Two-Dimensional Ordering of Poly(p-phenylene-terephthalamide) on the Ag(111) SurfaceInvestigated by Scanning Tunneling Microscopy

Christoph H. Schmitz,* Julian Ikonomov, and Moritz SokolowskiInstitut fur Physikalische und Theoretische Chemie, UniVersitat Bonn, Wegelerstr. 12, 53115 Bonn, Germany

ReceiVed: May 5, 2009; ReVised Manuscript ReceiVed: June 5, 2009

Long-range ordered monolayer domains of the polymer poly(p-phenylene-terephthalamide) were directlysynthesized on the Ag(111) surface and analyzed by scanning tunneling microscopy with submolecularresolution. The polymerization reaction of the monomers runs at room temperature and yields ordered domainswith a diameter of up to 50 nm. The lateral order of the polymer chains within these domains is similar tothat known for bulk crystals but reveals deviations that are caused by the role of the underlying Ag(111)surface. The polymer shows a chain folding to re-enter the ordered domains, which is in accordance withknown models of crystalline bulk polymers.

The modification of metallic or semiconducting surfaces byadsorption of organic substances and especially the formationof long-range ordered structures of organic adsorbates has beensubject to numerous recent studies.1 The intermolecular forcesthat lead to ordered structures are generally based on weak vander Waals forces,2 dipole interactions,3 hydrogen bonds,4,5 ormetal complexation.6,7 However, for possible applications, e.g.,in the field of organic electronics (organic field effect transis-tors,8 organic solar cells,9 etc.), layers in which the adsorbatesare interlinked by strong covalent bonds are advantageous. Suchlayers likely possess a larger chemical, thermal, and mechanicalstability. Besides these possible applications, covalent inter-linking of organic adsorbates on surfaces constitutes a principleapproach toward the synthesis of two-dimensional macromol-ecules and polymers,10 exploiting the template effect of well-defined crystalline surfaces.

So far, different concepts for the formation of monolayers withcovalent interlinking on the surface have been introduced.11-18 Forlarge organic adsorbates, the reaction is generally inducedthermally,12-17 while a reaction at room temperature requires thepresence of special functional groups, such as boroxine.18 Obvi-ously, the requirement of a thermal treatment to induce theinterlinking strongly limits the scope of possible substances;e.g., in the case of small organic molecules, desorption mayoccur before the reaction takes place. However, large, custom-synthesized molecules that circumvent the desorption problemare often too expensive for possible technical applications. Thus,procedures for the formation of covalent layers by interlinkingat room temperature are attractive and might provide a moreuniversal concept.

In the here reported approach, we utilize small, commerciallyavailable substances, which form polymer chains on a sur-face at room temperature by step-growth polymerization. Wehave chosen p-phenylenediamine (PPD) and terephthaloylchloride (TPC) with the aim to form the polyamide poly(p-

phenylene terephthalamide) (PPTA, trademarks Kevlar andTwaron). This system is of broad scientific and industrial interestand has been extensively characterized as a bulk material.19 Inthis context, the novel aspect of the present work is that we areable to image the polymer directly on the surface (in the firstmonolayer) by scanning tunneling microscopy (STM) withresolution of individual chain segments, which provides mi-croscopic insight into the role of the surface on the structuralorder and the two-dimensional growth morphology of thepolymer domains.

The preparation of the polymer monolayer on the surface isdone by the “vacuum deposition polymerization” (VDP) tech-nique.20 This method has so far been successfully used for thepreparation of PPTA films with a layer thickness of a fewhundreds of nanometers.21,22 Here, we perform VDP underultrahigh vacuum (UHV) conditions in order to obtain a polymermonolayer on a clean Ag(111) surface. Both monomers, PPDand TPC, are dosed simultaneously into the UHV system bymeans of two variable leak valves to perform the step-growthpolymerization on the surface at room temperature. The byprod-uct of the polymerization, hydrogen chloride (c.f., Scheme 1),is detected by a quadrupole mass spectrometer (QMS) duringthe deposition process, unambiguously proving the progress ofthe reaction.23 Earlier experiments on thicker PPTA films (>100nm) have revealed that the reaction to form the polyamide layerduring VDP takes place in the condensed phase after adsorptionand diffusion of the monomers.21,24 We note that this reactionis however not limited to the surface of the Ag(111) substratealone but likely occurs on all surfaces of the UHV chamber.

Figure 1a shows a constant current STM image after thesimultaneous deposition of both monomers. During scanning,

* Corresponding author. Phone: +49 (228) 73 2520. Fax: +49 (228) 732551. E-mail: christoph.schmitz@pc.uni-bonn.de.

SCHEME 1: Reaction Scheme

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we do not detect any further change of the appearance of thelayer, concluding that the polymerization reaction is completeddirectly after the deposition process. This has already beenshown by means of X-ray photoelectron spectroscopy andinfrared absorption measurements for a thicker film of acomparable polyamide, evaporated on a gold substrate at roomtemperature.24

The Ag(111) surface is covered by two different species:small, compact, point-like objects and extended chains that areimaged with larger apparent height (Figure 1). The chains, whichpossess a sawtooth-like intramolecular contrast, are the PPTApolymers that have been formed during the deposition process.The polymer chains show a length distribution from trimers (i.e.,three monomeric building units) up to a maximum length of45 nm, which corresponds to approximately 70 monomericbuilding units. The chains consist of long, linear segments andare randomly kinked between these. Interestingly, the directneighborhood of two adjacent chains causes a linear conforma-tion of both chains, probably due to attractive lateral forces

between them, which stabilize the linear geometry of the doublestrand (see below). In this case, the longer chain is not kinkeduntil the shorter chain is terminated. As expected, we do notfind any branched chains.

In between the polymer matrix, adsorbed molecules of theexcess monomer are embedded. In larger interspaces of morethan about 5 nm × 5 nm, the monomers form ordered domainswith a skewed hexagonal arrangement. These single moleculesof the excess monomer generally hinder the ordered agglomera-tion of the polymer chains. To remove them from the surface,the sample was annealed for 5 min at 420 K. At this temperature,monomers as well as short oligomers desorb from the surfaceand diffusion processes lead to a rearrangement of the remainingpolymer chains.

Notably, we do not find evidence for considerable furtherpolymerization progress during annealing. The typical as wellas the maximal length of the polymer chains is not significantlychanged. Further annealing steps have also no influence on theordering of the polymer monolayer. Deposition of the monomersat higher sample temperatures leads to a reduction of theadsorption rate without an improved ordering of the polymerchains. Above T ) 420 K, only short chains are formed, whichdo not form ordered domains, possibly due to partial competitivebonding of decomposed chain ends to the Ag(111) substrate.

Figure 1b shows the resulting layer directly after annealing.Monomers are no longer found on the surface, while theremaining polymer chains have now formed two-dimensional,long-range ordered domains, consisting of linear, parallel chains.The domains show six different orientations as expected fromthe symmetry of the Ag(111) surface (three different rotationsand the corresponding mirror domains). The direction of chainpropagation inside the domains is at an angle of approximately10-15° with respect to the close-packed rows of Ag atoms([101j] direction). The dimensions of the unit cell are a ) 1.4( 0.1 nm, b ) 1.5 ( 0.1 nm, and γ ) 91 ( 1°, with the cellvector a parallel to the chain direction. Within the givenaccuracy of the STM images, and taking into account theorientation of the domains, the measurement points to acommensurate superstructure with respect to the Ag(111) surfacethat is described by the unit cell matrix

which corresponds to the unit cell dimensions a ) 1.32 nm, b) 1.53 nm, and γ ) 90°. The theoretical angle of a with respectto the [101j] direction of the substrate is 11°. The unit cellcontains two parallel chain segments oriented along the vectora, with one AB building block each. A model of the adsorbatestructure is presented in Figure 2. The length of a ) 1.32 nm,corresponding to the periodicity of the polymer chain on thesurface, agrees nearly exactly (within 2%) with the periodicityof the polymer chain in the crystal structure, which is 1.29nm.25-27 Hence, the conformation of the polymer on the Ag(111)surface shown in Figure 2 was directly extracted from the crystalstructure of PPTA26 and was only very slightly elongated by2% along the chain axis in order to fit the observed periodicityin the monolayer on Ag(111).

Interestingly, while the periodicity of the polymer remainsnearly unchanged, the intermolecular distance between twoadjacent chains (i.e., along b) is increased by nearly 50%compared to the PPTA bulk crystal. In the crystal structure,the arrangement of the polymer chains is mainly controlled byattractive hydrogen bonds between the amide groups of adjacent

Figure 1. (a) STM image of the monolayer directly after deposition.Polymer chains are imaged with a larger apparent height than the excessmonomer. (b) After annealing of the monolayer at 420 K. The excessmonomer has been desorbed from the surface. Polymer chains formlong-range ordered domains. Tunneling parameters are (a) Usample )-1.2 V, I ) 560 pA; (b) Usample ) -3.2 V, I ) 32 pA.

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chains.25-27 For PPTA on Ag(111), the lateral arrangement ofthe polymer chains is a consequence of the direct interactionbetween the chains in combination with the interaction betweenthe corrugated Ag surface and the adsorbate, which can beinferred from the commensurability of the polymer unit cellwith the Ag(111) surface. Intermolecular hydrogen bonds(O · · ·H-N) between adjacent chains apparently play a lesserrole here. These hydrogen bonds have a length of 4 Å, whichis quite long and allows only weaker interaction between thechains.

In the STM images, the ordered polymer chains are imagedwith a sawtooth shape, having the largest apparent height onthe flanks and hinted nodal planes perpendicular to the overallchain direction at the vertices of the sawtooth. The sawtoothshape originates from the alternately clockwise and anticlock-wise twisting of the phenyl rings out of the HNCO plane of theamide groups, as illustrated in Figure 2. Such a twisting isknown for the crystal structure and is caused by repulsive, stericinteractions between the R-hydrogen atoms of the phenyl rings

and the amide moieties.25-27 This twisting will be preserved onthe Ag(111) surface, albeit the twisting angle might be reduceddue to the interaction of the phenyl rings with the substrate.We further assign the positions of the tunneling maxima to thehigh density of states of the oxygen and nitrogen atoms in theamide moieties, since this explains the lateral distances betweenthe tunneling maxima parallel and perpendicular to the chainsmost consistently with the structure model shown in Figure 2.The nodal planes are consequently at the positions of the centersof the phenyl rings. On the surface, we find that the twisting ofthe phenyl rings of adjacent chains occurs in opposite directions,i.e., toward each, in contrast to the crystal structure, where thephenyl rings of adjacent chains are rotated in the same direction.Thus, the unit cell on the Ag(111) surface is doubled along bcompared to the crystal structure and expands over two polymerchains.

A defect inside a domain constituting a relative displacementbetween two adjacent chains along the chain direction resultsin an increase of their lateral distance. This can be seen in Figure3 for the double-folded chain in the middle and the chain tothe right (marked with an arrow). The reason is a mismatchof the amide moieties of the two adjacent chains, which makesthe formation of the already weak hydrogen bonds impossiblein this case.

Finally, we discuss the morphology of the ordered PPTAdomains. A minimum length of a chain seems to be requiredso that it is incorporated into an ordered domain. Chains witha length of less than 8 nm are either attached to the edge of anordered domain or form separate, disordered domains (notshown). Polymer chains that are longer than the dimension ofthe domains along the a direction, which is found to beindependent of the preparation parameters and typically about10 nm, can either leave the domain and terminate after a partlydisordered sequence or re-enter the domain after a chain foldinghas occurred once, or several times. The re-entry occurs atdirectly adjacent positions of the exit point or randomly at moreremote positions on the domain edge. This is illustrated in Figure3 with some colorized representative chains. The two differentre-entry possibilities can be in principle identified with the

Figure 2. Overlay of an STM image and the hard-sphere model ofthe PPTA structure on the Ag(111) surface (color assignment: carbon,gray; nitrogen, blue; oxygen, red). Carbon atoms that are located above/below the drawing plane are colored in light/heavy gray, according tothe protrusions in the STM image. Two unit cells of the adsorbate areindicated (a ) 1.31 nm; b ) 1.53 nm; γ ) 90°). The Ag(111) latticeis illustrated by black lines. The relative positions of the adsorbate andthe substrate are chosen arbitrarily. For further details, see the text.The tunneling parameters are Usample ) 1.7 V, I ) 26 pA.

Figure 3. STM image of a PPTA domain. Polymer chains that arelonger than the linear dimension of the ordered domain may leave(marked red) or fold and re-enter (marked yellow) the domain. A shiftof two adjacent chains results in a larger distance due to the lateralmismatch of the adjacent amide moieties of the two chains (whitearrow). The tunneling parameters are Usample ) 1.6 V, I ) 7.4 pA.

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“regular model” (direct re-entry) and the “switchboard model”(random re-entry), introduced by Flory for the morphology ofcrystalline (3D) bulk polymers.28 In continuative concepts andcalculations, it has been predicted that both models shouldcoexist.29,30 In the present system, the chain folding can bedirectly imaged in real space with an unambiguous molecularcontrast that clearly proves the coexistence of the two differentmodels in this analogue two-dimensional case.

In summary, we have successfully realized the synthesis ofpolymer chains by step-growth polymerization of TPC and PPDon the Ag(111) surface at room temperature. The PPTA polymerforms long-range ordered domains after removal of the excessmonomer. The lateral ordering is mainly ruled by thesubstrate-adsorbate interactions as found from a comparisonof the lateral order with that known for the crystal structure ofthe bulk material. This suggests that the growth on anisotropicsubstrates, e.g., Ag(110), should have strong implications onthe film morphology. The presented method can be extendedto combinations of other amides and acid chlorides, thus alteringthe motif of the covalent interlinking. As we have demonstratedhere, polymer monolayers on surfaces can possibly be used asanalogue, two-dimensional model systems for the study oftopological concepts developed to describe the morphology andgrowth of crystalline bulk polymers by real space imagingmethods with submolecular contrast, such as STM.

Acknowledgment. Financial support by the Deutsche For-schungsgemeinschaft through SFB 624 is gratefully acknowl-edged. We thank Sigurd Hoger for helpful discussions.

Supporting Information Available: Details of the experi-mental setup and the preparation procedure. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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