The size and shape of a metallic nanocluster have a significant impact on its electronic and optical properties, particularly in terms of absorption and emission spectra. This is due to the quantum confinement effect and the plasmonic properties of metallic nanoparticles.1. Quantum confinement effect: When the size of a metallic nanocluster is reduced to the nanometer scale, the motion of electrons becomes confined within the small dimensions of the cluster. This leads to the quantization of energy levels, which in turn affects the electronic and optical properties of the nanocluster. As the size of the nanocluster decreases, the energy gap between the quantized levels increases, leading to a blue shift in the absorption and emission spectra. This means that smaller nanoclusters absorb and emit light at shorter wavelengths higher energies compared to larger ones.2. Plasmonic properties: Metallic nanoparticles, particularly noble metals like gold and silver, exhibit unique plasmonic properties due to the collective oscillation of their conduction electrons. This phenomenon, known as localized surface plasmon resonance LSPR , leads to strong absorption and scattering of light at specific wavelengths. The position and intensity of the LSPR peak in the absorption spectrum depend on the size and shape of the nanocluster.- Size: As the size of the metallic nanocluster increases, the LSPR peak redshifts i.e., the peak moves to longer wavelengths . This is because larger nanoparticles have a higher density of conduction electrons, which results in a stronger interaction with the electromagnetic field of the incident light.- Shape: The shape of the metallic nanocluster also plays a crucial role in determining its plasmonic properties. Different shapes, such as spheres, rods, cubes, and stars, exhibit distinct LSPR peaks due to the variation in the distribution of conduction electrons and the local electromagnetic field enhancement. For example, anisotropic shapes like nanorods have two LSPR peaks corresponding to the transverse and longitudinal modes of electron oscillation, while isotropic shapes like nanospheres exhibit a single LSPR peak.In summary, the size and shape of a metallic nanocluster significantly influence its electronic and optical properties, including absorption and emission spectra. Smaller nanoclusters exhibit blue-shifted spectra due to quantum confinement, while the position and intensity of LSPR peaks depend on both the size and shape of the nanocluster. By controlling these parameters, it is possible to tune the optical properties of metallic nanoclusters for various applications, such as sensing, imaging, and photothermal therapy.