Metal Nanoclusters Modify the Band Gap and Maintain the Ultrathin Nature of Semiconducting Two-Dimensional Materials

Modifying the band gap of semiconducting two-dimensional materials (S2DM) such as monolayer molybdenum disulfide (MoS₂) is useful in ultrathin optoelectronic applications. Electron doping is an efficient technique to alter the electronic band gap and change exciton binding energy of MoS₂, thus modifying the optical band gap. Photoexcited silver nanoclusters (AgNCs) can produce a large number of energetic hot electrons with a lifetime in the hundreds of picosecond timescale. These hot electrons can inject into the conduction band of a single-layered MoS₂, thereby modifying its optoelectrical properties when AgNCs come in contact with the sheet. Additionally, increasing AgNC coverage density on the MoS₂ surface increases the electron doping density. At low AgNC coverage density, the absorption and photoluminescence (PL) spectrum of MoS₂ are red-shifted as a result of band gap renormalization. The magnitude of the red shift increases as the coverage density of AgNCs is increased before blue-shifting remarkably at a high AgNC coverage. The blue shift is attributed to the population of the high-energy dark excitonic states. The optical band gap of monolayer MoS₂ is also tuned by integration with silver nanodisks (AgND). Unlike the high efficiency and controllable modification of band gap of MoS₂ by AgNCs, photoexcited AgNDs exhibit opposing effects on the band gap of MoS₂. Photoexcited AgNDs produce a strong electromagnetic field, which changes the spin-orbital coupling inside the MoS₂ and so the electronic band gap of MoS₂. The plasmon field decays generating hot electrons which cross the nanoparticle/MoS₂ Schottky barrier and inject into the conduction band of MoS₂ within a hundred femtoseconds. The limitation of hot electrons of AgNDs is ascribed to the delay in the electron injection process leading to the relaxation of hot electrons, which generates heat that induces the transformation of 2H semiconducting MoS₂ into metallic 1T.