PHYSICAL REVIEW B, VOLUME 64, 153405

Intramolecular relaxation observed in the surface of the quasi-one-dimensional organic conductor␤ -„BEDT-TTF… 2 PF6Masahiko Ishida,1,* Osamu Takeuchi,1 Takehiko Mori,2 and Hidemi Shigekawa1,†
1

Institute of Applied Physics, CREST, Japan Science and Technology Corporation (JST), University of Tsukuba,
Tsukuba 305-8573, Japan2
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8552, Japan
共Received 11 May 2001; published 18 September 2001兲
In addition to the intermolecular symmetry breaking which is generally observed during the surface
relaxation/reconstruction of atomic crystals, intramolecular relaxation is expected to exist in the case of molecular crystals, which was confirmed on the surface of ␤ -共BEDT-TTF兲2PF6 关BEDT-TTF is bis共ethylenedithio兲tetrathiafulvalene兴 by using scanning tunneling microscopy 共STM兲. Two types of molecular images
obtained by STM were comprehensively explained by the spatial distributions of the highest-occupied molecular orbital calculated for the two types of the molecular structures. One is the structure appearing in the crystal
that is derived from the x-ray diffraction measurement, which was used in the previous calculations to reproduce the STM images. The other is the relaxed molecular structure calculated by optimization of the molecularstructure.
DOI: 10.1103/PhysRevB.64.153405

PACS number共s兲: 68.35.Bs, 68.37.Ef, 61.66.Hq

On the surface of materials, crystal symmetry breaks inorder to stabilize the surface structure by reducing the surface energy.1–5 Generally, the total energy of the ideal surfaces is increased by the occurrence of chemical bond breaking, and great changes such as atomic rearrangement ormodulation of the periodicity are induced, in order to reduce
the surface energy. From this standpoint, structures that do
not undergo chemical bond breaking on the surface, such as
molecular crystals or layered materials, are considered to bestable. In fact, in contrast with inorganic crystals, most of theother materials actually exhibit less change in the surface
structure.
However, recently, using scanning tunneling microscopy
共STM兲 and related techniques, the existence of marked structural modulations introduced by surface relaxation/reconstruction has been revealed on the surface of bis共ethylenedithio兲tetrathiafulvalene 共BEDT-TTF兲 based lowdimensional organic conductors.5–14
Figure1showsthecrystalstructureof␤ -共BEDT-TTF兲2PF6, 15,16 an organic conductor recognized
one-dimensional along the side-by-side 共transverse兲 array, inthe direction of the c axis. In the crystal, BEDT-TTF and PF6
layers are alternately stacked along the b axis, and each ofthem appears as the top layer of the surface on the a-c plane.
In STM images, contrast to the crystal structure, twofold
periodic modulation along the crystal a axis was observed
for both surfaces.5In the case of the surface terminated by PF6 molecules,
drastic change occurs, namely, PF6 molecular rows are alternately missing along the crystal a axis 关Fig. 1共b兲兴.5 On theother hand, for the surface with BEDT-TTF molecules, theunit cell of the BEDT-TTF layer consists of four BEDT-TTF
molecules, which form two dimmers 关A-B and C-D in Fig.
1共b兲兴. If the surface retains the ideal crystal structure, molecules A and C should be observed with the same shape andbrightness in the STM image of the crystal a-c plane because
they have the same structure and are the same distance fromthe surface. However, alternately modulated molecular rows
appear.
0163-1829/2001/64共15兲/153405共4兲/$20.00

The origin of the observed surface structures was explained by a mechanism similar to that which is used tounderstand the surface structures of inorganic materials consisting of atomic elements, such as Si共100兲 andGaAs共100兲.1–3 Through the transfer of 0.5 electrons fromeach BEDT-TTF molecules to anion layers, BEDT-TTF andanion layers become conductive and insulating, respectively.
However, in the surface top layer, one side layer does notexist, and charge transfer is half and incomplete similar tothe semiconductor surfaces.

FIG. 1. Schematic structure of ␤ -共BEDT-TTF兲2PF6 crystal 共a兲,and the structural models for the surface top layer terminated by 共b兲
PF6 and 共c兲 BEDT-TTF molecules.

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FIG. 3. 共Color兲 Structure of the molecule in the crystal projected
onto the a-c plane 共a兲 and the relaxed molecular structure derived
from the calculation 共b兲. Ethylene group at the one end of the molecule is colored red. 共c兲 and 共d兲 are the spatial distributions of
HOMO for the structures of two types corresponding to 共a兲 and 共b兲.

FIG. 2. 共Color兲 共a兲 Typical STM image of ␤ -共BEDT-TTF兲2PF6.
共b兲 Top and side views of the molecular arrangement in a unit cell.
共c兲 Magnification of the squared area in 共a兲. 共d兲 Cross sections along
the red and yellow lines in 共a兲.

In As-rich GaAs共100兲 surface, As dimmer rows are orderly missing which improves the charge transfer in the surface layers. Assuming a similar mechanism for the surface
terminated by PF6 molecules, when half of the PF6 molecules
are removed 关Fig. 1共b兲兴, the charge is exactly balanced
within the surface layer, resulting in stable surface.5 On theother hand, in the case of the surface terminated by BEDTTTF molecules, since the BEDT-TTF molecules strongly interact with the adjacent BEDT-TTF molecules, they are notdesorbed and only structural modulation occurs such as thebuckling of Si dimmers. Namely, dimmers A-B and C-D in aunit cell are pulled toward the surface and inside, respectively 关Fig. 1共c兲兴.5
The basic mechanism for the surface relaxation has been
considered to be established. However, since the elements
are molecules in the case of molecular crystals, further
change regarding the elemental molecules is expected to occur. Namely, structure of the elemental molecules is expected
to play a role in the formation of crystals or surface structures. In fact, we found a unique mechanism operating in thesurface relaxation process of molecular crystals of BEDTTTF compounds.
Figure 2共a兲 shows a three-dimensional view of the typical
STM image acquired over the a-c plane of␤ -共BEDT-TTF兲2PF6. For comparison, top and side views of
the molecular arrangement is shown in Fig. 2共b兲. STM measurements were performed in the topographic mode over thetwo-dimensional 共2D兲 BEDT-TTF molecular layer terminating the surface, under ambient conditions using a Pt/Ir tip.
The conduction band of cation salts consists of the highestoccupied molecular orbital 共HOMO兲, from which the otherenergy levels are separated enough. In fact, no bias dependence was observed, reflecting the fact that no other molecular orbital contributes to the energy levels near the Fermi
level, except for the HOMO.

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Dimensions of the surface unit cell estimated from theSTM image are a⫽0.65 nm and c⫽1.50 nm, and are in good
agreement with the lattice parameters determined by x-ray
diffraction, a⫽0.67 nm and c⫽1.50 nm. The molecular rows
aligned in the direction of the most conductive axis, the caxis, are alternately modulated in bright and dark 关indicatedby red and yellow lines in Fig. 2共a兲兴, as previously observed
by STM/AFM.3 The measured height difference of the two
dimers is 0.1 nm in the direction perpendicular to the surface, and consequently, the 1D feature along the c axis appears in the STM/AFM images. This change is the intermolecular symmetry breaking due to the surface relaxation as
mentioned above 关Fig. 1共c兲兴.5
In order to analyze the observed difference in the molecular images of the two types of molecular rows in more detail,a magnified image of the squared area in Fig. 2共a兲 and crosssections along the two adjacent types of dimmer rows indicated by yellow and red lines in Fig. 2共a兲 are shown in Figs.
2共c兲 and 2共d兲, respectively. As expected, there exists an additional difference. As the most noteworthy point, thebrighter molecule, molecule A, has only one protrusion, although two protrusions exist at the darker molecule, molecule C 共protrusions C 1 and C 2 兲. This difference is clearlyseen in the cross section in Fig. 2共d兲.
In previous studies, two different calculations have beenreported to interpret the STM images of the BEDT-TTF compounds. One is the tight-binding calculation, concluding thatthe surface HOMO density of the BEDT-TTF compounds isdominated by the topmost hydrogen-related orbital,10–12 andthe other focuses on the contribution of the sulfur orbital tothe HOMO.13 However, characteristics of the newly confirmed modulation, appearance of a single or a pair of twoprotrusions from a BEDT-TTF molecule, cannot be reproduced by either of these two calculations.
A possible mechanism explaining the observed characteristics in the STM image is to consider the intramolecularrelaxation on the surface. In fact, in the crystal, face-to-face
BEDT-TTF molecules are strongly dimerized,15 and the molecules are distorted from their relaxed structure as will bediscussed in detail later. The increase in the energy due to thedeformation is estimated to be 4% per molecule, and themolecular structure in the crystal is stabilized by the existence of the anion layers sandwiching the BEDT-TTF layer.
Therefore, occurrence of the intramolecular relaxation on
the surface due to the missing of the anion layer is mostly
probable.
In order to clarify the molecular structure on the surface,
we calculated the spatial distribution of the BEDT-TTF
HOMO using density-functional theory 共DFT兲 with the generalized gradient approximation 共GGA兲 of Perdew and Wang
共PW91兲. To make a comparison between the STM results
and the calculation, two types of molecular structures wereexamined. One is the structure appearing in the crystal that isderived from the x-ray diffraction measurement, which wasused in the previous calculations to reproduce the STM images. The other is the relaxed molecular structure calculated
by optimization of the molecular structure. In our calculation, we took into account the sulfur 3d orbital, which was
not considered in earlier calculations despite the fact that it

must make a great contribution to the side-by-side structureof the HOMO.15 As shown later, spatial distribution of the3d orbital plays an important role to reproduce the obtained
STM images.
Figures 3共a兲 and 3共b兲 illustrate the molecular structure in
the crystal projected onto the a-c plane and the relaxed molecular structure derived from the calculation, respectively.
As indicated by the part colored red in Fig. 3共b兲, the maindifference appears around the topmost ethylene group(uCH2—CH2u), which is considerably relaxed in the optimized structure. In the crystal, the four carbon orbitals related to the ethylene group exhibit an energy shift toward thelower binding energy 共⬃1 eV for the C 1s orbital兲 as compared with the other carbon orbitals. However, the sulfurrelated orbitals produce less difference between these structures even in the HOMO. This result indicates that theinfluence of crystallization is concentrated in the four carbon
atoms forming the ethylene groups.
Spatial distributions of the HOMO for the two types ofmolecule are shown in Figs. 3共c兲 and 3共d兲. The contour plots
are 0.5 nm away from the topmost hydrogen atom and areillustrated together with the molecular frames shown in Figs.
3共a兲 and 3共b兲, respectively. Despite the change in the densityof state with the distance from the surface, the general features of the plots were less dependent on the distance. Asshown in these figures, the two types of molecular structure
yield completely different distributions of the HOMO.
As shown in Fig. 3共c兲, the surface HOMO density for themolecule in the crystal has two distinct peaks divided by anode. The distance between the two peaks is ⬃0.40 nm,
which is in good agreement with the experimental value appearing in Fig. 2共c兲 as molecule C 共distance between C 1 andC 2 兲, 0.41 nm.
The origin of the upper peak of the HOMO density is thep- and d-␲ electrons from the sulfur just below the higher
density position. Another peak located in the lower part includes the ␲ electron of the C—C bond. The appearance ofthe ␲ characteristic in addition to the ␴ bond suggests theexistence of high strain in the ethylene group. As has beenshown, the sulfur-related orbitals account for the HOMO
density, which is higher than in the previous results. The
appearance of the sulfur component in this case is explainedby the introduction of S 3d into the calculation, because
S 3d in BEDT-TTF has a large distribution in the direction
normal to the surface.
On the other hand, for the optimized molecule, only one
large peak dominates the HOMO density 关Fig. 3共d兲兴, unlike
to the molecule in the crystal shown in Fig. 3共c兲. The result isin good agreement with the STM image of molecule A inFig. 2共c兲. The origin of the dominant peak appearing over the
top of the molecule is the same component of the sulfur
orbital referred to in Fig. 3共c兲. The low density peak locatedin the lower part is not from the p-␲-like orbital of the ethylene group but from the orbital of another sulfur near thesurface plane. Structural optimization of the molecule resultsin the disappearance of the ␲-like orbital of the ethylenegroup, which dominated a considerable part of Fig. 3共c兲.
Therefore, the STM image of the BEDT-TTF compounds isdominantly characterized by the distribution of the sulfur-

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related orbital near the surface, and the additional component
appears as protrusion depending on the local strain in theethylene group.
On the basis of the comparison between Fig. 2共c兲 and Fig.
3, as described above, molecules C and A in Fig. 2共c兲 can beattributed to molecules which have a molecular structure appearing in the crystal 关Fig. 3共c兲兴 and a relaxed molecularstructure 关Fig. 3共d兲兴, respectively. The obtained result indicates that the structural relaxation of the elemental molecules
is not negligible during the formation of the surface structures of BEDT-TTF compounds.
The
characteristicfeatures
observedfor␤ -共BEDT-TTF兲2PF6 have not been pointed out until now.
However, they can be seen in the STM images of previous
works on other BEDT-TTF-based compounds with the 1D
characteristic, such as ␣ -共BEDT-TTF兲2I3 共Ref. 10兲 and共BEDT-TTF兲2TlHg共NCS兲4. 11 Their STM images also show
the two types of molecular images: a single large protrusion
and a pair of two protrusions.
Since the deformation energy of the BEDT-TTF molecules is balanced by the intermolecular interaction, intermolecular and intramolecular symmetry breaking must have astrong relationship. In most of the BEDT-TTF compounds,BEDT-TTF molecules are stacked in a face-to-face structure
forming a molecular column, as shown in Fig. 1. It is due tothe anisotropic molecular interaction. In the case of theBEDT-TTF compounds with 1D-like structure, the conductivity in the BEDT-TTF compounds is characterized bystrong ␲-␲ interaction in the molecular column. Therefore,
the stronger anisotropic interactions between neighboring
molecules may suppress the flexible relaxation of the surface
structure, and the structural relaxation is considered to berestricted by the column structure. Further study is needed tomake a complete understanding of the relationship between
intermolecular and intramolecular relaxations.
In conclusion, we found a unique mechanism operating inthe surface relaxation process of BEDT-TTF-based molecular crystals. In addition to the intermolecular symmetry
breaking which is generally observed during the surface relaxation of atomic crystals, the intramolecular relaxation wasconfirmed to play a role in the formation of the surface structure. Since the physical properties of materials depend on thecharacteristics of the crystal elements, this mechanism is fundamentally important and expected to aid in designing anddeveloping multifunctional molecular devices in the future.

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poration Tsukuba, 305-8501, Japan.
†
Corresponding author. Email address: hidemi@ims.tsukuba.ac.jp
http://dora.ims.tsukuba.ac.jp
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