Nanocomposites ofpoly[(2-methoxy,5-octoxy)1,4-phenylenevinylene]-zinc selenide(MOPPV-ZnSe) are synthesized by mixing the polymerization of 1,4-bis(chloromethyl)-2-methoxy-5-octoxy-benzene in the presence of ZnSequantum dots. The resulting MOPPV-ZnSe nanocomposites possess awell-defined interfacial contact, thus significantly promoting thedispersion of ZnSe within the MOPPV matrix and facilitating theelectronic interaction between these two components. Raman andUV--visible absorption spectra are influenced by the incorporationof ZnSe nanocrystals. High-resolution transmission electronmicroscopic and tapping-mode atomic force microscopic results showclearly the evidence for phase-segregated networks of ZnSenanocrystals, which provide a large area of interface for chargeseparation to occur. Steady-state spectra of MOPPV-ZnSenanocomposites are markedly quenched by the introduction of intimatepolymer/ZnSe junctions. Time-resolved photoluminescencemeasurements show that the lifetime decays quickly, which furtherconfirms the occurrence of charge transfer in MOPPV-ZnSenanocomposites. 相似文献
We report results from optical spectroscopy such as photoluminescence (PL) and time resolved photo-luminescence (TRPL) techniques
from different well width MOCVD grown GaN/Al0.07Ga0.93N MQW samples. There is evidence of localization at low temperature in all samples. The decay time of all samples becomes
non-exponential when the detection energy is increased with respect to the peak of the emission. Localization of carriers
(excitons) is demonstrated by the “S-shape” dependences of the PL peak energies on the temperature. The time-resolved PL spectra
of the 3-nm well multi quantum wells reveal that the spectral peak position shifts toward lower energies as the decay time
increases and becomes red-shifted at longer decay times. There is a gradient in the PL decay time across the emission peak
profile, so that the PL process at low temperatures is a free electron-localized hole transition. 相似文献
Time‐resolved excitation spectra were measured at the same conditions at 10 K in the 4–20 eV energy range for the luminescence of self‐trapped and various localized excitons as well as of Ce3+‐, Pb2+‐, and Bi3+‐related centers in single crystals and single crystalline films of Lu3Al5O12 garnet. The decay kinetics of the intrinsic 4.9 and 3.65 eV luminescence of Lu3Al5O12 crystals were studied at 4.2–300 K in the 10−9–10−3 s time range. The origin and structure of the exciton states as well as the luminescence, energy transfer, and recombination processes, taking place under excitation in the exciton absorption region, were discussed. The influence of various defects on the scintillation characteristics of Ce3+‐ and Pr3+‐doped Lu3Al5O12 crystals was considered. 相似文献
The growth of semipolar AlN and AlGaN epilayers on m‐plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD) has been investigated for the first time. The implementation of pulsed MOCVD technique for the deposition of the AlN buffer, and the insertion of a strain relieving AlN/AlGaN short‐period superlattice structure proved instrumental in the growth of a thick, crack‐free, and single domain n‐Al0.56Ga0.44N ( ) films, which also exhibit a good crystal quality. To assess the suitability of this AlGaN/AlN/m‐sapphire template for practical use in deep‐ultraviolet light emitters, both polar (0001) and semipolar ( ) AlGaN multiple quantum wells (MQWs) were grown side‐by‐side. The room temperature photoluminescence (PL) spectra of the semipolar MQW structure peaked at 305 nm, which was approximately 10 nm longer than the PL peak originating from the reference polar MQWs. Besides, the semipolar MQWs showed a fairly excitation‐independent emission wavelength, which suggests the absence of any polarization‐induced electric fields. The intensity of the luminescence from the c‐oriented MQWs was, however, stronger than that of its semipolar counterpart. The result indicates that further reduction of the extended defects density in ( ) AlGaN is critical to significantly improving the optical properties of the overlying quantum heterostructures, and potentially achieving efficient optoelectronic devices. 相似文献
The time-resolved photoluminescence (TRPL) of sintered ZnO ceramics was measured at low temperatures. A broad luminescence band was observed in the visible region. The TRPL experiment shows that photoluminescence decay behaviour can be depicted as t-n(r). The decay rate n(r) and lifetime are wavelength dependent, and the former varies exponentially with wavelength. The power-lowering behaviour of the luminescence intensity indicates that the luminescence band originates from the recombination of donor-acceptor pairs. 相似文献
MOCVD AlN layers grown in c‐direction on PVT bulk AlN substrates are investigated by high‐resolution photoluminescence in the energy region of the band gap. The experiments allow for observation of different donor bound exciton lines with a full width at half maximum below 0.5 meV. Assignments are suggested for the substitutional shallow donors silicon and oxygen. The visibility of an excited state of the free exciton with a hole from the highest valence band allows to determine the exciton binding energy to 52.4 meV by use of a simple hydrogen model. The resulting band gap energy is 6.094 eV at 10 K. Comparison with earlier reports on homoepitaxial AlN leads to a new identification of the bands reported there. 相似文献
Freestanding GaN layers of various thicknesses grown by HVPE have been studied by time-resolved spectroscopy combined with structural and electrical measurements. We have observed an increase of the PL lifetime with increasing layer thickness; however, a saturation of the recombination times has been detected for the GaN layers thicker than 400 μm. We explain the observed thickness-dependent behavior of the decay times by competition of two nonradiative mechanisms; namely, for layers with thickness less than 400 μm the main nonradiative channel is related to the structural defects, while in thicker layers the recombination decay time is limited by impurities and/or vacancies. 相似文献
The lifetime of electrostatically trapped indirect excitons in a field-effect structure based on coupled AlGaN/GaN quantum wells has been theoretically studied. Within the plane of a double quantum well, indirect excitons are trapped between the surfaces of the AlGaN/GaN heterostructures and a semitransparent metallic top gate. The trapping mechanism has been assumed to be a combination of the quantum confined Stark effect and local field enhancement. In order to study the trapped exciton lifetime, the binding energy of indirect excitons in coupled quantum wells is calculated by finite difference method in the presence of an electric field. Thus, the lifetime of trapped excitons is computed as a function of well width, AlGaN barrier width, the position of double quantum well in the device and applied voltage. 相似文献
Amorphous silicon carbide films are deposited by the plasma enhanced chemical vapour deposition technique,and optical emissions from the near-infrared to the visible are obtained.The optical band gap of the films increases from 1.91 eV to 2.92 eV by increasing the carbon content,and the photoluminescence(PL) peak shifts from 1.51 eV to 2.16 eV.The band tail state PL mechanism is confirmed by analysing the optical band gap,PL intensity,the Stocks shift of the PL,and the Urbach energy of the film.The PL decay times of the samples are in the nanosecond scale,and the dependence of the PL lifetime on the emission energy also supports that the optical emission is related to the radiative recombination in the band tail state. 相似文献
The optical properties of single‐wall carbon nanotubes (SWNTs) are dominated by the excitonic character of the transitions even at room temperature. The very peculiar properties of these excitons arise from both the one‐dimensional (1D) nature of carbon nanotubes and from the electronic properties of graphene from which nanotubes are made. We first propose a brief qualitative review of the structure of the excitonic manifold and emphasize the role of dark states. We describe recent experimental investigations of this excitonic structure by means of temperature dependent PL measurements. We investigate the case of upper sub‐bands and show that high‐order optical transitions remain excitonic for large diameter nanotubes. A careful investigation of Rayleigh scattering spectra at the single nanotube level reveals clear exciton–phonon side‐bands and Lorentzian line profiles for all semi‐conducting nanotubes. In contrast, metallic nanotubes show an ambivalent behavior which is related to the reduced excitonic binding energy.
