APPLIED SCIENCES AND ENGINEERING Direct observation of ... · CVD graphene grain boundaries are...

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1Department ofMicro andNanosystems, School of Electrical EngineeringandComputerScience, KTHRoyal Institute of Technology,Osquldas väg10, 10044 Stockholm, Sweden.2Faculty of Electrical Engineering and Information Technology, Rheinisch-WestfälischeTechnische Hochschule (RWTH) Aachen University, Otto-Blumenthal-Str. 25,52074 Aachen, Germany. 3Institute of Physics, Chemnitz University of Technology,Reichenhainer Straße 70, 09126 Chemnitz, Germany. 4Gesellschaft für angewandteMikro- und Optoelektronik mbH (AMO GmbH), Advanced Microelectronic CenterAachen, Otto-Blumenthal Str. 25, 52074 Aachen, Germany.*Corresponding authors. Email: [email protected] (F.N.); [email protected] (M.C.L.)

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Direct observation of grain boundaries in graphenethrough vapor hydrofluoric acid (VHF) exposureXuge Fan,1 Stefan Wagner,2 Philip Schädlich,3 Florian Speck,3 Satender Kataria,2

Tommy Haraldsson,1 Thomas Seyller,3 Max C. Lemme,2,4* Frank Niklaus1*

The shape and density of grain boundary defects in graphene strongly influence its electrical, mechanical, andchemical properties. However, it is difficult and elaborate to gain information about the large-area distributionof grain boundary defects in graphene. An approach is presented that allows fast visualization of the large-areadistribution of grain boundary–based line defects in chemical vapor deposition graphene after transferringgraphene from the original copper substrate to a silicon dioxide surface. The approach is based on exposinggraphene to vapor hydrofluoric acid (VHF), causing partial etching of the silicon dioxide underneath the grapheneas VHF diffuses through graphene defects. The defects can then be identified using optical microscopy, scanningelectron microscopy, or Raman spectroscopy. The methodology enables simple evaluation of the grain sizes inpolycrystalline graphene and can therefore be a valuable procedure for optimizing graphene synthesis processes.

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INTRODUCTIONGraphene, an atomically thin two-dimensional (2D) sheet of covalent-ly bonded carbon atoms, presents new opportunities in fundamentalresearch and practical applications (1), due to its unusual electronic(2), optical (3), thermal (4), and mechanical properties (5). Large-areahigh-quality graphene growth has been intensively studied, and signif-icant advances have been made toward practical device application ofgraphene (6–9). Among the options for graphene synthesis, chemicalvapor deposition (CVD) can yield high-quality, polycrystalline single-layer graphene on catalytic substrates such as Cu (7). Typically, CVDgrowth yields a polycrystalline graphene structure with grain sizes inthe order of a few hundred nanometers (10), a few micrometers (11),or even a few hundred micrometers (12). Grain boundaries representdefects in the graphene lattice, because they typically deviate from theideal honeycomb structure. Grain boundaries occur when graphenedomains nucleate simultaneously at different locations on the growthsubstrate and then coalesce. In addition to these dislocation-like linedefects, point defects (such as Stone-Wales defects, adatoms, vacan-cies, or substitutional impurities) are observed (13, 14). Moreover,the transfer of graphene from the growth substrate to a target sub-strate, typically a silicon dioxide (SiO2) surface of a silicon (Si) sub-strate, introduces wrinkles (15), folds (16), and cracks (17) in CVDgraphene. In particular, the form and distribution of grain boundariesare critically affecting the electrical, thermal, mechanical, and chemicalproperties of the graphene, with smaller grain sizes potentially redu-cing electronic mobility (6), lowering thermal conductivity (18), andreducing ultimate mechanical strength (19). However, defects, such asgrain boundaries, might be beneficially exploited by controlled grainboundary engineering (20). Thus, the study of grain boundaries ingraphene is important for graphene characterization, for understand-ing the characteristic behavior of graphene in specific device applica-

tions, and for optimizing graphene growth conditions. A number ofreports demonstratemethods for characterizing graphene grain bound-aries using transmission electron microscopy, scanning tunnelingmicroscopy, or atomic force microscopy (AFM) (10, 11, 21, 22). Al-though these techniques can obtain atomic resolution at graphenegrain boundaries, they can be time-consuming and have limited capa-bilities in accessing large-scale information about the distribution ofthe grain boundaries. By contrast, techniques using optical micros-copy (23–27), 2D Raman imaging of the integrated intensities of theG andD peaks (28), or scanning electronmicroscopy (SEM) imagingof transitionmetal dichalcogenides that are epitaxially grown on gra-phene (29) are in principle suitable for characterizing graphene grainboundaries at larger scales. For instance, graphene grain boundarieshave been visualized using optical birefringence in graphene coveredby a liquid crystal (23). Graphene grain boundaries have also beenvisualized in graphene grown on a copper foil using optical micros-copy, by selectively oxidizing the underlying copper foil through thegrain boundaries in a moisture-rich atmosphere combined with ultra-violet irradiation (24), by selectively oxidizing the copper through thegrain boundaries by electron injection from an etching process using asodium chloride solution (25), by selectively oxidizing the copperin between grains of partially CVD grown graphene using a thermalannealing process (26), and by selectively etching the graphene grainboundaries in oxygen plasma and subsequently oxidizing theunderlying copper by thermal heating (27). However, these techniquesare either very elaborate or require the graphene to be placed on a cop-per substrate. Different characterization methods for observation ofCVD graphene grain boundaries are summarized in table S1. Here,we propose a simple method for visualizing grain boundaries in gra-phene placed on a SiO2 substrate with optical microscopy, SEM, orRaman spectroscopy. The method requires the exposure of the gra-phene to vapor hydrofluoric acid (VHF) before imaging (Fig. 1A),thereby partially etching the SiO2 underneath the graphene by diffusionof theVHF through the graphene grain boundaries. Thus, our approachallows for efficient and rapid large-scale visualization of graphene grainboundaries. As many graphene device applications are achieved usingCVD graphene that is transferred from the growth substrate (for exam-ple, a copper foil) onto a SiO2 surface, the proposed metrology is highlyrelevant for the development, optimization, and understanding of poly-crystalline graphene targeted at these types of applications.

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RESULTS AND DISCUSSIONExperiments were conducted using CVD-grown graphene on a copperfoil (Graphenea) that was transferred onto 125-nm-thick SiO2 layers onSi substrates using a wet transfer method (see Materials and Methods)(30). At this state, graphene grain boundaries are not visible usingoptical microscopy, SEM, or Raman spectroscopy because of the nano-scale dimensions of the line defects (Fig. 1, B and F) (24). After thegraphene on the SiO2 surface is exposed to VHF at 40°C for differenttime periods (30, 60, and 120 s), line patterns in the areas covered by thegraphene become visible in both optical microscopy images (Fig. 1, C toE) and SEM images (Fig. 1, G to I). These line patterns become moredistinct and increase in number with increasing time of exposure toVHF. When the time of exposure to VHF is extended from 30 to 60 s,more line patterns can be observed using optical microscope and SEMimaging (Fig. 1, C, D, G, and H). Upon further increasing the exposuretime to VHF from 60 to 120 s, the density of the line patterns does notincrease (Fig. 1, F and I), whereas the existing line patterns get morepronounced, that is, the lines get wider. The CVD graphene used inour experiments has a polycrystalline graphene structurewith grain sizesin the order of a few micrometers. The optical microscope and SEMimages in Fig. 1 show that the dimensions of the areas surrounded bythe line patterns are in the range of a few micrometers, which indicatesthat the line patterns are likely causedby, and represent, the grain bound-aries in the graphene.

To further demonstrate that the line patterns correspond to the grainboundaries of CVD graphene, the same type of CVD graphene on cop-per that was used in our VHF exposure experiments was characterizedby low-energy electron microscopy (LEEM) and low-energy electrondiffraction (LEED). The LEED pattern (Fig. 2A) shows the presenceof multiple rotated graphene domains on various facets of the copperfoil surface, similar to what has been reported previously (31). By suc-cessively selecting single diffraction spots for imaging (dark-fieldLEEM), the lateral distribution of domains becomes visible (Fig. 2B).The observed grain sizes range from ~2 to ~10 mm. Both the grain pat-tern and the dimensions are consistent with the observed patterns anddimensions visualized by our method. This further indicates that theline patterns in our method are mainly attributed to the grain bound-aries ofCVDgraphene.Wehave also attempted to characterize graphenesamples placed on a SiO2 surface with LEEM and LEED; however, it wasnot possible to obtain reasonable LEED patterns for aligning the instru-ment. This could be due to residues on the transferred graphene (for ex-ample, from the supporting layer necessary for transfer) as themethod ishighly surface-sensitive because of the short mean free path of electronsat the energies used for LEED.

Beside the line patterns, in the SEM images, we observed a gradualchange of the surface morphology in the areas surrounded by the linepatterns upon exposure to VHF (fig. S1). In these areas, corrugationswith a characteristic size of 50 to 100 nm in-plane were formed acrossthe surface of the samples (fig. S1). The SiO2 surface areas that had notbeen covered by graphene show very similar corrugations after expo-sure to VHF, whereas no line patterns appear in these areas (fig. S2).The evolution of the surface morphologies in the areas in betweenthe line patterns (the grain areas) that depend on the time of exposureto VHF illustrates that the reaction of VHF with the SiO2 substrate un-derneath the graphene is initiated near the graphene grain boundariesand spreads from there across the graphene crystallites. To verify thestability and repeatability of our proposed method, different graphenesamples were prepared, and very similar phenomena were observed.For instance, graphene transferred to different Si substrates containing

Fig. 1. Observation of grain boundaries in graphene after exposure to VHF.(A) Schematic illustrations of graphene on a SiO2 surface during exposure to VHF.(B to E) Optical images of graphene on a SiO2 surface of a Si substrate before expo-sure to VHF and after exposure to VHF for 30, 60, and 120 s, respectively. Line patternsare visible using optical microscopy after VHF exposure, and the patterns increase innumber and become more distinct with increasing time of exposure to VHF. For in-stance, very distinct line patterns can be observed after exposure to VHF for 120 s, asshown in (E). (F to I) SEMmicrographs of graphene on a SiO2 substrate surface beforeexposure to VHF and after exposure to VHF for 30, 60, and 120 s, respectively. Linepatterns are visible using SEM imaging after VHF exposure, and the line patterns in-crease in number and becomemore distinct with increasing time of exposure to VHF.When the time of exposure to VHF is increased to 120 s, the density of the linepatterns does not increase further, while the existing line patterns get morepronounced, that is, the lines get wider.

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1.4-mm-thick SiO2 surface layers were exposed to VHF for 60 and 120 s,respectively (fig. S3), which resulted in a comparable line pattern. Inaddition, single-layer graphene was transferred directly to a Si substratecovered only by a native SiO2 layer (~1 nm thick) and exposed to VHFfor 60 and 120 s, respectively (fig. S3). Line patterns with similardimensions and structure as in the experiments with the thick SiO2
layers were observed; however, the line patterns in the case of the nativeSiO2 are much less pronounced and not uniformly distributed. Theresults from these control experiments show that the presence of aSiO2 layer of sufficient thickness underneath the graphene is criticalfor creating pronounced line patterns after exposure to VHF.

The quality of the transferred graphene on SiO2was characterized byscanning micro-Raman spectroscopy before and after exposure of thegraphene to VHF for different exposure times of 0, 30, 60, and 120 s(Fig. 3). The absence of an appreciable D peak (1350 cm−1) showsrelatively high-quality single-layer graphene before exposure to VHF(Fig. 3A). Note that the Raman spectrum in Fig. 3 (A4) is obtainedon a monolayer region (Fig. 3A2), whereas the Raman spectrum inFig. 3 (A3) is obtained on amultilayer region (Fig. 3A2), thereby havinga markedly lower intensity ratio of the 2D and G peaks (I2D/IG). Afterexposure to VHF for 30 s, there is still an obvious G and 2D band, bothon the line patterns and in between the line patterns (within the grains)(Fig. 3B). This can be explained by the fact that the width of the linepatterns created during VHF exposure for 30 s is smaller than the di-ameter of the Raman laser spot (~300 nm) that is represented by theblue and purple circles in Fig. 3 (A2, B2, C2, and D2). Thus, the laserbeam reaches across the line, thereby at the same time characterizing thegraphene on the line patterns and beside the line patterns. After expo-sure to VHF for 60 and 120 s, the line patterns are found to widen, andthe G and 2D bands weaken substantially at the line patterns. By con-trast, the G and 2D bands remain in the areas between the line patterns(Fig. 3, C and D). The treatment of graphene in fluorine-containing at-mospheres (for example, F2 and XeF2) has been shown to result influorinated graphene (32–36), and an evolution of the Raman spectrasimilar to the onewe saw in our experiments (Fig. 3) was reported uponfluorination of graphene and graphite in gaseous atmospheres of XeF2


and F2 after several weeks of exposure to these gases (33, 34). The dis-appearance of characteristic peaks at the line patterns (Fig. 3D3)may beexplained as follows: Because of the enhanced chemical reactivity ofgraphene at grain boundaries (27, 37, 38), fluorination preferentiallyoccurs at these sites, thereby opening a relatively wide band gap in theenergy spectrum of the graphene (34). An increase of the time of expo-sure to VHF leads to an increase in the degree of fluorination of thesamples, thereby increasing the band gap (Fig. 3, B3, C3, and D3). Asshown in Fig. 3 (D3), when the width of the band gap of the graphene atthe grain boundaries is larger than the excitation photon energy of theRaman spectrometer (here, l = 532 nm), the fluorinated graphene at thegrain boundaries acts like a wide-band gap semiconductor and becomestransparent for the excitation radiation of the Raman tool laser (34). Inaddition, cracks may form at the graphene grain boundaries after expo-sure to VHF that can lead to decreased or vanished intensities of thecharacteristic peaks of the Raman signals. Energy-dispersive x-ray(EDX) analysis was carried out on the graphene after VHF exposure.Although fluorine sometimes appears in the EDX spectra, the resultsare not conclusive due to the low fluorine content and the low detectionsensitivity of EDX. TheRaman spectroscopymaps in Fig. 3 (A, B, C, andD) are not obtained from the same position of graphene before and afterVHF exposure, but from four different samples accordingly, whereVHFtime is varied systematically for a clear comparison. The ambiguous linepatterns in Fig. 3 (A1) are likely wrinkles, resulting from the graphenetransfer, and therefore do not correspond to the etched lines in Fig. 3(B1, C1, and D1) at all.

The microscopic features defining the line patterns in our sampleswere correlated with nanoscale information using AFM imaging beforeand after exposing the samples to VHF (Fig. 4). Before exposure toVHF, no topographical features can be observed in the transferredgraphene layers on the SiO2 substrate surface (Fig. 4, A and B).Whenthe VHF exposure time was 30, 60, and 120 s, obvious topographicfeatures outlining the line patterns start appearing with heights on theorder of 15 to 20 nm (Fig. 4, C and D), 25 to 40 nm (Fig. 4, E and F),and70 to80nm(Fig. 4,GandH), respectively. These topographic featuresin the SiO2 layer below the graphene are resulting from the exposure to

Fig. 2. LEED/LEEM characterization of graphene on copper. (A) LEED pattern of graphene on copper substrate. Diffraction spots corresponding to different rotational do-mains (1 to 6) are highlighted. The field of view for the LEED image is 7Å−1. (B) LEEMdark-field images acquiredby selecting thediffraction spots 1 to6 show the lateral distributionof rotated domains of graphene on copper substrate. The grain size ranges from ~2 to ~10 mm. The electron energy was 43.7 eV.

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VHF and they are the reason why the line patterns can be easily observedwith optical microscope and SEM imaging as shown in Fig. 1.

In addition to the structural investigations, we have carried outfour-point probe sheet resistance measurements of graphene placedon a 125-nm-thick SiO2 layer before and after exposure to VHF tomonitor the electrical properties of the graphene. The dependenceof the graphene sheet resistance on the time of exposure to VHF isplotted in fig. S4. The sheet resistance of the graphene before exposureto VHF is in the range from 1 to 1.5 kilohm/sq and in the range from1.5 to 5 kilohm/sq after exposure to VHF for less than 30 s. Thereafter,the sheet resistance increases sharply by six orders of magnitude afterexposure to VHF for 60 s, with an additional sheet resistance increaseby three orders of magnitude after exposure to VHF for 120 s. Thissharp increase of the sheet resistance from ~103 to ~1012 ohm/sq in-dicates the gradual formation of insulating regions (34, 39), possiblydue to the fluorination of graphene and/or the formation of cracks at


the graphene grain boundaries after exposure to VHF for more than15 s and up to 120 s, which is in good agreement with the results of theRaman spectroscopy experiments (Fig. 3). Similar resistance changeswere observed for graphene placed on a 1.4-mm-thick layer of SiO2 afterexposure to VHF (fig. S4). In another experiment, after exposure toVHF, the graphene was removed from the surface of several samplesby an O2 plasma etching process (1 min at 80 W with 80 sccm O2 at80 mtorr pressure), followed by 30 min in Piranha solution to removeresidues from the sample. This allowed a detailed investigation of thetopography of the underlying SiO2 substrate surface. Unexpectedly,the AFM topographic images show that the surface of the line patternsoutlining the graphene grain boundaries are at a higher level than thesurfaces of the areas previously covered by the pristine graphene crys-tallites, that is, the areas between the line patterns (Fig. 4I). Thesurfaces of the line patterns are at the same level as the SiO2 surfacesthat were not covered by graphene during VHF etching. This means

Fig. 3. Ramancharacterization. Raman spectroscopy map of graphene placed on a SiO2 surface before exposure to VHF (A): (A1) map of the intensities of the 2D band.(A2) is a close-up of (A1). (A3) and (A4) are Raman spectra of two areas in (A2), respectively. The corresponding data after VHF exposure for 30, 60, and 120 s, respec-tively, are displayed in (B) to (D). CCD, charge-coupled device.

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that the etching rate of SiO2 in areas that are not covered by grapheneis identical with the etching rate of the SiO2 at the line patterns (thatis, the graphene grain boundaries). Surprisingly, the etching rate of theSiO2 underneath the graphene crystallites (that is, the areas in betweenthe graphene grain boundaries) is higher than the etching rate of boththe SiO2 directly exposed to the VHF and the SiO2 at the line patterns.A similar observation of an increased etching rate of SiO2 underneathmechanically exfoliated graphene was reported by Stolyarova et al.(40). Our experiments demonstrate that the visibility of the linepatterns in areas covered by graphene is caused by a differentialetching of the underlying SiO2 during exposure to VHF. A similar


process of differential VHF etching of SiO2 around individual carbonnanotubes (CNTs) has been reported previously for visualizing CNTswith optical microscopy (41). Here, we provide a possible explanationconcerning the processes involved in forming the line patterns under-neath the graphene (Fig. 5A): First, VHF along with water moleculesevaporates from the liquid HF solution and preferentially interact withthe graphene grain boundaries due to their enhanced chemical re-activity (27, 37, 38), thereby forming open channels through whichmolecules can diffuse. Next, water condenses at the surfaces, and dis-solved HF interacts with the graphene and the SiO2 underneath thegraphene grain boundaries. Although etching of SiO2 with VHF is a

Fig. 4. AFM characterization. (A) AFM image of the surface topography of graphene placed on a SiO2 substrate surface before exposure to VHF. (B) Height profile of theselected area in (A). The corresponding data after VHF exposure for 30, 60, and 120 s, respectively, are displayed in (C) to (H). (I) AFM characterization of a SiO2 surface that has beencoveredwith graphene and a SiO2 surface that has not been coveredwith graphenewhile exposed to VHF. AFMdatawere taken after exposure of the sample to VHF for 120 s, O2

plasma etching for 60 s at 80Wwith 80 sccm of O2 at 80mtorr, and piranha cleaning for 30min. (I1) Optical microscope image of the SiO2 surfaces after the etching and cleaningprocedure. (I2) AFM image of the surface topography of the SiO2 surfaces after the etching and cleaning procedures. (I3) Height profile of the selected area in (I2), spanning SiO2

surfaces that were covered and that were not covered by graphene while exposed to VHF. (I4) and (I5) show an AFM image and a height profile of a SiO2 surface that was notcovered by graphene while exposed to VHF.

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very complex process with numerous sequential equilibrium reactions(42, 43), a simple illustration of the net reactions shows that an excessof H2Omolecules is produced during the etching reaction (see Materialsand Methods)

SiO2 þ 2H2O→SiðOHÞ4 ð1Þ

SiðOHÞ4 þ 4HF→SiF4↑þ 4H2O ð2Þ


The SiO2 etch rate inHF is intimately linked to the presence of water(40, 41, 43). Thus, SiO2 etching in a low water concentration environ-ment, such as SiO2 that is directly exposed toHF vapor (that is, the casesof SiO2 that is not covered with graphene and SiO2 in the areas of thegraphene grain boundaries), will be slower than etching of SiO2 that isexposed to liquid water with dissolved HF (that is, the case of the areasbeneath the graphene crystallites in between the line patterns, providedthat there is an efficient feed ofHF to thewater reservoir underneath thegraphene) (40). We speculate that VHF diffusing through the graphenegrain boundaries is absorbed by the water trapped underneath the

Fig. 5. Proposed etching mechanism during exposure to VHF. (A) Schematics of the proposed reaction processes when exposing graphene that is placed on a SiO2

substrate surface to VHF. (B) SEM micrograph of a SiO2 substrate surface that is covered by grains of CVD-grown graphene that are not fully grown to a sufficiently largesize to completely connect to neighboring grains, before exposure to VHF (blue lines around grains added for clarity). Thus, the grain boundary regions can beidentified in the areas where the blue lines of neighboring grains merge. (C) SEM micrograph of the sample shown in (B) after exposure to VHF for 17 s at roomtemperature and subsequent 13 s at 40°C. The SEM micrographs of (B) and (C) are taken from the same chip but not exactly from the same position.

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graphene in the regions between the grain boundaries, thereby effi-ciently transferring HF molecules across the graphene grain bound-aries to the liquid water reservoirs trapped underneath the graphenecrystallites, thereby effectively realizing liquid water reservoirs withdissolved HF (Fig. 5A). In this manner, higher etch rates are achievedunderneath the graphene crystallites as compared to the SiO2 areasthat are directly exposed to VHF and in which the etch rate dependson the water condensation rate. The etch rates of the SiO2 used in ourexperiments when exposed to VHF or liquid HF have been measuredand support the above hypothesis. That is, SiO2 was etched in VHF(that is evaporated from a 25% liquid HF solution) or in 10% liquidHF, with extracted etch rates of 45 and 80 nm/min, respectively. Thisconfirms the lower etch rates of SiO2 in VHF than in liquid HF solu-tions with comparably low concentrations.

Graphene grain boundaries are typically formed during the growthprocess by the joining of adjacent graphene islands initiated by separatenucleation seeds (44). To further verify our proposition that thegraphene grain boundaries are predominant for the reaction withVHF in forming the visible line patterns, we have used CVD graphenesamples where the growth time for the graphene was set to obtain par-tially isolated single grains of graphene. After transfer, the grains thusonly partially cover the surface of the chip substrate. In these samples,the graphene grain boundaries can be distinguished between differentgrains (Fig. 5B). After exposing these samples to VHF, the line pat-terns predominantly appear at the locations of grain boundariessurrounding grains where a grain merges with a neighboring grain(Fig. 5C and fig. S5). This is expected and consistent with the otherexperimental results. To evaluate what happens when double-layergraphene is present on the SiO2 surface instead of single-layer graphene,we exposed manually stacked double-layer CVD graphene transferredonto a SiO2 substrate surface to VHF for 120 s under the same con-ditions as the single-layer graphene. In these experiments, line patternscan still be observed by opticalmicroscope and SEM imaging in selectedareas (fig. S6), but the effect is significantly less pronounced than insingle-layer graphene. This is expected because effective diffusion ofVHF may happen only in areas where one of the stacked graphenelayers is defective. Finally, a control experimentwas performed inwhichsingle-layer graphene placed on a SiO2 substrate surface was immersedin 5% liquidHF at room temperature for 60, 120, 180, and 240 s, respec-tively. In these experiments, we did not observe the characteristic linepatterns after HF exposure. By contrast, in these experiments thegraphene was destroyed or lifted off the SiO2 surface during immer-sion in the liquid HF (fig. S7). This confirms that the proposed methodfor large-area imaging of graphene grain boundaries relies on the spe-cific chemical reactions that occur when exposing graphene on a SiO2

surface to VHF.Together, we have demonstrated a fast and simplemethod for large-

area visualization of grain boundaries in CVD graphene transferred to aSiO2 surface. We used indirect evidence from different characterizationapproaches along with control measurements, all yielding consistentresults supporting the proposition that our method visualizes grainboundaries in CVD graphene. The method only requires VHF etchingand optical microscope inspection, which both are standard processesin wafer fabs, and thus, themethod can be applied in these environmentswithout additional investment. Note that our method is invasive; that is,the graphene and the substrate aremodified during grain boundary visu-alization,which is in contrast to noninvasive grain boundary visualizationmethods, such as Raman spectroscopy (28), in which the graphene sam-ple is not modified (see table S1). However, despite its invasiveness, our


method has the clear benefits of simplicity, high speed, and throughput.Thus, this method could be very useful to speed up the process of devel-oping large-scale high-quality graphene synthesis because it offers a quickevaluation of the grain boundary density on large-area graphene samples.It can also be used for postmortem analysis of emerging graphene devicesthat use graphene patches placed on a SiO2 substrate, such as graphene-based pressure sensors, transistors, and gas sensors, to study the effects ofgrain boundary line defects on device performance.

CONCLUSIONIn conclusion, we report a fast and simple methodology for large-areavisualization of grain boundaries in CVD-grown graphene using opticalmicroscopy, SEM imaging, andRaman spectroscopy.Ourmethodologyrelies on the differential etching behavior of SiO2 near graphene grainboundaries and of SiO2 placed underneath graphene crystallites uponexposure to VHF. We attribute the observed realization of visible linepatterns to higher SiO2 etch rates in the areas covered by the grains. Thiscan be explained by amore efficient etching of SiO2 caused by reservoirscontaining liquid H2O with dissolved HF underneath the graphenegrains as compared to the areas in vicinity of the grain boundaries thatdo not contain trapped liquid H2O. The resulting topographical differ-ences and, hence, the grain boundaries can thus be easily observedvisually on a microscopic scale. The proposed approach is specificallyattractive for rapid and large-scale imaging of graphene grain bound-aries during development andmonitoring of graphene growth processes,ultimately contributing to improved understanding and optimization ofthe mechanical, electrical, and chemical properties of CVD graphene.

MATERIALS AND METHODSPreparation and transfer of grapheneCommercial (Graphenea) and in-house (45) CVD-grown single-layergraphene (Moorfield NanoCVD) was used in our experiments. Anoptimized wet transfer method was used to transfer single-layer CVD-grown graphene from the copper substrate to a SiO2 layer on a Si sub-strate (46). The thickness of the SiO2 layer was 125 nm. Beforetransferring the graphene, the SiO2 substrate surface was cleaned inpiranha solution (a mixture of sulfuric acid and hydrogen peroxidewith a volume ratio of 3:1) to remove possible organic contaminations.A poly(Bisphenol A carbonate) (PC) layer was spin-coated on the frontside of the graphene/copper foils at 2000 rpm for 5 s and 2500 rpmfor 30 s to deposit the polymer support layer (approximately 200 nmthick) during the graphene transfer process. Then, the samples werebaked for 15 min at 45°C on a hot plate to evaporate the solvent. Thecarbon residues on the backside of the copper foil were removed usingO2plasma etching at lowpower (50W).The copper foil was then placedon a surface of iron chloride (FeCl3) solution for wet etching of the cop-per, with the copper foil floating on the FeCl3 solution and the grapheneside facing away from the liquid. After about 2 hours, the PC/graphenestack without copper residues floating on the FeCl3 solution wastransferred with the aid of a Si chip onto the surface of deionized(DI) water, onto diluted HCl solution, and then finally back to DI waterfor cleaning and removing iron (III) residues and removing chlorideresidues, respectively. During this transfer process, it is important tokeep the PC/graphene stack floating on the surface of the liquids andto keep the graphene side on top to prevent wetting of the PC coveringthe graphene by the etch solution. After the cleaning procedure, thePC/graphene stack was picked up with a SiO2-coated Si chip and dried

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on a hot plate at 45°C for 10 min to improve graphene adhesion to thesubstrate. After drying, the chips were put into chloroform for 24 hoursto remove the PC, followed by an isopropanol dip for 5 min to removechloroform residues. A nitrogen gun was used to gently dry the chipsbefore baking them at 45°C for 10 min on a hot plate. The preparationof the double-layer graphene samples was based on an extension of thesingle-layer graphene transfer: Here, the PC/graphene stack floating onthe DI water (as described for the single-layer transfer process) wastransferred onto a second graphene film on copper foil and subsequent-ly placed on a hot plate at 45°C to increase the adhesion between the twographene layers. Carbon residues on the backside of the copper foil wereremoved using O2 plasma. To remove the copper foil from the double-layer graphene/PC stack, we used a process identical to the case ofsingle-layer graphene. Finally, the double-layer graphene/PC stackwas transferred onto the SiO2 substrate surface. All remaining stepswere the same as in the singe-layer graphene transfer process.

Experimental details of exposing the samples to VHFThe completed chips were mounted onto a custom-designed reactioncell with the graphene side facing toward the VHF chamber. VHF wasgenerated passively from a 25% liquid HF solution placed in a smallreservoir at the bottom of the reaction cell. The VHF penetrated theCVD graphene films at the grain boundaries and reacted with theSiO2 surface. During the reaction, volatile SiF4was formed. The reactionof VHF with SiO2 requires small amounts of H2O to be present at thesurface (42, 43). To obtain amicroscopic water film on the surface with-out producingdroplets and to accurately control the etchinghomogeneity,we gently heated the chips from the backside by a heater plate during theexposure to the VHF. In our experiments, the set temperature was 40°C.The expected reaction process was given by reactions (1) and (2) in themain text.

Characterization of the samplesThe graphene films were inspected in situ using optical microscopy(Olympus BX 51M) to obtain images with differentmagnifications dis-playing themorphologies of the graphene/SiO2 surfaces before and afterdifferent times of VHF exposure. A scanning electron microscope(Gemini, Zeiss, Ultra 55) was used to investigate the surface morphol-ogy of the samples at different acceleration voltages to obtain high-contrast images at different magnifications. EDX spectroscopy ofgraphene samples was also obtained with the same SEM tool to analyzetrace elements on the graphene samples after VHF exposure. Ramanspectroscopywas performedusing an alpha300R spectrometer (WITec)with al=532nm laser and a100×objective.AFMimageswere obtainedwith a PSIA XE-100 (Park Systems) in tapping mode to analyze themorphologies and dimensions of the graphene grain boundaries afterVHF exposure. A conventional probe station in connection with aSCS4200 parameter analyzer (Keithley) was used for four-probe mea-surements of the sheet resistances of the graphene samples before andafter VHF exposure. LEEM/LEED characterization of CVD grapheneon copper was done using a LEEM (FE-LEEM P90, Specs). The diffrac-tion spots for dark-field imaging were selected by means of an aperturein the back focal plane of the objective lens.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaar5170/DC1fig. S1. Initial formation of line defects after exposure to VHF for short time and formation ofcorrugations after exposure to VHF for a long time.


fig. S2. SiO2 without graphene coverage after exposure to VHF.fig. S3. Graphene placed on a 1.4-mm-thick SiO2 layer and on the native SiO2 layer of a Sisubstrate after exposure to VHF.fig. S4. Sheet resistance of graphene placed on SiO2 layers of different thicknesses afterexposure to VHF.fig. S5. SEM images of partially isolated single grains of CVD graphene with successivelyincreasing magnification after exposure to VHF.fig. S6. SEM images of double-layer graphene placed on a SiO2 surface after exposure to VHF.fig. S7. SEM images of graphene placed on a SiO2 surface after exposure to 5% liquid HF.table S1. Comparison of characterization methods of graphene grain boundaries in CVDgraphene.References (47–50)

REFERENCES AND NOTES1. A. K. Geim, K. S. Novoselov, The rise of graphene. Nat. Mater. 6, 183–191 (2007).2. A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, A. K. Geim, The electronic

properties of graphene. Rev. Mod. Phys. 81, 109–162 (2009).3. L. A. Falkovsky, Optical properties of graphene. J. Phys. Conf. Ser. 129, 012004 (2008).4. A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C. N. Lau, Superior

thermal conductivity of single-layer graphene. Nano Lett. 8, 902–907 (2008).5. C. Lee, X. Wei, J. W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic

strength of monolayer graphene. Science 321, 385–388 (2008).6. A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. S. Dresselhaus, J. Kong, Large area,

few-layer graphene films on arbitrary substrates by chemical vapor deposition. Nano Lett.9, 30–35 (2009).

7. X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc,S. K. Banerjee, L. Colombo, R. S. Ruoff, Large-area synthesis of high-quality and uniformgraphene films on copper foils. Science 324, 1312–1314 (2009).

8. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi,B. H. Hong, Large-scale pattern growth of graphene films for stretchable transparentelectrodes. Nature 457, 706–710 (2009).

9. S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim,Y. I. Song, Y.-J. Kim, K. S. Kim, B. Özyilmaz, J.-H. Ahn, B. H. Hong, S. Iijima, Roll-to-rollproduction of 30-inch graphene films for transparent electrodes. Nat. Nanotechnol.5, 574–578 (2010).

10. P. Y. Huang, C. S. Ruiz-Vargas, A. M. van der Zande, W. S. Whitney, M. P. Levendorf,J. W. Kevek, S. Garg, J. S. Alden, C. J. Hustedt, Y. Zhu, J. Park, P. L. McEuen, D. A. Muller,Grains and grain boundaries in single-layer graphene atomic patchwork quilts. Nature469, 389–392 (2011).

11. K. Kim, Z. Lee, W. Regan, C. Kisielowski, M. F. Crommie, A. Zettl, Grain boundary mappingin polycrystalline graphene. ACS Nano 5, 2142–2146 (2011).

12. L. Banszerus, M. Schmitz, S. Engels, J. Dauber, M. Oellers, F. Haupt, K. Watanabe,T. Taniguchi, B. Beschoten, C. Stampfer, Ultrahigh-mobility graphene devices fromchemical vapor deposition on reusable copper. Sci. Adv. 1, e1500222 (2015).

13. I. A. Ovid’ko, Mechanical properties of graphene. Rev. Adv. Mater. Sci. 34, 1–11 (2013).14. F. Banhart, J. Kotakoski, A. V. Krasheninnikov, Structural defects in graphene. ACS Nano

5, 26–41 (2011).15. N. Liu, Z. Pan, L. Fu, C. Zhang, B. Dai, Z. Liu, The origin of wrinkles on transferred

graphene. Nano Res. 4, 996–1004 (2011).16. K. Kim, Z. Lee, B. D. Malone, K. T. Chan, B. Alemán, W. Regan, W. Gannett, M. F. Crommie,

M. L. Cohen, A. Zettl, Multiply folded graphene. Phys. Rev. B 83, 245433 (2011).17. K. Kim, V. I. Artyukhov, W. Regan, Y. Liu, M. F. Crommie, B. I. Yakobson, A. Zettl, Ripping

graphene: Preferred directions. Nano Lett. 12, 293–297 (2012).18. A. Y. Serov, Z.-Y. Ong, E. Pop, Effect of grain boundaries on thermal transport in graphene.

Appl. Phys. Lett. 102, 033104 (2013).19. R. Grantab, V. B. Shenoy, R. S. Ruoff, Anomalous strength characteristics of tilt grain

boundaries in graphene. Science 330, 946–948 (2010).20. O. V. Yazyev, S. G. Louie, Electronic transport in polycrystalline graphene. Nat. Mater.

9, 806–809 (2010).21. H. I. Rasool, E. B. Song, M. Mecklenburg, B. C. Regan, K. L. Wang, B. H. Weiller,

J. K. Gimzewski, Atomic-scale characterization of graphene grown on copper (100) singlecrystals. J. Am. Chem. Soc. 133, 12536–12543 (2011).

22. G.-H. Lee, R. C. Cooper, S. J. An, S. Lee, A. van der Zande, N. Petrone, A. G. Hammerberg,C. Lee, B. Crawford, W. Oliver, J. W. Kysar, J. Hone, High-strength chemical-vapor–deposited graphene and grain boundaries. Science 340, 1073–1076 (2013).

23. D. W. Kim, Y. H. Kim, H. S. Jeong, H.-T. Jung, Direct visualization of large-area graphenedomains and boundaries by optical birefringency. Nat. Nanotechnol. 7, 29–34 (2012).

24. D. L. Duong, G. H. Han, S. M. Lee, F. Gunes, E. S. Kim, S. T. Kim, H. Kim, Q. H. Ta, K. P. So,S. J. Yoon, S. J. Chae, Y. W. Jo, M. H. Park, S. H. Chae, S. C. Lim, J. Y. Choi, Y. H. Lee, Probinggraphene grain boundaries with optical microscopy. Nature 490, 235–239 (2012).

8 of 9

http://advances.sciencemag.org/cgi/content/full/4/5/eaar5170/DC1

http://advances.sciencemag.org/cgi/content/full/4/5/eaar5170/DC1




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25. T. H. Ly, D. L. Duong, Q. H. Ta, F. Yao, Q. A. Vu, H. Y. Jeong, S. H. Chae, Y. H. Lee,Nondestructive characterization of graphene defects. Adv. Funct. Mater. 23, 5183–5189(2013).

26. C. Jia, J. Jiang, L. Gan, X. Guo, Direct optical characterization of graphene growth anddomains on growth substrates. Sci. Rep. 2, 707 (2012).

27. J. Y. Lee, J.-H. Lee, M. J. Kim, J. K. Dash, C.-H. Lee, R. Joshi, S. Lee, J. Hone, A. Soon, G.-H. Lee,Direct observation of grain boundaries in chemical vapor deposited graphene. Carbon115, 147–153 (2017).

28. T. Lee, F. A. Mas’ud, M. J. Kim, H. Rho, Spatially resolved Raman spectroscopy of defects,strains, and strain fluctuations in domain structures of monolayer graphene. Sci. Rep.7, 16681 (2017).

29. H. Ago, S. Fukamachi, H. Endo, P. Solís-Fernández, R. M. Yunus, Y. Uchida, V. Panchal,O. Kazakova, M. Tsuji, Visualization of grain structure and boundaries of polycrystallinegraphene and two-dimensional materials by epitaxial growth of transition metaldichalcogenides. ACS Nano 10, 3233–3240 (2016).

30. X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo, R. S. Ruoff,Transfer of large-area graphene films for high-performance transparent conductiveelectrodes. Nano Lett. 9, 4359–4363 (2009).

31. J. Kraus, S. Böcklein, R. Reichelt, S. Günther, B. Santos, T. O. Menteş, A. Locatelli, Towardsthe perfect graphene membrane?—Improvement and limits during formation of highquality graphene grown on Cu-foils. Carbon 64, 377–390 (2013).

32. D. W. Boukhvalov, M. I. Katsnelson, Chemical functionalization of graphene. J. Phys.Condens. Matter 21, 344205 (2009).

33. F. Withers, M. Dubois, A. K. Savchenko, Electron properties of fluorinated single-layergraphene transistors. Phys. Rev. B 82, 073403 (2010).

34. R. R. Nair, W. Ren, R. Jalil, I. Riaz, V. G. Kravets, L. Britnell, P. Blake, F. Schedin, A. S. Mayorov,S. Yuan, M. I. Katsnelson, H.-M. Cheng, W. Strupinski, L. G. Bulusheva, A. V. Okotrub,I. V. Grigorieva, A. N. Grigorenko, K. S. Novoselov, A. K. Geim, Fluorographene:A two-dimensional counterpart of Teflon. Small 6, 2877–2884 (2010).

35. S.-H. Cheng, K. Zou, F. Okino, H. R. Gutierrez, A. Gupta, N. Shen, P. C. Eklund, J. O. Sofo,J. Zhu, Reversible fluorination of graphene: Evidence of a two-dimensional wide bandgapsemiconductor. Phys. Rev. B 81, 205435 (2010).

36. Z. Wang, J. Wang, Z. Li, P. Gong, X. Liu, L. Zhang, J. Ren, H. Wang, S. Yang, Synthesis offluorinated graphene with tunable degree of fluorination. Carbon 50, 5403–5410 (2012).

37. N. A. Nebogatikova, I. V. Antonova, V. Y. Prinz, V. A. Volodin, D. A. Zatsepin, E. Z. Kurmaev,I. S. Zhidkov, S. O. Cholakh, Functionalization of graphene and few-layer graphene filmsin an hydrofluoric acid aqueous solution. Nanotechnol. Russia 9, 51–59 (2014).

38. K. Kim, H.-B.-R. Lee, R. W. Johnson, J. T. Tanskanen, N. Liu, M.-G. Kim, C. Pang, C. Ahn,S. F. Bent, Z. Bao, Selective metal deposition at graphene line defects by atomic layerdeposition. Nat. Commun. 5, 4781 (2014).

39. D. K. Samarakoon, Z. Chen, C. Nicolas, X.-Q. Wang, Structural and electronic properties offluorographene. Small 7, 965–969 (2011).

40. E. Stolyarova, D. Stolyarov, K. Bolotin, S. Ryu, L. Liu, K. T. Rim, M. Klima, M. Hybertsen,I. Pogorelsky, I. Pavlishin, K. Kusche, J. Hone, P. Kim, H. L. Stormer, V. Yakimenko, G. Flynn,Observation of graphene bubbles and effective mass transport under graphene films.Nano Lett. 9, 332–337 (2009).

41. M. A. Novak, S. Surwade, J. Prokop, K. Bolotin, J. Hone, L. Brus, C. Nuckolls, H. Liu,Visualizing individual carbon nanotubes with optical microscopy. J. Am. Chem. Soc.136, 8536–8539 (2014).

42. A. Witvrouw, B. Du Bois, P. De Moor, A. Verbist, C. A. Van Hoof, H. Bender, C. Baert,A comparison between wet HF etching and vapor HF etching for sacrificial oxide


removal, Proceeding Volume 4174, Micromachining and Microfabrication ProcessTechnology VI (SPIE, 2000), Santa Clara, CA, 25 August 2000.

43. C. R. Helms, B. E. Deal, Mechanisms of the HF/H2O vapor phase etching of SiO2. J. Vac. Sci.Technol. A 10, 806–811 (1992).

44. Q. Yu, L. A. Jauregui, W. Wu, R. Colby, J. Tian, Z. Su, H. Cao, Z. Liu, D. Pandey, D. Wei,T. F. Chung, P. Peng, N. P. Guisinger, E. A. Stach, J. Bao, S.-S. Pei, Y. P. Chen, Control andcharacterization of individual grains and grain boundaries in graphene grown bychemical vapour deposition. Nat. Mater. 10, 443–449 (2011).

45. S. Kataria, S. Wagner, J. Ruhkopf, A. Gahoi, H. Pandey, R. Bornemann, S. Vaziri, A. D. Smith,M. Ostling, M. C. Lemme, Chemical vapor deposited graphene: From synthesis toapplications. Phys. Status Solidi A 211, 2439–2449 (2014).

46. S. Wagner, C. Weisenstein, A. D. Smith, M. Östling, S. Kataria, M. C. Lemme, Graphenetransfer methods for the fabrication of membrane-based NEMS devices. Microelectron.Eng. 159, 108–113 (2016).

47. L. Gao, J. R. Guest, N. P. Guisinger, Epitaxial graphene on Cu(111). Nano Lett.10, 3512–3516 (2010).

48. E. Loginova, S. Nie, K. Thürmer, N. C. Bartelt, K. F. McCarty, Defects of graphene on Ir(111):Rotational domains and ridges. Phys. Rev. B 80, 085430 (2009).

49. P. W. Sutter, J.-I. Flege, E. A. Sutter, Epitaxial graphene on ruthenium. Nat. Mater.7, 406–411 (2008).

50. M. Ahmad, H. An, Y. S. Kim, J. H. Lee, J. Juang, S.-H. Chuan, Y. Seo, Nanoscale investigationof charge transport at the grain boundaries and wrinkles in graphene film.Nanotechnology 23, 285705 (2012).

Acknowledgments: We thank M. Otto for help with AFM characterizations. Funding: Thiswork was supported by the European Research Council through the Starting Grants M&M’s(277879) and InteGraDe (307311), the Swedish Research Council (GEMS, 2015-05112), theChina Scholarship Council, the German Federal Ministry for Education and Research(NanoGraM, BMBF, 03XP0006C), and the German Research Foundation (DFG; LE 2440/1-2).The Chemnitz group acknowledges support by the German Research Council (DFG) throughthe Priority Program SPP 1459 Graphene. Author contributions: X.F., F.N., and M.C.L.conceived and designed the experiments. X.F. performed the experiments and optical,electrical, and SEM characterizations and wrote the manuscript. S.W. performed the AFMcharacterization. S.W. and S.K. performed the Raman spectroscopy characterization. P.S.,F.S., and T.S. performed LEEM and LEED characterization. T.H. contributed to the analysis ofthe chemical reactions. F.N. and M.C.L. provided guidance in all the experiments andmanuscript writing. All authors analyzed and discussed the results and commented onthe manuscript. Competing interests: The authors declare that they have no competinginterests. Data and materials availability: All data needed to evaluate the conclusions in thepaper are present in the paper and/or the Supplementary Materials. Additional data relatedto this paper may be requested from the authors.

Submitted 17 November 2017Accepted 12 April 2018Published 25 May 201810.1126/sciadv.aar5170

Citation: X. Fan, S. Wagner, P. Schädlich, F. Speck, S. Kataria, T. Haraldsson, T. Seyller,M. C. Lemme, F. Niklaus, Direct observation of grain boundaries in graphene through vaporhydrofluoric acid (VHF) exposure. Sci. Adv. 4, eaar5170 (2018).

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exposureDirect observation of grain boundaries in graphene through vapor hydrofluoric acid (VHF)

Lemme and Frank NiklausXuge Fan, Stefan Wagner, Philip Schädlich, Florian Speck, Satender Kataria, Tommy Haraldsson, Thomas Seyller, Max C.

DOI: 10.1126/sciadv.aar5170 (5), eaar5170.4Sci Adv

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