Effects of local symmetry on upconversion emission mechanisms under pulsed excitation

Lowering the local symmetry around the lanthanide dopant represents a well-established strategy to enhance the quantum yield and colour selectivity of nanoparticle upconversion (UPC) emission. Here, we show that, under pulsed excitation at two near-infrared (NIR) wavelengths, the UPC emission of the Er activator (no Yb sensitizer) is comparable and more than two orders of magnitude (110 times) stronger in higher (tetragonal, D-2d) compared to in lower (monoclinic, C-1) local symmetry, a result that contrasts with the general trend observed for cw excited UPC. The much longer-lived reservoir levels of Er in the higher local symmetry host determine a higher contribution of the energy transfer mechanism (ETU) over excited state absorption (ESA), despite the fact that the two types of nanoparticles have an identical Er concentration and share similar structural properties. Time-resolved UPC excitation spectra (TREXS), which correlate the absorption in the spectral regions around 980 and 1530 nm and UPC emission during the timescale of Er green (around 550 nm), red (around 680 nm) and NIR (around 810 nm) emission, emerge as a valuable investigation tool. Using TREXS, we show that the local symmetry influences not only the emission intensity but also the relative contribution of the UPC mechanisms, which can be spectrally tuned by the excitation wavelength. Considering the advantages of pulsed over cw excitation of lanthanide based UPC nanoparticles, our results are relevant for bioimaging, thermometry and lifetime multiplexing applications.