J . Phys. Chem. 1992, 96, 2812-2820
2812
Physical Properties of Mixed Langmuir-Blodgett Conducting Films Based on a Tetrathiafulvalene Derivative C. Dourthe,+M. Izumi,' C. Garrigou-Lagrange,+T. Buffeteau,! B. Desbat,! and P. Delhaes*++ Centre de Recherche Paul Pascal, Avenue A. Schweitzer, 33600 Pessac, France, Department of Physics, Tokyo University of Mercantile Marine, Etchu-Jima, Koto- Ku, Tokyo 135, Japan, and Laboratoire de Spectroscopie MolCculaire et Cristalline, Universitl Bordeaux I , 33405 Talence Cedex, France (Received: July I , 1991; In Final Form: October 22, 1991)
Electroactive molecules such as (ethylenedithio)bis(octadecylthio)tetrathiafulvalene [EDTTTF(SC,,),] are useful molecular blocks for building up Langmuir-Blodgett conducting films when they are mixed with a fatty acid such as behenic acid and then doped with iodine vapors. To understand the molecular organization in these layers, we have investigated the IR electronic and vibrational absorption spectra, the resonance Raman scattering of the iodine species and the dc electrical conductivity as a function of the fatty acid content, and the number of deposited layers. These investigations have led us to propose a model of heterogeneous mixed film where clusters of EDTTTF(SC18)2are surrounded by fatty acid bilayers, as demonstrated by using a 3D percolation theory. Inside a conducting cluster we have shown that the iodine intercalation process occurs in two stages: The insertion process lends to a complete oxidation of the TTF-type molecules, which form charged dimers associated with 13- ions, without any noticeable structural modification. A thermally activated process leads to a structural modification, where regular chains of triiodide are associated with an array of mixed-valence EDTTTF(SC,.&, stacks. Besides using a vibronic model which considers both electron-molecular vibrations and electronic correlations in these systems, we have been able to interpret quantitatively the optical absorption spectra. For that purpose we have used a specific calculation which takes account of the multireflections always present in thin films. We have shown that we have a mixed-valence system with its own molecular organization, which can be used as a component in a chemical sensor or perhaps for nonlinear optical devices.
Introduction Langmuir-Blodgett (LB) films, with in-plane metallic conductivity, have been an attractive possibility associated with the prospect of molecular electronics. The current developments are derived from the knowledge gained on molecular conductors and superconductors, Le., the synthkis of charge-transfer complexes (CTC) or radical ion salts (RIS). Different strategies have been elaborated, based on the synthesis of ?r donor or acceptor molecules with amphiphilic character. The formation and stability of a monolayer at the liquid-air interface, the transfer onto a substrate, and the chemical stability of a mixed-valence multilayer are the fundamental stepping stones. Among the different series of electroactive molecules which have been investigated, the two main classes concern either the donor molecule of the tetrathiafulvalene (TTF) type or the acceptor ones, such as tetracyanoquinodimethane(TCNQ)* and metal (dmit)2 [H2 dmit, bis(4,5-dimercapt0-1,3-dithiole)-2-thione].~ The semiamphiphilic TCNQ salts are the largest class of compounds which have been examined. Associated with amphiphilic cations, they give rise to a mixed-valence LB film, thanks to a subsequent iodine ~xidation.~Nevertheless, most of the TCNQ-based conducting LB films suffer a nonthermodynamically stable monolayer at the water surface, associated with the occurrence of a kinetic plateau, which precludes complete control of the molecular organization. It must be pointed out that the recent proposal of a homodoping strategySis a significant improvement with this kind of molecule. The TTF-type molecules offer a large chemical flexibility, because it is possible to add to the electroactive core, on the one hand, a polar head with a functional group and, on the other hand, one, two, or even four alkane tails of different lengths. Several long-chain derivatives of TTF have been prepared which are6,' or are nots amphiphilic. The amphiphilic TTF are the most attractive because,usually, a stable floating monolayer is controlled on the Langmuir trough, but two main drawbacks are encountered: (i) A mixture with a fatty acid must be used for a successful transfer to a substrate with good built-up multilayers. (ii) A Centre de Recherche Paul Pascal. 'Tokyo University of Mercantile Marine. 8 Universitt Bordeaux.
mixed-valence state is obtained after iodine exposure and a subsequent chemical reaction, but the final conducting stage is not stable. The nonamphiphilic molecule (ethylenedithio)bis(octadecylthi0)tetrathiafulvalene [EDTTTF(SC,,),] (see Table I for its chemical formula) is actually one of the rare molecules which gives rise to a stable conducting phase after a simple oxidation with iodine vapors8 Thanks to a detailed structural investigation of mixed LB films of EDTTTF(SCl8)2 and behenic acid, we have recently shown that this solid-state chemical reaction is a rather complex process, associated with the existence of aggregates of TTF-type molecules, surrounded by the behenic acid m~lecules.~ By X-ray diffraction study, we have demonstrated that a stable monolayer transferred on a glass substrate forms a Y-type centrosymmetrical bilayer film, with a stacking periodicity of 55 A. Moreover, we have also shown that a controlled oxidation process in the presence of iodine vapor at 30 OC leads to a new brown insulating phase with a larger bilayer periodicity of 72 A. This intermediate phase can be transformed to a violet conducting one, after 2 h of annealing at 40 OC. This third phase exhibits a bilayer thickness of only 46 A. To fully understand this multistep oxidation process, which is accompanied by a large structural rearrangement, we have carried out the following detailed spectroscopic and electronic property investigations: resonance Raman (1 ) Delhab, P. In Lower Dimensional S y s t e m and Molecular Electronics; Metzger, R. M., Day, P., Papavassiliou, G. C., Eds.; NATO AS1 Series, Series B248; Plenum Press: New York, 1991; p 43. (2) Vandevyver, M. In Lower Dimensional Systems and Molecular Electronics; Metzger, R. M., Day, P., Papavassiliou, G. C., Eds.; NATO AS1 Series, Series B248; Plenum Press: New York, 1991; p 503. (3) Nakamura, T.; Tanaka, H.; Kojima, K.; Matsumoto, M.; Tachibana, H.; Tanaka, M.; Kawabata, Y. Thin Solid Films 1989, 179, 183. (4) Richard, J.; Delhah, P.; Vandevyver, M. New J . Chem. 1991,15, 137. ( 5 ) Ruaudel-Texier, A,; Vandevyver, M.; Roulliay, M.; Bourgoin, J. P.; Barraud, A.; Lequan, M.; Lequan, R. M. J . Phys. D: Appl. Phys. 1990,23, 987. ( 6 ) Dhindsa, A. S.; Ward, R. J.; Bryce, M. R.; Lvov, Y. M.; Munro, H. S.;Petty, M. C. Synrh. Mer. 1990, 35, 307. (7) Vandevyver, M.; Roulliay, M.; Bourgoin, J. P.; Barraud, A.; Gionis, V.; Kakoussis, V. C.; Mousdis, G . A.; Morand, J. P.; No& 0.J . Phys. Chem. 1991, 95, 246. (8) Richard, J.; Vandevyver, M.; Barraud, A,; Morand, J. P.; Delhabs, P. J . Colloid Inrerface Sci. 1989, 129, 254. (9) Izumi, M.; Dourthe-Lalanne, C.; Dupart, E.; Flandrois, S.; Morand, J. P.; DelhaEs, P. J . Phys. D: Appl. Phys. 1991, 24, 1141.
0022-3654/92/2096-28 12%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 2813
Mixed LB Conducting Films
TABLE 1: Experimental Conditions for the Deposit of Mixed LB Films with the Two Indicated Molecules
EDT TTF (SC18)2
substrate CaF2 and fused silica
physical property resonance Raman UV-visible absorption
behenic acid
surface treatment CHCI3, H 2 0 ultrasound, behenic acid precoating ( 5 layers)
no. of layers 30, 50, 100
transfer ratio =100%
IR absorption
ZnSe
Si wafer
(V > 1000 cm-I) electrical conductivity ESR IR absorption ( v > 600 cm-') I R absorption ( v > 200 cm-I)
+
EtOH, CHC13 behenic acid precoating sulfochromic acid, water, and behenic acid precoating
scattering experiments, to elucidate the nature of the iodine species; IR and visible transmission spectra, to detect the molecular conformation changes associated with the structural modifications, and to obtain information on the valence state of the different phases through their chargetransfer bands; electrical conductivity experiments on the conducting violet phase. The final purpose of this paper is to understand the conduction mechanism in these mixed LB films, by comparison with the insight already gained on conducting crystals.' In particular, the comparison of the basic interactions present in these narrow electronic band systems, namely, electronic correlations and electron-phonon interactions,' will be useful to understand how to produce pure highly conducting ultrathin films with organic compounds.
Experimental Section and Results (1) LB Film Preparation. High-purity EDTTTF(SCls)2has been synthesized, and the stability of mixed monolayers with behenic acid has been already described.8 The multilayer preparation has followed the classical method, which is the transfer of the mixed monolayer by vertically passing the substrate through the air-water interface (Atemeta LB 105 trough). In the present investigation, we have controlled the following pertinent variables: (i) the nature of the substrate; (ii) the behenic acid content mixed with the EDTTTF(SCl8)2 molecule; (iii) the number of deposited layers. Besides a glass substrate used for X-ray diffraction experim e n t ~we , ~ have also employed different hydrophilic substrates, as presented in Table I. A specific chemical etching has been applied in each case, in order to get a Y-type deposit with maximum transfer rate. Another parameter is the monolayer composition, because it has been shown that, in order to obtain a stable floating monolayer, it is necessary to add a long-chain fatty acid.8 Besides, the molecular organization appears to be dependent on the number of behenic acid molecules n mixed with one EDTTTF(SC18!2. We have selected therefore different behenic acid concentrations (n = 1, 3, 6, 8) for each physical property (see Table I). In the standard case, we have used a molar ratio of 1:l. Finally, the last parameter is the number of deposited layers. This factor would not be significant if we had a two-dimensional organization, but it should be crucial if the material has to be considered as tridimensional for a transport property, as for example the electrical conductivity presented later on. These neutral LB films are submitted to iodine vapor in a two-temperature step procedure. First, at 30 OC, we observe a color change toward a brown insulating phase, and then second, after annealing at =40 O C for 2 h, we follow a phase transformation to a more stable violet phase. As a reference, we have prepared a powder of EDTTTF(SC,,), doped with iodine gas, which yielded a violet phase on which we carried out a few physical characterizations. This is a paramagnetic compound which presents at room temperature both a
30, 50, 100 100
F25
100%
160%
static susceptibility [x, * emu (CGS) mol-'] and an ESR signal (with a line width AH = 35 G at g = 2.0065). IR absorption spectroscopy shows that it is a mixed-valence charge-transfer salt. These spectra will be presented later on in comparison with the doped LB films. (2) Resonance Raman Scattering. These experiments were performed on a Dilor 224 spectrometer equipped with a spinner to avoid any thermal decomposition, using the exciting line at 514.5 nm of an argon laser. On two characteristic samples representative of each phase after doping, we have determined the final nature of the reduced species. The results are presented in Figure 1: we observe for the brown phase on a fresh sample (b') three main lines situated at 165, 147, and 107 cm-' and then, after a delay, a stabilized form (b) with the resonance lines at 224, 168, and 112 cm-I. These spectra are characteristic of isolated triiodides which are solvated in iodine (I2). For the violet phase, a different Raman spectrum is identified, with three lines situated a t 333, 224, and 112 cm-I, characteristic of a regular (I;) chain.I0 It follows from this first set of experiments that a redox reaction takes place during the insertion process, followed by a structural reorganization leading to triiodide chains, as for example, in the starch-iodine complex." One point necessary to mention is the possible existence of I- species which cannot be detected by this technique. (3) Infrared and Visible Spectroscopy. The infrared spectra have been recorded with two FT spectrometers, a Nicolet 740 (resolution =4 cm-l) for room-temperature experiments and a Nicolet MX1 (resolution =2 cm-I) equipped with a low-temperature accessory. For the near-IR-visible spectra we have used a Perkin-Elmer 350 spectrometer. (a) Electronic Absorption Spectra. A first general picture emerges from the electronic absorption bands for both brown and violet phases presented in Figure 2, where we recognize the charge-transfer bands labeled A and B (following Torrance's notation used for charge-transfer salts'*), an intramolecular excitonic band around 20 000 cm-' characteristic of the TTF ring, and finally a band around 24000 cm-I attributed to the 1, species! The spectrum of the doped bulk compound presents both A and B charge-transfer bands, while the LB films exhibit either the B band characteristic of a completely ionized state (brown phase) or the A band of a mixed-valence state (violet phase). Nevertheless, the comparison between these spectra shows a red shift for both lines, which is really large for the A band. (b) IR Absorption Spectra of Bulk Compounds. Comparative transmission experiments of pellet samples of the compounds diluted in finely ground KBr, namely, the neutral molecule and the final violet phase, are presented in Figure 3. The undoped (10) Marks, J.; Kalina, D. W . In Extended Linear Chain Compounds; Miller, J. S.,Ed.; Plenum Press: New York, 1982; Vol. 1, p 197. (1 1) Reddy, J. M.; Knox, K.;Robin, M. B. J . Chem. Phys. IW, 40,1082. (12) Torrance, J. B.; Scott, B. A.; Kaufman, F. B. Solid Stare Commun. 1975, 17, 1369.
2814 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
Dourthe et al.
I 514.5 nm
A,,,
I
I
600
Kx)
I
I
400
1
300
I
200
-v
1
la, (cm-')
Figure 1. Resonance Raman spectra observed at room temperature for the brown (b' and b) and the violet phases (c). LB film characteristics: substrate CaF,, molar composition 1:1, N = 100 layers. intramolmlar excitation band
. Y
v
c
,.- .doptd p o d o r
n c ,n D
n
a
Figure 3. Room-temperature IR absorption spectra of the neutral EDTTTF(SCI,), molecule (- -) and the iodine-doped powder [EDTTTF(SC18)2]xtI,- (-). The absorption lines, located in the lower frequency range, are detailed in the inset.
-
neutral EDTTTF(SC,& spectrum is characterized essentially by a series of alkyl chain vibrations, with two intense bands at 2846 and 2917 cm-I (symmetric and antisymmetric stretching methylene bands) and a series of progression bands in the frequency range 1180-1300 cm-I, which are analogous to those well-known in behenic acid, for example,13J4attributed to the all-trans configuration; no crystal field splitting is observed. For the doped compound we observe a completely different spectrum, characterized by three intense bands, which are the signature of a mixed-valence state in TTF-type compounds:15 the
intense and large charge-transfer A band around 3600 cm-I; vibronic lines, which are ag modes becoming IR active through the classical electron-molecular vibration (e-mv) interactions; in particular, the agv4(vcentral) and agv10( v + ~ + internal) are identified around 1300 and 450 cm-I, respectively. For both the EDTTTF(SC& neutral and ionized molecules, we also observe several bands below lo00 cm-' (seeinset of Figure 3). The most intense are at 770 and 723 cm-', due respectively to a transition moment parallel to the cyclic larger axis and to the chain rocking modes.16 (c) IR Spectra of the LB Films. We have recorded the roomtemperature IR spectra of the neutral and oxidized LB films deposited on a CaF, substrate (Figure 4); consequently, for this first set of experiments, the frequency range is limited to above =1000 cm-l. In the inset, for the neutral film, we detect the alkyl stretching modes and the progressive bands as in the powder; these lines are mixed with those due to behenic acid. Nevertheless the v(0H) band of the behenic acid is not seen, and the stretching modes of the alkyl groups around 2800 cm-I are very strong; those
(13) Cholet, P. A.; Messier, J.; Rosilio, C. J . Chem. Phys. 1976, 64, 1042. (14) Cholet, P. A,; Messier, J. Thin Solid Films 1983, 99, 197; Chem. Phys. 1985, 97, 365.
(15) Garrigou-Lagrange, C.; Graja, A.; Coulon, C.; Delhab, P. J . Phys. C Solid State Phys. 1984, 17, 5437. (16) Bozio, R.; Zanon, I.; Girlando, A.; Pecile, C. J. Chem. Phys. 1979, 71, 2282.
. /
/
/
,
10 OOO
20 aw, v Icm-1)
Figure 2. Comparative electronic absorption spectra of LB films and doped powder of EDTTTF(SC18),salts.
Mixed LB Conducting Films
The Journal of Physical Chemistry, Vol, 96, No. 7, 1992 2815
WCI:
w,: 3800 cm-1 w l : 1469 cui-1 w 2 :500 cui-' y, : 50 cm-' y 2 I M cm-1
* C
n L . 0 I
n
a
1
4000
1
XI
A
L
3400
2600
A, I 0.2
A 2 I 0.1 I
2800
"O0 v
(cm-1)
400
I
1
101
1800 v
F
2Wcm-'
r, : 2 500 cm-1
U Y
(em-1)
p 4. Room-temperature IR absorption spectra of LB films: neutral
(a) (inset), brown (b), and violet phases (c) (substrate CaF,, molar concentration 1 : 1 , N = 100 layers).
0
I 1
I
4000
2800
1600
400
I
v (cm-1) Figure 6. IR absorption spectra (-) of the violet phase deposited on a Si wafer ( N = 50 layers, molar ratio 1:l) and recorded at 290 (a) and 80 K (b). The dotted lines are the calculated spectra, using the multilayer approach described in the text with the parameter values given in inset.
. I U
c *
0
L * 0
n 4
v (cm-1)
v (cm-1 I Figure 5. Comparative IR absorption spectra of neutral (-) and violet phase ( - - - ) LB films (substrate ZnSe, N = 50 layers, molar ratio 1:l)
in the high-frequency (a) and in the low-frequency range (b).
phenomena indicate that the chain axis and the OH groups are more or less normal to the interface plane. We will use these intensity arguments for the conformational analysis. A similar spectrum is detected for the brown phase with, in particular, a new line at 1335 cm-' which is characteristic of a TTF ionized dimer.I6 Finally, for the violet form, we observe a charge-transfer band around 2000 cm-I, associated with a vibronic mode at 1200
cm-l; both of them are red shifted when compared with the solid-state spectra (Figure 3). This result means that there is a stronger e-mv coupling in the LB film than in the powder." A complementary experiment has been carried out with a LB film deposited on a ZnSe substrate to complete the lower frequency range, below 1000 cm-' (Figure 5 ) . Under the same set of controlled deposition parameters, we observe a change of the line intensities between the neutral and the violet forms (but no change between the neutral and the brown forms). In the upper frequency range (see Figure 5a), this change is relative to the stretching modes of the alkyl chains, whereas in the lower range (Figure 5b), we observe a rocking CHI mode at 720 cm-l and a TTF ring mode at 775 cm-I which remains visible after iodine doping. These results indicate a conformational change, which will be analyzed. We have examined carefully the IR absorption spectra obtained on this conducting phase as a function of the molar concentrations and the number of deposited layers and, finally, their temperature dependences. We observe no significant change when either the number of layers grows from N = 6 to 100 or when the molecular concentrations are changed. We have also carried out temperature dependences of these absorption spectra using LB films deposited on CaF2 or on a silicon wafer; these results are identical. We present therefore those obtained on an Si substrate, which allows us to see the second vibronic mode agv10around 450 cm-l (see Figure 6 , the continuous lines). We do not observe any significant change between the two spectra obtained respectively at 300 and 80 K; moreover, preliminary experiments down to 15 K confirm that there is no indication of a structural phase transition. We observe only (17) Rice, J . M.; Yartsev, V. M.; Jacobsen, C. S. Phys. Rev. B 1980, 21, 3437.
2816 The Journal ofphysical Chemistry, Vol. 96, No. 7, 1992 I
1
s
/+-
-6t
-a
1
0
A
nr6
o
nr8
A
I
0
A 0
0 0 1 1
50 N (numbcr of
-l0II
- I2-
I
I
100
lam,)
I
I
Dourthe et al. EDTl'TF(SC,,),-TCNQF, LB films, in which a decrease of ESR line intensity associated with a g-factor shift has been recorded after iodine doping.19 The large spin-orbit coupling constant induced by the iodine species is supposed to be the origin of such a broadening effect, which may be more effective in the LB films than in the powder.
Analysis and Comments It appears that in the final violet phase we have a heterogeneous system, with clusters of EDTTTF(SCI8), radical cation salts embedded in the behenic acid layers. We have therefore developed a quantitative approach, based upon a two-step description. In the first part, we will describe, at the molecular level, the chemistry and structural changes associated with the doping process inside the clusters; a complementary approach will be the analysis of the electronic and vibronic absorption bands in the IR spectra. At a second level of description, we will examine the size and the distribution of these clusters by developing a percolation model based upon the dc electrical conductivity results. (1) chemical Imertion Model and IR Conformational Analysis. Two complementary points, redox potentials and steric hindrance, have to be taken into account. To elucidate the electron transfers in these films, we have to consider the oxidation potential of the A donor molecules. The cyclic voltammetry experiments carried out in solution have shown that the half-wave potentials for the EDT"F(SC& are respectively E t j z = 0.63 V and E;,2 = 0.94 V for the first-oxidation stepsM These values have to be compared with the iodine reduction potential 3/21z + e- 1,- occurring at 0.76 V under the same experimental conditions. Under iodine gas treatment at 30 OC, the neutral LB film is changed to an ionic phase, with a supposed stoichiometry [EDTTTF(SC,8)z']z(13-)2. This is the brown phase, which consists of more or less isolated dimers, as evidenced by the charge-transfer band at =10000 cm-I and the associated vibronic modes at 1335 cm-l (Figure 4) and confirmed by isolated (I
