- Email: [email protected]
Study of the interaction of polydimethylphenylene oxide with chromium vapours by X-ray photo-electron spectroscopy
Polymer Science U.S.S.R. Vol. 32, No. 1, pp. 18--22,1990 Printed in Great Britain.
0032-3950190 $10.00 + .00 © 1991 Pergamon Press pie
STUDY OF THE INTERACTION OF POLYDIMETHYLPHENYLENE OXIDE WITH CHROMIUM VAPOURS BY X-RAY PHOTO-ELECTRON SPECTROSCOPY* Yu. M. P A S H U N I N , A. I. P E R T S I N , V. A. S E R G E Y E V , L. I. VDOVINA and Yu. V. SMETANNIKOV Nesmeyanov Institute of Elemento-Organic Compounds, U.S.S.R. Academy of Sciences (Received 17 June 1988)
X-ray photo-electron spectroscopy has been used to study the interaction of poly-2,6-dimethyl-l,4phenylene oxide with Cr vapours. Binding of Cr into complexes of the b/sarene type with phenylene groups and degradation of the polymer chain via the oxygen atoms with the formation of Cr phenylates and oxides were found. The appearance of Cr in the surface layers was accompanied by increase in surface conductivity of the polymer expressed in the disappearance of the recharging of the surface with a photo-emission of electrons.
THE INTERACTIONof polymers with metal vapours has recently attracted considerable attention in connection with many fundamental and applied problems. This primarily includes the problem of the electronic structure of small metal clusters formed on a solid support at the initial stages of the vaporization of the metal. Because of the end (small) number of particles in the clusters their electronic structure offers an intermediate variant between the electronic structure of isolated metal atoms and the zonal structure typical of volumetric metal samples [1]. As compared with volumetric systems the clusters are characterized by a narrower valent zone and appreciable shift of the skeleton levels. The features of the electron structure of metal clusters form the subject of intensive research [2-5] in view of their enormous importance in heterogeneous catalysis. In the case of polymeric supports able to react with an atomic metal, study of the electron states reveals the nature of the chemical bond at the metal-polymer boundary [6]. Such information is necessary for understanding the mechanism of adhesion of metal coatings formed on the surface of polymer films by vacuum metal deposition. Work in this direction is being stimulated by the requirements of microelectronics where polymer films with a sprayed metal electrode (including optically transparent [7]) are being used ever more widely [8]. One may take as a separate trend the saturation of the surface layers of a polymer with dispersed metal to improve the surface conductivity [9, 10]. The final aim of such saturation may be to combat static electricity or produce materials with special electrical properties. Finally, the interaction of polymers with metal vapours is also of purely chemical interest as a possible method of synthesizing new organometallic polymers [11]. In particular, on interaction of poly-2,6-dimethyl-l,4-phenylene oxide (PDMPO) with Cr vapours binding of the Cr atoms to the phenylene groups of the polymer may be expected with the formation of complexes of the b/sarene type. The PDMPO samples were cast in film form onto the surface of a titanium holder from solution in
* Vysokomol. soyed. A32: No. 1, 20-24, 1990.
18
Study of the interaction of PDMPO with Cr
19
chloroform. Metallic Cr was sublimated onto the surface of the polymer using a thermal tungsten vaporizer in the preparation chamber of the Kratos XSAM-800 X-ray photo-electron (X-PE) spectrometer in a vacuum of the order 10 - 7 Pa. Since to obtain the vacuum we used diffusion pumps the danger existed of contamination of the PDMPO surface by diffusion oil vaporized from the surface of the components of the vaporizer. To avoid this, the vaporizer was first heated for 2-3 h with an "idle" tungsten heater (without Cr) additionally mounted in the shell of the vaporizer. To control the composition of the vaporized gases Cr was applied alternately to the sample and the free opposite surface of the titanium holder. Increase in the content of carbon on the surface of the sample through the diffusion oil practically made no difference to the results of the measurements. To analyse the X-PE spectra the sample was moved (without coming into contact with air) into the chamber of the analyser of the spectrometer with vacuum of the order 10 - 9 Pa. The spectra were recorded with use of Mg K, radiation in the regime of constant relative resolution. The potential scanning unit of the analyser was somewhat modified to improve the linearity of scanning and its stability in time. The spectrometer was calibrated against the lines Au4f (Eb = 84 eV), Ag3d ( E b = 368.3 eV), CuLMM (Eb = 335.0 eV), Cu2p (Eb = 932.7 eV) and Ni (EFerm i = 0); the instrument reproduced the values presented within the limits 0.1 eV. With the regime of the X-ray gun used (15 kV, 10 mA) recharging of the samples did not exceed 0.7 V and was estimated over the line Cls (Eb = 285 eV). Curves 1 in Fig. 1 depict the profiles of the Cls and Ols lines of the initial polymer. The Cls line is well described by the superposing of the components C - - C = 285.0 and C - - O = 286.3 eV in the ratio 3 : 1 (C--C and C - - O denote the carbon atoms valently unbound and valently bound to the oxygen atoms, respectively). On an enlarged scale (16:1) Fig. la, shows the "shake up" satellite (Eb=292 eV) due to excitation of the ,r-¢t*-phenylene groups. The Ols line is comparatively narrow (--1.6 eV) and almost symmetrical and the ratio O l s : C - - O practically matches the stoichiometric (1:2). Thus, the profiles of the Cls and Ols lines agree well with the chemical structure of PDMPO indicating absence of appreciable carbon- and oxygen-containing contaminations on the surface of the starting polymer. Curves 2 in Fig. 1 show the Cls and Ols lines of the sample obtained as a result of sublimation of Cr for 10 min at 1600°C. The total Cr content in the surface layers of the polymer is - 3 atm.% (12% by weight). As may be seen from Fig. la, the appearance of Cr on the surface of PDMPO is
,f\
",
2,3 ~hGk~
IzO i
Zgq
.'o0
28t7
2tt?
dd~
[Jo
Eb, eV
FIG. 1. (a) Cls and (b) Ols lines. (a) 1, Starting PDMPO; 2, after depositing Cr 2.9atm.%; 3, after holding for 18 h. (b) 1, Starting PDMPO; 2, 3, after depositing Cr 2.9 and 19.1 atm.%, respectively.
20
Yu. M. PASHUNINet al.
accompanied by appreciable fall in the intensity of the "shake up" satellite (up to 75% of the initial value) indicating binding of the ¢r electron systems of some of the phenylene groups with the formation of complexes of the bisarene type with simultaneous rise in the ratio C---C: C - - O from 3 to 3.5. This indicates fall in the number of carbon atoms valently bound to the oxygen atoms, i.e. degradation of the polymer chain over the oxygen atoms confirmed by change in the profile of the Ols line in Fig. lb, in which together with the peak of the starting polymer components appear with a binding energy 532.5 (20%) and 530.9 (6%) eV. The last component may be assigned to C r 2 0 3 (E8 = 530.8 eV [12]) while the first component may be due to Cr(III) phenylates (increase in Eb by 1-1.5 eV is typical of the transition from an oxide to the corresponding alcoholate [13]). These components of the Ols line are well resolved in the spectrum of the sample with high Cr content (Fig. lb, curve 3). The interpretation of the Cr2p lines shown in Fig. 2 calls for much caution in view of the fact that the electrical contact of the polymer with the particles of the metal-containing phase may not ensure thermal equilibrium of the electrons and equalization of the distribution of the electric charge on the surface [13]. As a consequence, the Cr2p line may be shifted in relation to the lines of the polymer because of the differences in the Fermi levels and the differential charging of the surface. To check on the presence of an electric link between the individual phases of the heterogeneous sample its surface is usually "soaked" with a flow of slow electrons or a shift potential is applied to the sample holder [13]. In both cases there is a redistribution of the potential in the sample and spectrometer chamber in an absence of good electric contact the lines of the individual phases shift by different values. Such a situation is illustrated in Fig. 3 showing the Cls and Pt4f lines of the PDMPO sample onto the surface of which Pt was sprayed. To spray Pt we use a beam of Ar ÷ ions (30 nA) with an energy 1 keV. Curves 1 were obtained with the holder earthed and curves 2 on applying the shift potential--10 V. As may be seen from the figure, the shift of the holder potential moves the Cls and Pt4f lines by different values (3.4 and 2.5 eV, respectively). Another notable feature is the appreciable widening of the Cls line of the earthed sample due to the low energy wing of the peak. This widening may be explained by the appearance in the Cls line of a low energy
(o)
(b)
3, 4 ev
2, 5ev
',,._. .:.: ..i,:i. I
"'"":'"""" I'"..... "'"":""
'~7~'
,~80 FIG, 2.
Eb, eV
I
~90
r
2£0
80
7? Eb, eV
FIG. 3.
FIG. 2. Cr2p line immediately after deposition (1), after 30 min (2), after 1 h (3) and after 18 h (4). FIG. 3. (a) Cls and (b) Pt4f lines obtained with earthed holder (1) and on applying the shift potential - 1 0 v (2).
Study of the interaction of PDMPO with Cr
21
component due to the regions of the polymer in the direct proximity of the Pt clusters. With shift in the holder potential this component shifts synchronously with the Pt4fline by 2.5 eV while the main Cls peak shifts by 3.4 eV. As a result the low energy component "moves" under the main maximum of the Cls line leading to the observed change in the form of the line. We would note that the shift of the lines of the polymer to the region of lower binding energies with the appearance on the surface of metal clusters has already been observed [3] in study of the Au-PI system. In the view of the authors of Ref. [3] the observed shift of the lines of the polymer matrix is due to injection of electrons emitted from the clusters into the polymer then caught in the traps. Similar experiments with shift potential were also run by us for PDMPO--Cr samples. In this case all the lines of the spectrum were shifted by a strictly identical value (1.6 eV), the form of the lines remaining virtually unchanged. The presence of electric contact between the metal-containing clusters and the PDMPO matrix is, in essence, justification for the calibration of all the lines of the spectrum against the Cls level with Eb = 285.0 eV. Thus, with an accuracy to indeterminacy in the position of the Cls line (+0.3 V [14]) the energies of the Cr2p line obtained may be regarded as absolute. From the above considered change in the Cls and Ols lines it may be expected that as well as the particles of metallic Cr the surface layers of the polymer contain bisarene Cr complexes and also Cr(III) oxides and phenylates. Good approximation of curve 1 in Fig. 2 (with standard deviation 1.4%) is given by superposing the components with Es = 574.3 (31%), 575.3 (30%), 576.5 (26%) and 578.1 (12%) eV. The first component corresponds to metallic Cr [15]. Possible shift of the line because of the small size of the particles must be low since the valent zone of Cr is roughly half-filled [1]. The position of the following component of the line with E b = 575.3 eV is typical of the bisarene Cr(0) complexes (for Cr(C6H6)2 gb = 575.2 eV [15], for Cr(C6H6CN)2, according to our findings, Eb -- 575.1 eV). The component with Ea = 576.5 eV may be assigned to the Cr(III) phenylates and, in addition, in this region peaks may be expected for the bisarene Cr(I) complexes. As for the component with EB -- 578.1 eV it is apparently due to Cr203 and multiplet splitting of the preceding component. An interesting feature of the system studied not previously observed in metal-polymer systems is the evolution of the state of the surface in time. Thus, on holding the sample with 3 atm.% Cr (Fig. la, curve 2) in conditions of ultrahigh vacuum there was a drop in the intensity of the "shake up" satellite from 75 to 60% (curve 3) indicating continuation of the reaction of binding of the phenylene groups into bisarene Cr complexes with simultaneous shift of the maximum of the Cr2p line to large Eb values (Fig. 2) due to the drop in the relative content of metallic Cr and increase in the content of the bound Cr forms. The drop in the content of metallic Cr in time is also confirmed by the spectra of the valent zone (Fig. 4) where drop in the density of the electron states at the Fermi level is clearly in evidence. Holding the sample for a day after deposition also appreciably reduced the total Cr content on the surface (by - 1 5 % of the original value). This might have been connected both with diffusion of Cr into the deeper layers of the polymer inaccessible to X-PES and with enlargement of the particles of the metal-containing phase. In the last case there would also be a fall in the overall surface of the metal-containing particles accessible to X-PES. Repeat sublimation of Cr onto the surface of the sample raised the intensity of the low energy components of the Cr2p line and shifted its maximum to smaller binding energy values with simultaneous drop in the intensity of the "shake up" satellite and rise in the intensity of the two high energy components of the Ols line. Subsequent holding was accompanied by the same changes in the Cls, Ols and Cr2p lines as discussed above.
22
Yu. M. PASHUNIN et al.
J
10 FIG. 4.
5
Eb, eV
Spectra of the valent zone of PDMPO immediately after depositing 2.9 atm.% Cr (1), after 1 h (2) and after 18 h (3).
In conclusion, we would note that the appearance of Cr in the surface layers of P D M P O was accompanied by rise in the surface conductivity of the sample expressed as a fall in the recharging of the surface on photo-emission of electrons. While in the starting P D M P O recharging amounted to 0.7 V, in the sample immediately after sublimation of Cr recharging was absent all together. As the holding of the sample was continued the content of metallic Cr dropped and recharging of the surface reappeared. In the sample corresponding to curve 4 in Fig. 2, recharging amounted to 0.3 V. Translated by A. CRozY
REFERENCES 1. P. H. CTIRIN and G. K. WERTHEIM, Phys. Rev. B27: 3176, 1983. 2. S. KOHIKI and S. IHEDA, Ibid. B34: 3786, 1986. 3. H. M. MEYER, S. G. ANDERSON, L. J. ATANASOSKA and J. H. WEAVER, J. Vac. Sci. Technoi. A6: 30, 1988. 4. G. K. WERTHEIM, S. B. DICENZO and S. E. YOUNGQUIST, Phys, Rev. Lett. 51: 2310, 1983. 5. J. S. SLATER and K. H. JOHNSON, Phys. Today 27: 34, 1974. 6. J. H. BURKSTRAND, J. Vac. Sci. Technol. 20: 440, 1952. 7. HATAKEYAMA HIRIOSHI, Konbatekku 20: 20, 1986. 8. J. K. LEE and J. D. CRAIG, J. Amer. Soc. Symp. Ser. 184: 107, 1982. 9. J. D. RANCOURT and R. H. BOGGES, J. Electrochem. Soc. 134: 85, 1987. 10. J. D. RANCOURT and L. T. TAYLOR, Macromolecules 20: 790, 1987. 11. Kriokhimiya (Cryochemistry) (Ed. J. B. Sergeyev). 594 pp., Moscow, 1979. 12. C. D. WAGNER, Analyt, Chem. 52: 1445, 1980. 13. Applied Electron Spectroscopy for Chemical Analysis (Eds. H. Windawi and F. F. Ho). 213 pp., N.Y., 1982. 14. P. SWIFT, Surf. Interface Anal. 4: 47, 1982. 15. V. I. NEFEDOV, Rentgenoelektronnaya spektroskopiya khimicheskikh soyedinenii (X-ray Electron Spectroscopy of Chemical Compounds). 256 pp., Moscow, 1984.
0032-3950190 $10.00 + .00 © 1991 Pergamon Press pie
STUDY OF THE INTERACTION OF POLYDIMETHYLPHENYLENE OXIDE WITH CHROMIUM VAPOURS BY X-RAY PHOTO-ELECTRON SPECTROSCOPY* Yu. M. P A S H U N I N , A. I. P E R T S I N , V. A. S E R G E Y E V , L. I. VDOVINA and Yu. V. SMETANNIKOV Nesmeyanov Institute of Elemento-Organic Compounds, U.S.S.R. Academy of Sciences (Received 17 June 1988)
X-ray photo-electron spectroscopy has been used to study the interaction of poly-2,6-dimethyl-l,4phenylene oxide with Cr vapours. Binding of Cr into complexes of the b/sarene type with phenylene groups and degradation of the polymer chain via the oxygen atoms with the formation of Cr phenylates and oxides were found. The appearance of Cr in the surface layers was accompanied by increase in surface conductivity of the polymer expressed in the disappearance of the recharging of the surface with a photo-emission of electrons.
THE INTERACTIONof polymers with metal vapours has recently attracted considerable attention in connection with many fundamental and applied problems. This primarily includes the problem of the electronic structure of small metal clusters formed on a solid support at the initial stages of the vaporization of the metal. Because of the end (small) number of particles in the clusters their electronic structure offers an intermediate variant between the electronic structure of isolated metal atoms and the zonal structure typical of volumetric metal samples [1]. As compared with volumetric systems the clusters are characterized by a narrower valent zone and appreciable shift of the skeleton levels. The features of the electron structure of metal clusters form the subject of intensive research [2-5] in view of their enormous importance in heterogeneous catalysis. In the case of polymeric supports able to react with an atomic metal, study of the electron states reveals the nature of the chemical bond at the metal-polymer boundary [6]. Such information is necessary for understanding the mechanism of adhesion of metal coatings formed on the surface of polymer films by vacuum metal deposition. Work in this direction is being stimulated by the requirements of microelectronics where polymer films with a sprayed metal electrode (including optically transparent [7]) are being used ever more widely [8]. One may take as a separate trend the saturation of the surface layers of a polymer with dispersed metal to improve the surface conductivity [9, 10]. The final aim of such saturation may be to combat static electricity or produce materials with special electrical properties. Finally, the interaction of polymers with metal vapours is also of purely chemical interest as a possible method of synthesizing new organometallic polymers [11]. In particular, on interaction of poly-2,6-dimethyl-l,4-phenylene oxide (PDMPO) with Cr vapours binding of the Cr atoms to the phenylene groups of the polymer may be expected with the formation of complexes of the b/sarene type. The PDMPO samples were cast in film form onto the surface of a titanium holder from solution in
* Vysokomol. soyed. A32: No. 1, 20-24, 1990.
18
Study of the interaction of PDMPO with Cr
19
chloroform. Metallic Cr was sublimated onto the surface of the polymer using a thermal tungsten vaporizer in the preparation chamber of the Kratos XSAM-800 X-ray photo-electron (X-PE) spectrometer in a vacuum of the order 10 - 7 Pa. Since to obtain the vacuum we used diffusion pumps the danger existed of contamination of the PDMPO surface by diffusion oil vaporized from the surface of the components of the vaporizer. To avoid this, the vaporizer was first heated for 2-3 h with an "idle" tungsten heater (without Cr) additionally mounted in the shell of the vaporizer. To control the composition of the vaporized gases Cr was applied alternately to the sample and the free opposite surface of the titanium holder. Increase in the content of carbon on the surface of the sample through the diffusion oil practically made no difference to the results of the measurements. To analyse the X-PE spectra the sample was moved (without coming into contact with air) into the chamber of the analyser of the spectrometer with vacuum of the order 10 - 9 Pa. The spectra were recorded with use of Mg K, radiation in the regime of constant relative resolution. The potential scanning unit of the analyser was somewhat modified to improve the linearity of scanning and its stability in time. The spectrometer was calibrated against the lines Au4f (Eb = 84 eV), Ag3d ( E b = 368.3 eV), CuLMM (Eb = 335.0 eV), Cu2p (Eb = 932.7 eV) and Ni (EFerm i = 0); the instrument reproduced the values presented within the limits 0.1 eV. With the regime of the X-ray gun used (15 kV, 10 mA) recharging of the samples did not exceed 0.7 V and was estimated over the line Cls (Eb = 285 eV). Curves 1 in Fig. 1 depict the profiles of the Cls and Ols lines of the initial polymer. The Cls line is well described by the superposing of the components C - - C = 285.0 and C - - O = 286.3 eV in the ratio 3 : 1 (C--C and C - - O denote the carbon atoms valently unbound and valently bound to the oxygen atoms, respectively). On an enlarged scale (16:1) Fig. la, shows the "shake up" satellite (Eb=292 eV) due to excitation of the ,r-¢t*-phenylene groups. The Ols line is comparatively narrow (--1.6 eV) and almost symmetrical and the ratio O l s : C - - O practically matches the stoichiometric (1:2). Thus, the profiles of the Cls and Ols lines agree well with the chemical structure of PDMPO indicating absence of appreciable carbon- and oxygen-containing contaminations on the surface of the starting polymer. Curves 2 in Fig. 1 show the Cls and Ols lines of the sample obtained as a result of sublimation of Cr for 10 min at 1600°C. The total Cr content in the surface layers of the polymer is - 3 atm.% (12% by weight). As may be seen from Fig. la, the appearance of Cr on the surface of PDMPO is
,f\
",
2,3 ~hGk~
IzO i
Zgq
.'o0
28t7
2tt?
dd~
[Jo
Eb, eV
FIG. 1. (a) Cls and (b) Ols lines. (a) 1, Starting PDMPO; 2, after depositing Cr 2.9atm.%; 3, after holding for 18 h. (b) 1, Starting PDMPO; 2, 3, after depositing Cr 2.9 and 19.1 atm.%, respectively.
20
Yu. M. PASHUNINet al.
accompanied by appreciable fall in the intensity of the "shake up" satellite (up to 75% of the initial value) indicating binding of the ¢r electron systems of some of the phenylene groups with the formation of complexes of the bisarene type with simultaneous rise in the ratio C---C: C - - O from 3 to 3.5. This indicates fall in the number of carbon atoms valently bound to the oxygen atoms, i.e. degradation of the polymer chain over the oxygen atoms confirmed by change in the profile of the Ols line in Fig. lb, in which together with the peak of the starting polymer components appear with a binding energy 532.5 (20%) and 530.9 (6%) eV. The last component may be assigned to C r 2 0 3 (E8 = 530.8 eV [12]) while the first component may be due to Cr(III) phenylates (increase in Eb by 1-1.5 eV is typical of the transition from an oxide to the corresponding alcoholate [13]). These components of the Ols line are well resolved in the spectrum of the sample with high Cr content (Fig. lb, curve 3). The interpretation of the Cr2p lines shown in Fig. 2 calls for much caution in view of the fact that the electrical contact of the polymer with the particles of the metal-containing phase may not ensure thermal equilibrium of the electrons and equalization of the distribution of the electric charge on the surface [13]. As a consequence, the Cr2p line may be shifted in relation to the lines of the polymer because of the differences in the Fermi levels and the differential charging of the surface. To check on the presence of an electric link between the individual phases of the heterogeneous sample its surface is usually "soaked" with a flow of slow electrons or a shift potential is applied to the sample holder [13]. In both cases there is a redistribution of the potential in the sample and spectrometer chamber in an absence of good electric contact the lines of the individual phases shift by different values. Such a situation is illustrated in Fig. 3 showing the Cls and Pt4f lines of the PDMPO sample onto the surface of which Pt was sprayed. To spray Pt we use a beam of Ar ÷ ions (30 nA) with an energy 1 keV. Curves 1 were obtained with the holder earthed and curves 2 on applying the shift potential--10 V. As may be seen from the figure, the shift of the holder potential moves the Cls and Pt4f lines by different values (3.4 and 2.5 eV, respectively). Another notable feature is the appreciable widening of the Cls line of the earthed sample due to the low energy wing of the peak. This widening may be explained by the appearance in the Cls line of a low energy
(o)
(b)
3, 4 ev
2, 5ev
',,._. .:.: ..i,:i. I
"'"":'"""" I'"..... "'"":""
'~7~'
,~80 FIG, 2.
Eb, eV
I
~90
r
2£0
80
7? Eb, eV
FIG. 3.
FIG. 2. Cr2p line immediately after deposition (1), after 30 min (2), after 1 h (3) and after 18 h (4). FIG. 3. (a) Cls and (b) Pt4f lines obtained with earthed holder (1) and on applying the shift potential - 1 0 v (2).
Study of the interaction of PDMPO with Cr
21
component due to the regions of the polymer in the direct proximity of the Pt clusters. With shift in the holder potential this component shifts synchronously with the Pt4fline by 2.5 eV while the main Cls peak shifts by 3.4 eV. As a result the low energy component "moves" under the main maximum of the Cls line leading to the observed change in the form of the line. We would note that the shift of the lines of the polymer to the region of lower binding energies with the appearance on the surface of metal clusters has already been observed [3] in study of the Au-PI system. In the view of the authors of Ref. [3] the observed shift of the lines of the polymer matrix is due to injection of electrons emitted from the clusters into the polymer then caught in the traps. Similar experiments with shift potential were also run by us for PDMPO--Cr samples. In this case all the lines of the spectrum were shifted by a strictly identical value (1.6 eV), the form of the lines remaining virtually unchanged. The presence of electric contact between the metal-containing clusters and the PDMPO matrix is, in essence, justification for the calibration of all the lines of the spectrum against the Cls level with Eb = 285.0 eV. Thus, with an accuracy to indeterminacy in the position of the Cls line (+0.3 V [14]) the energies of the Cr2p line obtained may be regarded as absolute. From the above considered change in the Cls and Ols lines it may be expected that as well as the particles of metallic Cr the surface layers of the polymer contain bisarene Cr complexes and also Cr(III) oxides and phenylates. Good approximation of curve 1 in Fig. 2 (with standard deviation 1.4%) is given by superposing the components with Es = 574.3 (31%), 575.3 (30%), 576.5 (26%) and 578.1 (12%) eV. The first component corresponds to metallic Cr [15]. Possible shift of the line because of the small size of the particles must be low since the valent zone of Cr is roughly half-filled [1]. The position of the following component of the line with E b = 575.3 eV is typical of the bisarene Cr(0) complexes (for Cr(C6H6)2 gb = 575.2 eV [15], for Cr(C6H6CN)2, according to our findings, Eb -- 575.1 eV). The component with Ea = 576.5 eV may be assigned to the Cr(III) phenylates and, in addition, in this region peaks may be expected for the bisarene Cr(I) complexes. As for the component with EB -- 578.1 eV it is apparently due to Cr203 and multiplet splitting of the preceding component. An interesting feature of the system studied not previously observed in metal-polymer systems is the evolution of the state of the surface in time. Thus, on holding the sample with 3 atm.% Cr (Fig. la, curve 2) in conditions of ultrahigh vacuum there was a drop in the intensity of the "shake up" satellite from 75 to 60% (curve 3) indicating continuation of the reaction of binding of the phenylene groups into bisarene Cr complexes with simultaneous shift of the maximum of the Cr2p line to large Eb values (Fig. 2) due to the drop in the relative content of metallic Cr and increase in the content of the bound Cr forms. The drop in the content of metallic Cr in time is also confirmed by the spectra of the valent zone (Fig. 4) where drop in the density of the electron states at the Fermi level is clearly in evidence. Holding the sample for a day after deposition also appreciably reduced the total Cr content on the surface (by - 1 5 % of the original value). This might have been connected both with diffusion of Cr into the deeper layers of the polymer inaccessible to X-PES and with enlargement of the particles of the metal-containing phase. In the last case there would also be a fall in the overall surface of the metal-containing particles accessible to X-PES. Repeat sublimation of Cr onto the surface of the sample raised the intensity of the low energy components of the Cr2p line and shifted its maximum to smaller binding energy values with simultaneous drop in the intensity of the "shake up" satellite and rise in the intensity of the two high energy components of the Ols line. Subsequent holding was accompanied by the same changes in the Cls, Ols and Cr2p lines as discussed above.
22
Yu. M. PASHUNIN et al.
J
10 FIG. 4.
5
Eb, eV
Spectra of the valent zone of PDMPO immediately after depositing 2.9 atm.% Cr (1), after 1 h (2) and after 18 h (3).
In conclusion, we would note that the appearance of Cr in the surface layers of P D M P O was accompanied by rise in the surface conductivity of the sample expressed as a fall in the recharging of the surface on photo-emission of electrons. While in the starting P D M P O recharging amounted to 0.7 V, in the sample immediately after sublimation of Cr recharging was absent all together. As the holding of the sample was continued the content of metallic Cr dropped and recharging of the surface reappeared. In the sample corresponding to curve 4 in Fig. 2, recharging amounted to 0.3 V. Translated by A. CRozY
REFERENCES 1. P. H. CTIRIN and G. K. WERTHEIM, Phys. Rev. B27: 3176, 1983. 2. S. KOHIKI and S. IHEDA, Ibid. B34: 3786, 1986. 3. H. M. MEYER, S. G. ANDERSON, L. J. ATANASOSKA and J. H. WEAVER, J. Vac. Sci. Technoi. A6: 30, 1988. 4. G. K. WERTHEIM, S. B. DICENZO and S. E. YOUNGQUIST, Phys, Rev. Lett. 51: 2310, 1983. 5. J. S. SLATER and K. H. JOHNSON, Phys. Today 27: 34, 1974. 6. J. H. BURKSTRAND, J. Vac. Sci. Technol. 20: 440, 1952. 7. HATAKEYAMA HIRIOSHI, Konbatekku 20: 20, 1986. 8. J. K. LEE and J. D. CRAIG, J. Amer. Soc. Symp. Ser. 184: 107, 1982. 9. J. D. RANCOURT and R. H. BOGGES, J. Electrochem. Soc. 134: 85, 1987. 10. J. D. RANCOURT and L. T. TAYLOR, Macromolecules 20: 790, 1987. 11. Kriokhimiya (Cryochemistry) (Ed. J. B. Sergeyev). 594 pp., Moscow, 1979. 12. C. D. WAGNER, Analyt, Chem. 52: 1445, 1980. 13. Applied Electron Spectroscopy for Chemical Analysis (Eds. H. Windawi and F. F. Ho). 213 pp., N.Y., 1982. 14. P. SWIFT, Surf. Interface Anal. 4: 47, 1982. 15. V. I. NEFEDOV, Rentgenoelektronnaya spektroskopiya khimicheskikh soyedinenii (X-ray Electron Spectroscopy of Chemical Compounds). 256 pp., Moscow, 1984.