TEM of Sensitive Organic Materials
Imaging and Analysis with Reduced Beam Damage
- TEM of Sensitive Organic Materials - Imaging and Analysis with Reduced Beam Damage
- Fig. 1: TEM overview of the co-deposited sample (A). Defocused image showing the 4-fold ordering of the PEN background (B). Diffractogram with 4-fold overlay of simulated PEN(001) diffraction pattern (C). AFM micrograph of a co-deposited PEN:PFP film (D). The boundaries of the image run along KCl<100> directions.
- Fig. 2: Images obtained via ASTAR method from the detached, freestanding film: virtual bright-field (A), color-coded orientation perpendicular to the film (B), virtual dark-field images (C,D) with two different virtual apertures (cf. inset).
In this article the adaption of a scanning-nanobeam diffraction technique for the transmission electron microscope (ASTAR system) to beam-sensitive organic materials will be discussed. This method not only offers additional functionality for the nanoscale characterization of organic films but is inherently advantageous for materials that are prone to beam-induced structural changes. As example of co-deposited films of pentacene (PEN) and perfluoropentacene (PFP) grown on KCl(100) were used.
Characterizing organic semiconductors by means of electron microscopy is a challenging task as these materials tend to need careful handling from preparation to investigation. In the transmission electron microscopy (TEM) only low doses of electrons can be tolerated before structural damage distorts the measurement or hinders it completely. On the other hand the growth of organic semiconductors is intricate and requires highly localized structural information that only TEM can provide. To meet the needs of research and development of organic semiconductors it is necessary to develop and adapt new techniques to reliably acquire local structural information while reducing beam damage. The adaption of a new method for automated crystal phase and orientation determination by means of scanning nanobeam diffraction to organic semiconductors is very promising as will be shown in the following. This method has been developed by Rauch et al.  and is available as software and hardware by NanoMegas  under the pseudonym ASTAR. As a material system the co-deposited equimolar intermixture of the organic semiconductors pentacene (PEN) and perfluoropentacene (PFP) has been used. These two molecules have been grown on a KCl(100) substrate which leads for both compounds to epitaxial growth and crystalline films.
Films of PFP  (Kanto Denka Kogoyo Co. LTD, purity ≥ 99%) and PEN (Sigma-Aldrich, purity ≥ 99.9%) were grown by organic molecular beam deposition on KCl(100) (Korth Kristalle) substrates.
Atomic force microscopy (AFM, Agilent SPM 5500) was performed in tapping mode.
For TEM a JEOL JEM-3010 TEM and a JEOL JEM-2200FS TEM with an attached ASTAR system were used at 300 and 200 kV, respectively. The samples have been cooled to liquid nitrogen temperatures.
For TEM measurements the samples were prepared by dissolving the KCl substrate in a droplet of ultrapure water and floating the freestanding molecular film on a holey-carbon coated copper grid. Further information can be found in .
Figure 1A depicts a TEM overview of a codeposited PEN:PFP sample. Very tall and seemingly arbitrarily shaped islands coexist with flat regions formed by segregated excess of either PEN or PFP which deviates from an equimolar ratio . At this point we will not cover the islands, which consist of an approximate 1:1 mixture of PEN:PFP , but just discuss the flat regions. When strongly defocused, the TEM reveals two 90°-rotated domains within these regions, as can be seen in figure 1B. In strongly defocused operation mode, a direct assignment of the actual image dimensions is not possible, thus we provide no scale marker in the figure.
Figure 1C shows a diffraction pattern of the sample with the overlay of two simulated diffraction patterns of PEN(001). For simulation the JEMS software package  was used. Clearly, the diffraction of the sample also exhibits a 4-fold symmetry of the structure. The two perpendicular PEN(001) diffractograms are superimposed by polycrystalline diffraction signatures of the unoriented fibers consisting of the mixed phase.
Figure 1D is an AFM micrograph of a co-deposited PEN:PFP film. 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 AFM with the structure and morphology information from TEM, the orientation of the PEN molecules relative to the KCl substrate can be determined, the normal to either the PEN (100) or (010) planes points along KCl<100> (ambiguity due to the superimposed 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. Consequently, it is not possible to determine which of the two orientations identified in the diffractogram corresponds to the long axis of the flakes. Therefore, these two questions will be answered using ASTAR analysis.
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 2A. Using the automated orientation mapping for the direction perpendicular to the film, figure 2B 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  and the  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 2C,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 2A correspond to the morphology, the short unit cell axis (the  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 PFP  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 AFM micrograph, it can be determined that the short axis of the PEN unit cell is oriented along KCl<100>. Moreover, when comparing figure 2C,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.
The role of scanning-nanobeam-based orientation and phase mapping for beam-sensitive materials is accentuated. 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.
Support of the German Science Foundation (DFG) in the framework of the Heisenberg program (Kerstin Volz) and the SFB 1083 and support by the Friedrich-Ebert-Stiftung (Tobias Breuer) are gratefully acknowledged. Reprinted (adapted) with permission from Crystal Growth and Design , Copyright (2014) American Chemical Society.
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Dr. Katharina I. Gries (corresponding author via e-mail request)
Dr. Tobias Breuer
Prof. Dr. Gregor Witte
Prof. Dr. Kerstin Volz
Philipps-University of Marburg
Faculty of Physics and Materials Science Center
Structure and Technology Research Laboratory
Current Address: Benedikt Haas
UMR-E CEA/UJF-Grenoble 1
INAC, SP2M, Minatec
Current Address: Ines Häusler
Bundesanstalt für Materialforschung und -prüfung
Dr. Ines Häusler
Institute of Physics
Humboldt University of Berlin