PHYSICAL REVIEW B 86, 155422 (2012)

Chemical tip fingerprinting in scanning probe microscopy of an oxidized Cu(110) surface

J. Bamidele,1 Y. Kinoshita,2 R. Turansky,3 S. H. Lee,2 Y. Naitoh,2 Y. J. Li,2 Y. Sugawara,2 I. Stich,3,* and L. Kantorovich1

1Department of Physics, King’s College London, The Strand, London, WC2R 2LS, United Kingdom2Department of Applied Physics, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

3Center for Computational Material Science, Institute of Physics, Slovak Academy of Sciences, 84511 Bratislava, Slovakia(Received 21 May 2012; revised manuscript received 6 August 2012; published 12 October 2012)

We present a joint experimental/theoretical study of the oxygen-terminated Cu(110) surface. Noncontact atomicforce microscopy (NC-AFM) identifies patches of p(2 × 1) and c(6 × 2) structures. For the latter reconstruction,we observe distinctly different NC-AFM images depending on the chemical tip identity, which were identifiedas either Cu, O, or Si terminated. Since knowledge of the chemical structure of the tip asperity in NC-AFM iscrucial for controlled imaging and manipulation of atoms and/or molecules on surfaces, this surface is proposedto be used for the tip termination “fingerprinting,” i.e., identifying the chemical species at its apex. Practicalways of enforcing the chemical structure of the tip asperity both before as well as at any stage of the imaging ormanipulation experiment when numerous tip changes may take place, are also discussed.

DOI: 10.1103/PhysRevB.86.155422 PACS number(s): 68.37.Ps, 81.16.Ta, 68.35.B−

I. INTRODUCTION

Scanning tunneling microscopy (STM) and noncontactatomic force microscopy (NC-AFM) are perhaps the mostimportant scanning probe (SPM) techniques. STM, basedon measuring tunneling current between an atomically sharptip and surface, has repeatedly and reproducibly achievedatomic contrast when imaging surfaces as well as atomsand molecules on them, identifying atomic species andperforming nanomanipulations.1,2 However, it is limited onlyto conducting surfaces and thin insulating films on conductingsubstrates. NC-AFM has proven an excellent tool for perform-ing similar tasks, i.e., atomic3 and chemical resolution,4 andnanomanipulation5 have been achieved by measuring forcesbetween the tip and surface.6,7 In addition, NC-AFM, unlikeSTM, can be performed on conducting, semiconducting andinsulating surfaces, allowing it to be the tool of choice forbuilding nanodevices on all types of surfaces.

We present here a NC-AFM study of the oxygen-terminatedCu(110) surface. As we shall propose below, this binaryoxidized surface, on which several STM experiments havealready been performed,8–13 may help in solving one keyoutstanding problem of the SPM, namely, identification ofthe chemical tip asperity during experiment. Interpretation ofexperimental NC-AFM images typically requires considerabletheoretical input, which is impossible without the clearchemical and structural model for the tip apex. However,whilst it is possible to image tips at the mesoscopic level with,e.g., transmission electron microscopy,14,15 this only helps inevaluating the macroscopic component of the force responsiblefor the overall tip force magnitude, but not for the lateralresolution; the species of the actual tip asperity determiningthe lateral resolution remains an experimentally unknownparameter that can never be known a priori. In many cases,this makes determining surface species and interpreting atomicstructures very problematic. Even with the aid of theoreticalanalysis the tip can only be determined a posteriori, if at all, butin real experiments it is most beneficial to know the asperitybefore and during scanning.

We find in our NC-AFM study of the structure of thepartially oxidized Cu(110) surface that it is solely the chemical

identity at the tip apex that is essential to the contrast forthis system, the actual structure of the apex is of secondaryimportance. Whilst this idea in itself may not be new, wehave also found that the contrast for this system can becontrollably changed fairly easily. In short, we have found thatthis system can be used for “fingerprinting” and enforcementof the chemical identity of the tip asperity both before as well asat any stage of the imaging or manipulation experiment. Hencewe propose that the oxygen-terminated Cu(110) surface,interesting in its own right, may also be used as a referencesystem in performing imaging and manipulation experimentsusing NC-AFM.

Experimental and simulation details are summarized inthe next section, followed by results of our experiments andtheoretical analysis. The paper is closed with a discussion andconclusions.

II. EXPERIMENTAL AND SIMULATION DETAILS

All experiments were carried out by a home-built NC-AFMoperating under ultrahigh vacuum conditions at 78 K. Thefrequency-modulation technique16 was used to detect thetip-sample force interaction. Low resistivity (0.01–0.025 �cm)n-doped commercial silicon cantilevers (Nanoworld, Switzer-land) were oscillated keeping the oscillation amplitude con-stant at 6.7 nm. The cantilevers typically had a Q factor of150 000, spring constant of 40 N/m, and resonance frequenciesof 150 kHz. The frequency shift of the oscillating cantileverwas measured by using phase-locked-loop-based commercialelectronics (easyPLL plus detector and controller, Nanosurf,Switzerland). Before experiments, the Cu(110):O surface wasprepared. First, the Cu(110) surface was cleaned by repeatedAr ion sputtering (1200 eV) and annealed at 550 ◦C. Then, itwas exposed to 2000 L of oxygen at 300 ◦C. The tip apex wascleaned by Ar ion sputtering (650 eV) in situ and was found tohave a radius of curvature less than 10 nm. Both the cantileverand the sample were always electrically grounded.

In order to verify the experimental conclusions, thenature of the interactions between Cu and O terminatedtips and the c(6 × 2) surface were studied theoretically. All

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theoretical calculations were performed using density func-tional theory (DFT) as implemented in the VASP code.17,18 Weemployed a plane-wave basis set and projector augmented-wave potentials18,19 considering eleven and six valence elec-trons on every copper and oxygen atom, respectively. Thechoice of the plane waves has been made since the use of thecomputationally much more efficient localized basis sets ledto a sizable, difficult to correct, tip-height-dependent basis setsuperposition error qualitatively modifying the force-distancecurves. Electronic exchange and correlation were describedusing the generalized gradient approximation (GGA) asemployed by the Perdew-Burke-Ernzerhof (PBE) densityfunctional.20 In calculations of the bulk Cu, the Brillouinzone was sampled using the Monkhorst-Pack method witha (30 × 30 × 30) k-points grid21 and fractional occupancieswere handled employing the broadening approach suggestedby Methfessel and Paxton21 with a width of σ = 0.2 eV. Forall subsequent calculations, the k-point sampling was scaledfrom that of the bulk and a plane-wave cutoff of 500 eV wasused. Typically, forces on atoms were converged to 0.01 eV/A.

Since experimentally the tip is crashed into a Cu-O clusterseveral times in order to form tip type I or II, we assume that aCu-O cluster terminated the Si cantilever. Therefore, for our tipmodel shown in Sec. III, we considered a microscopic Cu-Ocluster terminating a spherical macroscopic Si cantilever. Tomodel the O terminated tip cluster, we used a relatively stiff Cutetrahedron employing an O atom asperity in which the lowestfour atoms were allowed to relax upon interaction with thesurface. The same model was used for the Cu terminated tipwith a Cu atom asperity instead. Both tips consist of a 10-atomtetrahedral structure with either Cu or O atoms at their apex.Technical details of the simulation cell chosen for our studyas well as the model used to account for macroscopic van derWaals interactions due to the macroscopic tip are described inAppendices A and B, respectively.

III. RESULTS

At the experimental oxygen exposures (2000 L) theoxidized Cu(110) surface consists of p(2 × 1) and c(6 × 2)patches. The p(2 × 1) surface shown in Fig. 1(a) has analternating row/missing row conformation22,23 in the (110)direction, where along the (001) rows lie alternating Cu andO atoms. Alike the p(2 × 1) surface, STM,24 AFM22 and

FIG. 1. (Color online) (a) Ball model of the p(2 × 1) phase (topand side views), where light red, grey and light grey balls depict lowO, added Cu, and bulk Cu atoms, respectively. (b) Ball model of thec(6 × 2) phase, where additionally dark grey and red balls depict thesuper Cu and high O atoms.

low-energy electron diffraction (LEED)25 measurements allconfirm that the c(6 × 2) reconstruction, formed at a muchhigher oxygen exposure,22 also has a missing row conforma-tion along the (110) direction. The key difference is that itfeatures a double row/missing row structure, where, likewise,Cu and O atoms alternate in the (001) direction. However, inevery second missing row of the c(6 × 2) surface, additionalCu atoms are bonded between two oxygen atoms. Figure 1(b)illustrates this atomic arrangement with the “added strand”26

reconstruction. The additional Cu atoms lie higher than allother surface atoms and have thus acquired the name “super”Cu atoms.22,24 Also due to the attraction of the super Cu to thetwo O atoms on either side of them, those O atoms are raisedslightly out of plane and move closer to the super Cu they sur-round, acquiring the names “high”22 or “buckled”24 O atoms.This means that super Cu atoms have a different and distin-guishable ordering from the O atoms on the c(6 × 2) surface.

Initially, a pure silicon tip was prepared and used to scanthe surface, producing the image shown in Fig. 2(a). In thisimage, both Cu and O atoms are imaged: Cu atoms as brightprotrusions and O as dark depressions. We note that thecontinuous NC-AFM imaging with a pure Si tip terminationis extremely difficult as the tip is easily contaminated bysurface species (Cu and O atoms). It was noticed by takinga large area image of the oxidized Cu(110) surface shown inFig. 2(b), that there are several Cu-O clusters distributed on thesurface. These clusters [indicated by the arrow in Fig. 2(b)] arelocated mainly at domain boundary, kink and/or step edge sites,appearing as an aggregation of Cu and O atoms after the surfacecleaning process. To enforce a particular tip termination,the following procedure was used. The tip was intentionallycrashed into one such Cu-O cluster as schematically shown inFig. 2(c). Tip crashes reproducibly led to two different typesof images: I and II. The ratios of type I, type II, and otherswere 31%, 51%, and 19%, respectively. Images of both typestaken over the same scanning region, consisting of a c(6 × 2)edge on the p(2 × 1) surface are shown in Figs. 2(d) and 2(e),respectively. These clearly differ from the image taken withthe pure Si tip, Fig. 2(a), showing two different arrangementsof atoms with the same supercell symmetry. In the type Iimage, a single bright spot is seen at the expected positionsof the super Cu atoms as highlighted in the schematic of thec(6 × 2) surface below image (d). Conversely, in the type IIimage, two close spots are imaged instead, located around thesame position as in type I and hence must be attributed to thetwo high O atoms which surround each super Cu atom, ashighlighted in the schematic below image (e).

To further study these two different tip terminations, a largearea of the oxidized Cu(110) surface shown in Fig. 3 wasscanned with both tip types. In the left part of the two imagesone can see a p(2 × 1) reconstructed surface. It containsequidistant (001) rows of Cu and O atoms characteristic forthis phase. One can see that images of this particular phaselook the same with both tip types. In the right part of the twoimages, one can see a part of a big island with the characteristicc(6 × 2) reconstruction, while an additional layer of atomson top of the island can be seen in the far bottom-rightcorner of the two images; note that we do not know preciselythe structure of this additional layer. One can clearly seethe characteristic signatures of the tip types I and II in the

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FIG. 2. (Color online) NC-AFM measurements of (a) the purec(6 × 2) surface with the Si tip before contamination with thesurface species, and (b) a large area of the oxidized Cu(110) surfacecontaining a Cu-O cluster (indicated by an arrow) used to modifythe tip apex. The experimental procedure is depicted by a cartoon(c), illustrating the Si tip crashing into a Cu-O cluster on the surface,thereby resulting in tips of either types I or II. NC-AFM topographyimages of the pure c(6 × 2) surface performed with tip type I (d) andtip type II (e) have the schematic of the top layer of the c(6 × 2)surface overlaid in each case. At the bottom of images (a), (d), and(e), the corresponding schematic of the c(6 × 2) surface is given tohighlight the imaged atoms.

images (a) and (b) of the c(6 × 2) reconstructed island, namely,single (a) and double (b) spots in place of the “high O-superCu-high O” chain of atoms (super Cu region). Several defectsin the structure are also clearly visible in both images. Lessbright features at the edge of the c(6 × 2) reconstructed island(indicated by arrows) correspond to the reconstruction of theCu-O (001) rows underneath the super Cu atoms. Note thatif in the p(2 × 1) phase the Cu-O (001) rows are equidistantwith a single missing row between any two adjacent rows, inthe c(6 × 2) reconstructed surface, a double row/missing rowstructure is featured instead.

(a) (b)

FIG. 3. (Color online) NC-AFM topography images of the samearea of the oxidized Cu(110) surface performed using tip type I (a)and II (b). An elongated island of (most likely) Cu and O atoms ontop of the Cu(110) c(6 × 2) surface is clearly visible in the bottomright corner of the images.

A much clearer indication of this restructuring of the Cu-Orows can be seen in Fig. 4 in the circled areas as well as at theedges of the c(6 × 2) island as indicated by the white arrows.Additionally, here, the registry of the bright features imagedby both tip types can be seen more clearly, indicated by theblack arrows. For the type I tip, the large bright protrusionon the c(6 × 2) island is out of registry with the protrusionsseen on the p(2 × 1) surface, as shown in Fig. 4(a). This isin line with the deduction that the type I tip images only Cuatoms, strongly indicating that the large bright protrusion mustbe super Cu atoms. It then follows that the protrusions on thep(2 × 1) surface are the Cu atoms that run along the Cu-O(001) rows. The super Cu atoms appear out of registry becausethey are bonded to two oxygen atoms each, which are notimaged as bright by this tip. Therefore the super Cu atomsare out of registry with the Cu atoms in the Cu-O (001) rowsand are in registry with the O atoms, which are not imaged.This supports our findings. Conversely, in Fig. 4(b), the brightprotrusions appear in registry with the bright spots on thep(2 × 1) surface. This can be explained by the same logic,assuming that the type II tip images only O atoms, which liewithin the Cu-O (001) rows, the high O atoms would naturallyappear in registry with the p(2 × 1) rows. This shows that eachtip indeed only images one species: the type I tip images onlyCu atoms and the type II tip images only O atoms. Thus whenscanning the p(2 × 1) surface, which is made up of rows with

(a) (b)

FIG. 4. (Color online) NC-AFM topography images of the samearea of the oxidized Cu(110) surface performed using tip types I(a) and II (b). The circled area in both images shows initiation ofthe reconstruction of the Cu-O (001) rows from missing row to thedouble row/missing row structure. A reduced distance between theCu-O (001) rows can also be seen at the edges of the c(6 × 2) island(indicated by white arrows). Black arrows indicate the registry ofp(2 × 1) and c(6 × 2) features.

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(a) (b)

FIG. 5. (Color online) (a) NC-AFM topography image, wheretype I contrast changes into type II. (b) NC-AFM topography scanof a c(6 × 2) island on the p(2 × 1) surface in which a Cu atomis deposited from the tip onto the p(2 × 1) surface (indicated byarrows). In both images, schematics of some of the super Cu atomssurrounded by high O atoms are overlaid for clarity and the slow scandirection is from the top to the bottom.

alternating equidistant Cu and O atoms, with both tip types,the topographies appear identical.

To summarize, the two types of images should be attributedto two different tip terminations (I and II) obtained fromthe pure Si tip when crashing it into a Cu-O cluster. Thisconclusion is further supported by the observations madeduring scanning, when the tip changes between the two types.A typical example is shown in Fig. 5(a). One can see that,initially, super Cu atoms were imaged (type I), then two tipchanges occurred during the same scan resulting in a change ofcontrast in which high O atoms were imaged (type II). Clearly,as Cu and O are the most abundant species on the surface,these results are highly reproducible (similar results obtainedin separate experiments), and since usually no impurities areinitially imaged near the location of where the tip is crashedinto the Cu-O cluster, one has to assume that tip types I and IIcorrespond to the tips terminated with either Cu or O atoms.

It is essential to point out that these results were gatheredfrom different sets of experiments. Furthermore there is nochange in contrast even when the tip deposits an atom ontothe surface, see Fig. 5(b). Here, a type I tip was used asonly Cu atoms are imaged. Then undoubtedly a Cu atom,as indicated by arrows, was deposited to the surface fromthe tip (i.e., a structural change must have occurred in thetip). However, we see that the contrast did not change afterthe deposition, meaning that the tip apex had also not beenchemically altered, i.e., it remained type I (stable before,during and after the manipulation). Therefore, it is not the fullstructure, but specifically the chemical species terminating thetip that is essential to the contrast, as these results were found tobe highly reproducible, with little change in contrast betweenexperiments. Thus we conclude that the images showing typesI and II were obtained with two different classes of tips,terminated either with Cu or O, although in each particularinstance the actual structures of the tips having the sametermination might have been different.

These conclusions are corroborated by scan lines, Figs. 6(a)and 6(d), taken along the (110) direction for both tip termina-tions. It can be seen that the type I tip images the super Cuatoms as bright, and the high O atoms as darker than the hollowsites features. For the same region, the type II tip images thehigh O only as bright and interacts significantly less with both

FIG. 6. (Color online) Experimental (black solid curves) andtheoretical (red dashed curves) scan lines of the Cu(110) c(6 × 2)surface. (a) Scan lines with experimental tip type I and the theoreticalO terminated tip; (b) the corresponding tip model and (c) theexperimental topography image. (d) Scan lines with experimental tiptype II and the theoretical Cu terminated tip; (e) the correspondingtip model and (f) the experimental topography image. All scan linesgo across three missing rows, over the super Cu, high O, and hollowsites as shown in the schematic beneath (d). The same color codeis used as in Fig. 1. Experimental results for type I and II tipswere obtained using �f values of −19.5 Hz and and −24.4 Hz,respectively. Theoretical results for O and Cu terminated tips wereobtained using �f values of −22 Hz and 24 Hz, respectively.

the super Cu and hollow site Cu atoms, imaging them both asdarker features.

Using the model introduced in Sec. II and Appendix B, wehave calculated the topography scan lines along the same direc-tion as in experiment, passing through the O-Cu-O sequence ofatoms. Comparison of the theoretical and experimental resultsin Fig. 6(a) and 6(d) strongly suggests that the type I tip is infact the O terminated tip, imaging super Cu atoms as shownin Fig. 6(c), and the type II tip is Cu terminated, imaging highO atoms as shown in Fig. 6(f). Indeed, our theoretical analysisshows without doubt that before and after the deposition,shown in Fig. 5(b), the tip must have been O terminated andthe deposited Cu atom originated from further along the tipand not from the apex as one might naively assume.

The key feature observed so far was the ability to experi-mentally modify the tip apex reproducibly and reversibly bydipping it into the Cu-O clusters abundantly present on thesurface to produce two distinctly different NC-AFM images ofthe c(6 × 2) surface. Although this method does seem to ensureonly two possible chemical terminations of the tip (either Cuor O), the actual tip structure as well as chemical identity ofother species present above the apex atom cannot be controlledby this procedure and hence, one may expect, this may affectthe measured NC-AFM image. Surprisingly, however, we findthat although occasionally there are indeed secondary features

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FIG. 7. (Color online) (a) Experimental (left panels) and (b)theoretical (right panels) spectroscopies of the Cu(110) c(6 × 2)surface over super Cu atom (red lines), high O atom (blue lines), andhollow site (cyan lines) for type I apex. The theoretical tip modelis shown in (c). (d) The corresponding experimental image withspectroscopic sites indicated by crosses. Note that bright spots aroundsuper Cu atoms have oval-like shapes. (e)–(h) The same informationfor type II apex.

present in our images, the main characteristics of the images(either bright spots around super Cu atoms or two brightspots above high O atoms) remain clearly identifiable. InFig. 7(d), we show images which feature elliptical, as opposedto circular, images of the super Cu atoms with type I apex:compare, for instance, images in Figs. 6 and 7. In spite of thisdifference, the two cases (bright spots on the super Cu versuson high O atoms) are still easily recognizable.

To illustrate additionally that the primary features are notaffected by the actual tip structure and determined solelyby the chemical identity of the tip apex atom, we comparein Fig. 7 experimental and simulated spectroscopy [�f (z)]curves over the most important surface sites: super Cu atom,high O atom, and the hollow site. In spite of the fact thatour theoretical models are unlikely to be exactly the same asthe actual experimentally used tip apices, the gross featuresof both experimental and simulated contrast are the same andhence not affected by the possible secondary structural tipdifferences, i.e., type I (oxygen terminated) apex yields superCu atoms as brightest spots and type II (Cu terminated) apexyields high O sites as brightest. We reiterate here that prelim-inary results indicated a larger tip model is required for softercontact, leading to better quantitative agreement, however,qualitatively, the results produced by our theoretical modelare in very good agreement with experimental measurements.

So, for tip-surface separations at which NC-AFM imagingis normally done (≈0.2–0.3 nm), our theoretical results agreewell with experiment: (1) both Cu and O terminated tips imageonly one of the two surface species each and (2) Cu terminatedtips image a different surface species than O terminated tips.

Moreover, theoretical calculations helped interpretation of theexperimental images and provided unambiguous characteriza-tion of the chemical nature of the tip apex. Our results havealso indicated that care should be taken when interpreting theoutcome of manipulation experiments, as a usually made as-sumption that atoms deposited on the surface from the tip musthave come from the tip asperity may not necessarily be true.

To summarize, three unique image signatures are foundwhen imaging the c(6 × 2) oxidized Cu(110) surface, where,depending on whether the tip apex is terminated with Si, Cuor O, three types of contrast are obtained; (i) Si: both surfacespecies are imaged with super Cu appearing as protrusionsand high O as holes, (ii) type I: only super Cu are imagedas bright, and (iii) type II: only high O are imaged as bright.A Si termination is distinctly different and therefore clearlyidentifiable from the type I and II terminations. The contrastobtained by the type I and II tip terminations shows differentatoms, and this surface allows for controllable switchingbetween the two tip terminations with high reproducibility.These experimental findings are supported by theoreticalsimulations, which were able to identify the type I and IItips as being O and Cu terminated, respectively.

IV. DISCUSSION AND CONCLUSIONS

In conclusion, we have performed a joint experimen-tal/theoretical study of the oxygen-terminated Cu(110) sur-face. NC-AFM identified patches of two coexisting phases,p(2 × 1) and c(6 × 2), and the latter was found to exhibit areversal of the image contrast with respect to the chemical, asopposed to structural, tip apex identity. Although changes ofchemical contrast after modification of the chemical identity ofthe apex have been previously reported,27–29 it is clear that theoxidized Cu(110) surface offers some advantages over othersystems studied so far: it allows for much more control over thechanges of the chemical species at the tip apex (the tip class),and also, if required, the apex can be changed to one of the twostable terminations by locating a Cu-O cluster on the surfaceand crashing the tip into the cluster. It is still not possible tochange the tip apex to the desired termination, either Cu orO termination is obtained and there is little control over this;however, our theoretical simulations suggest without doubtthat the knowledge of the chemical nature of the tip asperitycan be obtained immediately after the dipping protocol byeither scanning across a small region of a c(6 × 2) patch orperforming spectroscopy above the three main sites (hollow,high O and super Cu). The protocol we implemented wasshown to make it possible to fingerprint/enforce the chemicaltip identity of the tip before, during and after scanning, andwhilst this idea may not be new, the realization of it may quitepossibly be, at least on this system.

Once the desired termination is obtained (NC-AFM topog-raphy image is seen), measurements or controlled manipula-tions of this surface can proceed with clear knowledge of the tipapex. In the case of tip change (which can easily be recognizedexperimentally in most cases by a sudden change of contrast),one should still be able to tell the tip apex termination. If thetermination does not correspond to any of these three knownsignatures (e.g., a foreign contaminant is present on the apex),then one of the two terminations (Cu or O) can be enforced

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again by performing the crashing protocol. Surprisingly, al-though the concept is not new, to our knowledge the procedureproposed here has not been vigorously pursued in the past.

Alternative methods for identifying the tip have beenpreviously proposed, such as: determining the tip polarity byscanning30 or performing spectroscopy measurements31 on anionic system, performing a laborious search in the “tip phasespace” to reproduce the results of NC-AFM29 or STM/NC-AFM measurements,32 ramping the bias voltage to probe thetip chemical identity33 and performing frequency shift versusbias measurements to verify if the tip is metallic.34 However,these methods can be either too cumbersome, limited, andmay not allow for stable and reversible tip enforcement. Moreimportantly, the mentioned methods either require substantialtheoretical effort, which can only be exercised after the actualexperiment, or provide only qualitative information about thetip apex structure (e.g., polarity). To control the tip terminationwhen performing NC-AFM imaging, impressive results havebeen achieved by functionalizing the tip apex35 to guaranteethe known tip termination, however, this can frequently beunstable when performing manipulation experiments.

We believe that the experimental protocol successfullyperformed in this study may help in resolving the key problemfacing the NC-AFM, namely identification of the chemical na-ture of the tip asperity during the course of experiment. Whilstseveral methods exist for ensuring a single species termination,the protocol shown here allows for controlled switchingbetween two different terminations in NC-AFM experiments.

In addition, we believe that this particular surface may haveeven wider utilization and appeal. Firstly, we propose that theCu(110) surface, partially oxidized, be used as the main surfaceor substrate for nanomanipulation and building nanodevices,e.g., by adsorbing molecules onto the either clean or oxidizedCu(110) surface.12,13,36–39 This is because Cu-O clusters arecommonly available on it after surface preparation40 and soare islands of the p(2 × 1) missing row phase with some superCu atoms in the missing rows, deforming the nearest twoO atoms either side of them as in the c(6 × 2) phase.22,24,40

Hence, the tip identification protocol successfully used in thisstudy, may be performed at any stage of the experiment usingthe oxidized part of the surface. Secondly, significant progresshas been recently reported by Liljeroth et al. who performedan NC-AFM experiment in which a tip was prepared on theCu(111) surface, before transferring it to the actual surface ofinterest inside the same (cold) chamber.41 Although extremelychallenging, this method, if and when successful, may opennew avenues in NC-AFM experiments, and then the oxidizedCu(110) surface we believe could be a good choice for areference system for controlling the chemical nature of thetip apex when scanning other surfaces.

For the surface studied here, the key feature required for itto serve as a reference is the ability to distinguish betweenthe atomic arrangements of the metal and oxygen atoms.Since this is the only criterion, identification of a muchlarger class of reference systems of this type with a supermetal atom in a missing row or having other characteristicarrangements of several atomic species, which may also showvariable signatures for different tip terminations, may alsobe possible. For instance, these could be binary oxidizedmetal surfaces42 such as oxidized Cu(100),26,43 Ag(100),43

Ag(110),44 Ag(111),45 Ni(110),23,46,47 and Au(110),48 whichhave either a missing row reconstruction (on which there maybe super metal atoms), or distinct and different arrangementsof metal and oxygen atoms.

We hope this study will stimulate more research in thedirections proposed to investigate other surfaces with similarproperties, which may serve as convenient references. Also,we believe an advance is required on the instrumentationside which would enable performing controlled scans atany point during experiment of a reference system. Theprotocol and a reference system designed for the chemical tipidentification/enforcement as outlined here may well be just afirst step in the effort to build a whole database for chemicaltip fingerprinting.

ACKNOWLEDGMENTS

Work supported by the Grant-in-Aid for Scientific Re-search of Japan, APVV projects ESF-EC-0007-07 underthe Nanoparma ESF FANAS and APVV-0207-11, VEGA(2/0007/12), and the Engineering and Physical SciencesResearch Council (EPSRC). This research was supported inpart also by ERDF OP R & D, project CE meta-QUTE ITMS26240120022, and via CE SAS QUTE. J.B., in particular,thanks EPSRC for studentship funding. We also acknowledgethe support of King’s College London’s HPC Facility managedby Alessio Comisso and the Material Chemistry consortiumfor the computer time allocation on the HECTOR UK NationalFacility.

APPENDIX A: SIMULATION CELL

To model the surface, a six-layer slab was used with 78atoms in the unit cell; the bottom two layers were kept

a1

a2

a1

Plan View

Side View

FIG. 8. (Color online) Schematic of the cell model used in ourtheoretical calculations. Lattice vectors a1 = 8.53 A and a2 = 14.55 Aand the angle between them α = 64.76◦. Plan view of the surface ison the left and side view on the right. Light grey, grey, and dark greyballs correspond to bulk, added, and super Cu atoms. Light red andred balls correspond to low and high O atoms as in Fig. 1.

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fixed to simulate the missing bulk; the simulation cell isshown in Fig. 8. This cell was chosen to provide adequateseparation of neighboring tip structures. The lattice vectorsshown in Fig. 8 for the slab were chosen so as to maximize thenearest-neighbor distance between tip images for the givennumber of atoms in the cell. The nearest-neighbor distancebetween tip images chosen was 4.1 A, which was found to bevery reasonable and introduces little finite size effects due toperiodic boundary conditions. This was tested against a celltwice as large (extended in the lattice direction a1) with a tipnearest neighbor distance of 10.1 A, and was shown to havenegligible differences in the vertical forces acting on the tipatoms of less than 0.02 eV/A.

APPENDIX B: MODEL OF MACROSCOPIC TIPVAN DER WAALS INTERACTION

To realistically reproduce the experimental scan lines andspectroscopy curves theoretically, contributions due to macro-scopic van der Waals (vdW) interaction due to a macroscopictip must be included. We accounted for this using a spherical tipmodel49,50 shown in Fig. 9: a large sphere was assumed for themacroscopic portion of the tip, terminated by the microscopictip considered atomistically in our DFT calculations. Theoverall force acting on the whole tip is evaluated from acombination of the macroscopic vdW forces due to the largespherical part and the microscopic forces on the atomistic partcalculated by DFT. The two parts of the tip are separated fromeach other by some unknown offset distance which serves asa parameter of the theory. If the offset is negative then themicroscopic tip sits inside the sphere as in Fig. 9; in this case,there is a double counting of the vdW interaction due to theoverlapping part; however, the effect of this must be negligible.If the offset is positive the two portions of the tip have a gap,which assumes a larger microscopic tip (not considered in theactual DFT calculations). In this case, the interaction with thesurface due to this missing part of the tip is not accountedfor; again, this is not important as this part is adequately

Radius

Macroscopic Tip

Microscopic Tip

Surface

Offset

FIG. 9. (Color online) Schematic of the tip model used tocalculate theoretical scan lines/spectroscopy curves Fig. 4/Fig. 5.It consists of a macroscopic sphere terminated with a microscopic(atomistic) model. The portion of the microscopic tip that sits insidethe macroscopic sphere is defined by the offset.

separated from the surface and the missing contribution mustbe also negligible. In our calculations, the macroscopic vdWcontributions were found to be small, this was due to a rathersmall radius of the sphere used of 25 A and an offset of +3.2 A,indicating that for more quantitatively accurate calculations alarger microscopic (atomistic) tip model might be used. Notethat the value of H = 1.16 eV for the Hamaker constant(corresponding to silicon) has been used in our calculationsof the vdW contribution to the force. We optimized the tipradius and the offset distance to achieve the best agreementwith the experimentally measured scan lines shown in Fig. 4,and experimental values were used for all other parameters.

*ivan.stich@savba.sk1D. M. Eigler and E. K. Schweizer, Nature (London) 344, 524(1990).

2A. J. Heinrich, C. P. Lutz, J. A. Gupta, and D. M. Eigler, Science298, 1381 (2002).

3A. Sweetman, S. Jarvis, R. Danza, J. Bamidele, S. Gangopadhyay,G. A. Shaw, L. Kantorovich, and P. Moriarty, Phys. Rev. Lett. 106,136101 (2011).

4Y. Sugimoto, P. Pou, M. Abe, P. Jelinek, R. Perez, S. Morita, andO. Custance, Nature (London) 446, 64 (2007).

5Y. Sugimoto, M. Abe, S. Hirayama, N. Oyabu, O. Custance, andS. Morita, Nat. Mater. 4, 156 (2005).

6F. J. Giessibl, Rev. Mod. Phys. 75, 949 (2003).7S. P. Jarvis, H. Yamada, S. I. Yamamoto, H. Tokumoto, and J. B.Pethica, Nature (London) 384, 247 (1996).

8M. Wagner, S. Surnev, M. G. Ramsey, G. Barcaro, L. Sementa,F. R. Negreiros, A. Fortunelli, Z. Dohnalek, and F. P. Netzer,J. Phys. Chem. C 115, 23480 (2011).

9N. Tsukahara, K. I. Noto, M. Ohara, S. Shiraki, N. Takagi, Y. Takata,J. Miyawaki, M. Taguchi, A. Chainani, S. Shin, and M. Kawai, Phys.Rev. Lett. 102, 167203 (2009).

10A. Gumbsch, G. Barcaro, M. G. Ramsey, S. Surnev, A. Fortunelli,and F. P. Netzer, Phys. Rev. B 81, 165420 (2010).

11A. Gumbsch, G. Barcaro, M. G. Ramsey, S. Surnev, A. Fortunelli,and F. P. Netzer, J. Phys.: Condens. Matter 22, 222202 (2010).

12Y. Hu, K. Maschek, L. Sun, M. Hohage, and P. Zeppenfeld, Surf.Sci. 600, 762 (2006).

13M. Oehzelt, S. Berkebile, G. Koller, J. Ivanco, S. Surnev, and M. G.Ramsey, Surf. Sci. 603, 412 (2009).

14R. B. Marcus, T. S. Ravi, T. Gmitter, K. Chin, D. Liu, W. J. Orvis,D. R. Ciarlo, C. E. Hunt, and J. Trujillo, Appl. Phys. Lett. 56, 236(1990).

15Y. Naitoh, Y. Kinoshita, Y. J. Li, M. Kageshima, and Y. Sugawara,Nanotechnology 20, 264011 (2009).

16T. R. Albrecht, P. Grutter, D. Horne, and D. Rugar, J. Appl. Phys.69, 668 (1991).

155422-7

J. BAMIDELE et al. PHYSICAL REVIEW B 86, 155422 (2012)

17G. Kresse and J. Furthmuller, Phys. Rev. B 54, 11169 (1996).18G. Kresse and J. Furthmuller, Comput. Mater. Sci. 6, 15 (1996).19P. E. Blochl, Phys. Rev. B 50, 17953 (1994).20J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865

(1996).21M. Methfessel and A. T. Paxton, Phys. Rev. B 40, 3616 (1989).22S. Kishimoto, M. Kageshima, Y. Naitoh, Y. J. Li, and Y. Sugawara,

Surf. Sci. 602, 2175 (2008).23L. Ruan, F. Besenbacher, I. Stensgaard, and E. Laegsgaard, Phys.

Rev. Lett. 70, 4079 (1993).24R. Feidenhans’l, F. Grey, M. Nielsen, F. Besenbacher, F. Jensen,

E. Landgsgaard, I. Stensgaard, K. W. Jacobsen, J. K. Norskov, andR. L. Johnson, Phys. Rev. Lett. 65, 2027 (1990).

25W. Liu, K. C. Wong, and K. A. R. Mitchell, Surf. Sci. 339, 151(1995).

26X. Duan, O. Warschkow, A. Soon, B. Delley, and C. Stampfl, Phys.Rev. B 81, 075430 (2010).

27S. H. Ke, T. Uda, I. Stich, and K. Terakura, Phys. Rev. B 63, 245323(2001).

28J. Tobik, I. Stich, and K. Terakura, Phys. Rev. B 63, 245324 (2001).29R. Bechstein, C. Gonzalez, J. Schutte, P. Jelinek, R. Perez, and

A. Kuhnle, Nanotechnology 20, 505703 (2009).30A. S. Foster, C. Barth, A. L. Shluger, and M. Reichling, Phys. Rev.

Lett. 86, 2373 (2001).31R. Hoffmann, L. N. Kantorovich, A. Baratoff, H. J. Hug, and H. J.

Guntherodt, Phys. Rev. Lett. 92, 146103 (2004).32G. H. Enevoldsen, H. P. Pinto, A. S. Foster, M. C. R. Jensen,

A. Kuhnle, M. Reichling, W. A. Hofer, J. V. Lauritsen, andF. Besenbacher, Phys. Rev. B 78, 045416 (2008).

33A. S. Foster, A. Y. Gal, J. M. Airaksinen, O. H. Pakarinen, Y. J.Lee, J. D. Gale, A. L. Shluger, and R. M. Nieminen, Phys. Rev. B68, 195420 (2003).

34G. Teobaldi, K. Lammle, T. Trevethan, M. Watkins, A. Schwarz,R. Wiesendanger, and A. L. Shluger, Phys. Rev. Lett. 106, 216102(2011).

35L. Gross, F. Mohn, N. Moll, P. Liljeroth, and G. Meyer, Science325, 1110 (2009).

36T. Djuric, T. Ules, H.-G. Flesch, H. Plank, Q. Shen, C. Teichert,R. Resel, and M. G. Ramsey, Cryst. Growth Des. 11, 1015 (2011).

37K. Muller, A. Kara, T. K. Kim, R. Bertschinger, A. Scheybal,J. Osterwalder, and T. A. Jung, Phys. Rev. B 79, 245421 (2009).

38N. Atodiresei, V. Caciuc, S. Blugel, and H. Holscher, Phys. Rev. B77, 153408 (2008).

39X.-D. Ma, D. I. Bazhanov, O. Fruchart, F. Yildiz, T. Yokoyama,M. Przybylski, V. S. Stepanyuk, W. Hergert, and J. Kirschner, Phys.Rev. Lett. 102, 205503 (2009).

40D. Coulman, J. Wintterlin, J. V. Barth, G. Ertl, and R. J. Behm, Surf.Sci. 240, 151 (1990).

41P. Lijeroth et al., Towards Reality in Nanoscale MaterialsConference, 2012 (unpublished).

42H. Over and A. P. Seitsonen, Science 297, 2003 (2002).43N. Bonini, A. Kokalj, A. D. Corso, S. de Gironcoli, and S. Baroni,

Surf. Sci. 600, 5074 (2006).44O. Nakagoe, K. Watanabe, N. Takagi, and Y. Matsumoto, J. Phys.

Chem. B 109, 14536 (2005).45J. Knudsen, N. M. Martin, E. Granas, S. Blomberg, J. Gustafson,

J. N. Andersen, E. Lundgren, S. Klacar, A. Hellman, andH. Gronbeck, Phys. Rev. B 84, 115430 (2011).

46M. Busch, D. Blauth, and H. Winter, Surf. Sci. 602, 2808 (2008).47W. B. Zhang and B. Y. Tang, Appl. Phys. Lett. 94, 091901 (2009).48H. Shi and C. Stampfl, Phys. Rev. B 77, 094127 (2008).49H. C. Hamaker, Physica 4, 1058 (1937).50J. Israelachvili, Intermolecular and Surface Forces, 2nd ed.

(Academic Press, London, 1992), Chap. 11.

155422-8