J. Phys. Chem. 1995,99, 16516-16518
16516
Photoinduced Cross-Linking and Decomposition of Cm in the Presence of Nitric Oxide (NO) K. B. Lyons,* A. F. Hebard, D. Innis, R. L. Opila, Jr., H. L. Carter, Jr., and R. C. Haddon AT&T Bell Laboratories, Murray Hill, New Jersey 07974
Received: June 27, 1995; In Final Form: September 11, 1995@
We report observations of an enhancement of at least 3 orders of magnitude in the rate for photoinduced cross linking of C a films under an ambient NO atmosphere. Under extended illumination (for as long as 48 h at 8 W/cm2) the fullerene clusters disappear, and the film evolves toward an amorphous mixture of C, N, and 0. Raman and XPS data show signatures of C-N bond formation. We propose a mechanism for the role of NO based on stabilization of the diradical intermediate formed in the primary photochemical step. This mechanism suggests a new generic pathway for photoinduced transformation of fullerene films. Related experiments with samples removed from NO and exposed to air also show a residual polymerization effect with a decay time on the order of an hour.
HF and amorphous films if a surface oxide remains on the Si.*
Introduction Although Cm in its ground state has a closed shell of electrons and hence is very stable, under photoirradiation it can undergo a variety of interesting transformations. A photodimerization reaction which yields a pair of Cm molecules linked by a 4-membered was discovered in the initial Raman studies of this materiaL3 In the presence of oxygen this reaction pathway is quenched: but a different sort of photosensitivity is observed5which results from photoinduced oxygen diffusion. In the first case, the pentagonal pinch mode seen in Raman scattering shifts from 1468 to 1458 cm-1,6 while in the latter case the shift does not exceed 2 cm-l. Thus, Raman scattering is a particularly useful tool for studying the dynamics of crosslinking reactions. In this study we report on the photoinduced transformations of C a films that occur in a nitric oxide (NO) ambient. We have discovered that under an NO atmosphere the films undergo a cross-linking reaction with startling rapidity when illuminated by green or blue laser light, under conditions where the dimerization reaction should occur very slowly, by comparison, in the pure oxygen-free material. The response to UV light is enhanced by a similar factor. Upon extended visible-light exposure, we have also observed photoinduced decomposition of the cross-linked C ~ into O an amorphous complex of C, N, and 0 which we have characterized using Raman scattering, UV-visible absorption, XPS, and AFM. NO was chosen since it is a small molecule and should therefore readily occupy the interstitial sites of the C a solid, and it is the simplest thermally stable odd-electron molecule known. Although NO normally inhibits radical-initiated polymerization reactions: in the case of a diradical reaction, as we discuss below, it is possible for the reaction with NO to prolong the lifetime of the reactive species, thus enhancing the reaction rate. This combination of circumstances made it a likely candidate for increasing the cross-linking response in Cm.
Experimental Method The Cm samples employed were thin films, -1000 %, thick, deposited at a rate of 15 k s onto room-temperature (1OO)Si substrates. Substrate preparation is important, giving crystalline films if the Si surface is hydrogen terminated by dipping in 5% @
Abstract published in Aduunce ACS Abstracts, October 15, 1995.
0022-365419512099-16516$09.00/0
Both kinds of samples behaved similarly in the studies reported here. The Raman spectra reported here were excited using an argon laser, operated at either 5145 or 4880 A. The incident beam was focused using a cylindrical lens to provide an elliptical focal spot, approximately 100pm x 2 mm on the sample, with power density about 1 W/cm2. The angle of incidence was 65", and the scattered light was collected in an j72 cone centered about the normal. The incident polarization lay in the scattering plane, and an analyzer in the scattered light path selected light in the scattering plane (hh geometry) or normal to it (hv geometry). The spectra were resolved using a Spex Triplemate spectrometer with CCD detection. The spectra were all corrected using a standard lamp, calibrated against an NBS-calibrated lamp in a separate experiment. The resolution typical of this arrangement was about 4 cm-'. The samples were studied under 1 atm of NO. A smallvolume cell was designed to minimize the toxicity risk from accidental breakage. The Si substrate was mounted on a BeCu holder pressed against the window by another Be-Cu plate spring. The cell body was stainless steel, with a Pyrex window. Under these conditions we found that the response of the sample was stable for days at a time, so long as we began with a fresh, unexposed spot for each series of measurements.
Results and Discussion The scattering intensity was such that the main Cm line could be located to an accuracy of 1 cm-I in an exposure of 20 s, and a very high quality spectrum could be acquired in about 10 min. Typical spectra obtained on a single spot during extended exposure are shown in Figure 1. The prominent narrow features evident in all the traces are the Ai (near 1468 cm-I) and the Hg lines (near 1430 and 1565 cm-I). The A: line shifts to 1457 cm-I, and also broadens and weakens, as the polymerization reaction takes place, as is evident in the figure. Under similar conditions no observable cross linking takes place in the absence of NO even in an hour, as shown by the inset trace in the upper left block of the figure. Under the NO atmosphere, the reaction is nearly complete in less than 2 min. The elastic scattering increases at the same time, far more so than when the material is cross-linked in the absence of NO. When this treatment was extended to periods of 8-24 h, the film evolves from its initial rose color to a bright blue and then 0 1995 American Chemical Society
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J. Phys. Chem., Vol. 99, No. 45, 1995 16517 3000,
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Figure 2. AFM line scan over a range of 100 pm, extending from the center of the spot (on the right) to a region well outside the exposure spot (on the left). The slope of the baseline has been removed. Note the smooth nature of both the unexposed and heavily exposed material, as contrasted with the material in between.
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Figure 1. Raman spectra obtained from amorphous Cm sample studied under an atmosphere of pure NO. Each block shows a pair of hh and hv spectra, with the total exposure time indicated at the left. For comparison, the corresponding spectrum of a similarly exposed pure Cm film is shown (hh only, displaced 75 units for clarity) at upper left. A gain expansion of x 10 is used at the right. The hh and hv spectra are, in each case, displayed on properly normalized scales, so the relative intensity is meaningful. In each pair, the hh trace is the more intense of the two. The instrumental respose has been removed independently for each polarization.
to pink, when viewed in reflection on a Si substrate. In the final product the Raman intensity is substantially depressed and the elastic scattered light nearly vanishes, while the visible luminescence increases substantially. Under the extended exposure that leads to this color change, we observe the appearance of broad high-frequency Raman bands (1500-2000 cm-I) which may indicate C-N bond formation. The bands are weak and broad, which makes them hard to discern against the luminescence background. There is also a curious feature of the spectra shown in Figure 1 which bears noting: the broad background feature has a polarization ratio p substantially different from unity. This suggests it is not a wing of the luminescence peak near 6000 cm-I, which exhibits p = 1. It might instead represent a scattering into some large continuum of states, possibly electronic in origin. As noted above, the weak feature observable in the spectra near 1750 cm-' provides some hint of C-N bond formation. To study bond changes quantitatively, we carried out an XPS study on another sample on Si(100) prepared using 4880 8, light at about 4 W/cm2 over an area of several mm2 for 18 h. Visually, the spot was graded from pink in the center through a blue ring to the usual rose color of the unexposed material. After this irradiation XPS performed on a region 400 pm x 400 pm in the center of the exposed spot showed the presence of C, N, and 0 with the approximate atomic concentrations of 54%, lo%, and 36%, respectively. A study using higher spatial resolution showed a clear evolution reflecting gradual inclusion of N in the carbon matrix, reaching a maximum at the spot center, with the strength of the C-N bond showing dramatic increase (by a factor of 3) in both the C(ls) and N(ls) spectral regions. In the final material, only about 15% of the N in the film can be attributed to the presence of NO, to the depth accessible by XPS, indicating that a large majority of the nitrogen present is bound to carbon. The change in elastic scattering suggests significant changes in surface morphology. To study these changes in greater detail,
we also characterized a heavily exposed sample using atomic force microscopy (a Figure ). 2 shows * an AFM line scan across the edge of the exposed spot. Moving from left to right in the figure corresponds to moving from the unexposed to the exposed region. This surface profile suggests that both the unexposed and heavily exposed regions are smooth. There is a -30 p m thick band between the two regions, suggesting "pileup" or expansion has occuned. In this region, there is also evidence that the film is rougher, at least on this scale. These conclusions are consistent with the elastic scattering observed visually. There is substantial roughness in the edge region, with an in-plane correlation length of 1-2 pm, which is no doubt responsible for the increased elastic scattering observed. Higher resolution images (not shown) demonstrate that the characteristic size of the surface features evolves quite continuously from the -20 nm islands characteristic of the unexposed film to the much larger regions characteristic of the edge region. The mechanism responsible for this increasing length scale remains a puzzle. From the images obtained we cannot characterize the final stage of the process definitively. The film appears to flatten quite suddenly, but whether this constitutes a continuation of the growth mentioned above or a new smoothing mechanism is not clear. The final material appears to be a highly disordered mixture of C, N, and 0,since the Raman spectra manifest no sharp features but only a very broad band, covering many hundreds of cm-I, with only a hint of structure. Scratch tests showed that the irradiated film had somewhat higher cohesion than the unirradiated material. The adhesion of the film to the substrate, though, was in no case sufficient to withstand even mild abrasion. However, the XPS evidence that C-N bonds are clearly formed suggests that, with further development, this approach might yield a route for photochemical preparation of carbon nitride films, which might have much higher hardness and abrasion resistance. We also performed UV irradiation tests in the same cell by changing the window to silica. At an exposure rate of about 1 mW/cm2 from a mercury lamp, we found that an exposure of 1 min was sufficient to produce patterning upon subsequent immersion in toluene, in which the un-cross-linked material is soluble. This corresponds to a total W dose of about 0.1 J/cm2, to be compared with the total dose of 50 J/cm2 used in previous work. This improvement of exposure time by a factor of > 100 over that reported previously5 reflects not only an accelerated phototransformation but also a different mechanism; the enhanced cohesion of the W-irradiated NO-infused films is due
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16518 J. Phys. Chem., Vol. 99,No. 45, 1995
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ate triplet diradical competes with dissociation, and this twostep process provides the primary pathway for dimerization. The enhanced reactivity of excited state Cm toward NO may be attributed to the inherent strain relief of about 8 kcal/mol for each of the four carbon atoms involved in the cycl~addition.~ Although NO reacts with radicals to give nitroso compounds and inhibits radical polymerization reactions, there appears to be no precedent for the attack of NO at triconjugate carbon atoms.I0 Similarly, nitroso compounds are known to undergo reaction with radicals to produce nitroxides." Thus, in accounting for this aspect of Cm reactivity, it is clear that the strain in the molecule confers a degree of biradical character on the excited states. This fact may make Cm an interesting starting material for a number of synthetic pathways. The reaction of 2 and 3 with NO to form 4 greatly increases the rate of the reaction by prolonging the lifetime of the reactive species. Further reaction of the nitroso compound might involve excited state Cm to produce 6, or another nitroso compound to yield 7. The possible final products 5 and 8 provide a means to incorporate NO into the matrix. Similar reactions involving dimers could lead to more complex cross-linking.
Conclusions 'N-0 8
9 (CSO)?
Figure 3. Schematic of the reaction pathway discussed in the text for the photoinduced cross-linking of Cm molecules. Only a fragment of each molecule is shown in the diagrams.
to cross linking of Cm rather than photoinduced diffusion of oxygen molecules into the octahedral sites of the host struct~re.~ In an extension of these experiments to samples removed from the NO and exposed in ambient air, we found that the polymerizationxesponse decays initially on a time scale of about an hour, and there is a long-lived residual photosensitivity which lasts at least for days. This behavior is in contrast to that of deoxygenated samples, where any air exposure is sufficient to completely destroy the photopolymerization effect' and results instead in a photooxygenation effect which stabilizes the film . in a different manners5 We note, moreover, that the behavior under NO is the same with or without prior oxygen exposure, thus suggesting that NO effectively displaces oxygen in the matrix. In Figure 3 we outline a proposed mechanism for the effect of nitric oxide on the photoinduced cross-linking and degradation of Cm, together with some of the possible intermediates in the reactions. The primary photochemical step is the formation of the excited singlet (2), which undergoes rapid intersystem crossing to the triplet (3). The latter has a lifetime of microsecond^.^ Although the excited singlet can undergo an allowed [d,d,] electrocyclic reaction with another groundstate Cm molecule to give the dimer in a concerted reaction,2 this does not constitute the major polymerization pathway. Since pairs of double bonds do not adopt the required suprafacial geometry with high probability, the short lifetime of the excited singlet limits the contribution of this reaction pathway. In fact, oxygen, which quenches the triplet state, greatly slows the polymerization reaction.' We propose here that the reaction proceeds via a two-step mechanism, which has the additional advantage that spin selection rules may be properly satisfied in the reaction from the triplet state. From kinetic studies2 it appears that the dominant reaction pathway does not involve pairs of Cm molecules in their initial photoexcited state. In our view, then, spin dephasing and cyclization of the intermedi-
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In conclusion, our results show that in the presence of visible irradiation NO catalyzes a very rapid dimerization of the fullerene in the first stage of the reaction. Under extended irradiation, the fullerene clusters disappear from the sample. Raman and XPS data suggest a picture of the material as a bonded, amorphous C-N-0 matrix. During this multistage chemical process, there appears to be a smooth evolution in both the chemical composition and the surface morphology of the film. The N and 0 content of the film increases, with the N clearly bonding predominantly to C, while the surface exhibits growing flat plates which eventually become very flat and smooth in the final product film. We have proposed a reaction scheme for the chemical reactions involved, where NO plays a role by prolonging the lifetime of the reactive species and thus becomes incorporated into the final product. Accordingly, our results indicate a new (third) mechanism for phototransformation of fullerene films. Further understanding of the dramatic changes in the reactivity of the fullerene molecule in its excited state may in the future open up many possibilities for useful photochemistry.
Acknowledgment. We are indebted to R. Fleming, L. Rothberg, and 0. Zhou for assistance with related measurements and to E. Chandross for helpful discussions of the results. References and Notes (1) Wang, Y.; Holden, J. M.; Dong, Z.-H.; Bi, X.-X.; Eklund, P. C. Chem. Phys. Lett. 1993, 211, 341. (2) Zhou, P.; Dong, Z.-H.; Rao, A. M.; Eklund, P. C. Chem. Phys. Lett. 1993, 211, 337.
(3) Eklund, P. C.; Zhou, P.; Wang, K.-A,; Dresselhaus, G.; Dresselhaus, M. S. J. Phys. Chem. Solids 1992, 53, 1391. (4) Arbogast, J. W.; et al. J . Phys. Chem. 1991, 95, 1 1 . (5) Hebard, A. F.; et ai. Appl. Phys. A 1993, 57, 299. (6) Zhou, P.; et al. Appl. Phys. Lett. 1992, 60, 2871. (7) Calvert, J. G.;Pitts, Jr., J. N. Photochemistry; John Wiley: New
York, 1967. (8) Hebard, A. F.; Zhou, 0.; Zhong, Q.; Fleming, R. M.; Haddon, R. C. Thin Solid Films 1995, 257, 147. (9) Haddon, R. C. Science 1993, 261, 1545. (10) Maruthamuthu, M.; Scaiano, J. C. J. Phys. Chem. 1978, 82, 88. Guyot, A.; Mordini, J. J . Polym. Sci. 1971, C33, 65. (1 1) Chalfont, G. R.; Perkin, M. J.; Horshfield, A. J . Am. Chem. SOC. 1968, 90, 7141. Jp95 1778N
