Photoluminescence Reduction Research Articles

Morphological modulation of optoelectronic properties of organic–inorganic nanohybrids prepared with a one-step co-fed chemical vapor deposition polymerization process

A co-fed chemical vapor deposition polymerization (CVDP) process was developed to successfully prepare poly(p-phenylene vinylene)–CdS (PPV–CdS) nanohybrids in one step. The PPV and CdS were formed from the corresponding precursors, di-chloro-p-xylene and cadmium dipropyldithiocarbamate, respectively. The morphology of the nanohybrids can be readily tuned, from nanocomposites containing well dispersed nanocrystals to continuous bulk heterostructures, by controlling the precursor temperature and carrier gas flow rate, from which the optoelectronic properties of the nanohybrids can be effectively modulated. The p–n junctions formed by the p-type PPV and n-type CdS within the nanohybrids enhanced charge separation, giving rise to a one order of magnitude reduction in photoluminescence (PL) quantum yields and enhancement of current densities. The PL emissions first red-shifted and then blue-shifted back with increasing CdS loading, caused by the competition between the polarization and conjugation length reduction effects. The present work demonstrates a new way of designing organic–inorganic nanohybrids for applications requiring specific optoelectronic properties.

Sonochemical deposition of nanosized Au on titanium oxides with different surface coverage and their photocatalytic activity

Different surface coverage of nanosized Au on the TiO 2 matrix was simply fabricated in an ultrasound-driven cell by adjusting the loading volume of gold precursor. Non-annealed Au–TiO 2 composites with low surface coverage of nanosized Au exhibited a significantly enhanced photocatalytic activity in comparison to pristine TiO 2. However, a further increase of nanosized Au loading consequently led to a decrease of photocatalytic efficiency of Au-deposited TiO 2, probably due to the reduction of available surface area for organic pollutant adsorption and light absorption. The X-ray photoelectron spectroscopy (XPS) analysis of Au-deposited TiO 2 indicated the coexistence of metallic Au o and non-metallic Au species that might play a role as an electron scavenger. The reduction of photoluminescence (PL) intensity also indicated the efficient separation of electrons and holes even in Au–TiO 2 composites without heat treatment.

Influence of As4 flux on the growth kinetics, structure, and optical properties of InAs∕GaAs quantum dots

We report the effects of variations in As4 growth flux on the evolution of molecular beam epitaxy grown InAs quantum dots (QDs) and their structures and optical properties. For InAs QDs grown under As-stable conditions, evaluated through photoluminescence and atomic force microscopy (AFM) measurements, it is evident that QD size increases with As4 pressure along with improvement in size uniformity. Furthermore, transmission electron microscopy measurements for InAs layers of critical thicknesses (∼1.7 ML) showed decreasing QD density with increasing As4 pressure accompanied by a strong reduction in photoluminescence (PL) integral intensity. These show that high As4 fluxes suppress InAs QD formation while the decreasing PL intensity seems to indicate cluster formation that features nonradiative recombination. AFM measurements show larger and denser QDs for samples grown at higher As4 pressures. These are explained on the basis of adatom condensation during surface cooling and the influence of As4 pressure on indium incorporation.

Coherent population trapping in diamond N-V centers at zero magnetic field

Coherent population trapping at zero magnetic field was observed for nitrogen-vacancy centers in diamond under optical excitation. This was measured as a reduction in photoluminescence when the detuning between two excitation lasers matched the 2.88 GHz crystal-field splitting of the color center ground states. This behavior is highly sensitive to strain, which modifies the excited states, and was unexpected following recent experiments demonstrating optical readout of single nitrogen-vacancy electron spins based on cycling transitions. These results demonstrate for the first time that three-level Lambda configurations suitable for proposed quantum information applications can be realized simultaneously for all four orientations of nitrogen-vacancy centers at zero magnetic field.

Separation of the Raman Spectral Signatures of Bioapatite and Collagen in Compact Mouse Bone Bleached with Hydrogen Peroxide

We studied the effectiveness of hydrogen peroxide in removing visible laser-induced photoluminescence from the Raman spectra of compact bone of mice. In testing various bone sample preparations, we found that hydrogen peroxide bleaching was most effective when applied directly to fresh or fresh-frozen bone samples. The extent of the reduction in photoluminescence in the peroxide-bleached bone was such that the Raman spectrum could be readily recorded with 532-nm laser excitation. A comparison of bone samples before and after hydrogen peroxide bleaching shows that the Raman shifts of all the collagen and bioapatite bands are unaffected by the peroxide bleach. Moreover, the low spectral backgrounds of the peroxide-treated bone samples permit the Raman spectra of these two major components of bone to be fully separated. The Raman spectrum of collagen-rich periosteum was subtracted from the Raman spectrum of compact bone, which isolated the Raman spectral signature of the bioapatite fraction of the bone. This derived spectrum of bioapatite was then used, through spectral subtraction, to generate the spectral signature of the collagen component of the bone. All the major and minor Raman bands of collagen and bioapatite can be identified in these separate spectra.

Dendritic sidegroups as three-dimensional barriers to aggregation quenching of conjugated polymer fluorescence

We study the photophysics of poly-phenylenevinylene (PPV) polymers with large dendritic sidegroups under both isolated chain and aggregated chain conditions. Aggregation causes a substantial reduction in photoluminescence (PL) quantum yield but that decrease is ameliorated by the attachment of an additional generation of dendron to the PPV backbone. Three-dimensional separation of chromophores can be achieved with these sidegroups and the formation of nearly non-emissive interchain excitations leads to a factor of four enhancements in solid-state PL.

Organic heteromultilayers: electronic structure of sexithienyl/ thin films grown in ultra-high vacuum

Conjugated polymers have among the largest optical nonlinearities and interesting charge generation and transport properties. The combination of both these characteristics is appealing for optoelectronics. We report here on the spectroscopic characterization of alternated multilayers of and with the aim of understanding the electronic structure of a prototype donor-acceptor multilayer. We demonstrate the absence of a stable charge-transfer state at the interface. The strong photoluminescence reduction and a much faster radiative lifetime are indirect evidence of the formation of a metastable charge transfer. Deliberate doping of the fullerene layer with causes strong photoluminescence trapping into -induced traps.

Strong room-temperature infrared emission from Er-implanted porous Si

This communication demonstrates a strong, room-temperature (RT), infrared (IR) (1.54 μm) emission from Er-implanted red-emitting (peaked at 1.9 eV) porous silicon (Er:PSi). Erbium was implanted into porous Si, bulk Si, and quartz with a dose of 1015/cm2 at 190 keV and annealed for 30 minutes in N2 at temperatures ranging from 500 °C to 900 °C under identical conditions. No RT IR emission was observed from Er implanted quartz and silicon after annealing at 650 °C (although after annealing at 900 °C very weak emission was observed from quartz at 9 K). The highest RT emission intensity at 1.54 μm was from Er:PSi with a peak concentration of 1.5×1020/cm3 and annealed at 650 °C. Even the luminescence intensity from Er:PSi annealed at 500 °C was 26 times higher than that observed from Er-implanted quartz at 400 keV and annealed at 900 °C. A reduction in photoluminescence (PL) intensity of about a factor of two from Er:PSi over the 9 to 300 K temperature range was observed which is consistent with Er in wide band gap materials.