Structure formation in bis(terpyridine) derivative adlayers: molecule-substrate versus...

Structure Formation in Bis(terpyridine) Derivative Adlayers:Molecule-Substrate versus Molecule-Molecule Interactions

Harry E. Hoster,*,† Matthias Roos,† Achim Breitruck,† Christoph Meier,‡ Katrin Tonigold,†,§

Thomas Waldmann,† Ulrich Ziener,‡ Katharina Landfester,‡ and R. Ju¨rgen Behm*,†

Institute of Surface Chemistry and Catalysis, Institute of Organic Chemistry III- MacromolecularChemistry and Organic Materials, and Institute for Theoretical Chemistry, Ulm UniVersity

D-89069 Ulm, Germany

ReceiVed May 12, 2007. In Final Form: August 2, 2007

The influence of the substrate and the deposition conditionssvapor deposition versus deposition from solutionsonthe structures formed upon self-assembly of deposited bis(terpyridine) derivative (2,4′-BTP) monolayers on differenthexagonal substrates, including highly oriented pyrolytic graphite (HOPG), Au(111), and (111)-oriented Ag thin films,was investigated by high-resolution scanning tunneling microscopy and by model calculations of the intermolecularenergies and the lateral corrugation of the substrate-adsorbate interaction. Similar quasi-quadratic network structureswith almost the same lattice constants obtained on all substrates are essentially identical to the optimum configurationexpected from an optimization of the adlayer structure with C-H‚‚‚N-type bridging bonds as a structure-determiningfactor, which underlines a key role of the intermolecular interactions in adlayer order. Slight distortions from theoptimum values to form commensurate adlayer structures on the metal substrates and the preferential orientation ofthe adlayer with respect to the substrate are attributed to the substrate-adsorbate interactions, specifically, the lateralcorrugation in the substrate-adsorbate interaction upon lateral displacement and rotation of the adsorbed BTP molecules.The fact that similar adlayer structures are obtained on HOPG under ultrahigh vacuum conditions (solid|gas interface)and on HOPG in trichlorobenzene (solid|liquid interface) indicates that the intermolecular interactions are not severelyaffected by the solvent.

1. Introduction

The structure of ordered organic adlayers formed on single-crystalline substrates is known to be largely determined by theinteractions between these admolecules, which are often domi-nated by hydrogen bonds.1-3 Nevertheless, the interaction withthe substrate may also play an important role. The variation ofthe adsorption energy upon lateral displacement or rotation ofthe adsorbed molecule (i.e., the corrugation of the adsorptionpotential) may result in a more or less severe modification of theoptimum configuration of the free adlayer depending on theextent of this energy corrugation and its relation to theintermolecular spacing determined by the molecule-moleculeinteractions.3,4 When the influence of the substrate-adsorbateinteractions becomes more pronounced, the adsorbates will showan increasing tendency to attain a favorable adsorption geometrywith respect to the substrate. The minimization of the substrate-adsorbate interaction favors the formation of commensurateadlayer structures5 with a high fraction of molecules onenergetically favorable sites. Adlayer phases dominated byadsorbate-adsorbate interactions, however, will often exhibitincommensurate or higher-order commensurate,5-7 coincident

adlayer structures, with larger, nonprimitive unit cells wherefewer molecules are adsorbed on their optimum adsorption sites.8

Hence, the structure associated with the minimum total energyresults from a subtle balance between adsorbate-adsorbate andadsorbate-substrate interactions, which in many cases gives riseto small distortions of the adlayer structure to attain a higherdegree of coincidence with the substrate lattice.9 Furthermore,minimization of the interface energy will also affect the orientationof the adlayer to optimize the number of favorable adsorptionsites, which is known as “rotational” or “orientational” epit-axy.3,6,10 For the case of organic adlayers (“organic epitaxy”),these concepts were developed further and refined by Forrest,3,9

Hillier and Ward,4,11 Fritz,12-15 and others,16,17 discriminating,for example, between different classes of coincidence structuressuchaspoint-on-point, point-on-line,orgeometric concidence.12,15

For a single substrate-adsorbate system, it is generally hardto discriminate the effects related to molecule-molecule andmolecule-substrate interactions on the basis of the resultingadlayer structure alone3,12-17because the optimum configurationof the adlayer is not known. This is different when comparingthe ordered structures formed by the same adsorbate on differentsubstrates. Such comparative studies give access to trends in theadsorption potential on the respective surfaces. Furthermore,they can provide an experimental basis for calculations of (i) the* Authors to whom correspondence should be addressed. E-mail:

[email protected], [email protected].† Institute of Surface Chemistry and Catalysis.‡ Institute of Organic Chemistry III- Macromolecular Chemistry and

Organic Materials.§ Institute for Theoretical Chemistry.(1) Schmittel, M.; Venkateshwarlu, K. Functional, Discrete, Nanoscale

Supramolecular Assemblies. InFunctional Molecular Nanostructures; Schluter,A. D., Ed.; Springer: Berlin, 2005; Vol. 245, p 1.

(2) Barlow, S. M.; Raval, R.Surf. Sci. Rep.2003, 50, 201.(3) Forrest, S. R.Chem. ReV. 1997, 97, 1793.(4) Hillier, A. C.; Ward, M. D.Phys. ReV. B 1996, 54, 14037.(5) Ying, S. C.Phys. ReV. B 1971, 3, 4160.(6) Kern, K.; David, R.; Palmer, R. L.; Comsa, G.Appl. Phys. A1986, 41, 91.(7) Weimer, W.; Knorr, K.; Wiechert, H.Phys. ReV. Lett. 1988, 61, 1623.

(8) Zangwill, A. In Physics at Surfaces; Cambridge University Press:Cambridge, U.K., 1988; p 234.

(9) Zhang, Y.; Forrest, S. R.Phys. ReV. Lett. 1993, 71, 2765.(10) Novaco, A. D.; McTague, J. P.Phys. ReV. Lett. 1977, 38, 1286.(11) Last, J. A.; Hooks, D. E.; Hillier, A. C.; Ward, M. D.J. Phys. Chem. B

1999, 103, 6723.(12) Hooks, D. E.; Fritz, T.; Ward, M. D.AdV. Mater. 2001, 13, 227.(13) Mannsfeld, S. C. B.; Fritz, T.Phys. ReV. B 2004, 69, 075416.(14) Mannsfeld, S. C. B.; Fritz, T.Phys. ReV. B 2005, 71, 235405.(15) Mannsfeld, S. C. B.; Fritz, T.Mod. Phys. Lett. B2006, 20, 585.(16) Eremtchenko, M.; Scha¨fer, J. A.; Tautz, F. S.Nature2003, 429, 602.(17) Eremtchenko, M.; Bauer, D.; Schaefer, J. A.; Tautz, F. S.New J. Phys.

2004, 6, 1367.

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10.1021/la701382n CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 10/03/2007

adsorption configurations on the different substrates, (ii) the trendsin the lateral corrugation of the adsorption potential on thesesubstrates, and (iii) the adsorbate-adsorbate interactions. Suchinvestigations have been performed, for example, for PTCDAmonolayer and multilayer crystallite structures on varioussubstrates, which on HOPG,3,9,18-20 MoS2,19,20 Ag(111),16 Au-(111),21 Cu(111),21 and Cu(110)21 revealed in-plane structureswith a “herringbone” geometry similar to that of bulk PTCDAcrystals. The 2D unit cells of the molecular adlayers on the metalsurfaces were shown to be smaller than on HOPG and MoS2 orin bulk PTCDA. Furthermore, they were commensurate with therespective substrate, which was explained by a stronger lateralcorrugation in the molecule-substrate interaction.16,22 Finally,on Ag(110), PTCDA monolayers form a highly ordered,commensurate network with a different geometry that isdominated by the molecule-substrate interactions.12,20,22

The question of the substrate effect on the resulting adlayerstructures is also the topic of this article. Here we report resultsof a comparative, high-resolution scanning tunneling microscopy(STM) study of the structure-modifying effects induced bydifferent hexagonal substrates by comparing the structures formedby the oligopyridine 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(py-ridine-4-yl)pyridine-2-yl)pyrimidine (in the following, abbrevi-ated as 2,4′-BTP; see Figure 1)23,24on highly oriented pyrolyticgraphite (HOPG), on Au(111), and on thin Ag films on a Ru-(0001) substrate, which are close to a Ag(111) surface.25,26 Inprevious studies,23,24these molecules were shown to form orderedplanar networks on HOPG in which the molecules are connectedby C-H‚‚‚N-type hydrogen bonds in a structure mainlydetermined by the positions of the peripheral N atoms. Referringto previous studies23,24 at the solid-liquid interface, we havestudied the influence of the environment by investigating the2,4′-BTP adlayers formed on HOPG both in an ultrahigh vacuum(UHV) environment and in trichlorobenzene. The experimental

studies were complemented by calculations of the interactionenergies between neighboring adsorbed molecules using interac-tion energies between pyridine molecules determined viatheoretical calculations and by an analysis of the changes in theenergetics induced by the position and orientation of the adsorbedmolecule relative to the substrate as function of the rotationalangle of the adsorbate.

2. Methods

STM measurements at the solid|gas interface were performed ina standard ultrahigh vacuum (UHV) system equipped with ahomebuilt “pocket size” STM and facilities for sample preparationand characterization such as an Ar ion sputter gun, a sample heater,evaporators for metal and BTP deposition, Auger electron spec-troscopy (AES), and a quadrupole mass spectrometer for residualgas analysis.27,28 STM measurements at the solid|liquid interfacewere performed using a low-current RHK-1000 system. In bothcases, a commercialI-Vconverter (109 V A-1, type IVP, RHK) wasused for STM imaging with picoampere currents. Most imagespresented in this article were acquired with tunneling currents of2-100 pA, and the sample was negatively biased at 100-3000 mV.All images were recorded in constant-current mode. STM imagesare presented as top-view representations, with brighter gray levelscorresponding to higher apparent heights. The calibration of bothSTMs was verified in atomically resolved images of HOPG andRu(0001).

HOPG (SPI-grade 3, SPI) substrates were prepared by cleavingin air, using adhesive tape. For UHV measurements, the freshlycleaved HOPG substrate was transferred to the UHV system via aload-lock system. Subsequently, it was degassed at 670 K for atleast 30 min. Clean Au(111) surfaces were achieved by repetitiveAr sputtering at 0.5 kV (10 min, 5µA cm-2 ion current), with thecrystal at 300 K and subsequent annealing at 770 K. Ag substrateswere prepared by depositing thin Ag films on a Ru(0001) substrate.The preparation of the Ru(0001) crystal followed standard proce-dures.29 Before Ag deposition, it was cleaned by three to four cyclesof argon sputtering (0.5 kV Ar+ ions, 5µA cm-2, 10 min), followedby flash annealing to 1650 K. The remaining carbon impurities wereremoved by oxidation, involving oxygen adsorption (10 L of O2 -1 L ) 1.33× 10-6 mbar s) and subsequent repeated flash annealingto 1650 K. The freshly prepared surface exhibits 50-200 nm wide,atomically flat terraces separated by monolayer steps. Contaminationlevels are generally far below the AES detection limit of about 0.01monolayers (ML), and the high surface purity is confirmed byatomically resolved images. The Ag thin films were evaporatedfrom a resistively heated Knudsen cell (tectra/WA-Technology, 0.1-0.3 ML min-1), with the sample kept at about 500 K. The resulting

(18) Ludwig, C.; Gompf, B.; Petersen, J.; Strohmaier, R.; Eisenmenger, W.Z. Phys. B1994, 93, 365.

(19) Ludwig, C.; Gompf, B.; Glatz, W.; Petersen, J.; Eisenmenger, W.; Mo¨bus,M.; Zimmermann, U.; Karl, N.Z. Phys. B1992, 86, 397.

(20) Hoshino, A.; Isoda, S.; Kurata, H.; Kobayashi, T.J. Appl. Phys.1994,76, 4113.

(21) Wagner, Th.; Bannani, A.; Bobisch, C.; Karacuban, H.; Sto¨hr, M.; Gabriel,M.; Moller, R. Org. Electron.2004, 5, 35.

(22) Glockler, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.;Bohringer, M.; Berndt, R.; Schneider, W.-D.Surf. Sci.1998, 405, 1.

(23) Ziener, U.; Lehn, J. M.; Mourran, A.; Mo¨ller, M. Chem.sEur. J.2002,8, 951.

(24) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P.J. Phys. Chem. B2005, 109, 21015.

(25) Hwang, R. Q.; Hamilton, J. C.; Stevens, J. L.; Foiles, S. M.Phys. ReV.Lett. 1995, 75, 4242.

(26) Ling, W. L.; Hamilton, J. C.; Thu¨rmer, K.; Thayer, G. E.; de la Figuera,J.; Hwang, R. Q.; Carter, C. B.; Bartelt, N. C.; McCarty, K. F.Surf. Sci.2006,600, 1735.

(27) Kopatzki, K. E. Ph.D. Thesis, Universita¨t Munchen, 1994.(28) Buatier de Mongeot, F.; Scherer, M.; Gleich, B.; Kopatzki, E.; Behm, R.

J. Surf. Sci.1998, 411, 249.(29) Diemant, T.; Hager, T.; Hoster, H. E.; Rauscher, H.; Behm, R. J.Surf.

Sci.2003, 541, 137.

Figure 1. (a) Molecular structure of the 2,4′-BTP oligopyridine. (b) Model of the molecule as used in Figure 2.

Structure Formation in 2,4′-BTP Adlayers Langmuir, Vol. 23, No. 23, 200711571

coverages and evaporation rates were determined by a quantitativeevaluation of STM images and by AES.

For measurements at the solid-liquid interface,24 the molecularfilms were deposited from a 3 mMsolution of 2,4′-BTP in 1,2,4-trichlorobenzene (TCB) by placing a droplet of that solution on thesubstrate while the tip was kept in tunneling contact. For UHVmeasurements, deposition was performed by evaporation via ahomebuilt, resistively heated Knudsen cell (deposition rate, 0.1molecules nm-2 s-1; cell temperature, 528 K). The network structureinvestigated in this article was prepared by depositing about 0.7molecules nm-2 and subsequent removal of the excess by heatingthe sample to 500 K. Upon heating to this temperature, desorptionreduces the coverage of 2,4′-BTP molecules to 0.4 molecules nm-2.The annealing step is not required for structure formation but wasfound to improve the resulting film structure and to reach the desiredcoverage in a more reproducible way.

The geometry of the molecules used for the structure models wasdetermined by the internal force field in Hyperchem 7. The interactionenergies between free pyridine and benzene rings, which were usedto model the molecule-molecule interactions in the adlayer, weredetermined as a function of the N‚‚‚H and H‚‚‚H distances using theGaussian 03 program package (Gaussian, Inc., Wallingford, CT,2004) with the MP2/6-31G(d,p) level of theory. The total interaction

energies of the adlayer were calculated by adding the interactionenergies of the different N‚‚‚H and H‚‚‚H distances in the adlayer.

3. Results and Discussion

3.1. STM Results.2,4′-BTP deposition on the Ag(111) filmsubstrate and subsequent annealing as described in section 2lead to the formation of the ordered adlayer resolved in the STMimage in Figure 2a. This and similar submolecular resolutionimages point to the planar adsorption of molecules in anarrangement indicated by the structural model superimposed ontothe STM image in Figure 2a (for details, see also Figure 2b,c),although slight distortions of the molecules or tilting at low tiltangles cannot be excluded from the STM images. It should benoted that adlayer structures with different densities are formedwhen varying the coverage.30 The unit cell of the adlayer itselfis indicated by the white square in Figure 2a. Within a givendomain, four different rotational orientations of the moleculesexist. The recurring building block of the network structure is

(30) Roos, M.; Hoster, H. E.; Meier, C.; Breitruck, A.; Ziener, U.; Behm, R.J. Phys. Chem. Chem. Phys., 2007, DOI: 10.1039/b708578h.

Figure 2. Quasi-quadratic network (QQN) formed by 2,4′-BTP on Ag(111)/Ru(0001) in UHV. (a) STM image (20× 20 nm2; UT ) -2.8V, IT ) 7.9 pA). (b) Large-scale model of the windmill-like building block stabilized by C-H‚‚‚N bonds (black elliptical bars). (c) Structuralmodel: smallest possible unit cell (a1 anda2) and rectangular unit cell used for calculations (b1 andb2).

11572 Langmuir, Vol. 23, No. 23, 2007 Hoster et al.

a windmill-like arrangement of four 2,4′-BTP molecules withinternalC4h symmetry in the center of each unit cell (Figure 2b).Each of these four molecules is also part of a neighboring, mirror-inverted building block (i.e., the adlayer itself is not chiral). Inthis structure, each molecule forms hydrogen bonds betweenC-H and N (black elliptic bars in Figure 2b) with each of itssix nearest neighbors, which add up to four C-H‚‚‚N-typehydrogen bonds per molecule (Figure 2c). The intermolecularinteraction energy mainly results from these hydrogen bondsand, depending on the local geometry, the repulsion or weakattraction between H atoms of neighboring molecules (blacklines in Figure 2b).

In addition to lattice vectorsab1 andab2 of the smallest possibleunit cell, we can also use vectorsbB1 ) ab1 + ab2 andbB2 ) ab2 -ab1 to describe the adlayer structure. These are more suitable forthe energy calculations described below because they are parallelor perpendicular to the (vertical) mirror plane of the 2,4′-BTPmolecules. Experimentally, 2D lattice vectorsbB1 and bB2 weredetermined from large-scale STM images such as that in Figure3, which yields lattice constants ofb1 ≈ b2 ) 4.5 ( 0.07 nm(equivalent toa1≈ a2 ) 3.18( 0.05 nm). To reduce drift effects,the lattice constants were evaluated only in directions close tothe fast-scan direction in the STM images. Because the samplealignment varied for different STM measurements, both latticeconstants could be evaluated this way and were found to differby no more than 3%. Because a truly quadratic adlayer networkwould be incommensurate to a hexagonal substrate whereas ourfollowing analyses point to coincident adlayer-substrate align-ment, we refer to the structure as a “quasi-quadratic network”(QQN). STM images showing two or more domains of the QQNon Ag(111) (cf. Figure 3b) revealed that the domains differ in

their rotational orientation by either 60 or 120° and that onlythree experimentally distinguishable orientations of the adlayerexist. This is possible only if the 2D lattices of the substrate andadlayer share two mirror planes because in all other cases onewould expect six different rotational domains. The mirror planesof the substrate are parallel and perpendicular to the close-packedsubstrate directions, whereas the four mirror planes of the QQNstructure are parallel to adlayer lattice vectorsab1, ab2, bB1, andbB2

(Figure 2). The sharing of two mirror planes is possible only ifeitherab1 or bB1 is parallel or perpendicular to the close-packedlattice directions.

When an appropriate amount of 2,4′-BTP is deposited on Au-(111), the QQN phase can also be prepared on this substrate,with the same molecular arrangement and dimensions of the unitcell as for Ag(111) (Figure 4). Again, higher and lower coverageslead to other network structures that are still under investigation.As previously observed for other systems,31 the (22x3) her-ringbone reconstruction of the Au(111) substrate32 is visible“through” the ordered molecular adlayer. The epitaxial relation-ship of the QQN unit cell and the (22x3) reconstruction can bederived from the low-pass-filtered STM image in Figure 4a. Thetwo adjacent pairs of domain boundaries (reconstruction lines),which are marked by dotted lines, are separated by 22× 0.2884nm) 6.345 nm. Lattice vectorsab1 andab2 are oriented orthogonaland parallel to the [112h] direction, respectively. Orthogonal tothe [112h] direction, two unit cells fit between the reconstructionlines (i.e., the QQN structure is commensurate with thereconstruction). Lattice vectorab1 is equal to 11 nearest-neighbordistances along the [11h0] direction of the unreconstructed Au-(111) plane (i.e.,|ab1| ) 3.172 nm). Within the limits of theexperimental precision, the unit vectorab2 of the QQN has the

(31) Schmitz-Hu¨bsch, T.; Fritz, T.; Sellam, F.; Straub, R.; Leo, K.Phys. ReV.B 1997, 55, 7972.

(32) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J.Phys. ReV. B1990, 42, 9307.

Figure 3. Two large-scale STM images used for measurements of(a) the lattice constant (55× 35 nm2; UT ) -1.1 V, IT ) 5.0 pA)and (b) the angular alignment of neighboring domains of the quasi-quadratic network (QQN). The visible step and the holes belong tothe underlying Ag film (54× 34 nm2; UT ) -1.1 V, IT ) 5.0 pA).

Figure 4. QQN on reconstructed Au(111) in UHV. (a) Simultaneousimaging of QQN and the underlying (22x3) reconstruction (54×30 nm2; UT ) -1.9 V, IT ) 7.1 pA). The image is low-pass filteredfor better visibility of the reconstruction. (b) QQN imaged withsubmolecular resolution (27× 16 nm2; UT ) -3.0 V, IT ) 7.1 pA).


same length. In total, the structures of the molecular adlayers onthe two different metal substrates are remarkably similar, withthe symmetry planes of the substrate being parallel or orthogonalto either adlayer lattice vectorab1 or bB1 for the Ag(111) films andto ab1 for Au(111).

A similar QQN-type layer is formed on HOPG, for which the2,4′-BTP molecules were deposited either by evaporation in UHV(Figure 5) or, as already described previously,23,24out of a 1,2,4-

trichlorobenzene solution (Figure 6). In this case, STM imagesreveal not only multiples of 60° but also angles in the range of6( 1° between neighboring, differently oriented domains (Figure5). Furthermore, the rotational angle between the adlayer andsubstrate could be directly measured by scanning the same areatwice with different tunneling parameters, resolving either theadlayer (higher gap resistance,IT ) 40 pA, sample biasUT )-810 mV, Figure 6b) or the substrate lattice (lower gap resistance,IT ) 100 pA, sample biasUT ) -10 mV, inset of Figure 6b).Note that the image in the inset, which shows the hexagonalHOPG lattice, is enlarged for better visibility. The dotted andsolid lines in Figure 6b show the directions of a substrate latticevector and adlayer lattice vectorbb2, respectively. The angle of

Figure 5. Quasi-quadratic network of 2,4′-BTP on HOPG preparedby vapor deposition in UHV. (a) Large domain (111× 111 nm2;UT ) 2.8 V, IT ) 5.6 pA). (b) Image with submolecular resolution(20 × 20 nm2; UT ) -2.0 V, IT ) 5.6 pA). (c) Border of twodomains differing in azimuth angle by∼6 ( 1° (27 × 27 nm2; UT) 2.0 V, IT ) 5.6 pA).

Figure 6. (a) QQN structure formed by 2,4′-BTP dissolved in 1,2,4-trichlorobenzene solution at the solid/liquid interface on HOPG (20× 20 nm2, UT ) -1000 mV,IT ) 80 pA). (b) STM images of theHOPG substrate (inset) and the molecular network sequentiallyrecorded at the same position by switching from high (inset;UT )-10 mV, IT ) 100 pA) to low (UT ) -810 mV, IT ) 40 pA)tunneling currents. A difference in azimuth angle of 3.5° betweenthe densely packed rows of the HOPG substrate and the rectangularadlayer unit cell indicates a low degree of coincidence in the adlayer-substrate alignment. The lattice parameters are found to beb1 ) b2) 4.51( 0.02 nm (large image, 21.5× 21.5 nm2; inset, 5.4× 5.4nm2).


3.5( 0.5° between the substrate and adlayer lattice orientation(dotted line in Figure 6b) well fits the small angles possiblebetween differently oriented domains (6( 1°). Lattice constants|ab1| and |ab2| of the QQN structure on HOPG were determinedto be 3.19( 0.03 nm, both at the solid|gas and solid|liquidinterfaces, which is very close to the value found on the twometal substrates.

3.2. Molecule-Molecule and Substrate-Molecule Interac-tions. In the following text, we will discuss the adlayer structureand the underlying molecule-substrate and molecule-moleculeinteractions in three consecutive steps, first deriving informationon the alignment of the adlayer structure with respect to thesubstrate and then comparing the resulting adlayer structure andthe length of the N‚‚‚H-C hydrogen bonds and the C-H‚‚‚H-C distances in that structure with the results of semiempiricalcalculations on the ideal length of these interactions. Finally, wewill estimate the lateral corrugation of the adsorption potentialon the different substrates from a simple model based on themetal-N interaction of pyridine adsorbed on Au(111).

Adlayer-Substrate Alignment.The behavior observed on thetwo metal surfaces indicates a clear effect of the molecule-substrate interaction, whose corrugation is sufficiently strong toaffect the resulting adlayer structure. As derived from thesimultaneously imaged surface reconstruction, the lattice vectorab1 of the QQN phase on Au(111) is perpendicular to the brightlines of the reconstruction, which are separated by 22sb1, with sb1

being the lattice vector of the unreconstructed Au(111) surfacealong the [11h0] direction. Because a pair of reconstruction linesis separated by two adlayer lattice vectorsab1 (see above), weattainab1) 11sb1. Though there is no comparable direct informationabout the relation of|ab2| with respect to the [112h] direction (i.e.,the direction connecting next-nearest neighbors at a distance ofx3 × 0.2884 nm), it is possible to suggest coincident structureswith dimensions close to those observed experimentally. Thelattice parameters are|ab1| ≈ |ab2| ≈ 3.19( 0.03 nm and 89° <â < 91°. On the Ag(111) film surface, these parameters areidentical, but the data leave two possibilities for the actual angulardifferenceθ betweenab1 andsb1: θ ) 0 or 45°. The latter casemeans thatbB1||sb1. In Table 1, we have listed two structures (Me1and Me2) with coincident adlayer unit cells that fulfill theseconditions and would correspond to “geometrical” and “point-on-line” coincident structures,3,12 respectively. Both unit cellsare also illustrated in Figure 7a. The first structure (Me1) wouldfit the observations on both the Ag films and Au(111) (the nearest-neighbor distances in Ag(111) and Au(111), 0.2889 and 0.2884nm, are virtually identical), whereas the second one (Me2) wouldagree only with the results for the Ag films. Nonprimitive cellsof adlayer structures Me1 and Me2 commensurate to the substratecan be constructed by (1× 3) and (2× 2) supercells, respectively(cf. Figure 7a). For Me2, an even smaller commensurate,nonprimitive unit cell can be constructed by using vectorsbB1 andbB2 instead ofab1 andab2, comprising eight molecules instead offour (Figures 2c and 7a). On the basis of reasons discussed below,we find it more likely that the rotational angle of the QQN withrespect to the substrate is the same on Au(111) and the Ag films,which would leave only the first candidate (Me1) withab1||sb1. It

should be noted that in this analysis we neglected the subtlechanges in interatomic distances in the outermost Au layerassociated with the Au(111) herringbone reconstruction. Becausethis contraction changes at every elbow and these changes willeven occur underneath individual adlayer molecules, this is almostimpossible to include in a model.

An analogous analysis was performed for the QQN adlayerphase on HOPG. If we assume that the same adlayer alignmentis found under UHV conditions and at the solid/liquid interface,then the two coincident structures with the best agreement to theexperimentally determined lattice parameters are those listed asG1 (point-on-line coincident3,12,15) and G2 (geometricallycoincident or coincident II12,15) in Table 2. In contrast to the unitcells on the metal surfaces, the QQN unit cells on HOPG do nothave a mirror plane in common with the substrate. In the G1structure, the smallest angle between vectorsbB1 andbB1′ of twodomains is 5.5°, which results from two domains oriented(2.73° to a mirror plane vertical to substrate lattice vectorsb1,whereas for corresponding vectorsbB2 andbB2′ the orientationsdiffer by 7.34°. For G2, which is illustrated in two mirrorsymmetric versions in Figure 7b, the corresponding angles are6.77 and 6.5°, respectively. Three of these four values would bewithin the error limits of the experimental observations (Figure5), making these structures equally probable with respect to theangular differences. Only for G2, however, does the angle betweenbB1 and sb1 correspond, within the margins of precision, to thealignment observed at the solid-liquid interface (Figure 6).Including both substrate and adsorbate, commensurate unit cellsare reached for (3× 1) and (4× 3) supercells of the adlayerlattice in the G1 and G2 structures, respectively.

In summary, the dimensions of the QQN unit cells, includingtheir internal anglesR orâ, are essentially identical on all surfaces,whereas the rotational orientation of the adlayer is different forHOPG on one hand and the two metal surfaces on the other hand.This also affects the orientation of the vertical adlayer symmetryplanes, where one set is oriented along or perpendicular to theclose-packed orientations of the substrate for the metal surfaces,whereas they deviate slightly from these directions on HOPG.This shows that the internal QQN adlayer structure is mainlygoverned by the intermolecular forces, whereas the corrugationof the substrate-adsorbate interaction is responsible for theazimuthal orientation and possibly also for small distortionsrelative to an ideal, relaxed adlayer structure produced bymolecule-molecule interactions only. The latter structure isexperimentally inaccessible but can be estimated from the simpleaddition of the distance-dependent interaction energies betweenthe molecules (below).

Molecule-Molecule Interactions.The energy of the C-H‚‚‚N-type hydrogen bonds formed between peripheral pyridylmoieties of neighboring molecules in the QQN structure wasapproximated by theoretical calculations of the interactionbetween two free pyridine molecules in vacuum, in a configurationsimilar to that of the pyridyl moieties of the neighboring moleculesin the QQN structure.24 By analogy, the interaction energybetween a pyridyl and a phenyl group was approximated bysimilar calculations for pyridine and benzene rings in vacuum

Table 1. Candidates for Coincident QQN Unit Cells on Au(111) and Ag(111)a

no. a1/nm a2/nm aangleab1,sb1 b1/nm b2/nm b

anglebB1, sb1 [M]

Me1 3.178 3.169 90° 0° 4.488 4.488 90.2° 15.1° [ 11 0-19/3 38/3]

Me2 3.227 3.227 91.5° 44.3° 4.504 4.622 90° 90° [ 7/2 9-25/2 9]

a See Figures 2 and 7a for definitions of unit cell dimensions and angles.


(Figure 8a). The energies of both pairs were calculated as afunction of the H‚‚‚N and H‚‚‚H distances (dotted lines, Figure8a). The resulting interaction energies (Figure 8b) were used toestimate the molecule-molecule interaction within the QQNphase. The H‚‚‚N and H‚‚‚H distances within the network werederived from the positions of the H and N atoms within the2,4′-BTP molecule and the relative positions of the moleculeswithin the unit cell. The structure of the molecules was determinedby the internal force field in Hyperchem 7. (Note that for thesemodel calculations we assume that the adsorbed molecules arein a planar configuration.) The total interaction energy wascalculated as the sum of the interaction energies of all nearest-neighbor pyridyl and phenyl moieties. In this approximation, wedo not include (i) possible anisotropies in the C-H‚‚‚N-C andC-H‚‚‚H-C interaction energies, (ii) minor distortions of the

molecular geometry due to intermolecular interactions, and (iii)changes arising from the differences between free pyridine andbenzene molecules on one hand and pyridyl and phenyl moietiesas part of oligopyridine molecules on the other hand.

The 2D plot in Figure 9 shows the calculated interaction energyin the QQN adlayer as a function of the length of adlayer latticevectorsbB1 andbB2, with â kept at 90°. (See also Figure 2c.) Foreach calculated set of parameters|bB1| and|bB2|, the positions ofthe molecules within the unit cell were optimized by translationalongbB1 andbB2. The lowest energy of-0.47 eV per moleculeis reached for|bB1| ) |bB2| ) 4.55 nm (see also Table 2, bottomline), which is very close to the experimentally observed valuesof |bB1| ) |bB2| ≈ 4.5( 0.07 nm. This underlines the dominatingrole of the hydrogen bonds in the unit cell dimension of the QQNstructure and the arrangement of the molecules within the unitcell. Though we cannot exclude possible influences of the differentsubstrates on the interaction potentials, they do not substantiallyaffect the optimum values of the N-H and H-H distances.

Molecule-Substrate Interactions.The interactionof theadlayerwith the metal substrate or, more specifically, the lateralcorrugation in the adsorption potential, can affect the structureof the adlayer in two different ways: (i) by favoring specificsites and orientations of an individual molecule (minimizationof the adsorption energy of an individual molecule) and (ii) byfavoring specific structures of the entire adlayer (minimizationof the system energy) minimization of the adsorption energywithin the system unit cell). These two aspects are of courseclosely related to each other but are not identical because themost favorable structure and orientation of the adlayer may notnecessarily include the most favorable molecular adsorptionconfigurations. A first principles calculation of the adsorptionpotential, which would allow for a quantitative description of theadlayer energetics, would require extensive calculations, whichare beyond the scope of the present study. (Note that the adsorptionpotential of a single molecule is highly multidimensional becausefor each position of the molecule all possible rotationalorientations have to be considered.) Here we applied a simplemodel, where we assume that the most favorable orientation ofa single 2,4′-BTP molecule is largely determined by the positionof the N atoms with respect to the metal substrate lattice (i.e.,by the average (lateral) distance of the N atoms in the moleculeto the optimum adsorption site). Furthermore, we assumed thatthe interaction energy decreases linearly with lateral distancebetween the N atom and the optimum adsorption site, which isa reasonable approximation for lateral distances that are not toolarge. This approach is supported by recent DFT calculationsthat indicated a distinct role of the position of the N atoms withrespect to the underlying substrate for pyridine adsorbed in aplanar configuration on Au(111).33The study revealed substantialvariations of the adsorption energy for planarly adsorbed pyridine,with an energy difference of 0.022 eV between the strongest andweakest planar adsorption positions. In the former configuration,the N atom is positioned on top of a surface Au atom, and in thelatter, above a 3-fold hollow site.33 It is likely that planarlyadsorbed oligopyridines will also prefer such positions associatedwith as many N atoms right above the Au atoms as possible.

The optimum adsorption configuration of the 2,4′-BTPmolecule was determined by minimizing the average lateraldistance between a fixed point in the substrate lattice (e.g., thesurface atoms) and all N atoms in the molecule for all relativemolecule-substrate positions within the range of one substrateunit cell for each rotational angle between 0 and 60°. The resultingaverage lateral distances of the lowest- (highest-) energy

(33) Bilic, A.; Reimers, J. R.; Hush, N. S.J. Phys. Chem. B2002, 106, 6740.

Figure 7. Coincident unit cells of the QQN on different surfaces.(a) Ag(111) or Au(111), where commensurate supermeshes aremarked by dotted lines. (b) HOPG, with most probable candidateG2 shown in two mirror symmetric variations to illustrate the expecteddifferences in rotational angle.


configuration for each orientation are plotted in the two curvesin Figure 10. The pronounced minima in the resulting lowest-energy plot indicate that the best registry between N atoms andthe substrate lattice can be achieved for rotational anglesθ ofabout 16, 44, 76, and 104°. Because of the hexagonal symmetryof the substrate, this sequence repeats forθ + 120° and θ +240°, yielding 12 orientations in total. In a single rotational domainof the QQN phase, 4 molecular orientations are found, whichgives 12 orientations for all 3 domains. The good agreementbetween prediction and experiment is illustrated in Figure 4b,where we indicated the expected orientations of the molecules

in a single domain of 44°, 136° (120+ 16°), 224° (120+ 104°),and 316° (240 + 76°).

It should be noted that this procedure for determining theoptimum adsorption configuration of the molecule does not relyon or allow for an absolute determination of the optimumadsorption site of the N atoms. This could be somewhere in theunit cell, although surface metal atoms would be the most plausiblecandidates.33 Furthermore, as described above, we are using alinear relation between the lateral displacement of the N atomrelative to the optimum adsorption site and the metal-Ninteraction energy, which can be considered to be a good

Table 2. Candidates for Coincident QQN Unit Cells on HOPGa

no. a1/nm a2/nm aangleab1,sb1 b1/nm b2/nm b

anglebB1, sb1 [M]

G1 3.146 3.197 90.1° 48.3° 4.481 4.49 89.1° 2.73° [ 3 -1144/3 10 ]

G2 3.206 3.198 90.34° 48.5° 4.515 4.542 90.1° 3.39° [ 3 -45/444/3 10 ]

FR 3.217 3.217 90° n.a. 4.55 4.55 90° n.a. n.a.

a See Figures 2 and 7b for definitions of unit cell dimensions and angles. FR denotes a fully relaxed adlayer as revealed by the calculations shownin Figures 8 and 9.

Figure 8. Interaction of free benzene and pyridine rings as a model for the interaction of 2,4′-BTP moieties. (a) Two models used for thecalculation. (b) Interaction energy as function of the N-H/H-H distance as determined by MP2 calculations.

Figure 9. Total intermolecular energy of the QQN calculated by means of the potentials shown in Figure 7. (a) Energy as function ofb1andb2, with molecules in optimized positions. (b) Cross section along the dotted line in part a (i.e., minimum energy of the QQN as a functionof b1 ) b2.


approximation for displacements that are not too large. In theabsence of detailed calculations on the interaction of BTPmolecules with Ag(111) and Au(111), this approach yieldssemiquantitative structural information that is surprisingly closeto the experimental result. Because the nearest-neighbor distancesin Ag(111) and an unreconstructed Au(111) (see second lastsection) are nearly identical (0.2889 vs 0.2884 nm, respectively),the angles of the optimized adlayer orientation are identical forboth substrates.

Figure 10 illustrates the optimum orientations and thedifferences between different orientations as a function of theaverage distance between the substrate lattice and the positionof the N atom. The related variation in the adsorption potentialcan be estimated only from the potential energy variation of asingle metal-N interaction. Because 2,4′-BTP contains eight Natoms, the maximum corrugation for the entire molecule couldreach up to 0.173 eV (8 times the energy variation for a singlemetal-N bond in pyridine on Au(111)). Because of the fixedgeometry of the molecule, however, not all N atoms reach thestable and unstable sites simultaneously. If we compare theamplitude of the average N displacement in the lower curve inFigure 10 (0.05 nm) with the distance between the on-top and3-fold hollow sites on the Au(111) surface (0.166 nm), whichwere found to be the most and least stable adsorption sites forpyridine,33 respectively, the difference in the average lateral Ndisplacement between the best and worst adsorption sites isdecreased to∼30% of the pyridine value. On the basis of theassumed linear relation between lateral displacement and energyvariation (corrugation of the adsorption energy), this translatesinto a reduction of the energy corrugation for a BTP admoleculeto 30% of the maximum value for eight N atoms (0.173 eV).Comparable energy amplitudes would also be obtained uponvarying the lateral position of the molecule at fixed rotationalorientation (compare the upper and lower curves in Figure 10).

The resulting amplitude of the adsorption potential of about0.05 eV would also be compatible with the structure-dominatingrole of the adsorbate-adsorbate interactions, which wereestimated before to be around 0.47 eV per molecule in theoptimum configurations. Nevertheless, substrate-adsorbateinteractions of this magnitude could be responsible for smalldistortions from the ideal (free) adlayer lattice. According to thecalculated intermolecular interaction energies (Figure 9), thedifference in intermolecular energy between structure Me1 (|bB1|

) |bB2| ) 4.488 nm) and the fully relaxed (FR, Table 2) QQNstructure (optimized C-H‚‚‚N interactions at|bB1| ) |bB2| ) 4.55nm) would only be about 0.05 eV per molecule. Hence, thepossible energy gain due to an optimized substrate registry is ofa similar order of magnitude as the loss in intermolecularinteraction energy and is thus sufficient to compensate for thelatter in the case of such adlayer distortions.

On HOPG, the energy variation for the translation and rotationof an individual BTP molecule is assumed to be even smallerthan on the metal substrates. Therefore, the possible influenceof the substrate on the intermolecular forces themselves shouldalso be smaller on HOPG than on the latter surfaces, and theQQN geometry observed on HOPG should be very close to theoptimum configuration resulting only from intermolecular forces.Nevertheless, despite the weaker interactions between substrateand adsorbate, the substrate effect is still sufficiently strong toresult in preferred rotational orientations of the adlayer relativeto the substrate. In this case, however, it is likely to result fromthe coincident adaptation of the adlayer unit cell to the substratelattice rather than from the orientation of the individual molecules.Comparing coincident adlayer unit cells G1 and G2 listed inTable 2, the energy differences with respect to the optimumconfiguration are 0.06 and 0.008 eV per molecule, respectively.

The behavior observed for the BTP adlayers closely resemblesthat of PTCDA adlayers, which form the same ordered structureon HOPG3,9,20and Ag(111)34 but with a compression of 2% onthe latter substrate to optimize its substrate registry.34For PTCDAon HOPG, two point-on-line coincident unit cells were reportedthat were rotated by 3.1 and 9.9° with respect to the substrate.20

In a theoretical study using Buckingham atom-atom potentialsto simulate van der Waals interactions between a rigid PTCDAadlayer and an underlying HOPG substrate and integrating overa 10× 10 adlayer supercell, the energy per PTCDA moleculewas found to vary by less than 0.004 eV with the rotationalangle.9 Hence, in this case the orientation of the adlayer is alsopresumably determined by the optimization of the total substrate-adsorbate registry rather than by that of a single admolecule,with only minor effects of the orientation of the individualmolecules.

(34) Umbach, E.; Glo¨ckler, K.; Sokolowski, M.Surf. Sci.1998, 402-404, 20.

Figure 10. Adaptation of single 2,4′-BTP molecules to Au(111) (or Ag(111), see the text). (a) Average lateral distance between N and Auatoms as a function of angle. For each angleθ, the maximum (dotted line) and minimum (straight line) values attainable for any translationare plotted. (b) Visualization of the two mirror-inverted optimum angles between the mirror plane and the [112h] lattice vector, for whichall N atoms can reach positions close to the Au atoms.


4. Conclusions

We have shown by high-resolution STM imaging that thesame type of adlayer structure, a quasi-quadratic network (QQN)structure, is formed by 2,4′-BTP oligopyridines adsorbed onboth weakly (HOPG) and more strongly (Au(111) and (111)-oriented Ag films on Ru(0001)) interacting surfaces. Structureformation is dominated by intermolecular interactions, specif-ically, by C-H‚‚‚N-type hydrogen bonds and by C-H‚‚‚H-Cinteractions between adjacent molecules. The experimentallyobserved structure agrees qualitatively and quantitatively withpredictions from model calculations based on a summation ofthe corresponding distance-dependent pairwise interaction po-tentials derived from MP2 calculations. Substrate-adsorbateinteractions, though they do no determine the structural char-acteristics of the adlayer, are important because they determinethe registry to the substrate lattice. This results in higher-ordercommensurate, coincident adlayer structures on all substrates,for which we have derived the most probable unit cells. Themore pronounced substrate-adsorbate interactions on the metalsurfaces compared to HOPG are reflected by (i) a strongercompression of the adlayer, relative to a fully relaxed adlayercalculated from the optimization of the pairwise C-H‚‚‚N andC-H‚‚‚H-C interactions and (ii) the orientation of the individual

BTP molecules with respect to the substrate lattice. On the metalsubstrates, the adlayer lattice constants, derived from thecomparison of experiment and model calculations, are compressedby 1.4% relative to the fully relaxed adlayer structure. On HOPG,the resulting adlayer lattice constant is even closer to the fullyrelaxed value (compression 0.5%). Furthermore, on the metalsubstrates the 2,4′-BTP molecules assume orientations that arevery close to those of the ideal configurations based on theminimization of the average lateral displacement of the N atomswith respect to the underlying metal surface lattice, thusunderlining the importance of the metal-N interactions for theconfiguration of the adsorbed 2,4′-BTP molecule.

In total, this comparative study showed that simple models forthe overall adsorbate-adsorbate and substrate-adsorbate in-teractions, based on the summation of the distance-dependentenergies of individual C-H‚‚‚N hydrogen bonds and of metal-Ninteractions, lead to correct structural predictions and a semi-quantitative understanding of the resulting BTP adlayer structureand the contributing interactions.

Acknowledgment. Financial support by the Deutsche For-schungsgemeinschaftwithintheSFB569isgratefullyacknowledged.

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