Lu doping nickel oxide thin films using sol-gel spin coated and density functional theory: optoelectronic and magnetic properties

Abstract

For the first time, sol-gel spin coating was used to fabricate thin films of NiO doped with lutetium. The films were characterized to determine their crystalline structure, surface morphology, and optical properties as a function of Lu doping concentration. The investigations revealed that the Lu-doped NiO films consisted of nano-polycrystalline particles with a cubic bunsenite structure and (200) preferential orientation. Optical studies indicated that the optical band gap of pure NiO widened with low levels of Lu incorporation before narrowing with higher concentrations. The Urbach energy value for pure NiO initially decreased with 1 at. % Lu-content, from 224 meV to 190 meV, and then continuously increased to 380 meV with more Lu-level. To investigate the effects of Lu doping on the electronics, magnetic, and optical properties of NiO, first-principle computations were performed. The results showed that bulk magnetization underwent significant modifications due to a high hybridization between the Lu-f/d and Ni-d states. This study suggests that NiO doped with lutetium could be used for spin-polarized transport devices and other spin-dependent applications.

1. Introduction

Nickel oxide (NiO) is a widely used material in various products such as inorganic light-emitting diodes, solar cells, electrochromic, and spintronic devices [[1], [2], [3]]. This is due to its unique properties, including a large band gap of 3.6–4.0 eV, high thermal conductivity, excellent optical transparency, chemical stability, and antiferromagnetism [4,5]. NiO has been extensively studied for its potential use in electrochromic devices such as smart windows, display applications, and automotive rearview mirrors [6,7]. In its stoichiometric form, NiO exhibits remarkable insulator qualities with a resistivity of approximately 1013 Ω cm [8]. However, non-stoichiometric NiO behaves as a p-type semiconductor due to the presence of nickel defects and oxygen interstitials [9].

Additionally, NiO nanostructures refer to a class of metal oxide materials that have a variety of uses, including cathodes in electrochemical capacitors [12], lithium batteries [13], and opening collectors in solar cells thanks to NiO's p-type nature [10,11]. NiO nanostructures, which are primarily nanoparticles, have also generated a great deal of interest because of the potential applications connected with the capacity to modify their chemical and physical characteristics [14]. Pure NiO exhibits low p-type conductivity due to its large band hole, with the conduction system mostly being represented by openings brought on by nickel opportunities, doping, and oxygen interstitial iotas. Nickel oxide nanoparticles scattered in a diffuse silica lattice remain antiferromagnetic, according to recent analysis of nanosized NiO materials. Sol gel burning was employed to create these NiO nanoparticles [15,16]. By combining outside components including lithium, copper, cobalt, potassium, phosphorous, and europium, NiO movies' optical-electrochromic highlights, geological structure, electrical properties, and attractiveness can all be changed and subsequently limited. Because of its high rate of light transmission, broad band gap of 3.6–4.0 eV, and high-rise thermal and chemical steadiness [4,5], nickel oxide is widely used in LEDs, solar cells, gas sensors, and electrochromic applications [[1], [2], [3]]. Although non-stoichiometric nickel oxide (NiO), which is associated to nickel vacancies and oxygen interstitials, has a p-type semiconductor feature [6,7], the stoichiometric form of nickel oxide (NiO) has a great insulator character with a resistance of 10¹³ Ω⋅cm [6]. By adding contributing elements including lithium (Li), copper (Cu), cobalt (Co), potassium (K), phosphorous (P), lutetium (Lu), and others, NiO films' crystal and topography structure, electrical and magnetic characteristics, and optical-electrochromic characteristics can be changed and regulated. Herein we consider lutetium (Lu) as a dopant element for the NiO structure.

According to our knowledge, there is not any study on Lu-incorporated NiO films. Therefore, it is very helpful to make several works to understand clearly impacts of Lu incorporation on characters of nickel oxide. Due to its ease of use, safety, affordability, and capacity to deposit homogenous and highly qualified films [[8], [9], [10]], the sol-gel approach, which has been utilized for the current investigation, is a very qualified thin film deposition method. In this investigation, pure and Lu-incorporated NiO thin films were deposited on glass substrates with sol-gel spin coating method and Lu contribution effects on crystalline, topographic, and optical properties of NiO were studied.

2. Materials and experimental methods

Undoped and Lu-doped NiO samples were fabricated on glass substrates via a sol-gel spin coating method. NiO coating solution was prepared by employing nickel (II) acetate tetrahydrate (Ni(OCOCH3)₂4H₂O), methanol (CH₃OH) and monoethanolamine (C₂H₇NO, MEA). Lutetium doping was achieved by inserting of lutetium (III) chloride (LuCl₃) into precursor coating solution. To produce a transparent and homogeneous solution, the solutions were agitated at 50 °C for 2 h. Using an ultrasonic cleaner and acetone, deionized water, and methanol, the microscopic glass substrates were cleaned before being dried with nitrogen. During the spin coating procedure, liquids were dropped on glass substrates that were rotated for 25 s at a speed of 3500 rpm. For 10 min, the as-coated layer was sintered at 200 °C. The coating/sintering procedure was repeated ten times, and the films were then annealed in the air for 1 h at 550 °C. NLu-0, NLu-1, NLu-2, NLu-3, and NLu-5, respectively, are the names of the pure, 1 at.%, 2 at.%, 3 at.%, and 5 at.% Lu integrated NiO samples.

When employing a Rigaku/SmartLabdiffractometer with CuK_α radiation (λ = 0.154059 nm) operated at 40 kV and 30 mA, X-ray diffraction (XRD) patterns were used to determine the crystal structure of pure and Lu-contributed NiO films. The measurements were made using linked θ-2θ geometry with steps of 0.02° between 15° and 85°. The atomic force microscope was used to examine the surface features of movies (AFM-Nanomagnetic Instruments). The AFM images were captured at 1.5 m × 1.5 m planar, in 512 × 512 resolutions, and in tapping mode at a scan rate of 1.97 Hz. The UV-VIS spectrophotometer (PerkinElmer, Lambda 35), which operates in the 190–1100 nm range, was used to conduct the optical examination.

2.1. Theoretical modeling and computational details

Using the Quantum Atomistix ToolKit (QuantumATK) software [17] based on the local combination of atomic orbitals method [[18], [19], [20]], Density Functional Theory (DFT) calculations were performed using the Generalized Gradient Approximation (GGA) of the Perdew, Burke, Ernzerhof (PBE) exchange-correlation potential. The norm-conserving PseudoDojo pseudopotential was used to define the interaction between ion nuclei and valence electrons [21]. Self-consistent field simulations were conducted with a tolerance limit of 10⁻⁸ Ha to assess energy convergence. The mesh cut-off energy was set at 90 Ha, and the basis set was PseudoDojo-medium. By utilizing the Broyden-Fletcher-Goldfarb-Shanno (LBFGS) approach, the lattice characteristics and stability arrangement of atoms were determined by lowering the system's overall energy until a force on each atom was reduced below 0.05 eV/Å. Electronic property calculations were performed using an 8 × 8 × 2 Monkhorst-Pack [22] k-grid, while a 10 × 10 × 8 grid was used for geometry optimization. Furthermore, for more accurate results, the electronic structure of Lu_xNi_1-xO (x = 12.5%, 3.125%, and 1.25%) was calculated using a hybrid spin-polarized density functional theory (DFT) computation [17], employing the GGA + Hubbard potential (U) (for the d/f orbitals).

Since the Lu partially filled localized orbitals (4f) cannot be implemented by a standard DFT-GGA, the Hubbard potential, more specifically, GGA + U was employed to better explain the on-site coulomb interaction of the electrons in the Lu-4f/5d and Ni-4d states in order to enhance the correlation effects. J was the exchange parameter [23]. The Lu⁺ ion has entirely filled the f shell (spin up) in these compounds, while the empty shell (spin down) causes the function of the J parameter to decrease and renormalize the value of U. According to various research [24,25], it is demonstrated that the Hubbard potential effective value and reduction energy are linearly related.

To look into the electronic structure of Lu doped NiO compounds, on-site Hubbard parameters of 5.0 eV and 4 eV for the monovalent Lu⁺ ion and Ni were utilized, respectively. This study's objective is to accurately perform a DFT analysis of the electrical characteristics of Lu_xNi_1-xO systems. Three alternative supercell techniques were used in DFT calculations to simulate the doping impact of Lu in NiO thin films at varied concentrations, i.e., 2 × 1 × 1 supercell containing 7 O, 8 Ni and 1 Lu atoms with cubic structure (P 4/m m) leading to 12.5 % of Lu:NiO, as illustrated in Fig. 1. The 2nd supercell has 2 × 2 × 2 with Pm-3 m cubic symmetry and contains 31 O, 32 Ni and 1 Lu in the center leading to 3.125% of doping concentration as illustrated in Fig. 1(a–c). Furthermore, we reached a low concentration of Lu doping with 1.25% by considering 3 × 3 × 2 of supercell containing 71 O, 72 Ni and 1 Lu in the center.

Fig. 1.

Structures of Lu doped NiO in the center with concentrations: (a) 12.5%, (b) 3.125% and (c) 1.25%.

3. Results and discussions

3.1. Crystal features

X-ray diffraction pictures for pure and Lu-incorporated nickel oxide samples are given in Fig. 2. The films are polycrystalline with cubic bunsenite NiO (JCPDS card no: 47–1049). The preferential direction is detected as (200) and the additional peaks are also found as (110), (220) and (222). The (311) peak is observed for pure, 1 and 2 at. % Lu-incorporated NiO samples not for others. The similar crystal property has been adhered in sol-gel spin coating studies [2,11].

Fig. 2.

The XRD graph of Lu-incorporated NiO films.

The peak intensity of pure NiO initially become strong with 2 at. % Lu content, then it slides to lower values with further Lu doping. An increase in the crystallinity of NiO with low Lu-content may result from a proper substitution of Lu with Ni atoms in NiO structure. The further Lu atoms may locate on inter-grain places and improper lattice positions and this can cause a decrement in the crystallinity. Table 1 contains the values for the inter planer distances of films as determined by Bragg's law. The observed “d” values matchup with those that are typical. The lattice constant values of films are calculated by following relation (1) [12]:

$$\frac{1}{d^2}=\left(\frac{h^2+k^2+l^2}{a^2}\right)$$ (1)

where h, k, l are miller indices and d is inter-planer distance. The calculated lattice constant values are given in Table 2 and they agree with standard one of 4.1771 Å (JCPDS card no: 47–1049).

Table 1.

The values of pure and Lu-incorporated NiO films' inter-planer distances.

hkl	dstandard (Å)	NLu-0	NLu-1	NLu-2	NLuO-3	NLu-5
		d (Å)	d (Å)	d (Å)	d (Å)	d (Å)
(110)	2.4120	2.4162	2.4176	2.4201	2.4238	2.4185
(200)	2.0890	2.1205	2.1205	2.1167	2.1214	2.1186
(220)	1.4768	1.4780	1.4772	1.4793	1.4815	1.4810
(311)	1.2594	1.2600	1.2619	1.2632	–	–
(222)	1.2058	1.2047	1.2054	1.2018	1.2044	1.2029

Table 2.

A few NiO:Lu film crystalline, topographic, and optical characteristics.

Sample name	Lattice Constant (Å)	D (nm)	δ (x1015 lines/m2)	RMS (nm)	Eg (eV)	Eu (meV)
NLu-0	4.2410	14.88	4.52	0.68	3.72	224
NLu-1	4.2410	15.71	4.05	0.88	3.77	190
NLu-2	4.2334	16.07	3.87	1.77	3.79	207
NLu-3	4.2428	17.82	3.15	1.97	3.74	226
NLu-5	4.2372	20.50	2.38	2.22	3.70	380

The average crystallite size (D) for samples are identified by following equation (2)

$$D=\frac{0.9\lambda }{\left(\beta \mathit{cos}\theta \right)}$$ (2)

where $\beta$ is full width at half of the peak maximum (FWHM). The mean crystallite size of 14.88 nm consistently rises to 15.71 nm, 16.07 nm, 17.82 nm, and 20.50 nm for 1 at. %, 2 at. %, 3 at. %, and 5 at. % Lu-contribution contents. This can be associated with higher ionic radius of Lu³⁺ (86.1 p.m.) than Ni²⁺ (69 p.m.) [13]. The density of dislocation (δ) is found by relation (3) [14]:

$$\mathrm{\delta }=1/{\mathrm{D}}^2$$ (3)

δ value of pure-NiO generally goes down from 4.52 × 10¹⁵ lines/m² to the values of 4.05 × 10¹⁵ lines/m², 3.87 × 10¹⁵ lines/m², 3.15 × 10¹⁵ lines/m², and 2.38 × 10¹⁵ lines/m² for 1 at. %, 2 at. %, 3 at. %, and 5 at. % Lu-contribution contents.

3.2. AFM analysis

The 2D and 3D AFM micro pictures are given in Fig. 3, Fig. 4. As noticed from these pictures, the whole films have nano-sized particles. The nickel oxide nanoparticles have an almost homogenous distribution. The similar particle structure was also observed in literature [2]. As seen clearly from 2D and 3D images, the particle size of pure NiO goes up with Lu-level. The AFM images' particle size variation and the XRD data correspond well. The surface roughness value for pure NiO film is 0.68 nm and it increases to the values of 0.88 nm, 1.77 nm, 1.97 nm and 2.22 nm values for 1 at. %, 2 at. %, 3 at. %, and 5 at. % Lu-contribution contents. The Lu substance causes a forming rough surface.

Fig. 3.

2D AFM images of pure and Lu-contributed NiO films.

Fig. 4.

3D AFM images of pure and Lu-incorporated NiO.

3.3. Optical properties

The transmission graph for undoped and Lu-doped NiO films is given in Fig. 5. At first, the transmittance of pure NiO rises with low content Lu until 2 at. %, then it declines with more Lu additive content. Pure NiO film's absorption edge initially slides to short wavelengths with a 1 and 2 at.% Lu level. The longer values with greater Lu content are then used. Films' optical band gap values (${\mathrm{E}}_{\mathrm{g}}$) are dictated by the relations (4) and (5) [15]:

$$\mathrm{\alpha }=\ln (1/\mathrm{T})/\mathrm{d}$$ (4)

$$\mathrm{\alpha }\mathrm{h}\mathrm{\nu }=\mathrm{K}{\left(\mathrm{h}\mathrm{\nu }-{\mathrm{E}}_{\mathrm{g}}\right)}^{1/2}$$ (5)

The terms “absorption coefficient,” “transmittance,” “film thickness constant” are used in these relationships as, α, T, d and K respectively. The E_g value of NiO film first increases from 3.72 eV to 3.77 eV and 3.79 eV with 1 and 2 at.% Lu-doping contents, and subsequently decreases to 3.74 eV and 3.70 eV for 3 and 5 at.% Lu-contents, respectively, according to the graph of ${\left(\mathrm{\alpha }\mathrm{h}\mathrm{\nu }\right)}^2$ vs. $\mathrm{h}\mathrm{\nu }$ in Fig. 6. The band gap values in this study and those in other analyses [1,2,16] are in good agreement. A decrease in the optical band gap is likely connected to degrade NiO crystallinity, while an increase in optical band gap values with low Lu can be caused by increased crystallinity [17,18].

Fig. 5.

Spectra of pure and Lu-doped NiO films' transmittance.

Fig. 6.

${\left(\mathrm{\alpha }\mathrm{h}\mathrm{\nu }\right)}^2$ vs. ($\mathrm{h}\mathrm{\nu }$) curves of pure and Lu doped NiO films.

The absorption tail is explored for the current investigation. The Urbach tail [19], sometimes referred to as the absorption edge, is written as follows in equation (6):

$$\mathrm{\alpha }\left(\mathrm{E},\mathrm{T}\right)={\mathrm{\alpha }}_0\exp \left(\frac{\mathrm{E}-{\mathrm{E}}_0}{{\mathrm{E}}_{\mathrm{u}}\left(\mathrm{T},\mathrm{X}\right)}\right)$$ (6)

The Urbach energy (E_u) and crystal diseases are related to temperature (T) (X). Cody et al.'s indication of a non-thermal component associated with the structural diseases contrasts this [20]. E_u values are so established through relation (7)

$$\ln \mathrm{\alpha }=\mathrm{E}\frac{1}{{\mathrm{E}}_{\mathrm{u}}}\bullet \left(\ln \left({\mathrm{\alpha }}_0\right)+\frac{{\mathrm{E}}_0}{{\mathrm{E}}_{\mathrm{u}}}\right)$$ (7)

E_u = Δ(hν)/Δ(lnα) and it is based only on the degree of structural disorders. The plots of lnα versus hν are given in Fig. 7. The Urbach energy value of undoped NiO initially decreases from 224 meV to 190 meV with 1 at. % Lu-content, which can be associated with increasing crystallinity of NiO [8]. Then they start to increase to the values of 207 meV, 226 meV, and 380 meV with 2 at. %, 3 at. %, and 5 at. % Lu-levels, respectively. This increase is probably originated from deteriorated crystallinity, which results from structural imperfections [9,10]. A band gap narrowing with high Lu-content is due to the transitions from tail and band to tail.

Fig. 7.

The curves of lnα vs. photon energy.

3.4. Electronic and magnetic properties of Lu doped NiO

To further comprehend the unique characteristics of Lu doped NiO thin films, it is necessary to first illustrate the typical characteristics of the electronic structure of both the ferromagnetic and antiferromagnetic concentrations of Lu-doped NiO. In Fig. 8, the computed band structures are displayed.

Fig. 8.

Spin polarized band structures of Lu doped NiO for the three considered concentrations: (a) 12.5%, (b) 3.125% and (c)1.25%. The red curves represent the spin-up band structure and the black color denotes the spin down band structure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Pure NiO is believed to have an indirect band gap, according to our earlier research [26]. We found that the band gap, which is too narrow for plane L(S)DA, has been modified by the LDA + U approximation utilizing the Hubbard potential to pure NiO and the local spin density approximation. NiO with no impurities has an indirect band gap of 0.468 eV (L(S)DA) and 2.832 eV (LDA + U). The key characteristic of the LDA + U technique is the qualitative change in the structure of the orbitals in the valence band maximum (VBM) and conduction band minimum (CBM), as well as the change in band dispersion close to the Fermi level.

As shown in Fig. 8, Fig. 9, where we summarized the new band gaps and their nature with change in concentrations, the current Lu-doping of NiO causes a significant change in its electronic structure that introduces defect bands in the band gap and as a result leads to transitions from indirect band gap (2 eV for 12.5%) to direct band gap for the concentration 3.125%. The Lu-f states, which are above the Fermi level, cause the insulating properties of Lu:NiO in the majority spin to disappear. Because impurity bands can easily enter the host band gap while in minority spin, the band structure is still an insulator, resulting in a narrow band gap.

Fig. 9.

Band gap for the majority spin population (UP) versus Lu-concentration.

According to these findings, NiO becomes ferromagnetic at the concentration under consideration, changing from antiferromagnetic, as was the case with Lu-doping. We presented the total and partial densities of states curves for the pure and related 3.125% Lu-doping concentration in Fig. 10 in order to better understand the function of each atom in the new alloys Lu: NiO. Here, the CBM of this material is primarily made up of Ni-3d orbitals, which are qualitatively higher in intensity, with minor contributions from other Lu/O and Ni orbitals. While the valence band maximum is generated by the Lu-5d-4f orbitals, Fig. 10 shows that the 4f and 5d orbitals break into two powerful peaks, one of which crosses the Fermi level.

Fig. 10.

Spin polarized density of states of Lu doped NiO for the concentration 3.125%.

The interaction of the Ni-4s and Lu-6p orbitals produced the region above the Fermi level. Additionally, regions below the Fermi level are mostly home to the oxygen 2p orbitals and nickel 3d orbitals, with small contributions from other lutetium-derived orbitals. With the exception of the region above 9.0 eV, where Lu-4f/5d orbitals predominate, the greater intensity peaks in the PDOS above the Fermi level in the CB are brought about by the 3d/5d orbitals of the Ni/Lu atoms (found virtually flat bands in the band structures). These are non-bonding states in the CB, and Fig. 10 shows that their energy is unaffected.

In order to explore the magnetic properties changes due to the Lu doping, we present in Fig. 11 (a, b) the electron localization function (ELF) and spin density contours in 2D. ELF contours is showing the behaviors of different shared-electron interactions mainly between Ni and O atom whereas the interaction with Lu is very weak.

Fig. 11.

(a) Electronic localization function (ELF) and (b) spin difference density 2D maps.

ELF's involvement of ground state properties, which only depends on the system's degree of approximation (quality of the physical description), is one of its key characteristics. This property serves as a bridge between quantum mechanical calculations and the understanding of atomic interactions (binding) [27]. In addition to information concerning binding, ELF also provides information about the positions of bonds, the atomic structure of the system, the electron pairs, and the strength of the bond by integrating the electron density of the ELF zone.

This can be used to establish whether the connection is of the physical binding kind or of the chemical bonding type (covalent bonds, metallic bonds). We can see the difference in the topology of ELF between spin-up and spin-down population. In fact, there is high localization around the Ni in spin up compared to the contours in spin down where they appear distorted, as can be seen there are six very smooth edges around the ELF basin related to Ni with two O atoms [28]. In Fig. 11(b) we can see that Ni and O are polarized in the lattice of NiO that carries a high degree of polarization than the host Ni and O atoms [29,30]. Whereas we can't see any contribution of Lu. In fact, Lu is paramagnetic and hence has no magnetic moment. Furthermore, compared to Lu atoms, Ni and O-atoms' degree of spin polarization is higher and has completely transformed into a dipole, leading to stronger magnetism. The anions of the oxygen in the vicinity that are immediately coordinated to Ni are where the magnetization is primarily concentrated. Fig. 12 displays the computed average magnetic moment for each element at the same concentration. The magnetic moment of NiO is well known to exist, however, doping with rare-earth atoms will alter this magnetic moment. Ni is principally responsible for magnetic, however the oxygen atoms that are close to Ni and Lu are polarized. Unexpectedly, the magnetic moment of Lu is negative, supporting the inverse spin polarization shown in Fig. 9's spin-density isosurface.

Fig. 12.

Magnetic moment per element for the 3.125% concentration of Lu doping.

When an atom of Lu is added, the nickel atoms in its immediate surroundings have Ni = 1.788 μ_B and those farther away have Ni = 1.775 μ_B. The integration of Lu induces a magnetic moment in oxygen atoms (between 0.021 and 0.200 μ_B) for both circumstances (close to Lu and far from it) [31,32]. The total magnetic moment was estimated to be 61.070 μ_B for the current concentration and the hypothetical supercell with a concentration of 3.125% with 63 atoms.

4. Conclusion

Lutetium-contributed NiO thin films have been fabricated by sol-gel spin coating for the first time. The crystalline structure, surface morphology, and optical features of Lu-contributed films have been characterized as a function of Lu doping concentration. The XRD and AFM investigations have shown the Lu-contributed NiO films have consisted of nano-polycrystalline particles and they had a cubic bunsenite structure with (200) preferential orientation. At 1% Lu-content, the Urbach energy value of undoped NiO initially falls, which is consistent with NiO's growing crystallinity. Then they start to rise, which is likely the outcome of deteriorating crystallinity, which is brought on by structural flaws. The transitions from band to tail and from tail to tail cause a band gap to decrease when the Lu content is high. The influence of lutetium doping NiO on the electronic and magnetic structures was clarified by additional theoretical study utilizing first-principles calculations based on density functional theory within GGA + U. The change in polarization of NiO in response to the concentration of lutetium is demonstrated by the computation of the total magnetic moment and the contribution of each atom for various concentrations. This study confirms that spin-polarized transport devices and other spin-dependent applications may be successfully implemented on NiO doped Lutetium. The ability to modify magnetic ordering in nanostructures has been demonstrated.

Author contribution statement

Souraya Goumri-Said, Güven Turgut, Mohammed Benali Kanoun: Conceived and designed the experiments and the simulation works; Performed the experiments and computational modeling; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Acknowledgements

S. Goumri-Said thanks office of research at Alfaisal University for the support with internal project (IRG) number 22413.

Data availability statement

No data was used for the research described in the article.

Declaration of interest’s statement

The authors declare no conflict of interest.