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 $A{A}^{\prime }$ 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 $A{A}^{\prime }$ 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.

中文翻译：

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几层同位素纯正六方氮化硼的结构和电子跃迁

六方氮化硼（hBN）作为范德华异质结构的重要组成部分引起人们极大的兴趣，这是由于其提供弱相互作用界面的能力以及其大的带隙。尽管大多数理论计算得出标准$\mathrm{一种}{\mathrm{一种}}^{\prime }$堆叠几层hBN至今，尚未确切确定其结构和电子性能。在这里，我们提供对同位素纯净剥落的几层中结构和电子跃迁的直接观察${\mathrm{H}}^{11}\mathrm{国阵}$片状。我们的纳米角度分辨光发射光谱测量结果与密度泛函理论计算相结合，表明叠层和能带结构会受到膜厚的强烈影响。${\mathrm{H}}^{11}\mathrm{国阵}$。因此，我们表明hBN呈现出$\mathrm{一种}{\mathrm{一种}}^{\prime }$以散装形式堆叠，再堆叠三层和四层。我们的发现为理解和控制hBN中的堆叠开辟了前景，这对于光电应用可能会引起极大的兴趣。