Planar Silicene: A New Silicon Allotrope Epitaxially Grown by Segregation

2D sheets of graphene‐like silicon, namely planar silicene, are synthesized. This new silicon allotrope is prepared on Au(111) thin films grown on a Si(111) substrate in the process of surface segregation. Owing to its almost perfectly flat geometry it shares the atomic structure with graphene rather than with low‐buckled silicene. Scanning tunneling microscopy measurements clearly display an atomically resolved planar silicene honeycomb lattice. Ab initio density functional theory calculations fully support the experimental findings and predict a pure sp2 atomic configuration of Si atoms. The present work is the first experimental evidence of epitaxial planar silicene. Planar silicene, a novel silicon allotrope, is synthesized on a Au(111) substrate in the process of surface segregation. The lattice constant of planar silicene is almost 6% longer and the buckling lower by an order of magnitude with respect to the freestanding silicene values. Density functional theory calculations predict a perfectly flat honeycomb Si lattice with a pure sp2 atomic configuration.