Grazing Incidence X-ray Diffraction Studies Research Articles

Electrocompression of the Au(111) Surface Layer during Au Electrodeposition

In situ grazing-incidence x-ray diffraction studies of reconstructed Au(111) electrodes in aqueous electrolyte solutions are presented, which reveal a significantly increased compression of the Au surface layer during Au electrodeposition as compared to Au(111) surfaces under ultrahigh vacuum conditions or in the Au-free electrolyte. The compression increases towards more negative potentials, reaching 5.3% at the most negative potentials studied. It may be explained within a simple thermodynamic model by a release of potential-induced surface stress.

Grazing Incidence X-ray Diffraction Study on Surface Crystal Structure of Polyethylene Thin Films

Crystal structure in thin films of melt-crystallized and annealed polyethylene (PE) was examined by grazing incidence X-ray diffraction measurements. Choosing appropriate incident angles of X-rays to the films, surface and bulk molecular aggregation states were successfully extracted. Consequently, it was found that chain packing structure in the surface region was different from that in the bulk one. Based on paracrystalline analysis for the (110) reflection and its higher-order ones of PE orthorhombic crystal, it was clarified that the ordering for the crystalline lattice was lower in the surface region. Also, apparent crystallinity in the surface was lower than the bulk one. These results indicate that disordered crystallites were preferentially formed in the surface region.

Structural Changes of Phospholipid Monolayers Caused by Coupling of Human Serum Albumin: A GIXD Study at the Air/Water Interface

Phase behavior and structural changes were studied for phospholipid monolayers coupled with human serum albumin (HSA) at the air/buffer (pH 3.8) interface by film balance and grazing incidence X-ray diffraction (GIXD) measurements. For comparison, the lipid headgroup was varied from small ionic L-α-dipalmitoyl-phosphatidic acid (DPPA) to large ionic L-α-dipalmitoyl-phosphatidyl-L-serine (DPPS) and zwitterionic L-α-distearoyl-phosphatidylcholine (DSPC). The presence of HSA leads to a more or less pronounced plateau in the pressure/area isotherms of all phospholipids. GIXD diffraction patterns illustrate that the phase sequences of DPPS and DPPA on buffer (oblique−rectangular−hexagonal) are changed because of the binding of HSA. The oblique−rectangular phase transition disappears and the rectangular−hexagonal phase transition shifts to a lower surface pressure. However, there is no effect of HSA on the structure of the DSPC monolayer (NN tilted rectangular packing). Phase diagrams of DPPA/HSA, DPPS/HSA, and DSPC/HSA have been established. A tilt angle decrease in both DPPA/HSA and DPPS/HSA monolayers is observed, but no change of the tilt angle for DSPC molecules in the mixed DSPC/HSA monolayer is found. Thus, whereas HSA binding stabilizes the liquid-expanded phase at low pressures most probably via penetration, it condenses the ordered phases of anionic monolayers via reducing the headgroup repulsion. The positional correlation lengths ξi parallel and perpendicular to the tilt direction change because of the binding of HSA.

Lateral and Vertical Nanophase Separation in Langmuir−Blodgett Films of Phospholipids and Semifluorinated Alkanes

It has recently been found that monodisperse surface micelles (hemimicelles) were formed in Langmuir monolayers of the semifluorinated alkane C8F17C16H33 (F8H16) after transfer onto silicon wafers. Grazing incidence X-ray diffraction studies have demonstrated that compression of mixed Langmuir monolayers made from combinations of dipalmitoyl phosphatidylethanolamine (DPPE) and diblock F8H16 in various molar ratios resulted in the complete expulsion of the diblock molecule at high surface pressure. F8H16 then formed a second layer on top of a DPPE-only monolayer, demonstrating a novel type of reversible, pressure-induced, vertical phase separation. Using atomic force microscopy and X-ray reflectivity, we show now that mixed DPPE/F8H16 (1:1.3) Langmuir-Blodgett films transferred onto silicon wafers below 10 mN m(-1) are laterally phase separated and consist of domains of F8H16 surface micelles in coexistence with a monolayer of DPPE. The density of the network of F8H16 surface micelles increases when the surface pressure of transfer increases. Around 10 mN m(-1), the F8H16 surface micelles start to glide on the DPPE monolayer, progressively overlying it, until total coverage is achieved.

Grazing incidence X-ray diffraction studies on the structures of polyurethane films and their effects on adhesion to Al substrates

Grazing incidence X-ray diffraction was carried out to analyze the structure of polyurethane films as a function of X-ray penetration depth by varying the angle of incidence. Coherence lengths, interplanar spacings, and crystallinities were obtained for non-aged and aged films of OH numbers of 120, 375, and 600 bonded to an aluminum substrate. Aging led to the improvement of bulk crystallinity of all the samples, particularly in the case of the aged PU-375 film, for which a dramatic increase of the bond strength was observed. The crystallinity of non-aged samples varied from the air/polymer surface down to the polymer/aluminum interface. Invariance of coherence length from air surface to interface was observed for PU-120 and PU-375 samples, implying that substrate-induced ordering has little effect on the average crystallite size. As the X-ray penetration increases near the polymer/Al interface, interplanar spacing of (021) reflection in all the films approach the value found for a bulk polyurethane-urea by Ishihara et al. The bond strength of the polyurethane film to the aluminum was exponentially proportional to the crystallinity including the crystalline interphase formed near the substrate. It is also found that the polymer film containing more (100) planes provided higher bond strength.

Grazing-incidence x-ray diffraction study of Langmuir films of amphiphilic monodendrons.

We have used pressure-area isotherms and grazing-incidence x-ray diffraction to study structures of Langmuir films of first-generation monodendrons with two or three peripheral alkyl chains. Unlike the structures observed in their bulk liquid crystalline mesophases, these multichain monodendrons form either a centered rectangular lattice with molecular axes tilted toward nearest neighbors or an oblique lattice with molecular axes tilted in low-symmetry directions.

Interplay between Anisotropic Strain Relaxation and Uniaxial Interface Magnetic Anisotropy in Epitaxial Fe Films on (001) GaAs

Grazing incidence x-ray diffraction study of Fe epitaxial ultrathin films (1.5-13 nm) on GaAs (001) reveals an anisotropy of both domain shape and strain, with [110] and [1-10] as the principal directions. It is shown that the observed thickness-dependent strain anisotropy, together with a uniaxial interface term, can provide an unambiguous explanation to the usual in-plane magnetic anisotropy and its thickness dependence observed in this magnetic thin-film system.

Measuring surface stress discontinuities in self-organized systems with X rays.

We have performed a grazing incidence x-ray diffraction study of the self-organized N/Cu(001) system. Diffraction satellites associated with self-organization are particularly intense around Bragg conditions of the bulk crystal. Bulk elastic relaxations due to surface stress discontinuities at domain boundaries are responsible for this feature. A quantitative analysis shows that these relaxations, computed by molecular dynamics or continuum elasticity, explain very well the whole diffraction study. A difference in surface stress of 7 N m(-1) between uncovered and N-covered regions of the Cu surface is shown to be the driving force for self-organization.

Characterization of Niobium Oxide Films Prepared by Reactive DC Magnetron Sputtering

Niobium oxide films have been prepared on glass and Si(100) substrates by reactive dc magnetron sputtering of a metallic Nb target in an argon-oxygen atmosphere. The properties of the films as a function of oxygen partial pressure have been studied systematically by Rutherford backscattering spectroscopy, X-ray diffraction, X-ray reflectometry and optical spectroscopy. Rutherford backscattering studies show that the films are substoichiometric at lower oxygen flow while stoichiometric films can be formed above 7.5 sccm O2 flow. Grazing incidence X-ray diffraction studies show that the films are amorphous. X-ray reflectometry was used to determine the sputter rate, density and surface roughness of the films. The films prepared at lower oxygen flow rates had a higher density than those prepared at higher oxygen flow rates. For appropriate oxygen flow rates fully transparent niobium oxide films can be grown with rates up to 0.5 nm/s for a constant cathode current of 900 mA. From optical spectroscopy measurements of reflectance and transmittance we have determined the optical constants such as the refractive index (n), extinction coefficient (k) and band gap (Eg) as well as the film thickness. For the fully transparent films prepared above 7.5 sccm O2 flow rate a band gap of 3.5 to 3.6 eV is obtained. Optical spectroscopy measurements show that the refractive index of the films decreases upon increasing oxygen flow rate and this finding is related to a decrease of density upon increasing oxygen flow.

Investigation of the morphology of the initial growth of the aromatic molecule p-quaterphenyl on NaCl (001)

Investigations of the structural properties of the initial layers of p-quaterphenyl (p-4P) vapor deposited onto NaCl (001) have been carried out using atomic force microscopy (AFM), grazing incidence x-ray diffraction (GIXRD), and helium atom scattering (HAS). A series of AFM images reveal accumulations of p-4P molecules around surface defects. Film thicknesses determined from height analyses of these images are in reasonable agreement with those found using GIXRD. The GIXRD studies indicate that p-4P films vapor-deposited at ambient temperature are composed of crystallites oriented with the long molecular axis nearly perpendicular to the NaCl (001) surface. For a nominally three-monolayer film, the b axis of the crystallites appears preferentially oriented along the substrate’s [110] azimuth, but with increasing thickness the x-ray features resemble those from two-dimensional (2D) powders. The diffraction peaks found in the HAS investigation at ∼50 K for a thick film grown at ∼200 K are similarly consistent with a 2D powder. The principal difference from the room temperature GIXRD experiments on thick films is that the a and b lattice parameters of the monoclinic unit cell appear doubled in length at ∼50 K.

Interaction of dipyridamole with phospholipid monolayers at the air–water interface: Surface pressure and grazing incidence X-ray diffraction studies

Incorporation of dipyridamole (DIP) in Langmuir monolayers of the zwitterionic phospholipid dipalmitoylphosphatidylcholine (DPPC) has been investigated using surface pressure isotherm and grazing incidence X-ray diffraction (GIXD) measurements. An expansion in the pressure–area isotherms of the DPPC monolayer was observed, which was maximum in the liquid-expanded (LE) to liquid-condensed (LC) transition region and at a DIP concentration of 2 mol%. Accordingly, the GIXD measurements indicated an expansion of the 2D lattice in the LC phase up to a surface pressure of ca. 30 mN/m. A condensation effect on the DPPC packing for high DIP concentrations, as previously suggested, can be ruled out.

Grazing Incidence X-ray Diffraction Studies of Thin Films at the air-liquid Interface

AbstractMonomolecular films at the air-water interface can be investigated on the subnanometer scale with grazing incidence X-ray diffraction (GIXD) using synchrotron radiation. This surface semsitive technique utilizes the property of total external reflection of X-rays from a water surface : an evanescent wave generated within the film diffracts in the surface plane giving an image of the film reciprocal lattice. Three applications of GIXD are presented ranging from poorly to highly crystalline thin films. (i) Cholesteryl-L-glutamate forms a crystalline monolayer at the air-water interface within which the glutamate moieties are not closely packed. This system specifically incorporates hydrophobic amino acids from the subphase. (ii) Long-chain cholesteryl esters deposited on the water surface spontaneously self assemble, forming crystalline interdigitated bilayers. The molecular structure, solved at the atomic resolution, was found to be similar to the 3D counterpart. (iii) According to 2-D diffraction theory, the shape of Bragg peaks is related to the mechanical constants of the film. Rigidity of the film can be deduced from a detailed peak analysis for secondary short chain alcohols showing a softening of the monolayer close to melting.

Real-Time grazing incidence X-ray diffraction studies of polymerizing N-octadecyltrimethoxysilane Langmuir monolayers at the air/water interface.

Real-Time grazing incidence X-ray diffraction studies of polymerizing N-octadecyltrimethoxysilane Langmuir monolayers at the air/water interface.

Two-Dimensional Mixtures of Stearic Acid and Partially Fluorinated Amphiphilic Molecule: A Grazing Incidence X-ray Diffraction Study

We report a grazing incidence X-ray diffraction (GIXD) study of mixed Langmuir monolayers of stearic acid (ST) and a fluorinated amphiphilic molecule (FEP). The mixing behavior of the two components was investigated at two different surface pressures, 5 and 30 mN/m. The ST molecules are found to be in the L2 mesophase at 5 mN/m and in the LS mesophase at 30 mN/m, whereas the FEP molecules appear disordered. In the mixture, the GIXD measurements, associated with previous thermodynamical and optical microscopy results, show that in the whole range of molar ratio, the two molecules are completely phase-separated. Also, we observed at 5 mN/m, relaxation in the pure ST monolayer and in the mixtures rich in ST; the tilt angle of the molecules and the rectangular distortion of the 2D-lattice decrease. By comparing mixed films and pure ST films, this relaxation could be correlated to the size of the domains.

Corrosion behaviour of nitrogen implanted titanium in simulated body fluid

AbstractTitanium is one of the most important materials for medical applications, as a result of its uniquely high biocompatibility. The effect of nitrogen implantation on the biocompatibility and the corrosion resistance of cp titanium are reported. Grazing incidence X-ray diffraction studies showed that implantation formed a δ-TiNx phase. Electrochemical tests in HBSS showed an optimal decrease in corrosion current density for specimens implanted with 3 × 1017 ions cm-2 at 25 keV, compared with unimplanted titanium. Following implantation and immersion in a commercial physiological solution, phases are precipitated which are rich in calcium and phosphorus, and these hydroxyapatite precursors indicate that this implantation regime confers optimal properties of corrosion resistance and biocompatibility.

Transformation of (0002)-oriented Ti film into (111)TiN through nitrogen implantation

Nitrogen implantation into a (0002)-oriented Ti film has resulted in a systematic deformation of the hcp host with implantation dose. Grazing-incidence X-ray diffraction study shows an increase in interplanar spacing of the (0002) and the (10\(\)3) planes as a function of nitrogen concentration. The diffractograms indicate a smooth change of (0002) plane of the host into (111) of the δ-TiN. Strain in the hcp cell was found to be proportional to the nitrogen concentration. The results provide evidence for a strain-driven ‘martensite-like restacking’ formation of TiN compound.

Nanocrystalline TiO 2 studied by optical, FTIR and X-ray photoelectron spectroscopy: correlation to presence of surface states

Nanophase TiO 2 has been synthesized in an organic medium by two different routes, yielding two different structural phases viz. rutile and anatase. These differences are reflected in the different characterization techniques used to study the nanoparticles. UV-vis spectroscopy shows absorption-peak related particle sizes of ∼2.5 nm and agrees with transmission electron microscopy (TEM) estimates of 2.5–5.0 nm. Grazing incidence X-ray diffraction shows rutile and anatase phase with an overlay of Ti 2O 3 for the different routes considered. Differences in the nature of the transition from absorption plots not withstanding, a direct transition is confirmed. Photo-luminescence (PL) spectra for the two particulate structures shows prominent red shifted peaks at 314 nm and 399 nm, respectively (0.81 eV from the excitation), and also reveals vibrational features around the maximum PL signal. In addition a Ti 3+ (PL) state is observed at 617.5 nm for both cases, a feature governed by the Auger process. Fourier transform infrared (FTIR) studies reveal weak complex vibrations between the Ti–O oxide species and also additional unsaturated sites (Ti 3+) through incorporation of (OH) groups, not otherwise seen in bulk titania. A surface consisting of 6Ti 3+–OH for coordinative saturation (octahedral site), along with 4Ti 4+–O (tetrahedral) is thus necessary. Grazing incidence X-ray diffraction studies shows the presence of the rutile phase of TiO 2 and also a sub-oxide phase of Ti (Ti 2O 3). X-ray photoemission spectra (XPS) of thin films of TiO 2 confirms the oxide phase and also the presence of sub-valence states. The XPS and FTIR spectra confirm the presence of adsorbed sites for coordinative saturation of sub-valence states (Ti 2+, Ti 3+), through hydroxyl incorporation. These sites are amplified as the particle size is reduced, opening avenues for additional coordination, leading to important applications. In this case, a Ti 2O 3 overlay saturates the surface of titania. Subtle differences are observed in the data vis-a-vis literature reports.

Effect of Headgroup Dissociation on the Structure of Langmuir Monolayers

We present results of grazing incidence X-ray diffraction studies concerning the effect of pH (and thus, of headgroup dissociation) on Langmuir monolayers of fatty acids, in the absence of any divalent cations in the aqueous subphase. An increase in pH transforms the distorted-hexagonal S phase first to the partially disordered Rotator-I phase with less distortion, and then to the completely disordered Rotator-II phase with an undistorted hexagonal structure. The S−Rotator-I and Rotator-I−Rotator-II transitions are pushed to lower temperatures with increase in pH. The fact that the effects of pH increase are almost identical to the effects of increasing temperature indicates the important role of headgroup−headgroup interactions in these monolayer phases.

Two-dimensional freezing in the liquid-vapor interface of a dilute Pb:Ga alloy.

We report the results of x-ray reflectivity and grazing incidence x-ray diffraction studies of the liquid-vapor interface of a dilute alloy of Pb in Ga over the temperature range of 23-76 degrees C. Our data show that the liquid-vapor interface of this alloy is stratified for several atomic diameters into the bulk liquid and that a monolayer of Pb forms the outermost stratum of the interface. Over the temperature range of 23-56 degrees C, the monolayer of Pb is in an ordered hexagonal phase. At about 58 degrees C, this monolayer undergoes a first-order transition to a hexatic phase, which remains stable to 76 degrees C. An analogy between the observed transition and the first-order melting transition in a one-component classical plasma is suggested.

Anisotropic optical properties of doped poly(3,4-ethylenedioxythiophene)

Anisotropic optical properties of doped poly(3,4-ethylenedioxythiophene) (PEDOT) thin films have been determined by using variable-angle spectroscopic ellipsometry together with intensity reflectance and transmission spectrophotometry. The optical anisotropy of the PEDOT films are of uniaxial character with the optic axis normal to the film surface. The (ordinary) index of refraction in the plane shows a metallic state behavior while the out of plane (extraordinary) index of refraction shows a near dielectric character. The metallic state behavior in the surface plane agrees well with the high electrical conductivity measured for this material. The anisotropic optical properties of doped PEDOT thin films in terms of the complex indices of refraction together with results from grazing incidence X-ray diffraction studies allow us to deduce a preferential orientation of the polymer chains.