Optical properties of (oxy)nitride materials : a review

(Oxy)nitride materials, consisting mainly of transition metal and ionic-covalent (oxy)nitrides, show a vast number of interesting physical and chemical properties due to their substantial structural diversity. The optical properties of these (oxy)nitrides, in combination with their excellent mechanical strength, thermal properties, and chemical stability, enable (oxy)nitrides to be used in a variety of industrial fields, such as photovoltaic, photothermal, photocatalytic, pigment, lighting and display, optoelectronic, and defense industries. The optical properties are extremely related to the electronic band structure of (oxy)nitrides, and can be varied significantly by changing the chemical composition (e.g., the oxygen to nitrogen ratio) and preparation/processing conditions. This article overviews the optical properties (including refractive index, reflectance, absorbance, band gap, photoluminescence, and transmittance) of (oxy)nitride materials that are in the form of thin films, powders, or bulk ceramics, and highlights their applications as antireflection coatings, solar spectral selectivity coatings, visible-light-driven photocatalysts, ecological pigments, phosphors for light-emitting diodes, and transparent window materials

Electronic structure of the alkaline-earth silicon nitrides M2Si5N8 (M = Ca and Sr) obtained from first-principles calculations and optical reflectance spectra

Results of first-principles band-structure calculations for the ternary alkaline-earth silicon nitrides M2Si5N8 (M = Ca and Sr) are presented. In the structures of M2Si5N8 (M = Ca, Sr and Ba), the N atoms show connections to two (N[2]) and three (N[3]) neighbouring silicon tetrahedral centres. Calculations show that the local electronic structure is strongly dependent on the local chemical bonding. The valence band is dominated by N 2p hybridized with the s, p states of the alkaline-earth-metal and silicon atoms. The upper part of the valence band is dominated by the 2p states of N[2] atoms, while the N[3] 2p states lie about 2 eV below the Fermi level. The bottom of the conduction band consists of the N 3s characters hybridized with s orbitals of the alkaline-earth metals, while the s character of Si atoms is higher in energy. Sr2Si5N8 is a semiconductor with a direct energy gap at Γ, while Ca2Si5N8 is an indirect semiconductor. Optical diffuse reflectance spectra show an energy gap of 4.9 eV for Ca2Si5N8, 4.5 eV for Sr2Si5N8, as well as 4.1 eV for Ba2Si5N8, in fair agreement with the calculated values.

First-principles electronic structure calculations of Ba5Si2N6 with anomalous Si2N6 dimeric units

First-principles band structure calculations were performed for the ternary alkaline-earth silicon nitride Ba5Si2N6 using the LSW method. The calculations show that both the (zero-)dimensionality of the [Si2N6]10- dimeric units present in this structure and the coordination number of nitrogen by silicon have strong influences on the local electronic structure of these atoms. The interaction between the semicore-level states Ba 5p and N 2s is significant. Finally the compound is calculated to be a semiconductor with an indirect gap of 0.7 eV. The top of the valence band is dominated by the 2p states of the N[1] atoms. The conduction bands are determined by N 3s states hybridized with Ba 6s states.

Electronic structure and photoluminescence properties of Eu2+-activated Ca2BN2F

The electronic structure of undoped and luminescence properties of Eu2+-doped Ca2BN2F have been investigated. First-principles calculations for Ca2BN2F show that the valence band is mainly composed of F and N 2p, B 2s and 2p orbitals, while the Ca 4s and 3d are almost empty, indicating that Ca2BN2F is a very ionic compound. The valence band close to the Fermi level is dominated by the N 2p states, and the bottom of the conduction band is determined by the Ca 3d and N/B 3s orbitals. The direct energy gap is calculated to be about 3.1 eV, in fair agreement with the experimental data of ~3.6 eV derived from the diffuse reflection spectrum. Due to the high degree of ionic bonding of the coordinations of Eu with (N, F) on the Ca sites, Ca2BN2F:Eu2+ shows strong blue emission with a maximum at about 420 nm upon UV excitation in the absorption range of 330-400 nm. © 2009 Elsevier Inc. All rights reserved

Photoluminescence properties of rare-earth activated BaSi7N10

The photoluminescence properties of Eu2+-, Ce3+- and Tb3+-activated BaSi7N10 have been studied. The transitions of f ¿ d of the Eu2+ and Ce3+ ions occur at relatively high energies for nitrides which are principally attributed to weak bonding strength of Ba-N and small crystal field splitting of 5d excitation levels of Eu2+ and Ce3+ due to a large Ba crystallographic site in BaSi7N10. As a result, it was found that BaSi7N10:Eu2+ showed blue emission at about 475 nm originating from the 4f65d1 ¿ 4f7 transition of Eu2+ upon UV excitation. While BaSi7N10:Ce3+, Li+ exhibited UV-blue emission with a maximum at about 400 nm caused by the 5d ¿ 4f transition of Ce3+. In BaSi7N10:Tb3+, Li+, the luminescence spectrum of Tb3+ was dominated by green line emission arising from the 5D4 ¿ 5F J (J = 3, 4, 5, 6) transitions