Electronic structure and vibrational spectra of ${\mathrm{C}}_2{\mathrm{B}}_{10}$-based clusters and films

The electronic structure, total energy, and vibrational properties of ${\mathrm{C}}_2{\mathrm{B}}_{10}{\mathrm{H}}_{12}$ (carborane) molecules and ${\mathrm{C}}_2{\mathrm{B}}_{10}$ clusters formed when the hydrogen atoms are removed from carborane molecules are studied using density functional methods and a semiempirical model. Computed vibrational spectra for carborane molecules are shown to be in close agreement with previously published measured spectra taken on carborane solids. Semiconducting boron carbide films are prepared by removing hydrogen from the three polytypes of ${\mathrm{C}}_2{\mathrm{B}}_{10}{\mathrm{H}}_{12}$ deposited on various surfaces. Results from x-ray and Raman scattering measurements on these films are reported. Eleven vibrationally stable structures for ${\mathrm{C}}_2{\mathrm{B}}_{10}$ clusters are described and their energies and highest occupied and lowest unoccupied molecular orbital gaps tabulated. Calculated Raman and infrared spectra are reported for the six lowest-energy clusters. Good agreement with the experimental Raman spectra is achieved from theoretical spectra computed using a Boltzmann distribution of the six lowest-energy free clusters. The agreement is further improved if the computed frequencies are scaled by a factor of 0.94, a descrepancy which could easily arise from comparing results of two different systems: zero-temperature free clusters and room-temperature films. Calculated energies for removal of hydrogen pairs from carborane molecules are reported.

Figure 1

Comparison of DFT-calculated Raman spectra for the meta-carborane molecule with corresponding experimental results for the room-temperature solid (Ref. 7).

Figure 2

Comparison of DFT-calculated Raman spectra for the meta-carborane molecule with corresponding experimental results for the room-temperature solid (Ref. 7).

Figure 3

Comparison of DFT-calculated infrared spectra for the meta-carborane molecule with corresponding experimental results for the room-temperature solid (Ref. 7).

Figure 4

Comparison of DFT-calculated infrared spectra for the meta-carborane molecule with corresponding experimental results for the room-temperature solid (Ref. 7).

Figure 5

X-ray diffraction data for a sample on a ${\mathrm{Al}}_2{\mathrm{O}}_3$ (sapphire) substrate.

Figure 6

X-ray diffraction data for a sample on a silicon substrate.

Figure 7

Raman spectra from three randomly selected positions in the silicon substrate sample.

Figure 8

Raman spectra obtained using three different laser frequencies for the sample with a sapphire substrate.

Figure 9

(Color online) GS structure.

Figure 10

Vibrational spectrum for the GS structure.

Figure 11

(Color online) S1 structure.

Figure 12

Vibrational spectrum for the S1 structure.

Figure 13

(Color online) SG1 structure.

Figure 14

Vibrational spectrum for the SG1 structure.

Figure 15

(Color online) S2 structure

Figure 16

Vibrational spectrum for the S2 structure.

Figure 17

(Color online) SG2 structure.

Figure 18

Vibrational spectrum for the SG2 structure.

Figure 19

(Color online) SE1 structure.

Figure 20

Vibrational spectrum for the SE1 structure.

Figure 21

(Color online) Structure obtained by relaxing from the meta-carborane positions using the empirical model (left) and after further relaxation using DFT and NRLMOL (SE3, right).

Figure 22

(a) Computed Raman spectrum for a Boltzmann distribution of the six lowest-energy ${\mathrm{C}}_2{\mathrm{B}}_{10}$ clusters (solid line), those including only the planar structures (dashed line), and the six lowest-energy clusters with frequency scaled by 0.94 (heavy solid line) and (b) experimental data for a ${\mathrm{C}}_2{\mathrm{B}}_{10}{\mathrm{H}}_{ϵ}$ film (thin solid line), fitted Gaussian background (dashed line), and data with background removed (thick solid line).

Figure 23

Raman spectra for the GS structure with parallel and perpendicular scattering angles.

Figure 24

(Color online) Removal energies for two hydrogens from meta-carborane computed using the PM3-NDO semiempirical model. Values marked $\times$ for (4,10), (1,2), and (1,4) were computed using DFT and NRLMOL.