Microstructural Characterization of Organic Heterostructures by

May 23, 2014 - Phone: +49 (0)6421 2822249. ... Codeposited samples of pentacene (PEN) and perfluoropentacene (PFP) on a KCl(100) substrate have been ...
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Microstructural Characterization of Organic Heterostructures by (Transmission) Electron Microscopy Benedikt Haas,†,§ Katharina I. Gries,*,† Tobias Breuer,† Ines Haü sler,‡,∥ Gregor Witte,† and Kerstin Volz† †

Faculty of Physics and Materials Science Center, Philipps-University Marburg, 35032 Marburg, Germany Institute of Physics, Humboldt University of Berlin, 12489 Berlin, Germany § UMR-E CEA/UJF-Grenoble 1, INAC, SP2M, Minatec, 38054 Grenoble, France ∥ Bundesanstalt für Materialforschung und -prüfung, 12205 Berlin, Germany ‡

ABSTRACT: Transmission electron microscopy can be a powerful tool to characterize organic heterostructures, if suitable methods are applied. Here, we present with the example of codeposited films of pentacene (PEN) and perfluoropentacene (PFP) different techniques, which can also be applied to the radiation-sensitive organic materials. The structure and morphology of codeposited films of PEN and PFP on a KCl(100) substrate have been investigated by different (transmission) electron microscopy techniques. When prepared by stoichiometrically equivalent coevaporation of both compounds, the films exhibit an intermixed phase that consists of a 1:1 mixture of the two molecules. Unavoidable excess of one of the two molecules was shown to lead to a spread-out film consisting of the respective molecule, which was verified for the case of PEN excess and whose growth was shown in detail. Interestingly, the film of segregated PEN exhibits a distinctly distinguishable growth from the pure PEN on the same substrate material, forming a 4-fold symmetry aligned with the KCl⟨001⟩ directions. The adaption of an automated nanocrystal orientation and phase mapping technique for the transmission electron microscope, commercially available as the so-called ASTAR system, to these beam-sensitive materials will be discussed as this method not only offers additional functionality for the nanoscale characterization of the films but is inherently advantageous for materials that are prone to beam-induced structural changes.

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INTRODUCTION Organic semiconductors have had a rapid development in materials science throughout the past decades, e.g., as light sources, photovoltaic materials, or flexible electronics. Besides the possibility of inexpensive device fabrication via convenient methods not accessible to inorganic materials (like spin-coating and low process temperatures), the most intriguing characteristics of these materials are connected to the emission and absorption of light. Using organic semiconductors as lightemitting devices was very successful1,2 as can be seen in the commercial application as organic light-emitting diodes (OLEDs) in displays of different kinds (mostly mobile devices at the time). Also, the absorption of light and its conversion into usable energy, i.e., electrical current, works increasingly well.3−5 Heterostructures of organic semiconductors are particularly interesting because their properties are oftentimes beneficial for device applications.6 As of recently, it has been theoretically validated that the characteristics of the interfacial region between donator and acceptor molecules in organic solar cells determines the dissociation of electron−hole pairs and, therefore, is a key parameter for the device efficiency.7,8 This is only one example of the importance of interfacial regions in organic heterostructures that are especially difficult to control and characterize. © XXXX American Chemical Society

Therefore, characterization methods that are highly local and provide information about phase segregation, crystallinity, and element distribution are required to accompany the development of these structures. Despite the interest in and development of these organic materials, the successful direct nanoscale characterization of the structures, most prominently by transmission electron microscopy (TEM), is rarely reported. This is a direct consequence of the pronounced beam damage evoked by the high-energy electron irradiation. Therefore, it is much harder to acquire significant data and the methods of sample preparation and microscope operation are very different from the investigation of robust materials such as metals or inorganic semiconductors. The intermixture of codeposited pentacene (PEN) and perfluoropentacene (PFP) on a molecular level within an arising mixed crystal seems to provide a prototypical material to investigate effects arising from intermolecular coupling.9 This cocrystallized phase can only be obtained in a rather small thermodynamical window but is stable under standard conditions.10−12 Though some efforts have been made to investigate the structure of the cocrystallized material and phase Received: February 26, 2014 Revised: April 15, 2014

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structure and orientation of a nanometer-sized region. These diffractograms can be manipulated later on either by setting virtual apertures or by automatic comparison with simulated diffractograms from crystal files. By acquiring diffraction data via a CCD camera, the strong forward scattering is analyzed with a high signal-to-noise ratio, compared to other detectors and scattering angles (cf. high-angle annular dark-field scanning transmission electron microscopy21). This technique, therefore, seems to be optimal to investigate the sensitive and intricate films formed by molecular solids.

separation of excess material was observed, a number of questions remain, especially as a precise structural analysis of the crystalline phase by X-ray diffraction is hampered due to the complex small-scale segregation.10 The substrate material KCl has been chosen because, in the case of both compounds, it led to epitaxial growth and crystalline films.13,14 Therefore, the combination of PEN and PFP is interesting not only for the investigation of this material but also for the development of organic semiconductors in general to provide a route for characterizing the combination of structure and morphology on a nanoscale. This is of particular interest, because these materials are prone to complicated growth in contrast to the inorganic semiconductors for which a set of transmission electron microscopy methods is well-established. Previous work has investigated the issue of beam damage for PFP and established methods of coping with it.15 It has been shown that, with moderate irradiation intensities, the morphology is nearly unchanged even after extended exposure to the electron beam while the crystal structure is lost rapidly. Moreover, the amorphous state is reached without intermediary beam-induced structures. Interestingly, the critical dose that the material can tolerate until the diffraction intensities have diminished to 1/e (where e is Euler’s constant) of the initial value depends on the dose rate (cf. Kolb et al.16 and references therein). This concept will not be utilized in this article as it depends on material, shape, thickness, electron energy, sample coatings, temperature, and other parameters. The behavior of materials irradiated in the TEM is determined by the concentrations, diffusion, and chemical reactions of the molecule fragments produced by radiolysis. For aromatic molecules, it has been demonstrated that an increase in dose rate enhances the critical dose applicable to the material.17 Therefore, focused beam methods in which the beam is rapidly scanned across the sample, such as scanning transmission electron microscopy (STEM), are advantageous. The problem with STEM is that the beam diameter has to be smaller than the projected distance between atoms to obtain structural information at an atomic scale. For many materials, sufficiently short acquisition times to provide an adequate signal-to-noise ratio are experimentally not accessible. To cope with the pronounced beam sensitivity of the materials during the TEM measurements, the samples were cooled to liquid nitrogen temperatures and carbon-coated to enhance electrical and thermal conductivity and to lower the diffusion of split-off molecule fragments (created by radiolysis during electron irradiation) out of the sample.18 Additionally, the maximum acceleration voltage of the TEMs has been used, which has previously been shown to be advantageous for PFP.15 This is in accordance with the theory that predicts that materials that are prone to radiolysis profit from the lower effective cross section due to the higher velocity of the electrons. A relatively new and promising technique for crystal structure and orientation mapping, used up to now mostly for inorganic materials, is even more powerful for materials whose investigation is limited by degradation: the automated nanocrystal orientation and phase mapping technique developed by Rauch et al.19 and available as software and hardware from NanoMegas20 under the pseudonym ASTAR. Here, a nanometer-sized (almost) parallel beam is scanned across the sample and, for each position, a diffraction pattern is acquired via a high-speed camera. This method yields thousands of diffraction patterns within minutes, each revealing the local

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EXPERIMENTAL SECTION

Films of PFP22 (Kanto Denka Kogoyo Co. LTD, purity ≥ 99%) and PEN (Sigma-Aldrich, purity ≥ 99.9%) were grown under ultra-highvacuum (UHV) conditions by organic molecular beam deposition from alumina crucibles of a resistively heated Knudsen cell at deposition rates of 8 Å/min, monitored by a quartz crystal microbalance. The samples were grown at 343 K onto freshly cleaved and under UHV conditions annealed (001) faces of KCl (Korth Kristalle) with a nominal total thickness of approximately 50 nm for the TEM investigations and 10 nm in the case of atomic force analysis. The film morphology was characterized by atomic force microscopy (AFM, Agilent SPM 5500) operated in tapping mode at ambient conditions and room temperature. AFM tips with resonance frequencies of about 260 kHz, radii of 7 nm, and force constants of 26.1 N/m were used. Electron microscopic investigations were performed using a JEOL JEM-3010 TEM, a JEOL JIB-4610F scanning electron microscope (SEM) (implemented in a SEM/focused ion beam dual beam system) equipped with a Bruker XFlash 5010 energy-dispersive X-ray detector (EDX), and a JEOL JEM-2200FS TEM with an attached ASTAR system. During the TEM measurements, the samples were cooled to liquid nitrogen temperatures and the TEM was operated at the maximum acceleration voltages of 300 and 200 kV, respectively. The samples were prepared for TEM measurements by dissolving the KCl substrate in a droplet of ultrapure water and floating the freestanding molecular film on a copper grid coated with a holey carbon film. For better stability during this transfer and enhanced robustness against electron irradiation, the film was vapor-coated with some nanometers of amorphous carbon.

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RESULTS AND DISCUSSION Figure 1a depicts a secondary electron (SE) SEM image of a PEN:PFP film, still attached to the KCl(100) substrate, viewed under an angle of 55° relative to the substrate normal. Very tall (compared to the nominal thickness) and seemingly arbitrarily shaped islands that will be shown to consist of the PEN:PFP mixed phase coexist with flat regions formed by segregated excess of either PFP or PEN that deviated from the 1:1 ratio10 (horizontal clouds of smear arose from charging effects due to the low conductivity of the substrate). The perspective of the SEM image in combination with the low kinetic energy (3 keV) of the impinging electrons reveals an ordering of the flat region, which will be discussed in detail during the TEM analysis. For the EDX analysis, the film was removed from the substrate by the aforementioned technique. Figure 1b shows an SE SEM image of the PEN:PFP film transferred to the carboncoated TEM grid recorded at normal incidence and at 10 keV primary electron energy. At this electron energy, which is needed to yield sufficient X-ray intensity, the film becomes more transparent. On the left-hand side, the boundary of a hole in the underlying carbon film can be seen, which covers the whole right side. Therefore, the spectra taken at the two points do not include the carbon film, but only the freestanding sample. It can be cleary seen from the corresponding EDX B

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Figure 1. SEM images of (a) the codeposited PEN:PFP on KCl(100) tilted by 55° relative to the probe direction at 3 kV acceleration voltage and (b) the detached film perpendicular to its surface at 10 kV acceleration voltage. Part (c) shows the corresponding EDX spectra from the two indicated measurement points obtained under the same conditions as indicated in (b). For comparison, also an EDX spectrum obtained from pure, freestanding PFP is presented.

Figure 2. TEM overview of (a) the codeposited sample, and (b) defocused image showing the 4-fold ordering of the PEN background. (c) Diffractogram with 4-fold (cf. the colors) overlay of simulated PEN(001) diffraction using the JEMS software package.23 (d) Atomic force micrograph of a codeposited PEN:PFP film with a nominal thickness of 10 nm. The boundaries of the image run along KCl⟨100⟩ directions.

spectra in Figure 1c that the carbon-to-fluorine ratio is very different for the two measurement points. A quantitative evaluation of these data is difficult since carbon contamination occurs during the measurement, changing the carbon-tofluorine ratio. Nevertheless, it becomes clear that the measurement point 1 (PEN:PFP mixed-phase fiber) contains a quite high PFP fraction, whereas the amount of PFP for measurement point 2 (background film) is zero within the error margins (which arise mainly due to the weak signal of the light elements C and F). Figure 1c contains also for comparison an EDX spectrum of a pure, freestanding PFP film that was recorded under similar imaging conditions. Comparing the carbon-to-fluorine ratio of this spectrum with the one of the measurement point 1 reveals for the latter a lower amount of fluorine. We can, therefore, verify that the fibers consist of a mixture of PEN and PFP and that the background solely consists of one molecular species that was in excess of the 1:1 ratio during the growth, in this case, PEN. Figure 2a depicts a TEM overview of the same codeposited sample as shown in Figure 1b. The image looks similar to the SEM image at high electron energy with the thick PFP:PEN fibers, the PEN background, and holes in the underlying carbon film. When strongly defocused, the TEM reveals two 90°-rotated domains of the thin PEN film, as can be seen in Figure 2b. As in strongly defocused operation mode, a direct assignment of the actual image dimensions is not possible, and we provide no scale marker in the figure. The diffraction of the sample shown in Figure 2c also exhibits a 4-fold symmetry of the structure; the superposition of two perpendicular PEN(001) diffractograms together with polycrystalline diffraction signatures of the unoriented fibers consisting of the mixed phase is observed. Figure 2d is an atomic force micrograph of a codeposited PEN:PFP film with a 10 nm nominal thickness. In the background, elongated islands of PEN molecules are clearly visible, which are aligned along the substrate ⟨100⟩ directions

and exhibit monomolecular steps of 1.5 nm in height. By contrast, tall fibers of intermixed PEN and PFP are found, which exhibit heights of up to 100 nm. By combining this information from atomic force microscopy with the structure and morphology information from TEM, the orientation of the PEN molecules relative to the KCl substrate can be determined, either the normal to either the PEN (100) or (010) planes points along KCl⟨100⟩ (ambiguity due to the superposed diffractograms). Because of the low critical dose of the material, it is not possible to determine the texture of the film by conventional selected area diffraction and dark-field imaging. This hampers the determination of which of the two orientations seen in the diffractogram corresponds to the long axis of the flakes. Therefore, these two questions will be answered after the discussion of the PEN polymorph using ASTAR data. Simulated diffractograms (utilizing the JEMS software package23) for the Campbell bulk phase24 and thin-film phase25 overlaid with an experimental diffractogram of the PEN background (seemingly dominated by the diffraction of a single grain) are shown in Figure 3a,b, respectively. These are the two phases that have so far been observed on nonmetallic substrates. They exhibit the same triclinic space group and differ only slightly in unit cell dimensions and angles. Despite the blurry nature of the reflections, both polymorphs do not seem to match well; however, the agreement with the thin-film data is slightly better. Interestingly, this is different from the findings for pure PEN deposited onto KCl(100) under otherwise comparable growth conditions by Kiyomura et al.13 In their analysis, it was found that the pentacene crystallites adopt two different lateral orientations, whereas, in our investigations, only indications for one were found. There, it was claimed that the PEN crystallites C

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Figure 3. Overlay of simulated PEN (a) Campbell bulk and (b) thinfilm phase diffraction over an experimental diffractogram of the PEN(001) background of a codeposited PEN:PFP film on KCl(100). The dot diameter corresponds to the relative intensity of kinematical conditions. The arrows indicate Bragg reflections for which both polymorphs do not match perfectly, although the thin-film phase seems to be in better agreement.

were oriented with the normal of their (120) planes pointing along KCl⟨100⟩ directions (which was said to be the undisturbed growth) and the PEN(100) planes aligned perpendicular to KCl⟨100⟩ (attributed to step edges). The film exhibited a less clearly defined morphology with dendritic growth. They found the thin-film phase with roughly the same dimensions as previously reported for the growth on SiO2/Si by Ruiz et al.26 (later on more precisely determined by Schiefer et al.25). We note that PEN exhibits better wetting of the KCl surface than PFP. Why the pure PEN grows differently from the segregated PEN during the formation of PEN:PFP fibers remains a subject to further research. The ASTAR system was used to clarify details of the PEN film. To obtain optimal results, the following conditions were used: 6 nm step size between successive diffractograms with a beam diameter of about 2 nm and 35 diffractograms per second. The virtual bright-field image, where every diffractogram (100 × 100 measuring points in this case) is represented by the intensity value of the direct beam, can be seen in Figure 4a. Using the automated orientation mapping for the direction perpendicular to the film, Figure 4b was obtained. There, the color-coded stereographic projection inset shows that the almost white image with mostly light shades of pink indicates that the (001) orientation of PEN is ubiquitary with only small tilts toward in between the [100] and the [010] directions and some noise due to the (compared to materials stable under electron irradiation) suboptimal diffractograms. The automatic mapping of the in-plane orientation has not yet been completely satisfactory due to problems with the simulation of diffraction from the oblique unit cells, as the method has yet been predominately used and optimized for orthogonal systems found in most metal alloys. By setting virtual apertures around the reflections in the reciprocal space (indicated by the red circles in the insets of Figure 4c,d) and mapping the intensities inside the apertures, the two perpendicular grains of PEN could be visualized. This analysis shows that, as the features seen in Figure 4a correspond to the morphology, the short unit cell axis (the [100] direction for the thin-film phase) points indeed along the elongation of the film. This growth can be rationalized by the larger contact area of the molecules when arranging in this direction, similar to the growth of PFP15 but less pronounced as the interaction between PEN molecules is weaker and the intermolecular angle smaller (reducing the difference in contact areas for the two directions). When combined with the information from the atomic force micrograph, it can be determined that the short axis of the

Figure 4. Images obtained via ASTAR method from the detached, freestanding film: (a) virtual bright-field, (b) color-coded orientation perpendicular to the film (see inset), and (c, d) virtual dark-field images with two different virtual apertures (cf. inset).

PEN unit cell is oriented along KCl⟨100⟩. Moreover, when comparing Figure 4c,d at the boundary between the two grains, it becomes obvious that regions exist that can be attributed to both orientations, indicating an overlap of the two grains perpendicular to the film. When the TEM is used in a very swift operation mode (where the exposure to the beam is minimized by, e.g., optimizing the imaging conditions in adjacent areas), it is even possible to achieve phase contrast images of organic material. As the fringes observed in these images originate from the relative phase shift of the electron wave function at positions of different crystal potential, i.e., at and in between atomic columns, their periodicity corresponds to atomic planes and thus yields information about them.27 Figure 5 shows phase contrast of a mixed PEN:PFP fiber exhibiting lattice planes running along it. The planes are separated by approximately 13.1 Å, and together with their fundamental orientation relative to the morphology, this large value clearly indicates that it probably corresponds to a lattice vector of the PFP:PEN unit cell. This finding is especially interesting because previous investigations of this material have shown that X-ray reflectivity measurements are not able to determine lattice parameters, because they are hindered by the intricate phase segregation of the film.10 We were not able to acquire diffractograms that can be clearly attributed to the mixed fibers, because the fibers are too narrow to dominate selected area diffraction and too sensitive to electron irradiation to target them directly via nanobeam diffraction. It was not possible to separate crystallographic data of the mixed phase from pure PEN or PFP in normal diffraction mode as no lattice spacings were found that deviated significantly from the ones of the pure molecular solids. D

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 (0)6421 2822249. Fax: +49 (0)6421 2828935. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support of the German Science Foundation (DFG) in the framework of the Heisenberg program (K.V.) and the SFB 1083 and support by the Friedrich-Ebert-Stiftung (T.B.) are gratefully acknowledged.

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Figure 5. High-resolution phase contrast image exhibiting lattice fringes parallel to the orientation of the PFP:PEN fiber. A contrast profile perpendicular to the fiber and integrated along its length (cf. marker) shows lattice planes with a periodicity of about 13.1 Å. The small image indicates that the lattice planes continue to run along the fiber until its tip.

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SUMMARY A variety of electron-microscopy-based investigations of beamsensitive materials, such as molecular solids, have been demonstrated with an emphasis on the potential of the automated nanocrystal orientation and phase mapping technique developed by Rauch et al.19 It could be demonstrated that, with the combined use of SEM, EDX, and conventional and scanning-nanobeam TEM, a detailed analysis of the organic material is achievable with the example of the PEN:PFP mixed system. Besides validating the mixed composition of cocrystallized PEN:PFP fibers, crystallographic information was extracted via phase-contrast methods. The texture of an underlying film made from excess material of the codeposition (in this case PEN) was determined, and the epitaxial relation between molecular solid and the KCl(100) substrate was clarified. Interestingly, the growth of the segregated excess PEN is different (orientation and morphology) from films obtained by pure PEN deposition on the same substrate at the same conditions. Although the arise of a 4-fold-ordered PEN film is observed all along the sample, the reason for this growth during the codeposition of PEN and PFP is unknown. An interaction between the PEN:PFP fibers and the segregated PEN must be the reason for this difference in growth and may be a hint on how to change the growth of organic molecules on specific substrates to an energetically advantageous state. Enduring improvement of the preparation protocol to produce mixed fibers of enhanced dimensionality and further minimization of the beam exposure during analysis may allow closer structural characterization of the mixed phase of PEN and PFP. The role of scanning-nanobeam-based orientation and phase mapping for beam-sensitive materials is stressed, although the software is yet to be optimized for complicated and oblique unit cells. The inherent properties of this method make it ideal for investigating intricate nanoscale growth that is often exhibited by organic materials, as it conserves the integrity of the sample optimally while extracting the maximum amount of information. E

dx.doi.org/10.1021/cg5002896 | Cryst. Growth Des. XXXX, XXX, XXX−XXX