ABSTRACT
Monolayer transition metal dichalcogenides (TMDs) constitute a family of materials, in which coupled spin-valley physics can be explored and which could find applications in novel optoelectronic devices. However, before applications can be designed, a scalable method of monolayer extraction is required. Liquid phase exfoliation is a technique providing large quantities of the monolayer material, but the spin-valley properties of thus obtained TMDs are unknown. In this work, we employ steady-state and time-resolved photoluminescence (PL) to investigate the relaxation dynamics of localized excitons (LXs) in liquid exfoliated WS2. The results reveal that the circular polarization lifetime of the PL exceeds by at least an order of magnitude the PL lifetime. A rate equations model allows us to reproduce quantitatively the experimental data and to conclude that the observed large and long-lived PL polarization originates from efficient trapping of free excitons at localization sites hindering the intervalley relaxation. Furthermore, our results show that the depolarization process is inefficient for LXs. We discuss various mechanisms leading to this effect such as suppression of intervalley scattering of the LXs or inefficient spin relaxation of the holes.
ABSTRACT
The family of organic-inorganic tri-halide perovskites including MA (MethylAmmonium)PbI3, MAPbI3-xClx, FA (FormAmidinium)PbI3 and FAPbBr3 are having a tremendous impact on the field of photovoltaic cells due to the combination of their ease of deposition and high energy conversion efficiencies. Device performance, however, is known to be still significantly affected by the presence of inhomogeneities. Here we report on a study of temperature dependent micro-photoluminescence which shows a strong spatial inhomogeneity related to the presence of microcrystalline grains, which can be both bright and dark. In all of the tri-iodide based materials there is evidence that the tetragonal to orthorhombic phase transition observed around 160 K does not occur uniformly across the sample with domain formation related to the underlying microcrystallite grains, some of which remain in the high temperature, tetragonal, phase even at very low temperatures. At low temperature the tetragonal domains can be significantly influenced by local defects in the layers or the introduction of residual levels of chlorine in mixed halide layers or dopant atoms such as aluminium. We see that improvements in room temperature energy conversion efficiency appear to be directly related to reductions in the proportions of the layer which remain in the tetragonal phase at low temperature. In FAPbBr3 a more macroscopic domain structure is observed with large numbers of grains forming phase correlated regions.
ABSTRACT
Transition metal dichalcogenides (TMD) hold promise for applications in novel optoelectronic devices. There is therefore a need for materials that can be obtained in large quantities and with well understood optical properties. In this report, we present thorough photoluminescence (PL) investigations of monolayer tungsten disulphide obtained via liquid phase exfoliation. As shown by microscopy studies, the exfoliated nanosheets have dimensions of tens of nanometers and thickness of 2.5 monolayers on average. The monolayer content is about 20%. Our studies show that at low temperature the PL is dominated by excitons localized on nanosheet edges. As a consequence, the PL is strongly sensitive to the environment and exhibits an enhanced splitting in magnetic field. As the temperature is increased, the excitons are thermally excited out of the defect states and the dominant transition is that of the negatively charged exciton. Furthermore, upon excitation with a circularly polarized light, the PL retains a degree of polarization reaching 50% and inherited from the valley polarized photoexcited excitons. The studies of PL dynamics reveal that the PL lifetime is on the order of 10 ps, which is probably limited by non-radiative processes. Our results underline the potential of liquid exfoliated TMD monolayers in large scale optoelectronic devices.
ABSTRACT
Optical spectroscopy in high magnetic fields B ≤ 65 T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe2, the exciton emission shifts linearly with the magnetic field and exhibits a splitting that originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac Fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe2 where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model.
ABSTRACT
Magneto-photoluminescence measurements of individual zinc-blende GaAs/AlAs core/shell nanowires are reported. At low temperature, a strong emission line at 1.507 eV is observed under low power (nW) excitation. Measurements performed in high magnetic field allowed us to detect in this emission several lines associated with excitons bound to defect pairs. Such lines were observed before in epitaxial GaAs of very high quality, as reported by Kunzel and Ploog. This demonstrates that the optical quality of our GaAs/AlAs core/shell nanowires is comparable to the best GaAs layers grown by molecular beam epitaxy. Moreover, strong free exciton emission is observed even at room temperature. The bright optical emission of our nanowires in room temperature should open the way for numerous optoelectronic device applications.
