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Synthetic Metals 70 (1995) 935-936

M o l e c u l a r and e l e c t r o n i c p r o p e r t i e s of B - ( B E D T - T T F ) 2 P F 6s t u d i e d by s c a n n i n g t u n n e l i n g m i c r o s c o p y
Hidemi Shigekawa, Koji Miyake, Yoshiaki Aiso, and Haruhiro Oigawa
Institute of Materials Science, University of Tsuknba, Tsukuba 305 Japan
Takehiko Mori
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152 Japan
Akiko Kobayashi
Department of Chemistry, The University of Tokyo, 113 Japan
Two different structures based on one-dimensional molecular rows were observed by scanning tunneling microscopy (STM) in theB -(BEDT-TTF)2PF 6 crystal surface. The temperature dependence of tunneling current-bias voltage curves showed the property of ametal-insulator phase transition. However, metallic electronic states were observed even at -230K between the energy gap,indicating fluctuation of the charge density wave. The metallic property was recovered when the STM tip was softly brought into
contact with the sample surface, where the charge density wave is considered to be fonnd.
Figure l(a) shows a current scanning tunneling microscope(STM) image obtained over the a-c plane of a one-dimensionalorganicconductor,
B -(BEDT-TTF)2PF 6 (BEDT-TIT:bIbis(ethylenedithio)-tetrathiafulvalene), which has a metalinsulator transition at 297K. The observed size of the unit cellin Fig. l(a), 0.71nm x 1.46nm, agrees well with thatddetermined by X-ray diffraction, 0.664nm x 1.496nm. I'2) A
schematic o f the structure of the crystal a-c plane is shown inFig. l(b), where the upper two rings of the B E D T - T T F
molecules which are closer to the crystal surface are drawn
together with the anions, similarly to those reported in refs.
3-5. Ellipses indicate the lobes of the highest occupied
molecular orbital (HOMO) around the S atoms, and BEDT-TTF
molecules with the ellipses are about 0.1nm higher than othermolecules along the b-axis in the a-c plane. Therefore, STM
images are expected to reflect the arrangement of the ellipses
which are equivalent to each other.
The obtained molecular arrangement agrees well with thatof the crystal: however, molecular rows along the c-axis arealternately bright and dark, as indicated by a-b and c-d in Fig.
l(a). A similar structt,re was observed previously on a TTFT C N Q (tetrathiafulvalene-tetracyanoquinodimethane) crystal
surface below its critical temperature for a charge density wave
(CDW) formation. The double lattice constant was formed bybright molccular rows with twofold periodicity, however, thetwofold periodicity, which is expected to appear along themolecular rows, was not observed. 6)
Figure 2 shows a series of I-V (tunneling current - biasvoltage) curves measured at (a) 304K, (b) 230K, and (c) 202K,
under the conditions of (Vt, Is) (a) (50mV, 1.0nA), (b)(200mV, 0.5hA), and (c) (200mV, 0.5nA). As shown in Fig. 2,0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved
0379-6779(94)02707-6

SSD!

Fig. 1. (a) STM image obtained overthe a-c plane of B -(BEDT-TTF)2PF 6 crystal at ~280K (Vt=-40.0mV, Is---0.9nA,
l l n m x l l n m ) , (b) molecular arrangement of the crystal
projected on the crystal a-c plane ( l l : p , +: F, O : S, 0 : C).

936

H. Shigekawa et aL / Synthetic Metals 70 (1995) 935-936(a) 20.0
le.~ ...............i................~-400.0(b)-200.0~0.0

200.0

400.

Uolta~ (mU)

Fig.3

2.0

STM image obtained over the a-c plane of p-(BEDT-qq'F)2PF 6 crystal (Vt=-40.0mV, ls=0.9nA, 1 hun x 1 hun).

1.0Figure 3 shows another stnlcture observed at -280K. Here,one dark and three bright molecular rows form a new unit in themodulation, as indicated by two different kinds of arrows in
Fig. 3. The observed unit cell is ~0.63nm x ~l.65nm. Ratherextensive structural change may be observed in this cellcompared to that shown in Fig. l(a).
In the case of (BEDT-TTF)2Cu(NCS) 2, which has twodimensional conductivity, modulation was observed to form-2.0,/-500.0-250.00.0
250.0
Uoltage (mU)by uniting BEDT-TTF dimers as expected. 5) Therefore, further
study will be necessary in order to understand the detailed
properties of the one-dimensional conductive materials on an
atomic scale.

500.

(c)

J

0.50

.............. :................ :................ i ..............

0 " 00.

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-51~.0.

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We wish to thank UNISOK corp. lbr their help in themeasurement at low temperatures. This work was supported inpart by a Grant-in-Aid for Scientific Research from theMinistry of Education, Science and Culture of Japan. Supports
from IZUMI, KURATA, and MIKITANI Research Foundations
are also acknowledged.

i ..............

REFERENCES

0.0
250.0
Uolta~ (mU)

500.

Fig. 2. Series of I-V curves measured at (a)304K, (b)230K, and(C)202K.
the electronic property at 304K is metallic, and conductivity
decreased with temperature as expected. However, metallic
electronic states partially remained on an atomic scale even at230K between the energy gap, indicating fluctuation of theCDW. When the STM tip was softly brought into contact withthe sample surface at 132K, where CDW was thought to beformed, the metallic character was recovered again, as wasobserved for the (BEDT-TTF)2 Cu(NCS)2 crystal surface. 5)[1] H. Kobayashi, T. Mori, R. Kato, A. Kobayashi,
Y. Asaki, G. Saito, and H. Inokuchi, Chem. Lett.,(1983) 581.
[2] T. Mori, A. Kobayashi, Y. Sasaki, R. Kato, andH. Kobayashi, Solid State Commun., 53,8 (1985)
627.
[3] M. Yoshimura, H. Shigekawa, H. Nejo, G. Saito,Y. Saito, and A. Kawazu, Phys. Rev. B43(1991) 13590.
[41 M. Yoshimura, H. Shigekawa, H. Yamochi, G. Saito,and A. Kawazu, Phys.Rev.B44 (1991) 1970.
[5] H. Shigekawa, K. Miyake, H. Oigawa, T. Mori, andY. Saito, to be published. CDW is formed by uniting
two BEDE-TrF molecules in (BEDT-TTF)2Cu(NCS) 2.
[6) S. Pan, A.L. Delozanne, and R. Fainchtein,
J. Vac. Sci. Technol. B9 (1991) 1017.