The size of quantum dots QDs significantly affects their photochemical properties, including their quantum yield and stability. Quantum dots are semiconductor nanoparticles that exhibit size-dependent electronic and optical properties due to quantum confinement effects. The underlying mechanisms behind these effects are mainly related to the changes in energy levels, surface-to-volume ratio, and surface defects as the size of the QDs varies.1. Quantum confinement effect: As the size of the QDs decreases, the energy levels of the electrons and holes become more discrete due to the quantum confinement effect. This leads to a larger bandgap, which in turn results in a blue shift in the absorption and emission spectra. The quantum yield, which is the ratio of the number of emitted photons to the number of absorbed photons, can be affected by the size-dependent bandgap. Smaller QDs with larger bandgaps typically have higher quantum yields due to the reduced probability of non-radiative recombination processes.2. Surface-to-volume ratio: The surface-to-volume ratio increases as the size of the QDs decreases. This means that smaller QDs have a larger proportion of their atoms on the surface, which can affect their photochemical properties. Surface atoms have unsaturated bonds, which can act as trapping sites for charge carriers, leading to non-radiative recombination processes and reduced quantum yields. However, proper surface passivation with organic or inorganic ligands can help minimize these surface-related effects and improve the quantum yield.3. Surface defects: The presence of surface defects, such as vacancies, interstitials, and dangling bonds, can also affect the photochemical properties of QDs. These defects can act as non-radiative recombination centers, reducing the quantum yield. Smaller QDs are more susceptible to the influence of surface defects due to their larger surface-to-volume ratio. Proper surface passivation and synthesis techniques can help reduce the number of surface defects and improve the quantum yield.4. Stability: The stability of QDs is influenced by their size as well. Smaller QDs generally have higher surface energies, making them more prone to aggregation and degradation. This can lead to a decrease in their quantum yield and photostability over time. However, appropriate surface passivation and the use of stabilizing agents can help improve the stability of QDs.In summary, the size of quantum dots plays a crucial role in determining their photochemical properties, such as quantum yield and stability. The underlying mechanisms behind these effects are mainly related to the quantum confinement effect, surface-to-volume ratio, and surface defects. By controlling the size of QDs and employing proper surface passivation and synthesis techniques, it is possible to optimize their photochemical properties for various applications, such as solar cells, LEDs, and bioimaging.