Growth, structural, and optical properties of self-assembled (In,Ga)as quantum posts on GaAs.

Self-assembled quantum dots embedded in semiconductor heterostructures have proved to be a rich system for exploring the physics of three dimensionally confined charges and excitons. We present here a novel structure, which allows adjusting the level of confinement between 3D and 2D for electrons and holes, respectively. The quantum post consists of a quantum dot connected to a short quantum wire. The molecular beam epitaxy deposition of these self-assembled structures is discussed, and their structural and chemical compositions are presented. Their optical properties measured by photoluminescence are compared to an eight-band strain-dependent k.p model incorporating detailed structure and alloy composition. The calculations show electron delocalization in the quantum wire part of the quantum post and hole localization in the strain-induced regions at the ends of the quantum post. The quantum post offers the possibility of controlling the dipole moment in the structure and opens up new means for tuning the intra-subband transitions by controlling its dimensions.

Exciton fine structure splitting in InP quantum dots in GaInP.

We have investigated the electronic structure of excitons in InP quantum dots in GaInP. The exciton is theoretically expected to have four states. Two of the states are allowed to optically decay to the ground (vacuum) state in the dipole approximation. We see these two lines in photoluminescence (PL) experiments and find that the splitting between the lines (the fine structure splitting) is 150(± 30) µeV. The lines were perpendicularly polarized. We verified that the lines arise from neutral excitons by using correlation spectroscopy. The theoretical calculations show that the polarization of the emission lines are along and perpendicular to the major axis of elongated dots. The fine structure splitting depends on the degree of elongation of the dots and is close to zero for dots of cylindrical symmetry, despite the influence of the piezoelectric polarization, which is included in the calculation.

Core-shell nanorods for efficient photoelectrochemical hydrogen production.

We propose core-shell InP-CdS and InP-ZnTe nanorods as photoelectrodes in the efficient photoelectrochemical hydrogen production. On the basis of our systematic study using strain-dependent k.p theory, we find that in these heterostructures both energies and wave function distributions of electrons and holes can be favorably tailored to a considerable extent by exploiting the interplay between quantum confinement and strain. Consequently, these core-shell nanorods with proper dimensions (height, core radius, and shell thickness) can simultaneously satisfy all criteria for effective photoelectrodes in solar-based hydrogen production.

Accuracy of circular polarization as a measure of spin polarization in quantum dot qubits.

A quantum dot spin light emitting diode provides a test of carrier spin injection into a qubit and a means for analyzing carrier spin injection and local spin polarization. Even with 100% spin-polarized carriers the emitted light may be only partially circularly polarized due to the geometry of the dot. We have calculated carrier polarization-dependent optical matrix elements for InAs/GaAs self-assembled quantum dots (SAQDs) for electron and hole spin injection into a range of quantum dot sizes and shapes, and for arbitrary emission directions. Calculations for typical SAQD geometries with emission along [110] show light that is only 5% circularly polarized for spin states that are 100% polarized along [110]. Measuring along the growth direction gives near unity conversion of spin to photon polarization and is the least sensitive to uncertainties in SAQD geometry.

A comprehensive, barcoded system for the management of animal information in a research facility.

The authors describe an easy-to-use barcode-based animal tracking system that has improved record keeping and data retrieval and proved instrumental in the containment of an outbreak.

Ethanol-induced modifications to membrane lipid structure: effect on phospholipase A2-membrane interactions.

Chronic ethanol intoxication leads to the development of a resistance to lipid disordering by ethanol, a phenomenon known as "membrane tolerance". In the absence of the added ethanol, the lipid order, as measured by ESR and fluorescence techniques, does not necessarily change as a result of chronic ethanol ingestion (as in liver microsomes, for example). This suggests that the spectroscopic techniques detect tolerance somewhat indirectly, in that the modification responsible may reside in a region distinct from that being probed and also raises the question of whether membrane tolerance is necessarily associated with an alteration in the membrane lipid structure. Here we show that liver microsomes from rats treated chronically with ethanol are rendered relatively resistant to the hydrolytic action of exogenous phospholipase A2, compared to preparations from control animals. This resistance persists in reconstituted lipid vesicles prepared from extracted phospholipids. Since the same substrate (1-palmitoyl-2-N-(4-nitrobenzo-2-oxa-1,3-diazole)amino caproylphosphatidylcholine) was used in both membranes from ethanol-treated animals and controls, the modification appears to reside in the structure and/or organization of the membrane. Further evidence that the lipid structure is modified by chronic ethanol treatment is provided by the observation that perturbance of the membrane structural integrity by increasing levels of oleic acid led to a progressive loss of the ethanol-induced relative resistance to hydrolysis by phospholipase A2. The results of this study support the idea that membrane tolerance involves a modification to lipid structure probably at the bilayer surface. The use of exogenous phospholipase A2 provides a new method for probing the structural modifications induced by chronic ethanol ingestion.

The effects of dietary (n-3) fatty acid supplementation on lipid dynamics and composition in rat lymphocytes and liver microsomes.

Rats were fed diets devoid of (n-3) fatty acids (olive oil supplementation) or high in (n-3) fatty acids (fish oil supplementation) for a period of 10 days. In spleen lymphocytes and liver microsomes derived from animals fed fish oil diets, relatively high levels of (n-3) eicosapentaenoic (20:5), docosapentaenoic (22:5) and docosahexaenoic acids (22:6) were obtained compared to minimal levels when fed the olive oil diet. When the average lipid motional properties were examined by measuring the fluorescence anisotropy of diphenylhexatriene, no significant different was found between intact liver microsomes from animals fed the two diets. However, when lipid motion was examined in vesicles of phosphatidylcholine, isolated from the microsomes from fish oil fed animals (21.4% (n-3) fatty acids), the fluorescence anisotropy was significantly less than the corresponding phosphatidylcholine from olive oil fed animals (5.6% (n-3) fatty acids), indicating a more disordered or fluid bilayer in the presence of higher levels of (n-3) fatty acids. Phosphatidylethanolamine (n-3) fatty acids were also elevated after fish oil supplementation (41.3% of total fatty acids), compared to the level after olive oil supplementation (21.4%). The major effect of the fish oil supplementation was a replacement of (n-6) arachidonic acid by the (n-3) fatty acids and when this was 'modeled', using liposomes of synthetic lipids, 1-palmitoyl-2-arachidonyl(n-6) or docosahexaenoyl(n-3)-phosphatidylcholine, significant differences in lipid motional properties were found, with the docosahexaenoate conferring a more disordered or fluid lipid environment. Thus it appears that although lipid order/fluidity can be significantly decreased by increases in the highly unsaturated (n-3) fatty acid levels, alterations in membrane domain organization and/or phospholipid molecular species composition effectively compensated for the changes, at least as far as average lipid motional properties in the intact membranes was concerned.

Intraocular metastases from cutaneous malignant melanoma.

We present a consecutive series of ten patients (12 eyes) with symptomatic intraocular metastases from primary cutaneous malignant melanoma, with emphasis on the variation in clinical features and extent of intraocular involvement. An investigation of each patient's history and the presence of other concurrent metastatic foci help to establish the diagnosis. Palliative radiation therapy may help to control the intraocular metastatic foci and limit the visual loss.

Aggregation, lipid exchange, and metastable phases of dimyristoylphosphatidylethanolamine vesicles.

A new fluorescent lipid analogue, bimanephosphatidylcholine, has been synthesized for use in lipid bilayers. This probe is well suited as an energy-transfer donor with N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine as the acceptor. Dimyristoylphosphatidylethanolamine vesicles are prepared by sonication at pH 9 and characterized by electron microscopy and other methods. Resonance energy transfer between separately labeled donor and acceptor vesicles is monitored during HCl-induced aggregation to determine the kinetics of lipid randomization. Light scattering is also monitored to measure the kinetics of aggregation. The light scattering shows a marked reversal with NaOH while the energy transfer does not, indicating lipid exchange during a reversibly aggregated state; the extent of energy transfer suggests that only lipids in the outer monolayers exchange. The gel to liquid-crystalline phase transition temperature in HCl-treated vesicles is found to be 47 degrees C with diphenylhexatriene. The initial sonicated dispersion does not show a sharp phase transition. In vesicles labeled with both donor and acceptor probes, a small, irreversible increase in energy transfer is obtained upon lowering and then restoring the pH. These results suggest a metastable phase in the sonicated vesicles containing a randomized distribution of lipid and probes within the bilayers; the thermodynamically favored phase, whose formation is triggered by the pH shock, contains domains within which the probe lipids are more highly concentrated.