The work function of palladium is known to be sensitive to hydrogen by the formation of a surface dipole layer or Pd hydride. One approach to detect such a change in the work function can be based on the formation of a Schottky barrier between the palladium metal and a semiconductor. Here, we study the hydrogen sensitivity of Schottky barrier field-effect transistors made for the first time from diameter- and chirality-sorted semiconducting single-walled carbon nanotubes (s-SWNTs) in contact with Pd electrodes. We observe an unrivaled 100-fold change in the on-state conductance at 100 ppm H2 compared to air for devices with s-SWNT and diameters between 1 and 1.6 nm. Hydrogen sensing is not observed for devices of Pd-contacted few-layer graphene (FLG), as expected due to the absence of a significant Schottky barrier. Unexpectedly, we observe also a vanishing sensitivity for small-diameter SWNTs. We explain this observation by changes in the nanotube work function caused by spillover and chemisorption of atomic hydrogen onto small-diameter nanotubes. We also observe that long-term sensing stability is only achieved if the gate voltage is inverted periodically. Under constant gate bias, the sensitivity reduces with time, which we relate to gate screening by accumulated charges in the substrate.

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Recent success in the high-purity separation of semiconducting single-walled carbon nanotubes (sSWNTs) allows for the development of large-scale nanotube electronics. Here, it is demonstrated that alternating-current dielectrophoresis can assemble high-density, multidirectional arrays of sSWNTs. A 99% pure solution of arc-discharge sSWNTs (mean diameter of 1.4 nm), fractionated by density-gradient ultracentrifugation, is used in the process because it has been shown to have the most desirable performance for nanotube electronics. Annealing is used to degrade the surfactant after assembly, resulting in reduced nanotube-metal contact resistance and higher on-off ratio. Devices fabricated on a hydrophobic parylene surface show ambipolar transport and reduced hysteresis in the transfer characteristics. Together, these steps lead to scalable, CMOScompatible integration of ambipolar sSWNT devices.SWNTs form a family of versatile quasi 1D systems, [ 1 ] exhibiting electronic structure ranging from zero bandgap metal to semiconducting with bandgaps up to 1.5 eV. [ 2 ] Carbon nanotubes (CNTs) have been shown to outperform conventional copper interconnects and silicon fi eld-effect transistors (FETs) as individual devices. [ 3 , 4 ] As a result, high-performance all-nanotube electronic circuits could be fabricated from metallic SWNT interconnects and semiconducting SWNT transistors. The practical realization of such circuits, however, is hindered by limited available routes for large-scale integration [ 5 , 6 ] and the polydispersity of as-grown SWNTs. SWNTs are typically synthesized as a mixture of multiple chiralities, with varied electronic properties due to strong structure-property correlation, [ 7 ] i.e., their bandgap is related directly to chirality. The ( n , m ) notation of chirality is used here to identify SWNTs. Individual SWNTs have been directly grown with desired location and orientation by an in situ electric-fi eld, [ 8 , 9 ] gasfl ow, [ 10 ] or substrate-mediated alignment. [ 11 ] While some control over growth conditions has been achieved in limiting the diameter distribution of the SWNTs and consequently the possible chiralities, [ 12 , 13 ] the polydispersity still remains. This, coupled with the high growth temperatures required, which are incompatible with CMOS technology, rules out directly growing chirality-specifi c SWNTs at predetermined locations in an integrated circuit.The alternate approach is bottom-up assembly of a controlled number of SWNTs with desired electronic properties at specifi c locations and orientation. The challenge was defi ned by McEuen in 2000 [ 6 ] and has to be achieved to realize SWNT-based integrated microelectronic circuits. Signifi cant recent advances have been made in individually dispersed SWNT suspensions [ 14 ] and solution-phase sorting of SWNTs [15][16][17][18][19][20][21][22] by length, diameter, whether they are metallic or semiconducting, and even chirality; therefore, a combination of solution-phase sorting with advanced bottom-up assembly presents a pr...

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