Band Gap Opening in 8-Pmmn Borophene by Hydrogenation

A first-principles calculation has been performed to explore the adsorption and dissociation of hydrogen on 8-Pmmn borophene. Different hydrogen adsorption sites, coverage, and dissociation reaction pathways have been considered. The results show that for one hydrogen atom adsorption the top site is the most stable adsorption site with an adsorption energy of 3.13 eV/H. Under high hydrogen coverage, the adsorption energy of hydrogen in BE1/2 is of the largest value (3.44 eV/H) among the five different hydrogen coverages (BH1/48, BH1/24, BH1/4, BH1/2, and BH3/4). Before the hydrogenation, 8-Pmmn borophene is a gapless semiconductor. Unexpectedly, BH1/4 and BH1/2 are both semiconductors. More specifically, BH1/4 is an indirect semiconductor with a 0.82 eV band gap while BH1/2 is a direct semiconductor with a 0.78 eV band gap. The band gap opening for 8-Pmmn borophene has been achieved by hydrogenation. Furthermore, the electronic band gap of BH1/2 is sensitive to mechanical strains, and more interestingly the direct to indirect band gap electronic phase transition in BH1/2 has been found under the three applied tensile strains.