  A field effect transistor (FET) measurement of a SWNT shows a transition from
a metallic one to a p-type semiconductor after helical wrapping of DNA. Water
is found to be critical to activate this metal-semiconductor transition in the
SWNT-ssDNA hybrid. Raman spectroscopy confirms the same change in electrical
behavior. According to our ab initio calculations, a band gap can open up in a
metallic SWNT with wrapped ssDNA in the presence of water molecules due to
charge transfer.
