A heating stage up to 1173 K for X-ray diffraction studies in the whole orientation space

Memory Effect by Melt Crystallization Observed in Polymorphs of a Benzothieno-Benzothiophene Derivative

This work provides a comprehensive illustration of a crystalline melt memory effect recorded for three solvates of the 2,7-bis(2-(2-methoxyethoxy)ethoxy)benzo[b]benzo[4,5] thieno[2,3-d]thiophene (OEG-BTBT) molecule with dichloromethane (DCM) molecules. Combined optical microscopy and X-ray diffraction measurements at different temperatures are used to get an overview of the structural and morphological properties like melting points, isotropic transition temperatures, induction times, and crystallization kinetics of the three forms. An outstanding observation is made upon annealing the three polymorphs at temperatures well above their respective melting points as well as above the optical clearance temperature. After cooling back to room temperature, recrystallization results in the formation of the initial phase present before the annealing process. This melt memory effect is observed for all three solvates. These observations can be correlated to the strong interaction between the DCM molecules and the oligoethylene glycol side chains, even in the molten state. This conclusion rationalizes the experimental observation made upon solvent vapor annealing of the crystalline sample with DCM, which unambiguously transformed the system into a disordered state.

Polymorph screening at surfaces of a benzothienobenzothiophene derivative: discovering new solvate forms

The discovery of new polymorphs opens up unique applications for molecular materials since their physical properties are predominantly influenced by the crystal structure type. The deposition of molecules at surfaces offers great potential in the variation of the crystallization conditions, thereby allowing access to unknown polymorphs. With our surface crystallization approach, four new phases are found for an oligoethylene glycol-benzothienobenzothiophene molecule, and none of these phases could be identified via classical polymorph screening. The corresponding crystal lattices of three of the new phases were obtained via X-ray diffraction (XRD). Based on the volumetric considerations together with X-ray fluorescence and Raman spectroscopy data, the phases are identified as solvates containing one, two or three solvent molecules per molecule. The strong interaction of dichloromethane with the oligoethylene glycol side chains of the molecules may be responsible for the formation of the solvates. Temperature-dependent XRD reveals the low thermal stability of the new phases, contrary to the thermodynamically stable bulk form. Nevertheless, the four solvates are stable under ambient conditions for at least two years. This work illustrates that defined crystallization at surfaces enables access to multiple solvates of a given material through precise and controlled variations in the crystallization kinetics.

Humidity Response of Cellulose Thin Films

Cellulose–water interactions are crucial to understand biological processes as well as to develop tailor made cellulose-based products. However, the main challenge to study these interactions is the diversity of natural cellulose fibers and alterations in their supramolecular structure. Here, we study the humidity response of different, well-defined, ultrathin cellulose films as a function of industrially relevant treatments using different techniques. As treatments, drying at elevated temperature, swelling, and swelling followed by drying at elevated temperatures were chosen. The cellulose films were prepared by spin coating a soluble cellulose derivative, trimethylsilyl cellulose, onto solid substrates followed by conversion to cellulose by HCl vapor. For the highest investigated humidity levels (97%), the layer thickness increased by ca. 40% corresponding to the incorporation of 3.6 molecules of water per anhydroglucose unit (AGU), independent of the cellulose source used. The aforementioned treatments affected this ratio significantly with drying being the most notable procedure (2.0 and 2.6 molecules per AGU). The alterations were investigated in real time with X-ray reflectivity and quartz crystal microbalance with dissipation, equipped with a humidity module to obtain information about changes in the thickness, roughness, and electron density of the films and qualitatively confirmed using grazing incidence small angle X-ray scattering measurements using synchrotron irradiation.

Phase Transition toward a Thermodynamically Less Stable Phase: Cross-Nucleation due to Thin Film Growth of a Benzothieno-benzothiophene Derivative

The molecule 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene is an organic semiconductor, with outstanding properties in terms of molecular packing and its use in organic electronics. The asymmetric shape of the molecule causes a double layer crystal structure at room temperature. In this work we report its thin film growth by physical vapor deposition starting from the monolayer regime up to thick films. The films are studied in terms of their morphology, crystallographic properties, and thermal stability by atomic force microscopy and X-ray diffraction methods. It is found that the bulk molecular packing of the bilayer is formed at the initial thin film growth stage. After a thickness of one double layer, a transition into a new polymorph is observed which is of metastable character. The new phase represents a single layer phase; the crystal structure could be solved by a combination of X-ray diffraction and molecular dynamics simulations. The observed thin film growth is outstanding in terms of surface crystallization: the formation of a metastable phase is not associated with the initial thin film growth, since the first growth stage represents rather the bulk crystal structure of this molecule. Its formation is associated with cross-nucleation of one polymorph by another, which explains why a metastable phase can be formed on top of a thermodynamically more stable phase.

Molecular Disorder in Crystalline Thin Films of an Asymmetric BTBT Derivative

The molecule 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) is an organic semiconductor with outstanding performance in thin-film transistors. The asymmetric shape of the molecule causes an unusual phase behavior, which is a result of a distinct difference in the molecular arrangement between the head-to-head stacking of the molecules versus head-to-tail stacking. Thin films are prepared at elevated temperatures by crystallization from melt under controlled cooling rates, thermal-gradient crystallization, and bar coating at elevated temperatures. The films are investigated using X-ray diffraction techniques. Unusual peak-broadening effects are found, which cannot be explained using standard models. The modeling of the diffraction patterns with a statistic variation of the molecules reveal that a specific type of molecular disorder is responsible for the observed peak-broadening phenomena: the known head-to-head stacking within the crystalline phase is disturbed by the statistic integration of reversed (or flipped) molecules. It is found that 7-15% of the molecules are integrated in a reversed way, and these fractions are correlated with cooling rates during the sample preparation procedure. Temperature-dependent in situ experiments reveal that the defects can be healed by approaching the transition from the crystalline state to the smectic E state at a temperature of 145 °C. This work identifies and quantifies a specific crystalline defect type within thin films of an asymmetric rodlike conjugated molecule, which is caused by the crystallization kinetics.

In-Situ XRD Study of Phase Transformation Kinetics in a Co-Cr-W-Alloy Manufactured by Laser Powder-Bed Fusion

The additive manufacturing process of laser powder-bed fusion (L-PBF) is an increasingly popular approach for patient-specific production of dental frameworks made from Co-Cr alloys. Macroscopically, frameworks produced in this way exhibit high anisotropy especially in Young’s modulus, and are missing standardized requirements. Microscopically, pronounced texture and high residual stresses are characteristic. To reduce resulting detrimental effects, the as-built (AB) parts are heat treated. Dependent on the treatment temperature, effects like the transformation of the γ-phase matrix in the AB condition to ϵ-phase, precipitation, stress relief, and grain growth were observed. While the existence of these processes was established in the past, little is known about their kinetics. To fill this gap, these effects were studied with in-situ X-ray diffraction (XRD) methods in isothermal heat treatments (HTs) at four different sample surface temperatures TS reaching from 650∘C to 900∘C. Furthermore, room temperature ex situ XRD and SEM/EDS measurements completed the analysis. An evaluation of the datasets, with single peak fitting and QXRD methods, yielded the following results. In the HTs below a certain threshold, a γ-to-ϵ transformation was observed in the sample bulk and close to the sample surface. In the latter case, evidence for a partially strain-induced transformation related to oxide formation was present. Above this threshold and possibly slightly below, σ- and Laves-phase precipitated. Additionally, peak profile evolutions hinted at a drop of inter- and intragranular stresses within the first 30 to 60 min. Therefore, an HT of about 30 to 60 min slightly above the threshold is proposed as optimal for reducing residual stresses while retaining a predominantly single-phased microstructure, possibly superior in corrosion properties and likewise in bio-compatibility.

Cold Crystallization of the Organic n-Type Small Molecule Semiconductor 2-Decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene S,S,S',S'-Tetraoxide

The asymmetric n-type Ph-BTBT-C₁₀ derivative 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene S,S,S',S'-tetraoxide is structurally investigated in the thin film regime. After film preparation by spin coating and physical vapor deposition, a rather disordered structure is observed, with a strong change of its internal degree of order upon heating. At 95 °C, a transition into a layered structure of upright standing molecules without any in-plane order appears, and at 135 °C, crystallization takes place. This phase information is combined with surface morphological studies and charge carrier mobility measurements to describe the structure and thin film transistor applicability of this molecule.

Multilayer Density Analysis of Cellulose Thin Films

An approach for the multilayer density analysis of polysaccharide thin films at the example of cellulose is presented. In detail, a model was developed for the evaluation of the density in different layers across the thickness direction of the film. The cellulose thin film was split into a so called "roughness layer" present at the surface and a "bulk layer" attached to the substrate surface. For this approach, a combination of multi-parameter surface plasmon resonance spectroscopy (SPR) and atomic force microscopy (AFM) was employed to detect changes in the properties, such as cellulose content and density, thickness and refractive index, of the surface near layer and the bulk layer. The surface region of the films featured a much lower density than the bulk. Further, these results correlate to X-ray reflectivity studies, indicating a similar layered structure with reduced density at the surface near regions. The proposed method provides an approach to analyse density variations in thin films which can be used to study material properties and swelling behavior in different layers of the films. Limitations and challenges of the multilayer model evaluation method of cellulose thin films were discussed. This particularly involves the selection of the starting values for iteration of the layer thickness of the top layer, which was overcome by incorporation of AFM data in this study.

Reversibility of temperature driven discrete layer-by-layer formation of dioctyl-benzothieno-benzothiophene films

Film forming properties of semiconducting organic molecules comprising alkyl-chains combined with an aromatic unit have a decisive impact on possible applications in organic electronics. In particular, knowledge on the film formation process in terms of wetting or dewetting, and the precise control of these processes, is of high importance. In the present work, the subtle effect of temperature on the morphology and structure of dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films deposited on silica surfaces by spin coating is investigated in situ via X-ray diffraction techniques and atomic force microscopy. Depending on temperature, bulk C8-BTBT exhibits a crystalline, a smectic A and an isotropic phase. Heating of thin C8-BTBT layers at temperatures below the smectic phase transition temperature leads to a strong dewetting of the films. Upon approaching the smectic phase transition, the molecules start to rewet the surface in the form of discrete monolayers with a defined number of monolayers being present at a given temperature. The wetting process and layer formation is well defined and thermally stable at a given temperature. On cooling the reverse effect is observed and dewetting occurs. This demonstrates the full reversibility of the film formation behavior and reveals that the layering process is defined by an equilibrium thermodynamic state, rather than by kinetic effects.

Multiple scattering in grazing-incidence X-ray diffraction: impact on lattice-constant determination in thin films

Dynamical scattering effects are observed in grazing-incidence X-ray diffraction experiments using an organic thin film of 2,2':6',2''-ternaphthalene grown on oxidized silicon as substrate. Here, a splitting of all Bragg peaks in the out-of-plane direction (z-direction) has been observed, the magnitude of which depends both on the incidence angle of the primary beam and the out-of-plane angle of the scattered beam. The incident angle was varied between 0.09° and 0.25° for synchrotron radiation of 10.5 keV. This study reveals comparable intensities of the split peaks with a maximum for incidence angles close to the critical angle of total external reflection of the substrate. This observation is rationalized by two different scattering pathways resulting in diffraction peaks at different positions at the detector. In order to minimize the splitting, the data suggest either using incident angles well below the critical angle of total reflection or angles well above, which sufficiently attenuates the contributions from the second scattering path. This study highlights that the refraction of X-rays in (organic) thin films has to be corrected accordingly to allow for the determination of peak positions with sufficient accuracy. Based thereon, a reliable determination of the lattice constants becomes feasible, which is required for crystallographic structure solutions from thin films.

Interfacial Morphology and Effects on Device Performance of Organic Bilayer Heterojunction Solar Cells

The effects of interface roughness between donor and acceptor in a bilayer heterojunction solar cell were investigated on a polymer-polymer system based on poly(3-hexylthiophene) (P3HT) and poly(dioctylfluorene-alt-benzothiadiazole) (F8BT). Both polymers are known to reorganize into semicrystalline structures when heated above their glass-transition temperature. Here, the bilayers were thermally annealed below glass transition of the bulk polymers (≈140 °C) at temperatures of 90, 100, and 110 °C for time periods from 2 min up to 250 min. No change of crystallinity could be observed at those temperatures. However, X-ray reflectivity and device characteristics reveal a coherent trend upon heat treatment. In X-ray reflectivity investigations, an increasing interface roughness between the two polymers is observed as a function of temperature and annealing time, up to a value of 1 nm. Simultaneously, according bilayer devices show an up to 80% increase of power conversion efficiency (PCE) for short annealing periods at any of the mentioned temperatures. Together, this is in agreement with the expectations for enlargement of the interfacial area. However, for longer annealing times, a decrease of PCE is observed, despite the ongoing increase of interface roughness. The onset of decreasing PCE shifts to shorter durations the higher the annealing temperature. Both, X-ray reflectivity and device characteristics display a significant change at temperatures below the glass transition temperatures of P3HT and F8BT.

A new micro-furnace forin situhigh-temperature single-crystal X-ray diffraction measurements

A new micro-furnace equipped with an H-shaped resistance heater has been developed to conductin situsingle-crystal X-ray diffraction experiments at high temperature. The compact design of the furnace does not restrict access to reciprocal space out to 2θ = 60°. Therefore, unit-cell parameters and intensity data can be determined to a resolution of 0.71 Å with Mo radiation. The combined use of mineral phases with well characterized lattice expansion (e.g.pure Si and SiO2quartz) and a small-diameter (0.025 mm) K-type thermocouple allowed accurate temperature calibration from room temperature to about 1273 K and consequent evaluation of thermal gradients and stability. The new furnace design allows temperatures up to about 1273 K to be reached with a thermal stability better than ±5 K even at the highest temperatures. Measurements of the lattice thermal expansion of pure silicon (Si), pure synthetic grossular garnet (Ca3Al2Si3O12) and quartz (SiO2) are presented to demonstrate the performance of the device. Its main advantages and limitations and important considerations for using it to perform high-temperature diffraction measurements are discussed.

A compact high vacuum heating chamber for in-situ x-ray scattering studies

A very compact multi purpose high vacuum heating chamber for x-ray scattering techniques was developed. The compact design allows the chamber to be installed on high precision diffractometers which usually cannot support heavy and/or large equipment. The chamber is covered by a Be dome allowing full access to the hemisphere above the sample which is required for in-plane grazing incident x-ray diffraction and out-off plane wide angle x-ray diffraction.

Lateral gradients of phases, residual stress and hardness in a laser heated Ti0.52Al0.48N coating on hard metal

The influence of a local thermal treatment on the properties of Ti-Al-N coatings is not understood. In the present work, a Ti_0.52Al_0.48N coating on a WC-Co substrate was heated with a diode laser up to 900 °C for 30 s and radially symmetric lateral gradients of phases, residual stress and hardness were characterized ex-situ using position-resolved synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nanoindentation. The results reveal (i) a residual stress relaxation at the edge of the irradiated area and (ii) a compressive stress increase of few GPa in the irradiated area center due to the Ti-Al-N decomposition, in particular due to the formation of small wurtzite (w) AlN domains. The coating hardness increased from 35 to 47 GPa towards the center of the heated spot. In the underlying heated substrate, a residual stress change from about - 200 to 500 MPa down to a depth of 6 μm is observed. Complementary, in-situ high-temperature X-ray diffraction analysis of stresses in a homogeneously heated Ti_0.52Al_0.48N coating on a WC-Co substrate was performed in the range of 25-1003 °C. The in-situ experiment revealed the origin of the observed thermally-activated residual stress oscillation across the laser heated spot. Finally, it is demonstrated that the coupling of laser heating to produce lateral thermal gradients and position-resolved experimental techniques opens the possibility to perform fast screening of structure-property relationships in complex materials.

Interface induced crystal structures of dioctyl-terthiophene thin films

Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.

High-temperature residual stresses in thin films characterized by x-ray diffraction substrate curvature method

A new x-ray technique to determine temperature dependencies of macroscopic stresses in thin films by characterizing the substrate curvature is introduced. The technique is demonstrated on polycrystalline TiN and Al thin films deposited on Si(100) wafers. The structures are thermally cycled in the temperature range of 25-400 degrees C using a newly developed heating chamber attached to a commercial x-ray diffractometer. The curvature of the freestanding samples was determined by the rocking curve measurement of substrate Si 400 reflections at different lateral positions of the samples, and the stresses are calculated using Stoney's formula. The results show that the magnitude of the stress is in good agreement with the results obtained by other techniques. For the practical application of the technique, the sample mounting and the temperature control are of great importance.