Energy transfer and luminescent properties in Tb3+ and Eu3+ co-doped CaMoO4/SrMoO4 thin films

A series of Eu3+ and Tb3+ co-doped CaMoO4/SrMoO4 luminescent thin films were prepared by a facile solution method, and they were annealed at 550 °C for 2 h. The luminescent properties of the thin films were studied, which involve the energy transfer from Tb3+ to Eu3+. The emission color can be changed from green to red, with increasing Eu3+ doping concentration in Tb3+-doped CaMoO4/SrMoO4 thin films. In addition, it was observed that the PL intensity of Eu3+ will enhance when Tb3+ ions are incorporated into Eu3+-doped CaMoO4/SrMoO4 thin films. The optical band gaps of the luminescent thin films are found to be in the range of 4.49 to 4.72 eV. These results revealed that Eu3+ and Tb3+ co-doped CaMoO4/SrMoO4 luminescent thin films have a significantly potential application in electroluminescent devices.

The optical band gap of Eu 3+ and Tb 3+ co-doped CaMoO 4 /SrMoO 4 luminescent materials are significantly narrower than that of rare earth doped oxides, which means they may be used as the emitting layer in electroluminescent thin film devices. The luminescent thin films have a high potential application in plasma display panel [26], field emission display [27,28], cathode ray tubes [29]. Therefore, developing an efficient and facile solution approach to deposit highly luminescent rare earth doped CaMoO 4 /SrMoO 4 thin films is desirable. According to the previous reports, numerous methods were adopted to synthesize rare earth doped CaMoO 4 /SrMoO 4 phosphors, such as solid-state reaction [30,31], sol-gel method [32] and coprecipitation [33], but only few papers were focused on rare earth ions-doped CaMoO 4 /SrMoO 4 thin films.
In this work, we developed a facile solution method to fabricate Eu 3+ and Tb 3+ co-doped CaMoO 4 /SrMoO 4 luminescent thin films. Their luminescent properties and energy transfer between Tb 3+ and Eu 3+ were investigated. Tb 3+x precursor solution. Next, the thin film was deposited on a quartz substrate by spin-coating the precursor solution at 2000 rpm for 15 s and was sintered at 250°C for 1 min. Then, the thin film was further annealed at 550°C   Tb x 3 thin films, the emission peaks are attributed to the transition of Eu 3+ ions from 5 D 0 to 7 F 0 , 7 F 1 , 7 F 2 , 7 F 3 and 7 F 4 levels, which locate at 545, 594, 616, 656, and 704 nm, respectively [34,35]. But it was observed that the PL intensity at 616 nm (Eu 3+ : from 5 D 0 to 7 F 2 ) firstly increases and then decreases with the increase of Tb 3+ doping concentration, indicating the existence of the energy-transfer from Tb 3+ to Eu 3+ . We found that the optimal composition (the highest PL intensity) is

Materials and experimental
Under the irradiation of UV254 light, the emission colors of the thin films can be changed from red to orange with the increase of Tb 3+ doping concentration, as shown in figure 1(b figure 1(d). Apart from Tb 3+ and Eu 3+ characteristic emission peaks, a broad emission peak at around 400 nm was observed, which can be ascribed to the oxygen vacancy-related intrinsic defects.
Apart from CaMoO 4 host, the luminescent properties and energy transfer were also investigated for Tb 3+ and Eu 3+ ions co-doped SrMoO 4 thin films. The PL spectra of   2groups. The strongest absorption peaks locate at around 230 nm for the four samples. In addition, the prominent absorption peaks should be due to the band-to-band transition rather than the transition of impurity levels. The (αhv) 2 ∼hv curves of four samples are displayed in figures 4(a)-(d), respectively. the optical band gap (E g ) can be obtained by plotting (αhv) 2 ∼hv curves, where hv is the light energy, and α is the absorption efficient [38,39]. By extrapolation of linear portion of (αhv) 2 ∼hv curves, the E g of +

Structure
The crystal structures of the thin films were determined by x-ray diffraction (XRD). (b) thin films, respectively. It was found that the XRD patterns of thin films are in good agreement with JCPDS card 29-0351 [10] and 08-0482 [40], respectively. Therefore, it can be concluded that the doping Tb 3+ and Eu 3+ ions do not alter the crystal structures of CaMoO 4 and SrMoO 4 , i.e. the Eu 3+ and Tb 3+ ions prefer to occupy the Ca 2+ or Sr 2+ sites in CaMoO 4 or SrMoO 4 lattice.

SEM analysis
The surface morphology of In this work, all of the characterizations were conducted for the thin films spin-coated once except for the cross-sectional SEM image. It can be found that the thin film is consisted of dense nanoparticles with a mean particle size of 45 nm.

Conclusions
In conclusion, the luminescent properties of Eu 3+  thin films. These rare-earth ions co-doped luminescent thin films have a huge potential application in light-emitting diodes as the emitting layer or the photo-conversion layer.