Composition profile at PtxNi100−x alloy surfaces by ideas (incidence dependent excitation for Auger spectroscopy)

223

Surface Science 233 (1990) 223-238 North-Holland

COMPOSITION PROFILE (INCIDENCE DEPENDENT D. DUFAYARD,

R. BAUDOING

Laboraroire de Spectromktrie

Received

17 November

AT Pt,Ni,,_ x ALLOY SURFACES BY IDEAS EXCITATION FOR AUGER SPE~ROSCOPY) and Y. GAUTHIER

Physique, associk au CNRS,

1989; accepted

for publication

Universith Joseph Fs~urier, BP X7, 38402 Saint Martin &H&es

23 February

Gdex,

France

1990

The IDEAS method is applied to measure the composition profile of very different Pt-Ni monocrystalline surfaces: Pt,,Ni,, (110) and (100). and Pt,,Ni, (110). The basic ingredients of the method are carefully discussed and an optimized procedure is presented. The results are in very good agreement with previous results obtained mainly from LEED but also from ISS and RBS under channeling and blocking conditions. The accuracy is typically of 5 to 10 at% Pt and 10 to 20 at% Pt on the determination of C, and C, respectively: this is well suited to segregation studies for comparison with model predictions or in conjunction with catalysis work. Being a spectroscopic instead of a scattering method, it is possible to apply it to a wide range of situations: less ordered surfaces, high temperature, compounds made of chemical elements with close atomic numbers. Although no geometrical information is accessible, IDEAS seems a valuable complementary tool to scattering methods in a complete surface crystallographic study.

1. Introduction Auger spectroscopy, particularly in the incidence dependent excitation version (IDEAS), is one of the very few efficient tools to study the imposition profile two layers deep at complex surfaces like bimetallic alloys. This is of crucial interest in two different scientific contexts: (i) In itself, the knowledge of the composition profile is of considerable help for the segregation theories, and very few systems are actually quantitatively known yet. (ii) Catalytic properties of alloy surfaces may strongly depend on the actual composition profile of the surface, not only of the top layer but also of the few underlying layers via a modification of the electronic properties. The IDEAS method is based on several ingredients, discussed in a previous paper [l], only briefly recalled here: (a) Auger spectra are measured in the entire range of incidence, from normal to grazing; this is the key to weigh the topmost layers differently and hence to reach the composition profile. 0039-6028/~/$03.50

6~ 1990 - Elsevier Science

Publishers

(b) The solid angle of detection must be as large as possible, not only for a good sensitivity. but also to average out diffraction effects on Auger electrons emitted from monocrystalIine samples. fc) The Auger lines of the elements constituting the compound should be close in energy, to avoid the use of poorly known energy dependencies of the parameters entering the model of Auger emission. The advantage of the Auger spectroscopy, over LEED for instance, is the possibility to deal with less ordered or poorly ordered surfaces; this will be illustrated with a bombarded surface. Another possibility is to work with heated surfaces to a point where they emit too much light for LEED to be used, if Debye-Waller effects did not weaken the LEED pattern yet! The thermodynamical parameters should thus be within reach, nicely complementing the ISS [2] or RBS [3,4] techniques, used for this purpose up to now. Finally, contrary to techniques based on diffusion cross sections (LEED or RBS), spectroscopic techniques like Auger emission can deal with compounds of elements with close atomic numbers: Ni-Cu, Fe-

B.V. (North-~~ol~and)

Co or Pt-Ir alloys for instance; we show in this paper that even if the Auger lines strongly overlap one can extract the contributions of each element and thus the composition profile. In ref. [I] we have introduced the IDEAS method and illustrated it on one case, namely the Pt50Ni50(111) surface. Here we apply it to very different composition profiles encountered in the Pt-Ni system, for which a large amount of results has been produced by LEED [5-91 and to a smaller extend by ISS [2]: we show that in all cases IDEAS yields good results and we discuss the various aspects of the method, including the problem of the reference spectra corresponding to the pure metals. In addition we have also put some emphasis on the (110) face of Pt ,Ni, ~, since it presents interesting problems [7,8]: (1) The composition profile is reversed with respect to the (111) face, i.e. there is a Ni enriched top layer above a Pt enriched second layer. (2) LEED has recently shown the existence of a metastability [8]: it is mainly the second layer composition which changes when the bulk platinum content is increased from 10% to 50%; can Auger spectroscopy confirm this behaviour and contribute to this study? This is a hard test for the sensitivity of the technique. (3) The pure Pt(ll0) reference surface is reconstructed in a missing row model; will the composition profile given by our analysis depend too much on the fact that atoms are missing in the top layer of the reference? Finally we have also looked at the Pt 50Ni 50(100) surface to cover the full face orientation dependency of the composition profile for the 50% bulk alloy. As a whole this represents the investigation of four different surfaces, after a thorough discussion of the method. The paper is organized as follows: section 2 recalls the basic ingredients of the method; section 3 describes the experiment; we discuss in section 4 the various aspects of the quantitative analysis with some emphasis on the choice of the references and summarize the results obtained for the Pt-Ni system; we then illustrate the possibility to deal with poorly ordered surfaces by analyzing a surface after bombardment with argon ions and

finally we comment on the performances of the IDEAS method in comparison with other methods and conclude on the results obtained on the various faces and bulk concentrations on the Pt-Ni alloys.

2. Extraction

of the composition

profile

We only recall here the major aspects of the method which have been discussed in ref. [l]. Each spectrum measured from the alloy is synthesized as a linear combination of reference spectra according to: I;,,, = al,,, + PIN,.

(1)

where 1y and ,8 are the relative contributions of the alloy constituents. A possible limitation of this description is the occurrence of a strong “alloy effect”; the small differences between the experimental and the synthesized spectra in our analysis are good indications that Pt-Ni exhibits little “alloy effect”. We then calculate the ratio of these contributions. for each polar angle 8, from normal to grazing incidence: Rcxp( 6, ) = N/P,

(2)

this gives us an “experimental” curve. To extract the composition profile we model the corresponding theoretical ratio R th( 8, ), using the following hypotheses [l]: (1) The crystal is treated layer by layer [lo], the composition (at% Pt) of which are C,, C,, C, for the first, second and bulk layers respectively: (2) The secondary electron source, contributing via inelastic cascade processes to the Auger signal, as well as the Auger electron source, are assumed to be isotropic. This is fairly reasonable: although high energy collisions exhibit a so-called “focusing is much effect”. meaning that forward scattering more probable, cascades of such effects are very efficient to “defocus” the secondary electrons and thus to yield a much more isotropic angular distribution, as used in the model presented in ref. [l]; (3) The incident electrons contribute to ionize the metal atoms at depth z via two processes: a direct one due to the damped incident beam and

D. Dujayard er al. / Composition profile ar Pt, Ni,,

_

II ulky surjaces b-vIDEAS

22s

an indirect one via cascades of secondary electrons of energy intermediate between the primary beam energy and an ionization cut-off energy; this is accounted fos by an effectiue meun ionisation pa& Li to describe the z distribution of the ionized utorns

in the alioy;

(4) The damping of Auger electrons, by inelastic collisions along their escape path, is accounted for by a mean free path X, given in the jelhum approximation. A generaiisation of Tokutaka’s work [12], given in ref. 111, has shown that we can still use the following expression: ‘I’ =”

N(Ni)

1 -p(Pt)

N(Pt)

1 -p(Ni)

x [C, + C,p(all) X [(lo0 t-(100-

+ C,p2(all)/(l

-p(all))]

- C,) + (100 - C,)p(all) C,)p2(all)/(l

-~(all))]-‘,

(3)

where: r0 is the ratio of the backscattering coefficients of the two constituents, (1 + rNi)/(l + rP,f, which is equal to about 0.8 in our case and for the Auger lines of interest; N(Ni) and N(Pt) are the number of atoms per unit area in the references; here their ratio is about 1.24; p(X) = exp[-d(X) (l/I, + I/& cos S,)] is a damping term and d(X) is the interlayer distance in the references (X = Pt, Ni) and in the alloy [14] (X = all) respectively; I, is the average escape depth for the Auger electrons, defined as f, = kX,, k being a geometrical factor related to the detector (it is equal to 0.82 for a standard RFA Auger analyser [ll] but to 0.7 in our goniometer where the solid angle of collection is 2~ steradians); d may be relaxed for surface layers (d,,

f: d,“,k). The ratio (1 -p(Pt))/(l - p(Ni)) depends on the face and on I_,, X, matter of fact it varies very little parameters in our case and is about The sensitivity of the method relies term:

in principle and Bi; as a with these equal to 1.1. on the last

ICI + C, p tall) + C,p2(all)/(l -P(all))] X ((100 - C,) + (100 - C,)p(all) +(lOO-

C~)p2~all)/~~

-p(all))]-‘,

(4)

0

Fig. 1. Variation, as a function of the polar angle of incidence 8,, of the ratio R(B,) between the Pt and Ni contributions to the alloy spectra. The four PtNi surfaces exhibit a very different behaviour.

which, on the contrary, varies very quickly with C, and C,. We illustrate this point on fig. 1, where the ratio R ~rxporth~(8i) of the respective contributions of Pt and Ni is compared in the case of four different PtNi surfaces (the first one is Pt,oNi,,, (110) while the three others are the (110), (111) and (100) faces respectively of the same Pt soNi5o bulk alloy). We observe very different shapes in the range 60”-85O (curvature at large angle and/or average level at small incidence) for surfaces with very different composition profiles, as seen later on in section 4.4. To measure the level of agreement between experimental and theoretical R( 8) functions, we

226

D. DuJuyurd et al. / Composition profile at Pt, Ni ,oo

use the “relative D =

c [ R,,(~,) 1

deviation”

D defined

as:

- R,,I,(U]‘j l/2 (5 1

.

3. Experiment 3.1. Apparatus In the present paper we report on work performed with our LEED Auger goniometer [13], the geometry of which is optimized for this purpose: whatever the incidence direction, the optics always collect the emitted electrons over the full half space above the sample, which remains fixed (fig. 2). The moving gun allows to span the polar angle of incidence from normal (0”) to real grazing (90 o ) while the azimuth can be varied from 0 o to 360”. With such a geometry we are free from the subtle (but not negligible!) changes on the solid angle of collection occurring in the standard RFA experiments: the limited angle of collection certainly contributed to the larger dispersion seen in the (111) experimental curve (from ref. [l]) compared to the other curves (present work). 3.2. Surface preparation All the surfaces studied in this paper, Pt(lll), Ni(ll0) for the references and

, alloy surfuces by IDEAS

(110) Pt,,Ni,,(llO) and PtS,Ni,,(lOO) for the alloys, were prepared with the same procedure: spark erosion from monocrystal rods, X-ray orientation by means of Bragg reflection, and mechanical polishing down to a 1 pm diamond grain size. The comparison between X-ray and laser reflections gives a difference less than or equal to 0.2“ between the true atomic surface layers and the polished surfaces. The defects due to mechanical polishing and the impurities segregation are cured by many sequences of ion bombardment (10 PA of 350 eV argon ions during 15 min), and annealing under ultra high vacuum, the final annealing temperature being in the range of 800” C to 1000°C depending on the sample [l&9]. The alloy samples are monocrystalline but exhibit a substitutional disorder in bulk; care was taken to check the absence of an ordered phase. In all cases we obtained a clean and well organized surface: (a) For Pt,,Ni,,(llO), we observe a weak structured background in the LEED pattern, which is most probably related to partial chemical ordering in the surface region [8]. (b) For Pt(llO), we obtain the standard (1 X 2) reconstruction and LEED I(E) spectra in very good agreement with the published data [15,16]. (c) Finally, for PtsONisO(lOO), the surface exhibits a (1 X 19) commensurate reconstruction [9] which is under LEED quantitative analysis. 3.3. Data collection

Pt(llO), Pt,,Ni,,

Fig. 2. The LEED Auger goniometer: schematic view of the experimental set-up. The movable gun allows normal to grazing incidence, while the azimuth can be varied from 0 to 360 o with the sample holder. The hemispherical grid system integrates the signal over the 277 solid angle of collection.

All the Auger spectra, collected in the second derivative mode, were taken: (1) with an incident beam current (18 PA, E, = 800 eV) optimized for a maximum Auger yield for the transitions under study and for a mean free path as low as possible to enhance the surface sensitivity, (2) in the energy range 32-120 eV, with a modulation of 0.5 V rms, low enough to separate very close Pt and Ni contributions. This range is very convenient for several reasons: (i) it corresponds roughly to the minimum escape mean free path, about 5 _t 2 A, and hence to the best surface sensitivity; (ii) we use two intense lines (OYV at 43 eV and N,,VV at 66 eV) for platinum and one

D. Dufayard et al. / Composilion profile at Pt, Ni,,, _ x alloy surfaces by IDEAS

227

(llO), and will be reported below. For the other surfaces, we applied the conclusions drawn from this complete investigation. 4. I. Synthesis

50

E (eVi

Fig. 3. Auger spectra (d N( E)/d E) for the PtNi alloy the pure metal reference surfaces, taken after annealing angle of incidence (t? = 700), in arbitrary but coherent For the Pt,,Ni,,(llO) surface, we show the effect bombardment (- - - - - -), and of annealing (-

100

and for at large units. of ion ).

intense line (M,,W at 61 eV) for nickel; (iii) no significant energy dependence is expected for the main parameters entering the Auger model. Typical spectra are shown in fig. 3, for the references, for three annealed alloy samples and for a bombarded alloy sample; accumulation was performed to improve the signal to noise ratio, and the spectra were then smoothed out over 5 neighbouring points, with the respective weights 1, 2.5, 5, 2.5, 1.

4. Quantitative

analysis

In the course of our analysis, we have investigated various aspects concerning: the calculation of Rexp(Bi), by synthesis of the alloy spectra from the reference spectra; the calculation of Rth( e,), and the influence of L, and X,; the procedure to fit both curves and to extract the concentrations C, and C,. This was done mainly with Pt soNi

of

the alby spectra

The synthesis of the alloy spectra is performed as a linear combination of the reference spectra, from which we extract the ratio R,,,(f?,) = ai,/& (section 2). It can be made directly with the smoothed spectra or after “background substraction”. This second possibility was dropped down after a thorough investigation, in the case of Pt-Ni alloys, and in the following we only report on the treatment of “as taken” spectra. Each spectrum depends on the face orientation, on the alloy bulk composition and on the angles of incidence ei and (pi. The question is whether the reference spectra must also be taken in the same conditions: this concerns the angles of incidence, on one hand, and the choice of the reference samples, some of which being reconstructed, on the other hand; this will be extensively discussed in section 4.1.1. In addition, we may have to allow for a small energy shift of the reference spectra, either to account for the slight work function difference between the reference samples, or to account for a small alloy effect (see below, section 4.1.4). 4.1.1. Choice of the reference spectra The first question raised above is quite general: we could think it preferable, in principle, to use as many reference spectra as there are incidence conditions for the alloy. In addition to the fact that this would seriously limit the app~~ability of the method, we immediately meet the problem of “diffraction effects” related to the incoming primary beam. For single crystals, many measurable quantities (crystal-to-ground current, losses of various kinds, secondary and Auger emission for example) will vary in similar ways, essentially correlated to the mutations visible in a Kikuchi diagram. For secondary or Auger emission, there is a smoothed dependency upon the angle of incidence to which “diffraction effect” features are superimposed as shown for Al(100) by AlliC et al. 1171 and illustrated in fig. 4 for Ni(ll0).

228

D. Dufaycrrd et ul. / Composition profile at PI ~Ni,,,

I

.5

_ _~alloy surfaces by IDEAS

angular

dependent

references

1

50

0

Bi (“)

3

Fig. 5. Pt,,Ni,,(llO). Experimental ratto R,,,(B,) obtained with angular dependent references taken from Ni and Pt surfaces. Ni: (110) face, azimuth along the [OOl] direction; Pt: (a) (110) face, azimuth along the [OOl] direction, ( -); (b) (110) face, azimuth along the [liO] direction, (- - -); (c) (111) face, azimuth along a symmetry direction, (- - - - -).

C 5’0 Fig. 4. Ni(ll0). Correlation of the variation the crystal to ground current ( -) and Auger peak to peak amplitude (X), as incidence angle (both curves are plotted origin).

8i (“)

!30

(arbitrary units) of of the Ni (64 eV) a function of the with the same y

Since the unit meshes of platinum and nickel differ by about lo%, the diffraction effects will not be exactly the same. Consequently the weights of the constituents (Y~and pi of the synthesized alloy spectra exhibit angular variations which do not cancel by taking the ratio Rexp = q//Ii, as can be seen in fig. 5. On the opposite, using reference spectra at a fixed angle of incidence 8, = 0” and 70 o respectively, one sees in fig. 6 that the diffruction peaks have practically been washed out by using the ratio R,,,,. (a) The same shape is found for Rexp ( ei) with fixed or angular dependent references (figs. 5 and 6): the composition profile (C,, C,) is practically unchanged, but the level of agreement is two to three times better for fixed references. (b) Changing from 0” to 70” the angle at which fixed references are measured amounts to

only a slight change in the optimum profile: (C,, C,) = (20%, 100%) and respectively.

a

0

l._

.5..

composition (25%, 100%)

f i xed

references

50

hi I’)

Fig. 6. Pt,,Ni,,(llO). Experimental ratio R,,,(B,) obtained with references taken at specific fixed angles. (a) Brcr = O” for Pt(ll0) and Ni(llO), (); (b) Brer = 70° along the [OOl] direction, for Pt(ll0) and Ni(llO), (- - -); (c) f&r = O” for Pt(lll)andNi(llO), (------).

(c) In the same way, results obtained with references taken on Pt(ll0) with two azimuthal angles 90” apart (figs. 5a and 5h) are very similar. The second question is more specific to the (110) face under study, although a similar problem may be more general: can we valuably synthesize the spectrum of a non-reconstructed alloy face using the spectrum of the Pt(l10) face which is reconstructed (50 at% Pt are missing in the top layer!) and the spectrum of Ni(ll0) which is not reconstructed? The same question would be raised with different bulk structures for the references and the alloy; this would be the case of Pt-Fe or even of Pt-Co since Fe and Co have bee and hcp lattices respectively. Using the Pt(ll1) sample instead of the reconstructed Pt(ll0) sample to collect reference spectra at normal incidence, results in quite a small change: (C,, C,) = (25%, 100%) instead of (20%, 100%) (see figs. 6c and 6a). It can thus he concluded that fixed incidence nngle refererl~es can be used safely and the attune angles at whjch references are measured have littie influence and wiN produce ueq similar concentration profiles. 41.2. Angular range for the collecfiof? of the aBoy spectra As shown in ref. [l], it is important to collect the alloy spectra down to an incidence as grazing as possible, for a good sensitivity (the larger the angle the better the sensifivity to surface layers) and for a good accuracy on the composition profile. The question is to check the validity of the spectra against possible artefacts. For instance, if the sample is not accurately positioned at the center of the LEED Auger optics, beyond a cut-off value the incident beam could hit the sample on its side and induce irrelevant contributions to the Auger emission (the secondary electron emission from a flat surface increases with 8,; when the incident beam starts to miss the crystal surface, the crystal-to-ground current decreases abruptly). The cut-off angle is about 80°, and in the best cases, we can go to 85 or 86 O. Similarly, the weights cy, and pi in the synthesized spectra should not exhibit an abrupt decrease at large incidence, even i’f the ratio CX/& may decrease

0

i

50

i

go

Eli (“) Fig. 7. Pt,,,Ni50(110). Angular dependence of the Pt (- -I - - -) and Ni (.a*) contributions ( LYand j3 respectively) to the alloy Auger spectra, of the ratio Relp = a/@ ( -) and of the crystal to ground current (- - -) (St,, = O” for Pt(llO) and Ni(ll0)).

because of the particular composition profile under study: for instance, in fig. 7, the point at Bi = 85 * should be rejected, because (Y and /3 decrease. Finally, if one uses two fixed guns, one with the LEED-Auger optics and another one at grazing incidence like in ref. [I], the angle between both guns must be accurately known. Such problems can be detected (or calibrated) by looking at the crystal-to-ground current. Another question is the azimuthal angle of incidence. We have collected alloy spectra at a fixed polar angle e = 70 O, and at a variable azimuth, each lo’, from the [00 l] to the [l T 0] directions in the surface. The ratio Rcxp depends very little on the azimuth, as seen in fig, 8: in this case, R ex!, = 1.10 + 0.04 (this dispersion is smaller than the uncertainty on each value I&,(8,)). As a final check, we have also analyzed complete sets of data taken in two different symmetrical azimuths, 90” apart on the Pt,,Ni,,(llO) sample, 45” apart on the Pt 50Ni 50(100) sample; the composition profile (C,, C,) are essentially identical.

transition metal-platinum alloys Pt-Fe or Pt-Co for instance.

I

of ___*___*---..-__ -*---*_. . . .._.*..... r***-#****e* . . . . . . . . . . . -w .

l

.=

l

l l

.-*---*.__ ;:.-••**..

l

p

bill 0

. 50

pi

(‘1

. .. .. .

of interest

like

4.1.4. Alloy effect, and energy shift of the reference spectra We have shown that Pt-Ni alloy spectra are well synthesized by a linear combination of Pt and Ni spectra (fig. 9). This certainly means that the electronic rearrangements in the valence states upon alloying are quite small. However, we have included in our synthesis, but not yet discussed, possible independent energy shifts of the reference spectra (0.5 eV steps), to account for a slight difference in the work functions and for a possible small rigid energy shift of the local density of states upon alloying. For Pt s,Ni 5,,(110), R,,,(Bi) depends very little on these shifts; however, the agreement is twice better when they are optimized (0.5 eV towards smaller energies for the Pt peaks, and 0.5 eV towards higher energies for the Ni peak). For the other surfaces the situation is similar as shown in table 1.

1

[ITo] 3’

Fig. 8. Same as fig. 7, versus the azimuth. The relative variations of R ( i 3.5%) are smaller than those of CYand p (5% and 7% respectively) and can be considered as negligible; (8, = 700).

Because the result (C,, C,) is independent of the azimuth, we can choose the azimuth in which the data are ualid to the most grazing incidence as checked from the crystal-to-ground current variations.

I

4.1.3. Energy range for the analysis of the spectra The alloy spectra were recorded in a rather wide energy range (32-120 eV); in a preliminary step we tried to know whether we could equally well use a shorter range (50-80 eV) which contains the most intense Auger lines for Pt and Ni but with a strong overlap. It turns out that there is not enough information to get an accurate value of both weights LY~and pi, and we obtained too large a dispersion in the experimental curve R cxJ e 1. Actually the energy range used to measure and synthesize fhe spectra, is of more importance than the facfors discussed aboue, even more than the choice of the references. It is particularly favourable in the case of Pt-Ni alloys, since it corresponds to the ~nimum of the electron mean free path, and hence to a maximum sensitivity to the superficial composition profile and requires no correction for the energy variation of this mean free path. These conclusions are valid for other

lN(E)/dE

-1

b

E(eV)

-_.-L--

Fig. 9. Pt~~Ni~~(llO). dN(E)/dE Auger spectra at normal (a) and large (b) angle of incidence: as measured ( ---)X synthesized (- - -) and their difference ( ’ . . . ’ ). The arrows indicate the direction of the shift to apply to the reference spectra ( - 0.5 eV for Pt, i 0.5 eV for Ni) necessary to minimize the difference.

D. Dufayard et al. / Composition profile at Pt, Ni,,, Table 1 Energy shift of the Pt and Ni reference spectra, synthesized alloy spectra (see also fig. 9) C, (bulk at’% Pt)

Face

50

(100)

10

for optimum

Shift (eV) Pt

Ni

(110)

0 -0.5

+1 to +1.5 +0.5 to +1

(110)

-1

to -1.5

0

In spite of the improvement, there still remains some shape difference in the spectra, particularly in the composite peak at 60-65 eV for grazing incidence (see the difference curve in fig. 9). Two possibilities can occur: (i) The energy step, 0.5 eV, may be slightly too large; we can correct for this, by interpolating R,,,(d,). The deviation between R,, and R,.+ is 25% better with interpolation but no difference is found in the composition profile. (ii) There exists an alloying effect, which is not fully accounted for by a simple rigid shift of the valence band density of states; for instance, some narrowing of the Pt 4f surface levels of Pt,,Ni,,

I

Rexp

.

r alloy surfaces by IDEAS

231

(111) was observed by photoemission [18], and this observation is compatible with the results discussed in section 4.4. The top layer is an almost pure Pt layer, located above a rich nickel layer: the Pt atoms have less Pt neighbours than in pure Pt, and consequently the 4f band is narrower. A possible indirect argument for surface electronic rearrangements is the change in the reactivity of the Pt very rich Pt,,Ni,,l(lll) surface compared to the pure Pt(ll1) surface [19]. In summary, although the shape of the composite peak around 60 eV is not fully well reproduced, particularly at grazing incidence, an energy shift of the reference spectra in the synthesis of the day spectra has very little effect on the optimum composition profile C,, C, (a few percent on C, ) but reveals possible electronic chunges in the bulk or at the surface of the alloy. 4.1.5. Conclusions on the synthesis of the Auger alloy spectra To illustrate our conclusions, we have reported on fig. 10 all the values R_(di), found for PtS,Ni,,(llO) using different references. Most of the experimental points lie within a region defined by two solutions (C,, C,) = (17%, 100%) and C,, C,) = (28%, 100%); the average value (C,, C,) = (22.5%, 100%) is in good agreement with our previous LEED results [7] and with the final values given in table 2. This justifies a posteriori the following procedure, which avoids unnecessary complications: use reference spectra at fixed angles, to minimize diffraction effects; proceed with a synthesis of the spectra as taken; use a large enough energy range, 32-120 eV in this case, but small enough to keep negligible energy variations of Li and X,; allow for a small energy shift of the reference spectra. 4.2. Two layer analysis

I

50

Bi (‘1

Fig. 10. Pt,,Nis,(llO). Summary of the experimental points R,,,(Bi) obtained with different choices of references (compiled from figs. 5 and 6): most of them lie between two limit theoretical curves Rth( ei), corresponding to (C,, C,) = (17 at%, 100 at% Pt) (lower curve) and (28 at%, 100 at% Pt) (upper curve) respectively, calculated for the parameters L, = 20 p\ and A, = 6 A (optimized values as discussed in section 4.2).

We now turn to the discussion of the theoretical expression, R th( 8, ), and of the relative deviation D between both functions R.._ and Rth, defined in section 2. Apart from the interesting quantities C, and C,, R,,, depends on the following parameters (eq. (3)): the bulk concentration C,, the polar angle of incidence 8;. the interlayer

232 Table 2 Concentration alloy surface5

D. Dufayurd er ~1. / C’ompositron profile ut Pr, NI ,,,(,

profile

and

relative

deviation,

for the Pt-Ni

Ch (bulk at’% Pt)

Face

C,

C,

D

50

(100)

107+ 10 llOi 5 95* 5 20+ 5 70

3Oi20 25*15 OklO lOO+lO 100

0.05 0.06 0.12 0.02

45*10

0.035

(111) (110)

10

(110)

5*

5

a) b) C) d) e) f)

” Unreconstructed (100) model without relaxation on d,,, and total number of atoms normalized to 100% in the top layer. h’ Quasi hexagonal top layer model with 45% outward relaxation on d,,, and total number of atoms normalized to 116% in the top layer; note the very similar composition profile in both cases (a) and (b). ” From ref. [I]. ” Well ordered surface, after ion bombardment and annealing. e’ Bombarded surface. ” L, = 18 P\. A, = 5 d instead of 20 and 6 respectively for Pt,,Ni,,.

distances (db in the bulk, and d,, at the surface), and, last but not least, on the mean free paths of the electrons. We have thus studied the influence of the escape mean free path of the Auger electrons X, (between 4 and 7 A in our case) and of the effective mean ionisation path of the incident electrons L,. 4.2.1. Limit values for L, Primary electrons of 800 eV are expected to inelastic collision mean free have a “ universal” path of about 16 A [20], while one gets about 15 A in platinum and 13 A in nickel [21]. We point out that these values are characteristic of the first inelastic collision suffered by the incident electrons, but do not take into account the cascade of the secondary electrons which have still enough energy to ionize the atoms and hence contribute to the measured Auger signal. On one hand, this cascade can be crudely accounted for by a longer effective mean ionization path Li for the excitation of Auger electrons, as discussed first by Tokutaka et al. [12] for normal incidence, and generalized to arbitrary incidences [l]. On the other hand, Deckers et al. [22] have measured a true escape mean free path of about 8 to 10 A for the Ni 842 eV Auger electrons through a Pt film.

, cllloy surfmes

by IDEAS

This value, smaller than the universal length, can be considered as an extreme low limit for L,Oand consequently we limited the range from 10 A up to about 30 A. 4.2.2. Influence of A, and L, We have used two complementary methods to study the influence of these two parameters on the optimum concentration profiles (C,, C,): (a) we calculate a “map” of the relative deviation D( C,, C,) when C, and C, span the whole range of concentrations (O-100%), and we then look for the minimum value; (b) we can also look directly for the minimum of D(C,, C,) by means of a gradient method. Each method has advantages and drawbacks, and they will be used concurrently: method (b) is faster and allows to calculate the correlations between parameters, but it may lead to local minima and/or to unphysical values for C, or C,; method (a) is well suited to visualize deep and elongated valleys which may exhibit several weak local minima among which we may hesitate to select the best answer. The results of method (b) are shown for two faces Pt,,Ni,,(llO) (fig. lla) and Pt,,Ni,,(llO) (fig. lib): C,, C, and D are plotted as a function of L, and X,. For both (110) faces, D varies somewhat with X, but only little with L,, which yields some uncertainty on the composition profile (C,, C,). Nevertheless, it appears clearly that a strong oscillation of composition is present. We thus need additional considerations to focus on the final answer, for instance very similar values of the mean paths for all the 50% alloy surfaces. It appears, from fig. 11, that the strongest restriction on the determination of (C,, C,) is L, 5 20 A and X, I 6 A for Pt,,Ni,,(llO), otherwise we end up with C, > 100% (this is totally consistent with the values discussed in section 4.2.1: L; = 20 A corresponds to about twice the value found by Deckers for the true escape mean free path for 840 eV Ni Auger electrons). The only acceptable exception to C, 5 100% occurs for the (100) face: there exists a superstructure which is due to a quasi-hexagonal top layer above the square (100) substrate layers [9]. The quasi-hexagonal layer exhibits a higher density of atoms (+ 16%), so that

D. Dujayard et al. / Composition profile at PI, Ni,,,)

233

x alloy surjaces by IDEAS

100

.04 Ci(at%lV)

t

.02

01

\-. \ L

Fig. 11. Pt,Ni, _,(llO). concentrations

30

(a) x = 50% and (bj x = 10%: influence of the effective (L,) and Auger mean free path (A,), on the optimum C, and C, (at% Pt) and on the corresponding relative deviation D between Rexp( 8,) and R th(8, ).

scaled to the substrate square lattice, the Pt concentration has an upper limit of 116%. In the region L, from 10 to 30 A, we conclude that: (i) C, and C, depend rather weakly on X, compared to the dependence upon Li. In a first approximation, X, can be attributed a mean value of 6 A; (ii) C, and C, depend almost linearly on L,; eliminating Li, one sees that C, and C, tend to be linearly correlated (table 3) as C,+A

C,=B,

in fig. 12, for the three faces and for Li = 20 A and h, = 6 A. They illustrate the results obtained from the gradient method: there is a strong quasi Table 3 Best values for the coefficients linear correlation between C, surfaces C, (bulk at% Pt)

Face

C,+AC2=B

50

(100) (111) (110)

C, C, c, C,

(110)

C, +0.65

(6)

where A depends essentially on the face via the interlayer distance (this is related to the respective contributions to the Auger signal of layers 1 and 2): the same value A = 0.65 is found for the (110) faces of the 50 and 10 at% Pt alloy; and B depends on the face and on the bulk composition C, (this has to do with the overall segregation in the two outermost layers). The corresponding maps D(C,, C,) are shown

A and B which describe the and C, for the Pt-Ni alloy

10

+0.44 +0.30 io.27 +0.65

C, C, c, C,

=118 =118 = 95 = 85

a) b) c)

C, = 33

‘) Unreconstructed (100) model. ” Quasi hexagonal top layer model with 45% outward relaxation of d,,, and total number of atoms normalized to 116% in the top layer. Although A is significantly different from case (a), the composition profile (table 2) is only weakly affected. ‘) From ref. [l].

234

D. Dufuyord et 01. / Composition profile at Pt )(Ni lCm I alloy surfaces by IDEAS

CZ(at%Pt) 0

IO

20

30

40

50

60

70

80

90

Q(at%Pt)

100

20

30

40

50

60 -I

0 IO 20 30 40 50 60 70 80 90 10

Cl(at%Pt)

Cl(at%Pt)

Cz(at%Pt) 0

IO

20

30

40

50

Cl(at%Pt) Fig. 12. Pt,Ni,_,(llO). (a) x= 50%; (b) X= 10% and (c) Pt,,Ni,,(lOO); isovalue contour maps of the relative deviation D, for L, = 20 A and A, = 6 A. For the Pt,,Ni,,(lOO) face, we introduced a 45% outward relaxation of the interlayer distance d,z and a larger amount of atoms in the top layer (normalized to 116%) than in the (100) substrate layers, to take into account the observed reconstruction. The levels have been chosen according to: D=,, + CD,,,>,,,- D,,,,) 2”/100.

linear correlation between C, and C, along the bottom of the minimum valleys. With both methods we find very similar results, listed in table 2, which need some remarks. Concerning the Pt~~~i~~(lOO) face, to see the effect of a quasi-hexagonal top layer: we have to scale to 116% instead of 100% the total amount of

atoms in the top layer, to account for a quasi hexagonal layer denser than a bulk (100) layer; we have to include a 45% outward relaxation of the interlayer distance d,,: (in a hard sphere model, a flat quasi-hexagonal layer would be at d,, = (1 + 0.45)d, on top of the substrate); although it changes the coefficient A of the linear relationship

D. B&yard

et al. / Co~~osit~o~ profire at Pt, Ni,,, _ x alloy surfaces & IDEAS

between C, and C, (eq. (6)), this relaxation induces only a small change on the composition profile (table 2). surface: the Concerning the Pt,,Ni,,(llO) gradient method leads to (C, = O%, slightly negative actually, C, = 53%) with a minimum of the deviation D = 0.034, for Li = 20 A and X, = 6 A. On the map of the metric distance versus (C,, C,), we observe a minimum almost equivalent at C, = 4 f 5% and C, = 46 5 10% with P, = 0.035. To conclude on this two layer analysis, the gradient method is faster to study the influence of the mean paths, but one must draw maps P)(C,, C,) to visualize the relative deviation in the whole range of concentrations. Both methods lead to the same determination of the two constants A and B of eq. (6) with a very good accuracy; the choice of Li and h,, if possible by other considerations, can then lead to C, and C, separately with a good_ accuracy. In our case, Li = 20 A and X, = 6 A are good average values, consistent for the whole set of 50% bulk composition. 4.3. composition

chunges induced by ion

bombardment

One of the possible applications of IDEAS is to follow in situ treatments like ion bombardment of alloy surfaces, with preferential sputtering effects, or annealing of the bombarded surfaces to come back to a stable composition profile and follow temperature changes. We only show here one such example to illustrate the feasability. After a typical gentle ion bombardment of the alloy surface to clean it, the spectra collected on the Pt~~Ni~~(llO) surface show a large increase of the Pt Auger peaks and a large decrease of the Ni Auger peak compared to the well annealed surface (fig. 3): there is a marked preferential sputtering of the nickel atoms, also noticed for the other surfaces. The analysis of the alloy spectra show that we can synthesize them as linear combination of the pure metal spectra, and the level of agreement with the experimental spectra is about the same before and after the sputtering. The best agreement is obtained with no energy shift for Pt and + 1 eV for Ni, while for the annealed surface we

235

R,,~ (B+A) 1’1 oc (B) lb--_,...--

+___*_---*---*--

(I,.*...*. . . . . l ,..... /3 (B) l

l

l*******. **fl* . . . . l****..* . . . . -*

, 0 50 90 Fig. 13. Pt50Ni50(110). Comparison of the bombarded surface (B) with the bombarded and annealed surface (B+A). The ratio R exp(oi) and the platinum and nickel contributions (a and j3) to the alloy Auger signal of the bombarded surface clearly show a Pt enrichment, visible at grazing incidence, compared to the annealed surface.

obtain -0.5 eV for Pt and +0.5 eV for Ni. Platinum atoms are the majority species in the bombarded outermost layer, and their electronic properties are apparently closer to the pure Pt ones, while Ni atoms are more perturbed. After bombardment, the weight 01of the relative Pt contribution to the alloy signal is multiplied by 1.3 to 1.4 while the Ni contribution /3 is multiplied by 0.6 (compare figs. 7 and 13). Consequently the ratio R,,,(8) is much larger for the bombarded surface than for the annealed surface (2.5 instead of 1.1) (fig. 13). An estimation of the effective composition profile (effective in the sense that the bombarded layers are assumed without defects!) yields C, about 70% and C, about lOO%, instead of 20 and 100% respectively, after annealing (see table 2). 4.4. Summary other methods

of the results and comparison

with

As can be seen in fig. 14, the IDEAS results are very close to those reported from LEED [5-91, KS [2] and XPS [23]. This work clearly confirms several main conclusions of the LEED studies: (i)

236

D. Dufuwrd

CI

01. / Compo.rmon profile

(100)

(II

Pt, NI ,,,,,

x crlloy .surfuce.r by IDEAS

(111)

UlO>

Fig. 14. Pt,Ni,_, composition profiles obtained with IDEAS compared to other methods: shaded areas stand for the IDEAS with their uncertainty, black circles for the LEED results [5-91. open squares for the ISS results [2]), and the black rectangles angular resolved XPS results [23].

the general oscillating behaviour, (ii) the segregation reversal from the (111) to the (110) face. In addition, very recently, RBS experiments on the (111) face in the channeling and blocking mode with medium energy ions [4], and predictions of segregation theories (TBIM [24,25] and EAM [26]) yielded conclusions in very good agreement with the present results. As a general remark on the quantitative comparison between composition profiles obtained from diffraction techniques (LEED) and collision or spectroscopic techniques (RBS, Auger, X-ray photoelectron diffraction, . . .) we point out that LEED probes essentially the long range ordered parts of the surface while the others also probe the defect regions. This certainly explains the generally slightly larger oscillation of composition profile found by LEED, which may be in this sense closer to the true behaviour of the ideal monocrystalline surface. Coming back to fig. 1, the genrrul &UJW of the function R(B) is oery different for the three faces of the 50 at% alloy sample; obviously, IDEAS dutu exhibit from the very first glance u drustic the composition profile:

chunge

in

(1) The level at angles of incidence close to normal (0 to 50” ) is almost the same for the (110) and (111) faces, indicating a similar composition of the outermost layers taken as a whole; this has to do with the constant B of eq. (6) listed in table 3.

(2) At larger

results for the

angles of incidence (60 to 85”) for the (111) face, indicating a Pt enrichment in the top layer, while R(0) decreases for the (110) faces, indicating a Pt depletion in the top layer; (3) For the faces (100) and (111) the general shape is similar, with a large Pt enrichment in the outermost layers. However, the small angle level is very different: (a) one reason is that the (111) data (ref. [1]) were collected at 1500 eV primary energy instead of the optimum value of 800 eV used in the present work, but this would only contribute to a change from 1.2 to about 1.4; (b) a second reason is that there is (relatively to the substrate) more Pt in the outermost layers at the (100) surface than at the (111) surface; (c) moreover, the weight of the enriched top layer is increased because of the large outward relaxation of the interlayer distance d,,, which produces a damping of the deeper layers contribution. In spite of a significant dispersion in some of the experimental points (see the function R,,,(8) for the (111) and the (100) faces on fig. (1) we obtain quite a good accuracy on C, and C,, and a very good accuracy on the constants A and B in eq. (6). This dispersion has more effects on the minimum value of the relative deviation D which can vary from 0.12 to 0.02 depending on the faces (table 2) than on the optimum composition profile. In table 1, we have listed the energy shifts of R( 13) increases

D. Dufayard

et al. / Composition

profile at Pt, Ni,,,

the reference spectra leading to the best synthesized alloy spectra; these shifts show that, in addition to differences between the geometry, the composition and possibly the 2D order, there may exist differences in electronic density from one face to the other and also with respect to the pure metals. These differences may appear more clearly on XPS or UPS data and should be investigated in correlation with the new chemical and catalytic properties observed on these alloy surfaces: the most recent results report an order of magnitude enhancement in the activity of the (110) face of Pt,,Ni,, alloy compared to the (111) face [27], for instance.

5. Conclusion We gave a detailed discussion of the IDEAS method itself, particularly the way one must choose and collect reference spectra from the pure elements. From this investigation we proposed an optimized procedure to collect IDEAS data and to extract composition profiles. We then applied this procedure to study three different Pt-Ni alloy surfaces which show very different composition profiles, in particular: (a) The influence of the face orientation, for the 50 at% alloy: the composition profile is clearly reversed on the (110) face, compared to the (100) face (present work) and to the (111) face (ref. [l]). (b) The influence of the bulk Pt content on the composition profile at the (110) face: the main change lies in the composition of the second layer: C, is about 50% for the Pt,,Ni,, alloy and almost 100% for the Pt,,Ni,, alloy, while the top layer concentration C, is changed slightly. The IDEAS method has major advantages: since it smoothes out anisotropic emission effects, the RFA detector is well adapted to single crystal studies in contrast with the CMA detector; it gives a quick answer for the composition of two layers; it is a very cheap method compared to others (LEED, ISS, RBS, . . .); it provides us with values accurate enough to allow good tests of segregation predictions and to monitor possible changes of surface composition in catalysis studies; it is a spectroscopic method: contrary to scattering

_ x alloy surfaces by IDEAS

231

methods, it is possible to study compounds of elements with similar atomic numbers, and poorly ordered surfaces. This analysis could equally well be applied to other compounds than alloys or to ultrathin layer deposition problems, provided reference spectra can be obtained. A possible drawback is the need of reference spectra for the pure elements constituting the compound to study.

Acknowledgements

We wish to acknowledge Professor J. Kirschner, Berlin (FRG), for lending the pure Pt samples; J.C. Bertolini, J. Massardier and A. Renouprez, Institut de la Catalyse (Lyon) for lending the PtNi samples and for useful discussions; D. Aberdam and J.P. Segaud, for their help in some computing aspects; P. Beccat, for his help in some experimental aspects and G. d’Assenza and C. Brun for their contributions to the preparation of the samples. This work was done as part of the scientific program run by the Groupement Surface Rh6ne Alpes.

References

I11 J.P. Segaud, E. Blanc, C. Lauroz and R. Baudoing.

Surf. Sci. 206 (1988) 297. 121L. de Temmerman, C. Creemers, H. Van Hove, A. Neyens, J.C. Bertolini and J. Massardier, Surf. Sci. 178 (1986) 888. and 131 T.M. Buck and E.G. McRae, Surface Modifications Coatings, Ed. R.D. Sisson (Am. Sot. Metals, New York, 1986) p. 337. W. van der Weg, A.W. van der [41 S. Deckers, F. Habraken, Con, B. Pluis, J.F. van der Veen and R. Baudoing, Phys. Rev. B, in press. ISI Y. Gauthier, Y. Joly, R. Baudoing and J. Rundgren, Phys. Rev. B 31 (1985) 6216. I61 R. Baudoing, Y. Gauthier, M. Lundberg and J. Rundgren, J. Phys. C 19 (19X6) 2825. R. Baudoing, M. Lundberg and J. Rundgren, I71 Y. Gauthier, Phys. Rev. B 35 (1987) 7867. 181Y. Gauthier, R. Baudoing and J. Jupille. Phys. Rev. B 40 (1989) 1500. R. Baudoing, in: Surface Segregation and [91 Y. Gauthier, related phenomena, Eds. P. Dowben and A. Miller (CRC Press. Boca Raton. 1990) Ch. 14.

[IO] T.E. Gallon, Surf. Sci. 17 (1969) 486. [Ill T.R. Weher, C.E. RI-J”“, P.J. D&son and D. Chadwck, Surf. Sci. 105 (1981) 20. [12] H. Tokutaka, K. Nishimori, K. Takashima and T. Ichinokawa, Surf. Sci. 133 (19X3) 547. [I 31 L. de Bersuder. Rev. Sci. Instrum. 45 (1974) 1569. 1141 C.E. Dahmani. Thesis, 1985, Strasbourg. France, p. 3X. [I 51 E.C. Sowa. M.A. Van Hove and D.L. Adams, Surf. %I. 199 (1988) 174. 1161 P. Fery. W. Moritz and D. Wolf, Phys. Rev. B 38 (1988) 7275. [17] G. Allie, E. Blanc, D. Dufayard and R.M. Stern, Surf. Sci. 46 (1974) 188. [1X] J.C. Bertolini, J. Massardier, Y. Jugnet, G. Grenet and J. Lecante. Rapport d’Activite LURE (198331985) 286.

[19] J. Massardier and J.C. Bertolini, J. Catal. 90 (1984) 358. [20] M.P. Seah and W.A. Dench, Surf. Interface Anal. 1 (1979) L.

[21] A.D. van Langeveld, Thesis, 1983, Leiden, The Netherlands. [22] S. Deckers, private communication. 1231 Y. Jugnet, G. Grenet, N.S. Prakash, Tran Min Due and H.C. Peon, Phys. Rev. B 38 (1988) 5281. [24] G. Treglia and B. Legrand, Phys. Rev. B 35 (1987) 4338. [25] B. Legrand, G. Treglia and F. Ducastelle, Phys. Rev. B 41 (1990) 4423. [26] M. Lundberg, Phys. Rev. B 36 (1987) 4692. [27] J.C. Bertolini, J. Massardier, Ph. Ruiz and B. Tardy, Surf. Sci. 211/212 (1989) 1053.