The quantum confinement effect plays a significant role in determining the electronic and optical properties of CdSe/ZnS core-shell quantum dots QDs of different sizes. This effect occurs when the size of the quantum dot is comparable to or smaller than the exciton Bohr radius, which leads to the spatial confinement of charge carriers electrons and holes within the quantum dot. This confinement results in discrete energy levels and changes in the electronic and optical properties of the QDs.1. Bandgap and energy levels: As the size of the CdSe/ZnS QDs decreases, the quantum confinement effect becomes more pronounced, leading to an increase in the bandgap energy. This is because the confinement restricts the movement of the charge carriers, causing the energy levels to become more discrete and separated. Consequently, a larger energy is required for an electron to transition from the valence band to the conduction band.2. Absorption and emission spectra: The increase in bandgap energy due to quantum confinement leads to a blue shift in the absorption and emission spectra of the QDs. Smaller QDs have higher bandgap energies, which result in higher energy shorter wavelength absorption and emission peaks. This size-dependent tunability of the optical properties is a key feature of quantum dots and enables their use in various applications, such as light-emitting diodes LEDs , solar cells, and bioimaging.3. Photoluminescence quantum yield PLQY : The CdSe core is often passivated with a ZnS shell to improve the photoluminescence quantum yield PLQY of the QDs. The ZnS shell reduces the surface defects and non-radiative recombination pathways, leading to enhanced PLQY. The PLQY can also be size-dependent, with smaller QDs generally exhibiting lower PLQY due to a higher surface-to-volume ratio and increased surface defects.4. Exciton binding energy: The exciton binding energy, which is the energy required to separate an electron-hole pair exciton into free carriers, also increases with decreasing QD size due to the quantum confinement effect. This can influence the efficiency of charge separation in applications such as solar cells and photocatalysis.In summary, the quantum confinement effect significantly impacts the electronic and optical properties of CdSe/ZnS core-shell quantum dots of different sizes. The size-dependent tunability of these properties enables their use in a wide range of applications, from optoelectronics to biotechnology.