Oier Bikondoa

Structure of a model TiO2 photocatalytic interface

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis.

Structured oligo(aniline) nanofilms via ionic self-assembly

Conducting polymers have shown great potential for application in electronic devices. A major challenge in such applications is to control the supramolecular structures these materials form to optimise the functionality. In this work we probe the structure of oligo(aniline) thin films (of sub-μm thickness) drop cast on a silicon substrate using synchrotron surface diffraction. Self-assembly was induced through doping with an acid surfactant, bis(ethyl hexyl) phosphate (BEHP), resulting in the formation of well-ordered lamellae with the d-spacing ranging from 2.15 nm to 2.35 nm. The exact structural characteristics depended both on the oligomer chain length and film thickness, as well as the doping ratio. Complementary UV/Vis spectroscopy measurements confirm that such thin films retain their bulk electronic properties. Our results point to a simple and effective ionic self-assembly approach to prepare thin films with well-defined structures by tailoring parameters such as the oligomer molecular architecture, the nanofilm composition and the interfacial roughness.

Structure of the clean NiAl(1 1 0) surface and the Al2O3/NiAl(1 1 0) interface by measurements of crystal truncation rods

The clean NiAl(1 1 0) surface and the Al2O3/NiAl(1 1 0) interface have been investigated by synchrotron X-ray diffraction experiments. In the case of the oxide surface the analysis of the NiAl(1 1 0) crystal truncation rods (CTR) provide information about the interface between the Al2O3 film and the NiAl substrate. The analysis of the CTR-data shows clearly a rippled Ni-Al topmost surface with an amplitude value of RNi/Al = 0.16 ± 0.01 A for the clean surface and RNi/Al = 0.18 ± 0.02 A for the oxide covered surface. On the clean surface the Al sites are expanded by +3.8\% (outwards) and the Ni sites are contracted by -3.2\% (inwards) respect to the unrelaxed interlayer separation. For the oxide covered surface an increase of the expansion of the outermost Al atoms (+7.3\%) relative to their bulk positions has been found, while the Ni atoms remain (-0.9\%) at the bulk position. On both cases, an ideal surface stoichiometry (1:1) was obtained. However, some intermixing (chemical disorder) of one specimen in the sites of the other and vice versa was present (less than 4\%). This chemical disorder was not enhanced by the presence of the Al2O3 overlayer. Neither rippling nor oscillatory relaxation in deeper layers was detectable.

Absence of Structural Impact of Noble Nanoparticles on P3HT:PCBM Blends for Plasmon-Enhanced Bulk-Heterojunction Organic Solar Cells Probed by Synchrotron GI-XRD

The incorporation of noble metal nanoparticles, displaying localized surface plasmon resonance, in the active area of donor-acceptor bulk-heterojunction organic photovoltaic devices is an industrially compatible light trapping strategy, able to guarantee better absorption of the incident photons and give an efficiency improvement between 12% and 38%. In the present work, we investigate the effect of Au and Ag nanoparticles blended with P3HT: PCBM on the P3HT crystallization dynamics by synchrotron grazing incidence X-ray diffraction. We conclude that the presence of (1) 80 nm Au, (2) mix of 5 nm, 50 nm, 80 nm Au, (3) 40 nm Ag, and (4) 10 nm, 40 nm, 60 nm Ag colloidal nanoparticles, at different concentrations below 0.3 wt% for Au and below 0.1% for Ag in P3HT: PCBM blends, does not affect the behaviour of the blends themselves.

Grain rotation and lattice deformation during perovskite spray coating and annealing probed in situ by GI-WAXS

We report for the first time on grain rotation in CH3NH3PbI3 perovskite films for ∼12% efficient planar solar cells and present a new method for investigating their texture evolution during thermal annealing. Our technique is based on in situ 2D grazing incidence wide-angle X-ray scattering (GI-WAXS) and employs a 10 keV wide-focussed X-ray beam to simultaneously probe a large number of grains. The ability to track the texture dynamics from a statistically relevant number of spots diffracting from single grains during thermal annealing and in grazing incidence geometry can have applications understanding the processing dynamics of a range of new materials.

In situ simultaneous photovoltaic and structural evolution of perovskite solar cells during film formation

Metal-halide perovskites show remarkably clean semiconductor behaviour, as evidenced by their excellent solar cell performance, in spite of the presence of many structural and chemical defects. Here, we show how this clean semiconductor performance sets in during the earliest phase of conversion from the metal salts and organic-based precursors and solvent, using simultaneous in situ synchrotron X-ray and in operando current–voltage measurements on films prepared on interdigitated back-contact substrates. These structures function as working solar cells as soon as sufficient semiconductor material is present across the electrodes. We find that at the first stages of conversion from the precursor phase, at the percolation threshold for bulk conductance, high photovoltages are observed, even though the bulk of the material is still present as precursors. This indicates that at the earliest stages of perovskite structure formation, the semiconductor gap is already well-defined and free of sub-gap trap states. The short circuit current, in contrast, continues to grow until the perovskite phase is fully formed, when there are bulk pathways for charge diffusion and collection. This work reveals important relationships between the precursors conversion and device performance and highlights the remarkable defect tolerance of perovskite materials.

Ion beam sputtered surface dynamics investigated with two-time correlation functions: a model study.

Ion beam sputtering is a widely used technique to obtain patterned surfaces. Despite the wide use of this approach on different materials to create surface nanostructures, the theoretical model to explain the time evolution of the erosion process is still debated. We show, with the help of simulations, that two-time correlation functions can serve to assess the validity of different models. These functions can be measured experimentally with the x-ray photon correlation spectroscopy technique.

Ageing dynamics of ion bombardment induced self-organization processes

Instabilities caused during the erosion of a surface by an ion beam can lead to the formation of self-organized patterns of nanostructures. Understanding the self-organization process requires not only the in-situ characterization of ensemble averaged properties but also probing the dynamics. This can be done with the use of coherent X-rays and analyzing the temporal correlations of the scattered intensity. Here, we show that the dynamics of a semiconductor surface nanopatterned by normal incidence ion beam sputtering are age-dependent and slow down with sputtering time. This work provides a novel insight into the erosion dynamics and opens new perspectives for the understanding of self-organization mechanisms.

Interactions of Nanoparticles with Purple Membrane Films

Lamellar structures self-assembled from purple membranes (PM) of Halobacterium salinarum are promising building units for bio-electronic devices, due to proton pumping ability of the PM. The functionality and durability of such devices are hinged on the structural integrity of PM lamellae. Using X-ray diffraction, we examined the structure of PM multilayers on silicon when challenged with two types of nanoparticles (NPs): carboxymethyl-dextran coated magnetite (2.4 nm core size) and citrate-stabilised gold (5 nm core size). We tried to infiltrate the PM multilayers with the NPs using two alternative routes: facile penetration (FP) and co-assembly (CS) by solution mixing. We found that under all conditions the NPs did not disrupt the overall lamellar structure of the PM films or enter the inter-lamellar space, although the presence of NPs affected the self-assembly process of the PM films. This caused an increase in the disorder in the film structure, as assessed by the decreasing number of layers in the multilayer stack as the NP concentration increased. Despite this, UV-Vis spectroscopic measurements showed that the conformation of the retinal residue within the protein was intact so the proton pumping functionality of PM multilayers would be retained in all samples with added NPs. Our results show that the effects of NPs on the PM structure and functionality are subtle and complex, and we will discuss the structural integrity of lipid-protein composite PM films against NP infiltration in terms of their high bending modulus as compared with that of fluid lipid bilayers.

Versatile vacuum chamber for in situ surface X-ray scattering studies

A compact portable vacuum-compatible chamber designed for surface X-ray scattering measurements on beamline ID01 of the European Synchrotron Radiation Facility, Grenoble, is described. The chamber is versatile and can be used for in situ investigation of various systems, such as surfaces, nanostructures, thin films etc., using a variety of X-ray-based techniques such as reflectivity, grazing-incidence small-angle scattering and diffraction. It has been conceived for the study of morphology and structure of semiconductor surfaces during ion beam erosion, but it is also used for the study of surface oxidation or thin film growth under ultra-high-vacuum conditions. Coherent X-ray beam experiments are also possible. The chamber is described in detail, and examples of its use are given.