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Experimental Determination of the Excited-State Polarizability and Dipole Moment in a Thin Organic Semiconductor Film Mary P. Steele, Michael L. Blumenfeld, and Oliver L.A. Monti* Department of Chemistry and Biochemistry, The University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721
ABSTRACT We report the evolution of the electronic structure of excited (sub)monolayer films of vanadyl naphthalocyanine (VONc) at the interface with highly oriented pyrolytic graphite (HOPG). Using two-photon photoemission spectroscopy, an unoccupied state corresponding to the lowest unoccupied molecular orbital of VONc is observed. The energy of this state shows a significant dependence on coverage, interpreted in the context of the electrostatic environment at the interface. On the basis of a simple electrostatic model, we were able to determine the excited-state polarizability and dipole moment of VONc at the interface with HOPG. The results suggest that local electric fields may have a major influence on interfacial energy level alignment in the excited-state manifold, with direct consequences for interfacial charge-transfer dynamics. SECTION Surfaces, Interfaces, Catalysis
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rganic semiconductor interfaces play a significant role in charge-transfer processes in organic lightemitting diodes, photovoltaic cells, and thin-film transistors. Often a considerable interface dipole is developed through, for example, partial charge transfer or exchange correlation interactions,1-4 resulting in large and generally short-range electric fields with direct impact on the interfacial energy level alignment. Governed by the ground-state molecular polarizability, these fields may induce additional dipole moments, thereby altering the interfacial and molecular electronic structure.5,6 Recent calculations by Linares et al. suggest that molecular multipoles and the significant molecular polarizability of many organic semiconductors may be responsible for the interface dipole at organic/organic heterojunctions,7 thus potentially providing a molecular basis for the “dark dipoles” thought to be responsible for facilitating exciton dissociation at interfaces.8 The presence of strong induced dipole moments may however not only lead to changes in the energy level alignment but also cause a noticeable nonclassical Stark effect:9 The rate of charge-transfer processes may also be strongly influenced by interfacial electrostatic fields. This suggests that interfacial charge transfer may be manipulated in a controlled fashion if electric fields can be deliberately and controllably modified. A first step in this direction has already been achieved using well-defined thin films of oriented dipolar molecules, where the response of the vacuum level and the molecular ground-state manifold have been explored.5,6,10 Much less is however known about the excited-state manifold. In the excited state, the molecular polarizability R is correlated with the exciton size, although the precise quantitative relationship is still not clear. B€ assler and co-workers used a
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simplified hydrogenic model for the exciton, excluding electron correlation and dielectric screening effects, to show that ð1Þ R ¼ 4r3XC where rXC is the exciton radius.11 While this relationship is not quantitatively correct, there is nonetheless an approximate linear relationship between exciton volume and polarizability as found, for example, in polymers.12,13 The excited-state polarizability informs therefore qualitatively on the exciton size, of critical importance to understanding charge-transfer characteristics. This suggests the importance of obtaining a quantitative assessment of excited-state polarizabilities at interfaces of organic semiconductors. While bulk groundand excited-state polarizabilities can be obtained from Stark spectroscopy in solution or glassy matrices,9,14-16 reports of interfacial excited-state polarizabilities and dipole moments have however not yet been available. Recently, the measurement of the ground-state polarizability and dipole moment at highly defined interfaces of oriented dipolar organic semiconductors was reported.5,6 The choice of dipolar molecules provides a path to measuring the polarizability in an internal field established by the dipolar molecules themselves. We report here for the first time measurements of the excited-state polarizability and dipole moment of an organic semiconductor at an electrode interface. We use twophoton photoemission (TPPE) of vanadyl naphthalocyanine (VONc), a near-planar heterocyclic molecule with a permanent Received Date: May 17, 2010 Accepted Date: June 8, 2010 Published on Web Date: June 14, 2010
2011
DOI: 10.1021/jz1006466 |J. Phys. Chem. Lett. 2010, 1, 2011–2016
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but reported elsewhere. A broad feature is clearly visible near 5 eV, partially overlapping with the secondary electron cutoff (SECO) at the lower photon energies used. The intensity of this feature shows that it appears to be moving out of resonance at photon energies above 4.3 eV. The shape of the feature and its width of 270 meV is uncharacteristically broad for annealed VONc/HOPG films, where spectral peaks are more commonly around 150 meV wide.5 Upon closer inspection, it reveals in fact the presence of two close-lying peaks labeled L0 and L1. Using two Gaussian features with widths of 180 (L0) and 150-170 meV (L1), a global fit of the TPPE spectra at all coverages and wavelengths investigated was able to identify the final state energies of both features reliably, despite a certain amount of spectral overlap between the two features. The result of these fits is also shown in Figure 1b. By measuring the slope of Efinal versus the excitation energy hν as shown in Figure 1c, L0 (m = 0.99(3)) and L1 (m = 0.99(3)) are identified as unoccupied states since Efinal = Ees þ hν - EF for unoccupied states. Here, Ees is the excited state intermediate state energy relative to the Fermi energy EF. Given a HOMO position of -0.81 eV and L0 and L1 energies of 0.98 and 1.12 eV versus EF, these peaks can be assigned to vertical Frenkel excitons of VONc. They are unlikely to correspond to molecular anion resonances because of the resonant character near 4.13 eV, typical for molecular excitations. As can be seen in Figure 1a, comparison with density functional theory (DFT) calculations indicates that L0 and L1 correspond to states with one electron promoted to the LUMO since no other states are predicted to be nearby. This yields a surface optical gap of 1.8 eV, in good agreement with the bulk optical gap of 1.5 eV.17 The origin of the two LUMO peaks L0 and L1 is at present unclear but may result from a surface-induced splitting of the degenerate LUMO.18 We note in passing that at the photon energies of >4 eVemployed here, the observed transition is not HOMO f LUMO but corresponds in fact to (HOMO-n) f LUMO, reflecting the substantial density of occupied states several eV below the HOMO. This is likely also the origin of the resonance enhancement at excitation energies below 4.3 eV. Shown in Figure 2 is a sequence of TPPE spectra for films ranging from 0 to 1 ML of VONc, acquired at an excitation energy of 4.43 eV and displayed in terms of Ees. The SECO at around Ees ≈ 0 eV shifts to progressively higher energies, indicating an increase of the vacuum level with coverage. This change arises from the increased potential step eΔV at the surface in the presence of an increased density of dipolar VONc with O atoms directed toward vacuum and is described quantitatively by the Topping model19 eμgs Fgs ð2Þ eΔV ¼ gs 3=2 E0 ð1 þ f~ Rzz Fgs Þ
Figure 1. (a) Molecular structure of VONc from density functional theory (DFT) and experimental and theoretical energy level diagrams for VONc and HOPG. The DFT results are referenced to the experimental HOMO in the 0 ML limit. H: HOMO; L: LUMO; Lþ1: LUMOþ1. (b) TPPE spectra of 1 ML VONc at different excitation energies. The colored curves represent global nonlinear leastsquares fits of the two features L0 and L1. The feature in the topmost spectrum labeled with * is the result of one-photon photoemission at this wavelength. (c) Photon dependence of final state energy of L0 and L1.
dipole moment perpendicular to the plane of the molecule (Figure 1a), to investigate the evolution of the interfacial excited-state electronic structure as a function of coverage on highly oriented pyrolytic graphite (HOPG). We show experimentally that changes in the energy level alignment at this interface can be used to obtain an experimental estimate of the vertical molecular polarizability and dipole moment component in the lowest unoccupied molecular orbital (LUMO) region. A simple electrostatic model is used to obtain Rzz in this excited state of VONc at the interface with HOPG. The procedure outlined here can be used in principle to determine these quantities for any observable dipolar interfacial excited state with a similar film structure. Figure 1b shows a sequence of TPPE spectra in terms of final state energy Efinal of an annealed 1 ML film of VONc acquired at photon energies ranging from 4.04 to 4.54 eV. In this film, VONc molecules reside on the HOPG surface predominantly in the oxygen-terminated “O up” orientation.5 Peaks arising from the highest occupied molecular orbital (HOMO), image states, and some highly excited unoccupied states are located at approximately 7.5 eV, not shown here
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where μgs is the ground-state molecular dipole moment at zero coverage, Fgs is the surface density of ground-state VONc, gs ɛ0 is the vacuum permittivity, R ~ zz is the zz-component of the ground-state polarizability tensor in units of m3, and f = 9.03 is a geometrical factor representing the adsorption structure of VONc.5 The slow secondary electrons are sensitive to the coverage-dependent potential step at distances of >10 Å in
2012
DOI: 10.1021/jz1006466 |J. Phys. Chem. Lett. 2010, 1, 2011–2016
pubs.acs.org/JPCL
Figure 3. (a) Thickness dependence of the L0 intermediate state energy and nonlinear least-squares fit by eq 3 in red, yielding R ~ es zz = 6.1(6) 10-28 m3 and μes = 2.7(1) D. (b) Near-field electrostatic model in the low-excitation density limit, with the potential measured as a function of coverage in the near field at 2.7 Å at an excited molecule (red) and at a ground-state molecule (blue).
Figure 2. Thickness-dependent TPPE spectra of L0 and L1 in the range of 0-1 ML, displayed in terms of the intermediate state energy Ees and acquired at 4.43 eV. The two gray lines serve to guide the eye.
polarizability and dipole moment of an interfacial dipolar organic semiconductor layer. A detailed measurement of the intermediate state energy of L0 as a function of coverage is shown in Figure 3a, showing an overall stabilization of this state by approximately 120 meV between 0 and 1 ML. This is larger than the observed HOMO shift, consistent with the notion that excited states typically have a higher polarizability. In the limit of low excitation probability, the intermediate state energies of the “O up” VONc, Ees up(Fgs), can be fit by a simple phenomenological expression as a function of coverage, in analogy to a related model of the ground-state electronic structure (see Supporting Information for the derivation)5 0 1 3=2 es R ~ f F μ gs zz es es gs @ es A Ees up ðFgs Þ ¼ ðEdown þ ΔEμ ¼ μ0 Þ þ ΔEμ ¼ μ0 gs 3=2 μgs 1 þ R ~ zz f Fgs
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the far-field of the surface. Note that eq 2 and the model developed below are predicated on the assumption that the intermolecular distance varies smoothly with coverage, a situation sometimes referred to as “2D gas”. This is indeed supported by spectroscopic findings for VONc/HOPG in the HOMO and vacuum level region,5 the vacuum level shift of TiOPc/HOPG,6 and spot profile analysis LEED of SnPc on Ag (111).20 Surprisingly, the intermediate state energy Ees of L0 and L1 decreases with coverage between 0 and 1 ML, in contrast to the common assumption that the molecular electronic structure is unaffected by the change in vacuum level, shifting therefore in the same direction as the vacuum level. We demonstrated recently that in the case of annealed VONc films, the occupied states shift to progressively higher binding energies relative to EF, in the opposite direction of the vacuum level but the same direction as the LUMO in Figure 2.5 A simple electrostatic model showed that in the near-field region at heights of
