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Hydrogen Storage Performance Enhancement and Bandgap Opening of M-Decorated ($\mathrm{M}=\mathrm{Li}$, Na and K) III\textsubscript{4}-V\textsubscript{4} Monolayer by Fluorine Functionalization

The effects of fluorine functionalization on the hydrogen storage capability of alkaline decorated III\textsubscript{4}-V\textsubscript{4} monolayers is studied. This structure can store up to two hydrogen molecules per alkaline atom. Here, we demonstrate that functionalizing alkaline decorated monolayer with a high electronegative element of fluorine can significantly enhance both binding energy and the maximum number of the stored hydrogen molecules. In this regard, the hydrogen molecule storage capability is notably improved from two to four and when absolute binding is considered. In addition, F-functionalization of M-decorated III\textsubscript{4}-V\textsubscript{4} can demonstrate bandgap opening effect, introduce semiconducting characteristics and forming a new two-dimensional semiconductor structure. The bandgap for Li-Al$_4$P$_4$-F is 1.23 eV which is very close to the solar peak. The resulted bandgap in the M-III\textsubscript{4}-V\textsubscript{4}-F structure is even significantly larger than that of pristine III\textsubscript{4}-V\textsubscript{4} monolayer. The binding of the hydrogen molecule to the alkaline atom is also improved from 0.114 to 0.272 eV by the fluorine functionalization.