NANO LETTERS
Proton Irradiation of Carbon Nanotubes Bishun Khare,†,§ M. Meyyappan,*,† Marla H. Moore,‡ Patrick Wilhite,† Hiroshi Imanaka,† and Bin Chen†,|
2003 Vol. 3, No. 5 643-646
NASA Ames Research Center, Moffett Field, California 94035 and NASA Goddard Space Flight Center, Greenbelt, Maryland 20771 Received January 31, 2003; Revised Manuscript Received March 4, 2003
ABSTRACT Single-walled carbon nanotubes (SWNTs) were irradiated with 1 MeV protons and the samples were analyzed using Fourier transform infrared spectroscopy, UV−vis−NIR spectroscopy, Raman spectroscopy, and secondary ion mass spectrometry. There is clear evidence that the irradiated sample contains C−H bonds.
Carbon nanotubes (CNTs) have been receiving much attention for their potential in future nanoelectronics, computing and data storage, sensors, detectors, and other systems. The electronic properties of CNTs seem to be influenced through atomic scale defects either introduced during growth or induced by mechanical strain.1,2 Irradiation of the CNTs also introduces structural defects, and in this context the effects of electron and heavy ion irradiation on electronic and chemical properties have been studied.3-7 For example, electron irradition has been reported to generate point defects exclusively and to modify the electronic properties near the Fermi level.3 Irradiation with heavy Ar ions over a range of 50-3000 eV introduces a number of structural defects with the most common being vacancies, according to molecular dynamics simulations.4 Effects of gamma radiation on the mechanical properties (such as glass transition temperature, modulus) and dielectric properties of the CNT composites have also been studied.8 The dielectric properties of CNT/ polymer composites were found to be more susceptible to gamma radiation than mechanical properties. While the knowledge generated in the above studies can lead to effective tailoring of CNT properties through irradiation, they also aid device design for radiation-resistant systems. Among possible sources, the effect of proton irradiation of nanotubes has received very little attention.9 Protons comprise an important component in radiation belts about the Earth and outer planets such as Jupiter. The integral omnidirectional flux of protons can be up to 108 particles/ cm2 s at 1 MeV depending on the altitude in the Earth’s proton belt.10 Trapped protons in radiation belts have been known to cause single-event upsets and global radiation * Corresponding author. E-mail:
[email protected] † NASA Ames Research Center ‡ NASA Goddard Space Flight Center. § Also from SETI Institute. | Also from ELORET Corporation. 10.1021/nl034058y CCC: $25.00 Published on Web 03/14/2003
© 2003 American Chemical Society
damage. In silicon devices, typical proton damage effects have been threshold shift in MOS transistors and degradation of gain and leakage current in bipolar transistors.10 As interest in CNT-based nanoelectronics continues to gain momentum, it is of interest to study proton radiation effects on the properties of carbon nanotubes. We report here on the change in properties of single-walled carbon nanotubes (SWNTs) upon irradiation with 1 MeV protons. We used SWNTs from the high-pressure carbon monoxide (HiPCo) process11 which were purified according to a protocol reported previously.12 Prior to proton bombardment, the purified sample was heated at 700 °C under argon atmosphere to remove absorbed CH4, if any. A 1 in. diameter aluminum mirror coated with the SWNT sample was exposed at 300 K to the proton beam. The thickness of the film, as measured by Tencor P-10 profilometer, was 0.5 µm. We also used another sample with a 16.75 µm thick xenon layer deposited at 15 K on top of the 0.5 µm SWNT film which was exposed to the proton beam at 15 K. Protons were generated from a Van de Graaf accelerator (NASA Goddard),13 and the total dosages on the two samples were 9.6 × 1014 and 5.5 × 1014 protons/cm2, respectively. These samples and a control sample were analyzed using several techniques. Fourier transform infrared spectroscopy (FTIR) of the sample was done using a Nicolet 670 FTIR with a MCT detector cooled by liquid nitrogen. UV-vis-NIR data on the samples were taken using PE Lambda 900 model spectrometer. This is a double beam, double monochromator, ratio recording spectrometer in the range 200-3300 nm. Raman spectroscopy was performed with Nicolet 670 FTIR with Raman attachment. The excitation wavelength was 1064 nm from a Nd:YAG laser with a laser power of 0.37 W. A Nicolet InGaAs detector was used. The number of scans was 128 in 266.4 s with a resolution of 4 cm-1. Raman analysis was also performed using a Renishaw (System 2000) microRaman spectrometer in the backscattering configuration. A
Figure 1. FTIR Spectra of 1 MeV proton bombarded SWNT film.
633 nm He-Ne laser was used for excitation in this case, and a resolution of 4 cm-1 in the 100-4000 cm-1 spectral window was achieved. The samples were also analyzed by secondary ion mass spectrometry (SIMS) using 10 keV Cs+ ions (Cameca IMS-3f). Finally, structural changes were studied using transmission electron microscopy (TEM) (JEOL 2000); it is important to note that electron irradiation from TEM may alter the structure and it may be difficult to discriminate the source of observed changes, if any. The proton implant effects were modeled using SRIM 2000 (stopping and range of ions into matter) Monte Carlo code, assuming a carbon density of 1.5 and 2.26 g/cm-3 (to provide a range). No structural details of SWNTs were considered in the simulation. This analysis is similar to the routine ion implantation modeling of semiconductor doping processes.14 The 0.5 µm thick straight SWNT film (with no Xe) after proton bombardment was first analyzed with FTIR by rotating the sample by 180° within a vacuum under reflection. The IR beam goes through the film and reflects back from the back of the film, aluminum surface. Thus the beam passes through the film twice, giving a transmission spectrum through double the film thickness (∼ 1 µm). The observed spectrum was not different from that of the control sample. According to Monte Carlo simulations using SRIM-2000, 1 MeV protons implant in carbon mostly at lengths 16-18 µm. This was the reason to use a SWNT film with a thick Xe layer on top at 15 K. As discussed below, this sample exhibits a C-H band after irradiation and all further discussion is restricted to this sample. Figure 1 shows the FTIR results where a C-H stretching mode is clearly seen for the proton-bombarded SWNT film. The control sample does not have this feature. A direct comparison with our previous report15 on atomic hydrogen functionalization of SWNTs using a microwave discharge reveals that the peaks at various wavelength positions compare well, though the intensities cannot be compared since they are not at a common scale. The reflection spectra were also taken at different angles (not shown here) for the proton-bombarded SWNT film. The integrated area of C-H peak at different angles of incidence indicates fewer C-H bonds on the surface reflection at 80° than at lower angles. For example, the highest magnitudes were seen at 55 and 644
Figure 2. Surface H concentration measured by SIMS.
30 degrees of reflection, which look deeper into the film. This is consistent with the SIMS results shown in Figure 2. The concentration of H in Figure 2 is much higher in the case of proton-bombarded sample compared to the control sample as expected. For the UV-vis-NIR analysis, no direct transmission is possible with the SWNT sample on aluminum substrate. With the SWNT sample being dark and having low reflection, measurements were made in both specular (governed by Fresnel equations) and diffuse reflectance modes using integrating sphere. The diffuse reflectance mode provides information about the interior of the sample through penetration of a portion of the incident flux into the interior of the sample.16 The results are shown in Figure 3. The SWNT film (control) is dark throughout the region investigated. However, the film becomes more transparent upon proton bombardment (see Figure 3a). The diffuse reflection data give information about the molecules on the surface (Figure 3b). Just as in the specular case, the diffuse spectrum also shows that the SWNT film becomes transparent upon proton bombardment, except below 400 nm. This is because the dominant specular part is eliminated and the light beam hitting the surface reflects from the spots where elemental C broken off from the SWNT is scattered; this is in agreement with the TEM data in ref 9 for proton-bombarded nanotubes. The imaginary part of the refractive index (which affects the transmission) of amorphous carbon below 400 nm is high and increases at lower wavelengths. The functionalized SWNT is expected to have a much lower imaginary part of the refractive index compared to neat SWNT and very low compared to amorphous carbon. This is the reason that in the diffuse reflection we notice that the curve for protonated SWNT goes below (for < 400 nm) the unprotonated one. The Raman spectra (Figure 4) show C-H stretching vibration at around 2800 cm-1 which is not present in the control sample. It is noted that the tangential region undergoes vibrational changes. The SWNT G-band shows a red shift of about 5 cm-1: 1554 cm-1 shift to 1558 cm-1 and 1590 cm-1 to 1596 cm-1. This possibly could result from Nano Lett., Vol. 3, No. 5, 2003
and some curving of the nanotubes. Our TEM analysis is unable to confirm their observation since we see curved nanotubes in both pristine and irradiated samples. In summary, we have exposed SWNTs to 1 MeV proton and analyzed the irradiated samples using various techniques. There is clear evidence that irradiation results in C-H bond formation in carbon nanotubes. It is important to study if this chemical modification is accompanied by a change in electronic properties, which will be the subject of future work. The effect of proton and other sources of radiation on functional devices is a subject of interest because CNT-based nanoelectronics and data storage devices, if successful, will be used in future Earth orbit and solar system explorations. Future work would include the study of electronic conductivity changes and analysis of simple device geometries such as CNT diodes and field effect transistors upon irradiation. Monte Carlo simulations using SRIM indicate that vacancies form throughout the CNT film, though detailed modeling is necessary to confirm and further elucidate mechanisms. Our results indicate that proton irradiation can be used to achieve atomic hydrogen functionalization of CNTs when needed for any applications, but this can be more readily and economically accomplished using simple glow discharges.15 It is noted that hydrogen in general acts as a good radiation shield,17 and in this regard hydrogen-functionalized CNTs in composites may be desirable in spacecraft design.
Figure 3. UV-vis-NIR spectra (a) specular reflectance and (b) diffuse reflectance. R is the reflection.
Acknowledgment. The authors acknowledge Lance Delzeit for the TEM analysis and Charlie Bauschlicher, Jim Arnold, and Mark Loomis for a critical review of the manuscript. Discussion with E. T. Arakawa, Shoba Singh, and Scott Corzine for providing the Monte Carlo results is acknowledged. Work by SETI and ELORET authors was supported by NASA contracts to these organizations. Professor Richard Smalley and his HiPCo team at Rice University are acknowledged for providing SWNTs. References
Figure 4. Raman spectra of proton bombarded SWNT film using 1064 nm excitation; similar results were also obtained with 633 nm excitation.
the H attachment to the sidewall. The D-band intensity also increases simultaneously after proton bombardment, providing further evidence to creation of defects. Inspection of radial breathing mode of the spectra from the Renishaw system results (not shown here) reveals that the range of diameters remains the same (0.9-1.3 nm) before and after proton bombardment; but the distribution of 1.25 nm diameter tubes increases after irradiation. TEM results from ref 9 indicate only some form of welding of the nanotubes Nano Lett., Vol. 3, No. 5, 2003
(1) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, Ph.; Carbon Nanotube: Synthesis, Structure, Properties and Applications; SpringerVerlag: Berlin, 2001. (2) Yang, L.; Anantram, M. P.; Han, J.; Lu, J. P. Phys. ReV. B 1999, 60, 13874. (3) Beuneu, F.; l’Huillier, C.; Salvetat, J.-P.; Bonard, J.-M.; Forro, L. Phys. ReV. B 1999, 59, 5945. (4) Krasheninnikov, A. V.; Nordlund, K.; Sirvio¨, M.; Salonen, E.; Keinonen, J. Phys. ReV. B 2001, 63, 245405. (5) Krasheninnikov, A. V.; Nordlund, K.; Keinonen, J. Phys. ReV. B 2002, 65, 165423. (6) Ni, B.; Andrews, R.; Jacques, D.; Qian, D.; Wijesundara, M. B. J.; Choi, Y.; Hanley, L.; Sinnott, S. B. J. Phys. Chem. B 2001, 105, 12719. (7) Stahl, H.; Appenzeller, J.; Martel, R.; Avouris, Ph.; Lengeler, B. Phys. ReV. Lett. 2000, 85, 5186. (8) O’Rourke Muisener, P. A.; Clayton, L.; Angelo, J. D.; Harmon, J. P.; Sukder, A. K.; Kumar, A.; Cassell, A. M.; Meyyappan, M. J. Mater. Res. 2002, 17, 2507. (9) Basiuk, V. A.; Kobayshi, K.; Kaneko, T.; Negishi, Y.; Basiuk, E. V.; Saniger-Blesa, J.-Mi Nano Lett. 2002, 2, 789. (10) Edmonds, L. D.; Barnes, C. E.; Scheik, L. Z. An Introduction to Space Radiation Effects in Microelectronics, JPL Publication 0006; NASA Jet Propulsion Laboratory: Pasadena, 2000. 645
(11) Nikolev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (12) Khare, B. N.; Meyyappan, M.; Cassell, A. M.; Nguyen, C. V.; Han, J. Nano Lett. 2002, 2, 73. (13) Moore, M. H.; Hudson, R. L.; Gerakines, P. A. Spectrochim. Acta A 2001, 57, 843. (14) Ziegler, J. F.; Biersack, J. P.; Littmarck, U.; Stopping and Range of Ions into Matter; Pergamon Press: New York, 1985.
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(15) Khare, B. N.; Meyyappan, M.; Kralj, J.; Wilhite, P.; Sisay, M.; Imanaka, H.; Koehne, J.; Bauschlicher, C. W. Appl. Phys. Lett. 2002, 81, 5237. (16) Wendlandt, E. W.; Hecht, H. G.; Reflectance Spectroscopy: Interscience Publishers: New York, 1966. (17) Wilson, J. W.; Cucinotta, F. A.; Kim, M. H. Y.; Schimmerling, W. Phys. Medica 2001, 17, 67.
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