Ludmila Bakueva

Size-tunable infrared (1000–1600 nm) electroluminescence from PbS quantum-dot nanocrystals in a semiconducting polymer

Nanocomposites consisting of PbS nanocrystals in a conjugated polymer matrix were fabricated. We report results of photo- and electroluminescence across the range of 1000 to 1600 nm with tunability obtained via the quantum-size effect. The intensity of electroluminescence reached values corresponding to an internal quantum efficiency up to 1.2%. We discuss the impact of using different-length capping ligands on the transfer of excitations from polymer matrix to nanocrystals.

Efficient excitation transfer from polymer to nanocrystals

We quantify experimentally the efficiency of excitation transfer from a semiconducting polymer matrix to quantum dot nanocrystals. We study 5±0.5 nm PbS nanocrystals embedded in MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)]) polymer. We determine the excitation transfer efficiency from normalized photoluminescence excitation measurements. When the composites are made using as-synthesized PbS nanocrystals capped by oleate ligands, the excitation transfer efficiency is about 20%. Replacing these ligands with shorter chains results in a factor-of-3 enhancement in the excitation transfer efficiency. Our findings provide guidance to the realization of efficient electroluminescent devices.

Room-temperature amplified spontaneous emission at 1300 nm in solution-processed PbS quantum-dot films

We report room-temperature amplified spontaneous emission and spectral narrowing at infrared wavelengths in solution-processed films made up of PbS quantum-dot nanocrystals. The results are relevant to optical amplification and lasing integrated upon a variety of substrates. The active optical medium operates at room temperature without any additional matrix material, providing an optical gain of 260 cm(-1) and a pump threshold of 1 mJ/cm(-2). Nanocrystals synthesized in an aqueous solution and stabilized by use of short ligands result in high quantum-dot volume fractions in solid films and in a redshift emission relative to absorption.

Luminescence from processible quantum dot-polymer light emitters 1100–1600 nm: Tailoring spectral width and shape

Electroluminescent devices combining two families of PbS colloidal quantum dots to achieve spectrally tailored two-color emission are reported. Depending on device structure selected—the use of two separated layers versus a mixture of nanocrystals—the structures demonstrated light emission either in two infrared frequency peaks corresponding to the spectral region 1.1–1.6 μm or in a wide band spanning this same spectral region. Separated-layer devices exhibit wide tunability in the relative intensity of the two peaks by varying excitation conditions. Replacing oleate with octodecylamine ligands increases the internal electroluminescence efficiency to 3.1%.

Luminescent properties and electronic structure of conjugated polymer-dielectric nanocrystal composites

We report results of experimental investigations of the luminescent properties of two different conjugated polymers in which we embedded nanocrystals of Al2O3, Y2O3, ZnO, and SnSbO. The dielectric nanocrystals result in a blueshifting and broadening of luminescence spectra of poly(p-phenylene vinylene) with a simultaneous disappearance of its vibronic structure. The same nanocrystals in a poly[2-(6-cyano-6′-methylheptyloxy)-1,4-phenylene] matrix cause redshifting and spectral broadening. These observations are explained by referring to a model that accounts for the change in the polarization component of the carrier and exciton energy in the vicinity of inclusions.

Negative capacitance in polymer-nanocrystal composites

The effect of negative capacitance (NC) was observed and investigated in conducting polymers with embedded PbS nanocrystals. NC occurred for signal frequencies up to 1MHz and was absent in control samples without nanocrystals. The amplitude of NC increased with the applied bias and was accompanied by a superlinear current–voltage characteristic. A theoretical description of NC is presented based on a voltage-induced redistribution of carriers in a nonuniform sample with nanocrystals playing the role of potential wells for carriers. The characteristic time of carrier capture by nanocrystals, determined from the frequency dispersion of NC, was ∼3×10−5s.

Electronic properties of semiconducting poly(ferrocenylsilane) thin films with vapor-phase iodine diffusion doping

We report diffusion doping of the semiconducting polymer poly (ferrocenylsilanes), or PFS. We have obtained a steady-state conductivity change of greater than eight orders of magnitude effected through iodine vapor doping of PFS. The sign of thermoelectric power measurements indicates p-type conductivity. The conductivity exhibits an activation energy of ΔE=(0.45–0.65) eV for moderately-doped samples and ΔE=(0.8–0.99) eV for heavily-doped samples. We report sample photoconductivity, which we find to be principally of bolometric origin. We also study the evolution of electrical properties over time: during several days after fabrication, the samples exhibit an irreversible decrease in conductivity which may be attributable to partial iodine desorption. After this aging process, especially noticeable in heavily doped material, the sample properties stabilize, suggesting promise in a range of prospective device applications.

Enhancement of third harmonic contrast with harmonophores in multimodal non-linear microscopy of histological sections

Histological investigations of biological tissue benefited tremendously from staining different cellular structures with various organic dyes. With the introduction of new imaging modalities such as second harmonic generation (SHG) and third harmonic generation (THG) microscopy, the demand for novel dyes that enhance the harmonic signals has arisen. The new labels with high molecular hyperpolarizability have recently been termed harmonophores. In this study, we demonstrate that hematoxylin, the standard histological stain used in H&E (hematoxylin and eosin) staining, enhances the microscopic THG signal. Hematoxylin has an affinity for basophilic structures such as the cell nucleus, ribosomes and mitochondria, while eosin stains structures such as the cytoplasm, collagen and red blood cells. The histological sections of H&E stained cancerous prostate tissue found in transgenic adenocarcinoma of the mouse prostate (TRAMP) have been investigated with the multimodal SHG, THG and multiphoton excitation fluorescence (MPF) microscope. Strong THG signal revealed intracellular structures originating where the hematoxylin stain resides, while SHG imaging of the tissue showed the presence of collagen fibrils in the extracellular matrix. The MPF was mostly present in the extracellular matrix. The spectrally and temporally resolved MPF revealed that most of the fluorescence originates from the eosin. The THG image did not correlate with MPF confirming that the harmonic signal originates from hematoxylin. Multimodal nonlinear microscopy adds invaluable information about cellular structures to the widely used bright field investigations of H&E stained histological sections, and can be efficiently used for morphological studies as well as cancer diagnostics.

Influence of nanocrystals on the energy levels and luminescent properties of the polymer matrix in conjugated polymer–dielectric nanocomposites

Abstract We report results of experimental investigations into the luminescent properties of conjugated polymers in which are embedded dielectric nanocrystals. In poly( p -phenylene vinylene), embedding nanocrystals results in a blue shift and broadening of luminescence spectra with a simultaneous disappearance of its vibronic structure. The same nanocrystals in poly(2-(6-cyano-6 ′ -methylheptyloxy)-1,4-phenylene) cause a red shift and spectral broadening. The results are explained with reference to a model accounting for the change in the polarization component of carrier and exciton energy in the vicinity of nanocrystals. The polymer–dielectric nanocomposites demonstrated increased electroluminescence effectiveness compared with pure polymers.

Fabrication and investigation of nanocomposites of conducting polymers and GaSb nanocrystals

Nanocomposites consisting of GaSb nanocrystals in conducting polymer matrices were fabricated and investigated. Optical absorption and electrical properties of the samples were analyzed. Electroluminescence with a spectral maximum near 1600 nm was observed for both bias polarities. Infrared electroluminescence from GaSb/polymer nanocomposites points to possible applications in fibre-optic communications.