Color Point Tuning for (Sr,Ca,Ba) Si2O2N2:Eu2+ for White Light LEDs

Color point tuning is an important challenge for improving white light LEDs. In this paper, the possibilities of color tuning with the efficient LED phosphor Sr1−x−y−zCaxBaySi2O2N2:Euz2+ (0 ≤ x, y ≤ 1; 0.005 ≤ z ≤ 0.16) are investigated. The emission color can be tuned in two ways: by changing Eu2+ concentration and by substitution of the host lattice cation Sr2+ by either Ca2+ or Ba2+. The variation in the Eu2+ concentration shows a red shift of the emission upon increasing the Eu concentration above 2%. The red shift is explained by energy migration and energy transfer to Eu2+ ions emitting at longer wavelengths. Along with this (desired) red shift there is an (undesired) lowering of the quantum efficiency and the thermal quenching temperature due to concentration quenching. Partial substitution of Sr2+ by either Ca2+ or Ba2+ also results in a red-shifted Eu2+ emission. For Ca2+ this is expected and the red shift is explained by an increased crystal field splitting for Eu2+ on the (smaller) Ca2+ cation site. For Ba2+, the red shift is surprising. Often, a blue shift of the fd emission is observed in case of substitution of Sr2+ by the larger Ba2+ cation. The Eu2+ emission in the pure BaSi2O2N2 host lattice is indeed blue-shifted. Temperature dependent luminescence measurements show that the quenching temperature drops upon substitution of Sr by Ca, whereas for Ba substitution, the quenching temperature remains high. Color tuning by partial substitution of Sr2+ by Ba2+ is therefore the most promising way to shift the color point of LEDs while retaining the high quantum yield and high luminescence quenching temperature

A new route towards polarized luminescence: 0D/2D nanocomposites

Combining wide bandgap 2D inorganic materials and blue-light-emitting 0D carbon dots in 0D/2D heterojunction nanocomposites was shown to give rise to unique optical properties and a multifunctional prototype device was developed, capable of polarized light luminescence, modulation and detection

Ultrabright near-infrared light

The recent race for bright and broadband near-infrared (NIR) light sources has focused on new luminescent materials (phosphors) for efficient conversion of blue light emitting diode (LED) emission to broadband NIR. For extreme brightness, blue laser diodes (LDs) are superior to LEDs as they maintain efficiency at high current density. However, so far NIR phosphors suffered from severe efficiency droop at these high pumping powers. The team of Zhiguo Xia from South China University of Technology reported a record high 6 Watt NIR output from LD-pumped MgO:Cr3+ translucent ceramics emitting around 810 nm. Interestingly, MgO:Cr3+ is an old phosphor, extensively studied in the past century. The success is based in spark plasma sintering of translucent MgO:Cr3+ ceramics using nanosilica as flux, incorporated at grain boundaries. The refractive index contrast between MgO and SiO2 induces scattering of blue light, increasing light absorption, and the dense ceramic has excellent thermal conductivity, reducing thermal quenching of NIR emission. The new scheme using NIR-emitting translucent ceramics for laser-driven NIR light sources will propel photonics applications in night vision, biomedical imaging and sensing

Identification and Quantification of Charge Transfer in CaAl2O4:Eu2+,Nd3+ Persistent Phosphor

Reversible charge transfer between lanthanide ions is identified as storage mechanism for the current workhorse persistent phosphors. Present evidence relies on sophisticated X-ray absorption spectroscopy to detect the reversible charge transfer. Here, simple optical spectroscopy is used to study the charge transfer in one of the benchmark persistent phosphors CaAl2O4:Eu2+,Nd3+. Based on the observation that both the trapping and de-trapping processes in CaAl2O4:Eu2+,Nd3+ are thermally activated, forward charge transfer from Eu2+ to Nd3+ and backward charge transfer from Nd2+ to Eu3+ are identified. The percentages of Eu2+ and Nd3+ involved in the charge transfer are >10%, which exceeds the previous estimates in other persistent phosphors. Furthermore, this strategy offers additional advantage of site selectivity, which enables the identification of the distinct contributions of different Nd3+ sites to the charge transfer. These findings underline the significance of reversible charge transfer in persistent phosphors and move towards a complete understanding of persistent luminescence mechanism

Dual functionality luminescence thermometry with Gd2O2S:Eu3+,Nd3+ and its multiple applications in biosensing and in situ temperature measurements

Luminescence thermometry using sharp line emission of lanthanide ions has become an active area of research as it offers the advantages of remote temperature sensing with high sensitivity and superior spatial resolution. The most widely applied method relies on the temperature dependence of the luminescence intensity ratio of emission lines from two thermally coupled levels. However, the usable temperature range for this type of Boltzmann thermometer is limited. In addition, the weak and narrow line absorption of the parity forbidden 4f-4f transitions of lanthanides forms a serious drawback. To solve both problems, we here report a new dual functionality luminescence thermometer: Gd2O2S co-doped with Eu3+ and Nd3+. This material combines Boltzmann and energy transfer thermometry to extend the temperature range and uses the strong and broad charge transfer absorption band of Eu3+ for sensitization. In the T-range of 300–500 K efficient energy transfer from Eu3+ to Nd3+ allows for charge transfer-sensitized luminescence thermometry using near infrared emission from the thermally coupled 4F3/2 and 4F5/2 levels of Nd3+. Above 500 K a high temperature sensitivity is obtained using the strong temperature dependence of the luminescence intensity ratio of red Eu3+ to near infrared Nd3+ emission. The dual-functionality provides a single thermometer combining strong absorption and high relative sensitivity (0.6 – 1.4%) over a wide temperature range (300 to 650 K). Finally, it is proposed that this dual-function luminescent thermometer has promising potential for multifunctional applications in biosensors and in situ temperature measurements of chemical reaction process

Understanding enormous redshifts in highly concentrated Mn2+ phosphors

Broad band near infrared (NIR) emission has recently been reported for a wide variety of concentrated Mn2+ phosphors. Typically, Mn2+ emits in the green to red spectral region, depending on local coordination. The enormous redshift to the NIR was explained by exchange coupling between Mn2+ neighbours at high Mn2+ dopant concentrations. However, the reported redshifts are an order of magnitude larger than expected for exchange coupling and also the absence of a shift in excitation spectra suggests that exchange coupling cannot explain the observations. Here, extensive concentration, temperature and time dependent luminescence studies are reported for Mg1−xMnxAl2O4 (x = 0.01-0.5). The results show that the broad band NIR emission originates from NIR emitting trap centers, possibly Mn3+. High Mn2+ dopant concentrations enable efficient energy migration over the Mn2+ sublattice to these traps, consistent with the same excitation spectra for the green Mn2+ and NIR trap emission. Upon cooling to cryogenic temperatures energy migration is hampered and the green Mn2+ emission increases, especially in the most concentrated systems. Finally, the relative intensity of the NIR emission was varied by changing synthesis conditions providing further support that the NIR emission in concentrated Mn2+ phosphors originates from NIR emitting centers and not exchange coupled Mn2+ pairs

Impact of Noise and Background on Measurement Uncertainties in Luminescence Thermometry

Materials with temperature-dependent luminescence can be used as local thermometers when incorporated in, for example, a biological environment or chemical reactor. Researchers have continuously developed new materials aiming for the highest sensitivity of luminescence to temperature. Although the comparison of luminescent materials based on their temperature sensitivity is convenient, this parameter gives an incomplete description of the potential performance of the materials in applications. Here, we demonstrate how the precision of a temperature measurement with luminescent nanocrystals depends not only on the temperature sensitivity of the nanocrystals but also on their luminescence strength compared to measurement noise and background signal. After first determining the noise characteristics of our instrumentation, we show how the uncertainty of a temperature measurement can be predicted quantitatively. Our predictions match the temperature uncertainties that we extract from repeated measurements, over a wide temperature range (303-473 K), for different CCD readout settings, and for different background levels. The work presented here is the first study that incorporates all of these practical issues to accurately calculate the uncertainty of luminescent nanothermometers. This method will be important for the optimization and development of luminescent nanothermometers