Tuning of solid-to-solid structural transitions in amorphous carbon films by optical pumping and chemical modification

In amorphous carbon (a-C) structures, the carbon atoms are mainly sp²- and partially sp³-hybridized and are exposed to structural clustering and hydrogenation.^1–3 As a hard thin film, a-C is used for a manifold of scientific and technological purposes including the study and engineering of friction and wear,^4–7 the design of anti-bacterial surfaces,^8,9 surface finishing and product refinements,¹⁰ and food storage,¹¹ which benefit from its rich electrical, optical, and mechanical properties.^3,12–14 The proportion of carbon hybridization, which is closely linked to its spatial density, is tailored by deposition parameters of the growth technique, specifying, in turn, the mechanical features of the a-C structure.^2,13,15–19 Hydrogenated amorphous carbon (a-C:H) possesses a hybridized network similar to that of a-C but with a few key differences: on the one hand, hydrogen stabilizes the carbon sp³ hybridization by the formation of strong C–H bonds, and, on the other hand, high hydrogenation acts as a barrier to the sp² clustering by defining carbon chains.²⁰ This variety opens up a broad spectrum for employing a-C thin films.

A key drawback of a-C and a-C:H structures, however, is their thermal instability, potentially leading to an allotropic conversion of the carbon network.¹ Apart from thermal impacts,²¹ such solid-to-solid transitions including restructuring of a-C could be initiated by transferring energy in terms of electromagnetic waves, such as microwave radiation, laser light, or x-ray pulses to the sample.^22–27 Recent studies demonstrate that both sp²- as well as sp³-dominant a-C structures have an intricate structural transition behavior when they experience significant temperature variations and changes in the chemical conditions of the growth process.^28,29 Both issues limit the application of a-C. Until now, the mechanisms including structural transitions within the a-C network were not elucidated in dependence on a continuously varied thermal energy transfer or on a chemical element modification of the a-C structure.

As an exemplary hardware platform, we exploit micrometer-thin a-C films with an sp³/sp² ratio of about 10%.¹⁷ Due to their high thermal sensitivity, they are ideal candidates to realize an optical-spectroscopic method, studying solid-to-solid structural transitions in a tailored way. We developed a pump- and probe-like technique, allowing for optically inducing heat into a local area of a thin film and probing its structural characteristics using confocal Raman scattering. The laser-induced deposition of heat in thin films combined with confocal Raman scattering represents a sophisticated method to distinctly introduce and analyze structural changes in solid-state materials at the microscopic scale. Following the scattering processes, in which optical phonons and defects are involved, allows for probing the thermal stability, solid-to-solid structural transitions, and bonding distortions. For different types of a-C based thin films, we figure out five different stages depending on the pump-laser power density in which the atomic carbon network is optically transformed from amorphous carbon via an enrichment in graphitic structures to clusters of long-range ordered graphite. This structural evolution is modified chemically with Si, Cu, Ag, or W having concentrations of a few atom percent (at. %). For a-C:Si, the thermal stability is enhanced, while the hydrogenation of a-C inhibits a nano-crystalline graphite enrichment. The Raman scattering microscopy combined with a nanosecond-pulsed laser excitation allows for simulating the thermal conditions of a tribological contact between opaque amorphous materials. The energies and dispersions measured as well as efficiencies of the interatomic vibrations provide insight into mechanisms responsible for thermo-tribological interactions. They lie at the heart of nanotribology, which is an emerging and a highly innovative topic at the interface of mechanical engineering, chemistry, and physics.

The experimental key components are a nanosecond-pulsed pump laser providing average power densities of several kW/cm², a continuous-wave single-frequency probe laser, and a confocal microscopy setup with high spectral and spatial resolution, as sketched in Fig. 1(a). The thin film is illuminated in vacuum with the pump laser light (532 nm) of a definite power and illumination time. Simultaneously, the surface temperature T_surf is gathered by a fiber-coupled (FC) two-color pyrometer (PM) working in the near-infrared regime. The probe laser light (532 nm) is inelastically scattered from the central illumination part of the pump-laser spot area. An exemplary Raman scattering spectrum is depicted in Fig. 1(b). It predominantly consists of the D and G Raman peaks. The D peak is explained by a double-resonance mechanism involving a phonon and a defect and is related to the breathing modes of sp² atoms coordinated in rings. The G peak is described by an ordinary Raman process given by bond stretching of all pairs of sp² atoms in both rings and chains. The spectrum also includes both their second-order scattering signatures with the dominant 2D peak³⁰ as well as the mainly sp¹-related L₁ and L₂ peaks at low Raman shifts. The D and G peaks are modeled by pseudo-Voigt functions, allowing for differentiating between Gaussian and Lorentzian line shapes.

The a-C films were synthesized by physical vapor deposition in an industrial magnetron sputtering device. Purified graphite targets (purity: 99.99%) were used with 2 kW cathode power and 20 kHz mid-frequency in bipolar-pulsed mode. The bias voltage was set to −100 V, and the Ar regulated pressure was 3 × 10⁻³ mbar. The deposition time was set to achieve a layer thickness of 1 µm.^4,31,32 The chamber was evacuated below 1 × 10⁻⁴ mbar and heated above 100 °C for humidity reduction. The substrate is case-hardened steel AISI 5115 or heat-treated AISI H11, which was surface-etched by Ar ions for cleaning. An interlayer of CrC with a gradient from Cr- to C-dominant concentration was grown for improved film adhesion.³³ 

The surface temperature T_surf was measured by a fiber-coupled two-color pyrometer working in the near-infrared regime. The system was supported by an achromatic reflective fiber coupler and a 10× near-infrared microscope objective. It detected the unbalanced intensity at two different wavelengths (1.675 and 1.945 µm) and calculated the ratio. For conversion between the intensity ratio and temperature, a calibrated black-body radiation source was measured, and the data were fitted with a polynomial function of the fifth degree.

The D, G, and 2D Raman peaks were fitted with a pseudo-Voigt function $I(Δν̃)$⁠, which is appropriate for both borderline cases of amorphous and graphitic structures.^34,35 The equation is given by

with $σ̃=σ/2ln2$⁠, FWHM = 2σ, the Raman shift $Δν̃$⁠, peak position Δν, amplitude A, and the form factor α ∈ [0, 1].

The tuning of the average power density P_pump of the pump laser from a few tens of W/cm² to about 10 kW/cm² induces significant changes in the line widths, Raman shifts, and intensities of the first- and second-order scattering peaks of the a-C and a-C:H thin films. The respective Raman scattering spectra are shown as contour plots in Fig. 2. At about 0.6 kW/cm², the D and G peaks are strongly shifted, while above 2 kW/cm², a significant line narrowing correlates with the appearance of the 2D peak. The Raman peaks are sensitive to laser-induced temperature impacts so that they indicate structural changes in the atomic carbon network. In the following, the peak parameters obtained from pseudo-Voigt curve fittings are studied as a function of P_pump and, in turn, T_surf. They are shown in Fig. 3.

Stage I. At low pump power densities leading to surface temperatures of maximum 350 K, the D and G Raman scattering peaks do not exhibit changes for a-C and a-C:H. For a-C, the broad D (G) peak remains at about 1380 cm⁻¹ (1550 cm⁻¹) with FWHM ≈ 410 cm⁻¹ (175 cm⁻¹); see Figs. 3(b), 3(d), and 3(e). The narrow G peak is Gaussian-like characterized by the shape form factor α(G) = 0, while the D peak is described by Lorentzian- and Gaussian-like contributions yielding α(D) = 0.75; see Fig. 3(c). The intensity ratio I(D)/I(G) stays at 1.45, as demonstrated in Fig. 3(a). For the a-C:H film that contains a hydrogen content below 20 at.%, most peak parameters agree with that of the a-C film. Only I(D)/I(G) amounts to 0.75, and the D peak is Gaussian-like with α(D) = 0.

This difference between a-C and a-C:H is attributed to the effect of hydrogen on the a-C bonding. The low intensity ratio for a-C:H is qualitatively caused by the higher sp³ content.³⁶ Hydrogen further saturates dangling bonds, especially close to the surface, and limits the size of possible graphitic clusters. Taking into account the intensity ratio of 1.45, an sp³ content of about 12%, a mass density of 2 g/cm³, an optical extinction coefficient of 0.7, and an equivalent graphitic cluster size of about 1.5 nm are obtained.^{13,17,32,37,38} In terms of the activation-relaxation technique,^15,39 the weak heat impact at low P_pump does not trigger crossings of energetic barriers or structural relaxation processes, for the small-sized graphitic clusters. The initial a-C and a-C:H structures are maintained, including saturation of dangling carbon bonds by hydrogen at the a-C:H surface,^2,40 while the laser-light induced heat is diffused within the illuminated and absorbing top surface (ranges of 100 nm).²⁷ 

Stage II. From 0.2 to 0.9 kW/cm² with a maximum surface temperature T_surf of 455 K, the intensity ratios are increased, and the G peak is narrowed, while its spectral position shifts to large Δν. The width and position of the D peak slightly decrease at the end of stage II. For a-C, the intensity ratio increases up to a maximum value of 2.1, while it goes up to about 1.25, for a-C:H; see Fig. 3(a). The enhancement factors are 1.44 and 1.67, respectively. Interestingly, for reaching the maximum intensity ratios, the G peaks of the a-C and a-C:H structures approach each other; the same holds true for the D peaks. Up to P_pump = 0.9 kW/cm², a further intriguing change is demonstrated by the form factor of the D peak for a-C, which becomes more Gaussian-like.

The rise in the intensity ratios, which is particularly governed by increasing D peak intensities, may indicate an expansion of existing clusters (to a limited extent) or enrichment of ordered nano-crystalline graphitic clusters embedded in the disordered a-C matrix. A spatial expansion of clusters would imply a significant narrowing of the D peak, which is inversely proportional to their micro- and nanostructural sizes. The spatial enrichment of aromatic clusters increases independent relaxation lifetimes and, in turn, the Gaussian part, which contributes to the shape of the D peak. In agreement with the experimental data, for stage II, the presence of non-sixfold rings becomes less probable, and clustering of the sp² phase into ordered rings takes place, which corresponds to the formation of nanometer-sized graphitic structures. On the contrary, for a-C:H, the form factors α indicate only Gaussian-like line shapes so that an aromatic enrichment is likely impeded by the C–H terminal bonds. The narrowing of the G peak, instead, displays a possible removal of sp³ bonds from in-plane carbon sp² bond stretchings and a reduction in the number of lattices and topological defects.⁴¹ 

Stage III. It ranges from 1 kW/cm² to about 3 kW/cm² and covers surface temperatures from 455 to about 1200 K. The D and G peaks spectrally move further away from each other, and the intensity ratio decreases to 1.2, in particular for the a-C film. The D peak is remarkably narrowed from 400 to about 100 cm⁻¹, as illustrated in Fig. 3(d). The line shape of the D as well as the G peaks switches to a Lorentzian description; however, the G peak remains slightly Gaussian-like characterized by α(G) ≈ 0.9. Moreover, the 2D peak appears in the spectra whose width is comparable to the width of the D peak.

The significant changes in the linewidth and shape clearly indicate a structural transition from amorphous carbon to defected graphite including a spatial expansion of graphitic nanocrystals in the presence of lattice defects characterized by non-hexagonal lattice arrangements. The appearance of the 2D peak is, moreover, clear proof that these graphitic structures obtain a long-range ordering.⁴² At high surface temperatures induced by a pump laser excitation above 1 kW/cm², the sp² bonds become homogenized due to thermally activated structural relaxation processes. Accordingly, the degree of structural order is much higher than in the amorphous carbon stages I and II. Besides that, the number of phonon states becomes smaller, and the spectral phonon density becomes narrower so that less phonons are able to facilitate the double-resonance mechanism. Accordingly, the scattering frequency of, in particular, the D peak is only weakly dispersive. Furthermore, at the solid-to-solid transition, the intensities of the D and G Raman peaks increase remarkably. It is due to the predominant formation of hexagonal lattice regions so that the Raman relaxation length between hexagonal lattice regions and defects is reduced for the electrons leading to efficient scattering processes.

Interestingly, for the a-C:H film, the intensity ratio changes only slightly, and the 2D peak is not detected. Above 700 K, the effusion of hydrogen and hydrocarbons is expected to begin.⁴³ The still present hydrogen atoms prevent the formation of defective graphite; it is thermally activated at higher temperatures, in comparison to the a-C film; see stage IV.

The power-dependent delayed transformation of the G peak profile into a Lorentzian-like shape shows that the sp² bondings in carbon pairs are stronger/shorter than in aromatic rings. This is in line with the hardening of the G phonon mode (increase in the Raman shift) and a softening of the D phonon mode.

Stage IV. Stage IV is present at pump power densities between 3 and 7 kW/cm². By the end of the stage, temperatures of T_surf ≈ 3800 K are reached, which is the limit of sublimation for graphite-like carbon.⁴⁴ The intensity ratio increases to about 1.4 and subsequently drops to 0.6, while the ratio I(2D)/I(G) is strongly increased due to the intensity enhancement of the Lorentzian-shaped 2D peak. The FWHM of the D and G peaks is further reduced to about 50 cm⁻¹. As shown in Fig. 3(c), the shape of the G peak is contributed by the Gaussian part of the pseudo-Voigt function yielding α = 0.7.

The line narrowing combined with the strong intensity growth of the 2D peak clearly outlines that the surface of the films becomes further graphitized, providing long-range-ordered hexagonal lattice patterns in agreement with the International Union of Pure and Applied Chemistry (IUPAC) definition of graphite in distinction to nanocrystalline or defective graphite.⁴⁵ The reduction in α(G) expresses that the expansion of the graphitic nanoclusters faces barriers, namely the different clusters approach each other, thus preventing further enlargement. For reaching the sublimation limit, the graphitic clusters are fused together and, in turn, contribute to larger and ordered structures, as underlined by the high 2D peak intensity and the decreases in the FWHM of the D and G peaks.

Stage V. At the end of stage V, the Raman scattering signals vanish due to laser-induced ablation of the a-C (a-C:H) film with surface temperatures exceeding 3800 K.

Element modification of a-C with Si, Cu, Ag, or W. For the modification with Cu (9.7 at. %), Ag (5.6 at. %), or W (16.0 at. %), the pump power dependences of the Raman shifts are similar to that of the a-C and a-C:H films. Only for a-C films modified by Si (8.6 and 11 at. %), an initial reduction in the D and G peak positions is detected, and spectral separation of both peaks occurs at higher power densities with increasing Si content, as shown in Fig. 3(f). The intensity ratios, for a-C:Si, start to change also at higher P_pump; in particular, for a-C:Si (11 at. %), the evolution of the intensity ratio is strongly shifted toward high P_pump. For the Cu and Ag modification, the intensity ratio significantly increases in stage III; see Fig. 3(g).

This observation hints at a reduction in the nanocrystalline graphite cluster size, or, in other words, the formation of larger hexagonal lattice patterns is initiated only at higher pump power densities. Since Ag is known to dissolve in the amorphous carbon matrix without the formation of phases or crystallites,⁴⁶ its behavior practically corresponds to that obtained for the a-C:Si films. The power-dependent changes for the a-C:Si films are attributed to a stabilizing effect of the Si atoms on the sp³-hybridized carbon single bonds, leading, in turn, to a reduction in the sp² cluster sizes and numbers.^31,47–49 The evolutions of I(D)/I(G) for the Si-modified films are similar to that of the a-C film; however, they are shifted along the horizontal axis toward higher pump power densities. The I(D)/I(G) curves are brought into agreement with the respective curve for the a-C film, using a multiplication factor of three. Thus, we conjecture that the thermal stability of the a-C:Si films is enhanced up to a factor of three. Additionally, Si may create a competing network of amorphous silicon,^50,51 and Si–C bonds could be formed having a stress reducing and sp³-stabilizing effect, respectively. The a-C:W film demonstrates a similar behavior like the a-C film. Since W supports the sp² hybridization in a-C,^31,52 the carbon clusters are gained in number and size, which explains the high intensity ratio at the amorphous carbon stages and the low intensity ratio at the defective graphite stages III and IV. Thus, the graphitic clusters are enriched more efficiently, which is not the case for the catalytically active elements Ag and Cu.

The pump- and probe-like Raman scattering microscopy supported by a pseudo-Voigt fitting model allows for initiating and probing solid-to-solid structural transitions in a-C films. As schematically shown in Fig. 4, at low surface temperatures and, correspondingly, low pump power densities, the a-C films remain a short-range-ordered network of mainly sp²-hybridized carbon atoms. With increasing thermal impact on the surfaces, sixfold rings of carbon atoms obtain an aromatic character by delocalized π electrons. The system is considered an amorphous carbon matrix with progressive enrichment of aromatic seeds. Further increasing the temperature leads to the solid-to-solid structural transition from amorphous carbon to defective graphite including the effusion of possible hydrogen or hydrocarbons. A saturation of carbon dangling bonds at the film surface due to a hydrogen effusion and/or a rearrangement of local sp² or sp³ substructures caused by the breaking of C–H bonds may be studied in the future. Above 1200 K, the graphitic lattice arrangements are enlarged followed subsequently by graphitization. Finally, laser-light induced surface temperatures of about 3800 K yield an ablation of the carbon films. The modification of the a-C films by Si significantly enhances the thermal stability by a factor of three, while, for example, the a-C:W film demonstrates a more efficient enrichment of the nanocrystalline graphitic clusters. The Raman scattering microscopy combined with a nanosecond-pulsed laser excitation provides a sophisticated method to study changes in the composition as well as the thermal stability of amorphous solid-state structures without the need for an advanced x-ray method.

The authors acknowledge the financial support from the German Research Foundation (DFG) within the frame of the SPP 2074 under the project “Fluid-free lubricant layers for the heavily loaded and unsynchronized operation of dry-running screw machines” (Project No. 407710554).

The authors have no conflicts to disclose.

Carl Arne Thomann: Formal analysis (lead); Investigation (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (equal). Adrian Wittrock: Investigation (supporting). Alexandra Wittig: Resources (equal). Nelson Filipe Lopes Dias: Resources (equal). Dominic Stangier: Resources (equal). Wolfgang Tillmann: Funding acquisition (lead); Supervision (lead). Jörg Debus: Conceptualization (lead); Funding acquisition (lead); Supervision (lead); Writing – original draft (supporting); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.