Thin Solid Films 518 (2009) 299–304

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Thin-film phases of organic charge-transfer complexes formed by chemicalvapor deposition

Hiroshi Wada a, Dominique de Caro b,c, Lydie Valade b,c, Tatsuhiko Ozawa a,Yoshimasa Bando a, Takehiko Mori a,⁎a Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, 152-8552 Tokyo, Japanb CNRS; LCC (Laboratoire de Chimie de Coordination); 205, route de Narbonne, F-31077 Toulouse, Francec Université de Toulouse; UPS, INPT; LCC; F-31077 Toulouse, France

⁎ Corresponding author.E-mail address: mori.t.ae@m.titech.ac.jp (T. Mori).

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.06.040

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 March 2009Received in revised form 27 May 2009Accepted 22 June 2009Available online 27 June 2009

Keywords:Chemical vapor deposition (CVD)Organic conductorsElectrical properties and measurementsConductivity

Thin films of organic charge-transfer salts, (TTF)(TCNQ) and (TTF)(DMDCNQI), are prepared by the chemicalvapor deposition method, where TTF is tetrathiafulvalene, TCNQ is tetracyanoquinodimethane, andDMDCNQI is dimethyldicyanoquinonediimine. Depending on the substrate temperatures, we have obtainedrandomly oriented polycrystalline phases composed of relatively large crystals and microcrystalline thin-filmphases, which sometimes contain well-grown nanowires. The latter shows much different conductingproperties from the bulk crystals, and particularly the (TTF)(DMDCNQI) film is nearly as conductive as the(TTF)(TCNQ) film in spite of the bulk insulating property coming from the mixed-stack crystal structure.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, a great deal of attention to organic semiconductorshas been aroused due to their application to organic electronics such asorganic light-emitting diode, organic field-effect transistors (OFETs),and organic solar cells [1]. In contrast, organic charge-transfercomplexes have been studied mainly due to the high conductivity andsuperconductivity, in which the bulk crystals have been used in theinvestigation. Recently, organic charge-transfer salts are utilized inorganic devices [2,3], so that thin films of organic charge-transfer saltshave attracted considerable attention. In order to form thinfilms, severaldifferent methods have been investigated including thermal evapora-tion, dipping process, drop-casting process including spin coating,electrodeposition, and the Langmuir-Blodgettfilm technique [4]. Amongthem, thinfilms of two-component organic charge-transfer salts such as(TTF)(TCNQ) are easily vacuum deposited from a single source (TTF:tetrathiafulvalene, and TCNQ: tetracyanoquinodimethane) [2–10], andparticularly when alkali halide substrates are used, epitaxial growth isobserved. However, we cannot obtain a film of a similar charge-transfercomplex (TTF)(DMDCNQI) by the same method, probably due to thedifference of the evaporation temperatures of the two components(DMDCNQI: dimethyldicyanoquinonediimine). Chemical vapor deposi-tion (CVD) is another useful method to form thin films. Preparation ofCVD-deposited magnetic complexes based on tetracyanoethylene

ll rights reserved.

(TCNE) has been investigated, and the metal dependence of the M(TCNE)2 films has been discussed [6,7]. (TTF)(TCNQ) films have beenalso studied [11–15], where the clear Peierls transition around 53 Kdisappears. In this communication, thin films of TTF-based charge-transfer complexes have been prepared by the CVD method. We haveobtained not only (TTF)(TCNQ) but also (TTF)(DMDCNQI) thin films,and the morphological differences induced by the acceptor species andthe deposition conditions are investigated. On a relatively high-temperature substrate, large polycrystals are obtained, but on a low-temperature substrate, growth of nanowires is observed.

2. Experimental details

TTF and TCNQ were purchased from Tokyo Kasei Co. and usedwithout further purification. DMDCNQI was prepared by following thepreviously reported procedure [16]. Thin films of TTF-based charge-transfer complexes were fabricated by using a conventional hot-wallCVD equipment described in Ref. [17]. In the CVD apparatus, severalpieces of Si(001) wafers washed with acetone, and starting materialswere appropriately loaded. After the glass crucibles were evacuated, allparts were charged with He once, and then He gas flow started underreduced pressures as listed inTable 1. The startingmaterialswere heatedto the vaporizing temperatures by using oil baths and transported by Hecarrier gas, whose flow rate was regulated by using a flow gauge. Theconditions of the vaporization chambers of the starting materials werecontrolled independently for the donor and the acceptor, as listed inTable 1. At the same time, the temperatures of the mixing zone and the

Fig. 2. (a) Raman spectrum of Sample U of the (TTF)(TCNQ) thin film. XRD profiles for(b) Sample U, and (c) Sample L.

Table 1Typical deposition conditions.

Sample Precursor Sourcetemperature

He flowrate⁎

Mixingzone

Totalpressure

TTF 100 mg 78 °C 60 sccm 170 °C 850 PaTCNQ 101 mg 145 °C 80 sccmTTF 44 mg 78 °C 80 sccm 155 °C 460 PaDMDCNQI 35 mg 130 °C 70 sccm

⁎ sccm: cc/min under the condition of 1 atm.

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carrier gas linewere adjustedat20–30°Cabove the source temperaturesso as to avoid the deposition of the source materials on the line. Thinfilms were formed onto the Si wafers in the reactor.

Thin films were characterized by infrared (Perkin-Elmer SpectrumGX spectrophotometer) and Raman (LabRAM-HR800 (Jobin Yvon),source: 632.8 nm line of a He–Ne laser) spectroscopies, and X-raydiffraction (XRD) (XPert Pro Panalitical diffractometer, Cu Ka radiation(λ=1.54187 Å) in the θ–2θ configuration). The morphologies wereinvestigated by SEM (JEOL JSM 6700F scanning electron microscopeequipped with a field-effect gun operated at 10 kV). Themeasurementswere conducted for two types of samples obtained from the upper andlower positions of the deposition zone, designated as Samples U and L inFig.1. Samples U and L corresponded to relatively high and low substratetemperatures, respectively. Sample U was thick but fragile, and suitablefor optical measurements, while Sample L was thin but tight, andparticularly appropriate for transport measurements.

3. Thin films of (TTF)(TCNQ)

The charge transfer fromTCNQ toTTF is investigated by IR and Ramanspectra [11]. In the IR spectra of the thin films grown on the Si substrates(Sample U), the absorption band attributed to mCN of TCNQ is observed at2202 cm-1, which corresponds to the (TTF)(TCNQ) phase with a chargetransfer ρ=0.57 (ρ=0.59 in the single crystal phase), in good agree-mentwith the previous value, 2203 cm-1 [11]. Raman spectrum shown inFig. 2(a) also shows several peaks attributed to mC=C at 1415 cm-1,1514 cm-1, and1603 cm-1. Thefirst and the thirdpeaks are assigned to theTCNQ moiety and correspond to the m4 ag and m3 ag modes [18]. Thesecond peak corresponds to the m2 ag mode of TTF, which sensitivelychanges depending on the TTF charge (from 1555 cm-1 for neutral TTF to1505 cm-1 for TTF+) [19]. The observed stretching frequency is close tothe latter, verifying that charge transfer occurs from TTF to TCNQ.

XRD patterns of Samples U and L are shown in Fig. 2 (b) and (c).Sample L gives only strong peaks (Fig. 2(c)), which are in goodagreement with the (002) and (004) reflections of the previouslyreported (TTF)(TCNQ) single crystals [11]. For Sample U, several smallpeaks in addition to the two strongpeaks are observed.Mostof the smallpeaks are reasonably indexed by the crystal data of (TTF)(TCNQ),indicating the presence of crystalline phase of (TTF)(TCNQ) withrandom orientation. The two strong peaks come from thewell-orientedthin-film phase, but do not perfectly agree with the crystal data. Whenthe peakA is assumed as (004), dspace is estimated as 17.6 Å, which is notin accord with the (TTF)(TCNQ) single crystal phase (the c-axis is18.2 Å). These two strong peaks, however, originate from the c-axisorientation essentially, and the difference between dspace and the c-axis

Fig. 1. Schematic image of the CDV furnace, a

might be caused by the very small tilt of the thin-film phase, where thetilt angle is estimated to be 14.8°.

The SEM images are shown in Fig. 3. Sample U (Fig. 3 (a) and (b)) iscovered by plate-like crystals with the size of up to 20mm in diameter.Single crystals of (TTF)(TCNQ) are generally in a plate type, and this isin good agreement with the present observation. In addition, XRDprofiles also support the single-crystal-like phase. It is, however,

nd chemical structures of the materials.

Fig. 3. SEM images of thin films of (TTF)(TCNQ): (a) and (b) for Sample U, and (c)–(e) for Sample L.

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noteworthy that this crystal size is even larger than the typical domainsize, 5 μm, of the epitaxial films [13,14]. On the other hand, Fig. 3(b)shows a different underlying phase. At the earlier stage of deposition,the molecules are well arranged, similar to the previous report [11].Afterwards single crystals start to grow randomly on the well-arranged film. This abrupt change of morphology has been noted byVollmann et al. [20], and Figueras et al. have also reported this changefor the (TTF)(TCNQ) film formed by the CVD method [12].

Sample L exhibits various morphologies as shown in Fig. 3(c)–(e).It should be noted that the scales of these images are about one ordersmaller than Fig. 3 (a) and (b). The substrate is mainly covered bymicrocrystals as observed in Fig. 3(c), whose crystal size, typically lessthan 1 mm, is considerably smaller than that of Sample U. In addition,the shape is not rectangular plate but acute triangles. For Sample L(Fig. 3(d)), nearly the same underlying morphology as Sample U isoccasionally observed, though the domain size is a little different. Thissuggests that there is no difference in the early stage of the thin filmformation irrespective of the substrate position while the greatdifference occurs in the later stage. In other areas of Sample L,

Fig. 4. Process of nanowire formation ((a)–(c)

interestingly, nanowires grow on the edge of crystals (Fig. 3(e)). Thedimensions of the nanowires are typically 100 nm in diameter and5 mm in length.

The formation of the nanowires is explained as follows. As the leftarrow in Fig. 4 (a) shows, branching happens at the edge of the plate-like crystals on account of the varying growth rates. A nanowire growsstarting from this branching point, as indicated by the right arrow inthe same figure. Further development of the branching point results innanowires as indicated in Fig. 4 (b) and (c).

More variation of the crystal-growth rate prompts the growth of adifferent morphology as shown in Fig. 4 (d) and (e), which has beenobserved at lower side of Sample L. Fig. 4(d) shows that nanowiresgrow in different orientations leading to a kind of “carnivorous plant”.When themorphology is investigated from the upper side to the lowerside, the shape changes from plate-like form as observed in Fig. 3 (a)and (b), to fibers and nanowires as observed in Fig. 4. A slight changeof the deposition conditions strongly affects the crystal growth.

Temperature dependence of the resistivity was measured (Fig. 5).Thin films of (TTF)(TCNQ) deposited by thermal evaporation on SiO2

), and images of characteristic nanowires.

Fig. 5. Temperature dependence of the linear and the Arrhenius plots of resistivity for (TTF)(TCNQ) thin films: (a) and (b) films formed by the CVD (Sample L), and (c) and (d) formedby thermal evaporation on SiO2 substrates.

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were also investigated. The crystal of (TTF)(TCNQ) shows the Peierlstransition at 53 K. In the evaporated films, the Peierls transitionappears as an inflection point of the Arrhenius plot (Fig. 5 (c) and (d))[21], but the present CVD film does not show any clear transition. Thethin film resistance is mostly governed by the resistance of domainboundaries, so that the metal-insulator transition is obscured inSample L composed of microcrystals. This may suggest that theaverage domain size is smaller than the coherence lengths of thechange density waves. The room-temperature resistivity 2 ω cm is alittle higher than 0.12 ω cm of the films evaporated on SiO2 [22],because the former is randomly oriented, but the conducting plane ofthe latter is parallel to the substrate. Since the resistivity of a bulkcrystal is 0.0025ω cm (//a), 0.4ω cm (//b), and 2.5ω cm (//c) [23], thepresent value is comparable to the last one. Activation energy above100 K is 21 meV and 20 meV for the CVD and the evaporation films,respectively. These values are close to the value of 14 meV for theprevious CVD films [11].

4. Thin films of (TTF)(DMDCNQI)

DMDCNQI is known as a slightly weaker acceptor than TCNQ,where the reduction potentials are 0.21 V and 0.39 V, respectively [24].Single crystal structure of (TTF)(DMDCNQI) has been reported byAumüller et al., where the donor and the acceptor are alternatelystacked along the a-axis [25]. (TTF)(DMDCNQI) is regarded as a“neutral” charge transfer salt, inwhich the charge transfer from TTF toDMDCNQI in the crystal is close to zero. However, there is no attemptto prepare thin films of (TTF)(DMDCNQI), though metal DMDCNQIfilms have been reported [26–28].

The CVD thin films of (TTF)(DMDCNQI) show dark gray color. Fig. 6(a) shows Raman spectra of the deposited thin film, and the singlecrystal obtained from an acetonitrile solution together with single-crystal neutral TTF and single-crystal neutral DMDCNQI. The thin film

shows three peaks at 1453 cm-1, 1516 cm-1 and 1620 cm-1 as shown inFig. 6(b). The third peak (1620 cm-1) originating from n5 of DMDCNQI ishelpful to determine the charge transfer ρ. Yamamoto et al. havereported the correlation between peak position of m5 and ρ for thecharge-transfer complex (DMDCNQI)2CuxLi1-x [27]. They have con-cluded that the peak position of m5 shifts linearly with the value of x,directly reflecting the charge transfer ρ. The charge transfer of thecrystal (TTF)(DMDCNQI) is estimated as approximately zero from m5 at1628 cm-1 [27]. Interestingly, the peak for the CVD film is observed at1620 cm-1, and shifted downward by 8 cm-1. However, the second peak(1516 cm-1) assigned to m3 for the C=C stretching in TTF, which is verysensitive to the charge transfer: 1518 cm-1 (TTF) to 1420 cm-1 (TTF+),indicates that TTF in the CVD film is close to neutral [19]. XRD profile ofthe thin film (TTF)(DMDCNQI) is shown in Fig. 6(b). These peaks areindexed according to the single crystal data of (TTF)(DMDCNQI).

The SEM images show ribbon-like crystals both for Samples U andL, in contrast to the plate-like crystals of (TTF)(TCNQ) (Fig. 7). Theseribbon-like crystals are in good agreement with the ribbon-like (TTF)(DMDCNQI) single crystals obtained from solution. The ribbons inSample U have larger widths than those of Sample L; the former are upto 20 mm (Fig. 7(b)). Sample L is composed of relatively thin crystals(Fig. 7(c)) and fibers (Fig. 7(d)). Large crystals for Sample U andmicrocrystals for Sample L are entirely the same situations as (TTF)(TCNQ).

Mechanism of formation of these nanofibers is not certain, but itdoes not seem the same as the (TTF)(TCNQ) nanowires because thebranched and split parts are not observed on the ribbons. In addition,the nanowires grow up to over 50 μm in length, which is much longerthan at most 10 μm of (TTF)(TCNQ). This difference may come fromthe stronger one-dimensional nature of (TTF)(DMDCNQI) than that of(TTF)(TCNQ) with plate-like crystals. The difference of the evapora-tion temperatures between TCNQ and DMDCNQI is also crucial to thedeposition mechanism. DMDCNQI evaporates at a lower temperature

Fig. 6. (a) Raman spectra and (b) XRD of (TTF)(DMDCNQI) (Sample U).

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than TCNQ, as obvious from the source temperatures in Table 1, andeven re-evaporation happens under vacuum conditions [28]. This mayinfluence the morphology and composition of the final thin films.

Fig. 7. SEM images of thin film (TTF)(DMDCNQI): (a) and (b) for

To control the morphology, we have to investigate the depositionconditions carefully. In the present study, the thin film bands of bothcharge-transfer complexes seem to be associated with the thermalgradient. High-temperature Sample U reduces the nuclear formationbecause the vapor pressures of TTF and TCNQ are high, giving largecrystals. On the other hand, Sample L gives microcrystals because ofthe lower temperature. From the standpoint of crystal dimension, lowtemperature reduces the crystal dimension, and finally yields one-dimensional crystal such as nanowires and nanofibers. Similarthermal sensitivity of the crystal growth has been reported for thevacuum evaporated films of (TTF)(TCNQ) as well [5].

Temperature dependence of resistivity and conductivity is shownin Fig. 8. The room-temperature resistivity of the (TTF)(DMDCNQI)film is 10 ω cm, which is remarkably smaller than the bulk valueN107 ω cm, reported for the pellets [29]. This high conductivity is onlyby one order smaller than that of the (TTF)(TCNQ) thin film. This issurprising because the bulk (TTF)(DMDCNQI) crystal is an insulatorwith a mixed-stack structure [25]. Interestingly, two humps areobserved in the heating process while no such behavior is observed inthe cooling process, and this behavior is reproduced in another sampleas shown in Fig. 8. Recently, it has been reported that interface of TTFand TCNQ single crystals conducts electricity [30], and charge-transferfrom a self-assembled monolayer on an organic semiconductor crystallargely enhances the conductivity [31]. These examples demonstratethat the interfacial conductivity in organic materials is sometimeslargely different from the bulk properties. It has been also reportedthat organic transistors based on (TTF)(TCNQ) nanowire show n-channel performance, probably due to the local dominance of TCNQ[32,33]. These results suggest the possibility that the local chargetransfer at the (TTF)(DMDCNQI) interface may significantly enhancethe conductivity of the thin film, despite of the mixed-stack bulkcrystal structure.

5. Conclusion

The CVD method gives charge-transfer thin films with character-istic morphologies. The present thin films are not as flat as used inorganic devices such as OFETs. Instead, nanowires are observed bothin (TTF)(TCNQ) and (TTF)(DMDCNQI). Organic nanowire transistors[34] and many other organic nanowires for organic electronics havebeen developed [35]. However, electronic properties of these thin

Sample U, and (c) and (d) for (TTF)(DMDCNQI) (Sample L).

Fig. 8. (a) Linear and (b) Arrhenius plots of the temperature dependence of theresistivity for duplicates of Sample L.

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films prepared by the CVD method are remarkably different from thebulk crystals and thin films obtained by other methods. The CVDmethod gives various nanostructures including nanowires based onorganic charge-transfer complexes, which await future applications toorganic electronics.

Acknowledgments

H.W. is grateful to Integrated Doctoral Education Program at TokyoInstitute of Technology for supporting this work. Collaboration

between the Japanese and French groups is supported by the CNRS-GDRI 91 program onMultifunctional Molecular Materials and Devices.

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