Biaxial strain enhanced piezoelectric properties in monolayer g-$\mathrm{C_3N_4}$.

Graphite-like carbon nitride (g-$\mathrm{C_3N_4}$) is considered as a promising candidate for energy materials. In this work, the biaxial strain (-4\%-4\%) effects on piezoelectric properties of g-$\mathrm{C_3N_4}$ monolayer are studied by density functional theory (DFT). It is found that the increasing strain can reduce the elastic coefficient $C_{11}$-$C_{12}$, and increases piezoelectric stress coefficient $e_{11}$, which lead to the enhanced piezoelectric strain coefficient $d_{11}$. Compared to unstrained one, strain of 4\% can raise the $d_{11}$ by about 330\%. From -4\% to 4\%, strain can induce the improved ionic contribution to $e_{11}$ of g-$\mathrm{C_3N_4}$, and almost unchanged electronic contribution, which is different from $\mathrm{MoS_2}$ monolayer (the enhanced electronic contribution and reduced ionic contribution). To prohibit current leakage, a piezoelectric material should be a semiconductor, and g-$\mathrm{C_3N_4}$ monolayer is always a semiconductor in considered strain range. Calculated results show that the gap increases from compressive strain to tensile one. At 4\% strain, the first and second valence bands cross, which has important effect on transition dipole moment (TDM). Our works provide a strategy to achieve enhanced piezoelectric effect of g-$\mathrm{C_3N_4}$ monolayer, which gives a useful guidence for developing efficient energy conversion devices.