Multiexciton generation (MEG) has been indirectly observed in colloidal quantum dots, both in solution and the solid state, but has not yet been shown to enhance photocurrent in an optoelectronic device. Here, we report a class of solution-processed photoconductive detectors, sensitive in the ultraviolet, visible, and the infrared, in which the internal gain is dramatically enhanced for photon energies Ephoton greater than 2.7 times the quantum-confined bandgap Ebandgap. Three thin-film devices with different quantum-confined bandgaps (set by the size of their constituent lead sulfide nanoparticles) show enhancement determined by the bandgap-normalized photon energy, Ephoton/Ebandgap, which is a clear signature of MEG. The findings point to a valuable role for MEG in enhancing the photocurrent in a solid-state optoelectronic device. We compare the conditions on carrier excitation, recombination, and transport for photoconductive versus photovoltaic devices to benefit from MEG.

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...

Sources of coherent, monochromatic short-wavelength infrared (1-2 mum) light are essential in telecommunications, biomedical diagnosis, and optical sensing. Today's semiconductor lasers are made by epitaxial growth on a lattice-matched single-crystal substrate. This strategy is incompatible with integration on silicon. Colloidal quantum dots grown in solution can, in contrast, be coated onto any surface. Here we show a 1.53 mum laser fabricated using a remarkably simple process: dipping a glass capillary into a colloidal suspension of semiconductor quantum dots. We developed the procedures to produce a smooth, low-scattering-loss film inside the capillary, resulting in a whispering gallery mode laser with a well-defined threshold. While there exist three prior reports of optical gain in infrared-emitting colloidal quantum dots [1,2,3], this work represents the first report of an infrared laser made using solution processing. We also report dlambda(max)/dT, the temperature-sensitivity of lasing wavelength, of 0.03 nm/K, the lowest ever reported in a colloidal quantum dot system and 10 times lower than in traditional semiconductor quantum wells.

We engineer colloidal quantum dot nanocrystals through the choice of biomolecular ligands responsible for nanoparticle nucleation, growth, stabilization, and passivation. We systematically vary the presence of, and thereby elucidate the role of, phosphate groups and a multiplicity of functionalities on the mononucleotides used as ligands. The results provide the basis for synthesis of nanoparticles using precisely controlled synthetic oligonucleotide sequences.