- Email: [email protected]
c(8×2)
Surface Science 486 (2001) 55±64
www.elsevier.nl/locate/susc
Synchrotron radiation photoelectron spectroscopy study of Pb-Pc thin ®lms on InSb(1 0 0)-(4 2)=c(8 2) L. Giovanelli a,*, H. Von Schenck b, M. Sinner-Hettenbach c, thelid b, G. Le Lay a N. Papageorgiou d, M. Go a
b
CRMC2-CNRS, Campus de Luminy, 13288 Marseille, France Department of Material Physics, Royal Institute of Technology, Teknikringen 14, 10044 Stockholm, Sweden c University of Tubingen, IPC, Auf der Morgenstelle 8, 72076 Tubingen, Germany d PIIM UMR 6633, Universit e de Provence, 13397 Marseille, France Received 9 November 2000; accepted for publication 29 March 2001
Abstract The electronic properties and the thermal stability of a thin ®lm of lead-phthalocyanine deposited on the InSb(1 0 0)(4 2)=c(8 2) surface were studied by synchrotron radiation core level and valence band photoelectron spectroscopy. The interaction between the overlayer and the substrate was determined by analyzing the photoemission spectra of a thin ®lm and of a single monolayer of adsorbed molecules. Subsequently the monolayer was annealed at increasing temperatures, leading ®rst to a gradual change of the oxidation state of the central lead atom, then to a fragmentation of the macrocycle itself. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Synchrotron radiation photoelectron spectroscopy; Aromatics; Indium antimonide; Heterojunctions; Semiconductor± semiconductor thin ®lm structures
1. Introduction Metallo-phthalocyanines (M-Pc's) are organic semiconductors with interesting physical and technological properties. They ®nd applications in the ®eld of gas sensor, non-linear optics and molecular electronics [1]. Their structure (Fig. 1) is
* Corresponding author. Present address: APE beamline INFM, c/o Lab. Elettra Sincrotrone Trieste, S.S. 14 Km 163.5, Area Science Park, I-34012 Basovizza, Trieste, Italy. Tel.: +3940-3758408; fax: +39-40-3758400. E-mail address: [email protected] (L. Giovanelli).
made of four benzene±pyrrole moieties (isoindole moieties) connected together by four meso-aza nitrogens. The whole structure contains a metal atom at the center, coordinated to the pyrrole nitrogens. Compared to more largely studied transition metal Pc's, the lead-phthalocyanine (Pb-Pc) represents quite a particular case. In fact, because the Pb atom does of its large dimension (1.75 A), not ®t inside the central coordination core but lies above the molecular plane. According to the crystal form, the steric repulsion of the Pb atom is in the monoclinic phase and 1.28 A in the 0.91 A triclinic phase [2,3]. One interesting property of Pb-Pc's is that when they crystallize in the monoclinic phase the d.c. conductivity along the vertical
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 0 6 2 - 7
56
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Fig. 1. The lead-phthalocyanine molecule. The diagonal length The height of Pb protrusion is 0.91±1.28 A above is about 13 A. the molecular plane depending on the crystallographic form.
stacks becomes relatively high (r 10 4 S cm 1 [4]). The study of the interaction of these organic materials with semiconductor and metal surfaces appears to be of great interest for a planned use of Pc's as a constituent of electronic devices. In this view many attempts have been recently made to control the growth of both metal and metal free Pc's on a number of dierent substrates. The results are very encouraging since well ordered single- or multi-layers of molecules have been deposited on both metallic and semiconducting surfaces [5±13]. Many authors have emphasized that to improve the control of the molecular deposition, the proper surface reactivity is essential. This is normally accomplished with the less reactive substrates, which allow for a good lateral mobility. It was also found correlatively that a good matching between the substrate and overlayer lattice size plays a non-secondary role for the growth of well-ordered layers. An interesting case is represented by the M-Pc thin ®lms grown on III±V semiconductor compounds. Recently Cox and coworkers have reported a detailed structural investigations of Cu±Pc deposited on a variety of III±V semiconductors with dierent chemical compositions and surface reconstructions [8,12,13]. Despite the presence of surface dangling bonds, ordered molecular domains were found after deposition on In-terminated InSb(1 0 0) and InAs(1 0 0)-(4 2)=c(2 8) reconstructed surfaces. The results show that both the lattice parameter and the chemical composition of the reconstructed surfaces in¯uenced good
growth conditions. For instance, the indiumterminated InSb(1 0 0)-(4 2)=c(2 8) surface allowed the creation of a well-ordered molecular layer whereas this was not the case for the antimony rich c(4 4) reconstruction of the same substrate. In order to better understand the mechanisms responsible for Pc adsorption on this kind of surfaces we performed a synchrotron radiation highresolution photoelectron spectroscopy (PES) studies of a thin ®lm of Pb-Pc (see Fig. 1) deposited on the indium-rich InSb(1 0 0)-(4 2)=c(8 2) surface. The interaction of the organic molecules with the substrate was studied by means of both core level (CL) and valence band (VB) spectroscopy. The composition and the electronic properties of the molecules in contact with the substrate were then followed as a function of annealing temperatures up to 450°C. 2. Experimental All measurements were performed under ultrahigh vacuum at Beamline I-311, at the MAX-Lab synchrotron radiation facility (Lund, Sweden). The preparation chamber was equipped with an ion gun, a low energy electron diraction apparatus (LEED) and with a Knudsen evaporation cell for molecular sublimation. The n-type doped InSb(1 0 0) single crystal was prepared by cycles of Ar sputtering (500 eV) followed by annealing at 400°C. The clean substrate showed a sharp (4 2)=c(8 2) LEED pattern characteristic of a well reconstructed surface [12]. The Pb-Pc molecules were puri®ed ex situ at a temperature of 344°C resulting in an ultra-pure powder (99.9%). The molecules were then deposited on the substrate held at room temperature (RT) by sublimation from the carefully degassed Knudsen cell at about 300°C. The evaporation rate was about one monolayer (ML) per minute. The number of deposited layers was estimated by X-ray photoelectron spectroscopy (XPS). The experimental chamber was equipped with a large hemispherical analyzer (200 mm radius) of Scienta type. The overall energy resolution of monochromator and analyzer was better than 30 meV for VB spectra
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
whereas it was below 80 meV for CLs. All the binding energies (BEs) are referenced to the Fermi level of a clean tantalum foil in direct contact with the substrate. All spectra were taken in an angleintegrated mode and in normal emission. 3. Results and discussion A thin layer, 1 nm thick, corresponding to about 3 ML of Pb-Pc, was ®rst deposited at RT onto the clean InSb(1 0 0)-(4 2)=c(8 2) reconstructed surface. After deposition the substrate LEED pattern was no longer visible and no new structure due to the overlayer were detectable. The thin ®lm was then annealed at increasing temperatures. At the annealing temperature of 300°C, the molecules from the third and second layer, not in direct contact with the substrate, started to desorb, but no LEED pattern was observed. When the temperature was raised to 325°C a sharp LEED pattern with a rectangular unit cell, similar to the one previously reported by Cox et al. [13] for CuPc deposited on InSb(1 0 0)-(4 2)=c(8 2), was observed. In fact, as already proved on other substrates [14], the annealing temperature was high enough to desorb the two uppermost layers but not the molecules of the ®rst ML in direct contact with the surface. We thus identi®ed the system annealed at 325°C as corresponding to one ordered ML of Pb-Pc on the InSb(1 0 0) surface. Later on the single layer was further annealed at 340°C and 450°C. The LEED pattern due to the overlayer gradually vanished while at the same time the substrate reconstructed structure became visible again. In Fig. 2 we compare the CL and VB spectra of the as-deposited 3 ML ®lm of Pb-Pc on InSb(1 0 0) at RT (bottom curves) to some previously reported results of thick Pb-Pc ®lms (upper curves) [15,16]. The C 1s and N 1s spectra are almost the same for our 3 ML ®lm and for the thick ®lms. Compared to the thick ®lms, the Pb 4f spectrum of our 3 ML ®lm presents a small shoulder at higher BEs, this point will be addressed later. The dierences seen in the VB spectra are essentially imputable to dierent collection geometries of the photoelectrons with analyzers of dierent types
57
and to changes in the photoionization cross-section for the dierent VB features at the two different photon energies used. Hence, we can conclude that for the as deposited 3 ML ®lm the interaction of the molecules with the substrate is weak and that at this stage the ®lm is already representative of the Pb-Pc bulk material. To derive the electronic properties of the molecules adsorbed directly at the interface between the organic ®lm and the substrate, we then analyze the spectral dierences between the 3 ML ®lm and the ML ®lm obtained after annealing at 325°C. Later on, the behavior of this ML ®lm itself is followed as a function of increasing annealing temperatures. 3.1. Core levels In this section we present the CLs of the central Pb atom and of the macrocycle. The bottom spectra of Figs. 3 and 4 concern the as-deposited thin ®lm at RT, while the other spectra refer to the system annealed at increasing temperatures. All spectra are normalized to the incident photon ¯ux. 3.1.1. Pb 4f core levels We ®rst focus on the CL spectra of the central metal atom. It has been shown that the metal's chemical state strongly aects the behavior of the molecule upon adsorption on dierent surfaces [5,6,16±18]. As already mentioned, because of its large size, the Pb atom does not ®t inside the macrocycle but instead lies as a protrusion above the horizontal plane (see Fig. 1). As a result the Pb atom is less strongly bound to the molecule than some smaller atoms like Cu or Co. In fact, after the adsorption of thin layers of Pb-Pc on the Pt(1 1 1) surface it was found that the Pb atoms migrate away from the molecule, changing their oxidation state, and form an alloy with the metallic substrate [18]. It is thus interesting to check the stability of the molecule with respect to a potentially less reactive surface such as InSb(1 0 0)(4 2)=c(8 2). In Fig. 3 we display the Pb 4f CLs for the thin ®lm deposited at RT and then annealed at increasing temperatures. A strong 4f doublet dominates the spectrum of the 3 ML. The BE for the
58
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Fig. 2. Comparison between 3 ML of Pb-Pc/InSb(1 0 0) (bottom spectra) and thick Pb-Pc ®lm (upper spectra) [15,16]: (a) C 1s; (b) N 1s, (c) Pb 4f, (d) VB. The photon energies are indicated in the ®gure. The intensities have been normalized to the peak maxima.
7/2 component is 138.6 eV and the full width at half maximum (FWHM) is 0.9 eV. These values correspond to the doubly oxidized Pb(II) species in the Pb-Pc molecules [16,18]. A small component is present at lower BE and may be assigned to the molecules directly in contact with the substrate. A strong modi®cation to the Pb CLs appears when the system is annealed at 300°C: two doublets are now clearly resolved, revealing the presence of Pb
atoms in two dierent oxidation states. The doublet at higher BEs has the same BE position and width as the one attributed to the Pb(II) species in the pristine molecules. The new component that grew at lower BEs is sensibly narrower (FWHM 0:6 eV) and lies at 136.9 eV BE; these values are representative of fully reduced Pb0 species [19]. As a matter of fact, the 300°C spectrum corresponds to an intermediate situation
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Fig. 3. Pb 4f CL spectra of, from bottom to top, 3 ML of PbPc deposited at RT on InSb(1 0 0); the system after 5 min annealing at 300°C; an ordered single layer obtained by annealing the 3 ML at 325°C, the same system annealed at 340°C and 450°C. The photon energy was 250 eV. All spectra are normalized to the photon ¯ux.
between the original 3 ML ®lm and the ML ®lm obtained after annealing at 325°C. This is con®rmed by the intensities: the total area under the 300°C spectrum, while smaller, indeed, than that of the RT thin ®lm, is higher than that of the 325°C ML ®lm (see Fig. 5). In the spectrum corresponding to this adsorbed ML, the oxidized component has completely disappeared while the reduced one has grown in intensity. This is due on one side to the transformation of Pb(II) to Pb0 , and on the other to the lack of attenuation from the now desorbed second layer. It is worth noting that upto 340°C the Pb-Pc molecules are stable since they were puri®ed at 344°C (see Section 2). The presence of the
59
InSb(1 0 0) surface must then have an active role in the changing of oxidation state of the lead atoms. The question arises whether or not the Pb atoms are in direct contact with the substrate surface. A recent photoemission experiment in which a subML ®lm of lead was deposited on the InAs(1 0 0)(4 2)=c(8 2) surface, forming an InAs(1 0 0)1 4-Pb reconstruction [20], reveals the same zero oxidation state for the lead atoms. Since Pb also adsorbs in bulk InSb as well as at its surface as neutral entities [21] we assume that in the ML regime the lead atoms bind directly to the InSb(1 0 0) surface. In this view, the simultaneous presence of reduced and oxidized components in the 300°C spectrum re¯ects the Pb atoms interacting directly with the substrate and those which are not in direct contact with the substrate. Finally, when the system is annealed to 340°C the amount of Pb left on the surface has considerably diminished and it has completely disappeared at the annealing temperature of 450°C. We emphasize here that the desorption of lead is preferential to that of carbon and nitrogen. This is shown in Fig. 5, where the intensity of the C 1s (in the case of N 1s a similar result was found) and Pb 4f CLs as a function of annealing temperatures, are compared (both series of data are normalized to the intensity of the spectra of the as-deposited 3 ML ®lm). The higher desorption rate of lead is already noticeable at 300°C annealing temperature, but it becomes even more remarkable at 340°C. The present data are then consistent with a picture in which the lead atoms desorb predominantly from the surface after having changed their oxidation state from two to zero. 3.1.2. Core levels of the macrocycle: C 1s and N 1s The C 1s and N 1s CL spectra of the organic macrocycle are shown in Fig. 4. Their BEs and shapes help to determine the chemical state of the molecules [5,16±18]. The C 1s spectrum of the thin ®lm is composed of three main components. The most intense feature is related to the 24 carbon atoms of the benzene sub-structures (denoted C2± C7 in Fig. 1). The second component is assigned to the eight pyrrolitic carbon atoms bonded to the nitrogen atoms (C1 and C8 in Fig. 1). We found a value of 284.6 eV for the BE of the benzene
60
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Fig. 4. C 1s (left panel) and N 1s (right panel) CL spectra of, from bottom to top, 3 ML of Pb-Pc deposited at RT on InSb(1 0 0); the system after 5 min annealing at 300°C; an ordered single layer obtained by annealing the 3 ML at 325°C, the same system annealed at 340°C and 450°C. The photon energy was 350 and 450 eV for the C 1s and N 1s respectively. All spectra are normalized to the photon ¯ux.
component whereas the pyrrole related peak is displaced by 1.3 eV towards higher BEs from the main component. These values are in fair agreement with those already reported for a thick ®lm of Pb-Pc [16], again con®rming the view that our 3 ML thin ®lm is already representative of a thick Pb-Pc layer (see Fig. 2). The third component is a p±p shake-up of the pyrrole peak, shifted by 1.85 eV to higher BE from it. It corresponds to the electronic excitation from the highest occupied to the lowest unoccupied molecular orbital
(HOMO±LUMO electronic transition). The N 1s spectrum is also similar to the previously reported spectra [16]. The spectrum relative to the 3 ML ®lm displays a single main component centered at 398.55 eV. In fact the two chemically non-equivalent nitrogen sites of the molecule could not be distinguished. The rather broad structure present at 1.8 eV higher BE is probably due to a ®nal state excitation of p±p character. It is interesting to note the dierence in relative intensity of this shake-up structure between the spectra of the C 1s
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
61
Fig. 5. Peak area of C 1s and Pb 4f as a function of annealing temperature. The data are divided by the peak area of the 3 ML to emphasize the rate of desorption after each annealing.
and N 1s. One possible reason for this eect is the high localization of the HOMO in the macrocycle. In fact, as shown by the theoretical calculation of Orti and Bredas [22] on H2 Pc, the HOMO is strongly localized on the isoindole moieties and no contribution from the nitrogens to this molecular orbital was found. The formation of a nitrogen core hole would then be less eective in perturbing the HOMO electrons and an excitation to an unoccupied molecular state would have less probability of occurrence. When the system is annealed at 300°C and 325°C the intensities of the CLs of the macrocycle decrease because of the gradual desorption of the upper layers; the CL lines broaden and shift to higher BEs. The CL spectra of the adsorbed monolayer display an overall broadening of 0.1
eV compared to the spectra of the thin ®lm. This re¯ects the bonding of the molecules to the surface. However the interaction appears to be weaker than that observed when the same molecule is deposited on a more reactive surface like Si(1 1 1) [16]. In fact, for Pb-Pc/Si(1 1 1) the broadening in going from a multilayer to a ML ®lm was such that for the latter the pyrrole peak in the C 1s spectrum was hardly distinguishable, just appearing as a shoulder of the main benzene peak, whereas the p±p shake-up was not detectable at all. Along with the broadening, the annealings at 300°C and 325°C induce also a shift to higher BEs of all the macrocycle related structures. In the case of the C 1s core level this shift is almost the same for the benzene and pyrrole related peaks but somewhat larger for the p±p satellite. In fact the
62
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
shake-up distance in energy from the pyrrole peak is now 1.9 eV and it may be due to a change of the HOMO±LUMO separation in the adsorbed ML. Annealing to 340°C did not result in a signi®cant change of the C 1s and N 1s CL line shape. Yet when the system was annealed at 450°C for 15 minutes the spectral intensities diminished appreciably as a consequence of the thermal desorption of the molecules. A shift of 0.2 eV to lower BEs was also measured and we notice a further broadening of all features. Moreover, the shake-up structures for both the C 1s and N 1s CL totally disappeared. All the observed changes in the CL spectra of the macrocycles point to strong chemical modi®cations when the temperature of the system is raised to 450°C. It is likely that such alterations reveal a breaking of the molecules left on the surface. The breaking of the molecular frame would cause the lost of the molecular orbitals proper of the macrocycle (e.g. the HOMO, cf. Section 3.2) and this would in turn explain the absence of the shake-up structures in the CL spectra. On the other hand, a total decomposition of the molecules would strongly aect the intensity ratio between the benzene and pyrrole component of the C 1s spectrum resulting in a single broad component as observed, for example, when a single layer of Pb-Pc was annealed at 150°C on the Pt(1 1 1) surface [18]. Because the intensity ratio between the two main peaks of the C 1s spectrum remains essentially unchanged, we thus conclude that the breaking of the molecules leads to fragmentation rather than to complete decomposition. 3.2. Valence bands In Fig. 6 we present VB electron distribution curves (EDCs) of the as deposited 3 ML ®lm at RT (bottom spectrum) and after successive annealings at the same temperatures as previously. The top spectrum is that of the clean InSb(1 0 0)(4 2)=c(8 2) surface. All spectra are normalized to the incident photon ¯ux. As already pointed out in Fig. 2, the VB of the thin 3 ML ®lm is already representative of the bulk material without any evident sign of interaction with the substrate.
Fig. 6. VB spectra of, from bottom to top, 3 ML of Pb-Pc deposited at RT on InSb(1 0 0); the system after 5 min annealing at 300°C; an ordered single layer obtained by annealing the 3 ML at 325°C, the same system annealed at 340°C and 450°C, the clean InSb(1 0 0)-(4 2)=c(8 2). The photon energy was 50 eV. All spectra are normalized to the photon ¯ux.
When the system was annealed to 300°C the VB spectrum did not show signi®cant dierences with respect to the 3 ML, RT spectrum. On the contrary, when a single ordered ML of molecules was left on the surface after annealing at 325°C, the EDC of the VB appeared somehow modi®ed (Fig. 6). Apart from the enhanced contribution of the
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
signal coming from the substrate, there is in fact a sharpening and increasing of intensity of the HOMO and HOMO-1 related structures and the appearance of a new structure at about 5 eV BE. These two evidences may be related to the change in oxidation state of the Pb atoms and to the reduced dimensionality of the system with respect to the previous spectra. In fact, in the highly ordered 1 ML Pb-Pc/InSb(1 0 0)-(4 2)=c(8 2) there is probably a reduced overlap of the wave functions of neighboring molecules which are consequently less perturbed than in the 3 ML system. This will in turn increase the density of states of the less bound orbitals as observed for the HOMO and HOMO-1. On the other hand a precise assignment of the Pb contribution to the VB is not straightforward due to the small spectroscopic weight of the sole Pb atom with respect to the entire macrocycle. Annealing the system at 340°C, a temperature of preferential desorption of the lead atoms, did not produce a strong change of the VB features. However, the VB spectrum changes dramatically when the annealing temperature is raised to 450°C. In fact the HOMO is no longer distinguishable within the highest part of the substrate VB and, in general, all the molecular features are strongly aected by the interaction with the substrate. These drastic modi®cations con®rm the scenario proposed in the discussion of the CLs of the macrocycle, that is, annealing at 450°C causes a fragmentation of the molecules remaining on the surface. 3.3. Heterojunction band oset In our analysis of the Pb-Pc/InSb(1 0 0) interface we now focus on the VB oset at the interface of the organic±inorganic heterojunction. Before deposition (top spectrum in Fig. 6) the valence band maximum (VBM) of the InSb(1 0 0) clean reconstructed surface is at 0.16 eV below the Fermi level (EF ). After deposition of 3 MLs of Pb-Pc, and subsequent annealing, a detailed deconvolution of the substrate CLs (not shown here) evidenced a shift of 0.17 eV towards higher BEs caused by band bending. Therefore the VBM of the substrate in the presence of the molecular adlayer is now
63
located at 0.33 eV below EF . When a ML of molecules remains on the surface the simultaneous presence of substrate and adlayer related photoemission structures allows us to determine the VB oset at the interface. The top of the HOMO band was found by interpolation of the HOMO-peak leading edge to the zero emission line [23]. For the ML we found a value of 0.98 eV from EF , consequently, the VB oset is then estimated to be 0.65 eV. It is noteworthy that in the spectrum relative to the ordered ML the top of the InSb VB appears to shift by 0.11 eV towards lower BEs. This is due to the ®lling of the density of states near the InSb VBM caused by the bonding of the molecules to the substrate [23]. 4. Conclusions We have studied the interfacial behavior and the thermal stability of Pb-Pc thin ®lms on the InSb(1 0 0)-(4 2)=c(8 2) surface by synchrotron radiation PES. The electronic properties at the interface between the Pb-Pc molecules and the substrate have been determined upon comparison of a single ordered ML and a thin ®lm of 3 ML, still representative of bulk like properties. The results can be summarized as follows. The CLs of the macrocycle, when in direct contact with the substrate surface, are slightly aected by the interaction, which leads to a small broadening of all peaks. Upon moderate annealing, the Pb atoms change their oxidation state from two to zero and they directly bind to the substrate. The changes observed in the VB EDC support the picture in which the molecules of the ordered ML are weakly bound to the surface. Moreover, the observed sharpening of the HOMO and HOMO-1 suggests that the molecular states are now less perturbed by the interaction between neighboring molecules. Further annealing of the system upto 450°C led to a preferential desorption of the Pb atoms and to an enhanced interaction of the organic macrocycles remaining on the surface which now appear to be strongly chemisorbed. This chemisorption aects the molecular structure promoting a fragmentation of the macrocycle.
64
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Finally a value of 0.65 eV was determined for the VB oset at the organic±inorganic heterojunction.
Acknowledgements We wish to thank J.M. Layet and G. Terzian for actively participating in the measurements and Prof. P. Perfetti for careful reading of the manuscript. We also thank J.C. Mossoyan, M. Mossoyan-Deneux for supplying the Pb-Pc molecules. The work of L. Giovanelli is supported by grant no. 9811/2271 of the PACA region. H. von Shenck would like to thank the Ernst Johnson foundation for ®nancial support. We acknowledge the European Union for ®nancial support in the context of the ARI program.
References [1] S.R. Forest, Chem. Rev. 97 (1997) 1793. [2] K. Ukey, Acta Cryst. B 29 (1978) 2290. [3] Y. Iyechika, K. Yakushi, I. Ikemoto, H. Kuroda, Acta Cryst. B 38 (1982) 766. [4] K. Ukei, Ph.D. Thesis, University of Tokyo, 1975. [5] L. Ottaviano, S. Di Nardo, L. Lozzi, M. Passacantando, P. Picozzi, S. Santucci, Surf. Sci. 373 (1997) 318.
[6] F. Rochet, G. Dufour, H. Roulet, N. Motta, A. Sgarlata, M.N. Piancastelli, M. De Crescenzi, Surf. Sci 319 (1994) 10. [7] I. Chizhov, G. Scoles, A. Kahn, Langmuir 16 (2000) 4358. [8] J.J. Cox, T.S. Jones, Surf. Sci. 457 (2000) 311. [9] M.D. Upward, P.H. Beton, P. Moriarty, Surf. Sci. 441 (1999) 21. [10] T. Shimada, A. Suzuki, T. Sakurada, A. Koma, Appl. Phys. Lett. 68 (1996) 2502. [11] M. Nakamura, Y. Morita, H. Tokumoto, Appl. Surf. Sci. 113/114 (1997) 316. [12] J.J. Cox, S.M. Bayliss, T.S. Jones, Surf. Sci. 425 (1999) 326. [13] J.J. Cox, S.M. Bayliss, T.S. Jones, Surf. Sci 433±435 (1999) 152. [14] J.-Y. Grand, T. Kunstmann, D. Homann, A. Haas, M. Dietsche, J. Seifritz, R. M oller, Surf. Sci. 366 (1996) 403. [15] E.E. Koch, W.D. Grobman, J. Chem. Phys. 67 (1977) 837. [16] L. Ottaviano, L. Lozzi, S. Santucci, S. Di Nardo, M. Passacantando, Surf. Sci. 392 (1997) 52. [17] G. Dufour, C. Poncey, F. Rochet, H. Roulet, M. Sacchi, M. De Santis, M. De Crescenzi, Surf. Sci 319 (1994) 251. [18] N. Papageorgiou, J.C. Mossoyan, M. Mossoyan-Deneux, G. Terzian, E. Janin, M. G othelid, L. Giovanelli, J.M. Layet, G. Le Lay, Appl. Surf. Sci. 162±163 (2000) 178. [19] B.V. Crist, Handbook of Monochromatic XPS spectra, Vol. 1, XPS International, Kamiasao, Asao-ku, Kawasaki, 1999. [20] J.M. Layet, M. Carrere, H.J. Kim, R.L. Johnson, R. Belkhou, V. Zhilin, V.Yu. Aristov, G. Le Lay, Surf. Sci. 402±404 (1998) 724. [21] M. Oszwaldowski, T. Berus, J. Phys. Chem. Solids 61 (2000) 875. [22] E. Orti, J.L. Bredas, J. Chem. Phys. 89 (1988) 1009. [23] Y. Hirose, W. Chen, E.I. Haskal, S.R. Forest, A. Kahn, Appl. Phys. Lett. 64 (1994) 3482.
www.elsevier.nl/locate/susc
Synchrotron radiation photoelectron spectroscopy study of Pb-Pc thin ®lms on InSb(1 0 0)-(4 2)=c(8 2) L. Giovanelli a,*, H. Von Schenck b, M. Sinner-Hettenbach c, thelid b, G. Le Lay a N. Papageorgiou d, M. Go a
b
CRMC2-CNRS, Campus de Luminy, 13288 Marseille, France Department of Material Physics, Royal Institute of Technology, Teknikringen 14, 10044 Stockholm, Sweden c University of Tubingen, IPC, Auf der Morgenstelle 8, 72076 Tubingen, Germany d PIIM UMR 6633, Universit e de Provence, 13397 Marseille, France Received 9 November 2000; accepted for publication 29 March 2001
Abstract The electronic properties and the thermal stability of a thin ®lm of lead-phthalocyanine deposited on the InSb(1 0 0)(4 2)=c(8 2) surface were studied by synchrotron radiation core level and valence band photoelectron spectroscopy. The interaction between the overlayer and the substrate was determined by analyzing the photoemission spectra of a thin ®lm and of a single monolayer of adsorbed molecules. Subsequently the monolayer was annealed at increasing temperatures, leading ®rst to a gradual change of the oxidation state of the central lead atom, then to a fragmentation of the macrocycle itself. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Synchrotron radiation photoelectron spectroscopy; Aromatics; Indium antimonide; Heterojunctions; Semiconductor± semiconductor thin ®lm structures
1. Introduction Metallo-phthalocyanines (M-Pc's) are organic semiconductors with interesting physical and technological properties. They ®nd applications in the ®eld of gas sensor, non-linear optics and molecular electronics [1]. Their structure (Fig. 1) is
* Corresponding author. Present address: APE beamline INFM, c/o Lab. Elettra Sincrotrone Trieste, S.S. 14 Km 163.5, Area Science Park, I-34012 Basovizza, Trieste, Italy. Tel.: +3940-3758408; fax: +39-40-3758400. E-mail address: [email protected] (L. Giovanelli).
made of four benzene±pyrrole moieties (isoindole moieties) connected together by four meso-aza nitrogens. The whole structure contains a metal atom at the center, coordinated to the pyrrole nitrogens. Compared to more largely studied transition metal Pc's, the lead-phthalocyanine (Pb-Pc) represents quite a particular case. In fact, because the Pb atom does of its large dimension (1.75 A), not ®t inside the central coordination core but lies above the molecular plane. According to the crystal form, the steric repulsion of the Pb atom is in the monoclinic phase and 1.28 A in the 0.91 A triclinic phase [2,3]. One interesting property of Pb-Pc's is that when they crystallize in the monoclinic phase the d.c. conductivity along the vertical
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 0 6 2 - 7
56
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Fig. 1. The lead-phthalocyanine molecule. The diagonal length The height of Pb protrusion is 0.91±1.28 A above is about 13 A. the molecular plane depending on the crystallographic form.
stacks becomes relatively high (r 10 4 S cm 1 [4]). The study of the interaction of these organic materials with semiconductor and metal surfaces appears to be of great interest for a planned use of Pc's as a constituent of electronic devices. In this view many attempts have been recently made to control the growth of both metal and metal free Pc's on a number of dierent substrates. The results are very encouraging since well ordered single- or multi-layers of molecules have been deposited on both metallic and semiconducting surfaces [5±13]. Many authors have emphasized that to improve the control of the molecular deposition, the proper surface reactivity is essential. This is normally accomplished with the less reactive substrates, which allow for a good lateral mobility. It was also found correlatively that a good matching between the substrate and overlayer lattice size plays a non-secondary role for the growth of well-ordered layers. An interesting case is represented by the M-Pc thin ®lms grown on III±V semiconductor compounds. Recently Cox and coworkers have reported a detailed structural investigations of Cu±Pc deposited on a variety of III±V semiconductors with dierent chemical compositions and surface reconstructions [8,12,13]. Despite the presence of surface dangling bonds, ordered molecular domains were found after deposition on In-terminated InSb(1 0 0) and InAs(1 0 0)-(4 2)=c(2 8) reconstructed surfaces. The results show that both the lattice parameter and the chemical composition of the reconstructed surfaces in¯uenced good
growth conditions. For instance, the indiumterminated InSb(1 0 0)-(4 2)=c(2 8) surface allowed the creation of a well-ordered molecular layer whereas this was not the case for the antimony rich c(4 4) reconstruction of the same substrate. In order to better understand the mechanisms responsible for Pc adsorption on this kind of surfaces we performed a synchrotron radiation highresolution photoelectron spectroscopy (PES) studies of a thin ®lm of Pb-Pc (see Fig. 1) deposited on the indium-rich InSb(1 0 0)-(4 2)=c(8 2) surface. The interaction of the organic molecules with the substrate was studied by means of both core level (CL) and valence band (VB) spectroscopy. The composition and the electronic properties of the molecules in contact with the substrate were then followed as a function of annealing temperatures up to 450°C. 2. Experimental All measurements were performed under ultrahigh vacuum at Beamline I-311, at the MAX-Lab synchrotron radiation facility (Lund, Sweden). The preparation chamber was equipped with an ion gun, a low energy electron diraction apparatus (LEED) and with a Knudsen evaporation cell for molecular sublimation. The n-type doped InSb(1 0 0) single crystal was prepared by cycles of Ar sputtering (500 eV) followed by annealing at 400°C. The clean substrate showed a sharp (4 2)=c(8 2) LEED pattern characteristic of a well reconstructed surface [12]. The Pb-Pc molecules were puri®ed ex situ at a temperature of 344°C resulting in an ultra-pure powder (99.9%). The molecules were then deposited on the substrate held at room temperature (RT) by sublimation from the carefully degassed Knudsen cell at about 300°C. The evaporation rate was about one monolayer (ML) per minute. The number of deposited layers was estimated by X-ray photoelectron spectroscopy (XPS). The experimental chamber was equipped with a large hemispherical analyzer (200 mm radius) of Scienta type. The overall energy resolution of monochromator and analyzer was better than 30 meV for VB spectra
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
whereas it was below 80 meV for CLs. All the binding energies (BEs) are referenced to the Fermi level of a clean tantalum foil in direct contact with the substrate. All spectra were taken in an angleintegrated mode and in normal emission. 3. Results and discussion A thin layer, 1 nm thick, corresponding to about 3 ML of Pb-Pc, was ®rst deposited at RT onto the clean InSb(1 0 0)-(4 2)=c(8 2) reconstructed surface. After deposition the substrate LEED pattern was no longer visible and no new structure due to the overlayer were detectable. The thin ®lm was then annealed at increasing temperatures. At the annealing temperature of 300°C, the molecules from the third and second layer, not in direct contact with the substrate, started to desorb, but no LEED pattern was observed. When the temperature was raised to 325°C a sharp LEED pattern with a rectangular unit cell, similar to the one previously reported by Cox et al. [13] for CuPc deposited on InSb(1 0 0)-(4 2)=c(8 2), was observed. In fact, as already proved on other substrates [14], the annealing temperature was high enough to desorb the two uppermost layers but not the molecules of the ®rst ML in direct contact with the surface. We thus identi®ed the system annealed at 325°C as corresponding to one ordered ML of Pb-Pc on the InSb(1 0 0) surface. Later on the single layer was further annealed at 340°C and 450°C. The LEED pattern due to the overlayer gradually vanished while at the same time the substrate reconstructed structure became visible again. In Fig. 2 we compare the CL and VB spectra of the as-deposited 3 ML ®lm of Pb-Pc on InSb(1 0 0) at RT (bottom curves) to some previously reported results of thick Pb-Pc ®lms (upper curves) [15,16]. The C 1s and N 1s spectra are almost the same for our 3 ML ®lm and for the thick ®lms. Compared to the thick ®lms, the Pb 4f spectrum of our 3 ML ®lm presents a small shoulder at higher BEs, this point will be addressed later. The dierences seen in the VB spectra are essentially imputable to dierent collection geometries of the photoelectrons with analyzers of dierent types
57
and to changes in the photoionization cross-section for the dierent VB features at the two different photon energies used. Hence, we can conclude that for the as deposited 3 ML ®lm the interaction of the molecules with the substrate is weak and that at this stage the ®lm is already representative of the Pb-Pc bulk material. To derive the electronic properties of the molecules adsorbed directly at the interface between the organic ®lm and the substrate, we then analyze the spectral dierences between the 3 ML ®lm and the ML ®lm obtained after annealing at 325°C. Later on, the behavior of this ML ®lm itself is followed as a function of increasing annealing temperatures. 3.1. Core levels In this section we present the CLs of the central Pb atom and of the macrocycle. The bottom spectra of Figs. 3 and 4 concern the as-deposited thin ®lm at RT, while the other spectra refer to the system annealed at increasing temperatures. All spectra are normalized to the incident photon ¯ux. 3.1.1. Pb 4f core levels We ®rst focus on the CL spectra of the central metal atom. It has been shown that the metal's chemical state strongly aects the behavior of the molecule upon adsorption on dierent surfaces [5,6,16±18]. As already mentioned, because of its large size, the Pb atom does not ®t inside the macrocycle but instead lies as a protrusion above the horizontal plane (see Fig. 1). As a result the Pb atom is less strongly bound to the molecule than some smaller atoms like Cu or Co. In fact, after the adsorption of thin layers of Pb-Pc on the Pt(1 1 1) surface it was found that the Pb atoms migrate away from the molecule, changing their oxidation state, and form an alloy with the metallic substrate [18]. It is thus interesting to check the stability of the molecule with respect to a potentially less reactive surface such as InSb(1 0 0)(4 2)=c(8 2). In Fig. 3 we display the Pb 4f CLs for the thin ®lm deposited at RT and then annealed at increasing temperatures. A strong 4f doublet dominates the spectrum of the 3 ML. The BE for the
58
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Fig. 2. Comparison between 3 ML of Pb-Pc/InSb(1 0 0) (bottom spectra) and thick Pb-Pc ®lm (upper spectra) [15,16]: (a) C 1s; (b) N 1s, (c) Pb 4f, (d) VB. The photon energies are indicated in the ®gure. The intensities have been normalized to the peak maxima.
7/2 component is 138.6 eV and the full width at half maximum (FWHM) is 0.9 eV. These values correspond to the doubly oxidized Pb(II) species in the Pb-Pc molecules [16,18]. A small component is present at lower BE and may be assigned to the molecules directly in contact with the substrate. A strong modi®cation to the Pb CLs appears when the system is annealed at 300°C: two doublets are now clearly resolved, revealing the presence of Pb
atoms in two dierent oxidation states. The doublet at higher BEs has the same BE position and width as the one attributed to the Pb(II) species in the pristine molecules. The new component that grew at lower BEs is sensibly narrower (FWHM 0:6 eV) and lies at 136.9 eV BE; these values are representative of fully reduced Pb0 species [19]. As a matter of fact, the 300°C spectrum corresponds to an intermediate situation
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Fig. 3. Pb 4f CL spectra of, from bottom to top, 3 ML of PbPc deposited at RT on InSb(1 0 0); the system after 5 min annealing at 300°C; an ordered single layer obtained by annealing the 3 ML at 325°C, the same system annealed at 340°C and 450°C. The photon energy was 250 eV. All spectra are normalized to the photon ¯ux.
between the original 3 ML ®lm and the ML ®lm obtained after annealing at 325°C. This is con®rmed by the intensities: the total area under the 300°C spectrum, while smaller, indeed, than that of the RT thin ®lm, is higher than that of the 325°C ML ®lm (see Fig. 5). In the spectrum corresponding to this adsorbed ML, the oxidized component has completely disappeared while the reduced one has grown in intensity. This is due on one side to the transformation of Pb(II) to Pb0 , and on the other to the lack of attenuation from the now desorbed second layer. It is worth noting that upto 340°C the Pb-Pc molecules are stable since they were puri®ed at 344°C (see Section 2). The presence of the
59
InSb(1 0 0) surface must then have an active role in the changing of oxidation state of the lead atoms. The question arises whether or not the Pb atoms are in direct contact with the substrate surface. A recent photoemission experiment in which a subML ®lm of lead was deposited on the InAs(1 0 0)(4 2)=c(8 2) surface, forming an InAs(1 0 0)1 4-Pb reconstruction [20], reveals the same zero oxidation state for the lead atoms. Since Pb also adsorbs in bulk InSb as well as at its surface as neutral entities [21] we assume that in the ML regime the lead atoms bind directly to the InSb(1 0 0) surface. In this view, the simultaneous presence of reduced and oxidized components in the 300°C spectrum re¯ects the Pb atoms interacting directly with the substrate and those which are not in direct contact with the substrate. Finally, when the system is annealed to 340°C the amount of Pb left on the surface has considerably diminished and it has completely disappeared at the annealing temperature of 450°C. We emphasize here that the desorption of lead is preferential to that of carbon and nitrogen. This is shown in Fig. 5, where the intensity of the C 1s (in the case of N 1s a similar result was found) and Pb 4f CLs as a function of annealing temperatures, are compared (both series of data are normalized to the intensity of the spectra of the as-deposited 3 ML ®lm). The higher desorption rate of lead is already noticeable at 300°C annealing temperature, but it becomes even more remarkable at 340°C. The present data are then consistent with a picture in which the lead atoms desorb predominantly from the surface after having changed their oxidation state from two to zero. 3.1.2. Core levels of the macrocycle: C 1s and N 1s The C 1s and N 1s CL spectra of the organic macrocycle are shown in Fig. 4. Their BEs and shapes help to determine the chemical state of the molecules [5,16±18]. The C 1s spectrum of the thin ®lm is composed of three main components. The most intense feature is related to the 24 carbon atoms of the benzene sub-structures (denoted C2± C7 in Fig. 1). The second component is assigned to the eight pyrrolitic carbon atoms bonded to the nitrogen atoms (C1 and C8 in Fig. 1). We found a value of 284.6 eV for the BE of the benzene
60
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Fig. 4. C 1s (left panel) and N 1s (right panel) CL spectra of, from bottom to top, 3 ML of Pb-Pc deposited at RT on InSb(1 0 0); the system after 5 min annealing at 300°C; an ordered single layer obtained by annealing the 3 ML at 325°C, the same system annealed at 340°C and 450°C. The photon energy was 350 and 450 eV for the C 1s and N 1s respectively. All spectra are normalized to the photon ¯ux.
component whereas the pyrrole related peak is displaced by 1.3 eV towards higher BEs from the main component. These values are in fair agreement with those already reported for a thick ®lm of Pb-Pc [16], again con®rming the view that our 3 ML thin ®lm is already representative of a thick Pb-Pc layer (see Fig. 2). The third component is a p±p shake-up of the pyrrole peak, shifted by 1.85 eV to higher BE from it. It corresponds to the electronic excitation from the highest occupied to the lowest unoccupied molecular orbital
(HOMO±LUMO electronic transition). The N 1s spectrum is also similar to the previously reported spectra [16]. The spectrum relative to the 3 ML ®lm displays a single main component centered at 398.55 eV. In fact the two chemically non-equivalent nitrogen sites of the molecule could not be distinguished. The rather broad structure present at 1.8 eV higher BE is probably due to a ®nal state excitation of p±p character. It is interesting to note the dierence in relative intensity of this shake-up structure between the spectra of the C 1s
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
61
Fig. 5. Peak area of C 1s and Pb 4f as a function of annealing temperature. The data are divided by the peak area of the 3 ML to emphasize the rate of desorption after each annealing.
and N 1s. One possible reason for this eect is the high localization of the HOMO in the macrocycle. In fact, as shown by the theoretical calculation of Orti and Bredas [22] on H2 Pc, the HOMO is strongly localized on the isoindole moieties and no contribution from the nitrogens to this molecular orbital was found. The formation of a nitrogen core hole would then be less eective in perturbing the HOMO electrons and an excitation to an unoccupied molecular state would have less probability of occurrence. When the system is annealed at 300°C and 325°C the intensities of the CLs of the macrocycle decrease because of the gradual desorption of the upper layers; the CL lines broaden and shift to higher BEs. The CL spectra of the adsorbed monolayer display an overall broadening of 0.1
eV compared to the spectra of the thin ®lm. This re¯ects the bonding of the molecules to the surface. However the interaction appears to be weaker than that observed when the same molecule is deposited on a more reactive surface like Si(1 1 1) [16]. In fact, for Pb-Pc/Si(1 1 1) the broadening in going from a multilayer to a ML ®lm was such that for the latter the pyrrole peak in the C 1s spectrum was hardly distinguishable, just appearing as a shoulder of the main benzene peak, whereas the p±p shake-up was not detectable at all. Along with the broadening, the annealings at 300°C and 325°C induce also a shift to higher BEs of all the macrocycle related structures. In the case of the C 1s core level this shift is almost the same for the benzene and pyrrole related peaks but somewhat larger for the p±p satellite. In fact the
62
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
shake-up distance in energy from the pyrrole peak is now 1.9 eV and it may be due to a change of the HOMO±LUMO separation in the adsorbed ML. Annealing to 340°C did not result in a signi®cant change of the C 1s and N 1s CL line shape. Yet when the system was annealed at 450°C for 15 minutes the spectral intensities diminished appreciably as a consequence of the thermal desorption of the molecules. A shift of 0.2 eV to lower BEs was also measured and we notice a further broadening of all features. Moreover, the shake-up structures for both the C 1s and N 1s CL totally disappeared. All the observed changes in the CL spectra of the macrocycles point to strong chemical modi®cations when the temperature of the system is raised to 450°C. It is likely that such alterations reveal a breaking of the molecules left on the surface. The breaking of the molecular frame would cause the lost of the molecular orbitals proper of the macrocycle (e.g. the HOMO, cf. Section 3.2) and this would in turn explain the absence of the shake-up structures in the CL spectra. On the other hand, a total decomposition of the molecules would strongly aect the intensity ratio between the benzene and pyrrole component of the C 1s spectrum resulting in a single broad component as observed, for example, when a single layer of Pb-Pc was annealed at 150°C on the Pt(1 1 1) surface [18]. Because the intensity ratio between the two main peaks of the C 1s spectrum remains essentially unchanged, we thus conclude that the breaking of the molecules leads to fragmentation rather than to complete decomposition. 3.2. Valence bands In Fig. 6 we present VB electron distribution curves (EDCs) of the as deposited 3 ML ®lm at RT (bottom spectrum) and after successive annealings at the same temperatures as previously. The top spectrum is that of the clean InSb(1 0 0)(4 2)=c(8 2) surface. All spectra are normalized to the incident photon ¯ux. As already pointed out in Fig. 2, the VB of the thin 3 ML ®lm is already representative of the bulk material without any evident sign of interaction with the substrate.
Fig. 6. VB spectra of, from bottom to top, 3 ML of Pb-Pc deposited at RT on InSb(1 0 0); the system after 5 min annealing at 300°C; an ordered single layer obtained by annealing the 3 ML at 325°C, the same system annealed at 340°C and 450°C, the clean InSb(1 0 0)-(4 2)=c(8 2). The photon energy was 50 eV. All spectra are normalized to the photon ¯ux.
When the system was annealed to 300°C the VB spectrum did not show signi®cant dierences with respect to the 3 ML, RT spectrum. On the contrary, when a single ordered ML of molecules was left on the surface after annealing at 325°C, the EDC of the VB appeared somehow modi®ed (Fig. 6). Apart from the enhanced contribution of the
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
signal coming from the substrate, there is in fact a sharpening and increasing of intensity of the HOMO and HOMO-1 related structures and the appearance of a new structure at about 5 eV BE. These two evidences may be related to the change in oxidation state of the Pb atoms and to the reduced dimensionality of the system with respect to the previous spectra. In fact, in the highly ordered 1 ML Pb-Pc/InSb(1 0 0)-(4 2)=c(8 2) there is probably a reduced overlap of the wave functions of neighboring molecules which are consequently less perturbed than in the 3 ML system. This will in turn increase the density of states of the less bound orbitals as observed for the HOMO and HOMO-1. On the other hand a precise assignment of the Pb contribution to the VB is not straightforward due to the small spectroscopic weight of the sole Pb atom with respect to the entire macrocycle. Annealing the system at 340°C, a temperature of preferential desorption of the lead atoms, did not produce a strong change of the VB features. However, the VB spectrum changes dramatically when the annealing temperature is raised to 450°C. In fact the HOMO is no longer distinguishable within the highest part of the substrate VB and, in general, all the molecular features are strongly aected by the interaction with the substrate. These drastic modi®cations con®rm the scenario proposed in the discussion of the CLs of the macrocycle, that is, annealing at 450°C causes a fragmentation of the molecules remaining on the surface. 3.3. Heterojunction band oset In our analysis of the Pb-Pc/InSb(1 0 0) interface we now focus on the VB oset at the interface of the organic±inorganic heterojunction. Before deposition (top spectrum in Fig. 6) the valence band maximum (VBM) of the InSb(1 0 0) clean reconstructed surface is at 0.16 eV below the Fermi level (EF ). After deposition of 3 MLs of Pb-Pc, and subsequent annealing, a detailed deconvolution of the substrate CLs (not shown here) evidenced a shift of 0.17 eV towards higher BEs caused by band bending. Therefore the VBM of the substrate in the presence of the molecular adlayer is now
63
located at 0.33 eV below EF . When a ML of molecules remains on the surface the simultaneous presence of substrate and adlayer related photoemission structures allows us to determine the VB oset at the interface. The top of the HOMO band was found by interpolation of the HOMO-peak leading edge to the zero emission line [23]. For the ML we found a value of 0.98 eV from EF , consequently, the VB oset is then estimated to be 0.65 eV. It is noteworthy that in the spectrum relative to the ordered ML the top of the InSb VB appears to shift by 0.11 eV towards lower BEs. This is due to the ®lling of the density of states near the InSb VBM caused by the bonding of the molecules to the substrate [23]. 4. Conclusions We have studied the interfacial behavior and the thermal stability of Pb-Pc thin ®lms on the InSb(1 0 0)-(4 2)=c(8 2) surface by synchrotron radiation PES. The electronic properties at the interface between the Pb-Pc molecules and the substrate have been determined upon comparison of a single ordered ML and a thin ®lm of 3 ML, still representative of bulk like properties. The results can be summarized as follows. The CLs of the macrocycle, when in direct contact with the substrate surface, are slightly aected by the interaction, which leads to a small broadening of all peaks. Upon moderate annealing, the Pb atoms change their oxidation state from two to zero and they directly bind to the substrate. The changes observed in the VB EDC support the picture in which the molecules of the ordered ML are weakly bound to the surface. Moreover, the observed sharpening of the HOMO and HOMO-1 suggests that the molecular states are now less perturbed by the interaction between neighboring molecules. Further annealing of the system upto 450°C led to a preferential desorption of the Pb atoms and to an enhanced interaction of the organic macrocycles remaining on the surface which now appear to be strongly chemisorbed. This chemisorption aects the molecular structure promoting a fragmentation of the macrocycle.
64
L. Giovanelli et al. / Surface Science 486 (2001) 55±64
Finally a value of 0.65 eV was determined for the VB oset at the organic±inorganic heterojunction.
Acknowledgements We wish to thank J.M. Layet and G. Terzian for actively participating in the measurements and Prof. P. Perfetti for careful reading of the manuscript. We also thank J.C. Mossoyan, M. Mossoyan-Deneux for supplying the Pb-Pc molecules. The work of L. Giovanelli is supported by grant no. 9811/2271 of the PACA region. H. von Shenck would like to thank the Ernst Johnson foundation for ®nancial support. We acknowledge the European Union for ®nancial support in the context of the ARI program.
References [1] S.R. Forest, Chem. Rev. 97 (1997) 1793. [2] K. Ukey, Acta Cryst. B 29 (1978) 2290. [3] Y. Iyechika, K. Yakushi, I. Ikemoto, H. Kuroda, Acta Cryst. B 38 (1982) 766. [4] K. Ukei, Ph.D. Thesis, University of Tokyo, 1975. [5] L. Ottaviano, S. Di Nardo, L. Lozzi, M. Passacantando, P. Picozzi, S. Santucci, Surf. Sci. 373 (1997) 318.
[6] F. Rochet, G. Dufour, H. Roulet, N. Motta, A. Sgarlata, M.N. Piancastelli, M. De Crescenzi, Surf. Sci 319 (1994) 10. [7] I. Chizhov, G. Scoles, A. Kahn, Langmuir 16 (2000) 4358. [8] J.J. Cox, T.S. Jones, Surf. Sci. 457 (2000) 311. [9] M.D. Upward, P.H. Beton, P. Moriarty, Surf. Sci. 441 (1999) 21. [10] T. Shimada, A. Suzuki, T. Sakurada, A. Koma, Appl. Phys. Lett. 68 (1996) 2502. [11] M. Nakamura, Y. Morita, H. Tokumoto, Appl. Surf. Sci. 113/114 (1997) 316. [12] J.J. Cox, S.M. Bayliss, T.S. Jones, Surf. Sci. 425 (1999) 326. [13] J.J. Cox, S.M. Bayliss, T.S. Jones, Surf. Sci 433±435 (1999) 152. [14] J.-Y. Grand, T. Kunstmann, D. Homann, A. Haas, M. Dietsche, J. Seifritz, R. M oller, Surf. Sci. 366 (1996) 403. [15] E.E. Koch, W.D. Grobman, J. Chem. Phys. 67 (1977) 837. [16] L. Ottaviano, L. Lozzi, S. Santucci, S. Di Nardo, M. Passacantando, Surf. Sci. 392 (1997) 52. [17] G. Dufour, C. Poncey, F. Rochet, H. Roulet, M. Sacchi, M. De Santis, M. De Crescenzi, Surf. Sci 319 (1994) 251. [18] N. Papageorgiou, J.C. Mossoyan, M. Mossoyan-Deneux, G. Terzian, E. Janin, M. G othelid, L. Giovanelli, J.M. Layet, G. Le Lay, Appl. Surf. Sci. 162±163 (2000) 178. [19] B.V. Crist, Handbook of Monochromatic XPS spectra, Vol. 1, XPS International, Kamiasao, Asao-ku, Kawasaki, 1999. [20] J.M. Layet, M. Carrere, H.J. Kim, R.L. Johnson, R. Belkhou, V. Zhilin, V.Yu. Aristov, G. Le Lay, Surf. Sci. 402±404 (1998) 724. [21] M. Oszwaldowski, T. Berus, J. Phys. Chem. Solids 61 (2000) 875. [22] E. Orti, J.L. Bredas, J. Chem. Phys. 89 (1988) 1009. [23] Y. Hirose, W. Chen, E.I. Haskal, S.R. Forest, A. Kahn, Appl. Phys. Lett. 64 (1994) 3482.