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

Ultra-high-resolution optical microscopic techniques are used to measure the reflectance and photoluminescence (PL) spectrum of individual monolayered MoS2 of dimensions below 200 x 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 A(1)' Raman band of MoS2 was measured on top of CPs, which is sensitive to the electron injection.