Luet al.Reply:

Abstract

Lu et al. Reply:The authors of the preceding Comment [1] argue that the patterns we measured on highly oriented pyrolytic graphite (HOPG) by electric field microscopy (EFM) [2] were a result of surface contamination (e.g., water film) and cannot be attributed to the potential fluctuations of graphite. We disagree with their arguments.The method we used is indeed suitable to measure absolute contact potential differences (CPD), and the voltage dependence shown in Fig. 1 in Ref.[2] is necessary for the calibration. The curves shown in that figure are normalized at the minimum; that is why they coincide. Even if they were not normalized, the difference in CP between the two chosen areas in that figure is about 0.1 V, which in the scale of Fig. 1(a) is within the order of the symbol width [2]. We measured relative values of potentials with the same calibration in a surface region atomically flat as observed by atomic force microscopy. In a homogeneous metal there are no CP fluctuations, neither in a HOPG sample of 0.8 mosaicity measured under similar conditions [2] as the 0.4 -HOPG, a fact that speaks against contamination. Independently done measurements using Kelvin force microscopy in HOPG as well as in Pd-Au structures provide the same CPD as the EFM parabola shift, supporting our interpretation.The authors [1] do not seem to be aware of a key point to understand the EFM experiments. Graphite is not a metal; it conducts in-plane but is practically insulating normal to the planes [3]. Therefore, ideal graphite screens poorly [4] and has a positive dielectric constant [5], an insulator property. Therefore, the applied electric field penetrates 100 nm, and the potential information is not only from the surface but from far inside the bulk. Imagine a condenser consisting of an air gap and 100 nm graphite. In EFM, we are measuring the fluctuation in the energy of this condenser. In this case, it is clear that the role played by an impurity dielectric layer of 1 nm thickness is irrelevant. In particular, one should understand the point of the screening of the electric field. The fluctuation patterns observed are a consequence of regions with different graphite structures or defect densities, which also may link graphite planes changing the density of states at Fermi level.Although the regions with different potentials appear rounded, they are not circles but nearly polygons. Electron backscatter diffraction (EBSD) provides an independent confirmation of the observed patterns on a similar HOPG sample; see Fig. 1. The EBSD patterns look similar to the EFM ones [2] and are related to different dislocation or crystallographic defect density and orientation, which originate the CPD.There have been recent angle-resolved photoemission spectroscopy experiments [6] on clean surfaces of graphite samples in UHV, etc., where regions with a different density of states at the Fermi level were clearly observed. These results provide an indication for the existence of fluctuations of the potential on the sample surface, in agreement w...

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