Exciton emission

Excitonic emission, F state, 35 380-381 Exogenous ligand binding, [M-3Fe0134S] clusters, 38 365-368 Explosion calorimetry, 24 10 Extended fullerenes, 44 2... 

Electron-hole pairs may bind to each other to form excitons, which can be either free or tied to impurities or defects.1218 The decay of such excitons can lead to light emission that may be tunable by, for example, quantum confinement. Such excitonic emission is thus under active investigation in quantum well, wire, and dot... 

Abstract. The spectral dependence of photoluminescence and optical conductivity for the solid C6o and Cd-Q)0 films (with the admixture of C70 fullerenes) are studied under irradiation by argon ions with different doses. The fragmentation of the C6o molecules and the formation of the radiation defects, which are accompanied by appearance and increase in the intensity of the new component of the excitons emission, by decrease in the high-frequency optical conductivity spectral dependence to an analogous characteristic for the amorphous carbon films are observed with an increase in the radiation dose. This testifies that with the destruction of the molecules structure by ions the growth of the number of electrons, which are in the sp2 -hybridized state takes place. Furthermore, with the appearance of radiation defects the formation of the traps of the free charge carriers, which lead to a total decrease in the optical conductivity occurs. 

Figure 2. Spectra of the excitons emission for the Cd-C6o films (with the admixture of C70 Mlerenes) under irradiation hy the argon (Ar+) ions 1 - the initial, nonirradiated state 2 -the radiation dose is 8xl014 ion/cm2 3 - 27xl014 ion/cm2. The substrate is the stainless steel, d= 1200 nm, Tmelt = 473 K.

A (002) XRC width of 9 arcmin in 0-20 scan has been achieved for MBE grown epilayers [36], Sharp and flat interface structures have been reported using the HRTEM technique [21], For the optical properties, low temperature PL excitonic emission widths around 18 meV [4,34] and a photoreflectance (PR) spin-orbit splitting structure [36] have been reported. Concerning the electrical quality, there have been almost no reliable data, because 3C-SiC layers used as substrates are usually conductive. 

Figure 15 The kinetic scheme illustrating the interplay between exciton (S) and charge carrier (q) trapping by crystal defects (S0t)-The PL spectrum of the crystal contains the excitonic emission (kr, hvm) and the trap center emission (kj., hi ). the latter being controlled by the number of the defect sites available for excitation. The exciton capture process (yst) competes directly with charge carrier trapping (yqt). The defects filled with charge reduce the emission resulting from radiative relaxation of the excited states produced at defect sites. For further explanations, see text.

Abstract. The spectral dependence of photoluminescence and optical conductivity for the solid C60 and Cd-C6o films (with the admixture of C70 fullerenes) are studied under irradiation by argon ions with different doses. The fragmentation of the C60 molecules and the formation of the radiation defects, which are accompanied by appearance and increase in the intensity of the new component of the excitons emission, by decrease in the high-frequency optical conductivity o e) and... 

Excitonic emission from the F state should be a sharp line (almost resonant with the onset of the band gap), whereas emission from the S state should be a Stokes-shifted broad band, characteristic of localized deexcitation. Closer consideration suggests that the stable state changes abruptly from F-like to S-like when the ratio g = EulB exceeds a certain value (g - 1). The emission of Ti02 and CsVOa is an... 

Double band-emitting ZnSe Mn-ZnS colloidal quantum dots solubilized with various mercaptoacids demonstrate the different response in the emission intensity to pH level in a solution. In the case of mercaptoacetic acid both the excitonic and Mn emission bands increase the intensity with increased pH. For mercaptoundecanoic acid only the excitonic emission band was found to be sensitive to pH. The mechanism proposed is based on the competition between three recombination channels of an excited exciton direct radiative recombination and energy transfer to Mn ion. 

In all cases we started from original pH level of each solution which was varied from 8 for QDs solubilized with TGA to 9.4 for MUA. Then, we increased pH step-by-step and passed through nearly two pH. Then, with acidic titration we tried to go back to the initial pH level. For TGA solubilized QDs we obtained the largest changes in PL intensities of both excitonic and Mn bands with increased pH (Fig. lb). QDs solubilized with MUA show about twice less effect for excitonic band and almost no effect for Mn band (a weak decrease in Mn band intensity in Fig. Ic relates to the dilution of QDs with titrating solution). These results are in contrast to published earlier data where CdTe QDs capped with TGA shown the increase in the excitonic emission at acidic pH [5,6]. QDs solubilized with MPS were the least sensitive to the pH variations. 

From the data collected in Fig. 1 we may conclude that the Intensity of excitonic emission increases with an increased surface charge due to dissociation of TGA and MUA carboxyl groups at basic conditions. The dissociation of MPS sulfate groups is not influenced by the pH level. Also, this demonstrates that the presence of OH-groups themselves in the solution of QDs does not affect their PL emission. 

An explanation for the observed effects can be based on the assumption that carboxyl groups in protonated form may be a good photoelectron acceptor. This may be associated with the known easier electrochemical reduction of organic acids at low pH [8]. Excited electron-hole pairs in ZnSe core may recombine in few possible ways. First, a direct recombination results in the appearance of excitonic emission band at A,=408 nm. The second possible pathway is the energy transfer to Mn ion followed by Mn emission at A.=590 nm. At the neutral and acidic pH an additional recombination channel may be realized via trapping of photoelectrons by carboxyl groups (prior to the energy transfer to Mn ions)... 

FIGURE 2.3. A schematic representation of the elementary processes for charge carrier recombination, production of molecular excitons, emission, and external emission. 

The emission peak of MEH-PPV films at A,max 580 nm is predominately due to the single-chain exciton emission, whereas that at 630 nm is most likely due to an interchain species (the Ex-I species). The devices with a stronger red (630 nm peak) emission usually have higher quantum efficiency. 

In retrospect, by inspecting the literature, we find a confirmation of this variance (see for instance Ref. [67]). Peak intensities of bands originally assigned to Franck-Condon components of the excitonic emission have random relative intensities. This would not be possible if the bands were intrinsically vibronic. Since we know that the excitonic emission, as it is observed in single crystals, is rather sharp at low temperatures, we were forced to reconsider the assignment of the fluorescence of thin films. From the temperature dependence of the fluores-... 

In AgBr the free exciton emission is manifest as a number of very weak sharp lines between 2.60 eV (477 nm) and 2.70 eV (460 nm) [66], The free exciton zero phonon line, which is extremely weak, is at 2.685 eV and there are a series of phonon replicates at lower energies usually involving TO, TA, LA, and LO phonons and some combinations. The TO phonon lines are usually the strongest lines in a series (see, for example, the insert in Figure 5, which probably represents enhanced free exciton emission). 

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