996
J. Phys. Chem. B 2002, 106, 996-1003
Scanning Tunneling Microscopy, Orbital-Mediated Tunneling Spectroscopy, and Ultraviolet Photoelectron Spectroscopy of Nickel(II) Octaethylporphyrin Deposited from Vapor L. Scudiero, Dan E. Barlow, and K. W. Hipps* Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: June 26, 2001; In Final Form: September 19, 2001
Thin films of vapor-deposited Ni(II) octaethylporphyrin (NiOEP) were studied supported on gold. Thin films thermally deposited onto flame-annealed Au (111) were analyzed by ultraviolet photoelectron spectroscopy (UPS) using He I radiation and by X-ray photoelectron spectroscopy (XPS) using Mg KR radiation. Reflectance absorption infrared spectroscopy (RAIRS) was used to study NiOEP films on polycrystalline gold. Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (STM-OMTS) were performed on submonolayer films of NiOEP supported on Au(111). The highest occupied π molecular orbitals of the porphyrin ring were seen in both STM-OMTS and in UPS at about 6.4 and 6.8 eV below the vacuum level. The lowest unoccupied π* molecular orbital of the porphyrin ring also was observed by STM-OMTS to be located near 3.4 eV below the vacuum level.
Introduction Metalloporphyrins are intensively studied for many reasons. They play an important role in a wide variety of biological processes ranging from oxygen transport to pigmentation changes. They can act as catalysts,1 and there has been significant recent interest in structural distortions from planar geometry.2-35 Metalloporphyrins can undergo reversible redox reactions in which the site of electron transfer may be localized on the porphyrin ring, or on the central metal ion. Both reaction types are important in natural processes.6 Thin porphyrin films on metal and semiconductor surfaces are also of great interest. Chemical sensors made from porphyrin films have been reported.7 In the electrochemical environment, infrared (IR) spectroscopy,6 Raman spectroscopy,8 and scanning tunneling microscopy (STM)9-15 studies have provided a wealth of information on the structure of porphyrins at the electrode surface. Langmuir-Blodgett films,16 self-assembled monolayers,9,17 and self-organized structures18,19 of porphyrins have been studied by a wide variety of techniques. Vapor-deposited porphyrins have received less attention. IR transmission spectra and electron diffraction studies have been performed on thin films of metal free tetraphenylporphyrin (H2TPP) vapor deposited onto KCl.20 Ultrahigh-vacuum (UHV) studies of vapor-deposited Cu(II) tetra-(3,5-di-tertiary-butylphenyl) porphyrin (CuTTBPP) on Cu(100) have been reported by Gimzewski,21,22 who has also provided a single image of a mixed monolayer of CuTPP and CuTTBPP on Cu(100).22 Recently, Scudiero et al. reported an XPS, IR, STM, and STM orbital-mediated tunneling (STM-OMT) study of submonolayer films of CoTPP and NiTPP on Au(111) under UHV conditions.23,24 Scudiero et al. analyzed IR and XPS spectra of thin films of Co(II)TPP, Cu(II)TPP, and Ni(II)TPP in terms of oxidation state, chemical composition, and orientation.23 It was found that these compounds may be thermally deposited onto gold without change in chemical composition or oxidation state. Molecular * To whom correspondence should be addressed. E-mail:
[email protected].
resolution STM images were reported and chemical specificity in STM imaging for these complexes was demonstrated.23,24 As had been previously shown for metal phthalocyanine (MPc) complexes,25-27 varying the metal ion at the center of a metal(II) tetraphenylporphyrin (MTPP) produced huge variations in the constant current STM images. This was interpreted as indicating large changes in tunneling probability associated with occupancy of the dz2 orbital of the transition metal ion. Scudiero et al. also provided electronic spectroscopic properties of metallotetraphenylporphyrins.24 In particular, they determined the energies of the highest occupied and lowest unoccupied π orbitals, and the highest occupied d metal orbital. For the first time, results from STM and tunnel-diode-based orbital-mediated tunneling spectroscopy, and from ultraviolet photoemission spectroscopy (UPS) measurements on the same species were reported and compared. All three types of spectra were in agreement in their regions of overlap.24 An electrochemical model for estimating orbital-mediated tunneling bands was found to give good qualitative agreement with experiment, and good quantitative agreement for transient reduction processes and oxidative processes near the Fermi energy EF. The quantitative predictions of the model were poor for oxidative processes far from EF. In the present study attention is turned to a different metalloporphyrin, Ni(II) octaethylporphyrin (NiOEP). A space filling model (CPK) of a typical first row M(II)OEP is shown in Figure 1 in both top and side views. This model is based on the crystal structure of NiOEP,28 but with the ethyl groups all curled in the same direction. This configuration was chosen because it is consistent with the STM images we have obtained (vide infra). In the crystal, four curl up and four curl down. NiOEP offers the possibility of a significant change in geometry for the adsorbed species relative to the nickel tetraphenylporphyrin (NiTPP). Consider NiTPP and NiOEP adsorbed on Au(111). Because the phenyl groups of NiTPP are roughly perpendicular to the porphyrin ring, they force the central ring plane to lay about 0.34 nm above the gold surface. NiOEP, on the other hand, can have the porphyrin ring in direct contact with the metal surface by adopting the conformation shown in
10.1021/jp012436m CCC: $22.00 © 2002 American Chemical Society Published on Web 11/02/2001
NiOEP Deposited from Vapor
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Figure 2. Schematic of critical energy levels of a porphyrin relative to the vacuum level, standard calomel electrode (sce), and Fermi energy. Ac and Af are electron affinity levels for crystalline and for thin film on metal phases, respectively. A is the energy required to take an electron from a negative ion state to the vacuum level and leave a neutral molecule. Ic and If are the corresponding ionization energies and are the energies required to take an electron from an orbital of a neutral molecule to the vacuum level. Figure 1. CPK model of a typical metal(II) OEP. The four nitrogens that bind the central metal ion are shown in light gray. The molecule is shown both in top and side view. The configuration of the ethyl groups is chosen to be consistent with the structure of NiOEP adsorbed on Au(111).
Figure 1. The present paper was motivated by our desire to understand the effects on conformation and electronic structure of this potentially stronger interaction between NiOEP and Au(111). XPS and Fourier transform infrared (FT-IR) spectroscopy (RAIRS and transmission modes) will be used to establish that the chemical composition of NiOEP thermally deposited on gold is unchanged by the preparation process. UPS and STM-based orbital-mediated tunneling spectroscopy (STM-OMTS) will be used to probe the highest occupied and lowest unoccupied molecular orbitals. To contrast and compare experimental results from STMOMTS and UPS, we will utilize the energy of the vacuum level (zero kinetic energy electron) as a unifying reference point. STM-OMTS data are conventionally presented with the zero taken at the Fermi level EF and peak positions are reported as shifts (∆o and ∆r). Once EF is determined, however, these can be converted to energies below the vacuum level as shown in Figure 2. Similarly, ultraviolet photoelectron spectroscopic determination of occupied orbital energies are normally measured relative to the Fermi energy and these binding energies can also be converted (using Figure 2) into ionization energies I (orbital energies below the vacuum level). Electron affinities A, the energy gained by adding an electron to an unoccupied orbital, also are given conventionally relative to the vacuum level. Reduction and oxidation potentials in solution, often given relative to sce, may also be converted to energies below the vacuum level by using a procedure given by Richardson.29 Key to many of these conversions is a knowledge of the Fermi energy relative to the vacuum level. The Fermi energies for the gold samples used in this work were determined in a previous paper24 and will be used to convert the variously measured quantities to energies below the vacuum level.
Experimental Section Materials. Metalloporphyrins were purchased from Porphyrin Products and used as supplied. NiOEP was sent to Desert Analytics for elemental analysis and the results were (C) 73.1% expected, 73.4% found; (H) 7.5% expected, 7.3% found; (N) 9.5% expected, 9.5% found. Gold and aluminum metals were >99.99% purity. The porphyrins were deposited from Ta metal sources for all STM, XPS, and UPS samples, while a quartz crucible was used for RAIRS sample preparation. STM Sample Preparation and Data Acquisition. Epitaxial Au(111) films with well-defined terraces and single atomic steps were prepared on mica by previously described methods.25,27 These films were 0.1-0.2 µm thick and had a mean single grain diameter of about 0.3 µm. Unlike true single-crystal gold,30 these small crystal grains showed reconstruction line spacing ranging from 6.3 to about 9.0 nm. The gold films were transferred via air-lock into the UHV STM chamber (working pressure ∼5 × 10-10 Torr) where the porphyrin was thermally deposited and then studied without exposure to air. The thickness of the metalloporphyrin layers was determined with a quartz crystal thin film monitor. The STM head used was produced by McAllister Technical Services (Coeur d'Alene, ID) and is of the inertial approach type. A Digital Instruments Nanoscope III controller was used to acquire and process the reported data. Constant current images are reported and any filtering is indicated in the appropriate figure caption. All images were acquired at about 21 °C. Both etched W and cut Pt0.8Ir0.2 tips were used. Generally, the tips required a UHV cleaning step (electron beam bombardment) in order to produce high-quality images. Spectroscopy was performed by using the Digital Instruments software to measure current as a function of sample bias voltage I(V) at fixed tip-sample separation (feedback off). I(V) curves were recorded for different x and y coordinates, where x and y were manually changed by steps of 1 nm. The resulting curves were then averaged and dI/dV was obtained as a numerical derivative of the average I(V). The dI/dV data presented here are the average of 8 to 12 spectra. STM spectra
998 J. Phys. Chem. B, Vol. 106, No. 5, 2002 (dI/dV at fixed height) were measured both on islands of the porphyrin and over the clean gold surface as pairs. Spectra having anomalous structure over gold indicated that tip artifacts were present and both the porphyrin and the Au surface dI/dV curves were discarded. IR Sample Preparation and Spectral Acquisition. In a glass bell jar deposition system (base pressure ∼7 × 10-8 Torr) 200 nm of Al was first deposited on clean glass microscope slides. Then, about 30 nm of Au was deposited at a rate of 0.17 nm/s on the Al film. Three such substrates were made at one time. On two of these substrates, 24 nm (as determined by a quartz crystal microbalance) of NiOEP was deposited at a rate of 0.2 nm/s. A density of 1.3 g/cm3 was assumed. The third substrate was used as the RAIR reference. Substrates were maintained near 21 °C during NiOEP deposition. Samples and reference were then removed from the deposition system and transferred to the vacuum (roughing pressure) bench of a Bruker IFS113 Fourier transform IR spectrometer. These samples were studied by reflectance-absorption IR (RAIR) with an 86° angle of incidence. Alternatively,
