Simultaneous Monitoring of Molecular Thin Film Morphology and Crystal Structure by X-ray Scattering (original) (raw)
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Physical Review Letters, 2006
We study kinetically controlled orientational and structural transitions of molecular thin films during growth in situ and in real time, using diindenoperylene (DIP) as an example. By time-resolved surfacesensitive x-ray scattering (out of plane and in plane), we follow the organic molecular beam deposition of DIP on silicon oxide, on stepped sapphire, and on rubrene as an organic model surface. We identify transitions for the few-monolayer (ML) regime, as well as for thick (several 10's of ML) films. We show that the differences in the interaction of DIP with the substrate change the thickness as well as temperature range of the transitions, which include (transient) strain, subtle changes of the orientation, as well as complete reorientation. These effects should be considered rather general features of the growth of organics, which, with its orientational degrees of freedom, is qualitatively different from growth of inorganics.
Physical Review B
Understanding the growth of organic semi-conducting molecules with shape anisotropy is of high relevance to the processing of optoelectronic devices. This work provides insight into the growth of thin films of the prototypical rodlike organic semiconductor diindenoperylene on a microscopic level, by analyzing in detail the film morphology. We model our data, which were obtained by high-resolution grazing incidence small angle x-ray scattering (GISAXS), using a theoretical description from small angle scattering theory derived for simple liquids. Based on form factor calculations for different object types we determine how the island shapes change in the respective layers. Atomic force microscopy measurements approve our findings.
Polarized X-ray scattering reveals non-crystalline orientational ordering in organic films
Nature Materials, 2012
Molecular orientation critically influences the mechanical, chemical, optical and electronic properties of organic materials. So far, molecular-scale ordering in soft matter could be characterized with X-ray or electron microscopy techniques only if the sample exhibited sufficient crystallinity. Here, we show that the resonant scattering of polarized soft X-rays (P-SoXS) by molecular orbitals is not limited by crystallinity and that it can be used to probe molecular orientation down to size scales of 10 nm. We first apply the technique on highly crystalline small-molecule thin films and subsequently use its high sensitivity to probe the impact of liquid-crystalline ordering on charge mobility in polymeric transistors. P-SoXS also reveals scattering anisotropy in amorphous domains of all-polymer organic solar cells where interfacial interactions pattern orientational alignment in the matrix phase, which probably plays an important role in the photophysics. The energy and q-dependence of the scattering anisotropy allows the identification of the composition and the degree of orientational order in the domains.
Solid State Communications, 2014
Depth profiling studies in 200 nm organic semiconductor (OSC) films on quartz substrate have been carried out using slow positron beam and X-ray reflectivity (XRR) techniques with the objective of examining structural inhomogeneities in as-deposited film and those annealed at high temperature. Grazing incidence X-ray diffraction and atomic force microscopy measurements are carried out to examine the crystallinity and surface morphology, respectively. In general, annealing is seen to modify the morphology and nanostructure. However, a significant inhomogeneity in nanostructure, marked by a disordered layer with low density region is observed in the film annealed at 200 1C from positron as well as XRR measurements. This study highlights the sensitivity of these techniques to defects and inhomogeneities in nanoscale that may have profound influence on device performance.
Thin Solid Films, 2006
The electronic structure of thin films of the organic semiconductors copper and vanadyl (VO) phthalocyanine (Pc) has been measured using resonant soft X-ray emission spectroscopy and resonant inelastic X-ray scattering. For Cu -Pc we report the observation of two discrete states near E F . This differs from published photoemission results, but is in excellent agreement with density functional calculations. For VO -Pc, the vanadyl species is shown to be highly localized. Both dipole forbidden V 3d to V 3d*, and O 2p to V 3d* charge transfer transitions are observed, and explained in a local molecular orbital model. D
In situ X-ray synchrotron study of organic semiconductor ultra-thin films growth
2006
In this work we present an X-ray diffraction study of the early stages of growth of an organic semiconductor (sexithiophene, T 6) thin film prepared by high vacuum sublimation. Specular reflectometry and grazing incidence X-ray diffraction were used to monitor the formation of T 6 films on silicon oxide. Our results show that T 6 grows as a crystalline layer from the beginning of the evaporation. The reflectometry analysis suggests that, in the range of rates and temperatures studied, the growth is never layer by layer but rather 3D in nature. In-plane GIXD has allowed us to observe for the first time a thin film phase of T 6 formed of molecules standing normal to the substrate and arranged in a compressed unit cell with respect to the bulk, i.e. the unit cell parameters b and c are relatively smaller. We have followed the dynamics of formation of this new phase and identified the threshold of appearance of the bulk phase, which occurs above %5-6 monolayers. These results are relevant to the problem of organic thin film transistors, for which we have previously demonstrated experimentally that only the first two monolayers of T 6 films are involved in the electrical transport. The layers above the second one do not effectively contribute to charge mobility, either because they are more ''disordered'' or because of a screening of the gate field.
Applied Surface Science, 1999
Based on photoelectron forwarding-scattering analysis, we present a new method using angle-resolved XPS to investigate Ž. Ž. the molecular packing structure of Langmuir-Blodgett LB films and self-assembled monolayers SAMs. It was found that Ž. Ž. the average tilt angle with surface normal of a bis 4-diethyannodithiobenzil nickel BDN molecule is about 408 for the Ž. BDN-SA stearyl alcohol LB films, which is in agreement with the results of FTIR. The thickness and orientation of the molecular for 12-alkyl thiols SAMs on gold substrate were also estimated by using ARXPS. Estimated tilt angle with surface normal for 12-alkyl thiols molecule is about 308, which was confirmed by other literature results.
The journal of physical chemistry. C, Nanomaterials and interfaces, 2015
Photoelectron emission microscopy (PEEM) and differential (optical) reflectance spectroscopy (DRS) have proven independently to be versatile analytical tools for monitoring the evolution of organic thin films during growth. In this paper, we present the first experiment in which both techniques have been applied simultaneously and synchronously. We illustrate how the combined PEEM and DRS results can be correlated to obtain an extended perspective on the electronic and optical properties of a molecular film dependent on the film thickness and morphology. As an example, we studied the deposition of the organic molecule α-sexithiophene on Ag(111) in the thickness range from submonolayers up to several monolayers.
Arxiv preprint arXiv:1102.2676, 2011
Synchrotron-based x-ray reflectivity is increasingly employed as an in situ probe of surface morphology during thin film growth, but complete interpretation of the results requires modeling the growth process. Many models have been developed and employed for this purpose, yet no detailed, comparative studies of their scope and accuracy exists in the literature. Using experimental data obtained from hyperthermal deposition of pentane and diindenoperylene (DIP) on SiO2, we compare and contrast three such models, both with each other and with detailed characterization of the surface morphology using ex-situ atomic force microscopy (AFM). These two systems each exhibit particular phenomena of broader interest: pentacene/SiO2 exhibits a rapid transition from rough to smooth growth. DIP/SiO2, under the conditions employed here, exhibits growth rate acceleration due to a different sticking probability between the substrate and film. In general, independent of which model is used, we find good agreement between the surface morphology obtained from fits to the in situx-ray data with the actual morphology at early times. This agreement deteriorates at later time, once the root-mean squared (rms) film roughness exceeds about 1 ML. A second observation is that, because layer coverages are under-determined by the evolution of a single point on the reflectivity curve, we find that the best fits to reflectivity data-corresponding to the lowest values of χ 2 ν-do not necessarily yield the best agreement between simulated and measured surface morphologies. Instead, it appears critical that the model reproduce all local extrema in the data. In addition to showing that layer morphologies can be extracted from a minimal set of data, the methodology established here provides a basis for improving models of multilayer growth by comparison to real systems.