Ag and Au nanoclusters

Gold and silver nanoclusters are a new class of materials that bridges the “missing properties” of atomic and plasmonic nanoparticles of noble metals. The novel properties such as ultra-small sizes, multiple absorption peaks, and enhanced luminescence promises various applications in catalysis, sensor, biology, and energy harvesting. Herein, this chapter especially focuses on the fundamental developments in Ag and Au nanoclusters carried out over several decades. The size-dependent variation in the band structure and photophysical properties from bulk to nano to quantum sizes are discussed in detail. The nanoclusters are unique in terms of their surface coverage (thiols, phosphines, biomolecules, polymers, and dendrimers) or surface forces (electrostatic, micelle, or bare surfaces) and various types of these nanoclusters are introduced. The factors to stabilize the nanoclusters are complex but their understanding is advanced through various well-designed synthetic routes and experimental studies. The factors stabilizing the nanoclusters include electronic, geometric stability, the nature of solvent polarity, and the charge-state of a nanocluster. In addition, the influence of external surroundings such as temperature and light radiation could degrade the nanoclusters. The major contribution of stabilizing a particular nanocluster with controlled crystalline structures (BCC, FCC, HCP) is highly dependent on the structure and steric hindrance of a bulky ligand. Recently, advances in naming the typical sizes of gold nanoclusters Au25(SR)18, Au38(SR)24, and Au144(SR)60 offer in-depth topology and structure-based studies possible.