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.

Rapid progress in tailoring the size and shape of semiconductor nanocrystals (quantum dots) enables a high degree of control over their optical and electronic properties. This control can be harnessed for applications in biological assays, optoelectronic integration, and wireless-systems engineering. In optoelectronics, size tunability of quantum dots permits control over the spectrum of absorption for photovoltaic and photoluminescent, stimulated emission, and electroluminescence applications, [1±4] whereas in biological applications, this allows spectral multiplexing and coding. [5] In both applications, luminescence in the near-infrared region is urgently needed: in optoelectronics to bridge interconnects with shortand medium-haul networks, and in biological applications to exploit the spectral windows in water and biological absorption. An ideal nanocrystal (NC) synthetic route combines features that have so far been demonstrated in isolation. Synthesis in aqueous solution, or transfer from organic media to water, is highly desirable for compatibility with biological assays.[5] It also enables multilayer polymer±nanocrystal device fabrication for heterostructure engineering through the use of alternating aqueous/organic solvents in sequential layers.Other crucial features of synthetic route are simplicity, use of non-toxic solvents, and moderate reaction temperatures. [6,7] From the point of view of the resulting material properties, the NCs desired in the above-mentioned applications would be size-tunable in the near-IR spectral range; have a high quantum yield, and exhibit stable, narrow fluorescence peaks [8] requiring no yield-reducing size-selective precipitation.To date, no route to synthesis of quantum dots (QDs) has been reported that accommodates all of these features simultaneously. We report herein a one-stage water-based synthesis of stable and monodisperse PbS NCs that show photo-and electroluminescence in the spectral range 1000±1400 nm. It is the combination of the simplicity of water-based synthesis, a one-stage process with no need in encapsulation or size-selective precipitation, relatively low toxicity, [9,10] stability, size tunability, and excellent performance that make these NCs promising in optoelectronic and biological applications. PbS nanocrystals were prepared in aqueous solutions using a mixture of thiols as a stabilizing agent. We admit that waterbased synthesis and thiol ligands have been used in II±VI nanoparticle synthesis. [11,12] This approach, however, has not been tested in the preparation of PbS NCs: previous reports on PbS nanoparticles synthesis in aqueous solutions employed poly(vinyl alcohol), poly(vinyl pyrrolidone), gelatin, and DNA as stabilizers. [13,14] These reports and our experiments indicate that these methods result in agglomerates of poly/ nanocrystals of very small diameter (~1 nm) and absorption and luminescence in the visible spectral range. We examined a number of different capping agents, such as thiobutanol, thiohexanol, triglycolic acid, thioglycero...

We quantify experimentally the efficiency of excitation transfer from a semiconducting polymer matrix to quantum dot nanocrystals. We study 5Ϯ0.5 nm PbS nanocrystals embedded in MEH-PPV ͑poly͓2-methoxy-5-͑2Ј-ethylhexyloxy-p-phenylenevinylene͔͒͒ 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.

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.

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 m 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%.