J. Phys. Chem. B 2006, 110, 20789-20793
20789
Molecular Interface Effect of Heterogeneous Molecular Junctions Consisting of Nonfluorinated and Fluorinated Phthalocyanines Yuhong Liu,†,§ Deliang Yang,† and Chen Wang*,‡ Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China, and National Center for NanoScience and Technology, China, Beijing 100080, China ReceiVed: January 27, 2006; In Final Form: August 17, 2006
We report the tunneling behavior of homogeneous and heterogeneous molecular junctions using p-type molecules of iron phthalocyanine (FePc), phthalocyanine (H2Pc), and copper(II) octaalkoxyl substituted phthalocyanine (CuPcOC8) and n-type molecule of copper hexadecafluorophthalocyanine (F16CuPc). The molecular films formed on the electrode surfaces were inspected by X-ray photoelectron spectroscopy (XPS). The measured characteristic tunneling curves of single-component phthalocyanines revealed comparable energy gaps for homogeneous tunneling junctions using the photoemission method. In contrast, for the heterogeneous tunnel junctions of mixed phthalocyanines including fluorinated phthalocyanine a distinctive offset of the energy gaps to the positive bias voltage direction can be clearly identified. It is suggested that the substitution of phthalocyanines and surface affinity of phthalocyanines could contribute to the controlled phase separation within the heterogeneous tunneling junctions. The apparent shift of the tunneling spectra is attributed to the existence of an internal electric field originated with the phase separation of the binary mixture of p-type and n-type phthalocyanines within the tunneling junction.
1. Introduction Design, synthesis, and construction of molecular devices based on rich chemical and physical properties of various molecules have been a stimulating field of study. Herein, we report a method to study the electron transportation behavior of both homogeneous junctions and heterogeneous junctions made by a series of phthalocyanine derivatives with different ligand metal and functional substitutes, sandwiched between electrodes. An apparent shift of the tunneling spectra is identified for junctions consisting nonfluorinated and fluorinated phthalocyanines and is attributed to the rectification effect that originated with the phase separation of the binary mixture of p-type and n-type phthalocyanines within the tunneling junction. Earlier explorations proposed that a single molecule with a donor-spacer-acceptor structure could develop rectification properties and behave as a diode.1,2 Such structures could have wide applications in fast switches, oscillators, and frequencylocking circuits. Since then, a variety of molecules have been synthesized and relevant detection techniques have been developed.3 The techniques for preparing molecular tunneling junctions include evaporation processes,4 the breakpoint method,5,6 the mercury column method,7-10 the nanowire method,11 nanolithographically defined pores,12 capillary molecular junctions,13 the cross-wire method,14 metallic nanoparticle based contacts,15 junctions prepared by using the electromigration effect,16 and so on. The results obtained from these junctions mainly represent ensemble averages. The usage of scanning probe microscopy (SPM)17-25 further made it possible to address individual * To whom correspondence should be addressed. Fax: (86)-10-62562871. E-mail:
[email protected]. † Chinese Academy of Science. ‡ National Center for NanoScience and Technology, China. § Also at the State Key Laboratory of Tribology, Department of Precision Instruments and Mechanology, Tsinghua University, Beijing 100084, China.
molecules. The substrates used to assemble molecules are mostly metals such as Au, Ag, Hg, and Cu.8 The reported results revealed the electron decay lengths for a series of alkanethiolate on gold and mercury surface,7,9,11,19,20,25 which was found to depend on tip bias.20 The resistance of chemically bonded single octanedithiol molecules is at least 4 orders of magnitude lower24 than that of nonbonded molecules, which is significantly higher than that of the molecules containing phenylene groups.5 In addition, thin film morphologies are a strong function of the methods and conditions of film preparation, including molecular vacuum deposition, Langmuir-Blodgett films, molecular beam epitaxy, self-assembly technology, and high-speed spin-casting. It is noticed that self-assembly technology has grown as a promising approach for preparing molecular films because of its high efficiency. The interest in the field of thin organic films has evolved dramatically due to their successful application in optical and electronic devices, such as light emitting diodes, photovoltaic solar cells, chemical sensors, and field effect transistors. An important topic is the carrier transportation across heterogeneous molecular layers formed by n-like and p-like molecular semiconductor layers (n-MSL and p-MSL), as well as moleculeelectrode interfaces. Such transportation processes are key to the understanding of electrical rectification and light emission of the organic devices.26,27 The highly localized phase separation of the component materials is crucial to the optical and electrical properties of such junctions through nanoscale or mesoscale molecular interfaces. The family of phthalocyanines (Pc’s) represents promising candidates for ordered organic thin films in organic electron electronics, owing to their chemical stability, excellent film growth, and electronic properties.28 Both p- and n- type conductivity have been demonstrated within this substance class, and the preferred type can be chosen by chemical modifications. For instance, copper phthalocyanine (CuPc) is known as a p-type material in air,29 whereas its
10.1021/jp0605799 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/04/2006
20790 J. Phys. Chem. B, Vol. 110, No. 42, 2006 perfluorinated derivative, copper hexadecafluorophthalocyanine (F16CuPc), is one of the n-type organic semiconductors.30 Interface dipoles for dissimilar molecular junctions have been studied by using photoelectron spectroscopy. Such interface dipoles are originated from the different highest occupied orbital (HOMO) positions, ionization potentials, and work functions for dissimilar molecules. Earlier studies using combined X-ray and ultraviolet photoemission (XPS, UPS) revealed a significant interface dipole for ZnPc/PTCDA while the ClInPc/PTCDA contact was essentially dipole free.27 Such studies of molecular interfaces are key to the understanding of the rectification properties of heterogeneous thin films, and in the processes of photovoltage formation and exciton dissociation. The present study is aimed at studying the electron tunneling behavior of homogeneous junctions and heterogeneous junctions made by a series of phthalocyanine derivatives with different metal and functional substitutes, sandwiched between electrodes. The electron tunneling characteristics of the gallium/molecule/ gallium junctions can be studied with high reproducibility. Characterizations of the bare electrode surfaces were carried out by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The molecular films formed on gallium electrode surface were examined by XPS. 2. Experimental Section 2.1. Materials. Metallic gallium (Acros Organics, purity 99.99%) was used as-received without further purification. Phthalocyanine (denoted as H2Pc, purity 99%) and iron phthalocyanine (denoted as FePc) were purchased from Acros Organics. Copper(II) 2,3,9,10,16,17,23,24-octakis(octyloxy)29H,31H-phthalocyanine (denoted as CuPcOC8) and copper(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro29H,31H-phthalocyanine (denoted as F16CuPc)were obtained from Aldrich. The reagents were >95% pure and used without any purification. Ether, tetrahydrofuran, acetone, and ethanol were purchased from Beijing Jinxin Chemical Factory (AR grade). High-purity silver paint was obtained from SPI (Division of Structure Probe, Inc.). 2.2. Junction Preparation. The tunnel junctions were formed inside capillary fibers between electrodes of solidified gallium as schematically illustrated in the Supporting Information (Figure 1; description of the procedure can also be found in a previous work13). Gallium was selected as the electrode material for its relatively low melting point (29.78 °C). The tunnel junctions were formed within a commercially available capillary fiber of 100 µm diameter. The capillary fiber was pretreated by supersonic rinsing in distilled water (MilliQ) for 30 min followed by 30 min rinsing in ethanol. The melted gallium was filled into the fiber to the preset height at temperatures slightly higher than the melting point of gallium. When it was cooled to room temperature, the fiber was filled up with solid gallium. The gallium wire contained in the fiber was supersonically rinsed in ethanol for at least 20 min to remove possible contaminants. The end point of the fiber was then immersed in 0.01 M tetrahydrofuran solution of phthalocyanine (as well as other phthalocynanines) to allow formation of molecular films. In accordance with the reported data on thiol assembling on Au and Ag surfaces, a typical immersion time of 72-120 h was allowed to form dense self-assembled monolayers (SAMs).31,32 The end of the fiber was then thoroughly rinsed in acetone for 5 min followed by 30 min immersion in ether to remove residual molecules. When the solvent was completely vaporized, the open end of the capillary fiber was covered with the assembled molecules. The SAM-
Liu et al. modified fiber was subsequently inserted into another fiber with larger inner diameter to seal off the molecular films inside the capillary fiber with gallium. We wish to point out that the molecular films were prepared from the adsorption of specimens from saturated solutions. From our extended experience with preparing SAMs for scanning tunneling microscopic (STM) studies on substrates, adsorption of molecular species is more preferential compared with larger aggregates such as microparticles. We have learned from the studies that tetrahydrofuran can be used for preparing highquality SAMs of fluorinated phthalocynanines, and can be considered as supportive evidence to the presence of molecularly dissolved Pc’s in the solutions. 2.3. Apparatus and Measurements. A commercial lock-in amplifier (Stanford Research System, Model SR810 DSP) was used in the home-designed electrical conductance measurement setup. The current-voltage (I-V) curves and dI/dV versus voltage curves were measured and recorded simultaneously. Parallel experiments were performed on gallium-covered glass slides by XRD to detect the composition of gallium film and XPS to confirm the existence of SAM and the ratios of components absorbing on the electrode surface. The gallium electrode surface was analyzed using XRD in the θ-2θ scan mode. The experiment was performed on the Rigaku Dlmax2500 XRD analyzer (Cuprum target, 45 kV, 300 mA). In this mode, only diffracting planes parallel to the plane of the substrate produce significant diffraction intensity. The peaks present in the XRD profile represent the dominant crystallographic constants of the crystals. The XPS experiment was carried out by an ESCALAB 220I-XL X-ray photoelectron spectrometer. The monochromatic Al KR line (1486.6 eV) and Mg KR line (1253.6 eV) were used as the excitation source. The binding energy was calibrated against the C(1s) peak at 284.6 eV of the sample carbon. 3. Results and Discussion For characterizing the gallium surface and molecular films on electrode surface, parallel experiments by XRD and XPS were performed. The XRD analysis revealed that, besides gallium, gallium oxides were also present in the substrate. These are considered naturally formed oxides in the electrode preparation process of solidification of melt gallium metal. The molecular-film-modified Ga films were also characterized by XPS, and the results confirmed the existence of the molecular films of phthalocyanine derivatives. For the adsorption of mixed phthalocyanines, we cannot detect the presence of F16CuPc in the mixed samples of CuPcOC8/F16CuPc and H2Pc/F16CuPc (prepared by the immersion and rinsing process described in the Experimental Section) by XPS experiments. This result is a clear indication that, for the mixed systems, CuPcOC8 and H2Pc are the main components on the gallium electrode surface rather than F16CuPc. Using the preparation method illustrated in the Experimental Section, we have measured the tunneling characteristics of a series of molecular junctions of phthalocyanine derivatives on gallium. Repeated measurements on independently prepared capillary junctions produce comparable results. Figure 1a,b show the characteristic nonlinear tunneling behavior in junctions of the four types of molecules. This behavior closely resembles the data on phthalocyanines obtained by scanning tunneling spectroscopy (STS) as previously reported.33 The measured I-V curves display breakdown voltages higher than 1.5 V at both polarities for the above-mentioned junctions, indicative of robust junctions with low defect concentrations.7,8 In addition, repeated
Molecular Junctions Containing Phthalocyanines
J. Phys. Chem. B, Vol. 110, No. 42, 2006 20791
Figure 1. Electrical conductance of the homogeneous and heterogeneous tunnel junctions. (a) Measured I-V curves for phthalocyanine derivatives. (b) dI/dV versus V characteristics showing the apparent gap regions for phthalocyanine derivatives. (c) Measured I-V curves for mixed phthalocyanine derivatives. (d) dI/dV versus V characteristics showing the apparent gap regions for phthalocyanine derivatives.
TABLE 1: Energy Gaps and Offsets of Fermi Level (Center Position of the Gap Region) of Phthalocyanine Derivative Junctions on Gallium According to Figure 1a,b H2Pc FePc F16CuPc CuPcOC8
energy gap (∆), eV
Fermi level offsets (δ), eV
1.83 ( 0.20 1.88 ( 0.10 1.90 ( 0.10 2.28 ( 0.20
-0.22 ( 0.10 -0.15 ( 0.05 -0.25 ( 0.05 -0.24 ( 0.10
control experiments under the same preparation conditions without molecular films showed linear I-V characteristics in our measurement regime. Therefore, the observed nonlinear behavior of the molecular decorated gallium electrode was attributed to the molecular films and the oxide layer has negligible effect on the observed nonlinear I-V characteristics of the molecular junctions. The typical conductance curves of four phthalocyanine molecular films display discernible gap regions around zero bias in Figure 1b. Similar to the method described earlier by Feenstra,34 the edges for energy gaps are determined by the position of abrupt change in slope in dI/dV versus V curves. We determined the values for the apparent energy gaps and the offsets of Fermi level (the center position of the gap region). The energy gaps and the offsets of Fermi level of four phthalocyanine molecules listed in Table 1 were summarized from the statistic of several tens of different samples and hundreds of measurements. Their energy gaps range from 1.8 to 2.3 eV, which agree well with the STS results on SAMs of phthalocyanines obtained by using STM.35 Furthermore, the offsets of Fermi level of four phthalocyanines, including perfluorinated F16CuPc, are negative, indicating the asymmetric tunneling characteristics around zero bias. It is noticed that the magnitude of the apparent energy gaps and the shift of center position may differ among the species, and the fluorination does
TABLE 2: Energy Gaps and Offsets of Fermi Level (Center Position of the Gap Region) of Mixed Phthalocyanine Derivative Junctions on Gallium According to Figure 1c,d energy gap (∆), eV Fermi level offsets (δ), eV CuPcOC8&H2Pc FePc&H2Pc F16CuPc&CuPcOC8 F16CuPc&H2Pc
2.10 ( 0.20 1.86 ( 0.10 1.80 ( 0.10 2.22 ( 0.10
-0.25 ( 0.10 -0.13 ( 0.05 0.00 ( 0.05 0.21 ( 0.05
not affect significantly the electron density distribution as revealed by photoemission experiments.36 We also tested the tunneling properties of molecular junctions of mixed phthalocyanines using gallium as electrode material. Similar to the junctions of homogeneous phthalocyanines, the heterogeneous junctions of phthalocyanine derivatives also display nonlinear tunneling characteristics, as shown in Figure 1c. The measured I-V curves also displayed breakdown voltages higher than 1.5 V at both polarities for the junctions of the four types of mixed phthalocyanine molecules. Figure 1d shows that the typical conductance curves of four mixed phthalocyanine molecular films display apparent gap regions around zero bias. The edges for the energy gaps are determined by the position of abrupt change in slope in dI/dV versus V curves. The energy gaps and the offsets of Fermi level of four mixed phthalocyanine molecules are summarized in Table 2. Comparing with the data in Table 1, we can find that the energy gaps have little change in the tunneling junctions of mixed phthalocyanines as shown in Figure 2a,b. More interestingly, the offsets of Fermi level of four heterogeneous phthalocyanine junctions have distinctive differences. It can be concluded that when F16CuPc is mixed with other nonfluorinated phthalocyanine species, e.g., CuPcOC8 and H2Pc, the Fermi level is always shifted to the positive voltage direction. This effect can be clearly distinguished from the statistical distributions of the positive and negative edges for the energy gaps in the homogeneous and heterogeneous
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Figure 3. Schematic of self-assembled structure of CuPcOC8 and F16CuPc molecules on gallium electrode.
Figure 2. Distributions of positive and negative edges for the gap regions of homogeneous and heterogeneous junctions based on gallium electrode. (a) CuPcOC8, F16CuPc, and mixed Pc’s. (b) H2Pc, F16CuPc, and mixed Pc,s.
junctions, as shown in Figure 2. In contrast, when nonfluorinated phthalocyanine molecules are mixed with each other, the Fermi levels of the heterogeneous phthalocyanine junctions remain as nearly unchanged as those of homogeneous phthalocyanine junctions as indicated in Table 2. It is considered that the reproducible shift of the Fermi level position should be originated from the phase separation of the mixed molecular films. Phase separation behaviors in binary molecular mixtures, for compounds such as phthalocyanines and porphyrins,37,38 have been identified and are associated with differing molecular-substrate and molecule-molecule interactions. Such principles are also applicable to the mixtures in this study, where we expect the dissimilar molecules to form separated regions with the junction region. As presented for the mixed system of CuPcOC8 and F16CuPc from XPS experiments, the dominant adsorption of alkyl-substituted phthalocyanine and H2Pc to the gallium electrode surface and the phase separation of fluorinated and nonfluorinated phthalocyanine molecules determine the dominant configuration of the sandwich structure, as illustrated in Figure 3. Because of the effect of perfluorinated substitution, the electron affinity increased prominently,36 which means a stronger ability to accept electrons rather than nonfluorinated phthalocyanines. Due to the difference in electron affinity, an internal electric field (Eint) may develop at the interface of the phase separation due to interface dipoles, which is directed opposite to the direction of the external electric field (Eext). It would result in the decrease of the effective electric field (Eeff; Eeff ) Eext - Eint) experienced by the molecular junction. As a consequence, a positive shift of the offsets of
the conduction curves will occur. One may notice, from comparing the measured apparent energy gap values in Tables 1 and 2, that the measured energy gap values for binary molecular junctions involving perfluorinated phthalocyanine may be slightly different from those of the individual components. We consider such a variation to be insignificant, and therefore the mean-field mechanism can be applied to such a heterogeneous junction. The observed slight variation of the energy band edges may be associated with the possible asymmetry in the barrier for carrier transportation at the interface of heterogeneous junctions. More systematic experiments and detailed theoretical investigations are needed to provide quantitative descriptions of these phenomena. The observed asymmetries in gap offset for pure samples are associated with the intrinsic conductive nature of the phthalocyanines. CuPc has been known to be a p-type semiconductor in air; therefore the Fermi level is expected to offset from the center position of the gap region. In addition, the photoemission spectroscopic (UPS and XPS) characterization shows fluorinated Pc’s have the same energy gap between the highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) in comparison with the unfluorinated Pc’s.36 The XPS and valence band UPS studies also revealed that the most important influence of fluorination is the increase of ionization potential (IP): the IP of F16CuPc increases more than 1 eV in comparison with CuPc, but the compositions of the HOMO and LUMO remain nearly unchanged. According to their reports, the increased IP caused by the fluorination only weakly affects the position of the top of the HOMO relative to the Fermi level. The above observations suggest that one would expect that the Fermi levels within the gap region for bulk Pc’s would not be appreciably altered, as is observed in the current molecular tunneling junction study. The above-mentioned asymmetries in tunneling characteristics also exist for single-component Pc’s as revealed by STS experiments on single molecules. The main observation in this study is that the direction of asymmetry is dramatically different for junctions with binary components, even though the magnitude is comparable for both types of junctions. The above discussions suggest that the tunneling characteristics of the molecular junctions could be jointly affected by molecular electronic structures, molecular assembly structure, and surface conditions. Our studies also suggested that tunneling characteristics of heterojunctions could provide a complementary view to photoelectron spectroscopy to reveal the interface electronic structures. We wish to emphasize that knowledge of chemical composition is critical for characterizing the heterogeneous junctions. At this stage of research, there is a clear lack of proper methods to characterize the localized compositions for heterogeneous interfaces of molecular systems. Supported by the observed phase separation behavior and preferential adsorption by STM studies, we believe that the phase separation behaviors in binary molecular mixtures for compounds with dramatically different solubilities could provide relatively reliable candidates for forming stable heterogeneous
Molecular Junctions Containing Phthalocyanines structures such as the ones in this study. More rigorous experimental and theoretical investigations on these systems are required for deeper understanding of the observed behavior. 4. Conclusions In this work, the tunneling characteristics of four phthalocyanine species including fluorinated copper phthalocyanine were studied. On the basis of these results, we further investigated the electric transport properties of the heterogeneous molecular junctions. Appreciable variations of the offsets of Fermi level are observed among the mixed molecular junctions of the fluorinated phthalocyanine with other nonfluorinated phthalocyanine junctions. This effect is attributed to the rectification of carrier transportation at the interface of different phthalocyanines. The preferential molecular absorption structure on electrode surface is important for the direction of the observed rectification effect. Acknowledgment. The authors thank the National Natural Science Foundation (Grants 90406019 and 20473097) and the Foundation of the Chinese Academy of Science for financial support. Supporting Information Available: Table showing XPS binding energies, figures showing a schematic of the preparation procedure of a capillary junction and an XRD spectrum of a bare gallium electrode surface, and scheme showing the molecular structure of phthalocyanine derivatives. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Molecular Electronic Designs I & II; Carter, F. L., Ed.; Marcel Dekker: New York, 1987. (2) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (3) Metzger, R. M. Chem. ReV. 2003, 103, 3803. (4) Mann, B.; Kuhn, H. J. Appl. Phys. 1971, 42, 4389. (5) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (6) Kergueris, C.; Bourgoin, J. P.; Palacin, S.; Esteve, D.; Urbina, C.; Magoga, M.; Joachim, C. Phys. ReV. B 1999, 59, 12505. (7) Rampi, M. A.; Whitesides, G. M. Chem. Phys. 2002, 281, 373. (8) Haag, R.; Rampi, M. A.; Holmin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895. (9) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075. (10) Slowinski, K.; Fong, H. K. Y.; Majda, M. J. Am. Chem. Soc. 1999, 121, 7257. (11) Mbindyo, J. K. N.; Mallouk, T. E.; Mattzela, J. B.; Kratochvilova, I.; Razavi, B.; Jackson, T. N.; Mayer, T. S. J. Am. Chem. Soc. 2002, 124, 4020.
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