Chemical vapor deposition synthesis of N-, P-, and Si-doped single-walled carbon nanotubes.

Abstract

Here we report the synthesis of single-walled carbon nanotube bundles by chemical vapor deposition in the presence of electron donor elements (N, P, and Si). In order to introduce each dopant into the graphitic carbon lattice, different precursors containing the doping elements (benzylamine, pyrazine, triphenylphosphine, and methoxytrimethylsilane) were added at various concentrations into ethanol/ferrocene solutions. The synthesized nanotubes and byproduct were characterized by electron microscopy and Raman spectroscopy. Our results reveal intrinsic structural and electronic differences for the N-, P-, and Si- doped nanotubes. These tubes can now be tested for the fabrication of electronic nanodevices, and their performance can be observed.

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Chemical Vapor Deposition Synthesis of
N-, P-, and Si-Doped Single-Walled
Carbon Nanotubes
Jessica Campos-Delgado,†Indhira O. Maciel,‡David A. Cullen,§David J. Smith,§Ado Jorio,‡,?
Marcos A. Pimenta,?Humberto Terrones,?and Mauricio Terrones?,?,*
†AdvancedMaterialsDepartment,IPICYT,CaminoalaPresaSanJose ´ 2055,Col.Lomas4aseccio ´n,78216,SanLuisPotosı ´,SLP,Mexico,‡Divisa ˜odeMetrologiadeMateriais,
InstitutoNacionaldeMetrologia,Normalizac ¸a ˜oeQualidadeIndustrial(INMETRO),DuquedeCaxias,RJ,25250-020Brazil,§SchoolofMaterialsandDepartmentofPhysics,
ArizonaStateUniversity,Tempe,Arizona85287,?DepartamentodeFı ´sica,UniversidadeFederaldeMinasGerais,BeloHorizonte,MG,31270-901Brazil,?SociedadMexicana
deNanocienciasyNanotecnología,SOMENANO,SierradelaCruz144,Col.Lomas4asección,C.P.78216,SanLuisPotosí,S.L.P.,Mexico,and?TheDepartmentofPhysicsand
Mathematics,DivisionofScience,ArtsandTechnology,UniversidadIberoamericana,AvenidaProlongaciónPaseodelaReforma880,SantaFé012100,DF,Mexico
T
ductor industry. The p-type and n-type dop-
ing of carbon nanotubes is possible if carbon
atoms are substituted with other atomic spe-
ciescontaininglessormoreelectrons,respec-
tively. The most common dopants of carbon
nanotubes have been boron and
nitrogen,1?10which are the nearest neigh-
borsofcarbonintheperiodictable,ingroups
13 and 15, respectively.
Nitrogen doping in multi-walled carbon
nanotubes (MWNTs) induces the so-called
“bamboo-like” morphologies,5?10and theo-
retical calculations of substitutional nitrogen
doping in single-walled carbon nanotubes
(SWNTs) have revealed a different electronic
structure, caused by introducing donor-like
features into the conduction band.6
The production of N-doped SWNTs has
been achieved by arc discharge involving
graphite?metal?melamine electrodes9and
through chemical vapor deposition (CVD)10
by adding benzylamine to a ferrocene/etha-
ailoring the electronic structure of
materials by adding foreign atoms
has long been known in the semicon-
nol solution. In this work, we demonstrate
the use of an alternative nitrogen precursor
(pyrazine) in order to synthesize N-doped
SWNTs, which results in improved nitrogen
doping and better quality material.
In the literature, one can also find experi-
mental reports related to phosphorus?
nitrogen heterodoping of MWNTs.11Theo-
retically, the substitutional doping of SWNTs
by phosphorus is energetically favorable, and
experimentally, it has been studied by Ra-
manspectroscopy.12,13However,thedetailed
synthesis and electron microscopy character-
ization of P-doped SWNTs have not been re-
ported hitherto.
Silicon doping of fullerenes and fullerene-
likenanostructureshasbeenachievedexperi-
mentally and was reported in the late
1990s.14,15The incorporation of silicon spe-
cies into the hexagonal lattice of SWNTs was
proposed theoretically by Baierle et al. in
2001.16These calculations showed that sub-
stitutional Si doping of SWNTs introduces
donor-like states above the Fermi level. To
the best of our knowledge, the synthesis of
Si-doped SWNTs has not been reported.
While electron microscopy and elemental
analysis provide direct evaluation of sample
morphology and chemical content, respec-
tively, Raman spectroscopy has proven to be
a powerful nondestructive tool to character-
ize SWNTs.17?19For example, the radial
breathing mode (RBM) signal (?100?400
cm?1) allows the determination of the tube
diameter distribution in bundles of SWNTs
when different laser lines are used to excite
the samples. Interestingly, the morphology
of the tangential mode (G band), observed
at ?1500?1600 cm?1, changes for metallic
*Address correspondence to
mtterrones@gmail.com.
Received for review November 11, 2009
and accepted February 15, 2010.
Published online March 4, 2010.
10.1021/nn901599g
© 2010 American Chemical Society
ABSTRACT Here we report the synthesis of single-walled carbon nanotube bundles by chemical vapor
depositioninthepresenceofelectrondonorelements(N,P,andSi).Inordertointroduceeachdopantintothe
graphiticcarbonlattice,differentprecursorscontainingthedopingelements(benzylamine,pyrazine,
triphenylphosphine,andmethoxytrimethylsilane)wereaddedatvariousconcentrationsintoethanol/ferrocene
solutions.ThesynthesizednanotubesandbyproductwerecharacterizedbyelectronmicroscopyandRaman
spectroscopy. Our results reveal intrinsic structural and electronic differences for the N-, P-, and Si- doped
nanotubes.Thesetubescannowbetestedforthefabricationofelectronicnanodevices,andtheirperformance
canbeobserved.
KEYWORDS: SWNTs · doping · phosphorus · silicon · nitrogen
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and semiconducting nanotubes. The D band
(observed at ?1350 cm?1for Elaser? 2.41 eV)
appears when the symmetry of the hexago-
nal network in sp2-hybridized carbon is bro-
ken. Imperfections in the hexagonal lattice,
such as defects, dopants, and amorphous-
like domains, could thus cause the incre-
ment of this band. Recently, the presence
of a defect-induced feature (G=Defband) in
the immediate vicinity of the second-order
G=Prisband (Pris for pristine, located at
?2600?2700 cm?1for Elaser? 2.41 eV)
has provided valuable information about
charged defects present in SWNT struc-
tures.20
CVD represents a low-cost route to synthe-
sizecarbonnanotubesinthepresenceofmetal
catalysts, which are responsible for achieving
nanotube growth. The production of SWNTs
with the floating catalyst method using orga-
nometallic compounds (nickelocene (NiCp2),
cobaltocene (CoCp2), ferrocene (FeCp2)) stands
out as a practical one-step technique to ob-
tain long strands of SWNT bundles.
In this paper, we report the floating cata-
lyst CVD synthesis of N-, P-, and Si-doped
SWNTs in the presence of precursors contain-
ing such carbon electron donor elements. For
nitrogen doping, we compared the use of dif-
ferent precursors (benzylamine and pyrazine)
in the resulting N-doped SWNTs. The presence
of SWNTs was confirmed in each case by carry-
ing out electron microscopy characterization
(SEM and TEM) and by analyzing the RBM sig-
nal and other modes using Raman
spectroscopy.
Although the expected doping level in our samples
is below the detection limit of most elemental analysis
techniques (e.g., ?1 atom %), we have used the sensi-
tivity of Raman spectroscopy to probe changes in the
electronic structure induced by the incorporation of for-
eign atoms in the hexagonal network via the careful in-
spection of the G= band spectra of the synthesized ma-
terials. The appearance of the G=Defpeak, induced by
negatively charged defects, and its relative intensity
compared to the G=Prispeak (IG=Def/IG=Pris) provides insight
into the doping atoms within the samples. A detailed
comparative analysis indicates the effects of the differ-
ent dopant atoms on SWNT behavior.
RESULTS AND DISCUSSION
ElectronMicroscopyAnalysis.SiliconDoping. Experiments
with methoxytrimethylsilane (MTMS) resulted in the
formation of SWNT bundles mixed with byproduct,
which were analyzed to determine their morphology
and composition.
Figure 1 shows representative TEM images of the
nanostructures produced using MTMS. At the lowest
concentration of MTMS (0.05 wt %), catalytic Fe nano-
particles were present, embedded in the SWNT
bundles, very similar to the pristine material (see Fig-
ure 1a,b). Figure 1c shows the material synthesized at
0.1 wt % of MTMS. The top panel depicts a low-
magnification image, and the bottom one is a higher
magnification image. Using this MTMS concentration,
the SWNTs appear with spherical nanoparticles embed-
ded in a matrix. Our elemental analysis measurements,
using EELS and HAADF STEM (high-angle annular dark-
field scanning transmission electron microscopy) imag-
ing, confirmed that the spherical nanoparticles were
composed purely of iron, embedded in a Si?O?C ma-
trix (see Supporting Information).
A higher concentration of silicon (0.2 wt % of MTMS)
promoted the formation of short nanorods composed
of Si and O with metallic Fe?Si?O hemispherical tips
(see Figure 1d and Supporting Information for compo-
sition analysis). In these samples, it was difficult to find
Figure 1. High-resolution transmission electron micrographs of SWNTs produced with
different concentrations of methoxytrimethylsilane, (a) pristine SWNTs, (b) 0.05 wt %, (c)
0.1 wt %, and (d) 0.2 wt %; the arrow points to a SWNT embedded in the byproduct.
(b?d) Top panels contain low-magnification images, and the bottom panels contain
higher magnification images, picturing the carbon nanotubes and the morphology of
the accompanying byproduct.
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numerous SWNTs; nevertheless, in the bottom panel
of Figure 1d, an individual SWNT can be observed along
with the byproduct synthesized for this MTMS
concentration.
It appears that the formation of pure Fe particles,
which serve as SWNT catalysts during growth, was not
favored when using these experimental conditions. Ac-
cording to the phase diagram, the formation of binary
Fe?Si alloys is highly probable at this synthesis temper-
ature; hence, the catalytic activity of Fe was reduced,
and the formation of nanotubes was not abundant.
However, due to the high sensitivity of Raman spec-
troscopy for identifying SWNTs, their presence was con-
firmed by the observation of RBM signal (see Figures 4
and 5).
Phosphorus Doping. The introduction of triphenylphos-
phine (TPP) led to the synthesis of SWNTs and nanopar-
ticles, as depicted in Figure 2. Figure 2a?d displays
SEM micrographs of SWNTs produced at 0.1, 0.15, 0.2,
and 0.25 wt %, respectively. The nanoparticles accom-
panying the nanotubes, as observed in Figure 2a?d,
are dense when imaged by HAADF STEM (see Support-
ing Information Figure S7). The high proportion of
SWNTs in this sample indicates that mainly Fe was cata-
lyzing their growth. Figures 4 and 5 show Raman spec-
tra analysis of these samples (discussed below).
NitrogenDoping.Thenatureandmorphologyofthema-
terialsproducedwithbenzylaminewillnotbediscussed
here; the interested reader is referred to ref 10 for mi-
croscopy characterization and further details. Briefly,
this material contains SWNTs accompanied by Fe nano-
particles. Extensive characterization of these samples
using X-ray photoelectron spectroscopy (XPS)21has re-
vealed that the synthesized N-doped nanotubes exhibit
a maximum concentration at 0.3 atom % of N. EDX or
EELS is unable to detect such low concentrations, and
Raman characterization thus becomes the most ad-
equate tool to observe the doping effects in SWNTs.
When pyrazine was used as the nitrogen precursor
in the N-doped SWNT synthesis, metallic (Fe) nanopar-
ticles were also observed in the samples (see Figure
3a?d). At low pyrazine concentrations, the amount of
Fe nanoparticles present in the sample is scarce and it
increases gradually with increasing pyrazine concentra-
tion in the sprayer solution. Raman results for these
samples are discussed in the following section.
Micro-Raman Spectroscopy Analysis. We carried out an
RBM evolution study as a function of doping level with
spectra recorded at Elaser ? 1.96 eV. In addition, the
IG=Def/IG=Prisrelative intensities at Elaser? 2.41 eV were
used to determine the effects of doping on the elec-
tronic and vibrational structure of SWNTs. It has been
theoretically predicted and experimentally demon-
strated that the incorporation of heavier elements in
the hexagonal carbon lattice is energetically favored in
narrower nanotubes that exhibit higher radii of
curvature.12,13,22
In our experiments, the production of narrower nan-
otubes when P, N, and Si atoms are substitutionally
doping the SWNTs is illustrated in Figure 4. In particu-
lar, the RBM region of the Raman spectra recorded at
Elaser? 1.96 eV is plotted for the P-, N-, and Si- doped
samples (Figure 4a?c, respectively). The red arrows
help to visualize the RBM features that significantly de-
Figure 2. SEM images of SWNTs and byproduct deposited over copper grids synthesized with (a) 0.1, (b) 0.15, (c) 0.2, and
(d) 0.25 wt % of TPP; (e) TEM image of the material synthesized with 0.2 wt %, (f) higher resolution TEM images at 0.2 wt %
(left panel) and 0.25 wt % (right panel). Scale bars in panels a?d represent 200 nm. Scale bars in panels e and f represent 10
and 5 nm, respectively.
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crease in relative intensity as the doping precursor con-
centration in the ferrocene/ethanol solution is in-
creased. This decrease can be correlated with the ab-
sence of large diameter doped SWNTs.
Figure 5a shows a plot of
the RBM and G= band re-
gions, recorded at Elaser?
2.41 eV, for the samples syn-
thesized with TPP, pyrazine,
benzylamine, and MTMS and
of pristine SWNTs annealed
at 400 °C (HT Pristine). This
plot reveals that SWNTs were
successfully grown in the dif-
ferent environments and at
every concentration used.
A two-peak Lorentzian fit-
ting of the G= band of all the
spectra shown in Figure 5a
was carried out according to
previous studies.13,20,23This
procedure has to be carried
out with care because the
G=Prisfrequency depends on
the tube diameter, which
changes upon doping (the
more doping, the more nar-
row tubes). The ratio of the
relative intensities (ampli-
tudes) of the G=Defband and
G=Prisband (IG=Def/IG=Pris) was
computed, and the resulting
values are plotted in Figure
5b as a function of doping at-
oms available per carbon
atom during the synthesis in
the precursor ferrocene/etha-
nol solution. The solid
squares represent the set of
P-doped samples synthe-
sized with triphenylphos-
phine; the solid circles and
solid triangles represent the N-doped samples synthe-
sized with pyrazine and benzylamine, respectively, and
the solid inverse triangles are for the Si-doped materials
synthesized using methoxytrimethylsilane as precur-
Figure 3. (a?d) SEM images of SWNTs synthesized in the presence of pyrazine at con-
centrations of 0.5, 1, 1.5, and 2.5 wt %, respectively. (e,f) HRTEM images of the material
at 1.5 wt % of pyrazine.
Figure 4. RBM spectra recorded at Elaser? 1.96 eV, (a) P-doped SWNTs synthesized with different concentrations of TPP, (b) N-doped
SWNTs synthesized with different concentrations of benzylamine and pyrazine, and (c) Si-doped SWNTs synthesized with methoxytri-
methylsilane at different concentrations. The spectra were normalized to the G band, and the red arrows point to RBM features that de-
crease in intensity as the precursor concentration is increased.
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sor.Forallprecursors,anincreaseinthenumberofdop-
ing atoms per carbon atom available in the synthesis re-
sults in the increase of the IG=Def/IG=Prisrelative intensity,
thus suggesting the presence of higher doping levels
as the precursor concentration increases.
The lowest IG=Def/IG=Prisrelative intensity values
were obtained for the Si-doped samples. How-
ever, these were also the samples with lowest
doping atoms per carbon atom. We are confi-
dent that higher Si doping will result from the
use of higher concentrations of MTMS during
the synthesis. However, other experimental con-
ditions should be varied to avoid the suppres-
sion of the catalytic effect of Fe when Fe?Si al-
loys are formed. We believe that the variation of
flow rates and temperatures will lead us in the
right direction (these experiments are under-
way).
Although the highest concentration of ben-
zylamine used in this work (11 wt %) provided
thehighestnumberofdopingatomspercarbon
atom during synthesis, the IG=Def/IG=Prisrelative in-
tensityvalueswerebelowthoseobtainedfor1.5
and 2.5 wt % of pyrazine. This demonstrates
more efficient N doping when pyrazine was
used as the nitrogen precursor. As pointed out
earlier, XPS analysis of N-doped SWNTs using
benzylamine as the precursor resulted in SWNTs
doped with the maximum content of 0.3 atom
%. Our Raman results suggest that pyrazine-
produced N-doped nanotubes could increase
this limit. A careful XPS characterization of our
pyrazine N-doped materials is in progress.
Further inspection of Figure 5b leads to the
conclusion that the most efficient doping can
be attributed to phosphorus because the
amount of P atoms available per C atom is rela-
tively small (below the amount of N atoms avail-
able) and yet the IG=Def/IG=Prisrelative intensity
shows its highest values.
However, it is not yet clear whether the IG=Def/IG=Pris
relative intensities can be directly compared for differ-
ent dopants. As discussed in ref 13, different atoms dis-
turb the SWNT lattice differently. In order to address
this issue, the samples with highest IG=Def/IG=Prisrelative in-
Figure 5. (a) Raman spectra of the synthesized materials normalized to the G= band, acquired with Elaser? 2.41 eV, showing
the RBM and G= band regions. The spectrum of pristine SWNTs annealed at 400 °C as described in ref 13 is included for com-
parison. (b) Plot of the IG=Def/IG=Prisrelative intensities as a function of doping atoms (P, N, and Si) per carbon atoms intro-
duced in the synthesis environment (calculated from the precursor concentration, such concentrations are also included
next to each corresponding symbol).
Figure 6. Raman spectra in the G= band region at Elaser? 2.41 eV of
the samples with maximum IG=Def/IG=Prisrelative intensities for each
precursor (corresponding to the highest concentrations used in this
work). The blue and cyan lines correspond to the two-peak Lorentz-
ian fitting of the G=Defand G=Prisbands, respectively. The spectrum
of annealed pristine SWNTs at 400 °C is included for comparison. (b)
Plot corresponding to the G= band splitting (?G=Pris??G=Def) as a func-
tion of precursor concentration.
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tensities from Figure 5b (i.e., 0.25 wt % of TPP, 2.5 wt
% of pyrazine, 11 wt % of benzylamine, and 0.2 wt % of
MTMS) were selected. The G= band region and the corre-
sponding two-peak fitting of the above-mentioned
samples are depicted in Figure 6a. In this graph, it is
very easy to visualize the G=Defbands (blue lines) and
the G=Prisbands (cyan lines).
The information of the fitted spectra was further used
to calculate the frequency splitting ?G=Pris??G=Defbetween
thetwoG=peaksforthedopedsamples(seeFigure6b).It
isclearfromthisfigurethatthereisarangeofvaluesspe-
cific to each doping element. The splitting values for
phosphorus are in the range of ?31?33 cm?1, the split-
tingvaluesfornitrogenareintherangeof?35?40cm?1,
and Si reported the highest values in the range of
?41?45 cm?1. These results suggest that the G= band
splitting is more related to the nature of the doping ele-
ment rather than to the doping level.
CONCLUSIONS
The synthesis of SWNTs from solutions containing
nitrogen, phosphorus, and silicon precursors was car-
ried out via aerosol-assisted CVD. Precursors contain-
ing the target doping element were mixed in ethanol/
ferrocene solutions at different concentrations.
Triphenylphosphine was used in the case of phospho-
rus; methoxytrimethylsilane was used as the silicon pre-
cursor, and benzylamine and pyrazine were both used
as nitrogen precursors. Electron microscopy studies of
the samples as well as RBM Raman signals confirmed
the presence of SWNTs in the synthesized materials.
Electron microscopy analysis revealed that most of the
materials consisted of doped SWNTs entangled with
metallic nanoparticles. For Si, unusual morphologies
were also observed, such as Si nanorods with metallic
tips. As the doping precursor concentration in the
sprayer solutions was increased, narrower diameter
tubes were favored, according to our RBM analysis. The
latter result is consistent with theoretical calculations
indicating that dopants of heavier elements embedded
in the hexagonal carbon lattice are more energetically
favored in narrower tubes exhibiting higher radii of cur-
vature. The IG=Def/IG=Prisrelative intensities were used as a
direct doping index. Si-doped samples showed low
IG=Def/IG=Prisrelative intensity values that are directly re-
lated to the small amount of silicon atoms available per
carbon atoms during the synthesis. Nitrogen doping
was more effective when pyrazine was used instead of
benzylamine, and phosphorus doping was very effec-
tive even at low TPP concentrations. Our Raman results
showed that increasing precursor concentration led to
higher doping levels, increasing the IG=Def/IG=Prisrelative in-
tensities, and that the frequency splitting of the G=
band depended more on the doping element than the
doping amount. Further synthesis experiments are be-
ing performed in order to have a larger number of
samples per doping element, to be able to run Raman
spectroscopy measurements, and to elucidate the na-
ture of the G= band splitting features observed in this
work. XPS studies of N-doped SWNTs using benzyl-
amine as a precursor have been carried out recently.21
These confirm the presence of N within the SWNT ma-
terial studied here. Additional XPS studies on P-doped, Si-
doped, and N-doped using pyrazine SWNTs are currently
underwayincollaborationwithtwoexpertlaboratoriesin
order to be able to perform such challenging measure-
ments. Preliminary results reveal the presence of phos-
phorus in the nanotube lattice (results not shown here).
EXPERIMENTAL SECTION
An aerosol-assisted CVD method with floating catalyst was
used, based on ferrocene (Fe(C5H5)2) and ethanol (C2H5OH) solu-
tions.24The doping element was then introduced into the solu-
tion by inserting minute amounts of an appropriate compound
in order to synthesize the doped SWNTs (such compounds are
called precursor compounds in this work). The precursors were
chosenbasedontheirphysicalproperties(i.e.,meltingpointand,
most of all, solubility in ethanol).
The concentration of ferrocene (as source of catalytic iron)
was kept constant at 1.25 wt % for all of the experiments reported
here,andtheconcentrationoftheprecursorwasvaried.Anaerosol
of the solution was generated using an ultrasonic sprayer. Argon
(or an Ar?H (95/5) mixture) was used as carrier gas inside a quartz
tube to direct the aerosol to the hot zone of a tubular furnace op-
eratedat950°C.After30min,theaerosolgeneratorwasturnedoff
and the system was allowed to cool to room temperature, and
the quartz tube was removed from the furnace. A web-like ma-
terialcontainingSWNTsandbyprod-
uctwascollectedfromthezoneout-
side the furnace, as illustrated in ref
24.
Nitrogen-doped SWNTs were
synthesized reproducing the results
reported by Villalpando-Paez and
co-workers10using benzylamine
(C7H7NH2) as the N precursor in the
ethanol/ferrocene solution at con-
centrations of 7 and 11 wt %. An-
other N precursor, namely, pyrazine
(C4H4N2),wasusedatconcentrations
of 0.5, 1, 1.5, and 2.5 wt %.
Phosphorus was inserted into
the system using triphenylphos-
TABLE 1. Summary of the Precursor Compounds and Experimental Conditions Used
in the Synthesis of N-, P-, and Si-Doped SWNTs
doping
element
precursorcompound
concentration
(wt%)
synthesis
temperature
duration
carrier
gas
flow
rate
Nbenzylamine
C7H7NH2
pyrazine
C4H4N2
triphenylphosphine
P(C6H5)3
methoxytrimethylsilane
CH3OSi(CH3)3
7,11950°C30minAr1.2L/min
N0.5,1,1.5,2.5950°C30minAr?H2
1.2L/min
P0.1,0.15,0.2,0.25950°C30minAr?H2
0.8L/min
Si 0.05,0.1,0.2950°C30min Ar0.6L/min
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phine (P(C6H5)3) at concentrations of 0.1, 0.15, 0.2, and 0.25 wt %.
The production of SWNTs in the presence of silicon was
achieved by adding different concentrations of methoxytrimethyl-
silane (CH3OSi(CH3)3) as a Si precursor. The concentrations used in
our experiments were 0.05, 0.1, and 0.2 wt %.
The conditions (temperature, carrier gas, and flow rate) for
the different experiments are summarized in Table 1.
Scanning electron microscopy (SEM) characterization was
carried out using a XL-30 FEI-SFEG-STEM operated at 10?15 kV.
High-resolution transmission electron microscopy (HRTEM) ob-
servations were performed with a TECNAI F20 FEI operated at
200 kV and a Phillips CM200 FEG TEM. We also used a JEOL 2010F
STEM equipped with a Gatan Enfina spectrometer for recording
electron energy loss spectroscopy (EELS).
Micro-Raman spectroscopy measurements were recorded at
room temperature using a Renishaw InVia equipment. The spec-
tra were recorded with an Ar line, ? ? 514.5 nm (Elaser? 2.41
eV), and a He?Ne line, ? ? 633 nm (Elaser? 1.96 eV), at a power
of ?0.3 mW in the backscattering geometry using a 100? objec-
tive lens to focus the laser beam. No less than three measure-
ments were recorded per sample, and the spectra shown here
are the resulting averages.
Acknowledgment. The authors are thankful to D. Ramı ´rez-
Gonza ´lez, G. Pe ´rez-Assaf, and K. Go ´mez-Serrato for technical as-
sistance. This work was supported in part by CONACYT-Mexico
Ph.D. Scholarship (J.C.D.). A.J. acknowledges financial support
from Rede Nacional de Pesquisa em Nanotubos de Carbono
(MCT) and PNPq (Brazil) and AFOSR (USA). M.A.P. acknowledges
MCT and Instituto Nacional de Ciencia e Tecnologia de
Nanoestruturas de Carbono (Fapemig and CNPq, Brazil).
Supporting Information Available: Information about the
chemical composition of the byproduct obtained when MTMS
was used at 0.1 and 0.2 wt % is included (HAADF STEM images
and EELS spectra). SEM images, bright field, and HAADF STEM
images of materials synthesized with TPP are included, which
confirm the composition by heavier atoms of the co-products
present in the samples. This material is available free of charge
via the Internet at http://pubs.acs.org.
REFERENCES AND NOTES
1. Han, W.; Bando, Y.; Kurashima, K.; Sato, T. Boron-Doped
Carbon Nanotubes Prepared through a Substitution
Reaction. Chem. Phys. Lett. 1999, 299, 368–373.
2. Goldberg, D.; Bando, Y.; Han, W.; Kurashima, K.; Sato, T.
Single-Walled B-Doped Carbon, B/N-Doped Carbon and
BN Nanotubes Synthesized from Single-Walled Carbon
Nanotubes through a Substitution Reaction. Chem. Phys.
Lett. 1999, 308, 337–342.
3. Blase, X.; Charlier, J.-C.; De Vita, A.; Car, R.; Redlich, Ph.;
Terrones, M; Hsu, W. K.; Terrones, H.; Carroll, D. L.; Ajayan,
P. M. Boron-Mediated Growth of Long Helicity-Selected
Carbon Nanotubes. Phys. Rev. Lett. 1999, 83, 5078–5081.
4. McGuire, K.; Gothard, N.; Gai, P. L.; Dresselhaus, M. S.;
Sumanasekera, G.; Rao, A. M. Synthesis and Raman
Characterization of Boron-Doped Single-Walled Carbon
Nanotubes. Carbon 2005, 43, 219–227.
5. Sen, R.; Satishkumar, B. C.; Govindaraj, A.; Harikumar, K. R.;
Raina, G.; Zhang, J.-P.; Cheetham, A. K.; Rao, C. N. R.
B?C?N, C?N and B?N Nanotubes Produced by the
Pyrolysis of Precursor Molecules over Co Catalysts. Chem.
Phys. Lett. 1998, 287, 671–676.
6. Terrones, M.; Ajayan, P. M.; Banhart, F.; Blase, X.; Carroll,
D. L.; Charlier, J. C.; Czerw, R.; Foley, B.; Grobert, N.;
Kamalakaran, R.; et al. N-Doping and Coalescence of
Carbon Nanotubes: Synthesis and Electronic Properties.
Appl. Phys. A: Mater. Sci. Process. 2002, 74, 355–361.
7. Droppa, R., Jr.; Hammer, P.; Carvalho, A. C. M.; dos Santos,
M. C.; Alvarez, F. Incorporation of Nitrogen in Carbon
Nanotubes. J. Non-Cryst. Solids 2002, 299?302, 874–879.
8. Glerup, M.; Castignolles, M.; Holzinger, M.; Hug, G.; Loiseau,
A.; Bernier, P. Synthesis of Highly Nitrogen-Doped Multi-
Walled Carbon Nanotubes. Chem. Commun. 2003,
2542–2543.
9. Glerup, M.; Steinmetz, J.; Samaille, D.; Ste ´phan, O.; Enouz,
S.; Loiseau, A.; Roth, S.; Bernier, P. Synthesis of N-Doped
SWNT Using the Arc-Discharge Procedure. Chem. Phys.
Lett. 2004, 238, 193–197.
10. Villalpando-Paez, F.; Zamudio, A.; Elias, A. L.; Son, H.;
Barros, E. B.; Chou, S. G.; Kim, Y. A.; Muramatsu, H.; Hayashi,
T.; Kong, J.; et al. Synthesis and Characterization of Long
Strands of Nitrogen-Doped Single-Walled Carbon
Nanotubes. Chem. Phys. Lett. 2006, 424, 345–352.
11. Cruz-Silva, E.; Cullen, D. A.; Gu, L.; Romo-Herrera, J. M.;
Mun ˜oz-Sandoval, E.; Lo ´pez-Urı ´as, F.; Sumpter, B. G.;
Meunier, V.; Charlier, J.-C.; Smith, D. J.; et al. Heterodoped
Nanotubes: Theory, Synthesis, and Characterization of
Phosphorus?Nitrogen Doped Multiwalled Carbon
Nanotubes. ACS Nano 2008, 2, 441–448.
12. Sumpter, B. G.; Huang, J.; Meunier, V.; Romo-Herrerea,
J. M.; Cruz-Silva, E.; Terrones, H.; Terrones, M. A Theoretical
and Experimental Study on Manipulating the Structure
and Properties of Carbon Nanotubes Using Substitutional
Dopants. Int. J. Quantum Chem. 2009, 109, 97–118.
13. Maciel, I. O.; Campos-Delgado, J.; Cruz-Silva, E.; Pimenta,
M. A.; Sumpter, B. G.; Meunier, V.; Lo ´pez-Urı ´as, F.; Mun ˜oz-
Sandoval, E.; Terrones, H.; Terrones, M.; et al. Synthesis,
Electronic Structure, and Raman Scattering of Phosphorus-
Doped Single-Wall Carbon Nanotubes. Nano Lett. 2009, 9,
2267–2272.
14. Kimura, T.; Sugai, T.; Shinohara, H. Production and
Characterization of Boron- and Silicon-Doped Carbon
Clusters. Chem. Phys. Lett. 1996, 256, 269–273.
15. Ray, C.; Pellarin, M.; Lerme ´, J. L.; Vialle, J. L.; Broyer, M.;
Blase, X.; Me ´linon, P.; Ke ´ghe ´lian, P.; Perez, A. Synthesis and
Structure of Silicon-Doped Heterofullerenes. Phys. Rev. Lett.
1998, 80, 5365.
16. Baierle, R. J.; Fagan, S. B.; Mota, R.; da Silva, A. J. R.; Fazzio,
A. Electronic and Structural Properties of Silicon-Doped
Carbon Nanotubes. Phys. Rev. B 2001, 64, 085413.
17. Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.;
Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.;
Thess, A.; et al. Diameter-Selective Raman Scattering from
Vibrational Modes in Carbon Nanotubes. Science 1997,
275, 187–191.
18. Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A.
Raman Spectroscopy of Carbon Nanotubes. Phys. Rep.
2005, 409, 47.
19. Souza Filho, A. G.; Jorio, A.; Samsonidze, Ge. G.;
Dresselhaus, G.; Saito, R.; Dresselhaus, M. S. Raman
Spectroscopy for Probing Chemically/Physically Induced
Phenomena in Carbon Nanotubes. Nanotechnology 2003,
14, 1130–1139.
20. Maciel, I. O.; Anderson, N.; Pimenta, M. A.; Hartschuh, A.;
Qian, H.; Terrones, M.; Terrones, H.; Campos-Delgado, J.;
Rao, A. M.; Novotny, L.; et al. Electron and Phonon
Renormalization near Charged Defects in Carbon
Nanotubes. Nat. Mater. 2008, 7, 878–883.
21. Elı ´as, A. L.; Ayala, P.; Zamudio, A.; Grobosch, M.; Cruz-Silva,
E.; Romo-Herrera, J. M.; Campos-Delgado, J.; Terrones, H.;
Pichler, T.; Terrones. M. Spectroscopic Characterization of
N-Doped Single-Walled Carbon Nanotube Strands: An XPS
and Raman Study. J. Nanosci. Nanotechnol. In press.
22. Sumpter, B. G.; Meunier, V.; Romo-Herrera, J. M.; Cruz-Silva,
E.; Cullen, D. A.; Terrones, H.; Smith, D. J.; Terrones, M.
Nitrogen-Mediated Carbon Nanotube Growth: Diameter
Reduction, Metallicity, Bundle Dispersability, and Bamboo-
like Structure Formation. ACS Nano 2007, 1, 369–375.
23. Maciel, I. O.; Pimenta, M. A.; Terrones, M.; Terrones, H.;
Campos-Delgado, J.; Jorio, A. The Two Peaks G= Band in
Carbon Nanotubes. Phys. Status Solidi B 2008, 245, 2197–
2200.
24. Lupo, F.; Rodrı ´guez-Manzo, J. A.; Zamudio, A.; Elı ´as, A. L.;
Kim, Y. A.; Hayashi, T.; Muramatsu, M.; Kamalakaran, R;
Terrones, H.; Endo, M.; et al. Pyrolytic Synthesis of Long
Strands of Large Diameter Single-Walled Carbon
Nanotubes at Atmospheric Pressure in the Absence of
Sulphur and Hydrogen. Chem. Phys. Lett. 2005, 410, 384–
390.
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