Carbon nanotubes have shown promise as a possible medium for post-CMOS electronics, and some research projects have used them as conductors, resistors and semiconductors in novel approaches to transistor and circuit design. Although the tubes are versatile electronic components, integrating them can be a problem. Now researchers at Columbia University's Nanoscience and Engineering Center have come up with a promising approach by blending carbon nanotubes with organic molecules that naturally bind to carbon. The team reported successfully inserting several types of organic molecules and gave detailed test results for an application-specific organic molecule that changed its conductivity in response to pH, enabling that molecular transistor to act as a pH sensor. The researchers first positioned a carbon nanotube on a silicon substrate with metal contacts at each end. The tube was then covered with PMMA, a mask material used in lithography. Using high-resolution electron-beam equipment, they cut a 10-nanometer-long window in the PMMA. Oxygen plasma was then applied to vaporize the section of nanotube exposed in the window. The resulting gap is so small that it cannot be seen with an electron microscope. Even an atomic-force microscope has problems resolving the gap. Chemically, the oxygen plasma creates a particular molecular end structure on both sides of the gap consisting of attached carboxylic-acid molecules that narrow the gap to about 2 nm. These molecules bind readily to a wide variety of organic molecules, which makes the technique flexible in terms of different molecular operations.