Multiexciton Recombination Research Articles

Carrier Multiplication and Its Reduction by Photodoping in Colloidal InAs Quantum Dots

Carrier (exciton) multiplication in colloidal InAs/CdSe/ZnSe core−shell quantum dots (QDs) is investigated using terahertz time-domain spectroscopy, time-resolved transient absorption, and quasi-continuous wave excitation spectroscopy. For excitation by high-energy photons (2.7 times the band gap energy), highly efficient carrier multiplication (CM) results in the appearance of multi-excitons, amounting to 1.6 excitons per absorbed photon. Multi-exciton recombination occurs within tens of picoseconds via Auger-type processes. Photodoping (i.e., photoinjection of an exciton) of the QDs prior to excitation results in a reduction of the CM efficiency to 1.3. This exciton-induced reduction of CM efficiency can be explained by the twofold degeneracy of the lowest conduction band energy level. We discuss the implications of our findings for the potential application of InAs QDs as light absorbers in solar cells.

Radiative Recombination of Triexcitons in CdSe Colloidal Quantum Dots

Recent experimental observations of multiexciton recombination in colloidal CdSe quantum dots have revealed a high-energy emission band that has been attributed to the radiative recombination of a p-like electron with a p-like hole in a triexciton complex (three holes + three electrons). The reason for the occupation of p-like hole states in the triexciton, however, has remained elusive. Using atomistic pseudopotential calculations, we show here that p-like hole states are populated even at low temperature because of the relatively small Coulomb repulsion between s-like and p-like hole states, which leads to a non-Aufbau occupation sequence of the hole levels. We also show that the observed temperature dependence of the p-p emission band originates from the dark-bright splitting of the triexciton ground state. Our results provide a consistent explanation of the physical origin of the triexciton emission lines in CdSe quantum dots.

Carrier Multiplication and Its Reduction by Photodoping in Colloidal InAs Quantum Dots

Carrier (exciton) multiplication in colloidal InAs/CdSe/ZnSe core−shell quantum dots (QDs) is investigated using terahertz time-domain spectroscopy, time-resolved transient absorption, and quasi-continuous wave excitation spectroscopy. For excitation by high-energy photons (∼2.7 times the band gap energy), highly efficient carrier multiplication (CM) results in the appearance of multi-excitons, amounting to ∼1.6 excitons per absorbed photon. Multi-exciton recombination occurs within tens of picoseconds via Auger-type processes. Photodoping (i.e., photoinjection of an exciton) of the QDs prior to excitation results in a reduction of the CM efficiency to ∼1.3. This exciton-induced reduction of CM efficiency can be explained by the twofold degeneracy of the lowest conduction band energy level. We discuss the implications of our findings for the potential application of InAs QDs as light absorbers in solar cells.

Nonlinear differential ultrafast laser absorption spectroscopy observation of charge carrier dynamicsof CdSxSe1−x nanocrystal doped glasses

Based on femtosecond transient absorption spectroscopy, the charge carrier dynamics ofCdSxSe1−x nanocrystal doped glasses was investigated by analyzing the nonlinear differential transientabsorption spectra. Time constants were extracted, which are different from the literatureresults using similar samples: they are pump laser intensity-dependent and decreasewith increasing pump laser intensity, whereas risetime constants are pump laserintensity-independent and high-order exciton recombination can be identified from thespectra. The threshold pump intensity was determined, which is lower than the literatureresult. Reasons for high-order recombination were analyzed, and broad size, statedistribution and high pump intensity are found as key factors. Auger recombination mayplay a key role in the multi-exciton recombination. Theoretical calculations andanalysis support this conclusion. A schematic model for electron transport and theformation of triexcitons in our samples are proposed to explain these results.

Nonlinear optical approach to multiexciton relaxation dynamics in quantum dots

Unlike the majority of molecular systems quantum dots can accommodate multiple excitations, which is a particularly important attribute for potential lasing applications. We demonstrate in this work the concept of using nth order nonlinear spectroscopies in the transient grating configuration as a means of selectively exciting (n-1)/2 excitons in a semiconductor and probing the subsequent relaxation dynamics. We report a direct observation of multiparticle dynamics on ultrashort time scales through comparison of third and fifth order experiments for CdSe colloidal quantum dots. Time constants associated with multiexciton recombination and depopulation dynamics are reported. Deviation from a Poisson model for the distribution of photoexcited excitons, biexcitons, and triexcitons is also discussed.

Predicting the electronic properties of 3D, million-atom semiconductor nanostructure architectures

The past ∼10 years have witnessed revolutionary breakthroughs both in synthesis of quantum dots (leading to nearly monodispersed, defect-free nanostructures) and in characterization of such systems, revealing ultra narrow spectroscopic lines of <1 meV width, exposing new intriguing effects, such as multiple exciton generation, fine-structure splitting, quantum entanglement, multiexciton recombination and more. These discoveries have led to new technological applications including quantum computing and ultra-high efficiency solar cells. Our work in this project is based on two realizations/observations: First, that the dots exhibiting clean and rich spectroscopic and transport characteristics are rather big. Indeed, the phenomenology indicated above is exhibited only by the well-passivated defect-free quantum dots containing at least a few thousand atoms (colloidal) and even a few hundred thousand atoms (self assembled). Understanding the behavior of nanotechnology devices requires the study of even larger, million-atom systems composed of multiple components such as wires+dots+films. Second, first-principles many-body computational techniques based on current approaches (Quantum Monte-Carlo, GW, Bethe-Salpeter) are unlikely to be adaptable to such large structures and, at the same time, the effective mass-based techniques are too crude to provide insights on the many-body/atomistic phenomenology revealed by experiment. Thus, we have developed a set of methods that use an atomistic approach (unlike effective-mass based techniques) and utilize single-particle + many body techniques that are readily scalable to ∼103−106 atom nanostructures. New mathematical and computational techniques have also been developed to accelerate our calculations and go beyond simple conjugate gradient based methods allowing us to study larger systems. In this short paper based on a poster presented at the DOE SciDAC06 conference we will present the overall structure as well as highlights of our computational nanoscience project.

Optical properties of a coherent phase in electron-hole systems: Stimulated light scattering and multibeam processes

A theoretical study is reported of stimulated light scattering, including wave-vector reversal and anomalous transmission, by a coherent phase in electron-hole (e-h) systems of low and high charge-carrier density. For these two cases the coherent phase is taken to be a Bose-Einstein condensate of excitons or a BCS-like state of e-h pairs, respectively. The scattering mechanism is laser-induced coherent recombination of two excitons or two coherent e-h pairs, respectively. The e-h system is assumed to exist within a GaAs/AlGaAs double quantum well or bulk GaAs. The emission rate of two photons with antiparallel momenta is estimated. Multiphoton emission due to multiexciton coherent recombination is covered. Methods for detecting the effects predicted are proposed.

Efficient Inverse Auger Recombination at Threshold in CdSe Nanocrystals

We apply the semiempirical nonlocal pseudopotential method to the investigation of prospects for direct carrier multiplication (DCM) in neutral and negatively charged CdSe nanocrystals. In this process, known in the bulk as impact ionization, a highly excited carrier transfers, upon relaxation to the band edge, its excess energy Δ to a valence electron, promoting it across the band gap and thus creating two excitons from one. For excess energies just a few meV above the energy gap Eg (the DCM threshold), we find the following: (i) DCM is much more efficient in quantum dots than in bulk materials, with rates of the order of 1010 s-1. In conventional bulk solids, comparable rates are obtained only for excess energies about 1 eV above Eg. (ii) Unlike the case in the bulk, in both neutral and charged nanocrystals the DCM rate is not an increasing function of the excess energy but oscillates as Δ moves in and out of resonance with the energy of the discrete spectrum of these 0D systems. (iii) The main contribution to the DCM rates is found to come from the dot surface, as in the case of Auger multiexciton recombination rates. (iv) Direct radiative recombination of excited electron−hole pairs and phonon-assisted decay are slower than DCM, but (v) the rate of Auger cooling (where the relaxation energy of an excited electron is used to excite a hole into deeper levels) can be of the same order of magnitude as that of the DCM process. Furthermore, for excess energies well above the DCM threshold, the presence of an energy gap within the hole manifold considerably slows DCM compared to Auger cooling (AC), which is not affected by it. Achieving competitive DCM processes will, therefore, require the suppression of Auger cooling, for example, by removing the hole from the dot or by trapping it at the surface.

Pseudopotential theory of Auger processes in CdSe quantum dots.

Auger rates are calculated for CdSe colloidal quantum dots using atomistic empirical pseudopotential wave functions. We predict the dependence of Auger electron cooling on size, on correlation effects (included via configuration interaction), and on the presence of a spectator exciton. Auger multiexciton recombination rates are predicted for biexcitons as well as for triexcitons. The results agree quantitatively with recent measurements and offer new predictions.

A second isoelectronic multiexciton center in annealed Czochralski silicon

A recently reported group of photoluminescence lines observed in thermally annealed Czochralski silicon is studied using photoluminescence, excitation, and transient spectroscopies. The properties of these lines are remarkably similar to those of another, higher energy, series reported earlier. As for this previous case, we conclude that the new lines result from bound exciton and bound multiexciton complex recombination on an inhomogeneously broadened distribution of isoelectronic binding centers.