If the collagen triple helix is so built as to have one set of NH ⃛ O hydrogen bonds of the type N3H3(A) ⃛ O2(B), then it is possible to have a linkage between N1H1(B) and O1(A) through the intermediary of a water molecule with an oxygen O 1W leading to the formation of the hydrogen bonds N1(B) ⃛ O 1W and O 1W (A). In the same configuration, another water molecule with an oxygen O 1W can link two earbonyl oxygens of chains A and B forming the hydrogen bonds O 2W O1(A) and O 2W O0 (B). The two water oxygens also become receptors at the same time for CH ⃛ O hydrogen bonds. Thus, the neighboring chains in the triple helix are held together by secondary valence bond linkages occurring regularly sit intervals of about 3 Å along the length of the protofibril. The additional water molecules occur on the periphery of the proto‐fibril and will contribute their full share towards stabilizing the structure in the solid state. In solution, they will be disturbed by the medium unless they are protected by long side groups. It appears that this type of two‐bonded structure, in which one NH ⃛ O bond is to a water molecule, can explain several observations on the stability and hydrogen exchange properties of collagen itself and related synthetic polypeptides. The nature of the water bonds and their strength are found to be better in the one‐bonded structure proposed from Madras than in the one having the coordinates of Rich and Crick.

Interface structure and the chemical states of Pt film on polar-ZnO single crystalThis paper presents the combined use of mathematical modeling and Auger depth profiling to study and quantify the oxidation of Ta films over a wide range of temperatures. The thermal oxidation of tantalum films ͑ϳ700 nm͒ is studied using direct measurements of species concentration by means of Auger depth profiling. The oxidation temperature range of this study extends from 300 to 700°C and the oxidation period varies from 5 s to 12.5 h. The Auger depth profiles revealed that the metallic film oxidizes to first form low valence oxides of Ta that progressively convert to tantalum pentoxide with increasing temperature and time. A first-order reaction diffusion model is used to quantify the diffusion of oxygen through a film that is evolving in composition. The Auger depth profiling and reaction-diffusion model are used to estimate the actual diffusivity values for oxygen in the evolving Ta/Ta-oxide thin-film matrix, rather than more conventional techniques that estimate either the initial diffusion of oxygen through a semi-infinite metal or give a depth-and time-integrated value for the diffusivity. A comparison between the actual diffusivity values estimated in this work and the depth-and time-integrated version using the same model revealed that the integrated values are higher than the actual diffusion values by greater than 300% for the temperature range tested. Moreover, these depth-and time-integrated values for diffusivity values match over the applicable temperature ranges the diffusivity values given in the literature, which are essentially integrated average values for Ta/Ta oxide matrix. Furthermore, using the Auger depth profiles, the oxide growth rates are quantified as a function of temperature and compared with available literature. The growth rate of the oxide that is observed to be logarithmic at 300°C is seen to have a parabolic growth at 500°C and then a multistep growth behavior ͑a combination of parabolic and linear growth͒ at 700°C. These growth rates and the transition from one growth type to another strongly correlate to the change in surface and film morphology and also the transition from amorphous to crystalline Ta 2 O 5 .

No abstract

No abstract

Ultrananocrystalline diamond film deposition by direct-current plasma assisted chemical vapor deposition using hydrogen-rich precursor gas in the absence of the positive columnInteresting trends in direct current electrical conductivity of chemical vapor deposited diamond sheetsThe electrical conduction behavior of undoped ultrananocrystalline diamond ͑UNCD͒ and its dependence on deposition temperature and chemical structure are presented. UNCD films were grown using a microwave plasma-enhanced chemical vapor deposition technique at deposition temperatures of 400°C and 800°C. The chemical structure of the UNCD films is characterized with several tools including: Elastic recoil detection analysis, Fourier transform infrared spectroscopy, electron energy loss spectroscopy, Raman spectroscopy, and environmental scanning electron microscope. The results show a higher content of sp 2 -bonded carbon for the 800°C deposition samples ͑ϳ65%͒ in comparison with the 400°C samples ͑ϳ38%͒. In both kinds of films, the hydrocarbon bonds have the saturated sp 3 structures, while there is lower hydrogen content in the 800°C samples ͑ϳ8%͒ than in the 400°C samples ͑ϳ10%͒. For conduction properties, experiments are conducted using a probe station and conductive-atomic force microscopy. Experimental data show that the samples deposited at 800°C are several orders of magnitude more conductive than the 400°C samples. The conduction occurs primarily along the grain boundary for both types of samples. The conductivity of both types of films also shows field dependent nonlinear behavior. Both the Poole-Frenkel models and single and overlapping Coulombic potential models show that the conduction is directly correlated with the sp 2 bond carbon density, and the role of the hydrocarbon bonds in the conduction path is formed by the network of the sp 2 bonded carbon.