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Morphology and Structure Transitions of Copper Hexadecafluorophthalocyanine (F16CuPc) Thin Films J. L. Yang, S. Schumann, and T. S. Jones* Department of Chemistry, UniVersity of Warwick, CoVentry, CV4 7AL, United Kingdom ReceiVed: August 28, 2009; ReVised Manuscript ReceiVed: NoVember 23, 2009
The thickness-dependent morphology and structure transitions of pristine and thermally annealed F16CuPc thin films deposited from the vapor phase by organic molecular beam deposition have been studied with use of atomic force microscopy, scanning electron microscopy, and X-ray diffraction. Pristine films show clear morphology transitions with increasing thickness, changing from spherical-like crystals to flexible-fiber-like crystals via standing-up needle-like crystals. Two different roughening processes are identified: kinetic roughening for films 30 nm is not easily explained by conventional mound growth. Recent studies of the growth of metal-free phthalocyanine (H2Pc) and diindenoperylene (DIP) films reported large β values of 1.02 and 0.75, respectively,26,27
Figure 5. XRD of F16CuPc pristine films grown on ITO substrates with thickness D ) 56 nm (a), 75 nm (b), 120 nm (c), and 255 nm (d). The main diffraction peaks show a gradual change in peak shape where new peaks at higher 2θ values emerge with increasing film thickness. The inset shows fitted curves (dotted lines) for the (001) diffraction peaks. The diffraction peak of the (221) plane of the ITO substrate is included for reference.
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and in the case of DIP, the observed rapid roughening was explained by spatial inhomogeneities due to the presence of different tilt domains and grain boundaries. More recent analysis of a range of molecular thin film systems suggests that the large β values arise from the intrinsic anisotropy of the molecules, resulting in different stacking directions between crystallite domains.10 However, the rapid roughening observed for F16CuPc thin films is less straightforward to rationalize because of the curious transition in morphology that accompanies the change in growth mechanism. There are two possible reasons: (i) the growth induced film strain relaxes gradually as the film thickness increases, and thus the diffusion process and orientation of the F16CuPc molecules should change; (ii) it is the intrinsic characteristics of the F16CuPc molecular crystals, in particular an apparent flexibility, which manifest themselves in the bending of the elongated crystals and formation of the flexible-fiberlike crystals without breaking (Figure 2e,f). XRD was also used to follow the change of thin film structure with increased thickness (Figure 5). The (001) diffraction peak changes and a new peak emerges at a slightly larger 2θ value as D increases up to 120 nm (inset in Figure 5). This suggests that the molecules in the stand-up configuration and the outof-plane structure adopt a similar packing arrangement with the molecular layer spacing decreasing very gradually. Similar results were observed for F16CuPc films formed by WEG.23 However, thicker films, e.g. D ) 255 nm, exhibit a much broader diffraction peak with the peak width at half-height (PWHH) of about 0.5°. This results from the formation of a crystal film with the two morphology layers (Figure 3). For D < 120 nm, the S-layer dominates the XRD measurements and shows a decrease in molecular layer spacing. For thicker films, the S-layer and F-layer both contribute to the XRD, the more flexible-fiber crystals resulting in a greater distribution of layer spacings and the resulting much broader XRD peak. 3.2. Effects of Thermal Annealing. Thermal annealing is an effective way of controlling thin film morphology and structure, and is often used to improve the performance of organic semiconductor devices. When compared with pristine F16CuPc films, thermal annealing leads to very significant changes in both morphology and structure (Figure 6). For a constant annealing time of 2 h at 265 °C, large crystals with heights of ca. 80 nm were formed for relatively thin films (Figure 6, panels a, c, and d, for D ) 30 nm), while thicker films show a transition to larger-sized crystals which coexist with the spherical crystals (Figure 6b). The flexible-fiber-like crystals disappear after annealing. XRD measurements (Figure 6e) show a shift in the main diffraction peak after annealing, from 2θ ) 6.10° (dplane ) 14.47 Å) to 2θ ) 6.31° (dplane ) 13.99 Å), indexed as the (001) and (001)′ planes, respectively. There is also a weak shoulder associated with the (001)′ diffraction peak at low 2θ values indicating a transition of the thin film structure, consistent with the morphology seen in Figure 6b. The (001)′ diffraction peak is much sharper, and a second diffraction peak labeled (002)′ (2θ ) 12.64°, dplane ) 6.99 Å) also emerges. These results suggest that the crystal quality and size both improve significantly after annealing. The effects of different annealing times on the film structure and morphology can be seen in Figure 7, which shows SEM images of 48 nm films after annealing at 265 °C for (a) 1, (b) 2, and (c) 3 h. There is a clear transition from spherical crystals to large-size crystals, and after 2 h of annealing the spherical crystals have almost disappeared. Most of the crystal clusters transform into large-size crystals, with some also disappearing due to evaporation, a trend that continues with prolonged
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Figure 6. Morphology of F16CuPc films after thermal annealing at 265 °C for 2 h: (a, b) SEM images of 30 and 120 nm films; (c) AFM image and (d) corresponding height cross-section of a 30 nm film; (e) XRD data for a 120 nm film before and after annealing: the inset shows fitted curves for the (001) diffraction peak of the annealed films. The (221) ITO diffraction peak is included for reference.
annealing times. The XRD results in Figure 7d confirm this observation. The main (001) diffraction shifts gradually to higher 2θ values from 5.95° (dplane ) 14.84 Å) to 6.21° (dplane ) 14.22 Å), with no diffraction peaks seen in the film produced after 3 h of annealing due to significant evaporation and material loss. The formation of new large crystals involves the creation of new nuclei, which are small molecular aggregates released from the pristine films by the annealing process, and subsequent molecular reorganization around the nuclei.14,28 After nucleation, the crystals first grow by attachment of molecules from the nearest pristine crystals until the area around them is vacant. Crystal growth then proceeds through an additional pathway similar to Ostwald ripening, where there is no direct mass transport to the new crystals from the pristine crystals and the driving force is the elimination of unstable interfaces. The Gibbs-Thomson equation, ai) a∞e2γiΩ/RkT, forms the basis for the classical theory of Ostwald ripening, where ai is the
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Figure 7. SEM images of a 48 nm F16CuPc film after annealing at 265 °C for (a) 1, (b) 2, and (c) 3 h. (d) Corresponding XRD data for the films (black a, no annealing; blue b, 1 h annealing; red c, 2 h annealing; green d, 3 h annealing).
equilibrium vapor concentration, R is the cluster radius, γ is the energy per unit area of the cluster surfaces, k is Boltzmann’s constant, T is the temperature, and the factor a∞ represents the equilibrium vapor concentration above the flat surface of the condensed phase.29 It is derived by assuming that the molecular cluster is in equilibrium with the surrounding vapor and it states that the concentration of vapor is exponentially higher around small molecular clusters. Consequently, smaller clusters or crystals will evaporate very easily and larger ones will attract evaporated molecules from different sized clusters and crystals in the surrounding area. Thermal annealing is therefore advantageous from a thermodynamic perspective to convert pristine films of small crystals into films with much larger crystals. It is also interesting to note that the large-size single crystals bend easily under irradiation of the electron beam during imaging in the SEM (Figure 8). The linear single crystals formed after thermal annealing change to a helical appearance after irradiation, with adjacent crystals becoming interlinked. This suggests that the annealed F16CuPc crystal films are very flexible, similar to previous reports of CuPc single crystals and F16CuPc nanoribbons fabricated by organic physical vapor transport.30,31 The theory of the elastic properties of anisotropic
nanoribbons suggests that the formation of nanohelices and twisted nanoribbons is energetically favorable when compared to a flat geometry for certain conditions, the actual process being a competition between the elastic energy, surface-polarizationinduced energy, the volume energy, and defect-induced energy.32,33 The flexible characteristics of the annealed F16CuPc films, which are composed of large-size crystals, are potentially very attractive for the fabrication of devices on flexible substrates, i.e. plastic electronics. As shown in Figure 5, the (001) diffraction peaks for the pristine F16CuPc films broaden with further shifts occurring after annealing (Figure 6e). This means that the molecular arrangement changes and the (001) interplane separation decreases, with the stacking angle between the molecular plane and the substrate decreasing. A schematic showing the change in molecular arrangement with increasing film thickness and after thermal annealing is shown in Figure 9. These suggest that the molecular stacking in the annealed films is the most stable arrangement with the lowest energy. This is consistent with previous studies of p-type phthalocyanine films, such as CuPc and H2Pc, which can transform from R-phase films with large stacking angles
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Figure 8. SEM images of F16CuPc crystals, formed initially by thermal annealing, after irradiation with the SEM electron beam (10 kV): (a) t + 0 s, (b) t + 52 s, and (c) t + 130 s. The inset in panel c shows an example of a helical crystal.
shown to be thermodynamically favorable. The large crystals in the annealed films are very flexible and appear promising for potential device fabrication on flexible substrates to meet the requirements of plastic electronics. Our studies show that controlled thermal annealing is an excellent means for improving the crystalline properties of F16CuPc thin films, which may further improve device performance. Future studies will focus on investigating charge transport properties and device fabrication. Acknowledgment. This work was funded by the Engineering and Physical Sciences Research Council (EPSRC), UK, through the Basic Technology Project “Molecular Spintronics”. The authors thank Dr. D. Walker for his help with the XRD experiments. References and Notes
Figure 9. Schematic of the F16CuPc molecular stacking in (a) pristine films, (b) pristine films with increased thickness, and (c) films formed after annealing. The layer distance, h, and the angle, R, between the molecular plane and the stacking axis both change in going from panels a-c: R1 > R2 > R3 and h1 > h2 > h3.
and smaller crystals to β-phase films with small stacking angles and large crystals when treated by thermal annealing.34,35 4. Conclusions The morphology and structure transitions of pristine and thermally annealed F16CuPc films have been studied in detail with use of AFM, SEM, and XRD. Pristine F16CuPc films show a clear transition with increasing film thickness from sphericallike crystals to flexible-fiber-like crystal networks via upright needle-like crystals, with at least two different roughening processes involved, i.e. kinetic roughening for film thicknesses
