The chemisorption of polyimide precursors and related molecules on metal surfaces

Journal of EL&on Spectroscopy and Related Pherwmena, 54/55 (1990) 1133-1142 Elsevier Science Publishers B.V., Amsterdam

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The chemise tion of polyimide precursors and related molecules on metal su 2 aces. M.R. Ashton’, T.S. Jones’, N.V. Richardson’, R.G. Mackb and W.N. Unertlb.

‘Surface Science Research Centre, University of Liverpool, PG Box 147, Liverpool, L69 3BX, UK.

b Laboratory for Surface Science and Technology, University of Maine, Grono, Maine, 04469, USA.

Abstract A series of molecules including pyromellitic dianhydride (PMDA), m-phenylene diamine (m-PDA), succinic anhydride, phthalic anhydride and aniline were, separately, either chemisorbed at room temperature or condensed at -150 K on a nickel surface and subsequently investigated by X-ray photoelectron spectroscopy and high resolution electron energy loss spectroscopy with a view to determining the nature of the adsorbed species and its orientation at the surface. The anhydride species interact through the anhydride unit, though for dianhydride, PMDA, the second anhydride unit remains intact and directed away from the metal. The amine species interact through a nitrogen atom which undergoes loss of hydrogen. The aromatic ring is aligned almost parallel to the metal surface.

1. INTRODUCTION Polyimides

are formed in the condensation

reaction of polyimide precursors, a di-anhydride and a di-amine, see Figure 1, via the formation of a polyamic acid and subsequent high temperature imidisation. We have used high resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) to study the adsorption on Ni( 110) of the polyimide precursors, m-phenylenediamine (m-PDA) and pyromellitic dianhydride @MDA). Two simpler anhydrides (phthalic and succinic anhydrides) and an amine, aniline, (see Figure 1) were also

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0 1990 Elsevier Science Publishers B.V.

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adsorbed on the nickel crystal and their observed behaviour is used to asist our understanding

of

that of the larger, polymer precursors. In some cases, both deuterated and fully protonated compounds were used. Although much work has been published on the adsorption of polyimides [l-3] and of some of the individual precursors [4-61 onto a variety of substrates, we know of no previous studies of adsorption of the simpler anhydrides we describe in this paper. However, previous reports have appeared in the literature on the surface characterisation of aniline. Ramsey et al. used angleresolved ultra-violet photoelectron spectroscopy (ARUPS), thermal desorption spectroscopy ( TDS), low energy electron diffraction, LEED, and work function measurements to show that aniline deprotonates on adsorption on Pd( 110) [7]. They conclude that the molecule bonds to the surface via the electrons of the benzene ring, and via the nitrogen atom. The ring plane of the molecule is said to lie close to the metal surface although the possibility of some tilting of the molecule could not be ruled out. This is consistent with previous XPS results taken for aniline on evaporated nickel and iron films [8]. TDS and reflection absorption infra-red spectroscopy (RAIRS) were used by Schoofs and Benziger to suggest that a thin film of the high temperature polymer, polyaniline, is formed on exposing Ni( 100) to aniline at 170 K [9]. The infrared spectra showed that if formed, the polymer is orientated with the benzene ring parallel to the surface and the N-H bond at a substantial angle to the surface.

2. EXPERIMENTAL Experiments at the University of Liverpool were carried out in an ultra-high vacuum (UHV) system (base pressure c5 x 10-*’mbar) equipped with HREELS, low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and mass spectrometer. The HREEL spectrometer (VS W Scientific Instruments Ltd.) consists of a tixedmonochromator and rotatable analyser, both 180’ hemispheres with four element lenses. For the rather disordered systems described in this paper, the energy resolution was routinely 8-10 meV (64-80 cm’) full width half maximum (FWHM). HREEL spectra were collected with incident angles of 60” and off specular data was collected at an angle of 50”, all angles measured relative to the surface normal. The XPS measurements were performed at the University of Maine using a hemispherrical analyser (VSW Scientific Instruments Ltd., model HA-100) operated in the constant resolution mode with a pass energy of 25 eV. The analyser accepted electrons emitted between about 13O and 20” from the surface normal. The energy resolution in this mode was measured to be about 1.6 eV FWHM using Ag 3d,, emission. The binding energy scale was calibrated using the spectra of clean Ag, Au and Cu. The base pressure of the analysis chamber was -6 x lO*Ombar. Two Ni (110) crystals were used in this study, both cut from the same boule. One was used for the HREELS work in Liverpool, the other for the XPS in Maine. Both crystals were mechanically polished, chemically etched, and then cleaned in UHV using standard argon ion sputtering and annealing (900-1000 K) procedures. Surface cleanliness was monitored by the appearance of a sharp p( 1 x 1) LEED pattern, by the absence of any loss peaks in the tail of the elastic HREELS peak, and/or by the absence of Auger or XPS lines other than those of the metal substrate. Carbon, oxygen and nitrogen impurity levels were less than a few percent of a monolayer.

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An independently pumped (base pressure -2 x 107 mbar) side chamber, which could be isolated from the main chamber, was used to house the solid samples before deposition. Succinic anhydride (Aldrich Chemical Co. Ltd., 99’8 Aldrich Gold Label), phthalic anhydride (Aldrich, 99+% purity) pyromellitic dianhydride (Lancaster Synthesis Ltd., 97% purity), and m-PDA (Lancaster Synthesis Ltd., 98% purity) were deposited in this way. Each sample in turn was contained in a ceramic dish and heated using external heating tape while being pumped. Pressures in the dosing line typically rose to 2 x 104 mbar upon heating. Temperatures were monitored externally using chromel-alumel thermocouple wire, and source temperatures were kept at approximately 25 “C for succinic anhydride, -30 “C for phthalic anhydride, 75 “C for PMDA and 40 “C for m-PDA. Each sample was loaded as quickly as possible into the dosing chamber to minimise exposure to moisture in the air. This is particularly important for the anhydrides which hydrolyse easily in air. The samples were continuously heated and pumped for several hours before opening the valve to the main chamber for dosing. Knudsen sources were also used [6]. Typical dosing pressures in the main chamber were l-5 x lO* mbar for succinic anhydride, 2 x lo-* mbar for phthalic anhydride, 2 x 107 mbar for PMDA and 2 x 106 mbar for m-PDA; following a typical dose, up to several hours are required to recover the base pressure because of slow desorption of the samples from the walls of the vacuum chamber. This effect makes it very difficult to study the coverage dependence of adsorption of these compounds. The liquid samples (d.,- and h,-aniline, Aldrich Chemical Co. Ltd., 99% purity) were dosed from a second dosing line after having been subjected to several freeze-pump-thaw cycles to remove any dissolved gases. No sample heating was necessary. Typical dosing pressure was -6 x 10-*mbar. The dosing line was exposed to with D,O several times before using the deuterated aniline to reduce the extent of D/H exchange of dramline in the dosing line. In order to condense layers of each anhydride and amine onto the substrate, liquid nitrogen was used to cool the crystal to -170 K. The crystal was flashed to above 275 K to desorb any condensed water immediately prior to adsorption of the intended sample.

3. RESULTS

AND DISCUSSION

No attempt will be made here to give full assignments of each spectrum, but attention will be brought to the main features of the spectra. The results will be discussed in more detail in future publications [ 101. 3.1 Anydrides (i) Succinic anhydride HREEL spectra taken for Ni( 1 lo), after exposure to fully protonated succinic anhydride and with an incident electron beam energy of 12 eV, can be seen in Figure 2 (a-c). A thick layer of succinic anhydride was condensed onto the Ni( 110) sample at at 170 K. The corresponding HREEL spectrum can be seen in Figure 2 (c). There is a strong feature 18 10 cm’ with a shoulder at 1880 cm-l which is in the C=O stretching region and which we assign to the asymmetric and symmetric C=O stretching modesrespectively of the complete anhydride unit. The corresponding gas-phase molecule has bands at 1812 cm’ and 1872 cm-‘. Other bands in the spectrum also correlate well with features in the spectrum of gaseous succinic anhydride. Specular and off specular spectra were recorded after an exposure to l-5 x lo* mbar succinic anhydride for 5

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minutes at 300 K (Figure 2(a,b)). There are major changes compared with the condensed phase spectrum. In particular, there are IZObands in the C=O stretching region. There are m major intensity changes between specularandoff specular spectra, although a band at 1420 cm“ does show some increased intensity in the specular direction, i.e., some dipole 4-L . x 20 activity. The absence of marked intensity = 3-changes may result from the chemisorbed layer being highly disordered. Strong B bands in this region of the spectrum are .* 2
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(ii) Phthalic anhydride Figure 3 (a-c) shows specular andoff specular HKEEL spectra taken after the Ni( 110) substrate was exposed to 2 x lO_’mbar phthalic anhydride at 300 K for 20 minutes. Again, as was true for the succinic anhydride adsorbed at 300 K, there is no evidence of any C=O stretch at 18001900 cm-‘. There is, however, a band at 1410 cm-‘. This dominates the specular spectrum while being more weakly present off specular, and is clearly a strongly dipole active mode. We assign this to the symmetric O-C-O stretch of a carboxylate species or the symmetric (O-C-O-C-O) stretch of an intact anhydride unit and further that the plane of the group must be at a large angle with respect to the Ni( 110) surface. This was not so clear for the succinic anhydride, for which the band at 1420 cm“ was only weakly dipole active. The band at 760 cm-’ is assigned as the inphase, out-of-plane C-Hdeformation. This is present in both spectra, but is relatively more intense with respect to the other spectral features for the specular spectrum. Thus this suggests that this mode has a significant dipole perpendicular to the surface and therefore that the plane of the benzene ring is not perpendicular to the metal. The HREEL spectrum in Figure 3 (c) was taken for a condensed layer of phthalic anhydride deposited on Ni( 110) at a substrate temperature of 170 K. This suectrum auuears much simuler than those recorded after room temperature adsorption. Perhaps worth a mention here is the surprisingly small intensity in the C-H stretching region, which, for spectra taken with an incident electron beam energy of 8 eV is barely visible above the baseline, although other spectral features are quite strong. At 5 eV incident energy the C-H stretch has approximately the same intensity as the C=O band. However, there is obvious intensity in the C=O stretching region (1760 cm-‘) and there is a good correlation with other features of the gas phase spectrum. This confirms that we are seeing the molecular form of the phthalic anhydride after adsorption at 170 K. XPS results on phthalic anhydride adsorption are not yet available but the I-IREELS results suggest that bonding of phthalic anhydride toNi( 110) at 300 Kis similar to that of succinic anhydride in that a surface carboxylate or similar (C,O;) unit is formed on adsorption. Energy loss
1138 conclusions drawn from the dipole activity of the modes associated with the ‘carboxylate’ and aromatic parts of the adsorbed phthalic anhydride species are correct, this would imply that there has been significant perturbation of the molecule, which would have been expected to be planar. (iii) PMDA Having looked at the model compounds, the polyimide precursor is now studied. The molecular anhydride in this case has two anhydride rings. Figure 4 (a-c) show spectra taken following the adsorption of PMDA on Ni(ll0) at both 300 K and 170 K. Similar loss features can be seen in both spectra taken at room temperature, although some changes in relative band intensities can be distinguished. Again, because of the size and complexity of the molecule, a full band assignment is Energy loss not attempted here and these results Figure 4. HREEL spectra of Ni( 110) after exposure are discussed in more detail elsewhere to PMDA (a) for specular and (b) 10” off specular [ 10, 111. In the spectra taken at both scattering geometries, both at 300 K, and (c) at 300 and 170 K, a weak band can be 170 K. The incident energy was 10 eV. seen at -1820 cm-‘, in the C=O stretching region. This was not seen for either succinic or phthalic anhydride at 300 K. There is a band at 1420 cm-’ which was also present for the other, smaller anhydrides. This band is again assigned to the C-O stretch of a carboxylate or similar unit. Since not all of the C=O character is lost, this suggests that only one of the anhydride groups interacts with the surface. XPS results confii this [5,6]. While there is a band at 890 cm-’ which we assign to the in-phase, out-of-plane C-H deformation, this is not a dipole active mode of vibration. This therefore implies that the plane of the benzene part of the molecule does not lie parallel to the surface but is aligned nearly perpendicular to the surface. This again would imply that bonding to the surface is through only one of the two anhydride groups for steric reasons. 3.2 Amines (i) Aniline The Ni( 110) was exposed to 6 x l@ mbar h,-aniline at 300 K for 5 minutes. Specular and off specular spectra were taken and can be seen in Figure 5 (a-c). The results will be treated in the same way as the anhydrides, i.e. only the main features will be discussed and a more detailed

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discussion with a fuller band assignment will appear elsewhere [lo]. (In off specular and condensed spectra, the band seen at -2280 cm-’ is in the C-D stretching region from residual deuterated aniline or indicating that some HiD exchange has occurred ). The band at 760 cm’ which is moreintense for specularthanoff specular geometry is the out-of-plane C-H deformation of the benzene ring. This band moves down to - 550 cm1 on full deuteration of the molecule (daniline). Since this is therefore a dipole active mode, it suggests that the benzene ring is aligned roughly parallel to the surface. This is consistent with results from ARUPS and TDS work on aniline on Pd(ll0) [7]. The C-H stretching modes are present in the expected energy region, and there is a clear doublet at -3000 and 3040 cm”. The observation of two C-H stretching frequencies is a clear indication that hydrogen atoms are in two different environments. Hydrogen atoms which are closer to the metal are known to give rise to a band of lower frequency band than that for hydrogens which do not interact with the metal surface [12]. This also indicates that the aromatic ring is not completely parallel to the metal surface. Perhaps the most unusual feature of the resultant spectra is the weakness of N-H stretches which would be at -3300 cm’. For the adsorbate formed on exposure of the metal to aniline at 300 K, this could be explained by the deprotonation of the amino group, the remainder of the molecule interacting with the Ni(ll0) through the nitrogen atom [7]. Figure 5 (c) shows an HREEL spectrum taken after adsorption of h,aniline at a substrate temperature of 170 K. There is very little evidence of N-H stretching modes in this spectrum either, although the gas phase spectrum of aniline also shows that the N-H features are weak compared to the C-H stretching bands. One possible argument may be that there is a considerable degree of hydrogen bonding occurring between aniline molecules. This would be expected to weaken the N-H bonds and the energy of the stretching vibration would shift down. A thick multilayer of dimethylamine on Cu( 110) at 100 K gives spectra with the N-H bend at 724 cm’, some -160 cm-’ lower than in the gas phase [13]. This shift was attributed to hydrogen bonding within the condensed layer, but no reference is made to the N-H stretching Energy loss (cm- 1) band. Whether the shift wouldbe sufficient Figure 5. HREEL spectra of Ni( 110) after exposure so that the band is coincident with the to aniline (a) for specular and (b) 10” off specular C-H stretching bands is debatable. scattering geometries, both at 300 K, and (c) at Secondly, these results would not be 170 K. The incident beam energy was 7 eV. wholly inconsistent with the formation of

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the polymer, polyaniline, as suggested by Schoofs and Benziger [9]. However, the polymer structure still has one hydrogen atom per nitrogen atom, and this would be expected to give some intensity in the N-H stretching region. A similar anomaly was found in an EELS study of condensed ammonia on Ag(311) [14]. Here also, the N-H stretch can hardly be seen on a spectrum taken for 24L q adsorbed on Ag(311) at 90 K.

4ooo 3000 2000

loo0 0

=3ooOO

x40

B -ZOO00 .e

H 1OOoO (ii) m-PDA s /J:"j-;A+ After exposure to 3 x 1tY mbar m0 PDA at a substrate temperature of 300 K, specular and off specular HREEL spectra were taken and can be seen in Figure 6 (ac). In direct comparison with the aniline/ ZI;[iL_,i@! Ni( 110) system, there is a strongly dipole active band at -760 cm-’ which is again assigned to the out-of-plane C-H (a) deformation. Therefore, it follows that othe benzene ring of this molecule is also 3000 0 1000 2000 more parallel than perpendicular to the Energy Loss (cm- 1) metal surface. Also, as for the C-H Figure 6. HREEL spectra of Ni( 110) after exposure stretching region, there are two C-H to m-PDA (a) for specular and (b) 10” off specular stretches for m-PDA after adsorption on scattering geometries, both at 300 K, and (c) at the Ni( 1 IO) surface. This again would 170 K. The incident beam energy was 4 eV. seem to imply that rather than being completely parallel, the interaction of some of the hydrogen atoms with the metal is greater than the others, and that therefore some of the hydrogens are closer to the metal than others. The major difference between spectra taken for m-PDA and aniline is in the N-H stretching region. While there was very little evidence for a band in this region for the aniline-Ni adsorption system, it is clearly present for m-PDA on Ni( 110). The spectrum in Figure 6 (c) is from the condensed layer of m-PDA formed on exposing the metal to the polyimide precursor at 170 K. Here also, the N-H stretching bands are observed as expected from the molecular form of the molecule. There is some decrease in intensity of this N-H band on moving to the monolayer, but the remaining intensity, with indications of a doublet assignable to symmetric and antisymmetric components of N-H stretch in an NH, unit, suggest that one amine unit remains intact while the other is partially, at least, deprotonated. XPS data for a thick layer of m-PDA on Ni( 110) show a Cls peak which is not very different to the Cls taken for a thin layer of m-PDA at room temperature. The Nls line is very different for the two adsorption temperature states of the m-PDA. The Nls shifts by about 1.3 eV to lower

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binding energy andtheFWI-IMisdoubledfor the room temperature adsorbate compared with that for the condensed layer. This indicates that at 300 K, the nitrogen is in at least two different states [6, 111. This is consistent with the I-IREELS results. It would seem that m-PDA adsorbs via one of the two nitrogen atoms, with loss of hydrogen from the nitrogen which interacts with the metal, and that the benzene ring is almost parallel to the surface.

4. CONCLUSIONS Anhydrides: adsorption at 300 K leads to significant ring perturbation such that the C=O stretching intensity is significantly lower than would have been expected if the anhydrides had adsorbed molecularly. In the cases of both succinic and phthalic anhydrides, there was no carbonyl stretching band, but there was a band in this region for adsorbed PMDA. A band at -1420 cm-’ is created for each anhydride in the absence of the C=O stretch, which is reminisent of a symmetric stretch of the O-C-O of a carboxylate group. This band is highly dipole active for both of the aromatic adsorbates (though less so for succinic anhydride) and this leads us to conclude that the carboxylate (or C,O;) group is perpendicular to the surface. The PMDA spectra show the 1420 cm-’ feature but also the C=O stretch which implies that some of the carbonyl of the parent molecule is still intact after reaction with the metal surface. Both of the aromatic systems have benzene rings which are not parallel to the surface. Thus the adsorbed phthalic anhydride and PMDA are bonded to the surface via a carboxylate or similar species and have their benzene rings inclined. Amines: The out-of-plane C-H deformation at -760 cm” in spectra from both aniline and mPDA is strongly dipole active. This suggests that the plane of the aromatic part of the molecule is at a large angle from the surface normal. Interaction with the metal is through the nitrogen for both amines, with some loss of hydrogen. The interaction of m-PDA with Ni( 110) is through one of the two amine groups, and there is only partial deprotonation. Although the expected N-H stretching band is present for the condensed layer of m-PDA on the substrate, it is very weak in the case of aniline.

5. ACKNOWLEDGMENTS NATO are thanked for the award of a travel grant and Courtaulds Coatings plc are thanked for the award of a studentship to MRA. WNU and RGM acknowledge support from the Office of Naval Research.

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4. J.J. Pireaux, C. Gregoire, P.A. Thiry, R. Caudano and T.C. Clarke, J. Vat. Sci. Technol., A5 (1987) 598. 5. M. Grunzc and R.N. Lamb, Surf. Sci., 204 (1988) 183; M. Grunze. W.N. Unertl, S. Gnarajan and J. French, Mat. Res. Sot. Symp. Proc., 108 (1988) 189. 6. T.S. Jones, M.R. Ashton, N.V. Richardson, R.G. Mack and W.N. Unertl, J. Vat. Sci. Technol., A8(3) (1990) 2370. 7. M.G. Ramsey, G. Rosina, D. Steinmtiller, H.H. Graen and F.P. Netzer, Surf. Sci., 232 (1990) 266. 8. K. Kishi, K. Chinomi, Y. Inoue and S. Ikeda, J. Catal., 60 (1979) 228. 9. G.R. Schoofs and J.B. Benziger, J. Phys. Chem., 92 (1988) 741. 10. M.R. Ashton, T.S. Jones, N.V. Richardson, R.G. Mack and W.N. Unertl, to be published. 11. T.S. Jones, M.R. Ashton, N.V. Richardson and W.N. Unertl, J. Phys.: Condens. Matter, 1 (1989) SB139. 12. R. Raval and M.A. Chesters, Surf. Sci., 219 (1988) L505. 13. J.A. Kelber, J.W. Rogers, Jr., B.A. Banse and B.E. Koel, Appl. Surf. Sci., 44 (1990) 193. 14. S.T. Ceyer and J.T. Yates, Jr., Surf. Sci., 155 (1985) 584.