Cobalt Xanthate Thin Film with Chemical Bath Deposition

Cobalt xanthate thin films (CXTFs) were successfully deposited by chemical bath deposition, onto amorphous glass substrates, as well as on pand n-silicon, indium tin oxide, and poly(methyl methacrylate). The structure of the films was analyzed by farinfrared spectrum (FIR), mid-infrared (MIR) spectrum, nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM).These films were investigated from their structural, optical, and electrical properties point of view. Electrical properties were measured using four-point method, whereas optical properties were investigated via UV-VIS spectroscopic technique. Uniform distribution of grains was clearly observed from the photographs taken by scanning electronmicroscope (SEM).The transmittance was about 70–80% (4 hours, 50C). The optical band gap of the CXTF was graphically estimated to be 3.99–4.02 eV. The resistivity of the films was calculated as 22.47–75.91Ω⋅cm on commercial glass depending on film thickness and 44.90–73.10 Ω ⋅cm on the other substrates. It has been observed that the relative resistivity changed with film thickness.TheMIR and FIR spectra of the films were in agreement with the literature analogues. The expected peaks of cobalt xanthate were observed in NMR analysis on glass. The films were dipped in chloroform as organic solvent and were analyzed by NMR.


Introduction
Cobalt xanthates are similar to hybrid materials, because they have an organic sulfide part and inorganic metal part as iron xanthate.These organometallic compounds have wide ranging properties such as optical, electrical, and magnetic characteristics [1][2][3][4].It has been shown that these thin films show different properties such as an antibacterial agent, magnetic and semiconductor material, which allowed them to be used for data storage, solar cell production, and water purification [5][6][7][8][9][10].Neither cobalt isopropyl xanthate thin films production nor their optical, electrical properties and structural analysis have been studied yet.These thin films and their bulk compounds may be useful in many areas, especially in solar cells.Chemical vapor deposition and physical vapor deposition are included among thin film deposition methods used to produce metal sulfide thin films [5,11].These methods are expensive and need a variety of instruments unlike the chemical bath deposition method.
The aim of this paper was to produce cobalt xanthate thin film by chemical bath deposition method and to examine its structural, optical, and electrical properties.Metal xanthate complexes rapidly occur when the precipitate rises to the surface in chemical bath deposition.Moreover, due to the nature of organometallic thin films, it is difficult to analyze them.The metal xanthates thin films were produced with difficulty.

Experimental
2.1.Reagents.Isopropyl xanthate was synthesized first as described in the literature, and the stock solution of 0.1 M was prepared.High purity reagents were used for all the prepared solutions.The stock solution was diluted each time when required.The other stock solution of iron nitrate salt was prepared from high purity compound (99.9%,E. Merck, Darmstadt, Sigma Aldrich).All laboratory glassware and substrates were cleaned by soaking in diluted nitric acid and rinsing with alcohol and deionized water prior to use.

Synthesis of Isopropyl Xanthate.
Isopropyl xanthate was synthesized by dissolving 3.74 g of KOH (0.067 mol) in a mixture of 4.5 mL CS 2 , 6 mL isopropyl alcohol and 9 mL benzene and heated under a reflux condenser.The mixture was mixed   for 20 minutes at 35  electron detector.Electrical properties were measured using four-point measurements technique, and accordingly the resistivity was calculated.The film thicknesses were measured with atomic force microscopy (AFM).The optical measurements were conducted by a Hach Lange DR 5000 spectrophotometer at room temperature by placing an uncoated identical glass substrate in the reference beam.The optical spectra of the thin films were recorded in the wavelength range of 250-700 nm.

Results and Discussion
We used the FIR, MIR, and 1 H-NMR spectra of cobalt xanthate thin film on glass.The spectra are shown in Figures 2, 3, 4, 5, and 6. Figure 1 illustrates that in the cobalt xanthate thin film, (i) the -OH peak occurred at 3200-3600 cm −1 , (ii) the symmetric stretching vibration of the aliphatic groups (-CH 3 ) was seen at 2976-2932 cm −1 , (iii) the bending vibration of -CH 3 was seen at 1448-1373-    In Figure 3, the stretching vibrations of Co-S were observed at 359 cm −1 , similar to the other functional groups of cobalt xanthate's organic parts [31][32][33][34][35][36][37][38].We did not consider any measurements below 200 cm −1 .Figure 3   In the 1 H-NMR spectrum (Figures 4-6), CXTF was dipped into a chloroform solution for spectrum analysis.The scanning number was 16 for all samples.The solvent peak (chloroform) was observed at 7.2 ppm, whereas trace concentration of the water peak was observed at 1.5 ppm when the triplet peak of the -CH 3 group occurred at 1.52 and 1.41 ppm.Since proton number was one, integration number was 1.05.The group of -CH NMR peaks was divided into five at between 5.50-5.60due to neighboring -CH 3 groups, and the integration number was 1.05.The -CH group had an oxygen atom as a neighbor which caused a decreased electron shielding, so its peak was observed in the low area [39][40][41].No impurities were observed in the spectra.The 1 ppm peak was expected to split into a double peak, but actually it was divided into triplet peak.This meant that (i) the rotation of the isopropyl groups can be obstructed, and the methyl groups give doublet-doublet resonance, (ii) the isopropyl groups can be bonded to different cobalt shapes, and the methyl groups give doublet-doublet resonance, (iii) magnetic properties of cobalt atom can also be affected by the NMR spectrum, which is come together doublet-doublet peak.
The doublet peaks can be combined with each other, thus we saw a triplet peak [42].
The transmittance () and absorbance () for CXTF can be used for the calculation of reflectance () from the following expression [43]: Transmittance and absorbance measurements were performed at room temperature in the range of 300-1100 nm.The films were deposited at different deposition temperatures and deposition times as shown in Figures 7, 8 values for deposition time and temperature were selected as 4 hours at 50 ∘ C, respectively (∼70% transmittance).The highest deposition temperature and time were taken as 50 ∘ C and 19 hours (at 30 ∘ C) for upper limit of the parameters, respectively.Metal xanthates are known to decompose beyond these limits [36].Cobalt xanthates, in a similar way, decomposed at 60 ∘ C and 21 hour of deposition time to give iron oxide, oxide chloride, or sulfide as decomposition product in this study.The cobalt xanthate decomposed into cobalt oxide or sulfide above 50 ∘ C and 19 hours.
The refractive index and extinction coefficient for the films are given by the following equations [43]: The refractive index was not regularly affected by deposition temperatures at 30, 40, 50 ∘ C, and its values were reported as 1.80, 2.60, and 1.80 in Figure 10.The refractive index was sputtered at 40 ∘ C.Moreover, the extinction coefficient behaved in the same way and the extinction coefficients were, respectively, 0.023, 0.025, and 0.082 at 30, 40, 50 ∘ C (in 550 nm wavelength).The n-k graphic shows anomalous dispersion.
According to literature, this anomalous behavior is due to the resonance effect between the incidental electromagnetic radiation and the electrons polarization, which leads to the electron coupling in the structure of the oscillating electric field [44].The optic band gap energy (Eg) was determined from the absorption spectra of the films using the following relation [43][44][45][46][47][48]: where  is a constant,  is the absorption coefficient, ℎ] is the photon energy, and  is a constant, which is equal to 1/2 for the direct band gap semiconductor.The plot of (ℎ]) 2 versus ℎ] is illustrated in Figure 11.
The band gaps (Eg) of the films changed as 3.98, 3.99, and 4.01 in Figure 11.Film thickness was increased with deposition temperature, whereas the band gap of the films was not decreased with film thickness.Both electrical and optical bandwidths of a material are correlated with the magnitude of columbic interactions.To be more precise, the size of atoms and electronegativity values are the two most important factors, affecting bandwidth.Bandwidth of a material is expected to get higher when the atoms get smaller, bonds getting stronger, and with higher electronegativity of the atoms.Although there are some exceptions, nitrides and oxides, which are the elements from III-V groups of periodic table, are generally known with their high optic band ranges.The high optic bandwidth of the organometallic compound used in our study is in line with the literature, since it is formed by small atoms with high electronegativity [49].
It was found that the film thicknesses were not increased nearly linearly with increasing deposition temperature whereas the band gap of the films decreased with increasing film thickness as expected.Similarly, resistivity increased with film thicknesses which was contradictory to the literature [50].The film thickness of the films is shown at Figure 12, according to deposition time.
The resistivity of the films was determined by four-point measurements of the films using the following relation [51]: where  is film thickness,  is the voltage,  is the current, and  is the distance between the probes.The resistivity was measured in dark, at room temperature.In the measurements, the distance of the probes was in millimeter, whereas the film thickness was in nanometer scale.The resistivities of the films were measured as 22.47, 42.71, and 75.91 Ω⋅cm for the film thicknesses 198.14, 267.15, and 454.07 nm, respectively.The resistivity of the films deposited increased with the film thickness as can be seen from the plot of film thickness versus resistivity (Figure 13).Although Moualkia et al. and Kasap et al. found that the film thicknesses between 250-900 nm were slightly affected by the resistivity of the film, the resistivity of cobalt xanthate increased with the film thickness [48,50].It is a known fact that the aliphatic groups in the structure increase the resistance, they are not conductive.Thus, they are causing an increase on the resistance with the increase of the film thickness.
Thin fims were also produced on n-silicium, p-silicium, poly(methyl methacrylate), and indium tin oxide as different substrates in this work.The film resistivity on these substrates was found to be different than commerical glass studied previously.The resistivities of the poly(methyl methacrylate) (PMM), indium tin oxide (ITO), n-silicon (n-Si), and psilicon (p-Si) substrates were measured as 67.15, 50.98, 44.90, and 73.10 Ω⋅cm at the optimized parameters of deposition time and temperature, respectively (Figure 14).It was found that CXTF on p-Si substrate had the highest resistivity, whereas the resistivity of the n-Si was the lowest [52][53][54].It is a well-known fact that these kinds of materials are stuck to the surface with -bonds or with idle electrons of electronegative atoms [6].When such a material is deposited on n-Si, which is rich on electrons, the electron doublets of these bonds as well as idle doublets will apply a tension to the material; thus electron transmission between the substrate and thin film will be easier.So the resistance will decrease and the conductivity will increase.In the literature, there are examples about the impact of substrates on the resistance [55].
Figures 15(a) and 15(b) show SEM photos of cobalt xanthate thin film.It is given strange exposure.At 5 and 10 m, SEM photo of cobalt xanthate thin film was dispersed like biologic cells.The grains were resembling cancer cells in micron scale.It was assumed that the grains formed colonies.Spaces appeared very frequently at 10 m scale.The grains scattered very much.Any connection did not appear between the cells.These large spaces affect the homogeneity of the thin films, which might be the reason of their high resistivity.Also it may be the reason of anomalous behaviors of optical properties.The elemental analysis results were calculated using the ratio of sulfide and cobalt according to EDX data in Figure 16.The ratio of sulfide and cobalt changed with deposition temperature.The ratios of sulfide and cobalt in different deposition temperatures were 1.262, 1.776, and 2.112.In a cobalt   xanthate molecule, the theoretical ratio of the sulfide and cobalt is approximately 3.255.The elemental results were closer to the theoretical value at 50 ∘ C (Co-tris isopropyl xanthate).This shows that the cobalt cation bonds to the three xanthate molecule, as per the literature [37].

Conclusion
Cobalt isopropyl xanthate thin film was prepared first time on glass, poly(methyl methacrylate), indium tin oxide, n-silicon, SEM images of the thin films gave insight about their resistivity and refractive index.It can be seen that the film deposited on n-Si had lower resistivity than other ones.Although further investigation is needed for this new thin film, it seems to be a material which may be very useful for solar cells, dedectors, or sensors.

Figure 1 :
Figure 1: Probable steps related with complex formation (ion-ion mechanism).(a) Particles of L-M-L complexes are spread towards the substrate.(b)The complex is stuck to the substrate as an LML composite, still being in the solution.(c) LML grains grow with the formation of more complexes and absorption.(d) Grains form the film by sticking to each other (M: metal, L: ligand) [12].
illustrates that in the cobalt xanthate thin film, (i) the vibration of -C-S at 662-521 cm −1 , (ii) the bending vibration of -C-O-C at 463 cm −1 , (iii) the stretching vibration of O-C-S at 291 cm −1 , and (iv) the bending vibration of S-C-S at 238 cm −1 .

Figure 10 :
Figure 10: n-k graphic according to deposition temperature.

Figure 16 :
Figure 16: Sulfide and cobalt elemental ratio according to deposition temperature.

hour at 60 ∘ C. Approximately 9 grams of the reaction product was then purified by rinsing with acetone and drying in the oven at 30 ∘ C for 48 hours [13-30].
∘ C and for 45 minutes at 45 ∘ C.Then, it was mixed for 1