Glatzel, Th.

Sadewasser, S.

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Comment on ‘‘Electrostatic Force Microscopy onOriented Graphite Surfaces: Coexistence of Insulatingand Conducting Behaviors’’In a recent Letter [1], Lu et al. claimed to have identifiedinsulating and conductive regions on a highly orientedpyrolytic graphite (HOPG) surface by differences in thesurface potential. Because of the reasons given below, theresults presented do not give sufficient evidence to supportthe claims made.(1) The minimum of the force gradient signal corre-sponds to the contact potential difference (CPD) and ap-pears to be at the same bias voltage for the tip on the darkand the bright regions [Fig. 1(a) in Ref. [1]]. This is indisagreement with the statement that the CPD is differentby 0.25 V between the two regions. The square root of thiscurve does not result in a suitable calibration curve, as itsslope depends on the tip-sample distance [Fig. 5(d) inRef. [1]] and on the dielectric constant of the sample [2].Variations in the tip-sample distance due to locally differ-ent CPD [3] or dielectric constant [2] will result in a changeof this slope. The quantification of the CPD from thecontrast in the electrostatic force microscopy (EFM) im-ages is therefore not justified.(2) It was shown by measurements under ultrahighvacuum (UHV) conditions that graphite peeled off in am-bient air shows a CPD lower by 0.5 eV compared to asample peeled off in UHV (Fig. 1 in Ref. [4]). This resultclearly proves that, besides the conditions during measure-ment, the conditions during sample preparation are alsorelevant.(3) The shape of the majority of the regions with a higherEFM contrast appears to be circular. This could indicatethe presence of a contamination film (i.e., water or hydro-carbon) on the surface, stemming either from the samplepreparation in air or from the tip itself, which likely con-tains a contamination layer that could be transferred to thesample surface during the measurement. In fact, the imagesresemble the water films observed in Ref. [2]. Additionalevidence can be observed in Figs. 2 and 4 in Ref. [1], wheresubsequent images show variations in the shape and size ofthe regions exhibiting the EFM contrast. This demonstratesthat the sample is modified by simply scanning the tipacross it. The authors explain the EFM contrast by thepresence of defects in the graphite structure. In order tochange the defect structure, more energy is likely requiredas could be provided by scanning with an AFM tip.However, modifying a contamination film on the samplerequires less energy and is the more likely explanation.(4) The inversed EFM contrast for positive and negativesample bias (Fig. 3 in Ref. [1]) does not exclude thepresence of a contamination layer. In the present case ofa conductive substrate (graphite), changing the polarity ofthe applied bias would change the polarization of thecontamination layer (i.e., by reorientation), resulting inthe observed contrast inversion. This effect is absentwhen an insulating substrate is used, as shown in Ref. [2].(5) The topography as shown in Figs. 2, 4, and 5 inRef. [1] shows a curvature which is proposed to be a resultof the presence of defects in single graphene layers. Thiscurvature appears only along the x axis in all images and islikely an artifact occurring for tube piezos at large scansizes. Additionally, the length scale of the topographycurvature is on the order of several micrometers, whereasthe length scale of the EFM contrast variations is on theorder of 1 m and below. If the same origin, namely, thepresence of defects in the graphene structure, is maderesponsible for both effects, then both the topographyand the EFM contrast should vary on the same length scale.In addition, the vertical resolution of the topography scan isfairly low; monolayer steps in graphite (0:34 nm) or theheight of a thin contamination layer cannot be resolved [thedigital steps in Fig. 2(b) in Ref. [1] have a height of0.5 nm].(6) The electrostatic signal obtained in EFM is ex-tremely sensitive to the sample surface and tip condition.The experiments conducted by the authors are subject touncontrolled conditions, especially when operating in am-bient air. Also, for the measurements in Ar atmosphere, thesample and the tip are introduced from ambient air and arelikely contaminated. Even for measurements conductedunder UHV conditions [4], the sample preparation influ-ences the result (see point 2 above). As differences in thelaboratory environment are not controllable, it is not sci-entifically justified to consider the absence of the EFMpattern on the HOPG-0.8 sample as a proof of differentproperties of the HOPG-0.8 and the HOPG-0.4 samples.The arguments given above strongly indicate that theobserved EFM contrast is the result of a surface contami-nation and cannot be attributed to an electronic property ofthe graphite sample.S. Sadewasser1 and Th. Glatzel21Hahn-Meitner InstitutGlienicker Strasse 100D-14109 Berlin, Germany2Institute of PhysicsUniversity of BaselKlingelbergstrasse 82CH-4056 Basel, SwitzerlandReceived 28 November 2006; published 28 June 2007DOI: 10.1103/PhysRevLett.98.269701PACS numbers: 73.25.+i, 73.43.Fj, 79.60.Jv, 81.05.Uw[1] Y. Lu et al., Phys. Rev. Lett. 97, 076805 (2006).[2] J. Hu, X.-D. Xiao, and M. Salmeron, Appl. Phys. Lett. 67,476 (1995).[3] S. Sadewasser and M. Ch. Lux-Steiner, Phys. Rev. Lett.91, 266101 (2003).[4] Ch. Sommerhalter et al., Appl. Phys. Lett. 75, 286 (1999).PRL 98, 269701 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending29 JUNE 20070031-9007=07=98(26)=269701(1) 269701-1 © 2007 The American Physical Society

Comment on  Electrostatic force microscopy on oriented graphite surfaces  Coexistence of insulating and conducting behaviors

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Comment on Electrostatic force microscopy on oriented graphite surfaces Coexistence of insulating and conducting behaviors

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