As a result of quantum-confinement effects, the emission colour of semiconductor nanocrystals can be modified dramatically by simply changing their size. Such spectral tunability, together with large photoluminescence quantum yields and high photostability, make nanocrystals attractive for use in a variety of light-emitting technologies--for example, displays, fluorescence tagging, solid-state lighting and lasers. An important limitation for such applications, however, is the difficulty of achieving electrical pumping, largely due to the presence of an insulating organic capping layer on the nanocrystals. Here, we describe an approach for indirect injection of electron-hole pairs (the electron-hole radiative recombination gives rise to light emission) into nanocrystals by non-contact, non-radiative energy transfer from a proximal quantum well that can in principle be pumped either electrically or optically. Our theoretical and experimental results indicate that this transfer is fast enough to compete with electron-hole recombination in the quantum well, and results in greater than 50 per cent energy-transfer efficiencies in the tested structures. Furthermore, the measured energy-transfer rates are sufficiently large to provide pumping in the stimulated emission regime, indicating the feasibility of nanocrystal-based optical amplifiers and lasers based on this approach.

We observe ultrafast 1P-to-1S intraband relaxation in PbSe and CdSe nanocrystals (NCs) that have distinct energy spectra. While ultrafast dynamics in CdSe NCs has typically been interpreted in terms of electron-hole energy transfer, this mechanism is not active in PbSe NCs because of sparse densities of states in the conduction and valence bands. Our observations of temperature activation and confinement-enhanced relaxation in PbSe NCs can be explained by efficient multiphonon emission triggered by nonadiabatic electron-phonon interactions and are indicative of large, size-dependent, intraband Huang-Rhys parameters.

Recently, we demonstrated that PbSe nanocrystal quantum dots can efficiently produce multiple electron-hole pairs (excitons) in response to a single absorbed photon. To address the generality of this carrier-multiplication phenomenon to other materials, we perform a comparative study of multiexciton generation in PbSe and CdSe nanocrystals that have distinctly different electronic structures. We find that both materials exhibit high-efficiency carrier multiplication and the activation threshold is lower in CdSe nanocrystals than in PbSe nanocrystals (∼2.5 vs ∼2.9 energy gaps). Furthermore, the efficiencies of multiexciton generation are nearly identical for both materials despite a vast difference in both energy structures and carrier relaxation behaviors, strongly suggesting that this phenomenon is general to quantum-confined semiconductor nanocrystals.

We study spectrally resolved dynamics of Förster energy transfer in single monolayers and bilayers of semiconductor nanocrystal quantum dots assembled using Langmuir-Blodgett (LB) techniques. For a single monolayer, we observe a distribution of transfer times from ~50 ps to ~10 ns, which can be quantitatively modeled assuming that the energy transfer is dominated by interactions of a donor nanocrystal with acceptor nanocrystals from the first three "shells" surrounding the donor. We also detect an effective enhancement of the absorption cross section (up to a factor of 4) for larger nanocrystals on the "red" side of the size distribution, which results from strong, inter-dot electrostatic coupling in the LB film (the light-harvesting antenna effect). By assembling bilayers of nanocrystals of two different sizes, we are able to improve the donor-acceptor spectral overlap for engineered transfer in a specific ("vertical") direction. These bilayers show a fast, unidirectional energy flow with a time constant of ~120 ps.

Here, for the first time, we demonstrate amplified spontaneous emission (ASE) from PbSe nanocrystals (NCs) with emission energies tunable in the near-infrared (IR). We show that despite complications associated with a high, 8-fold degeneracy of the lowest quantized states and fast, nonradiative Auger recombination, optical gain parameters of PbSe NCs are comparable to those of CdSe NCs used for light amplification in the visible. These results indicate that previous unsuccessful attempts to realize the lasing regime in NCs of lead salts were not due to intrinsic physical reasons but likely resulted from material quality issues. By using a novel sol-gel procedure that provides both good quality surface passivation and high NC filling factors (>15%), we fabricate PbSe NC/sol-gel nanocomposites that produce ASE, which is tunable via NC size, in the near-IR. This finding indicates the feasibility of NC-based amplifiers and lasers tunable in the near-IR range and, in particular, in the range of telecommunication windows.Chemically synthesized nanocrystals (NCs) exhibit widerange size-controlled tunability of the emission color and high photoluminescence (PL) quantum yields (QYs). These characteristics make NCs attractive materials for light-emitting applications ranging from bio-labeling 1 and solid-state lighting 2 to optical amplification and lasing. 3,4 Similar to such "soft" light emitters as organic dyes and conjugated polymers, colloidal NCs offer chemical flexibility and processibility and can be easily incorporated into photonic structures or optical waveguides. 5,6 However, while organic molecules are poor infrared (IR) light emitters, III-V 7 and IV-VI 8 NCs show efficient IR emission with high, up to near-unity QYs. 9 Here, we analyze the fundamental physics of light amplification in PbSe NCs with emission energies in the near-IR. We experimentally verify that the lowest quantized states of PbSe NCs exhibit a high, 8-fold degeneracy. We find that despite this high degeneracy and fast, nonradiative Auger recombination, 10,11 PbSe NCs exhibit optical gain parameters that are similar to those of CdSe NCs, which have been shown to produce light amplification at visible wavelengths. 3 Using a novel sol-gel procedure that maintains good surface passivation and high NC filling factors, we demonstrate size-tunable ASE from PbSe NC/sol-gel nanocomposites prepared in the form of "active" planar waveguides.To date, light amplification and lasing have only been demonstrated for II-VI CdSe NCs. 3-6 However, because of the large energy gap (>1.75 eV), these NCs cannot be used for light amplification in the IR spectral range. On the other hand, tunable NC-based media capable of producing emission (see Figure 1a) and optical gain at a specific near-or mid-IR wavelength (within, e.g., telecommunication and atmospheric transparency windows) are highly desirable for a number of applications ranging from optical communications and remote sensing to a recent proposal of coherent plasmon generation. 12 Lead salts such as PbSe ha...

Numerous technologies including solid-state lighting, displays, and traffic signals can benefit from efficient, color-selectable light sources that are driven electrically. Semiconductor nanocrystals are attractive types of chromophores that combine size-controlled emission colors and high emission efficiencies with excellent photostability and chemical flexibility. Applications of nanocrystals in light-emitting technologies, however, have been significantly hindered by difficulties in achieving direct electrical injection of carriers. Here we report the first successful demonstration of electroluminescence from an all-inorganic, nanocrystal-based architecture in which semiconductor nanocrystals are incorporated into a p-n junction formed from GaN injection layers. The critical step in the fabrication of these nanocrystal/GaN hybrid structures is the use of a novel deposition technique, energetic neutral atom beam lithography/epitaxy, that allows for the encapsulation of nanocrystals within a GaN matrix without adversely affecting either the nanocrystal integrity or its luminescence properties. We demonstrate electroluminescence (injection efficiencies of at least 1%) in both single- and two-color regimes using structures comprising either a single monolayer or a bilayer of nanocrystals.

In this communication, we demonstrate a new approach to sensitization of Ru-polypyridine complexes by using semiconductor nanocrystal quantum dots (NQDs). When mixed in solution, the complexes functionalized by carboxylic groups adsorb onto the surface of the NQDs. Excitation of NQDs by 400 nm light leads to fast, 5 ps hole transfer from the photoexcited NQDs to the surface-adsorbed complexes. This result indicates that Ru complexes can be sensitized by CdSe NQDs, which opens interesting opportunities for designing new types of photocatalytic materials for solar energy conversion applications. These materials will take advantage of broad size-controlled absorption spectra and large extinction coefficients of NQDs as well as the unique property of NQDs to respond to absorption of a single photon by producing multiple electron-hole pairs.

We study different emission regimes in close-packed films of chemically synthesized CdSe nanoparticles [nanocrystal quantum dots (NQDs)]. We observe that the NQD photoluminescence is dominated by excitons and biexcitons, respectively, before and after the threshold for stimulated emission. Furthermore, we demonstrate the regime of microring lasing into sharp, whispering-gallery modes using NQD solids incorporated into microcapillary tubes. This result indicates a feasibility of miniature, solid-state laser devices based on chemically synthesized NQDs.