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Photophysical properties of conjugated polymers strongly correlate with chain morphology and interactions between chains. Here, we observed two characteristic types of spectra of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)] (F8T2) in both single-chain and bulk cases. Despite the similarities in emission spectra, fluorescence dynamics and intensities corresponding to each type of spectrum are radically different in these cases. We hypothesize that the origin of the spectrum with a suppressed 0-0 vibronic band in photoluminescence is chain bending in the single-chain experiments, while the same phenomenon is caused by interchain aggregation in bulk samples. We propose a microscopic interpretation of the single-chain result in terms of bending that we expect to be characteristic of flexible conjugated polymers with five-membered rings. Caution must be exercised in drawing inferences about the meaning of single-chain spectra by analogy with bulk data.

We report the detection of antigen capture by immobilized antibodies using a simple, label-free version of monochromatic reflective interferometry. The technique is implemented on silicon with its native oxide and relies on choosing an incident angle between the Brewster angles for the air/oxide and oxide/silicon interfaces. We demonstrate sensitivity to anti-human and anti-rabbit immunoglobulin (anti-IgG) concentrations less than 100 nM using only 10 nL droplets of the analyte. We have introduced a protocol using a model sugar to reduce nonspecific binding and have been able to detect anti-IgG even in the presence of 100-fold larger concentrations of bovine serum albumin. The limit of detection is not yet associated with the optical method but is imposed by nonspecific binding. Evaluated in terms of pg/mm2, our sensors are comparable in sensitivity to surface plasmon resonance (SPR) but are advantaged with respect to SPR in the tolerance of the optical components and alignment, the low material usage, and the ability to exploit multiplex detection without modification. The simplicity and convenience of the method are promising for eventual application to portable diagnostic applications.

Long lived stimulated emission is observed from conventional organic light-emitting diodes in extended cavities with onset at current densities of less than 1 mA/cm2. These current densities are more than six orders of magnitude lower than those previously reported for transient stimulated emission in an elaborate structure. The wavelengths at which stimulated emission occurs are determined by a balance between transition strength and population inversion. Because the latter is temperature-sensitive, the electroluminescence spectrum varies as the device progresses toward thermal equilibrium. We explain the spectral dynamics and achieve a good quantitative understanding of the onset current density. At the low operating currents used here, the ability to produce stimulated emission is not limited by absorptive losses, only by the Boltzmann population of vibrational states of the electronic ground state that serve as the lower levels for the emission

Molecules that undergo reverse intersystem-crossing (RISC), which enables thermally activated delayed fluorescence, represent an important advance in the development of organic-based light-emitting...

We study the delayed fluorescence following geminate recombination of photogenerated polaron pairs (PPs) in films of a model conjugated polymer F8BT. Doping with tiny gold nanoparticles also enables us to observe phosphorescence from triplet states that arise via intersystem crossing from the directly photoexcited singlet state. Small magnetic fields ∼5 mT have large effects on the delayed fluorescence by causing demixing of the hyperfine coupled singlet and triplet PP configurations, implying PP spin equilibration times much shorter than 1 μs. Nevertheless, the magnetic field effect on the fluorescence persists to tens of microseconds, and we argue that this apparent inconsistency implies that photogenerated polarons reside primarily as uncoupled spin half particles with long spin memory times. We consider the implications for whether recombination of charges in organic light-emitting diodes is likely to be governed by quantum spin statistics.

Phosphorescent white organic light emitting diodes (WOLEDs) with a multi-layer emissive structure comprising two separate blue layers and an ultra-thin red and green co-doped layer sandwiched in between have been studied. With proper host and dopant compositions and optimized layer thicknesses, high-performance WOLEDs having a power efficiency over 40 lm/W at 1000 cd/m 2 with a low efficiency roll-off have been produced. Through a systematic investigation of the exciton confinement and various pathways for energy transfer among the hosts and dopants, we have found that both the ultra-thin co-doped layer and two blue emitting layers play a vital role in achieving high device efficiency and controllable white emission.

Fluorescence quantum yields of conjugated polymer films are systematically lower than their counterparts in dilute solution. Films also exhibit a long "temporal tail" in their fluorescence decay dynamics not present in solution. We study the spectroscopy, excitation wavelength dependence, temperature dependence, and electric field quenching of the temporal tail of the photoluminescence in MEH-PPV on a nanosecond time scale to elucidate the relationship between those observations. We conclude that the tail represents emission from H-like aggregated regions in the polymer. Using a simple model of the photophysics, we estimate the formation yield of the aggregates responsible for the tail emission to be

Enhancement of solar photovoltaics by integrating cells with metal nanoparticles is of potential interest to reduce the usage of semiconductor material. Appropriately shaped metal particles to optimize spectral response have inadequate thermal stability to withstand standard semiconductor processing. We synthesize silica-capped gold nanorods that maintain nonspherical shapes to over 600 °C and show that they can increase photoconductivity in thin films of amorphous silicon by much more than a factor of 2 across the entire visible spectrum. We also report mechanistic studies of this phenomenon that show that much of this effect is primarily due to strong near-field light concentration rather than scattering as has often been assumed.

A blue phosphorescent emitter based on tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III), Ir(iprpmi)3, as the dopant and 3,3′-bis(N-carbazolyl)biphenyl, mCBP, as the host have been evaluated in OLED devices. By optimizing the dopant concentration and the materials for the electron and hole-transport layers, external quantum efficiencies greater than 20% have been achieved. Improved device lifetimes over those using the classic light-blue dopant FIrpic have also been achieved. These improvements can be attributed to the control of the electron-hole recombination and emission regions within the emitter layer as well as the choice of material for the transport layers.

By incorporating prospective materials for blue PHOLED emissive layers into model OLEDs, we have investigated how hole transport in a prototypical blue phosphorescent emitter, FIrpic (bis(4,6-difluorophenyl-pyridinato- N ,C2) picolinate Iridium) doped mCBP (4′-bis(3-methylcarbazol-9-yl)-2,2′-biphenyl), can impact PHOLED device operational stability. We found the host mCBP to be stable in supporting hole transport, but unstable with respect to electron–hole recombination. As a dopant, FIrpic was found to be unstable with respect to both hole transport and charge recombination processes. Our results indicate that FIrpic doped mCBP is unsuitable for use as an emissive layer in OLED devices and we provide a general strategy to screen materials and better understand their stability.

The spatial distribution of charge recombination in blue phosphorescent organic light-emitting diodes with linearly graded, step-graded and uniformly mixed host architectures was identified by selective doping of the emissive layer. Using TCTA and UGH-3 for hole and electron transporting hosts in the emitting layer, the recombination zones were found to be near the interface with electron transport layer due to the relatively high hole mobility of TCTA. For linearly graded host concentration profiles, however, the recombination region extends much further into the emissive layer. Expansion of the recombination region increases device efficiency by reducing quenching at the interface with the electron transport layer or by reducing triplet diffusion into the electron transport layer.

A blue PhOLED with a linearly-graded mixed host architecture is compared to a uniformly-mixed host device. We show that the recombination region is moved into the interior of the emissive layer and results in higher efficiency through avoiding quenching of the phosphor by species in the electron transporting layer.

The conjugated polymer MEH-PPV consists of two basic types of chromophores that we characterize as “isolated” and “aggregated.” Because the aggregated chromophores have lower emissive yields but are favored by energy migration, we show that preferential bleaching of the aggregated chromophores can result in substantially improved fluorescence quantum yield. We study a wide variety of sample types including solutions, dilute films, neat films, and core-shell particles and show that the degree to which luminescence improvements are possible depends on the organization of the chromophores.

Under certain conditions, single-stranded DNA adsorbs to negatively charged gold nanoparticles in a colloid whereas double-stranded DNA does not. We present evidence that this phenomenon can be explained by the difference in their electrostatic properties that in turn reflects conformational differences. The ability to discriminate the hybridization state of DNA on the basis of adsorption behavior can be utilized to design simple colorimetric and fluorimetric assays that take advantage of plasmon resonance in the gold nanoparticles. We present examples where we detect specific target sequences in oligonucleotides and in genomic DNA. Because conformational changes in special DNA sequences can also be induced by analytes such as potassium, we report a potassium ion detection scheme based on the same principle.

Two novel polynorbornenes 11 and 12 functionalized with electronically active conjugated oligomer units in the side chain were synthesized by the ring-opening metathesis polymerization (ROMP) method. Both polymers showed good optical characteristics, thermal stability, film-forming properties, and interesting electrochemical properties. The photophysical and redox behaviors of 11 and 12 are markedly different due to variation in the structure of the pendant oligomers. Polymer 11 with phenyl end-capped oligothiophene co-oligomer in the side chain showed much higher stability toward electrochemical oxidation than 12 with a sexithiophene in the side chain. This was demonstrated by in-situ study of the changes in absorption spectra of the polymer films while varying the potential in electrochemical experiments. During the p-doping process, polymer 11 exhibited highly reversible changes in its absorption peaks when monitored at 430 and 650 nm, and the p-doping/dedoping processes can be repeated many cycles. In...

A new class of semi-flexible dendrimers with oligothiophene (OT) arms up to the third generation have been synthesized and investigated. The synthetic methods employed include a combination of palladium-catalyzed Stille cross-coupling reactions for oligothiophenes, Sonogashira cross-coupling reactions for building blocks, and carbodiimide-mediated esterification for building up the various dendrimers. The optical and electrochemical properties of this series of oligothiophenes-based dendrimers are shown to be strongly influenced by their morphologies as demonstrated by their pronounced solvatochromic and thermochromic responses under different environmental conditions. Introducing rigid oligothiophene arms to shape non-persistent ester-linked dendrimers causes higher generation dendrimers (G2 and G3) to exhibit solvatochromism and thermochromism, while their oligomeric counterpart (3b) and lower generation (G1) analogue do not. Spectroscopic changes due to both intramolecular and intermolecular aggregations are observed.

We report surface-enhanced Raman scattering (SERS) from 4-mercaptopyridine adsorbed on nanotextured silver surfaces as the coverage of silver is varied. The degree of surface enhancement is strongly dependent on silver coverage and correlated to the extinction of the surface at the Raman excitation wavelength, that extinction being determined by multiparticle surface plasmon resonances. The coverage dependence of the Raman intensity is consistent with signals being dominated by molecules at junctions inside nanoparticle aggregates where electromagnetic energy is localized into “hot spots” by interactions of the incident and scattered fields with the surface plasmons. The Raman intensity drops precipitously near the conductivity percolation threshold because these hot spots are destroyed when conducting paths allow plasmons to propagate. Our approach to substrate preparation provides clean surfaces with average enhancements ≥107, an order of magnitude larger than typical for SERS.

The study of proteins and the molecules with which they interact on an organismwide scale is critical to understanding basic biology, and understanding and improving human health. New platform technologies allowing label-free, quantitative array-based analysis of proteins are particularly desirable. We have developed an analytical technology, reflective interferometry (RI), which provides specific, rapid, and label-free optical detection of biomolecules in complex mixtures. In order to evaluate the suitability of RI for proteomics, we have prepared a series of arrays bearing the extracellular domain of the secreted enteropathogenic Escherichia coli (EPEC) protein Translocated Intimin Receptor (Tir). These arrays are able to selectively detect the extracellular domain of the protein Intimin, Tir's natural binding partner. Furthermore, we demonstrate the use of RI and Tir-functionalized arrays for the selective detection of EPEC directly from culture.

We have used the disparity in adsorption rates for single- and double-stranded RNA on ionically coated gold nanoparticles suspended in a colloid to design a rapid sequence identification assay. Unlabeled target RNA and a probe sequence are mixed prior to exposure to the gold nanoparticles to enable efficient hybridization. We have designed assays based on either color changes or fluorescence that are sensitive to a few picomoles of target. Single-base mutations on RNA sequences can be detected even in complex oligonucleotide mixtures. The assay requires less than 10 min so that RNA degradation problems are avoided.

We observe more than a 200-fold increase in the photoexcited phosphorescent emission of PtOEP (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum II) in a polystyrene film on nanotextured silver surfaces, coincident with a reduction in the triplet state lifetime by a factor of 5. The large enhancement results in films with apparent luminescence quantum yields much greater than unity and can be understood in terms of increased radiative rates due to interactions between the molecules and the electron plasma in nearby silver nanoparticles. We study the photoluminescence efficiency, excitation spectrum, and decay dynamics as a function of film thickness and silver density. We use a model of the photophysics to decompose the phosphorescent enhancement into contributions from increases in absorption, emissive rate, and quenching. Quenching increases in importance for very thin films, and we conclude that approximately 3 nm spacing between metal and chromophore leads to the largest photoluminescence enhancement.

We observe spectral and intensity fluctuations in Raman scattering from single molecules of 4-mercaptopyridine (4-Mpy) adsorbed on textured silver surfaces. We present evidence that the root cause of these fluctuations is thermal. Analysis of the spectra shows that in some cases the spectral changes are associated with molecular reorientation and in others with chemical reactions of the 4-Mpy. Analogous surfaces fully covered with 4-Mpy exhibit similar behavior, indicating that monolayer Raman spectra are dominated by a few molecules at most.

We report sensitive label-free detection of DNA oligonucleotide sequences using ac impedance measurements. The surface attachment chemistry is critical, and using mixed self-assembled monolayers on a gold electrode results in much better performance than homogeneous self-assembled monolayers. Contrary to expectations, binding of the target sequence reduces rather than increases the charge transfer resistance. Similar behavior is observed on indium tin oxide electrodes, and we ascribe it to the hydrophilicity and rigidity of the DNA duplex that cause it to reside further from the electrode surface and facilitate the approach of negatively charged redox moieties to the interface.

Simple, fast, economical, and sensitive detection of specific DNA sequences is crucial to pathogen detection and biomedical research. We have designed a novel fluorescent assay for DNA hybridization based on the electrostatic properties of DNA. We exploit the ability to create conditions where single-stranded DNA adsorbs on negatively charged gold nanoparticles while double-stranded DNA does not. Dye-tagged probe sequences have their fluorescence efficiently quenched when they are mixed with gold nanoparticles unless they hybridize with components of the analyte. Subfemtomole amounts of untagged target are detected in minutes using commercially available materials. Target sequences in complex mixtures of DNA and single-base mismatches in DNA sequences are easily detected.

We document the surprising result that single-stranded DNA adsorbs on negatively charged gold nanoparticles (Au-nps) with a rate that depends on sequence length and temperature. After ss-DNA adsorbs on Au-nps, we find that the particles are stabilized against salt-induced aggregation. These observations can be rationalized on the basis of electrostatics and form the basis for a colorimetric assay to identify specific sequences and single nucleotide polymorphisms on polymerase chain reaction (PCR)-amplified DNA. The assay is label-free, requires no covalent modification of the DNA or Au-np surfaces, and takes on the sensitivity of PCR. Most important, binding of target and probe takes place in solution where hybridization occurs in less than 1 min. As an example, we test PCR-amplified genomic DNA from clinical samples for single nucleotide polymorphisms (SNPs) associated with a fatal arrhythmia known as long QT syndrome.

We use charge modulation spectroscopy (CMS) in model holes-only devices to address the reason for decreasing quantum efficiency with increasing drive current in organic light-emitting diodes based on Alq3 doped with the red dye DCJTB. We are able to measure the density of DCJTB+ and correlate it with the diminution of the photoluminescence. The magnitude and DCJTB+ density dependence of the photoluminescence reduction are roughly consistent with quenching of the singlet-excited-state of DCJTB via Forster energy transfer to DCJTB+. At low DCJTB+ densities, the quenching is stronger than theory would predict and we propose that singlet energy transport among DCJTB molecules extends the distance over which quenching by DCJTB+ is effective. Since the Forster mechanism can be responsible for charge-induced PL quenching as large as 70% even though DCJTB+ is very weakly absorbing where DCJTB singlets fluoresce, it may be difficult to avoid quenching by electrically generated species.

New chip-based methods for the detection of unmodified biomolecular targets have significant potential as enabling technology in fundamental biology and biomedical analysis. We report a method based on changes in reflectivity from specially fabricated substrates that is capable of detecting the binding of as little as an average of 0.2 nm (i.e., a fraction of a monolayer) of biomolecules. We demonstrate the method on detection of femtomole quantities of untagged oligonucleotides in an array format, showing that the amount of target bound can be determined quantitatively. The simplicity of the approach promises to make it broadly applicable for any biomolecule for which suitable molecular recognition chemistry is available.

In conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field-assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field-independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.

We present fluorescence spectra, quantum yields and structural data from 1 H NMR studies on solutions of a di-alkoxy substituted pentamer of p -phenylenevinylene (5PV) and the corresponding polymer poly-( p -phenylenevinylene) (PPV). We vary the solvent quality continuously using miscible good and poor solvents to study the effects of polymer aggregation as is relevant to properties of films and of the concentrated solutions from which films are formed. We observe a large drop in quantum yield with reduced solvent quality that we attribute to formation of interchain excitations with negligible luminescence when the polymer aggregates. Concomitant with the drop in fluorescence, we observe a red shift in the spectra. The NMR data suggest the red shift is due to an increase in the steric hindrance of the backbone motion that leads to an increase in effective conjugation length. In each material, our data can be described as a linear combination of only two species with distinct spectroscopy and quantum yield, an isolated and aggregated form of the polymer. Studies of the properties of mixtures of the pentamer and polymer demonstrate conclusively that these species mix and that the spectroscopic changes are due to aggregation and not conformational changes of single chains. They also provide insight into recent results from electroluminescent devices based on analogous conjugated polymer blends.

We report transient photoluminescence (PL) measurements in a conjugated polymer over more than six decades in time and intensity. Long-lived luminescence by the singlet exciton arising from charge photogeneration and subsequent recombination represents a significant fraction of the total emission. Comparison between neat films and dilute films in polystyrene illustrates the importance of interchain charge separation in the photophysics of these materials. The data permit quantitative estimates of polaron pair and excimer formation.

We find that giant surface-enhanced Raman scattering for adsorbates on silver surfaces is present only on surfaces that exhibit self-similar fractal topology as inferred from atomic force microscopy. The fractal character results in localizing the energy of incident photons to volumes of a few nanometers on a side, millions of times smaller than the diffraction limit. Consistent with this finding, we have found an enhancement in spontaneous Raman cross section of >13 orders of magnitude for adsorbates on silver surfaces demonstrated to be fractal. The location of “hot spots” on the fractal surfaces is found to be hypersensitive to incident wavelength and polarization even though the observed Raman scattering is strictly linear in incident intensity. These observations are consistent with localization of the photon energy facilitated by the disordered nature of fractal organization through interference between the incident wave and scattered radiation from silver nanoparticle surface plasmons. We also present a surface preparation method that consistently produces fractal topologies that support single-molecule Raman scattering.

We have demonstrated the feasibility of sensing adsorption of untagged molecular analytes in an electrochemically fabricated porous aluminum oxide template with regular nanometer dimension pores by monitoring reflectivity spectra. Combining this with chemistry to functionalize Al2O3 with DNA oligonucleotides, we are able to sense 2 nmol/cm2 of complementary DNA as a 3.4 nm shift in a spectral minimum. We have modeled the change in reflection spectrum versus uptake of analytes to quantify binding. This technique can achieve subpicomole sensitivity in straightforward fashion with 0.1 mm probe spots.

This paper reports the effects of polydispersity in conjugation length on polymer light-emitting diode (PLED) device performance. For this study a di-alkoxy substituted pentamer of p-phenylenevinylene (5PV) was blended with a small amount of a lower band gap poly(p-phenylenevinylene) (PPV) or a small amount of a di-alkoxy substituted p-phenylenevinylene type nonamer (9PV). We also fractionated two different di-alkoxy substituted oxadiazole-PPVs and blended the high molecular weight, lower band gap fraction, into a matrix of the lower molecular weight, higher band gap, polymer. It was found that incorporation of a small amount of the low band gap material significantly degraded PLED device performance. To further test the significance of polydispersity we synthesized a low molecular weight narrow polydispersity PPV. Devices made with this PPV were about 2 orders of magnitude better in LED external quantum efficiency when compared to a high molecular weight, higher polydispersity PPV of identical structure....

We study the molecular weight dependence of the photophysics of a soluble electroluminescent conjugated polymer DOO-PPV in dilute chloroform solution. The absorption and the fluorescence maxima red-shift progressively with increasing weight as the average chain length increases from approximately 9 to 30 monomer units. At the same time, the fluorescence yield of the polymer decreases even though the radiative rate is found to increase. This behavior is caused by a rapid increase in nonradiative decay as the optical gap of the polymer decreases, and we present a quantitative theory that satisfactorily accounts for the dominance of multiphonon relaxation via internal conversion. We also find that the polymer length at which the optical properties saturate is considerably longer than predicted by theoretical calculations of electronic coherence length or by modeling of absorption data using the Strickler-Berg relationship.

We report on measurements of electric field induced fluorescence quenching Φ and optical charge generation η by using the time-delayed-collection-field technique (TDCF). The material under investigation is the methyl-substituted ladder-type poly(para-phenylene) (MeLPPP), a π-conjugated polymer characterized by low intrachain disorder. Fluorescence quenching Φ of up to 50% is observed. The field dependence of both Φ and η is associated with the primary step in charge generation, i.e., exciton dissociation into geminate electron–hole pairs rather than their full dissociation into free charge carriers. This is at variance with the basic assumption in Onsager's theory of charge generation.

A series of spiro-linked oligofluorenes were synthesized that exhibit blue emission with photoluminescence quantum yields in some cases exceeding 0.50 in neat spin-coated films. Differential scanning calorimetry revealed highly variable glass transition temperatures. A longer conjugation length is desired for better stability against thermally activated crystallization, and a shorter pendant is desired for a higher glass transition temperature at a given conjugation length. In contrast to poly(2,7-(9,9-di-n-octylfluorene)), prolonged heating of spiro-oligo(fluorene) films resulted in no changes in morphology, emissive color, and photoluminescence quantum yield. The propensity of spiro-oligo(fluorene)s to thermally activated crystallization was rationalized with molecular shapes predicted by molecular mechanics simulations.

A series of fully spiro-configured terfluorenes were synthesized and characterized to be amorphous materials showing a Tg of 296 °C with dendritic aromatic pendants, the highest value ever reported for organic molecular systems. X-ray single-crystal analysis of key structural moieties indicated nearly perpendicular orientations at all spiro-links. The spectroscopic comparison of solid film and dilute solution suggested absence of aggregates in the ground state and no increase in torsion angle with an increasing volume of the aromatic pendant. All these conformational characteristics are consistent with molecular mechanics simulation. Thermal annealing in argon caused a decrease in photoluminescence quantum yield with phenyl pendants but an increase with dendritic pendants. These can be understood in terms of local morphological reorganization of molecules to enhance or inhibit interactions between the backbone chromophores.

We measure transient absorption, gain and bleaching of a model conjugated polymer in solution where aggregation of the polymer has been deliberately controlled. Evidence is presented that photoexcitation of aggregated polymer chains leads to creation of interchain non-emissive polaron pairs in large quantum yield. This is shown to be consistent with film behavior and accounts for low luminescence quantum yields in the solid state. It also explains the recent reports of rapidly formed species with characteristic infrared active vibrations. Our data support a picture where polaron pairs mostly recombine to form the ground state directly, in stark contrast to their presumed recombination mechanism in electroluminescent devices where they have been postulated to form only excited singlet and triplet states.

We report the design and testing of a versatile biosensor exclusively using silicon. The device structure consists of a microcavity resonator made of various porous silicon layers. Porous silicon contains silicon nanocrystals that can luminesce efficiently in the visible, depending on the size and passivation conditions. When a luminescent porous silicon layer is inserted between two Bragg reflectors (also made of porous silicon), the broad luminescence band is altered and multiple and very narrow peaks are detected. The position of these peaks is extremely sensitive to a small change in refractive index, such as that obtained when a biological object is attached to the large internal surface of porous silicon. We have demonstrated a DNA sensor that displays appropriate sensitivity, selectivity and response speed. The device fabrication procedure and the results of extensive testing are presented. An extension of the DNA biosensor has been made to include the detection of viral DNA. This work will lead to the development of an all-silicon sensor array for the detection of biomacromolecules.

Nanosecond electroluminescence (EL) spikes observed at the voltage turn-off when multilayer blue DPVBi-based organic light-emitting devices (OLEDs) are excited by rectangular voltage pulses are described. The spikes exceed the cw brightness by up to an order of magnitude. Time-resolved images of the devices demonstrate that the emission from most of the sample surface decays with a single time constant τ 1 = 13 ± 3 ns. This decay is attributed to recombination of charges, which accumulate at the interface of the electron and hole transporting layers, possibly at intrinsic trapping sites. In areas of increased electron injection and EL such as cathode edges and morphological defects, a second slower decay time 20 ns < τ 2 < 1?s is observed, apparently due to release of carriers from localized trap states in the organic/cathode interface. Only marginal variations in τ 1 are found between bright and dim areas of the devices. At a bias of 10 V, the amplitude of the spike is found to peak at a pulse duration of ∼ 20 μs.

We report the effects of solvent-induced aggregation on photoluminescence spectra, decay dynamics and quantum yield in trimers of MEH-PPV and compare these to analogous experiments on the high molecular weight polymer. Nuclear magnetic resonance spectra imply that torsional motion is quenched by aggregation leading to a conjugation length increase that explains red shifts in the photoluminescence. We ascribe reduction of fluorescence quantum yield to interchain effects and observe anomalously long-lived luminescence ascribed to geminate recombination of polaron pairs. The decay dynamics suggest that, unlike the polymer case, we are able to time resolve the formation of polaron pairs in the aggregated trimer.

We report transient and steady state photoluminescence results along with absorption and NMR data to support the existence of two distinct morphological species in MEH−PPV solutions. NMR data provide evidence for the close packing of polymer chains, a consequence of solvent quality reduction. These data are correlated with optical properties of the aggregated species in poor solvents and the isolated chains in good solvents. We infer that steric hindrance of backbone motions increases effective conjugation length and leads to a spectral red shift in absorption and emission. At the same time, interchain excitations with negligible luminescence can be formed, leading to a dramatic reduction in photoluminescence quantum yield. While spectral changes are observed as packing is induced, we show that interchain state formation and its subsequent back-transfer to excitons are particularly sensitive to the interchain registry of the highly packed chains.

A mixture consisting of a nematic acrylate and a chiral diacrylate lightly doped with Exalite 428, a laser dye, was prepared into thin films for in situ photopolymerization. The laser dye served to regulate the photocuring intensity profile into the film and as a light emitter. With optimized chemical composition, photocuring intensity, and temperature, a gradient-pitch film with a broad resonance band was produced to cover the emission peak of the laser dye. The SEM image provided new insight into the distribution of pitch length across the film thickness, consistent with the selective reflection spectra. Upon UV excitation, the observed sense and degree of circularly polarized photoluminescence were qualitatively interpreted on the basis of the photoexcitation intensity received by the dopant molecules and by the wavelength-dependent interaction of emitted light with an asymmetric structure.

We report subnanosecond photoluminescence (PL) decay dynamics of dendron decorated poly(phenylene vinylenes) (PPVs) in solutions and as neat films. Previous PL studies revealed that two conformations of the polymer, with distinct luminescence quantum yields and PL, are responsible for the photophysics of these polymers. These conformations are characteristic of isolated and aggregated chains. From the decay dynamics, energy transfer between the two species is evident and repopulation of exciton states occurs in aggregated regions of the sample.

Porous silicon microcavity resonators possess the unique characteristics of line narrowing and luminescence enhancement. The emission peak position is completely tunable by changing the properties of the central layer. Increasing the thickness of the central active layer introduces multiple narrow peaks in the luminescence spectrum. Narrow and visible luminescence peaks are observed with a full width at half the maximum (FWHM) value of 3 nm. The microcavity structure is highly sensitive and any slight change in the effective optical thickness modifies the reflectivity spectra, causing a spectral shift in the interference peaks. This makes porous silicon microcavities an ideal host for sensor applications. We demonstrate the usefulness of this microcavity resonator structure as a biosensor. A DNA biosensor has been fabricated based on a porous silicon multiple peak microcavity structure. Shifts in the luminescence spectra are observed and detected for various DNA concentrations. When exposed to a non-complementary DNA strand, no shifts are observed. An extension of the DNA biosensor has been made to include the detection of viral DNA. Both sensor fabrication and results of extensive testing are presented in this report.

We compare the photoluminescence (PL) of poly-p-phenyl-enevinylene (PPV) to that of one of its soluble derivatives, poly (2-methoxy, 5-(21-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV). We show that excimers due to π-orbital overlap of adjacent polymer backbones are formed in MEH-PPV in high quantum yield. They do not emit light at room temperature, potentially explaining the reduction of PL yield in MEH-PPV films relative to solution. The herringbone packing of polymer chains in PPV prevents π-π stacking, but low PL yields are observed anyway. We conclude that polymer design to inhibit interchain registration is a questionable strategy to increase luminescence yields. The low PL yield in PPV is ascribed to bound polaron pair formation and preliminary data suggest that these may also form upon photoexcitation of MEH-PPV films.

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.

The influence of the indium tin oxide (ITO) electrode surface acido basicity on organic electroluminescent device characteristics is studied. As measured by photoelectron spectroscopy, acid and base treatments produce large work function shifts of the oxide compared to standard ITO treated by an oxygen plasma or water. The current onsets for triphenyldiamine (TPD) single layer diodes sandwiched between ITO and a silver electrode are in qualitative agreement with the work function of the hole injecting oxide electrode. However, saturated photovoltage measurements on single layer diodes built on acid and water treated ITO disagree with the work function obtained from photoelectron experiments. This is explained by protonation of the diamine layer close to the electrode surface producing a charged double layer. It suggests that even acidic surface hydroxyl groups of the oxide electrode are able to protonate the diamine layer, explaining the importance of plasma cleaning to remove surface hydroxyls. The same ...