Surface excitons emission

The excitation spectra of surface emission show, besides the a-polarized surface-exciton Davydov component, vibronic surface structures which... 

In this theory, the dynamics of the intrinsic-surface-confined excitons account surprisingly well—in a natural way, without introducing ad hoc parameters—for the surface emissive properties, and they allow, a contrario, a very sensitive probing of various types of surface disorders, whether residual, accidental, or induced. The disorder may be thermal, substitutional, chaotic owing to surface chemistry, or mechanical owing to interface compression. It may be analyzed as a specific perturbation of the surface exciton s coherence and of its enhanced emissive properties. 

Excitation spectroscopy Monitoring of the surface emission allows one to discriminate the upper excited surface states and their relaxation dynamics. Problems such as surface reconstruction, or quantum percolation of surface excitons upon thermal and static disorder, are connected with high accuracy to changes of the exciton spectra.61118,119,121... 

The emission of the surface exciton, coupled to the bulk polaritons, may also be calculated using the above scheme, which, here also, coincides exactly with the quantuum-mechanical results. Instead of the lorentzian emission (3.20), we obtain an emission proportional to... 

Fig. 3.17 shows the b-emission excitation spectrum, with an a-polarized excitation, in the region E°° + 220 cm "1 and above. We observe a strong and broad peak, attributed here to the a surface component, following refs. 117,67. This peak is strongly asymmetric, broadened on the high-energy side. The value of the component indicated on the maximum (25 523 cm 1, at 222 cm 1 above the b component) may be falsified by the asymmetry of the band. (This will be examined in Section III.B.3 below.) We find that the surface-exciton Davydov splitting is quite comparable with its bulk counterpart (222 cm"1 vs 224 cm-1), to the accuracy of our experiments. Furthermore, the two 0-0... 

Figure 3.22. Model relaxation of the surface excitons created inside or near the threshold of a two-particle-state continuum (illustrated for the 390-cm 1 mode). After excitation (1) at the energy Ekh + 390 cm. a two-particle state is created (2) by fission. Then the exciton may relax along two competing paths an intrasurface channel (3) leading to emission, and a nonradiative channel (3 ) to the bulk (eventually to its fluorescence), with respective probabilities ris and rB. Therefore, the surface emission efficiency depends on the ratio rjrB, which determines the observed profile. When the excitation occurs at exactly 390 cm 1 above the detection, we observe the very narrow Raman peak.

This section has been devoted to the study of the surface excitons of the (001) face of the anthracene crystal, which behave as 2D perturbed excitons. They have been analyzed in reflectivity and transmission spectra, as well as in excitation spectra bf the first surface fluorescence. The theoretical study in Section III.A of a perfect isolated layer of dipoles explains one of the most important characteristics of the 2D surface excitons their abnormally strong radiative width of about 15 cm -1, corresponding to an emission power 10s to 106 times stronger than that of the isolated molecule. Also, the dominant excitonic coherence means that the intrinsic properties of the crystal can be used readily in the analysis of the spectroscopy of high-quality crystals any nonradiative phenomena of the crystal imperfections are residual or can be treated validly as perturbations. The main phenomena are accounted for by the excitons and phonons of the perfect crystal, their mutual interactions, and their coupling to the internal and external radiation induced by the crystal symmetry. No ad hoc parameters are necessary to account for the observed structures. 

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. 

Apparently, in the near future there will be developed (a) a detailed theory of surface excitons not only at the crystal boundary with vacuum but also at the interfaces of various condensed media, particularly of different symmetry (b) a theory of surface excitons including the exciton-phonon interaction and, in particular, the theory of self-trapping of surface excitons (c) the features of surface excitons for quasi-one-dimensional and quasi-two-dimensional crystals (d) the theory of kinetic parameters and, particularly, the theory of diffusion of surface excitons (e) the theory of surface excitons in disordered media (f) the features of Anderson localization on a surface (g) the theory of the interaction of surface excitons of various types with charged and neutral particles (h) the evaluation of the role of surface excitons in the process of photoelectron emission (i) the electronic and structural phase transitions on the surface with participation of surface excitons. We mention here also the theory of exciton-exciton interactions at the surface, the surface biexcitons, and the role of defects (see, as example, (53)). The above list of problems reflects mainly the interests of the author and thus is far from complete. Referring in one or another way to surface excitons we enter into a large, interesting, and yet insufficiently studied field of solid-state physics. 

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

Photoluminescence could be due to the radiative annihilation (or recombination) of excitons to produce a free exciton peak or due to recombination of an exciton bound to a donor or acceptor impurity (neutral or charged) in the semiconductor. The free exciton spectrum generally represents the product of the polariton distribution function and the transmission coefficient of polaritons at the sample surface. Bound exciton emission involves interaction between bound charges and phonons, leading to the appearance of phonon side bands. The above-mentioned electronic properties exhibit quantum size effect in the nanometric size regime when the crystallite size becomes comparable to the Bohr radius, qb- The basic physics of this effect is contained in the equation for confinement energy,... 

Fig. 2 illustrates (he room-temperature photoluminescence (PL) spectra recorded from the as-prepared ZnO colloidal solution and the ZnO nanostructure formed after deposition of the colloid on the silicon substrate. An UV band at 385 nm was detected from all ZnO products. In addition, a broad orange-red photoluminescence band centered at around 620 nm could be also observed in some materials. The UV photoluminescence peak at 385 nm is well known to be related to the exciton emission, ihe mechanism of visible emission is suggested mainly due to the present of various point defects, either extrinsic or intrinsic, which can easily form recombination centers. Photoluminescence measurements show that the deposited ZnO nanostructures have the stronger UV emission than the ZnO nanoparticles in the colloidal solutions. The better UV emission characteristic of deposited ZnO is suggested to be due to the lower defect density and oxygen vacancies in ZnO nanocrystals in the first case. Similar results have also been reported previously [8]. In addition, the aqueous surrounding can change the surface states of ZnO nanocrystals. It is well known that surface states may... 

Bandis C, Pate BB. Photoelectric emission from negative-electron-affinity dia-mond(lll) surfaces exciton breakup versus conduction-band emission. Phys Rev B 1995 52 12056-12071. 

It should be underlined that the luminescence spectra of low-defect density nonpolar GaN material grown by optimized ELOG templates are dominated by the exciton-related emissions, and the defect-related recombinations have been significantly reduced [70]. Furthermore, for the bulk GaN material with nonpolar surfaces sliced from boules, which have been grown in the c direction, the low-defect density is reflected in a high optical quality [86, 100]. Namely, low temperature PL spectra in the NBE region show free-exciton emissions... 

Suppression of the defect-related emissions and enhancement of the NBE exciton emission has been found in layers produced by different ELO techniques. Such layers have an improved structural quahty owing to reduced density of the TDs and BSE in the overgrowth regions if the ELO stripes are with optimal orientation, that is the orientation perpendicular to the c axis or to its projection. In addition, the ELO layers possess a smooth surface morphology and can serve as templates for subsequent growth of device structures. 

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