Since the global positioning system (GPS) is not applicable underwater, source localization using wireless sensor networks (WSNs) is gaining popularity in oceanographic applications. Unlike terrestrial WSNs (TWSNs) which uses electromagnetic signaling, underwater WSNs (UWSNs) require underwater acoustic (UWA) signaling. Received signal strength (RSS)-based source localization is considered in this paper due to its practical simplicity and the constraint of low-cost sensor devices, but this area received little attention so far because of the complicated UWA transmission loss (TL) phenomena. In this paper, we address this issue and propose two novel semidefinite programming (SDP) approaches which can be solved more efficiently. The numerical results validate our proposed SDP solvers in underwater environments, and indicate that the placement of the anchor nodes influences the RSS-based localization accuracy similarly as in the terrestrial counterpart. We also highlight that adopting traditional terrestrial RSS-based localization methods will fail in underwater scenarios.@en

RE3+ (RE = Pr, Sm, Tb, Er, Dy)-activated Y4Si2O7N2 samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. The absorption band located at about 250 nm is attributed to the host absorption. The 5d bands of Pr3+ and Tb3+ are at rather low energy in Y4Si2O7N2 compared to oxide. The direct Pr3+ 4f2 → 4f15d1 excitation at 275 nm leads to typical 4f2 → 4f2 line emissions (450–700 nm) and strong 4f15d1 → 4f2 broad band emission (300–450 nm), respectively. The charge transfer (N3− → Sm3+) band of Sm3+ was observed at a somewhat lower energy of 4.68 eV compared to oxide, and Sm3+-activated sample shows a bright red emission originating from 4G5/2 → 6HJ (J = 5/2, 7/2 and 9/2) transitions. For Tb3+-doped sample, the direct Tb3+ 4f8 → 4f75d1 excitation leads to 5D3 → 7FJ (J = 6, 5, 4, 3) (blue) and 5D4 → 7FJ (J = 6, 5, 4, 3) (green) line emissions, the cross-relaxation depended on Tb concentration has happened. The incorporation of Er3+ (or Dy3+) into Y4Si2O7N2 resulted in a typical Er3+ (or Dy3+) f-f line absorptions and emissions. Moreover, the energy transfer from the host lattice to the luminescent activators (Pr3+, Tb3+ and Sm3+) is observed. The energy level diagram containing the position of 4f and 5d energy levels of all Ln2+ and Ln3+ ions relative to the valence and conduction band of Y4Si2O7N2 has been established and studied based on the data presented in this work, and further provides a platform for studying the photoluminescence properties as well as the valence stability of the lanthanide ions.@en

RE3+ (RE = Pr, Sm, Tb, Er, Dy)-activated Y4Si2O7N2 samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. The absorption band located at about 250 nm is attributed to the host absorption. The 5d bands of Pr3+ and Tb3+ are at rather low energy in Y4Si2O7N2 compared to oxide. The direct Pr3+ 4f2 → 4f15d1 excitation at 275 nm leads to typical 4f2 → 4f2 line emissions (450–700 nm) and strong 4f15d1 → 4f2 broad band emission (300–450 nm), respectively. The charge transfer (N3− → Sm3+) band of Sm3+ was observed at a somewhat lower energy of 4.68 eV compared to oxide, and Sm3+-activated sample shows a bright red emission originating from 4G5/2 → 6HJ (J = 5/2, 7/2 and 9/2) transitions. For Tb3+-doped sample, the direct Tb3+ 4f8 → 4f75d1 excitation leads to 5D3 → 7FJ (J = 6, 5, 4, 3) (blue) and 5D4 → 7FJ (J = 6, 5, 4, 3) (green) line emissions, the cross-relaxation depended on Tb concentration has happened. The incorporation of Er3+ (or Dy3+) into Y4Si2O7N2 resulted in a typical Er3+ (or Dy3+) f-f line absorptions and emissions. Moreover, the energy transfer from the host lattice to the luminescent activators (Pr3+, Tb3+ and Sm3+) is observed. The energy level diagram containing the position of 4f and 5d energy levels of all Ln2+ and Ln3+ ions relative to the valence and conduction band of Y4Si2O7N2 has been established and studied based on the data presented in this work, and further provides a platform for studying the photoluminescence properties as well as the valence stability of the lanthanide ions.@en