Semiconducting Carbon Nanotubes: low-cost Separation for Flexible Electronics – NanoTSConde

The preparation of the semi-conducting SWNT material is based first on the solubilization of SWNT in high quantity, and second on the coupling of SWNTs with diazoether in aqueous solution, according to our patented process. The metallic SWNTs in the mixture get functionalized with aryl groups covalently coupled, which makes them non conducting. The chemical reaction has been thoroughly studied to get a high control and reproducibility on the reaction as well as a way to adapt the grafted aryl.
The solutions are then used for deposition on substrate, either by water thin film or spray or inkjet. THe high concentration of the solutions we prepare is a great advantage in the formulation of inks and the deposition in a single step.
The semi-conducting character of the material is evaluated in transistor devices, either in low cost electronics or in high frequency transistors on plastic. The microfabrication of the devices is under way (UV lithography and metal evaporation for contacts, spray through meccanical masks for SWNT and gate dielectric).

The preparation of SWNTs in solution at high concentration has been demonstrated. The solutions are obtained by a high scale/low cost process (concentrations in the range 0.3-1g/L). The SWNT purity is high and the surfactant concentration is maintained low, so that the concentration in SWNTs in the dry material is high (5 to 35%). This is an interesting point for spray and inkjet printing, even for the preparation of conducting films (before the diazoether treatment).
The diazoether/SWNT coupling reaction has been studied in detail (absorbance, luminescence, Raman, conductivity, XPS) to extract the mechanism and control the selectivity. This work has just been published (L. Darchy et al., Carbon 2013).
The SWNT solutions have been prepared and formulated for spray. Spray deposition has been optimized for conductivity, homogeneity and repeatability.
The compatibility of the chemical treatment with the microfabrication processes has been verified, in particular the resistance to high temperature and metal evaporation.
Dimensioning tests in transistors have been realized to optimize the high frequency device design.
A strategy for the microfabrication of low cost transistors on plastic has been designed according to the former results, in particular an adapted top gate. The microfabrication means have been designed and ordered (masks, positioning patterns).

A study for the physical separation of functionnalized m-SWNT and sc-SWNT after coupling has been started. The chemical structure of the diazoether and the role of cosolvents has been studied. This did not provide a satisfactory separation method yet. This work will be pursued in 2014 by a hired postdoc.

The project has to date produced a paper in the peer reviewed journal Carbon (impact factor 5.9): A highly selective non-radical diazo coupling provides low cost semi-conducting carbon nanotubes
Léa Darchy, Nassim Hanifi, Fabien Vialla, Christophe Voisin, Pierre-Alain Bayle, Luigi Genovese, Caroline Celle, Jean-Pierre Simonato, Arianna Filoramo, Vincent Derycke, Pascale Chenevier Carbon, 2013.

The work was also presented in 4 international conferences through one oral communication and 3 posters, among which one was awarded the best poster price. The work was also presented in a national conference by an oral communication.

The initial french patent has been extended to an international patent: WO 2012/168899 « METHOD FOR SELECTIVE FUNCTIONALIZATION OF SINGLE-WALLED CARBON NANOTUBES » .

Single wall carbon nanotubes (SWNT) are produced as a mixture of metallic (m-SWNT) and semiconducting (sc-SWNT) nanotubes, whatever the growth process. In applications using sc-SWNTs, the presence of m-SWNTs leads to damageable shortcuts, precluding industrial development in electronics so far. SWNT sorting is recognized as a major challenge and several beautiful solutions have been published since 2000. Yet commercial separated SWNTs are available only at high price and low concentration, incompatible with such applications as low cost printable electronics or organic photovoltaic. A high throughput low cost process is still demanded.

A chemical differentiation process patented by partner IRAMIS in 2009 destroys m-SWNT conductivity through a selective chemical functionalization. A mixture of SWNTs after differentiation can therefore be directly used as a source of sc-SWNTs, for example for transistor fabrication. The aim of the present project is to optimize and scale up the differentiation process and to demonstrate its utility in electronic applications. Indeed, SWNTs are of particular interest in organic electronics because they show low operation/ threshold voltages and high mobility, compatible with high frequency operation needed in telecommunications, because SWNT devices are transparent and flexible and finally because they are compatible with organic high throughput, low temperature and low cost processes like printing or roll-to-roll.

Differentiated sc-SWNTs will be made at the 0.5g scale and the process will be optimized through transistor performance feedback. Sc- and m-SWNTs separation will also be studied. Two electronic demonstrations are targeted in the present project: printed electronics on plastic, and high frequency flexible electronics. First, series of sc-SWNT transistors will be fabricated on plastic support by spray, then by ink jet printing and the project will end up with the design of a logic circuit demonstrator. Second, sc-SWNT transistors for high frequency operation will be made on flexible, plastic substrate with the differentiated sc-SWNT material. The project also plans to explore other uses for differentiated sc-SWNTs by making the material available in large enough quantity for other projects in such domains as energy, sensing and photonics.