Nanostructure and enhancement of the optical properties of Tb-doped NiO for photodiode applications

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Nanostructure and enhancement of the optical properties of Tb-doped NiO for photodiode applications A.A.M. Farag , I.S. Yahia , M.S. Al-Kotb PII: DOI: Reference:

S0577-9073(20)30005-8 https://doi.org/10.1016/j.cjph.2019.12.024 CJPH 1048

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Chinese Journal of Physics

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19 August 2019 7 December 2019 24 December 2019

Please cite this article as: A.A.M. Farag , I.S. Yahia , M.S. Al-Kotb , Nanostructure and enhancement of the optical properties of Tb-doped NiO for photodiode applications, Chinese Journal of Physics (2020), doi: https://doi.org/10.1016/j.cjph.2019.12.024

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Nanostructure and enhancement of the optical properties of Tbdoped NiO for photodiode applications A.A.M. Farag1,2, , I. S. Yahia

3,4,5

, M.S. Al-Kotb6

1

Physics Department, Faculty of Science and Arts, Jouf University, Jouf, Saudi Arabia Thin-film Laboratory, Physics Department, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt 3 Research Center for Advanced Materials Science (RCAMS), King Khalid University, Abha 61413, P.O. Box 9004, Saudi Arabia 4 Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia. 5 Metallurgical Lab., Nanoscience Laboratory for Environmental and Bio-medical Applications (NLEBA), Semiconductor Lab., Physics Department, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt 6 Physics Department, Faculty of Science, Ain Shams University, Abbassia, 11566, Cairo, Egypt 2

Highlights 

High-quality thin films of pure NiO and doped Tb% were obtained by low cost dip-coating.

 

The plasma frequency of these NiO thin films was extracted for the first time. The remarkable optical conductivity of the Tb-doped NiO films confirms their applicability for photodiodes.



The nonlinear optical parameters of the prepared NiO nanocrystalline films were optimized.

Abstract In the present work, pure and 1, 2.5, 5 and 10% Tb-doped NiO nanostructures were fabricated in the form of thin-films by the sol-gel spin coating process. The prepared structures were identified by an X-ray diffraction pattern and atomic force microscopy. The results of the X-ray diffraction indicate that the prepared films are polycrystalline with a cubic lattice face-centered for wholly Tb-doped concentration films. The surface topography of the films was studied by atomic force microscopy, and surface mapping was introduced to check the quality of the surface for optical investigations. The measured optical transmission indicated a high transmission that exceeds 80% through the visible region depending on the Tb-doping concentrations. It is affirmed that the measured optical bandgap and the index of refraction are strongly

influenced by the Tb-doping concentrations. The parameters of nonlinearity were also critically affected by the Tb-doping concentrations. This innovative result can hopefully be applied in an industrialized approach for the field of photodiode devices.

Keywords:

Spin coating; Thin film; Nanostructure; Optical properties;

Spectrophotometric measurements

1. Introduction Thin films of the type of materials that include transparent conducting properties of the well-known transition metal oxides have revealed distinctive landscapes for several utilizations, such as energy-efficient technologies that include electrochromic and optoelectronic devices [1-3]. These types of materials are characterized by small electrical resistivity, high optical transmittance and many other unique features that have been described in detail [4,5]. One of the most significant current discussions is devoted to nickel oxide, due to its persistence for each thermal and chemical effect, low-price, remarkable physical properties, such as electrical and optical, as well as the exceptional antiferromagnetic consequences [6-7]. This structure is also characterized by a high value for the bandgap, and it behaves as a semiconductor of p-type due to some non-stoichiometry performance [8], and furthermore it can be manufactured as a thin film by a diversity of techniques [2,4]. Among the well-known methods is the sol-gel spin coating technique, which is characterized by some unique influences, including a low-price and the accessibility of controlling the condition for obtaining the specific film quality and characterization.

Ou et al. [9] have reported on a study on the optoelectronic applications based on the nanostructure and recorded a strong depletion in hybrid perovskite p–n junctions induced by local electronic doping. They studied the hybrid organic-inorganic perovskites . They recorded the enhancement of the built-in electric field as well as a large electric permittivity. They referred this behavior to the influence of ionic polarizability that gives an additional contribution to the creation of a strangely wide depletion region. Pei et al. [10] have discussed the nano-structured hybrid perovskite media and their optical cavities for photonics and optoelectronics. They concluded that the properties of these types of structures can strongly be influenced by the bandgap, defect, and environmental engineering, and they studied the mechanism of these factors for the optoelectronic applications. Huang et al. [11] have discussed a facile fabrication and characterization of two-dimensional bismuth(iii) sulfide nanosheets

for

high-performance

photodetector

applications

under

ambient

conditions. They also studied the photoresponse enhancement under the influence of applying an external bias potential and increasing of the alkaline concentration. Zhang et al. [12] have reported on the photonics and optoelectronics characteristics by means of nanostructured hybrid perovskite media and the related optical cavities. They concluded that these structures offer achievable strategies to harmonize with the optical properties of perovskites and their excitation subtleties. They remarked on an improved light–material reaction towards several optoelectronic and photonic applications based on this type of structure. However, there has been no detailed investigation of the nanocrystalline structure of NiO, especially those prepared by the sol-gel spin coating technique. In addition, there has been little quantitative analysis of the optical properties of the prepared films. To date, there are few studies that have investigated the association between the influence of the Tb concentration as a dopant and the linear and nonlinear optical characteristics of NiO thin films. To the best of our information, there are no available studies for the plasma frequency detection of nanocrystalline NiO and the influence of Tb-dopant content. Accordingly, the present paper attempts to show the applicability for obtaining high-quality thin films of NiO by a low-cost technique and study its morphological and crystalline structure. The other purpose of this study is to explore the plasma frequency by using the spectrophotometric measurements with a unique professional program and assessing the extent to which the Tb-dopant concentration factor affects this property. Moreover, the absorption and dispersion

parameters and other linear related optical characteristics are extracted. Finally, the optimization of the nonlinear optical parameters of the prepared NiO nanocrystalline films in light of the provided literature is examined

2.Experimental Details 2.1. Materials Nickel (II) nitrate hexahydrate, Ni (NO)3.6 H2O (99.99% trace metals basis) of 0.5 M, purchased from Merk, was dissolved in a suitable solvent 2-methoxy ethanol (anhydrous 99.8%) (10 ml). The initial amounts of 0.5 M nickel nitrate hexahydrate was taken and dissolved in 10 ml of 2-methoxyethnaol as a solvent. Nickel nitrate hexahydrate is a highly water-soluble crystalline nickel source for uses compatible with nitrates and lower (acidic) pH and has been specified by various researchers [13,14]. For the Tb doping process, terbium nitrate is added to the above solution as an origin of Tb+3 with various molar ratios of Tb/ Ni (1, 2.5, 5, and 10 wt %). After which a continued stirring (at a temperature of 343 K for approximately 1h) is performed for obtaining uniform and transparent solutions. For obtaining the gel, all the above reactions are performed in the dark and approximately 300 μml monoethanolamine was added to the solutions as a stabilizing agent with continuity of stirring for 1 h.

2.2. Thin-film preparation A spin coating process methodology by means of type Brewer Science, Cee, precision spin coaters was chosen for obtaining the elevated features of the NiO thin films deposited with strong adherence on the glass substrates. All the detailed procedures for obtaining a good adherent thin film of high quality suitable for the application are as stated in the literature [15].

2.3. Characterizations The crystalline characterization of the prepared films was examined by utilizing an X-ray diffractometer type, Shimadzu Lab X XRD-6000, with CuKα radiation and a Ni filter. The surface topography characteristics were done by using an atomic force microscope AFM-NT-MDT, type Next, Russia.

The optical measurements were investigated using a JASCO V-570 spectrophotometer throughout the full spectral range of UV–Vis–NIR at 300 K.

3. Results and discussion 3.1. The morphological and crystalline structure Figs. 1(A-D) show the 2 D and 3D images of the AFM and its roughness tracer analysis of NiO with different terbium doping concentrations of 1%, 2.5 %, 5%, and 10 %, respectively. We observed from these figures the influence of doping on both the particle size and the surface roughness of the prepared films. Moreover, a nearly homogeneous distribution of the particles through the area of the substrates with the appearance of aggregation characteristics was observed. The particle size was calculated through various positions that are not similar due to this accumulation, and it was found to be hard to accurately determine, but the main feature is the generally increasing of the particle size with increasing the terbium doping concentrations. The average diameters of the particle were estimated utilizing the image analysis software of AFM and found to be 79, 85, 92, and 105 nm for NiO with the different terbium doping concentrations of 1%, 2.5 %, 5%, and 10 %, respectively. This behavior of increasing particle size with increasing doping can be due to the increase of agglomeration as a result of the increasing Tb doping concentration, which is like those published by various authors [6,15,16]. In addition to that, the surface roughness was measured for all the studied samples and shown in Figs. 1(A-D)-c using a roughness tracer analysis. The main result indicates an increase of the roughness of the surfaces as the concentration of Tb doping increases, which can be attributed to the increased agglomeration probability which makes the surface tougher, in agreement with results published in the literature [16-18]. The crystalline structure and the influence of the doping concentration on the features of NiO can be observed from the X-ray diffraction patterns, shown in Fig. 2(a-d). All the observed preferred orientation peaks indicate only one characteristic structure of the lattice face-centered cubic with a space group of Fm-3m, using JCPDS no.89-5881. The detected structures are a single phase for all the studied films without any other impure materials, and the basic preferred peaks are oriented about (222), (422), (440) and (531) for the pure and Tb-doped NiO films with a high intensity for (222) in all the films. Moreover, a gradual improvement for the crystallinity of the films is recorded as the concentration of Tb increased.

The main important crystallite parameter,

the mean crystallite size, D, is

calculated using the wavelength of the CuK, , and Scherrer's constant, K, and the FWHM, , according to the following formula [19]:

D

K .  cos 

(1) According to Eq. (1) other important parameters are calculated, such as the microstrain, which can be obtained according to the following relation [15,19]:



 cos  4

(2) The above crystalline parameters were estimated to record the enhancement due to the Tb-dopant of NiO as well as other parameters like the dislocation density, , and the number of crystallites per unit area, N. These parameters are tabulated in Table 1. As observed from Table 1 and Fig. 3(a-d), an increase of mean crystallite size and microstrain was followed by a decrease in the dislocation density as the concentration of Tb-doped NiO increased, which means an enhancement of the structural characteristics due to Tb-doping for NiO. As published in the literature by various authors [20,21], the doping can affect the expansion of the lattice volume, and can then induce an increase of the microstrain. However, the calculated crystallite size is found to be smaller than those obtained from the AFM. This can be attributed to the accumulation processes that led to the fact that the particle is composed of more than one crystal. The obtained results are in accordance with those published in the literature by various authors [15,18,19].

3.2. Optical characterizations Fig. 4 shows the average measured T% and R% of pure and Tb-doped NiO (1, 2.5, 5 and 10 %) as a function of wavelength in the range of 200–2500 nm. The figure shows that the films have their maximum values of T% in the wavelength region of 500-1200 nm and shows a high dependence on the Tb-dopant %. The transmission edge of all the studied films varied from 250 nm to 400 nm. Moreover, the reflectance shows minimum values at a certain wavelength depending on the Tb dopant %, as shown in Fig. 5, after which the reflectance increased sharply. This behavior is accompanied by a decrease in the transmittance. The demonstration of the smallest

value of the reflectance is predicted to occur for the material nearby the frequency identified as the plasma frequency, ωp. This performance was documented for some metal oxides, like Al-doped ZnO [22] and CuO[23]. This type of frequency can be obtained based on the minimum frequency, min , and the high-frequency dielectric constant,   , using the following expression [22]: 1

    p  min   .   1   

(3) The values of min and ωp for all the studied Tb-doped NiO are listed in Table 2 and displayed in Fig. 4(b), (C). The obtained values of ωp are found to decrease with increasing Tb-dopant concentration, except for the concentration of Tb-2.5 %, due to the high value of both the reflectance and dielectric constant of this structure as compared to the others. Another plot is taken into consideration, which gives an indication of the expected value of the energy gap and is extracted from the first derivative of both T and R [23]. Figs. 5 (a) and (b) show the plot of the spectral dependence of both dT/dλ and dR/dλ, respectively. The observed maximum peak in each curve gives an indication of the energy gap, which lies between 3.45 from dR/dλ and 3.78 eV from dT/dλ and is in agreement with the range of those published for NiO of different dopant concentrations [19-22]. The optical absorption coefficient, , of the films with film thickness d were calculated for all the studied films using the following expression [24]: 1/2 2 4    1  1  R   1  R  2    ln   R   4T 2   d  2T    

1

.

(4) The analysis of the absorption coefficient in the region of the band edge and in the UV region is used to obtain the energy bandgap under consideration for band theory using the following formula [25]:

 h  B  h  E g 

s

(5)

where B is a constant

and Eg is the bandgap energy; the exponent s gives an

indication for the type of transition ( s = 1/2 or 2 for a direct or indirect allowed transition, respectively). For obtaining the type and the value of the bandgap of Tbdoped NiO thin films, the dependence of (αhν)2 and (αhν)1/2 on the photon energy is plotted, as shown in Figs. 6(a) and (b). A comparison between the two plots is taken into consideration, according to the best straight-line fit, the range of fitting, as well as the well –known established and published energy gap of NiO. Accordingly, the best fit is for the indirect transition, and the energy gap can easily be taken out from the extrapolation of the fitted linear part in each case to the value of (αhν)1/2=0. The energy band gap of pure and Tb-doped NiO thin films is tabulated in Table 2 and plotted in Fig. 7. The range of the determined band gap is consistent with those published in the literature by various authors [ 23, 24, 26]. As observed in Fig. 7, the high value of Eg is for the pure NiO , and it decreases with increasing Tb-NiO, due to the influence of the grain boundary, till the concentration of 2.5 %, after which the value of Eg increases with a small rate, which can be attributed to the influence of scattering of the carriers and/or the presence of some porosities, which causes some broadening for the energy band gap [25,27]. To complete the view of the study of the optical characteristics of pure and Tbdoped NiO, there are the main important parameters which should be considered. First, are the optical constants, refractive index, n, and the extinction coefficient, k. The extinction coefficient, k, can be estimated using the following relation [28]:

k  (

 ), 4

(6) while the refractive index, n, can be estimated from the reflectance and extinction coefficient using the following relation [29]:

1/2

  1  R   4R n  k 2 .    2   1  R   1  R  

(7) The photon energy dependence of both k and n is illustrated in Fig. 8. As observed from this figure, the deviation in their values corresponds to the influence of the altered Tb-dopant concentrations. Another important observation from the results is

that there is a characteristic maximum value of n and the corresponding minimum in k at a value of the photon energy near to that recorded for the energy gap of the obtained films. This observation is supported by the results published by Yahia et al. [23]. The other important optical parameter is the dielectric constant, ε, which can be identified by the real ε1 (= n2 − k2) and imaginary ε2 (= 2n k) parts as follows: ε = ε1 − i ε2

.

(8) The plot of the photon energy dependence of both ε1 and ε2 is depicted in Fig. 9. The variation of both ε1 and ε2 depends on the behavior of both n and k, which is strongly influenced by the Tb-doped concentration. Moreover, the value of ε1 is still larger than those for ε2 throughout the photon energy range. The values of both ε1 and ε2 are nearly stable in the region of lower photon energy. In addition, the loss tangent tan(=

2 ) is also calculated for pure and Tb-doped NiO of different concentrations 1

and is shown in Fig. 10. The figure shows a clear dependence of tan on the Tb-doped concentration, especially at the higher photon energy range (E3.5 eV). There are two other important optical parameters related to the dielectric, called the volume energy loss function, VELF, and the surface energy loss functions, SELF, which can be expressed as follows [30]:

 2  VELF   2 2 2  ,  1   2  (9)

   22 SELF   2 . 2 2   (1  1)   2  (10) Fig. 11 (a) and (b) show the photon energy dependence of VELF and SELF, respectively. The observed values of VELF are larger than those of SELF for all the studied films, which gives an indication for the energy losses as probably those occuring inside the material (bulk material) being larger than those at the surface [31].

An optical parameter that has a relation with the dielectric constant of the material and the electronic state density is the optical conductivity, the real (σ1) and the imaginary (σ2) can be defined as follows [31-33]: σ1=ωεoε2,

σ2=ωεoε1

.

(11) Fig. 12(a) and (b) illustrates the variation of both σ1 and σ2 as a function of the photon energy for the pure and Tb-dopant concentrations. The high value of the optical conductivity indicates a remarkable photoresponse behavior of the studied material for the optoelectronic device applications [32,33]. Moreover, the high influence of the Tb-dopant is observed at a higher energy range ( 3.5 eV) as compared to the lower one. At higher wavelength (lower energy), the refractive energy shows a single mode of vibration, which can be explained by a single oscillator model, related to the oscillator energy Eo and the dispersion energy Ed using the formula of WempleDiDomenico as follows [33]:

 Eo   E 2   n  1   E    E E  .  d  o d 2

2

(12) Fig. 13 shows the relation of (n2-1)-1 versus E2, which shows a remarkable dependence on the Tb-doping concentration through the entire energy range; the values of Eo, Ed were calculated from the straight-line to fit through the slope and intercepts and tabulated in Table 2 and represented in Figs. 14(a) and (b). As observed, the values of Eo and Ed are found to decrease with an increase of the Tbdoping concentration. Furthermore, they show the presence of a peak for each of Eo and Ed at a certain Tb-doping concentration (2.5%) due to the energy loss, as compared to the other structures. There is are moments related to E0 and Ed, called the rth moments, denoted by M−1and M−3, which can be expressed as follows [34]:

E o2  (13)

M 1 M 3

E d2 

M 31 M 3

(14) The values of M−1 and M−3 are extracted using Eqs. (13) and (14) and listed in Table 2 as well as represented in Fig. 15(a) and (b) as a function of the Tb-doping concentration. As observed, the values of M−1 are larger than those of M−3, and the presence of a peak for the Tb-doping concentration can be attributed to the contribution of both Eo and Ed. Another analysis was done in the high wavelength region, which is called the transparent region, to obtain some important parameters, like the high-frequency dielectric constant, , as well as the ratio of free carrier concentration to the effective mass, N/m*, according to the following relationship [32-34]:

 e 2  N  n 2      2  *   2 ,  c   m  (15) where c is the light speed and e is the electronic charge. Fig. 16 shows the plot of the dependence of n2 on λ2, which was fitted to a straight line to obtain the related parameters like ε∞ and (N/m*) from the extrapolation of these curves to the corresponding λ2=0 and from the slopes of the straight-line fit, respectively. These values are extracted and tabulated in Table 2 as well as represented as a function of Tb-doping concentration, shown in Fig. 17(a), (b).

3.3. Nonlinear optical properties In addition to the above optical studies, we will also shed some light on the nonlinear optical characteristics. The field of nonlinear optics symbolized a milestone of numerous optical applications, including communications, ultrafast switches, data processing, ultra-short pulsed lasers, optical computation, and other related photonic applications [35]. The characteristics of nonlinearity can be observed using the bulk and thin films of both the crystalline and amorphous organic and inorganic materials, and another state of the materials such as liquid, gas, and plasma [36-37]. For the nonlinear characteristics of the studied films, empirical expressions founded on the widespread Miller's law can be utilized for the extraction of the wellknown third-order nonlinear optical susceptibility, χ(3). The calculations are based on a combination of the Miller's rule [38] and the linear refractive index at the low-

frequency range, using the Wemple–DiDomenico formula deduced under the condition of the single oscillator model [39]. The χ(3) yields some important information about the self-phase modulation, photorefractive nonlinear waveguides, and the bistability of the optical behavior[36,37]. Various studies of the nonlinear optical characteristics were done for the material deposited on a conductive substrate like ITO and FTO for improvement applications in the field for optical devices [40,41]. It is well known that for the linear optical characteristics, the induced polarizability (p) is proportional to the electric field (E) according to the following expression [35]: p = (1)E (16) where (1) is the well-known linear optical susceptibility, which can be expressed as follows:



(1)

 no2  1     4 

(17)

For a high-intensity field, the nonlinear optical characteristics are observed, and the third-order optical susceptibility can be written as follows [42]:



(3)

 no2  1   A(  )  A    4 

4

(1) 4

(18) where A is a constant ( 1.7 × 10−10 esu [42]). The photon energy dependence of both (1) and (3) are illustrated in Figs. 18(a) and (b), respectively. As observed, the influence Tb-doping NiO on (1) is limited for photon energies less than 2.5 eV, but a high dependence is observed for (3) when the photon energy is higher than 3 eV, which can be attributed to the condition of how the photon energy generates a high polarizability through the material. Figs. 19(a) and (b) show the values of (1) and (3), respectively at a photon energy of 5 eV. As observed, the high values of both (1) and (3) are recorded for the pure NiO, and a reduction occurred for the other Tb-doped NiO, which is in

agreement with the findings published by Shkir et al. [15]. In addition, the recorded values of (1) and (3) are found to be higher than those published by Shkir et al. [15] and Chtouki et al. [43], which gives an opportunity for various related applications. Investigating the consequences of (3) in Table 2 in comparison with those published for various structures [43-47], one can observe that its values are higher than those obtained by approximately two orders. Some factors can affect the results, such as the methods of preparation, the doping type, and the substrate type. The obtained results support that the FTO substrate can improve the optical nonlinearity of Tb-doped NiO, which gives an opportunity for the possibility, as Han et al. [48] and Nagaraja et al. [49] have discussed, for this enhancement to the probability of the interdiffusion process of the FTO substrate and the NiO film, and consequently expanding the localized states and increasing the nonlinear absorption characteristics. Nagaraja et al. [49] and Irimpan et al. [50] have suggested that the role of particle size, that is affected by Tb-doping, on the improvement of the third-order nonlinear optical characteristics is due to the increases of the nonlinear absorption that affects the optical limiting power properties of the device.

4. Conclusions The preparation of Tb-doped NiO with various dopant contents of 1, 2.5, 5 and 10 % was well accomplished as thin films and identified by XRD and AFM. The influence of increasing Tb-doping showed a remarkable effect on the features of the films and their morphological and crystalline parameters. The measured particle size confirms a nanostructure behavior, and its values increase from 17 nm to 18.8 nm with increasing Tb-doping content. The prepared films showed a plasma frequency behavior for the first time, and the plasma frequency was detected and found to decrease with increasing Tb-doping concentration, except for the Tb-dopant content of 2.5 %, due to its surface characteristics that affect the optical features. The optical band gap was checked for direct and indirect allowed, and the indirect allowed is found to be more predominant with values depending on the Tb-dopant content. The values of the energy gap were measured and found to decrease with increasing Tbdoping content from 3.85 to 3.54 eV. The dispersion characteristics were also studied based on a single oscillator model, which enables us to extract most of the main parameters that confirm the possibility for the films for optoelectronic devices. The

values of the oscillating energy were found to decrease from 3.12 to 3.02 eV, while the dispersion energy decreases from 7.02 to 4.79 eV with increasing Tb-doping content. The nonlinear optical parameters were also studied and compared with those published elsewhere to give support for the enhancement of the characteristics in the light of the scope of the obtained results for photodiode applications.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment One of the authors (I.S. Yahia) extends their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through research groups program under Grant No. R.G.P.1/12/39..

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Nanostructure and improvement of the optical properties of Tb-doped NiO for photodiode applications

Nanostructure and improvement of the optical properties of Tb-doped NiO for photodiode applications

B)

Fig. 1. (A-D) 2 D and 3D of AFM images and its roughness tracer analysis of Tb-doped NiO (A) 1%, (B) 2.5 %, (C) 5% and (D) 10 %.

(a)

(222)

40 0

50 2

60

70

30

(531)

(440)

40

60

70

o

120

(422)

Intensity (counts/s)

(222)

120

(422)

Intensity (counts/s)

(d)

160

80

50 2

(c)

160

40

o

80

(531)

40

(222)

30

(440)

0

80

(531)

(440)

40

120

(422)

Intensity (counts/s)

80

(531)

120

(440)

(222)

(b)

160

(422)

Intensity (counts/s)

160

40 0

0 30

40

50 2

o

60

70

30

40

50 2

60

70

o

Fig. 2. (A-D) X-ray diffraction analysis of Tb-doped NiO (a) 1%, (b) 2.5 %, (c) 5% and (d) 10 %.

19

0.46

(b)

(a)

0.44

D (nm)

FWHM

0.45

0.43

18

0.42 0.41

17

0

2

4

6

0

Tb-dopant content % 2.0x10

-3

2.0x10

-3

1.9x10

-3

1.9x10

-3

1.8x10

-3

1.8x10

-3

2

4

6

Tb-dopant content % 0.0034

(c)

(d)



-2

 (nm )

0.0032

0.0030

0.0028 0

2

4

6

0

2

4

6

Tb-dopant content %

Tb-dopant content %

Fig. 3. Plot of (a) Full-Width at half maximum, FWHM vs. Tb-dopant content % (b) Mean Crystallite size, D vs. Tb-dopant content % , (C) Strain,  vs. Tb-dopant content % and (d) Dislocation, vs. Tbdopant content % .

23

80 80 T%

R%

60

60

40

T%

R0 T0 R1 T1 R2.5 T2.5 R5 T5 R10 T10

40 20

R%

20

0 500

1000

1500

2000

0 2500

 (nm)

Fig. 4(a) Plot of spectral analysis of T% and R% of pure and Tb-doped NiO thin films.

24

8x104 8.5x104

(b)

(c)

8x104

7.5x10

4

7.0x10

4

7x104 p(cm-1)

min(cm-1)

8.0x104

7x104 6x104

6.5x104 6.0x104

6x104

5.5x104

5x104 0

2

4

6

8

10

2

4

6

8

10

Tb-dopant content %

Tb-dopant content %

Fig. 4(b),(c) Plot of  min and

0

 p vs. Tb-doping content of NiO thin films.

25

1

(a)

1.2

0

(b)

T/

R/

0.8

0.4

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

-1 Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

0.0

-2 500

1000

500

 (nm)

1000

 (nm)

Fig. 5. Plot of spectral analysis of (a) R/λ and (b) T/λ of pure and Tb-doped NiO thin films .

26

(b)

(a)

3x10

12

2x10

12

800 Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

1/2

(h) , (eV/cm)

2

(h) , (eV/cm)

2

1/2

1200

1x10

12

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

400

0

0 0

2

4

6

0

2

E (eV)

4

6

E (eV)

Fig. 6. Plot of (a) (αh)2 vs. E and (b) (αh)1/2 vs. E of pure and Tb-doped NiO thin films.

3.9 3.8

Eg (eV)

3.7 3.6 3.5 3.4 3.3 0

2

4

6

Tb-doping content (%)

27

8

10

Fig. 7. Plot of Eg vs. Tb-doping content of NiO thin films.

(a)

0.6

(b)

12 10 Eg

8

Eg

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

n

k

0.3

6 0.0

4 Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

0

2

4

2

6

0

2

4

6

E (eV)

E (eV)

Fig. 8. Plot of (a) k vs. E and (b) n vs. E of pure and Tb-doped NiO thin films.

28

160

(a)

(b)

8

2

1

120

80 Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

40

4

0 Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

0 0

2

4

6

0

2

4

6

E (eV)

E (eV)

Fig. 9. Plot of (a) 1 vs. E and (a) 2 vs. E of pure and Tb-doped NiO thin films.

29

0.5 0.4

tan

0.3 0.2 0.1 0.0

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

-0.1 1

2

3

4

5

E (eV)

Fig. 10 Plot of tan vs. of pure and Tb-doped NiO thin films.

30

6

0.25

7x10

-5

6x10

-5

5x10

-5

4x10

-5

3x10

-5

2x10

-5

1x10

-5

(b)

(a)

VELF

0.20

SELF

0.15 0.10 Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

0.05

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

0

0.00 2

4

2

6

4

6

E (eV)

E (eV)

Fig. 11. Plot of (a) VELF vs. E and (a) SELF vs. E of pure and Tb-doped NiO thin films.

31

6.0x10

3

4.0x10

3

2.0x10

3

(a)

8.0x10

4

6.0x10

4



2

3



1

8.0x10

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

4.0x10

4

2.0x10

4

0.0

(b)

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

0.0 2

4

6

E (eV)

2

4 E (eV)

Fig.12. Plot of (a)  vs. E and (a) 2 vs. of pure and Tb-doped NiO thin films.

32

6

0.7 Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

0.6

0.4

2

(n -1)

-1

0.5

0.3 0.2 0.1 0.0 2

4

6 2

E , (eV)

Fig. 13. Plot of (n2-1)-1 vs. E2

3.30

8

2

of of pure and Tb-doped NiO thin films.

8

(a)

(b)

3.25 7

3.15

Ed (eV)

E0 (eV)

3.20

3.10

6

3.05 5 3.00 2.95 4 0

2

4

6

8

10

Tb-doping content (%)

0

2

4

6

8

10

Tb-doping content (%)

Fig. 14. Plot of (a) E0 vs. Doping content % and (a) E0 vs. Doping content % of Tb-doped NiO thin films

33

0.24 0.50

(a)

(b)

0.23 0.45 0.22 0.40 0.35

M-3

M-1

0.21 0.20

0.30 0.19 0.25 0.18 0.20 0.17 0.15 0

2

4

6

8

0

10

2

4

6

8

10

Tb-doping content %

Tb-doping content %

Fig. 15. Plot of (a) M-1 vs. Doping content % and (a) M-3 vs. Doping content % of Tb-doped NiO thin films

34

10 Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

8

n

2

6

4

2

0 0.30

0.35

0.40

0.45 2

0.50

2

 (m ) Fig. 16. Plot of n2 vs. λ2

of pure and Tb-doped NiO thin films.

35

0.55

0.60

8 (b)

(a)

N/m x10 (m /Kg)

8

6

*

57



3

7

6

4 5 0

2

4

6

8

10

0

Tb-doping content %

2

4

6

8

10

Tb-doping content %

Fig. 17. Plot of (a)  vs. Doping content % and (a) N/m*vs. Doping content % of Tb-doped NiO thin films.

36

160

(a)

(b)

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

120

Pure 1 % Tb 2.5% Tb 5 % Tb 10 % Tb

-5

6.0x10

-5

4.0x10



1

80

-5

2.0x10

40

0.0

0 2

4

6

Energy (eV)

2

4

Energy (eV)

Fig. 18. Plot of (a) 1 vs. E and (a) 1 vs. E of pure and Tb-doped NiO thin films.

37

6

80

(b)

(a) -5

2.0x10

60 -5

(3)

40

-5

1.0x10





(1)

1.5x10

-6

20

5.0x10

0.0

0 0

2

4

6

8

10

0

Tb-dopant content %

2

4

6

8

10

Tb-dopant content %

Fig. 19. Plot of (a) 1 vs. Doping content % and (a) 3 vs. Doping content % of Tb-doped NiO thin films.

38

Table 1. Main crystallite parameters of pure NiO and various Tb-dopant NiO thin films. Structure Pure NiO 1% Tb–NiO 2.5% Tb–NiO 5% Tb–NiO

(hkl) 222 222 222 222

FWHM

D (nm)





0.458 0.433 0.431 0.418

17.199 18.180 18.263 18.847

0.00181 0.00187 0.00188 0.00199

0.00338 0.00303 0.00300 0.00282

Table 2. Main absorption and dispersion parameters of pure NiO and various Tb-dopant NiO thin films. . Structure

min (cm-1)

Pure NiO 1% Tb– NiO 2.5% Tb–NiO 5% Tb– NiO Pure NiO

64.94x103 62.93x103 83.32x103 59.52x103 58.24x103

 p (cm-1) 59829.963 57804.896 77180.380 53491.638 59829.963

Eg(eV)

M-3 N/m*x1057  (eV−2) (Kg−1 cm−3 ) 0.2305 6.614 5.39

Ed

M-1

3.85

3.12 7.02

0.18

3.44

3.06 5.64 0.362 0.1966 6.401

6.59

3.38

3.26 7.09 0.484 0.2134 7.045

8.59

3.44

2.97 5.49

0.38

0.208

5.202

3.75

3.54

3.02 4.79 0.274

0.173

5.748

5.04

39

E0

Table 3. The third polarizability of pure NiO and various Tb-dopant NiO thin films.

Structure

χ(3) esu

References

Pure NiO

2.0 x10-5

Present work

1% Tb–NiO

8.1 x10-7

Present work

2.5% Tb–NiO

1.0 x10-6

Present work

5% Tb–NiO

2.4 x10-6

Present work

Pure NiO

2.0x10-6

Present work

10-8

[ 15]

10-12

[43]

10-12

[44]

10-9

[45]

10-8

[46]

10-9

[47]

NiO doped Cr+3 NiO M Xene Ti3C2Tx (T = F, O, or OH)

Bismuthene black phosphorous Anitmonene

40