X-ray irradiation-induced structural changes on Single Wall Carbon Nanotubes

N. Bardi a, I. Jurewicz a, A. K. King b, M. Alkhorayef c, D. Bradley a and A. B. Dalton b,*

a Department of Physics, University of Surrey, GU2 7XH, Guildford, United Kingdomb Department of Physics, University of Sussex, BN1 9RH, Brighton, United Kingdomc Department of Radiological Sciences, College of Applied Medical Sciences, King Saud University, Riyadh 11433, Saudi Arabia

*Electronic mail: A.B.Dalton@sussex.ac.uk

Key words: Carbon Nanotubes, X-ray radiation, ionizing radiation, nanomaterials, XPS, Raman

Highlights Effects of 20cGy and 45 cGy X-ray doses on Single Wall Carbon Nanotubes. Defects in the nanotube structure are created due to X-ray irradiation dose. X-ray selectively destroy some SWCNT diameters. X-ray radiation induces sp3 defects on Single Wall Carbon Nanotubes.

Abstract

Dosimetry devices based on Carbon Nanotubes are a promising new technology. In particular using devices based on single wall carbon nanotubes may offer a tissue equivalent response with the possibility for device miniaturisation, high scale manufacturing and low cost. An important precursor to device fabrication requires a quantitative study of the effects of X-ray radiation on the physical and chemical properties of the individual nanotubes. In this study, we concentrate on the effects of relatively low doses, 20 cGy and 45 cGy, respectively. We use a range of characterization techniques including scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy to quantify the effects of the radiation dose on inherent properties of the nanotubes. Specifically we find that the radiation exposure results in a reduction in the sp2 nature of the nanotube bond structure. Moreover, our analysis indicates that the exposure results in nanotubes that have an increased defect density which ultimately effects the electrical properties of the nanotubes.

1. Introduction

The X-ray dose for potential health hazards should be monitored. Acute health effects, like skin burns or acute radiation syndrome, can occur when the dose is high. Also, the risk of longer term effects, like cancer, can be increased by even relatively low doses of ionising radiation [1]. In particular, developing a radiation dosimeter based on tissue equivalent materials has a variety of uses, including routine quality assurance and quality control in both diagnostic and therapeutic applications.

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Single Wall Carbon Nanotubes (SWNTs) are one of the most promising materials for a range of applications in sensing and detection due to their unique quasi-one-dimensional nature and superior mechanical and electrical properties. Depending on how the precursor graphene sheet is rolled up into a nanotube, they can be either metals or narrow-band semiconductors, which, along with their inherent nm-size, makes them ideal candidates for the use in a range of potential applications [2]. Moreover, these tissue equivalent materials could be used in composite systems as X-ray detectors and effective with high sensitivities to long-term effects from ionising radiation [3].

The unique properties of SWNTs arise from their perfect atomic structure. Previous studies have shown that, more generally, carbon nanostructures are very sensitive to irradiation [4] and that their low atomic weight results in sufficient radiation doses modifying their inherent properties by atom dislocation [5]. The mechanism behind damage creation in SWNTs is different than the that associated with bulk carbon systems, such as graphite and diamond, as they have large surface and are highly anisotropic in nature [6]. Therefore, a systematic study of the X-ray effects on the structure of SWNTs is needed.

During a course of radiation therapy a patient can be exposed to an integrated radiation dose of 50 Gy conducted during several sessions [7]. In this study we exposed raw SWNT powders to relatively low doses (20cGy and 45 cGy) of X-ray irradiation. We find that even at such low doses, there are significant structural and chemical modifications. The development of such structure-property relationships during X-ray irradiation of the raw material is an important precursor to using these materials in active device configurations going forward.

2. Materials and Methods

2.1 Sample preparation and handling

The SWCNTs used in this study were commercially produced using the well-known high pressure decomposition of carbon monoxide (Lot#R23-106C purchased from Nanointegris). Typically, the raw product contains bundles of individual nanotubes assembled into rope-like structures. The raw material also contains iron catalytic particles which are used during the fabrication process.

2.2 Irradiation setup

The raw SWCNT materials were irradiated using a MXR-225/22 X-ray tube (Type No. 915326.51). It comprises of a dual focal spot, a tungsten anode and a directional bream. The dose rate was mapped for 60 sec with an ionisation chamber 2571 with sensitivity 41.171Gy /μC. Samples were irradiated with120 kV , 18.8 mA at a distance of 10 cm for 15 min and 30 min. The exposure doses were estimated as 20cGy and 45 cGy, respectively.

2.3 Characterization

SEM images were taken with a JSM-7100F SEM at vacuum 5 ∙10−4 Pa, a working distance of 10 mm, an accelerating voltage of 5kV and a probe current 8 mA . The resulting magnifications was 550, 5000 and 20000 respectively. The raw SWNT material is deposited onto carbon tape and attached onto a 0.5 Aluminium Specimen Stubs pins (Agar Scientific) using a typical specimen holder having a diameter of 32 mm.

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Raman spectra of the SWNTs were recorded using a NT-MDT Raman spectrometer with software Nova Px34.0, under ambient conditions. The Raman microscope was calibrated using a silicon wafer. All spectra were recorded using a He-Ne laser 633 nm (1.96 eV ) excitation. Fityk software was used for the baseline subtraction and the fitting of all the samples. Each spectrum was normalised to its maximum intensity and offset for clarity.

X-Ray photoelectron spectra were recorded with a ThermoProbe equipped with an X-ray gun with an Al Ka source. All spectra were recorded with an X-ray gun control using a 400 μm spot size, under pressure 1 ∙10−7mB. Current and voltage settings were 6.25 nA and 16kV (100 W power) respectively. The elemental peaks were acquired with 20 scans with a pass energy of 50 000 eV , dwell time 50ms and energy step size 0.2 eV . Thermo Avantage Software was used for the peak synthesis. The background was subtracted using the Shirley method. Spectra were later corrected to 285 eV which is the reference position for the Carbon C1s peak.

3. Results and Discussion

3.1 Scanning Electron Microscope

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Figure 1 : SEM images of HiPCo SWCNTs (a) before and after (b) 20 cGy and (c) 45 cGy X-ray irradiation at magnification (i) 550, (ii) 5000 and (iii) 20000

Figure 1 shows the SEM images of as-produced SWNTs before and after 20 cGy and 45 cGy X-ray irradiated dose respectively. The average length and diameter of individual tubes and the effects of the X-ray cannot be deducted, as they are close-packed bundles with various diameters and chiralities. As previously stated, the raw material is produced using Fe nanoparticles as catalytic seeds and some metal nanoparticles are distinguishable imaged under the highest magnification.

3.2 Raman SpectroscopyRaman spectroscopy has proven to be a powerful tool in the study of one-dimensional (1D) SWCNTs. SWNTs have several distinct features in their associated Raman spectra. At low frequency, there exists the radial breathing modes (RBM) which are associated with the synchronous vibrational movement of carbon atoms in the radial direction which generates an effect similar to "breathing". Figure 2 (a) shows the associated RBM spectra before and after X-ray doses of 20cGy and 45 cGy. There are clear modifications to the RBM features after exposure to radiation. The frequency of individual RBM modes have a direct inverse relationship to the diameter of individual nanotubes in the raw material. At the excitation wavelength chosen for these experiments, small diameter nanotubes are in resonance [8]. Both features associated with semiconducting and metallic SWNTs are observed. Moving from low to high frequency, the peaks at 103−143 cm−1 correspond to semiconducting tubes, 145−211cm−1 to metallic tubes and at 213−349 cm−1 to semiconducting SWCNTs respectively [9]. The modal features associated with semiconducting tubes are initially prominent in the pristine sample. However, after X-ray irradiation there is a clear modification to the spectrum with these features becoming less prominent. For the 45 cGy sample, they have completely disappeared.

Another prominent mode in the SWNT Raman spectrum is the so-called G-band. This mode corresponds to planar vibrations of carbon atoms and is present in most graphite-like materials. There is also a so called D-band which originates from structural defects[10]. Figure 2 (b) shows the G and D modes of the SWCNTs before and after X-ray doses of 20 cGy and 45 cGy respectively. The G peak is split into two components. The main G+ is associated with Carbon atoms vibrations along the CNT direction, and the G -, correlates to vibrations of atoms along the circumferential axis of the tube [11]. Before the X-rays, the G+

peak is at 1586 cm−1. The G- peak is soft and broad with an asymmetric spectral lineshape [12]. After 20cGy X-rays, G+ peak is found at 1587.63 cm−1 and the G- at 1559.79 cm−1. After 45 cGy X-rays, G+ peak is at 1586 cm−1 and the G- at 1554.86 cm−1. By fitting the G band, four asymmetric features are observed initially and five peaks are found after the X-rays. They arise from phonons with different symmetries [13]. In addition, D-band becomes more intense after irradiation indicating that the defect density is increasing as a result of structural modifications to the nanotube sidewalls [13].

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The I D /IG ratio (the ration of intensities between the D-band and the G-band) is an indicator of how great the X-ray induced damage on the SWCNTs is. This ratio dramatically increases from an initial value of 0.008 to 0.102 after 20 cGy X-ray and finally to 10.57 after 45 cGy X-rays. Therefore, damage has been observed for both 20 cGy and 45 cGy X-ray irradiation with marked increases in the effect at higher doses.

Figure 2 : Fitting of the Raman peaks of HiPCo SWCNTs (laser 633nm) before and after 20 cGy and 45 cGy X-ray irradiation (a) RBM peak and (b) D and G peaks

3.3 X-Ray Photoelectron SpectroscopyThe surface chemistry of the SWNTs can be estimated by X-ray photoelectron spectroscopy (XPS). Elemental analysis of the Carbon peak before and after the X-ray irradiation is shown in Figure 3. The C1 s region is composed of a symmetrical peak and a satellite feature that corresponds to the sp2 hybridisation of the carbon bond structure.

The C1 s region shows a primary C peak at 285 eV , associated with the C=C bonds. This binding energy corresponded to non-functionalised sp2 carbons. A second peak at 285.4 eV for the irradiated samples can be attributed to the presence of defects in the CNT structure [14, 15]. These binding energies could also correspond to sp3 free radical defects; since the Carbon peak of nanodiamond is situated there [16]. The presence of sp3 type bonding in the nanotube structure is a direct sign of induced damage to the SWCNTs [17]. The ratio sp2/sp3 of the C1 s spectra corresponds to the ratio sp2/sp3 in the nanomaterial [18].

In the irradiated SWCNTs present a broad shoulder centred at 287.4 eV for 20cGy and at 286.8 eV for 45 cGy that corresponds to the presence of C=O (carbonyl groups). Bond breakage or ionisation of the SWNTs due to the radiation could form oxides on the surface [19]. Oxidative processes is anticipated to be concentrated at the outer surface of the CNTs, in dislocations and defects caused by irradiation [20].

It is clear from the combined SEM, Raman and XPS studies that the SWCNTs can rapidly degrade under irradiation. X-rays have been found to disrupt the extended π-network of the bare sp2 –hybridised SWNTs by inserting defects in the lattice structure [4]. The bonding character changes from sp2 to sp3 in nature.

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Figure 3 : C1s peak based on X-ray Photoelectron spectroscopy of HiPCo SWCNTs before and after 20 cGy and 45 cGy X-ray irradiation

4. Conclusion

The effects of 20 cGy and 45 cGy X-ray irradiative doses on raw SWNT powder were studied. The irradiated samples were compared to the pristine material using a range of characterisation techniques. SEM analysis indicates that there is no obvious change to the microscopic morphological structure of the SWNT samples. For the 45 cGy dose, the Raman ratio I D /IG was found to be 790 % higher than for the pristine and X-rays were found to selectively destroy certain diameter SWNTs. XPS measurements confirmed the modifications to the structure of the nanotube surfaces. Raman spectroscopy and XPS indicated that the degree of disorder in the CNT structure correlates with the X-ray irradiation dose.

5. Acknowledgments

This research was supported by the Defence Science and Technology Laboratory (dstl), UK. We thank Dr Steven Hinder who provided expertise that greatly assisted with the XPS characterization.

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7. Declaration of interest All authors declare that Niki Bardi had financial support from dstl for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

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