Spin-allowed and spin-forbidden fd emission from Er3+ and LiYF4
ABSTRACT Luminescence measurements in the vacuum ultraviolet of 4f11-->4f10 5d transitions on Er3+ in LiYF4 have shown the presence of a weak fd band. The weakness of the band is explained by the spin-forbidden character of transitions from the ground state to this fd state, which is at lower energy than the spin-allowed fd bands. Upon excitation in the spin-allowed fd bands, both fast spin-allowed fd emission (tau=5 ns) and slow spin-forbidden fd emission (tau>2 mus) are observed. These results can be understood with a configurational coordinate diagram analogous to the Jablonski diagrams that are used to explain the fluorescence and phosphorescence from organic molecules.
- Optics Express 01/2008; 16(16). · 3.55 Impact Factor
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ABSTRACT: Luminescent materials with the quantum efficiency (QE) higher than unity could be playing a significant role in the progress of lighting industry and certain electronic display systems. The recent demonstration of an efficient visible quantum cutting (QC) in vacuum ultraviolet (VUV)-excited LiGdF4:Eu phosphors [Wegh RT, Donker H, Oskam KD, Meijerink A. Visible quantum cutting in LiGdF4:Eu3+ through downconversion. Science 1999; 283: 663–6] has provided an exciting and interesting trends in the development of several potentially important luminescent materials and devices. The possibility of the higher QE depends on the principle of QC in phosphors which could generate two or more low-energy photons for every incident high-energy photon that is being absorbed by phosphors. Investigation on QC systems has started on single ions doped-fluorides capable of a cascade emission from ions such as Pr3+, Tm3+, Er3+ and Gd3+. The focus has now been shifted to the combination of two ions, where the energy of the donor ion could be transferred stepwise to two acceptor ions via a downconversion. A well-known example is the Gd3+–Eu3+ dual ions. QC via downconversion has now been widely witnessed in many rare earths (RE)-based phosphors, the interesting and appreciable QE in the visible spectral region has earlier been reported from LiGdF4:Eu (190%) and BaF2:Gd,Eu (194%) phosphors. QC materials could also be used in solar cells, if conversion of one UV–visible photon into two near-infrared (NIR) photons is realized, and energy loss due to thermalization of electron–hole pairs is minimized. The present article reviews on the recent progress made on: (a) materials and developments in the fields of UV–visible QC phosphors and the mechanism involved, including QC in single RE ion activated fluorides- and oxides-based phosphors, energy transfer and downconversion, QC in dual/ternary ions activated phosphors; and (b) NIR QC in RE3+–Yb3+ (RE=Tb, Tm, and Pr) dual ions doped phosphors via cooperative energy transfer. Appropriate discussions have been made on materials, materials synthesis and characterization, the structural and luminescence properties of various QC luminescent materials via different synthesis techniques. In addition, applications, challenge and future advances of the visible- and NIR-QC phosphors have also been dealt with.Progress in Materials Science - PROG MATER SCI. 01/2010; 55(5):353-427.
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ABSTRACT: The current interest in returning human exploration to the Moon and Mars makes cost-effective and low-mass health monitoring sensors essential for spacecraft development. In space, there are many surface measurements that are required to monitor the condition of the spacecraft including: surface temperature, radiation dose, and impact. Through the use of tailored phosphors, these conditions can be monitored. Practical space-based phosphor sensors will depend heavily upon research investigating the resistance of phosphors to ionizing radiation and their ability to anneal or "self-heal" from damage caused by ionizing radiation. For the present research, a group of lutetium orthophosphate (LuPO<sub>4</sub>) crystals with dopants including europium, erbium, and neodymium were characterized. Cathodoluminescence (CL) testing was performed using the low energy electron system located at the NASA Marshall Space Flight Center in Huntsville, Alabama. The data were collected using an Ocean Optics HR4000 spectrometer and a fiber optic feed-through. Previous research has shown that increases in both beam energy and current density improved the CL fluorescence yield. While the total electron dose was small, the intention was to maximize the number of irradiated materials. Additionally, these samples were evaluated using a PTI Quantum Master Spectrophotometer to determine the photoluminescence emission spectra.IEEE Transactions on Nuclear Science 07/2008; · 1.22 Impact Factor