Ronit Popovitz-Biro

Structural Phase Control in Self-Catalyzed Growth of GaAs Nanowires on Silicon (111)

Au free GaAs nanowires with zinc blende structure, free of twin planes and with remarkable aspect ratios, have been grown on (111) Si substrates by molecular beam epitaxy. Nanowires with diameters down to 20 nm are obtained using a thin native oxide layer on the Si substrates. We discuss how the structural phase distribution along the wire length is controlled by the effective V/III ratio and temperature at the growth interface and explain how to obtain a pure twin plane free zinc blende structure.

Ice Nucleation by Alcohols Arranged in Monolayers at the Surface of Water Drops

Monolayers of aliphatic long-chain alcohols induced nucleation of ice at temperatures approaching 0�C, in contrast with water-soluble alcohols, which are effective antifreeze agents. The corresponding fatty acids, or alcohols with bulky hydrophobic groups, induce freezing at temperatures as much as 12�C lower. The freezing point induced by the amphiphilic alcohols was sensitive not only to surface area per molecule but, for the aliphatic series (CnH2n + 1OH), to chain length and parity. The freezing point for chains with n odd reached an asymptotic temperature of 0�C for an upper value of n = 31; for n even the freezing point reached a plateau of -8�C for n in the upper range of 22 to 30. The higher freezing point induced by the aliphatic alcohols is due to formation of ordered clusters in the uncompressed state as detected by grazing incidence synchrotron x-ray diffraction measurements. The diffraction data indicate a close lattice match with the ab layer of hexagonal ice.

Guided growth of millimeter-long horizontal nanowires with controlled orientations.

Long, horizontal gallium nitride nanowires are controllably grown on different faces of a sapphire substrate. The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. We report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.

Method for suppression of stacking faults in Wurtzite III-V nanowires.

The growth of wurtzite GaAs and InAs nanowires with diameters of a few tens of nanometers with negligible intermixing of zinc blende stacking is reported. The suppression of the number of stacking faults was obtained by a procedure within the vapor-liquid-solid growth, which exploits the theoretical result that nanowires of small diameter ( approximately 10 nm) adopt purely wurtzite structure and are observed to thicken (via lateral growth) once the axial growth exceeds a certain length.

Friction mechanism of individual multilayered nanoparticles

Inorganic nanoparticles of layered [two-dimensional (2D)] compounds with hollow polyhedral structure, known as fullerene-like nanoparticles (IF), were found to have excellent lubricating properties. This behavior can be explained by superposition of three main mechanisms: rolling, sliding, and exfoliation-material transfer (third body). In order to elucidate the tribological mechanism of individual nanoparticles in different regimes, in situ axial nanocompression and shearing forces were applied to individual nanoparticles using a high resolution scanning electron microscope. Gold nanoparticles deposited onto the IF nanoparticles surface served as markers, delineating the motion of individual IF nanoparticle. It can be concluded from these experiments that rolling is an important lubrication mechanism for IF-WS2 in the relatively low range of normal stress (0.96±0.38 GPa). Sliding is shown to be relevant under slightly higher normal stress, where the spacing between the two mating surfaces does not permit free rolling of the nanoparticles. Exfoliation of the IF nanoparticles becomes the dominant mechanism at the high end of normal stress; above 1.2 GPa and (slow) shear; i.e., boundary lubrication conditions. It is argued that the modus operandi of the nanoparticles depends on their degree of crystallinity (defects); sizes; shape, and their mechanical characteristics. This study suggests that the rolling mechanism, which leads to low friction and wear, could be attained by improving the sphericity of the IF nanoparticle, the dispersion (deagglomeration) of the nanoparticles, and the smoothness of the mating surfaces.

New reactor for production of tungsten disulfide hollow onion-like (inorganic fullerene-like) nanoparticles

Abstract MS2 (M=Mo, W) hollow onion-like nanoparticles were the first inorganic fullerene-like (IF) materials, found in 1992. Understanding of the IF-MS 2 growth mechanism in 1996 enabled us to build a rather simple reactor, which produced about 0.4 g per batch, of an almost pure IF-WS2 powder. Soon after, it was found that the new powder showed better tribological properties compared with the regular MS2 (M=Mo, W) powder, which is a well-known solid lubricant. The present work shows a new synthetic approach, which allows for a scale-up of IF-WS2 production by more than two orders of magnitude. The falling-bed and, especially, fluidized-bed methods, which are presented here, pave the way for an almost ideal growth condition of the IF synthesis from an oxide precursor. As a result, the presently produced IF has a more uniform (spherical) shape and can grow to a larger size (up to 0.5 μm). It is expected that the relatively spherical IF-WS2 nanoparticles, which are produced by the falling (fluidized) bed reactor, will exhibit superior tribological properties, than reported before.

Stacking-Faults-Free Zinc Blende GaAs Nanowires

Stacking-faults-free zinc blende GaAs nanowires have been grown by molecular beam epitaxy using the vapor-liquid-solid gold assisted growth method. Two different approaches were used to obtain continuous low supersaturation in the vicinity of the growing wires. A double distribution of gold droplets on the (111)B surface in the first case, and a highly terraced (311)B growth surface in the second case both avoided the commonly observed transition to wurtzite structure.

Superior tribological properties of powder materials with solid lubricant nanoparticles

Abstract Friction and wear of powder materials impregnated with commercially available layered (platelets) WS 2 (2H) and inorganic fullerene-like WS 2 nanoparticles (IF) were studied. Bronze–graphite, iron–graphite and iron–nickel–graphite samples were used in this experiment. The linear wear of powder materials (in situ) was measured. It was shown that the IF nanoparticles impregnated into the pores improve the tribological properties of powder materials in comparison to a reference sample or the sample impregnated with 2H solid lubricant particles. The mechanisms of friction and wear of the IF nanoparticles have been considered. The tribological role of the wear particles and nanoparticles of solid lubricants has been analyzed in the framework of a third body lubrication model. The state of the IF nanoparticles before and after the wear test was studied. It was found that the shape of the IF nanoparticles is preserved during the friction tests under high loads. Thin wear debris surrounded by spherical IF nanoparticles appear to be formed and provide easily sheared lubrication film (low friction coefficient) during friction experiments of powder materials containing IF nanoparticles.

Synthesis of Copious Amounts of SnS2 and SnS2/SnS Nanotubes with Ordered Superstructures

Nanoparticles of layered (two-dimensional, 2D) compounds are known to be unstable in the planar form, forming instead closed polyhedral inorganic fullerene-like (IF) nanoparticles and also inorganic nanotubes (INT). Their formation was attributed to the annihilation of the dangling bonds of the rim atoms. IF nanoparticles either of SnS2 or having a SnS2–SnS ordered superstructure and short tubules thereof were reported in the past. Very recently, SnS2 INTs were synthesized by the vapor–liquid–solid (VLS) technique utilizing bismuth as a liquid catalyst. INTs of misfit-layered compounds have been known for a long time. In analogy to chrysotile (asbestos) nanotubes, the driving force for the formation of nanotubes of misfit compounds stems from the asymmetry along the c axis of the unit cell. The phase diagram of the Sn–S system shows three main compounds: tin monosulfide SnS, Sn2S3, and tin disulfide SnS2. a-SnS (herzenbergite) has a GeS structure with an orthorhombic (pseudo-tetragonal highly distorted NaCl) unit cell (a = 1.118, b = 0.398, c = 0.432 nm; Pnma). a-SnS2 has a CdI2 layered (trigonal-1T) structure with a pseudo-hexagonal unit cell (a = 0.36486 and c = 0.58992 nm). 9] The Sn–S system can be regarded as a misfit-layered compound and the tubular morphology 5] is a result of the lattice mismatch between the two alternating layers of SnS2 and SnS sublattices, which leads to intrinsic stress in the SnS2/SnS superstructure sheets. This driving force comes in addition to the already established closure mechanism, that is, annihilation of dangling bonds at the periphery of the layers of the INT nanostructures. The combination of these driving forces leads to the formation of nanoscrolls and nanotube morphologies as shown in Figure 1. The Sn–S system was used in the past to demonstrate a solar battery, that is, a solar cell in parallel with a Sn–S electrode which delivers constant power day and night.

Synthesis of fullerene-like MoS2nanoparticles and their tribological behavior

Further understanding of the growth mechanism and the detailed structure of fullerene-like MoS2 (IF-MoS2) nanoparticles was achieved by using a new kind of reactor. The annealed nanoparticles consist of >30 closed layers and their average diameter is 50–80 nm although a small (<5%) fraction of larger IF nanoparticles was discernible. The majority of the nanoparticles are found to have an oval (pitta-bread or flying-saucer) shape rather than being quasi-spherical. The (002) peak of the powder diffraction pattern reveals only a small (0.3%) shift to lower angles as compared to the bulk (2H) phase. This observation suggests that the structure of the nanoparticles produced in the present reactor is more relaxed as compared to the previously synthesized IF-MoS2 powder, which exhibited up to 2% shift. The present reactor also permitted scaling up of the production of the IF-MoS2 to more than 0.6 g/batch. Impregnation of such nanoparticles in metallic coatings is shown to endow these surfaces with excellent tribological behavior, which suggests numerous applications.