Excitation Energy Dependence of Photoluminescence Quantum Yields in Semiconductor Nanomaterials with Varying Dimensionalities.

The excitation energy dependence (EED) of the photoluminescence quantum yield (ΦPL) of semiconductor nanoparticles with varying dimensionalities is reported. Specifically, the EEDs of CdSe quantum dots, CdSe quantum platelets, CdSe quantum belts, and CdTe quantum wires were determined via measurements of individual ΦPL values and photoluminescence efficiency (PLEff(E)) spectra. There is a general trend of overall decreasing efficiency for radiative recombination with increasing excitation energy. In addition, there are often local minima in the PLEff(E) spectra that are most often at energies between quantum-confinement transitions. The average PL lifetimes of the samples do not depend on the excitation energy, suggesting that the EED of ΦPL arises from charge carrier trapping that competes efficiently with intraband carrier relaxation to the band edge. The local minima in the PLEff(E) spectra are attributed to excitation into optically coupled states that results in the loss of carriers in the semiconductor. The EED data suggest that the PLEff(E) spectra depend on the sample synthesis, preparation, surface passivation, and environment.

Two-dimensional semiconductor nanocrystals: properties, templated formation, and magic-size nanocluster intermediates.

CONSPECTUS: Semiconductor nanocrystals having an extended length dimension and capable of efficiently transporting energy and charge would have useful applications in solar-energy conversion and other emerging technologies. Pseudocylindrical semiconductor nanowires and quantum wires are available that could potentially serve in this role. Sadly, however, their defective surfaces contain significant populations of surface trap sites that preclude efficient transport. The very large surface area of long wires is at least part of the problem. As electrons, holes, and excitons migrate along a nanowire or quantum wire, they are exposed to an extensive surface and to potentially large numbers of trap sites. A solution to this dilemma might be found by identifying "long" semiconductor nanocrystals of other morphologies that are better passivated. In this Account, we discuss a newly emerging family of flat semiconductor nanocrystals that have surprising characteristics. These thin, flat nanocrystals have up to micrometer-scale (orthogonal) lateral dimensions and thus very large surface areas. Even so, their typical photoluminescence efficiencies of 30% are astonishingly high and are 2 orders of magnitude higher than those typical of semiconductor quantum wires. The very sharp emission spectra of the pseudo-two-dimensional nanocrystals reflect a remarkable uniformity in their discrete thicknesses. Evidence that excitons are effectively delocalized and hence transported over the full dimensions of these nanocrystals has been obtained. The excellent optical properties of the flat semiconductor nanocrystals confirm that they are exceptionally well passivated. This Account summarizes the two synthetic methods that have been developed for the preparation of pseudo-two-dimensional semiconductor nanocrystals. A discussion of their structural features accounts for their discrete, uniform thicknesses and details the crystal-lattice expansions and contractions they exhibit. An analysis of their optical properties justifies the sharp photoluminescence spectra and high photoluminescence efficiencies. Finally, a bilayer mesophase template pathway is elucidated for the formation of the nanocrystals, explaining their flat morphologies. Magic-size nanocluster intermediates are found to be potent nanocrystal nucleants, allowing the synthesis temperatures to be decreased to as low as room temperature. The potential of these flat semiconductor nanocrystals in the form of nanoribbons or nanosheets for long-range energy and charge transport appears to be high.

Excitation Energy Dependence of the Photoluminescence Quantum Yields of Core and Core/Shell Quantum Dots.

The photoluminescence (PL) intensity of semiconductor quantum dots (QDs) is routinely monitored to track the chemical and physical properties within a sample or device incorporating the QDs. A dependence of the PL quantum yields (QYs) on the excitation energy could lead to erroneous conclusions but is commonly not considered. We summarize previous evidence and present results from two methodologies that confirm the possibility of a dependence of the PL QYs on the excitation energy. The data presented indicate that PL QYs of CdSe and CdSe/ZnS QDs suspended in toluene are highest for excitation just above the band gap, Eg, of each. The PL QYs decrease with increasing excitation energies up to 1 eV above Eg. The PL intensity decay profiles recorded for these samples at varying emission and excitation energies indicate that the changes in the PL QYs result from the nonradiative relaxation pathways sampled as the charge carriers relax down to the band edge.

Bound 1D Excitons in Single CdSe Quantum Wires.

Photogenerated electron-hole pairs are observed to be bound as 1D excitons in CdSe quantum wires (QWs) at room temperature. Microscopy experiments performed on dilute samples of CdSe QWs prepared on coverslips with patterned electrodes reveal that there is no change in either the overall photoluminescence (PL) intensity or the distribution of the PL intensity with the application of an external electric field. Changes in the PL intensity, and thus evidence for separate charge carriers within the QWs, are observed only for concentrated samples. In these concentrated samples, a thin film of other compounds, including trioctylphosphine oxide and a bismuth salt formed in the synthesis, is observed to encompass the QWs. The separate charge carriers that influence the PL intensity are attributed to the other compounds in the sample.

Effect of carbohydrate-protein supplementation postexercise on rat muscle glycogen synthesis and phosphorylation of proteins controlling glucose storage.

To examine whether addition of protein to a carbohydrate supplement enhances muscle glycogen synthesis, we compared the muscle glycogen concentrations of rats that had been depleted of their muscle glycogen stores with a 3-hour swim and immediately supplemented with a placebo (Con), carbohydrate (CHO), or carbohydrate plus protein supplement (C+P). Rats were given either 0.9 g carbohydrate per kilogram body mass for the CHO group or 0.9 g carbohydrate + 0.3 g protein per kilogram body mass for the C+P groups. Muscle samples of the red and white quadriceps were excised immediately, 30 minutes, or 90 minutes postexercise. Glycogen concentration of the C+P group was greater than that of the CHO group at 90 minutes postexercise in both red (C+P, 28.3 ± 2.6 µmol/g vs CHO, 22.4 ± 2.0 µmol/g; P < .05) and white (C+P, 24.9 ± 2.4 µmol/g vs CHO, 17.64 ± 1.5 µmol/g; P < .01) quadriceps. Protein kinase B phosphorylation was greater in the C+P-30 group (the number following treatment group abbreviation refers to time [in minutes] of euthanasia following exercise) than the sedentary control and exercised control groups in red quadriceps at 30 minutes and in white quadriceps at 90 minutes postexercise. This difference was not observed in the CHO group. Phosphorylation of glycogen synthase was significantly reduced 30 minutes postexercise and returned to baseline levels by 90 minutes postexercise in both CHO- and C+P-supplemented groups, with no difference between supplements. These results demonstrated that the addition of protein to a carbohydrate supplement will enhance the rate of muscle glycogen restoration postexercise and may involve facilitation of the glucose transport process.

Adding protein to a carbohydrate supplement provided after endurance exercise enhances 4E-BP1 and RPS6 signaling in skeletal muscle.

To examine the role of both endurance exercise and nutrient supplementation on the activation of mRNA translation signaling pathways postexercise, rats were subjected to a 3-h swimming protocol. Immediately following exercise, the rats were provided with a solution containing either 23.7% wt/vol carbohydrates (CHO), 7.9% wt/vol protein (Pro), 31.6% wt/vol (23.7% wt/vol CHO + 7.9% wt/vol Pro) carbohydrates and Pro (CP), or a placebo (EX). The rats were then killed at 0, 30, and 90 min postexercise, and phosphorylation states of mammalian target of rapamycin (mTOR), ribosomal S6 kinase (p70(S6K)), ribosomal protein S6 (rpS6), and 4E-binding protein 1 (4E-BP1), were analyzed by immunoblot analysis in the red and white quadriceps muscle. Results demonstrated that rat groups provided with any of the three nutritional supplements (CHO, Pro, CP) transiently increased the phosphorylation states of mTOR, 4E-BP1, rpS6, and p70(S6K) compared with EX rats. Although CHO, Pro, and CP supplements phosphorylated mTOR and p70(S6K) after exercise, only CP elevated the phosphorylation of rpS6 above all other supplements 30 min postexercise and 4E-BP1 30 and 90 min postexercise. Furthermore, the phosphorylation states of 4E-BP1 (r(2) = 0.7942) and rpS6 (r(2) = 0.760) were highly correlated to insulin concentrations in each group. These results suggest that CP supplementation may be most effective in activating the mTOR-dependent signaling pathway in the postprandial state postexercise, and that there is a strong relationship between the insulin concentration and the activation of enzymes critical for mRNA translation.

Selective ET(A) receptor antagonism with ABT-627 attenuates all renal effects of endothelin in humans.

++Endothelin (ET-1) acts as a potent vasoconstrictor in the human kidney, and this vasoconstriction could contribute to the ischemia seen in acute renal failure. In animal studies, the vasoactive properties of ET-1 are known to be ET(A) receptor-and/or ET(B) receptor-mediated; however, the receptor subtype involved in the human kidney remains to be defined. In a phase I, single-center, double-blind, randomized, three-period, crossover design, the effects of orally administered ABT-627, a selective ET(A) receptor antagonist, on renal hemodynamics during ET-1 infusion were evaluated. Two doses of ABT-627 (5 and 20 mg) were compared with placebo and nifedipine. For each dose level of ABT-627, a cohort of nine subjects was studied. A para-aminohippuric acid/inulin clearance test was performed once at the end of each 7-d treatment period. Infusion of ET-1 significantly decreased effective renal plasma flow, GFR, sodium excretion, and urine flow. Pretreatment with 20 mg of ABT-627 significantly decreased mean arterial pressure. In contrast, 7 d of treatment with both doses of ABT-627 did not affect baseline renal parameters. However, because mean arterial pressure decreased, a tendency toward a reduction of renal vascular resistance could indeed be demonstrated. Compared with placebo, both doses of ABT-627 were equally effective in blocking all renal effects caused by ET-1 infusion. In the model of exogenous ET-1 infusion, ABT-627 had a tendency to prevent ET-1-induced renal changes more effectively compared with nifedipine. The contribution of endogenous ET-1 and the ET(A) receptor in maintaining basal renal vascular tone in the human kidney is small. In addition, compared with placebo, selective ET(A) receptor antagonism with both doses of ABT-627 completely prevented all renal changes caused by ET-1 infusion.