Search

The effects of uniaxial tensile strain on the structural and electronic properties of positively charged oxygen vacancy defects in amorphous silica (a-SiO2) are systematically investigated using ab-initio calculation based on density functional theory. Four types of positively charged oxygen vacancy defects, namely the dimer, unpuckered, and puckered four-fold (4 ×), and puckered five-fold (5 ×) configurations have been investigated. It is shown by the calculations that applying uniaxial tensile strain can lead to irreversible transitions of defect structures, which can be identified from the fluctuations of the curves of relative total energy versus strain. Driven by strain, a positively charged dimer configuration may relax into a puckered 5× configuration, and an unpuckered configuration may relax into either a puckered 4× configuration or a forward-oriented configuration. Accordingly, the Fermi contacts of the defects remarkably increase and the defect levels shift under strain. The Fermi contacts of the puckered configurations also increase under strain to the values close to that of center in a-SiO2. In addition, it is shown by the calculations that the relaxation channels of the puckered configurations after electron recombination are sensitive to strain, that is, those configurations are more likely to relax into a two-fold coordinated Si structure or to hold a puckered structure under strain, both of which may raise up the thermodynamic charge-state transition levels of the defects into Si band gap. As strain induces more puckered configurations with the transition levels in Si band gap, it may facilitate directly the development of oxide charge accumulation and indirectly that of interface charge accumulation by promoting proton generation under ionization radiation. This work sheds a light on understanding the strain effect on ionization damage at an atomic scale. [ABSTRACT FROM AUTHOR]

The characteristics of organic acids in plant organs (roots, stems and leaves) , and root exudates of seedlings of Orychophragmus violaceus and Brassica napus were investigated under the stress of Zn deficiency and excessive bicarbonate by ion chromatography, the two plant species were hydroponically grown and cultured in four different treatments: + Zn0 (the treatment of adequate Zn and none HCO3-), + Zn10 (the treatment of adequate Zn and HCO3- addition), —Zn10 (the treatment of Zn deficiency and none HCO3-) , and —Zn10 (the treatment of Zn deficiency and HCO3- addition) , respectively. The results were as follows: (1) The total content of organic acids in plant organs and root exudations of the two plant species were significantly increased under excessive bicarbonate treatment, particularly under the dual treatment of Zn deficiency and excessive bicarbonate conditions (—Zn10 treatment) ,the organic acids in organs and root exudates of O. violaceus were more sensitive than that of B. napus , oxalic, citric and malic acids were the dominant organic acids in organs and root exudates of 0. violaceus , the content of these three organic acids took account for more than seventy five percent of the total content of organic acids in organs and root exudates of O. violaceus; (2) The leave was the main region of organic acids production of the two plant species, the content and the allocated proportion of organic acids decreased from aboveground parts (leaves and stems) to belowground parts; (3) The variation trends of organic acids contents of organs were identical with that of root exudates in O. violaceus and B. napus , and the source of organic acids in leaves of the two plant species was derived from the processes of dark respiration and photorespiration, while the source of organic acids in other organs such as stems and roots, as well as the source of organic acids in root exudates were come from the process of dark respiration; (4) The adaptability to the environment of low Zn and excessive HCO3- of O. violaceus was higher than that of B. napus ,which provided evidences for the adaptability to karst environment of 0. violaceus , and ecological restoration in environments with low Zn and excessive HCO3-, such as a karst area. [ABSTRACT FROM AUTHOR]