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

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

Temperature quenching of Cr3+ in ASc(Si1-xGex)2O6 (A=Li/Na) solid solutions

Blue absorbing near infrared (NIR) emitting phosphors are a promising class of materials for phosphor converted NIR LEDs, which can be used in compact NIR spectrometers. Preferably, these phosphors have a broad emission spectrum and show negligible luminescence quenching at LED operating temperatures (100 °C). Here, we investigated ASc(Si1-xGex)2O6 (A = Li/Na, x = 0,0.2,0.4,0.6,0.8,1) solid solutions doped with Cr3+ to tune and optimize the emission maximum and bandwidth to cover the full 700–1100 nm range. With increasing Ge content an emission redshift was observed, along with emission band broadening at intermediate Ge/Si ratio, which is explained by disorder around Cr3+ in the second coordination sphere (mixed Si/Ge). Temperature dependent emission spectra and luminescence decay curves were measured between 90 K and 670 K to determine the quenching temperature TQ. With increasing Ge content TQ drops from 550 K to below 400 K. Interestingly, Cr3+ emission in the highly symmetric site in LiScSi2O6 shows a strongly temperature dependent lifetime before thermal quenching sets in. DFT calculations on LiScSi2O6 indicate that asymmetric vibrations at the Sc site are involved and calculated phonon energies were confirmed by measuring FTIR. Our study indicates that a solid solution is a promising way to increase the emission bandwidth. However, with increasing Ge content TQ decreases. An optimum Ge-content in LiSc(Si1-xGex)2O6:Cr3+ is x = 0.2–0.4 as it redshifts the NIR band maximum close to 900 nm and offers a FWHM bandwidth around 180 nm, while keeping the thermal quenching temperature high enough for application in NIR-LEDs

Transient photoluminescence enhancement as a probe of the structure of impurity-trapped excitons in CaF2_2:Yb2+^{2+}

We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF$_2$:Yb$^{2+}$. The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation.Comment: Minor corrections. arXiv admin note: text overlap with arXiv:1107.4888v

Engineering of lipid-coated PLGA nanoparticles with a tunable payload of diagnostically active nanocrystals for medical imaging

Polylactic-co-glycolic acid (PLGA) based nanoparticles are biocompatible and biodegradable and therefore have been extensively investigated as therapeutic carriers. Here, we engineered diagnostically active PLGA nanoparticles that incorporate high payloads of nanocrystals into their core for tunable bioimaging features. We accomplished this through esterification reactions of PLGA to generate polymers modified with nanocrystals. The PLGA nanoparticles formed from modified PLGA polymers that were functionalized with either gold nanocrystals or quantum dots exhibited favorable features for computed tomography and optical imaging, respectively.National Heart, Lung, and Blood Institute (Programs of Excellence in Nanotechnology (PEN) Award, Contract #HHSN268201000045C))National Institutes of Health (U.S.) (R01 EB009638)National Institutes of Health (U.S.) (R01 CA155432)National Institutes of Health (U.S.) (K99 EB012165)Netherlands Organization for Scientific Research ((NWO) ECHO.06.B.047

Finite-Size Effects on Energy Transfer between Dopants in Nanocrystals

Many phosphor materials rely on energy transfer (ET) between optically active dopant ions. Typically, a donor species absorbs light of one color and transfers the energy to an acceptor species that emits light of a different color. For many applications, it is beneficial, or even crucial, that the phosphor is of nanocrystalline nature. Much unlike the widely recognized finite-size effects on the optical properties of quantum dots, the behavior of optically active ions is generally assumed to be independent of the size or shape of the optically inactive host material. Here, we demonstrate that ET between optically active dopants is also impacted by finite-size effects: Donor ions close to the surface of a nanocrystal (NC) are likely to have fewer acceptors in proximity compared to donors in a bulk-like coordination. As such, the rate and efficiency of ET in nanocrystalline phosphors are low in comparison to that of their bulk counterparts. Surprisingly, these undesired finite-size effects should be considered already for NCs with diameters as large as 12 nm. If we suppress radiative decay of the donor by embedding the NCs in media with low refractive indices, we can compensate for finite-size effects on the ET rate. Experimentally, we demonstrate these finite-size effects and how to compensate for them in YPO4 NCs co-doped with Tb3+ and Yb3+