Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8

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Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8-Tetracyanoquinodimethane Single-Component and Charge-Transfer Complex Single Crystals: Nucleation, Crystal Growth, Morphology, and Charge Transport Yukihiro Takahashi, Tomohiro Mikasa, Kei Hayakawa, Seiya Yokokura, Hiroyuki Hasegawa, Jun Harada, and Tamotsu Inabe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05376 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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The Journal of Physical Chemistry

Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8-Tetracyanoquinodimethane Single-Component and Charge-Transfer Complex Single Crystals: Nucleation, Crystal Growth, Morphology, and Charge Transport Yukihiro Takahashi,*,†,‡ Tomohiro Mikasa,† Kei Hayakawa,†,# Seiya Yokokura,†,‡ Hiroyuki Hasegawa,‡ Jun Harada,†,‡ and Tamotsu Inabe*,†,‡

†

‡

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan

Corresponding authors to whom inquiries about the paper: Tamotsu Inabe, Tel/Fax: +81-11-706-3511, E-mail: [email protected] Yukihiro Takahashi, Tel: +81-11-706-3534, Fax: +81-11-706-3511, E-mail: [email protected]

#

Present Address: DENSO Research Laboratories, Nisshin, Aichi 470-0111, Japan

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ABSTRACT Electrically conducting TTF-TCNQ thin films are fabricated on various molecular crystals containing 7,7,8,8-tetracyanoquinodimethane (TCNQ) by exposing a tetrathiafulvalene (TTF) vapor under ambient conditions. To systematically investigate the properties of the films, mixed-stack TCNQ charge-transfer (CT) complex crystals with nine kinds of donors have been prepared as the substrates, and the morphology change of the films on the surfaces at the initial stage of the TTF vapor contact has been observed. When the substrate is a TCNQ single-component crystal, randomly oriented TTF-TCNQ nanometer-size needle crystals are grown by the reaction with a TTF vapor. However, when the substrate is a TCNQ CT complex crystal, TTF-TCNQ crystals are grown with alignment of their needle axis along the mixed-stack direction of the substrate. The surface roughness, the size of the needle crystals, and the degree of the dense packing of the needles have been found to systematically depend on the strength of the CT interactions in the substrate, and the sheet resistance also exhibits a systematic change. The resistance drop is rapid and remarkable when the donor of the substrate CT complex is weak. The difference in the morphology and the properties is considered to arise from the difference in the ease of nucleus formation and the rate of crystal growth of the TTF-TCNQ nanocrystals.


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INTRODUCTION In recent years, contact interfaces between organic crystal substrates and molecules or molecular crystals have attracted attention because interesting phenomena, such as charge transfer, have been observed. For example, metal-like charge transport was observed at the contact

interfaces

between

rubrene

1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide)

and single

N,N’-bis(n-alkyl)-(1,7 crystals1

and

and between

(phthalocyaninato)nickel(II) and 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane single crystals.2 In both cases, the charge carriers are considered to originate from pure charge injection at the interfaces. However, in the first report of the conducting contact interface formed between tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) single crystals,3 it was suggested that the formation of highly conducting TTF-TCNQ complex nanocrystals at the interface might also contribute to the charge transport.4 In the case of TTF/TCNQ single crystal contact interfaces, a heterojunction between the conducting film and a semiconductor is automatically formed on the surfaces of organic crystals, so it is proposed that this phenomenon may become a useful tool for organic device fabrication. Formation of TTF-TCNQ nanocrystals at the interface between the TTF and TCNQ single crystals is considered to result from the volatile nature of TTF. Indeed, it has been confirmed in this study that the formation of TTF-TCNQ on the surfaces of TCNQ crystals occurs under 3 ACS Paragon Plus Environment


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ambient conditions when TTF and TCNQ crystals are placed in a closed container without being allowed to contact each other. In the “mechanochemical method”, originally proposed as solid-solid reactions, it was suggested that the vapor–solid reactions might contribute to the progression of the reaction.5,6 This phenomenon is considered to be an effective method to fabricate a conducting thin film on organic semiconductors. Indeed, this methodology was applied to the fabrication of electrodes for tetramethyltetrathiafulvalene-based organic field effect transistors by evaporating TCNQ, and found to be effective to reduce the contact resistance (“self-contact” by Mori, et al.).7 So far, this vapor contact method has mainly been applied to single-component crystal surfaces. However, for the TTF vapor contact, when TCNQ is relatively loosely bound to the crystal lattice, TTF is expected to extract TCNQ from the TCNQ containing crystals to form TTF-TCNQ on the surfaces. As semiconductor crystals, TCNQ charge-transfer (CT) complex crystals with neutral ground states are considered to be promising candidates. If the complexes contain much weaker donors compared with TTF, the degree of charge transfer becomes close to zero, yielding mixed-stack complexes. As a preceding experiment, we performed the TTF vapor contact with the surfaces of an anthracene-TCNQ complex crystal.8 It was found that the fine-needle TTF-TCNQ crystals were grown on the surfaces of the substrate CT complex crystal with highly ordered alignment along the mixed-stack axis of the substrate. The electrical 4 ACS Paragon Plus Environment

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conductivity of the thin film measured along the needle axis showed metallic behavior down to 150 K. This phenomenon may be interpreted by the preferential surface deconstruction of the substrate and TTF-TCNQ crystal growth along the direction of the CT interaction in the substrate. If this interpretation is correct, it is expected that the growth rate, size, and density of the TTF-TCNQ nanocrystals in the thin films formed on the substrate surfaces depend on the strength of the CT interaction in the substrate. Therefore, systematic changes of the sheet resistance of the films resulting from the strength of the CT interaction in the substrate are expected. To investigate this proposal, TCNQ CT complex crystals containing nine kinds of polycyclic aromatic hydrocarbons (PAHs) have been prepared and are reacted with the TTF vapor. The morphology and transport properties of the TTF-TCNQ thin films formed on the surfaces of the crystals have been systematically investigated. In this paper, we present the difference of the morphology of the thin films resulting from the various donor species and discuss the correlation between the features of the film and the donor strength.

EXPERIMENTAL METHODS Materials. TTF, TCNQ and nine kinds of PAHs (biphenyl (1), phenanthrene (2), hexamethylbenzene (3), acenaphthene (4), coronene (5), pyrene (6), anthracene (7), perylene (8), 5 ACS Paragon Plus Environment


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and tetracene (9) in Figure 1a), used for the preparation of CT complexes with TCNQ, were obtained commercially (Tokyo Chemical Industries Co., Ltd.). All these compounds were purified by vacuum sublimation before use. TCNQ single component crystals were prepared by simple recrystallization in chlorobenzene. The nine kinds of PAH-TCNQ crystals were obtained as follows. Single crystals of the TCNQ complexes with 1, 2, and 6–9 were obtained by a co-sublimation method.9 Single crystals of the complexes with 3–5 were obtained by crystallization from mixed PAH and TCNQ solutions. Crystal structures of all the obtained CT complexes and TCNQ were determined by X-ray structure analyses. The composition was found to be 1:1 for all the complexes, and the structures of 1- to 8-TCNQs were found to be practically the same as those previously reported.10-17 Since the room-temperature structure of 9-TCNQ has not been determined, the structure was determined in this study. The details of the X-ray structure analyses, crystal data, molecular arrangements in the lattices, and the crystallographic information file of 9-TCNQ have been deposited as Supporting Information. The crystal axes and Miller indices represented in this paper are based on these parameters. The donor strength is usually evaluated from the redox potential for the first oxidation process (E1/2ox) in the solution, and the E1/2ox values are summarized in Table 1.18-21 The strength of the CT interaction in the complex crystal can be approximated from the difference between E1/2ox of the donor and the redox potential for the first reduction process of the acceptor (E1/2red), 6 ACS Paragon Plus Environment

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namely, ∆Eredox = E1/2ox – E1/2red, and it has been proposed by Torrance that a linear correlation of hνCT = ∆Eredox + C (hνCT: energy of the CT band; C: constant) can be assumed for the complexes with neutral ground states.22 In our study, since the acceptor is always TCNQ, one can expect that a linear correlation of hνCT = E1/2ox + C’ (C’: constant) will appear. The CT bands observed in the Vis-NIR spectra (Figure 1b) indeed show a systematic energy shift, and the plot of hνCT versus E1/2ox shown in Figure 1c, reveals a good linear correlation. From this figure, as the donor strength changes, the strength of the CT interaction in the complexes used as the substrates in this study covers a certain range in the neutral ground state region. TTF vapor contact method. In the standard experiments, the TCNQ single-component or CT complex crystal was fixed on a Si plate, and the TTF powder was placed near to the crystal. The whole set was placed in a capped polystyrene container (30 × 30 × 10 mm3). All the procedures were performed under ambient atmosphere, and the container was kept at room temperature for a certain period, unless otherwise stated.



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Figure 1. (a) PAH donors used for the TCNQ CT complex substrate crystals. (b) Diffuse reflectance spectra of the TCNQ complexes in the present study. The reflectance was converted to the Kubelka–Munk function, f(R), corresponding to the absorbance. The arrows indicate the CT band. (c) CT band energy (hνCT) versus E1/2ox plot of the TCNQ complexes in the present study.


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Table 1. Oxidation potential of the donor, energy of the CT band of each TCNQ complex, and Miller index of the surface plane of each TCNQ complex crystal E1/2ox / Va [reference]

donor

1 2 3 4 5 6 7 8 9 a

hνCT of the TCNQ

surface plane of the TCNQ complex

complex /eV

crystal [reference]

1.78 [18]

2.38

(001) [10]

1.58 [19] 1.38 [20]

2.34 1.97

(001) [11] (011) or (01-1) [12]

1.34 [21] 1.23 [19]

1.89 1.77

(011) [13] (100) [14]

1.16 [19] 1.09 [19] 0.85 [19]

1.67 1.53 1.34

(011) [15] (110) [16] (011) [17]

0.77 [19]

1.19

(110) [this study]

Half-wave oxidation potential versus SCE in CH3CN

Measurements. Raman scattering spectra were obtained with an RMP-510 spectrometer (JASCO Co., Ltd.). Fourier transform infrared (FT-IR) spectra were obtained for KBr pellet specimens with a Spectrum One spectrometer (Perkin Elmer). Diffuse UV-Vis-NIR reflectance spectra were measured on a Jasco V-570 spectrometer equipped with an integrating sphere accessory (ILN-472). A KBr powder was used as the diluent. A source-measurements unit R6243 (Advantest Co., Ltd.) was used to measure the current-voltage characteristics with a DC two-probe method. The temperature dependence of the sheet resistance was measured by a DC four probe method with using R6243 as the current source and a nanovoltmeter 2182 (Keithley Co., Ltd.) as a voltage monitor. Atomic force microscope (AFM) images were obtained using Nanocute (SII Nanotechnology Inc.) with a tapping mode. The conductivity of the surfaces was 9 ACS Paragon Plus Environment


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measured using the AFM equipped with an ARTCAM-300MI-HHS-WOM unit (ARTRAY Co., Ltd.).

RESULTS AND DISCUSSION In situ Observation of the Thin Film Formation on the Crystal Surfaces. An example of the TTF vapor contact is shown in Figure 2. The substrate crystal was fixed on a Si plate covered with a SiO2 thin layer by exposing the flat plane to its external environment, and the TTF powder was placed near to the crystal. The whole set was placed on the AFM stage opened in air.

Figure 2. Microscope image of the typical setup for the in situ observation of the TTF vapor contact experiments under ambient conditions (the substrate crystal is 1-TCNQ).

Using this setup, the sequential scanning of the AFM images was performed to monitor the changes occurring on the crystal surfaces by the TTF vapor contact. Since it was not possible to take an image at the moment the TTF vapor contact commenced, owing to adjustment of the scan


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conditions, the first image was obtained typically five minutes after contacting with the TTF vapor; this moment is defined as t0. Figure 3a shows the sequential AFM images of the flat (001) plane of the TCNQ single-component crystal scanned every 160 s after t0. At t = t0, an irregular shaped lump appeared on the flat surface image (indicated by the arrow). After 160 s, the lump grew irregularly with gutters forming around it. At t = t0 + 320 s, nanometer-size needle crystals started to grow along the gutters. Three main directions of the growth can be recognized, and all of them extend out from the original lump. At the same time, the size of the irregular shaped lump becomes smaller (the periphery is changed to needle crystals). At t = t0 + 480 s, needle crystals are crowded and some of them obstruct the growth of other crystals. As shown in Figure 3b, the TCNQ (001) surface plane is completely covered after 6 hours with randomly oriented needle crystals with a typical width of 200 nm and height of 5 nm.



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Figure 3. (a) Sequential AFM images (every 160 s) for the TTF vapor contact on the TCNQ single component substrate crystal. The white arrow and blue arrow in the image at t = t0 indicate an initially formed lump and the a + b direction of the substrate (corresponding to the TCNQ stacking axis), respectively. (b) AFM image at t = t0 + 6 hours and the height profile in the area indicated by the black square.

The events occurred on the (001) surface of the TCNQ single-component crystal may be summarized as follows. Lumps with irregular shape are formed after contacting with the TTF vapor. They are proposed to be noncrystalline clusters that play a role of crystal nucleation. As will be mentioned in the next section, the final products on the surface are confirmed to be TTF-TCNQ, and TTF and TCNQ are considered to be paired in these clusters. Therefore, the surface area around the lump is scooped out by extracting the TCNQ molecules from the substrate, forming gutters around the lump. When the lump of clusters grows to a certain size, 12 ACS Paragon Plus Environment

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crystallization of TTF-TCNQ begins. The growth direction of the needle crystals seems to be dominated by the shape (outline) of the cluster rather than the structure (molecular arrangement) of the substrate surface. Next, the sequential AFM images on the (001) surface of 1-TCNQ after contacting with the TTF vapor are shown in Figure 4a. At t = t0, a shapeless lump appeared on the surface (indicated by the arrow). After 120 s, the lump resembled a well-shaped needle crystal that grew along the mixed-stack axis in the substrate 1-TCNQ (a axis). At t = t0 + 240 s, the needle crystal grew along the a axis of 1-TCNQ. At the same time, one can recognize that gutters formed around the crystal. The crystal continued to grow exactly along the a axis of 1-TCNQ at t = t0 + 360 s. After 24 hours, the (001) surface of 1-TCNQ was completely covered with fine-needle crystals that were perfectly aligned along the molecular stacking axis in 1-TCNQ. The typical width of the needles was 200 nm, and the height was 10 nm (Figure 4b).



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Figure 4. (a) Sequential AFM images (every 120 s) for the TTF vapor contact on the (001) surface plane of the 1-TCNQ substrate crystal. The white arrow and blue arrow in the image at t = t0 indicate an initially formed lump and the a direction of the substrate (corresponding to the mixed-stack axis), respectively. (b) AFM image at t = t0 + 24 hours and the height profile in the area indicated by the black square.

The CT interaction in 1-TCNQ is rather weak, since the electron donating ability of 1 is the weakest among the donors adopted in this study. When the CT interaction in the substrate TCNQ CT crystal becomes stronger, the change of the crystal growth on the surface is shown in Figure 5. Sequential AFM images of the 6-TCNQ surface after contacting the TTF vapor are shown in Figure 5a, and the image at t0 + 24 hours is shown in Figure 5b. The direction of the crystal growth of the needles is completely aligned along the mixed-stack axis of the substrate, and the surface is fully covered by such needles after 24 hours. The width of the needles is nearly the 14 ACS Paragon Plus Environment

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same as that obtained on the 1-TCNQ surface, while the height is increased by approximately four times compared with that on 1-TCNQ. Sequential AFM images after contact of the TTF vapor with the 9-TCNQ surface, in which the CT interaction in the substrate is the strongest among the TCNQ complexes used in this study, is shown in Figure 5c. The crystal-growth rate is obviously much slower compared with those for 1-TCNQ and 6-TCNQ. Though the surface at t0 + 24 hours in Figure 5d is also covered by the needle crystals, the thickness of the crystallites is increased; the width becomes twice that of the crystallites on the 1-TCNQ surface and the height is increased eight times. However, the length of the individual needles is shortened compared with those grown on the 1-TCNQ and 6-TCNQ surfaces. Compared with the images of the (001) plane of 1-TCNQ, no new lumps that play a role of nucleation of crystallization appear in the scanned area for the (011) plane of 6-TCNQ and the (110) plane of 9-TCNQ. The occurrence frequency of the nucleation on the 6-TCNQ and 9-TCNQ surfaces is considered to be much lower, and the surface defects may play a preferential role as nucleation sites.



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Figure 5. (a) Sequential AFM images (every 420 s) for the TTF vapor contact on the (011) surface plane of the 6-TCNQ substrate crystal. The blue arrow in the image at t = t0 indicates the a direction of the substrate (corresponding to the mixed-stack axis). (b) AFM image at t = t0 + 24 hours for the TTF vapor contact on the (011) surface plane of the 6-TCNQ substrate crystal and the height profile in the area indicated by the black square. (c) Sequential AFM images (every 420 s) for the TTF vapor contact on the (110) surface plane of the 9-TCNQ substrate crystal. The blue arrow in the image at t = t0 indicate the c direction of the substrate (corresponding to the mixed-stack axis). (d) AFM image at t = t0 + 24 hours for the TTF vapor contact on the (110) surface plane of the 9-TCNQ substrate crystal and the height profile in the area indicated by the black square.

The correlation between the strength of the CT interaction and the change of crystal growth on the surface is now considered. For all of the 1-, 6-, and 9-TCNQ CT complex substrate crystals, the direction of the crystal growth of needles on the surface is regulated to be along the mixed-stack axis in the substrate. This point contrasts with the random growth observed on the TCNQ single-component crystal substrate, shown in Figure 3. As mentioned later, the needle crystals on all of the substrates are of the TTF-TCNQ complex. Therefore, TCNQ has to be extracted from the surface of the substrate for the crystal growth. In the TCNQ single-component 16 ACS Paragon Plus Environment

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crystal, the intermolecular attractive interaction arises solely from the van der Waals force that operates in an isotropic manner. Therefore, when a TCNQ molecule is pulled out from the surface layer, there is no preferential direction in which the next TCNQ molecule is extracted. Consequently, gutters form along various directions with needle crystals growing along them (since TCNQ is planar, the dispersion interaction is slightly larger along the face-to-face stacking direction than those along the other directions. Some of the needles in Figure 3b thus grow along the stacking direction). However, TCNQ in all the CT complexes is always sandwiched between the PAH donors, and the CT interaction along the mixed-stack direction participates in addition to the normal van der Waals force as the intermolecular interactions. As a result, the probability of extracting the TCNQ molecule from the CT complex crystals is considered to become lower than that from the TCNQ single-component crystal, and it is expected to depend on the strength of the CT interaction. Since donor 1 is rather weak, TCNQ is easily extracted from the CT crystal. Therefore, the formation of lumps, which are precursors of the crystal nucleation, was observed even in a narrow scan area. However, when the CT interaction becomes stronger, as in 6- and 9-TCNQ, it is difficult for TCNQ to be extracted from the CT crystals. As a result, both the growth rate of the needle crystals and the occurrence frequency of the nucleus formation are decreased, resulting in an increase of the thickness of the needles. The regulation of the growth direction of the needles has been fully discussed for 7-TCNQ.7 Initially, the evaporated TTF 17 ACS Paragon Plus Environment


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molecules are in contact with the mixed-stack CT crystal surface and react with the TCNQ molecules in the CT crystal. Then, cluster-like TTF-TCNQ nucleuses are formed on the crystal surface. By forming TTF-TCNQ on the surface, the donor molecules in the substrate lose their paired acceptor molecules. The donor molecules’ cohesive energies become considerably weaker and the donor becomes labile. Subsequently, the TCNQ molecules that lost a neighborhood donor molecule also become labile. As a result, the CT crystal surface is further deconstructed. Because the CT interaction operates in one dimension, the deconstruction of the molecular packing only occurs along the direction of the intermolecular CT interactions. In these processes of nucleation and crystal growth, the alignment direction is determined. The morphology of the thin films after 24-hr exposure of the TTF vapor to the substrates other than 1-, 6-, and 9-TCNQ is shown in Figure 6a. The arrow in each image indicates the mixed-stack axis in the substrate CT crystal. The needles formed by contact with the TTF vapor grow clearly along this direction for the 2-, 4-, 7-, and 8-TCNQ substrates. However, when the substrate is 3-TCNQ, there are many needles that grow independently on the mixed-stack axis. This is because the film becomes a multi-layer structure. Since 3 has the highest vapor pressure among the donors in this study,23 there may be a certain amount of isolated TCNQ molecules around the surfaces. This will lead to the formation of a second TTF-TCNQ layer on the first


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layer without any orientational relationship between them. One can observe the needle crystals grown along the mixed-stack axis in the lowermost layer of the image. When 5-TCNQ was used as the substrate, we can observe rod-like crystallites grown along the mixed-stack axis. However, their growth is interrupted by the foreign crystalline substances projected from the substrate. These foreign crystalline substances grow independently to the molecular arrangement in the substrate. We tentatively assume that they are re-condensed coronene (5) crystals. Since coronene has an extremely low vapor pressure,24 it may have a chance to re-crystallize as single-component crystals on the substrate surfaces. These coronene crystals are considered to become obstacles to the growth of the TTF-TCNQ needle crystals. The root mean square (RMS) value of the height profile can be used as a measure of the surface roughness, which reflects the thickness of the surface crystallites. The RMS values are plotted in Figure 6b, and the RMS shows some correlation with E1/2ox except 5-TCNQ which is a special situation as mentioned above. This correlation can be interpreted as follows. When the CT interaction in the substrate becomes stronger, TCNQ isolation becomes less efficient. This leads to a disruption of the rapid growth of the needles, and allows the needles to grow fatter instead.



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Figure 6. (a) AFM images of the surface morphology of 2-, 3-, 4-, 5-, 7-, and 8-TCNQ substrates after 24-hr exposure of the TTF vapor. The blue arrows indicate the mixed-stack axis in the substrate. (b) RMS (root mean square) roughness (obtained from the images observed after 24-hr exposure of the TTF vapor) versus E1/2ox plot of the TCNQ complexes.


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Confirmation of the Chemical Entity of the Thin Films. The crystalline films on the TCNQ single-component and 7-TCNQ CT-complex substrates after contacting TTF were investigated in detail by AFM, Raman scattering, and IR spectroscopic measurements.7 As a result, the chemical entity of the film was confirmed as TTF-TCNQ. For the present systems, the same analyses were performed. Figure 7a shows the Raman scattering spectra of the surface of substrates 2-, 3-, 4-, 5-, 6-, 7-, and 8-TCNQs after reaction with TTF vapor. Except for 8-TCNQ, the samples were prepared under ambient conditions. The sample on 8-TCNQ was prepared at 323 K, since the reaction was too slow. Spectra of neutral TCNQ and TTF-TCNQ complex are also shown for reference. It is well known that the ν4, C=C stretching mode of TCNQ, is sensitive to the CT degree.25 The band is observed at 1400 cm–1 for TCNQ–1 and at 1450 cm–1 for TCNQ0. The ν4, C=C stretching band of TTF-TCNQ in which the formal charge of TCNQ is –0.59, is observed at 1420 cm–1.25 In each spectrum of the TCNQ complex substrate surfaces, one main peak was observed around 1450 cm–1. Although the peak is slightly shifted from 1450 cm–1 in each spectrum reflecting the different circumstances of TCNQ in each crystal, all peaks are considered to arise from TCNQ0 in the neutral ground state. An additional weak peak was observed in the region of 1410–1425 cm–1 in each spectrum. Some of these peaks are not so clear, but it is reasonable to assign these peaks to TCNQ–0.59 in TTF-TCNQ. 21 ACS Paragon Plus Environment


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For the samples 1- and 9-TCNQs, the Raman scattering experiments were failed because the substrate crystals absorbed the Raman laser (568 nm). Therefore, for confirmation, the formation of TTF-TCNQ was examined by FT-IR spectroscopy. In these measurements, a mixed powder of TTF and 1- or 9-TCNQ was used. The FT-IR spectra are depicted in Figure 7b and are focused on the C≡N stretching band of TCNQ. The band at 2204 cm–1 of TTF-TCNQ was observed in both 1- and 9-TCNQ spectra, confirming the generation of TTF-TCNQ at the contact interface between TTF and PAH-TCNQ.


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Figure 7. (a) Raman scattering spectra of the surface of substrates 2-, 3-, 4-, 5-, 6-, 7-, and 8-TCNQs after reaction with the TTF vapor under ambient conditions for 2-, 3-, 4-, 5-, 6-, and 7-TCNQs and at 323 K for 8-TCNQ. The bottom panel indicates the corresponding spectra of TCNQ and TTF-TCNQ. (b) FT-IR spectra of the mixed powders of TTF and 1- and 9-TCNQs. The bottom panel indicates the corresponding spectra of TCNQ and TTF-TCNQ.



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Transport Properties. The sheet resistance at room temperature of the TTF-TCNQ thin films grown on the TCNQ complex substrates are plotted against E1/2ox of the donor in Figure 8a. These plots are for the samples for which conductance did not show any changes with further contact of the TTF vapor, namely the surface was fully reacted with TTF vapor. For reference, the TTF-TCNQ thin film similarly grown on the TCNQ single-component crystal substrate is shown in Figure 8a. The sheet resistance is monotonically increased with decreasing E1/2ox of the donor in the substrates except for 5-TCNQ. As shown in Figure 6, the film morphology and roughness of the thin film on 5-TCNQ were found to be different from those formed on the other PAH-TCNQ substrates owing to the formation of the obstacles composed of 5. The thin film on 5-TCNQ showed a high sheet resistance, and it is consistent with the film morphology; the obstacles interfered with the formation of the conduction path of TTF-TCNQ crystals in the film. Here, one should note that the TTF-TCNQ thin film on the TCNQ single-component crystal also shows a high sheet resistance (about 5 × 104 Ω sq–1), regardless of its very small roughness (RMS roughness = 2.1 nm). The high sheet resistance is obviously caused by the random orientation of the crystallites in the thin film. Therefore, one-directional crystallite alignment is important to obtain a low-resistance TTF-TCNQ film, and the present method is useful to fabricate a highly conductive TTF-TCNQ thin film.


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The changes of the sheet resistance during the contact of TTF vapor with the surfaces of 1-, 6-, and 9-TCNQ substrate crystals were monitored (Figure 8b). Initial sheet resistance of the three substrates surface exceeded 1 × 1010 Ω sq–1. The sheet resistance of the surface of 1-TCNQ rapidly decreased to 1 × 104 Ω sq–1 in 10 minutes. This rapid drop of sheet resistance suggests the formation of rather firm networks of TTF-TCNQ crystallites between the electrodes. The sheet resistance further decreased to less than 1 × 103 Ω sq–1 at 1000 minutes. The decrement of sheet resistance of the surface of 6-TCNQ commenced after 30 minutes. A gradual decrease up to 65 minutes and then rapid decrease to about 1 × 105 Ω sq-1 was observed. The sheet resistance also showed a gradual decrement to approximately 1 × 104 Ω sq–1 at 1000 minutes. The decrement of the sheet resistance of the 9-TCNQ surface occurred more slowly than that observed for the other crystals, which started at 150 minutes, and the resistance decreased to 7 × 105 Ω sq–1 and then decreased gradually to 1×105 Ω sq–1 at 1000 minutes. As shown in Figure 6, the roughness of the surface and the size of the TTF-TCNQ crystallites on PAH-TCNQ substrates were increased with increasing E1/2ox of the donor. To clarify the effect of the size of the TTF-TCNQ crystallites in the thin film, the sheet resistance was measured using the conductive AFM. Figure 8c is the current map of the surfaces of 1-, 6-, and 9-TCNQ substrates after TTF vapor exposure for 1000 minutes. The surface of 1-TCNQ is fully covered by densely grown small TTF-TCNQ crystallites, and the whole surface shows a highly 25 ACS Paragon Plus Environment


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conductive feature (orange to yellow in Figure 8c). However, sparsely grown large TTF-TCNQ crystallites are observed on the surface of 9-TCNQ, and the current map shows many large insulating areas (blue in Figure 8c). The current map for the surface of the 6-TCNQ substrate consists of both intermediate conducting and insulating areas. The density of the effective conducting paths in the TTF-TCNQ thin film is therefore strongly influenced by the CT interaction in the substrate crystal.

Figure 8. (a) Sheet resistance at room temperature derived from the ohmic current density-electric field characteristics versus E1/2ox of the donor plot. (b) Time dependence of the sheet resistance by contacting of TTF vapor for 1-, 6-, and 9-TCNQs. (c) Current maps of the surfaces of 1-, 6-, 9-TCNQs after exposure of the TTF vapor for 1000 minutes.


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The density of the effective conducting paths in the TTF-TCNQ thin film substantially affects the temperature dependence of the sheet resistance. Figure 9 shows the temperature dependence of the sheet resistance of PAH-TCNQ and TCNQ crystal surfaces after fully reacted with a TTF vapor (when no further resistance drop was observed, i.e., typically after 1–2 days). For the TTF-TCNQ thin films in which the thin needle crystals are densely packed with aligning in one direction, namely, for the films on the 1-, 2-, 4-, 6-, 7-TCNQ substrates, metallic temperature dependence around room temperature was observed. However, for the films composed of randomly oriented TTF-TCNQ thin needles, namely, for the films on the 3-TCNQ and TCNQ single-component crystal substrates, the sheet resistance shows a thermally activated temperature dependence at room temperature. When the TTF-TCNQ crystals in the film were thick and the roughness of the film was large, that is, for 5-, 8-, 9-TCNQ substrates, the temperature dependence of the sheet resistance exhibited a more pronounced thermally activated behavior, reflecting a larger contact resistance between the TTF-TCNQ needle crystals. Therefore, it was clarified that the control of size and alignment of crystallites is required to obtain thin films with an efficient inherent functionality.



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Figure 9. Temperature dependence of the sheet resistance of the TTF-TCNQ thin films grown on PAH-TCNQ and TCNQ crystals.

CONCLUSIONS To study the TTF-TCNQ thin film growth on the TCNQ containing single-crystalline substrates, TTF vapor contact experiments were performed for various mixed-stack TCNQ CT complex crystals with the neutral ground state as well as TCNQ single-component crystal. In contrast to the random orientation of the TTF-TCNQ needle crystals on the TCNQ single-component crystalline substrate, a highly ordered alignment of the needles in the thin films was achieved on the CT-complex crystalline substrates.


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The surface roughness, the size of the needles, and the degree of dense packing of the needles in the TTF-TCNQ thin films were found to depend on the strength of the CT interaction in the substrate CT complexes. When the CT interaction in the substrate crystal is weak, TTF-TCNQ thin needle crystals are densely packed with perfect alignment of the needles along the mixed-stack axis in the substrate CT complex. The charge transport in these films is metallic around room temperature. However, when the CT interaction becomes stronger (still in the neutral ground state) or the donor becomes nonvolatile, the surface roughness and the thickness of the needle crystals in the TTF-TCNQ films become large. Consequently, the charge transport in these films is poorer compared with the films on the TCNQ complexes with weak CT interactions. This study indicates that one can fabricate highly conducting CT complex thin films on mixed-stack CT complexes with neutral ground states by a simple vapor contact method. The heterojunctions formed on the surface may be used to design device functionalities of organic electronics.

ASSOCIATED CONTENT Supporting Information. The supporting Information is available free of charge on the ACS Publications website at DOI: 29 ACS Paragon Plus Environment


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X-ray structure analysis procedures, crystal data of TCNQ and PAH-TCNQs, molecular arrangements in the lattices of TCNQ and PAH-TCNQs (PDF) Crystallographic data of 9-TCNQ (CIF)

ACKNOWLEDGMENTS The authors would like to thank Profs. T. Nakamura and S. Noro at Hokkaido University for their help with the Raman spectroscopy measurements. This work was supported in part by a Grant-in-Aid for Scientific Research JSPS KAKENHI Grant No. 26288029, and JST, CREST.

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Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8

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