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.
We study the photoluminescence of Alq3 on nanotextured silver surfaces to investigate the feasibility of using plasmon
enhancement of radiative rates to improve the performance of organic light-emitting diodes. We observe more than 20-fold increases in photoluminescence and separate the contributions of absorption and emissive efficiency increases. With
optimized silver coverage, the fraction of excited states created that fluorescence can be made nearly 100 %, an increase
of more than 3 times relative to thin films without silver.
The effects of thin silver films embedded in a tandem pentacene/C60 photovoltaic cell are investigated. A 2 nm Ag film
improves the device's power efficiency under white light illumination from 0.32% to 1.11% by almost doubling its open
circuit voltage and enhancing its short circuit current density. The doubled open circuit voltage is due to the formation of
two separated photovoltaic pentacene/C60 cells in series where discontinuous silver clusters provide carrier
recombination centers. The increased photocurrent density is partly ascribed to improved charge separation and transport
associated with the silver layer. In addition, wavelength dependent measurements suggest that plasmon-enhanced light
absorption by pentacene due to surface plasmon resonance of silver nanoparticles contributes as much as a factor of 4 to
the power efficiency near the plasmon resonance around 450 nm.
Plasmon-enhanced vibrational Raman spectroscopy enables us to learn about the conformations of single molecules.
Here, we report surface-enhanced Raman studies of single chains of the widely studied model conjugated polymer, MEH-PPV. We will provide evidence that field localization on nanotextured silver surfaces is so strong that,
remarkably, scattering from a single chromophore out of hundreds dominates the Raman spectrum. We observe wide
variation of spectra from chain to chain and strong temporal fluctuations in the Raman spectrum and intensity for
individual chains of MEH-PPV. In some cases, we are able to clearly associate these fluctuations with
photochemistry and polaron formation. In most cases, however, we believe that local conformational changes in the
polymer are responsible for the fluctuations.
Up to fifty fold increases in water soluble conjugated phenylenevinylene polymer fluorescence are observed when these polymers are adsorbed onto silver nanoparticle treated surfaces with layer-by-layer deposited polyelectrolyte spacers. The silver particle density and spacer thickness dependence of the enhancement are investigated. Using absorption, fluorescence, fluorescence excitation and transient photoluminescence measurments, we infer the relative importance of absorption and emissive rate increases in explaining the observed enhancement. Large blue shifts due to interactions of the molecular excited states with the silver particle plasmons are observed.
Nanosecond electroluminescence (EL) spikes observed at the voltage turn-off when multilayer blue 4,4'-bis(2,2'diphenyl vinyl) -1,1'-biphenyl (DPVBi)-based organic light-emitting devices (OLED's) 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 (tau) 1 equals 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 < (tau) 2 < 1 microsecond is observed, apparently due to release of carriers from localized trap states in the organic/cathode interface. Only marginal variations in (tau) 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 approximately 20 microseconds. It is noted that similar OLED's, in which the DPVBi was replaced by tris-(8- hydroxyquinoline Al) (Alq3), did not exhibit such spikes.
Porous silicon nanostructures are ideal hosts for sensor applications because of their large internal surface area, which implies strong adsorbate effects. The average pore size can be easily adjusted to accommodate either small or large molecular species. When porous silicon is fabricated into a structure consisting of two high reflectivity multilayer mirrors separated by an active layer, a microcavity is formed. Multiple narrow and visible luminescence peaks are observed with a full width at half the maximum value of 3 nm. These multiple peak microcavity resonators are very sensitive structures. Any slight change in the effective optical thickness induces a change in the reflectivity spectra, causing a shift in the interference peaks. We demonstrate the usefulness of this microcavity resonator structure as a biosensor. Biosensors are devices that exploit the powerful recognition capability of bioreceptors. We have fabricated a DNA biosensor based on a porous silicon multiple peak microcavity structure. An initial strand of DNA is first immobilized in a porous silicon substrate and then subsequently exposed to its complementary DNA strand. Shifts in the luminescence spectra are observed and detected for DNA concentrations less than 1 (mu) M. When exposed to a non- complementary DNA strand, no shifts are observed. A detailed study on the selectivity and sensitivity issues of porous silicon microcavity biosensors is presented.
Indium tin oxide (ITO) is commonly used as a hole-injecting electrode in organic electroluminescent devices due to its transparency and good electrical conductivity. Various solutions of acid and base have been used to modify the work function of ITO. Depending on the solution, a negative or positive work function shift with respect to that of the standard ITO has been obtained. We have investigated the interface formation between treated ITO and N,N'-bis-(1- naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB), an organic materials often used as hole transport layer in OLEDs, using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The barrier for hole injection, which is defined as the energy difference between the Fermi level and the highest occupied molecular orbital of NPB, decreases from base-, to standard, to acid-treated ITO. No significant reactions is observed for NPB deposited on standard ITO. For the acid- treated ITO substrate, the XPS results point to the reaction between the proton of the adsorbed acid layer and the NPB nitrogen. XPS results also suggest that at low NPB coverage, islanding is occurred at the standard ITO surface.
We studied photoconductivity in particle dispersions and vacuum evaporated films of benzimidazole perylene (BZP) pigment in contact with a tetraphenyldiamine (TPD) hole transport layer. Xerographic photoreceptors based on evaporated BZP photogenerator layers showed about two times higher carried generation efficiency than photoreceptors containing generator layer in the form of a particle dispersion. Time resolved fluorescence measurements were performed on these two types of structures. Fluorescence time evolution was compared in samples with and without TPD in a polycarbonate overcoat layer. Significantly larger decrease in average fluorescence lifetime was observed in structures containing thin evaporated films compared to the structures with the pigment in the form of particle dispersion. This indicates that reduced carrier generation efficiency in particle dispersion sin a consequence od exciton bulk trapping which significantly reduces the number of excitons reaching BZP/TPD interface. As carrier generation in BZP/TPD system is a result of exciton dissociation at the interface to produce electron-hole pairs. Thus a smaller number of excitons reaching the interface leads to reduction of carrier generation efficiency.
We report microcavity efficiency enhancement of organic electroluminescent devices based on the hole transporter bis(triphenyl)diamine and the electron transporter and light emitter tris(8-hydroxyquinoline)aluminum. Microcavity organic light emitting diodes are described which emit 4 times the light measured in the forward direction, or almost twice the total light of a non-cavity organic light-emitting diode for identical electrical drive conditions.
Transient photoluminescence, photoluminescence excitation and picosecond photoinduced absorption studies on stretch oriented phenylenevinylene polymer films are presented. A coherent picture of the processes occurring after light absorption emerges which can account for the data. The implications of this picture for photoconductivity and electroluminescence are considered.
We report the femtosecond laser studies of exciton decay kinetics in stretch oriented poly(paraphenylenevinylene) (PPV) films. Stimulated emission of excitons and stimulated Raman gain have been observed after 500 nm photoexcitation with 100 fs laser pulses. These are not observed when higher energy photons are used to excite the polymer.
Picosecond vibrational spectroscopy is used to address a model problem in protein dynamics, that of how carbon
monoxide and dioxygen reach the functional group in myoglobin. The spectra indicate that there is a metastable binding
position for CO at 300 K with 60 ns lifetime. We use polarization dependent measurements and molecular dynamics software
to assign the ligand's metastable location to a particular pocket in the protein. This location does not coincide with that which
is determined by freezing out intermediates at low temperatures. The implications for the entry and exit trajectories of ligands
in myoglobin are considered.
Vibrational energy relaxation of the symmetric C-H stretching mode of methyl thiolate on a Ag(l 1 1) surface is measured by picosecond infrared-visible sum frequency generation. Vibrational relaxation lifetime components of ~3 PS and 63 PS are observed at 300 K. The long lifetime component shows a moderate temperature dependence. Both population relaxation components are assigned to intramolecular energy transfer on the basis of comparisons with other measurements and the predicted temperature dependence of intramolecular relaxation rates. Direct energy transfer to electronic or vibrational degrees of freedom in the substrate is not found to be important for this vibrational mode.
We use nanosecond and picosecond optical spectroscopic techniques
to isolate and study the photodynamics of the single particle spectrum of
35A CdSe nanoclusters exhibiting three-dimensional quantum confinement.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.