Heavily doped semiconductor nanocrystal quantum dots

Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices.     

DNA-templated carbon nanotube field-effect transistor

The combination of their electronic properties and dimensions makes carbon nanotubes ideal building blocks for molecular electronics. However, the advancement of carbon nanotube-based electronics requires assembly strategies that allow their precise localization and interconnection. Using a scheme based on recognition between molecular building blocks, we report the realization of a self-assembled carbon nanotube field-effect transistor operating at room temperature. A DNA scaffold molecule provides the address for precise localization of a semiconducting single-wall carbon nanotube as well as the template for the extended metallic wires contacting it.     

Methodology of generation and characteristics of simple sequence repeat DNA markers in avocado (Persea americana M.)

Summary Generation of Simple Sequence Repeat (SSR) DNA markers was based on the construction of genomic DNA library of avocado (Persea americana M.). The library was screened with the four dinucleotide probes (AG), (AT), (GC) and (CA). Positive clones were sequenced to validate the presence of simple sequence repeats (SSR) and to generate polymerase chain reaction (PCR) primers based on the sequences flanking the simple sequence repeat. Twenty six different pairs of primers which yield a PCR product in the initial screening were synthesized. The SSR A1E11 was found to have eleven alleles while A3F8 has eight alleles. The SSRs in avocado were found to be inherited in a Mendelian fashion.Contribution from the Agricultural Research Organization, The Volcani Center, Bet-Dagan, Israel. No. 1335-E, 1994 series.     

Selective growth of metal tips onto semiconductor quantum rods and tetrapods

We show the anisotropic selective growth of gold tips onto semiconductor (cadmium selenide) nanorods and tetrapods by a simple reaction. The size of the gold tips can be controlled by the concentration of the starting materials. The new nanostructures display modified optical properties caused by the strong coupling between the gold and semiconductor parts. The gold tips show increased conductivity as well as selective chemical affinity for forming self-assembled chains of rods. Such gold-tipped nanostructures provide natural contact points for self-assembly and for electrical devices and can solve the difficult problem of contacting colloidal nanorods and tetrapods to the external world.     

Sequence-specific molecular lithography on single DNA molecules

Recent advances in the realization of individual molecular-scale electronic devices emphasize the need for novel tools and concepts capable of assembling such devices into large-scale functional circuits. We demonstrated sequence-specific molecular lithography on substrate DNA molecules by harnessing homologous recombination by RecA protein. In a sequence-specific manner, we patterned the coating of DNA with metal, localized labeled molecular objects and grew metal islands on specific sites along the DNA substrate, and generated molecularly accurate stable DNA junctions for patterning the DNA substrate connectivity. In our molecular lithography, the information encoded in the DNA molecules replaces the masks used in conventional microelectronics, and the RecA protein serves as the resist. The molecular lithography works with high resolution over a broad range of length scales from nanometers to many micrometers.     

A Note on Soil Erosion and Its Environmental Consequences in the United States

Soil erosion has both on-farm and off-farm impacts. Reduction ofsoil depth can impair the land's productivity, and the transportof sediments can degrade streams, lakes, and estuaries. Since1933, soil conservation policies have existed in the UnitedStates. Originally they focused on the on-farm benefits ofkeeping soil on the land and increasing net farm income.Beginning in the 1980s, however, policy goals increasinglyincluded reductions in off-site impacts of erosion. As aconsequence of conservation efforts associated with explicitU.S. government policies, total soil erosion between 1982 and1992 was reduced by 32% and the sheet and rill erosion ratefell from an average of 4.1 tons per acre per year in 1982 to 3.1 tons per acre in 1992. Wind erosion rate fell from anaverage of 3.3 tons per acre per year to 2.4 tons per acre peryear over the same period. Still, soil erosion is imposingsubstantial social costs. These costs are estimated to be about$37.6 billion annually. To further reduce soil erosion andthereby mitigate its social costs, there are a number of policyoptions available to induce farmers to adopt conservationpractices including education and technical assistance,financial assistance, research and development, land retirement,and regulation and taxes.     

Avocado rootstock influences scion leaf mineral content

ABSTRACT

To test the influence of avocado rootstocks on the scion leaf ionome, the nutritional status of avocado cultivars Hass and Ettinger grafted onto 15 rootstocks was compared over 3 years. The rootstocks were of different genetic origins (West Indian or Mexican) and were clonally or reproductively propagated. The trees were grown in a high-density orchard, and were continually fertigated as is common in modern avocado orchards. Leaf mineral composition was analysed and found to be correlated with crop load. ‘Hass’ leaves had significantly higher levels of B, Ca, Mg, Na, P and K than ‘Ettinger’ leaves. Rootstocks of Mexican origin produced higher foliar Cl levels, but lower levels of Mg and Mn. Rootstocks grown from seedlings conferred higher foliar K and lower B, Ca and Mg. The results demonstrate that avocado rootstocks affect the nutritional status of the tree, by a differential mineral transport, which is indicated by the scion leaf ionome.     

Measuring the costs of improving the quality of the environment in the United States

The analysis in this paper is directed at comparing the relative effectiveness of meeting the objectives outlined in the recently passed Clean Air Act Amendments of 1990 through taxation and through regulation. The analytical vehicle used in the analysis consists of a computable general equilibrium model composed of 12 producing sectors, 13 consuming sectors, 6 household categories classified by income, a foreign sector and a government. We find first that, irrespective of what type of strategy is followed, both output and consumption decline. Hence, there is an identifiable and quantifiable tradeoff between economic activity (economic growth) and the quality of the environment. Beyond this, the aggregate loss in production and economic welfare (measured by consumption expenditures and utility) is less under a policy which stresses reliance on alternative fuels than through one that requires the installation of pollution abatement devices.     

ACQUIRED MEGAESOPHAGUS IN A HEIFER

An 18-month-old pregnant heifer, hitherto normal, was noticed to regurgitate wet, partially digested roughage several times a day. On clinical examination, the heifer was losing body weight. There was no obstacle to passing a stomach tube to the rumen. A barium study of the esophagus was performed. Megaesophagus involving the cervical and cranial thoracic esophagus was diagnosed. Postmortem findings were a dilated esophagus, with no gross pathology seen. Histopathology showed extensive submucosal cellular infiltration of eosinophils. The classification and possible etilogy of the case is discussed.