XPS studies of donor intercalation compounds

Synthetic Metals, 12 (1985) 2 5 1 - 2 5 6

251

XPS STUDIES OF DONOR INTERCALATION COMPOUNDS

S.B. DiCENZO

A T & T Bell Laboratories, M u r r a y Hill, N J 07974 (U.S.A.)

I N T R O D U C T I ON

This p a p e r p r e s e n t s examples of X-ray p h o t o e l e c t r o n spectroscopy (XPS) d a t a on g r a p h i t e i n t e r c a l a t i o n c o m p o u n d s (GICs), with e m p h a s i s on b o t h the capabilities and t h e l i m i t a t i o n s of this s p e c t r o s c o p y

All d a t a were t a k e n on freshly-cleaved, liquid

nitrogen-cooled samples, in an E S C A L A B MK II s p e c t r o m e t e r with a m o n o c h r o m a t i z e d Al K a

source

(ht/=1486

eV)

The

base pressure

is less t h a n

10 - 1 °

torr

The

p h o t o e l e c t r o n ' s kinetic energy is m e a s u r e d in a h e m i s p h e r i c a l analyzer; this d e t e r m i n e s the e l e c t r o n ' s b i n d i n g energy, as EB = h ~ , - E K + ¢, where ¢ is the s p e c t r o m e t e r ' s work function.

Because ¢ is c o n s t a n t , X P S directly m e a s u r e s the electron's b i n d i n g energy

relative to EF, the F e r m i level of the s a m p l e V A L E N C E B A N D XPS: KC8 T h e p h o t o e m i s s i o n i n t e n s i t y is p r o p o r t i o n a l to t h e n u m b e r of p h o t o e m i t t i n g a t o m s a n d to t h e i r p h o t o e m i s s i o n cross section.

T h u s t h e valence b a n d s p e c t r u m m a p s o u t

t h e d e n s i t y of states, with each atomic orbital w e i g h t e d by its cross section. example of t h e usefulness of valence b a n d X P S is found in F i g . K C 8 at low E B

An

1, t h e s p e c t r u m of

T h e 7r o r b i t a l s of g r a p h i t e have essentially zero cross section at our

p h o t o n energy[l], a n d t h u s any i n t e n s i t y o b s e r v e d near the F e r m i level (E B = 0) is due to electrons o c c u p y i n g s t a t e s derived from K o r b i t a l s i n t e r c a l a n t charge to occupy t h e K 4s orbital. the

potassium

controversial[2,3]

4s

charge

is

transferred

In p a r t i c u l a r , one expects a n y

To date, t h e question of how m u c h of to

graphite

bands

in

KC s

remains

By c o m p a r i n g t h e yield near E F with t h a t of t h e K 3p peak (at

a b o u t E B = 2 0 eV), a n d n o t i n g the relative 3p a n d 4s cross sections[l], we d e d u c e the p r e s e n c e of a p p r o x i m a t e l y 0.1 valence 4s electron per K atom.

Previously published

dural2 ] on K C 8 were o b t a i n e d with an u n m o n o c h r o m a t i z e d source; the d e c o n v o l n t i o n Elsevier Sequoia/Printed in The Netherlands

252

u

x 1oo

Q.

]

0

o¢

U

8

I 20

I 10 BINDING

ENERGY

I 0 (eV)

Fig. 1. X - r a y p h o t o e m i s s i o n s p e c t r u m of KCs, e m p h a s i z i n g the c o m p a r i s o n b e t w e e n t h e K 4s yield at E B = 0 a n d t h e K 3p a t " 18 eV. These d a t a were collected in a 40 m i n u t e scan b e g u n less t h a n one h o u r after t h e nitrogen-cooled sample was cleaved. needed

to

remove

the

satellite

spectrum

of the

u n d e s i r a b l e increase in noise at the F e r m i energy.

aluminum

anode

produced

an

O u r use of a m o n o c h r o m a t o r

p r o d u c e s a m u c h cleaner s p e c t r u m and, hence, more definitive results.

Despite this

i m p r o v e m e n t , t h e r e is more to consider in i n t e r p r e t i n g these data. E l e c t r o n s with a kinetic energy of 1 keV have a m e a n free p a t h in solids of 12-20A . T h u s t h e electrons d e t e c t e d in the typical X P S m e a s u r e m e n t originate in t h e first few atomic layers of the sample, a n d t h i s surface sensitivity m u s t be b o r n e in m i n d w h e n i n t e r p r e t i n g p h o t o e m i s s i o n data.

For GICs in p a r t i c u l a r , d a t a are always t a k e n in

u l t r a - h i g h v a c u u m , w i t h samples cleaved in situ.

To verify t h a t the d a t a t a k e n are

r e p r e s e n t a t i v e of the bulk, it is useful to c o m p a r e s p e c t r a t a k e n a t d i f f e r e n t emission angles. A p p l i c a t i o n of this t e c h n i q u e to K C 8 finds excess p o t a s s i u m ( < at t h e surface.

1 monolayer)

F u r t h e r m o r e , t h e K 4s signal is mostly associated with t h i s surface

p o t a s s i u m , so t h a t these X P S d a t a establish an u p p e r limit of 0.05 4s electron r e t a i n e d per K atom.

T h e s t r e n g t h of this d e t e r m i n a t i o n is t h a t t h e m e a s u r e m e n t of t h e K 4s

charge is direct a n d t h e i n t e r p r e t a t i o n of the d a t a is s t r a i g h t f o r w a r d . CORE LEVEL SPECTROSCOPY:

KC24

T h e more c o m m o n use of X P S is in core level spectroscopy. consists

of

identifying

elements

present

in

the

sample

In its simplest form, this and

determining

the

s t o i c h i o m e t r y , relying on each e l e m e n t ' s unique s p e c t r a l f i n g e r p r i n t of core electron

253

i, U

I-

2

0

U 0 0

I

I 288

290

I

I 286

I

BINDING ENERGY

I 284

(eV)

Fig. 2. C Is s p e c t r u m of KC24 , including t h e r e s u l t of a f i v e - c o m p o n e n t least-squares fit. T h i s is a sum of seven 9 - m i n u t e scans. p e a k s of k n o w n b i n d i n g energy and cross section.

Beyond this, e x p e r i m e n t a l i s t s often

infer t h e charge s t a t e of an atom from the shift in the core electron b i n d i n g energy, which

can vary by several eV d e p e n d i n g on the a t o m ' s e n v i r o n m e n t .

Although

Siegbahn[4] has offered c a l i b r a t i o n s of shift vs charge state, E B is n o t simply a linear f u n c t i o n of charge.

T h e r e are e x t e r n a l c o n t r i b u t i o n s such as the M a d e l u n g p o t e n t i a l ,

a n d t h e r e are final-state effects.

In t h e final state, t h e valence electrons' response to

the core hole reduces t h e m e a s u r e d b i n d i n g energy, by an a m o u n t d e p e n d i n g on, e.g., t h e valence electron configuration.

A l t h o u g h large shifts, especially, can clearly signal

a c h a n g e in the a t o m ' s charge state, core level spectroscopy is often c o m p l i c a t e d by our inability to d i s t i n g u i s h b e t w e e n initial a n d final s t a t e c o n t r i b u t i o n s to tile shift. T h e d e p e n d e n c e of EB on t h e a t o m ' s e n v i r o n m e n t is i l l u s t r a t e d in F i g . s p e c t r u m of a liquid nitrogen-cooled KC24 sample.

2, t h e C l s

In t h i s s t a g e - t w o c o m p o u n d the

c a r b o n a t o m s m u s t occupy a v a r i e t y of sites relative to the K + ions[5]. In p a r t i c u l a r , if t h e K ions are registered, i.e., if t h e y occupy sites above t h e centers of the g r a p h i t e hexagons, t h e n half of t h e C a t o m s will be i m m e d i a t e l y a d j a c e n t to K ions; this is likely to r e s u l t in a unique value of EB for these C atoms.

T h e C ls s p e c t r u m has

r e p r o d u c i b l e s t r u c t u r e , with the b r e a k n e a r the m a x i m u m i n d i c a t i n g a m a j o r d i s t i n c t c o m p o n e n t at lower EB. Indeed, in a five peak fit to t h e data, the c o m p o n e n t at low E B has a p p r o x i m a t e l y half (0.48) the total C ls intensity.

This result offers s t r o n g

s u p p o r t for the notion of registry of K ions in KC24. T E R N A R Y GICs: KHgC 8 and KHgC 4 T h e r e are as yet no b a n d s t r u c t u r e calculations to s u p p l e m e n t the e x p e r i m e n t a l results

on

these

interesting

compounds.

KHg-intercalated

graphite[6]

is

s u p e r c o n d u c t i n g , with a h i g h e r TC in the s t a g e - t w o c o m p o u n d KHgC 8 t h a n in t h e

254 stage one

compound

K H g C 4.

Thus

it is surprising that

the

specific heat

measurement[7] shows a higher density of states for stage one. We hope for valence band XPS to give insight into the nature of these states near EF. It is also interesting to compare the ternary compounds to KC 8 and KC24. The K atoms in KHg-GIC have the same 2x2 in-plane structure as in KC 8, and the amount of charge in the r bands is similar in the binary and ternary compounds[8]. Because the

K/C

ratio is higher in the ternaries, there is the question of whether the excess K 4s

charge is retained by the K atoms, is transferred to the Hg layer, or occupies hybridized states of both Hg and K origin.

Also, as the likely structure of the

intercalant layer is only a slight distortion of the structure of the intermetallic compound KHg 2, one expects strong K-Hg interactions in KHgC4,s.

~

AKHgC

Fig. 3. The core electron spectra of KHgC4, KHgCs, and KCs. including the C ls peak at lower EB and the K 2p doublet. Each spectrum was accumulated in "50 minutes.

.,-.J ,

i

i

It 290

i

i

I

i

}1

i

i

800

BINDINGENERGY(eV)

Figure 3 compares the core level data fromKHgC4, KHgCs, a n d K C 8. The decreased K 2p binding energy in the ternary compounds, noted in reference 9, suggests more K 4s charge in these compounds.

The high resolution of our data permits a lineshape

analysis of the K 2p doublet[10].

The significant difference between these spectra,

besides the binding energy shift, is the increasing asymmetry in the sequence KCs, KHgC8, and KHgC 4. Core line asymmetry in XPS of metals results from the finalstate screening response of the conduction electrons. A higher asymmetry is typically due to an increase in the density of conduction electrons occupying the valence orbitals of the core-ionized atom or occupying hybridized states derived from that atom's orbitals.

Thus the simplest interpretation of the increasing asymmetry is an

increasing K 4s occupancy in the sequence KCs, KHgCs, KHgC 4.

255

Figure 4 compares the valence band spectra of KHgC 4 and KHgCs, normalized to the Hg 5d doublet.

The most obvious difference between the two spectra is the change in

the K-to-Hg ratio, caused mainly by the evaporation of Hg from the cleaved surface of the stage-one sample.

Such problems with surface composition are in our experience

confined to stage-one s a m p l e s carbon

layers,

so t h a t

the

Stage two samples cleave almost entirely between surface

consists

mainly

of relatively

inert

carbon-

intercalant-carbon sandwiches and allows little or no Hg evaporation or K migration. A comparison of Hg and K intensities for KHgC s yields the correct stoichiometry to within 10°7o.

'

L . . . .

--

KHgC 4 KHgC

8

¢ Z

&

!

I-

o

' BINDING

1'o

i

I

ENERGY

i

t

(,v)

i

i

i

Fig. 4. The valence band spectra of KHgC 4 and KHgCs, normalized to the Hg 5d peak at " 1 0 eV. Each s p e c t r u m was accumulated in " 3 . 5 h, beginning " 1 . 5 h after each cooled sample was cleaved.

A more interesting feature of Figure 4 is the additional intensity near EF in the stage two data; this intensity is " 4 0 % higher per Hg atom than in stage one. As in the KC8 data, this signal can be due only to intercalant orbitals, but these d a t a do not refute the lower K 4s occupancy in KHgC8 t h a t was tentatively inferred above from the core level data.

The increased intensity may reflect an increase in intercalant charge, or a

transfer of charge from orbitals of relatively low cross section to orbitals of higher cross section.

The relevant intercalant orbitals are Hg 8s, Hg 6p, and K 4s, which have

cross-section ratios of 7:2:1, so t h a t the d a t a are most easily explained by the stage-two compound having more charge in the Hg 6s orbital or, more likely, in hybridized states of significant 6s character.

This is at least consistent with the inference, drawn from

the K 2p spectra, of less K 4s charge in the stage two compound, but given the 7:1 ratio of Hg 6s and K 4s cross sections, these valence band data cannot directly test for changes in the K 4s occupancy.

A t this point the lack of theoretical results[ll] is

sorely felt, as these d a t a could best set bounds on such quantities as the a m o u n t of Hg 6s charge in each compound.

256

SUMMARY The XPS data on donor GICs presented here provide insights that are a valuable complement to data obtained using other techniques. Valence band spectra show that the potassium in KC 8 is nearly fully ionized. The C l s s p e c t r u m of KC24 is consistent with the K ions being registered with the graphite lattice. The core level spectra of KHgC4 and KHgC8 suggest the presence of K 4s charge in these compounds, with more K 4s charge present in the stage 1 compound. The valence band spectra for the ternaries suggest more Hg 6s character in KHgC8, which has a higher T¢; this raises the possibility that the Hg s electrons play a key role in the superconductivity of KHg-GIC. The data presented here were taken in experiments done in collaboration with J. E. Fischer, D. Neumann, and P. A. Rosenthal.

REFERENCES 1

J.H. Seofield, J. Electron Spectrosc.,8 129(1976).

2

M. E P r e i l a n d J.E. Fischer, Phys. Rev. Lett., 52 1141(1984).

3

A summary of the conflicting results can be found in reference 2.

4

K. Siegbahn, C. Nordling, G. Johansson, J. Heden, P. F. Heden, K. Hamrin, U. Gelius, T. Bergmark, L. O. Werne, R. Manne, and Y. Baer, in ESCA Applied to Free Molecules (North-Holland, Amsterdam, 1969).

5

S.B. DiCenzo, S. Basu, G. K. Wertheim, D. N. E. Buchanan, and J. E. Fischer, Phys. Rev.,B25 620(1982).

6

c.f., e.g., Intercalated Graphite, Vol. 20 of Materials Research Society Symposium Proceedings, edited by M. S. Dresselhaus, G. Dresselhaus, J. E. Fischer, and M. J. Moran (North-Holland, New York, 1983).

7

M. Alexander, D. P. Goshorn, D. Gu~rard, P. Lagrange, M. El Makrini, and D. G. Onn, Solid State Commun., 38 103(1981).

8

M. E Preil and J. E. Fischer, Solid State Comm., 44 357 (1982); M. E Preil, L. A. Grunes, J J. Ritsko, and J.E. Fischer, Phys. R e v . , B 3 0 5852(1984).

9

M. E P r e i l a n d J.E. Fischer, SyntheticMetals,8 149 (1983).

10

For a description of the analysis procedure, see G. K. Wertheim and S. B. DiCenzo, J. Electron Spectrosc. (in press).

11

Calculations for KHgC4, at least, are in progress. See R. J. Brown and N A. W. Holzwarth, B. A. P. S., 30 284(1985).