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Chemistry and Electronics of Carbon Nanotubes Go Together.

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Modifying Nanotubes
Chemistry and Electronics of Carbon Nanotubes
Go Together
Ernesto Joselevich*
chemical reactivity · electronic structure · molecular
electronics · nanotechnology · nanotubes
ne of the critical issues for the
application of single-wall carbon nanotubes (SWCNTs) in nanoelectronics is
the control of their electronic properties,
which can be either metallic or semiconducting in their pristine form, depending on their diameter and chirality.[1] Since all known preparative methods yield mixtures of metallic and semiconducting nanotubes, extensive research has been devoted to the
modification of the nanotube electronic
structure and to the separation between
metallic and semiconducting carbon
nanotubes. Recently, metallic and semiconducting nanotubes have been physically separated by electric fields, based
on their different dielectric constants.[2]
Recently, two groups simultaneously
reported how chemical reactions involving covalent bonds can modify the
electronic structure of single-wall carbon nanotubes,[3] and have a high selectivity for metallic versus semiconducting
carbon nanotubes,[4] thus opening a
potential avenue to their complete separation by chemical manipulation. Beyond the practical implication of this
breakthrough for nanotechnology, the
fundamental interplay between electronic structure and chemical reactivity
in carbon nanotubes begins to be revealed.
Previously, the electronic properties
of carbon nanotubes were modified by
ionic doping,[5] and recent advances
have enabled significant discrimination
[*] Dr. E. Joselevich
Department of Materials and Interfaces
Weizmann Institute of Science
Rehovot 76100 (Israel)
Fax: (+ 972) 8-934-2350
between metallic and semiconducting
nanotubes to be made, based on differences in physical properties, such as
their dielectric response to electric fields
and electrical breakdown,[2, 6, 7] or their
selective ability to adsorb surfactants
and DNA.[8–11] What is new in the work
of Kamaras et al.[2] and Strano et al.[4] is
that they directly exploit the connection
between the chemical reactivity and the
electronic structure of metallic and
semiconducting carbon nanotubes,
which are treated as molecular species.
The effect of covalent modification
on the electronic structure of single-wall
carbon nanotubes is demonstrated by
Kamaras et al.,[3] who treated the nanotubes with dichlorocarbene (Figure 1 a).
This covalent sidewall functionalization
is found to convert metallic nanotubes
into semiconducting ones, as shown by a
strong decrease in intensity in the farinfrared absorption spectrum, assigned
to intraband transitions near the Fermi
level, and a simultaneous increase in the
intensity in the visible region. The effect
is opposite to that of ionic doping, which
turns semiconducting nanotubes into
conducting (metallic) nanotubes, by injecting electrons or holes into the valence or conduction bands, respectively.
Covalent chemistry, on the other hand,
breaks up the all-conjugated system into
a series of smaller condensed aromatic
systems by introducing saturated sp3
carbon atoms in the nanotube backbone.
The concomitant electronic localization
and loss of translational symmetry open
a gap at the Fermi level of the metallic
carbon nanotubes, turning them semiconducting (the Fermi level is the chemical potential of the electrons, in metals
it is the topmost filled level at zero
temperature, whereas in semiconductors
it lies within the bad gap, where no
states are allowed).
The effect of electronic structure on
chemical reactivity, on the other hand, is
demonstrated by Strano et al.,[4] who
treated single-wall carbon nanotubes
with diazonium reagents (Figure 1 b),
while monitoring spectral changes. UV/
Vis-NIR spectra indicates that 4-chlorobenzenediazonium reacts preferentially
with metallic nanotubes, while Raman
spectroscopy spectra provide details
about the electronic structure of specific
nanotubes under reaction. Low-wavenumber Raman spectra, associated with
radial breathing modes, allow for the full
DOI: 10.1002/anie.200301715
Angew. Chem. Int. Ed. 2004, 43, 2992 –2994
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Two covalent reactions of single-wall
carbon nanotubes. a) Result of the reaction
with dichlorocarbene, which turns a nominal
double bond into a cyclopropane ring (circled). b) Reaction with 4-chlorobenzenediazonium tetrafluoroborate, which releases nitrogen and adds a 4-chlorophenyl group to the
structural assignment of a few metallic
and semiconducting nanotubes, and
show that the reaction is completely
selective for the metallic nanotubes,
leaving the semiconducting ones intact.
Raman peaks associated with tangential
stretching modes show a decrease in
intensity, because the reaction disrupts
the resonance that gives rise to enhanced Raman scattering. On the other
hand, the intensity of peaks associated
with a symmetry-forbidden transition
sharply increases, because the reaction
breaks the translational symmetry that
forbade the transition, and then decreases, along with the tangential stretching
peaks, owing to the same loss of resonance. In addition to this metallic/semiconducting selectivity, the reactivity of
the metallic nanotubes is inversely related to their diameter.
Although the electronic band structure of carbon nanotubes has been
recognized from the time of their discovery, previous discussions on the
chemical reactivity of carbon nanotubes
have attributed a major role to local
strain, which arises from curvature-induced pyramidalization and misalignment of p orbitals.[11] The diameter selectivity found by Strano et al. can be
correctly predicted by this formalism.
However, the metallic/semiconducting
selectivity underscores the role of the
electronic band structure in determining
the chemical reactivity of the single-wall
carbon nanotubes. To account for this
selectivity, Strano et al. propose a reaction mechanism in which the diazonium
reagent forms a charge-transfer complex
at the nanotube surface. Electron donation stabilizes the transition state that
leads to the decomposition of the diazonium functional group and the covalent
attachment of the aryl group to the
nanotube. The extent of electron donation correlates with the density of states
at the Fermi level, which is larger for
metallic nanotubes.
The role of electronic structure in
determining the chemical reactivity of
molecules is central to organic-chemistry reaction mechanisms. One of the
difficulties in rationalizing the chemical
properties of carbon nanotubes in terms
of their electronic structure is that the
electronic structure is usually represented by band structure diagrams in reciprocal space, which are better suited for
Angew. Chem. Int. Ed. 2004, 43, 2992 –2994
describing materials properties than
molecular reactions. We have recently
shown a chemist's view of carbon nanotubes that merges the solid-state physics
description with the molecular orbitals
picture of chemical reactions, by applying traditional concepts of organic
chemistry, such as aromaticity, orbital
symmetry, and frontier orbitals.[12] Metallic nanotubes are only slightly less
aromatic than semiconducting ones in
terms of overall resonance energy.[13]
However, from a frontier-orbitals perspective, the kinetic stability of polycyclic aromatic molecules is more related
to their HOMO–LUMO energy gap,[14]
than to their resonance energy, which
determines their thermodynamic stability. The selectivity towards addition
found by Strano et al. underscores the
role of the HOMO–LUMO energy gap
in determining the reactivity of singlewall carbon nanotubes.
A more detailed perspective that
could be useful to describe the p chemistry of carbon nanotubes, is Roald
Hoffmann's frontier-orbitals picture of
chemisorption.[15] Molecule-surface twoorbital interactions are classified according to the total number of electrons
in the two orbitals involved. These
include the usual HOMO–LUMO and
LUMO–HOMO two-electron interactions that normally dominate the overall
interaction between two molecules, but
also interactions between two occupied
orbitals (four-electron) or between two
empty orbitals (zero-electron), which
are usually repulsive or ineffective but
become attractive after charge transfer
to or from the surface, respectively.
Charge transfer depends on the relative
position of the Fermi level with respect
to the mixed molecule–surface LUMO.
This picture, when applied to carbon
nanotubes in a system such as the one
reported by Strano et al. (Figure 1 b),
could be represented as in Figure 2. In
this case, a metallic nanotube (Figure 2 a) can establish attractive interactions with the molecule that a semiconducting nanotube (Figure 2 b) cannot. As in heterogeneous catalysis,
chemisorption weakens the inner bonds
inside the molecule and activates it
towards chemical reaction.
In conclusion, the results of Strano
et al.[3] and Kamaras et al.,[4] could be
summarized in the familiar terms of
organic chemistry: A higher reactivity
for smaller diameter carbon nanotubes
towards the addition reactions can be
attributed to increased steric strain,
while a higher reactivity for metallic
versus semiconducting nanotubes is related to the large differences in their
HOMO–LUMO gap. In addition, metallic nanotubes can act as redox catalysts
by providing a reservoir of electrons.
Upon reaction, the all-conjugated system of each metallic nanotube breaks
into smaller polycyclic aromatic systems
with larger HOMO–LUMO gaps. The
road is now open for the manipulation
and integration of carbon nanotubes
into functional devices by chemical
strategies. On the theoretical front,
new models of carbon-nanotube chemistry will have to take into account both
the localized and delocalized electronic
structure, and blend together the quantitative power of the solid-state physics
Figure 2. Frontier-orbital representation of zero-electron two-orbital interactions between a molecule and a metallic (a) or a semiconducting (b) single-wall carbon nanotube. The molecule frontier orbitals are designated as HOMO and LUMO, and the nanotube orbitals are represented by
the density of states plots.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
framework with the rich traditional
knowledge of organic chemistry.
Published Online: April 30, 2004
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Angew. Chem. Int. Ed. 2004, 43, 2992 –2994
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