Optical properties of nanostructure boron doped NiO thin films

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parameters of the NiO film are controlled with boron doping. Keywords Nickel oxide а Boron ..... Urbach F (1953) Phys Rev 92:1434. 14. Adachi S (1999) Optical ...

J Sol-Gel Sci Technol DOI 10.1007/s10971-012-2909-1

ORIGINAL PAPER

Optical properties of nanostructure boron doped NiO thin films H. Aydin • Sh. A. Mansour • C. Aydin • Ahmed A. Al-Ghamdi • Omar A. Al-Hartomy Farid El-Tantawy • F. Yakuphanoglu



Received: 3 September 2012 / Accepted: 20 October 2012  Springer Science+Business Media New York 2012

Abstract Boron doped NiO films were prepared by sol–gel method. The effects of B content on the morphological and optical properties of NiO films were studied with atomic force microscopy, and optical characterization method. The average transmittance at the visible region is reached to 75 % for lower doped films (0.1 and 0.2 % B), whereas, the recorded average value of transmittance was about 62 % for doped film with 1 % B throughout the region. The optical energy gap value for pure NiO film was found to be 3.73 eV. These values were affected by B doping with non-monotonic variation and reached to 3.64 eV for 0.1 % B doped NiO. Also, the refractive index dispersion and dielectric constants of the NiO films were studied throughout the investigated range of

H. Aydin  Sh. A. Mansour (&)  F. Yakuphanoglu Physics Department, Faculty of Science, Firat University, Elazig, Turkey e-mail: [email protected] Sh. A. Mansour Basic Engineering Science Department, Faculty of Engineering, Menofia University, Shebin El-Kom, Egypt C. Aydin Metallurgy and Materials Engineering Department, Faculty of Technology, Firat University, Elazig, Turkey A. A. Al-Ghamdi  O. A. Al-Hartomy  F. Yakuphanoglu Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia O. A. Al-Hartomy Department of Physics, Faculty of Science, Tabuk University, Tabuk 71491, Saudi Arabia F. El-Tantawy Department of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypt

wavelengths. The obtained results indicate that the optical parameters of the NiO film are controlled with boron doping. Keywords index

Nickel oxide  Boron  Sol–gel  Refractive

1 Introduction Materials in nano-scale exhibit novel properties in comparison with its macro-scale counterparts. Among these materials, transition metal oxides have been extensively studied in nano-scale and showed a potential applications in electronic, optic, magnetic and electrochromic (EC) devices [1, 2]. EC materials are able to reversibly and persistently change their optical properties under an applied electric field [3]. Due to this unique behavior, EC materials have being extensively studied by many authors to develop high-efficiency, low cost and durable devices that can be used in large area surfaces [3]. Among the various materials, nickel oxides thin films stand out as particularly viable and combine: (1) a large span in optical density between full bleached and full colored states; (2) good durability; (3) relatively low materials cost, etc… [4]. To enhance the EC properties of the NiO thin films, many approaches, such as substitution for other elements (c.f. [5] and references cited therein), adding protective layers [6] and optimizing the process of heat-treatment [7, 8] were used. Recently, many researchers begin to investigate the doping of NiO with metal ions. Avendano et al. [9] doped NiO with a wide variety of additives to enhance optical properties and durability. Significantly fewer papers have discussed boron (B) doped NiO films [10, 11], and none to date have studied the effect of B addition on the refractive index dispersion and dielectric constants of NiO films.

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Accordingly, this work focuses on sol–gel synthesis of thin films of pure NiO and doped by boron (B) particularly at 0.1, 0.2 and 1 % of B. Also, the variation of some optical parameters (optical energy gap, refractive index and dielectric constants) with different doping concentrations of B doped NiO nanosturcture thin films were studied.

of boric acetate. For those samples, a B2O3 phase was observed. This confirms that the NiO samples doped with high percentages of boric acetate are boron oxide doped NiO samples and this result is in also agreement with the results of Lou et al. [11]. Thus, we have evaluated that the samples doped with low percentages of boric acetate are B doped NiO samples.

2 Experimental details

3.2 Optical properties of the undoped and doped NiO nanostructure thin films

Undoped and B-doped NiO thin films have been deposited by sol–gel spin coating method onto glass substrates. As a starting material and dopant source, nickel acetate tetrahydrate [C4H6NiO44H2O) and boric acid were used. 2-Methoxethanol and monoethanolamine (MEA) were used as a solvent and stabilizer, respectively. These solutions of 0.5 M were mixed together in nominal ratios of the B/NiO (0, 0.1, 0.2 and 1 %). The obtained sols were stirred at 60 C for 2 h to yield a clear and homogeneous solution. The solutions were coated onto the substrate at 1,000 rpm for 0.5 s. Then the films were coated with five layers. Finally, all the films were annealed at 400 C for 1 h. The structural properties of the films were investigated by Park System XE-100E atomic force microscopy (AFM). The UV–Vis spectra of the films were recorded from 200 to 1,000 nm wavelength using SHIMADZU UV-3600 UV– Vis–NIR spectrophotometer at room temperature.

3 Results and discussion 3.1 Morphological characterization of the prepared thin films Figure 1 shows 2D (40 lm 9 40 lm), 2D (5 lm 9 5 lm) and 2D (1 lm 9 1 lm) AFM images of undoped and doped NiO films with different doping percentages of B. The obtained values of grain size (D), roughness (Rq) and thickness (t) for all the samples calculated using a PARK system XEI software programming are listed in Table 1. As seen in Table 1, the values of D for all the samples are very close to each other and its mean value was found to be 32.1 nm. Also, AFM images for such nanostructure films showed a small value of Rq with average value 48.8 nm. The chemical structure of NiO doped with low percentages (0.1, 0.2 and 1 %) of boron was controlled by FTIR spectra. The FTIR spectra of the films exhibited the absorption bands at 402, 415 and 442 cm-1. These bands correspond to the stretching vibrations of Ni–O bond of nickel oxide. We did not observe any peak of B2O3 phase in FTIR spectra. Also, in order to confirm the presence of boron oxide phase in the samples, we prepared another series of samples with high percentages (10, 30, 40, 50 %)

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The measured UV–Visible transmittance (T) and reflectance (R) spectra of the films at room temperature are shown in Figs. 2 and 3, respectively. The average transmittance at the visible region was reached to about 75 % for undoped and doped (0.1 and 0.2 % B) films. Whereas, for the highest doped NiO film with 1 % B shows lower transmittance than all other films in that range and its average value was observed about 62 %. This relative low transmittance value for NiO doped with 1 % film may be due to the high thickness film respect to the other films (Table 1). It is well known that the low transmittance of the deposited films mainly results from the high film thickness. The obtained average transmittance value for the undoped NiO film (75 %) is in good agreement with the bleached undoped NiO film (80 %) prepared by sol–gel method at 350 C for 2 h obtained by Lou et al. [11]. Figure 3 clearly indicated that the NiO films doped with 1 % B and 0.1 % exhibit the higher reflectance spectra than other two samples for whole investigated range. Also, Fig. 3 emerges that the reflectance spectra for both of the NiO films doped with 1 % B and 0.1 % have nonmonotonic change in wavelength range (250:600 nm), i.e., anomalous dispersion. Then, they varied in a normal dispersion manner after 600 nm. The observed anomalous behavior can be attributed to the resonance effect between the incident electromagnetic radiation and the electrons polarization, which leads to the coupling of electrons in the films to the oscillating electric field [12]. However, for undoped and 0.2 % B doped films their reflectance spectra varied in a normal dispersion manner after &390 nm. 3.2.1 Determination of the optical band gap of undoped and doped films The absorption coefficient (a) was used to determine the optical band gap of the films using the following relation [13]: ahm ¼ bðhm  Eg Þm

ð1Þ

where b-1 is the band edge parameter constant, Eg is the energy band gap between the valence band and the conduction band, m is a number characterizing the transition process which takes values 1/2, 3/2, 2 or more depending

J Sol-Gel Sci Technol

Fig. 1 2D (i 1 lm 9 1 lm, ii 5 lm 9 5 lm and iii 40 lm 9 40 lm) AFM images for a pure NiO, b 0.1 %, c 0.2 % and d 1 % B-doped NiO films

Table 1 The grain size, roughness, thickness and optical parameters of undoped and doped NiO nanostructure films D (nm)

Rq (nm)

t (nm)

Eg (eV)

E0 (eV)

Ed (eV)

Undoped NiO

30.8

51.5

171

3.73

4.5

5.18

0.1 % B doped NiO

31.1

45.5

125

3.64

5.62

9.37

0.2 % B doped NiO 1 % B doped NiO

33.3

49.5

145

3.7

5.42

8.64

33.1

48.5

198

3.77

5.76

9.84

Samples

on whether the transition is direct or indirect and allowed or forbidden, respectively. For direct transitions, the m index values are 1/2 and 3/2. Whereas, for the indirect transitions, m values are 2 and 3 [14, 15]. The optical band gaps of all the samples are determined by plotting energy (ht) versus (aht)2, as shown in Fig. 4. The Eg values of the films were determined for each sample by extrapolating the linear portion of the plot to (aht)2 = 0 and are given in Table 1. The Eg of NiO undoped film was found to be 3.73 eV, which is close to the value (3.71 eV) for nanocrystalline NiO film deposited by magnetron sputtering in pure argon atmosphere reported by Guziewiciz et al. [16]. Also, the

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100

6.0x10

2 -1

eV)

14

4.0x10

2

60

(αhν) (cm

Transmittance (%)

80

40

Undoped NiO NiO doped by 0.1% B NiO doped by 0.2% B NiO doped by 1% B

20

0 200

14

2.0x10

0.0 2.6

300

400

500

600

700

800

Fig. 2 The transmittance spectra for undoped and B doped NiO films

11 Undoped NiO NiO doped by 0.1% B NiO doped by 0.2% B NiO doped by 1% B

10 9 8

2.8

3.0

3.2

3.4

3.6

3.8

4.0

hν (eV)

Wavelength (nm)

Reflectance (%)

Undoped NiO NiO doped by 0.1% B NiO doped by 0.2% B NiO doped by 1% B

Fig. 4 Plots of (aht)2 versus ht for undoped and B doped NiO films

interaction, (2) line and planar defects in the crystalline film and (3) the crystalline size which effect on band bending at the particle boundaries. Here, the defects in the crystalline due to boron content may be a predominant effect on the Eg values.

7

3.2.2 Study of the refractive index dispersion and dielectric constants of the films

6 5 4 3 2 300

400

500

600

700

800

Wavelength (nm)

Fig. 3 The reflectance spectra for undoped and B doped NiO films

Study of the complex refractive index and dielectric function of any solid material plays an important role in the field of optical materials research due to their significant factor in optical communication and in designing devices for spectral dispersion. The complex refractive index of the films can be expressed by the following relation [27]: n ¼ nðkÞ þ ikðkÞ

obtained value of Eg is in a good agreement with the values reported by Srivastava et al. [17] and Song et al. [18], Eg = 3.70 eV, for pure NiO nanostructure film prepared by sol–gel method and annealed at 400 C for 2 h and NiO nanorods assisted by microwave heating, respectively. Whereas, it is greater than that obtained (3.51 eV) for NiO nanopowder prepared by sol–gel method and calcinated at 400 C for 3 h [19]. On the other hand, the obtained value for the undoped NiO nanostructure film (3.73 eV) is smaller than the value of bulk material (4.0 eV) [20]. It is well known that it causes a blue shift in the spectra of semiconductors in nano size due to the quantum confinement effects. However, the undoped sample is an Eg smaller than the bulk one. This effect is likely due to the chemical defects or vacancies present in the intergranular regions generating new energy level to reduce the band gap energy [21, 22]. As seen in Table 1, there is non-monotonic variation in the Eg values with different boron percentage values. Generally, the variation of the energy band gap values for crystalline films can be attributed to [23–26]: (1) a stress induced distortion of the band by the film/substrate

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ð2Þ

where n is the real part and the imaginary part k, k = ak/4p, called is the extinction coefficient. Using the 2

2

þk reflectance, Fresnel formulae, R ¼ ðn1Þ , can solve to ðnþ1Þ2 þk2

calculate refractive index (n) in the following expression [28]:   sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1þR 4R ð3Þ  k2 þ n¼ 1R ð1  RÞ2 The variation of the calculated n and k values with wavelength are shown in Figs. 5 and 6, respectively. As seen in Fig. 5, the value of n for undoped NiO film varies from 1.82 to 1.42 in the studied wavelength range. Also, the variation of n values with wavelength indicates a nonmonotonic change in whole studied wavelength range especially for doped films. Moreover, the k values for undoped and doped films increase up to certain value of wavelength around 285 nm and then, are decreased. The decrease and increase in n and k values in visible region can be attributed to the decrease and increase of optical absorption with increasing B content [29].

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1.9 1.8

Undoped NiO NiO doped by 0.1% B NiO doped by 0.2% B NiO doped by 1% B

0.60

0.55

n

2

1/(n -1)

1.7 1.6

Undoped NiO NiO doped by 0.1% B NiO doped by 0.2% B NiO doped by 1% B

1.5 1.4 300

400

500

0.50

0.45

600

700

0.40 6.0

800

6.5

7.0

7.5

Wavelength (nm)

Fig. 5 The variation of refractive index with wavelength for undoped and B doped NiO films

8.5

9.0

2

Fig. 7 Plots of (n2 - 1)-1 versus E2 for undoped and B doped NiO films

0.7

4.0

(a)

Undoped NiO NiO doped by 0.1% B NiO doped by 0.2% B NiO doped by 1% B

0.6 0.5

3.5

3.0

k

ε1

0.4 0.3

2.5 Undoped NiO NiO doped by 0.1% B NiO doped by 0.2% B NiO doped by 1% B

0.2 2.0

0.1 0.0

8.0

2

E (eV )

1.5

300

400

500

600

700

800

1.5

2.0

2.5

3.0

3.5

4.0

hν (eV)

Wavelength (nm) 2.0

Fig. 6 The variation of extinction coefficient with wavelength for undoped and B doped NiO films

The refractive index dispersion of the films can be analyzed by Wemple and Didomenico (WD) model. This model [30, 31] is based on a single oscillator description of the frequency-dependent dielectric constant and it is defined by a relation between the refractive index (n), and E0 oscillator energy. The single oscillator model can be expressed by the following relation, E0 Ed n 1¼ 2 E0  E 2 2

ð4Þ

where Ed is the dispersion energy, which is a measure of the intensity of the inter band optical transitions and E is the incident photon energy. Thus, E0 and Ed can be determined from the plot of (n2 - 1)-1 versus E2. Figure 6 shows such a relation for all the films. As seen in Fig. 7, the undoped NiO sample shows the best fitting to WD model with R2 = 0.99. The values of E0 and Ed for all the films were calculated from the slope, (E0Ed)-1, and intercept, (E0/Ed), on the vertical axis of each line and are given in Table 1. As seen in Table 1, the obtained value of E0 for undoped NiO nanostructure film is the lower value

ε2

1.5

(b)

Undoped NiO NiO doped by 0.1% B NiO doped by 0.2% B NiO doped by 1% B

1.0

0.5

0.0 1.5

2.0

2.5

3.0

3.5

4.0

hν (eV)

Fig. 8 The variation of real and imaginary part of the dielectric constant with energy for undoped and B doped NiO films

compared with others films. This refers that E0 is affected by boron doping. It is well known that polarizabiltiy of any solid is proportional to its dielectric constant which is related to the density of states within the forbidden gap. In this respect, it is important to study the real and imaginary parts of the complex dielectric constant. The dielectric constant is defined as, e ¼ e1 þ ie2 and real and imaginary parts of

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complex dielectric constant are expressed by the following relation [27]: e 1 ¼ n2  k 2 ;

and

e2 ¼ 2nk

ð5Þ

The dependence of e1 and e2 on the photon energy are shown in Fig. 8a, b. It is seen that the average e1 values are higher than those of e2. Also, the values of e1 and e2 affected by B incorporation in NiO.

4 Conclusions The nanostructure thin films of undoped and B doped NiO films were prepared using the sol–gel method. The optical transmittance spectrum of the lower doped films (0.1 and 0.2 % B) is nearly the same values for undoped ones in the whole investigated range of wavelength. The optical parameters (optical energy gap, refractive index, extinction coefficient, dispersion energy and dielectric constants) of the films exhibit a considerable variation with B content. The obtained results indicate that the optical parameters of the NiO thin films can be controlled by the boron doping. Acknowledgments One of the authors (Sh.A. Mansour) is grateful to the Scientific and Technological Research Council of Turkey, (TUBITAK)–BIDEB for providing him a fellowship to work in Turkey via Research Fellowship Programme for Foreign Citizens. The present study is a result of an international collaboration program between University of Tabuk, Tabuk, Saudi Arabia, University of King Abdulaziz, Jeddah, Saudi Arabia and Firat University, Elazig, Turkey. The authors gratefully acknowledge the financial support from the University of Tabuk, Project number 4/1433.

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