The Internet of Things (IoT) is a promising paradigm to accommodate massive device connections in 5G and beyond. To pave the way for future IoT, the spectrum should be planed in advance. Spectrum sharing is a preferable solution for IoT due to the scarcity of available spectrum resource. In particular, mobile operators are inclined to exploit the existing standards and infrastructures of current cellular networks and deploy IoT within licensed cellular spectrum. Yet, proprietary companies prefer to deploy IoT within unlicensed spectrum to avoid any licence fee. In this paper, we provide a survey on prevalent IoT technologies deployed within licensed cellular spectrum and unlicensed spectrum. Notably, emphasis will be on the spectrum sharing solutions including the shared spectrum, interference model, and interference management. To this end, we discuss both advantages and disadvantages of different IoT technologies. Finally, we identify challenges for future IoT and suggest potential research directions.
The present study describes the synthesis of monodispersed PbSe and PbS nanocrystals by a facile, less hazardous and inexpensive approach. In this study, Se and S powder are used as chalcogenide precursors instead of trioctylphosphine-selenium (TOPSe) and hexamethyldisilthiane (TMS), which are toxic and expensive. The chalcogenide precursors used in this method are inexpensive and air-stable, which not only reduces the cost of the experiment, but also simplifies the synthesis process. Monodispersed PbSe and PbS nanocrystals with spherical, cubic and cuboctahedral shapes were obtained, and the size of the nanocrystals can be tuned in a wide range (6-25 nm for PbSe and 10-80 nm for PbS) via tuning the concentration of oleic acid (OA) and oleyamine (OAm). The mechanism of the size control and shape evolution is discussed. The concentration of OA and OAm is found to play an important role in deciding the size and shape of the nanocrystals in the nucleation and growth process.
Metal Chalcogenides (MCs) have emerged as an extremely important class of nanomaterials with applications ranging from lubrication to energy storage devices. Here we report our discovery of a universal, ultrafast (60 seconds), energy-efficient, and facile technique of synthesizing MC nanoparticles and nanostructures, using microwave-assisted heating. A suitable combination of chemicals was selected for reactions on Polypyrrole nanofibers (PPy-NF) in presence of microwave irradiation. The PPy-NF serves as the conducting medium to absorb microwave energy to heat the chemicals that provide the metal and the chalcogenide constituents separately. The MCs are formed as nanoparticles that eventually undergo a size-dependent, multi-stage aggregation process to yield different kinds of MC nanostructures. Most importantly, this is a single-step metal chalcogenide formation process that is much faster and much more energy-efficient than all the other existing methods and can be universally employed to produce different kinds of MCs (e.g., MoS2, and WS2).
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