopower canprovide a more sensitive alternative to the ubiquitousresistivity method for studying phase transitions andordering processes in metals and alloys, and haveapplied it specifically to a study of the ordering kineticsof CuAu and Au,Cu (Metal Sci. ,7., 1973, 7, (July),147-152). These are apt choices inasmuch as CuAupossesses two ordered phases both of which haveresistivities lower than that of the disordered phase,while Au3Cu is anomalous in that it has a slightlyhigher resistivity in the ordered state. It is in this sortof situation that the resistivity method of studyingphase changes has such obvious limitation.

The total thermoelectric power of metals and alloysis usually dominated by the diffusion component, SD ,

which is a bulk property of any metal and whose valueis of prime interest in this context for reasons whichwill become apparent.

Measurements were made using a simple thermo-couple arrangement whereby the sample material

and a reference metal were subjected to a temperaturegradient. The Seebeck e.m.f., e, thus produced isrelated to the absolute thermopower by the equationde/dT = S„ (y mp , e) - SD (ref) . It is clear, therefore, thatprovided S„ (ref) is a monotonic function of tempera-ture, then changes in c can give a direct insight of thestructural changes taking place in the sample. It isobviously desirable that the sample be kept in avirtually isothermal environment, -but to generate ameasurable e.m.f. of even a few microvolts a smalltemperature gradient must be established.

With this in mind apparatus was constructed whichpermitted thermal e.m.f. measurement while enablingthe temperature of the sample to be controlled,especially with respect to means of reducing thesample temperature to a stable level below the transi-tion temperature as quickly as possible.

Samples were made up in the form of wire. WithAu,Cu a conventional drawing process was employedbut with CuAu it was necessary to cast the wire in afine quartz tube.

Results obtained by this method are very encourag-ing, particularly for Au,Cu for which evidence came tolight for the occurrence of short range ordering attemperatures considerably higher than those pre-viously reported, as well as the existence of finestructure in the ordering curve. A. D. M. K.

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