Observation of Ultrathin Precursor Film Formation during Ge-Si Liquid-Phase Epitaxy from an Undersaturated Solution.

Our in situ X-ray study shows that a silicon substrate in contact with an undersaturated In(Ge) solution is wetted by an approximately 1 nm thin germanium film, which does not grow any thicker. The results can be understood by the use of thickness-dependent correlated interfacial energies. This near-equilibrium heterogeneous interface structure marks the initial stage of crystal growth before the formation of bulk material, which can only form under conditions of supersaturation. This finding uncovers a fundamental aspect of the thermodynamics at solid-liquid interfaces relevant for understanding the transition from equilibrium to supersaturation and is of importance for nanoscale solution growth methods.

Atomic structure and composition of the yttria-stabilized zirconia (111) surface.

Anomalous and nonanomalous surface X-ray diffraction is used to investigate the atomic structure and composition of the yttria-stabilized zirconia (YSZ)(111) surface. By simulation it is shown that the method is sensitive to Y surface segregation, but that the data must contain high enough Fourier components in order to distinguish between different models describing Y/Zr disorder. Data were collected at room temperature after two different annealing procedures. First by applying oxidative conditions at 10- 5 mbar O2 and 700 K to the as-received samples, where we find that about 30% of the surface is covered by oxide islands, which are depleted in Y as compared with the bulk. After annealing in ultrahigh vacuum at 1270 K the island morphology of the surface remains unchanged but the islands and the first near surface layer get significantly enriched in Y. Furthermore, the observation of Zr and oxygen vacancies implies the formation of a porous surface region. Our findings have important implications for the use of YSZ as solid oxide fuel cell electrode material where yttrium atoms and zirconium vacancies can act as reactive centers, as well as for the use of YSZ as substrate material for thin film and nanoparticle growth where defects control the nucleation process.

Reversible shape changes of Pd nanoparticles on MgO(100).

We studied the interaction of oxygen with MgO(100) supported Pd nanoparticles at 10(-5) mbar oxygen pressure and a sample temperature of 570 K. We employed high-resolution X-ray reciprocal space mapping, which allows us to resolve the average particle shape from the quantitative analysis of intensity diffraction rods running perpendicular to corresponding facet surfaces. We identified the oxygen induced formation of nanosized (112) facets which is reversible in a CO atmosphere. Our results give direct evidence for the microscopic evolution of the nanoparticle shape under reactant exposure, which is essential for an atomistic understanding of catalytic reactions on nanoparticles.

Tunable symmetry and periodicity in binary supramolecular nanostructures.

We present a route to change the "compositional" order of highly crystalline binary layers comprising diindenoperylene and copper-phthalocyanines from two- to one-dimensional periodicity. This is achieved by exchanging fluorine with hydrogen atoms in the phthalocyanines, thereby reducing the C-F···H-C interactions and allowing the interplay of long-range electrostatic interactions in mesoscopic phases. Linear patterns are thus obtained, whose periodicity can be additionally tuned by an appropriate stoichiometry of the components.

Growth mechanisms of phthalocyanine nanowires induced by Au nanoparticle templates.

We combine X-ray reflectivity and scanning electron microscopy measurements to investigate the mechanisms involved in the growth of vertical arrays of phthalocyanine nanowires directed by templates of Au nanoparticles. The study has been carried out for H(16)CuPc at different substrate temperatures. It is shown that three organic morphologies evolve during the growth: 1D nanostructures on top of the Au nanoparticles, a multilayer film on the substrate and a layer wetting the gold nanoparticles. For substrate temperatures below 100 °C there is a coexisting and competing growth of the three structures, whereas beyond this temperature the 1D growth on the nanoparticles is predominantly favored. The observance of two regimes with the temperature is characterized by two different activation energies. Both the length of the 1D structures and the thickness of the multilayer film can be precisely controlled by the substrate temperature which is of importance for application of vertical organic nanowires as donor/acceptor architecture in organic solar cells.

Strain-effect for controlled growth mode and well-ordered structure of quaterrylene thin films.

We investigated the evolution of quaterrylene thin films on SiO(2) and on an octadecyltrichlorosilane self-assembled monolayer (OTS-SAM) to examine the impact of film strains on the growth processes and evolving structure. Surface modification by SAMs allowed tailoring of the growth process from a Stranski-Krastanov (SK) mode (layer-plus-island) on the SiO(2) surface to a Frank-van der Merwe mode (layer-by-layer) on the OTS surface. Detailed structural analysis by x-ray diffraction techniques confirmed that the SK mode was driven by lattice strain in the initial wetting layers on the SiO(2) surface. On the other hand, strain-free wetting layers were already formed at the beginning of growth on the OTS surface, thereby suppressing three-dimensional island formation. Moreover, the films on the SiO(2) surface were found to incorporate high microstrain induced by crystal defects such as dislocations and a mosaic structure. In contrast, few crystal defects were present in the films on OTS surface, demonstrating that OTS treatment enables marked improvement of the molecular alignment. These results clearly indicate that the lattice strain induced by the molecular-substrate interaction is essential for controlling the overall growth process.

On the origin of the hydrophobic water gap: An X-ray reflectivity and MD simulation study.

The density deficit of water at hydrophobic interfaces, frequently called the hydrophobic gap, has been the subject of numerous experimental and theoretical studies in the past decade. Recent experiments give values for the interfacial depletion that consistently correspond to less than a monolayer of water. The main question which remained so far unanswered is its origin and the mechanisms affected by the chemistry and molecular geometry of a particular hydrophobic coating. In this work, we present a combined high-energy X-ray reflectivity and molecular dynamics simulation study of the water depletion at a perfluorinated hydrophobic interface with a spatial resolution on the molecular scale. A comparison of our experimental and computational results elucidates the underlying mechanisms that affect the extent of the interfacial depletion. The complex interplay between surface chemistry and topography precludes the existence of a direct and universal relation between the macroscopic contact angle and the nanoscopic water depletion.

Evidence for a layer-dependent Ehrlich-Schwöbel barrier in organic thin film growth.

We report direct experimental evidence for a layer-dependent step-edge barrier in organic thin film growth, investigating di-indenoperylene on SiO_{2} as an archetypical system. In particular, we show that a noticeable Ehrlich-Schwöbel effect emerges only beyond the 3rd molecular layer, accompanied by mass step-upward diffusion. We further disclose that this thickness dependence of the interlayer transport is directly related to molecular reorientations during the first stages of the growth. This is ultimately responsible for a morphological transition from layer-by-layer growth to surface rapid roughening. These experimental findings should compel further development of molecular-scale models for organic thin film growth.

Layering of [BMIM]+-based ionic liquids at a charged sapphire interface.

The structure of two model room temperature ionic liquids, [BMIM](+)[PF(6)](-) and [BMIM](+)[BF(4)](-), near the solid/liquid interface with charged Al(2)O(3)(0001) (sapphire) was determined with subnanometer resolution by high energy (72.5 keV) x-ray reflectivity. [BMIM](+)[PF(6)](-) exhibits alternately charged, exponentially decaying, near-surface layering. By contrast, the smaller-anion compound, [BMIM](+)[BF(4)](-), shows only a single layer of enhanced electron density at the interface. The different layering behaviors, and their characteristic length scales, correspond well to the different bulk diffraction patterns, also measured in this study. Complementary measurements of the surface and interface energies showed no significant different between the two RTILs. The combined bulk-interface results support the conclusion that the interfacial ordering is dominated by the same electrostatic ion-ion interactions dominating the bulk correlations, with hydrogen bonding and dispersion interactions playing only a minor role.

Supramolecular assembly of a 2D binary network of pentacene and phthalocyanine on Cu(100).

A crystalline nanoporous molecular network was tailored by supramolecular assembly of pentacene and F16CuPc on Cu(100). The structure and self-assembly mechanisms of the pure and binary layers were analyzed by STM. F16CuPc films and mixed layers of pentacene/F16CuPc in a ratio of 2:1 show two enantiomorphic chiral domains with high structural order in contrast to pentacene which exhibits no long-range order in pure films. A model of the epitaxial relationship on Cu(100) is given, which suggests C-F...H bonding as a possible driving force for the bimolecular self-assembly in addition to the still strong interaction between the substrate and the organic bilayer.

A spin-echo resolved grazing incidence scattering setup for the neutron interrogation of buried nanostructures.

We present a dedicated experimental spin-echo resolved grazing incidence scattering (SERGIS) setup for the investigation of surfaces and thin films exhibiting large lateral length scales. This technique uses the neutron spin to encode one in-plane component of the wave-vector transfer in a grazing angle scattering experiment. Instead of the scattering angle, the depolarization of the scattered beam is measured. This allows one to achieve a very high in-plane momentum resolution without collimation of the incident neutron beam in the corresponding direction. SERGIS can therefore offer an alternative or complementary method to conventional grazing incidence neutron scattering experiments. We describe the experimental setup installed at the neutron sources ILL (Grenoble) and FRM II (Garching) and present data obtained with this setup on various samples exhibiting characteristic mesoscopic length scales in the range of several hundred nanometers. We also derive general formulas and error margins for the analysis and interpretation of SERGIS data and apply them to the cases of a one-dimensional structure and of an island morphology.

X-ray cross correlation analysis uncovers hidden local symmetries in disordered matter.

We explore the different local symmetries in colloidal glasses beyond the standard pair correlation analysis. Using our newly developed X-ray cross correlation analysis (XCCA) concept together with brilliant coherent X-ray sources, we have been able to access and classify the otherwise hidden local order within disorder. The emerging local symmetries are coupled to distinct momentum transfer (Q) values, which do not coincide with the maxima of the amorphous structure factor. Four-, 6-, 10- and, most prevalently, 5-fold symmetries are observed. The observation of dynamical evolution of these symmetries forms a connection to dynamical heterogeneities in glasses, which is far beyond conventional diffraction analysis. The XCCA concept opens up a fascinating view into the world of disorder and will definitely allow, with the advent of free electron X-ray lasers, an accurate and systematic experimental characterization of the structure of the liquid and glass states.

Nanoconfinement effects in the self-assembly of diindenoperylene (DIP) on Cu(111) surfaces.

The structure of monolayer films of diindenoperylene (DIP) on Cu(111) single crystals has been studied by means of scanning tunneling microscopy (STM). Pronounced film confinement effects are observed, resulting in distinctly different film structures depending upon the substrate terrace width. On wide terraces (exceeding approximately 15 nm in width) a short range ordered structure forms with the DIP molecules aligned along three discrete directions related by 60 degrees rotations. On the contrary, on narrow terraces (<15 nm), the confinement enhances the role of the step edges and leads to the formation of a long range ordered structure with co-directionally oriented molecules, tentatively ascribed to an energy minimization by optimization of the surface coverage.

Molecular layering of fluorinated ionic liquids at a charged sapphire (0001) surface.

Room-temperature ionic liquids (RTILs) are promising candidates for a broad range of "green" applications, for which their interaction with solid surfaces plays a crucial role. In this high-energy x-ray reflectivity study, the temperature-dependent structures of three ionic liquids with the tris(pentafluoroethyl)trifluorophosphate anion in contact with a charged sapphire substrate were investigated with submolecular resolution. All three RTILs show strong interfacial layering, starting with a cation layer at the substrate and decaying exponentially into the bulk liquid. The observed decay length and layering period point to an interfacial ordering mechanism, akin to the charge inversion effect, which is suggested to originate from strong correlations between the unscreened ions. The observed layering is expected to be a generic feature of RTILs at charged interfaces.

Three-dimensional molecular packing of thin organic films of PTCDI-C8 determined by surface X-ray diffraction.

We have determined the full molecular 3D packing of thin organic films of the archetypical organic n-type semiconductor N, N'-dioctyl-3,4:9,10-perylene tetracarboxylic diimide (PTCDI-C 8) by surface X-ray crystallography. We show that PTCDI-C 8 forms smooth layered films on Al 2O 3 (11-20) with an outstanding degree of molecular order. The thin-film structure is found to consist of a triclinic unit cell with the plane of the aromatic core tilted by 67 +/- 2 degrees with respect to the surface plane, which differs significantly from the bulk structure. The 3D crystallites extend with vertical coherent order across the entire film thickness.

Self-assembled molecular nanowires of 6,13-Bis(methylthio)pentacene: growth, electrical properties, and applications.

We demonstrate a comprehensive study of self-assembled molecular nanowire, including molecular design, one-dimensional crystal growth, resistivity measurement of individual wire, and application to a field-effect transistor. Appropriate molecular design and control of interfacial interactions lead to single crystalline wire growth with an extensive pi-stacking motif. Resistivity measurements of an individual molecular wire indicate that these structural features are advantageous for electrical transport. Finally, field-effect transistors with single- and double-wire channels were fabricated to give some indication of the potential application of the molecular wires.

Water and ice in contact with octadecyl-trichlorosilane functionalized surfaces: a high resolution x-ray reflectivity study.

We present a high energy x-ray reflectivity study of the density profiles of water and ice at hydrophobic and hydrophilic substrates. At the hydrophobic water/octadecyl-trichlorosilane (water-OTS) interface, we find clear evidence for a thin density depletion layer with an integrated density deficit corresponding to approximately 40% of a monolayer of water molecules. We discuss the experimental results in terms of a simple model of hydrophobic/hydrophilic solid-liquid interfaces. Our results also exclude the presence of nanobubbles. A detailed study of possible radiation damage induced by the intense x-ray beam at the dry OTS surface and at the ice-OTS, as well as at water-OTS interfaces, discloses that noticeable damage is only induced at the water-OTS interface, and thus points to the dominant role of highly mobile radicals formed in bulk water close to the interface.

Organic n-channel transistors based on core-cyanated perylene carboxylic diimide derivatives.

Five core-cyanated perylene carboxylic diimides end-functionalized with fluorine-containing linear and cyclic substituents have been synthesized and employed in the fabrication of air-stable n-channel organic thin-film field-effect transistors with carrier mobilities up to 0.1 cm2/Vs. The relationships between molecular structure, thin-film morphology, substrate temperature during vacuum deposition, transistor performance, and air stability have been investigated. Our experiments led us to conclude that the role of the fluorine functionalization in the air-stable n-channel operation of the transistors is different than previously thought.