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Polymers in Special Uses
Published in Manas Chanda, Plastics Technology Handbook, 2017
Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where there is registry between the overlayer and the substrate. The overlayer is called an epitaxial film or epitaxial layer. Epitaxial films may be grown from gaseous or liquid precursors (e.g., carbon oxides or hydrocarbons) by pyrolysis. Because the substrate acts as a seed crystal, the deposited film may lock into one or more crystallographic orientations with respect to the substrate crystal. In some cases, epitaxial graphene layers are coupled to surfaces weakly enough (by Van der Waals forces) to retain the 2D electronic band structure of isolated graphene, an example being epitaxial graphene grown on SiC. Epitaxial graphene on SiC has played a pivotal role in its development as an electronic material and it was the first to be proposed as a platform for graphene-based electronics. In a later development, the epitaxial growth of large-area, single-domain graphene on hexagonal boron nitride (h-BN) by plasma-assisted deposition has been reported [349], while h-BN has emerged as an excellent substrate for graphene nanodevices.
A new interpretation of the √7×√7 R19.1° structure for P adsorbed on a Ni(111) surface
Published in Science and Technology of Advanced Materials, 2019
Elizabeth Barrow, Grant S. Seuser, Hiroko Ariga-Miwa, Donna A. Chen, Jochen Lauterbach, Kiyotaka Asakura
In the literature, the is usually interpreted in terms of a hexagonal overlayer. In the LEED analysis of of P/Ni(111), 4 surface Ni atoms out of 7 were replaced with 4 P atoms to produce the mixed overlayer of Ni and P [26]. The overlayer for S adsorbed on Cu(111) [61,62], Rh(111) [63–65] and Pd(111) [66,67] had similar structures, maintaining the hexagonal unit cell. The rectangular lattice structure we propose is unique and explains the relationship between the structure and the two incommensurate structures of pattern B and pattern C. In the literature we found one other example of the formation of an overlayer with a different lattice system. This example was the pattern on Ni(111), where the surface overlayer had a square lattice [24]. S was adsorbed on the square-reconstructed Ni surface to increase the number of S-Ni bonds.
Understanding the fundamentals of TiO2 surfaces
Published in Surface Engineering, 2022
The depth explored during a photoemission experiment depends on the angle of electron emission, that is, the spectrum collected at 0° to the surface normal originates from atoms (or ions) in approximately the first 10 layers (≈ 20 Å) of the solid with a small contribution from the uppermost atomic layer, while the 70–85° spectrum has a relatively large contribution of the first 2–3 atomic layers (≈ 6 Å). As a consequence, Sham et al. [229] proposed to use angle-resolved XPS measurements in order to increase the surface sensitivity of the XPS signal. This method was then used to qualitatively characterize the surface chemistry of oxygen-depleted rutile single crystals without going into the quantitative details of surface structures. These authors have studied in-situ the chemistry of rutile (001) single crystal surfaces before and after exposure to water vapour. It is well known that the surface of titanium dioxide, which consists of unsaturated Ti4+ and O2− sites, reacts rapidly with moisture in ambient air to form a hydroxylated surface, following Pauling’s rule of electroneutrality [363]. It was demonstrated by Boehm [297] and also by other researchers using e.g. thermal desorption and infrared spectroscopy studies [18,22], that acidic OHbr groups, those bonded to two Ti4+ ions, and basic groups Ti-OH, those bonded to one Ti4+ acidic site, complete the preferred sixfold coordination of Ti, formed from H2O dissociative chemisorption. The reactivity of these groups has also been characterized [e.g. 5,8,33,57,104,120,156,297]. Adsorption of, for example, CO2 and O2 along with a physisorbed overlayer of H2O through hydrogen bonding occurs on top of hydroxyl groups under ambient conditions [e.g. 5,104,210,297].