Optical and other spectroscopic studies of lead, zinc bismuth borate glasses doped with CuO

Optical and other spectroscopic studies of lead, zinc bismuth borate glasses doped with CuO

Physica B 406 (2011) 4366–4372 Contents lists available at SciVerse ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb Optical...

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Physica B 406 (2011) 4366–4372

Contents lists available at SciVerse ScienceDirect

Physica B journal homepage: www.elsevier.com/locate/physb

Optical and other spectroscopic studies of lead, zinc bismuth borate glasses doped with CuO Ch. Rajyasree a, P. Michael Vinaya Teja a, K.V.R. Murthy b, D. Krishna Rao a,n a b

Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522510, AP, India Department of Applied Physics, M.S. University of Baroda, Baroda 390001, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 February 2011 Received in revised form 19 July 2011 Accepted 24 August 2011 Available online 30 August 2011

10MO  20Bi2O3  (70  x)B2O3  xCuO [M ¼ Pb, Zn] with x ¼ 0, 0.4 and 0.8 (wt%) glasses were synthesized by the melt-quenching technique and were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Physical parameters, like density, and spectroscopic studies (optical absorption, EPR, FTIR and photoluminescence) were used to understand the role of modifier oxide and CuO in the glass matrix. A red shift of the absorption band corresponds to 2B1g-2B2g transition of Cu2 þ ions from P2 to Z4 samples and the increase of hyperfine splitting factor (A:) from P2 to Z2 shows that with the integration of PbO by ZnO the electron density around copper ion is increased. It is also supported by the gradual increase in theoretical optical basicity values of ZnO mixed glasses, as compared to that of PbO mixed glass matrix. Reduced bismuth radicals are found in undoped and 0.4% CuO doped glasses of both the series. Analysis of the absorption and emission studies indicates that the concentration of luminescence centers of bismuth ions (Bi3 þ ions in UV region) is decreased by the integration of ZnO as well as by increasing the dopant concentration. In lead series PbO4 and BiO3 units are increased from P2 to P4 and in zinc series BiO3 units are decreased from Z0 to Z4. The conductivity of the glass matrices is increased in both the series with the dopant of CuO. & 2011 Elsevier B.V. All rights reserved.

Keywords: Melt-quenching Electron paramagnetic resonance FTIR Bismuth borate glasses

1. Introduction Recently considerable attention has been paid to bismuth borate glasses for their potential applications such as long infrared cutoff and higher third order nonlinear optical susceptibility, which make these glasses potential candidates for making infrared transmission components, ultrafast optical switches and photonic devices, besides scope of other applications like, thermal, mechanical sensors and reflecting windows [1–4]. Borate glasses containing bismuth have been studied intensively in the recent decades for electrical, optical and thermal properties [5,6] as well as important applications in piezoelectric, ferroelectric, pyroelectric and nonlinear optical device materials [7,8]. Investigations of the secondorder non-linear optical (NLO) properties on BiB3O6 glasses showed [9,10] a very good effective frequency conversion and the effective second-order NLO susceptibility d222 is larger than that of many other crystals being widely used in NLO applications. For broad band optical amplification and laser material in the wavelength region from 1100 to 1300 nm 75B2O3–25 Bi2O3 glass is observed to be a good candidate [11]. The applications of bismuth borate glasses

n Corresponding author. Tel.: þ91 863 6458142 (res.)/ þ 91 863 2346380 (off.)/ þ91 9440712142 (mob.); fax: þ 91 863 2293378. E-mail address: [email protected] (D. Krishna Rao).

0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.08.082

are substantially extendable, in particular, during the cationic doping, like Cu2 þ ions [9,10]. In view of the potential applications of bismuth borate glasses, attempts were made to prepare MBiBCuO (M¼ Pb, Zn) glasses and also to examine their physical and structural properties with the help of optical absorption, ESR, FTIR and luminescence studies as they are very sensitive to the local symmetry, in terms of the chemical bond. In the present investigation PbO and ZnO have been chosen as network modifiers because Pb2 þ is isoelectronic ˚ is almost equal to that with Bi3 þ and ionic radius of Zn2 þ (0.74 A) ˚ The results have been presented and of dopant Cu2 þ (0.73 A). discussed in this paper.

2. Experimental Reagent grade pure PbO, ZnO, Bi2O3, H3BO3 and CuO were taken in stoichiometric proportion to get the glass system of composition 10MO  20Bi2O3  (70  x)B2O3  xCuO [R¼ Pb, Zn] with x¼0, 0.4 and 0.8 (wt%). Mechanically homogenized 15 g of the above batches was melted for 12 min in sintered porcelain crucibles at 1123 K using PID temperature controlled furnace. The molten material was quenched to room temperature and subsequently annealed at 573 K. Transparent yellow colored pure

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3. Results and discussion 3.1. Physical parameters Table 1 summarizes the physical parameters like concentration of Cu2 þ ions, their interionic separation and polaron radius calculated from the measured densities of the as prepared samples. The densities and inter ionic separations of lead series samples have been observed to be slightly higher than that of zinc series, which may be due to atomic weight of Pb2 þ being higher than that of Zn2 þ . An increase in density from P0 to P4 has been ascribed to the replacement of low atomic weight of B3 þ ions with higher Cu2 þ ions. In zinc series the density is found to decrease at lower concentration (0.4 wt%) and again it is increased at higher concentration (0.8 wt%) than Z0.

3.3. Optical absorption 3.3.1. Absorption bands Fig. 3a and b and its inset shows the optical absorption spectra of the as prepared glasses ranging from 450 to 1100 nm. P0 sample has exhibited a band at 821 nm and is quenched and blue shifted, by the integration of ZnO, to 815 nm along with a small additional peak around 685 nm. Interestingly, addition of CuO to both the host series of a broad absorption corresponds to 2B1g–2B2g transition of Cu2 þ ions that was observed in the region 740–784 nm with a shoulder at 690 nm. At higher concentration of CuO a red shift was observed in the characteristic transition of Cu2 þ ions to 791 nm with the absence of shoulder around 690 nm as shown in Table 2. Su et al. [12] observed absorption band around 680 nm in Bi: SrB4O7

Intensity (arb units)

and greenish colored doped samples obtained were characterized by means of X-ray diffraction with an X’pert pro MPD diffractometer. SEM images of the samples were obtained using Carl Zeiss, EVO MA 15, OXFORD INSTRUMENTS, Inca Penta FET  3 Scanning Electron Microscope. By standard Archimedes’ method with O-Xylene (99.99% pure) as immersion liquid the density ‘r’ of as-prepared samples was measured. The FTIR transmittance spectra of the glasses were recorded with a JASCO FT-IR 6200 spectrometer using the KBr pellet technique in the spectral range 400–1600 cm  1. The samples of about 0.5070.02 mm thickness were used for the absorption and emission spectra measurements. The absorption spectra of the samples in the range 200–3200 nm were measured by double beam UV–visible spectrophotometer (JASCO V-670) with an error of 70.1 nm. The photoluminescence (excitation and emission spectra) were recorded at room temperature by Shimadzu RF-5301 PC model spectrophotofluorometer. The electron paramagnetic resonance on 100 mg glass powder taken in quartz tube was performed using JEOL-FE-1X ESR spectrometer in the X-band frequency (E9.400 GHz) with 100 kHz field modulation frequency and 100 mW microwave power.

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10

20

30

40 50 2(θ) (degree)

60

Fig. 1. XRD pattern of the as-prepared Z4 glass.

3.2. XRD and SEM Fig. 1 shows XRD pattern of Z4 sample. The lack of long range structural order is usually defined by the technique of XRD patterns. For the samples taken from different regions of the bulk specimen, the absence of sharp Bragg peaks in the XRD pattern (Fig. 1) confirmed that the prepared samples are amorphous and homogeneous in nature. Moreover, the broad humps indicate that there is an existence of short range order in the glass network. Fig. 2 presents the SEM image of P0 sample, which also supports the amorphous nature of investigated samples. Similar results were noticed in remaining samples also.

Fig. 2. SEM image of P0 glass sample.

Table 1 The density r, molar volume Vm, Cu2 þ ion concentration Ni, mean Cu2 þ ion separation Ri and Polaron radius Rp of the glasses 10MO  20Bi2O3  (70  x)B2O3  xCuO [M¼ Pb, Zn]. Sample

x (wt%)

r (g/cm3) (7 0.0001)

Vm (cm3/mol) (7 0.001)

Ni  1021 ions/cm3

˚ Ri (A)

˚ Rp (A)

P0 P2 P4 Z0 Z2 Z4

0.0 0.4 0.8 0.0 0.4 0.8

4.8569 4.8793 5.1495 4.5831 4.5792 4.9205

33.816 33.669 31.910 32.743 32.779 30.514

– 0.072 0.151 – 0.073 0.158

– 24.08 18.78 – 23.87 18.50

– 9.703 7.567 – 9.619 7.455

70

80

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1.0 0.25 821

(P0)

0.20

789.5 Absorbance

0.15 600

700

800

900

0.5 690 740

P4 821 P2 P0

0.0 600

800 λ (nm)

1000

1.0 685

815 (Z0)

0.18

Therefore, both pure and 0.4 wt% CuO doped glass network consists of reduced Bi þ radicals along with Bi3 þ ions. At higher concentrations of CuO the absence of shoulder at about 690 nm perhaps due to either convolution of the peak with the characteristic Cu2 þ ions transition or decreasing of Bi þ ions concentration. Generally Cu2 þ (3d9) ion is rarely found in regular octahedral sites and due to the Jahn–Teller effect the ground state 2Eg splits into 2A1g and 2B1g and the 2T2g level into 2Eg and 2B2g, the ground state being 2B1g [16]. From the optical absorption spectra of asprepared glasses the broad band corresponds to the 2B1g–2B2g transitions of Cu2 þ ions, in both the series. The observed red-ward shift from 740 to 791 nm with increase in dopant concentration can be explained as follows: By dissolving trace concentrations of CuO in host glasses it coordinates with the ligand field oxide anions leading to the formation of Molecular Orbitals (MOs), which increases the electron density of inner shells. Consequently, nuclear charge shielding affects the energy involved in the 2 B1g–2B2g transitions, which decreases with increase in inner shell electron density. The electronic charge density distribution plays the principal role on the bonding–antibonding pB–pO molecular orbital. The interactions with the localized 3d Cu states should give additional Jahn–Teller splitting determining some additional contributions to the optical polarizabilities. The competition between the different localized states leads to different charge transfers, which is crucial for nonlinear optics.

0.17 0.16

Absorbance

600

700

800

900

0.5

690

Z4

784

Z2 Z0

where C is a constant and DE is the Urbach’s energy interpreted as the energy required for optical transitions between widths of the localized tail states in the forbidden gap. DE values have been calculated by taking the reciprocals of the slopes of the linear

0.0 600

800 λ (nm)

3.3.2. Optical band gap and Urbach energy To obtain optical band gaps of the as prepared samples from UV absorption edge, plots between (ahu)1/2 and hu were drawn and are presented in Fig. 4. By extrapolating the linear region of these curves towards the energy axis at (ahu)1/2 ¼0, the values of optical energy band gap (Eg) have been evaluated and are listed in Table 2 for all compositions. Eg is observed to be minimum for P2 and maximum for Z0 glass. Urbach’s energy is determined from the values of a(u) lying between102 and 104 cm  1, which is defined as Urbach’s exponential tail region [17] by the following equation (1):   hn ð1Þ aðnÞ ¼ C exp DE

1000

Fig. 3. (a) Optical absorption spectra of PbBiBCuO glasses; inset shows the absorption peak of P0 glass. (b) Optical absorption spectra of ZnBiBCuO glasses; inset shows the absorption peaks of Z0 glass.

Sample lc (nm) (7 0.1)

2

B1g-2B2g band position (nm) (70.1)

Eg (eV) (7 0.0001)

(7 0.0001)

P0 P2 P4 Z0 Z2 Z4

– 740.0 789.5 – 784.0 791.0

3.3512 3.0023 3.0975 3.4245 3.1858 3.1337

0.1787 0.2484 0.2114 0.1704 0.2590 0.2948

343.0 380.0 372.5 339.0 361.5 369.0

DE (eV)

Lth

0.42900 0.42906 0.42912 0.43260 0.43266 0.43272

3.4

20 ( αhν)1/2 (cm-1 eV)1/2

Table 2 Cutoff wavelength (lc), optical band gap (Eg), Urbach energy (DE) and theoretical optical basicity (Lth) of the glasses 10MO  20Bi2O3  (70  x)B2O3  xCuO [M¼ Pb, Zn].

25 Eg (eV)

791

Pb Zn

3.3 3.2

P2

P4 Z4

P0 Z0

Z2

3.1 3.0 0.0

15

0.4 % of CuO

0.8

10

5

0 (2.0% mol and 5.0% mol bismuth-doped) glasses and ascribed to the electronic transitions of 3P0 to 3P1 of Bi þ ions [13]. The absorption band at 815 nm in Z0 glass sample has been attributed to the electronic transition of 3P0 to 3P2 of Bi þ radicals [14,15].

2.9

3.0

3.1

3.2

3.3 3.4 hν (eV)

3.5

3.6

3.7

Fig. 4. Urbach plots for optical band gap of as prepared glasses. Inset shows the variation of optical band gap energy with composition of CuO.

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given in Table 2.

5.0 P2

P4 Z4

Lth ¼

P0 Z0

Z2

4.5

ð2Þ

where ‘n’ is the number of cations present, Zi is the oxidation number of the ith cation, ri is the ratio of the number of ith cation to the number of oxides present and gi is the basicity moderating parameter of the ith cation. To calculate the basicity moderating parameter ‘gi’ the following Eq. (3) has been used:

4.0 3.5

gi ¼ 1:36ðxi 0:26Þ 3.0 Pb Zn

0.3 ΔE (eV)

2.5 2.0

0.2

0.0

1.5 3.1

3.2

3.3

3.4 hν (eV)

3.5

0.4 % of CuO

3.6

0.8

3.7

Fig. 5. ln a vs. hu (photon energy) of investigated glasses. Inset shows the variation of Urbach’s tail gap energy with composition of CuO.

portion of the ln a(u) vs. hu curves shown in Fig. 5 and are also incorporated in Table 2. From table 2 it is clear that the DE value increases form Z0 to Z4 and P0 to P2 but in lead series at higher concentration of dopant DE value is found to decrease as shown in the inset of Fig. 5. Both the values of Eg and DE calculated for as-prepared glasses are of the same order of copper borate glasses [18] and bismuth borate glasses [17] reported in the literature. It has been observed that by the addition of CuO content the Eg values are decreased because CuO introduces additional defect states [like color centers and wrong bonds] in the matrix [19] and there is some overlap between the delocalized quasi-band states and localized d Cu states, which may give substantial changes to the optical excitation transfer. The density of localized states was found to be proportional to the concentration of these defects [20] and consequently to CuO content. The localized states of the color centers or wrong bonds overlap and extend in to the mobility gap [21], which decreases Eg with increasing CuO content. As the content of Cu2 þ ions increases in the borate network, BO3 units with non-bridging oxygen (NBOs) atoms are formed. As NBOs are more excited than bridging oxygens (BOs), the band gap is found to decrease as shown in Table 2. The band gap of P0 glass is lower than that of corresponding Z0 glass because Pb2 þ ions are highly polarizing than Zn2 þ . The Urbach energy of P0 (0.1787 eV) is higher than that of Z0 (0.1704 eV). Pb2 þ ions therefore, have much modifying effect on bismuth borate network than the Zn2 þ ions because Pb2 þ and Bi3 þ ions are isoelectronic with each other. The increase in tail gap energy from Z0 to Z4 (Table 2) with addition of CuO indicates that defects increase with Cu2 þ content in Zn series. Where as in Pb series, the observed decrease in Urbach energy at higher concentration of CuO indicates that localized states are decreased in forbidden gap and it may be due to the decrease of NBOs.

3.3.3. Theoretical optical basicity Using Eq. (2) by Duffy and Ingram. [22] theoretical optical basicity (Lth) of as prepared samples has been calculated and

ð3Þ

where ‘xi’ is the Pauling electro negativity [23] of the ith cation. From Table 2 it has been observed that with the increase in copper content as well as with integration of ZnO, the values of theoretical optical basicity (Lth) are found to increase linearly in both the series. Generally, optical basicity of a chemically complex glass represents the mean polarization state of the ligands [oxides in the present study] and their ability to transfer fractional electronic charges to the central cation. Increasing Lth value suggests that localized donor pressure on cations increases or in other words, the ionic nature of the glass increases. Bismuth always has several kinds of valence states simultaneously existing in glass and crystal [24,25]. On the basis of the optical basicity of Duffy and Ingram [26] higher basicity favors higher valence states of multivalent metal ions, as a result in P0 glass Bi3 þ ions may partially be converted into low valence states such as Bi2 þ and Bi þ ions nevertheless Zn series favors to form higher valance states. 3.4. Electron paramagnetic resonance Fig. 6 shows EPR spectra of MBiBCuO glass samples for different concentrations of CuO at room temperature. From the spectra at lower concentration of CuO, three parallel with one perpendicular component has been observed in both the series where as at higher concentration, intensity of the signal is increased with two parallel components. In order to understand the environment around Cu2 þ ions the Hamiltonian parameters

g

|| g

|

P2 Z2

First derivative of absorption

ln (α)

n X Zi ri 9Z0 9gi i¼1

P4 Z4

2400

2600

2800 3000 3200 Magnetic field B (Gauss)

3400

3600

Fig. 6. EPR spectra of Cu2 þ ions in MO  Bi2O3  B2O3 (M¼ Pb, Zn) glass matrices.

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Table 3 Spin-Hamiltonian parameters and molecular orbital bonding coefficients for P2 and Z2 glass samples. Sample gJ g? AJ  10–4 cm  1 (7 0.0001) (7 0.0001) (70.01)

a2

P2 Z2

0.7597 0.7773

2.3531 2.3345

2.0794 2.0529

120.91 127.59

b21

( 70.0001) (7 0.0001)

426

0.9204 0.8220

470 510

3.4.1. Molecular orbital coefficients Molecular orbital bonding coefficients like a2 (in plane s 2 bonding) and b1 (in plane p bonding) of Cu2 þ complex have been evaluated by correlating the optical absorption and EPR data [28,29] using the following formula:   7 AJ A 2 5 6 g?  ð4Þ  þ gJ  a2 ¼ 4 P P 3 21 7 where P ¼0.036 cm  1 and A ¼ ½ð1=3ÞAJ þ ð2=3ÞA?  " # 2 4la2 b1 gJ ¼ 2:0023 1 E1

1244 1353

678

924 P2

588

1474 P4

1600

1400

1200

1000

800

600

400

cm-1

425 517

470

758 677 Z0 %T

like g:, g? and A: for P2 and Z2 samples were calculated using formulae reported elsewhere [27] and are presented in Table 3. From Table 3, it is observed that g values of lead series are slightly higher than that of zinc series by satisfying g: 4g? 4ge, which confirms the ground state of unpaired electron is 2B1g and the site symmetry of Cu2 þ ions is tetragonally elongated octahedral (D4h) [27]. In general, we will observe hyperfine splitting in the EPR signal whenever magnetic moment of unpaired electron interacts with the magnetic moment of its own nucleus. The hyperfine splitting factor, hence indicates how close the electrons are to the nucleus, in other words, the electron density around the nucleus. In the present investigated samples the A: of Z2 is higher than that of P2. The electron density therefore, around Cu2 þ nuclei is higher in ZnO mixed glasses than that of PbO mixed glasses. The increase in theoretical optical basicity (Table 2) from P2 to Z2 also suggests that the ability of oxide ions to transfer fractional electron charges to the cations has to increase in ZnO mixed glasses.

%T

756 P0 1018

1238 1340

1013 932

Z2

Z4 ð5Þ

where E1 is the energy corresponding to the transition 2B1g-2B2g and l is spin–orbit coupling constant (¼  828 cm  1). For a complete ionic bond a2 should be 1 and if overlapping integral is vanishingly small then a2 ¼0.5 then the bond would be 2 completely covalent. The values of a2 and b1 hence lie between 0.5 and 1.0, which are the limits of pure covalent and ionic bonds, respectively. By the integration of ZnO, from Table 3, increase in a2 and decrease in b21 have been observed suggesting that inplane s-bond is more ionic whereas in-plane p-bond is slightly covalent in ZnO mixed glasses. 3.5. Fourier transform infrared spectroscopy Fig. 7a and b presents FTIR transmittance spectra of PbO and ZnO mixed glasses, respectively at room temperature. The bands observed from the spectra and their corresponding assignments are shown in Table 4, which reveal the presence of various structural units corresponding to B2O3 and Bi2O3. From the figure one can clearly observe three broad and intense absorption bands centered around 1353, 924 and 678 cm  1 have been observed in P0 sample. However, some shoulders and feeble bands located at 1244, 1018, 756, 510, 470 and 426 cm  1 are found. The broad band around 1353 cm  1 and its shoulder around 1244 cm  1 are due to symmetric stretching vibrations of B–O bonds

617

1457 1600

1400

1200

1000

800

600

400

cm-1 Fig. 7. (a) FTIR spectra of PbBiBCuO glass samples. (b) FTIR spectra of ZnBiBCuO glass samples.

in BO3 units from varied types and pyro, ortho borate groups, respectively, whereas the band at 924 cm  1 is due to asymmetric stretching vibrations of the B–O bonds in BO4 units from diborate groups [30–32]. The absorption band found at 678 cm  1 is due to B–O–B bending vibration and the feeble band at 756 cm  1 is due to O3B–O–BO4 bending vibration [30]. The absorption band at 510 cm  1 is assigned to both Bi–O stretching vibrations of BiO6 units [33,34] and Pb–O symmetrical bending vibration [35]. The peak at 470 cm  1 is assigned to characterise both the totally symmetric bending vibrations of BiO3 pyramids [33] and of the Pb–O stretching vibration of PbO4 structural units [36]. The band at 426 cm  1 is ascribed to Bi–O bending vibrations in BiO6 units. In both the doped series the band at 756 cm  1 has not been found, suggesting that there is a breakthrough in O3B–O–BO4 chains occuring with CuO addition. At lower concentration of CuO, in lead mixed glasses, a slight blue shift has been observed in B–O–B bending vibration, which may be ascribed to the formation of B–O–Cu linkages [32]. However, in ZnO mixed glasses no such

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Table 4 Assignment of absorption bands in the infrared spectra (with a probable error of 70.1 cm  1) of the glasses 10MO  20Bi2O3  (70  x)B2O3  xCuO [M ¼Pb, Zn]. P0

P2

P4

Z0

Z2

Z4

Assignment

426 470 510 – 678 756 924 1018 1244 1353 –

425 469 527 588 682 – 931 1020 1235 1337 –

425 470 528 – 691 – 954 1026 1220 1331 1474

425 470 517 – 677 758 932 1013 1238 1340 –

430 469 520 – 677 – 927 1016 1230 1330 –

428 469 523 617 679 – 921 1017 1221 1329 1457

Bi–O bonds in BiO6 units A totally symmetrical bending vibration of BiO3 units and aPb–O stretching vibrations of PbO4 units Bi–O stretch in BiO6 units and aPb–O symmetrical bending vibrations Bi–O  stretch in BiO6 units B–O–B bend O3B–O–BO4 bend B–O stretch in BO4 units from diborate groups B–O stretch in BO4 units from tri-, tetra- and penta-borate groups B–Osym stretch in BO3 units from pyro and ortho borate groups B–Osym stretch in BO3 units from varied types of borate groups B–O  stretch in BO2O  units from varied types of borate groups

For PbO mixed glasses only.

shift has been observed. The intensity and area of the band at 470 cm  1 is increased in P4 than P2 sample and is gradually decreased from Z0 to Z4 sample in ZnO mixed samples. Either PbO4 or BiO3 units, therefore, are higher in P4 than P2, while BiO3 units are gradually decreasing from Z0 to Z4. It is also supported by increase in optical band gap of P4 than P2. At 0.8 wt% of CuO a shoulder peak around 1474 and 1457 cm  1 have been observed in P4 and Z4 samples, respectively, which are due to the B–O  stretching vibrations of NBOs in BO2O  units. Interestingly small bands at 588 and 617 cm  1 in P2 and Z4 glass matrices, respectively were observed due to Bi-O  stretching vibrations of BiO6 units as shown in Fig. 7(a) and (b).

120

λex = 254 nm 100 80

398

60 40 20

3.6. Photoluminescence

0 200

300

400

500

600

700

λ (nm) 80

Z0 Z2 Z4

387

70

λex = 254 nm

60 50 Intensity

Fig. 8a and b shows the luminescence spectra of as prepared glasses excited at 254 nm. UV luminescence has been observed nearly at 372 nm (3.33 eV) for P0 sample. By the addition of CuO the intensity and area of the peak were quenched with a slight red shift in peak position has been observed. A comparison between emission spectra of P0 and Z0 glasses reveals that integration of ZnO also decreases the intensity and area of the peak and shifted the peak position to 387 nm (3.204 eV). However, in ZnO mixed glasses no substantial changes have been observed with the addition of Cu2 þ ions. The observed near UV luminescence well coincided with the reported value [13] and it is ascribed to Bi3 þ ions as follows: Depending upon different host lattices, the emission peak of Bi3 þ ions varies from UV to red region and is not located in one characteristic spectral region. The energy level diagram of Bi3 þ ions and the mechanism underlying UV emission were studied well by various researchers [13,14]. Bordun and Dmitruk [37] observed the emissions at 2.40, 3.03, 3.15 and 3.75 eV related to 3 P1–1S0 transition of Bi3 þ ions in various sites with point symmetries C2, C3i and D2. In the PbO mixed glasses, the observed red shift in peak position clearly indicates that the environment of Bi3 þ ions is strongly affected by the addition of CuO. However, the absence of such shift in ZnO mixed glasses shows Cu2 þ ions addition has no much affect on Bi3 þ site symmetry. It may be due ˚ being very close to that of to the ionic radius of Cu2 þ (0.73 A) ˚ ions. In the present investigation, therefore, the Zn2 þ (0.74 A) shift in peak position from P0 to Z4 is ascribed to site-to-site variations in the environment of emission centers. The quenching of emission peak intensity and area clearly suggests the decrease in Bi3 þ luminescence centers. Murata and Mouri [38] and Ren et al. [39] found that the increase in the optical basicity of the glasses decreases the luminescence centers and also the intensity of the emission peak. The increase in theoretical optical basicity from P0 to Z0 (Table 2) decreases the emission peak intensity in Z0 sample.

P0 P2 P4

372

140

Intensity

a

40 30 20 10 0 350

400

450

500 λ (nm)

550

600

650

Fig. 8. (a) UV luminescence of PbBiBCuO glass samples when excited at 254 nm and (b) UV luminescence of ZnBiBCuO glass samples when excited at 254 nm.

4. Conclusions Spectroscopic studies like optical absorption, EPR, FTIR and photoluminescence were carried out to examine the effect of modifier oxide (MO) (M¼Pb, Zn) on bismuth borate glasses with

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increasing dopant concentration of CuO. From the analysis of the results the following conclusions were drawn:

 The environment of Cu2 þ ions is more covalent in lead series 



and ionic in zinc series, which is in good agreement with the theoretical and experimental values. Absorption and emission studies indicate that the concentration of luminescence centers of bismuth ions (Bi3 þ and Bi þ ions) is decreased by the integration of ZnO as well as by increasing the dopant concentration. Therefore, such observations indicate that the P0 glass is a potential candidate among all investigated samples for the applications of both UV and IR amplifiers. In lead series PbO4 and BiO3 units are increased from P2 to P4 whereas in zinc series BiO3 units are decreased from Z0 to Z4. Hence, at higher concentration of CuO optical band gap increased in PbO mixed glasses and is decreased in ZnO mixed glasses. The conductivity of the glass matrix is increased in both the series with the dopant of CuO.

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