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Presents a study which investigated Cd[sub1-x]zn[subx]Te crystals grown by the traveling heater method. Determination of excitonic energy gap as a function of the alloy composition; Methodology in the characterization of crystals; Theoretical calculations.

We present theoretical aspects of the exchange interaction between the ground optically-forbidden “dark” exciton state and surface dangling bonds in colloidal CdSe spherical nanocrystals and nanoplatelets. The influence of the dangling bond spins on the radiative recombination and spin splitting of the dark exciton is shown. Processes of optically-driven and external magnetic field-driven formation of the dangling bond magnetic polaron (DBMP) are considered. Thermodynamic and dynamic polarization mechanisms of the DBMP formation within these two processes and corresponding critical temperatures are compared. Experimental manifestations of the DBMP formation in CdSe nanocrystals and nanoplatelets are discussed. [ABSTRACT FROM AUTHOR]

The envelope function method is applied to hole and exciton states in wurtzite and zinc-blende structure cylindrical nanowires using the axial Kohn-Luttinger Hamiltonian including a crystal-field splitting for the wurtzite case. The quantized states are found to be mixtures of heavy- and light-hole band derived states. Explicit expressions for the mixing coefficients and the conditions giving the quantized energy states are derived. These are then applied to obtain the relative intensity of light emitted polarized parallel and perpendicular to the nanowire for the lowest-energy excitons. Slightly different expressions are obtained in the weak quantization and strong quantization limits. In the latter case, an overlap integral of the hole and electron envelope function is involved, while in the former case only an integral over the hole envelope function is required. An expression for the degree of polarization of the spectrally integrated photoluminescence as a function of temperature is obtained by summing over the excitons within the integration range. The degree of polarization as a function of temperature is explored numerically as a function of nanowire radius and crystal-field splitting for GaAs and GaN nanowires. Examples of complete reversal of the polarization are illustrated. [ABSTRACT FROM AUTHOR]

We performed spin-polarized resonant Raman and resonant photoluminescence excitation spectroscopy (also known as ‘fluorescence line narrowing’) on ZnS-capped CdSe nanocrystal quantum dots in high magnetic fields to 33 Tesla and temperatures down to 1.7K, which allows detailed investigation of the excitonic spin states. In these experiments, spin-polarized electrons and holes are resonantly injected by circularly polarized light into colloidal quantum dots of specific size, using a narrowband tunable dye laser and a fiber-coupled probe that is specially-designed for use in high-field magnets. In addition to the expected broad features associated with excitonic recombination and Raman-like peaks associated with quantized acoustic phonons, the photoluminescence spectra measured at magnetic fields larger than 10 Tesla develop a sharp peak, which moves roughly linearly with applied magnetic field. Further, the energy of this high-field peak varies systematically as a function of nanocrystal size. However, unlike typical electron spin flip transitions, the mode energy extrapolates to a finite value at zero magnetic field, suggesting the existence of an additional size-dependent exchange mechanism. [ABSTRACT FROM AUTHOR]

We present a theoretical description of excitons and positively and negatively charged trions in "giant" CdSe/CdS core-shell nanocrystals (NCs). The developed theory provides the parameters describing the fine structure of excitons in CdSe/CdS core/thick shell NCs as a function of the CdSe/CdS conduction band offset and the CdSe core radius. We have also developed a general theory describing the fine structure of positively charged trions created in semiconductor NCs with a degenerate valence band. The calculations take into account the complex structure of the CdSe valence band and interparticle Coulomb and exchange interaction. Presented in this paper are the CdSe core size and CdSe/CdS conduction band offset dependencies (i) of the positively charged trion fine structure, (ii) of the binding energy of the negatively charged trion, and (iii) of the radiative decay time for excitons and trions. The results of theoretical calculations are in qualitative agreement with available experimental data. [ABSTRACT FROM AUTHOR]

CdSe nanoplatelets show perfectly quantized thicknesses of a few monolayers. They present a situation of extreme, yet well defined quantum confinement. Due to a large dielectric contrast between the semiconductor and its ligand environment, interaction between carriers and their dielectric images strongly renormalize bare single particle states. We discuss the electronic properties of this original system in an advanced tight-binding model, and show that Coulomb interactions, including self-energy corrections and enhanced electron-hole interaction, lead to exciton binding energies up to several hundred meV. [ABSTRACT FROM AUTHOR]