  Density-functional theory calculations with spin-polarized generalized
gradient approximation and Hubbard $U$ correction is carried out to investigate
the mechanical, structural, electronic and magnetic properties of graphitic
heptazine with embedded $\mathrm{Fe}$ atom under bi-axial tensile strain and
applied perpendicular electric field. It was found that the binding energy of
heptazine with embedded $\mathrm{Fe}$ atom system decreases as more tensile
strain is applied and increases as more electric field strength is applied. Our
calculations also predict a band gap at a peak value of 5 tensile strain but at
expense of the structural stability of the system. The band gap opening at 5
tensile strain is due to distortion in the structure caused by the repulsive
effect in the cavity between the lone pairs of edge nitrogen atoms and
$\mathrm{d}_{{xy}}/\mathrm{d}_{x^2-y^2}$ orbital of Fe atom, hence the
unoccupied $\mathrm{p}_z$-orbital is forced to shift towards higher energy. The
electronic and magnetic properties of the heptazine with embedded $\mathrm{Fe}$
system under perpendicular electric field up to a peak value of 10
$\mathrm{V/nm}$ is also well preserved despite obvious buckled structure. Such
properties may be desirable for diluted magnetic semiconductors, spintronics,
and sensing devices.
