Instabilidades estruturais e eletrônicas em nanofios de silício com metais encapsulados

Abstract

In this work, we employ an ab initiomethodology to studymonomers, dimers and nanowires of silicon clusters containing encapsulated transition metals. The encapsulation of transition metals of 4d and 5d series leads to the stabilization of clusters with a structure consisting of atwelve-atom hexagonal prism, formed by two hexagonal faces of silicon atoms with a transition metal in its center, whose chemical formula is denoted M@Si12 (where M is the metal atom), wich was synthesized in an ealier experimental work. In general, we observe trends in structural properties of these systems, strongly related to the filling of the metal d orbitals. In the case of monomers, we observe that structures withgreater stability are those with metals that have semi-filled d shells. The greater stability of the monomers in these cases, suggests a lower propensity to form extended structures such as nanowires, which is the behavior that we observed for the wires, which have lower bindingenergies when such metals are encapsulated along the wire axis.We also study nanowires formed from monomers and dimers used as fundamental units of single and double period wires, respectively. The single-period wires basically display two types of structures, which we classify as ZZPS in the case of two types of single-period zigzaglikedistortion of the monomer units, and regular, where the monomer unti remains with its geometry essentially unaltered. All single-period wires display metallic behavior. In the case of wires formed by dimers, we work with structures with Zr, Nb and Ta. We see that these wires undergo a Peierls distortion, with energy gaps ranging from0.03 eV (Ta) to 0.21eV (Zr). Finally, we identify a dimer with stoichiometry M2@Si18 that has formation energy lower than theM2@Si24 dimer. The study of nanowires formed by dimers of the typeM2@Si18, and a more complete investigation of the occurrence of Peierls instability in nanowires of thistype is forthcoming.