L. Marciniak

White emission of lithium ytterbium tetraphosphate nanocrystals.

An efficient anti-Stokes white broadband emission induced by 976 nm laser diode in lithium ytterbium tetraphosphate (LiYbP4O12) nanocrystals was investigated. The emission occurs at room temperature and atmospheric pressure. Its intensity demonstrates an evident threshold dependence on the temperature and excitation density characteristic to avalanche process. The white emission is accompanied by very efficient photoconductivity characterized by microampere photocurrent which increases with the fourth order of applied incident light power (~P4). We show that this emission is critically dependent on temperature and increases significantly in vacuum. It is concluded that the anti-Stokes white emission is associated with theYb3+- CT luminescence.

Anti-Stokes bright yellowish emission of NdAlO3 nanocrystals

Infrared laser diode–induced anti-Stokes bright yellowish emission of NdAlO3 nanocrystalline powder was observed at room temperature in ambient atmosphere. The emission intensity was found to be unaltered with lowering temperature and to increase by two orders of magnitude in vacuum. The temperature of bright emission under ambient atmosphere was determined to be 350 °C. It was found that the yellowish emission was accompanied by a giant photocurrent of 0.5 microamperes at relatively low applied voltage. The power dependence of the photocurrent was governed by an avalanche-like mechanism. The origin of the bright emission is discussed in terms of charge transfer luminescence of Nd3+.

A new generation of highly sensitive luminescent thermometers operating in the optical window of biological tissues

A new type of luminescent thermometer based on highly temperature dependent d–d Cr3+ transitions related to barely temperature dependent f–f Nd3+ transitions is reported for the first time, showing exceptionally high sensitivity and shifting current paradigms behind the physics and chemistry of luminescent nanothermometers. The highest sensitivity in the physiological temperature range was found for LiLaP4O12:1%Cr,10%Nd and reached 4.89%/°C – three times, up to over one order of magnitude higher than most luminescent thermometers reported to date. Moreover, the brightness of such probes based on Cr ions was around one order of magnitude higher than the Stokes Nd emission. Even higher sensitivities, up to 30%/°C, were found above 200 °C, indicating the importance of a rational approach to the design of chemical composition and smart involvement of photophysical processes, to significantly enhance the properties of luminescent thermometers. The thermal dependence of the luminescence intensity ratio from the two ions was investigated for different Cr3+ and Nd3+ ion concentrations, which actually had no severe impact, either on the LIR or on the sensitivity of such luminescent thermometers.

Controlling luminescence colour through concentration of Dy3+ ions in LiLa1−xDyxP4O12 nanocrystals

Luminescence and excitation spectra of LiLa1−xDyxP4O12 nanocrystals are reported. In particular, the impact of dopant concentration on the luminescence properties of LiLa1−xDyxP4O12 nanocrystals was investigated. It was found that increase in Dy3+ concentration results in a strong quenching of luminescence. The mechanism of concentration quenching was discussed in terms of the Yokota–Tanimoto model. It is concluded that with increasing Dy3+ concentration, the donor–acceptor interaction leads to a significant concentration quenching resulting in nonexponential luminescence decay, whereas at a high concentration limit the donor–donor interaction becomes dominant and exponential decay is observed.

Size dependent sensitivity of Yb3+,Er3+ up-converting luminescent nano-thermometers

The non-contact temperature sensing using the luminescence intensity ratio (LIR) between 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 electronic transition in upconverting Yb3+ and Er3+ co-doped nanocrystals has been known for two decades. This phenomenon is governed by the simple Boltzmann formula, which however does not consider materials properties other than energy gap between the respective energy states. For example, the temperature sensitivity has never been shown to depend on the size of nanoparticles, however our studies, performed on two different NaYF4:Er3+,Yb3+ and LiLaP4O12:Er3+,Yb3+ nanocrystals clearly show evidences of such a relationship. Upon constant excitation intensity, the changes in the LIR emission intensity ratio in response to rising external temperature become consistently more rapid for smaller nanocrystals. It was found that the sensitivity of an up-converting luminescent thermometer changes proportionally to the nanoparticle size, i.e. raised from 1.1% K−1 @ 240 nm to 1.8% K−1 @ 20 nm for tetraphosphate nanocrystals and similarly raised from 1.1% K−1 @ 200 nm to 2.1% K−1 @ 8 nm for fluoride nanocrystals at 200 K. These phenomena were manifested mainly at low temperatures and can be neglected at temperatures above 250 K. The observed changes were discussed in terms of ratio of superficial to bulk ions. Phenomenological models have been proposed to describe both the size dependent luminescence lifetimes and temperature sensitivity.

Temperature of broadband anti-Stokes white emission in LiYbP4O12: Er nanocrystals

The up-conversion emission of LiYb0.99Er0.01P4O12 nanocrystals was investigated in vacuum upon the focused beam of infrared laser excitation. The intense white broad band emission was observed above the threshold power of 800 mW. An increase of excitation power results in enhancement of temperature of emitting nanocrystals. The temperature of nanocrystals was determined following the thermally equilibrated f-f emission transitions of Er3+ ions. The highest measured temperature of white emission was determined to be about 900 K. It is concluded that the black body radiation is not responsible for the white broad band emission.

Lanthanide 9-anthracenate: solution processable emitters for efficient purely NIR emitting host-free OLEDs

Searching for new NIR emitting materials, lanthanide 9-anthracenates Ln(ant)3 were synthesized and thoroughly characterized. Ytterbium 9-anthracenate Yb(ant)3, that demonstrated the highest NIR luminescence efficiency, was successfully used as an emission layer of a host-free OLED and its electroluminescence quantum efficiency, corresponding to the sole band at 1000 nm, reached 0.21%. This performance could be achieved due to the high quantum yield of Yb(ant)3, which reached 1.5% and was increased up to 2.5% by partial Yb3+ substitution with Lu3+, as well as its high electron mobility due to the extended stacking in its crystal structure. The first gadolinium-based PHOLED was prepared based on Gd(ant)3.

Influence of grain size on optical properties of Sr2CeO4 nanocrystals

The absorption, excitation, and emission spectra of the Sr2CeO4 nanocrystals prepared by the modified sol-gel method were investigated. The impact of the average grain size of Sr2CeO4 nanocrystals on their optical properties was investigated. It was observed that with increasing the average grain size of Sr2CeO4 nanocrystals, the emission decay times decreased significantly. A similar behavior was observed for the emission quantum efficiencies and the Huang-Rhys factors. The grain size dependence of optical parameters of Sr2CeO4 nanocrystals was found well fitted by functions of the reciprocal of the grain diameter. It was shown that this dependence may be rationalized assuming that the correction for electric local field associated with effective refractive index affecting the spherical nanoparticle is governed by its shell.

Laser induced white lighting of graphene foam

Laser induced white light emission was observed from porous graphene foam irradiated with a focused continuous wave beam of the infrared laser diode. It was found that the intensity of the emission increases exponentially with increasing laser power density, having a saturation level at ca. 1.5 W and being characterized by stable emission conditions. It was also observed that the white light emission is spatially confined to the focal point dimensions of the illuminating laser light. Several other features of the laser induced white light emission were also discussed. It was observed that the white light emission is highly dependent on the electric field intensity, allowing one to modulate the emission intensity. The electric field intensity ca. 0.5 V/μm was able to decrease the white light intensity by half. Origins of the laser-induced white light emission along with its characteristic features were discussed in terms of avalanche multiphoton ionization, inter-valence charge transfer and possible plasma build-up processes. It is shown that the laser-induced white light emission may be well utilized in new types of white light sources.

Synthesis and up-conversion luminescence of Er3+ and Y b3+ codoped nanocrystalline tetra- (KLaP4O12) and pentaphosphates (LaP5O14)

The up-converting nanocrystals of KLa0.95Er0.05Y bxP4O12 and La0.95-xEr0.05Y bxP5O14 were prepared using co-precipitation method. The spectroscopic properties of these materials were investigated in a function of Y b(3+) concentration. The up-conversion emission, power dependence of emission intensities, and the luminescence decay times were investigated. It was found that the green to red and (2)H11/2 → (4)I15/2 to (4)S3/2 → (4)I15/2 emission intensity ratio were strongly affected by the Y b(3+) concentration. Moreover, the order of up-conversion emission and threshold power rises up with Y b(3+) concentration for (4)S3/2 → (4)I15/2 transition. The luminescence decay time of the (4)S3/2 → (4)I15/2 emission increases with Y b(3+) concentration while the (4)F9/2 → (4)I15/2 emission is independent of dopant concentration. The influence of the Y b(3+) concentration on the up-conversion emission intensities was discussed in terms of concentration dependent hetero looped photon avalanche process. A comparison of the up-conversion properties of KLa0.95Er0.05Y bxP4O12 and La0.95-xEr0.05Y bxP5O14 nanocrystals was presented.