  Engineering atomic-scale structures allows great manipulation of physical
properties and chemical processes for advanced technology. We show that the B
atoms deployed at the centers of honeycombs in boron sheets, borophene, behave
as nearly perfect electron donors for filling the graphitic $\sigma$ bonding
states without forming additional in-plane bonds by first-principles
calculations. The dilute electron density distribution owing to the weak
bonding surrounding the center atoms provides easier atomic-scale engineering
and is highly tunable via in-plane strain, promising for practical
applications, such as modulating the extraordinarily high thermal conductance
that exceeds the reported value in graphene. The hidden honeycomb bonding
structure suggests an unusual energy sequence of core electrons that has been
verified by our high-resolution core-level photoelectron spectroscopy
measurements. With the experimental and theoretical evidence, we demonstrate
that borophene exhibits a peculiar bonding structure and is distinctive among
two-dimensional materials.
