Multi-color solar absorption as a synergetic UV up-conversion enhancement mechanism in LiYF 4 :Yb 3+ ,Tm 3+ nanocrystals

Motivated by the technologically important spectral conversion of sunlight for efficient photocatalysis, we present a detailed analysis of the multicolor excitation of LiYF4:Yb3+,Tm3+ nanocrystals leading to UV emission through upconversion. We demonstrate in particular that the combination of blue and IR light for generating upconversion UV emission is a linear mechanism that occurs at rather low density of excitation (a few mW/cm2). This upconversion efficiency is fully compatible with the broadband excitation from the sun, with comparable density similar to daylight excitation. It demonstrates that an appropriate design combining photocatalyst and a dedicated upconverter leads to a drastic improvement of the photocatalytic effect.