Electronic structure of partially fluorinated copper phthalocyanine (CuPCF4) and its interface to Au(100)

doi:10.1016/S0039-6028(02)01967-2

Surface Science

Volume 515, Issues 2–3, September 2002, Pages 491-498

https://doi.org/10.1016/S0039-6028(02)01967-2 Get rights and content

Abstract

We present a study of the organic semiconductor copper tetrafluorophthalocyanine (CuPCF₄) on single crystalline Au(1 0 0) using photoemission spectroscopy and compare the results to the unsubstituated copper phthalocyanine (CuPC) complex, which is a well-established molecular organic semiconductor. The observed satellite structures in the C 1s core level photoemission spectra are discussed in detail and we show that the fluorination considerably changes the ionization potential while leaving the energy separation of the occupied electronic levels almost unchanged. Concerning the metal/organic interface, we observe the formation of an interface dipole, whereas the observed dipole is very different for CuPCF₄ (0.6 eV) and CuPC (1.2 eV). Furthermore, the energy position of the highest occupied molecular orbital with respect to the gold Femi level is similar in both cases.

Introduction

In recent years there has been growing interest in the field of thin organic films due to their successful application in optical and electronic devices, such as light emitting diodes or field effect transistors. Especially for the realisation of complementary logic circuits both p- and n-channel materials are required. Since however the most organic semiconductors are p-type materials or unstable in air, very extensive research has been carried out to find suitable n-type materials.

The family of the phthalocyanines (PC's) represents one of the most promising candidates for ordered organic thin films, as these systems possess advantageous attributes such as chemical stability, excellent film growth and electronic properties [1]. Whereas thin PC films are known as p-type materials [1] in air, it has been shown recently that perfluorinated copper phthalocyanine (CuPC) may be favourable as n-channel material for organic electronics [2]. For both substituted and unsubstituted PC's it was shown that the high degree of ordering and the orientation of the organic molecules is advantageous for the device performance.

Furthermore, the interfaces between the electrodes and the active organic layers control the injection of the carriers in any devices and thus they have a dramatic effect on the efficiency. The knowledge of barrier heights between organic solids and metals is of enormous importance for an understanding and improvement of organic semiconductor devices. In this work, we present a photoemission study of the influence of the introduction of fluorine substituents into CuPC on both the electronic structure and the energy level alignment at the interface to Au(1 0 0).

Section snippets

Experimental

The Au(1 0 0) single crystal, which was used as substrate has been cleaned by cycles of sputtering and annealing. The cleanliness of the gold surface was checked using core level X-ray photoemission spectroscopy (XPS) and low-energy electron-diffraction (LEED), no sign of contamination could be detected and a sharp LEED pattern (5×20 superstructure) was obtained. Organic films of CuPC and copper tetrafluorophthalocyanine (CuPCF₄) (Syntec GmbH) were evaporated in a step-wise manner. The pressure

Bulk electronic structure

The introduction of fluorine atoms into CuPC occurs via the substitution of hydrogen atoms, in the case of CuPCF₄ four of the outermost atoms are substituted, i.e. one F atom is directly bonded to the benzene ring of each of the four ligands (see inset in Fig. 1). Since the strongly electronegative fluorine decreases the electron density especially at the neighboring carbon atom, the BE of the corresponding C 1s core level is increased, as was shown for the perfluorinated, related compound CuPCF

Conclusion

In this work we have reported, for the first time, high resolution photoemission spectra of partially fluorinated CuPCF₄. We have shown, that for the description of the C 1s spectra the consideration of a satellite structure arising from the aromatic carbon is necessary. The CuPCF₄/Au(1 0 0) interface was investigated in detail and compared to CuPC/Au(1 0 0). We found no evidence for strong interactions at these interfaces. The fluorination mainly causes an increase of the IP and a lowering of the

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The n-type conduction behaviour of MPcF16 films originates from the strongly stabilized electronic π-system due to the electron withdrawing character of F-substituents [15,16]. In spite of numerous investigations of unsubstituted (MPc) and hexadecafluorosubstituted (MPcF16) phthalocyanine films [17–21], only sporadic studies reporting investigations of tetrafluorosubstituted metal phthalocyanine (MPcF4) films are available [22–24]. Ultra thin films of CuPcF4 deposited on the Au(100) and ITO surfaces were studied by photoemission spectroscopy [22,23].
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Hybrid organic-inorganic systems formed by self-assembled gold nanoparticles in CuPcF<inf>4</inf> molecular crystal
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Only the atoms, which take part in the spectra, are tinted. The C 1s spectrum (see Fig. 9(a)), taken from pristine CuPcF4 film, has a complex structure, indicating high quality of organic thin film [36,37]. The decomposition reveals basically three main components: B, which originates from the aromatic carbons of the benzene rings of CuPcF4 (blue positions in inset), P, shifted by 1.2 eV toward higher binding energy (red positions), which is originated from the pyrrole carbon linked to the more electronegative nitrogen N1 (four nitrogen atoms linked to the central metallic atom) and N2 (four pirrole nitrogen atoms), and F, which is originated from the benzene ring carbon atoms linked to fluorine (orange positions).
In this work we have fabricated and studied hybrid organic-inorganic nanocomposite system formed by gold nanoparticles self-assembled in organic semiconductor thin film - copper tetrafluorophthalocyanine (CuPcF₄). By means of Photoelectron Spectroscopy and Transmission Electron Microscopy (TEM) the evolution of the morphology and electronic structure of the system as a function of nominal gold content have been investigated. The gold atoms, deposited onto the CuPcF₄ surface, diffuse into the organic matrix and self-assemble to nanoparticles in a well-defined manner with a narrow size distribution, which depends on the amount of deposited gold. Using High-Resolution TEM, we were able to observe the atomic planes of single gold nanoparticles and their coalescence processes. Photoelectron spectroscopy has not revealed any detectable chemical reaction between gold and organic. However, the strong upward band bending, induced by gold nanoparticles in the organic film, takes place.
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It can be observed from the figure that the N 1s peak position is similarly shifted to lower binding energy at lower coverage. It should be noted that the chemical states of the aza-bridging nitrogens and the pyrrolic nitrogens (inner four nitrogens bonded with the carbon atoms and the central Cu metal atom) are similar, and thus they cannot be separated in the peak [29]. For this reason, the peak shape was almost unchanged with the thickness of the deposited film.
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Electronic structure of partially fluorinated copper phthalocyanine (CuPCF4) and its interface to Au(100)

Abstract

Introduction

Section snippets

Experimental

Bulk electronic structure

Conclusion

J. Electron Spectrosc. Relat. Phenom.

Surf. Sci.

Surf. Sci.

Surf. Sci. Rep.

Org. Electron.

Surf. Sci.

Appl. Phys. Lett.

J. Appl. Phys.

J. Appl. Phys.

Electronic structure of partially fluorinated copper phthalocyanine (CuPCF₄) and its interface to Au( $1 0 0$ )