L86 Surface Science I 15 (I 982) L86-L9 I North-Holland Publishing Company

SURFACE SCIENCE LETTERS

SURFACE SEGREGATION AND SURFACE ELECTRONIC INTERACTIONS IN PtCu *

M.L. SHEK, P.M. STEFAN, D.L. WEISSMAN-WENOCUR, B.B. PATE, I. LINDAU and W.E. SPICER

Stanford Electronics Laboratories, Stanford University, Stanford, California 94305, USA

and

V.S. SUNDARAM

lnstituto de Fisica, Universidade Estadual de Campinas, 13.100 Campinas, Sao Paulo, Brazil

Received 28 August 1981; accepted for publication I December 1981

Photoemission and Auger electron spectroscopy on Pto.9sCu0.o2 show that the (110) face has over twice as much Cu surface segregation as the (I I I) face. The Cu 3d-derived surface "density of states" differ strikingly in peak shape and in width (by 0.5 eV). The centroids, compared with bulk C u d states, are shifted by more than 0.3 eV towards the Fermi level. This is the first experimental correlation between surface segregation and surface bonding.

Both the surface electronic structure and the surface segregation of Cu are found to differ significantly on two different faces [(111) and (110)] of a Pt0.gsCu0.02 single crystal. This is the first experiment which shows the interde- pendence of surface segregation, surface atomic arrangement and electronic structures. The results also suggest that the effects of surface bonding on surface segregation should be considered in a more fundamental manner than the usual thermodynamical or statistical approaches [ 1,2]. The implications of surface electronic interactions for surface segregation have generally been overlooked, except for some recent theoretical investigations [3].

The preparation and cleaning procedures for our alloy samples have been reported elsewhere [4]. Near-surface compositions were generally characterized by Auger electron spectroscopy (AES) and photoemission. The derivative peak-to-peak height ratios between the Pt 237 eV and the Cu 920 eV Auger transitions were used as a first approximation. Since the 920 eV electrons have an escape depth of 12 to 15 .~, the AES results are lower estimates of the Cu surface concentrations. Surface valence band structures were studied by photo-

* This work was supported by NSF Grant INT-78-07268 and CNPQ (Brazil). The experiments were done at SSRL which is supported by the NSF through the Division of Materials Research.

0039-6028/82/0000-0000/$02.75 © 1982 North-Holland

M.L. Shek et aL / Segregation and electronic interactions in PtCu L87

emission experiments performed at the Stanford Synchrotron Radiation Laboratory. Data were collected using a double-pass cylindrical mirror analyzer at 0.4 eV resolution. We shall show the spectra taken at hp = 150 eV, where the Pt 5d photoionization cross-section is only 0.1 -+ 0.025 that of Cu 3d. Hence, the Cu 3d emission is preferentially observed [5]. Since the escape depth of the photoelectrons collected is only about 5 ~,, it is primarily the Cu 3d surface "density of states" that is observed.

The valence band structures of PtCu(110) and PtCu(l 11) are shown in fig. l a. Under the anneal conditions given in the caption, the Cu near-surface concentrations did not change upon further anneal. The Cu 3d states are identified through the hv-dependence of the valence band features [6]. At hv= 150 eV, the PtCu(l l0) surface gives rise to a 3d peak which has a maximum at - 2 . 2 eV (referenced to the Fermi level EF) and a full width at half maximum of 1.6 eV. The PtCu(111) surface, however, shows a broader and skewed structure. Fig. lb shows the difference curve between the PtCu(111) valence spectrum and a Pt ( l l 1) valence spectrum. The peak maximum lies at approximately - 2 . 3 eV. There is some uncertainty in peak position, owing to the broadness of the peak. The width at half maximum is approximately 2.1 eV. A shoulder is apparent around -3 .1 eV. The corresponding spectrum for pure Cu yields a 3d peak maximum at approximately - 2 . 5 eV, and shoulders around - 3 . 5 eV and - 4 . 7 eV [6,7]. Hence the Cu 3d-derived states in the alloy are significantly shifted towards smaller binding energy [8].

The Cu concentrations in the top 3 to 4 A of the surface region, [9], as given by the ratios of the Cu 3d to the Pt 5d emissions, are 9 . 8 - 3.2% for the PtCu(l l0) sample and only 4.4-- + 1.5% for the P tCu( l l l ) sample [10]. This difference is also reflected in the AES measurements. The AES estimates

corresponding to the data in fig. 1, averaged over the top 9 to 11 A, [8], are 5.9 -4- 1.2% Cu for the PtCu(110) surface and 3.5 - 0.6% Cu for the PtCu(111) surface. Although the crystal face dependence of Cu surface segregaton was not studied in previous investigations on PtCu alloys [11,12] this phenomenon has been reported for CuNi [13,14]. However, there was no accompanying photoemission study of the CuNi valence bands of the two different crystal surfaces.

The differences in the Cu 3d "density of states" of the two PtCu surfaces agree with a crude model based on surface atomic coordination. As shown in the inset of fig. 1, a Cu atorn in the topmost layer of a fcc(111) surface, can interact with 9P t nearest neighbors, whereas for an ideal (110) surface, the atomic coordinations in the top two layers are only 7 and 11, respectively. Hence, the smaller width of the Cu 3d structure on PtCu(110) reflects a weaker P t -Cu interaction due to a reduced number of nearest neighbors. The actual situation is slightly more complicated in the case of the PtCu(l l0) surface, which shows a reconstructed (1 × 3) structure in low-energy electron diffrac- tion (LEED). However, the interaction with the nearest neighbors is still expected to be smaJler than that on a close-packed (111) surface [4]. The shift

L88 M.L. Shek et al. / Segregation and electronic interactions in PtCu

u

u')

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0 0

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6 0 0 0

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-8 -6 -4 -2 EF INITIAL STATE ENERGY (eV)

F~g. I. (a) UPS electron energy distribution N ( E ) at h ~,= 150 eV. ( I ) PtCu(110), annealed at 600°C for 90 min. (2) PtCu(1 l.I ), annealed at 500°C for 120 min. In the inset, a darkened circle denotes a Cu atom in the top layer; a full circle denotes a Pt atom in the top layer; a broken circle denotes a Pt atom in the second layer; for (110), the nearest neighbor atom in the third layer, directly below the Cu atom, is not drawn here. Both spectra were taken with a CMA, and a geometry such that electrons are collected over all polar angles between 36 ° and 48 ° with respect to the sample surface normal. (b) UPS difference curve between PtCu(l 11) and Pt(I 1 I), h i ,= 150 eV. Note that (a) and (b) have different vertical scales.

of the alloy surface Cu 3d states towards E F, relative to the pure Cu 3d states, is reminescent of a previous study of the bulk electronic structure of PtCu [15].

We suggest that the effects of unlike-atom bonding on surface segregation can be more important than usually assumed. There is a large discrepancy between our surface segregation data and model predictions based on the c o m m o n assumption that pure elemental surface energies apply to the alloy constituents• The widely used broken-bond model provides a convenient basis for discussion [1]. The top-layer Cu concentration ( y ) can be calculated for a bulk concentration (x) , given a knowledge of the enthalpy difference ( A H ) of

M.L. Shek et al. / Segregation and electronic interactions in PtCu L89

exchanging a Pt atom for a Cu atom at the surface:

A H = 2WB(--m--½1+ 2mx+lx) + 2wS[½(l+m)--ly--mx ]

--AEn(rn+½1 ) + AEs[½( I+ m) ] . (1)

In eq. (1), c0 B = [Evtcu i 1 - i E c u c u - iEptPt ] where Ei/ (kcal /mole) denotes the enthalpy of a bulk bond between constituents i and j . co s i~ the correspond- ing quantity for the surface bonds. A E B = 2 (Ecucu- -Ep tP t ) for the bulk. AE s = (1 + 8)AE B is the corresponding quantity for the surface, where 8 is the surface bond enthalpy relaxation parameter. The surface bonds include the intralayer I lateral bonds, as well as the rn interlayer bonds to the second layer of atoms. Comparison between table lb and lc shows that the use of pure elemental sublimation energies to calculate AE B yields far too much Cu surface segregation, even when surface bond relaxation is included. The overestimate of Cu surface segregation in PtCu also arises in a different surface segregation model [16]. We note that the AE B and AE s terms in eq. (1) correspond to the difference in the energies associated with the uncoordinated Cu and Pt bonds, or the "surface energies". The usual assumption is that the "surface energies" of the constituent atoms in the alloy are the same as those of the atoms in the pure solids. However, because of the significant electronic interactions between Pt and Cu, such an assumption of independent and transferable bond energies should be invalid. To proceed, we may use eq. (1) to determine AE B from the experimental results on PtCu(111) and then calculate the top-layer Cu con- centration on PtCu(l l0) . The AE B values are shown in the bot tom row of table 1. The calculated values for the Cu surface concentration on PtCu(110), shown in table ld, are in fair agreement with experiment. Alternatively, the effect of P t - C u bonding on Cu surface segregation may be seen directly from the Cu 3d-deri.ved "density of states" (fig. 1). We have noted that the Cu 3d structures on both alloy surfaces lie closer to the Fermi level than do the pure Cu 3d states. This suggests that bringing a Cu atom to the surface layer does not lower the surface electronic energy as much as would be expected if the binding energy [8] of a Cu atom in the alloy were the same as in pure-Cu. Therefore, the driving force for Cu surface segregation is less than that expected from the electronic structures of the pure elements. We hope that this tentative suggestion will be pursued by an examination of the detailed dif- ferences between the valence bands of the two alloy surfaces, which show different Cu surface segregation behavior. Theoretical calculations on the valence density of states, and excitation spectra, on various PtCu alloy surfaces would be helpful.

In summary, we have shown the influence of surface atomic coordination on both the valence electronic structures and surface segregation of Cu in PtCu. On the basis of comparison between our surface segregation results with the predictions of the broken-bond model, we suggest that it is not appropriate to ignore the effects Of unlike-atom bonding on the uncoordinated bonds of an

Tab

le I

T

he u

se o

f th

e br

oken

-bon

d m

odel

sho

win

g th

at t

he s

urfa

ce e

nerg

ies

of p

ure

Pt

and

pure

Cu

are

not

tran

sfer

able

to

the

allo

y: c

alcu

lati

on f

or P

to.g

aCuo

.02

(a)

(b)

(c)

(d)

Sur

face

C

uob s

(%

) (P

ES

) C

u~al

c (%

) C

u (%

) (A

E e

fit

)

• (i)

Av

(ii)

Top

(i

) 8

=0

(i

i) 8

>0

(i

) 8

=0

(i

i) 8

>0

(I 1

I)

4.4-

- + 1.

5 8.

0-+

4.0

80.3

25

.5

8.0

(4.0

-12.

0)

8.0

(4.0

-12.

0)

e~

t-,

......

(110

) 9.

8+3.

2 28

.6-'

- 10

.9

>99

.9

95.4

18

.4 (

6.2-

32.5

) 34

.9 (

9.5-

59.7

) ~-

A E

n (

kcal

/mol

e)

9.08

9.

08

2.55

( 1

.61-

3.17

) 4.

57 (

2.56

-5.9

1 )

e~

Hea

t of

mix

ing

per

bond

, ~0

B =

-

0.4

kcal

/mol

e, T

~l e f

~ = 8

23 K

, an

d T~

tH~

= 87

3 K

. (a

) D

iffe

rent

pla

nes.

(b

) S

urfa

ce C

u% o

btai

ned

by p

hoto

emis

sion

; (i

) va

lues

ave

rage

d ov

er t

he d

epth

sam

pled

: (i

i) v

alue

s de

rive

d w

ith

the

assu

mpt

ion

that

onl

y th

e C

u% i

n th

e ~"

to

pmos

t la

yer

devi

ates

fro

m t

he b

ulk

CuV

o.

.x

(c)

Top

-lay

er C

u% c

alcu

late

d by

Wil

liam

and

Nas

on's

app

roac

h in

ref

. [I

]: t

he h

eats

of

subl

imat

ion

of C

u an

d P

t ar

e 80

.5 a

nd 1

35 k

cal/

mol

e, r

espe

ctiv

ely:

(i

) w

itho

ut s

urfa

ce r

elax

atio

n, 8

= 0

; (i

i) w

ith

surf

ace

rela

xati

on p

aram

eter

s 8~

ll >

= 0.

174

and

8~ll0

~ = 0

.236

. ~.

(d

) T

he r

esul

ts o

f pa

ram

eter

izin

g A

E n

in

eq.

(I);

A E

n i

s ob

tain

ed f

rom

CU

obf~

for

(ll

I)

and

then

use

d fo

r pr

edic

ting

CuC

g fo

r (I

10)

; (i

) w

itho

ut s

urfa

ce

rela

xati

on,

8:0

: (i

i) w

ith

surf

ace

rela

xati

on p

aram

eter

s 8 o

it>

=0.

174

and

8~11

0~ =

0.23

6.

2 T

he b

otto

m r

ow s

how

s th

e A

E a

val

ues

asso

ciat

ed w

ith

the

vari

ous

Cu%

giv

en i

n th

e co

rres

pond

ing

colu

mns

. A

Ea

valu

es o

btai

ned

by f

it a

re 2

to

3 ti

mes

sm

alle

r th

an A

E a

obt

aine

d fr

om t

he d

iffe

renc

e in

Pt,

Cu

subl

imat

ion

ener

gies

.

M.L. Shek et al. / Segregation and electronic interactions in PtCu L91

alloy surface. Much work is needed to clarify the fundamental relationships between surface segregation and surface electronic density of states.

References

[I] F,L. Williams and D. Nason, Surface Sci. 45 (1974) 377. [2] R.G. Donnelly and T.S. King, Surface Sci. 74 (1978) 89. [3] G. Kerker, J.L. Moran-Lopez and K.H. Bennemann, Phys. Rev. BI5 (1977) 638. [4] M.L. Shek, P.M. Stefan, D.L. Weissman-Wenocur, B.B. Pate, I. Lindau and W.E. Spicer, J.

Vacuum Sci. Technol. 18 (1981) 533. [5] I. Lindau, W.E. Spicer0 J.N. Miller, D.T. Ling, P. Pianetta, P.W. Chye and C.M. Garner,

Phys. Scripta 16 (1977) 388. [6] The Pt emission is relatively flat and is spread over a larger width in energy. That the Pt

emission does not contribute to the structure assigned as Cu 3d states in the h I, = 150 eV data, can be seen from the photon energy dependence of the valence band spectra. We have taken data (unpublished) at h~,=80, 110 and 130 eV as well. The strengths of the Cu-derived features increase with respect to the rest of the valence band, in accordance with the photon energy dependence of the Pt 5d and Cu 3d photoionization cross-sections.

[7] J. Stohr, F.R. Feely, G. Apai, P.S. Wehner and D.A. Shirley, Phys. Rev. BI4 (1976) 4431. [8] By "binding energy" we mean the energy difference between the Fermi level E F and the

average energy of the d band as seen in photoemission. [9] The depth which is sampled is the electron escape depth multiplied by cos 42 ° .

[10] The relative strengths of Cu 3d and Pt 5d emissions are estimated as follows. For the (111) alloy sample, the peak area under the difference spectrum PtCu(l 11) -P t ( l l l ) [i.e. fig. Ib] is divided by the area under the pure P t ( l l l ) valence band spectrum (with the inelastic background subtrated). For the (110) alloy sample, the Pt contribution is assumed to be similar to the P t ( l l l ) valence emission. The use of Pt( l l0) as reference for the PtCu(110) sample wouJd be unlikely to lead to significantly different results at h~,= 150 eV. Moreover, the emissions integrated over the Pt bandwidths (about 7 eV) should be rather insensitive to the shape of [he Pt valence electronic density of states.

[11] H.H. Brongersma, M.J. Sparnaay and T.M. Buck, Surface Sci. 71 (1978) 657. [12] M.J. Kelley, D.G. Swatzfager and V.S. Sundaram, J. Vacuum Sci. Technol. 16 (1979) 664. [13] Y.S. Ng, S.B. McLane, Jr. and T.T. Tsong, J. Vacuum Sci. Technol. 17 (1980) 154. [14] C.R. Brundle and K. Wandelt , J. Vacuum Sci. Technol. 18 (1981) 537. [15] G.G. Kleiman, V.S. Sundaram, C.L. Barreto and J.D. Rogers, Solid State Commun. 32 (1979)

919. Unfortunately, it is beyond the scope of this letter to compare and contrast the present results with these authors' X-ray photoemission results on polycrystalline PtCu foils.

[16] A.R. Miedema, Z. Metallk. 69 (1978) 455.