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1.
The nature and relative contributions (RCs) of various radiative recombination processes to the photoluminescence (PL) spectra for bulk nanostructured Pb0.30Cd0.70I2 solid solutions have been established at different temperatures. The analysis indicates that the PL is caused by free, bound and self-trapped excitons as well as by donor-acceptor pairs emission with the participation of shallow and deep acceptor centers. It was shown that X-ray luminescence (XRL) spectra are also determined by these recombination processes. However, their RC and the temperature evolution are considerably different. Besides, XRL spectra contain an intense long-wavelength band associated with the emission of many LO-phonons. It was shown that the deep luminescence surface states, associated with the self-trapped excitons and deep intrinsic defects, mainly determine the intensity of XRL spectra both at 80 K and room temperature. The results obtained open the way to the optimization of the scintillator properties of the investigated materials. 相似文献
2.
Exciton states with a large oscillator strength (transverse-longitudinal splitting of 50 meV) and binding energy of 290 meV have been observed in PbGa2S4 crystals. The ground exciton states are stable up to the room temperature. The ground (n = 1) exciton state and two excited (n = 2 and n = 3) states of the short-wavelength B-excitons series were observed at low temperature (T = 10 K). The ground (n = 1) and the excited (n = 2) states of the long-wavelength A-exciton series were also revealed. The exciton Bohr radius of the ground exciton state was determined to be 70 Å for the A-exciton and 10 Å for the B-exciton. The band splitting and the main parameters of excitons have been determined from the calculation of the reflectivity spectra contours. The group theory analysis of the band symmetry was performed and the scheme of bands responsible for exciton transitions in the center of the Brillouin zone was proposed. 相似文献
3.
B. Škipina D.Lj. Mirjani?S.M. Vu?enovi? J.P. Šetraj?i?I.J. Šetraj?i? A.J. Šetraj?i?-Tomi?S.S. Pelemiš B. Markoski 《Optical Materials》2011,33(11):1578-1584
We have formulated a microscopic theory of optical properties of ultrathin molecular films (nanofilms), i.e. quasi 2D systems parallel to XY planes bounded by two surfaces. Exposure of nanofilms to the external electromagnetic fields has result in creation of excitons - but different than bulk ones. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green’s functions. It has been shown that two types of optical excitations can occur: bulk and surface exciton states. Exciton energy dispersion law shows discrete behavior with non-zero values. Analysis of the dielectric properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. In addition, permittivity shows very narrow and discrete dependence of external electromagnetic field frequency, which is a consequence of both resonance and quantum size effects. Influences of boundary conditions on optical characteristics (through analyses of dynamical absorption coefficient) of these nanostructures were specially and in details explored. 相似文献
4.
T. Nishioka K. Suruga N. Natsume K. Ishikawa H. Takezoe A. Fukuda 《Synthetic Metals》1996,80(3):315-318
A trans-polyacetylene film was directly synthesized to reduce defects introduced during usual thermal isomerization treatment. A photocurrent excitation spectrum of this film showed very low efficiency around a reflection peak which was accompanied with phonon side bands. This strongly indicates that the lowest optically allowed state of trans-polyacetylene is a 11Bu exciton state like other conjugated polymers, such as polydiacetylenes. Misinterpretations of the experimental results under the SSH model, because of the use of a film of low quality, are pointed out and reinterpretations are made according to the present conclusion. 相似文献
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Thin films of microcrystalline (CnH2n + 1NH3)2PbBr4 (n = 4, 5, 7 and 12) have been prepared by a modified spin-coating method, and the effect of the number of carbon atoms of the alkyl chain length (n) on optical properties has been investigated. Absorption spectra reveal that (CnH2n + 1NH3)2PbBr4 films show stable excitons with a binding energy of a few hundred meV. The excitonic structure of (CnH2n + 1NH3)2PbBr4 varies with the number of carbon atoms. The lowest-energy exciton splits into a few fine-structure levels at low temperature. (CnH2n + 1NH3)2PbBr4 films (n = 5, 7 and 12) show not only singlet excitons but also triplet excitons at low temperature, while (C4H9NH3)2PbBr4 films show only singlet excitons. The intersystem crossing from excited singlet state to triplet state plays an important role in the relaxation process of excitons. 相似文献
8.
A. Rivera J. Olivares G. García 《Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms》2010,268(13):2249-24257
We present a MonteCarlo approach to the non-radiative exciton-decay model recently proposed to describe ion-beam damage in LiNbO3 produced in the electronic excitation regime. It takes into account the statistical (random) spatial distribution of ion impacts on the crystal surface. The MonteCarlo approach is necessary to simulate the evolution of the damage morphology with irradiation fluence from the single track regime to the overlapping track regime. A detailed comparison between the morphologies found for sub-threshold and above threshold irradiations is presented. Moreover, a good representation of the Avrami’s type kinetics for amorphization has been achieved and it is in fair accordance with experiment. For moderate fluences where homogeneous amorphous layers are generated, the new approach predicts that the amorphous and crystalline layers are separated by a diffuse (thick) boundary that includes a mixed amorphous-crystalline composition. 相似文献
9.
S. Nedilko P. Nagornii I. Nedyelko V. Scherbatskii N. Stus O. Gomenyuk V. Sheludko V. Boyko R. Boyko 《Optical Materials》2009,31(12):1828-1830
The results of the photoluminescence (PL) investigation of pure and chromium-doped MAlP2O7 (M = Na, K, Cs) compounds are presented. The spectra of the intrinsic luminescence of MAlP2O7 crystals consist of a separated UV band at a peak position near 330 nm and a complex wide band which covers the region of visible light up to 750 nm at excitation by VUV synchrotron radiation. The “red” band in 600–1000 nm diapason appears in the PL spectra of crystals doped with chromium ions. The effect of the temperature on the structure, the peak positions and intensities of the luminescence bands was studied. An assumption about the nature of the intrinsic PL was made. The “red” luminescence was considered as a result of the 4Т2 → 4А2 radiation transitions in the impurity Cr3+ ions located in the intermediate crystal field. 相似文献
10.
Johan F. Prins 《Materials Research Innovations》1998,1(4):243-253
Two identical, high purity, natural type IIa diamonds, which displayed the ubiquitous blue cathodoluminescence (CL) band
at ≈ 2.9 eV, as well as an indication of the corresponding green band at ≈ 2.4 eV, have been equivalently doped by using extremely
low dose B+- and C+-ion CIRA-implantations respectively. Comparative CL measurements showed changes in the intensities of the 2.9 and 2.4 eV
bands and the generation of bands at ≈ 4 eV, as well as at ≈ 3.5 and ≈ 4.6 eV (the latter two in the B+-CIRA diamond). The results are commensurate with the model (proposed previously) in which the 2.9 and 4 eV bands are generated
respectively by electron-hole recombinations at negatively charged acceptor- and positively charged donor-like, intrinsic
defects. The present results indicate that Coulomb interactions between the latter defects and (at least partially) compensated,
negatively charged, boron acceptors, generate the 3.5 and 4.6 eV bands, which may be considered as higher energy (≈ 0.6 eV)
replicas of the 2.9 and 4 eV bands. In both cases, two electrons and a hole interact just before the hole combines with an
electron. Such a configuration of charges seems related to, and could possibly be described as, a type of ”ionised exciton
molecule”, where the ”bonding” of two negative ”nuclei” is facilitated by the presence of the hole. The CL measurements further
indicate that the 2.4 eV band forms when a high enough density of, in this case, neutral acceptors are present. These neutral
acceptors compete with the valence band to supply holes for recombination at the negatively charged, acceptor-type, intrinsic
defects which are, in the absence of the boron, responsible for the generation of the blue, 2.9 eV band.
Received: 5 December 1997 / Accepted: 13 December 1997 相似文献