ARTICLE pubs.acs.org/JPCC
Energy-Level Alignment in 40 -Substituted Stilbene-4-thiolate Self-Assembled Monolayers on Gold Michaz Malicki,*,†,|| Georg Heimel,*,‡ Ze-Lei Guan,§ Sieu D. Ha,§ Stephen Barlow,† Antoine Kahn,§ and Seth R. Marder† †
School of Chemistry and Biochemistry and Center for Organic Electronics and Photonics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ Institut f€ur Physik, Humboldt-Universit€at zu Berlin, Brook-Taylor-Strasse 6, D-12489 Berlin, Germany § Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, United States
bS Supporting Information ABSTRACT: Molecular energy-level alignment around the Fermi level in self-assembled monolayers of E-stilbene-4-thiolate (SAM1), E40 -(ethoxy)stilbene-4-thiolate (SAM2), and E-40 -(dimethylamino)stilbene-4-thiolate (SAM3) on Au(111) was studied by ultraviolet photoelectron spectroscopy. A comparison of the measured hole-injection barriers into SAM13 with the electrochemically estimated molecular ionization energies reveals that the influence of the 40 -substituent in the stilbene backbone on the hole-injection barrier is greatly suppressed. While predicted theoretically for a variety of densely packed π-conjugated thiolate monolayers before, we here provide conclusive experimental evidence for this phenomenon. The obtained results are compared to periodic density-functional theory calculations and are discussed in the light of electrostatic properties of dipolar monolayers.
’ INTRODUCTION The development of new organic materials as alternatives to traditional inorganic semiconductors for (opto-)electronic applications has been continuously accelerating in recent years. The major design principles regarding the electronic properties of organic electronic materials have been based on specific requirements for the energies of the relevant electronic levels of all components in an electronic device.13 For example, all other parameters being held constant, the efficiency of charge injection from a metallic electrode into an organic-semiconductor layer is increased if the Fermi level of the metal is matched with the energy of the chargetransport states of the organic material.1,2 In principle, this can be achieved by the proper choice of the metal,46 and/or by changing the electronic properties of the organic semiconductor.1,79 If one particular combination of organic semiconductor and metal is desirable, energy-level matching can still be achieved by changing the effective work function of the latter, for example, by UV/ozone treatment,1012 or by precoverage with (sub)monolayers of strong electron donor or acceptor molecules that undergo charge-transfer reactions with the underlying metal.1318 Perhaps a more versatile strategy to tune the effective work function of a surface is the use of self-assembled monolayers (SAMs).1922 The last approach has been facilitated by fundamental studies of electronic properties of molecular monolayers on a variety of metals.1 The fact that the work function of an electrode can be tuned by merely depositing an ultrathin film of organic molecules onto its surface highlights the r 2011 American Chemical Society
importance of interfacial phenomena in the context of electronics applications. While the judicious use of SAMs has already been shown to improve device characteristics,2022 our understanding of the influence of the molecular structure of the monolayer on the charge injection or collection processes at electrode/organicsemiconductor interfaces is incomplete. It has been shown that the rate of electron transfer through an organic SAM, a quantity related to electrical conductance,23,24 depends on the structure of the monolayer.2328 Both in resonant and in tunneling regimes, the alignment of the molecular electronic energy levels in the SAM with the Fermi level (EF) of a metallic electrode is thus an important parameter that governs the characteristics of charge transport through the interface.23,24 Consequently, a number of studies directed toward establishing the electronic energetics of organic molecules adsorbed on the surface of metals have been reported.2934 The energy-level alignment in π-conjugated thiols on gold is of particular interest for electronics applications due to the relatively high conductance of these systems as compared to, for example, nonconjugated alkane thiols.25,27 SAMs can be used to tune the work function of metal electrodes by creating surface dipoles through introduction of polar substituents into the SAM.20,35,36 Thus, especially if Received: December 15, 2010 Revised: February 11, 2011 Published: March 25, 2011 7487
dx.doi.org/10.1021/jp111900g | J. Phys. Chem. C 2011, 115, 7487–7495
The Journal of Physical Chemistry C π-conjugated thiols are to be used, one must gain an understanding of the influence of polar substituents on both the extent of the work-function modification and the molecular energy-level alignment of the SAM with the Fermi level of the electrode. While prior experimental studies have been concerned with, for example, the influence of side-group substitutions in the πconjugated backbone on the work function modification and the energy-level alignment29 and the impact of end-group substituents on the work-function modification only,20,35,37 the influence of polar end-group substituents in densely packed SAMs of π-conjugated thiols on the energy-level alignment has only been addressed theoretically so far.3842 These computational studies are intriguing insofar as they predict that, while the presence of polar π-donor and π-acceptor substituents at the far end of the molecular backbone, that is, on the top of the monolayer, has a significant impact on the work function of the sample and on the ionization potential of the SAM, the position of highest-energy molecule-derived occupied orbitals relative to EF is hardly affected at all in densely packed SAMs,3841 regardless of the backbone polarizability.42 This behavior is rather puzzling from the point of view of well-established molecular orbital theory, which predicts that the electronic coupling of π-donors and π-acceptors with a π-conjugated molecule should lead to significant changes in the energies of the frontier molecular orbitals.43 It is important to emphasize that, thus far, the computational results for densely packed monolayers have not been experimentally confirmed. To test these computational predictions, we conducted an experimental study in which we examined structureelectronic property relationships in densely packed SAMs of stilbene thiolates on Au(111) surfaces. The structural motifs of the ordered monolayers (Figure 1) were chosen to allow for investigation of the influence of polar π-donating end-group substituents on the relative energies of the highest-lying molecular orbitals and the metal Fermi energy, that is, the influence of the substituents on the hole-injection barrier from the gold electrode into the SAM. To this end, we used cyclic voltammetry (CV) measurements on the monolayer constituents in solution and ultraviolet photoelectron spectroscopy (UPS) measurements on the SAMs. While we find that the ionization energies of the isolated molecules depend significantly on the substituents (as expected), we confirm experimentally that their impact on the hole-injection barriers is strongly suppressed. Our findings are corroborated by periodic density-functional theory (DFT) calculations, and the reasons behind this chemically counterintuitive phenomenon are discussed in terms of the electrostatic characteristics of dipole layers.
’ EXPERIMENTAL SECTION The preparation and characterization of the compounds and SAMs (see Figure 1) were described in detail in ref 35. All solvents and chemicals were purchased and used without further purification apart from dichloromethane, which was dried by passing through columns of activated alumina in the same manner as that described in the literature.44 Atomically flat Au(111) on mica substrates acquired from Agilent were used for the UPS and scanning-tunneling microscopy (STM) studies. The substrates were delivered under an argon atmosphere, were unpacked in a glovebox, and placed directly into an ca. 0.5 mM ethanolic solution of the appropriate acetylthio-stilbene for 20 h.35 After that time, each sample was
ARTICLE
Figure 1. Schematic representations of the studied monolayers (SAM13) and S-methylated stilbene thiolates (M13).
washed copiously with ethanol, dried under a stream of nitrogen, and stored under nitrogen for later analysis. UPS measurements were performed in an ultrahigh vacuum system (base pressure 20 Å was introduced between periodic replica of the slab in the direction of the surface normal (z-direction) to suppress spurious electronic interaction. An artificial dipole layer in this vacuum region compensates for the net dipole of the asymmetric slab, thus preserving periodic boundary conditions in the potential in z-direction.49 Valencecore interactions were treated within the projector augmented-wave scheme,50,51 allowing for the low kinetic-energy cutoff of 20 Ryd for the plane-wave expansion of the valence KohnSham pseudo wave functions. An 8 5 1 MonkhorstPack grid of k-points52 was used for Brillouin-zone sampling together with a MethfesselPaxton smearing scheme (broadening 0.2 eV).53 The positions of all atoms in the SAM and the top two gold layers were relaxed until the remaining forces were
