Organic Thin Films - American Chemical Society

Chapter 1

Downloaded by DUESSELDORF LIBRARIES on December 18, 2013 | http://pubs.acs.org Publication Date: August 6, 1998 | doi: 10.1021/bk-1998-0695.ch001

Some Emerging Organic-Thin-Films Technologies J. D. Swalen Department of Physics, University of California at Santa Cruz, Santa Cruz, CA 95064

Ordered thin organic films in the thickness range of a few to several hundred nanometers currently hold considerable technological promise. Electronic and optical devices incorporate structures that are in this thickness range, and organic thin films have been proposed, and in some cases applied, as passive or active components traditionally fabricated with other materials. Some proposed, but not all, applications of organic thin films will be discussed.

Organic thin films, which have some designed molecular order, can exhibit different material properties from the collective behavior of the molecules in a restricted geometry. This is materials science from the molecular point of view. Molecular solids are now being designed into organized films to perform new and special functions. This has stimulated many intensive scientific investigations in the preparation of new films and their characterizations. Many of the newer surface science techniques, designed and developed for semiconductors, metals, and dielectrics, are addressing specific details about structure and morphology of these organic films. In the past, organic films were considered to be too fragile and not of sufficient purity to give reliable and consistent properties to make them of much use. This is changing with new materials. We are seeing many new compounds and polymers being synthesized and made into thin films by a variety of techniques. These films are carefully constructed to avoid the common problems. Extensive scientific studies have revealing a wealth of knowledge useful in the development of these new thin film materials, exhibiting specifically desired behaviors. A number of books, reviews, and general articles have been published about organic thin films, both Langmuir-Blodgett-Kuhn films (LBK) and self-assembled films (SAM). (1-9) Other related topics cover polymer surfaces and interfaces (10) and the optical properties of organic thin fi\ms(ll-14). The reader is directed to these 2

©1998 American Chemical Society

In Organic Thin Films; Frank, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


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references for more details. Although the preparation and study of L B K films are still very active areas of research, (15) self-assembled films and polymeric films, either deposited as a polymeric film or polymerized in situ, are rapidly overtaking the field. Fluorinated surfaces are being applied to lower the surface energy. Much of the scientific research on organic thin films has been on determining the orientation of the molecules and their packing. Is there order in the films? What are the domain sizes, i f any? Does the substrate or superstrate induce any structure? Molecular motion is also being studied. This includes lateral diffusion, gas or ionic diffusion through the film, phase transitions, melting, and other relaxation processes. Interface properties between two layers of different materials are important. This is studied often between two different polymeric layers. Surface functionalization for specific interactions is an active area. This leads to wetting phenomena, adsorption, adhesion, and lubrication. Much work is being done in the characterization of the optical, spectroscopic, and electrical properties, including energy transfer between molecules in the film and between layers. Clearly, one needs to know the morphology of the substrate and its influence. The alignment of molecules and the order parameter are needed. Cooperative effects can change the behavior of any film significantly, and the extent of interaction, both laterally and vertically between layers or substrate is important. With this base of knowledge it is hoped that this will lead to our understanding of intermolecular interactions, energy transfer, and dynamic behavior for best optimizing a film for a specific research study or application. The following articles in this ACS monograph report research along these topics and for the most part will not be covered in this review. These topics include new self-assembled layers, new Langmuir-Blodgett-Kuhn films, polymeric thin films, fluorinated surfaces, the alignment of liquid crystals by surface treatment, nonlinear optical films, light emitting diodes, photoresists, low dielectric films for faster electronic circuit speeds, sensors, and nano-particles. The reader is referred to these current scientific studies. Current Applications of Organic Thin Films There has been an increased utilization of organic thin films in many new electronic, optical, and mechanical devices; for example, organic photoconductors are being use in copiers and printers. The first organic photoconductors were charge transfer polymers which performed both the charge generation and conduction processes. Later versions separated these roles into different layers, each of which could be optimized. Liquid crystal displays are now common in watches and laptops. Not only have the liquid crystals been improved, but the surface treatment and manufacturability have been significantly advanced. Related to this is surface modification by organic materials for other specific applications. Photoresists and e-beam resists are the keys to the success of very large scale integrated (VLSI) electronic circuits. Without these resists, most electronic equipment we know today would not exist. These polymers are spun onto the semiconductor and the circuit pattern is exposed, leading to main chain scission or cross linking; with either a wet process or a dry one, sections are removed. Further


4 treatment can include either diffusion of various semiconductor elements or metalization for conduction lines. Layer by layer, the total package is developed. Current research is directed towards finer features in the patterns and changes in the surface characteristics for subsequent layers. Polyimide and other related high temperature polymers are being used extensively in VLSI as insulating layers or for packaging. Previously, sputtered S i 0 films were used. This process, however, required a large sputtering chamber, the films did not planarize well, and they were subject to cracks. Polyimide was, however, not without problems. Imidization gives water as a product which could be detrimental to semiconductor devices. Also, the adhesion of metal lines to polyimide was at times not good. Both of these problems now seem to be under control. To increase the storage density on magnetic disks, efforts to reduce the bit size by flying the read-write head closer to the disk surface have generated new problems called "stiction." Here, the head would stick or have high friction with the disk. To overcome these problems, lubricants were added to the disk surface. The most popular have been fluorinated polymeric ethers, usually on top of an amorphous carbon coating, allowing the read-write head to fly much closer to the magnetic storage disk surface. A number of groups are studying friction, lubrication and adhesion from a fundamental point of view (16-22). On another topic, electrolytic capacitors were introduced by Sanyo in 1983 based on TCNQ (23). Since that time, a number of other capacitors have been introduced into the marketplace. Polypyrrole, polyaniline and polythiophene have all been used. These capacitors range in values from 0.1 |iF to 200 |iF and have low equivalent series resistance and high frequency impedance with good reliability and lifetimes. Some ten years ago, the Department of Energy realized that, in their materials research efforts, they were supporting very little on organic thin films. As a consequence of all this new activity in the applications of organic thin films, they commissioned a review panel, and the results of our discussions and concurrences were published in the journal Langmuir(24). The applied topics, which were chosen because of their perceived significance, were thin film optics, sensors and transducers, protective layers for packaging and insulating, patterning of surfaces of electronic circuit components, functionalized surfaces, and coatings for electrodes. In addition, the scientific research on intermolecular forces, intermolecular order, and analytical methods for the characterizing thin films were addressed. This paper will be an update of these applications of organic thin films and add some new ones that have come into vogue.


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New Applications of Organic Thin Films Light Emitting Diodes. Some ten years ago Tang and VanSlyke (25) reported a two layer organic light emitting diode, consisting of holes ejected from an anode of indium-tin-oxide (ITO), a hole-transport layer, an electron-transport and lightemitting layer and a metal, e. g., magnesium with 10% silver, having a low work function. Electrons and holes are produced at opposite electrodes and diffuse to the middle of the film where they recombine to produce light. The light emitter was a quinoline complex of aluminum and the hole transport layer was a



5 triphenyldiamine. Shortly thereafter, Friend and collaborators (26) made a single layer diode with the polymer, poly(p-phenylenevinylene). Although the light emission was rather low and the diodes had only a short term stability, many research groups entered the field and the progress has been significant (27-29). Organic light emitting diodes are being considered for display application because with recent advances the intensity has been increased approximately to that of a fluorescent lamp, and most colors have been obtained with a great variety of fluorescing dyes. The life-time has also been increased to several thousands of hours; however, degradation is still a major problem. Oxygen, probably coming from the ITO anode, is believed to be one of the culprits (30,3 J); water is another (29). Charges tend to be trapped. In the two-layer diode the aluminum complex appears to diffuse into the amine layer causing dark spots (30). Research is continuing to increase intensity and l i f e t i m e ^ / New compounds are being tried and additional layers are being added to enhance performance. Nonlinear Optical Materials. The research on nonlinear optical polymers has made significant advances over the last ten years (13). These polymers contain a dye with an electron donor at one end and a charge acceptor at the other end. This chromophore dye can be a guest in the polymer host or be chemically attached. The "drosophila" of these N L O chromophores is the dye, disperse red No. 1. It is an azo dye with two phenyl groups, one on each side of the -N=N- group. In the para position on one benzene ring is the donor, an aliphatic substituted amine, and on the other benzene ring, also in the para position, is a nitro group. It has the chemical formula: (02Η -)ΗΟ02Η -Ν-φ-Ν=Ν-φ-Νθ2, where the symbol φ represents a phenyl ring. A majority of the N L O experiments have been done with this dye, but recently many more chromophores have been synthesized. The optical nonlinearity has been increased by changing the commonly used charge transfer donor, the nitro group, to a tricyanovinyl group. Surprisingly, the increased unsaturated bonds in the tricyanovinyl group do not significantly degrade the thermal stability. In fact, they perform well. In addition, it was found (32) that by changing the aliphatic amine to an aromatic amine improved the thermal stability of the dye by 50 to 100 °C. This is important for fabricating any N L O device exhibiting long lifetimes. Further, by substituting a heterocyclic ring for one of the phenyl groups, it has been found that the nonlinearity increases, but the thermal stability is slighted degraded. Hence, with these improvements the technology is here for the production of modulators and switches made from N L O polymers. As yet, to the best of my knowledge, none have appeared in the market. In fact, a number of research laboratories have stopped or reduced research activities in N L O polymers. Hopefully, a viable product will be forthcoming to spur continued activity in this area. 5

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Photorefractives. Although interest in nonlinear optical polymers for electrooptic modulators and deflectors seems to be waning, the activity on organic photorefractives for optical storage is vigorous (33-36). In this material both photoconductivity and electrooptic activity must be present. Charges are generated by light and they become mobile and separate (An electric field has been found to


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enhance the effect.). Commonly only one charge moves and is trapped, and the other remains rather stationary. An internal electric field is thus generated by these separated charges which produces a difference in the index of refraction. Two mechanisms cause this effect: the electro-optic coefficient and a molecular rotational term from the anisotropy; both produce an index change. These indices of refraction patterns are then observable, usually as a holographic image. Recently, an organic glass was developed as a photorefractive material which had a higher concentration of active chromophores (36). This material has a higher refractive index change but is still somewhat slow. Liquid Crystals. Ferroelectric liquid crystals can be switched much faster than the conventional nematic liquid crystal display. They consist of a Smectic C mesogen with a chiral center which causes each layer to be twisted with respect to neighboring layers. Clark and Lagerwall (37) were able to unwind the helical structure and stabilize it with a surface treatment of the cell walls. Although they have a fast response time and low power requirements, they suffer from poor mechanical stability to shock(38). During the same period of time, Ringsdorf and his associates (39) were studying polymers with liquid crystals attached as a sidechain. They found that for the systems to exhibit liquid crystal behavior a flexible chemical link was needed between the polymer backbone and mesogen. These systems have a slow response time but are mechanically stable. Therefore, the current efforts are to make a polymer with flexible spacers to which ferroelectric mesogens can be attached as side-chains (38). In this way, the polymer can give mechanical stability to the fast switching ferroelectric mesogens for displays and sensors. Nematic liquid crystals dispersed in poly (vinyl alcohol) are being developed for paper-like displays (40). Normally, the liquid crystal droplets scatter light such that a film will appear translucent and white. Under an electric field, ordering leads to a clearer film. The scattering of these nematic droplets depends on the difference in the indices of refraction between the polymer and the liquid crystal and on the anisotropy in the index of the liquid crystal. This technology promises to become an inexpensive display material. Organic Transistors. In the DOE report (24) on electronic circuit components, the panel concurred that "the use of molecular monolayers for circuit elements seems to be too speculative " Further, "the members of this panel have some reservations concerning the applicability of organic films to high speed switching...." This point of view has to be corrected. Recent advances (41) reported at the Device Research Conference by Gamier at CNRS in Thiais, France, Phillips at Lucent and Jackson at Penn State indicate that films of pentacene or α-hexathiophene can exhibit mobilities approximately the same as amorphous silicon. Jackson reported that with a slow vacuum evaporation better quality films with fewer chargetrapping defects could be made. Also, these organic materials show low loss characteristics. Hence, in spite of a negative prediction 10 years ago, significant progress has been made in adapting thin organic films to electronic devices.


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Conclusions Not too many years ago, polymers were identified with cheap, molded plastics. These materials were not very pure and, as such, were not consistent from batch to batch. Although inexpensive, they were soft and rather unstable in heat and sunlight. Their electrical and optical properties were not good. On the positive side, nylon, polyethylene, polystyrene, poly(methyl methacrylate), and polycarbonate all were active in commerce; they had a use. The major breakthrough in the electronic industry came with polyimide as an insulating layer in electronic chips, organic photoconductors in copiers and printers, photoresists in layer-by-layer circuit deposition, and liquid crystals for displays. Except for liquid crystals, most application, even the new applications discussed above, were with amorphous films. Many other applications not covered in the discussion are also under development. Many are proprietary and not published in the open literature, and others are optimized configurations of known applications. As time progresses, these additional technologies will become apparent by the announcement of new products or new patents. With ordered films produced by, for example, self assembly, we can expect to discover new phenomena and effects which can be utilized both scientifically and technically. It is clear that the future is bright for organic thin films. New materials are needed and an interdisciplinary approach is required. Chemists, chemical engineers, applied physicists, materials scientists, and electrical engineers all must work together in a concerted effort to solve these problems in thin film materials science research and development at the molecular level of these organic and polymeric systems. Literature Cited 1. Roberts, G.; Eds.; Langmuir-Blodgett Films; Plenum: New York, NY, 1990. 2. Ulman, Α., An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. 3. Swalen, J. D.; Annu. Rev. Mater. Sci. 1991, 21, 373. 4. Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 437. 5. Whitesides, G. M.; Ferguson, G. S.; Allara, D.; Scherson, D.; Speaker, L; Ulman, A. Crit. Rev. Surf. Chem. 1993, 3, 49. 6. Bohn, P. W.; Annu. Rev. Phys. Chem. 1993, 44, 37. 7. Whitesides, G. M. Sci. Am. 1995, 273, 146. 8. Ulman, Α., Ed.; Organic Thin Films and Surfaces: Directions for the Nineties, in Thin Films; Academic: San Diego, CA, 1995. 9. Allara, D. L. Biosensors & Bioelectronics; Elsevier: Amsterdam, 1995; Vol. 10, pp. 771-783. 10. Sanchez, I. C., Ed.; Physics of Polymer Surfaces and Interfaces; ButterworthHeinemann: Boston, MA, 1992. 11. Swalen, J. D. J. ofMolecular Electronics 1986, 2, 155. 12. Knoll, W. Encyclopedia of Applied Physics; VCH Publisher: Brooklyn, NY, 1996; Vol. 14, pp. 569-605.



8 13. Organic Thin Films for Waveguiding Nonlinear Optics; F. Kajzar, F.; Swalen J. D.; Eds.; Advances in Nonlinear Optics; Gordon and Breach Publisher: Amsterdam, 1996; Vol. 3. 14. Knoll, W. In Handbook of Optical Properties, Optics of Small Particles, Interfaces, and Surfaces; Hummel, R. E. and Wiβmann, P., Eds.; CRC Press: Boca Raton, FL, 1997; Vol. 2, pp. 373-400. 15. For recent results on organic thin films see the proceedings of the conference "Organized Molecular Films" Gabrielli, G. and Rustichelli, F., Guest Eds.; in Thin Solid Films 1996, 284. 16. Kendall, K., Science 1994, 263, 1720. 17. Yoshizawa, H.; Israelachvili, J.; Thin Solid Films 1994, 246. 18. Yoshizawa, H., Chen, Y. L.; Israelachvili, J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1993, 34, 282. 19. Yoshizawa, H.; Chen, Y.L.; Israelachvili, J. Wear: 1993, 168, 161. 20. Reiter, G.; Demirel, Α.; Levent, Α.; Peanasky, J.; Cai, L. L.; Granick, S. J. Chem. Phys. 1994, 101, 2606. 21. Peanasky, J.; Cai, L. L.; Granick, S.; Kessel, C. R. Langmuir 1994, 10, 3874. 22. Thomas, R..C.; Houston, J. E.; Crooks, R. M.; Kim, T. M.; Terry A. Am. Chem. Soc. 1995,117,3830. 23. Miller, J. S. Adv. Mater. 1993, 5, 671. 24. Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, Ε. Α.; Garoff, S. J.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. 25. Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. 26. Halls, J. J. M.; Baigent, D. R.; Cacialli, F.; Greenham, N. C.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Thin Solid Films 1996, 276, 13. 27. Kido, J. Bull. Electrochem. 1994, 10, 1. 28. Sheats, J. R.; Antoniadis, H.; Hueschen, M.; Leonard, W.; Miller, J.; Moon, R.; Roitman, D.; Stocking, A Science 1996, 273, 884. 29. Service, R. F. Science 1996, 273, 884. 30. Fujihira, M; Do, L.-M.; Koike, Α.; Han, E.-M. Appl. Phys. Lett. 1996, 68, 1787. 31. Scott, J. C.; Karg, S.; Carter, S. A. Appl. Phys. Lett. 1997, 70. 32. Miller, R. D. In Reference 13, pp. 329-456. 33. Donckers, M. C. J. M.; Silence, S. M.; Walsh, C. Α.; Hache, F.; Burland, D. M.; Moerner, W. E.; Twieg, R. J. Opt. Lett. 1993, 18, 1044. 34. Moerner, W. E.; Silence, S. M.; Hache, F.; Bjorklund, G. C. J. Opt. Soc. Amer. 1994, B11, 320. 35. Meerholz, K.; Volodin B. L.; Sandalphon; Kippelen, B.; Peyghambarian N. Nature 1994, 371, 497. 36. Lundquist, P. M.; Wortmann, R.; Gelentneky, C.; Twieg, R. J.; Jurich, M.; Lee, V. Y.; Moylan C. R.; Burland, D. M. Science 1996, 274, 1182. 37. Clark, Ν. Α.; Lagerwall, S. T.; Appl Phys. Lett. 1980, 36, 899. 38. Blackwood, K. M., Science 1996, 273, 909. 39. Finkelmann, H.; Happ, M.; Portugall, M.; Ringsdorf, H. Makromol. Chem. 1978, 179, 2541. 40. Drzaic, S.; Gonzales A. M. Appl. Phys. Lett. 1993, 62, 1332. 41. Service, R. F. Science 1996, 273, 879.


Organic Thin Films - American Chemical Society

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