Yezdi B. Pithawalla

Synthesis and characterization of nanocrystalline iron aluminide particles

Abstract In this work, we report the synthesis of intermetallic FeAl nanoparticles using the laser vaporization controlled condensation technique. Nanoparticles of iron aluminides are expected to enhance the room temperature ductility and the high temperature strength of these materials. The nanoparticles have an average particle diameter between 6 and 9 nm. Measurements of the d-spacing from X-ray (XRD) and electron diffraction studies confirm that the nanoparticles have the same crystal structure (B2) as the bulk FeAl. High-resolution TEM reveals that the nanoparticles consist of a crystalline core encased within a ≈1 nm amorphous layer formed upon the exposure of the particles to air. XPS results indicate that the naoparticles have an Al-rich surface composition, which supports the assumption that the surface coating is Al oxide. The FeAl oxide nanoparticles prepared in the presence of O 2 have the hercynite FeAl 2 O 4 composition and exhibit a different XRD pattern from that of the surface-oxidized FeAl nanoparticles. The current results demonstrate that, by controlling the experimental conditions, it is possible to prepare a variety of intermetallic nanoparticles with selected size, morphology and composition. These particles can serve as the building blocks for advanced, high performance materials for several industrial and technological applications.

Evidence for changes in the electronic and photoluminescence properties of surface-oxidized silicon nanocrystals induced by shrinking the size of the silicon core

Abstract Web-like aggregates of Si nanocrystals produced by laser vaporization–controlled condensation technique are allowed to oxidize slowly in air and the photoluminescence (PL) is measured. A significant shift in the PL red band from 1.83 to 1.94 eV is observed. The bonding structure is established by correlating the PL data with the photon-yield electronic structure measurements using soft-X-ray fluorescence (SXF) and photon-yield near-edge X-ray absorption fine structure (NEXAFS) techniques. The results indicate that as the nanoparticles oxidize, the radius of the crystalline core decreases, which gives rise to a larger bandgap and consequently to the observed blue shift in the PL band.

Effect of atmospheric oxidation on the electronic and photoluminescence properties of silicon nanocrystal

Web-like aggregates of coalesced Si nanocrystals produced by a laser vaporization-controlled condensation technique show luminescence properties that are similar to those of porous Si. The results are consistent with a quantum confinement mechanism as the source of the red photoluminescence (PL) in this system. The oxidized Si nanoparticles do not exhibit the red PL that is characteristic of the surface-oxidized Si nanocrystals. The nanoparticles are allowed to oxidize slowly, and the PL is measured as a function of the exposure time in air. A significant blue shift in the red PL peak is observed as a result of the slow oxidation process. The dependence of quantum size effects on the bonding structure is established by correlating the PL data with the photon-yield electronic structure measurements made at the Advanced Light Source. The results indicate that as the nanoparticles oxidize, the radius of the crystalline core decreases in size, which gives rise to a larger bandgap and consequently to the observed blue-shift in the PL band. The correlation between the PL, SXF, and NEXAFS results provides further support for the quantum confinement mechanism as the origin of the visible PL in Si nanocrystals.

Morphology, photoluminescence and electronic structure in oxidized silicon nanoclusters

Abstract The dependence of quantum size effects on bonding structure in oxidized silicon nanoclusters is established by correlating photoluminescence data with photon-yield electronic structure measurements at the advanced light source. The nanoclusters were synthesized using a laser ablation technique that utilizes a convective He environment to control the size of the particles. After removal from the growth chamber, our ex situ photoluminescence (PL) results indicate that, as the nanoclusters oxidize, the main PL peak moves from 1.83 to 1.94 eV in energy. The central focus of the present work is to establish the origin of the main PL peak, and to determine why its energy shifts as the nanoclusters are allowed to oxidize slowly in air. The changes in the morphology and bonding structure of the clusters was established using soft-X-ray fluorescence spectroscopy (SXF) and photon-yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, which probe the element-specific density of occupied (SXF) and unoccupied (NEXAFS) electronic structure. Our conclusion is that the as-synthesized nanoclusters consist of a pure, crystalline Si core within a nearly pure SiO2 shell, with little or no sub-oxides present. As the nanoclusters oxidize, the radius of the crystalline core decreases in size, which gives rise to the change in the position of the PL signal.

Comparative study of the binary clusters of acetic acid–water and acetic acid–benzene using electron impact and multiphoton ionization techniques

Abstract Electron impact and multiphoton ionization techniques are used for a comparative study of the acetic acid–water and acetic acid–benzene clusters generated by supersonic beam expansion. In acetic acid–water clusters, hydrogen-bonding interaction is the driving force in determining the structures of the clusters. The protonated and the methyl cation containing clusters are characterized by 6-membered cyclic and 8-membered bicyclic structures. The similar magic number patterns observed for the protonated and methyl cation containing clusters suggest that the cyclic structures are stabilized by the charge interaction. A remarkable periodicity in the ion intensity of benzene (acetic acid) n clusters is observed. The clusters containing an even number of acetic acid molecules exhibit enhanced ion intensities. This effect is attributed to the formation of multiple cyclic dimers as a result of clustering from a dimer-rich vapor phase. Protonated acetic acid clusters are generated following the three-photon absorption by the binary clusters, which leads to dissociative charge transfer followed by proton transfer within the ionized acetic acid clusters. Evidence is presented for the special stability of the B 2 A 8 ion, which is proposed to consist of a benzene dimer cation entrapped between two acetic acid tetramers.