Alexander Mikhailovsky

Efficient and Color-Tunable Oxyfluoride Solid Solution Phosphors for Solid-State White Lighting

A solid solution strategy helps increase the efficiency of Ce{sup 3+} oxyfluoride phosphors for solid-state white lighting. The use of a phosphor-capping architecture provides additional light extraction. The accompanying image displays electroluminescence spectra from a 434-nm InGaN LED phosphor that has been capped with the oxyfluoride phosphor.

Scalable Routes to Gold Nanoshells with Tunable Sizes and Response to Near‐Infrared Pulsed‐Laser Irradiation

A simplified synthesis of hollow gold nanoshells 20-50 nm in diameter via the well-established templated galvanic replacement reaction of silver for gold is presented. The surface plasmon resonance absorbance of the nanoshells is tuned using basic colloid chemistry to control the size of the silver templates. The gold nanoshells have an aqueous core and are varied in size and shell thickness depending on the silver/gold reagent ratios. The template replacement chemistry is rapid, highly scalable, uses minimal amounts of toxic reagents, and in many cases is a true one-pot synthesis. The smallest nanoshells (20-nm diameter, 7-nm wall thickness) reach the highest temperature on irradiation with femtosecond light pulses in the near infrared and anneal to form spherical nanoparticles fastest, even though their plasmon resonance does not overlap as well as the larger nanoshells (50-nm diameter, 7-nm wall thickness) with 800-nm wavelength excitation.

Molecular doping enhances photoconductivity in polymer bulk heterojunction solar cells

Addition of low concentrations (<1:100, dopant:donor) of a fluorinated p-type dopant, F4-TCNQ leads to a considerable enhancement of the photocurrent in PCDTBT:PC70 BM bulk heterojunction solar cells. As a result, the power conversion efficiency increases from 6.41% to 7.94 %.

Nitric oxide releasing materials triggered by near-infrared excitation through tissue filters

Novel materials for the phototherapeutic release of the bioregulator nitric oxide (nitrogen monoxide) are described. Also reported is a method for scanning these materials with a focused NIR beam to induce photouncaging while minimizing damage from local heating. The new materials consist of poly(dimethylsiloxane) composites with near-infrared-to-visible upconverting nanoparticles (UCNPs) that are cast into a biocompatible polymer disk (PD). These PDs are then impregnated with the photochemical nitric oxide precursor Roussins black salt (RBS) to give UCNP_RBS_PD devices that generate NO when irradiated with 980 nm light. When the UCNP_RBS_PD composites were irradiated with NIR light through filters composed of porcine tissue, physiologically relevant NO concentrations were released, thus demonstrating the potential of such devices for minimally invasive phototherapeutic applications.

Conjugated polyelectrolyte-metal nanoparticle platforms for optically amplified DNA detection.

2010 WILEY-VCH Verlag Gm -Ag NP platform. The chemical structures of PFBT, PDDA and PSS, and the he silver NP array on the glass slide. A variety of materials have been applied as platforms that increase the signal intensities of fluorescent detection techniques. The goals of this integration are to increase sensitivity and to achieve lower detection limits, as compared to those obtained with conventional fluorescent dye reporters. One class of materials used for such optical amplification is light-harvesting conjugated polyelectrolytes (CPEs). These macromolecules contain p-delocalized backbones and pendant ionic functionalities. The conjugated electronic structure leads to large optical cross-sections and allows efficient fluorescence resonance energy transfer (FRET) between optical segments and to signaling dyes, while the charged groups are useful for triggering FRET events after a specific recognition event. Such features have led to the successful use of CPEs in either homogeneous or heterogeneous detection formats that are responsive to a variety of targets including DNA, RNA, proteins, and other small molecules. A different approach to increase the signal intensity of dye-based assays involves metal-enhanced fluorescence (MEF). MEF is the result of interactions between fluorophores and surface plasmons in metallic nanostructures, most typically of Ag and Au. Enhanced emission can be obtained as a result of an amplification of the incident electric field, which effectively increases the fluorophore’s absorption cross-section, or by an acceleration of the radiative decay rate. These desirable effects need to counterbalance possible emission suppression by energy transfer to the metallic surfaces. Nevertheless, differences in spatial and orientational dependencies between amplification and quenching mechanisms allow one to find appropriate emitter-surface environments and organizations where improved performance is obtained. Examples include MEF-based assays for DNA, RNA, and immunological applications. Herein, we report a multi-component sensing platform that allows DNA detection on a CPE surface that also permits MEF fine-tuning. The manuscript is organized as follows. We first demonstrate the enhancement of CPE emission when deposited atop a polyelectrolyte multilayer thin film built on substrates containing Ag nanoparticles (NPs). Further, the two nearly equal absorption bands of the selected CPE allow us to establish that the influence of the NPs is to increase the incident field. Finally, we take advantage of a neutral peptide nucleic acid (PNA) probe and the charged nature of the sensing surface to demonstrate a simple to use single stranded DNA (ssDNA) sensing scheme. Scheme 1 shows the MEF platform design and the molecular structures of the polymeric components. Poly[9,9-bis(9-N,N,Ntrimethylammoniumethoxyethoxyethyl)fluorene-alt-4,7-(2,1,3benzothiadiazole) dibromide] (PFBT) was selected as the CPE for this study. Substrate preparation starts with the immobilization of negatively charged Ag NPs onto NH3 þ-functionalized glass slides. A polyelectrolyte layer-by-layer (LbL) process using poly(diallyldimethyl ammonium chloride) (PDDA) and poly(sodium 4-styrenesulfonate) (PSS) is then used to modify the distance between the Ag NPs and the PFBT layer. In a final step, a thin layer of PDDA is adsorbed to increase the positive charge density on the top surface. The Ag NPs were synthesized by using a modified Lee–Meisel method. After purification by centrifugation and filtration, the

A green-yellow emitting oxyfluoride solid solution phosphor Sr2Ba(AlO4F)1−x(SiO5)x:Ce3+ for thermally stable, high color rendition solid state white lighting

A near-UV excited, oxyfluoride phosphor solid solution Sr1.975Ce0.025Ba(AlO4F)1−x(SiO5)x has been developed for solid state white lighting applications. An examination of the host lattice, and the local structure around the Ce3+ activator ions through a combination of density functional theory, synchrotron X-ray and neutron powder diffraction and total scattering, and electron paramagnetic resonance, points to how chemical substitutions play a crucial role in tuning the optical properties of the phosphor. The maximum emission wavelength can be tuned from green (λem = 523 nm) to yellow (λem = 552 nm) by tuning the composition, x. Photoluminescent quantum yield is determined to be 70 ± 5% for some of the examples in the series. Excellent thermal properties were found for the x = 0.5 sample, with the photoluminescence intensity at 160 °C only decreased to 82% of its room temperature value. Phosphor-converted LED devices fabricated using an InGaN LED (λmax = 400 nm) exhibit high color rendering white light with Ra = 70 and a correlated color temperature near 7000 K. The value of Ra could be raised to 90 by the addition of a red component, and the correlated color temperature lowered to near 4000 K.

Solvent Effects on the Architecture and Performance of Polymer White‐Light‐Emitting Diodes with Conjugated Oligoelectrolyte Electron‐Transport Layers

Multilayered structures improve the function of organic optoelectronic devices. For example, operational voltages can be substantially decreased, and the light output to current ratio increased, upon insertion of electron-injection/transport layers (ETLs) or hole-injection/transport layers at the cathode and anode interfaces, respectively. These layers can reduce the barriers to charge injection from the electrodes into the organic semiconductor, and improve efficiencies via mechanisms that have been summarized in the literature. One practical challenge concerns methods for fabricating multilayered systems via procedures that are efficient and that provide for control of activelayer thicknesses and organic/organic interfaces widths. The fabrication of small-molecule-based devices benefits from the ability of vacuum depositionmethods to sequentially depositmaterials, with excellent precision over the individual layer thickness and without large disruptions of underlying organic coatings. The situation with conjugated polymer-based devices, such as polymer light-emitting diodes (PLEDs), is different. PLEDs offer the possibility of solution-based fabrication options and of tailoring blends that incorporate multiple components in a single layer. However, multilayer fabrication is challenging if one wishes to deposit layers atop of each other when the materials have similar solubility characteristics. Under these circumstances, one observes poorly defined interfaces and a disruption of the underlying polymer layers. It is in the context of PLED preparation that conjugated polyelectrolytes can offer new options to build well-defined multilayered structures. These materials are described as presenting a backbone with an electronically delocalized structure, that is, the semiconducting component, and pendant groups bearing ionic functionalities. Such structural attributes lead to higher solubility

An Efficient, Thermally Stable Cerium-Based Silicate Phosphor for Solid State White Lighting

A novel cerium-substituted, barium yttrium silicate has been identified as an efficient blue-green phosphor for application in solid state lighting. Ba9Y2Si6O24:Ce(3+) was prepared and structurally characterized using synchrotron X-ray powder diffraction. The photoluminescent characterization identified a major peak at 394 nm in the excitation spectrum, making this material viable for near-UV LED excitation. An efficient emission, with a quantum yield of ≈60%, covers a broad portion (430-675 nm) of the visible spectrum, leading to the blue-green color. Concentration quenching occurs when the Ce(3+) content exceeds ≈3 mol %, whereas high temperature photoluminescent measurements show a 25% drop from the room temperature efficiency at 500 K. The emission of this compound can be red-shifted via the solid solution Ba9(Y(1-y)Sc(y))(1.94)Ce(0.06)Si6O24 (y = 0.1, 0.2), allowing for tunable color properties when device integration is considered.

Average and Local Structural Origins of the Optical Properties of the Nitride Phosphor La3–xCexSi6N11 (0 < x ≤ 3)

Structural intricacies of the orange-red nitride phosphor system La(3-x)Ce(x)Si6N11 (0 < x ≤ 3) have been elucidated using a combination of state-of-the art tools, in order to understand the origins of the exceptional optical properties of this important solid-state lighting material. In addition, the optical properties of the end-member (x = 3) compound, Ce3Si6N11, are described for the first time. A combination of synchrotron powder X-ray diffraction and neutron scattering is employed to establish site preferences and the rigid nature of the structure, which is characterized by a high Debye temperature. The high Debye temperature is also corroborated from ab initio electronic structure calculations. Solid-state (29)Si nuclear magnetic resonance, including paramagnetic shifts of (29)Si spectra, are employed in conjunction with low-temperature electron spin resonance studies to probes of the local environments of Ce ions. Detailed wavelength-, time-, and temperature-dependent luminescence properties of the solid solution are presented. Temperature-dependent quantum yield measurements demonstrate the remarkable thermal robustness of luminescence of La2.82Ce0.18Si6N11, which shows little sign of thermal quenching, even at temperatures as high as 500 K. This robustness is attributed to the highly rigid lattice. Luminescence decay measurements indicate very short decay times (close to 40 ns). The fast decay is suggested to prevent strong self-quenching of luminescence, allowing even the end-member compound Ce3Si6N11 to display bright luminescence.

Widely Tunable Infrared Antennas Using Free Carrier Refraction.

We demonstrate tuning of infrared Mie resonances by varying the carrier concentration in doped semiconductor antennas. We fabricate spherical silicon and germanium particles of varying sizes and doping concentrations. Single-particle infrared spectra reveal electric and magnetic dipole, quadrupole, and hexapole resonances. We subsequently demonstrate doping-dependent frequency shifts that follow simple Drude models, culminating in the emergence of plasmonic resonances at high doping levels and long wavelengths. These findings demonstrate the potential for actively tuning infrared Mie resonances by optically or electrically modulating charge carrier densities, thus providing an excellent platform for tunable metamaterials.