Twist Morphology of Organic Crystal (BPE-DMB)IBr2

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Twist Morphology of Organic Crystal (BPE-DMB)IBr2

Kazushige Kawabata, Tomonori Kumagai, Makoto Mizutani, Takashi Sambongi

To cite this version:

Kazushige Kawabata, Tomonori Kumagai, Makoto Mizutani, Takashi Sambongi. Twist Morphology of Organic Crystal (BPE-DMB)IBr2. Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.1575-1580.

�10.1051/jp1:1996175�. �jpa-00247266�

Twist Morpl~ology of Organic Crystal (BPE-DMB)IBr2

Kazushige

Kawabata

(~~*),

^Tomonori

Kumagai (~),

Makoto Mizutani

(~)

and Takashi

Sambongi (~)

(~)

Department

of

Physics,

Graduate School of

Science,

Hokkaido University,

Sapporo

060,

Japan

(~)

PRESTO,

JRDC

(Idemitsu

Kosan Co.

LTD), Sodegaura

Chiba 299-02, Japan

(Received

26

April

1996,

accepted

1

July1996)

PACS.61.66.Hq

Organic compounds

PACS.68.70.+w Whiskers and dendrites

(growth,

structure, and nonelectronic

properties)

PACS.61.72.Ff Direct observation of dislocations and other defects

(etch

pits, decoration electron microscopy, _x-ray

topography, etc.)

Abstract.

Crystals

of

(BPE-DMB)IBr2,

where BPE-DMB denotes

1,4-bis[2-(pyrene-1-yl)

vinyl]-2,5-dimethylbenzene,

_are found to be twisted

along

the

growing

_axis. Their

morphology

was examined in detail

by

X-ray examination, _scanning electron and

optical

microscopic ob- servations. It _was found that trie twisted

crystals

_are

quasi-stable

and that the

twisting

_is net discrete but rather continuous in sub-micron scale. Trie relation between _a twist

pitch

and trie cross-section _{area was} obtained and

compared

with trie dislocation mortel

proposed

by Eshelby.

1. Introduction

In

general, crystals

show _various

morphology

_on

growing

under

unequilibrium conditions,

for

example,

dendritic

growth

of succinomtrile

crystals iii,

island formation of

evaporated

thin films [2] and

twisting growth

of metal whiskers [3].

Morphology

_is

expected

to give us a lot of

important information both of constitutional materials and of

crystal growth

_processes ^under

unequilibrium

conditions.

Visser and deBoer bave

reported

that _some

needle-shaped crystals

of substituted

morpholin-

ium

TCNQ complexes

_{[4] are} twisted _or curved. This _is trie first observation of unusual _mor-

phology

of

organic crystals, though

such bas been

frequently

^{found in whiskers of metals and}

morgamc

compounds.

Webb and coworkers

reported

that whiskers of

a-A1203, silver,

_copper and

palladium

_grown

by

chemical reaction _{are m} trie form of hehces and twisted _prisms

[3, 5,6].

We found that _some

crystals

of

(BPE-DMB)IBr2

exhibit

twisting along

trie growing axis over

a macroscopic scale.

(BPE-DMB)IBr2

is _a

conducting charge

transfer

complex,

where BPE-

DMB is

1,4-bis[2-(pyrene-1-yl) vinyl]-2,5-dimethylbenzene [î].

Trie donor molecule consists of _{two pyrenes}

conjugated by divinyl-dimethylbenzene,

as shown _m

Figure

1. It is

expected

that _a

large

7r-molecular orbit is extended _over trie molecule.

(BPE-DMB)IBr2

shows

high conductivity

of 100 S

/cm

at _room

temperature along

trie growmg axis. Chemical

compositional analysis

revealed that ratio of

(BPE-DMB):IBr2

m

smgle crystals

is 1:1 [8].

Though

its

crystal

(*)

Author for

correspondence je-mail: kaw@skws.phys.hokudai.ac.jp)

©

Les Editions de

Physique

1996

1576 JOURNAL DE

PHYSIQUE

I N°12

1,4-bis[ 2-(pyrene-1-yl) vinyl ]-2,5-dimethylbenzene

Fig.

l. Molecular _structure of BPE-DMB.

structure bas not been determined yet, we estimated from

X-ray

oscillation

photograph

that

trie

period along

trie

growing

axis is 1.08 _nm.

In this

work,

_we

study morphology

of trie

t~N.isting crystals

of

(BPE-DMB)IBr2 _{by scanning}

electron and

optical microscopic

observations and also

by X-ray

examination. Trie results

are

compared

with trie

finding by Eshelby

who

proposed

that trie twist is stabilized

by

_screw dislocations with

giant Burger

vector

parallel

to trie growmg axis [9]. Trie

preliminary

result of electrical

conductivity

measurement of trie twisted

crystal

is

compared

with that of trie

non-twisted _one.

2.

Experimental

Molecule of BPE-DMB _was

synthesized

_as described in reference [7].

Crystals

of

(BPE-DàlB)

IBr2

_{~N.ere grown}

by

trie standard electrochemical

growth

method from

tetrabutyl-ammonium

IBr2

and BPE-DMB.

CH2Br2

_was used _{as a} solvent. Trie obtained

crystals

_are of needle

shape

with trie

typical

_size 3 _x 0.01 _x 0.oo3

mm~.

_Chemical

compositional analysis

for C and H and fluorescent

X-ray analysis

for Br _were carried _out. Trie results confirmed that trie ratio of

(BPE-DMB):IBr2

in

smgle crystal

_is 1:1 and solvent molecule of

CH2Br2

_was not detected

within

experimental sensitivity

of 5

at.$io.

Twisting morphology

of trie

crystals

was observed

by scanning

electron

(SEM)

and

optical microscopes

at _room

temperature. Especially,

trie twist

pitch

and trie cross-section _{area were}

measured

by

SEM with _an accuracy

of1o%,

_{where trie twist}

pitch

is defined _as trie

length along

trie needle axis for 27r rotation of trie

crystal plane. X-ray

oscillation and

~veissenberg pattems

were recorded

using imaging plate.

Data _were

processed

with

Rigaku

Automated

K-ray Imagmg System

at _room

temperature,

with resolution of 50 _x 50

~m~

in _area. Electrical

conductivity

of trie

crystal

_was measured

by

a de four

probe

method. As

electrodes,

fine

gold

wires _were attached with

gold

_poste.

3. Results and Discussion

Many crystals

of

(BPE-DMB)IBr2

_were exammed

by optical microscope

and about 5$io of trie

crystals

_are twisted

visibly.

Some

crystals

_are twisted clockwise while others

counter-clockwise,

with

approximately equal proportions.

Trie _twist

pitch

is not uniform in _a

crystal;

it is _coarse in trie

tips

^{and fine} m trie center.

Figure

2 is _a SEM

image

of _a

typical

twisted

(BPE-DMB )IBr2

50~m

Fig.

2. SEM

image

of twist

morphology

of

(BPE-DMB)IBr2 crystal.

Bar indicates 50 _~lm in size.

crystal

which is twisted about 180° in 80 _~m

length

in trie center: its

pitch

is o.16 _mm. Trie non-uniform

pitch

is

apparent

^{in this}

figure.

We measured trie

piton

in trie

crystals

which were

relatively large

and

capable

of

being

handled. In most

crystals

trie twist

pitch

near trie center

spreads

from o.1 to 8 _mm. Because _some t~visted

crystals

were grown

perpendicular

to trie electrode _wire, it is not

likely

that trie twist is _a result of _some electrostatic force

applied by

trie electrode wire

during gro~N.th.

We made _an

experiment

^{in which extemal twist} stress was

applied

to _one

tip

n>hile trie other

was fixed. After

releasing

trie _stress, trie

crystal

recovered trie

original

distribution of trie

pitch.

In another

experiment,

trie twisted

crystals

_were heated _up to trie

melting temperature

^{of about} 250 °C and

quenched

into

liqmd nitrogen.

Trie distribution of trie t~N.ist

angle along

trie needle axis _is not affected

by

both

heating ^aùd coohng

treatments. This result is _m contrast with trie

finding

by. ^Visser et ai. that

morpholimum TCNQ complexes

show a reversible

development

of twisted form _on

cooling.

From trie

present

^results,

thermally

induced residual stress is not trie origm of trie twist; ^trie twist

morphology

_is

quasi-stable-

Figure

3 shows _a

X-ray

o-th

layer Weissenberg image

^{of trie}

typical

twisted

crystal.

~vhich

was rotated around trie

growing

axis

during

_exposure. From

optical

and SEàl observations trie twist

pitch

of trie examined

crystal

_was 8 mm (~

).

All diffràction

spots

are found to

elongate along

trie

rotating

_axis; ^trie diffraction condition

is satisfied at any

angle

^{of trie}

crystal

rotation. Trie

X-ray

^{film, whose diameter} is 58.1 _mm,

is

displaced synchronously ^by

mm while trie

crystal

is rotated

by

2°, and trie

length

of trie

elongated

spots is

approximately

8 _mm

irrespective

of trie indices. From these values, trie twist

angle

is 16° within trie

sample

volume irradiated

by X-ray

beam

(colhmator

diameter o-î

mm)

In

Figure 4,

trie

X-ray intensity profiles

of _an

elongated spot along

and

perpendicular

to trie growmg axis are

plotted. Intensity

of trie reflection spot

along

trie growing axis shows

a continuous variation without any break _{or any}

step

^within

experimental

_accuracy.

Though

its functional form

depends

on trie

experimental arrangement, asy.mmetry

^{of trie}

intensity

(~)In crystals

of smaller twist

pitch,

X-ray diffraction spots are so broadened that intensity

profile

coula not be

separated

from the

background.

1578 JOURNAL DE

PHYSIQUE

I N°12

1'

'

; _'

)~

~

+ :

.'

a

Î j

i '~

'

.

.

.

.

:_)

;

~ Î

' .

<

~

.

.Î ..

.,.

.

*

~

.

,

distribution is

explained by

trie non-uniform

pitch along

^trie

growing

axis and

by

trie

edge-like shape

of this

sample

within trie

X-ray

beam. If trie

crystal

is twisted in discrete _manner,

X-ray intensity

should show a discrete variation in trie

profile.

We simulated if _a discrete rotation of trie

crystal

_{axes can} be detected from

Figure 4, assuming

that trie instrumental width of diffraction

spot

is

given by

that

perpendicular

to trie

elongated

^{direction. We found}

that _a

stepwise

and

periodic

rotation of trie

crystal

_axes

by

_an

angle larger

than o.2° coula be resolved in

Figure

4 _{as an}

oscillatory

variation of trie

intensity.

Careful studies _were

repeated

and therefore _{we can} conclude that trie

crystal

is twisted rather

continuously.

In whiskers of

inorganic

materials such _as ZnS

[loi

and Pd

[3],

^twisted ones bave been

frequently

observed. Most of them _{are m a}

shape

of

spiral. Eshelby proposed

that trie twist is mduced

by

trie stress field

by

an

agglomerate

of _screw dislocations

parallel

to trie needle axis [9]

Webb and

Forgeng found,

in many

crystals,

_a

cavity

of micron size mside trie whisker [6].

Existence of such _a

cavity

can reduce trie _core energy of trie

giant

screw dislocation.

According

to

Eshelby

_[9], trie twist

pitch

_p is

expressed by

trie relation

p =

27rk(S/b)

where b is trie

Burgers

vector and S is trie cross-section _area. Trie constant k

depends

on trie

shape

of _a whisker but is not much different from 1. Trie above relation bas been derived for

isotropic

^medium but Frank bas

proved

that it is

usually applicable

to

crystals having

anisotropic

elastic constants [11]

In

Figure 5,

trie

pitches

of trie

relatively large crystals

are

plotted against

their _cross- section

area S. In most

crystals

trie

pitch

is not uniform, hence trie

averaged piton

in center

part

^{of trie}

crystals

is

plotted

in trie

figure. Though

_some other

crystals

show

larger pitch

of trie

twist, they

are

clearly separated

from those

plotted

in trie

figure.

Trie data show _a rather

large

scatter,

mainly

because of limited accuracy of S.

Nevertheless,

it is found that trie

pitch

is

proportional

to S. This result _means that trie

giant Burgers

vector b is

independent

of trie

samples

and is as

large

_as 240 _nm. Since trie real _space

period

of trie lattice

along

^trie

growing

axis _is determined

as 1.08 nm

by

trie

X-ray

oscillation

photographie image,

each

crystal

contains

approximately

230 _screw

dislocations,

which _are distributed _near trie center of trie cross-section. It is

quite

unexpected

that _every

crystal

contains trie _same number of dislocations.

According

to

Frank,

screw dislocations bave been introduced

during growth,

_{some m}

pair,

and those of trie hke-

handed _were

self-trapped

while others were

expelled

from trie

crystal.

However there is _no mechanism to

keep

_a constant number of dislocations with trie _same

sign

m trie

crystal

_m

spite

of trie

crystal growth

^{under different conditions of trie} electrochemical method. Furthermore trie observed

pitch

is not uniform and

changes gradually

from _one

tip

and another _m all

crystals

of

(BPE-DMB)IBr2;

trie

pitch

is fine in trie central

part

^{of trie}

crystal

and _coarse in both ends, _as illustrated in

Figure

2. This _means that _screw dislocations _were introduced and then

expelled

as trie

crystal

_grows m

spite

that trie cross-section _area is uniform.

Trie electrical

conductivity

^of trie twisted

crystal

_was also measured and

compared

with that of trie non-twisted _one. Trie former is _one order of

magnitude

^{smaller than trie latter. Trie dif-}

ference is

dependent

on temperature ^and

just

^{outside trie}

uncertainty

limit of trie

sample

_size.

Trie

conductivity

mcreases with

increasing temperature. Assuming

that

(BPE-DMB)IBr2

is _a

semiconductor,

trie energy gap is about 400 K. Both

temperature dependences

^{of trie twisted} and trie non-twisted

crystals

_are

approximately

trie _same. This _means that trie

twisting

is

correlated with many

scattering

centers for _{carriers m} trie

crystal

without

significant

influence

on trie electromc structure. This result _may

imply

that there is considerable amount of im-

perfection

_or

impurity

m trie

crystal.

It is trot

likely

m metals that

scattering

is associated

with _screw

dislocations; highly

stramed _region localized _near dislocations does not contribute

1580 JOURNAL DE

PHYSIQUE

I N°12

to such reduction of trie conductance since

they

are

parallel

to trie current

patin

and small in volume. Further

investigation

_is needed for

clarifying

trie relation.

In

conclusion,

_we found that _some

crystals

of

(BPE-DMB)IBr2

_are twisted. Trie

twisting

is

quasi-stable against

extemal

twisting

force and thermal treatments and is rather continuous in sub-micron scale. Trie observed twist is

explained quahtatively by

trie

Eshelby's

_screw

dislocation model.

However, assuming

^this

model,

_an unusual result should be derived that all twisted

crystals

bave trie _same number of dislocations

m it.

Therefore,

while trie dislocation model for trie twist is

plausible

in

general.

it should be _more

carefully

examined trie present system ^{if trie} twist

morphology

_m trie orgamc

crystals

_cari be

explained.

Acknowledgments

We would like to thank Mr. S. Terada for _a technical assistance of SEM observation and also thank Mr. Sasaki of

Rigaku

Co. LTD for technical support of computer software.

References

iii Huang

S.C. and Glicksman

M.E.,

Act. Metai. 29

(1981)

îol.

[2] for

example,

Barabasi A. and

Stanley H.E.,

Fractal concepts m surface

growth (Cambridge

Univ. press,

1995)

_p. 167.

[3j

Riebling

E.F. and Webb

W.W.,

Science 126

(1957)

309.

[4j Visser R.J.J. and De Boer

J.L.,

J.

Phys. Coiioq.

France 44

(1983)

C3-1219.

[5j Webb

W.W., Dragsdorf

R.D. and

Forgeng W.D., Phys.

Rev. 108

(1957)

498.

[6j ^{Webb W-W-} and

Forgeng W.D.,

J.

Appi. Phys.

28

(1958)

1449.

[7] Mizutam M. and Shimane

Y., Synth.

Met. 55

(1993)

2185.

[8] Mizutani

M.,

to be

published.

[9]

Eshelby J.D.,

J.

Appi. Phys.

24

(1953)

1î6.

[loi

Mardix

S., Lang A.R.,

Kowalski G. and

Makepeace

A.P.W., Philos.

Mag.

A56

(1987)

251.

[Il]

Frank

F.C.,

Philos.

Mag.

A56

(1987)

263.