Surface structure of graphite intercalation compounds resolved in real space by scanning tunneling microscopy

SlttthcticMctals, 34(1989) 175 185

lvb

SURFACE STRUCTURE OF GRAPHITE INTERCALATION COMPOUNDS RESOLVED IN

R.

REAL SPACE BY SCANNING

WIESENDANGER,

and

H.-3.

D.

ANSELHETTI,

TUNNELING MICROSCOPY

V.

GEISER,

H.R.

HIDBER

GUNTHERODT

Institut

fdr

CH-#056

Basel

Physik,

Universit~t

Basel,

Klingelbergstrasse

~2,

(Switzerland)

ABSTRACT Scanning freshly

compounds reveal

tunneling

cleaved at

been

defects

as w e l l

observed

measurements a grapbitic

the

performed

so

surface

a lithium

layer

cleavinq.

The l i t h i u m

the

graphite

being

lattice

two d i m e n s i o n a l

on

top

forms

or

study

on a s u b m i c r o m e t e r

with

of

ordered with

surface

We s u g g e s t on t h e of

the

In

first

graphite

of

that

lattice STM

CsK h a v e the

of

revealed

different

C6L1 a r e

graphitic

supertattices

incommensurate,

scale

spatial

contrast,

surface

the

superlattices

the

C6Li.

the

either

an

to

intercalation

unprecedented

on

locally

used

STM i m a g e s

surface

far

been

graphite

steps

terraces,

structure.

observed

has

1 donor

incommensurate

on

superlattices

as

flat

as

(STM)

stage

such

On a t o m i c a l l y

commensurate have

of

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

surface

resolution.

microscopy

surfaces

layer

due

to

after

commensurate

nearly

with

close-packed

overlayer.

INTRODUCTION In

recent

years

intercalation resolution little

is

micrometer Setti

transmission known

eta].

freshly

about

scale. /2/

20 nm l a t e r a l

of

the

microscopic

compounds

electron the

bulk

has

have used

structure

been

work

a scanning

ion

determining

SbC15-intercalated

in

/]/.

structure

pioneering

for

of

studied

microscopy

surface

their

resolution

cleaved

0379-6779/89/$3.50

In

(GIGs)

of in

graphite detail

On t h e GIGs

this

graphite.

surface

Since

high

other

hand

on a s u b -

field,

microprobe the

by

R.

capable

Leviof

microstrueture

the

invention

© Elsevier Sequoia/Printed in The Netherlands

176

of

the

scanning

it

has

become p o s s i b l e

in

real

space

technique the

tunneling even

here

as

literature

of

several

in

to

distances.

the

it

is

the

process

of

to

the

electronic

report S.

donor

of

two

t~e w i l l

GZCs

lattices

/3/,

the

STM

available

in

lattice

have

been

surface

structure

as

of

mainly They

GICs

C8K by

observed was

as

on

found

for

frequency

play

the

short an

Fermi and

surface

to

acceptor

CICs

level,

its

of

a graphitic

impor-

STM i s

suitable

the

tool

for

CICs

/5/.

has

been

surface

their

experimental

/7/. studies

of

room temperature.

incommensurate

the

also

since

a highly

resolution

STM a t

as w e l l

the

graphite

interpreted

depletion

is over

of

at

found

and

it

near

of

constant4

be d i f f e r e n t

which

states

waves

/6/.

atomic

probe,

Thirdly,

STN i s

density

al.

might

steps

study

several

lattice

structure

intercalation.

acceptor

and

which

determination

such

for the

the

el

commensurate

interest

a local

STH a p p l i e d

present

C6Li

of

determine

surface

Finally,

acceptor

by a s u r f a c e

Here,

al.

directly

describe

already

the

density

charge

Cauthier

for

results

et

surfaces

not

are

surface

a comparative

compounds.

A first

structure

STM a l l o w s

for

is to

STM i s of

in

possible

by

Since

changes

defects

intercalation

given

the

surface

suited

detecting

GICs

of

well

Binnig

of

We w i l l

articles

be u s e d at

bulk. for

Secondly,

sensitive

STN t o

STM can

look

and character role

by C.

structure

scale.

revlew

superlattiees

those

tant

of

Firstly,

possible

(SIN)

the

/4/.

possible

from

study

on an a t o m i c

The a p p l i c a t i o n reasons.

microscope to

surface

of

the

stage

Ordered

with

the

1

super-

graphite

C6Li

while

a graphitic

the

study

of

C8K.

EXPERIHENTAL The s c a n n i n g GICs are

has

less

not steel

A gas to

already

extreme]y

could

far

tunnellng been

glove

box

puriflcation

etching

of

cleaved

in-situ

described

sensitive

be p e r f o r m e d

below

mleroscope

i

to in

eontaining

system ppm.

a 0.25

to

or

in

a poor

a high have

/8/.

Since

been

series

purity

of

All

donor

donor

GICs

SIN s t u d i e s

We u s e d

argon such

prepared

wire.

the

vacuum.

impurities

tungsten each

for

contamination,

reduced

STH t i p s

diameter

prior

elsewhere

surface air

used

a stain-

atmosphere.

as

H20,

02 and N2

by e l e c t r o c h e m i c a l samples

have

STM m e a s u r e m e n t s .

been

177 STM r e s u l t s For host

on

with

the

oriented

surface

by

the

over of

cleaved

shown

in

(HOPE)

(lO0

high

the

length

scale

compounds,

graphite

several

terraces,

STH a s

lateral

intercalation

pyrolytic

extending

these

graphite

calibrating

for

Atomically

usually

lattice

of

comparing

studied

After

graphite

flat

terraces on

the

zooming can

the

highly

be o b s e r v e d

HOPE s a m p l e s .

hexaoonal Fig.

by STH.

nm) 2 c a n

quality

and

we f i r s t

into

be r e s o l v e d

l.

Fig. 1. STM t o p - v i e w image of a 1.8xl.8 2 surface a r e a o f HOPE o b t a i n e d

nm

by r e c o r d i n g tunneling value

current

of

voltage

changes

[=l

nA

In

this

where

grey-scale the

top-view

tunneling

correspond

to

Within

elemental

the

bright, This

one

can

within

the

which

is

surface

found

atom

atom

is

have

shown

that

level,

going the In

which

the

carbon view

the

is

of

an

a-site

to

graphite,

the

next

the

spots

ABAB...

one

carbon

al.

STM / 1 1 / , the

the

of

density is

higher

tunneling

a b-site

belonging

to

a-

on

GICs

and

the

b-sites which

bright:cst. atoms

sequence

has

a carbon

the

hexagon Tomanek

the is

be

near

other

in

et

the

al. the

b-sites

changing

vertical may

are

appears

carbon

states

for

current

although

5TH s t u d i e s

of

locations

whereas

a carbon D.

see.

spots

stacking atom

and

to

appear two

(a-site)

/9/

i

increased.

which

the

mean

Data

bright

which of

center et

the

the bias

about

correspond

atoms

layer

electronic

by

to

three

mV).

current

carbon

inequivalency

Batra

local

spots

around

took

whereas

tunneling

six

Due

above I.P.

Therefore

atoms the

in

probed

a-sites.

from

for

located

(b-site).

the ceil.

directly

layer

the of

identify by

unit

next

for

where

clearly

dark

decreased

hexagon

explained

usually

neighbour carbon

locations

can

be

image

current

the

(sample

U= + 1 0 0

acquisition

of

/10/ FermJ

than by

positions

of

identical.

described

below

it

is

178 important

to

note

gives

an e x a c t

atomic

scale.

hexagons

C6Li with

appear

on

molten

acts

between

dark

in

a reference

the

the

system

STH s c a n n i n g

centers

STM i m a g e s

were

lithium

2 4 h and

obtained

is

by a l i q u i d

(reaction

60 h ) .

of

which

unit

on

neighbour

known

of

X-ray

diffraction.

The s u r f a c e

by

on a s u b m i c r o m e t e r

STH f i r s t

atomically

regions

as

ponding

bird-vlew

flat

shown

in

of

terraces

the

to

the

carbon

be 0 . 2 4 6

(Fig.

samples

freshly

nm.

exposure

plot

£6Li

Typical

with

HOPC times by

was c h a r a c t e -

STM i m a g e s

highly

(Fig.

of

was d e t e r m i n e d

cleaved

scale.

scan

reaction

250°0,

the

together

STH l i n e

image

phase

temperature:

The s t a g e

rized reveal

as

of

C6Li

samples

between

graphite calibration

The s p a c i n g

which

STM r e s u l t s

that

lateral

defective

2a)

and

the

corres-

2b).

14nrn 7rim

Fig.

2a.

250x250 obtained

STM l i n e scan plot 2 nm s u r f a c e area of in

the

constant

mode o f

operation

U= - 2 0 0

mV).

After ordered

lattice and

locating

the

superlattices not

types constants

0o49~0.02

Fig.

C6Li

was as

eurrent

of

tip

above

could only

one

be

of

the of

nm ( F i g .

3c).

latter

image which

from

the

same d a t a

2a.

acquisition

took

about

min.

free

on an

terraces, atomic

superlattiee

could

0.35~0.02 The

Bird-view

generated Fig.

defect

observed

type

superlattiees

nm ( F i g .

2b.

Data

hA,

i

Interestingly, different

(l=l

a

of

be

with

0.42~0.02 lattice

(Fig.

structure

identified

3a), two

well

scale

but

3a-c) three

in-plane

nm ( F i g .

constants

3b)

corres-

]79

(a)

(b) Fi 9. 3. STM top-view 2 1.8x1.8

nm

showing

three

with a)

surface

e)

rim,

b)

scale

was c a l i b r a t e d

on g r a p h i t e .

atomic

by c u r r e n t

U= - 1 0 0

images)

in

t=l

mV f o r

about

nm

by

resolut Data

imaging

current

of

The l a t e r a l

with

bias

C6[ i

0.42t 0 . 0 2

nm.

STM i m a g e s

tunne]ing

on

constants

comparison

obtained

of

superlattiee:

lattice

0.49~0.02

length

areas

different

in-plane

0.35t0.02

and

images

nA,

all

[on were

(mean

sample

three

1 see.

(c) pond tile

to

commensurate

bulk

constant of

nearly

the

increase

C6Li,

of

0.35t0.02

]ithium lithium this

STM i m a g e s as

lattice this

shown

in

constant constants superlattice.

on

0.311 the

rim.

The

0.35

which

are

However,

an

if

lattice

of

somewhat

be e x p l a i n e d

also

in-plane

neighbour

4.

observed

there

is

superlattice

a charge

repulsion

the

values around

by t h e

transfer

observed

three

than

in

will

The s p r e a d

larger

in

lattice

distance

constants

around

Fig.

is

incommensurate

electrostatic

grouped

nm i s can

to

in-plane

histogram

which

mentioned

The n e a r e s t

graphite,

CgLi

the of

The f i r s t

llthlum.

spacing.

other

~rxlr~,

nm c o r r e s p o n d s

is to

above lattice

and 2 x 2 .

close-packed

hexagonal from

of

superlattices:

for

the

in

mentioned the other

two

incommensurability

of

180

Fig.

4. Histogram showing the

d i s t r i b u t i o n of the observed i n - p l a n e l a t t i c e constants on ,l,,,oln,,rl,,,,l,,,,l,,,,l, 0.3 0.4 0.5 Into]

We w o u l d

like

observed

within

that

surface

or

H20 w i l l

of

the

after at

to

mention a few

that

hours

limit

atomic

this

lithium

However,

STM i m a g e s

sample

defective

superlattices cleaving

e.g.

time.

resolution the

the after

contamination,

cleaving

highly

the C6Li surface.

suggests

surface

the

the

only

sample.

reacted

residual

before

the

02

good quality

terraces

surface

be

We b e l i e v e

with

surprising

on t h e that

regions

could

obtained

directiy

contamination

starts

defect

free

terraces

become contaminated.

STM r e s u l t s For

on C~K

comparison

The s a m p l e s

had

characterized surface

of

Terraces of

as

separated

one elemental

defect After

the

superlattices seen At

the

to

an

moment

it

remains

surface

of

during

the on a bird-

surface

regions.

(not

slmply

as w e l l

of

multiples as

surface

other

defects

is

host. free

Fig. of

8.

C6Li,

identical

open whether

also

top-view,

defect in

STM.

different

and g r o o v e s

above

was a l w a y s

cleaving, extensively

heights

the

C8K by

and w e r e

we p r e s e n t

The f r e q u e n c y

as shown

on t h e

property

conditions

various

value),

STM t i p

in-situ

three

on HOPC as

lattlce

C8K s u r f a c e

intrinsic

non-idea]

than

observed

on t h e

of

5-7

of

of

reaction

investigated

Fig.

be s e e n .

the

atomic

In

surface

phase

After

been

images

height

can

locating

resolve

STM. steps

step

much h i g h e r

has

scan by

structures

usua]ly

by

line

the

by a gas

diffraction.

C8K s a m p l e scale

as w e l l

studied

obtained

by X - r a y the

submicrometer view

we h a v e a l s o been

this of

terraces,

we c o u l d

In

contrary

the

lattice

to

that

observation

the

surface

the

STM m e a s u r e m e n t s

of

to

the

constant graphite.

might

C8K c o m p o u n d s on C8K.

or

be due due

to

18]

(a) [ig.

(b) 5.

surface ([=

STM t o p - v i e ~ area

1 hA,

the

surfac'e

mV).

bird-vim~

scale

reqfons appears

in

In the

by a g r e y

deepest relief

and

C8K o b t a i n e d

U= +200

represented and

of

(a)

with

the

image

constant

top-vie~

appearing under

the

(b)

the

current

image

highest

black,

In

illumination

of

the

mode o f

third

regions the

a 250x250

appearing

bird-view

Fig.

6.

STH t o p - v i e w nm 2 s u r f a c e

(a) area

and of

bird-view CsK

(I=

(b) 1 nA,

image of U= +100

another

mV).

[s

~hite

Linage t h e

by an a r t i f i c i a l

(b)

250x250

opePation

dimension

Source.

(a)

rim'

I ight

182

2nm lnm

(a)

(b)

Fig.

7.

image

STM l l n e

(b)

of

scan

a third

U= + I 0 0

mV).

Fig.

STM t o p - v i e w

8.

obtained is

the

by c u r r e n t same as

in

plot

(a)

and

the

corresponding

2 5 0 × 2 5 0 nm 2 s u r f a c e

image of imaging.

STM i m a g e s

a 2.7x2.7

area

nm

The o b s e r v e d of

a graphlte

2

of

bird-view

C8K ( I =

surface

in-plane surface.

area lattice

1 hA,

of

C8K constant

183

DISCUSSION In

the

tation

of

the

following the

surface

of

electronic the

C6Li.

what

et

al.

C6li

/12/

have looked

should

the

tunneling

is

in

agreement

small

voltage

Kirezenow

covering

far

is

as

in

the

the

top

from

bulk

layer

1

of

two

with

in

or

has

always

llke

the

work

been

been

C6Li ,

omitted

seems

of

the

the

work

carbon

site charge

reported

so

superlattiee and

/13/.

be a c a r b o n

same

Secondly, although

after

will

should

the

layer,

that

intercalant

theoretical

at and

layers

GICs

be f i x e d

obvious

CGLi

Qin

transferred

completely to

on

structure.

STM on

to

X.

and

of

Firstly,

due

prediction

STM c o r r u g a t i o n

graphite

subband

assumed

it

part

Future

amount

about

assumed

viewpoint

surface. that

lithium

the

have

range.

of

been

mobility

may n o t study

physical

our

cleaving

be l e f t

focus

of

lattice

can

necessarily local

order

properties

exist. of

on t h e

on t o p

on

on t h e s e

of

surface,

STH as a l o c a l

probe

also two

order

be u s e f u l

dimensional

to

after

of

HOPG

as w e l l struc-

and c o v e r a g e that

due

different

incommensurate although

quasi

on t o p

temperature

the

or

surface

make a c o m p a r i s o n

incommensurate

be r e a l i z e d

and w i l l

to

on C 6 L i , we s u g g e s t

commensurate

locally

CGLi

long-range

close-packed

depending

lithium

the

overlayers

wlthout

STM r e s u l t s

either

of

may be u s e f u l

lithium

hexagonal

observed,

on t o p

it

overlayers

forming

superlattiees

graphite

indeed

evaporated

where

To e x p l a i n

high

is

STM e x p e r l m e n t ~

/15/

overlayers

tures

to

/12/

LEED s t u d i e s

substrates

the

to

surface

of

C6Li

voltage

This

predicted.

number

to

been

bias

on

STM m e a s u r e m e n t s

measured

on

difference

3a-e)

STM c o r r u g a t i o n

the

made

Selloni

STM i m a g e s

state.

than

layer,

has

Fig.

of

points.

cleaving

of

the to

assumptions:

surface

has

GlCs the

Assuming

as

the

the

larger

the

of

sample

dependent

surface

been

A.

(height

in

on

explanation

have

dependence spots

interpre-

the

the

/12,13/.

interlayer

although

theoretical

an e x p e r i m e n t a l

stage top

and

for

increasing

empty

mV) i s

guest

on d r a s t i c

at

with

the

graphite,

role

predictions

darkest

that

on

superlattices

STM c o r r u g a t i o n

voltage

/i#/

predict

all

based

structure

the

of

pure

voltage

be s e n s i t i v e

first

Unfortunately,

the

zero

our

distribution

of

important

measured

(,300

/13/

should the

its

with

case

mainly

kinds

by STM on G I C ' s

smooth

elsewhere

bias

asymmetry and

to the

focus

theoretical

and

fall into

as r e p o r t e d

the

at

the

brightest

rapidly

to

C.

that

to

different

an

be o b s e r v e d

and f o u n d

between

in

plays

Recently,

should

like

of

As

structure

STM i m a g e s .

of

we w o u l d

observation

with

long-range is

ideally

investigate

metallic

to kinds

the order suited the

overlayers

184 in d e t a i l .

Future

transitions on t h e

experimental

work

between d i f f e r e n t

investigation

of

will

kinds

surfaces

of

of

further

focus

superlattiees

other

on

the

as w e l l

as

d o n o r GICs.

ACKNOWLEDGEMENTS We w o u l d l i k e kindly

to

provldlng

A.W.

Moore

HOPG, M. Baur f o r

thank

skilful

microscope,

P.

steel

box and L.

glove

Reimann f o r

STM d a t a a c q u i s i t i o n Prof.

Dr.

and T.

assistance

Rosenthaler

(Union

in

for

K. L ~ d e r s f o r

his

help

in

proofreading

operating

writing

Swiss N a t i o n a l

the

the

Science

Carbide)

machining of

and d a t a p r o c e s s i n g .

Richmond f o r

from the

Dr.

the stainless

the software

We a l s o early is

for

thank

stage

manuscript.

Foundatlon

for the

of

thls

Financial

gratefully

work support

acknowledged.

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