Guinther Kellermann

Structure of PbTe(SiO(2))/SiO(2) multilayers deposited on Si(111)

The structure of thin films composed of a multilayer of PbTe nanocrystals embedded in SiO2, named as PbTe(SiO2), between homogeneous layers of amorphous SiO2 deposited on a single-crystal Si(111) substrate was studied by grazing-incidence small-angle X-ray scattering (GISAXS) as a function of PbTe content. PbTe(SiO2)/SiO2 multilayers were produced by alternately applying plasma-enhanced chemical vapour deposition and pulsed laser deposition techniques. From the analysis of the experimental GISAXS patterns, the average radius and radius dispersion of PbTe nanocrystals were determined. With increasing deposition dose the size of the PbTe nanocrystals progressively increases while their number density decreases. Analysis of the GISAXS intensity profiles along the normal to the sample surface allowed the determination of the period parameter of the layers and a structure parameter that characterizes the disorder in the distances between PbTe layers.

Structure of PbTe(SiO2)/SiO2 multilayers deposited on Si(111)

The structure of thin films composed of a multilayer of PbTe nanocrystals embedded in SiO2, named as PbTe(SiO2), between homogeneous layers of amorphous SiO2 deposited on a single-crystal Si(111) substrate was studied by grazing-incidence small-angle X-ray scattering (GISAXS) as a function of PbTe content. PbTe(SiO2)/SiO2 multilayers were produced by alternately applying plasma-enhanced chemical vapour deposition and pulsed laser deposition techniques. From the analysis of the experimental GISAXS patterns, the average radius and radius dispersion of PbTe nanocrystals were determined. With increasing deposition dose the size of the PbTe nanocrystals progressively increases while their number density decreases. Analysis of the GISAXS intensity profiles along the normal to the sample surface allowed the determination of the period parameter of the layers and a structure parameter that characterizes the disorder in the distances between PbTe layers.

Formation of an extended CoSi2 thin nanohexagons array coherently buried in silicon single crystal

A Co‐doped silica film was deposited on the surface of a Si(100) wafer and isothermally annealed at 750 °C to form spherical Co nanoparticles embedded in the silica film and a few atomic layer thick CoSi2 nanoplatelets within the wafer. The structure, morphology, and spatial orientation of the nanoplatelets were characterized. The experimental results indicate that the nanoplatelets exhibit hexagonal shape and a uniform thickness. The CoSi2 nanostructures lattice is coherent with the Si lattice, and each of them is parallel to one of the four planes belonging to the {111} crystallographic form of the host lattice.

Investigation of indirect structural and chemical parameters of GeSi nanoparticles in a silica matrix by combined synchrotron radiation techniques

The formation of GeSi nanoparticles on an SiO2 matrix is studied here by synchrotron-based techniques. The shape, average diameter and size dispersion were obtained from grazing-incidence small-angle X-ray scattering data. X-ray diffraction measurements were used to obtain crystallite sizes and composition via resonant (anomalous) measurements. By using these techniques as input for extended X-ray absorption fine structure analysis, the local composition surrounding the Ge atoms is investigated. Although the results for each of the methods above are commonly analyzed separately, the combination of such techniques leads to an improved understanding of nanoparticle structural and chemical properties. Crucial indirect parameters that cannot be quantified by other means are accessed in this work, such as local strain, the possibility of forming core–shell structures, the fraction of Ge atoms diluted in the matrix (not forming nanoparticles), the amorphous and crystalline Ge fractions, and the relative population of nanoparticles with single and multiple crystalline domains.

Determination of the melting and freezing temperatures of Pb nanoparticles embedded in a PbO–B2O3–SnO2 glass by using only the SAXS method

Melting and freezing of metallic nanoparticles embedded in glass matrices usually occur at temperatures lower than for the same metal in the bulk state. In situ small-angle X-ray scattering (SAXS) measurements using a synchrotron beamline and a specially designed high-temperature chamber allowed the determination of the temperature dependence of the SAXS intensity produced by a dilute and nearly monodisperse set of spherical Pb nanoparticles, with an average radius 〈R〉 = 16.1 nm, embedded in a homogeneous lead–borate oxide glass. The temperature dependences of the nanoparticle volume V(T) and nanoparticle radius of gyration Rg(T) derived from SAXS results exhibit clear discontinuities during the cooling and during the heating processes, thus allowing for precise determinations of the melting and freezing temperatures of the studied Pb nanoparticles. Additional features observed in both V(T) and Rg(T) curves showed that during the heating cycle the frozen Pb nanoparticles suffer a transition to a more compact phase at 433 K before melting at 580 K. The results of this work demonstrate that the melting and freezing temperatures of nanoparticles in a very diluted state – for which the X-ray diffraction technique is not sensitive enough – can be precisely determined by applying only the SAXS method.

In situ study of the endotaxial growth of hexagonal CoSi2 nanoplatelets in Si(001)

This investigation aims at studying–by in situ grazing-incidence small-angle x-ray scattering–the process of growth of hexagonal CoSi2 nanoplatelets endotaxially buried in a Si(001) wafer. The early formation of spherical Co nanoparticles with bimodal size distribution in the deposited silica thin film during a pretreatment at 500 °C and their subsequent growth at 700 °C were also characterized. Isothermal annealing at 700 °C promotes a drastic reduction in the number of the smallest Co nanoparticles and a continuous decrease in their volume fraction in the silica thin film. At the same time, Co atoms diffuse across the SiO2/Si(001) interface into the silicon wafer, react with Si, and build up thin hexagonal CoSi2 nanoplatelets, all of them with their main surfaces parallel to Si{111} crystallographic planes. The observed progressive growths in thickness and lateral size of the hexagonal CoSi2 nanoplatelets occur at the expense of the dissolution of the small Co nanoparticles that are formed during the pr...

Melting and freezing temperatures of confined Bi nanoparticles over a wide size range

The size dependences of the melting and freezing temperatures, Tm and Tf, respectively, of spherical Bi nanoparticles embedded in a sodium borate glass were determined by applying a new experimental procedure based on the combined and simultaneous use of small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). This experimental procedure is particularly useful for materials in which a widely polydisperse set of nanoparticles are embedded. The results provide additional and stronger evidence supporting the main previous conclusions: (i) the melting and freezing temperatures both decrease linearly for increasing reciprocal radius (1/R); and (ii) the effect of undercooling is suppressed for Bi nanoparticles with radii smaller than a critical value equal to 1.8 nm. These results confirm a previously proposed low-resolution structural model for Bi nanocrystals below their melting temperature and with radius R > 1.8 nm, which consists of a crystalline core surrounded by a disordered shell. In the present work, a number of samples with different and partially overlapping radius distributions were studied, allowing the determination of Tm(R) and Tf(R) functions over a wide range of radii (1 < R < 11 nm). Comparison of the experimentally determined Tm(R) and Tf(R) functions corresponding to different samples indicates good reproducibility of the experimental results. This allowed the verification of the robustness of the experimental procedure based on in situ combined use of SAXS and WAXS for determination of the radius dependence of the melting and freezing temperatures of spherical nanoparticles in dilute solution.

X-Ray Diffraction and Scattering by Nanomaterials

This chapter contains basic concepts and applications of wide-angle X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) to studies of nanostructured materials. The first part includes an introduction to the technique of X-ray powder diffraction (XPD), which is commonly applied to phase identification of powdered materials, characterization of unit cells, determinations of lattice parameters and, in some cases, also coordinates of atoms inside the unit cells. The main aspect described here is the analysis of the shape of X-ray diffraction peak profiles, which allows one to determine additional and also valuable structural information of nanomaterials, such as average crystallite sizes and crystallite microstrains. Detailed deductions of the equations used for different applications of X-ray diffraction and basic concepts of crystallography (such as those related to symmetries, Bravais and reciprocal lattices, etc.) are not described. The second part presents the basic equations related to the SAXS method and their applications to several biological systems (proteins in solution). Classical SAXS is an experimental procedure that is employed in transmission mode and is also applied to the study of many nanostructured inorganic materials. The third part includes two variants of the classical SAXS procedure, namely grazing incidence small-angle X-ray scattering (GISAXS) and anomalous small-angle X-ray scattering (ASAXS). The basic concepts of the GISAXS method and several applications to studies of nanostructured materials deposited on flat substrates and buried nanostructures are presented. The basic concepts of the ASAXS method are described, together with its applications to complex materials that cannot be properly studied using the classical SAXS technique, such as, for example, materials modeled by three phases with different electron densities. Most of the experiments described in this chapter were performed by the authors using X-ray beam lines of the National Synchrotron Light Laboratory (LNLS), Campinas, Brazil.

Radius-dependent transition temperatures of spherical Bi nanoparticles

The radius dependences of the melting and freezing temperatures, Tm(R) and Tf(R), respectively, associated with sizepolydisperse and dilute sets of spherical Bi nanoparticles embedded in a sodium-borate glass, were previously determined by applying an experimental method based on combined and simultaneous use of small-angle X-ray scattering (SAXS) and wideangle X-ray scattering (WAXS) techniques [1]. This procedure combines size information from analyses of SAXS curves and determinations of temperature-dependent fractions of crystalline volume derived from integral values of Bragg peaks in WAXS patterns. For dilute sets of nanoparticles with approximately same size, an alternative and simpler procedure can be applied, which requires only SAXS measurements of several samples each of them containing nanoparticles with different average radius [2]. Previous determinations of the radius dependences of melting and freezing temperatures of Bi referred to nanoparticles with radii varying from 1 nm up to 4 nm circa [1]. The present study aims at obtaining additional and stronger evidences regarding the conclusions derived from previous work [1]. This purpose was expected to be achieved by determining the radius dependences of the melting and freezing temperatures of spherical Bi nanoparticles over a radius range much wider than in previous work. Thus, we have studied a series of samples containing dilute and size-polydisperse sets of spherical Bi nanoparticles embedded in a sodium-borate glass, the sets of nanoparticles in each sample having different average radius, and wide partially overlapping radius distributions. SAXS and WAXS measurements were conducted at the Brazilian National Synchrotron Light Laboratory (LNLS). The simultaneous SAXS and WAXS measurements were performed in situ, on sample heating and cooling cycles, using a specially designed high-temperature chamber [3] and two independent position sensitive Xray detectors. By combining the results derived from SAXS and WAXS measurements referring to seven different samples, we have determined the radius dependences of the melting and freezing temperatures of spherical Bi nanoparticles with radii ranging from 1 up to 10 nm, that is, over a radius range much wider than in previous work. The results of SAXS/WAXS studies of a series of samples covering a wide radius range provided additional and stronger evidences supporting previous main conclusions, namely (i) the temperatures of melting and freezing of spherical Bi nanoparticles both decrease linearly for increasing reciprocal radius (1/R), and (ii) the effect of undercooling is absent for Bi liquid nanodroplets with radius smaller than a critical value Rc=1.8nm. The observed agreements of Tm(R) and Tf(R) functions derived from measurements corresponding to different samples, with partially overlapping radius ranges, indicated a good reproducibility of the experimental results and, consequently, established the robustness of the method that combines information derived from SAXS and WAXS. This SAXS/WAXS procedure is particularly useful for studies of very dilute and size-polydisperse sets of nanoparticles for which the classical alternative method, based on size analysis of usually weak WAXS Bragg peak profiles, does not provide precise results.

DIFRAÇÃO E ESPALHAMENTO DE RAIOS X POR NANOMATERIAIS

Este capitulo descreve os metodos experimentais de difracao e de espalhamento de raios X e suas aplicacoes em estudos de materiais nanoestruturados. A primeira parte contem os aspectos basicos da difracao de raios X de po cristalino (XPD) e suas aplicacoes para determinacoes de parâmetros estruturais relevantes de nanopos cristalinos, tais como tamanho medio e microdeformacoes dos cristalitos. Na segunda parte sao apresentadas as equacoes basicas do espalhamento de raios X a baixo ângulo (SAXS), metodo comumente utilizado em pesquisas estruturais de baixa resolucao de nanoparticulas inorgânicas e macromoleculas biologicas, e sao descritos exemplos de estudos de proteinas em solucao. Finalmente, a terceira parte trata sobre a teoria basica e as aplicacoes de duas variantes da tecnica SAXS: SAXS com incidencia rasante (GISAXS), para pesquisas de nanoestruturas em filmes finos suportados ou em partes superficiais de materiais, e SAXS anomalo (ASAXS), aplicada em estudos de nanoestruturas de maior complexidade.