Self-Assembled Heterostructures of Electroactive Polymers: New Opportunities for Thin-Film Devices

Abstract

A process involving the alternate spontaneous adsorption of oppositely charged polymers onto substrate surfaces has been utilized to fabricate a number of new thin film multilayer heterostructures with electrical and optical properties that can be tuned at the molecular level. Examples of what can be accomplished with this new approach include the fabrication of thin film light emitting devices based on multilayer heterostructures of poly(p-phenylene vinylene) (PPV) and various polyanions and the fabrication of ultra-thin, electrically conductive multilayers of polyaniline. In the former case, light emitting devices with high brightness (>100 cd/m 2 in the range of 8-10 volts) and tunable emission wavelengths have been created through the use of multi-bilayer "slab" systems that are used to control the charge injection and transport characteristics of the device. It was also demonstrated that the presence of very thin (about 20 Å thick) insulating layers at the Al/polymer interface improves device efficiency by a factor of 2 -4. In the latter case, the layer-by-layer processing of polyaniline was accomplished by using hydrogen bonding interactions. This represents the first time that such secondary forces have been used to fabricate multilayer structures of polyaniline.The ability to control molecular and supramolecular structure at the nanoscale level is now recognized as one of the most promising means of exploiting the incredibly diverse electrical and optical properties of conjugated polymers and related electroactive organic materials. This is particularly true when one considers the use of these materials in thin film heterostructure devices where molecular-level control over layer thickness, interfaces (both internal and electrode interfaces) and thin film architecture is essential to realize the full potential of these systems. Recently, a very simple and yet powerful technique for manipulating charged polymers into multilayer thin films has been described (7,2). This approach, which involves the alternate deposition of oppositely charged polymers from dilute aqueous solutions, makes it possible to process conjugated polymers in a layer-by-layer manner with nanometer level control over the thickness of the individual layers. To date, our group has demonstrated that this process can be used to manipulate a wide variety of materials into multilayer thin films including, conjugated polyions (3,4), electrically conducting polymers (5,6), light

Polyaniline Langmuir–Blodgett films: formation and properties

“…From a practical perspective, LB films of PAN have already found several different applications, ranging from optical gas sensors for detecting H 2 S and NO x , 32,[46][47][48] to organic light emitting diodes. [49][50][51] Optimisation of these, and other applications, requires a detailed understanding of film structure and how it relates to function.…”

Enhanced optical and electrical properties of layer-by-layer luminescent films

“…Moreover, POMA was adsorbed in a layer-bylayer ͑LBL͒ fashion with nonself-limited adsorption, 11 which holds great promise for molecular control of the final su-a͒ Author to whom correspondence should be addressed; electronic mail: balogh@if.sc.usp.br pramolecular structure. The use of LBL films and the modification of electrodes to enhance luminescent properties of PPV polymers reported here take advantage of previous works by Rubner and co-workers, [12][13][14][15][16][17][18] as well as from other authors ͑see for instance 18 -22 and references therein͒, who sought to combine distinct materials in convenient means allowed by the LBL technique. Figure 1 shows the chemical structure of POMA in the emeraldine salt form, which can be dissolved in water and be used as polycation in the LBL process.…”

Patterning Thin Films of Poly(ethylene imine) on a Reactive SAM Using Microcontact Printing

“…Patterned thin films of polymers have a wide range of applications, for example, in preventing etching, in molecular electronics, − in optical devices, , in biological and chemical sensors, − and in tissue engineering ,− Crooks, Bergbreiter, Graigner, Rubner, Hammond, and others have developed methods for attaching polymers to SAMs using, for example, electrostatic adsorption of polyelectrolytes to an oppositely charged surface, − chemisorption of polymers containing reactive groups to a surface, , and covalent attachment of polymers to reactive SAMs; ,,,,− this work extends these techniques to the formation of patterned surfaces. There are presently only a few methods available for patterning thin films of polymers on SAMs; these include procedures based on photolithography, , templating the deposition of polymers using patterned SAMs, ,, and templating phase separation in diblock copolymers. , Patterned thin films of polymers attached covalently to the surface are more stable than are ones only physically adsorbed.…”

“…16 Thin films of polymers that incorporate reactive functional groups provide a surface that can be further modified by chemical reactions. 15,[17][18][19][20] Crooks, Bergbreiter, Graigner, Rubner, Hammond, and others have developed methods for attaching polymers to SAMs using, for example, electrostatic adsorption of polyelectrolytes to an oppositely charged surface, [21][22][23][24] chemisorption of polymers containing reactive groups to a surface, 25,26 and covalent attachment of polymers to reactive SAMs; 13,14,17,18,[27][28][29] for patterning thin films of polymers on SAMs; these include procedures based on photolithography, 28,29 templating the deposition of polymers using patterned SAMs, 23,24,30 and templating phase separation in diblock copolymers. 31,32 Patterned thin films of polymers attached covalently to the surface are more stable than are ones only physically adsorbed.…”