Thin Solid Films 534 (2013) 308–311

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A scanning tunneling microscopy study on self-assembled Fe(III)meso-tetra(4-carboxyphenyl) porphyrin chloride chains

Dylan Nicholls, Benjamin Ware, Zhe Zhang, Nuri Oncel ⁎Department of Physics and Astrophysics, University of North Dakota, 101 Cornell Street, Grand Forks, North Dakota, 58203, United States

⁎ Corresponding author. Tel.: +1 701 777 3529.E-mail address: nuri.oncel@und.edu (N. Oncel).

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.01.108

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 September 2012Received in revised form 18 January 2013Accepted 30 January 2013Available online 16 February 2013

Keywords:Scanning tunneling microscopySolid liquid interfacePorphyrins

Fe(III) meso-tetra(4-carboxyphenyl) porphyrin chloride (Fe-TCP chloride) molecules co-deposited togetherwith 5-(octadecyloxy) isophthalic acid (5-OIA) molecules on a highly-ordered pyrolytic graphite were studiedwith the help of a scanning tunnelingmicroscope (STM) under ambient conditions. Themeasurementswere car-ried out at solid–liquid interface. STM images show the formation of one-dimensional Fe-TCP chloride/5-OIAchains stretching tens of nanometers across the surface. The driving mechanism behind the formation of thesechains is the physical interaction between the molecules.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Porphyrins have a nearly square core conformation, with atwo-dimensional delocalized conjugated π-electron system. Metalporphyrins are in the focus of extensive studies as they have uniqueproperties that make them valuable for device applications in areassuch as light-emitting diodes, organic displays, and thin film transis-tors [1,2]. In addition to this, metal porphyrins constitute the activecenters of biologically important molecules such as heme and chlo-rophyll [3,4]. There are extensive studies of thin films of these mol-ecules in ultra-high vacuum and at the solid–liquid interface [5–7].The feasibility of these technological applications partly dependson the quality and properties of the thin films of these molecules.In order to contrive building molecular structures of porphyrins;one can employ physical interactions between adsorbed moleculesand between molecules and substrate. These physical interactionsinclude non-directional van der Waals forces and directional hydro-gen bonds. On a chemically inert surface such as highly-ordered py-rolytic graphite (HOPG), the adsorption of molecules occurs via vander Waals interactions which hinder the control over the morphol-ogy of the thin film. Even if the interaction between molecules andsubstrate is via non-directional van der Waals forces, it is still possi-ble to compile large and uniform lamellae of molecules with thehelp of directional intermolecular forces, such as hydrogen bonding[8–10].

Formation of porphyrin chains has been reported before [11–14].However, these porphyrin chains form with the help of long carbon

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tails attached to the porphyrin core via chemical reaction. However,in this particular work, we show an alternative technique that em-ploys only physical interactions between porphyrin molecules andlong chain acids to control and build porphyrin structures. Thispaper presents experimental data that proves the idea of using justthe physical interactions to build molecular chains out of completelysymmetric porphyrins.

For this study, we specifically chose Fe(III) meso-tetra(4-carboxyphenyl) porphyrin chloride (Fe-TCP chloride) moleculesbecause of their carboxyl groups. Carboxyl groups offer the possi-bility to make two dimensional networks of hydrogen-bondedstructures. Studies on various derivatives of TCP molecules showedthat the phenyl rings of TCP do not lay parallel to the surface. This geom-etry of phenyl groups weakens the van derWaals interactions betweenTCP molecules and the HOPG surface. This makes them highly mobileon the surface and prevents those forming stable thin films. In orderto overcome this problem and break the symmetry of the porphyrinmolecules, we co-deposited Fe-TCP chloride molecules togetherwith secondary molecules that readily form a highly stable monolay-er on HOPG surface. The secondary molecules are usually organicacids with long chains of carbon that forms two dimensional do-mains with the help of van der Waals interactions and hydrogenbonding [15,16]. In this study, we chose 5-(octadecyloxy)isophthalic acid (5-OIA) as a secondary molecule. 5-OIA forms ex-tremely stable monolayers on HOPG by taking the advantage ofstrong van der Waals interactions between HOPG and the long car-bon chain [17,18]. In addition to that, 5-OIA molecules have two car-boxylic acid groups bonded at the 3- and 5-positions of its phenylring head group. Since there are two opportunities for hydrogenbonding at one end of the molecular chain, 5-OIA molecules forminteresting supramolecular structures on HOPG.

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2. Experimental details

Substrates of HOPG and platinum-iridium wire for the scanningtunneling microscopy (STM) tips (Pt0.8Ir0.2, 25 mm diam.) were pur-chased from Nanoscience Instruments. Fe-TCP chloride was pur-chased from Frontier Scientific. 5-OIA (98%) and 1-phenyloctane(98%) were both purchased from Aldrich. All were used without fur-ther purification. Schematic diagrams of Fe-TCP chloride (top) and5-OIA (bottom) are shown in Fig. 1a. Before each measurement,HOPG substrate was cleaved to obtain an atomically clean surface.Experiments were carried out under ambient conditions and atsolid–liquid interface with the NanoSurf easyScan 2 STM.

Solutions used in the experiments 5-OIA was dissolved in1-phenyloctane until saturation occurred. Then Fe-TCP chloride mole-cules were added to create a solution saturated with Fe-TCP chloride.

3. Results and discussion

Fig. 1b shows that three domains i.e. 5-OIA domain, Fe-TCP chloride/5-OIA chains and an oblique domain co-exist on the surface. Fig. 2a andb shows high resolution STM images of domains of 5-OIA and Fe-TCPchloride/5-OIA molecular chains, respectively. A comparison betweenFig. 2a and b reveals that Fe-TCP chloride/5-OIA molecular chains fol-low the head groups of 5-OIA molecules. In addition to that the carbontails of 5-OIA molecules are visible in trough between the Fe-TCP chlo-ride and 5-OIA chains. The size of the unit cell vectors measuredon Fe-TCP chloride/5-OIA molecular chains are (2.21±0.21 nm)x×(3.30±0.14 nm) y and the angle between the lattice vectors isθ=86.9º±0.2° This translates into a coverage of about 0.14 Fe-TCPchloride molecules per nm2. The measured 3.3 nm spacing betweenthe Fe-TCP chloride/5-OIA chains is about 0.4 nm larger than the spac-ing between the rows of 5-OIA molecules [17]. This is possible only ifthe chloride ion at the center of a porphyrin molecule moves in be-tween the head groups of 5-OIAmolecules and pins the porphyrin mol-ecules into the superlattice. A model based on the measurements isproposed in Fig. 2c. It is not possible to find out the orientation of thephenyl groups from the STM images. Therefore, in order to keep themodel perceptible we ignore the rotational freedom of the phenylgroups and show them lying flat on the surface. However, if we consid-er the rotational freedom of phenyl groups, we can speculate aboutpossible hydrogen bonds between Fe-TCP chloride molecules and theunderlying 5-OIA layer. Unlike smaller 5-OIA domains, the domainsof Fe-TCP chloride/5-OIA chains can stretch up to tens of nanometers(see Fig. 1b). This also provides additional proof that the chains ofFe-TCP chloride molecules are not simply formed by Fe-TCP chloride

Fig. 1. (a) Schematic diagram of Fe-TCP chloride and 5-OIA molecules. The atoms are colortunneling current is 0.4 nA.

molecules adsorbed on the head groups of 5-OIA molecules butinstead these two molecules with rather different geometries worktogether to form these molecular chains.

As we mentioned before, the chains of Fe-TCP chloride/5-OIA mol-ecules are not the only domain that exists on the surface. Fig. 3ashows high-resolution STM images of one of these oblique domains.The lattice vectors for the uniform oblique domain of Fe-TCP chloridemolecules are (4.0±0.1 nm) x×(3.0±0.17 nm) y with θ=113.5º±1.5°. In an oblique domain, the surface coverage becomes approxi-mately 0.08 Fe-TCP chloride molecules per nm2. The oblique ringsthat we attribute to porphyrin molecules are separated with 5-OIAmolecules (see Fig. 3b). The blue dots indicate the position ofcarboxyphenyl groups surrounding the porphyrin ring. The greendots indicate the position of carboxyphenyl group of 5-OIA molecules.A combination of these six carboxyphenyl groups gives a rather un-usual hexagonal appearance to the Fe-TCP molecule.

In our previous study on Copper (II) meso-tetra(4-carboxyphenyl)porphyrin (Cu-TCP) molecules, we observed that the phenyl rings of5-OIA molecules act like a potential barrier and anchor Cu-TCP mole-cules. This reduces the mobility of the Cu-TCP molecules and leads tothe formation of Cu-TCP chains and 2-D domains [19]. The importanceof the central metal atom in the morphology of thin films of porphyrinmolecules has been shown before in a sequence of studies on platinum,nickel and vanadium-oxide octaethyl-porphyrin molecules adsorbedon bare and 5-OIA modified HOPG surfaces [20,21]. The difference inthe morphology of Cu-TCP chains and Fe-TCP chloride/5-OIA chainsclearly supports this theory. Studies involving other porphyrin derivativesonvarious substrateswithno co-adsorbedmolecules showthat porphyrinscan form close-packed hexagonal domains with lattice spacing of about1 nm [22,23]. Such close-packed domains were also observed when por-phyrins are co-adsorbed with 5-OIA molecules [20,21]. This happens onlywhen the molecule–molecule interaction is much stronger than the mole-cule substrate interaction. Absence of closed-packed hexagonal domains ofFe-TCP chloridemolecules indicates that Fe–Cl complex at the center of theporphyrin molecules enhances the interaction of the whole molecule withthe underlying substrate and leads a better control over themorphology ofthe thin film.

In order to provide experimental proof for the proposed modelsabove, we came up with the following experiment. We deposited5-OIA molecules on freshly cleaved HOPG surface from a saturatedsolution of 5-OIA dissolved in 1-phenyloctane. We confirmed the for-mation of 5-OIA layer with the STM. Then a small drop of Fe-TCP chlo-ride solution (solvent is acetone) was placed on the STM tip. Once theacetone was evaporated, the tip had miniscule amount of Fe-TCPchloride molecules. The STM images measured right after immersing

coded. (b) is 75 nm×75 nm STM images of the surface. The tip bias is 1.1 V and the

Fig. 2. (a) is a 9.5 nm×9.5 nm STM image. (b) is a 10 nm×10 nm STM image. (c) Proposed model of Fe-TCP chloride molecules aligning on head groups of 5-OIA. For both imagesthe tip bias and the tunneling current are 1.1 V and 0.4 nA, respectively.

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the tip into the thin film of 5-OIA solution (solvent is 1-phenyloctane)on HOPG showed that the surface is uniformly covered by oblique do-mains (see Fig. 4a). Since the surface was initially covered with 5-OIAlayer, the observation of oblique domains of Fe-TCP chloride moleculesproves that for this particular domain Fe-TCP chloride moleculessimply lie down on top of 5-OIA molecules. We repeated this exper-iment with different tips and freshly prepared samples but we neverobserved the formation of Fe-TCP chloride/5-OIA chains indicatingthat the formation of these chains requires co-adsorption of Fe-TCPchloride and 5-OIA molecules. In order to reveal 5-OIA molecules

Fig. 3. (a) is a 30 nm×30 nm STM image Fe-TCP chloride structure with oblique symmetry.the tunneling current are 1.1 V and 0.18 nA, respectively.

underneath of Fe-TCP chloride, we scanned the surface with lowvoltage-high current settings (specifically 0.1 V and 1 nA). Underthese conditions, tip moves closer to the surface and pushes porphy-rin molecules away. Once we zoomed out and scanned the surface atusual tunneling settings, we observed that the oblique domain ofFe-TCP chloride is partly removed and 5-OIA layer became visible(see Fig. 4b). The resolution of the image in Fig. 4b is somewhat lim-ited due to the rough treatment of the tip to remove Fe-TCP chloridemolecules and the rapidly diffusing Fe-TCP chloride molecules on thesurface.

(b) shows a proposed schematic model of the surface. For both images the tip bias and

Fig. 4. (a) is a 100 nm×100 nm STM image Fe-TCP chloride structure with oblique symmetry. (b) is a 100 nm×100 nm STM image of the same region. For both images the tip biasand the tunneling current are 1.2 V and 0.18 nA, respectively. Between measuring these two images, a 25 nm×25 nm region (indicated by red square) of the surface was scannedat 0.1 V and 1 nA. The two red ovals are placed on the corner of the STM images in order to indicate existing 5-OIA domains and to prove that the two images were measured on thesame region

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4. Conclusion

In summary, solutions containing both Fe-TCP chloride and 5-OIAwere measured with an STM at solid–liquid interface under ambientconditions.We showed that 5-OIAmolecules facilitate ameans to anchorFe-TCP chloride molecules on HOPG surface to form Fe-TCP chloridechains. The formation of porphyrin chains opens up the possibility ofdesigning low-dimensional molecular structures by employing simplephysical interactions.

Acknowledgment

This work was supported by the North Dakota EPSCoR office (NSFgrant #EPS-814442.) and the University of North Dakota. We alsothank Prof. S. L. Bernasek from Princeton University for providing us5-OIA molecules.

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