Gradient Structures of Nanoparticles Prepared on Chemical Template
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nanotube field-effect transistors (FETs). Their main approach involved converting p-type FETs in an array into n-type FETs by a local electrical manipulation under a vacuum. As reported in the September issue of Nano Letters, Ali Javey, Hongjie Dai, and co-workers performed the fabrication of the p-type transistor device depositing single-wall nanotubes (SWNTs) between metal electrodes formed on the photolithographically patterned SiO2 layer. Device fabrication involves the synthesis of SWNT arrays by chemical vapor deposition of methane on substrates pre-patterned with catalyst and bottom-W-gate arrays. Thus-obtained 4 × 4 mm2 chips contain about 100 devices. The yield of the acceptable SWNT FETs was found to be in the range of 20–30%. The researchers achieved a p-type FET conversion into n-type by applying a high local gate voltage (-40 V) combined with a large source-drain (20 V) bias for a certain duration under vacuum (10-8 Torr). The yield of this technique results in ~50% p–n conversion. The researchers suggested a mechanism for this conversion based on reversible desorption of molecular oxygen from SWNTs, leading to significant n-channel conduction. The ability of local manipulation of SWNT FETs into n-type has enabled the researchers to obtain multiple n-FETs and p-FETs on a SWNT chip and construct complementary logic devices and ring oscillators. The researchers concluded that simple computing operations are possible by using the high percentage of semiconducting SWNTs and the ability for local gating, manipulation, and doping of individual SWNT FETs. ANDREI A. ELISEEV
AFM Enables Study of Biomolecules’ Interaction with Minerals Treavor Kendall, a graduate student in the mineral-microbe group in the Department of Geological Sciences at the Virginia Polytechnic Institute and State University, has demonstrated the use of an atomic force microscope (AFM) to study how biomolecules extract minerals. At the 12th Annual V.M. Goldschmidt Conference, an international geochemistry conference held August 18–23 in Davos, Switzerland, Kendall described his experiments on how the bacteria Azotobacter vinelandii acquire iron. Kendall said that the bacteria release the organic molecules siderophores, which have an affinity for iron. While studies have shown how siderophores
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interact with iron in water, Kendall’s research explored how they acquire iron that is embedded in a mineral structure. “This is important,” said Kendall, “because minerals are a primary source of iron in the environment.” He specifically looked at the affinity between azotobactin and the mineral goethite, an important iron oxide in soils worldwide, he said. The research team covalently attached the molecule to the AFM tip using a common protein-coupling technique. The activated tip was then used to probe various minerals including goethite and diaspore, goethite’s isostructural aluminum equivalent. The sensitivity of the AFM allowed the forces of interaction associated with the azotobactin and each mineral surface to be measured. A two- to thr
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