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XPS investigation of the photon degradation of Znq2 green organic phosphor
Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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XPS investigation of the photon degradation of Znq2 green organic phosphor Mart-Mari Duvenhage, Jacobus J. Terblans, Martin Ntwaeaborwa, Hendrik C. Swart n Department of Physics, University of the Free State, Bloemfontein, South Africa
art ic l e i nf o
a b s t r a c t
Article history: Received 14 May 2015 Received in revised form 17 July 2015 Accepted 18 July 2015
By substituting Al with Zn to form bis-(8-hydroxyquinoline) zinc (Znq2), the device performance of organic light emitting diodes (OLED) can be improved. Znq2 also has a more closed packed crystal structure that makes it less vulnerable to reactions with atmospheric oxygen and moisture leading to more stable and longer lasting devices. In this work the effect of photon degradation of Znq2 in air was investigated. Znq2 powder was synthesized using a co-precipitation method and recrystallized in acetone. The structure of the sample was confirmed to be Znq2 2H2O by X-ray diffraction. The photoluminescence (PL) emission data also confirmed that the Znq2 2H2O crystal form of Znq2 was present. To study the photon degradation, the sample was irradiated with a UV lamp for 400 h. The emission data was collected and the change in PL intensity with time was monitored. X-ray photoelectron spectroscopy was performed on the as prepared and photon-degraded samples. The Zn2p and N1s peaks showed no change after degradation. The O1s and C1s peaks confirmed that the phenoxide ring ruptured and that C ¼O and C–O species had formed. & 2015 Elsevier B.V. All rights reserved.
Keywords: OLED Znq2 Photoluminescence Photon degradation X-ray photoelectron spectroscopy
1. Introduction Although tris-(8-hydroxyquinoline) aluminum (Alq3) is used as a green emissive layer in organic light emitting diodes (OLED) [1], it tends to degrade with time leading to a decrease in device performance and efficiency. Bing-she et al. [2] have reported that by substituting Al with Zn to form bis-(8-hydroxyquinoline) zinc (Znq2), the Znq2 shows advantages over the Alq3 in electron transport and higher quantum yields in device performance which would result in lower operating voltages. Alq3 is very sensitive to the atmospheric environment and the performance of Alq3 is effected by oxygen, moisture and light exposure. Duvenhage et al. [3] showed that the luminescence intensity will decrease by 50% within the first 24 h of exposure to UV light. A decrease of 90% in luminescence intensity was observed after 300 h of irradiation. This decrease was ascribed to the rupturing of the phenoxide ring due to oxidation. The oxygen and moisture in the atmosphere reacted with the phenoxide ring by breaking the C–C bond at the C-7 position. C–O and C¼ O bonds will form at this position. The highest occupied molecular orbital n
Corresponding author. E-mail address: [email protected] (H.C. Swart).
(HOMO) is mainly situated on the phenoxide ring and the lowest unoccupied molecular orbital (LUMO) on the pyridyl ring. The emission of metal quinolates originates from the ligand's electronic π–π* transitions [4]. If the ring is broken these transitions can not take place anymore and the molecule is rendered nonluminescent. By substituting Al with Zn the molecular structure and molecular packing of the molecule is changed. Photoluminescence (PL), charge transport properties, energy band offset and environmental stability are strongly coupled to molecular structure and bulk molecular packing characteristics [5]. In this study the effect of molecular packing on the stability of the PL intensity of Znq2 is studied.
2. Experimental 2.1. Synthesis Znq2 phosphor powder was synthesized using the co-precipitation method [6]. 0.555 g of 8-hydroxyquinoline (8-Hq) was added to a mixture of 6.5 ml H2O and 6.5 ml of acetic acid. It was stirred for 15 min. 0.5 g Zn(NO3)2 was added to 20 ml H2O and was
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Please cite this article as: M.-M. Duvenhage, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.07.019i
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2
500
300000 Alq3 Znq2
Znq2 Alq3
I ntens ity (arb unit s )
Count s (arb units )
400 200000
100000
300
200
100
0 0
10
20
30
40 50 2θ (degrees)
60
70
0 420
80
440
460
480
500 520 540 560 Wavelength (nm)
580
600
620
Fig. 1. (a) XRD spectra and (b) PL spectra of Alq3 and Znq2.
1.0
1800
Znq2 Alq3
Before deg After 10 hours After 20 hours After 100 hours After 200 hours After 400 hours
1600 1400 I ntens ity (arb unit s )
I n t en s it y ( arb u n it s )
0.8
0.6
0.4
1200 1000 800 600 400
0.2
200
0.0
0
0
100
200 Time (hours)
300
400
440
480
520 560 Wavelength (nm)
600
640
Fig. 2. Degradation of Znq2 under UV exposure (λ ¼365 nm). (a) Quenching of luminescence with time and (b) evolution of the emission band with time.
stirred for 15 min. The Zn(NO3)2 solution was added drop wise to the 8-Hq solution with vigorous stirring. The resulting brown mixture was stirred for 15 min. 5 ml of NH4OH was added drop wise to the mixture while stirring. A yellow green precipitate formed. The precipitate was filtered and washed several times with distilled water. The precipitate was left to dry overnight at 80 °C. After drying the precipitate was ground to get a fine powder. The powder was dissolved in 10 ml acetone and left to recrystallize in atmosphere at room temperature. 2.2. Characterization The crystal structure of the samples were determined by XRD (X-ray diffraction) using a Bruker D8 Advance Diffractometer equipped with a Cu Kα source. The excitation and emission PL data were collected with a Cary Eclipse fluorescence spectrophotometer equipped with a Xenon flash lamp. To study the photon degradation, the sample was irradiated with an 8 W Matelec UV lamp for 400 h. The emission data was collected every 10 min using a HR4000CG-UV-NIR Ocean Optics spectrometer. The chemical composition of the samples was analyzed using a PHI 5000 versa probe X-ray photoelectron spectrometer (XPS). Monochromatic Al Kα radiation (hν ¼1486.6 eV) was used for the XPS measurements. High resolution scans were recorded using a
0.1 eV/step and a step time of 100 ms with a pass energy of 11.75 eV. The sample area analyzed was 1 mm2 and the pressure during data acquisition was in the order of 1 10 8 Torr.
3. Results and discussion XRD measurements were performed on the Znq2 sample and the results were compared to that obtained for Alq3 (Fig. 1(a)). It can be seen that the crystal structure of the two samples are not the same. The main reason for this is that Znq2 only has two quinoline rings compared to the three of Alq3. The main diffraction peaks of Znq2 occurs at 2θ ¼ 6.96°, 16.5°, 18.26°, 21.0°, 23.5°, and 28.96°. The peaks can be indexed to be the Znq2 2H2O crystal form of bis(8-hydroxyquinoline) zinc [2,7]. Fig. 1(b) shows the PL spectra of Alq3 and Znq2 excited at 330 nm. Znq2 will crystallize in either the Znq2 2H2O or (Znq2)4. The latter was observed from the samples prepared at temperatures of 135 °C or higher. The PL spectrum of (Znq2)4 showed a peak at 542 nm whereas the PL spectrum of Znq2 2H2O showed a PL peak at 506 nm [8]. This indicated that the HOMO–LUMO gap in Znq2 2H2O is broader. As can be seen from Fig. 1, the Znq2 PL peak has a maximum at 506 nm indicating that the Znq2 2H2O crystal form of Znq2 formed during synthesis. The PL intensity of Znq2 is
Please cite this article as: M.-M. Duvenhage, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.07.019i
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3
Fig. 3. High resolution XPS peaks of the as-prepared Znq2 sample.
also 2 times higher than that of Alq3. The change in crystal structure for the Znq2 sample can lead to more π–π overlaps between neighboring molecules giving rise to an increase in PL intensity. OLED devices made of Znq2 might lead to more efficient devices. Fig. 2(a) shows the normalized PL intensity of Znq2 and Alq3 during the 400 h of UV exposure. Both bands showed a decrease in intensity with time. A decrease of 80% was observed for the Alq3 band while a decrease of only 30% was observed for the Znq2 band. This indicates that the Znq2 sample was more stable during photon degradation. The fact that Znq2 only has two quinoline ligands, and therefore a different molecular structure than Alq3, might lead to this improvement in stability [5]. From Fig. 2 (b) it can be seen that the luminescence intensity decreased rapidly ( 10%) in the first few hours after exposure, but slowed down for longer exposure times. XPS measurements were done on the as-prepared and degraded samples of Znq2. Fig. 3 shows the high resolution XPS spectra of the as-prepared sample. The Zn2p peak consists of two
peaks at 1021.5 eV and 1044.6 eV. These peaks are attributed to the O–Zn–O bond. The O1s peak consists of two peaks. The one at 530.9 eV is attributed to the C–O–Zn bond and the one at 532.2 eV is attributed to chemisorbed species such as –CO3, adsorbed O2, or adsorbed H2O [9]. Only one peak is observed for N1s at 399.3 eV and this peak is attributed to the C ¼N–C bond [10,11]. Fig. 4(a) shows the high resolution C1s XPS peak of the asprepared Znq2 sample. Carbon appears in Znq2 in five chemical environments (Fig. 5(b)) resulting in five identifiable binding energies for the C1s transition. Each chemical state and the proportions it occurs in are summarized in Table 1. The sixth peak is attributed to chemisorbed species such as CO and CO2. Fig. 5 shows the fitted XPS peaks of the high resolution scans of the degraded samples of Zn2p, O1s and N1s. The fitted peaks for Zn2p at 1021.9 eV and 1045.0 eV correspond to the O–Zn–O bond. Three peaks were fitted for the O1s peak. The peak at 530.8 eV corresponds to the C–O–Zn bond and the one at 531.5 eV is attributed to chemisorbed species. The intensity of the C–O–Zn is higher than that of the chemisorbed species, indicating that most
Please cite this article as: M.-M. Duvenhage, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.07.019i
M.-M. Duvenhage et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
H H H
H
4
C C
C
C
C
C
6
5
C C
2
H H
N H
C O
Zn O
1
H C
C
C C
3
H
H
C
H
C
N C
C
C H
Fig. 4. (a) High resolution C1s XPS peak of as-prepared Znq2 and (b) the as-prepared Znq2 molecule showing the different carbon bonds.
Fig. 5. High resolution XPS peaks of the degraded Znq2 sample.
M.-M. Duvenhage et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
5
Table 1 Carbon bonds for as-prepared Znq2. Bond number (eV)
Carbon bond
1 (284.7)
HC
Number of bonds
CH2
C
2
O HC
C
C
10
HC
H C
CH
2
HC
C C
H
5 (285.8)
HC
N C
N
6 (284.1)
Chemisorbed C
2 (284.95)
3 (283.4)
4 (285.3)
2
2
C
O
H H
4
C
C
H
O
5 2
C O
1
3
C C
3
H
C
O
C N
Zn
C
+
H
C H
Fig. 6. (a) High resolution C1s XPS peak of P2 and (b) part of the degraded Znq2 molecule (P2) showing the different carbon bonds.
of the chemisorbed species have reacted with the ozone produced by the UV light and have left the surface. The third peak at 532.8 eV is attributed to the carbonyl and methoxy O1s species that are present in the degraded products [12]. Only one peak is fitted for the N1s peak at 399.6 eV and it corresponds to the C– N ¼C bond.
Degraded Znq2 can also form the four degraded products proposed by Rosseli [13] when the phenoxide ring ruptures. Fig. 6 (a) and (b) shows the fitted carbon peaks of one of these four products ( Table 2). It can be seen that C¼ O and C–O bonds had formed after prolonged exposure of the sample to UV photons. Although the
Please cite this article as: M.-M. Duvenhage, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.07.019i
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6
Table 2 Carbon bonds for degraded Znq2. Bond number (eV)
Carbon bond
1 (283.9)
O
C
C
2 (285.75)
HC
O C
N
3 (284.7)
4 (284.9)
5 (284.5)
HC
Number of bonds
C C
4
CH
HC
C C
C
HC
H C
H
2
2
8
2
N degradation of the sample was much less severe than that of Alq3, the sample still reacted with the oxygen and moisture in the atmosphere. The closer packed crystal did contribute to a lesser effect in degradation, but a physical barrier, like encapsulation, is still needed to remove the effect of atmospheric reaction with Znq2.
4. Conclusion Znq2 powder was successfully synthesized using the co-precipitation method. XRD measurements confirmed that the Znq2 2H2O crystal had formed during the synthesis. The PL data
also confirmed that the Znq2 2H2O crystal form was present with a broad emission peak centered at 506 nm. Under prolonged UV exposure it was observed that the luminescence intensity decreased with time. Only a 30% decrease in intensity was observed compared to an 80% decrease for Alq3 powder under the same conditions. XPS studies done on the degraded powder confirmed the presence of C–O and C ¼O bonds after prolonged exposure to UV. This is an indication that the phenoxide ring had ruptured during UV exposure due to oxidation. There was no change to the C–N ¼C bond confirming that the pyridyl ring stayed intact.
Acknowledgments This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology, and the National Research Foundation of South Africa (Grant no. 84415).
References [1] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913–915. [2] X. Bing-she, H. Yu-ying, W. Hua, Z. He-feng, L. Xu-guang, C. Ming-wei, Solid State Commun. 136 (2005) 318–322. [3] M.M. Duvenhage, O.M. Ntwaeaborwa, H.C. Swart, Phys. B-Condens. Matter 407 (2012) 1521–1524. [4] M.M. Shi, J.J. Lin, Y.W. Shi, M. Ouyang, M. Wang, H.Z. Chen, Mater. Chem. Phys. 115 (2009) 841–845. [5] G. Yu, S. Yin, Y. Liu, Z. Shuai, D. Zhu, J. Am. Chem. Soc. 125 (2003) 14816–14824. [6] J.G. Mahakhode, B.M. Bahirwar, S.J. Dhoble, S.V. Moharil, in: Proceedings of ASID, 2006, 237 p. [7] X. Wang, M. Shao, L. Liu, Synth. Met. 160 (2010) 718–721. [8] R. Reisfeld, E. Zigansky, T. Saraidarov, Opt. Mater. 30 (2008) 1706–1709. [9] L.L. Yang, Q.X. Zhao, M. Willander, X.J. Liu, M. Fahlman, J.H. Yang, Appl. Surf. Sci. 256 (2010) 3592–3597. [10] V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, R.E. Kroon, J.J. Terblans, S.K.K. Shaat, A. Yousif, M.M. Duvenhage, Mater. Lett. 101 (2013) 57–60. [11] L.L. Yang, Q. Zhai, M. Willander, X. Liu, M. Fahlman, J.H. Yang, Appl. Surf. Sci. 11 (2010) 3592–3597. [12] L.P. Buchwalter, G. Czornyj, J. Vac. Sci. Technol. A 8 (1990) 781–784. [13] F.P. Rosseli, W.G. Quirino, C. Legnani, V.L. Calil, K.C. Teixeira, A.A. Leitao, R. B. Capaz, M. Cremona, C.A. Achete, Org. Electron. 10 (2009) 1417–1423.
Please cite this article as: M.-M. Duvenhage, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.07.019i
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
XPS investigation of the photon degradation of Znq2 green organic phosphor Mart-Mari Duvenhage, Jacobus J. Terblans, Martin Ntwaeaborwa, Hendrik C. Swart n Department of Physics, University of the Free State, Bloemfontein, South Africa
art ic l e i nf o
a b s t r a c t
Article history: Received 14 May 2015 Received in revised form 17 July 2015 Accepted 18 July 2015
By substituting Al with Zn to form bis-(8-hydroxyquinoline) zinc (Znq2), the device performance of organic light emitting diodes (OLED) can be improved. Znq2 also has a more closed packed crystal structure that makes it less vulnerable to reactions with atmospheric oxygen and moisture leading to more stable and longer lasting devices. In this work the effect of photon degradation of Znq2 in air was investigated. Znq2 powder was synthesized using a co-precipitation method and recrystallized in acetone. The structure of the sample was confirmed to be Znq2 2H2O by X-ray diffraction. The photoluminescence (PL) emission data also confirmed that the Znq2 2H2O crystal form of Znq2 was present. To study the photon degradation, the sample was irradiated with a UV lamp for 400 h. The emission data was collected and the change in PL intensity with time was monitored. X-ray photoelectron spectroscopy was performed on the as prepared and photon-degraded samples. The Zn2p and N1s peaks showed no change after degradation. The O1s and C1s peaks confirmed that the phenoxide ring ruptured and that C ¼O and C–O species had formed. & 2015 Elsevier B.V. All rights reserved.
Keywords: OLED Znq2 Photoluminescence Photon degradation X-ray photoelectron spectroscopy
1. Introduction Although tris-(8-hydroxyquinoline) aluminum (Alq3) is used as a green emissive layer in organic light emitting diodes (OLED) [1], it tends to degrade with time leading to a decrease in device performance and efficiency. Bing-she et al. [2] have reported that by substituting Al with Zn to form bis-(8-hydroxyquinoline) zinc (Znq2), the Znq2 shows advantages over the Alq3 in electron transport and higher quantum yields in device performance which would result in lower operating voltages. Alq3 is very sensitive to the atmospheric environment and the performance of Alq3 is effected by oxygen, moisture and light exposure. Duvenhage et al. [3] showed that the luminescence intensity will decrease by 50% within the first 24 h of exposure to UV light. A decrease of 90% in luminescence intensity was observed after 300 h of irradiation. This decrease was ascribed to the rupturing of the phenoxide ring due to oxidation. The oxygen and moisture in the atmosphere reacted with the phenoxide ring by breaking the C–C bond at the C-7 position. C–O and C¼ O bonds will form at this position. The highest occupied molecular orbital n
Corresponding author. E-mail address: [email protected] (H.C. Swart).
(HOMO) is mainly situated on the phenoxide ring and the lowest unoccupied molecular orbital (LUMO) on the pyridyl ring. The emission of metal quinolates originates from the ligand's electronic π–π* transitions [4]. If the ring is broken these transitions can not take place anymore and the molecule is rendered nonluminescent. By substituting Al with Zn the molecular structure and molecular packing of the molecule is changed. Photoluminescence (PL), charge transport properties, energy band offset and environmental stability are strongly coupled to molecular structure and bulk molecular packing characteristics [5]. In this study the effect of molecular packing on the stability of the PL intensity of Znq2 is studied.
2. Experimental 2.1. Synthesis Znq2 phosphor powder was synthesized using the co-precipitation method [6]. 0.555 g of 8-hydroxyquinoline (8-Hq) was added to a mixture of 6.5 ml H2O and 6.5 ml of acetic acid. It was stirred for 15 min. 0.5 g Zn(NO3)2 was added to 20 ml H2O and was
http://dx.doi.org/10.1016/j.physb.2015.07.019 0921-4526/& 2015 Elsevier B.V. All rights reserved.
Please cite this article as: M.-M. Duvenhage, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.07.019i
M.-M. Duvenhage et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
500
300000 Alq3 Znq2
Znq2 Alq3
I ntens ity (arb unit s )
Count s (arb units )
400 200000
100000
300
200
100
0 0
10
20
30
40 50 2θ (degrees)
60
70
0 420
80
440
460
480
500 520 540 560 Wavelength (nm)
580
600
620
Fig. 1. (a) XRD spectra and (b) PL spectra of Alq3 and Znq2.
1.0
1800
Znq2 Alq3
Before deg After 10 hours After 20 hours After 100 hours After 200 hours After 400 hours
1600 1400 I ntens ity (arb unit s )
I n t en s it y ( arb u n it s )
0.8
0.6
0.4
1200 1000 800 600 400
0.2
200
0.0
0
0
100
200 Time (hours)
300
400
440
480
520 560 Wavelength (nm)
600
640
Fig. 2. Degradation of Znq2 under UV exposure (λ ¼365 nm). (a) Quenching of luminescence with time and (b) evolution of the emission band with time.
stirred for 15 min. The Zn(NO3)2 solution was added drop wise to the 8-Hq solution with vigorous stirring. The resulting brown mixture was stirred for 15 min. 5 ml of NH4OH was added drop wise to the mixture while stirring. A yellow green precipitate formed. The precipitate was filtered and washed several times with distilled water. The precipitate was left to dry overnight at 80 °C. After drying the precipitate was ground to get a fine powder. The powder was dissolved in 10 ml acetone and left to recrystallize in atmosphere at room temperature. 2.2. Characterization The crystal structure of the samples were determined by XRD (X-ray diffraction) using a Bruker D8 Advance Diffractometer equipped with a Cu Kα source. The excitation and emission PL data were collected with a Cary Eclipse fluorescence spectrophotometer equipped with a Xenon flash lamp. To study the photon degradation, the sample was irradiated with an 8 W Matelec UV lamp for 400 h. The emission data was collected every 10 min using a HR4000CG-UV-NIR Ocean Optics spectrometer. The chemical composition of the samples was analyzed using a PHI 5000 versa probe X-ray photoelectron spectrometer (XPS). Monochromatic Al Kα radiation (hν ¼1486.6 eV) was used for the XPS measurements. High resolution scans were recorded using a
0.1 eV/step and a step time of 100 ms with a pass energy of 11.75 eV. The sample area analyzed was 1 mm2 and the pressure during data acquisition was in the order of 1 10 8 Torr.
3. Results and discussion XRD measurements were performed on the Znq2 sample and the results were compared to that obtained for Alq3 (Fig. 1(a)). It can be seen that the crystal structure of the two samples are not the same. The main reason for this is that Znq2 only has two quinoline rings compared to the three of Alq3. The main diffraction peaks of Znq2 occurs at 2θ ¼ 6.96°, 16.5°, 18.26°, 21.0°, 23.5°, and 28.96°. The peaks can be indexed to be the Znq2 2H2O crystal form of bis(8-hydroxyquinoline) zinc [2,7]. Fig. 1(b) shows the PL spectra of Alq3 and Znq2 excited at 330 nm. Znq2 will crystallize in either the Znq2 2H2O or (Znq2)4. The latter was observed from the samples prepared at temperatures of 135 °C or higher. The PL spectrum of (Znq2)4 showed a peak at 542 nm whereas the PL spectrum of Znq2 2H2O showed a PL peak at 506 nm [8]. This indicated that the HOMO–LUMO gap in Znq2 2H2O is broader. As can be seen from Fig. 1, the Znq2 PL peak has a maximum at 506 nm indicating that the Znq2 2H2O crystal form of Znq2 formed during synthesis. The PL intensity of Znq2 is
Please cite this article as: M.-M. Duvenhage, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.07.019i
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3
Fig. 3. High resolution XPS peaks of the as-prepared Znq2 sample.
also 2 times higher than that of Alq3. The change in crystal structure for the Znq2 sample can lead to more π–π overlaps between neighboring molecules giving rise to an increase in PL intensity. OLED devices made of Znq2 might lead to more efficient devices. Fig. 2(a) shows the normalized PL intensity of Znq2 and Alq3 during the 400 h of UV exposure. Both bands showed a decrease in intensity with time. A decrease of 80% was observed for the Alq3 band while a decrease of only 30% was observed for the Znq2 band. This indicates that the Znq2 sample was more stable during photon degradation. The fact that Znq2 only has two quinoline ligands, and therefore a different molecular structure than Alq3, might lead to this improvement in stability [5]. From Fig. 2 (b) it can be seen that the luminescence intensity decreased rapidly ( 10%) in the first few hours after exposure, but slowed down for longer exposure times. XPS measurements were done on the as-prepared and degraded samples of Znq2. Fig. 3 shows the high resolution XPS spectra of the as-prepared sample. The Zn2p peak consists of two
peaks at 1021.5 eV and 1044.6 eV. These peaks are attributed to the O–Zn–O bond. The O1s peak consists of two peaks. The one at 530.9 eV is attributed to the C–O–Zn bond and the one at 532.2 eV is attributed to chemisorbed species such as –CO3, adsorbed O2, or adsorbed H2O [9]. Only one peak is observed for N1s at 399.3 eV and this peak is attributed to the C ¼N–C bond [10,11]. Fig. 4(a) shows the high resolution C1s XPS peak of the asprepared Znq2 sample. Carbon appears in Znq2 in five chemical environments (Fig. 5(b)) resulting in five identifiable binding energies for the C1s transition. Each chemical state and the proportions it occurs in are summarized in Table 1. The sixth peak is attributed to chemisorbed species such as CO and CO2. Fig. 5 shows the fitted XPS peaks of the high resolution scans of the degraded samples of Zn2p, O1s and N1s. The fitted peaks for Zn2p at 1021.9 eV and 1045.0 eV correspond to the O–Zn–O bond. Three peaks were fitted for the O1s peak. The peak at 530.8 eV corresponds to the C–O–Zn bond and the one at 531.5 eV is attributed to chemisorbed species. The intensity of the C–O–Zn is higher than that of the chemisorbed species, indicating that most
Please cite this article as: M.-M. Duvenhage, et al., Physica B (2015), http://dx.doi.org/10.1016/j.physb.2015.07.019i
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4
H H H
H
4
C C
C
C
C
C
6
5
C C
2
H H
N H
C O
Zn O
1
H C
C
C C
3
H
H
C
H
C
N C
C
C H
Fig. 4. (a) High resolution C1s XPS peak of as-prepared Znq2 and (b) the as-prepared Znq2 molecule showing the different carbon bonds.
Fig. 5. High resolution XPS peaks of the degraded Znq2 sample.
M.-M. Duvenhage et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
5
Table 1 Carbon bonds for as-prepared Znq2. Bond number (eV)
Carbon bond
1 (284.7)
HC
Number of bonds
CH2
C
2
O HC
C
C
10
HC
H C
CH
2
HC
C C
H
5 (285.8)
HC
N C
N
6 (284.1)
Chemisorbed C
2 (284.95)
3 (283.4)
4 (285.3)
2
2
C
O
H H
4
C
C
H
O
5 2
C O
1
3
C C
3
H
C
O
C N
Zn
C
+
H
C H
Fig. 6. (a) High resolution C1s XPS peak of P2 and (b) part of the degraded Znq2 molecule (P2) showing the different carbon bonds.
of the chemisorbed species have reacted with the ozone produced by the UV light and have left the surface. The third peak at 532.8 eV is attributed to the carbonyl and methoxy O1s species that are present in the degraded products [12]. Only one peak is fitted for the N1s peak at 399.6 eV and it corresponds to the C– N ¼C bond.
Degraded Znq2 can also form the four degraded products proposed by Rosseli [13] when the phenoxide ring ruptures. Fig. 6 (a) and (b) shows the fitted carbon peaks of one of these four products ( Table 2). It can be seen that C¼ O and C–O bonds had formed after prolonged exposure of the sample to UV photons. Although the
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6
Table 2 Carbon bonds for degraded Znq2. Bond number (eV)
Carbon bond
1 (283.9)
O
C
C
2 (285.75)
HC
O C
N
3 (284.7)
4 (284.9)
5 (284.5)
HC
Number of bonds
C C
4
CH
HC
C C
C
HC
H C
H
2
2
8
2
N degradation of the sample was much less severe than that of Alq3, the sample still reacted with the oxygen and moisture in the atmosphere. The closer packed crystal did contribute to a lesser effect in degradation, but a physical barrier, like encapsulation, is still needed to remove the effect of atmospheric reaction with Znq2.
4. Conclusion Znq2 powder was successfully synthesized using the co-precipitation method. XRD measurements confirmed that the Znq2 2H2O crystal had formed during the synthesis. The PL data
also confirmed that the Znq2 2H2O crystal form was present with a broad emission peak centered at 506 nm. Under prolonged UV exposure it was observed that the luminescence intensity decreased with time. Only a 30% decrease in intensity was observed compared to an 80% decrease for Alq3 powder under the same conditions. XPS studies done on the degraded powder confirmed the presence of C–O and C ¼O bonds after prolonged exposure to UV. This is an indication that the phenoxide ring had ruptured during UV exposure due to oxidation. There was no change to the C–N ¼C bond confirming that the pyridyl ring stayed intact.
Acknowledgments This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology, and the National Research Foundation of South Africa (Grant no. 84415).
References [1] C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913–915. [2] X. Bing-she, H. Yu-ying, W. Hua, Z. He-feng, L. Xu-guang, C. Ming-wei, Solid State Commun. 136 (2005) 318–322. [3] M.M. Duvenhage, O.M. Ntwaeaborwa, H.C. Swart, Phys. B-Condens. Matter 407 (2012) 1521–1524. [4] M.M. Shi, J.J. Lin, Y.W. Shi, M. Ouyang, M. Wang, H.Z. Chen, Mater. Chem. Phys. 115 (2009) 841–845. [5] G. Yu, S. Yin, Y. Liu, Z. Shuai, D. Zhu, J. Am. Chem. Soc. 125 (2003) 14816–14824. [6] J.G. Mahakhode, B.M. Bahirwar, S.J. Dhoble, S.V. Moharil, in: Proceedings of ASID, 2006, 237 p. [7] X. Wang, M. Shao, L. Liu, Synth. Met. 160 (2010) 718–721. [8] R. Reisfeld, E. Zigansky, T. Saraidarov, Opt. Mater. 30 (2008) 1706–1709. [9] L.L. Yang, Q.X. Zhao, M. Willander, X.J. Liu, M. Fahlman, J.H. Yang, Appl. Surf. Sci. 256 (2010) 3592–3597. [10] V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, R.E. Kroon, J.J. Terblans, S.K.K. Shaat, A. Yousif, M.M. Duvenhage, Mater. Lett. 101 (2013) 57–60. [11] L.L. Yang, Q. Zhai, M. Willander, X. Liu, M. Fahlman, J.H. Yang, Appl. Surf. Sci. 11 (2010) 3592–3597. [12] L.P. Buchwalter, G. Czornyj, J. Vac. Sci. Technol. A 8 (1990) 781–784. [13] F.P. Rosseli, W.G. Quirino, C. Legnani, V.L. Calil, K.C. Teixeira, A.A. Leitao, R. B. Capaz, M. Cremona, C.A. Achete, Org. Electron. 10 (2009) 1417–1423.
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