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Paul Meredith

Publications and source records attributed to Paul Meredith.

25 records · Page 2Linked to original sources
arXiv2005

A Quantum Yield Map for Synthetic Eumelanin

The quantum yield of synthetic eumelanin is known to be extremely low and it has recently been reported to be dependent on excitation wavelength. In this paper, we present quantum yield as a function of excitation wavelength between 250 and 500 nm, showing it to be a factor of 4 higher at 250 nm than at 500 nm. In addition, we present a definitive map of the steady-state fluorescence as a function of excitation and emission wavelengths, and significantly, a three-dimensional map of the specific quantum yield: the fraction of photons absorbed at each wavelength that are subsequently radiated at each emission wavelength. This map contains clear features, which we attribute to certain structural models, and shows that radiative emission and specific quantum yield are negligible at emission wavelengths outside the range of 585 and 385 nm (2.2 and 3.2 eV), regardless of excitation wavelength. This information is important in the context of understanding melanin biofunctionality, and the quantum molecular biophysics therein.

physics.chem-ph↗
arXiv2005

Quantitative Fluorescence Excitation Spectra for Synthetic Eumelanin

Previously reported excitation spectra for eumelanin are sparse and inconsistent. Moreover, these studies have failed to account for probe beam attenuation and emission reabsorption within the samples, making them qualitative at best. We report for the first time quantitative excitation spectra for synthetic eumelanin, acquired for a range of solution concentrations and emission wavelengths. Our data indicate that probe beam attenuation and emission reabsorption significantly affect the spectra even in low-concentration eumelanin solutions and that previously published data do not reflect the true excitation profile. We apply a correction procedure (previously applied to emission spectra) to account for these effects. Application of this procedure reconstructs the expected relationship of signal intensity with concentration, and the normalised spectra show a similarity in form to the absorption profiles. These spectra reveal valuable information regarding the photophysics and photochemistry of eumelanin. Most notably, an excitation peak at 365 nm (3.40 eV), whose position is independent of emission wavelength, is possibly attributable to a DHICA component singly linked to a polymeric structure.

cond-mat.soft↗
arXiv2004

Carrier Transport in PbS Nanocrystal Conducting Polymer Composites

In this paper we report the first measurements of carrier mobilities in an inorganic nanocrystal: conducting polymer composite. The composite material in question (lead sulphide nanocrystals in the conducting polymer MEH-PPV was made using a new single-pot, surfactant-free synthesis. Mobilties were measured using time of flight (ToF) and steady-state techniques. We have found that the inclusion of PbS nanocrystals in MEH-PPV both balances and markedly increases the hole and electron mobilities - the hole mobility is increased by a factor of ~105 and the electron mobility increased by ~107 under an applied bias of 5kVcm-1. These results explain why dramatic improvements in electrical conductivity and photovoltaic performance are seen in devices fabricated from these composites.

cond-mat.mtrl-sci↗
arXiv2004

Lead Sulphide Nanocrystal: Conducting Polymer Solar Cells

In this paper we report photovoltaic devices fabricated from PbS nanocrystals and the conducting polymer poly MEH-PPV. This composite material was produced via a new single-pot synthesis which solves many of the issues associated with existing methods. Our devices have white light power conversion efficiencies under AM1.5 illumination of 0.7% and single wavelength conversion efficiencies of 1.1%. Additionally, they exhibit remarkably good ideality factors (n=1.15). Our measurements show that these composites have significant potential as soft optoelectronic materials.

cond-mat.mtrl-sci↗
arXiv2004

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.

physics.bio-ph↗
arXiv2003

Radiative Relaxation Quantum Yields for Synthetic Eumelanin

We report absolute values for the radiative relaxation quantum yield of synthetic eumelanin as a function of excitation energy. These values were determined by correcting for pump beam attenuation and emission re-absorption in both eumelanin samples and fluorescein standards over a large range of concentrations. Our results confirm that eumelanins are capable of dissipating >99.9% of absorbed UV and visible radiation through non-radiative means. Furthermore, we have found that the radiative quantum yield of synthetic eumelanin is excitation energy dependent. This observation is supported by corrected emission spectra, which also show a clear dependence of both peak position and peak width upon excitation energy. Our findings indicate that photoluminescence emission in eumelanins is derived from ensembles of small chemically distinct oligomeric units which can be selectively pumped. This hypothesis lends support to the theory that the basic structural unit of eumelanin is oligomeric rather than hetero-polymeric.

cond-mat.soft↗