High density molecule-like species formed in surface layers of C8K

Synthetic Metals, 12 (1985) 269 274

269

HIGH DENSITY MOLECULE-LIKE SPECIES FORMED IN SURFACE LAYERS of C8K

K. MIKI, H. SUEMATSU and A. KOMA I n s t i t u t e of M a t e r i a l s Science, University of Tsukuba, lbaraki 305 (Japan)

ABSTRACT The s u r f a c e of C8K is very a c t i v e at room t e m p e r a t u r e and it is easily d e t e r i o r a t e d even under ultrahigh vacuum a t m o s p h e r e . Freshly c l e a v e d surfaces of C8K were exposed to H2., H 2 0 or O2 gas s y s t e m a t i c a l l y , and i n v e s t i g a t e d with lowenergy e l e c t r o n energy loss spectroscopy and Auger e l e c t r o n spectroscopy. It is proved t h a t a t room t e m p e r a t u r e the CsK s u r f a c e r e a c t s strongly to oxygen, and a high density of molecular species, presumably K 2 0 or its r e l a t e d compound molecules, are f o r m e d on the surface. This results in t h e very easy d e t e r i o r a tion of t h e C8K surface. The s u r f a c e of C8K a t 140 K, on t h e o t h e r hand, is much less a c t i v e when it is exposed to gasses.

INTRODUCTION The surface

of C8K is unusually a c t i v e and a freshly c l e a v e d s u r f a c e d e t e r i o -

r a t e s within a few hours a t room t e m p e r a t u r e under ultrahigh vacuum of 2 x 10 -8 Pa.

Thus

surface

spectroscopy,

has

sensitive been

measurement

made

with

the

on

C8K ,

specimens

such

as

cooled

X-ray

by

photoemission

liquid

nitrogen

in

order to avoid s u r f a c e d e t e r i o r a t i o n . [1] We have electron It

investigated

energy

is found t h a t

sharp

and

similar features face

the

deterioration

are that

region of C8K.

of d e t e r i o r a t e d

structures.

obtained

The

regardless

dense and d e f i n i t e In order

of C8K s y s t e m a t i c a l l y

(LEELS) and Auger

LEELS s p e c t r a

characteristic

spectra imply

the

loss spectroscopy

to

of

electron

of CgK

are

reproducible

deterioration

m o l e c u l a r species

identify

the

low-energy

surfaces

spectra the

with

spectroscopy

formed

are

procedure. formed

molecular

(AES).

show many and

the

Those

on the sur-

species,

LEELS

and AES s p e c t r a of CgK s u r f a c e s exposed to H 2, 0 2 and H 2 0 have been m e a s u r e d at room t e m p e r a t u r e and 140 K. 0379-6779/85/$3.30

© Elsevier Sequoia/Printed in The Netherlands

270

EXPERIMENTAL Since the or

C8K s u r f a c e is very active, special c a r e was taken to obtain clean

gas-exposed s u r f a c e s

was

soldered

to

a

in a

w e l l - c o n t r o l l e d manner.

The

intercalated

specimen

s p e c i m e n holder with indium in a glove box filled with pure

helium gas, and then put into a small vacuum vessel having a g a t e valve.

After

the vacuum vessel was a t t a c h e d to the s p e c i m e n t r a n s f e r mechanism, it was evacua t e d down to 5 x 10 -6 Pa. chamber

through

t r a n s f e r r e d to

the

Then the s p e c i m e n was t r a n s f e r r e d into the measuring

transfer

mechanism.

Thus

the

intercalated

specimen

the measuring c h a m b e r without any exposure to air.

was

The pressure

in t h e measuring c h a m b e r was less than 2 x l0 -8 Pa and the s p e c i m e n was c l e a v e d under this vacuum condition just b e f o r e m e a s u r e m e n t . The

gas-exposure

measurement

was

cally pure gas into the UHV system.

accomplished

by

introducing

spectroscopi-

The s p e c i m e n was kept at room t e m p e r a t u r e

or 140 K in order to s e e the e f f e c t of the t e m p e r a t u r e on the gas adsorption. The Auger and LEELS s p e c t r a w e r e m e a s u r e d with a p e r s o n a l - c o m p u t e r - c o n t r o l l e d s p e c t r o m e t e r using pulse count d e t e c t i o n . where. [2] merical very beam

The details have been published e l s e -

The N(E) s p e c t r a w e r e s t o r e d in the c o m p u t e r , and averaging and nu-

differentiation

high

sensitivity

current

irradiation.

as

were and

low as

Actually

accomplished

is especially possible to

the

afterwards.

useful

for

The

reducing

present the

system

primary

has

electron

avoid s u r f a c e change due to e l e c t r o n beam

measuring e l e c t r o n beam

has

been

proved

to

give

no

e f f e c t on the s p e c i m e n surface.

EXPERIMENTAL RESULTS AND DISCUSSIONS In o r d e r

to check how easily the freshly c l e a v e d s u r f a c e of C8K d e t e r i o r a t e s ,

the changes in AES and LEELS s p e c t r a w e r e measured with t i m e a f t e r the cleavage. Figures 1 and 2 show time-dependent AES and LEELS spectra of the specimen kept

under 2 x 10 -8 Pa at room t e m p e r a t u r e . t i m e elapsed a f t e r

the cleavage.

The numbers in the figures indicate the

As shown in Fig.

f r e e from c o n t a m i n a t i o n just a f t e r

t h e cleavage.

potassium

increases

signal

appears a f t e r spectra 300 Those Fig.

about

in Fig.

rain,

and

to 2.

This was

signal

300 rain.

A

the change in t h e s p e c t r u m

characteristic

structures measured

gradually.

slight

the r a t i o oxygen

of

signal

T i m e - d e p e n d e n t change is also seen in the LEELS

Here again

structures

characteristic 2.

carbon

1, the s p e c i m e n s u r f a c e is

As t i m e elapses,

for

are a

of a deteriorated

clearly

seen

in

the

is seen a f t e r

about

s u r f a c e b e g i n t o appear bottom

spectrum

in

typical " d e t e r i o r a t e d " s u r f a c e of a specimen,

which had been kept in the ultrahigh vacuum c h a m b e r for two days a f t e r cleavage. Similar LEELS s p e c t r a w e r e observed in any d e t e r i o r a t e d s u r f a c e of C8K including

271

C8K

I

0

K,,C

Ep: 1 O0 eV

22

76

70rain

i

(,1 ¢,,:3

.d

160 min 240!min

..d t,,-

0 v

Z

~...__.~_.~"~90

295min

LLI "10

min

360 min

~J

Z 10 ~

5

0

m

i

620 mi n

n

189~min

•

~-.,..,._~/~510

min

deteriorat

jv V ,~/F__ 0

I

10 20 30 40 ENERGY LOSS( eV )

0

t r a n s f e r r i n g into the m e a -

This indicates t h a t those s t r u c t u r e s c o m e from the s a m e origin.

It must be noted t h a t sharp

600

Fig. 2. Time-dependent change of the AES spectrum o~ C ~ surface kept under 2 x ]O-U Pa at room temperature after fresh cleavage.

n o t - c l e a v e d s u r f a c e of a C8K s p e c i m e n just a f t e r

suring c h a m b e r .

400

ENERGY(eV)

Fig. I. Time-dependent change of the LEELS spectrum of C~K surface kept under 2 x ~0 -8 Pa at room temperature after fresh cleavage.

the

200

structures

the LEELS s p e c t r a in Fig. 2 are n o n - d i f f e r e n t i a t e d ones

and

such

with

strong

the

n o n - d i f f e r e n t i a t e d LEELS s p e c t r a

intensities

of o t h e r

are

usually

materials.

not

presented

in

Those sharp s t r u c t u r e s

show t h a t

some molecular species, which give sharp LEELS s t r u c t u r e s due to the

electronic

transitions

s u r f a c e of a C8K.

between

their

narrow

energy

levels,

are

formed

The strong i n t e n s i t i e s of the s t r u c t u r e s imply t h a t

on

the

the den-

sity of molecular species is c o m p a r a b l e to that of t h e a t o m s in a solid and that they exist not only in t h e tent.

Actually

the

top layer but also in the d e e p e r region to some ex-

LEELS m e a s u r e m e n t on t h e d e t e r i o r a t e d s u r f a c e of C8K , in

which the probing depth was changed b e t w e e n 2 A and 10

by changing the inci-

dent e l e c t r o n energy b e t w e e n 100 eV and 1000 eV, has shown t h a t t h e s p e c t r a are

272

j/ .~~ ~ d e ,.t e r i o r a t e d

Ep=

"C~ % A t ' , .

100eV]

/e×posed

to

I

r

UJ

~,l,i~ "

Ixio'ePo HzO

1000 eV

~ 3D 2.5 7.9 12.717/, 21.7 28.7 37.2

[ 0

10 20 30 40 0 ENERGY LOSS(eV) Fig. 3. The depth-profile of deteriorated surface of C ~ by raring the incident electron energy in LEELS.

, , ,C8K 10 20 30 z~ ENERGY LOSS( eV )

Fig. 4. LEELS spectra of cleaved face of CSK exposed t~ H 2, H ~ and 02 gas of 1 x 10 -v Pa for 10 min. at room temperature.

nearly the s a m e in t h e m e a s u r e d probing d e p t h region, as shown in Fig. 3. T h e r e fore t h e molecular species are f o r m e d a t least in a few layers from t h e s u r f a c e of C8K even under t h e u l t r a h i g h vacuum of 10 -8 Pa r a n g e within two days a f t e r a fresh cleavage. Figure 4 shows t h e

LEELS s p e c t r a of c l e a v e d faces of C8K , which were t a k e n

a f t e r the exposure to H2, 0 2 or H 2 0 gas of 1 x 10 -6 Pa for 10 rain a t room t e m perature.

As shown in t h e figure, t h e s p e c r a are a l m o s t unchanged when t h e spe-

c i m e n is exposed to H 2 or H 2 0 gas.

But p r o m i n e n t c h a n g e in t h e s p e c t r a is seen

a f t e r the exposure to 0 2 gas, and it b e c o m e s just t h e s a m e as t h e s p e c t r u m of t h e d e t e r i o r a t e d

surface.

The c h a n g e

only in the s p e c i m e n exposed to 0 2 gas.

in AES s p e c t r a has also been observed The AES s p e c t r u m for t h e s p e c i m e n ex-

p o s e d to 0 2 gas is the same as the "deteriorated" surface o f C8K, w h i c h cons i s t s of a s t r o n g p o t a s s i u m p e a k and w e a k carbon and o x y g e n peaks. Those experimental

results

indicate

clearly

that

freshly

cleaved

surface

of

C8K

reacts

273

CsK

exposed to lx10-6 PQ02

:3

Jd

exposed to 1 x 10-SpQ £h

m

ex.l~_e~:l to lx10- PQ02

8

deterionsted CsK

,6

0

2b

3b

ENERGYLOSS (eV)

Fig. 5. LEELS spectra of a cleaved face of C ~ exposed to 02 gas of the pressure shown in the figure for 10 min. at 1~0 K.

strongly and s e l e c t i v e l y to oxygen. the

It is also presumed from

those results t h a t

molecular species formed in the d e t e r i o r a t e d s u r f a c e layers of CgK are K20

or its r e l a t e d compound molecules. The LEELS and AES s p e c t r a have also been measured for the C8K s u r f a c e s exposed to oxygen

gas at

140 K.

Figure 5 shows the LEELS spectra of the cleaved

f a c e of C8K exposed to 0 2 gas of 1 x 10-6 Pa, 1 x 10-5 Pa or 1 x 10 -4 Pa for 10 rain. at

140 K.

As shown in the figure, the LEELS s p e c t r u m is almost unchanged

even upto the 10 min. 0 2 exposure of 1 x 10 -4 Pa, which is two orders of magnitude larger than the exposure inducing the large change in t h e s p e c t r u m at room temperature.

Thus the c l e a v e d f a c e o f C8K is much less a c t i v e at

room t e m p e r a t u r e .

140 K than at

Although t h e LEELS s p e c t r u m has been almost unchanged, the

AES s p e c t r u m of C8K has changed substantially with the exposure of 1 x 10-4 Pa oxygen for

10 rain. at

140 K.

The observed oxygen signal is c o m p a r a b l e to t h a t

in the AES s p e c t r u m of CBK s u r f a c e exposed to 1 x 10 -6 Pa o x y g e n at room t e m perature,

which shows a c o m p l e t e l y changed LEELS s p e c t r u m .

tion occurs on t h e C8K s u r f a c e at without forming c h e m i c a l species.

Thus only physisorp-

140 K and oxygen exsists on the C8K s u r f a c e

274

The s a m e gas-exposure m e a s u r e m e n t was made at room t e m p e r a t u r e also for fresh surface

of

deposited

metal

potassium,

but

the

LEELS s p e c t r u m

was

almost

changed even a f t e r the s a m e exposure to oxygen gas as the c a s e in C8K. indicates t h a t

the

r e a c t i v i t y to oxygen is unusually high on the s u r f a c e of CsK ,

which results in its very easy d e t e r i o r a t i o n under ultrahigh vacuum perature.

unThis

at

room t e m -

Thus the potassium a t o m s seem to exist in very r e a c t i v e forms in C8K.

CONCLUSIONS The c l e a v e d s u r f a c e of C8K is unusually active at room t e m p e r a t u r e . It r e a c t s s e l e c t i v e l y to oxygen and a high density of molecular species, presumably K 2 0 or its r e l a t e d compound molecules, are of

f o r m e d in the s u r f a c e layers. The e x s i s t e n c e

high density molecular species s e e m s to b e c o m e possible only in the

lation compound such as C8K.

interca-

Strong r e a c t i v i t y to oxygen is the cause of easy

d e t e r i o r a t i o n of C8K s u r f a c e at room t e m p e r a t u r e .

At

140 K, on t h e o t h e r hand,

the s u r f a c e of C s K is much less active, and physisorption only occurs when it is

exposed to gasses.

REFERENCES 1

M.E. Preil and J.E. Fischer, Phys. Rev. L e t t . , 5 2 (1984) 1141.

2

A. Koma and K. Yoshimura, Jpn. J. Appl. Phys.~ 22 (1984) L173.