Structural and electronic transitions in few layers of isotopically pure hexagonal boron nitride

Hexagonal boron nitride (hBN) is attracting tremendous interest as an essential component in van der Waals heterostructures due to its ability to provide weakly interacting interfaces and because of its large bandgap. Although most of theoretical calculations yield the standard $A{A}^{\ensuremath{'}}$ stacking for few-layer hBN, the exact determination of its structural and electronic properties remains unrevealed to date. Here, we provide the direct observation of structural and electronic transitions in few layers of isotopically pure exfoliated ${\mathrm{h}}¹¹\mathrm{BN}$ flakes. Our nanoscopic angle-resolved photoemission spectroscopy measurements combined with density-functional theory calculations indicate that the stacking and the band structure can be strongly affected by the thickness of ${\mathrm{h}}¹¹\mathrm{BN}$. Hence, we show that hBN presents an $A{A}^{\ensuremath{'}}$ stacking in its bulk form and another more exotic stacking for three and four layers. Our findings open perspectives in understanding and controlling the stackings in hBN, which could be of great interest for optoelectronic applications.