Optical control of the mobility of a MODFET with a layer of self-assembled quantum dots

We report an experimental study of GaAs/Al_0.33Ga_0.67As modulation doped field effect (MODFET) transistors, in which an InAs layer of self-assembled quantum dots is placed in one of the Al_0.33Ga_0.67As barrier layers close to the two-dimensional electron gas (2DEG). We find the source–drain resistance is bistable with the two states controlled by illumination and applied gate bias. Brief illumination induces a large, persistent drop in the resistance, which can be recovered by applying a positive gate bias. Magneto-transport measurements show that while illumination causes only a relatively small change in the 2DEG density, it can greatly enhance its mobility. We suggest this is because the 2DEG mobility is limited by percolation of the electrons through the rough electrostatic potential induced by the charged dots. Illumination reduces the negative charge trapped in the dots, thus smoothing the conduction band potential, which produces a large increase in the mobility.     

Routes to Higher Nuclearity Mixed-Metal Carbonyl Clusters Using the [Rh(η<Superscript>5</Superscript>-C<Subscript>5</Subscript>Me<Subscript>5</Subscript>)(NCMe)<Subscript>3</Subscript>]<Superscript>2+</Superscript> Dication as a Building Block

Reaction of the [Rh(η⁵-C₅Me₅)(NCMe)₃]²⁺ (1) dication with the hexaosmium [Os₆(CO)₁₇]²⁻ (2) dianion leads to the initial formation of [Os₆(CO)₁₇Rh(η⁵-C₅Me₅)] (3). This cluster readily adds CO to form [Os₆(CO)₁₈Rh(η⁵-C₅Me₅)] (4) which has been characterised crystallographically. 3 also adds dihydrogen to give [Os₆H₂(CO)₁₇Rh(η⁵-C₅Me₅)] (5) and undergoes a substitution reaction with PPh₃ to form [Os₆(CO)₁₆(PPh₃)Rh(η⁵-C₅Me₅)] (6). With the [Ru₆(CO)₁₈]²⁻ (7) dianion, [Rh(η⁵-C₅Me₅)(NCMe)₃]²⁺ (1) reacts to form three mixed-metal clusters [Ru₅(CO)₁₅Rh(η⁵-C₅Me₅)] (8), [Ru₆(CO)₁₈Rh(η⁵-C₅Me₅)] (9) and [Ru₆(CO)₁₈Rh₂(η⁵-C₅Me₅)₂] (10). The clusters have been characterised spectroscopically and the structures of 8 and 10 have been confirmed crystallographically. The cluster 8 undergoes a substitution reaction with P(OMe)₃ to form the disubstituted product [Ru₅(CO)₁₃(P(OMe)₃)₂Rh((η⁵-C₅Me₅)] (11) which has also been characterised crystallographically.     

Subcellular localization of the antidepressant-sensitive norepinephrine transporter

Background  

Reuptake of synaptic norepinephrine (NE) via the antidepressant-sensitive NE transporter (NET) supports efficient noradrenergic signaling and presynaptic NE homeostasis. Limited, and somewhat contradictory, information currently describes the axonal transport and localization of NET in neurons.     

Biodegradation of Crude Oil from the BP Oil Spill in the Marsh Sediments of Southeast Louisiana, USA

The significant challenges presented by the April 20, 2010 explosion, sinking, and subsequent oil spill of the Deepwater Horizon drilling platform in Canyon Block 252 about 52 miles southeast of Venice, LA, USA greatly impacted Louisiana??s coastal ecosystem including the sea food industry, recreational fishing, and tourism. The short-term and long-term impact of this oil spill are significant, and the Deepwater Horizon spill is potentially both an economic and an ecological disaster. Microbes present in the water column and sediments have the potential to degrade the oil. Oil degradation could be enhanced by biostimulation method. The conventional approach to bioremediation of petroleum hydrocarbon is based on aerobic processes. Anaerobic bioremediation has been tested only in a very few cases and is still considered experimental. The currently practiced conventional in situ biorestoration of petroleum-contaminated soils and ground water relies on the supply of oxygen to the subsurface to enhance natural aerobic processes to remediate the contaminants. However, anaerobic microbial processes can be significant in oxygen-depleted subsurface environments and sediments that are contaminated with petroleum-based compounds such as oil-impacted marshes in Louisiana. The goal of this work was to identify the right conditions for the indigenous anaerobic bacteria present in the contaminated sites to enhance degradation of petroleum hydrocarbons. We evaluated the ability of microorganisms under a variety of electron acceptor conditions to degrade petroleum hydrocarbons. Researched microbial systems include sulfate-, nitrate-reducing bacteria, and fermenting bacteria. The results indicated that anaerobic bacteria are viable candidates for bioremediation. Enhanced biodegradation was attained under mixed electron acceptor conditions, where various electron-accepting anaerobes coexisted and aided in degrading complex petroleum hydrocarbon components of marsh sediments in the coastal Louisiana. Significant degradation of oil also occurred under sulfate-reducing and nitrate-reducing conditions.     

Hydration of the bisulfate ion: atmospheric implications

Using molecular dynamics configurational sampling combined with ab initio energy calculations, we determined the low energy isomers of the bisulfate hydrates. We calculated the CCSD(T) complete basis set (CBS) binding electronic and Gibbs free energies for 53 low energy isomers of HSO(4)(-)(H(2)O)(n=1-6) and derived the thermodynamics of adding waters sequentially to the bisulfate ion and its hydrates. Comparing the HSO(4)(-)/H(2)O system to the neutral H(2)SO(4)/H(2)O cluster, water binds more strongly to the anion than it does to the neutral molecules. The difference in the binding thermodynamics of HSO(4)(-)/H(2)O and H(2)SO(4)/H(2)O systems decreases with increasing number of waters. The thermodynamics for the formation of HSO(4)(-)(H(2)O)(n=1-5) is favorable at 298.15 K, and that of HSO(4)(-)(H(2)O)(n=1-6) is favorable for T < 273.15 K. The HSO(4)(-) ion is almost always hydrated at temperatures and relative humidity values encountered in the troposphere. Because the bisulfate ion binds more strongly to sulfuric acid than it does to water, it is expected to play a role in ion-induced nucleation by forming a strong complex with sulfuric acid and water, thus facilitating the formation of a critical nucleus.     

Isomer stability and bond-breaking energies of N8C8H8 cages

Molecules consisting entirely or predominantly of nitrogen have been extensively investigated for their potential as high-energy density materials (HEDM). Such molecules react to produce N2 and large amounts of energy, but many such molecules are too unstable for practical applications. In the present study, cage isomers of N8C8H8 are studied using theoretical calculations to determine the structural features that lead to the most stable cages and determine the energetics of dissociation for the various isomers. The isomers are evaluated for thermodynamic (isomer vs isomer) stability and kinetic (with respect to dissociation) stability. Density functional theory (B3LYP), perturbation theory (MP2), and coupled-cluster theory [CCSD(T)] are employed, in conjunction with the cc-pVDZ basis set of Dunning. Trends in isomer stability and dissociation energies are calculated and discussed.     

Prediction of accurate anharmonic experimental vibrational frequencies for water clusters, (H2O)n, n=2-5

Accurate anharmonic experimental vibrational frequencies for water clusters consisting of 2-5 water molecules have been predicted on the basis of comparing different methods with MP2/aug-cc-pVTZ calculated and experimental anharmonic frequencies. The combination of using HF/6-31G* scaled frequencies for intramolecular modes and anharmonic frequencies for intermolecular modes gives excellent agreement with experiment for the water dimer and trimer and are as good as the expensive anharmonic MP2 calculations. The water trimer, the cyclic Ci and S4 tetramers, and the cyclic pentamer all have unique peaks in the infrared spectrum between 500 and 800 cm-1 and between 3400 and 3700 cm-1. Under the right experimental conditions these different clusters can be uniquely identified using high-resolution IR spectroscopy.     

A Method for the Determination of Octa- and Nonachloro-2-phenoxyphenols in Chicken Tissue

Abstract

A gas-liquid chromatographic method was developed capable of determining octa-and nonachloro-2-phenoxyphenols in chicken liver or muscle at 0.25 ppb and fat at 2.5 ppb. The method involves extraction with acidified acetone:hexane, cleanup with concentrated H₂SO₄ and Florisil column chromatography, methylation with diazomethane, and quantitation by capillary column gas-liquid chromatography with electron capture detection. Fortification of liver and muscle at 0.25 or 0.5 ppb and fat at 2.5 or 5.0 ppb and subsequent analysis yielded recoveries averaging 91% for octa-and 97% for nonachlorophenoxyphenol.     

Benchmark structures and binding energies of small water clusters with anharmonicity corrections

For (H(2)O)(n) where n = 1-10, we used a scheme combining molecular dynamics sampling with high level ab initio calculations to locate the global and many low lying local minima for each cluster. For each isomer, we extrapolated the RI-MP2 energies to their complete basis set limit, included a CCSD(T) correction using a smaller basis set and added finite temperature corrections within the rigid-rotor-harmonic-oscillator (RRHO) model using scaled and unscaled harmonic vibrational frequencies. The vibrational scaling factors were determined specifically for water clusters by comparing harmonic frequencies with VPT2 fundamental frequencies. We find the CCSD(T) correction to the RI-MP2 binding energy to be small (<1%) but still important in determining accurate conformational energies. Anharmonic corrections are found to be non-negligble; they do not alter the energetic ordering of isomers, but they do lower the free energies of formation of the water clusters by as much as 4 kcal/mol at 298.15 K.