Monolayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mn>7</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">N</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math>: Room-temperature excitons with large binding energies and high thermal conductivities
Yu Wu, Y. Chen, Congcong Ma, Zixuan Lu, Hao Zhang, Bohayra Mortazavi, Bowen Hou, Ke Xu, Haodong Mei, Timon Rabczuk, Heyuan Zhu, Zhilai Fang, Rongjun Zhang, Costas M. Soukoulis
Abstract
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.