Re-examination of the Size-Dependent Absorption Properties of CdSe Quantum Dots (original) (raw)
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ACS Photonics, 2015
We calculated the absorption coefficient of colloidal wurtzite CdSe/CdS nanocrystals (NCs), ranging from spherical to strongly elongated shapes (aspect ratio up to 21) by correlating the NC absorbance spectrum to the NC elemental composition and overall dimensions. We compared experimental data with numerical finite-element calculations of the NC absorption coefficient, which can account for the nonspheroidal NC shape and the influence of the organic ligand shell. The results unveiled that quantum confinement effects influence the NC absorption properties, even at relatively high photon energies (∼4.2 eV). From a practical point of view, we provide a general expression for the absorption coefficient that only requires the knowledge of the NC aspect ratio and CdSe/CdS volume ratio, giving access to a fast and nondestructive estimate of NC concentrations in solution and single-particle absorption cross sections, relevant for a wide range of photonic applications. More fundamentally, the unique aspect ratio-dependence of the absorption coefficient allowed us to derive the real (ε r) and imaginary (ε i) parts of the dielectric constant, demonstrating that even at energies far above the band gap the NC absorption coefficient differs from bulk due to a reduction of both ε r and ε i. As the methods presented are general, our analysis can be applied to a wide range of materials of varying composition and yields comprehensive insight into the optical constants of colloidal nanocrystals.
Multiexciton Absorption Cross Sections of CdSe Nanocrystals at Band-Edge Energy
Epj Web of Conferences, 2013
Picosecond transient absorption signals of two kinds of cadmium selenide quantum dots were measured at various excitation intensities. The average number of excitons per quantum dot was calculated from a Poisson model, which together with kinetic parameters was used to determine exciton population kinetics. Exciton and multiexciton absorption cross sections were determined and analyzed in terms of the electronic states of the quantum dots.
A Theoretical-Experimental Comparison of CdSe Quantum Dot Optical Properties
Proceedings of the 2nd World Congress on Recent Advances in Nanotechnology, 2017
In this work, we have studied the optical properties of CdSe quantum dots (QDs) with different sizes. Using the SIESTA code and the Kramers-Kronig relations, we have computed the imaginary part of the dielectric constant and the density of states (DOS). The absorption spectra are compared to experimental results from samples fabricated using the thermal decomposition method and a good agreement was obtained. The experimental band edge absorption could be associated to a specific optical transition in our QDs. A well defined second absorption band has been observed in our theoretical results. The energy maximum of these bands follow the expected quantum size effect. However, we do not observed the increase of the energy difference between them, reported by other authors. The reducing of absorption band intensity when the quantum dot size increases, has been seen. Preliminary density of states calculations, also reported in this work, allowed the association of Cd-or Se-character to the energy states in our samples.
Small, 2006
The tuning of CdSe quantum dot (QDs) sizes, and consequently their corresponding two-photon absorption (TPA) cross section, has been systematically investigated. As the size (diameter) of the quantum dots increases, the TPA cross section is found to be empirically related via a power-law proportionality of 3.5 AE 0.5 and 5.6 AE 0.7 to the diameters of CdSe and CdTe QDs, respectively. The results are tentatively rationalized via a theoretical model of two-photon excitation properties in a system incorporating excitons and defects.
ChemInform, 2001
Nearly monodisperse CdSe quantum dots of 1.8 nm in size were synthesized and capped with a surface monolayer of 1-thioglycerol. The optical properties of thin films made from these matrix-free close-packed quantum dots were studied at different temperatures and electric field strengths and compared with isolated quantum dots of the same size embedded in a PMMA matrix. The broadening and red shift of the optical transitions observed in absorption and excitation spectroscopy for the ensemble of close-packed quantum dots is explained by the formation of collective electronic states between interacting nanocrystals. The reversible collapse of these extended electronic states into localized states was demonstrated for the close-packed quantum dots by applying an external electric field.
Spectroscopic Study of Electronic States in an Ensemble of Close-Packed CdSe Nanocrystals
The Journal of Physical Chemistry B, 2000
Nearly monodisperse CdSe quantum dots of 1.8 nm in size were synthesized and capped with a surface monolayer of 1-thioglycerol. The optical properties of thin films made from these matrix-free close-packed quantum dots were studied at different temperatures and electric field strengths and compared with isolated quantum dots of the same size embedded in a PMMA matrix. The broadening and red shift of the optical transitions observed in absorption and excitation spectroscopy for the ensemble of close-packed quantum dots is explained by the formation of collective electronic states between interacting nanocrystals. The reversible collapse of these extended electronic states into localized states was demonstrated for the close-packed quantum dots by applying an external electric field.
STRUCTURAL AND OPTICAL INVESTIGATION OF CdSe QUANTUM DOTS
CdSe semiconducting Quantum dots were prepared by a chemical method at a room temperature. X-ray powder diffraction and transmission electron microscope measurements conformed a hexagonal cubic crystalline phase of Cdse semiconducting Quantum dots of about 15 nm average size with nearly spherical shape. The absorption and photoluminescence spectra of the CdSe quantum dots were strongly shown blue shifted due to size quantization. The present study describes a simultaneous and highly reproducible large scale synthesis of highly luminescent CdSe Quantum dots.
Ukrainian Journal of Physical Optics, 2010
Peculiarities of the absorption and photoluminescence (PL) spectra of colloidal solution of CdSe nanoparticles have been studied in the process of size-selective photoetching. The values of homogeneous broadening and lifetime of resonantly excited exciton states in nanoparticles of definite sizes have been estimated. A model explaining the nature of Stokes shift has been suggested, which takes into account inhomogeneity of energy of the electron-hole pair in the volume of nanoparticle. The Stokes shift values have been determined for different samples. Different changes in the PL spectra observed during the process of photoetching could be caused by the influence of different initial defect structures of the samples.
Spectroscopic study of oscillator strength and radiative decay time of colloidal CdSe quantum dots
Optical and Quantum Electronics, 2018
Characterization of samples of cadmium selenide quantum dots (CdSe) QDs dissolved in toluene colloidal solutions at a concentration of 1.4 mg/ml was carried out through UV-Vis absorption and photoluminescence (PL) spectroscopy. The size-dependent absorption and red-shifted PL emission peak wavelengths could be tuned between 510-576 and 545-606 nm respectively. Optical absorption spectral measurements yielded CdSe QDs having diameters about * 2.44-3.69 nm with energy gaps 2.32-2.08 eV which are higher than the bulk CdSe (1.74 eV) reminiscent of quantum confinement. This is found to be in good agreement with the semi-empirical pseudopotential model. In addition, the first excitonic absorption transition 1S (e) 1S 3/2(h) oscillator strength and the corresponding fluorescence radiative decay time of CdSe QDs are assessed using relevant Einstein relations for absorption and emission in a two-level system. The elaborated calculations would anticipate that the transition oscillator scale with the CdSe QD radius as * R 2.54. Correspondingly, the calculated radiative decay times decrease from 56.4 to 23.2 ns which scale with CdSe QDs radius as * R-2.155 in fairly good agreement with experimental values reported in the literature.
Optical Properties of CdSe Nanoparticle Assemblies
2004
We report on three-dimensional fluorescence imaging of micron-size faceted crystals precipitated from solutions of CdSe nanocrystals. Such crystals have previously been suggested to be superlattices of CdSe quantum dots . Possible applications for these materials include their use in optical and optoelectronic devices. The micron-size crystals were grown by slow evaporation from toluene solutions of CdSe nanocrystals in the range of 3-6 nm, produced by traditional wet-chemistry techniques. By using a confocal microscope with laser illumination, three-dimensional raster-scanning and synchronized hyper-spectral detection, we have generated spatial profiles of the fluorescence emission intensity and spectrum. The fluorescence data of the micro-crystals were compared with spectra of individual nanocrystals obtained from the same solution. The results do not support the assertion that these microcrystals consist of CdSe superlattices.