Reflectivity is typically carried out in the hard X-rays to study morphology and structure of layered systems, where electron density contrast between materials is exploited. In the soft X-rays, reflectivity at resonance provides additional advantages in terms of atomic and depth-resolved investigation of the chemical, structural and magnetic properties of a variety of systems, including organic materials. The technique is non-destructive. The sampling depth is not limited to the near-surface region, as for electron yield spectroscopies, but deep buried interfaces can be accessed as well (several tenths of nm). We developed a protocol [1,2] to get simultaneous quantitative information on the structure, interface morphology, chemical properties and optical anisotropies of layered materials with sub-nm depth resolution. The method is based on the quantitative prediction of the spectral line-shape at specific elemental edges through: 1) the simulation (from the first principles) of the dielectric tensor of each material in a stack; 2) the simulation of the propagation of the electromagnetic field in the layers and the computation of the (anisotropic) optical properties (reflectivity); 3) the comparison and fitting of the simulation to the experiment.
Soft X-ray reflectivity at carbon K-edge with linearly polarized photons is used here to study Pentacene thin films produced by Supersonic Molecular Beam Deposition (SuMBD) on SiO2. SuMBD allows a precise control of molecular directionality and velocity during growth, with the formation of highly flat organic layers that present enhanced electronic transport properties in organic thin film transistors . Films of different thickness, up to 20 nm, are examined. It is observed that molecules adopt a standing orientation, with the tilt angle of the long molecular axis with respect to the substrate normal that progressively reduces with the increasing film thickness. This is expected to influence the molecular reciprocal interaction and the charge transport parallel to the dielectric substrate in devices. The effect of the deposition of an Au electrode overlayer on top of the organic film is also studied, as far as the organic molecular organization below the metal contact is concerned. It is found that the pentacene molecules at the buried interface with Au assume a flat orientation, that propagates two-tree layers into the organic film. This can have a strong impact on charge injection at electrodes. We believe that studies of this type, aiming at understanding the molecular orientation at deep buried interfaces, are fundamental to finely tune organic thin film thickness and growth modes and to guide optimization of devices architectures.
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