prof. dr. A. Meijerink (Andries)

Luminescence spectroscopy of lanthanides and quantum dots

Andries Meijerink leads an active research group that focuses on the optical spectroscopy of lanthanide ions in solids and of colloidal semiconductor quantum dots. In the field of lanthanide ions his work involves fundamental research on the energy level structure of both 4fn and 4fn-15d states, energy transfer and finding new concepts related to applications in solar cells, LEDs, scintillators and thermometry. His research on quantum dots is aimed at unraveling the influence of quantum confinement and surface effects on the electronic structure and exciton dynamics of quantum dots through optical spectroscopy and using the quantum dots as labels in bio-imaging. Research on luminescence of doped nanocrystals integrates the two themes.

Some research highlights include:

  • Extending the energy level structure of lanthanides (Dieke diagram) into the vacuum ultraviolet and unravelling the energy levels of lanthanides above 40 000 cm-1 to 70 000 cm-1
  • Insight in the energy level structure of 4fn-15d states of lanthanides by combining high resolution (VUV) spectroscopy with energy level calculations to validate the rich fine structure observed.
  • Unraveling the origin of the efficient green emission from ZnO using quantum confinement effects in ZnO nanocrystals.
  • Discovery of downconversion where two-step energy transfer between lanthanides can result in splitting of one high energy photon into two (the reverse process of upconversion).
  • Understanding exciton dynamics in quantum dots and the role of the dark state in conventional II-VI and IV-VI quantum dots as well as, more recently, perovskite quantum dots.
  • Excited state excitation (ESE) spectroscopy for both intra-4fn and intra-4fn-15d transitions to provide insight in high energy levels of lanthanides, combined with (ab initio) energy level calculations to understand the experimental results.
  • Quantum cutting through cooperative energy transfer. This process, predicted in 1957 by Dexter, was discovered for the (Tb, Yb) couple through modelling decay dynamics using Monte Carlo simulations.
  • Quantum cutting for solar cells through downconversion in lanthanide couples. Internal quantum efficiencies close to 200% were demonstrated for the (Pr, Yb) and (Er, Yb) couples.
  • Insight in the quenching mechanism and color tuning of white light LED phosphors as Ce3+-doped garnets and Eu2+-doped (Ca,Sr,Ba) oxynitrides.
  • Understanding of the influence of photonic effects on decay rates for electric dipole and magnetic dipole transitions as well as Förster energy transfer using lanthanide doped nanocrystals as probes.
  • Insight in phonon quenching processes in Ln-doped nanocrystals based on energy transfer shell models relying on exact distributions of lanthanide ions in nanocrystals.
  • Combining experiments and theory to provide a basis for better understanding and optimizing the accuracy of luminescence thermometry and fulfilling conditions for Boltzmann equilibrium.

Luminescence thermometry with (upconversion) nanocrystals
Luminescence (nano)thermometry is an increasingly important field for remote temperature sensing with high spatial resolution. Most typically, ratiometric sensing of the luminescence emission intensities of two thermally coupled emissive states based on a Boltzmann equilibrium is used to detect the local temperature. Dependent on the temperature range and preferred spectral window, various choices for potential candidates appear possible. In this project a strong theoretical basis is provided for a better understanding of basic principles that govern thermometry which enables optimization of the luminescent probe and host lattice. Applications are investigated, also in cooperation with other research groups, for example for temperature sensing in catalysis and microfluidics.

New phosphors for white light LEDs
The future of lighting is white light LEDs. Since the discovery of the blue LED and combining the blue LED with the yellow emitting YAG:Ce phosphor by Nichia in 1996, a revolution in lighting has started and extensive research is aimed at better phosphors for spectral conversion of the blue LED light. An important topic is finding narrow band red and narrow band green phosphors as well as phosphor with superior thermal stability and reduced ‘droop’ under high power densities in ultrabright light sources such as laser diode driven LEDs and concentrator rods. In all these topics research is conducted, also in cooperation with companies including Seaborough, Nichia and Signify, and provides insight in quenching mechanisms, new avenues for narrow band red and green phosphors and processes involved in saturation and droop behavior.

Lanthanide doped nanocrystals as probes for photonic effects and quenching mechanisms
Photonic effects on radiative decay processes and also non-radiative processes (energy transfer, multi-phonon relaxation) are not easily investigated in bulk materials because changing the dielectric properties of the host also induces other changes which influence radiative and non-radiative processes. Nanocrystals of sizes much smaller than the wavelength of light and doped with lanthanide ions are perfect probes for these effects. The research has provided understanding of the influence of photonic effects on radiative decay rates for electric dipole and magnetic dipole transitions, Förster energy transfer between lanthanide ions and phonon quenching processes in Ln-doped (upconversion) nanocrystals. Crucial in the analysis is using energy transfer shell models relying on exact distributions of lanthanide ions in nanocrystals. The outcome also has practical implications, for example for tuning energy transfer efficiencies and predicting shell thicknesses in core-shell nanocrystals to effectively suppress solvent quenching.

Optical properties of doped and undoped quantum dots
Quantum confinement effects in semiconductor nanocrystals continue to fascinate researchers worldwide. New challenges involve the newly discovered class of halide perovskite quantum nanocrystals (and double perovskites) and doping luminescent ions such as transition metals and lanthanides into these quantum dots. Understanding the decay dynamics and the role of the dark state in quantum dots, energy transfer from exciton to dopant and the chemistry of doping luminescent ions are some of the topics covered.