Iko Hyppänen

Environmental and Excitation Power Effects on the Ratiometric Upconversion Luminescence Based Temperature Sensing Using Nanocrystalline NaYF4:Yb3+,Er3+

The luminescence intensity ratio (LIR) of the green emissions of the near-infrared excited NaYF4 :Yb3+ ,Er3+ nanocrystals is a promising method for temperature sensing. Here, the influence of excitation power density, excitation pulse length, excitation wavelength, silica shell, and solvent on the LIR and its temperature response is reported. The primary objective is to study the LIR mechanism and the impact of measurement and environmental parameters on the calibration and precision of the LIR. The LIR value is demonstrated to be unaffected by the excitation intensity in the studied range. This result is essential, considering the application feasibility of the LIR method as temperature sensor, where the effective excitation power density depends on the sample matrix and the distance excitation light travels in the sample. The pulsed excitation, however, results in an increase in the LIR value upon short pulse width. Silanization of bare nanocrystals has no effect on the LIR values, but the local warming of H2 O samples under laser exposure results in slightly increased LIR values compared to other solvents; D2 O, oleic acid, and dimethyl sulfoxide. The thermal quenching of luminescence lifetimes of Er3+ emission is proved to be too weak for sensing applications.

Frequency-Domain Measurement of Luminescent Lanthanide Chelates

The sinusoidal modulation of excitation intensity and phase-sensitive detection of emission is ideally suitable for the accurate determination of the lifetime and intensity of lanthanide luminescence. In this work we elaborate on the general mathematical and instrumental techniques of the frequency-domain (FD) measurements in the low-frequency domain below 100 kHz. A modular FD luminometer is constructed by using a UV-LED as the excitation source, proper light filters in the excitation and emission paths, a photomultiplier with a fast preamplifier, and a conventional dual-phase lock-in amplifier. Starting from the set of linear differential equations governing the excited-state processes of the lanthanide chelates, an equation linking the luminescence intensity to the general form of the excitation modulation was derived. Application to the sinusoidal modulation in the Eulers exponential form gives the expression for the in-phase and out-of-phase signals of a dual-phase lock-in amplifier. It is shown that by using a relatively large number of logarithmically equidistant modulation frequencies it is possible to use the Kramers-Kronig relation for checking the compatibility of the out-of-phase and in-phase signals. As an example, the emission from two different europium(III) chelates were measured by using 200 modulation frequencies between 10 Hz and 100 kHz. In addition to the conventional transition between (5)D(0) and (7)F(2) levels emitting at 615 nm, also the emission from the transition between (5)D(1) and (7)F(1) levels at ca. 540 nm was measured. The latter emission was also measured at different temperatures, yielding the energy difference between the (5)D(1) and (5)D(0) levels. The relatively large number of modulation frequencies allows also an accurate determination of lifetimes and corresponding amplitudes by using an appropriate nonlinear regression method. Comparison of the time-domain and frequency-domain methods shows that the weighting of data is different and both methods have application areas of their own.

Mechanisms of Tenebrescence and Persistent Luminescence in Synthetic Hackmanite Na8Al6Si6O24(Cl,S)(2)

Synthetic hackmanites, Na8Al6Si6O24(Cl,S)2, showing efficient purple tenebrescence and blue/white persistent luminescence were studied using different spectroscopic techniques to obtain a quantified view on the storage and release of optical energy in these materials. The persistent luminescence emitter was identified as impurity Ti(3+) originating from the precursor materials used in the synthesis, and the energy storage for persistent luminescence was postulated to take place in oxygen vacancies within the aluminosilicate framework. Tenebrescence, on the other hand, was observed to function within the Na4(Cl,S) entities located in the cavities of the aluminosilicate framework. The mechanism of persistent luminescence and tenebrescence in hackmanite is presented for the first time.

Preparation and Characterization of Nanocrystalline ZrO2:Yb3+,Er3+ Up-conversion Phosphors

Nanocrystalline up‐converting phosphors with zirconium oxide (ZrO2) as the host lattice were prepared with combustion and sol–gel methods. Impurities were analyzed with Fourier transform infrared (FT‐IR) spectroscopy. Yb3+ absorption was studied in the wave number region 10,000–11,500 cm−1 at room temperature and at 10 K. The whole‐blood absorption was measured in the region 9100–41,600 cm−1 at room temperature. Up‐conversion luminescence was excited at room temperature with an IR‐laser at 977 nm. The up‐conversion luminescence spectra showed red (650–685 nm) and green emission (520–560 nm) due to the 4F9/2 → 4I15/2 and (2H11/2, 4S3/2) → 4I15/2 transitions of Er3+, respectively. The materials prepared with combustion synthesis were found to yield the most efficient up‐conversion intensity and the longest luminescence decay.

Frequency-Domain Measurements

In the frequency-domain measurements of luminescence, the excitation intensity is modulated sinusoidally and the emission detected using a phase-sensitive amplifier. The present availability of conveniently modulatable light sources, such as light-emitting diodes and diode lasers, and relatively inexpensive lock-in amplifiers makes this technique well suited for the determination of lanthanide luminescence. The mathematical theory of luminescent lanthanide systems involves the application of matrix and complex analysis to the set of linear differential equations of the rate processes. The general solution is derived for the temporal populations of the excited species in the presence of an arbitrary functional form of excitation. The sinusoidal excitation and dual-phase lock-in detection of the emission provide a signal which can be expressed as a complex quantity with real and imaginary parts. It is shown that the imaginary part of the signal, i.e., the out-of-phase signal of the lock-in amplifier, is less prone to the interference from organic prompt fluorophores and external sources. The Kramers–Kronig relation can be used for checking the mutual compatibility of the real and imaginary parts of the signal. Two examples are given for the instrumentation and data treatment. The first example is the resonance energy transfer from a europium chelate to an organic acceptor held at a constant distance from the donor by oligonucleotide hybridization. The second example deals with the upconversion material, a mixture of lanthanide compounds. In both cases, the signal-to-noise ratio is excellent, allowing even the estimation of the continuous lifetime distribution.