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Polymorphism in Phthalocyanine Thin Films: Mechanism of the r f β Transition S. Heutz, S. M. Bayliss, R. L. Middleton, G. Rumbles, and T. S. Jones* Centre for Electronic Materials and DeVices and Department of Chemistry, Imperial College of Science, Technology and Medicine, London SW7 2AY, U.K. ReceiVed: January 6, 2000; In Final Form: May 9, 2000
Two different polymorphic forms of free base phthalocyanine films have been grown on glass substrates by ultrahigh vacuum organic molecular beam deposition. Postgrowth annealing of films grown at room temperature leads to transformation from the R to the β1 phase. The effects of annealing lead to a number of transition states whose morphological, structural, and spectroscopic properties can be identified using atomic force and optical interference microscopy, X-ray diffraction, and Raman and electronic absorption spectroscopy. Detailed morphological studies indicate that the transition occurs via a discrete number of nucleations and is preceded by an elongation of the R crystallites. Quantitative analysis of the crystallites and domain size shows that the individual β1 crystals grow but are confined to domains of similar orientation. The film thickness plays a critical role and three regimes have been identified. The R f β1 transformation is only complete for films thicker than ∼940 Å, and thick films lead to a higher degree of orientation and larger domains.
1. Introduction There is considerable interest in the growth of thin films of molecular materials as an alternative to more conventional inorganic technology for applications in electronics and optoelectonics. Phthalocyanine films have been particularly well studied and are found as components in solar cells, field effect transistors,1 and organic light emitting devices (OLEDs), in the last case as a buffer layer at the ITO anode.2 Their structure is highly conjugated and the polyaromatic ring (abbreviated Pc) can be bound to a divalent transition metal (MPc) or to hydrogen (H2Pc).3 The metal-free phthalocyanines (H2Pc) crystallize as a variety of polymorphs, and the two main phases identified as R and β. They are both characterized by a herringbone structure with the molecules stacked along the b-axis, but the polymorphs are differentiated by the angle between the plane of the molecule and the stacking axis.4 The two phases are monoclinic and crystallize in the C2/c and P21/a space groups for the R and β polymorphs, respectively.5,6 The R and β phases can be grown in ultrahigh vacuum by organic molecular beam deposition (OMBD). The R phase is obtained by growth at room temperature on weakly interacting substrates such as glass.7 High-temperature growth or postgrowth annealing of an R film leads to the formation of β films.8 The β-phase is commonly accepted as the stable form above a critical crystal size.9 The two polymorphs differ in terms of their morphological, spectroscopic, and structural properties. The R phase grows as small spherical crystallites, while the β crystals are large and slender.10 The β-crystals can be randomly oriented or highly ordered over a short range (about 100 µm2) for growth at high substrate temperature or annealing of an R film, respectively. The high substrate temperature and annealed film can also be differentiated structurally. These two structures have recently been characterized by powder X-ray diffraction and are labeled β1 and β2, respectively.10,11 The different films have been fully characterized as pure phases, but little is known about the actual transition between * Corresponding author. Tel: +44 (0)20-7594-5794. Fax: +44 (0)207594-5801. E-mail:
[email protected].
the two phases. In this paper, we study the evolution of an R film submitted to annealing for different times by a variety of ex situ techniques. The early stages of the transformation are identified and an elongation of the R crystallites is seen in atomic force microscopy (AFM) studies. The sizes of the crystallites (either R or β1) and of the oriented domains of β1 crystallites have been analyzed quantitatively and used to determine the mechanism of the transition. The influence of the thickness of the film on its morphology has also been studied and three different regimes have been observed. These show that continuity of the films is a critical factor for the completion of the transition. 2. Experimental Section The films were grown in an OMBD chamber with a base pressure of about 2 × 10-9 Torr. The H2Pc powder (SynTech, 99%) was outgassed for 15-20 h before growth and sublimed onto the substrate using a miniature Knudsen effusion cell. The cell temperature was ∼330 °C, which corresponds to a growth rate of ∼5 Å s-1, as determined by a quartz crystal microbalance positioned near the substrate. The film thickness was also calibrated ex situ using scanning electron microscopy measurements on cleaved silicon samples. The substrates were cut from glass microscope slides (BDH super premium) and cleaned thoroughly in a methanol sonic bath for 15 min before being mounted and transferred into the chamber. No in situ cleaning was performed, since this appeared to have no obvious effect on the morphology and spectroscopic characteristics of the deposited films. The R f β1 transition was affected by in situ annealing of samples grown at room temperature. Two series of samples were investigated. First, 2330 Å thick films were submitted to an annealing at 325 °C for various times, up to a maximum of 2 h. Second, the annealing temperature and time were kept constant (325 °C for 2 h), but the thickness of the initial R film was varied. Five thicknesses have been considered: 560, 660, 940, 2330, and 4810 Å. Ex situ morphology analyses were performed using a Nomar-
10.1021/jp0000836 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/08/2000
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Figure 1. Powder XRD 2θ scans recorded from 2330 Å thick H2Pc films after annealing for (a) 0, (b) 0.75, (c) 1.0, (d) 1.25, and (e) 1.5 h. The last two scans have been scaled by a factor of 20. The transition from R to β1 characteristics is very abrupt and occurs between 1 and 1.25 h annealing.
ski interference optical microscope (Olympus BH2-UMA) and an AFM (Burleigh instruments) operated in the tapping (noncontact) mode. The electronic properties of the films were assessed using a Unicam UV4 spectrometer and a Siemens D5000 powder diffractometer was used to record 2θ scans. Raman spectra were also obtained by excitation with a 632.8 nm laser source and detection by a microscope, spectrograph and CCD detection system (ISA-Horiba infinity). 3. Results and Discussion The film properties were probed using powder X-ray diffraction and electronic absorption spectroscopy. Each data point for a different annealing time represents a different sample. It was observed that gradual sublimation of the films occurred with increased annealing time; for example, annealing for 3 h resulted in complete sublimation of the material from the substrate. XRD 2θ scans are shown in Figure 1, for (a) a reference R film, and samples annealed for (b) 0.75, (c) 1.0, (d) 1.25, and (e) 1.5 h. It should be noted that the intensities of the samples annealed for 1.25 and 1.5 h have been scaled by a factor of 20 for the sake of clarity. Films annealed up to 1 h (a-c) show peaks characteristic of the R phase, with diffraction from the (200) and (400) planes giving 2θ values at 6.82 and 13.6°, respectively.12 Longer annealing (d,e) results in a diffractogram typical of the β1 phase, where the peaks due to diffraction from the (001), (002) and (41h3h) planes appear at 2θ ) 7.0, 14.1, and 15.5°.13 The transition is abrupt, with no intermediate state containing diffraction signals typical of both polymorphs. The fwhm of the diffraction peaks do not change with increased annealing time, indicating little or no growth of the crystal size for either the R or β1 form. The same abrupt transition is also seen in electronic absorption spectroscopy. The spectra of the samples submitted to different annealing times are shown in Figure 2; (a) no anneal and (b) 0.75, (c) 1.0, (d) 1.25, (e) 1.5, and (f) 2.0 h annealing. A rather sharp transition from R character to β character is again observed between 1 and 1.25 h annealing. Samples that are annealed for 1 h or less (a-c) show absorption features characteristic of the R phase, with a peak at 630 nm and a high-
Figure 2. Electronic absorption spectra recorded from 2330 Å thick H2Pc films after annealing for (a) 0, (b) 0.75, (c) 1.0, (d) 1.25, (e) 1.5, and (f) 2.0 h. The R features are retained for up to 1 h annealing, and films annealed for 1.25 h or more have β1 characteristics. Spectrum f is less intense due to partial sublimation of the film.
intensity shoulder at about 690 nm. Samples annealed for longer times (d-f) show characteristics of the β1 phase, with distinct peaks at 650 and 710 nm. There is some evidence that the relative intensities of the two bands in the β1 phase spectrum are not the same. The broad higher wavelength Q-band (710 nm) becomes slightly more intense with prolonged annealing with respect to the equally broad, lower wavelength Q-band (650 nm). A similar phenomenon has been observed by Loutfy14 and has been explained in terms of the size of the crystallites, with larger crystals having more intense bands at higher wavelength. This assumption will be confirmed by the AFM analysis presented in this paper. Resonance Raman spectra were also recorded for the films after different annealing times. Here, the focused laser meant that a 1 µm2 area of the sample was measured and spectra were recorded from 8 to 10 different points of the surface. The Raman spectra presented in Figure 3 show a representative spectrum for each sample. Typical spectra of the pure β1 and R phase are shown as a reference in (a) and (b), respectively. The regions that allow for distinction between the two phases are shaded in gray. In the 600-800 cm-1 region, corresponding to the vibrations of the macrocycle,15 the high-energy peak at 791 cm-1 is more intense than the low-energy one at 677 cm-1 for the β1 phase and this intensity ratio is reversed in the R phase. The transitions around 1500 cm-1 are present as a doublet in the R phase and a triplet in the β phase and correspond to vibrations of the isoindole modified by the macrocycle. After annealing for 0.75 h, the Raman spectra showed that the film was still R across the whole surface (c). Annealing for 1 h leads to a mixture of the R and β1 phase coexisting on the surface (d,e). This observation shows the advantage of a microscopic probe such as Raman over the “bulk” film measurement techniques (XRD and electronic absorption spectroscopy), which were not sensitive to the onset of β1 phase formation. The ratio of R phase spectra to β1 phase spectra after 1 h annealing is approximately
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Figure 5. Nomarski micrographs of (a) an R H2Pc film and films annealed for (b) 1.0, (c) 1.25, and (d) 2.0 h. The film thickness was 2330 Å.
Figure 3. Resonance Raman spectra of 2330 Å H2Pc films acquired with a 632.8 nm laser source. (a) and (b) are reference spectra for β1 and R films, respectively. Different annealing times are shown: (c) 0.75 h, (d) 1.0 h, R phase, (e) 1.0 h, β1 phase, (f) 1.25 h, (g) 1.5 h, and (h) 2.0 h. The ratio of R (d) to β1 (e) for the film annealed for 1 h is approximately 5:2.
Figure 4. AFM images of 2330 Å thick H2Pc films as a function of annealing time: (a) no anneal, (b) 1.0 h, R phase, (c) 1.0 h, β1 phase (minor component), (d) 1.25 h and (e) 2.0 h.
5:2, indicating that the surface is still predominantly R phase, as previously observed. Annealing for 1.25, 1.5, and 2 h (f-h) leads to spectra characteristic of the β1 phase only with the appearance of a new peak at 1525 cm-1, the intensity of which increases with annealing time. The morphology of the samples annealed for different times was assessed using AFM (Figure 4). A film grown at room
temperature with no annealing is shown as a reference in (a) and displays the high density of spherical islands that characterizes the R phase.10 The first changes become apparent after annealing for 0.75 h. The underlying surface morphology is similar to that seen for growth at room temperature, but closer inspection reveals some elongation of the spheres and the presence of several larger islands on the surface, with a typical diameter of 0.2-0.3 µm. Elongation of the R spheres becomes more pronounced after annealing for 1 h, (b). This elongation prior to transformation has been reported in a previous electron microscopy study of CuPc films on muscovite.14 Larger islands are also present on the surface, although these have not increased in size and number (number density of islands = 4 µm-1). A small proportion of the sample displays regions covered in oriented slender crystallites (c). These areas are at least 49 µm,2 the typical area of an AFM image, and are characteristic of a pure β1 phase film.10 The organized growth of the β1 phase starts therefore after 1 h annealing, consistent with the Raman spectra shown in Figure 3. After 1.25 h annealing the surface is completely covered with the long thin crystals of the β1 phase. The crystallites are parallel to each other over large areas, and a boundary between two domains of different orientations can be seen in (d). Annealing for longer times leads to no appreciable change in the surface morphology until after 2 h (e). At this point, the appearance of smooth areas between remaining islands of β1 crystals indicates that the β1 phase crystals have grown together and there only remain a few well-defined thin β1 crystallites, mostly arranged in columns. However, the domain boundaries are still present and there are some features reminiscent of the oriented crystallites in the smooth areas, indicating that these have been formed by the merging of the slender β1 crystallites. Nucleation and growth of the β1 phase with prolonged annealing can also be seen morphologically on a larger scale using optical interference microscopy, as shown in Figure 5. The first appearance of distinctive features occurs after annealing for 1 h (b), where isolated islands are seen superimposed on the smooth background that characterizes the R phase (a).10 These islands are thought to be nucleation sites of domains of β1 phase crystals, corresponding to the areas that have β1 characteristics in Raman (Figure 3e) and that display oriented slender crystallites in AFM (Figure 4c). Annealing for 1.25 h results in complete coverage of the surface with the irregular
Polymorphism in Phthalocyanine Thin Films
Figure 6. Mean crystallite size, as obtained by AFM, as a function of annealing time for 2330 Å thick H2Pc films. The R spheres and islands become slightly larger with annealing time, but the effect is particularly pronounced for the β1 crystallites, which grow from 0.10 to 0.51 µm2 when the annealing time is increased from 1.0 to 2.0 h.
shaped domains of the β1 phase (c). It has been verified by a large scale AFM analysis that the domains correspond to regions where the crystallites have a similar orientation. The boundary seen by AFM (Figure 4d,e) corresponds to a separation between two zones of different contrast in the optical micrograph. The main characteristics of the Nomarski images are retained with increased annealing time (d). The sizes of the crystallites measured by AFM and of the domains detected on the Nomarski micrographs have been analyzed to obtain quantitative information concerning the R-β1 transition. Images were taken at four to five different areas of the sample, and all the crystallites or domains present were measured to ensure a statistical and representative picture of the total sample. Figure 6 shows the evolution of the mean size of the main features appearing in AFM images: (i) the R phase crystallites, (ii) the ill-defined islands that appear during the transition (Figure 4c), and (iii) the slender β1 crystallites. The elongation of the R spheres prior to the transition is accompanied by an increase in mean size from 0.015 to 0.022 µm2. The β1 crystallites undergo a clear linear increase in size from 0.10 to 0.51 µm2. This confirms the earlier observation that the ratio of the two Q-bands in the electronic absorption spectra change as a function of annealing time due to the growth of the crystals (Figure 2). It is likely that the size of the crystallites will continue to increase until sublimation reduces the film thickness to a level where gaps between the crystallites will appear. Closer analysis of the size evolution for the β1 phase can be made by evaluating the shape of the crystals. We define a shape parameter S, which is the length/width ratio of the crystallites. S is plotted as a function of annealing time in Figure 7 and it is clear that the crystallites become progressively more elongated. This elongation has also been noted for R crystallites. Since the lattice vector b is the common vector that lies parallel to the substrate for both phases of H2Pc, it can be concluded that the crystals, either R or β1, grow along their b axis. A quantitative size analysis can also be made using the Nomarski micrographs. Figure 8 shows the evolution of the domains as a function of annealing time. The mean size of the domains increases until an annealing time of between 1.25 and 1.5 h, at which point the size saturates at approximately 100 µm2. The different domains observed have been shown to be regions where the β1 crystals are oriented in one particular direction. It is improbable that the boundaries between two regions where the crystals are oriented differently would break
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Figure 7. The shape parameter, S, corresponding to the length/width ratio of the β1 crystallites, as a function of annealing time for 2330 Å thick H2Pc films. The crystallites become more elongated and grow along their b axis. The solid line is simply a guide to the eye.
Figure 8. Mean size of the domains observed by Nomarski microscopy as a function of annealing time for 2330 Å thick H2Pc films. The domain size increases and saturates at about 100 µm2 after 1.25-1.5 h annealing. The solid line is simply a guide to the eye.
down to allow for the merging of the domains, and hence the saturation of the domain size is to be expected. The Nomarski image presented in Figure 5b indicates that nucleation occurs at many sites. As these domains grow in size, their final and finite size is inversely related to the number of nucleation sites that can be achieved before boundaries are formed between the domains. The finite number of nucleation sites can be obtained by evaluating the number of domains in a totally β1 sample, and this number should remain constant after complete coverage. Here, the number of sites are 1.2 ( 0.3 µm-1. It is apparent that when the crystallites and hence the domain boundaries have all merged, a totally smooth β1 film is produced. A high thickness for the film initially grown at room temperature is therefore required to compensate for the progressive sublimation of material during the annealing transformation. A qualitative analysis of the influence of the film thickness on the R f β1 transformation was carried out by studying the surface morphology. It is first important to note that there is a film thickness constraint on the R f β1 phase transition. It is not possible to affect the transformation on films that are thinner than 560 Å, either by growth at an elevated substrate temperature, or by annealing. Ashida et al.16 suggested that complete transformation of films below a certain thickness did not occur because there was not enough material present on the surface to ensure a continuous film during the transition. An alternative explanation is that below a certain film thickness, and therefore below a certain R phase crystal size, the R phase is more stable than the β1 phase, leading to the transformation being energetically unfavorable; the figure quoted for the point at which the stability of the two polymorphs is equal is 2 × 104 molecules.8 If the R phase crystals are assumed to be spherical, this corresponds to a crystal radius of 177 Å, or a diameter of 345 Å. This is the theoretical sphere diameter below which the R
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Figure 10. Nomarski micrographs of H2Pc films annealed for 2 h at 325 °C, for different film thicknesses: (a) 560, (b) 990, and (c) 4880 Å. The thinnest film shows nucleation of the β1 phase and a fern-like morphology. Increasing thickness leads to an increase in the domain size. Figure 9. AFM images of H2Pc films annealed for 2 h at 325 °C, for different film thicknesses: (a) 560, (b) 990, and (c) 4880 Å. Partial transformation with discrete domains of R and β1 phases is observed for the thinnest film. The crystallites are more ordered for thicker films.
phase will be more stable than the β1. This compares well with the observed film thickness of 560 Å, below which the transformation does not appear to occur. For films less than 560 Å, sublimation of material occurs rather than transformation for equivalent annealing conditions. The effects of film thickness are also apparent in the electronic absorption spectra. Samples with a film thickness greater than 940 Å are characteristic of the β1 phase whereas thinner films give a weak signal that is mainly β1 but still shows traces of R. AFM images of the annealed films (2 h) at different thicknesses is presented in Figure 9. The thinner films (
