Large-Scale Layer-by-Layer Synthesis of Borophene on Ru(0001)

Borophene, a two-dimensional (2D) layer of elemental boron, has attracted attention as a metallic 2D material with rich polymorphism, as well as potentially interesting electronic, optical, chemical, and catalytic properties. While controllable adjustments of the thickness beyond a single layer promise a further tunability of the properties, the growth of borophene on metal substrates has largely remained limited to monolayers. Recently, bilayer borophene has been realized on Ag(111), albeit so far limited to domains a few tens of nanometers in size. The broader exploration of the properties of borophene calls for the development of synthesis methods that produce borophene with controlled thickness beyond a single layer. Here, we report layer-by-layer borophene growth on Ru(0001) substrates up to a coverage of two atomic layers using surface segregation of interstitial boron during cooling from high temperatures. Real-time surface microscopy follows borophene nucleation and growth, establishes the growth mode of the first and second layers, and demonstrates coverage across large surface areas. Diffraction shows highly crystalline borophene layers that grow in a high-order superstructure on the Ru(0001) surface lattice. Our results add to the capabilities of synthesizing borophene and point out avenues for growing high-quality few-layer borophene on metal substrates.