Photoluminescence spectral shifts

In addition to the photoluminescence red shifts, broadening of photoluminescence spectra and decrease in the photoluminescence quantum efficiency are reported with increasing temperature. The spectral broadening is due to scattering by coupling of excitons with acoustic and LO phonons [22]. The decrease in the photoluminescence quantum efficiency is due to non-radiative relaxation from the thermally activated state. The Stark effect also produces photoluminescence spectral shifts in CdSe quantum dots [23]. Large red shifts up to 75 meV are reported in the photoluminescence spectra of CdSe quantum dots under an applied electric field of 350 kVcm . Here, the applied electric field decreases or cancels a component in the excited state dipole that is parallel to the applied field the excited state dipole is contributed by the charge carriers present on the surface of the quantum dots. 

Figure 17.4 (A) Photoluminescence spectral shifts (AX,) of a solution of CdSe quantum dot aggregates during heating-cooling cycles photoluminescence spectral maxima were recorded at 298 K during cooling and 353 K during heating. Reversibility of the photoluminescence spectral shift was attained after four heating-cooling cycles. (B) Photoluminescence spectra of a solution ofCdSe...

We report on the efficient photoluminescence up-conversion in colloidally synthesized CdTe nanocrystals. We demonstrate that the efficiency of photon energy up-conversion and the magnitude of the spectral shift can be controlled by (i) the size of the nanocrystals (ii) the temperature dependence of the excited state absorption coefficient (iii) the excitation intensity. We suggest that intrinsic gap states are involved as intermediate states in the up-conversion rather than nonlinear two-photon absorption or Auger processes,... 

Rogers JK, Seiferth F, Vaez-Iravani M (1995) Near field probe microscopy of porous silicon observation of spectral shifts in photoluminescence of small particles. Appl Phys Lett 66(24) 3260... 

Photoluminescence measurements are inherently more sensitive than absorption, enabling detection limits of 10 mol dm to be readily achieved. Luminescence intensity and lifetime are the most commonly monitored properties however fluorescence anisotropy, spectral shifts, and changes in vibrational fine-structure may all be used as probing parameters. 

A soluble silole-containing polyplatinayne 51 was prepared.47 As compared to 2,5-dibromo-l,l-diethylsilole (Xmax = 326 nm), the positions of the low-lying shoulder bands (Xmax = 504 nm in CH2C12) are remarkably red-shifted by 178 nm for 51 after the inclusion of heavy-metal chromophores. This is possibly due to the intramolecular D-A interaction between the electron-rich metal ethynyl unit and the electron-poor silole ring. The Ee value is impressive at 2.10 eV for 51, and it is significantly lowered by 1.0 eV relative to 53 (3.10 eV).48 Compound 51 is photoluminescent with the singlet emission band at 537 nm. No room temperature emission from the Ti state was detected over the measured spectral window. 

In summary, the photoluminescence of CdSe quantum dots can be strongly enhanced by nearby metal nanoparticles, where most of the enhancement results from excitation effects. We observed that the shape of the PLE spectra of the quantum dots near a metal nanoparticle is significantly altered for both gold and Ag nanoparticles, and shows a new PLE peak coincident with the LSPR peak of the metal nanoparticle. Although the absolute enhancement factor varies from one metal nanoparticle to another, the wavelength dep>endence of the total enhancement factor still mirrors the line shape of the metal nanoparticle s scattering spectrum. There may be a small offset in the maximum excitation enhancement from the nanoparticle s scattering peak (as was described for the total fluorescence in Section 4.3 above), but at present our experiments have not had sufficient spectral resolution to identify any such shift. 

Figures 2.27(b) shows the spectral dependence of the long-time photoluminescence integrated over 30-90 ns after excitation as a function of temperature. At these times the initial exciton population has completely decayed and all the remaining emission originates from the long-lived exciplex states. At low temperatures, we see the red-shifted emission characteristic of the exciplex. For higher temperatures, however, the emission spectrum increasingly acquires excitonic...

The luminescence properties of siloxene have now been studied in great detail. A typical siloxene PL spectrum is shown at the bottom of Figure 15.4 (b). It has a maximum in the yellow-green spectral range at around 2.4 eV. At low temperatures, a radiative lifetime of 10 ns and a polarization memory were observed. These properties and the small Stokes shift between the photoluminescence and its excitation spectra provide experimental evidence for the hypothesis that siloxene does indeed have a direct band gap. Details of the excited states in siloxene leading to the luminescence have also been obtained from measurements of optically detected magnetic resonance... 

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