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We propose a guideline for exploring substrates that stabilize the monolayer honeycomb structure of silicene and germanene while simultaneously preserving the Dirac states: in addition to having a strong binding energy to the monolayer, a suitable substrate should be a large-gap semiconductor with a proper work function such that the Dirac point lies in the gap and far from the substrate states when their bands align. We illustrate our idea by performing first-principles calculations for silicene and germanene on the Al-terminated (0001) surface of . The overlaid monolayers on Al-terminated (0001) retain the main structural profile of the low-buckled honeycomb structure via a binding energy comparable to the one between silicene and Ag(111). An unfolded band structure derived from the -projection method reveals that a gapped Dirac cone is formed at the K point due to the structural distortion and the …