J. Phys. Chem. C 2009, 113, 14467–14473
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Single Wall Carbon Nanotube Response to Proton Radiation Peter J. Boul,*,† Kathryn Turner,‡ Jing Li,§ Merlyn X. Pulikkathara,| R. C. Dwivedi,⊥ Edward D. Sosa,† Yijiang Lu,§ Oleksandr V. Kuznetsov,| Padraig Moloney,‡ R. Wilkins,⊥ Mary Jane O’Rourke,‡ Valery N. Khabashesku,| Sivaram Arepalli,† and Leonard Yowell‡ ERC Inc. at NASA Johnson Space Center, Houston, Texas 77058, NASAsJohnson Space Center, Houston, Texas 77058, NASA Ames Research Center, Moffett Field, California 94035, Department of Chemistry, Department of Physics and Astronomy, and the Smalley Institute for Nanoscale Science and Technology, Rice UniVersity, Houston, Texas 77005, and Center for Applied Radiation Research, Prairie View A&M UniVersity, Prairie View, Texas 77446 ReceiVed: September 26, 2008; ReVised Manuscript ReceiVed: May 5, 2009
In the interest of developing a highly sensitive, low power radiation dosimeter, a series of tests were performed on single-wall carbon nanotube (SWCNT)-based nanomaterials to monitor their response to 10 and 30 MeV proton radiation. The SWCNT materials were deposited on an interdigitated electrode (IDE) that was developed at NASA Ames for chemical sensing. In order to investigate the effects of nanotube functionalization on the sensor properties, the SWCNTs were covalently or noncovalently functionalized prior to their incorporation into the devices. The functionalized nanotubes which were assayed included fluorinated SWCNT (F-SWCNT), alkylated F-SWCNT (F-SWCNT-C11H23), refluorinated alkylated F-SWCNT (F-SWCNT-C11F23), palladium doped SWCNTs (Pd-SWCNTs), and nanotubes noncovalently associated with cellulose (Cel-SWCNTs). These five functionalized nanotube types and pristine carbon nanotubes were investigated for their responses to proton radiation. The device response to irradiation, measured as a change in resistance, was found to vary with the type of functional group attached to the SWCNT. The samples were also characterized by Raman spectroscopy in order to observe changes in the disorder band (at 1350 cm-1) of the nanotube materials. Depending on nanotube functionalization, the devices showed a real-time response to radiation at the energy levels tested. The nature of the response indicates that these nanomaterials may potentially be used to produce a dosimeter that is memory-free, reusable, and reversible. 1. Introduction Space radiation is a significant concern for astronauts/ cosmonauts outside the Earth’s magnetosphere. Ionizing radiation can be damaging to equipment in addition to posing a biological hazard to humans. Sources of ionizing radiation in low Earth orbit (LEO) include galactic cosmic rays (GCRs), trapped particles in the Earth’s radiation belts, and solar particle events (SPEs). The Earth’s magnetosphere shields the surface of the Earth from the most damaging GCRs. Beyond LEO, GCRs become a major concern. Missions can be timed to occur during solar maximum, when the solar wind deflects GCRs; but during solar maximum, there is a significant risk of solar storms and SPEs, during which a huge radiation dose can be delivered in a relatively small period of time. The most common form of radiation in these events is proton radiation.1 There are, at this time, very few dosimeters that can monitor the radiation environment in real time. Of the dosimeters with time resolution, none are conveniently portable or can be used as personal dosimeters. Radiation detection aboard the International Space Station (ISS) is currently carried out with a suite of detectors, each with its own set of advantages and drawbacks. Passive detectors, such as thermoluminescent detectors (TLDs), * Corresponding author. Phone: (281) 483-8117. E-mail: peter.j.boul@ nasa.gov. † ERC Inc. at NASA Johnson Space Center. ‡ NASAsJohnson Space Center. § NASA Ames Research Center. | Rice University. ⊥ Prairie View A&M University.
are occasionally worn by crewmembers, in addition to generally being the detector type of choice on Earth, in locations such as nuclear plants and hospitals. The disadvantage of passive systems is that the radiation dose cannot be measured until the device is processed and analyzed, which is time-consuming. Another disadvantage of passive detectors such as TLDs is that they have a limited energy response. The Johnson Space Center Tissue Equivalent Proportional Counter (JSC-TEPC) is the principal radiation detection instrument for crew dose aboard the shuttle and ISS.1b A TEPC is an active device that uses a gas volume to simulate a small cell volume such that the response to radiation can be directly extrapolated to biological tissue.1c The data collected by the TEPC is analyzed with an electronic spectrometer, consisting of electronic hardware and software, which reports a local dose on the order of every minute. The TEPCs, while active, cannot be employed as personal dosimeters because of their size and weight but are considered useful as area radiation monitors. The ISS also has charged particle directional spectrometers (CPDSs), both intravehicular (IV) and extravehicular (EV). A CPDS uses silicon to detect charged particles. It includes several detectors arranged in parallel so that a single particle’s strikes on the detectors can be used to extrapolate the particle’s path. Like a TEPC, a CPDS is too large to function as a personal dosimeter.1 Carbon nanotube-based devices are excellent candidates for lightweight personal dosimeters. They are highly sensitive as sensors, display particularly low power consumption,3 and have already shown promise in X-ray dosimetry.4 Chemical func-
10.1021/jp808553u CCC: $40.75 2009 American Chemical Society Published on Web 07/22/2009
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tionalization of SWCNTs can be used to increase a sensor’s physical and chemoselective sensitivity. There has been substantial effort in the functionalization of SWCNTs to alter both their electronic properties and dispersibility.5 Previously developed functionalization techniques enable the tailoring of the SWCNT material properties to those desired for radiation sensing.5a In the interest of developing a real time, memoryfree sensor, radiation tolerance of the semiconductor materials is a key factor. It has been demonstrated previously that unfunctionalized SWCNTs are radiation tolerant5b,6 because of the inherent strength of the aromatic C-C bonds. In the present work, two groups of covalently functionalized SWCNTs were studied. The first group was fluorinated SWCNTs (F-SWCNTs) that were then further derivatized by alkylating their sidewalls with C11H23 long-chain radicals (F-SWCNT-C11H23) according to a previously published procedure.7 The second group was F-SWCNT-C11F23 nanotube derivatives that were prepared by refluorination of F-SWCNT-C11H23 at room temperature for 0.5 h to convert the alkyl chains on the side walls to -C11F23 chains.8 Noncovalently functionalized nanotube materials, which have previously been used as gas sensors, were also used. These materials were palladium-doped SWCNTs (Pd-SWCNTs) and nanotubes noncovalently associated with cellulose (CelSWCNTs).9 It is found that the addition of these functional groups to the SWCNTs significantly alters the radiation response of electrical devices that incorporate the functionalized SWCNT compared to that of similar devices with pristine SWCNTs. 2. Experimental Section A nanosensor technology which was previously developed for ppb level gas sensing9 has been adapted in this study for proton radiation detection. This sensor is an array of interdigitated electrode (IDE) pairs, fabricated using photolithography and thin-film metallization techniques. Suspensions of covalently and noncovalently derivatized SWCNTs were subsequently deposited across IDEs to form nanotube thin films. Similar IDEs have been successfully employed as sensors for methane, ammonia, NO2, and other gases in the past.10 The radiation effects were measured as a change in the conductivity of nanotube material as a function of radiation fluence and particle energy. The measurements may be termed real-time in the sense that the response during irradiation was observable in a short enough time that a human observer would report a real-time change in the data. The actual data acquisition time was 15 s for all 32 channels on each array. The effects of proton radiation on the conductivity of a variety of different functionalized carbon nanotube IDEs are discussed below. 2.1. Sensing Materials Preparation. Molecular nanotube functionalization was achieved as follows: fluorinated nanotubes (F-SWCNTs) of approximately C2F stoichiometry were acquired from Carbon Nanotechnologies Inc. and used as received. The F-SWCNTs were then alkylated with free undecyl radicals produced by thermal decomposition of lauroyl peroxide according to a previously published procedure.7 Some of these tubes (designated as F-SWCNT-C11H23) were then refluorinated at room temperature to convert the side alkyl chains on the nanotubes into perfluorinated C11F23 chains to yield a new SWCNT derivative, F-SWCNT-C11F23. The covalently functionalized SWCNTs were characterized by Fourier transform infrared spectroscopy, Raman spectroscopy, thermal gravimetric analysis, and X-ray photoelectron spectroscopy. Supramolecular nanotube functionalization was achieved by the following steps: Cel-SWCNTs were prepared through the following method. Hydroxypropyl cellulose (0.791 g) was
Boul et al.
Figure 1. Picture of a 32-channel sensor chip in a PLCC 68 chip carrier. All 32 sensors were wire-bonded to the connecting pads on the chip carrier for the signal measurement.
dissolved in 25 mL of chloroform in order to coat the nanotube surface. In each case, an aliquot of 5 nL of polymer solution was drop-deposited onto the SWCNT network of one of the elements in Figure 1 to coat the corresponding SWCNTs. PdSWCNTs were prepared as follows: a layer of 10 nm thick metallic Pd was sputter coated onto a pile of SWCNT powder (98%, Carbon Nanotechnologies Inc., Houston, TX.) and mixed with SWCNTs through shaking. The Pd-doped SWCNTs (∼1% wt.) were then dispersed in distilled deionized (dd) water (0.1 mg of Pd-SWCNTs in 10 mL of dd water). The solution was sonicated and drop-deposited onto the interdigitated electrodes to create a sensor with an initial resistance in the range of 1-10 kΩ. Scanning electron microscope (SEM) images show Pd nanoparticles with an average diameter of 10 nm deposited on SWCNT bundles. 2.2. Radiation Source and Experiment Setup. The proton energies and the total fluence covered in this study are consistent with the expected long-term radiation exposure in LEO, as well as other space radiation environments. Protons found in orbits designated as LEO (usually considered
