We report the first microporous cobalt imidazolate glass obtained from a meltable cobalt-based zeolitic imidazolate framework, ZIF-62(Co).
We present a detailed investigation of the exciton and trion dynamics in naturally doped MoSe 2 and WSe 2 single atomic layers as a function of temperature in the range 10-300K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied with strong changes in PL including suppression of the trion PL and decrease of the trion PL life-time, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and appearance of a pronounced * To whom correspondence should be addressed † TU Dortmund ‡ University of Sheffield ¶ University of Manchester 1 slow PL decay component. In MoSe 2 and WSe 2 studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at T ≈10K. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in MoSe 2 and WSe 2 , sample doping, trion and exciton localization and various temperature-dependent non-radiative processes.
Hybrid structures synthesized from di erent materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg)Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an e ective p-d exchange that is mediated by elliptically polarized phonons. E xchange interactions are the origin for correlated magnetism in condensed matter with multi-faceted behaviour such as ferro-, antiferro-or ferrimagnetism. In magnetic semiconductors (SCs), the exchange occurs between free charge carriers and localized magnetic atoms 1-4 and is determined by their wavefunction overlap. To control this overlap, hybrid structures consisting of a ferromagnetic (FM) layer and a semiconductor quantum well (QW) are appealing objects because they allow wavefunction engineering. Furthermore, the mobility of QW carriers will not be reduced by inclusion of magnetic ions in the same spatial region in these systems.More specifically, for a two-dimensional hole gas (2DHG, the p-system) in a QW the overlap of the hole wavefunction with the magnetic atoms in a nearby ferromagnetic layer (the d-system) is believed to result in p-d exchange interaction 5-8 . This exchange interaction may cause strong coupling between the SC and FM spin systems 9 , through which the ferromagnetism of the unified system, as evidenced by its hysteresis loop, can be tuned. In particular, the 2DHG spin system becomes polarized in the effective magnetic field from the p-d exchange 5,8 . Recently 10 , it was shown that in addition to this equilibrium 2DHG polarization there is an alternative mechanism involving spin-dependent capture of carriers from the SC into the FM. For ferromagnetic (Ga,Mn)As on top of an (In,Ga)As QW, electron capture induces electron spin polarization in the QW, representing a dynamical effect in contrast to the exchange-induced equilibrium polarization.Here we study a different FM/QW hybrid, consisting of a Co layer and a CdTe II-VI semiconductor QW, separated by a nanometrethick barrier. Owing to the negligible hole tunnelling through the barrier, this hybrid combination shows mostly a quasi-equilibrium proximity effect due to p-d exchange interaction between magnetic atoms and holes in the QW. Surprisingly, howev...
The dynamics of exciton recombination in an ensemble of indirect band-gap (In,Al)As/AlAs quantum dots with type-I band alignment is studied. The lifetime of confined excitons which are indirect in momentum-space is mainly influenced by the sharpness of the heterointerface between the (In,Al)As quantum dot and the AlAs barrier matrix. Time-resolved photoluminescence experiments and theoretical model calculations reveal a strong dependence of the exciton lifetime on the thickness of the interface diffusion layer. The lifetime of excitons with a particular optical transition energy varies because this energy is obtained for quantum dots differing in size, shape and composition. The different exciton lifetimes, which result in photoluminescence with non-exponential decay obeying a power-law function, can be described by a phenomenological distribution function G(τ ), which allows one to explain the photoluminescence decay with one fitting parameter only.
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