Two-Dimensional Ordering of Poly(p-phenylene-terephthalamide) on

Jun 17, 2009 - Long-range ordered monolayer domains of the polymer poly(p-phenylene-terephthalamide) were directly synthesized on the Ag(111) surface ...
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2009, 113, 11984–11987 Published on Web 06/17/2009

Two-Dimensional Ordering of Poly(p-phenylene-terephthalamide) on the Ag(111) Surface Investigated by Scanning Tunneling Microscopy Christoph H. Schmitz,* Julian Ikonomov, and Moritz Sokolowski Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t 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 directly synthesized on the Ag(111) surface and analyzed by scanning tunneling microscopy with submolecular resolution. The polymerization reaction of the monomers runs at room temperature and yields ordered domains with a diameter of up to 50 nm. The lateral order of the polymer chains within these domains is similar to that 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 with known models of crystalline bulk polymers. The modification of metallic or semiconducting surfaces by adsorption of organic substances and especially the formation of long-range ordered structures of organic adsorbates has been subject to numerous recent studies.1 The intermolecular forces that lead to ordered structures are generally based on weak van der Waals forces,2 dipole interactions,3 hydrogen bonds,4,5 or metal complexation.6,7 However, for possible applications, e.g., in the field of organic electronics (organic field effect transistors,8 organic solar cells,9 etc.), layers in which the adsorbates are interlinked by strong covalent bonds are advantageous. Such layers likely possess a larger chemical, thermal, and mechanical stability. Besides these possible applications, covalent interlinking of organic adsorbates on surfaces constitutes a principle approach toward the synthesis of two-dimensional macromolecules and polymers,10 exploiting the template effect of welldefined crystalline surfaces. So far, different concepts for the formation of monolayers with covalent interlinking on the surface have been introduced.11-18 For large organic adsorbates, the reaction is generally induced thermally,12-17 while a reaction at room temperature requires the presence of special functional groups, such as boroxine.18 Obviously, the requirement of a thermal treatment to induce the interlinking strongly limits the scope of possible substances; e.g., in the case of small organic molecules, desorption may occur before the reaction takes place. However, large, customsynthesized molecules that circumvent the desorption problem are often too expensive for possible technical applications. Thus, procedures for the formation of covalent layers by interlinking at room temperature are attractive and might provide a more universal concept. In the here reported approach, we utilize small, commercially available substances, which form polymer chains on a surface at room temperature by step-growth polymerization. We have chosen p-phenylenediamine (PPD) and terephthaloyl chloride (TPC) with the aim to form the polyamide poly(p* Corresponding author. Phone: +49 (228) 73 2520. Fax: +49 (228) 73 2551. E-mail: [email protected].

10.1021/jp9041777 CCC: $40.75

SCHEME 1: Reaction Scheme

phenylene terephthalamide) (PPTA, trademarks Kevlar and Twaron). This system is of broad scientific and industrial interest and has been extensively characterized as a bulk material.19 In this context, the novel aspect of the present work is that we are able to image the polymer directly on the surface (in the first monolayer) by scanning tunneling microscopy (STM) with resolution of individual chain segments, which provides microscopic insight into the role of the surface on the structural order and the two-dimensional growth morphology of the polymer domains. The preparation of the polymer monolayer on the surface is done by the “vacuum deposition polymerization” (VDP) technique.20 This method has so far been successfully used for the preparation of PPTA films with a layer thickness of a few hundreds of nanometers.21,22 Here, we perform VDP under ultrahigh vacuum (UHV) conditions in order to obtain a polymer monolayer on a clean Ag(111) surface. Both monomers, PPD and TPC, are dosed simultaneously into the UHV system by means of two variable leak valves to perform the step-growth polymerization on the surface at room temperature. The byproduct of the polymerization, hydrogen chloride (c.f., Scheme 1), is detected by a quadrupole mass spectrometer (QMS) during the deposition process, unambiguously proving the progress of the reaction.23 Earlier experiments on thicker PPTA films (>100 nm) have revealed that the reaction to form the polyamide layer during VDP takes place in the condensed phase after adsorption and diffusion of the monomers.21,24 We note that this reaction is however not limited to the surface of the Ag(111) substrate alone but likely occurs on all surfaces of the UHV chamber. Figure 1a shows a constant current STM image after the simultaneous deposition of both monomers. During scanning,  2009 American Chemical Society

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

we do not detect any further change of the appearance of the layer, concluding that the polymerization reaction is completed directly after the deposition process. This has already been shown by means of X-ray photoelectron spectroscopy and infrared absorption measurements for a thicker film of a comparable polyamide, evaporated on a gold substrate at room temperature.24 The Ag(111) surface is covered by two different species: small, compact, point-like objects and extended chains that are imaged with larger apparent height (Figure 1). The chains, which possess a sawtooth-like intramolecular contrast, are the PPTA polymers 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 of 45 nm, which corresponds to approximately 70 monomeric building units. The chains consist of long, linear segments and are randomly kinked between these. Interestingly, the direct neighborhood of two adjacent chains causes a linear conformation of both chains, probably due to attractive lateral forces

J. Phys. Chem. C, Vol. 113, No. 28, 2009 11985 between them, which stabilize the linear geometry of the double strand (see below). In this case, the longer chain is not kinked until the shorter chain is terminated. As expected, we do not find any branched chains. In between the polymer matrix, adsorbed molecules of the excess monomer are embedded. In larger interspaces of more than about 5 nm × 5 nm, the monomers form ordered domains with a skewed hexagonal arrangement. These single molecules of the excess monomer generally hinder the ordered agglomeration 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 surface and diffusion processes lead to a rearrangement of the remaining polymer chains. Notably, we do not find evidence for considerable further polymerization progress during annealing. The typical as well as the maximal length of the polymer chains is not significantly changed. Further annealing steps have also no influence on the ordering of the polymer monolayer. Deposition of the monomers at higher sample temperatures leads to a reduction of the adsorption rate without an improved ordering of the polymer chains. Above T ) 420 K, only short chains are formed, which do not form ordered domains, possibly due to partial competitive bonding 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 the remaining polymer chains have now formed two-dimensional, long-range ordered domains, consisting of linear, parallel chains. The domains show six different orientations as expected from the symmetry of the Ag(111) surface (three different rotations and the corresponding mirror domains). The direction of chain propagation inside the domains is at an angle of approximately 10-15° 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 cell vector a parallel to the chain direction. Within the given accuracy of the STM images, and taking into account the orientation of the domains, the measurement points to a commensurate superstructure with respect to the Ag(111) surface that is described by the unit cell matrix

( ) 1 5 6 2

which corresponds to the unit cell dimensions a ) 1.32 nm, b ) 1.53 nm, and γ ) 90°. The theoretical angle of a with respect to the [101j] direction of the substrate is 11°. The unit cell contains two parallel chain segments oriented along the vector a, with one AB building block each. A model of the adsorbate structure is presented in Figure 2. The length of a ) 1.32 nm, corresponding to the periodicity of the polymer chain on the surface, agrees nearly exactly (within 2%) with the periodicity of the polymer chain in the crystal structure, which is 1.29 nm.25-27 Hence, the conformation of the polymer on the Ag(111) surface shown in Figure 2 was directly extracted from the crystal structure of PPTA26 and was only very slightly elongated by 2% along the chain axis in order to fit the observed periodicity in the monolayer on Ag(111). Interestingly, while the periodicity of the polymer remains nearly unchanged, the intermolecular distance between two adjacent 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 by attractive hydrogen bonds between the amide groups of adjacent

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

Figure 2. Overlay of an STM image and the hard-sphere model of the 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 to the protrusions in the STM image. Two unit cells of the adsorbate are indicated (a ) 1.31 nm; b ) 1.53 nm; γ ) 90°). The Ag(111) lattice is illustrated by black lines. The relative positions of the adsorbate and the substrate are chosen arbitrarily. For further details, see the text. The tunneling parameters are Usample ) 1.7 V, I ) 26 pA.

chains.25-27 For PPTA on Ag(111), the lateral arrangement of the polymer chains is a consequence of the direct interaction between the chains in combination with the interaction between the corrugated Ag surface and the adsorbate, which can be inferred from the commensurability of the polymer unit cell with the Ag(111) surface. Intermolecular hydrogen bonds (O · · · H-N) between adjacent chains apparently play a lesser role here. These hydrogen bonds have a length of 4 Å, which is quite long and allows only weaker interaction between the chains. In the STM images, the ordered polymer chains are imaged with a sawtooth shape, having the largest apparent height on the flanks and hinted nodal planes perpendicular to the overall chain direction at the vertices of the sawtooth. The sawtooth shape originates from the alternately clockwise and anticlockwise twisting of the phenyl rings out of the HNCO plane of the amide groups, as illustrated in Figure 2. Such a twisting is known for the crystal structure and is caused by repulsive, steric interactions between the R-hydrogen atoms of the phenyl rings

and the amide moieties.25-27 This twisting will be preserved on the Ag(111) surface, albeit the twisting angle might be reduced due to the interaction of the phenyl rings with the substrate. We further assign the positions of the tunneling maxima to the high density of states of the oxygen and nitrogen atoms in the amide moieties, since this explains the lateral distances between the tunneling maxima parallel and perpendicular to the chains most consistently with the structure model shown in Figure 2. The nodal planes are consequently at the positions of the centers of the phenyl rings. On the surface, we find that the twisting of the phenyl rings of adjacent chains occurs in opposite directions, i.e., toward each, in contrast to the crystal structure, where the phenyl rings of adjacent chains are rotated in the same direction. Thus, the unit cell on the Ag(111) surface is doubled along b compared to the crystal structure and expands over two polymer chains. A defect inside a domain constituting a relative displacement between two adjacent chains along the chain direction results in an increase of their lateral distance. This can be seen in Figure 3 for the double-folded chain in the middle and the chain to the right (marked with an arrow). The reason is a mismatch of the amide moieties of the two adjacent chains, which makes the formation of the already weak hydrogen bonds impossible in this case. Finally, we discuss the morphology of the ordered PPTA domains. A minimum length of a chain seems to be required so that it is incorporated into an ordered domain. Chains with a length of less than 8 nm are either attached to the edge of an ordered domain or form separate, disordered domains (not shown). Polymer chains that are longer than the dimension of the domains along the a direction, which is found to be independent of the preparation parameters and typically about 10 nm, can either leave the domain and terminate after a partly disordered sequence or re-enter the domain after a chain folding has occurred once, or several times. The re-entry occurs at directly adjacent positions of the exit point or randomly at more remote positions on the domain edge. This is illustrated in Figure 3 with some colorized representative chains. The two different re-entry possibilities can be in principle identified with the

Letters “regular model” (direct re-entry) and the “switchboard model” (random re-entry), introduced by Flory for the morphology of crystalline (3D) bulk polymers.28 In continuative concepts and calculations, it has been predicted that both models should coexist.29,30 In the present system, the chain folding can be directly imaged in real space with an unambiguous molecular contrast that clearly proves the coexistence of the two different models in this analogue two-dimensional case. In summary, we have successfully realized the synthesis of polymer chains by step-growth polymerization of TPC and PPD on the Ag(111) surface at room temperature. The PPTA polymer forms long-range ordered domains after removal of the excess monomer. The lateral ordering is mainly ruled by the substrate-adsorbate interactions as found from a comparison of the lateral order with that known for the crystal structure of the bulk material. This suggests that the growth on anisotropic substrates, e.g., Ag(110), should have strong implications on the film morphology. The presented method can be extended to combinations of other amides and acid chlorides, thus altering the motif of the covalent interlinking. As we have demonstrated here, polymer monolayers on surfaces can possibly be used as analogue, two-dimensional model systems for the study of topological concepts developed to describe the morphology and growth of crystalline bulk polymers by real space imaging methods with submolecular contrast, such as STM. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft through SFB 624 is gratefully acknowledged. We thank Sigurd Ho¨ger for helpful discussions. Supporting Information Available: Details of the experimental setup and the preparation procedure. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201. (2) Hipps, K. W.; Scudiero, L.; Barlow, D. E.; Cooke, M. P. J. Am. Chem. Soc. 2002, 124, 2126. (3) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619.

J. Phys. Chem. C, Vol. 113, No. 28, 2009 11987 (4) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem. 2000, 112, 1285. (5) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (6) Dmitriev, A.; Spillmann, H.; Lin, N.; Barth, J. V.; Kern, K. Angew. Chem. 2003, 115, 2774. (7) Tait, S. L.; Langner, A.; Lin, N.; Chandrasekar, R.; Fuhr, O.; Ruben, M.; Kern, K. ChemPhysChem 2008, 9, 2495. (8) Horowitz, G. J. Mater. Res. 2004, 19, 1946. (9) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924. (10) Sakamoto, J.; van Heijst, J.; Lukin, O.; Schlu¨ter, A. D. Angew. Chem. 2009, 121, 1048. (11) Grim, P. C. M.; Feyter, S. D.; Gesquie`re, A.; Vanoppen, P.; Ru¨cker, M.; Schryver, F. C. D.; Valiyaveettil, S.; Moessner, G.; Mu¨llen, K. Angew. Chem. 1997, 109, 2713. (12) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687. (13) In’t Veld, M.; Iavicoli, P.; Haq, S.; Amabilino, D. B.; Raval, R. Chem. Commun. 2008, 1536. (14) Matena, M.; Riehm, T.; Sto¨hr, M.; Jung, T. A.; Gade, L. H. Angew. Chem. 2008, 120, 2448. (15) Weigelt, S.; Busse, C.; Bombis, C.; Knudsen, M. M.; Gothelf, K. V.; Lægsgaard, E.; Besenbacher, F.; Linderoth, T. R. Angew. Chem. 2008, 120, 4478. (16) Treier, M.; Fasel, R.; Champness, N. R.; Argent, S.; Richardson, N. V. Phys. Chem. Chem. Phys. 2009, 11, 1209. (17) Lipton-Duffin, J. A.; Ivasenko, O.; Perepichka, D. F.; Rosei, F. Small 2009, 5, 592. (18) Zwaneveld, N. A. A.; Pawlak, R.; Abel, M.; Catalin, D.; Gigmes, D.; Bertin, D.; Porte, L. J. Am. Chem. Soc. 2008, 130, 6678. (19) Tanner, D.; Fitzgerald, J. A.; Phillips, B. R. Angew. Chem. 1989, 101, 665. (20) Takahashi, Y.; Iijima, M.; Inagawa, K.; Itoh, A. J. Vac. Sci. Technol., A 1987, 5, 2253. (21) Sakata, J.; Mochizuki, M. Thin Solid Films 1996, 277, 180. (22) Takahashi, Y.; Iijima, M.; Oishi, Y.; Kakimoto, M.; Imai, Y. Macromolecules 1991, 24, 3543. (23) Details of the sample preparation can be found in the Supporting Information. (24) Kruse, A.; Thu¨mmler, C.; Killinger, A.; Meyer, W.; Grunze, M. J. Electron Spectrosc. Relat. Phenom. 1992, 60, 193. (25) Gardner, K. H.; English, A. D.; Forsyth, V. T. Macromolecules 2004, 37, 9654. (26) Liu, J.; Cheng, S. Z. D.; Geil, P. H. Polymer 1996, 37, 1413. (27) Plazanet, M.; Fontaine-Vive, F.; Gardner, K. H.; Forsyth, V. T.; Ivanov, A.; Ramirez-Cuesta, A. J.; Johnson, M. R. J. Am. Chem. Soc. 2005, 127, 6672. (28) Flory, P. J. J. Am. Chem. Soc. 1962, 84, 2857. (29) Frank, F. C. Faraday Discuss. 1979, 68, 7. (30) Hoffman, J. D.; Miller, R. L. Polymer 1997, 38, 3151.

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