SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION Linking catalyst composition to chirality distributions of as-grown singlewalled carbon nanotubes by tuning Ni x Fe 1-x nanoparticles Supplementary Information Wei-Hung Chiang and R. Mohan Sankaran Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH Microplasma synthesis of Ni x Fe 1-x nanoparticles Figure 1S presents a summary of aerosol size classification results for various compositions of Ni x Fe 1-x nanoparticles synthesized in a microplasma. Each particle size distribution was fitted to a log-normal function in order to obtain the geometric mean diameter (D g ) and standard deviation ( g ). Below a particle diameter of 2.0 nm, size distributions could not be obtained because of the lower detection limit of the DMA-CPC system; however, we suspect that particles smaller than 2.0 nm can be synthesized in the microplasma. The particle mean diameter shifts to larger values as the metallocene vapor concentration is increased in the microplasma, allowing tunable diameters of particles with narrow size distributions to be prepared. At higher precursor concentrations, the density of radical moieties generated from dissociation of the metallocene vapor increases, accelerating particle nucleation and growth. To obtain NiFe bimetallic nanoparticles with tunable chemical composition at constant particle size, it was necessary to adjust both the relative concentrations of the metallocene vapors and the total metallocene flow rate 1. In the range of metallocene vapor concentrations explored here ( nature materials 1

2 supplementary information ppm), the microplasma was stable and the particle size distributions were highly reproducible. The mean diameter of as-grown metal particles acquired from aerosol measurements is found to depend linearly on the total metallocene vapor concentration in the microplasma (C m ). As indicated by the error bars, there is some overlap in the particle size distributions as the particle mean diameter is varied. However, the mean diameters are clearly differentiable at the different metallocene vapor concentrations used in the microplasma. EDX and XRD characterization of Ni x Fe 1-x nanoparticles To evaluate the chemical composition of individual Ni x Fe 1-x nanoparticles synthesized in a microplasma, we have carried out EDX analysis. Initially, a background spectrum was acquired for a bare TEM grid under the same working conditions used for particle samples and subtracted. The background was found to contain peaks for C and Cu, from the carbon-coated copper grids, in addition to spurious peaks for Fe, as a result of interference from the microscope lens elements 2. Spectra were then obtained from 3-5 nanoparticles, averaged, and background subtracted. Figure S2 shows spectra for three different compositions of Ni x Fe 1-x nanoparticles. The atomic composition was estimated from the ratio of peak intensities for Ni K and Fe K radiation in the EDX spectra corrected by their respective k factors. A peak attributed to O is present due to sample transfer in air from the reactor to the TEM. The Si peak is attributed to the Li-drift detector. Based on in situ aerosol measurements and ex situ EDX microcharacterization, we find that the composition of nanocatalysts is controlled by the microplasma synthesis route. The crystalline structure of the bimetallic nanoparticles was characterized by XRD. Broadening of the diffraction peaks was observed for all thin films, indicating that samples were composed of nanoparticles. In Fig. S3, diffraction peaks at o, o, and o of 2 2 nature MATERIALS

3 supplementary information value are ascribed to the (111), (200), and (220) crystalline planes of single-phase -Ni with facecentered cubic (fcc) structure. The corresponding lattice spacings calculated from Bragg s law are 0.20, 0.17, and 0.13 nm. This result is in good agreement with HRTEM analysis of pure Ni particles where the lattice spacing for the (111) plane was observed. Lattice constants can be calculated from d a/ (h k l ), where a is the lattice constant and h, k, l are the index hkl numbers of the Miller indices of crystalline plane (hkl). On the basis of the lattice parameter calculated from XRD, the nearest neighbor atom distance (i.e. Ni-Ni bond length) is obtained from the equation a fcc R 2, where a fcc is the fcc lattice constant and R is the bond length. The XRD patterns of Ni 0.67 Fe 0.33 and Ni 0.27 Fe 0.73 particles can be similarly indexed to a fcc structure with slight shifting of the Bragg peaks to lower angles, which is consistent with previous reports for bulk 3 and nanoparticle 4,5,6 alloys of NiFe. Using Bragg s law, the calculated lattice spacing for the (111) planes of Ni 0.27 Fe 0.73 particles is 0.21 nm, slightly larger than that obtained for Ni particles, and agrees with HRTEM observation. The absence of peaks at 2 values of o, o and o which are the expected positions of body-centered cubic (bcc) structure of -Fe confirms that the Ni 0.67 Fe 0.33 particles are truly alloyed. As the Fe content in the Ni x Fe 1-x particles increases, the respective fcc lattice constant and bond length increases, illustrating the expansion of the crystal as a result of Fe atom substitution. In the case of Ni 0.27 Fe 0.73 particles, two peaks slightly shifted from the bcc peaks for α-fe located at 2 equal to o and o in the XRD pattern were also observed. The bcc lattice constant is calculated from the diffraction peaks and a nearest neighbor atom distance (i.e. bond length) is obtained from the equation a bcc 2R/ 3, where a fcc is the bcc lattice constant and R is the bond length. The existence of both bcc α and fcc γ phases is expected at higher Fe atomic fractions in NiFe alloys from the reported phase diagram 7. Interestingly, the decrease of the bcc lattice constant nature materials 3

4 supplementary information has similarly been observed in NiFe nanoparticle alloys 4, but not in the bulk 3. A summary of XRD data and calculated results for lattice constants and bond lengths of various compositions and crystal structures of Ni x Fe 1-x nanoparticles are shown in Table S1. TEM of SWCNTs Thin films of SWCNTs for TEM analysis were obtained by either drop casting a suspension of SDS-dispersed SWCNTs [Fig. S4(a)] or depositing nanotubes exiting the flow furnace onto carbon-coated copper grids with an electrostatic precipitator (TSI Inc., Model 3089) [Fig. S4(b)]. Figure S4(a) shows CNTs grown using Ni catalyst particles with an average diameter of 2.0 nm; the sample consist primarily of SWCNTs with no larger diameter CNTs visible. The HRTEM image in Fig. S4(b) shows the presence of SWCNT bundles with overall diameters ranging from 5 to 10 nm and individual nanotubes with an outside diameter of approximately 1 nm. Micro Raman characterization of SWCNTs grown with Ni and Ni x Fe 1-x nanoparticles The high frequency region of micro Raman spectra obtained for SWCNTs grown at 600 o C with Ni and NiFe bimetallic nanoparticles are shown in Fig. S5(a). The relatively high and similar G to D band ratio confirms the presence of a large fraction of amorphous-free SWCNTs in each sample, independent of catalyst composition 8. The micro Raman spectrum obtained for commercial HiPCO product (purified) is included for comparison. 4 nature MATERIALS

5 supplementary information Micro Raman characterization of CNTs grown with Fe nanoparticles Micro Raman spectra of CNTs synthesized with Fe nanoparticles at the lower growth temperature (600 o C) are shown in Fig. S5(b). The lack of radial breathing mode (RBM) peaks for Fe-catalyzed samples and the relatively small G to D-band ratio indicates the presence of a large fraction of MWCNTs 8. Fe particles may not produce SWCNTs at the same process conditions used for Ni x Fe 1-x nanoparticles (e.g. particle size, reactor temperature, etc.) because of the higher carbon solubility in Fe as compared to Ni 9. The growth of SWCNTs depends on many process parameters including particle size, growth temperature, carbon feedstock gas, and pressure that must be tuned to obtain SWCNTs from Fe catalysts. We have found that SWCNTs are grown with Fe catalysts at 700 o C, albeit with low yields. SWCNT diameter estimation from RBM peaks in micro Raman spectra The radial breathing mode (RBM) region of the micro Raman spectra can be analyzed to determine the SWCNT diameter distribution in the various samples. For typical SWCNT bundles, the diameter, d t (nm) has been related to the RBM frequency, R (cm -1 ), using a correlation of the form RBM = A/d t + B, where A and B are empirical parameters 8,10,11,12,13. Based on the similarity of our experimental synthesis and sample preparation to that reported by Bachilo et al., we have used their parameters of A=223.5 and B=12.5 to obtain the diameters of nanotubes detected by Raman measurements (see Table S2). Photoluminescence spectroscopy of SWCNTs We applied a semi-quantitative analysis to the PL results to estimate the relative abundance of individual semiconducting nanotubes. Although the PL brightness has been found nature materials 5

6 supplementary information to be approximately the same for different semiconducting structures 14, we have used a recently reported model that predicts the quantum efficiency and absorption extinction coefficients for specific chiralities 15. To minimize the effect of a varying background, the fraction of SWCNTs is estimated from the partial derivative of the PL peak intensity 16. The calculated abundances of semiconducting SWCNTs (n,m) in the samples grown with varying compositions of NiFe nanocatalysts are summarized in Table S3 and plotted in Fig. S7(a). The NiFe nanocatalysts with a Ni concentration more than 50 at% clearly produce broad distributions of semiconducting SWCNTs including (7,6), (9,4), (8,4), (10,3), (8,6), (9,5), (7,5) and (8,7), while those with 50 at% Ni or less exhibit a narrower chirality distribution with mostly (6,5), (7,5), and (8,4). Remarkably, the abundance of (8,4) in the Ni 0.27 Fe 0.73 sample is nearly 40%, significantly higher than that found in either HiPCO (4.2%) or CoMoCAT 17 (14%) products. Figures S7(b) and (c) show the calculated distributions of the diameter and the chiral angle, respectively, for the as-grown SWCNT samples. It is evident that the distributions of nanotubes in the samples grown from nanocatalysts with lower Ni atomic concentration (Ni 0.5 Fe 0.5 and Ni 0.27 Fe 0.73 ) are narrow not only in diameter, but also in chiral angle. The average nanotube diameter in Ni 0.5 Fe 0.5 and Ni 0.27 Fe 0.73 nanocatalyst samples is approximately 0.82 nm, significantly smaller than the average of 0.90 nm found in the Ni and Ni 0.67 Fe 0.33 nanocatalyst samples. The result indicates that an increase in the Fe content in NiFe alloyed catalysts leads to a shift in the diameter distribution to smaller values, which agrees with Raman analysis (see Fig. 3). In comparison, the chiral angles of the SWCNTs in our samples are predominately distributed in the higher angle region close to armchair type, especially in the case of nanocatalyst samples with higher Fe atomic concentrations. Despite the diameter dependence, only small variations are found in the chiral angle distribution as a function of Fe content in the nanocatalyst. Previous experimental findings by Bachilo 17 and Miyauchi 18 for CoMo and FeCo 6 nature MATERIALS

7 supplementary information catalyst, respectively, are in accordance with these results even though our reactor conditions are drastically different. Therefore, it appears that while the nanotube diameter is sensitive to catalyst composition, the preferred chiral angle of SWCNTs in catalytic CVD processes is the armchair structure. nature materials 7

8 supplementary information Fe Ni Ni 0.67 Fe 0.33 Ni 0.27 Fe 0.73 D g (nm) Particle size for SWCNT growth C m (ppm) Figure S1. Aerosol size classification of as-grown Ni x Fe 1-x nanoparticles. Geometric mean diameter (D g ) of nanoparticles obtained from aerosol size classification versus total metallocene vapor concentration (C m ) in the Ar microplasma. The atomic fraction of Ni and Fe were calculated from the vapor concentrations of nickelocene and ferrocene, respectively. 1 Error bars indicate the standard deviations of the particle size distributions. 8 nature MATERIALS

9 supplementary information Ni 0.27 Fe 0.73 Ni Ni Fe Ni Fe Counts (A. U.) Ni 0.67 Fe 0.33 Ni C Si Cu Cu O Energy (kev) Figure S2. EDX spectra of Ni x Fe 1-x nanoparticles. nature materials 9

10 supplementary information (110) Fe Intensity (A. U.) (111) (111) (110) (200) (200) Ni 0.27 Fe 0.73 Ni 0.67 Fe 0.33 (111) (200) Ni (220) (200) (220) (220) (degree) Figure S3. XRD spectra of Ni x Fe 1-x nanoparticles. 10 nature MATERIALS

11 supplementary information Catalyst fcc bcc Diffraction peak Bond Diffraction peak (degree) a fcc length (degree) a bcc (nm) (nm) (111) (200) (220) (nm) (110) (200) Ni Ni 0.67 Fe Bond length (nm) Ni 0.27 Fe Fe Table S1. Summary of XRD results and analysis. The lattice constant is calculated for facecentered cubic (fcc) and body-centered cubic (bcc) crystal structures from the respective diffraction peaks and indicated as a fcc and a bcc, respectively. The bond lengths are the nearest neighbor atom distances estimated from the respective lattice constants using the equations a fcc 2R for fcc anda bcc 2R/ 3 for bcc structures. nature materials 11

12 supplementary information (a) 10 nm (b) 5 nm Figure S4. TEM images of SWCNTs. a,b Low (a) and high magnification (b) TEM images of SWCNTs nature MATERIALS

13 supplementary information Ni 0.27 Fe 0.73 (a) Ni 0.5 Fe 0.5 (b) D G Intensity (A. U.) Ni 0.67 Fe 0.33 Ni Intensity (A. U.) RBM HiPCO G D Raman shift (cm -1 ) Raman shift (cm -1 ) Figure S5. Micro Raman characterization of CNTs. a,b Micro Raman spectra for CNTs catalyzed at 600 o C by monometallic Ni and NiFe bimetallic nanoparticles (a) and monometallic Fe nanoparticles (b). nature materials 13

14 supplementary information Excitation wavelength (nm) Temperature ( o C) Catalyst RBM (cm -1 ) d t (nm) Ni Ni 0.67 Fe Ni 0.5 Fe Ni 0.27 Fe Ni 0.27 Fe Fe nature MATERIALS

15 supplementary information Ni Ni 0.27 Fe Ni Ni 0.27 Fe Table S2. Summary of results for SWCNT diameters obtained from micro Raman spectra. Estimation of the diameters of SWCNTs grown with Ni x Fe 1-x catalysts using the correlation RBM = 223.5/d t nature materials 15

16 supplementary information Catalyst (n.m) Diameter (nm) Chiral angle (Deg.) Amplitude of the partial derivative of PL intensity Calculated PL Intensity Relative Abundance (%) Ni (6,5) (8,3) (7,5) (8,4) (10,2) (7,6) (9,4) (10,3) (8,6) (9,5) (8,7) Ni 0.67 Fe 0.33 (6,5) (8,3) (7,5) (8,4) (10,2) (7,6) (9,4) (10,3) (8,6) (9,5) (8,7) Ni 0.5 Fe 0.5 (6,5) (8,3) (7,5) (8,4) (10,2) nature MATERIALS

17 supplementary information (7,6) Ni 0.27 Fe 0.73 (6,5) (8,3) (7,5) (8,4) (10,2) (7,6) Table S3. Summary of results for the relative chirality abundance of SWCNTs. Calculations for the amplitude of the partial derivative of the PL intensities, corrected PL intensities using the electro-phonon interaction model 15, and (n,m) relative abundance of SWCNT samples grown with Ni x Fe 1-x catalysts at a constant particle size of 2.0 nm. nature materials 17

18 supplementary information (a) Relative abundance (%) Ni 0.27 Fe 0.73 Ni 0.5 Fe 0.5 Ni 0.67 Fe 0.37 Ni 0 (6,5) (8,3) (7,5) (8,4) (10,2) (7,6) (9,4) (10,3) CNT Chirality (n,m) (8,6) (9,5) (8,7) (b) 40 Ni 0.27 Fe 0.73 (c) 40 Ni 0.27 Fe Ni 0.5 Fe Ni 0.5 Fe 0.5 Abundance (%) Ni 0.67 Fe 0.33 Abundance (%) Ni 0.67 Fe Ni 0 40 Ni Diameter (nm) Chiral angle (Deg.) Figure S6. Chirality distributions of SDS-dispersed SWCNTs. a-c, The relative chiral abundance (a), diameter (b) and chiral angle (c) distributions of SDS-dispersed SWCNT samples grown with compositionally-tuned Ni x Fe 1-x nanocatalysts at a constant mean particle diameter of 2.0 nm. 18 nature MATERIALS

19 supplementary information References 1 Torres-Gomez, L. A., Barreiro-Rodriguez, G., and Mendez-Ruiz, F., Vapor-pressures and enthalpies of sublimation of ferrocene, cobaltocene and nickelocene. Thermochim. Acta 124, (1988). 2 Williams, D. B. and Carter, C. B., Transmission Electron Microcopy. (Plenum Press, New York, 1996). 3 McKeehan, L. W., The crystal structure of iron-nickel alloys Phys. Rev. 21, (1923). 4 Li, X. G., Chiba, A., and Takahashi, S., Preparation and magnetic properties of ultrafine particles of Fe-Ni alloys. J. Magn. Magn. Mater. 170 (3), (1997). 5 Kuhrt, C. and Schultz, L., Phase formation and martensitic-transformation in the mechanically alloyed nanocrystalline Fe-Ni. J. Appl. Phys. 73 (4), (1993). 6 Shafi, K., Gedanken, A., Goldfarb, R. B., and Felner, I., Sonochemical preparation of nanosized amorphous Fe-Ni alloys. J. Appl. Phys. 81 (10), (1997). 7 Hansen, M. and Anderko, K., Constitution of Binary Alloys. (McGraw-Hill Book Company, New York, 1958). 8 Dresselhaus, M. S., Dresselhaus, G., Saito, R., and Jorio, A., Raman spectroscopy of carbon nanotubes. Phys. Rep.-Rep. Sec. Phys. Lett. 409 (2), (2005). 9 Rummeli et al., Catalyst volume to surface area constraints for nucleating carbon nanotubes, J. Phys. Chem. B 111 (28), (2007). 10 Yu et al., (n,m) Structural assignments and chirality dependence in single-wall carbon nanotube Raman scattering, J. Phys. Chem B 105 (29), (2005). 11 Alvarez et al., Resonant Raman study of the structure and electronic properties of single-wall carbon nanotubes, Chem. Phys. Lett. 316, (2000). 12 Bachilo et al., Stucture-assigned optical spectra of single-walled carbon nanotubes, Science 298, (2002). 13 J. Roberston et al., Growth and characterization of high-density mats of single-walled carbon nanotubes for interconnects, Appl. Phys. Lett. 93, (2008). nature materials 19

20 supplementary information 14 Tsyboulski, D. A. et al., Structure-dependent fluorescence efficiencies of individual singlewalled cardon nanotubes. Nano Lett. 7 (10), (2007). 15 Oyama, Y. et al., Photoluminescence intensity of single-wall carbon nanotubes. Carbon 44 (5), (2006). 16 Arnold, M. S. et al., Sorting carbon nanotubes by electronic structure using density differentiation. Nat. Nanotechnol. 1 (1), (2006). 17 Bachilo, S. M. et al., Narrow (n,m)-distribution of single-walled carbon nanotubes grown using a solid supported catalyst. J. Am. Chem. Soc. 125 (37), (2003). 18 Miyauchi, Y. H. et al., Fluorescence spectroscopy of single-walled carbon nanotubes synthesized from alcohol. Chem. Phys. Lett. 387 (1-3), (2004). 20 nature MATERIALS