Grazing Incidence Synchrotron X-ray Diffraction Research Articles

Determination of the epitaxial growth of zinc oxide nanowires on sapphire by grazing incidence synchrotron x-ray diffraction

This letter shows that aligned zinc oxide (ZnO) nanowires growth on sapphire substrates is epitaxial and demonstrates the crystallographic relation between the two using grazing incidence synchrotron x-ray diffraction (XRD). The in-plane lattice match between the sapphire and the nanowires was directly probed by using XRD at grazing angles of incidence, where the lattice match between the (0001) plane of the sapphire and the (11−20) plane of the ZnO were observed simultaneously. It will also be shown that gold acts as a catalyst to initiate ZnO nanowire growth, but it does not interfere with the epitaxial mechanism between the nanowires and the sapphire substrate.

Crystal Nucleation, Growth, and Morphology of the Synthetic Malaria Pigment β-Hematin and the Effect Thereon by Quinoline Additives: The Malaria Pigment as a Target of Various Antimalarial Drugs

The morphology of micrometer-sized beta-hematin crystals (synthetic malaria pigment) was determined by TEM images and diffraction, and by grazing incidence synchrotron X-ray diffraction at the air-water interface. The needle-like crystals are bounded by sharp {100} and {010} side faces, and capped by {011} and, to a lesser extent, by {001} end faces, in agreement with hemozoin (malaria pigment) crystals. The beta-hematin crystals grown in the presence of 10% chloroquine or quinine took appreciably longer to precipitate and tended to be symmetrically tapered toward both ends of the needle, due to stereoselective additive binding to {001} or {011} ledges. Evidence, but marginal, is presented that additives reduce crystal mosaic domain size along the needle axis, based on X-ray powder diffraction data. Coherent grazing exit X-ray diffraction suggests that the mosaic domains are smaller and less structurally stable than in pure crystals. IR-ATR and Raman spectra indicate molecular based differences due to a modification of surface and bulk propionic acid groups, following additive binding and a molecular rearrangement in the environment of the bulk sites poisoned by occluded quinoline. These results provided incentive to examine computationally whether hemozoin may be a target of antimalarial drugs diethylamino-alkoxyxanthones and artemisinin. A variation in activity of the former as a function of the alkoxy chain length is correlated with computed binding energy to {001} and {011} faces of beta-hematin. A model is proposed for artemisinin activity involving hemozoin nucleation inhibition via artemisinin-beta-hematin adducts bound to the principal crystal faces. Regarding nucleation of hemozoin inside the digestive vacuole of the malaria parasite, nucleation via the vacuole's membranous surface is proposed, based on a reported hemozoin alignment. As a test, a dibehenoyl-phosphatidylcholine monolayer transferred onto OTS-Si wafer nucleated far more beta-hematin crystals, albeit randomly oriented, than a reference OTS-Si.

Synchrotron x-ray scattering study of thin epitaxial Pr2O3 films on Si(001)

We investigate the structural and interfacial properties of thin Pr2O3 films on Si(001) substrates grown by molecular beam epitaxy using synchrotron grazing incidence x-ray diffraction and reflectivity measurements in ultrahigh vacuum. The epitaxial films consist of two orthogonal [101]-oriented cubic domains, with equal proportion. The average in-plane domain sizes are larger than the film thickness. Scans along crystal truncation rods confirm the cubic Mn2O3 structure of the epitaxial layer. A small amount of hexagonal Pr2O3 is found in thin films. Its fraction increases with increasing layer thickness indicating that it is not confined to the interface. Reflectivity measurements reveal an additional layer at the Pr2O3∕Si(001) interface, which is extended by in situ annealing. Transmission electron microscopy of the samples confirms the structural properties of the films found by the x-ray measurements and shows that the interfacial layers are nonuniform with a coexistence of crystalline and amorphous regions.

Synchrotron X-ray investigation of tetracene thin films grown at different deposition fluxes

Grazing incidence synchrotron X-ray diffraction was used to investigate the structure of vacuum-sublimated tetracene thin films (6–50nm thick) deposited on silicon dioxide. The films were found to be polycrystalline with crystallite texturing and size increasing with the deposition flux. This last parameter was found to have a great influence on the relative amounts of the polymorphs (thin film α and β phases) composing the films. These two different phases are characterised by different spacings of the (00ℓ) planes, the spacing of the α phase being closer to that of the bulk. The thicknesses of the two phases in the film change as a function of the deposition flux. Finally, we discuss the role of the deposition flux on the charge-carrier mobility in tetracene films used as active layers in field-effect devices.

Structure and thermal stability of Langmuir–Blodgett films of barium arachidate

The structures of multilayer Langmuir–Blodgett films of barium arachidate before and after heat treatment have been investigated using both atomic force microscopy (AFM) and grazing incidence synchrotron X-ray diffraction (GIXD). AFM gave information on surface morphology at molecular resolution while GIXD provided quantitative details of the lattice structures of the films with their crystal symmetries and lattice constants. As-prepared films contained three coexisting structures: two triclinic structures with the molecular chains tilted by about 20° from the film normal and with 3 × 1 or 2 × 2 super-lattice features arising from height modulation of the molecules in the films; a rectangular structure with molecules perpendicular to the film surface. Of these, the 3 × 1 structure is dominant with a loose correlation between the bilayers. In the film plane both superstructures are commensurate with the local structures, having different oblique symmetries. The lattice constants for the 3 × 1 structure are a s = 3 a = 13.86 Å, b s = b = 4.31 Å and γ s = γ = 82.7°; for the 2 × 2 structure a s = 2 a = 16.54 Å, b s = 2 b = 9.67 Å, γ s = γ = 88°. For the rectangular structure the lattice constants are a = 7.39 Å, b = 4.96 Å and γ = 90°. After annealing, the 2 × 2 and rectangular structures were not observed, while the 3 × 1 structure had developed over the entire film. For the annealed films the correlation length in the film plane is about twice that in the unheated films, and in the out-of-plane direction covers two bilayers. The above lattice parameters, determined by GIXD, differed significantly from the values obtained by AFM, due possibly to distortion of the films by the scanning action of the AFM tip.

In situ investigation of MnAs/GaAs(001) growth and interface structure using synchrotron x-ray diffraction

Using grazing-incidence synchrotron x-ray diffraction, we have quantitatively studied the strain evolution of epitaxial MnAs films in situ during growth by molecular-beam epitaxy. We find that the MnAs layer grows in four distinct stages, being compressively strained at the beginning of growth and relaxing as the thickness increases. The evolution of the in-plane grain size is determined as a function of layer thickness. We find a one-dimensional ordered array of periodic misfit dislocations at the interface, having a periodicity of 4.9±0.05 nm along GaAs[110]. An annealing of the film increases the grain size as well as improves the ordering of dislocation array.

Adaptive chemistry of bifunctional gold nanoparticles at the air/water interface. A synchrotron X-ray study of giant amphiphiles.

A series of ligand stabilized gold nanoparticles with diameters close to 3 nm were studied as Langmuir monolayers at the air/water interface by synchrotron X-ray diffraction and reflectivity. Alkylthiols with different length and/or terminal functional group (hydrophilic or hydrophobic) were introduced into the ligand shell by ligand place exchange reactions. Synchrotron grazing incidence X-ray diffraction (GIXD) and specular X-ray reflectivity reveal the well known hexagonally packed monolayers. In addition the mixed hydrophilic/hydrophobic ligand shell nanoparticles show a high degree of environmental responsiveness, as they adapt to an amphiphilic distribution of ligands around the gold core when spread at the water surface. Likewise nanoparticles of mixed long and short alkyl chains respond to lateral pressure by adopting a structure where the short alkyl chains determine the in-plane nearest neighbor distance while the long alkyl chains determine the film thickness. Based on X-ray reflectivity measurements, which quantitatively account for the electron density in the monolayers, combined with GIXD we calculate the average size and number of atoms of the individual gold particle cores, and estimate the number of passivating ligand on the particle surface. Typical values are dAU-core = 16 A, dnanoparticle = 26-37 A depending on the ligands, Mw = 30-40000 g mol(-1) number of ligands = 40-60. The thickness of the monolayers was determined by AFM after transfer of the monolayers to a solid support using the Langmuir Schaefer technique. The combination of the different techniques produce a very consistent picture of the structure and adaptive chemical nature of the nanoparticles studied, and reveal a surprisingly monodisperse particle distribution centered around 140 atoms in the gold core.

Grazing Incidence Synchrotron X-ray Diffraction of Polymerizing Langmuir Monolayers

Octadecyltrimethoxysilane (OTMS) was spread on the water surface of a Langmuir trough. Utilizing the high-intensity synchrotron X-ray radiation at the Advanced Photon Source at Argonne National Laboratory, the kinetics of film growth of OTMS as a function of the subphase pH was studied. In situ grazing incidence X-ray diffraction experiments at the air/water interface revealed that OTMS self-assembles into small domains and that the number of these ordered domains increases as OTMS undergoes hydrolysis/condensation reactions. The hexagonal packing of alkyl chains found in the OTMS monolayer is consistent with the ordering found in bulk OTMS by powder X-ray diffraction.

Interface directed assembly of cyanide-bridged Fe–Co and Fe–Mn square grid networks

Two isostructural square grid networks were formed by reacting a Langmuir monolayer of an amphiphilic pentacyanoferrate (3+) complex with aqueous Co 2+ or Mn 2+ ions dissolved in the subphase. Confinement of the reactants to the air–water interface discriminates against the formation of higher dimensional products and directs the lateral propagation of a polymeric two-dimensional cyanide-bridged network. The network can be transferred to a variety of supports to form monolayer or multilayer lamellar films by the Langmuir–Blodgett (LB) technique. Characterization of both the Fe–Co and Fe–Mn LB film extended networks by FTIR spectroscopy, SQUID magnetometry, and grazing incidence synchrotron X-ray diffraction (GIXD) reveals face-centered square grid structures directed by the defined bond angles of the octahedral metal complexes and the linear cyanide bridges. Magnetic measurements on both samples indicate the presence of anisotropic low temperature magnetic exchange interactions between the paramagnetic centers. The results illustrate the potential utility of an interface as a structure director in the assembly of low dimensional coordinate covalent network solids.

GIXRD of nanoscale strain patterning in wafer bonding

Synchrotron grazing incidence X-ray diffraction is used to study tuneable lateral dislocation superlattices obtained by Si wafer bonding. Deformation satellites peaks around the {2 2 0 } reflections are measure for pure twist and mixed (twist–tilt) grain boundaries. The twist of the two crystals induces a square network of screw dislocations whose features are calculated by continuum elasticity theory and kinematical approximation. The tilt of the crystals is accommodated by alternated mixed dislocations and the interactions between these two networks is discussed from the X-ray diffraction measurements.

Grazing incidence synchrotron X-ray diffraction study of crystal orientation in microporous films

Thin microporous films with MFI type structure were deposited on silicon wafers via the seed method and spin coating of a stable colloidal mixture containing nanosized zeolites. The seed method includes adsorption of nanosized MFI seed crystals followed by hydrothermal synthesis of the seeds in order to obtain an intergrown film with a thickness of about 250 nm. In addition, thin MFI porous films with similar thicknesses were prepared by spin coating of stable colloidal suspensions and are compared with the seeded and grown layers. The diffractograms of the MFI films at grazing incidence and exit geometry were measured, which reflect the distribution of the crystal orientation along the film thickness. Preferred orientation of porous materials in the films is attributed to the deposition techniques and morphological features of the zeolite nanoparticles. The MFI adsorbed seeds and spin-deposited crystals are preferentially oriented with their a-axis perpendicular to the substrate, while the crystals in the grown film show an orientation change from the a- to the b-axis perpendicular to the substrate as a function of the film thickness.

Supramolecular assembly at interfaces: formation of an extended two-dimensional coordinate covalent square grid network at the air-water interface.

Reaction of a Langmuir monolayer of an amphiphilic pentacyanoferrate(3+) complex with Ni(2+) ions from the subphase results in the formation of a two-dimensional iron-nickel cyanide-bridged network at the air-water interface. The network can be transferred to various supports to form monolayer or multilayer lamellar films by the Langmuir-Blodgett (LB) technique. The same network does not form from homogeneous reaction conditions. Therefore, the results demonstrate the potential utility of an interface as a structure director in the assembly of low dimensional coordinate covalent network solids. Characterization of the LB film extended networks by X-ray photoelectron spectroscopy (XPS), FT-IR spectroscopy, SQUID magnetometry, X-ray absorption fine structure (XAFS), and grazing incidence synchrotron X-ray diffraction (GIXD) revealed a face-centered square grid structure with an average domain size of 3600 A(2). Magnetic measurements indicated that the network undergoes a transition to a ferromagnetic state below a T(c) of 8 K.

Influence of palmitic acid and hexadecanol on the phase transition temperature and molecular packing of dipalmitoylphosphatidyl-choline monolayers at the air–water interface

Palmitic acid (PA) and 1-hexadecanol (HD) strongly affect the phase transition temperature and molecular packing of dipalmitoylphosphatidylcholine (DPPC) monolayers at the air–water interface. The phase behavior and morphology of mixed DPPC/PA as well as DPPC/HD monolayers were determined by pressure-area-isotherms and fluorescence microscopy. The molecular organization was probed by synchrotron grazing incidence x-ray diffraction using a liquid surface diffractometer. Addition of PA or HD to DPPC monolayers increases the temperature of the liquid-expanded to condensed phase transition. X-ray diffraction shows that DPPC forms mixed crystals both with PA and HD over a wide range of mixing ratios. At a surface pressure (π) of 40 mN/m, increasing the amount of the single chain surfactant leads to a reduction in tilt angle of the aliphatic chains from nearly 30° for pure DPPC to almost 0° in a 1:1 molar ratio of DPPC and PA or HD. At this composition we also find closest packing of the aliphatic chains. Further increase of the amount of PA or HD does not change the lattice or the tilt.

Packing of Ganglioside-Phospholipid Monolayers: An X-Ray Diffraction and Reflectivity Study

Using synchrotron grazing-incidence x-ray diffraction (GIXD) and reflectivity, the in-plane and out-of-plane structure of mixed ganglioside-phospholipid monolayers was investigated at the air-water interface. Mixed monolayers of 0, 5, 10, 20, and 100 mol% ganglioside GM 1 and the phospholipid dipalmitoylphosphatidylethanolamine (DPPE) were studied in the solid phase at 23°C and a surface pressure of 45 mN/m. At these concentrations and conditions the two components do not phase-separate and no evidence for domain formation was observed. X-ray scattering measurements reveal that GM 1 is accommodated within the host DPPE monolayer and does not distort the hexagonal in-plane unit cell or out-of-plane two-dimensional (2-D) packing compared with a pure DPPE monolayer. The oligosaccharide headgroups were found to extend normally from the monolayer surface, and the incorporation of these glycolipids into DPPE monolayers did not affect hydrocarbon tail packing (fluidization or condensation of the hydrocarbon region). This is in contrast to previous investigations of lipopolymer-lipid mixtures, where the packing structure of phospholipid monolayers was greatly altered by the inclusion of lipids bearing hydrophilic polymer groups. Indeed, the lack of packing disruptions by the oligosaccharide groups indicates that protein-GM 1 interactions, including binding, insertion, chain fluidization, and domain formation (lipid rafts), can be studied in 2-D monolayers using scattering techniques.

Near-critical two-dimensional smectic-C to solid-like phase transition in azobenzene-derivative Langmuir monolayers

We describe the results of a detailed study of two-dimensional (2D) smectic-C to solid-like phase transition in azobenzene-derivative Langmuir monolayers by the surface pressure vs area-per-molecule isotherm measurement, polarizing optical microscopy, and the grazing incidence synchrotron x-ray diffraction. The phase transition was observed to be weakly first order up to slightly above room temperature with a transition entropy less than 1.0 kB per molecule, accompanied by proper characteristics of a nearly ideal horizontal coexistence line on the isotherm, an abrupt change in optical texture, and the emergence of a conspicuous x-ray diffraction peak in the solid-like phase, which indicates a weak positional order with a correlation length of 10–20 nm. Analysis of the x-ray diffraction data within the framework of distorted hexagonal lattice suggests that the solid-like phase may be regarded as a 2D analog of smectic-L phase that is a hexatic smectic phase with the molecules tilted toward a direction between the nearest and the next-nearest bonds. At higher temperatures, the transition became less discontinuous and entirely disappeared above 40 °C in all these experimental aspects. We argue that the transition is viewed as a 2D induced hexatic–hexatic transition (analog of smectic-C to smectic-L transition), which has an isolated critical point where the first-order transition comes to an end.

Positional order and thermal expansion of surface crystalline N-alkane monolayers.

We report a high-resolution synchrotron grazing incidence x-ray diffraction measurement of a surface crystalline monolayer at the liquid-vapor interface of the n-alkane eicosane (C20H42) just above its melting temperature. The peak width of the surface monolayer rotator phase is shown to be resolution limited and implies positional correlations of at least approximately 1 microm. The high resolution allowed determination of the temperature dependence of the peak position over the narrow (3 degrees C) temperature range of the surface crystal phase. The two-dimensional thermal expansion was determined to be (dA/dT)/A=1.8(+/-0.1)x10(-3) degrees C-1, which is comparable to the expansion in similar chain length bulk n-alkane rotator phases. Our data are consistent with the power-law shaped scattering tails expected from quasi-long-range order in two dimensions.

X-ray Scattering Studies of Mixed Monolayers of Tetrakis(3,5-di-tert-butylphenyl)porphinatocopper(II) with Cadmium Arachidate at the Air/Water Interface

The structures of mixed monolayers of cadmium arachidate (CdAr) with tetrakis(3,5-di-tert-butylphenyl)porphinatocopper(II) (TDBPC) (0.13 mole fraction) at the air/water interface have been studied by grazing incidence synchrotron X-ray diffraction (GIXD) and specular X-ray reflectivity. Only the hexagonal structure of CdAr was observed in the GIXD measurements, with d11 = 4.16 Å, giving an area per chain of 20.0 Å2. The chains are perpendicular to the subphase surface and the positional correlation length is 45 Å. For comparison, GIXD measurements of CdAr alone show a hexagonal in-plane structure with d11 = 4.13 Å and an area per chain of 19.7 Å2. The acyl chains in the film are perpendicular to the surface of the subphase and have a correlation length of 140 Å. Reflectivity measurements on the monolayer of arachidic acid on a CdCl2 solution confirm that the chains are normal to the surface of the subphase and suggest that the headgroups have the composition −COOCdOH. For the mixed film reflectivity measurements indicate that, at a surface pressure of 25 mN m-1, the porphyrin rings have a diameter of about 15.2 Å and are perpendicular to the surface. At 2 mN m-1 the rings are either tilted from the normal by about 28°, or normal to the surface leaving some areas of low density in the film. The two compounds are immiscible in the mixed monolayer, but the formation of CdAr domains is hindered by the presence of the porphyrin.

Synchrotron Radiation Diffraction from Two-Dimensional Protein Crystals at the Air/Water Interface

Protein structure determination by classical x-ray crystallography requires three-dimensional crystals that are difficult to obtain for most proteins and especially for membrane proteins. An alternative is to grow two-dimensional (2D) crystals by adsorbing proteins to ligand-lipid monolayers at the surface of water. This confined geometry requires only small amounts of material and offers numerous advantages: self-assembly and ordering over micrometer scales is easier to obtain in two dimensions; although fully hydrated, the crystals are sufficiently rigid to be investigated by various techniques, such as electron crystallography or micromechanical measurements. Here we report structural studies, using grazing incidence synchrotron x-ray diffraction, of three different 2D protein crystals at the air-water interface, namely streptavidine, annexin V, and the transcription factor HupR. Using a set-up of high angular resolution, we observe narrow Bragg reflections showing long-range crystalline order in two dimensions. In the case of streptavidin the angular range of the observed diffraction corresponds to a resolution of 10 Å in plane and 14 Å normal to the plane. We show that this approach is complementary to electron crystallography but without the need for transfer of the monolayer onto a grid. Moreover, as the 2D crystals are accessible from the buffer solution, the formation and structure of protein complexes can be investigated in situ.

Crystalline Self-Assembly into Monolayers of Folded Oligomers at the Air-Water Interface

Insertion of the 1,3-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 Å, induces chain folding at the air-water interface. These folded molecules self-assemble into crystalline monolayers at this interface, with the plane of the folding unit almost perpendicular to the water surface, as determined by synchrotron grazing-incidence X-ray diffraction. Three distinct molecular shapes, of the types U, inverted U, and M, were obtained in the two-dimensional crystalline state, depending upon the number of spacer units, and the number and position of the hydrophilic groups in the molecule. The molecules form ribbons with a higher crystal coherence in the direction of stacking between the molecular ribbons, and a lower coherence along the ribbon direction. A similar molecule, but with a spacer unit that imposes a 5 Å separation between alkyl chains, yields the conventional herringbone arrangement.

Crystalline self-assembly into monolayers of folded oligomers at the air-water interface

Insertion of the 1,3-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 A. induces chain folding at the air-water interface. These folded molecules self-assemble into crystalline monolayers at this interface, with the plane of the folding unit almost perpendicular to the water surface, as determined by synchrotron grazing-incidence X-ray diffraction. Three distinct molecular shapes, of the types U, inverted U, and M, were obtained in the two-dimensional crystalline state, depending upon the number of spacer units, and the number and position of the hydrophilic groups in the molecule. The molecules form ribbons with a higher crystal coherence in the direction of stacking between the molecular ribbons, and a lower coherence along the ribbon direction. A similar molecule, but with a spacer unit that imposes a 5 A separation between alkyl chains, yields the conventional herringbone arrangement.