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J. Phys. Chem. B 2006, 110, 1256-1260
Charge-Transfer Effect at the Interface of Phthalocyanine-Electrode Contact Studied by Scanning Tunneling Spectroscopy Sheng-Bin Lei,† Ke Deng,‡ De-Liang Yang,† Qing-Dao Zeng,† and Chen Wang*,‡ Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, PRC, and National Center for Nanoscience, Beijing, 100080, PRC ReceiVed: June 28, 2005; In Final Form: NoVember 15, 2005
Scanning tunneling microscopy (STM) and spectroscopy (STS) are used in this work to investigate the chargetransfer effect at the molecule-substrate interface of substituted metal phthalocyanines. STS results revealed that the apparent energy gaps for both fluorinated phthalocyanines and unsubstituted phthalocyanines are essentially the same, which agree with the hybrid density functional calculations. More interestingly, there is a systematic shift of the energy level of valence bands, possibly as the result of charge-transfer effect at the molecule-substrate interface.
Introduction Molecule-substrate contact is key to the molecular electronic devices such as light-emitting medium and field-effect transistors.1 Molecular adsorption and the charge-transfer effect at electrode surfaces have been widely studied by spectroscopic techniques such as X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS)2-3 as well as scanning tunneling microscopy (STM)4-7 and spectroscopy (STS)8-10 methods, and also STM/STS in combination with reflectance absorption infrared spectroscopy (RAIRS).11 Among the techniques for the investigation of molecular electronic structure, XPS and UPS are proved to be powerful for the investigation of electronic structure and interface properties of organic thin films. On the other hand, the disadvantage is that these methods can detect only bulk properties of the materials, rather than single molecular structure and properties. STM and STS enable the complementary investigations on the structure and properties of a single molecule at the surfaces and interfaces due to its ultrahigh space resolution. Many researchers have used this technique to study the electronic structure and transport properties of adsorbed organic molecules.9-17 Bias-dependent visualization of molecules containing both electron-donor and -acceptor moieties has been demonstrated,15-17 which reveals the sensitivity of STM for the electronic properties of the adsorbed molecules. In the same time, Hipps and co-workers have combined STM/STS, tunnel diode-based orbital-mediated tunneling spectroscopy, and UPS measurements to investigate the electronic properties of a series of metal porphyrin and phthalocyanines.9,10 The obtained spectra demonstrate the reliability of the STS measurements. The advantage of STS is that it allows the investigation of molecular electronic structure and properties on a single-molecule scale, for example, electronic properties of different molecular moieties in a nanophase segregation15 or even different parts of a single copper phthalocyanine (CuPc) molecule.8 In addition, STS can focus the measurement on the single molecular layer in direct contact with * Author to whom correspondence should be addressed. Fax: 86 10 6256 2871. E-mail:
[email protected]. † Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences. ‡ National Center for Nanoscience.
the substrate, allowing investigations on the possible interface effects. The family of phthalocyanines (Pc’s) represents one of the promising candidates for ordered organic thin films in organic electronics due to their significant chemical stability and electronic properties. An important advantage for this class of substance is that both p- and n-type conductivity can be attained by chemical modifications, and this is of special importance for realization of complementary logic circuits.18 For example, CuPc has been known to be a p-type semiconductor in air, but fluorinated copper(II) hexadecafluorophthalocyanine (F16CuPc) shows high performance and stability in air for n-channel operation,18 suggesting a strong effect of substituent polarity on the electronic structure of phthalocyanine. Photoemission spectroscopic (UPS and XPS) characterization shows fluorinated Pc’s have nearly the same energy gap between the highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) in comparison with the unfluorinated Pc’s, that is, rigid band structures prevail for the fluorination process.2 The XPS and valence-band UPS study by Peisert et al.2 revealed that the most important influence of fluorination is the increase of ionization potential (IP); the IP of F16CuPc increases for more than 1 eV in comparison with CuPc, but the composition of the HOMO and LUMO remains 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 Ef. As a result, either downward or upward band bending have been observed on the Pc-gold interface and were attributed to a charge-transfer process at the interface. Charge transfer between molecules and surfaces has been extensively pursued by various techniques, such as photoemission spectroscopy (UPS and XPS), STM/ STS, and so forth.2,8,19-20 A charge-transfer effect due to molecular adsorption has been reported for NH3 intercalated transition metal dichalcogenide TiS2.19 In that work, the adsorption of NH3 causes discernible shift of the substrate Fermi level. In a recent study, a charge-transfer effect between donor (porphyrin) and acceptor (C60) molecules was identified through the contrast variation under opposite bias using a C60-decorated STM tip.20 The STS study on the evolution of electronic structure as a single CuPc molecule comes into contact with Au atomic chains shows
10.1021/jp0535036 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/04/2006
Charge Transfer at Phthalocyanine-Electrode Contact
J. Phys. Chem. B, Vol. 110, No. 3, 2006 1257
Figure 1. Different assembling structures formed by C18SH with differently substituted CuPc’s: (a) CuPc, (b) Cl16CuPc, (c) F16CuPc.
that the LUMO orbital of the CuPc molecule shifts to lower energy and, in contrast, the Au states shift to higher energy when they come in contact with CuPc. This result clearly demonstrates charge-transfer effect on a single-molecule level.8 The charge transfer can be expected with the direct shift of molecular orbitals as the result of the difference in electron affinity or ionization potential between the adsorbate molecule and the substrate Fermi level. Here we report a direct observation of the electron chargetransfer effect for adsorbed molecules on graphite surfaces. It is demonstrated that the STS method, together with highresolution images of adsorbed molecules, can provide insights into the molecular-substrate contacts, complementary to the assemble averaged results by means of UPS and PES. Experimental Section Copper(II) phthalocyanine (CuPc), copper(II) hexadecachloric-phthalocyanine (Cl16CuPc), and copper(II) hexadecafluorophthalocyanine (F16CuPc) were purchased from Acros Co. and used as received. The assemblies of phthalocyanines were prepared by depositing a drop of mixture of phthalocyanine with n-octadecyl mercaptan (C18SH) (about 1:3 in molar) (in toluene) directly onto a freshly cleaved graphite surface. STM and STS measurements were performed in ambient conditions with a Nanoscope IIIA SPM system (Digital Instruments, Santa Barbara, CA). The tips were mechanically formed from Pt/Ir wires (90/10). The typical tunneling conditions are bias: 500 to 800mV, setpoint: 500 pA to 1.0 nA. Spectroscopy was performed by adding a dithering modulation (peak-to-peak 20-30 mV) to the bias voltage, and the bias was scanned through the designated voltage range. A lock-in amplifier was used to collect the dI/dV signal. The dI/dV characteristic curves are the average of about 10 spectra. The feedback of the STM control was turned off while taking spectroscopy measurements. Tunneling spectra were also measured on the alkane derivative lamellae and over the clean HOPG surface as a control experiment. Hybrid density functional theory (DFT) calculations are performed for the electronic properties of CuPc, Cl16CuPc, and F16CuPc using B3LYP in GAUSSIAN 03 software.21 The B3LYP employs the Becke exchange gradient correction22 with the Lee-Yang-Parr gradient-corrected correction energy functional.23 Symmetry-constrained geometry optimizations are performed for the three phthalocyanines with the BroydenFletcher-Goldfard-Shanno technique.24,25 The Pople’s basis set 6-31++G(d) is adopted in calculations, which combines valence, diffuse, and polarization functions. Results and Discussion In previous studies, it has been shown that by attaching alkyl substituents to the periphery of conjugated ring or by coad-
sorption with alkane derivatives, planar molecules such as phthalocyanine and porphyrin can be ideally immobilized and assembled on graphite surfaces at ambient conditions.4,6,26,27,29 In the current study, we choose to use n-octadecyl mercaptan (C18SH) as the coadsorption agent for immobilization of phthalocyanines. An additional merit for such heterogeneous binary system is that one can use the assembled alkane lamella as the sites for performing control measurements of tunneling spectroscopy. Figure 1 shows the assembling structures formed by three differently substituted phthalocyanines when co-deposited with C18SH on the HOPG surface. It can be seen that CuPc and Cl16CuPc form uniform single-molecule arrays which is interdigitated with C18SH lamellae. These assembling structures are the same as reported in our previous work.6,28-29 However, in the case of F16CuPc, the assembling structure changed dramatically in comparison with CuPc and Cl16CuPc to two-dimensional crystallization, as was shown in the earlier report that peripheral polarity, that is, the polarity of the groups attached to the phthalocyanine ring, could dramatically change the assembling behavior of metal phthalocyanines with alkane derivatives.26 This dramatic change in assembling behavior is considered to be associated with the large electron affinity of fluorine atoms attached to the phthalocyanine ring. We further utilized STS to investigate the influence of substituents polarity on the electronic structure of copper phthalocyanine in contact with the graphite surface. The apparent gap values between HOMO and LUMO as well as the positions of HOMO and LUMO (edges) are estimated by using STS results. Figure 2b shows the typical dI/dV versus V spectrum obtained by locating the STM tip on top of individual phthalocyanine molecules. The spectrum obtained on bare graphite and alkane lamellae is shown in Figure 2a for comparison. The spectrum obtained from phthalocyanine shows a characteristic energy gap of about 2 eV. In the control experiments for alkane and graphite, the spectra appear with a characteristic parabola shape, and no apparent energy gap regions can be observed. In respect to the differently substituted phthalocyanines, Figure 2b indicates that the center position of dI/dV versus V spectrum of F16CuPc is shifted to a negative direction in comparison with that of Cl16CuPc and CuPc. The dI/dV versus V curve of Cl16CuPc shows only a negligible shift to the negative direction compared with that of CuPc. The dI/dV versus V curve is considered to reflect the density of states for the adsorbate. At an appropriate bias applied to the substrate, the Fermi energy of the substrate will come into energetic resonance with either HOMO or LUMO of the molecule, resulting in a rapid slope change in the dI/dV versus V spectra. Thus the gap edge defined by cross-point of the
1258 J. Phys. Chem. B, Vol. 110, No. 3, 2006
Lei et al.
Figure 2. Typical dI/dV curves obtained on bare graphite and alkane lamellae (a) and differently substituted CuPc’s (b). The dI/dV curves in (b) are shifted for clarity. These dI/dV spectra were acquired at fixed height with given set point current and sample bias, in (a) 500 mV, 1.06 nA for graphite and 1.06 V, 648 pA for alkane, (b) -1.00 V, 380 pA for CuPc, -1.02 V, 350 pA for Cl16CuPc and -1.08 V, 220 pA for F16CuPc.
Figure 3. (a) A histogram of the gap edges of differently substituted phthalocyanines. The solid lines show a Gauss fit of the columns. (b) Variation of the averaged gap edge position with the changing of the substituents.
tangents of the platform and uplifted part of the dI/dV spectra are considered to represent the edge of HOMO and LUMO. The apparent energy gap between HOMO and LUMO is estimated from the separation between these two edges. Determined from the dI/dV spectra, all the phthalocyanine molecules show an apparent gap value of near 2 eV, the difference of the gap value is estimated to be less than 0.2 eV. As for alkanes, quantum chemical calculations show that no molecular orbital lies within the scanned voltage range that can come into resonance with the substrate’s Fermi level.15 As a result, no rapid slope change was expected and their dI/dV spectra show the same parabola shape as the graphite substrate. The histogram in Figure 3a shows a statistic of the HOMO/ LUMO edges measured from a large number of spectra. Though the histogram shows that the distribution of apparent gap edges measured from the dI/dV spectrum for each species is rather wide (in a range from 0.3 to 0.6 eV) and some overlapping exists for different species, a discernible shift of the spectrum can still be observed from the averaged value of gap edges and energy gaps obtained by a Gaussian simulation (Figure 3b). An appreciable negative shift of HOMO/LUMO edges for F16CuPc can be identified, while the HOMO/LUMO edges of Cl16CuPc shift only slightly to the negative direction. Another noticeable feature of the histogram is that the LUMO edge of F16CuPc shifted more apparently in comparison with that of
the HOMO, this means the energy gap for F16CuPc is slightly smaller than that of Cl16CuPc and CuPc. This trend can be better seen in Figure 3b. The apparent energy gaps of these molecules are estimated to be 2.19 ( 0.3 eV, 2.17 ( 0.2 eV, and 1.98 ( 0.3 eV for CuPc, Cl16CuPc, and F16CuPc, respectively. Relative to the Fermi level of the substrate (0 bias location in dI/dV vs V spectrum), the energy gap centers for CuPc and Cl16CuPc are estimated to be shifted positively for 0.12 and 0.05 eV respectively, while for F16CuPc, the energy gap center is shifted negatively for 0.21 eV relative to the Fermi level of substrate. This observation is consistent with the previous reports that copper phthalocyanine (CuPc) is known as a p-type material in air, whereas its perfluorinated derivative, copper hexadecafluorophthalocyanine (F16CuPc), is one of the n-type organic semiconductors.18 It should be noted that previous reports have shown that the STM tip material (etched W and cut Pt0.8Ir0.2 tips),9 as well as tip-sample separation,10 do not have significant effect on the band positions measured by STS. In consideration of the good consistency of the current results obtained on graphite with that reported on other substrates,8,30 the substrate material also does not significantly change the band positions measured by STS. These observations indicate that the dI/dV spectrum reflects the intrinsic electronic properties of the adsorbed molecules independent of the tip material.
Charge Transfer at Phthalocyanine-Electrode Contact
J. Phys. Chem. B, Vol. 110, No. 3, 2006 1259
SCHEME 1: Schematic Illustration of the Tunneling Process and the Energy Alignment of the Molecular Orbitals of CuPc and F16CuPc
TABLE 1: Electronic Properties Measured by STS and Theoretical Calculation of Investigated CuPc’s inflection I inflection II gap (STS)/eV Ef offset/eV LUMO/eV HOMO/eV gap (theoretical)/eV
F16CuPc
Cl16CuPc
CuPc
0.78 -1.20 1.98 -0.21 -3.58 -5.69 2.11
1.13 -1.04 2.17 0.05 -3.66 -5.78 2.12
1.21 -0.98 2.19 0.12 -2.75 -4.94 2.19
a The shift of E is defined as shift of the gap center relative to the f zero bias.
We have also performed the hybrid DFT calculations to investigate the electronic properties of CuPc, Cl16CuPc, and F16CuPc. The calculated energy gaps of these molecules are 2.19, 2.12, and 2.11 eV, respectively. (See Table 1 and Scheme 1.) These data agree well with the experimental values and suggest that the fluorinated and chlorinated substitution affect very little for the energy gap of the copper phthalocynine. To understand the different behaviors of the charge transfer on the interface, that is, p-type semiconductor for CuPc and n-channel operation for F16CuPc, we have calculated the ionization potential (IP) of these molecules. We find that the IP of CuPc is about 6.00 eV, while the IPs of the chlorinated and fluorinated Pc’s increase to about 6.70 and 6.73 eV, respectively. It confirms that the substitution of Pc’s causes the molecules to exhibit high IPs, which are responsible to the different charge-transfer effect of these copper phthalocynines.2 Discussion Hipps et al. carried out a series of investigations on the electronic spectroscopic properties of metalloporphyrins.9-11 Results from STM, tunnel diode, and UPS of the same compound are compared. Their studies show that the STMbased orbital-mediated tunneling spectrum (OMTS) can provide more information than UPS or tunnel junction-based OMTS. STM-based OMTS can provide information both from the highest occupied π molecular orbital as well as from the lowest unoccupied π* molecular orbital, and another advantage is that it can provide this information at single-molecule resolution. In consideration of the structural similarity of phthalocyanine with porphyrin, our results obtained in ambient conditions show reasonable consistency with their results. The dI/dV spectra of CuPc also agree well with the Nazin et al. report8 though their measurements are conducted under UHV and low temperature
and only the positive bias part of the spectra were estimated. This indicates that by statistics of an adequate amount of spectra, reliable STS results could be acquired in ambient conditions. Taking the average work function for carbon as 4.81 eV and the Fermi level of molecules at the center of the HOMO and LUMO, the charge transfer between molecule and substrate can be expected. For the influence of the substituent polarity on the electronic structure of phthalocyanine, our STS results show that the HOMO edge of F16CuPc shifts for only 0.22 eV in comparison with that of CuPc, in agreement with the UPS result on fluorinated copper phthalocyanines using XPS and valenceband UPS on gold.2 However, the STS data indicate a larger shift of the LUMO edge of F16CuPc (∼0.43 eV) which was not observed by UPS. This demonstrates that fluorination may have a larger impact on the lowest unoccupied orbital than on the highest occupied orbital. Conclusion The influence of substituent polarity on the electronic structure of copper phthalocyanine was investigated by STS. The STS results suggest a systematic shift of the valence bands and a slight narrowing of the apparent energy gaps between HOMO and LUMO as the polarity of substituents increases. Our results are in general accordance with the previous reports obtained by XPS and UPS, and additional information provided by STS is that the substituent seems to cause a larger shift to the LUMOs than the HOMOs. The information provided by the STS method may be useful for fabrication of the organic semiconductor device and also for the understanding of the mechanism of carrier transportation in these devices. Acknowledgment. The authors are grateful for the financial support from National Natural Science Foundation (Grant Nos. 90406019, 20303023, 20121301, 20573116). References and Notes (1) Forrest, S. R. Chem. ReV. 1997, 97, 1793. (2) Peisert, H.; Knupfer, M.; Schwieger, T.; Fuentes, G. G.; Olligs, D.; Fink, J.; Schmidt, Th. J. Appl. Phys. 2003, 93, 9683. (3) Schwieger, T.; Peisert, H.; Golden, M. S.; Knupfer, M.; Fink, J. Phys. ReV. B 2002, 66, 155207. (4) Xu, B.; Yin, S.; Wang, C.; Qui, X.; Zeng, Q.; Bai, C. J. Phys. Chem. B 2000, 104, 10502. (5) Yoshimoto, S.; Higa, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8540. (6) Lei, S. B.; Wang, C.; Yin, S. X.; Bai, C. L. J. Phys. Chem. B 2001, 105, 12272.
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