  Single ion detection has, for many years, been the domain of large devices
such as the Geiger counter, and studies on interactions of ionized gasses with
materials have been limited to large systems. To date, there have been no
reports on single gaseous ion interaction with microelectronic devices, and
single neutral atom detection techniques have shown only small, barely
detectable responses. Here, we report the first observation of single ion
adsorption onto individual carbon nanotubes (CNTs), which, due to the severely
restricted one-dimensional current path, experience discrete, quantized
resistance increases of over two orders of magnitude. Only positive ions cause
changes, by the mechanism of ion potential induced carrier depletion, which is
supported by density functional and Landauer transport theory. Our observations
reveal a new single-ion/CNT heterostructure with novel electronic properties,
and demonstrate that as electronics are ultimately scaled towards the
one-dimensional limit, atomic scale effects become increasingly important.
