Tip-Sample Distance Dependence in the STM-Based Orbital-Mediated Tunneling Spectrum of Nickel(II) Tetraphenylporphyrin Deposited on Au(111) Wenli Deng and K. W. Hipps* Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: April 6, 2003; In Final Form: July 23, 2003 Orbital-mediated tunneling spectra obtained in the STM environment are reported for nickel(II) tetra- phenylporphyrin, NiTPP, as a function of molecule-tip separation. Spectra were acquired over a range of tip motion of 0.42 nm. Spectra did not show the variation in band splitting with tip distance predicted by several models. It appears for molecules such as NiTPP that the average potential at the molecule is essentially the same as at the metal substrate, at least for gap resistance values greater than 500 MΩ. Thus, at least for molecules of the height of NiTPP, the STM-OMT spectra should give reliable occupied and unoccupied orbital energies over a wide range of tip-molecule distances. An unexpected small shift of all orbital energies of NiTPP relative to the vacuum level is observed as a function of tip-molecule separation. Introduction Metalloporphyrins are intensively studied. They play an important role in biological processes such as oxygen transport and photosynthesis. They can act as catalysts 1 and 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. 2 Recently, it has become known that structural distortions from planar geometry are fairly common and may also play a role in their solution phase chemistry. 3-6 Thin porphyrin films on metal and semiconductor surfaces are also of great interest. Chemical sensors made from porphyrin films have been reported. 7 Infrared (IR) spectroscopy, 2 Raman spectroscopy, 8 and scanning tun- neling microscopy (STM) 9-15 studies have provided a wealth of information on the structure of porphyrins at the solution- electrode surface under controlled potential conditions. Lang- muir-Blodgett films, 16 self-assembled monolayers, 9,17 and self- organized structures 18,19 of porphyrins have been studied by a wide variety of techniques. Vapor-deposited porphyrins have received less attention. Metal-free tetraphenylporphyrin (H 2 TPP) thin films vapor- deposited onto KCl were studied by IR transmission spectros- copy and electron diffraction. 20 Ultrahigh-vacuum (UHV) studies of vapor-deposited Cu(II) tetrakis(3,5-di-tert-butyl-phenyl) por- phyrin (CuTTBPP) on Cu(100) have been reported by Gimze- wski, 21,22 who has also provided an image of a mixed monolayer of CuTPP and CuTTBPP on Cu(100). 22 Recently, Scudiero et al. reported a XPS, IR, STM, and STM-orbital-mediated tunneling (STM-OMT) studies 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 Molecular 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) tetraphenylpor- phyrin (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 d z 2 orbital of the transition metal ion. Scudiero et al. also provided electronic spectroscopic properties of metallotetraphenylpor- phyrins and nickel(II) octaethylporphyrin. 24,28 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. 24,28 An electro- chemical model for estimating orbital-mediated tunneling bands was found to give good qualitative agreement with experiment, and good quantitative agreement for transient reduction pro- cesses and oxidative processes near the Fermi energy, E F . In all of these reports, STM-based orbital-mediated tunneling spectra, STM-OMTS were reported as if the observed transient oxidation and reduction bands were independent of tip-sample separation. The excellent agreement reported between the observed energies and those reported from UPS and electro- chemistry justified the interpretations proposed but does not provide assurance that spectra taken at different set points (tip- sample separation) would provide identical results. The pos- sibility that changing the distance between the tip and the molecular film might also change the OMT peak positions is raised by previous theoretical 29-33 and experimental 34-41 studies. For example, the tip-dot separation in STM studies of quantum dots plays a major role in the observed tunneling spectra. 34,35 When the tip-dot distance is large such that the tip-dot impedance is greater than the dot-substrate impedance, the electronic affinity levels of the dot are measured. As the tip- dot distance shrinks such that the tunneling rate from tip to dot far exceeds that from the dot to the substrate, a coulomb blockade is observed and the apparent positions of the affinity levels shift. This apparent shift is due to the fact that the local potential at the dot is less than the potential applied between tip and substrate. The peripheral phenyl groups on tetraphen- * Corresponding author. E-mail: hipps@wsu.edu. 10736 J. Phys. Chem. B 2003, 107, 10736-10740 10.1021/jp034900o CCC: $25.00 © 2003 American Chemical Society Published on Web 09/06/2003