The influence of Ce3+ codoping and excitation scheme on spectroscopic properties of NaYF4:Yb3+,Ho3+

Abstract The possibility to design and intentionally tune the luminescent properties of phosphors, e.g. their emission colour, quantum yield and kinetic behavior, is of fundamental significance and a challenge for numerous applications. Usually, to obtain multicolour emission, multiple nanomaterials with different co-dopants combinations, sizes or chemical architectures are synthesized and characterized. Of special interest would be those materials, where on-demand and in-situ modifications of multicolour emission can be obtained by changing the external stimulus, such as magnetic field, temperature, excitation intensity or pulse width. Here we synthesize and evaluate luminescent NaYF4 nanoparticles doped Yb3+/Ho3+, whose red and green emission colour is modulated by variable concentration of Ce3+ admixture, excitation pulse duration and temperature. In the first scenario (Yb3+/Ho3+ only doping), such materials exhibit strong and stable green upconversion emission. Upon Ce3+ co-doping, the red to green ratio (RGR) changed from 0,43 (for pristine dopants) to 4,23 (for 15%Ce3+ co-doped samples). Moreover, the measurement performed with excitation pulse width modulation (PWM) enabled to increase contribution of red emission in-situ. For short excitation pulses, the green emission was observed, while for longer pulses the red emission started to dominate. Finally, the energy transfer in this tri-doped system demonstrated to be susceptible to external temperature as well, with relative thermal sensitivity of 0,8%/K in biological range for 5%Ce3+ doped sample. Detailed spectroscopic analysis including temperature dependent emission spectra, rise times and emission kinetics allow us to understand and propose the possible energy transfer mechanisms in this system.