1882
J. Phys. Chem. 1994,98, 1882-1887
Structure and Electrical Properties of the Metallic Langmuir-Blodgett Film without Secondary Treatments Takayoshi Nakamura,' Gen Yunome, Reiko Azumi, Motoo Tanaka, Hiroaki Tachibana, and Mutsuyoshi Matsumoto National Institute of Materials and Chemical Research, 1 - 1 , Higashi, Tsukuba, Ibaraki 305, Japan
Sachio Horiuchi, Hideki Yamochi, and Gunzi Saito Department of Chemistry, Faculty of Science, Kyoto University, Sakyo- ku, Kyoto 606,Japan Received: July 2, 1993; In Final Form: November 16, 1993'
A metallic Langmuir-Blodgett (LB) film which does not require any secondary treatments obtained by using a charge-transfer complex of bis(ethy1enedioxy)tetrathiafulvalene (BO) and decyltetracyanoquinodimethane (CloTCNQ) was examined. A strong band in the I R region was observed due to the conduction electron. The average charges of the CloTCNQ and BO in the LB film were -1 and +0.4,respectively, determined from the shift of the specific bands. The room-temperature conductivity of the film was 10 S/cm. The temperature dependence of conductivity of the film was well fitted by the formula u = A P exp(-Eg/2ker), which is related to an activation-type conduction within the domain boundaries of a granular-structured film. The temperature dependence of the thermoelectric power clearly shows the metallic nature of the film. The thermoelectric power was positive and in the range of 15 kV/K at room temperature and tends to 0 pV/K at 0 K: the behavior of the temperature dependence of the thermoelectric power was determined by that of the metallic domains of partially-charge-transferred BO.
SCHEME 1
Introduction This paper reports on the physical properties of the metallic Langmuir-Blodgett (LB) film without secondarytreatments, the LB film based on a charge-transfercomplex of bis(ethy1enedioxy)tetrathiafulvalene(BO) and decyltetracyanoquinodimethane((210TCNQ) . The LB technique has been used to fabricate thin-film molecular assemblies.' Conducting LB films have been reported, in the initial stage, consisting of charge-transfer salts of TCNQ, TTF, or metal complex derivatives.2 The one-dimensionalnature of these charge-transfer complexes has made the electric conduction of the above LB films susceptibleto structural defects and disorders present in the LB films.3 Macroscopic conductivities of these films are of activation type even when the complexes are expected to be metallic in a single crystalline state.4 This difficulty has beenovercome by using metal-dmit (Hzdmit = 4,5-dimercapto-l,3-dithiol-2-thione) complexes, which can form a conduction network of more than one dimension^.^ We have reported that the LB films of the tridecylmethylammoniumsalt of the gold-dmit complex show a metallicconductivity.6 However, during the preparation, secondary treatments are required such as electrochemical oxidation in aqueous electrolyte solutions.6 The next step to exploit the possibilities of conducting LB films is to obtain metallic LB film which do not require secondary treatments. Among the conducting solid charge-transfer complexes, BO complexes are worthy of remark. This donor has provided a number of metallic complexes not only with inorganic anions7 but also with a large variety of organicacceptormolecules.* The striking aspect of these complexes is that they exhibit metallic behaviors in the conductivity even when measured on the compressed pellets of the powdered samples,indicating the strong ability of BO to give metallic conductivities even in disordered systems. This ability is assumed to come from the strong selfassembling ability of BO which forms a two-dimensional network. Recently, we have found that the BO forms conducting complexes with long-chain derivatives of TCNQ9 and that the a
Abstract published in Advance ACS Absfracfs, January 15, 1994.
0022-365419412098-1882$04.50/0
BO
C1 oTC NQ
complexes could be fabricated in the forms of LB films.IO We have used three kinds of TCNQ derivatives which are different in the length of side chains ((210, C14, and CIS). In the case of the BO-CloTCNQ complex, the LB film showed a "metallic" temperature dependence of conductivity. However, the conduction mechanism of the films is not clear. In this paper, we will reveal the physical properties of the LB films by means of IR spectroscopy, conductivity measurements, and thermoelectric power measurements. The intrinsic metallic nature of the film is confirmed, although the results show that the "metallic" temperature dependenceof conductivityis related to the activation-type conduction within the domain boundaries in the granular structure of the film.
Experimental Section Preparation of Solid Complex. The complex of BO and CloTCNQ was prepared by mixing an acetonitrile solution of each component. The composition of the complex was determined by the elemental analysis as B0:ClOTCNQ:HzO = 10:4:1. The conductivity at room temperature was 3.5 S/cm for the compaction sample. The temperature dependence of the conductivity was metallic around room temperature and reached a maximum conductivity of 4.5 S/cm at around 150 K for the compaction sample. Preparation of LB Films. An acetonitrile solution of BOCloTCNQ (5 X 10-4mol/L based on the amount of CloTCNQ) was mixed at the volume ratio of 1:l with a benzene solution of icosanoic acid ( C ~ O(5 ) X 10-4 mol/L) and spread onto a water subphase. The barrier speed was 2 cm/min. The deposition of 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1883
Metallic Langmuir-Blodgett Films
2222
-'?, I
4000
I
I
2000
I
1000
Wavenumber I cm-1
Figure 1. IR spectrum of the LB film of BO-CloTCNQ and icosanoic
acid on CaF2 substrate.
the films was carried out at 20 mN/m and 290 K. The conductivity of the LB films was strongly affected by the preparation conditions, e.g., spreading solvents or deposition methods. The film obtained by the horizontal lifting method from the monolayer on a water subphase spread from a benzene acetonitrile solution gave the highest conductivity.I0 This LB film was subjected to further measurements. IR Measurements. IR spectra were taken using a Jasco FT8300 Fourier transform spectrometer with TGS detector in the range 4600 to 400 cm-1. The resolution was 0.5 cm-1. In order to obtain a good signal-to-noiseratio, 1000 interferograms were accumulated for each spectrum. The spectra of the powder samples were taken in the form of a KBr pellet. The LB films for IR measurements were prepared onto CaF2 substrates by a vertical dipping method for the first three monolayers followed by horizontal lifting method for the subsequent 42 layers. A CaF2 plate was used as a reference for the measurements of the LB films. The spectrum below 800 cm-1 could not be recorded due to the strong absorption of CaF2. Electrical Measurements. The glass plates and poly(ethy1ene terephthalate) (PET) films (0.1 mm thick) were used as the substrates of LB films for conductivity and thermoelectric power measurements, respectively. The glass plates and PET films were hydrophobized by hexamethyldisilazaneand by precoating with five monolayers of cadmium icosanoate, respectively. Gold was evaporated onto each substrate before deposition of the film to form electrodes (05" gap distance). The electrical conductivity measurements were carried out by a dc four-probe method using evaporated gold electrodes mentioned above for the 20-layer samples. Copper wires were attached using silver paste onto electrodes as leads. The samples for the thermoelectric power measurements were attached by gold paste to the copper blocks. The details of the measurement of thermoelectric power of conducting LB films were reported elsewhere.1' RMults
IR Spectra. Figure 1 shows the IR spectra of the LB film of CaF2. A broad band due to the conduction electron appears over the whole range, which is consistent with the high conductivity of the film. The spectrum is somewhat complicated because the film is composed of three components, BO and CloTCNQ, which form the charge-transfer complex, and C ~ O as the matrix. However, there are relatively strong bands below 1100cm-' which are attributed to the vibrations concerning C-0 bonds of B0.12 Two bands indicated by arrows show pronounced antiresonant behaviors, which are assigned as A, modes with an electronmoleculevibrationcoupling (EMVC).Q13 This suggeststhedimer formation of the BO molecules: the antisymmetric vibrations of the BO dimer induce the EMVC."
CloTCNQ
I-&/']\
BO-CIOTCNQ
,
(powder)
\
7
BO-CIOTCNQ (LB film)
I
2300
2200
2100
Wavenumber / cm-1
Figure 2. Vibrational spectra of CN stretchingmode for different phases of CloTCNQ.
The vibration bands of BO and CloTCNQ are shifted compared with those of the neutral molecules when the charge transfer takes place. The positions of the specific bands reflect the degree of the charge transfer. The amount of charge on the TCNQ molecule has been estimated by examining the position of the CN stretching mode,14 which appears around 2200 cm-l. We have prepared the potassium salt of CloTCNQ, in which the TCNQ moiety should be an anion radical. Using the wavenumber of the CN stretching band of the neutral and the potassium salt of TCNQ and assuming the linear relationship between wavenumber shift and the amount of charge on TCNQ, we can estimate the charge transferred onto the CloTCNQ part of the complexes. The CloTCNQ moiety of the complex is considered to be in a full-charge-transferred state in the forms of both powder and LB film. Figure 2 shows the CN stretching bands for the BOCloTCNQcomplex in the two forms, together with those of the neutral and the potassium salt of CloTCNQ. The bands of the complexes appear at almost the same position as that of the potassium salt,14 indicating that the CloTCNQ of the complex should be an anion radical. This means that the CloTCNQ part behaves as a counteranion. A small peak around 2200-2220 cm-1 was observed only in the LB film correspondingto the charge on CloTCNQof at around -0.25 by assuming the maximum at 2210 cm-l of the peak.15 The charge-transfer ratio can also be estimated by the shift of thevibration bands of BO. Recently, Moldenhauer et al. reported that the frequencies of thevibration bandsconcerning C - O bonds of BO are directly related to the average charge on the donor molecules.16 They have used four vibrational bands in the wavenumber region of 1100-800 cm-l, two Bzu modes and two A,, modes indicated in Figure 3. They have reported that the frequenciesof these bands decrease with an increase in the transferred charge. The relationship between the frequency and the degree of charge transfer is not linear. However, two salts with the same charge of +0.5 on BO (BOzReOdH20 and B02C1) have shown identicalshifts, indicating the validity of the estimation of the degree of the charge transfer from the frequency shifts.16 The two A, bands showed antiresonant behaviors in the spectra of BO complexes as mentioned above, with the peak frequency being ambiguous due to the broadening of the bands. On the other hand, BZubands have symmetrical shapes, and we have adopted these two bands for the estimation of the degree of the transferred charge.
Nakamura et al.
1884 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994
U
6 9.6 200
220
240
260
280
300
Temperature / K 0 0
50
I
I
I
I
100
150
200
250
300
Temperature / K
Figure 4. Temperature dependence of the conductivity for a 20-layer sampleof 1:1 mixture of BO-CloTCNQandicosanoicacid. Theoriginate in the inset is enlarged to show the crossover point. The solid curve is a theoretical fit (see text). BO-C~OTCNQ
(LB film)
0
IO Wavenumber I cm-1
Figure 3. Vibrationalspectra concerning C-O bonds in different phases
of BO.
TABLE 1: Frequency Shift of Vibrational Mod@ from Neutral BO shift of Bzubands' comwund B2.(48) Bd50) charge of BO ob oe 0 BO 0 3 +0.33 BO~CU~(NCS)~~ e BOCloTCNQ (powder) 2 4 3 5 e BO-CloTCNQ(LB film) 4 8 +0.42 B02.4hd 5 8 +0.5 BOzReOd2Od 5 8 +0.5 B02Cld a In cm-1. b At 1082 cm-l. At 962 cm-l. Taken from ref 16. This work. The bands appear a t 1080 and 958 cm-1 for the BO complex of the KBr pellet and 1079 and 957 cm-I for the LB film. The difference of 1 cm-I could be an experimental err0r.I' The frequency shifts compared to the neutral BO are summarized in Table 1 together with the reported values.16 The frequency shifts are 3 and 5 cm-1 in case of LB film, which are between the values of 0 and 3 cm-3 for B03Cu*(NCS)3 (average charge per BO is +0.33) and 4 and 8 cm-I for BO2.413 (average charge per BO is +0.42). The results are in good agreement with the average charge per BO of +0.4 calculated by assuming the charge of -1 for CloTCNQ, taking into account the composition of the complex (BO:CloTCNQ = 10:4) from the elemental analysis. Conductivity. The LB films showed a metallic temperature dependence of conductivity from room temperature down to around 250 K as is shown in Figure 4. The conductivity at room temperature is 10 S/cm by assuming that the thickness of the monolayer is 2.5 nm. The conductivity of the film increases with a decrease in temperature down to around 250 K and decreases with decreasing temperature below the crossover point. At low temperatures, the conductivity obeys an activation-type conduction mechanism. The temperature dependence of the conductivity is well fitted by the model proposed by Miller and Epstein,ls u = A F exp(
-
&)
where ks and E, are the Boltzmann constant and the band gap, respectively. This formula assumes that the conductivity is
u
50
100
150 200 Temperature / K
250
Figure 5. Temperature dependence of thermoelectricpower of the LB film from liquid nitrogen temperatureto room temperature. The broken and dotted lines are a fit of S = SO+ BT above 200 K and below 170
K (see text).
expressed by the product of carrier density n a exp(-E,/2k~T), mobility p a T-a,and charge per carrier e, and it has been applied to explain the temperature-dependent conductivity of the narrowgap semiconductors.~~J9 The solid line in Figure 4 represents the curve with the value of a = 2.34 and E, = 1140 K. Thermoelectric Power. The temperature dependence of thermoelectric power of the LB film down to liquid nitrogen temperatures is shown in Figure 5.20 Below this temperature, the voltage could not be measured due to the high impedance of the film. The thermoelectric power of the film was positive and in the range of 15 pV/K at room temperature. No anomaly was observed corresponding to the crossover around 250 K in the conductivity measurements. The results clearly deny the possibility that the LB film is a narrow-gap semiconductor such as TCNQ complex salts in which a t high temperatures the thermoelectric power approaches the value determined by spin entropy.z1 The thermoelectric power tends to 0 pV/K at 0 K. However, the thermoelectric power does not show single linear relationship
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1885
Metallic Langmuir-Blodgett Films with Tin contrast to usual metals. Linear fits could be applied in two parts above 200 K and below 170 K, S=So+CT (2) with the values SO= -7.5 pV/K and C = 7.6 X 1 t 2pV/K2 for T > 200 K and So = -1.5 pV/K and C = 4.4 X pV/K2 for T < 170 K.
Discussion The electronic structure of the LB film is examined by IR spectra which indicate the high conductivity of the film and partially-charge-transferred state of the BO radicals: the high conductivity of the film is governed by the BO part. However, the conductivity suggested by the optical measurements should not appear directly in the electrical conductivity measurements. The film has a granular structure, and the gap distance between the electrodes (0.5 mm) is large enough compared to the domain size (less than 1 pm), which is revealed by AFM.l0 This means that no selective paths of single domain giving a high conductivity through these conduction paths are present between the electrodes. We consider a “uniform” granular structure, where the size of the domains and the thickness of the domain boundaries are almost constant in the whole region, with conductivities u, and Ob for domains (m) and domain boundaries (b), respectively. From the argument on the conducting polymers,22 the conductivityu in the one-dimensionalsystem with two types of materials in series is written as
is uniform in the whole area of the film. In the case of the onedimensionalsystem, the temperature drops is proportional to the length of each part,
s = (1 - rb)Sm+ r p b
(64 When we apply the same assumption with the discussion on conductivity, the thermoelectric power in two-dimensional granular structure is represented by (see Appendix)
It should be noted that y is a temperature-independent term. Formula 6b shows that the temperature dependence of the thermoelectric power is mainly explained by that in highlyconducting domains with perturbation from that in domain boundaries. The observed thermoelectric power, nearly linear with T and tending to 0 pV/Kat 0 K, indicates that the behavior is essentially metallic in origin. As the thermoelectric power is positive, the carrier of this metallicbehavior should be holes, which is consistent with theresults of IR measurementsindicatingthat the conduction in the LB film is achieved by the partially-charge-transferred BO part. In common metals, the thermoelectric power is represented by the single linear expressionz4
(3) by assuming rbu, >> (1 - rb)ub, where r b is the fraction of the domain boundaries. In the two-dimensional granular structure, the 1/rb could not be an exact scaling factor. Instead, we should use a scaling factor l / y which is defined as
r = (3 1
(4)
where rb( is a fraction of the domain boundaries at the ith cross section (see Appendix). The behavior of temperaturedependent conductivity is not governed by the highly conducting domains but is determined by the domain boundaries, although the total conductivity may be much larger than that in the domain boundaries scaled by l / y . The energy gap which appears in formula 1 is due not to the intrinsic gap of the complex forming each domain but to the conduction within the domain boundaries. It is not clear at present whether the CloTCNQ species giving a small peak of CN stretching band around 2200-2220 cm-1 in the IR spectrum of the LB film concerns the conductionin the domain boundarie~.~’ The electrical properties would be examined more precisely when combined with the thermoelectric power measurements. The electrical resistance in the domain boundaries will have less effect on the thermoelectric power than that on electrical conductivity because the temperature drop in the domain boundaries will usually be much less significant than the voltage drop. The general expression for the thermoelectric power for the two types of the materials in series is written asz2 AT,
s=-s,+AT
ATb AT’,
where S, and S b are the characteristic thermoelectric power of the domains and the domain boundaries, respectively, and AT, and ATb are the temperature drops across each part and AT = AT, i- ATb. As we deal with the LB films on the thick substrate (0.1-mm PET film), the temperature gradient in the film is determined by the heat resistance of the PET film; that is, the heat resistivity
The thermoelectric power of the LB film showed an overlinear behavior with crossover point at around 170-200 K. We measured the thermoelectric power of the compaction sample in which the structure and the properties of the domain boundaries could be different from those in the LB film and observed a similar overlinear behavior. Thisstronglysuggeststhat the thermoelectric power originates from the metallic domains, and the overlinear behavior is not due to the effect of the domain boundaries which is represented by ysb in formula 6b. The phonon-drag thermoelectric power may be neglected in the measured region. We suggest two possible causes for the overlinear behavior of the thermoelectric power of the LB film. One explanation is that the observed behavior is due to the anisotropic thermoelectric power of the BO complex. It is wellknown that the two-dimensionalconductors like BO or BEDTTTF salts give anisotropic thermoelectric power in some cases: completely different results were observed in the measurement along different crystal a ~ e s . 2 ~In ~ ~the 5 LB film, the metallic domains are randomly oriented in two dimensions. We only observe an average of the thermoelectric power in different directions in the two-dimensional domains, which may lead to a nonlinear behavior of the thermoelectric power. The other possibility is that a certain kind of transition in the domains results in the crossover of the thermoelectric power. Such a transition was not detected in the conductivity measurements. However, if the conductivityof the domain is high enough even after the transition, no anomalies should be observed in the conductivity measurements at the transition point, since the conductivity is still governed by domain boundaries at lower temperatures. The conduction within the domains is metallic, achieved by the partially-charge-transferredBO which dominatesthe behavior of the thermoelectric power. Due to the high conductivityof the domains, the behavior of temperature-dependent conductivity within the domains could not be observed in the conductivity measurements. The domain boundaries are semiconducting,and the behavior is observable in the conductivity measurements.
1886 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 Summary and Conclusion We have shown the physical properties of the newly developed conductingLB films of the BO-CloTCNQ complex. The results are summarized as follows: 1. The film had a broad absorption in the IR region due to the conduction electrons. The A, modes of the BO molecules showed strong antiresonant behaviors due to the EMVC, suggesting that the BO molecules form dimers in the film. The CloTCNQ moiety in the film was in a fully-charge-transferred state for the most part, and the average charge on BO was +0.4 estimated by the frequency shifts of the vibrational modes. 2. Theroom-temperatureconductivityofthefilmwas10S/cm. The temperature dependence of the conductivity was well fitted by the formula u = A P exp(-Eg/2k~T), which should be governed by the less conducting domain boundaries. 3. The thermoelectric power is determined mainly by the highly-conducting metallic domains. The thermoelectric power was positive at room temperature (15 pV/K). The temperature dependence was fitted by two lines of S = SO+ CT above 200 K and below 170 K. The behavior of the thermoelectric power may be explained by the anisotropic thermoelectric power of the BO complex or by a certain transition in the domain. In conclusion, we have demonstrated the fabrication of the metallicLB film without secondarytreatments based on a chargetransfer complex of BO and CloTCNQ. This metallic LB film will open up new possibilities for the fabrication of bioelectronic and molecular electronic devices in the future.
Acknowledgment. The authors express their thanks to Prof. Toshiaki Enoki, Tokyo Institute of Technology,Dr. Heikki Isotalo, Semiconductor Laboratory, Technical Research Centre of Finland, and Dr. Keiichi Ikegami, Electrotechnical Laboratory, for their valuable comments and fruitful discussionson thermoelectric power.
Nakamura et al. domain boundary, we assume rblam>> (1 - rbi)Ub, and formula A2 becomes
Using the term y defined in the text as l / y = [X(l/rw)]/n = ( l/rbi) and the equation 1/R = Au/L, the total conductivity is written as
= (l/yIub (A4) In the "uniform" granular structure, y is almost the same with the fraction of domain boundary defined as rb = (rw), and mathematically always Q 1 y. Thermoelectric Power. The thermoelectricpower of each strip is written as
+
si= (1 - qJSm rbpb (A51 because the temperaturedrops shouldbe proportional to the length of each part as mentioned in the text. The total voltage between the electrodes (V)and that between each strip (Vi)are expressed using the temperature drop ATand the total thermoelectricpower S as
As the total current between the electrodes should be 0 at the steady state, the formula
C Y --0 should hold. Using 1/R = C(l/Ri) and formulas A6 and A7, the thermoelectric power becomes
Appendix In this Appendix we examine the conductivity and the thermoelectric power in the two-dimensional granular structure. The structure is relatively uniform: (1) the domains are almost of the same size in the whole film with nearly two-dimensional symmetricalshape,and (2) the thickness of the domain boundaries are almost constant. The size of the domains is small enough compared to the width between the electrodes; that is, no conduction paths of single domain are present between the electrodes. We consider the two-dimensionalsheet as a sum of one-dimensional strips, which means that we neglect the nondiagonal terms in the conductivity tensor. The assumption is appropriate when we deal with relatively uniform granular structures. The anisotropy in conductivity within domains or domain boundaries is also neglected. The conductivity of the domains and the domain boundaries are represented by the terms umand Ub, respectively. Conductivity. We divide the two-dimensional film into n strips parallel to the measured direction. When the strip is narrow enough compared to the domain size, the resistance of the ith strip (RJ is given using the conductivitiesof the domain and the domain boundary as
-
R, - [ n ( l
[,I($)
irbiIL](L) + nrbiL
(Al) urn where L and A are the total length and the cross-sectionalarea of the sample, respectively. The total resistance R is written as
Since the conductivity in the domain is much larger than in the
s=C(R/RJSi
(A91
The contribution of the thermoelectric power of each strip to the total thermoelectric power is determined by the conductance of each strip. Using formulas AI, A2, and A5, and assuming rwum >> (1 - rbi)Ub,S is exhibited as
s = (1 - y)sm+ rsb
(A 10)
The term y is smaller than Q as mentioned above, which is also explained as follows: the domain boundaries parallel to the measured direction have a smaller contribution to the total thermoelectric power compared to those in the perpendicular direction due to the small conductivityof the domain boundaries.
References and Notes (1) Recent progress in the LB films appears in: Proceedings of 5th International Conference on Langmuir-Blodgett Films. Thin Solid Films 1992,210-211. (2) (a) Ruaudel-Teixier, A.; Vandevyver, M.; Barraud, A. Mol. Crysr. Liq. Cryst. 1985, 120, 319. (b) Nakamura, T.; Matsumoto, M.; Takei, F.; Tanaka, M.; Sekiguchi, T.; Manda, E.; Kawabata, Y. Chem. Lett. 1986,709. (c) Kawabata, Y.; Nakamura, T.; Matsumoto, M.; Sekiguchi, T.; Komizu, H.; Manda, E.; Saito, G. Synrh. Mer. 1987,19,663. (d) Bertho, F.; Talham, D.; Robert, A.; Batail, L.; Megtert, S.;Robin, P. Mol. Crysr.Liq. Crysr. 1988, 156,339. (e) Dhindsa, A. S.; Brice, M. R.; Lloyd, J. P.; Petty, M. C. Thin Solid Films 1988,165, L97. (f) Richard, J.; Vandevyver, M.; Barraud, A,; Morand, J. P.; Lapouyade, R.; Delhacs, P.; Jaquinot, J. F. J. Chem. Soc., Chem. Commun. 1988,754. (g) Matsumoto, M.; Nakamura, T.; Manda, E.; Kawabata, Y.; Ikegami, K.; Kuroda, S.;Sugi, M.; Saito, G. ThinSolid Films 1988,160,61. (h) Galchenkov, L. A,; Ivanov, S. N.; Nad, F. Ya.; Chernov, V. P.; Berzina, T. S.; Troitsky, V. I. Synrh. Met. 1991,12, 1471. (i) Gionis, V.; Ficbet, 0.;Izumi, M.; Garrigou-Lagrange, C.; Amiell, J.; Papavassiliou, G.; Delhaes, P. Chem. Leu. 1991, 871. (3) Ikegami, K.; Kuroda, S.; Saito, M.; Saito, K.; Sugi, M.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Phys. Rev. B 1987, 35, 3667. (4) Ikegami, K.; Kuroda, S.;Saito, K.; Saito, M.; Sugi, M.; Nakamura, T.; Matsumoto, M.; Kawabata, Y.; Saito, G. Synrh. Mer. 1988, 27, B587.
Metallic Langmuir-Blcdgett Films ( 5 ) Bousseau, M.; Valade, L.; Legros, J.; Cassoux, P.; Garbauskas, M.; Interrante, L. V. J. Am. Chem. SOC.1986, 108, 1908. (6) Nakamura, T.; Kojima, K.; Matsumoto, M.; Tachibana, H.; Tanaka, M.; Manda, E.; Kawabata, Y. Chem. Lett. 1989, 367, (7) (a) Yamochi, H.; nakamura, T.; Saito, G.; Kikuchi, T.; Saito, S.; Nozawa, K.; Kinoshita, M.;Sugano,T.; Wudl, F.Synrh. Mer. 1991,42,1741. (b) Suzuki, T.; Yamochi, H.; Isotalo, H.; Fite, C.; Kasmai, H.; Liou, K.; Srdanov, G.; Wudl, F.; Coppens, P.; Maly, K.; Frost-Jensen, A. Synth. Me?. 1991,42,2225. (c) Beno, M. A.; Wang, H. H.; Kini, A. M.; Carlson, K. D.; Geiser,U.;Kwok, W. K.;Thompson, J. E.; Williams,J. M.; Ren, J.; Whangbo, M.-H. Inorg. Chem. 1990,29,1599. (d) Kahlich, S.;Schweitzer, D.; Heinen, I.; Lan, S.E.; Nuber, B.; Keller, H. J.; Winzer, K.; Helberg, H. W. SolidStore Commun. 1991,80, 191. (81 Yamochi, H.; Horiuchi, S.; Saito, G. Phosphorus, Sulfur Silicon Re1at:Elem. 1992, 67, 305. (9) Yamochi, H.; Horiuchi, S.;Saito, G.; Kusunoki, M.; Sakaguchi, K.; Kikuchi, T.; Sato, S.Synth. Met. 1993, 56, 2096. (10) Nakamura, T.; Yunome, G.; Azumi, R.; Tanaka, M.; Yumura, M.; matsumoto, M.; Horiuchi, S.;Yamochi, H.; Saito, G. Synrh. Mer. 1993,57, 3853. (1 1) Miura, Y. F.; Takenaga, M.; Kasai, A.; Nakamura, T.; matsumoto, M.; Kawabata, Y . Jpn. J . Appl. Phys. 1991,30, 3503. (12) Pokhodnia, K. I.; Kozlov, M. E.; Onischenko, V. G.; Schweitzer, D.; Moldenhauer, J.; Zamboni, R. Synrh. Mer. 1993, 56, 2364. (13) (a) Rice, M. J. SolidSrote Commun. 1979,31,93. (b) Painelli, A.; Girlando,A.; Pecile, C. SolidSrate Commun. 1984,52,801. (c) Moldenhauer, J.; Pokhodnia, K. I.; Schweitzer, D.; Keller, H. J. Synrh. Mer. 1993,56,2554. (14) The CN stretching of the complex appears at several wavenumbers lower than that of the potassium salt. This does not mean a charge transfer greater than unity on CloTCNQ which should be prohibited by large on-site Coulomb repulsive energy. This shift can be regarded as the result of the screening of the cationic charge on the BO molecules by the conduction electrons, in contrast to the situation in the alkali metal-TCNQ in which the
The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 1887 absorption bands appear in the higher-wavenumber regions because of the ineffective screening: Chappell, J. S.;Bloch, A. N.; Bryden, W. A.; Maxfield, M.; Poehler, T. 0.;Cowan, D. 0. J. Am. Chem. SOC.1981,103,2442. (1 5) The linear relationship between wavenumber shift and the transferred charge from the neutral CloTCNQ (2222 cm-I) to monovalent species in the LB film (2179 cm-l) was assumed for the estimation. (16) Moldenhauer, J.; Pokhodnia, K. I.;Schweitzer, D.; Heinen, I.; Keller, H. J. Synrh. Mer. 1993, 56, 2548. (17) The accuracy in the wavenumber is much less than 0.5 cm-I in these spectra. However, the position of peak top is influenced by the shape of the base line, which contributes largely to the inaccuracy of the peak frequency. (18) Miller, J. S.;Epstein, A. J. Angew. Chem., In?. Ed. Engl. 1987, 26, 287. (19) (a) Epstein, A. J.; Conwell, E. M. Solid Srate Commun. 1977, 24, 627. (b) Epstein, A. J.; Conwell, E. M.; Sandman, D. J.; Miller, J. S.Solid Srate Commun. 1977, 33, 355. (20) As the thermoelectric power was measured relative to copper block,
the contribution of the thermoelectric power of copper was corrected: Maddison, D. S.;Unsworth, J.; Roberts, R. B. Synrh. Mer. 1988, 26, 99. (21) Chaikin, P. M.; Kwak, J. F.; Epstein, A. J. Phys. Rev. Len. 1979, 42, 1178. (22) Kaiser, A. B. Phys. Rev. B. 1989, 40, 2806. (23) The origin of the species of [ C ~ O T C N Q in ] ~the , ~ film is not clear
at present. However, the dissolution of the some part of the BO molecules at the domain edge to the subphase during the film-forming process at the air-water interface is most probable for the explanation of the formation of the CloTCNQ molecules in the partially-charge-transferred state. In this case, CloTCNQ species should concern the conduction within domain boundaries. (24) Mortensen, K.; Williams, J. M.; Wang, H. H. SolidStore Commun. 1985, 56, 105. (25) Mori, T.; Inokuchi, H.; Mori, H.;Tanaka,S.;Oshima, M.;Saito, G. J. Phys. SOC.Jpn. 1990, 59, 2624.
