Hubertus T. Hintzen

Research progress and application prospects of transition metal Mn4+-activated luminescent materials

Phosphor converted white light-emitting diodes (pc-WLEDs) are an alternative choice for general lighting due to their superior features such as high efficiency, durability and reliability. However, most pc-WLEDs in the market suffer from problems resulting from the lack of red emission, which can be resolved by adding red-emitting phosphors. The currently dominant red-emitting phosphors are Eu2+-doped nitrides, but the requirement of elevated temperature during synthesis makes them costly, and moreover the over broad emission may result in loss of lumen efficiency. Recently, transition metal Mn4+ doped materials showing very narrow red emission have attracted tremendous interest for compositions based on abundant resources and mild production processes, resulting in a highly efficient way to obtain phosphors with favorable luminescence properties. In this work, we describe the recent progress on transition metal Mn4+-doped inorganic luminescent materials, including oxides (subdivision into alkaline-earth germanates, arsenates, aluminates, titanates, pyrosilicates, phosphates, zirconates, gallates, simple oxides and others), fluorides and nitrides. More specifically, the review focuses on Mn4+ activated red-emitting phosphors that can be effectively excited by NUV and blue LED chips. The excitation and emission spectra as well as the preparation process of some representative phosphors are given and discussed. Meanwhile, the merits and drawbacks of several kinds of matrix materials in applications for white LEDs, as well as some problems, development trends and application prospects in this field of Mn4+ doped phosphors are summarized.

Photoluminescence properties of Pr3+, Sm3+ and Tb3+ doped SrAlSi4N7 and energy level locations of rare-earth ions in SrAlSi4N7

RE3+ (RE = Pr, Sm, and Tb)-doped SrAlSi4N7 samples were synthesized by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. It is noticeable that the 5d bands of Pr3+ and Tb3+ are at rather low energy in SrAlSi4N7 compared to oxides. Typical 4f2 → 4f2 emission lines (480–800 nm) of Pr3+ under 4f2 → 4f15d1 excitation were observed in Pr3+-doped SrAlSi4N7. Sm3+-doped SrAlSi4N7 shows red emission originating from 4G5/2 → 6HJ (J = 5/2, 7/2 and 9/2) transitions, and the charge transfer band of Sm3+ was observed at an unusually low energy of 3.98 eV. The Tb3+-doped sample exhibits 5D3 → 7FJ (J = 6, 5, 4, 3, 2, 1) (blue) and 5D4 → 7FJ (J = 6, 5, 4, 3) (green) line emissions in the wavelength range of 375–650 nm under the direct Tb3+ 4f8 → 4f75d1 excitation. The bands at about 256 nm in the excitation spectra are attributed to the host lattice absorption. In addition, there is energy transfer from the host lattice to the luminescent activators (Pr3+, Sm3+, and Tb3+). The energy level diagram containing the position of the 4f and 5d levels of all divalent and trivalent lanthanide ions relative to the valence and conduction band of SrAlSi4N7 has been constructed and discussed.

Transition of Emission Colours as a Consequence of Heat-Treatment of Carbon Coated Ce3+-Doped YAG Phosphors

To modify the luminescence properties of Ce3+-doped Y3Al5O12 (YAG) phosphors, they have been coated with a carbon layer by chemical vapor deposition and subsequently heat-treated at high temperature under N2 atmosphere. Luminescence of the carbon coated YAG:Ce3+ phosphors has been investigated as a function of heat-treatment at 1500 and 1650 °C. The 540 nm emission intensity of C@YAG:Ce3+ is the highest when heated at 1650 °C, while a blue emission at 400–420 nm is observed when heated at 1500 °C but not at 1650 °C. It is verified by X-ray diffraction (XRD) that the intriguing luminescence changes are induced by the formation of new phases in C@YAG:Ce3+-1500 °C, which disappear in C@YAG:Ce3+-1650 °C. In order to understand the mechanisms responsible for the enhancement of YAG:Ce3+ emission and the presence of the blue emission observed for C@YAG:Ce3+-1500 °C, the samples have been investigated by a combination of several electron microscopy techniques, such as HRTEM, SEM-CL, and SEM-EDS. This local and cross-sectional analysis clearly reveals a gradual transformation of phase and morphology in heated C@YAG:Ce3+ phosphors, which is related to a reaction between C and YAG:Ce3+ in N2 atmosphere. Through reaction between the carbon layer and YAG host materials, the emission colour of the phosphors can be modified from yellow, white, and then back to yellow under UV excitation as a function of heat-treatment in N2 atmosphere.

On the relations between the bandgap, structure and composition of the M–Si–N (M = alkali, alkaline earth or rare-earth metal) nitridosilicates

Relations between the bandgap and structural properties and composition of the M–Si–N nitridosilicates (M = alkali, alkaline earth or rare earth metal) have been obtained, using experimental data collected from literature; and qualitative models are presented to explain the observed trends. Compounds with a higher degree of condensation, i.e. a higher Si/N ratio, generally have longer M–N bonds and shorter Si–N bonds. The observations can be explained based on the effective charge of N, dependent on its coordination with Si (NSix). With increasing Si/N ratio the coordination number of N by Si increases, making the effective charge of the nitrogen atom less negative, resulting in a longer and less covalent M–N bond. This also shifts the N 2p levels down in energy, lowering the top of the valence band (mainly composed of N orbitals); while decreasing the Si–N distance shifts the bottom of the conduction band (mainly composed of Si and M orbitals) upward. Some nitridosilicates show deviations to the general trends, such as γ-Si3N4 and several Li-containing compounds. These deviations have been discussed and possible explanations have been given based on peculiarities in their structural characteristics.

Performance improvement by alumina coatings on Y3Al5O12:Ce3+ phosphor powder deposited using atomic layer deposition in a fluidized bed reactor

To improve the thermal stability, Al2O3 has been successfully coated on a Y3Al5O12:Ce3+ (YAG:Ce) phosphor powder host by using the Atomic Layer Deposition (ALD) approach in a fluidized bed reactor. Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray spectroscopy (EDX) analysis indicate that coating an Al2O3 thin layer by ALD is highly feasible. The luminescence properties (such as excitation and emission as well as quantum efficiency and UV-absorption of the coated YAG:Ce phosphor) were systematically analysed, with the further examination of the thermal resistance characteristics. The Al2O3 thin layer coating with precisely controlled thickness by ALD can obviously improve the luminescence intensity and greatly enhances the thermal stability of the YAG:Ce phosphor. It is suggested that the alumina coating with tailoring thickness seems not only to act like a barrier to decrease the thermal quenching, but also as a great help to promote the light absorption and transfer.

Dependence of the photoluminescence properties of Eu2+ doped M–Si–N (M = alkali, alkaline earth or rare earth metal) nitridosilicates on their structure and composition

Optical data of the Eu2+ doped nitridosilicates (MxSiyNz) have been collected from the literature and have been analysed with regard to their dependence on structure and composition. Nitridosilicates with a higher degree of condensation, i.e. a higher Si/N ratio, generally have a higher Eu2+ 4f–5d absorption energy, a higher 5d–4f emission energy and a larger Stokes shift. The higher absorption and emission energies are due to the increase of the N by Si coordination number with increasing Si/N ratio. This results in more electrons on N that participate in the bonding with Si, and thus less electrons are available for Eu–N bonding, reducing the covalency of the Eu–N bonds. The lower covalency gives a weaker nephelauxetic effect, reducing the centroid shift of the 5d level. The lowest 4f–5d absorption energy further increases due to the reduction of the crystal field splitting of the 5d levels, as the Eu–N bonds become longer with increasing Si/N ratio. The Stokes shift increases with increasing degree of condensation despite an increase of lattice rigidity, ascribed to a decrease of local rigidity around the Eu2+ ion caused by the larger Eu–N bond lengths. Some nitridosilicates show deviations from the general trends attributed to peculiarities in their crystal structure and the way Eu2+ is substituted in the lattice. The relationships established in the present work will be helpful for the design and exploration of new Eu2+ doped nitride-based luminescent materials for practical applications.