Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 3823–3826
Part 1, No. 6B, June 2000
c
°2000
The Japan Society of Applied Physics

Comparison of the Symmetry Breaking in the Surface Molecular Structuresof One- and Two-Dimensional Bis(ethylenedithio)tetrathiafulvalene Compounds
Masahiko I SHIDA1 , Osamu TAKEUCHI1 , Takehiko M ORI2 and Hidemi S HIGEKAWA1,3, ∗1 Institute of Applied Physics, CREST, Japan Science and Technology Corporation, University of Tsukuba, Tsukuba 305-8573, Japan
2 Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8552, Japan
3 Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo, Tokyo 113-8656, Japan(Received March 6, 2000; accepted for publication April 13, 2000)

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) were performed on the cationic molecular surfaces of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)-based organic conductors with one-dimensional (β-(BEDTTTF)2 AsF6 ) and two-dimensional (κ-(BEDT-TTF)2 Cu(NCS)2 ) conductivity. For the β-(BEDT-TTF)2 AsF6 surface, highly
similar STM/AFM images to those of β-(BEDT-TTF)2 PF6 were observed. Two types of alternately aligned molecular rows
existed in the direction of the crystal a-axis; one is characterized by bright and distinct protrusions, and the other is a little
darker with less distinct rows forming a zigzag structure. On the other hand, in the case of κ-(BEDT-TTF)2 Cu(NCS)2 , similar
one-dimensional modulation was confirmed by AFM for the first time in addition to the STM observation. However, the observed difference in the molecular images was only in their brightness, and their shapes were quite similar. Since the surface
structures were confirmed by both STM and AFM, the obtained results indicate that the observed modulations are associatedwith the structural changes in the surface molecular arrangements. The modulations were discussed in relation to the crystal
structures.
KEYWORDS: scanning tunneling microscopy, atomic force microscopy, bis(ethylenedithio)tetrathiafulvalene, surface structure,surface reconstruction, surface relaxation, molecular crystal, charge transfer salt

1. Introduction
On the surface of inorganic crystals, it is well known
that the structures are modulated from the ideal ones due
to surface reconstruction or surface relaxation. Generally,
the total energy of the ideal surfaces is increased by theoccurence of chemical bond breaking, and marked changes
such as atomic arrangement or modulation of the periodicity are induced, in order to reduce the surface energy. From
this standpoint, structures that do not undergo chemical bondbreaking on the surface, such as molecular crystals or layered materials, are considered to be stable. In fact, in contrast with inorganic crystals, most of the other materials
actually exhibit less changes in the surface from the ideal
structure.1) However, recently, using scanning tunneling microscopy (STM) and atomic force microscopy (AFM),2–8)
structural modulation introduced by surface relaxation andsurface reconstruction has been observed on the surfacesof bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET)
based low-dimensional organic conductors such as β-(BEDTTTF)2 PF6 6–8) and κ-(BEDT-TTF)2 Cu(NCS)2 .5) In particular,
in the case of β-(BEDT-TTF)2 PF6 , a marked change is induced by the formation of ordered missing molecular rows.
These results indicate that the ideal surface is not stable evenfor organic materials. The origin of the observed surface
structures is related to the characteristics of these surfaces,such as polarity and charge transfer,9, 10) and the modulations
were explained comprehensively by the same mechanism
such as that used to describe the surface structures of inorganic material surfaces such as GaAs(111),11) MnO(111),12)
Si(001)13) and GaAs(001).14)
However, some uncertainty still remains. In the case ofβ-(BEDT-TTF)2 PF6 , which has one-dimensional (1D) conductivity, 1D characteristic modulation was observed by STM
and AFM.6–8) There exist two types of molecular rows alternately aligned in the direction perpendicular to the quasi∗ E-mail address: hidemi@ims.tsukuba.ac.jp http://dora.ims.tsukuba.ac.jp

1D axis (c-axis). One is characterized by the bright and distinct protrusions, and the other is a little darker and featuresless distinct rows forming a zigzag structure. On the other
hand, in the case of κ-(BEDT-TTF)2 Cu(NCS)2 , for example, which has two-dimensional (2D) conductivity, the existence of the modulation is still a topic of controversy.3–5)
One-dimensional modulation formed by the two types ofET molecular rows alternately aligned was pointed out by
Shigekawa et al,5) however, according to Yoon et al, modulation was observed for the surface terminated with the anion
layer, and no structural change occurred on the surface terminated with ET molecules.3, 4)
As is well known, ET compounds are formed by the alternate stacking of two-dimensional (2D) cation and anion
molecular layers, and through the charge transfer of 0.5 electrons from each ET molecule to the anion layers, ET andanion layers become conductive and insulating, respectively.
The conductive cation (ET) layers govern the electronic properties of the materials, such as conductivity and phase transitions,15) and their characteristic properties are strongly related to the molecular structure of the ET layers. Therefore, it
is very important to study the molecular and electronic structures of the ET layers on an atomic scale. In consideration of
the fact that the conductivity of the materials is 1D and 2D forβ-(BEDT-TTF)2 PF6 and κ-(BEDT-TTF)2 Cu(NCS)2 , respectively, the observed difference in modulation may be related
to the dimension of the material conductivity.
From these points of view, we performed STM/AFM observation on the cationic molecular surfaces of β-(BEDTTTF)2 AsF6 16) which has 1D conductivity like β-(BEDTTTF)2 PF6 ,17) and κ-(BEDT-TTF)2 Cu(NCS)2 ,18) the structureof which is still controversial as described above.
2. Experimental and Results
2.1 Experimental
Single crystals of β-(BEDT-TTF)2 AsF6 and κ-(BEDTTTF)2 Cu(NCS)2 were grown by the standard electrochemical
oxidation method and fixed onto metallic plates with conduc-

3823

3824

M. I SHIDA et al.

Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 6B

tive silver epoxy. STM/AFM measurements were performedon the surfaces terminated with the ET layer, in the topographic mode under ambient conditions using a mechanically
sharpened Pt/Ir tip and a pyramidal Si3 N4 tip, respectively.
β-(BEDT-TTF)2 AsF6 16) has a crystal structure and physicalproperty similar to those of β-(BEDT-TTF)2 PF6 ;17) conductivity is 1D and metal-insulator transition occurs at ∼264 K.
But its conductivity is more isotropic in the ET layers, because of the stronger interaction between 1D chains whichis induced by the larger difference in the lattice parameterscaused by As substitution. On the other hand, κ-(BEDTTTF)2 Cu(NCS)2 is one of the typical ET compounds with 2D
conductivity and shows normal-superconducting phase transition at 10.4 K. Molecular arrangements in the ET layers ofthese crystals are schematically shown in Fig. 1.

of ET which forms the conduction band of this material. A
cross section along A-B is shown in the lower part of Fig. 2(a).
Since the molecules are not aligned in a straight line along the
crystal a-axis, the values were averaged over the unit cells inthe square represented by the white dots in the figure.
As is shown in Fig. 2(a), the STM image is remarkably
similar to that of β-(BEDT-TTF)2 PF6 . There are two typesof molecular rows alternately aligned in the direction perpendicular to the quasi-1D axis (c-axis). One is characterized bybright and distinct protrusions (represented by open circles),and the other is a little darker and features less distinct rows

1.0

2.2 Results of the β-(BEDT-TTF)2 AsF6 surface
Figure 2(a) shows a topographic STM image obtained over
the a-c plane of β-(BEDT-TTF)2 AsF6 terminated with the ET
layer. Dimensions of the unit cell estimated from the STM
image, a = 0.66 nm, c = 1.56 nm, are in good agreementwith the lattice parameters determined by the X-ray diffraction measurement, a = 0.67 nm, c = 1.51 nm.19) No bias
dependence was observed, reflecting the fact that no othermolecular orbital contributes to the energy levels near the
Fermi level, except for the highest occupied molecular orbital

0
(nm)

(a)

(nm)

0.2

c

B
A

C

0 A
0

B
5 (nm)

1.0Do

0.903/11.902
4.98/0.7587/10=0.6564
5.70/0.7587/5=1.502

a

(b)

b

B C

A

D

o

c

0
(nm)
(nm)

0.4
0
Fig. 1. Molecular arrangements of the ET layers of (a) β-(BEDTTTF)2 AsF6 and (b) κ-(BEDT-TTF)2 Cu(NCS)2 . The upper two rings in
the ET molecule, which are closer to the surface, are shown in these models. Dotted circles indicate dimerization of the molecules in a unit cell.
Molecules depicted by dotted lines are about 0.2 nm lower from the surface compared to the other molecules.

C0D
5 (nm)

Fig. 2. (a) A topographic STM image obtained over the a-c plane ofβ-(BEDT-TTF)2 AsF6 (Vs = 40 mV, It = 0.3 nA). Two types of molecular images are denoted by circles and a zigzag line. (b) A topographic AFM image obtained over the a-c plane of β-(BEDT-TTF)2 AsF6
(FREF = 0.39 nN). Two types of molecular images were also observed.

Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 6B

M. I SHIDA et al.

3825

forming a zigzag structure, as shown in the figure. Since theSTM image is strongly influenced by the electronic structurenear the Fermi level, we performed AFM observation on thissurface.
Figure 2(b) shows an example of the AFM image. As in
Fig. 2(a), a cross section C-D, along the crystal a-axis, averaged over the unit cells, is shown. Dimensions of the obtained unit cell are a = 0.66 nm and c = 1.50 nm. The 1D
modulation appearing in the STM image (Fig. 2(a)) was also
observed by AFM, as shown in Fig. 2(b). If the ET surface
retains its ideal structure, two molecular rows are expected
to appear with the same height and shape in the AFM image. Therefore, the observed differences in the brightness andshape indicate direct evidence of a structural difference between the two types of molecular rows.
2.3 Results of the κ-(BEDT-TTF)2 Cu(NCS)2 surface
Figure 3(a) shows a topographic STM image obtained
over the b-c plane of κ-(BEDT-TTF)2 Cu(NCS)2 . Dimensions of the obtained unit cell are estimated as b = 1.31 nm,
c = 0.83 nm, and agree well with the lattice parameters,
b = 1.31 nm, c = 0.84 nm, determined from the X-ray
diffraction patterns. An interesting characteristic of this image is that the four molecules in a unit cell (denoted in the inset) are observed with almost identical shape, but their brightness differs. The observed difference of the four molecules in
the unit cell is quite similar to the previous results obtained by
Yoshimura et al.,22) as opposed to the results of Yoon et al.,4)in which the two dimers in a unit cell were imaged similarly.
As a result of the modulated brightness, 1D modulation along
the b-axis appears in spite of its 2D crystal structure. In order to confirm the observed modulation, we performed AFM
observation on this surface.
Figure 3(b) shows a typical topographic AFM image of thesurface. Dimensions of the unit cell were estimated to beb = 1.30 nm, 4c = 0.93 nm. Contrary to the result of Yoonet al.,4) the obtained AFM image in Fig. 3(b) shows a striking similarity to the STM image in Fig. 3(a), not only in thearrangement of the protrusions but also in the modulated 1D
structure. We confirmed the reproducibility of this structure
through observations of a mica surface using the same AFM
tip.
The height difference of the four molecules observed by
STM/AFM is summarized in Fig. 3(c). They are the values acquired at the highest positions on each of the fourmolecules, which were averaged over the unit cells in the
STM/AFM images. This result indicates that the modulated
1D structure in this 2D material also includes a structuralchange similar to that observed for the ET surface with 1D
conductivity, β-(BEDT-TTF)2 AsF6 . However, in the case of
κ-(BEDT-TTF)2 Cu(NCS)2 , the observed breaking of symmetry in the molecular structure was recognized only by thebrightness of the protrusions. The additional modulation in
the molecular shape that appeared in the case of β-(BEDTTTF)2 AsF6 was not observed.
3. Discussion
From the STM and AFM investigations of the surfacesof the ET compounds with different anisotropy, β-(BEDTTTF)2 AsF6 and κ-(BEDT-TTF)2 Cu(NCS)2 , the 1D modulation consisting of two types of ET dimers appears to be a common characteristic of typical ET-surface structure. The mod-Fig. 3. (a) A topographic STM image obtained over the b-c plane of
κ-(BEDT-TTF)2 Cu(NCS)2 (Vs = −30 mV, It = 0.55 nA). Four different types of molecules in a unit cell are denoted by A-D in the inset. (b) A topographic AFM image obtained over the b-c plane of
κ-(BEDT-TTF)2 Cu(NCS)2 (FREF = 0.27 nN).

ulations are considered to originate from the breaking of symmetry of the ET dimer structure in a unit cell. However, there
exists a noteworthy structural difference between the modulations observed in the two kinds of crystals with 1D- and 2Dconductivity. In the case of the β-(BEDT-TTF)2 AsF6 surface,
the difference between two kinds of dimer rows is not only inthe brightness but also in their shape in the molecular image;molecules in the bright dimer rows are imaged as a distinct
protrusion, but molecules in the darker rows are imaged asa continuous protrusion along the 1D-axis. The modulation
observed here can be seen in the STM images of other previously reported ET compounds with 1D anisotropy, for example, α-(BEDT-TTF)2 I3 ,20) (BEDT-TTF)2 TlHg(NCS)4 ,21) and3826

Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 6B

β-(BEDT-TTF)2 PF6 .6–8) Taking into account these facts, theobserved modulation is considered to be a common phenomenon in the ET-based compounds with 1D anisotropy. On
the other hand, in the case of κ-(BEDT-TTF)2 Cu(NCS)2 , theobserved breaking of symmetry of the molecular images isrecognized only by the brightness of the protrusions.
In consideration of the fact that the conductivity of the materials, the observed difference in their surface modulations
may be related to the symmetry of the material conductivity.
In order to understand this point in further detail, we compare
the obtained results with that of β-(BEDT-TTF)2 I3 with 2D
conductivity, reported in previous papers.2, 23, 24)
In the previous studies on the β-(BEDT-TTF)2 I3 surface
terminated with ET molecules, the existence of surface reconstruction was noted,2) but no 1D-type modulations were observed in contrast to the case of κ-(BEDT-TTF)2 Cu(NCS)2 .
The surface reconstruction observed for β-(BEDT-TTF)2 I3
appears to occur dominantly in the surface plane.2, 24) Sinceboth κ-(BEDT-TTF)2 Cu(NCS)2 and β-(BEDT-TTF)2 I3 have
2D-conductivity, some other mechanism is required to explain the difference between them. In order to consider thispoint, let us see the molecular structures of these two crystals.
Figure 4 shows the molecular structures of the ET layerof (a) β-(BEDT-TTF)2 I3 and (b) κ-(BEDT-TTF)2 Cu(NCS)2 .
The arrangement of the ET molecules in β-(BEDT-TTF)2 I3
appears less isotropic compared to that of κ-(BEDTTTF)2 Cu(NCS)2 . Therefore, the difference in the structuralmodulation may be understood as being related to the structural isotropy. Namely, the stronger anisotropic interactions
between neighboring molecules may suppress the flexible relaxation of the surface structure. Further study is necessary to
clarify the relation between the anisotropic crystal structures
and the types of symmetry breaking on the surfaces.

(a)
A
B

c

b

o

(b)
C

b
B

D

A

o

c

Fig. 4. Molecular arrangements of the ET layers of (a) β-(BEDT-TTF)2 I3and (b) κ-(BEDT-TTF)2 Cu(NCS)2 .

M. I SHIDA et al.

4. Conclusion
We performed STM and AFM observations on β-(BEDTTTF)2 AsF6 and κ-(BEDT-TTF)2 Cu(NCS)2 , which have 1Dand 2D conductivity, respectively. Characteristic 1D structures were observed for both cases, but the details were remarkably different. In the case of β-(BEDT-TTF)2 AsF6 , wefound two types of molecular rows alternately aligned in the
direction perpendicular to the quasi-1D conductive axis (caxis). One is characterized by bright and features distinct
protrusions, and the other is a little darker and less distinct
rows forming a zigzag structure. The modulation structureis quite similar to that previously observed on the surface of
1D materials. On the other hand, in the case of κ-(BEDTTTF)2 Cu(NCS)2 , the modulation was observed only in thebrightness of the molecules, and the molecules were imaged
in a similar shape. In comparison with the result on the caseof β-(BEDT-TTF)2 I3 , molecular arrangement which is related
to the anisotropy of the molecular interaction must be taken
into account in the case of 2D materials. Further study is necessary.
Acknowledgements
This work was supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports andCulture of Japan. One of the authors (M. I.) was financially
supported by the Japan Society for the Promotion of Science(JSPS).
1) T. Sleator and R. Tycko: Phys. Rev. Lett. 60 (1988) 1418.
2) M. Yoshimura, H. Shigekawa, H. Yamochi, G. Saito, Y. Saito and A.
Kawazu: Phys. Rev. B 44 (1991) 1970.
3) R. Fainchtein, S. T. D’Archangelis, S. S. Yang and D. O. Cowan: Science 256 (1992) 1012.
4) S. Yoon, W. F. Smith, M. Yoo, A. L. deLozanne, R. Fainchtein, T. J.
Kistenmacher, S. T. D’Arcangelis and D. O. Cowan: Phys. Rev. B 47
(1993) 4802.
5) H. Shigekawa, K. Miyake, H. Oigawa, Y. Nannichi, T. Mori and Y. Saito:
Phys. Rev. B 50 (1994) 15427.
6) M. Ishida, K. Hata, T. Mori and H. Shigekawa: Phys. Rev. B 55 (1997)
6773.
7) M. Ishida, T. Mori and H. Shigekawa: Surf. Sci. 433-435 (1999) 147.
8) M. Ishida, T. Mori and H. Shigekawa: Synth. Met. 103 (1999) 2105.
9) R.W. Nosker and P. Mark: Surf. Sci. 19 (1970) 291.
10) M. Tsukada and T. Hoshino: J. Phys. Soc. Jpn. 51 (1982) 2562.
11) K.W. Haberern and M.D. Pashley: Phys. Rev. B 41 (1990) 3226.
12) K. Kimoto and I. Nishida: Jpn. J. Appl. Phys. 6 (1967) 1047.
13) R. M. Tromp, R. J. Hamers and J. E. Bemuth: Phys. Rev. Lett. 55 (1985)
1303.
14) M. D. Pashley, K. W. Haberern, W. Friday, J. M. Woodall and P. D.
Kirchner: Phys. Rev. Lett. 60 (1988) 2176.
15) M. Ishida, T. Mori and H. Shigekawa: Phys. Rev. Lett. 82 (1999) 596.
16) R. Laversanne, J. Amiell, P. Delhaes, D. Chasseau and C. Hauw: Solid
State Commun. 52 (1984) 177.
17) T. Mori, A. Kobayashi, Y. Sasaki, R. Kato and H. Kobayashi: Solid
State Commun. 53 (1985) 627.
18) H. Urayama, H. Yamochi, G. Saito, S. Sato, A. Kawamoto, J. Tanaka,
T. Mori, Y. Maruyama and H. Inokuchi: Chem. Lett. 1988 (1988) 463.
19) H. Kobayashi, T. Mori, R. Kato, A. Kobayashi, Y. Sasaki, G. Saito and
H. Inokuchi: Chem. Lett. 1983 (1983) 581.
20) S. N. Magonov, G. Bar, A. Y. Gorenberg, E. B. Yagubskii and H.-J.
Cantow: Adv. Mater. 5 (1993) 453.
21) S. N. Magonov, G. Bar, H.-J. Cantow, J. Paradis, J. Ren, M.-H. Whangbo
and E. B. Yagubskii: J. Phys. Chem. 97 (1993) 9170.
22) M. Yoshimura, H. Shigekawa, H. Nejoh, Y. Saito, G. Saito and A.
Kawazu: Phys. Rev. B 43 (1991) 13590.
23) S. N. Magonov, G. Bar, E. Keller, E. B. Yagubskii and H.-J. Cantow:
Systh. Met. 40 (1991) 247.
24) K. Hashimoto, T. Nakayama, N. Yoshimoto, M. Yoshizawa, M. Aonoand I. Yamaguchi: Jpn. J. Appl. Phys. 38 (1999) L464.