Theoretical Study of Structural and Electronic Properties of H-Silsesquioxanes

Abstract

The results of first principles calculations on H-silsesquioxanes (i.e., (HSiO 3/2 ) n with n ) 4, 6, 8, 10, 12, 14, and 16) are reported here. Double numeric basis sets and local and nonlocal density approximations to density functional theory are employed for calculations. It is shown that use of the nonlocal density approximation is required for the reliable prediction of the most stable isomer for silsesquioxanes. Furthermore, a progression of the preferred building unit with the increase in size of the T cage is revealed. The smaller T cages prefer four-and five-member rings while the larger cages are found to prefer four-and six-member rings. Analysis of the energy of the hydrolysis reaction, binding energy, and fragmentation paths finds the relative stability of the silsesquioxane cages containing four-, five-, and six-member rings in agreement with experimental observations. For the (HSiO 3/2 ) 16 cage, the calculated results predict the stability of the D 2d -6 4 5 0 4 6 configuration over the D 4d -6 0 5 8 4 2 configuration in contradiction to suggestions based on 29 Si NMR measurements. We find a consistent picture for the highest occupied molecular orbitals (HOMOs) of all silsesquioxanes considered showing them to be composed of (lone-pair) oxygen p-type atomic orbitals. On the other hand, the lowest unoccupied molecular orbitals (LUMOs) show size dependence in their composition which appears to cause the presence of a state in the HOMO-LUMO gap for higher silsesquioxane cages. Density of states plots and analysis of molecular orbitals reveal this state to be due to the terminal hydrogens bonded to silicon atoms.

Calculations of Linear and Nonlinear Optical Properties of H−Silsesquioxanes

“…The energies of these peaks can therefore correspond to the calculated HOMO-LUMO gap energies of the T n cages. 8 Overall, the calculated largest susceptibility elements show a decreasing tread from T 4 to T 16 at below the frequency of 3.0 eV/h and have a very small dispersion with almost constant value at below 3 eV/h for each case.…”

“…The largest susceptibility element is the xyz (-2ω;ω,ω) of -0.85 pm/V for T 4 with T d symmetry and the second one is the yzy (-2ω;ω,ω) of 0.19 pm/V for T 8 with C 2V symmetry. Moreover, we can see from Figure 4 that the first peaks for T 8 and T 16 are localized about 4 eV.…”

“…Although, the calculations predict the largest value of 〈n〉 for T 4 and T 6 , these silsesquioxanes are not yet prepared and are found to be relatively unstable in our previous theoretical work. 8 Table 1 also lists the average molecular polarizabilities 〈R〉 of the isolated T n cages showing its variation with the cage size.…”

“…[1][2][3][4] The structure of this polymer is based on the siloxane-containing cages that are formed from the building block unit of a trifunctional monomer (HSiO 3/2 ), designated as T. The cross-linking of various siloxane-containing cages leads to a wide range in the distribution of molecular masses up to 200 kDa for the polymer. Furthermore, the T 8 (i.e., H 8 Si 8 O 12 ) cage has shown to be a suitable starting molecule for the synthesis of higher substituted octanuclear silsesquioxanes. 5 There has been considerable interest in the fundamental understanding of the structure-property relationship of H-silsesquioxanes due to either their importance in achieving the atomic scale control of the Si/SiO 2 interface 6 or their role in manufacturing of integrated circuits as interlayer dielectric.…”

“…5 There has been considerable interest in the fundamental understanding of the structure-property relationship of H-silsesquioxanes due to either their importance in achieving the atomic scale control of the Si/SiO 2 interface 6 or their role in manufacturing of integrated circuits as interlayer dielectric. 7 Recently, we have performed a detailed theoretical study 8 on H-silsesquioxanes (i.e., (HSiO 3/2 ) n with n ) 4, 6, 8, 10, 12, 14 and 16) where cages of various sizes and conformations were investigated in the framework of density functional theory. One of the revealing feature of this study was the prediction of the presence of a gap state for the large T cages suggesting that larger size cages in H-silsesquioxanes could affect the dielectric properties of such materials.…”