All forms of water are essential for life and a wide range of scientific and technological processes. When adsorbed on metal surfaces, water usually forms a variety of clusters with a broad size distribution. Here we report the monodisperse water clusters formed on the semimetallic Bi(111) surface. High-resolution scanning tunneling microscopy images combined with density functional theory calculations demonstrate that the monodisperse clusters are built exclusively from hexagonal arrangement of six water molecules with slight buckling. All the water hexamers are oriented at the same direction and reveal the identical motif: three-lobed protrusions at high bias and a trigonal hole at low bias. The monodispersity of cyclic water clusters can be rationalized by the fact that hydrogen bonding is the dominant interaction responsible for water cluster stability on Bi(111). The water hexamer is the most favorable because of the maximum strength of H bonds. The decrease of H bonds (the presence of double acceptors) makes it difficult for water molecules to form a smaller (larger) cluster.

• Received 19 March 2018
• Revised 19 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.165409