Structural studies of tetrathiafulvalene–tetracyanoquinodimethane thinfilms by scanning tunneling microscopy
Norihiko Araa) and Akira Kawazu
Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

Hidemi Shigekawa
Institute of Material Science, University of Tsukuba, Tsukuba 305, Japan

Kiyoshi Yase
Department of Polymer Physics, National Institute of Materials and Chemical Research, 1-1 Higashi,Tsukuba, Ibaraki 305, Japan

Masamichi Yoshimura
Department of Electrical Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan~Received 15 August 1994; accepted for publication 27 March 1995!
Thin films of tetrathiafulvanene–tetracyanoquinodimethane ~TTF-TCNQ! grown on mica substrates
by vacuum deposition were studied by scanning tunneling microscopy ~STM!. STM images
displayed the usual arrangement of alternative TTF and TCNQ columns aligned parallel to thecrystal b axis. However, in addition to the same phase as that of a TTF-TCNQ bulk crystal, a newphase is observed. In this new phase the tilt angles the TCNQ and TTF molecular planes make withthe a3b axis are different from those observed in the normal phase. This new phase can beexplained by the introduction of a stacking fault on the surface. © 1995 American Institute ofPhysics.

Organic materials are highly designable on a molecularlevel, especially with regards to their electronic properties.
For this reason molecular electronic devices which use molecular films have been considered as strong candidates tosucceed semiconductors as the next generation of electronicdevices.1 In order to utilize the high potentiality of organic
materials, thin films structures must be designed to produce
ideal or new functions. However, because the properties oforganic films strongly depend on their molecular structure,
characterization of an organic thin film’s molecular or electronic structures on an atomic scale is very important fromboth a practical and fundamental standpoint.
Scanning tunneling microscope ~STM! has recently beenused to characterize the organic materials. In particular, STM
molecular images can be compared with the theoreticalmodels.2– 6
In this letter, we present the results of an STM study ofthin films of the organic molecular metal, tetrathiafulvalene–tetracyanoquinodimethane ~TTF-TCNQ!, grownon mica substrates by vacuum deposition.
Figure 1~a! shows the crystal structure of TTF-TCNQ
determined by x-ray diffraction.7 The crystal is monoclinicwith lattice constants: a51.2298 nm, b50.3819 nm,
c51.8468 nm, and b5104.46°. Since the molecules form
homologous columns along the b axis and molecular orbitals
between adjacent molecules in each column overlap along
this direction, the crystal shows one-dimensional electronic
conductivity along the b axis. TTF and TCNQ molecularplanes are set at an angle of 58.5° to one another. They areoppositely directed by 24.5° and 34.0°, respectively, relative
to the axis c*5a3b, as shown in Fig. 1~b!.

Mica substrates were cleaved in air and were cleaned byheating at 373 K in a vacuum of 331024 Pa for 1 h.
TTF-TCNQ was deposited in high vacuum of 131024 Paon the mica substrate at room temperature by heating aquartz crucible containing TTF-TCNQ crystals. The deposition rate was 10 nm/min and the thickness of the films grown
was about 100 nm. STM measurements were performed inair at room temperature by the constant height mode using aa!

FIG. 1. ~a! Crystal structure of TTF-TCNQ determined by x-ray diffraction.
~b! Angular relation between TTF and TCNQ molecules.

Present address: Frontier Research Program, RIKEN, Wako, Saitama 35101, Japan.

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0003-6951/95/66(24)/3278/3/$6.00© 1995 American Institute of Physics

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FIG. 3. A 6.836.8 nm2 image showing the domain structure at a tip bias ofV t 524.9 mV and an average tunneling current I t 50.42 nA.

FIG. 2. ~a! A 6.036.0 nm2 image of a TTF-TCNQ film on a mica substrateobtained over the ab plane at a tip bias of V t 524.9 mV and an averagetunneling current I t 50.42 nA. ~b! Upper half and lower half of the molecules are drawn by solid and broken lines, respectively.

NanoScope II ~Digital Instruments, Inc., Santa Barbara, CA!
with 80% Pt/20% Ir tips. The STM images presented hereare filtered to remove higher frequency noise with the lowpass filter.
Figure 2~a! shows an STM image obtained at the tip bias
voltage V t of 24.9 mV and average tunneling current I t of
0.42 nA. Figure 2~b! shows the molecular arrangement of thecrystal, which was determined by x-ray diffraction,7 projected onto the ab plane. The measured size of the unit cell
indicated in Fig. 2~a! is 1.23 nm30.40 nm and this agrees
well with an ab plane x-ray measurement of a bulk crystal,
which yielded 1.2298 nm30.3819 nm @Fig. 2~b!#. Two kindsof columns appear alternately in Fig. 2~a!; one type of column consists of triplet protrusions, indicated by ‘‘A,’’ and theother type of column consists of columns of single protrusions, indicated by ‘‘B.’’ The triplets have a chevron shapeand the center balls in the triplets are brighter than the outerones. This image is similar to the STM image of the surface
of a TTF-TCNQ single crystal ab plane obtained by Sleatorand Tycko.2 According to their assignment, the triplets correspond to the TCNQ molecules and the single balls correspond to the TTF molecules. These results indicate that theTTF-TCNQ thin film was grown with its ab plane parallel tothe mica substrate.
Figure 3 shows an STM image taken under the samescanning conditions as Fig. 2~a!. The STM images of TCNQ
molecules on the left-hand side ~region II! of the boundary,indicated by the arrows, are quite different from those on theright-hand side ~region I!. In numerous such studies, we haveAppl. Phys. Lett., Vol. 66, No. 24, 12 June 1995

found that there exist only two such types of images. Because the STM images in region I are very similar to those ofFig. 2~a! as well as those of a single crystal surface obtainedby Sleator and Tycko2 and these two types of STM images
coexist in a single picture, we believe the image in region II
is not an artifact. The differences of the images in these tworegions originate not from a difference of measurement coditions, such as different tip shapes, but from a difference inthe surface structures. It is reasonable to think that a packingstructure different from that of bulk is formed under the nonequilibrium conditions which occur during vacuum deposition. This new packing structure has not been reported yet in
the bulk. Such a structure must be specific to the film. The
main differences between the images in the two regions are
as follows: ~1! the TCNQ triplets are oppositely directed in
the two regions; ~2! the central angle of the triplet in regionII, 156°68°, is larger than that in region I, 145°610°; ~3! thecenter ball in the triplet in region I is brighter than the outerones, but in region II they have almost the same intensity or
the center one is darker than the outer ones; and ~4! there isa displacement between the two lines drawn on the TTF
molecules along the a axis in regions I and II as shown inFig. 3.
Difference ~1! is the most striking one and can be understood to be the result of two neighboring domains in which
TCNQ molecules are oppositely directed, relative to the c*axis. We cannot conclude that the region II is a new phase
only from the difference of the STM image because STM
images sometimes depend strongly on the direction of ascan. Namely, it is possible that the two domains are simple
twins and the differences between their images originate
from the direction dependence of a scan. However, the STM
images for a scan from the right-hand side to the left-handside had the similar characteristics as those for a scan withthe opposite direction and the characteristics of STM images
did not depend on the direction of a scan. Since the angle ofTTF molecular plane relative to the axis c*5a3b is different
from that of TCNQ molecular plane, simple twins in whichmolecules are oppositely tilted must not be able to exist.
Therefore, it is concluded that two domains are not simple
twins and the region II is a new phase specific to thin film.
This new phase did not disappear with time and was obAra et al.

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FIG. 4. TCNQ molecules with two different tilt angles ~balls show theelectron distribution of TCNQ LUMOs! and the expected change of theprojections of a TCNQ triplet onto the ab plane.

served on the sample after annealing at 50 °C for 10 min.
Differences ~2! and ~3! can be interpreted as follows:
When the tilt angle a TCNQ molecular plane makes with thec* axis becomes smaller, their projection of the TCNQ triplets onto the ab plane will change, as is shown in Fig. 4, andthe central angle of the TCNQ triplet becomes larger. Whenthe tilt angle of the TCNQ molecular plane becomes smaller,
the distances from the center robe and outer ones of theTCNQ triplet to the STM tip become larger and smaller,respectively. Tunneling current strongly depends on the distance between a tip and a substrate surface. Therefore, theimage of the center ball of the triplet becomes darker, and theouter balls become brighter.
In this study these two STM images were often simultaneously observed at many places on the sample surface. We
have also observed similar images in STM studies of TTFTCNQ films on alkali halide substrates. The frequent coexistence of these two phases suggests that these two phases
differ only slightly in energy. Therefore, the TTF and TCNQ
molecules also in region II must be oppositely directed, relative to the c* axis, as those in region I. The TTF molecules atthe boundary indicated by the arrow in Fig. 3 belong to region II because those are on the line drawn on the TTF
molecules in region II. These boundary TTF molecules mustbe parallel to the TCNQ molecules in the adjacent column inregion I to decrease the strain at the boundary, while the tilt
angle a TTF molecular plane makes with the c* axis are3280

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different from that of a TCNQ molecular plane in a normal
phase. This is why no particular distortion and no change in
the a-axis lattice constant in the image were observed aroundthe boundary, as can be seen in Fig. 3. This boundary effectresults in the displacement of the TTF balls in the different
regions @difference ~4!#, and the change in the angular orientation of the molecular planes. Further studies on the originof such a domain structure are in progress.
In conclusion, TTF-TCNQ thin films grown on mica
substrates were found to consist of two types of domains; inaddition to the normal phase with STM images similar to
those previously reported for TTF-TCNQ crystals, a newphase was observed. This new phase can be explained by theintroduction of a stacking fault formed in the thin films. In
this new phase the tilt angles the TCNQ and TTF planes
make to the c* axis are different from those in the normal
phase. This study shows that organic thin films can havedifferent structures in surfaces from those in bulks. Suchstructural differences must be clearly understood before thethin films are applied to molecular electronic devices. Therefore, STM is essential to understand the structures or electronic properties of the surfaces of organic conductors on anatomic scale.
One of the authors ~N.A.! would like to acknowledge Dr.
Jason P. Sokoloff in the Frontier Research Program, RIKEN
for helpful discussions.

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