Excitons luminescence

Nishimura, H., Yamaoka, T., Hattori, K., Matsui, A. and Mizuno, K. (1985) Wavelength-dependent decay times and time-dependent spectra of the singlet-exciton luminescence in anthracene crystals./. Phys. Soc. Jpn., 54, 4370-4381. Matsui, A. and Nishimura, H. (1980) Luminescence of free and self trapped excitons in pyrene. J. Phys. Soc. Jpn., 49, 657-663. 

Fig. 17 Photoluminescence spectra covering the no-phonon and TA phonon-replica energy regions taken at 4.2 K. The spectra show the bound exciton luminescence of samples implanted with B, In, and T1 before (a, c, e) and after (b, d, f) treatment in atomic H. Bound exciton luminescence due to the implanted impurities has been shaded in to distinguish it from the substrate luminescence. From Thewalt et al. (1985).

The photoluminescence (PL) spectrum in Figure 1.7 shows a number of lines related to nitrogen-bound excitons and free excitons. SiC has an indirect bandgap, thus the exciton-related luminescence is often assisted by a phonon. Bound exciton luminescence without phonon assistance can, however, occur because conservation in momentum can be accomplished with the help of the core or the nucleus of the nitrogen atom. That is why the zero phonon lines of the nitrogen atom are seen, denoted and Q , in the spectrum but not the zero phonon line of the free exciton. 

Addition of alkylamines dramatically increased exciton luminescence of the semiconductor particles... 

Fig. 15. (a) Schematic Er excitation model, showing the electronic band structure of Si nanocrystall-doped Si02 and the Er 4f energy levels. An optically generated exciton (dotted line) confined in the nanocrystal can recombine and excite Er3+. (b) Schematic representation of SiC>2 containing Er (crosses) and nanocrystals (circles). The nanocrystals that couple to Er (filled circles) show no exciton luminescence (redraw after (Kik and Polman, 2001)). 

The 1150°C annealed sample PL is comparable to the reference except that its exciton luminescence has not recovered, which suggests that implant damage has not been fully removed by annealing. 

In order to increase the efficiency and suppress the slow excitonic luminescence, the effect of the incorporation of Ce3+ in BaF2 has been examined, the presence of the Ce3+ 4f- 5d absorption bands in the wavelength range of the excitonic luminescence allowing energy transfer [48,49],... 

Time dependent fluorescence depolarization is influenced by the exciton annihilation which occurs in confined molecular domains . Photoemission results from singlet exciton fusion as shown by the excitation intensity dependence which occurs in anthracene crystals. Reabsorption of excitonic luminescence is an effect which has been shown to occur in pyrene crystals. The dynamics of exciton trapping in p-methylnaphthalene doped naphthalene crystals involves phonon assisted detrapping of electronic energy. Ps time resolved spectroscopy was the experimental technique used in this work. 

These conditions lead to simple and rather general relations between the excitonic absorption spectrum and the spectrum of excitonic luminescence. 

The number of photons of the excitonic luminescence, created in the crystal per second, depends on the frequency u> as follows... 

Fig. 3.3. Ground state potential and asymmetric double-well potential associated with the phenomenon of exciton self-trapping, as a function of the coordinate rj that undergoes a strong displacement upon self-trapping. F is the bottom of the free-exciton band, in which the lattice is not distorted (77 = 0), S denotes the lowest self-trapped exciton state, and U is the barrier height. The luminescence from the self-trapped state is red-shifted relative to the free-exciton luminescence. Upon photoexcitation of the system, two pathways towards the self-trapped state occur. The first possibility is that the created excitons first relax towards the bottom of the free-exciton well, after which they may further relax to the self-trapped state through tunneling or a thermoactivated process. This pathway is indicated by the filled arrows. The second possibility is that high-energy (hot) excitons relax directly to the self-trapped state, as indicated by the open arrow. Reprinted with permission from Knoester et al. (47). Copyright Elsevier (2003).

For studies of the dynamics of energy transfer from a semiconductor quantum well to organics it is very important to know the origin of exciton luminescence of the semiconductor quantum well under electric or optical nonresonant pumping... 

However, the process of free excitons binding in a deep local state need not be taken into consideration if no account is taken of the processes of nonradiative decay of single excitons, whereby the energy A goes over into the phonon energy. In crystals, where the quantum yield of exciton luminescence is close to unity (for instance, in anthracene crystals), the nonradiative decay of excitons cannot be realized within the exciton lifetime (otherwise we cannot regard the number of excitons in the crystal in consideration of collective processes as specified). 

Fig. 2.37 Electroluminescence from the same PFB/F8BT bilayer device as in Fig. 2.31 measured at 43 K and 9V (black curve), 10V (red), and 13 V (green) bias. Current densities were 0.02, 0.14, and 11 mA/cm2. At higher fields, injection overthe heterojunction becomes feasible and capture occurs in the bulk resulting in both yellow-green F8BTand blue PFB exciton luminescence. Because of the high noise level, the 9- curve has been smoothed using an adjacent averaging algorithm and both the 9- and 10-V curves have been truncated below 510nm.

Surface plays an important role in excited state relaxation processes. In the ideal case of a three-dimensionally confined exciton, one expects to see strong exciton luminescence due to enhanced overlap of the electron and hole wavefunction. The radiative rate of the exciton should increase with increasing cluster size. In reality, this is generally not observed. Most of the luminescence spectra of semiconductor nanoclusters consist of a stokes-shifted broad luminescence band, usually attributed to emission from surface defects. Sometimes near the band edge, an exciton-like luminescence band can be observed. Various passivation procedures have been used to enhance the exciton luminescence. These are discussed in Section III. 

For semiconductor clusters with larger Bohr radius, such as CdS and CdSe, the observation of the superradiant effect proves to be more elusive. This is mainly due to the difficulty of preparing high-quality samples of varying sizes of clusters that exhibit exciton luminescence. The spectra and kinetics of the luminescence are usually very complicated, which makes the positive identification of exciton luminescence difficult. For example, sharp band-edge luminescence with well-resolved vibronic structures was observed from 32-A CdSe clusters [60]. The decay kinetics of the luminescence is multiexponential and only the first 100-psec decay is identified with the exciton luminescence. The lifetime of this luminescence, however, is tempera-... 

Sometimes luminescence with long lifetimes can be observed near the absorption edge. The lifetimes of these luminescences are too long to be attributed directly to the exciton luminescence. Several studies have sought to elucidate the nature of band-edge luminescences [50, 60, 62, 63]. 

CdS clusters of narrow size distribution were studied by Eychmuller et al. [63], In this case a rather narrow luminescence band can be observed near the absorption band. The decay kinetics of this excitonic luminescence is multiexponential with a typical lifetime on the order of nanoseconds, much longer than the expected exciton lifetime. The temperature dependence of the excitonic luminescence shows complex behavior. Again, the authors use the three-level thermal equilibrium model to explain the data. The excitonic luminescence is identified as delayed luminescence occurring by detrapping of trapped electrons. Furthermore, they invoke the concept... 

Surface defects such as dangling bonds and vacancies may, in principle, be removed by chemical passivation techniques. A successful passivation procedure should result in the disappearance of defect luminescence and the appearance of strong exciton luminescence. Many chemicals have been tried for this purpose but with few successes. The two most successful passivation techniques developed so far involve the use of ammonia [50] and hydroxide ion [35]. In both cases, defect luminescence is greatly reduced with the... 

Cuba, V., Niki, M. 2011. Radiation-assisted preparation of powder materials and their exciton luminescence. In Exciton Quasiparticles, ed. R. M. Bergin, pp. 181-210. New York Nova Science. 

Wilkinson, J. Ucer, K. B., Williams, R. T. 2004. Picosecond excitonic luminescence in ZnO and other wide-gap semiconductors. Radiation Measurements 38 501-505. 

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