Autoassemblage de monocouches organiques à faible température

At ambient temperature monolayer phases of most alkane chain molecules exhibit phases, characterised by the rotation of the chain around the molecular axis. Consequently, these phases are only weakly ordered rather than crystalline. In order to achieve crystalline ordering, the energy of the system needs to be reduced. New cryo-protective liquid subphases extend the accessible range to much lower temperatures than the freezing threshold of water. Monolayers of common surfactants like fatty acids and phospholipids were prepared under constant volume/constant surface pressure conditions and their phase behaviour was studied upon cooling. First insights into film stability and evolution upon cooling were achieved by Langmuir isotherms and GIXOS, while the in-plane ordering was investigated by GIXD. The molecules were found to adapt crystalline phases by a purely entropy driven self-assembly process, reaching packing densities similar to three dimensional single crystals. The evolution of the structure parameters compare to those of bulk alkanes undergoing the rotator-crystalline phase transition. XPCS was employed to relate the surface dynamics of the system to the appearance of crystalline phases. The phase transition was found to be accompanied by a change of the dynamics from propagating to overdamped capillary waves.A température ambiante, la plupart des monocouches constituées de molécules formées de chaînes alcanes présentent des phases caractérisées par la rotation des chaînes autour de leur axe moléculaire. Afin d'obtenir un ordre cristallin, il est impératif de réduire l'énergie du système. De nouvelles sous-phases liquides utilisant des agents cryoprotectifs permettent d'accéder à des températures plus faibles que la température de glace de l'eau. Les monocouches de surfactants tels que les acides gras ou les phospholipides ont été préparées à volume et pression surfacique constants. L'étude de la stabilité des films par refroidissement a été effectuée par des isothermes de Langmuir et GIXOS et celle de l'ordre dans le plan par GIXD. Les molécules adoptent des phases cristallines qui sont induites par un processus d'autoassemblage d'origine exclusivement entropique et dont la densité de compactage est similaire à celle d'un monocristal à trois dimensions. La technique d'XPCS a enfin été employée pour relier la dynamique de surface du système étudié à la formation des phases cristallines

Low temperature self-assembly of organic monolayers

Monolagen von Alkan Molekülen besitzen bei Raumtemperatur Phasen, die durch eine Rotation der Alkanketten um ihre Längsachse charakterisiert sind. Als eine Folge dieser Rotation sind die entsprechenden Phasen eher schwach geordnet als kristallin. Um eine kristalline Ordung der Moleküle zu erreichen, muss die Energie des Systems verringert werden. Neue flüssige Subphasen erweitern den verfügbaren Temperaturbereich zu Temperaturen tiefer als der Gefrierpunkt von Wasser. Monolagen von gewöhnlichen Tensiden wie Fettsäuren wurden bei konstantem Spreitvolumen/konstantem Oberflächendruck prepariert, um anschliessend das Phasenverhalten beim Abkühlen zu studieren. Erste Einsichten bezüglich der Stabilität der Filme und die Entwicklung ihrer Struktur während des Abkühlens wurden durch Langmuir Isothermen und diffuser Streuung unter streifendem Einfall (GIXOS) erreicht. Die kristalline Organisation in der Ebene der Monolagen wurde mit Röntgendiffraktion unter streifendem Einfall (GIXD) untersucht. Die Moleküle nehmen durch einen entropie- gesteuerten Selbstorganisationsprozess kristalline Phasen an, in denen die Packungsdichte die von dreidimensionalen Einkristallen erreicht. Die Entwicklung der Strukturparameter verläuft ähnlich zu denen von Volumensystemen von Paraffinen während des Rotator-Kristall Phasenübergangs. Das für die kristalline Phase charakteristische „Fischgrätenmuster“ der Alkanketten konnte für die Fettsäueremonolage anhand von Strukturfaktorrechnungen nachgewiesen werden. Mittels Röntgen Korrelations Spektroskopie (XPCS) wurde die Oberflächendynamik der Systeme mit dem Auftreten einer kristallinen Phase in Relation gesetzt. Es stellte sich heraus, dass der Phasenübergang von einem Wechsel der Dynamik von propagierenden zu überdämpften Kapillarwellen begleitet wird

Local step-flow dynamics in thin film growth with desorption

Desorption of deposited species plays a role in determining the evolution of surface morphology during crystal growth when the desorption time constant is short compared to the time to diffuse to a defect site, step edge or kink. However, experiments to directly test the predictions of these effects are lacking. Novel techniques such as \emph{in-situ} coherent X-ray scattering can provide significant new information. Herein we present X-ray Photon Correlation Spectroscopy (XPCS) measurements during diindenoperylene (DIP) vapor deposition on thermally oxidized silicon surfaces. DIP forms a nearly complete two-dimensional first layer over the range of temperatures studied (40 - 120 $^{\circ}$C), followed by mounded growth during subsequent deposition. Local step flow within mounds was observed, and we find that there was a terrace-length-dependent behavior of the step edge dynamics. This led to unstable growth with rapid roughening ($\beta>0.5$) and deviation from a symmetric error-function-like height profile. At high temperatures, the grooves between the mounds tend to close up leading to nearly flat polycrystalline films. Numerical analysis based on a 1 + 1 dimensional model suggests that terrace-length dependent desorption of deposited ad-molecules is an essential cause of the step dynamics, and it influences the morphology evolution.Comment: 21 pages, 9 figures, and one tabl

Autoassemblage de monocouches organiques à faible température

At ambient temperature monolayer phases of most alkane chain molecules exhibit phases, characterised by the rotation of the chain around the molecular axis. Consequently, these phases are only weakly ordered rather than crystalline. In order to achieve crystalline ordering, the energy of the system needs to be reduced. New cryo-protective liquid subphases extend the accessible range to much lower temperatures than the freezing threshold of water. Monolayers of common surfactants like fatty acids and phospholipids were prepared under constant volume/constant surface pressure conditions and their phase behaviour was studied upon cooling. First insights into film stability and evolution upon cooling were achieved by Langmuir isotherms and GIXOS, while the in-plane ordering was investigated by GIXD. The molecules were found to adapt crystalline phases by a purely entropy driven self-assembly process, reaching packing densities similar to three dimensional single crystals. The evolution of the structure parameters compare to those of bulk alkanes undergoing the rotator-crystalline phase transition. XPCS was employed to relate the surface dynamics of the system to the appearance of crystalline phases. The phase transition was found to be accompanied by a change of the dynamics from propagating to overdamped capillary waves.A température ambiante, la plupart des monocouches constituées de molécules formées de chaînes alcanes présentent des phases caractérisées par la rotation des chaînes autour de leur axe moléculaire. Afin d'obtenir un ordre cristallin, il est impératif de réduire l'énergie du système. De nouvelles sous-phases liquides utilisant des agents cryoprotectifs permettent d'accéder à des températures plus faibles que la température de glace de l'eau. Les monocouches de surfactants tels que les acides gras ou les phospholipides ont été préparées à volume et pression surfacique constants. L'étude de la stabilité des films par refroidissement a été effectuée par des isothermes de Langmuir et GIXOS et celle de l'ordre dans le plan par GIXD. Les molécules adoptent des phases cristallines qui sont induites par un processus d'autoassemblage d'origine exclusivement entropique et dont la densité de compactage est similaire à celle d'un monocristal à trois dimensions. La technique d'XPCS a enfin été employée pour relier la dynamique de surface du système étudié à la formation des phases cristallines

des Fachbereichs Physik der Universität Dortmund

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Efficient end-to-end simulation of time-dependent coherent X-ray scattering experiments

Physical optics simulations for beamlines and experiments allow users to test experiment feasibility and optimize beamline settings ahead of beam time in order to optimize valuable beam time at synchrotron light sources like NSLS-II. Further, such simulations also help to develop and test experimental data processing methods and software in advance. The Synchrotron Radiation Workshop (SRW) software package supports such complex simulations. We demonstrate how recent developments in SRW significantly improve the efficiency of physical optics simulations, such as end-to-end simulations of time-dependent X-ray photon correlation spectroscopy experiments with partially coherent undulator radiation (UR). The molecular dynamics simulation code LAMMPS was chosen to model the sample: a solution of silica nanoparticles in water at room temperature. Real-space distributions of nanoparticles produced by LAMMPS were imported into SRW and used to simulate scattering patterns of partially coherent hard X-ray UR from such a sample at the detector. The partially coherent UR illuminating the sample can be represented by a set of orthogonal coherent modes obtained by simulation of emission and propagation of this radiation through the coherent hard X-ray (CHX) scattering beamline followed by a coherent-mode decomposition. GPU acceleration is added for several key functions of SRW used in propagation from sample to detector, further improving the speed of the calculations. The accuracy of this simulation is benchmarked by comparison with experimental data