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...

Thiol-capped water-soluble PbS nanocrystals (NCs) stabilized with 1-thioglycerol, dithioglycerol, or a mixture of 1-thioglycerol/dithioglycerol (TGL/DTG) were prepared via one-stage synthesis at room temperature. We found that NCs stabilized with a TGL/DTG mixture show efficient and stable infrared photoluminescence centered in the second "biological window" (1050-1200 nm). Under optimized conditions, full width at half-maximum of the PL emission peak was from 70 to 100 nm. PbS NCs were stable to precipitation and aggregation for the time period from 2 to 3 months when stored in the dark under room temperature. Room-temperature photoluminescence quantum efficiency of NCs was from 7 to 10%. When NCs were stored at 37 degrees C, their PL emission red-shifted, consistent with the NC growth.

Quantum-dot nanocrystals have been used to label single molecules during living-cell assays [1] and provide direct visual guidance and real-time confirmation of complete resection during cancer surgery in an animal model. [2] The use of quantum dots for deep-tissue imaging accompanied by low autofluorescence in vivo requires emission in the second infrared biological window of 1000±1200 nm combined with stability in biological media. Surface chemistry determines the chemical and optical stability of quantum dots. A stabilizing outer shell minimizes diffusion of oxygen to the surface of the core of the nanoparticle, as demonstrated using a high-bandgap semiconductor shell [3] and a dielectric shell.[4] Silanized nanoparticles have been shown to be water soluble and to retain the absorption and emission spectra of the original particles; however, the nanoparticles lost 60±80 % of their original quantum efficiency in this process. Recently, oligomeric phosphines [5] have been employed to form three thin concentric sublayers around quantum dots: an inner phosphine layer for dot-surface passivation, a linking layer for protection, and an outer functionalized layer for miscibility and subsequent chemical modification or conjugation to biomolecules. In the infrared, the application of this multistep synthetic method to type-II core±shell nanoparticles has resulted in quantum dots that show modest degradation in 37 C plasma over the course of half an hour.[3]Here we adopt an entirely different strategy: we report the first growth of efficient infrared photoluminescent quantum dots directly on a DNA template. Our infrared-emitting quantum dots grown on the biomolecular template are efficient and stable in water, serum, and blood plasma.DNA has previously been decorated with metal nanoparticles 5±10 nm in diameter through the use of thiol linkages. [6] DNA has also been used as a long-term stabilizer and template in the growth of CdS nanocrystals, but with no reports of a photoluminescence quantum efficiency.[7±10] Related progress has also been made in synthesizing CdS nanoparticles in which growth was carried out at room temperature followed by annealing at 80 C to improve photoluminescent properties; quantum efficiencies of 10 ±4 were estimated.[11] The only previous report of DNA-templated growth on PbS has yielded materials with no detectable luminescence in the infrared.[12]We worked instead at a synthesis temperature at which chemical interaction was possible between the metal cations used in PbS growth and at least two classes of sites on DNA: the phosphate backbone, and also DNA's purine and pyrimidine bases. The bases provide an additional opportunity for control over the growth of nanoparticles and the passivation of their surface states. The synthesis reported herein is simple, reproducible, and yields PbS nanoparticles with exceptional stability and photoluminescence quantum efficiency. Energyfiltered transmission electron microscopy (EFTEM) reveals cubic-latticed PbS quantum dots 4 nm in diameter on a network...

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.