Large Area Extreme-UV Lithography of Graphene Oxide via SpatiallyResolved PhotoreductionS. Prezioso,*,†,§ F. Perrozzi,† M. Donarelli,†,§ F. Bisti,† S. Santucci,†,§ L. Palladino,†,§ M. Nardone,†

E. Treossi,‡ V. Palermo,‡ and L. Ottaviano†,§

†Dipartimento di Fisica, Universita dell’Aquila, Via Vetoio, 67100, L’Aquila, Italy‡CNR-ISOF, Via Gobetti 101, 40129 Bologna, Italy§Dipartimento di Fisica, Universita dell’Aquila, g.c. LNGS-INFN, 67100, L’Aquila, Italy

*S Supporting Information

ABSTRACT: The ability to pattern graphene over large areaswith nanometer resolution is the current request for nano-device fabrication at the industrial scale. Existing methods donot match high throughput with nanometer resolution. Wepropose a high-throughput resistless extreme-UV (EUV)photolithographic approach operating with sub-micrometerresolution on large area (∼10 mm2) graphene oxide (GO)films via spatially resolved photoreduction. The efficiency ofEUV photoreduction is tested with 46.9 nm coherent light produced by a table top capillary discharge plasma source. Irradiatedsamples are studied by X-ray photoemission spectroscopy (XPS) and micro-Raman Spectroscopy (μRS). XPS data show that 200mJ/cm2 EUV dose produces, onto pristine GO, a 6% increase of sp2 carbon bonds and a 20% decrease of C−O bonds. μRS datademonstrate a photoreduction efficiency 2 orders of magnitude higher than the one reported in the literature for UV-assistedphotoreduction. GO patterning is obtained modulating the EUV dose with a Lloyd’s interferometer. The lithographic featuresconsist of GO stripes with modulated reduction degree. Such modulation is investigated and demonstrated by μRS on patternswith 2 μm periodicity.

■ INTRODUCTIONGraphene, the two-dimensional hexagonal allotrope of carbonisolated in 2004 by Geim and Novoselov,1,2 has rapidlyachieved increasing consideration in materials science andcondensed matter physics because of the huge number ofpotential applications allowed by its peculiar properties.3

Important branches of technology like electronics oroptoelectronics, dominated for decades by silicon, are todayopening the doors to graphene. The rising carbon-basedtechnology is proving to be a valid alternative to the silicon-based one in an era of challenges like the extrememiniaturization of devices and eco-sustainability. The fabrica-tion of graphene-based nanostructures has been demonstratedwith several approaches. Conventional electron beam lithog-raphy (EBL) combined with plasma etching has beensuccessfully used to fabricate carbon-based nanodots,4−6

nanorings,7,8 and nanoribbons9,10 from graphene sheets. Gate-defined graphene quantum dots have been isolated from EBLpatterned nanoribbons.11 Graphene quantum dots have beenobtained also by current assisted rupture of suspendedgraphene under proper cryogenic conditions.12 Graphenenanorings have been patterned by local anodic oxidation(LAO).13 Graphene nanoribbons have been derived withchemical ways,14 synthesized by chemical vapor deposition-(CVD),15 cut by crystallographic etching,16 patterned byLAO,13,17 fabricated through atomically resolved bottom-up

approaches,18,19 or stretched out in different ways by chemicalunzipping of carbon nanotubes.20−23

Despite the excellent ability of scientists to fabricate isolatedgraphene-based nanostructures, the way these objects can beassembled into miniaturized electronic devices or even intomore complex microcircuits is still a challenging issue. Manystrategies have been proposed to accomplish this task, but all ofthem adopt local approaches that limit the number ofassembled elements to few units. Graphene-based microcircuitshave been recently realized on graphene oxide (GO) films viadirect visible laser reduction.24,25 Nanopatterns of reduced GO(rGO) have been obtained also using atomic force microscopy(AFM) by tip-induced local thermal reduction26 or electro-chemical reduction.27 AFM lithography has been used directlyon graphene films to engrave nanopatterns via local hydro-genation/oxidation by just controlling the tip bias.28 Atomicallystructured graphene features have been obtained both byelectron beam lithography29 and scanning tunneling micros-copy lithography.30 All these strategies are good choices for afine control of shape and dimensions of nanostructures but notfor large-scale processing, their throughput being intrinsicallysmall.

Received: November 24, 2011Revised: February 22, 2012Published: February 29, 2012

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The need of high-throughput processes is driving scientiststoward solutions inspired by existing technologies. Dimiev atal.31 have recently demonstrated the possibility of fabricating agraphene-based device drawing inspiration from standardlithography; in their approach, the lithographic steps consistin the layer-by-layer etching of grapheneor graphene-likematerialsselectively masked by predesigned zinc patterns; thecorrect sequence of steps gives the desired graphene pattern,analogously to what happens when a silicon wafer is processedin a standard lithographic stepper. Another possible ap-proachdrawing inspiration from standard lithography aswellis to use light for photoinduced patterning of graphene-based layers. The effects of light have been widely studied forgraphene and GO at various wavelengths. In particular, UVwavelengthstypically used in the processes of photolithog-raphyare known to have a reducing effect on GO but most ofthe works report about bulk processes where GO sheets aresuspended in liquid solutions containing nanoparticles catalysts,typically TiO2,

32,33 ZnO,34,35 or metals (Au, Pt, and Pd).36

Catalyst-free UV photoreduction of GO dispersed in aqueoussolution has been demonstrated using intense UV light, whichcreates high temperature and reactive environment.37

An example of UV-assisted photopatterning has been alsoreported demonstrating that micrometer size conductivepatterns can be obtained on insulating GO.38 An original ideais to use extreme-UV (EUV) instead of UV light for GOphotopatterning. EUV light is considered since long the naturalcandidate to go beyond the resolution limits imposed by UVradiation in standard photolithography (currently operatingwith silicon-based materials only). Once EUV is aimed to aneffective photolithography of GO, one has to cope with twolimitations, namely the occurrence of possible in plane breakingof carbon−carbon bonds (that introduces defects in thegraphene layer) and also photothermal effects (that limit the

lithographic spatial resolution). Indeed, GO photoreduction isavailable in a range of photon energies that covers the entireEUV range, i.e., beyond the threshold for photoreduction (3.2eV)39 and below the energy threshold that causes breaking ofthe in-plane carbon−carbon bonds in single-layer exfoliatedgraphene (200−300 eV).40 Thus, the choice of EUV radiationof a few tens of electronvolts nicely fits into these two boundaryenergy limits. Moreover, the condition for the photopatterningresolution limit to scale with the diffraction limit (down to tensof nanometers or lower) is the absence of photothermal effects.Up to now, such effects have been explicitly exploited for visiblelaser-induced photopatterning,24,25 or they have been found, inany case, to play an important role in UV-assisted processes.37

Nonetheless, the loss of spatial resolution owed to such thermaleffects can only be avoided by using small EUV radiation doses(namely, several orders of magnitude lower than those used byZhang et al.24 and by Zhou et al.25).Here we propose a high-throughput EUV (26.4 eV)

lithographic approach for large area (∼10 mm2) GO sub-micrometer scale photopatterning. Besides lying in theopportune range for GO photoreduction,39,40 the photonenergy we adopt here for our purposes matches well the energyof the O 2s electronic states in the GO valence band,41 an idealcondition to improve the absorption of the EUV photons andto enhance the efficiency of the photoreduction process. GOphotoreduction is studied by X-ray photoemission spectroscopy(XPS) and micro-Raman spectroscopy (μRS). The reductionprocess is catalyst-free and consists in a highly efficientdissociation of epoxy and hydroxy groups due to the directaction of the EUV photons. The values of irradiance used in ourexperiments are small enough (20 mW/cm2, i.e., 109−1010times smaller values than in refs 24 and 25) to be confident thatphotothermal effects are negligible. Patterns with micrometerand sub-micrometer sized features are obtained by interference

Figure 1. (a) Sketch of the procedure used to prepare the samples. Single-layer GO flakes are deposited by spin-coating on 72 nm thick Al2O3/Si(100) and reduced by EUV laser exposure. (b) CCD image of the laser beam acquired from a phosphorus screen placed in the sample position(top) and intensity profile (bottom). The large and regular beam section allows an homogeneous photoreduction over large areas (∼10 mm2). (c, d)20 × optical microscope images of as-deposited samples prepared respectively for μRS and XPS analysis by single (c) and double (d) spin-coatingdeposition.

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lithography. The reduction degree of the micrometer sizedfeatures is studied by μRS, giving a direct evidence of amodulation of the local GO reduction. The possibilityheredemonstratedof using EUV light to pattern GO is a relevantupgrade for the graphene-based technology that can takeadvantage, in this way, from the entire know-how of the EUV-based technology.

■ EXPERIMENTAL SECTIONLarge area (100s of μm2) GO sheets are prepared via a modifiedHummers method42 starting from graphite flakes of 500 μm maximumsize. The resulting GO is dispersed in water and then spin-coated at2000 rpm on 72 nm thick Al2O3/Si(100) previously sonicated inNH4−OH solution (Figure 1). The thickness of the Al2O3 film isspecific for best optical contrast of the deposited GO sheets.43

Deposited monolayers are identified by simple optical inspection onthe basis of the calibration given in ref 43. The thickness of theobserved monolayers is 1.1 ± 0.1 nm.44 A standard optical microscope(Olympus BH2-UMA) equipped with four different objectives (5×,20×, 40×, and 100× Olympus objective) and a scanning electronmicroscope (SEM) (ZEISS-GEMINI LEO 1530) have been used toobserve the surface of the samples. Optical images (20× magnifica-tion) of the as-deposited GO sheets are reported in Figure 1, at twodifferent surface coverage conditions. The surface density of GO flakesis, in general, increased by multiple depositions.44 In our case, a doubledeposition is enough for an almost complete paving of the substratesurface, as shown in Figure 1d. An image of sample prepared by singledeposition is reported in Figure 1c.The EUV light source used to reduce GO is a compact table-top

laseralmost unique in the world scenarioemitting at λ = 46.9 nm.The laser pulse (1.5 ns duration, 150 μJ energy) is produced by thesingle pass amplification of the 3p−3s transition in Ne-like Ar in anelongated plasma column created by a fast capillary discharge.45 Effectsof the exposure of GO sheets to EUV light have been studied both byroom temperature micro-Raman spectroscopy (μRS) and X-rayphotoemission spectroscopy (XPS). In both cases the sample surfacehas been investigated both before and after EUV exposure. ALABRAM spectrometer (Horiba-Jobin Yvon, λ = 633 nm, 1 μm spatialresolution, and ∼2 cm−1 spectral resolution) equipped with a confocaloptical microscope (100× MPLAN objective with 0.9 numericalaperture and 0.15 mm work distance) has been used for μRS, while aPHI 1257 spectroscometer (monochromatic Al Kα source, hν =1486.6 eV) has been used for XPS (base pressure 2 × 10−10 Torr).Samples have been exposed to EUV light under high-vacuumconditions (∼10−5 Torr) and then investigated in air for μRS analysisor in ultrahigh vacuum (UHV) for XPS analysis.Interference lithography has been performed using a simple Lloyd’s

interferometer (illustrated in Figure 5a), exploiting the excellentcoherence properties of the EUV laser source.46,47 The formation ofperiodic patterns is the outcome of the modulation of the EUV doseover the samples surface. No photoresists are used in the lithographicprocess. In the Lloyd’s configuration, half of the laser beam (∼2 mmspot radius on the sample, Figure 1b) grazes the Lloyd’s mirror (0.5nm rms Si reflecting surface) at an angle θ. The reflected half-beamoverlaps the undeflected one forming an interference pattern over a∼10 mm2 area with half-moon shape. The period p of the periodicpattern is easily tunable by varying the angle θ, p being defined by thesimple relation p = λ/(2 sin θ).46,48,49 Using λ = 46.9 nm, the periodcan be scaled down to 23 nm.50

■ RESULTS AND DISCUSSION

μRS spectra have been collected on monolayer GO both beforeand after EUV exposure (Figure 2). Samples have been exposedto a 200 mJ/cm2 EUV dose, that is, the same dose hereinafterused for XPS analysis. Intensities are normalized to the G peak.The left-hand peak (D peak) is commonly related to thepresence of defects while the right-hand peak (G peak) is

characteristic of graphene.51 The ratio ID/IG between D and Gpeak intensities is related to the average size La of sp2-hybridized carbon domains (i.e., graphene-like domains) by theTuinstra−Koenig equation.52 Here, ID/IG passes from 1.350 ±0.005 (as-deposited GO) to 1.310 ± 0.005 (irradiated GO),corresponding to a 3% increase in La (that passes from 6.1 to6.3 nm). Hence, the evolution of the Raman spectrum indicatesthat GO is reduced as a consequence of EUV laser exposure.A systematic study of the properties of GO reduced by

thermal annealing in UHV has been recently reported by ourgroup, demonstrating the possibility of recovering almostcompletely the optical contrast of graphene at 670 °C.53 Thedata reported in ref 53 will be used here for a calibration of theGO properties in terms of the annealing temperature.According to this calibration, the above measured variation ofID/IG is found to correspond to a 60 ± 10 °C thermally inducedreduction.A dose-dependent μRS analysis has been performed to

investigate the effects that higher doses have on monolayer GO.A plot of ID/IG as a function of the EUV dose is reported inFigure 3. As expected for a photoreduction process, the ratio

Figure 2. Room temperature micro-Raman spectra measured on as-deposited (solid line) and EUV-exposed (dashed line) monolayer GO(EUV dose ∼200 mJ/cm2). Spectra (normalized to the G peakintensity) are reported over the entire range (a) and in a narrowerenergy range (b). The D peak intensity of the EUV-exposed sample islower than the as-deposited one, indicating that GO has been reducedby the EUV light.

Figure 3. Dose-dependent plot of ID/IG for EUV-exposed monolayerGO.

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ID/IG regularly decreases as the EUV dose increases. At theseregimes neither saturation nor degradation processes areobserved. The slope obtained from a linear fit of theexperimental data (dashed line) is −3 ± 2 (J/cm2)−1,corresponding to a 2 ± 1% variation of ID/IG per J/cm2. Thisvalue of photoreduction efficiency is 2 orders of magnitudehigher than the one measured in the case of UV-assistedphotopatterning.38 Such a huge enhancement of the efficiencycan be ascribed to a selectively enhanced EUV absorption ofGO at 26.4 eV due to the perfect match of the photon energywith the energy of the O 2s atomic resonance.41

XPS has been performed on samples prepared by double GOdeposition. Double deposition has been preferred to the singleone because enhancing the signal of GO with respect to thesubstrate. Spectra have been acquired both before and afterEUV exposure (respectively Figures 4a and 4b). Irradiated

samples have been exposed to the same dose used for micro-Raman analysis (∼200 mJ/cm2) for eventual comparison.Deconvolution of XPS spectra into elementary componentsreveals a dominant behavior governing the evolution of sp2

carbon and oxygen contents. This behavior is accompanied byminor spurious effects that have been quantitatively taken intoaccount.54 Further details are reported in the SupportingInformation. The spectrum measured before exposure is wellreproduced (according to the literature55−57) by the con-volution of three peaks (Figure 4a): the Csp

2 peak (related tothe presence of sp2 carbon domains), the C−O peak (mainlyascribed to the presence of epoxy and hydroxyl groups), andthe CO peak (gathering the contributions of the CO andCO(OH) bonds). After exposure, the Csp

2 peak increases,the C−O peak decreases, and the CO peak remainsapproximately the same (Figure 4b). A fourth peak appearsthat can be attributed to the formation of defects (hereafterreferred to as Cdef peak).

58

The evolution of C−O and Csp2 peaks indicates that epoxy

and hydroxyl groups are dissociated as a consequence of EUV

irradiation and that sp2 carbon bonds are recovered. A similarevolution of the C 1s spectrum is reported in ref 37 fordispersed GO under intense UV light (as in our case, thephotoreduction process is catalyst-free). The describeddynamics unambiguously identifies a process of GO reduction.In particular, the decrease of the oxygen-related component ofthe XPS spectrum indicates that the oxygen atoms are the mainresponsible of the GO EUV light absorption, whose crosssection is large enough to allow an efficient process ofphotoreduction. Due, in fact, to the resonance (at ∼27 eV) ofthe EUV photons of our setup with the O 2s electrons, thephotoabsorption at the oxygen atoms is enhanced withconsequent break of the various C−O bonds. A quantitativeestimate of the amount of oxygen atoms dissociating from GOsheets under the effect of the EUV light is obtained from theevolution of the areas of C−O and Csp

2 peaks. A 6% increase ofsp2 carbon bonds and a 20% decrease of C−O bonds are found.Such values correspond to a ∼50 °C thermally inducedreduction (in UHV),53 in good agreement with the resultobtained by μRS (60 ± 10 °C).The demonstration of a high-efficiency photoreduction effect

of EUV light on as-deposited monolayer GO suggests itsapplication to photolithography. Here we report GO patterningvia spatially resolved photoreduction. We demonstrate a simpleperiodic patterning modulating the EUV dose with a Lloyd’sinterferometer (Figure 5a). No photoresists are used in theprocess. The patterns, as they are observed with the opticalmicroscope, consist of alternated darker/lighter stripes (Figure6a,b). The exposed monolayer GO sheets are deposited on 72nm thick Al2O3/Si(100). Images of patterns with 2 μm and 500nm periodicity are reported respectively in Figure 5b,c.Corresponding features are 1 μm and 250 nm sized,respectively. A detail of fringes is reported in both cases(bottom of each box). Folds of the single-layer GO sheets(spread out as a tablecloth on the substrate) can be easilydistinguished. Patterns cover a wide area (∼10 mm2) whereinthey preserve the same period and good contrast. The 250 nmsized features are less resolved than the 1 μm sized ones butthey are still visible.The micrometer scale patterns of Figure 5b have been used

to investigate the reduction degree of the single periodic featureby μRS. A first evidence of reduction degree modulation isgiven by the modulation of the color contrast (dark and lightstripes) exhibited by the EUV-exposed monolayer GO whenobserved at the optical microscope (Figure 6a,b). In particular,darker stripes are those with a higher reduction degree.59 Thewidth of the stripes (1 μm) matches well the lateral resolutionof the visible laser equipping the Raman spectrometer. Themeasure has been repeated on several stripes to achievestatistical significance. Collected spectra (Figure 6c,d) areidentical to the spectrum reported in Figure 2 for photoreducedmonolayer GO but with slight variations in the D peak intensity(as in the case of Figure 2, spectra are normalized to the G peakintensity). The gap here observed is much smaller than inFigure 2 but it is still significant. ID/IG passes from 1.315 ±0.005 (lighter stripes) to 1.305 ± 0.005 (darker stripes).Because of a background superimposed to the dosemodulationand increasing with the dose itselfGO isreduced in both dark and light stripes.

■ CONCLUSIONSIn this work, we have reported the first demonstration ofresistless EUV lithography on monolayer GO. In particular, we

Figure 4. (a, b) XPS spectra of GO on Al2O3/Si(100) before (a) andafter (b) EUV exposure (∼200 mJ/cm2 dose). Deconvolution intomultiple elementary peaks is reported for each spectrum.

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have demonstrated that supported GO (GO on 72 nm thick

Al2O3/Si(100)) can be reduced with a high efficiency process

by direct EUV light exposure and that single monolayers can be

patterned over large areas (∼10 mm2) with sub-micrometer

resolution via spatially resolved photoreduction. Despite the

choice of a specific substrate (motivated by the need of

enhancing the optical contrast of the patterned features on top

of it), the results are of general validity. The technique has

proved to be a promising candidate to fabricate integrated

graphene-based nanocircuits because parallel, operating on

large areas, and resistless. Given this proof of concept, the

process can be very likely significantly improved with the use of

high-performance sources like synchrotron or free electron

laser.

■ ASSOCIATED CONTENT*S Supporting InformationQuantitative estimation of the variation of sp2 carbon bondsand C−O bonds due to photoreduction. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: stefano.prezioso@aquila.infn.it.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was partly supported by the Istituto Nazionale diFisica Nucleare (INFN) project XILOPHON. S. Preziosoacknowledges individual grant from CARISPAQ (Cassa diRisparmio Provincia dell’Aquila). Authors acknowledge M.

Figure 5. (a) The Lloyd’s interferometer used to pattern the GO flakes: detail of the patterning area (top) and schematic top view (bottom). In theLloyd’s configuration, half of the laser beam grazes the Lloyd’s mirror at an angle θ and overlaps the undeflected half-beam to form the interferencepattern. (b, c) SEM images of the patterned monolayer GO (white grids are enclosed in (c)). Patterns with 1 μm (b) and 250 nm (c) sized featureshave been realized.

Figure 6. (a, b) Optical observation of the features: 100× image (a) and detail (b). (c−e) Room temperature micro-Raman analysis: spectra havebeen collected on dark (c) and light (d) stripes (both normalized to the G peak intensity), as schematically illustrated in (e). (f) Difference betweenthe two spectra (Ilight − Idark). Red and blue colors have been used to associate each spectrum to the selected stripe.

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Giammatteo and L. Arrizza (Centro di Microscopie, Universita dell’Aquila) for technical support.

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