The Influence of Doping Perimidine Ruthenium Complexes on Structural, Optic, and Residual Stress Properties of ZnO Thin Films

Perimidine ruthenium complexes ([Ru(L1-2)(p-cymene)Cl]Cl) were synthesized and were used to produce Ru-doped ZnO thin films by the sol–gel spin-coating method. These films were characterized using XRD, SEM, FT-IR, UV–Vis, and photoluminescence (PL) spectra. It was determined by XRD analysis that all films have a hexagonal wurtzite structure. The residual stress was generally compressive and ranged from − 2.117 to 1.4 GPa as the ruthenium content increased from 0 to 6%. The crystal quality tended to increase, and the residual stress reduced with increasing Ru content. In the visible range, all the films exhibited transmittance greater than 70%. Further, the optical bandgap was 3.20 eV for pure film and bandgap of Ru-doped films varied between 2.76 and 3.18 eV. This research shows that new high conjugated structures are promising as dopant materials for the ZnO thin film, and good crystalline and optical properties could be obtained from the Ru-doped ZnO films prepared for nano-optoelectronic devices.


Introduction
One of the most important multifunctional semiconductor oxides is zinc oxide (ZnO) due to its unique characteristics and a variety of applications such as microelectronic devices [1], sensors [2], batteries [3], and optoelectronics [4]. It is a desirable substance for material science due to its wide bandgap (3.37 eV), high bond energy (60 meV), and elevated thermal and mechanical performance at room temperature [5]. Doped and undoped zinc oxide is used in sol-gel derived thin film production, and it attracts the attention of researchers due to its structural, optical and electrical properties [6][7][8]. The dopant materials are used to improve electrical, structural, and optical properties of ZnO thin films [9]. There are a lot of thin film production methods such as evaporation [10], sputtering [11], pulsed laser deposition [12], sol-gel [13], and chemical bath deposition [14]. Among these methods, the most preferred one is sol-gel which has simple stages of (i) exhausting, (ii) excess deposit processing, and (iii) drying which provides good control over the chemical composition with low temperature, low cost, and low energy consumption [15].
Ruthenium (Ru) is one of the dopant materials used to produce ZnO thin film. It is a unique transition metal that belongs the platinum group on the periodic chart. Ruthenium is extremely scarce as the 74th most common ingredient of the Earth's crust, occurring at around 0.001 ppm [16]. This element is commonly present in ores in the Ural Mountains and in North and South America, along with the other platinum group metals such as osmiridium, iridiosmium, and laurite [17]. This metal is used in several fields, such as the medicine, mechanical and chemical industries [18,19]. It has special chemical and physical characteristics, such as high conductivity, high melting point, hardness, and catalytic effects. Due to these outstanding properties, ruthenium is used as a dopant material in the production of thin films.
There are several Ru compounds in the literature. Ruthenium (III) chloride is one of them and is used to produce Rudoped ZnO thin films in some studies [20][21][22][23]. Vettumperumal et al. [20] prepared nanocrystalline Ru-doped ZnO thin film using sol-gel spin-coating with ruthenium (III) chloride and investigated structural properties and UV characteristics. They observed an improvement in sheet resistance with an increase in the dopant amount of Ru (1-2 mol%) and recorded an improved I-V characteristic response at 1 mol%. Gómez-Pozos et al. [21] deposited ruthenium and chromium-doped zinc oxide thin films using sol-gel dip-coating on soda-lime glass substrates for gas sensing application. They used ruthenium (III) chloride for Rudoped thin film with a non-alkoxide route. The doping process was done as one, three, and five immersions. They observed that Ru was better than Cr in five immersions for C 3 H 8 gas detection.
Besides the commercially available ruthenium (III) chloride, there are ruthenium complexes that have notable growth and improvement in the area of coordination and organometallic chemistry. Recently, several publications have appeared on the creation and applications of Ru-based complexes in fields such as nanoscience, catalysis, medicine, genetics, redox, and photoactive materials. The fact that Ru has outstanding potential to appear in several oxidation states may be connected to these advances. Three key properties of ruthenium-based complexes have been stated by Allardyce et al. [24] to make them suitable for drug application. Lida et al. [25] explored the role of ruthenium complexes on cancer inhibitory activity. Li et al. [26] investigated the antimicrobial activity of ruthenium complexes and discussed the relationship between biological processing and chemical composition. Although there are several studies related to different application areas, there is no study related to ZnO thin film produced by using ruthenium complexes. The aim of this study is to investigate the usability of ruthenium complex in ZnO thin films. These complexes were synthesized using high conjugation perimidine ligand system (L 1 and L 2 ) as mentioned in previously published procedures [27][28][29][30].
It is known that adding materials in nanoscale increases the various properties of materials [31][32][33]. The incorporation of Ru into ZnO at nanoscale sizes can change the stress in the lattice of ZnO as a result of the tensile or compressive stress occurring in the nanostructured films [34]. For this reason, we synthesized perimidine ruthenium complexes ([Ru(L 1-2 )(p-cymene)Cl]Cl) from 2-pyridinecarboxaldehyde/ quinoline-2-carboxaldehyde and 1,8-diaminonaphthalene [35], and these Ru-complexes were added to ZnO at the content of 2, 4, 6 wt% to fabricate Ru-doped ZnO thin films by the sol-gel spin-coating method. The stress measurements of the fabricated films were performed with the XRD method and compared with the crystal sizes obtained from SEM and XRD. Furthermore, structural and optical properties of Rudoped ZnO thin films were investigated by FTIR, UV-vis, and PL spectra.

Synthesis of Ligands and [Ru(L 1-2 )(p-Cymene)Cl] Cl Complexes
The ligands and ruthenium complexes were synthesized according to our previous studies by making some modifications [29,30,36]. All ligands and metal complexes were synthesized with a high yield. As summarized in Fig. 1, ligands (L 1-2 ) were prepared in moderate yield with the optimized conditions by one-pot reaction via 1,8-diaminonaphthalene refluxing the 1:1 molar ratio with 2-pyridinecarboxaldehyde and 2-quinolinecarboxaldehyde in anhydrous ethanol (EtOH). The targeted ligands (L 1-2 ) were produced in good yields as solids upon recrystallization (80-85%), and all ligands had satisfactory spectroscopic results. The metal complexes ([Ru(L 1-2 )(p-cymene)Cl]Cl) were obtained by reactions of ligands (L 1-2 ) with [RuCl 2 (p-cymene)] 2 as yellow solids following recrystallization. The synthesis and spectroscopic data of the ligands (L 1-2 ) and their metal complexes ([Ru(L 1-2 ) (p-cymene)Cl]Cl) were given as supporting information. The 1 H and 13 C NMR spectra and LC MS/MS with elemental analysis results support 1:1 ratio of metal/ligand, as expected. The ligands and their compounds confirmed the indicated structures according to multinuclear NMR spectra and elemental analyses.

Preparation of Ru-Doped ZnO Thin Films
Sol-gel spin-coating method was used to synthesize Ru-doped ZnO thin films on a glass substrate. The starting material and stabilizer were zinc acetate dihydrate [Zn(CH 3 COO) 2 .2H 2 O] and monoethanolamine (MEA). Ethanol, dimethyl sulfoxide (DMSO), and isopropanol were used as the solvent. Commercially available ruthenium (III) chloride (RuCl 3 ·xH 2 O) and synthesized [Ru(L 1-2 )(p-cymene)Cl]Cl complexes were used as dopant materials. The first step is to dissolve the Zinc acetate dihydrate in a mixture of MEA and isopropanol. Zinc acetate had a molar content of 0.5 M. The molar ratio of monoethanolamine to zinc acetate was kept constant at 1:1. Using a magnetic stirrer (MR Hei-Tech, Heidolph), the solution was first stirred at 65 °C for 30 min to get a clear and uniform sol. At this stage, ruthenium solutions were added to the zinc acetate solution and mixed for another 90 min. Ruthenium solutions were prepared by dissolving ruthenium compounds in 20 µl DMSO and 0.5 ml ethanol. Three different dopant concentrations (2, 4, 6 wt% with respect to zinc acetate) were used. The notations of nine different thin films are shown in Table 1.
The final solutions were aged at room temperature for 24 h. Before the deposition process, the glass substrates were washed with soapy water and ultrasonically cleaned to remove any dirt, oil, and grease in deionized water, isopropanol, and acetone for 10 min each, respectively. The thin films were prepared using a spin coater (POLOS SPIN200i-NPP-INT) at 3000 rpm and 30 s, followed by a 10-min drying process at 200 °C on a hot plate to extract solvents. The deposition and drying procedures were repeated 10 times to get a thin film. Finally, the films were annealed at 450 °C for 1 h in a furnace at air atmosphere. The flow chart for the deposition procedure of Ru-doped ZnO thin films is shown in Fig. 2.

Characterization of Ru-Doped Zno Thin Films
X-Ray diffraction (XRD) studies were performed to obtain the information on phases, crystalline quality, and crystallite size. CuK alpha radiation was used to do the XRD tests (Rigaku Dmax 2000 Diffractometer) at 40 kV and 30 mA. The range of 2θ scanning speed (0.02) was 20° to 70°. All measurements were done on coated surfaces of glasses.  Residual stress analyses were performed using sin 2 ψ method. Rietveld refinements were conducted using match software. Optical transmission spectra were conducted with a scanning speed of 0.02 in a 200--800-nm wavelength range (Shimadzu UV-1700). The surface characteristics and thickness measurements of the films were analyzed using SEM microscope (Zeiss Evo 50). Photoluminescence (PL) measurements were carried out using a spectrometer at room temperature (Perkin Elmer LS 55) with a Xenon flash lamp (at line frequency 50 or 60 Hz) as an excitation source in the range of 200 to 800 nm with a step of 1 nm. FT-IR spectra in the range of 4000-500 cm −1 were acquired using a Perkin Elmer Two FT-IR spectrophotometer and the ATR accessory with a diamond ATR crystal. Figure 3 displays the XRD patterns of the deposited Rudoped ZnO thin films. All peaks were indexed well with the zinc oxide (pdf number: 01-075-0576) or zincite (pdf number: 01-075-1526) structure with orientation along (002). No diffraction peaks that belong to ruthenium are observed in all samples. The peak is a little bit wider for the low content of Ru. But the positions of peaks slightly shift to a little bit higher at 2-theta values with ruthenium engaging into the matrix, and it becomes sharper with ruthenium doping due to differences in the ionic radii of Zn 2+ and Ru 3+ [37]. Further, an increasing tendency in the intensity of (002) and (101) peaks has occurred with the increase of Ru for B and C films, while an increase in the intensity of (100) and (101) peaks of A films has been observed. It is known that increases in concentration attribute to an increase in film thickness. The primary XRD peaks were used to derive the lattice constants a and c, which are presented in Table 2. These parameters have shown a slight difference for Ru-doped samples as compared to undoped ZnO. Ghosh et al. [37] expressed that this was due to differences in the ionic radii of Zn 2+ and Ru 3+ . Furthermore, Fig. 3 clearly shows that the Ru complex of the L 1 ligand gave higher peaks compared to those with the L 2 ligand. The complete width at half-maximum of the (002) reflection has been evaluated to calculate the mean crystallite size by using conventional DebyeScherrer's formula (D  [38]. Furthermore, samples with greater concentrations may age faster and reach peak grain size faster than those with lower amounts [39]. The Rietveld refinement has been performed by using the Zincite and Zinc Oxide structure as starting model structure. An increase has occurred in the a and c lattice parameters after Rietveld refinement. The Young's modulus of films is given in the range of 76-257 GPa in the literature [40]. The interval of the Young modulus can be attributed to geometrical effects caused by microfabrication. Young modulus can be calculated via Eq. (1).
Residual stress is a significant characteristic of films that has a direct impact on the conductivity and surface morphology of the film [42]. Thus, residual stress values and properties of films have been investigated in this study. As the ruthenium content increased from 0 to 6%, the residual stress values ranged from − 2.117 to 1.4 GPa, and the measured stress was determined to be compressive except for A4. It can be due to stability problems and decreasing fiber radius. Compared to %6 doping, C6 has demonstrated the lowest residual stress value. For ZnO films, Al-Khawaja et al. [43] have found that when crystalline quality improves with film thickness, the stress concentration reduces rapidly in increasing thickness and stays unchanged. The influence of dopants on crystallinity may be described by the growing process, which can be clarified by the desorption possibility of dopant atoms [42]. The thermoelectric capabilities of the ZnO thin films with the least compressive stress are enhanced by higher power factor value. Residual stress values have shown a lower value than pure ZnO film. It has been discovered that as the thickness of the produced films grows, the crystalline quality improves and the residual stress decreases. These results indicate the positive effect of the perimidine ligand derivatives for Ru-doped ZnO thin films.

SEM Analysis
The surface structure of Ru-doped ZnO thin film was shown in SEM images in Fig. 4. Images were taken at 10.00 K magnification with an EHT value of 15.00 kV. The Ru-doped ZnO thin film was mechanically stable and demonstrated very strong adherence to the substrate at room temperature. After deposition, no peel or crack was observed in the thin films except for C films. In these films, a slight decrease in stability was observed from C4 to C6 films as the addition rate increased. As can be seen from the SEM images, while there is a leafy structure in undoped thin film, a nano fiber-like structure was observed in Ru-doped ones and the density of this structure increased as the amount of additive increased. The nano fiber-like structures were shown in the upper right corner of SEM image in Fig. 4. The average thickness of these fibers calculated by taking different measurements was shown in Table 2. The coating thicknesses of thin films were obtained from the SEM image of the cross-sectional area. Both increased as the doping rate increased except for C6 film. In this sample, it was seen that as the coating thickness increased, the fiber thickness decreased. This reveals that the [Ru(L 2 )(p-cymene)Cl]Cl compound used for the ruthenium doping process has a stability problem at 6 wt%.

UV-Vis Analysis
The optical property of pure ZnO and Ru-doped ZnO films has been determined by UV-vis spectrometer from 300 to 800 nm wavelength range. The UV-vis absorption spectra show a broad absorption in the ultraviolet region around 300-370 nm (Fig. 5). All of the films have relatively high transmittance of more than 70% in the visible spectrum. The absorption edges have been found between 392 and 425 nm. The dramatic rise in absorbance at wavelengths of 300 nm is due to inter-band transitions near the fundamental edge [37]. Undoped ZnO thin film has lower absorption in the visible range of the spectrum and the absorption edge increases for B and C films depending on dopant content. Further, the absorption wavelength increases from 403 to 417 nm for B and 403 to 420 nm for C and shifts to blue. Blue emissions are often caused by impurities and imperfections in the ZnO lattice [44].
The bandgap is a key indicator of electrical conductivity and is influenced by layer thickness, degree of crystallinity, and temperature. Tauc's relationship was used to calculate the optical bandgaps of the coatings (Eq. 2).
Here, A is constant, hv is photon energy, and a is the absorption coefficient. The optical bandgap of all films are given in Fig. 6. The optical bandgap is 3.20 eV for pure ZnO and all films have a bandgap between 2.76 and 3.20 eV. The bandgaps of the A and B films have decreased with the increase of Ru content. The s-d and p-d contacts that lead to the bandgap cause the shrinkage, making these materials more suitable for nano-optoelectronic devices [37,45]. Because the wave patterns of the electrons coupled to the impure atoms begin to overlap as the Ru doping concentration level grows, the bandgap decreases at high doping densities. Allowable shallow states in the bandgap are created by impurities and these shallow levels have very weak ionization energies [41]. Also, the film thickness and particle size have an effect on bandgap. Lower particle size has a higher bandgap due to quantization. The reason for the contraction in the Eg bandgap of Ru-doped ZnO films may be that Ru doping formed new recombination centers with lower emission energy. According to Kumar Fig. 5 UV-vis absorption spectra of Ru-doped ZnO thin films Fig. 6 Optical bandgaps of Ru-doped ZnO thin films et al. [45], the smaller bandgap value in Ru-doped ZnO nanorods can be related to the materials' oxygen-deficient and non-stoichiometric nature, whereas the bandgap is a bit increased at C4, and then decreased at C6 film. The reason for the increase in Eg values in C films may be that the increased carrier concentration prevents low energy states in the transmission band and causes the Burstein-Moss effect [46,47]. This explains the decrease in fiber thickness at the transition from C4 to C6, although fiber sizes increase as ruthenium contribution increases. The transmittance spectra of Ru-doped ZnO thin films can be seen in Fig. 7. All of the films have shown high transmittance of more than 70% in the visible spectrum. The ZnO films have around 70% transmittance level, while ruthenium doping has caused an increase in the transmittance of B and C samples. This increase has also been observed for A samples except for A6. Since the electron density in the substituents attached to the B and C molecule is high, delocalization has occurred in these complexes. Therefore, it is thought that this causes an increase in the transmittance of the B and C films compared to A films [36].

Photoluminescence Analysis
Photoluminescence (PL) analysis is a non-destructive and sensitive analysis method used to examine defects in semiconductors and the energy levels of these defects. Roomtemperature PL spectra of pure and Ru-doped ZnO films are given in Fig. 8. The interface, surfaces, contamination levels, interface smoothness, and structural flaws in crystals can all affect photoluminescence (PL) intensity and wavelength [48].
Photoluminescence (PL) spectroscopy has been used to investigate the luminescence characteristics of individual and collective ZnO nanostructures [49]. Three bands in PL spectra has been obtained in our study. One excitation band had been observed around 326 nm and the low-intensity and broad excitation peak centered at 471 nm corresponding to blue emission for B samples. This peak level decreases depending on the Ru content. This excitation peak is slightly shifted with the effect of annealing [50]. At 650 nm, a red emission peak has been seen. The red PL band has attributed either to the oxygen intermediate atoms (Oi) or the hydroxyl group (OH) due to the excess oxygen at the ZnO surface. The PL spectra of Ru-doped ZnO films have shifted to higher wavelengths depending on Ru content. The emission wavelength has obtained around 650 nm for 2, 4, and 6 wt% Ru dopants while it is around 641 nm for pure ZnO. The change from Zn interstitial (Zni) to oxygen interstitial (Oi) states caused the red emission. Tsay and Hsu have expressed that there is a red-shift if the optical bandgap energy of thin films decrease [51]. Bandgap energy has decreased depending on Ru content. Further, the peak intensities have shown an increase depending on Ru content. These kinds of occurrences are most common in oxygen-rich environments.
Other reports show that receiver defects associated with VZn are assumed to be responsible for deep-level emissions close to 650 nm [52]. Crystal defects in ZnO are caused by the substitution of divalent Zn 2+ ions with trivalent Ru 3+ ions, such as zinc interstitials (Zni), zinc spaces (VZn), oxygen interstitials (Oi), and oxygen vacancies (VO) [50]. As the concentration of Ru ions increased, the intensity of UV emission increased due to increasing the crystal's quality of ZnO. The strong emissions have been observed in 6% Rudoped ZnO films.

FT-IR Analysis
FT-IR spectra of all synthesized thin films have been recorded in the range of 400-4000 cm −1 using KBr pellets and are shown in Fig. 9. The commercial ruthenium additive (A films) has caused to form new absorption peaks around 3672 cm −1 attributed to O-H stretching vibrations, and 1394 cm −1 attributed to bending vibration of O-H groups [53]. The peak observed around 2849 and 2918 cm −1 at Z0 film, which is attributed to CH 2 and CH groups and other associations by H-bond, has become more intense with the commercial ruthenium doping, and has shifted to 2920 and 2988 cm −1 , respectively. The υ(C = O) ester peak, seen around 1730 cm −1 for Z0 and 2% films, has disappeared with the increase in ruthenium amount. A shift occurred in the peaks observed around 1581, 1139, and 872 cm −1 with ruthenium addition. Sing et al. [54] have expressed that the reason why a shift occurs in peaks of Zn-O stretching vibrations with the increasing doping of Ru might be due to the increased replacement of Zn 2+ ions by Ru 3+ ions as the percentage of Ru increases in the ZnO lattice. The peaks have been observed at 2920, 2846, 1571, 1080, and 867 cm −1 in the spectrum of C films, similar to Z0 films. Unlike Z0 films, there has been an increase in the intensity of the peaks seen at 2920 and 2846 cm −1 , and the peak at 1137 cm −1 has shifted towards 1080 cm −1 . The new peaks have been observed around 3672, 1377, 1056, and 665 cm −1 in the spectrum of B films compared to Z0 films. A shifting for both organic ruthenium doped film has occurred in peaks seen at 2923, 2853, 1581, and 892 cm −1 of pure film.

Conclusions
In this study, Ru(II) complexes ([Ru(L 1-2 )(p-cymene)Cl]Cl) derived from perimidine ligands (L 1-2 ) were synthesized and used as dopant materials to produce the Ru-doped ZnO thin film prepared by sol-gel spin coating. The produced thin films were characterized by using XRD, SEM, FTIR, UV, and photoluminescence spectra. The film thicknesses were found to depend on Ru concentration and ranged from 1045 to 2586 nm. XRD analysis showed that all films had a hexagonal wurtzite structure and particle size has an increase with the rise of Ru amount. Further, the residual stress values decreased with the synthesized Ru complexes and therefore the crystalline quality increased. Also, it was found that all films had a bandgap between 2.76 and 3.20 eV. The PL spectra of the Ru-doped ZnO films showed a strong excitation centered at 326 nm and a low intensity and broad excitation at 471, with a peak centered at around 650 nm. High excitations were found in 6% of Ru doping with A, C, and B, respectively. The Ru complex of the L 1 ligand (B films) usually gave higher results compared to those with the L 2 ligand (C films). This study has shown that Ru complexes can be used in the sol-gel production of ZnO thin films, and these new high conjugated structures are promising as dopant materials for the ZnO thin film.
Author Contribution All the authors have contributed equally to the manuscript and permit it for submission.

Data Availability
The raw data required to reproduce these findings are available to download.

Declarations
Competing Interests The authors declare no competing interests.