We report a synthesis of highly luminescent (CdSe)ZnS composite quantum dots with CdSe cores ranging in diameter from 23 to 55 A. The narrow photoluminescence (fwhm ≤ 40 nm) from these composite dots spans most of the visible spectrum from blue through red with quantum yields of 30−50% at room temperature. We characterize these materials using a range of optical and structural techniques. Optical absorption and photoluminescence spectroscopies probe the effect of ZnS passivation on the electronic structure of the dots. We use a combination of wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, small and wide angle X-ray scattering, and transmission electron microscopy to analyze the composite dots and determine their chemical composition, average size, size distribution, shape, and internal structure. Using a simple effective mass theory, we model the energy shift for the first excited state for (CdSe)ZnS and (CdSe)CdS dots with varying shell thickness. Finally, we characterize the...

(CdSe)ZnS Core-Shell Quantum Dots - Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites

Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.

Excitons in nanoscale systems

The techniques of colloidal chemistry permit the routine creation of semiconductor nanocrystals1,2 whose dimensions are much smaller than those that can be realized using lithographic techniques3,4,5,6. The sizes of such nanocrystals can be varied systematically to study quantum size effects or to make novel electronic or optical materials with tailored properties7,8,9. Preliminary studies of both the electrical10,11,12,13 and optical properties14,15,16 of individual nanocrystals have been performed recently. These studies show clearly that a single excess charge on a nanocrystal can markedly influence its properties. Here we present measurements of electrical transport in a single-electron transistor made from a colloidal nanocrystal of cadmium selenide. This device structure enables the number of charge carriers on the nanocrystal to be tuned directly, and so permits the measurement of the energy required for adding successive charge carriers. Such measurements are invaluable in understanding the energy-level spectra of small electronic systems, as has been shown by similar studies of lithographically patterned quantum dots3,4,5,6 and small metallic grains17.

A single-electron transistor made from a cadmium selenide nanocrystal

Nanocrystals or quantum dots of the IV−VI semiconductors PbS, PbSe, and PbTe provide unique properties for investigating the effects of strong confinement on electrons and phonons. The degree of confinement of charge carriers can be many times stronger than in most II−VI and III−V semiconductors, and lead salt nanostructures may be the only materials in which the electronic energies are determined primarily by quantum confinement. This Account briefly reviews recent research on lead salt quantum dots.

Lead Salt Quantum Dots: the Limit of Strong Quantum Confinement

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

CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

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Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions

Spectra and decay kinetics of radiative recombination in CdS microcrystals

Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots : assignment of the first electronic transitions

Erratum: ``Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions'' [J. Opt. Soc. Am. B 10, 100 (1993)]

Navigation receivers based on the software defined radio (SDR) concept need significant computing power in the tracking unit. In this work, we discuss data reduction algorithm for solution of this problem. This algorithm allows reduction of computing load; however, it has some limitations. Several methods of algorithm correction have been developed, which make it possible to increase time periods when the tracking process can be disabled, therefore, computing resources might be saved. Two approaches are considered: a method based on previous data and another one, based on external motion data. Both suggested methods improve stability and save computing resources. The most practical outcome can be achieved in tasks where no frequent position determination is needed. In addition, it may be helpful for a cloud receiver when network throughput is limited.

I. A. Kudryavtsev

Papers

Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions

Spectra and decay kinetics of radiative recombination in CdS microcrystals

Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots : assignment of the first electronic transitions

Erratum: ``Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions'' [J. Opt. Soc. Am. B 10, 100 (1993)]

Navigation Receiver Signal Tracking Module Correction Based on Motion Data