Paper NS-ThA9
A Hydrogen-Sensitive Polymer Nanostructure with Reversible Conductance

Thursday, October 18, 2007, 4:40 pm, Room 616

There is considerable interest in the use of polymer nanostructures for flexible electronics and sensors. We have previously written poly(3-dodecylthiophene-2,5-diyl) (PDDT) nanostructures between gold electrodes using thermal Dip-Pen Nanolithography (tDPN). tDPN uses a heatable atomic force microscopy cantilever to directly deposit "inks" that are solid at room temperature. When a PDDT-coated tip is heated close to or above the PDDT glass transition temperature, PDDT flows from the tip to the substrate surface with molecular-scale order.¹ We have discovered a novel property of such PDDT nanostructures: their conductivity can be increased by more than five orders of magnitude (from <10^-4 S cm^-1 to 10 S cm^-1) by exposure to energetic electrons (5 keV). In contrast, spin-coated polymer thin films deposited on similar gold electrodes do not show any electron-induced conductivity. Interestingly, the electron-induced conductivity of the PDDT device reverts to a semi-insulating state by exposure to H₂, with a measurable change in conductance occurring following exposures as low as 6×10^-4 Torr·s. Repeated shifting between the high and low conductivity states is possible. Significantly, exposure to oxygen, argon, or ambient air does not affect the conductance, suggesting that PDDT nanostructure devices deposited by tDPN have potential as H₂ sensors. We propose a mechanism for the conductance modulation based on H desorption and re-sorption from the side-chains of the polymer. This mechanism is supported by a two terminal response theory based on Green’s-function-based Landauer-Büttiker multichannel formalism.

¹ Yang et al., J. Amer. Chem. Soc. 128, 6774 (2006).