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Photoemission studies of functional organic materials and their interfaces
JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
Invited paper
Photoemission studies of functional organic materials and their interfaces K. Seki*, H. Ishii Department of Chemistry, Nagoya University, Furocho, Nagoya 464-01, Japan
Abstract Recent studies of large functional organic materials by ultraviolet photoelectron spectroscopy (UPS) are reviewed focusing on the studies of interfaces including organics and the development of angle-resolved UPS (ARUPS) studies. The development of organic electroluminescent devices stimulated the studies on metal/organic, organic/metal, and organic/organic interfaces. Much chemistry occurs when metal vapor is deposited on organics, while interfaces formed by depositing on metals have much smaller reactivity, and are more suitable for studying the fundamental aspects of metal/organic contact. In contradiction to the traditional assumption of vacuum level alignment of vacuum level, vacuum level shifts at the interface were found in most interfaces. From the studies of various systems, it was found that both electronic polarization by image charge and charge transfer across the interface can occur. The effect of oxygen was also examined and ascribed to the change of the metal surfaces. The ARUPS studies of oriented samples have become a powerful tool by the development of reasonably reliable theoretical methods based on the independent-atomic-center (IAC) approximation. The emission angle dependence of peak intensities has been used for determining the molecular orientations, for detailed studies of molecular electronic structures of model systems of polymers, and the study of peak intensity oscillation with photon energy for fullerenes. © 1998 Elsevier Science B.V. Keywords: Interface; Electroluminescent device; ARUPS; Energy band dispersion; Molecular orientation
1. Introduction Recently many organic functional materials have attracted attention in relation to the possible application to electronic and display devices. UV photoemission spectroscopy (UPS) has already been used for probing the electronic structures of bulk states of these materials [1]. Recent studies have also opened new applications of UPS for studying the interfacial electronic structures. Such studies are important for devices using interfacial phenomena. In particular, the rapid progress in the development of * Corresponding author.
electroluminescent devices [2] stimulated much research activity in this field [3-6]. In these studies, interesting phenomena such as the shift of the vacuum level [5] and chemical reactions at the interface [3,6] were found. Another recent advance in the UPS studies of organic materials is the detailed studies of well oriented samples by the combination of angleresolved UPS (ARUPS) and reasonably reliable intensity calculations [7]. Although A R U P S studies have already been performed for more than a decade [8,9], the discussion was mainly based on the selection rules based on symmetry and k-conservation in regularly repeating systems. The recent progress in
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the preparation of well oriented specimens and the possibility of discussing the peak intensities opened a new area for ARUPS studies such as the detailed analysis of the intensity as a function of photon energy [10,11] and the determination of molecular orientation [7,12] from the dependence of the peak intensity on electron emission angles. The latter make ARUPS useful to deduce the geometrical structure of organics as well as their electronic structure. The information available is complimentary to the knowledge of the surface lattice structure obtained by LEED measurements. In the following we will focus our discussion on these two subjects, taking examples mainly from the work of the authors and collaborating laboratories, and mentioning the results of other groups where appropriate. We also note that other interesting subjects in the field of organic materials are covered in this conference (although we do not cite each of them) including various techniques such as inverse photoemission, two photon photoemission, Penning ionization electron spectroscopy, and high resolution electron energy loss spectroscopy.
2. Interfaces including organic materials Recently, various electronically functional organic materials have been studied in relation to applications to various devices and the potential development of molecular devices. Many are based on the electronic phenomena at interfaces such as: (1) carrier injection from electrodes into the organic layer in an organic electroluminescent device [2]; (2) carrier separation at the Schottky barrier in an organic solar cell [13]; and (3) electron injection from adsorbed dye into silver halides in photography [14]. Thus the elucidation of the interfacial electronic structure forms the basis of understanding these phenomena and the function of the devices, but the interfacial electronic structures have been mostly estimated from either (a) macroscopic electric measurements such as current-voltage characteristics or interfacial capacitance, or (b) the combination of the separately measured electronic structures of the two materials assuming vacuum level alignment at the interface, as shown in Fig. l(a). When the film thickness is large and there are sufficient charge
A=0
.........3.
.0
Z F ,
e
N [ ~""HOM'o metal
organic (a)
o
-
HOMO" metal
organic (b)
Fig. 1. Models for the electronic structure of an organic/metal interface; (a) traditional method of estimation where a common vacuum level is assumed; (b) currently observed electronic structure at the organic/metal interface. 'I'm, work function of the metal; EFm, Fermi energy of the metal; Is, ionization threshold energy of the organic layer; evF, energy of the HOMO level of the organic layer, and A, the shift of the virtual vacuum level at the interface.
carriers, band bending to align the Fermi levels may occur [15], with alignment of hypothetical vacuum levels at the interface. However, the validity of such models has not been experimentally examined, and direct studies of real interfaces have been greatly needed. Recently there have been several such studies of real interfaces formed by stepwise deposition of one component on the other, with UPS measurements at each step of deposition. They can be classified into: 1. Deposition of metals on organic materials [3,6,16]; 2. Deposition of organic material on inorganic material (metals [5,17-19], semiconductors [20], and ionic solids [21-23]; 3. Deposition of organic material on other organic material [19,23]. We will examine these systems in the following. As for the second category, we will concentrate on organic/metal interfaces where recent studies have been extensively carried out. 2.1. Metals on organics
The deposition of metals on organic film systems has been extensively studied, since the deposition of low work function metals forms a step in the real fabrication of EL devices [3,6,16]. These cases are
K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998)821-830
characterized by various degrees of interfacial chemical reaction between the metal and organic material. The hot metal atoms with high reactivity from the evaporation source attack the organic molecules, often causing the formation of covalent bonds or charge-transfer complex formation. Thus the interface cannot be simply regarded as the contacting plane between the two components. This situation resembles the reaction between metal and inorganic semiconductor such as silicide formation at the Au/Si interface. For example, the deposition of A1 on poly(p-phenylenevinylene) (PPV), causes a dramatic decrease in the intensity of the top peak of the valence band, accompanied by the disappearance of the shakeup satellite in the C ls XPS spectra corresponding to excitation in the vinylene part [24]. These findings were interpreted to be the result of covalent bond formation between the Ca atoms and the carbon atoms of the vinylene group. The v-conjugation among the constituent units is broken, and the width of the intramolecular r-band formed by conjugation is much reduced, leading to the disappearance of the peak at the top of this band.
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This shift clearly demonstrates that the assumption of a common vacuum level in Fig. l(a) and Fig. l(b) is invalid. Instead, a model shown in Fig. l(b) emerges. Such a shift of the vacuum level is already known in the field of surface science, but is not common knowledge for people working on organic electronic materials. From the data shown in Fig. 2, the interfacial energy diagrams for Alq3 on Au and A1 in Fig. 3(a) and Fig. 3(b) are derived. We note that the observed injection barrier for electron at the interface evF is very small, although the accurate value is not clear due to the uncertainty of the LUMO energy of Alq3. This result is consistent with the efficient carrier injection at the Alq3/A1 interface. On the other hand, a value more than 1 eV is derived if we align the data of Alq3 and A1 by assuming a common vacuum level. Thus the formation of such a double layer is of fundamental importance for the understanding of the performance of the devices. Such a shift of vacuum level
2.2. Organics on metals
These systems are more suitable for detailed studies of the contact between organic and inorganic materials. The reactivity of the metal in the solid state is usually not so high as that of vapor, although care must be taken when examining the possible reactions. Fig. 2 depicts the change of the UPS spectra with the deposition of tris(8-hydroxyquinolino)aluminum (Alq3), which is a representative light-emitting material in an EL device, on Au substrate [5]. The spectral features can be assigned in detail by the molecular orbital (MO) calculations [Sugiyama and Ishii, unpublished data]. At the deposition on Au, the valence electron structures of Alq3 grow up, with a significant shift of the low energy cutoff at the left side of the spectra. This corresponds to lowering of the vacuum level (decrease of the work function) by the deposition of Alq3. This indicates that an interfacial dipole layer is formed with the organic side positively charged. For the metals with not very small work functions (Mg, A1, Ag, and Au), most systems showed such lowering of the work function except for the cases described below.
e"
m
w
-5
0
5
10
15
20
Retarding Voltage / V Fig. 2. UPS spectra of Alq3 with increasing thickness on Au substrate.
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was also observed for merocyanine dyes/silver halide interfaces [21,22], and plays an important role in determining the sensitizing behavior of dyes adsorbed on silver halide surfaces in photography. In general, there are several possible microscopic mechanisms in interfacial dipole layer, such as (a) orientation of permanent dipole of a polar molecule, (b) electron transfer (CT) between the molecule and the substrate, and (c) reorganization (or polarization) of electron clouds of the molecule and the substrate. Mechanism (a) applies only to polar molecules. This may occur in the case of merocyanine dyes on AgBr and AgC1 [21,22]. The examination of molecular orientation by NEXAFS also indicated at least some part of dipole formation comes from this factor [22]. For non-polar molecules, such as Alq3 and porphyrins, we have to consider mechanisms (b) and (c). To determine whether mechanism (c) is operative, the examination of a non-polar system with little possibility of charge transfer is useful. Such a study was performed on the system of a long chain alkane n-CH3(CH2)a2CH3 on metals by the combination of UPS and Penning ionization electron spectroscopy (PIES) [27]. The alkanes are typical organic insulators with a wide gap of about 9.6 eV resulting from the large ionization energy (9.1 eV) [28] and the small (even negative) electron affinity (-0.5 eV) [29]. The lowering of the vacuum level was also observed
in these cases, suggesting that the interfacial dipole is formed by the polarization of the electronic cloud at the interface. A similar lowering of the work function of metals examined here has been observed at the deposition of Xe, which is also a good insulator [30]. This was interpreted as the effect of image charge polarizing the electron cloud in the Xe atoms to be attracted to the interfacial side [31]. This makes the vacuum side positively charged, in agreement with the observation. This mechanism of electronic polarization seems to apply also to organic/metal interfaces. If CT mechanism (b) operates, the lowering of the vacuum level corresponds to the electron transfer from organic molecule to the substrate. This is consistent with results for molecules with small or medium values of ionization energy, such as Alq3 (5.9 eV), TPD (5.3 eV), and porphyrins (5.3-6 eV). When an electron accepting molecule with a large electron affinity is deposited on a low work function metal, we expect that the direction of CT may be reversed, resulting in the negative charging of the vacuum side. This was actually observed in the case of depositing N,N'-diphenyl-l,4,5,8-naphthyltetracarboxylimide (DPNTCI) on A1 with an upward shift of the vacuum level A = -0.2 eV. Although the deposition of DPNTCI on Au showed the lowering of the vacuum level, a still stronger acceptor,
[•"=
1.0eV
V .... E v a o
m=,.sv II/ v ao=a.SeV 3.3eV E Fm
LUM0
LUMO EFm LleV
eV
HOMO HOMO AI
AIq3 (lnm)
vacuum
Au
AIq3 (lnm)
vacuum
Fig. 3. Observed interfacial energy diagrams of (a) Alq3/AI and (b) AIq3/Au interfaces obtained from UPS experiments.
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tetracyanoquinodimethane (TCNQ) showed a lowering of the vacuum level by 0.2 eV [19]. A similar rise of work function was observed for the deposition of tetracyanoquinodimethane (TCNQ) on Cu [26]. Thus we see that both polarization by image charge and charge transfer can take place. More systematic studies on the magnitude of the shift were carried out for three porphyrins deposited on metals with various work functions (Au, Ag, Cu, A1 and Mg) [18]. The spectra of deposited porphyrins with 5 nm thickness showed little change in shape from those of bulk samples, but they are rigidly shifted on the energy scale as shown by the filled circles in Fig. 4. In Fig. 5 we show the dependence of the vacuum level shift on the work function of metals. For each porphyrin, we see a linear relationship with the metal work function. The slope S is near unity for Zn tetraphenyl porphyrin (ZnTPP), while
ZnTPP/Metal in UHV
tl
N(~i~::
o after 0 2 expos ure
~o
B
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.,
% ~ ~ "" ~ . ~
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about one-half for metal-free tetraphenyl porphyrin (H2TPP) and metal-free tetrapyridyl porphyrin (H2TPyP). The independence of the shift on metal for ZnTPP indicates that the origin of the interfacial double layer is of physical nature rather than chemical, suggesting the dominant role of polarization. The observed S near unity for ZnTPP is rather unique, in view of the much smaller S for semiconductors such as Si, Ge, and GaAs [32]. On the other hand, the linear relation between A and cI~ m for H2TPP and H2TPyP with S ~ 0.5 suggests the existence of some interface state, as seen in the semiconductors mentioned above [33]. 2.3. Organics on organics
The organic/organic systems are important in simulating carrier injection in the multilayer devices in EL display devices and electronic photography. For example, a layer of hole transport material, such as N,N'-diphenyl-N,N'-(3-methylphenyl)- 1,1 '-biphenyl4,4'-diamine (TPD) is often inserted between the transparent indium tin electrode and the light emitting layer. From the discussion above, we expect that the shift of the vacuum level will be small except for the system with some charge transfer. Actually only a very small shift of the vacuum level was found for the combination of organics with comparable electron donating/accepting abilities such as Alq3/TPD [18,23]. On the other hand, a small but definite shift was observed for a combination of a strong electron donor (tetrathianaphthacene TTN) and TCNQ, with the direction of dipole being consistent with the one expected from CT picture [Sugiyama and Ishii, unpublished data]. 2.4. Other factors
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6
I
5
,
I
I
4
3
,
d
2
f
I
n
1
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O=E F
Binding Energy / eV Fig. 4. UPS spectra in the uppermost valence region for Zn tetraphenyl porphyrin(ZnTPP) filmsevaporatedon variousmetals (Mg, AI, Ag, and Au) in ultrahigh vacuum (UHV) (filled circles) and after exposure to oxygen (4 Torr, 5 min) (open circles). The solid and dotted vertical lines indicate the onset of peak A measured before and after exposure to oxygen, respectively. The energy shift by exposure to oxygenis denoted by an arrow.
The results described above were mostly obtained by experiments under ultrahigh vacuum (UHV) conditions. On the other hand, the fabrication of real devices are made under much lower vacuum. Therefore we should be careful about matching experimental conditions in the comparison between the UPS and electrical results. In view of this factor, the studies of organic interfaces can be classified in two groups. One is the extension to more sophisticated and well defined
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systems, as described in the next section. The other is studies with a good match to real devices. As an example of the latter case, in Fig. 4 we show the UPS spectra of ZnTPP film prepared under UHV after exposure to 4 Torr of 02 for 5 min (open circles). The spectral shapes do not change, but they are rigidly shifted. The magnitude and even the direction of the shift strongly depends on the substrate metal. From this finding we can immediately exclude the possibility of doping of the organic layer, since it means that the effect of oxygen will not depend on the metal. Rather, we ascribe the shift to the change of the metal surface underlying the organic layer by oxygen molecules penetrating to the interface through defects, grain boundaries, and so on. Actually the result of shifts are roughly parallel with the work function change of clean metal surfaces by oxygen exposure.
materials. Recently well-ordered films on substrates became available, and the study of these samples with the angle-resolved UPS (ARUPS) technique, supported by the recent development of quantitatively reliable formalism of theoretical simulation, opened a new chapter of UPS studies of organic materials. Such improvement of thin film samples is essential, since organic single crystals cannot be normally used due to the difficulty of charging. The ARUPS spectra of a well-ordered organic film usually show significant dependence on experimental parameters such as incidence angle of photons or, emission angle of electrons (0, 40, photon energy hu, and the polarization of the incident photons [9]. This kind of ARUPS data has been successfully used for (1) determining the orientation of adsorbed molecules on surfaces based on the selection rules for the emission angles of high symmetry [34], and (2) probing the energy band dispersion relation in systems with translational symmetry, based on the k-reservation rule [35-40]. Besides these, however, there was no practical tool to calculate the emission intensity for a given set (~, 0, and hu). Although the sophisticated calculations applied to small molecules were successful, they were difficult to apply to large organic molecules.
3. Geometrical and electronic structures of ordered systems based on ARUPS studies combined with theoretical simulation
So far the studies of organic systems have been mostly carried out for poorly- or moderately-ordered systems, providing the density of states of bulk
(b)
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,
-0.2
,
,
,
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.
ZnTPP/Metal
.....
........ H2TPP/Metal O H2T(4-Py)P/Metal []
"'"'•' -0.4 >
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~:~
"........ "'"',. '""
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....
-0.8
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,
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4.6
I
4.8
,
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5.0
Au I
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5.4
/eV 133
Fig. 5. Plot of the observedshift of the vacuumlevel by depositing porphyrins against the work function of metals. ZnTPP, Zn tetraphenylporphyrin; H2TPP, metal-freetetraphenylporphyrin,and HaT(4-Py)P,metal-freetetra(4-pyridyl)porphyrin.
K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
To improve such a situation, Grobman formulated the emission intensity from a MO under independent atomic center (IAC) approximation [41 ], in which the initial state wavefunction is LCAO (linear combination of atomic orbitals)--MO deduced by an appropriate method, and the photoemission from a MO is expressed as the sum of emission of electrons from each AO to free electron state weighted by the contribution of the AO to the MO. Emission from each AO is regarded as the sum of transitions into partial waves, with specific phase shifts for each combination of the initial AO and the partial wave. Thus the outgoing electron wave is the sum of various partial waves from the AOs in the molecule, and the interference among these electron waves from the atoms results in the angular distribution of photoelectrons. However, serious experimental examination was not performed. Recently, Hasegawa and Ueno [7,12] constructed a versatile program of calculating photoemission intensity with reasonable accuracy, based on the formulation by Grobman [41]. The various parameters were evaluated, and the resultant formulation has been combined with MO calculations at both semiempirical (MNDO) and ab initio levels. They have also developed the formulation of taking account of the scattering of photoelectrons by surrounding atoms [42], and have found that the effect of scattering significantly depends on the shape and orientation of the molecule. As described below, this method of simulation has been used in conjunction with the ARUPS experiments using synchrotron radiation as a powerful tool for the quantitative analysis of the ARUPS spectra. 3.1. Molecular orientation
The molecular orientation and packing on the substrate surface has so far been studied mainly by diffraction techniques such as low-energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED). Although they offer lattice constants, the molecular arrangement and orientation in a unit cell could not be determined directly, and needed supplemental data by other techniques such as Penning ionization electron spectroscopy and infrared spectroscopy. The development of theoretical simulation enabled the estimation of molecular orientation in ordered
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films by comparing the observed 0- and ~b-dependence with simulated ones for various combinations of the experimental parameters. Thus ARUPS offers complimentary information with other techniques such as LEED. Ueno and Hasegawa performed this kind of study to determining the molecular orientation of bis(1,2,5-thiaziazolo)-p-quinobis(1,3-dithiol) (BTQBT) [7], Cu- and H2 phthalocyanine [43-45], chloroaluminum phthalocyanine [46], naphthacene [42], and perylenetetracarboxylic anhydride (PTCDA) [47] on the cleaved surfaces of layer compounds such as MoS 2 and graphite. The advantage of this approach is that (1) it is much less destructive compared with electron diffraction, (2) it can offer information about azimuthal as well as polar orientation, and (3) geometrical and electronic information can be simultaneously obtained from the same set of data from a single sample. 3.2. Intramolecular electronic structure o f model systems o f polymers
An organic solid is formed by two stages of aggregation of atoms. The first step is the formation of a molecule from atoms, and the second is the aggregation of molecules to form the solid. In both, we can suppose the occurrence of translational symmetry and the observation of the energy band dispersion by ARUPS. As for the first step, an extended chain of polymer, which is formed by repeating a small unit, can be regarded as a one-dimensional crystal with intramolecular translational symmetry. The latter corresponds to the usual packing of molecules in a crystal. The dispersion relation for such intramolecular bands has been most extensively studied for alkanes, which are good model compounds of polyethylene (CHz)n. Samples vertically [35,36], parallel [37], and even obliquely [38] oriented relative to the substrate surface have been examined. Recently we have reexamined the ARUPS spectra of a long chain alkane n-CH3(CH2)42CH3 deposited on a clean Cu(100) surface using synchrotron radiation, with the help of IAC calculation [48] to examine the applicability of this method to systems without 7r electrons. The LEED patterns revealed that the molecules are lying parallel to the substrate surface, with azimuthal order of forming two domains with the molecular
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axes parallel to the [110] and [1 i0] directions. When we assumed that a molecule oriented parallel to the substrate with the plane formed by the C atoms parallel to the surface, an excellent correspondence between the observed and simulated spectra was obtained for a set of spectra of varying emission angle of electrons, as shown in Fig. 6. This corrects a similar study by other workers who concluded that the carbon plane is vertical to the surface [37]. Another interesting subject in short model compounds of polymers (oligomers) is the validity of the k-vector concept. It is a good quantum number for a system of infinite repeating units, while it has no sense for a system of only one unit. How does its significance decrease with the decrease of the number of repeating units N? This subject was studied by
examining the relaxation of the k-conservation rule in the normal emission spectra of sexiphenyl (N = 6) oriented vertically to the substrate surface [11]. It is a model molecule of poly(p-phenyl) formed by linearly connecting benzene rings. The sample was prepared by vacuum evaporation on a substrate heated to 90°C. The topmost part of the valence state is formed by the 7r band formed by the highest occupied molecular orbital (HOMO) of benzene. The low binding energy peaks of the spectra corresponding to this region changed with h~,, indicating the enhancement of these peaks at specific photon energies. These enhancements occur at hu matching the k-conservation rule, but become much broader compared with the similar enhancement for the long chain alkane n-CH3(CH2)34CH3. This is due to the
IAC "flat-on" h ~,=40eV ot=70deg.
TTC/Cu(100)
ARUPS h~=40eV o~=70deg. O/deg. 68 65 62.5 6O 57.5 55 52.5 50 47.5 45 42.5 40 37.5 35 32.5 30 27.5 25 22.5 2O 17.5 15 12.5 10 7.5 5 2.5 0
c-
e-
20
15
10
5
Eb (vs. EF)/eV
0
n-C32H66.,,L~±
i l
e/deg. 67,5 65 62.5 60 57.5 55 52.5 50 47.5 45 42.5 40 37.5 35 32.5 30 27.5 25 22.5 20 17.5 15 12.5 lO 7.5 5 2,5 o
.._ZI I
,Y i 4""
I
~
LX__./' A • I1'
-~.
L
I
30
a
I
J
I
A
i
I
25 20 15 Eb ( v s . E v a c ) / e V
I
10
Fig. 6. Observed and simulated polar emission angle dependence of the UPS spectra of the monolayer of an alkane n-CH3(CH2)42CH3 on Cu(100) surface. The analyzer moved in a plane which is vertical to the substrate surface and parallel to the [110] direction.
K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-9l (1998) 821-830 blurring of the k-vector in the initial state wavefunction, by an amount almost comparable to the width of the Brillouin zone. In this case, again the IAC calculations well reproduced the he dependence of the peak intensities, confirming both the molecular orientation and the physical picture described above. 3.3. Intensity oscillations with photon energy in C6o The intensity oscillation of photoemission intensity from the upper part of the valence band [25] has attracted much attention. Originally it was ascribed to the parity of the occupied levels, but later calculations excluded this origin. Hasegawa applied IAC calculations to C60, and found that the oscillation can be reproduced by only assuming a simple radial wavefunction which is large on a thin spherical shell with appropriate angular wavefunction [10]. This shows that the essential origin is the spherical shape of the electron cloud. This example shows another type of application of IAC approximation.
Acknowledgements The authors wish to thank Professors Yeshiya Harada and Hiroo Inokuchi, and the late Professor E.E. Koch for the introduction to the fruitful field of UPS of organic materials. They also thank Professor Nobuo Ueno of Chiba University for the constant collaboration. The collaboration with Drs Satoru Narioka, Eisuke Ito, Messrs. Kiyoshi Sugiyama, Daisuke Yoshimura, and Professor Yukio Ouchi at Nagoya University, Dr Koji Kamiya-Okudaira and Masaru Aoki of Chiba University, Dr Shinji Hasegawa and Takayuki Miyamae of IMS, and Dr Takafumi Miyazaki is gratefully acknowledged. The ARUPS studies using synchrotron radiation were performed as part of the Joint Studies Program of the Institute for Molecular Science. We thank the staff of UVSOR Facility at the Institute for Molecular Science for their support. The studies described above were supported in part by the COE Program of the Grant in Aid for Scientific Research 'Molecular Chirality' (No. 07CE2004) from the Ministry of Education, Science, Sports and Culture of Japan, and the Venture Business Laboratory Project
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'Advanced Nanoprocess Technologies' of Nagoya University.
References [1] For reviews and fundamental aspects, e.g.W.D. Grobman, E.E. Koch, Photoemissionin Solids, vol. 2, Springer, Berlin, 1979, p. 261; W.R. Salaneck, CRC Crit. Rev. Solid State Mater. Sci., 12 (1985) 267; K. Seki, Optical Techniques to Characterize Polymer Systems, Elsevier, Amsterdam, 1989, p. 115. [2] N.C. Greenham, R.H. Friend, Solid State Phys. 49 (1995) 1. [3] W.R. Salaneck,S. Stafstroem, J.-L. Br6das, ConjugatedPolymer Surfaces and Interfaces, Cambridge University Press, Cambridge, 1996, and references therein. [4] A. Schmidt, M.L. Anderson,N.R. Armstrong, J. Appl. Phys. 78 (1995) 5619. [5] H. Ishii, K. Seki, IEEE Trans. Electron Devices 44 (1997) 1295, and references therein. [6] Y. Hirose, A. Kahn, V. Aristov, P. Soukiassian, Appl. Phys. Lett. 68 (1996) 217. [7] S. Hasegawa, S. Tanaka, Y. Yamashita, H. Inokuchi, H. Fujimoto, K. Kamiya, K. Seki, N. Ueno, Phys. Rev. B 48 (1993) 2596. [8] T. Permien,R. Engelhardt, C.A. Feldmann,E.E. Koch, Chem. Phys. Lett. 98 (1983) 527. [9] For a recent review, P.A. Dauben,Z. Phys. Chem. 202 (1997) 227. [10] S. Hasegawa, T. Miyamae, K. Yakushi, H. Inokuchi,K. Seki, N. Ueno, 7th InternationalConference on Electron Spectroscopy, Chiba, 1997, 4P-58. [11] S. Narioka, H. Ishii, K. Edamatsu, K. Kamiya, S. Hasegawa, T. Ohta, N. Ueno, K. Seki, Phys. Rev. 52 (1995) 2362. [12] N. Ueno, J. Electron Spectrosc. 78 (1996) 345 and references therein. [13] J. Simon, JJ. Andre, Molecular Semiconductors, Springer, Berlin, 1985. [14] T. Tani, Photographic Sensitivity, Oxford University Press, Oxford, 1995. [15] K.C. Kao, W. Hwang, Electrical Transport in Solids, Pergamon, Oxford, 1981. [16] W.R. Salaneck, J.L. Br6das, Adv. Mater. 8 (1996) 48-52. [17] S. Narioka, H. Ishii, M. Sei, Y. Ouchi, K. Seki, S. Hasegawa, T. Miyazaki, Y. Harima, K. Yamashita, Appl. Phys. Lett. 67 (1995) 1899. [18] H. Ishii, S. Hasegawa, D. Yoshimura, K. Sugiyama, S. Narioka, M. Sei, Y. Ouchi, K. Seki, Y. Harima, K. Yamashita, Mol. Cryst. Liq. Cryst. 296 (1997) 427, and references therein. [19] H. Ishii, K. Sugiyama, K. Seki, Proc. SPIE 3148, in press. [20] Y. Hirose, W. Chen, E.I. Haskal, S.R. Forrest, A. Kahn, Appl. Phys. Lett. 64 (1994) 3482. [21] K. Seki, H. Yanagi, Y. Kobayashi, T. Ohta, T. Tani, Phys. Rev. B 49 (1994) 2760. [22] K. Seki, T. Araki, E. Ito, K. Oichi, S. Narioka, T. Yokoyama, T. Ohta, S. Watanabe, A. Matsunaga, T. Tani, J. Im. Sci. Tech. 37 (1993) 589.
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[23] N. Sato, M. Yoshikawa, J. Electron Spectrosc. 78 (1996) 387. [24] M. Fahlman, D. Beljonne, M. Loeglund, P.L. Burn, A.B. Holmes, R.H. Friend, J.L. Br~das, W.R. Salaneck, Chem. Phys. Lett. 214 (1993) 327. [25] P.J. Benning, D.M. Poirier, N. Troullier, J.L. Martins, J.H. Weaver, R.E. Haufler, L.P.F. Chibante, R.E. Smalley, Phys. Rev. B 44 (1991) 1962. [26] W. Erley, H. Ibach, Surf. Sci. 178 (1986) 565. [27] K. Seki, E. Ito, H. Ishii, Proc. Int. Conf. Electroluminesc., Kitakyushu, 1997. [28] K. Seki, S. Hashimoto, N. Sato, Y. Harada, H. Ishii, H. Inokuchi, J. Kanbe, J. Chem. Phys. 66 (1977) 3644. [29] N. Ueno, K. Sugita, K. Seki, H. Inokuchi, Phys. Rev. B 34 (1986) 6386. [30] A. Zangwill, Physics at Surfaces, Cambridge University Press, Cambridge, 1988, and references therein. [31] K. Wandelt, J.E. Hulse, J. Chem. Phys. 80 (1984) 1340. [32] R. Moench, Surf. Sci. 299/300 (1994) 928. [33] S. Kurtin, T.C. McGilI, C.H. Mead, Phys. Rev. Lett. 22 (1969) 1433. [34] N.V. Richardson, A.M. Bradshaw, in: C.R. Brundle, A.D. Baker (Eds.), Electron Spectroscopy, Vol. 4, Academic Press, New York, 1982, Ch. 3. [35] K. Seki, N. Ueno, U.O. Karlsson, R. Engelhardt, E.E. Koch, Chem. Phys. 105 (1986) 247. [36] N. Ueno, K. Seki, N. Sato, H. Fujimoto, K. Sugita, H. Inokuchi, Phys. Rev. B 41 (1990) 1176, and references therein. [37] R. Dudde, B. Reihl, Chem. Phys. Lett. 196 (1992) 91.
[38] Ch. Zubraegel, F. Schneider, M. Neumann, G. Haehner, Ch. Woell, M. Grunze, Chem. Phys. Lett. 219 (1994) 127. [39] D. Schmeisser, W. Jaegermann, C.H. Pettenkofer, H. Wachtel, A. Jimenez-Gonzales, J.U. von Schuetz, H.C. Wolf, P. Erk, H. Meixner, S. Huenig, Solid State Commun. 81 (1992) 827. [40] S. Hasegawa, T. Mori, K. Imaeda, S. Tanaka, K. Yamashita, H. Inokuchi, H. Fujimoto, K. Seki, N. Ueno, J. Chem. Phys. 100 (1994) 6969. [41] W.D. Grobman, Phys. Rev. B 17 (1978) 4573. [42] S. Hasegawa, K. Seki, H. Inokuchi, N. Ueno, J. Electron Spectrosc. 78 (1996) 391. [43] N. Ueno, K. Suzuki, S. Hasegawa, K. Kamiya, K. Seki, H. Inokuchi, J. Chem. Phys. 99 (1995) 7169. [44] K. Kamiya, M. Momose, A. Kitamura, Y. Harada, N. Ueno, T. Miyazaki, S. Hasegawa, H. Inokuchi, S. Narioka, H. Ishii, K. Seki, Mol. Cryst. Liq. Cryst. 267 (1995) 211. [45] K. Kamiya, M. Momose, A. Kitamura, Y. Harada, N. Ueno, S. Hasegawa, T. Miyazaki, H. Inokuchi, S. Narioka, H. Ishiii, K. Seki, J. Electron Spectrosc., in press. [46] M. Aoki, S. Masuda, Y. Einaga, K. Kamiya, A. Kitamura, M. Momose, N. Ueno, Y. Harada, T. Miyazaki, S. Hasegawa, H. Inokuchi, K. Deki, J. Electron Spectrosc. 76 (1995) 259. [47] Y. Azuma, T. Hasebe, T. Miyamae, K.K. Okudaira, Y. Harada, K. Seki, E. Morikawa, V. Saile, N. Ueno, J. Syn. Rad., in press. [48] D. Yoshimura, H. Ishii, Y. Ouchi, E. Ito, T. Miyamae, S. Hasegwa, N. Ueno, K. Seki, 7th International Conference on Electron Spectroscopy, Chiba, 1997, 2P-72.
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
Invited paper
Photoemission studies of functional organic materials and their interfaces K. Seki*, H. Ishii Department of Chemistry, Nagoya University, Furocho, Nagoya 464-01, Japan
Abstract Recent studies of large functional organic materials by ultraviolet photoelectron spectroscopy (UPS) are reviewed focusing on the studies of interfaces including organics and the development of angle-resolved UPS (ARUPS) studies. The development of organic electroluminescent devices stimulated the studies on metal/organic, organic/metal, and organic/organic interfaces. Much chemistry occurs when metal vapor is deposited on organics, while interfaces formed by depositing on metals have much smaller reactivity, and are more suitable for studying the fundamental aspects of metal/organic contact. In contradiction to the traditional assumption of vacuum level alignment of vacuum level, vacuum level shifts at the interface were found in most interfaces. From the studies of various systems, it was found that both electronic polarization by image charge and charge transfer across the interface can occur. The effect of oxygen was also examined and ascribed to the change of the metal surfaces. The ARUPS studies of oriented samples have become a powerful tool by the development of reasonably reliable theoretical methods based on the independent-atomic-center (IAC) approximation. The emission angle dependence of peak intensities has been used for determining the molecular orientations, for detailed studies of molecular electronic structures of model systems of polymers, and the study of peak intensity oscillation with photon energy for fullerenes. © 1998 Elsevier Science B.V. Keywords: Interface; Electroluminescent device; ARUPS; Energy band dispersion; Molecular orientation
1. Introduction Recently many organic functional materials have attracted attention in relation to the possible application to electronic and display devices. UV photoemission spectroscopy (UPS) has already been used for probing the electronic structures of bulk states of these materials [1]. Recent studies have also opened new applications of UPS for studying the interfacial electronic structures. Such studies are important for devices using interfacial phenomena. In particular, the rapid progress in the development of * Corresponding author.
electroluminescent devices [2] stimulated much research activity in this field [3-6]. In these studies, interesting phenomena such as the shift of the vacuum level [5] and chemical reactions at the interface [3,6] were found. Another recent advance in the UPS studies of organic materials is the detailed studies of well oriented samples by the combination of angleresolved UPS (ARUPS) and reasonably reliable intensity calculations [7]. Although A R U P S studies have already been performed for more than a decade [8,9], the discussion was mainly based on the selection rules based on symmetry and k-conservation in regularly repeating systems. The recent progress in
0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved Pll S0368-2048(97)00168-0
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the preparation of well oriented specimens and the possibility of discussing the peak intensities opened a new area for ARUPS studies such as the detailed analysis of the intensity as a function of photon energy [10,11] and the determination of molecular orientation [7,12] from the dependence of the peak intensity on electron emission angles. The latter make ARUPS useful to deduce the geometrical structure of organics as well as their electronic structure. The information available is complimentary to the knowledge of the surface lattice structure obtained by LEED measurements. In the following we will focus our discussion on these two subjects, taking examples mainly from the work of the authors and collaborating laboratories, and mentioning the results of other groups where appropriate. We also note that other interesting subjects in the field of organic materials are covered in this conference (although we do not cite each of them) including various techniques such as inverse photoemission, two photon photoemission, Penning ionization electron spectroscopy, and high resolution electron energy loss spectroscopy.
2. Interfaces including organic materials Recently, various electronically functional organic materials have been studied in relation to applications to various devices and the potential development of molecular devices. Many are based on the electronic phenomena at interfaces such as: (1) carrier injection from electrodes into the organic layer in an organic electroluminescent device [2]; (2) carrier separation at the Schottky barrier in an organic solar cell [13]; and (3) electron injection from adsorbed dye into silver halides in photography [14]. Thus the elucidation of the interfacial electronic structure forms the basis of understanding these phenomena and the function of the devices, but the interfacial electronic structures have been mostly estimated from either (a) macroscopic electric measurements such as current-voltage characteristics or interfacial capacitance, or (b) the combination of the separately measured electronic structures of the two materials assuming vacuum level alignment at the interface, as shown in Fig. l(a). When the film thickness is large and there are sufficient charge
A=0
.........3.
.0
Z F ,
e
N [ ~""HOM'o metal
organic (a)
o
-
HOMO" metal
organic (b)
Fig. 1. Models for the electronic structure of an organic/metal interface; (a) traditional method of estimation where a common vacuum level is assumed; (b) currently observed electronic structure at the organic/metal interface. 'I'm, work function of the metal; EFm, Fermi energy of the metal; Is, ionization threshold energy of the organic layer; evF, energy of the HOMO level of the organic layer, and A, the shift of the virtual vacuum level at the interface.
carriers, band bending to align the Fermi levels may occur [15], with alignment of hypothetical vacuum levels at the interface. However, the validity of such models has not been experimentally examined, and direct studies of real interfaces have been greatly needed. Recently there have been several such studies of real interfaces formed by stepwise deposition of one component on the other, with UPS measurements at each step of deposition. They can be classified into: 1. Deposition of metals on organic materials [3,6,16]; 2. Deposition of organic material on inorganic material (metals [5,17-19], semiconductors [20], and ionic solids [21-23]; 3. Deposition of organic material on other organic material [19,23]. We will examine these systems in the following. As for the second category, we will concentrate on organic/metal interfaces where recent studies have been extensively carried out. 2.1. Metals on organics
The deposition of metals on organic film systems has been extensively studied, since the deposition of low work function metals forms a step in the real fabrication of EL devices [3,6,16]. These cases are
K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998)821-830
characterized by various degrees of interfacial chemical reaction between the metal and organic material. The hot metal atoms with high reactivity from the evaporation source attack the organic molecules, often causing the formation of covalent bonds or charge-transfer complex formation. Thus the interface cannot be simply regarded as the contacting plane between the two components. This situation resembles the reaction between metal and inorganic semiconductor such as silicide formation at the Au/Si interface. For example, the deposition of A1 on poly(p-phenylenevinylene) (PPV), causes a dramatic decrease in the intensity of the top peak of the valence band, accompanied by the disappearance of the shakeup satellite in the C ls XPS spectra corresponding to excitation in the vinylene part [24]. These findings were interpreted to be the result of covalent bond formation between the Ca atoms and the carbon atoms of the vinylene group. The v-conjugation among the constituent units is broken, and the width of the intramolecular r-band formed by conjugation is much reduced, leading to the disappearance of the peak at the top of this band.
823
This shift clearly demonstrates that the assumption of a common vacuum level in Fig. l(a) and Fig. l(b) is invalid. Instead, a model shown in Fig. l(b) emerges. Such a shift of the vacuum level is already known in the field of surface science, but is not common knowledge for people working on organic electronic materials. From the data shown in Fig. 2, the interfacial energy diagrams for Alq3 on Au and A1 in Fig. 3(a) and Fig. 3(b) are derived. We note that the observed injection barrier for electron at the interface evF is very small, although the accurate value is not clear due to the uncertainty of the LUMO energy of Alq3. This result is consistent with the efficient carrier injection at the Alq3/A1 interface. On the other hand, a value more than 1 eV is derived if we align the data of Alq3 and A1 by assuming a common vacuum level. Thus the formation of such a double layer is of fundamental importance for the understanding of the performance of the devices. Such a shift of vacuum level
2.2. Organics on metals
These systems are more suitable for detailed studies of the contact between organic and inorganic materials. The reactivity of the metal in the solid state is usually not so high as that of vapor, although care must be taken when examining the possible reactions. Fig. 2 depicts the change of the UPS spectra with the deposition of tris(8-hydroxyquinolino)aluminum (Alq3), which is a representative light-emitting material in an EL device, on Au substrate [5]. The spectral features can be assigned in detail by the molecular orbital (MO) calculations [Sugiyama and Ishii, unpublished data]. At the deposition on Au, the valence electron structures of Alq3 grow up, with a significant shift of the low energy cutoff at the left side of the spectra. This corresponds to lowering of the vacuum level (decrease of the work function) by the deposition of Alq3. This indicates that an interfacial dipole layer is formed with the organic side positively charged. For the metals with not very small work functions (Mg, A1, Ag, and Au), most systems showed such lowering of the work function except for the cases described below.
e"
m
w
-5
0
5
10
15
20
Retarding Voltage / V Fig. 2. UPS spectra of Alq3 with increasing thickness on Au substrate.
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K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
was also observed for merocyanine dyes/silver halide interfaces [21,22], and plays an important role in determining the sensitizing behavior of dyes adsorbed on silver halide surfaces in photography. In general, there are several possible microscopic mechanisms in interfacial dipole layer, such as (a) orientation of permanent dipole of a polar molecule, (b) electron transfer (CT) between the molecule and the substrate, and (c) reorganization (or polarization) of electron clouds of the molecule and the substrate. Mechanism (a) applies only to polar molecules. This may occur in the case of merocyanine dyes on AgBr and AgC1 [21,22]. The examination of molecular orientation by NEXAFS also indicated at least some part of dipole formation comes from this factor [22]. For non-polar molecules, such as Alq3 and porphyrins, we have to consider mechanisms (b) and (c). To determine whether mechanism (c) is operative, the examination of a non-polar system with little possibility of charge transfer is useful. Such a study was performed on the system of a long chain alkane n-CH3(CH2)a2CH3 on metals by the combination of UPS and Penning ionization electron spectroscopy (PIES) [27]. The alkanes are typical organic insulators with a wide gap of about 9.6 eV resulting from the large ionization energy (9.1 eV) [28] and the small (even negative) electron affinity (-0.5 eV) [29]. The lowering of the vacuum level was also observed
in these cases, suggesting that the interfacial dipole is formed by the polarization of the electronic cloud at the interface. A similar lowering of the work function of metals examined here has been observed at the deposition of Xe, which is also a good insulator [30]. This was interpreted as the effect of image charge polarizing the electron cloud in the Xe atoms to be attracted to the interfacial side [31]. This makes the vacuum side positively charged, in agreement with the observation. This mechanism of electronic polarization seems to apply also to organic/metal interfaces. If CT mechanism (b) operates, the lowering of the vacuum level corresponds to the electron transfer from organic molecule to the substrate. This is consistent with results for molecules with small or medium values of ionization energy, such as Alq3 (5.9 eV), TPD (5.3 eV), and porphyrins (5.3-6 eV). When an electron accepting molecule with a large electron affinity is deposited on a low work function metal, we expect that the direction of CT may be reversed, resulting in the negative charging of the vacuum side. This was actually observed in the case of depositing N,N'-diphenyl-l,4,5,8-naphthyltetracarboxylimide (DPNTCI) on A1 with an upward shift of the vacuum level A = -0.2 eV. Although the deposition of DPNTCI on Au showed the lowering of the vacuum level, a still stronger acceptor,
[•"=
1.0eV
V .... E v a o
m=,.sv II/ v ao=a.SeV 3.3eV E Fm
LUM0
LUMO EFm LleV
eV
HOMO HOMO AI
AIq3 (lnm)
vacuum
Au
AIq3 (lnm)
vacuum
Fig. 3. Observed interfacial energy diagrams of (a) Alq3/AI and (b) AIq3/Au interfaces obtained from UPS experiments.
K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
tetracyanoquinodimethane (TCNQ) showed a lowering of the vacuum level by 0.2 eV [19]. A similar rise of work function was observed for the deposition of tetracyanoquinodimethane (TCNQ) on Cu [26]. Thus we see that both polarization by image charge and charge transfer can take place. More systematic studies on the magnitude of the shift were carried out for three porphyrins deposited on metals with various work functions (Au, Ag, Cu, A1 and Mg) [18]. The spectra of deposited porphyrins with 5 nm thickness showed little change in shape from those of bulk samples, but they are rigidly shifted on the energy scale as shown by the filled circles in Fig. 4. In Fig. 5 we show the dependence of the vacuum level shift on the work function of metals. For each porphyrin, we see a linear relationship with the metal work function. The slope S is near unity for Zn tetraphenyl porphyrin (ZnTPP), while
ZnTPP/Metal in UHV
tl
N(~i~::
o after 0 2 expos ure
~o
B
(D +-"
.,
% ~ ~ "" ~ . ~
Ag
825
about one-half for metal-free tetraphenyl porphyrin (H2TPP) and metal-free tetrapyridyl porphyrin (H2TPyP). The independence of the shift on metal for ZnTPP indicates that the origin of the interfacial double layer is of physical nature rather than chemical, suggesting the dominant role of polarization. The observed S near unity for ZnTPP is rather unique, in view of the much smaller S for semiconductors such as Si, Ge, and GaAs [32]. On the other hand, the linear relation between A and cI~ m for H2TPP and H2TPyP with S ~ 0.5 suggests the existence of some interface state, as seen in the semiconductors mentioned above [33]. 2.3. Organics on organics
The organic/organic systems are important in simulating carrier injection in the multilayer devices in EL display devices and electronic photography. For example, a layer of hole transport material, such as N,N'-diphenyl-N,N'-(3-methylphenyl)- 1,1 '-biphenyl4,4'-diamine (TPD) is often inserted between the transparent indium tin electrode and the light emitting layer. From the discussion above, we expect that the shift of the vacuum level will be small except for the system with some charge transfer. Actually only a very small shift of the vacuum level was found for the combination of organics with comparable electron donating/accepting abilities such as Alq3/TPD [18,23]. On the other hand, a small but definite shift was observed for a combination of a strong electron donor (tetrathianaphthacene TTN) and TCNQ, with the direction of dipole being consistent with the one expected from CT picture [Sugiyama and Ishii, unpublished data]. 2.4. Other factors
J
6
I
5
,
I
I
4
3
,
d
2
f
I
n
1
I
O=E F
Binding Energy / eV Fig. 4. UPS spectra in the uppermost valence region for Zn tetraphenyl porphyrin(ZnTPP) filmsevaporatedon variousmetals (Mg, AI, Ag, and Au) in ultrahigh vacuum (UHV) (filled circles) and after exposure to oxygen (4 Torr, 5 min) (open circles). The solid and dotted vertical lines indicate the onset of peak A measured before and after exposure to oxygen, respectively. The energy shift by exposure to oxygenis denoted by an arrow.
The results described above were mostly obtained by experiments under ultrahigh vacuum (UHV) conditions. On the other hand, the fabrication of real devices are made under much lower vacuum. Therefore we should be careful about matching experimental conditions in the comparison between the UPS and electrical results. In view of this factor, the studies of organic interfaces can be classified in two groups. One is the extension to more sophisticated and well defined
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K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
systems, as described in the next section. The other is studies with a good match to real devices. As an example of the latter case, in Fig. 4 we show the UPS spectra of ZnTPP film prepared under UHV after exposure to 4 Torr of 02 for 5 min (open circles). The spectral shapes do not change, but they are rigidly shifted. The magnitude and even the direction of the shift strongly depends on the substrate metal. From this finding we can immediately exclude the possibility of doping of the organic layer, since it means that the effect of oxygen will not depend on the metal. Rather, we ascribe the shift to the change of the metal surface underlying the organic layer by oxygen molecules penetrating to the interface through defects, grain boundaries, and so on. Actually the result of shifts are roughly parallel with the work function change of clean metal surfaces by oxygen exposure.
materials. Recently well-ordered films on substrates became available, and the study of these samples with the angle-resolved UPS (ARUPS) technique, supported by the recent development of quantitatively reliable formalism of theoretical simulation, opened a new chapter of UPS studies of organic materials. Such improvement of thin film samples is essential, since organic single crystals cannot be normally used due to the difficulty of charging. The ARUPS spectra of a well-ordered organic film usually show significant dependence on experimental parameters such as incidence angle of photons or, emission angle of electrons (0, 40, photon energy hu, and the polarization of the incident photons [9]. This kind of ARUPS data has been successfully used for (1) determining the orientation of adsorbed molecules on surfaces based on the selection rules for the emission angles of high symmetry [34], and (2) probing the energy band dispersion relation in systems with translational symmetry, based on the k-reservation rule [35-40]. Besides these, however, there was no practical tool to calculate the emission intensity for a given set (~, 0, and hu). Although the sophisticated calculations applied to small molecules were successful, they were difficult to apply to large organic molecules.
3. Geometrical and electronic structures of ordered systems based on ARUPS studies combined with theoretical simulation
So far the studies of organic systems have been mostly carried out for poorly- or moderately-ordered systems, providing the density of states of bulk
(b)
o.o
,
-0.2
,
,
,
J
A
~OMg ~
.
ZnTPP/Metal
.....
........ H2TPP/Metal O H2T(4-Py)P/Metal []
"'"'•' -0.4 >
Mg
-0.6
(D
~:~
"........ "'"',. '""
Mg A
A~ . . . . .
....
-0.8
A~'Ag "'"...
-I.0 -1,2
"'i.. I
3.6
,
I
3.8
,
I
4.0
,
I
4.2
,
I
,
4.4 ¢,
,
4.6
I
4.8
,
,
5.0
Au I
5.2
5.4
/eV 133
Fig. 5. Plot of the observedshift of the vacuumlevel by depositing porphyrins against the work function of metals. ZnTPP, Zn tetraphenylporphyrin; H2TPP, metal-freetetraphenylporphyrin,and HaT(4-Py)P,metal-freetetra(4-pyridyl)porphyrin.
K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
To improve such a situation, Grobman formulated the emission intensity from a MO under independent atomic center (IAC) approximation [41 ], in which the initial state wavefunction is LCAO (linear combination of atomic orbitals)--MO deduced by an appropriate method, and the photoemission from a MO is expressed as the sum of emission of electrons from each AO to free electron state weighted by the contribution of the AO to the MO. Emission from each AO is regarded as the sum of transitions into partial waves, with specific phase shifts for each combination of the initial AO and the partial wave. Thus the outgoing electron wave is the sum of various partial waves from the AOs in the molecule, and the interference among these electron waves from the atoms results in the angular distribution of photoelectrons. However, serious experimental examination was not performed. Recently, Hasegawa and Ueno [7,12] constructed a versatile program of calculating photoemission intensity with reasonable accuracy, based on the formulation by Grobman [41]. The various parameters were evaluated, and the resultant formulation has been combined with MO calculations at both semiempirical (MNDO) and ab initio levels. They have also developed the formulation of taking account of the scattering of photoelectrons by surrounding atoms [42], and have found that the effect of scattering significantly depends on the shape and orientation of the molecule. As described below, this method of simulation has been used in conjunction with the ARUPS experiments using synchrotron radiation as a powerful tool for the quantitative analysis of the ARUPS spectra. 3.1. Molecular orientation
The molecular orientation and packing on the substrate surface has so far been studied mainly by diffraction techniques such as low-energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED). Although they offer lattice constants, the molecular arrangement and orientation in a unit cell could not be determined directly, and needed supplemental data by other techniques such as Penning ionization electron spectroscopy and infrared spectroscopy. The development of theoretical simulation enabled the estimation of molecular orientation in ordered
827
films by comparing the observed 0- and ~b-dependence with simulated ones for various combinations of the experimental parameters. Thus ARUPS offers complimentary information with other techniques such as LEED. Ueno and Hasegawa performed this kind of study to determining the molecular orientation of bis(1,2,5-thiaziazolo)-p-quinobis(1,3-dithiol) (BTQBT) [7], Cu- and H2 phthalocyanine [43-45], chloroaluminum phthalocyanine [46], naphthacene [42], and perylenetetracarboxylic anhydride (PTCDA) [47] on the cleaved surfaces of layer compounds such as MoS 2 and graphite. The advantage of this approach is that (1) it is much less destructive compared with electron diffraction, (2) it can offer information about azimuthal as well as polar orientation, and (3) geometrical and electronic information can be simultaneously obtained from the same set of data from a single sample. 3.2. Intramolecular electronic structure o f model systems o f polymers
An organic solid is formed by two stages of aggregation of atoms. The first step is the formation of a molecule from atoms, and the second is the aggregation of molecules to form the solid. In both, we can suppose the occurrence of translational symmetry and the observation of the energy band dispersion by ARUPS. As for the first step, an extended chain of polymer, which is formed by repeating a small unit, can be regarded as a one-dimensional crystal with intramolecular translational symmetry. The latter corresponds to the usual packing of molecules in a crystal. The dispersion relation for such intramolecular bands has been most extensively studied for alkanes, which are good model compounds of polyethylene (CHz)n. Samples vertically [35,36], parallel [37], and even obliquely [38] oriented relative to the substrate surface have been examined. Recently we have reexamined the ARUPS spectra of a long chain alkane n-CH3(CH2)42CH3 deposited on a clean Cu(100) surface using synchrotron radiation, with the help of IAC calculation [48] to examine the applicability of this method to systems without 7r electrons. The LEED patterns revealed that the molecules are lying parallel to the substrate surface, with azimuthal order of forming two domains with the molecular
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K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
axes parallel to the [110] and [1 i0] directions. When we assumed that a molecule oriented parallel to the substrate with the plane formed by the C atoms parallel to the surface, an excellent correspondence between the observed and simulated spectra was obtained for a set of spectra of varying emission angle of electrons, as shown in Fig. 6. This corrects a similar study by other workers who concluded that the carbon plane is vertical to the surface [37]. Another interesting subject in short model compounds of polymers (oligomers) is the validity of the k-vector concept. It is a good quantum number for a system of infinite repeating units, while it has no sense for a system of only one unit. How does its significance decrease with the decrease of the number of repeating units N? This subject was studied by
examining the relaxation of the k-conservation rule in the normal emission spectra of sexiphenyl (N = 6) oriented vertically to the substrate surface [11]. It is a model molecule of poly(p-phenyl) formed by linearly connecting benzene rings. The sample was prepared by vacuum evaporation on a substrate heated to 90°C. The topmost part of the valence state is formed by the 7r band formed by the highest occupied molecular orbital (HOMO) of benzene. The low binding energy peaks of the spectra corresponding to this region changed with h~,, indicating the enhancement of these peaks at specific photon energies. These enhancements occur at hu matching the k-conservation rule, but become much broader compared with the similar enhancement for the long chain alkane n-CH3(CH2)34CH3. This is due to the
IAC "flat-on" h ~,=40eV ot=70deg.
TTC/Cu(100)
ARUPS h~=40eV o~=70deg. O/deg. 68 65 62.5 6O 57.5 55 52.5 50 47.5 45 42.5 40 37.5 35 32.5 30 27.5 25 22.5 2O 17.5 15 12.5 10 7.5 5 2.5 0
c-
e-
20
15
10
5
Eb (vs. EF)/eV
0
n-C32H66.,,L~±
i l
e/deg. 67,5 65 62.5 60 57.5 55 52.5 50 47.5 45 42.5 40 37.5 35 32.5 30 27.5 25 22.5 20 17.5 15 12.5 lO 7.5 5 2,5 o
.._ZI I
,Y i 4""
I
~
LX__./' A • I1'
-~.
L
I
30
a
I
J
I
A
i
I
25 20 15 Eb ( v s . E v a c ) / e V
I
10
Fig. 6. Observed and simulated polar emission angle dependence of the UPS spectra of the monolayer of an alkane n-CH3(CH2)42CH3 on Cu(100) surface. The analyzer moved in a plane which is vertical to the substrate surface and parallel to the [110] direction.
K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-9l (1998) 821-830 blurring of the k-vector in the initial state wavefunction, by an amount almost comparable to the width of the Brillouin zone. In this case, again the IAC calculations well reproduced the he dependence of the peak intensities, confirming both the molecular orientation and the physical picture described above. 3.3. Intensity oscillations with photon energy in C6o The intensity oscillation of photoemission intensity from the upper part of the valence band [25] has attracted much attention. Originally it was ascribed to the parity of the occupied levels, but later calculations excluded this origin. Hasegawa applied IAC calculations to C60, and found that the oscillation can be reproduced by only assuming a simple radial wavefunction which is large on a thin spherical shell with appropriate angular wavefunction [10]. This shows that the essential origin is the spherical shape of the electron cloud. This example shows another type of application of IAC approximation.
Acknowledgements The authors wish to thank Professors Yeshiya Harada and Hiroo Inokuchi, and the late Professor E.E. Koch for the introduction to the fruitful field of UPS of organic materials. They also thank Professor Nobuo Ueno of Chiba University for the constant collaboration. The collaboration with Drs Satoru Narioka, Eisuke Ito, Messrs. Kiyoshi Sugiyama, Daisuke Yoshimura, and Professor Yukio Ouchi at Nagoya University, Dr Koji Kamiya-Okudaira and Masaru Aoki of Chiba University, Dr Shinji Hasegawa and Takayuki Miyamae of IMS, and Dr Takafumi Miyazaki is gratefully acknowledged. The ARUPS studies using synchrotron radiation were performed as part of the Joint Studies Program of the Institute for Molecular Science. We thank the staff of UVSOR Facility at the Institute for Molecular Science for their support. The studies described above were supported in part by the COE Program of the Grant in Aid for Scientific Research 'Molecular Chirality' (No. 07CE2004) from the Ministry of Education, Science, Sports and Culture of Japan, and the Venture Business Laboratory Project
829
'Advanced Nanoprocess Technologies' of Nagoya University.
References [1] For reviews and fundamental aspects, e.g.W.D. Grobman, E.E. Koch, Photoemissionin Solids, vol. 2, Springer, Berlin, 1979, p. 261; W.R. Salaneck, CRC Crit. Rev. Solid State Mater. Sci., 12 (1985) 267; K. Seki, Optical Techniques to Characterize Polymer Systems, Elsevier, Amsterdam, 1989, p. 115. [2] N.C. Greenham, R.H. Friend, Solid State Phys. 49 (1995) 1. [3] W.R. Salaneck,S. Stafstroem, J.-L. Br6das, ConjugatedPolymer Surfaces and Interfaces, Cambridge University Press, Cambridge, 1996, and references therein. [4] A. Schmidt, M.L. Anderson,N.R. Armstrong, J. Appl. Phys. 78 (1995) 5619. [5] H. Ishii, K. Seki, IEEE Trans. Electron Devices 44 (1997) 1295, and references therein. [6] Y. Hirose, A. Kahn, V. Aristov, P. Soukiassian, Appl. Phys. Lett. 68 (1996) 217. [7] S. Hasegawa, S. Tanaka, Y. Yamashita, H. Inokuchi, H. Fujimoto, K. Kamiya, K. Seki, N. Ueno, Phys. Rev. B 48 (1993) 2596. [8] T. Permien,R. Engelhardt, C.A. Feldmann,E.E. Koch, Chem. Phys. Lett. 98 (1983) 527. [9] For a recent review, P.A. Dauben,Z. Phys. Chem. 202 (1997) 227. [10] S. Hasegawa, T. Miyamae, K. Yakushi, H. Inokuchi,K. Seki, N. Ueno, 7th InternationalConference on Electron Spectroscopy, Chiba, 1997, 4P-58. [11] S. Narioka, H. Ishii, K. Edamatsu, K. Kamiya, S. Hasegawa, T. Ohta, N. Ueno, K. Seki, Phys. Rev. 52 (1995) 2362. [12] N. Ueno, J. Electron Spectrosc. 78 (1996) 345 and references therein. [13] J. Simon, JJ. Andre, Molecular Semiconductors, Springer, Berlin, 1985. [14] T. Tani, Photographic Sensitivity, Oxford University Press, Oxford, 1995. [15] K.C. Kao, W. Hwang, Electrical Transport in Solids, Pergamon, Oxford, 1981. [16] W.R. Salaneck, J.L. Br6das, Adv. Mater. 8 (1996) 48-52. [17] S. Narioka, H. Ishii, M. Sei, Y. Ouchi, K. Seki, S. Hasegawa, T. Miyazaki, Y. Harima, K. Yamashita, Appl. Phys. Lett. 67 (1995) 1899. [18] H. Ishii, S. Hasegawa, D. Yoshimura, K. Sugiyama, S. Narioka, M. Sei, Y. Ouchi, K. Seki, Y. Harima, K. Yamashita, Mol. Cryst. Liq. Cryst. 296 (1997) 427, and references therein. [19] H. Ishii, K. Sugiyama, K. Seki, Proc. SPIE 3148, in press. [20] Y. Hirose, W. Chen, E.I. Haskal, S.R. Forrest, A. Kahn, Appl. Phys. Lett. 64 (1994) 3482. [21] K. Seki, H. Yanagi, Y. Kobayashi, T. Ohta, T. Tani, Phys. Rev. B 49 (1994) 2760. [22] K. Seki, T. Araki, E. Ito, K. Oichi, S. Narioka, T. Yokoyama, T. Ohta, S. Watanabe, A. Matsunaga, T. Tani, J. Im. Sci. Tech. 37 (1993) 589.
830
K. Seki, H. lshii/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 821-830
[23] N. Sato, M. Yoshikawa, J. Electron Spectrosc. 78 (1996) 387. [24] M. Fahlman, D. Beljonne, M. Loeglund, P.L. Burn, A.B. Holmes, R.H. Friend, J.L. Br~das, W.R. Salaneck, Chem. Phys. Lett. 214 (1993) 327. [25] P.J. Benning, D.M. Poirier, N. Troullier, J.L. Martins, J.H. Weaver, R.E. Haufler, L.P.F. Chibante, R.E. Smalley, Phys. Rev. B 44 (1991) 1962. [26] W. Erley, H. Ibach, Surf. Sci. 178 (1986) 565. [27] K. Seki, E. Ito, H. Ishii, Proc. Int. Conf. Electroluminesc., Kitakyushu, 1997. [28] K. Seki, S. Hashimoto, N. Sato, Y. Harada, H. Ishii, H. Inokuchi, J. Kanbe, J. Chem. Phys. 66 (1977) 3644. [29] N. Ueno, K. Sugita, K. Seki, H. Inokuchi, Phys. Rev. B 34 (1986) 6386. [30] A. Zangwill, Physics at Surfaces, Cambridge University Press, Cambridge, 1988, and references therein. [31] K. Wandelt, J.E. Hulse, J. Chem. Phys. 80 (1984) 1340. [32] R. Moench, Surf. Sci. 299/300 (1994) 928. [33] S. Kurtin, T.C. McGilI, C.H. Mead, Phys. Rev. Lett. 22 (1969) 1433. [34] N.V. Richardson, A.M. Bradshaw, in: C.R. Brundle, A.D. Baker (Eds.), Electron Spectroscopy, Vol. 4, Academic Press, New York, 1982, Ch. 3. [35] K. Seki, N. Ueno, U.O. Karlsson, R. Engelhardt, E.E. Koch, Chem. Phys. 105 (1986) 247. [36] N. Ueno, K. Seki, N. Sato, H. Fujimoto, K. Sugita, H. Inokuchi, Phys. Rev. B 41 (1990) 1176, and references therein. [37] R. Dudde, B. Reihl, Chem. Phys. Lett. 196 (1992) 91.
[38] Ch. Zubraegel, F. Schneider, M. Neumann, G. Haehner, Ch. Woell, M. Grunze, Chem. Phys. Lett. 219 (1994) 127. [39] D. Schmeisser, W. Jaegermann, C.H. Pettenkofer, H. Wachtel, A. Jimenez-Gonzales, J.U. von Schuetz, H.C. Wolf, P. Erk, H. Meixner, S. Huenig, Solid State Commun. 81 (1992) 827. [40] S. Hasegawa, T. Mori, K. Imaeda, S. Tanaka, K. Yamashita, H. Inokuchi, H. Fujimoto, K. Seki, N. Ueno, J. Chem. Phys. 100 (1994) 6969. [41] W.D. Grobman, Phys. Rev. B 17 (1978) 4573. [42] S. Hasegawa, K. Seki, H. Inokuchi, N. Ueno, J. Electron Spectrosc. 78 (1996) 391. [43] N. Ueno, K. Suzuki, S. Hasegawa, K. Kamiya, K. Seki, H. Inokuchi, J. Chem. Phys. 99 (1995) 7169. [44] K. Kamiya, M. Momose, A. Kitamura, Y. Harada, N. Ueno, T. Miyazaki, S. Hasegawa, H. Inokuchi, S. Narioka, H. Ishii, K. Seki, Mol. Cryst. Liq. Cryst. 267 (1995) 211. [45] K. Kamiya, M. Momose, A. Kitamura, Y. Harada, N. Ueno, S. Hasegawa, T. Miyazaki, H. Inokuchi, S. Narioka, H. Ishiii, K. Seki, J. Electron Spectrosc., in press. [46] M. Aoki, S. Masuda, Y. Einaga, K. Kamiya, A. Kitamura, M. Momose, N. Ueno, Y. Harada, T. Miyazaki, S. Hasegawa, H. Inokuchi, K. Deki, J. Electron Spectrosc. 76 (1995) 259. [47] Y. Azuma, T. Hasebe, T. Miyamae, K.K. Okudaira, Y. Harada, K. Seki, E. Morikawa, V. Saile, N. Ueno, J. Syn. Rad., in press. [48] D. Yoshimura, H. Ishii, Y. Ouchi, E. Ito, T. Miyamae, S. Hasegwa, N. Ueno, K. Seki, 7th International Conference on Electron Spectroscopy, Chiba, 1997, 2P-72.