Structural Order in Perfluoropentacene Thin Films ... - ACS Publications

Johannes Kepler UniVersität Linz, Altenbergerstrassw 69, A-4040 Linz, Austria, and Institute for. Molecular Science, Myodaiji, Okazaki 444-8787, Japa...
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Langmuir 2008, 24, 7294-7298

Structural Order in Perfluoropentacene Thin Films and Heterostructures with Pentacene Ingo Salzmann,† Steffen Duhm,† Georg Heimel,† Ju¨rgen P. Rabe,† Norbert Koch,*,† Martin Oehzelt,‡ Youichi Sakamoto,§ and Toshiyasu Suzuki§ Humboldt-UniVersita¨t zu Berlin, Institut fu¨r Physik, Newtonstrasse 15, D-12489 Berlin, Germany, Johannes Kepler UniVersita¨t Linz, Altenbergerstrassw 69, A-4040 Linz, Austria, and Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan ReceiVed February 25, 2008. ReVised Manuscript ReceiVed April 23, 2008 Synchrotron x-ray diffraction reciprocal space mapping was performed on perfluoropentacene (PFP) thin films on SiO2 in order to determine the crystal structure of a novel, substrate-induced thin film phase to be monoclinic with unit cell parameters of a ) 15.76 ( 0.02 Å, b ) 4.51 ( 0.02 Å, c ) 11.48 ( 0.02 Å, and β ) 90.4 ( 0.1°. Moreover, layered and co-deposited heterostructures of PFP and pentacene (P) were investigated by specular and grazingincidence x-ray diffraction, atomic force microscopy, and Fourier-transform infrared spectroscopy. For a ca. threemonolayers-thick PFP film grown on a P underlayer, slightly increased lattice spacing was found. In contrast, codeposited P/PFP films form a new mixed-crystal structure with no detectable degree of phase separation. These results highlight the structural complexity of these technically relevant molecular heterojunctions for use in organic electronics.

Introduction Molecular thin films of conjugated organic compounds have attracted significant attention in recent decades because of their applicability in novel electronic devices including organic fieldeffect transistors (OFETs), organic light-emitting diodes, and photovoltaic devices.1–3 The possibility to alter the electronic and structural properties of a certain compound through chemical substitution is regarded as the key advantage of organic compounds over inorganic semiconductors.4 Pentacene (P) (C22H14) is one of the most thoroughly investigated conjugated organic molecules with a high application potential because of the hole mobility in OFETs of up to 5.5 cm2 V-1 s-1 (almost comparable to amorphous silicon).3,5 However, the electron mobility of P reported so far6 is only 0.04 cm2 V-1 s-1. Thus, to achieve ambipolar behavior in OFETs, one promising approach is combining P with compounds of high electron mobility (i.e., n-type organic semiconductors) such as fullerene (C60)7,8 or perfluoropentacene (PFP) (C22F14).9,10 The latter, with a maximum electron mobility of 0.22 cm2 V-1 s-1 in OFETs appears to be particularly well suited.9–11 The fundamental electronic properties and the crystallographic structure of PFP have already been * Corresponding author. E-mail: [email protected]. † Humboldt-Universita¨t zu Berlin. ‡ Johannes Kepler Universita¨t Linz. § Institute for Molecular Science.

(1) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (2) Forrest, S. R. Nature 2004, 428, 911. (3) Koch, N. ChemPhysChem 2007, 8, 1438. (4) Anthony, J. Chem. ReV. 2006, 106, 5028. (5) Lee, S.; Koo, B.; Shin, J.; Lee, E.; Park, H.; Kim, H. Appl. Phys. Lett. 2006, 88, 162109. (6) Singh, T. B.; Senkarabacak, P.; Sariciftci, N. S.; Tanda, A.; Lackner, C.; Hagelauer, R.; Horowitz, G. Appl. Phys. Lett. 2006, 89, 033512. (7) Wang, S. D.; Kanai, K.; Ouchi, Y.; Seki, K. Org. Electron. 2006, 7, 457– 464. (8) Cosseddu, P.; Bonfiglio, A.; Salzmann, I.; Rabe, J. P.; Koch, N. Org. Electron. 2008, 9, 191. (9) Inoue, Y.; Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Tokito, S. Jpn. J. Appl. Phys 2005, 44, 3663. (10) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Inoue, Y.; Tokito, S. Mol. Cryst. Liq. Cryst. 2006, 444, 225. (11) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126, 8138.

investigated to some extent.11–14 For potential device applications, both aspects are highly important because charge injection efficiency and transport properties critically depend on the molecular orientation in organic thin films.15–19 Thin films of PFP grow fiber-textured on SiO2 with a (100) lattice spacing (d100) of 15.7 Å, which is notably larger than the value of 15.5 Å determined for a single crystal by x-ray diffraction.11,13 This suggests the existence of a PFP thin film phase on SiO2 with yet unknown lattice parameters. An analogous growth behavior has been observed for P, where several polymorphs have been observed, including a long-debated thin film phase that was successfully solved by x-ray diffraction only recently.20–22 In addition, the structural properties of devices based on a combination of P and PFP in layered and blended structures, reported to exhibit ambipolar behavior in OFETs,9 are still unexplored. In this study, we applied x-ray diffraction reciprocal space mapping (RSM)23–25 using synchrotron radiation to derive the lattice parameters of the PFP thin film phase. This highly surface(12) Koch, N.; Vollmer, A.; Duhm, S.; Sakamoto, Y.; Suzuki, T. AdV. Mater. 2007, 19, 112. (13) Hinderhofer, A.; Heinemeyer, U.; Gerlach, A.; Kowarik, S.; Jacobs, R.; Sakamoto, Y.; Suzuki, T.; Schreiber, F. J. Chem. Phys. 2007, 127, 194705. (14) Kowarik, S.; Gerlach, A.; Hinderhofer, A.; Milita, S.; Borgatti, F.; Zontone, F.; Suzuki, T.; Biscarini, F.; Schreiber, F. Phys. Status Solidi 2008, 2, 120. (15) Chen, X.; Lovinger, A.; Bao, Z.; Sapjeta, J. Chem. Mater. 2001, 13, 1341. (16) Sreearunothai, P.; Morteani, A. C.; Avilov, I.; Cornil, J.; Beljonne, D.; Friend, R. H.; Phillips, R. T.; Silva, C.; Herz, L. M. Phys. ReV. Lett. 2006, 96, 117403. (17) Koch, N.; Elschner, A.; Schwartz, J.; Kahn, A. Appl. Phys. Lett. 2003, 82, 2281. (18) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (19) Duhm, S.; Heimel, G.; Salzmann, I.; Glowatzki, H.; Vollmer, A.; Johnson, R. L.; Vollmer, A.; Rabe, J. P.; Koch, N. Nat. Mater. 2008, 7, 326. (20) Schiefer, S.; Huth, M.; Dobrinevski, A.; Nickel, B. J. Am. Chem. Soc. 2007, 129, 10316. (21) Yoshida, H.; Inaba, K.; Sato, N. Appl. Phys. Lett. 2007, 90, 181930. (22) Nabok, D.; Puschnig, P.; Ambrosch-Draxl, C.; Werzer, O.; Resel, R.; Smilgies, D. M. Phys. ReV. B 2007, 76, 235322. (23) Smilgies, D. M.; Blasini, D. R. J. Appl. Cryst. 2007, 40, 716. (24) Lengyel, O.; Haber, T.; Werzer, O.; Hardeman, W.; de Leeuw, D. M.; Wondergem, H. J.; Resel, R. J. Appl. Cryst. 2007, 40, 580. (25) Yoon, J.; Choi, S.; Jin, S.; Jin, K. S.; Heo, K.; Ree, M. J. Appl. Cryst. 2007, 40, s669.

10.1021/la800606h CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

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sensitive technique allows to solve crystal structures of thin films while minimizing radiation damage issues.23 Moreover, using specular and grazing incidence x-ray diffraction (GIXD), we investigated (i) a PFP on P bilayer structure of ca. three monolayer thickness to explore the growth behavior in the region of the organic-organic interface as well as (ii) a P:PFP (1:1) codeposited film in order to investigate the potential of such blended films as a bulk heterojunction for organic photovoltaic devices. Complementary investigations were performed with atomic force microscopy (AFM) and Fourier-transform infrared spectroscopy (FTIR); both methods have been helpful in exploring polymorphism and phase separation in organic thin films.26–28

Experimental Details Pentacene (Fluka, purum grade 99.9%) and perfluoropentacene11 (purity 99%) were used without further purification. Films were obtained by vacuum deposition (substrate at room temperature, base pressure 3 × 10 -7 mbar, 10 Å/min deposition rate) from resistively heated ceramic crucibles. The film mass thickness ( χ) was measured in situ by a quartz microbalance; the error in the χ measurement was determined to be below 10% via AFM investigations of P submonolayer covered samples. Substrates were (100) p-doped silicon wafers (Siegert Consulting, prime grade) with a native oxide layer cut into 10 × 10 mm2 coupons. The root mean square (rms) roughness of the substrates was determined to be below 2 Å by AFM. The substrates were used as received, and the cleanliness was confirmed prior to film deposition by AFM. X-ray diffraction (XRD) measurements were performed at the W1 beamline at the synchrotron radiation source HASYLAB (Hamburg, Germany); the wavelength was set to λ ) 1.1808 Å. The upper limit of instrumental broadening of the setup was estimated from the 2Θ width of the (111) reflection of a Ag(111) single crystal to 0.0382 ( 0.0003°; line profiles were fitted using pseudo-Voigt functions. The GIXD measurements were performed at an angle of incidence of the primary beam (Ri) of 0.15°; the detector angle (Rf) was set to 0.5°. The RSM measurement was performed in pseudo-z-axis geometry23 using a scintillating point detector with square entry slits set to a 1 mm side length. The angle of incidence Ri was set to 0.15°, and the correction of the deviation of the diffractometer and sample coordinate systems was performed ex situ via data treatment; the simulation of the peak positions and the visualization were performed on the basis of STEREOPOLE29 using IDL (interactive data language) of ITT Visual Information Solutions. The reciprocal space map is composed of 120 longitudinal scans (450 points each). AFM investigations were done with a Veeco Nanoscope III in tapping mode. FTIR measurements (resolution 2.0 cm-1, near-normal transmission geometry) were performed with a Bruker IFS-66v spectrometer; the reference scan was done on a SiO2 substrate covered with PFP powder.

Results and Discussion Structure: XRD Measurements. Thin films of pure P, PFP, and co-deposited P:PFP (total χ ) 300 Å) as well as a thin layered film of PFP on P ( χ ) 50 Å P + 50 Å PFP) were investigated by specular x-ray diffraction and GIXD; the results are summarized in Figure 1. The pure P film was in the P thin film phase and exhibited (001) fiber texture (d001 ) 15.45 ( 0.05 Å). In addition, weak contributions of (1-10) and (022) orientations of the P bulk phase30 were observed in the specular (26) Salzmann, I.; Opitz, R.; Rogaschewski, S.; Rabe, J. P.; Koch, N. Phys. ReV. B 2007, 75, 174108. (27) Koch, N.; Vollmer, A.; Salzmann, I.; Nickel, B.; Weiss, H.; Rabe, J. P. Phys. ReV. Lett. 2006, 96, 156803. (28) Salzmann, I.; Duhm, S.; Opitz, R.; Rabe, J. P.; Koch, N. Appl. Phys. Lett. 2007, 91, 051919. (29) Salzmann, I.; Resel, R. J. Appl. Cryst. 2004, 37, 1029. (30) Mattheus, C. C.; Dros, A. B.; Baas, J.; Oostergetel, G. T.; Meetsma, A.; de Boer, J. L.; Palstra, T. T. M. Synth. Met. 2003, 138, 475.

Figure 1. Specular (a) and GIXD (b) scans of P, PFP, and co-deposited P/PFP films of χ ) 300 Å and of a 50 + 50 Å thick layered PFP:P film. The peaks are labeled with the respective indices; qz and qxy denote the perpendicular and lateral momentum transfer with respect to the substrate plane, respectively. The inset in part (a) shows the result of a Williamson-Hall analysis of the specular peaks series, and β/ and d/ denote the 2Θ integral peak breadth and the lattice spacing, respectively, expressed in reciprocal units.

scan, which agrees with previous studies.26,31–34 The pure PFP film exhibited a similarly textured growth behavior, however, with a larger lattice spacing of d100 ) 15.75 ( 0.05 Å, which deviates from the respective value in the PFP single-crystal structure.9,13 These data clearly reveal that thin PFP films on SiO2 grow in a crystal structure that is different from that of the PFP single crystal (i.e., in a substrate-induced PFP thin film phase). To determine this unknown structure, an x-ray diffraction reciprocal space map study was performed; the results are shown in Figure 2 together with a simulated pattern. The crystal structure was found to be monoclinic with unit cell parameters a ) 15.76 ( 0.02 Å, b ) 4.51 ( 0.02 Å, c ) 11.48 ( 0.02 Å, and β ) 90.4 ( 0.1°. The most significant deviation from the singlecrystal structure is the 0.25 Å elongated unit cell axis a as well as the reduced monoclinic angle, which is only slightly different from 90°.55 The cell volume is V ) 816.0 Å3 and therefore (31) Oehzelt, M.; Resel, R.; Suess, C.; Friedlein, R.; Salaneck, W. R. J. Chem. Phys. 2006, 124, 054711. (32) Bouchoms, I. P. M.; Schoonveld, W. A.; Vrijmoeth, J.; Klapwijk, T. M. Synth. Met. 1999, 104, 175. (33) Minakata, T.; Imai, H.; Ozaki, M.; Saco, K. J. Appl. Phys. 1992, 72, 5220. (34) Fritz, S.; Martin, S.; Frisbie, C.; Ward, M.; Toney, M. J. Am. Chem. Soc. 2004, 126, 4084.

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Figure 3. FTIR spectra of a P, PFP, a co-deposited P/PFP film, and a layered PFP:P film in comparison to a PFP reference. Vertical lines mark the peaks of the pure P and PFP films.

Figure 2. Experimental (bottom) result of an RSM investigation on a 300 Å thick PFP film on SiO2 below a simulated pattern (top); q| and q⊥ denote the in-plane and out-of-plane components of momentum transfer q. The series of different indices h and equal indices k, (l are found to be vertically aligned along q⊥. The split between peaks with indices (l stems from the monoclinic unit cell angle β being slightly off 90°; the respective peak with negative index l appears at lower q⊥. The experimental map was divided into three parts because different scaling had to be applied as a result of the high background at low values of momentum transfer.

slightly larger than in the case of the single-crystal structure. Hence, by analogy to the PFP single-crystal structure, we assume space group P21/c and two nonequivalent molecules per unit cell (Z ) 2) for the PFP thin film phase. These changes from bulk to thin film phase are analogous to the P case. The specular scan of the co-deposited P:PFP film (Figure 1a) also shows a series of Bragg reflections up to fourth order with a lattice spacing of d001 ) 15.95 ( 0.05 Å as well as a pronounced peak at qz ) 0.945 ( 0.007 Å-1 momentum transfer (marked with a star in Figure 1a), which corresponds to a lattice spacing of 6.65 ( 0.05 Å and exhibits an out-of-plane crystalline coherence length (D) of 130 ( 13 Å (as estimated by the Scherrer formula35). The observed d value cannot be explained by any known crystal structure of P or PFP, hence it cannot be decided whether it stems from an additional distinct polymorph or a different orientation of the specific polymorph, which yields the (100) series.56 The microstructure of the pure P, PFP, and the P:PFP codeposited films was investigated with Williamson-Hall analysis (WHA);28,36,37 the results are shown in the inset of Figure 1. The total integral breadth (β2Θ), (i.e., the ratio of peak area to height) (35) Scherrer, P. Nachr. Ges. Wiss. Go¨ttingen 1918, 98. (36) Williamson, G. K.; Hall, W. H. Acta Metall. 1953, 1, 22. (37) Oehzelt, M.; Koller, G.; Ivanco, J.; Berkebile, S.; Haber, T.; Resel, R.; Netzer, F.; Ramsey, M. AdV. Mater. 2006, 18, 2466.

is attributed to (i) diffraction-order-independent broadening (βS) due to the average finite size of the crystallites, (ii) orderdependent broadening (βD) due to strain fields introduced by dislocations (microstrains), and (iii) instrumental broadening. / With WHA, βS can be separated from βD by a plot of β2Θ ) βS/ + βD/ ) 1/〈L〉V + 2ed/, where 〈L〉V denotes the volume-weighted average thickness of the crystallites along a certain direction [hkl] and e denotes the maximum (upper limit) strain, which is proportional to the distortion of the crystal lattice;38–40 parameters marked with * are expressed in reciprocal units.57 From WHA, values of 312 ( 9, 216 ( 8, and 219 ( 17 Å were observed for 〈L〉V of the P, PFP and P:PFP films, respectively. The film of PFP shows 5 times more strain (e ) 4.4 × 10-4) than the pure P film, and the co-deposited film shows even 10 times more. From these findings, it can be concluded that the PFP film (prepared under the same conditions as the P film) contains a significantly larger number of defects that leads to the elevated microstrain compared to the P film. The reduced value of 〈L〉V at equal χ in case of PFP also points to a reduced crystalline quality of the film. The codeposited P:PFP film shows the same 〈L〉V value as the pure PFP film, however, with a dramatically increased value of the microstrain. Together with the increased lattice spacing determined from the specular scan, this finding of severely disturbed crystal growth can be seen as evidence for intercalated growth of P and PFP (i.e., the growth of a mixed-crystal structure in case of co-deposition). This assumption is, in fact, corroborated by GIXD (Figure 1) as well as FTIR (Figure 3), where the mixed film exhibited a severely altered spectrum with no detectable features of the pure films. As the thin layered sample of nominally 50 Å PFP on 50 Å, P shows thickness oscillations (Kiessig fringes41) corresponding to a layer thickness of 52 ( 5 Å and a series of three Bragg peaks that correspond to a lattice spacing of d100 ) 16.2 ( 0.2 Å. This demonstrates that the thin PFP film on the P underlayer grows with a more upright molecular arrangement than in case of the thick PFP film on SiO2. Similar growth behavior has been reported for ultrathin P layers on SiO2.34,42 It is well known that P films (38) Langford, J. I. Accuracy in Powder Diffraction II; National Institute of Standards and Technology Special Publication; National Institute of Standards and Technology: Gaithersburg, MD, 1992; Vol. 846. (39) Snyder, R.; Fiala, J.; Bunge, H. J. Defect and Microstructure Analysis by Diffraction; Oxford University Press: New York, 1999. (40) Birkholz, M. Thin Film Analysis by X-Ray Scattering; Wiley-VCH: Weinheim, Germany, 2006. (41) Kiessig, H. Ann. Phys.-Berlin 1931, 402, 715. (42) Wang, S. D.; Dong, X.; Lee, C. S.; Lee, S. T. J. Phys. Chem. B 2005, 109, 9892.

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follow the Stranski-Krastanov growth mode on SiO2, characterized by islands growing on closed underlayers.42,43 Specular x-ray diffraction on a reference sample of the P underlayer yielded Kiessig fringes that correspond to a slightly lower layer thickness of 40 ( 5 Å (not shown). Therefore, we assume that the P underlayer dominates the thickness oscillations with a PFP top layer dominantly forming islands.58 GIXD investigations were performed on the same set of samples to investigate the in-plane structure of the films; the results are shown in Figure 1b. The P film exclusively shows reflections originating from the thin film phase, and the peak positions agree with the calculated values. The GIXD spectrum of the pure PFP film is similar to a calculated powder pattern of the single-crystal structure; however, all measured peaks deviate by an amount of at least one peak half-width from this structure. Therefore, it appears reasonable to assume a similar structure of the PFP thin film phase with analogous indexing, which could be justified by the results of the RSM measurement (Figure 2). The GIXD pattern of the co-deposited P:PFP film shows reflections that can not be explained by any known P or PFP structure. (They do, however, resemble the pattern of the P thin film phase.) This is strong evidence that P and PFP form a mixed crystal structure in the case of co-deposition, which is corroborated by our FTIR results (Figure 3). However, it is not possible to derive the unit cell dimensions from the present data; an RSM investigation in this regard will be the subject of a forthcoming study. The majority of the P-GIXD reflections can also be found in the pattern of the thin layered PFP/P sample, which points to the high crystalline quality of the P underlayer. In addition, the most intense PFP reflection of the pristine film (marked with a square in Figure 1b) also occurs in the spectrum of the layered sample, although it is slightly shifted to higher values of momentum transfer. This corresponds to a decreased d value, which fits the increase in the c/ value deduced from the specular scan if the unit cell volume is expected to be constant. Moreover, the PFP peak is much broader than the vicinal peaks of P, which corresponds to a lower crystalline coherence length in the film plane, as expected in the case of island growth. Morphology: AFM Measurements. To investigate the film morphology, we performed AFM measurements on all investigated samples; representative micrographs are shown in Figure 4. The PFP thin film morphology matches the results of previous studies.9,13 We found pronounced steps with a height (h) of 16 ( 2 Å, and we measured a visible integrated film volume of 9×10-3 µm3 per µm2 area (Figure 4a), which is less than 1/3 of the nominal deposited volume. Therefore, the PFP layer can be considered to be completely closed at this value of χ without significant voids reaching down to the substrate. The co-deposited P:PFP film (Figure 4b) exhibits a morphology that is very different from that of the pure P and PFP films. We found a highly corrugated morphology with a needlelike network and terraced areas (inset in Figure 4b) with h ) 17 ( 2 Å, which we attribute to steps of the (100) series found in the specular x-ray diffraction scan (i.e., nearly upright-standing molecules). The mean height of the needlelike crystallites above the terraced structure was 110 ( 20 Å, which agrees well with the value of D ) 130 ( 10 Å derived for the peak corresponding to d ) 6.65 ( 0.05 Å. On closer inspection, the needlelike crystallites exhibit characteristic angles of 132 ( 10° and even a zigzag shape at several positions. Together with the low lattice spacing, this is evidence of the long molecular axes being close to parallel to the substrate

in this morphology. It points to domain boundaries between symmetry-equivalent domains at the vertices, as also found for the morphology of lying R-sexithiophene.44 The terraced structure possibly grows substrate-induced up to a certain critical thickness at which the film grows, preferentially forming the needlelike structure. (Note that significant strain is present in the film.) This hypothesis, however, needs to be further investigated for films with various χ values. The morphology of the layered PFP/P structure is shown in Figure 4d, and the P underlayer is depicted in Figure 4c. PFP on P exhibits a terraced morphology very similar to that of PFP grown on SiO2. The analysis of the film volume above the height level marked with an arrow in Figure 4d yields a volume of 3 × 10-3 µm3, which is evidence that at least one layer of PFP is fully closed under the islands. Note that the thickness oscillations found in the specular scan in Figure 1a corresponding to a layer thickness of 52 ( 5 Å cannot stem from P alone, for which we found 40 ( 5 Å. It equals χ for the P film, although the AFM micrographs (Figure 4c) proved the growth of islands. This indicates that PFP forms a closed interface on the thin P layer (i.e., a wetting of the underlayer that gives rise to the thickness oscillations corresponding to an increased layer thickness). This finding is highly relevant for device applications based on bilayer structures because it points towards maximum contact area at the PFP/P interface. Vibrational Spectroscopy Measurements: FTIR. In addition, all samples were investigated by FTIR because vibrational spectroscopy is very sensitive to changes in the molecular environment,45–47 (i.e., the crystallization in different polymorphs and the molecular intercalation in case of mixed crystal structures). A characteristic part of the FTIR spectra is shown in Figure 3

(43) Ruiz, R.; Nickel, B.; Koch, N.; Feldman, L. C.; Haglund, R. F.; Kahn, A.; Scoles, G. Phys. ReV. B 2003, 67, 125406.

(44) Koller, G.; Berkebile, S.; Krenn, J.; Netzer, F.; Oehzelt, M.; Haber, T.; Resel, R.; Ramsey, M. Nano Lett. 2006, 6, 1207.

Figure 4. AFM micrographs of a PFP (a) and a co-deposited P:PFP (b) film of χ ) 300 Å, of a P film of χ ) 50 Å (c), and of a layered PFP/P film of χ ) 50 + 50 Å (d). The images show an area of 3 × 3 µm2, and colors correspond to height levels of a 200 Å range in parts a and b and a 100 Å range in parts c and d. The inset in part b shows a 1 × 1 µm2 enlarged area. The arrow in part b points to a region with steps of various molecular height; in part d, the basis height level for a volume analysis of the PFP layer is marked (see text).

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and is compared to a reference spectrum of PFP powder. In the region of C-H out-of-plane bending and stretching modes,48,49 we found that the most prominent P absorption (903.5 cm-1, peak 1 in Figure 3) in the spectrum of the co-deposited P/PFP film shifted by +2.0 cm-1 with respect to the pure P film. This points to a different molecular environment of P molecules in the P thin film phase and the co-deposited film because this shift is too low to be attributed to strong interactions such as intermolecular charge transfer. Moreover, in a recent study a comparable shift of this vibration of P in its bulk and thin film phase was observed.26 Vibrations of the pure PFP film were found at 920.0, 933.0, 974.5, and 980.0 cm-1 (peaks labeled 2-5 in Figure 3), which we could assign to C-F in-plane stretching modes through comparison with a calculated theoretical spectrum of PFP.50,60 Peaks 2 and 3 are significantly shifted with respect to the PFP powder reference, which can be explained by the structural difference between the PFP thin film phase and the bulk crystal structure. These peaks are also noticeably shifted with respect to the thin PFP film on a P underlayer, which supports our finding of the slightly different layer spacing observed by specular x-ray diffraction. The PFP single-crystal structure exhibits a herringbone arrangement with two inequivalent molecules in the unit cell; the same holds for the PFP thin film structure. Therefore, we suggest that peaks 2-5 are components of a Davydov split peak in both PFP powder and thin film.51–53 For the co-deposited P:PFP film, only two single peaks can be observed in the vicinity of peaks 2 and 4, which is direct evidence of the intercalation of P and PFP in the case of the co-deposited film. Because this film has been shown to be crystalline by x-ray diffraction, this provides evidence of the growth of a mixed (45) Heimel, G.; Somitsch, D.; Knoll, P.; Zojer, E. J. Chem. Phys. 2002, 116, 10921. (46) Heimel, G.; Cai, Q.; Martin, C.; Puschnig, P.; Guha, S.; Graupner, W.; Ambrosch-Draxl, C.; Chandrasekhar, M.; Leising, G. Synth. Met. 2001, 119, 371. (47) Heimel, G.; Puschnig, P.; Cai, Q.; Martin, C.; Zojer, E.; Graupner, W.; Chandrasekhar, M.; Chandrasekhar, H. R.; Ambrosch-Draxl, C.; Leising, G. Synth. Met. 2001, 116, 163. (48) Szczepanski, J.; Wehlburg, C.; Vala, M. Chem. Phys. Lett. 1995, 232, 221. (49) Hudgins, D. M.; Sandford, S. A. J. Phys. Chem. A 1998, 102, 344. (50) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, reVision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (51) Schwoerer, M.; Wolf, H. C. Organic Molecular Solids; Wiley-VCH: Weinheim, Germany, 2006. (52) Snigur, A. V.; Rozenbaum, V. M. Opt. Spectrosc. 2003, 95, 685. (53) Vogel, J.-O.; Salzmann, I.; Opitz, R.; Duhm, S.; Nickel, B.; Rabe, J. P.; Koch, N. J. Phys. Chem. B 2007, 111, 14097.

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crystal of P:PFP with one PFP molecule per unit cell. Moreover, this result indicates that there exist no relevant film portions of pure and crystalline PFP because even at χ ) 50 Å the missing peaks (3 and 5) are very well pronounced in the spectrum of the thin-layer film.

Summary We determined the unit cell parameters of the PFP thin film phase on SiO2 and explored the growth behavior of PFP in heterostructures with P. On a P underlayer, PFP exhibits a slightly larger lattice spacing compared to the PFP thin-film phase and forms a large area interface with P. Importantly, PFP was shown to form a mixed crystal if co-deposited with P with no detectable phase separation. The dependence of the PFP thin film crystal structure on the substrate has direct implications for the application of n-type semiconducting material PFP in thin film transistors. For P, it is already well established that the electron bandwidth depends on the specific crystal polymorph, which directly impacts the charge carrier mobility.54 By analogy, a dependence of the charge-carrier mobility on the specific PFP polymorph can be expected. Therefore, our results demonstrate that polymorphism in PFP has to be taken into account when comparing charge carrier mobility values from different device structures. Acknowledgment. We thank W. Caliebe and O. Seeck (HASYLAB, Hamburg, Germany) for experimental support and S. Kowarik and A. Gerlach (Universita¨t Tu¨bingen, Germany) for fruitful discussions. This work was supported by the Sfb448 (DFG). N.K. acknowledges support from the Emmy NoetherProgram (DFG). Supporting Information Available: Table of calculated indices and respective peak positions of the reciprocal space map in Figure 2. This material is available free of charge via the Internet at http://pubs.acs.org. LA800606H (54) Troisi, A.; Orlandi, G. J. Phys. Chem. B 2005, 109, 1849. (55) Peaks that differ by indices of (l cannot be resolved individually because of the low deviation of β from 90°. However, the oval peak shape and the dependency of its longer diameter on the peak order allows us to determine the split corresponding to a maximum error of ∆β ) 0.1°. (56) In the case of the PFP series in the specular scan, the labels are chosen differently from those in the case of P because the PFP single-crystal structure (most similar to the PFP thin film structure) is defined with the lattice parameter a as the longest edge length (instead of c in case of the P thin film structure). The GIXD spectrum of the P:PFP film resembles the P-GIXD spectrum; therefore, we followed the labeling order of P in this case. / (57) The parameters β2Θ and d are expressed in reciprocal units: β2Θ ) β2Θ cos(θ)/λ, d/ ) 2 sin(θ)/λ. (58) Note that, in contrast, a layered PFP/P film of χ ) 500 + 500 Å shows a simple superposition of the pristine spectra with a PFP lattice spacing equal to the PFP thin film phase (not shown). (59) Using computational image analysis (Veeco NanoScope v5.3,) the observed volume aboVe a certain height level was analyzed and compared to the nominally deposited volume per unit area (µm2) of films with χ ) 300 Å and 50 Å (3 × 10-2 and 5×10-3 µm3, respectively). (60) Calculation was performed with Gaussian 03, reVision C.02 using the modified Perdew-Wang (mPW) exchange functional, the Lee-Yang-Parr correlation functional (LYP), and the 4-31G** basis set (mPWLYP/431G**).