Surface excitons experimental observations

One of the most obvious perturbations of the surface excitons by the bulk crystal is the short-range coulombic interactions causing surface-exciton transfer to the bulk. We discard these perturbations on the grounds both of theoretical calculations27 and of the experimental observations, which show the presence of a surface exciton (the second subsurface exciton S3 see Fig. 3.2) resolved at about 2cm-1 above the bulk-exciton resonance. 

To explain the observed width, it is necessary to look for strong surface-to-bulk interactions, i.e. large magnitudes of surface-exciton wave vectors. Such states, in our experimental conditions, may arise from virtual interactions with the surface polariton branch, which contains the whole branch of K vectors. We propose the following indirect mechanism for the surface-to-bulk transfer The surface exciton, K = 0, is scattered, with creation of a virtual surface phonon, to a surface polariton (K / 0). For K 0, the dipole sums for the interaction between surface and bulk layers may be very important (a few hundred reciprocal centimeters). Through this interaction the surface exciton penetrates deeply into the bulk, where the energy relaxes by the creation of bulk phonons. The probability of such a process is determined by the diagram... 

After these remarks let us return again to the first monolayer of the anthracene crystal. Note, first of all, that the width of the exciton band in this crystal for wavevectors directed along the C -axis (i.e., along the normal to the (a, b) plane) is very small ( 5 cm-1) in comparison with bandwidth of excitons with wavevectors parallel to the (a, b) plane and it is very small in comparison with the blue-shift of the exciton level in the outermost monolayer. For this reason, first of all, the mobility of excitons located in monolayers in the direction towards the surface is rather small. On the other hand, as the energy of he exciton located at the outermost monolayer is larger than the energy of the exciton in the bulk the interaction of bulk excitons with the surface is repulsive. We have here a type of dead layer for the bulk exciton. Qualitatively this picture helps explain why the experimental observation of photoluminescence of excitons located at the outermost monolayer of the anthracene crystal was possible. 

At T = 2 K, in the jt-polarised reflection spectrum of the (OOl)-cleavage planes of anthracene crystals, one observes spectrally-resolved surface excitons (Fig. 6.17). The Davydov splittings of their 0,0 transitions are only 10-20% smaller than those of the volume excitons (Sect. 6.7 and Ref [39]). Discuss this experimental result qualitatively (cf Sect. 6.5.2). 

Experimental data on unoccupied surface states are again only indirectly obtained via NEXAFS [68]. As for the clean diamond (100) surface, a surface core exciton with an excitation energy 4.8 eV lower than that of the bulk core exciton is observed that can be taken as a qualitative confirmation for the existence of unoccupied surface states within the band gap (S in Figure 10.15a). For C(lll)l x 1 H only the... 

Measurements of the optical properties in this range of wavelengths can probe the fundamental electronic transitions in these nanostructures. Some of the aforementioned effects have in fact been experimentally revealed in this series of experiments (90). As mentioned above, the IF nanoparticles in this study were prepared by a careful sulfidization of oxide nanoparticles. Briefly, the reaction starts on the surface of the oxide nanoparticle and proceeds inward, and hence the number of closed (fullerene-like) sulfide layers can be controlled quite accurately during the reaction. Also, the deeper the sulfide layer in the nanoparticle, the smaller is its radius and the larger is the strain in the nanostructure. Once available in sufficient quantities, the absorption spectra of thin films of the fullerene-like particles and nanotubes were measured at various temperatures (4-300 K). The excitonic nature of the absorption of the nanoparticles was established, which is a manifestation of the semiconducting nature of the material. Furthermore, a clear red shift in the ex-citon energy, which increased with the number of sulfide layers of the nanoparticles, was also observed (see Fig. 21). The temperature dependence of the exciton... 

Comparison of these calculated exciton transitions with the experimental data in Table V shows that the main features of the results are reproduced. The energies for the <100) surface (5-coordinated ions) are only slightly shifted from the bulk, whereas those transitions corresponding to the higher index planes are much closer to the experimental data. On an atomic scale this means that ions whose coordination numbers are 4 and 3 are involved in the observed transitions, whereas 5-coordinated ions at the surface will absorb at higher energies closer to the bulk band edge. This theoretical treatment is approximate since it considers only an ideal surface and assumes that the electron affinity and ionization potential are constant for the different planes. In fact, the evidence already presented on electron transfer in Section VI,A indicates that the ionization potential varies with the coordination of the ion. 

Experimental study of the size-dependent oscillator strength, which determines the radiative rate, is discussed in Section I1I.A. There has been no systematic study of the size dependence of the absorption cross section of semiconductor nanoclusters. The enhancement in the exciton absorption cross section with decreasing cluster size has been observed for CdS qualitatively (e.g., Figure 1) [47,50]. Quantitative study is still not available because of problems such as size inhomogeneity and surface defects. One of the most puzzling observations is that many of the semiconductor clusters synthesized so far show no exciton absorption bands at all, in spite of the positive identification of their existence by X-ray diffraction [2, 11, 51]. While size inhomogeneity is usually conveniently invoked as the explana-... 

This paper is devoted to the presentation of a brief overview of a recently-developed "relaxation-localization" model of localized molecular-ion and exciton states in polymers and molecular glasses. This model was proposed initially to interpret photoemission measurements from two pgn ant-group polymers polystyrene and p ly(2-vinyl pyridine.) It ext was utilized in the prediction and subsequent observation of surface states of molecular solids as well as of the temperature dependence of photoe iss on and UV absorption linewidths of molecular films. Having proven successful in describing the spectroscopic properties of typical pendant-group polymers and molecular glasses, the model most recently has been extended to provide a description of electron-transfer processes in both these materials and molecularly-doped polymers. Therefore it affords a unified and experimentally-verified microscopic description of electron ionization, excitation and transfer processes in a variety of molecular and polymeric materials. 

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