Strikingly, we also observe that the linear fits to the three datasets are offset with respect to the xaxis, and intercept the y-axis at the same point, -25 nm ± 1 nm. As this offset is in the order of the expected tip radius (Rtip) this is reminiscent of the theory used to describe tunneling in a STM, where a spherically symmetric tip is effectively equivalent to a point located at distance Rtip from the apex .