Langmuir 1990, 6, 1680-1682
1680
Infrared Spectroscopic Studies on the Structure and Ordering of Hexadecanoyltetrathiafulvalene Conducting Langmuir-Blodgett Multilayers A. S. Dhindsa,tJ M. R. Bryce,t H. Ancelin,t M. C. Petty,$ and J. Yarwood*lt Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, Great Britain, and Molecular Electronics Research Group, School of Engineering and Applied Sciences, University of Durham, South Road, Durham DHl 3LE, Great Britain Received February 14, 1990 Fourier transform infrared spectroscopy has been used to study molecular ordering and conductivity processes of a HDTTF charge-transfer salt deposited in Langmuir-Blodgett multilayers onto silicon and metal substrates. Both the chemistry and the degree of conductivity may be easily monitored "in situ" as a function of doping level. Linear dichroism measurements confirm the vertical alignment of hydrocarbon chains needed to produce lateral electron mobility via well-ordered,fulvalene rings. Both 12 and Brz readily act as oxidizing dopants for this particular donor, but only 12 produces an organic conductor.
Introduction There is strong current interest in the preparation' and characterization2J of low-dimensional organic conductor^^^^ including charge-transfer (CT) salts. It has been that quasi-one-dimensional organic conductors of this type can be produced by the Langmuir-Blodgett (LB) deposition t e ~ h n i q u e 6and , ~ that room temperature conductivities in the region of lo-' S cm-' may be obtained by using, for example, CT salts of tetracyanoquinodimethane (TCNQ) or TTF. In order to achieve such high lateral conductivity, well-ordered molecular arrays are essential, and this is why the LB technique offers distinct advantages over more conventional methods of depositing organic thin films. One of the most important means of characterizing molecular order in such films is vibrational (infrared) linear d i c h r o i ~ m . ~In, ~situ monitoring of the molecular alignment on a solid substrate as a function of deposition conditions,8J0 temperature," or doping12 may be employed to understand the relationships between chemical structure a n d / o r molecular ordering and conduction level, as well as providing information about the mode and extent of the conduction process. As will be demonstrated in this paper, TTF derivatives provide a particularly good example of how such information is obtainable and provide a "textbook" demonstration of the value of the infrared technique. + Department of
Chemistry.
* Molecular Electronics Research Group.
(1)Dhindsa, A. S.; Bryce, M. R.; Lloyd, J. P.; Petty, M. C. Synth. Met. 1987,22,185-189; 1988,27,B563-B568. (2)Bozio, R.; Zanon, I.; Girlando, A.; Pecile, C. Chem. Phys. Lett. 1977, 52,503-508J . Chem. Phys. 1979,71,2282-2293. (3)Richard, J.; Vandevyver, M.; Lesieur, P.; Ruaudel-Teixier. A.; Barraud, A.; Bozio, R.; Pecile, C. J. Chem. Phys. 1987,86,2428-2438. (4)Heeger, A. J.; Garito, A. F. Low Dimensional Cooperatiue Phenomena; Keller, H. J., Eds.; Plenum: New York, 1975. (5)Proceedings of the International Conference on Science and Technology of Synthetic Metals, Santa Fe, Synth. Met. 1988,1989,2729. (6)Roberts, G.G.Adv. Phys. 1985,34,475-512. ( 7 )Agarwal, V. K. Physics Today 1988,40-46and references therein. (8) Swalen, J. D. J.Mol. Electron. 1987,2,155-181;Thin Solid Films 1987,152,151-154. (9)Rabolt, J. F.;Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J.Electron Spectrosc. Relat. Phenom. 1983,30,34-39. (IO)Davies, G. H.; Yarwood, J. Spectrochim. Acta 1987,43A, 16191623. (11)Naselli, C.;Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985,82, 2136-2140. (12)Dhindsa, A. S.;Davies, G. H.; Bryce, M. R.; Yarwood, J.; Lloyd, J. P.; Petty, M. C.; Lvov, Y.M. J. Mol. Electron. 1989,5,135-142.
Experimental Section The TTF derivative (Figure 1) was prepared as described previously.' For examination by attenuated total reflection (ATR),it was deposited by the LB technique onto a silicon ATR crystal (2 X 1 X 0.25 mm) and sampled in the micro-ATR unit of our Mattson Sirius 100 FTIR instrument. Samples for examination by reflection/absorption spectroscopy (RAIRS)I3 were deposited onto aluminized glass slides and examined by using our SPECAC variable-angle grazing incidence accessory. Infrared spectra were collected as a function of time after doping by exposure to either Ip or Brz under ambient conditions. Samples were allowed to stand in air (for this purpose) for up to 17 h following the initial oxidation. Results and Discussion Figure 2 shows the ATR spectra of t h e H D T T F derivative (Figure 1) in the 1200-4000-cm-' region, both doped and undoped, and as a function of exposure to iodine vapor (the oxidizing dopant). As may be observed, a very broad and intense conduction band arises in t h e 2000-4000-cm-' region, but only after doping and standing (under ambient conditions) for about 1h. (No such band is present immediately after doping.) The spectrum shows clearly that this band arises after some of the iodine has been with the production of a mixed valence chargetransfer complex which results in an increase in the conductivity. Careful examination of the very low v(CH2) band intensities in the corresponding RAIRS spectrum (which couples only transition dipoles perpendicular to the substrate)l3is highly illuminating in terms of the ordering of the alkyl chains. Firstly, the vs(CH~)and ve(CHz) bands near 2850 and 2920 cm-l are very weak, and they get even weaker on doping (compare A and B at 2950 cm-' in Figure 3). This means that the alkyl chain orientational order gets better on doping. Further confirmation of this type of ordering is afforded by the relatively high intensities of the v,(CH3) and v,(CH3) bands a t 2880 and 2970 cm-'. The molecule contains only one CH3 group, but for perpendicular alignment the transition dipoles have a significant component perpendicular to the substrate. The improvement in molecular ordering perpendicular to the substrate on doping is related to (and probably driven by) the onset of conduction in a lateral direction (parallel to (13)Allara, D.; Swalen, J. D. J. Phys. Chem. 1982,86,27OC-2704. (14)Such a loss of 12 is supported by ESCA data (Dhindsa, A. S:; Ward, R. J.; Bryce, M. R.; Lvov, Y. M.; Munro, H. S.; Petty, M. C. Thin Solrd Films. In press.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 11,1990 1681
IR Study of Conducting LB Multilayers
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Figure 3. RAIRS spectra of HDTTF LB film material as a function of time after doping with 12 vapor: (A)before doping, (B) immediately after doping, (C) 17 h after doping. 1.001
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Figure 2. ATR spectra of HDTTF LB film material as a function of time after doping with 12 vapor: (A) before doping, (B) immediately after doping (t = 0), (C) t = 1h, (D) t = 2 h. the substrate) since the fulvalene rings need t o be preferentially aligned parallel to each other for maximum conduction efficiency. It is also very obvious that the electron mobility is parallel to the substrate since the conduction band has its associated transition dipole in this direction, and its intensity in the RAIRS spectrum (Figure 3) is very low (compare spectral intensities in Figures 2 and 3 a t -3800 cm-l). It should be noted that in a recent study using lowangle X-ray diffraction14 we reported that the d-spacing of the HDTTF multilayer f i i decreased significantly upon doping with either iodine or bromine (from 6.1 t o approximately 4.1 nm). Our interpretation of these data was that, upon doping, the molecules become interdigitated, tilted, or both. Clearly, from the infrared studies, no evidence for the tilting of the aliphatic chains upon exposure to iodine is revealed (in fact, the opposite is noted). However, it may still be possible for the long axes of the TTF moieties to be a t an angle to the substrate normal (i.e., no longer parallel to the aliphatic chains) in the doped LB films. The dichroism of the conduction band and of the vg(CH2) and v,(CH2) bands in providing information about
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Figure 4. ATR spectra of HDTTF LB f i i material as a function of time after doping with 12 vapor: (A) before doping, (B) immediately after doping, (C) t = 1 h, (D) t = 2 h. the molecular alignment is supported by observations in the v(C=O) and v(C=C) region of the spectrum (see Figures 4 and 5). Reference to Figure 1 shows that, for perfect alignment perpendicular to the substrate, the v(C= 0)mode has its transition dipole orientated parallel to the substrate while that of the v(C=C) mode (of the ring) is perpendicular to the substrate. Figure 4 shows that, as expected, the latter band (-1530 cm-l) is quite weak in ATR (both in doped and undoped material). In the RAIRS spectrum, the intensity ratio is reversed (Figure 5). Notice that several "v(C=C)"bands are expected in the 1500 cm-' region? and indeed the band is a multiplet. The observed dichroism indicates that the major intensity component has its transition dipole roughly perpendicular to the substrate. These bands reflect strongly the chemistry of the initial oxidation and subsequent loss of 1 2 to produce an organic conductor. Immediately after doping, the v(C= 0)band moves to high frequency (B in Figure 4) because of electronic changes associated with oxidation of the HDTTF to HDTTF'+. The v(C=C) band more or less disappears, showing that the radical cation produced carries a largely C-C single bond. The band reappears again (C and D in Figure 4) when iodine molecules leave the system t o form a mixed valence complex (of t h e t y p e HDTTF'+(I-), or HDTTF'+(I3-),, where x < 11, with some fulvalene rings in the ground state (i.e., none in the chargetransfer state). The v(C=O) band likewise moves back
1682 Langmuir, Vol. 6, No. 11, 1990 1.00 0.88
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Figure 6. Comparison of ATR and RAIRS spectra of HDTTF LB film material 2 h after doping with 12 vapor: (A) ATR, (B) RAIRS. to almost its original position when conduction accprs, implying that the conducting material has an electronic distribution closer to that of the unoxidized precursor fulvalene. The charge transfer in such systems often leads to vibronic excitation2p3 of originally infrared-forbidden vibrations, and this salt is no exception. As can be seen from Figure 4, a very intense band arises near 1350 cm-l immediately after doping with IZ (B in Figure 4). This band, which arises from the coupling of motion of electrons with the u3, alg, mode of the fulvalene ring (in D 2 h symmetry;2 usually called electron-molecule vibronic coupling) becomes very much broader and more intense in the conducting material (C and D in Figure 4). The dichroic behavior-shown in the comparison of ATR and RAIRS spectra of Figure 5-shows clearly that CT occurs parallel to the substrate surface (since this band is not observed a t all in RAIRS). One might expect that similar chemistry and structural changes would occur if Brz vapor were used as the oxidizing agent. However, electrical measurements reveal that the resulting material is an insulator, and spectral analysis (Figures 6 and 7 ) shows that the structure is indeed compatible with that of a fully ionic CT salt (i.e., an insulator). T h e ATR spectrum (Figure 6) shows no conduction band in the 4000-cm-1 region, although the methylene chains are well aligned with their transition dipoles almost parallel to the substrate. The u(C=O) band near 1650 cm-1 shifts to a higher frequency on doping, and the vibronically excited band arises near 1350 cm-l. So CT is clearly evident from HDTTF and Brz to produce HDTTF'+Br-. The v(C=O) band does not move back to 1650 cm-I on standing (as is the case for IZ doping; compare trace D in Figures 4 and 7 ) , and the band a t 1350 cm-' remains strong throughout the doping process. However, on standing the material remains an insulator.
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Conclusion The IR studies reported here demonstrate clearly that doping of LB films of HDTTF with iodine or bromine produces, immediately after doping, CT salts of the type HDTTF'+(I3-), HDTTF*+(I-), or HDTTF'+(Br-). On
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Figure 6. ATR spectra of HDTTF LB film material aa a function of doping with Brz vapor: (A) before doping, (B) immediately after doping (t = 0), (C) t = 1 h, (D) t = 2 h.
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Figure 7. Same as Figure 6 but in the 1200-1800-~m-~ region. standing, the iodine CT salt(s) are unstable and decompose t o produce mixed valence compounds of t h e type HDTTF'+(I-), or HDTTF'+(I3-), (where x < 1). This process is the one responsible for the production of a conducting LB film material. On the other hand, the salt HDTTF'+(Br-) is stable under ambient conditions, and the film remains essentially insulating. The power of infrared linear dichroism in elucidating the structure and chemistry of ultrathin organic films is definitively illustrated by this work. These materials lend themselves particularly well to investigations of this type, and the data provide a clear insight into the details of the molecular processes leading to conduction. Our studies continue on these (and other) exciting device-oriented materials.
Acknowledgment. SERC are thanked for equipment grants and a research assistantship (to A.S.D.).
