Published on Web Date: November 13, 2009

r 2009 American Chemical Society 190 DOI: 10.1021/jz900146f |J. Phys. Chem. Lett. 2010, 1, 190–194

pubs.acs.org/JPCL

Structure and Epitaxial Registry on Graphite of a Series ofNanoporous Self-Assembled Molecular MonolayersClaire Arrigoni,§,† Guillaume Schull,§,‡ David Bl�eger,† Ludovic Douillard,§C�eline Fiorini-Debuisschert,§ Fabrice Mathevet,† David Kreher,† Andr�e-Jean Attias,*,† andFabrice Charra*,§

§Nanophotonics Laboratory, Service de Physique et Chimie des Surfaces et Interfaces, IRAMIS, CEA, F-91191 Gif-sur-YvetteCedex, France, †Laboratoire de Chimie des Polym�eres;UMR7610, Universit�e Pierre et Marie Curie, 4 place Jussieu, case 185,75252 Paris Cedex 05, France, and ‡Institut de Physique et de Chimie de Strasbourg, Universit�e Louis Pasteur, CNRS UMR 7504,23 rue du Loess, 67034 Strasbourg, France

ABSTRACT We have analyzed by STM the detailed structures of a series ofnanoporous honeycomb networks stabilized by alkyl chain interdigitation ongraphite at the liquid-solid interface, that is, clip-like noncovalent bonding. Thevariations observed as a function of the length of the peripheral aliphatic chainsshow that the assembly is directed not only by lateral intermolecular interactionsbut also by the adsorption site on the substrate. We derive an atomically accuratemodel for the registry with graphite of our nanoporous model series of systems. Infull agreement with the quantitative model, the pore areas vary step-by-step bymore than one order of magnitude along the whole series while preserving thedetailed features of the graphite-induced alkyl chain interdigitation. The largestpores observed correspond to a ratio of uncovered substrate area as large as 35%.

SECTION Surfaces, Interfaces, Catalysis

M olecular self-assembly is a promising route for thebottom-up manufacturing of nanostructures onatomically flat surfaces1. The mechanisms of mo-

lecular self-organization have been the subject of numerousinvestigations aimed at the realization of building blocksdesigned so as to spontaneously form 2D monolayers withspecific topologies. Intermolecular interaction may be drivenby various molecular recognition processes, from steric hin-drance 2,3 to hydrogen bonding,4-9 metal coordination, 10-13

or interdigitation of alkyl chains, 14-16 The latter may bestrongly favored by adsorption on the surface of highlyoriented pyrolytic graphite (HOPG). As a matter of fact, thevalue of the alkyl-chain period allows an adsorption in registrywith HOPG along its Æ1,0,0æ axis according to the Groszekmodel.17-20 It has been noticed that the distance whichminimizes the energy of side interaction between alkylchains, 4.26 Å, exactly matches a period of Æ1,0,0æ rows.20Thus,molecularmoieties able to “clip” together by interdigita-tion on HOPG have been designed by finely adjusting thelateral distance between chains.15

Among the various geometries based on alkyl chain clipsdemonstrated so farat the solution-solid interface,15,21 one22

forms a nanoporous matrix, a generic structure which cur-rently attracts strong interest.23 This specific nanoporoussystem presents intriguing molecular sieve and dynamicproperties.22,24,25 The structure of its building block moleculeis represented in Figure 1 together with the honeycombstructure that it forms. The possibility to finely tune the poredimensions constitutes a prerequisite toward the control and

exploitation of these properties. From the structure of theabove-mentionednetwork (Figure 1), onemayanticipate thatvarying the length of peripheral alkyl chains will result inchanges of pore dimensions. Yet, although the influence of thesubstrate is seldom considered in interpreting self-assembledmolecular structures, in the present case, the core role ofHOPG on clip formation requires a more careful account ofthe substrate. In the above series of molecules, the step-by-step variation of alkyl chain lengths offers a unique opportu-nity to analyze the influence of adsorbate-adsorbent inter-actions.

In this paper, we analyze the evolution of the epitaxialrelationships of nanoporous networks formed on HOPG byinterdigitation of alkyl side-chains with various lengths. Weshow that the molecular lattices stay in registry with HOPGfollowing a common atomically accurate model while poreareas vary by more than one order-of-magnitude.

Molecules of 1,3,5-tristyrylbenzene substituted by alkoxyperipheral chains presenting n = 6, 8, 10, 12, or 14 carbonatoms (TSB3,5-Cn; see Figure 1) were deposited on the HOPGsurface. They have been synthesized and purified by adaptingtheproceduredescribed in the literature,26using thecorrespond-ing 3,5-dialkyloxybenzaldehydes. The solvent was 1-phenyloc-tane (98%, Aldrich), which is well suited for in situ scanningtunneling microscopy (STM) because it is hydrophobic, highly

Received Date: October 15, 2009Accepted Date: November 9, 2009

r 2009 American Chemical Society 191 DOI: 10.1021/jz900146f |J. Phys. Chem. Lett. 2010, 1, 190–194

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insulating, and poorly volatile. The substrate was a freshlycleaved HOPG (Goodfellow). The self-assembled monolayersformed from TSB3,5-C6, TSB3,5-C8, and TSB3,5-C10 weredeposited from 10-3 to 10-4 mol 3 L

-1 solutions immediatelyafter cleaving the substrate and were then imaged by STM.Special caremust be taken for the deposition of TSB3,5-C12 andTSB3,5-C14, which result in large pores and thus low-densitymonolayers. Solutions with lower concentrationwere then used(10-5 to 10-6 mol 3 L

-1) to avoid formation of alternate densernetworks 27. Moreover, those solutions were deposited ontogently heated substrates (60 �C) before the systemwas progres-sivelycooleddown to roomtemperature for STMobservation. Inturn, this method happens to favor the formation of largedomains. As an example, a typical large-area image of a self-assembled monolayer is presented in Figure 1. By applying theabove procedure, domains larger than typically 100 nm wereroutinely observed. The tips used for STM imaging were me-chanically formed in a 250 μm Pt-Ir wire (Pt80/Ir20, Good-fellow). The STM was a homemade fully digital system. Theimages reported here were obtained in the current mode, withslow height regulation, at a sample bias of ∼-1000 mV andtunnel current setpoints in the rangeof3-17pA.Thescananglewas adjusted in such a way as to keep the fast-scan axis nearlyperpendicular to the sample slope. Images acquired simulta-neously in forward and backward fast-scan directions weresystematically recorded and compared. All images were cor-rected by homemade software for the drift of the instrument,which was evaluated by correlating successively acquiredimages with the opposite slow-scan direction. For angle mea-surements between domains, a hexagonal lattice with 60�angles between the lattice directions was assumed. Then, thecombination of themeasurementswith the three axes, togetherwith averaging over all domains present in the image, permittedaccuracies of typically 0.5�.

For molecules comprising both aliphatic and conjugatedparts, the conjugated moieties appear much brighter in STMimages. On the basis of the observation of the star-shapedconjugated cores, it appears that each of the five moleculesself-assembles on HOPG, forming large domains of honey-comb network (see Figure 2). The unit cells appear to havehexagonal symmetry. Their parameters evolvemonotonouslyfrom 3.1 (TSB3,5-C6) to 4.5 nm (TSB3,5-C14)with increasing

alkyl chain lengths. In each case, only two kinds of domainsare formed, displaying mirror-symmetric STM images. Speci-fically, it can be observed that, depending on the orientationof the domain, two neighboring molecules are tilted eitherboth clockwise or both counterclockwise with respect to theaxis defined by the pair of molecule centers. The anglesformed by the unit cell axes of two domains of opposite left-handed and right-handed orientations grown on the sameterrace are shown in Figure 2 for TSB3,5-C6, TSB3,5-C8, andTSB3,5-C10 and reported below their respective images.TSB3,5-C12 is a specific case where both types of domainshave the same direction, within experimental uncertainties(0.8�). A thorough observation of STM images of the unit cellstill permits one to discern two distinct mirror-symmetricvariants of domains, their unit cell axes being parallel one tothe other. Thus, the angle θ between the two sets of domainsvaries uniformly from23.6 (TSB3,5-C6) to 0� (TSB3,5-C12). Inthe case of TSB3,5-C14, the only operational depositionprocedure, employing the lowest concentration and highsubstrate temperature during the self-assembly, yields verylarge single domains occupying entire terraces. Then, theangle can not be reproducibly measured, but the two sets ofmirror-symmetric domains are still observed. The measuredlattice parameters, that is, the unit cell dimensions a and theangles θ, are reported in Figure 2.

The qualitative features of self-assembled monomolecularlayers can often be explained solely in termsof intermolecularinteractions, the substrate contribution to lateral order beinglimited to the interplay between coverage maximization andsteric hindrance. Yet, the existence of fixed lattice orienta-tions, observed even for disjoined domains, can only beexplained by interactions with the underlying HOPG sub-strate. To understand quantitatively the structure of themolecular assembly, an important point is whether theabove-described lattice variations with alkyl chain length aregoverned only by intermolecular distances or are quantifiedby conditions of registry of the molecular lattice with HOPG.

To address this question, let us assume an optimal registrywith the substrate. Thismeans that allmolecules are adsorbedin the same, most stable, geometry relative to the surface ofthe HOPG substrate. This hypothesis is consistent with thehigh similarity between STM images of different molecules,

Figure 1. Molecular formula and structure of the self-assembly on HOPG. Left: molecular formula of TSB3,5-Cn. R represents the alkylchain-CnH2nþ1. For the studiedmolecules n=6, 8, 10, 12, or 14.Middle: example of a self-assembledmonolayer as observed by STMat thesolid-liquid interface forn=10. The scanned area,within a single domain, is 70 nm�70nm.Right: schemeof alkyl chain interdigitation asobserved for the case of n = 10.22 Two pores are separated by four interdigitated alkyl chains forming the so-called clip structure.15

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including between those having different orientations. As amatterof fact, it hasbeenpredicted28,29 andobserved30,31 thatthe appearance in STM images of such graphene-like conju-gated structures changes considerably with the adsorptionsite. This imposes a six-fold rotation invariance around thecenter of a pore, for both the molecular layer and the topmostatomic layer of the HOPG substrate. The molecular pores arethus necessarily centered over alveoli of HOPG as sketched inFigure 3a (points labeled C6). Notice however that, if oneaccounts for the second underlying atomic layer, the six-foldsymmetry is broken and reduced to a three-fold rotationinvariance. Then, two molecules rotated by 60� cannot bestrictly equivalent. The stability of the adsorption is howevergoverned mainly by interactions with the substrate surface,which justifies considering only one atomic layer of HOPG.

Similarly, the centers of the molecules correspond to athree-fold symmetry axis of the molecular layer (pointslabeled C3 in Figure 3) and are thus necessarily aligned onat least a three-fold symmetry axis of HOPG. There are threepossible such situations, represented below Figure 3a(referred to as C3-1, C3-10, and C3-2). Each type corre-sponds to a different adsorption geometry and stability of themolecule. If we still assume that the molecules systematicallyarrange themselves so as to adopt the most stable one, thenthis point must remain of the same type when varying thelength of the alkyl chains. This quantifies and reduces thepossible periods of the lattice structures adopted for thevarious lengths, as shown in Figure 3b in the case of a C3-1type. A detailed examination of the few possibilities thus leftshows that the experimentally observed series of latticeparameters and angles between mirror-symmetric domains

exactly matches the series of possible periods displayed inFigure 3b. This series of lattice vectors is fully consistent withthemolecularmodel depicted in Figure 3a extrapolated to theother chain lengths. Specifically, it allows preservation of boththe Groszek-type adsorption of the alkyl chains and their“clip”-like interdigitation over the whole series. The latticeepitaxial relation with HOPG follows the general form [(91 þ15nþ3n2)1/2� (91þ15nþ3n2)1/2Rθ] and tanθ=31/2[(n-6)/(3nþ 16)], where n is the number methylene pairs in the alkylchain (from n=3 for TSB3,5-C6 to n=7 for TSB3,5-C14). Thecorresponding data are reported in Figure 2 below each STMimage. Notice that the angle between the lattice vectors ofmirror-symmetric domains is 2θ. The agreement betweenexperimental and theoretical data fully confirms the principlesdeveloped here.

The agreement with the model of epitaxial adsorptionenables a derivation of the pore dimensions as a function ofthe chain length. As reported in Figure 2 (last row), thediameter (respectively area) of the circumcircle of the poresincreases monotonously from 0.62 (0.30 nm2) to 2.31 nm(4.2 nm2) from TSB3,5-C6 to TSB3,5-C14. The ratio of theHOPG area not covered by molecules also increases, from∼5% for TSB3,5-C6 up to∼35% for TSB3,5-C14. In the lattercase, the energy cost corresponding to the lack of moleculeadsorption on a considerable proportion of the availablesubstrate surface explains the difficulty in forming the layerand the necessity of a thorough control of the thermody-namics and kinetics of the self-assembly process.

Finally, the observation that the molecule adsorptionstrictly preserves the Groszek model structure for alkyl chaininteractionwithHOPG further supports the roleof the so-called

Figure 2. Analysis of molecular lattice evolution with increasing alkyl chain lengths from-C6H13 to-C14H29. Row 1: alkyl chain formula.Row 2: selected STM images showingmirror-symmetric domains imaged simultaneously. For easier comparison, the five images have beenclipped to represent the same dimensions, 26 nm� 26 nm. The domain boundaries are highlighted by dotted lines. In each case, one latticedirection on each domain is superimposed to the image, and the angle that they form is represented. Row 3: measured lattice parameters,distance a, and angle 2θ between domains (in parentheses, theoretical values from the model depicted in Figure 3; see text for discussion).Row 4: epitaxial relationship with HOPG. The period is represented as a green arrow in STM images of the second row. Last row:representation of pore dimension and diameter of the largest circle circumscribed to the van-der-Waals spheres of frontier H atoms (see textfor discussion). This circle is also reported in green in the STM images of the second row.

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clip structure on the organization of themolecular layer, up theatomically accurate scale.

In conclusion,wehave analyzed thedetailed structures ofaseries of nanoporous honeycomb networks formed by tristil-bene derivatives on HOPG by STM at the liquid-solid inter-face. The variations observed as a function of the length of theperipheral aliphatic chains shows that the assembly is direc-ted not only by lateral intermolecular interactions, that is,steric hindrance and chain interdigitation, but also by thesearch for themost stable adsorption site on the substrate.Wehave thus been able to derive an atomically accurate modelfor the registry with HOPG of our nanoporousmodel series ofsystems. The pore areas vary step-by-step by more than oneorder of magnitude along the whole series, while preservingthe detailed features of the HOPG-induced alkyl chain inter-digitation. The largest pores observed correspond to a ratio ofuncovered substrate area as large as 35%.

SUPPORTING INFORMATION AVAILABLE Direct com-parison of the molecular lattice orientation and the HOPGÆ110æaxis and derivation of the lattice epitaxial registry formula.This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author:*Towhomcorrespondence should be addressed. E-mail: andre-jean.attias@upmc.fr. Phone: þ33/144275302. Fax: þ33/144277089(A.-J.A.); E-mail: fabrice.charra@cea.fr. Phone: þ33/169089722.Fax: þ33/169088446 (F.C.).

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Figure 3. Epitaxial relationship with HOPG. (a) Close-up showingthe molecular positioning relative to HOPG (superimposed lightgray chicken wire). For clarity, the case of shorter chains, TSB3,5-C6, has been represented. One lattice vector connecting two C6points is represented (solid arrow). Two vectors connecting C6 andC3 points are also represented (dotted arrow). By symmetry, theymust form a 120� angle and have the same lengths. (b) Variationsof the lattice vector with chain length, from TSB3,5-C6 to TSB3,5-C14, constrained by the preservation of the nature of the C3 pointrelative to HOPG. (Here a C3-1 type). The mirror-symmetriclattice is also represented in the upper part, showing the variationsof the angle between simultaneously observed domains. In allschemes, the centers of molecular pores, forming six-fold sym-metry axes, are labeled C6. The centers of the molecules areanalogously labeled C3.

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