RESUMO
GaN is a technologically indispensable material for various optoelectronic properties, mainly due to the dopant-induced or native atomic-scale point defects that can create single photon emitters, a range of luminescence bands, and n- or p-type conductivities. Among the various dopants, chromium and manganese-induced defects have been of particular interest over the past few years, because some of them contribute to our present-day light-emitting diode (LED) and spintronic technologies. However, the nature of such atomistic centers in Cr and Mn-doped GaN is yet to be understood. A comprehensive defect thermodynamic analysis of Cr- and Mn-induced defects is essential for their engineering in GaN crystals because by mapping out the defect stabilities as a function of crystal growth parameters, we can maximize the concentration of the target point defects. We therefore investigate chromium and manganese-induced defects in GaN with ab initio methods using the highly accurate exchange-correlation hybrid functionals, and the phase transformations upon excess incorporation of these dopants using the CALPHAD method. We also investigate the impact of oxygen codoping that can be unintentionally incorporated during crystal growth. Our analysis sheds light on the atomistic cause of the unintentional n-type conductivity in GaN, being ON-related. In the case of Cr doping, the formation of CrGa defects is the most dominant, with an E +/0 charge transition at E VBM + 2.19 eV. Increasing nitrogen partial pressure tends to enhance the concentration of CrGa. However, in the case of doping with Mn, several different Mn-related centers can form depending on the growth conditions, with MnGa being the most dominant. MnGa possesses the E 2+/+, E +/0, and E 0/- charge transitions at 0.56, 1.04, and 2.10 eV above the VBM. The incorporation of oxygen tends to cause the formation of the MnGa-VGa center, which explains a series of prior experimental observations in Mn-doped GaN. We provide a powerful tool for point defect engineering in wide band gap binary semiconductors that can be readily used to design optimal crystal growth protocols.
RESUMO
Lithium-sulfur (Li-S) batteries have attracted much attention due to their superior theoretical specific capacity and high theoretical energy density. However, rapid capacity fading originating from the shuttle effect, insulating the S cathode and the dendrite formation on the Li anode restrict the practical applications of Li-S batteries. Herein, we suggest novel coatings on glass fiber separators to satisfy all high-performance Li-S battery requirements. A conductive Ti3C2Tx (MXene) nanosheet/Fe-MOF or Ti3C2Tx (MXene) nanosheet/Cu-MOF layer was coated on a glass fiber separator to act as a polysulfide trapping layer. The MXene layer with high conductivity and polar surface functional groups could confine polysulfides and accelerate the redox conversions. The porous MOF layer acts as a Li ion sieve, thereby leading to the interception of polysulfides and mitigation of Li dendrite growth. The cells with the Cu-MOF/MXenes and Fe-MOF/MXene separators display superior capacities of 1100 and 1131 mA h g-1 after 300 cycles, respectively, whereas the cell with a pure glass fiber separator delivers a very low capacity of 309 mA h g-1 after 300 cycles. With Fe-MOF/MXene and Cu-MOF/MXene configurations, the discharge capacity, coulombic efficiency, cycling stability, and electrochemical conversion reactions are significantly improved. Our ab initio calculations demonstrate that the MXene layer dissociates lithium polysulfides into adsorbed S and mobile Li ions, which explains the experimental findings.
