Band Gap Opening of Graphene by Noncovalent π-π Interaction with Porphyrins

Arramel  , Andres Castellanos-Gomez, Bart Jan van Wees

Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands.
Physics of Nanodevices, Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands; Kavli Institute of Nanoscience, Delft University of Technology, Delft, The Netherlands.

DOI: 10.4236/graphene.2013.23015                

Abstract

Graphene has been recognized as a promising 2D material with many new properties. However, pristine graphene is gapless which hinders its direct application towards graphene-based semiconducting devices. Recently, various ways have been proposed to overcome this problem. In this study, we report a robust method to open a gap in graphene via noncovalent functionalization with porphyrin molecules. Two type of porphyrins, namely, iron protoporphyrin (FePP) and zinc protoporphryin (ZnPP) were independently physisorbed on graphene grown on nickel by chemical vapour deposition (CVD) resulting in a bandgap opening in graphene. Using a statistical analysis of scanning tunneling spectroscopy (STS) measurements, we demonstrated that the magnitude of the band gap depends on the type of deposited porphyrin molecule.The π-π stacking of FePP on graphene yielded a considerably larger band gap value (0.45 eV) than physisorbed ZnPP (0.23 eV). We proposed that the origin of different band gap value is governed due to the metallic character of the respective porphyrin.

Keywords

Band Gap; Graphene; Scanning Tunneling Spectroscopy; Porphyrins; π-π Stacking

1. Introduction

The development of novel techniques to produce highly crystalline monolayer graphene on the surface of transition metals is bringing graphene-based applications a step closer [1-6]. However, the absence of band gap around the Fermi level in graphene is hampering its application in nanoelectronic and photonic devices [2], since an energy gap is necessary to fabricate field effect transistors with a large current on-off ratio [7,8]. This fact has motivated the study of different methods to open a band gap in graphene. The band structure of graphene is mainly governed by its special electronic properties where π and π^* bands are crossing at the so called Dirac neutrality point. The opening of a gap in graphene can be realized by breaking its symmetry in various approaches: defect formation [9], chemical dopants [10-12], electric fields [13-15], and interaction with gases [16].

The noncovalent stacking of aromatic organic molecules on graphene through π-π interaction is emerging as a promising route to tailor the electronic properties of graphene [17], motivated by the study of the interaction between large aromatic molecules and graphene [18-20]. Among all possible aromatic molecules, porphyrins are of primary interest in molecular electronics due to their rich electronic/photonic properties (including charge transport, energy transfer, light absorption or emission) [21]. Individual porphyrin molecules physorbed on different substrates have shown that their structure can be wellresolved using scanning tunneling microscopy (STM) [22-24]. In particular, metalloporphyrins (with a metal ion at the central part of the porphyrin) have attracted much interest due to their peculiar electronic properties [21]. However, a microscopic knowledge of the topographic and the local electronic structure of physisorbed porphyrin molecules on monolayer graphene is still lacking.

In this work the π-π stacking of porphryins on graphene and its effect on the electronic properties of graphene is investigated. Two different types of porphyrin molecules, namely, iron protoporphyrin and zinc porphryins were physisorbed on CVD graphene grown on nickel. By means of a systematic statistical analysis of scanning tunnelling spectroscopy measurements, we probe the opening of an energy bandgap in graphene in the vicinity of physisorbed porphyrins. Although optical experiments (photo electron spectroscopy or UV-VIS experiments) can be useful to probe the electronic properties of a fully functionalized graphene surface, scanning tunneling spectroscopy allows one the chance to study even single molecules and to investigate of the electronic properties of the sample with high accuracy and spatial resolution.

2. Experiments

STM measurements were carried out using a PicoLE STM (Agilent) equipped with low tunneling current STM scanner head (type: N9501-A) and digital instrument from Agilent Technologies model N960A. STM Tips were obtained by mechanically cutting 0.25 mm Pt_0.8Ir_0.2 wire (Goodfellow). The STM images were acquired in constant current operating mode, in air at room temperature. Typical scanning parameters to obtain STM images are in the range of 10 to 15 pA with the tip bias voltage ranging from −0.8 to −1.2 V. The STM images were treated by standard flattening procedure without further image processing treatment. The porphyrins were used without any further purification from Aldrich. A solution 1.63 mM of FePP in uvasol grade chloroform was prepared. Further, a droplet of 10 µL of the porphyrin solution was dropcasted onto CVD-graphene surface and the solvent was left to evaporate for 3h in open air condition. The same procedure was also applied for the deposition of ZnPP.

3. Results and Discussion

We have studied and used commercially available graphene/multi-layered graphene grown by chemical vapour deposition on polycrystalline nickel [25]. The as received samples have been first characterized by scanning tunnelling microscopy and spectroscopy. Figure 1 shows the STM topography, measured in constant current mode, of the surface of the CVD graphene on nickel, showing a characteristic rippled structure (a Moiré pattern) due to mismatching between the graphene and nickel lattices. The topographic line profile, measured along the black line in Figure 1(c), shows the graphene atomic corrugation superimposed to the Moiré pattern of graphene (Figure 1(d)). Iron porphyrins (FePP) are then deposited onto the CVD graphene surface by drop casting.

Figure 2(b) shows the STM topography of the CVD graphene surface after the deposition of FePP molecules. There are two main features associated with the presence of FePP molecules on the surface: small and chain-like protuberances (see Figure 2(c)). While chain-like structures present typical lengths ranging from 4.5 nm to 7 nm and 0.6 nm in height, higher resolution images reveals that the small protuberances have a star-like shape with

Conflicts of Interest

The authors declare no conflicts of interest.

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