  The emergence of two-dimensional (2D) materials launched a fascinating
frontier of flatland electronics. Most crystalline atomic layer materials are
based on layered van der Waals materials with weak interlayer bonding, which
naturally leads to thermodynamically stable monolayers. We report the synthesis
of a 2D insulator comprised of a single atomic sheet of honeycomb structure BeO
(h-BeO), although its bulk counterpart has a wurtzite structure. The h-BeO is
grown by molecular beam epitaxy (MBE) on Ag(111) thin films that are
conveniently grown on Si(111) wafers. Using scanning tunneling microscopy and
spectroscopy (STM/S), the honeycomb BeO lattice constant is determined to be
2.65 angstrom with an insulating band gap of 6 eV. Our low energy electron
diffraction (LEED) measurements indicate that the h-BeO forms a continuous
layer with good crystallinity at the millimeter scale. Moir\'e pattern analysis
shows the BeO honeycomb structure maintains long range phase coherence in
atomic registry even across Ag steps. We find that the interaction between the
h-BeO layer and the Ag(111) substrate is weak by using STS and complimentary
density functional theory calculations. We not only demonstrate the feasibility
of growing h-BeO monolayers by MBE, but also illustrate that the large-scale
growth, weak substrate interactions, and long-range crystallinity make h-BeO an
attractive candidate for future technological applications. More significantly,
the ability to create a stable single crystalline atomic sheet without a bulk
layered counterpart is an intriguing approach to tailoring novel 2D electronic
materials.
