Correspondence: Reply to ‘On the nature of strong piezoelectricity in graphene on SiO2'

In our paper1 we provided an experimental evidence that the single-layer graphene (SLG) deposited on SiO2 grating substrate exhibits very strong out-of-plane piezoelectric effect, several times greater than that of the best piezoceramics such as lead-zirconate titanate. Simultaneously, the in-plane strain distribution was measured by micro-Raman scattering in an attempt to relate such unusual activity to the strain gradient expected in suspended graphene near the ridges. We appreciate the comment by Stampfer and Reichardt2 who noticed that our calculations of the in-plane strain based on ref. 3 were incorrect. The overestimation of the strain values, however, does not change the main conclusion of our paper, since the piezoresponse force microscopy measurements give the value of out-of-plane a.c. deformation, fully decoupled from the d.c. in-plane strain measured by micro-Raman. We recalculated the strain map and corrected the strain values that vary now in the range from 0.078 to þ 0.078%. Nevertheless, the strain ratio between supported and suspended graphene is still 2.5 (strain for suspended graphene is about þ 0.02%). Figure 1a,b displays both the corrected strain map and cross-section of the position of Raman line and recalculated strain. Below we reply in detail to other issues raised by Stampfer and Reichardt in their communication2. Gruneisen parameter suggested by Stampfer and Reichardt2 is related to graphene produced by mechanical exfoliation, whereas in our work1 we used graphene sheets prepared by CVD. These two materials differ by the number of intrinsic defects and, therefore, their behaviour under uniaxial or biaxial strain is different. This was exactly demonstrated in ref. 3, where the opposite sign of Gruneisen parameter for G-band was revealed for CVD graphene with respect to the exfoliated one. Therefore, we used the value reported in ref. 3 for our calculations which gave us a maximum strain of 0.078%. Stampfer and Reichardt2 questioned why we used the Gruneisen parameter for uniaxial strain. We believe that using the value for uniaxial strain is dictated by the nature of the substrate. Being periodical in one direction only, this feature naturally defines the symmetry of the graphene deformations as well. Therefore, we considered uniaxial strain in our work. Stampfer and Reichardt2 claim that our paper ‘ydirectly linked the observed piezoelectricity in graphene to the supposed high in-plane strain induced by the substratey’.