Showing papers by "Shlomo Efrima published in 2007"

Raman Spectroscopy of Ultranarrow CdS Nanostructures

Abstract: CdS semiconductor nanoparticles, with dimensions above and below the Bohr radius (∼2.5 nm) of bulk material, were prepared in a single-step benchtop procedure. The degree of quantum confinement in these nanoparticles was determined from their optical absorption and photoluminescence spectra. The size-dependent properties of the nanocrystals were studied by resonance Raman spectroscopy as a function of excitation wavelength and temperature. The spectra were composed of the fundamental longitudinal optical (LO) mode around 300 cm-1, along with the first and second overtones. The shapes and positions of the Raman peaks exhibited only a weak dependence on particle size even for the two extreme cases of bulk and nanostructures. We show that the ratio of the overtone to the fundamental LO frequency was sensitive to the particle diameter and decreased upon reduction of the particle diameter to values below the Bohr radius. Temperature-dependent Raman measurements of ultranarrow nanorods showed a small red shift ...

Synthesis, two-dimensional assembly, and surface pressure-induced coalescence of ultranarrow PbS nanowires.

Abstract: Ultranarrow (1.8 nm) PbS nanowires are synthesized in a single step, under benchtop conditions at relatively low temperature (90 °C). The nanowires exhibit a nearly perfect crystal lattice, high width uniformity, and tight side-by-side registry. Two-dimensional (2D) assembly over large areas (>15 μm2) is achieved using the Langmuir Blodgett method. The wire width can be readily controlled in the range 1.8−10 nm by a surface pressure-induced coalescence reaction, as monitored by transmission electron microscopy and Raman spectroscopy. The fluorescence of the 2D assembly shows strong polarization dependence along the long axis of the wires, making the system potentially suitable for orientation-sensitive devices.

Synthesis, assembly, and optical properties of shape-and phase-controlled ZnSe nanostructures

Abstract: Shape-, size-, and phase-controlled ZnSe nanostructures were synthesized by thermolysis of zinc acetate and selenourea using liganding solvents of octadecylamine (ODA) and trioctylphosphineoxide (TOPO) at different molar ratios. Materials synthesized in pure ODA resulted in uniform ultranarrow nanorods and nanowires of 1.3 nm in diameter. Morphological change from nanowire to spherical particle of larger diameter occurs with increasing TOPO/ODA ratio. Variation of the TOPO content in the mixed solvent also allows control of the crystallographic phase of ZnSe (wurtzite or zinc blende). The conditions and mechanisms of shape and phase control are discussed. Ultra-high-density networks of the ordered wires are achieved using the Langmuir-Blodgett technique in a single step as an essential stage on the route to ultra-high-density semiconductor nanocircuit fabrication.