## 3 Results  
### Quantum Confinement Effect in the PbS QDs and PbS/MnS Core Shell QDs  
The effect of MnS shells on the PbS QDs as synthesized was investigated by observing the PL emissions of all samples at room temperature. Hence, different shell thicknesses of MnS shells showed different PL emissions compared to PbS QDs.  
Figure 1: The schematic diagram of the preparation of PbS/MnS core shell quantum dots (QDs). (**a**) Preparation of Mn\({}^{2+}\) precursor and (**b**) preparation of PbS/MnS core shell QDs.  
From Figure 2, it can be observed that the PL peak energy was blueshifted as the thickness of the MnS shell increased. The PL peak energies of the PbS QDs were centered at E\({}_{\text{PbS}}\) = 1.020 eV and gradually increased to 1.040, 1.051, 1.074, 1.080 and 1.082 eV for CS 0.3, CS 0.6, CS 0.9, CS 1.2 and CS 1.5, respectively. The significant blueshifts in PL emissions confirmed that MnS shells with different thicknesses were successfully grown, thus showing the effect of strong quantum confinement. In this case, overcoating the narrow band gap core material with the wider band gap shell material increased the confinement of charge carriers inside the core, causing the PL peak to shift to the higher energy level [29]. The blueshift of PL emissions after the presence of the shell was previously reported for type-I core shell QDs (CuInS\({}_{2}\)/Zn) [30]. In their case, the blueshift of a PL emission with the value of 130 nm was achieved due to the reduction of core size after cation exchange between the shell and core precursors. As in our PbS/MnS type-I core shell QDs, the charge carriers were concentrated in the PbS core and sandwiched between the large potential energy well of MnS, leading to fewer interactions of core-localized electron-hole pairs with surface traps than with bare QDs and thus increasing the PL peak energy. The band alignment of PbS/MnS core shell QDs is shown in Figure 3.  
Figure 3: Schematic representation of the band alignment of the PbS/MnS core shell QDs.  
Figure 2: Room temperature photoluminescence (PL) spectra of the colloidal PbS QDs and PbS/MnS core shell QDs. The presence of the MnS shell causes PL peak energy to blueshift.  
As the thickness of the MnS shell increased, the localization of charge carriers was increased, which translates to a strong carrier confinement and this is proven by a blueshift of PL peak energies. However, when optimum shell thickness is reached, the high energy excitons can tunnel through the core shell interface causing delocalization. This causes the confinement strength to be reduced as shown between CS 1.2 and CS 1.5 where their peak position shows a small shift of 0.2 meV.  
Previous research also reported this effect on PbS/CdS type-I core shell QDs by using effective mass approximation with the single-band model [31]. It was stated that the addition of a CdS shell onto the PbS core QDs causes the radial probability of charge carriers to be shifted to the center of the core shell QD region, signaling stronger localization. This stronger carrier localization proves the stronger confinement which translates to higher transmission energy in PbS/CdS core shell QDs compared to their PbS core QD counterparts.