The pH value and urea concentration of precursor dramatically impact on the particle size, morphology and upconversion luminescence of Y2O3:Yb3+, Er3+ nanophosphors. The lower pH leads to the smaller granula size of the final product, for instance, the crystalline size reduces from similar to 30 nm to similar to 16 nm as pH value varies from 9.3 to 6.7. The intensity of green (21111/2, 453/2 -> 4115/2) and red emission (4F9/2 -> 4115/2) can be effectively and readily tuned by adjusting the pH value and urea content of precursor solution. The sample derived at pH of 6.7 and urea content of 384 mg/mL exhibits the highest red to green ratio, as a result of more surface defects and hydroxyl residues. Besides, the upconversion lifetime investigation further proved that the quenching and nonradiative relaxation from 21111/2 and 453/2 to 4F9/2 caused by the surface defects could be responsible for the relative enhancement of the red emission.

Upconversion NaErF4:Yb,Gd nanocrystals with bright red emissions were prepared via a facile solvothermal method.The crystalline phase,size and the relative intensity of upconversion luminescence can be simultaneously manipulated by adjusting Gd3+ ions contents.The introduction of Gd3+ can effectively promote the cubic to hexagonal phase transformation,size reduction and obviously upconversion luminescence (UCL) intensity improvement of NaErF4:Yb,Gd nanocrystals.XRD,TEM and UCL spectra results reveal that the sample co-doped with 25% Gd3+ ions (mole fraction) of NaErF4:Yb system exhibits the optimized structural and optical properties.Meanwhile,the mechanism involving upconverting photon excitation and energy transfer between Yb3+ ions and Er3+ ions were investigated under the excitation of 980 nm diode laser.

Upconverting NaErF4:Yb3+,Gd3+ nanoparticles (NPs) and nanorods (NRs) with improved red emission have been successfully achieved via a facile hydrothermal route using oleic acid as the assistant surfactant. The crystalline phase, morphology even the size are simultaneously tuned by controlling the reaction temperatures and Gd3+ doping contents. The higher synthesis temperature leads to the morphology evolution from NPs to NRs. The integrated intensity ratio of red to green emissions is much improved for Gd3+ codoping nanostructures. The microstructure characterizations along with the steady and transient spectroscopy are performed to better understand the underlying mechanisms of phase evolution and emission enhancement. For the different states of Er3+, i.e. H-2(11/2) and F-4(9/2), the radiative/non-radiative transition probabilities could be affected by Gd3+ doping in different ways as for NPs and NRs, based on the lifetime and emission intensity data. NaErF4:Yb3+,Gd3+ nanosctructures are expected to have promising applications in multimodal bioimaging for deeper tissue penetration.

Mn2+/Fe3+ co-doped upconverting nanocrystals were prepared by hydrothermal method. The crystal phase, size and fluorescence intensity were modulated via changing the doping concen-trations. The maximum intensity of upconversion luminescence ( UCL) was observed in current work at the reaction time of 4 h when coating the silica shell of different thickness onto the NaYF4∶Yb, Er,Fe nanocrystals through facilely altering the reaction time. The integrated overall intensity of up-conversion luminescence was 3. 7 and 4. 5 times enhanced by co-doping Mn2+ and Fe3+, respective-ly. In particular, the red emission of NaYF4∶Yb,Er,Fe nanocrystals was 7 times intensified selec-tively compared to the green band of Er3+. The intensified visible UCL especially the dominant red emission is mainly ascribed to the energy transfer from 2 F7/2 , 4 T1g> ( Yb3+-Mn2+/Fe3+ dimer) to 4 F9/2 ( Er3+) states as well as the distortion of the crystalline field symmetry. The quenching of UCL as increasing dopants is stemming from the exchange interaction between 3d5 metal ions.