Graphitic carbon nitride (g-C₃N₄), a metal-free semiconductor with a band gap of 2.7 eV, has received considerable attention owing to its fascinating photocatalytic performances under visible-light. g-C₃N₄ exhibits high thermal and chemical stability and non-toxicity such that it has been considered as the most promising photocatalyst for environmental improvement and energy conservation. Hence, it is of great importance to obtain high-quality g-C₃N₄ and gain a clear understanding of its optical properties. Herein, we report a high-yield synthesis of g-C₃N₄ products via heating of high vacuum-sealed melamine powder in an ampoule at temperatures between 450 and 650 °C. Using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), the chemical composition and crystallization of the as-produced g-C₃N₄ are demonstrated. A systematic optical study of g-C₃N₄ is carried out with several approaches. The optical phonon behavior of g-C₃N₄ is revealed by infrared and Raman spectroscopy, and the emission properties of g-C₃N₄ are investigated using photoluminescence (PL) spectroscopy, while the photocatalytic properties are explored by the photodegradation experiment.

By using density-functional theory and many-body perturbation theory based first-principles calculations, we have systematically investigated the electronic and optical properties of monolayer group-IV monochalcogenides $MX$ ($M=\mathrm{Ge},\mathrm{Sn}$; $X=\mathrm{S},\mathrm{Se},\mathrm{Te}$). All $MX$ monolayers are predicted to be indirect gap semiconductors, except the GeSe monolayer, which has a direct gap of 1.66 eV. The carrier mobilities of $MX$ monolayers are estimated to be on the order of ${10}^3$ to ${10}^5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, which is comparable to, and in some cases higher than, that of phosphorene using a phonon-limited scattering model. Moreover, the optical spectra of $MX$ monolayers obtained from GW-Bethe-Salpeter equation calculations are highly orientation dependent, especially for the GeS monolayer, suggesting their potential application as a linear polarizing filter. Our results reveal that the GeSe monolayer is an attractive candidate for optoelectronic applications as it is a semiconductor with a direct band gap, a relatively high carrier mobility, and an onset optical absorption energy in the visible light range. Finally, based on an effective-mass model with nonlocal Coulomb interaction included, we find that the excitonic effects of the GeSe monolayer can be effectively tuned by the presence of dielectric substrates. Our studies provide an improved understanding of electronic, optical, and excitonic properties of group-IV monochalcogenides monolayers and might shed light on their potential electronic and optoelectronic applications.

The two-dimensional layer of molybdenum disulfide (MoS₂) exhibits promising prospects in the applications of optoelectronics and valleytronics.