Semiconductor quantum dots relaxation

Scheme 16.2 Relationship between absorption and emission energies for molecules (discrete, left) and for semiconductor quantum dots (right). Upon the absorption of a photon, an electron is lifted from the ground state (1) to an electronically excited state (2). The bond order decreases, because excited states are antibinding and the atoms relax to larger intemuclear distances (3). From the lowest excited state (only one is shown here), emission of a photon (4) and relaxation to the ground state occurs. In semiconductor quantum dots (e.g. CdSe), more possibilities for the absorption of photons exist, because several/many orbitals can be found. (Reproduced with permission from E. M. Boatman et al., 2005. J. Chem. Ed. 82 1697-1699. Copyright 2005 American Chemical Society.)...

A.J. Nozik, Spectroscopy and hot electron relaxation dynamics in semiconductor quantum wells and quantum dots, Ann. Rev. Phys. Chem. 52 (2001) 193-231. 

Bavykin, Dmitry V. is a Ph.D. researcher in the Laboratory of photocatalysis on semiconductors at the Boreskov Institute of Catalysis, Novosibirsk, Russia. The title of his PhD thesis (1998) Luminescent and photocatalytic properties of CdS nanocolloids . Area of his interests is the photophysical-photochemical properties of nanosized sulfide semiconductors, including synthesis of particles with definite size and surface properties, their characterisation the study of the photoexcited states dynamics, relaxation in quantum dots by the luminescence and flash photolysis measurements studies of the interfacial charge transfer from colloidal semiconductor particles by the steady state photolysis, luminescence quenching method. 

Kamalov, V. F. Little, R. Logunov, S. L. El-Sayed, M. A. Picosecond electronic relaxation in CdS/HgS/CdS quantum dot quantum well semiconductor nanoparticles, J. Phys. Chem. 1996, 100, 6381. 

A QD Hamiltonian includes both Coulomb and electron-phonon interactions. Apparently, the phonon modes (denoted as QD) in the quantum dot are different from the semiconductor ones. The electron-phonon interaction determines relaxation processes in quantum dot (hot electrons or excitons). Thus, the QD Hamiltonian yields... 

Electron-phonon interaction in a semiconductor is the main factor for relaxation of a transferred electron. There are two different relaxation processes that decrease the efficiency of light conversion in a solar system (1) relaxation of an electron from a semiconductor conduction band to a valence band and (2) a backward electron transfer reaction. The forward and backward electron transfer processes have been already included in the tunneling interaction, HSm-qd, described by Eq. (108). However, the effect of SM e-ph interaction is important for the correct description of electron transfer in the SM-QD solar cell system. In the previous section, we have gradually considered different types of interactions in the quantum dot and obtained the exact expression for the photocurrent (128) where the exact nonequilibrium QD Green s functions determined from Eq. (127) have been used. However, in... 

In the following, we consider in some detail the transition from discrete to continuum spectra for the case of luminescence from highly excited semiconductor nanostructures. We wiU restrict ourselves to undoped semiconductors so that all carriers in conduction and valence band are optically excited. The luminescence is preceded by a fast carrier relaxation [76], so the recombination takes place when the electron and hole gases are in their respective ground states. In quantum wells, luminescence from high-density optically created electron-hole gases was studied in Refs. [77-79]. In confined structures, such as quantum dots, electrons and holes fill size-quantization energy states up to their respective Fermi... 

An interest to intraband relaxation in quantum dots (QDs) with discrete energy levels is conditioned by the application of QDs as effective active media for semiconductor lasers. 

Absorption of and Emission fiom Nanoparticles, 541 What Is a Surface Plasmon 541 The Optical Extinction of Nanoparticles, 542 The Simple Drude Model Describes Metal Nanoparticles, 545 Semiconductor Nanoparticles (Quantum Dots), 549 Discrete Dipole Approximation (DDA), 550 Luminescence from Noble Metal Nanostructures, 550 Nonradiative Relaxation Dynamics of the Surface Plasmon Oscillation, 554 Nanoparticles Rule From Forster Energy Transfer to the Plasmon Ruler Equation, 558... 

Quantum confinement is defined as the space where the motions of electrons and holes in a semiconductor are restricted in one or more dimensions. This quantum confinement occurs when the size of semiconductor crystallites is smaller than the bulk exciton Bohr radius. Quantum wells, quantum wires, and quantum dots are confined in one, two, and three dimensions, respectively [1, 2]. The confinement can be created due to electrostatic potentials, the presence of an interface between different semiconductor materials, and the presence of a semiconductor surface. A valence band and a conduction band are separated by an energy range known as the band gap ( g). These amounts of energy will be absorbed in order to promote an electron from the valence band to the conduction band and emitted when the electron relaxes directly fi om the conduction band back to the valence band. By changing the size of the semiconductor nanoparticles, the energy width of the band gap can be altered and the optical and electrical responses of these particles are changed (Fig. 1). 

Solar cells based on hot carrier extraction and CM rely on precise control of hot carrier relaxation were expected to be realized in nanostructured semiconductors e.g. QDs) because of enhanced carrier arrier interactions and discretized energy levels. As will be shown below, TRTS is capable of probing charge carrier dynamics at early times after photoexdtation, including intraband relaxation and CM in bulk materials and quantum dots. As such, TRTS represents a powerful technique for evaluating novel semiconductor systems that may be used in the design of more efficient solar cells. 

---