Monolayer SixCy constitutes an important family of 2D materials that is predicted to feature a honeycomb structure and appreciable bandgaps. However, due to its binary chemical nature and the lack of bulk polymorphs with a layered structure, the fabrication of such materials has so far been challenging. Here, the synthesis of atomic monolayer Si9C15 on Ru (0001) and Rh(111) substrates is reported. A combination of scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and density functional theory (DFT) calculations is used to infer that the 2D lattice of Si9C15 is a buckled honeycomb structure. Monolayer Si9C15 shows semiconducting behavior with a bandgap of ≈1.9 eV. Remarkably, the Si9C15 lattice remains intact after exposure to ambient conditions, indicating good air stability. The present work expands the 2D‐materials library and provides a promising platform for future studies in nanoelectronics and nanophotonics.

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