Formation of condensed films of nanoparticles having small ratio of metal-core-diameter to organic-shell-thickness is desired for several applications in nanotechnology. We report here results of a X-ray scattering study carried out to understand structure and morphology of monolayer of such nanoparticles having gold-core and thiol-shell directly on the water surface before the monolayer undergoes a continuous transition to a bilayer. Our results demonstrate buckling of the monolayer over a large surface pressure range ( 1 to 15 mN/m). The buckled state exhibits reversibility on decompression and can be annealed with temperature. We also show that condensed monolayer films of nanoparticles can be formed by annealing the buckled monolayer before transferring to solid substrates. Copyright (C) EPLA, 2007.

The three-tailed amphiphile ferric stearate molecule, which forms a bimolecular layer on water surface with molecules in the lower and upper layers in different conformations, has been studied to understand transfer and growth of bimolecular films on the surface of hydrophilic silicon substrates. This bimolecular film forms a two-dimensional lattice on water with a slightly distorted hexagonal lattice where both the in-plane and out-of-plane domain sizes are small. The film also showed larger microscopic rigidity compared to its macroscopic mechanical response. This asymmetric bimolecular layer was found to be preserved when the film is transferred on the substrates at different values of surface pressures ranging from 1 mN/m to near-collapse (55 mN/m). Both the upper and lower layers become denser and interfaces between these layers become sharper with increase in deposition pressure but the growths have different natures. The lower layer of transferred film is dense from 1 mN/m and, except for a steplike increase between 20 and 30 mN/m, changes slowly in density. The density of the upper molecular layer grows continuously with surface pressure.

We have demonstrated by x-ray diffuse scattering that a bimolecular layer of a preformed three-tailed amphiphile, ferric stearate, drastically enhances capillary wave fluctuations on water surface due to a reduction in surface tension to 1 mN/m . The bimolecular layer is composed of molecules in symmetric configuration, on top of molecules in asymmetric configuration with ferric ions in contact with water. Unlike the usual Langmuir monolayers, this layer of molecules does not rupture under compression, but becomes thicker. This behavior mimics folding of a membrane on a liquid surface and is closely related to the cohesive interaction brought by the ferric ions. The low effective tension of this artificial membrane depends on the available area and reduces as the microscopic excess area increases.

We have investigated Langmuir films at the water surface using grazing-incidence diffuse X-ray scattering. We show that beyond structural parameters like the film thickness and density, diffuse scattering also gives access to phase transitions and elastic properties (surface tension and bending rigidity). We find that the surface tension measured with X-rays is consistent with the Wilhelmy plate measurements, at least for tilted phases. The bending rigidity is found to be on the order of 20-30k(B)T whatever the phase. The underlying molecular mechanisms are discussed using different models.

We have investigated the small-scale structure of the liquid-vapor interface using synchrotron x-ray scattering for liquids with different molecular structures and interactions. The effective momentum-dependent surface energy first decreases from its macroscopic value due to the effect of long-range forces, and then increases with increasing wave vector. The results are analyzed using a recent density functional theory. The large wave-vector increase is attributed to a bending energy for which local and nonlocal contributions are equally important.

X-ray scattering techniques are increasingly used for the study of liquid surfaces and interfaces. We give here scattering cross-sections for diffraction or diffuse scattering from liquid interfaces and we discuss the examples of the liquid–vapour interface and of films at a liquid–liquid interface. We show that the interfacial structure can be understood as resulting of the interplay between thermal fluctuations, van der Waals forces, and elastic properties.

Liquid–vapour interfaces, particularly those involving water, are common in both natural and artificial environments. They were first described as regions of continuous variation of density1, caused by density fluctuations within the bulk phases2,3,4. In contrast, the more recent capillary-wave model5,6 assumes a step-like local density profile across the liquid–vapour interface, whose width is the result of the propagation of thermally excited capillary waves. The model has been validated for length scales of tenths of micrometres and larger7,8, but the structure of liquid surfaces on submicrometre length scales—where the capillary theory is expected to break down—remains poorly understood. Here we report grazing-incidence X-ray scattering experiments that allow for a complete determination of the free surface structure and surface energy for water and a range of organic liquids. We observe a large decrease of up to 75% in the surface energy of submicrometre waves that cannot be explained by capillary theory, but is in accord with the effects arising from the non-locality of attractive intermolecule interactions as predicted by a recent density functional theory9. Our data, and the results of comparable measurements on liquid solutions, metallic alloys, surfactants, lipids and wetting films should thus provide a stringent test for any new theories that attempt to describe the structure of liquid interfaces with nanometre-scale resolution.

Grazing incidence x-ray diffraction and Brewster-angle microscopy measurements have been undertaken for a mixed Langmuir monolayer of octadecanoic acid and methyl octadecanoate. For the composition studied (49.7 mol % ester) there are two noncrystalline tilted phases, one at low pressure in which the chains point to their nearest neighbors (L2 phase) and a higher-pressure phase in which the tilt is toward next-nearest neighbors. The higher-pressure phase arises from a merger of the L2′ and Ov phases, which are separated in the pure acid. A continuous change between the two regions is observed rather than a first-order transition, as had been suggested. The results are discussed with reference to different proposals for the origin of two distinct next-nearest neighbor tilted mesophases: the presence of one-dimensional chain backbone order [V. M. Kaganer and E. B. Loginov, Phys. Rev. E 51, 2237 (1995)] and a coupling between tilt and distortion [E. Sirota, Langmuir 13, 3849 (1997)].

Grazing incidence x-ray surface scattering has been used to investigate liquid surfaces down to the molecular scale. The free surface of water is well described by the capillary wave model ( ~ q-2 spectrum) up to wavevectors > 10^8 m^-1. At larger wavevectors near-surface acoustic waves must be taken into account. When the interface is bounded by a surfactant monolayer, it exhibits a bending stiffness and the bending rigidity modulus can be measured. However, bending effects generally cannot be described using the Helfrich Hamiltonian and the characteristic exponent in the roughness power spectrum can smaller than 4. Finally, upon compression, tethered monolayers formed on a subphase containing divalent ions are shown to buckle in the third dimension with a characteristic wavelength on the order of 10^8 m^-1.

The interfacial structure and fluctuations of a L-alpha-dipalmytoilphosphatidylcholine monolayer at the hexadecane/water interface were investigated using grazing-incidence surface scattering of X-ray synchrotron radiation. The interfacial structure of the densely packed monolayer is consistent with that of a similar monolayer at the air/water interface. The fluctuations are shown to be limited by interfacial tension on length-scales down to in-plane wavelengths of a few tens of nanometers. This result at the alkane/water interface demonstrates the power of the X-ray grazing-incidence technique applied to the study of liquid-liquid interfaces.

The linear compressibility of two-dimensional fatty acid mesophases has been determined by grazing incidence X-ray diffraction. The unit cell parameters of the , , , S and phases of behenic acid and of the phase of myristic acid were determined as a function of surface pressure and temperature. Surface pressure versus molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion along a given direction for a two-dimensional isotropic applied stress) was determined both in the sample plane and in a plane normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with different molecular mechanisms. The largest compressibilities (10 m/N) are observed in the tilted phases. They are apparently independent on the chain length and could be related to the reorganization of the headgroup hydrogen-bounded network, whose role should be revalued. Intermediate compressibilities are observed in phases with quasi long-range order (directions normal to the molecular tilt in the or phases, S phase, and could be related to the ordering of these phases. The lowest compressibilities are observed in the solid untilted phase and for one direction of the S and phases. They are similar to the compressibility of crystalline polymers and correspond to the interactions between methyl groups in the crystal. Finally, negative compressibilities are observed in the transverse plane for the and phases and can be traced to subtle reorganizations upon untilting.

The diffuse scattering of x rays by the thermally excited out-of-plane fluctuations of different amphiphilic films was measured for in-plane wavelengths down to the nanometer range, giving access to nontrivial bending effects. The Helfrich Hamiltonian applies on pure water and in the solid phase of an arachidic acid monolayer a large bending rigidity constant was measured. When formed on a subphase containing divalent cadmium ions, the height-height fluctuation spectrum (z(q)z(-q)) is greatly modified: no longer consistent with a q(-1) law at large wavelengths but rather with a q(-3.3+/-0.2) law, revealing a very different physical mechanism whose origin is discussed.

X-ray diffraction analyses of good-quality single crystals of HgBa2Ca2Cu3O8+delta synthetized by a closed-vessel technique at low pressure, have lead to more definite results with respect to previous structural studies. Samples with T-c=135 K showed a Cu occupancy of 16.1% on the Hg site at the origin of the unit cell. Excess oxygen appears to be present only in the basal plane at the interstitial site 1/2, 1/2, 0 with an occupancy delta = 0.190 +/- 0.015. This compound has the shortest copper oxygen apical distance within the mercury family (2.696(3)Angstrom) together with an almost complete planarity of the CuO2 planes. Variations in the excess oxygen content upon different heat treatments were analyzed by thermogravimetry and magnetic-susceptibility measurements. A correlation between delta and T-c could be derived, showing in particular that superconductivity exists down to a very low interstitial oxygen content.

Monolayers of a polymerisable phospholipid on water have been studied both before and after polymerisation. Before polymerisation, the phase diagram is established by isotherm measurements and optical microscopy (epifluorescence and direct observation between crossed polariser and analyser). This allows us to bring into evidence a coexistence region between a condensed and an expanded phase, above a triple point temperature T t =20 o C. The dramatic influence of impurities on the sise of coexistence domains between the condensed phase and the expanded one is clearly demonstrated, even at a very low concentration of impurities. Structural and morphological modifications during the polymerisation were investigated using X-ray surface scattering together with atomic force microscopy. Whatever the polymerisation conditions (constant area or constant pressure), X-ray reflectivity clearly shows the reorientation of the diacetylenic links. Only constant area polymerisation leads to a viscoelastic behavior of the film, as shown by talcum decoration. The topochemical nature of the polymerisation of diacetylenic groups induces strong constraints on the monolayers and, when the polymerisation is achieved at constant area, leads to the collapse of the films evidenced by both techniques

Superconducting single crystals of YBa2Cu3O7−δ displaying a secondary phase pattern superimposed on the orthorhombic bulk phase pattern have been investigated by X-ray diffraction. The crystal structure of the satellite phase material has been solved and shown to correspond to a new alkaline earth oxocuprate (II) BaCu3O4. It has orthorhombic symmetry. The space group is D192h-Cmmm with a=10.986Å, b=5.503Å, c=3.923Å and Z=2. All the oxygen atoms of the structure are involved in a 2-D network of copper-oxygen square planar coordination polyhedra having edge linkage. Short distances d(Cu-Cu)=2.752 Å are found along infinite 1-D copper chains. Structural similarities with YBa2Cu3O7−δ may explain the frequent occurence of BaCu3O4 within the host phase, and consequently its possible role as a weak link between superconducting regions.

A procedure is described for the determination of the phases of waves scattered by a multilayer structure using interference between surface reflections and structure diffraction. The applicability of the method to Langmuir-Blodgett and metallic sputtered multilayers is discussed.

Low-field (0.4 G⩽H⩽3 G) magnetization measurements have been performed on small single crystals of superconducting YBa2Cu3O7−δ (Tc⋍95 K) using a SQUID magnetometer. They revealed anisotropic properties in the temperature dependences of the shielding and the Meissner effects. A sharp unique transition at 95 K is observed with the field parallel to c. In the perpendicular direction a second transition line seems to be crossed at T∗=84 K. This temperature, T∗, decreases slightly as a function of the applied field in the range of fields investigated.

Low field (H ≤ 3G) magnetization measurements have been performed on small single crystals of superconducting YBa2Cu3O7-δ (Tc ≃ 95K) using a SQUID magnetometer. They revealed anisotropic properties in the temperature dependence of the magnetization. The Meissner effect (always smaller than 10 % ) is 2 to 3 times larger along the c direction than along one of the perpendicular directions. A sharp unique transition at 95K is observed with the field parallel to c. In the perpendicular direction a second transition line seems to be crossed at T = 84K with an applied field of 3G. In all three crystals studied, a very small ferromagnetic polarization, pointing approximately along the c direction, is observed.

A new periodic antiphase structure is proposed concerning the ordering of the counterions in the 1D Pt chain compounds Mx[Pt(C2O4)2]yH2O. Its diffraction spectrum, which is superimposed on the overall modulation spectrum previously described, is shown to be in agreement with the observed extinction rules. This type of ordering further supports the idea, introduced some years ago, postulating a fragmentation of these structures into periodic microdomains locally commensurate, but statistically incommensurate with the underlying lattice.