A study of fullerite ablation with energetic pulsed electrons

Nuclear Instruments and Methods in Physics Research B 269 (2011) 1097–1102

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Experimental Methods and Equipment

A study of fullerite ablation with energetic pulsed electrons Sumaira Zeeshan a,b,⇑, Sumera Javeed a,b, Kashif Yaqub a,b, Mazhar Mehmood b, Sohail Ahmad Janjua a, Shoaib Ahmad b,c a

Accelerator and Carbon Based Nanotechnology Laboratory, PINSTECH, P.O. Box Nilore, Islamabad, Pakistan Pakistan Institute of Engineering and Applied Sciences (PIEAS), P.O. Box Nilore, Islamabad, Pakistan c Government College University, CASP, Church Road, Lahore 45000, Pakistan b

a r t i c l e

i n f o

Article history: Received 12 April 2010 Received in revised form 25 February 2011 Available online 16 March 2011 Keywords: C60 Ablation Fragmentation Pulsed discharge Emission spectroscopy

a b s t r a c t We report the ablation of fullerite films deposited on metallic substrates with 3 keV electron pulses generated in a specially designed pulsed discharge tube. During ablation the fragmented species were detected by emission spectroscopy. The emission spectra of C2 and C1⁄ (CII) provide the signatures of C60 fragmentation. The vibrational temperature of the C2 emitted from the ablated fullerite is 12,700 ± 1160 K compared with 18,230 ± 1150 K for the graphite sample under similar conditions. The fullerite films were produced by vacuum sublimation on Aluminum, Iron and Copper substrates and characterized by Atomic force microscope, X-ray diffraction; Raman and Fourier transform infrared spectroscopy. The comparisons of electron ablation of fullerite films with that of graphite show the similarities and differences of carbon bonding in the caged structure of C60 with that of the planar graphene sheets of graphite. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Ever since the discovery of C60 [1] the formation mechanisms of this highly stable molecule have been actively investigated. The processes and the dynamics of fragmentation of C60 are considered to follow the same route as formation. It has been proposed that C60 and other smaller and larger fullerenes may be formed by the insertion of C2 in successive steps. Similarly, the emission of the C2 during the fragmentation of fullerenes has been observed. However, the insertion or removal of C2 introduces a large number of possible isomer structures with diverse symmetries. This aspect has been dealt with in detail in the Atlas of Fullerenes [2]. In the regenerative sooting discharge the role of C2 in the carbon cluster growth and fragmentation was discussed by Ahmad [3]. A model has been proposed based upon the C2 addition route leading to cage closure and the series of sequences for the formation of C60 [4]. Theoretical as well as the experimental studies have established C2 as a basic unit during the self-assembly of C60 and single walled carbon nanotubes [5]. In the present study, the role of C2 in fragmentation as a result of electron pulse induced ablation of the condensed films of C60 is presented. Electron induced fragmentation of C60 has been shown in the time of flight mass spectrometric ⇑ Corresponding author at: Accelerator and Carbon Based Nanotechnology Laboratory, PINSTECH, P.O. Box Nilore, Islamabad, Pakistan. Tel.: +92 51 2207276; fax: +92 51 9248808. E-mail address: [email protected] (S. Zeeshan). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.03.005

experiments to be due to the sequential loss of C2 [6–8]. C2 was also observed as the dominant fragment in the high energy proton impacts by Tsuchida et al. [9]. Lykke studied the fragmentation of C60 by photo-excitation and reported neutral C2 along with C1, C3 and C4 [10]. O’Brien et al. studied photo-fragmentation of C60 and observed fragments consisting of C1, C2 and C3. It was observed that the clusters (Cn) of sizes n > 30 fragmented by the emission of C1 and C2 while the lower ones by the ejection of C3 [11]. The collisions of energetic alkali metals (Li+ and Na+) with C60 produce endohedral fullerenes and subsequently fragment by the ejection of C2 [12]. The method reported here for the fragmentation of the condensed C60 is based on energetic pulsed electron ablation. The electron ablation technique has certain advantages over other ablation and fragmentation techniques that will be discussed later. Electron ablation of C60 is compared with that of graphite under similar conditions and geometry. Graphite is a layered structure with conducting properties; the condensed C60 – the fullerite on the other hand, is an insulator. We report the observed differences between the two allotropes towards electron ablation. The emission spectra of C1 and C2 during ablation are used to determine their number densities. The differences in the emission spectra for the ablated graphite and C60 films can be related to the state of carbon bonding in the two structures. In graphite, carbon atoms are less tightly bound as compared to those in C60 that may affect the relative bond breaking efficiency of the ablated surfaces.

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Fig. 1. Experimental setup to study the ablation of C60 using pulsed discharge.

4000

8000

(a) Intensity (a.u)

2000

Iron

6000 5000 4000 3000

Copper

2000

1000

Iron

20

2θ

25

d = 2.8980

30

d = 2.7392

d = 3.1794

d = 4.1087

d = 4.2836

d = 5.0195

15

1000

Aluminium

Copper

0 500

35

1000

1500

2000

2500

3000

-1

Raman Shift (cm )

1182.169

(c) 1538.941

526.4798

2192.219

40

576.1386

% Absorbance

60

20

2327.696

10

d = 8.2175

d = 8.6940

Aluminuim 0

(b)

7000

1429.017

Intensity (a.u)

3000

Iron Copper

Aluminium

0 600

900

1200

1500

1800

2100

2400

-1

Wavenumber (cm ) Fig. 2. Spectra of C60 film deposited on aluminum, copper and iron for (a) XRD (b) Raman (c) FTIR.

2. Experimental Fullerite films of thickness 1 micron were deposited on polished Aluminum, Copper and Iron substrates by vacuum sublimation of pure C60 powder using an Edwards coating unit. Substrates were discs of 15 mm diameter. The films were characterized by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Atomic force microscopy (AFM) after vacuum sublimation and prior to electron ablation. Raman spectroscopy was performed with green laser k = 532 nm (R-3000 Raman system, USA). The XRD analysis ensured crystallin-

ity of the sublimed films (D8 Discover by Bruker axs, Germany). FTIR spectroscopy further characterized the films (NICOLET 6700 by Thermoelectron Corporation, USA). AFM of the fullerite films sublimed on Aluminum, Copper and Iron revealed the surface morphology (Quesant Universal SPM by Ambios Technology, USA). Fig. 1 shows the experimental setup used for the electron ablation of the condensed C60 films. A pulsed discharge between the two coaxial cylindrical electrodes in the presence of the support gas Ne, at pressure of 5 mbar, is generated by discharging a 12 lF capacitor. Anode discs of diameter 15 mm coated with C60 films are mounted on a 50 mm long cylinder that is surrounded

S. Zeeshan et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 1097–1102

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Fig. 3. AFM images of C60 coated on substrates (a) aluminum (b) copper and (c) iron.

by an outer cylindrical aluminum cathode with 30 mm diameter and 300 mm length. A dc voltage 3 kV is applied across the coaxial electrodes separated by a radial distance of 7 mm. A dc power supply with controllable RC time constant charges the capacitor which is subsequently discharged manually. The technique delivers high energy density pulses of duration 150 ls. Average energy per pulse is 54 ± 0.5 J. In situ emission spectroscopic data are collected through an optical fiber from the front aperture. The optical fiber collects the data by looking straight at the electron irradiated spot. From the spectra we identify the fragmenting atomic and molecular species. The same setup is used to study the graphite ablation; with the difference that the anode is replaced by a graphite rod of similar dimension. The graphite ablation data provide the reference for comparison with that of the C60 thin films ablation. The electron ablation technique reported here can be compared with the laser ablation of graphite developed by Kroto et al. [1] where intense photon beam ablates the graphite surface. In the case of laser ablation the energy transfer takes place in photon– electron interaction while in the electron ablation mode, the energy is transferred in electron–electron collisions. There are however, major differences between laser ablation of graphite to form fullerenes and the electron ablation of similar targets. Laser ablation takes place at higher pressures of the support gases (bar). During electron ablation few mbar pressure is maintained for the glow discharge mode. In our case, we observe the onset of ablation of the irradiated surfaces that leads to fragmentation

of the surface structures during the impact of the electron beam while in the case of laser ablation the clusters are formed from the fragmented species in and around the region of the conical extractor were observed in the time of flight mass spectrometry. The emission lines from the excited and ionic fragmented species resulting from the electron pulse ablated graphite and thin fullerite films are observed with Ocean Optics spectrometers. Two spectrometers that cover (200–1100 nm) and (400–600 nm) ranges collect the data simultaneously. The first spectrometer scans the entire emission range from UV to the NIR while the second spectrometer with higher resolution focuses at specific range where Q 3Q the C2 Swan bands ½d g  a3 u  appear [13]. The spectral resolution of the first spectrometer 1 nm while it is 0.16 nm for the second one. A calibrated Halogen light source (DH 2000) and the Ocean Optics Irradiance software calibrate the emission intensities after the background subtraction by using the differential curves dI/dk in units of lW/cm2-nm. Level densities Nu of the upper excited levels (electronic or vibrational) are obtained from the emission spectrum by using the relation

Iul ¼ Nu Aul htul

ð1Þ

where Iul is the calibrated intensity calculated from the spectrum for the relevant transition, htul is the energy difference between the upper (u) and the lower (l) levels and Aul is the Einstein transition probability of spontaneous emission for the transitions [14]. The upper level density Nu is related inversely with the vibrational temperature (Tvib) by the Boltzmann relation

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2.0 466.4 Al II

(a)

HI

2

Intensity (µW/cm .nm)

1.5

C2

585.29 Ne I 589.3 Ne I

558.2 559.5 Al II 563.1 569.8 572.4

550.7 554.2

516.55 513

510.7 Cu I

467.7 469.9 473.5 471.7

Δν = − 1

521.85 Cu I

453.5 448.2 451.2

422.9 426.9 C II 434.02 415.3

0.5

C2

Δν = 0

486.39

C2

Δν = + 1

1.0

0.0 400

450

500

550

600

C2

C2

Δν = + 1

481.17 486.31 494.27

501.88 507.03

HI

512.85

C2

Δν = − 1 550 553.99 558.51 563.58 569.76

468.11 469 471.65

436.63 448.25

0.5

434.06 H I

410.32 415.34

1.0

426.85 CII

473

1.5

467.7

466.4 Al II

2

Intensity (µW/cm .nm)

2.0

(b)

516.54

422.85

Δν = 0

589.21

2.5

Ne I

Wavelength (nm)

0.0 400

450

500

550

600

Wavelength (nm) Fig. 4. In situ emission spectrum taken in pulsed discharge in the presence of Ne for ablation of (a) C60 (b) graphite.

Nu ¼

  gu N Eu exp  UðTÞ kB T v ib

ð2Þ

3. Results and discussion 3.1. Characterization of the fullerite films

where N and Nu are the total density of the particles and the density of particles in the u state, respectively; gu is the statistical weight of the upper state; U(T) is the internal partition function; Eu is energy of the respective upper levels relative to the ground state. From the emission spectra a range of upper level of the desired band  densities  heads are calculated. By plotting ln Ng u as a function of Eu/kB one u can determine Tvib from the slope (1/Tvib) by the Boltzmann graphical method [15].

Fig. 2a shows the XRD spectra of the fullerite films on Aluminum, Copper and Iron substrates, respectively. All of the peaks at 2h = 10.2, 10.9, 17.7, 20.7, 21.8, and 28.1 in the spectra identify a hexagonal closed pack structure. The lattice parameters of [the] structure are: a = b = 10.039 and c = 16.435. All the substrates show similar structure for the deposited fullerite films; hence no effect of the substrate material is seen on the structure formed. The Raman spectra of the fullerite films are shown in Fig. 2b for the three substrates, i.e. Aluminum, Copper and Iron. The most

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(a)

(b) 29.6

ln N(Number Density)

ln N(Number Density)

28.8

28.5

28.2

29.4

29.2

29.0

28.8

27.9

28.6

28

30

32

34

28

36

30

32

34

36

E/k B x 1000 (K)

E/k B x 1000 (K)

Fig. 5. Calculation of vibrational temperature of C2 using Boltzmann graphical method for ablation from (a) C60 (b) graphite.

3.2. Ablation through pulsed discharge In situ emission spectrum of ablation of the condensed C60 film on copper is shown in Fig. 4a. The prominent emission features are the C2 Swan bands. The band heads at 563, 516, and 474 nm are clearly observed [13]. The emission lines from ionized monatomic carbon CII and the excited neutral neon NeI at 427 nm and 589 nm, respectively, are also present. Neon is the discharge gas. The peaks corresponding to the substrate such as CuI are also visible at 510 and 521 nm. Similarly, the atomic lines for Al II at 466 and 559 nm are part of the emission spectrum [16]. By using the Boltzmann graphical method in Fig. 5a, Eu/kB is plotted against ln(Nu) for the peaks corresponding to m0, m1, m2, m3 of the C2 Swan bands. From the slope of the linear fit one obtains the vibrational temperature Tvib 12,700 ± 1160 K. The C2 and CII lines in the emission spectra identify the mode of fragmentation of C60 under the energetic electron pulse bombardments. The results highlight the importance of C2 as an essential component among the fragments of the ablated fullerenes. However, C2 may not be the only fragmented component because the emission spectroscopy of higher carbon molecules e.g. the detection of the Swings bands of C3 is much more difficult. We have plans for mass spectrometry of the sputtered and ablated fullerite fragments in the near future. The mass spectrometry may be able to complement and complete the picture. From the detection of

C2 among the ablated C60 fragments one may conjecture that the Buckyball electron pulse ablative fragmentation follows the usual route i.e. C60 ? C58 + C2. The C2 may further break into C1 through the reaction C2 ? C1 + C1. However all of the C1 seen in the spectra may not have come from this route; a direct emission is also possible. Fig. 4b has the emission spectrum for ablation of the graphite sample. The C2 Swan bands with Dm = 1, 0, +1 are labeled. Here too, C2 is the dominant ablated fragment. Graphite has unsaturated carbon bonds on the edges as compared to the closed cages of C60. That may explain the observed higher densities of the ablated species from graphite than those from the condensed films of C60. Fig. 5a and b shows the vibrational temperatures calculated from the C2 swan bands for C60 and graphite, respectively. Vibrational temperature for the plasma formed by the C60 fragments is 12,700 ± 1160 K and that for the graphite fragments is 18,230 ± 1150 K. The carbon bond energy in graphite is lower than that for C60 thus making larger fraction of the deposited energy appear in the form of the vibrational energy of C2. The AFM images of the fullerite on copper demonstrate that the agglomerates of condensed C60 are formed on the surface separated from each other. The fullerite films are insulators but there are localized spots from where the electrons find conducting surfaces that help

20

15

% Absorbance

prominent peak in all of the spectra is at 1465 cm1 that is the landmark of C60. A minor peak corresponding to the G band (1570 cm1) in the spectra indicate the existence of a small fraction of the graphitic structure. Fig. 2c has the FTIR spectra of the fullerite films on the same substrates. Absorption at 525.7, 576.0, 1182.0, and 1430.2 cm1 manifest the presence of C60 while a broad peak at 2327 cm1 corresponds to the graphitic structure. The Raman and FTIR spectra indicate that the condensed films of C60 have similar Raman and FTIR peaks as those of the individual C60 molecule. However there are the signature of the graphitic structure at 1570 cm1 in Raman and a broad hump at 2327 cm1 in the FTIR spectra. Fig. 3(a–c) exhibit the AFM images of the fullerite films on Aluminum, Copper and Iron substrates, respectively. Fullerite on iron shows equally dispersed particles as compared to the other substrates. On the copper substrate the agglomerates of clusters are prominent. There is higher carbon affinity in the case of iron as opposed to the carbon immiscibility towards copper and aluminum.

10 Nonablated film Ablated film 5

0

500

1000

1500

2000 -1

Wavenumber (cm ) Fig. 6. FTIR spectra of ablated and nonablated C60 film.

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hexagonal closed pack structure for the films. The AFM images revealed that the uniformity of the films is dependent upon the miscibility of carbon with the three metallic substrates. In situ emission spectroscopy detected the atomic and the diatomic carbon species during the energetic electron ablation of the fullerite films. C2 is found to be the major fragment of C60 along with the traces of the atomic carbon in the ionized form i.e. CII. The same fragmented components were observed during electron ablation of the graphite sample. The vibrational temperature calculated from the Swan band heads of the C2 ablated from the graphite is about 40% higher than that from the fullerite films. The difference in the ablated fragments, number densities and their vibrational temperatures may be due to the bond energies in graphite and C60.

5000

Intensity (a.u)

4000

3000

Nonablated film 2000

Ablated film 1000

Acknowledgements 500

1000

1500

2000

2500

-1

Raman Shift (cm ) Fig. 7. Raman spectra of ablated and nonablated C60 film.

in forming of the pulse. This may be the reason for the greater number densities of the fragmented carbon atoms and molecules obtained as a result of the electron ablation from the graphite sample as compared to those from the fullerite. In the case of other substrates there are fewer such localized spots seen in the AFM images. The fullerite films after exposure to the energetic electron bombardment are further characterized by using FTIR and Raman spectroscopies. Fig. 6 shows the Raman spectra from the ablated and the unablated films. Sharp differences can be observed; the C60 characteristic peak at 1465 cm1 has significantly reduced in the case of the ablated sample. Similarly, in the FTIR spectrum displayed in Fig. 7, the intensities of the C60 peaks have been reduced. The electron ablated fullerite films are structurally deformed. The C60 molecules that comprise the condensed fullerite films seem to have fragmented by emitting monatomic and diatomic carbon components due to the energetic electron pulse ablation. 4. Conclusion Vacuum sublimation of the C60 powder on various metallic substrates forms 1 micron thick fullerite films. The XRD indicated

The authors gratefully acknowledge technical support provided by Mr. Qaiser for coating apparatus. The authors also acknowledge Mr. Rizwan, Mr. Faisal for technical help and GSD workshops for fabrication of different components. References [1] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature 318 (1985) 162. [2] P.W. Fowler, D.E. Manopoulos, An Atlas of Fullerenes, Clarendon Press, Oxford, 1995. [3] S. Ahmad, A. Qayyum, M.N. Akhtar, T. Akhtar, Nucl. Instr. and Meth. B 171 (2000) 551. [4] S.D. Khan, S. Ahmad, Nanotechnology 17 (2006) 4654. [5] S. Ahmad, Nanotechnology 16 (2005) 1739. [6] P. Scheier, B. Dunser, R. Worgotter, D. Muigg, S. Matt, O. Echt, M. Foltin, T.D. Mark, Phys. Rev. Lett. 77 (1996) 13. [7] G. Senn, D. Muigg, B. Dunser, P. Scheier, T.D. Mark, Hyperfine Interact. 108 (1997) 95. [8] S. Matt, D. Muigg, A. Ding, C. Lifshitz, P. Scheier, T.D. Mark, J. Phys. Chem. 100 (1996) 8692. [9] H. Tsuchida, A. Itoh, Y. Nakai, K. Miyabe, M. Imai, N. Imanishi, Focused on Nanoparticles, clusters, RIKEN 17 (1998) 57. [10] K.R. Lykke, Phys. Rev. A 52 (1995) 1354. [11] S.C. O’Brien, J.R. Heath, R.F. Curl, R.E. Smalley, J. Chem. Phys. 88 (1988) 1. [12] Z. Wan, J.F. Christian, S.L. Anderson, Phys. Rev. Lett. 69 (1992) 1. [13] R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, 4th edition., John Wiley & Sons, Inc., New York, 1976. [14] I.I. Sobelman, Introduction to the Theory of Atomic Spectra, Vol. 40, Pergamon Press, 1972 (Chapter 9). [15] M. Venugopalan, Reactions under Plasma Conditions, Vol. 1, John Wiley, 1971 (Chapter 7). [16] NIST Atomic Spectra Database at http://physics.nist.gov/asd3.