X-ray spectroscopy reveals high symmetry and electronic shell structure of transition-metal-doped silicon clusters

Size-selected cationic transition-metal-doped silicon clusters have been studied with x-ray absorption spectroscopy at the transition-metal $L_{2,3}$ edges to investigate the local electronic structure of the dopant atoms. For ${\text{VSi}}_{16}^{+}$, the x-ray absorption spectrum is dominated by sharp transitions which directly reveal the formation of a highly symmetric silicon cage around the vanadium atom. In spite of their different number of valence electrons, a nearly identical local electronic structure is found for the dopant atoms in ${\text{TiSi}}_{16}^{+}$, ${\text{VSi}}_{16}^{+}$, and ${\text{CrSi}}_{16}^{+}$. This indicates strongly interlinked electronic and geometric properties: while the transition-metal atom imposes a geometric rearrangement on the silicon cluster, the interaction with the highly symmetric silicon cage determines the local electronic structure of the transition-metal dopant.

Figure 1

(Color online) $L_{2,3}$ x-ray absorption spectra of size-selected ${\text{VSi}}_n^{+}$ clusters $\left(n=14–18\right)$. The ${\text{VSi}}_{16}^{+}$ spectrum is characterized by the sharpest lines, indicating a highly symmetric cage with high degeneracy of the electronic levels. The double peak structure can be linked to the electronic density of states in tetrahedral symmetry.

Figure 2

(Color online) Transition-metal $L_{2,3}$ x-ray absorption spectra of ${\text{TiSi}}_{16}^{+}$, ${\text{VSi}}_{16}^{+}$, and ${\text{CrSi}}_{16}^{+}$ cage clusters in the lower panel, and of bare ${\text{Ti}}^{+}$, ${\text{V}}^{+}$, and ${\text{Cr}}^{+}$ ions [29] in the upper panel. The spectra are aligned at the first peak position. While bare ions show differing spectra, the relative excitation energy in doped silicon clusters is nearly identical. This surprising fact indicates almost identical local electronic structure at the dopant atom site. Relative positions of the $L_2$ edge shift because of decreasing $2p$ spin-orbit splitting from chromium to titanium. In the titanium spectrum, the 5 eV feature of the $L_3$ line is partially masked by overlapping $L_2$ intensity.

Figure 3

(Color online) Silicon $L_{2,3}$ x-ray absorption spectra of ${\text{Si}}_{16}^{+}$ (dashed line) and ${\text{VSi}}_{16}^{+}$ (solid line) clusters. The high symmetry of doped silicon cage clusters is again reflected in the highly structured x-ray absorption spectrum, with sharp transitions as compared to pure silicon clusters. ${\text{TiSi}}_{16}^{+}$ and ${\text{CrSi}}_{16}^{+}$ show features similar to ${\text{VSi}}_{16}^{+}$.

Figure 4

(Color online) Schematic view of the ${\text{VSi}}_{16}^{+}$ density of states (DOS). The total DOS is shown in the left panel while the local $l$-projected DOS at the dopant site [9] is shown in the right panel. In x-ray absorption spectroscopy at the spin-orbit split transition-metal $2p$ levels, electronic transitions are allowed only into local, $d$- or $s$-projected states. Therefore, only states marked with an arrow $(3d,4s)$ are probed. The splitting of the metal $3d$ states into bonding and antibonding states is indicated schematically.