Upconversion of infrared to visible light in rare-earths doped phosphate phosphors for photodynamic therapy application
Mokoena, Puseletso Pricilla
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Phosphate phosphors have emerged as an important family of luminescent material due to their low sintering temperature, broad band gaps, high thermal and chemical stability, and moderate phonon energies. Their structure can provide a wide range of possible cationic substitutions since there are different inequivalent sites of metal ions presenting a large-scale of size and coordination spheres. Rare earth ions doped phosphate compounds as luminescence materials have been widely investigated in different host lattices including phosphates. In this study, the luminescent properties of different phosphate phosphors doped with Er3+, Eu3+, and Yb3+ were investigated. Er3+ and Yb3+ singly doped, and Er3+/Yb3+ co-doped Ba5(PO4)3OH phosphor powders were successfully synthesized by the urea combustion method. The X-ray Diffraction (XRD) patterns exhibited hexagonal structure for Ba5(PO4)3OH referenced in the ICDD (International Center for Diffraction Data) Card Number 00-024-0028. There were no peak shifts nor secondary peaks observed suggesting that pure phases were crystallized. The Scanning Electron Microscope (SEM) image showed that the particles were agglomerated together forming ellipsoidal shapes. The Energy Dispersive x-ray Spectroscopy (EDS) spectra with intense peaks of Ba, P, and O were observed confirming the formation of Ba5(PO4)3OH. The particle size distribution of the Ba5(PO4)3OH powder was estimated from a statistical analysis by measuring approximately 10 particles. The average particles length and width were 867 and 169 nm, respectively. Upon excitation using a 980 nm laser, multiple emission peaks in the green region and red region were observed corresponding to the transition of the Er3+ ion. By further co-doping with Yb3+ the red emission was enhanced due to energy transfer from Yb3+ to Er3+. Ba5(PO4)3OH co-doped with Eu3+ and Yb3+ phosphors were prepared by the urea combustion method. The diffraction peaks of Ba5(PO4)3OH were indexed to the pure hexagonal phase, referenced in ICDD Card Number 00-024-0028. The SEM images showed a change (ranging from rods, spherical, needle-like to non-uniform particles) in surface morphology which was due to annealing and addition of dopants. The size of the particles appeared to be larger/bigger when comparing as-prepared and annealed phosphor powders. This could be due to the annealing-induced expansion. The broad intense excitation peak at 240 nm and other excitation peaks located at ~319, 360, 382, 395 and 465-537 nm were assigned to transitions of Eu3+ ion. The emission peaks were observed at ~589, 614, 651 and 699 nm. Upon co-doping with Yb3+, the strong emission peak was observed at 657 nm assigned to the Eu3+ transitions. This was due to the cooperative energy transfer process. Er3+ and Yb3+ co-doped Ca5(PO4)3OH samples were synthesized by urea the combustion method. The XRD patterns of Ca5(PO4)3OH powders for both as-prepared and those annealed at 800 0C were attributed to the hexagonal phase of Ca5(PO4)3OH referenced in ICDD Card No. 00-073-0293. The SEM micrographs exhibited rod or plate-like morphology forming flowers, plate-like structures and small agglomerated particles on top of the plates. For Er3+ singly doped phosphors emission peaks were observed in the green region ranging from 517 -573 nm and red region in the range of 653- 679 nm. Ca5(PO4)3OH:Er3+ phosphors were prepared using different concentrations of Er3+ ranging from 1-7 mol.%. The photoluminescence intensity increased with increasing concentrations from 1 to 3 mol%, and decreased at high concentrations of 5 and 7 mol.% due to concentration quenching effects. Adding different concentrations (5-15 mol.%) of Yb3+. The emission intensities on both the green and red region increased with increasing concentrations of Yb3+ ions. The enhancement of green emission can be due to increasing of the three-photon energy transfer process probability between the Yb3+ and Er3+ ions. Ca5(PO4)3OH:Eu3+, Yb3+ phosphor powders were synthesized by the combustion method using urea as a fuel. The XRD patterns of Ca5(PO4)3OH powders for both as-prepared and those annealed at 800 0C were assigned to the hexagonal phase of Ca5(PO4)3OH referenced in ICDD Card No. 00-073-0293. The crystal sizes calculated for as-prepared and annealed powders were found to be 27 and 44 nm, respectively. UC emission spectrum of Ca5(PO4)3OH:Eu3+,Yb3+ phosphor powder was observed under 980 nm excitation. Prominent red emission from Eu3+ ion was clearly observed at 613 nm together with minor emission peaks at 547, 591, 654 and 697 nm. The prominent red emission from Eu3+ was due to energy transfer from Yb3+ ion. A cooperative energy transfer from Yb3+ ion pair to a single Eu3+ ion occurred by fast non-radiative relaxation to the metastable 5D0 state, and the red Eu3+ emission was observed. Sr5(PO4)3OH co-doped Er3+/Yb3+ phosphor powders were synthesized by combustion method. The XRD pattern diffraction peaks were consistent with the standard data referenced in ICDD Card No. 00-033-1348. The average crystallite size calculated was 43 ± 2 nm. The SEM micrographs showed that the powder was composed of agglomerated particles with edges forming hexagonal shapes. The agglomeration showed a porous structure resulting from the nature of the combustion reaction associated with the evolution of large volume of gases. Upon 980 nm excitation, Sr5(PO4)3OH:Er3+ exhibited multiple emission bands in the green region and a less intense peak in the red region. The strong red emission peak with two minor splits were observed at 661 nm, and (651 and 679 nm), by co-doping with Yb3+ ion. Sr5(PO4)3OH co-doped Eu3+/Yb3+ phosphor powders were synthesized by the combustion method. All the diffraction patterns matched with the standard data referenced by ICDD Card No. 00-033-1348. The SEM image showed that the powder composed of a network of particles with irregular shapes and small bright particles encrusted on the surface of the bigger particles. The particles containing heavy atoms in backscattered electron detector were stronger than light particles and they appear brighter. UC emission spectrum of Sr5(PO4)3OH:Eu3+,Yb3+ phosphor powder was observed under 980 nm excitation. Prominent red emission from Eu3+ ion was clearly observed at 658 nm due to cooperative energy transfer process. Photodynamic therapy uses special drugs, called photosensitizers, along with light to kill cancer cells. The drugs only works after been activated by certain kinds of light. Most drugs are activated by red light. The enhanced red luminescence from the above mentioned phosphors suitable to activate different photosensitizers for treatment of cancer or photodynamic therapy. Photodynamic therapy activity was performed using red emitting phosphors prepared in this study together with phthalocyanine as a photosensitizer. Phthalocyanine is activated by the wavelength ~670 nm. The activity results are discussed in chapter 10.