Thin Solid Films 590 (2015) 324–329

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Nanostructures formed on carbon-based materials with different levelsof crystallinity using oxygen plasma treatment

Tae-Jun Ko a,b, Wonjin Jo a, Heon Ju Lee a, Kyu Hwan Oh b, Myoung-Woon Moon a,⁎a Institute for Multidisciplinary Convergence of Matter, Korea Institute of Science and Technology, Seoul 136-791, Republic of Koreab Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea

⁎ Corresponding author at: Hwarangno 14-gil 5, Seongbof Korea. Tel.: +82 2 958 5487; fax: +82 2 958 5509.

E-mail address: mwmoon@kist.re.kr (M.-W. Moon).

http://dx.doi.org/10.1016/j.tsf.2015.02.0400040-6090/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Available online 20 February 2015

Keywords:NanostructureCarbon-based materialsOxygen plasmaMetal co-deposition

Nanostructure formation was explored for various carbon-based materials, such as diamond, carbon fiber, poly-ethylene terephthalate and poly (methylmethacrylate), which have different levels of crystallinity, ranging fromperfect crystal to polymeric amorphous. After treatment of oxygen plasma glow discharge, the nanostructures onthese carbon-based materials were found to evolve via preferential etching due to the co-deposition of metalelements sputtered from the metal cathode plate. Local islands or clusters formed by the metal co-depositionhave a low etching rate compared to pristine regions on each material, resulting in anisotropic patterns on thecarbon-based materials. This pattern formation mechanism was confirmed by covering the cathode orpreventing the co-deposition ofmetallic sourceswith a polymericmaterial. Regardless of the level of crystallinityof the carbon-based materials, no patterns were observed on the surfaces covered with the polymeric material,and the surfaces were uniformly etched. It was found that the materials with low crystallinity had a high etchingrate due to low carbon atom density, which thus easily formed high-aspect-ratio nanostructures for the sameplasma treatment duration.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Functional nanostructures on metals, semiconductors, or polymershave been continuously developed for various applications, such asphotovoltaic devices, biomaterials, optical lenses and catalysis [1–4].The configuration of nanostructures is considered as a governing factorfor functionalization of nanostructures, which depends on materialcharacteristics such as softness or hardness, crystalline or amorphous,or chemical reaction behavior. The nanostructure configuration can betailored by various fabrication methods, such as top-down approaches(e.g., photolithography, wet etching, molding, and stamping) orbottom-up approaches (e.g., oblique angle deposition, vapor-phase de-position, and ion beam or plasma treatment) [5–7]. Among thesemethods, ion beam or plasma treatment has attracted considerable at-tention due to the high throughput capability, eco-friendly nature, andlow temperature processing. Additionally, ion beam or plasma treat-ment can easily fabricate nanostructures of many different configura-tions, such as nanodot, nanobump, nanopillar, or nanohair, withoutany pre-patterning technique [7,8].

Recent studies have been conducted to reveal the mechanism ofnanostructure formation via the ion beam or plasma treatment onmany kinds of polymeric materials. It is insisted that as shown in

uk-gu, Seoul 136-791, Republic

Fig. 1, co-deposition of other elements originating from chamber wallor cathode plate in vacuum system may play a role as an etching maskfor reactive ion etching on the polymer surfaces [9,10]. Due to thenature of anisotropic etching of ion beam or plasma treatment, a poly-meric surface is etched by plasma or ion-beam exposure, except in theregions with etching inhibitors, which induces large differences in theetching rate. Thus, high-aspect-ratio nanostructures can be fabricated[10–12].

In addition to polymeric materials, diamond, one of the carbon-based materials, has been demonstrated to exhibit high-aspect-rationanoscale structures after reactive ion etching with air, or oxygenplasma [6,13]. Carbon fiber, a semi-crystalline carbon-based materialmade via carbonization of polymers, exhibited nanoscale features ofnanopillars or hairy structures formed by oxygen plasma treatment[7]. Thus, the pattern formation on carbon-based materials associatedwith oxygen plasma treatment may be understood by a similar mecha-nism as that for polymers.

In thiswork,we explored thepattern formation behavior for carbon-based materials induced by reactive oxygen plasma. We investigatedthe relationship between crystallinity and nanostructuring behaviorby performing surface etchings using glow discharge of oxygen gas oncarbon-based materials having different crystallinity levels rangingfrom perfect crystal to polymeric amorphous, such as diamond, carbonfiber, polyethylene terephthalate (PET) and poly (methylmethacrylate)(PMMA). It was found that co-deposition originated from a stainlesssteel cathode and formed local metallic compounds, on which oxygen

Fig. 1. A schematic for pattern formation of the preferential etching on a carbon-based material under oxygen plasma treatment.

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plasma etching was inhibited, while the other regions were rapidlyetched by the oxygen plasma. The morphological evolution of variouscarbon-basedmaterials was also studiedwith respect to oxygen plasmatreatment duration. The change in the chemical compound on the sur-face was discussed for each material. The later part of this work provid-ed a detailed discussion of pattern formation on carbon-basedmaterialsplaced on a cathode covered with and without polymeric materials ofpolystyrene (PS), which would block the metal sputtering from thestainless steel cathode. When the cathode plate was covered by poly-meric materials, regardless of the crystallinity of the carbon-basedmaterials, no patterns were observed on the surfaces and the surfacewas uniformly etched. Additionally, we analyzed the flux effect of co-deposited metal atoms on nanostructuring with respect to the distancebetween the metal source and PET under oxygen plasma treatment.

Fig. 2. SEMmicrographs of oxygenplasma-treateddiamond, carbonfiber, PET and PMMA for duthe diamond and the carbon fiber network.

2. Experimental section

2.1. Sample preparation

Oxygen plasma treatment was performed in glow discharge at aradio-frequency of 13.56 MHz. Four different carbon-based materialswere prepared: an artificial diamond (Iljin Diamond Co., Ltd., Rep.Korea) with a diameter of 700 μm, carbon paper (Ce Tech Co., Ltd.,Taiwan) in which the diameter of each carbonized polyacrylonitrilefiber is 7–8 μm, and flat sheet coupons of PET (LG Chemical, Rep. Korea)and PMMA (LG MMA, Rep. Korea) with sizes of 20 × 20 × 1.3 mm3. In avacuum chamber, samples were placed on a stainless steel cathode of160mm in diameter. Next, the chamberwas evacuated to a base pressureof lower than 0.1 Pa. For the oxygen plasma treatment, the duration was

rations of 3 to 60min. Insets are lowmagnification SEM images showing the entire shape of

Fig. 3. (a) The diameter and height and (b) the etching rate and aspect ratio measured ondiamond, carbon fiber, PET and PMMA surfaces for 30 min of oxygen plasma treatment.

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varied from 3 to 60min at a negative bias voltage of−400 V. The operat-ing pressure and flow rate of oxygen gas were kept at 2.67 Pa and20 sccm, respectively. To explore metal co-deposition from the plasmachamber environment, the stainless steel cathode was covered with aPS thin plate having a thickness of 1 mm, and the result was comparedwith that of the case preformed without covering. PS with the molecularformula of (C8H8)n was chosen because the materials were made of car-bon and hydrogen in their polymer chains, which would not affect theetching behavior of each carbon-basedmaterial. To investigate theflux ef-fect of metal co-deposition, PET samples were placed next to a stainlesssteel coupon on the cathode covered with the PS thin plate. Surface mor-phological evolution on PET surfaces was observed with respect to thedistance from the stainless steel coupon.

2.2. Surface analysis

The surface morphology of the materials was characterized using ascanning electron microscope (SEM, Nova NanoSEM 200, FEI) with10 kV of electron accelerating voltage. Prior to observation, 10-nm-thick Pt film was coated on the samples to prevent electron chargingon the samples during the SEM observation. The roughness of the sur-face was measured in five areas of 1 × 1 μm2 using an atomic force mi-croscope (AFM, XE-70, Park systems Co.) in the non-contact mode.Compositional analysis was performed using X-ray photo-electronspectroscopy (XPS, PHI 5800, ESCA System) to investigate the chemicalchange of the surfaces of the carbon-based materials by oxygen plasmatreatment. An Al Kα (1486.6 eV) X-ray sourcewas used as the excitationsource for XPS, and the X-ray source anode was operated at 250 W,10 kV, and 27 mA with a beam spot size of 400 μm × 400 μm. The XPSpeak position was calibrated using the C1s peak at 284.6 eV.

3. Results and discussion

3.1. Nanostructure formation on carbon-based materials

As introduced earlier, the co-deposition of the other element of thehard materials would be a crucial factor to cause a preferential etchingon carbon-based materials under ion beam or plasma etching. Dia-mond, carbon fiber, PET, and PMMA were etched with oxygen plasmaas those samples were placed on a stainless steel electrode in a plasmaglow discharge chamber. The surface morphologies after oxygen plas-ma treatment are shown in Fig. 2, showing nanostructures formed onall samples regardless of thematerial's crystalline nature, while the pat-tern configurations were varied from dot or straight pillars to curlyhairs.

As the oxygen plasma treatment duration increased from 3 to60 min, various patterns were observed: nanodots for diamond andcarbon fiber or nanopillars for polymers for 3 min duration weretransited into pillar or hairy structures. The sizes of the nanostruc-tures for 30 min oxygen plasma etching were measured, as shownin Fig. 3. The height or length of the nanostructure increased from132 nm to 2177 nm with the decrease of the material's crystallinity,while the diameter or width of the patterns was slightly different oneach material (Fig. 3a). Therefore, the aspect ratio of the nanostruc-ture, defined as the ratio of length over diameter, is dependent onthe crystallinity, which exhibits a strong correlation with the etchingrate of each material. This result shows that the materials with lowercrystallinity had the higher aspect ratio (Fig. 3b). Overall, the oxygenplasma treatment produced nanopatterns on the carbon-based ma-terials with relatively similar size in width or diameter, while theheight or length of the patterns is dependent on the material's prop-erties. It can be considered that the etching rate for crystalline carbonwith higher carbon density in the matrix is much lower than thosefor amorphous carbonaceous materials [14].

It was suggested that under oxygen plasma conditions, the etchingrate of polymericmaterials can be analyzed in terms of the total number

of carbon atoms in a monomer unit. The etching rate of the polymers,mainly composed of oxygen, carbon, and hydrogen, is determined bythe effective carbon content in a material under ion bombardment[15]. The empirical parameter of the Ohnishi number for etching rate(V) of polymeric materials is defined by

V∝NNC

where N is the total number of atoms in a monomer unit, and NC is thenumber of carbon atoms in a monomer unit [15]. As NC increases, theetching rate decreases. Although this equation was developed for or-ganic materials, the etching rate for diamond and carbon fiber can beconsidered with this equation because diamond and carbon fiber aremainly composed of carbon atoms. Therefore, as the etching ratewould depend on the number of carbon atoms in the unit, moreatoms or higher crystallinity would correspond to a lower etching rate.

3.2. Mechanism of nanopatterning

According to the above SEM images, the morphology of nanofeatureson the four different materials seems similar, except the variation inheight, which comes from the different etching rates, as mentioned be-fore. To examine the chemical composition of the oxygen plasma-etched surfaces, XPS analysis was performed to explore the change inatomic concentration on the carbon-based materials before and afterthe oxygenplasmaetching for 30min. As presented in Table 1, the spectraon the fourmaterials placed on a stainless steel cathode revealed theme-tallic compounds of Fe or Cr peaks after the oxygen plasma etching re-gardless of the substrate materials. Although the materials had differentatomic concentrations, the total metal elements were measured in therange of 3 to 6 at.% for the 30 min oxygen plasma treatment duration. Itwas reported that in the ion beam or plasma treatment process, the co-deposition of metallic materials could be originated from several sources

Table 1Atomic concentration (at.%) of pristine and oxygen plasma treated materials.

Material Plasmaduration(min)

Atomic concentration (at %)

C O Fe Cr

Diamond Pristine 91.2 8.8 – –30 76.0 20.9 2.93 0.09

CarbonFiber Pristine 95.8 4.2 – –30 64.6 31.4 3.21 0.77

PET Pristine 74.2 25.8 – –30 44.8 49.6 4.22 1.34

PMMA Pristine 73.4 26.6 – –30 48.5 46.0 4.25 1.25

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such as the chamber walls, ion beamwalls, or cathodes [16,17]. In partic-ular, the cathodeplate in the glowdischarge systemmaybe amain sourcefor co-deposited metallic elements [10]. It was reported that the fluxof metallic compounds has a critical value for initiation of anisotropicpattern formation on Si under Ar ion beam irradiation [18]. Recentworks reported that the co-deposition of the metal elements inducednano-patterns on Si surface, while surface smoothening was dominantwithout the metal co-deposition [16,17,19,20]. In the case of reactiveion etching on polymeric materials, simultaneous deposition of themetal element would induce non-etchable regions on the surface andcause etching anisotropy, resulting in columnar structures [10,21]. Duringthe oxygen plasma treatment, not only does reactive ion etching occurthat forms the volatile species but co-deposition of etching inhibitor si-multaneously occurs, which induces a large difference in the etchingrate [9,16,21,22]. In addition, once difference in height occurs due toetching inhibitor deposition, the metal elements are deposited on theupper part of the patterns by the shadoweffect. As a result, the nanostruc-tures with high-aspect-ratio are fabricated on polymers, as shown inFigs. 1 and 2 [10].

Fig. 4. (a) A schematic of the plasma treatment on a substrate placed on the cathode coveredwitwith the PS for the 3 min of oxygen plasma etching. Scale bars are 300 nm. (c) The roughnesscathode cover. (d) XPS analysis of 30 min of oxygen plasma-treated materials with or without

To prevent metal atoms from being sputtered out from the cathodeplate and co-depositing onto the substrate during oxygen plasmaetching, the stainless steel cathode plate was covered with a PS plateof 1-mm thickness, as shown in Fig. 4a. Because the PS plate coveredthe metal cathode, the sputtering by the oxygen plasma was notallowed and no metallic compound was expected to co-deposit ontothe carbon-based materials. Furthermore, as the PS is composed of car-bon and hydrogen, no hard inhibitor for etching resistance was formedon the surfaces. As a result, the PET surfaces exhibited dramatically dif-ferent surface morphologies according to the PS plate presence: high-aspect ratio patterned surface at the uncovered cathode and flat surfaceat the covered cathode (Fig. 4b). The root–mean–square roughness (Rq)wasmeasured by AFM for the four types of carbon-based material after3 min of oxygen plasma etching with or without cathode cover, asshown in Fig. 4c. In the case of the uncovered cathode, the Rq values ofdiamond, carbon fiber, PET, and PMMA were 1.6 nm, 6.1 nm, 5.8 nm,and 21.3 nm, respectively. However, in the case of the PS-covered cath-ode, theRq valueswere decreased to 1.5 nm, 2.3 nm, 1.2 nm, and3.1 nm,which is similar to the Rq values on the pristine condition measured as0.4 nm, 0.2 nm, 1.4 nm, and 0.9 nm, respectively.

After the 30min oxygen plasma etching, XPS analysiswas performedto investigate the effect of cathode covering with the PS by measuringand comparing the atomic concentration of the four carbon-basedmate-rials. Fe and Cr atomswere not detected on all materials on the cathodecoveredwith PS, while theywere detected on all of the original stainlesssteel surfaces in Fig. 4d. Two peak components in the Fe2p spectra with-in the range of 705–735 eV corresponding to Fe2p 1/2 and Fe2p 3/2were shown in the nanostructured surface of the carbon-based mate-rials on the stainless steel cathodewithout PS cover. Likewise, two com-ponents related to Cr2p 1/2 and Cr2p 3/2 located at 587.5 eV and577.3 eV, respectively, were only detected on the substrate placed onthe stainless steel cathode. However, in the case with the cathode cov-ered with PS, no metallic components were detected.

hout orwith a PS plate. (b) AFM images of PET surface on the cathode coveredwithout andof the pristine materials and the 3 min of plasma-treated materials with or without thethe cathode cover.

Fig. 5. (a) A schematic of the experimental design for investigating the dependence on distance from themetal source. (b–e) SEMmicrographs of oxygen plasma-treated PET surfaces fordifferent positions with respect to distance from metal sources: (b) 0 mm, (c) 10 mm, (d) 30 mm, and (e) 50 mm from the steel source. (f) An SEM micrograph of the oxygen plasma-treated PET surface on the cathode covered with PS. Scale bars are 200 nm.

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3.3. Dependency of metal concentration on nanopatterning

The pattern formation on Si was reported to have a critical valuewith re-depositedmetal ionflux, overwhich the nanopattern formationmayoccur [18,23,24]. To examine theflux effect of the co-depositedme-tallic compounds for plasma treatment, a PET samplewas placed next tothe stainless steel, a source for metallic compounds, which is similar tothe experiment in the previous report [17]. As shown in Fig. 5, after10min of oxygen plasma treatment, the PET surface nearby the stainlesssteel had nanopatterns with higher aspect ratios because of the highermetal flux. However, as the distance from the stainless steel increased,the aspect ratio of the PET nanopattern decreased because of the rela-tively lower supply of metallic elements. In the case of the surfaceslocated more than 50 mm (Fig. 5e) away from the metal source,nanopatterns were not evolved significantly.

We further performed the roughness and XPS analysis on severaldifferent positions frommetal source to obtain a quantitative measure-ment of the metal compounds, as shown in Fig. 6. It shows that theroughness of the nanostructured surfaces (Fig. 6a) wasmeasured to de-crease with the reduction of metallic compounds on the PET surface, as

shown in Fig. 6b. Similar to the results from Fig. 4d, the peaks associatedwith Cr and Fe components, which are themain components of stainlesssteel, were detected on all of the positions. The atomic concentrationwas tracedwith respect to the distance from themetal source, revealingthat both Cr and Fe were gradually reduced due to the lower density ofimpurity co-deposited for the far position from the stainless steel, asshown in Fig. 6b. In particular, the atomic concentration of Cr is foundto be lower than 0.5 at.%, at which the roughness (Rq) of thenanopatterns was found to reduce to a comparable range that is similarto those measured on the pristine PET or oxygen plasma-treated PETwith PS cover. Therefore, the total amount of metallic compoundswould affect the behavior of pattern formation and configuration onthe carbon-based materials [17].

4. Conclusion

Plasma induced pattern formation was explored on carbon-basedmaterials of diamond, carbon fiber, PET, and PMMA, which have differ-ent levels of crystallinity, ranging from perfect crystal to polymericamorphous. With oxygen plasma treatment, it was found that the

Fig. 6. (a) The roughness of nanostructures and (b) the Cr and Fe atomic ratiomeasured atthe different positions on oxygen plasma-treated PET surfaces with respect to distancefrom the metal source.

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nanostructures on the carbon-based materials were produced by co-deposition of metal elements coming from a stainless steel cathodesputtered by the oxygen plasma. This pattern formation mechanismwas confirmed by covering the cathode or preventing co-deposition ofmetallic sources with PS. It was found that regardless of the crystallinityfor carbon-based materials, no patterns were observed on the surfacescovered with PS. It was found that the material with low crystallinityhad a high etching rate due to the low carbon atom density, whichthus easily formed high-aspect-ratio nanostructures. In this work, wedemonstrated that the morphological configuration or density of thenanopatterns could be controlled through the pattern formation mech-anism under reactive oxygen plasma. Because the plasma treatment oncarbon-based materials is significantly applicable in various fieldsincluding superhydrophobic surfaces, anti-reflective surfaces, bio-adhesion controllable surfaces, microfluidic channels, and controllableadhesive surfaces, our findings could be quite helpful and valuable tofind the optimal treatment conditions and produce the most effectivesurfaces.

Acknowledgments

Thisworkwas supported by theKIST internal project (2E22790) andby the Ministry of Knowledge Economy (No. 10040003, MWM), theRepublic of Korea. This research was also supported by Ministry of

Culture, Sports and Tourism(MCST) and Korea Creative ContentAgency(KOCCA) in the Culture Technology(CT) Research & Develop-ment Program (R2014040016, LHJ).

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