film induced by synchrotron radiation white beam - American

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J. Phys. Chem. 1992,96, 5917-5921 (58) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 429. (59) Siiman, 0.;Bumm, L. A.; Blatchford, C. G.;Kerker, M. J. Phys. Chem. 1983,87, 1014. (60) Kerker, M. J. Colloid Interface Sci. 1985, 105, 297. (61) Doremus, R. H.J. Appl. Phys. 1964, 35, 3456. (62) Mie, G. Ann. Phys. 1908, 25, 377. (63) Barnickel, P.; Wokaun, A. Mol. Phys. 1990, 69, 1. (64) Skillman, D. C.; Berry, C. R. J. Chem. Phys. 1968, 48, 3297. (65) Meier, M.; Wokaun, A. Opt. Lett. 1983, 8, 581. (66) Morris, R. H.; Collins, L. F. J . Chem. Phys. 1964, 41, 3357. (67) Avnir, D., Ed. The Fractal Approach to Heterogeneous Chemistry: Surfaces, Colloids, Polymers; Wiley: Chichester, 1989. (68) Family, F.; Landau, D. P. Kinetics of Aggregation and Gelation; North Holland: Amsterdam. 1984. (69) Mesinger, B. J.; vondaben, K. U.; Chang, R. K.; Barber, P. W. Phys. Rev. B 1981, 24, 649.

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(70) Wokaun, A. Mol. Phys. 1985, 56, 1. (71) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120. 435. (72) Creighton, J. A.; Alvarez, M. S.;Weitz, D. A.; Garoff, S.;Kim, M. W. J . Phys. Chem. 1983,87, 4793. (73) von Smoluchowski, M. Z . Phvs. Chem. 1917. 92. 129. (74) Sonntag, H.; Strenge, K. Coa&ation kinetics'and structure formation;Plenum Press: New York, 1987. (75) Jones, L. H.; Penneman, R. A. J . Chem. Phys. 1954, 22, 965. (76) Kunimatsu, K.; Seki, H.; Golden, W. G.Chem. Phys. Lett. 1984,108, 195. (77) von Raben, K. U.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. 1981, 79, 465. (78) Lange, Handbook of Chemistry, 10th 4.;McGraw-Hill: New York, 1967. (79) Mulvaney, P.; Linnert, T.; Henglein, A. J. Phys. Chem. 1991, 95, 7843.

Spatially Resolved Electronic Modifications of a Thin Poly(acrylon1trile) Film Induced by a Synchrotron Radiation White Beam Daniel G u y , GGrard Tourillon,* LURE, Britiment 209D, UniversitP Paris-Sud, Orsay, 91 405, France

Pascal Viel, and Grard Lecayon DPpartement de Physico- Chimie, Section d'Erude des Surfaces CEN-Saclay. 91 191 Gij-sur- Yvette, France (Received: September 18, 1991; In Final Form: February 26, 1992)

NEXAFS experiments at the C and N K-edges, Fourier transform infrared spectroscopy, and photoemission measurements at variable excitation energy have been used to characterize the electronic and structural modifications occurring in thin poly(acrylonitri1e)(PAN) films when they are irradiated by a synchrotron radiation white beam. Upon irradiation, the nitrile groups are rapidly removed from the film and the H content is substantially reduced. The final chemical composition of the layer is similar to that of graphite, with highly delocalized electrons located just below the Fermi level. The mechanism which leads to this structure is different from that observed when PAN is annealed. The extent of transformation is controlled by the exposure time to the synchrotron white beam and is spatially localized to the irradiated area. New applications of the synchrotron white beam in the synthesis of materials may be envisioned.

Introduction In the past, the microelectronicindustry has looked for an ever increasing integration of its circuit elements. This has prompted a rapid development of the lithography technique, using mainly conventional ultraviolet light sources. More recently, tightly focused laser light beams were used to drive local chemical reactions on a small spot size at the surface of the material,' and thus, the prospect of a "direct writing" technologyZhas attracted a lot of attention. Synchrotron radiation white beam may offer an interesting alternative to the laser source. The light issuing from a synchrotron storage ring is intense, well collimated, and spans the whole electromagneticspectrum. So,various research groups have foreseen its usefulness in high-resolution X-ray lithography. The first results were so promising that synchrotron rings dedicated only to this application are presently being built around the world. However, up to now, the use of synchrotron white beam as a tool to generate materials with unusual properties is not yet common and only a few papers have been devoted to this area of research: synthesis of Si,NyH, of carbon films from n-butane5and Mo metal deposition from Mo(CO)~adsorbed on Si(l1 1).6 It is well-known that the electronic properties of a variety of polymers can be modified by annealing. At moderate temperatures, an insulating polymer becomes a semiconductor with conductivity ).( between 1O-Io and $2-I cm-', while at higher temperature a conducting state is reached with u of about 10 W cm-'.'J At very high temperature (>lo00 "C), most of the non-carbon atoms are generally driven out of the material and a 'graphitic-like" state is attained.9 In that respect, and owing to its general physicochemical properties, poly(acrylonitri1e)

((C,H3N),,,PAN) has been the subject of a number of studies.'*"* Its intrinsic conductivity can be increased up to 1 0'cm-' upon annealing. PAN fibers are also the precursor of the well-known carbon fibers, obtained by pyrolysis. Moreover, thin films of PAN can be synthesized electrochemi~ally,'~J~ and recently, the effect of the film thickness and annealing treatments on the molecular orientation and electronic structure of the polymer has been investigated. This paper describes the electronic and structural modifications occurring in thin PAN films when they are irradiated with a synchrotron radiation white beam for various exposure times. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the C and N K edges, Fourier transform infrared (FTIR) technique, and photoemission spectroscopy at variable excitation energy were used to follow the physicochemical transformations induced by the white beam. We show that, by varying the exposure time to the synchrotron white beam, the electronic properties of the film can be tailored from the insulating to the conducting state, due to the formation of extended electronic states near the Fermi level. NEXAFS spectroscopy is a new method, well suited to give information on the electronic and structural characteristics of organic molecules since the two ll* and u* resonanca are probing the unoccupied density of states of the material and the intermolecular distance, respectively.I6-l8

'

Experimental Section The poly(acry1onitrile) used in this study (average molecular weight 86000) was purchased from Aldrich and used without any further purification. Thin films (300 nm thick) were obtained

0022-365419212096-5917S03.OO/O 0 1992 American Chemical Society

5918 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992

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Figure 1. C K-edge NEXAFS absorption spectra obtained on a 300 nm thick PAN film after irradiation by the synchrotron white beam at various exposure time: (a) 0 s, (b) 1 s, (c) 20 s, (d) 50 s, and (e) 1800 S.

w23,

0 390

N Kedge

5 ,6 1 , 8 400 410 ENERGY leVl

,

Figure 2. N K-edge NEXAFS absorption spectra obtained on a 300 nm thick PAN film after irradiation by the synchrotron white beam at various exposure time: (a) 0 s, (b) 1 s, (e) 20 s, (d) 50 s, and (e) 1800 s. Curves d and e have been multiplied by a factor of 2 and 5, respectively.

by spin-coating a 3% solution of PAN in dimethylformamide TABLE I: Energies and Proposed Assignments of the Features (DMF) onto a platinum substrate. Slow evaporation of the DMF Observed in the C and N K-Edge NEXAFS Spectra of a 300 nm in air was allowed to take place during 3 days. After that, the Thick As-Deposited PAN Film (Figure 1, Curve a) and after film was placed in a vacuum system (base pressure 10” Torr) for Irradiation with the Synchrotron White Beam (Figure 1, Curve e) 3 more days. As evidenced by FTIR measurement, this procedure energy, eV enables the complete evaporation of the DMF. Very thin PAN feature C K-edge N K-edge assignment films (50-500 A thick) were also electrochemically deposited on 1 385.0 395.6 r*a’lC=N) Ni or Pt substrates, following a procedure already p ~ b 1 i s h e d . l ~ ~ ‘ ~ r*(C=C) and/or r*(C=N) 2 286.2 397.5 Irradiation of the sample by the synchrotron radiation white 3 287.8 399.0 r*a”(C=N) beam as well as the photoemission studies and NEXAFS ex4 289.4 a*(C-H) periments was conducted in an ultrahigh-vacuum (UHV) system 5 402.5 (base pressure of about 1O-Io Torr). They were carried out at the 6 294.0 405.8 a*(C-C) and a*(C-N) W V Super-ACO storage ring on the SACEMOR beam line using I 301.7 408.0 a*(C=C) and/or a*(C=N) a high-energy TGM monochromator (resolution 0.2 eV at the C 8 309.1 411.2 a*(C=N) K-edge). For NEXAFS, the incident photon beam Io was monitored by collecting the total electron yield from an 85% transcording the spectrum of a clean Pt substrate. The angle of inmission copper metal grid, freshly coated with gold. The total cidence of the IR beam was nearly normal with respect to the Pt electron yield from the sample, I, was then normalized with respect substrate. to Io. The spectra presented therein were normalized at 320 and Irradiation of the sample by the synchrotron white beam was 420 eV for the C and N K-edges, respectively, since the nonreachieved by moving the grating of the monochromator to zero sonant continuum at these energies is atomic-like.20 order. The photoemission and the NEXAFS spectra presented The photoemission data were recorded at two excitation energy therein were obtained in independent experiments. Each set of values: 40.8 eV (He I1 discarge lamp, Model UVL HI-VG Co.) measurements were recorded by irradiating the same area of the and 180 eV (by using the TGM monochromator). They were film, and so the exposure time indicated in the figures represents collected using a hemispherical electron energy analyzer CLAM the cumulative exposure to the synchrotron white beam. Ex situ 2 from VG with a band pass energy of 10 eV. The valence band FTIR spectroscopy has been performed after removal of the film spectra were referenced against the Fermi level, knowing that the from the high-vacuum system. In that case, different areas of work function of the analyzer (against the vacuum level) was equal the film were irradiated during the exposure time indicated in the to 4.5 eV. figures. The C 1s core level spectra of the sample were recorded to Results determine if any variation of the charging effect was occurring Direct observation and all the spectroscopic studies indicate upon irradiation. This was done using the 400-eV excitation that the radiation induced transformations were confined to the energy from the TGM monochromator. By doing so,the excitation irradiated area, thus eliminating thermal effects. beam has the same size as the white beam used to irradiate the C and N K-Edge NEXAFS Characteristics. Figures 1 and 2 sample. This procedure ensures that only the core level spectrum show the evolution of the C and N K-edge NEXAFS spectra of of the modified film is recorded. a thin PAN film upon irradiation with a synchrotron white beam The infrared spectra were measured with an infrared Fourier for different exposure times. Curves a of both figures, which transform Brucker spectrometer (IFS 6 6 ) . In order to measure correspond to the as-deposited PAN film, are similar to those the FTIR spectrum of sample of small dimensions, the optical obtained from an electropolymerized samplels and the assignment bench of the spectrometer was coupled to a microscope. An of the various features is summarized in Table I. infrared spot size of 250 Fm was thus achieved. Each spectrum Features 1 and 3 a t both C and N K-edges are assigned to was measured by recording 256 scans between 650 and 4000 cm-I, transitions of the 1s electron to the a*a’(C%N) and a*a”(C=N) with a deuterated triglycine sulfate type detector, used at a resorbitals, respectively. The a* orbitals of the CEN groups are olution of 2 cm-I. Transmittance spectra were obtained by re-

Thin Poly(acrylonitri1e) Films normally degenerate so the NEXAFS spectra should exhibit only one transition, in agreement with the experimental data obtained for CH3CN in the gas phase, or condensed as a monolayer or multilayers on Ag( 1lo).% However, when molecules like HCN and C2N2are adsorbed on a Pd( 111) surface in the "lying down" configuration, the interaction of the C=N groups with the substrate leads to a splitting of the r* orbitals and the appearance of two 1s r* transitions in the NEXAFS Since thick PAN films do not show any polarization-dependent effects in their C K- and N K-edges NEXAFS spectra,I5one can hardly invoke an intermolecular interaction to explain the existence of two T* transitions. In fact, a splitting of 2-3 eV was calculated for the degenerate r ( C = N ) valence states, when intramolecular interactions occur between adjacent C=N.22 Thus, features 1 and 3 arise from the splitting in energy of the r * ( m N ) orbitals, due to this intramolecular interaction. Features 4,6, and 8 at the C K-edge NEXAFS spectrum are assigned to the transition of the C 1s core electron to the a*(C-H), u*(C-C), and u*(C=N) orbitals, respectively, in agreement with the results of Outka et al.23and Hitchcock et al." Feature 8 in the N K-edge NEXAFS spectrum is assigned to a transition to the a*(C=N) orbitals. As soon as the film is irradiated with the white beam, the pressure in the analysis chamber rises from the lower to upper 1O-IoTorr scale, indicating that some material is vaporized. This is a transient phenomenon and the analysis chamber gradually returns to the initial base pressure. This loss of material upon exposure to the white beam is accompanied by a gradual and substantial decrease of the absorption coefficient at the N K-edge. This indicates that the N content of the film is reduced. No attempt was made to analyze the nature of the gas-phase species. Dramatic changes at the C and N K-edge NEXAFS spectra were observed after white beam irradiation (curves b-e of Figures 1 and 2): After irradiation for 2 s, features 1 and 3 (T*(C=N) transition) and 8 (a*(-N) transition) decrease and a new resonance (feature 2) appears at 286.2 (C K-edge) and 397.5 eV (N K-edge). This feature is assigned to the transition of the 1s electron to r*(C=C) and/or r*(C==N)orbitals. The appearance of the same transition has been observed when thin electropolymerized PAN films are annealed at 500 OC.15 These modifications clearly reveal that the major part of the C E N groups is very quickly removed from the PAN film and unsaturated C - C bonds are formed. After irradiation for 50 s, the intensity of feature 2 increases and the a*(C-H) resonance vanishes. New transitions appear at both edges: feature 7 at 301.7 eV (C K-edge) and features 6 and 7 at 405.8 and 408.0 eV (N K-edge), which are assigned to u*(C=C), a*(C=N), and a*(C-N) resonances. However, due to the very low N content of the irradiated film, most of the intensity of feature 7 at the C K-edge is ascribed to a transition to the a*(C=C) orbital. At this stage, the film is mainly composed of C atoms with unsaturated bonds. After 15 min exposure to the white beam, the C K-edge NEXAFS spectrum exhibits only three transitions (?r*(C=C), u*(C=C), and a*(C-C)), similar to those recorded on a freshly cleaved carbon sample. No more change is observed if the irradiation is continued. FTIR Spectroscopy. Figure 3, curve a, shows a infrared transmittance spectrum of the as-deposited PAN film. No absorption peak is observed at 1760 cm-l, indicating no residual DMF in the film. Strong and sharp absorption peaks are observed at 2243 and 1455 cm-I, which are attributed to the u(CN) and 6(CH2) vibrations of the polymer. Absorption in the 2900-cm-I region corresponds to the v(CH2) vibrations, while features below 1400 cm-l are assigned to various vibration modes of CH, CH2, and CC. After 15 min of irradiation (Figure 3, curve b) the characteristic CN and CHI vibrations disappeared. The sharp and well-defined peaks characteristic of the molecular nature of PAN are replaced by broad absorption bands centered at 1610 and 1389 cm-l. These bands have been observed in PAN after pyrolysis under vacuum and are attributed to the vibrations of C - C and C-N bonds

The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5919

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Figure 3. Fourier transform infrared transmittance spectra of a 300 nm thick PAN film: (a) as-deposited and (b) irradiated 15 min with the synchrotron white beam.

h II

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1600

I

I

I

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Figure 4. Fourier transform infrared transmittance spectra in the 1600-cm-l region of a 300 nm thick PAN film exposed to the synchrotron white beam during (a) 0 s, (b) 15 s, (c) 50 s, and (d) 15 min.

embedded in the highly delocalized ~ t r u c t u r e . ~ *The - ~ ~very weak band at 2213 cm-'is attributed to a nitrile function in an aromatic environment.26 This chemical group is not detected by NEXAFS. The two techniques do not probe to the same depth. NEXAFS probes close to the surface (30-50 A) of the sample, while IR probes the bulk. This indicates that the composition of the bulk of the film is somewhat different from that of its surface. Figure 4 shows the evolution of the 16OO-cm-' region of the spectra with the exposure time to the white beam: (a) 0 s, (b) 15 s, (c) 50 s, and (d) 15 min. The 1640-cm-' band, which is assigned to an isolated C = C vibration, continuously shifted to lower frequency, and was located at 1610 cm-' after 15 min exposure. This displacement indicates that an aromatic structure developed as the CN, CH, and CHI groups disappeared. The NEXAFS results indicate that the chemical structure of the film at the end of the process corresponds to a "graphitic-like" material. If this indeed occurs then major changes are expected in the photoemission characteristics, especially the appearance of occupied states near the Fermi level due to the delocalized r electrons. Photoemission Studies. Figure 5, curves a-d, depicts the evolution of the photoemission spectra with exposure time (0,2, 5, and 1000 s, respectively) of a thin PAN film irradiated with the synchrotron white beam. To get some information about the atomic deep energy levels, these spectra were recorded with an excitation energy of 180 eV.

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Figure 5. Photoemission spectra of a 300 nm thick PAN film irradiated by the synchrotron white beam at various exposure time: (a) 0 s, (b) 2 s, (c) 5 s, and (d) 1000 s. The excitation energy was 180 eV. The reference energy, 0 eV, corresponds to the Fermi level. TABLE II: Energies and Proposed Assignments of the Features Observed in the Valence Band Spectra of a 300 nm Thick As-Deposited PAN Film (Figure 5, Curve a) and after 15 min Exposure to the White Beam (Fieure 1. Curve d)“ feature energy, eV assignment A -0.3 photoemission edge B -3.5 c 2P C -8.5 *(C=N) N lone electron pair D -12.2 o(C-H) o(C-C) E -13.5 c 2sb F -17.0 c 2sb G -20.5 c 2sb H -25.3 N 2sb “Excitation energy of 180 eV; the reference energy -0 eV corresponds to the Fermi level. No correction was made to take into account the shift in energy due to the charging effect of the as-deposited PAN film. The work function of the analyzer was 4.5 eV. *This assignment on ref 29.

Curve a of Figure 5 is similar to the ultraviolet photoelectron spectroscopy (UPS)and X-ray photoelectron spectroscopy (XPS) valence spectra of PAN.z7,28 The differences observed in the intensity of the peaks are due to variation of the photoemission cross section with the excitation energy.29 From the molecular orbital calculations performed on PAN,22*30 peak C is assigned to the localized n orbitals of the W N group and to the electron lone pair of the N atom. Peak D, which is not well resolved at this excitation energy, comes from the u bonds of the C atoms (CH and C-C). Finally, features F, G, and H originate from C 2s and N 2s orbitals (see Table 11). Dramatic changes were observed upon exposure of the PAN film to the synchrotron radiation white beam (curves b, c, and d). For the shortest exposure time (2 s), a rigid shift of the whole spectrum to lower energy was observed, due to the elimination of the charging e f f e ~ t . ~No . ~further ~ shift in energy was observed upon prolonged irradiation. For longer exposure time, several modifications were observed: (i) the intensities of peaks C and H were greatly reduced, which confirmed the removal of N atoms from the film and, (ii) a band appeared at about -3.5 eV (feature B) which shifted the emission threshold toward the Fermi level (feature A). Other changes in the photoemission spectra included the appearance of features E at about -13.5 eV and the growth of feature F at -17.5 eV, both features being assigned to C 2s orbitals. No further change in the photoemission spectrum was observed after a 15-min exposure to the white beam. The same photoemission characteristics were obtained irrespective of whether the irradiation was done continuously for 15 min or by a succession of short exposures up to a total exposure time of 15 min. The

Guay et al. same modifications were also observed on 50 and 500 A thick electropolymerized PAN films. The photoemission curves of irradiated PAN were also recorded using an excitation energy of 40.8 eV. These spectra were similar to those recorded with an excitation energy of 180 eV, except that the resolution of feature B was better. Since the photoemission cross section of 2p orbital is larger a t -40 eV than at 180 eV,29 this indicates that the new states are associated to the formation of delocalized n bonding arising mainly from the C atoms. The C 1s core level excitation spectrum of the as-deposited consists of a broad peak centered a t about 288.1 eV. After 15 min irradiation, the peak is shifted by 2.7 eV toward the lower binding energy. From the energy shift recorded in Figure 3, the elimination of the charging effect can account for roughly 1.5 eV. The 1.2-eV energy difference is due to a variation in the chemical nature of the carbon atoms. As the C 1s core level of the nitrile group occurs at a higher binding energy than that of the CH and CH2 groups,28this shift is consistent with the lost of the nitrile group. In fact, the position of the C 1s core level excitation spectrum of the irradiated film is consistent with that expected for an unsaturated carbon structure.32 Thus, the electronic properties of the PAN film evolve from an insulating to a conducting state. Moreover, the progressive transformations of the film reveal that it is possible to tailor its conductivity by controlling the exposure time to the synchrotron white beam.

Discussion Based on different analytical techniques (infrared,33ultraviolet, and X-ray photoemi~sion,~’*~**~~ electron energy ultraviolet absorption,I2 and metastable deexcitation spectroscopiesz7)a model has been proposed to explain the structural modifications that lead to the large increase of conductivity observed in heat-treated PAN. There is general agreement that three steps occur: At temperatures below 300 OC, a cyclization process of the nitrile groups occurs upon pyrolysis and the polymer is transformed to a single-conjugated ring structure. At higher pyrolysis temperatures (but below 600 “C), hydrogen is removed from the material leading to the formation of a conjugated structure (poly(pyridinopyridine)),which is often referred to as a double-conjugated ring structure. Finally, a complete loss of the non-carbon atoms occurs at much higher temperature (800-1000 “C) and a “graphitic-like” phase is synthesized. AU the spectroscopic data collected on the white beam irradiated PAN films show that, after a few minutes of exposure to the white beam, they are essentially composed of C atoms with a graphite-like structure. However, the mechanisms leading to this evolution are different from those observed when the film is pyrolyzed: The NEXAFS, IR, and photoemission spectra clearly indicate that the first step corresponds to the removal of the nitrile group without any cyclization of the N atoms. More prolonged irradiation causes a progressive loss of the H atoms leading to the formation of highly conjugated carbon structure. This evolution is similar to that observed when thin electrochemically synthesized PAN films are annealed at temperature around 500 OC.15 In that case too, a decrease in the N content of the film was observed, although hydrogen was still present in the film. Finally, after 15 min exposure time, the physicochemical characteristics of the film and especially the appearance of occupied states near the Fermi level is consistent with the synthesis of a graphitic-like material with highly delocalized A electrons. Theoretical calculations of photoemission valence band spectra, based on the molecular structures proposed within the model describing the thermal evolution of PAN, fail to predict the appearance of states just at the Fermi level. They are best predicted assuming a N-free polymer like p o l y a ~ e n e .Indeed, ~~ the photoemission curves of the irradiated PAN most closely resemble those of a glow-discharge carbon film annealed to 500 OC, and a one-to-one correspondence can be established between features

Thin Poly(acrylonitri1e) Films

The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5921

G, A, H, and I of Figure 1 and photoemission spectra for the carbon films.35 These authors have also shown that the emission peak at about -3.5 eV is absent in the asdeposited hydrogenated carbon film and only appears upon annealing, when hydrogen is lost from the film. The use of the synchrotron white beam permits precise control of the modifications induced in the film and hence of the electronic properties of the modified material. Moreover, since the transformation is limited to the area of the film that has been directly illuminated, spatial resolution can be achieved. Different mechanisms can be envisioned to explain the interaction of the synchrotron white beam with the polymer. After irradiation, a dark-brown spot was observed on the PAN film, whose size was identical to that of the white beam. This spatially defined modification of the polymer is hardly compatible with a thermal process. In that hypothesis, one would expect the irradiated spot to be surrounded by a more or leas extended region of less transformed material. A mechanism based on the photoionization and a photodiswiation of the PAN is more plausible, since it conveys the required spatial definition. A timedependent analysis of the gas evolving from the PAN upon irradiation should help in determining the nature of the chemical processes leading to the formation of this highly conjugated dehydrogenated carbon film.

( 5 ) Ohashi, H.; Inoue, K.; Saito, Y; Ywhida, A,; Ogawa, H.; Shobatake, K. Appl. Phys. Lett. 1989.55, 1644. (6) Zanoni, R.; Piancastelli, M. N.; McKinley, J.; Margaritondo, G.Appl. Phys. Letr. 1989, 55, 1020. (7) Meier, H. Organfc Semiconductors; Verlag-Chemic: Weinheim, 1974. (8) Brom, H. B.; Tomkiewicz, Y.; Aviram, A,; Broers, A.; Sunners, B. Solid State Commun. 1980, 35, 135. (9) Reynolds, W. N. Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A,, Eds.; Dekker: New York, 1973; Vol. 11. (10) Teoh, H.; Metz, P. D.; Wilhelm, W. G.Mol. Cryst. Liq, Cryst. 1982, 83, 297. (11) Ritsko, J. J.; Crccelius, G.;Fink, J. Phys. Rev. B 1983, 27, 2612. (12) Chung, T.4.; Schlesinger, Y.;Etemad, S.; MacDiarmid, A. G.; Hecger, A. J. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 1239. (13) Teng, F. S.; Mahalingam, R. J. Appl. Polym. Sci. 1979, 23, 101. (14) Lecayon, G.;Bouizem, Y.; Le Gressus, C.; Juret, C.; Reynaud, C.; Boiziau, C.; Juret, C. Chem. Phys. Lett. 1982, 91, 506. (1 5) Tourillon, 0.;Garrett, R.; Lazarz, N.; Raynaud, M.; Reynaud, C.; Lbayon, G.;Viel, P. J. Electrochem. Soc. 1990, 137, 2499. (16) Stohr, J.; Outka, D. A. J . Vuc. Sei. Technol. A 1987, 5, 919. (17) Sette, F.; Stohr, J.; Hitchcock, A. P. J . Chem. Phys. 1984,81,4906. (18) Hitchcock, A. P.; Stohr, J. J. Chem. Phys. 1987,87, 3253. (19) Outka, D. A.; Stohr, J. J. Chem. Phys. 1988,88, 3539. (20) Stohr, J.; Outka, D. A. Springer Series in Surface Science; Vanstlow, R., Howe, R., Eds.; Springer: New York, in press. (21) Kordesch, M. E.; Lindner, Th.; Somers, J.; Stenzel, W.; Conrad, H.; Bradshaw, A. M.; Williams, G.P. Spectrmhim. Actu 1987, 43A, 1561. (22) Raynaud, C. Ph.D. Thesis,Universit6 Paris-Sud, Orsay, France, 1989. (23) Outka,D. A,; Stohr, J.; Rabe, J. P.; Swalen, J. D.; Rotermund, H. H. Phys. Rev. Lett. 1987, 59, 1321. (24) Hitchcock, A. P.; Tronc, M.: Modelli, A. J . Phys. Chem. 1989, 93,

Conclusion

3068. (25) Leroy, S.; Boidau, G.;Lecayon, C.; Le Gressus. C.; Vigouroux, J. P. Ma&. Lett. 1985, 3, 239. (26) Socratts, G.Infrared Characteristic Group Frequencieq Wiley: New York. ...~ , 1980. ---(27) Reynaud, C.; Boidau, C.; Juret, C.; Leroy, S.; Perreau, J.; Lecayon, G.Synth. Met. 1985. 11, 159. (28) Wu, C. R.; Salaneck, W. R.; Ritsko, J. J.; Bredas, J. L. Synrh. Met. 1986. 16. 147. (29) Yeh, J. J.; Lindau, 1. At. Dura Nucl. Dura Tables 1985, 32, 1. (30) Bredas, J. L.; Salaneck, W. R. J. Chem. Phys. 1986,85, 2219. (31) Reynaud, C.; Juret, C.; Boiziau, C. Surf. Sci. 1982, 126, 733. (32) Handbook of X-ray Photoelectron Spectroscopy; Wagner, C. D.,

Synchrotron radiation white beam was used to induce specific transformations of PAN that yield a highly conjugated dehydrogenated carbon structure. The local nature of the transformation induced by the beam and the ability to tailor the electronic properties of the material by varying the exposure time allow us to envision new applications of the synthrotron radiation in the microelectronic area. Registry No. PAN (homopolymer), 25014-41-9.

Referencea and Notes (1) Ehrlich, D. J.; Tsao, J. Y. J . Vac. Sci. Technol. B 1983, 1 , 969. (2) Ehrlich, D. J.; Tsao, J. Y. V U I Electronics: Microstructure Science; Einspruch, N. G.,Ed.; Academic: New York, 1983/1984. (3) Kyuragi, H.; Urisu, T. J . Appl. Phys. 1987, 61, 2035. (4) Urisu, T.; Kyuragi, H. J. Vac. Sci. Technol. B 1987, 5, 1436.

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