Adsorption study of thin films of terephthalic acid and substituted benzoic acids on HOPG studied by Metastable Induced Electron Spectroscopy and Ultraviolet Photoelectron Spectroscopy

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Highlights

  • Adsorption Study of 4-alkoxybenzoic acids and terephthalic acid (TPA) on HOPG with MIES.

  • Flat adsorption of the molecules in the monolayer range.

  • For 4-decoxy benzoic acid, a reorientation of the long alkyl chains in transition to the multilayer.

Abstract

The adsorption behavior of thin films of 4-alkoxybenzoic acids (with either hydroxy-, methoxy-, propoxy-, pentoxy- and decoxy-chains) and terephthalic acid, evaporated on a Highly Oriented Pyrolytic Graphite (HOPG) surface was studied. The orientation of the molecules was characterized by Metastable Induced Electron Spectroscopy (MIES) and Ultraviolet Photoelectron Spectroscopy (UPS(HeI)). On HOPG the 4-alkoxybenzoic acids and the terephthalic acid show a layer like growth at the beginning. Since the MIE spectra for the molecules on HOPG show all molecular orbitals, especially the π orbitals of the phenyl ring, a flat laying orientation on the graphite surface is assumed. The only exception is observed for 4-decoxy-benzoic acid. After a coverage of approximately 1 monolayer (ML) the spectrum is dominated by the spectral features of the alkyl structures.

Introduction

Molecular self-assembly of organic molecules on surfaces is nowadays an intensively studied field [1,2]. New kinds of 2D and 3D materials could be generated on surfaces due to the self-assembly process of molecules [2,3]. For graphitic materials such as carbon nanotubes (CNTs) or graphene the chemical functionalization with organic molecules, like aromatic compounds, is an important way to adjust the physical and chemical properties of these materials [4]. For CNTs it has been shown, that this non-covalent approach does not change the geometric structure of the CNTs considerably. Peng et al. have used a functionalization of single-walled carbon nanotubes with carboxylic acids to improve the solubility in polar solvents [5]. In case of graphene Wang et al. have used carboxylate-terminated perylene molecules to achieve an uniform Al2O3 layer on the graphene sheet [6]. In all cases the interactions between the aromatic molecules and these graphitic materials are referred mainly to π−π interactions. Using density functional theory (DFT) Rochefort and Wuest have shown that the functional groups attached to an aromatic molecule can modify the nature and the degree of the interaction between the molecules and a graphene surface [7]. By studying the adsorption of benzene, benzoic acid, isophthalic acid and trimesic acid on graphene with DFT they showed that a strong adsorption occurs from attractive COOH-graphite, π-π* interactions and directional intermolecular interactions between the molecules [7]. Moriguchi et al. have claimed as early as 1970 that benzoic acids adsorb from aqueous solution via plane-to-plane stacking on a graphite surface [8]. Thus, the investigation of the adsorption of benzoic acids on a graphitic surface is of interest.

Here we report on adsorption behavior of 4-alkoxybenzoic acids (with either hydroxy-, methoxy-, propoxy-, pentoxy- and decoxy-chains) and terephthalic acid, evaporated on a Highly Oriented Pyrolytic Graphite (HOPG) surface. We have chosen HOPG as model surface for a graphitic system where an interplay between π−π interaction and influence of the COOH-group of the benzoic acids and the TPA has to be expected. In addition, TPA for example is known to form strong and directional linear hydrogen bonds and is also able to serve as organic linker in highly organized supramolecular systems [9,10]. By substitution of one COOH-group by another functional group such as a hydroxyl group this kind of molecule molecule interaction can be varied.

In our study we used the Metastable Induced Electron spectroscopy (MIES) and Ultraviolet Photoelectron Spectroscopy (UPS(HeI)). Hereby, MIES exhibits an extremely high surface sensitivity for detecting the electronic structure of the outermost molecular orbitals. The method has been used for example to investigate the molecular composition and the orientation of organic films adsorbed on different surfaces. Yamazaki et al. have used MIES for the investigation of 2,9-dihexylpentacene films on graphite (0001) [11]. By means of fingerprint spectra for different orientations they have found, that the molecules lie flat in the monolayer and stand up in thicker films with the methyl ends of the chains exposed to the vacuum. For the interaction of metastable atoms with benzene molecules in the gas phase it was already shown that this method is very sensitive with respect to π orbitals [12]. This is due to the extension of the π orbitals out of the molecular plane [12]. For the adsorption of benzene on metal surfaces such as Mo(100) and Ru(100) it was shown by MIES that during benzene adsorption a phase transition takes place. At the beginning of the adsorption the benzene molecules are orientated parallel to the surface. This is followed by a transition to molecules, which adopt an approximately perpendicular orientation to the surface [13]. Recently, we have studied the adsorption of TPA on gold (Au(111)) and oxidized aluminum with MIES and UPS [14]. On Au(111) we exhibited a more flat orientation, whereas a perpendicular adsorption of the TPA can be observed on the oxidized aluminum surface.

Section snippets

Experimental

The experiments were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure below 5 × 10−10 mbar, described in detail previously [15,16]. The chamber is equipped with a combined MIES/UPS source, a commercial X-ray source (Fisons XR3E2-324) and a hemispheric analyzer. A cold cathode gas double discharge source is used for the production of metastable He*(3S/1S) (E* = 21.2 eV) as a source for MIES and UPS. The intensity ratio 3S/1S is found to be 7:1 (Stracke et al. 2001 [17]). A

Results

Fig. 2a shows the MIE spectra and Fig. 2b shows the simultaneously recorded UP spectra of the investigated 4-alkoxybenzoic acids and TPA on HOPG. In our previous work concerning the interaction of TPA with Au(111) surfaces we have already shown that the structures at around 6–7 eV, 10 eV and 12 eV can be attributed to the COOH group of the TPA (marked in Fig. 2a,b) [14]. Additionally, the spectrum shows the ring-based molecular π1,2 orbitals in the region of 3–5 eV (marked in Fig. 2a,b). For

Discussion

The spectrum of the TPA evaporated on the HOPG surface is in a good agreement with previous studies of TPA on an Au(111) surface. Since for both substrates in the MIE spectra all molecular features can be observed, especially the π1,2 orbitals are clearly visible, a more flat lying arrangement of the molecules can be supposed. The same conclusion is given by Addou and Batzill in their study “Defects and Domain Boundaries in Self-Assembled Terephthalic Acid (TPA) Monolayers on CVD-Grown Graphene

Conclusion

We have shown the non-destructive evaporation of 4-alkoxybenzoic and terephthalic acid on a HOPG surface. We found hints for a more flat lying orientation of all used benzoic acids in the sub monolayer region, due to π−π interaction and stabilized by the attached functional at the phenyl ring. Considering the MIE spectra of the 4-decoxy benzoic acid, we assume at least a reorientation of the alkyl chains in transition to the multilayer.

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