Effect of erbium nanoparticles on structural and spectroscopic properties of bio-silica borotellurite glasses containing silver oxide

Effect of erbium nanoparticles on structural and spectroscopic properties of bio-silica borotellurite glasses containing silver oxide

Materials Chemistry and Physics 236 (2019) 121795 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.el...

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Materials Chemistry and Physics 236 (2019) 121795

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Effect of erbium nanoparticles on structural and spectroscopic properties of bio-silica borotellurite glasses containing silver oxide M.K. Halimah a, *, A.M. Hamza a, b, F.D. Muhammad a, K.T. Chan a, S.A. Umar c, I. Umaru d, I. G. Geidam a, e a

Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400, Upm Serdang, Selangor, Malaysia National Agency for Science and Engineering Infrastructure, Idu, Abuja, Nigeria Federal University Lafia, Nasarawa State, Nigeria d Nasarawa State University, Keffi, Nasarawa State, Nigeria e Yobe State University, Damaturu, Nigeria b c

H I G H L I G H T S

� Er3þ NPs doped TBSE glasses were prepared using melt quenching technique. � The glass structure was investigated using XRD and FTIR. � Emission cross-sections were obtained from McCumber theory. � The laser media performance was evaluated using the optical gain. � The results show that TBSE glasses are promising materials for solid state lasers. A R T I C L E I N F O

A B S T R A C T

Keywords: Erbium nanoparticles Optical gain McCumber theory Near infrared spectrum

Er3þ NPs doped bio-silica borotellurite glasses containing silver oxide were prepared using melt quenching techniques. The silica used was extracted from rice husk by acid leaching process. Optical absorption and photoluminescence properties of the glass samples were studied. The samples were confirmed to be amorphous in nature from the XRD results. The FTIR results reveal the existing functional groups in the glass structure. XRF result shows that 98.6% silica was obtained from the rice husk. The optical absorption spectra revealed that fundamental absorption edge shifts to longer wavelength as the content of erbium NPs increases. The value of band gap had been calculated and shown to be increased with an increase content of erbium NPs. The Urbach energy was shown to be linearly decreased with an increase in content of erbium NPs oxides. The introduction of silver oxides modifies the optical properties of the glass. The emission cross-section (δe ) and gain bandwidth (FWHM � δe ) for 4I13/2→4I15/2 transition were calculated from the McCumber theory. The photoluminescence spectra revealed three emission bands corresponding to 4H11/2-4I15/2 (470 nm), 4S3/2-4I15/2 (561 nm) and 4F9/ 4 2→ I15/2 (758 nm) transitions assigned to green and red emissions respectively. The results suggest that these glasses might be used as a promising candidate as well as gain media for solid state lasers.

1. Introduction Rare earth (RE) doped glass has become a significant class of solids that attracts much attention to researchers in recent years, as a result of their future applications in many areas, such as laser, optical amplifi­ cation for communication colour display and sensor [1]. A considerable amount of research has been conducted on characteristics of laser

transition in rare earth doped solid materials. However, due to the better glass forming ability and wider applications of RE doped oxide host glasses like borates, silicates, tellurites and phosphates were synthesized and published [2]. The RE are most cherished for their sole properties particularly as optically vigorous elements in their ionized lasers state. When fused in amorphous or crystalline hosts, the R.E exist as ionþ3 [3]. The optimization of Er3þ ion allowed the understanding of laser

* Corresponding author. E-mail address: [email protected] (M.K. Halimah). https://doi.org/10.1016/j.matchemphys.2019.121795 Received 10 April 2019; Received in revised form 20 June 2019; Accepted 26 June 2019 Available online 27 June 2019 0254-0584/© 2019 Published by Elsevier B.V.

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4000

Intensity (a.u)

[8]. Similarly, Borate-based glasses in particular have demonstrated both increased and full conversion rates to hydroxy-carbonate apatite when compared to silicate-based glasses due to their lower chemical durability [9]. Currently, they have also shown great promise in wound healing applications which is attributed to their: (1) high degradation rate, (2) greater increase in local pH compared to silicate-based bioac­ tive glasses, and (3) the ability of boron to stimulate angiogenesis [10]. Silicate glasses which are mechanically and chemically stable with a very weak upconversion luminescence because of their large phonon energy [11] are inexpensive. Tellurite based glasses have many advan­ tages among oxide glasses such as good optical transparency, high dielectric constants and good thermal stability. Their phonon energy is low (around 700 cm 1) and have large refractive index than other oxide glasses (like borate phosphate and silicate), which are all useful for high radiative transition of ions. Tellurite glasses are used to produce a planar waveguides and optical fibers [12]. Moreover, the existence of TeO2 in borosilica glasses minimizes their hygroscopic state, increases their IR transmission, decreases their phonon energy and increases refractive index [13]. Borotellurite glasses also gained attention because of their good thermal stability and wide transmittance window [14]. Silicate glasses are primarily based on SiO2 (the glass network former); other modifying oxides are also included as Na2O, CaO and P2O5 to adjust the properties of the produced glass. These oxides are usually included in specific molar ratios to produce a biologically active glass [15]. To enhance these properties, the addition of silver oxide (Ag2O) has been investigated. When Ag2O is added to tellurite glass, it transforms the structure from TeO4 trigonal bipyramids (tbp) to TeO3 trigonal pyramids (tp) [16]. The structural change through intermediate, TeO3þ1 polyhedra, and leads to the formation of nonbridging oxygens (NBOs) atoms. A large number of glass compositions containing different amount of glass formers such as tellurite, phosphate and borate oxides, with silver oxides as modifier, were investigated some years back for their outstanding future applications as optical materials. Earlier studies on the binary silver tellurite glasses revealed that the structural transi­ tion and the formation of more NBOs is responsible for the enhancement in the glass conductivity when silver oxide is added [17]. Some specific metallic ions including silver (Agþ), boron (B3þ), zinc (Zn2þ), copper (Cuþ and Cu2þ), cobalt (Co2þ), gallium (Ga3þ), cerium (Ce3þ), and strontium (Sr2þ) are now being incorporated in glasses to improve their biological impact [18]. Based on the above facts, the aim of this research was to determine the spectroscopic and structural properties of Er3þ NPs doped bio-silica borotellurite glass using the optical absorption spectra and photo­ luminescence techniques. The (δemi) and (FWHM � δe) were evaluated successfully from the emission spectral and McCumber theory. The ef­ fect of erbium nanoparticle on the structure of the glass samples were studied using XRD and FTIR techniques.

0.00 Er NPs+Ag 0.01 Er NPs+Ag 0.02 Er NPs+Ag 0.03 Er NPs+Ag 0.04 Er NPs+Ag 0.05 Er NPs+Ag

5000

3000

2000

1000

20

30

40

50

60

70

80

2 (degree) Fig. 1. X-ray diffraction of TBSE glass system.

BO4

100

BO3

BO3

Trnsmittance (%)

80

TeO4

60

TeO3

0.01 Er NPs+Ag 0.02 Er NPs+Ag 0.03 Er NPs+Ag 0.04 Er NPs+Ag 0.05 Er NPs+Ag

40 20 0

500

1000

-1

Wavenumber (cm )

1500

Fig. 2. FTIR spectra of Er2O3 NPs doped TBSE glasses. Table 1 Indirect band gap, Eopt (eV) and Urbach energy, ΔE (eV) of erbium nano­ particles doped TBSE glass system. Mol fraction

EΔ (eV)

Eopt (eV)

0.01 0.02 0.03 0.04 0.05

0.79 0.34 0.34 0.17 0.18

2.89 2.83 2.82 2.75 2.29

2. Experimental details 2.1. Glass fabrication Erbium nanoparticles doped silica borotellurite bioactive glass sys­ tem containing silver oxide with chemical structure [{[(TeO2)0.8 (B2O3)0.2]0.8 (SiO2)0.2}0.99 (Ag2O)0.01]1-y (Er2O3)y, represented by TBSE, where y ¼ 0:01; 0:02; 0:03; 0:04 and 0:05 molar fraction were pre­ pared using melt quenching techniques. The compositions of the starting materials were weighted and thoroughly mixed, the mixture was then preheated in alumina crucible using an electric furnace at 400 � C for 1 h 30 min in order to remove moisture. The mixture as then transferred into another electric furnace and melted at a temperature of 1100 � C for 3 h. The melt was quickly transferred into cylindrical stainless mould maintain at 200 � C to evade excess thermal shocks and annealed at 400 � C for 1 h 30 min to reduce mechanical and thermal stress [19]. The achieved glasses were cut and well-polished using various grades of sand paper to meet the optical spectra measurements requirements.

amplifiers with outstanding feature when compared with other laser amplifiers [4]. Er3þ-doped glasses possess large possibility to be used in wide band internet due to their nonuniform broadening for glass network with erbium ions. Hence, much consideration has been paid to studies on erbium-doped fiber amplifier (EDFAs) which is the main element in eye safe laser systems [2,5,6]. Er is a famous ion with (4I13/2 → 4I15/2) transitions at infrared region around 1550 nm wave­ length and in the green region around 550 nm (4S3/2 → 4I15/2) [7]. Borate glasses have gained much attention among the oxide glasses due to their optical applications, like in luminescent materials, solid state and waveguide lasers. Particularly, a borate glass is an appropriate optical material due to its good thermal stability, low melting point, high optical transparency and moderate solubility of the rare earth ions 2

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12

)1/2 (cm-1 eV)1/2

10 0.01 Er NPs+Ag 0.01

8

0.02 Er NPs+Ag 0.02 0.03 Er NPs+Ag 0.03

6

0.04 Er NPs+Ag 0.04 0.05 Er NPs+Ag 0.05

(

4 2 0

0

1

2

3

4

5

6

(eV) Fig. 3. Optical band gap of Er2O3 NPs doped TBSE glasses.

Table 2 Refractive index, n, molar volume, Mv, optical electronegativity, χ, molar refraction (Rm), optical basicity (Ʌ) and the cationic polarizability (αm). MV (cm3 mol

2.2967 2.1338 2.6705 2.2333 2.2378

36.6456 37.0641 35.8842 32.1908 32.8015

1

)

χ

Ʌ

Rm

αm

0.5461 0.6974 0.3252 0.5998 0.5958

1.4269 1.3513 1.5373 1.4001 1.4021

23.4469 27.9481 23.8983 21.4744 21.8817

9.30 1.11 9.48 8.52 8.68

1260

Extinction coefficient, k x 10-2

n

1470

3.5

Skin depth, 10-8 cm

3.0 2.5 2.0

cut-off

0.5

1

2

3

4

Photon energy (eV)

5

630 420 210

0

300

600

900

1200

1500

wavelength (nm)

1800

2100

mounting the transparent pallet in to the spectrometer. Transmission emission microscopy (TEM LEO 912AB) with � 1% accuracy was used to snap the image of the erbium NPs. A fine powder of the glass was dis­ solved in an acetone solution. The scanning process took place with the solution placed in the sample holder. The absorption spectra of the Er3þ NPs doped glasses were recorded within the range of 300–2000 nm with Shimadzu, model: UV 1650 spectrometer. A xenon light flash was utilized in characterizing a sample of 0.2 cm thickness. The photoluminescence spectra measurements were performed using Parkin-Elmer (LS 55) spectrometer containing Argon laser beam. The samples were excited in the visible range with 380 nm excitation wavelength. The two instruments have a standard error of � 0:1 nm. All the measurements were carried out at room temperature.

1.0

0

840

Fig. 5. Extinction coefficient of Er2O3 NPs doped TBSE glasses.

1.5

0.0

1050

0

0.01 Er NPs 0.02 Er NPs 0.03 Er NPs 0.04 Er NPs 0.05 Er NPs

0.01 Er NPs 0.02 Er NPs 0.03 Er NPs 0.04 Er NPs 0.05 Er NPs

6

Fig. 4. Skin depth of Er2O3 NPs doped TBSE glasses.

The XRD spectra were obtained using X’part pro pan analytical system at room temperature with CuKα as radiation source (λ ¼ 1:5418 � A), at 40 kV. Fourier transform infrared (FTIR) was carried out

3. Results and discussion

using KBr pallet technique with the transmission spectra recorded in the range of 400–4000 cm 1. Approximately, 0.005 g of the sample powder was thoroughly mixed with a highly pure 0.2 g KBr powder and then pressed at 0.1–0.2 kbar pressure. The scanning process took place by

3.1. X-ray diffraction analysis The XRD spectroscopy of the prepared samples was conducted at 3

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About six vibrations bands are observed within the range of 321–1373 cm 1. The vibration band around 321 cm 1 is possibly attributed to the formation of (TeO3) trigonal pyramidal (tp) units with non-bridging oxygen (NBO) [21]. Vibration bands around 1059 cm 1 could be due to B–O stretching of BO4 tetrahedral units [22]. Addi­ tionally, the band at 1059 cm 1 can be attributed to Si–O–Si anti-symmetric stretching of bridging oxygen within tetrahedral [20]. Another vibration seen around 1234 cm 1 is attributed to the B–O bond of trigonal BO3 units. Moreover, the peak observed around 1373 cm 1 might be assigned to the B–O asymmetric stretching vibration of BO3 structural units [20].

x 1012 s-1

80000 70000 60000

Optical conductivity,

50000 40000 30000

3.3. Optical studies

20000

Optical absorption is an important technique to investigate the induced transitions and also provides data on the structure of the bands and band gap energy of amorphous solids. The absorption spectra of erbium doped bio-silica borotellurite glasses are obtained from the UV–Vis analysis recorded in the range of 300–2000 nm. The energy band gap (Eg), molar refraction (Rm), refractive index (n), optical electro­ negativity (χ) and optical basicity (Ʌ) were evaluated using the relations [23].

10000 00

300

600

900

1200

1500

1800

2100

Wavelength (nm) Fig. 6. Optical conductivity of the host TBSE glass.

room temperature and the results are given in Fig. 1. The nondistinguishable intensity peaks observed with broad halo diffraction around the low angle between 20� to 30� confirms that the glasses are in amorphous state. It also identifies clearly, the absence of a long range arrangement of atoms [20].

αðωÞ ¼ B

ðħω

EoptÞq ħω

(1)

where B is a constant and q is an integer with values 0.5 and 2 for in­ direct and direct transitions respectively. The indirect band gap of the glass samples were evaluated using Tauc plot of (αhv)1/2 against Eopt as displayed in Fig. 1. The indirect band gap Eopt is used to calculate the refractive index, n using the equation in Ref. [24].

3.2. Fourier transform infrared (FTIR) analysis The FTIR analysis was conducted in order to find the vibrations that are assigned to the various functional groups that exist in the glass network. The FTIR spectra for all the samples are illustrated in Fig. 2.

(a)

(b)

(c)

(d)

Fig. 7. (a) Typical TEM image with resolution of 200 nm, (b) TEM image with resolution of 100 nm (c) TEM image with resolution of 50 nm and (d) TEM image with resolution of 20 nm of Er2O3 NPs doped TBSE glasses. 4

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Materials Chemistry and Physics 236 (2019) 121795

4.40E-007

8.00E-007

polaron radius inter ionic dist

4.20E-007

7.50E-007

Polaron radius (Å)

3.80E-007 3.60E-007

6.50E-007

3.40E-007

6.00E-007

3.20E-007 5.50E-007

3.00E-007

inter ionic distance (Å)

4.00E-007 7.00E-007

2.80E-007

5.00E-007

2.60E-007

4.50E-007 0.01

0.02

0.03

0.04

0.05

Er NPs+Ag molar fraction

2.40E-007

Fig. 8. Variation of polaron radius with interionic distance of Er2O3 NPs doped TBSE glasses.

3

Absorbance (a.u)

the changes in the structure and the formation of non-bridging oxygens that occur within the glass structure [26]. The increasing values of op­ tical band gap of erbium nanoparticles doped TBSE glasses could be attributed to the formation of TeO4 units which subdue the formation of TeO3 in the glass network [27]. The increase of the indirect band gap is attributed to the decrement in non-bridging oxygen (NBO) [3]. The band gap obtained are found to be lower compare to Zinc borosilicate [28] glasses. The decrement in the values of the Urbach energy suggests a decrease in the tendency to change weak bond into defect within the glass network. The refractive index, n as seen in Table 2 shows a decreasing trend with an increase in concentration of erbium. This could be linked to the high polarity of erbium Er3þ in the TeO2 glass system which causes the breakage of Te–O–Te bridging oxygen and increase the number of nonbridging oxygen [23]. The decrease in the value of the Urbach energy as shown in Table 1 is due to the decrement in the amount of disorderliness in the glass structure with less defect in the glass network [2]. From Table 2 above, the optical band gap is observed to increase at 0.02 M fraction of Er3þ to 2.5954 eV and later decreases with the increment of Er3þ concentration. The increase could be attributed to the glass compactness, which increases with addition of R.E oxide [29]. The increase also suggests that the glass matrix more covalent in nature with the increase in concentration of Er3þ ions [30]. The decrease in the value of band gap observed may be attributed to the polarity that leads to the breaking of bridging oxygen to non-bridging oxygen. The extinction coefficient (k) of a material is given by the relation­ ship

Er NPs+Ag 0.00 Er NPs+Ag 0.01 Er NPs+Ag 0.02 Er NPs+Ag 0.03 Er NPs+Ag 0.04 Er NPs+Ag 0.05

4

2

G9/2 4 H11/2

2

4

F7/2

1

4

F9/2

S3/2

0 200

400

4

4

600

4 4

I9/2

800

I13/2

I11/2

1000 1200 1400 1600 1800 2000

Wavelength (nm)

Fig. 9. Absorption spectra of Er2O3 NPs doped TBSE glasses.

n2 1 ¼1 n2 þ 2

rffiffiffiffiffi Eg 20

(2)

The optical electronegativity (χ), molar refraction (Rm), optical ba­ sicity (Ʌ) and the cationic polarizability (αm) of the glasses were eval­ uated using the expressions in Refs. [23,25].

χ ¼ 0:2668 � Eg where Eg is the energy band gap of the sample glass. � 2 � n 1 Rm ¼ 2 � Vm n þ2

(3)

0.5χ þ 1.7 � � 3 αm ¼ � Rm 4π NA

(7)

(4)

3.4. Optical conductivity and skin depth (δ)

(5)

These are some typical parameters which are related to the photon’s absorption within the texture of the glass. Skin depth is the thickness when optical photon density is equal to 1/ e of the surface value. It depends also on the glass conductivity and the incident photon’s frequency. The skin depth (δ) and absorption coeffi­ cient (α) are related using the expression below

where Vm is the molar volume of the glass Ʌ ¼

αλ 4π



(6)

The value of αm is given in (� A) and NA is the Avogadro’s number. Urbach energy and band gap values are given in Table 1. The values of the energy band gap changes with the addition of erbium nano­ particles concentration as shown in Fig. 3. The changes could be due to

δ¼

1

α

(8)

The optical conductivity depends on various parameters, that include absorption coefficient, the incident photons frequency, 5

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Materials Chemistry and Physics 236 (2019) 121795

Emission cross section (x 10-21 cm2)

12 ErNPs+Ag 0.01 10

ErNPs+Ag 0.02 ErNPs+Ag 0.03

8

ErNPs+Ag 0.04 6

ErNPs+Ag 0.05

4 2 0 1400

1450

1500

1550

1600

Wavelength (nm) Fig. 10. Normallized emission cross section of Er2O3 NPs doped TBSE glasses. Table 3 Absorption cross section, δa , emission cross section, δe , full width half maximum (FWHM) and bandwidth quality factor, (FWHM � δe ) of UV–Vis bands for 4I13/2 → 4I15/2 laser transition of Er3þ ion. δe ( � 10 21 cm2 )

FWHM (nm)

FWHM � δe (� 10 28 cm 3 )

Reference

14.13

11.50

67.81

779.82

This work

14.83

12.11

66.15

801.08

This work

15.01

11.79

69.41

818.34

This work

11.95

9.68

64.15

620.97

This work

11.05

8.10

70.65

623.13

This work

6.33 – 6.80

7.93 8.30 5.50

65.1 54.0 40.0

515.92 448.20 220.00

[40] [1] [41]

8 6 4 -1

0.01 Er NPs 0.02 Er NPs 0.03 Er NPs 0.04 Er NPs 0.05 Er NPs Ag 0.5 Tellurite Silicate

δa ( � 10 21 cm2 )

10

Gain (cm )

Samples

αnc 4πk

2 0 -2 -4 -6 -8 -10 -12 1300

1400

1500

1600

1700

Wavelength (nm) Fig. 11. Gain coefficient for 4I13/2 → 4I15/2 transition of 0.01 Er2O3 NPs doped TBSE glass.

refractive index and extinction coefficient. The optical conductivity is evaluated using the relation (Hassanien and Akl, 2015).

σ¼

P=0.1 P=0.2 P=0.3 P=0.4 P=0.5 P=0.6 P=0.7 P=0.8 P=0.9 1

12

coefficient increases with the increment in the concentration Er3þ ions, this is attributed to the appearance of some asymmetrical internal scattering with increase in the absorption coefficient [19]. The value of k is higher due to the random scattering of waves inside the solid mate­ rial’s grain boundaries [33]. The optical conductivity of the glass strongly depends on the energy band gap of the material, it is also dependent on the refractive index, absorption coefficient, photon energy frequency and extinction coeffi­ cient. Equation (9) is used to estimate the optical conductivity of the studied glass composition. Fig. 8 shows the increase in optical conduc­ tivity of the host glass with increase in wavelength. The optical con­ ductivity is high at higher photon energy. This could be attributed to the high absorbent nature of the glass within the region, it could also be connected to the electrons excitation by the incoming photon [34]. The distribution of erbium nanoparticles in the present glass is shown in Fig. 7. The size of Er NPs is obtained to be approximately 25–28 nm as shown in Fig. 7 (d). The average diameter of pure Er NPs is in the range of 20–25 nm [35]. The image shows agglomeration of the

(9)

where c is the speed of light in air. The value of the skin depth decreased as Er3þ concentration is increased. The increment in the percentage of Er3þ ions in the glass compositions causes the colour of the glass samples to be turning dark, as a result, it results to a shortage in the glass transparency. Similar findings were reported in the literature [31]. This behavior of the skin depth also depends strongly on the crystal structure of the glass [32]. It is obvious from Fig. 4 that, the skin depth (δ) decreases with increase in the photon energy. This behavior is observed in almost all the samples. For wavelengths higher than the cut-off wave length (hv ¼ 3:2 eV), the absorption effect disappears and the decrease in amplitude occurs after passing a substantial distance as shown in Fig. 6. Fig. 5 shows the graph of the extinction coefficient k as a function of the wavelength, λ. Generally, the extinction coefficient is related to the absorption properties of a material. It is seen that the extinction 6

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Materials Chemistry and Physics 236 (2019) 121795

where N is the concentration of Er3þ in the glass system. The polaron radius and interionic distance are decreasing as the Er3þ ions increased as illustrated in Fig. 8. The decrease could be connected to the decrease in the number of non-bridging oxygens (NBOs). The dop­ ants ions in the glass network forms bonds with NBOs where it acts as network modifier and decreases the number of NBOs in the studied glass structure. The decrease in inter ionic distances improves the glass den­ sity at higher concentration of Er3þ ions [38]. Moreover, the decrement in the inter-ionic distance will result to a stronger field strength inside the glass network [39]. 3.5. Absorption (δabs) and emission ðδem) cross section The cross sections are calculated using the measured absorption spectra given in Fig. 9 and McCumber theory so as to understand 1.5 μm emission property. Fig. 10 displays the stimulated emission cross sec­ tions of these study glasses at 4I15/2 → 4I13/2 transition of Er3þ ions. Moreover, from the absorption spectra, the δabs could be obtained using Fig. 12. NIR spectrum for 4I13/2 → 4I15/2 transition of Er3þ ions of TBSE glass system.

δabs ðλÞ ¼

=

ε hcλ 1

δem ðλÞ ¼ δem ðλÞe

(10)

� �13 1 N

(13)

kT

where ε is the net free energy needed to excite an Er3þ ion from the ground state 4I5/2 to 4I13/2 state at a temperature T, K and h are Boltz­ mann and Planck’s constants respectively. Another significant factor required to evaluate the laser media per­ formance is the optical gain. The values are evaluated from the δabs and δemi cross sections at room temperature using the expression below

=

(11)

(14)

G(λ) ¼ δemis(λ)NP – δabs(λ)N(1-)P

1200

1000

800

1000 800

Intensity (a.u)

ri ð� AÞ ¼

(12)

where AB is the absorbance (obtained from the absorption spectra), N is the concentration of Er3þ ions, and l is the sample thickness. The stim­ ulated emission cross section is evaluated using the McCumber theory [2,30]. � �

particles. This could be due to the increase in the size of Er NPs after the formation of the glass that is caused by the Ostwald ripening mechanism where the smaller particles dissolve or coalesce while the larger ones grow [36]. The agglomeration is also due to the high force of attraction between the Er NPs [37]. The polaron radius and interionic distance rp (Å)and ri (Å) were evaluated using the expressions 1 1 �π �2 rp ð� AÞ ¼ 2 6N

2:203AB ðλÞ Nl

4

4

F9/2 - I15/2

600

400

Er NPs 0.01 Er NPs 0.02 Er NPs 0.03 Er NPs 0.04 Er NPs 0.05

200

600 0

0.01

0.02

0.03

0.04

0.05

Er NPs + Ag concentration (molar fraction)

400 200

4 2

4

H11/2 - I15/2

4

S3/2 - I15/2

0 500

600

700

800

Wavelength (nm)

Fig. 13. PL spectra of Er2O3 NPs doped TBSE glasses. 7

900

1000

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Materials Chemistry and Physics 236 (2019) 121795

around 561 nm are attributed to 4S3/2→4I15/2 Er3þ ions transitions [43]. The peak seen around 470 nm wavelength corresponds to 4H11/2→4I15/2 transition. Furthermore, another peak is observed around 758 nm cor­ responding to 4F9/2→4I15/2. The intensity of 4S3/2→4I15/2, 4H11/2→4I15/2 and 4F9/2→4I15/2 transitions show an increasing trend with Er3þ con­ centration and later decrease at 0.04 and 0.05 concentrations, the decrease is attributed to quenching effect [44–46]. At the higher Er3þ NPs concentration, the distance between the Er3þ NPs ions decrease which also increases the ionic interaction. The change in the emission intensity with Er3þ NPs concentrations is illustrated, it is also observed that the increment in Er3þ NPs concentration results to the increase in luminescence intensities, further increase in erbium decrease the in­ tensities. The decrease is caused by concentration quenching which occurs as a result of transfer of energy between the Er3þ to Er3þ ions [9, 43]. Fig. 14 displays the energy level diagram of the erbium nano­ particles doped silica borotellurite bioactive glass excited at 380 nm, the diagram shows the excitation of Er3þNPs from 4I15/2 ground state to 4 G11/2 excited state, the excited ions at 4G11/2 are further excited to 2 H11/2 through excited state absorption (ESA) process [47,48]. The Er3þ NPs ions in these state decay non radiatively to lower state through 4S3/2 - 4I15/2 (green emission). The Er3þ NPs ions furthermore decay non-radiatively to 4S3/2 → 4F9/2 state accompanied by 4F9/2→4I15/2 transition (red emission) [49]. 4. Conclusion

Fig. 14. Schematic energy level diagram of Er3þ NPs doped TBSE glasses.

In summary, this research described the structural and optical properties of Er3þ NPs doped silica borotellurite bioactive glasses con­ taining silver oxide. The XRD result confirmed the glasses are in amor­ phous state. The particle size of erbium nanoparticles obtained from TEM image varies from 22 to 28 nm with average size of ~20 nm. The FTIR spectra show the structural modifications of the glass with the addition of Ag2O to Er3þ NPs. McCumber theory is used to obtain the emission cross section. The optical gain coefficient to the population inversion of the 4I13/2 → 4I15/2 laser transition of Er3þ NPs was analysed. It is revealed that the Er3þ NPs doped bio-silica borotellurite glass shows a high emission cross section of (12.11 � 10 21 cm2). Moreover, the broadband width emission with FWHM of 69.41 nm is obtained with high gain bandwidth of about 818 � 10 28 cm3 . The calculated laser parameters values showed that Er3þ NPs doped bio-silica borotellurite glass is a future glass candidate for broadband amplifiers as well as a potential erbium based laser glass which could have an application in medical and dental practice due to its provision of appropriate energy delivery without possible heat buildup in human tissue during diagnosis.

where P is the population inversion for 4I 15/2→2H 11/2, 4I15/2→4S3/2, 4I 4 3þ NPs ions. 15/2 → F9/2 transitions of Er The δemi (λ) for 4I13/2 → 4I15/2 laser transition of Er3þ NPs ion was assessed using McCumber theory and the result is presented in Table 3. The δe(λ) for 4I13/2 → 4I15/2 laser transition is presented in Fig. 10. The δa (λ) and δemi (λ) values of the studied glasses are compared with previous literature [19]. It is well acknowledged that the larger values obtained δemi (λ) shows low threshold and high gain laser performance. Therefore, these studied glasses might be suitable for laser applications particularly in the near infrared (NIR) region. The quality factor bandwidth is measured generally by the product of FWHM � δemi. The properties of the amplifier are said to be better when the product is larger [46]. The amplification quality factors with relevant spectral parameters of the Er3þ NPs doped TBSE glasses are listed in Table 3. Fig. 11illustrates the gain cross-section for 0.01 Er3þ-doped bio-silica borotellurite glass with different population inversion. It is observed from the figure that the value of the population inversion is positive for factor 0.5, which covers C and L communication bands. However, for the population inversion around 85%, the laser emission would shift to the shorter wavelength when the pump power is increased. The peak of the gain cross-section also shifts towards shorter wavelengths with increasing inversion. The FWHM � δe product of the 4I13/2 state is a critical quality parameter for solid state laser evaluation [40]. Larger values of FWHM � δe product indicates broader gain bandwidth with lower pump threshold power as illustrated in Fig 12. The experimental results show that 0.03 Er NPs glass has large bandwidth quality factor (818 � 10 28 cm3) compare to those reported in the literature for silicate (220 � 10 21 cm2 nm) [41], tellurite (448.2 � 10 21 cm2 nm) [42], which propose that the present glass could be future candidate host material for solid state laser.

Acknowledgement This research was financially supported by Geran Putra Berimpak, Universiti Putra Malaysia through the grant number (vot no: 9597200). References [1] W. Stambouli, H. Elhouichet, C. Barthou, M. F�erid, Energy transfer induced photoluminescence improvement in, J. Alloy. Comp. 580 (2013) 310–315. [2] C.R. Kesavulu, et al., In fl uence of Er 3 þ ion concentration on optical and photoluminescence properties of Er 3 þ -doped gadolinium-calcium silica borate glasses, J. Alloy. Comp. 683 (2016) 590–598. [3] N.A. Nabilah Razali, et al., The physical and optical studies of erbium doped borosilicate glass, J. Phys. Conf. Ser. 1083 (1) (2018) 12004. [4] M.N. Azlan, M.K. Halimah, H.A.A. Sidek, “Author ’ s Accepted Manuscript Linear and nonlinear optical properties of erbium doped zinc borotellurite glass system, J. Lumin. 181 (2016) 400–406. [5] K. Kopczynski, et al., Er 3 1 and Yb 3 1 Doped Active Media for ‘eye Safe’ Laser Systems vol. 301, 2000, pp. 398–406. [6] Y. Tian, X. Jing, S. Xu, “Spectrochimica Acta Part A : Molecular and Biomolecular Spectroscopy Spectroscopic Analysis and Efficient Diode-Pumped 2 . 0 l m emission, vol. 115, SAA, 2013, pp. 33–38. [7] Y. Tian, R. Xu, L. Zhang, L. Hu, J. Zhang, Observation of 2 : 7 μ M Emission from Diode-Pumped Er 3 þ ¼ Pr 3 þ -codoped Fluorophosphate Glass, vol. 36, 2011, pp. 109–111, 2.

3.6. Luminescence spectra Fig. 13 shows the emission spectra of Er3þ NPs doped bio-silica borotellurite glasses with 380 nm excitation and the dependence of in­ tensity peak for 4F9/2 – 4I15/2 transition on the Er3þ ion concentrations. The result reveals three emission bands that are assigned to 4S3/2-4I15/2, 4 H11/2-4I15/2 and 4F9/2→4I15/2. Thus, the green emissions observed 8

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