the 2 stm setup presented 
 in figure 1a was built specifically for this 
 study by dme , danish micro engineering a / s , herlev , denmark . it consists 
 of two standard dme s stm scanners mounted on a custom base 
 that holds the scanners on opposing sides of the sample . this base 
 was designed so that each scanner and the sample can be removed and 
 replaced with a position accuracy of a few ( less than five ) micrometers , 
 and a ccd microscope camera can be placed into the position of each 
 scanner . each scan head is connected to one dual scope c-26 controller 
 ( dme ) . they were controlled by two instances of software dme - scantool . 
 in order to reach the same alignment for both of the scan heads , the 
 following procedure was used : ( 1 ) the sample holder is placed in a 
 solid support (  guide  ) in between the two scanners . ( 2 ) ccd camera is attached to one side of the guide and adjusted to 
 have the sample area in the field of view . the exact position of the 
 graphene membrane is marked with a cross hair on a monitor . ( 3 ) the 
 sample is removed from the guide and the first scan head ( stm 1 ) is 
 placed opposite to the camera and the probe is aligned with micrometer 
 screws to match with the cross hair ( in the focal plane of the camera ) . ( 5 ) steps 1 and 2 are repeated for the 
 second scan head ( stm 2 ) . ( 6 ) finally , the camera is removed and both 
 scanners are attached . the tip of one scanner is now within the scan 
 range ( 5 m ) of the other scanner , and the exact alignment of 
 the tips is possible by using the piezos since the tip - induced deformation 
 of the membrane by one stationary tip is visible in the scanned image 
 of the other tip . our sample preparation 
 follows well - known 
 procedures for the transfer of graphene sheets from silicon / silicon 
 dioxide to a target substrate by using a dissolvable plastic film 
 as an intermediate carrier . in our case , we first transfer the mechanically 
 exfoliated few - layer graphene sheets to plastic - only quantifoil transmission 
 electron microscopy ( tem ) grids with 1.2 m sized holes . this 
  perforated  plastic film has the added advantage that 
 those graphene areas that coincide with holes are never in contact 
 with the polymer . because these grids are nonconductive and not rigid 
 enough for stm experiments , we then transferred the samples to 2.65 
 mm  2.65 mm  200 m silicon window frames purchased 
 from silson ltd . those chips , originally intended for tem use , 
 have a single rectangular window at the center with sizes ranging 
 from 2 to 25 m , from which we selected the smaller 
 ones . to ensure conductivity , we coated the silicon frames with a 
 thin gold layer prior to the transfer . in the single - tip experiment 
 presented in figure 2 , we did not dissolve 
 the plastic film ( stopping the process at step iii in figure 1b ) and thereby obtained graphene membranes on a 
 circular support . in other experiments , the plastic film was dissolved 
 in chloroform followed by methylpyrrolidone ( heated to 100 c ) , 
 leaving the few - layer graphene suspended on the rectangular hole of 
 the silicon window . we used 0.35 mm polycrystalline 
 tungsten wire and a 2 m koh solution as the electrolyte . tem images 
 of an as - prepared stm tip are shown in the supporting 
 information . the thickness of the graphene samples used in 
 these experiments was between 5 and 8 layers and was determined optically . the graphene structures 
 used in the atomistic 
 simulations consisted of either 11040 carbon atoms ( 17 nm  17 
 nm ; figure 5 , supporting 
 information figure s1 ) or 276  800 atoms ( 84 nm  
 84 nm ; figure 6 ) . to mimic the experimental 
 conditions , we initially put the structures under 2% compression and 
 then carried out molecular dynamics simulation at a finite temperature 
 ( 300 k ) for 5 ps . after this step , we obtain a rippled structure as 
 shown in figures 5a or 6c . the carbon  carbon interaction was modeled using the reactive 
 bond - order potential by brenner et al . , and the temperature of the system was maintained according to the 
 scheme of berendsen et al . for 
 simulations presented in figure 5 , all of the 
 atoms were allowed to move , whereas for those presented in figure 6 and supporting information figure s1 the outermost atoms ( atoms within 1 nm from the boundary ) 
 were fixed to model the edge of the substrate . in the scan simulation 
 presented in figure 5 , the effect of the scanning 
 probe was modeled by applying a force of 1 ev /  on those atoms 
 exactly under the tip ( assuming a tip diameter of 3  ) . the tip 
 was moved from y = 75  to y = 75  with a step size of 5  . for each of the tip positions , the atomic structure was optimized with conjugate gradient energy 
 minimization scheme while maintaining the force on the atoms under 
 the tip . then , a 20 ps molecular dynamics simulation at 300 k was 
 carried out before moving the tip further . in the simulations 
 presented in supporting 
 information figure s1 and supplementary video 2 , we first applied 
 an increasing ( upward ) force on one of the atoms in the middle of 
 the graphene sheet until the structure optimization resulted in a 
 convex shape for the optimized structure instead of a concave shape 
 which was obtained with no force . then , an increasing opposite force 
 was applied to the same atom in the convex structure until the force 
 was high enough to break the membrane . after this , we modeled the 
 effect of scanning on the membrane , as described above , by moving 
 the tip gradually over the whole sample area and applying a downward 
 force on the atoms under the tip . as previously , the force was applied 
 to 10 atoms under the tip , which prevented the membrane from breaking 
 even when the total force was increased . the resulting decrease of 
 the height of the membrane after four subsequent scans is presented 
 in the supporting information . to 
 obtain structures with locally different ripple configurations 
 used to study the point and mountain features ( figure 6 ) , we ran extended md simulations at a high temperature ( 2500 
 k ) for a total of 31 ps , which allowed the local ripple structure 
 of the membrane to evolve without creating defects in the lattice . 
 during this simulation , next , 
 the effect of the stationary tip was again modeled by applying different 
 static out - of - plane forces ( between 0.1 and 20.0 ev /  ) to 10 
 atoms in the middle of each of the structures to study the local changes 
 to the shape of the membrane as a function of the applied force . we point out that our model system is a simplification of the experimental 
 setup , since both the membrane size and thickness are scaled down 
 in order to make simulations computationally feasible . however , our 
 models capture the most important aspects of the experimental situation 
 in a manner that provides a reasonably good qualitative understanding .
in two - dimensional samples , all atoms 
 are at the surface and thereby 
 exposed for probing and manipulation by physical or chemical means 
 from both sides . here 
 , we show that we can access the same point on 
 both surfaces of a few - layer graphene membrane simultaneously , using 
 a dual - probe scanning tunneling microscopy ( stm ) setup . at the closest 
 point , 
 the two probes are separated only by the thickness of the graphene 
 membrane . 
 this allows us for the first time to directly measure the 
 deformations induced by one stm probe on a free - standing membrane 
 with an independent second probe . 
 we reveal different regimes of stability 
 of few - layer graphene and show how the stm probes can be used as tools 
 to shape the membrane in a controlled manner . 
 our work opens new avenues 
 for the study of mechanical and electronic properties of two - dimensional 
 materials .