Highly aqueous soluble CaF2:Ce/Tb nanocrystals: effect of surface functionalization on structural, optical band gap, and photoluminescence properties

Abstract

The design of nanostructured materials with highly stable water-dispersion and luminescence efficiency is an important concern in nanotechnology and nanomedicine. In this paper, we described the synthesis and distinct surface modification on the morphological structure and optical (optical absorption, band gap energy, excitation, emission, decay time, etc.) properties of highly crystalline water-dispersible CaF:Ce/Tb nanocrystals (core-nanocrystals). The epitaxial growth of inert CaF and silica shell, respectively, on their surface forming as CaF:Ce/Tb@CaF (core/shell) and CaF:Ce/Tb@CaF@SiO (core/shell/SiO) nanoarchitecture. X-ray diffraction and transmission electron microscope image shows that the nanocrystals were in irregular spherical phase, highly crystalline (~20 nm) with narrow size distribution. The core/shell nanocrystals confirm that the surface coating is responsible in the change of symmetrical nanostructure, which was determined from the band gap energy and luminescent properties. It was found that an inert inorganic shell formation effectively enhances the luminescence efficiency and silica shell makes the nanocrystals highly water-dispersible. In addition, Ce/Tb-co-doped CaF nanocrystals show efficient energy transfer from Ce to Tb ion and strong green luminescence of Tb ion at 541 nm(D→F). Luminescence decay curves of core and core/shell nanocrystals were fitted using mono and biexponential equations, and R regression coefficient criteria were used to discriminate the goodness of the fitted model. The lifetime values for the core/shell nanocrystals are higher than core-nanocrystals. Considering the high stable water-dispersion and intensive luminescence emission in the visible region, these luminescent core/shell nanocrystals could be potential candidates for luminescent bio-imaging, optical bio-probe, displays, staining, and multianalyte optical sensing. A newly designed CaF:Ce/Tb nanoparticles via metal complex decomposition rout shows high dispersibility in aqueous solvents with enhanced photoluminescence. The epitaxial growth of inert CaF shell and further amorphous silica, respectively, enhanced their optical and luminescence properties, which is highly usable for luminescent biolabeling, and optical bioprobe etc.

Silica‐modified luminescent LaPO4:Eu@LaPO4@SiO2 core/shell nanorods: Synthesis, structural and luminescent properties

“…The emission intensity of silica functionalized core/shell/Si NRs was remarkably suppressed due to the surface‐modified silanol(Si–OH) groups. These surface‐modified silanol (Si–OH) molecules enhanced the non‐radiative transition rate and, as a result, quenched the emission intensity . This situation is the main drawback of silica surface coating, as silica surface‐modified luminescent NRs are hydrophilic in nature, whereas core/shell NRs are hydrophobic in nature.…”

“…The sol–gel Stober method was employed for silica surface modification. A solution containing 312 mg LaPO 4 :Eu@LaPO 4 NRs, 140 ml C 2 H 5 OH, 35 ml H 2 O and 2 ml NH 4 OH was ultrasonicated and later vigorously stirred on a hot plate at room temperature for 30 min to get homogenous mixture . Subsequently, 1 ml TEOS was added slowly into the foregoing reaction mixture and the reaction was allowed to proceed for 5–6 h under constant magnetic stirring.…”

“…Growth of a crystalline layer of an analogous material over the luminescent seed core NRs to form a core/shell structure has been regarded as an effective strategy to improve luminescence efficiency . These hybrid luminescent nanostructured materials have altered properties that can be tailored according to need, and show improved thermal and photochemical resistance, high chemical rigidity and good luminescence efficiency .…”

Facile Synthesis Method for the Preparation of Large‐scale Ultra‐small GdPO4:Tb and GdPO4:Tb@LaPO4 Nanowires

“…The excitation spectra of both samples were recorded with monitoring the 542 nm emission in the spectral range 300–500 nm (Figure ). The excitation spectra display several sharp excitation lines in the center of the UV–vis region at 482, 377, 367, 350, 339, 326, 317, 310, and 304 nm assigned to 7 F 6 → 5 D 4 , 7 F 6 → 5 D 3 , 7 F 6 → 5 G 6 , 7 F 6 → 5 L 10 , 5 G 5 , 7 F 6 → 5 L 8 , 7 F 6 → 5 D 1 , 7 F 6 → 5 D 0 interconfigurational 4f–4f transitions of the Tb 3+ ion . Figure shows the well‐known emission peaks of Tb 3+ at 487, 542, 585, and 618 nm, corresponding to 5 D 4 → 7 F 6 , 5 D 4 → 7 F 5 , 5 D 4 → 7 F 4 , and 5 D 4 → 7 F 3 intraconfigurational parity‐forbidden 4f–4f excitation electronic transitions originating from 5 D 4 ground state to the labeled excited state of Tb 3+ ions under monitoring excitation at 368 nm .…”

“…The excitation spectra display several sharp excitation lines in the center of the UV–vis region at 482, 377, 367, 350, 339, 326, 317, 310, and 304 nm assigned to 7 F 6 → 5 D 4 , 7 F 6 → 5 D 3 , 7 F 6 → 5 G 6 , 7 F 6 → 5 L 10 , 5 G 5 , 7 F 6 → 5 L 8 , 7 F 6 → 5 D 1 , 7 F 6 → 5 D 0 interconfigurational 4f–4f transitions of the Tb 3+ ion . Figure shows the well‐known emission peaks of Tb 3+ at 487, 542, 585, and 618 nm, corresponding to 5 D 4 → 7 F 6 , 5 D 4 → 7 F 5 , 5 D 4 → 7 F 4 , and 5 D 4 → 7 F 3 intraconfigurational parity‐forbidden 4f–4f excitation electronic transitions originating from 5 D 4 ground state to the labeled excited state of Tb 3+ ions under monitoring excitation at 368 nm . Among the observed transitions, 5 D 4 → 7 F 5 is the most prominent one, which is the so‐called hypersensitive transition, because it provides more information about the change in chemical environment during the formation of the new chemical bond between the host and the Tb 3+ ion .…”