Quantitative photoluminescence of broad band absorbing melanins: a procedure to correct for inner filter and re-absorption effects

We report methods for correcting the photoluminescence emission and excitation spectra of highly absorbing samples for re-absorption and inner filter effects. We derive the general form of the correction, and investigate various methods for determining the parameters. Additionally, the correction methods are tested with highly absorbing fluorescein and melanin (broadband absorption) solutions; the expected linear relationships between absorption and emission are recovered upon application of the correction, indicating that the methods are valid. These procedures allow accurate quantitative analysis of the emission of low quantum yield samples (such as melanin) at concentrations where absorption is significant. (c) 2004 Elsevier B.V. All rights reserved.

Surface morphology dependent photoluminescence from colloidal silicon nanocrystals

Monodisperse 1-2 nm silicon nanocrystals are synthesized in reverse micelles and have their surfaces capped with either allylamine or 1-heptene to produce either hydrophilic or hydrophobic silicon nanocrystals. Optical characterization (absorption, PL, and time-resolved PL) is performed on colloidal solutions with the two types of surface-capped silicon nanocrystals with identical size distributions. Direct evidence is obtained for the modification of the optical properties of silicon nanocrystals by the surface-capping molecule. The two different surface-capped silicon nanocrystals show remarkably different optical properties.

Rice growth and post-rice mungbean in relation to two puddling intensities under glasshouse conditions

The effect of soil puddling on growth of lowland rice (Oryza sativa) and post-rice mungbean (Vigna radiata) was investigated using mini rice beds under controlled glasshouse conditions. Each mini rice bed was approximately 1 m(3) in size. Three different soil types were used: a well-drained, permeable loam; a hardsetting, structurally unstable silty loam; and a medium clay. Rice yields were reduced by low puddling compared with high puddling intensity on the loam but not affected on the heavier textured soils (silty loam and clay). Yield of mungbean was reduced on highly puddle, structurally unstable soil, indicating that puddling should be reduced on structurally unstable soils. Under glasshouse condition where crop establishment was not a limiting factor and plant available water in 0.65 m of soil was 100 mm, mungbean yields of >1 t/ha were achieved. However, under conditions where subsoil water reserves were depleted for the production of vegetative biomass during initial optimal growing condition, grain yield remained well below 1 t/ha.

Time-resolved photoluminescence spectroscopy of ligand-capped PbS nanocrystals

PbS nanocrystals are synthesized using colloidal techniques and have their surfaces capped with oleic acid. The absorption band edge of the PbS nanocrystals is tuned between 900 and 580 nm. The PbS nanocrystals exhibit tuneable photoluminescence with large non-resonant Stokes shifts of up to 500 mcV. The magnitude of the Stokes shift is found to be dependent upon the size of PbS nanocrystals. Time-resolved photoluminescence spectroscopy of the PbS nanocrystals reveals that the photouminescence has an extraordinarily long lifetime of 1 mus. This long fluorescence lifetime is attributed to the effect of dielectric screening similar to that observed in other IV-VI semiconductor nanocrystals.

Non-linear photoluminescence from purified aqueous PbS nanocrystals

A simple and effective method for purifying photoluminescent water-soluble surface passivated PbS nanocrystals has been developed. Centrifuging at high speeds removes PbS nanocrystals that exhibit strong red band edge photoluminescence from an original solution containing multiple nanocrystalline species with broad photoluminescence spectra. The ability to purify the PbS nanocrystals allowed two-photon photoluminescence spectroscopy to be performed on water-soluble PbS nanocrystals and be attributed to band edge recombination. (c) 2006 Elsevier B.V. All rights reserved.

Quantitative fluorescence spectra and quantum yield map of synthetic pheomelanin

Spectroscopic studies of pheomelanin and its constituents have been sparse. These data present what is by far the most complete description of the fluorescence characteristics of synthetic pheomelanin. Emission spectra between 260 and 600 nm were acquired,for excitation wavelengths between 250 and 500 nm at 1-nm intervals. A quantum yield map is also presented, correcting the fluorescence intensities for differences in species concentration and molar absorptivity. These fluorescence features exhibit interesting similarities and differences to eumelanin, and these data are interpreted with respect to possible chemical structures. Overall, these data suggest that pheomelanin oligomers may be more tightly coupled than those of eumelanin. Finally, the quantum yield is shown to be on the order of 10(-4) and exhibit a complex dependence on excitation energy, varying by a factor of 4 across the energies employed here. (c) 2006 Wiley Periodicals, Inc.