Strongly bound excitons in monolayer PtS₂ and PtSe₂

Based on first-principles calculations, the structural, electronic, and optical properties of monolayers PtS₂ and PtSe₂ are investigated. The bond stiffnesses and elastic moduli are determined by means of the spring constants and strain-energy relations, respectively. Dynamic stability is confirmed by calculating the phonon spectra, which shows excellent agreement with experimental reports for the frequencies of the Raman-active modes. The Heyd-Scuseria-Ernzerhof functional results in electronic bandgaps of 2.66 eV for monolayer PtS₂ and 1.74 eV for monolayer PtSe₂. G₀W₀ calculations combined with the Bethe-Salpeter equation are used to predict the optical spectra and exciton binding energies (0.78 eV for monolayer PtS₂ and 0.60 eV for monolayer PtSe₂). It turns out that the excitons are strongly bound and therefore very stable against external perturbations.