High dielectric-energy storage and ferromagnetic-superparamagnetic properties: tetra-doping CuO nanocompositions

Tetra-doping by (Mn, Fe, Co, Ni) ions strongly boosted the room temperature dielectric constant and the ferromagnetic-superparamagnetic characteristics of monoclinic CuO structure. In this study, undoped CuO, Cu0.98Mn0.005Fe0.005Co0.005Ni0.005O, Cu0.96Mn0.01Fe0.01Co0.01Ni0.01O and Cu0.94Mn0.015Fe0.015Co0.015Ni0.015O nanocompositions were synthesized through coprecipitation technique. The crystal structure analysis verified that all samples have a pure single phase, corresponding to monoclinic CuO structure. The substitution of Cu2+-sites into CuO lattice by Mn2+, Fe2+/3+, Co2+ and Ni2+ ions has been deduced from the expansions of lattice constant, shifts of XRD diffraction peaks and band gap energy alteration. The additions of (Mn, Fe, Co, Ni) ions lead to the formation of homogenous distributed very fine spherical nanoparticles, especially at large concentrations of dopants (Cu0.94Mn0.015Fe0.015Co0.015Ni0.015O sample). The tetra-doping by (Mn, Fe, Co, Ni) ions reduced the intensity of the diffuse reflectance alongside red shifted the absorption edge and the band gap energy of monoclinic CuO structure. Cu0.98Mn0.005Fe0.005Co0.005Ni0.005O exhibits a high relative permittivity value of 6096 at low frequency of 42 Hz with small dielectric loss tangent (tan δ) compared to pure one. The tetra-doping by (Mn, Fe, Co, Ni) dopants induced excellent intrinsic ferromagnetic and superparamagnetic hysteresis loops into monoclinic CuO structure with full saturation loops shape and variable coercivity values.


Introduction
In recent years, the p-and n-types metal oxide semiconductors have gained more attention owing to their applicable dependent electrical, optoelectronic, magnetic and dielectric energy storage properties [1][2][3]. The modern renewable resources such as solar power and wind enable the electrical energy to being produced in a mass amount [4]. For economic and environmental benefit, storing the electrical energy to be consumed efficiently later is an important issue [4]. For these reasons, the quality of the electronic systems that can store energy plays a leading role in the storage of the electrical energy [5]. The dielectric capacitors as storage devices have many advantages including high power, fast charging and long life time as well as can deliver the electrical energy almost instantly [6,7]. These remarks make the dielectric capacitors a perfect choice for great energy storage. Also, the fast development of the microelectronic devices for aerospace engineering and electrical vehicles make immediately need for dielectric materials with high energy storage performance [8][9][10]. The dielectric properties (dielectric constant and dielectric loss tangent, tan d) of the fabricated materials are very important for its applicability in the electronics industry [8]. To investigate the permittivity or the dielectric characteristics of any structure, the dielectric constant (e 0 ) is a fundamental aspect that determines the capacity or amplitude of the material for charge storage [11][12][13][14]. Colossal relative permittivity materials are utilized as memory cells, gate dielectrics in metal oxide semiconductor (MOS) transistors, capacitors and supercapacitors [15][16][17][18]. In the same context, low relative permittivity materials have advanced applications in high-speed integrated circuits and electrical devices [19]. The metal oxides semiconductors (MOS) as dielectric substances are main components for varied thin-film electronic chips due to their remarkable mechanical and dielectric properties [20]. On the other hand, the metal oxides semiconductors are looked upon as promising materials in diluted magnetic semiconductor (DMS) systems for spin-electronics and magnetic-medical applications [21][22][23]. Reinforce the ferromagnetic and superparamagnetic characteristics of the metal oxides semiconductors alongside the dielectric properties can inspire their multi-functionality in spin-electronics and medical technology [24,25]. Spin-electronics is a novel future technology that utilizes the electron spin together with its charge to upsurge the storage capacity, speed and efficiency [26][27][28]. For spin-electronics technology, one of the essential requirements is the synthesis of single phase structure which holds both the ferromagnetic-semiconducting characteristics at room temperature [26][27][28]. The metal oxides semiconductors with room temperature ferromagnetism are promising materials for advanced spin electronics devices [26][27][28]. Besides, prompting superparamagnetic features in specified materials is a vital subject owing to their use in hyperthermia, magnetic sensors, magnetic resonance imaging and drug delivery [29][30][31][32][33]. Copper oxide (CuO) structure is an interesting p-type semiconductor material with remarkable electrical, magnetic, optical and thermal characteristics as well as inexpensive and nontoxic [34]. CuO semiconductor is technologically well-known material having multifunctional properties with promising applications in magnetic storage media [35], gas sensor [36], optical devices [37], catalysts [38], lithium-ion batteries [39], p-n diode [40], solar energy [41] and superconductors [42]. The physical electrical, dielectric, magnetic and optical properties of CuO can be controlled or enriched through incorporation of appropriate dopants into its lattice. Previous studies on the influences of the transition metals doping and codoping on the optical, electrical and magnetic properties of CuO were carried out [43][44][45][46][47][48][49]. As far we known, no available study on the competition effect of the four dopants including Mn, Fe, Co and Ni ions on the optical, electrical, dielectric, ferromagnetic and superparamagnetic characteristics of CuO was detected. The combination between variable dopants can induce and enhances many different features. Compared to single-doping, the multi-doping can help in improving the concentration and stability of the defects, tuning the dopants populations as well as the durable bonding between dopants considerably decreases the formation energy. The ionic radii of Mn, Fe, Co and Ni ions are analogous to that of Cu 2?sites; hence the replacement can take place without causing much lattice distortion. Furthermore, 3delectrons of Mn, Fe, Co and Ni ions can interact into CuO and enhance the optical, electrical and magnetic properties. In this study, Mn 2? , Fe 2?/3? , Co 2? and Ni 2? as multi-substituted ions were used to tune the optical, dielectric and magnetic properties of monoclinic CuO semiconductor for multi-functional applications. In this work, equal concentration was used for each element equal to 0.5%, 1 and 1.5 wt% to achieve the following compositions Cu 0.98 Mn 0.
where b, h, K, k, D, e and hkl symbolize the width at half maximum (radian), position of the peaks (radian), K is the Scherrer constant, wavelength of incident X-ray (0.15406 nm), crystallite size, microstrain and Miller indices, respectively. The relation between the 4sinh hkl (X-axis) and bcosh hkl (Y-axis) provides the microstrain by estimating the slope of the fitting line and the size of crystallite (D) was calculated using the intercept value on bcosh hkl axis (D = Kk/intercept) as illustrated in Fig. 2. The microstrain of the 0%TC sample was found to be 0.000043, Fig. 3   3.2 Scanning electron microscope (SEM) Figure 5 reveals the scanning electron microscopy (SEM) and the 3D pictures of 0%TC, 2%TC, 4%TC and 6%TC powders. The micrograph of the synthesized 0%TC powder, Fig. 5a, demonstrates the formation of asymmetrical grains having variable sizes with appearance of some agglomeration. The SEM image of 2%TC sample has shown very fine grains (major) with occurrence of some rod grains (minor) as illustrated in Fig. 5b. The SEM image of 4%TC powder clearly shows the presence of some asymmetrical large grains besides very fine (major) and rods particles, Fig. 5c. In case of 6%TC sample, homogenous distributed very fine grains are formed, Fig. 5d. These results show that Mn 2? , Fe 2?/3? , Co 2? or Ni 2? dopants have remarkable roles in improving the homogenous shape of the grains besides reducing their sizes through acting as restriction ions for the growth of the grains. Figure 6 Fig. 7a. Also, the absorption edge was noticeably changed and red shifts for higher wavelengths were observed. The reflectance (%) was converted to absorptivity through applying the Kubelka-Munk function (F (R) = (1-R) 2 /2R = a/ S) [51]. The normalized diffuse reflectance spectra (Kubelka-Munk function vs. wavelength) are shown in Fig. 7b. The normalized diffuse reflectance spectra display a marked difference in terms of infrared absorption between pure and tetra-doped CuO samples, in relation with the amount of Mn, Fe, Co and Ni ions introduced. To calculate the band gap energy of 0%TC, 2%TC, 4%TC and 6%TC samples from reflectance data, the Kubelka-Munk and Tauc's equations were used. The band gap energy is estimated from the plot between (F(R) ht) 2 and ht [52]. The Kubelka-Munk function and the Tauc's equations can be writing as shown below [52][53][54][55]: R is the reflectance, h is known as a plank's constant, ht is the photon energy in eV, Eg is the band gap and n = 2. From the (F(R) hm) 2 and hm plot, the band gap was found through the intercept of the linear part of the curve with X-axis as illustrated in Fig. 7c

Dielectric properties
The relative permittivity (dielectric constant, e 0 ) and the dielectric loss tangent (tan d) are two fundamental electrical parameters that give information concerning the conduction behavior in CuO structure. The real part of relative permittivity symbolizes the polarization of the substance while the dielectric loss tangent (tan d) signifies the energy loss due to the polarization and ionic conduction [58]. The relative permittivity and dielectric loss tangent of the synthesized compositions are related to the occurrence of space charge polarization alongside on structural defects, temperature, morphology, frequency and chemical composition [59,60]. Figure 8a illustrates the dependence of the relative permittivity on frequency for 0%TC, 2%TC, 4%TC and 6%TC pellets at room temperature. The values of the relative permittivity were reduced with growing the applied frequency. Herein, the curves of the dielectric constant show three regions with frequency; from 42 Hz to 1 kHz the samples reveal strong decreases, from 1 to 100 kHz the relative permittivity slowly decreased and from 1 kHz to 5 MHz the relative permittivity is nearly not affected by frequency. At 42 Hz, 0%TC sample has a relative permittivity value of 850 while 2%TC, 4%TC and 6%TC samples exhibit high relative permittivity values of 6096, 3014 and 2328 respectively. The relative permittivity values of the 0%TC,  seems that the hopping mechanism between Mn 2? , Fe 2? , Co 2? , Ni 2? and Cu 2? is predominant and leads to higher values of relative permittivity while at greater concentration, the dopants ions existing in the CuO lattice may behave as deep donor. Thus, it may inhibit the conduction mechanism and does not contribute to the conduction process but provides the impedance to it and hence decreases the relative permittivity values. The measured relative permittivity behavior can be discussed by Maxwell-Wagner effect [61,62]. The Maxwell-Wagner view considers the dielectric substance as composed of highly conducting grains disconnected by the weak conducting grain boundaries. Under ac electric field, by hopping mechanism the electrons pass from the grains to the grain boundaries and crowd owing to the large resistance which yield polarization. As a result, the relative permittivity values are very high at the lower frequencies. When the frequency growth, the hopping of the electric charge cannot track the electric field, consequently the electrons have weak chance to reach the grain boundary, leading to less polarization (low relative permittivity value). Figure 8b displays the dependence of dielectric loss tangent (tan d, energy dissipation) on frequency for 0%TC, 2%TC, 4%TC and 6%TC samples. At low frequency, the dielectric loss tangent (tan d) has high value especially for 0%TC sample owing to interfacial polarization and with increasing the frequency the loss tangent decreases until reach a constant value. Interestingly, at lower frequencies the (Mn, Fe, Co, Ni) tetra-doping strongly reduces the dielectric loss tangent of pure CuO to large extent owing to the influence of the space charge polarization. This means that the 2%TC, 4%TC and 6%TC samples can store more energy in the low frequency region. The domain wall resonance can be used to explain the reduction in the dielectric loss tangent with frequency [59]. At low frequency, additional energy is needed due to the high grain boundary resistance while at high frequency the resistance decreased and the electrons required a small energy to transfer. The high e 0 and the low tan d values make the 2%TC composition suitable for energy storage applications.

Electrical modulus analysis
To explore the nature of the relaxation, the real and imaginary components of the complex modulus have been investigated [63]. The electrical modulus M* (M' and M'') is correlated to the complex permittivity e * (e 0 and e 00 ) by the next equations [64]: M 00 ¼ e 0 =½ðe 0 Þ 2 þ ðe 00 Þ 2 Figure 10 demonstrates the log frequency-dependent the real part (M') and the imaginary part (M'') of 0%TC, 2%TC, 4%TC and 6%TC samples. The real part (M 0 ) has nearly zero value in the low frequency region, indicating a negligible influence from electrodes [65]. At higher frequency, the real part (M 0 ) of the complex modulus increases throughout the frequency range used. In the range of 100 kHz to 5 MHz region, both (M') and (M'') show relaxation peaks at certain frequency. The observed peaks point out to that the relaxation phenomenon in CuO samples is due to the dielectric relaxation. At higher frequency, (M') and (M'') have higher values for 4%TC and 6%TC samples compared to 0%TC and 2%TC samples. Figure 11 displays the room temperature ac electrical conductivity (r ac ) with log frequency (F) for 0%TC, 2%TC, 4%TC and 6%TC samples, within range from 42 Hz to 5 MHz. Approximately, the r ac independent on frequency up to 10 kHz followed by steady or gradual increases up to 100 kHz, and after that the r ac reveals exponential increases. The variation of the r ac with frequency displays two regions from frequency-independent (region of the low frequency) to frequency-dependent (region of the high frequency), indicating relaxation phenomenon of r ac . The gradual increases of r ac with frequency can be discussed based on interfacial Maxwell-Wagner influence [59,60]. The weak conducting grain boundaries are more effective at lower frequency, which consecutively decreases the r ac of the CuO samples. When frequency growth, the electron hopping is improved, consequently the r ac was increased.  Table 2. The M-H relation of 0%TC powder illustrates full ferromagnetic performance with saturation magnetization of 0.045 emu/g and coercivity of 150 Oe, attributed to the uncompensated spins on the CuO surface [66,67]. Upon incorporation of 2 wt% (Mn, Fe, Co, Ni) into CuO structure (2%TC), the ferromagnetic performance converts to nearly superparamagnetic with coercivity value close to  Fig. 11 Dependence of room temperature ac electrical conductivity on frequency for 0%TC, 2%TC, 4%TC and 6%TC samples zero (14 Oe). As well, the saturation magnetization value is greatly boosted to 0.59 emu/g. With increasing (Mn, Fe, Co, Ni) concentration to 4 wt% (4%TC), the magnetization increased to 0.88 emu/g and the coercivity growth to 29 Oe (close the ferromagnetic performance). At 6 wt% (Mn, Fe, Co, Ni) concentration (6%TC), the S-hysteresis shape reveals a magnetization of 1.06 emu/g and a coercivity value of 15 Oe, close to the superparamagnetism. As a result, the synthesized compositions exhibits two distinctive kinds of magnetic characteristic based on the coercivity values. The 0%TC and 4%TC compositions display nearly ferromagnetism at room temperature, whereas 2%TC and 6%TC compositions reveal approximately room temperature superparamagnetism with coercivity value close to zero (14 or 15 Oe). The crystal structure analysis, optical properties and band gap reengineering of 0%TC, 2%TC, 4%TC and 6%TC samples recommended the effective replacement of Cu 2? -sites by Mn 2? , Fe 2?/3? , Co 2? and Ni 2? ions. The expanded XRD pattern of 2%TC, 4%TC and 6%TC compositions demonstrates notable 2h angle shifts for the (-111) and (111) crystallographic planes compared to 0%TC sample. As results, these shifts and the changes of lattice constant agree with Vegard's law and point to the substitutional tetra-doping of Mn 2? , Fe 2? , Co 2? and Ni 2? ions at the Cu 2? -sites [68]. Based on XRD peaks, no secondary phases associated with any impurities oxides were detected. Linked to these comments, we  can deduce that the room temperature ferromagneticsuperparamagnetic characteristics of 0%TC, 2%TC, 4%TC and 6%TC compositions have intrinsic causes and not associated with any impurities. The accurate cause of the ferromagnetic and superparamagnetic characteristics in the undoped or low doped oxides semiconductor (diluted magnetic semiconductor) is still in uncertainty [69]. Many scientists proposed diverse theories or models to illuminate the main cause of the ferromagnetism or superparamagnetism in undoped or low-doped oxides semiconductor including direct interactions like super-exchange and double-exchange interactions [70], Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange mechanism [71], p-d Zener mechanism [72], F-center-based oxygen vacancies interaction and the bound magnetic polarons (BMPs) theory [73]. In this context, Coey et al. [74] reported that the observed ferromagnetism at room temperature in lightly doped or multi-doped materials by transition or rare earth metals like Mn, Fe, Co, Ni and Gd dopants could be interpreted via BMP model. The BMP model was proposed for oxides semiconductor and it is fundamentally based on the formation of a spin polarized cloud nearby a region holding both magnetic impurities (Mn, Fe, Co, Ni) and donor defects (vacancies). When the size or the number of these spin polarized clouds increases, the BMPs can join which produces magnetically ordered areas. Sequentially, these magnetically ordered areas can powerfully overlap and extent over the total structure which producing a condition comparable to a ferromagnetic state and this type can continue to room temperature or above [75]. There is an additional mechanism termed F-center (FC, oxygen vacancies mediated ferromagnetism) that it is a sub-mechanism or an equivalent path to the BMPs mechanism to describe the ferromagnetic characteristics at room temperature in undoped or transition (Mn, Fe, Co, Ni) or rare earth (Gd, Ce, Sm) metals lightly doped oxides semiconductor [76][77][78][79]. In the F-center mechanism, the electrons trapped by the oxygen vacancies can act as coupling F-centers, over which the doped magnetic ions like Mn, Fe, Co, Ni, Gd and Sm align in ferromagnetic order. The F-center expects that M n? -V O -M n? states (M n? = doped magnetic ions and V O = oxygen vacancies) will be popular in the composition and the electrons trapped in the oxygen vacancies produce F-centers; where the electron occupies an orbital which overlaps the d-orbitals of both magnetic ions neighbors.

Magnetic properties analysis
According to RKKY concept [80], the superparamagnetic characteristics at room temperature can be assigned to the interactions of the spin-polarized localized electrons with the conductive one. Spin polarization of the conductive electrons take place due to these exchange interactions. As a result, the spin-polarized conductive electrons can make exchange interactions with the spin-polarized localized electrons of the compositions, single phase with monoclinic structure was detected via XRD analysis. The expansions of lattice constants, shifts of (-111) or (111) XRD crystallographic planes and the reengineering of the band gap structure verified the effective replacement of Cu 2? -sites by Mn 2? , Fe 2?/3? , Co 2? and Ni 2? ions. The SEM results demonstrated that Mn 2? , Fe 2?/3? , Co 2? or Ni 2? dopants have remarkable roles in improving the homogenous shape of the grains besides reducing their sizes through acting as restriction ions for the growth of the grains. Obvious red shifts for the absorption edge of CuO were detected due to insertion of Mn 2? , Fe 2?/3? , Co 2? and Ni 2? ions into its lattice. The 0%TC, 2%TC, 4%TC and 6%TC compositions revealed a band gap energy of 1.445, 1.39, 1.35 and 1.37 eV, respectively. Remarkably, 2%TC sample exhibits a high relative permittivity of 6096 at low frequency of 42 Hz with small loss tangent values compared to 0%TC sample. The synthesized compositions exhibit two distinctive kinds of magnetic characteristics based on the coercivity values. The 0%TC and 4%TC compositions display full ferromagnetic behaviour at room temperature with saturation magnetization of 0.045 and 0.88 emu/g, whereas 2%TC and 6%TC compositions reveal approximately room temperature superparamagnetism with coercivity value close to zero (14 or 15 Oe).

Author contributions
The submission of the manuscript has been approved by all coauthors. All authors have equal contributions in preparing the manuscript including experimental, writing, discussion and revision.

Funding
Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Conflict of interest 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.
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