1. Introduction

The miniaturization demands in the electronic industry required development of new materials that allow the creation of these miniature devices. A good candidate material is carbon, which has a wide variety of allotropic forms with interesting physical and chemical properties. Although the properties of diamond and graphite have been extensively investigated, there are other allotropic forms (fullerenes) that are of great interest today. Since its discovery, carbon nitride (CN) has aroused great interest in theoretical [1] and experimental studies, given its physical and chemical properties. Carbon nitrides have attracted attention owing to their potential applications in anti-corrosion photo-electrochemical technology [2], photo-degradation [3], and photo-catalysis [4, 5], which can be used in optoelectronics [6] and the manufacture of biosensors [7, 8]. From an experimental point of view, the most important material successes have been in the synthesis of carbon nitrides in the form of thin films by chemical and physical deposition of vapor. Among the physical deposition methods, the use of plasmas [9], especially the sputtering method [10], has been successfully implemented for deposition of carbon nitrides. Recently, a hybrid coating process that combines RF and DC sputtering systems has been successfully used: DC magnetron or DC-pulsed discharge [11, 12]. The advantages of these hybrid coating processes are that the various deposition parameters (e.g., working pressure, sputtering power, current, substrate bias voltage, and target material) can be adjusted separately to produce films with excellent mechanical and tribological properties.

The physical properties of carbon-based films depend upon different forms of hybrid bonds of the carbon atom: sp¹, sp², and sp³. The incorporation of nitrogen into the system enables the formation of different kinds of carbon configurations, including sp³ (tetrahedral bond), sp² (triple bond) and C=N and N=C-type chains. These configurations improve the mechanical and physical properties of CN_x compounds, increasing the application range of the films [13]. To the best of our knowledge, there are no publications in the literature reporting attempts to synthesize C-N compounds by CH₃CH₂OH-N₂ DC-pulsed sputtering. Even more significantly, there still exist various unanswered queries about how the growth process impacts the resulting structure in amorphous CN (a-CN_x) films. The purpose of this article is to investigate the morphological and microstructural properties of a-CN_x films deposited by DC-pulsed sputtering in a CH₃CH₂OH-N₂ plasma mixture. Characterization of the bond structure of the thin films was performed by Raman spectroscopy, and their morphology and microstructure were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Plasma characterization during thin-film deposition was also carried out using optical emission spectroscopy (OES), and these findings are correlated to variations in microstructure and morphology of the thin films.

2. Materials and Method

Reactive sputtering is a suitable coating technique for the preparation of thin films. The pulsed DC sputtering deposition system used in this work (Figure 1) consists of a cylindrical stainless-steel chamber (33-cm long, 20-cm radius, and a volume of 1.04 × 10³ cm³) and two mobile Cu electrodes (5-cm in diameters) placed in the center of the vacuum chamber with a separation space of 20 mm. The power supply consisted of Pinnacle Plus+ generators, which delivered DC power in a pulsing configuration to enable reactive sputtering. Pirani pressure sensors (MKS, model 103170027SH) were used to determine the total pressure in the chamber, and a flowmeter (Matheson Tri. Gas FM-1050) was used to regulate the gas flow within the system. An ocean optics spectrometer monitored the optical emission spectra generated within the discharge and the chamber vacuum was formed using a mechanical pump (Varian DS302). The background and working pressure of the vacuum chamber were 2 × 10⁻² and 1 Torr, respectively.

High-purity CH₃CH₂OH (99.9%) was used as the carbon precursor, and high-purity nitrogen (99.9%) was used both as the sputtering ion source, which bombards the target, and as the nitrogen species, which is introduced into the thin films with carbon ions. The relation of the mixture was 20%-CH₃CH₂OH to 80%-N₂. To generate plasma, the vacuum chamber is first evacuated with a mechanical pump until a base pressure of 2 × 10⁻² Torr is reached; next, it is filled with CH₃CH₂OH-N₂ mixture gas and discharge is generated by applying a pulsed current (300 mA at 450 V) between two electrodes. Then, the CH₃CH₂OH-N₂ mixture gas is ionized and accelerated by the electrical field produced by the electrodes, forming an energetic ion beam that provides both sputtering ions and reacting nitrogen and carbon precursors. A Cu disk was used as a substrate for the growth of CNx films. Pulsed discharge was performed for 1 hour. The intention of growing CNx films on the surface of Cu

The bond structure and microstructure of the thin films was investigated using Raman spectroscopy (micro-Raman, LabRam HR 800 system). Raman measurements were recorded using a He–Ne laser, which was focused using a 50X lens. During the measurements, 60 recoded data were collected every 60 s. SEM (Jeol IT-100 coupled to a Bruker X-Ray microscope) and AFM (EasyScan 2 Flex AFM) were used to determine the thin-film morphology. The SEM microscope was operated in high voltage mode with an accelerating voltage of 20 kV. SEM studies were carried out on the electrolytic Cu (99.99%) substrate to observe the formation of attached carbon structures and thus corroborate their existence.

To characterize the plasma during film deposition, the species generated within the plasma discharge was observed by optical emission spectroscopy (OES). An ocean optics HR 4000CG-UV-NIR spectrometer was used, and spectra were observed in a wavelength range of 200 to 850 nm, with a resolution of 0.75-nm FWHM.

Figure 1: Experimental apparatus

3. Results and Discussion

3.1.  Raman spectroscopy

 Raman spectroscopy is commonly used to analyze the bond structure of carbon-related materials. Visible Raman spectroscopy is 50 to 230 times more sensitive to sp² sites because p electrons are preferentially excited with visible photons [12]. Thus, the Raman spectra obtained for the deposited CN_x films provided double-link information, in addition to indication of changes in film microstructure.

Figure 2: Raman spectra of thin CN_x deposited by pulsed discharge at a pressure of 1 Torr.

Figure 2 shows Raman spectra for CN_x films deposited onto Cu at 300 mA, 550 V and 120 W at a working pressure of 1 Torr. Raman bands appear at 692, 1,362 (D peak), 1,588 (G peak), and 2,250 cm⁻¹ [14]. The 692 cm⁻¹ band is characteristic of CN films deposited under the influence of a reactive gas. This band may also appear owing to structural disorder in the film. The G peak arises from stretching vibrations at all sp² sites, both in olefinic chains and in aromatic rings, while the existence of the D peak is associated with breathing vibrations in the aromatic rings. These peaks, therefore, indicate the presence of distorted-ring structure, or the lack of coherence between adjacent planes of graphite-like structures [15, 16]. This is due to the disorder in the link angles, despite the graphite-like micro-domains induced by the binding of sp³ carbon atoms or the finite size of the sp² micro-domains.

The D peak is only related to the presence of aromatic rings. The D peak arises in materials that exhibit formations of small clouds or clusters organized into such rings (graphite clusters) [17]. The band centered at about 2250 cm⁻¹ is due to the stretching vibration of a C-N triple bond, suggesting that the C and N atoms are chemically bonded in the film. The position of the D and G peaks, together with their widths and the relationship between their inetnsities (I_D/I_G) provide valuable information concerning the deposited films.

The position of the G peak is shifted toward higher energies as a result of the current used within our system (300 mA). This current value guarantees an increase in the system temperature. The position of the G peak, independent of the composition of the film or the growth method, is related to the vibrational energy of the present bonds. Therefore, the shift toward high frequencies is due to a decrease in sp³ bonds and an increase in sp² links [18]. This indicates that a conversion of sp³ to sp² bonds occurred in the a-CN_x films due to the increase in temperature. This conversion process could form graphite domains with larger size and/or a number of sp² clusters, eventually leading to graphitization.

The Raman shift of the D peak at high frequencies suggests a densification of sp²-ringed structures in the atomic network, which also leads to increased sp²-link content [15]. On the other hand, I_D/I_G = 1.11, which suggests that C sp² are more likely to form C=C and/or C=N bonds in rings and less likely to form C=C and/or C=N bonds in chains. Additionally, the I_D/I_G value indicates the growth of graphite domains or an increase in their number [19–21], i.e., graphitization by heating with an increase in the disorder of the bond angles of C atoms and N sp² in the films [20, 22]. The width of the D peak also reflects the crystallinity of the structure: a narrower band suggests the structure is more crystalline, but if it is wider, the structure is more amorphous. For the obtained spectrum, it can be seen that peak D is wider, which suggests derangement of the structure due to the creation of defects, possibly through the introduction of C or N atoms between the graphite-like layers. This may indicate that the structures formed in the thin-film are more amorphous.

3.2.  SEM analysis

The SEM images (Figure 3) acquired at different magnifications reveal the morphology of the deposited thin-film. In Figure 3(a), the carbon clusters deposited onto the Cu surface form a non-homogeneous surface. It is likely that this lack of uniformity is due to the distance between the electrodes (20 mm) because the deposition rate is proportional to this distance [23]. The agglomerations observed in the SEM image cause the formation of large islands with small grains, in turn generating an uneven surface [24]. Images of the thin-film at different magnifications, shown in Figures 3(b) and 3(c), were collected to better observe the surface. Thus, it is inferred that internal stresses decrease within the material, allowing the diffusion of particles within the film surface. This uniformity on the surface can be explained by two factors: the energy with which the plasma interacts with the target and the high internal strain generated within the samples as a result of the formation of amorphous structures.

Figure 3.  SEM micrographs of thin CN_x deposited by pulsed discharge at a pressure of 1 Torr, taken at (a) 10X, (b) 50X, and (c) 1000X magnification..

3.3.  AFM analysis

Figure 4 shows the three-dimensional (3D) AFM images of the deposited thin-film using a current of 300 mA. Figures 4(a), 4(b), and 4(c) show results obtained by AFM at different scales in contact mode that is, at swept lines of 50, 25, and 10 mm, respectively. The morphology that occurs in the deposited films is determined by the non-uniformity in the surface morphology, which can be explained by the presence of large graphitic domains and a polymer component of CN.

Higher polymer components lead to a smoother film, whereas higher graphite-cluster contents and larger cluster sizes lead to more agglomerations [23]. A large polymeric component results in low roughness, whereas a greater number of graphitic clusters induce the formation of more or larger agglomerations. Although no defined grains were observed on the surfaces of the films, we have established that the CN_x films had linear and graphite-like polymer components, which could lead to the surface non-uniformity. These results agree with those presented by A. Stanishevsky [25] regarding the morphology of CN_x films.

The microstructure, in addition to the morphology formed during thin-film formation, are important issues in determining the mechanical (hardness and elastic recovery) and tribological properties (friction coefficient and wear rate) of CN_x films [26]. Therefore, such thin films could be important materials for application in many tribological uses, because of their mechanical properties, and in solar cells, due to their optical and electrical properties [27]. Another interesting application of the present work is that the observed morphology determined the non-uniformity in the surface that effect could be reproduce using phase-field model for planer film surface [28], by the corresponding different nanoscale effects [29], or by thin planar interfacial models [30, 31].

3.4.  Plasma analysis

Figure 5 display the OES spectrum for CH₃CH₂-N₂ in the spectral range of 200–900 nm. In the mixture, decomposition of CH₃CH₂OH takes place by breaking the C-H chemical bonds in the discharge, producing atomic and molecular impacts and excitation reactions between radicals and N atoms. The species observed are as follows: Swan bands (d³P_g – a³P_u), which are characteristic of the C₂ molecule at 473.71 nm (Dn = +1), 516.52 nm (Dn = 0), and 563.55 nm (Dn = −1) [16], and CN violet bands (B²S⁺ – X²S⁺), for which the band heads appear at 359.04 nm (Dn = +1), 388.34 nm ( ), and 421.60 nm (Dn = −1). Here Dn = n′ − n″ is the difference in vibrational quantum numbers between the upper (n′) and lower (n″) transition states. Also seen are CH (B²S-X²P) at 390 nm and CH (A²D-X²P) at 426.76 nm.

Figure 6 shows CN, N₂, and H_b emissions observed in the plasma discharge. The N₂ (B³P_g–C³P_u) band, which corresponds to the second positive system (SPS, 337.1 nm), may be mainly due to the presence of two consecutive reactions: (1) the electron impact excitation of the N₂ molecular energy ground state and (2) radiative decay to a lower level with the emission of a characteristic photon. These reactions are respectively expressed as

Figure 4. AFM images of thin CN_x deposited by pulsed discharge at a pressure of 1 Torr, taken at different scales. (a) 50 mm; (b) 25 mm; (c) 10 mm

N₂( ₁ ⁺_g)  N₂( ³ _u) + e                    (1)

and

N₂( ³ _u)  N₂( ³ _g) + hv (337.1 nm)                 (2)

Figure 5. OES CH₃CH₂OH/N₂ plasma.

The interactions of N₂ species with C₂H₅OH lead to the formation of CN species [32]. The violet and red spectral systems of CN are shown in Figure 6.

The violet spectral system of CN may be produced by the following reaction:

Figure 6. OES spectrum CN, N₂ and CH species.

which corresponds to Dn = 0 and a radiative lifetime t = 60 ns. The resulting vibrational distribution of CN (X²S⁺) can be modified by relaxation processes that include vibrational collision relaxation within each electronic state, inter-electronic transfer by collision between vibrational levels of the states X²S⁺ and A²P, and the radiative deexcitation of the vibrational levels of A²P toward vibrational levels lower than the X²S⁺ state. The latter of these relaxation processes result in the appearance of the red spectral system, as shown in Figure 7, and is expressed as

with Dn = 3 and a radiative lifetime t = 6000 ns.

Conversely, from Figure 5 it can be seen that the CN species at 388.3 nm and the N₂⁺ species at 390 nm overlap, implying that the production of CN radicals in that interval is mainly linked to the excited state of N₂ (C³P_u). This phenomenon has two causes: (i) resonant excitation in collisions between highly excited neutral N₂ molecules (3.26 eV) and CN radicals (3.19 eV) or (ii) the high energy of the excited N₂ (11.1 eV) favoring the generation of atomic N, which in turn favors CN formation. The reaction of N atoms with C or C-containing species is exothermal, which facilitates these reactions [33–35]. N atoms within the discharge could not be detected because their lifetimes are shortened by bonding with N atoms and C-containing species.

Its peaks are also covered up by CN compounds with higher intensities. The presence of CN radicals within the discharge is mainly related to the excited state of N₂ [32]. C₂ Swan band emissions arise from transitions between electronic states (d³P_g–a³ P_u) as a probable result of the dissociation of CH₃CH₂OH in the following way:

Figure 7. OES spectrum CN (red spectral system), N₂ and O atom species.

As can be seen in (5), atomic hydrogen is removed from CH₃CH₂OH [32, 36] and the C-C bond finally dissociates by direct electron impact, which leads to the formation of solid carbon. The presence of C₂ can be taken as an indication of solid carbon formation, as it is known that the C₂ species initially nucleates in solid carbon clusters [37]. This would result in the presence of C₂ Swan bands in the range of 480–530 nm.

Swan band heads correspond to (0,0), (1,1), (2,2), (3,3), and (4,4) with a Dn = 0. Carbon molecules react with N atoms in the following way:

N + C₂  CN + C                                      (6)

We assume that generation of C₂ molecules was caused by collisions in the plasma, which promote recombination of the dissociated C of the CH₅ species. The C₂ species and the CN radicals generated within the mixture contribute to the growth of the CN thin films. The availability of C₂ species provides a considerable fraction of sp² hybridization, as opposed to sp³ hybridization, which is an important factor in the formation of graphite-like or fullerene-like structures with sp² hybridization in deposited CN films.

The emission of CH (B²S⁺–X²P) at 388.9 nm becomes embedded in the CN signal, as seen in Figure 5. The presence of CH in the CH₃CH₂OH discharge can be considered an indication of solid carbon and higher order hydrocarbon formation. This is because CH species are typically found as intermediate radicals in the formation of these products [36]. CH species react with nitrogen atoms in the following process:

However, CH radicals can arise as intermediate reaction products to generate CH₄, C₂H₂, C₂H₄, C₂H₆, or C₃H₈ [38, 39] when hydrogen atoms (H_a and H_b) react with solid carbon fragments [40–42]. The presence of H_a and H_b in the discharge is a result of (7), which forms the hydrogen species. In Figure 5, these species are seen as the most intense lines of the Balmer series.

The presence of H_a and H_b  is related to dissociation of the CH and OH within the discharge, as seen in (7) and (8), which could also be a new source for hydrogen generation.

4. Conclusions

CN_x thin films were deposited on Cu substrates using a pulsed discharge method with CH₃CH₂OH as the source of carbon and N₂ as the sputtering ion and nitrogen source.

Through Raman analysis, it was demonstrated that the structure of the deposited films was amorphous. The characteristic Raman D and G peaks for carbon were identified. The position of the G peak is related to the vibrational energy of the present bonds; therefore, there is a shift toward higher frequencies due to the decrease of sp³ bonds and hence the increase in sp² bonds. This indicates that a conversion of sp³ bonds to sp² bonds occurred in the CN_x films owing to the increase in temperature. Such a conversion process could form large graphite domains with large size and/or an increased number of sp² clusters, eventually leading to graphitization. The observed high-frequency D peak shift, on the other hand, suggested densification of ringed sp² in the atomic network, also increasing the sp²-bond content. In addition, the Raman analysis showed an additional band centered at approximately at 2250 cm⁻¹, which appears because of the stretching vibration of a triple bond C-N. This suggests that the C and N atoms are chemically bonded in the film. An I_D/I_G value of 1.32 suggested that C sp² is more likely to have formed in the film, considering that C=C and/or C=N bonds can be formed in rings and there are fewer opportunities to form C=C and/or C=N links in chains. In addition, this I_D/I_G value indicated the growth of the graphitic domains or the increase of their number, i.e., graphitization by heating with an increase in disorder of the angles of the bonds of the C and N atoms sp² in the films.

Through SEM analysis, we were able to observe the thin-film morphology, wherein carbon clusters deposited on the surface of the Cu were observed. The lack of uniformity of the non-homogeneous surface can be attributed to the distance between the electrodes during the deposition process. Since the deposition rate is considered proportional to the distance, these observed agglomerations cause the formation of large islands with small grains, in turn generating an irregular surface. These results were later corroborated with AFM results, from which we determined the morphology of the CN_x film. The surface was found to lack uniformity, which can be explained by the presence of large graphite domains and a polymeric component of CN. This reveals that the greater the content of graphite groups or the larger their size, the greater the number of agglomerations. Finally, we conclude that the films grow via island growth, very similar to the results presented by A. Stanishevsky [25].

OES measurements confirmed the decomposition of the CH₃CH₂OH molecule as a result of electron impact dissociation in the plasma discharge. Products obtained from the dissociation collide with the N species to generate CN radicals and form violet

and red emission spectra characteristic of CN. CN radicals are important because of their variety of industrial applications, mainly in materials science. In addition, the C₂ Swan band, a species that also plays an important role in the appearance of CN radicals, was identified. The presence of CH indicated the formation of solid carbon. Hydrogen species were also detected in the CH₃CH₂OH-N₂ plasma mixture. Therefore, the mixture plasma is not only useful for producing CN but also for generating hydrogen.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We are grateful to the Advanced Physics Laboratory (FC/UAEM), Spectroscopy laboratory (ICF/UNAM) and TESJo for their infrastructure provided to carry out the research. This research was supported by CONACyT scholarship for doctoral study, DGAPA[IN102222], CONACyT [225991 and 268644] and UAEM [4307/2017/CI].

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