The size of quantum dots significantly affects their photochemical properties due to the phenomenon known as quantum confinement. Quantum dots are semiconductor nanoparticles with dimensions typically ranging from 1 to 10 nanometers. At this scale, the electronic and optical properties of the material become highly dependent on the size and shape of the nanoparticles.1. Bandgap energy: As the size of the quantum dots decreases, the bandgap energy increases. This is because the confinement of the electrons and holes in the smaller-sized quantum dots leads to a greater separation between the energy levels. Consequently, a higher energy is required for an electron to transition from the valence band to the conduction band, resulting in a larger bandgap.2. Absorption and emission spectra: The size-dependent bandgap energy directly influences the absorption and emission spectra of quantum dots. Smaller quantum dots absorb and emit light at higher energies shorter wavelengths compared to larger quantum dots. This size-tunable property allows for the precise control of the color of emitted light, making quantum dots ideal for applications in light-emitting diodes LEDs , solar cells, and bioimaging.3. Photoluminescence quantum yield: The photoluminescence quantum yield PLQY is a measure of the efficiency of the light emission process. The PLQY of quantum dots is highly dependent on their size, as well as the quality of their surface and core-shell structure. Generally, smaller quantum dots exhibit lower PLQY due to a higher probability of non-radiative recombination processes. However, by optimizing the surface passivation and core-shell structure, high PLQY can be achieved even for small-sized quantum dots.4. Exciton dynamics: The size of quantum dots also affects the exciton dynamics, including the exciton lifetime and diffusion length. Smaller quantum dots typically exhibit shorter exciton lifetimes due to the increased probability of non-radiative recombination processes. The exciton diffusion length, which is the distance an exciton can travel before recombining, is also influenced by the size of the quantum dots and can impact the efficiency of devices such as solar cells.In summary, the size of quantum dots plays a crucial role in determining their photochemical properties, including bandgap energy, absorption and emission spectra, photoluminescence quantum yield, and exciton dynamics. By controlling the size of quantum dots, their properties can be tuned for various applications in optoelectronics, photovoltaics, and bioimaging.