The photochemical properties of quantum dots QDs are highly dependent on their size and composition. Quantum dots are semiconductor nanocrystals with unique optical and electronic properties due to their quantum confinement effects. When the size and composition of QDs are altered, it can significantly impact their absorption and emission spectra, quantum yield, and photostability.1. Size: As the size of a quantum dot increases, the energy levels of the electrons in the QD become less discrete, and the energy gap between the highest occupied molecular orbital HOMO and the lowest unoccupied molecular orbital LUMO decreases. This results in a redshift in the absorption and emission spectra. Conversely, as the size of the QD decreases, the energy gap increases, leading to a blueshift in the absorption and emission spectra. Smaller QDs generally have higher quantum yields due to the larger energy gap, which reduces the probability of non-radiative relaxation pathways.2. Composition: The composition of a quantum dot, including the type of semiconductor material and any surface modifications, can also greatly affect its photochemical properties. Different semiconductor materials have different bandgaps, which determine the energy required for an electron to transition from the valence band to the conduction band. For example, CdSe QDs have a smaller bandgap than CdS QDs, resulting in a redshift in their absorption and emission spectra. Surface modifications, such as the addition of a shell or ligands, can also influence the photochemical properties of QDs. A shell can improve the quantum yield and photostability by passivating surface defects and reducing non-radiative recombination pathways. Ligands can affect the solubility, stability, and surface charge of the QDs, which can, in turn, impact their photochemical properties.In summary, the photochemical properties of quantum dots are highly sensitive to changes in their size and composition. By controlling these factors, it is possible to fine-tune the optical and electronic properties of QDs for various applications, such as solar cells, LEDs, and biological imaging.