•The emission intensity of red and green UCL show a completely opposite effect with increasing Yb3 + ion.•The number of upconverting photons from a four and three-photon process change to one to two-photon processes with the increase in Yb3 + ion concentration.•The modification of UC emission through the interaction between Ln3 + ions and nano host.We reported the upconversion luminescence (UCL) properties of Er3 +-Yb3 + co-doped BiOCl semiconductor nanosheets synthesized by hydrothermal method. Under 980 nm excitation, the red and green UCL of Er3 + ions were observed to be populated by a four and three-photon process in the case of absent or low concentration Yb3 + dopant. However, an increase of Yb3 + dopants show a completely opposite effect on the emission intensity of red and green one, accompanying with the change of upconverting process. It indicates that the red-shifting absorption edge of semiconductor and the super saturation UC processes involved with Yb3 + and Er3 + doping in BiOCl semiconductor nanosheets, respectively, are mainly responsible for the above UC phenomena.Energy level diagram of Er3 +-Yb3 + co-doped BiOCl nanosheets with low Yb3 + concentration and possibly UC mechanisms under 980 nm excitation.Download high-res image (142KB)Download full-size image

AbstractThe exploration of novel long lasting red phosphors is still of importance due to expected commercial applications and scientific interests. In this work, we reported the red long lasting phosphorescence (LLP) from Eu3+ doped BiOCl semiconductor polycrystals. The LLP property of the red phosphor is relatively weak due to less trap density, but the excitation band of LLP stems from the energy gap transition of semiconductor, offering experimental evidence for energy transfer between BiOCl semiconductor and Eu3+ ions. Although the afterglow duration of Eu3+ doped BiOCl was short temporarily, this work may open a novel kind of red LLP phosphors.Schematic diagram of the long afterglow mechanism in the BiOCl:Eu3+

Photon avalanche (PA) is a seldom discovered phenomenon in upconversion (UC) materials on account of the requirement for high pumping intensities and strict construction conditions. Herein, we present the controllable PA emission behavior of violet, green, red and near infrared emission from the Er3+ ion in Er3+–Yb3+ co-doped BiOCl crystals. We show that the generation of “looping” and the cycle numbers of PA processes could be precisely controlled through the control of doped Yb3+ concentrations. Fluorescent probe effects from doping Eu3+ ions indicates that the manipulation of PA behaviors may arise from a strong dependence on the magnitude of the internal polarizable chemical environment of BiOCl crystals that are weakened on doping with Yb3+ ions rather than other rare-earth dopants. Our results may offer a new insight into the mechanism for constructing PA process and promote the potential applications of PA UC phosphors in solid state lasers and optical switches.

•Emissions centered at 1150 and 1300 nm originate from Bi0 and Bi+, respectively.•Energy transfer processes between Bi emitting centers and Yb3 + have been discussed.•Yb3 + ion can only sensitize for the emission of Bi0.Glasses with the composition 37.5SiO2–12.5P2O5–25Al2O3–25PbO–0.5Bi2O3–xYb2O3 (x = 0–1 mol%) were prepared and characterized. When excited by 690 and 808 nm, broadband near infrared emissions of Bi ions centered at 1150 and 1300 nm, respectively, as well as the transition of Yb3 +: 2F5/2 → 2F7/2 could be observed simultaneously, and the emission intensity of Yb3 + was enhanced with increasing of Yb2O3 concentration. Energy transfer processes between Bi emitting centers and Yb3 + were discussed according to the effects of Yb2O3 addition on the near infrared emissions from Bi ions.

The luminescent properties of Bi activated M5(PO4)3Cl (M = Ca, Sr and Ba) prepared under the air and mild reducing conditions (5% H2 and 95% N2) were investigated. Results show that all samples present ultraviolet (UV) emissions of Bi3+ besides Ba5(PO4)3Cl prepared in reducing condition, which also shows broadband yellow–white and near infrared (NIR) luminescence attributed to univalent bismuth (Bi+). Emission data with the size available lattice sites of samples prepared in air suggests there are two types of Bi luminescent center, and each located in one of the two available M2+ lattice sites. In the case of Ba5(PO4)3Cl crystal, Bi3+ incorporated on Ba2+(1) sites can be reduced to Bi+ for reasons of charge compensation and size match of ionic radius.Highlights► Spectroscopic properties of Bi activated chlorophosphates are investigated. ► Only Ba5(PO4)3Cl:Bi prepared in reducing air show broadband visible and NIR emissions. ► Broadband yellow–white and NIR emission are ascribed to originate from Bi+. ► Existence of Bi+ is related to the accommodation and structure of crystal lattices.

A transparent glass ceramic tri-doped with Ce3+/Er3+/Yb3+ was fabricated by the high-temperature melting technique and following heat-treatment. X-ray diffraction and transmission electron microscope results demonstrated that Ca5(PO4)3F(FAP) nanocrystals, possessed with preferable emission performances for the 1.54 μm transition for doping Er3+, were homogeneously precipitated among the glass matrix with a mean size of 30 nm. Addition of Ce3+ greatly enhanced 1.54 μm fluorescence of Er3+ by the cross relaxation energy transfer between Er3+ and Ce3+. Meanwhile, incorporation of Ce3+ dramatically decreased the visible upconversion emission intensity of glass ceramic than that of glass, suggesting that Ce3+ might incorporate into the FAP nanocrystals. The properties of this transparent glass ceramic showed the potential application as an efficient 980 nm pumped infrared laser medium.NIR emission spectra of Er3+ in glass ceramic samples with different contents of CeO2, excited at 980 nm

•Transparent Eu2+–Yb3+ co-doped glass ceramic containing nano-Ca5(PO4)3F was fabricated.•Downconversion luminescence from Eu2+ to Yb3+ was enhanced by precipitating FAP nanocrystals.•Phonon energy of FAP crystal remedy energy gap between Eu2+ to Yb3+.Eu2+–Yb3+ co-doped transparent glass–ceramic containing nano-Ca5(PO4)3F (FAP) was prepared in reducing atmosphere. XRD and TEM analysis indicated that nano-FAP about 40 nm precipitated homogeneously in glass matrix after heat treatment. Confirmed by spectroscopy measurements, the crystal-like absorption and emission of Eu2+ indicated the partition of Eu2+ into FAP nanocrystals in glass ceramic. NIR emission due to the transition 2F5/2→2F7/2 of Yb3+ ions (about 980–1100 nm) was observed from glasses under ultraviolet excitation, ascribed to downconversion from Eu2+ to Yb3+, which can be enhanced by precipitating nano-FAP crystals. The results indicated that Eu2+–Yb3+ co-doped glass–ceramic embedding with nano-FAP is a promising candidate as downconversion materials for enhancing conversion efficiency of solar cells.

Bismuth ions doped Ba10(PO4)6Cl2 was synthesized by solid state reaction and its spectral properties were investigated. The results showed that the samples prepared in air only presented typical ultraviolet (UV) luminescence of Bi3+ at 370 nm, however broadband yellow–white and near infrared (NIR) emission could be observed in the samples prepared in reductive atmosphere, indicating those broadband emissions are related to the presence of low valent bismuth species. Correlating excitation and emission data with the charge of available lattice sites suggest that Bi3+ coexist with Bi+, and each locate on one of the two available Ba2+ lattice sites. We propose that broadband yellow–white and NIR emissions are ascribed to the related transitions of Bi+ ion.Highlights► Broadband yellow–white and NIR emissions are found from Bi-doped Ba10(PO4)6Cl2. ► Bi+ is considered as the origin of broadband visible and NIR emission. ► The stabilization of Bi+ is attributed to the unique structure of apatite crystal.

Glasses with compositions of 34.5SiO2–25AlPO4–12.5Al2O3–12.5B2O3–15R2O–0.5Bi2O3 (R = Li, Na, K) were synthesized. Characteristic absorption bands of bismuth centered at 500, 700, and 800 nm and broadband near infrared (NIR) emissions peaked at 1100 and 1300 nm were observed. With the change of alkali ion (Li+ → Na+ → K+), the intensity of absorption bands at 500 and 800 nm increase, while the intensity of two NIR emissions decrease. The dependence of NIR emission peaked at 1300 nm on the radius of the alkali ion is found to be opposite to that in previous reports, being ascribed to the abnormal reduction of Bi ion in the glasses investigated herein. The results of this work is helpful for understanding the nature of Bi-NIR-emission, and the NIR emission peaked at 1300 nm was proposed to originate from Bi+ according to the influence of the alkali ions sizes.Highlights► NIR emission from Bi-doped aluminophosphsilicate glasses increase with increase in alkali ion radius. ► Tetrahedral anion groups in glass host can stabilize low valent Bi ions. ► NIR emission at 1300 nm was proposed from Bi+ according to the charge and the radius of alkali ion.

Tetragonal structure La1–xBixOCl polycrystals were prepared by solid state synthesis. With increase in bismuth content, the morphology of the prepared samples changed from non-oriented particles to layered crystals and the broadband blue-white emission from LaOCl polycrystals decreased dramatically. Comparative experiments and X-ray photoelectron spectroscopy (XPS) analysis indicated that the host emission more possibly originated from Cl self-doping in oxychloride crystals. According to a close composition relationship of the Cl self-doping behavior and crystal morphology, it was suggested that the La composition strengthened the interlayer interaction between Cl− anion and crystal cell lamellar, hindering the orientation of LaOCl crystals. The results of our work would deepen the understanding of the orientation structure of tetragonal oxyhalides and offer an insight into the origin of its host luminescence.Emission spectra of prepared samples under excitation at 270 nm (the above inset is the excitation spectra recorded at 350 nmDownload high-res image (137KB)Download full-size image

Photon avalanche (PA) is a seldom discovered phenomenon in upconversion (UC) materials on account of the requirement for high pumping intensities and strict construction conditions. Herein, we present the controllable PA emission behavior of violet, green, red and near infrared emission from the Er3+ ion in Er3+–Yb3+ co-doped BiOCl crystals. We show that the generation of “looping” and the cycle numbers of PA processes could be precisely controlled through the control of doped Yb3+ concentrations. Fluorescent probe effects from doping Eu3+ ions indicates that the manipulation of PA behaviors may arise from a strong dependence on the magnitude of the internal polarizable chemical environment of BiOCl crystals that are weakened on doping with Yb3+ ions rather than other rare-earth dopants. Our results may offer a new insight into the mechanism for constructing PA process and promote the potential applications of PA UC phosphors in solid state lasers and optical switches.