The structure and growth mechanism of Si nanoneedles prepared by plasma-enhanced chemical vapor deposition.

Silicon nanowires and nanoneedles show promise for many device applications in nanoelectronics and nanophotonics, but the remaining challenge is to grow them at low temperatures on low-cost materials. Here we present plasma-enhanced chemical vapor deposition of crystalline/amorphous Si nanoneedles on glass at temperatures as low as 250 °C. High resolution electron microscopy and micro-Raman spectroscopy have been used to study the crystal structure and the growth mechanism of individual Si nanoneedles. The H(2) dilution of the SiH(4) plasma working gas has caused the formation of extremely sharp nanoneedle tips that in some cases do not contain a catalytic particle at the end.

Creating nanocrystals in amorphous silicon using a conductive tip.

Field-enhanced metal-induced solid phase crystallization (FE-MISPC) of amorphous silicon is scaled down to nanoscale dimensions by using a sharp conductive tip in atomic force microscopy (AFM) as one of the electrodes. The room temperature process is driven by the electrical current of the order of 100 pA between the tip and the bottom nickel electrode. This results in energy transfer rates of 30-50 nJ s(-1). Amplitude of the current is limited by a MOSFET transistor to avoid electrical discharge from parasitic parallel capacitance. Limiting the current amplitude and control of the transferred energy (approximately 100 nJ) enables formation of silicon crystals with dimensions smaller than 100 nm in the amorphous film. Formation of the nanocrystals is localized by the AFM tip position. The presence of nanocrystals is detected by current-sensing AFM and independently corroborated by micro-Raman spectroscopy. The nanocrystal formation is discussed based on a model considering microscopic electrical contact, thermodynamics of crystallization and silicide formation.

Extraction current transients: new method of study of charge transport in microcrystalline silicon

The transport properties of microcrystalline silicon, namely, mobility and conductivity, are investigated by a new method, for which the simple theory as well as numerical modeling is presented. The basic idea of the new method is verified on amorphous hydrogenated silicon by comparison with the widely used time-of-flight method. Contrary to time of flight, the new method can be used even for relatively conductive materials. Preliminary results on microcrystalline silicon clearly indicate the critical role of amorphouslike tissue in transport in microcrystalline silicon.

Mycoplasma lactucae sp. nov., a sterol-requiring mollicute from a plant surface.

Strain 831-C4T (T = type strain), isolated from the surface of lettuce plants (Lactuca sativa) obtained from a retail food market, was shown to be a sterol-requiring mollicute. Morphological examination of this organism by electron and dark-field microscopic techniques showed that it consists of small, nonhelical, nonmotile, pleomorphic coccoid cells, with individual cells surrounded by a single cytoplasmic membrane. No evidence of a cell wall was observed. The organism grew rapidly in all conventional culture medium formulations for mollicutes in either aerobic or anaerobic environments. The optimum temperature for growth was 30 degrees C, but multiplication occurred at 18 to 37 degrees C. Strain 831-C4T catabolized glucose, but hydrolysis of arginine or urea could not be demonstrated. The genome size of strain 831-C4T was determined to be about 569 megadaltons, while the base composition (guanine-plus-cytosine content) of the DNA was 30.0 mol%. Recent studies in which we compared the 16S rRNA sequences of strain 831-C4T with those of more than 40 other mollicutes indicated that this organism is phylogenetically related to the Spiroplasma-Mycoplasma mycoides clade. Strain 831-C4T was serologically unrelated to the type strains of previously described Mycoplasma species and to 18 other unclassified sterol-requiring isolates cultivated from various animal, plant, or insect sources. Strain 831-C4T (= ATCC 49193) is the type strain of Mycoplasma lactucae sp. nov.

Induction by streptococcal L-forms of two reproductive isolating mechanisms in Drosophila paulistorum.

The six semispecies of the Drosophila paulistorum complex do not normally interbreed, but when hybrids are produced in the laboratory, the males are sterile. In earlier work, cytoplasmic components responsible for this sterility had been transferred by injection of streptococcal L-forms isolated from the Mesitas and Santa Marta flies. To extend these findings, and L-form from the Centro-American flies was injected into three semispecies. Transfer of L-form from one semispecies to another resulted in sterile male progeny. The L-form isolated from Mesitas was injected into all six semispecies. Results simulated "hybrid inviability," i.e., only the Mesitas semispecies produced the expected number of offspring, and the total number of progeny from the other five semispecies was considerably reduced. Despite the small numbers of males, reflecting induced inviability, there was evidence that the transfer of L-forms from one semispecies to another simultaneously caused sterility in male progeny.

Streptococcal L-forms isolated from Drosophila paulistorum semispecies cause sterility in male progeny.

The Drosophila paulistorum complex contains six semispecies that do not normally interbreed. In the laboratory, crosses between semispecies produce fertile daughters and sterile sons. Microbial endosymbionts have been observed in all D. paulistorum flies that display this male sterility. Streptococcal L-forms have been isolated from the Andean-Brazilian (Mesitas) and Transitional (Santa Marta) semispecies and cultured in artificial medium. Transfer of these L-forms from their native hosts into reciprocal semispecies resulted in sterile male progeny. When L-forms were inoculated into the semispecies from which they had been isolated, most of the male progeny were fertile. Control streptococcal L-forms did not show this sterility pattern.

Isolation of acholeplasmas and a mycoplasma from vegetables.

The isolation of Mollicutes from food has not been reported. To isolate Mollicutes in the presence of high levels of unwanted bacteria, we first incubated fresh vegetables in liquid culture media containing lysozyme, ampicillin, and thallous acetate. Culture fluids were than separated from the vegetable samples, subjected to one freeze-thaw cycle, and passed through a filter of 0.4-micron porosity. Filtered samples were cultured in SP4 medium and in a conventional medium containing horse serum. With this procedure 21 acholeplasma isolations representing three species were obtained from endive, broccoli, and kale. Of 35 food samples tested, 11 were positive for acholeplasmas; acholeplasmas isolated from 6 of these samples were recovered only in SP4 medium. In seven single vegetable samples, two or more Acholeplasma spp. were isolated. A. laidlawii was isolated from all three vegetables and A. axanthum was found in broccoli and kale. Four isolates were serologically identified as A. oculi. Mycoplasma verecundum was the only Mycoplasma species recovered. Several isolates could not be typed serologically, as they reacted with antisera to both A. morum and A. hippikon. these isolates may include new Acholeplasma spp.

Characteristics of the Drosophila paulistorum Male Sterility Agent in a Secondary Host, Ephestia kuehniella.

Crosses among the six semispecies of Drosophila paulistorum produce sterile male hybrids. This sterility is caused by an agent which has characteristics of a microorganism. It is pathogenic in a secondary host, the larvae of the Mediterranean meal moth, Ephestia kuehniella, and can be serially passaged in Ephestia, where it is lethal. The agent was passaged back into D. paulistorum, where it induced sterility in males of a semispecies different from that of origin of the agent. Infectious particles were obtained from an extract of infected Ephestia by ultracentrifugation in a sucrose-Ficoll-metrizamide gradient. Both crude and purified extracts were lyophilized and stored indefinitely without loss of killing power. The agent was destroyed by low pH, lipid solvents, ultraviolet light, and exposure to a temperature of 56 degrees C for 30 min. It appeared to be sensitive to tetracycline and insensitive to penicillin, suggesting that the agent is not a virus, but more likely a cell wall-deficient bacterium or mycoplasma-like organism.