Synthetic two-dimensional (2D) materials have no bulk counterparts and typically exist as single atomic layers due to substrate-stabilized growth. Multilayer formation, although broadly sought for structure and property tuning, has not yet been achieved in the case of synthetic 2D boron: that is, borophene^1,2. Here, we experimentally demonstrate the synthesis of an atomically well-defined borophene polymorph beyond the single-atomic-layer (SL) limit. The structure of this bilayer (BL) borophene is consistent with two covalently bonded α-phase layers (termed BL-α borophene) as evidenced from bond-resolved scanning tunnelling microscopy, non-contact atomic force microscopy and density functional theory calculations. While the electronic density of states near the Fermi level of BL-α borophene is similar to SL borophene polymorphs, field-emission resonance spectroscopy reveals distinct interfacial charge transfer doping and a heightened local work function exceeding 5 eV. The extension of borophene polymorphs beyond the SL limit significantly expands the phase space for boron-based nanomaterials.

合成二维 (2D) 材料没有大块对应物，并且由于衬底稳定生长，通常以单原子层的形式存在。多层形成虽然广泛用于结构和性能调整，但在合成二维硼的情况下尚未实现：即硼烯^1,2. 在这里，我们通过实验证明了超出单原子层（SL）限制的原子定义明确的硼烯多晶型物的合成。从键分辨扫描隧道显微镜、非接触原子力显微镜和密度泛函理论计算证明，这种双层 (BL) 硼烯的结构与两个共价键合的 α 相层（称为 BL-α 硼烯）一致。虽然 BL-α 硼烯的费米能级附近的电子态密度与 SL 硼烯多晶型物相似，但场发射共振光谱显示出明显的界面电荷转移掺杂和超过 5 eV 的局部功函数。超出 SL 限制的硼烯多晶型物的扩展显着扩展了硼基纳米材料的相空间。