Measurement of Wavelength-Dependent Polarization Character in the Absorption Anisotropies of Ensembles of CdSe Nanorods

Abstract

Transient absorption (TA) and photoluminescence excitation (PLE) anisotropy measurements were used to investigate the polarization of band-edge and above-band-edge excitonic states in ensembles of CdSe nanocrystals with aspect ratios of 1:1, 3:1, and 10:1, dispersed in hexanes. The 1:1 nanocrystals (quantum dots) are isotropic absorbers and emitters. The 10:1 nanorods have a nonzero but featureless anisotropy spectrum above the band edge due to heterogeneity in the crystal structure and, therefore, electronic structure within single nanorods. The nanocrystals with an aspect ratio of 3:1, which are largely single crystals, have PLE and TA anisotropy spectra with features that correspond to those in the absorption spectrum. Direct measurement of the TA anisotropy spectrum of the nanorods and comparison with the PLE anisotropy spectrum reveal that the band-edge absorptive and emissive transitions contain both linear (z) and planar (xy) character. The degree of planar character at the band-edge states, modulated by classical local field effects arising from the dielectric contrast between the nanorod and the solvent, limits the degree of photoselection at this wavelength. The variation in the magnitude of the xy projection of the absorptive transitions within states above the band edge is responsible for the wavelength dependence of the absorption and emission anisotropies.

Efficient hot-electron transfer by a plasmon-induced interfacial charge-transfer transition

“…5A, inset), which indicates that the band-edge-absorption transition dipole has 70% axial and 30% radial components ( fig. S12; see the supplementary materials for details), consistent with previous reports (45). The intraband transition between 1s e and 1p e showed negligible anisotropy ( fig.…”

Excitation wavelength dependence of the photoluminescence quantum yield and decay behavior of CdSe/CdS quantum dot/quantum rods with different aspect ratios

“…15,16 In addition, their emission is anisotropic or polarized, whereas the photoluminescence (PL) from QDs is isotropic. 17,18 Hence, in the last years, QDQRs have increasingly replaced QDs, and many of the commercially available II/VI semiconductor nanocrystals (SCNCs) or materials incorporated into SCNC-based products are nowadays QDQRs with typical aspect ratios of up to about five. 19,20 For QDs and QDQRs, the size of the spectroscopic key parameter ΦPL 21 is particularly relevant as this value provides also a direct measure for particle and shell quality.…”

Quantification of Anisotropy-Related Uncertainties in Relative Photoluminescence Quantum Yield Measurements of Nanomaterials – Semiconductor Quantum Dots and Rods

“…[2,18,19] Moreover, rod-shaped nanomaterials like elongated semiconductor nanocrystals (quantum rods, QRs) typically display an anisotropic emission. [20][21][22][23] The increasing importance in measuring PL of anisotropic emitters like dye-biomolecule conjugates and luminescent nanomaterials with e.g., a shapeinduced anisotropy encouraged us to study polarization-induced changes of the detection efficiency of a commercial fluorometer and to quantify polarizationrelated uncertainties of PL measurements. For this purpose, we chose quantum dot-rods (QDRs), a novel class of semiconductor nanocrystals increasingly used as reporters for bioanalytical applications and medical diagnostics [28][29][30] and active materials for plasma displays, which consist of a spherical core and an elongated shell.…”

“…Moreover, most importantly for our study, the emission anisotropy of QDRs depends on their aspect ratios. [20][21][22][23] In order to assess anisotropy-related uncertainties, we compared PL measured without and with polarizers using magic angle conditions and studied systematically the dependence of the detected emission intensities on the polarizer settings used for samples with different anisotropies and the effect on relatively determined PL . This included a dispersion of spherical QDs with ideally isotropic emission, the solution of a small organic dye in a fluid non-interacting solvent (Rh101 in ethanol) with nearly isotropic emission, a dispersion of elongated QDRs with significant anisotropic emission due to the elongated shape of the QDRs, and a small organic dye embedded in a solid polymer matrix showing nearly the highest possible emission anisotropy.…”