Laboratoire Charles Coulomb UMR 5221 CNRS/UM2 (L2C)

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Accueil > La Recherche > Axes & Equipes > Nanostructures & Spectroscopies > Equipe : Nanomatériaux > Thème : Elaboration et étude de réseaux, films et composites à base de nanotubes

Dopage, Confinement et fonctionnalisation dans les nanotubes

par Sébastien LAYSSAC, Webmaster - publié le , mis à jour le

The controlled tuning of carbon nanotubes physical properties constitutes one of the major challenges for the application of these materials. We aimed at modifying either the electronic or the structural properties. We explored four different directions that look particularly interesting :
The first one consists in the doping of carbon nanotube with other chemical elements like boron (B) or nitrogen (N).
The second one concerns the functionalization of the carbon nanotubes. Carbon nanotubes can be fonctionalized either by oxidation or by attaching covalently organic molecules via nitrenes, carbenes or radicals...
The third one deals with the intercalation by alkali metals or halogens in order to induce a charge transfer between the host material and the nanotubes.
The last one is the trapping of foreign species into the hollow core of nanotube that can give rise to fascinating confinement effects since the behavior of the confined phases can differ significantly from the bulk phases. Our group has been investigating for many years confinement effects of several species A (A=iodine chains, C60 molecules, conjugated polymer ...) inserted into the inner core of SWNT. The main objective is to induce an efficient coupling between the host species and the SWNT in order to modify either the electronic or the optical properties of SWNT. Indeed, the strong interaction between the enclosed specie and the SWNT cavity allows to finely tune the characteristics of the whole system.

Nitrogen doping of carbon nanotubes

Involved researchers : J.L. Bantignies, L. Alvarez, R. Baaba
Collaboration : B. Doyle - ELECTRA (Trieste, Italie), A. Loiseau - LEM, ONERA, F. Lenormand - IPCMS Strasbourg, Ph. Parent - LCPMR Orsay, O. Stéphan - LPS Orsay

The electronic properties of undoped carbon nanotubes can be either semiconducting or metallic, depending on their chirality. Doping carbon nanotubes with nitrogen or boron could be a particularly interesting way for tuning their electronic properties. This has led to an increasing interest in preparing doped carbon nanotubes. For instance, N-doped nanotubes show n-type behavior regardless of a tube chirality
In our group, the nitrogen doping of carbon nanotubes is both investigated by high energy resolution NEXAFS and spatial resolved EELS (collaboration with LPS). Most of the tubes appear to be N-doped. In addition different electronic structures of nitrogen are clearly evidenced (pyridine-like, and pyrollic-like). High resolution NEXAFS experiments show that most of the nitrogen atomic content of N-MWNT seems to arise from "molecular N2 encapsulated" inside the tubes in a small fraction of the N-MWNT (G feature on figure 1).

Figure 1 : N K-edge NEXAFS spectra of multi-walled CNx nanotubes - © L2C

S. Enouz et al. J. Nanosci. Nanotechnol. 7 (2007)

Functionalization of carbon nanotubes

Involved researchers : J.L. Bantignies, M. Paillet
Collaboration : J.P. Lère-Porte, F. Serein-Spirau - ICG Montpellier)

Structural modification of carbon nanotubes offers the possibility to confer them new properties and to facilitate their processing and assembly. Among the large opportunities given by functionalization chemistry, we are studying the effect of covalent grafting and non-covalent stacking of optically active species on carbon nanotube walls. We are interested in the investigation and the characterization of the structural and physical properties of functionalized carbon nanotubes. We are varying parameters such as the type of chemical reaction used for functionalization, the density and chemical nature of the addends and the diameter/chirality distribution of the nanotube samples. Beyond the fundamental aspect of such studies, we aim to design new hybrid nanostructures with extended functionalities.

Raman (left) and absorption (right) spectra of raw and iodo-phenyl functionalized nanotubes. - © L2C

J. Cabana et al. Langmuir in press

Functionalization of carbon nanotubes with thermoplastic polymers

Involved researchers : J.L. Bantignies, L. Alvarez, R. Baaba, V. Leon
Collaboration : B. Doyle - ELECTRA (Trieste, Italie), F. Despetis - GES Montpellier, F. Lenormand - IPCMS Strasbourg, Ph. Parent - LCPMR Orsay

Processing of carbon nanotubes (NTs) has been difficult due to their insolubility in the common solvents. Considerable research efforts have been focused on the chemical modification of NTs surface which can provide not only a facility for manipulation by improving the solubility in solvent, but also which gives the possibility to create functional nanomaterials. Functionalization of NTs gives an approach to develop superior composite materials with significant physical properties. The grafting of functional polymers on nanotubes can improve both the ability to disperse the nanotubes homogeneously throughout the matrix and load transfer from the matrix to nanotubes. Functionalization of multiwalled carbon nanotubes (MWNTs) surface by thermoplastic and thermoset polymers is studied. The study of the surface chemistry of carbon nanotubes is still a serious task.
Combined microscopy and spectroscopic (NEXAFS, XPS, IR, Raman) investigations are used to give evidence of covalent functionalization of NTs by macromolecules. In the case of sulfonated poly (ether ether ketone) (SPEEK) chains (thermoplastic polymer) grafted on NTs, Transmission electron microscopy (TEM) shows that tubes are wrapped by polymer chains (figure 1).

Figure 1 : TEM images of a) pristine MWNTs and b) SPEEK-MWCNTs - © L2C

M.-R. Babaa et al. J. Nanosci. Nanotechnol. 7, (2007)

Structure and electronic properties of nanotubes in polyelectrolyte solutions

Involved researchers : F. Dragin (PhD), E. Anglaret
Collaboration : A. Pénicaud (CNRS Bordeaux) and R. Martel (Dpt Chemistry, U. Montréal

Nanotubes salts (charge transfer systems nanotube-alkali atoms) are soluble in polar aprotic solvents and form polyelectrolyte solutions. We studied the structure and optical properties of nanotubes in polyelectrolyte solutions. Coupled AFM and X-ray diffraction studies demonstrated that the nanotubes are essentially individual in the solutions. We showed that redox titration could be used to control the global density of charges in the solutions. The Raman signatures of the nanotubes change continuously during the titration and the results indicate a direct charge-transfer reaction from the acceptor molecules to the nanotubes, leading to their gradual neutralization.
We also studied charge transfer in an electrochemical cell. We found that charge transfer can be achieved selectively on different (n,m) nanotubes because of their different electronic structures. We used Raman spectroscopy to study selective (n,m) undoping and p-doping as a function of electrochemical potential (see figure). We found that the potentials for undoping and doping nanotubes depend on the diameter and chiral angle. These measurements allowed us to measure the work function of individual (n,m) nanotubes

Raman spectra of a polyelectrolyte solution for various electrochemical potentials - © L2C

A. Pénicaud et al, JACS 127(1), 8 (2005) ; A. Pénicaud et al, Comp. Science and Tech. 67, 795 (2007)
E. Anglaret et al., J. Phys. Chem. B 110, 3949 (2006)
Funding : ANR P-NANO TRICOTRA, CRSNG-Canada and Lavoisier-France grants

Structural properties of rubidium intercalated single walled carbon nanotubes

Involved researchers : J.L. Bantignies, L. Alvarez, J.L.Sauvajol, A. Zahab, R. Aznar, E. Alibert
Collaboration : F. Villain, V. Briois (SAMBA, Soleil, Saint Aubin)

Alkali metal insertion into SWNT bundle is shown to lead to a significant increase of the SWNT conductivity but the structure of the intercalated phases as a function of the doping rate is still an open question. In particular, the intercalation sites as a function of the doping rate are still not identified. The different sites available are displayed on figure 1. In the aim to light these points, we are currently developing an experimental set-up devoted to multi-spectroscopy investigations during an in situ intercalation of alkali metal vapour phase (figure 2).
We want to probe the local arrangement around the dopant by X-ray Absorption experiment (EXAFS) at Soleil synchrotron Facility, the electrical conductivity and the vibrational properties by Raman spectroscopy. More precisely, the study consists in determining the correlation between the electrical and the structural properties of intercalated single walled nanotubes. However, because alkali metals are extremely air sensitive, encapsulation into the inner core of SWNTs is a mandatory requirement for further potential applications (in fact, most of the alkali metal ions are naturally inserted inside the nanotubes). Our final aim is the identification of insertion phases of controlled and chosen electronic properties that could be stable under normal conditions.

Figure 1 : Interstitial possible sites - © L2C

JL. Bantignies et al, Phys. Rev. B, 71, 195419, (2005)

Figure 2 : Home made in situ experimental set-up - © L2C

Structural properties of carbon peapods

Involved researchers : L. Alvarez, J.L.Sauvajol, R. Almairac, M.Chorro, S. Rols, J. Cambedouzou, J.L. Sauvajol
Collaborations : M. Mezouar, ID 27 (ESRF, Grenoble), H. Kataura (AIST, Japan)

Recently, much effort have been focussed on the so-called "carbon peapods", consisting in C60 molecules trapped into single-walled carbon nanotubes (figure 1). Structural properties have been studied under high-pressure/high-temperature conditions (HPHT) by in situ X-ray diffraction on the ID27 beamline at ESRF. Under ambient conditions, the results showed that fullerenes form a 1D crystal inside the tubes with lattice parameter of 9.8 Å in good agreement with van der Waals interaction between the C60 molecules. At 4 GPa, increasing the temperature up to 1023 degrees leads to a progressive C60 polymerization by cyclo-addition reaction that is associated to a shortening of C60-C60 distance down to 8.7 A. Back to ambient conditions, the C60-C60 chain remains polymerized, emphasizing the high stability of this material.

M. Chorro et al, Phys.Rev.B , 74, 205425 (2006)

Structural properties of iodine intercalated carbon nanotubes

Involved researchers : L. Alvarez, J.L. Bantignies, J.L.Sauvajol, R. Leparc, A. Zahab, R. Aznar
Collaborations : A. San Miguel, D. Machon (LPMCN, Lyon), M. Chorro, O. Mathon (BM 29, ESRF, Grenoble), P. Lagarde, J.P. Itié, A.M. Flank (SLS, Villigen, Switzerland), P. Launois, J. Cambedouzou, (LPS, Orsay)

Insertion of SWNT with halogens (iodine) improves the SWNT conductivity by charge transfer. Our works by X-ray absorption spectroscopy (EXAFS) in collaboration with the beamline BM29 at the ESRF synchrotron investigated the poly-iodides structure inside the nanotube (figure 1) and evidenced the presence of mainly disordered I5- chains. More recent investigations carried out by Raman and X-ray absorption spectroscopy at SLS suggest a strong modification of the charge transfer on the poly iodides chains under high pressure ( 8 GPa). Raman results could even be interpreted by considering a transformation of I5- chains into I3- chains under pressure.

Figure 1 : Polyiodide chains in the inner core of SWNTs - © L2C

Michel et al, EXAFS investigations of iodine-doped carbon nanotubes : PHYS.REV.B , 73, 195419, (2006)

Quater-thiophene encapsulated into single walled carbon nanotubes

Involved researchers : L. Alvarez, J.L. Bantignies, J.L.Sauvajol, R. Leparc, A. Zahab
Collaborations : P. Hermet (FUNDP, Namur, Belgique), R. Babaa, P. Jegou (CEA, Saclay), S. Rols (ILL, Grenoble)

Encapsulation of conjugated polymers into SWNT can extend the optical properties of SWNT to the UV domain. Thus, the whole system could be used as a nanodiode being able of absorbing light over a wide range, from the UV to the near infrared.
Very recently, we undertook studies in collaboration with CEA saclay on quaterthiophene encapsulated into SWNT (figure 1) by Raman spectroscopy and inelastic neutrons scattering on the beamlines IN4 and IN6 at ILL. An important charge transfer between the guest oligothiophenes and the host tube has been evidenced. This strong interaction significantly changes the optical properties of the quaterthiophene. Inelastic neutrons scattering evidenced a phase transition for temperatures below 150 K.

Figure 1 : Quater-thiophene encapsulated into SWNT - © L2C

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