radial deformation of single-walled carbon nanotubes … · on quartz substrates and the resultant...

Nano Res

1

Radial deformation of single-walled carbon nanotubes

on quartz substrates and the resultant anomalous

diameter-dependent reaction selectivity

Juan Yang, Yu Liu, Daqi Zhang, Xiao Wang, Ruoming Li, and Yan Li ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0811-1

http://www.thenanoresearch.com on May 9, 2015

© Tsinghua University Press 2015

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DOI 10.1007/s12274-015-0811-1

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Juan Yang, Yu Liu, Daqi Zhang, Xiao Wang, and Yan Li*

Beijing National Laboratory for Molecular Sciences, Key

Laboratory for the Physics and Chemistry of Nanodevices,

State Key Laboratory of Rare Earth Materials Chemistry

and Applications, College of Chemistry and Molecular

Engineering, Peking University, Beijing 100871, China.

Radial-deformed single-walled carbon nanotubes on quartz substrates

distribute anomalous diameter-dependent reaction selectivity in

treatment with iodine vapor.

Provide the authors’ webside if possible.

Juan Yang, http://www.chem.pku.edu.cn/page/liy/labhomepage/yangjuan.html

Yan Li, http://www.chem.pku.edu.cn/page/liy/labhomepage/index.html




Juan Yang, Yu Liu, Daqi Zhang, Xiao Wang, Ruoming Li, and Yan Li ( )

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Single-walled carbon

nanotubes, reaction

selectivity, interaction,

quartz substrate, radial

deformation

ABSTRACT

Owing to the unique conjugated structure, chemical reaction selectivity of

single-walled carbon nanotubes (SWNTs) has attracted great attentions. By

utilizing the radial deformation of SWNTs caused by the strong interactions

with the quartz lattice, we achieve an anomalous diameter-dependent reaction

selectivity of quartz lattice-oriented SWNTs in treatment with iodine vapor,

which is distinctly different from the widely reported and well accepted higher

reaction activity in small-diameter tubes over the large ones. The radial

deformation of SWNTs on quartz substrate is verified by detailed Raman

spectra and mappings in both G band and RBM. Due to the strong interaction

between SWNTs and the quartz lattice, large-diameter tubes present larger

degree of radial deformation and more delocalized partial electrons are

distributed at certain sidewall sites with high local curvature. It is thus easier

for the carbon-carbon bonds at those high curvature sites on large-diameter

tubes to break down upon reaction. This anomalous reaction activity offers a

novel approach for selective removal of small-bandgap large-diameter tubes.

1 Introduction

Ever since the discovery, single-walled carbon

nanotubes (SWNTs) have drawn great attention due

to their remarkable mechanical, optical, electrical,

and chemical properties [1]. Of those properties,

many are closely related to the tube diameter (dt) of

SWNTs. For example, the symmetry allowed

interband transition energies between the

corresponding van Hove singularities (vHSs), which

govern the various photophysical processes of

SWNTs, is approximately inversely proportional to

dt.

As a large π conjugated system, SWNTs are of

special interests to many scientists in their reaction

activities. The chemical reaction selectivity of SWNTs

plays an important role in sorting them on a large

scale for further applications in many areas, such as

Nano Research

DOI (automatically inserted by the publisher)

Address correspondence to Yan Li, [email protected]

Research Article

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2 Nano Res.

nanoelectronic devices [2], chemical and biosensors

[3, 4]. Previously, it was widely reported in many

literatures that small-diameter tubes show higher

reaction activity over the large ones. The reactants

included nitronium ions [5], methane plasma [6],

NO2 [7], fluorine gas [8], benzenediazonium salts [9],

lithium vapor [10], O2 [11], dichlorocarbene [12], etc.

As a representative example, it was found that three

diameter-dependent regimes exist for SWNTs in

treatment with methane plasma [6]. In the

small-diameter regime, both metallic (M) and

semiconducting (S) tubes were etched

nondiscriminately. In the medium-diameter regime

only M-SWNTs were selectively removed, whereas in

the large-diameter regime both M- and S-SWNTs

were not etched [6]. Therefore, diameter is a general

key factor for the chemical reactivity of SWNTs. This

higher reactivity of small-diameter tubes can be

mainly attributed to the higher curvature of

small-diameter tubes and the consequent

curvature-induced strain in sp2-hybridized graphene

sheet, leading to decreased stability of the

carbon-carbon bonds [6, 10, 11].

In all previous cases, small-diameter tubes showed

higher chemical reactivity over the large ones, and

the small-diameter tubes were always preferentially

etched from the sample via various chemical

reactions. However, photoelectronic applications of

SWNTs require tuning of bandgaps in various

spectral region. In some special cases, selective

removal of large-diameter small-bandgap tubes are

strongly needed. In order to preferentially etch those

normally less reactive tubes, it is necessary to

increase the reactivity of the large-diameter tubes

over the small ones. Our design is to change the

electron density distribution by varying the local

curvature of tubes with different diameter. This can

be realized by the strong interaction between SWNTs

and the substrate lattice, e.g. quartz lattice.

Previously, strong interaction between SWNTs and

the quartz lattice was reported by many researchers

[13-17]. A nonuniform axial compressive strain

arising from the difference in coefficients of thermal

expansion (CTE) between quartz and SWNTs was

believed to exist in horizontally aligned SWNTs

grown on quartz substrate [13]. However, axial strain

cannot lead to large changes in local curvature and

the electron density distribution. Only radial

deformation of SWNTs will result in variations in

local curvature and distorted partial electron density.

Therefore, radial deformation of SWNTs is necessary

to achieve different chemical reaction selectivities of

SWNTs.

Here in this article, we report that radial

deformation is indeed present for quartz

lattice-oriented SWNTs based on detailed Raman

mappings. We also reveal that the radial deformation

of SWNTs is non-destructive and recoverable. More

importantly, we demonstrate that due to this radial

deformation of SWNTs, more delocalized partial

electrons are distributed at high curvature sidewall

sites on large-diameter tubes. Consequently, those

large-diameter tubes distribute higher reaction

priority over the small ones in treatment with iodine

vapor (Scheme 1), which is distinctly different from

the widely reported and well accepted higher

reaction activity in small-diameter tubes over the

large ones. This anomalous reaction activity thus

offers a novel approach to selectively remove those

small-bandgap large-diameter tubes.

Scheme 1 Schematic diagram of iodine reacting with the radial-deformed SWNTs on quartz substrate. Red dimers denote iodine molecules and red dots denote iodine radicals.

2 Experimental

2.1 Preparation of SWNT arrays

The SWNT arrays are directly grown on single

crystal quartz substrate (ST-cut, Hoffman Materials

Inc.) using chemical vapor deposition (CVD) method

described elsewhere [18, 19].

2.2 Iodine treatment procedure

The lattice-oriented SWNT arrays on quartz substrate

prepared by chemical vapor deposition (CVD) were

placed in the central heating zone of a quartz tube in

a 1 inch tube furnace. Iodine powders were put in a

small porcelain boat at the entrance of the tube

furnace inside the quartz tube. The entire system was

first purged with an argon flow of 500 sccm for 30

min to maximally remove the residual oxygen, then

heated up to 900 °C in an argon flow of 500 sccm. At

this high temperature iodine powders were

sublimated into vapor, and the iodine vapor was

mixed with the argon flow into the quartz tube. The

heating stopped after 15-30 min and the system was

then cooled down to room temperature in argon

atmosphere.

2.3 Raman and SEM measurements

The Raman spectra of SWNTs on quartz substrate

were collected using a Jobin Yvon LabRam ARAMIS

spectrometer with 532 nm laser excitation. The

Raman spectra were taken in a backscattering

configuration by a microscope using a 100× objective

with laser focal spot of ~1 μm in diameter and a

charge coupled device (CCD) detector. All laser

power was attenuated to be less than 1 mW to avoid

heating effects.

The SEM images were taken on a Hitachi S4800 at

acceleration voltage of 1 kV.

3 Results and Discussion

Fig. 1a shows the Raman G band mapping for several

lattice-oriented SWNTs on quartz substrate in the

spectral region of 1580-1620 cm-1, from which irregular variations in both G band frequency (ωG)

and intensity (IG) are clearly observed. In the

direction of nanotube growth, which is along the

quartz [100] direction, some parts of the SWNT arrays distribute low ωG at ~1590 cm-1 (position A),

close to that of a suspended SWNT free of interaction.

Figure 1 Raman G band mapping for several lattice-oriented SWNTs on quartz substrate (a) and a typical individual SWNT grown on

silicon substrate (b). The mapping regions are 1580-1620 cm-1 for (a) and 1570-1600 cm-1 for (b), respectively. (c) Normalized Raman G band spectra for the marked positions A, B, and C, respectively. (d) Raman G band mapping and the corresponding G band frequency for a partially bent individual SWNT on quartz substrate. The red arrow indicates the quartz [100] direction.

However, some other parts of the SWNT arrays show distinctly upshifted ωG at above 1600 cm-1 (position B).

In very rare cases ωG as high as ~1620 cm-1 is also

observed. As a comparison, SWNTs grown on silicon

substrate distribute nearly uniform IG and the ωG is

typically observed in the 1580-1590 cm-1 range (Fig.

1b, position C). Therefore, lattice-oriented SWNTs

directly grown on quartz substrate differ from

SWNTs grown on silicon substrate in significant ωG

upshift and irregular IG variation. This distinct ωG

upshift and irregular IG variation for quartz

lattice-oriented SWNTs are also reported previously

by many others [13-17].

According to literatures, at constant experimental

parameters such as temperature and laser power, the ωG upshift and IG variation of SWNTs on substrate

may arise either from defects [20, 21], doping or

charge carrier implantation [22-24], or mechanical

deformation [14, 15, 25, 26]. Since the quartz

lattice-oriented SWNTs do not show clear

disorder-related D band, and doping or charge

carrier implantation could hardly cause a G band

upshift of more than 10 cm-1 [22-24], the only possible reason accounting for the significant ωG upshift and

IG variation is mechanical deformation of SWNTs.

For a one-dimensional SWNT, the mechanical

deformation can be either in the axial direction or in

the radial direction. Previously, it is believed that an

axial compressive strain is resulted from the large

difference in CTE of SWNTs and quartz. A 27 cm-1 G

band upshift per 1% compressive strain with a

maximum compressive strain of up to ~1.1% at room

temperature is reported [15]. However, no radial

deformation of quartz lattice-oriented SWNTs is

reported previously. With this strong interaction

between SWNTs and the quartz lattice, is any

deformation in the radial direction of SWNTs also

present?

To answer this question, we first perform the G

band mapping for a partially bent individual SWNT

on quartz substrate. The G band mapping and the

corresponding G band frequency with respect to

positions of this SWNT are illustrated in Fig. 1d. The

red arrow indicates the quartz [100] direction. It is

evident that all the SWNT parts along the [100]

direction distribute large ωG upshifts with frequency

higher than 1600 cm-1, whereas all the rest parts along

other directions correspond to ωG lower than 1600

cm-1. As the interaction between SWNTs and the

quartz [100] direction is the strongest of all, the

upshifts in ωG thus can be directly related to the

SWNT-quartz interaction. More importantly, because

the CTE of SWNTs is expected to be at least one order

of magnitude less than that of single crystal quartz

[27-29], and the CTE of quartz for all directions only

differs from each other by a ratio of about 2 [27, 28],

if the ωG upshifts are resulted all from axial

compressive strain due to the large difference in CTE

of SWNTs and quartz, it would be expected that the ωG variation of this SWNT is more or less uniform in

all directions. The nonuniformity of ωG variations in

different directions then indicates that deformation

of SWNT other than axial compressive strain is

present, which is most likely to be deformation in the

radial direction.

As G band of SWNTs corresponds to tangential

vibrational modes, it is quite insensitive to the

changes in the radial direction. The radial breathing

mode (RBM), on the other hand, is a

diameter-dependent Raman active mode

corresponding to all carbon atoms moving

simultaneously in the radial direction. Thus it is a

very sensitive mode to study changes of SWNTs

happened in the radial direction.

Previously, it is reported that the RBM intensity (IRBM) but not the frequency (ωRBM) changes

significantly when axial strain is applied [25, 30, 31]. In situ Raman measurements of suspended

individual SWNTs under tensile strain do not show evident change in ωRBM, however, IRBM reduces

rapidly with increasing strain [32]. Force constant

calculations based on molecular dynamics (MD)

simulations suggest that ωRBM is insensitive to axial

strain and that the shift in ωRBM is very small (less

than ±1 cm-1) with ±1% axial strain [33]. A critical

compressive strain, at which the tube buckles and a

sudden lateral deflection appears, is found to be

inversely proportional to tube diameter (dt) and also

dependent on tube chirality. For all tubes with dt>0.7

nm, the calculated critical compressive strain is less

than -10%. Above the critical compressive strain the

tube undergoes unstable changing and consequently ωRBM drops rapidly [33]. Therefore, a maximum

compressive strain of ~1.1% at room temperature

arising from the CTE difference in SWNTs and quartz

would lead to little ωRBM variation but significant IRBM

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

5 Nano Res.

variation. Meanwhile, it is also reported that both IRBM and

ωRBM vary significantly when radial deformation is

applied, for example, by hydrostatic pressure [13, 34].

In the low pressure regime, upshifts in ωRBM are

linear and reversible, and the pressure derivative of

ωRBM increases with increasing dt [13]. Classical

constant-pressure MD simulations and the force

constants model indicate the existence of a critical

pressure, below which the cross-section of SWNTs

remains circular but above which the tube shape

changes to ellipse [35]. Calculations also show that

ωRBM increases linearly with increasing pressure blow

the critical pressure [35]. Although the strong

interaction between SWNT and the quartz lattice will

probably cause asymmetric radial deformation of the

SWNT (presumably forming an asymmetric elliptical

cross-section, flatter on the side facing quartz), the

experiments and calculations based on uniform

deformation of SWNTs under pressure may still

serve as important guides for our quartz

lattice-oriented SWNTs. And because the asymmetric

deformation caused by the strong interaction

between SWNTs and quartz lattice is in the radial

direction, both IRBM and ωRBM are expected to vary

significantly.

According to the above experimental and

theoretical analysis in literatures, the variation in ωRBM is a key factor in order to determine whether or

not radial deformation is present for quartz lattice-oriented SWNTs. Therefore, detailed in situ

Raman mappings for the RBM are performed.

However, due to the strong interaction between

SWNTs and the quartz lattice, only very weak or

nearly no RBM signal can be observed, allowing very

limited in situ RBM data to be collected. Fig. 2a plots

the in situ RBM mapping of a typical quartz

lattice-oriented SWNT as an example. Significant variations in IRBM are evidently observed for this

SWNT. More importantly, ωRBM also distributes

significant variations, the data of which are shown in

green dots with ωRBM at 154±4 cm-1 in Fig. 2b. A 5.2%

large relative RBM shift is observed for this particular

SWNT. In Fig. 2b, the variations in ωRBM with respect

to positions for 5 individual quartz lattice-oriented

SWNTs (colored dots) and 3 individual SWNTs

transferred from quartz to silicon substrate (black

squares) with different diameters are illustrated.

Clearly, the 5 quartz lattice-oriented SWNTs all show

significant ωRBM variations whereas the 3 transferred

Figure 2 (a) In situ RBM mapping for a typical quartz lattice-oriented SWNT. The mapping region is 140-168 cm-1. (b) In situ RBM frequency variations for 5 individual quartz lattice-oriented SWNTs (colored dots) and 3 individual SWNTs transferred from quartz to silicon substrate (black squares) with different diameters. The RBM mapping in (a) corresponds to data in green dots with RBM frequencies at 154±4 cm-1. (c) Relative RBM shifts for the 5 individual quartz lattice-oriented SWNTs with respect to their average

RBM frequencies.

SWNTs on silicon substrate distribute no shift at all.

This means the deformation of SWNTs disappears

when transferred from quartz to silicon substrate. In

other words, it demonstrates the deformation of

SWNTs on quartz are non-destructive and recoverable. Assuming that this large ωRBM variations

for quartz lattice-oriented SWNTs all arise from axial strain, in order to cause an ωRBM variation as large as

±4 cm-1 the axial strain needs to reach at least an

order of ±10% according to MD calculations [33]. On

one hand, a compressive stain as large as -10% on

SWNTs is far beyond the reported maximum

compressive strain of up to ~1.1% at room

temperature [15]. On the other hand, at this large

compressive strain almost all SWNTs would have

already buckled and undergone unrecoverable

structural transitions. Therefore, the large ωRBM

variations can only be resulted from radial

deformation of SWNTs.

Based on the above experimental results, we

conclude that the mechanical deformation of SWNTs

on quartz is mainly radial deformation, arising from

the strong interaction between SWNTs and the

quartz lattice. This radial deformation is found to be

nonuniform along the nanotube growth direction.

Previously, the nonuniformity of G band was

believed to arise from the polishing induced surface

roughness of the quartz substrate [15]. We also

believe nanometer scale surface fluctuation of quartz

substrate can be caused by parallel scratches during

the standard surface polishing procedure. Stronger

interactions between SWNTs and the quartz lattice

are expected for the surface convexes and weaker

interactions for the surface concaves.

In Fig. 2b, one may also notice that the ωRBM

variations for large-diameter tubes are slightly larger than that for the small ones, given that ωRBM is

linearly related to the reciprocal of dt [36]. As the

curvature change of a nanotube can be reflected by

Δdt/dt, which is closely related to the relative RBM shift defined as ΔωRBM/ωRBM, we then plot the relative

RBM shifts for the 5 individual quartz

lattice-oriented SWNTs with respect to their average

ωRBM in Fig. 2c. It is found that the relative RBM shifts

for large-diameter SWNTs (roughly with ωRBM<180

cm-1) are higher than that for the small ones (roughly with ωRBM>180 cm-1). Calculations indicate that large

relative RBM shifts correspond to large degree of

radial deformation [35]. Therefore, large-diameter

SWNTs are found to distribute larger degree of radial

deformation than the small ones.

As mentioned earlier, small-diameter tubes

showed higher chemical reactivity over the large

ones in all previous cases. In order to selectively

remove those normally less reactive large-diameter

tubes, our design is to utilize the strong interaction

between SWNTs and the quartz lattice. Now that we

already demonstrate radial deformation do exist for

quartz lattice-oriented SWNTs, and that larger degree

of radial deformation is present for large-diameter

tubes, we would like then to further explore the

possible changes in chemical reaction selectivities of

the radial-deformed SWNTs.

The quartz lattice-oriented horizontally aligned

SWNT arrays are heated at 900 °C in iodine vapor for

a time period of 15-30 min, the detailed procedure of

which is described in the experimental section. We

do not observe any encapsulation of iodine into the

tubes [37]. It is found by scanning electron

microscopy (SEM) that after a heating time of 30 min

at 900 °C in iodine vapor, all SWNT arrays on quartz

substrate disappear, however, there are still some

SWNTs remaining if the heating time is reduced to 15

min (Fig. 3a and 3b). As a control, SWNT arrays on

quartz substrate treated under exactly the same

experimental conditions, only without introducing

iodine vapor, do not show any distinguishable

changes (Fig. S1a and S1b), suggesting that iodine

vapor somehow reacts with the SWNT arrays on

quartz substrate at 900 °C.

Figure 3 SEM images of SWNTs on quartz substrate. Lattice-oriented SWNTs before (a) and after (b) a heating time of 15 min at 900 °C in iodine vapor. A specifically prepared sample

with both lattice-oriented SWNT arrays and floating-growth SWNTs on the same quartz substrate before (c) and after (d) a heating time of 30 min at 900 °C in iodine vapor.


7 Nano Res.

Moreover, it is observed in a specifically prepared

sample with both lattice-oriented SWNT arrays and

floating-growth SWNTs on the same quartz substrate,

that with a heating time of 30 min at 900 °C in iodine

vapor, only the lattice-oriented SWNT arrays

disappear whereas the floating-growth SWNTs are

kept intact (Fig. 3c and 3d). Further experiments

demonstrate that all SWNTs on silicon substrate, no

matter the floating-growth ultralong tubes, or

random tubes, or SWNT arrays transferred from

quartz substrate with the nanotransfer printing

technique [38], as well as bulk SWNT samples, do not

react with iodine under the previously described

experimental conditions. Therefore, it can be

summarized that in all above SWNT samples only

quartz lattice-oriented SWNTs react with iodine

under the previously described experimental

conditions.

In Fig. 3b, after a reaction time of 15 min at 900 °C

in iodine vapor, the sample shows much lower

SWNT density in the middle area (position 2) than in

the area (position 1) close to the pattered catalyst

regions. On the other hand, the control sample

treated under exactly the same experimental

conditions only without introducing iodine vapor

distributes similar SWNT density at the

corresponding positions 1’ and 2’ (Fig. S1b). This

demonstrates that the reaction of SWNTs with iodine

starts from the tip of the SWNTs, and that some

SWNTs react faster, or are of higher reactivity, than

others. Therefore, special reaction selectivity might

exist for those SWNTs in reaction with iodine.

Hereafter, we refer position 1 (or 1’) to the area close

to the pattered catalyst regions and refer position 2

(or 2’) to the middle area away from the pattered

catalyst regions.

In order to obtain information about the selectivity

of this reaction, Raman spectra in the RBM region are

collected. Again the strong interaction between

SWNTs and the quartz lattice quenches most RBM

signals. We then have to transfer the SWNT arrays

from quartz to silicon substrate with the nanotransfer

printing technique [38]. After transferring, the

sample gives more and stronger RBM signal, and a

statistical analysis of the RBM shifts (ωRBM) can thus

be obtained. Fig. 4a and 4b plot the statistical

distributions of the observed ωRBM with 532 nm laser

excitation at positions 1 and 2, respectively, for the

sample after a reaction time of 15 min at 900 °C in

iodine vapor. These two distributions clearly differ in the percentage of SWNTs with ωRBM <175 cm-1, i.e.,

50% at position 1 and only 15% at position 2. As a

comparison, the corresponding values are 50% at

position 1’ (Fig. 4c) and 41% at position 2’ (Fig. 4d) for

the control sample treated under exactly the same

experimental conditions only without introducing

iodine vapor. Therefore, it is found that after the

reaction with iodine the percentage of SWNTs with ωRBM<175 cm-1 is largely decreased in the middle area,

where more SWNT arrays react with iodine.

According to our specifically derived relation [39] of

ωRBM=222.0/dt+8.0 for SWNTs transferred from quartz

to silicon substrate by nanotransfer printing

technique, the statistical data of ωRBM indicate that the

reaction rates of large-diameter tubes with dt>1.33 nm

is predominant. In other words, large-diameter tubes

indeed distribute higher reactivity over the small

ones in reaction with iodine as we expected. This

reaction priority in large-diameter tubes is distinctly

differs from the previously reported reaction

priorities in small-diameter tubes. For 633 nm

excitation, with only very limited SWNTs excited by

the 633 nm laser, we still observe a similar trend that

the percentage of SWNTs with ωRBM <175 cm-1 is

much less at the middle position 2 (0%) than at

position 1 (62%) close to the pattered catalyst regions,

shown as in Fig. S2. Because semiconducting tubes

are expected to appear in the ωRBM regions of roughly

about 125-195 cm-1 for 532 nm excitation, and of

about 170-215 cm-1 for 633 nm excitation, this reaction

selectivity is clearly diameter-dependent, not

conductivity-dependent. Unambiguously, the ωRBM

boundary of 175 cm-1 nearly perfectly matches with

the previously mentioned boundary value of ~180

cm-1 between large and small relative RBM shifts in

Fig. 2c, indicating the intrinsic connections between

the anomalous reactivity and the radial deformation

of SWNTs, as we expected in our initial design.


8 Nano Res.

S MM

50%

S MM

15%

S MM

50%

S MM

41%

Figure 4 Statistical distributions of RBM frequencies with 532 nm exciation at positions 1 (a) and 2 (b) in Fig. 3b for quartz lattice-oriented SWNTs after a reaction time of 15 min at 900 °C

in iodine vapor, and at the corresponding positions 1’ (c) and 2’

(d) in Fig. S1b for the control sample treated under exactly the same experimental conditions only without introducing iodine vapor. Metallic and semiconducting tubes are denoted as M and S, respectively.

To further explore this anomalous reactivity,

detailed Raman mappings for the tangential G band

are performed. The G band mapping for a sample of

quartz lattice-oriented SWNT arrays clearly

distributes irregular G band variations in both

frequency and intensity (Fig. 5a). For the same

sample after a heating time of 15 min at 900 °C in

iodine vapor, the G band mapping shows evident

disappearance of G band at many locations (Fig. 5d).

Comparing to the G band mapping of the same

sample before reaction, the percentage of SWNTs

with large G band upshifts (in the spectral region of

1600-1620 cm-1) reduces greatly, indicating the

reaction happens selectively from the SWNT parts

with large G band upshifts, i.e., the SWNT parts with

large degree of radial deformation. Randomly

selected G band spectra of some individual tubes

from the stacked spectra in Fig. 5e and 5f are listed in

Fig. S3.

Figure 5 G band mapping and spectra for quartz lattice-oriented SWNTs before (a-c) and after (d-f) a reaction

time of 15 min with iodine. The G band mapping regions are

1580-1620 cm-1 for (a) and (d), and 1600-1620 cm-1 for (b) and (d), respectively. As can be seen, disappearance of G band at many locations is clearly observed, and the percentage of SWNTs with G band intensity in 1600-1620 cm-1 reduces greatly after

reaction. The black and red guiding lines in (c) and (f) correspond to Raman frequencies at 1590 and 1610 cm-1,

respectively.

To verify how radial deformation of a SWNT

affects its reaction activity, Fig. 6 plots the electron

localization functions (ELF) of a circular and a radial-deformed (11,11) tubes with dt of 1.49 nm.

Higher values of ELF implies more localized electron

density distribution. As can be seen, the circular tube

distribute uniform curvatures whereas for the

radial-deformed tube, sidewall sites with high and

low local curvatures are introduced. At the sites with

low local curvature, most electron density focuses

between the two neighboring carbon atoms. At the

high local curvature sites, however, more partial

electron density distributes around the carbon atoms

and less in the carbon-carbon neighboring section

than at the low local curvature sites. Those

delocalized partial electrons are more active and

easier to be attacked upon chemical reactions [40]. Meanwhile, as radial deformation affects the s and p

orbital hybridization on carbon atoms, there is less sp2 component on the carbon-carbon bonds at the

high local curvature sites than at the low local

curvature sites, resulting in weaker carbon-carbon

bonds and higher reaction priority at those high local


9 Nano Res.

curvature sites. Once the reaction is initiated from

those high local curvature sites, the SWNTs become

much more reactive. On the other hand, uniform

curvatures on a circular tube without radial

deformation lead to uniform electron density on the

sidewalls, and consequently the reactivity is low.

Figure 6 ELF plots of (a) a circular and (b) a radial-deformed (11,11) tubes.

For the reaction of SWNTs with iodine vapor, we

have previously concluded that quartz

lattice-oriented SWNTs can react with iodine and that

large-diameter tubes distribute higher reaction

priority over the small ones. Based on the above

experimental data and analysis, we now believe that

our design of varying the reaction selectivity of

SWNTs by the radial deformation of SWNTs and the

consequent changes in the local curvature of tubes

with different diameter is successful. As radial

deformation results in distorted partial electron

distribution and weaker carbon-carbon bonds at

certain high local curvature sites, it is thus easier for

those weaker carbon-carbon bonds to break down

upon reaction. Since large-diameter tubes present

larger degree of radial deformation, the local

curvature of their high curvature sites could become

even higher than that for the small ones. Therefore, it

is easier for the carbon-carbon bonds at the sidewall

sites with high local curvature on large-diameter

tubes to break down upon reaction. And the

anomalous reaction priority for large-diameter

quartz lattice-oriented SWNTs over the small ones

can be readily expected.

The reaction of iodine with the radial-deformed

SWNTs on quartz is likely a radical addition reaction.

For the lattice-oriented SWNTs on quartz substrates,

different degree of radial deformation in SWNTs, i.e.,

larger for large-diameter tubes and smaller for the

small ones, is present due to the strong SWNT-quartz

interaction. Consequently, the SWNTs with different

degree of radial deformation distribute different

chemical reaction activities. When iodine vapor

which serves as a mild reactant, is introduced to the

SWNTs, first, iodine molecules break down into

iodine radicals by homolysis at 900 oC. Second, the

iodine radicals selectively attack the radial-deformed

SWNT sidewall sites with high local curvature,

presumably on the large-diameter tubes. Then,

iodine leaves with carbon in a form of CIx from the

SWNT, and the thermally unstable CIx quickly

decomposes upon heating. As a consequence,

large-diameter tubes are selectively etched by iodine

vapor, and the percentage of small-diameter tubes

increases significantly after this reaction.

4 Conclusion

Based on the large variations in both intensity and frequency from the in situ Raman RBM mappings of

SWNTs, we distinctly observe radial deformation for

quartz lattice-oriented SWNTs, and attribute this

radial deformation to the strong interaction between

SWNTs and the quartz lattice. We also find

large-diameter tubes distribute larger relative RBM

variations than the small ones, indicating larger

degree of radial deformation for the large-diameter

tubes. As larger degree of radial deformation

introduces higher local curvature sites on the SWNT

sidewalls, more delocalized partial electron

distributions are present at those higher local

curvature sites, resulting in weaker and more

reactive carbon-carbon bonds at those higher local

curvature sites on large-diameter tubes. Therefore,

the normally less reactive large-diameter tubes

distribute anomalous higher reactivity upon reaction

with iodine vapor, and this is in good accordance

with the statistical Raman RBM distribution data. In

this reaction, iodine vapor serves as a mild reactant

that can distinguish the different reactivities of

SWNTs with different degree of radial deformation

on quartz substrate. The observation of radial

deformation of quartz lattice-oriented SWNTs is

important for understanding the behavior of such

SWNTs, and the anomalous diameter-dependent

reaction activity offers a novel approach for selective

removal of small-bandgap large-diameter tubes.


10 Nano Res.

5 Computational Methods

Density functional theory (DFT) computations were

performed with the Perdew-Burke-Ernzerhof (PBE)

[41] exchange correlation function using the Castep

module [42]. Projector augmented wave (PAW) [43]

was chosen to describe the interaction between

atomic core and electrons with the cut-off energy for

plane wave set 400 eV. General gradient

approximation (GGA) [44] was used to include the

electronic exchange and correlation. The Periodic

boundary conditions (PBC) were implemented and

the supercell box was 30.00×30.00×4.91 Å 3 with at

least 15 Å of vacuum along nonperiodic directions to

preclude interactions between SWNT and its images.

Special 5 k-points uniformly were sampled along the

1D Brillouin zone with the energy convergence less

than 1 meV/atom. The electron density distribution

was determined by electron localization function

(ELF) [45, 46].

Acknowledgements

The authors would like to thank Ministry of Science

and Technology of China (Project 2011CB933003),

National Natural Science Foundation of China

(Projects 21125103, 91333105, 11179011), and Beijing

Higher Education Young Elite Teacher Project (No.

YETP0007) for financial support.

Electronic Supplementary Material: Supplementary

material (SEM images, statistical RBM distributions,

Raman spectra of SWNTs) is available in the online

version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher). References

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Nano Res.

Electronic Supplementary Material




Juan Yang, Yu Liu, Daqi Zhang, Xiao Wang, and Yan Li ( )

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

SUPPLEMENTARY FIGURES S1-S5

Figure S1 SEM images of quartz lattice-oriented SWNTs before (a) and after (b) a heating time of 15 min at

900 °C without introducing iodine, serving as a control to Figures 4a and 4b. No distinguishable changes before

and after heating are observed.

Figure S3 SEM images of quartz lattice-oriented SWNTs before (a) and after (b) a heating time

of 15 min at 900 °C without introducing iodine, serving as a control to Figures 4a and 4b. No

distinguishable changes before and after heating are observed.


Nano Res.

Control

MS

62%

MS

0%

Figure S2 Statistical distributions of RBM frequencies with 633 nm exciation at positions 1 (a) and 2 (b) in

Figure 3b for quartz lattice-oriented SWNTs after a reaction time of 15 min at 900 °C in iodine vapor. Metallic

and semiconducting tubes are denoted as M and S, respectively.

Figure S3 Randomly selected Raman G band spectra of some individual tubes from Figures 6e (before) and 6f

(after).

Address correspondence to Yan Li, [email protected]

radial deformation of single-walled carbon nanotubes … · on quartz substrates and the resultant...

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