Masayoshi Mikami - Academia.edu (original) (raw)

Papers by Masayoshi Mikami

First-principles total-energy calculations have been performed for Methyl-group-absorbed surface ... more First-principles total-energy calculations have been performed for Methyl-group-absorbed surface models during organometallic vapor phase epitaxy (OMVPE). The surface models were based on the As-rich (1x2)-CH3 GaAs (001) surface proposed by Creighton and Baucom (Surf. Sci. 409, 372 (1998)). A phase diagram regarding methyl-group coverage was obtained from the surface formation energies. The phase diagram predicted several possible methyl-group-absorbed models besides

CrystEngComm, 2014

As new light sources for next-generation illumination, white light-emitting diodes (LEDs) have be... more As new light sources for next-generation illumination, white light-emitting diodes (LEDs) have been developed extensively and are commercially available due to their excellent advantages. However, the current white LEDs present in the market based on the combination of a blue chip and a yellow phosphor cannot satisfy the need for indoor illumination or some other colourful fields due to the lack of a sufficient red spectral component. Here we report a green phosphor, NaBaScSi 2 O 7 :Eu 2+ , which can be effectively excited using a near-ultraviolet chip and emit bright green light with extremely excellent thermal stability. The electronic structure and characteristic photoluminescence and cathodoluminescence properties as well as the thermal quenching properties were investigated in detail. The origin of the desired green luminescence was also determined by analyzing the crystal structure and measuring fluorescence lifetimes and the site-selective excitation and emission spectra. In addition, to investigate its application in field emission displays, the cathodoluminescence (CL) spectra of NaBaScSi 2 O 7 :Eu 2+ as a function of the accelerating voltage, probe current and the electron radiation time were also measured and discussed in detail. The current results indicate that NaBaScSi 2 O 7 :Eu 2+ can serve as a potential green phosphor for application in high-power white LEDs and field emission displays.

Physical Review B, 1999

We present density functional calculations of the structural, electronic and magnetic properties ... more We present density functional calculations of the structural, electronic and magnetic properties of various intrinsic defects in (Ga,Mn)As. Our main finding is that As excess generally weakens the ferromagnetic coupling between Mn ions. Moreover, we find that the interaction is strongly dependent on the microscopic configuration of the Mn ions and the defects, suggesting that a mean field description is not always appropriate. r

Journal of Light & Visual Environment, 2008

High-luminous green phosphors and a red phosphor have been newly developed for white LEDs. The de... more High-luminous green phosphors and a red phosphor have been newly developed for white LEDs. The developed green phosphors are Ca3(Sc,Mg)2Si3O12:Ce made by replacing with Mg a part of Sc of green phosphor Ca3Sc2Si3O12:Ce, a new green phosphor CaSc2O4:Ce, and a new host material doped with a rare-earth element, Ba3Si6O12N12:Eu. The new red phosphor is (Sr,Ca)AlSiN3:Eu made by replacing a part

MRS Proceedings, 2007

ABSTRACT

J. Mater. Chem. C, 2015

ABSTRACT The structural properties of wadeite K2ZrSi3O9 have been investigated using the high-res... more ABSTRACT The structural properties of wadeite K2ZrSi3O9 have been investigated using the high-resolution transmission electron microscopy and X-ray powder diffraction refinement, and the luminescence properties of Eu2+ activated K2ZrSi3O9 have been studied to explore the new materials for phosphor-converted white light near ultraviolet light-emitting diodes (NUV-LEDs). Eu2+ was introduced into the K2ZrSi3O9 host in the reducing atmosphere, and the special crystallographic positions of the Eu2+ were determined based on XRD, photoluminescence emission spectra, temperature dependence properties, time-resolved photoluminescence etc..The calculated band gap is about 4.7 eV. The CIE chromaticity coordinates and FWHM of the blue phosphor K2ZrSi3O9: 1%Eu2+ are (0.1538, 0.1857) and 57 nm. The photoluminescence properties of co-doped Eu2+-Al3+and Eu2+-Sc3+ charge compensation pairs phosphors were investigated. The Eu2+ single-doped K2ZrSi3O9 phosphor shows blue emission with the broad-band peaking at 465 nm upon 400 nm NUV excitation. By Eu2+-Al3+ as charge compensation, the photoluminescence properties do not change distinctly, while the photoluminescence emission spectrum shifts to red area about 39 nm and becomes green emission by using Eu2+-Sc3+ pair. Different occupying situations with different charge compensation pairs were discussed. Blue and green emission can be yielded in K2ZrSi3O9: Eu2+ compound by different charge compensation mechanism. It reveals that K2ZrSi3O9: Eu2+ possesses remarkable optical properties and can be used in NUV-LEDs.

Physical Review B, 2002

Lattice vibrations and dielectric properties of the anatase phase of titanium dioxide are investi... more Lattice vibrations and dielectric properties of the anatase phase of titanium dioxide are investigated from a theoretical perspective. A first-principles pseudopotential band calculation based on density-functional perturbation theory is performed to evaluate ...

Physical Review B, 1998

The electronic structure of yttrium oxysulfide (Y 2 O 2 S) is investigated using first-principles... more The electronic structure of yttrium oxysulfide (Y 2 O 2 S) is investigated using first-principles pseudopotential calculations by means of the local-density approximation within density-functional theory. It is found that ͑i͒ the material is an indirect band-gap semiconductor, ͑ii͒ the calculated bulk modulus ͑142 GPa͒ is between the one of yttrium oxide and that of yttrium sulfide, ͑iii͒ the top of the valence bands shows anisotropic behavior, and ͑iv͒ substantial covalent bonds between Y and O and less covalent bonds between Y and S coexist in the material. ͓S0163-1829͑98͒01016-9͔

Optical Materials Express, 2014

ABSTRACT A novel green emitting phosphor of Eu2+ doped Ca7(PO4)2(SiO4)2 was synthesized and its p... more ABSTRACT A novel green emitting phosphor of Eu2+ doped Ca7(PO4)2(SiO4)2 was synthesized and its photoluminescence properties were investigated for application in UV LEDs. XRD was used to identify sample phase. Diffuse reflection spectra and photoluminescence spectra were used to investigate its photoluminescence properties. Ca7(PO4)2(SiO4)2:Eu2+ showed an absorption ranging from 240 to 440 nm in ultraviolet range and a broad green emission band peaked at 522 nm. The concentration quenching mechanism and the key parameters for the fabrication of WLEDs, such as the temperature dependent photoluminescence and CIE value had also been studied.

MRS Proceedings, 2014

ABSTRACT A series of Ce3+ doped Ca8La2(PO4)6O2 phosphors with tunable emission were successfully ... more ABSTRACT A series of Ce3+ doped Ca8La2(PO4)6O2 phosphors with tunable emission were successfully synthesized by traditional solid state reaction. The crystal structure and photoluminescence properties were studied through X-ray diffraction, photoluminescence excitation and emission spectra. The results indicated that Ca8La2(PO4)6O2:Ce3+ exhibited color-tunable emission due to the 5d-4f transitions of Ce3+ ions under different wavelength excitation. The optimal doping content of Ce3+ ions in Ca8La2(PO4)6O2 was found to be 5 mol%. The site-selective photoluminescence property and the reason for red-shift of the emission band along with Ce3+ content and the excitation wavelength were also studied in detail.

Dalton transactions (Cambridge, England : 2003), Jan 28, 2015

A novel Ce(3+) activated Lu3MgAl3SiO12:Ce phosphor was synthesized and found to crystallize in th... more A novel Ce(3+) activated Lu3MgAl3SiO12:Ce phosphor was synthesized and found to crystallize in the garnet structure. The crystal structure of the synthesized phosphor has been characterised by X-ray diffraction and Rietveld refinement. Both room and high temperature photoluminescence spectra are utilized to investigate the luminescence properties and crystal field splitting. The high temperature quenching of these phosphors and their quantum efficiency (QE) are also studied using both the prepared YAG:Ce and the commercial YAG:Ce phosphor named P46-y3 as the reference. Upon excitation with blue light, the composition-optimized Lu3MgAl3SiO12:Ce phosphor exhibited strong yellow light with a high QE of 81.2% and better thermal stability than that of the commercial phosphor. The results indicate that the Lu3MgAl3SiO12:Ce phosphor can serve as a candidate for blue chip LEDs.

Rare-earth oxysulfides are known as wide-gap semiconductors and have been utilized as efficient h... more Rare-earth oxysulfides are known as wide-gap semiconductors and have been utilized as efficient host materials for phosphorescent use about a quarter of a century. They are expected to be rather ionic materials. However, little is known about electronic structure of the hosts and of their intrinsic defects in both experimental and theoretical meaning. Thus we have employed self-consistent first-principles total-energy

The lattice vibrations of yttrium oxysulfide (Y_2O_2S) were studied with the combination of Raman... more The lattice vibrations of yttrium oxysulfide (Y_2O_2S) were studied with the combination of Raman scattering experiment and first-principles calculation. To obtain as clean Raman spectra as possible, relatively large-sized (about 1 mm^3) pure single crystals of Y_2O_2S were synthesized by sintering method at a temperature just below the melting point of the compound. The observed Raman spectra revealed four Raman frequencies, which appeared close to the previous Raman measurement with the powder samples (Yokono et al., J. Phys. Soc. Jpn. 46, 1882 (1979)),within 3 cm-1. Furthermore, density functional perturbation theory based on first-principles pseudopotential method was employed to predict the lattice vibration frequencies and to assign the vibration modes. The calculated Raman frequencies appeared close to our experimental result as well as Yokono's result, within 50 cm-1. All of the phonon modes of Y_2O_2S were also assigned owing to our calculated result. Our calculation also revealed several anisortopic optical properties due to the anisotropic crystal structure.

ABSTRACT Yellow-emitting phosphors activated by Ce3+ are key components of white light-emitting d... more ABSTRACT Yellow-emitting phosphors activated by Ce3+ are key components of white light-emitting diodes (LEDs) based on the blue (Ga,In)N LED. In these phosphors, the electronic environment around Ce3+ determines its photoluminescence wavelengths. Placing Ce3+ in octahedral sites in oxides can potentially lead to yellow or orange phosphors that can compete with the important commercial phosphor, Y3Al5O12:Ce. However, there are very few examples of such materials. In this article, we explore the photoluminescent behavior of Ce3+ in oxides with the olivine structure, whereby two distorted octahedral doping sites exist. We demonstrate that the promising new yellow phosphor γ-Ca2SiO4:Ce requires a second dopant, Al3+, to avoid the formation of the undesired β polymorph. Our results indicate that the yellow emission is indirectly caused by the complex polymorphism of Ca2SiO4, particularly the low temperature formation of the γ-phase. The dramatic shift in emission from yellow to light-blue when this phosphor is heated to 800 °C is explained in terms of the redistribution of Ce3+ in the lattice. A similar effect is observed when Si4+ is substituted by Ge4+. The light-blue emission of Ca2GeO4:Ce,Al is reported as well as the photoluminescent properties of the Ca2(Ge,Si)O4:Ce,Al solid solution.

M2SiO4-based phosphors (M: alkaline-earth metal) that emit red to blue light are expected to offe... more M2SiO4-based phosphors (M: alkaline-earth metal) that emit red to blue light are expected to offer high color rendering to white light-emitting diodes (LEDs) in combination with blue or near-UV excitation sources. It is very important for the complete control of the emission color to understand the crystal field around the active elements (rare-earth and transition metals). XAFS spectroscopy is applied

New square-planar complexes [Pt(1 −H ) 2 ] (2a) [1 −H = (oxazolin-2-yl)bis(diphenylphosphino)meth... more New square-planar complexes [Pt(1 −H ) 2 ] (2a) [1 −H = (oxazolin-2-yl)bis(diphenylphosphino)methanide] and [Pd(1 −H ) 2 ] (2b), of general formula [M{(Ph 2 P) 2 C---C---NCH 2 CH 2 O} 2 ] (M = Pt, 2a; M = Pd, 2b), result from deprotonation of 2-{bis(diphenylphosphino)methyl}oxazoline (1) at the PCHP site. The new, functionalized dppm-type ligand 4-{bis(diphenylphosphino)methyl}pyridine, (Ph 2 P) 2 CH(4-C 5 H 4 N) (4), was prepared by double lithiation and phosphorylation of 4-picoline. In the presence of NEt 3 , the reactions of 2 equiv of 4 with [PtCl 2 (NCPh) 2 ] and [Pd(acac) 2 ] (acac = acetylacetonate) afforded [Pt(4 −H ) 2 ] (5a) [4 −H = bis(diphenylphosphino)(pyridin-4-yl)methanide] and [Pd(4 −H ) 2 ] (5b), of general formula [M{(Ph 2 P) 2 C(4-C 5 H 4 N)} 2 ] (M = Pt, 5a; M = Pd, 5b), respectively.