Spectroscopic Evidence for a Substrate Dependent Orientation of Sexithiophene Thin Films Deposited onto Oriented PTFE

Quasi-one-dimensional cyano-phenylene aggregates: Uniform molecule alignment contrasts varying electrostatic surface potential

The epitaxial growth of the mono-functionalized para-quaterphenylene molecule CNHP4 on muscovite mica is investigated. The vacuum deposited molecules aggregate into nanofibers of varying morphology. Due to muscovite's cm symmetry, almost mutually parallel fibers grow. Polarized light microscopy together with X-ray diffraction resolves the projected orientation of the molecules on the substrate surface and within the fibers. Several different contact planes with the substrate are detected. For all of them, the molecules orient with their long molecule axis approximately perpendicular to the grooved muscovite direction, so that the alignment of the molecules on the substrate is uniform. Kelvin probe force microscopy finds vastly different electrostatic properties of different fiber types and facets.

Directed growth of mixed self-assembled monolayers on a nanostructured template: a step toward the patterning of functional molecular domains

We report on the elaboration of networks of SAM domains. More precisely, we show the feasibility in making arrays of functionalized alkylthiol nanodomains bordered with an alkylthiol matrix. The several step process relies on the replication of a self-organized cobalt array grown on Au(111). The SAM process takes place in solution. The chemical affinity of thiol for gold leads to the selective grafting of molecules on the surface. After having removed the inorganic array, alkylthiol functionalized with a terthiophene unit is grafted in free gold areas. The efficiency of the replication of the initial template depends on the stability of the first SAM. We also investigate electronic tunnel transport through oligothiophene islands with the STM. The variation of the molecular contrast with bias voltage between the two molecular species indicates a potential resonant tunneling mechanism through the orbitals of the aromatic compound.

Guiding crystallization around bends and sharp corners

Control over the molecular orientation in organic thin films is demonstrated with precise in-plane spatial resolution over large areas. By exploiting the differential crystallization rates on substrates with different surface energies, the radial symmetry of spherulitic growth can be disrupted by preferentially selecting the molecular orientations that promote growth along the paths of the underlying patterns.

Orientation of α-sexithiophene on friction-transferred polythiophene film

Controlling the molecular orientation of the conjugated oligomer, α-sexithiophene (6T), is crucial to improve organic optoelectronic device performance. Most 6T molecules evaporated onto quartz and SiO(2)/Si substrates orient nearly perpendicular to the substrate. Here, we report the formation of oriented thin films of 6T on in-plane-oriented polythiophene (PT) films formed by the friction-transfer method. 6T was evaporated onto oriented PT films under vacuum. The films were investigated by polarized optical microscopy, polarized ultraviolet-visible light (UV-vis) absorption spectroscopy, and grazing incidence X-ray diffraction measurement (GIXD). In all spectra, larger absorbance derived from PT and 6T was observed, in parallel polarization to the friction direction, compared to that of orthogonal polarization. These results indicate that the 6T molecular axis is aligned in the friction direction (PT chain direction) of PT films. GIXD also confirmed that the 6T molecular axis was aligned parallel to the PT chain axis. In contrast, 6T molecules evaporated onto quartz and poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-coated silicon substrates aligned nearly perpendicular to the substrate. These results indicate that oriented PT films induce 6T orientation parallel to the PT chain direction.

Photoluminescence Study of Sexithiophene Thin Films

To determine the exciton diffusion length of sexithiophene (6T) thin films, quenching of the photoluminescence (PL) of vacuum-deposited 6T films on TiO2 and on quartz has been investigated. For films with a thickness of more than 22 nm and at temperatures below 100 K, additional PL lines appear in luminescence spectra. This feature is related to the structural properties of 6T films. The PL intensity is thermally activated with an activation energy of 18 meV on TiO2 and 6 meV on quartz. When 6T is applied on TiO2, exciton quenching occurs for films up to 120 nm. For 6T on quartz this value is reduced to 60 nm. By comparing the relative luminescence intensities of 6T on quartz and on TiO2 substrates, an exciton diffusion length of 60 +/- 5 nm is derived.