Grazing Incidence X-ray Research Articles

PEG-induced orientation enhancement of supported lipid multi-bilayers revealed by surface X-ray diffraction

Polyethylene glycol (PEG) is one of the most widely used polymers in cosmetic and biomedical products. The supported lipid multi-bilayer (SLMB) is an important model membrane, which is highly advantageous to surface diffraction techniques. In this study, in-situ grazing-incidence X-ray diffraction, X-ray reflectivity, and X-ray rocking scans are combined to systematically study the effects of PEG on SLMBs. A structure restoring effect of PEG is revealed, which is indicated by the reduced bilayer periodicity, improved molecular packing, and enhanced membrane orientation of the SLMBs. Moreover, this effect is found to be irreversible; i.e., removing PEG does not eliminate this effect. This finding may contribute to a better understanding of PEG-membrane interaction and the further biomedical applications of PEG.

Characterizing Interfacial Structures of Dye-Sensitized Solar Cell Working Electrodes.

In this feature article, we discuss the fundamental use of materials-characterization methods that directly determine structural information on the dye···TiO2 interface in dye-sensitized solar cells (DSCs). This interface is usually buried within the DSC and submerged in solvent and electrolyte, which renders such metrological work nontrivial. We will show how ex-situ X-ray reflectometry (XRR), atomic-force microscopy (AFM), grazing-incidence X-ray scattering (GIXS), pair-distribution-function analysis of X-ray diffraction data (gaPDF), and in-situ neutron reflectometry (NR) can be used to deliver specific structural information on the dye···TiO2 interface regarding dye anchoring, dye aggregation, molecular dye orientation, intermolecular spacing between dye molecules, interactions between the dye molecules and the TiO2 surface, and interactions between the dye molecules and the electrolyte components and precursors. Some of these materials-characterization techniques have been developed specifically for this purpose. We will demonstrate how the direct acquisition of such information from materials-characterization experiments is crucial for assembling a holistic structural picture of this interface, which in turn can be used to develop DSC design guidelines. Moreover, we will show how these methodologies can be used in the experimental-validation process of "design-to-device" pipelines for big-data- and machine-learning-based materials discovery. We conclude with an outlook on further developments of this design-to-device approach as well as the materials characterization of more dye···TiO2 interfacial structures that involve known DSC dyes using the methods described herein. In addition, we propose to combine these formally disparate metrologies so that their complementary merits can be exploited simultaneously. New metrologies of this kind could serve as a "one-stop-shop" for the materials characterization of surfaces, interfaces, and bulk structures in DSCs and other devices with layered architectures.

The magnetic anisotropy of Tb-phthalocyanine films effected by molecular orientation

Tb-phthalocyanine (TbPc2) films have been grown on 25 °C and 150 °C glass substrates. The temperature dependence of magnetic hysteresis loops for the TbPc2 films was measured and analyzed. The out-of-plane and in-plane magnetic hysteresis loops are nearly overlapped for the TbPc2 film grown at 25 °C, indicating the film is basically magnetically isotropic. In contrast, the TbPc2 film grown at 150 °C shows a strong magnetic anisotropy, and its out-of-plane magnetic moment at 30 kOe is about 13% larger than that of in-plane value for the measurement at 2 K. To explore the inter-relation between the molecular stacking structure and magnetic properties of the TbPc2 films, the X-ray diffractometer and grazing-incidence wide-angle X-ray scattering were used to characterize the crystalline structures of TbPc2 films. The results demonstrate that TbPc2 film grown at 25 °C is nearly amorphous. While, TbPc2 film grown at 150 °C shows a clear crystalline texture. The relationship between molecular orientation and magnetic properties has been theoretically analyzed. Based on the consideration of magnetic anisotropy axis of TbPc2 molecules, the out-of-plane and in-plane magnetic loops of the present two films, it can be concluded that the TbPc2 molecules prefer to lie flat close to the substrate surface on 150 °C glass substrate. This work is significant to guide the design of electron devices based on single-molecule magnets.

Thickness dependent structural and magnetic properties investigation of Co film interfaced with Hf

Thickness dependent structural and magnetic properties of Co films interfaced with Hf have been studied. X-ray reflectivity (XRR) and Grazing Incidence X-ray Diffraction (GIXRD) provides the structural properties of as-deposited samples. Film thicknesses as observed from XRR analysis are further implemented to study the magnetic properties of samples as function of Co film thickness, using magneto optical Kerr effect (MOKE) and polarized neutron reflectivity (PNR). MOKE results show the variation of magnetic coercivity with Co film thickness which exhibits a maximum value around 6 nm. Effective magnetic moment of samples as obtained from PNR fitting displays a linear variation with film thickness. Extrapolation of such variation on thickness axis show presence of magnetically dead layer (MDL) at interface, of thickness below 1 nm, in agreement with the reported values in literature for magnetic films interfaced with 5d heavy metals.

State of the art of ultra-thin gold layers: formation fundamentals and applications.

Fabrication of ultra-thin gold (Au) layers (UTGLs) has been regarded as the key technique to achieve applications with tunable optical response, flexible sensors and electronic devices. Various strategies have been developed to optimize the wetting process of Au, resulting in the formation of UTGLs at a minimum thickness. The related studies on UTGLs attracted huge attention in recent years. On the one hand, the growth processes of UTGLs on different substrates were in-depth probed by advanced in situ characterization techniques and the effects of optimization strategies on the growth of UTGLs were also revealed. On the other hand, based on the understanding of the growth behavior and the assistance of optimization strategies, various applications of UTGLs were realized based on optical/plasmon responses, surface-enhanced Raman scattering and as electrodes for various sensors and electronic devices, as well as being seed layers for thin film growth. In this focused review, both the fundamental and practical studies on UTGLs in the most recent years are elaborated in detail. The growth processes of UTGLs revealed by in situ characterization techniques, such as grazing-incidence small-angle X-ray scattering (GISAXS), as well as the state of the art of UTGL-based applications, are reviewed.

Understanding, quantifying, and controlling the molecular ordering of semiconducting polymers: from novices to experts and amorphous to perfect crystals.

Molecular packing and texture of semiconducting polymers are often critical to the performance of devices using these materials. Although frameworks exist to quantify the ordering, interpretations are often just qualitative, resulting in imprecise use of terminology. Here, we reemphasize the significance of quantifying molecular ordering in terms of degree of crystallinity (volume fractions that are ordered) and quality of ordering and their relation to the size scale of an ordered region. We are motivated in part by our own imprecise and inconsistent use of terminology in the past, as well as the need to have a primer or tutorial reference to teach new group members. We strive to develop and use consistent terminology with regards to crystallinity, semicrystallinity, paracrystallinity, and related characteristics. To account for vastly different quality of ordering along different directions, we classify paracrystals into 2D and 3D paracrystals and use paracrystallite to describe the spatial extent of molecular ordering in 1-10 nm. We show that a deeper understanding of molecular ordering can be achieved by combining grazing-incidence wide-angle X-ray scattering and differential scanning calorimetry, even though not all aspects of these measurements are consistent, and some classification appears to be method dependent. We classify a broad range of representative polymers under common processing conditions into five categories based on the quantitative analysis of the paracrystalline disorder parameter (g) and thermal transitions. A small database is presented for 13 representative conjugated and insulating polymers ranging from amorphous to semi-paracrystalline. Finally, we outline the challenges to rationally design more perfect polymer crystals and propose a new molecular design approach that envisions conceptual molecular grafting that is akin to strained and unstrained hetero-epitaxy in classic (compound) semiconductors thin film growth.

Selective CO2 adsorption and bathochromic shift in a phosphocholine-based lipid and conjugated polymer assembly.

We assemble a film of a phosphocholine-based lipid and a crystalline conjugated polymer using hydrophobic interactions between the alkyl tails of the lipid and alkyl side chains of the polymer, and demonstrated its selective gas adsorption properties and the polymer's improved light absorption properties. We show that a strong attractive interaction between the polar lipid heads and CO2 was responsible for 6 times more CO2 being adsorbed onto the assembly than N2, and that with repeated CO2 adsorption and vacuuming procedures, the assembly structures of the lipid-polymer assembly were irreversibly changed, as demonstrated by in situ grazing-incidence X-ray diffraction during the gas adsorption and desorption. Despite the disruption of the lipid structure caused by adsorbed polar gas molecules on polar head groups, gas adsorption could promote orderly alkyl chain packing by inducing compressive strain, resulting in enhanced electron delocalization of conjugated backbones and bathochromic light absorption. The findings suggest that merging the structures of the crystalline functional polymer and lipid bilayer is a viable option for solar energy-converting systems that use conjugated polymers as a light harvester and the polar heads as CO2-capturing sites.

Solid-solvent hybrid additive for the simultaneous control of the macro- and micro-morphology in non-fullerene-based organic solar cells

Most organic solar cells based on a bulk-heterojunction active layer are fabricated with the assistance of high boiling point solvent additives to optimize the phase separation of the donor and acceptor. The macroscopic phase separation can be controlled by this solvent additive. However, the control of the microscopic morphology (e.g., π-π stacking, orientation) of the inside phase is still dependent on the interaction and self-assembly characteristics of each donor and acceptor. In this work, we introduce a solid-solvent hybrid additive on PM6:Y6 solar cells to optimize both the macroscopic phase separation and the microscopic morphology at the same time. For the solvent additive, the well-known 1-CN solvent additive was used. For the solid additive, newly synthesized 3D star-shaped solid additives (Star-A and Star-F), which were delicately designed to achieve adequate electrical properties, electrostatic potential, and geometrical structure, were used to further optimize the microscopic morphology. Grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements showed that the star additives not only induce the dense and enhanced microscopic intermolecular π-π stacking within the phase, but also further optimize the phase separation. By adding only 1% star-series solid additives, a significantly enhanced efficiency was achieved from PM6:Y6 solar cells.

Improved electrical performance of solution-processed zinc oxide-based thin-film transistors with bilayer structures

A bilayer thin-film transistor (TFT) structure with ZnO and Al-doped ZnO (AZO) was fabricated using a solution process. The film thickness and sintering atmosphere of ZnO and AZO were controlled and then evaluated by measuring their electrical characteristics. By changing the sintering atmosphere, carriers attributed to oxygen vacancies in the thin film increased. Moreover, the ZnO single-layer TFT sintered in N2 exhibited good electrical characteristics. In the ZnO/AZO bilayer TFT laminated with the ZnO sintered in N2 and high-resistance AZO thin films, electrical characteristics, such as the On/Off ratio and subthreshold swing, improved compared to those of the ZnO-TFT. The On/Off ratio of the ZnO/AZO-TFT using the AZO thin film sintered in an O2 atmosphere notably improved to 3.7 × 105 compared to that of the ZnO-TFT (1.7 × 104). In addition, the subthreshold swing in the ZnO/AZO-TFT was 0.36 V/dec, while the field-effect mobility was 2.2 × 10−1 cm2/Vs, thereby exhibiting the best electrical characteristics among the fabricated samples. According to the grazing-incidence X-ray diffraction measurement, the ZnO particle size and crystallinity of the ZnO/AZO bilayer were higher than those of the ZnO single layer. Therefore, improvement of the electrical characteristics was confirmed.

Order-to-Disorder Transition of Cylinder-Forming Block Copolymer Films Confined within Neutral Interfaces

We present the phase behavior of cylinder-forming block copolymer (BCP) films confined within neutral interfaces through theory and experiment. The Langevin field-theoretic simulation (L-FTS) is utilized to account for the compositional fluctuation effect on the order-to-disorder transition (ODT), and the ultraviolet divergence in the L-FTS result is eliminated by renormalization of the Flory–Huggins parameter χ, allowing accurate comparison of thickness-dependent (χN)ODT, where N is the polymerization index. The L-FTS is accelerated by utilizing a deep learning method, and we managed to achieve a 50–60% reduction in simulation time. The L-FTS results show that (χN)ODT decreases as the film thickness decreases at a very large value of the invariant polymerization index N̅, and BCPs undergo a phase transition from disordered to cylindrical phases through a spherical phase as predicted by the self-consistent field theory calculation. By performing the L-FTS at experimentally relevant low N̅, we show that the BCPs undergo a direct transition from disordered to cylindrical phases, and the (χN)ODT value of cylinder-forming BCPs confined within neutral interfaces increases as the film thickness decreases. To verify and complement the theoretical results, we investigate the ODT temperature (TODT) of cylinder-forming polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) films supported on a neutral mat of P(S-r-2VP) random copolymers using grazing-incidence small-angle X-ray scattering. Our film experiments with low-N̅ PS-b-P2VP confirm the direct transition from disordered to cylindrical phases and the decrease in TODT with decreasing thickness below the onset thickness above which the TODT levels off.

Kinetic viscoelasticity during early polymer-polymer spinodal dewetting

The dewetting kinetics of a supported polymer bilayer were measured in situ using coherent grazing-incidence x-ray scattering. X-ray photon correlation spectroscopy provides both the two-time correlation functions and the cross-correlation function which measures the average spatial shift of the speckles produced by the coherent x rays. The stress in the ultrathin top dewetting film can be directly observed due to the exquisite sensitivity to sample curvature changes provided by the x-ray speckle correlation functions. The hole-opening events in the film are found to be associated with significant changes to the stress. These results are interpreted through an analogy between viscoelastic spinodal dewetting and early-stage bulk viscoelastic phase separation. The frequency of hole-initiation events during dewetting decreases with time as a power law, and the power-law exponent can be linked to nonlinear viscoelastic effects, showing similarity in their stress relief dynamics to aftershock decays.

Hyper 100 °C Langmuir-Blodgett (Langmuir-Schaefer) Technique for Organized Ultrathin Film of Polymeric Semiconductors.

In this study, we advanced the conventional Langmuir-Blodgett (LB) method to a high-temperature range (above 100 °C) using a newly manufactured LB machine, which is adaptable to a high-boiling-point subphase, as a universally usable apparatus. A sophisticated trough design, with homogeneous heating capability up to approximately 200 °C, together with automatic film compression and Langmuir-Schaefer type film transfer, enabled the fabrication of highly aligned thin films of polymeric semiconductors with uniaxial alignment of polymer backbones, which is desirable for efficient charge transport. Herein, ultrathin films of semicrystalline thiophene-based semiconductors were prepared on ethylene glycol and heated to 80 °C. The analyses of the transferred films with pressure-area isotherms, atomic force microscopy (AFM), polarized optical microscopy (POM), and grazing-incidence wide-angle X-ray scattering (GIWAXS) indicated that the proposed high-temperature LB method allows ideal deposition of high-quality ultrathin films with molecular layer precision at the selected high-temperature conditions. Furthermore, preparing thin-film donor-acceptor-type copolymers in ionic liquids at high temperatures (up to 140 °C) was a challenging task that was successfully demonstrated in this study. Highly ordered thin films of donor-acceptor polymers with a uniaxial backbone orientation were obtained only at 140 °C. The obtained semicrystalline thin films with uniaxially aligned polymer backbones significantly contribute to the two-dimensional overlap of molecular orbitals, which is likely to promote charge transport. The use of the manufactured automatic LB machines is advantageous for better quality films prepared at higher temperatures (even above 100 °C) from various technical viewpoints, including homogeneous heating, constant compression, and automatic film transfer. The novel methodology proposed herein expands the possibilities of the Hyper 100 °C Langmuir-Blodgett technique, which has not been accessible by the conventional LB method with the aqueous subphase.

Green-Solvent-Processable Organic Photovoltaics with High Performances Enabled by Asymmetric Non-Fullerene Acceptors.

In this work, two asymmetric non-fullerene acceptors (NFAs), BTP-EHBO-4F and BTP-PHD-4F, are designed to be applied in green-solvent-processable organic photovoltaics (OPVs). BTP-EHBO-4F and BTP-PHD-4F show good solubilities in green solvent o-xylene. As a result, PM6:BTP-EHBO-4F-based devices exhibit outstanding photovoltaic performances using o-xylene as a solvent. By comparison, due to the poor solubility of Y6 in o-xylene, PM6:Y6-based devices show poor performances. Owing to the favorable phase separation, molecule packing, and orientation observed from atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, PM6:BTP-PHD-4F-based devices demonstrate a PCE of 15.91% with a VOC of 0.87 V, a JSC of 25.64 mA/cm2, and an FF of 71.34%. Moreover, PM6:BTP-EHBO-4F-based devices exhibit an impressive PCE of 16.82% with a VOC of 0.85 V, a JSC of 26.12 mA/cm2, and an FF of 75.78%, which is outstanding for OPVs using o-xylene as a solvent.

Effect of various additives on aluminum oxide thin films prepared by dip coating, thermal behavior, kinetics and optical properties

Aluminum oxide thin films attract research interest due to their properties. Aluminum acetate was used as an Al source with acetic acid, oxalic acid, and nitric acid as additives. The transmittance and the thickness of the films strongly depend on the additives, with the approximate bandgap energy changing from 5 ev to 5.4 ev. The aluminum oxide film deposited by dip-coating is presented great uniform surface morphology. The knowledge of the degradation kinetics of materials is essential for investigating the thermal stability of compounds. The acetic acid thin film proved to be the most efficient additive by demonstrating interesting optoelectronic properties. The thin films deposited by dip-coating were characterized by using X-ray grazing incidence diffraction, SEM, UV-Visible spectroscopy. Gamma aluminum oxide thin films prepared by acetic acid can be a good candidate for a wide range of optical applications.

Absolute radiation tolerance of amorphous alumina coatings at room temperature

In this study structural and mechanical properties of a 1 μm thick Al2O3 coating, deposited on 316L stainless steel by Pulsed Laser Deposition (PLD), subjected to high energy ion irradiation were assessed. Mechanical properties of pristine and ion-modified specimens were investigated using the nanoindentation technique. A comprehensive characterization combining Transmission Electron Microscopy and Grazing-Incidence X-ray Diffraction provided deep insight into the structure of the tested material at the nano- and micro-scale. Variation in the local atomic ordering of the irradiated zone at different doses was investigated using a reduced distribution function analysis obtained from electron diffraction data. Findings from nanoindentation measurements revealed a slight reduction in hardness of all irradiated layers. At the same time TEM examination indicated that the irradiated layer remained amorphous over the whole dpa range. No evidence of crystallization, void formation or element segregation was observed up to the highest implanted dose. Reported mechanical and structural findings were critically compared with each other pointing to the conclusion that under given irradiation conditions, over the whole range of doses used, alumina coatings exhibit excellent radiation resistance. Obtained data strongly suggest that investigated material may be considered as a promising candidate for next-generation nuclear reactors, especially LFR-type, where high corrosion protection is one of the highest prerogatives to be met.

SRG X-ray orbital observatory

The orbital observatory Spectrum-Roentgen-Gamma (SRG), equipped with the grazing-incidence X-ray telescopes Mikhail Pavlinsky ART-XC and eROSITA, was launched by Roscosmos to the Lagrange L2 point of the Sun–Earth system on July 13, 2019. The launch was carried out from the Baikonur Cosmodrome by a Proton-M rocket with a DM-03 upper stage. The German telescope eROSITA was installed on SRG under an agreement between Roskosmos and the DLR, the German Aerospace Agency. In December 2019, SRG started to perform its main scientific task: scanning the celestial sphere to obtain X-ray maps of the entire sky in several energy ranges (from 0.2 to 8 keV with eROSITA, and from 4 to 30 keV with ART-XC). By mid-June 2021, the third six-month all-sky survey had been completed. Over a period of four years, it is planned to obtain eight independent maps of the entire sky in each of the energy ranges. The sum of these maps will provide high sensitivity and reveal more than three million quasars and over one hundred thousand massive galaxy clusters and galaxy groups. The availability of eight sky maps will enable monitoring of long-term variability (every six months) of a huge number of extragalactic and Galactic X-ray sources, including hundreds of thousands of stars with hot coronae. In addition, the rotation of the satellite around the axis directed toward the Sun with a period of four hours enables tracking the faster variability of bright X-ray sources during one day every half year. The chosen strategy of scanning the sky leads to the formation of deep survey zones near both ecliptic poles. The paper presents sky maps obtained by the telescopes on board SRG during the first survey of the entire sky and a number of results of deep observations performed during the flight to the L2 point in the frame of the performance verification program, demonstrating the capabilities of the observatory in imaging, spectroscopy, and timing of X-ray sources. It is planned that in December 2023, the observatory will for at least two years switch to observations of the most interesting sources in the sky in triaxial orientation mode and deep scanning of selected celestial fields with an area of up to 150 square degrees. These modes of operation were tested during the performance verification phase. Every day, data from the SRG observatory are dumped onto the largest antennas of the Russian Deep Space Network in Bear Lakes and near Ussuriysk.

Optical Reflection from Unforbidden Diffraction of Block Copolymer Templated Gyroid Films.

We present material substitutions and optical characterization of block copolymer (BCP)-templated gyroid structures that are obtained from a volume-asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA). In addition to the structural analyses reported earlier, we elucidate the optical responses to the nonaffine gyroid planes, in which the PMMA channels are complexed with Al2O3 by sequential infiltration synthesis and the organic components are further eliminated to produce an inorganic air-Al2O3 gyroid film. Grazing-incidence small-angle X-ray scattering measurements show that three-dimensional gyroid lattices are retained in both in-plane and out-of-plane directions through these material substitution processes. Our BCP-templated gyroid films respond to the middle UV wavelength from 200 to 300 nm, and peculiar optical reflectance peaks correlate with the unforbidden {110} diffraction spots. Together with the red- and blue-shifts of the reflectance peaks by the component substitutions, the air-Al2O3 gyroid structure reveals the high-amplitude spectrum due to the large refractive-index difference between channel and matrix.

Deposition of the tin sulfide thin films using ALD and a vacuum annealing process for tuning the phase transition

Tin sulfide (SnSx) thin films deposited by atomic layer deposition (ALD) under various deposition conditions have unique properties. Such films have two phases, tin monosulfide (SnS) and tin disulfide (SnS2), that depend on the composition ratio of the thin film, and these materials can be applied to a wide range of technologies, such as solar cells, optical sensors, and transistors. In this study, we deposit amorphous tin sulfide thin films at 100 °C using ALD and perform post annealing at various temperatures to control the phase transition from SnS2 to SnS. The post annealing temperature-dependent phase transition was analyzed through Raman spectroscopy, grazing-incidence X-ray diffraction (GI-XRD), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV–vis) spectroscopy, and ultraviolet photoelectron spectroscopy (UPS). Thermodynamic calculations were performed to determine that the initial temperature of the phase transition correspond to the post annealing temperature of 350 °C. The phases and crystal structures of the films annealed at different temperatures were investigated by Raman and GI-XRD. The chemical bonding of the films was analyzed through XPS, revealing that the binding energy of Sn4+ states shifted to the lower binding energy of Sn2+ states with increasing annealing temperatures. The electronic band structures of the films were calculated by UV–vis and UPS. The band gaps of the 300 °C, 350 °C, 400 °C annealed films were 2.26 eV, 2.05 eV, 1.63 eV, respectively. The tin sulfide thin films had n-type characteristics when annealed at 300 °C, which changed to p-type characteristics when annealed at 350 °C and 400 °C. Through this study, we can investigate phase transition of tin sulfides and correlate fabrication conditions to both n-type and p-type characteristics, which can be applied to synthesize the desired films for application in various devices.

Investigation of Active Catalysts at Work.

ConspectusEven after being in business for at least the last 100 years, research into the field of (heterogeneous) catalysis is still vibrant, both in academia and in industry. One of the reasons for this is that around 90% of all chemicals and materials used in everyday life are produced employing catalysis. In 2020, the global catalyst market size reached $35 billion, and it is still steadily increasing every year. Additionally, catalysts will be the driving force behind the transition toward sustainable energy. However, even after having been investigated for 100 years, we still have not reached the holy grail of developing catalysts from rational design instead of from trial-and-error. There are two main reasons for this, indicated by the two so-called “gaps” between (academic) research and actual catalysis. The first one is the “pressure gap”, indicating the 13 orders of magnitude difference in pressure between the ultrahigh vacuum lab conditions and the atmospheric pressures (and higher) of industrial catalysis. The second one is the “materials gap”, indicating the difference in complexity between single-crystal model catalysts of academic research and the real catalysts, consisting of metallic nanoparticles on supports, promoters, fillers, and binders. Although over the past decades significant efforts have been made in closing these gaps, many steps still have to be taken. In this Account, I will discuss the steps we have taken at Leiden University to further our fundamental understanding of heterogeneous catalysis at the (near-)atomic scale. I will focus on bridging the pressure gap, though we are also working on closing the materials gap. Over the past years, we developed state-of-the-art equipment that is able to investigate the (near-)atomic-scale structure of the catalyst surface during the chemical reaction using several surface-science-based techniques such as scanning tunneling microscopy, atomic force microscopy, optical microscopy, and X-ray-based techniques (surface X-ray diffraction, grazing-incidence small-angle X-ray scattering, and X-ray reflectivity, in collaboration with ESRF). Simultaneously with imaging the surface, we can investigate the catalyst’s performance via mass spectrometry, enabling us to link changes in the catalyst structure to its activity, selectivity, or stability. Although we are currently investigating many industrially relevant catalytic systems, I will here focus the discussion on the oxidation of platinum during, for example, CO and NO oxidation, the NO reduction reaction on platinum, and the growth of graphene on liquid (molten) copper. I will show that to be able to obtain the full picture of heterogeneous catalysis, the ability to investigate the catalyst at the (near-)atomic scale during the chemical reaction is a must.

Quasi-Homoepitaxial Junction of Organic Semiconductors: A Structurally Seamless but Electronically Abrupt Interface between Rubrene and Bis(trifluoromethyl)dimethylrubrene

Single-crystalline organic semiconductors exhibiting band transport have opened new possibilities for the utilization of efficient charge carrier conduction in organic electronic devices. The epitaxial growth of molecular materials is a promising route for the realization of well-crystallized organic semiconductor p–n junctions for optoelectronic applications enhanced by the improved charge carrier mobility. In this study, the formation of a high-quality crystalline interface upon “quasi-homoepitaxial” growth of bis(trifluoromethyl)dimethylrubrene (fmRub) on the single-crystal surface of rubrene was revealed by using out-of-plane and grazing-incidence X-ray diffraction techniques. Ultraviolet photoelectron spectroscopy results indicated abrupt electronic energy levels and the occurrence of band bending across this quasi-homoepitaxial interface. This study verifies that the minimization of the lattice mismatch enhances the crystalline qualities at the heterojunctions even for van der Waals molecular condensates, potentially opening an untested route for the realization of high-mobility organic semiconductor optoelectronics.