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Role of Water in the Molecular Reorientation on Thermal Annealing of Bis(n-propylimido)perylene Films Alicia P. Kam and Ricardo Aroca* Materials and Surface Science Group, Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario N9B 3P4, Canada
James Duff and Carl P. Tripp*,† Xerox Research Center of Canada, 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada Received November 19, 1998. In Final Form: September 16, 1999 The change in morphology and molecular organization occurring in bis(n-propylimido)perylene thin solid films exposed to various solvents in the gas phase were investigated using transmission and reflectionabsorption infrared spectroscopy. Specifically, infrared transmission and reflection spectroscopy has been used to study the effect of solvent vapor annealing on the organization of bis(n-propylimido)perylene films. It is shown that the environmental source of molecular reorientation arises from the penetration of molecular water at the air/molecular film interface. The effect of solvent vapor annealing of several polar molecules on the orientation of bis(n-propylimido)perylene films is also reported.
Introduction Organic molecular thin films are becoming increasingly popular in the electronic, optical, and imaging industries. Their synthetic diversity and inexpensive production costs have made such thin films a viable and competitive substitute for inorganic thin films. Moreover, the investigation into the relation between molecular orientation and film morphology is of significant interest because of its importance in dictating the physical properties of these thin films. Perylenetetracarboxylic acid (PTCD) and derivatives thereof are a class of organic materials often used in the fabrication of thin film devices because of their unique optical and electronic properties and because of their ability to form highly ordered films on various substrates. The physical properties and structural stability of thin organic films are dependent on both molecular structure and orientation/morphology. Molecular structure is defined by synthetic routes whereas orientation/morphology are dependent on molecular structure and organization, deposition/processing protocols, and other less understood effects. Moreover, changes in morphology of the organic films with time are common and often lead to a drop in device performance and eventually device failure. As such, much emphasis has been placed on identifying the factors that induce change on a molecular scale. For example, it is well-known that the performance degradation of electroluminescence devices is accelerated at higher operating temperatures1 and that this can be slowed by encapsulating the device in a protective coating.2 By use of optical microscopy, it has been shown that both the temperature and encapsulation dependence correlated with the appearance and growth of crystalline domains in the film † Present address: Department of Chemistry and Laboratory for Surface Science and Technology, 5764 Sawyer Research Center, University of Maine, Orono, ME 04469-5764.
(1) Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.; Roitman, D.; Stocking, A. Science 1996, 273, 884. (2) Burrows, P. E.; Bulovic, V.; Forrest, S. R.; Sapochak, L. S.; McCarty, D. M.; Thompson, M. E. Appl. Phys. Lett. 1994, 65, 2922.
Figure 1. Structure of bis(n-propylimido)perylene.
arising from a humidity-induced change in morphology.3 More recently, we used reflection infrared spectroscopy to show that a change in orientation of perylene films occurs with thermal annealing.4 The infrared study provided the link between molecular detail and morphological changes in these films, which in turn have been correlated to device performance.5 In this paper, we now apply this infrared tool to the identification of the environmental source and the molecular basis for reorientation of the perylene films. Experimental Section Materials. Bis(n-propylimido)perylene (denoted as bis-PTCD, Figure 1) was prepared by condensing perylene-3,4,9,10-tetracarboxylic acid dianhydride with propylamine in an appropriate solvent.4 The bis-PTCD was then purified by temperaturegradient sublimation. Vacuum Deposition. The substrates used were transparent and precleaned borosilicate slides (Baxter Catalog M6145). Metal deposition was performed in an Edwards vacuum system evaporator. Silver shots (Aldrich 20,436-6) were thermally evaporated from a baffled tungsten boat, using an evaporation source supply and a BSV 080 glow discharge/evaporator unit. The background pressure was nominally 10-6 Torr as measured by the Penning cold cathode gauge. Silver was deposited on a glass slide at a rate of 0.5 nm/s to a total mass thickness of 100 nm. This deposition rate was allowed (3) Aziz, H.; Popovic, Z.; Xie, S.; Hor, A.-M.; Hu, N.-X.; Tripp, C. P.; Xu, G.; Appl. Phys. Lett. 1998, 72, 756. (4) Kam, A.; Aroca, R.; Duff, J.; Tripp, C. P. Chem Mater. 1998, 10, 172. (5) Aziz, H.; Popovic, Z.; Tripp, C. P.; Hu, N.-X.; Hor, A.-M.; Xu, G. Appl. Phys. Lett. 1998, 72, 2642.
10.1021/la9816252 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/07/1999
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to stabilize before the shutter was opened. Film thickness and deposition rate were monitored using a quartz crystal oscillator. Bis-PTCD was deposited on continuous silver films (mass thickness 100 nm), in turn supported on a glass substrate. The evaporation system used was a Cooke’s vacuum system, model CVE30 IFR. The perylene pigment was evaporated from tantalum boats at room temperature using an evaporation source supply, model CV301 EPS. The deposition rate for the pigment was approximately 0.2 nm/s to a mass thickness of 20 nm. As for the metal deposition, the rate of the organic deposition was allowed to stabilize prior to opening the shutter. Spectroscopy. The transmission spectrum of bis(n-propylimido)perylene (bis-PTCD) dispersed in potassium bromide was recorded on the Bomem DA3 FTIR spectrometer with a 1 cm-1 resolution. The reflection absorption infrared spectra were recorded on an in-house-modified Bomem 110-E equipped with a Judson midrange (650 cm-1) 1 mm × 1 mm MCT detector.6 The entire system was purged and maintained under a closed cell of dry air. The sample holder was adapted to include four concave mirrors which focused the IR beam onto the substrate at a 80° angle of incidence.7 The substrate holder consisted of a flat aluminum heating element, which was thermally controlled by a 100 V variastat. The temperature of the substrate was monitored using a thermocouple. Vapor annealing of the sample during the reflection/absorption infrared spectroscopy (RAIRS) measurements was performed in situ on the heating plate. Infrared spectra were recorded in the region of 4000-200 cm-1 with 4 cm-1 resolution. Encapsulation and Thermal Annealing. Annealing of the samples under various atmospheres was done at the specified temperature for 30 min using standard vacuum line techniques and cells. Encapsulation of the Ag/glass or bis-PTCD/Ag/glass was done by placing the sample into a freshly prepared 10-3 M octadecyltrichlorosilane (OTS) solution in cyclohexane for 10 min, rinsing with cyclohexane, and then drying in a N2 stream. The OTS reacts with the adsorbed water on the surface of the film to produce an ill-defined cross-linked polysiloxane coating. The OTS film was subjected to a final annealing at 200 °C for 30 min.
Results and Discussion Changes in orientation are deduced from the well-known surface selection rules for reflection spectroscopy from a smooth metal surface. The description of infrared experiments on molecules adsorbed onto reflecting metal surfaces follows Greenler’s8 realization that the light at the reflecting surface is highly polarized, and at the appropriate angle of incidence the p-polarized (E⊥) component of the electromagnetic wave is 3 orders of magnitude larger than the parallel component. In brief, the infrared active mode whose dynamic dipole moment is perpendicular to the metal surface is more intense than those modes with dynamic dipole moments parallel to the surface.9 The bis-PTCD chromophore is planar (see Figure 1), and thus changes in orientation can be easily determined from a comparison of modes with dynamic dipoles located within the plane of the chromophore (i.e., within the plane of the aromatic moiety) with fundamental vibrational modes with dynamic dipole moments perpendicular to the aromatic plane. Specifically, orientational changes are deduced from a comparison of the in plane CdO stretching vibrational bands at 1662 and 1697 cm-1 relative to the out-of-plane C-H wagging vibrational band associated with the aromatic ring located at 810 cm-1. For brevity, we abbreviate this band intensity ratio as CdO/C-H. In a previous study,4 it was shown that the CdO/C-H band intensity was higher than the same ratio (6) Tripp, C. P.; Hair, M. L. Appl. Spectrosc. 1992, 46, 100. (7) Guzonns, D. A.; Hair, M. L.; Tripp, C. P. In Fourier Transform Infrared Spectroscopy in Colloid & Interface Science; Scheuing, D. R. ed.; 1990, 237-250. (8) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (9) Debe, M. K. Prog. Surf. Sci. 1987, 24.
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Figure 2. Reflection-absorption spectra of bis(n-propylimido)perylene film before and after thermal annealing. Notice the change in relative intensity of the out-of-plane wagging mode.
obtained for the transmission spectrum of bis-PTCD dispersed in a KBr pellet. This intensity difference is the result of the sublimed bis-PTCD having, on average, a preferred orientation in which the aromatic rings extend perpendicular from the surface (edge-on adsorption). Heating the sample in air at 125 °C or below did not alter the spectrum. However, when heated to 200 °C in air, the C-H out of plane modes increased in intensity by a factor of 7 (see Figure 2). This has been interpreted as a change in orientation where the bis-PTCD assumes an orientation where, on average, the aromatic rings are face-on relative to the surface. The nature and origin of the induced change in the molecular organization of bis(n-propylimido)perylene films described earlier,4 are explored using reflection infrared experiments as a sensor for detecting the onset in reorientation due to solvent vapor annealing. In this manner, the change in the spectrum could be used as a probe in testing strategies for improving film robustness. In essence, a film that was more resistive to changes in morphology would likely exhibit a higher transition temperature or require longer heating times to produce a detectable band intensity change in the reflection infrared experiment. As mentioned earlier, it is wellknown that encapsulation of organic thin film devices in a protective sheath often increases device lifetimes. In this paper we have used temperature as a probe in identifying the role of a protective coating by observing the orientational change of bis-PTCD films, as measured by reflection infrared spectroscopy. The Origin of Molecular Reorganization. The origin of the induced reorientation under thermal annealing was assumed to stem from external factors, in this case environmental conditions. The primary step in determining the factors which influence and induce reorientation on thermal annealing was to exclude those factors found in the matrix of air, which are nominally nitrogen, oxygen, carbon dioxide, and water vapor. To determine whether reorientation originated from one or a combination of these factors, a thin film of bis-PTCD on smooth silver was annealed in a vacuum, employing a range of annealing temperatures, 50-200 °C. Results indicate that there was no molecular reorientation, as no significant changes were observed in the RAIRS spectra obtained both prior to and after annealing, in contrast to the findings shown in Figure 2.4 This experiment was then repeated in dry air, concluding in a similar result as obtained under vacuum; that is, no reorientation was observed as shown in Figure 2. The latter results suggest that the origin of the reorientation was due to the presence of water in the atmosphere. To validate this theory, the thermal annealing
Morphology of Thin Films by Solvent Vapors
Figure 3. Reflection-absorption spectra of bis(n-propylimido)perylene films in the presence of methanol and 1-propanol vapors.
of a thin film of bis-PTCD was performed solely in the presence of water vapor. A comparison of the RAIRS spectra taken of the film both prior to and post-watervapor annealing showed the out-of -plane C-H bands, at 746 and 810 cm-1, increased in intensity as annealing progressed. This indicated an average reorientation of the molecules on the surface to a face-on organization on the substrate. The source of molecular reorientation with thermal vapor annealing would appear to be the penetration of water molecules from the air/film interface into the layered structure of the bis-PTCD film. Supporting evidence for the water effect in the bis-PTCD layers was provided by thermal vapor annealing the films using several organic solvents. Solvent vapor annealing of the bis-PTCD films at 200 °C using methylene chloride, tetrahydrofuran, thionyl chloride, and carbon tetrachloride did not produce spectral changes in the RAIRS spectra. However, RAIRS spectral changes did occur when the samples were vapor annealed in the presence of alcohols such as methanol, ethanol, propanol, butanol, and 2-propanol. The effect observed with methanol and 1-propanol is shown in Figure 3. The RAIRS spectrum of a bis-PTCD film annealed in a vacuum at 150 °C is also included for comparison. It can be seen that the relative intensity of the out-of-plane C-H bands increases for films annealed in the presence of methanol and 1-propanol. The vibrational bands observed above 1300 cm-1 are securely assigned to in-plane modes of the chromophore PTCDA ring.10,11 Therefore, the increase in the C-H wags confirms the average reorientation (higher face-on contribution) produced by solvent annealing. Furthermore, an interesting trend was observed with the size of the alcohol. The onset of spectral change was dependent on the size of the alcohol. A higher temperature or longer incubation time was needed to cause a change in the spectrum, and both increased as the size of the alcohol increased. A reorientation requiring alcohols or water and not observed with other solvents suggests a mechanism involving hydrogen bonding interactions, whereas the alcohol size dependence is indicative of a process where water is more efficiently intercalated into the bis-PTCD layers. The most likely occurrence of hydrogen bonding is with the polar imido moieties. Aziz et al.3 have observed humidity-induced crystallization in electron transporting layers and not in the hole transporting layers of electroluminescence devices.5 We note that the electrontransporting molecules contain polar functionalities (10) Johnson E.; Aroca, R. Langmuir 1995, 11 (5), 1693. (11) Maiti, A. K.; Aroca, R.; Nagao, Y. J. Raman Spectrosc. 1993, 24, 351.
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whereas the hole transporting molecules are generally nonpolar in nature. The intercalation of water via hydrogen bonding with the imido group would increase the spacing between perylene molecules and thus decrease the π-π ring overlap. This decrease in π-π interaction (or increase in intermolecular spacing) would weaken the coupling between molecules enabling a reorientation to occur more readily. Finally, the atomic force microscopy (AFM) images of bis-PTCD films before and after annealing revealed a substantial change in the film morphology as can be seen in Figure 4. A vacuum evaporated bis-PTCD film of 200 nm mass thickness on glass is not a smooth film. It is an island film as can be seen in the AFM image where the size and height are shown. Direct comparison of the island size is possible since the spatial dimensions represented in Figure 4 are kept constant. The cross section in Figure 4 shows that the island or disk-type structures of the film surface after annealing are greater than 300 nm in width. After our work had been completed, a report on the effect of vapor annealing on thin-film molecular semiconductor bilayer (TiOPc/ PPEI, titanyl phtahlocyanine and perylene phenethylimide) was published.12 The spectroscopic work was conducted in the visible region of the spectrum, and there is no report of the infrared spectra. However, the AFM data also show that treatment of the vacuum-deposited films results in the crystalline transformation of the materials and severely alters the contact between the TiOPc/PPEI layers. Film Encapsulation. A series of experiments were carried out in order to study the annealing of protected “encapsulated” bis-PTCD films from the effect of atmospheric gases. Therefore, the reflection-absorption infrared spectra of bis-PTCD films before and after coating with a hydrophobic OTS film were recorded. The presence of a polymerized OTS layer is evidenced by the methylene modes at 2920, 2850, and 1466 cm-1. It is important to note that the CdO/C-H ratio for bis-PTCD is the same in both spectra; the OTS treatment did not cause a change in the orientation of the underlying bis-PTCD film. More importantly, no change in orientation of the bis-PTCD is observed when the OTS-coated sample was heated in air up to 200 °C. The fact that no spectral change is observed is consistent with the expected suppression of morphological changes by encapsulation. The RAIR spectra obtained for an encapsulated sample annealed at four different temperatures are presented in Figure 5. One possible explanation is that the OTS overlayer freezes the location of the bis-PTCD molecules hindering any possible reorientation. A second explanation is that the protective coating prevents a component found in the vapor phase from interacting with the bis-PTCD layer. To test these two possibilities, a second series of experiments was conducted in which the bis-PTCD films were annealed under varying atmospheric conditions. In the first experiment, the bis-PTCD film was annealed under vacuum (10-6 Torr). No spectral changes were observed when the sample was heated to 150 or 200 °C. Next, the bis-PTCD was heated in dry air (water vapor and CO2 removed). Again, there were no spectral changes at 150 or 175 °C. Finally, a bis-PTCD film was heated solely in the presence of water vapor (2 Torr). In this case, spectral changes occurred at 150 °C akin to those observed when the sample is heated at the same temperature in air. These experiments showed that water vapor was needed for reorientation of the film and that the role of the OTS film is to prevent water from penetrating into this layer. (12) Conboy, J. C.; Olson, E. J. C.; Adams, D. M.; Kerimo, J.; Zaban, A.; Gregg, A. B.; Barbara, P. F. J. Phys. Chem. B 1998, 102, 4516.
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Figure 4. AFM images for bis-PTCD film before annealing (A) and after heating at 200 °C in air (B).
PTCD deposition. This has two effects. First, it is unlikely that water is penetrating at the OTS/bis-PTCD interface, and second, the OTS isolates the bis-PTCD from direct contact with any oxidative changes to the metal surface. On thermal annealing of this modified film in air, reorientation occurred at the same temperature as that of a non-OTS coated substrate. This suggests that the origin of molecular reorientation is not due to the penetration of water at the silver/bis-PTCD interface. Conclusions
Figure 5. Reflection-absorption spectra of a bis-PTCD film on smooth silver coated with OTS. Spectra taken after annealing the sample at 25, 50, 120, and 200 °C.
While water is clearly needed to induce reorientation, a molecular basis for this change has yet to be determined. Clearly one possibility is that the water reacts with the bis-PTCD at 150 °C. However, this is highly unlikely because the DSC decomposition temperature reported for this compound is above 450 °C. Furthermore, we did not detect any reaction when bis-PTCD powder was heated at 150 °C in a high humidity chamber for a period of 1 h. A second possibility is that the water does not interact with the bis-PTCD but rather modifies the metal/organic interface. Water penetrating at this interface could either oxidize the metal or interpenetrate at the metal/organic interface resulting in the delamination of the bis-PTCD from the surface. To test this possibility, OTS was again used. An OTS film was deposited on silver prior to bis-
The effect of water and alcohols vapor annealing on thin solid films of bis-PTCD has been identified using vibrational spectroscopy. The well-known surface selection rules of RAIRS provided the basis to show that water, methanol, and 1-propanol induced molecular reorientation on bis-PTCD films under thermal annealing. The bisPTCD films (200 nm mass thickness) were fabricated on reflecting silver substrates, and the most likely cause of the reorientation caused by water is a hydrogen bonding mechanism with the polar moieties of the bis-PTCD that disrupt the packing in molecular solid film. It was further shown that film organization may be protected by encapsulation. Experiments of solvent vapor annealing of bis-PTCD films with a silane protective coating have shown that the molecular organization of the evaporated films is preserved. Acknowledgment. We gratefully acknowledge the funding provided by NSERC of Canada. We wish to thank Dr. Rich MacAloney from the Department of Chemistry, University of Toronto, for performing AFM measurements. LA9816252
