graphene is likely 
 to play a key role in a broad range of electronic , 
 electrochemical , and structural applications . because the study of 
 these systems continues to focus on the nanoscale , understanding the 
 interaction of graphene with ultrathin films , molecular assemblies , 
 and even individual molecules becomes increasingly important . to reliably investigate such structures with techniques , such as 
 atomic force microscopy and scanning tunneling microscopy , smooth 
 substrates , devoid of contamination and large features , similarly , surface force measurements , which 
 require smooth surfaces over macroscopic areas , are also constrained 
 by these requirements . graphene can be 
 prepared directly from graphite , either mechanically or via chemical exfoliation ; however , the characteristics of the graphene flakes produced are 
 largely uncontrollable , and the mean flake size is limited to only 
 a few micrometers . conversely , graphene synthesized by chemical vapor 
 deposition ( cvd ) provides much larger surface areas with control over 
 the number of graphene layers ( vlassiouk et al . nonetheless , difficulties 
 arise in transferring graphene from the growth substrate to the target 
 material , with polymer contamination of the graphene surface common 
 when using current transfer methods . furthermore , graphene is generally wrinkled because of a difference 
 in thermal expansion coefficients of graphene and the growth substrate 
 upon cooling after synthesis . here , we present 
 a new and facile method of transferring cvd graphene 
 to produce clean , molecularly smooth surfaces on the order of 1 cm in area . double - transfer  
 procedure , where an intermediate step uses freshly cleaved and atomically 
 smooth mica as a template to flatten the graphene . mica is a naturally 
 occurring mineral , which can be cleaved to reveal a perfectly clean 
 and flat surface over macroscopic areas , making it ideal for this 
 process . for sfb applications , it is imperative that graphene exhibits 
 a close to molecularly smooth roughness and polymer contamination 
 is explicitly avoided . as a final step in this work , we demonstrate 
 that the resulting graphene surfaces are indeed sufficiently smooth 
 to allow for force measurements in a surface force balance ( sfb ) , 
 by presenting a proof - of - principle measurement . the modified instrument 
 is referred to as a graphene surface force balance ( gsfb ) . some features 
 of the resulting force profiles across aqueous solution are as yet 
 unexplained , and future work in our laboratories will investigate 
 the interaction between graphene sheets with externally applied potential . the sfb [ also called surface force apparatus ( sfa ) ] has provided 
 pioneering measurements over the past half - century of surface and 
 colloidal forces in liquids : electrostatic surface forces , lifshitz / van der waals forces , solvation 
 forces , forces because of adsorbed and 
 grafted polymers , and forces because of surfactants / lipids 
 and biological molecules were all first 
 characterized using this technique . the power of the method arises 
 from the molecular ( often sub - molecular ) resolution in separation 
 between two identical and atomically smooth mica sheets of precisely 
 known contact geometry , obtained using white - light interferometry . although a number of modifications have been made to explore 
 different substrate materials , including the hugely desirable move 
 to use conducting materials , it has not been possible to replace both 
 surfaces with conducting layers of ( sub)molecular roughness over the 
 square - centimeter - sized areas as required . the difficulty arises from the simultaneous need 
 for optical transparency , good conductivity , and molecular - scale smoothness 
 over macroscopic ( cm ) areas . it is the aim of 
 this work to provide such surfaces using cvd graphene and demonstrate 
 their potential for future surface force measurements . schematic of the conventional 
 mica sfb illustrating the crossed - cylinder 
 geometry of the mica - covered lenses . semi - transparent silver mirrors 
 behind the mica form a white light interferometer , which is used to 
 measure the surface separation with sub - nanometer precision . forces 
 at the mica surface are simultaneously measured by recording the deflection 
 of a spring as the surfaces are brought together . in summary , we propose a new ultraflat transfer for graphene , 
 which 
 produces molecularly smooth , clean model surfaces . although graphene 
 has attracted a huge amount of attention in recent years , it has never 
 before been used in sfb measurements . this technique facilitates a 
 modification to the classical sfb to produce a gsfb that allows for 
 graphene  graphene interactions to be measured across air and 
 across liquids with simultaneous detection of their separation with 
 sub - nanometer resolution . few - layer graphene was synthesized 
 using atmospheric pressure chemical vapor deposition of ch4 in the presence of h2 on 10  25 mm cu foils ( alfa 
 aesar , 99.8% , 25 m thick ) . the cvd setup consisted of a quartz 
 tube ( 20 mm inner diameter ) located inside a horizontal cylindrical 
 furnace and connected to an acetone bubbler . contamination from supplier 
 processing was removed before synthesis by sonicating copper in acetic 
 acid for 10 min and then rinsing with deionized water . copper foil was placed inside the quartz tube 
 at 1035 c and annealed for 30 min in the presence of 500 standard 
 cubic centimeters per minute ( sccm ) h2 to further clean 
 the copper surface and increase the copper grain size . hydrogen flow 
 was then reduced to 300 sccm , and 5 sccm ch4 was introduced 
 for 50 min for few - layer graphene growth . graphene was transferred 
 from the copper foil by drop - coating poly(methyl methacrylate ) ( pmma ; 
 molecular weight = 996  000 , dissolved in chlorobenzene ) onto 
 one side of the foil and then etching the copper in 0.1 m ammonium 
 persulfate solution . the pmma / graphene was then removed from the etchant 
 solution and washed thoroughly with ultrapure water [ 18.2 m 
 resistivity , < 1 ppb total organic carbon ( toc ) ] . it was then placed 
 onto freshly cleaved mica , graphene side down , and pressed between 
 glass microscope slides at 110 c for 30 min . pmma was removed 
 by submerging the sample in glacial acetic acid ( fisher , extra pure ) 
 for 2 h. a total of 40 nm of ag ( alfa aesar , 
 99.999% ) was thermally evaporated under vacuum ( 0.3  s , 10 mbar ) onto hemispherical quartz lenses ( 1 
 cm diameter , 1 cm radius of curvature ) . fringes arising from silver 
 mirrors evaporated onto quartz lenses were similar to fringes arising 
 from back - silvered mica and smooth enough to resolve long - range forces . 
 alternatively , the mirrors can also be prepared via the template stripping 
 method , which produces atomically smooth mirrors with potentially 
 increased resolution . buelher epothin epoxy diluted with ethanol ( sigma - aldrich , 
 > 99.8% , 1:2 ratio by volume ) was spin - coated onto the ag surface 
 with 
 a laurel ws650 mz-23npp / lite spin coater . the following spin cycle 
 was used : accelerate at 100 rpm s for 5 s and 
 then accelerate to 3000 at 300 rpm s for 30 s. graphene on mica was then laid onto the surface , and the epoxy 
 was left to cure overnight . the mica could then be removed , and the 
 ultraflat graphene was exposed immediately prior to use in the sfb . 
 submerging the lens in ultrapure water facilitated removal of the 
 mica . the sfb used has been described in detail elsewhere ; a full paper describing the setup and operation 
 of the modified gsfb will be communicated in due course . essential 
 details of the setup relating to this proof - of - principle experiment 
 are as follows . two hemicylindrical lenses , with graphene sheets attached 
 as described in the text , were mounted in crossed - cylinder configuration 
 inside the sfb . one lens was translated toward or away from the other , 
 in the perpendicular direction , using a motor and a differential spring 
 mechanism to achieve fine control . forces were detected via the bending 
 of a spring upon which one surface is mounted . under high applied 
 loads , the glue layer was compressed , leading to a systematic error 
 in the separation distance under high load ; this was corrected by 
 subtraction of a linear fit to the compressive behavior from the measured 
 data points . fringes arising from reflection 
 between the silver layers on the lenses [  fringes of equal 
 chromatic order  ( feco ) ] were observed in a grating spectrometer 
 and captured using a charge - coupled device ( ccd ) camera . the shape 
 and location of the fringes within these images were extrapolated 
 using a method similar to that used by quon et al . in this method , each row of the pixels is fitted separately 
 to map the fringe . however , in contrast to previous work , the algorithm 
 here used a one - dimensional center of mass centroid fit instead of 
 a gaussian fit . by approximating that the two glue layers are 
 of the same thickness 
 and noting that the thickness of the glue is far greater than that 
 of graphene , the interferometer was estimated to behave as a three - layer 
 symmetric interferometer . this allowed for the fringe locations from 
 within the images to be converted into distance separation using the 
 standard analytical solution for a three - layer interferometer described 
 elsewhere . overbeek 
 ( dlvo ) fitting process and was found to be 2 cm in this case . the potential of the graphene surfaces was controlled with a metrohm 
 autolabiii potentiostat , with pt wire as the reference electrode , 
 and the effective surface potential at the region of interaction was 
 measured from the minimum point of an electrocapillary curve obtained 
 through electrochemical impedance spectroscopy ( see figure s1 of the supporting information ) . electrical connections 
 to the graphene surfaces were made with silver conductive paint ( rs 
 components ) . cvd graphene 
 was characterized 
 by scanning electron microscopy ( sem ) , raman spectroscopy and atomic 
 force microscopy ( afm ) . for sem raman spectroscopy was conducted at room temperature using 
 a jy horiba labram aramis raman spectrometer equipped with a 532 nm 
 laser . atomic force micrographs were recorded with a nanoscope multimode 
 afm in contact mode ( nanosensors pointprobe plus contact mode probes ) . we present a new 
 five - layer interferometer setup , analogous to the traditional mica  film mica 
 interferometer used in sfb , consisting of epoxy  graphene  film  graphene  epoxy 
 ( where the film can be air , vapor , or a liquid ) , as shown in figure 2a . the key to the success of the graphene - coated 
 lenses for force measurement and interferometry are ( i ) chemical cleanliness 
 ( e.g. , absence of polymer residues from graphene transfer ) and ( ii ) 
 absence of  positive protrusions  in the graphene , as 
 indicated in panels b and c of figure 2 . this 
 was achieved by way of a new double - transfer procedure , making use 
 of an atomically smooth and clean mica surface as a template . individual 
 steps in this double - transfer procedure were as follows ( see the materials and methods for full details , and a schematic 
 of the procedure is shown in figure 3 ) : graphene was synthesized using 
 atmospheric - pressure cvd of ch4 in the presence of h2 on cu foils at 1035 c . gas concentrations and experimental 
 times were tailored to synthesize high - quality , few - layer graphene . graphene was transferred 
 from the cu foil by adapting and incorporating additional steps to 
 a well - established conventional polymer transfer method . pmma was deposited onto the graphene / cu foil , followed by copper 
 substrate etching . the pmma / graphene was then removed from the etchant 
 solution , thoroughly rinsed with ultrapure water ( 18.2 m cm 
 resistivity , < 1 ppb total organic content ) , and placed onto freshly 
 cleaved mica . the mica was cleaved such that it was thin and flexible 
 enough to lie over a curved sfb lens . following this , the pmma / graphene 
 was pressed into the mica surface at 110 c , after which pmma 
 was removed with glacial acetic acid . to detect separation distance 
 and interaction force , the sfb requires each of the hemicylindrical 
 optical lenses to be covered by a partially reflective mirror , which 
 was achieved by evaporating 40 nm of silver onto the sfb lenses . furthermore , the silver mirrors must still be separated by 210 
 m when the graphene surfaces are in contact . to achieve this 
 precise separation of the mirrors , the epoxy used to fix the graphene 
 to the mirror was spin - coated . the resulting epoxy thickness ( 110 
 m ) was precisely controlled by diluting the epoxy and varying 
 the speed and duration of the spin cycle . at this stage , after the epoxy had cured , 
 the mica was peeled off , revealing an ultraflat graphene surface , 
 which mirrored the low roughness of the mica template . submerging the lens in ultrapure water greatly facilitated the mica removal , 
 presumably because of favorable interactions between water and the 
 high - energy mica surface . the mica removal step was performed immediately 
 prior to using the lenses in the gsfb to minimize exposure to airborne 
 contamination and ensure that the surface was as clean as possible . 
 for electrochemical experiments , electrical connections to the graphene ( a ) illustration of gsfb lens structure detailing the 
 five - layer 
 interferometer setup consisting of epoxy  graphene  film  graphene  epoxy . ( b and c ) two sets of graphene lens surfaces showing the detrimental 
 effect of positive protrusions on finding a contact point . after cvd growth of graphene 
 on copper , the graphene surface is coated with pmma to provide a mechanical 
 support for the graphene film during etching . after etching , the graphene 
 is laid onto freshly cleaved mica . ( right ) preparation of sfb lens with the ultraflat graphene 
 surface suitable for the feco interference method . a total of 40 nm 
 of silver is evaporated onto a clean sfb lens . subsequently , the epoxy 
 adhesive is spin - coated onto the silver layer onto which the graphene 
 mica stack is placed . after curing , removal of the mica with water 
 exposes a clean , smooth , graphene surface with no positive protrusions . cvd - synthesized graphene 
 allows for the production of large area graphene films with precise 
 control over the number of graphene layers . panels a and b of figure 4 show typical sem 
 images of fully covered copper substrates with few - layer ( approximately 
 35 ) cvd graphene used in these experiments . figure 4a demonstrates the uniform coverage of the few - layer 
 graphene across several copper grains . as highlighted by a white arrow 
 in figure 4b , wrinkles were observed in these 
 sem images as expected for dense coverage of few - layer graphene . regardless 
 of this level of roughness , the proposed transfer technique still 
 allows for molecularly smooth surfaces to be obtained . ( a and b ) sem images 
 of few - layer graphene ( white arrow highlights 
 an example of a wrinkle ) . the absence of a d peak at 1350 cm is indicative 
 of a low - defect density . ( d ) sem image of graphene transferred to 
 a glass slide using the ultraflat technique ( white arrow highlights 
 a now inverted wrinkle , embedded in the epoxy below , as in the schematic 
 in figure 2b ) . figure 4c shows a typical raman spectrum 
 of the graphene used in this study . the ratio of the 2d / g peak is 
 indicative of few - layer graphene . there is no appearance 
 of a d peak in the spectrum ( 1350 cm ) , 
 indicating high - quality ( low - defect ) graphene . figure 4d shows a sem image of graphene 
 that was transferred onto a glass slide , as opposed to a sfb lens , 
 via the ultraflat transfer process as outlined in figure 3 . a white arrow highlights a wrinkle that can be 
 seen protruding into the epoxy below , as shown schematically in figure 2c . a raman spectrum of the graphene after transfer 
 onto a sfb lens is shown in figure s2 of the supporting 
 information . the resistance across the lens , measured 
 with a two - point probe , 
 was 0.52 k , indicating that moderate currents ( up to 
 1 a ) could be passed through the lens without significant 
 voltage drops or joule heating occurring . full coverage of graphene 
 across the copper foil was preferred 
 for sfb purposes to ensure that only graphene  graphene interactions 
 were probed rather than graphene  epoxy interactions . the benefits 
 of few - layer graphene in sfb experiments are expected to be twofold : 
 increased screening of intermolecular forces from the underlying epoxy 
 and increased conductivity of the surface . it has been shown that monolayer graphene is essentially  transparent  
 to the van der waals forces of the underlying support material ; however , 
 as the number of graphene layers increases , the contact angle of a 
 water droplet approaches that of water on graphite . conductivity of the surface is of interest because 
 a conductive 
 surface permits external control of the surface potential . the recorded 
 resistance affirms the viability of using the gsfb for electrochemical 
 and other potential - dependent experiments . figure 5a shows cvd graphene 
 transferred onto mica before transfer to the lens ( see the left - hand 
 side of figure 3 for transfer details ) . this 
 image is representative of cvd graphene , which has been transferred 
 to an arbitrary substrate , such as a silicon wafer , using the conventional 
 polymer transfer method . such a surface is unsuitable for surface force measurements 
 because the protrusions prevent close contact of the surfaces ( see 
 figure 2c ) . ( top ) afm micrographs of ( a ) 5 m scan of the 
 top surface of cvd graphene transferred onto mica using a conventional 
 polymer transfer method , with 4.43 nm rms roughness , ( b ) 5 m scan of ultraflat cvd graphene transferred onto sfb lens 
 using a double - transfer procedure , with 0.44 nm rms roughness , and 
 ( c ) 1 m scan of ultraflat cvd graphene transferred 
 onto sfb lens using a double - transfer procedure , with 0.19 nm rms 
 roughness . when the graphene is 
 flipped , adhered onto a quartz lens , and the 
 mica is removed , the resulting graphene surface replicates the flatness 
 of the mica . indeed , from the afm images ( figure 5 ) , it is clear that graphene as laid onto the sfb lens using 
 the transfer method ( panels b and c of figure 5 ) is significantly smoother than graphene transferred using a conventional 
 transfer method ( figure 5a ) . the root - mean - square 
 ( rms ) roughness of the double - transferred surface is 0.44 nm over 
 5 m and 0.19 nm over 1 m , approaching 
 the roughness of mica itself . this is 
 an order of magnitude less than the roughness of the non - templated 
 surface ( 4.43 nm ) . furthermore , the dominant features in figure 5b , as seen from the height profile , are negative 
 protrusions or  valleys  , enabling surfaces to come 
 into contact during sfb measurements ( figure 2b ) . low surface roughness is essential for insightful sfb experiments 
 because the roughness essentially limits the resolution of the experiment : 
 if the surface is rough , forces arising from liquid film structure 
 at small surface separation will be obscured by the force required 
 to compress surface asperities ( figure 2 ) . the resultant graphene surface is also free from commonly encountered 
 polymer residue contamination because this side of graphene is never 
 in contact with pmma . furthermore , there 
 is no evidence of the presence of metal nanoparticle contamination 
 arising from incomplete etching of the copper . finally , the surface is free from airborne particulates . this is because , unlike conventional transfer methods , the final graphene 
 surface has had only brief exposure to the ambient environment . the 
 graphene surface is protected by the mica up until the point at which 
 it is required , and the mica is removed . top  side of 
 the graphene , the side exposed to the air after synthesis and any 
 polymer support . to confirm 
 the feasibility of measuring surface forces between graphene sheets 
 with external control of the surface potential in the gsfb , the interaction 
 between graphene sheets across an aqueous solution of 0.1 mm kclo4 was investigated ( figure 6 ; procedure 
 detailed in the materials and methods ) . the 
 force , as a function of surface separation in the range of 200 
 nm down to 0 nm , was first measured at open circuit . at large separation , no force is detected ( within the resolution of the measurement ) , and 
 then very weak repulsive double - layer forces are detected in the range 
 from tens of nanometers to the point of surface contact . the surfaces 
 were then retracted , and a potential of 210  10 mv versus 
 the point of zero charge ( see figure s1 of the 
 supporting information ) was applied to both graphene surfaces . the subsequent force profile shows a large repulsive interaction , 
 exponentially increasing with decreasing surface separation . the magnitude 
 of force is much greater with the large applied surface potential , 
 as expected within the standard dlvo theory . the data adheres closely 
 to the solution of the nonlinear poisson  boltzmann ( pb ) equation 
 for a 1:1 electrolyte between identical , charged surfaces with a sphere - on - flat 
 geometry at 210 mv and with 0.6 mm salt concentration . the higher salt concentration ( steeper gradient ) 
 of the pb fit compared to that expected for 0.1 mm ionic strength 
 indicates the presence of additional ions in the double - layer region . this current will have an insignificant effect on the overall concentration 
 ( approximately 10 nm increase in the concentration over the duration 
 of the experiment ) , but there may be an increase in the concentration 
 at the surface where the electrochemical reaction occurs . in the absence 
 of an applied potential , the pb fit agrees with the expected 0.1 mm 
 ionic strength . at small separations , the fit overestimates the force 
 compared to the measured data ; the origin of this non - pb behavior 
 is not yet clear , although we note it is also present in the electrochemical 
 sfb measurements between gold surfaces by kasuya and kurihara . crucially , upon relaxation of the surface potential 
 ( a return to open circuit ) , the original surface force profile was 
 recovered , illustrating the reversibility of the system . the non - zero 
 forces detected at open circuit potential indicate a weak charging 
 of the graphene in the aqueous electrolyte environment , possibly because 
 of specific adsorption . ( a ) force  distance profile of graphene 
 surfaces under potential 
 control in the gsfb with 0.1 mm kclo4 solution . blue triangles 
 show the system at open circuit ; red circles show a 210 mv 
 surface potential ; and green triangles show the system at open circuit 
 after the 210 mv potential had been switched off . the black 
 line corresponds to the theoretically predicted force profile for 
 a 0.6 mm 1:1 electrolyte solution with 210 mv surface potential . the solid green and blue lines correspond to fits for 0.1 mm 1:1 electrolyte 
 solution . ( c ) diagram of the experimental setup . with regard to the objective of this work , viz . 
 demonstrating the 
 feasibility of graphene  sfb measurements , three key points 
 are noted : ( i ) the appearance of double - layer forces ( and no additional 
 repulsive force ) down to nanometer separation between the two graphene 
 sheets indicates that the surfaces are indeed sufficiently smooth , 
 over the whole interacting area , for gsfb measurements . ( ii ) the strong 
 repulsive force observed when symmetric potential is applied to the 
 graphene surfaces indicates that the graphene sheets are continuous 
 across the surfaces and able to support the external potential as 
 expected . ( iii ) the successful use of feco interference fringes to 
 calculate the surface force and surface separation , with nanometer 
 precision , demonstrates the feasibility of using the procedures outlined 
 above for the preparation of graphene lenses and subsequent force 
 measurement in the gsfb . a new double - transfer 
 procedure has been demonstrated for preparation 
 of ultraflat graphene surfaces over macroscopic areas . this was achieved 
 by modifying the conventional polymer transfer method with an additional 
 transfer step using freshly cleaved mica as a perfectly clean and 
 flat support . graphene surfaces produced by the double - transfer procedure 
 have a rms roughness of less than half a nanometer ( 0.44 nm ) over 
 micrometer - sized areas . in comparison , surfaces produced by the conventional 
 polymer transfer method have a rms roughness an order of magnitude 
 higher ( 4.43 nm ) . furthermore , the double - transfer procedure yields 
 graphene surfaces that are free from polymer residues commonly encountered 
 when using conventional transfer methods . these ultraflat graphene 
 surfaces have been integrated into a new 
 apparatus , the gsfb , an instrument for the measurement of surface 
 forces and surface liquid structure at the nanoscale . we show that 
 the graphene surfaces can be brought into clean contact and their 
 separation and interaction force measured with subnanometer resolution 
 by white - light interferometry . graphene adhesion 
 and friction , liquid film structure at the graphene surface , and owing 
 to the high conductivity of graphene , potential - dependent and electrochemical 
 effects on normal interaction forces and shear forces . we note 
 that the graphene transfer method reported here , the key 
 enabling step in the process , is widely applicable for graphene and 
 other two - dimensional ( 2d ) material applications where macroscopic 
 graphene sheets are required and where cleanliness and roughness are 
 key hurdles .
we report a method for transferring 
 graphene , grown by chemical 
 vapor deposition , which produces ultraflat graphene surfaces ( root - mean - square 
 roughness of 0.19 nm ) free from polymer residues over macroscopic 
 areas ( > 1 cm2 ) . 
 the critical step in preparing such 
 surfaces 
 involves the use of an intermediate mica template , which itself is 
 atomically smooth . 
 we demonstrate the compatibility of these model 
 surfaces with the surface force balance , opening up the possibility 
 of measuring normal and lateral forces , including friction and adhesion , 
 between two graphene sheets either in contact or across a liquid medium . 
 the conductivity of the graphene surfaces allows forces to be measured 
 while controlling the surface potential . 
 this new apparatus , the graphene 
 surface force balance , is expected to be of importance to the future 
 understanding of graphene in applications from lubrication to electrochemical 
 energy storage systems .