Growth and properties of Si–N–C–O nanocones and graphitic nanofibers synthesized using three-nanometer diameter iron/platinum -catalyst

H. Cuia) Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 X. Yang Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996 H.M. Meyer and L.R. Baylor Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 M.L. Simpson Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996; and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 W.L. Gardner and D.H. Lowndes Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 L. An and J. Liu Department 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 graphitic nanofibers were synthesized on silicon substrates using iron/platinum alloy as the catalyst in a direct-current plasma enhanced chemical vapor deposition reactor. The catalyst nanoparticles were monodisperse in size with an average diameter of 3 (±1) nm. The nanocones were produced on laterally widely dispersed catalyst particles and were oriented perpendicular to the substrate surface with an amorphous internal structure. The nanocones were produced by gas phase mixing and deposition of plasma-sputtered silicon, nitrogen, , and oxygen species on a central backbone nucleated by the Fe–Pt catalyst particle. Field emission measurements showed that a very high turn-on electric field was required for electron emission from the nanocones. In contrast, the graphitic nanofibers that were produced when silicon sputtering and redeposition were minimized had the “stacked-cup” structure, and well-defined voids could be observed within nanofibers nucleated from larger catalyst particles.

I. INTRODUCTION that a single VACNF can be grown wherever an appro- Growth of isolated and vertically aligned carbon nano- priately sized catalyst particle is formed, generally by using e-beam lithography (EBL) for patterning small fibers (VACNFs) is interesting for fundamental under- 6–8 standing of nanomaterial growth mechanisms and for po- evaporated catalyst-metal dots, and complete VACNF ∼ 7,9 tential practical applications that include field-emission arrays can be grown at temperatures 700 °C. The wall cathodes for vacuum nanoelectronic devices1 highly par- structure of VACNFs is quite imperfect compared to 2 3 concentric hollow carbon nanotubes, consisting of disor- allel e-beam lithography, cellular membrane mimics, 2 4 dered layers of sp -bonded graphitic carbon with a “bam- electrochemical probes of viable cells, and scanning 7,10–13 probe tips.5 Extensive studies using a plasma-enhanced boo-like” or “stacked-cup” cross-section. The in- ∼ chemical vapor deposition (PECVD) process have dem- dividual VACNFs have a tip radius ra typically 15 nm and heights h of a few microns, depending on the growth onstrated that VACNF growth is highly deterministic in 7,14 duration. Arrays of VACNFs with aspect ratios (h/ra) a)Address all correspondence to this author. >100 have been grown and are moderately good field e-mail: [email protected] emitters as measured by a movable-probe method with DOI: 10.1557/JMR.2005.0106 threshold fields (for 1 nA current) of 12–60 V/␮m.15

850 J. Mater. Res., Vol. 20, No. 4, Apr 2005 © 2005 Materials Research Society H. Cui et al.: Growth and properties of Si–N–C–O nanocones and graphitic nanofibers

As device size shrinks, however, scaled-down VACNFs composition information, the growth products were in with sharp tips are desired. The growth of VACNFs with situ sputter-etched using 3.5 keV Ar ions for 1 min. The smaller tip diameters requires catalyst particles much etch rate was calibrated at 250 Å/min using a standard smaller than those formed by the present EBL-patterned SiO2 film. During the in situ sputter etch, the substrate evaporation technique. Consequently, we investigated also was rotated at 1 rpm to increase sputter uniformity VACNF growth from very small and nearly monodis- and minimize redeposition.18 Field-emission properties perse iron-based nanoparticles16 that are two to three of the nanocones were measured in a vacuum chamber orders of magnitude smaller in volume than the EBL- with a base pressure of 2 × 10−6 Torr. A tungsten probe patterned catalyst particles previously used.17 Here we with a tip diameter of about 2 ␮m was used as an anode. report results demonstrating that, as the size of catalyst The probe position was computer controlled with a mini- particles gets smaller, graphitic nanofibers still can be mum step size of 75 nm in the X-Y-Z directions, and the nucleated and grown by the direct-current PECVD (dc- probe-substrate distance was 3 ␮m, as determined after PECVD) process, but their morphology and structure field emission measurement. strongly depend on their size and the local packing den- sity of the catalyst nanoparticles. III. RESULTS AND DISCUSSION Figure 1 shows a FESEM image of well-separated II. EXPERIMENTAL growth products taken at 45° tilt angle. The growth con- Iron/platinum (Fe/Pt) alloy nanoparticles were pro- ditions are listed in Table I(a). The growth products have duced following the procedures reported by Sun et al.16 a distinctly cone-shaped morphology and are thus called The nanoparticles dissolved in hexane were spin-coated nanocones to distinguish them from VACNFs. They on a 5-cm-diameter silicon wafer and dried in air. The grow perpendicular to the substrate with a smooth outer concentration of the nanoparticles was controlled by di- surface and with the catalyst nanoparticles forming sharp lution such that the average separation between the nano- tips (see Fig. 2), except a few with rounded tips that particles was about 300 nm. The Fe/Pt alloy nanopar- resulted from losing their nanoparticles and subsequent ticles have an average diameter of 3 (±1) nm and a com- plasma etching. Table I(b) shows the geometrical param- position of ∼75 at.% iron and ∼25 at.% platinum, as eters of the nanocones at the specific growth conditions determined using energy dispersive x-ray (EDX) analy- listed in Table I(a). For comparison, VACNFs have as- sis. Each crystalline nanoparticle is stabilized by ligands pect ratios ranging from 150 to 700 and tip cross-section (oleic acid and oleyl amine). The ligands suppress reac- full angles less than 10°.15 As Table I(b) shows, the tions or aggregation of the nanoparticles in hexane. The nanocones have smaller aspect-ratio and larger tip cross- wafer then was cut into 0.5 × 1 cm pieces and transferred section full-angle than typical VACNFs. onto the center of a 5-cm-diameter heating susceptor in Figure 2(a) shows a nanocone that separated from the the dc-PECVD chamber, which was subsequently evacu- substrate and is protruding out of amorphous resin intro- ated down to ∼30 mtorr. During the following heating duced during transmission (TEM) stage, a flow of ammonia (99.99%) was maintained in the cross-section preparation. A Fe/Pt nanoparticle is located chamber. After the desired temperature was reached at the tip of the nanocone so that its diameter defines the (measured using a thermocouple attached to the bottom tip sharpness. In contrast to VACNFs, the body of the of the susceptor), a mixed gas flow of acetylene (99.6%) nanocone lacks a hollow core and is highly defective. and ammonia was introduced into the chamber. A dc voltage then was applied between the susceptor which acted 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 products also were scratched off the substrate onto holey carbon grids and imaged using a high-resolution transmission elec- tron microscope (HRTEM; Hitachi HF2000, 200 keV). Cross-section samples were also prepared for HRTEM imaging. Composition information was acquired using scanning Auger microanalysis (SAM; Physical Elec tronics Phi680 Nanoprobe, Chanhassen, MN). The pri- mary beam energy was 20 keV at 1 nA current, resulting FIG. 1. FESEM image of nanocones on a silicon substrate viewed at in a probe beam diameter of 150 Å. To obtain bulk 45° tilt.

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TABLE I. (a) Growth conditions of the sample in Fig. 1; (b) geo- as-received material and the bulk composition of the cone metrical parameters of the corresponding nanocones surveyed over ten following the Ar ion sputter cleaning. The table clearly nanocones. A value of 1.5 nm is used as the tip radius. shows that the majority component of the nanocone is (a) silicon, followed by nitrogen, carbon, and oxygen. The silicon must be from the silicon substrate, which is sput- C2H2/NH3 Temperature Pressure flow rate Current Growth time tered and etched during the growth process, since there is (°C) (Torr) (sccm) (mA) (min) no active silicon supply gas. Subsequently, the silicon 700 3 30/80 100 8 atoms are re-deposited on the growth structure. The sili- con substrate etching is further supported by the obser- (b) vation of the silicon cone formed beneath the nanocone Base Ratio of Ratio of Tip [see Figs. 2(c) and 2(d)] since the nanocone behaves like Height diameter height/tip height/base cross-section a physical mask, shielding the area it covers from etching (nm) (nm) radius radius full angle (°) during the growth process. The relative compositions of 163 ± 9 nm 47 ± 3 nm 109 ± 6 7.0 ± 0.4 16.3 ± 0.9 nanocone between the surface and the bulk, however, are different. The surface appears to have a richer composi- tion of nitrogen, carbon, and oxygen than the bulk. This Figure 2(b) shows a section of a nanocone with a round is most likely caused by gas-phase deposition of the am- tip and no Fe/Pt nanoparticle attached to it. Figure 2(c) monia and acetylene gas during the period after turning shows another nanocone with its base attached to the off the dc glow discharge and complete evacuation, silicon substrate. Due to its own amorphous structure, it which stops the substrate sputtering and, consequently, is difficult to distinguish the nanocone from the sur- the silicon supply. The oxygen could be from both leak- rounding amorphous resin, but its base area has a darker ing during the growth and exposure to the atmosphere contrast than its body. A HRTEM image [Fig. 2(d)] when the sample was taken out of the growth chamber. reveals that the dark contrast is caused by a well- Growth of nanocones on the silicon substrate has not crystallized internal silicon cone, whose structure is con- been observed without the presence of Fe/Pt nanopar- tinuous with the silicon substrate. The composition of the ticles. In combination with the TEM data, it is clear that nanocone, as revealed by SAM in the next paragraph, the nanoparticles define nucleation and growth sites for may further contribute to the contrast difference between the nanocones. The nanoparticles are crystalline after the crystalline internal silicon cone and the amorphous growth, as shown in Fig. 2(e). In the HRTEM image, the nanocone body. nanoparticle is located on an amorphous holey carbon Table II shows composition information acquired us- film, and the crystalline structure of the nanoparticle is ing SAM from an as-received nanocone (about three clearly seen. Our experiments reveal that the Fe/Pt nano- times as large as those shown in Fig. 1) and after 1 min particles are highly active and can nucleate nanocones of sputter cleaning (Fig. 3). Because the Auger analysis even at temperatures as low as 400 °C. However, the technique is surface sensitive, Table II reveals the sur- growth rate is low, compared with the nanocones grown face composition information of the nanocone for the at 700 °C. These small nanoparticles do not appear to

FIG. 2. TEM images of nanocones nucleated by 3-nm Fe–Pt nanoparticles dispersed on a silicon substrate: (a) a nanocone with its tip section hanging over a hole, with the inset showing an enlarged view of the nanoparticle; (b) a nanocone with rounded tip due to plasma etching following loss of its nanoparticle; (c) a nanocone attached on the silicon substrate; (d) the base region of the nanocone in (c); and (e) a catalyst nanoparticle on an film after nanocone growth.

852 J. Mater. Res., Vol. 20, No. 4, Apr 2005 H. Cui et al.: Growth and properties of Si–N–C–O nanocones and graphitic nanofibers

TABLE II. Surface and bulk composition of a nanocone in atomic nanoparticles with different sizes. These images clearly percent, determined by scanning Auger microanalysis at a location indicate that the catalyst nanoparticles are located at near the center of a nanocone. the tips of the nanofibers. These nanofibers are composed Elements Surface Bulk of graphitic planes with “cup-like” morphology, which Si 42.39 79.78 are stacked along their growth axis. For comparison, N 20.84 10.94 Fig. 4(d) shows a nanofiber structure nucleated by a pure C 18.37 5.48 iron nanoparticle. O 18.40 3.80 The large catalyst particles in Fig. 4 have a “pear” or “tear-drop” shape with their pointed end toward the di- rection of precipitation, but the small catalyst nanoparticles are nearly round. The graphitic planes in all the nanofibers have curvature similar to their correspond- ing catalyst particles. Central voids in the can be seen and become more visible in the larger nano- fibers nucleated by a 15-nm-diameter Fe–Pt nanoparticle and by a 23-nm-diameter Fe nanoparticle. It has been suggested that the bamboo-like structure is formed when the catalyst particle is pushed upward out of its graphite sheath to the top of the tube19 by carbon precipitation and thus leaves voids in the structure. This abrupt movement of the catalyst particle has recently been observed via time-resolved in-situ TEM.20 Our results suggest that the abrupt movement of the catalyst particle and the subse- quent void formation are dependent on the size of the catalyst particles: the size of the void increases with that of the catalyst particle. A possible explanation for this is FIG. 3. SEM image of a nanocone taken at 45° tilt after 1 min sputter that the stress built up between the catalyst particle and cleaning. The nanocone is about three times as large as those shown in the precipitated graphite is larger for the large catalyst Fig. 1. The large size of the nanocone prevents the contribution of the particle; consequently the catalyst particle jumps a larger substrate to the Auger signal. distance, resulting in a larger void. The above observations show dramatic differences be- break down into smaller nanoparticles during the heat- tween the structures due to the influence of silicon sub- ing process; thus one nanoparticle nucleates only one strate etching and redeposition. In an experiment similar nanocone. Their catalytic properties also are retained to that above but without acetylene present, nanocones even after a growth process is terminated and then were not observed after growth, suggesting that silicon restarted. from substrate cannot produce vertical growth and car- To understand the growth mechanism, we also used bon is required for the vertical growth of the nanocone the tip of an atomic force microscope (AFM) as a sub- structure. We believe that carbon atoms diffuse through strate. The small size of the AFM cantilever facilitates or upon the catalyst nanoparticle and precipitate in gra- easy manipulation in TEM. The pyramid-shaped tip pro- phitic form, accounting for the vertical growth of both vides edges that can be used as a platform to hold catalyst nanocone and graphitic nanofiber. However, silicon re- particles. The wedge-shape of the tip also reduces rede- deposition and ion bombardment have a strong impact on position of the sputtered silicon on the nanostructures the graphitic structure during nanocone growth, eventu- nucleated by the catalyst particles. Concentrated solu- ally destroying its crystalline nature. With other ele- tions of the Fe–Pt catalyst particles were dropped on the ments (N, O) from the gas phase, this surface deposition tip and formed a dense layer of catalyst particles on the results in the amorphous structure and cone-like mor- tip surface, further reducing silicon substrate etching by phology, accounting for the lateral growth of the nano- reducing substrate exposure. As a result, the influence of cone structure. the substrate sputtering and redeposition was minimized. Previous studies revealed a strong influence of the

At the same time, the closely packed Fe–Pt nanoparticles C2H2:NH3 ratio on the growth of the carbon nanofibers, tend to agglomerate into a variety of larger particles at which changed from cylinder-like to cone-like morphol- 21 elevated temperature during (or preceding) the dc- ogy with increasing C2H2:NH3 ratio. This observation PECVD growth, so the dependence on catalyst par- also was explained as a result of competition between ticle size could be observed. Figures 4(a)–4(c) show vertical growth and lateral growth; the dominance of lat-

TEM images of the nanofibers nucleated by Fe–Pt eral growth from carbon deposition at high C2H2:NH3

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FIG. 4. TEM images (a), (b), and (c) show carbon nanofibers nucleated using Fe–Pt nanoparticles with different sizes. For comparison, (d) shows a nucleated using a pure iron nanoparticle. ratio resulted in cone-shaped morphology. Although sili- software (Integrated Engineering Software, Inc., Win- con substrate etching was also observed22,23 and silicon nipeg, Canada). The simulation gives a turn-on voltage was found in the carbon nanofibers structures,24,25 due to of about 60 V/␮m for a pure graphite emitter with di- the large catalyst particles and consequently large carbon mensions taken from Table I and our measurement con- nanofiber length (a few micrometers), the influence of ditions, which is much lower than the experimental value silicon on the growth morphology was not considered. for nanocones. However, the calculation is consis- However, as reported here, nanoconesare observed at tent with the field emission behavior of truly graphitic low C2H2:NH3 ratio (10:80 sccm), strongly suggesting VACNFs of this shape. This is believed to be a direct that, as the size of the catalyst particles is reduced, silicon consequence of the insulating nature of the Si–N–C–O etched from the substrate has a dramatic influence on the composition of the nanocones, causing high local field morphology, structure, and consequently properties of required for field emission. the nominally carbon nanofiber growth. Figure 5 shows a current–voltage characteristic of IV. CONCLUSIONS electric field emission measurement on the nanocones, In summary, cone-shaped nanostructures have been while the inset is a Fowler–Nordheim plot showing a nucleated and grown using 3-nm-diameter Fe/Pt alloy linear behavior indicative of a field emission process. catalyst nanoparticles with high lateral separation on sili- The turn-on voltage for 1 nA current is 270 V/␮m, ob- con substrates. The nanocones grow perpendicular to the tained by dividing the applied voltage between the substrate surface and have uniform geometries with a nanocones and the probe anode by their separation. To smooth outer surface and the catalyst nanoparticles lo- understand the field emission behavior, field emission cated at their tips. The silicon substrate is etched dur- simulations were performed using the Lorentz 2D ing nanocone growth and the resulting Si flux supplies the main component of the nanocone’s composition. On the other hand, graphitic nanofibers are grown using the Fe–Pt catalyst particles as silicon sputtering and redepo- sition is minimized. The conical shape of the nanocones is thus formed by gas phase deposition of amorphous Si–N–C–O on a core nucleated by the catalyst particle during its growth. However, the nanocone has an amor- phous internal structure due to the damaging impact of ion bombardment and silicon deposition during growth. Field emission measurements show that a large turn-on electric field is required for the nanocones to emit electrons.

ACKNOWLEDGMENTS The authors would like to thank J.T. Luck for TEM FIG. 5. Field emission I-V characteristic of the nanocones. cross-section sample preparation. This research was

854 J. Mater. Res., Vol. 20, No. 4, Apr 2005 H. Cui et al.: Growth and properties of Si–N–C–O nanocones and graphitic nanofibers supported by the Office of Basic Energy Sciences, Divi- 10. H. Cui, O. Zhou, and B.R. Stoner: Deposition of aligned bamboo- sion of Materials Sciences, United States Department of like carbon nanotubes via microwave plasma enhanced chemical vapor deposition. J. Appl. Phys. 88, 6072 (2000). Energy (U.S. DOE), and by the Defense Advanced 11. C. Bower, O. Zhou, W. Zhu, D.J. Werder, and S.H. Jin: Nuclea- Research Projects Agency under Contract No. 1868HH26X1 tion and growth of carbon nanotubes by microwave plasma chemi- with Oak Ridge National Laboratory (ORNL). The re- cal vapor deposition. Appl. Phys. Lett. 77, 2767 (2000). search was carried out at ORNL, managed by UT- 12. H. Cui, X. Yang, M.L. Simpson, D.H. Lowndes, and M. Verela: Battelle, LLC, for the U.S. DOE under Contract No. Initial growth of vertically aligned carbon nanofibers. Appl. Phys. Lett. 84, 4077 (2004). DE-AC05-00OR22725. This research also was supported 13. Y.Y. Wang, G.Y. Tang, F.M. Koeck, B. Brown, J.M. Garguilo, in part by an appointment (H. Cui) to the ORNL Post- and R.J. Nemanich: Experimental studies of the formation process doctoral Research Associates Program, which is spon- and morphologies of carbon nanotubes with bamboo mode struc- sored by ORNL and administered jointly by ORNL and tures. Relat. Mater. 13, 1287 (2004). by the Oak Ridge Institute for Science and Education 14. V.I. Merkulov, A.V. Melechko, M.A. Guillorn, D.H. Lowndes, and M.L. Simpson: Sharpening of carbon nanocone tips during under Contract Nos. DE-AC05-84OR21400 and DE- plasma-enhanced chemical vapor growth. Chem. Phys. Lett. 350, AC05-76OR00033, respectively. 381 (2001). 15. L.R. Baylor, V.I. Merkulov, E.D. Ellis, M.A. Guillorn, D.H. Lowndes, A.V. Melechko, M.L. Simpson, and J.H. Whealton: Field emis- REFERENCES sion from isolated individual vertically aligned carbon nanocones. J. 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