Quantum Mechanics Calculations of the Thermodynamically Controlled Coverage and Structure of Alkyl Monolayers on Si(111) Surfaces

The heat of formation, ΔE, for silicon (111) surfaces terminated with increasing densities of the alkyl groups CH_3- (methyl), C_2H_5- (ethyl), (CH_3)_2CH- (isopropyl), (CH_3)_3C- (tert-butyl), CH_3(CH_2)_5- (hexyl), CH_3(CH_2)_7- (octyl), and C_6H_5- (phenyl) was calculated using quantum mechanics (QM) methods, with unalkylated sites being H-terminated. The free energy, ΔG, for the formation of both Si−C and Si−H bonds from Si−Cl model componds was also calculated using QM, with four separate Si−H formation mechanisms proposed, to give overall ΔG_S values for the formation of alkylated Si(111) surfaces through a two step chlorination/alkylation method. The data are in good agreement with measurements of the packing densities for alkylated surfaces formed through this technique, for Si−H free energies of formation, ΔG_H, corresponding to a reaction mechanism including the elimination of two H atoms and the formation of a C C double bond in either unreacted alkyl Grignard groups or tetrahydrofuran solvent

Covalent Attachment of Acetylene and Methylacetylene Functionality to Si(111) Surfaces: Scaffolds for Organic Surface Functionalization while Retaining Si−C Passivation of Si(111) Surface Sites

Si(111) surfaces have been functionalized with Si−C⋮C−R species, where R = H or –CH_3, using a two-step reaction sequence involving chlorination of H−Si(111) followed by treatment with Na−C⋮C−H or CH_3−C⋮C−Na reagents. The resulting surfaces showed no detectable oxidation as evidenced by X-ray photoelectron spectroscopic (XPS) data in the Si 2p region, electrochemical measurements of Si−H oxidation, or infrared spectroscopy. The Si−C⋮C−R-terminated surfaces exhibited a characteristic C⋮C stretch in the infrared at 2179 cm^(-1), which was strongly polarized perpendicular to the Si(111) surface plane. XPS measurements in the C 1s region showed a low binding energy peak indicative of Si−C bonding, with a coverage that was, within experimental error, identical to that of the CH_3-terminated Si(111) surface, which has been shown to fully terminate the Si atop sites on an unreconstructed Si(111) surface. The Si−C⋮C−H-terminated surfaces were further functionalized by exposure to n-C_4H_9Li followed by exposure to para Br−C_6H_5−CF_3, allowing for introduction of para –C_6H_5CF_3 groups while maintaining the desirable chemical and electrical properties that accompany complete Si−C termination of the atop sites on the Si(111) surface

Fabrication of Free-Standing Nanoscale Alumina Membranes with Controllable Pore Aspect Ratios

Porous alumina films with controllable pore sizes and having submicrometer film thicknesses were fabricated by the anodization of Al overlayers. The Al was deposited by sputtering onto either glass or onto silicon that had been coated with a layer of silicon nitride. Alumina membranes having thicknesses between 300 and 1000 nm were prepared analogously using a lithographic process to produce free-standing porous alumina films that were peripherally supported on a 500-μm-thick silicon substrate

Chemical and Electrical Passivation of Silicon (111) Surfaces through Functionalization with Sterically Hindered Alkyl Groups

Crystalline Si(111) surfaces have been alkylated in a two-step chlorination/alkylation process using sterically bulky alkyl groups such as (CH_3)_2CH− (iso-propyl), (CH_3)_3C− (tert-butyl), and C_6H_5− (phenyl) moieties. X-ray photoelectron spectroscopic (XPS) data in the C 1s region of such surfaces exhibited a low energy emission at 283.9 binding eV, consistent with carbon bonded to Si. The C 1s XPS data indicated that the alkyls were present at lower coverages than methyl groups on CH_3-terminated Si(111) surfaces. Despite the lower alkyl group coverage, no Cl was detected after alkylation. Functionalization with the bulky alkyl groups effectively inhibited the oxidation of Si(111) surfaces in air and produced low (<100 cm s^(-1)) surface recombination velocities. Transmission infrared spectroscopy indicated that the surfaces were partially H-terminated after the functionalization reaction. Application of a reducing potential, −2.5 V vs Ag^+/Ag, to Cl-terminated Si(111) electrodes in tetrahydrofuran resulted in the complete elimination of Cl, as measured by XPS. The data are consistent with a mechanism in which the reaction of alkyl Grignard reagents with the Cl-terminated Si(111) surfaces involves electron transfer from the Grignard reagent to the Si, loss of chloride to solution, and subsequent reaction between the resultant silicon radical and alkyl radical to form a silicon−carbon bond. Sites sterically hindered by neighboring alkyl groups abstract a H atom to produce Si−H bonds on the surface

Thermal Conductivity of Mesoporous Titania Films Made From Nanocrystalline Building Blocks and Sol-Gel Reagents

This paper reports the cross-plane thermal conductivity of amorphous and crystalline mesoporous titania thin films synthesized by evaporation-induced self-assembly. Both sol-gel and nanocrystal-based mesoporous films were investigated, with average porosities of 30% and 35%, respectively. The pore diameter ranged from 7 to 30 nm and film thickness from 60 to 370nm while the average wall thickness varied from 3 to 50 nm. The crystalline domain sizes in sol-gel films varied from 12 to 13 nm while the nanocrystal-based films consisted of monodisperse nanocrystals 9 nm in diameter. The cross-plane thermal conductivity was measured at room temperature using the 3w method. The average thermal conductivity of the amorphous sol-gel mesoporous titania films was 0.37 ± 0.05 W/m.K. It did not show strong dependence on pore diameter, wall thickness, and film thickness for sol-gel amorphous mesoporous titania thinfilms. This result can be attributed to the fact that heat is carried, in the amorphous matrix, by localized non-propagating vibrational modes. The thermal conductivity of crystalline sol-gelmesoporous titania thin films was significantly larger at 1.06 ± 0.04 W/m.K and depended on the organic template used to make the films. The thermal conductivity of nanocrystal-based thin films was 0.48 ± 0.05 W/m.K and significantly lower than that of the crystalline sol-gel mesoporous thin films. This was due to the fact that the nanocrystals were not as well interconnected as the crystalline domains in the crystalline sol-gel films. These results suggest that both connectivity and size of the nanocrys-tals or the crystalline domains can provide control over thermal conductivity in addition to porosity

Thermal Conductivity of Mesoporous Titania Films Made From Nanocrystalline Building Blocks and Sol-Gel Reagents

This paper reports the cross-plane thermal conductivity of amorphous and crystalline mesoporous titania thin films synthesized by evaporation-induced self-assembly. Both sol-gel and nanocrystal-based mesoporous films were investigated, with average porosities of 30% and 35%, respectively. The pore diameter ranged from 7 to 30 nm and film thickness from 60 to 370nm while the average wall thickness varied from 3 to 50 nm. The crystalline domain sizes in sol-gel films varied from 12 to 13 nm while the nanocrystal-based films consisted of monodisperse nanocrystals 9 nm in diameter. The cross-plane thermal conductivity was measured at room temperature using the 3w method. The average thermal conductivity of the amorphous sol-gel mesoporous titania films was 0.37 ± 0.05 W/m.K. It did not show strong dependence on pore diameter, wall thickness, and film thickness for sol-gel amorphous mesoporous titania thinfilms. This result can be attributed to the fact that heat is carried, in the amorphous matrix, by localized non-propagating vibrational modes. The thermal conductivity of crystalline sol-gelmesoporous titania thin films was significantly larger at 1.06 ± 0.04 W/m.K and depended on the organic template used to make the films. The thermal conductivity of nanocrystal-based thin films was 0.48 ± 0.05 W/m.K and significantly lower than that of the crystalline sol-gel mesoporous thin films. This was due to the fact that the nanocrystals were not as well interconnected as the crystalline domains in the crystalline sol-gel films. These results suggest that both connectivity and size of the nanocrys-tals or the crystalline domains can provide control over thermal conductivity in addition to porosity