Highly tunable and strongly bound exciton in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>MoSi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">N</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math> via strain engineering
Dan Liang, Shicai Xu, Pengfei Lu, Yongqing Cai
Abstract
Motivated by the recently synthesized layered material ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$, we investigate excitonic response of quasiparticle of monolayer ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ by using ${G}_{0}{W}_{0}$ and Bethe-Salpeter equation calculations. With a dually sandwiched structure consisting of a central ${\mathrm{MoN}}_{2}$ layer analog of $2H\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ capped with silicon-nitrogen (SiN) honeycomb outer layers, ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ possesses frontier orbitals confined at the central ${\mathrm{MoN}}_{2}$ layer with similar subvalley at the $K$ point as $2H\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$. The valley splitting ($\ensuremath{\sim}130\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$) due to the spin-orbital coupling gives rise to a doublet in the spectrum. Excitons in ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ show a strong binding energy up to 0.95 eV with the optical band gap of 2.44 eV. Both electronic and optical gaps are highly sensitive to tensile strains and become redshifted, albeit a marginal change of exciton binding energy. With the protection of capped SiN layers, quantum confined excitons in ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ without the need of additional passivation layer like BN would provide a bright platform for robust emission with partially screened disturbance from environment.