Fig. 2: Investigation of energy migration in a sensitizer-activator codoped system.
From: Manipulating energy migration in nanoparticles toward tunable photochromic upconversion
a Schematic of the competition between energy migration and energy transfer in the NaYF4:Yb/Tm@NaYF4:Yb/Er@NaYF4:Nd core-shell-shell nanostructure under 808 nm excitation. The control sample structure without doping Er3+ in the interlayer is also presented. b Upconversion emission spectra of the NaYF4:Yb/Tm(30/1 mol%)@NaYF4:Yb/Er(20/0,2 mol%)@NaYF4:Nd(20 mol%) core-shell-shell nanoparticles under 808 nm excitation. c Emission intensities of Tm3+ as a function of Er3+ concentration in the interlayer for (b) samples. d The characteristic ratio parameter (γEM) versus Er3+ concentration for (c) samples and the simulation result of a normalized number of excited Yb3+ (Nex) traveling into the core region under steady-state excitation with an energy transfer rate of 9500 s−1. The error bars are defined as the average of γEM for the 450, 475, and 695 nm emissions. e, f The measured and simulated time-dependent upconversion emission profiles of Tm3+ at 695 nm for (c) samples under pulsed 808 nm excitation. g CIE chromatic coordinates of the upconversion emissions from (c) samples. Insets show the emission photographs with Er3+ concentrations from 0 to 2 mol%. h Upconversion emission spectra of NaYF4:Yb/Tm(30/1 mol%)@NaYF4:Yb/Er(20/0.1 mol%)@NaYF4:Nd(20 mol%) core-shell-shell nanoparticles by tuning pulse width of 808 nm laser with a frequency of 100 Hz. i Time-dependent upconversion emission profiles for (h) sample. j Schematic of a possible mechanism for the color-tunable upconversion under non-steady-state excitation. RET and NRET represent resonant and non-resonant energy transfer, respectively.
