Chemi-Inspired Silicon Allotropes—Experimentally Accessible Si9 Cages as Proposed Building Block for 1D Polymers, 2D Sheets, Single-Walled Nanotubes, and Nanoparticles

Abstract

Numerous studies on silicon allotropes with three-dimensional networks or as materials of lower dimensionality have been carried out in the past. Herein, allotropes of silicon, which are based on structures of experimentally accessible [Si9]4− clusters known as stable anionic molecular species in neat solids and in solution, are predicted. Hypothetical oxidative coupling under the formation of covalent Si–Si bonds between the clusters leads to uncharged two-, one- and zero-dimensional silicon nanomaterials not suffering from dangling bonds. A large variety of structures are derived and investigated by quantum chemical calculations. Their relative energies are in the same range as experimentally known silicene, and some structures are even energetically more favorable than silicene. Significantly smaller relative energies are reached by the insertion of linkers in form of tetrahedrally connected Si atoms. A chessboard pattern built of Si9 clusters bridged by tetrahedrally connected Si atoms represents a two-dimensional silicon species with remarkably lower relative energy in comparison with silicene. We discuss the structural and electronic properties of the predicted silicon materials and their building block nido-[Si9]4– based on density functional calculations. All considered structures are semiconductors. The band structures exclusively show bands of low dispersion, as is typical for covalent polymers.

Structural characteristics of mixed nido‐[Si9−xGex]4− (x=1, 2) Zintl clusters in solution and within solvent crystals

“…Furthermore, computational investigations of dimeric, oligomeric, and polymeric modifications derived from [Si 9 ] 4À and [Ge 9 ] 4À clusters have revealed new semiconducting structures with interesting electronic properties. [39,40] Single-crystal structure determination of the mixed cluster Rb 4 [Si 7.8 Ge 1.2 ](NH 3 ) 5 shows that each Si atom position within the cluster has a different probability for being substituted by Ge, with some positions being favored over others (Figure 1). [33] To complete the study and establish a basis for further Si/Ge material discoveries, we investigated this system with quantum chemical methods to explain the site preferences and structural characteristics within the molecular cluster and the solvated cluster in the Rb/NH 3 crystal environment.…”

“…Especially the preference for Ge to substitute at certain cluster sites is a valuable contribution to this field of research, as the formation of clathrates [34] or nano‐ [35] and mesoporous [36–38] materials using [Ge 9 ] 4− clusters as precursors is known. Furthermore, computational investigations of dimeric, oligomeric, and polymeric modifications derived from [Si 9 ] 4− and [Ge 9 ] 4− clusters have revealed new semiconducting structures with interesting electronic properties [39,40] …”