First-principles investigation of positively charged and neutral oxygen vacancies in amorphous silica.

The structural parameters, electron localization functions, electron paramagnetic resonance (EPR) parameters, formation energies, and thermodynamic transition levels of various oxygen vacancy defects in amorphous silica are comprehensively and integrally investigated by using density functional theory. The trends of changes in the oxygen vacancy defect structure and electron localization induced by the increase in distance between defective silicon atoms are clearly identified. It is shown that the dimer configuration may be the potential structure of the Eδ' center. For the back-projected unpuckered configuration and the puckered configuration, whose EPR parameters are more consistent with the experimental values of the Eγ' center, the unpaired electron localized on the sp3 hybridized silicon atom is a common feature. Due to the three-coordinated oxygen atom in the forward-oriented configuration, the EPR parameters are closest to those of the Eα' center. Transformations of oxygen vacancy defects under different charge states are studied by sequentially adding and removing electrons. The thermodynamic transition level analysis reveals that the dimer and forward configurations may behave as deep traps for electron accumulation. The back-projected puckered fourfold-coordinated and fivefold-coordinated configurations are comparatively stable and may be able to function as shallow traps for electron transport. The neutral double unpuckered, neutral back-projected puckered fourfold-coordinated, and neutral back-projected unpuckered configurations are more likely to lose electrons during hole trapping. As the bias voltage is repeatedly changed, the defect density of the puckered configuration may reduce, while that of the dimer and unpuckered configuration may take an opposite trend.

Theoretical insights into volatile iodine adsorption onto COF-DL229.

COF-DL229 is one of the promising sorbents for the capture of volatile radioiodine due to its large adsorption capacity. However, the interaction mechanism between them remains unclear. In the present work, the adsorption of volatile iodine onto COF-DL229 was systematically investigated using periodic density functional theory and crystal orbital Hamilton population calculations. The "soft" characters of COF-DL229 have been theoretically demonstrated. Furthermore, the adsorption energies are extremely large (-8.38 to -9.26 eV), which mainly originate from the framework deformation energies, accounting for 90% at least. The I2 interacts with the skeleton mainly through the N atoms of the imine linkers or the C atoms of the phenyl rings. And, the I-N bond is the strongest bond among all the potential secondary bonds formed between the skeleton and I2. The electrons could be transferred from the skeletons to the iodine atoms and from the near iodine atom to the far one. It is also found that the energy gap becomes narrow after iodine adsorption and the skeletons mainly interact with the bonding orbital σp of I2. The present work could provide reasonable theoretical explanations to the corresponding experimental investigations and contribute to the design and screening of better sorbents for the capture of volatile radioiodine.

Dynamics Studies of Nitrogen Interstitial in GaN from Ab Initio Calculations.

Understanding the properties of defects is crucial to design higher performance semiconductor materials because they influence the electronic and optical properties significantly. Using ab initio calculations, the dynamics properties of nitrogen interstitial in GaN material, including the configuration, migration, and interaction with vacancy were systematically investigated in the present work. By introducing different sites of foreign nitrogen atom, the most stable configuration of nitrogen interstitial was calculated to show a threefold symmetry in each layer and different charge states were characterized, respectively. In the researches of migration, two migration paths, in-plane and out-of-plane, were considered. With regards to the in-plane migration, an intermediated rotation process was observed first time. Due to this rotation behavior, two different barriers were demonstrated to reveal that the migration is an anisotropic behavior. Additionally, charged nitrogen Frenkel pair was found to be a relatively stable defect complex and its well separation distance was about 3.9 Å. Part of our results are in good agreement with the experimental results, and our work provides underlying insights of the identification and dynamics of nitrogen interstitial in GaN material. This study of defects in GaN material is useful to establish a more complete theory and improve the performance of GaN-based devices.