Hiroaki Onuma

Large-scale electronic structure calculation on blue phosphor BaMgAl10O17:Eu2+ using tight-binding quantum chemistry method implemented for rare-earth elements

In this study, we carried out large-scale electronic structure calculations on blue phosphor BaMgAl10O17:Eu2+ (BAM) using an self-consistent charge (SCC) tight-binding quantum chemistry method with an improved convergence for the 4 f orbitals of rare-earth elements. Calculation results obtained by the present method are in good agreement with first-principles and experimental results. We first compared the thermodynamic stability and the electronic structures for three different Eu sites. A first-principles calculation showed that the Beevers–Ross site was the most stable site for the Eu atom. Large-scale electronic structure calculations by the improved SCC tight-binding quantum chemistry method suggested that the electronic structures of Eu 5d orbitals are dependent on the shape of Eu 5d orbitals and the positions of oxygen atoms around the Eu atom. We also investigated the effects of an oxygen vacancy (VO) on the luminescence properties of BAM. We found that energy levels of molecular orbitals (MOs) with main contributions from Eu 5d orbitals were shifted lower by the VO near the Eu atom, and thus our results suggested that the formation of the VO near the Eu atom leads to the red shift of the luminescence color.

Host emission from BaMgAl10O17 and SrMgAl10O17 phosphor: Effects of temperature and defect level

— Understanding the mechanism of blue-light emission in Eu-doped BAM phosphor as well as its sensitive degradation is required because this is a very important material in fluorescent lamps and plasma-display panels. In this study, both theoretical and experimental investigations on the host emissions in BaMgAl10O17 and SrMgAl10O17 were performed. Host emissions from BaMgAl10O17 and SrMgAl10O17 by photoluminescence and thermoluminescence spectra were observed. Photoluminescence spectra suggested that the host emission from SrMgAl10O17 was easily quenched by thermal vibrations. The thermoluminescence spectra showed the existence of shallow and deep defect levels in BaMgAl10O17 and SrMgAl10O17 phosphors. It was shown that SrMgAl10O17 and its conduction plane could undergo degradation during irradiation of vacuum-ultra-violet (VUV) lights based on the calculated energy of formation of an oxygen vacancy. Moreover, the structural defects, such as oxygen vacancies, would cause localizing levels in the upper level in the valence band and in theconduction band. The results suggest the contribution of the host emission to the energy transfer to the Eu atoms would not be significant and the oxygen vacancies would act as the traps for excited carriers.

Quantum Chemistry Study of Surface Structure Effects on Secondary Electron Emission in MgO Protecting Layers for Plasma Displays

MgO protecting layers, which have high ion induced secondary electron emission coefficient (γ), are required in order to decrease the firing voltage of plasma displays. Theoretical estimation of ideal γ value is needed for a design of better protecting layers. In this study, we report our developed γ calculation method based on a tight-binding quantum calculation and application to an estimation of γ values of MgO protecting layers. From our calculation results, it was revealed that electron trap sites arising from surface roughness would work as an effective emission sites and increase γ value. Especially for Xe+ species as induced ion, the γ value changed drastically by the presence of the trapped electron. It is also suggested that a presence of chemisorbed water on the MgO surface decreases the γ values because of the contribution of the electrons at the low energy levels originated from surface OH groups.

Electronic structure and electrical conductivity of MgO protecting layer in plasma-display panels: A tight-binding quantum chemical study

— The tight-binding quantum chemical molecular dynamics code, Colors, has been successfully applied to the electronic-structure calculations of the MgO-protecting-layer model in plasma-display panels (PDPs). The code succeeded in reproducing the band-gap energy of the MgO crystal structure. The energy gap between the bottom of the conduction band (CB) and the top of valence band (VB) was 7.45 eV, which is in quantitative agreement with the experimental and previous theoretical results. The electronic structure of the undoped MgO model and Si-doped MgO model was also calculated. The impurity level was 2.15 eV lower than that for the bottom of the CB. This result was in qualitative agreement with recent cathodoluminescence measurements. In addition, we have already succeeded in developing a novel electrical conductivity simulator using the spatial distribution of the probability density of wave functions obtained from the tight-binding quantum chemical molecular dynamics code, Colors. The electrical conductivity of the MgO-protecting-layer model was estimated with and without an oxygen defect and a significant change in the electrical conductivity of the MgO-protecting-layer materials was observed with the introduction of oxygen defects.

Quantum chemistry and QSPR study on relationship between crystal structure and emission wavelength of Eu2+‐doped phosphors

— The relationship between crystal structures and emission properties has been computationally investigated for Eu2+-doped phosphors. The electronic structure of the Eu2+-doped BaMgAl10O17 phosphor was analyzed by using the quantum chemistry method. The different effects of O and Ba atoms on the Eu 5d states were determined. The presence of O and Ba atoms increases and decreases the energy level of the Eu 5d orbital by forming anti-bonding and bonding interactions, respectively. According to the electronic-structure analysis, the structure index that represents the local geometrical information of the Eu atom was defined. The relationship between the crystal structures and the emission wavelengths of the 1 6 Eu2+-doped oxide phosphors were studied by using the quantitative structure-property relationship (QSPR). The QSPR model suggested that the both O and alkaline-earth atoms around the Eu atom are of importance in the determination of the emission wavelength. The interaction between the Eu and the nearest O atoms make the Eu2+ emission wavelength short. On the other hand, the interaction from the alkaline-earth atoms around the Eu atom lengthens the Eu2+emission wavelength. This evaluation method is useful in selecting the host material that indicates a desirable emission wavelength of the Eu2+-doped phosphors.

A Theoretical Study of Initial Deposition Processes of Mg on MgO: A Novel Quantum Chemical Molecular Dynamics Approach

Initial deposition processes of Mg on MgO were studied using our novel quantum chemical molecular dynamics (QCMD) simulator. This novel simulator was developed on the basis of our tight-binding quantum chemical molecular dynamics (TB-QCMD) simulator and our molecular dynamics simulator. The justification of the novel QCMD simulator was shown by the excellent agreement of the QCMD result with the first-principles results and experimental results for the physicochemical properties of the MgO crystal. Dynamic behaviors of Mg atom deposited on the MgO(001) and (111) surfaces were simulated by the novel QCMD method. It was shown that the mobility of Mg deposited on the MgO(001) surface was larger than that on the (111) surface, which suggests the difference between the interaction of the deposited Mg atom with the MgO(111) surface and that with the MgO(001) surface.

Computational Analysis of Electron Injection on Light-Emitting Polymer/Cathode Interface

Tohoku Univ., Dept. Chem. Eng., Grad. School Eng., 6-6-10-205 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Phone: +81-22-795-7237 E-mail: itaru@aki.che.tohoku.ac.jp Tohoku Univ., Dept. Appl. Chem., Grad. School Eng., 6-6-10-205 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Tohoku Univ., NICHe, 6-6-10-205, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Tohoku Univ., FRI, Grad. School Eng., 6-6-11-701 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

Development of Computational Analysis Method for Carrier Transport Pathway in Light-Emitting Polymers

Tohoku Univ., Dept. Chem. Eng., Grad. School Eng., 6-6-11-1302 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Phone: +81-22-795-7237 E-mail: itaru@aki.che.tohoku.ac.jp Tohoku Univ., Dept. Appl. Chem., Grad. School Eng., 6-6-11-1302 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Tohoku Univ., NICHe, 6-6-11-1302 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Tohoku Univ., FRI, Grad. School Eng., 6-6-11-701 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Univ. of Utah, Salt Lake City, Utah, USA.

A Large Scale Quantum Chemistry Study for the High γMgO Protecting Layer of Plasma Display Panels

1 Tohoku Univ., Dept. Appl. Chem., Grad. Sch. Eng., 6-6-11-1302 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Phone: +81-22-795-7237 e-mail: kazumi@aki.che.tohoku.ac.jp 2 Hiroshima Univ., Dept. of Semiconductor Elec. and Integration Sci., Grad. Sch. Adv. Sci. Matt., 1-3-1 Kagamiyama, Higashihiiroshima, Hiroshima 739-8530, Japan 3 Tohoku Univ., Dept. Chem. Eng., Grad. Sch. Eng., 6-6-11-1302 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan 4 Tohoku Univ., NICHe, 6-6-11-1302 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan 5 Tohoku Univ., FRI, Grad. Sch. Eng., 6-6-11-701 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

19.3: Electronic Structure Calculation and QSPR Analysis of Eu2+‐doped Oxide Phosphors: Relationship between Emission Wavelength and Crystal Structure

In this study, we have computationally revealed the relationship between crystal structure and emission wavelength for Eu2+-doped oxide phosphors. The quantum chemistry calculations and quantitative analysis of structure-property relationship suggested that both O and alkaline-earth (A) atoms around the Eu atom in phosphors are of importance for emission wavelength. This extension of the consideration range for interaction of A atoms gave the simple guideline for the emission wavelength control of Eu2+-doped phosphors: 1) The host materials which have many A atoms and shorter/longer A-A/A-O distances are suitable to get longer Eu2+ emission wavelength, 2) The host materials which have less A atoms and longer/shorter A-A/A-O distances are suitable to get shorter Eu2+ emission wavelength.