STM Observations at the Atomic Scale of a Tilt Grain Sub-Boundary on Highly Oriented Pyrolytic Graphite

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STM Observations at the Atomic Scale of a Tilt Grain Sub-Boundary on Highly Oriented Pyrolytic Graphite

C. Daulan, A. Derré, S. Flandrois, J. Roux, H. Saadaoui

To cite this version:

C. Daulan, A. Derré, S. Flandrois, J. Roux, H. Saadaoui. STM Observations at the Atomic Scale

of a Tilt Grain Sub-Boundary on Highly Oriented Pyrolytic Graphite. Journal de Physique I, EDP

Sciences, 1995, 5 (9), pp.1111-1117. �10.1051/jp1:1995184�. �jpa-00247122�

STM Observations~at the Atonùc Scale of

_a

Tilt Grain

Sub-Boundary

_on

Highly Oriented Pyrolytic Graphite

C.

Daulan,

A.

Derré,

S.

Flandrois,

J-C- Roux and H. Saadaoui

Centre de Recherche Paul

Pascal, CNRS,

Université

Bordeaux-I,

_av. A. Schweitzer, 33 600

Pessac,

France

(Received

11

April

1995, revised 5

July1995, accepted

19

July1995)

Résumé. Des observations à l'échelle atomique ^d'un

sous-joint

de

grains

de flexion ont été réalisées _pour la

première

fois _par

microscopie

à effet tunnel

(STM).

Des modèles

géométriques

ont permis

d'interpréter

le réseau de dislocations mis _en évidence

sur la

ligne

de

jonction.

Ces

images

STM révèlent

également

la modification locale de la densité

volumique

de

charge près

du niveau de Fermi dans la _zone de

jonction.

Abstract. We report ^here the first observations ai the atomic scale of _a symmetrical tilt

grain sub-boundary

with _a STM. Trie

edge

dislocations observed _ai the atomic scale

along

trie boundary fine _can be understood in the trame of

geometrical

models. These STM images ^also reveal _a local modification of the

charge density

_near the Fermi level in trie

junction

_area.

Grain boundaries and

grain

sub-boundaries _are

crystallographic

defects charactenstic of _zone transitions between _two

crystals

with dilferent orientations. The manifestations of this kind of defects _at the

"macroscopic

scale" _were the

object

of _numerous studies

(using

TEM

essentially).

Other

techniques

such _as

High-Resolution

Electron

Microscopy (HREM) Iii, Grazing-Incidence X-ray Scattering technique (GIXS)

_{[2] or} Fresnel _contrast

analysis

in low-resolution

images

_(3]

provide,

in _some cases, information at _a smaller scale.

However,

these

techniques

are

heavy

to use and Iimited in their

spatial

resolution

(lateral

_as well _as

vertical). Owing

to trie

development

of

Scanning Tunneling Microscope (STM)

defects such _as

grain

boundaries _can be studied at the atomic scale. The lateral resolution achieved with this

type

of

microscope (<

1

À)

allows

one to

study

the _very fine _structure of these defects

(atomic displacements

in the

junction area).

This is also of

particular

interest when the interatomic distances of the studied

crystal

are less than the lateral resolution of _a HREM for

example (2 À),

_in the _case of

graphite

^for

instance

(in-plane

C-C distance of1.42

À).

Although graphite

is _one of the most STM studied

compound,

few observations of

grain

boundaries _were

reported

_on

Higuly

Oriented

Pyrolytic Graphite (HOPG).

These stems

mainly

©

Les Editions de

Physique

1995

ll12 JOURNAL DE

PHYSIQUE

I N°9

Fig.

l.

Low-pass

filtered STM

image

20 _{nm x} 20 _nm of _a tilt

grain sub-boundary

ai trie surface of HOPG; b1as 30 mV; outrent 2

nÀ;

constant current mode;

electrochemically sharpened

tungsten tip.

have the _{mean size} of the

grains

in HOPG

(50x50 pm~),

_a size far above the size of the

maximum STM field

required

for atomic resolution

(20x20 nm2).

Whereas dislocations in

various solids _were

successfully

observed with _a STM

(4, Si,

^{the atomic}

periodicity

_as well _as the dislocations network _{on a}

grain boundary

_were

rarely

observed

[6].

On

graphite surface,

a twist

grain sub-boundary

with _a

honeycomb

structure

I?i

^{and tilt boundaries} _(8]

(associated

with

multiple tip elfects)

_were indeed observed

by STM,

but atomic resolution observations

were not

reported

in these studies. In the

following,

_we

report

observations at the atomic scale of _a tilt

sub-boundary

on HOPG surface.

The observations

reported

here occurred in the _course of _an electrochemical

oxydation study

of _a HOPG

graphite.

Tue

experimental

set up is described elsewhere

(9]. Figure

1 shows _a 400

nm2 portion

of the surface wuere _{we can see}

that, separated by

_a Iine of

"holes",

the

global

orientations of the atomic _{rows on} the left and tue

right

_are

diflerent,

tue misorientation

angle being roughly

13°. The 2D Fourier transform of this

image displays

2 arrays of 6 spots, each

being

characteristic of the

hexagonal

^network

(lattice

parameter 2.46

À)

of

graphite.

The orientation

relationship

between the two

grains

is

simply

_a rotation around the < Dol _> axis shared

by

both

grains,

thus the

boundary

is characteristic of _a tilt _grain misorientation. The

misorientation

angle

is 13° ~ l.5°.

The direction of the

grain boundary

is

symmetrical

with

respect

to the two networks: it makes _a 6.5°

angle

with each. This

junction

_zone

spanning

_on 9 _to II observed _at smaller scale

(Fig. 2)

shows _numerous structural defects: ₁₎ The emergence on the surface

plane

of

edge

dislocations

fines, regularly spaced,

which is _an essential landmark of the connection of _two tilted networks.

ii)

Furtuermore both networks _are

slightly

distorted in tuis _{zone m}

order to minimize tue _energy of tuis

boundary.

Tue

grain sub-boundary (the

connection

zone between tue two

networks)

is cuaracterized

by

_a succession of

large elongated

"voles"

with _a

period

of 21 ~ 2

À.

_The

origm

^{of these} "holes" is

purely

electronic and

by

_{no way}

topographie. They

^{result from the} local modification of the

charge density

in the

vicinity

bi=

_a

l1001

a

loiol

Fig.

2. STM filtered

image (4.5

_nm x 4.5

nm)

obtained from trie 2D Fourier spectrum of _a

(not shown)

_{raw image}

(taken

_on trie _same

grain

sub-

boundary;

_same experimental conditions _as

Fig. l).

Two

edge

dislocations _are outliued _ou trie picture ^and sketched _ou the

nght

side.

Table I. Some

properties of

the three CSL

of

interest.

Rotation

angle

II.64° 13.17° 15.18°

Multiplicity

73 19 43

Periodicity

21.00 18.58 16.13

Burgers

vector 4.259 4.261 4.261

of defects

(edge dislocations,

network

distortions)

in the

grain boundary

_area. Trie observed electronic

periodicity

reflects

directly

the defect

periodicity (specially

the dislocation's

one).

Geometrical models have been

proposed (10-12]

in order to understand the

boundary

between

hexagonal

networks.

They rely

_on the occurrences of coincidence sites observed when two misoriented networks _are

superimposed.

This

description, generalized

to _a

great variety

of

grain boundaries,

is called Coincidence Sites Lattice

(CSL).

Tables

giving

the

multiplicity (Z)

of the unit cells _{as a} function of

privileged

rotation

angles

around the < 001 _> axis have been established. The

angle

observed in this

study (13°

~

l.5°)

_can fit the

angles

11.64°

(273),

13.17°

(Zig)

_or 15.18°

(243)

calculated for exact coincidences of

hexagonal

networks. The

corresponding

unit cells of the CSL _are shown

Figure

^3. On these schematic

representations

the

multiplicity

Z _can be understood _as the number of atoms of each network enclosed inside the unit cell.

Figure

4 shows _a detailed

representation

of the unit cells for the two

crystals

and for the so-called

bi-crystal

for the Zig situation. The

symmetrical grain sub-boundary

_is, for

geometrical

_reasons, either _a

diagonal (Zig)

_{or a} lateralside

(273, 243)

^of the so-called bi-

crystal.

Of _course in the actual situation the _atoms of the network _{on one} side of the

boundary

do not exist _on the other side of the

boundary.

Table I

gives,

for the three

previously

mentioned

angles 9,

the

multiplicity

Z and the

periodicity

D of the _exact coincidences

along

the

grain

boundary.

Dur

experimental

results

(9=13°

~ l.5° and D=21

À

_{~ 2}

À)

_are in

agreement

with

1114 JOURNAL DE

PHYSIQUE

I N°9

&)

b)

C)

Fig. 3. Unit cells for trie

bi-crystals

E73

la),

E19

16), E43(c).

Nodes of the two

supenmposed

uetworks _are

respectively represented

by fuit aud empty ^dots. A unit cell of each of these uetworks _is also

represented.

~~ j

<210>(Z19) 0>(II)

<l00>(Z19)

<oie>jz19)

Fig.

4. Axis of trie two

"elementary"

crystals _in the E19 situation.

~

a)

. . °

o ~

o ~

o ~

o ~

o

~ o

o

ll16 JOURNAL DE

PHYSIQUE

I N°9

4

il)

Fig.

6. STM

passband images

reconstructed from trie 2D Fourier trausform of _a

(not shown)

_raw

image.

a)

3.2 _x 3.2

um~; b)

2.5 x 2.5

um~.

In

regard

of each

image

is shown _an

"adapted"

schematic

representatiou

of the theoretical

cases E73 aud E19. The hard-drawn fines _on trie filtered

images

matenalize trie

bouudanes,

their

lengths

_are trie

leugths

of the

correspondiug

"motif'.

face

(one

atom out of

two)

would be

complicated

if _we took into account the actual network.

Actually

_our restricted

interpretation

^leads _us to merge four situations into _a

single

one. How-

ever, these

ignored

situations would not show _up at the surface but

could,

_may

be, help

_m

understanding

the

positions

of the visible _atoms in the

boundary

_zone.

Furthermore,

_as the observed STM

graphitic

network results from the interaction of _at least two

graphene planes,

observations in the

boundary

_{zone may} ^thus result from

reorganizations

of ail these

planes,

and

our

"adapted"

schematic

representation

must be understood

simply

_{as an}

attempt

to mimic the actual

image

and

by

_{no way as} the result of _any

speculations concerning

the actual dis-

placements

of the _atoms in this _zone. However _our

experimental observations,

at

large scale, indicate

that the

periodicity along

^trie

boundary

_is not

perfect,

which confirms the tentative

conclusions drawn from

Figure

6.

It remains that _we did

observe,

for the first

time,

at the atomic

scale,

a

symmetrical,

or

nearly symmetrical,

tilt grain

sub-boundary

with _a misorientation

angle compatible

with the

geometrically preferred angles

9 of II.64° and 13.17°.

Furthermore,

the STM

images

^of _a tilt

grain sub-boundary clearly

revealed the local modi- fications of the

charge density

of states _near the Fermi level in this

perturbed

_area.

Acknowledgments

The authors thank L.

Bonpunt

and F. Nallet for

helpful

cornments

during

the revision of the

manuscript.

[5]

Zheng N-J-,

Wilson

I.H., Knippiug U.,

Burt

D.M., Krinsley

D.H. aud

Tsoug I.S.T., Phgs.

Reu. B 38

(1988)

12780.

[GI Kazmerski

L.L., "Polycristalline semiconductors,

Grain boundaries and

interfaces", Springer

Proc.

m

Physics

35, H. J.

Môller,

H. P. Strunk and J. H.

Werner,

Eds.

(Spriuger-Verlag Berlin, 1989)

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J.,

Lacaze

E.,

Faivre

G.,

Gauthier S. aud Schott

M.,

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Mag.

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833.

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T.R.,

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C.F.,

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Nysten B.,

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J-C-,

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