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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]

We show theoretically that spins of dangling bonds (DBSs) at the surface of colloidal nanostructures can be responsible for the dark exciton radiative recombination governing low temperature photoluminecence. The DBS-assisted dark exciton recombination is efficient when all spins are randomly oriented. The DBS polarization caused by external magnetic field or by the formation of the dangling bond magnetic polaron (DBMP) at low temperatures suppresses the DBS-assisted recombination. The temperature or magnetic field control on the DBS-polarization allows to detect experimentally the presence of DBS and DBS-assisted recombination in CdSe nanocrystals and 2D nanoplatelets. [ABSTRACT FROM AUTHOR]

Applications of semiconductor nanocrystals such as biomarkers and light-emitting optoelectronic devices require that their fluorescence quantum yield be close to 100%. However, such quantum yields have not been obtained yet, in part, because non-radiative Auger recombination in charged nanocrystals could not be suppressed completely. Here, we synthesize colloidal core/thick-shell CdSe/CdS nanocrystals with 100% quantum yield and completely quenched Auger processes at low temperatures, although the nanocrystals are negatively photocharged. Single particle and ensemble spectroscopy in the temperature range 30-300 K shows that the non-radiative Auger recombination is thermally activated around 200 K. Experimental results are well described by a model suggesting a temperature-dependent delocalization of one of the trion electrons from the CdSe core and enhanced Auger recombination at the abrupt CdS outer surface. These results point to a route for the design of core/shell structures with 100% quantum yield at room temperature. [ABSTRACT FROM AUTHOR]

The syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still greatly underdeveloped. Here, we demonstrate the formation of atomically flat quasi-two-dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well-defined thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have the electronic properties of two-dimensional quantum wells formed by molecular beam epitaxy, and their thickness-dependent absorption and emissionspectra are described very well within an eight-band Pidgeon-Brown model. They present an extremely narrow emission spectrum with full-width at half-maximum less than 40?meV at room temperature. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1?ns at 6?K, two orders of magnitude less than for spherical CdSe nanoparticles. This makes the nanoplatelets the fastest colloidal fluorescent emitters and strongly suggests that they show a giant oscillator strength transition. [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]

Using first principles calculations we describe the energy band structure of bulk CuInS2. The energy band parameters for the multiband effective mass approximation that describes the band edges of this semiconductor are obtained by fitting them to the first principles spectra. Within the multiband effective mass approximation we develop a theoretical description for the structure of band-edge levels and optical properties of the CuInS2 nanocrystals. For the nanocrystals of spherical shape, the optical transitions are weakly allowed between the electron and hole ground states due to the tetragonal symmetry of the crystal lattice, resulting in a large Stokes shift of photoluminescence up to 300 meV in the smallest nanocrystals. This theory of the band-edge optical transitions in CuInS2 NCs can be applied to spherical NCs made of other chalcopyrite compounds. [ABSTRACT FROM AUTHOR]