Rare-earth elements are strong candidates for upconverting materials due to their relatively long-lived excited states.1-3 However, they are generally not efficient light absorbers.4-6 One of the approaches to enhance the optical absorption is adding a sensitizer layer (i.e.an extra semiconductor layer or using organic dyes as sensitizer with stronger absorption properties).7 On the other hand, the surface quenching effect decreases the efficiency of optical emission. This project explores the creation of undoped shells, sensitizer shells on rare earth nanoparticles and studies the effect of the size of the shell, semiconductor shells and dyes on the optical properties.NaYF4:Yb0.20, Er0.02 nanoparticlesare investigated specifically to reveal the effect of sensitizers on the absorption and emission properties. A solvothermal method is used to synthesize core NaYF4:Yb0.20, Er0.02 nanoparticles and NaYF4doped core-undoped shell particles.8-13 XRD and TEM indicated that pure β-NaYF4nanoparticles were synthesized. The size of NaYF4nanoparticles was monitored using reaction time. The emission spectrum revealed that growth of undoped NaYF4shells enhanced the emission intensity of doped core-undoped shell particles. This is presumably because the shell inhibits the nonradiative transition and the surface quenching on the surface ofNaYF4:Yb0.20, Er0.02core nanoparticles. CdS shells and ligand exchange with a dye absorbing in NIR were investigated as potential methods to enhance the absorption properties of NaYF4:Yb0.20, Er0.02 nanoparticles. TEM revealed that CdS segregated to form heterostructurewith NaYF4:Yb0.20, Er0.02 core nanoparticlesinstead of core-shell structure. This is likely due to the mismatch of CdS lattice to β-NaYF4.A NIR absorbing dye was coated to the NaYF4:Yb0.20, Er0.02 corenanoparticles via ligand exchange method. A color change was noticed after the ligand exchange with the nanoparticles. However, the emission properties and energy transfer process need further studies since the intensity at 542 nm and 660 nm are not pronounced under 806 nm excitation and 980 nm excitation.