in the sample preparation , heavily 
 p - doped si substrate with a thermally oxidized layer ( sio2 layer ) of 285 nm thickness on top is employed . first , a small metallic 
 pattern is deposited on the sio2 substrates used as alignment 
 markers between graphene flakes . graphene flakes are then exfoliated 
 mechanically from a graphite piece and printed on the substrate . the 
 number of layers of graphene is identified from measurement of the 
 green light intensity and the 2d - band 
 of raman spectra . then , for the metal 
 thin film method , ni films are deposited by an electron beam evaporator . 
 or for the sog method an sog material ( accuglass t-11 ( 211 ) , honeywell 
 corp . ) is deposited on graphene by spin coating at 3000 rpm for 40 
 s followed by baking to form a cross - linked structure of organic materials . the resultant thickness of sog is 200 nm . the metal markers , ni films , 
 and the sog materials are patterned by electron beam lithography ( elionix 
 7700 system ) using the electron beam resist nano 950 pmma a6 ( microchem 
 corp . ) spin - coated at 4000 rpm for 50 s on the samples , followed by 
 electron beam evaporation and lift - off process and chemical etching . 
 to remove redundant sog , we dipped our sample in the buffered hydrofluoric 
 acid ( bhf ) and rinsed it by purified water . typically , 
 we repeated the procedure bhf etching of 10 s and checking the etched 
 state until sog is removed from the surface of the substrate . to introduce 
 biaxial strain , electron beam ( typically 80 kv of acceleration voltage 
 and 2 na of beam current ) is irradiated on all the surface areas of 
 the ni films in figure 1a , c and figure 2a or on the two regions of the sog film around the 
 edges of the graphene flakes as shown in figure 3c . to introduce in - plane isotropic compressive strain , the sample 
 is coated with the sog film and heated on a hot plate with different 
 baking temperatures at 190 , 235 , and 300 c in the laboratory 
 atmosphere . raman 
 spectra are measured 
 with the excitation laser wavelength of 532 nm , and the estimated 
 diameter of the laser spot is 23 m . the laser power 
 is kept less than 1 mw to avoid damaging the graphene .
theoretical works suggest the possibility 
 and usefulness of strain 
 engineering of graphene by predicting remarkable properties , such 
 as dirac cone merging , bandgap opening and pseudo magnetic field generation . 
 however , most of these predictions have not yet been confirmed because 
 it is experimentally difficult to control the magnitude and type ( e.g. , 
 uniaxial , biaxial , and so forth ) of strain in graphene devices . here 
 we report two novel methods to apply strain without bending the substrate . 
 we employ thin films of evaporated metal and organic insulator deposited 
 on graphene , which shrink after electron beam irradiation or heat 
 application . 
 these methods make it possible to apply both biaxial 
 strain and in - plane isotropic compressive strain in a well - controlled 
 manner . 
 raman spectroscopy measurements show a clear splitting of 
 the degenerate states of the g - band in the case of biaxial strain , 
 and g - band blue shift without splitting in the case of in - plane isotropic 
 compressive strain . in the case of biaxial strain application , 
 we 
 find out the ratio of the strain component perpendicular to the stretching 
 direction is at least three times larger than what was previously 
 observed , indicating that shrinkage of the metal or organic insulator 
 deposited on graphene induces both tensile and compressive strain 
 in this atomically thin material . 
 our studies present for the first 
 time a viable way to apply strain to graphene without the need to 
 bend the substrate .