Up-conversion Emissions And Energy Transfer Processes In 12CaO·7Al₂O₃: Gd³⁺/Er³⁺/Yb³⁺ Phosphor

Short-wavelength compact solid-state laser has gained much more attention dueto the wide range of applications, such as high-density information storage,high-resolution printing, undersea communication, etc. Up-conversion (UC) emissionis researched in many rare earth (RE) ions dopant materials; it can convert the infraredto shorter wavelengths radiation. The RE ions can be excited by multi-photon UCprocess due to the long lifetime metastable levels of RE ions. Therefore, UC emissionis an efficient ways to achieve short-wavelength laser.The gadolinium ions (Gd³⁺) can be an activator for the UV emission. But it is cannot be excited by the visible or the near-infrared radiation directly, because of the wideenergy gap about 32000 cm-1. The erbium ions (Er³⁺) and the thulium ions (Tm³⁺) haveample metastable levels; they can be the sensitizer to transfer the energy to the Gd³⁺ions with the 980 nm laser in the fluorides host materials. However, the chemicaldurability and mechanical strength of the fluorides are low and cause serious problemsfor the applications. Thus, an alternative host material with comparable performanceshould be developed. The 12CaO·7Al₂O₃ (C12A7) is an important cement materialwith excellent mechanical characteristic, so that, it is a potential host material due tothe high chemical durability and the low phonon energy.The Gd³⁺-Er³⁺-Yb³⁺ codoped C12A7 nano-powders are prepared by sol-gelmethod and excited by 980 nm laser. The XRD results reveal the samples are C12A7and the SEM shows the distribution rage of the particle is 350 nm-500 nm,respectively. The ultraviolet emissions centered at 312 nm of Gd³⁺, corresponding to6P7/2â†'8S7/2, are observed. The emissions from Er³⁺ ions are also observed at 324 nm,365 nm, 380 nm, 410 nm, 480 nm, 522 nm, 550 nm and 660 nm assigning to2P3/2â†'4I15/2, 2G9/2â†'4I15/2, 4G11/2â†'4I15/2, 2H9/2â†'4I15/2, 4F3/2/4F5/2â†'4I15/2, 2H11/2â†'4I15/2,4S3/2â†'4I15/2, 4F9/2â†'4I15/2. The 6P7/2 levels of Gd³⁺ ions can be efficiently excited byenergy transfer process of Yb³⁺â†'Er³⁺â†'Gd³⁺ and the energy back transfer processenhanced UC emissions peaked at 365 nm, 410 nm and 480 nm of Er³⁺ ions. Theexcitation and emission spectra of C127A: Gd³⁺/Er³⁺/Yb³⁺ prove the emission band at312 nm is attributed to Gd³⁺ ions. The pump power dependence ofC127A:Gd³⁺/Er³⁺/Yb³⁺ on UC luminescence suggest two-photon, three-photon,four-photon, and five-photon process are occurred.