Carbon nanotubes (CNTs) are going to be very useful materials in the future thanks to their excellent optical and electronic properties with potential applications that include transparent electrodes for touch screens, flat panel displays and solar cells. Unfortunately, as-made carbon nanotubes cannot be used directly in such devices because their electrical resistance is too high and their electrical conductivities too low.

Happily, the electronic properties of carbon nanotubes can be tailored by doping, which increases the number of free charge carriers (electrons and holes) in the materials. Doping also reduces the resistance of tube junctions as it allows carriers to pass more easily between metallic and semiconducting CNTs. (As-made CNTs typically contain a mix of metallic and semiconducting tubes – a problem that researchers are also actively working on).

However, there is a hitch: the most commonly used doping methods, such as oxygen adsorption (which p-dopes CNTs) and partial chemical oxidation, can often be difficult to control and can produce significant numbers of defects in the materials that ultimately reduce electrical conductivity even further. Other dopants, such as alkali metals and halogens, and acids and nitrates, produce tubes that are subsequently unstable in air even though the electronic and optical properties of the tubes remain good.

Zhenan Bao and colleagues are now saying that thermally-activated molybdenum oxides (MoOx) may be ideal dopants for CNTs. “We have discovered that certain processing conditions – in particular those involving a heating step at around 450 °C – drive charge transfer between MoOx and CNTs,” team member Sondra Hellstrom told “Once this charge transfer has been induced, the treated CNTs are then fairly insensitive to subsequent oxidation or even to reheating and retain their excellent electronic and optical properties.”

To demonstrate how good such heat-activated MoOx is for doping CNTs, the researchers showed that they could fabricate transparent conductors from the materials. These doped CNTs had DC-to-optical conductivity ratios of as high as 2.3.

“We hope that our doping process will be useful for making transparent nanocarbon conductors for thin-film electronics and in any other applications where strong chemical doping of CNTs is needed, as in transistor electrodes for example,” said Hellstrom.

MoOx could also be used to dope graphene, she adds.

The work is reported in Nano Letters.