The present invention is generally in the area of the fabrication of multilayer thin films of electrically conducting polymers.
Ultrathin organic films are currently gaining interest in many areas such as integrated optics, sensors, friction reducing coatings or surface orientation layers, as described, for example, by Swalen, et al., Langmuir 3, 932 (1987) and the Special Issue on Organic Thin Films, Adv. Mater. 3 (1991). Most of these functions require the preparation of well defined films composed of molecules with appropriate properties in a unique geometrical arrangement with respect to each other and to the substrate. Molecularly thin layers, particularly those deposited one layer at a time, offer the possibility to construct multilayer assemblies in which the distance between two molecules can be controlled in the Angstrom range. As a result, the processing and manipulation of polymer monolayers has recently emerged as a viable means to create ultra thin polymer coatings with well defined molecular organizations.
The surfaces of various materials can be dramatically modified by the deposition of monolayers of polymers onto the substrate. Such surface modification, for example, can be used to promote adhesion and lubrication, prevent corrosion, modify the electrical and optical properties of the material or create electroactive monolayers suitable for various optical and electronic sensors and devices. In many cases, however, the deposition of a single monolayer is not sufficient to achieve the desired changes in surface characteristics and it is necessary to coat the substrate with multiple layers of the polymer. This presents a significant problem as typical adsorption processes (both chemical and physical) are self-limiting, thereby only allowing the deposition of a single monolayer with thicknesses in the range of 5-30 .ANG.. Thus, one would like to have the ability to deposit multilayer thin films onto substrates.
There are four principal methods for the preparation of ultrathin multilayered films: solution casting, Langmuir-Blodgett technique, chemisorption, and the method of Decher, et al. Solution casting of preformed bilayer aggregates and annealing of spin coated films of copolymers yields layered structures, but the alignment of the layers and the positioning of molecules with respect to each other is limited. In the Langmuir-Blodgett (LB) technique, a film is prepared on the surface of water and then transferred onto solid substrates. This method, however, is inconvenient for automation and large scale application and is generally only applicable to flat substrates. Another method is based on chemisorption but requires exacting conditions and oftentimes multiple chemical reactions.
Recently, Decher, et al., Thin Solid Films 210/211, 831 (1992) and DE 4026978 (WO92-073188/10), have demonstrated that it is possible to build up multilayer thin films of polymers onto charged surfaces via the alternating deposition of polycations and polyanions. The basis for this multilayer assembly process is the ionic attraction of the permanently fixed charges that exist on the polycations (positive charge) and polyanions (negative charge). In essence, the excess charge of a polyion adsorbed onto a substrate surface is used to attract a polyion of the opposite charge onto the surface. Multilayer thin films are fabricated by simply alternating the dipping process.
The self-assembly process as described by Decher and coworkers is illustrated in FIG. 1. In this case, a positively charged glass substrate 10, created by suitable silane chemistry, is first immersed into a dilute solution of a polyanion 12 followed by immersion in a dilute solution of a polycation 14. As indicated in the figure, repetition of this cycle produces a multilayer thin film comprised of alternating layers of polycations 14 and polyanions 12. The thickness and conformation of each polymer layer deposited are determined by the chemistry of the depositing solution. For example, solutions with relatively high polyion concentrations or high ionic strengths favor the formation of thicker monolayers deposited in the form of random coils whereas very dilute solutions produce thinner monolayers with polymer chains adopting a more extended chain conformation.
This approach can be used to manipulate a variety of different polyions, including conjugated polyions (conjugated polymers fitted with ionizable sidegroups). These latter materials, frequently referred to as conducting polymers, are of interest due to their unusual electrical and optical properties which have their origin in the delocalized electronic states of the polymer's conjugated backbone. Although layer-by-layer deposition is possible with conjugated polyions, such materials simply do not exhibit the range of properties found in conjugated polymers that do not contain ionizable sidegroups. In short, the addition of ionizable sidegroups to the repeat structure of a conjugated polymer significantly compromises the level of conductivity achievable with the polymer and lowers its environmental stability. It is therefore much more desirable and useful to be able to fabricate more conventional conjugated polymers such as polyaniline and polypyrrole into ultrathin multilayer thin films.
Although these nonderivatized conjugated polymers have been identified as the source of many potentially useful electrical and optical properties, it is extremely difficult to process the electrically conductive forms of these materials into technologically useful forms. For example, many applications proposed for conducting polymers, such as microelectronic devices, chemical and biochemical sensors, electrochromic displays, anti-corrosion coatings and transparent antistatic coatings, require thin films of electrically conductive polymers with precisely controlled thicknesses and molecular organizations. Indeed, it is apparent that significant progress could be made towards the application of these materials if they could be obtained in large area, thin film forms in which both the thickness and molecular organization of the film were controllable at the molecular level.
Some attempts have been made to accomplish this important goal. For example, Milliken Corp. has disclosed a procedure for coating various textile fibers with uniform, electrically conductive films of polypyrrole and polyaniline. Specifically, the deposition of an electrically conductive coating of polypyrrole onto the fibers is accomplished by placing the fibers into a dilute aqueous solution of pyrrole that also contains an oxidizing agent such as ferric chloride and negative counterions suitable for enhancing the conductivity and conductivity stability of the polymer. The counterions are typically added in the form of sulfonic acids such as naphthalene disulfonic acid. A typical coating solution contains about 10 g/l ferric chloride anhydride, 5 g/l toluenesulfonic acid and 0.2 g of pyrrole monomer.
Although this chemistry is well suited for coating fibers with thin films of conducting polymers such as polypyrrole and polyaniline, it is not possible to use this process to fabricate multilayer thin films with precisely controlled thicknesses and layer sequences. Since the chemistry used to deposit a conducting polymer coating cannot be controlled at the molecular level it is extremely difficult to reproducibly deposit ultra-thin coatings in the range of 10-50 .ANG. thick. In short, the Milliken process and related processes for creating thin film coatings of conducting polymers simply do not provide layer-by-layer molecular level control over the deposition process nor the structure of the film. The fabrication of multilayer heterostructures of conducting polymers is therefore not possible with currently known techniques.
It is therefore an object of the present invention to provide a method for producing multilayered thin films of conducting polymers having high electrical conductivities which are environmentally stable.
It is another object of the present invention to provide multilayered thin films of conducting polymers with high electrical conductivities which are environmentally stable.
It is still another object of the present invention to provide methods for solubilizing p-doped conjugated polymers, and the solutions, for use in making multilayered thin films.