The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.

Femtosecond transient absorption in the visible and infrared spectral ranges has been applied to study carrier dynamics and mechanisms for resonant optical nonlinearities in CdSe nanocrystals (NCs) with a variety of surface passivations. Sequential filling of the 1S, 1P, and 1D atomic-like electron orbitals, governed by Fermi statistics, is clearly observed in the NC bleaching spectra recorded at progressively higher pump intensities. We observe that electron-hole (e-h) spatial separation strongly affects electron intraband dynamics. Such dependence indicates a nonphonon energy relaxation mechanism involving e-h interactions. A strong difference in electron and hole relaxation behavior in the stage following initial intraband relaxation is observed. In contrast to electron relaxation, which is sensitive to the quality of surface passivation (i.e., is affected by trapping at surface defects), depopulation dynamics of the initially-excited hole states are identical in NCs with different surface properties, suggesting that these dynamics are due to relaxation into intrinsic NC states. In the regime of multiparticle excitation, a quantization of relaxation rates corresponding to different multiple e-h pair states is observed. This effect is explained in terms of quantum-confined Auger recombination.

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...

Because of the strong spatial confinement of electronic wave functions and reduced dielectric screening, the effects of carrier-carrier Coulomb interactions are greatly enhanced in semiconductor nanocrystals (NCs) compared with those in bulk materials. These interactions open a highly efficient decay channel via Auger recombination, which represents a dominant recombination pathway for multiexcitons in NCs. Furthermore, strong Coulomb coupling between charge carriers leads to extremely efficient direct photogeneration of multiexcitons by single photons via carrier (or exciton) multiplication. This review focuses on spectral and dynamical properties of multiexcitons in semiconductor NCs. The specific topics discussed here include the structure of NC electronic states, spectral signatures of multiexcitons in transient absorption and photoluminescence, exciton-exciton interaction energies, Auger recombination, and carrier multiplication. This chapter also briefly reviews the implications of multiexciton effects for practical technologies, such as NC lasing and photovoltaics.