Structure and properties of ultrathin iron films on Ru(1010) : The formation of metastable surface phases of γ-Fe

Surface Science 201 (1988) 393-407 North-Holland, Amsterdam

393

STRUCTURE A N D PROPERTIES OF ULTRATHIN IRON FILMS O N Ru(10]0): THE FORMATION OF METASTABLE SURFACE PHASES OF ¥-Fe

Kevin HARRISON * Robert H. PRINCE ** and Richard M. LAMBERT ** * Department of Physical Chemistry, University of Cambridge, Cambridge CB2 IEP, UK Received 18 January 1988; accepted for publication 24 March 1988

The chemisorption and desorption of Fe at the (10]0) surface of Ru has been investigated by

LEED, Auger speetloscopy, &O and thermal desorption measurements over a substrate temperature range of 300-900 K. The growth mode of the iron deposit was found to be strongly dependent on the temperature. At 300 K up to seven iron monolayers could be grown, these adopting the configuration of the fcc (111) plane of bulk ,/-iron. The layc,~ were metastable and heating of such films or deposition at elevated temperatures resulted in nucleation and growth of crystallites. Only a single Fe desorption peak ( E d ~ 250 kJ moi - t ) was observed over the :,hole coverage regime; this is assigned to the evaporation of iron atoms in contact with the ruthenium substrate, either pre-existing in the first monolayer or suppfied from the Fe crystallites.

1. Introduction

In the context of chemical catalysis, the ruthenium/iron system is of particular interest with respect to the synthesis of ammonia from nitrogen and hydrogen. Fe-based catalysts have long been employed in chemical technology and high activity Ru catalysts have recently been developed for this process [1]; this raises the possibility of achieving further gains in performance through the application of bimetallic R u / F e c~.talysts. Indeed such materials are effective catalysts in Fischer-Tropsch synthesis, recent work in this particular area having been reviewed by Guczi [2]. On the other hand, very few model studies appear to have been carried out on the R u / F e system, although well-characterised Fe films have been investigated on Ni, Cu and Ag single [a K1 l~e.~,,,~at a!. r-n have ..o,,o.,,h, ,,,.ht;,~,,.a br:e ¢ ,-o~,,,,-¢ * Permanent address: BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TWl6 7LN, UK. ** Permanent address: Department of Physics, York University, Faculty of Science, 4700 Keele Street, North York, Ontario, Canada l~'3J 1P3. *** To whom correspondence should be addressed.

0039-6028/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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K. Harrison et al. / Properties of uitrathin iron films on Ru(1010)

on the Ru(0001)/Fe system which bears an interesting relationship to the present paper. They observed Frank-van der Merwe (FM) layer-by-layer growth of the Fe deposit, with the first layer adopting a structure which corresponds to an expanded (111) plane of fcc iron (i.e. ~,-Fe; stable as a bulk phase only above 1033 K). However, deposition of even one more layer resulted in complete relaxation to (ll0)-oriented normal bcc iron (a-Fe). In this article we report on the growth mode, structural transformations and thermal properties of ultrathin Fe films on Ru(1010). In common with Egawa et al. [7] w e find that in-registry structures are formed which are based on (111)-oriented fcc y-Fe layers. However, these can be grown to much greater thickness than on the basal plane of Ru; moreover, in this case thermal relaxation does not lead to the formation of bcc a-Fe structures but to microcrystallites of (111)-oriented fcc y-Fe. This ability of Ru(10]0) to stabilise the other~yi'se metastable ~,-Fe phase is of some interest in regard to ammonia synthesis catalysis, given the well known structure sensitivity of this reaction on Fe surfaces [8,9].

2. E x p e r i m e n t a l

The apparatus and extensive crystal cleaning procedures have been described in a previous publication [10]. Briefly, experiments were performed in a diffusion and ion pumped UHV chamber, operating at a typical base pressure of 3 x 10-10 Torr. AES and LEED data were obtained using a 4-grid RFA and integral electron gun, the latter also being utilised to provide work function measurements using the Anderson retarding potential method [11]. Thermal desorption (TD) data were taken by a quadrupole mass spectrometer positioned - 3 cm distant from the Ru(1010) surface. The Ru sample was cut, cleaned and polished to within 0.5 o of the (1010) plane by standard methods and was roughly elliptical in shape, with a front face area of 30 mm2. A Pt/Pt-10%Rh thermocouple spot-welded to its edge monitored the crystal temperature, this being variable over the range 295-1900 K by direct resistive heating. Removal of C, S and O involved [10] high temperature annealing. N e / A r ion bombardment and oxidation/reduction cycles (5 X 10 -7 Ton" O 2 / H 2 a t 1300 K). Iron overlayers were deposited onto the specimen surface using an evaporation source which is shown schematically in fig. 1. This contained a very fine, [dgh purity, iron wire (0.025 mm, 99.99 + %), wound around a 0.2 nun tungsten spiral; during operation a constant current of 3.0 A was passed through this spiral, producing a monolayer of iron on the crystal in - 125 s. The deposition rate tended to decrease slightly with prolonged use as the amount of available iron was depleted, and the heating current was increased appropriately to compensate for this. A copper plated nickel collimator

K. Harrison et aL / Properties of ultrathin iron films on Ru(lO'lO)

Ceramtek Constant current

395

Cu-coated

support block

collimator

I

supply

--~_. Iron wrapped N filament Zan gauge ~ ' ~ - 1 ' ~ power supply

I

Outgasslng filament

Fig. 1. Schematic diagram of Fe evaporation source.

surrounded the spiral assembly and could be separately outgassed by electron bombardment from a second external tungsten filament; this precaution minimised codeposition of gaseous impurities during operation of the source. After an initial period of outgassing, during which large quantities of sulphur, presumably originating from the collimator, were deposited on the specimen, a clean and reproducible quantity of iron could be evaporated onto the sample. The chamber pressure rose by no more than 2 × 10-z0 Torr during even the longest dosing periods. Auger spectroscopy control experiments revealed that the source produced an even flux over the entire crystal area (< 5% variation).

3. Results: Fe dosing at 300 K

3.1. Work function changes The work function remained unche.nged within the detection limits of + 0.1 eV over the entire deposition range of 0-900 s, indicating that negligible charge transfer occurred between substrate and ovedayer film at all coverages. Little difference between the absolute ~ values for the clean surface and thick iron deposits was expected in any case, in view of ~,he almost identical values of 4.6 eV for Ru(1010) [12] and 4.6-4.8 eV for various iron surfaces [13].

3.2. Thermal desorption measurements Fig. 2 presents a series of 56 ainu (Fe +) thermal desorption spectra following dosing at 300 K and covering the dose range 0-900 s (heating rate - 115 K/s). Only a single peak was ever observed but i: always included a small shoulder on the lower temperature side. Control experiments indicated that desorption from thc specimen supports was responsible for this latter feature. The desorption maximum of the main peak shifted downwards from

396

If. Harrison et ai. / Properties of uitrathin iron films on au(lOlO)

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l

1100

I

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1300

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~-[

I ~T=="

1500

"~101

~700

TemDerature / ~<

Fig. 2. Fe thermal desorption spectra following iron deposition on Ru(1010) at 300 K. Dose times are in seconds.

1580 to 1500 K over the dosing range 0-120 s, but then increased slowly, reaching 1560 K at 900 s does. A plot of the area under this peak against deposition time is shown in fig. 3; it is essentially linear, indicating a constant sticking probability for Fe on Ru(1010) at 300 K.

3.3. Auger spectroscopy The growth mode of the iron deposit was studied using Auger spectroscopy. The occurrence of layor-by-layer (Frank-van der Merwe) growth was tested for by using a model which has been extensively developed previously [14]. Following this procedure the parameter aR,,Fe is used to denote the fractional transmission of_ the .natheniium substrate Auger signal ~o,.,.vu,,t . . . . . a derivative peakto-peak height) by an evenly dispersed monolayer of iron atoms. The equivalent parameter relating to the transmission of the iron Auger signal through an overlying iron monolayer is denoted are.F*Fig. 4 is a plot of the Ru(231 eV) and Fe(651 eV) Auger intensities as a function of the integrated thermal desorption yields for Fe dosing at 300 K: the solid lines drawn through the data points are the results of a calculation (see below). Desorption yields were

K. Harrison et al. / Properties of ultrathin iron films on Ru(lOlO)

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398

K. Harrison et al. / Properties of ultrathin iron films on Ru(lO'lO)

used in preference t o dose times in order t o eliminate the effects of any slight changes in dosing flux between experiments, the Ru(231 eV) signals being normalised to the height of the same peak from the clean surface. A break is indicated in the ruthenium signal at a coverage of 4 units as the first monolayer is completed, corresponding to a dose time of 130 s. These data yield a value of 0.48 for aRu(231 Fe eV), the attenuation factor of the Ru substrate intensity by an iron overlayer. Plots of the Ru(150 eV) and Ru(273 eV) intensities (not shown) exhibited the same general shape and gave attenuation factors of 0.35 and 0.53 respectively. Using the formula [14] = d / ( 1 - a ) cos 0,

where ~,--inelastic mean free path (IMFP) of the Auger electron, d--iron monolayer thickness, 0 -- effective mean acceptance angle of the RFA, IMFPs of 2.1, 2.6 and 2.9 monolayers for the 150, 231 and 273 eV electrons are therefore calculated, the ratio of these values being in excellent agreement with the E ~/2 dependence of the empirical formula of $eah and Dench [15]. Furthermore, this agreement provides confirmation of the absence of ~ignificant levels vf C and S in the deposited Fe films. The correlated behaviour of all three ruthenium peaks clearly defines the completion of an initial i:on ~onolayer, and a knee in the Fe~651 eV) signal at the same coverage is also discernible, though less distinct, because of the scatter in the initially weak iron intensities. It can be seen that a second break occurs in both the Ru and Fe plots at a coverage of 8 units, exactly twice that corresponding to completion of the first layer, suggesting completion of a second iron monolayer with identical atomic packing to that of the first. There are insufficient d.~a points thereafter tc~ make furthel firm conclusions regarding the growth rnede., though the observation that the Ru signal decays to zero strongly suggests a continually thickening iron oveflayer up to the maximum dose of seven equivalent monolayers. Also plotted on fig. 4 are the Ru and Fe intensity variations calculated according to the simple power attenuation lnodel using the aR~(231 F~ eV) value of 0.48 calculated at the first break, and a Fe aF~(651 eV) value of 0.67, which is again consistent with the E ~/2 dependence of the IMFP. That is, the data have been fitted in piecewise linear fashion, each slope being an integral power of the appropriate a factor. This calculation fits the data very well, though it should be noted that the model is very simplistic, assuming equal packing densities in all layers and completely neglecting backscattering and interracial effects on the Auger yields. Nonetheless it appears that at 300 K, at least two, and possibly up to a total of seven, iron overlayers grown on Ru(1010) by the Frank-van der Merwe mechanism. Higher coverage structures were found to be metastable and fig. 5 shows the effect of heating such an overlayer (nominal loading seven monolayers) to 870 K for a few seconds. Initially, the Ru suhstrate signal was almost

K. Harrison et al. / Properties of ultrathin iron films o n Ru(lO'lO)

399

A o (5t

/ Fe {598ev)

Fe [?03eV)

Fe (851eV) Ru (198eV)

B Heat to 870K

Ru (150ev)

Ru (273eVJ

Ru (231eV)

Fig. 5. Fe and Ru Auger spectra for an iron film grown at 300 K (A) showing the effect of heating '.o 870 K (B).

undetectable, but after heating the normalised Ru(231 eV) intensity ratio was increased from 0.03 to 0.30, with a concomitant fall in the Fe(651 eV) signal height from 50 to 35 arbitrary units. No additional change occurred on annealing at 870 K for 30 rain, suggesting that equilibrium had been reached during the initial heating. By depositing sufficient further quantities of iron at room temperature it was possible to almost completely extinguish the Ru Auger signals once more and increase the iron intensities to their previous, larger, levels. Subsequent heating resulted in intensity changes identical to those outlined above, and repeated deposition/heating cycles induced equivalent excursions in the Auger s~gnals. Temperature effects on the growth mode were further investigated by performing sequential uptake experiments in which the iron overlayers were built up by cumulative addition of short doses (at 300 K) to the previous accumulation - i.e. without flashing off the iron in between. The crystal was

400

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Iron deposition time (s) Fig. 6. Fe uptake and growth morphology monitored by Auger spectroscopy. Showing adsorbate and substrate signals after deposition at 300 K and heating 870 K after each dose. Dotted line shows expected behaviour for FM growth.

heated to 870 K after each dose so that the growth morphology of the annealed film was monitored, and the resulting uptake curves are shown in fig. 6. A break is clearly visible in both the Fe(651 eV) and Ru(231 eV) plots at around 120 s dose, this being the same time at which the first knee appears in fig. 4; the aRF~ value of 0.52 is slightly larger here, however, and this small effect may reflect reduced levels of adventitiously adsorbed CO due to the thermal treatment employed in this instance. The complete absence of detectable oxygen and carbon Auger signals in the corresponding spectra is in keeping with this (oxygen < 0.02 ML). A second linear segment persisting up to a dose of 360 s is also observed, the normalized Ru(231 eV) intensity having fallen to a value of 0.16 at this point. This dose time corresponds to a loading of three monolayers, though there is no obvious second break at the expected second monolayer completion dose of 240 s. It should also be noted that the decay of the Ru(231 eV) intensity is less rapid than predicted by the simple FM growth model, the latter being shown as a dotted line which was calculated with the aR,Fe value of 0.52 deduced above. At deposition times of 360 s and greater the Ru intensity actually increases and that of the iron falls, even though more iron is being deposited. This is definitely a real effect since it was apparent for all four Ru peaks (at 150, 198, 231 and 273 eV) and all three iron peaks (at 598, 651 and 703 eV). 3.4. LEED

In contrast to the wealth of patterns often seen to result from metal deposition on the similarly furrowed bcc (211) [16-18] and fcc (110) [19l

K. Harrison et al. / Properties of ultrathin iron films on Ru( lO'fO)

401

surfaces, no new diffraction spots were observed at any coverage in the present case. Up to a monolayer dose (120 s) the substrate features became progressively less sharp and the background intensity increased; flashing to 870 K resulted in general im~rovcments in sharpness and contrast, indicating an increase in ordering of the iron deposit. This behaviour suggests an initially random distribution of Fe followed by (tkermally assisted) pseudomorphic growth. At coverages greater than a monolayer the background intensity of the diffraction pattern continued to rise, but some of the substrate spots remained visible, persisting until the maximum dose of seven equivalent monolayers, while the others gradually faded into the background. In fact the resulting pattern could be indexed in terms of a c(1 × 1) structure (relative to the substrate mesh vectors) - every other substrate spot being completely extinguished at all electron energies. Heating to 870 K always reduced the background intensity and restored the missing spots; this effect was most pronounced at the highest coverages, i.e. in the same regime where very pronounced thermally induced changes in Auger intensities were observed.

4. Discussion

4.1. As dosed, unheated iron films Inspection of the Auger-derived uptake curves suggests the growth of at least two, and perhaps up to a total of seven, iron monolayers by the FM, layer-by-layer growth mechanism. LEED data indicate either a pseudomorphic or disordered first layer, while on further deposition a c(1 × 1) structure evolved. It is commonly observed [18,20,21] that epitaxially grown metal films tend to adopt a configuration closely resembling a high index plane of the bulk material, especially in the outermost layers of thick films, which are relatively unaffected by the underlying substrate potential. However the initially deposited layers, especially the first, feel the substrate potential modulations much more strongly and may grog' in a distorted manner in comparison to the corresponding bulk material. Atomically rough surfaces such as Ru(1010), tend to exert more influence on the growth morphology than atomically flat planes, and Bauer lists several examples in his review on this subject [22]. Strain forces may thus be set up in the growing layer, resulting in dislocations and perhaps a complete structural reorganisation - - ' " - - ~ amount of deposit has been '---"" OUSlt u p . x,,, v t e ._1,-_ t a t u t ; iroi-t z: .t_~_ci~t e x i s t s when a cuu~at in three allotropic forms as given in table 1. A simple and appealing explanation of all the LEED observations is suggested by an analysis of the LEED pattern which corresponds to a c(1 × 1) periodicity an which is observed following the largest doses ¢f up to seven

402

K. Harrison et ai. / Properties of ~trathin iron films on Ru(lO-fO)

Table 1 Phase

Transition temperature (K)

Structure

Bond length (~,)

a

1043 1~1 1803 1808

bcc b~ f~ b~

2.48 2.48 2.58 2.54

y 8

monolayers. The Ru Auger features are completely extinguished by this stage, and it is therefore reasonable to assume that the pattern is due to the iron overlayer alone. Fig. 7 illustrates the simplest way of generating the required c(1 x 1) periodicity in terms of a single Fe monolayer on the Ru(1010) substrate; this Fe monolayer corresponds to a slightly distorted (111) plane of fcc iron. It is of course unlikely that such a "flat" monolayer would be formed in direct contact with the Ru substrate: some rumpling would be expected. However, it is reasonable to suppose that this "flat" c(1 × 1) structure could form for thick overlayers - in agreement with experiment. F M growth therefore involves a transition from a pseudomorphic layer ((1 x 1) LEED) to an epitaxial structure (c(1 x 1) LEED) which corresponds to a somewhat distorted (111) layer of fcc iron. The structure of the Fe is thus seen to evolve in a manner which accords with expectation and compares interestingly with the recent results of Egawa et al. [7] for Fe on the basal plane of Ru. These authors report the occurrence of FM growth in which formation of an initially distorted (111) first layer of fcc iron is immediately followed by relaxation to (110) layers of bcc iron. This difference in properties between Ru(1010) and Ru(0001) may be of significance from the point of view of R u / F e bimetallic catalysts in that the two Ru surfaces appear to stabilise Fe overgrowths which [oool] Fe on Ru (lOgO]

~

57"42' .

Bulk

rd

fcc

Fe(lil}

. Unit

cells

Fig. 7. Proposed structure for c(1 × 1) phase exhibited by thick Fe films on Ru(1010) showing comparison with (111) plane of bulk fcc Fe. (a 2 is the [0001] direction).

K. Harrison et al. / Properties of ultrathin iron films on Ru(lOlO)

403

are structurally very different. This is of some interest, given the marked structure sensitivity of N H 3 synthesis over iron surfaces [9]. Fig. 7 shows a comparison of the idealised e(1 x 1) unit mesh with that of (111) fee Fe. Growth of undistorted (111) fee Fe on Ru(1010) would produce LEED pattern very similar to that due to the in-registry e(1 x 1) phase, as inspection of the corresponding reciprocal lattice vectors shows. e(1 x 1) in-registry structure:

b~' --a~' -a~',

b~' -- 2a~;

fully relaxed (111) fee Fe: b~' -- 1.05a~' - 0.96a~,

b~' -- 1.91a~'.

The quality of the thick film LEED data does not permit a decision between these two possibilities. For thinner films the occurrence or non-occurrence of broadening for certain LEED reflections (due to multiple scattering between Ru and Fe meshes) would enable a distinction to be made. Once again, however, the resolution of the relevant diffraction patterns is inadequate for this purpose. It is therefore not possible to decide whether the (111)-oriented fee Fe overgrowth consists of fully relaxed or slightly distorted y-Fe. Desorption data give further information on the nature of the iron deposit, although the possible effects of thermally induced changes which occur during the temperature sweep must be borne in mind. LEED and Auger data indicate that heating of coverages below one monolayer has relatively little effect on the adsorbed iron, except for some increase in the structural order within the layer and possibly the removal of adventitiously adsorbed CO. At higher coverages, however, the Auger results clearly indicate that rearrangements can occur even during a single excursion to 870 K, these being most marked at the highest Fe loadings ( - 7 ML). In the submonolayer regime the Fe desorption peak temperature shifts with increasing coverage from 1580 to 1500 K; treating these results in terms of first-order desorption according to Redhead's formula with a pre-exponential factor of 1013 s-1 yields a desorption energy of - 378 k J/tool at near zero coverage falling to - 362 kJ mol-1 at monolayer coverage. A more reliable estimate is however provided by carrying out an Arrhenius plot over an appropriate temperature interval using data obtained with low initial Fe loadings (specimen support effects minimised). Thus for an initial coverage of - 0.5 monolayers plotting the data according to a ~irst-order process over the range 1350-1500 K yields a value of - 250 kJ mol-1. This point will be returned to below.

4.2. Effects of heating to 870 K A number of possible effects may be invoked to account for the very pronounced Auger intensity changes which occur on heating: desorption of

404

K. Harrison et at / Properties of ultrathin iron fil'ms on Ru(lO'[O)

Fe, dissolution of Fe, surface alloy formation or agglomeration into 3D crystailites. High sensitivity desorption measurements failed to produce any evidence for evaporation of 56 (Fe+), 112 (Feq+), 168 amu (Fe~') or other iron containing species over the temperature range in question (300-900 K). In addition, 56 amu desorption yields could always be correlated with the initially measured Auger intensities and not those obtained after heating: i.e. for a given Ru(231 eV) Auger intensity measured after heating, the Fe thermal desorption yield was much greater than expected on the basis of fig. 4 - the discrepancy increasing as more deposition/heating cycles were performed. Thus iron was not lost from the crystal on heating, but a large fraction originally detectable by Auger spectroscopy was removed from the sampled volume whilst remaining visible in TDS. This could have resulted from Fe dissolution into the Ru substrate, partial agglomeration of the initial overlayers into 3D crystallites, or formation of a surface alloy. Using fig. 4 as a guide to coverage, the effects depicted in fig. 5 correspond to the loss of around 5 equivalent monolayers of iron. It seems rather improbable that such an amount of iron could be transported far enough into the bulk to evade detection by AES within the few seconds taken to heat to 870 K. Two possibilities thus remain: a surface alloying process and crystallite formation. Careful consideration of the available data suggests that the growth of iron crystallites is the more plausible explanation. Comparison of figs. 4 and 6 shows that the growth mode of the firs~ complete monolayer is tittle changed by flashing to 870 K. Above this coverage differences begin to appear: Auger signals from the heated surfaces exhibit no clear second layer break and the Ru intensities decay less rapidly than expected for growth of a second layer in FM fashion. This suggests partial aggregation of Fe into a 3D crystaUite phase while the second layer is growing. The unusual intensity changes for doses greater than 360 s are probably related to an accelerated growth of nuclei at the expense of Fe previously distribution in a more even manner.

A crystallite growth mechanism also accounts for the results shown in fig. 5. Heating the initially uniformly distributed layers resulted in rapid coalescence to 3D crystallites: further iron was deposited as uniform layers, but on supplying the necessary thermal energy this too agglomerated. The Auger intensities would remain constant if the crystallites formed were relatively large normal to the surface, thereby only covering a small fraction of the available area. The reappearance of the "missing" substrate LEED spots is ~, 1_ accounted for in t1",h.,~ same manner: the s-~r, t.o_ (1 × 1ty pattern ot,1,.....~" v,OUSly reflects the periodicity of the substrate and is also consistent with the presence of ,/-Fe [fcc(lll) I!Ru(1010)] crystallites as discussed above. Rationalising the LEED observations within an alloying model requires considerable conjecture and therefore seems less plausible. The alloy w6uld have to exist either as several monolayers above the Ru(1010) surface aed with ~' same unit cell, or as

K. Harrison et al. / Properties of ultrathin iron films on Ru( lOlO)

405

agglomerated crystallites, again with the Ru(1010) unit cell, or with a structure close to the ~-Fe fcc (111) layers proposed for the pure iron overlayers. The thermal desorption results lend strong support to the view that heating leads to the nucleation and growth of crystallites and that these crystalfites consist of Fe rather than an alloy phase. They are somewhat unusual in that only ~ ~ingie major feature is observed at all coverages but associated kinetic behaviour changes in passing from submonolayer to multilayer loadings. In the former regime (see above) elementary analysis indicates that chemisorbed Fe in the first layer evaporates from Ru(1010) with an activation energy of -- 250 kJ mol-1; downward shift of peak temperature with increasing coverage suggests that this quantity may be somewhat dependent on Fe coverage. In the multilayer regime the peak shifts to higher temperature as coverage increases, indicating a kinetic order of less than unit. Such behaviour can be rationalised in terms of evaporation from a dispersed phase (chemisorbed adatoms) in dynamic equifibrium with islands of a condensed phase (2D or 3D crystallites) [23]. Detailed analysis of the data is not warranted in the present case because the spectra are somewhat perturbed by desorpt:on from the specimen supports. Nevertheless, an Arrhenius plot of the multilayer results over the interval 1350-1500 K yields an activation energy of ~ 250 kJ tool-1. essentially identical with that derived for the submonolayer regime, despite the qaite different desorption kinetics. Given that the sublimation energy of Fe from bulk iron is 415 kJ mo1-1 [13] these observations are consistent with evaporation of Fe from Ru being the rate limiting step at all Fe loadings. When multilayers are heated they coalesce to form crystallites from which evaporation occurs via transport of Fe onto the Ru surface foUowed by desorption. Such a model is in good accord with all the observations by LEED and Auger spectroscopy. In order to confirm the essential correctness of this overall picture, some Auger uptake data were also obtained with the crystal continually maintained at a temperature of 860 K, thereby enabling the iron overlayer to form in its equilibrium configuration. The sticking probability was approximately 20% smaller than at room temperature, but the shape of the plot was almost identical to that of fig. 6, as would be expected. Once again the minimum normalised Ru(23~ eV) intensity attainable was 0.20, after which a slight increase in this parameter began to occur. A negligible quantity of oxygen species (< 0.02 ML) were coadsorbed during these experiments, confirming the rearrangement process to be an inherent property of the iron film itself. Fe "'-"~ t . . . .~ ., O . l J .~A ,~. ¢~,~ snbstrate temperature U , ~ . J L L ~ J.~ , ~ . w.e r t.~ , QL,3Id .l~,~.i ~,,... lU~t ~.LI. L L Z ~ , ;..o,~..,,,,a~t,~ tZaL~.ZZJLa~,~a~t~" of 550 K, and a typical example is presented ,as fig. 8 (LEED and TDS results were essentially identical to those obtained at 300 K). As observed at room temperature, there is clear evidence for formation of two complete monolayers (anur:e= 0.50). In this case, however, further iron deposition then results in the slow formation of crystallites. Thus the be-

406

K. Harrison et al. / Properties of ultrathin iron films on Ru(lO-lO) i. Oi

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100

200

300

400

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500 time

600

700

800

0

(s)

Fig. 8. Fe uptake on Ru(1010) at 550 K monitored by Auger spectroscopy; dependence of Ru(231 eV) and Fe(651 eV) signals on dose time.

haviour at 550 K is intermediate between that occurring at 300 K (up to seven monolayers) and 860 K (one monolayer followed by slow crystallite growth). At 300 K the kinetics of the metastable layers-to-crystallite transition is sufficiently slow as to give no time-dependent effects. At 550 K, however, more energy is available to the iron atoms to overcome any diffusion barriers, and the results could be slightly altered by using differing deposition rates. By using a high dose rate it was possible to almost completely attenuate the ruthenium Auger transitions with a single extended dose; large intensity shifts would then oc~cur on heating to 860 K, comparable to those shown in fig. 5. Even at 550 K, slow time-dependent changes in signal amplitudes were observed after completion of Fe dosing on standing. The behav~our observed in the present case somewhat resembles that reported for the W / F e system [24] which also exhibits FM growth with significant changes in adsorbate mobility over the range 300-500 K. However it seems possible that with this bee substrate, solid solution formation may have occurred in certain circumstances. 5. Conclusions

,-, . . . . . . . . . . . . . . . ~. . . . j ,~ .,,,,, ~ ,eau~ to the growth of m~Lastaoie ......... continuous layers of ( l l l ) - o r i e n t e d fee iron by the Frank-van der Merwe mechanism. (2) Upon heating the system, the interface undergoes a major reorganisation involving the nucleation and growth of three-dimensional crystallites of iron; alloy formation does not appear to be significant.

K. Harrison et al. / Properties of uitrathin iron films on Ru( l O'fO)

407

(3) Thermal evaporation of iron occurs as atomic Fe and the process is characterised by an activation energy of - 250 kJ mol-t over a wide range of metal loadings - despite changes in kinetic behaviour. Iron crystallite evaporation is mediated by the Ru surface, the rate limiting step being desorption of chemisorbed Fe from the substrate. (4) Adsorption at elevated temperatt,-es produces essentially identical behaviour to that which is observed to result from heating the same Fe loading deposited at 300 K.

Acknowledgements K.H. thanks the Science and Engineering Research Council and ICI plc for the provision of a CASE Studentship. We are grateful to Johnson Matthey Ltd for a loan of precious metals.

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