Synthetic Metals 71 (1995) 2077-2078
of conducting single crystal [EDT-TTF(SC,),],I,
H.Ohnuki”, K.Kitamura”, MJzumi”, H.Yamauchib and R.Kato’ “Department of Physics, Tokyo University of Mercantile Marine, 2-l-6, Etchujima, Koto-ku, Tokyol35, Japan bSuperconductivity Research Laboratory, l-10-13, Shinonome, Koto-ku, Tokyo135, Japan %stitute for Solid State Physics, University of Tokyo, 7-22-1, Roppongi, Minato-ku, Tokyo106, Japan Abstract An approach is described for the investigation of the physical properties on the conducting Langmuir-Blodgett (LB) film, in which single crystals having the identical structure with the LB film were prepared. The single crystals of [EDT-‘M’F(SC,,),],I, were obtained by electrochemical oxidation and its structure and electrical resistivity were studied. The unit cell parameters are; orthorhombic, a=lOS, b=5.4 and c=45A. These values are in good agreement with those of packing model. It indicates that the lateral packing of alkyl chains is of critical importance for the unit cell formation. The possible stacking manners of EDT-TI’F part are also discussed. The temperature dependence of the electrical resistivity shows semiconducting behavior below room temperature. The metal-insulator transition is also observed at 310 K. Introduction In many experiments on Langmuir-Blodgett (LB) films, the difficulties to obtain intrinsic physical properties are caused by their in-plane structure which are constructed of randomly arranged 2D domains [ 11.Especially, macroscopic properties such as dc conductivity are directly influenced by the domain boundaries and the domains with different crystallographic orientations that make it difficult to observe the inherent property of the LB film. One of the most desirable approach to avoid such domain effects is to study comparatively on the single crystal which has the identical crystal structure with the LB film. The purpose of this paper is to show first step for such approach to extract the essential properties by preparing single crystal. We have investigated the electrical and structural properties upon the conducting LB film which is the 1:l mixture of EDT‘TTF(SC,,)2 (Fig. 1) and behenic acid. After the two-step oxidation process with iodine vapor, the film shows dc conductivity up to -1 S/cm . In spite of relatively higher conductivity, its temperature dependence remains semiconducting (A=O.O2 eV).The X-ray diffraction profile revealed the existence of two types of domains which are formed of [EDT-TTF(SC,,),]$, and behenic acid, respectively. To understand the mechanism of the conductivity, two important informations seemed to be lack as follows. The first one was on the origin of the semiconducting behavior, where two possible origins were predicted: a) the carrier hopping between conducting (metallic) clusters under the thermal activation process, b) the intrinsic semiconducting property of [EDT-‘ITF(SC,,),],I,.
The second one concerns in-plane stacking manner of EDT-TTF. As well known in the case of (BEDT-TIF),I, , the conducting property strongly depends on the stacking manner of donor molecules. To obtain clear information about two points, the preparation of the single crystal of [EDT-TTF(SC,,),],I, was carried out. Sample preparation The dark brown and very thin needle crystals (typically 1.5 x 0.02 x SO.005 mm) were obtained by electrochemical oxidation of EDT-TTF(SC,,), (10 mg) in a dichloroethane solution (20 ml) on a platinum anode under a constant current of 2yA. Tetra-nbutylammonium triiodide (100 mg) was used as the supporting electrolyte. The result of elemental analysis on the obtained crystals showed the I,/donor ratio to be close to l/2. Figure 2a shows the X-ray diffraction profile of the pressed crystals on the glass plate. Under such configuration, the thin
Fig. 1. Molecular structure of EDT-TTF(SC,,),.
All rights reserved
Fig. 2. (a) X-ray diffraction profile of crystals and LB film (purple phase ). Arrows indicate 001reflections. (b) Morphology and setting of axes for single crystal of [EDT-lTF(SC,,)&
H. Ohnuki et al. / Synthetic Metals 71 (1995) 2077-2078
Fig. 3. Monochromatic
in the a-b plane.
needle crystals tend to be oriented uniaxially along the c axis (Fig. 2b) like a LB film. The diffraction profile of LB film is also presented in Fig. 2a. The crystals showed similar 001 diffraction profile to that of LB film. This result indicates the existence of a common structure between the crystals and LB film (bilayer structure).
Experiment and Results To investigate the in-plane (a-b plane) structure, the monochromatic Laue method was performed on the single crystal. The sample was mounted on polyimide film (thickness 7.5 pm) and irradiated with monochromatic MO Ka radiation along the c axis. Figure 3 exhibits the obtained X-ray photograph. Combining the previous results, the unit cell of [EDT-T’TF(SC,,),],13 was determined to be: orthorhombic, a=lOS, b=5.4 and c=45 A. It is difficult to determine the in-plane stacking manner of EDT-‘M’F part from only the unit cell parameter. However, from the viewpoint of the lateral packing of alkyl chains, the two possible stacking manners of EDT-TTF part can be expected as follows. In most cases, the subcell type for alkyl chains which forms the orthorhombic cell is 0 type . Assuming the 0[+3,0] layer , the cell dimension will be a=10.62 and b=4.96 A (Fig. 4a) which well reproduce the observed cell. It is also notable that the inclination angle of alkyl chains calculated from the 0[+3,0] layer model is 45.8” which is substantially in agreement with the observed angle of 50”  by FT-IR experiments on LB film. The next step is determination of the position of EDT-T’TF part under the O[f3,0] layer model. The EDT-TTF part has to be located between two alkyl chains and also its position has to keep the same unit cell dimension and symmetry as those of alkyl chains. The EDT-TTF which fulfills such conditions occupies the position between the chains at comer and center in the 0[*3,0] cell (FigAa). However, the above consideration is for only one side of the bilayer structure. Figure 4b shows the two types of different stacking manners of EDT-TTF part in bilayer system. In both of them, the interdigitation between EDT-TTF parts which belong to
Fig. 4. (a) In-plane structure model based on the 0[?3,0]
Fig. 4. (b) Two possible stacking manner of EDT-‘ITF part (inplane). The dark and white patterns indicate EDT-TTF positions belonging to the upper layre and lower one, respectively. each side of bilayers is brought about to realize the shorter intermolecular distance along the b axis, which gives the conducting property in this direction. The temperature dependence of the resistivity along the b axis exhibits semiconducting behavior below room temperature (Fig.5). The remarkable feature is occurrence of the metalinsulator transition at 310 K as shown in the inset. However, the mechanism of the transition is not clear at the present stage. Further investigations are necessary for understanding the conducting property especially in its relation to the in-plane structure.
Fig. 5. Temperature dependence of the normalised electrical resistance along the b axis.
References 1. J.Gamaes, N.B.Larsen, T.Bjornholm, M.Jorgensen, K.Kjaer, J.Als-Nielsen, J.F.Jorgensen and J.A.Zasadzinski, Science, 264 (1994) 1301. 2. C.Dourthe, M.Izumi, C.Garrigou-Lagrange, T.Buffeteau, B.Desbat and P.Delhaes, J. Phys. Chem., 96 (1992) 2812.; M.Izumi, H.Ohnuki, H.Yamaguchi, H.Oyanagi and P.Delhaes, Synth. Metals, 55-57 (1993) 2560.; H. Ohnuki, M.Izumi, K.Kitamura, H.Yamaguchi, H.Oyanagi and P.Delhaes, Thin Solid Films, 243 (1994) 415. 3. Proceeding of the International Conference on Synthetic Metals, Synth. Met., 55-57 (1993). 4. A.I.Kitaigorodskii, Organic Chemical Crystallography, Consultants Bureau, New York, 1961,; A.I.Kitaigorodskii, Molecular Crystals and Molecules, Academic Press, London, 1973.