Quantum dots QDs are nanoscale semiconductor materials that exhibit unique photochemical properties due to their size and composition. These properties include size-tunable bandgap, high quantum yield, and broad absorption with narrow emission spectra. The photochemical properties of quantum dots can be altered by varying their size and composition, which in turn affects their electronic structure and optical properties.1. Size: As the size of the quantum dots decreases, the confinement of the electrons and holes charge carriers within the QDs increases. This leads to an increase in the energy difference between the valence and conduction bands, known as the bandgap. Consequently, the absorption and emission wavelengths of the QDs shift towards the blue shorter wavelengths as the size decreases, a phenomenon known as the quantum confinement effect. Smaller QDs exhibit higher energy emissions blue-shift , while larger QDs exhibit lower energy emissions red-shift .2. Composition: The composition of quantum dots, including the choice of semiconductor materials and the presence of any dopants or surface ligands, can also significantly influence their photochemical properties. Different semiconductor materials, such as CdSe, CdTe, or InP, have different inherent bandgaps and electron affinities, which affect the QDs' absorption and emission properties. Additionally, the presence of dopants or alloying of different materials can modify the band structure and energy levels within the QDs, leading to changes in their optical properties.3. Surface properties: The surface properties of quantum dots, such as the type and density of surface ligands, can also impact their photochemical properties. Surface ligands can passivate surface defects, which reduces non-radiative recombination and enhances the quantum yield of the QDs. Moreover, the choice of surface ligands can affect the solubility and stability of the QDs in different solvents, which is crucial for their application in various fields.4. Shape and structure: Apart from size and composition, the shape and structure of quantum dots can also influence their photochemical properties. For example, core-shell quantum dots, where a core semiconductor material is surrounded by a shell of another semiconductor material, can exhibit improved quantum yield and stability compared to their core-only counterparts. This is due to the passivation of surface defects and the reduction of non-radiative recombination processes.In summary, the photochemical properties of quantum dots are highly dependent on their size, composition, surface properties, and structure. By carefully controlling these factors, researchers can tailor the optical properties of quantum dots for various applications, such as solar cells, LEDs, and bioimaging.