Vladimir Ezersky

New nanocrystalline materials: a previously unknown simple cubic phase in the SnS binary system.

We report a new phase in the binary SnS system, obtained as highly symmetric nanotetrahedra. Due to the nanoscale size and minute amounts of these particles in the synthesis yield, the structure was exclusively solved using electron diffraction methods. The atomic model of the new phase (a = 11.7 Å, P2(1)3) was deduced and found to be associated with the rocksalt-type structure. Kramers-Kronig analysis predicted different optical and electronic properties for the new phase, as compared to α-SnS.

Synthesis and properties of nanocrystalline π-SnS – a new cubic phase of tin sulphide

We report on the synthesis of the newly discovered cubic phase of tin sulfide π-SnS and compare its properties to the well-known phase of tin sulfide, α-SnS. Shape control was achieved by the variation of synthesis parameters, resulting in cubic, rhombic dodecahedral and tetrahedral shapes of the π-SnS nanoparticles. X-ray diffraction provided authentication of the proposed model and refined determination of the lattice parameter a = 11.595 A. Raman spectroscopy showed a substantial shift towards higher energies and peak splitting for π-SnS. Optical absorption spectroscopy indicated an indirect band gap of 1.53 eV, in good agreement with density functional theory (DFT) calculations indicating a band gap greater than that of α-SnS. DFT total energy calculations show that the π-SnS phase is energetically similar to α-SnS, and is significantly more stable than the hypothetical ideal rocksalt structure of SnS.

Nanoparticles and nanotubes induced by femtosecond lasers

In this paper, we suggest the creation of a nanoparticles and nanotubes by using the interaction of a femtosecond laser with a solid target in a vacuum. A simple model is used to predict the optimal target and the laser parameters for the production of efficient nanoparticles. At the Soreq laboratory, experiments are performed with aluminium and carbon targets using a femtosecond laser. The irradiated targets are composed of either a thin layer of aluminium or of carbon, deposited on a transparent heat-insulating glass substrate. The nanoparticle debris is collected on a silicone wafer for X-ray diffraction (XRD), for scanning electron microscopy (SEM), and for atomic force microscopy (AFM). For transmission electron microscopy (TEM), the debris is caught on a copper grid covered on one side with a carbon membrane. Our experiments confirm the creation of crystal nanoparticles for aluminium and nanotubes for carbon experiments.

A new nanocrystalline binary phase: synthesis and properties of cubic tin monoselenide

A new nanometric cubic binary phase of the tin mono-selenide system, π-SnSe, was obtained as cube shaped nanoparticles. Its structure and atomic positions were adopted from previously reported π-SnS (P213, a0 = 11.7 A). The proposed structure model of π-SnSe, with 64 atoms per unit cell, was refined against experimental X-ray diffraction using Rietveld method (a0 = 11.9702(9) A; Rp = 1.65 Rwp = 2.11). The optical properties of this new cubic SnSe phase were characterized by Raman and optical absorption spectroscopies. The optical band gap was assessed to be indirect, with Eg = 1.28 eV (in the near infrared), compared to Eg = 0.9 eV (indirect) and 1.3 eV (direct) for the conventional orthorhombic phase of α-SnSe. Raman spectroscopy indicated significant phonon restraining, which is likely to be beneficial for thermoelectric applications. Since the new cubic phase belongs to a class of non-centrosymmetric crystals, interesting and potentially useful properties may arise. Density functional theory calculations have been applied in order to validate phase stability and evaluate the energy bandgap. These results, together with the recently discovered cubic phase of π-SnS, confirm the existence of a new class of nanoscale materials in the tin chalcogenide system.

Identification of the structure of a new Al-U-Fe phase by electron microdiffraction technique

The particles of an unknown intermetallic phase with the approximate composition Al10Fe2U were observed in a ternary Al–Fe–U alloy. The structure of this phase was investigated in a transmission electron microscope using a microdiffraction technique based on analysis of the symmetry and relative positions of reflections in the zero-order and high-order Laue zones. The phase has an orthorhombic C-centered unit cell with lattice parameters a=8.900, b=10.190 and c=8.993 A; its crystal symmetry can be described by the Cmcm space group.

Transmission electron microscopy of epitaxial PbS nanocrystals on polydiacetylene Langmuir films

Highly oriented arrays of lead sulfide (PbS) nanocrystals have been prepared in ambient conditions by the exposure of crystalline polydiacetylene (PDA) Langmuir films to H2S gas at the air–PbCl2 solution interface. The nanocrystals were studied by transmission electron microscopy (TEM), electron diffraction (ED), dark field TEM imaging (DF), high resolution TEM (HRTEM) and fast Fourier transform (FFT) analysis of HRTEM images. Three distinct, coexisting orientations were observed with typical nanocrystal morphologies corresponding to each orientation. The direction was aligned parallel to the PDA linear direction for all orientations, indicating a template-directed growth for the PbS/PDA system.

High loading of short W(Mo)S2 slabs inside the nanotubes of SBA-15. Promotion with Ni(Co) and performance in hydrodesulfurization and hydrogenation.

Abstract Layered nanoslabs of a M0S2 and WS2 phases with a well-defined hexagonal crystalline structure were inserted into the nanotubular channels of SBA-15 at loadings up to 60 wt%. Sonication of a slurry containing SBA-15 in a W(Mo)(CO)6-sulfur-diphenylmethane solution yielded an amorphous W(Mo)S2 phase inside the mesopores that was transformed into hexagonal crystalline W(Mo)S2 nanoslabs by further sulfidation. The nanoslabs were distributed exclusively inside the mesopores in a uniform manner (HRTEM, local quantitative microanalysis), without blocking the pores (N2-sorption). The Ni(Co) promoters were introduced into the W(Mo)S2/SBA-15 composites by impregnation from an aqueous solution of nickel (cobalt) acetate. The activity (based on the volume of the catalyst loaded into reactor) of the optimized Ni-W-S/SBA-15 catalyst in hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of toluene was 1.4 and 7.3 times higher, respectively, than that of a sulfided commercial CO-MO/Al2O3. The HDS activity of Co-Mo- S/SBA-15 catalyst was 1.2 times higher than that of commercial catalyst. After promotion with Co, the directly introduced M0S2 slabs and M0S2 slabs prepared by sulfidation of Mo- oxide monolayer spread over SBA-15 displayed similar HDS performance.

Enhanced SWIR absorption in chemical bath deposited PbS thin films alloyed with thorium and oxygen

We report on chemically deposited thin films of PbS alloyed with thorium. Control over the thorium content in the films was achieved by lowering the solution pH and compensating by adding tri sodium citrate as a co-complexant. Homogeneous distribution of thorium was achieved, accompanied by substantial oxygen content, up to concentrations of 9 at% thorium and 20 at% oxygen. Regardless of these relatively high concentrations, a single phase of alloyed PbS was found in X-ray and electron diffraction, indicating complete solubility of the species within the lattice. Physical properties such as the optical band gap and transmission spectra showed a strong dependence on thorium content due to chemical variations and size dependent quantum confinement. This new system is a promising candidate for electro-optic applications due to ease of band-gap tuning and enhanced optical absorption in the short wave infrared (SWIR) range.

The stability of structures with icosahedral local order in Al-based alloys with transition metals

Abstract The paper considers the stability of the phases formed in alloys of aluminum with transition metals (TM) in terms of the theory of coordination compounds. The structural stability is related to the degree of distortion of the first-neighbor coordination icosahedra commonly found around TM atoms. Changes in the symmetry of coordination polyhedrons are explained in terms of the Jahn–Teller theorem. The stability of the structures may be correlated with the effective charge of the TM atom and with the electronegativity of its neighbors: the phase transforms to the structure with lower symmetry when the electronegativity or effective charge decrease.

Nanostructure Synthesis at the Solid–Water Interface: Spontaneous Assembly and Chemical Transformations of Tellurium Nanorods

Bottom-up synthesis offers novel routes to obtain nanostructures for nanotechnology applications. Most self-assembly processes are carried out in three dimensions (i.e. solutions); however, the large majority of nanostructure-based devices function in two dimensions (i.e. on surfaces). Accordingly, an essential and often cumbersome step in bottom-up applications involves harvesting and transferring the synthesized nanostructures from the solution onto target surfaces. We demonstrate a simple strategy for the synthesis and chemical transformation of tellurium nanorods, which is carried out directly at the solid-solution interface. The technique involves binding the nanorod precursors onto amine-functionalized surfaces, followed by in situ crystallization/oxidation. We show that the surface-anchored tellurium nanorods can be further transformed in situ into Ag2Te, Cu2Te, and SERS-active Au-Te nanorods. This new approach offers a way to construct functional nanostructures directly on surfaces.