The graphitic carbon nitride ($g$-C$_3$N$_4$) is a promising layered two-dimension material with an opened bandgap. It is of interest to explore the tunability of the bandgap together with the magnetism by doping transition metal atoms. In this work, we investigated the transition metals (Mn, Fe, Co, Ni) and their hydroxides doped $g$-C$_3$N$_4$ monolayers. The electron correlations between the 3$d$ electrons of the doped transition metal atoms are self-consistently calculated and analyzed based on the density functional theory. The magnetism, electronic band structures and optical properties are systematically investigated. It reveals that the transition metal doped $g$-C$_3$N$_4$ is ferromagnetic (FM) state at small doping concentration, where the two spins show different bandgaps. When the doping is high enough, it turns to metallic antiferromagnetic (AFM) state except that Mn doped $g$-C$_3$N$_4$ is metallic FM state. On another hand, the system shows variable absorption spectra at different doping level. When the vacancy sites are fully occupied, a large absorption peak appears around 1.5 eV suitable for visible light. Moreover, within the transition metal hydroxides doped $g$-C$_3$N$_4$, the global ground state shows as AFM, and the absorption spectra within low energy range is distinct due to the presence of hydroxyl group. Therefore, doping with transition metal atoms and hydroxides can effectively tune the bandgap, magnetism and optical properties of $g$-C$_3$N$_4$ so as to promote its applications.

中文翻译：

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过渡金属和氢氧化物掺杂单层 g-C3N4 的电子、磁性和光学性质，第一原理研究


石墨氮化碳（$g$-C$_3$N$_4$）是一种有前途的带隙开放的层状二维材料。通过掺杂过渡金属原子来探索带隙和磁性的可调性是很有趣的。在这项工作中，我们研究了过渡金属（Mn、Fe、Co、Ni）及其氢氧化物掺杂$g$-C$_3$N$_4$单层。基于密度泛函理论，自洽地计算和分析了掺杂过渡金属原子的3$d$电子之间的电子相关性。系统地研究了磁性、电子能带结构和光学性质。结果表明，过渡金属掺杂$g$-C$_3$N$_4$在小掺杂浓度下呈铁磁（FM）态，其中两个自旋表现出不同的带隙。当掺杂足够高时，它会变成金属反铁磁（AFM）态，除了Mn掺杂$g$-C$_3$N$_4$是金属FM态之外。另一方面，该系统在不同的掺杂水平下显示出不同的吸收光谱。当空位被完全占据时，在 1.5 eV 左右出现适合可见光的大吸收峰。此外，在掺杂$g$-C$_3$N$_4$的过渡金属氢氧化物中，整体基态显示为AFM，并且由于羟基的存在，低能范围内的吸收光谱是明显的。因此，掺杂过渡金属原子和氢氧化物可以有效调节$g$-C$_3$N$_4$的带隙、磁性和光学性质，从而促进其应用。