Modification of Ag shell on upconversion populating paths of NaYF4:Yb3+,Er3+@Ag nanocomposites

NaYF4:Yb3+,Er3+@Ag core–shell nanocomposites were prepared. Due to the outer shell of Ag, some modifications were found on the upconversion (UC) processes of NaYF4:Yb3+,Er3+ core nanoparticles (NPs) caused by the co-interaction of surface effect, surface plasma absorption effect and local thermal effect under infrared radiation. Upon 980 nm excitation, the relative UC intensity of the green (2H11/2, 4S3/2–4I15/2) to red (4F9/2–4I15/2) and the slope of power-dependence increased intensively, and three-photon population process for the green level appeared.