  The family of graphynes, novel two-dimensional semiconductors with various
and fascinating chemical and physical properties, has attracted great interest
from both science and industry. Currently, the focus of graphynes is on
graphdiyne, or graphyne-2. In this work, we systematically study the effect of
acetylene, i.e., carbon-carbon triple bond, links on the electronic and optical
properties of a series of graphynes (graphyne-n, where n = 1-5, the number of
acetylene bonds) using the ab initio calculations. We find an even-odd pattern,
i.e., n = 1, 3, 5 and n = 2, 4 having different features, which has not be
discovered in studying graphyne or graphdyine only. It is found that as the
number of acetylene bonds increases, the electron effective mass increases
continuously in the low energy range because of the flatter conduction band
induced by the longer acetylene links. Meanwhile, longer acetylene links result
in larger redshift of the imaginary part of the dielectric function, loss
function, and extinction coefficient. In this work, we propose an effective
method to tune and manipulate both the electronic and optical properties of
graphynes for the applications in optoelectronic devices and photo-chemical
catalysis.
