Control of the Graphite Femtosecond Ablation Plume Kinetics by

Article pubs.acs.org/JPCC

Control of the Graphite Femtosecond Ablation Plume Kinetics by Temporal Laser Pulse Shaping: Effects on Pulsed Laser Deposition of Diamond-Like Carbon Florent Bourquard, Teddy Tite, Anne-Sophie Loir, Christophe Donnet, and Florence Garrelie* Université de Lyon, CNRS UMR 5516, Laboratoire Hubert Curien, Université Jean Monnet, 42023 Saint-Étienne, France ABSTRACT: This study proposes to consider the effect of temporal laser pulse shaping on graphite femtosecond laser ablation plume composition and kinetics, and its potential for depositing diamond-like carbon (DLC) films. Double pulses and long pulses with up to 100-ps delay and 10-ps duration, respectively, are used. The plasma composition and kinetics are investigated by using an optical emission spectroscopy system with a spatial resolution along the ejection axis. Temporal pulse shaping is shown to strongly modify the quantities of the different ejected species in the plume. In particular, it reduces the number of slow C2 or C3 radicals and increases the proportion of monomers, adding ionized species in front of the plume. At the same time, the different components of the multimodal plasma plume maintain their average speed whatever temporal pulse shape is used, thus demonstrating the kinetic modifications. The multiwavelength Raman study of DLC films deposited using different temporal pulse shapes does not reveal significant structural differences between the films. This was found to be in accordance with a prevalence of neutral C atoms and radicals in all the generated plumes, the former being responsible for the subplantation process.

1. INTRODUCTION During the past few years, picosecond and subpicosecond lasermatter interaction have given access to new mechanisms of laser ablation, impacting several applications such as micromachining and pulsed laser deposition (PLD).1 Various thin films can be grown by femtosecond and picosecond PLD (fsPLD and ps-PLD), such as oxides,2 nitrides,3 quasicrystals4 or diamond-like carbon (DLC),5 exhibiting new states and structures when compared to classical deposition methods such as nanosecond PLD (ns-PLD). In the case of DLC deposition, it has long been admitted that ns-PLD could produce films with higher sp3 contents (up to 80%)6−9 than fs-PLD (up to 60−70%), which in turn would generate micrometric carbon fragments on the samples ’surface. The advantages of ultrashort interaction rely on the enhancement of several other properties, notably adherence of the films,10 tribological behavior5,11 and electro-thermal characteristics.12,13 For classic physical deposition methods such as nsPLD14and ion beam deposition15 the sp3 bonding is admitted to arise through subplantation process.16 Carbon ions impact the growing DLC structure with sufficient energy to penetrate inside the material (optimal energy is around 100 eV). Evaluating the impact of subplantation process during fs-PLD is difficult because of the general lack of information on the neutral species.7 Nevertheless, the presence of high kinetic ions (≈10 keV) which could cause the destruction of sp3 bonding6 while enhancing adherence8 (≈1 keV) has been shown. These differences in growth processes of ns and fs-PLD (or ps-PLD) mostly rely on different ablated plume spatial and temporal distributions.7,17 The lack of significant spatial © 2014 American Chemical Society

expansion of matter during the irradiation with ultrafast laser and the consequent absence of plasma screening result in a free expansion of the irradiated matter. Studies in molecular dynamics consider two different mechanisms for graphite ablation,18−20 both relying on energy absorption by electrons and further fast electron−phonon coupling. At very low fluences (0.21 J/cm2 for a 20 fs laser pulse), interaction induces strong homogeneous vibrations of graphite planes, eventually leading to removal of intact graphite sheets. When fluence gets higher (above 0.36 J/cm2), carbon atoms oscillate in and out of plane, bonding with atoms from other planes, leading to the ultrafast melting of graphite and the ejection of monomers and few element radicals. Observations confirm the presence of multiple species with a multimodal kinetic distribution in the ablation plume. Monomers constitute the fastest component of the ablation plume with speeds around 106 to 107 cm/s,7,21 while some experiments also observe ultrafast non thermal ionic components6,22 caused by Coulomb explosion. C2 and C3 radicals23 come after and appear through direct ejection and monomer recombination, but also by fragmentation of larger clusters, especially in gaseous environments.24 Large clusters constitute the slowest component (103 to 104 cm/s) and can be observed through their blackbody radiation23 and more recently through third harmonic generation.25 Received: September 13, 2013 Revised: January 22, 2014 Published: February 7, 2014 4377

dx.doi.org/10.1021/jp409191s | J. Phys. Chem. C 2014, 118, 4377−4385

The Journal of Physical Chemistry C

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interferometer with the mobile arm controlled by a micrometric resolution translation stage, ensuring delays from hundreds of femtoseconds up to a few nanoseconds and the conservation of the original duration and Gaussian profile of the split pulse. Since an interference pattern between the beams will appear if both arms of the interferometer are put at equal distance, one arm of the interferometer is shunted and the energy is adjusted to get only single femtosecond pulses. The pulses’ duration and delay are controlled by the complementary use of frequency-resolved optical gating (FROG) and background-free second-order autocorrelation. All duration values are given for fwhm. The laser beam enters the PLD vacuum chamber (≈10−5Pa) through a thin quartz window. Ablation targets, which can be rotated, consist of 99.98% pure graphite. Average fluence is obtained by measuring the irradiation-modified region for various laser energies.36 During all experiments, the target was on the focus point of the focusing lens. The laser spot waist at the surface was found to be around 7500 μm2. Laser energy was modified through either the use of optical density filters or by changing the polarization direction before sending the laser onto a polarizing mirror. Ablation depths could be measured by optical microscopy, assessing the vertical displacement of the objective during successive focusing on the sample surface and the crater bottom. The ablation plume is observed through a UV−visible 1-D spatially resolved optical-emission spectroscopy system.31 A 20 mm x20 mm square close to the ablation target is imaged on the 2 mm ×0.1 mm optical fibers array through spherical and cylindrical lenses. The array consists of 15 fibers, giving a 1.33 mm spatial resolution along the ejection axis. The 15 spots are then imaged on the slits of a spectrometer (Princeton Acton). 1-D-resolute spectra are then acquired by an ultrafast intensified charge-coupled device camera (Princeton PIMAX3) allowing precise delays of acquisition relative to pulse impact on the target, up to 10 ns resolution. Carefully choosing these delays permit to adapt the setup to either total emission acquisition or kinetics measurement. All measurements are averaged from five CCD recordings. Each recording consists in the accumulation of signal from several successive irradiations the number of which will be given in the following sections. Deposition of DLC films is performed at room temperature on silicon wafers facing the ablation spot. Pressure is around 5.10−5Pa and deposition duration is 20 min at 1 kHz repetition rate for all samples. Growth rate is measured by contact profilometry. The samples are analyzed through a LabRAM ARAMIS multiwavelength Raman spectrometer. The G band is due to the bond stretching of all pairs of sp2 carbon atoms in both rings and chains. The D band is due to the breathing mode of sp2 atoms in rings. The use of several excitation frequencies allows to further characterize the wavelengthdependent intensity ratio ID/IG and the G peak dispersion versus the Raman laser wavelength, both important features in carbon-based structural analysis.37 A Nova NanoSEM FEGSEM permits one to observe the surface aspect of the deposited samples.

This plume composition is different from that obtained during metal ablation, where the main two constituents are a vaporized monomer plasma and liquid-phase-generated nanoparticles,26−28 aside from spallated micrometric fragments. Metallic femtosecond ablation plumes have been proven to strongly depend on the temporal distribution of irradiating energy. In particular, either the use of double pulses up to hundreds of picosecond delays or the stretching of the pulse up to tens of picosecond duration produce higher ionization of the plasma component, while decreasing quantity of nanoparticles.29−31 More complex temporal tailoring of the pulses has also been used to obtain higher ionization ratio through the use of adaptive loops.32,33 Theoretical investigations show that extending the overall irradiation time on the picosecond time scale leads to less deep light penetration in matter and overheating of superficial layers, thus favoring hot plasma generation mechanisms over droplet formation.28,34 Concerning DLC deposition by fs-PLD, it has been proven that when using glassy carbon targets, important structural changes could be induced by the use of double pulses. Increasing the delay between the pulses enhanced or reduced the sp3 contents depending on the laser wavelength and pulse duration (500 fs at 248 nm or 180 fs at 800 nm respectively).35 Such ablation targets are however unusual for DLC films deposition. In this study, we explore the possibilities of using temporal laser pulse shaping to modify the composition and kinetics of plumes in graphite ablation, in order to explore the modifications of the structural properties of deposited DLC layers. Thus, we expect to better understand the laser interaction with graphite and the growth processes involved in fs-PLD and ps-PLD of DLC.

2. EXPERIMENTAL SETUP The experimental setup for the deposition and the optical emission analysis is depicted in Figure 1. The amplified

Figure 1. Schematic description of the experimental setup.

femtosecond laser system (Coherent Legend) produces Gaussian-profile pulses with 50 fs fwhm (Full Width at Half Maximum) centered at an 800 nm wavelength. The stretching of the laser pulses up to a 10 ps duration is ensured by slightly turning the compression gratings at the end of the amplification line, thus changing the compression efficiency while conserving the same total energy and Gaussian temporal profile. The creation of double pulses is achieved through a Michelson

3. OBSERVATIONS 3.1. Carbon Plasma Plume. The ablation plume spectral analysis reveals the presence of multiple types of optical emissions corresponding to various species (Figure 2) acquired from 200 to 300 ns (30000 accumulations) after the laser impact consisting of a 1 ps pulse at 2.4 J/cm2, conditions for 4378



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two most intense Swan bands with heads at 469 and 513 nm will be further studied in this paper. The wide band starting around 405 nm and degrading to the red is referred to as the “λ4050 Comet-Head Group”, emitted by C3 radicals, already observed in the literature.23 At longer distances from the target, some spectral lines corresponding to neutral carbon atoms (C I) and once-ionized ions (C II) appear.39 With a higher fluence and shorter delay after interaction (40−60 ns after irradiation, 2000 accumulations), a C III line at 405 nm farther from the rest of the plasma indicates the existence of C2+ ions. This example shows the multimodal structure of the ablation plume, with ions moving faster than neutrals, and heavier radicals coming last. Our first set of analyses consisted in the acquisition of plume emission spectra for various fluences and a 75 fs duration pulse (short pulse SP) in order to determine the threshold of appearance of the various species contained in the plumes. At fluences around 1 J/cm2, the signal can be only attributed to initial bremsstrahlung and C2 or C3 radicals. Slightly increasing the fluence triggers the detection of neutral monomers. Ions appear around 2.5 J/cm2 and twice-ionized ions at about 4 J/ cm2. The use of double pulses (DP) and long pulses (LP) with respectively various delays and durations did not permit to significantly modify the plume emission when using low fluences around 1 J/cm2 and only C2 and C3 could be observed.

Figure 2. Map of the spectrally dispersed carbon ablation plume produced by a 1 ps laser pulse at a fluence of 2.4 J/cm2. The target surface is in the upper part and species ejection is directed toward the bottom. Acquisition was recorded between 200 and 300 ns after irradiation.

which most emission lines and bands are visible. Several diffuse emission bands appear in the vicinity of the target that can be identified.38 Most of them are found to be in agreement with the well-known Swan System produced by C2 molecules. The

Figure 3. Intensity variation of the spectral emission of several lines and bands using (a, c) DPs and (b, d) LPs at (a, b) 4 J/cm2 and (c, d) 2.4 J/cm2. All values are the ratio of the observed yield divided by the yield obtained with SP. Durations and delays scale are not linear, ratio to SP scale is logarithmic. 4379



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Figure 3 represents the results of using DPs and LPs, at fluences of 2.4 and 4 J/cm2, in terms of emission and ablation depth. Except for C III, the signal was accumulated 10000 times from 100 to 700 ns starting from laser impact, in order to avoid initial bremsstrahlung and gather all the emission. For C III, the signal was accumulated 2000 times from 40 to 60 ns after irradiation. Only selected lines and band emissions are represented here for visibility purpose, since there are no observable differences between enhancement ratios of different Swan bands and different C I lines. The Comet-head emission corresponding to C3 cannot be seen at the highest fluence, while the C III line does not exist for the lowest. It should be noted that the Y-axis always represents a ratio of the signal or depth measured with DPs or LPs over the signal or depth measured with the original single 70 fs pulse (SP) respectively. Concerning the ablation quantity, there were no significant changes of the ablated surface at constant fluence; therefore only depth was relevant. The increase of LP duration or DP delay lead to a general decrease of the quantity of ablated matter. In the case of DP, it essentially reminds a metallic-like behavior.30 This correlates with a decrease of the emission due to C2 and C3 radicals. It is interesting to note that for the same decrease in ablation depth, radicals’ emission will be decreased more efficiently by LP than by DP. The DP and LP can have different effects on monomer emission. For both fluences, increasing DP separation leads to an optimal delay for which neutral and ionized species’ emissions are maximized. At 2.4 J/cm2, this corresponds to a delay of 16 ps between the two pulses. At 4 J/cm2, it can be observed at a delay of 24 ps for C and C+ emissions, and 16 ps for C2+ emission. When increasing the delays over these optimal values, the ion emission decreases. Ultimately, it reaches a value inferior to what is observed with SP. At 2.4 J/ cm2, this corresponds to a delay of 48 ps for C+ and, at 4 J/cm2, to delays of 96 ps for C+ and 48 ps for C2+. Such behavior has been attested in the literature for metal ablation.29 The use of LP also enhances the emission of monomers, even allowing higher emission yields for C2+ at 4 J/cm2 and C2+ at 2.4 J/cm2 to be reached, in accordance with what is observed on aluminum.32 At 2.4 J/cm2 a maximum yield for a 1 ps duration LP appears, whereas no optimum can be observed at 4 J/cm2. However, in the last case, it should be noted that the maximum duration is only 10 ps, which is short compared to the maximum DP delays. Furthermore, comparing LP and DP effects, it appears that for the same enhancement of C I emission, LP produces a higher enhancement of C II and C III emissions. In the case of delays and durations below 1 ps, neither DPs nor LPs modify plume emission at 4 J/cm2, while having important effects at 2.4 J/cm2. The analysis of the plasma kinetics has been carried out for all DPs and LPs presented on Figure 3, with acquisition-time windows of 150 to 200 ns and 200 to 300 ns, respectively for 4 and 2.4 J/cm2 and 30000 on-CCD accumulations in order to obtain both sufficient signal and distance from the target. Examples of the obtained speed distributions are presented in Figure 4. They reveal a multimodal structure of the plasma plume, as expected. Note that all the plasma components are created for both fluences. At 4 J/cm2, the use of DPs or LPs permits to generate C2+ ions in front of the plasma, while they appear to be in the C2+ component with SP. At 2.4 J/cm2, a classic femtosecond interaction only permits one to create few

Figure 4. Mapping of the speed distribution of the observed species in the ablation plume at (a) 4 J/cm2 and (b) 2.4 J/cm2. Intensity is normalized for each species and fluence.

ions mixed with the neutral species, and DPs or LPs here create a once-ionized component further away from the target. The impact of LPs and DPs on each component’s general speed seems to be very limited. For all the delays and durations investigated in this work, the different plume components, when existing, had roughly the same average speed. Only the fluence increase induces an increase of the monomers’ speed. The speed values and the corresponding kinetic energies are summed up in Table 1. Average speed was computed through a maxwellian fitting of the optical emission. The kinetic variations in the plume are essentially due to variations of the quantity of each component rather than changes in their speed. 3.2. Diamond-Like Carbon Deposition. Seven conditions of fluence and temporal pulse shapes were used to achieve DLC depositions, corresponding to the distributions in Figure 4. The modification of deposition rate was found to be in good agreement with the evolution of the ablation depth, as can be seen from the results of profilometry analysis given in Table 2. The structural study of DLC coatings was performed through multiwavelength Raman spectroscopy. Parts a and b 4380



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characterize the samples’ surface, SEM analysis was performed and the most relevant images are presented in Figure 6. The film obtained with SP at 4 J/cm2 presents micrometric and nanometric fragments on a smooth matrix (Figure 6a). The 6ps LP (Figure 6b) and 24-ps delay DP (Figure 6c) tend to diminish the number of fragments and clusters observed at the surface. At 2.4 J/cm2 (Figure 6d−f), a much higher number of nanometric and micrometric structures are visible on the sample’s surface. It can be observed that 16-ps delay DP and 1ps LP generate fewer fragments and clusters than SP, LP being more efficient in this case.

Table 1. Average Speed and Kinetics Energy Computed for Each Plume Species 4 J/cm2 average speed (cm/s)

species

− 1.0 3.3 5.8 8.0

C3 C2 C C+ C2+

× × × ×

2.4 J/cm2

average kinetic energy by atom (eV) − 6.27 68.2 211 400

6

10 106 106 106

average speed (cm/s)

average kinetic energy by atom (eV)

× × × ×

3.81 4.53 42.4 163 −

7.8 8.5 2.6 5.1 −

105 105 106 106

4. DISCUSSION 4.1. Impact of Increasing the Interaction Duration. As seen in the previous section, femtosecond interaction with graphite leads to the generation of a multimodal ablation plume, containing slow radicals and fast monomers, with ions coming in front of the plasma. This is generally explained by the fact that laser-matter interaction ends before significant thermal diffusion and spatial dilatation of matter come into play. In metallic materials, free electrons are responsible for light extinction, and the process roughly follows the Beer− Lambert law.40 Theoretical works19 show that during the femtosecond irradiation, even at a fluence close to the ablation threshold, graphite quickly acquires a metallic-like electronic density of state, allowing a sufficient density of free electrons to compare it to a metal when describing light absorption. After an adiabatic heating by fast electron−phonon coupling, Beer− Lambert law immediately generates a temperature gradient under the irradiated surface. Depending on the reached temperature, different layers of matter will undergo different relaxation processes which, in the case of metal ablation, successively lead to plasma fragmentation, nanoparticles nucleation, sputtering and shrapnel spallation deeper in the material.40 Part of the irradiated graphite target actually undergoes ultrafast melting. However, this liquid is very unstable and does not exist long enough to generate nanoparticles. During graphite expansion, monomers and short carbon chains are ejected from the liquid layer.18−20 Molecular agitation is responsible for bond breaking and prevents recombination as well as ionization. It becomes obvious that ions will arise from the top of the irradiated volume with the rest of the monomers below them and radicals in the depth. Hydrodynamic expansion of the plume will then keep this order, thus explaining the spatial distribution of species observed in this work with short single pulses (SP). When increasing the duration of interaction or using double pulses within the 10-ps time scale studied in this work, it is generally considered that the beginning of the pulse modifies the optical properties of matter before the interaction ends. These modifications lead to an increase of absorption due to electronic temperature rise, generating a hot optically thick plasma28,31,33 on the top of the irradiated volume. The first consequence of delaying the arrival of the laser power is that a part of the energy is prevented from penetrating farther than the heated target surface. Thus, the energy density needed for matter removal cannot be reached deep inside the material as in SP ablation, accounting for the lower ablation depths for longer interaction durations as seen on Figure 3. The second consequence is the stronger heating of superficial layers. Matter can reach higher ionization states on the top of the ablated volume and the breaking of atomic bonds can occur in a thicker layer. This leads to the observed higher emission from

Table 2. Film Thickness and Growth Rate for Seven Different Fluences and Temporal Pulse Shapes (DPx or LPx: DP with x ps Delay or LP with x ps Duration) fluence (J/cm2)

temporal shape

film thickness (nm)

growth rate (nm/min)

2.4

SP DP16 LP01 SP DP08 DP24 LP06

90 75 78 121 97 73 85

4.5 3.75 3.9 6.05 4.85 3.75 4.25

4

Figure 5 show the Raman spectra of DLC films at two different laser excitation wavelengths for a deposition fluence of 4 J/cm2

Figure 5. (a, b) Raman spectra for DLC deposited with four different temporal shapes (DPx or LPx: DP with x ps delay or LP with x ps duration) at a fluence of 4 J/cm2 with two different excitation wavelengths. (c) Illustration of the G and D peak fitting in the case of SP pulse at 4 J/cm2.

with four different temporal pulse shapes. The G-band dispersion and ID/IG ratios calculated from the spectra on Figure 5c correspond to DLC films already obtained in previous works9 using nonshaped femtosecond pulses, revealing an sp3 content around 50%. Furthermore, this corresponds to the presence of rather large clusters of sp2 cycles, not found using ns-PLD where this hybridization appears in chains. As displayed by parts a and b of Figure 5, the Raman spectra performed on films deposited with different DPs, LPs and fluences proved to be almost identical whatever excitation wavelengths were used, implying no structural differences are created between the samples. Raman spectra obtained for 2.4 J/ cm2 are also identical to those displayed here. In order to 4381



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Figure 6. 20 × 20 μm2 secondary electron SEM images of DLC thin films deposited at different fluences (a−c) 4 J/cm2 and (d−f) 2.4 J/cm2 and using (a, d) SP, (b) 6 ps LP, and (c) 24 ps delay DP, (e) 16 ps delay DP, and (f) 1 ps LP.

the lattice and that this optimum is immediately reached for SP at 4 J/cm2 while there can still be a progression at 2.4 J/cm2. 4.2. Impact of Using Different Temporal Shapes. Although LPs and DPs with durations and delays on the picosecond time scale globally tend to produce the same effects, there are several differences. Considering the same total ablated volume, the use of LPs favors the production of ionized monomers and reduces the generation of radicals more efficiently than DPs, as seen in Figure 3. In DPs the two pulses have the same duration, as in the SP case. Hence, the first pulse contains sufficient energy for in-depth material penetration, while preparing the arrival of the second pulse. In LPs, on the contrary, the energy density is spread over a longer time, ensuring only a moderate power affects the surface at each moment during the interaction, limiting penetration. Preheating of metals at the picosecond time scale has been shown to induce the highest temperatures of superficial layers.34 It becomes obvious that for LPs, a much higher part of the pulse ablating energy can be used for superficial heating, while with DPs, the pulses, especially the first one, affect deeper layers. It is well established for metals that precise and accurate temporal pulse shaping permits optimal energy coupling at the material’s surface28 and a more efficient ionization.33,41 The differences between DPs and LPs observed here make graphite a good candidate for such plume optimization attempts. If the disappearing of ionized species when decreasing fluence can be explained easily by the lower temperature reached on the target surface, the disappearing of C3 radicals’ emission when increasing energy is more challenging. We suggest that their number, and consequently their emission, decreases below the detection threshold of our setup, which is limited by the saturation of our ICCD by strong Swan band and C II emissions. 4.3. Plume Kinetics. The main feature observed here about plasma kinetics is certainly the relative constancy of plasma components’ speeds. While modifying the quantity of ejected species in each state (ions, neutrals, radicals), DPs and LPs of picosecond delays and durations do not affect much the speeds reached by each species, as seen in Figure 4. This result is a typical feature of low gas density flows.42,43 One could expect

ions and neutral monomers. These two processes decrease the quantity of observed radicals, which are now created between an enlarged overheated superficial layer and a shallower crater. The effects of increasing interaction duration up to several picoseconds can be roughly summarized as exchanging part of the total ablated volume for a more excited plume. To a certain extent, an increase of the total interaction duration will enhance those effects, as the plasma starts to expand, limiting heat conductivity and adapting its density to homogeneously absorb the whole energy, thus favoring plasma overheating. This is however limited depending on the pulse type. In the case of DP, for delays longer than a few tens of picoseconds, excessive matter expansion after the first pulse leads to a decrease of plasma absorption and optical thickness, as seen in Figure 3. A higher fraction of the second pulse will go through the plasma toward deeper layers, limiting re-excitation, thus leading to the existence of observed maxima for the emission of monomers. Further delaying, up to hundreds of picoseconds, only decreases matter re-excitation without ensuring deep matter removal due to energy dissipation by heat conduction inside the material. Reaching a lower temperature after the first pulse, matter irradiated at lower fluence will undergo de-excitation faster. This explains the general decrease of delays for optimal excitation or deexcitation when depositing energy density of 2.4 J/cm2 rather than 4 J/cm2. It is interesting to note that the same behavior is observed with metal ablation,29 but with characteristic delays one order longer. An analogous phenomenon happens when using LPs, but it should be considered that it is the beginning of the interaction which fails to excite matter and compete with heat dissipation, hence limiting the total impact of the pulse. We only observe this for 2.4 J/cm2, but we suggest it could occur at 4 J/cm2 for durations longer than 10 ps. It is however unclear why DPs and LPs below 1 ps delay and 1 ps duration, respectively, do not affect the plume at 4 J/cm2 whereas they produce higher excitation at 2.4 J/cm2. Considering that electron−phonon relaxation times are of the picosecond order, it might explain a change of interaction regime when dealing with shorter times. It seems that free electrons can only absorb so much energy before coupling with 4382



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that a huge C+ component in front of the plume would generally slow down the other components due to backward collisions. This means in our case that the plasma densities involved are too low for sufficient collisions, which is consistent with the usual absence of thermal equilibrium in femtosecond plasma plumes and consequent privileged expansion along the ejection axis.44 Speed then mainly depends on the initial kinetic energy of the particle related to its temperature. Thus, there is a link between a component’s speed and its corresponding ionization state. The slight differences between the two considered fluences might be associated with a modification of the ablated surfaces which can impact the gas flow dynamics. It should be noted that the spatially broad global structure of each plume component with the mutual relation between the components in a continuous flow reflect only thermal ablation. Our diagnostic setup never detected any products of Coulomb explosion, despite the use of many different delays of acquisition. It may come from the fact that the ejected ions do not meet the required temperature and density conditions for our setup sensitivity. One of the main goals of this study was to modify the ablation plume kinetics. Indeed, the use of DPs and LPs of picosecond delays and durations permits to modify the quantities of each plasma component (C3, C2, C, C+, C2+) and in certain cases to recreate these components. Then, using DPs and LPs will notably modify plasma kinetics. This allows us to populate various plume components corresponding to a precise speed in a controlled manner. When compared to SP, it was possible to obtain ablation plumes with much higher kinetic energy species and less heavy radicals at the end of the plasma, as seen in Figure 4. Since plasma kinetics is important for the structure and contents of DLC layers produced by PLD,16 changes were expected to occur in the layers. The subplantation process, generally admitted to be responsible for sp3 contents, is optimal at a kinetic energy per monomer around 100 eV. This value is also relevant for radicals,16 requiring only a little more energy to break C2 bonds. In the plume, the kinetic energy of C2 or C3 radicals is much lower than that optimum, while C and C+ have energies distributed around it. Diminishing the fraction of radicals in the plume while raising the fraction of species with kinetic energy around 100 eV appeared to be a good way to increase the sp3/sp2 ratio in the coatings. Nevertheless, LPs and DPs also increase the fraction of species with an energy much higher than 200 eV that can have a negative impact on this ratio through enhanced relaxation of the growing film. 4.4. DLC Thin Films Properties. Despite the effects mentioned above, the Raman spectra obtained on DLC coatings while using DPs and LPs of picosecond delays and durations (Figure 5a,b), chosen for the drastically different kinetic distributions of generated ablation plumes, exhibit no structure modification. Although Raman spectroscopy is not an ideal way to characterize sp3 content, there is virtually no possibility of changing it without observing variations of the ID/ IG ratio or G peak dispersion,37 especially with multiwavelength Raman analysis down to UV range. Given the variety of kinetic structures employed, it is very unlikely that the positive impact of the decrease of radical quantities would exactly compensate for the negative effect of the increase of highly kinetic monomers. More probably, this means that at least one of the two components we always observe, i.e., C2 and C, represents the vast majority of species contained in the plasma. Then, variations of C+ and C2+

quantities do not modify enough the subplantation process. It should be considered however that the ratio between C and C2 components strongly vary. The Raman results are in good agreement with those obtained with classic fs-PLD of DLC9 (Figure 5c). If we refer to the literature15 where different deposition methods and calculations for the subplantation process efficiency are presented,16 sp3 proportions around 35− 50% correspond quite well to the 42 to 68 eV average kinetic energy of the C monomer component presented in Table 1, keeping in mind its kinetic broadness. Radicals should represent a big part of the ablated material, since their emission follows the ablation rate evolutions, as seen in Figure 3. They do not possess a kinetic energy suitable for efficient subplantation. Thus, they can reasonably be expected to be deposited as a high content sp2 layer. These sp2 contents can be converted into sp3 thanks to subplantation by incoming monomers generated by the next ablation pulses during the whole deposition process involving the generation of thousands of ablation plumes. This hypothesis can explain the existence of a few nanometer-thick sp2-rich layers on the top of DLC films achieved by fs-PLD, not found in ns-PLD.45 Indeed, at the end of the deposition, there are no sufficient monomer plasmas left to successfully subplant sp2 contents in the last deposited layers. It is thus clear that at the fluences employed here, no control over the sp3 content of DLC layer was obtained using double femtosecond pulses or long pulses on the picosecond time scale. This can be fully explained from the correlation between plasma kinetics and growing mechanisms. The use of DPs and LPs with durations and delays on the picosecond time scale at lower fluences did not impact the plume contents and kinetics. Hence, it is not recommended to use DPs and LPs with lower laser energy to modify the coatings’ structure. A case should be made for the use of higher fluences, especially since the fraction of ionized species would be enhanced even for SP. It should be kept in mind however that temporal pulse shaping effects tend to decrease when increasing fluence,32,46,47 and we did not see much improvement when almost doubling laser fluence here. Specifically, due to the temperature profile of femtosecond irradiated volume, there is always a large proportion of the ejected products issued from heated layers corresponding to the observed C2 and C plume components. Those should roughly keep the same kinetics, save for geometrical changes of the irradiation spot, since no sufficient gas density is reached. The surface analysis of the deposited films shows different results depending on the fluence. At 4 J/cm2, an apparently smooth surface with micrometric fragments seems to indicate that the film mainly originates from vaporization and spallation process. The shockwave intensity, responsible for the spallation of films, tends to decrease when using picosecond pulses48 or double pulses49rather than single femtosecond pulses during metal ablation. The observed decrease of the number of micrometric fragments on DLC films surface suggests that this is also true for graphite ablation. At 2.4 J/cm2, a lot of nanoclusters in the range of a few tens to hundreds of nanometers can be observed. Simulations of graphite ablation in the literature18 evidenced two mechanisms responsible for the formation of such clusters during the ablation process, either by large “flakes” ejection for a low energy density or as long carbon chains ejected from within the unstable liquid phase for higher energy density. The morphology and size of the clusters here suggest that they arise through the first mechanism. This is coherent with the disappearing of these structures when increasing either fluence or pulse length or 4383



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modifications of DLC coatings and no control over the sp3 contents was acquired. It was shown that femtosecond and picosecond regimes are very similar. Additional studies should be performed in order to prove that the important mechanical and tribological properties as well as other physical properties of fs-PLD produced DLC are conserved when using ps-PLD, considering that cheaper material could be used for obtaining such films.

using double pulses. In all three cases, a larger fraction of the total ablated volume is submitted to higher energy density. Concerning the applications, several perspectives appear. First, this shows that in the perspective of DLC deposition from a graphite target, the only sound advantage of going from picosecond interaction to femtosecond is the deposition speed. This should be taken into account for cost evaluation of an ultrafast PLD setup. Experiments to verify the conservation of important tribological and mechanical fs-PLD-produced DLC properties5,11 are necessary to ensure the interest of ps-PLD. It should be noted that the better adhesion of the films was shown to result from very energetic species (≈1 keV),50 which are greatly enhanced by the temporal shapes used here, especially by the long pulses.

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AUTHOR INFORMATION

Corresponding Author

*(F.G.) Telephone: + 33 4 77 91 58 01. E-mail: florence. [email protected]. Notes

The authors declare no competing financial interest.

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5. CONCLUSION This study features the use of temporal pulse shaping in the form of double pulses and long pulses in an fs-PLD setup for the generation of DLC films from graphite targets. It relies on the use of 1-D spatially resolved optical emission spectroscopy in order to characterize both the contents of the ablation plumes and their kinetics. We observed numerous species: C3, C2, C, C+, and C2+, spatially separated along the ejection axis (safe for radicals) and here sorted from the slowest to the fastest. These components were shown to keep their speed almost constant regardless of their quantities due to the gas flow properties of the plume, their kinetic energy only related to their ejection temperature also responsible for their excitation state (bonding, ionization). Increasing interaction duration on the picosecond time scale was found to decrease ablation depth and emission of radicals, while increasing the emission of monomers up to a certain optimal duration after which it also decreases. This is essentially due to the beginning of irradiation that changes the optical properties of the matter toward higher absorption, switching the localization of absorbed energy toward surface layers and thus modifying the temperature distribution of the ablated volume. Specifically, surface layers are overheated, enhancing ionization, while temperature decreases faster in the depth, reducing total ablation quantity. Long pulses up to several picoseconds are more adapted to overheating and ionization due to the fact that spreading energy over longer durations typically limits its penetration and ensures absorption close to the surface. Changing the quantity of species in each component while keeping their speed constant authorizes the modification of general plume kinetics. Raman analysis of DLC produced from graphite targets with classic femtosecond pulses and several DPs or LPs proved that no structural changes, especially sp3 contents, were induced by any observed kinetic modification. The films’ properties indeed confirm a prevalence of both the neutral C and radicals’ components over ionized species. The former is responsible for the subplantation process while the latter can explain the existence of high sp2-content layers over the films. In general, we elucidate the role of neutral C as the main tool for subplantation process, since their average kinetic energy is that expected from the literature for obtaining our sp3 contents. The use of double pulses and long pulses proved useful in reducing both nanoclusters and the number of micrometric fragments on the DLC films’ surface. In this study, we proved that a substantial modification over the plasma kinetics could be obtained by using temporal pulse shaping. However, we did not get significant structural

ACKNOWLEDGMENTS The authors would like to acknowledge Dr Jean-Philippe Colombier for discussions and insights on the femtosecond interaction theory. We also acknowledge the support of the French Region Rhône-Alpes via the CIBLE program.

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Control of the Graphite Femtosecond Ablation Plume Kinetics by

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