Structural and Optical Interplay in Ultrafast-Decay Alkaline-Earth Rare-Earth Fluoride Nanoparticles for Novel Gamma Ray Scintillators

Alkaline-earth rare-earth fluoride (MLnF) nanoparticles have been extensively studied over the past decade for a plethora of applications ranging from energy storage to biolabeling and bioimaging. Here, we investigate MLnF core-shell nanoparticles doped with lanthanide ions with a fast spontaneous emission rate as promising building blocks for scintillators. Scintillators are a key component of various medical imaging systems including positron emission tomography (PET) and they consist of materials that can efficiently convert high energy photons to the UV-Vis regime, more specifically from gamma rays to optical frequency light in the case of PET. To tune the emission to the UV-Vis regime and minimize the decay lifetime, we systematically investigate the MLnF nanoparticles and the impact of various compositional factors including host lattice, choice of dopant and concentration of dopant.<br/>To fulfill the criteria for an ideal scintillator (high Z, high light yield, short rise, and decay time), we synthesize monodisperse, sub 20nm diameter MLnF core-shell nanoparticles doped with high Z trivalent lanthanides with concentrations systematically varying from 5%-45%. We select SrLuF with effective Z number of 54.5 as the host lattices of choice, an undoped SrLuF as the inert shell composition of choice and Ce3+, Pr3+ trivalent lanthanide ions with allowed interconfigurational f-d transitions (fast spontaneous emission rate) as choice of dopants. To reduce surface quenching, we shell these nanoparticles with varying thicknesses of 5-15nm. During the two-step synthesis, the core nanoparticles serve as seed crystals with surface nucleation sites for epitaxial growth of shell layers via dropwise hot injection approach.<br/>We employ a range of optical characterization techniques to identify the compositional dependence of Pr3+, Ce3+ 4f→5d (2F7/2+ 2F 5/2) decay lifetime and emission wavelength via photoluminescence (PL) and time-correlated single photon counting (TCSPC) techniques. The measured decay lifetime (τ) for Ce³⁺ 4f→5d varies between ~20ns to ~288ns with the shortest lifetime belonging to the 5% Ce3+ sample and longest lifetime belonging to the 45% Ce3+ sample.In addition, high resolution transmission electron microscopy (HRTEM) in conjunction with cathodoluminescence (CL) are used to correlate the microstructural properties with localized optical response. Finally, various assembly pathways for building 3D artificially designed nanophotonic structures using the MLnF nanoparticles as basic building blocks with the ability to manipulate light and create a highly directional scintillation process will be discussed. This understanding is a key step to develop next-generation highly efficient scintillator materials.