Modulating the Optical Band Gap of Small Semiconducting Two-Dimensional Materials by Conjugated Polymers

Ultra-high-resolution optical microscopic techniques are used to measure the reflectance and photoluminescence (PL) spectrum of individual monolayered MoS₂ of dimensions below 200 × 200 nm, while placed on top of a thin film conjugated polymer (CP). p-type and n-type CPs such as poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM), respectively, modified the optical band gap of the MoS₂ sheet differently. However, the optical band gap is decreased after integration with P3HT, while it is increased after being combined with PCBM. The acceptable reason for the modification of the band gap of MoS₂ by CPs is the generation of interlayer excitons (ILE) at their interface. The optical band gap of MoS₂ is further changed by introducing an inert polymer spacer of different thickness to separate MoS₂ from the CP. This is attributed to the reduction of the efficiency of excitonic interactions and lowering the exciton binding energy, which is induced by the increase of the dielectric function at the CP-MoS₂ interface. No sign of electron injection to the conduction band of MoS₂ after integration with P3HT or PCBM, as no significant shift of the A₁^' Raman band of MoS₂ was measured on top of CPs, which is sensitive to the electron injection.