## 1 Introduction  
Colloidal quantum dots (QDs) have attracted great attention over the last decades because of their unique optical properties related to quantum confinement effects where their band gap can be tuned by adjusting their particle size [1, 2]. This effect gives a significant impact on the energy band gap of the QDs. As the size of the particles reaches nanoscale, the movement of excitons is impeded. However, the total energy of excitons is significantly unaffected until the size of particles is smaller than the exciton Bohr radius. At this point, the excitation energy required to create electron-hole pairsstarts to increase, and this inversely proportional phenomenon of excitation energy and QD size is known as quantum confinement. Furthermore, the effect of quantum confinement can be seen in the core shell QDs. It was reported that increasing thickness of the shell causes the photoluminescence (PL) peak to be shifted to a higher energy level [3]. Thus, properties of QDs such as photostability, bright luminescence, size dependent PL, narrow emission and broad absorption bands have caused colloidal QDs to receive extensive attention for the past decades [4]. Lead sulphide (PbS) QDs belong to the group of IV-VI semiconductors with a direct narrow band gap of 0.41 eV at room temperature [5]. Therefore, the band structure and electronic properties have distinguished PbS from II-VI (e.g., cadmium sulphide (CdS)) and III-V (e.g., indium arsenide (InAs)) semiconductors. PbS may offer unique opportunities to study the fundamental physics of quantum confinement due to the fact that it has a large exciton Bohr radius (\(\sim\)18 nm) which can achieve a strong quantum confinement effect with a small QD diameter (\(<\)10 nm) as compared to other groups of material [6; 7]. This extreme confinement of charge carriers is predicted to enhance the quantization energy of nearly all material properties dependent on size [8]. Certainly, PbS QDs exhibit blueshifted PL onsets compared to the bulk crystals. In terms of technological perspective, PbS shows promising interest as it can provide size-tunable optical transitions at important near infrared (NIR) wavelengths. This may be advantageous to a variety of applications, including solar cells [9; 10; 11], dye imaging [12] and light emitting diodes [13] operating at NIR wavelengths. From the examples above, practical applications of QDs are mainly related to the interaction of QDs with light, thus investigating the effects of laser light irradiation on the PL properties of QDs is one of the crucial tasks. Under ambient conditions, there are two possible effects on the influence of light on the QDs: (i) increase of luminescence intensity due to QD surface modification and (ii) degradation of PL spectra due to the photooxidation process. The combination of both effects is expected during prolonged light exposure and has been investigated experimentally [14; 15; 16; 17; 18]. It is clear that fluorescence enhancement is achieved through the use of appropriate surface passivation, which can eliminate surface defects that cause non-radiative relaxation [19]. One of the strategies is growing a wider bandgap semiconductor material towards the core QDs, so-called type-I core shell QDs. In such type of core shell QD, the exciton is confined in the core where the electron populates the energy in the conduction band while the hole is in the valence band [20; 21]. It was reported that PL intensity is enhanced due to stabilization with surfactant molecules or surface ligand passivation [22; 23; 24]. The surfactant molecules such as trioctylphosphine oxide (TOPO) and hexadecylamine in organic solvents passivate the defects on the surface of the QDs resulting in enhancement. In addition, in terms of sample stability, attachment of capping ligands on core QDs results in unique properties, where the QDs will isolate from each other thus preventing them from agglomeration.