Evaluation of organic sub-monolayers by X-ray based measurements under gracing incident conditions

Werzer, Oliver; Stadlober, Barbara; Haase, A.; Flesch, Heinz-Georg; Resel, Roland

doi:10.1051/epjap/2009038

Cited by 9 publications

(10 citation statements)

References 29 publications

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“…The wetting layer, with a thickness of 0.6 nm, could be accounted for only if the molecules in this layer are considered to lie flat on the substrate, in contrast to the upright‐standing configuration of the bulk film, though it is not clear if molecules are ordered within this layer. For films of 1 , it was also found that the nucleation density of the wetting layer in submonolayer films (≈1 nm nominal thickness) is very sensitive to the nature of the substrate . Evidence of a wetting layer at the interface with the substrate was also found in films of 6a where the SIP was present but not those where the bulk form was present, while a wetting layer was also observed beneath the monolayer structure of 6b …”

Section: Origins Of Sipsmentioning

confidence: 85%

Substrate‐Induced and Thin‐Film Phases: Polymorphism of Organic Materials on Surfaces

Jones

Chattopadhyay

Geerts

et al. 2016

Adv Funct Materials

228

280

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2233wileyonlinelibrary.com to that of the bulk liquid; [ 7 ] order typically exists over a few molecular layers before rapidly decaying to bulk behavior as the distance from the wall increases. [ 8 ] Similar ordering effects can also be observed in the quasiliquids which form on the surface of many solids due to surface melting (also known as premelting): [ 9,10 ] the existence of a liquid-like layer a few molecules thick on the surface of a solid below the melting point of the bulk material. [ 11 ] The structure within this layer is also found to be different to that of the bulk isotropic liquid, [ 12 ] highlighting the ordering occurring at the interface. A converse effect, surface freezing, is also observed in some liquids (primarily n -alkanes with 16 ≤ n ≤ 50), where a solid monolayer may exist at an interface up to ≈30 °C above the bulk melting point while the rest of the compound is molten. [13][14][15] Such phase behavior is strongly dependent on molecular shape and has only been observed for chain molecules, which then form layers similar to self-assembled monolayers (SAMs) at the interface with the liquid bulk. [ 16 ] Further examples of molecular ordering at interfaces can be seen in materials which display liquid crystal (LC) phases, a phase behavior also dependent on molecular shape and generally observed for molecules with a highly anisotropic shape (e.g., rod-like, disc-like or bowl-like molecules). [ 17 ] LCs have properties of both solids and liquids and display long range order which may be orientational and/or positional; various types of LC phases (mesophases) are possible and normally exist only over a certain temperature range (i.e., they are thermotropic); a further property of LCs is that they generally reach thermodynamic equilibrium in the bulk and at interfaces as a result of their short relaxation time. Nematic (N) phases of calamitic LCs (rod-like molecules with cylindrical symmetry) have no positional order but an orientational order which can be described by a unit vector, n , known as the director, which represents the preferred molecular orientation ( Figure 1 , left). A scalar order parameter can also be defi ned which is related to the average angle the long molecular axes make to the director, n .Increased order is seen in smetic phases (Sm) which have both orientational and positional order; for example in a smetic A phase (SmA) molecules form layers (positional ordering of the molecular mass centers) which are oriented normal to the plane of the layers (orientational order), while smetic C phases (SmC) form layers where molecules are oriented with the long molecular axes tilted at an angle to the molecular layer normal However, the factors that drive such a process are not clearly understood or studied in depth. In this feature article, we review the current state of understanding concerning SIPs, giving examples of systems where SIPs have been observed, discussing their origins, and which questions remain to be answered. The role of the substrate in controlling the growth a...

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Section: Origins Of Sipsmentioning

confidence: 85%

Substrate‐Induced and Thin‐Film Phases: Polymorphism of Organic Materials on Surfaces

Jones

Chattopadhyay

Geerts

et al. 2016

Adv Funct Materials

228

280

View full text Add to dashboard Cite

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“…26 Finally, for PEN even the formation of an amorphous molecular 'wetting layer' decorating the SiO x substrate below thin-film phase was proposed. 27 The aim of the present work is, first, to challenge the common belief that the film thickness itself is the central parameter for thin-film phase formation, and, second, to identify further parameters influencing its formation. As model compound for our investigations we chose dihexylterthiophene (DH3T), which is composed of a thiophene backbone and hexyl end-substitutions that allow processing the material from solution.…”

Section: Introductionmentioning

confidence: 99%

Crystallisation kinetics in thin films of dihexyl-terthiophene: the appearance of polymorphic phases

et al. 2012

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The presence of surface-induced crystal structures is well known within organic thin films. However, the physical parameters responsible for their formation are still under debate. In the present work, we present the formation of polymorphic crystal structures of the molecule dihexyl-terthiophene in thin films. The films are prepared by different methods using solution-based methods like spin-coating, dip-coating and drop-casting, but also by physical vapour deposition. The thin films are characterised by various X-ray diffraction methods to investigate the crystallographic properties and by microscopy techniques (atomic force microscopy and optical microscopy) to determine the thin film morphologies. Three different polymorphic crystal structures are identified and their appearance is related to the film preparation parameters. The crystallisation speed is varied by the evaporation rate of the solvent and is identified as a key parameter for the respective polymorphs present in the films. Slow crystallisation speed induces preferential growth in the stable bulk structure, while fast crystallisation leads to the occurrence of a metastable thin-film phase. Furthermore, by combining X-ray reflectivity investigations with photoelectron spectroscopy experiments, the presence of a monolayer thick wetting layer below the crystalline film could be evidenced. This work gives an example of thin film growth where the kinetics during the crystallisation rather than the film thickness is identified as the critical parameter for the presence of a thin-film phase within organic thin films.

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“…It is known that surfaces are able to induce specific polymorphs upon deposition. 5 , 17 , 18 Even the occurrence of surface mediated phases can be observed with distinct properties from the bulk. 5 …”

mentioning

confidence: 99%

Surface Mediated Structures: Stabilization of Metastable Polymorphs on the Example of Paracetamol

Ehmann

Werzer

2014

Crystal Growth & Design

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The preparation of typically thermodynamically unstable polymorphic structures is a challenge. However, solid surfaces are well established aids for the formation and stabilization of polymorphic structures within, for instance, organic electronics. In this study, we report the stabilization of a pharmaceutically relevant substance via a solid surface at ambient conditions. Form III of paracetamol, which is typically unstable in the bulk at standard conditions, can be stabilized with a model silica surface by a standard spin coating procedure followed by rapid heat treatment. Such a preparation technique allows the use of atomic force microscopy and grazing incidence X-ray diffraction measurements revealing detailed information on the morphology and structure of the polymorph. Furthermore, the results exhibit that this polymorph is stable over a long period of time revealing surface mediated stabilization. These findings demonstrate a novel approach to provide thermodynamic stability when applied to similar molecules with specific applications.

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Evaluation of organic sub-monolayers by X-ray based measurements under gracing incident conditions

Cited by 9 publications

References 29 publications

Substrate‐Induced and Thin‐Film Phases: Polymorphism of Organic Materials on Surfaces

Substrate‐Induced and Thin‐Film Phases: Polymorphism of Organic Materials on Surfaces

Crystallisation kinetics in thin films of dihexyl-terthiophene: the appearance of polymorphic phases

Surface Mediated Structures: Stabilization of Metastable Polymorphs on the Example of Paracetamol

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