Chapter 10 Metallic Films on Metallic Substrates

Chapter 10 Metallic Films on Metallic Substrates

371 Chapter 10 METALLIC FILMS ON METALLIC SUBSTRATES K. JACOB1 1 INTRODUCTION There i s p r a c t i c a l and basic i n t e r e s t i n t h i n m e...

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371

Chapter 10 METALLIC FILMS ON METALLIC SUBSTRATES K. JACOB1

1

INTRODUCTION There i s p r a c t i c a l and basic i n t e r e s t i n t h i n m e t a l l i c f i l m s on m e t a l l i c

substrates. F i r s t , one might t h i n k o f such processes as m e t a l l i z a t i o n , by which r e a c t i v e m e t a l l i c compounds l i k e s t e e l o r brass are protected against c o r r o s i o n by t h i n l a y e r s o f chromium or n i c k e l . The thickness o f such m e t a l l i c coatings i s i n the range o f several pm, i.e.

the f i l m s have the p r o p e r t i e s o f bulk met-

als. The aim o f t h i s review goes beyond t h i s thickness range t o thicknesses of up t o several monolayers (ML), a range which can be q u a n t i f i e d by most o f t h e known s u r f a c e - a n a l y t i c a l techniques, As we w i l l show, i t i s a l s o t h e range i n which the t r a n s i t i o n i n e l e c t r o n i c s t r u c t u r e from atom t o b u l k occurs. Recently, w i t h extension o f the development o f e l e c t r i c a l devices i n t o t h e submicron

range, one r e a l i z e s the importance o f understanding and c o n t r o l l i n g t h e growth

o f u l t r a t h i n metallic films.

The

same i s t r u e

also

for

photovoltaic

and

magnetic devices w i t h t h e i r great promise. I t seems t h a t t h e f i e l d o f u l t r a t h i n m e t a l l i c f i l m s i s bound t o become ever more important. M e t a l l i c f i l m s on m e t a l l i c substrates have been i n v e s t i g a t e d so f a r under various aspects, i n c l u d i n g the geometrical s t r u c t u r e o f t h e adlayer,

t h e bind-

i n g energy o f t h e adsorbed metal atom, or number d e n s i t i e s and morphology o f

the n u c l e i . Auger e l e c t r o n spectroscopy (AES),

low-energy e l e c t r o n d i f f r a c t i o n

(LEED), and thermal desorption spectroscopy (TDS) have been used f o r t h i s purpose. The pioneering work was performed by Palmberg and Rhodin (1). Studies o f t h i s k i n d have been undertaken since then by many other groups, e t a l . (2-4),

Bauer e t a l . (5-9) o r Venables e t a l .

e.g.

by Rhead

( 1 0 , l l ) . The reader i s r e -

f e r r e d t o several e x c e l l e n t review a r t i c l e s (3,6,9,10).

R e l a t i v e l y few atttempts have been made t o evaluate t h e e l e c t r o n i c s t r u c -

t u r e o f d i f f e r e n t m e t a l l i c adsorbates, e.g.

by photoemission. A pioneering work

was performed by Eastman and Grobman i n studying t h i n Cu and Pd f i l m s on an Ag substrate (12). It has been w e l l recognized t h a t t h e e l e c t r o n i c s t r u c t u r e p l a y s an important r o l e i n several nucleation phenomena. Thus,

i n discussing the

a s t o n i s h i n g l y l a r g e d i f f e r e n c e s between Ag and Au on W(110) and Mo(ll0) subs t r a t e s Bauer and Poppa (7) have mentioned the e l e c t r o n i c s t r u c t u r e as a poss i b l e explanation.

For basic research one important aspect i n studying t h i n m e t a l l i c f i l m s i s the t r a n s i t i o n from t h e atomic t o t h e bulk state.

There a r e several examples

372

f o r which layerwise growth has been found,

i n which case two-dimensional

(20)

c l u s t e r s can be a n t i c i p a t e d i n t h e sub ML region. Such 20 c l u s t e r s represent an intermediate s t a t e between atom and bulk. intermediate state.

Surface-analytical

The complete ML i s a l s o such an

methods l i k e LEED o r AES supply some

i n f o r m a t i o n on t h e average s i z e and shape o f 2D c l u s t e r s . Also, more r e c e n t l y , imaging methods have been developed such as t h e Scanning Tunneling microscopy

(13) and LEED microscopy (14) which open up many new ways t o study s i z e and shape o f 2D c l u s t e r s . These developments make t h e i n v e s t i g a t i o n o f m e t a l l i c f i l m s on s i n g l e c r y s t a l surfaces down i n t o t h e sub ML r e g i o n very promising f o r g a i n i n g some i n s i g h t i n t o the atom-to-bulk

t r a n s i t i o n . We w i l l show how angle-resolved UP

photoelectron spectroscopy (ARUPS)

c o n t r i b u t e s t o answer t h i s question.

This

method makes i t possible t o d i s c r i m i n a t e between atomic l i n e s , 2D bands and 30 bands i n the conduction-electron

region.

I t seems t h a t t h e t r a n s i t i o n from

atomic l i n e s t o 30 bands occurs between 0.1 and 10 ML.

I n studying t h i n m e t a l l i c f i l m s , one furthermore encounters the problem o f

n u c l e a t i o n and f i l m qrowth. We can touch upon t h i s important p o i n t o n l y b r i e f l y , since the i n v e s t i g a t i o n o f t h e e l e c t r o n i c s t r u c t u r e i s n o t the o n l y key t o

understand t h i s phenomenon. I t may be i n t e r e s t i n g t o note t h a t t h e s i t u a t i o n i s q u i t e d i f f e r e n t f o r the condensation o f r a r e gases (e.g.

Xe),

where one can

d i f f e r e n t i a t e beween s i n g l e Xe atoms, 2D Xe i s l a n d s and Xe i s l a n d s i n t h e sec-

ond and t h i r d l a y e r on t o p o f the complete f i r s t l a y e r from t h e b i n d i n g energy (BE) o f t h e Xe 5p l e v e l s along (15-17).

Very r e c e n t l y s i m i l a r observations have

been made f o r a l k a l i metals as we w i l l discuss below. The growth o f t h i n m e t a l l i c f i l m s i s governed by l a t t i c e and svmmetry matching o f the adlayer w i t h t h e supporting substrate. From t h e case s t u d i e s we

w i l l see which range o f l a t t i c e mismatch has been observed. The r e s u l t o f mismatch i s a r a t h e r l a r g e amount o f s t r a i n w i t h i n the f i r s t l a y e r s o f t h e f i l m .

So f a r i t i s n o t c l e a r whether t h i s s t r a i n can be visualized,

e.g.

i n photo-

emission. We w i l l make t h e p o i n t t h a t f o r the ML t h e question o f s t r a i n has t o be disucssed anew i n t h e l i g h t o f a possible i n t r i n s i c ( i n general smaller) l a t t i c e constant w i t h i n the ML. Nevertheless, f o r t h i c k e r layers, p e r f e c t epit a x i a l growth c e r t a i n l y needs a l a t t i c e match o f 1 % o r b e t t e r .

It has been demonstrated already i n the foregoing chapters t h a t ARUPS i s e s p e c i a l l y w e l l s u i t e d t o evaluate a 2D e l e c t r o n i c s t r u c t u r e . Therefore, t h e ML and t h e t r a n s i t i o n from sub ML t o supra ML coverage i s very accessible t o

ARUPS.

It also has been noted t h a t the ARUPS signal comes o n l y from very few

l a y e r s near the surface ( c e r t a i n l y l e s s than 10) due t o t h e very small escape depth o f t h e photoelectrons i n the accessible range o f photon energies. Therefore, a 10 ML t h i c k f i l m , which i s s t i l l an u l t r a t h i n f i l m , i s t h i c k enough f o r studying p r o p e r t i e s o f t h e bulk metal.

373

An i n t e r e s t i n g problem a r i s e s i n t h i s context, namely t h e existence o r non-

existence o f i n t e r f a c e states.

There are o n l y some very recent examples o f

experimentally v e r i f i e d i n t e r f a c e states. The main problem here i s t o separate i n t e r f a c e from ML states,

n e i t h e r o f which i s known y e t f o r most systems.

It

may be necessary t o note t h a t i n order t o i n v e s t i g a t e ML as w e l l as i n t e r f a c e s t a t e s one needs n o t only p e r f e c t epitaxy b u t a l s o s i n g l e domains o f ordered adlayers. This i s n o t a t a l l a t r i v i a l problem, since most ML are ordered w i t h -

i n hexagonal close-packed (hcp) phases which tend t o a l i g n t h e i r t i g h t l y packed

rows along or n e a r l y along t i g h t l y packed rows o f the substrate. Therefore,

a

f o u r f o l d symmetric substrate surface l i k e t h e f c c (001) surface mostly gives

r i s e t o two domains which are r o t a t e d by 90" against each other. One p o s s i b i l i t y t o circumvent t h i s d i f f i c u l t y i s t o use the twofold symmetric f c c (110) sur-

face. This attempt can be successful only i n p a r t , since many o f these (110) faces are reconstructed, preventing a f l a t metal f i l m from growing

or promoting

a l l o y i n g o f t h e adatoms w i t h the substrate. On a f c c (111) substrate surface very o f t e n t h e r e i s a small r o t a t i o n a l angle a between t h e adlayer and subs t r a t e hcp l a y e r s g i v i n g r i s e then t o two adlayer domains r o t a t e d by +a against t h e substrate.

The choice o f t h e substrate i s important f o r several reasons. I t determines

t h e growth mode, enables or prevents i n t e r d i f f u s i o n , and obscures t h e adsorbate emission, i f substrate emission evolves a t t h e same energy.

One p o s s i b l e way t o

solve t h i s problem i s t o use an sp metal w i t h i t s nearly f l a t sp-band as subs t r a t e i n s t e a d o f a d-band metal.

I n an attempt t o evaluate the t r a n s i t i o n from atom t o bulk from t h e study o f t h i n m e t a l l i c f i l m s we l i k e t o compare t h i s study w i t h c l u s t e r studies. This technique, w h i l e being most useful for a f i r s t look, encounters l i m i t i n g d i f f i c u l t i e s . So f a r i t has n o t been possible t o prepare c l u s t e r s o f one s i z e only;

i n s t e a d one prepares a d i s t r i b u t i o n o f c l u s t e r s w i t h d i f f e r e n t sizes. There i s no i n - s i t u

-

i.e.

d u r i n g spectrocopy

-

access t o shape and s i z e of t h e c l u s t e r s

being analyzed. Furthermore, spectroscopy mostly i s done on c l u s t e r s f r o z e n i n t o a m a t r i x o f i n e r t gases i n order t o increase the c l u s t e r density. T h i s i n troduces t h e problem o f the so-called m a t r i x - e f f e c t ,

i.e.

how t h e m a t r i x i n f l u -

ences shape and e l e c t r o n i c s t r u c t u r e o f t h e i n d i v i d u a l c l u s t e r . Thus, c l u s t e r s on a s i n g l e - c r y s t a l l i n e surface are f o r a longer time amenable t o a wider range o f spectroscopies. But the i n f l u e n c e from the support i s a more severe problem i n t h i s case.

I n t h e f o l l o w i n g we w i l l discuss the e l e c t r o n i c s t r u c t u r e o f t h i n m e t a l l i c f i l m s on metals, which i s mainly evaluated by ARUPS.

I n a f i r s t step the t h i n

m e t a l l i c l a y e r s can be analyzed i n terms o f BE and o p t i c a l d e n s i t y o f s t a t e s and t h e i r development w i t h thickness. This step can be performed a l s o i n an angle-integrated photoemission experiment. For t h e ordered ML, ARUPS a f f o r d s a

374

wave-vector-resolved analysis of the conduction band. For this purpose the known relation for k,,

can be applied. klis the wave vector parallel to the surface layer, Ekin the kinetic energy of the emitted photoelectrons in the vacuum, and 0 the angle of emission with respect to the surface normal. Using the trivial relation between Ekin and the binding energy related to EF EsF, hw

=

I E B ~ I+ @ + Ekin,

the 20 band structure EBF(kl) can be determined. Cp denotes the work function of the surface. For greater thickness the bulk (3D) metal develops. The conduction band changes, in general it broadens. The photoemission process changes dramatically. Now the photoelectrons are excited into empty bulk states under k-conservation in the reduced Brillouin zone. As a consequence the transition becomes strongly hw dependent. This dependence is a reliable proof that the photoexcitation process occurs in the 30 bulk. From (2) one easily recognizes how the work function can be determined by photoemission. If we ask for the energy of the Fermi edge, we have IEBFI = 0 by definition, and Ekin is the kinetic energy of the photoelectrons excited from EF. Therefore, Cp is given by the difference between hw and Ekin at EF. Using a discharge lamp hw is exactly given to a few meV only. Therfore, how precisely @ can be determined depends on the precision in determining Ekin of EF. The latter is given by the difference between energies of the Fermi and the secondary-electron edge. In an AR mode these edges can be measured with an accuracy of ?lo meV, and the width of the secondary-electron edge turned out to be as sharp as the Fermi edge (20). Therefore, for an AR mode and normal emission, @ can be determined absolutely to 9 0 meV. It should be stressed also that there i s no need for any extrapolation at the secondary edge (or threshold), as is commonly performed in the A1 mode, since in the AR mode one always collects secondary electrons with k, = 0 at normal emission. For an ideal system we expect the following changes with thickness i n the ARUP spectra from conduction electrons. For isolated atoms in the sub ML region we expect atomic lines without any k-dependence of EB and being only broadened by 0.3 to 1.0 eV, as is known for the condensed state mainly due to life-time effects. If these atoms then coagulate into 20 islands, we expect a 2D band structure to build up. For two or more ML the ARUP spectra may be smeared out due to overlapping features from the 20 bands and the beginning of 3D transi-

375

t i o n s . Then, a t even higher thicknesses, we expect t h e 3D ARUP spectrum f o r a t h i c k s i n g l e - c r y s t a l l i n e f i l m t o evolve i d e n t i c a l l y t o t h a t from a bulk sample.

To our knowledge such an i d e a l system has n o t y e t been found. Only some aspects have been v e r i f i e d so far,

and several problems should be mentioned b r i e f l y .

One problem i s t o d i f f e r e n t i a t e between 20 band s t r u c t u r e and i n t e r f a c e states.

We w i l l show by comparing our own r e s u l t s w i t h others t h a t t h e character o f t h e s u b s t r a t e i s very important. I t seems t h a t d-metal substrates e x h i b i t d-derived i n t e r f a c e s t a t e s which are as intense as t h e adlayer and substrate s t a t e s themselves.

Our conclusion w i l l be t h a t the best approximation o f t h e 20 band

s t r u c t u r e o f conduction d s t a t e s i s obtained by s t a r t i n g w i t h an sp substrate. For t h e development o f an s p - l i k e ML band there e x i s t s a t the moment o n l y t h e example o f Cs. The next problem i s t o understand,

a t which thickness t h e 3D

band s t r u c t u r e develops. There are r e l a t i v e l y l a r g e d i f f e r e n c e s found between d i f f e r e n t systems.

I t w i l l be shown t h a t the work f u n c t i o n i s very i n d i c a t i v e

f o r t h i s transition. The o u t l i n e o f t h i s chapter i s as follows.

From the given arguments i t has

become c l e a r t h a t t h e c h a r a c t e r i z a t i o n o f t h e t h i n f i l m s i s important w i t h spect t o t h e i r growth mode,

thickness and alignment t o t h e substrate.

re-

There-

fore, we b r i e f l y discuss t h e experimental aspect o f determining t h e ML coverage by AES.

Then we present an overview o f t h e d i f f e r e n t systems known so f a r ,

ordered according t o t h e k i n d o f t h e adlayer metal.

We s t a r t w i t h t h e noble

metals Cu, Ag, Au, go then t o the 3d t r a n s i t i o n metals, mention Pd and P t and t u r n f i n a l l y t o t h e sp metals, e s p e c i a l l y t o the a l k a l i n e and e a r t h - a l k a l i n e

metals. We do n o t discuss r a r e - e a r t h metals,

since t o o l i t t l e i s known about

these systems a t t h e moment.

We w i l l see t h a t from t h e small number o f systems studied so f a r o n l y very

f i r s t and t e n t a t i v e conclusions can be drawn.

2

AUGER ELECTRON SPECTROSCOPY FOR THICKNESS ANALYSIS Most o f our knowledge o f the e l e c t r o n i c s t r u c t u r e o f t h i n m e t a l l i c f i l m s

o r i g i n a t e s from ARUPS. The discussion o f such studies w i l l f i l l t h e g r e a t e s t p a r t o f t h i s chapter.

However,

besides ARUPS data, a d d i t i o n a l i n f o r m a t i o n i s

c o l l e c t e d a l s o from o t h e r surface-analytical

techniques. The geometrical s t r u c -

t u r e and t h e alignment o f t h e adlayer i s very o f t e n determined by LEED. I n t h e f u t u r e , methods which analyze only t h e topmost l a y e r may become more important.

Such methods as i o n s c a t t e r i n g spectroscopy (ISS) and photoelectron d i f f r a c t i o n

(PED) are n o t w i d e l y used a t the moment. For our t o p i c the most important subs i d i a r y technique i s AES. However, we w i l l not discuss t h a t i n g r e a t d e t a i l here, f o r we only want t o show how s t r u c t u r a l i n f o r m a t i o n can be e x t r a c t e d from

AES.

376

Mostly, the AES data are taken from the energy-distribution curve N ( E ) of the secondary electrons in its derivative mode dN/dE. For coverages up to several ML the peak-to-peak amplitude in the dN/dE curve of an Auger transition is a quantitative measure of the coverage. For the analysis of a metallic film A (from adsorbate or adlayer) on a substrate S one has available both the signal from the metal and from the substrate. The adlayer signal IA will increase and the substrate signal Is will decrease with coverage. The rate of these changes depends characteristically on the morphology of the adlayer. Let us consider first the Frank van der Merwe (FM) growth mode for which the adlayer grows layer by layer. Assuming that the change of Is(z) with thickness z is proportional to Is(z), i.e.

and that each adsorbed atom is smeared out into an infinitely thin 20 layer, one obtains Is(z)

Is(o) exp(-z/Xs).

=

(4)

A S is the mean free path for inelastic scattering of the outgoing electrons which originate from the substrate. For reasons of simplicity this and the following formulae are given for an emission direction normal to the surface. For an emission under an angle 0 with respect to the surface normal the effective layer thickness will increase by a factor ([email protected])-l. The curve according to equation ( 4 ) i s given in fig. 1 by a broken line. The model calculations given in fig. 1 are performed for a ML thickness of d = 2.5 and X s = 4.0 8, the latter value being near the minimum for most substances (18). In reality the adlayer will grow laterally atom by atom and not by adding thin 20 slabs. This gives rise to straight lines with a change in slope after completion of every ML, as shown in fig. 1 by a line. The growth is described then by:

Is’(z)

=

x Is(o) exp(-nd/Xs) + (1-x) I s ( o ) ,

(9

with x the fraction of the ML, by which the substrate is covered. d is the thickness of the ML and n = 1,2, the number of the adsorbed layers. Equation (5) describes a sequence of straight lines with breaks in slope after completion of the nth layer. These breaks lie on the broken curve from equation ( 4 ) , i . e . just at the ML breaks, the continuous model ( 4 ) and the discrete model ( 5 ) coincide. In the continuous model dJ/dz is larger at the beginning and smaller

...

377

when the ML is nearly completed compared to the constant dJ/dz value for the discrete model (5). In the continuous model every additional slab weakens somewhat less than the foregoing, whereas in the discrete model each additional atom adds a whole package of thin slabs with lateral extension of only one atom and with their different weakening power thus adding a constant weakening contribution for each atom.

''hi '\

,

1

3 COVERAGE 0 (MLI I

2

L

Fiq. 1: Auger intensities I(@) of a Auger transition of the substrate S as function of coverage @of the adlayer A in units of monolayers (ML) for the three different growth modes as indicated by the insets. With d = 2.5 A and X s = 4.0 A the model calculations of fig. 1 exhibit nearly the strongest deviation from exponential decrease which i s possible for normal emission. For larger exit angles this deviation can be larger. Considering 0 = 80", which would be really an extreme case, z/X = 3.60 compared to value down to 0.027 at the 0.63 for 0 = 0". This would decrease the I(@)/I(o) 1 ML break resulting in a very strong deviation from the exponential curve. Under the same assumptions the adsorbate Auger intensity is given by:

for the continuous model and by

IA(z)

=

x IA(~) [ l - exp(-nd/XA)l

(7)

for the discrete model. One gets these curves by mirroring the curves of fig. 1 at the I(O)/I(o) = 0.5 line. Two useful formulae can be deduced from the above equations. From (4) the thickness of the ML can be deduced as:

Furthermore,

This equation holds for adlayer and substrate Auger intensities, By (9) deviations from the layer-by-layer growth mode can be easily detected. Besides the FM mode two other growth modes are indicated in fig. 1. In the Stranski-Krastanov mode (SK) only the first ML grows as layer on top of which 3D cluster growth continues. For the SK mode the ML break is well developed. The slope beyond the ML break depends on the size of the 3D clusters. For large clusters a horizontal line is found beyond the ML break. This can be understood easily. Let us assume that one atom covers 5 %', i.e. approximately 2x1013 atoms cover lmd. If we condense these atoms into one cube, it will cover 3.7~10-5 mm2 and is therefore far below the limit which can be detected by AES (which is 10-2 to 10-3 mm2 probing a surface area of 1 mm2). Thus, the slope in fig. 1 indicates much smaller clusters. From the change of slope a mean size of the clusters can be calculated. This is reasonable if, as sketched in fig. 1, a constant slope over several layers indicates a constant cluster size distribution. The Vollmer Weber (VW) mode is the same as the SK mode without a complete ML in the first layer. In fig. 1 a rather similar slope is taken arbitrarily for the VW and SK modes in the cluster range. In general, the cluster distribution can be quite different. It should be noted that from the experiments the SK mode turns out to be the most common case. From basic thermodynamics this is quite understandable. The quantity to be considered here is the specific surface free energy T. Neglecting edge energies as well as shape and size dependencies one has to consider

which is the difference between the specific surface-free energies of the adsorbate (A), the interface (I), and the substrate ( S ) . For A > o the VW mode is the equilibrium mode, whereas A s o holds for the SK and the FM mode. Bauer

379

(6) has pointed t o the f a c t t h a t f o r the FM ( l a y e r by l a y e r ) mode A s o has t o be f u l f i l l e d f o r each new layer. This w i l l be the case o n l y when adsorbate and s u b s t r a t e are very s i m i l a r :

TA

= TS, TI = 0. From t h i s simple argument i t be-

comes c l e a r t h a t t h e SK mode i s t h e most l i k e l y one.

It has t o be noted t h a t

f o r p r a c t i c a l cases the above argument i s not very h e l p f u l : Exact

‘I

values are

n o t known i n most cases, and the e q u i l i b r i u m c o n f i g u r a t i o n may n o t be reached, since t h e m o b i l i t y o f the incoming atoms may n o t be l a r g e enough. Thus, pending on substrate temperature, observed,

de-

d i f f e r e n t metastable c o n f i g u r a t i o n s may be

and i t has t o he proven experimentally which growth mode occurs i n

each case.

Fiq. 2: Auger i n t e n s i t i e s from Mo substrate and Pd adl a y e r as f u n c t i o n o f evapor a t i o n steps. From (8).

0

5

10 15 NUMBER OF 1 mnn DOSES OF Pd

20

25

From an experimental p o i n t o f v i e w i t i s most d i f f i c u l t t o v e r i f y t h e FM mode, since some s c a t t e r o f experimental p o i n t s may round o f f t h e breaks i n t h e Auger

curves.

The most important p o i n t

i s to

have a l l

settings f o r

the

evaporation source, the AES detector and sample p o s i t i o n w e l l reproducible. The best curves are published by Bauer and h i s coworkers. One example i s shown i n f i g . 2. I t e x h i b i t s r e a l l y s t r a i g h t l i n e s f o r the Is(Mo) as w e l l as f o r IA(Pd). I n o t h e r cases, where a d d i t i o n a l data are c o l l e c t e d depending on thickness, t h e s c a t t e r of t h e data may increase g r e a t l y . One o f these examples i s presented i n

3, where a k i n d o f SK mode i s demonstrated f o r Ag/A1(111)

fig.

(19).

from our group

One c l e a r l y recognizes a break a f t e r one ML and a s t r a i g h t l i n e up t o

about 4 ML. By evaluating the data i n d e t a i l we found a completion o f t h e M L up

t o 87% before the second l a y e r s t a r t s growing and estimated the c l u s t e r on t o p

o f t h e f i r s t ML t o be 3-4 l a y e r s t h i c k . As Rhead e t a l .

(3) have pointed out t h e r e are several o t h e r more com-

p l i c a t e d growth modes possible besides t h e three simple cases o f f i g .

1. For

380

rt

I

1

I

I

I

I

Fiq. 3: Auger intensities for Ag layers vapor deposited onto Al(111) as function of evaporation time. The kinetic energies of the Ag and A1 transitions are given in parentheses. The sample temperature was 300 K. From (19).

I

I C

200

-

LOO 600 EVAPORATION TIME I s

800

instance, it is quite possible that real exponential behavior is exhibited over the whole thickness range or starting with the second layer for the SK mode, if there is simultaneous multilayer growth. This mode can be operative for high supersaturation and low substrate temperature when the atoms may adsorb at the site, where they hit the surface, Furthermore, the discussion above breaks down, if the sticking coefficient varies with thickness (which is, on the other hand, very unlikely for metallic adsorbates and rather low substrate temperatures). Another underlying assumption is the neglect o f interdiffusion, which has to be checked in each case. 3

NOBLE METALS The noble metals Cu, Ag and Au are the most frequently studied among the thin metal film systems. This is certainly due to the importance of these metals as electrical conductors. Besides this, the investigations are facilitated by the chemical inactivity of these metals. In the following we discuss each of the three metals separately. Some conclusions are drawn in section 3.4, where some tables of BE are presented. 3.1 Comer There are two major ARUPS studies of Cu on Ag(001) (21,22) which confirm pseudomorphic growth up to a thickness of 2-3 Cu layers. These Cu layers are expanded by 13 % relative to bulk Cu. The large misfit induces more strain than can be sustained by the adlayer during FM (layer-by-layer) growth. Therefore, after 2-3 ML the system switches to the SK mode as can be seen from the deterioration of the LEE0 pattern and the shallow slope of the AS curves.

381

Fiq. 4: Series of AR spectra taken along the Z azimuth for 1 ML Cu on Ag(001). The photon energy is 30 eV. Three bands of features (indicated by the tick marks) are attributed t o Cu 3d states and are seen to disperse in energy as 0 i s varied. The Cu 3d origin of the features is established by comparison with data from clean Ag and from Ag with 2-6 monolayers of Cu coverage, Emission at 0 = 40" originates near the M symmetry point. From (21).

-6

-a

-4

Energy

-2

(eV)

0

Fig. 4 exhibits a series of ARUP spectra taken in the TM azimuth for different angles of emission 0 (21). Especially, above the upper edge of the Ag 4d emission of the substrate at about 4 eV, well resolved Cu-derived peaks can be seen exhibiting some dispersion. The 20 band structure is presented in fig. 5, as it is deduced by the authors from all measured spectra including those of fig. 4. By using well suited angles and photon energies also states near 5 eV are identified as Cu states which overlap with the Ag bands from the -

-

-

Ag(OO11* 1 ML

Cu

-I

Fiq. 5: 2D band structure of the Cu monolayer. Features displaying clear A 1 or ’c2 symmetry are indicated by solid circles while open circles represent A1 or Z7 states. The remaining features are plotted as triangles or, in the case o f unresolved peaks, as error bars soannina the Drobable ’peak posit\ons. Bands are drawn through the points in qualitative agreement with theory (23,24). Bands with C2 or A2 symmetry are indicated by dashed lines. The shaded area indicates the projection o f the Ag 4d bulk band structure. From L

4

l

2

'.

I -4

-5 I

I5

10

05

0

05

I0

-

(21)

-

382

substrate. For the ML only peaks are taken, which increase in intensity also for the 2 and 3 ML thicknesses. Other Cu-induced features are not discussed, since in the Ag 4d region there is no way to differentiate between redistributed Ag features and real interface states. The former effect may arise from increased scattering due to the higher density of defects at the interface compared to the clean Ag(001) surface. Thus, it is interesting to note that (a) the derived band width of the Cu ML (3.15 eV) is essentially identical to that reported for bulk Cu (3.20 e V ) , and (b) the bands are more tightly bound than bulk Cu bands by only 0.25 eV. These findings agree best with a calculation for a Cu ML on Ni(001) (23) indicating that the lattice constant is only o f minor importance for the 20 band structure. For the same system (Cu/Ag(001)) Smith et al. (22) arrive at a somewhat different 20 band structure, which i s shown in fig. 6. There is a further band resolved nearer to EF and, more important, the total width of the ML bands is only 1.5 eV and lying completely above the substrate 4d emission. This finding led the authors to vary the result of a ML calculation (24) in order to explain their result as pointed out in the caption of fig. 6. Fiq. 6: Energy bands of the

mono1 ayer on Cu (100) Ag(100). Full curves, LCAO bands calculated by Smith et al. (24) for an isolated Cu(100) monolayer, after reducing the energy dispersion by a factor o f 1.8 in order to correct for the Cu lattice expansion and rigidly shifting the bands 1.2 eV away from the Fermi level. From (22).

I

R

r

1 X

The second well defined Cu adlayer system is Cu/Ru(0001) (25-27), which has the advantage of being composed of two immiscible components. Cu grows pseudomorphically up to one ML on the Ru(0001) surface with a 5 % tensile strain with respect to the Cu(ll1) bulk lattice, which is largely reduced compared to the Ag(001) surface. Houston et al. (25) have been able to separate a true interface state which is, according to their slab calculations, localized in the Cu and outermost Ru layers. Guided by their calculations they have been able to separate this state from the Ru substrate emission near the ?t point at 1.5 eV

383

Fiq. 7: ARUP spectra taken with He1 radiation at normal incidence and an electron emission angle of 52" for Cu on Ru(0001) are shown as functions of Cu coverage. The intensity of the various curves has been normalized at the Fermi level, EF. The individual curves are matched to their corresponding Cu coverages in monolayers by the solid lines while the saturating behavior of the interface state at approximately -1.4 eV is identified by the dashed lines. From

(25).

-6.0 -5.0 4.0 -3.0

-2.0

-1.0

0.0

BINDING ENERGY-OV

below EF, as shown in fig. 7. From this figure it is also seen that the Cu ML emission evolves at nearly the same energy as the Cu 3d emission for higher coverages. Houston et al. did not try further t o extract a 2D band structure. They stressed that for 1 ML or less "the Cu 3d levels mix stronqly with the Ru 4d states". This may become even more clear from their calculated bands as shown in fig. 8. They point out that without a strong interaction between the first Cu layer and the Ru substrate a pseudomorphic growth with a 5 % tensile strain would be difficult to understand. They felt this to be in contradiction

aa

9 s w

-20

F

-

persions along the r - K s.ymEfmetry line for a five-layer Ru(0001) film covered on both ’faces by a 1 ML 1x1 Cu overlayer. States indicated by ,. heavy lines and arrows are ’strongly weighted on the outer Cu overlayers and first underlying Ru layers of the film.

384

Fiq. 9: ARUP spectra (normal emission, mixed s/p polarization) from Cu/Ru bilayers as a function of the Cu coverage (OCU= 0.38... ? 5 ML), at a fixed photon energy of 30 eV. Since only the Cu d-band position is to be shown, the curves have not been normalized with respect to their absolute intensity. The dotted line represents the spectrum o f the uncovered Ru(001) surface. The Cu was evaporated with the substrate at 1000 K. From (26).

\ .- _.

ML

038

, - 00 I

-1 E -0 F initial energy lev 1

-8 -7 -6 -5 - L -3 -2

to the analysis o f Vickerman et al. (26), which found the Cu 3d and Ru 4d emission just added "without any hint of a stronq electronic interaction". Fig. 9 presents some of their data (26). They evaluated also a 20 band structure as shown in f i g . 10 and found their results of the ML to be in agreement with the work of Richter e t al. (27). q.0

-

-% -1

,

I

1

Fiq. 10: 20 band structure of the Cu film on Ru(0001). The open circles refer to a 0.82 ML film ( h a = 28 eV). The squares (ha= 30 eV) and the diamonds ( R w = 50 eV) refer to a 2 ML Cu film. The solid circles indicate the position of the Cu d-bands as obtained experimentally in (28) for Cu on NixCui-x(ll1). From (26).

%

m

$ -2 e

-2 -3 .-

.-C

-c

-5

P parallel momentum

ii,,

385

In order to separate the adlayer from the substrate state an interesting experiment was performed by Shek et al. (29) for Cu on Pt(ll1). From LEEO and AES they argued that the ML grows pseudomorphically, giving rise to a tensile stress of 9 %.By using synchrotron radiation at ho= 150 eV they suppressed the Pt 5d emission, since the Cooper minimum lies at this energy for Pt. They observed the Cu emission at 2.65 eV and a weak shoulder at 3.5 eV developing for coverages between 0.75 and 1.0 ML. From their core-level measurements they concluded that the Cu adatoms are essentially neutral in spite of the large electronegativity difference between Cu and Pt. The work function, as measured from the PE EDC’s, decreased by 1.26 eV during completion of the ML. We turn now to what we believe is another class of substrates - the spmetal substrates. Fig. 11 presents the result of the pioneering work of Abbati et al. (30). Cu was deposited onto a freshly cleaved Zn(0001) surface. LEEO indicated a pseudomorphic adlayer, but an AES study was not performed at that time. In agreement with their tight-binding calculation they found for the Cu ML a shift of 1.2 eV to larger binding energies and an appreciable narrowing of the 3d emission. These results have been quite consistently interpreted: The band narrowing in the ML is due to the decreased number of neighbors from 12 to 6 with respect to bulk Cu. The distance between EF and the 3d band is increased. since the average density of states in the lower part of the conduction band is decreased. Thus, the shift to higher BE may be seen as a consequence of the SP charqe decompression at the surface with respect to the bulk. The final d occupancy was found to be 9.963, i.e. larger than the value 9.886 obtained for the bulk. Thus the Cu species is more atomic in the over1 ayer

.

Fiq. 11: Energy distribution curves for (a) one Cu ordered monolayer on Zn(0001) face, (b) about one ML’ of Cu on a polycrystalline Zn film, and (c) thick Cu layer. Theoretical results are given for (d) the local density of states of a Cu overlayer and the total density o f states of (e) Cu and Zn, (f) bulk Cu, and (9) the isolated monolayer. From

-

(30)

386

It is worthwhile to explain in more detail what is meant by "sp charge decompression". It is connected to the normalized atom approach (31) pointing back to the beginning of band structure calculations (31). For example, let us consider the transition from the Cu atom with its 3d10 4s1 configuration to bulk Cu. By the interaction in the bulk both the 3d10 and the 4sl level broaden into bands and overlap strongly. Furthermore, the center of gravity of the d band is shifted to smaller energies by more than 5 eV so that the bottom of the s band (which is actually an sp band due to the admixture of states) falls well below the bottom of the d band. The local charge distribution around the atom is also changed. The sp states which are spatially more extended than the d states, are somewhat compressed into the atomic unit cell in the bulk. Introducing now a surface means a "sp charge decompression" into the vacuum. Within this model the d band should move back to higher energies as it is observed for the Cu ML here (30). It is interesting to note that a quite similar result has been found for Cu on different surfaces o f Al, which belongs also to the group of sp-metal substrates. Di Castro and Polzonetti (32) have found a ML emission peaked at 4.2 eV, which shifts to 2 eV for thicker layers. They measured on polycrystalline A1 films. From their Auger intensities as function of thickness they deduced some interdiffusion between A1 and Cu for the first layer. An ARUPS investigation for Cu on Al(111) was performed by Barnes et al. (33). From their Auger intensities versus thickness curves they concluded that

INITIAL

ENERGY

EiieV)

'

Fiq. 12: ARUP spectra at normal emission from Cu films grown on Al(111) at Ts = 300 K. Also shown is emission from a semi-infinite Cu(ll1) single crystal. Incident radiation: He1 (ha= 21.22 eV). From (33).

e s s e n t i a l l y l a y e r growth takes place below 300

K besides some i n t e r f a c i a l mix-

i n g i n t h e ML. Fig. 12 e x h i b i t s t h e main r e s u l t o f Barnes e t a l . (33). There i s a whole t r a n s i t i o n r e g i o n from the ML t o about 10 ML w i t h i n which t h e Cu 3d

emission i s s h i f t e d towards EF by about 1.5 eV. The C u ( l l 1 ) 3d band has been

developed o n l y a t about 10 ML. The broad ML and sharp 10 ML-ARUP spectra a r e i n good agreement w i t h t h e long-range disorder f o r t h e ML and a weak C u ( l l 1 ) LEED p a t t e r n f o r 10 ML. Finally,

i t i s i n t e r e s t i n g t o note t h a t these r e s u l t s are i n very good

agreement w i t h o l d e r angle-integrated XPS measurements (34). 3.2 S i l v e r Among t h e noble metals, Ag i s most i n t e n s i v e l y studied and t h e Ag ML i s best defined w i t h respect t o i n t e r d i f f u s i o n and a l l o y formation. Tobin e t a l . (35,36)

i n v e s t i g a t e d Ag on Cu(OO1). Ag forms a hcp ML g i v i n g r i s e t o a ~ ( 1 0 x 2 )

s t r u c t u r e as i n d i c a t e d i n f i g . 13. Normally two domains develop so t h a t t h e FR and % d i r e c t i o n s

cannot be separated i n t h e (110) plane.

For one surface t h e

authors c l a i m t h a t they have been able t o prepare a s i n g l e domain Ag adlayer (36).

I n fig.

14 some normal emission spectra are shown f o r C u ( l l l ) ,

Ag(ll1)

and Ag overlayers o f d i f f e r e n t thickness. The BE are given i n t h e 4 t o 5 eV interval for different

photon energies. For normal emission

kll= 0 and kl

is

REAL SPACE

RECIPROCAL SPACE

F i q . 13: Depiction o f one o f t h e two o r thogonal domains o f c(10x2)Ag/Cu(001) in r e a l space. The Ag atoms are shown as f i l l e d c i r c l e s and t h e Cu(OO1) surface l a t t i c e as squares. The a c t u a l r e g i s t r y w i t h t h e subs t r a t e i s unknown. The s u r f a c e - B r i l l o u i n zones of Cu(OO1) and both u n d i s t o r t e d hexagonal Ag domains as w e l l as t h e paths across each zone taken when r o t a t i n g o f f normal i n t h e Cu(OO1) planes (110) and (100) are shown. Only t h e domain associated w i t h (c) was observed w i t h LEED. From (35).

388 s-pol

23-eV Photon energy

He I

m

,

r

.-VI

L

o(

L

U C

-e Q

h .In f

m

c

E -

Binding energy (eV)

I l l 10

5

I

,

,

I

I

I 1

EF

Binding energy (eV) Fiq. 14 (left): Mapping of the binding energies (BF) o f the silver features vs photon energy for (a) Ag(lll), (b) 5 ML, (c) 4 ML, and (d) 2 ML o f c(lOxZ)Ag/ Cu(OO1). The band i i i states (BF > 4.5 eV) at 2 ML become bands 4, 5, and 6 in Ag(ll1). The weak leading shoulder in Ag(ll1) at BF near 4.2 eV is shown with open circles and i s due to band iv. (e) Normal-emission spectra collected with hw = 23 eV for curve A Ag(lll), curve B 5 ML of c(10x2)Ag/Cu(001), curve C 4 ML, curve 0 2 ML, and curve E clean Cu(OO1). From (35). F i q . 15 (riqht): ARUP spectra taken of clean Cu(OO1) (lower member o f each pair) and 1) ML of c(lOxZ)Ag/Cu(001) (upper member of each pair), with s-polarized He1 radiation. The angle listed is the polar emission angle 0 versus the surface normal. Each spectrum is normalized to the largest Cu d-band peak. From

(36).

389

Fiq. 16: 20 band structure for Ag on Cu(100) observed at near-monolayer coverages. The triangles at BF near 4.8 eV at ?; are the averaged values of the spin-orbit split peaks observed with s- and p-polarized He1 and NeI radiation.

I

Y

10

08

06

k II

-8

a4

02

02

I

-6

I

06

04

08

I

-4

I

I

-2

I

INITIAL ENERGY(&)

10

I Z Z 14



k II



L 6 0 I

EF

,

Fiq. 17 (left): ARUP spectra from 1.2 A (1/2 monolayer) of Ag on Ni(001) taken with a photon energy ho= 22 eV and kl along Ni[llO]. The polar emission angles 0 are indicated. Features which are due to the presence of the Ag overlayer are marked with arrows. The inset shows a schematic drawing of the Brillouin zone of monolayer Ag(ll1). From (37). Fiq. 18 (riqhtl: Comparison of the 20 band structure a 0.5 ML Ag on Niflll) (data indicated by crosses: energy scale on the left-hand side) to the 2D dispersion relations from a near-monolayer coverage o f Ag on Cu(OO1) (data indicated by squares; energy scale on the right-hand side). In both cases k, is along the direction o f the Ag overlayer. The smooth solid and dashed curves are proposed band dispersions of bands 1-6. From (37).

390

varied with ho. It is necessary for a 2D band structure that the dispersion relation be independent of kl. Fig. 14 indicates that this requirement is nicely fulfilled for the 2 ML features between 4 and 5 eV. Fig. 15 presents some ML spectra for s-light from a He1 laboratory light source. The ML spectra are compared with the clean Cu(OO1) spectra indicating a fairly large overlap between Ag and Cu states down to an energy of 5 eV. Thus, this system certainly demanded some care to unambiguously figure out the 2D band structure of the Ag ML as shown in fig. 16. The investigations of this kind seem to be at the very beginning, where trends have to be figured out first. Therefore, the study of Shapiro e t al. (37) is very useful, in which they measured the Ag films on Ni(ll1) and Ni(100) substrates. Surprisingly, they found the 2D band structure to be practically identical for both substrates. With respect to the results for Ag on Cu(100) note the normal emission spectrum of fig. 17, which exhibits only one strong peak for both substrates. Fig. 17 demonstrates also that Ni is better suited as substrate than Cu, since its 3d states are better separated from the Ag 4d adlayer states. Interestingly, Shapiro et al. have been able to demonstrate that the 20 band structures of Ag on Ni and Ag on Cu are very similar, if one admits a rigid shift in energy of only 0.32 eV to higher BE for Ag on Cu as shown in fig. 18.

Fiq. 19: Normal-emission ARUP spectra taken with a photon energy of 22 eV from Cu(ll1) covered by various thicknesses of Ag as indicated. From (38).

J

1.8

I

0.6

l

0.4

l

0.2

I

EF

Binding Energy (eV)

Besides the 20 band structure of the adlayer, the evolution of surface states is very important for an understanding o f the electronic structure o f thin metal films. This question was studied carefully for Ag on Cu(ll1) (38)

391

which grows e p i t a x i a l l y w i t h a 13 % l a t t i c e contraction. We w i l l comment on t h e very l a r g e amount o f t h i s c o n t r a c t i o n l a t e r . Fig. 19 e x h i b i t s the surface s t a t e a t t h e L-gap and i t s changes from C u ( l l 1 ) t o A g ( l l 1 ) . For the 0.5 ML f i l m two peaks can be seen, one from the bare C u ( l l 1 ) surface and one f o r t h e Ag ML. The continuous s h i f t o f the Ag surface s t a t e w i t h l a y e r thickness i s explained by t h e degree o f i t s l o c a l i z a t i o n . Two o t h e r substrates have been used t o support Ag f i l m s , namely Pd(100) and

P t . Supported by Pd(100) Ag forms a p ( l x 1 ) e p i t a x i a l adlayer i n t h e range between 1 and 10 ML (39). The ML spectrum e x h i b i t s one strong Ag peak near 4.5 eV (Fig. 20). One recognizes t h a t t h e bulk spectrum develops r a t h e r l a t e a t about 10 ML. Capehart e t a l . (40) have found a two-peak spectrum f o r Ag on Pd(100) a t the

7

point.

By comparing these data w i t h t h e i r c a l c u l a t i o n s they have been

a b l e t o d e r i v e t h e symmetry o f the peak, which i s a l ( = d3z2-r2) peak and e(= dxy,yz)

f o r the 4.6

f o r t h e 6 eV

eV peak. A s i m i l a r double peak i s found on

Pt(100) and P t ( l l 1 ) (41).

F i q . 20: ARUP spectra measured normal t o t h e surface o f an Ag/Pd(100) overlayer system a t v a r i o u s coverages o f Ag. Photon beam o f 21.22 eV was i n c i d e n t a t t h e angle o f 15" r e l a t i v e t o t h e surface normal i n t h e (001) plane. From (39).

-800

-600

-LW

E, leVi

-200

J

-OW

Now we t u r n again t o t h e sp-metal substrates.

I n f i g . 21 we show the A1 XPS

r e s u l t o f Egelhoff Jr. (42) f o r Ag on Al(100). For t h e sub ML species the Ag 4d emission i s centered a t about 6.4 eV. These measurements agree very n i c e l y w i t h more r e c e n t r e s u l t s f o r Ag on A l ( 1 1 1 )

from our l a b o r a t o r y (19).

From c a r e f u l

AES a n a l y s i s we found SK growth mode w i t h a w e l l defined Ag ML. e x h i b i t s t h e t r a n s i t i o n from 0.8 ML t o bulk Ag(111),

evidenced by LEED. Between the ML, which i s pseudomorphic, Ag(ll1) f i l m a t

Fig.

22

the l a t t e r being f u r t h e r and t h e e p i t a x i a l

O > 10 ML t h e r e i s a broad t r a n s i t i o n r e g i o n w i t h o u t any LEED

392

Binding Energy, eV Fiq. 21: Valence band XPS spectra f o r Ag on Al(100) a t t h e i n d i c a t e d Ag t h i c k nesses. From (42).

F i q . 22: ARUP spectra f o r Ag l a y e r s o f d i f f e r e n t thickness on an A l ( 1 1 1 ) subs t r a t e . The spectra were taken a t normal emission. Angle o f incidence o f t h e l i g h t (ho=21.2 eV) was 45" w i t h respect t o t h e surface normal. From (19).

ENERGY BELOW EF(eV)

393

i

I

ii -

1.5

;

@ I0

1 ML A g on A l ( 1 l l )

D

HF

uI

:

212 ev

I

: 6 a e~

r

0.5

0

4

1

M

05 k,, [k’l

10

1.5

Fiq. 23: 20 band s t r u c t u r e o f an Ag ML on Al(111). From (19).

p a t t e r n and w i t h a broad PE spectrum as shown i n f i g . 22. The ML s t a t e e x h i b i t s r a t h e r sharp, d i s p e r s i n g peaks. From t h e whole s e t o f d a t a a 20 band s t r u c t u r e was derived as shown i n f i g . 23. The 20 Ag band s t r u c t u r e found f o r A l ( 1 1 1 )

is

much s i m p l e r than those found f o r t h e Cu and N i substrates as shown above.

3.3 G o l d The e l e c t r o n i c s t r u c t u r e o f Au f i l m s i s l e s s f r e q u e n t l y studied than t h a t o f Cu or Ag f i l m s . As d-metal substrates o n l y Pd (43), P t (41), (45-48)

have been used. The data from Au on P d ( l l 1 )

W (44) and Cu

(43) are measured i n an

angle-integrated mode and are n o t backed by an a d d i t i o n a l AES o r LEE0 analysis. The ML i s c h a r a c t e r i z e d by peaks a t 1.5,

3.1,

4.3,

and 5.9. From (43) one r e c -

ognizes t h e strong overlap between the Pd substrate- and Au o v e r l a y e r - s t a t e s so t h a t o n l y t h e peak a t 5.9 eV i s w i t h o u t doubt mainly Au 5d derived. The s t r o n g i n t e r a c t i o n between Au and Pd manifests i t s e l f a l s o by Au d i f f u s i o n i n t o Pd,

which can be achieved by a 30 s anneal t o 670 K. The r e s u l t i s a s h i f t o f t h e

Au 5d emission t o smaller BE by several t e n t h s o f an eV. From t h i s change t h e authors (43) assume i m p l i c i t l y t h a t t h i s d i f f u s i o n s t a r t s o n l y a t e l e v a t e d temp e r a t u r e s and n o t already a t 300 K. The spectra f o r A u / P t ( l l l )

(41) taken i n an angle-integrated mode e x h i b i t

f e a t u r e s a t 1, 3, 4 and 5.8 eV BE f o r t h e Au ML. I t i s i n t e r e s t i n g t o note t h a t t h e r e s u l t s f o r Pt(100) and Pt(997) substrates g i v e n by t h e same authors look v e r y much t h e same as f o r P t ( l l 1 ) .

On W(110)

Au forms an ordered p ( l x 1 )

394

overlayer which is compressed by 3.4 %. This compression is released at a thickness of 3-4 ML, when the LEED pattern vanishes. By XPS a rather broad twopeak structure is found for the ML with peaks at about 4.0 and 6.2 eV (44). In this work also the Au 4f levels have been studied carefully as shown in fig. 24. For the ML the 4f BE is shifted by 0.31 eV to higher energies, i.e. opposite t o the surface core-level shift. The reason for this shift is not clear at the moment. It may be due to a strong hybridization o f the Au and W valence orbitals or - with respect to bulk Au - to a reduced shielding by the 2D Au 6s band.

Fiq. 24: Gold 4f spectra for various coverages of Au on W(100). (a) 0.8 ML; (b) 2.4 ML; (c) thick Au layer. From (44).

BlNOlNG ENERGY teV1

We turn now to the Au/Cu(001) system, which has been studied very recently different groups (45-48). The investigation of this pair goes back to the pioneering work of Palmberg and Rhodin (1) who interpreted their ~(2x2)structure found at room temperature as a Cu Au surface alloy. They also showed by LEED that the transition into the ~(2x2) structure proceeds at temperatures above 250 K. Very recently Graham (45) found by low energy ion scattering that the Cu(lOO)-c(2~2) Au surface is very similar to the Cu3Au(100) surface. Wang et al. (46) came to the same conclusion by a LEEO analysis. In fig. 25 (45) some spectra are compared for Cu(lOO), Cu(lOO)-c(2~2)Au, and Cu3Au(100). For He1 and O = 0” (normal emission) the intensity of the Au 5d states is weak compared to the Cu 3d substrate emission. Furthermore, the similarity between the Cu(lOO)-c(2x2)Au and the Cu3Au emission is obvious. At the r point the B E ’ S are 5.2 and 6.3 eV. These states exhibit a weak dispersion of by

-

395 I

I

I

I

I

I

I

I

He1

"Cu 1100l-c~ZX21Au'

i'i W

-

t

cu11001

-

I I I I I I I I I

8

4

0

BINDING ENERGY ( e V )

8 4 0 BINDING ENERGY (eV)

Fiq. 25: ARUP spectra o f Cu3Au(100), "Cu(lOO)-c(2xZ)Au" and Cu(100). The emission i s normal t o the surface ( l e f t ) and off-normal i n the (010) plane ( r i g h t ) From (45).

about 0.4 eV. A t t h e M p o i n t a surface s t a t e i s found a t 1.6 eV which i s known t o l i e a t 1.8 eV f o r Cu(100). The greater width o f the Au 5d bands, i.e. extension down t o g r e a t e r BE,

their

i s i n agreement w i t h r e s u l t s from o t h e r authors

(46) f o r photon energies between 24 and 40 eV. For the same system Knapp e t a l . (47) have v a r i e d the thickness o f t h e Au layer. They have found an ordered ML a t 220 K e x h i b i t i n g a ~ ( 1 2 x 2 ) LEE0 p a t t e r n i n analogy t o t h e c(1Ox2) p a t t e r n f o r Ag on Cu(100).

As shown i n f i g .

have analyzed t h e B E ' S as f u n c t i o n o f Rw. Obviously,

26 they

the sample temperature

was 300 K throughout these measurements so t h a t a l l t h i c k e r l a y e r s grow on t o p o f t h e ~ ( 2 x 2 ) reconstructed one. V a r i a t i o n o f the photon energy is very h e l p f u l i n separating 20 from 30 bands. For normal emission the 20 d i s p e r s i o n is e l i m i -

BE i n d i c a t e s emission from a bulk (30) sample. Therefore, f o r 30 bands a d i s p e r s i o n w i t h hw i s expected and can be c l e a r l y seen f o r Cu(100) (OML) and A u ( l l 1 ) i n f i g . 26. For 2 ML, a s t a t e a t 4.7 eV evolves, which i s 2D i n character. The s t a t e a t 6.5 eV i s 20 f o r a 1 ML coverage b u t not f o r 2 o r 3 ML. According t o t h e authors t h i s nated, since t h e momentum i s zero. Thus, any v a r i a t i o n o f

i s due t o a change i n mean f r e e path o f t h e outgoing photoelectrons. It i s i n -

t e r e s t i n g t o note t h a t the bulk A u ( l l 1 ) photoelectron spectra are seen o n l y f o r thicknesses 2 12 ML.

396

-% 5-

. . . . . . . . ... . + ......... ................................

1 .-

I '

lm[.

..... . .. ..*...... .. .... ....:

2.01..

8 -

w

.

..

L. ..........?...........L...........

2MLiML;:.

*

3M_t it . 3

*.

6Mcj

1 2W 12w[

6 Mc'

i

Ad11111 J

J

. ................... .......... "’.I ......... .: . ....... ...... i ..... ........ ...... '. i ......:" . .. . . . . ........ ._ j . . . . . . ..

.(

4.0-

6.0-

a.

.

I

1..

1

PHOTON D E W Y (eVJ

Fiq. 26: P l o t o f Au/Cu(100) BE r e f e r r e d t o EF versus photon energy f o r 0, 1, 2, 3, 6 and 12 ML coverages. From (47).

I n a recent paper t h e same group (48)

has measured t h e unreconstructed

Au ML on a Cu(100) surface e x h i b i t i n g t h e ~ ( 1 4 x 2 ) s t r u c t u r e below 220 K. As a r e s u l t o f t h e 2D band mapping three bands are observed a t 6.3 eV

(7)

(2).

and 3.5 eV

Furthermore,

(T), 4.7 eV

the Au 4 f core l e v e l s s h i f t by 0.33

higher BE during the t r a n s i t i o n from p(lx1)

eV t o

a t 190 K t o ~ ( 2 x 2 ) a t 300 K

observed f o r the 0.5 ML coverage. I

1

I

I

1 1 - 1

I

1

I

.. . .,-.. ,

.. , .. .

.

.

. . .. ...... . . .._..-.. . ..... : . .... . . . . . ... . ...-. . . .. -. ,..._-.._ ..,.,........... .__.... . . -.r.:

F i q . 27: ARUP spectra f o r Au l a y e r s on A l ( 1 1 1 ) . The spectra are taken a t normal emission f o r W o = 21.2 eV. (a) 1 ML o f A12Au on top o f Al(111); (b) 1 ML A12Au

i

I

p l u s 1 ML Au; (c) intermediate s t a t e ; (d) bulk Au thickness l a r g e r than 10 ML. From

(49).

.......... I

I

I

I

I

5

I

I

...-. I

1

L

OPE,

BINDING ENERGY lev)

(a1

397

Now we t u r n t o the second group o f substrates, the sp metals. Egelhoff has found q u i t e d i f f e r e n t spectra f o r Au adlayers on Al(100) (34). The sub ML emiss i o n i s centered a t 7 eV w i t h an FWHM o f 4 eV i n p a r t due t o t h e reduced energy r e s o l u t i o n o f XPS. Recently, we have found several d i f f e r e n t Au 5d spectra on a

A1 (111) surface depending on coverage and evaporation c o n d i t i o n s as i n d i c a t e d i n f i g . 27 (49). The LEED p a t t e r n i n combination w i t h r e f l e c t i o n e l e c t r o n mic-

r o s c o p i c studies f o r the bulk Al2Au a l l o y from the l i t e r a t u r e i n d i c a t e d t h a t i n the ML an A12Au(lIO) l a y e r was formed. The r e s u l t o f the ARUPS study o f f i g . 27 i s t h a t t h e Au 5d l e v e l s s h i f t t o smaller BE w i t h an increasing l a y e r t h i c k ness,

i.e.

an increasing Au 5d overlap.

Thus,

s t a t e (b) i n f i g .

understood as an Au ML on top o f the f i r s t A12Au a l l o y layer.

27 can be

T h i s geometry

leads t o a Au 5d overlap s i m i l a r t o t h a t i n a AuAl a l l o y . State (c) i s i n t e r mediate between the Au ML and bulk Au, as was found i n a s i m i l a r k i n d o f Ag on

Al(111)

(19).

F i n a l l y , a t about 10 ML t h e bulk A u ( l l 1 ) spectrum i s measured.

Q u i t e obviously, t h e l a t t i c e mismatch between A12Au and Au prevents an ordered phase i n t h e t r a n s i t i o n region.

3.4 Conclusions about noble metals We have noted already t h a t o f a l l metals the noble metals have been most e x t e n s i v e l y studied as t h i n m e t a l l i c f i l m s . Therefore, one can t r y t o evaluate some general conclusions. For t h i s purpose we have summarized the BEF values i n t a b l e s 1 t o 3.

For the angle-resolved mode the values a t

are taken. We are

t a k i n g i n t o account mainly the experimental r e s u l t s here and comment on theor e t i c a l r e s u l t s i n t h e summary below. As we have already pointed out, there are two groups o f substrates, t h e dand sp-metal substrates, which induce a q u i t e d i f f e r e n t behavior o f t h e noblemetal adlayer.

For the sp-metal

substrates the BEF are s h i f t e d g e n e r a l l y t o

higher values. S o , t a k i n g t h e most intense peak a t 2.7

eV t o 4.2

7.0 eV f o r Au.

eV f o r Cu, from 4.6 eV t o 5.7

?, these

mean s h i f t s a r e from

eV f o r Ag,

and from 6.2

eV t o

I t seems t h a t t h e i n t e r a c t i o n o f t h e noble metals w i t h d-metal

substrates i s stronger than w i t h sp-metal substrates. For most o f t h e Ag and Au adlayers t h e r e i s a c o n t r a c t i o n o f the ML w i t h respect t o the b u l k i n t e r a t o m i c distances. The reason f o r t h i s c o n t r a c t i o n may be two-fold.

(a) I t may be i n -

duced by a tendency t o b i n d the adatom t o s p e c i f i c substrate-surface s i t e s . (b)

It may a l s o w e l l be t h a t a metal ML tends t o develop i t s own next-neighbor d i s tance, which i s smaller than i n the bulk. This can be understood by the reasona b l e i d e a t h a t t h e bonding t o neighbor atoms i n t h e l a y e r s above and below t h e ML weakens t h e bond strength w i t h i n the ML. We have found an example which exh i b i t s such behavior. Ag grows pseudomorphically on an A l ( 1 1 1 )

surface,

until

a t about t h e complete ML coverage i t becomes f u r t h e r contracted from about 1 % i n t h e pseudomorphic phase t o 5.6 % i n t h e compressed phase (5O).This

tendency

398

o f the Ag ML t o c o n t r a c t by about 6 % may a l s o e x p l a i n the unusually l a r g e c o n t r a c t i o n o f 13 % f o r Ag on C u ( l l 1 ) which was reported above. There are only few examples f o r a compression i n a r e l a t i v e l y weakly bound ML (51),

which can

be due t o the f a c t t h a t most experimental examples are f o r d-metal substrates.

It may be i n t e r e s t i n g t o note t h a t by a recent t o t a l energy c a l c u l a t i o n a l a t -

an unsupported A1 l a y e r (52). I n an experiment one i s n o t able t o work w i t h an unsupported ML. Therefore, a d e l i c a t e t i c e c o n t r a c t i o n o f 7 % was found f o r energetic balance has t o be sustained.

I n order t o prepare a ML t h e r e must be a

l a r g e enough adatom-substrate i n t e r a c t i o n t o prevent 3D c l u s t e r growth; b u t n o t

so l a r g e as t o prevent t h e overlayer from contracting. Coming back t o t h e d i f f e r e n c e i n BEF on d- and sp-metal substrates, we propose t h a t t h i s m a y be i n p a r t due t o the l a r g e r c o n t r a c t i o n o f t h e adlayer on a d-metal

substrate.

This c o n t r a c t i o n counteracts the decompression o f t h e sp

electrons, which i s a general property o f the monolayer and which t o Abbati e t a l .

(30)

-

-

according

induces the downward s h i f t o f t h e BE. This counteract-

i n g compression seems t o be so e f f e c t i v e t h a t t h e observed 2D band widths are, s u r p r i s i n g l y , as l a r g e as the 30 ones, as noticed by many authors. A t t h e moment, we have t o admit t h a t i t i s n o t clear, t o which e x t e n t t h i s explanation i s c o r r e c t o r whether other mechanisms are more important.

One c e r t a i n l y has

a l s o t o consider the d i r e c t adatom d-level t o substrate d-level case o f u n f i l l e d d bands

-

interaction I n

which are n o t considered i n t h i s s e c t i o n

-

t e r a c t i o n can be so strong t h a t t h e adlayer i s - also e l e c t r o n i c a l l y

t h i s in-

-

just a

c o n t i n u a t i o n o f the bulk substrate and cannot be discussed as a separate 2D i d e n t i t y . On the other hand, f o r the Cu, Ag, Au case one would expect a decrease o f

the

adatom-substrate

interaction with

the

increasing

separation

between EF and t h e d band from Cu t o Au. For t h e cases discussed here such a tendency cannot be deduced. I n t h e noble-metal case studies we have presented many examples

-

which may

be somewhat confusing a t f i r s t . But the reason i s q u i t e simple. I t i s j u s t n o t p o s s i b l e a t the moment t o c l a s s i f y a s i n g l e experimental r e s u l t as more o r l e s s representative. Furthermore, i t i s n o t so simple t h a t t h e 20 band s t r u c t u r e i s completely d i f f e r e n t between d i f f e r e n t substrates. There i s a n i c e example presented i n f i g . 18 showing t h a t t h e Ag

on Cu(100) and Ag on Ni(100) e x h i b i t the

same 2D band s t r u c t u r e t a k i n g i n t o account o n l y a small energy s h i f t o f 0.3 eV.

There are s i m i l a r i t i e s between t h e 2D band s t r u c t u r e s o f a noble metal on d i f f e r e n t substrates, and we have t r i e d t o f i g u r e them out. F i n a l l y , we want t o s t r e s s t h e f o l l o w i n g important observation which p a r t l y explains t h e complicated spectra. We b e l i e v e t h a t a l a r g e p a r t o f the observed peaks o r bands are i n t e r f a c e s t a t e s having a l a r g e substrate d character. This p o i n t was made by Houston e t a l . a t all.

(25) who d i d n o t e x t r a c t a 20 band s t r u c t u r e

It seems t o us t h a t t h i s p o i n t becomes r a t h e r evident f o r t h e best

399

documented Ag case. I f one compares the 2D band s t r u c t u r e s f o r Ag on N i and Cu

23, one i s l e d t o the conclusion t h a t t h e

i n f i g . 18 and f o r Ag on A1 i n f i g .

two h i g h - l y i n g bands between 5.7 and 7.0 eV are t h e pure Ag bands, whereas t h e o t h e r l o w - l y i n g bands on Cu and N i may be l a r g e l y substrate-derived.

We b e l i e v e

t h a t t h i s i d e a should be used as a guide i n f u t u r e work i n order t o shed l i g h t on t h i s r a t h e r complicated phenomenon. Our many examples i n d i c a t e f i n a l l y t h a t one should n o t i n v e s t i g a t e systems w i t h a complete overlap i n energy o f t h e adlayer and substrate d-states.

TABLE

1

Binding energies r e f e r r e d t o EF o f Cu 3d e l e c t r o n s i n ML f i l m s o f Cu on d i f f e r e n t substrates.

Angle-integrated ( A I )

and angle-resolved

(AR)

(at

7;) modes

are indicated.

Substrate Ag,polyc.

Ag(100) Ag (100)

Ru (1000) Ru (1000)

Pt(ll1)

Zn (1 000) Al, polyc.

AI (1 11)

Al (100)

Ref.

12

(21 22

Al

IAR AR

26

AR

29

Al

27

Al

30

Al

33

AR

31

34

hw (eV)

Mode

A1 At

1

8.6 30 21.2 30

21.2

150

21.2 21.2 21.2

1300

BEF(eW)

I

2.5 2.47

2.66 2.75 2.7

2.75

2.65

I

3.33 3.42 3.5 3.5

3.8

4.2

4.15

4.5

400

TABLE 2

Binding energies referred to EF of Ag 4d electrons in ML films of Ag on different substrates. Angle-integrated (AI) and angle-resolved (AR) (at 7) modes are indicated.

TABLE 3

Binding energies referred to EF of Au 5d electrons in ML films of Au on different substrates. Angle-integrated (AI) and angle-resolved (AR) (at 7) modes are indicated. Substrate

Pd (1 11)

1R (111)

w (1 10) c u (1 00) c(2x2)

hw lev)

Ref. Mode 43

AI

141 I A I

4.4 45

Al

AR

BEF (eV)

40.8

I

21.2

1

1.5

3.1

4.3

5.9

1.0

3.0

4.0

5.8

4.0

6.2

1550 21.2

5.2

6.3

4.8

6.5

4.7

6.3

I

~

c u (100) c( 14x2)

48

AR

A1 (100)

34

Al

IA12Au

49

AR

21.2

lAuML

49

AR

21.2

IAuML(l2OK) 49

AR

21.2

3.5 (at 2) 1300

7

Al (1 11)

6.0 4.55

6.4

7.9

4.9

7.0

5.75

7.5

~

401

3d TRANSITION METALS Thin films of ferromagnetic materials epitaxially grown on non-magnetic substrates have been of interest for a long time (53-56). The electronic properties of such films are of great importance, although naturally the main impact is on in-situ studies of magnetization and spin-resolved ARUP spectra which are reported in another chapter of this monograph. Here we discuss the electronic properties with respect to the conduction bands of the non-magnetic thin films. Therefore we report briefly first results for Ni, Co, Fe and Mn. It seems to us that there will be great progress especially in this field in the next years. Thompson and Erskine (57) have studied Ni on Cu(OO1). F o r this system perfect epitaxy is known within the first few layers, which is not surprising in view of the small lattice mismatch of 3.2 %, by which amount the Ni NN distance is enlarged on Cu. The ARUP spectra of fig. 28 show Cu-substrate-derived peaks between -2 and -5 eV and very sharp Ni peaks between EF and -2 eV. In contrast to the Cu features the Ni features do not depend on hoproving the essentially 20 character of these features. The 2D band structure was evaluated as shown in fig. 29 both for even and odd symmetric states by appropriately chosen directions of the outgoing electrons and the vector potential of the incoming light. The magnetic exchange splitting is resolved at the M point for even symmetry as shown in fig. 29. Co/Cu(OOl) is studied rather extensively. Co grows pseudomorphically with a 2.3 % expanded lattice (58-61). For this system LEED intensity analysis is performed (60) for one ML and 8 ML films. For the monolayer film the interlayer spacings between the Co layer and the first Cu layer as well as between the first and second Cu layers are contracted by 6 % compared to bulk Cu. Co adsorbs in the fcc Cu site. This investigation yields the very reasonable result that the Co lattice is vertically contracted but it is laterally expanded. These findings show that pseudomorphy means only the same lateral lattice size and position, as it does in most cases, since the vertical distances are mostly unknown. The ARUP spectra for the ML Co/Cu(OOl) (59) system are not as sharp as those for Ni/Cu (57). Fig. 30 presents two examples. One reason for the rather broad features may lie in the large magnetic exchange splitting of 0.80 2 0.15 eV, which makes it more difficult to assign the related bands. Nevertheless, Miranda et al. (59) have been able to deduce a first 20 band structure for this system. Following their discussion the bands are slightly shifted to higher BE indicating a larger d-band filling. Continuing the path through the 3d transition metals from right to left in the periodic table we arrive at Fe. In principle it is not evident whether ARUP spectra can be measured to a better solution o r not for 3d metals. The magnetic 4

402

-7

-6

-5 -4 -3 -2 -1 BINDING E NERGY(eV)

E,

Fiq. 28 (left): ARUP spectra for a p(lx1) Ni film on Cu(100) along the Tfi direction of the 2D surface Brillouin zone. Features below -2 eV belong to the Cu substrate 3d states. From (57). Fiq. 29 (riqht): Dispersion of 20 energy bands for the p(lx1) Ni overlayer on Cu(100) along -i'x and %.Upper panel, even-symmetry states; lower panel, odd-symmetry states. From (57).

Fiq. 30: ARUP spectra taken at normal emission for: (a) 0.3 and (b) 1.2 f 0.2 monolayers of cobalt on Cu(100). The continuous line represents the fit of the experimental data at monolayer completion with the following contributions: Two pairs of Lorentzians for the exchange-split bands o f cobalt (dashed lines), the s-p band of Cu (dashed-dotted line) and a background of secondary electrons. Arrows indicate spin up and spin down assignment. From (59).

403

exchange s p l i t t i n g i s g r e a t l y increased up t o 2.6 eV f o r b u l k Fe. On t h e o t h e r hand,

t h e problem o f empty d states

enhanced.

Binns e t a l .

and l i f e t i m e broadening may even be

(62) p o i n t t o the d i f f i c u l t y o f separating 5 c l o s e l y

spaced Fe 3d bands. Also instrumental r e s o l u t i o n plays an important r o l e here, i f one wants t o measure a t ho= 120 eV i n order t o suppress t h e 4d i n t e n s i t y

from Ag or Pd substrates

against the adlayer 3d i n t e n s i t y v i a t h e Cooper

m i n i mum.

Binns e t a l .

present one normal emission spectrum from a f c c Fe ML on

Ag(001) taken a t a photon energy o f 120 eV which e x h i b i t s only weak wiggles r a t h e r than w e l l resolved peaks (62). Nevertheless,

i n order t o e x t r a c t i n f o r -

mation on t h e magnetic order i n the t h i n - l a y e r Fe f i l m s they measured the Fe 3s core l e v e l . As shown i n f i g . 31 they measured a m u l t i p l e t s p l i t t i n g

of 4.4

5

0.1 eV, which i s s i m i l a r t o bulk Fe i n d i c a t i n g a s i m i l a r l o c a l moment i n t h e Fe ML. Furthermore, they compared Fe/Ag(001)

(63) and Fe/Pd( 111). The overlayer-

s u b s t r a t e i n t e r a c t i o n should be very d i f f e r e n t i n t h e two systems, since i n t h e former case t h e Fe 3d s t a t e s overlap w i t h t h e Ag 5sp band, w h i l e i n the l a t t e r case they overlap w i t h the Pd 4d states.

I n agreement w i t h these ideas they

found a ferromagnetic Fe l a y e r on Ag(001) b u t a "dead" l a y e r on P d ( l l 1 ) .

l n l FelAgllOOl

hr I 160eV

....

-..- ..::. *

-

.

I 8

Fiq. 31 Fe 3s photoelectron spectrum from a Fe ML L, Ag(001) a f t e r s u b t r a c t i o n o f t h e Ag 4s peak. BE i s measured r e l a t i v e t o t h e main Fe 3s peak. From (62).

.......... . .....

*

. -... . .

. ..

I

! l l 4 0 Relohve Binding Energy IeVI

More r e c e n t l y , some e f f o r t was d i r e c t e d t o t h e Fe/Cu(001) system (64, 65). For t h i s system a l s o two LEE0 i n t e n s i t y analyses have been performed (66, 67), b o t h i n d i c a t i n g t h a t Fe on Cu(OO1) grows pseudomorphically,

i.e.

s t r u c t u r e w i t h t h e Cu i n t e r l a y e r separation e x a c t l y w i t h i n ? 0.05

i n an f c c

A.

Using

ARUPS, O n e l l i o n e t a l . (65) have analyzed the Fe ML. Some spectra o f t h i s study

404

a r e shown i n f i g .

32, and the evaluated 20 band s t r u c t u r e i s shown i n f i g . 33.

With respect t o t h e r e s u l t s o f Binns e t a l . (62) one c e r t a i n l y has t o take t h i s as a f i r s t step only, since the Fe s t a t e w i l l extend below 2 eV and t h e r e f o r e overlap s t r o n g l y w i t h the Cu substrate states. Mn i s a very i n t e r e s t i n g 3d element, since i t s atomic c o n f i g u r a t i o n i s 3d5, and according t o Hund's r u l e s i t should have i t s f i v e spins aligned. There are some reasons t o associate a l a r g e magnetic moment o f Mn w i t h a l o o s e l y bound Mn atom. I t i s t h e squeezing o f the Mn atom i n the bulk which causes i t t o disobey Hund's r u l e s . So t h e question a r i s e s as t o what happens i n Mn adlayer on metall i c substrates. Up t o now, there have been only very f e w i n v e s t i g a t i o n s o f t h i s problem (68-70).

On Cu(OO1) the Mn growth goes through the f o l l o w i n g stages:

l a t t i c e gas and ~ ( 2 x 2 ) i n t h e submonoalyer regime and a disordered l a y e r growth above 1 ML (68). The ~ ( 2 x 2 ) i s believed t o be t h e @ = 0.5-coverage

s t r u c t u r e on

t o p o f the Cu l a t t i c e instead o f a Cu3Mn(100) surface a l l o y (68). weak s t r u c t u r e s are found i n UPS. For coverages 0.18 5 0 2 0.25 resonance i s found a t -1.3

Only very

a weak surface

eV discussed as a v i r t u a l bound s t a t e (71, 72). The

work f u n c t i o n decreases from about 4.8 t o 4.2 eV w i t h i n one ML.

For Mn on Ru(001) a d 3 s t r u c t u r e i s found f o r coverages up t o 8 ML, which

transforms t o a (1x1)

pseudomorphic one upon annealing.

ARUPS i n d i c a t e s how

marginal t h e Mn adsorbate UPS p a t t e r n is against a 4d substrate l i k e Ru (70).

So f a r no 2D band mapping has been performed. Instead, t h e Mn 4s l e v e l s p l i t t i n g was measured i n d i c a t i n g a l a r g e a t o m i c - l i k e magnetic moment i n t h e ML

(69)

-

5

PALLADIUM AND PLATINUM Pd has been even more f r e q u e n t l y studied than the noble metals Cu, Ag, Au.

Among d i f f e r e n t substrates N b ( l l 0 ) was used by El-Batanouny e t a l . several reasons.

(73-79) f o r

For the Pd/Nb(llO) system the hydrogen-uptake r a t e was found

t o increase r a p i d l y and roughly l i n e a r l y w i t h Pd coverages i n excess o f one ML, w h i l e i t i s n e g l i g i b l e f o r clean N b ( l l 0 ) and N b ( l l 0 ) w i t h up t o one ML coverage o f Pd (80).

The f i r s t Pd l a y e r adsorbs i n a commensurate (1x1) arrangement,

which w i l l be r e f e r r e d t o as Pd*(llO)

A

(74).

I n t h i s l a y e r t h e NN distance i s

A

f o r P d ( l l 1 ) . As t h e second and t h e next f e w successive

Pd l a y e r s are deposited,

LEED shows the development o f a beat p a t t e r n charac-

2.86

instead o f 2.75

t e r i s t i c o f an incommensurate overlayer w i t h an atomic arrangement presumably being t h a t o f Pd i n P d ( l l 1 ) l a y e r (74, 79). F i n a l l y , the P d ( l l 1 ) LEED p a t t e r n i s observed. There i s a simple advance o f the Nb substrate since a t h i n (25 pm) Nb f o i l can be r e c r y s t a l l i z e d by annealing under vacuum w i t h t h e f i n a l r e s u l t t h a t t h e surface c o n s i s t s o f (110) f a c e t s e x h i b i t i n g a f a i r LEED p a t t e r n . Fig.

34 presents a s e t o f ARUP spectra f o r increasing Pd coverages.

For

coverages g r e a t e r than the ML ( y > 7) a peak a t 0.7 eV evolves and the o v e r a l l

405

p (IX I ) MONOLAYER Fe/Cu(IOOl

EVEN SYMMETRY

B E,

150 1.00 050

r

o

a50

x

100

050 I00 150

200

ODD SYMMETRY

e E,

1.50

ioo am

r

o

-X 050

100

050 I00

I50 2 00 2ML 0

D

-7

-6

-5 -4 -3 -2 -I BINDING ENERGY (OW

..

IYL

CILCULITEO h"

16Ra.V

21.22,"

IML

- YIJORITY ---- MINORITY

-.-

2ML

-..-.

E,

Fiq. 32 ( l e f t ) : ARUP spectra f o r one- and two-layer p ( l x 1 ) Fe f i l m on Cu(100). Values o f kll correspond t o t h e TR d i r e c t i o n . From (65). Fiq. 33 ( r i q h t ) : 20 band s t r u c t u r e o f p(lx1)Fe on Cu(100). The two broad curves i n d i c a t e t h e regions o f binding energy and k, where a prominent s t r u c t u r e r e s u l t i n g from t h e Cu sp band i s observed. L i g h t s o l i d and dashed curves represent c a l c u l a t e d surface Fe bands having over 50 % surface character. Data are represented by empty (two-monolayer f i l m s ) and s o l i d (one-monolayer f i l m s ) c i r c l e s ( h a = 16.85 eV) and rectangles ( h a = 21.22 eV). From (65).

spectrum changes t o become more s i m i l a r t o a Pd(l11) spectrum. The authors (79) do n o t take t h i s as a l a r g e change, but t a k i n g spectra a t other photon energies,

e.g.

a t ha= 90 eV as shown i n f i g .

35, o r other emission angles,

the

change i s q u i t e dramatic. Thus, t h e r e i s a c l e a r c o r r e l a t i o n between t h e change i n hydrogen uptake as i n d i c a t e d by the i n s e t i n f i g .

35,

and t h e e l e c t r o n i c

s t r u c t u r e as i n d i c a t e d by t h e ARUP spectra. The work f u n c t i o n changes w i t h coverage o f Pd. It goes through a minimum a t about 1 ML and reaches a f i n a l value a t 3 ML (73). strongly.

Figs.

34 and 35 i n d i c a t e t h a t the Pd and Nb 4d s t a t e s overlap

I n order t o suppress the c o n t r i b u t i o n s from t h e substrate one can

choose h w s o t h a t one works i n the Cooper minimum. The s i z e o f t h i s e f f e c t can be recognized from f i g . 35.

406

Fiq. 34: ARUP spectra for normal emission from a Nb(ll0) surface for various coverages o f Pd. At the Pd coverage of parameter y = 7, 1 ML Pd is deposited. Photon energy is 21.2 eV. From (79).

Fig. 36 presents the 20 band structure of the Pd 4d states in the pseudomorphic Pd*(llO) layer (77). The Ei states (i = 1...4) are numbered by their index. The two-dimensional Brillouin zone is shown in Fig. 36. The (110) face of a bcc crystal has CzV symmetry, so at the zone center (and at N) the states can be classified as Xi, We note that s. d,?, and dx2-y2 belong to Z1, dxy belongs to 22, dYZIXZ belongs to Z3,4. Along I'-N the only symmetry is a vertical reflection plane, so E l and Z4 can mix, as can Z2 and Z3. Similarly, along r-H, Z1 and Z3 mix, as do C 2 and Z4. States along these lines can be classified by even or odd symmetry under reflection. In the isolated monolayer there i s an additional symmetry, reflection in the plane (z * -z), which prevents these mixings. This fact is important in explaining the difference between the single-layer and the multi-layer systems. The authors performed a self-consistent linear augmented plane-wave calculation on a five-layer Nb film with a Pd layer on each side of the film. The Pd features overlap Nb bands and are resonances rather than surface states. How-

407

.:- 3

-0

-4

'\

ICI -0

-6

-4

-2

1.

0-E.

ENERGY OF INITIAL STATE lev1

N

r

H

Fiq. 35 (left): Normal-emission spectra at hw = 90 eV: Curve A, a Pd(ll1) overlayer on Nb(ll0); curve B, a Pd*(llO) overlayer on Nb(ll0); and curve C, Nb(ll0). The insets schematically show the hydrogen-uptake curves, where the change in resistance (AR) of the Nb foil is plotted against time. Hydrogen uptake is measured by the change in the resistance of the Nb f o i l with hydrogen bulk concentration. From (75).

Fiq. 36 (riqht): Experimental dispersion of Pd-induced states on Nb(ll0). Also shown is the surface Brillouin zone. Solid lines are states which are even and dashed lines states which are odd under reflection. Symmetry labels are explained in the text. From (37). ever, the Pd ML states overlap mainly s-like Nb bands and the coupling is not strong, which leads to sharp resonances. Finally and most importantly, the authors (77) stress that the Pd 4d-derived states lie below E F as in a noble metal. Comparison with the ML calculation shows that the noble-metal confiquration i s a consequence of the interaction with Nb. They point out that the isolated layer does not have a noble-meta1 configuration, because the large s component on the lowest state at ’ J implies that EF must fall below the top of the d complex. The center of gravity of the d states drops by 2 eV as a result of charge rearrangement. This noblemetal character then is in accordance with the missing interaction with H2. This argument is analogous to the observation that Ni does and Cu does not interact with Hp. The noble-metal character o f the Pd*(llO) layer is also in accordance with the missing Fano-type profile for the Pd 4p core-level emission in this system (76).

408

Now we turn to the noble-metal substrates. For Cu(ll1) fig. 37 exhibits the coverage dependence for normal emission (81). First of all, one recognizes the strong overlap of adlayer Pd 4d and substrate Cu 3d states. For thickness of 2 2.3 ML the spectra look bulk-like. Actually, Pessa and Jylha observed strong dependences from kl even for a 2 ML film. From LEED and AES it is concluded that, up to 1 ML, Pd grows pseudomorphically with a misfit o f 8 %. For 0 > 1 the LEED pattern relaxes into the Pd(ll1) one, and the growth mode is FM (layer by layer). The work function goes through a minimum at 0.3 ML and then rises linearly to the bulk-like value at 2.5 ML. From fig. 37 one may realize that the leading peak at 1.3 eV is resonantly enhanced, similar t o the Pd/Nb case (73). The 2D band structure o f the ML is not deduced by Pessa and Jylha. For Pd/Cu(001) a ~(2x2)structure is observed (82). It i s not clear at the moment whether this is an eventually buckled Pd ML or a Cu3Pd(100) surface alloy. The Auger curves tend to indicate a Pd layer. The electronic state of the ML is noble-metallike, in accordance with a chemisorptive behavior for CO which lies between Cu(OO1) and Pd(001) (82). Pd on both Ag(001) and (111) surfaces was studied in detail in the group of C. Norris (83,84) by using ARUPS, AES, LEED and work function measurements. At 300 K Pd grows in the layer mode. Retention of the p(lx1) pattern in the ML region indicates pseudomorphic growth giving rise to a 5.1 % expansion in the epitaxial Pd layer with respect to the bulk Pd. An increased background and reduced sharpness of the diffraction spots between 3 and 8 ML indicate a relaxation of the overlayer to the bulk Pd.

Fiq. 37: ARUP spectra obtained normal to the (111) surface o f the Pd overlayer on Cu system for various coverages R (in units of monolayer). Structures ST and S denote a satellite line in the incident radiation (no=21.22 eV) and the Shockley respectively. surface state of Cu(lll), From (81).

-M

-LD -20 E, I N I T I A L ENERGY ( e V )

-60

409

Fig.

38 presents normal emission spectra as a f u n c t i o n o f coverage which

e x h i b i t several

i n t e r e s t i n g aspects.

The overlap i n energy between t h e 4d

s t a t e s o f Ag and Pd i s small, rendering t h i s system w e l l s u i t e d t o the study of the e l e c t r o n i c s t r u c t u r e o f the Pd overlayer. This i s f u r t h e r supported by t h e observation t h a t t h e substrate emission decreases r a t h e r smoothly,

indicating

t h a t (1) no major charge t r a n s f e r takes place which would weaken a s p e c i f i c (bonding) band, and (2) no a d d i t i o n a l surface vectors are a v a i l a b l e f o r a r e d i s t r i b u t i o n o f emission i n space due t o the pseudomorphic growth. A t the ML, w e l l defined features are observed which have been used f o r t h e

I

I

i

38: Normal-emission photoe l e c t r o n spectra f o r d i f f e r e n t coverages 0 ( i n monolayers) o f Pd on Ag(100). h w = 21.2 eV. The i n set shows i n more d e t a i l s p e c t r a f o r 0 < 1.0 ML i n t h e energy r e gion occupied by t h e Pd 4d band. The weak f e a t u r e a t -3.2 eV i n t h e spectra f o r O < 0.1 ML i s a s a t e l l i t e of the s i l v e r peak at -5.06 eV e x c i t e d by the He1 5 emission l i n e . From (83).

Fiq.

I

1

I

1

-0

1

I

I

t

-6 -4 -1 EF Energy of inihalrtate leVl

L -I

.I

.,

r,

I

20 band mapping (see f i g . 39). A s i m i l a r r e s u l t i s found f o r t h e (111) surface: the main d i f f e r e n c e l i e s i n a smaller band width.

The authors c l a i m agreement

w i t h the band c a l c u l a t i o n f o r the free-standing ML o f Noffke and F r i t z s c h e (85) which i s remarkable i n view o f the 5 % expanded Pd l a t t i c e a t t h e Ag(001) surface. Although i t i s n o t easy t o recognize from t h e i r data, Smith e t a l .

(83)

argue t h a t t h e i r ML has noble-metal character s i m i l a r t o t h a t found f o r Pd on Nb (77).

Again, t h e bulk spectra evolve a t r a t h e r l a r g e thicknesses. T h i s cor-

r e l a t e s w i t h t h e development o f the work f u n c t i o n f o r which a minimum i s observed a t t h e (001) surface.

For (111) no minimum i s found,

which c o r r e l a t e s

w i t h t h e model o f t h e authors t h a t a t (111) 20 c r y s t a l l i t e s develop from the very beginning, whereas a t (001) a frozen Pd l a t t i c e gas i s expected due t o a higher d i f f u s i o n b a r r i e r p a r a l l e l t o the surface plane. For Pd on A u ( l l 1 ) l a y e r growth i s found (86). The angle-integrated UP spect r a (86) are much l e s s informative than the ARUPS data o f Pd/Ag(001) (83). Also angle-integrated

spectra from Pd/Ta( IIO), which was prepared by r e c r y s t a l -

l i z a t i o n o f Ta f o i l ,

i n d i c a t e the noble-metal

character o f the Pd ML (87).

4 10

Fiq. 39: The experimental 20 band structure (circles) for the Pd monolayer on silver (100). Open circles denote weak features. The full curves are the calculated energy bands of an isolated palladium (100) monolayer (85). From (83).

First results of Graham (88) for Pd on W(110) and (100) indicate similarities to the case o f Pd on Nb(ll0) (77). Finally, we turn to an s p substrate. Fig. 40 shows the normal emission spectrum for 0.7 ML of Pd on Al(111) (19). This result shows, better than any o f the d-metal substrate results discussed above, that the Pd ML has noblemetal character. This means that the atomic 4d10 5so configuration has not changed into the 4d9-6 550-4 configuration of bulk Pd; instead, a more atomiclike 4dlO-x 5sX (with x small) configuration is retained in the ML state. In all condensed states there is certainly also some admixture o f 5p states not noted in the formula above (87). From AES results, SK growth mode was concluded at 300 K (89). The 4d density of states develops as shown in fig. 41 over a rather long transition region up to about 10 ML. The center of gravity is shifted to EF by about 2 eV before the Pd 4d emission touches EF and the anticipated 4d + 5s redistribution can take place. At the same coverage empty 4d states evolve as shown in fig. 42 (90). It is interesting t o note that the work function changes (fig. 43) quite similarly to the case Pd/Ag(001). It seems that the development of the bulk work function and band structure are highly correlated. The development o f the 30 Pd band structure is connected with the 4d into 5s redistribution. This may explain the work function change also, since the 5s states leak considerably into the space i n front of the surface, while the localized d states do not, which may therefore increase the dipole barrier very effectively (89).

41 1

%

. . , . . . .

~

.

.. ,

0 7 ML Pd on AI(III)

. . . , 17

Fiq. 40: ARUP spectra f o r the clean Al(111) substrate and a 0.7 ML t h i c k Pd layer. The spectra are measured a t normal emission w i t h h w = 21.2 eV. From (19).

AI(II1). CLEAN

W/AI

.................

-+ ..... vI

25 _..... ......................-.

...............

3 .. m -3> ...... ..................1?............ ............... ............... t ................. 10 ......... : 5 ...... ............. .... ............... 0 ..... ............. ............................ vI

C..

..I.*

Pd/Alllll) hw = 9 5 eV

,

-6

, . . 4 -2 E, BINDING ENERGY (eV)

.a.

0

I

I

,

2

.

,

l

,

L

l

l

,

l

6

F i q . 41 ( l e f t ) : Normal emission ARUP spectra from Pd f i l m s on A l ( 1 1 1 ) . The e x c i t a t i o n was done w i t h He1 (21.22 eV) i n c i d e n t under 45 w i t h respect t o t h e surface normal. A reference t a b l e f o r comparison o f the evaporation time, the Auger i n t e n s i t y r a t i o and t h e thickness i n ML i s given. From (89).

Fiq. 42 ( r i q h t ) : e l e c t r o n beam f o r Pd (330 eV) t o A1 1.0 i s e q u i v a l e n t From (90).

Inverse photoemission spectra a t normal incidence o f the t h e clean Al(111) surface and Pd deposits on t o p o f it. The (68 eV) Auger i n t e n s i t y r a t i o i s given as parameter. Pd/A1 = t o about 1 ML, Pd/Al = 5.9 = 5.4 ML, PD/Al = 25 = > 10 ML.

412

Fiq. 43: The work function of Pd deposited on Al(111) as a function of Pd evaporation time. The reference values from the literature are indicated by stars. The data have been taken simultaneously with the measurements in Fig. 41. From ( 89).

z u z

7!!!LA U

L.0 0

EVAPORATION 10 20TIME bin)30

Interestingly, it was demonstrated recently by Rutherford backscattering that Pd interdiffuses into the bulk A1 at the Al(111) as well as at the Al(100) surfaces (130). The spectrum of fig. 40 is then more similar to a diluted alloy of Pd in Al. It is easy to verify that all other findings, i.e. the development of filled and empty bands and the chemisorption behavior, are well explainable under this assumption. We plan to recheck this for our own samples by inelastic ion scattering. Thin Pt films are studied only rarely. Recently it was found that Pt forms 2D commensurate islands on Nb(ll0) at sub ML coverage which become incommensurate prior to ML coverage (91). The Pt ML differs from Pt(ll1) which i s thought by the authors to be induced by the strong overlap between Pt and Nb and not by a change in electronic configuration, which is 5d9 6sl in the atom and not very much different in the metal. It is interesting to note that these authors therefore doubt the noble-metal interpretation for the Pd/Nb system. 6 sp METAL ADLAYERS 6.1 Hq, T1 and Pb The heavy sp metals Hg, T1 and Pb have some interesting properties and present their own problems in the evaluation of their electronic structure. Except for alkali and earth-alkaline metals, they are the only examples for this group o f metals. On the other hand, thin Pb films are well studied on many substrates, which is certainly due to the low melting point o f Pb which facilitates evaporation and the relatively large size of Pb which hinders interdiffusion for most systems. The problem in evaluating the electronic structure of a sp-metal adlayer is envisaged in fig. 44. For Pb on Ni(ll1) the SK growth mode has been found with several well ordered structures in the submonmolayer region (92). So, the full, densely packed ML is well characterized by AES as well as by photoemission from

413

-._...._.._.

........ ,...._..-'.'-

I

10

I

I

I

8

I

I

I

6

-Ei/eV

,

I

I

L

I

I

I

2

,

10..ML ......

'-...

.......

,

I

0

clean

Fiq. 44: He1 (21.2 eV) excited photoelectron spectra for the bare and Pb-covered Ni(ll1) surface. The binding energy EgF is referred to the Fermi edge. The full curve is for bulk lead from (93). The spectra are taken in the angle-integrated mode using a double-pass cylindrical minor analyser (CMA). The surface normal is tilted by an angle of 42 ' with respect to the CMA axis so that the mormal emission spectra is fully transmitted. From (92).

the shallow Pd 5d core level. Nevertheless, the ML does not show up in photoemission from the Pb valence band. Only at coverages of about 10 ML i s a typical (93) Pb valence band spectrum observed. Pb possesses a 6s2 6p2 structure in the atom, which leads to a 6s-derived band around 6 eV and a 6p-derived band below EF. Also Sn layers on Ni(ll1) and Sn as well as Pb layers on Al(111) do not exhibit well resolvable valence-band peaks against the substrate emission (92). It seems that sp-derived adlayer states cannot be easily discriminated against substrate emission by ARUPS. There are only two exceptions concerning Tl/Cu(OOl) and Cs/Al(lll), which we will discuss below. In this section we will briefly report on few results known thus far for Hg, T1 and Pb. Recently, Hg adlayers have been studied by ARUPS on Ag(001) (94). Interestingly, the first two layers grow in fcc structure differently from the rhombohedral structure in the bulk. Nothing is seen from the s band expected for the Hg 6s2 configuration. Instead the shallow Hg 5d states are observed as shown in fig. 45. Besides the 5d3/2-5d5/2 spin-orbit split a third peak is observed which is marked by an arrow in fig. 45. This state is obviously not an interface state, since it is not weakened at higher Hg coverages. Instead, it is believed by the authors to be of itinerant character, i.e. a Hg 5d band is formed. It may be interesting to note that there is some similarity to the Au 5d emission found for the Au ML on Al(111). Furthermore, the Hg 5d states can be compared with Zn 3d at BE 9 to 11 eV below EF for which itinerant character was deduced from ARUPS for the bulk metal (95).

414

Fiq. 45: Normal emission ARUP spectra (hw= 50 eV) f o r various exposures o f Hg ( i n langmuirs) on Ag(100). Arrow denotes new feature. I n s e t shows spectrum for 20 L exposure (-7 monolayer) o f Hg. From (94).

1

-I

--

n

c

* 3

=>

In

z

Y

c

z

12

10

8

6

4

BINDING ENERGY

2

E,

eV)

Recently, a l s o t h a l l i u m was studied on Cu(OO1) (96, 97).

I n the ML the T1

atoms are very l i k e l y t o be arranged i n long chains l y i n g i n t h e (110) furrows o f t h e Cu(OO1) substrate (see f i g .

5.12

A

46).

The spacing between t h e chains i s

and the mean atomic spacing along the chains i s 3.41

A.

The r e g i s t r y i s

t h r e e T1 atoms t o f o u r Cu atoms and the u n i t mesh is ~ ( 4 x 4 ) . Higher coverage i s achieved by t h e rows moving c l o s e r together, (96).

u n t i l a dense hcp ML i s formed

From the d i f f e r e n t distances along the rows and between t h e d i f f e r e n t

rows t h i s system was supposed t o be a v e r i f i c a t i o n o f a 1D e l e c t r o n i c system. This idea was substantiated by ARUPS as shown i n f i g . 46.From t h e deeper-lying T1 6s2 band one peak i s found t o disperse and one t o be f i x e d i n energy under v a r i a t i o n o f t h e emission angle

0. Following Binns e t a l .

(96)

both t h e

d i s p e r s i o n along the chains and the missing dispersion normal t o the chains are observed here, since two domains o f chains oriented along e i t h e r o f two perpen-

d i c u l a r axes [ O l l ] and [Oll] are b u i l t up a t the surface. I n a f u r t h e r very de-

t a i l e d i n v e s t i g a t i o n they looked f o r t h e 6p band near EF which i s expected according t o t h e atomic 6s2 6 p l c o n f i g u r a t i o n o f T1. They were able t o l o c a l i z e t h i s weak band as demonstrated i n f i g s .

47 and 48.

One phase o f t h e l i n e a r

chains undergoes a spontaneous ( P e i e r l s ) d i s t o r t i o n . As expected f o r a 10 elec-

t r o n i c system, t h i s i s connected w i t h t h e opening o f a gap a t EF which was deduced here t o be about 0.3 eV. The d i s t o r t i o n along t h e chains has t h e p e r i o d o f 8 T1 atoms i n accordance w i t h LEED observation. example o f a 1D e l e c t r o n i c system a t a surface.

Up t o now, t h i s . i s t h e o n l y

415

I

i

E='

ioiii

4

10111

1

8

9 10 11 Bindingenergy lev1

12

F i q . 46 ( l e f t ) : The l i n e a r chain arrangement o f T1 atoms (shaded) adsorbed on t h e (100) surface o f copper (ppen c i r c l e s ) a t a coverage o f 0.6 ML. The separat i o n between chains i n 5.12 A and the mean atomic spacing along the chains i s 3.41 A. The ~ ( 4 x 4 ) u n i t mesh i s i n d i c a t e d by t h e square. The s t r u c t u r e i s i d e a l i s e d and does n o t show rumpling along the chains.

Fiq. 46 ( r i q h t ) : Photoelectron d i f f e r e n c e spectra (copper background subs t r a t e d ) i n the r e g i o n o f t h e T1 6s f e a t u r e from t h e ~ ( 4 x 4 ) l i n e a r chain s t r u c t u r e as a f u n c t i o n o f t a k e - o f f angle (analyzer r o t a t e d i n t h e (011) plane). From (96). F i n a l l y , t u r n i n g t o Pb, we have mentioned already t h e Pb on N i ( l l 1 ) Al(111)

systems (92).

and

Whereas UPS from t h e conduction s t a t e s d i d n o t p r o v i d e

any information, i n t e r e s t i n g r e s u l t s have been deduced from t h e a n a l y s i s o f t h e Pb 5d shallow-core state. The two substrates behave q u i t e d i f f e r e n t l y . Fig. 49 presents t h e Pb 5d emission from Pb l a y e r s o f d i f f e r e n t thickness on A l ( 1 1 1 ) . The BE stays constant w i t h i n the experimental accuracy o f about 50 meV. The P b / N i ( l l l ) looks very d i f f e r e n t as shown i n f i g . 50 i n which a continuous s h i f t o f t h e BE t o l a r g e r values can be seen. Both d i r e c t i o n and c o n t i n u i t y o f t h e s h i f t were explained i n a Born-Haber-Cycle model as shown i n f i g . 51. The e x c i t a t i o n energy !$(Z) tation i n t o different,

i s c a l c u l a t e d on a second p a t h by separating t h e e x c i otherwise known, energies.

For t h i s purpose, w i t h i n a

"gedanken experiment" the Z atom i s desorbed from the N i ( l l 1 ) surface by applyi n g EA(Z).

The gas phase Z-atom i s then photoionized by applying EBV(Z,gas)

which i s r e f e r r e d t o t h e vacuum l e v e l V. Then the so-called equivalent core approximation i s used t o set the core-ionized Z-atom (Z*) i o n i z e d Z+1-atom ((Z+l)+).

equal t o a valence-

The l a t t e r i s rendered n e u t r a l by f i l l i n g i n one

e l e c t r o n and gaining t h e i o n i z a t i o n energy for the Z+1 atom (11(2+1)), finally

the neutral

(Z+1)

atom is adsorbed again a t

the

Ni(ll1)

and

surface

416

-2.

--

;- 0 6 -

', .-..#-."'.. @.=Lo-

,,

.

-

I

4 =O

J

c" - 1 0 w

-12-

nlo

I

1

TI 6p

[second zanel

%

A.

*

-----....--.-... . . . . .

b.42.

k

5 -08-

.-.......-.

'-1%.

I

,/

Energy of mktd state (eVl

-2

10

12

k Ib"1

1L

1 2n1o

Fiq. 47 ( l e f t ) : (a) Photoemission spectra around the Fermi l e v e l a t an e l e c t r o n from t h e clean substrate ( 0 ) and the T1 chains a t 0 c o l l e c t i o n angle o f 38 = 0.53 (.). (b) D i f f e r e n c e spectra showing the behavior o f the T1 6p feature c l o s e t o t h e Fermi l e v e l . The sudden drop i n i n t e n s i t y between Oe = 40 and Oe = 42 i s taken as an i n d i c a t i o n t h a t the 6p band has crossed t h e Fermi l e v e l . From (97). O

O

48 ( r i q h t ) : 6p band s t r u c t u r e o f the T1 chains ( @ = 0.53) a t T = 80 K. The i n s e t shows the i n t e n s i t y o f the T1 6p f e a t u r e close t o the Fermi l e v e l . The Fermi l e v e l crossing i s i n f e r r e d from the sudden drop i n i n t e n s i t y . From (97).

Fiq.

r e l e a s i n g E ~ ( 2 + 1 ) . W i t h i n t h i s BHC, EBV(Z,gas)

- Ii(Z+l)

-

EBF i s c a l c u l a t e d as EBF(Z)

= EA(Z)

+

E A ( Z + ~ ) . For t h i s w e l l defined system EA(Z) and E A ( Z + ~ )

were measured by a thermal desorption experiment (98). The gas phase

EBF value

was measured r e c e n t l y w i t h the important r e s u l t t h a t about 85 % o f t h e whole i n t e n s i t y was confined i n t o one l i n e which then could be used o u t o f a f i n a l s t a t e m u l t i p l e t , spreading over more than 2 eV. ments i s given i n f i g .

52,

The r e s u l t o f these measure-

i n d i c a t i n g a very good agreement w i t h the measured

values. D i f f e r e n t conclusions could be drawn from t h i s r e s u l t :

(1) The s h i e l d -

i n g o f t h e f i n a l - s t a t e hole i n Pb i s narrow i n space and complete, e l e c t r o n i s f i l l e d i n t o t h e valence s h e l l o f the (Z+1) s h i f t i n t h e case o f P b / N i ( l l l ) BE i n t h e

as if one

atom. (2) The continuous

i s caused by a continuous s h i f t o f the atomic

M L range both f o r Pb and B i . This continuous s h i f t i s b a s i c a l l y

caused by a l a t e r a l l y r e p u l s i v e i n t e r a c t i o n between the Pb and B i atoms.

(3)

The s h i f t t o higher BE i s caused by the higher atomic BE o f B i t o N i ( l l 1 ) g i v i n g t h e s i g n t o the EA difference.

It should be noted f i n a l l y t h a t t h e BE i s

considered here w i t h respect t o EF, so t h a t the Born-Haber-cycle i s independent

o f work-function changes.

417

49: The Pb 5d s p i n - o r b i t s p l i t doublet f o r Pb on A l ( 1 1 1 ) . The parameter i s t h e thickness o f the Pb layer. The b i n d i n g energy EgF i s r e f e r r e d t o t h e Fermi edge. Photon energy i s He11 r a d i a t i o n ( h w = 40.8 and 21.2 eV). The step a t 19.6 eV i s the Fermi edge from the 21.2 eV c o n t r i b u t i o n o f t h e r a d i a t i o n e x c i t e d by the He lamp i n the He11 mode. From (92). Fiq.

Fiq. 50: Pb 5d c o r e - l e v e l spectra f o r Pb on N i ( l l 1 ) . Other d e t a i l s as f o r f i g . 49. From (92).

21

20

19

18

17

-EkieV

Fiq. 51: Born-Haber c y c l e f o r t h e calculation o f the core-1 eve1 binding energy EgF referenced t o the F e r m i l e v e l . EA is t h e adsorpt i o n ( o r sublimation) energy, Egv the core-level b i n d i n g energy f o r t h e atom referenced t o the vacuum l e v e l and I 1 t h e f i r s t i o n i z a t i o n p o t e n t i a l . The a s t e r i s k i n Z* i n d i c a t e s a 2 atom w i t h a core hole. From (92). AOLAYER OF Z-ATOMS

418

I

1e.o.C

17.8

#

*

52: Pb 5d5/2 binding energies referenced to the Fermi level as a function of coverage in units ML. (.) From the UPS experiment (x) Calculated (hv.=.40.8 eV). for the Born-Haber cycle model including the desorption energies from the TDS experiment. From

PblNilllll

Eg

Pb5d5i2

(98)*

11 4i x I

1

i

3

COVERAGE 6 / H L

6.2 Alkali metals Thin films of alkali metals on metals have been studied extensively. The interest in these systems arises from technological aspects such as photodetection or promotion in catalysis as well as from more fundamental considerations. Alkali atoms represent a simple adsorption system, the properties of which can be calculated self-consistently in the framework of the jellium model. Such calculations using the Kohn-Sham local-density approximation have been pioneered by Lang and Williams (99). A wealth of experimental data has been collected using mainly LEED, electron energy loss spectroscopy, contact potential and TDS (100). Not so much is known about the electronic structure, in part certainly due to the small cross section of the s electrons in photoemission. On the other hand, the difficulties in elaborating the electronic properties arise also from the high vapor pressures of these metals, which prevent the easy production of single crystals, and their high chemical reactivity. Due to their high vapor pressure the substrates have to be cooled below room temperature in order to stabilize more than the first ML. These difficulties are nicely represented in fig. 53 (101). Before the first very small 02 dose i s given to the Cs film, a contamination is already visible, which exceeds the 6s band emission below EF by a factor of four. One furthermore recognizes, how sensitive this surface is against very small amounts of 02. From the great number of studies (100) a consistent picture has emerged on alkali adsorption: the work function drops rapidly, reaches a minimum and increases slightly again at ML coverage approximately to reach the value of the alkali-metal surface. At full ML the alkali atoms form hcp structures. The

419

CS FILM NO I

hv: 30 eV

Fiq. 53: ARUP spectra of clean and oxygen-exposed Cs at ho = 30 eV. Some oxygen was incorporated into the Cs film prior to the intended exposure (bottom curve). Note that emission from the valence band of Cs is visible in all spectra. From (101).

0

10

BINDING ENERGY ( r V )

initial decrease of the work function arises, because the electronegativity of the alkali metals i s small compared to that of the substrate, and charge is transferred towards the substrate, as it i s well described in the jellium model by Lang (102). In this model it was shown that for increasing coverage the electronic charge, which is peaked at the adsorbate substrate interface, moves towards the adsorbate resulting in a depolarization of the adsorbate-induced dipole moment (103). UI

P

-C

Fiq. 54: ARUP spectra of electrons emitted normally from a Cu(ll1) crystal at different Cs coverages. The peak labelled V i s due to electrons emitted from Cs valence states. @ = 0.25 is defined by the close-packed ~ ( 2 x 2 ) Cs overlayer as observed by LEED. From

c

U

% c

L

a + 0

(104).

I P,

P

5

2

-2

-1

0

Initial energy (ev)

420

I t seems t o be common b e l i e f a t the moment t h a t i n a q u a l i t a t i v e p i c t u r e

t h e bonding o f t h e a l k a l i atom i s more i o n i c a t small coverages and m e t a l l i c a t t h e ML. Therefore,

during the t r a n s i t i o n from atom t o ML the charge i n the

conduction band has t o reappear. This was f i r s t c l e a r l y observed by Lindgren and Wallden (104) as shown i n f i g . 54. Recently, t h i s was n i c e l y reproduced f o r Na on Cu(OO1) (105) and f o r a l k a l i metals on Al(111) even f o r l a r g e r photon energies (106,107). above EF

-

5 5 i n d i c a t e s t h a t i n the r e g i o n o f empty bands, i.e.

Fig.

as analyzed by inverse photoemission - also a s t r u c t u r e moves down

which i s assigned t o the empty p band by these authors. The idea t h a t t h e cond u c t i o n s e l e c t r o n i s l a r g e l y f i l l e d back i n the ML s t a t e i s confirmed

by an

experiment o f metastable 3s He* d e e x c i t a t i o n i n f r o n t o f a Cs ML on C u ( l l 0 ) (108).

This process involves o n l y the outermost atomic l a y e r and t h e r e also

mainly t h e most far-reaching o r b i t a l s .

56 t h e Cs 6s signal

fig.

orbitals,

The l a t t e r f a c t also explains why i n

i s much stronger than t h a t o f

t h e Cs 5 p - f i l l e d

which i s more l o c a l i z e d towards the center o f the atom.

Fig.

56

demonstrates again how s e n s i t i v e t h e surface i s w i t h respect t o oxygen. I t was only r e c e n t l y t h a t the shallow p-core states were a l s o investigated.

From these states the t r a n s i t i o n from ML t o m u l t i l a y e r can c l e a r l y be recognized. This was f i r s t observed by Rotermund and Jacobi (109) b u t not discussed

11.1 eV i n t h e It i s i n t e r e s t i n g t o

i n d e t a i l a t t h a t time. Fig. 57 i n d i c a t e s t h a t EgF (Cs 5p3/2) ML and i s then s h i f t e d t o 11.9 eV f o r the t h i c k Cs f i l m .

=

note t h a t the 5 ~ 3 1 2l e v e l i s composed from two s t r u c t u r e s separated by about

0.2 eV. I t was shown r e c e n t l y by Domke e t a l . (110) t h a t t h i s s p l i t t i n g , which has the exact value o f 0.23

eV,

i s due t o a surface core-level

shift.

This

s p l i t t i n g cannot be as w e l l observed f o r the 5p1/2 peak since l i f e t i m e e f f e c t s

due t o a 5p 5p 6s Auger t r a n s i t i o n broaden i t considerably. I n t h e meantime t h e s h i f t i n BE f o r t h e t r a n s i t i o n from ML t o m u l t i l a y e r s has been observed by sev-

e r a l groups (109-113). up t o 2 ML f o r

Fig. 58 shows the t r a n s i t i o n from submonolayer coverages

Cs on Al(111) (113). A t t h i s surface Cs b u i l d s w e l l ordered ad-

l a y e r s , from which t h e coverage can be determined exactly. I t i s i n t e r e s t i n g t o note t h a t there i s also a s h i f t i n BE i n the ML region. The i n t e r p r e t a t i o n o f these s h i f t s sial.

-

i n the ML as w e l l as from ML t o m u l t i l a y e r - i s s t i l l controver-

Hohlfeld e t al.

densities,

(113) a t t r i b u t e d the s h i f t s t o d i f f e r e n c e s i n e l e c t r o n

s h i e l d i n g t h e photoemission final. s t a t e hole, s i m i l a r l y t o Xe m u l t i -

l a y e r s on metals (15-17).

Contrary, Domke e t a l .

(110) discuss these s h i f t s i n

terms o f a Born-Haber c y c l e (see f i g . 51 f o r Pb on N i ( l l l ) ) ,

i.e.

i n terms o f

atomic BE d i f f e r e n c e s i n t h e i n i t i a l state.

F i n a l l y , we b r i e f l y note t h a t there are recent attempts t o go beyond t h e j e l l i u m model (99) and include the atomic s t r u c t u r e o f the substrate, l i k e surface s t a t e s and also the Cs 5p states (115,116). found f o r Cs on W(OO1)

Wimmer e t al.

i t s d-

(115)

t h a t Cs forms a m e t a l l i c overlayer w i t h t h e valence

421

Fiq. 55: ARUP spectra (left side) and inverse photoemission spectra (right side) for K on Al(111) measured at different coverage as indicated. From (107).

AR

-2

-1

EF-O

EF=O

1

2

3

4

Energy /eV

Cs-65

S’ He' Fiq. 56: Electron energy distribution

curves induced by deexcitation of metastable 35 He* from (a) a clean monolayer of Cs adsorbed on a Cu(ll0) surface, and after exposure to (b) 0.2 and (c) 0.5 L of 02. From (108).

422

hw=21.2 eV T =20K

..._......."... . .*

I

1

I

-1L

-12

I

I

-10

. . . I .

"

-8

.."..-.__ 14

13 12 11 H) Energy below EF (eV)

Fiq. 57 ( l e f t ) : ARUP spectra i n t h e r e g i o n o f the 5p l e v e l s a t normal emission f o r (A) b u l k p o l y c r y s t a l l i n e Cs and (B) a Cs ML. From (109). F i q . 58 ( r i q h t ) : UP spectra o f Cs adsorbed on A l ( 1 1 1 ) i n t h e Cs 5p energy range. The ML spectrum i s compared w i t h the Cs gas phase spectrum from (114). From (113).

e l e c t r o n s p o l a r i z e d t o the also p o l a r i z e d Cs 5p shallow-core

electrons.

The

admixture o f t h e d i r e c t i o n a l character o f Cs d - l i k e charge and t h e p e r s i s t e n t dominance o f W d - l i k e covalent Cs(s,d)-W

surface states near EF i n d i c a t e a tendency towards a

(surface, d) band. Furthermore, f o l l o w i n g Wimmer (116) t h e

Cs 5p e l e c t r o n s are p o l a r i z e d even i n a Cs ML, as shown i n f i g .

59. Further-

more, the Cs NN distance i n the ML i s contracted by about 11 % compared t o t h e

corresponding distance i n bulk Cs. This i s i n e x c e l l e n t agreement w i t h t h e

experimental r e s u l t s (117,118)

f o r Cs adsorption on W.

Also a c o n t r a c t i o n b u t

423

r

Fiq. 59: Charge d e n s i t y o f t h e 5p-derived s t a t e s i n a hexagonal Cs monolayer i n a plane perpendicular t o the slap i n u n i t s o f 10-5 e/bohr3. I n s i d e t h e atomic spheres t h e s p h e r i c a l l y symmetr i c ( 1 = 0) components have ben subtracted and outside contours up t o a maximum value of 76x10-5 e/bohr3 are shown. From (116).

I

I

w i t h a smaller amount was observed f o r Cs on Al(111) (113) which should be even b e t t e r modelled by t h e ML. Thus, i t seems t h a t p a r t o f the compression observed on W i s due t o i n t e r a c t i o n w i t h the substrate, and t h i s e f f e c t seems somewhat t o o l a r g e i n the c i t e d c a l c u l a t i o n (116). The c o r r e l a t i o n w i t h the f i n d i n g s f o r t h e Ag ML on A l ( 1 1 1 ) 7

i s noteworthy.

CALCULATIONS OF ELECTRONIC STRUCTURE OF METAL MONOLAYERS

A r a t h e r l a r g e amount o f the t h e o r e t i c a l work i s already mentioned i n d i s cussing the case studies. We w i l l very b r i e f l y mention some o f these c o n t r i b u t i o n s i n c l u d i n g a d d i t i o n a l studies but without commenting on the various comput a t i o n a l methods. As a f i r s t guide, c a l c u l a t i o n s o f the band s t r u c t u r e f o r an unsupported ML are o f g r e a t value. Comparing b e t t e r t o t h e experimental r e s u l t s are s t u d i e s o f supported ML where the i n t e r a c t i o n o f the ML w i t h the substrate i s taken i n t o account. Most i n f o r m a t i v e would be a s e l f - c o n s i s t e n t band s t r u c t u r e c a l c u l a t i o n f o r a supported ML which takes i n t o account a l l s t r u c t u r a l parameters o f the adlayer, i.e. t h e substrate,

t h e NN distances w i t h i n the ML, the adlayer p o s i t i o n r e l a t i v e t o and the v e r t i c a l distance from the substrate. No such complete

calculation exists

so far,

mainly due t o the

fact

that

not

all

of

the

424

parameters are known e i t h e r from experimental work o r from t o t a l energy calcul a t i o n s which became f e a s i b l e more r e c e n t l y . It seems t o us t h a t also t h e vert i c a l distance between adlayer and substrate should be taken i n t o considerat i o n . This i s because o f recent i n s i g h t i n t o atomic rearrangement during reconstruction,

for instance a t the f c c (110) surfaces. For a l a t e r a l c o n t r a c t i o n

some release i n t h e v e r t i c a l distance i s t o be expected so t h a t the v e r t i c a l distance between adlayer and substrate should n o t be simply approximated by t a k i n g sums o f m e t a l l i c r a d i i . There were very e a r l y attempts t o non-self-consistently s t r u c t u r e o f a free-standing ML e.g. performed

a

self-consistent

o f Cu (119).

band-structure

c a l c u l a t e t h e band

Later Jepsen e t al.

calculation

for

(120)

free-standing

Cu(100) and (111) ML f i l m s . They found t h e d-band width W depending on NN i n t e r a c t i o n : W(100) = 2.0 eV, W(111)

=

2.7 eV and W(bu1k) = 3.4 eV. Such a reduc-

t i o n i n width i s expected from simple t i g h t - b i n d i n g theory. W(bu1k)

The r a t i o W ( l O O ) /

should be n e a r l y equal t o the square r o o t o f t h e r a t i o s between the

numbers o f nearest neighbors, i.e.

(4/12)1/2

=

0.58 and (6/12)1/2 = 0.71.

These

values compare q u i t e w e l l t o W(100)/W(bulk) = 0.59 and W(111)/W(bulk) = 0.79. Noffke and F r i t s c h e (86) have calculated s e l f - c o n s i s t e n t l y t h e band s t r u c t u r e of unsupported ML o f Fe, Co, N i and Pd using NN distances o f the b u l k meta l s . They c a l c u l a t e d spin-dependent d e n s i t i e s o f states and s p i n magnetic moments per atom which we do n o t discuss here. We f i n d it i n t e r e s t i n g t h a t they have g e n e r a l l y found an increased 3d and 4s occupancy a t t h e expense o f 4p occupancy f o r the ML. Wimmer (121) has performed a systematic study o f such k i n d f o r the a l k a l i , the a l k a l i n e - e a r t h and 3d t r a n s i t i o n metals. Much a t t e n t i o n was paid t o

self-consistent

calculation o f

the

surface

e l e c t r o n i c s t r u c t u r e f o r a great number o f surfaces, o f which we mention o n l y Cu(100) (24,122).

From these c a l c u l a t i o n s layer-resolved d e n s i t y o f s t a t e s and

band s t r u c t u r e were obtained,

i.e.

kll-resolved

states being h i g h l y l o c a l i z e d

i n the surface plane. Such c a l c u l a t i o n s mainly designed t o study surface s t a t e s

o f the bulk metal were also very h e l p f u l i n analyzing t h e e l e c t r o n i c s t r u c t u r e o f t h i n metal f i l m s . The features o f the f i r s t l a y e r are taken t o be i n d i c a t i v e f o r the e l e c t r o n i c p r o p e r t i e s o f the surface. For Cu(100) i t i s found t h a t i n the surface l a y e r t h e d e n s i t y o f s t a t e s i s r a t h e r increased i n the upper p a r t o f the d band (24). This seems t o be a Sene r a l t r e n d found f o r many other surfaces. What i s also i n t e r e s t i n g t o note i s t h a t the d band moves t o higher energies (away from EF) w i t h an increasing number o f layers. Considering the t r a n s i t i o n from atom t o bulk, which was b r i e f l y discussed i n reviewing t h e experimental r e s u l t s on Cu, the d i r e c t i o n o f t h i s s h i f t i s not understandable. I t was argued there t h a t t h i s s h i f t should be from t h e deep-lying atomic l i n e t o the h i g h - l y i n g bulk band. F i n a l l y , noted t h a t the f i n a l

i t should be

r e s u l t f o r the distance between d-band edge and EF i s

425

1.5 eV in (24), which is too small compared to the experimental result of 2.0 eV, which may be seen from the examples in section 3.1 above. These deficiencies seem t o be quite common to calculations of this kind at the moment, as one can realize comparing the references (119) and (123) with (24) for the Cu case. More recently several self-consistent calculations were performed for ordered ML on single-crystal surfaces. Some results for Cu on Ru have been presented in fig. 8 (25). Very elaborate calculations have been performed also for Ni on Cu(100) (123), Ag on Rh(100) and (111) (124), and Pd on Fe(100) (125). We are aware that this is not a complete list, but additional information can be found in the references of the given examples. The Ag on Rh study o f Feibelman and Hamann (124) is remarkable, since several physical entiti’es are calculated and discussed which can be evaluated in the experiment: not only the surfaceband dispersion but also surface core-level shifts and absolute work-function values are given. The same authors (126) performed calculations on the variation of work function with thickness of free-standing and adsorbed thin metal films. This work continued the jellium calculation of Schulte (127). They found that a strong surface state at EF, as for Cr(100), stabilizes the charge-density profile in the surface region and makes the work function independent of film thickness. Furthermore, they pointed out that metals with low state densities have low surface energies, and are thus not apt to be thermodynamically stable substrates for high-density-of-states metals. They finally remarked that there is no obvious candidate for a metal-on-a-metal system, for which strong thickness-dependent work-function variations can be expected. Interestingly, such a variation was recently reported for Li adlayers on W(0ll) and Mo(Ol1) (128). Finally, we would like to mention a calculation for Au, Pt, Ir and 0s overlayers on Cr(100) (129). It was found that neither Au nor Pt could remove the strong surface-state intensity near EF of the Cr(100) surface, whereas 0s was able to do so. This was explained by the different symmetries of the d bands near EF. For Au and Ir the top of the mostly filled d bands lies near EF. The wave functions are of anti-bonding character there, whereas, for Cr, EF intersects the d band in the lower half of the d band, where the symmetry of the wave function is bonding or non-bonding. This finding has strong implications on the adsorption test for surface states: only those adsorbates, whose valence orbitals have the right symmetry to interact with the surface-state band, can remove the surface states. 8

SUMMARY There i s no doubt that the investigation of metallic films on metallic substrates has a large impact on many fields of scientific as well as technologi-

426 c a l importance. A f i r s t glance a t the presented m a t e r i a l shows t h e impressive v a r i e t y o f systems which has been studied up t o now. Compared t o t h i s l a r g e number o f studied systems and the even l a r g e r number o f systems which are n o t investigated, the number o f papers, which are given reference t o ter,

i s r e l a t i v e l y small.

.11

t h i s chap-

This i n d i c a t e s t h a t t h e m e t a l l i c f i l m s on m e t a l l i c

substrates are j u s t s t a r t i n g t o be investigated. This becomes even more s t r i k i n g i n view o f t h e l a r g e number o f papers on a s i n g l e adsorbate l i k e CO. Therefore,

we are s t i l l

looking f o r c o r r e l a t i o n s between t h e e l e c t r o n i c

s t r u c t u r e - which i s the 30 band structure,

t h e 2D band s t r u c t u r e ,

surface s t a t e s and resonances, and the t r a n s i t i o n between them l i k e work function,

-

including

and q u a n t i t i e s

s-d e l e c t r o n t r a n s f e r or real-space bonding.

We have t o

l e a r n much more about t h e c o r r e l a t i o n between geometric and e l e c t r o n i c s t r u c t u r e , before we can t h i n k o f e l e c t r o n i c - i n t e r f a c e t a i l o r i n g . For each system several surface-analytical

techniques have t o be adopted,

before r e l i a b l e conclusions can be drawn. Even such a well-known technique as q u a n t i t a t i v e AES can be misleading, cate. Therefore,

as the case of Pd/A1(111)

tends t o i n d i -

i n f u t u r e studies, more s t r u c t u r a l methods have t o be used t o -

gether w i t h photoemission. There i s good hope t h a t t h i s may be f a c i l i t a t e d by t h e great progress which has been made i n Scanning Tunneling and LEED microsCOPY.

The question may be r a i s e d as t o whether or not the 20 band s t r u c t u r e i s a

workable conception.

This i s c e r t a i n l y t r u e f o r c a l c u l a t i o n s b u t may n o t be

t r u e f o r experiments. I n the r e a l world o f experiments one needs a substrate as a support f o r t h e ML. To spread t h e M L on top of the substrate t h e adatom-sub-

s t r a t e i n t e r a c t i o n needs t o be l a r g e r than the adatom-adatom i n t e r a c t i o n . Thus,

a d e l i c a t e balance has t o be established t o have the adatom-substrate i n t e r a c t i o n l a r g e enough t o enable the spreading-out o f the ML and t o have i t as small as possible i n order not t o overlay the l a t e r a l i n t e r a c t i o n s . From t h e overview given above i t seems t h a t n e i t h e r system o f the d-metalon-d-metal

k i n d can serve as an example f o r a 2D-electronic system. The i n t e r -

a c t i o n w i t h the substrate i s t o o strong r e s u l t i n g i n a m i x t u r e o f i n t e r f a c e and 20-adlayer states. The only examples could be Ag or t h e a l k a l i - m e t a l adlaye r s on A l ,

i.e.

on an sp-metal substrate. We have given f u r t h e r arguments above

and have presented the Ag/A1(111) case i n d e t a i l . The a l k a l i - m e t a l adlayers are o n l y o f l i m i t e d value, since t h e i r conduction band i s h a r d l y t o be observed i n most cases due t o the small cross s e c t i o n i n photoemission f o r the s-derived states. The Ag/A1(111) case e x h i b i t s the i n t e r e s t i n g r e s u l t o f being compressed by 5.6 % i n t o a 20 l a t t i c e a t t h e f u l l ML coverage. This can be understood as one of t h e r a r e cases i n which a separate E D - l a t t i c e constant could be observed.

For the d-metal

substrates the adlayer-substrate

mostly t o determine the l a t t i c e constant,

i.e.

i n t e r a c t i o n seems

seems t o o v e r r i d e the l a t e r a l

427

i n t e r a c t i o n . Therefore,

c o n t r a c t i o n as w e l l as d i l a t i o n are observed.

A t the

moment t h e o v e r a l l impression i s t h a t the e l e c t r o n i c s t r u c t u r e seems r a t h e r i n dependent t o changes i n l a t t i c e parameter by about ?5 %.This s i t u a t i o n i s r a t h e r u n s a t i s f a c t o r y . It could w e l l be t h a t one has t o look f o r more s e n s i t i v e experimental parameters. We have elucidated one general d i s t i n c t i o n , i . e . a1 substrates.

t h a t between sp and d-met-

The sp-metal substrates seem t o be appropriate t o make t h e 2D

band s t r u c t u r e observable i n some cases. The d-metal substrates, on t h e o t h e r hand, a r e l a r g e l y dominated by a mixture o f adlayer and i n t e r f a c e states. I n t e r e s t i n g l y , t h e energy distance o f the d bands from EF move q u i t e d i f f e r e n t l y w i t h thickness f o r these two cases: f o r sp-metal

substrates i t moves

towards EF, f o r d-metal substates i t moves away from EF w i t h increasing t h i c k ness. I t i s a l s o n o t understood a t t h e moment. To deduce some general trends from t h e experiments i s r a t h e r d i f f i c u l t . This i s t r u e a l s o f o r most c a l c u l a t i o n s . We have somewhat enlightened t h a t f o r t h e Cu case. There are very few c o n t r i b u t i o n s , i n which shortcomings o f t h e own method and d i f f e r e n c e s w i t h recent r e s u l t s from other authors are discussed. From most work no g u i d e l i n e s f o r discussing experiments can be gained. For t h e d-metal

substrates the work f u n c t i o n seems t o go t o t h e new value

w i t h i n t h e f i r s t ML. This i s d i f f e r e n t on sp substrates, where a r a t h e r long t r a n s i t i o n o f up t o 10 ML i s found i n some cases. I n p a r t t h i s may be due t o a l l o y formation i n t h e f i r s t layer. Cu, Pd and Au on A1 and Au on Cu seem t o be such cases. I n f u t u r e work magnetic material, rare-earth metals and compounds w i l l g a i n some i n t e r e s t together w i t h a l k a l i and e a r t h - a l k a l i n e s i n g l e c r y s t a l s .

Photo-

emission should be c o r r e l a t e d more s t r o n g l y w i t h geometric s t r u c t u r e . Calculat i o n s should be performed f o r more r e a l i s t i c geometry i n c l u d i n g the v e r t i c a l distance o f t h e adlayer. The important parameters and c a l c u l a t i o n methods have t o be made more understandable.

ACKNOWLEDGEMENT

The author i s g r a t e f u l t o M. Reimers, J. R e i f f e l and I. Reinhardt f o r care-

f u l t y p e w r i t i n g , proofreading and composing o f t h e manuscript. Discussion w i t h K. Kambe and some comments o f S. D. Kevan are appreciated.

428

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