| Zinc sulfide, as an importantâ…¡-â…¥semiconductor with wide band gap energy 3.7 eV, is particularly suitable for use as host materials for a large variety of dopants. As a well-known luminescent material, it shows various luminescence properties such as photoluminescence (PL), electroluminescence (EL), thermal luminescence, and which has been extensively studied for a variety of applications, e.g. in solid-state solar window layers, electrooptic modulators, field effect transistors, optical sensors and other light-emitting materials. In fact, ZnS has found special importance in thin-film electroluminescent devices, lasers, and flat-panel displays when doped with divalent manganese ions. It is well known that the properties of nanomaterials are sensitive to its structure and morphologies. And surprising, wurtzite ZnS is much more desirable for its optical properties than the sphalerite phase. It is believed that one-dimensional (1D) ZnS nanostructures, especially doped nanostructures, such as nanorods, nanowires and nanotubes, will also exhibit characteristic optical properties, and which has attracted much attention. The synthesis of low-dimensional ZnS nanocrystal, especially doped nanocrystal and the physical properties, especially the photoluminescence property are still a challenging topics in this field. Lwo-dimension nanomaterials, such as nanorods, nanobelts and nanowores, have a wide range of electrical and opertical properties and variable mechanical stability and phase-transition mechanisms that show a sensitive dependency on size, shape and structure. For wurtzite phase ZnS, the lack of structural stability limits its technological application.Due to its potential application in biological and biomedical field, photoluminescence up-conversion (UCPL) in semiconductor nanocrystal quantum dots has attracted much attention recently. Many studies have focused on the physical nature of the phenomenon. However, the underlying mechanisms are still under debate. One question of the mechanism is the up-conversion process, and generally held that UCPL is through a single photon absorption process. But from very recently studies, a near quadratic power dependence of the UCPL was observed and a two-photon absorption process has been suggested. The models published so far cannot explain the red shift of the UCPL related to the normal PL observed in some QDs system. The originate energy states of the UCPL in semiconductor QDs are very complicated, and it would be very hard to figure out these states at ambient condition. High pressure can effectively tune the energy of electronic states and different energy state has its own characteristic pressure dependence. It has been employed to study the normal PL from semiconductor QDs. To the best of our knowledge, no study on UCPL from QDs under high pressure has been reported. CdSe/ZnS core/shell QDs is a typical material in the family of UCPL QDs. The epitaxial ZnS shell around the CdSe core can significantly improve QDs'chemical and thermal stabilities, as well as the optical properties. Therefore, the high pressure study of the UCPL in CdSe/ZnS QDs can provide new information on the UCPL mechanism of the semiconductor QDs.To the best of our knowledge there are no reports on synthesis of wurtize phase ZnS:Mn nanorods via wet chemical methods. In this work, we have synthesized wutzite phase ZnS:Mn nanorods with varying Mn concentration in powder form via a solvothermal process. The systhesised nanorods with a narrow size distrubition, the diameter within 15~25 nm, is good crystallization. A strong photoluminescence peak at 590 nm was observed from the systhesised nanorods, and the intensity of the PL increased with Mn concentrations, which promises valuable application of the prepared ZnS: Mn nanorods in optical device. And structure transition of the synthesized ZnS:Mn nanorods was also investigated by synchrotron angle dispersive X-ray-diffraction. From X-ray-diffraction study, we found that the morphology of ZnS:Mn nanorods leads to a very high mechanical stability to 17 GPa and then transformed from wurtzite to rocksalt phase.In this work, the efficient UCPL from CdSe/ZnS core/shell QDs was observed excited at both visible and NIR lasers, 633 and 830 nm, respectively. The UCPL were studied under hydrostatic pressure up to ~8 GPa compared with pressure dependence of normal PL excited at 514 nm. A blue shift of the PL and UCPL peaks position with increasing pressure were observed, and the emission peaks are basically quenched above ~7.0 GPa due to the phase transition from wutzite to ralksalt phase. The most striking feature is that the pressure dependence of the UCPL excited at 633 nm is obviously different from that of normal PL, but the pressure dependence of the UCPL excited at 830 nm is quite similar to that of PL. Our data indicate that the UCPL excited at 633 nm originates from the surface states while the UCPL excited at 830 nm originates from the band edge. Our results also help us to understand the red shift of UCPL related to that of normal PL peak, for the UCPL excited at 633 nm is due to its originate energy state while excited at 830 nm is due to the selective excitation of larger particles. For further investigating the up-conversion mechanism, we carried out measurement for the power dependence and temperature dependence of the UCPL excited at 633 nm and 830 nm, respectively. The linear power dependence and the UCPL intensity decreases at low temperature indicate that the UCPL excited at 633 nm originates from phonon populated surface states. And a near-quadratic power dependence, not linear dependence occurs in the UCPL process excited at 830 nm, indicating that the UCPL mechanism is two-photon absorption in nature in this case. Our investigation reveals that the UCPL excited at 633 nm and 830 nm originate from two different energy states with two different up-conversion processes. And our study also suggests that high pressure provides a new approach to understand the UCPL mechanism.
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