We demonstrate the preferential formation and self-assembly of monodisperse Si magic clusters ͑X 4 ͒ of size ϳ13.5Ϯ 0.5 Å on Si͑111͒-͑7 ϫ 7͒ surface using scanning tunneling microscope. The growth process is observed to occur via a stepwise assembly of planarized Si tetramers ͑X 1 ͒ formed from Si adatoms deposited at room temperature, leading to Si tetraclusters ͑X 2 ͒ ͑size ϳ4.6Ϯ 0.5 Å͒ and culminating in tetracluster dimer ͑X 3 ͒ and trimer ͑X 4 ͒ formations as the surface is being annealed progressively to 150°C. The respective cluster species density distribution at each annealing temperature also shows the preferential formation of X 1 → X 2 → X 3 → X 4 at higher temperatures, which we describe using surface reaction schemes; X 1 → X 2 , X 2 + X 2 → X 3 , and X 2 + X 3 → X 4 . We determine the activation and formation energies for respective cluster species and elucidate the formation energetics and dynamics of tetraclusters which function unequivocally as fundamental building blocks in the self-assembly of stable Si magic clusters. Finally, we resolve the structure of the Si magic cluster to comprise three tetraclusters or n = 12 Si atoms taking into consideration ͑i͒ cluster symmetry and alignment, ͑ii͒ close packing, and ͑iii͒ minimization of dangling bonds.
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