Revealing the nature of low-temperature photoluminescence peaks by laser treatment in van der Waals epitaxially grown WS2 monolayers.

Monolayers of transition metal dichalcogenides (TMD) are promising materials for optoelectronics devices. However, one of the challenges is to fabricate large-scale growth of high quality TMD monolayers with the desired properties in order to expand their use in potential applications. Here, we demonstrate large-scale tungsten disulfide (WS2) monolayers grown by van der Waals Epitaxy (VdWE). We show that, in addition to the large structural uniformity and homogeneity of these samples, their optical properties are very sensitive to laser irradiation. We observe a time instability in the photoluminescence (PL) emission at low temperatures in the scale of seconds to minutes. Interestingly, this change of the PL spectra with time, which is due to laser induced carrier doping, is employed to successfully distinguish the emission of two negatively charged bright excitons. Furthermore, we also detect blinking sharp bound exciton emissions which are usually attractive for single photon sources. Our findings contribute to a deeper understanding of this complex carrier dynamics induced by laser irradiation which is very important for future optoelectronic devices based on large scale TMD monolayers.

Visible to infrared low temperature luminescence of Er(3+), Nd(3+) and Sm(3+) in CaSnO3 phosphors.

Novel stannate phosphor, orthorhombic CaSnO3 phosphors doped with Er(3+), Nd(3+) and Sm(3+) have been synthesized by a conventional solid-state method under N2+H2 gas flow. Visible and near-infrared photoluminescence (PL) properties were investigated as function of laser power and temperature. It was observed that all dopant ions are well incorporated in CaSnO3 and are responsible for the optical emission in the temperature range of 10-300K. PL peaks at 490, 546, 656, 696, 894, 1065, and 1344nm were observed for the CaSnO3:Nd(3+) phosphor and associated to f-f transition of Nd(3+) ion. Emissions at 564, 600-607, 646-656 and 714nm were detected for the CaSnO3:Sm(3+). The strongest one, observed at 600nm, was associated to (4)G5/2→(6)H7/2 of Sm(3). Emission lines at 528, 548, 662 at 852nm were also seen for CaSnO3:Er(3+) and correspond to Er(3+) intra-4f(n) shell transitions. In addition, at low temperatures, a stark splitting of the 4f electron energy levels of the Er(3+) ions were observed in infrared region (1520-1558nm) and assigned to the transition between the (4)I13/2 state and the (4)I15/2 state. Finally, our results show that the rare earth doped CaSnO3 has remarkable potential for applications as optical material since it exhibits efficient and sharp emissions due to rare earth ions.