monolayers in large-scale on on-axis Au*

Ultrathin two-dimensional transition-metal dichalcogenides (TMDs) with sizeable bandgaps have been pursued in recent years for their potentials in electronics, optoelectronics, spin- and valley-electronics. To this end, it is needed to synthesize crystalline TMD monolayers at large-scale. Lately, remarkable progresses have been made, where wafer-size metal disulfides (MoS₂ and WS₂) have been realized by chemical vapor deposition (CVD) on various vicinal substrates and explained by a step-guide mechanism. ^[1-5] Here, we report growth of large-scale single crystalline MoSe₂ and WSe₂ monolayer by molecular-beam epitaxy (MBE) at low temperature on on-axis Au substrates. Electron diffraction measurements reveal consistent three-fold symmetry across the whole sample, signifying single crystalline MSe₂ (M = Mo, W). Optical measurements reveal high spatial uniformity of the epifilms. Defect density is estimated to be in the range of low 10¹² cm^-2. Mirror twin domain boundaries (MTBs), which are commonly seen in MBE-grown MoSe₂ on graphene or HOPG, is rarely observed in films on Au. Moreover, we show that MSe₂ grows on Au via the van der Waals (vdW) epitaxy mechanism, where a continuous film extends across the whole surface, hangs over atomic-layer steps on substrate. By following the initial stage nucleation, we identify that the highly crystalline MSe₂ is not by the guidance of Au steps but rooted from a moderate Au-MSe₂ interaction.