Controlling the surface silanol density in capillary columns and planar silicon via the self-limiting, gas-phase deposition of tris(dimethylamino)methylsilane, and quantification of surface silanols after silanization by low energy ion scattering.

Surface silanols (Si-OH) play a vital role on fused silica surfaces in chromatography. Here, we used an atmospheric-pressure, gas-phase reactor to modify the inner surface of a gas chromatography, fused silica capillary column (0.53 mm ID) with a small, reactive silane (tris(dimethylamino)methylsilane, TDMAMS). The deposition of TDMAMS on planar witness samples around the capillary was confirmed with X-ray photoelectron spectroscopy (XPS), ex situ spectroscopic ellipsometry (SE), and wetting. The number of surface silanols on unmodified and TDMAMS-modified native oxide-terminated silicon were quantified by tagging with dimethylzinc (DMZ) via atomic layer deposition (ALD) and counting the resulting zinc atoms with high sensitivity-low energy ion scattering (HS-LEIS). A bare, clean native oxide - terminated silicon wafer has 3.66 OH/nm2, which agrees with density functional theory (DFT) calculations from the literature. After TDMAMS modification of native oxide-terminated silicon, the number of surface silanols decreases by a factor of ca. 10 (to 0.31 OH/nm2). Intermediate surface testing (IST) was used to characterize the surface activities of functionalized capillaries. It suggested a significant deactivation/passivation of the capillary with some surface silanols remaining; the modified capillary shows significant deactivation compared to the native/unmodified fused silica tubing. We believe that this methodology for determining the number of residual silanols on silanized fused silica will be enabling for chromatography.

Area-Selective Atomic Layer Deposition of ZnO on Si\SiO2 Modified with Tris(dimethylamino)methylsilane.

Delayed atomic layer deposition (ALD) of ZnO, i.e., area selective (AS)-ALD, was successfully achieved on silicon wafers (Si\SiO2) terminated with tris(dimethylamino)methylsilane (TDMAMS). This resist molecule was deposited in a home-built, near-atmospheric pressure, flow-through, gas-phase reactor. TDMAMS had previously been shown to react with Si\SiO2 in a single cycle/reaction and to drastically reduce the number of silanols that remain at the surface. ZnO was deposited in a commercial ALD system using dimethylzinc (DMZ) as the zinc precursor and H2O as the coreactant. Deposition of TDMAMS was confirmed by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and wetting. ALD of ZnO, including its selectivity on TDMAMS-terminated Si\SiO2 (Si\SiO2\TDMAMS), was confirmed by in situ multi-wavelength ellipsometry, ex situ SE, XPS, and/or high-sensitivity/low-energy ion scattering (HS-LEIS). The thermal stability of the TDMAMS resist layer, which is an important parameter for AS-ALD, was investigated by heating Si\SiO2\TDMAMS in air and nitrogen at 330 °C. ALD of ZnO takes place more readily on Si\SiO2\TDMAMS heated in the air than in N2, suggesting greater damage to the surface heated in the air. To better understand the in situ ALD of ZnO on Si\SiO2\TDMAMS and modified (thermally stressed) forms of it, the ellipsometry results were plotted as the normalized growth per cycle. Even one short pulse of TDMAMS effectively passivates Si\SiO2. TDMAMS can be a useful, small-molecule inhibitor of ALD of ZnO on Si\SiO2 surfaces.

Highly Sensitive Detection of Surface and Intercalated Impurities in Graphene by LEIS.

Low-energy ion scattering (LEIS) is known for its extreme surface sensitivity, as it yields a quantitative analysis of the outermost surface as well as highly resolved in-depth information for ultrathin surface layers. Hence, it could have been generally considered to be a suitable technique for the analysis of graphene samples. However, due to the low scattering cross section for light elements such as carbon, LEIS has not become a common technique for the characterization of graphene. In the present study we use a high-sensitivity LEIS instrument with parallel energy analysis for the characterization of CVD graphene transferred to thermal silica/silicon substrates. Thanks to its high sensitivity and the exceptional depth resolution typical of LEIS, the graphene layer closure was verified, and different kinds of contaminants were detected, quantified, and localized within the graphene structure. Utilizing the extraordinarily strong neutralization of helium by carbon atoms in graphene, LEIS experiments performed at several primary ion energies permit us to distinguish carbon in graphene from that in nongraphitic forms (e.g., the remains of a resist). Furthermore, metal impurities such as Fe, Sn, and Na located at the graphene-silica interface (intercalated) are detected, and the coverages of Fe and Sn are determined. Hence, high-resolution LEIS is capable of both checking the purity of graphene surfaces and detecting impurities incorporated into graphene layers or their interfaces. Thus, it is a suitable method for monitoring the quality of the whole fabrication process of graphene, including its transfer on various substrates.

Surface composition/organization of ionic liquids with Au nanoparticles revealed by high-sensitivity low-energy ion scattering.

High-sensitivity low-energy ion scattering (HS-LEIS) analysis was used to elucidate the outermost layer of both functionalized and non-functionalized imidazolium ionic liquids (ILs). The IL outermost layer is composed of all atoms of both cations and anions. The HS-LEIS analyses also allow for quantitative measurement of the thickness of IL overlayers on Au nanoparticles prepared by sputter deposition, which was shown to be a monolayer of ions, as predicted by density functional theory calculations.

Probe molecule chemisorption-low energy ion scattering study of surface active sites present in the orthorhombic Mo-V-(Te-Nb)-O catalysts for propane (amm)oxidation.

Methanol and allyl alcohol chemisorption and surface reaction in combination with low energy ion scattering (LEIS) were employed to determine the outermost surface compositions and chemical nature of active surface sites present on the orthorhombic (M1) Mo-V-O and Mo-V-Te-Nb-O phases. These orthorhombic phases exhibited vastly different behavior in propane (amm)oxidation reactions and, therefore, represented highly promising model systems for the study of the surface active sites. The LEIS data for the Mo-V-Te-Nb-O catalyst indicated surface depletion for V (-23%) and Mo (-27%), and enrichments for Nb (+55%) and Te (+165%) with respect to its bulk composition. Only minor changes in the topmost surface composition were observed for this catalyst under the conditions of the LEIS experiments at 400 degrees C, which is a typical temperature employed in these propane transformation reactions. These findings strongly suggested that the bulk orthorhombic Mo-V-Te-Nb-O structure may function as a support for the unique active and selective surface monolayer in propane (amm)oxidation. Moreover, direct evidence was obtained that the topmost surface VO(x) sites in the orthorhombic Mo-V-Te-Nb-O catalyst were preferentially covered by chemisorbed allyloxy species, whereas methanol was a significantly less discriminating probe molecule. The surface TeO(x) and NbO(x) sites on the Mo-V-Te-Nb-O catalyst were unable to chemisorb these probe molecules to the same extent as the VO(x) and MoO(x) sites. Our findings suggested that different surface locations for V(5+) ions in the orthorhombic Mo-V-O and Mo-V-Te-Nb-O catalysts may be primarily responsible for vastly different catalytic behavior exhibited by the Mo-V-O and Mo-V-Te-Nb-O phases. Although the proposed isolated V(5+) pentagonal bipyramidal sites in the orthorhombic Mo-V-O phase may be capable of converting propane to propylene with modest selectivity, the selective 8-electron transformation of propane to acrylic acid and acrylonitrile may require the presence of several surface VO(x) redox sites lining the entrances to the hexagonal and heptagonal channels of the orthorhombic Mo-V-Te-Nb-O phase. The study of allyl alcohol oxidation over the Mo-V-O and Mo-V-Te-Nb-O catalysts further suggested that water plays a critical role during the oxidation of acrolein intermediate to acrylic acid over the orthorhombic (M1 phase) Mo-V-Te-Nb-O catalysts. Finally, the present study strongly indicated that chemical probe chemisorption combined with low energy ion scattering (LEIS) is a novel and highly promising surface characterization technique for the investigation of the active surface sites present in the bulk mixed metal oxides.

Roles of surface Te, Nb, and Sb oxides in propane oxidation to acrylic acid over bulk orthorhombic Mo-V-O phase.

The outermost surfaces and subsurface layers of the orthorhombic (M1) Mo-V-O catalysts promoted with Te, Nb, and Sb oxide species at submonolayer surface coverage were examined by low-energy ion scattering (LEIS). This study indicated that the Nb oxide species was preferentially located at the topmost surface, while the subsurface Te and Sb concentrations declined gradually into the bulk. Although the original Mo-V-O catalyst was essentially unselective in propane oxidation to acrylic acid, significant improvement in the selectivity to acrylic acid was observed when Te, Nb, and Sb oxides were present as the surface species at submonolayer coverage. These findings further suggested that the formation of the surface V-O-M bonds (M = Nb, Te, or Sb) was highly beneficial for both the activity and selectivity of the orthorhombic Mo-V-O catalysts in propane oxidation to acrylic acid. The highest selectivity was observed when both Nb and Te (or Sb) oxide species were present at the surface. The selectivity trends established for the surface-promoted Mo-V-O catalyst parallel those found previously for the corresponding bulk Mo-V-M-O catalysts. These results further indicated that the introduction of surface metal oxide species is a highly promising method to prepare well-defined model catalysts for studies of the structure-activity/selectivity relationships as well as optimize the catalytic performance of the bulk mixed Mo-V-M-O catalysts for selective (amm)oxidation of propane.

A study of the surface region of the Mo-V-Te-O catalysts for propane oxidation to acrylic acid.

The bulk mixed Mo-V-Te oxides possess high activity and selectivity in propane oxidation to acrylic acid and represent well-defined model catalysts for studies of the surface molecular structure-activity/selectivity relationships in this selective oxidation reaction. The elemental compositions, metal oxidation states, and catalytic functions of V, Mo, and Te in the surface region of the model Mo-V-Te-O system were examined employing low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). This study indicated that the surfaces of these catalysts are terminated with a monolayer, which possesses a different elemental composition from that of the bulk. The rates of propane consumption and formation of propylene and acrylic acid depended on the topmost surface V concentration, whereas no dependence of these reaction rates on either the surface Mo or Te concentrations was observed. These findings suggested that the bulk Mo-V-Te-O structure may function as a support for the unique active and selective surface monolayer in propane oxidation to acrylic acid. The results of this study have important practical consequences for the development of improved selective oxidation catalysts by introducing surface metal oxide components to form new surface active V-O-M sites for propane oxidation to acrylic acid.