Synthesis of Quantum-Confined Borophene Nanoribbons

Borophene refers to synthetic two-dimensional (2D) boron, which has attracted significant attention due to its anisotropic metallicity, correlated electron phenomena, and diverse superlattice structures.^[1] Reducing the dimensionality of nanomaterials imposes additional quantum confinement effects that unlock new physical phenomena, such as one-dimensional (1D) confined plasmons, spin-polarized edge states, and Luttinger liquid behavior. However, the realization of quantum-confined borophene nanoribbons (BNRs) is hampered by limited boron precursor options for bottom-up synthesis. In this work, we present a substrate mediation strategy to synthesize quantum-confined BNRs on vicinal Ag(977) substrates. The resulting BNRs inherit the high degree of polymorphism present in borophene, resulting in <i>v</i>_1/5 and <i>v</i>_1/6-BNR lattice configurations in addition to phase intermixing.^[2] Through atomic-scale imaging, spectroscopy, and first principles calculations, the edge structures of BNR polymorphs are shown to possess reconstructed armchair edges for <i>v</i>_1/6-BNRs and sawtooth zigzag edges for <i>v</i>_1/5-BNRs. The confined electron wave functions in 1D BNRs lead to the observation of energy level quantization and spatial nodes characteristic of quantum-well states.<br/><br/><b>Acknowledgement:</b><br/>This work was supported by the Office of Naval Research (ONR N00014-17-1-2993) and the National Science Foundation Materials Research Science and Engineering Center (NSF DMR-1720139).<br/><br/><b>References:</b><br/>[1] Mannix, A. J., <i>et al.</i>, Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs. <i>Science</i>, <b>2015</b>, <i>350 </i>(6267), 1513-1516.<br/>[2] Liu, X., <i>et al.</i>, Intermixing and periodic self-assembly of borophene line defects. <i>Nature Materials</i>, <b>2018</b>, <i>17 </i>(9), 783-788.