Chemical Fingerprints of Large Organic Molecules in Scanning Tunneling Microscopy:Imaging Adsorbate-Substrate Coupling of Metalloporphyrins

Florian Buchner,† Karl-Georg Warnick,‡ Thorsten Wolfle,‡ Andreas Gorling,‡,§

Hans-Peter Steinruck,†,§ Wolfgang Hieringer,*,‡ and Hubertus Marbach*,†

Lehrstuhl fur Physikalische Chemie II, Lehrstuhl fur Theoretische Chemie, and Interdisciplinary Center ofMolecular Materials (ICMM), UniVersitat Erlangen-Nurnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany

ReceiVed: May 19, 2009

In this combined experimental and theoretical work we demonstrate at the example of tetraphenylporphyrinson Ag(111) how differences in individual adsorbate orbitals and their interaction with the substrate can beexploited to switch the appearance of the adsorbate in scanning tunneling microscopy (STM) experiments,such that electronically and chemically very similar large molecules become distinguishable in STM. Inparticular, an intermixed layer consisting of 2HTPP (TPP ) tetraphenylporphyrin), CoTPP, and FeTPPmolecules on Ag(111) was investigated, and it is demonstrated that STM images acquired with different biasvoltages constitute fingerprints of the different molecules within the intermixed layer. By means of densityfunctional calculations the observed features could be explained in detail and traced back to a direct orbitalinteraction of the adsorbed molecule with the surface. The explicit consideration of the surface in the calculationstherefore turned out to be decisive to achieve good agreement with experiment.

Introduction

The investigation of large organic molecules and transitionmetal complexes on surfaces by scanning tunneling microscopy(STM) in ultrahigh vacuum (UHV) has gained significantimportance due to a large variety of proposed applications, fromlife science to organic electronics. A prominent class of suchmolecules is the family of porphyrins.1 On the one hand therigid structure of the porphyrin core often triggers long-rangeorder and therefore makes them ideal building blocks for thegeneration of self-assembled functional devices;2 on the otherhand, the so-called metalloporphyrins exhibit versatile func-tionalities in biological and artificial systems, which are mainlydetermined by their central metal atom. For example, modelstructures based on monolayer assemblies of porphyrins mightbe used as active elements in chemical sensors,3-5 as photo-voltaic devices,6 or as catalysts.7

One route toward the generation of tailor-made supramo-lecular porphyrin networks is the variation of the side groupsof individual molecules.8-17 Additionally, the functionality ofporphynoids can be modified by the choice of the central metalatom. Recently, it has been demonstrated that metal atoms likeZn,18 Co,19,20 and Fe10,21-23 can be inserted into adsorbed freebase porphyrins or phthalocyanines by in situ metalation inUHV, which opens a new route for the preparation of well-defined mixed layers of free base porphyrins and metallopor-phyrins. Alternative routes are the successive thermal evapo-ration of different porphyrins8 or the thermal evaporation of themixed raw material.11,13 Considering the large variety of possiblesubstituents and/or central metal atoms, it is obvious thatmolecular arrays of high structural and chemical complexitycan be prepared. In addition, interactions with the surface canbe used to change the properties of the adsorbed molecules. As

a consequence, the chemical identification of the differentmolecular species on a surface becomes a major challenge.

STM is presently the most important experimental tool tostudy the inter- and intramolecular structure of molecularassemblies.24 In many cases, rather simple considerations basedon the topography of the molecule are sufficient to interpretthe observed apparent heights in the STM images. Such intuitiveinterpretations work, for example, for the discrimination of freebase and metalloporphynoids at certain tunneling conditions,10,13

typically at bias voltages around -1.0 eV. Two examples canbe found in Figure 1, parts b and c, where STM images ofdifferent mixtures of 2HTPP (TPP ) tetraphenylporphyrin) andCoTPP on Ag(111) are depicted, with CoTPP appearing asprotrusions (bright spots). However, it should be noted that forCoTPP one observes a contrast inversion by changing thepolarity of the bias voltage,13 which clearly shows that the simple“topographic approach” is limited. It has indeed long beenknown that STM images in general represent a delicate interplayof electronic and topographic effects, a prominent example beingthe appearance of graphite in STM.25

For different metalloporphynoids with basically identicaltopography, the specific electronic structure can be exploitedto distinguish them. Scudiero et al. were able to discriminateCoTPP and NiTPP on Au(111) based on an increased apparentheight of the CoTPP at a bias voltage of -1 V.8 They interpretedthis observation as due to the half-filled dz2 orbital of the centralCo atom (3d7 electron configuration). However, it is not possibleto distinguish, e.g., the very similar porphynoids CoPc and FePcin this way, because the FePc is assumed to have a triplet groundstate in the tetrapyrrolic environment and thus has a half-filleddz2 orbital.26 In order to distinguish topographically and chemi-cally similar molecules additional degrees of freedom likechanges in the applied bias voltage can be helpful tools. Sucha strategy requires, however, a detailed understanding of thecontrast mechanism. In this contribution we demonstrate howdifferences in individual adsorbate orbitals and their interactionwith the substrate can be exploited to switch the appearance of

* To whom correspondence should be addressed.† Lehrstuhl fur Physikalische Chemie II.‡ Lehrstuhl fur Theoretische Chemie.§ Interdisciplinary Center of Molecular Materials (ICMM).

J. Phys. Chem. C 2009, 113, 16450–1645716450

10.1021/jp904680c CCC: $40.75 2009 American Chemical SocietyPublished on Web 08/24/2009

the adsorbate in STM experiments, such that even electronicallyand chemically very similar molecules become distinguishable.

In order to understand the contrast mechanism of theinvestigated systems in STM additional information from theoryis required in general. Although in some cases simple compu-tational approaches such as extended Huckel theory calculationscan in certain cases be sufficient to reproduce the main featuresof the STM observations,12,27-29 there are also cases where evensophisticated theoretical approaches do not lead to satisfyingagreement with the experimental findings.30 In general, anoversimplification of the problem such as the neglect of themetal surface can present problems if the surface is an activepart of the system. For larger adsorbate molecules, the adequateinclusion of the surface within first-principles approaches is stilla challenge in terms of computational complexity. On the otherhand, the importance of surface-molecule interactions isdiscussed in the literature for metalloporphinoids with a singlyoccupied dz2 orbital of the metal ion,1,8,26,31 which indicates thenecessity to consider the surface in a theoretical approach.

In this study, we report the characteristic bias-dependentappearance of different metalloporphyrins in mixed layers onAg(111) in STM images. The investigation of mixed layersallows us to compare the different appearances in situ underidentical conditions. To understand the experimental observa-tions, the electronic structures and the STM images of theporphyrins were calculated both in the gas phase and in theadsorbed phase via periodic density functional calculations.32-34

These calculations reproduce the experimental images with highaccuracy for all studied porphyrins, and it turns out that thebias dependence of the STM images can only be explained interms of the electronic structure of the adsorbed moleculesincluding the surface.

Results and Discussion

To allow the in situ comparison of the different porphyrinderivatives the preparation of intermixed 2H/Co/FeTPP layerswith controlled stoichiometry was established. The correspond-ing preparation route is sketched in Figure 1 and described indetail in the Experimental Techniques section.

Figure 2 shows a constant current STM image acquired onan intermixed layer prepared with a nominal composition of50% 2HTPP, 30% FeTPP, and 20% CoTPP at a moderatenegative bias voltage (UGap ) -0.81 V and ISet ) 30 pA). Atthese tunneling conditions one can distinguish only two differentshapes of the molecules, as indicated by the arrows in Figure2b. This bimodal appearance is observed over a wide biasvoltage range of approximately -0.4 to -1.5 V (larger negativevalues were not tested). The left molecule in Figure 2b, whichappears with a central depression, can be identified as 2HTPP.10,20

The other two exhibit a dumbbell-like shape, as is typicallyobserved for Fe- and Co-porphyrin derivates at medium negativebias voltages;12,21,30 they are thus assigned accordingly. Thisinterpretation is corroborated by the fact that the fraction ofmolecules with the dumbbell-like appearance roughly matchesthe expected amount of 50% metallotetraphenylporphyrin(MTPP). The saddle shape can be explained by the topographyof the molecule. Naively one might expect a conformation ofTPP with phenyl substituents perpendicular to the porphyrinmacrocycle, as depicted in Figure 3a for 2HTPP.35 However, itis well-established that upon adsorption TPP and also othersimilar molecules exhibit a conformational adaptation: forCoTPP on Ag(111)11 and CoTPP on Cu(111)12 a considerablerotation of the phenyl substituents is concluded, as shown inFigure 3b. As a consequence the central porphyrin macrocycleis deformed such that two opposing pyrrole groups are tiltedupward and the other two downward, which is commonly

Figure 1. Schemes and STM images illustrating the preparation routeto generate the intermixed porphyrin layer consisting of the threedifferent species: 2HTPP, CoTPP, and FeTPP. (a) Scheme showingthe evaporation of a mixture of 2HTPP and CoTPP onto the Ag(111)substrate. (b and c) Medium-resolution STM images of mixed 2HTPP/CoTPP layers generated from mixtures with different ratios of the twoporphyrin species as indicated. Both images were acquired at negativebias voltages around -1 V where the metalloporphyrin (CoTPP) appearsas a protrusion. The ratio of the molecules appearing as protrusions isin excellent agreement with the CoTPP portion in the original mixture.(d) Scheme of the in situ evaporation of Fe onto a mixed 2HTPP/CoTPP layer. Fe will metalate available 2HTPP under the release ofH2 with almost 100% yield. By evaporating a defined amount of Fethe fraction of the resulting FeTPP within the intermixed layer can becontrolled. Thus, the whole process enables the generation of anintermixed 2HTPP, CoTPP, and FeTPP layer with defined ratios ofthe corresponding species.

Figure 2. STM constant current images of the same surface area ofan intermixed 2H-, Co-, and FeTPP at different bias voltages. Obviouslyat the higher bias voltage as shown in panels a and b a bimodalappearance of the three porphyrin species is observed. The 2HTPPappears with a central depression, whereas the two metalloporphyrinspecies, namely, CoTPP and FeTPP, appear both with the typical saddleshape, which means that they are undistinguishable at the actualtunneling condition. In panels c and d the same surface areas as inpanels a and b, respectively, are imaged at reduced bias voltage. It isapparent that at this tunneling condition three different appearances ofindividual molecules can be observed. Whereas the 2HTPP imagesremain essentially the same, a fraction of the saddle-shaped moleculesnow appear with a central protrusion. These molecules can be identifiedas CoTPP, which means that (in contrast to 2HTPP and FeTPP) CoTPPexhibits a strong bias-dependent appearance in STM. Thus, thediscrimination of the three species becomes possible at reduced biasvoltages.

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referred to as a saddle shape conformation (indicated with yellowin Figure 3b).

A main challenge now is the discrimination of CoTPP andFeTPP, since both obviously appear with an oblong (dumbbell)shape at the typical tunneling conditions used in Figure 2, partsa and b. As pointed out in the Introduction, both metallopor-phyrins appear to have a half-filled dz2 orbital, which has beenassumed to mediate the tunneling from the tip to the surface.26

Due to its characteristic out-of-plane “club shape”, the singlyoccupied dz2 molecular orbital might be expected to appear inSTM as a well-defined protrusion centered in the porphyrinmacrocycle. However, this is not the case, at least at thetunneling conditions of Figure 2, parts a and b, i.e., in the rangefrom -1.5 to -0.4 eV. Similar conditions have been used innearly all STM studies of adsorbed porphyrins in the literature.

At a reduced negative voltage of Ubias ) -0.24 V, however,an additional third shape is observed in Figure 2, parts c and d,which can be described as the expected central protrusion orbright spot. When the enlarged images in Figure 2, parts a andc, are compared it becomes obvious that a certain fraction ofthe molecules changed their appearance from the saddle shapeat -0.81 V to the bright spot at -0.24 V. These molecules canbe identified as CoTPP, since experiments on a pure CoTPPmonolayer exhibit exactly the same bias-dependent shape. Inaddition, counting the molecules over larger scan areas resultsin roughly 20% of the bright spots, which matches the expectedCoTPP content. It should be noted that this bias dependencewas reproduced for several freshly prepared mixed monolayers,with only slight variations in the different shapes (possibly dueto an altered tip shape). The transition from bright spot to saddleshape and vice versa was found to occur at roughly -0.4 V,again with only slight variations from experiment to experiment(possibly also due to an altered tip). A possible structuraldeformation of the CoTPP due to a change in bias around -0.4V is very unlikely, since (1) the tunneling conditions can beregarded as too “mild” to induce such a change, (2) no

dependence of the tunneling current was observed, and (3)FeTPP and 2HTPP do not exhibit such dramatic changes.Therefore, this characteristic bias-dependent shape change isassigned to an electronic effect. Even though CoTPP is quitewell investigated by STM, this specific appearance with acentered protrusion was not reported in literature so far. This isespecially remarkable since the observed shape intuitivelyresembles the proposed enhanced tunneling through the half-filled dz2 orbital.

To obtain a detailed understanding of the experimentalfindings, STM images of CoTPP, FeTPP, and 2HTPP have beensimulated via periodic density functional calculations for variousbias voltages. To corroborate the topographic interpretationgiven above for each species two different molecular geometrieshave been considered, with phenyl-ring dihedral angles (i.e.,angle between the planes of the phenyl group and the porphyrinmacrocycle) of Θ ) 90° (compare Figure 3a) and Θ ) 60°(compare Figure 3b). This results in a planar and a saddle shapeof the central porphyrin core, respectively (see Figure 3). Tostudy the influence of the surface, all calculations wereperformed in the gas phase (i.e., at an infinite distance from thesurface) and on a five-layer Ag(111) slab with a fixedmolecule-substrate distance of 0.53 nm (for CoTPP also otherdistances were considered, see the Supporting Information).

In the following the calculations for CoTPP on Ag(111) willbe discussed in detail because the most pronounced biasdependence was experimentally found for this system. In Figure4 experimental high-resolution STM images of CoTPP onAg(111) at low and high bias voltages are compared withcorresponding calculated STM images using various settings.A comparison of the images for low bias voltage in Figure 4(upper row) shows that the bright feature in the molecular centeris only reproduced in the calculations if the surface is included,which indicates its important contribution to the observed biasdependence. On the other hand, the STM images for 2HTPPand FeTPP exhibit only minor differences when comparing thecalculations with and without including the substrate (notshown); this implies that for those molecules the incorporationof the surface is not essential to achieve reasonable agreementwith the experiment.

Second, the geometry of the adsorbed CoTPP molecule isimportant to reproduce the rodlike shape in the correspondingSTM image as can be seen from the data at high bias voltageshown in Figure 4 (lower row). The dihedral angle of the phenylring of 60° (cf. Figure 3b) goes along with a saddle-typedeformation of the porphyrin core with two opposing pyrrolerings bent toward the surface while the other two pyrrole ringsare bent away from the surface and hence come closer to thetip of the STM. The rod-shaped appearance of the STM imageis thus mainly a consequence of the fact that the two lowerpyrrole rings are less “visible” in the STM image for topographicreasons (compare Figure 3b). On the other hand, for a flatgeometry with a dihedral angle of 90° a more or less square-shaped image is obtained from the calculation, because allpyrrole groups have the same distance to the tip. However, sucha “square-shaped” STM image is not observed in the experi-ments. It is therefore concluded that a dihedral angle Θ closeto 60° or at least with a significant deviation from 90° has tobe assumed as the real adsorption geometry. The same appear-ance is also found for 2HTPP and FeTPP in the present study;similar results were also obtained for CoTPP on Cu(111) andwere reproduced by gas-phase calculations of the molecularorbitals with extended Huckel theory.12

Figure 3. Space-filling models of the free base tetraphenylporphyrin(2HTPP) and a metallotetraphenylporphyrin (MTPP). (a) Top view andside view of 2HTPP with the naive but unrealistic conformation. Herethe plane of the phenyl substituents is perpendicular to the porphyrinmacrocycle (dihedral angle between porphyrin plane and phenyl ringis Θ ) 90°). (b) Top view and side view of MTPP with the realisticabsorption geometry, i.e., Θ ) 60°. As a result of the depictedconformation of the phenyl rings two opposing pyrrole groups are bentaway from the surface and the other two toward the substrate. Thetopography of the porphyrin ring can then be described as saddle-shaped.

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Parts a and b of Figure 5 show results from an optimizedimage preparation process in which the images for the individualmolecules were calculated separately, then merged (left, 2HTPP;middle, CoTPP; right, FeTPP), and in the final step a Gaussiansmooth was applied. These calculated images are juxtaposedto three-dimensional (3D) images (Figure 5, parts c and d) whichwere extracted from experimental data. In total, Figure 5 showsan almost perfect agreement of the main features of thetheoretical and the experimental data drawn as 3D plots. Theremaining small differences might be attributed to thermalmotion effects or tip irregularities, all of which are not includedin the theoretical model.

Overall, the most striking feature observed experimentallyis the pronounced bias dependence of the STM images ofCoTPP. This behavior is also reproduced by the calculated STMimages as is evident from Figures 4 and 5. Since the molecularconformation of the porphyrin molecules is the same in thesesimulations, the agreement between theory and experimentproves that the bias-dependent appearance of CoTPP is solelydue to an electronic effect. An analysis in terms of single-particlestates (orbitals) is performed in order to explain the generalshape of the STM images and the pronounced bias dependencefor CoTPP.

First, the relevant features of the electronic structure of anisolated CoTPP molecule in the gas phase are briefly discussed:The two highest occupied molecular orbitals (HOMOs, labels115� and 116� in Figure 6) are almost degenerate, with largecontributions from Co(dxz/dyz) and π-type orbitals located onthe TPP ring, cf. Figure 6. As the Co(dz2) orbital is singlyoccupied in CoTPP two spin channels have to be distinguished.The occupied Co(dz2) spin orbital (111R) is computed at -1.8eV, i.e., at a significantly lower energy than the frontier orbitals,while the corresponding unoccupied spin orbital is found to bethe lowest unoccupied molecular orbital (LUMO, 117�). In thepresence of the surface, the character of the frontier orbitals ischanged. The highest occupied orbital (i.e., the new HOMO,label 941�, cf. Figure 6) is now composed of the Co(dz2) orbitaland of Ag 5s atomic orbitals from the silver slab (see Figure7). Note that the gap between the corresponding orbitals isreduced. Generally speaking, it thus becomes evident that thereis a direct orbital interaction of CoTPP with the substrate asindicated by the new HOMO 941� close to the Fermi energy.This goes along with findings by photoelectron spectroscopy(XPS, UPS) that show a strong electronic interaction of CoTPPwith the underlying Ag(111) substrate, mediated by the dz2

orbital.31,36

On the basis of this analysis, the experimental observationof a single bright spot in the STM image of CoTPP at low biasvoltage and a dumbbell-like structure at high bias voltage canbe explained as follows. At low bias (i.e., below ∼0.3 eVaccording to the calculations, cf. Figure 6), the Ag(5s) + Co(dz2)

Figure 4. Experimental (a and b) and calculated (c-j) STM images of a single CoTPP molecule on Ag(111) at low negative (top row) and highnegative bias voltages (bottom row). As indicated in the images the calculations were performed for the porphyrin adsorbed on Ag(111) and for theporphyrin in the gas phase as well as for different conformations (Θ ) 60° and Θ ) 90°). The experimental observation of the central protrusionat low bias voltages in panel a could only be reproduced in the simulations in panels c, e, and g, where the CoTPP is adsorbed on Ag(111), i.e.,this characteristic feature could not be reproduced for the gas-phase calculations as shown in panel i even though the same molecular geometry hasbeen used. The experimentally observed saddle shape in the high-bias regime in panel b was reproduced assuming Θ ) 60° in panels d, f, and j.Thus, a good agreement for both bias regimes could only be achieved considering both the molecule-surface interaction as well as the dihedralangle Θ ) 60°. In the calculations an indefinitely sharp tip is assumed, which results in unrealistic details in the calculated STM images. Toexemplary address this problem a Gaussian smooth was applied to the images e and f; the results are shown in panels c and d, respectively.

Figure 5. Comparison of calculated (a and b) and experimental (cand d) constant current STM images and the corresponding scaledmolecular models (e). The calculations were performed individuallyfor the single molecules with the porphyrins adsorbed on Ag(111) andwith a phenyl-ring twist angle of Θ ) 60° (saddle shape of porphyrincore). The resulting calculated data was then merged in the shownarrangement, and after that a Gaussian smooth was applied (compareFigure 4, parts c and d).

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HOMO (label 941�) is the only orbital within the energywindow defined by the bias voltage and the Fermi level whichhas a significant amplitude on the adsorbed CoTPP moleculeand is therefore close enough to the STM tip to contribute tothe tunneling current. (Note that the levels of the silver surfacebelow the molecule are not visible in the STM image due totheir large separation from the tip.) This orbital thereforedominates the STM image, i.e., it exclusively makes up for thecentral spot in both the simulated and the experimental STMimage observed a low bias. Side views for different density

contour values of this crucial orbital 941� are plotted in Figure7. Here the dz2 character of the orbital becomes evident. Athigher bias voltages (which means that lower-energy bands alsocontribute to the STM image) the CoTPP orbitals (938� and939� in Figure 6) with a large contribution from Co(dxz/dyz)atomic orbitals and π-type orbitals on the carbon skeletondominate the simulated STM image. The shape of these orbitalsclosely follows the topography of the porphyrin ring, i.e., thesaddle shape for the conformation with Θ ) 60° and two pyrrolerings at opposite sides of the TPP core bent toward the surface

Figure 6. Density functional single-particle energy levels of CoTPP (left column), FeTPP (right column), Ag(111) (center, black), and thecorresponding adsorbate systems MTPP/Ag(111) (second and fourth columns). The energy levels have been shifted for each column such that theFermi energy (EF) is zero; spin label � corresponds to minority spin channel; energy levels of orbitals which significantly contribute to the STMimage (cf. Figure 4) are color-coded in red; black arrows indicate the contribution of MTPP and Ag(111) orbitals to the corresponding level in theadsorbate system; contour images of salient orbital densities of MTPP have been generated based on a density contour value of 2.5 × 10-3 e/Å3.

Figure 7. Side view of the orbital density of the highest occupied orbital (labeled 941� in Figure 6) in the CoTPP/Ag(111) adsorbate systems atthree different density contour values, showing that the dz2 orbital on the cobalt center of CoTPP has a constructive overlap with the silver 5s band:left, 17.5 × 10-4 e/Å3; center, 1.5 × 10-4 e/Å3; right, 0.5 × 10-4 e/Å3.

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and the other two away from it, as discussed above. (Note thatassuming a hypothetical, perfect D2d or D4h symmetry of theCoTPP molecule, these orbitals descend from the degenerateset of orbitals corresponding to a two-dimensional E irreduciblerepresentation.) Hence, the two orbitals with labels 938� and939�, while being almost degenerate, contribute to the STMimage to a different extent due to the saddle shape of adsorbedCoTPP, i.e., due to topographic effects. This results in thedumbbell-like shape of the submolecularly resolved STM imageat high bias voltages, in full agreement with the experimentalimage (Figures 4 and 5) and the appearance of CoTPP and alsoFeTPP reported in literature so far (at high negative biasvoltages).

Note that the central spot is still observed at low bias in STMimages of the hypothetical, flat structure (D4h symmetry, Θ )90°) of CoTPP, see Figure 4, parts g and h. Again, at high biasthe image is dominated by density covering the whole adsorbatemolecule. The reason is that in the now flat D4h symmetricmolecule the orbitals corresponding to 938� and 939� fromFigure 6, which form a degenerate orbital set of E symmetry inthe molecules without the surface, no longer are discriminatedtopographically, and consequently a dumbbell-shaped image isnot obtained at any bias voltage. In summary, this means thatfor CoTPP the STM image is dominated by the details of theelectronic structure at low bias voltages, whereas the topographyof the adsorbed molecule becomes important at high biasvoltages.

In contrast to the situation for CoTPP, the shape of theSTM images of 2HTPP and FeTPP shows no pronouncedbias dependence in neither experiment nor computation. Thisobservation can be rationalized based on their differentelectronic structures as compared to CoTPP. On the right-hand side of Figure 6 the orbital energy levels of gas-phaseFeTPP and the Ag(111) slab are juxtaposed to the levels ofthe FeTPP + Ag(111) adsorbate system. Once again, thehighest occupied level (label 940�) is a Ag band with a smalladmixture of an orbital centered mostly on the Fe center ofFeTPP. Only slightly below in energy the HOMO-1 (label939�) is computed. It is built from the Fe(dxz/dyz) orbitalsand a π-type orbital located on TPP, very similar to orbitals115�/116� for CoTPP. About 0.1 eV below is another orbital(938�) with Fe(d) contribution. (Note that there is nodegeneracy due to the deviation from D2d symmetry aspredicted by the gas-phase geometry optimizations; see theSupporting Information for details.) The important differencebetween adsorbed CoTPP and FeTPP is that for FeTPP +Ag(111) the π-type orbital (939�) is very close in energy tothe HOMO so that the STM image is always determined byboth orbitals and thus never displays a central protrusion asfor CoTPP, not even at low bias. In other words the relevantenergy levels in the FeTPP adsorbate system are energeticallytoo close to each other to be resolved experimentally, incontrast to the situation for CoTPP (left part of Figure 6).

Finally, for the 2HTPP adsorbate system (energy levels andorbitals not shown), the situation is quite trivial because thereis no central metal and therefore no central protrusions areobserved in the STM images. Hence, the shape of the STMimage is dominated by the π-levels of the porphyrin ring andorbitals on the peripheral phenyl rings.

Summary and Conclusions

The main goal of the present study is to understand thebias dependence of the appearance of various tetraphenylpor-phyrins on a Ag(111) surface in STM images. To allow for

a direct in situ comparison of different porphyrins, apreparation route to generate an intermixed layer with adefined composition of 2HTPP, CoTPP, and FeTPP moleculeson Ag(111) was successfully introduced. In addition, densityfunctional slab calculations were performed which explicitlyinclude the interaction of the porphyrins with the substrate.They provide a consistent interpretation of the bias-dependentSTM contrast in terms of the electronic structure of theadsorbate complex. Taking into account the coupling to thesubstrate is essential to obtain a good agreement betweenexperimental and simulated STM images and to reproducetheir bias-dependent shape for negative bias voltages, from-1.5 to -0.1 V. This is particularly true for low negativebias voltages, where the highest occupied orbitals of theadsorbed porphyrins dominate the STM images.

The most peculiar observation in the STM images is thepronounced bias dependence of the appearance of CoTPP: atlow negative bias voltages (roughly -0.1 to -0.4 V), thismolecule appears with a central protrusion, which transformsinto a dumbbell shape at larger negative bias voltages (below-0.4 V). In contrast, 2HTPP appears with a central depression,while FeTPP also features a dumbbell-like shape, but with nopronounced bias dependence in the investigated bias range. Thecharacteristic appearance of CoTPP featuring a central protrusionat low bias voltage could be tracked down to an orbitalpredominantly composed of Ag 5s orbitals of the silver slaband the Co(dz2) orbital of CoTPP.

CoTPP/Ag(111) can thus be regarded as a prototypeexample where the STM image is clearly dominated by anelectronic effect under certain conditions, i.e., at low negativebias voltages. For 2HTPP and FeTPP in the whole investi-gated bias range as well as for CoTPP at high negative biasvoltages the appearance of the molecules is found to bemainly determined by topographic effects, i.e., the conforma-tion of the porphyrin molecules. Here a dihedral angle of Θ) 60° between the planes of the porphyrin macrocycle andthe phenyl substituents, resulting in a saddle shape deforma-tion of the porphyrin core, led to a good agreement ofsimulated and experimentally determined STM images, inaccordance with the literature.

It is important to note that those STM images which aredominated by topographic effects could also be simulatedwithout explicitly considering the molecule-substrate inter-actions. However, the strong bias dependence found forCoTPP clearly demonstrates the necessity to consider thewhole adsorption complex in order to achieve an accurateand complete understanding. This system is one of the fewclear-cut cases where the electronic and the topographiccontributions can be unequivocally distinguished. Thus, adetailed understanding of the appearance of the large organicmolecules in STM could be accomplished, and it is further-more demonstrated that by a bias-dependent switching ofSTM images a clear distinction of topographically andelectronically similar molecules is possible. The nature ofthe unusual metallotetraphenylporphyrin-surface interactionthat makes the difference here is of special interest and clearlydeserves further investigations.

Methods

Experimental Techniques. All experiments and samplepreparation were performed in a two-chamber UHV system,at a background pressure in the low 10-10 mbar regime. Themicroscope is an RHK UHV VT STM 300 with RHK SPM100 electronics. All given voltages are referred to the sample,

STM of Metalloporphyrins on a Ag(111) Surface J. Phys. Chem. C, Vol. 113, No. 37, 2009 16455

and the images have been taken in constant current mode.Moderate low-pass filtering has been applied for noisereduction. The Ag(111) single crystal was purchased fromSurface Preparation Laboratory; CoTPP and 2HTPP with aspecified purity of 98% were from Porphyrin Systems. Thepreparation of the mixed Co/Fe/2H-porphyrin monolayers wasachieved in a four-step procedure: (1) preparation of the cleanAg(111) surface by repeated cycles of Ar+-ion sputtering (500eV) and annealing up to 850 K; (2) preparation of a Co/2H-porphyrin layer by thermal sublimation of the mixed CoTPPand 2HTPP raw material with a home-built Knudsen cell ontothe Ag substrate held at room temperature (RT) (sketched inFigure 1a). The evaporation time was around 20 min withthe Knudsen cell at a temperature of 330 °C, yielding acoverage in the multilayer regime; (3) preparation of amonolayer by heating the substrate to 280 °C for 30 s, whichis sufficient to desorb excess multilayer molecules but toolow to desorb or thermally decompose the porphyrins in themonolayer. The STM images of the resulting monolayers oftwo different 2HTPP/CoTPP mixtures in Figure 1, parts band c, demonstrate the ability to control the composition;(4) in situ metalation of a controlled amount of 2HTPP withiron, by the deposition of iron atoms onto the mixed 2HTPP/CoTPP monolayer with an Omicron EFM 3 electron beamevaporator. The evaporant was an iron rod with a diameterof 2 mm and a purity of 99.99% from MaTecK.

Computational Methods. The computations presented arebased on density functional theory using the ab initio programVASP (Vienna ab initio simulation program)32-34,37 developedat the Institut fur Materialphysik of the Universitat Wien.The adsorbate systems are treated as a periodic slab arrange-ment. The orthorhombic unit cell is of the size 14.4 × 15.0× 35.4 Å3. It contains a single MTPP molecule (M ) 2H,Co, Fe) plus a five-layer silver slab with a total of 150 silveratoms and comprises a vacuum layer of approximately 18Å. The atomic basis vectors in the unit cell defining theadsorbate surface system were constructed as follows. In aseparate, first run on the isolated five-layer silver slab (i.e.,without MTPP, but the same unit cell dimensions) the twotopmost layers of the slab were optimized, while the bottomthree layers were kept fixed at the positions in the face-centered cubic (fcc) silver crystal [d(Ag-Ag) ) 2.89 Å].Likewise, the MTPP molecule was optimized separately usingthe nonperiodic Turbomole38 program in two different ways.On one hand, MTPP was optimized within D4h point groupsymmetry. This implies a flat porphyrin core and peripheralphenyl rings which are perpendicular to the core. This is notfound to be the global minimum structure in the gas phase,however.35 Alternatively, an MTPP geometry has been usedwhere the twist angles of the phenyl rings were constrainedto 60°39 during the geometry optimization. This leads to anMTPP structure with a saddle-shaped porphyrin core and isbelieved to resemble the actual adsorbate geometry moreclosely. Differences in simulated STM images due to thechosen MTPP structure are discussed in the Results andDiscussion section. Note that a full optimization in the gas-phase results in a similar geometry, although with a slightlylarger twist angle. All these optimized MTPP geometries werefound to slightly deviate from D2d point group symmetry (seethe Supporting Information for details).35 To construct theunit cell basis in either case, the molecular structure has beenplaced on top of the Ag(111) surface at a distance of 5.32 Åaccording to data from NI-XSW measurements40 withoutfurther geometry optimization. The distances were varied in

the computations to test the sensitivity of the results on thisparameter. It turned out that slight variations ((0.5 Å) ofthe distance imposed do not affect the main findings presented(see the Supporting Information for details). Within each unitcell, the CoTPP is oriented such that the peripheral phenylrings of one CoTPP molecule face those of the CoTPPmolecules in the adjacent unit cells in a T-shape fashion (seethe Supporting Information). Further details on the latticeand the geometric arrangement in the crystal (coordinates ofbasis, unit cell lattice vectors) can be found in the SupportingInformation.

The PBE41 generalized gradient approximation (GGA)functional has been used as an approximation to the unknownexchange-correlation functional in the density functionalcalculations, and the projector-augmented wave method(PAW)42,43 has been employed to treat the atomic cores. Theprecision setting “accurate” was used in the VASP calcula-tions, which corresponds to a global plane-wave energy cutoffof 400.0 eV (29.40 Ry, 5.42 au; 139 377 plane waves perk-point). The size of the grid was 94 × 98 × 230 points perk-point, and a single k-point has been used. The convergencecriterion for the self-consistent field procedure for theelectronic wave functions was set to 10-6 au. For CoTPPand FeTPP spin-unrestricted calculations have been carriedout (final magnetic moment -0.9994 µB for CoTPP, 2.000µB for FeTPP). Here, the minority spin channel is labeled �.The simulations of the STM images have been performedbased on the Tersoff-Hamann model44,45 using the programSTRender46 and self-written software.

Acknowledgment. The authors thank the German ScienceFoundation (DFG) for financial support through SFB 583 andthe Excellence Cluster “Engineering of Advanced Materials”granted to the University of Erlangen-Nurnberg. Furthercredits go to J. Michael Gottfried for sharing data of NI-XSW measurements and to Bernd Meyer for helpful discus-sions on STM theory.

Supporting Information Available: Additional computa-tional details, computed orbital energy levels for the CoTPP/Ag(111) adsorbate system, calculated STM images for CoTPPon Ag(111), and the arrangement of MTPP molecules on theAg(111) surface. This material is available free of charge viathe Internet at http://pubs.acs.org.

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STM of Metalloporphyrins on a Ag(111) Surface J. Phys. Chem. C, Vol. 113, No. 37, 2009 16457