Surface Science 433–435 (1999) 147–151
www.elsevier.nl/locate/susc

Modulation of b-(BEDT-TTF ) PF crystal surface structure
2 6
induced by charge redistribution in surface layers
Masahiko Ishida a, Takehiko Mori b, Hidemi Shigekawa a,c, *a Institute of Materials Science, CREST, Japan Science and Technology Corporation (JST), University of Tsukuba,Tsukuba 305-8573, Japanb Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8552, Japanc Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo,Hongo, Tokyo 113-8656, JapanAbstract
Scanning tunneling microscopy (STM ) and atomic force microscopy (AFM ) were used to characterize the surface
superstructures of a quasi-one-dimensional organic conductor, b-(BEDT-TTF ) PF [where BEDT-TTF denotes
2 6bis(ethylenedithio)-tetrathiafulvalene]. The existence of periodic modulation formed by alternate missing rows of
PF molecules, which was previously observed by STM on the PF top layer, was confirmed using AFM. For a66surface with a BEDT-TTF top layer, periodic modulation was observed to be formed by the combination of molecularand electronic structural changes in the surface layer. The phenomena observed are comprehensively explained by thecharge-compensation model. © 1999 Elsevier Science B.V. All rights reserved.
Keywords: Atomic force microscopy (AFM ); Low index single crystal surface; Other molecules; Scanning tunneling microscopy;
Surface relaxation and reconstruction

1. Introduction
It is well known that the structure of material
surfaces is unstable due to symmetry breaking,which induces surface relaxation/reconstruction inthe surface layers. Understanding the surfacereconstruction is essential from both fundamental
and practical points of view, and has been studied
extensively by scanning tunneling microscopy(STM ) and atomic force microscopy (AFM ) on
an atomic scale. Non-stoichiometry often occurs
on polar surfaces of ionic crystals such as the* Corresponding author. Home page:
http://dora.ims.tsukuba.ac.jp
E-mail address: hidemi@ims.tsukuba.ac.jp (H. Shigekawa)(111) surface, resulting in a reduction of charge
imbalance in the surface layers. Similarly, charge
redistribution in the surface layers induced by theformation of ordered defects with an excess ordeficient charge has been observed to stabilize thesurface structure of compound semiconductors [1].
In the case of silicon surfaces, an example is the
buckling of silicon dimers of the Si(100) which
induces a charge redistribution that stabilizes thesurface structure.
It has been thought that organic materials do
not show the surface reconstruction describedabove. However, recently a structural modulation
involving a drastic change in the moleculararrangement was observed by STM on the surfaceof a b-(BEDT-TTF ) PF crystal [where BEDT2 60039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.
PII: S0 0 39 - 6 0 28 ( 99 ) 0 01 3 4 -X

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M. Ishida et al. / Surface Science 433–435 (1999) 147–151

TTF denotes bis(ethylenedithio)-tetrathiafulvalene]. Previous work concluded that this arosefrom alternate missing rows of PF molecules
6
forming the outer layer [2]. A similar but lessdrastic modulation was also observed on the surface with a BEDT-TTF top layer [3–6 ]. If theobserved surface modulations can be explained by
the charge-compensation mechanism, i.e., bycharge and molecular rearrangements to reducethe charge imbalance in the surface layers, surface
reconstruction of these organic surfaces can besystematically understood.
In order to clarify the mechanism of the structural modulations observed on the b-(BEDTTTF ) PF surface, we performed STM and AFM
2 6measurements on the surface.

2. Experimental
Fig. 1 shows the crystal structure ofb-(BEDT-TTF ) PF determined by X-ray diffrac2 6tion [7]. The crystal c-axis corresponds to the onedimensional conductive axis of the crystal. Single
crystals of b-(BEDT-TTF ) PF were grown by the2 6
standard electrochemical oxidation method andfixed to metallic plates with a conductive silver
epoxy. STM and AFM images were taken overthe crystal a–c plane with a mechanically sharpened Pt/Ir tip and a pyramidal Si N tip, respec3 4
tively. All images reported in this paper are thetopographic images taken under ambient temperature and pressure conditions.

3. Results and discussion
Since BEDT-TTF (cation) and PF (anion)
6sheets are alternately stacked along the crystal aaxis, two types of surface with a BEDT-TTF or a
PF top layer can be observed on the a–c plane.
6The BEDT-TTF and PF molecular layers are 1.3
6and 0.3 nm thick, respectively, and are quite
different from one another. Therefore, we canidentify and observe each layer selectively bychecking the step height of the layers on thesurface. First, let us examine the surface structureof the PF top layer.
6Fig. 2 shows STM (a) and AFM (b) images
obtained on the surface. As is shown in Fig. 2a,alternate missing rows of PF molecules in the6direction of the crystal a-axis exist on the surface
as indicated by the white arrows. Similar structure
was also observed by AFM as shown in Fig. 2b.
Since the AFM image should reflect the structure
related to the total electron density, i.e., the atomic
positions, the results obtained strongly indicatethat the surface modulation is actually formed by
the ordered missing molecular rows.
Next, let us examine the structure of a BEDTTTF top layer. Fig. 3a is an STM image obtainedon the surface. The surface unit cell consists offour BEDT-TTF molecules forming dimers A–Band C–D as shown in Figs. 1 and 3c. If the surface
has an ideal structure, upper molecules of eachdimer should be observed with the same brightness
by STM because they are at the same distancefrom the STM tip. However, only one molecule ina unit cell (A or C ) was brightly imaged as clearlyshown in the cross-sectional profile along the aaxis. The periodic modulation on the surface wasalso observed by AFM as shown in Fig. 3b. In
consideration of the fact that an AFM image
reflects the structure of the total electron density,
close to the atomic position, we conclude that not
only a change in the electronic structure but alsoa molecular rearrangement occurred on this surface as has been observed in inorganic crystal
surfaces.
Next, let us discuss the surface modulation
produced by the charge-compensation mechanism[8,9]. In the surface layer of the BEDT-TTF
compounds, a half amount of the ionic charge
should be reduced. As shown in Fig. 4a, the PF
6molecular rows are alternately desorbed from thesurface and each surface PF molecule can have6
the favorable valence state (PF− ). Since the inter6
action between PF molecules is weak because of6
their large intermolecular distance in the anion
sheet, the PF molecules are considered to be6
removed relatively easily.
On the other hand, since BEDT-TTF moleculesin the cation sheet strongly interact with eachother, it is difficult for them to be desorbed fromthe surface. Therefore, only structural modulation,
which induces charge redistribution, occurs on theM. Ishida et al. / Surface Science 433–435 (1999) 147–151

149Fig. 1. (a) Crystal structure of b-(BEDT-TTF ) PF (a=1.4960, b=3.2643, c=0.6664 nm), and molecular arrangements of its anion
2 6(b) and cation (c) layers.

surface. When the structure of the BEDT-TTF
molecular layer is relaxed on the surface, thedegenerated energy levels of the molecules must

split, and charge transfer is expected to be induced.
In such a case, ionic interactions between thesurface BEDT-TTF molecules and PF molecules
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M. Ishida et al. / Surface Science 433–435 (1999) 147–151

Fig. 2. (a) STM image (7 nm×7 nm,V =30 mV, I =0.46 nA,st
ET: BEDT-TTF ) and (b) AFM image (7 nm×7 nm,
F =9.7 nN ) of the b-(BEDT-TTF ) PF surface with a PF
REF
2 6
6top layer. (c) Molecular arrangement of the PF layer.
6in the underlying layer are no longer uniform, andBEDT-TTF molecules that have greater charge(holes) must be pulled towards the PF layer due
6to the ionic interactions.
On the basis of the experimental results shownin Fig. 3, a model can be constructed where BEDT-Fig. 3. (a) STM image (9 nm×9 nm, V =−10 mV, I =st0.25 nA) and (b) AFM image (9 nm×9 nm, F =0.5 nN ) ofREF
the b-(BEDT-TTF ) PF surface with a BEDT-TTF top layer.
2 6(c) Molecular arrangement of the BEDT-TTF layer.

M. Ishida et al. / Surface Science 433–435 (1999) 147–151

151

the alternate missing rows of PF molecules was
6confirmed to exist on the anion layer of theb-(BEDT-TTF ) PF surface. For the surface
2 6with a BEDT-TTF top layer, similar periodic
modulation was observed to be formed by thecombination of molecular and electronic structuralchanges. The observed surface modulations can beunderstood in terms of a charge-compensation
mechanism similar to that found for surface reconstruction of inorganic surfaces.

Fig. 4. Schematic model of the surface modulation for the surface with a (a) PF and (b) BEDT-TTF top layers.
6TTF molecules are rearranged in the dimer unit
as shown schematically in Fig. 4b. Since the positional change observed is about 0.02 nm, upper
dimers are expected to be imaged brightly.
However, as described above, charge (holes) isconsidered to be transferred from the lower to theupper dimers. Therefore, when the charge transfer
is completed, the upper dimers are ideally considered to become semiconductive.
The results obtained suggest that the surface
modulation is governed not only by the charge
compensation, but also by some other factor suchas the elastic energy change induced by the modulation. This mechanism is generally accepted forthe surface reconstruction in inorganic materials.
b-(BEDT-TTF ) PF is known to show a metal–
2 6insulator transition with a charge density wave
(CDW ) at ~297 K [10]. Characteristic properties
of CDW phase, such as periodicity of superstructures and critical temperature, depend on theamount of electronic charge and the dispersion ofphonon modes. Since molecular arrangement muststrongly influence the electronic and vibrational
structures, the metal–insulator transition andrelated properties such as the formation of charge
density wave may show new characteristics on
this surface.
In summary, periodic modulation formed by

Acknowledgements
This work was supported by a Grant-in-Aidfor Scientific Research from the Ministry ofEducation, Science, Sports and Culture of Japan.
Support from the Shigekawa project of TARA,
University of Tsukuba, is also acknowledged. Oneof the authors (M.I.) was financially supported by
the Japan Society for the Promotion of Science(JSPS) Fellowships for Japanese Junior Scientists.

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