PHYSICAL REVIEW B

VOLUME 55, NUMBER 1115 MARCH 1997-I

Surface reconstruction formed by ordered missing molecular rows observedon the quasi-one-dimensional organic conductor b-„BEDT-TTF…2PF6M. Ishida, K. Hata, T. Mori,* and H. Shigekawa†
Institute of Materials Science, Center for TARA, Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba 305, Japan~Received 9 September 1996!
The surface structure of a quasi-one-dimensional organic conductor b-~BEDT-TTF!2PF6 crystal @BEDTTTF is bis~ethylenedithio!-tetrathiafulvalene# was studied by scanning tunneling microscopy ~STM!. Twofold
periodic modulation along the crystal a axis was observed on the crystal a-c plane with a PF6 top layer,
similarly as the previous result obtained for the surface with a BEDT-TTF top layer. With the analysis using
high-resolution STM images including the second BEDT-TTF molecular layer, the surface superstructure wasfound to be formed by alternate missing rows of PF6 molecules. To our knowledge, this is the first observationof an organic surface reconstruction involving such a drastic change in the molecular arrangement, which wascomprehensively explained using a proposed model based on the charge balance in the surface layers.
@S0163-1829~97!02408-9#

It is well known that material surfaces induce reconstructions to stabilize their surface structures by reducing surface
energy. With the progress of the techniques of surface physics, surface reconstructions have been studied extensively toclarify the mechanism for the processes, however, whichhave been subjected to only inorganic materials for a longtime. Recently, thin films of TTF-TCNQ @tetrathiafulvalenetetracyanoquinodimethane# and b-~BEDT-TTF!2I3 @BEDTTTF: bis~ethylenedithio!-tetrathiafulvalene# have been foundto have different phase transition properties in contrast withthose of the bulk crystals; critical temperatures of phase transition became higher for the thin films, and the importantrole of the surface effect has also been identified for theorganic materials.1,2 In addition, according to the recent
scanning tunneling microscopy ~STM! studies, surface superstructures were actually confirmed to exist on the materials such as b-~BEDT-TTF!2I3 and b-~BEDT-TTF!2PF6 .3–6
However, the observed structures could be understood as achange mainly in the electronic structures, and no drastic
structural change with reconstruction of elemental moleculesin the surface of organic materials has been observed until
now.
In this work, we present the STM observation of a surface
reconstruction which consists of missing molecular rows inan organic material surface of b-~BEDT-TTF!2PF6 crystal. In
consideration of the electronic structure including second
layer molecules, the observed reconstruction is concluded tobe caused by charge redistribution in the surface layers.
b-~BEDT-TTF!2PF6 is the first organic conductor inwhich quasi-one-dimensional conductivity has been observed in the direction parallel to the molecular plane, along
the c axis. The crystal structure of b-~BEDT-TTF!2PF6 determined by x-ray diffraction is shown in Fig. 1. The crystalstructure is orthorhombic with space group Pnna, anda51.4960~4!, b53.2643~7!, c50.6664~2! nm andV53.2546 nm3. The structure consists of a stack of alternate
BEDT-TTF layers and PF6 layers parallel to the b axis.
According to calculations based on the Hückel method,strong dimerization of BEDT-TTF molecules is expected toresult in uniform half-filled metallic chains along the c axis

above T c , and metal-insulator transition caused by low dimensionality has been observed at ;297 K ~5T c !.7 Sincecharge density waves ~CDW’s! cause modulation along theconductive axis in one-dimensional conductors, the lattice
constant along the c axis is expected to be doubled below T c
due to the corresponding 2k F CDW. However, according tothe results of recent STM measurements, the surface mostoften observed showed twofold charge modulation along thea axis, which remained, even in the image of a sample considered to be in the CDW phase.5 Since this material is acharge transfer salt, which is not balanced in the top layer,
the observed modulation is expected to be related to amechanism in which the surface structure is stabilized bycharge redistribution.
b-~BEDT-TTF!2PF6 crystals were grown using the electrochemical oxidation method via a layer-by-layer mechanism. They are black, flake-shaped, and elongated along theconductive c direction. For STM observation, the crystals
were fixed onto copper plates using conductive silver epoxy
resin, and STM observations were performed at room temperature in air. All images were taken by the constant currentmode.
Figure 2~a! shows a STM image of the crystal a-c plane.
As shown in Fig. 2~a!, there are two terraces separated by astep. A cross section along A-B in Fig. 2~a! is shown in Fig.
2~b!. The height difference between the two terraces is about
1.3 nm, which is close to half of the lattice parameters in thedirection of the crystal b axis, 1.6 nm. Since the crystal con-0163-1829/97/55~11!/6773~4!/$10.00

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FIG. 1. Crystal structure of b-~BEDT-TTF!2PF6 determined by
x-ray diffraction.
© 1997 The American Physical Society

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FIG. 2. ~a! ~Color! STM image of the a-c plane ofb-~BEDT-TTF!2PF6 ~20320 nm2, V s 52100 mV, I t 52.1 nA!. ~b!
Cross section along A-B in ~a!.

sists of alternate BEDT-TTF and PF6 molecular layers
stacked along the b axis, i.e., normal to the surface as shownin Fig. 1, the present result indicates that the crystal growthoccurs via a layer-by-layer mechanism, as expected.
When the crystal is grown layer by layer, the surface hasboth BEDT-TTF- and PF6-terminated areas. Figure 3~a!
shows a STM image of a terrace shown in Fig. 2~a!, where
two domains with different structures exist. The domainboundary is indicated by white dashes in Fig. 3~a!, and theunit cells in both areas are indicated by U ET and U PF , respectively. Both the two unit cells in Fig. 3~a! have dimensions of 1.6 nm30.62 nm, which agree well with the dimensions in Fig. 1, obtained by x-ray diffraction, of 1.4960~4!
30.6664~2! nm. Cross sections along A-B and C-D in Fig.
3~a! are shown in Fig. 3~b!. The step height is about 0.1 nm,
which is comparable to the height of one PF6 layer ;0.2 nm.
Therefore, the two domains separated by a small step on aterrace are probably surfaces with BEDT-TTF and PF6 molecular top layers.
A schematic structure for the surface in Fig. 3~a! is shownin Fig. 3~c!. The upper half shows a schematic structure of aBEDT-TTF layer projected onto the crystal a-c plane. In thelower half, a PF6 molecular layer overlaps the BEDT-TTFlayer. For the BEDT-TTF molecules, the upper two rings ofthe molecules, which are closer to the surface, are shown.
Since the lobes of the highest occupied molecular orbital~HOMO! around the S atoms in the hexagonal ring are large
according to ab initio calculations, and alternate BEDT-TTF
molecules along the a axis are about 0.1 nm from the surface, the positions of the HOMO lobes of the S atoms in theuppermost rings of alternate BEDT-TTF molecules are indicated by ellipses in the direction of the a axis, which should
be compared with the STM image obtained. On the other
hand, since top-layer PF6 molecules are considered to occupy

FIG. 3. ~a! ~Color! STM image of a terrace in Fig. 2~a! ~10310
nm2, V s 5244 mV, I t 50.53 nA!. ~b! Cross sections along A-B andC-D in ~a!. ~c! ~Color! Schematic structure of surface shown in ~a!.
BEDT-TTF and PF6 molecules are colored by blue and red, respectively.

equivalent sites, all of them in the lower half of the schematic diagram are indicated by the same circles. The observed image of PF6 molecules is in fact similar to that obtained on the surface of ~TMTSF!2PF6 crystal.8 Here,
twofold modulation of the surface, which was previouslyobserved,5 is not taken into account. The two unit cells indicated by U ET and U PF in Fig. 3~a! are shifted by an amountof D. The observed value of D was about 0.4 nm, whichagrees well with the value determined by x-ray diffraction,
d50.37 nm, shown in Fig. 3~c!. The agreement between theobserved image and the structure shown in Fig. 3~c! is excellent.
Figures 4~a! and 4~c! show high-resolution STM images

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FIG. 4. ~Color! High-resolution STM images ofb-~BEDT-TTF!2PF6 surface with a top layer consisting of ~a!
BEDT-TTF, and ~c! PF6 layers ~838 nm2, V s 5230 mV, I t 50.46
nA!. Schematic structures corresponding to STM images ~a! and ~c!
are shown in ~b! and ~d!, respectively, where BEDT-TTF andPF66 molecules are colored by blue and red.

of the structures on the upper and lower terraces in Fig. 3~a!,respectively. The structure in Fig. 4~a! is the same as thatobserved for the surface with a BEDT-TTF molecular toplayer. A schematic of a BEDT-TTF layer in the a-c plane isshown in Fig. 4~b!. Here, twofold modulation in the surface

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electronic structure along the crystal a-axis is taken into account, and alternate ellipses in the direction are indicated bydark ellipses. The molecular arrangement in Fig. 3~a! agrees
well with the structure shown in Fig. 3~b!.
The STM image of the PF6 terminated surface in Fig. 4~c!
is considered to show a superposition of the PF6 and thesecond BEDT-TTF molecular layers, the unit cells of which
are indicated by U ET and U PF , respectively. As shown inFig. 4~c!, the observed PF6 molecules have similar, but different twofold periodic modulation along the crystal a axis.
The unit cell of the PF6 layer in the crystal has centeredsymmetry, as indicated in the schematic shown in Fig. 3~b!.
In Fig. 4~c!, however, the molecular rows of PF6 are at intervals twice as large as those in the schematic, and alternate
rows are completely absent. For the BEDT-TTF molecules
shown in Fig. 4~a!, alternate rows of BEDT-TTF are darkened, but never disappear. For the PF6 molecules, no protrusion is observed at the center of the unit cell, U PF . The
height difference between the BEDT-TTF and PF6 moleculesin Fig. 4~c! is about ;0.1 nm, which is similar to the value
obtained from Fig. 3~b!. Since this characteristic was observed for all surfaces with PF6 top layers, regardless of thepolarity of the bias voltage applied to the sample, we conclude that the surface superstructure is formed by alternatelyordered missing PF6 molecular rows. A schematic structureof the surface in Fig. 4~c! is shown in Fig. 4~d!.
One possible explanation for the results obtained for theb-~BEDT-TTF!2PF6 surface is that the reconstruction iscaused by electronic deficiency in the surface. It is known
that half an electron per BEDT-TTF molecule transfers into
the PF6 molecular layers, which makes the BEDT-TTF molecular layers conductive, and the PF6 layers insulating.
Since the surface layer has a counter molecular layer only on
one side, when the top layer consists of PF6 molecules, they
are supplied with only half the number of electrons. Onsemiconductor surfaces, the symmetry of the surface structure is broken to compensate for the instability due to thecharge imbalance. For example, on Si~100! and GaAs~110!
surfaces, Si-Si and Ga-As dimers buckle, which inducescharge transfer which stabilizes the surface structure.9,10 In
GaAs~100! surfaces, As dimer rows are orderly missing
which improves the charge balance in the surface layers.11
Assuming a similar mechanism, when half of the PF6 molecules are removed, the charge is exactly balanced within thesurface layer, resulting in stable surface. The ordering of thealternate missing rows of PF6 molecules may be due to theone-dimensional conductivity in the BEDT-TTF layer along
the c axis. In addition, PF6 molecules coordinate mostclosely along the c axis.
Similar modulation is observed in a BEDT-TTFterminated surface, as shown in Fig. 4~a!. In this case, since
the BEDT-TTF molecules strongly interact with the adjacent
BEDT-TTF molecules, they are not desorbed and only
charge transfer is considered to occur.
When the surface is covered by a reconstructed PF6 molecular layer, the charge of the BEDT-TTF molecules in thesecond layer is considered to be balanced. Therefore, theelectronic structures of the BEDT-TTF molecules in the second layer must be equivalent, unlike those of the BEDT-TTF

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FIG. 5. Schematic of the electronic structures in the surface
BEDT-TTF layer: ~a! uniform, ~b! with twofold periodic modulation along the a axis, ~c! CDW phase, and ~d! with a top layer ofPF6 molecules:
and s indicate 1, 1/2, 1/4, and 0

holes per BEDT-TTF molecule. The large d in ~d! indicate
PF6 molecules that are completely filled.

molecules in the top layer. In fact, five BEDT-TTF molecules, indicated by arrows in the unit cell, UPF , in Fig. 4~c!,are imaged equivalently.
For a BEDT-TTF-terminated surface with a CDW phase,
twofold periodicity along the a axis still exists.5 This canalso be explained using our model. A schematic diagram of
the charge balance in the BEDT-TTF molecules on the surface is shown in Fig. 5: ~a! 1/4-filled uniform metallic phase,~b! with twofold periodic modulation along the a axis, ~c! a*Permanent address: Department of Polymeric Materials, Tokyo

Institute of Technology, Tokyo 152, Japan.
†
Corresponding
author.
Electronicaddress:
hidemi@mat.ims.tsukuba.ac.jp
1
N. Ara-Kato, K. Yase, H. Shigekawa, M. Yoshimura, and A.
Kawazu, Syn. Metals 70, 1245 ~1995!.
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K. Kawabata, K. Tanaka, and M. Mizutani, Solid State Commun.
74, 83 ~1990!.
3
S. Pan, A. L. Delozanne, and R. Fainchtein, J. Vac. Sci. Technol.
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M. Yoshimura, H. Shigekawa, H. Yamochi, G. Saito, Y. Saito,and A. Kawazu, Phys. Rev. B 44, 1970 ~1991!.
5H. Shigekawa, K. Miyake, A. Miyauchi, M. Ishida, H. Oigawa,55

CDW phase, ~d! with a top layer of PF6 molecules.
The amount of holes introduced into the BEDT-TTF layer
is indicated by four different types of circles:
and s for 1, 1/2, 1/4, and 0 holes per BEDT-TTF molecule,respectively. The large d in Fig. 5~d! indicates PF6 molecules completely filled. It had been reported in earlier investigations that the transition at T c is due to 4k F modulation
in a highly correlated band. According to the recent bandcalculation, strong dimerization of BEDT-TTF molecules occurs, which results in a phase transition at T c associated with2k F CDW’s. Considering the charge balance in our model,
the previously observed surface superstructure shown in Fig.
5~c! is considered to support the 2k F interaction in this material instead of 4k F .6
In conclusion, a superstructure consisting of alternate
missing molecular rows was observed in the surface of aquasi-one-dimensional organic conductor, b-~BEDTTTF!2PF6 , with a PF6 top layer. This is the first observationof a surface reconstruction with such a drastic change in themolecular arrangement of the surface. By comparing theelectronic structures of the BEDT-TTF molecules in the top
and second layers, the charge transfer in the surface layers
expected for this crystal structure was confirmed. Thischarge transfer is considered to play an important role ininducing the observed surface reconstruction.
This work was supported by the Shigekawa Project ofTARA, University of Tsukuba. Support from a Grant-in-Aidfor Scientific Research from the Ministry of Education, Science and Culture of Japan is also acknowledged.

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