In situ charge-transfer-induced transition from metallic to semiconducting single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are regarded to be potential building blocks for future electronics, because of their exceptional electrical, physical, and mechanical properties. A major obstacle to their practical use for high-performance nanoelectronics is the presence of both metallic SWNTs (M-SWNT) and semiconducting SWNTs (S-SWNT) in as-grown SWNT samples. Most metallicity-based SWNT sorting techniques involve suspension of the nanotubes in solutions, which generally also result in nanotube defects. As-grown SWNTs typically have far superior conductivity or carrier mobility than solution-suspended SWNTs; however, thus far, there is no simple or reproducible method to remove the electronic inhomogeneity in these as-grown nanotubes. We present a simple in situ method using an organic electron-acceptor compound, Acid Yellow (AY), to convert SWNTs from metallic to semiconducting to improve device field-effect behavior. By simply immersing the as-synthesized SWNTs (still attached to a wafer) into an AY solution, the originally metallic nanotubes behave similar to semiconducting ones. Using Raman spectroscopy, atomic force microscopy and single nanotube transistor device measurements, we show that the charge-transfer interaction between SWNTs and the organic electron-acceptor compound AY is diameter-dependent; in the large-diameter regime (1.60-2.60 nm), modulated metallic SWNTs exhibit semiconducting behavior, as evidenced by a pronounced field effect in the current-voltage (I-V) characteristics and up to 3 orders of magnitude increase in the on/off ratio of single M-SWNT field-effect transistors. This method presents a simple viable route toward the fabrication of switchable transistors with as-synthesized SWNTs.