We demonstrate for the first time that impact ionization (II) [the inverse of Auger recombination (AR)] occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than three times the NC band gap energy. Generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) timescale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells. [2,6]. II is an Auger-type process whereby a high-energy exciton, created in a semiconductor by absorbing a photon of energy ≥2E g , relaxes to the band edge via energy transfer of at least 1E g to a valence band electron, which is excited above the energy gap ( Fig. 1(a)). The result of this energy transfer process is that two excitons are formed for one absorbed photon. Thus, this process converts more of the high photon energy portion of the solar spectrum into usable energy.Here, for the first time, we demonstrate carrier multiplication via II in NCs. By directly monitoring exciton conversion to biexcitons in the time domain, we show that II in PbSe NCs is highly efficient, extremely fast, and occurs in a wavelength range that has potential to provide significantly increased solar cell power conversion efficiency.2 Schaller et al.Auger recombination (AR), the opposite of II, is a process in which an exciton recombines via energy transfer to an electron (or hole) that is excited to a higher energy state within or outside a NC (Fig. 1(b)). Because of restrictions imposed by energy and momentum conservation, AR is inefficient in bulk materials. However, AR becomes efficient in NCs due to enhanced Coulomb interactions and relaxation of momentum conservation [7][8][9][10]. Because of symmetry of the matrix elements that describe both II and AR, the former can also be very efficient in quantum-confined systems.Here, we use transient absorption (TA) to monitor carrier population dynamics in high quality, oleic acid-passivated, colloidal PbSe NC samples [11] (size dispersity was ~5-10%, studied NC diameters were ~4 to 6 nm). Pump pulses (50 fs) from an amplified Ti-sapphire laser (pump photon energies, ћω = 1.55 or 3.10 eV) or from a tunable optical parametric amplifier (OPA) excited NCs dissolved in hexane. The absorption change, ∆α, within the photo-excited spot is probed with 100 fs pulses that are tuned via another OPA to the band-edge (A 1 ) absorption maximum. As a measure of excitation density, we use an average number of photo-generated e-h pairs per NC, N eh , produced by the pump pulse that we can accurately calculate and experimentally verify [9]. One important problem in experimental studies of II is to rel...

There is much current interest in the optical properties of semiconductor nanowires, because the cylindrical geometry and strong two-dimensional confinement of electrons, holes and photons make them particularly attractive as potential building blocks for nanoscale electronics and optoelectronic devices, including lasersand nonlinear optical frequency converters. Gallium nitride (GaN) is a wide-bandgap semiconductor of much practical interest, because it is widely used in electrically pumped ultraviolet-blue light-emitting diodes, lasers and photodetectors. Recent progress in microfabrication techniques has allowed stimulated emission to be observed from a variety of GaN microstructures and films. Here we report the observation of ultraviolet-blue laser action in single monocrystalline GaN nanowires, using both near-field and far-field optical microscopy to characterize the waveguide mode structure and spectral properties of the radiation at room temperature. The optical microscope images reveal radiation patterns that correlate with axial Fabry-Perot modes (Q approximately 10(3)) observed in the laser spectrum, which result from the cylindrical cavity geometry of the monocrystalline nanowires. A redshift that is strongly dependent on pump power (45 meV microJ x cm(-2)) supports the idea that the electron-hole plasma mechanism is primarily responsible for the gain at room temperature. This study is a considerable advance towards the realization of electron-injected, nanowire-based ultraviolet-blue coherent light sources.

The optical properties of stoichiometric copper chalcogenide nanocrystals (NCs) are characterized by strong interband transitions in the blue part of the spectral range and a weaker absorption onset up to ~1000 nm, with negligible absorption in the near-infrared (NIR). Oxygen exposure leads to a gradual transformation of stoichiometric copper chalcogenide NCs (namely, Cu(2-x)S and Cu(2-x)Se, x = 0) into their nonstoichiometric counterparts (Cu(2-x)S and Cu(2-x)Se, x > 0), entailing the appearance and evolution of an intense localized surface plasmon (LSP) band in the NIR. We also show that well-defined copper telluride NCs (Cu(2-x)Te, x > 0) display a NIR LSP, in analogy to nonstoichiometric copper sulfide and selenide NCs. The LSP band in copper chalcogenide NCs can be tuned by actively controlling their degree of copper deficiency via oxidation and reduction experiments. We show that this controlled LSP tuning affects the excitonic transitions in the NCs, resulting in photoluminescence (PL) quenching upon oxidation and PL recovery upon subsequent reduction. Time-resolved PL spectroscopy reveals a decrease in exciton lifetime correlated to the PL quenching upon LSP evolution. Finally, we report on the dynamics of LSPs in nonstoichiometric copper chalcogenide NCs. Through pump-probe experiments, we determined the time constants for carrier-phonon scattering involved in LSP cooling. Our results demonstrate that copper chalcogenide NCs offer the unique property of holding excitons and highly tunable LSPs on demand, and hence they are envisaged as a unique platform for the evaluation of exciton/LSP interactions.

Many potential applications of semiconductor nanocrystals are hindered by nonradiative Auger recombination wherein the electron-hole (exciton) recombination energy is transferred to a third charge carrier. This process severely limits the lifetime and bandwidth of optical gain, leads to large nonradiative losses in light-emitting diodes and photovoltaic cells, and is believed to be responsible for intermittency ("blinking") of emission from single nanocrystals. The development of nanostructures in which Auger recombination is suppressed has recently been the subject of much research in the colloidal nanocrystal field. Here, we provide direct experimental evidence that socalled "giant" nanocrystals consisting of a small CdSe core and a thick CdS shell exhibit a significant (orders of magnitude) suppression of Auger decay rates. As a consequence, even multiexcitons of a very high order exhibit significant emission efficiencies, which allows us to demonstrate optical amplification with an extraordinarily large bandwidth (>500 meV) and record low excitation thresholds. This demonstration represents an important milestone toward practical lasing technologies utilizing solution-processable colloidal nanoparticles.Colloidal semiconductor nanocrystals (NCs) have been the subject of intense research due to potential applications in low-threshold lasers, biological tags, third-generation photovoltaics, and light-emitting diodes (LEDs). 1,2 All of these technologies can benefit from the unique properties of NCs such as a size-tunable energy gap, high photoluminescence (PL) quantum yields, good stability, and chemical processability. However, many of these potential applications are hindered by Auger recombination, wherein the energy of one electron-hole pair (exciton) is nonradiatively transferred to another charge carrier. 3 In NCs, this process occurs on subnanosecond time scales and reduces optical gain lifetime, 4 restricts the available time to extract multiple excitons generated via carrier multiplication, 5 limits LED brightness due to the build-up of charged NCs, 6 and leads to PL intermittency ("blinking") that is typically observed in single-NC studies. 7,8 While the physics underlying Auger recombination in NCs is still not fully understood, general considerations suggest that the rate of this process is directly dependent upon the strength of carrier-carrier Coulomb coupling and the degree of spatial overlap between the electron and hole wave functions involved in the Auger transition. 9-11 Previous approaches to reducing Auger recombination rates have utilized the manipulation of both of these parameters. For example, using elongated NCs (quantum rods), one can separate interacting excitons along the rod axis, which leads to decreased exciton-exciton Coulomb coupling. 12 Also, one can reduce the rate of Auger transitions by separating electrons and holes between the core and the shell

Infrared-emitting nanocrystal quantum dots (NQDs) have enormous potential as an enabling technology for applications ranging from tunable infrared lasers to biological labels. Notably, lead chalcogenide NQDs, especially PbSe NQDs, provide efficient emission over a large spectral range in the infrared, but their application has been limited by instability in emission quantum yield and peak position on exposure to ambient conditions. Conventional methods for improving NQD stability by applying a shell of a more stable, wider band gap semiconductor material are frustrated by the tendency of lead chalcogenide NQDs toward Ostwald ripening at even moderate reaction temperatures. Here, we describe a partial cation-exchange method in which we take advantage of this lability to controllably synthesize PbSe/CdSe core/shell NQDs. Critically, these NQDs are stable against fading and spectral shifting. Further, these NQDs can undergo additional shell growth to produce PbSe/CdSe/ZnS core/shell/shell NQDs that represent initial steps toward bright, biocompatible near-infrared optical labels.

One consequence of strong spatial confinement of electronic wave functions in semiconductor nanocrystals (NCs) is a significant enhancement in carrier-carrier Coulomb interactions. This effect leads to a number of novel physical phenomena including ultrafast decay of multiple electron-hole pairs (multiexcitons) by Auger recombination and high-efficiency generation of mutiexcitons by single photons via carrier multiplication (CM). Significant recent interest in multiexciton phenomena in NCs has been stimulated by studies of NC lasing, as well as potential applications of CM in solar-energy conversion. The focus of this Account is on CM. In this process, the kinetic energy of a "hot" electron (or a "hot" hole) does not dissipate as heat but is, instead, transferred via the Coulomb interaction to the valence-band electron, exciting it across the energy gap. Because of restrictions imposed by energy and translational-momentum conservation, as well as rapid energy loss due to phonon emission, CM is inefficient in bulk semiconductors, particularly at energies relevant to solar energy conversion. On the other hand, the CM efficiency can potentially be enhanced in zero-dimensional NCs because of factors such as a wide separation between discrete electronic states, which inhibits phonon emission ("phonon bottleneck"), enhanced Coulomb interactions, and relaxation in translational-momentum conservation. Here, we investigate CM in PbSe NCs by applying time-resolved photoluminescence and transient absorption. Both techniques show clear signatures of CM with efficiencies that are in good agreement with each other. NCs of the same energy gap show moderate batch-to-batch variations (within approximately 30%) in apparent multiexciton yields and larger variations (more than a factor of 3) due to differences in sample conditions (stirred vs static solutions). These results indicate that NC surface properties may affect the CM process. They also point toward potential interference from extraneous effects such as NC photoionization that can distort the results of CM studies. CM yields measured under conditions when extraneous effects are suppressed via intense sample stirring and the use of extremely low pump levels (0.02-0.03 photons absorbed per NC per pulse) reveal that both the electron-hole creation energy and the CM threshold are reduced compared with those in bulk solids. These results indicate a confinement-induced enhancement in the CM process in NC materials. Further optimization of CM performance should be possible by utilizing more complex (for example, shaped-controlled or heterostructured) NCs that allow for facile manipulation of carrier-carrier interactions, as well as single and multiexciton energies and dynamics.

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