PL spectrum

Figure 2 PL spectra of MBE grown GaAs at 2 K near the fundamental gap, showing C-...

The absorption and photoluminescence (PL) spectra of a-6T measured at 80 K are shown in Figure 7-24. There is an onset of absorption at 2.1 eV, with several sharp peaks and a maximum at 2.8 eV. A second absorption band is seen at 4.5 eV and is due to direct excitation of the thiophene ring. We take the first peak as the 0-0 transition and calculate an optical gap j,=2.4eV. The PL spec-... 

Figure 7-32. PL decay of CW) DOO-PPV al several doping concentrations, (b) Change in PL intensity, correlated to the delect spacing. Inset PL spectra vs doping concentration.

Figure 8-2. (a) Normalized PL spectra of m-LPPP films al 7=77 K for exeila ion ai 3.2 eV (390 am) with a circular spot al Iwo different flu-ences 0.3 mJ/eni2 (solid line) and 2.4 mJ/cm2 (dashed line). The inset shows ihe room lempcralure absorption spectrum of nt-LPPP. (b) Normalized PL spectrum for excitation with a rectangular spot at a fluence of 0.055 mJ/cm2 (from Ref. 125J with permission). 

Starting from the assumption that the geometry relaxation after excitation is of primary importance with respect to the luminescence response, we decided to employ a solid polymer matrix to suppress conformational changes of the oligomers. For the measurements, dilute blends with polysulfone as the transparent host matrix were prepared. In Figure 16-13, the PL decay curves for the two cyano compounds in both chloroform and polysulfone are presented, as are the PL spectra of Ooct-OPV5-CN in chloroform and polysulfone [69J. 

Figure 10-5. Transient transmission changes AV/Po in PPV for different lime delays between the pump and probe pulse. The pump pulse is a 100 fs laser pulse at 325 nm obtained by frequency doubling ol amplified dye laser pulses, (a) and (b) correspond to different sides of a PPV-film. The spectra in (a) were obtained lor the unoxidized side of the sample while the set of spectra in (b) was measured for the oxidized side of the same sample. The main differences observed are a much lower stimulated emission effect for the oxidized side. The two bottom spectra depict the PL-spectra for comparison. The dashed line indicates the optical absorption (according to Kef. (281).

Fig. 4. PL spectra as a function of emission peak energy (i.e., wavelength) measured at room temperature.

PL spectra of Mn-doped ZnS nanoparticles optically annealai in air (a) and in vacuum (b) are shown in Fig. 2. For Mn-doped ZnS nanoparticles, the PL band is seen at around 585mn. When Mn-doped ZnS nanoparticles were annealed in air, PL intensity is increased more significantly with UV irradiation time compared with ones ann ed in vacuum. PL spectra of Pr-doped ZnS nanoparticles axe shown in Fig. 3. The broad emission at 430 nm corresponds to the emission of the undoped ZnS nanoparticles. The other peak is relaftrii to the Pr-related complexes. The effect of the optical aimealing in air is more notable than in vacuum on the enhancement of luminescent intensity. The incre e of PL intensity for Pr-doped ZnS nanoparticles in mr is more rapid than undoped or Mn-doped ZnS nanoparticles. 

Fig. 1. PL spectra of undoped ZnS nanoparticles optically annealed in air (a) and in vacuum (b)...

Fig. 4 shows PL spectra of Mn and Pr-codoped ZnS nanoparticles opdcaily aimealed in air and vacuum. Mn and Pr-codoped ZnS nanoparticles emit light of white color. The PL intoisity of the Pr-related peaks incirasrf more rapidly than that of Mn-related peak, for the codoped ZnS nanoparticles ann ed in air. The different rates may be assodated with the luminescent ions. Pr-related oimplaces are incaeased with the incrrasing UV irradiation time, but Mn ions are constant. In case of the arni ing in vacuum, Pr-related peaks are initially weaker in intensity than Mn-related peaks due to small Pr-related complexes. 

Fig. 6 shows PL spectra of CdS nanoparticles and CdS-ZnS core-shell nanoparticles. In PL spectrum of CdS nanoparticles, the emission band is seen at around 400nm. The emission band of CdS-ZnS core-shell nanoparticles is higher dian that of CdS ones at around 400nm. The PL enhancement of CdS-ZnS core-shell nanoparticl is due to passivation which means that surface atoms are bonded to the shell material of similar lattice constant and much larger band gap [9], Althou the sur ce treatment conditions are different, the ranission band of CdS-ZnS core-shell nanoparticles is same in PL spectra of Fig. 6(b). This indicates that interfacial state between CdS core and shell material was unchan d by different surfaKs treatment. 

The use of doped and undoped silica aerogels as multifunctional host materials for fluorescent dyes and other luminescent materials for display and imaging applications has been reported.278 Results have been presented on the PL spectra of undoped silica aerogels and aerogels doped with Er3+, rhodamine, and fluorescein.278... 

The same group recently reported that the TBB defects can be brought below the nuclear magnetic resonance (NMR) detection limit by employing similar polymerization conditions (i-BuOK in THF at room temperature) in the synthesis of naphthyl-substituted PPVs 51-53 [112]. Although the absorption and PL spectra of all three polymers are similar, the EL can be finely tuned between 486 nm (for 52) and 542 nm (for 53). The external QE (studied for ITO/PEDOT/polymer/Ba/Al device) is also sensitive to the substituents pattern in the naphthyl pendant group 0.08% for 51, 0.02% for 52, and 0.54% for 53. 

The PL spectra of the PFs show well-resolved structural features with maxima at 420,445, and 475 nm assigned to the 0-0, 0-1, and 0-2 intrachain singlet transition, respectively (the 0-0 transition, the most intense) [247]. Due to the tail emission spectrum of PFs, the thin films emit bright sky-blue light. The QE of the PFs is very high, typically in the range of 40 to 80% and, as shown for PFO 196, it depends substantially on the morphology of the polymer [248]. 

Star-like PFs 236 with a silsesquioxane core have been prepared by Ni-mediated copolymerization of 2,7-dibromo-9,9-dioctylfluorene with octa(2-(4-bromophenyl)ethyl)octasilses-quioxane [333]. The polymer is thermally stable up to 424°C (TGA). In both chloroform solution and films, its absorption and PL spectra are very close to that for PFO 196, although a somewhat higher PL efficiency is observed in films (64 and 55%, respectively). The polymer 236, however, demonstrates a better PL color stability during thermal annealing. An ITO/PEDOT/236/Ca/Ag device can be turned on at 6.0 V, and shows a brightness of 5430 cd/m2 (at 8.8 V) with F] =0.44%, almost twice as high as that for the corresponding PFO device (Chart 2.60). 

Leclerc s group [345,346] in Canada first synthesized PF copolymer 245 based on carbazole-2, 7-diyl units that, in contrast to the above examples, is a fully conjugated system. Just as in carbazole-3,6-diyl copolymers, polymer 245 showed absorption and PL spectra similar to those of PFO 196, with almost the same PL QE. However, there was no sign of the green emission band in this copolymer after thermal annealing (Chart 2.63). 

Introduction of electron-accepting hi thieno[3,2-6 2, 3 -e]pyri dine units resulted in copolymer 308 with ca. 0.5 V lower reduction potential compared to the parent homopolymer PFO 195 [398]. Upon excitation at 420 nm (A ax =415 nm), copolymer 308 exhibited blue-green emission with two peaks at 481 and 536 nm. Preliminary EL studies of an ITO/PEDOT/308/A1 device showed two peaks positioned as in the PL spectra. The PLED exhibited low turn-on voltage ( 4 V) but at higher voltages of 6-9 V, a slight increase in the green component was observed (Chart 2.83). 

A very efficient energy transfer (producing emission at 613 nm) was observed in PL spectra of the perylene end-capped polymer 361 in solid films. This material had the highest QE (>60%) among the fluorene- perylene polymers, although the performance of its PLED has not yet been reported [434],... 

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