Growth of pentacene on parylene and on BCB for organic transistors application , and DNA - based nanostructures studied by Amplitude – Modulation

This work reports the various aspects of the application of atomic force microscopy (AFM), for the characterization oforganic semiconductors and DNA-based arrays, for organic electronics and biological applications. On these soft surfaces, theamplitude modulation AFM mode was chosen. This choice is argued by a study of dissipative processes, performed on a particularsample, a DNA chip. We showed the influence of experimental parameters on the topographic and phase image quality. Bycalculating the dissipative energy, it was shown that the dissipation on the DNA chip was mainly induced by a viscoelastic tip-sample interaction.The AFM study of the "thickness-driven” pentacene growth was made to link the morphology to the nature of the substrateand to the electrical performance of created pentacene-based Organic Field Effect Transistor (OFET). Deposited on two polymersubstrates, parylene and benzocyclobutene (BCB), pentacene has been characterized for nanoscale film thicknesses between 6 and60nm. It has been shown that the larger grains were created for a deposited thickness of 30nm. Spectroscopic AFM mode was used asan alternative to the method of contact angles, to measure local surface energy. Decrease of surface energy is characteristic of a moreordered surface and was measured for a thickness of 30 nm of pentacene deposited on both substrates. Models of statistical analysisof spectral images, based on the Power Spectrum Distribution (PSD) have been used to explain the morphology of pentacene films.In addition, these models have provided a comprehensive description not only of the accessible surface of the sample, but also of itsinternal structural properties. Highlighted in the models, the critical thickness of 30 nm corresponds to a transition from theorthorhombic phase to the triclinic phase for pentacene molecules deposited on parylene. Similarly, a polymorphic transition occurson the BCB. On OFETs, based on pentacene on BCB, the largest mobility of 3.1x10 cm2/Vs corresponds to the pentacene layer of30nm, that shows a better ordering of the orthorhombic molecular packing in comparison with the triclinic packing.The molecular arrangement of X and Y structures based on DNA was observed, by AFM, in air and in two buffer solutions of Trisand HEPES on a mica substrate. It was shown that the treatment of the mica by Ni + ions increases the strength of the DNA/substrateinteraction and reduces the diffusivity of the molecules. In air, wired macromolecules containing one double-stranded structure areobserved on untreated mica and macromolecules with a 2D geometry on pretreated mica. Onto a non-treated, the greater thermalmotion of weakly bounded to mica DNA molecules leads to the rupture of intermolecular bonding and the forming structures aremore simple and not branched. The organization is different in solutions of Tris and HEPES. In the Tris solution, containing Mgcations, the arrangement leads to a well-organized 2D architecture. In the HEPES solution, containing Ni cations, the ionic strengthis 10 times lower, this leads to a breaking of the bonds previously formed between DNA and mica. However, DNA molecules arenear each other due to a partial substitution of already adsorbed Mg + cations by Ni 2 + cations of higher affinity with the mica. Theseresults show that the two liquids promote a 2D assembly. In air, the networks are not stable and the few observed ones remain in adendritic structure on the surface of pretreated mica and as a linear macromolecule on the untreated mica.