Grazing Incidence Angle X-ray Diffraction Research Articles

Structural characterization of crystalline ternary inclusion compounds at the air-water interface.

Crystalline ternary inclusion monolayers consisting of a two-dimensional hydrogen-bonded host network of guanidinium (G) ions and organosulfonate (S) amphiphiles, and biphenylalkane guests, can be generated at the air-water interface through synergistic structural enforcement by hydrogen bonding and host-guest packing. Surface pressure-area isotherms of the 4'-hexadecylbiphenyl-4-sulfonate (C16BPS) amphiphile in the presence of G, with or without guest, are characterized by lift-off molecular areas expected for the GS sheet based on single-crystal X-ray structures of homologous bulk crystals. Intercalation of biphenylalkane guests (4-C(n)()H(2)(n)()(+1)-C(6)H(4)-C(6)H(5), n = 1, 4, 6, 10, 16; denoted CnBP) between organosulfonate hydrophobes, which define pocketlike cavities in the GS monolayer host, afford ternary inclusion monolayers with a 1:1 host-guest stoichiometry. These inclusion monolayers are less compressible than the guest-free host, consistent with dense packing of the biphenylalkane moieties of the host and the biphenylalkane guests. The inclusion monolayers are distinguished from the amorphous guest-free host and from selected guanidinium-free mixed monolayers by structural characterization with grazing-angle incidence X-ray diffraction (GIXD). The GIXD data for the ternary (G)C16BPS:C16BP and (G)C16BPS:C6BP inclusion monolayers obtained upon compression are consistent with a rectangular unit cell. The dimensions of these unit cells and refinement of the GIXD data suggest a "rotated shifted ribbon" GS hydrogen-bonding motif similar to that observed in some bulk GS crystals, including (G)(ethylbiphenylsulfonate). GIXD reveals that (G)C16BPS:C16BP and (G)C16BPS:C6BP are more crystalline than the corresponding guanidinium-free mixed monolayers. The (G)C16BPS:C6BP inclusion monolayer is stable upon compression, even though the alkyl-alkyl host-guest interactions are reduced due to the shorter hexyl substituents of the guest, demonstrating an important reinforcing role for the hydrogen-bonded GS sheet. The structure of a C16BPS:tetracosane (C24) mixed monolayer is independent of G; the unit cell symmetry and dimensions suggest a structure governed by alkyl-alkane interactions that prohibit formation of a GS network. These results illustrate that the existence of ternary inclusion monolayers with an intact GS network requires guest molecules that are structurally homologous with the hydrophobes of the host, in this case biphenylalkanes. The observation of these inclusion compounds suggests an approach for introducing functional nonamphiphilic molecules to an air-water interface through inclusion in a well-defined host.

Growth of AlN on lattice-matched MnO substrates by pulsed laser deposition

We have grown AlN on MnO (111) substrates with pulsed laser deposition (PLD) and characterized their structural properties using various techniques. We have found that hexagonal AlN successfully grows epitaxially on MnO with the use of PLD. Grazing incidence-angle X-ray diffraction (GIXD) measurements have revealed that the epitaxial relationship between MnO and AlN is [0001] AlN//[111] MnO and [11−20] AlN//[1−10] MnO, which minimizes the lattice mismatch (0.8%). We have also found that the heterointerface for AlN/MnO is less abrupt than that for AlN/Al 2O 3 due to the chemical vulnerability of MnO.

Grazing-angle incidence X-ray diffraction by Si 1−α( x) Ge α( x) thin layer if the composition coefficient α( x) is varying harmonically along the flat entrance surface

Presented theoretical paper concerns the investigation of a principal possibility of the Grazing-angle incidence hard and soft X-ray diffraction (GIXD) owing to long-period harmonic variations of the composition coefficient of the strained or relaxed crystalline, as well as amorphous SiGe layer grown on a thick silicon perfect-crystal substrate. The evaluation of the coherent part of X-radiation scattered by mentioned SiGe layer is pointed out the possibility of the direct GIXD experimental investigations of the long-period structured intermediate transformation states of SiGe layer, which are emerging due to periodicity of the strain field along the substrate–layer boundary.

Room-temperature epitaxial growth of AlN films

We have grown AlN films on nearly-lattice-matched (Mn,Zn)Fe2O4 (111) substrates by pulsed-laser deposition at room temperature and investigated their structural properties using reflection high-energy electron diffraction (RHEED), grazing incidence angle x-ray reflectivity (GIXR), grazing incidence angle x-ray diffraction (GIXD), and atomic force microscope. We have found that AlN grows epitaxially even at room temperature. In situ RHEED observations have shown that the room-temperature growth of AlN starts with the two-dimensional mode followed by a transition to the three-dimensional mode at the film thickness of 2 nm. GIXR and GIXD measurements have revealed that the heterointerface between AlN and (Mn,Zn)Fe2O4 is abrupt and approximately 90% of the lattice mismatch is released at the interface due to the introduction of crystalline defects such as misfit dislocations. These results indicate that the present technique solves one of the two major problems with heteroepitaxial growths of group III nitrides (mismatch in the thermal expansion coefficients) and alleviates the other problem (mismatch in the lattice constants).

Grazing-angle Incidence X-ray Diffraction by the Si1-α(x)-β(x)Geβ(x)Cβ(x)/Si Heterojunction where the Germanium and the Carbon Concentrations are Periodically Varying along the Flat Layer Surface

AbstractPresented theoretical paper concerns the investigation of SiGeC/Si heterojunction by the Grazing-angle Incidence X-ray Diffraction (GIXD) method. We consider a possibility in principal of the GIXD by the specific long-range harmonic variations of the germanium and carbon compositions in the thin SiGeC layer. Evaluation of the theoretically calculated coherent part of x-radiation scattered by the SiGeC layer points the way to the experimental direct investigations of the long-period structured intermediate transformation states of SiGeC layer that emerge owing to inhomogeneity of the strain field along the heterojunction surface.

Effect of implantation voltage on phase composition and surface roughness of α-Fe surface layer implanted by nitrogen ion

The implantation of N ion into α-Fe surface was carried out by plasma-based ion implantation method at the elevated temperature of 360 °C and different implantation voltages of 15 and 35 kV. The phase composition and the surface roughness were studied by means of grazing incidence angle X-ray diffraction and atomic force microscopy, respectively. The results indicated that the implantation voltage has great effects on the iron nitride formation and surface roughness of the implanted layer. The lower the voltage, the greater is the nitrides in the layer, and the higher is the roughness of the surface. The influence mechanisms of implantation voltage on the phase composition of the layer and the roughness of the surface were also discussed.

Sphingomyelin at the air–water interface

X-ray reflectivity (XR) and grazing incident-angle x-ray diffraction (GIXD) reveal that sphingomyelin, although forming homogeneous monomolecular films at air–water interfaces, does not develop long-range crystalline in-plane order at any in-plane density, most notably at high surface pressures where many other membrane lipids order as two-dimensional crystals. Studies were carried out on the monodisperse synthetic C18-sphingomyelin where both hydrocarbon chains are saturated and on natural sphingomyelin which contains a distribution of chain lengths, with saturated and unsaturated hydrocarbon chains. The surface pressure versus molecular area (π–A) isotherm for the natural sphingomyelin is similar to that of the synthetic, but has a somewhat higher surface pressure at intermediate areas. The absence of in-plane crystalline order is attributed to competing interactions in the head group region: hydrogen bonding in the erythro region, electrostatic interactions among zwitterions of neighboring head groups, and van der Waals among acyl chains.

Photoelectrochemistry of the insertion compounds Na xInSe and Li xInSe

The insertion reaction of Li and Na in lamellar InSe single crystal was performed electrochemically in a non-aqueous electrolyte at room temperature. The insertion product was, at the electrode surface, an intercalation compound (Li x InSe, Na x InSe) or the result of the chemical decomposition of the electrode material (Li 2Se, Na 2Se). The development of the reaction was followed by photoelectrochemical spectroscopy and by cyclic photocurrent voltammetry performed in situ in the reaction electrolyte. A Fermi level shift in the intercalation compound was deduced by a flatband shift to lower potential and a small increase of the bandgap energy resulted from the intercalation reaction. The Na-inserted samples have been analyzed by grazing angle incidence X-ray diffraction and an increase in the interlayer distance was observed.

Structure of Electrode Surfaces in the Course of Electrocatalytic Reactions: Oxidation of CO, Glucose, and Formaldehyde on Reconstructed and Unreconstructed Au(100)

Grazing incident angle X-ray diffraction and linear sweep voltammetry were used to determine structure and reactivity of the Au(100) surface during the course of glucose, CO, and formaldehyde oxidations. The potential dependence of the diffracted intensity at the principal reconstruction peak was measured in HClO4 (for CO and glucose) and NaOH (for formaldehyde) solutions in the absence and in the presence of adsorption and oxidation of organic molecules. The oxidation occurs before or at the onset of the lifting of the reconstruction. No significant change in the kinetics of the reconstruction lifting or recovering is caused by the adsorption and oxidation of the above organic molecules. The more negative current peak potentials and the larger values of heterogeneous rate constants for the unreconstructed Au(100) surface indicate that the unreconstructed Au(100) surface is more active than the hexagonally reconstructed Au(100) for all three reactions.

Determination of the Fe layer structure in Fe/Ru superlattices by grazing incidence angle X-ray diffraction

The hexagonal structure of ferromagnetic Fe in Fe/Ru superlattices is still controversial. The unexpected magnetic behaviour [1] as well as the X-ray absorption near-edge structure (XANES) results have led Beaudelet [2] to question the hcp structure for the Fe layers and to suggest another model of the Fe hexagonal stacking in which the local environment of Fe atoms looks like a distorted bcc one, despite the epitaxy on (0001) hcp Ru. To verify the existence of this unusual hexagonal stacking, we have undertaken an X-ray diffraction study performed with grazing incidence angle. Unexpectedly, our results support convincingly the regular hcp structure rather than the ‘bcc-like’ packing.

The epitaxial growth of evaporated Cu/CaF2 bilayers on Si(111)

Successful and unexpected epitaxial growth of Cu/CaF2 bilayers on hydrogen terminated Si(111) wafers by thermal evaporation is reported. The bilayers were characterized with conventional x-ray diffraction experiments, grazing angle incidence x-ray diffraction experiments, rocking curves, and χ scans. The growth mode of Cu films on CaF2 epitaxially grown on Si(111) is completely different from that of the Cu film grown directly on Si(111).

High-temperature stability of heteroepitaxial Ir-silicide/SiGe layers co-deposited on Si(100) under ultra-high vacuum

Heteroepitaxial Ir-silicide/SiGe layers have been grown on top of p-Si(100) at a substrate temperature of 450 °C under ultra-high vacuum. The epitaxial Ir-silicide layer was determined to be Ir 3Si 4 with four types of epitaxial modes. Reflection high-energy electron diffraction was used to examine the film quality. The high-temperature stability of the epitaxial Ir 3Si 4/SiGe layers was investigated by rapid thermal annealing the Ir 3Si 4/SiGe layers at 550, 750 and 950 °C for 20 s. The chemical composition, microstructures and surface morphologies of the layers were characterized by using Auger electron spectroscopy, grazing angle incidence X-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. A critical transition temperature existed between 550 and 750 °C for the Ir 3Si 4/SiGe layers, beyond which the Ir 3Si 4/SiGe layers were unstable during the rapid thermal annealing process. The Ir 3Si 4 layer became a mixture of dual Ir-silicide phases containing Ge atoms as the instability occurred. Because of the thermal instability, the conventional guard-ring fabrication should be performed before deposition of epitaxial films.

The effect of temperature on the epitaxial quality of bilayers of iridium silicide and SiGe deposited on Si(100) in ultrahigh vacuum

Ir-silicide/SiGe layers were co-deposited on a p-Si(100) substrate at high temperatures. Reflection high energy electron diffraction, transmission electron microscope, and grazing angle incidence X-ray diffraction have been used in characterizing the co-deposited layers. Heteroepitaxial Ir-silicide/SiGe layers on top of p-Si(100) have been achieved at a substrate temperature of 450°C. The Ir-silicide layer was determined to be Ir 3Si 4 with four types of epitaxial modes. The three principal axes ( a = 18.551 A ̊ , b = 3.840 A ̊ , and c = 5.713 A ̊ ) were found to be strained, slightly different from those in the bulk Ir 3Si 4 ( a = 18.870 A ̊ , b = 3.679 A ̊ , and c = 5.774 A ̊ ). The Ir-silicide/SiGe layers co-deposited at 550 and 525°C showed dual phases and polycrystalline structures. corresponding to three-dimensional island growth.