In a crystalline matrix, the crysta

In a crystalline matrix, the crystal structure influences the emission of RE dopants. However, it is difficult to perform this investigation for commonly used powders or colloidal solutions with disordered arrangements of particles. While our luminescent single MC provides a good opportunity, particularly anisotropy of crystal. There are three crystallographic axes (a, b, and c) in a crystal structure, as a result, there should be three excitation configurations (σ, α, and π) (see Figure S8, Supporting Information). β-NaYF4 crystal lattice is thought to be uniaxial so that an optical axis does exist and the crystalline field is therefore expected to be anisotropic. In this situation, σ- and α-configuration are same for our single β-NaYF4 MC, so, π- and σ-configuration were chosen during the test process, as illustrated in Figure 3a. First, the UC PL spectra of a single β-NaYF4:20%Yb3+,3%Pr3+ microrod in each excitation configuration are included in Figure 3b. The nearly overlapped PL spectra in this figure manifest that the excitation configuration has no influence on emission bands for RE ion in β-NaYF4 structure. But from the UC PL spectra recorded at different excitation polarization angles and the polar plots of single β-NaYF4:20%Yb3+, x%Pr3+ (x = 3%, 5%, and 10%) MC, included in Figure 3d–g and Figure S9 (Supporting Information), we have observed obvious anisotropic emission spectra change depending on the excitation polarization orientation. In π-configuration, the UC luminescence intensity changes periodically with the excitation polarization angles varying from 0° to 360°, providing convergent polar plots, indicating clearly the presence of excitation polarization dependence. While in σ-configuration, the intensity of UC luminescence remains almost unchanged along the variation of excitation polarization orientation, consequently, an approximate circle is drawn in polar plates, demonstrating there is no excitation polarization dependence in this measurement.