3.1. Composition of Mo–S–C–H Films Obtained by RPLD
Figure 1 shows the experimental and simulated RBS and ERDA spectra for Mo–C and Mo–S–C–H films obtained by pulsed laser codeposition of molybdenum and carbon under vacuum conditions and in H
2S gas with different pressures. For the film obtained under vacuum, mathematical processing of the spectra showed that the composition of this film was described by the formula C
0.81Mo
0.16H
0.03. There was almost no sulphur in the bulk of Мo–С–H film. A small amount of sulphur was found at the boundary of the Mo–C film with the Si substrate and on the surface of the Mo–C–H film. This was possibly due to the fact that the walls of the deposition chamber were not subjected to any special treatment prior to the formation of the Mo–C–H films. The walls of the chamber were covered with a thin S-containing film, formed during previous experiments on RPLD of MoS
x films. Sulphur atoms desorbing from the walls of the chamber could have deposited on the surface of the Si plate during vacuum pumping of the chamber and on the surface of the Mo–C–H film—after its deposition and storage (for some time) in the chamber. The presence of a small amount of hydrogen in the Mo–C–H films was possibly due to the interaction of the growing film with residual water vapour in the deposition chamber.
According to the RBS data, the thickness of the Mo–C–H film obtained during 20 min of deposition was 8.2 × 1017 atom/cm2. In reactive H2S gas, S atoms penetrated the deposited Mo–C–S–H film. Despite this fact, the overall deposition rate of atoms decreased. For hydrogen sulphide pressures of 5.5, 9, and 18 Pa, the composition of the films was described by the formulas C0.49Mo0.13S0.28H0.1, C0.405Mo0.13S0.37H0.095, and C0.24Mo0.115S0.55H0.095 respectively. The thickness of these films was 6.2 × 1017, 5.8 × 1017, and 5.2 × 1017 atom/cm2. For ablation of graphite and molybdenum targets, 1.5 × 104 laser pulses were used, which were divided into series of 100 pulses for the C target and 200 pulses for the Mo target. The ablation of the C target resulted in the deposition of ~2 × 1015 atom/cm2 in hydrogen sulphide. This was sufficient for the formation of approximately one monolayer of amorphous carbon, given the atomic density in amorphous carbon of ~(1 ÷ 1.7) × 1023 atom/cm3. The same estimates for Mo showed that a 0.6 ÷ 1 MoSx monolayer could be formed after 200-pulse ablation of the Mo target in hydrogen sulphide.
Figure 2 shows a change in the rate of deposition of various elements in the Mo–C–H and Mo–S–C–H films following an increase in the hydrogen sulphide pressure. As it is shown, H
2S gas did not only ensure the saturation of the films with S atoms, but also had a significant effect on the rate of carbon deposition. At a pressure of 18 Pa, the rate of carbon deposition dropped almost fivefold compared to the rate of deposition in a vacuum. Under the same conditions, the rate of deposition of Mo atoms decreased only twofold. This could be explained by the difference of collisions when light (C) and heavy (Mo) atoms moved through H
2S gas. The relatively heavy Mo atoms changed their trajectory only slightly and slowly lost their kinetic energy in collisions with the relatively light H
2S molecules. When colliding with the H
2S molecules, the lighter C atoms are scattered at large angles and leave the area where the deposition on the substrate took place. The possibility of reactive collisions of carbon ions with H
2S molecules cannot be ruled out. These collisions may have resulted in the formation of volatile hydrocarbon molecules. This is a possible explanation as to why an increase in the H
2S pressure did not result in a discernible change in the rate of the saturation of the films with hydrogen.
The analysis of the RBS and ERDA data for a-C(S,H) films formed during pulsed laser ablation of graphite in H
2S confirmed the assumption about the effective interaction of the ablated flux of carbon atoms and H
2S molecules (see
Figure S1, Supplementary Materials). An increase in the pressure of hydrogen sulphide resulted in a noticeable increase in the concentration of sulphur in the a-C (S, H) films. The compositions of the films at hydrogen sulphide pressures of 5.5, 9, and 18 Pa were described by the formulas C
0.71S
0.15H
0.14, C
0.61S
0.26H
0.13, and C
0.42S
0.4H
0.18. The thicknesses of the films were 2.2 × 10
18, 1.7 × 10
18 and 1.0 × 10
18 atom/cm
2 respectively. During the PLD of carbon films in vacuum (residual gas pressure ~10
−3 Pa), their thickness was 2.88 × 10
18 atom/cm
2, and the H atoms concentration did not exceed 1 at.%.
3.2. Morphology, Structure, and Chemical State of Mo–S–C–H Films Obtained by RPLD
The use of RPLD for obtaining Mo–S–C–H thin films made it possible to produce sufficiently smooth and dense coatings, the morphology of which, according to the results of SEM studies (
Figure 3), did not depend on the H
2S pressure. There were individual round-shaped particles of a submicron size on the surface of the Mo–S–C–H coatings. These particles could have formed because of the deposition of droplets formed during the ablation of the Mo and graphite targets. Such particles were found in a-C(S,H) films produced by RPLD from a graphite target in H
2S gas (
Figure S2, Supplementary Materials).
The results of the XRD of Mo–S–C–H thin-film coatings (
Figure 4) show the deposition of Mo particles upon ablation of the Mo target. The X-ray diffraction pattern of the Mo–S–C–H_5.5 coating had a weak intensity peak, which corresponded to the (110) reflection for a body-centred cubic Mo lattice. The peak was practically invisible in the X-ray diffraction patterns of the Mo–S–C–H_9 and Mo–S–C–H_18 coatings. This could be attributed to the fact that with an increase in the pressure of H
2S gas, the surface of the Mo target interacted with the reactive gas. Following this interaction, molybdenum sulphides could form on the target surface; this changed to a certain extent the mechanism of pulsed laser ablation of the Mo target.
XRD studies showed that at all selected pressures of H
2S, the Mo–S–C–H thin-film coatings had an amorphous structure with a broadened diffraction peak in the angle range from 35° to 50°. With greater hydrogen sulphide pressure, the intensity of this peak noticeably weakened. This indicated an increased disordering of the structure following a rise in the sulphur concentration. This type of XRD pattern has been extensively described in the literature; in most experiments, the Mo–S–C coatings have been obtained by ion sputtering/codeposition from MoS
2 and graphite targets (for example, [
15,
38,
39,
40]). For TMD coatings having an amorphous structure, this broad peak is usually explained by the formation of nanosize inclusions with a hexagonal lattice of the 2H-MoS
2 type [
41]. In the cases when there was no peak at angles 2θ~13°, but there was a peak in the 2θ range from 35° to 50°, the turbostratic stacking of (10
L) planes into Type I texture was supposed. With this texture, the basal planes (002) are oriented perpendicular to the surface of the substrate [
42]. The absence of a peak at 2θ~13° shows that the reactive PLD of Mo–S–C–H_5.5 films in H
2S may not have caused the formation of a self-assembled multilayer structure MoS
x/a-C (doped with Mo/S/H) with a periodicity in the nanometer scale, as it was the case during the magnetron sputtering of graphite and MoS
2 targets in Ar/N
2 gases [
14,
39]. In XRD patterns for the Mo–S–C–H_18 coatings, a weak-intensity and a very broad band appears at 2θ~15°. This shows that a MoS
x nanophase with a high sulphur concentration (
х ≥ 3) formed in the structure of these coatings. Such coatings are characterized by an XRD pattern with two strongly broadened bands at 2θ~15° and 2θ~40° [
35,
43].
HRTEM studies of the Mo–S–C–H thin films confirmed their amorphous structure. Only in the Mo–S–C–H_5.5 films obtained at the lowest H
2S, pressure, MoS
2 nanocrystallites with laminar packing of atomic planes were found in some local regions. The size of these crystallites did not exceed 10 nm, and they were surrounded by an amorphous matrix. The concentration of MoS
2 nanocrystallites in the Mo–S–C–H_5.5 film was not high, and their structure probably had a turbostratic character with a high degree of disordered local atomic packing. This was confirmed by the SAED pattern, consisting only of diffusely broadened rings (
Figure 5), as well as by the results of the Raman studies of Mo–S–C–H films.
The Raman spectrum for the Mo–S–C–H_9 coating in the frequency range of 100–600 cm
−1 was in many respects similar to the spectrum of the Mo–S–C–H_5.5 coating (
Figure 6). Broad peaks at 350 and 402 cm
−1 indicated the formation of the MoS
2 nanophase with a disordered atomic packing. The appearance in the spectrum of the Mo–S–C–H_9 coating of weak-intensity and broad peaks at ~200 and ~500 cm
−1 suggested that, along with the MoS
x nanophase, Mo
3–S clusters could form. When such clusters are combined into a polymer-like network, MoS
x compounds are formed, in which
х ≥ 3 (Mo
3S
12/Mo
3S
13-type). The composition of Mo
3-S clusters includes three Mo atoms connected in the Mo
3–S triangle through monomers and/or dimers of S atoms (S
2−/S
22−). With a sufficiently ordered packing of atoms in such clusters, narrow peaks are observed in the indicated frequency range; they correspond to various sulphur ligands [
35,
43,
44,
45]. In the Raman spectrum for the Mo–S–C–H_18 coating, the Mo
3–S clusters corresponded to peaks at the following vibration modes: ν(Mo-Mo) at ~210 cm
−1, ν(Mo-S)
coupled at ~330 cm
−1, ν(Mo-S
apical) at ~450 cm
−1, ν(S-S)
terminal at ~520 cm
−1, and ν(S-S)
bridging at 550 cm
−1. In addition to these peaks, the spectrum of this coating exhibited peaks at 360, 380, and 401 cm
−1, which could be due to atomic vibrations in the defective MoS
2 nanophase.
When choosing a model for the decomposition of the Raman spectra for C-based nanophase in Mo–S–C–H films, we took into account the changes in the Raman spectra for a-C(S,H) films with increasing hydrogen sulphide pressure. The Raman spectra for a-C(S,H) films are shown in
Figure S3 (
Supplementary Materials).
Figure S3 shows that, as the H
2S pressure grows, i.e., with an increase in the concentration of sulphur and hydrogen in a-C(S,H) films, the contribution to the Raman spectra of the two peaks at frequencies of ~1220 cm
−1 and ~1440 cm
−1 rises as well. The intensity of these peaks in films having the highest concentration of S exceeded the intensity of the D (at ~1340 cm
−1) and G (at ~1530 cm
−1) peaks characteristic of pure a-C films. In this case, with an increase in the sulphur concentration, the
ID/
IG ratio grew, which indicated an increase in the disordering (defectiveness) of atomic packing in graphite clusters.
Comparative analysis of the Raman spectra for the C-based nanophase in Mo–S–C–H and a-C (S,H) films showed (
Figure 6) that the addition of Mo atoms to the depositing flux did not cause significant changes in the Raman spectra for the C-based nanophase. This was confirmed by the fact that the spectra of Mo–S–C–H coatings and a-C (S, H) films in the frequency range of 1000–1800 cm
−1 were similar in many respects. An increase in the H
2S pressure during RPLD of the Mo–S–C–H coatings led to an increase in the contribution to the spectrum of lines at ~1220 and ~1440 cm
−1. Our analysis of the published data on Raman studies of Mo–S–C films formed by codeposition under magnetron sputtering (including the reactive one in an Ar/CH
4 mixture) showed that the spectra of these films did not have properties characteristic of the spectra of the Mo–S–C–H films produced by RPLD. In the spectra of Mo–S–C films for the C-based nanophase, the positions of the D and G peaks, as well as the ratio of their intensity, tended to change, but new peaks did not appear [
14,
15,
38,
39,
46,
47]. Changes in the Raman spectra were caused by the influence of the MoS
2 nanophase on the sp
2/sp
3 ratio (graphitization) and the level of mechanical stresses in the a-C(H) nanophase. More significant changes in the Raman spectrum of the carbon component in the a-C(S,H) films were found when using chemical vapour deposition in H
2S, as well as during magnetron sputtering and pulsed laser ablation of composite targets made of a mixture of powders (MoS
2, sulphur, graphite) [
48,
49,
50]. Unfortunately, these works do not contain a sufficiently detailed analysis of the Raman spectra. Therefore, to investigate the C-based nanophase in the a-C(S,H) and Mo–S–C–H films obtained by RPLD, we used the approach proposed by Takeuchi et al. [
51] for organic carbon sulphur materials.
Takeuchi et al. [
51] have identified a class of organic carbon sulphur materials, the Raman spectrum of which has peaks at ~1250, 1350, 1440, and 1590 cm
−1. The position of each peak has a tolerance of ±50 cm
−1. The structure of such materials depends on the ratio of the intensities of these peaks. If the peak at 1400 cm
−1 is the most intense, there is a large amount of the sp
3 component of the G-band, and the majority of the carbon component form an undeveloped graphene (C-C) skeleton. Other peaks correspond to the sp
3 component of the D band (~1250 cm
−1), the sp
2 component of the D band (~1350 cm
−1), and the sp
2 component of the G band (~1590 cm
−1). The S‒S bond stretching vibration should peak at ~480 cm
−1. This peak is present in the Raman spectrum of the Mo–S–C–H_18 coating (
Figure 6c). The low intensity of this peak shows that RPLD was not effective for the formation of sulphur clusters. The process of dispersing sulphur in the carbon nanophase turned out to be more productive and caused a change in the local packing of carbon atoms and in the structure of the carbon skeleton. With a rise in the H
2S pressure, i.e., with an increase in the concentration of sulphur, the contribution from the sp
3 states caused by the introduction of sulphur in the structure of the carbon skeleton grew. At the same time, an increase in the intensity of the
ID peak at 1350 cm
−1 (compared to
IG at 1540 cm
−1) was indicative of a growing number of defects in the atomic packing of pure graphite clusters. Low intensity peak at 1080 cm
−1 should be introduced for better fitting of the Raman spectrum.
The RBS technique made it possible to determine the concentration of sulphur in the nanocomposite Mo–S–C–H coatings. This technique nevertheless does not distinguish between the sulphur content in MoS
x and a-C(S,H) nanophases. Therefore, XPS measurements were carried out.
Figure 7 shows the XPS spectra, revealing chemical bonds in the surface layer of the Mo-S-C-H coatings formed by RPLD at various H
2S pressures. Decomposition of the Mo 3d spectrum showed that the chemical state of Mo atoms did not undergo significant changes with the increasing pressure of H
2S. The Mo 3d spectra contained Mo3d
5/2‒Mo3d
/2 doublets, corresponding to Mo
2+, Mo
4+, Mo
5+, and Mo
6+. The electron binding energies for the Mo3d
5/2 peaks at such valences of molybdenum were 228.6, 229.2, 230.3, and 232.6 eV, respectively. The dominance of the Mo
4+ doublet indicated the effective formation of MoS
2 and/or MoS
x compounds (with packing Mo
3‒C), in which
х ≥ 3 [
35,
43,
52,
53]. The Mo
5+ doublet may have been a result of the binding in the MoS
3 compound with a linear packing of atoms into Mo‒S
3 clusters [
35,
53,
54]. The presence of a Mo
6+ doublet with a low relative peak intensity indicated weak surface oxidation and the formation of Mo–O compounds [
35,
55]. With increasing H
2S pressure, the intensity of the Mo
6+ doublet weakened even more due to the chemical properties of hydrogen sulphide, which is a strong reducing agent. An increase in the H
2S pressure caused a decrease in the contribution of the Mo
2+ doublet, which corresponded to the Mo–C (Mo
2C) bonds [
55,
56]. This was due to both an increase in the total sulphur concentration in the Mo–S–C–H films with a rise in the H
2S pressure and, probably, due to an increased chemical activity of radicals formed upon activation of H
2S by a laser plasma, compared with carbon atoms in a laser plasma from a graphite target. Considering the small contribution of the Mo‒C states, the effect of the carbide nanophase on the properties of M–S–C–H coatings was not considered in this work.
In the decomposition of the C 1s spectra for Mo–S–C–H coatings, it was assumed that C atoms could form chemical bonds with each other (C=C binding energy 284.6 eV and C‒C binding energy 285.5 eV), with S atoms (C‒S energy bonds 286.5 eV), and Mo atoms (C‒Mo binding energy 283.6 ÷ 284.2 eV) [
36,
37,
56,
57]. The peak with the highest binding energy (~289 eV) is usually attributed to C‒O bonds [
57]. The analysis of the C 1s spectra showed that, at all H
2S pressures, the peak corresponding to the sp
2 bonds of C atoms dominated. As the H
2S pressure grew, the contribution from the peak corresponding to С‒S bonds increased too. In this case, the contribution of the peak at 285.5 eV, corresponding to sp
3 bonds of carbon atoms, slightly decreased. A thin film of organic contaminants containing CH
x molecules may form on the surface of Mo–S–C–H coatings after being blown out of the PLD chamber. The presence of this film could have a definite effect on the results of studying the chemical state of carbon in Mo–S–C–H coatings, first of all, it could have increase the intensity of the XPS peak binding energy of ~284.5 eV.
The deconvolution of the S 2p spectra for Mo–S–C–H coatings allowed us to assume that S atoms can form chemical bonds with Mo atoms, which are characteristic of MoS
x compounds with different values of
x, as well as of chemical bonds with C atoms. The most common approach to analysing the chemical state of S atoms in molybdenum sulphides is the separation of two doublets S 2p
3/2‒S 2p
1/2 having “low” and “high” binding energies. A doublet with a low binding energy (the binding energy of the S 2p
3/2 peak does not exceed 162.3 eV, and the S 2p
1/2—163.5 eV) usually corresponds to the S
2− species characteristic of the MoS
x compound [
35,
58]. A doublet with a high binding energy (the binding energy of the S 2p
3/2 peak is ~162.8 ÷ 163.4.4 eV) corresponds to the S
22-species, which are characteristic of MoS
x compounds, where clusters of Mo‒S
3 and/or Mo
3‒S are formed due to a high concentration of sulphur (
х > 2), [
35,
43,
52,
53]. S atoms in the chemical bond with C atoms (‒S‒C‒S‒) correspond to the doublet S S 2p
3/2‒S 2p
1/2, in which the binding energy of the S 2p
3/2 peak is 163.6 eV, and the S 2p
1/2 peak equals 165.2 eV [
57]. The choice of a model for describing the chemical state of S in a carbon matrix seems to be quite problematic since the binding energies of S atoms can strongly depend on the configuration of the nearest atoms. Thus, for a certain configuration of chemical bonds, the spin-orbit splitting of the S 2p state does not occur. Our analysis of the published results of XPS studies of sulphur-doped carbon materials has shown that even in the absence of spin-orbit splitting, a band at ~163.5 eV can dominate in the XPS spectra of S 2p (for example, a configuration of the C‒S
1÷2‒C type), together with which a band at 165.0 eV (for instance, a configuration of the ‒C=S‒type) appears [
59,
60].
The application of the chosen model of the decomposition of the S 2p spectra showed that an increase in the H2S pressure resulted in a decrease in the concentration of S2− states, and the contribution to the S22− states increased. The contribution of the species corresponding to the C‒S bonds increased as well. The calculation of the ratio x = S/Mo, taking into account S species (S2− + S22−) associated with Mo, and Mo species (Mo4+ + Mo5+) associated with S, showed that it was approximately 1.8, 2.5, and 4.0 for the Mo–S–C–H coatings obtained at pressures of 5.5, 9, and 18 Pa respectively. The composition of the C component in these coatings was described by the approximate formulas С0.78S0.08H0.14, C0.73S0.11H0.16, and C0.62S0.18H0.2. We assumed that H atoms are concentrated mainly in the a-C(S,H) nanophase. The calculated composition of the a-C(S,H) nanophase for Mo–S–C–H composite films differed from the composition of a-C(S,H) coatings obtained by RPLD at similar H2S pressures. The concentration of S atoms in the nanophase was lower than in the monophase thin-film coating. This could be attributed to the fact that S atoms deposited on the surface of the growing layer from the gas phase during ablation of a graphite target can be captured during the formation of the MoSх nanophase in the course of the subsequent ablation of the Mo target. Our calculations have shown that, as a result of the codeposition of Mo and C in reactive gas, the ratio х = S/Mo exceeds the ratio obtained earlier for MoSx films produced by RPLD of molybdenum in H2S at the same gas pressures.