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Classical ultraviolet photoelectron spectroscopy of polymers
Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 3–9
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Classical ultraviolet photoelectron spectroscopy of polymers W.R. Salaneck ∗ Department of Physics (IFM), Linköping University, 58183 Linköping, Sweden
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Article history: Available online 11 April 2009 Keywords: Ultraviolet photoelectron spectroscopy (UPS) Polymers, Hybrid interfaces
a b s t r a c t Although X-ray photoelectron spectroscopy of polymers was well established by Clark and coworkers in the 1970s, ultraviolet photoelectron spectroscopy of polymer films, was developed later. Previous to the 1970s, the first attempts to use ultraviolet light on polymer films took the form of appearance potential (valence band edge) measurements. Only some years later could the full valence band region of thin polymer films, including insulating polymers, semiconducting polymers and electrically conducting polymers. The development of what might be termed “classical ultraviolet photoelectron spectroscopy” of polymer films may be loosely based upon a variety of issues, including adapting thin polymer film technology to ultra high vacuum studies, the widespread use of helium resonance lamps for studies of solid surfaces, the combined advent of practical and sufficient theoretical–computational methods. The advent of, and the use of, easily available synchrotron radiation for multi-photon spectroscopies, nominally in the area of the near UV, is not included in the term “classical”. At the same time, electrically conducting polymers were discovered, leading to applications of the corresponding semiconducting polymers, which added technologically driven emphasis to this development of ultraviolet photoelectron spectroscopy for polymer materials. This paper traces a limited number of highlights in the evolution of ultraviolet photoelectron spectroscopy of polymers, from the 1970s through to 2008. Also, since this issue is dedicated to Prof. Kazuhiko Seki, who has been a friend and competitor for over two decades, the author relies on some of Prof. Seki’s earlier research, unpublished, on who-did-what-first. Prof. Seki’s own contributions to the field, however, are discussed in other articles in this issue. © 2009 Elsevier B.V. All rights reserved.
1. Introduction This article is written for the Special Issue of the Journal of Electron Spectroscopy and Related Phenomena, dedicated to Prof. K. Seki, who passed away on 30 June 2008. Prof. Seki, Kazuhiko-san, was always very careful to reference who-did-what-when in his publications. I rely on some private conversations we have had on the UPS of polymer films in the text below. Prof. Seki’s own scientific works [1,2], on the other hand, are discussed in other articles in this special issue. Starting with a glimpse of the historical perspective, the text represents an overview of the evolution of the “classical” ultraviolet photoelectron spectroscopy of polymers, that is, as carried out using in-house sources such as helium discharge lamps. Some parts of this overview were presented at the 54th Fujihara Seminar, organized by Prof. Seki in Hokkaido, Japan, September 2005 [3], and validated in the discussions. The general historical points of view are those of the author, and hopefully any omissions will be overlooked. The next few paragraphs below represent a re-worked and
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up-dated version of a portion of the report to the Fujihara Foundation. As discussed earlier [3], the state-of-the-art in organic semiconductors was reviewed and celebrated already in 1989 [4]. In a historical overview at that time, and on that occasion, it was recognized that photoelectric emission “of negative charges” from organic dyes was noted already in 1888 [5,6], and from anthracene layers already in 1906 [7,8], as discussed by Eley [9]. For condensed molecular solids, photoionization thresholds were measured as a means of determining the first ionization potential, in the 1950s [10–13], as discussed by Seki [14]. An early pioneer in the field was W.C. Price. See, for example [15]. It appears that in general ultraviolet photoelectron spectroscopy, or UPS, involving the energy resolution of photoemitted electrons into “spectra”, emerged in the 1960s, from Price and coworkers as well as, for example, Vilesov et al. [16], and Turner et al. [17]. A review of the evolution of Xray photoelectron spectroscopy from the 1940s through the 1970s is provided by K. Siegbahn, where in the development of Electron Spectroscopy for Chemical Applications (ESCA) is also discussed, in detail [18]. The ESCA of polymers has been reviewed by Clark [19]. This brief overview is divided into three parts: the beginnings of the full UPS of polymers; a more-or-less state-of-the-art of polymer
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UPS today; and one view of the evolution of “UPS” for molecular and polymer hybrid interfaces in the future. 2. Early full UPS polymers Because electrons generated by ultraviolet light (or lower frequencies) are of low kinetic energies, the elastic mean-free-paths (i.e., “escape depths”) are very short, on the order of 1–2 nm (see, for example, discussions in Ref. [20]). Thus non-electrically conducting materials, including condensed molecular solids and insulating polymers, must be studied in ultra-thin film form in order to avoid deleterious effects of so-called sample-charging; the build-up of positive charges on the surface of such insulating films [20]. For polymers, in thin film form, the earliest works involved appearance potential spectroscopy, looking mostly at the photoelectron emission edge as a function of photon energy [10–13]. Difficulties in obtaining a full ultraviolet photoelectron spectrum involved ultra-thin film sample preparation, and the combination of (ultra-) high vacuum requirements of UPS with “sample handling”, wherein a clean polymer surface could not easily be prepared in situ. These experimental requirements came together at the Xerox Webster Research Center in New York in the 1970s, under the guidance of C.B. Duke. According to Prof. Seki, probably the first UPS spectra of (pendant group) polymers [21,22] are those shown in Figs. 1 and 2. These measurements brought together a basic corporate knowledge in polymer science (thin film technology) and new instrumentation for photoelectron spectroscopy in UHV [23]. From the 1970s, the UPS of polymers, in ultra-thin film form, has evolved to be a tool of central importance in determining the electronic structure of polymers and recently especially hybrid interfaces. Initial electronic structure determinations were made possible through a close collaboration of theory and experiment in the same studies. This combination, in close cooperation, although intuitively obvious. . .after the fact. . .was the basis upon which the UPS of polymers became a key factor in determining the electronic structure of polymers for electronics in the future. Starting with the Duke–Salaneck collaborations [23–26] at Xerox in the 1970s, and
Fig. 2. The UPS spectrum of a thin solid film of polyvinylpyridine (top) is compared with the UPS spectra model molecules for polyvinylpyridine: a condensed molecular solid film of ethyl pyridine (second from top) as well as ethyl pyridine and pyridine in the gas phase [21,22]. The spectra are aligned at the vacuum level, rather than the Fermi level of the substrate. Comparison among the spectra enables an estimate of the intermolecular relaxation energy in the solid phase (and the polymer), as discussed in detail in Ref. [55].
continuing (with enhanced emphasis) with the Brédas–Salaneck collaborations [20,27]. Today, a large number of researchers today are involved in the same type of studies. The combined theoretical approach [28–30] represents the added value obtainable through interdisciplinary research (if we may define theoretical physics/chemistry and experimental physics as “different disciplines”). 3. Relative standard of UPS of polymers today
Fig. 1. The UPS spectrum of a thin solid film of polystyrene (top) is compared with the UPS spectra model molecules for polystyrene: a condensed molecular solid film of ethyl benzene (second from top) as well as ethyl benzene and benzene in the gas phase [21,22]. The spectra are aligned at the vacuum level, rather than the Fermi level of the substrate. Comparison among the spectra enables an estimate of the intermolecular relaxation energy in the solid phase (and the polymer), as discussed in detail in Ref. [55].
Space, and an acute fear of missing some important references, prevents the discussion of the full evolution of the UPS of polymers in this paper. However, an important step was the application of UPS to the (then) new electrically conducting organic polymers [25,31–34], and eventually, especially, to studies of the electronic structure of semiconducting polymers for applications in light emitting devices [3,20], and polymer electronics in general [3,27,35,36]. Although the use of synchrotron radiation is most certainly the way UPS is going today and in the future, especially involving wave-length dependent and even time resolved studies, what is illustrated below is an example of conventional UPS work, using light sources that are generally available to “everyone”. What might be termed “conventional UPS” of polymer (thin films) today normally refers to spectra taken in the home laboratories with “conventional” UV light sources, such as the well-developed helium resonance lamp. Two particularly useful lines, He I (21.2 eV photons) and He II (40.8 eV), are available. In addition, even though the He-lines are well separated in energy, monochromatization with a simple grating filter may be used to remove the He I, resulting in clean background for the He II spectra, as discussed in Ref. [34]. In the example chosen, a thin film of a precursor polymer for poly(p-phenylenevinylene), or PPV, was thermally converted in ultra-high vacuum (UHV) overnight, result-
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enables a detailed interpretation for the electronic structure of the polymer material [30,38]. 4. Hybrid interfaces studied with UPS
Fig. 3. The chemical structure of the repeat unit of poly(para-phenylenevinyl), or PPV.
ing in a pure, thin film (approximately 10 nm in thickness), with an ultra pure surface [37], the chemical structure of which is shown in Fig. 3. The UPS spectra shown in Fig. 4 were obtained with both He I (21.2 eV) and He II (40.8 eV) radiation [37], and compared with the results of quantum chemical calculations [38]. The lower panel exhibits the one-dimensional band structure of PPV calculated using the Valence Effective Hamiltonian (VEH) method developed largely by André and Brédas and coworkers [28,29,39]. The band structure is converted to an effective density-of-valence-states (DOVS) by counting the number of energy states-per-unit-energy versus energy (the VEH curve in the figure). Finally, the differences in intensities of the He I and He II spectra are caused by the differential cross-sections in the valance band for He I and He II light (not included in the VEH estimates) [20]. Although many such “conventional” UPS spectra are to be found in the literature today, we suggest that those of Fig. 4 are an early representation of the array of spectra in the literature, and still reflect the state-of-theart in resolution of thin, solid polymer films today. The very good correspondence between the structural features in the experimental spectra and those of the theoretical density-of-valence-states
Fig. 4. Upper panel: shown are the UPS spectra of PPV, thermally polymerized in situ (10−5 mPa), recorded for He I (21.2 eV) and He II (40.8 eV) light [56]. In addition, a density-of-valence-states curve, labeled VEH, generated from the data in the lower panel, is shown at the bottom of the upper panel. Lower panel: the one-dimensional band structure of PPV, based upon the repeat unit illustrated in Fig. 1, is shown, as momentum k versus energy, as computed using the Valence Effective Hamiltonian (VEH) method [56,57].
Although the sampling depth of UPS is quite small, on the order to a few nanometers [20], much progress has been made in interpreting spectra of hybrid interfaces, i.e., interfaces between metallic electrode materials and semiconducting polymers, by studying the polymer film over-layer on the metallic substrate [40]. Although full electronic structure determinations are not easily obtainable, details of band edge alignment and charge transfer at interfaces are available [3,41]. One, of many, examples, involving polymers, is illustrated below. Note that the case of organic molecular layers on clean metallic substrates has been one area of expertise of Prof. Seki [1]. These issues are of particular interest in applications involving light emitting diodes, field effect transistors and photovoltaic cells based on semiconducting and conducting organic molecules and polymers [41]. Thin film organic devices, in particular organic light emitting devices (o-LEDs) [42] contain multiple inorganic–organic and organic–organic interfaces [27], the properties of which largely influence device performance. Major efforts are focused on understanding and controlling the electronic properties of such interfaces. The representative inorganic–organic interface consists of a conductive electrode and an organic material, either a semiconducting polymer or a hole (or electron) transporting layer. At such interfaces, under certain conditions, the magnitude of the barrier for charge injection depends on the energy level alignment between the Fermi level of the electrode and the highest (lowest) occupied (unoccupied) states of the organic layer (the socalled Schottky–Mott limit) [43]. However, under other conditions, because of the formation of chemical bonds, charge transfer, or an electronic “push-back effect” [44], an interfacial dipole is formed which results in an offset of the respective vacuum levels (). A generalized energy level diagram [3,36,45] is shown in Fig. 5, where the label LUMO/CB means either the lowest unoccupied state in a molecular film, or the conduction band edge in a semiconducting polymer film. Likewise, HOMO/VB means the highest occupied state in a molecular film, or the valence band edge in a semiconducting polymer. Here we focus on research that has been carried out over the past decade on hybrid interfaces formed under “device conditions”, that is to say, under inert atmosphere or even in air, by spincoating, followed by drying in UHV (prior to spectroscopic investigations by, e.g., UPS and XPS) [3]. Hybrid interfaces formed, for example in UHV via molecular deposition on ultra clean metal surfaces, Prof. Seki’s expertise, exhibit somewhat different characteristics, and are not covered here [1,2,46]. However, some of the work reviewed here was carried out on hybrid interfaces formed by the vapor deposition of molecules on passivated substrates, i.e., the same “air exposed” or at least non-ultra-clean, metallic substrates comparable to those that occur in the spincoating cases. Some details about sample preparation and the main technique employed, ultraviolet photoelectron spectroscopy, or UPS, may be found in Refs. [3,20,27,47]. However, the parameters ˚S , IP and EVF are directly addressable by UPS. Note that IP is the “first ionization potential”, and is a materials parameter, which should not vary with the value of the ˚S of the substrate. It is preferable to plot the parameter EFVAC = IP − EVF of Fig. 6, which is the value that the work function of the organic over-layer would have if it was determined only by the position in energy of the Fermi level of the metallic substrate [47]. The understanding of the electronic structure of any one hybrid interface is achieved by studying a range of interfaces [48], varying the parameters in a systematic way. In the present cases, the substrate material, the “electrode”, was varied according to the work
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Fig. 5. The band edge alignment schemes for (a) generalized semiconductor–metal interface and (b) a semiconductor on a high work function electrode, are shown. In (c), the charge transfer (polaronic) pinning states are sketched [3,36,37,41,45,53,54].
function of the surface (˚S ), measured under the same conditions of cleanliness that apply to the devices in which the hybrid interface would occur [47]. In an ideal world, one would like to be able to vary the magnitude of ˚S in a continuously variable way. Unfortunately, this not being possible, it is necessary to use different materials as substrates, each one chosen for the specific value of ˚S . For each substrate, ultra-thin films (in the range of 5–100 nm in thickness) are deposited on the substrate, either by spincoating or by vapor deposition in vacuum. Each sample is then studied individually, characterizing the electronic structure, repeating to be certain, and then moving on to the next substrate material. Thus a set of data points, as discussed below, consists of the results of a series of separate experiments, each repeated several times to assure consistency and reproducibility. An early example of the behavior of the interface parameters, measured using UPS, as the substrate is varied (varying ˚S ), under spincoated films of poly(dioctyl fluorine), some times known as F8 or PFO, is shown in Fig. 6 [49]. Note that the measured value
Fig. 6. The position of the effective Fermi level of a semiconducting alkyl-substituted polyflourene (F8 or PFO), or EFVAC , on metallic electrodes of varying work function is shown for different film thicknesses [49]. The data follow very closely the Schottky–Mott line (unity slope) even for films up to over 100 nm, indicating the samples are in electro-chemical equilibrium. The range of the substrate work functions is, however, within the known Conduction Band–Valence Band (forbidden) energy gap of the semiconductor.
of EFVAC follows exactly the value of the substrate work function ˚S . This is the so-called Schottky–Mott behavior (or the Schottky–Mott limit). The fact that the behavior of the band edges at the interface corresponds to the flat band condition is seen in the lack of any thickness dependence. Several materials have been studied, with the same results as in Fig. 6, Schottky–Mott behavior over a range of work function that does not, however, span the forbidden energy gap, denoted here as the “band gap”. This is just the HOMO–LUMO gap in a condensed molecular solid, or the gap of the valence band and the conduction band in the case of a semiconducting polymer. A more interesting case occurs when the range of the work function of the substrate is larger than the band gap of the organic over-layer. Although the first work on this area was carried out by Tengstedt et al. [50,51], this case illustrated here follows a wider range of substrate work function [52]. The polymer, “APFO-Green1”, is an especially good film former. The chemical structure is shown as the inset in Fig. 7. Note the wide range of substrate work function in the figure. For the case of PFO, Fig. 6, the range of substrate work function ˚S was within the forbidden energy band gap. In the case of the data of Fig. 7, the range of values of ˚S is larger than the band
Fig. 7. The parameter EFVAC = IP − EVF of Fig. 6 is plotted versus the work function of a metallic electrode substrate, for the polymer for which the chemical structure is illustrated in the inset of the figure. Note that the range of the values of the work function of the substrate is larger than the band gap of the semiconducting polymer, leading to pinning of the substrate Fermi level at the polaronic states, near both the conduction band edge and the valence band edge [52].
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gap width, spanning from less than the EA and to more than the IP. Changing the work function of the substrate over the larger range would bring the Fermi level of the substrate either above the conduction band edge or below the valence band edge (or above the LUMO and below the HOMO). However, as seen in Fig. 7, the Fermi level becomes pinned at high and low values of the substrate work function. As the work function of the substrate becomes large, EF becomes pinned near the valence band edge (or the HOMO in a condensed molecular solid) and as ˚S gets smaller, EF becomes pinned near the conduction band edge (or LUMO). The pinning states are, however, not precisely at the band edges, but lie within the band gap [52]. The difference in the two pinning energies seen in Fig. 7, at 4.0 and 4.6 eV, is smaller than the optical band gap of the organic overlayer, the polymer “APFO-Green1”. The effects of pinning on the interfacial energy level alignments are illustrated for the semiconducting polymer case in Fig. 8 [52]. Note that as the work function of the substrate, ˚S , becomes larger, the Fermi level of the substrate becomes pinned at a charge transfer state that lies within the energy gap. As ˚S approaches the state in the semiconducting polymer where charge transfer can occur (in this case, from the substrate to the organic layer (polymer), electrons are transferred until a charge potential difference is set up that prevents further charge flow. This potential difference is seen in Figs. 5 and 8 as , the difference in the energies of the vacuum levels of the substrate and the organic layer at the interface. Simple charge transfer to the band edge is not expected to lead to localized states such that a pinning could occur. Therefore, the energy state in the semiconducting polymer is believed to be a positive polaron-like state, P+ . In certain cases, bipolaron states may be involved. Such a level of detail is still under investigation. Similarly, as the work function of the substrate, becomes smaller, and “tries to pass the conduction band edge”, another charge transfer state in the polymer layer is met. Electrons transferred from the substrate to the polymer now reside in the negative polaron state ˜ or possibly a bipolaron state under certain in the semiconductor, P, circumstances. The charge transfer results in a in the opposite direction, compared with the case of high ˚S , above. This situation is illustrated in the right hand panel of Fig. 8. Certain molecular materials may be spin coated on electrode substrates, or vapor-deposited on the same type of substrates. Similar properties are observed, but the charge transfer states in the organic layer will have slightly different character, because of localization, and other effects that may preclude the formation of polarons or bipolarons. A series of molecules, used as charge
Fig. 8. Pinning configurations for the data of Fig. 7, as the value of the substrate work function tries to get out of the band gap of the semiconductor. The value of the interface dipole generated by the charge transfer, , is illustrated [52].
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Fig. 9. The chemical structures of model molecules for vapor deposition on electrode substrates with varying values of the work function, for which the parameter EFVAC is shown in Fig. 10 [53,54]. The chemical names of (a)–(c) are given in the text.
transport layers in molecular electronics, were studied, both spin coated and vapor-deposited films, on a range of substrates with varying work functions [45,53,54]. The molecules, as labeled in Fig. 9, span wide range of values of the first ionization potential, denoted IP: (a) CBP (4,4 -N,N -dicarbazolyl-biphenyl), with an IP = 6.1 eV; (b) NPB (N,N -bis-(1-naphthyl)-N,N -diphenyl1-1,1biphenyl1-4,4 -diamine), with IP = 5.35 eV; and (c) m-MTDATA (4,4,4 -tris[3-methyl-phenyl(phenyl)amino]triphenylamine), for which IP = 5.0 eV. The molecules are illustrated in Fig. 9, while the pinning diagrams are shown in Fig. 10. Clearly, the same behavior is observed for molecular films as for semiconducting polymer films. These molecular materials have, however, very large values of the HOMO–LUMO gap. Therefore, since it is difficult to find electrode substrates with work functions low enough to pass the LUMO level in energy, the pinning point for low work function substrates was not observed in these studies. Also, the pinning features observed have obvious consequences for device behavior. Studies at DuPont Corporation in the USA have verified such behavior [54]. Finally, although the charge transfer states at the surface of the semiconducting polymers are believed to be polaron-like (or, possibly, bipolaron-like), the corresponding state in organic condensed molecular solids will be, of course, radical cations and anions. The question arises as to whether or not the transferred charges can actually be observed, and what is the binding energy relative to a band edge or molecular level. This question was addressed by carefully choosing a material system which would allow such observation. Since deposition of molecules can be controlled on a monolayer scale, and spincoating is not controllable on the required scale for this purpose, the system chosen was a fluorine-substituted TCNQ (i.e., F4 -TCNQ) vapor-deposited on a natural oxide of aluminum surface [45]. The molecular structure is shown in the inset of Fig. 11. First, a UPS spectrum of the lowest binding energy occupied states of the substrate is recorded, seen the lower long-dash curve in Fig. 11. The substrate has a very low density of states near the Fermi level, and thus the signal will not affect the observation of structure on the UPS of the monolayer molecular solid film. The UPS spectrum
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(density of states) of a multi-layer of F4 -TCNQ is shown as the upper short-dash line in Fig. 11, where the film is thick enough (∼10 monolayers) that, with the short electron escape depths encountered in UPS, the interface with the substrate is not seen in the spectrum. Finally, the solid curve shows the spectrum for approximately 1–2 monolayers of F4 -TCNQ, where the charge transfer that occurs at the interface maybe be seen in the extra peak at lowest binding energy. This is a direct observation of the interfacial charge transfer state. These spectra of Fig. 11 also show that the charge is localized at the interface and resides in the organic over-layer. In the original publication, the nature of the charge transfer state has been verified by both model studies of charge transfer to F4 -TCNQ from sodium atoms (sodium doping of F4 -TCNQ), and by model quantum chemical calculations. These separate studies also verify that the charge transfer is integer [45]. Detailed discussions of charge transfer states, in the weak coupling limit, are to be found elsewhere [36,41]. Acknowledgements
Fig. 10. The numerical value of the parameter EFVAC , for vapor-deposited ultra-thin films of the molecules of Fig. 9, is plotted versus the work function of the metallic substrates for a variety of substrates with a wide range of the magnitude of the work function. Note that since the HOMO–LUMO gap of these condensed molecular solid films is large, only the region of the HOMO could be reached in this study [36,53,54]. Substrates with sufficiently low work functions to reach the region of the LUMOs were not available.
WRS is grateful to Prof. M. Falman and especially to Asst. Prof. S. Braun, as well as the past and present members of the Division of Surface Physics and Chemistry, at the Department of Physics at Linköping University. The first ultraviolet photoelectron spectroscopy of polymers by WRS was carried out under the auspices of Dr. C.B. Duke at the Xerox Webster Research Center, NY, USA. The collaborations with J.L. Brédas on a combined theoretical–experimental approach (and with C.B. Duke, before the terminology “combined theoretical–experimental approach” was ever used) are gratefully acknowledged. Research in Linköping, during the course of the research reviewed above, was generally supported by many grants from the Swedish Science Foundation (Vetenskaps Rådet), the Swedish Foundation for Strategic Research and the Commission of the European Union. References
Fig. 11. The low-binding energy portions of UPS spectra for F-substituted TCNQ are shown. The long-dashed curve corresponds to the bare substrate (lightly oxidized aluminum) which has very little intensity in the low-binding energy region. The short-dashed curve is for a condensed molecular solid film of just sufficient thickness that electrons generated at the hybrid interface do not reach the surface and contribute to the UPS signal. The solid curve is for a condensed molecular film of nominally one monolayer thickness, where electrons generated at the interface reach the surface and contribute to the UPS signal. The extra intensity in the solid curve near 2 eV corresponds to the transferred charge at the interface that results in pinning of the Fermi level of the substrate at just 2 eV [45,54].
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W.R. Salaneck / Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 3–9 [25] C.B. Duke, A. Paton, W.R. Salaneck, H.R. Thomas, E.W. Plummer, A.J. Heeger, A.G. MacDiarmid, Chem. Phys. Lett. 59 (1978) 146. [26] C.B. Duke, J.W.-P. Lin, A. Paton, W.R. Salaneck, K.L. Yip, Chem. Phys. Lett. 61 (1979) 402. [27] W.R. Salaneck, K. Seki, A. Kahn, J.-J. Pireaux (Eds.), Conjugated Polymer and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Applications, Marcel Dekker, New York, 2002. [28] J.L. Brédas, R.R. Chance, R. Silbey, G. Nicolas, P. Durand, J. Chem. Phys. 75 (1981) 255. [29] J.L. Brédas, R.R. Chance, R. Silbey, Phys. Rev. B 26 (1982) 5843. [30] J.L. Brédas, Adv. Mater. 7 (1995) 263. [31] W.R. Salaneck, H.R. Thomas, C.B. Duke, A. Paton, E.W. Plummer, A.J. Heeger, A.G. MacDiarmid, J. Chem. Phys. 71 (1979) 2044–2050. [32] W.R. Salaneck, H.R. Thomas, R.W. Bigelow, C.B. Duke, E.W. Plummer, A.J. Heeger, A.G. MacDiarmid, J. Chem. Phys. 72 (1980) 3674–3678. [33] W.R. Salaneck, CRC Crit. Rev. Solid State Mater. Sci. 12 (1985) 267–296. [34] W.R. Salaneck, R. Bergman, J.-E. Sundgren, A. Rockett, T. Motooka, J.E. Greene, Surf. Sci. 198 (1988) 461–472. [35] N. Ueno, S. Kera, Prog. Surf. Sci. 83 (2008) 490–557. [36] S. Braun, W.R. Salaneck, M. Fahlman, Adv. Mat. 21 (2009) 1450. [37] A. Andersson, T. Kugler, M. Lögdlund, A.B.H.X. Li, W.R. Salaneck, Surf. Interf. Anal. 28 (1999) 186. [38] M. Lögdlund, W.R. Salaneck, F. Meyers, J.L. Brédas, G. Arbuckle, R.H. Friend, A.B. Holmes, G. Froyer, Macromolecules 26 (1993) 3815. [39] J.M. André, J. Dekhalle, J.L. Brédas, Quantum Chemistry Aided. Design of Organic Polymers, World Scientific, Singapore, 1991. [40] J.L. Brédas, W.R. Salaneck, G. Wegner (Eds.), Organic Materials for Electronics: Conjugated polymer interfaces with metals and semiconductors, North Holland, Amsterdam, 1994. [41] M. Fahlman, A. Crispin, X. Crispin, S.K.M. Henze, M.P.D. Jong, W. Osikowicz, C. Tengstedt, W.R. Salaneck, J. Phys.: Condens. Mater. 19 (2007) 183202.
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[42] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A.D. Santos, J.L. Brédas, M. Lögdgdlund, W.R. Salaneck, Nature 397 (1999) 121–128. [43] N. Mott, Proc. Camb. Phil. Soc. 34 (1938) 568. [44] X. Crispin, V.M. Geskin, A. Crispin, J. Cornil, R. Lazzaroni, W.R. Salaneck, J.L. Brédas, JACS 124 (2002) 8132. [45] S. Braun, W.R. Salaneck, Chem. Phys. Lett. 438 (2007) 259. [46] C. Shen, A. Kahn, I. Hill, in: W.R. Salaneck, K. Seki, A. Kahn, J.J. Pireaux (Eds.), Conjugated Polymer and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Application, Marcel Dekker, New York, 2002. [47] T. Kugler, W.R. Salaneck, H. Rost, A.B. Holmes, Chem. Phys. Lett. 310 (1999) 391. [48] R.J. Murdey, W.R. Salaneck, Jpn. J. Appl. Phys. 44 (2005) 3751. [49] G. Greczynski, M. Fahlman, W.R. Salaneck, Chem. Phys. Lett. 321 (2000) 379. [50] C. Tengstedt, A. Crispin, C.-H. Hsu, C. Zhang, I.D. Parker, W.R. Salanec, M. Fahlman, Org. Electron 6 (2005) 21. [51] C. Tengstedt, W. Osikowicz, W.R. Salaneck, I.D. Parker, C.-H. Hsu, M. Fahlman, Appl. Phys. Lett. 88 (2006) 053502. [52] A. Crispin, X. Crispin, M. Fahlman, M. Berggren, W.R. Salaneck, Appl. Phys. Lett. 89 (2006) 213503. [53] S. Braun, M.P.d. Jong, W. Osikowicz, W.R. Salaneck, Appl. Phys. Lett. 91 (2007) 202108. [54] S. Braun, W. Osikowicz, Y. Wang, W.R. Salaneck, Org. Electron 8 (2007) 14. [55] C.B. Duke, W.R. Salaneck, T.J. Fabish, J.J. Ritsko, H.R. Thomas, A. Paton, Phys. Rev. B18 (1978) 5717. [56] N. Johansson, W.R. Salaneck, in: H.I. Starnberg, H.P. Hughes (Eds.), Electron Spectroscopies Applied to Low-dimensional Systems, Kluwer Academic Publishers, Drodrecht, 1999. [57] M. Lögdlund, P. Dannetun, W.R. Salaneck, in: T. Skotheim, J.R. Reynolds, R.L. Elsenbaumer (Eds.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1996.
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Classical ultraviolet photoelectron spectroscopy of polymers W.R. Salaneck ∗ Department of Physics (IFM), Linköping University, 58183 Linköping, Sweden
a r t i c l e
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Article history: Available online 11 April 2009 Keywords: Ultraviolet photoelectron spectroscopy (UPS) Polymers, Hybrid interfaces
a b s t r a c t Although X-ray photoelectron spectroscopy of polymers was well established by Clark and coworkers in the 1970s, ultraviolet photoelectron spectroscopy of polymer films, was developed later. Previous to the 1970s, the first attempts to use ultraviolet light on polymer films took the form of appearance potential (valence band edge) measurements. Only some years later could the full valence band region of thin polymer films, including insulating polymers, semiconducting polymers and electrically conducting polymers. The development of what might be termed “classical ultraviolet photoelectron spectroscopy” of polymer films may be loosely based upon a variety of issues, including adapting thin polymer film technology to ultra high vacuum studies, the widespread use of helium resonance lamps for studies of solid surfaces, the combined advent of practical and sufficient theoretical–computational methods. The advent of, and the use of, easily available synchrotron radiation for multi-photon spectroscopies, nominally in the area of the near UV, is not included in the term “classical”. At the same time, electrically conducting polymers were discovered, leading to applications of the corresponding semiconducting polymers, which added technologically driven emphasis to this development of ultraviolet photoelectron spectroscopy for polymer materials. This paper traces a limited number of highlights in the evolution of ultraviolet photoelectron spectroscopy of polymers, from the 1970s through to 2008. Also, since this issue is dedicated to Prof. Kazuhiko Seki, who has been a friend and competitor for over two decades, the author relies on some of Prof. Seki’s earlier research, unpublished, on who-did-what-first. Prof. Seki’s own contributions to the field, however, are discussed in other articles in this issue. © 2009 Elsevier B.V. All rights reserved.
1. Introduction This article is written for the Special Issue of the Journal of Electron Spectroscopy and Related Phenomena, dedicated to Prof. K. Seki, who passed away on 30 June 2008. Prof. Seki, Kazuhiko-san, was always very careful to reference who-did-what-when in his publications. I rely on some private conversations we have had on the UPS of polymer films in the text below. Prof. Seki’s own scientific works [1,2], on the other hand, are discussed in other articles in this special issue. Starting with a glimpse of the historical perspective, the text represents an overview of the evolution of the “classical” ultraviolet photoelectron spectroscopy of polymers, that is, as carried out using in-house sources such as helium discharge lamps. Some parts of this overview were presented at the 54th Fujihara Seminar, organized by Prof. Seki in Hokkaido, Japan, September 2005 [3], and validated in the discussions. The general historical points of view are those of the author, and hopefully any omissions will be overlooked. The next few paragraphs below represent a re-worked and
∗ Tel.: +46 013 28 12 46; fax: +46 013 28 89 69. E-mail address: [email protected]. 0368-2048/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2009.03.024
up-dated version of a portion of the report to the Fujihara Foundation. As discussed earlier [3], the state-of-the-art in organic semiconductors was reviewed and celebrated already in 1989 [4]. In a historical overview at that time, and on that occasion, it was recognized that photoelectric emission “of negative charges” from organic dyes was noted already in 1888 [5,6], and from anthracene layers already in 1906 [7,8], as discussed by Eley [9]. For condensed molecular solids, photoionization thresholds were measured as a means of determining the first ionization potential, in the 1950s [10–13], as discussed by Seki [14]. An early pioneer in the field was W.C. Price. See, for example [15]. It appears that in general ultraviolet photoelectron spectroscopy, or UPS, involving the energy resolution of photoemitted electrons into “spectra”, emerged in the 1960s, from Price and coworkers as well as, for example, Vilesov et al. [16], and Turner et al. [17]. A review of the evolution of Xray photoelectron spectroscopy from the 1940s through the 1970s is provided by K. Siegbahn, where in the development of Electron Spectroscopy for Chemical Applications (ESCA) is also discussed, in detail [18]. The ESCA of polymers has been reviewed by Clark [19]. This brief overview is divided into three parts: the beginnings of the full UPS of polymers; a more-or-less state-of-the-art of polymer
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UPS today; and one view of the evolution of “UPS” for molecular and polymer hybrid interfaces in the future. 2. Early full UPS polymers Because electrons generated by ultraviolet light (or lower frequencies) are of low kinetic energies, the elastic mean-free-paths (i.e., “escape depths”) are very short, on the order of 1–2 nm (see, for example, discussions in Ref. [20]). Thus non-electrically conducting materials, including condensed molecular solids and insulating polymers, must be studied in ultra-thin film form in order to avoid deleterious effects of so-called sample-charging; the build-up of positive charges on the surface of such insulating films [20]. For polymers, in thin film form, the earliest works involved appearance potential spectroscopy, looking mostly at the photoelectron emission edge as a function of photon energy [10–13]. Difficulties in obtaining a full ultraviolet photoelectron spectrum involved ultra-thin film sample preparation, and the combination of (ultra-) high vacuum requirements of UPS with “sample handling”, wherein a clean polymer surface could not easily be prepared in situ. These experimental requirements came together at the Xerox Webster Research Center in New York in the 1970s, under the guidance of C.B. Duke. According to Prof. Seki, probably the first UPS spectra of (pendant group) polymers [21,22] are those shown in Figs. 1 and 2. These measurements brought together a basic corporate knowledge in polymer science (thin film technology) and new instrumentation for photoelectron spectroscopy in UHV [23]. From the 1970s, the UPS of polymers, in ultra-thin film form, has evolved to be a tool of central importance in determining the electronic structure of polymers and recently especially hybrid interfaces. Initial electronic structure determinations were made possible through a close collaboration of theory and experiment in the same studies. This combination, in close cooperation, although intuitively obvious. . .after the fact. . .was the basis upon which the UPS of polymers became a key factor in determining the electronic structure of polymers for electronics in the future. Starting with the Duke–Salaneck collaborations [23–26] at Xerox in the 1970s, and
Fig. 2. The UPS spectrum of a thin solid film of polyvinylpyridine (top) is compared with the UPS spectra model molecules for polyvinylpyridine: a condensed molecular solid film of ethyl pyridine (second from top) as well as ethyl pyridine and pyridine in the gas phase [21,22]. The spectra are aligned at the vacuum level, rather than the Fermi level of the substrate. Comparison among the spectra enables an estimate of the intermolecular relaxation energy in the solid phase (and the polymer), as discussed in detail in Ref. [55].
continuing (with enhanced emphasis) with the Brédas–Salaneck collaborations [20,27]. Today, a large number of researchers today are involved in the same type of studies. The combined theoretical approach [28–30] represents the added value obtainable through interdisciplinary research (if we may define theoretical physics/chemistry and experimental physics as “different disciplines”). 3. Relative standard of UPS of polymers today
Fig. 1. The UPS spectrum of a thin solid film of polystyrene (top) is compared with the UPS spectra model molecules for polystyrene: a condensed molecular solid film of ethyl benzene (second from top) as well as ethyl benzene and benzene in the gas phase [21,22]. The spectra are aligned at the vacuum level, rather than the Fermi level of the substrate. Comparison among the spectra enables an estimate of the intermolecular relaxation energy in the solid phase (and the polymer), as discussed in detail in Ref. [55].
Space, and an acute fear of missing some important references, prevents the discussion of the full evolution of the UPS of polymers in this paper. However, an important step was the application of UPS to the (then) new electrically conducting organic polymers [25,31–34], and eventually, especially, to studies of the electronic structure of semiconducting polymers for applications in light emitting devices [3,20], and polymer electronics in general [3,27,35,36]. Although the use of synchrotron radiation is most certainly the way UPS is going today and in the future, especially involving wave-length dependent and even time resolved studies, what is illustrated below is an example of conventional UPS work, using light sources that are generally available to “everyone”. What might be termed “conventional UPS” of polymer (thin films) today normally refers to spectra taken in the home laboratories with “conventional” UV light sources, such as the well-developed helium resonance lamp. Two particularly useful lines, He I (21.2 eV photons) and He II (40.8 eV), are available. In addition, even though the He-lines are well separated in energy, monochromatization with a simple grating filter may be used to remove the He I, resulting in clean background for the He II spectra, as discussed in Ref. [34]. In the example chosen, a thin film of a precursor polymer for poly(p-phenylenevinylene), or PPV, was thermally converted in ultra-high vacuum (UHV) overnight, result-
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enables a detailed interpretation for the electronic structure of the polymer material [30,38]. 4. Hybrid interfaces studied with UPS
Fig. 3. The chemical structure of the repeat unit of poly(para-phenylenevinyl), or PPV.
ing in a pure, thin film (approximately 10 nm in thickness), with an ultra pure surface [37], the chemical structure of which is shown in Fig. 3. The UPS spectra shown in Fig. 4 were obtained with both He I (21.2 eV) and He II (40.8 eV) radiation [37], and compared with the results of quantum chemical calculations [38]. The lower panel exhibits the one-dimensional band structure of PPV calculated using the Valence Effective Hamiltonian (VEH) method developed largely by André and Brédas and coworkers [28,29,39]. The band structure is converted to an effective density-of-valence-states (DOVS) by counting the number of energy states-per-unit-energy versus energy (the VEH curve in the figure). Finally, the differences in intensities of the He I and He II spectra are caused by the differential cross-sections in the valance band for He I and He II light (not included in the VEH estimates) [20]. Although many such “conventional” UPS spectra are to be found in the literature today, we suggest that those of Fig. 4 are an early representation of the array of spectra in the literature, and still reflect the state-of-theart in resolution of thin, solid polymer films today. The very good correspondence between the structural features in the experimental spectra and those of the theoretical density-of-valence-states
Fig. 4. Upper panel: shown are the UPS spectra of PPV, thermally polymerized in situ (10−5 mPa), recorded for He I (21.2 eV) and He II (40.8 eV) light [56]. In addition, a density-of-valence-states curve, labeled VEH, generated from the data in the lower panel, is shown at the bottom of the upper panel. Lower panel: the one-dimensional band structure of PPV, based upon the repeat unit illustrated in Fig. 1, is shown, as momentum k versus energy, as computed using the Valence Effective Hamiltonian (VEH) method [56,57].
Although the sampling depth of UPS is quite small, on the order to a few nanometers [20], much progress has been made in interpreting spectra of hybrid interfaces, i.e., interfaces between metallic electrode materials and semiconducting polymers, by studying the polymer film over-layer on the metallic substrate [40]. Although full electronic structure determinations are not easily obtainable, details of band edge alignment and charge transfer at interfaces are available [3,41]. One, of many, examples, involving polymers, is illustrated below. Note that the case of organic molecular layers on clean metallic substrates has been one area of expertise of Prof. Seki [1]. These issues are of particular interest in applications involving light emitting diodes, field effect transistors and photovoltaic cells based on semiconducting and conducting organic molecules and polymers [41]. Thin film organic devices, in particular organic light emitting devices (o-LEDs) [42] contain multiple inorganic–organic and organic–organic interfaces [27], the properties of which largely influence device performance. Major efforts are focused on understanding and controlling the electronic properties of such interfaces. The representative inorganic–organic interface consists of a conductive electrode and an organic material, either a semiconducting polymer or a hole (or electron) transporting layer. At such interfaces, under certain conditions, the magnitude of the barrier for charge injection depends on the energy level alignment between the Fermi level of the electrode and the highest (lowest) occupied (unoccupied) states of the organic layer (the socalled Schottky–Mott limit) [43]. However, under other conditions, because of the formation of chemical bonds, charge transfer, or an electronic “push-back effect” [44], an interfacial dipole is formed which results in an offset of the respective vacuum levels (). A generalized energy level diagram [3,36,45] is shown in Fig. 5, where the label LUMO/CB means either the lowest unoccupied state in a molecular film, or the conduction band edge in a semiconducting polymer film. Likewise, HOMO/VB means the highest occupied state in a molecular film, or the valence band edge in a semiconducting polymer. Here we focus on research that has been carried out over the past decade on hybrid interfaces formed under “device conditions”, that is to say, under inert atmosphere or even in air, by spincoating, followed by drying in UHV (prior to spectroscopic investigations by, e.g., UPS and XPS) [3]. Hybrid interfaces formed, for example in UHV via molecular deposition on ultra clean metal surfaces, Prof. Seki’s expertise, exhibit somewhat different characteristics, and are not covered here [1,2,46]. However, some of the work reviewed here was carried out on hybrid interfaces formed by the vapor deposition of molecules on passivated substrates, i.e., the same “air exposed” or at least non-ultra-clean, metallic substrates comparable to those that occur in the spincoating cases. Some details about sample preparation and the main technique employed, ultraviolet photoelectron spectroscopy, or UPS, may be found in Refs. [3,20,27,47]. However, the parameters ˚S , IP and EVF are directly addressable by UPS. Note that IP is the “first ionization potential”, and is a materials parameter, which should not vary with the value of the ˚S of the substrate. It is preferable to plot the parameter EFVAC = IP − EVF of Fig. 6, which is the value that the work function of the organic over-layer would have if it was determined only by the position in energy of the Fermi level of the metallic substrate [47]. The understanding of the electronic structure of any one hybrid interface is achieved by studying a range of interfaces [48], varying the parameters in a systematic way. In the present cases, the substrate material, the “electrode”, was varied according to the work
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Fig. 5. The band edge alignment schemes for (a) generalized semiconductor–metal interface and (b) a semiconductor on a high work function electrode, are shown. In (c), the charge transfer (polaronic) pinning states are sketched [3,36,37,41,45,53,54].
function of the surface (˚S ), measured under the same conditions of cleanliness that apply to the devices in which the hybrid interface would occur [47]. In an ideal world, one would like to be able to vary the magnitude of ˚S in a continuously variable way. Unfortunately, this not being possible, it is necessary to use different materials as substrates, each one chosen for the specific value of ˚S . For each substrate, ultra-thin films (in the range of 5–100 nm in thickness) are deposited on the substrate, either by spincoating or by vapor deposition in vacuum. Each sample is then studied individually, characterizing the electronic structure, repeating to be certain, and then moving on to the next substrate material. Thus a set of data points, as discussed below, consists of the results of a series of separate experiments, each repeated several times to assure consistency and reproducibility. An early example of the behavior of the interface parameters, measured using UPS, as the substrate is varied (varying ˚S ), under spincoated films of poly(dioctyl fluorine), some times known as F8 or PFO, is shown in Fig. 6 [49]. Note that the measured value
Fig. 6. The position of the effective Fermi level of a semiconducting alkyl-substituted polyflourene (F8 or PFO), or EFVAC , on metallic electrodes of varying work function is shown for different film thicknesses [49]. The data follow very closely the Schottky–Mott line (unity slope) even for films up to over 100 nm, indicating the samples are in electro-chemical equilibrium. The range of the substrate work functions is, however, within the known Conduction Band–Valence Band (forbidden) energy gap of the semiconductor.
of EFVAC follows exactly the value of the substrate work function ˚S . This is the so-called Schottky–Mott behavior (or the Schottky–Mott limit). The fact that the behavior of the band edges at the interface corresponds to the flat band condition is seen in the lack of any thickness dependence. Several materials have been studied, with the same results as in Fig. 6, Schottky–Mott behavior over a range of work function that does not, however, span the forbidden energy gap, denoted here as the “band gap”. This is just the HOMO–LUMO gap in a condensed molecular solid, or the gap of the valence band and the conduction band in the case of a semiconducting polymer. A more interesting case occurs when the range of the work function of the substrate is larger than the band gap of the organic over-layer. Although the first work on this area was carried out by Tengstedt et al. [50,51], this case illustrated here follows a wider range of substrate work function [52]. The polymer, “APFO-Green1”, is an especially good film former. The chemical structure is shown as the inset in Fig. 7. Note the wide range of substrate work function in the figure. For the case of PFO, Fig. 6, the range of substrate work function ˚S was within the forbidden energy band gap. In the case of the data of Fig. 7, the range of values of ˚S is larger than the band
Fig. 7. The parameter EFVAC = IP − EVF of Fig. 6 is plotted versus the work function of a metallic electrode substrate, for the polymer for which the chemical structure is illustrated in the inset of the figure. Note that the range of the values of the work function of the substrate is larger than the band gap of the semiconducting polymer, leading to pinning of the substrate Fermi level at the polaronic states, near both the conduction band edge and the valence band edge [52].
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gap width, spanning from less than the EA and to more than the IP. Changing the work function of the substrate over the larger range would bring the Fermi level of the substrate either above the conduction band edge or below the valence band edge (or above the LUMO and below the HOMO). However, as seen in Fig. 7, the Fermi level becomes pinned at high and low values of the substrate work function. As the work function of the substrate becomes large, EF becomes pinned near the valence band edge (or the HOMO in a condensed molecular solid) and as ˚S gets smaller, EF becomes pinned near the conduction band edge (or LUMO). The pinning states are, however, not precisely at the band edges, but lie within the band gap [52]. The difference in the two pinning energies seen in Fig. 7, at 4.0 and 4.6 eV, is smaller than the optical band gap of the organic overlayer, the polymer “APFO-Green1”. The effects of pinning on the interfacial energy level alignments are illustrated for the semiconducting polymer case in Fig. 8 [52]. Note that as the work function of the substrate, ˚S , becomes larger, the Fermi level of the substrate becomes pinned at a charge transfer state that lies within the energy gap. As ˚S approaches the state in the semiconducting polymer where charge transfer can occur (in this case, from the substrate to the organic layer (polymer), electrons are transferred until a charge potential difference is set up that prevents further charge flow. This potential difference is seen in Figs. 5 and 8 as , the difference in the energies of the vacuum levels of the substrate and the organic layer at the interface. Simple charge transfer to the band edge is not expected to lead to localized states such that a pinning could occur. Therefore, the energy state in the semiconducting polymer is believed to be a positive polaron-like state, P+ . In certain cases, bipolaron states may be involved. Such a level of detail is still under investigation. Similarly, as the work function of the substrate, becomes smaller, and “tries to pass the conduction band edge”, another charge transfer state in the polymer layer is met. Electrons transferred from the substrate to the polymer now reside in the negative polaron state ˜ or possibly a bipolaron state under certain in the semiconductor, P, circumstances. The charge transfer results in a in the opposite direction, compared with the case of high ˚S , above. This situation is illustrated in the right hand panel of Fig. 8. Certain molecular materials may be spin coated on electrode substrates, or vapor-deposited on the same type of substrates. Similar properties are observed, but the charge transfer states in the organic layer will have slightly different character, because of localization, and other effects that may preclude the formation of polarons or bipolarons. A series of molecules, used as charge
Fig. 8. Pinning configurations for the data of Fig. 7, as the value of the substrate work function tries to get out of the band gap of the semiconductor. The value of the interface dipole generated by the charge transfer, , is illustrated [52].
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Fig. 9. The chemical structures of model molecules for vapor deposition on electrode substrates with varying values of the work function, for which the parameter EFVAC is shown in Fig. 10 [53,54]. The chemical names of (a)–(c) are given in the text.
transport layers in molecular electronics, were studied, both spin coated and vapor-deposited films, on a range of substrates with varying work functions [45,53,54]. The molecules, as labeled in Fig. 9, span wide range of values of the first ionization potential, denoted IP: (a) CBP (4,4 -N,N -dicarbazolyl-biphenyl), with an IP = 6.1 eV; (b) NPB (N,N -bis-(1-naphthyl)-N,N -diphenyl1-1,1biphenyl1-4,4 -diamine), with IP = 5.35 eV; and (c) m-MTDATA (4,4,4 -tris[3-methyl-phenyl(phenyl)amino]triphenylamine), for which IP = 5.0 eV. The molecules are illustrated in Fig. 9, while the pinning diagrams are shown in Fig. 10. Clearly, the same behavior is observed for molecular films as for semiconducting polymer films. These molecular materials have, however, very large values of the HOMO–LUMO gap. Therefore, since it is difficult to find electrode substrates with work functions low enough to pass the LUMO level in energy, the pinning point for low work function substrates was not observed in these studies. Also, the pinning features observed have obvious consequences for device behavior. Studies at DuPont Corporation in the USA have verified such behavior [54]. Finally, although the charge transfer states at the surface of the semiconducting polymers are believed to be polaron-like (or, possibly, bipolaron-like), the corresponding state in organic condensed molecular solids will be, of course, radical cations and anions. The question arises as to whether or not the transferred charges can actually be observed, and what is the binding energy relative to a band edge or molecular level. This question was addressed by carefully choosing a material system which would allow such observation. Since deposition of molecules can be controlled on a monolayer scale, and spincoating is not controllable on the required scale for this purpose, the system chosen was a fluorine-substituted TCNQ (i.e., F4 -TCNQ) vapor-deposited on a natural oxide of aluminum surface [45]. The molecular structure is shown in the inset of Fig. 11. First, a UPS spectrum of the lowest binding energy occupied states of the substrate is recorded, seen the lower long-dash curve in Fig. 11. The substrate has a very low density of states near the Fermi level, and thus the signal will not affect the observation of structure on the UPS of the monolayer molecular solid film. The UPS spectrum
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W.R. Salaneck / Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 3–9
(density of states) of a multi-layer of F4 -TCNQ is shown as the upper short-dash line in Fig. 11, where the film is thick enough (∼10 monolayers) that, with the short electron escape depths encountered in UPS, the interface with the substrate is not seen in the spectrum. Finally, the solid curve shows the spectrum for approximately 1–2 monolayers of F4 -TCNQ, where the charge transfer that occurs at the interface maybe be seen in the extra peak at lowest binding energy. This is a direct observation of the interfacial charge transfer state. These spectra of Fig. 11 also show that the charge is localized at the interface and resides in the organic over-layer. In the original publication, the nature of the charge transfer state has been verified by both model studies of charge transfer to F4 -TCNQ from sodium atoms (sodium doping of F4 -TCNQ), and by model quantum chemical calculations. These separate studies also verify that the charge transfer is integer [45]. Detailed discussions of charge transfer states, in the weak coupling limit, are to be found elsewhere [36,41]. Acknowledgements
Fig. 10. The numerical value of the parameter EFVAC , for vapor-deposited ultra-thin films of the molecules of Fig. 9, is plotted versus the work function of the metallic substrates for a variety of substrates with a wide range of the magnitude of the work function. Note that since the HOMO–LUMO gap of these condensed molecular solid films is large, only the region of the HOMO could be reached in this study [36,53,54]. Substrates with sufficiently low work functions to reach the region of the LUMOs were not available.
WRS is grateful to Prof. M. Falman and especially to Asst. Prof. S. Braun, as well as the past and present members of the Division of Surface Physics and Chemistry, at the Department of Physics at Linköping University. The first ultraviolet photoelectron spectroscopy of polymers by WRS was carried out under the auspices of Dr. C.B. Duke at the Xerox Webster Research Center, NY, USA. The collaborations with J.L. Brédas on a combined theoretical–experimental approach (and with C.B. Duke, before the terminology “combined theoretical–experimental approach” was ever used) are gratefully acknowledged. Research in Linköping, during the course of the research reviewed above, was generally supported by many grants from the Swedish Science Foundation (Vetenskaps Rådet), the Swedish Foundation for Strategic Research and the Commission of the European Union. References
Fig. 11. The low-binding energy portions of UPS spectra for F-substituted TCNQ are shown. The long-dashed curve corresponds to the bare substrate (lightly oxidized aluminum) which has very little intensity in the low-binding energy region. The short-dashed curve is for a condensed molecular solid film of just sufficient thickness that electrons generated at the hybrid interface do not reach the surface and contribute to the UPS signal. The solid curve is for a condensed molecular film of nominally one monolayer thickness, where electrons generated at the interface reach the surface and contribute to the UPS signal. The extra intensity in the solid curve near 2 eV corresponds to the transferred charge at the interface that results in pinning of the Fermi level of the substrate at just 2 eV [45,54].
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