Modifying the band gap of semiconducting two-dimensional materials (S2DM) such as monolayer molybdenum disulfide (MoS 2 ) is useful in ultrathin optoelectronic applications. Electron doping is an efficient technique to alter the electronic band gap and change exciton binding energy of MoS 2 , 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 MoS 2 , thereby modifying its optoelectrical properties when AgNCs come in contact with the sheet. Additionally, increasing AgNC coverage density on the MoS 2 surface increases the electron doping density. At low AgNC coverage density, the absorption and photoluminescence (PL) spectrum of MoS 2 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 MoS 2 is also tuned by integration with silver nanodisks (AgND). Unlike the high efficiency and controllable modification of band gap of MoS 2 by AgNCs, photoexcited AgNDs exhibit opposing effects on the band gap of MoS 2 . Photoexcited AgNDs produce a strong electromagnetic field, which changes the spinorbital coupling inside the MoS 2 and so the electronic band gap of MoS 2 . The plasmon field decays generating hot electrons which cross the nanoparticle/MoS 2 Schottky barrier and inject into the conduction band of MoS 2 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 MoS 2 into metallic 1T.

The two characteristic absorption peaks of semiconducting two-dimensional tungsten disulfide (WS2) are red-shifted after integrating with gold nanocube (AuNC) arrays. The amount of the red shift is reduced when the AuNCs are coated with a high concentration of Pd. A negligible shift was observed in the absorption peaks of WS2 when smaller amounts of Pd are introduced to the surface of AuNCs. Conversely, the photoluminescence (PL) of WS2 is blue-shifted when measured on top of AuNCs and AuNCs coated with different amounts of Pd. AuNC–Pd Janus nanoparticles are prepared by depositing Pd atoms asymmetrically on AuNCs assembled into 2-D arrays on the surface of a glass substrate by the chemical reduction of Pd ions. Due to the large AuNC or AuNC–Pd/WS2 Schottky barrier, the plasmon-induced hot electron transfer (PHET) from AuNCs and AuNCs coated with a high concentration of Pd is responsible for the red shift of the absorption spectrum of WS2. Introducing a lower concentration of Pd to AuNCs increases the Schottky barrier further due to the formation of the Au–Pd equilibrium Fermi level of lower energy, reducing the efficiency of PHET. The effect of Pd on the Fermi level of AuNCs vanishes at high Pd deposition. Pauli blocking and phase-space filling are responsible for the blue shift of PL of WS2 on top of AuNCs and AuNCs coated with Pd. The Pauli blocking effect is directly proportional to the PHET efficiency. This explains the significant blue shift of PL of WS2 after integrating with AuNCs and AuNCs coated with a high concentration of Pd. Additionally, depositing Pd onto AuNCs elongates the lifetime of the hot electrons and enhances the PHET efficiency.