We have systematically studied the structural, electronic, and optical properties of two types of guest-free Si46 silicon clathrates using first-principles density-functional theory calculations. The properties are compared between two different crystal structures with the same chemical formula of Si46, type-I, and type-VIII. We apply a hybrid functional approach with the Becke, Lee-Yang-Parr (BLYP) functional and compare the electronic properties with those by generalized gradient approximation (GGA-PBE). The band structures of both clathrates calculated within the GGA indicate semiconducting (insulating) properties with indirect bandgaps of 1.38 and 1.36 eV for type-I and type-VIII, respectively. These band gaps increase to 1.60 and 1.62 eV by using the BLYP hybrid functional. The calculated optical absorption of type-I clathrate is slightly larger than that of type-VIII. Lastly, we also performed a symmetry analysis of optical transitions for both Si46 clathrates. It is found that optical transitions are dipole-allowed at the band-edge located along the Γ–Η line for Si46-VIII, while the transitions for type-I Si46 are forbidden by symmetry. It is hoped that this finding will motivate further investigations of optical capabilities of the silicon based type-VIII clathrates for photovoltaic and photonic devices.

As the first initiative of its kind in Algeria, here is presented a solar-powered mobile “Ultraviolet Germicidal Irradiation” disinfection unit (UVGI) based on a special germicidal ultraviolet-C (UVC) radiator. The system was designed to disinfect several objects such as: medical tools, reusable masks, face-shields, gloves, phones, laptops, keys, money and many other portable devices that need to be disinfected/sterilized. It offers the advantage of complete autonomy through the built-in photovoltaic (PV) system that includes a solar panel, a gel battery, a charge controller and a power inverter. The system provides an extra 220V-50Hz outlet with 375VA maximum power to be used when energy is needed. The system is easily scalable to generate higher ultraviolet dosages by adding more UVC lamps. The chemical-free germicidal UVC sanitizing method employed by this device effectively disactivates a very wide range of microorganisms (microbes, bacteria and fungi and viruses including the actual SARS-CoV-2 that causes COVID-19 respiratory disease) and it has several advantages in comparison to chemical-based sanitizing methods. The total cost to make this open source device is below 1000 € and is easily customizable and scalable. This device is an open source, secure and fast equipment for objects and surface disinfection. The device will be fully automated by adding PIR sensors or remote control after further funding will be received.

By means of density functional theory (DFT) calculations, we have investigated the effect of hydrostatic pressure on the structural, electronic and elastic properties of the barium-doped silicon clathrate Ba8Si46 in the type-VIII structure (phase). Physical properties are calculated under different conditions of pressure (0 GPa to 45 GPa) using the GGA-PBE functional, those calculations have been performed using the Cambridge serial total energy package CASTEP code within the Materials Studio package. Electronic properties have shown that the type-VIII Ba8Si46 has metal-like properties with a fundamental bandgap of 1 eV. Under pressure the fundamental bandgap increases slightly and the positions of the valence band maximum VBM and the conduction band minimum CBM remain unchanged. We found that the compound is mechanically stable under the pressure range, but this needs to be confirmed experimentally through synthesis, a comparison with the type-I and the guest-free counterparts has exhibited promising features for the type-VIII Ba8Si46. I.

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