  Edge termination plays a vital role in determining the properties of 2D
materials. By performing compelling ab initio simulations, a lowest-energy
U-edge [ZZ(U)] reconstruction is revealed in the bilayer phosphorene. Such
reconstruction reduces 60% edge energy compared with the pristine one and
occurs almost without energy barrier, implying it should be the dominating edge
in reality. The electronic band structure of phosphorene nanoribbon with such
reconstruction resembles that of intrinsic 2D layer, exhibiting nearly edgeless
band characteristics. Although ZZ(U) changes the topology of phosphorene
nanoribbon (PNR), simulated TEM, STEM and STM images indicates it is very hard
to be identified. One possible identify method is IR/Raman analyses because
ZZ(U) edge alters vibrational modes dramatically. Beyond, it also increases the
thermal conductivity of PNR 1.4 and 2.3 times than the pristine and Klein
edges.
