Self-assembly, dynamics, and structure of Si magic clusters

Abstract

Listen

Translate article

We demonstrate the preferential formation and self-assembly of monodisperse Si magic clusters $({X}_{4})$ of size $\ensuremath{\sim}13.5\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{\AA{}}$ on $\text{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ 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 $\ensuremath{\sim}4.6\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{\AA{}}$) and culminating in tetracluster dimer $({X}_{3})$ and trimer $({X}_{4})$ formations as the surface is being annealed progressively to $150\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The respective cluster species density distribution at each annealing temperature also shows the preferential formation of ${X}_{1}\ensuremath{\rightarrow}{X}_{2}\ensuremath{\rightarrow}{X}_{3}\ensuremath{\rightarrow}{X}_{4}$ at higher temperatures, which we describe using surface reaction schemes; ${X}_{1}\ensuremath{\rightarrow}{X}_{2}$, ${X}_{2}+{X}_{2}\ensuremath{\rightarrow}{X}_{3}$, and ${X}_{2}+{X}_{3}\ensuremath{\rightarrow}{X}_{4}$. 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 (i) cluster symmetry and alignment, (ii) close packing, and (iii) minimization of dangling bonds.