1,721,288 research outputs found
Semiconductors
No full textThis chapter first provides an overview of experimental aspects of x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) most relevant to semiconductor research and some applications of XAS and XES to semiconductor science. XAS plays an important role in elucidating, the real local structure of dopants in semiconductors in fact, an XAS investigation of As dopants in hydrogenated amorphous silicon is one of the first studies to illustrate the advantages of the technique. The chapter then illustrates that these techniques are powerful tools to probe the local environment of dopants and dilute atoms, thin films and nanostructures and to relate the local and macroscopic physical properties. Finally, it presents the evolution from analysis approaches based on the use of empirical standards to the use of ab initio simulations, including those based on density functional theory and the recent emergence of XES as a complementary tool to XAS
Structure of nitrogen – hydrogen complexes from x-ray and synchrotron radiation techniques
No full textThis Chapter illustrates the role played by x-ray techniques, in particular synchrotron radiation-based ones, in the analysis of the structure of nitrogen-hydrogen complexes in dilute nitrides. Para-graph 6.2 introduces the x-ray diffraction experiments which gave the first evidence of a modification of the structure driven by hy-drogenation. Paragraph 6.3 provides a theoretical and experimen-tal framework for understanding the main results presented in this chapter, which were obtained by x-ray spectroscopy; the paragraph explains the physics at the basis of x-ray absorption spectroscopy, with specific reference to the application to the subject of impurities in semiconductors, diffraction anomalous fine structure, and x-ray emission spectroscopy. The possibility of achieving both unidimensional and threedimensional information on the local struc-
ture around the isoelectronic impurities and the structure of the complexes playing on different analysis strategy is explained; the importance of coupling spectroscopy data analysis with state-of-art theoretical calculations described in Chapter 4 is stressed. Para-graph 6.4 shows the experimental results and data analysis of different x-ray spectroscopy experiments performed on hydrogenated dilute nitrides, focusing on the local structural information ob-tained and placing it in its historical context with respect to the results of different characterization techniques reported at Chapters 5 and 7. The complementarity between different technical approaches is emphasized. Paragraph 6.5 draws the main conclusions of the chapter and future perspectives
Local structure in dilute nitrides probed by X-ray absorption spectroscopy
No full textWe describe the use of X-ray absorption spectroscopy (XAS) with synchrotron radiation to study the local structure in dilute nitrides. After a brief description of the advantages of XAS to probe local atomic arrangements in semiconductor alloys and nanostructures we focus our attention on (InGa)(AsN). We discuss data which demonstrate that atomic ordering (in the form of an excess of In-N over Ga-N bonds) is present, but is significantly weaker than predicted; also we show that the experimental values for the bond lengths are in agreement with recent models which take into account strain due to pseudomorphic growth
Applications of XAFS to nanostructures and materials science
No full textI firstly will illustrate the role and the characteristics of XAFS as a probe of the local structure in materials and nano science. I will then provide a brief review of the use of the method in these fields, basing the discussion both on results which have stood the test of time and recent papers. Specifically, I will discuss dopants and defect complexes in semiconductors, bulk and heterostructure semiconductor alloys, phase transitions, highly correlated oxides, thin films and interfaces, semiconductor quantum dots and metallic clusters
Local environment of metal ions in phthalocyanines: K-edge X-ray absorption spectra
No full textWe report a detailed study of the K-edge X-ray absorption spectra of four transition metal phthalocyanines (MPc, M = Fe, Co, Cu and Zn). We identify the important single and multiple scattering contributions to the spectra in the extended energy range and provide a robust treatment of thermal damping; thus, a generally applicable model for the interpretation of X-ray absorption fine structure spectra is proposed. Consistent variations of bond lengths and Debye Waller factors are found as a function of atomic number of the metal ion, indicating a variation of the metal-ligand bond strength which correlates with the spatial arrangement and occupation of molecular orbitals. We also provide an interpretation of the near edge spectral features in the framework of a full potential real space multiple scattering approach and provide a connection to the local electronic structure
The ancient glass production of the Medieval Val Gargassa glasshouse: Fe and Mn XANES study.
No full textGlass finds from the Medieval Val Gargassa glasshouse (Genova,Italy) were investigated by chemical and spectroscopic techniques (colorimetric analysis and X-ray absorption spectroscopy) with the aim to obtain information on the ancient glass production technique. Sampling covers all the typologies of archaeological findings unearthed by the digging,as well as fragments of finished glass-wares. The results indicated that the samples are soda-lime based glasses. The color variability was exerted by the glassmakers by both controlling the kiln atmosphere and/or adding MnO2 as decoloring agent. In both cases,the Fe2+/Fe3+ ratio was modified at a specific step of the production cycle
Synchrotron Radiation and Chemistry: Studies of Materials for Renewable Energy Sources
No full textWe present an overview of selected applications of synchrotron radiation methods to topical chemical research. The analysis is limited to the studies on materials for renewable energy sources, focussing on topics peculiar to chemical research, such as reactivity and synthesis routes; in particular, the paper takes into account subjects having some relevance for the production and storage of energy based on hydrogen. Hydrogen production and storage are taken into account in the sections concerning: (i) Dye-sensitized solar cells, (ii) Metal-organic frameworks and (iii) Hydrides for hydrogen storage; production of energy by fuel cell devices is treated in (iv) Oxide ion and proton conductors and in (v) Electrodes for fuel cells. These arguments allowed to give a coherent outline of the involvement of synchrotron radiation techniques in traditional branches of chemistry such as inorganic and organic chemistry, catalysis and electrochemistry
Electronic properties of epitaxial cerium oxide films during controlled reduction and oxidation studied by resonant inelastic X-ray scattering
No full textWe investigated the evolution of the electronic structure of cerium oxide ultrathin epitaxial films during reduction and oxidation processes using resonant inelastic X-ray scattering at the Ce L3 absorption edge, a technique sensitive to the electronic configurations at the 4f levels and in the 5d band thanks to its high energy resolution. We used thermal treatments in high vacuum and in oxygen partial pressure to induce a controlled and reversible degree of reduction in cerium oxide ultrathin epitaxial films of different thicknesses. Two dominant spectral components contribute to the measured spectra at the different degrees of oxidation/reduction. In ultrathin films a modification of the electronic properties associated with platinum substrate proximity and with dimensionality is identified. The different electronic properties induce a higher reducibility in ultrathin films, ascribed to a decrease of the surface oxygen vacancy formation energy
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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