  Two-dimensional boron (borophene) is featured by its structural polymorphs
and distinct in-plane anisotropy, opening opportunities to achieve tailored
electronic properties by intermixing different phases. Here, using scanning
tunneling spectroscopy combined with first-principles calculations, delocalized
one-dimensional nearly free electron states (NFE) in the (2,3) or \b{eta}12
borophene sheet on the Ag(111) surface were observed. The NFE states emerge
from a line defect in the borophene, manifested as a structural unit of the
(2,2) or \c{hi}3 sheet, which creates an in-plane potential well that shifts
the states toward the Fermi level. The NFE states are held in the 2D plane of
borophene, rather than in the vacuum region as observed in other
nanostructures. Furthermore the borophene can provide a rare prototype to
further study novel NFE behaviors, which may have potential applications on
transport or field emission nanodevices based on boron.
