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Diamond & Related Materials

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Laser-induced graphitized periodic surface structure formed on tetrahedralamorphous carbon films

Naoki Yasumarua,⁎, Eisuke Sentokua, Takafumi Toyab, Ryoya Tominagab, Toru Harigaib,Hirofumi Takikawab, Tsuyoshi Tanimotoc

aNational Institute of Technology, Fukui College, Sabae, Fukui 916-8507, Japanb Toyohashi University of Technology, Toyohashi, Aichi 441-8580, JapancNational Institute of Technology, Kochi College, Nankoku, Kochi 783-8508, Japan

A R T I C L E I N F O

Keywords:Femtosecond-laser processingLaser-induced periodic surface structureGraphitizationSwellingTetrahedral amorphous carbonNanocrystalline graphite

A B S T R A C T

Femtosecond laser-induced periodic surface structure (LIPSS), graphitization and swelling observed on ultra-hard, hydrogen-free tetrahedral amorphous carbon (ta-C) films are examined and compared with those on hy-drogenated amorphous carbon (a-C:H) films, nitride films, and glassy carbon plates. The threshold fluence forLIPSS formation on ta-C is approximately twice as high as that for other specimens, and the LIPSS period Λ nearthe threshold is very fine at ca. 80 nm. Λ gradually increases with increasing fluence, and rapidly increases to ca.600 nm at a high fluence. The ablation rate also increases rapidly at this fluence. In addition, ta-C and a-C:H aregraphitized by irradiation and expand in volume. The surface layer of ta-C film changes to nanocrystallinegraphite as the fluence increases and the crystallinity is improved; however, at higher fluence, the crystallinitydeteriorates suddenly similar to that at low fluence. At high fluence, the rapid increase in Λ and the ablation rate,and the sudden deterioration in crystallinity are determined as common phenomena for these disordered car-bons. LIPSS formation and swelling over a large area by scanned spot irradiation produces submicron height flathills with conductivity and surface functionality on the insulating surface.

1. Introduction

Amorphous carbon films with a significant fraction of sp3 bonds arereferred to as diamond-like carbon (DLC) films and have been used inthe industrial and medical-related fields in recent years [1–4]. DLC isclassified into four types according to high or low sp3 content andwhether or not hydrogen is included. Amorphous carbon that does notcontain hydrogen is classified into tetrahedral amorphous carbon with ahigh sp3 content (ta-C) and amorphous carbon with a low sp3 content (a-C), and those that contain hydrogen are classified into hydrogenated ta-C (ta-C:H) and hydrogenated a-C (a-C:H).

Among these, hydrogen-free ta-C films have extreme hardness andhigh density close to diamond, and exhibit excellent wear resistance,heat resistance and transparency. In the past, micron and/or sub-mi-cron droplets generated during deposition of such films used to be amajor problem, but with the use of advanced magnetic filtering sys-tems, these problems have been overcome, and the latest films aresmooth, dense, uniform and almost droplet-free [4]. These films havebeen recently used in the field of tribology such as for automobile parts,and their use has expanded and attracted significant attention [5,6].

Femtosecond (fs) lasers are capable of precision micromachiningwith extremely low thermal effects on difficult-to-process materialssuch as DLC, and fine laser-induced periodic surface structure (LIPSS)can be easily formed on the DLC surface [7,8]. The sp3 sites in DLC filmare also converted to sp2 sites by fs-laser irradiation, so that DLC ismodified to nanocrystalline graphite (nc-G) accompanied by an ex-pansion in volume [9–13]. Fs-LIPSS has been increasingly reported as asurface design technology to optimize the frictional, wetting, optical,biological, and reactivity properties of a specimen surface [8]. Gra-phitization and swelling occur at the same time in DLC; therefore, it isexpected to lead to new surface modification technologies.

In general, DLC films show high transmittance for infrared (IR)wavelengths [14–16], so research using ultraviolet (UV) nanosecond(ns) lasers has been reported to induce graphitization, swelling andimproved frictional properties in DLC films [17–21]. Nistor et al. [17]reported TEM observations that indicated diamond particles of 2–7 nmsize embedded in a low crystallinity graphite matrix of a-C:H films wereformed at higher fluence, and that nc-G grows from 2 nm to 4 nm withincreasing fluence. More recently, research on the effects of UV ns-laserirradiation on ta-C films with respect to graphitization and tribological

https://doi.org/10.1016/j.diamond.2020.107909Received 25 February 2020; Received in revised form 8 April 2020; Accepted 29 April 2020

⁎ Corresponding author.E-mail address: yasuma@fukui-nct.ac.jp (N. Yasumaru).

Diamond & Related Materials 107 (2020) 107909

Available online 01 May 20200925-9635/ © 2020 Elsevier B.V. All rights reserved.

T

properties has increased [22–25].The effects of fs-laser patterning on the frictional properties of ta-C

films have also been reported [26]. The topographical changes of a-C:Hfilms [27] and structural modification of ta-C films [28] by single-pulsefs-laser irradiation have been investigated. Laser surface texturing ofdiamond-like nanocomposite (DLN) film (a-C:H:Si:O) utilizing IR andvisible fs-lasers has also been reported to improve the tribologicalproperties of laser patterned films at both the micro and macroscale[29–31]. Research on graphitization and improvement of the frictionalproperties of DLC films by laser irradiation is increasing; however, thereare few detailed reports on graphitized LIPSS, especially for ta-C.

Graphitized LIPSS was formed on a droplet-free ta-C film with ex-cellent smoothness by fs-laser irradiation, and the associated swellingphenomena were analyzed in detail. The obtained irradiation resultswere compared with those for relatively soft a-C:H films and nitridefilms of TiN and CrN, and glassy carbon (GC) plates. Nitride films wereexamined to account for changes in the irradiated surface of hard thinfilms without such swelling phenomenon. GC is a kind of nc-G and hasbeen applied to electrodes and crucibles because of its conductivity,chemical inertness and heat resistance. GC plates were utilized to elu-cidate the properties of ta-C surface modified to nc-G by laser irradia-tion.

A planar scanned spot irradiation technology is then used to showthat graphitized submicron height flat hills with very fine LIPSS andproperties such as conductivity can be easily processed on a ta-C filmsurface.

2. Experimental procedure

Hydrogen-free ta-C films with very few droplets were prepared on2 mm thick super-hard alloy (WC-Co) substrates using a T-shaped fil-tered arc deposition system [32–34]. Hard thin films of a-C:H, TiN andCrN were deposited on 2 mm thick commercially available austenitictype 304 stainless steel plates using an unbalanced magnetron sput-tering system. The physical properties of the ta-C, a-C:H, TiN, CrN filmsand GC plates used for the present experiments are summarized inTable 1, where the surface roughness Ra was measured with a precisionsurface profiler (Taylor Hobson Ltd., Form Talysurf PGI800S). Thesurface hardness was measured by nano-indentation (Elionix Inc., ENT-1100a) with an ultralight load of 5 mN using a Berkovich diamond tip.The ta-C film density (ρ) was 3.0 g/cm3, as determined using an X-rayreflectometer. The ρ value of a-C:H was 2.2 g/cm3, which was estimatedfrom the relationship between the hardness and density [34,35]. A2 mm thick surface-polished GC substrate (Tokai Carbon Co., Ltd.) withρ = 1.5 g/cm3 was also utilized.

These specimens were irradiated in air with linearly polarized,800 nm, 180 fs laser pulses from a Ti:sapphire chirped-pulse amplifi-cation system operated at a repetition rate of 1 kHz. The fs laser pulseswith pulse energies of 150–800 μJ were focused using a 200 cm focal-length parabolic mirror. The Gaussian-like beam radius w0 (1/e2) at thespecimen surface was measured to be ca. 230 μm using a laser beamprofiler. The average laser fluence F on the target was estimated to beF = 0.06–0.32 J/cm2, as described in [36]. All irradiation processeswere performed under normal incidence.

The target plate was mounted on a precise X-Y stage. In the firstexperiment, the stage was fixed and a number of laser pulses(N = 10–500) was superimposed on the plate at a repetition rate of

1 kHz (fixed spot irradiation). In the second experiment, the target platewas continuously translated at a constant speed of 1.6 μm/pulse, wherethe parallel scan at a period of 120 μm was performed to ablate thesurface over an area of 2 × 5 mm2 (scanned spot irradiation). Thismethod of laser scanning has been fully described in our previous report[12]. Approximately 150 shots of fs laser pulses were effectively su-perimposed on the target point during scanned spot irradiation, as es-timated from the scan conditions. In the case of a thin film, irradiationconditions were set so that the film thickness of the irradiated areawould not be too thin due to ablation.

Optical microscopy, field-emission scanning electron microscopy(FE-SEM; JEOL Ltd., JSM-7001FA) and scanning probe microscopy(SPM; Seiko Instruments Inc., Nanopics 2100) were used to examine themorphological changes of the irradiated specimens. The mean period Λof LIPSS formed on the specimen surface was determined from the peakposition of the power spectrum obtained by the Fourier transform of anSEM image of the LIPSS using image processing software (ImageMetrology A/S, SPIP™) [37]. The bonding structures of the ablated ta-C,a-C:H and GC were analyzed using Raman spectroscopy (JASCO Cor-poration, NRS-7100) using a 531.92 nm Nd:YVO4 based solid state cwlaser with a spot size of ca. 1 μm on the specimen surface. The swellingheight or ablation depth of ta-C and a-C:H produced by scanned spotirradiation was measured with a precision surface profiler.

3. Results and discussion

3.1. Surface modification by fixed spot irradiation

Amorphous carbon films of ta-C and a-C:H, nitride films of TiN andCrN, and GC plates were irradiated with fs-laser pulses with a laserfluence of F = 0.06–0.32 J/cm2 and a pulse number of N = 10–500.The central part of the laser-irradiated spot was observed using SEM.

Fig. 1 shows SEM micrographs of ta-C surfaces irradiated by su-perimposed fs-laser pulses at F= 0.16, 0.20, 0.24, 0.28 and 0.32 J/cm2,where N = 500 for (a)–(d) and 10 for (e). The arrow indicates thepolarization direction of the fs-laser pulses. This figure shows that theLIPSS, which is elongated in the direction perpendicular to the polar-ization direction of the laser, was successfully formed and the thresholdfluence Fth for LIPSS formation was considered to be around 0.16 J/cm2; the LIPSS period Λ increases as the fluence increases.

The dependence of the period Λ on the pulse number N at F = 0.20,0.24, 0.28 and 0.32 J/cm2 was obtained as shown in Fig. 2. These re-sults indicate that the minimum value of Λ is ca. 80 nm and 1/10 of thelaser wavelength λ. Under the constant fluence condition, Λ graduallydecreased with the number of pulses, and the rate of decrease increasedwith the fluence. Furthermore, from (a) to (c), the period Λ graduallyincreased with the fluence, whereas in (d), Λ suddenly increased to ca.600 nm.

Fig. 3 shows SEM micrographs of a-C:H surfaces irradiated by su-perimposed fs-laser pulses at F= 0.06, 0.08, 0.12, 0.16 and 0.20 J/cm2,where N = 500 for (a)–(c), 50 for (d) and 10 for (e). LIPSS was alsoformed in a-C:H, and Λ gradually increased with increasing fluence, andincreased rapidly in (e). Although the change in the period Λ was si-milar to that for ta-C, the shape of the LIPSS for ta-C was slightlyrounder. In addition, the threshold fluence Fth for the LIPSS formationof a-C:H decreased to around 0.07 J/cm2, which is approximately halfof that for ta-C.

LIPSS formed on GC plate, which is a type of nc-G, was observed tocompare LIPSS formed between amorphous carbon and disorderedcarbon. Fig. 4 shows SEM micrographs of GC surfaces irradiated bysuperimposed fs-laser pulses at F = 0.10, 0.12, 0.16, 0.20 and 0.24 J/cm2 with N = 500. LIPSS was also formed in GC, and Λ gradually in-creased with increasing fluence, and increased rapidly in (d). Thechange in the period Λ was the similar to that for ta-C or a-C:H, and thethreshold fluence Fth for the LIPSS formation of GC was around 0.08 J/cm2, which is approximately half of that for ta-C.

Table 1Physical properties of ta-C, a-C:H, TiN, CrN thin films and GC plates.

ta-C a-C:H TiN CrN GC

Thickness [μm] 0.9 0.8 2.0 3.0 –Roughness, Ra [nm] 2.3 4.3 6.0 3.3 2.3Hardness, HIT [GPa] 65 25 25 18 4.6

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Nitride films of TiN and CrN were also irradiated under the sameconditions, and similar LIPSS was formed [9,13]. The threshold fluenceFth of LIPSS formation was 0.08 J/cm2 for TiN and 0.06 J/cm2 for CrN,which was almost the same as that for a-C:H and GC. The reason why Fthfor ta-C is approximately twice as large as that for the other specimensis considered to be its small extinction coefficient and properties similarto diamond with many sp3 bonds. Fig. 5 shows the change of the LIPSSperiod Λ formed in these four thin films and GC with respect to the

fluence, where the number of pulses N is the same as that used inFigs. 1, 3 and 4 for ta-C, a-C:H and GC, respectively. For TiN and CrN,N = 500 was used at lower fluence, and N = 200 and 100 were used athigher fluence of 0.16 and 0.2 J/cm2, respectively. The fluence range ofLIPSS formation observed for ta-C is approximately twice as high as thatfor a-C:H and GC, although the overall tendency for the change of ta-C,a-C:H and GC was almost the same, i.e., the minimum values of Λ weresmall at ca. 80 nm, 90 and 50 nm, respectively, and the increase withincreasing fluence was small at first, but then increased rapidly to ca.600 nm at high fluence. LIPSS was observed for TiN and CrN in thesame fluence range as that for a-C:H and GC, and Λ gradually increasedwith fluence from (1/5–1/4) λ; however, no sudden increase of Λ aswith ta-C, a-C:H and GC at high fluence was observed.

Recently, in order to optimize the optical properties of the diamondsurface, such as black diamond, research has been conducted to formLIPSS on a diamond thin film or crystal using a femtosecond laser[38,39]. Since the diamond is transparent and strongly bonded onlywith sp3, LIPSS is processed at a higher Fth than that of ta-C. It is re-ported that LIPSS of Λ = 200 nm is formed on n-doped diamond at1.3 J/cm2 near the ablation threshold, and LIPSS of Λ = 50 nm isformed at 0.5 J/cm2 near the graphitization threshold [38], which issimilar to the LIPSS on ta-C at low fluence.

Next, the crater profile formed by fs-laser pulses was measured bySPM and the cross-sectional shape was obtained. Fig. 6 shows cross-sectional curves for craters irradiated by N = 200 pulses at F = 0.28 J/cm2 for ta-C, 0.12 J/cm2 for a-C:H, TiN and CrN, and by N= 100 pulsesat 0.2 J/cm2 for GC. TiN, CrN and GC were excavated at processing

(a)

(b)

(c)

(d)

(e)

Fig. 1. SEM images of ta-C surfaces irradiated by superimposed fs-laser pulsesat F = (a) 0.16, (b) 0.20, (c) 0.24, (d) 0.28 and (e) 0.32 J/cm2. N = 500 for(a)–(d) and 10 for (e). The arrow indicates the polarization direction of the fs-laser pulses.

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600

,doirePSSPIL

Λ[n

m]

Laser Shot Number

(a) (b) (c) (d)

Fig. 2. Period Λ of LIPSS formed on ta-C as a function of the laser shot number(N) at F = (a) 0.20, (b) 0.24, (c) 0.28 and (d) 0.32 J/cm2.

Fig. 3. SEM images of a-C:H surfaces irradiated by superimposed fs-laser pulsesat F = (a) 0.06, (b) 0.08, (c) 0.12, (d) 0.16 and (e) 0.20 J/cm2. N = 500 for(a)–(c), 50 for (d) and 10 for (e).

N. Yasumaru, et al. Diamond & Related Materials 107 (2020) 107909

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depths of ca. 600 nm, 1300 nm and 1700 nm at the deepest part, re-spectively, whereas ta-C and a-C:H were swollen to ca. 200 nm and100 nm, respectively. Fig. 6(e) shows that the ablation rate of GC in-creased sharply above 0.2 J/cm2, and the processed crater had a well-shaped profile where the diameter did not change significantly in thedepth direction. Although this may be related to laser-induced spalla-tion [20,21], crater profiles processed at high fluence for ta-C and a-C:Hhad regular rounded hole shapes, as shown in Fig. 6(c) and (d).

The processing depth or height was measured in the same way forthe cross-sectional curves of all irradiation craters, and the ablation ratewas calculated by dividing by the number of irradiation pulses. Fig. 7shows the ablation rates obtained from the profiles of craters formed onta-C, a-C:H, TiN, CrN and GC, measured as a function of the laser flu-ence. The negative ablation rate indicates a swelling rate, and swellingis observed to occur around the threshold fluence Fth in all the speci-mens. Swelling for the nitrides and GC is considered to be caused by therelease of compressive residual stress or surface oxidation. Compared tota-C and a-C:H, the amount of swelling for the nitrides was small, andthe fluence range in which swelling occurred was also small. Amor-phous carbon films of ta-C and a-C:H are known to swell with de-creasing density because the sp3 bonds in the film become sp2 bonds andgraphitization occurs [9–13,17–31]. Therefore, swelling occurred overa wide range of fluence below 0.12 J/cm2 for a-C:H and below 0.28 J/cm2 for ta-C, and the amount of swelling increased with the fluence forta-C. In particular, ta-C, which has a large fraction of sp3 bonds, showedthe largest swelling with a maximum swelling rate of 0.9 nm/pulse. Theswelling rate for a-C:H was 0.3 nm/pulse, which was approximately 1/3that of ta-C. After passing the region where swelling occurs at the low

Fig. 4. SEM images of GC surfaces irradiated by superimposed fs-laser pulses atF = (a) 0.10, (b) 0.12, (c) 0.16, (d) 0.20 and (e) 0.24 J/cm2 with N = 500.

0

100

200

300

400

500

600

700

0.05 0.1 0.15 0.2 0.25 0.3 0.35

LIPS

S Pe

riod,

Λ[n

m]

Laser Fluence [J/cm2]

(a) (b) (c) (d) (e)

Fig. 5. Period Λ of LIPSS formed on (a) ta-C, (b) a-C:H, (c) TiN, (d) CrN and (e)GC measured as a function of the laser fluence.

0

500

1000

1500

2000

0 100 200 300 400

]mn[

htpeDl acitre V

Horizontal Distance [μm]

(a)

(b)

(c)

(d)

(e)

Fig. 6. Crater profiles formed on (a) ta-C, (b) a-C:H, (c) TiN, (d) CrN and (e) GCablated by superimposed fs-laser pulses with N = 200 at (a) F = 0.28 and (b, c,d) 0.12 J/cm2, and with (e) N = 100 at F = 0.20 J/cm2.

-5

0

5

10

15

20

25

0.05

]eslup/mn[

etaR

no italbA

Laser Fluence [J/cm2]

(a)

(b)

(c)

(d)

(e)

0.2 0.30.150.1 0.4

Fig. 7. Ablation rate obtained from the profiles of craters formed on (a) ta-C,(b) a-C:H, (c) TiN, (d) CrN and (e) GC measured as a function of the laserfluence.

N. Yasumaru, et al. Diamond & Related Materials 107 (2020) 107909

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fluence side, the ablation rate increased with fluence. As described forthe change of the LIPSS period Λ, a-C:H, TiN, and CrN increased in thesame fluence range; however, ta-C changed with fluence approximatelytwice as much. The dependence of the ablation rate on the fluence isgenerally known to be linear with an increase to the right in the fluencesemi-logarithm graph, and the ablation threshold can be obtained fromthe x-axis intercept of this straight line [40]. In this experiment, swel-ling occurred on the low fluence side, so that the ablation thresholdcould not be obtained by this method.

Raman spectroscopic measurement was performed on the irradiatedsurface to investigate the graphitization and variation of the crystal-linity by laser irradiation. Fig. 8 shows Raman spectra of the non-ir-radiated ta-C film and those irradiated by superimposed fs-laser pulsesat F = 0.16, 0.20, 0.28 and 0.32 J/cm2, where N = 500 for 8(b)–(d)and 10 for 8(e). In the Raman spectrum for disordered graphite, twodistinct peaks are observed; the D peak near 1350 cm−1 and the G peaknear 1580–1600 cm−1, and the intensity ratio I(D)/I(G), the peak po-sition and the full width at half maximum (FWHM) of these two peaksvary depending on the states of the disordered carbon [2,3]. Graphiti-zation proceeds by fs-laser irradiation, and the spectrum changes from atypical ta-C spectrum which has very small I(D)/I(G) value and a largeFWHM to a spectrum with two D and G peaks. The crystallinity and theclustering of the sp2 phase improves as the fluence increases, and thespectrum changes into two sharp peaks, which indicates that ta-C ismodified into typical nanocrystal graphite (nc-G) where the spectrumshows a high I(D)/I(G) ratio, a narrow FWHM and an increase of theRaman shift of the G peak, as shown in Fig. 8(c) and (d). These spectralchanges are considered to be caused by the conversion from sp3 to sp2

sites and an increase of the sp2 cluster size due to the laser irradiation[2,3]. However, at high fluence (Fig. 8(e)), the I(D)/I(G) ratio decreasesand the FWHM increases inversely, which resembles the spectrum forlow fluence in Fig. 8(b).

Fig. 9 shows Raman spectra of the non-irradiated a-C:H film andthose irradiated by superimposed fs-laser pulses at F= 0.07, 0.08, 0.12,0.16 and 0.2 J/cm2, where N = 500 for 9(b)–(d), 50 for 9(e) and 10 for9(f). As with ta-C, graphitization progresses by laser irradiation from

the typical a-C:H spectrum with a small I(D)/I(G) ratio and largeFWHM, and its crystallinity improves with increasing fluence and be-comes modified to nc-G. However, at high fluence (Fig. 9(e) and (f)),the I(D)/I(G) ratio decreases and the FWHM increases, which resemblesthe spectrum for low fluence in Fig. 9(b). A comparison of the spectra of(c) and (d) in Figs. 8 and 9, which show two sharp peaks, indicates theFWHM of nc-G formed in ta-C is narrower and the crystallinity is fur-ther improved.

From these results, it may be considered that the decrease in thecrystallinity indicated by the Raman spectra for disordered carbonobserved at high fluence irradiation is a common phenomenon ob-served during fs-laser irradiation. Furthermore, it should be noted thatat high fluence, the LIPSS period Λ increases significantly to ca. 600 nmand the ablation rate rapidly increases, as shown in Figs. 5 and 7, re-spectively.

Considering the deterioration of the crystallinity of the ta-C and a-C:H films observed at high fluence, laser irradiation of the GC plate wasinvestigated to determine whether the same phenomenon occurred.Fig. 10 shows Raman spectra of non-irradiated GC plate and those ir-radiated by superimposed fs-laser pulses at F = 0.16, 0.2 and 0.24 J/cm2 with N= 500. Fig. 10(a) shows a typical GC spectrum with a high I(D)/I(G) ratio and narrow FWHM. The spectrum in Fig. 10(b) shows adecrease of the I(D)/I(G) ratio that is very close to that shown inFig. 8(d) for ta-C. At higher fluences (Fig. 10(c) and (d)), the crystal-linity is significantly deteriorated, which is evidenced by a large de-crease in the I(D)/I(G) ratio and an increase in the FWHM. Thesespectra are similar to those for ta-C and a-C:H observed at high fluence.McCulloch et al. [41,42] reported that a similar phenomenon was ob-served in the Raman spectra for an experimental study on the ion im-plantation of GC, i.e., the amorphization of GC progresses with an in-crease in the ion dose, which is accompanied by a decrease in the I(D)/I(G) ratio and an increase in the FWHM. It may be considered that GC,

(a)

(b)

(c)

(d)

(e)GD

1100 1200 1300 1400 1500 1600 1700 1800Raman Shift [cm-1]

]stinu.bra [yti sne tnI

Fig. 8. Raman spectra for (a) non-irradiated ta-C film and ta-C film irradiatedby superimposed fs-laser pulses at F = (b) 0.16, (c) 0.20, (d) 0.28 and (e)0.32 J/cm2. N = 500 for (b)–(d) and 10 for (e). The red lines correspond to thedata of spectra fitting by the sum of Lorentz line for the D peak and BWF line forthe G peak. (For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

(a)

(b)

(c)

(d)

(e)

(f)

GD

1100 1200 1300 1400 1500 1600 1700 1800Raman Shift [cm-1]

]stinu.bra[ytisnetnI

Fig. 9. Raman spectra for (a) non-irradiated a-C:H film and a-C:H film irra-diated by superimposed fs-laser pulses at F = (b) 0.07, (c) 0.08, (d) 0.12, (e)0.16 and (f) 0.20 J/cm2. N = 500 for (b)–(d), 50 for (e) and 10 for (f). The redlines correspond to the data of spectra fitting. (For interpretation of the refer-ences to colour in this figure legend, the reader is referred to the web version ofthis article.)

N. Yasumaru, et al. Diamond & Related Materials 107 (2020) 107909

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ta-C and a-C:H irradiated by fs-laser pulses at high fluence undergo thesame phenomena as GC caused by high dose ion implantation, whichresults in reduced crystallinity or amorphization.

To analyze the D and G peaks of the Raman spectra more quanti-tatively, the intensity ratio I(D)/I(G), the peak position and the peakwidth (FWHM) for these two peaks were obtained from a conventionalfitting of the Raman spectra with a Lorentzian line for the D peak and aBreit-Wigner-Fano (BWF) line for the G peak [2,3]. The fitting results ofthe experimental spectra are shown by red lines in Figs. 8–10. Fig. 11shows the change in the I(D)/I(G) ratio derived from Raman spectra for(a) ta-C films, (b) a-C:H films and (c) GC plates as a function of the laserfluence. The intensity ratios for ta-C and a-C:H increased from smallvalues on the unirradiated surface to values between 1.2 and 1.4 withincreasing laser fluence. The ratios then dropped to 0.8–1.0 at highfluence. On the other hand, the intensity ratio for GC decreased gra-dually from 1.9 on the unirradiated surface to 1.2 with increasing flu-ence, and then decreased significantly from a high fluence of 0.2 J/cm2

to ca. 0.9, which is almost the same as that for ta-C and a-C:H.Fig. 12 shows the G peak width (FWHM) derived from the Raman

spectra for (a) ta-C films, (b) a-C:H films and (c) GC plates as a functionof the laser fluence. The peak widths for ta-C and a-C:H decreased from

280 cm−1 and 210 cm−1 on the unirradiated surface with increasingfluence to 60 cm−1 and 80 cm−1, respectively, and then increased to200 cm−1 and 110 cm−1 at high fluence. On the other hand, the peakwidth of the non-irradiated GC surface was 60 cm−1, which is near theminimum value for the irradiated ta-C or a-C:H surfaces. The peakwidth for GC changed little at low fluence, but was significantly in-creased from that at 0.2 J/cm2 to>130 cm−1, which is almost thesame as those for ta-C and a-C:H.

As described in detail in Refs. [2, 3, 43], the graphitic cluster size(La) can be estimated from the I(D)/I(G) ratio. This model shows thatthe relationship between the I(D)/I(G) ratio and La differs depending onwhether La is larger than 2 nm or smaller, and the following equationshold.

> − =L LFor 2 nm in nc G, I(D)/I(G) C(λ)/ ,a a (1)

< = ′L LFor 2 nm in amorphous carbon, I(D)/I(G) C (λ)a a2 (2)

Using C (λ) = 4.4 nm and C′ (λ) = 0.55 nm−2 for the excitationwavelength λ = 514.5 nm in Raman spectroscopy as described in Ref.[2], La for laser-irradiated ta-C and a-C:H indicate typical Ramanspectra for nc-G with a high I(D)/I(G) ratio, and the narrow peak widthcould be estimated to be 3 to 4 nm by the application of Eq. (1). On theother hand, for ta-C and a-C:H irradiated at low fluence with a rela-tively low I(D)/I(G) ratio and a wide peak width, La was estimated to befrom 1 to 1.5 nm from Eq. (2). Ta-C, a-C:H and GC irradiated at highfluence also have similar Raman spectra with low crystallinity; there-fore, La for these specimens may be estimated to be from 1 to 1.5 nmusing Eq. (2).

3.2. Surface modification by scanned spot irradiation

Scanned spot irradiation was performed for thin films of ta-C and a-C:H in a 2 × 5 mm area, and the irradiated surface was observed usingSEM. Fig. 13 shows SEM images of the ta-C surface irradiated in aplanar fashion at a fluence of F = 0.20, 0.24 and 0.32 J/cm2. A gran-ular LIPSS elongated in the plane perpendicular to the polarizationdirection of the laser was formed in the same manner as for fixed spotirradiation, and the LIPSS period Λ increased from 80 nm to 340 nmwith increasing fluence. The reason why the period Λ at F=0.32 J/cm2

in Fig. 13(c) is narrower than that generated by fixed irradiation asshown in Fig. 1(e) may be related to the irradiation while moving thelaser spot, i.e., while the laser spot passes through a certain point withthis method, the laser intensity at that point increases and then

(a)(b)

(c)

(d)GD

1100 1200 1300 1400 1500 1600 1700 1800Raman Shift [cm-1]

]stinu.bra[ytisnetnI

Fig. 10. Raman spectra for (a) non-irradiated GC plate and GC plate irradiatedby superimposed fs-laser pulses at F = (b) 0.16, (c) 0.20 and (d) 0.24 J/cm2

with N = 500. The red lines correspond to the data of spectra fitting. (Forinterpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.1 0.2 0.3 0.4

I(D)/I

(G)

Fluence [J/cm2]

(a) (b)

(c) (d)

Fig. 11. I(D)/I(G) ratio derived from the Raman spectra for (a) ta-C, (b) a-C:Hand (c) GC irradiated by fixed spot and (d) ta-C irradiated by scanned spot as afunction of the laser fluence.

0

50

100

150

200

250

300

0 0.1 0.2 0.3 0.4

FWH

Mof

G P

eak

[cm

-1]

Fluence [J/cm2]

(a) (b)

(c) (d)

Fig. 12. FWHM of the G peak derived from the Raman spectra for (a) ta-C, (b)a-C:H and (c) GC irradiated by fixed spot and (d) ta-C irradiated by scannedspot as a function of the laser fluence.

N. Yasumaru, et al. Diamond & Related Materials 107 (2020) 107909

6

decreases.To investigate the undulation on the surface of the rectangular ir-

radiated area (2 × 5 mm) by scanned spot irradiation, the surfaceprofile was measured by scanning parallel to the short 2 mm side forthis area using a precision surface profiler. Fig. 14 shows superimposedsurface profiles measured parallel to the short side (2 mm) for a rec-tangular area irradiated at fluence of F = 0.20, 0.24 and 0.32 J/cm2. InFig. 14(a), surface waviness was formed with a parallel period of120 nm for each scan of the laser pulse, and the irradiated area swelledby ca. 50 nm over approximately 2 mm. In Fig. 14(b), the 120 nmperiod waviness disappeared, and a hill with an almost flat height of ca.130 nm was formed. Fine LIPSS was uniformly formed on the sub-micron flat hill, of which the surface area is increased by LIPSS andmodified to nc-G with functionality such as conductivity. Therefore,this nanostructured and graphitized hill is expected to expand the ap-plication fields of ta-C. The planar irradiated area of a-C:H swelled inthe same manner, and the maximum height was ca. 75 nm, which isapproximately 60% of that for ta-C. At high fluence (Fig. 14(c)), thepart swollen due to graphitization and the part removed due to ablationare periodically formed every 120 nm of the scan period due to theintensity distribution of the laser pulse. Therefore, the surface wavinesswas produced in which undulations of several hundreds of nanometersoccurred periodically in the swollen part and the removed part. Thisremoval involved laser-induced spallation [20,21], and led to a non-

uniform removal-processed surface.Raman spectroscopic measurements of ta-C surface irradiated at

F = 0.16, 0.20, 0.24 and 0.32 J/cm2 were performed to clarify thegraphitization phenomenon by scanned spot irradiation. From the samecurve fitting of the D and G peaks, and the changes in the I(D)/I(G) ratioand the G peak width (FWHM) are shown in Figs. 11(d) and 12(d),respectively. The peak ratio for the ta-C surface modified by scannedspot irradiation was ca. 30% smaller than that modified by fixed spotirradiation; however, the tendency for the change in the peak ratio andpeak width with respect to the fluence was almost the same.

4. Conclusion

The graphitized LIPSS and swelling formed on ta-C film by fs-laserpulses was investigated and comparing with those on a-C:H, TiN, andCrN films and on GC plates. The LIPSS on ta-C has a fine period Λ of ca.80 nm (1/10 λ) near the threshold fluence Fth for LIPSS formation,which increased rapidly to ca. 600 nm at high fluence. The Fth for ta-Cwas ca. 0.16 J/cm2, which was approximately twice of that for theother specimens. The density decreased and swelling occurred due tographitization of the laser-irradiated amorphous carbon. The amount ofswelling was higher for ta-C and was ca. 3 times that for a-C:H. Laser-irradiated ta-C and a-C:H were changed to nc-G with increasing fluence,but nc-G modified from ta-C had better crystallinity. However, thecrystallinity suddenly deteriorated at high fluence and the same phe-nomenon was also observed for the other disordered carbon a-C:H filmsand GC plates. Furthermore, at high fluence, the ablation rates for ta-C,a-C:H and GC rapidly increased. It should be noted that the rapid in-crease in the LIPSS period Λ and the ablation rate, and the suddendeterioration in crystallinity observed at high fluence were commonphenomena for these disordered carbons.

Further, we demonstrated that almost flat nanostructured graphi-tized hills with a maximum height of ca. 130 nm could be formed on ta-C. These micro hills are expected to have the potential to introduce newfunctionality to the ta-C surface.

CRediT authorship contribution statement

Naoki Yasumaru:Writing - original draft, Conceptualization,Methodology.Eisuke Sentoku:Investigation, Validation, Formalanalysis.Takafumi Toya:Investigation, Data curation.RyoyaTominaga:Investigation, Data curation.Toru Harigai:Investigation,Validation, Resources.Hirofumi Takikawa:Supervision, Fundingacquisition.Tsuyoshi Tanimoto:Validation, Resources.

Declaration of competing interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

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