Vacancy and interstitial defects in hafnia

We have performed plane wave density functional theory calculations of atomic and molecular interstitial defects and oxygen vacancies in monoclinic hafnia (HfO2). The atomic structures of singly and doubly positively charged oxygen vacancies, and singly and doubly negatively charged interstitial oxygen atoms and molecules are investigated. We also consider hafnium vacancies, substitutional zirconium, and an oxygen vacancy paired with substitutional zirconium in hafnia. Our results predict that atomic oxygen incorporation is energetically favored over molecular incorporation, and that charged defect species are more stable than neutral species when electrons are available from the hafnia conduction band. The calculated positions of defect levels with respect to the bottom of the silicon conduction band demonstrate that interstitial oxygen atoms and molecules and positively charged oxygen vacancies can trap electrons from silicon.Peer reviewe

Structure and electrical levels of point defects in monoclinic zirconia

We performed plane wave density functional theory (DFT) calculations of formation energies, relaxed structures, and electrical levels of oxygen vacancies and interstitial oxygen atoms in monoclinic zirconia. The atomic structures of positively and negatively charged vacancies and interstitial oxygen atoms are also investigated. The ionization energies and electron affinities of interstitial oxygen atoms and oxygen vacancies in different charge states are calculated with respect to the bottom of the zirconia conduction band. Using the experimental band offset values at the interface of ZrO2 films grown on silicon, we have found the positions of defect levels with respect to the bottom of silicon conduction band. The results demonstrate that interstitial oxygen atoms and positively charged oxygen vacancies can trap electrons from the bottom of the zirconia conduction band and from silicon. Neutral oxygen vacancy serves as a shallow hole trap for electrons injected from the silicon valence band. The calculations predict negative U for the O− center and stability of V+ centers with respect to disproportionation into V2+ and V0 in monoclinic zirconia.Peer reviewe

Can H-2 inside C-60 communicate with the outside world?

The quenching rate constants of singlet oxygen by C-60, H-2@C-60, D-2@C-60, H-2, and D-2 in solution were measured. The presence of a hydrogen (H-2@C-60) or deuterium (D-2@C-60) molecule inside the fullerene did not produce any observable effect based on triplet lifetime or EPR measurements. However, a remarkable effect was found for the O-1(2) quenching by C-60, H-2@C-60, D-2@C-60, H-2, and D-2. Singlet oxygen was generated by photosensitization or by thermal decomposition of naphthalene endoperoxide derivatives. Comparison of the rate constants for quenching Of O-1(2) by H-2@C-60 and D-2@C-60 demonstrates a significant vibrational interaction between oxygen and H-2 inside the fullerene. The quenching rate constant for H-2 is 1 order of magnitude higher than that of D2, in agreement with the results observed for the quenching Of O-1(2) with H-2@C-60 or D-2@C-60