Ressonància magnètica: ESR, RMN ESR o EPR: Ressonància de Spin Electrònic, o Ressonància Paramagnètica Electrònica RMN: Ressonància Magnètica Nuclear

ESR Ressonància de Spin Electrònic In most molecules, all electrons are in pairs, each pair with identical quantum numbers For an unpaired e- spin transitions can occur. ESR detects defects with unpaired electrons. In the absence of an external magnetic field, B, the unpaired electron, does not have a preference for any of the possible spin states; is a degenerated level.

Based on the Zeeman effect, a magetic field B 0 splits the electronic energy levels, the splitting is proportional to B 0 An electromagnetic radiation can induce a transition, when his energy h is exactly  E, the resonance condition Electron Spin Resonance: ESR measuring B 0, Information on energy levels

X-band microwaves 9-10 GHz X-band ESR conditions B 0 estatic, varying slowly from 0 a to 6000 Gauss to split the energy levels degeneration (Zeeman effect). An electromagnetic field B 1, perpendicularly polarized to B 0 at a convenient frequency to induce transitions between the Zeeman levels Magnetic field B G At the resonance, there is a power microwaves absorption and the cavity factor Q diminishes. The ESR line observed is the first derivative of this absorption

Exemple: Carbon nanotubes 5 1. Carbon nanotubes: properties 2. Thin films of carbon nanotubes Preparation. Features 3. TGA Analysis -The method. Results -Verification: IR optical absorption 4. Impedance measurements : DC Resistance, cut-off frequency 5. Electron Spin Resonance: ESR The method Results 6. CNT-conducting polymer pH sensors CNT/polypyrrole (CNT/PPy), CNT/polyaniline (CNT/PA) Properties pH Sensor characteristics Conclusions

Carbon nanotubes: properties 6 ICNM 2009 Discovered by S. Iijima, 1991 Mechanical properties High young modulus (Y = TPa) Electrical/ thermal properties *Ballistic electronic conductivity Depending on their chirality's *metallic *Semiconductors *High electrical and thermal conductivity (  kS/cm,  T = 6 kW/mK) *Low expansion coefficient, CTE <7.5 ppm/K *High density current High aspect ratio

Thin films of carbon nanotubes 7 ICNM 2009 Transparent and flexible carbon nanotube transistor -E. Artukovic, M. Kaempgen, D.S. Hecht, S. Roth, G. Grüner, NanoLetters, 5, 757 (2005) Transparent and flexible electrodes -Z. Wu, Rinzler et al., Science, 305,1273 (2004). -N. Ferrer-Anglada, M. Kaempgen,V. Skákalová, U. Dettlaf-Weglikowska, S. Roth, Diamond and related Materials, 13, 256 (2004). Nanocomposites -N. Ferrer-Anglada,V. Gomis, Z. El-Hachemi, U, Dettlaff-Weglikowska, M, Kaempgen, S, Roth, Physica Status Solidi (a) 203 (6), 1082 (2006). Applications *Electronic devices *Flexible displays (Samsung) *Flexible solar cells *Transparent flexible electrodes *Transparent flexible transistors

Thin films preparation 8 ICNM 2009 Obtention On a transparent and/or flexible substrate: glass, quartz, PPC, etc. We use single wall CNT made by arc discharge (Nanoledge) or Laser ablation (MPI-Stuttgart) By spraying a light suspension of CNT in an aqueous solution of SDS, after sonicating it (1h, 40W) heating the substrate at ºC, to avoid drops. After that, the film is submerged in water and dried in air Objective - to obtain reproducible films with reproducible properties - High electrical conductivity % transparency - flexible

ESR spectra of SWCNTs obtained by different methods

As usually can be considered a superposition of 3 lines, assigned to: Very large line: magnetic impurities From magnetic ions, catalizers‏ Narrow, symmetric: defects Narrow, asymmetric line: conduction electrons from CNTs Carbon nanotubes ESR spectra

ESR spectrum fitted by three Lorentzian lines A Abiad, N Ferrer-Anglada, S. Roth, Phys. Status Solidi B 247, (2010)

Randomly distributed semiconducting and metallic SWCNTs The temperature dependence of the ESR linewidth (∆HPP) for the narrow asymmetric component (line 1 in Figure 1) of the three SWCNT samples.

Randomly distributed semiconducting and metallic SWCNTs The temperature dependence of the ESR linewidth (∆HPP) for the narrow asymmetric component (line 1 in Figure 1) of the three SWCNT samples.

ESR on selected CNTs Figure 2. Comparison of X-Band ESR spectra at room temperature of selected 99% semiconducting or metallic SWCNTs. Comparison of X-Band ESR spectra at room temperature of selected 99% semiconducting or metallic SWCNTs.

Figure 8. Temperature dependence of signal intensity and activation energies of semiconducting or metallic SWCNTs corresponding to the resonance field at the perpendicular and parallel orientation. ESR on selected semiconducting or metallic CNT Temperature dependence of signal intensity of semiconducting or metallic SWCNTs corresponding to the resonance field at the perpendicular and parallel orientation.

ESR on selected CNTs Temperature dependence of the ESR linewidth for selected semiconducting or metallic SWCNT corresponding to the resonance field at the perpendicular and parallel orientation.

Conclusions The ESR intensity of the asymmetric lines assigned to SWCNTs conduction electrons decrease when the T decreases from 300 K to 160 K in most samples, following an Arrhenius law. The temperature dependent lines should arise from semiconducting nanotubes. For carbon nanotubes produced by arc discharge using non-magnetic catalyst Pt/Rh, the intensity of the asymmetric line does not show an exponential behavior, it keeps constant with T. This one is the unique sample that shows a narrow linewidth of 4 G. The asymmetry factor (I 1 /I 2 ) shows clearly anisotropy. The asymmetry factor is higher for the selected 99% metallic SWCNTs, as expected, since this parameter is used to characterize the resonance line assigned to itinerant spins. For the 99% metallic SWCNTs, the temperature dependence of the ESR intensity does not correspond to a line due to conduction electrons. It should be temperature independent.