Time resolved photoluminescence anisotropy of CdSe/ZnS nanoparticles in toluene at 300 K

Abstract

For CdSe nanoparticles it has been theoretically and experimentally shown that at low temperatures the photoluminescence is circularly polarized in accordance with a wurtzite structure and the corresponding allowed optical transitions. In the present Letter, we report on related investigations on CdSe/ZnS colloids in toluene solution. From time resolved photoluminescence anisotropy we conclude that also at room temperature the results are in good agreement with structure and related electronic states as determined from crystals at low temperature.

Aggregated CdS Quantum Dots: Host of Biomolecular Ligands

“…The significant drop of the average PL decay time compared to that of the nanostructure without OX1 (1.23 ns) also confirms the energy transfer occurs from CdS QDs to OX1 due to dipolar coupling. Although theoretical calculations to predict dipolar emission of a QD with wurtzite crystal structure have been advanced, 32 very few experimental results 33 have been published on the existence of excited-state dipole moment in semiconductor quantum dots. In a recent study from our group 22 on CdS nanoparticles in reverse micellar solution at 300 K, we reported physical motion of the dipolar QDs (photoluminescence anisotropy) having different shapes.…”

Luminescence Depolarization Dynamics of Quantum Dots: Is It Hydrodynamic Rotation or Exciton Migration?

“…If isotropic diffusion and "stick" boundary conditions (where solvent molecules move with solute molecules) are considered, then the rotational correlation time is given by the SED equation. 2,11 To obtain the theoretical estimate of the rotational correlation time (τ r ), the temperature-dependent viscosity value of isooctane is taken from the literature 28 and the calculated value of τ r is plotted against temperature (Figure 4d). The observed 2 ns component of the r(t) decay fits well with the theoretical value (solid line of Figure 4d), revealing the origin of the time constant to be stick hydrodynamic rotation of the dots.…”

“…The rotational correlation time (τ r ) is calculated from the Stokes-Einstein-Debye (SED) theory 11 and defined as where V is the volume of the solute molecule, η is the shear viscosity of the solvent, T is the absolute temperature, and k B is the Boltzmann constant. The f factor accounts for the shape of the solute molecule, approximated as a spheroid.…”

“…In contrast to common organic chromophores with a simple 1D transition dipole, QDs (such as CdSe) have been shown to possess a degenerate transition moment (2D transition moment) at cryogenic 8 as well as room temperature. [9][10][11] This 2D transition moment is inherently linked to the crystal structure 10 and therefore allows the determination of the QD orientation via the analysis of the emission polarization anisotropy of single QDs. 12 A 2D transition dipole, projected as an elliptical shape on the sample plane, intrinsically contains 3D orientation information 9 that can provide information about the complex processes.…”

“…Although a few reports are in the literature that depict the measurement of a 2D emission transition dipole of QDs in solid films 10,14 at room temperature, to the authors knowledge no reports are available regarding the measurement of a 2D transition dipole in the liquid phase. A few attempts 11,15,16 to use QDs (with microsecond excited-state lifetime) as a probe for the detection of photoluminescence (PL) anisotropy have been made in this direction. However, due to the presence of various internal relaxation processes, it is difficult to recognize the dynamics of anisotropy of QDs merely in terms of its rotational depolarization.…”