不用聚合物转移石墨烯的方法

Versatile Polymer-Free Graphene Transfer Method and Applications Guohui Zhang,?Aleix G.Gu e ll,*,?,?Paul M.Kirkman,Robert https://www.wendangku.net/doc/5917916572.html,zenby,Thomas https://www.wendangku.net/doc/5917916572.html,ler,

and Patrick R.Unwin*

Department of Chemistry,University of Warwick,Coventry CV47AL,United Kingdom

*Supporting Information

arbitrary substrates,including three-dimensional architectures as

transmission electron microscopy(TEM)grids.Graphene produces a

production of tips for conductive AFM imaging.Graphene

electron-transparent substrates for TEM imaging.These substrates

and high-resolution wetting studies.By using scanning

electrochemical and wetting measurements at either a freestanding

determine any di?erences in behavior.

AFM,graphene TEM grids,electrochemistry

Since its discovery in2004,1the outstanding electrical,2,3 mechanical,4,5and chemical6,7properties of graphene have been revealed,highlighting it as a hugely promising material for the future.The production of pristine graphene?akes was initially achieved through a(Scotch-tape-based)mechanical exfoliation1 method.However,with this time-consuming approach typically yielding micrometer-sized?akes,it is considered unrealistic for scale-up applications,where much larger areas of graphene are needed.8,9

Recently,chemical vapor deposition(CVD)has shown considerable promise for the synthesis of large-scale(with sheets of30in.reported10),high-quality graphene.11?13Among the metals used to catalyze the CVD growth of graphene, copper(Cu)is the most popular,producing mostly monolayer graphene.14However,depending on the application,an e?ective methodology for the subsequent transfer of such ?lms to substrates of interest is still required.15This is far from easy,especially when a large,continuous sheet is desired or three-dimensional(3D)structures are to be covered.Polymer support routes have been extensively employed for such transfer,in which a thin layer of polymer is deposited as a new support(template)on the as-grown(metal-supported) graphene,to allow removal of the metallic substrate by wet etching or electrochemical delamination,ultimately producing a polymer-supported graphene?lm.16,17Poly(methyl methacry-late)(PMMA),18poly(dimethylsiloxane)(PDMS),19and polycarbonate20layers(among others)are reported as suitable templates for the transfer of graphene onto a wide variety of planar/?at substrates,with the polymer subsequently removed through dissolution with organic solvents.Despite intensive research into such methods,the resulting graphene surfaces commonly appear littered with stubborn polymer residues,21,22 which may have a detrimental e?ect on subsequent applications,including the electronic and electrochemical performance of graphene.23,24Consequently,alternative routes of transfer are being sought,with polymer-free methods recently emerging as a fresh and promising way for clean graphene transfer.25,26

Herein,we introduce a polymer-free biphasic(liquid/liquid) approach for the transfer of monolayer CVD graphene to a wide range of target substrates.Our approach makes use of an inert nonpolar and low-viscosity liquid organic layer(hexane) lying on top of an aqueous etchant layer[ammonium persulfate,(NH4)2S2O8]to stabilize and protect the free-standing graphene sheet that is produced during the Cu wet etching and water rinsing processes.Essentially,the hexane layer replaces the deposited polymer layers used in the majority of current graphene transfer methods(vide supra),ensuring the freestanding graphene produced after etching of the growth

Received:January18,2016

Accepted:March8,2016

substrate is not torn apart by the surface tension associated with the aqueous etchant solution.Crucially,the lack of heteroatoms and aromatic groups in hexane,as well as its volatility and rapid evaporation,ensures that no residues are left on the graphene surface and that there is no doping after transfer to the desired substrate.Note that although an organic/water interface was recently used to decorate CVD graphene ?lms with nano-particles,the process used still relied on polymer coating and removal.27

Additionally,we demonstrate the feasibility and versatility of our approach for coating graphene onto coarse surfaces and 3D structures,due to the gentleness of the polymer-free transfer method.Beyond ?at substrates (e.g.,Si/SiO 2),monolayer graphene membranes have been transferred to more topo-graphically challenging substrates,such as atomic force microscopy (AFM)tips and transmission electron microscopy (TEM)grids.The resulting graphene-coated AFM tips and graphene TEM grids open up novel scienti ?c avenues,for example,new capability for conductive AFM mapping and atomic-resolution TEM imaging of nanoparticles.Our method is also very suitable for production of suspended graphene layers,an important goal in graphene science and technology to understand substrate e ?ects on the resulting graphene proper-ties.28,29Indeed,facilitated by this transfer method,we introduce the ?rst studies on wettability and electrochemistry of suspended graphene sheets.

■

RESULTS AND DISCUSSION

Polymer-free Transfer of CVD Graphene.The polymer-free biphasic transfer method is illustrated schematically in Figure 1.Monolayer graphene was grown on polycrystalline Cu foils in a low-pressure commercial CVD system,using methane as the carbon source (see Materials and Methods ).After the

back of the Cu foil was polished (to remove the graphene grown on the back side),the sample was initially ?oated (graphene side up)atop a 0.1M (NH 4)2S 2O 8etching solution,which has been shown to minimize residues compared to the other commonly used FeCl 3and Fe(NO 3)3solutions.9,30At this point,a nonpolar hexane layer was gently added dropwise to the surface of the etchant solution with a syringe,so that the graphene/Cu sample was trapped at the resulting organic/aqueous biphasic interface,with the exposed face of the hydrophobic graphene in contact only with the hexane and the Cu foil exposed to the etchant solution.After su ?cient etching time (～12h),only the synthesized graphene sheet remained trapped at the interface.Note that the surface tension for the hexane/water interface is ca.45mN ·m ?1,26,31lower than that of the air/water interface,which prevents the water layer pulling the sheet apart,as would be the case if the nonpolar layer were not present.26The “soft support ”from the hexane layer also protects the graphene sheet by minimizing physical drift at the interface.

To further minimize any possible contamination from etchant salts produced,the monolayer graphene sheet was scooped out and transferred to a new hexane/pure water interface with the aid of an Si/SiO 2wafer.After this cleaning step,the freestanding graphene sheet was scooped out from the interface by use of an arbitrary substrate of interest (e.g.,Si/SiO 2wafers,AFM tips,and TEM grids for the studies herein)in a single swift motion,before being left to dry at room temperature (see Supporting Information,section S1).

Salient observations from an etching process are presented in Figure 2.As shown in Figure 2a ?c,there is a gradual etching of the copper foil,eventually leading to a complete and highly transparent graphene ?lm of large area ?oating at the interface and maintaining its integrity.At this stage,the graphene ?

lm

Figure 1.Schematic of polymer-free biphasic method for CVD graphene

transfer.

Figure 2.(a ?c)Optical images of an as-grown graphene/copper sample ?oating at the interface between a hexane layer and a 0.1M (NH 4)2S 2O 8aqueous solution during etching.(d)Optical image of initial moments of the graphene ?lm being scooped out by means of an Si/SiO 2substrate.A video in the Supporting Information shows the rest of the transfer process.

was ready to be transferred with an Si/SiO2wafer to a new hexane/pure water interface for5h,for removal of any excess etchant salts(Figure2d shows the start of this process).For a video demonstrating the?nal transfer onto an Si/SiO2wafer, see Supporting Information.

A clean and complete graphene?lm was transferred onto Si/ SiO2,as evident by the optical and AFM images obtained after transfer(see Supporting Information,section S2).Raman spectroscopic measurements were also carried out to character-ize the graphene samples(see Supporting Information,section S3).The Raman spectrum of graphene on copper showed a pronounced2D band at2664cm?1and a small G band at1587 cm?1,with almost no detectable D band observed.This indicates the CVD growth of relatively high-quality monolayer graphene.19,22,32When the graphene sheet was fully transferred onto an Si/SiO2wafer by our polymer-free transfer method,the intensity ratio of2D and G bands(I2D/I G)was>2,with an associated full width at half-maximum(FWHM)for the2D band of～28cm?1,rea?rming the monolayer nature of the graphene grown.There was a small D band(at1333cm?1)in the Raman spectrum of graphene on Si/SiO2,with a D band intensity to G band intensity ratio(I D/I G)of0.11,being relatively uniform on the transferred graphene,as shown by the Raman map.This value suggests that relatively low-defect CVD graphene33was obtained by our growth and transfer process,of similar structural quality to that from polymer-assisted transfer methods commonly used in the literature(see Supporting Information,section S4).28

Fabrication and Utilization of Conductive Graphene AFM Tips.Sheets of graphene?nd interesting use as an ultrathin template for characterization of nanoscale structures trapped on a substrate,including molecules,34nanoparticles,35 and biological entities(e.g.,bacteria36or viruses37).The polymer-free biphasic method is attractive for the coating of fragile,small,and coarse substrates.We exempli?ed this capability by coating AFM probes with freestanding graphene ?lms.

CVD graphene?lms were deposited onto AFM probes following the biphasic procedure described in the previous

section(also see Materials and Methods).After transfer,the presence of graphene on the AFM probe cantilever was observable under an optical microscope.The tips were further characterized with scanning electron microscopy(SEM;Figure 3a,b)and TEM(Figure3c),from which relatively few super?cial features can be assigned to folds and wrinkles of the monolayer graphene.The images prove that the layer of graphene conforms very well to the AFM tip geometry, appearing to coat the AFM tip entirely,as well as the back of the cantilever by wrapping around it.Importantly,for AFM probe applications,we were interested in determining that the tip apex was also coated continuously with graphene and to discard the possibility of a perforation of the graphene?lm by the very sharp end of the tip.TEM imaging(see Figure3c)of graphene-coated AFM tips con?rmed the presence of a continuous thin layer at the end of the tip,assigned to the graphene sheet.An attribute of the graphene coating is the thinness of the layer,so that there is little change of the tip radius of curvature after coating to produce a conductive tip. This contrasts with metal-coated AFM tips,where several tens of nanometers is typically deposited to make a conducting tip,38,39with an impact on the spatial resolution of the imaging probe.

We converted as-prepared graphene-coated AFM tips into conductive AFM probes by evaporating a continuous gold thin ?lm onto the back of the AFM tip chip,wrapped by the graphene layer,to which an electrical contact was made(see schematic in Figure3d).Simultaneous AFM maps of topography and electrical conductivity of highly oriented pyrolytic graphite(HOPG)were recorded.This substrate was chosen for the well-known structure and the electrical heterogeneity of its surface after exfoliation.40,41As shown in Figure3e,the surface presents several graphitic planes that show distinct electrical conductivity,in agreement with the behavior previously reported by employing metal-coated AFM probes for its characterization.40?43We found that a single tip could be used for more than50h for conductive AFM measurements without noticeable deterioration in performance (a total of>50images,each of a5μm×5μm area).Our transfer method brings to the fore a quick and easy approach for making these tips.Such conducting probes may also serve as a platform for molecular junctions,44and other applications,for example,in electrochemistry and electrochemical imaging. Graphene Coating on TEM Grids.There is currently considerable interest in using graphene?lms as supports for TEM measurements.45?47However,most processes to

deposit Figure3.(a,b)SEM images of two graphene-coated AFM tips.(c) TEM images of the end of a graphene-coated AFM tip.(d)Schematic illustration of production of a conductive AFM probe by coating graphene on a commercial tip,followed by gold evaporation on the back.(e)Topography and conductivity maps for a5μm×5μm area of high-quality highly oriented pyrolytic graphite(HOPG),utilizing a graphene-coated conductive AFM tip.Experimental details are given in Materials and Methods.

graphene on holey TEM grids use polymer-assisted routes.48One study that was free of polymer,however,involved the etching of an Si/SiO 2layer,but this is time-consuming and possibly introduces more contaminants to the graphene surface.49

In this study,we employed the biphasic graphene transfer method to produce TEM grids with one continuous single layer of CVD graphene as a support (see Materials and Methods ).This represents a simple,cheap,and quick route to obtain graphene TEM substrates.The original TEM grids were in the form of Cu meshes with holes (11.5μm ×11.5μm),so that the transfer of graphene resulted in sections with a suspended graphene membrane (across the holes)and a supported graphene ?lm (on the Cu grid).After the transfer,coverage was complete for the majority of the grid,and an area of the as-prepared graphene TEM grid was characterized by AFM and SEM (Figure 4).In the AFM image of Figure 4a,a partially

coated hole in the upper left corner is deliberately displayed to present the contrast between covered and uncovered regions.The whole layer of graphene is therefore well-coated across the grid,with regions of suspended graphene membrane slightly subsiding from the surrounding Cu bars but remaining continuous,due to its strong mechanical properties (Support-ing Information,section S5).

SEM images of a partially covered hole,at the edge of the graphene ?lm (Figure 4b ?d),show that the graphene ?lm provides an excellent conformal coating over the relatively coarse Cu surface,as was also found for AFM tips.An important factor responsible for the excellent coating is evaporation of water and hexane trapped between the graphene sheet and the TEM grid after the transfer,which can act to pull both materials into intimate contact.25,49Compared with transfer methods that are assisted by relatively rigid polymer ?lms,such as PMMA and PDMS,this new method directly utilizes a graphene ?lm that is more ?exible,while also being free from additional treatments (e.g.,heating)used to enhance the contact,which are often required for polymer-transferred graphene.9

Graphene Membrane as a Support for TEM Charac-terization.The two-dimensional ultrathin nature of graphene,

and its low atomic number,together with excellent mechanical,thermal,and electrical stability,presently make it the ultimate support ?lm for TEM studies.25,45?48,50,51Indeed,graphene supports are nearly transparent to electron beams and enable atomic-resolution imaging of objects,such as biological molecules,48gold nanocrystals and its citrate capping agents,50or small organic molecules,51which would otherwise be very di ?cult to observe by TEM with commercial carbon supports.Herein,we imaged gold nanoparticles (AuNPs)to demonstrate that the suspended graphene membranes obtained with our biphasic method can be used as TEM supports.A drop of solution containing AuNPs was deposited onto the graphene-coated TEM grid and left in air to dry before TEM imaging was carried out.Figure 5a shows several AuNPs loaded

on the freestanding graphene membrane.They are of regular shape and similar size (～10nm diameter),as expected.High-resolution TEM characterization was also performed,from which the gold atomic structure and ligands (citrate,blurred surroundings)of a single AuNP can be seen (Figure 5b).Wetting and Electrochemistry of Supported and Suspended Graphene.The graphene TEM substrate opens up further opportunities for investigating wetting electro-chemistry on suspended graphene,for the ?rst time,and comparing the response to that of Cu-supported graphene on the same sample.This is possible by use of scanning electrochemical cell microscopy (SECCM),which essentially brings a small-scale meniscus electrochemical cell and counter/reference electrodes to a surface (working electrode),allowing electrochemical measurements of unusual electrode materials (see Figure 6a).52?54

It is well-known that the properties of graphene may be strongly in ?uenced by the supporting substrate;hence,studies on freestanding graphene are of enormous interest.55?57The graphene TEM grid was electrochemically tested with two well-known redox couples:(ferrocenylmethyl)trimethylammonium (FcTMA +/2+)and hexaammineruthenium [Ru(NH 3)63+/2+].SECCM utilizes a tapered θpipet ?lled with a solution of interest,such that a meniscus is formed across the two barrels at the end of the pipet.A bias,V 1,is applied between the two quasi-reference counter electrodes (QRCEs,an Ag/AgCl wire inserted into each barrel),to produce an ion conductance/migration current (i DC )between the barrels.When the meniscus comes into contact with the surface of a substrate (working electrode),its potential is controlled by tuning V 2,so that ?(V 1/2+V 2)versus QRCE is the working

electrode

Figure 4.(a)AFM image (50μm ×50μm)of part of the fabricated graphene TEM grid (schematic in inset),with a partially coated window observed in the upper left corner.(b)Top and (c,d)side views of false-colored SEM images of a graphene partially coated window of a TEM grid (graphene in blue).True-color images can be found in Supporting Information,Figure S6

.

Figure 5.(a)Low-magni ?cation TEM image of gold nanoparticles capped by citrate and (b)high-resolution TEM image of a gold nanoparticle,on a suspended graphene membrane over a Cu TEM grid.

potential (E )and i EC is the corresponding electrochemical current due to any redox reactions.This platform con ?nes the electrochemical cell to submicrometer (nanoscale)dimensions and allows either the Cu-supported graphene or suspended graphene on the TEM grid to be assessed individually by careful positioning of the SECCM probe in di ?erent places of the sample.

The SECCM setup was mounted on an inverted microscope,to facilitate the precise navigation and landing of the meniscus onto the graphene ?lm.The pipet ?rst approached near the graphene sheet,without establishing meniscus contact,by means of a micropositioner.The di ?raction of light due to the presence of the pipet was clearly seen through the inverted microscope and was used to locate the position of the pipet with respect to the TEM grid (on suspended or supported graphene)(see Supporting Information,section S6).From this point,further ?ner pipet approach was achieved with high control of the z -piezo of the SECCM setup.The ion conductance current or i DC can be indicative of the size of the meniscus between pipet and substrate 41,52?54and was used here to diagnose landing of the meniscus on the surface and control of the pipet (as described previously).58

In Figure 6b,c,we show representative approaches of i DC versus z -piezo displacement against supported and suspended graphene (representative of >16experiments in each case).On supported graphene (Figure 6b),after ?rst contact of the meniscus with conductive substrate (detected through a current spike in the electrochemical current i EC ),the meniscus was squeezed against the solid surface,as deduced from the continuous decrease of i DC with the approach.59This value dropped by approximately 20%until a sudden increase in the current was detected at a piezo displacement of ca.33.9μm,attributed to the meniscus,under pressure,suddenly wetting the surface.In contrast,when the pipet meniscus came into contact with the suspended graphene sheet (Figure 6c),i DC decreased monotonically by up to ～30%,during squeezing of the meniscus.This provides some qualitative implications about the di ?erence in wettability of Cu-supported and suspended graphene.

The wettability of graphene is of considerable interest,given the increasing application of graphene-coated materials.Yet the relatively few studies available are not in agreement,especially on the e ?ect of the substrate.29,60,61To the best of our

knowledge,the intrinsic wettability of suspended graphene has only been predicted theoretically by molecular dynamics 62and has not been measured,due to experimental challenges.Our studies suggest that Cu-supported graphene exhibits stronger wettability compared with a freestanding graphene sheet.This is in line with theoretical studies showing that the contact angle of water on suspended graphene is higher than on Cu-supported graphene.29,62?64

To further investigate the wettability of the suspended graphene membrane,approach and retract experiments were carried out in which the meniscus of an SECCM pipet was pushed further against the graphene with the precise control of the z -piezo,while i DC against z -piezo displacement was recorded,and the reverse (pull-o ?)of the meniscus was also measured.An example of these approach and retract curves (with ion conductance current i DC normalized to initial value of the approach i ini ,i DC /i ini )is presented in Figure 7(which is typical of three di ?erent experiments).The pipet came into contact with the graphene sheet at position 1on the approach,and as the pushing continued,a gradual decrease of

ionic

Figure 6.(a)Schematic of an SECCM pipet landing on supported and suspended parts of a graphene membrane over a Cu TEM grid (not to scale).(Inset)SEM image of the end of the type of pipet used.(b,c)Typical approach curves demonstrating the change of ion currents (i DC )against z -piezo displacement when a pipet meniscus was landed on (b)supported and (c)suspended graphene.Dashed vertical lines indicate the position where the meniscus ?rst contacted the graphene surface (red)and wetted graphene (green,panel b).These approaches are representative of more than 16measurements carried out for each of these two

scenarios.

Figure 7.Plot of normalized ion conductance current as a function of z -piezo displacement during the approach and retract processes of an SECCM pipet on suspended graphene.

current is observed due to meniscus compression (as described for Figure 6).The decrease (by ～25%)stopped at position 2,after which there was a slight increase in current that we attribute to minor meniscus wetting.This is because the wettability of suspended graphene can be enhanced if strained,65and the force on the meniscus between pipet and graphene may also aid wetting.The pipet was pushed further until position 3,whereupon the translation of the pipet was reversed.Interestingly,there is clearly an attractive interaction between water molecules and the atomically thin carbon sheet,as when the pipet was pulled away from the substrate surface,an increase in i DC was observed (positions 4,5,and 6),due to expansion (pulling)of the meniscus formed between SECCM probe and graphene substrate.These observations are consistent with recent theoretical predictions.61The meniscus detached at position 7,and i DC (meniscus con ?ned to the pipet)decreased suddenly to its original value.

Suspended graphene devices obtained with our biphasic method,in combination with SECCM,were also employed to study electrochemistry at suspended graphene for the ?rst time.Upon meniscus contact with the graphene sheet,the pipet was held and three cyclic voltammograms (CVs)were recorded at a scan rate of 0.1V ·s ?1at each landing site for (i)FcTMA +/2+(oxidation)and (ii)Ru(NH 3)63+/2+(reduction)in separate experiments (Figure 8).The CVs show the sigmoidal response

of a microelectrochemical system with nonlinear (spherical segment)di ?usion 41,53,66and are highly reproducible.These data are representative of >6spot measurements for each of the two couples.For FcTMA +/2+,the values of the potential di ?erence between the 3/4and 1/4-wave potentials (E 3/4?E 1/4),which is indicative of reversibility of the system,66,67was similar on Cu-supported graphene (75±2mV)and suspended graphene (71±2mV).With respect to Ru(NH 3)63+/2+,the CVs on Cu-supported graphene ?lm led to 69±2mV for E 1/4?E 3/4,and an E 1/4?E 3/4value of 72±2mV was obtained for

suspended graphene.All the CVs observed are characteristic of relatively fast (but not reversible)electron transfer kinetics for FcTMA +/2+and Ru(NH 3)63+/2+on the CVD graphene prepared herein and are broadly in agreement with previous studies on Si/SiO 2-and Cu-supported CVD graphene with the same,and similar,redox species.22,68,69

Cu-supported and suspended graphene on the TEM grid,along with graphene transferred onto Si/SiO 2(see Supporting Information,section S7),behave in essentially the same way (within experimental error)toward the redox couples studied.There is no detectable substrate e ?ect on the electrochemistry of CVD monolayer graphene at the spatial resolution of this study.Note that the limiting currents of FcTMA +/2+and Ru(NH 3)63+/2+on suspended graphene are lower than those of Cu-supported graphene.This is due to the di ?erent wettability of supported and suspended graphene membranes,producing di ?erent meniscus contact (working electrode)areas and mass transport rates (vide supra,Figure 6).

■

CONCLUSIONS

A new and e ?cient polymer-free biphasic (liquid/liquid)method for transfer of monolayer graphene to a variety of substrates has been demonstrated that opens up new applications and avenues for graphene studies.Key advantages of the method are that the graphene ?lms produced are completely free from any polymer contamination and that detrimental treatments,often associated with polymer-supported transfer routes,are minimized.

The new polymer-free transfer process is easy to implement and we have shown the capability of the method for transferring graphene (of centimeter scale)onto arbitrary substrates,including complex 3D objects such as AFM tips and TEM grids.The transferred graphene has been shown to adapt well to the substrate surfaces,resulting in high-quality conductive graphene-coated AFM tips and graphene TEM grids.Graphene coating of AFM tips is advantageous compared to metal-coated tips in that the spatial resolution is not compromised,due to the thinness of the graphene layer.Note that although graphene transfer was exempli ?ed with single tips,it should be possible to coat batches (wafers)of AFM probes from the transfer of a single graphene sheet,when it is considered that large-area graphene ?lms can be produced by CVD growth.The resulting graphene-coated AFM probes would also be amenable to further covalent functionalization,for example,via the reduction of diazonium molecules,o ?ering a new platform by which to produce probes for molecular recognition applications,as an alternative to the standard thiol modi ?cation of gold-coated tips.It is expected that the probes could be further modi ?ed into ultramicroelectrodes for use in combined atomic force and scanning electrochemical micros-copy (AFM-SECM),among other applications.

Graphene-coated TEM grids have enabled the wettability and electrochemistry of suspended graphene to be explored for the ?rst time,and also provide a powerful platform for high-resolution imaging of nanostructures.The electrochemical activity of suspended graphene (no discernible di ?erence from supported graphene)makes it suitable for use in sensors and other devices.The electrode/TEM grid combination would serve as a powerful platform for the electrodeposition of nanomaterials for subsequent TEM characterization,and it may also be possible to use the transfer method to fabricate cells for in situ TEM measurements.Further work to explore

the

Figure 8.Cyclic voltammograms for oxidation of 1mM FcTMA +and reduction of 1mM Ru(NH 3)63+in 25mM KCl,recorded at 0.1V ·s ?1on (a)supported graphene and (b)suspended graphene.Three consecutive cycles are shown for each case:?rst (blue),second (black),and third (red)scans.The data are representative of measurements in >6di ?erent locations (spots)for each couple.

graphene coating of soft materials,in particular,could be very worthwhile.

■MATERIALS AND METHODS

CVD Growth of Graphene.Monolayer graphene was synthesized in a commercial low-pressure CVD system(nanoCVD8G,Moore?eld Nanotechnology).Copper foil(no.13382,25μm,99.8%;Alfa Aesar) was cut into～1cm×1cm square substrates and subsequently cleaned with acetone,propan-2-ol,and water before being put into the CVD growth chamber.A purge regime was performed,pumping the system to vacuum and back?lling with Ar,?ve times.Subsequently the sample was heated to900°C as quickly as possible,under a?ow of 120standard cubic centimeters per minute(sccm)Ar and10sccm H2, before a temperature of900°C was maintained for2min.The temperature was then quickly increased to1000°C under the same gas ?ow conditions.The pressure regime of the system was also changed and set to maintain a chamber pressure of10Torr.The system was left to stabilize for15min to anneal the copper foils,before17%(of total gas?ow)CH4was introduced for10min,promoting graphene growth.Postgrowth,the CH4?ow was halted,while a?ow of120sccm Ar and10sccm H2was still maintained,allowing the system to cool down to100°C,at which point the system was vented and the sample was taken into air to cool down to room temperature.

Polymer-free Graphene Transfer.Mechanical polishing with sandpaper(P4000,Buehler)was employed to remove the graphene layer that grew on the back side of copper,exposing the copper surface and facilitating subsequent etching.For this purpose,the graphene/Cu sample was gently placed onto the surface of a0.1M ammonium persulfate[(NH4)2S2O8,Sigma?Aldrich,≥98%]aqueous solution, and a layer of hexane(VWR Chemicals,99%)was slowly added on top by means of a syringe.After an appropriate etching time(～12h)to remove the copper substrate,the graphene layer was left trapped at the interface.The graphene sheet was scooped out with a clean Si/SiO2 substrate,completing the?rst transfer step.A second transfer to a new liquid/liquid interface was carried out to remove any residual salt particles and debris from the back of the graphene layer.This was readily achieved by bringing the graphene?Si/SiO2substrate to an interface between hexane and water.The graphene sheet was kept there for5h,to aid cleaning.For the coating of AFM tips(RFESP and SNL-10,Bruker)and copper TEM grids(3mm,1500meshes,SPI Supplies),these substrates were temporarily glued onto a small piece of Si/SiO2as a support to facilitate manipulation with tweezers and scoop out the graphene sheet.The substrates were left in air brie?y to dry before use.

Scanning Electrochemical Cell Microscopy.The main features of the SECCM setup are illustrated in Figure6and described elsewhere.52,70A double-barrel capillary(1.5mm o.d.,1.2mm i.d., TGC150-10,Harvard Apparatus)was pulled to a～400nm tapered end by use of a CO2-laser puller(P-2000,Sutter Instruments).The pipet was then silanized in dichlorodimethylsilane[Si(CH3)2Cl2,Acros Organics,99+%]to provide a hydrophobic outer wall,before it was ?lled with a solution containing the redox species of interest:either1 mM(ferrocenylmethyl)trimethylammonium hexa?uorophosphate (FcTMA·PF6)or1mM hexaammineruthenium(III)chloride[Ru-(NH3)6Cl3,Strem Chemicals,99%]in25mM KCl.A data acquisition rate of390points/s(each point the average of256readings)was achieved by use of an FPGA card(PCIe-7852R)with a LabVIEW 2013interface.

Sample Characterization.Raman spectra and map were acquired on a Renishaw inVia micro-Raman microscope?tted with a charge-coupled device(CCD)detector and a633nm HeNe laser.The laser power was employed through a50×magni?cation lens,resulting in a laser spot size on the graphene surface of～1μm in diameter.Field-emission SEM images of SECCM pipets,conductive graphene AFM tips,and graphene TEM grids were obtained with a Zeiss Supra55-VP microscope at an acceleration voltage of3kV,with a secondary electron detector.AFM imaging of HOPG was carried out with a home-modi?ed Innova AFM(Bruker).The HOPG sample was kindly provided by Professor R.L.McCreery(University of Alberta,Canada),originating from Dr.A.Moore,Union Carbide(now GE Advanced Ceramics).For TEM imaging,a JEOL JEM-2000FX TEM was used to image the graphene-coated AFM probes.High-resolution TEM images of gold nanoparticles(10nm diameter,in citrate bu?er,Aldrich)on graphene-coated TEM grids were taken on a JEOL JEM-2100LaB6

TEM.Both microscopes were operated at200kV accelerating voltage.■ASSOCIATED CONTENT

*Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acsami.6b00681.

Additional text and eight?gures with details of polymer-

transfer method,optical and AFM images of CVD

monolayer graphene transferred to Si/SiO2,Raman

spectra and map of CVD monolayer graphene,

comparison of graphene transferred by the PDMS-

assisted route and the new biphasic method,SEM images

of graphene-coated TEM grids,light di?raction from

SECCM pipet over graphene-coated TEM grid,and CV

of graphene transferred onto Si/SiO2(PDF)

Video demonstrating the transfer of graphene(AVI)■AUTHOR INFORMATION

Corresponding Authors

*E-mail a.g.guell@https://www.wendangku.net/doc/5917916572.html,(A.G.G.).

*E-mail p.r.unwin@https://www.wendangku.net/doc/5917916572.html,(P.R.U.).

Present Address

?(A.G.G.)School of Engineering and Built Environment, Glasgow Caledonian University,Cowcaddens Road,Glasgow, G40BA,United Kingdom.

Author Contributions

?G.Z.and A.G.G.contributed equally.

Notes

The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS

We appreciate the Chancellor’s International Scholarship from the University of Warwick awarded to G.Z.We acknowledge the EPSRC(EP/H023909/1)for funding A.G.G.and R.A.L. and?nancial support from Lubrizol to P.M.K.We also acknowledge support from the European Research Council through ERC-2009-AdG247143-QUANTIF.We thank Dr. Jonathan Newland for acquisition of photos and videos,Ashley Page and Minkyung Kang for writing LabVIEW programs and support,and Dr.Yang-Rae Kim for helpful discussions.We also thank Professor Julie Macpherson,Dr.Anatolii Cuharuc,and Dr.Jonathan Edgeworth(Moor?eld Nanotechnology)for advice,for general discussions on graphene growth,and for providing the CVD reactor.Some of the equipment used in this work was obtained through the Science City Advanced Materials project with support from Advantage West Midlands

and the European Regional Development Fund.

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