visible upconversion light emission in er o and yb o ...curresweb.com/mejas/mejas/2015/mejas special...

Middle East Journal of Applied Sciences ISSN 2077-4613

Volume : 05 | Issue : 05 | Oct.-Dec. | 2015 Pages: 45-57

Corresponding Author: Elhady, M.M., Department of Physics, Faculty of Science, Al-Azhar University, Nasr City 11884, Cairo, Egypt.

E-mail: [email protected]

45

Visible Upconversion Light Emission in Er2O3 and Yb2O3-Doped Fluoride Glass under 808 nm Excitation 1 M.M.Elhady, 1I.I. Shaltout, 1E.E. Shaisha, 2M.M. El-Desoky, 1A.A. Bahgat 1Department of Physics, Faculty of Science, Al-Azhar University, Nasr City 11884, Cairo, Egypt. 2Department of Physics, Faculty of Science, Suez University, Suez.

ABSTRACT Rare earths-doped germanium oxide and lead fluoride were prepared with composition of [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3 + xYb2O3] glasses, where x= 0, 0.5, 1.5 and 2.5 mol%. The differential thermal analysis (DTA) was used to determine thermal stability and thermal characteristic properties of the glasses such as glass transition temperature (Tg), and temperature corresponding to the maximum of the crystallization rate (Tp1, Tp2), were evaluated. The structure and vibrational modes of glass network were investigated by Infrared absorption. Absorption spectra of lanthanide-doped glasses were measured at room temperature and used to determine the optical band gap and Urbach energies which are calculated from the absorption spectra measured between 190 and 1100 nm at room temperature. The optical band gap varies from 3.12 to 3.22 eV when the PbF2 content increases in the glass matrix. Upconversion properties of the glass with the composition 70GeO2 + (29.5-x) PbF2 + 0.5 Er2O3 + x Yb2O3 glasses, where x= 0, 0.5 and 1.5 mol% under 808 nm excitation was investigated. The intense green (534 and 550 nm) and red (639 nm) emissions corresponding to the transitions 2H11/2 →4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively, were simultaneously observed at room temperature. The PbF2 content offered an important influence on upconversion luminescence emission effectiveness. With increasing PbF2 content, the intensities of the green (534 nm) and red (639 nm) emissions increase slightly, while the green (550 nm) emission increases significantly. The results were explained on the basis of electron-phonon interaction. Keywords: Light Emission, thermal analysis, thermal characteristic, glasses

1. Introduction Lead fluoride-germanate glasses singly doped with Er3+ and doubly doped with Er3+ and Yb3+ have been intensively investigated because of their very low phonon energy compared to oxide glasses, and resulting improved luminescence properties (Fernandes et al., 2011) can be promising luminescent and laser active materials (Poulain, 1996). These glasses have better mechanical strength (Yang et al., 2004), good optical properties of halide glasses (Sun et al., 2005) with better chemical stability (Singh et al., 2006) and temperature stability with good transmission in the infrared region up to 4.5 m (Lahoz et al., 2008), which make them promising materials for technological applications such as new lasing materials (Sun et al., 2005) because of the lower phonon energy than oxide glasses (Zhang and Hu, 2003). The reduced phonon energy increases the quantum efficiency of luminescence from excited states of rare earth ions (Balda et al., 2004), fluoride glasses are attractive host materials for rare earth-doped lasers and amplifiers (Pan et al., 2006). Rare earth ions, especially erbium, have played an important role in the development of broadband erbium-doped fiber amplifiers in optical communication technology during the past few decades (El-Mallawany et al., 2004). Erbium-doped glasses are promising host materials due to their important optical properties which make them suitable for applications in photonics such as lasers, frequency upconverters (Balda et al., 2006), optical waveguides, display monitors, X-ray imaging, scintillators, lasers, and upconversion and amplifiers for fiber-optic communications. The sensitization of Er3+-doped materials with Yb3+ ions is a well-known method for increasing the optical pumping efficiency because of the efficient energy transfer from Yb3+ to Er3+ ions (Sun et al., 2006). The addition of fluoride ions has been observed to lower melting temperatures, contribute to the elimination of OH- groups, decrease refractive index and provide fluorine ion conductivity in silica and germanate glasses (Bueno et al., 2005). Also fluoride ions having low phonon energy, for the reason that the reduction of multiphonon emission rates and enhancing the lifetime for the metastable levels make the upconversion emission more efficient (Ji. Zhu et al., 2005). The host material systems most widely studied are fluoride crystals and glasses because fluoride have low non-radiative relaxation rate (Pan et al., 2007; Balda et al., 2003). This can reduce the multiphonon relaxation and thus achieves strong upconversion luminescence

Middle East J. Appl. Sci., 5(5): 45-57, 2015 ISSN 2077-4613

46

(Sun et al., 2006) and lead to higher emission efficiency (Sun et al., 2005). Among oxide glasses, lead-germanate glasses combine high mechanical strength, high chemical durability and better temperature stability. The studies for appropriate host materials are also very important. Among the dopants as the absorption and emission centers, Er3+ ion has been mostly widely studied for its rich energy levels in the trivalent rare-earth ions. Yb3+ ion has unique advantages of high absorption cross-section (Suna et al., 2005) as well the efficient energy-transfer from Yb3+ to Er3+ ions. In addition to that, in Er3+/Yb3+ co-doped glass there exists optimal Yb3+ doping content, where it can perform as an efficient sensitizer to other rare-earth acceptor ions (Liang et al., 2006). In the present work, glass system of the main composition (GeO2+PbF2) was selected in order to study the effect of the amount of PbF2 content on the efficiency of Infrared to visible light upconversion when doped with Er2O3 + Yb2O3.

2. Experimental: The glass hosts with composition (in mol%): [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3 + xYb2O3] glasses, where x= 0, 0.5, 1.5 and 2.5 mol. % were prepared. The starting materials are anhydrous powders of germanium oxide of 99.999% purity, similar to that for all the other used oxides namely PbF2, Er2O3 and Yb2O3, respectively. All used oxides are productions of Aldrich Chemical Company, Inc. Batches of 10 g in size were thoroughly mixed and melted in air using a porcelain crucible with a closed lid in an electrically heated furnace at 1100 ◦C for 0.5 h. The melts then were removed from the furnace and quenched in air by pressing between two copper plates at room temperature. The glass system was examined by X-ray using (Philips – PW3719). The glass sample was used in the form of very fine homogeneous powder. A thin flat layer inserted in the path of X-ray beam. Cu target and Ni filter were used to obtain a monochromatic Cu-Kα X-ray beam with a wavelength of 1.542 oA. Differential thermal analysis (DTA) curves of the samples were recorded using Shimadzu Analyzer (model DTA-50) with a heating rate of 10 0C/min. DTA is a schematic for detecting the temperature difference between the sample and the reference which are exposed to the same heating at a specific rate, whereas the Tp temperature is measured at the peak of crystallization. Absorption spectra were measured on single beam scanning spectrophotometer (Jenway 4600 UV/Vis. Spectrophotometer, England) over the range of 190-1100 nm at room temperature. IR measurements were carried out on (IR-Magna 560) infrared spectrometer, using KBr as a reference material. IR measurements were conducted at room temperature with KBr to sample ratio of 100:1 mg. The measurements were performed over wavenumber range of 400-4000 cm-1. The upconversion emission spectra were obtained on excitation by 808 nm laser diode. The re-emitted fluorescence light from the samples was analyzed with monochromator type - Spex 750 m. While the upconverted light signals were detected by a photomultiplier and finally recorded and amplified by a lock-in amplifier (SR 510). All measurements were taken at room temperature. 3. Results and Discussion 3.1 X-Ray diffraction: X-ray diffraction (XRD) patterns of the prepared samples are shown in Fig. 1. It can be seen that XRD patterns of these samples exhibits one broad hump which is characteristic of the amorphous nature.

10 20 30 40 50 60 70

29.5 PbF2

29 PbF2

28 PbF2

27 PbF2

Inte

nis

ty (

A.U

)

2 (degree)

Fig. 1: X-Ray diffraction patterns of [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3] glasses, where x= 0, 0.5, 1.5

and 2.5 mol%.


47

3.2 Density and Molar volume: The density of the glass samples were determined at room temperature by the Archimedes method using Carbon tetrachloride (CCl4) which have density of 1.593 g/cm3 as the immersion medium. The density ρ was then calculated according to the formula;

l

la

a

MM

M

(1) where

la MM and ,, l are sample density, sample mass in air, liquid density and sample mass in liquid,

respectively. The Molar volume Vm (cm3/mol) for all glassy samples is calculated by using the formula (Shen and Jha, 2004);

)/( MV m (2)

where M and are the molecular weight in (g/mol) and the measured density (g/cm3) respectively.

Fig. 2 shows the variation of density and molar volume of [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3] glasses, where x= 0, 0.5, 1.5 and 2.5 mol%. The density of a multi-component material is commonly related to the density of each component in the material in the network structure, which results in a more open structure with lower density (Cheng et al., 2008).

26 27 28 29 30 31

4.75

4.80

4.85

4.90

4.95

Den

sit

y (

g/c

m3)

PbF2 mol%.

Vm

36

37

38

39

40

V

m (

cm

3/g

)

Fig. 2: The variation of density and molar volume of [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3] glasses,

where x= 0, 0.5, 1.5 and 2.5 mol%. The density decreases by increasing of led fluoride because fluoride ions have an important influence on the formation of glass network. The lead fluoride enters the glass matrix as a network former as well as network modifier, and increases the number of non-bridging oxygens in the glass network, and weakens the glass network, and make it more open structure (Xu et al., 2004). 3.3 Thermal stability: The DSC results show that the as-prepared glasses exhibit a glass transition and an exothermic crystallization peak which is dependent on the composition. The measured characteristic temperatures, the glass transition temperature (Tg), the crystallization temperatures (Tp1, Tp2), and thermal parameter (ΔT = Tg/Tm) of [70GeO2+ (29.5-x) PbF2+ 0.5 Er2O3 + x Yb2O3] glasses, where x= 0, 0.5, 1.5 and 2.5 mol. % are listed in Fig. 3, Fig. 4, and Table 1. It can be seen that with increasing the content of PbF2, the glass transition, crystallization temperatures and thermal stability decrease gradually. The decrease implies that PbF2 enters glass matrix as an intermediate (Klimesz et al., 2004) between a network former and a network modifier, increases the number of non-bridging oxygens, and weakens the glass network (Xu et al., 2006). Fluorine ions have closely similar ionic radii and field strength to oxygen ions (Mortier et al., 2007). When fluorides are added into germanium dioxide glass, fluorine ions replaced oxygen ions to act as either non-bridging species or as fluorine bridges between structural units (Bueno et al., 2005). Thus fluorine ions which connect two structural units must form a weak bond compared to that formed by a bridging, divalent oxygen ion (Mortier and Auzel, 1999). Thus the thermal stability of glasses should be decreased with the replacement of oxygen. 3.4 Absorption spectra: Fig. 5 illustrates the UV–Vis transmittance spectra of 70GeO2 + (29.5-x) PbF2 + 0.5 Er2O3 + x Yb2O3 glasses, where x= 0, 0.5, 1.5 and 2.5 mol%. The transmittance spectrum consists of six bands peaked at 380, 490, 520, 654, 788 and 980 nm corresponding to the transitions from the ground state 4I15/2 of the Er3+ ions to the states 2G11/2, 4F7/2, 2H11/2, 4F9/2, 4I9/2 and 4I11/2, respectively (Chen et al., 2004). In glass system, the ground state


48

of 4I11/2 energy level of the Er3+ ions overlaps the ground state of 2F5/2 level for Yb3+ at 980 nm (Nie et al., 2006). Therefore, the transmittance band around 980 nm has a strong optical intensity owing to the large spectral overlap between Yb3+ transition (2F7/2→2F5/2) and Er3+ transition (4I15/2→4I11/2) (Sun et al., 2005). The transmittance around 980 nm can be used to predict the spontaneous emission probabilities. The intensity of this band increases with increasing of Yb3+ content and that is because Yb3+ has a larger absorption cross-section than that of Er3+ at this band.

Fig. 3: DTA thermo grams for [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3] glasses, where x= 0, 0.5, 1.5 and

2.5 mol%. with high heating 10 o C/min.

Fig. 4: Variation of (Tg), the thermal stability factor Tg, Tp and T with PbF2 content [70GeO2+ (30-x) PbF2+

0.5Er2O3+ xYb2O3+] glasses, where x= 0, 0.5, 1.5 and 2.5 mol%. Table 1: (Tg), (TP) and ΔT = Tg/ Tm, for [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3] glasses, where x= 0, 0.5, 1.5 and 2.5 mol%.

X mol% Tg (OC) TP1 (OC) TP2 (OC) Tm (OC) T=Tg/Tm 27 PbF2 463.6 623.8 752.3 796.9 0.582 28 PbF2 427.9 585.9 700.2 793.4 0.539 29 PbF2 423.7 583.3 698.6 791.8 0.535

29.5 PbF2 421 594.3 708 816.2 0.515


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Fig. 5: Transmittance spectra for [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3] glasses, where x= 0, 0.5, 1.5

and 2.5 mol%. The optical transmission spectra of the samples were recorded using a spectrophotometer in the wavelength range 190-1100 nm. The absorption coefficient (α), related to the light that is transmitted out of a sample of thickness t, is given by

t

o

I

I

dh ln

1)( (3)

Where Io is the input intensity of the light is incident on the sample. Absorption coefficient follows the empirical formula (Yang et al., 2003):

rEhνhνα(hν).

g

B (4)

where hυ is the photon energy and Eg is the optical band gap energy. Fig. 6 reveals Urbach plots of (αhυ)1/2 versus hυ for glass samples. The value of optical band gap energy can be obtained from the above relation by extrapolating the absorption coefficient to zero absorption in the (αhυ) 1/2 versus hυ plot. The band tails associated with valence band and conduction band, which are developed due to the potential fluctuations in the material, extend into the band gap and normally show an exponential behavior. The band tails are characterized by the band tail parameter Er (Urbach energy) (El-Samanoudy et al., 1991) and given by

)exp()(rE

hh

(5) Where Er can be found as the inverse slope of the lnα versus hυ plot which shown in Fig. 7. The composition dependence of optical band gap Eg and band tails Er represented in Fig.8 and table 2 shows the expected nice simultaneous reversal behavior of the band tails width and the optical energy gap. Eg is decreasing and the band tails increase with the increase of PbF2 content. Attributable to PbF2 added into germinate glasses, the potential of F- is more than the potential of O2-, the bond intensity of Pb–F is more than that of Ge–O, the polarizability of F- is lower than that of O2-, this leads to increasing the excitation energy of the absorption band. Additionally, the cutoff reduced due to F- with higher potential. In other words (Xu et al., 2002), the addition of PbF2 to germinate glass cause an increase in the number of non-bridging oxygen, thereby extra lowering the band gap, and hence the excitation energy of the absorption band decreases. Then, the


50

excitation of electron needs much bigger energy, the mobility gap of glass is broadened, and cutoff band is shifted to shorter wavelength. Also addition of PbF2 into the glasses results in the maximum phonon energy of about 260 cm-1, which is lower than those of borate (1350 cm-1), phosphate (1100 cm-1), silicate (1000 cm-1), germinate (900 cm-1) and tellurite (800 cm-1) (Gonçalves et al., 2002).

1 .0 1 .5 2 .0 2 .5 3 .0 3 .5 4 .00

5

10

15

20

25

h1

/2 (

cm

-1eV1

/2

h (eV)

27 P bF2

28 P bF2

29 P bF2

29 .5 P bF2

Fig. 6: The relation between the photon energy (eV) and (αh) 1/2 for [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+

xYb2O3] glasses, where x= 0, 0.5, 1.5 and 2.5 mol%.

3 .0 3 .2 3 .4 3 .6 3 .8 4 .0

3 .0

3 .5

4 .0

4 .5

5 .0

Ln

h ( e V )

2 7 P b F2

2 8 P b F2

2 9 P b F2

2 9 .5 P b F2

Fig. 7: The absorption coefficient Ln α versus hfor [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3] glasses,

where x= 0, 0.5, 1.5 and 2.5 mol%.

Fig. 8: Variation of optical band gap Eg and band tail Er with composition for [70GeO2+ (29.5-x) PbF2+

0.5Er2O3+ xYb2O3] glasses, where x= 0, 0.5, 1.5 and 2.5 mol%.


51

Table 2: Values of optical band gap and band tails for [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3] glasses, where x= 0, 0.5, 1.5 and 2.5

mol%. Er (eV) Eopt (eV) PbF2 (Mol %)

0.52 3.22 27 0.54 3.18 28 0.55 3.15 29 0.56 3.12 29.5

3.5 FTIR measurement: Fig. 9 illustrates the results of the deconvolution of the glass system. It appears that the deconvolution bands in IR spectra shows relative intensities. It shows two broad bands centered at ~2356 cm-1 and ~3979 cm-1. These two bands are assigned to the OH- groups weakly hydrogen bonded to NBO and the hydrogen bond free OH- groups respectively (Zhang et al., 2001). Fig. 10 shows the FTIR spectra of the glass. The FTIR bands in the 460–491 cm−1 frequency region are observed, which correspond to Pb–O stretching vibrations of the [PbO4] structural units along with the deformation modes of the Ge–O glass network (Pisarski et al., 2005). While the band at 555–590 cm-1 is from Ge-O-Ge stretching vibration (Ahmed and Hogarth, 1984). The highest band at 780–820 cm-1 could be attributed to rather localized Ge-O-1 stretching modes of the metagermenate units (interconnected tetrahedral with two non-bridge [Geo4]2-) (Cheng et al., 2008). The observed FTIR bands in the 690–930 cm−1 frequency region are responsible for the vibrations of [GeO4], [GeO6] germanate units and interconnected through Ge–O–Ge bridges in [GeO4] structural units. The main FTIR band located at about 832 cm−1 can be due to [GeO4] tetrahedral structural units attributable to the Ge–O–Ge asymmetric stretching modes (Pan et al., 2010). The strongest broad band at around 900 cm-1 is due to vibrations of PbO4. Absorption band at 976 cm-1 is attributed to Pb–O asymmetric stretching vibrations in PbO4 tetrahedra. Symmetric mode of Pb–O bonds vibrations is seen at 730 cm-1 (Auzel, 1976). The position of height bands shifted to the lower bands, this suggests that the addition of PbF2 make some of the germanium could be in a six fold coordinated environment or, the Ge-O-Ge bond has been weakened by the formation of Ge-O-Pb bonds (Teja et al., 2013). The position of the highest phonon band is important, because the multiphonon decay of rare-earth ions in glass depends on the maximum phonon energy of the host glass. In this kind of glasses, the highest band could be attributed to rather localized Ge–O-1 stretching modes of [GeO4]2- tetrahedral (Nocun et al., 2005).

1000 2000 3000 4000

28 PbF2

29.5 PbF2

27 PbF2A

bso

rban

ce (

A.U

)

W avenum ber (cm -1)

29 PbF2

Fig. 9: The FT-IR absorption spectra of un doped and Er / Yb Co doped germanium led fluoride glass with

different Er : Yb Concentration ratios. 3.6 Upconversion emission: The upconversion fluorescence spectrum in the range of 500–700 nm is shown in Fig. 11 of the Er3+/Yb3+ co-doped lead fluoride germanate glasses under 808 nm excitation. Three intense emission bands centered at 534, 550, and 639 nm corresponding to the transitions 2H11/2 →4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively, were simultaneously observed. The broad emission bands at about 680 nm upon excitation at 808 nm are due to the overlap of the 639 nm red emission and the strong pump light tail of 808 nm of Infrared laser diode. It is important to show that the green emission is bright enough to be observed by the naked eye at excitation power as low as 5 W for Er3+/Yb3+ co-doped lead fluoride germanate glass at room temperature. The dependence of upconversion luminescence intensity on PbF2 content is shown in Fig. 12. The intensities of the green (534 nm) and red (639 nm) emissions slightly increased with increasing PbF2 content, while the green emission (550 nm) increases extensively as clearly shown. It was suggested that PbF2 has more influence on the


52

green (550 nm) emission than the green (534 nm) and red (639 nm) emissions. For our doped samples, no upconversion signal was observed from Yb3+ which may be due to the high phonon energy of the host structure. Upconversion emission (not showed here) was observed for Yb3+ - Er3+ co-doped samples indicating that the Yb3+ content play an important role in the dynamic of the Er3+ emission by using source having low phonon energy.

400 600 800 1000 1200

Ab

so

rban

ce

(A

.U

Wavenuber (cm-1)

27 PbF2

400 600 800 1000 1200

Ab

so

rba

nce

(A

.U)

Wavenumber (cm-1)

28 PbF2

400 600 800 1000 1200

Ab

so

rba

nc

e (

A.U

)

Wavenumber (cm-1)

29 PbF2

400 600 800 1000 1200

Ab

so

rba

nc

e (

A.U

)

Wavenumber (cm-1

)

29.5 PbF2

Fig. 10: The deconvolution infra red spectra for [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3+] glasses, where

x= 0, 0.5, 1.5 and 2.5 mol%. Table 3: The experimental IR absorption band position for [70GeO2+ (29.5-x) PbF2+ 0.5Er2O3+ xYb2O3+] glasses, where x= 0, 0.5, 1.5 and

2.5 mol%. X

(mol. %) Peak position (cm-1)

1 2 3 4 5 27 PbF2 488 587 718 814.70 947 28 PbF2 489 586 717 822.98 1026 29 PbF2 490 583 713 810.23 953

29.5 PbF2 497 590 716 814.50 952

In an upconversion mechanism, the upconversion emission intensity IUP will be proportional to mth power of the IR excitation intensity IIR; i.e. IUP α I m

IR Where m is the number of IR photons absorbed per visible photon emitted. A plot of log IUP versus log IIR produces a straight line with slope m (Sun et al., 2004). The results are depicted in log–log plots of Fig. 13. It shows such a plot for the 534 and 550 and 639 nm emissions respectively. Table 4 present the obtained values of the parameter m. As can be seen, all the three emissions may be described by an almost quadratic exponent


53

dependence on the excitation power, which indicates that two photon steps are involved for the upconversion process (Kassab et al., 2005). To explain the obtained results the two basic mechanisms of upconversion are due to excited state absorption and energy transfer between excited ions, i.e. through crossover transition. With the help of the energy diagram of Fig. 14 (Balda et al., 2003), the excitation processes for the green emissions at 534 nm could be explained as Er3+ ion excited initially from the ground state 4I15/2 to the 4I9/2 state in the ground-state absorption process under the 808 nm excitation. Absorption of 808 nm photons raises Er3+ ions from ground state to the 4I9/2 state, which undergoes multiphonon relaxation to its lower state of 4I11/2 with a high rate. The excited ion at the 4I11/2 state further absorbs a second photon, i.e. excited state absorption, and is promoted to the 4F3/2,5/2 state and then relaxes non-radiatively very fast to the intermediate state 2H11/2 due to a multiphonon relaxation process; finally, the transition 2H11/2→ 4I15/2 shows the 534 nm green emission. The excited state absorption from the 4I11/2 state should also be considered for the green emissions: Er3 + (4I11/2) + Er3 + (4I11/2) → Er3 + (4F7/2) + Er3 + (4I15/2), while its contribution is much smaller than the excited state absorption. Er3+ ion at the 4F7/2 state can also decay down to the 4S3/2 state, and the 4S3/2 → 4I15/2 transition reveals the 550 nm green emission. The estimated energy gaps between the 2H11/2 state and the next lowest state 4S3/2 is around 800 cm-1; therefore, the MRP rates are very large and the 534 nm emissions intensity is significantly reduced. Since the energy gap below the 4S3/2 state and the next lowest state 4F9/2 is larger (about 3200 cm-1), the multiphonon relaxation rate from 4S3/2 becomes smaller. Therefore, the accumulation of population at this state gives a very strong 550 nm green emission. For the red upconversion emission (Yinggang and Dongmei, 2013), Er3+ ion is first excited from the ground state 4I15/2 to the 4I9/2 state through the ground-state absorption process under excitation of 808 nm. The ion then decays to the 4I13/2 state due to the multiphonon relaxation process, and is promoted to the 4F9/2 state by another excited ion, and finally the 4F9/2 → 4I15/2 transition gives the 639 nm red emission (Miguel et al., 2014). The dominant population of the 4F9/2 state is the total result of excited state absorption and energy transfer from the 4I13/2 state, and also a contribution from a higher energy state through a non-radiative relaxation. The energy transfer process can be described by Er3+ (4I13/2) + Er3+ (4I11/2) → Er3+ (4F9/2) + Er3+ (4I15/2).

500 550 600 650 700

8 W

In

ten

sit

y (

a.u

.)

Wavelength (nm)

28 PbF2

29 PbF2

29.5 PbF2

Fig. 11: Upconversion luminescence spectra of Er3+/Yb3+-co-doped lead fluoride germanate glasses under 808

nm with a power 8 mw at room temperature.

The possible upconversion mechanism for the green–red emissions upon excitation at 808 nm Infrared laser diode could be written as follows: Ground state absorption: Er3+ (4I15/2) + a photon-Er3+ → (4I9/2), Excited state absorption: Er3+ (4I13/2) +a photon → Er3+ (2H11/2), and Er3+ (4I11/2) + a photon → Er3+ (4F3/2). The energy transfer: Er3+ (4I13/2) + Er3+ (4I11/2) → Er3+ (4F9/2) + Er3+ (4I15/2), and Er3+ (4I11/2) + Er3+ (4I11/2) → Er3+ (4F7/2) + Er3 + (4I15/2).

Table 4: Slope values of the upconversion emission bands for Er3+/Yb3+ co-doped lead fluoride germanate glasses.

λ nm M 28.0 PbF2 29.0 PbF2 29.5 PbF2

534 1.91 1.63 1.77 550 1.85 1.67 1.88 639 2.2 1.82 1.80


54

6 .5 7 .0 7 .5 8 . 0 8 .5 9 .0 9 .5

0 .0 0 8

0 .0 1 0

0 .0 1 2

0 .0 1 4

0 .0 1 6

0 .0 1 8

0 .0 2 0

0 .0 2 2

2 8 P b F2

I UP (

a. u

.)

PI R

( W )

6 3 9 n m

5 3 5 n m

5 5 0 n m

6 . 5 7 .0 7 . 5 8 .0 8 .5 9 .0 9 .5

0 .0 1 5

0 .0 1 8

0 .0 2 1

0 .0 2 4

0 .0 2 7

0 .0 3 0

0 .0 3 3

PIR

( W )

6 3 9 n m

5 3 5 n m

5 5 0 n m

I UP (

a. u

.)

2 9 P b F2

6 . 5 7 . 0 7 . 5 8 . 0 8 . 5 9 . 0 9 . 5

0 . 0 2 5

0 . 0 3 0

0 . 0 3 5

0 . 0 4 0

0 . 0 4 5

0 . 0 5 0

0 . 0 5 5

0 . 0 6 0

PIR

( W )

I U

P (

a. u

.)

6 3 9 n m

5 3 5 n m

5 5 0 n m

2 9 .5 P b F2

Fig. 12: Dependence of upconversion fluorescence intensities on excitation power under 808 nm excitation for

lead fluoride germanate glasses.

3 . 8 1 3 . 8 4 3 . 8 7 3 . 9 0 3 . 9 3 3 . 9 6 3 . 9 90 . 9 0

0 . 9 5

1 . 0 0

1 . 0 5

1 . 1 0

1 . 1 5

1 . 2 0

1 . 2 5

1 . 3 0

1 . 3 5

Lo

g I U

P (

a. u

.)

L o g PI R

( m W )

2 8 P b F2

6 3 9 n m s l o p e = 2 . 2

5 3 4 n m s l o p e = 1 . 9 1

5 5 0 n m s l o p e = 1 . 8 5


55

3 . 8 0 3 . 8 4 3 . 8 8 3 . 9 2 3 . 9 6 4 . 0 01 . 1 4

1 . 2 0

1 . 2 6

1 . 3 2

1 . 3 8

1 . 4 4

1 . 5 0

1 . 5 6

L o g PI R

( m W )

6 3 9 n m s lo p e = 1 . 8 2

5 3 4 n m s lo p e = 1 . 6 3

5 5 0 n m s lo p e = 1 . 6 7

Lo

g I U

P (

a. u

.)2 9 P b F

2

3 . 8 0 3 . 8 4 3 . 8 8 3 . 9 2 3 . 9 6 4 . 0 0

1 . 3 8

1 . 4 4

1 . 5 0

1 . 5 6

1 . 6 2

1 . 6 8

1 . 7 4

1 . 8 0

L o g PIR

( m W )

6 3 9 n m s lo p e = 1 . 7 7

5 3 4 n m s lo p e = 1 . 8 0

5 5 0 n m s lo p e = 1 . 8 8

Lo

g I U

P (

a. u

.)

2 9 . 5 P b F2

Fig. 13: Logarithmic plot of the integrated intensities of the upconverted emission from 4S3/2 (534 nm), 4I15/2

(550) and 4F9/2 (659 nm) levels obtained under excitation at 808 nm for the sample doped with PbF2

ions.

Fig. 14: Schematic energy level diagram of Er3+/Yb3+-co-doped lead fluoride germanate glasses under 808 nm

excitation.


56

4. Conclusion: With increasing fluoride content, the density decreases because fluoride ions increases the number of non-bridging oxygens in the glass network, and make it more open structure. The differential thermal analysis curves show that, with increasing the PbF2 contents, the thermal parameters Tg, ΔT, Tp1, Tp2, and To, are shifted towards lower temperatures and the thermal stability factor (ΔT = To-Tg) is lying between 0.506 and 0.538 0C. Absorption processes of the rare earth ions Er3+ and Yb3+ have been observed at ~380, 490, 520, 654, 788, and 980 nm and were assigned to the Er3+ ions 2G11/2, 4F7/2, 2H11/2, 4F9/2, 4I9/2 and 4I11/2, respectively, the ground state of 4I11/2 energy level of the Er3+ ions overlaps the ground state of 2F5/2 level for Yb3+ at 980 nm. Indirect allowed transitions have been adopted as the absorption mechanism and Eopt was strongly decreasing, and Er was increasing with the increase of PbF2 content. Optical band gap Eopt of glass system was found between 3.12 and 3.22 eV and band tails width Er were found in the range 0.52 to 0.56 eV which is caused by the addition of PbF2 to germinate glass increase in the number of non-bridging oxygen sites. The structure of Lead fluoride-germanate glasses by FTIR spectra, which indicates that fluoride ions plays an important role in the formation of glass network. Three upconversion emissions have been observed centered at around 534, 550 and 639 nm. The green emissions at 534 and 550 nm are due to the 2H11/2 and 4S3/2 transitions respectively. The red upconversion emission at 639 nm has been associated with the 4F9/2 transition of Er3+ ions. The quadratic dependence of fluorescence on excitation laser power confirms that two-photons contribute to the upconversion of the green and red emissions (Elhady et al., 2013). Acknowledgements The authors thank Prof. Dr. Y. Badr, National Institute of Laser Enhanced Sciences (NILES), Cairo University, for supporting Up-conversion measurements. References Ahmed, M.M., C.A. Hogarth, 1984. Journal of Materials Science, 19: 4040-4044. Auzel, F., 1976. "Multiphonon-assisted anti-Stokes and Stokes fluorescence of triply ionized rare-earth ions,"

Physical Review B (Solid State), 13: 2809-2817. Balda, R., A. Oleaga, J. Fernandez, J.M. Fdez-Navarro, 2003. Optical Materials, 24: 83-90. Balda, R., A. Oleaga, J. Fernández, J.M. Fdez-Navarro, 2006. Optical Materials, 24: 83-90. Balda, R., J. Fern_andez, M.A. Arriandiaga, J.M. Fdez-Navarro, 2004. Optical Materials, 25: 157-163. Balda, R., J. Fernandez, J.L. Adam, L.M. Lacha, M.A. Arriandiaga, 2003. Journal of Non-Crystalline Solids,

326&327: 330-334. Bueno, L.A., J.P. Donoso, C.J. Magon, I. Kosacki, F.A. Dias Filho, C.C. Tambelli, Y. Messaddeq, S.J.L.

Ribeiro, 2005. Journal of Non-Crystalline Solids, 351: 766-770. Bueno, L.A., Y. Messaddeq, F.A. Dias Filho, S.J.L. Ribeiro, 2005. Journal of Non-Crystalline Solids, 351:

3804-3808. Chen, Y., Y. Huang, M. Huang, R. Chen, Z. Luo, 2004. Optical Materials, 25: 271-278. Cheng, Y., H. Xiao, W. Guo, 2008. Ceramics International, 34: 1335-1339. Cheng, Y., H. Xiao, W. Guo, 2008. Materials Science and Engineering, A. 480: 56-61. Elhady, M.M., I.I. Shaltout, E.E. Shaish, M.M. El-Desoky, A.A. Bahgat, 2013. Journal of Applied Sciences

Reaserch, 9: 5120-5125. El-Mallawany, R., A. Patra, C.S. Friend, R. Kapoor, Paras N. Prasad, 2004. Optical Materials, 26: 267-270. El-Samanoudy, M.M., A.I. Sabry, E.E. Shaisha, A.A. Bahgat, 1991. Physics and Chemistry of Glasses, 32: 115-

120. Fernandes, R.G., C. Pereira, R. Bertholdo, 2011. Cassanjes FC, Poirier G, Journal of Non-Crystalline Solids,

357: 3345-3350. Gonçalves, M.C., L.F. Santos, R.M. Almeida, C.R. Chimie, 2002. 5: 845-854. Ji. Zhu, Y. He, Z. Li, L. Qiu, W. Shen, 2005. Journal of Non-Crystalline Solids, 351: 1619-1622. Kassab, L.R.P., A. de Oliveira Preto, W. Lozano, F.X. de Sa, G.S. Maciel, 2005. Journal of Non-Crystalline

Solids, 351: 3468-3475. Klimesz, B., G. Dominiak-Dzik, P. Solarz, M. ˙Z elechower, W. Ryba-Romanowski, 2004. Journal of Alloys

and Compounds, 382: 292-299. Lahoz, F., P. Haro-Gonzalez, F. Rivera-López, S. González-Pérez, I.R. Martín, N.E. Capuj, C.N. Afonso, J.

Gonzalo, J. Fernández, R. Balda, 2008. Appl Phys., A 93: 621-625. Liang, H., K. Yang, X. Zhang, K. Machid, J. Meng, 2006. Journal of Alloys and Compounds, 408-412: 835-

837. Miguel, A., R. Moreab, M.A. Arriandiaga, M. Hernandez, F.J. Ferrer, C. Domingo, J.M. Fernandez-Navarro, J.

Gonzalo, J. Fernandez, R. Balda, 2014. Journal of the European Ceramic Society, 34: 3959-3968.


57

Mortier, M., A. Bensalah, G. Dantelle, G. Patriarche, D. Vivien, 2007. Optical Materials, 29: 1263-1270. Mortier, M., F. Auzel, 1999. Journal of Non-Crystalline Solids, 256&257: 361-365. Nie, Q., C. Jiang, X. Wang, T. Xu, H. Li, 2006. Materials Research Bulletin, 41: 1496-1502. Nocun, M., W. Mozgawa, J. Jedlinski, J. Najman, 2005. Journal of Molecular structure, 744-747: 603-607. Pan, J., R. Xu, M. Wang, Guojun Gao, J. Chen, L. Hu, J. Zhang, 2010. Solid State Communications, 150: 78-80. Pan, Z., A. Ueda, M. Hays, R. Mu, S.H. Morgan, 2006. Journal of Non-Crystalline Solids, 352: 801-806. Pan, Z., A. Ueda, R. Mu, S.H. Morgan, 2007. Journal of Luminescence, 126: 251-256. Pan, Z., A. Ueda, S.H. Morgan, R. Mu, 2006. Journal of rare earths, 24: 699-705. Pisarski, W.A., T. Goryczka, B. Wodecka-Du´s, M. Pło´nska, J. Pisarska, 2005. Materials Science and

Engineering, B 122: 94-99. Poulain, M., 1996. Thermochimica Acta, 280/281: 343-351. Shen, S., A. Jha, 2004. Optical Materials, 25: 321-333. Singh, K., S.B. Rai, D.K. Rai, V.B. Singh, 2006. Appl. Phys., B 82: 289-294. Sun, H., J. Yang, L. Zhang, J. Zhang, L.Hu, Z. Jiang, 2005. Solid State Communications, 133: 753-757. Sun, H., L. Wen, Z. Duan, L. Hu, J. Zhang, Z. Jiang, 2006. Journal of Alloys and Compounds, 414: 142-145. Sun, H., L. Wen, Z. Duan, L. Hu, J. Zhang, Z. Jiang, 2006. Journal of Alloys and Compounds, 414: 142-145. Sun, H., L. Zhang, S. Zhao, J. Zhang, C. Yu, D. He, Z. Duan, L. Hu, Z. Jiang, 2005. Solid State

Communications, 133: 357-361. Sun, H., S. Dai, D. Zhang, S. Xu, J. Zhang, L. Hu, Z. Jiang, 2004. Physica, B 353: 248-254. Sun, H., S. Xu, S. Dai, J. Zhang, L. Hu, Z. Jiang, 2005. Journal of Alloys and Compounds, 391: 198-201. Sun, H., S. Xu, S. Dai, L. Wen, J. Zhang, L. Hu, Z. Jiang, 2005. Journal of Non-Crystalline Solids, 351: 288-

292. Suna, H., S. Xu, S. Dai, J. Zhang, L. Hu, Z. Jiang, 2005. Journal of Alloys and Compounds, 391: 198-201. Teja, P.M.V., A.R. Babu, P.S. Rao, R. Vijay, D. Krishna Rao, 2013. Journal of Physics and Chemistry of Solids,

74: 963-970. Xu, S., G. Wang, J. Zhang, S. Dai, L. Hu, Z. Jiang, 2002. Journal of Non-Crystalline Solids, 336: 230-233. Xu, S., Z. Yang, S. Dai, G. Wang, L. Hu, Z. Jiang, 2004. Materials Letters, 58: 1026-1029. Xu, T., G. Li, Q. Nie, X. Shen, 2006. Spectrochimica Acta Part, A 64: 560-563. Yang, J., N. Dai, S. Dai, L. Wen, L. Hu, Z. Jiang, 2003. Chemical Physics Letters, 376: 671-675. Yang, Z., S. Xu, L. Hu, Z. Jiang, 2004. Journal of Alloys and Compounds, 370: 94-98. Yinggang, Z., S. Dongmei, 2013. Journal of rare earths, 31: 857-863. Zhang, L., H. Hu, 2003. Journal of Non-Crystalline Solids, 326&327: 353-358. Zhang, L., H. Hu, C. Qi, F. Lin, 2001. Optical Materials, 17: 371-377.

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