The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

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Quantum dot solar cells

Here, we will first briefly summarize the general principles of QD synthesis using our previous work on InP as an example. Then we will focus on QDs of the IV-VI Pb chalcogenides (PbSe, PbS, and PbTe) and Si QDs because these were among the first QDs that were reported to produce multiple excitons upon absorbing single photons of appropriate energy (a process we call multiple exciton generation (MEG)). We note that in addition to Si and the Pb-VI QDs, two other semiconductor systems (III-V InP QDs(56) and II-VI core-shell CdTe/CdSe QDs(57)) were very recently reported to also produce MEG. Then we will discuss photogenerated carrier dynamics in QDs, including the issues and controversies related to the cooling of hot carriers and the magnitude and significance of MEG in QDs. Finally, we will discuss applications of QDs and QD arrays in novel quantum dot PV cells, where multiple exciton generation from single photons could yield significantly higher PV conversion efficiencies.

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Semiconductor quantum dots and quantum dot arrays and applications of multiple exciton generation to third-generation photovoltaic solar cells.

The properties of quasi-two-dimensional semiconductor quantum dots are reviewed. Experimental techniques for measuring the electronic shell structure and the effect of magnetic fields are briefly described. The electronic structure is analyzed in terms of simple single-particle models, density-functional theory, and "exact" diagonalization methods. The spontaneous magnetization due to Hund's rule, spin-density wave states, and electron localization are addressed. As a function of the magnetic field, the electronic structure goes through several phases with qualitatively different properties. The formation of the so-called maximum-density droplet and its edge reconstruction is discussed, and the regime of strong magnetic fields in finite dot is examined. In addition, quasi-one-dimensional rings, deformed dots, and dot molecules are considered. (Less)

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Electronic structure of quantum dots

This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrysta...

Low-dimensional systems: quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

Isolated single quantum dots of different size have been fabricated by laser-induced local interdiffusion of a GaAs/AlGaAs quantum-well structure. Microscopic photoluminescence (PL) reveals a splitting and a blueshift, which depend systematically on dot size. The distinct PL peaks separated in energy by up to 10 meV are attributed to recombination between zero-dimensional (0D) electron and hole states. Complete quantization and inherent exclusion of inhomogeneous broadening in a single dot structure cause PL linewidths below 0.5 meV. The strength of the higher-energy transitions indicates the slowed energy relaxation theoretically predicted for 0D systems.

Photoluminescence from a single GaAs/AlGaAs quantum dot

Optical spectroscopy in one-dimensional (ID) and zero-dimensional (OD) semiconductor structures has been applied in recent years to investigate the peaked density of states.1-4 However, inhomogeneous broadening of the photoluminescence (PL) line caused by fluctuations in composition and structure size within arrays of ID and 0D systems made a clear identification of lateral quantization difficult. This inhomogeneous broadening is eliminated in spectroscopy of a single OD system.

Resonant inelastic light scattering is used to probe electronic excitations in shallow etched GaAs/AlGaAs quantum dots and wires. In both types of structures, intersubband excitations are observed in depolarized scattering geometries. They show significant blueshift with decreasing confinement length. In dot structures these transitions appear as dispersionless, whereas in wires they show strong broadening and an intensity dip at the energetic center of the excitation with increasing wave vector parallel to the wires, as expected for single-particle excitations. Their line shape correlates with the linear wave vector dispersion of additionally observed intrasubband excitations.

Single-particle excitations in quasi-zero- and quasi-one-dimensional electron systems.

Photoluminescence measurements with combined spatial, temporal, and spectral resolution are performed on single GaAs/${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum dots. The complete spatial quantization leads to a spectrum of discrete emission lines. A series of structures with various confinement strength is investigated, as a function of excitation wavelength, excitation power, and temperature. In all cases, a fast rise of the luminescence is observed. Several independent results show that Coulomb scattering plays a major role within the early stage of energy relaxation. At liquid-helium temperature, a strikingly different recombination dynamics is observed for dots with various lateral potential. For weak lateral confinement, energy relaxation is directly observed in the time dependence of the luminescence spectrum. In contrast, in the sample with strongest confinement, independent recombination of the discrete lines occurs. Increasing the excitation power, higher-energy lines appear and the spectral weight shifts systematically from the lowest to the higher-energy lines. For this variation, which corresponds to an increase in the estimated number of electron-hole pairs in the single dot from about 1\char21{}2 to 200, the peak energies hardly change. We have also performed detailed calculations of the energy spectrum and the relaxation and recombination times of excitons in quantum dots. The experimental results are surprisingly well interpreted assuming the formation of an exciton gas obeying the Pauli exclusion principle.

Microphotoluminescence studies of single quantum dots. I. Time-resolved experiments

We report on microphotoluminescence experiments in magnetic fields on single quantum dots formed by width fluctuations in a narrow GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum well. The ground state as well as two excited states are discussed. We find a different diamagnetic shift and Zeeman spin splitting for these states. Resonantly exciting zero-dimensional excitons, we observe a strong magnetic-field dependence of the spin flip rates.

U. Bockelmann

Papers

Photoluminescence from a single GaAs/AlGaAs quantum dot

Photoluminescence from a single GaAs/AlGaAs quantum dot

Single-particle excitations in quasi-zero- and quasi-one-dimensional electron systems.

Microphotoluminescence studies of single quantum dots. I. Time-resolved experiments

Magnetooptical studies of a single quantum dot:ffExcited states and spin flip of excitons