Monolayer C 7 N 6 : Room-temperature excitons with large binding energies and high thermal conductivities

Two-dimensional (2D) carbon nitrides compounds have attracted wide attention in recent years due to their diverse structures and excellent electronic, thermal, and optical properties. Here, by using first-principles approach, we investigate in details the stability, many-body effect, electronic/thermal transport properties, and thermoelectric performance of monolayer ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$, as a new kind of 2D carbon nitride compounds composed of $s{p}^{2}\text{\ensuremath{-}}\mathrm{hybridized}$ carbon atoms forming hexagonal lattice. Our results show that ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ monolayer is a direct band-gap semiconductor with a band-gap value of 3.56 eV under the accurate ${G}_{0}{W}_{0}$ method. Ab initio molecular dynamics simulations demonstrate that ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ maintains stable up to $1500\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Two exciton absorption peaks can be observed within the band gap with the respective large binding energies of $0.84$ and $0.09\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, which means both excitons can exist at room temperature. Monolayer ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ possesses high carrier mobility with the order of ${10}^{2}--{10}^{3}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Moreover, we find that the lattice thermal conductivity for ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ is as high as $134.55\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{mK}$ at room temperature, thus the thermoelectric figure of merit for ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ is relatively low. Our work suggests that ${\mathrm{C}}_{7}{\mathrm{N}}_{6}$ is a promising candidate for nanoscale (opto-)electronic and heat transport devices.