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 B0 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 B0 ,

Information on energy levels

X-band microwaves 9-10 GHz

X-band ESR conditions

B0 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

B0 3300 - 3400G

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 = 1 - 2 TPa)

Electrical/ thermal properties

* Ballistic electronic conductivityDepending 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 preparation8 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 120-150 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

- 90-95% 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 impuritiesFrom 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 Randomly distributed semiconducting and metallic SWCNTsand 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 Randomly distributed semiconducting and metallic SWCNTsand 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

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

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 (I1/I2) 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.