Marulanda J.M. (ed.) Electronic Properties of Carbon Nanotubes

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Characterizing Functionalized Carbon Nanotubes

for Improved Fabrication in Aqueous Solution Environments

graphene oxides (Liu et al., 2008; Geim, 2009; Yan and Chou, 2010), for advanced

technological applications.

5. Acknowledgements

We gratefully acknowledge support from the Faculty Research Creative Activity Committee

(FRCAC) of Middle Tennessee State University awarded in 2011.

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Electronic Properties of Carbon Nanotubes

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Selective Separation of Single-Walled

Carbon Nanotubes in Solution

Hongbo Li and Qingwen Li

Suzhou Institute of Nano-Tech and Nano-Bionics, CAS

P. R. China

1. Introduction

SWCNT can be conceptualized by wrapping a one-atomic-layer thick graphene into a

hollow cylinder. As shown in Figure 1, the wrapping way is represented by the chiral vector

(n, m), which denote the number of unit vectors along two directions in the crystal lattice of

graphene sheet.

Because of the symmetry and unique electronic structure of graphene, the

structures of SWCNTs strongly affect their electrical properties. In particular, their band

gaps can vary from zero to about 2 eV and electrical conductivity can be in a range of a

metal or semiconductor. For a given (n,m) nanotube, when n = m, the nanotube is metallic;

when n − m is a multiple of 3, the SWCNTs are semiconducting with geometry-dependent

band gaps.

Fig. 1. A SWCNT is formed by rolling up a graphene along a chiral vector. The

circumference of the SWCNT is determined by a chiral vector Ch = na

+ ma

, where (n,m)

are integers known as the chiral indices and a

and a

are the unit vectors of the graphene

lattice. Reproduced from ref.1.

Metallic SWCNTs (m-SWCNTs) may show ballistic behaviors and are ideal conducting

connectors and electrodes for electronic devices due to their excellent electron transport

Electronic Properties of Carbon Nanotubes

behavior, optical transparency and flexibility.

2-5

Semiconducting SWCNTs (s-SWCNTs),

with mobilities as high as 79 000 cm

-1

are considered to be among the most promising

candidates for next-generation high-performance field-effect transistors (FETs). The unique

electronic properties of SWCNTs make them the ideal building blocks for nanoelectronics

and thin film devices.

7, 8

Unfortunately, the established CNT synthethetic methods often

lead to the growth of the mixture of nanotubes, typically with 1/3 m-tubes and 2/3 s-tubes,

hard to selectively harvest the CNTs with a specific electronic type (m- / s-) or with a

desired chirality, which has greatly hindered their widespread applications. Therefore, the

mass m/s separation of SWCNTs is of great significance for their advanced applications in

high-performance nanoelectronic devices. Chiral separation is also required for the uniform

property of electronic devices. Since none of current synthetic techniques produce identical

populations of SWCNTs, over the past two decades, many post-synthetic approaches have

therefore been developed in an attempt to separate monodisperse SWCNTs into different

fractions by their electronic types, diameters and chiralities. This chapter provides a brief

introduction of the recent progress in the separation of SWCNT, in particular with an

emphasis on the chromatographic approach.

2. Several important post-synthetic separation approaches

One of the earliest developed enrichment techniques of SWCNTs is based on the selective

removal strategy with bulk SWCNT powders and films. The enrichment of m- or s- type

SWCNTs can be obtained by removing the other type (usually m-SWCNTs) through

electrical breakdown

, plasma

10, 11

, irradiation

12, 13

effects or chemical reagents

11, 14-17

. These

‘‘dry’’ physical or chemical removal techniques are scalable and compatible with existing

semiconductor processing for SWCNT-based nano-devices. However, as the selective

removal is an irreversible process, the removed fraction cannot be used any more. Hence in

recent years, separation of monodisperse SWCNTs, such as selective dispersion,

dielectrophoresis, centrifugation, and chromatography has been greatly developed.

SWCNTs can be thus sorted by electronic types and chiralities to some extent. Great efforts

are still required to further understand the separation mechanism and therefore able to

optimize the SWCNT sorting at lower cost and higher purity.

2.1 Selective functionalization

Both theoretical calculations and experimental studies have shown that the electronic

properties, diameters and chiralities of SWCNTs may affect their interaction with some

compounds, containing small organic molecules, surfactants, DNA and conjugated

polymers. Such tan interaction in return leads to the differentiation and sorting of SWCNTs

of different types.

Metallic SWCNTs are usually believed to have a higher reactivity than semiconducting

SWCNTs due to the higher electron density of states (DOS) at their Fermi levels

and

smaller ionization potential

. By covalent reaction on the sidewalls of SWCNTs, one type of

SWCNTs (generally m-SWCNTs) tends to be functionalized first, leading to its selective

dispersion in a solution and leaving the other one insoluble in the pristine form. As

illustrated in Figure 2, Strano et al. first reported that diazonium reagents preferred to react

with m-fractions rather than s-fraction, making s-SWCNTs well suspended in aqueous

solution, and achieving SWCNT manipulation under controlled conditions.

Selectivity is

Selective Separation of Single-Walled Carbon Nanotubes in Solution

dictated by the availability of electrons near the Fermi level to stabilize a charge-transfer

transition state preceding bond formation. Later Doyle et al. found that several 4-substituted

benzenediazonium salts, Ar-R (Ar = N

+-C

and R = Cl, NO

, OMe) at pH 10 showed the

highest reactivities for s-SWCNTs with the largest band gaps.

Additionally, nitronium

ions

20, 21

and osmium tetroxide (OsO

)

were also found to react preferentially with m-

SWCNTs due to their larger electron density near the Fermi level. However, azomethine

ylides derived from trialkylamine-N-oxides exhibited selective reactions with s-SWCNTs by

cycloaddition, which was achieved by preorganizing the starting N-oxides on the nanotube

surface prior to generating the reactive ylide.

Separation of m-SWCNTs from

functionalized s-SWCNTs was also successfully accomplished by inducing solubilization of

s-SWCNTs in the presence of lignoceric acid. Although covalent functionalization seems an

easy approach for the separation of m- or s-SWCNTs, it remains a challenge as a result of

uncontrollable reaction selectivity and possible structural damages on CNTs themselves.

Fig. 2. (A) Diazonium reagents extract electrons, forming N

gas and leaving a stable C−C

covalent bond at the nanotube surface. (B) The extent of electron transfer is dependent on

the density of states in that electron density near E

leads to higher initial activity for

metallic and semimetallic nanotubes. (C) A functionalized nanotube may exist as a

delocalized radical cation, which tends to receive electrons from neighboring nanotubes or

react with fluoride or diazonium salts. Reproduced from ref.14.

In comparison with covalent functionality, non-covalent functionality is a more popular

separation method, as it is a non-destructive process. By interacting with the hydrophobic

surface of SWCNTs or matching their chiral structures, some organic molecules may

selectively adsorb onto or wrap along SWCNT species. In 2003, Chattopadhyay and

coworkers reported that octadecylamine (ODA) had a higher affinity to carboxyl

functionlized s-SWCNTs than m-fractions. S-SWCNTs were thus retained in the supernatant

due to the increased solubility by stronger ODA adsorption, whereas m-SWCNTs were

Electronic Properties of Carbon Nanotubes

selectively precipitated.

As such, through the selective interaction of amine with s-

SWCNTs, a self-sorted s-SWCNT network could be fabricated by spin-coating SWCNT

solution on amine-functionalized surfaces,

where the chirality separation of nanotubes and

simultaneous control of density and alignment may be achieved in one step. The field-effect

transistors fabricated based on this method showed an on/off ratio as high as 900,000.

Interestingly, in the case of neutral SWCNTs without carboxyl group, both theoretical

calculations and experiments have shown that the amine groups prefer to selectively

interact with m-SWCNTs. Maeda et al. showed that m-SWCNTs could be highly

concentrated to 87% by applying a dispersion-centrifugation process in a tetrahydrofuran

solution of propylamine.

Similar results were also obtained in the m/s separation of

SWCNTs by using bromine

and porphyrins

. The porphyrin and its derivates tend to

attach onto the sidewalls of s-SWCNTs, making them enriched in the supernatant. While

due to the formation of charge-transfer complex between bromine and m-SWCNTs, m-

SWCNTs can be thus effectively sorted out by centrifugation.

In addition to m/s recognition, control on the structures of dispersants may also lead to the

selective separation of SWCNTs by diameter and chirality. Ortiz-Acevedo et al. proposed a

novel approach to coat SWCNTs with reversible cyclic peptides (RCPs) that covalently wrap

around CNTs through the oxidation of thiols incorporated into the peptide backbone.

controlling the length of the RCPs, they demonstrated limited diameter-selective

solubilizations of SWCNTs. Some aromatic polymers were also found to selectively

solubilize certain nanotube species. As illustrated in Figure 3, Nish et al. reported that the

polymer poly(9,9-dioctylfluorenyl-2,7-diyl) was very selective to nanotubes.

They

suggested that the SWCNT–polymer bonding was strongly influenced by both the relative

orientation of the polymer chain to the nanotube structure, and the possible charge transfer

that occur from metallic tubes and lead to changes in the conformation of the polymer.

Fig. 3. (A) The PLE map shows the strength of the PL emission as a false color plot, with the

different species labeled by their (n,m) indices. The emission is dominated by a very

prominent (7,5) peak. (B) A comparison between the Raman spectra, taken using a 647 nm

light source, of surfactant-wrapped SWCNTs (red line) and PFO wrapped SWCNTs (black

line), normalized with respect to the semiconducting (7,5) nanotube. Reproduced from

ref.29.

Selective Separation of Single-Walled Carbon Nanotubes in Solution

Small organic moluecules are also designed for the diameter- selective and chiral separation.

Tromp et al. used oligo-acene adducts as a diameter-selective molecular anchor for

separation and functionalization of SWCNTs.

SWCNT field effect transistors fabricated

from diameter-sorted SWCNTs showed remarkably improved electrical properties

compared to non-sorted SWCNTs. More recently, Wang et al. reported that the enantiomers

of SWCNTs can be separated based on molecular recognition with chiral diporphyrin

nanotweezers.

The chiral nanotweezers consisting of phenanthrene spacer and two chiral

porphyrins may discriminate the diameter and handedness of CNTs simultaneously by

taking into account the relationship between the (n, m) selectivity and the structures of

previously reported chiral nanotweezers. Owing to the relatively narrow cleft made by two

porphyrins, the nanotweezers showed high selectivity toward (6,5)-SWCNTs possessing the

smallest diameter among the major components of SWCNTs grown from CoMoCAT.. In

addition, the single enantiomer of (6,5)-SWCNTs could be enriched through the molecular

recognition with 1. These results imply that it is crucial for the selective functionalization of

SWCNTs to generate a broad variety of molecular anchors and functional backbones with

excellent diameter selectivity.

2.2 Dielectrophoresis

Due to different dielectric constants of m- and s-SWCNTs under an alternating current (a.c.)

electric field, SWCNTs can be sorted by their electronic types via dielectrophoresis. Early in

2003, Krupke et al. developed an alternating current dielectrophoretic method to separate

m-SWCNTs from s-SWCNTs in suspension.

As shown in Figure 4A, when SWCNT

suspension was placed on an array of microelectrodes and applied with an alternating

current, an opposite movement of m- and s-tubes occurred along the electric field gradient.

M-SWCNTs were attracted toward the microelectrode array, leaving s-SWCNTs in the

solvent. Principlly, a complete separation between metallic and semiconducting SWCNTs

may be possible if all tubes in the suspension are dispersed as individual tubes (no bundles).

However, this method was only available for the sorting of m- and s-SWCNTs on a small

scale.

Fig. 4. (A) Schematic of dielectrophoresis of SWCNT suspension by using a microelectrode

array. Reproduced from ref.32. (B) The continuous separation mechanism based on

dielectrophoresis by using an H-shaped microfluidic channel. Reproduced from ref.33.

Later, Shin et al. developed this technique and established a nondestructive, scalable

method for SWCNT separation by using an H-shaped microfluidic channel, where highly

pure m-SWCNTs were continuously extracted from a suspension, as shown in Figure 4B.

Electronic Properties of Carbon Nanotubes

Two laminar streams were generated in an H-shaped microfluidic channel with two inlets

and two outlets. The flow conditions were carefully controlled to minimize diffusive and

convective transport across the boundary between the two flows. Dielectrophoretic force

from the embedded electrode at the junction extracted m-SWCNTs from a stream of

nanotube suspension toward the other stream of buffer solution without nanotubes. Thus,

the highly pure m-SWCNTs were obtained simultaneously at separate outlets. However, the

separation efficiency and purity of s-SWCNTs is still difficult to be enhanced. Moreover, this

method is only compatible with the m/s separation, ineffective for the chiral separation.

2.3 Centrifugation

Centrifugation is a process widely used in industry and in laboratory settings for the

separation of mixtures driven by the centrifugal force. More-dense components in the

mixture migrate away from the axis of the centrifuge, while less-dense components in the

mixture migrate towards the axis. Increasing the effective gravitational force on a test tube,

more dense components will be precipitated on the bottom of the tube. The less dense

components are collected in the supernatant solution. SWCNTs are usually synthesized in a

highly mixed status with a broad distribution of lengths, diameters and chiralities, leading

to the sorting a tough issue. However, centrifugation is found effective for the separation of

SWCNTs. To date with this simple technique, the SWCNTs can be sorted by the m/s,

diameter and chirality.

2.3.1 Density gradient ultracentrifugation

Density gradient ultracentrifugation (DGU) is a technique commonly utilized to separate

and isolate different sub-cellular components, DNA from RNA, and even different

sequences of DNA by their compositions. Arnold et al. firstly described the sorting of DNA-

wrapped SWCNTs by DGU,

this technique has been well developed for SWCNT sorting

with multiple purposes, such as separation by m/s, diameter and chirality, even the

separation of the mirror-image isomers of seven (n,m) species.

35-41

In this process, SWCNT

suspension is added to centrifuge tubes containing liquid mixtures arranged to form a

spatially varying density profile. Under strong centrifugation, SWCNT species are separated

by migrating to regions matching their individual densities. Hence, the sorting efficiency is

greatly dependent on the dispersants and DGU processing.

Salts like sodium cholate (SC) or cosurfactants are often added to optimize the fine sorting

of monodispersed SWCNTs. Arnold et al reported in 2006 that the sorting of SWCNTs by

diameter, bandgap and electronic type using structure-discriminating surfactants to

engineer subtle differences in their buoyant densities.

Density gradients were formed from

aqueous solutions of a non-ionic density gradient medium, iodixanol. Gradients were

created directly in centrifuge tubes by one of two methods: by layering of discrete steps and

subsequent diffusion into linear gradients or by using a linear gradient maker. As shown in

Figure 5, they have isolated the SWCNTs of narrow diameter distributions, more than 97%

of which are within a 0.02-nm-diameter range. Furthermore, by using cosurfactants, they

obtained bulk quantities of SWCNTs of predominantly a single electronic type. By a

hydrodynamic model, Nair et al. described the motion of surfactant-suspended single-

walled carbon nanotubes in a density gradient.

The theoretical results predicted that the

number of surfactant molecules adsorbed on each nanotube determined its effective density

and, hence, its position in the gradient after centrifugation has been completed.