H558 Journal of The Electrochemical Society, 161 (9) H558-H563 (2014)

Growth and Electrochemical Characterization of CarbonNanospike Thin Film ElectrodesLeah B. Sheridan,a,∗ Dale K. Hensley,a Nickolay V. Lavrik,a Sean C. Smith,aViviane Schwartz,a Chengdu Liang,a,∗ Zili Wu,a Harry M. Meyer III,band Adam J. Rondinonea,z

aCenter for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USAbMaterials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

We report the growth of a nanospiked, carbon thin film electrode by an inexpensive, non-catalytic plasma enhanced CVD processon Si substrates. The electron transfer kinetics for various redox probes of these carbon nanospikes (CNS) were determined andcompared with glassy carbon and CNS exposed to oxygen plasma or a high temperature ammonia soak. The results indicate thatCNS can be used as a practical alternative to GC for various electroanalysis applications. These electrodes also exhibited activityand stability toward the oxygen reduction reaction, suggesting there potential use as electrocatalyst supports. Finally, the ability todeposit conformal thin films of CNS on a 3-D architecture for use as an electrode was demonstrated.© The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/),which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in anyway and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0891409jes]All rights reserved.

Manuscript submitted May 7, 2014; revised manuscript received June 11, 2014. Published June 20, 2014.

Carbon electrodes are commonly used in electrochemical applica-tions such as sensing and electrocatalysis. This class of electrode isoften touted for being inexpensive, highly conductive, electrochem-ically stable, amenable to surface chemistry modifications, and hav-ing a wide electrochemical window. While the electrochemistries ofstandard carbon electrodes like glassy carbon and carbon black havebeen thoroughly studied, newly developed nanoscale carbon materialspresent a potent new class of carbon electrodes to explore. A greatdeal of attention has focused on highly ordered carbon nanomaterialslike graphene1–3 and carbon nanotubes,4 in addition to less orderedcarbons like pyrolized graphite,5 carbon foams6 and fibers.7

One approach to forming nanoscale carbon electrodes is throughthin film deposition, which can produce free standing electrodes foreasy incorporation into sensors, microelectronic components, and en-ergy storage and conversion devices. Carbon thin films having a rangeof structural and electrochemical properties have been formed by tech-niques including photoresist pyrolysis,8–10 sputtering,11,12 and pulsedlaser-arc deposition,13 among others.

Chemical vapor deposition (CVD), which is best known for its usein the microelectronics industry for fabricating thin films of semicon-ductors, metals, alloys and dielectrics, is another option for carbonthin film formation. Fundamental research into CVD technologies ledto the development of numerous variations, such as metal-organicCVD (MOCVD) and plasma-enhanced CVD (PECVD). CVD in itsvarious forms has been used since the mid-90’s to synthesize nanos-tructured carbons including carbon nanotubes,14 carbon nanofibers,15

single and multiple layer graphene,16 vertically aligned graphene17

and nanodiamond.18

Nanostructured electrodes - those with controlled architecturesat the nanoscale - often possess high surface area, tunable surfacefunctionality, increased electrocatalytic activity, spatial control andunique diffusion properties (which can lead to higher sensitivity).19

Here we report the formation and electrochemical characterization ofnanostructured carbon thin film electrodes, which we refer to as carbonnanospikes (CNS) due to the primary morphological characteristic oftapered spikes approximately 50 nm in length. The nanospikes hereinare grown on Si wafer using non-catalytic PECVD in the presence ofammonia which dopes the CNS with about 5% atm. nitrogen. The CNSmay be grown homogeneously over an entire 4-inch wafer, which isthen cleaved for use. The CNS surface was also exposed to an oxygenplasma or a high temperature ammonia soak in an attempt to elucidate

∗Electrochemical Society Active Member.zE-mail: rondinoneaj@ornl.gov

the effect of different surface functionalities on its electrochemicalproperties. Additionally, we demonstrate the formation of a three-dimensional (3-D) electrode via a conformal coating of CNS on a Siscaffold.

Materials and Methods

Growth and structural characterization of CNS.— CNS weregrown on n-type 4-inch Si wafers 〈100〉 with As doping (<0.005 �)via PECVD in the presence of acetylene (C2H2) and ammonia (NH3)at 650◦C for 10 to 30 min. A DC plasma was generated between thesubstrate (cathode) and the showerhead (anode) in a continuous streamof C2H2 and NH3 gas, flowing at 80 sccm and 100 sccm, respectively.The total pressure was maintained at 6 Torr with a plasma power of240 W. After formation, the CNS film was either left untreated, ex-posed to an O2 plasma for 1 minute using a plasma cleaner (Harrick,PDC-32 G) or soaked in NH3 in the PECVD chamber at 680◦Cfor 6 h.

3-D electrodes were fabricated by first patterning a Si wafer usingphotolithography and a mask of 15 × 15 arrays of 8 μm diametercircles with a 200 μm pitch. NFR negative microphotoresist (JSRMicro) was used for patterning and also served as the etch stop duringSi etching via the Bosch Process.20 After etching was complete, thephotoresist was removed by soaking the wafer in 1165 NMP (n-methylpyrrolidinone) at 70◦C for ∼30 minutes, followed by a soak in acetoneand IPA before final rinsing. The CNS were then deposited by the sameprocedure used for planar Si substrates.

Scanning electron microscopy (SEM) and energy dispersive X-rayspectroscopy (EDS) (Zeiss Merlin VP SEM) were used for imagingand elemental analysis of the thin films. C, H, and N content of CNSscraped from the Si wafer were determined by combustion analysisand O content was evaluated by pyrolysis using a Flash 2000 or-ganic elemental analyzer. Raman spectroscopy measurements weremade in air, at room temperature with a Princeton Instruments ActonTrivista 555 using a 532 nm laser excitation source. X-ray photoelec-tron spectra were collected using a Thermo Scientific K-Alpha XPS.Transmission electron microscopy (TEM) images of CNS, grown ona holey carbon grid (SPI Supplies) under the same conditions as thefreestanding electrodes, were collected using a Carl Zeiss Libra 120Plus TEM. A four-point probe (Lucas Labs Pro4 V1.2.1) was used formeasuring the film resistivity.

Electrochemical characterization.— Electrochemical measure-ments were made with a Biologic VSP potentiostat (Bio-logic USA,LLC) using a 3-electrode configuration, comprised of a Pt coil counter

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Journal of The Electrochemical Society, 161 (9) H558-H563 (2014) H559

electrode, a Ag/AgCl (3 M KCl, BAS) reference electrode and aCNS/Si working electrode (geometric surface area = 0.317 cm2). Allsolutions were made using 18 M� nanopure water (Milipore) anddeoxygenated by bubbling with N2 for a minimum of 15 min priorto each measurement. The redox probe solutions were as follows:1 mM K4[Fe(CN)6] in 1 M KCl, 1 mM Ru(NH3)Cl3 in 1 M KCland 1 mM (NH4)2Fe(SO4)2 in 0.1 M H2SO4. Heterogeneous electrontransfer (ET) rate constants (k◦) were determined by an extension ofthe Nicholson method, previously proposed by Magno et. al.21 Cyclicvoltammograms (CVs) of the various redox probes were measured atscan rates (ν) ranging from 10 to 500 mV/s and compared with a glassycarbon (GC) electrode (BASi, geometric surface area = 0.071 cm2).Prior to each experiment, the GC was polished sequentially in 1, 0.3and 0.05 micron Al2O3/nanopure H2O slurries using Bueler plosh-ing pads (CH Instruments, Inc.), rinsingwith copious amounts of 18M� nanopure water between each step. Oxygen reduction reaction(ORR) experiments were carried out in 0.1 M NaOH that was firstbubbled with O2 for >30 min and then blanketed with O2 duringeach measurement. Linear sweep voltammetry (LSV) at a rotatingdisk electrode (RDE) was collected using a Pine Instrument Companyrotator interfaced with the potentiostat. CNS/Si (geometric surfacearea = 0.16 cm2) was affixed to a GC RDE using carbon tape andelectrically connected to the RDE shaft with Cu tape, both of whichwere protected from solution using fingernail polish. The bare GCRDE (Pine Instrument Company) was used for comparison (geomet-ric surface area = 0.196 cm2). All CVs are plotted using currentdensity based on the electrode’s geometric surface area.

Results and Discussion

Structural characterization of CNS.— The PECVD grown CNSexhibited consistent morphology from sample to sample, indicatinga highly reproducible process. A representative SEM image of theCNS surface (Fig. 1A) shows a dense film of nanotextured carbonfully covering the underlying Si. The SEM is not capable of resolvingthe nanospikes but can image the base structure, which appears asoverlapping folds. TEM images of a CNS-coated holey carbon grid,depicted in Fig. 1B, reveal that the film is terminated by randomlyoriented nanospikes approximately 50–80 nm in length. TEM alsoindicates (Fig. 1B inset) that each nanospike consists of layers ofpuckered carbon ending in a curled tip, ∼2 nm wide. The uneven,puckered layers of carbon are indicative of a disordered structure,which agrees with the elemental analysis and Raman results discussedbelow.

Elemental analysis via pyrolysis indicates that the films have sig-nificant N doping (5.1 +/−0.2 atomic%) and hydrogen content (8.6+/−0.1 atomic%) but minimal O content (0.5 +/−0.03 atomic%).Nitrogen is expected due to the use of ammonia etchant but thereare no sources of oxygen in the PECVD apparatus or reagent gases,which could indicate that the oxygen atoms originate in the underly-ing native silicon oxide layer. Air leakage into the reaction chamberas a source of oxygen is unlikely because the chamber is pumpedand ultimate vacuum verified prior to introduction of reagent gases.Raman spectroscopy was used to assess hybridization and structure.The Raman spectra in Fig. 2 display the characteristic G band at∼1595 cm−1, indicating the presence of sp2 carbon, typical of allgraphitic carbons. The defect or D band at ∼1345 cm−1 is due to de-fects arising from grain boundaries, heteroatom doping, amorphouscarbon or edge sites.22,23 This peak is, at least in part, due to the pres-ence of N in the film. The high peak intensity of the D band relativeto that of the G band suggests the CNS have a disordered graphiticstructure consistent with the TEM imaging.24 There is also a broadpeak around 2800 cm−1 where the 2D, or G′, band occurs, which re-sults from the overtone of the D band. The broad peak shape suggestsmultiple layers of graphene (i.e. graphite) likely consisting of both2-D and 3-D graphite phases,24,25 also consistent with the TEM. Weconclude that the disorder is hence due to significant nitrogen dop-ing, which undoubtedly contributes to the interesting electrochemicalbehavior of the CNS discussed below.

Figure 1. SEM (Fig. 1, A) and TEM (Fig. 1, B) of CNS. Lower-resolutionSEM is not capable of resolving the spikes but indicates that the microstructureof the carbon coating presents as a regular, folded film. TEM shows that thefolds are populated or terminated by nanospikes of approximately 50–80 nmin length each, made up of multiple layers of puckered graphene.

Figure 2. Raman spectrum of a CNS grown on Si wafer indicates a defectivegraphitic structure according to D- and G- bands respectively. Broad 2D featureindicates that multiple layers are present, consistent with TEM.

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H560 Journal of The Electrochemical Society, 161 (9) H558-H563 (2014)

Figure 3. Redox behavior of CNS. Electrochemical window of CNS (Fig. 3, A) is comparable to glassy carbon (GC). Inner sphere redox couple using Fe(CN)63−/4−

(Fig. 3, B) indicates that CNS has enhanced kinetics over GC, possibly due to microstructure. Outer sphere redox couple using Ru(NH3)63+/2+ (Fig. 3, C) indicates

quasi-reversible ET likely due to lower conductivity and a greater density of states. Final redox couple using Fe3+/2+ (Fig. 3, D) probes electrode oxygenfunctionality. Variations in �Ep as a function of oxygen content of the CNS by either electrochemical oxidation (CNS-Used) or modification (CNS-NH3 soak, O2plasma) demonstrate nitrogen content tunability.

In order to understand the bonding nature of the nitrogen dopant,X-ray photoelectron core-level spectra for C 1s, O 1s, and N 1s werecollected and fitted. Fits of the nitrogen spectrum indicate that thenitrogen is bonded in 3 ways: pyridinic (∼25%), pyrrolic (∼19%), andgraphitic (∼55%). 7% of the nitrogen is also bonded to oxygen. Forpyridinic and pyrrolic nitrogens, an unbonded electron pair is availablefor coordination of electrophilic species which greatly increases theelectro-reactivity of the material.26,27

A similar class of nanostructured carbon thin films, often referredto as petal-like carbon or carbon nanowalls, have been formed bya variety of methods, including microwave PECVD,28–31 hydrogenarc discharge32 and inductively coupled radio-frequency PECVD.33

A thorough review of the synthesis and characterization of these ma-terials was written by Hiramatsu and Hori34 and is suggested forfurther details. Carbon nanowalls are, to our knowledge, the mate-rial most closely related to CNS, meriting some discussion. Mor-phological differences are evident in SEM images and TEM images,which further highlight the distinctions between the two materials.Carbon nanowalls are comprised of carbon sheets, with lengths rang-ing from 100’s of nanometers to several microns,35,36 made up ofuniform graphene layers37 in contrast to the layers of puckered carbonforming the CNS. This structural difference is also reflected in theRaman spectroscopy. The D and G band peaks for the CNS spectrumare broad and partially overlap, while these bands are sharper and

narrower for typical carbon nanowalls, which suggest a higher degreeof graphitization. The D′ band is also present in the carbon nanowallsspectra, while it is not evident in that of CNS, suggesting that carbonnanowalls have exposed graphene edges and relatively low disordercompared to CNS.35 It is thought that a large amount of H atoms arenecessary to form well-defined carbon nanowalls with higher graphi-tization, so most synthetic methods use a mixture of CH4 and H2,34

while CNS is synthesized in the absence of H2. The differences canbe summarized as CNSs having smaller, sharper and more disorderedstructure than the carbon nanowalls.

Electrochemical characterization of CNS thin films.— The elec-trochemical windows of CNS and GC, determined by CV in 1 MKCl (Fig. 3A), are comparable at nearly 2 V. This is narrower than the3 V window reported for carbon nanowalls in acidic electrolyte, whichis similar to B-doped diamond.34 The large double-layer capacitance,observed between the onset of the oxidation and reduction features,are evidence of the expected increase in surface area of the CNScompared to GC.

Three redox couples, from different kinetic classes, were usedto investigate the electrochemical behavior of CNS before and afterpost-treatment. The ET kinetics depends on the redox system beinginvestigated, as the activation mechanism varies with each system.Fe(CN)6

3−/4−, a commonly used standard, is considered a “non-ideal”

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Journal of The Electrochemical Society, 161 (9) H558-H563 (2014) H561

Figure 4. CVs of as grown and post-treated CNS compared to GC in (A) 1 mM K4[Fe(CN)6] in 1 M KCl and (B) 1 mM Ru(NH3)Cl3 at 0.1 V/s.

inner-sphere redox couple, meaning the ET kinetics are sensitive tothe carbon surface, but the nature of the interaction is not well-definedand depends on both surface chemistry and solution chemistry.38 Ac-cording to the CVs (Fig. 3B) and the k◦ values, which were calculatedas 0.002 cm s−1 for GC and 0.008 ± 0.002 cm s−1 for CNS, the ki-netics are enhanced for CNS compared to GC. Luais et al. reported ak◦ of 0.009 cm s−1 for carbon nanowalls on amorphous carbon, whichis similar to that of CNS. CVs comparing the effect of exposure toO2 plasma or a high temperature NH3 soak (Fig 4A) indicate that thekinetics of Fe(CN)6

3−/4− were unaffected. �Ep values greater than59/n mV, the theoretical value for a reversible (Nernstian) processwhere n is the number of electrons transferred, and an increase of�Ep with ν, indicate quasi-reversible behavior. Both CNS and GCsatisfy these criteria.

As is evident in Fig. 3C, the kinetics of CNS and GC are no-tably different in Ru(NH3)6

3+/2+, which is generally insensitive tothe electrode surface, referred to as an outer-sphere redox couple.39,40

The �Ep for GC is 59 mV and is nearly constant as a function ofν, suggesting electrochemical reversibility as expected from earlierreports,41 while all the CNS display quasi-reversible ET (Fig. 4B).Though the ET rate for Ru(NH3)6

3+/2+ is independent of the electrodesurface, it is affected by the density of electronic states (DOS). More-over, electronic properties (i.e. DOS near the formal potential) canaffect the ET rate of all redox systems39,41,42 and lower conductivitygenerally correlates with lower DOS.41 Thus, the slower kinetics islikely due to the lower conductivity of CNS (i.e. lower DOS).

The final redox couple, Fe3+/2+, is highly sensitive to oxygen func-tionalities. Surfaces with fewer O functionalities or O-free adsorbatesexhibit slower ET.39,40,42,43 CVs comparing several samples of CNSwith GC are shown in Fig. 3D. As-grown CNS have low O contentoverall, which is reflected by its large �Ep of 269 mV. In comparison,the �Ep for GC is 206 mV, which agrees with recent literature.12

Interestingly, a CNS sample previously used for CV has a smaller�Ep of 187 mV. This suggests that with continued use, the amountof surface oxide on CNS increases, most likely due to repeated elec-trochemical oxidation.41 The ET rate at the O2 plasma treated CNSis further improved, having a �Ep of 176 mV, again reflecting an in-crease in surface oxide coverage. In contrast, soaking CNS in NH3 athigh temperature significantly hinders the rate of reaction, as shownby the large �Ep of 326 mV. This results from a decrease in thesurface oxide coverage due to formation of N-functional groups. Athigh temperatures NH3 decomposes to form NH2, NH, and atomichydrogen, which are capable of attacking surface oxides to generateN containing functionalities.44,45 While not explored here in depth,this indicates that the CNS electrocatalytic functionality is tunablethrough the nitrogen functionality. A recent report by Jiao et al.27

detailed simulations on pyridinic nitrogen-doped carbon nanotubesfor electrochemical capture of CO2. The CNS reported here, withhigh nitrogen content and conformal coating of 3D structures as de-scribed below, present an interesting route to possible strategies forCO2 mitigation.

As is noted in the experimental section, GC requires polishing be-fore each experiment and its electrochemical activity has been shownto be highly dependent on the method of polishing, the history of theelectrode and the use of additional activation procedures.41,46,47 Forexample, reported k◦ values for GC in Fe(CN)6

3−/4− range at least from0.002 to 0.020 cm s−1 using the same polishing method.12,46,48 Alter-natively CNS exhibit reasonable reproducibility without the need forany pretreatment and the low cost of fabricating large areas of CNS onSi makes it possible for use as a disposable electrode when required.

The activity of CNS toward ORR was also examined. A compari-son of linear sweep voltammetry of CNS and GC collected at a rotationrate of 1600 rpm in O2 saturated basic solution (Fig. 5A) shows thatboth electrodes have a similar onset potential for the ORR, while CNShas a higher steady-state current density. The enhanced current at theCNS suggests higher electrocatalytic activity as compared to GC, butmay also be due to the real surface area of the CNS being larger thanits geometric surface area. GC exhibits two reduction features, whichhave been reported in the literature to result from the reduction of O2

to HO2− by the 2e− pathway at 2 different active sites, rather than the

more desirable 4e− pathway.49 Though the reaction mechanism forthe CNS was not determined, it is well documented that the presenceof N functionalities affects the mechanism and can enhance the ORRactivity of carbon electrodes.50 Therefore, the N content in the CNSoffers one possible explanation for the enhanced activity at high po-tential as compared to GC. The stability of the CNS during ORR wasexamined by performing 500 successive CVs between 0.2 and −0.7 Vat 0.1 V/s in O2 saturated 0.1 M NaOH (Fig. 5B). The voltammetrydid not change, indicating that CNS are stable under these conditionsand may be of interest as an electrocatalyst support for ORR.

To compare against the conductivity of glassy carbon, the resistiv-ity of CNS grown on an n-type Si wafer 〈100〉 with P doping (>1000�) was measured to be 0.016 � cm. This is about 4× more resis-tive than GC, indicating a slightly lower conductivity, but on the or-der of values reported for carbon black and boron-doped diamond.41

A wide range of resistivities, ranging from approximately 0.006 to0.2 � cm are reported for carbon nanowalls due to the variation ingrowth conditions.34

Three-dimensional CNS thin film electrodes.— The increased sur-face area of 3-D carbon electrodes, along with their high conductivityand electrolyte accessibility, has been exploited for applications such

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H562 Journal of The Electrochemical Society, 161 (9) H558-H563 (2014)

Figure 5. Oxygen reduction reaction in O2-saturated 0.1 M NaOH solution. CNS demonstrates the same onset potential as glassy carbon, with significantlyincreased current at higher potentials (Fig. 4A). Higher current at higher potentials is not due to reduction of the electrode as evidenced by 500-cycle stability(Fig. 4B) and is likely due to high nitrogen content combined with texture.

as supercapacitors,51 biosensors52 and nanoparticle supports for use ingas sensors53 and electrocatalysis.54 A 3-D electrode was fabricatedto demonstrate the capability of depositing conformal thin films ofCNS. The 3-D Si scaffold, consisting of a 15 × 15 array of 5 μmtall posts with a diameter of 8 μm, was coated with CNS usingthe same procedure used with planar substrates. The layer of CNShas the same general surface morphology (Fig. 6 and inset) and is∼1 μm thick, as determined by the radius of the post in Fig. 6 minusthat of the Si scaffold, making it nearly equivalent to that depositedon a planar substrate. CVs were taken in Fe(CN)6

3−/4 to comparethe electrochemical behavior of the 3-D versus the planar electrode(Fig. 7). Based on the �Ep of the two electrodes, the electron transferkinetics are comparable, another indication that the CNS structureis equivalent in both configurations. The peak currents are larger forthe 3-D electrode; however, the additional geometric surface area ofthe 3-D electrode is not sufficient to account for the increase. Thissuggests enhanced activity along the walls and edges of the post ordue to increased diffusion around the nanoscale posts,55,56 and wouldbe considered an advantage for certain applications and will be thesubject of future investigations.

Figure 6. Conformal coating of CNS on a silicon pillar demonstrating theability to create 3D carbon electrodes. Top edge of silicon pillar (inset) showsoutstanding texture and uniformity. The total thickness of the carbon coatingson the side of the pillar are similar to the thickness deposited on a planarsubstrate, calculated by subtracting the width of the coated pillar from theuncoated.

Figure 7. CV of 3D electrode created by coating Si pillars with CNS. Elec-trochemical properties are identical, with increased current due to the presenceof the pillars. Planar area of CNS and 3D-CNS are identical (0.71 cm2).

Conclusions

We have reported the formation of a new nanostructured carbonelectrode having a unique nanospike architecture using PECVD. Themethod is a straightforward approach for producing large-scale carbonelectrodes at a relatively low cost. The electrochemical activity ofCNS toward Fe(CN)6

3−/4 is greater than that of GC, though it isless active toward Ru(NH3)6

3+/2+. In the case of Fe3+/2+, the ET ratedepends on the post-treatment and history of the CNS electrode andcan be optimized by a simple O2 plasma treatment. These electrodesare amenable to one-time use, there is no need for complex pre-treatment or activation procedures, and the electrochemical windowis similar to that of GC, making CNS a potential alternative to GC forelectroanalysis applications. CNS also show catalytic activity for theORR and may be promising as an electrocatalyst support. In addition,the ability to conformally coat 3-D architectures with thin films ofCNS significantly broadens the scope of its potential electrochemicalapplications.

Acknowledgments

This research was conducted at the Center for Nanophase MaterialsSciences, which is sponsored at Oak Ridge National Laboratory by the

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Journal of The Electrochemical Society, 161 (9) H558-H563 (2014) H563

Scientific User Facilities Division, Office of Basic Energy Sciences,U.S. Department of Energy. The research was supported in part by anappointment to the ORNL Postdoctoral Research Associates Programadministered jointly by Oak Ridge Institute for Science and Education(ORISE) and ORNL.

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