Epitaxial TetrathiafulvaleneâTetracyanoquinodimethane Thin Films on

Article pubs.acs.org/crystal

Epitaxial Tetrathiafulvalene−Tetracyanoquinodimethane Thin Films on KCl(100): New Preparation Methods and Observation of InterfaceMediated Thin Film Polymorph Alexander Man̈ z, Tobias Breuer, and Gregor Witte* Molekulare Festkörperphysik, Philipps-Universität Marburg, D-35032 Marburg, Germany S Supporting Information *

ABSTRACT: Combining organic compounds of complementary ionization potential and electron affinity allows fabrication of charge-transfer complexes that exhibit remarkable properties, resulting, for example, in very high conductivity. Though the bulk properties of the prototypical organic conductor tetrathiafulvalene−tetracyanoquinodimethane (TTF-TCNQ) have been studied in detail, the influence of defects and crystallite size on the resulting electronic properties, as well as an integration of these materials in organic thin film devices, is barely explored. One important requirement for such a comprehension is the precise control over crystallite size and quality. In this study, we report on different strategies to prepare crystalline TTF-TCNQ thin films and compare their structural quality. While conventional organic molecular beam deposition of TTF-TCNQ onto KCl(100) substrates enables the growth of epitaxial thin films with grain dimensions of up to 2 μm, further enhancement of the crystallite dimensions by raising the growth temperature is thermally limited by vanishing sticking and onset of vaporization. Using more sophisticated methods like hot wall evaporation, however, allows one to overcome these limitations and yields crystalline islands with extensions enhanced by 2 orders of magnitude. Furthermore, we identify and provide a full structure solution of a yet unknown interface-mediated thin film polymorph of TTF-TCNQ, which is adopted in films of thicknesses below 1 μm.

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INTRODUCTION The promising potential of organic semiconductors for fabrication of electronic thin film devices has triggered major research efforts in the field of organic electronics in the last two decades. In addition to the synthesis of new molecular compounds, especially the controlled preparation and structural analysis of thin films have been studied extensively.1−4 Combining molecular compounds of low ionization energy with others of high electron affinity can lead to (partial) charge transfer between both compounds. Such charge transfer complexes can also form ordered crystals that are known as charge transfer (CT) salts. The charge transfer not only results in additional stabilization due to Coulomb interaction between donor and acceptor but also causes an electron correlation, which yields remarkable electronic properties such as high conductivity.5,6 A prominent representative of this class of materials is tetrathiafulvalene−7,7,8,8-tetracyanoquinodimethane (TTF-TCNQ, Figure 1), which is known to form a quasi one-dimensional conductor at room temperature7 and further reveals Peierls-transitions at low temperature8,9 that are accompanied by structural phase transitions.10,11 Though charge transfer salts have already been known for more than 40 years, related studies were essentially carried out for bulk crystals12,13 while the growth of CT films is by far less thoroughly studied than that of organic semiconductors. Only recently issues regarding the influence of film thickness and © 2014 American Chemical Society

Figure 1. Simplified scheme showing the epitaxial arrangement of TTF-TCNQ films on a KCl(100) substrate.15,16

defects on the Peierls transition have been addressed by growing TTF-TCNQ films onto different substrates.14 However, due to the overall poor crystallinity, no consistent correlation could be obtained. Remarkably, TTF-TCNQ can be Received: October 6, 2014 Revised: October 21, 2014 Published: November 11, 2014 395

dx.doi.org/10.1021/cg501484p | Cryst. Growth Des. 2015, 15, 395−403

Crystal Growth & Design

Article

degassing, evaporator and tube are slipped into a gasket on the sample holder yielding a sealed volume above the substrate. In that way the molecular vapor pressure above the sample can be greatly increased by raising the cell temperature (typically 390 K), which enables growth at elevated substrate temperatures close to the desorption temperature. For comparison, pure TCNQ (Sigma-Aldrich, purity 98%, CAS 1518-16-7) films that have been grown by hot wall deposition were studied. The thermal stability of the various films was characterized by thermal desorption spectroscopy (TDS), using a quadropole mass spectrometer (Balzer QMG 220) with a Feulner cup positioned close to the sample surface. The spectra were acquired by recording the mass signal of the molecule ion (M(TTF) = M(TCNQ), m/z = 204 amu) during a computer-controlled linear increase of the substrate temperature from 250 to 450 K with a heating rate of β = 0.5 K/s. To provide reliable temperature measurements, a K-type thermocouple, which had been carefully calibrated using various temperature standards, was attached directly to the sample surface. The film morphology was characterized by atomic force microscopy (AFM, Agilent SPM 5500) operated in tapping mode ( f res = 325 kHz) under ambient conditions. Simultaneously with the topography also the phase and the amplitude of the cantilever have been measured. The crystalline structure and orientation of the films were analyzed by means of X-ray diffraction (Bruker AXS Discover D8) using monochromized Cu Kα radiation (λ = 1.54056 Å) and a LynxEye silicon strip detector. A UV/vis spectrophotometer (Agilent 8453) and optical microscopy in combination with spectral and polarization filters were utilized to characterize the optical properties and the domain distribution within the films.

grown epitaxially on KCl(100) substrates, providing a rational way to prepare well ordered crystalline films.15,16 A simple way to prepare films with equimolar stoichiometry is thermal sublimation of milled TTF-TCNQ crystallites that have been grown before from solution.17,18 By contrast simultaneous coevaporation of TTF and TCNQ from separate sources is complicated by different evaporation enthalpies19,20 and sticking coefficients of both compounds, which can lead to inhomogeneous films.22 The effective growth from TTFTCNQ powder indicates that actually molecular pairs or clusters are vaporized like in the case of alkali halide film growth.23,24 However, it has been found that at elevated evaporator temperature also neutral TCNQ molecules are deposited, which limits the usable crucible temperature to about 115 °C.16,25 Though different experimental approaches such as physical vapor and hot wall deposition have been reported for preparation of TTF-TCNQ films26,27 a systematic comparison of the resulting films, as well as attempts to optimize the crystallinity, has yet not been performed. Regarding the crystalline structure of TTF-TCNQ films on KCl(100), small but distinct deviations from the bulk structure28 have been reported15 but have yet not been further analyzed. In view of the close correlation between the structural and electronic properties a precise analysis is necessary to enable detailed studies of the physical properties of this system. Here we provide a comprehensive study of the growth and microstructure of TTF-TCNQ films on KCl(100) with particular emphasis on an optimization of the crystallite size by exploring the accessible temperature range of film growth. The paper is organized as follows: First, we discuss the morphology of thin films prepared at different substrate temperatures by conventional organic molecular beam deposition (OMBD). Based on the analysis of the thermal stability of such films (TDS), we then discuss new preparation methods to enhance the grain sizes. Subsequently, we determine the crystalline structure (XRD) of the samples, where a new polymorph of TTF-TCNQ is identified and analyzed. Finally, these results are complemented by polarized optical microscopy, which allows identification of phase separated pure TCNQ crystallites in selective films.

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RESULTS AND DISCUSSION Morphology of Thin Films Grown by OMBD. First, we analyze the morphology of multilayer films with 50 nm nominal thickness. As presented in Figure 2a−d, TTF-TCNQ films consist of elongated crystallites whose long sides are oriented along the ⟨110⟩KCl directions. While the epitaxial growth of TTF-TCNQ on KCl(100) has been reported before,15,30,31 the degree of azimuthal order as well as the grain size clearly depends on the substrate temperature during deposition (Tsub). To accentuate the crystallite shape, the AFM data are shown as amplitude images, which have been measured simultaneously with the topography data. At temperatures below 250 K (Figure 2a), quite small and comparably rough islands are formed so that the preferred lateral alignment of the islands is barely visible. Stepwise increase of Tsub up to 290 K leads to larger crystallites that perfectly align along the substrate diagonals, that is, ⟨110⟩KCl azimuth directions. At Tsub = 290 K, the platelike crystallites are remarkably smooth and monomolecular steps on the crystallites can be observed in the corresponding topographic line scans III and IV in Figure 2f. Further elevation of the substrate temperature during deposition, however, did not result in smoother films, because above 295 K the sticking coefficient of TTF-TCNQ on KCl(100) is significantly reduced and no closed layers are formed. Consequently, films prepared at Tsub ≥ 295 K (Figure 2e) exhibit strong islanding. At Tsub = 300 K additional crystallites are observed with deviating lateral alignment, for example, exhibiting an angle of 11° with the ⟨100⟩KCl azimuth (cf. Figure 2e, green arrows). In optical micrographs, these crystallites exhibit strikingly different absorption properties than crystallites grown in common direction (cf. Figures S2 and S5, Supporting Information). We attribute these crystallites to phase separated TCNQ, as will be discussed later. At even higher substrate temperatures above 300 K, no molecules adsorb on the surface, reflecting a vanishing sticking coefficient. The abrupt change in film morphology occurring within such a small temperature window

EXPERIMENTAL SECTION

All molecular films were grown under high vacuum conditions onto KCl(100) surfaces that have been prepared by cleaving slices of about 2 mm from a single crystal rod (Korth Kristalle GmbH) in air. Remaining debris was removed by means of adhesive tape followed by rinsing off the surface with isopropyl alcohol and blow-drying in a nitrogen stream. Subsequently, the samples were quickly transferred into the vacuum system using a load lock system and heated at 450 K to remove adsorbed water. Pestled TTF-TCNQ crystal powder (Sigma-Aldrich, purity ≥97.0%, CAS 40210-84-2) was used as source material to fabricate the binary molecular films. Corresponding XRD powder diffractograms confirm that this raw material exhibits the known TTF-TCNQ bulk structure28 (see Table S1, Supporting Information). The films were grown by organic molecular beam deposition (OMBD) from an alumina crucible in a resistively heated Knudsen cell. The maximum cell temperature applied was 390 K to exclude the evaporation of neutral TCNQ species.25 This temperature yielded a deposition rate of 20 Å/min as monitored by temperature stabilized quartz crystal microbalance (QCM). Utilizing the cooling and heating capability of the sample manipulator, samples have been grown at various substrate temperatures ranging between 200 and 350 K. Additional TTF-TCNQ films were grown using a hot wall-type evaporator, which contains a separately heated glass tube between the evaporator cell and the sample holder.29 After evacuation and 396



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Figure 3. Thermal desorption spectra of TTF-TCNQ thin films (m/z = 204 u) with thicknesses of 1−15 nm, deposited at 260 K (blue) and 290 K (red) on KCl(100). The inset depicts an Arrhenius plot that is used to determine the onset of desorption.

Figure 3 shows series of TD-spectra of TTF-TCNQ films deposited at Tsub = 260 K (blue, top) and Tsub = 290 K (red, bottom). Clearly, films of different thickness exhibit similar leading edges and differ only in the position of the tailing edges of their desorption peaks, as expected for multilayer desorption.34 Interestingly, the onset temperature differs significantly for the chosen substrate temperatures during deposition. Although a definite starting point for multilayer desorption cannot be measured, we defined an onset temperature as the intersection of the background level with the increase of the desorption signal in an Arrhenius diagram, as shown in the inset of Figure 3. While films prepared at 260 K exhibit such an onset temperature at 304 K, this value, as well as the tailing edges of the desorption peaks for equivalent thickness, are shifted by about 6 K to higher temperatures for films prepared at 290 K. We attribute this effect to the distinctly smaller crystallite sizes in films prepared below room temperature (cf. AFM micrographs in Figure 2). Such a correlation between crystallite size and thermal stability has been observed before for other molecular films such as PTCDA35 and rubrene.29 We note further that upon heating of films that have been grown at low temperatures, their color appearance changes: while they exhibit a yellowish color appearance when deposited at 260 K or below, the apparent color changes to a gray tone upon heating around 280−290 K. We attribute this change to postdeposition crystallization and assign the desorption signal occurring at 284 K for samples prepared at 260 K to noncrystalline regions (see Figure 3, upper inset). This resembles the situation reported for thin films of perylene,36 HBC,37 rubrene,38 and several TTF-related compounds, for example, (BEDT-TTF)2I3,39 where also thermally induced crystallization upon low temperature deposition has been observed, which in turn leads to enhanced thermal stability as discussed before. On the basis of these findings, we first prepared a 10 nm TTF-TCNQ seed layer at Tsub−seed = 290 K (as shown in Figure 4a) and subsequently deposited additional 40 nm at Tsub−final = 315 K. As the AFM micrograph of the final film in Figure 4b shows, this strategy indeed allows us to prepare films at temperatures beyond the aforementioned maximal substrate temperature of 295 K. However, the film morphology as well as the crystallite size nearly completely resemble the situation found for Tsub = 290 K (cf. Figure 2d) using conventional OMBD. This shows that no significant enlargement of the grain

Figure 2. (a−e) AFM micrographs (amplitude images) of TTFTCNQ thin films (dnom = 50 nm) grown at different substrate temperatures on KCl(100) substrates together with (f) corresponding topographic line scans. All image borders coincide with the ⟨100⟩KCl directions, and the scale bars denote a length of 2 μm.

constitutes a challenge for the preparation of thin films of reproducible morphology and highlights the importance of precise temperature control during preparation. This is especially important because the transition from ideal epitaxial growth to noncovered substrates occurs close to room temperature (around 295 K). Seed-Layer Supported Preparation. While the vanishing sticking coefficient seems to define a strict limit to the achievable thin film quality, there are additional preparation strategies to overcome this limitation. The first approach is to deposit seed layers at lower temperatures and subsequently deposit the final thickness at elevated temperatures. This strategy exploits the fact that the sticking coefficient of TTFTCNQ on itself is probably higher than the one of TTFTCNQ on KCl due to the larger energy accommodation, as predicted by the model of adsorption by Kisliuk.32 Along this preparation strategy, there are two different crucial temperature parameters: T sub−seed, the substrate temperature during deposition of the seed layer and Tsub−final, the temperature during the final step. To get an estimate of the right choice for Tsub−final, we performed TDS measurements, which allow us to analyze the thermal stability of TTF-TCNQ multilayers under vacuum conditions. Clearly, Tsub−final has to be chosen close to but below the onset temperature of multilayer desorption.33 397



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Figure 4. AFM micrographs (amplitude images) of (a) 10 nm TTFTCNQ deposited at 290 K (seed layer), (b) 40 nm TTF-TCNQ deposited at Tsub−final = 315 K onto a 10 nm seed layer grown before at 290 K, together with corresponding topographic line scans in panel c. All image borders coincide with the ⟨100⟩KCl directions; scale bar = 2 μm.

size or increase of molecular ordering is achieved by this method. We further note that preparations at higher values for Tsub−final failed, because at these temperatures, the seedlayer starts to desorb during the deposition process of the final layer, as proposed by the TDS measurements. High Temperature Sample Preparation by Hot Wall Deposition. Our second strategy to further improve the film ordering by circumventing the thermal limitation upon OMBD is based on hot wall deposition. Here, elevation of the evaporator temperature and the sample holder to higher temperatures compared with conventional OMBD results in a steady molecular atmosphere of TTF-TCNQ. Accordingly, the high impinging rate of molecules results in considerable total adsorption despite a low sticking coefficient at high Tsub. Therefore, an adequate choice of parameters allows us to prepare films at very high substrate temperatures. Because this kind of preparation enables the provision of notable thermal energy during film formation, it can, for example, be applied to achieve highly crystalline films of rubrene, of which conventional OMBD is not capable.29 In our case, we have been able to prepare films at substrate temperatures up to 340 K using a hot wall evaporator. As presented in Figure 5, we again observe azimuthally well-ordered crystallites of TTF-TCNQ and find that the grain size increases with Tsub up to about 50 μm in their long axis at 340 K. According to this effect, these micrographs show a roughly 100 times larger area than those in Figures 2 and 4. To emphasize the largely improved crystallite size achieved by this method, Figure 5e,f provides a comparison of the resulting films prepared by means of OMBD and hot wall deposition in same magnification. Note that the surface roughness for hot wall deposited samples is significantly greater than for samples prepared by OMBD, which is probably related to the larger film thicknesses. Since the hot wall evaporation technique does not provide in situ deposition monitoring by QCM, the sample thicknesses can only be determined indirectly. Such analyses have yield thicknesses of about 2000 nm, which is clearly larger than for films prepared by OMBD. The higher thickness in turn also results in enhanced roughness.40 At temperatures higher than 340 K, we observe individual and nonconnected islands rather than a closed thin film. Like in the case of samples prepared upon OMBD with

Figure 5. (a−d) AFM micrographs (amplitude images) of TTFTCNQ films prepared by hot wall deposition at various substrate temperatures; scale bar = 20 μm. Panels e and f show a comparison of OMBD and hot wall deposited samples at the same scale (scale bar = 5 μm). All image borders coincide with the ⟨100⟩KCl directions. (g) Height profiles of denoted linescans.

Tsub close to the upper temperature limit, we observe small crystallites with different habitus, which will be discussed later. Comparison of Grain Sizes. To quantify the film structure resulting from the different preparation methods, we compared the lateral dimensions of the TTF-TCNQ grains by analyzing the AFM data. Figure 6 presents the average values for their long axis as a function of Tsub. Assuming that the crystallite size is essentially controlled by the diffusion length of surface molecules, which scales according to L ≈ L0·e−(E0/(kBT)),41 we expect that also the grain length follows such a temperature dependency. In fact, for growth temperatures Tsub ≥ 290 K, the experimental data are well described by such an exponential dependence (gray curve in Figure 6). The numerical fit of the experimental data yields an activation energy of E0 = 58 kJ/mol, which is comparable to activation energies of diffusion reported for molecules of similar sublimation enthalpy, such as anthracene.42 Interestingly, this trend is not found for films prepared at Tsub below room temperature. There, the observed 398



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Figure 6. Average grain size of TTF-TCNQ films derived from AFM data versus substrate temperature during deposition obtained for different preparation methods. The gray curve shows a fit according to a simple diffusion model.

sizes are larger than expected. This can be explained by the fact that we conducted our AFM measurements at room temperature while films that had been prepared below 290 K have already experienced thermally activated postdeposition crystallization (cf. discussion of TDS measurements above). Furthermore, we observe that grain sizes of those samples that have been prepared by first depositing a seed layer at 290 K and subsequently depositing the final thickness at higher temperatures are nearly constant independent of the substrate temperature during the final deposition process, as indicated by the AFM data shown in Figure 4. Analysis of Thin Film Crystallinity by X-ray Diffraction. To complement the morphological analyses, we applied X-ray diffraction to study also the crystalline structure of the films. Figure 7a compares X-ray reflectivity scans of differently grown TTF-TCNQ films. All films exclusively exhibit reflections corresponding to an interlayer distance of about 18 Å, hence confirming a (001) orientation of the crystalline TTF-TCNQ adlayer. To obtain also information about the azimuthal orientation of the adlayer, in-plane scans have been performed. For this purpose, the sample has been tilted such that the (012) reflection of the TTF-TCNQ lattice can be detected, and its azimuthal distribution was measured by an azimuthal φ-scan for a well ordered film prepared by hot wall deposition at Tsub = 345 K. As depicted in Figure 7b, reflections are found at azimuthal angles of 45°, 135°, 225°, and 315° relative to the edge of the supporting KCl, that is, the ⟨100⟩ azimuth direction of the substrate. Since the projection of the (012)-plane normal onto the (a,b)-plane lies parallel to the baxis of the TTF-TCNQ lattice (cf. Figure S4, Supporting Information), this confirms the epitaxial relation that the b-axis lies parallel to ⟨110⟩KCl15,43 as depicted schematically in Figure 1. Though out-of-plane scans reveal an exclusive (001) orientation for all TTF-TCNQ films grown by the various preparation protocols, a closer look reveals slight differences. One interesting detail in the diffractograms is the exact position of the (00n) reflections, which can be well resolved for higher order peaks as shown for the (008) reflection in Figure 7c). For films prepared by OMBD, these reflections are slightly shifted to lower angles (denoted by (008)′), reflecting a slight interlayer expansion to 18.16 Å. By contrast, for films prepared by hot wall deposition at larger substrate temperatures, a lattice

Figure 7. Summary of XRD data of TTF-TCNQ thin films prepared at different Tsub on KCl(100): (a) specular scans, (b) azimuthal scan of TTF-TCNQ(012) peak of a sample prepared at 345 K using hot wall deposition, and (c) magnification of TTF-TCNQ(008) peak. The region marked by * shows the Kβ satellite of (200)KCl and a spurious TCNQ(020) peak, which is discussed in Figure 11

spacing of 17.83 Å is found, which is in good agreement with the (001)-layer spacing of 17.88 Å for the bulk crystal structure reported by Kistenmacher et al.28 We note that the new peak positions cannot be explained by any other molecular plane of TTF-TCNQ and also a missalignment can be safely excluded on the basis of the simultaneously recorded (200)KCl substrate reflection. Hence this is evidence of a relaxed crystalline phase upon initial film growth. Such interface-mediated, relaxed phases (commonly denoted as thin film polymorphs) have been observed upon initial film growth on inert substrates for various aromatic molecules including pentacene,44 perfluoropentacene,45 sexithiophene,46 and p-sexiphenyl.47 In the case of pentacene, it has been found that the molecules predominantly crystallize in the thin film polymorph at low substrate temperatures and low thicknesses, while at elevated temperature or larger film thickness, the bulk phase becomes dominant.48 This temperature dependence clearly resembles the situation found for TTF-TCNQ (cf. Figure 7c). To study the thickness dependence of this new phase, an additional TTFTCNQ film of about 20 μm has been grown on KCl(100). Figure 8a compares the corresponding (008) reflections of TTF-TCNQ films of 200 nm and 20 μm prepared by OMBD at Tsub = 280 K and clearly reveals a transition from the initial thin film phase toward the bulk phase with increasing film thickness. One possible explanation for the appearance of the thin film phase of TTF-TCNQ on KCl(100) could be the lattice missmatch between substrate and the bulk phase of the adsorbate. Based on transmission electron diffraction, Yase et al.15 observed a slight compression along the b-axis from a value of 3.82 Å for the bulk structure to 3.78 Å for TTF-TCNQ films on KCl(100). This relaxation might be rationalized by higher order commensurability to reduce the lattice mismatch between 399



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This is enabled by the fact that the phase is maintained also for rather large film thicknesses of 20 μm, thus providing sufficient scattering intensity. In addition, the amorphous SiO2 surface enables the acquisition of a RSM without complication of specific selection rules and the requirement for a precise adjustment of the sample due to the isotropic distribution of crystalline islands. The unit cell parameters of this new polymorph have been derived from the peak positions obtained from the RSM by utilizing the algorithm implemented by Laugier and Bochu49 for a structural refinement and using the TTF-TCNQ bulk phase as starting structure.50 Table 1 summarizes the resulting Table 1. Crystal Structure of TTF-TCNQ Thin Film Phase Derived from RSM Data Compared with the Bulk Structure28a a b c α β γ cell volume

Figure 8. Specular X-ray diffractograms showing a magnified region around the (008) reflection of TTF-TCNQ films of different thickness deposited (a) on KCl(100) and (b) on SiO2 at Tsub = 280 K.

substrate and adlayer. In fact, the quasi commensurable relation 7brelax ≈ 6⟨110⟩KCl yields a mismatch of less than 1%. However, the unrelaxed lattice would lead to an even smaller mismatch of 0.15%, which casts this explanation into doubt. To examine whether the appearance of the new thin film phase depends on the crystalline nature of the supporting substrate, we have also analyzed TTF-TCNQ films grown by OBMD onto amorphous SiO2 substrates. As depicted in Figure 8b, the same situation is found as on KCl(100): initially the crystalline films exhibit an enhanced (001) interlayer spacing and, only for very large film thicknesses, adopt the layer spacing of the bulk structure. Therefore, we conclude that the new thin film polymorph is not induced by strain due to quasi-epitaxial growth. To derive further information on the exact structure of this thin film phase, a reciprocal space map (RSM) has been recorded for a TTF-TCNQ film on SiO2 as shown in Figure 9.

bulk phase

thin film

Δ (abs)

Δ (rel)

12.30 3.82 18.47 90 104.46 90 839.89

12.26 3.77 18.69 90 103.76 90 838.74

−0.04 −0.05 0.22

−0.3% −1.3% 1.2%

−0.70

−0.7%

−1.15

−0.1%

a

All lengths and angles are given in Å and deg, respectively, while the cell volume is given in Å3.

lattice parameters of the thin film phase and their differences from the bulk phase. The thin film phase features a compression along the b-axis and an expansion along the caxis, while the unit cell volume remains nearly unchanged. This indicates that essentially the tilt angle of the TCNQ molecules relative to the (001) plane is slightly increased in the thin film phase. We note further that the length of the b-vector of the thin film phase agrees favorably with the value determined by Yase et al. on the basis of transmission electron diffraction measurements of thin TTF-TCNQ films.15 Optical Microscopy. The size and orientation of TTFTCNQ domains was also characterized complementarily by optical polarization microscopy. Figure 10a shows an optical micrograph of a TTF-TCNQ film that has been grown by hot wall deposition onto KCl(100). Panels c and d show a magnified region (indicated by white box in panel a) that was illuminated by linearly polarized light. The molecular adlayer exhibits maximal contrast for polarization along ⟨110⟩KCl. At this condition, one TTF-TCNQ domain is bright, while the 90°-rotated domain absorbs maximally and appears dark. A comparison of these optical micrographs with an AFM image of the same region (cf. Figure 10b) demonstrates that each rotational domain consists of many parallel crystallites yielding an effective domain width of more than 20 μm. We note that these measurements allow us to judge the homogeneity of the domains and in particular show that oriented domains are vertically not overgrown by rotated domains. In fact, the azimuthal dependence of the absorbance of the individual domains, which was measured by rotating the sample (or polarization plane) and analyzing the average brightness of equally oriented crystallites shows a cos2(β) dependence on the angle β between the polarization plane and the ⟨100⟩ direction of the KCl substrate (see Figure 10e). The optical adsorption spectra of TTF-TCNQ films and ethanolic solution (see Figure 10f) are rather similar and reveal a prominent absorption

Figure 9. Reciprocal space map of TTF-TCNQ thin film phase on SiO2. To accentuate the weaker diffraction spots, the intensity is presented on a nonlinear scale. 400



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Figure 11. (a) Magnification of the * marked region in Figure 7a, XRD-scan of thin films prepared via hot wall deposition at 345 K (I) and 320 K (II) and via OMBD at 305 K (III) and 280 K (IV), (b) optical micrograph of TTF-TCNQ crystallites grown at Tsub = 340 K (red arrow) and phase separated pure TCNQ crystallites (green arrow), and (c) polarization dependence of optical absorption of two TCNQ crystallites (green, yellow) as indicated by the green arrow in panel b.

Figure 10. (a) Optical micrograph of TTF-TCNQ thin film on KCl(100), (b) AFM amplitude micrograph of white boxed area, (c, d) optical micrographs of white boxed area in which incident light is linearly polarized in given directions, (e) quantitative analysis of intensity dependence on field orientation for areas equivalent to red and blue bordered areas marked in panels c and d, and (f) UV/vis absorption spectra of a TTF-TCNQ film on KCl(100) (gray) and dissolved in ethanol (black).

Figure S5, Supporting Information). In contrast to TTFTCNQ, pure TCNQ films on KCl(100) surfaces exhibit a more complex arrangement, in which the crystallites are oriented in several directions as described by Uyeda et al.52 In such films, we also observed crystallites with similar habitus and polarization dependence of their optical absorption properties as the phase-separated crystallites in the aforementioned samples. In this context, it is interesting to note that in a previous work Caro et al. reported an additional epitaxial relation for TTFTCNQ crystallites on KCl(100) with an orientation along the ⟨100⟩ direction.30 However, their films were prepared by coevaporation of TTF and TCNQ from separate evaporators. Our experience with coevaporation indicates that it is rather difficult to achieve films of equimolar stoichiometry if they consist of compounds with different vapor pressure. Therefore, it is rather conceivable that also their observation can be explained as phase separation of pure TCNQ.

maximum around 3.2 eV. Based on polarization resolved absorption experiments on TTF-TCNQ single crystals, it was found that the transition dipole moment of this fundamental optical excitation is oriented along the b-axis.51 Comparing the optical absorption of individual rotational domains with the fiber structure detected by AFM thus shows that the elongated TTF-TCNQ crystallites are oriented along the b-axis. Phase Separated TCNQ. A closer inspection of the diffractogram of TTF-TCNQ films grown by hot wall deposition (cf. Figure 7a) reveals an additional reflection besides the Kβ satellite of the (200)KCl peak, which is replotted in a magnified region in Figure 11a. Notably, this peak is not observed for films prepared by OMBD. Its position does not fit to any lattice plane of TTF-TCNQ, neither for the bulk nor for the thin film phase, but instead can be identified as the (020) peak of pure TCNQ.52 Its low intensity indicates that it only corresponds to a small minority of the crystallites. Corresponding optical micrographs (cf. Figure 11b) show that for Tsub ≥ 340 K, the resulting molecular films are not homogeneous and exhibit individual small rectangular shaped islands whose long axes are oriented approximately along the ⟨100⟩ substrate azimuth. Because these are not found in films prepared by conventional OMBD at low temperatures, we attribute them to phase separated TCNQ. This assignment is further corroborated by polarization resolved micrographs, which show a distinctly different azimuthal dependency than the epitaxial TTF-TCNQ crystallites, which is shown in Figure 11c. We explain this phase separation by the small sticking coefficient and high volatility of TTF at elevated growth temperatures leading to the formation of pure TCNQ crystallites. To further support this hypothesis, we have also prepared pure TCNQ films (hot wall deposition at Tsub = 340 K, cf.

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SUMMARY While the epitaxial growth of TTF-TCNQ on KCl(100) has already been studied before in comparably thick films, we have thoroughly analyzed the structure of those films as a function of thickness and substrate temperature during deposition. Combining information from AFM and XRD, we have found very strong correlation between the grain sizes and the substrate temperature. While at substrate temperatures above 295 K, no films can be prepared using conventional OMBD due to vanishing sticking, we compared two different strategies that allow us to circumvent this temperature limit. Especially hot wall deposition has yield highly crystalline films with grain sizes of up to 50 μm. Furthermore, we found that though all films exhibit the molecular (001) orientation, the crystal structure of thin films is significantly modified compared with the bulk structure. Only at thicknesses greater than 20 μm, TTF-TCNQ crystallizes in 401



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the bulk structure on KCl(100) substrates. Surprisingly, this effect is not induced by the epitaxial relation between substrate and adsorbate, because it also occurs on amorphous SiO2 substrates. By conducting a detailed reciprocal space analysis, we have retrieved the unit cell dimensions of this thin film polymorph, which reveals a more upright molecular orientation in the thin film structure than in the bulk polymorph. Finally, we identified impurities occurring in thin films prepared at substrate temperatures above 320 K as phase separated TCNQ crystallites. Our analysis provides detailed insights into the dependence of TTF-TCNQ thin film structures on the preparation protocols. Since grain size as well as stoichiometry and crystal structure are closely related to electronic properties, the study of spectroscopic characteristics always has to be supported by structural analyses. Based on our new knowledge, a number of previous interpretations regarding the structure−property relationship in TTF-TCNQ with respect to, for example, the Peierls transition have to be reconsidered, which will allow for a better and deeper understanding of these effects.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data on substrate characterization, optical microscopy, and XRD analyses. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support provided by the German Science Foundation (DFG) through the collaborative research center “Structure and Dynamics of Internal Interfaces” (SFB 1083, TP A2).

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REFERENCES

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