Growth and properties of Si–N–C–O nanocones and graphiticnanofibers synthesized using three-nanometer diameteriron/platinum nanoparticle-catalystH. Cuia)Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831X. YangDepartment of Materials Science and Engineering, University of Tennessee,Knoxville, Tennessee 37996H.M. Meyer and L.R. BaylorOak Ridge National Laboratory, Oak Ridge, Tennessee 37831M.L. SimpsonDepartment of Materials Science and Engineering, University of Tennessee,Knoxville, Tennessee 37996; and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831W.L. Gardner and D.H. LowndesOak Ridge National Laboratory, Oak Ridge, Tennessee 37831L. An and J. LiuDepartment of Chemistry, Duke University, Durham, North Carolina 27708(Received 23 July 2004; accepted 3 January 2005)Cone-shaped nanostructures of mixed composition (nanocones) and largely graphiticnanofibers were synthesized on silicon substrates using iron/platinum alloynanoparticles as the catalyst in a direct-current plasma enhanced chemical vapordeposition reactor. The catalyst nanoparticles were monodisperse in size with anaverage diameter of 3 (±1) nm. The nanocones were produced on laterally widelydispersed catalyst particles and were oriented perpendicular to the substrate surfacewith an amorphous internal structure. The nanocones were produced by gas phasemixing and deposition of plasma-sputtered silicon, nitrogen, carbon, and oxygenspecies on a central backbone nucleated by the Fe–Pt catalyst particle. Field emissionmeasurements showed that a very high turn-on electric field was required for electronemission from the nanocones. In contrast, the graphitic nanofibers that were producedwhen silicon sputtering and redeposition were minimized had the “stacked-cup”structure, and well-defined voids could be observed within nanofibers nucleated fromlarger catalyst particles.I. INTRODUCTIONGrowth of isolated and vertically aligned carbon nano-fibers (VACNFs) is interesting for fundamental under-standing of nanomaterial growth mechanisms and for po-tential practical applications that include field-emissioncathodes for vacuum nanoelectronic devices1highly par-allel e-beam lithography,2cellular membrane mimics,3electrochemical probes of viable cells,4and scanningprobe tips.5Extensive studies using a plasma-enhancedchemical vapor deposition (PECVD) process have dem-onstrated that VACNF growth is highly deterministic inthat a single VACNF can be grown wherever an appro-priately sized catalyst particle is formed, generally byusing e-beam lithography (EBL) for patterning smallevaporated catalyst-metal dots,6–8and complete VACNFarrays can be grown at temperatures ∼700 °C.7,9The wallstructure of VACNFs is quite imperfect compared toconcentric hollow carbon nanotubes, consisting of disor-dered layers of sp2-bonded graphitic carbon with a “bam-boo-like” or “stacked-cup” cross-section.7,10–13The in-dividual VACNFs have a tip radius ratypically ∼15 nmand heights h of a few microns, depending on the growthduration.7,14Arrays of VACNFs with aspect ratios (h/ra)>100 have been grown and are moderately good fieldemitters as measured by a movable-probe method withthreshold fields (for 1 nA current) of 12–60 V/␮m.15a)Address all correspondence to this author.e-mail: [email protected]: 10.1557/JMR.2005.0106J. Mater. Res., Vol. 20, No. 4, Apr 2005 © 2005 Materials Research Society850As device size shrinks, however, scaled-down VACNFswith sharp tips are desired. The growth of VACNFs withsmaller tip diameters requires catalyst particles muchsmaller than those formed by the present EBL-patternedevaporation technique. Consequently, we investigatedVACNF growth from very small and nearly monodis-perse iron-based nanoparticles16that are two to threeorders of magnitude smaller in volume than the EBL-patterned catalyst particles previously used.17Here wereport results demonstrating that, as the size of catalystparticles gets smaller, graphitic nanofibers still can benucleated and grown by the direct-current PECVD (dc-PECVD) process, but their morphology and structurestrongly depend on their size and the local packing den-sity of the catalyst nanoparticles.II. EXPERIMENTALIron/platinum (Fe/Pt) alloy nanoparticles were pro-duced following the procedures reported by Sun et al.16The nanoparticles dissolved in hexane were spin-coatedon a 5-cm-diameter silicon wafer and dried in air. Theconcentration of the nanoparticles was controlled by di-lution such that the average separation between the nano-particles was about 300 nm. The Fe/Pt alloy nanopar-ticles have an average diameter of 3 (±1) nm and a com-position of ∼75 at.% iron and ∼25 at.% platinum, asdetermined using energy dispersive x-ray (EDX) analy-sis. Each crystalline nanoparticle is stabilized by ligands(oleic acid and oleyl amine). The ligands suppress reac-tions or aggregation of the nanoparticles in hexane. Thewafer then was cut into 0.5 × 1 cm pieces and transferredonto the center of a 5-cm-diameter heating susceptor inthe dc-PECVD chamber, which was subsequently evacu-ated down to ∼30 mtorr. During the following heatingstage, a flow of ammonia (99.99%) was maintained in thechamber. After the desired temperature was reached(measured using a thermocouple attached to the bottomof the susceptor), a mixed gas flow of acetylene (99.6%)and ammonia was introduced into the chamber. A dcvoltage then was applied between the susceptor whichacted as a cathode, and an anode located above it, gen-erating a glow discharge to start the growth process. Af-ter growth, the substrate was characterized using a high-resolution field emission scanning electron microscope(FESEM; Hitachi S4700, Japan). The growth productsalso were scratched off the substrate onto holey carbongrids and imaged using a high-resolution transmission elec-tron microscope (HRTEM; Hitachi HF2000, 200 keV).Cross-section samples were also prepared for HRTEMimaging. Composition information was acquired usingscanning Auger microanalysis (SAM; Physical Electronics Phi680 Nanoprobe, Chanhassen, MN). The pri-mary beam energy was 20 keV at 1 nA current, resultingin a probe beam diameter of 150 Å. To obtain bulkcomposition information, the growth products were insitu sputter-etched using 3.5 keV Ar ions for 1 min. Theetch rate was calibrated at 250 Å/min using a standardSiO2film. During the in situ sputter etch, the substratealso was rotated at 1 rpm to increase sputter uniformityand minimize