What Happens at the Interface between TTF and TCNQ Crystals (TTF

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What Happens at the Interface between TTF and TCNQ Crystals (TTF = Tetrathiafulvalene and TCNQ = 7,7,8,8-Tetracyanoquinodimethane)? Yukihiro Takahashi,*,†,‡ Kei Hayakawa,‡ Toshio Naito,†,‡,§ and Tamotsu Inabe*,†,‡ †

Department of Chemistry, Faculty of Science and ‡Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan

bS Supporting Information ABSTRACT: The interface between tetrathiafulvalene (TTF) and 7,7,8,8tetracyanoquinodimethane (TCNQ) crystals was prepared by treating a TCNQ single crystal surface with TTF powder. Optical measurements and atomic force microscopy (AFM) observation of the interface indicated that not only are TTF TCNQ nanocrystals formed at the interface, but also direct charge injection from TTF powder to the TCNQ single crystal surface may be responsible for the high conductivity of the interface.

Chart 1

1. INTRODUCTION TTF TCNQ (TTF = tetrathiafulvalene and TCNQ = 7,7,8,8-tetracyanoquinodimethane) is a charge-transfer complex and a prototype molecular conductor composed of the TTF electron donor and TCNQ electron acceptor (Chart 1).1 3 Charge transport occurs through the π π stacked partially oxidized donors and through the π π stacked partially reduced acceptors in the complex crystal. The electrical conductivity of the TTF TCNQ single crystal is metallic down to 53 K, while in 2008 it was reported that metallic conduction was realized at the interface between TTF and TCNQ single crystals.4 The origin was explained by the transfer of charge at the interface between the two crystals on a molecular level. The method of modifying electrical properties at the interface is simple and can be extended to various molecular materials and is therefore expected to provide a valuable tool for the design of novel functional electronic systems. However, in some cases, solid-state reaction is known to occur by simply grinding two component crystals (mechanochemical method).5 The formation of TTF TCNQ charge transfer complex crystals was reported to occur by simply grinding TTF crystals and TCNQ crystals using an agate mortar and pestle.6 The reaction may proceed not only through direct contact of the crystal surfaces but also by the uptake of TTF vapor at the TCNQ crystal surface, because TTF is somewhat volatile at room temperature. Therefore, the possibility of formation of TTF TCNQ at the laminated interface between TTF and TCNQ single crystals cannot be eliminated. We have investigated the interface between TTF and TCNQ crystals to clarify this point. First, the lamination of TTF and TCNQ single crystals was performed and the electrical properties were measured. Consequently, highly conducting features similar to those reported in ref 4 were observed. A bare interface was then prepared by removal of the TTF powder from the TCNQ crystal and examined using Raman spectroscopy and atomic force microscopy (AFM). Herein, we describe the species r 2011 American Chemical Society

observed at the interface and discuss a possible mechanism for charge conduction at the interface.

2. EXPERIMENTAL METHODS Commercially available TTF and TCNQ were obtained and purified by vacuum sublimation. Single crystals of TTF and TCNQ were grown by sublimation with a nitrogen carrier gas in a glass tube using a tube furnace with temperature gradient. Thin platelet TTF single crystals and thin sticklike TCNQ crystals were obtained. The molecular packing of TTF exhibits several polymorphs; in this work, a polymorph with P1 space group7 was obtained where TTF does not form face-to-face stacking. On the other hand, the molecular packing of TCNQ is a face-to-face arrangement and the single crystals are elongated along the direction of molecular stacking.8 For preparation of the single crystal lamination interface, a TTF single crystal was first placed on a polydimethylsiloxane (PDMS) support and a TCNQ single crystal was placed on top. Electrical contacts between the interface and Au lead wires (25 μm diameter) for electrical conductivity measurements were achieved using a water-based carbon paste applied by a similar procedure to that reported in ref 4. Electrical conductivity was Received: August 4, 2011 Revised: November 28, 2011 Published: November 30, 2011 700

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Figure 1. (a) Configuration for conductivity measurement of the laminated interface between TTF and TCNQ single crystals. (b) Temperature dependence of the resistance-per-square of the interface between TTF and TCNQ single crystals.

measured using a standard four-point probe setup. The contact resistance was confirmed to be sufficiently low (10 100 kΩ) for the four-probe measurements. In order to expose the interface, removal of the top TCNQ single crystal from the laminated sample was attempted. However, this was found to be extremely difficult, because both crystals were too thin to manipulate. Therefore, a bare interface was prepared by contacting the TCNQ single crystal surface with TTF powder. The TTF powder was manually pressed onto the TCNQ crystal using a flat spatula to ensure contact, and the TTF powder was then removed from the surface using an aerosol duster. The process was performed with monitoring of the surface resistance of the TCNQ single crystal. The surface resistance decreased when contacted with the TTF powder and was retained after removal of the TTF powder. Raman spectra were measured using a Jasco RMP-300 spectrometer at room temperature. The incident laser beam was focused on the sample with a beam area of 50  50 μm2, and the detector collected the scattering along the direction at 180° to the incident beam. AFM images were obtained by a Jeol JSPM5200 scanning probe microscope at room temperature.

Figure 2. (a) Schematic diagram of the procedure for preparation of the bare interface. (b) Raman spectra of the bare interface on the TCNQ single crystal, TCNQ, TTF TCNQ, and K TCNQ.

powder onto the surface of the TCNQ single crystal is expected to generate the same interface as that obtained with the laminated sample. If the powder were pressed only gently, it could be removed without damaging the TCNQ crystal surface and the interface formed by contact with the TTF powder. The change in resistance was monitored during the process by attaching probes to the TCNQ crystal surface in advance, as shown in Figure 2a. The surface resistance of the TCNQ crystal surface before contact with the TTF powder was 500 MΩ sq 1, which was decreased to 60 kΩ sq 1 after contact with the TTF powder. After confirming the low resistance state of the surface, the TTF powder was removed. The monitored resistance was not affected by this process, although the color of the surface turned slightly green. The bare interface thus obtained was investigated using Raman spectroscopy. The observed spectral data are presented in Figure 2b together with those obtained for TCNQ, K TCNQ and TTF TCNQ. The spectral region shown in Figure 2 corresponds to the CdC stretching Ag mode of TCNQ, which is sensitive to the charge on TCNQ.9 The spectrum of the bare interface consists of a very strong TCNQ0 band originated from the substrate TCNQ single crystal and additional two bands at 1420 and 1388 cm 1. These two bands indicate that there are two different charge states of TCNQ at the interface. By comparing them with the spectra of K TCNQ in which TCNQ is fully ionized (TCNQ ) and TTF TCNQ in which TCNQ is partially ionized (TCNQ0.59 ), it can be seen that the band at 1388 cm 1 corresponds to TCNQ and that at 1420 cm 1 corresponds to TCNQ0.59 , meaning formation of TTF TCNQ. Therefore, the formation of the TTF TCNQ complex was confirmed to occur by simply contacting TTF crystals with TCNQ crystals. Of further interest is the presence of TCNQ at

3. RESULTS AND DISCUSSION As reported in ref 4, metallic behavior was observed at the interface between TTF and TCNQ single crystals. However, the behavior was changed to thermally activated type below 230 K in our best sample, as shown in Figure 1. The resistance was strongly dependent on the crystal surface conditions, as reported in ref 4. The surface resistance of the single crystal prior to lamination was several orders higher than that observed at the interface, which confirmed that the conducting interface was realized by the simple contact of the TTF crystal surface with the TCNQ crystal surface. However, direct observation of the interface for the laminated sample was difficult; therefore, the interface was prepared using a TCNQ single crystal and TTF powder. Contact of the TTF 701

dx.doi.org/10.1021/jp2074368 |J. Phys. Chem. C 2012, 116, 700–703

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the TCNQ crystal surface is considered to play an important role in the charge transport at the TCNQ crystal surface.

4. CONCLUSIONS A bare interface between TCNQ and TTF crystals was prepared by treatment of a TCNQ single crystal surface with TTF powder, and the species on the surface after removal of the TTF powder were analyzed. Raman spectroscopy indicated the presence of TCNQ as well as TTF TCNQ species on the surface. AFM revealed needlelike projections and dotlike projections on the TCNQ single crystal surface, which were considered to be TTF TCNQ and TTF nanocrystals, respectively. The TTF TCNQ nanocrystals alone do not form conduction paths; therefore, charges injection from the TTF nanocrystals to the TCNQ single crystal surface is considered to contribute to the charge transport on the surface. The bare interface prepared in this study with TTF powder on a TCNQ single crystal may not be exactly the same as that formed at the laminated interface between TTF and TCNQ single crystals, because the orientation of the TTF crystals in the TTF powder is completely random. However, the formation of TTF TCNQ nanocrystals at the laminated interface between TTF and TCNQ single crystals may not be excluded. The important conclusion in the present study is that even though TTF TCNQ nanocrystals are formed at the interface, they are not sufficient for metallic conductivity. These results suggest that charge injection at molecular-scale contacts plays a crucial role in the charge transport at the interface.

Figure 3. AFM image of the bare interface on the TCNQ single crystal.

the interface. TCNQ as well as TTF TCNQ species are considered to play an important role in charge transport. AFM was used to observe the microscopic features of the bare interface (Figure 3). Some projections are evident on the smooth TCNQ crystal surface, which may be categorized into the following two groups: needlelike elongated projections and dotlike projections. The former is considered to be TTF TCNQ of which the presence was confirmed from the Raman spectra. TTF TCNQ is a typical one-dimensional system and usually grows with needlelike shapes. It should be noted that these needlelike projections are not contacting each other, which indicates that although TTF TCNQ nanocrystals are formed at the interface, they do not form complete conduction paths. The latter dotlike projections are supposed to be residual TTF nanocrystals from the initially applied TTF powder. The Raman spectra indicate the existence of TCNQ ; therefore, there must be cations that neutralize the surface charges. One of the candidates for this cationic component may be the residual TTF nanocrystals. If TTF molecules at the bottom of these nanocrystals are ionized to TTF+ by electron transfer from TTF to the TCNQ crystal surface, there must be strong electrostatic attraction forces between the surface and the nanocrystals. Such nanocrystals would be strongly adhered to the surface and would not be easily removed by blowing. From this image, the number of TCNQ0.59 ions in the incident beam area (2500 μm2) can be roughly estimated to be 1010. Similarly, the number of TCNQ ions in the beam area is estimated to be about 109 by assuming that 1% of the surface layer is ionized. Both numbers are sufficiently detectable by Raman spectroscopy in the present study. The TTF TCNQ nanocrystals in Figure 3 are randomly oriented. We have examined the other images obtained for different batches (see Supporting Information) and have found that there is no trend indicating epitaxial growth of TTF TCNQ nanocrystals. Since dense formation of TTF TCNQ nanocrystals is frequently observed at the defects or cracks on the surface, nucleation process is considered to be important for the formation of TTF TCNQ nanocrystals. Though the TTF TCNQ nanocrystals in a small area as shown in Figure 3 do not form a network, one might consider that metallic conduction could be achieved by networking of TTF TCNQ nanocrystals. However, it should be noted that c-axis oriented thin films of TTF TCNQ on alkali-halide substrates in which electrical contacts between the TTF TCNQ microcrystals are ensured did not show metallic behavior of the conductivity.10,11 Therefore, charge injection from TTF nanocrystals to

’ ASSOCIATED CONTENT

bS

Supporting Information. AFM and SEM images of the bare interfaces on the TCNQ single crystals prepared by different conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*(Y.T.) Phone: +81 11 7063534. Fax: +81 11 7063511. E-mail: [email protected] (T.I.) Phone: +81 11 7063511. Fax: +81 11 7063511. E-mail: [email protected]. Present Addresses §

Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan.

’ ACKNOWLEDGMENT The authors thank Professors T. Nakamura and S. Noro at Hokkaido University for their help with the Raman spectra measurements. This work was supported in part by a Grant-inAid for Scientific Research and by the Global-COE program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and by Japan Science and Technology Agency, CREST. ’ REFERENCES (1) Ferraris, J. P.; Cowan, D. O.; Walatka, V.; Perlstein, J. H. J. Am. Chem. Soc. 1973, 95, 948–949. 702

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