3 were observed at least on 10 devices made from sample S and 3 devices from sample M (see the histogram in Fig. 2c). The electrical measurements unambiguously prove that sample S essentially consists of s-SWNTs, whereas sample M (and to greater extent L, not shown) contain s-SWNTs, m-SWNTs and probably impurities. At this point it is important to stress that the elec- trical, optical absorption and Raman measurements pro- vide strong evidence that sample S consist of s-SWNTs without detectable traces of m-SWNTs and impurities, within our detection limits. Ultracentrifugation (opti- mum conditions 250,000 g for 60 min) is the key tech- nique that lead us to the selective extraction of s-SWNTs from nanotubes powders with PFO as extracting agent in toluene solution. Note that the centrifugation conditions (9,000 g, 3 min) described by Nish et al. are inadequate to remove traces of m-SWNTS. 14 The present achieve- ment is extremely importance for both basic and applied research. Next, prototypical FET consisting of random network of SWNTs were fabricated by spin-coating the sam- ple S on Si/SiO$_{2}$ wafer with pre-patterned drain and source electrodes. Remarkably, such device exhibits very promising characteristics rarely observed simultaneously. The on-off current is around 10 $^{5}$, the on-current is I$_{ON}$ = 15 . 9 µ A, the transconductance is G$_{m}$ = 1 . 75 µ S ( G$_{m}$ = δI$_{D}$/δV$_{G}$ ) and the threshold slop is S$_{p}$ =3.15V/decade ( S$_{p}$ = δV$_{DS}$/δLog ( I$_{D}$ )). In addition, I$_{D}$ vs V$_{DS}$ exhibits a well defined linear regime ( δI$_{D}$/δV$_{DS}$ = − 0 . 2 µ A/V) and a saturation regime ( I$_{sat}$ = − 7 . 2 µ A). Similar results were observed on 10 devices, which were recorded using the same device structure but not necessarily the same density of SWNTs. The histogram in figure 2c shows that 5 devices are in the range R =5-5.5 and 3 are in the range R =3.5-4 (with R the on-off ratio defined as R = Log$_{10}$ ( I$_{ON}$/I$_{OFF}$ ))). Admittedly, the characteris- tics of our devices are relatively scattered, but we believe that this technical challenge can be surmounted and re- producible FET can be fabricated. Interestingly by plotting the data related to sample S and M, the figure 2c can be divided in 3 areas indicated with circles. On one side, high R is correlated to low I$_{ON}$ and G$_{m}$ , on the other side low R is correlated to high I$_{ON}$ and G$_{m}$ , and in between there is an interme- diate situation. Remarkably, I$_{ON}$ and G$_{m}$ decreases lin- early with increasing R . In substance, this result suggest that FET devices can be fabricated with desired trans- fer characteristics depending on the targeted application, by simply tuning the centrifugation conditions (in other words, the ratio semiconducting/metallic SWNTs) and the spin-coating conditions (density of SWNTs). To improve the performances of the FET devices, s- SWNTs in NMP (similar to sample S) were deposited on pre-defined location and orientation by dielectrophore- We thank all the members of the Nano-Carbon Team (AIST). N. Izard thanks the Japan Society for the Pro- motion of Science for financial support. In conclusion, we have demonstrated the selective ex- tract s-SWNTs, without detectable traces of m-SWNTs and impurities, from carbon nanotube powders using PFO as extracting agent in toluene assisted in particu- lar by ultracentrifugation (250,000g). We anticipate that this method will lead to pure s-SWNTs produced in siz- able quantities and formulated as a printable ”ink”. We have also demonstrated that s-SWNTs processable so- lutions can realize high-performances p-type FET with very promising characteristics: the on-off current ratio around 10 $^{5}$, I$_{ON}$ around 10 µ A and the estimated hole mobility larger than 2 cm $^{2}$/vs. We believe that the present achievement pave the way to the future develop- ment of FET and TFT based on s-SWNT. Admittedly, further study and optimization of both the materials and the devices are still necessary to met the industrial re- quirements. How our technique and the performances of our FET compare with previous reports? Fabrication of FET based on individual s-SWNTs are technically challeng- ing, and methods involving selective break-down of m- SWNT occasionally damage the entire device. 7,8 In con- trast, deposition of already prepared s-SWNT process- able solutions are compatible with FET and TFT tech- nology. The performances of our device are comparable to those made using density gradient technique, larger than the state-of-the-art solution processable polymers and organic molecules, but lower than amorphous Si and vacuum deposited Pentacene. 1,16 sis (DEP). 5,17 This technique leads to a relatively dense network of s-SWNTs oriented perpendicular to the elec- trodes eventually bridging the electrodes with a few nanotube-nanotube junctions (Fig. 3c). Here again, the device present relatively good characteristics: R =4.8, I$_{ON}$ =6.5 µ A, G$_{m}$ =1.7 µ S, and S$_{p}$ =2.98V/decade. Simi- lar results were observed at least for 3 devices (2 devices with R =4.8 and 1 device with R =3.6 as shown in fig- ure 3c). The estimated field effect hole mobility is larger than µ =2 cm $^{2}$/vs. The latter was calculated using es- tablished equations 7,8,16 with the following parameters L = 5 µ m (source-drain gap), W = 60 µ m (channel width), t$_{ox}$ = 500nm (gate thickness) and ϵ = 34 . 5 pF/m (permittivity of silicon dioxide). If we normalize I$_{ON}$ and G$_{m}$ to the width of the electrode W , than the perfor- mances of the FET made by DEP are exceeding (by a fac- tor 400 µ m/60 µ m) those made by spin-coating method. 2 M. J. Biercuk, S. Ilani, C. M. Marcus, and P. L. McEuen, 1 J. Jang, Materials Today, 4 , 46 (2006). Acknowledgments