10824
J. Phys. Chem. 1994, 98, 10824-10831
Raman Spectroscopy of C ~ Solid O Films: A Tale of Two Spectra Kelly L. Akers, Constantine Douketis, Tom L. Haslett, and Martin Moskovits* Department of Chemistry and the Ontario Laser and Lightwave Research Centre, University of Toronto, Toronto MSS IAl, Canada Received: March 30, 1994; In Final Form: July 8, I994@
Two distinct Raman spectra have been reported for solid Cm. They differ in the exact position and relative intensities of the spectral lines. Specifically, the frequency of the pentagonal pinch mode occurs at 1469 and 1459 cm-' in the two spectra. Several explanations have been offered for the existence of a second spectrum, including the influence of interstitial oxygen, a change in phase, the coexistence of singlet c 6 0 with laserpumped triplet c 6 0 , and photopolymerization. We show that there are two Raman spectra for solid films of Cm with varying relative intensities according to the sample temperature even in the absence of oxygen. The reversibility of the two forms of c60 with temperature argues against photopolymerization. Finally pumpprobe experiments designed to detect either triplet absorption or singlet depletion indicate that neither of the two Raman spectra are due to triplet c 6 0 . We conclude that our previous explanation in terms of two phases of c 6 0 is still the most consistent with the observations.
Introduction The unambiguous identification of the Raman spectrum of solid C ~ has O been complicated by the following influences: oxygen exposure, sample temperature, and laser irradiance. To date there have been several studies on the Raman spectroscopy of Cm.1-9J1-15 In spite of the simplicity of the Raman spectrum there remains controversy concerning the exact frequencies of some of the vibrational modes of the molecule and, in particular the pentagonal pinch mode. Bethune et al.' reported the first Raman spectrum of a Cm film; it featured an intense peak at 1469 cm-' which was assigned to the totally symmetric pentagonal pinch mode on the basis of polarization measurements. In that study the c60 film was prepared by sublimation in vacuum but the Raman spectrum was recorded in air. Subsequently, other researchers reported a Raman spectrum of a Cm film sublimed and maintained in a vacuum en~ironment.~ The most notable difference between the above two reported spectra is in the position of both the intense pentagonal pinch mode and the highest frequency H, mode, which are both redshifted by ~ 1 cm-' 0 for films maintained in vacuum. While several other modes exhibit a red-shift they do so to a smaller degree ( ~ 3 - 6cm-'). Furthermore, a new peak is observed in the spectrum at 1625 cm-'. Other weaker modes are also found which have been attributed to solid-state effect^.^ Subsequent exposure of the film to oxygen or air resulted in a Raman spectrum identical to that reported by Bethune.'S2 On exposure to oxygen the spectrum of the film was found to exhibit higher absolute intensities for all modes, with emphasis on the pentagonal pinch mode, which increased by as much as a factor of 3. It was concluded that the Raman spectrum of a c 6 0 film sublimed in vacuum was the intrinsic Raman spectrum of c 6 0 and that exposure of the c 6 0 films to air or oxygen results in a structural modification of the film due to the uptake of molecular oxygen. Zhou et aL4 report the pinch mode at 1469 cm-l for an oxygen-free Cm film measured in a helium environment when irradiated with 488-nm laser light with an irradiance of 5 W/cm2. The appearance of a new mode at 1459 cm-I after 30 min of laser irradiation at 75 W/cm2 was ascribed to the formation of a phototransformed phase of c 6 0 . They report similar room temperature Raman spectra for both unexposed and air-exposed @
Abstract published in Advance ACS Absrructs, September 1, 1994.
0022-3654/94/2098- 10824$04.50/0
films. The authors do note, however, that the peak at 1625 cm-' disappears upon oxygen exposure. Furthermore, the oxygen-exposed film is hardened against phototransformation. In an oxygen environment, significant oxygen uptake by the film was observed at irradiances as low as 5 W/cm2. The peak at 1625 cm-' appears whenever the 1460-cm-I peak appears and we, therefore, assume that they belong to the same phase. The vibrational Raman spectrum of a single-crystal sample of C 6 o has been followed as a function of temperat~re.~ The pentagonal pinch mode was reported to shift from 1466 cm-I at temperatures below 50 K to 1459 cm-' at 400 K. It was suggested that the shift in frequency with temperature can be correlated to a combination of thermal and lattice expansion effects. Raman studies of c 6 0 films sublimed and maintained in an ultrahigh vacuum (UHV) environment revealed similar spectral changes upon varying the sample temperature.6 Specifically, the room temperature spectrum matched the one reported by Duclos et al. and the low-temperature (57 K) spectrum is very similar to the one reported by Bethune et al. Recording the spectrum as a function of sample temperature shows the gradual disappearance of the initial spectrum with the concurrent appearance of a new spectrum. Over a wide range of intermediate temperatures, both spectra are clearly seen in varying relative proportions indicating a coexistence of two different phases of c 6 0 . Bowmar et al.' investigated the Raman spectrum of c 6 0 crystals as a function of temperature and air exposure. Unexposed crystal samples displayed a Raman spectrum containing peaks at both 1459 and 1469 cm-'. As the temperature was lowered below 250 K the spectrum displayed only one mode at 1469 cm-'. They assigned the peak at 1459 cm-I to that of pure c 6 0 in the freely rotating phase and that at 1469 cm-' as originating from parts of the crystal containing oxygen. They suggest that the presence of oxygen inhibits rotation of the nearby c 6 0 molecules giving a Raman spectrum at room temperature identical to that obtained from pure Cm at temperatures below the structural phase transition. They also found that the peak at 1625 cm-I appears only in the 1459-cm-I spectrum. Experiments performed on air-exposed crystals revealed that above 200 K the initial peak appeared at 1469 cm-' but irreversibly changed upon laser irradiation in vacuum 0 1994 American Chemical Society
Raman Spectroscopy of Cm Solid Films to a final position of 1459 cm-'. Concurrently, the peak at 1625 cm-' grows in. However, the blue-shift of the pinch mode upon cooling does not occur above 200 K. Photoinduced polymerization of solid c 6 0 films has been suggested8 to explain the spectral changes observed upon exposure of C a films and crystals to moderate irradiation with 514.5- and 488-nm light. Increasing the laser power density above 5 W/cm2 irreversibly transformed the Raman spectrum; the pinch mode shifted from 1469 to 1460 cm-'. It was suggested that the reactive intermediate in such a polymerization reaction is the excited triplet state (TI). It was also reported that the 1459-cm-l peak is an unpolarized line and is, therefore, inconsistent with the A, symmetry of the pentagonal pinch mode. Although the authors do not give a value for the measured depolarization ratio for the 1459-cm-' mode, they state that the polarization ratio deteriorated from 100% in pristine C a to 80% in the phototransformed phase. For a mode to be 100% polarized, the depolarization ratio = 0. The measured value is reported9 as 0.10, which would correspond to a polarized mode (but not a completely polarized mode) and we therefore understand their statement to mean that the measured depolarization ratio for the 1459-cm-' mode = 0.20. For a mode to become depolarized the depolarization ratio would be 0.75.1° Hence, although the ratio is larger for the 1459-cm-' mode it is polarized and is, therefore, associated with a totally symmetric mode. Recently, the same group" reported the thermal decomposition of polymeric C a at temperatures above 100 "C in a sealed reaction cell containing 1 Torr of He gas. Van Loosdrecht et al. l2 reported low-temperature Raman spectra of single-crystal c 6 0 as a function of the spectral irradiance. The crystals were grown in vacuum but were briefly exposed to the atmosphere upon transfer to the low-temperature cell, which was subsequently evacuated to a pressure of 3 x Torr. For irradiances below 5 W/cm2 the pinch mode appears at 1469 cm-'. Increasing the irradiance to 50 W/cm2 resulted in the appearance of an additional band at 1459 cm-', which totally replaces the 1469-cm-' peak at 300 W/cm2. This behavior was found to be reversible; that is, low irradiance Raman spectra taken after each higher irradiance spectrum were identical to the initial low irradiance spectrum. All spectra were recorded at 40 K. This behavior contrasts with our lowtemperature results6 in which at low temperatures and high spectral irradiance (> 1000 W/cm2) the pinch mode appeared at 1469 cm-' and no change is found when the irradiance was subsequently reduced. It is, however, difficult to assess the oxygen content in the samples used in that work12 since no room temperature spectra were reported. In ref 12 it is argued that the shifting of the pinch mode upon exposure to high spectral irradiances is due to the population of an electronically excited state of c60. namely, the triplet state. It was suggested that the triplet is progressively populated with increasing laser power. The observed red-shifting of various modes reflects the reduction in the force constants associated with the molecular vibrations of triplet c 6 0 . The failure of oxygen- and air-exposed samples to exhibit similar spectral changes was ascribed to the efficient quenching of the triplet state by oxygen, thereby permitting measurement of Cm in the ground state even at high radiant flux densities. The peak at 1469 cm-' was assigned to the pentagonal pinch mode in the ground state of Cm. Raman studies on single-crystal c 6 0 at room temperature by Byme et al.13 found the pinch mode to red-shift continuously and reversibly as a function of excitation intensity at 514.5 nm, to a final position of 1463 cm-'. The shift was found to be nonlinearly dependent on the laser power. Interestingly, a nonlinear increase in Raman intensity was found as the power
J. Phys. Chem., Vol. 98, No. 42, 1994 10825 was increased above a threshold irradiance of 90 W/cm2. However, experiments performed on sublimed c60 films found the pinch mode to appear initially at 1469 cm-'. At low irradiance the intensity of this mode decreased considerably and a new mode appeared at 1459 cm-l.14 The authors support the model whereby population of the triplet state leads to a photochemical reaction which, in tum, degrades the c 6 0 in oxygen-free environment^'^ to some unspecified product. Subsequent experiments which followed the pinch mode as a function of excitation irradiance at 514.5 nm revealed the pentagonal pinch mode to appear initially at 1468 cm-' and under fairly low laser irradiance (50 W/cm2) quickly shift to 1459 cm-'.l5 Above a threshold intensity the pinch mode blueshifts from 1459 to 1463 cm-' and remains stable as the laser power is increased further. Upon reducing the laser irradiance the pinch mode reverts back to 1469 cm-'. They proposed that a metastable species is formed, which reverts back to the original C60. Pump-probe experiments on c 6 0 in solution have unequivocally identified the triplet excited state.16-18 Several photophysical properties of c 6 0 have been reported, including the triplet-triplet absorption spectrum of the molecule in benzene.l9 The small energy difference between the first excited singlet and triplet states (9 kcaVmo1) indicates a small electron-electron repulsion energy for C60.l~The high yield of triplet state c60 formed in these experiments and the failure to detect fluorescence in both benzene and hexane suggest that the excited singlet decays by intersystem crossing. It has been demonstrated that C a possesses the right combination of ground- and triplet-state properties to make it a good candidate for an optical limiter.*O Specifically, at wavelengths of 488.0 and 514.5 nm, the absorption cross section of the excited triplet state19 is much larger than the corresponding cross section in the ground state.21 Furthermore, the lifetime of the triplet state is relatively long in solution. Both of these properties give Cm an exceptionally low optical limiting threshold. The triplet excited state of Cm has been detected in single crystal c 6 0 by Groenen et a1.22 at 1.4 K where competing nonradiative decay processes are reduced by freezing out the molecular vibrations. Two distinct triplet species were detected, supporting the concept of a 2-a, face-centered cubic superstructure at low temperatures. From this, they concluded that the triplet excitation is delocalized over at least a pair and possibly a more extended chain of Cm molecules. Direct C a triplet lifetime measurements were performed only in Cm solutions. A triplet lifetime of 40 ps and an 0 2 quenching rate constant of kq(02) = 2 x lo9M-' s-' have been reported.19 Triplet lifetimes for Cm in toluene of 22.8 x s16 and 1 2 . 9 x s17 have also been reported. In this paper we report the results of two sets of experiments: a detailed UHV Raman study of thin solid films of c 6 0 and pump-probe transmission experiments designed to detect the population of the triplet. In an attempt to determine the extent of involvement of the triplet state of Cm in the observed Raman spectra, two sets of experiments were carried out on the same sample: Raman spectroscopy was used to probe the vibrational properties of Cm films as a function of temperature and laser irradiance, and pump-probe experiments were performed in order to measure directly the absorption of the triplet state under identical irradiance conditions as those used to record the Raman spectra. The study was performed on c 6 0 films which were deposited and maintained in an UHV environment. The triplet-triplet absorption expected for a 1000-hhick film at 676 nm was estimated to be 0.12 by using the reported
Akers et al.
10826 J. Phys. Chem., Vol. 98, No. 42, 1994 extinction coefficient for the triplet-triplet solution absorption spectra.ls This is well within our detection limits. Finally a second pump-probe experiment was performed to record the ground-state depletion of the c60 molecules in the film.
1600
35000
I
I1
I
128000
-21000
Experimental Section Experiments were performed in an UHV chamber ionpumped to a base pressure of 8 x lo-" Torr. c60 was either purchased (99.9% pure) and used with no further purification or synthesized by using the Kratschmer-Huffman techniquez3 and purified by using standard chromatographic techniques (99.99%). Samples were subjected to mass and UV/vis spectral analysis to verify purity prior to introduction to the UHV chamber. The c60 was evaporated from a tantalum boat fixed to water-cooled high current feedthroughs. The temperature of the boat was determined with an alumel-chrome1 thermocouple spot welded to it. Samples were carefully degassed at increasingly higher temperatures to a maximum temperature of 450 "C and evaporated for 15 min before depositing onto the substrate. Deposition temperatures between 400 and 450 "C were used. The substrate was held at 298 or 57 K. Film thickness was determined using UVhisible absorption and published optical absorption coefficient^.^^ For the laser pump-probe experiment a polished copper substrate was used which could be heated to 473 K by using tungsten wire woven through the sample plate and cooled to 50 K by using a copper braid attached to the cold end of a helium closed-cycle refrigerator. The sample was mounted onto an XYZO manipulator to allow movement of the sample to the various sources and positions needed to form the film and to perform the spectral analysis. For the ground-state depopulation experiments, c60 films were deposited on a sapphire ( 2 x 5 cm) slide suspended from a coolable copper block. The temperature was monitored with an Au(Fe doped)/chromel thermocouple attached to the substrate. For the temperaturedependent Raman study, a diode was press-fit into the sample block in addition to the thermocouple to obtain accurate temperature measurements. Raman spectra were excited with an argon ion laser (488- and 514.5-nm laser lines). This was also used as the pump beam in the pump-probe experiments. Triplet absorption was detected by using a diode laser emitting at 676 nm. In solution one of the triplet absorption bands of c60 is centered at 750 nm.I8 The laser, therefore, probed the tail of the triplet absorption. The spot size of the pump beam was made larger than that of the probe beam to ensure uniform illumination over the spot illuminated by the probe beam. The diode laser light was detected with a photodiode mounted outside the vacuum chamber. The photodiode output was monitored either in a "DC" mode by an oscilloscope or with a lock-in amplifier and using a chopped laser. The sensitivity to transmission differentials was estimated at 0.1%. The absorption spectrum of a Cm film deposited on a sapphire substrate was obtained under UHV conditions by using both deuterium and quartz tungsten halogen lamp sources and a SPEX double-pass monochromator equipped with photon counting. The reference spectrum was obtained from the fullerenefree portion of the substrate. With our gratings the monochromator cuts off at 300 nm, preventing measurement of the strongest absorbance peak centered at 270 nm. Spectra were recorded between 320 and 500 nm, encompassing the singletsinglet transitions at 340 and 440 nm. Using the quartz tungsten halogen lamp as a probe, the decrease in ground-state population as evidenced by changes in transmission at 440 nm was measured by irradiating the film with 488- and 5 14.5-nm laser light, the same wavelengths used to excite the Raman spectra.
57
%C
400
600
800
p14000
I000 1200 1400 1600 1800 2000 220: Wavenumber (cm-1)
Figure 1. Raman spectrum of a Cm film sublimed and maintained in UHV at 298 K (lower trace) and 57 K (upper trace) in the region 2002200 cm-I. Irradiance = 5000 W/cm2 at 488 nm.
To eliminate the possibility of misalignment of the pump and probe beams, a He-Ne laser and a quartz beamsplitter were used to send the He-Ne back through the optical path and directly back into the center of the window of the lamp. The experiment was performed in two ways. In the first the absorption spectrum was measured by using photon counting (with small slits), recording sequential spectra with the laser on and laser off. The peak at 340 nm was recorded by using a deterium lamp. The visible region of the spectrum was recorded in two smaller overlapping regions to reduce drift of the lamp intensity for the duration of the experiment. In the second the experiment was repeated by using the PMT in an analogue mode (high voltage reduced), chopping the laser beam at 12 Hz, and using lock-in detection of the voltage to detect any laser-induced change in transmitted light through the film as a function of wavelength. A plot of (Z(l=er on) - Z(laser off)/llaser was obtained by subsequently recording a spectrum of by chopping the probe lamp beam. The difference signal was recorded by the lock-in amplifier as a function of wavelength in the region 365-675 nm. Raman spectra were recorded by using laser power densities in the range 20-5000 Wkm2 (spot size = 150 p - 1 . 5 mm). The scan speed was 1 cm-'/s; therefore, exposure times ranged between 3 and 30 min for an average scan between 1400 and 1600 cm-' or 100 and 2100 cm-'.
Results and Discussion Raman Spectra of CSOFilms as a Function of Temperature. Raman spectra of a 1 0 0 0 - h i c k c60 film are presented in Figure 1. These spectra were recorded from a film sublimed and maintained under UHV conditions and using an irradiance of 5000 W/cmz. The upper trace was obtained from a film at 57 K and the lower trace from the same film at room temperature. At room temperature the pentagonal pinch mode is seen at 1460 cm-'. It should be noted that a weak peak, marked with an asterisk in Figure 1, appears at -525 cm-' independent of the substrate used. Due to the coincidence of this peak with a Raman peak of silicon it was not assigned to c60 by some worker^.^^^ The room temperature spectra are essentially identical to those reported previously3 for Cm films sublimed and maintained in vacuum at temperatures greater than 250 K. The most notable change upon cooling (Figure 1) is the 10-fold increase in intensity of the pentagonal pinch mode. At room temperature the pentagonal pinch mode appears at 1460 cm-', whereas at 57 K it is blue-shifted to 1469 cm-'. The high-frequency H, mode shifts from 1564 to 1575 cm-' and the intensity increases by a factor of approximately 2. Finally,
Raman Spectroscopy of
c60
Solid Films
J. Phys. Chem., Vol. 98, No. 42, 1994 10827
P
8
t
350-
8 50-
/-?
1751
7i40
1450
Wavenumbers ("1)
Figure 2. Series of Raman spectra of a Cw film at the following temperatures: (A) 212 K, (B) 220 K, (C) 225 K, and (D) 235 K. All of the spectra are on the same scale but are shifted vertically for clarity. Laser irradiance was 2000 W/cm2 at 488 nm.
the peak at 1627 cm-' is completely absent in the lowtemperature spectrum and several new modes of moderate intensity appear. Films evaporated and maintained at low temperature produce spectra identical to those evaporated at room temperature and subsequently cooled down. This process is found to be completely reversible. Figure 2 shows a series of Raman spectra of a c60 film in the 1400- 1700-cm-' range at increasing temperatures. The important point to note in this figure is the coexistence of two distinct pentagonal pinch modes of varying intensity ratios depending on the temperature of the sample. This implies the coexistence of two different phases of Cm over a wide temperature range. Previously we explained this behavior in terms of an order-disorder phase transition involving the percolation of a cluster of c60 molecules, which then produces coherent Raman scattering.6 In considering the apparently different behavior of the pinch mode for our low-temperature films and the single-crystal spectra reported by van Loosedrecht et al.,12we note in particular that their low-power, low-temperature spectra agree with our high-power, low-temperature data. However, upon increasing their laser flux to 370 W/cm2 the pentagonal pinch mode shifted to 1459 cm-'. This behavior is found to be completely reversible. For laser fluxes in excess of 500 W/cm2, however, the spectra were reported to change irreversibly, indicating optical damage to the crystals. The high thermal conductivity of the substrates employed in our experiments, the thinness of the films, and the good contact with the substrate allow the use of high spectral irradiances without the thermal degradation which has been reported for single-crystal samples.1 2 3 1 3 Second, the minimization of thermal gradients in thin film samples permits a more accurate measurement of the sample temperature. The high irradiance data of van Loosedrecht et al. is consistent with our results if one assumes that in their experiment the local temperature of the illuminated area rises due to the poor thermal conductivity of the crystal. This is consistent with the reversible spectral changes we observe as a function of temperature. In the high irradiance Raman experiments the laser spot size was approximately 150 pm, and although care was taken to ensure that the same spot was being probed at room and low temperatures, we were concerned that perhaps thermal expansion caused different positions of the sample to be probed in the low and room temperature experiments. To eliminate this possibility an unfocused laser beam (1.5-mm diameter) was used to record the room temperature Raman spectrum. Using low laser irradiance (28 W/cm2) the pentagonal pinch mode appears at 1459 cm-', exactly as in the high-irradiance spectra reported
i' ' \
i460 Wavenumber (cm-1)
1470
1 1i e o
Figure 3. Raman spectrum of a Cw film at room temperature (trace A) and at low temperature (57 K) (trace B). Irradiance = 28 W/cm2 at 514.5 nm. Trace C is at 57 K after 30 min of exposure to radiation with irradiance = 2800 W/cm2. -""
600-
P
8 c 1?
400-
a
s 200-
04 1400
1420
1440 1460 Wavenumbers ("1)
1480
I
1500
Figure 4. Raman spectra of a Cw film: at low temperature before (trace A) and after (trace B) 7 h of laser exposure. Irradiance = 65 W/cmZ at 514.5 nm.
previously. Upon cooling the sample to 50 K in the absence of laser irradiation, the laser spot size was reduced with an aperture by a factor of 10. The irradiance was maintained at 28 W/cm2; however, the area of the film probed was significantly reduced in order to ensure that the area sampled at the low temperature fell within the region irradiated and sampled at room temperature. The low-irradiance (28 W/cm2) Raman spectrum was identical to the high-temperature form (trace B of Figure 3). Short scans eliminated long exposures to the laser radiation. The film was then exposed to 30 min of high laser irradiance and the spectrum recorded at the higher irradiance (2800 W/cm2). The Raman spectrum of the low-temperature form (1469 cm-' pinch mode) was obtained (trace C of Figure 3). With the sample cold and using an unfocused beam the Raman spectrum remained initially similar to that recorded at room temperature. Specifically the frequency of the pentagonal pinch mode was 1459 cm-' (Figure 4A). However, the spectrum transformed in time, under continued irradiation, to the 1469-cm-' form. The initial transformation rate was found to be approximately proportional to the laser irradiance. A sample kinetic curve is shown in Figure 5. This was obtained by following the Raman intensity at 1469 cm-' as a function of time. With an irradiance of 65 W/cm2 the transformation was very slow. Figure 4B shows the result after 7-h irradiation with the sample maintained at 50 K throughout; essentially only the 1469-cm-I peak is now visible. However, at low irradiance saturation is not reached even after 10 h. High irradiance is needed to effect saturation in a reasonable period of time (6 h).
Akers et al.
10828 J. Phys. Chem., Vol. 98, No. 42, 1994
\."
0
1400 V '
0
1
2
3 4 Time (hours)
5
7
6
Figure 5. Time dependence of the intensity at 1469 cm-' at low
temperature. Irradiance = 65 W/cm2at 514.5 nm.
r400
1420
14iO 1460 Wavenumber (em-1)
1480
1sw
Figure 6. Raman spectrum of Cm film at room temperature using a reduced spot size. Irradiance = 680 W/cm2 at 514.5 nm.
It is important to contrast this behavior with what is observed when a sample which had previously been transformed to the 1469-cm-' form is heated. In that case the change to the 1459cm-I spectrum occurs immediately. To assess the stability of the low-temperature form in the absence of laser irradiation, the laser was tumed off and the sample maintained at low temperature for 12 h. The Raman spectrum remained unchanged even under low irradiance illumination, indicating that the 1469-cm-' form is stable at low temperature even in the absence of irradiation. The refrigerator was then turned off and the intensity of the pinch mode at 1469 cm-' was monitored as a function of time while the sample temperature rose slowly. As before, the laser spot size was reduced by focusing the beam down by a factor of 10. The intensity of the 1469-cm-' peak decreased steadily as a function of temperature. The Raman spectrum at room temperature was identical to the original room temperature spectrum (Le., the 1459-cm-' spectrum, Figure 3A). The reversion to the 1459-cm-' spectrum appeared to be immediate regardless of the laser irradiance used. an increased irradiance (680 W/cm2) was used to obtain the room temperature spectrum in the reduced spot (Figure 6) to compensate for the lower intensity of the 1459-cm-I (room temperature) spectrum. Laser power studies at low temperature indicate that the initial slope in the kinetics of the appearance of the 1469-cm-I spectrum (Figure 5) is linear in power. Extrapolating the plot of initial rate versus power to zero power yields a zero intercept. This indicates that laser radiation is needed to effect the transition from the high-temperature to the low-temperature form
1420
1440 1460 Wavenumber (cm-1)
I
1
1480
1500
Figure 7. Raman spectrum taken at 57 K of a spot which had never been exposed to laser radiation at room temperature: before (trace A) and after (trace B) a 6-h exposure to laser radiation. Irradiance = 28 W/cm2at 514.5 nm.
with no apparent power threshold. More important, it is clear that the 1469-cm-I spectrum is carried by the thermodynamically stable phase at low temperatures and the transition can be attained quickly at high laser irradiance. To recapitulate, at room temperature the Raman spectrum of the Cm films exhibits the pinch mode at 1459 cm-' at irradiances as low as 28 W/cm2. The spectrum was found to be stable even after being irradiated with light up to 5000 W/cm2. Upon cooling the same film to 50 K the pinch mode remained at 1459 cm-' only at low irradiance and for short exposure times. Unlike the behavior at room temperature, the intensity of the peak at 1459 cm-' was found to decrease upon exposure to laser irradiation and a peak at 1469 cm-' began to grow in. The kinetics of this transition was found to vary linearly with the laser irradiance. At high irradiance (4000 W/cm2) the 1459-cm-' spectrum is completely replaced with the 1469-cm-' spectrum within a few minutes. The demonstrated reversibility of the two spectra as a function of temperature is not consistent with the polymer model. It is unlikely that a depolymerization reaction would occur only at low temperatures. More compelling evidence against the identification of the 1459-cm-' spectrum with polymeric Cm was found by repeating the experiment at low temperatures on a spot that had never been exposed to laser radiation. It was reported that the photopolymerization of c60 can only occur at temperatures at which the molecules are still rotating, since at low temperature the simple cubic structure prevents the 2+2 cycloaddition of parallel double bonds necessary for the proposed reaction.25In the low-temperature structure the molecules are orientationally ordered with pentagon faces arranged opposite hexagonhexagon edges of any neighbor; this apparently precludes the favorable orientation or arrangement of molecules, and reorientation is unlikely at low temperature since the rotation is hindered or even stopped. A large spot size was used with an identical irradiance as that used in the room temperature experiment. If the 1469-cm-' spectrum is the Raman spectrum of the "unpolymerized" C a and polymerization occurs only above 250 K, then it should be impossible to obtain the 1460cm-' spectrum at low temperature on a film which had not previously been exposed to laser radiation at room temperature. The Raman spectrum obtained under these conditions was found to be identical to that found previously for samples irradiated at room temperature (Figure 7A), that is, the appearance of the high-temperature (1459 cm-') form. The intensity of the pinch mode at 1469 cm-' was monitored as a function of time. The low-temperature behavior was identical to that found for samples exposed to laser radiation at room tempera-
Raman Spectroscopy of
c60
Solid Films
ture. The spectrum transformed in time under laser illumination to the 1469-cm-' form (Figure 7B). The spot size was reduced while maintaining the irradiance constant and several smaller spots within the large spot were probed, yielding identical spectra. The sample was then warmed up to room temperature and a typical room temperature Raman spectrum was obtained (1459 cm-') with irradiances as low as 20 W/cm2. The sample was then cooled again and the experiment repeated in an attempt to detect differences in the kinetics of the growth of the 1469cm-* peak at low temperature. The intensity versus time curves do not show any significant difference in the kinetics of the growth of the 1469-cm-' peak after exposure to laser radiation at room temperature. The observation of the 1459-cm-' peak at low temperature on a sample that had no prior exposure to radiation argues against the interpretation of the 1459-cm-I spectrum as that of a polymer. Finally, we determined the depolarization ratios of the Raman bands for a c 6 0 film at room and low temperature. At 50 K the depolarization ratio for the 1469-cm-' mode was found to be 0.08, in agreement with values reported previously at and at low temperature.*' The ratio measured at room temperature for the 1459-cm-I mode was found to be slightly higher, with a value of 0.18. The value seems in agreement with the results of EMund for the 1459-cm-' mode; however, it is obvious that this mode is still polarized although somewhat less polarized than the 1469-cm-l mode. The supposition that the 1469-cm-' mode is due to a more ordered phase of C a is supported by these results. Oxygen-free C a samples at low temperatures and oxygen-containing samples at room temperature exhibit similar Raman spectra. The position and depolarization ratio of the pentagonal pinch mode for these two samples are essentially identical. The depolarization ratio of the peak at 1459 cm-l is larger in the rotationally disordered phase but is polarized and hence still associated with a totally symmetric pentagonal pinch mode. The thermodynamically stable phase at room temperature is that in which the CMmolecules are freely rotating. The ordered or nonrotating form is stable below a phase transition temperature or at room temperature when oxygen is incorporated in the c 6 0 solid. One can, therefore conclude that in oxygen-free films at room temperature the 1459-cm-' peak is the phonon associated with the pentagonal pinch mode. The disordered phase appears to be kinetically stable at low temperature, although it is not the thermodynamically stable phase. Kinetic stability implies the existence of an energy barrier. Laser illumination provides the energy needed for the activation barrier to be surmounted, allowing the thermodynamically stable phase to be attained. Although the effect of laser illumination could take the form of local heating, it is more likely that, in this case, equilibrium is achieved by photoactivation involving an electronically excited state. Involvement of the Triplet State. Another model which has been proposed to explain the Raman data of c 6 0 is the formation of an excited triplet state under irradiation by 488or 514.5-nm laser light. The idea being that with a sufficiently long triplet lifetime and with CW pumping, a substantial fraction of the c 6 0 molecules would exist in the triplet state. Pumping the C a film with 488- or 514.5-nm laser radiation while probing the corresponding triplet absorption of 676-nm light produced no observable change in the intensity of the probe beam. The film thickness was determined by recording its absorption spectrum film in the range 320-675 nm as shown in Figure 8. The population of the c 6 0 triplet was also investigated indirectly by following the depletion of the ground-state population by monitoring the decrease in the 440-nm absorption peak cor-
J. Phys. Chem., Vol. 98,No. 42, 1994 10829
'I
04
320
I'
380
440
500
560
620
40
680
Wavelength (nm)
Figure 8. Absorbance (log(l&lm) and e-p(lfidZ,,) spectra taken of a Cm film measured under UHV conditions in the range 320-675 nm.
-0.024 360
400
440
520 560 Wavelength (nm)
480
600
640
680
Figure 9. Plot of emmured versus wavelength in the range 365-675 nm. Irradiance = 30 W/cm2 at 514.5 nm.
-
responding to the SI So transition. Since both triplet- and ground-state c 6 0 absorb light at 340 nm with approximately equal extinction coefficients, this experiment was carried out on the weaker absorption peak centered at 440 nm. Minimal absorption is reported at this wavelength for triplet absorption of C a in solution.'8 It is, therefore, unlikely that the solid would have a substantial absorption at this wavelength. The mechanical limit of the spectrometer allowed spectral analysis in the range 300-675 nm, thereby preventing us from detecting the increase in the triplet absorption at its band center of 750 nm, simultaneously with the decrease in absorption at 440 nm; however, there is sufficient absorption by the triplet at 675 nm to render it detectable. A plot of Q = (Z(laser on) - Z(laser oft$/ versus wavelength is shown in Figure 9. Ground-state depletion would have manifested itself as a positive differential centered at 440 nm, whereas any laser-induced absorption would have appeared as a negative signal in the spectrum. The figure indicates no obvious singlet depletion nor triplet absorption in the range 363-675 nm using a laser irradiance of 30 W/cm2. The experiment was repeated with the 5 14.5-nm laser line at different spots on the film; similar results were obtained. The absorption cross section of the triplet state is much larger than that of the ground state for c 6 0 in solution at 488 and 514.5 nm.19,20It would then seem reasonable that if the triplet excited state of Cm is easily obtained in the solid phase it would have been detected in the pump-probe experiment. This was not found to be the case for the c60 films that we examined. Although this result presents prima facie evidence against the involvement of triplet C a , one must resolve a subtle point before concluding this unequivocally. It is possible that the Raman signal originates from a very thin portion of the film where
10830 J. Phys. Chem., Vol. 98, No. 42, 1994
Akers et al.
most of the c6O has been converted to the triplet while the bulk of the film remains in the singlet ground state and would, therefore, produce only a weak absorption differential in the pump-probe experiment. In order to determine the consistency of the Raman and the pump-probe results, we must, therefore, perform an analysis of the expected intensity changes. Let us first consider the ratio of Raman signals from the singlet and triplet with the pump beam on. The Raman scattering, R, from an elementary volume of thickness dy at a point y within the film is proportional to A dy nfS,TIb) where fS,T is equal to the fraction of scatterers in the singlet or triplet state, respectively, A is the cross-sectional area, dy is an element of thickness, n is the total number density, and I b ) is the intensity of the pump beam at y . Using a back-scattering geometry to record the Raman spectra results in the attenuation of the Raman light by the factor e-k@where kR represents the absorption at the Raman frequency including the necessary geometrical factors due to oblique incidence. Integration over the whole film of thickness 1 leads to an expression for R with RT and Rs representing the triplet and singlet Raman intensities, respectively.
R , = oTkino12e-k@dy/(oI
+ k)
(1)
where (T is the absorption cross section and k is the rate at which the excited singlet state is depopulated in units of s-l. When the Raman frequency is not too large kR x no and 1
RT
= oThAnoZ2e-"q dy/(oZ
+ k)
-dI/I = oxdA
+ k)
where N = An dy = xO
+ x1
+
(oZ k) dI/(oZ)= -kn dy
(4)
QR = (ol/os)[(l-y2)/2 - (1-y)/a - L/a2]/ [(l-y>/a
Z - Zo + ( k / o )ln[I/Io] = -nky
(6)
Considering the limit of strong absorption of the pump beam (i.e., o >> k),
Z = Io - kny
(7)
whereas in the limit of weak absorption of the pump beam (i.e., o e k), I = I e-OY '
(8)
Substituting (8) into (2) leads to the following expression: (u&Wo)[(l - y2)/2 - (1 - y)/a - Wa2] (9)
where
+ ~ / a (11) ~ ]
In the transmission experiment we pump and probe in the SI -transmitting SO region. The singlet state population is determined by a second, weak probe beam of visible light with
4 5
intensities and through the sample. The superscripts 1 and o refer to transmission through the c 6 0 film and the film-free region of the substrate, respectively. We investigated the spectra region 363-680 nm; however, special attention should be focused on the 440-nm region where solid C ~ has O a singlet absorption peak. Considering now the change in intensity of the probe beam Ip, gives
-dIdIp = (onk dy)/(oZ(y)
+ k)
(12)
In the limit of weak absorption of the pump beam we substitute (8) for I(y) and integrate from y = 0 to 1 in thickness to obtain:
@$= [(do + k)/(oI0e-""~+ K)]e-""'
(13)
We define Q as
Q = Il,(on>/Il,(ofl- 1
(14)
where I',(off), which refers to the intensity of the prove beam having traversed the film with the pump beam extinguished, is obtained by replacing Io with 0. For the weak absorption case we obtain
Q = oZo(l- e-no')/(oIoe-nul
(5)
Integration of this expression from 0 to y for thickness and from Zo to I for intensity yields
Q = a(1 - e+)/(,+
+ k)
+ 1)
(15)
(16)
For k large (Le., short triplet lifetime) Q tends to 0 as expected. For strong absorption of the pump beam (7) is substituted into (14) giving
+
Q = (dIonl)/(oZo k)( 1 - nol)
(17)
The quantity exp(-,8), obtained trivially by using (13) with the pump laser off, is simply the ratio I(film)lI(bl&) (Figure 8). Combining the data of Figures 8 and 9 we calculate a by using (16). As is shown in Figure 10 the value of la1 < 0.02 over the entire spectrum. This together with (11) and assuming OT/ OS 1 implies that QR should be 50.01, Le., that there would be essentially no signal observed due to Raman scattering by triplet C a . Alternatively this implies that OT/OS must be at least IO4 for so small a triplet population to produce the observed Raman result. There is no physical basis for so large an increase in the Raman cross section of the pentagonal pinch mode in the triplet state.
-
Conclusion
a = oZ$k, and
The quantity of interest is the ratio QR = R&,
(3)
N is the total number of absorbers, xo is the number in the ground state, and XI is the number in the triplet state. Substitution for xo into (3) leads to
RT=
Similarly, R, can be expressed as follows:
(2)
The intensity of monochromatic light decreases through a film of an absorbing and saturable material as follows:
xo = Nk/(aI
L = ln[(ay + l)/(a + l)]
,8 = nol,
y = e-p
The temperature dependence of the Raman spectrum of solid films of c 6 0 was measured under UHV conditions. The coexistence of two distinct phases over a wide temperature range
Raman Spectroscopy of Cm Solid Films
J. Phys. Chem., Vol. 98, No. 42, 1994 10831 appears to proceed with little or no barrier. Oxygen-doped c 6 0 samples show Raman spectra similar to the low-temperature spectra, implying that interstitial oxygen hinders the free rotation of the molecules producing a more ordered phase.
Acknowledgment. We thank NSERC and the Centres of Excellence in Molecular and Interfacial Dynamics for financial support. K.A. thanks Dr. K. Fu for providing the Cm samples used in this work. References and Notes -VI"-?,
360
400
440
460 520 560 Wavelength (nm)
do
640
do
Figure 10. Plot of a (calculated by using eq 16) versus wavelength in the range 365-385 nm. Irradiance = 30 W/cm2 at 514.5 nm.
has been demonstrated. The current results are consistent with our previous suggestion6 that the behavior of the Raman spectrum of solid Cm with temperature is indicative of an orderdisorder phase transition which is possibly related to the firstorder structural phase transition from fcc to sc at 250 KeZ8The behavior was found to be fully reversible. At room temperature and irradiances between 20 and 5000 W/cm2 the pentagonal pinch mode always appears at 1459 cm-'. At low temperature (50 K) two distinct Raman spectra were found. Initially and at low irradiance (28 W/cm2) the pentagonal pinch mode appears at 1459 cm-'. However, after exposure to laser radiation the spectrum changes and the pinch mode appears at 1469 cm-'. The 1469-cm-' spectrum is found to be the stable one at low temperature. On heating a sample previously converted to the 1469-cm-' form it reverted immediately to the 1459-cm-' form. The reversibility of the two Raman spectrum together with the observation of the 1459-cm-* spectrum at low temperatures in regions of the samples which had never been exposed to laser radiation is inconsistent with the photopolymer model. The influence of oxygen exposure on the Raman spectrum of C6o was found to be reversible upon laser irradiation in vacuum. We conclude that absorbed oxygen diffuses out of the solid upon laser irradiation. No direct triplet-triplet absorption of solid Cm was detected at 676 nm upon irradiation with 488- or 514.5-nm laser light using irradiances as high as 5000 W/cm*. Furthermore, no laserinduced changes indicative of ground-state depletion were found in the absorption spectrum (365-676 nm) upon exposure to 514.5- or 488-nm laser radiation at irradiances equivalent to those at which Raman spectra were recorded. We, therefore, conclude that triplet c 6 0 is not involved in either of the two Raman spectra reported for solid Cw. We maintain our original interpretation that the two Raman spectra shown in Figure 1 reflect an ordered phase of CK, molecules which are engaged in cooperative Raman scattering (low temperature) and a disordered phase (high temperature). However, it is obvious that there is a kinetic barrier to the transformation of the hightemperature form to the low-temperature form. This barrier is surmounted by photoactivation, likely due to the electronic excitation of the fullerene. The reverse process, in which the low-temperature form is converted to the high-temperature form,
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