Contacting single bundles of carbon nanotubes with alternating electric fields

1. Contacting single bundles of carbon nanotubes with alternating electric fields Marcella De Carlo Danilo Zampetti

2. Introduction Carbon nanotubes are quasi-one dimensional solid. They can be: • Multiwall (firstly observed in 1991) • Single-wall (observed in 1993) They play a crucial role in miniaturization at the nanometre scale thanks to their great properties.

3. Properties of CNTs They can: • Conduct electricity better than copper • Transmit heat better than diamond • Tolerate stress better than steel Now, we focus our attention on CNTs’ electrical properties.

4. Electrical properties We canhave: • Metallic nanotubes (ideal nanoscale wires) • Semiconducting nanotubes (enable us to build nanoscale field-effect transistor) This distinction depends on the way the grafite sheet, that constitutes the nanotube, is rolled up.

5. Molecular Wires Thanks to their high electrical conductivity and chemical stability, we can use nanotubes to realize molecular wires. HOW CAN WE CONTACT AND POSITION NANOTUBES IN A CONTROLLED WAY? There are some methods: • Spraying of nanotubes • Catalytic growth of nanotubes • Self-assembling on chemically modified surfaces

6. Why don’t we tell nanotubes where to go? Recently, a new method has been proposed… Applying an alternating electric field, we can place a bundle of carbon nanotubes between two metal electrodes!

7. Nagahara’ s Advantages As shown by Nagahara et al., this can be a viable technique for wiring up nanotubes into circuits. In fact: • After the initial patterning of the metal electrodes, no further lithography steps are necessary. • We can selectively place CNTs separating them from contaminant species.

8. Nagahara’s issue …he can’t control the number of trapped nanotubes. SOLUTION: Choose a different electrode material! Now, we are going to describe a process to contact a single bundle of carbon nanotubes…

9. Getting the nanotubes… In order to obtain single-walled nanotubes we can use a laser ablation system. The next step is to purify the material from contaminant species.

10. Purification Process The purification is performed following a procedure that uses: • Acids • Centrifugal machine • Sonication • Filtering At the end of this process we obtain CNTs having -COOH functional groups associated with the acid treatment.

11. Substrate’s Preparation • The silicon surface is thermally oxidized • Electrodes are prepared with an e-beam lithography and lift-off technique. …Both Ag and Au electrodes are prepared using this method.

12. Experimental Setup The structure has been bonded onto a chip carrier and wired up as sketched in the figure. The circuit is powered by a frequency generator with a dc-offset voltage. The electric current is monitored by the ac or dc voltage.

13. Let’s trap them… • Switch on the frequency generator • Apply a drop of nanotube suspension • Wait a minute … A single bundle of carbon nanotubes trapped on four Au electrodes

14. Frequency and Alignment In order to have an excellent alignment, it is necessary to use frequencies above 1kHz. This fact is due to effects of the suspension. In fact, at lower frequencies ions act as an electric shield, not allowing CNTs to align precisely.

15. What’s the Number of Trapped Bundles ? The control over the number of trapped bundles depends on the electrode material. • Au electrodes: N increases with time. • Ag electrodes: only a very few bundles are trapped indipendent of time and concentration.

16. What’s the difference? Looking at the previous figure, we can see that the two-terminal resistence is lower when using Ag instead of Au electrodes. Therefore: Lower resistence Lower voltage Electric field collapses No more CNTs are trapped

17. Resistence Measuraments The previous measuraments have been performed after removal of the solvent. BUT: WHAT HAPPENS IN THE PRESENCE OF THE SOLVENT? To answer this question, we need other measurements.

18. Wet Resistence Measurements After adding a small DC offset, the VDCis measured giving the following results: We can see that CNTs form an electric contact with Ag in the presence of the suspension, while they don’t with Au.

19. A Chemical Explanation The origin of this fact may be found in the strong affinity of the –COOH groups of acid-treated nanotubes to Ag surface. • In fact, it’s known that n-alkanoic acids form a SAM on Ag surfaces. • On the other hand, they have no affinity to Au surfaces.

20. Conclusions It is possible to control the number of trapped bundles because it depends on chemical bonds. Such chemical properties can be controlled using different electrode materials.

21. To Be Done... Try to repeat the experiment using isolated carbon nanotubes ( not bundles )!!! This method uses acid-treated nanotubes… …what about different treatments?

22. Other methods… Several methods have been proposed to position carbon nanotubes, but none of these has been efficient enough to supersede the others. For example, we have found that at least 38 people will speak about CCVD at the “International Conference on the Science and Application of Nanotubes” that will begin in July.