Fraunhofer Institute for Microstructure of Materials and Systems IMWSRed, green, blue - LEDs (light-emitting diodes) can only produce light in one color at a time. They are therefore not suitable per se for the most common applications: Various color mixing methods are used to obtain white light. For example, light in the red and green color spectrum is combined with the light of a blue diode. Improving the corresponding phosphors offers great potential for making LED lighting even more efficient. During the synthesis of a new type of red phosphor with excellent luminescence properties, the Fraunhofer IMWS supported the characterization of the new material with measurements using STEM, EDX and ToF-SIMS.
The new luminescent material Sr[LiAl2O2N2]:Eu2+ was developed by the University of Innsbruck in collaboration with OSRAM Opto Semiconductors and a patent application was filed; the Fraunhofer IMWS provided support in the characterization of the materials. The results of the research work were also published in the article “Sr[Li2Al2O2N2]:Eu2+- A high performance red phosphor to brighten the future” in the journal “Nature Communications”.
The newly developed phosphor is based on fluorescent nitrides doped with europium. Compared to previously available phosphors, it emits less light in the infrared range, which is invisible to the human eye, and instead has a higher blue component. This means that a higher proportion of visible light is generated with the same amount of energy - energy losses are reduced and the light yield is increased by up to a sixth.
To optimize the synthesis process in the production of the phosphor, which the inventors have named SALON, a single crystal was produced, among other things, in order to determine the structure of the new material. The crystal structure is decisive for the lower wavelength of the emitted light and the other desired properties of the phosphor, which also include high thermal stability and very good color rendering. This made high-resolution methods of material diagnostics all the more important in the course of development in order to characterize and further optimize the phosphor.
The single crystal was analyzed using X-ray diffraction data to determine the exact positions of Sr, Li and Al within the cube-like crystal lattice. The derived model, including the assumed positions of nitrogen and oxygen atoms within the material, was confirmed by calculations. In order to examine the composition of SALON even more precisely, an elemental analysis of the novel phosphor was carried out at the Fraunhofer IMWS. Scanning transmission electron microscopy (STEM) was combined with energy-dispersive X-ray spectrometry (EDX) for this purpose. A key factor for the feasibility of this analysis was suitable sample preparation in order to thin the material down to less than 100 nm and thus make it electron-transparent. The focused ion beam (FIB) technique was used for this purpose.
EDX spectroscopy immediately revealed that the sample under investigation is rich in nitrogen, aluminum and strontium and also contains oxygen. Quantification of the EDX spectra of the phosphorus particles revealed an element ratio of Sr:Al:O:N, which is approximately 1:1.8:2.1:1.7. Based on the X-ray diffraction data, a ratio of 1:2:2:2 would have been expected for the crystal, so within the limits of measurement accuracy, the composition could be considered confirmed.
As EDX analysis does not allow the detection of lithium, a very light element with a low cross-sectional area for the excitation of X-ray quanta, another method was used that is available at the Fraunhofer IMWS: time-of-flight secondary ion mass spectrometry (ToF-SIMS) - one of the few microstructure analysis techniques that allows the detection of lithium beyond doubt. Lithium could thus be detected clearly and with a homogeneous distribution within the phosphorus particles. The creation of depth profiles - by comparison with a reference sample - also provided quantitative information about the composition. It was found that the intensity of the Li+ signal in SALON was 2.4 times higher than that of the Sr[LiAl3N4] reference, i.e. close to 2 when matrix effects were taken into account. By combining the EDX and ToF-SIMS results, the only possible composition of SALON was thus determined as Sr[Li2Al2O2N2], which confirmed the results of the single-crystal X-ray diffraction.
