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High purity green up-conversion (UC) materials have wide applications in fluorescent labeling, plastic recycling and displays, etc. However, Yb3+/Er3+-based UC materials exhibit poor color purity of green light, and their output color is normally unstable under a variable excitation power density, which hinders their application in the aforementioned areas. Herein, we report the highly stable and efficient pure green UC of rod-like β-NaGdF4:Yb3+,Ho3+ submicro-crystals synthesized via a newly developed ion-exchange modified (IEM) method. IEM-derived NaGdF4:Yb3+,Ho3+ monodisperse submicro-crystals present not only a high-purity green output color (due to the large intensity ratio of green to red, IGreen/IRed ≈ 5) but also excellent color stability under varying conditions, such as excitation power density and doping concentration of Yb3+/Ho3+. More importantly, IEM arouses an enormously enhanced UC emission intensity of ∼3700-fold that of the precursor crystals (PC). The UC emission intensity of IEM β-NaGdF4:Yb3+,Ho3+ is ∼4 times stronger than that of its high temperature annealing (HTA) counterpart. The emission intensity of the single green band is ∼40% stronger than that of the IEM β-NaGdF4:20%Yb3+,2%Er3+ submicro-crystals. A proof-of-concept fluorescent labeling of clover patterns (15 × 12 mm) printed from NaGdF4:Yb3+,Ho3+ ink with stable and high purity green fluorescence is demonstrated under excitation with various power densities.

Scintillators used in X-ray detectors typically require the use of heavy metal atoms to efficiently harvest ionizing radiation. Now the use of halogens is shown to yield efficient, metal-free organic scintillators.

We report efficient dual-mode up-conversion (UC) and down-shifting (DS) emission in a single Yb3+/Er3+-co-doped β-NaYF4 microcrystals with controlled morphology and size via a simple Na+ ion-exchange modification (IEM) method. IEM well preserves the crystal structure and monodispersed morphology of hydrothermal-synthesized β-NaYF4. Meanwhile, IEM gives rise to the significant enhancement of UC emission intensity up to 3800 times and strongly enhanced DS emission intensity of Er3+ and Yb3+ by several times in β-NaYF4:Yb3+,Er3+ microcrystals. IEM also strongly prolongs the DS emission lifetimes of Er3+ and Yb3+ in visible and near-infrared region. The enhanced UC and DS emission intensities and prolonged lifetimes in β-NaYF4:Yb3+,Er3+ are mainly ascribed to the dispersing of localized Yb3+ and Er3+ clusters during IEM.

Monodisperse β-NaYF4:Yb3+,Tm3+ (NYF) hexagonal microplates with efficient near-infrared (NIR)-to-NIR up-conversion (UC) developed via a new ion-exchange modified hydrothermal method are reported. Ion exchange modification (IEM) not only significantly increases the UC intensity by up to 1000 times and prolongs the emission lifetimes of Tm3+ and Yb3+, but also enables the monodisperse morphology and size of hexagonal microplates to be maintained. A high UC quantum efficiency (QE) of 3.1% is obtained for IEM β-NaYF4:20%Yb3+,1%Tm3+ when excited with 980 nm light at a power density of 10 W cm−2. The UC emission properties can be finely tailored by changing the NaF/NYF molar ratio (NYF represents lanthanide doped β-NaYF4) and the doping concentration of Tm3+. The two photon NIR UC emission centered at ∼803 nm (arising from Tm3+: 3H4 → 3H6) dominates the UC emission spectrum and high concentrations of Tm3+ favor NIR UC further. A proof-of-concept optical image of printed patterns is demonstrated to verify their applications in security. These results suggest the promising applications of the newly developed monodisperse β-NaYF4:Yb3+,Tm3+ hexagonal microplates in security, luminescent labels, solid-state lasers, amplifiers, and biomedicine.

Efficient upconversion (UC) microcrystals are important for the development of novel photonic markers, solid-state lasers, displays, etc. Herein, we report on greatly enhanced UC emission in β-NaYF4:20Yb,2Er microcrystals via a simple ion-exchange reaction. The ion exchange accompanying with an ionic redistribution process is demonstrated. By simply controlling the amount of NaF (0–8.0 mmol), reaction temperature (140–220 °C), and duration (0–144 h), enormous enhancement of the UC emission up to ~4200 times is obtained compared with the pre-synthesized β-NaYF4:20Yb,2Er microcrystals. The decrease of body defects and change of local structure around lanthanide may account for the UC emission enhancement. This method may apply to other UC materials to increase the UC emission efficiency.

A series of single-phase Ca9Ln(PO4)7:Eu2+,Mn2+ (Ln = Gd, La, Lu) phosphors with enhanced quantum yields were successfully developed through a topochemical reduction reaction strategy by using elemental aluminum as the reducing agent. Changes were observed both in the spectral shapes and photoluminescence intensities. New broadband emission covering the whole red region and centered at 630 nm from the remote Al reduced Ca9Ln(PO4)7:Eu2+ phosphors was observed, and their PL intensity was found to be greatly enhanced. The remote Al reduced Ca9Gd(PO4)7:Eu2+ reaches 4.3 times higher PL than the phosphors prepared by the traditional reduction method under CO atmosphere with the optimal Eu2+ dopant content. Finally, enhanced white-light emissions were gradually obtained by co-doping Eu2+ and Mn2+ in Ca9Ln(PO4)7, and the PLQY value is raised from extremely low to 61.6%. The mechanism for the changes of luminescence behavior was studied and discussed. This research also provides an enlightening reference for the preparation and development of high efficiency single-phase white light emitting phosphors.

Bi-doped siliconoxynitride (SiON:Bi) thin films were prepared by a sputtering method and the photoluminescence (PL) properties were studied. Without any thermal treatments, broad Bi-related luminescence was observed in the near-infrared (NIR) range. The luminescence efficiency depended strongly on the film composition. It was found that N atoms play a crucial role for the formation of Bi NIR luminescence centers. The effect of annealing on the luminescence efficiency was also studied. The optimum annealing temperature to have the largest number of Bi NIR luminescence centers depended strongly on the film composition and it was lower for films with lower N concentration. The PL excitation spectra revealed that two different Bi NIR luminescence centers exist in the films.

Er3+/Tm3+/Yb3+ tricloped oxyfluoride glass ceramics was synthesized in a general way. Under 980 nm LD pumping, intense red, green and blue upconversion was obtained. And with those primary colors, multicolor luminescence was observed in oxyfluoride glass ceramics with various dopant concentrations. The red and green upconversion is consistent with F-4(9/2) -> I-4(15/2) and H-2(11/2), S-4(3/2) -> I-4(15/2) transition of Er3+ respectively. While the blue upconversion originates from (1)G(4) -> H-3(6) transition of Tm3+. This is similar to that in Er3+/Yb3+ and/or Tm3+/Yb3+ codoped glass ceramics. However the upconversion of Tm3+ is enhanced by the energy transfer between Er3+ and Tm3+. (c) 2006 Published by Elsevier B.V.

Fluorophosphate glasses with various content of Al(PO 3 ) 3 were prepared. With the increment of Al(PO 3 ) 3 content, density decreases while refractive index increases, and transition temperature, crystallization peak temperature and melt temperature increase which were suggested by differential scanning calorimetry. These glasses exhibit the best stability against crystallization with 7–9 mol% Al(PO 3 ) 3 content. Normalized Raman spectra were used to analyze structure and phonon state. The increment of Al(PO 3 ) 3 content does not affect phonon energy but results in the augment of phonon density. Absorption spectra were measured. 3 H 6 → 3 F 4 transition exhibits absorption at L band of the third communication window. Compared with the energy of Tm 3+ excited states in other glass system, 3 F 4 energy of Tm 3+ in these glasses is considerable higher and 3 H 4 energy is considerable lower, and it can be predicted that emission band of 3 H 4 → 3 F 4 transition is close to the amplified band of gain-shift Tm 3+ doped fiber amplifier. Analyses of Judd–Ofelt theory suggest when Al(PO 3 ) 3 content is no more than 7 mol%, Judd–Ofelt parameters Ω t and the lifetime of 3 H 4 energy level of Tm 3+ vary little with the increment of Al(PO 3 ) 3 content, and when Al(PO 3 ) 3 content is more than 7 mol%, Ω 2 and Ω 6 increase and radiative lifetime of 3 H 4 energy level of Tm 3+ drops sharply with the increment of Al(PO 3 ) 3 content.

Tm 3+ –Yb 3+ codoped oxyfluoride silicate glasses suitable for upconversion laser has been fabricated. In this paper, effect of CdF 2 addition on thermal stability and upconversion luminescence properties in Tm 3+ –Yb 3+ codoped oxyfluoride silicate glasses have been systematically investigated. The experimental results indicate that, with the substitution CdF 2 for PbF 2 , the glass thermal stability increases and the UV cutoff edge moves to short-wave band slightly. With increasing CdF 2 content, the blue and red upconversion luminescence intensity increases slightly at first, and then increases rapidly. While the near infrared (NIR) upconversion emission intensity increases notably at first and then increases slightly. However, the blue and NIR luminescence intensity are much stronger than that of red, indicating these oxyfluoride silicate glasses are more preferable for blue and NIR emissions than red emission. The possible upconversion mechanisms for the blue, red and NIR fluorescence are also estimated and evaluated.

Er 3+ -doped oxyfluoride germanate glasses have been synthesized by the conventional melting and quenching method. The Judd–Ofelt intensity parameters were calculated based on the Judd–Ofelt theory and absorption spectra measurements. With the substitution of PbF 2 for PbO, the Ω 2 parameter decreases, while the Ω 6 parameter increases. These change trends indicate that fluoride anions come to coordinate erbium cations and the covalency of the Er–O bond decreases. Structural and thermal stability properties were obtained by Raman spectra and differential thermal analysis, indicating that PbF 2 plays an important role in the formation of glass network and has an important influence on the maximum phonon energy and thermal stability of host glasses. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 , respectively, were simultaneously observed at room temperature. With increasing PbF 2 content, the intensity of red (657 nm) emissions increases significantly, while that of the green (525 and 546 nm) emission increases slightly. The results indicate that PbF 2 has more influence on the red (657 nm) emission than the green (525 and 546 nm) emissions in oxyfluoride germanate glasses. The possible upconversion luminescence mechanisms have also been estimated and discussed.

Er3+/Yb3+-codoped novel oxyfluoride bismuth–germanium glass was prepared and its up-conversion fluorescence property under 975 nm excitation has been studied. Intense green and weak red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2, respectively, were observed at room temperature. The possible up-conversion mechanism was also evaluated. The optimal Yb3+–Er3+ concentration ratio is found based on the direct lifetime measurements of excited levels for Er3+ ion. The structure of this novel oxyfluoride bismuth–germanium glass has been investigated by peak-deconvolution of FT-Raman spectrum, and the structural information was obtained from the peak wavenumbers. This novel oxyfluoride bismuth–germanium glass with relatively lower maximum phonon energy (∼731 cm−1) can be used as potential host material for up-conversion lasers.

In a three-components fluorophosphate glass system, the introduction of H3BO3 brings some valuable influence to the spectroscopic and thermal properties of the glasses. With H3BO3 increases from 2 to 20mol%, ohm(6), S-ed4113/2, FWHM, T-g and fluorescence lifetime change from 3.21 x 10(-20) cm(2), 1.77 x 10(-20) cm(2), 45 nm, 480 degrees C and 8.8 ms to 4.66 x 10(-20) cm(2), 2.11 x 10(-20) cm(2), 50 nm, 541 degrees C and 7.4 ms, respectively. sigma(abs), sigma(emi), FWHM x tau(f) x sigma(emi) has a maximum when H-3 BO3 is 11 mol%. T-g and T-x-T-g increases with H3BO3 introduction. Results showed that in fluorophosphate glasses, proper amount of B2O3 can be used as a modifier to suppress upconversion and improve spectroscopic properties, broadband property and crystallization stability of the glasses while keeps the fluorescence lifetime relatively high. (c) 2005 Elsevier B.V. All rights reserved.

Tm 3+ /Yb 3+ -codoped heavy metal oxide-halide glasses have been synthesized by conventional melting and quenching method. Structural properties were obtained based on the Raman spectra, indicating that halide ion has an important influence on the phonon density and maximum phonon energy of host glasses. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions 1 G4 → 3 H6 and 1 G4 → 3 H4, respectively, were observed at room temperature. The possible up-conversion mechanisms are discussed and estimated. With increasing halide content, the up-conversion luminescence intensity and blue luminescence lifetimes of Tm 3+ ion increase notably. Our results show that with the substitution of halide ion for oxygen ion, the decrease of phonon density and maximum phonon energy of host glasses both contribute to the enhanced up-conversion emissions. © 2005 Elsevier B.V. All rights reserved.

The upconversion properties of Er 3 + / Yb 3 + codoped tellurite glasses and glass fibers with D-shape cladding under 980 nm excitation were investigated. Intense emission bands centered at 531, 546 and 658 nm corresponding to the transitions Er 3 + : H 11 / 2 2 → I 15 / 2 4 , S 3 / 2 4 → I 15 / 2 4 and F 9 / 2 4 → I 15 / 2 4 , respectively, were observed at room temperature. Compared with that in Er 3 + / Yb 3 + codoped tellurite bulk glass, the upconversion luminescence becomes more efficient in the fiber geometry. The dependence of upconversion intensities on fiber geometry and possible upconversion mechanism are discussed and evaluated. The presented Er 3 + / Yb 3 + codoped tellurite fibers with intense upconversion luminescence can be used as potential host materials for upconversion fiber lasers.

Yb3+/Tm3+-codoped oxychloride germanate glasses for developing potential upconversion lasers have been fabricated and characterized. Structural properties were obtained based on the Raman spectra analysis, indicating that PbCl2 plays an important role in the formation of glass network and has an important influence on the maximum phonon energies of host glasses. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions 1G4 → 3H6 and 1G4 → 3H4, respectively, were observed at room temperature. With increasing PbCl2 content, the intensity of blue (477 nm) emission increases significantly, while the red (650 nm) emission increases slowly. The results indicate that PbCl2 has more influence on the blue emissions than the red emission in oxychloride germanate glasses. The possible upconversion mechanisms are discussed and estimated. Intense blue upconversion luminescence indicates that these oxychloride germanate glasses can be used as potential host material for upconversion lasers.

Structural and infrared-to-visible upconversion fluorescence properties in ytterbium-sensitized erbrium-doped novel lead-free germanium–bismuth–lanthanum glass have been studied. The structure of lead-free germanium–bismuth–lanthanum glass was investigated by peak-deconvolution of Raman spectrum, and the structural information was obtained from the peak wavenumbers. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 , respectively, were observed at room temperature. The quadratic dependence of the 525, 546, and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation. This novel lead-free germanium–bismuth–lanthanum glass with low maximum phonon energy (∼ 751 cm −1 ) can be used as potential host material for upconversion lasers.

Broadband and upconversion properties were studied in Er3+/Yb3+ co-doped fluorophosphate glasses. Large Omega(6) and S-ed/(S-ed + S-md) values and the flat gain profile over 1530-1585 nm indicate the good broadband properties of the glass system. And a premise of using Omega(6) as a parameter to estimate the broadband properties of the glasses is proposed for the first time to our knowledge. Results showed that fluorescence intensity, upconversion luminescence intensity, the intensity ratio of red/green light (656 nm/545 nm) are closely related to the Yb3+:Er3+ ratio and Er3+ concentration, and the corresponding calculated lifetime of F-4(9/2) and S-4(3/2) states for red and green upconversion samples proves this conclusion. The upconversion mechanism is also discussed. (c) 2005 Elsevier Ltd. All rights reserved.

Effect of PbF2 on Yb3+ -doped fluorophosphate glasses is studied. Results indicate that proper amount of PbF2 has absolute advantages in improving the crystallization stability of fluorophosphate glasses. T, value performs a decreasing and increasing tendency with 25 mol% PbF2 as the turning-point. And the spectroscopic properties such as absorption and emission cross section, effective fluorescence linewidth are apparently enhanced with PbF2 over 25 mol%. Lasing parameters beta, I-sat and I-min increase slightly with the addition of PbF2. Raman analysis proves that over 20 mol% PbF2, destroys the phosphate vibration groups greatly. (c) 2005 Elsevier Ltd. All rights reserved.

Tm 3+ -doped oxide–chloride germanate and tellurite glasses have been synthesized by conventional melting method. Intense up-conversion luminescence emissions were simultaneously observed at room temperature in these glasses. The possible up-conversion mechanisms are discussed and estimated. However, in these Tm 3+ -doped glasses, tellurite glass showed weaker up-conversion emissions than germanate glass, which is inconsistent with the prediction from the difference of maximum phonon energy between tellurite and germanate glasses. In this paper, Raman spectroscopy was employed to investigate the origin of the difference in up-conversion luminescence in the two glasses. Our results confirm that, besides the maximum phonon energy, the phonon density of host glasses is also an important factor in determining the up-conversion efficiency.

Tm3+/Yb3+-codoped gernianate-niobic (GN) and germanium-bismuth (GB) glasses have been synthesized by conventional ruching and quenching method. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions (1)G(4)->H-3(6) and (1)G(4)->H-3(4), respectively, were observed at room temperature. The possible Up-conversion mechanisms are discussed and estimated. GN glass showed a weaker up-conversion emission than GB glass, which is inconsistent with the prediction from the difference of maximum phonon energy between GN and GB glasses. In this paper, Raman spectroscopy was employed to investigate the origin of the difference in up-conversion luminescence in the two glasses. Compared with phonon side-band spectroscopy, Raman spectroscopy extracts more information including both phonon energy and phonon density. For the first time, our results reveal that, besides the maximum phonon energy, the phonon density of host glasses is also an important factor in determining the up-conversion efficiency. (c) 2005 Elsevier Ltd. All rights reserved.

Effect of fluoride ions introduction on structural, OH − content and up-conversion luminescence properties in Er 3+ -doped heavy metal oxide glasses have been investigated. Structure was investigated, indicating that fluoride has an important influence on the phonon density, maximum phonon energy of host glasses. With increasing fluoride content, the up-conversion luminescence intensity and quantum efficiencies increase notably, which could not be explained only by the maximum phonon energy change of host glasses. Our results show that, with the introduction of PbF 2 , the decrease of phonon density and OH − content contributes more to the enhanced up-conversion emissions than that of maximum phonon energy.

Er 3+ /Yb 3+ -codoped potassium–barium–strontium–lead–bismuth glasses for developing potential upconversion lasers have been fabricated and characterized. Based on the results of energy transfer efficiency, the optimal Yb 3+ /Er 3+ concentration ratio is found to be 5:1. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 , respectively, were observed. The quadratic dependence of the 525, 546, and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation. The long-lived 4 I 11/2 level is supposed to serve as the intermediate state responsible for the upconversion processes.

New lithium–barium–lead–bismuth glasses with low OH− concentration have been obtained. The role of the different components in the glass formation has been explored from the thermal, density, and refractive index measurements. The Tg, Tx, and Tx−Tg values of these glasses are in the range of 358–400, 453–575, and 87–197 °C, respectively. The densities (ρ) and refractive indices of these glasses are mainly affected by Bi2O3 and PbO contents. A wide transmitting window from visible to infrared (IR) regions for some compositions of these glasses has been observed, which makes them appealing candidates for different optical applications such as upconverting phosphors, new laser materials, optical waveguides, and crystal-free fibre drawing.

Er 3+ /Yb 3+ -codoped bismuthate glasses for developing potential upconversion lasers have been fabricated and characterized. The optimal Yb 3+ doping content was investigated in the glasses with different Yb 3+ –Er 3+ concentration ratios and the optimal Yb 3+ –Er 3+ concentration ratio is 5:1. Under 975 nm excitation, intense green and red emissions centered at 525, 546 and 657 nm, corresponding to the transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 , respectively, were observed at room temperature. The quadratic dependence of the 525, 546 and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation.

Infrared-to-visible upconversion fluorescence property of Er3+/Yb3+-codoped novel bismuth–germanium glass under 975 nm LD excitation has been studied. Intense green and red emissions centered at 525, 546 and 657 nm, corresponding to the transitions 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2, respectively, were observed at room temperature. The quadratic dependence of the 525, 546 and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs. The structure of the bismuth–germanium glass has been investigated by peak-deconvolution of FT-Raman spectrum, and the structural information was obtained from the peak wavenumbers. This novel bismuth–germanium glass with low maximum phonon energy (∼750 cm−1) can be used as potential host material for upconversion lasers.

Er3+ -doped oxychloride germanate glasses have been synthesized by conventional melting and quenching method. Structural and thermal stability properties were obtained based on the Raman spectra and differential thermal analysis, indicating that PbCl2 plays an important role in the formation of glass network and has an important influence on the maximum phonon energy and thermal stability of host glasses. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions H-2(11/2) -> I-4(15/2), S-4(3/2) -> I-4(15/2), and F-4(9/2) -> I-4(15/2), respectively, were observed at room temperature. With increasing PbCl2 content, the intensity of green (525 and 546 nm) emissions increases significantly, while the red (657 nm) emission increases slowly. The results indicate that PbCl2 has more influence on the green emissions than the red emission in oxychloride germanate glasses. The possible upconversion luminescence mechanisms has also been estimated and discussed. (c) 2005 Elsevier Ltd. All rights reserved.

Er3+-doped lithium-barium-lead-bismuth glass for developing upconversion lasers has been fabricated and characterized. The Judd-Ofelt intensity parameters Omega(t) (t = 2, 4, 6), calculated based on the experimental absorption spectrum and Judd-Ofelt theory, were found to be Omega(2) = 3.05 x 10(-20) cm(2), Omega(4) = 0.95 x 10(-20) cm(2), and Omega(6) = 0.39 x 10(-20) cm(2). Under 975 nm excitation, intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions H-2(11/2) -> I-4(15/2), S-4(3/2) -> I-4(15/2), and F-4(9/2) -> I-4(15/2), respectively, were observed at room temperature. The upconversion mechanisms are discussed based on the energy matching and quadratic dependence on excitation power, and the dominant mechanisms are excited state absorption and energy transfer upconversion for the green and red emissions. The long-lived I-4(11/2) level is supposed to serve as the intermediate state responsible for the intense upconversion processes. The intense upconversion luminescence of Er3+-doped lithium-barium-lead-bismuth glass may be a potentially useful material for developing upconversion optical devices. (c) 2004 Elsevier B.V. All rights reserved.

Structural and infrared-to-visible upconversion fluorescence properties of Er3+/Yb3+-codoped oxychloride lead–germanium–bismuth glass have been studied. The Raman spectrum investigation indicates that PbCl2 plays an important role in the formation of glass network, and has an important influence on the upconversion luminescence owing to lower phonon energy. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2, respectively, were observed at room temperature. The quadratic dependence of the 525, 546, and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation.

Er 3+ -doped strontium lead bismuth glass for developing upconversion lasers has been fabricated and characterized. The Judd–Ofelt intensity parameters Ω t ( t = 2, 4, 6), calculated based on the experimental absorption spectrum and Judd–Ofelt theory, were found to be Ω 2 = 2.95 × 10 −20 , Ω 4 = 0.91 × 10 −20 , and Ω 6 = 0.36 × 10 −20 cm 2 . Under 975 nm excitation, intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 , respectively, were observed. The upconversion mechanisms are discussed based on the energy matching and quadratic dependence on excitation power, and the dominant mechanisms are excited state absorption and energy transfer upconversion for the green and red emissions. The long-lived 4 I 11/2 level is supposed to serve as the intermediate state responsible for the upconversion processes.

Er 3+ /Yb 3+ -codoped strontium–lead–bismuth glasses for developing potential upconversion lasers have been fabricated and characterized. The optimal Yb 3+ doping content was investigated in the glasses with different Yb 3+ –Er 3+ concentration ratios and the optimal Yb 3+ –Er 3+ concentration ratio is 5:1. Under 975-nm excitation, intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 , respectively, were observed at room temperature. The quadratic dependence of the 525-, 546-, and 657-nm emissions on excitation power indicates that a two-photon absorption process occurs under 975-nm excitation.

Er3+/Yb3+-codoped natrium–barium–strontium–lead–bismuth glasses for developing potential upconversion lasers have been fabricated and characterized. Based on the results of energy transfer efficiency and upconversion fluorescence spectra, the optimal Yb3+–Er3+ concentration ratio is found to be 5:1. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions H 11 / 2 2 → I 15 / 2 4 , S 3 / 2 → I 15 / 2 4 and F 9 / 2 2 → I 15 / 2 4 , respectively, were observed at room temperature. The quadratic dependence of the 525, 546, and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation. The long-lived I 11 / 2 4 level is supposed to serve as the intermediate state responsible for the upconversion processes.

Er 3+ /Yb 3+ -codoped potassium–magnesium–lead–bismuth glasses for developing potential upconversion lasers have been fabricated and characterized. Based on the results of energy transfer efficiency and upconversion spectra, the optimal Yb 3+ –Er 3+ concentration ratio is found to be 5:1. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 , respectively, were observed. The quadratic dependence of the 525, 546, and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation. The high-populated 4 I 11/2 level is supposed to serve as the intermediate state responsible for the upconversion processes. The decay times of the emitting states and the corresponding quantum efficiency were determined and explained.

The humidity sensitivity of silica-coated α-hematite sintered pellets was investigated by volt-amperometric and impedance spectroscopy techniques. A 130 m2/g specific area α-hematite powder was dispersed in 3-amino-propyl-trihydroxy-silane solution to yield a 0–4 wt.% Si. To obtain a 0.5 molar ratio of Si/Fe, α-Fe2O3 powder was dispersed in tetraethyl orthosilicate (Si(OEt)4). Sintering was carried out at 50°C steps in the temperature range 850–1100°C. Pellets were characterised by SEM, HRTEM, mercury intrusion and nitrogen adsorption techniques and He-adsorption densimeter. The electrical response of Si-coated a-Fe2O3 was characterised at low temperatures (20–70°C), to evaluate the ionic conduction mechanism of water molecules adsorbed on the surface of the metal oxide and at high temperatures (

Fluorophosphate glasses codoped with Tm3+ and Yb3+ were prepared and their thermal stability, phonon states, and upconversion properties were studied. It is found that the increment of phosphate content is good for the thermal stability but increases the phonon density of states. However, the phonon density of states of these fluorophosphate glasses is very low due to the low phosphate content in their composition. The upconversion luminescence spectra were measured under excitation of 970 nm laser diode, and the intense blue (476 nm) and near infrared (794 nm) emission were simultaneous obtained at room temperature. The sensitizing mechanisms of Yb3+ to Tm3+ for blue and red emission contain both sequential and cooperative sensitization. The near infrared emission is a two-photon upconversion process. These researches suggest that when the phosphate content in the composition is low enough, fluorophosphate glass can be suitable host material of Tm3+ codoped with Yb3+ for blue and near infrared upconversion luminescence.

A 5 MHz AT-cut quartz-crystal resonator has been totally immersed in distilled water at different temperatures between 5 and 95°C. The oscillation has been characterized by an impedance analyzer around the fundamental frequency. The complex impedance has been found to increase with rising water temperature. An equivalent circuit including inductance, resistance and capacitance has been designed to simulate the experimental data within an error of 3.5%. The simulated parameters (resistance, capacitance, inductance, etc.) have been related to the physical properties of water by comparison with literature data. This fundamental approach results in the realization of a simple and cheap monitor for practical use in continuously detecting the general pollution of waste water in terms of the impedance modulus and resonance frequency of the immersed quartz resonator.

α-Fe 2 O 3 thin films have been coated on silicon and sapphire substrates with Ag, Au and Pt electrodes by sol-gel processing. Differential thermal analysis and X-ray diffraction have highlighted that the film crystallization occurs at temperatures higher than 400 °C. After heat treatments at 400, 500 and 600 °C for 30 min, the 200 nm-thick films have α-Fe 2 O 3 cystallites about 50 nm in mean size, estimated from the half-width of the (104) diffraction peak. The electrical response of the films to various gases including NO 2 and CO has been tested in the temperature range 100–300 °C. The sensors with Pt and Au electrodes show the largest resistance changes for NO 2 at 210 °C and for CO at 300 °C, respectively. For instance, the sensitivity, defined as the ratio between the resistances in gas and in air, is 36 for a Pt-electrode sensor in 100 ppm NO 2 , and 1.6 for an Au-electrode sensor in 600 ppm CO. No CH 4 and NO sensitivities have been found in the investigated samples. The electrode effect on the gas sensitivity has been reasoned in terms of the Pt and Au catalyst reaction to NO 2 and CO on α-Fe 2 O 3 films.

The NO, NO 2 and NOx gas-sensitivity properties of Nb-doped α-Fe 2 O 3 sintered compacts have been studied in the 0–100 ppm gas concentration and 150–300 °C temperature ranges, by d.c. and a.c. techniques. Sensors have been prepared by suspending a 130 m 2 g −1 α-Fe 2 O 3 powder in a standard Nb solution in order to yield Nb/Fe atomic percentages between 0.5 and 20 at.%. Sintering has been performed at 800 °C for 2 h. XRD patterns show corundum-type α-Fe 2 O 3 occurrence and FeNbO 4 phases when the Nb doping is greater than 10 at.%. The 20% doped material shows a gas sensitivity ( S ), defined as R G / R A , where R A and R G are the electrical resistances in air and in the sample gas, respectively, as high as 36 at 100 ppm NO 2 and 200 °C working temperature. No sensitivity to NO has been measured at 100 ppm NO by utilizing N 2 as a carrier gas. The NOx sensitivity has been discussed in terms of the actual NO and NO 2 gas concentrations measured by an electrolytic gas sensor at the exit of the sample chamber. The conversion of NO to yield NO 2 has been experimentally measured to be 6% if a residence time of 60 s and an O 2 concentration higher than 2 vol.% are chosen. The NO 2 response is unaffected by the presence of NO gas. An electrical equivalent circuit including a constant phase element (CPE), which can simulate the electrical response of the sensor in the 0–100 ppm NO 2 gas concentration range, is also presented.

Electromechanical resonance phenomena of liquid-immersed quartz resonators were found to be associated with the viscoelastic and dielectric properties of glycerol and water mixtures. For AT-cut resonators immersed in the liquid, frequency dependences of impedance, phase angle, capacitance and loss tangent were carefully measured in the frequency range of oscillation. The experimental results show that the frequency spectra of capacitance and loss tangent are favorable for determining the properties of the liquid. Some preliminary relationships between the liquid properties and the resonance patterns have been made by using resonant and non-resonant data, i.e. frequency, capacitance and loss tangent.

Er3+, Yb3+ and Tm3+ codoped fluorophosphate glasses emitting blue, green and red upconversion luminescence at 970 nm laser diode excitation were studied. It was shown that Tm3+ behaves as the sensitizer to Er3+ for the green upconversion luminescence through the energy transfer process: Tm3+:3H4 + Er3+:4I15/2 → Er3+:4I9/2 + Tm3+:3H6, and for the red upconversion luminescence through the energy transfer process: Tm3+:3F4 + Er3+:4I11/2 → Tm3+:3H6 + Er3+:4F9/2. Moreover, Er3+ acts as quenching center for the blue upconversion luminescence of Tm3+. The sensitization of Tm3+ to Er3+ depends on the concentration of Yb3+. The intensity of blue, green and red emissions can be changed by adjusting the concentrations of the three kinds of rare earth ions. This research may provide useful information for the development of high color and spatial resolution devices and white light simulation.

A systematic investigation on glass formation in the PbF 2 –InF 3 –BaHPO 4 ternary system has been carried out. These glasses have characterized by IR spectra, Raman spectra and differential thermal analysis. The results show that the structure of these glasses is mainly affected by BaHPO 4 and InF 3 contents. With decreasing BaHPO 4 content, the glass structure gradually transforms from metaphosphate to polyphosphate. When InF 3 content is low, it mainly acts as network modifier, when its content is high; it enters glass matrix and forms In(O,F) 6 groups connecting the polymerized phosphorus oxygen species. PbF 2 mainly acts as network modifier in this system. Systematic variations of the glass transition temperature and the thermal stability index agree well with these results. The most stable glass with Δ T = 230 °C and S = 21.79 K is obtained.

Ytterbium-sensitized erbium-doped oxide-halide tellurite and germanate-niobic-lead glasses have been synthesized by conventional melting method. Intense green and red emissions centered at 525, 546 and 657 nm, corresponding to the transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 , respectively, were simultaneously observed at room temperature in these glasses. The quadratic dependence of the 525, 546 and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs. Tellurite glass showed a weaker up-conversion emission than germanate-niobic-lead glass, which is inconsistent with the prediction from the difference of maximum phonon energy between tellurite and germanate-niobic-lead glasses. In this paper, Raman spectroscopy was employed to investigate the origin of the difference in up-conversion luminescence in the two glasses. Compared with phonon side-band spectroscopy, Raman spectroscopy extracts more information including both phonon energy and phonon density. Our results reveal that the phonon density and the maximum phonon energy of host glasses are both important factors in determining the up-conversion efficiency.

Novel ozone sensors based on In/sub 2/O/sub 3/ thin films on sapphire substrates with interdigitated comb structures have been prepared from indium isopropoxide solutions by the sol-gel process. Highly sensitive films have been obtained by using aged solutions. The annealing temperature was 500/spl deg/C. Compared with earlier thin film devices, the advantages of these sensors include significantly higher ozone sensitivity (as large as 6 at 45 ppb), lower working temperature (below 200/spl deg/C) and low production cost.

Multilayer structures consisting of bismuth (Bi)-doped silica thin films and different kinds of spacer, i.e., Si, silica, Si-rich silica, layers are grown and the luminescence properties are studied. When samples were annealed at a low temperature, Bi-related near infrared active centers (BRACs) were formed at interfaces between Bi-doped silica and Si-rich silica (or silicon) due to the reduction of Bi3+ to BRACs by silicon. On the other hand, films annealed at a high temperature showed similar emission behaviors to bulk glasses. The results demonstrated here establish a new strategy for the control of BRACs and building peculiar Bi activated film structures.

Yb–Bi co-doped zeolites are prepared by a method consisting of a simple ion-exchange process and subsequent high-temperature annealing. We observe two strong near-infrared (NIR) emission bands overlapping in the range 930–1480 nm, corresponding to the electronic transitions of bismuth-related active centres (BiRAC) and Yb3+ ions, respectively. In the obtained products, the excitation wavelength of Yb3+ is extended to the range 420–850 nm, and the lifetime reaches 665 µs. In the zeolite matrix, Bi ions exist as BiRAC and Bi compound agglomerates. The former act as a sensitizer of Yb3+ ions, and the latter act as a blockage to seal the pores of zeolites, which enables Yb3+ ions to show efficient NIR emission even if the zeolites contain a large amount of coordinated water. The excellent optical and structural properties make these NIR-emitting nanoparticles promising in applications as biological probes.

We have shown that Lewis-acidic halogenoaluminate ionic liquid (IL) containing subvalent bismuth can be used as a near-IR (NIR) luminescent material. Raman and absorption spectra evidence the coexistence of Bi(5)(3+) and Bi(+) in the liquid. The Bi(5)(3+) and Bi(+) emitters, stabilized by this Lewis-acidic liquid, demonstrate ultrabroad NIR photoluminescence with a lifetime of around 1 μs. We envisage that the bismuth activated ILs would not only enrich the well-established spectrum of soft luminescent materials but also might promote the design of novel photonic materials activated by other p-block elements.

Yb3+Er3+-codoped chloride-modified germanate-bismuth-lead glasses have been synthesized by the conventional melting and quenching method. Structural and thermal stability properties have been obtained on the basis of the Raman spectra and differential thermal analysis, which indicate that the PbCl2 addition has an important influence on the phonon density of states, maximum phonon energy, and thermal stability of host glasses. The Judd-Ofelt intensity parameters and quantum efficiencies were calculated on the basis of the Judd-Ofelt theory and lifetime measurements. For the 1.53 µm emission band, the full widths at the half-maximum increase and peak wavelengths are blueshifted with increasing PbCl2 content. Moreover, the effect of the PbCl2 addition on the phonon density of states, OH− content, and upconversion luminescence has been discussed and evaluated. Our results reveal that, with increasing PbCl2 content, the decrease of phonon density and OH− content contributes more to the enhanced upconversion emissions than that of maximum phonon energy.

Nd-Bi codoped zeolites were prepared by an ion-exchange process, and the optical properties were investigated by photoluminescence (PL) and PL excitation spectra, and decay time measurements. The results show that the NIR emission of Nd(3+) ions is significantly enhanced by the introduction of bismuth in codoped samples, and the lifetime reaches 246 μs. It is also observed that NIR-active Bi acts as a sensitizer of Nd(3+) ions. The energy transfer efficiency is also estimated. The peculiar optical properties make Nd-Bi codoped zeolites promising for potential application in biological probes.

The gas sensitivity, selectivity and stability properties of WO3 thin films for the detection of NO2 gas in the concentration range 0.2–5 ppm, have been evaluated and discussed in the light of the preparation conditions and working temperature. Thin films were obtained by evaporating high purity WO3 powder by an electrically heated crucible at about 5 × 10−4 Pa on sapphire substrates provided with Pt interdigital type sputtered electrodes and annealed for 1 h at 400, 500 and 600°C. The film morphology, crystalline phase and chemical composition were characterised through AFM, low angle XRD and XPS. The electrical response was measured by means of DC current mode. The annealed films showed crystallographic orientation belonging to the triclinic structure of WO3, while the as-deposited films were found to be amorphous. The binding energies of O 1s and W 4f confirmed the existence of the WO3 phase, with a stoichiometric ratio close to the theoretical one. All the films showed the highest sensitivity to NO2 at a working temperature of 200°C. The 500°C annealed film was found to be the most sensitive to NO2 gas, compared to those annealed at 400 and 600°C. No cross sensitivity effects were found by exposing the sensors to CO, CH4. WO3 films showed strong sensitivity to C2H5OH and H2O. Long term stability test at a working temperature of 350°C, performed by cycling the films in dry air and 5 ppm NO2 revealed no substantial change in the electrical properties in terms of drift and sensitivity.