Synchrotron Radiation Applied to Real-Time Studies of the Kinetics of Growth of Aluminum Nitride Thin Multilayers.

This article reports the design, construction, and first use of an experimental device consisting of a specially designed vacuum chamber equipped with a reactive sputtering magnetron (RSM) to be used for controlled deposition of thin films on a Si(100) flat substrate. The setup was designed to allow for in situ and real-time recordings of X-ray diffraction patterns during the growth of the deposited films and was installed in the X-ray diffraction and spectroscopy beamline emerging from a superconducting wiggler source at the Brazilian Synchrotron Light Laboratory. The first use of the RSM setup was an in situ and real-time X-ray diffraction study of processes of growth of multilayered aluminum nitride thin films, whereas the operation parameters of the reactor were sequentially changed. This sequential process led to the development of multilayered films. Alternate variations in chamber pressure and magnetron power density allowed us to obtain thin films composed of several micrometer thick layers, with alternate amorphous and (10·0), (00·2), or (10·1) textured polycrystalline structures.

Controlled growth of extended arrays of CoSi2 hexagonal nanoplatelets buried in Si(001), Si(011) and Si(111) wafers.

Because of their high electrical conductivity CoSi2 nanostructures are potential candidates for preparing ordered nano-arrays to be used as electrode interconnectors and contacts in microelectronic devices. We here describe a controlled procedure for the endotaxial growth of hexagonal CoSi2 nanoplatelets buried in differently oriented single crystalline Si wafers on which a Co-doped SiO2 thin film was previously deposited. These nanomaterials were obtained by a clean procedure consisting of isothermal annealing at 750 °C under a He atmosphere of Co-doped SiO2 thin films deposited onto the surface of three differently oriented flat Si substrates, namely Si(001), Si(011) and Si(111). Buried CoSi2 nanoplatelets are in all cases spontaneously formed as a consequence of the diffusion of Co atoms into the silicon wafer and their reaction with host Si atoms. Our TEM and GISAXS analyses demonstrated that these arrays, irrespective of host Si orientation, consist of CoSi2 hexagonal nanoplatelets in all cases parallel to Si{111} crystallographic planes. Additionally, the dimensions of the nanoplatelets were consistently determined by TEM and GISAXS for the three different host Si single crystal orientations.

X-ray reflectivity analysis of titanium dioxide thin films grown by cathodic arc deposition.

TiO2 thin films deposited by a vacuum arc on a glass substrate were characterized by X-ray reflectivity (XRR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Several thin films with different amounts of deposited TiO2 mass and different deposition and annealing temperatures were studied. A qualitative analysis of the XRD patterns indicated the presence of the anatase and/or rutile crystalline phases in most of the studied samples. From the analysis of the experimental XRR curves--which exhibited a wide angular range of oscillatory behavior--the thickness, mass density and interface roughness were determined. All XRR patterns were well fitted by modeled curves that assume the presence of a single and homogeneous TiO2 layer over which a very thin H2O layer is adsorbed. The thickest H2O adsorption layers were developed in films with the highest anatase content. Our overall results of the XRR analyses are consistent with those derived from the imaging techniques (SEM and AFM).

Reactive sputter magnetron reactor for preparation of thin films and simultaneous in situ structural study by X-ray diffraction.

The purpose of the designed reactor is (i) to obtain polycrystalline and∕or amorphous thin films by controlled deposition induced by a reactive sputtering magnetron and (ii) to perform a parallel in situ structural study of the deposited thin films by X-ray diffraction, in real time, during the whole growth process. The designed reactor allows for the control and precise variation of the relevant processing parameters, namely, magnetron target-to-sample distance, dc magnetron voltage, and nature of the gas mixture, gas pressure and temperature of the substrate. On the other hand, the chamber can be used in different X-ray diffraction scanning modes, namely, θ-2θ scanning, fixed α-2θ scanning, and also low angle techniques such as grazing incidence small angle X-ray scattering and X-ray reflectivity. The chamber was mounted on a standard four-circle diffractometer located in a synchrotron beam line and first used for a preliminary X-ray diffraction analysis of AlN thin films during their growth on the surface of a (100) silicon wafer.

Local structure and near-infrared emission features of neodymium-based amine functionalized organic/inorganic hybrids.

Nd(3+)-based organic/inorganic hybrids have potential application in the field of integrated optics. Attractive sol-gel derived di-urea and di-urethane cross-linked poly(oxyethylene) (POE)/siloxane hybrids (di-ureasils and di-urethanesils, respectively) doped with neodymium triflate (Nd(CF(3)SO(3))(3)) were examined by Fourier transform mid-infrared (FT-IR), Raman (FT-Raman), (29)Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and photoluminescence spectroscopies, and small-angle X-ray scattering (SAXS). The goals of this work were to determine which cation coordinating site of the host matrix (ether oxygen atoms or carbonyl oxygen atoms) is active in each of the materials analyzed, its influence on the nanostructure of the samples and its relation with the photoluminescence properties. The main conclusion derived from this study is that the hydrogen-bonded associations formed throughout the materials play a major role in the hybrids nanostructure and photoluminescence properties.

Structure characterization and mechanism of growth of PbTe nanocrystals embedded in a silicate glass.

A nanocomposite consisting of PbTe nanocrystals embedded in a silicate glass was studied by small-angle x-ray scattering during the early stage of isothermal annealing at 793 K. A theoretical function based on a model of spherical PbTe nanocrystals surrounded by a Pb and Te depleted shell fits well to all experimental curves. The time dependences of the nanocrystal radius and size of the depleted shell agree with the prediction of the theory of nucleation and growth by the classical mechanism of atomic diffusion.

Crystal structure of the dimeric phosphoenolpyruvate carboxykinase (PEPCK) from Trypanosoma cruzi at 2 A resolution.

ATP-dependent phosphoenolpyruvate carboxykinase (PEPCK) (ATP: oxaloacetate carboxylyase (transphosphorylating), EC 4.1.1.49) is a key enzyme involved in the catabolism of glucose and amino acids in the parasite Trypanosoma cruzi, the causative agent of Chagas' disease. Due to the significant differences in the amino acid sequence and substrate specificity of the human enzyme (PEPCK (GTP-dependent), EC 4.1.1.32), the parasite enzyme has been considered a good target for the development of new anti-chagasic drugs. We have solved the crystal structure of the recombinant PEPCK of T. cruzi up to 2.0 A resolution, characterised the dimeric organisation of the enzyme by solution small angle X-ray scattering (SAXS) and compared the enzyme structure with the known crystal structure of the monomeric PEPCK from Escherichia coli. The dimeric structure possesses 2-fold symmetry, with each monomer sharing a high degree of structural similarity with the monomeric structure of the E. coli PEPCK. Each monomer folds into two complex mixed alpha/beta domains, with the active site located in a deep cleft between the domains. The two active sites in the dimer are far apart from each other, in an arrangement that seems to permit an independent access of the substrates to the two active sites. All residues of the E. coli PEPCK structure that had been found to interact with substrates and metal cofactors have been found conserved and in a substantially equivalent spatial disposition in the T. cruzi PEPCK structure. No substrate or metal ion was present in the crystal structure. A sulphate ion from the crystallisation medium has been found bound to the active site. Solution SAXS data suggest that, in solutions with lower sulphate concentration than that used for the crystallisation experiments, the actual enzyme conformation may be slightly different from its conformation in the crystal structure. This could be due to a conformational transition upon sulphate binding, similar to the ATP-induced transition observed in the E. coli PEPCK, or to crystal packing effects. The present structure of the T. cruzi PEPCK will provide a good basis for the modelling of new anti-chagasic drug leads.

Commissioning and operation of the first brazilian synchrotron light source.

The synchrotron light source designed and constructed at the LNLS is composed of a 1.37 GeV electron storage ring and a 120 MeV linac for low-energy injection. The storage ring has been commissioned and has already reached the designed electron-beam energy, current and emittance. The electron lifetime is now 6 h at 60 mA, and is steadily increasing. Seven beamlines (TGM, SGM, SXS, XAFS, XRD, SAXS, PCr) have been constructed in parallel with the electron accelerators and are at present in operation. Beam time was allocated to 129 approved research projects for the second semester of 1997. A number of them are currently under way.

Set-up and experimental parameters of the protein crystallography beamline at the Brazilian National Synchrotron Laboratory.

The Brazilian National Synchrotron Light Laboratory (LNLS) has a dedicated protein crystallography beamline. The optical elements of the beamline include an elastically bent cylindrical mirror and a triangular bent-crystal monochromator, which focus synchrotron radiation at the position of the sample in the vertical and horizontal planes, respectively. The monochromatic radiation is tunable between 2.0 and 1.2 A with the optimum wavelengths from 1.3 to 1.6 A, chosen to maximize the photon flux from the bending magnets of the storage ring (1.37 GeV). Diffraction images are recorded on a 345 mm-diameter MarResearch image-plate detector system with on-line readout. The experimental parameters of the beamline, such as the integral monochromatic flux and focus size, have been measured. The size of the beam at the position of the focal point is 0.5 x 0.5 mm(2). The flux density is between 4.4 x 10(10) and 8 x 10(10) photons s(-1) mm(-2) for wavelengths from 1.28 to 1.6 A. The energy resolution is sufficient to measure absorption edges of elements between 1.28 and 2 A. The facility, intended to serve the national and international community, has been commissioned and is available for users.

The subdomain structure of human serum albumin in solution under different pH conditions studied by small angle X-ray scattering.

Small-angle X-ray scattering (SAXS) was used to study structural characteristics of human serum albumin (HSA) in solution under different pH conditions. Guinier analysis of SAXS results yielded values of the molecular radius of gyration ranging from 26.7 A to 34.5 A for pH varying from 2.5 to 7.0. This suggests the existence of significant differences in the overall shape of the molecule at different pH. Molecular models based on subdomains with different spatial configurations were proposed. The distance distribution functions associated with these models were calculated and compared with those determined from the experimental SAXS intensity functions. The conclusion of this SAXS study is that the arrangement of molecular subdomains is clearly pH dependent; the molecule adopting more or less compact configuration for different pH conditions. The conclusions of this systematic study on the modification in molecular shape of HSA as a response to pH changes is consistent with those of previous investigations performed for particular pH conditions.

SAXS study of crotapotin at low pH.

The structure of crotapotin, a protein extracted, from the venom of the Crotalus durissus terrificus, in solution at pH = 1.5, was studied by SAXS. The experimental results yield structural parameter values of the molecular radius of gyration Rg = 13.6 A, volume v = 16.2 x 10(3) A3 A3 and maximal dimension Dmax = 46 A. The distance distribution function deduced from the scattering measurements is consistent with an overall molecular shape of an oblate ellipsoid of revolution with asymmetry parameter v = 0.45.

SAXS study of the snake toxin alpha-crotamine.

alpha-crotamine is a small toxic protein (42 amino acid residues with three disulphide bridges) present in the venom of Crotallus durissus terrificus. Molecular parameters (Rg = 13.7 A, S = 3,000 A2, V = 9,200 A3 and Dmax = 40 A) were derived from SAXS curves obtained from a solution of this protein at pH = 4.5. An excellent agreement between the experimental distance distribution curve and that calculated from a model consisting of two lobes linked by the Cys(18)-Cys(30) disulphide bridge.

Saxs study of structure and conformational changes of crotamine.

The radius of gyration of crotamine is determined by the small angle x-ray scattering technique. Several molecular solutions have been studied to correct for concentration effects. The apparent molecular radius of gyration is also determined as a function of pH. An important change between pH 9.5 and 12.5 is attributed to a dominant effect of molecular aggregation.