Experimental and Theoretical Identification of Valence Energy Levels

J. Phys. Chem. C 2008, 112, 13215–13225

13215

Experimental and Theoretical Identification of Valence Energy Levels and Interface Dipole Trends for a Family of (Oligo)Phenylene-ethynylenethiols Adsorbed on Gold Chad Risko,†,* Christopher D. Zangmeister,‡ Yuxing Yao,§ Tobin J. Marks,† James M. Tour,§ Mark A. Ratner,† and Roger D. van Zee‡ Department of Chemistry and the Materials Research Center, Northwestern UniVersity, EVanston, Illinois 60332, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Smalley Institute for Nanoscale Science and Technology, Department of Chemistry, Rice UniVersity, Houston, Texas 77005 ReceiVed: March 17, 2008; ReVised Manuscript ReceiVed: May 16, 2008

Metal-molecule-metal junctions composed of organic molecular wires formed via self-assembly are of relevance in the empirical evaluation of single-molecule electronics. Key to understanding the effects of these monolayer structures on the transport through single molecules, however, is discerning how the molecular electronic levels evolve under the influence of the metal substrate and intermolecular interactions. We present a joint experimental and computational investigation of the electronic structure and electrostatic properties of a series of self-assembled donor- and acceptor-substituted (oligo)pheneylene-ethynylenethiols (OPEs) on gold. Photoemission spectroscopy is employed to determine the energy-level alignment for these monolayers. Isolated molecule and small cluster calculations are performed to assess changes in geometry, electronic structure, and charge distribution upon chemisorption. The calculated densities of electronic states allow assignment of the higher-lying occupied states mapped by experimental photoemission data. Calculated estimates of the surface, bond dipole, and image potential energies are used to estimate contributions to the measured work function changes; good correlations between the experimental and theoretical values are found. Importantly, these results point to a dependence of the dipole contributions on the orientational order of the SAM. I. Introduction Organic molecules bound to metal and inorganic semiconductor substrates are of technological interest with prospective applications in single-molecule electronic devices, molecular/ polymeric thin-film electronics, dye-sensitized solar cells, biochemical redox sensors, engineered molecular nanostructures, etc.1–15 Essential to describing the electrical performance of molecule-based wire systems is understanding how molecular adsorption and coupling to the electrode affects the electronic structure of the molecular components.8,16,17 Although molecular orbitals are not the channels by which charge moves though the molecular wire, the energies of the molecular states have been shown to strongly correlate with transmission characteristics;18,19 therefore, it is important to gain direct insight into these one-electron levels. The relative alignment of a self-assembled monolayer’s (SAMʼs) energy levels with respect to the electrode Fermi energy can have a profound impact on the current-voltage (I-V) characteristics of a device,20 with the formation of an interfacial dipole playing a key role in the alignment.7,16,17,21–37 A number of factors can lead the presence of these interfacial dipoles, including: native surface dipoles due to the leakage of electron density from the electrode surface;38 charge transfer between the molecule and substrate upon adsorption; image charge interactions due to the redistribution of electron density in the electrode; and any other physical phenomena that induce charge redistributions within or between the substrate and * E-mail: [email protected]. † Northwestern University. ‡ National Institute of Standards and Technology. § Smalley Institute for Nanoscale Science and Technology.

organic layer. By understanding the formation of these dipoles and the redistribution of charge, we may be able to control the interfacial interactions through molecular design. Ultraviolet photoemission spectroscopic (UPS) methods provide insight into the electronic structure and coupling to the inorganic substrate by directly probing the occupied molecular electronic states.7,16,21,39 In photoemission experiments, ionization energies are measured as electrons are photoemitted from the molecular orbital structure and a radical-cation is formed. The kinetic energy of the photoemitted electrons is slow enough to include both the relaxation of the electronic polarization of the surrounding molecules and the structural relaxation of the molecular ion;21 therefore, Koopmans’ theorem40 is not directly valid.21 Obtaining appropriate and relevant theoretical descriptions of these large structures, however, is a formidable challenge as: (i) simply the sheer number of chemical systems and interactions needed to be taken into account is immense, and (ii) we are at the boundary where approximations involved with band structures and densities of states (to deal with the delocalization of valence electrons in metals) encounter those approximations of molecular electronic structure theory (to describe the discrete molecular orbital configurations and energies of single molecules).41 Thiol-bound SAMs, especially (oligo)pheneylene-ethynylenethiol (OPE) SAMs,4–6,16,37,42–52 have undergone intense experimental and theoretical investigation and scrutiny;4–6,16,37,43,45,53–83 this class of molecules is the subject of this investigation. In this work, we use gas-phase and extended-molecule model calculations to explore the influence of the gold electrode on the electronic structure of a series of donor- and acceptorsubstituted OPEs (see Figure 1) and compare these results directly to UPS measurements of the SAMs on gold surfaces.

10.1021/jp8023183 CCC: $40.75  2008 American Chemical Society Published on Web 08/06/2008

13216 J. Phys. Chem. C, Vol. 112, No. 34, 2008

Figure 1. Chemical structures of substituted OPE and smaller systems.

We are especially interested in exploring electrostatic interactions between the SAMs and the gold surface, in particular, the surface potential energy due to the intrinsic dipole moment of the OPE systems and image dipole effects. II. Methodology a. Computational Methodology. Adsorption processes have been studied theoretically using various levels of theory and a number of different models, including dipped adcluster,84 local space approximation,85–87 embedded cluster,41 and slab calculations with periodic boundary conditions.55,60,80–83,88 Because of its simplicity, the extended-molecule (i.e., bare cluster) approach has been widely used as a starting point in single-molecule electronics to understand electrode-molecule interactions.45,62,63,68,89–91 Although pertinent results have been obtained for numerous structures and geometries of molecule-surface interactions, the model has shortcomings, including neglect of long-range contributions of importance in determining binding energies and relative stabilities.92 Taking note of these assumptions and limitations, we use the extended-molecule model herein to understand the influence of the gold contact on the molecular electronic structure to provide a basis for future slab calculations. We begin by examining the single molecule properties in various redox and deprotonated states for a series of donor and acceptor substituted OPE-thiols; benzenethiol (B) and 4-(phenylethynyl)benzenethiol (PEB) are included in the theoretical analysis to probe the consequences of conjugation length. Geometry optimizations using density functional theory (DFT) for the neutral, radical-ion, thiyl (neutral, open-shell dehydrogenated thiol), and thiolate (anionic, closed-shell deprotonated thiol) molecular states were carried out using the B3LYP functional93–95 and a 6-31G** basis set. We then examine an extended-molecule picture, where three gold atoms serve as a minimal representation for the gold electrode. The extendedmolecule model provides a static picture of the Au-molecule interaction in one of many possible geometries; the nature of the Au-S bond (e.g., binding site, number of atomic Au-S interactions, etc.) is not well defined experimentally or theoretically (i.e., the nature of the bond description depends on density functional and basis set choice) because of the relatively flat potential energy surface55 for binding; we note that a very recent crystallographic study of thiol-monolayer-protected gold nanoparticles suggests an interesting bonding motif for such interactions.96,97 The present calculations were carried out under the following assumptions: chemisorption of a deprotonated thiyl structure, frozen Au cluster with no interactions from other layers or bulk, predominately hollow-site sulfur position as the starting geometry,55 and no interaction with neighboring molecules. For the extended-molecule calculations, the same functionals and basis sets were utilized for all non-gold atoms,

Risko et al. with the exception of sulfur to which an extra diffuse function was added to extend the interaction with the gold cluster; the small-core SBKJC effective core potential (ECP) basis was used for the gold atoms to allow for possible chemistries with a larger number of core electrons. Electronic density of states (DOS) for the isolated thiols and extended-molecule systems were created by convoluting the one-electron energies with Gaussian functions characterized by a full width at half-maximum (fwhm) of 0.5 eV. All calculations were carried out with QChem (version 2.1).98 b. Monolayer Preparation and Chacterization. Monolayers were grown on polycrystalline gold films prepared by evaporation of ∼0.2 µm of Au onto a ∼20 nm Cr adhesion layer on Si(100) wafers. The substrates were cleaned prior to monolayer growth by exposure to ultraviolet light/ozone for ∼20 min, followed by rinsing with water that was treated to remove organic, ionic, and biological impurities (F ≈ 18.2 MΩ cm). Finally, the substrates were dried with a stream of ultrapure gaseous nitrogen. OPE, OPE-F, OPE-NH2, OPE-NO2, and OPE-bisNO2 were synthesized as previously described.46,99 Monolayers of OPE, OPE-F, OPE-NH2, OPE-NO2, and OPE-bisNO2 were grown from a ∼0.5 mM solution in a ratio of 2:1 (10 mL total volume) anhydrous CH2Cl2 to anhydrous C2H5OH.16,46,100 Monolayers of OPE-NO2 and OPE-bisNO2 were prepared from thioacetates and were deprotected with 120 µL of sulfuric acid prior to introduction of the substrate. Monolayers of OPE-F and OPENH2 were deprotected with NH4OH.5,16,100 Monolayer growth was carried out in an Ar-purged dry box ([O2] < 1 µmol/mol; [H2O] < 0.5 µmol/mol). The substrates were allowed to incubate for about a day. After removal from solution, the substrates were rinsed with copious amounts of CH2Cl2 in the dry box. Reflection-absorption infrared spectroscopy, contact angle measurements, and spectroscopic ellipsometry were acquired on samples grown concurrently with those used in the photoemission investigation.5,16,46,100 These data were consistent with results from previous reports of densely packed, ordered monolayers of similar molecules. Absorption spectra in the ultraviolet and visible range were also acquired for ∼10 µM solutions in CH4 using a commercial spectrophotometer. The thioacetate compounds were deprotected prior to measurement. Preparation of these solutions involved handling very small quantities of material in the glovebox, resulting in considerable uncertainty in the amount of compound going into solution. Repeated measurements suggest that the fractional standard deviation between measurements may be as great as 50%. c. Electronic Structure Characterization of Monolayers. Samples were mounted on a vacuum chuck immediately after being removed from the dry box. All measures were taken to reduce exposure of the monolayers to the ambient prior to being placed in vacuum. Each sample was measured after identical growth time, ambient exposure and handling, and vacuum conditions. The samples were placed in a load lock, pumped down, and held at ∼1 × 10-8 Torr (1 Torr ≈ 133 Pa) overnight. The next day, samples were moved into the analysis chamber. The base pressure of the analysis chamber was typically ∼1 × 10-9 Torr during the course of the experiments. Ultraviolet photoelectron spectra were obtained using a He(I) source (21.22 eV) in a commercial spectrometer also equipped with an X-ray source at room temperature. The light was incident at 45° and 54° from the surface normal for ultraviolet and X-ray photoemission, respectively. Photoelectrons were collected at normal emission with a hemispherical electrostatic analyzer.

(Oligo)Phenylene-ethynylenethiols Adsorbed on Gold Two quantities were extracted from the ultraviolet photoemission spectra: the work function and the binding energy of the molecular highest occupied π-state (EB,π). The work function of the monolayer covered surface (φmono) was determined by subtracting the width [half-maximum at the Fermi edge (EF) to the half-maximum of high-binding energy cutoff (EB,max)] of the photoemission spectrum from the source photon energy (hυ), that is, φmono ) hυ - (EB,max - EF). The work function of the bare gold (φAu) was determined in the same way. Bare samples were cleaned with ultraviolet light/ozone and were rinsed with water before being transferred into the vacuum; the samples were then sputtered until no contaminants could be detected using X-ray photoemission. Typically, φAu ≈ 5.2 eV. The work function shift is simply ∆φ ) φmono - φAu. The binding energy of the highest occupied π-state (EB,π) is the difference between EF and the centroid of the feature in the monolayer spectrum corresponding to the highest occupied orbital of the isolated molecule. The peak centroid was determined by fitting the appropriate energy interval with a Gaussian for the peak and a polynomial for the background. Each value of EB,π, φmono, and φAu is an average determined from multiple spectra recorded using two independently prepared samples. X-ray photoemission spectra (XPS) were acquired after collection of ultraviolet photoelectron data. Survey and highresolution spectra were acquired for each monolayer to collect elemental information, determine film quality, and ascertain monolayer coverage. Of particular importance was the position of the S 2p at ∼162.2 ( 0.2 eV, typical with the formation of a S-Au bond. No higher energy S 2p peaks were observed. Oxygen levels were OPE (> OPE-bisNO2) > OPE-NH2. Using unsubstituted OPE as the baseline, the larger (positive) values of µz for OPE-NO2 and OPE-F suggest that the acceptor substituents located in the center of the molecular structure strongly pull electron density from the conjugated end protruding above the central phenylene

(Oligo)Phenylene-ethynylenethiols Adsorbed on Gold

J. Phys. Chem. C, Vol. 112, No. 34, 2008 13223

Figure 9. Experimental versus theoretical estimate of ∆φ (eV) and least-squares fit of the data (line). Theoretical estimate includes summation of bond dipole and average surface and image potential energies for θ, φ ) 0, and (30°. Error bars represent one standard deviation of the data set.

Figure 10. Pictorial representation of the donor/acceptor biphenyl (left) and OPE (right) molecular systems, see Table 4 and Figure 11. The colored discs represent the π-conjugated backbone (purple), thiol linker (yellow), and placement of the donor/acceptor substituent (gray).

TABLE 4: B3LYP/6-31G**-derived x-, y-, and z-Components (µx, µy, and µz) and Total Dipole Moments (µ) for the OPE Structures and a Series 4′-Donor/Acceptor Substituted 4-Mercaptobiphenylsa π-backbone

substituent

µx

µy

OPE

biphenyl

µz

µ

bisNO2 NO2 F H NH2

0.94 -3.20 -0.10 0.91 2.54

0.00 0.00 0.00 0.00 0.00

0.29 1.54 1.37 0.69 0.14

0.98 3.55 1.37 1.14 2.55

NO2 CNb F Hb SHb NH2b

0.47 0.81 0.87 0.89 -0.49 0.21

0.67 0.02 0.01 0.00 0.65 0.82

-4.87 -4.58 -0.54 0.87 1.25 2.86

4.94 4.65 1.03 1.25 1.49 2.98

a All dipole moments are in Debye. See Figure 10 for orientation and Figure 11 for a graphical representation. b Substituents used in ref. 80.

ring, inducing the larger positive dipole moment along the z-direction and, therefore, larger (more negative) ∆φ, whereas the smaller (still positive) µz for OPE-NH2 reflects the electron donating ability of the amine substituent (although it is not strong

Figure 11. Graphical representation of the B3LYP/6-31G**-derived x- and z-components (µx and µz) and total dipole moments (µ) for the OPE structures (left) and series of 4′-donor/acceptor substituted 4-mercaptobiphenyls (right); all dipole moments are in Debye (see Table 4). Also depicted are the theoretical estimates of ∆φ (eV) for the OPE structures (triangles) calculated herein and 4′-donor/acceptor substituted 4-mercaptobiphenyls (circles) as reported in ref. 80. †Substituents used in ref. 80.

enough to change the direction of µz). These results for the singly substituted systems suggest that there indeed does exist a correlation between the strength of the donor/acceptor and ∆φ through µz (Figure 11), although the direction of ∆φ with the substitution may seem at first counterintuitive. We note that the µx components in the OPE structures follow the relative trends of the electron accepting (donating) strength for the singly substituted systems, with OPE-NO2 and OPE-F having a negative µx (with OPE-NO2 > OPE-F) and that of OPE-NH2 being positive; the positive µx values for OPE and OPE-bisNO2 are due to the asymmetric nature of the thiol end group, which has the hydrogen atom directed along the positive x-direction. These results also aid in addressing an apparent discrepancy with previous UPS investigations of alkylthiols versus fluorinatedalkylthiols;108 Alloway et al.108 showed that unsubstituted alkylthiols produce a much more significant change in ∆φ (-1.35 eV) than their fluorinated counterparts (∆φ ∼0.06 eV - 0.46 eV) and that the direction of ∆φ changes with fluorination. The results herein, however, show the opposite effect for both OPE-F and OPE-NO2 versus OPE; not only is the magnitude of ∆φ larger for these two acceptor-substituted systems (see Table 2 and Figure 9), but the values of ∆φ all point in the same direction (have the same sign). From the above analysis, however, we see that each system has a positive µz, with the magnitude following the relative acceptor strength of the substituent; hence, we obtain a larger ∆φ for each of these two acceptor-substituted systems that point in the same direction as the unsubstituted structure. Overall, these results point to a strong orientation dependence of the donor/acceptor substituent in the OPE series on both the relative magnitude and direction of ∆φ. We expect that future experimental and theoretical studies that probe these effects in a controlled manner could provide informative insight into fully managing ∆φ through precise adsorbate design. V. Conclusions We have presented here a joint experimental and computational investigation of the electronic structure and electrostatic properties of a series of donor- and acceptor-substituted OPEs on gold. Calculations show that the effect of chemisorption on

13224 J. Phys. Chem. C, Vol. 112, No. 34, 2008 the valence electronic structure is minimal, confirming experimental observations. The calculated densities of electronic states allow us to identify the higher-lying occupied states that may be of relevance for single molecule transport through these molecular wires. Simulated spectra agree well with measured spectra and guide assignment of the spectral features. Calculated estimates of the surface, bond dipole, and image potential energies correlate well with the measured change in work function for the self-assembled monolayers. The contribution of the charge asymmetry within the monolayer itself depends upon substitution and orientation, whereas the polarization along the Au-S bond contributes about -0.2 to -0.5 eV to the work function shift for all monolayers. The image potential energy is found to be a significant contributor to the work function shift when a polarizable functional group is attached to the molecule. We next must probe with more extensive slab models17,52,55,60,80–83,88 for these compounds to take into account molecular interactions with both a periodic metal and the molecular substrate to verify that our simple model systems are correct. In the future, special emphasis will need to be placed on the effects of substituent placement and molecular orientation to provide a more quantitative picture on the effects of the molecular contributions to the changes in the measured work function. Acknowledgment. The authors are appreciative to the reviewers of this manuscript; the insight they provided greatly added to the overall quality of the manuscript. C. R. would like to thank Gemma Solomon and Maxim Sukharev for invaluable discussions and Notker Ro¨sch for a detailed description of surface/image charge interactions. C. D. Z and R. D. v. Z would like to acknowledge Steve Robey for insight into the photoemission measurements. The work at Northwestern was supported by the NSF through the Northwestern University MRSEC (DMR-0520513), Chemistry Division of the NSF [CHE0719420], and by the Office of Naval Research [N00014-051-0766]. The work at Rice was supported by the Defense Advanced Research Projects Agency and Office of Naval Research. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (2) Joachim, C.; Ratner, M. A. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 8801. (3) Selzer, Y.; Allara, D. L. Annu. ReV. Phys. Chem. 2006, 57, 593. (4) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (5) Stapleton, J. J.; Harder, P.; Daniel, T. A.; Reinard, M. D.; Yao, Y.; Price, D. W.; Tour, J. M.; Allara, D. L. Langmuir 2003, 19, 8245. (6) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (7) Zhu, X. Y. Surf. Sci. Rep. 2004, 56, 1. (8) Kim, B.; Beebe, J. M.; Jun, Y.; Zhu, X. Y.; Frisbie, C. D. J. Am. Chem. Soc. 2006, 128, 4970. (9) McCreery, R. L. Chem. Mater. 2004, 16, 4477. (10) de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh, T.; Blom, P. W. M. AdV. Mater. 2005, 17, 621. (11) Qan, H.; Huang, Q.; Cui, J.; Veinot, J. G. C.; Kern, M. M.; Marks, T. J. AdV. Mater. 2003, 15, 835. (12) Donley, C. L.; Xia, W.; Minch, B. A.; Zangmeister, R. A. P.; Drager, A. S.; Nebesny, K.; O’Brien, D. F.; Armstrong, N. R. Langmuir 2003, 19, 6512. (13) Liu, F.; Meyer, G. Inorg. Chem. 2005, 44, 9303. (14) Benson, D. E.; Conrad, D. W.; Lorimier, R. M. d.; Trammell, S. A.; Hellinga, H. W. Science 2001, 293, 1641. (15) Ye, T.; Takami, T.; Wang, R.; Jiang, J.; Weiss, P. S. J. Am. Chem. Soc. 2006, 128, 10984.

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