Thrust 1 - Preparation of Well-defined Surfaces, Interfaces, and Functional Structures
The interfaces between the various materials of an organic electronic device play a crucial role, affecting not only charge transfer, for example from metal electrode to organic semiconductor, but also the growth and morphology of successive layers. Thus, a major challenge undertaken in Thrust 1 is to control the formation of interfaces relevant to subsequent device assembly and performance.
Effect of surface properties on morphology and growth of organic semiconductors. The crystal morphology of organic semiconductors dictates their charge transport properties. Bao developed a new method for templated growth of large arrays of organic single crystals by vapor phase growth. This method has been shown to be applicable to a variety of organic semiconductors, such as anthracene, tetracene, pentacene, rubrene, C60, and TCNQ. This method can potentially lead to production of organic transistors with better charge transport mobilities. (Figure 1). Knoll prepared ultra-flat Au surfaces using template-stripped Au (TSG) to investigate the effect of surface roughness on the templated growth. Bao and Knoll prepared devices with embedded TSG electrodes. A smooth transition from gold to the insulator polymer has been achieved. Moreover, AFM images after pentacene evaporation confirm a smooth interface between the gold electrode and dielectric layer. Higher charge carrier mobility is obtained compared to similar devices prepared with thermally evaporated Au electrodes.
Figure 1. (a) Organic transistors with single crystal rubrene. (b) Pentacene and (c) rubrene single crystals grown in designated locations.
The ordering and morphology of a dielectric surface has a dramatic impact on organic semiconductor growth and morphology, which in turn affects the charge carrier mobility. Bao, Scott, and Miller compared an octadecyl substituted silane monolayer on SiO2 prepared by LB film technique and vapor phase treatment. They found that pentacene deposited on the highly ordered LB films gave much higher field-effect mobility (3 cm2/Vs vs. 0.5 cm2/Vs)
Surface functionalization of organic transistors for bio-sensor applications. For operation in biological media, the materials used in organic transistor devices need to be protected against the attack of water, ions, small organic and inorganic molecules, as well as biopolymers found typically in bio-analyte buffer solutions. Knoll used pulsed plasma techniques to prepare ultrathin protective coatings containing F, NH2, or COOH functional groups. Bao and Knoll deposited these ultrathin plasma layers directly onto organic transistors. They found the F plasma layer did not cause any degradation of the device performance while the NH2 and COOH layers only resulted in a slight decrease in transistor mobility. Preliminary results with FETS where the plasma layer is modified with PNAs detected the binding of complementary DNAs.
Preparation of organic thin films towards monolayer transistors. The chemistry and crystal structure of polydiacetylenes (PDA) make them attractive for use in monolayer electronic devices such as field-effect transistors and (bio)chemical sensors. Miller and Scott have developed polarized microscopy techniques to visualize monolayer PDA films on solid substrates. The images (see Figure 2) reveal 2D crystalline domains extending over 100 mm. Scott and Miller have shown (see below) that the conjugated monolayer exhibits field-effect gating in a transistor device.
Figure 2. Dichroism of monolayer poly(10,12 pentacosa-diynoic-ethanolamide) revealed by polarization microscopy. Field of view is 1 mm and the polarizers are rotated 90 ° between the two images.
Fuller and Miller have investigated the use of extensional surface flows to improve the orientation of a film of the polymer polydiacetylene ethanolamide (PDA-EA). Experiments using linear dichroism have shown that extensional flow induced in the film significantly improves the orientation of the film. The surface flow pattern optimizes the orientation and geometry of the monomers for topochemical polymerization.
In another approach to the fabrication of monolayer transistors, Frechet and Bao have synthesized organo-silane amphiphiles with oligothiophenes and oligophenylene thiophenes, respectively, as the semiconductor. Scott and Miller prepared LB films using these molecules. A mobility of 10-3 cm2/Vs was obtained with the LB monolayer film. This is the first time that relatively high mobility is obtained with a monolayer over a large electrode gap (a few micrometers vs. tens of nanometers reported in the literature) indicating long range order is achieved in the LB film.
Figure 3: DNA decorated e-beam patterned surface.
Nanoscale patterning of functionalized surfaces for biomolecule adsorption. Frommer is writing features for site selective adsorption of DNA and other bio-compatible species. The goal is to produce surfaces capable of oriented binding of complex DNA structures, in collaboration with Paul Rothemund at Caltech. Frommer investigated two approaches. The first approach uses an electric field applied between an atomic force microscope (AFM) tip and a derivatized silicon wafer to selectively oxidize patterns on the wafer surface in ambient atmosphere. By varying the parameters of electric field, humidity, and write speed, the characteristic feature dimension is optimized to produce sub-100 nm lines. This patterned oxide provides a platform for selective functionalization, ultimately allowing placement of biomolecules at specific locations on the surface.
The second patterning method (in collaboration with Rettner, Bozano, Deline, IBM) uses an electron beam lithography tool to print a nanoscale space charge layer on an oxidized silicon surface. Fluorescently tagged DNA fragments (with Wallraff, IBM) are then selectively bound to the charge-patterned regions from buffered, aqueous solution. The resulting selective adsorption is observed using fluorescence microscopy and AFM, as shown in the Figure. While the adsorption predictably occurs specifically on regions exposed to the electron beam, surprisingly the fluorescing DNA-coated regions appear as topographical depressions in AFM images. Possible explanations of this unexpected result are being pursued, including the effect of a charged sample on an AFM probe and selective dissolution of the e-beamed silica surface. Work is on-going to shed light on the role of surface charge in aqueous adsorption of molecular species.
