Raman Laser Polymerization of C60 Nanowhiskers

Abstract

Photopolymerization of C₆₀ nanowhiskers (C₆₀NWs) was investigated by using a Raman spectrometer in air at room temperature, since the polymerized C₆₀NWs are expected to exhibit a high mechanical strength and a thermal stability. Short C₆₀NWs with a mean length of 4.4 𝜇m were synthesized by LLIP method (liquid-liquid interfacial precipitation method). The A_g(2) peak of C₆₀NWs shifted to the lower wavenumbers with increasing the laser beam energy dose, and an energy dose more than about 1520 J/mm² was found necessary to obtain the photopolymerized C₆₀NWs. However, excessive energy doses at high-power densities increased the sample temperature and lead to the thermal decomposition of polymerized C₆₀ molecules.

1. Introduction

C₆₀ nanowhiskers (C₆₀NWs) are the single crystal nanofibers composed of C₆₀ molecules [1] and can be synthesized by a facile method called “LLIP method” [2]. C₆₀NWs have a variety of applications and such as field-effect transistors (FETs) [3], solar cells [4], biosensors [5].

C₆₀ molecules can be polymerized by electron beam irradiation [6]. Although as-grown C₆₀NWs are composed of the C₆₀ molecules that are weakly bonded via van der Waals forces [7], the C₆₀NWs irradiated by electron beams showed the stronger thermal stability [8], the higher Young’s modulus [9] than pristine van der Waals C₆₀ crystals. Hence, it is of great importance to study the polymerization of C₆₀NWs in order to improve their mechanical and thermal properties.

Laser irradiation is a promising method to obtain the polymerized C₆₀ molecules [7, 10]. We first showed the photopolymerization of C₆₀NWs by using the Raman laser beam irradiation [7]. Rao et al. showed that the peak of A_g(2) pentagonal pinch mode of C₆₀ shifts downward from 1469 cm⁻¹ to 1459 cm⁻¹ upon the photopolymerization [11], showing that the shift of A_g(2) peak is a good indicator for the polymerization of C₆₀.

Alvarez-Zauco et al. studied the polymerization of C₆₀ thin films in air by the ultraviolet (UV) laser irradiation as a function of laser energy dose (= fluence) from 10 to 50 mJ/cm² in order to optimize the photopolymerization of C₆₀ films [12]. Likewise, the laser energy dose for the photopolymerization of C₆₀NWs should be optimized. Hence, the present study aims to reveal how the polymerization of C₆₀NWs proceeds as a function of the laser beam energy dose.

2. Experimental

C₆₀NWs were synthesized by a modified liquid-liquid interfacial precipitation method. Isopropyl alcohol (IPA) was gently poured into a toluene solution saturated with C₆₀ (MTR Ltd. 99.5%) in a glass bottle to make a liquid-liquid interface, and then the solution was subjected to ultrasonication and stored in an incubator at 10°C to grow short C₆₀NWs. The synthesized C₆₀NWs were filtered and dried in vacuum at 100°C for 120 min. to remove the solvents. In the Raman spectrometry analyses, the C₆₀NWs dispersed in ethyl alcohol were mounted on a slide glass and dried in air.

A Raman spectrometer (JASCO, NRS-3100) with a green laser of 532 nm excitation wavelength was used for the polymerization and structural analysis of C₆₀NWs in air. The power of laser light illuminated onto the specimens was measured by using a silicon photodiode (S2281, Hamamatsu Photonics K.K.). The laser beam power density (𝐷) and the energy dose of excitation laser beams in the Raman spectroscopy were controlled by changing ND (Neutral Density) filters, the defocus value of objective lens, and the exposure time of laser beam. 𝐷 is defined by the following formula in this paper,𝐷=mW/mm²Thepoweroflaserbeam(mW)theareaoflaserbeamexposedonthesamplemm².(1)

3. Results and Discussion

Figure 1 shows examples of scanning electron microscopy (SEM) images and the size distributions of the synthesized C₆₀NWs with a mean length of 4.4 ± 2.7 μm and a mean diameter of 540 ± 161 nm. The distribution of aspect ratios (length/diameter) is also shown. Most of the C₆₀NWs were found to possess the aspect ratios less than 15.

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Figure 1 

(a) SEM images, (b) length, (c) diameter, and (d) aspect ratio (length/diameter) distributions of the synthesized C60NWs.

The power of excitation laser beam can be changed by selecting ND filters. Figure 2 shows the relationship between the ND filter number and the power of laser beam irradiated on samples. The laser beam power could be widely changed between OD1 and OD3. The ND filters OD1 (attenuation rate 0.1), OD2 (0.01), and OD3 (0.001) were used in the experiment, since the other filters gave too strong or too weak laser beam energies. The excitation laser beam power density could be varied from about 0.53 to 11800 mW/mm² using the above ND filters and by controlling the irradiation area of the laser beams and the defocus value from 0 to 100 μm as shown in Figure 3. The defocus value is defined as the distance from actual image plane and was set to be positive as the distance between the objective lens and the sample surface decreased. The places of C₆₀NWs exposed to the excitation laser beams can be recognized as the green circular areas marked in Figures 3(a)–3(f). The area of laser beam on the samples could be changed from 63.8 to 9270 μm² by controlling the defocus value from 0 to 100 μm.

Figure 2 

Relationship between the neutral density (ND) filter number and the laser beam power.

Figure 3 

Optical microscopy images of the samples of C60NWs irradiated by the excitation laser beams for the defocus values (under focus) of (a) 100, (b) 80, (c) 60, (d) 40, (e) 20, and (f) 0 μm and for the arrowed exposed areas of (a) 9270, (b) 6630, (c) 3480, (d) 1470, (e) 617, and (f) 63.8 μm2, respectively. Graph (g) shows the relationship between the defocus value and the exposed area.

The exposed area (𝑦, μm²) and the defocus value (𝑥, μm) were plotted as shown in Figure 3(g). The plotted points can be approximated by the fitted quadratic curve, 𝑦=0.88𝑥²+6.8𝑥+36. Figure 4 summarizes the relationship among the laser beam power density, ND filter number, and the defocus value.

Figure 4 

Power density of the Raman excitation laser beam measured as a function of ND filter number and the defocus value.

Figure 5 shows examples of the Raman spectra of C₆₀NWs taken by using the ND filters of OD1, OD2, and OD3 for an exposure time of about 220 s, where the spot size of laser beam on samples was 9 μm in diameter. Each power density of the excitation laser beam was (a) 11800, (b) 1660, and (c) 71.5 mW/mm², respectively. The A_g(2) peak around 1468 cm⁻¹ sifted to the lower wavenumbers with increasing the laser beam power density.

Figure 5 

Raman spectra of C60NWs. The power density of laser beam (𝐷) is (a) 11800, (b) 1660, and (c) 71.5 mW/mm2, respectively.

Figure 6 shows the A_g(2) peak positions of the Raman spectra of C₆₀NWs as a function of energy dose of the laser beam for each defocus value from 100 μm to 0 μm (just focus). The power density of laser beam on samples was changed by changing the defocus value and the ND filter number as shown in Figure 4. The energy dose was changed by setting the beam exposure time at 215 ± 6 s, 441 ± 10 s, 665 ± 9 s, and 899 ± 29 s for each power density. Hence, as a whole, 72 data points are plotted in Figure 6. As shown in Figure 5, the Raman shifts are found to generally decrease to the lower values with increasing the energy dose. However, the Raman shifts were observed to increase along the red arrows for the high energy doses in Figures 6(c), 6(d), 6(e), and 6(f). These phenomena are supposed to be explained by the temperature rise of the C₆₀NWs exposed to the laser beams, since it is known that the photopolymerized C₆₀ molecules decompose into their primary monomers and dimers by heating at temperatures higher than about 100°C [13].

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Figure 6 

Ag(2) peak positions of the Raman spectra of C60NWs under various exposure conditions at the defocus values of (a) 100 μm, (b) 80 μm, (c) 60 μm, (d) 40 μm, (e) 20 μm, and (f) 0 μm (just focus), corresponding to (a) ~ (f) of Figure 3.

The data points obtained using the highest power densities are indicated in each graph of Figure 6 by the black arrows for the exposure time of about 220 s. Figure 7 shows the relationship between the laser beam energy dose and the A_g(2) peak position for the arrowed data points of Figure 6. The fitted curve of semilog plot is expressed as 𝑦=−2.2𝑥+1467, where 𝑥 represents log₁₀ (laser beam energy dose) and 𝑦 represents the Raman shift of A_g(2) peak. Using this experimental formula, the energy dose more than about 1520 J/mm² is found to be necessary for the photopolymerization of C₆₀NWs in air, when the laser light with a wavelength of 532 nm is used.

Figure 7 

Relationship between the Raman shift of Ag(2) peak and the energy dose of C60NWs irradiated by the excitation laser beams.

Since it is known that the photopolymerization of C₆₀ progresses through the formation of four-membered rings between adjacent C₆₀ molecules [11], it is considered that C₆₀ molecules are linearly polymerized by forming the four-membered rings along the growth axis of C₆₀NWs, as was shown in Figure 6 of [2].

In the gas chromatography-mass spectrometry (GC-MS) measurement of solvents contained in the C₆₀NWs that were prepared by use of toluene and IPA, the major residual solvent was toluene, and the content of IPA was very small compared with toluene [14]. Since the residual toluene of C₆₀NWs was measured to be about 0.2% after drying in an Ar atmosphere at 100°C for 30 min. [14], it is considered that the residual toluene of the vacuum-dried samples of C₆₀NWs in the present experiment is negligible and does not influence the Raman profiles.

4. Conclusions

The photopolymerization of C₆₀NWs was investigated by using the Raman laser beam of 532 nm wavelength at various exposure conditions for the power density and the exposure time in air.

The A_g(2) peak of C₆₀NWs shifted to the lower wavenumbers from that of the as-grown dried C₆₀NWs. However, the A_g(2) peaks were found to move to the higher wavenumbers from the polymerized positions by the irradiation of laser beams for high energy doses at high-power densities, indicating the thermal dissociation of polymerized C₆₀ molecules owing to the temperature rise.

An energy dose larger than about 1520 J/mm² was found to be necessary for the laser beam of 532 nm wavelength to obtain the photopolymerized C₆₀NWs.

Acknowledgment

Part of this research was supported by Health and Labour Sciences Research Grants (H21-Chemistry-Ippan-008) from the Ministry of Health, Labour, and Welfare of Japan.